Energy from the Biomass: Watt Committee: report number 5

Published by: The Watt Committee on Energy Ltd 1 Birdcage Walk London SW1H 9JJ Telephone: 01–222 7899 This edition published in the Taylor & Francis e-Library, 2004. © 1979 The Watt Committee on Energy Ltd

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THE WATT COMMITTEE ON ENERGY

REPORT NUMBER 5

Energy from the biomass

A series of papers presented to Watt Committee Consultative Council and since extended for publication, with the verbal and written discussion.

JUNE 1979

Contents Page

ii

Foreword

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THE PROSPECT OF A BIOLOGICALPHOTOCHEMICAL APPROACH FOR THE UTILISATION OF SOLAR ENERGY Prof. D.O.Hall and Dr. J.Coombs

1

Extracts from Discussion

14

FUEL CROPS—AN ASSESSMENT OF THE UK POTENTIAL Prof. C.R.W.Spedding, Mrs. D.M.Bather and Dr. J.M.Walsingham

17

Extracts from Discussion

23

ENERGY FROM BIOMASS BY ANAEROBIC DIGESTION Dr. D.A.Stafford, Mr. R.Horton and Mr. D.Hawkes

25

Extracts from Discussion

34

BIOGAS PRODUCTION—AGRICULTURAL WASTES Dr. P.N.Hobson

37

Extracts from Discussion

43

FUEL ALCOHOL PRODUCTION FROM BIOMASS Mr. A.N.Emery and Dr. C.A.Kent

45

Extracts from Discussion

55

CONVERSION OF BIOMASS TO FUELS BY THERMAL PROCESSES Dr. B.W.Hatt and Dr. A.V.Bridgwater

61

Extracts from Discussion

67

DEPARTMENT OF ENERGY: SOLAR BIOLOGICAL PROGRAMME—BIOFUELS Dr. G.H.King

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Closing Comments

75

THE WATT COMMITTEE ON ENERGY

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THE WATT COMMITTEE ON ENERGY

Foreword The Watt Committee on Energy, which brings together on matters related to energy some 60 Professional and Scientific Institutions, is concerned with all aspects of energy generation, storage, distribution and utilisation. Its first formal task early in 1977 was to provide for the Department of Energy a detailed independent and objective review of the ACORD report on Energy R & D Strategy. This it was able to do because of the very wide range of expertise within its membership. This work highlighted new aspects of the need to develop alternative sources of energy both for the expected increase in demand, and against the time when fossil fuels, particularly oil and gas, become depleted.

They were chosen to cover the range of processes whereby solar energy is utilised by biosystems, and the energy subsequently obtained in a useful form from the intermediate biomass. The papers were prepared and presented at the December 1978 Watt Committee Consultative Council Meeting by the specialists involved in research into these subjects, under the chairmanship of Dr. J.H.Chesters, OBE, FEng, FRS, Chairman of The Watt Committee on Energy. The Working Group was helped very considerably by the advice and assistance of Dr. G.H.King of the Energy Technology Support Unit at Harwell, who also gave the summary paper at the close of the meeting.

Of the possible alternative sources of energy the renewable sources are attractive because they have minimum environmental impact. One of these renewable sources is biomass, i.e. organic material of vegetable or animal origin, the generation of which is initially dependent on photosynthesis.

The papers and subsequent discussion were believed to be of much wider interest than to The Watt Committee only, and following this idea, some papers have been extended further for this publication. Now with the agreement of the authors and especially of the Department of Energy, for whom much of the work reported is being carried out, the whole proceedings are presented in this report.

A small working group drawn from The Watt Committee Executive was set up with the task of assembling a panel of speakers to talk on the subject of “Energy from Biomass” to brief the general Watt Committee membership.

The Working Group wishes to express its sincere appreciation for the help given by all concerned with the production of the papers and the subsequent compilation of this report.

The chosen forum was a one-day meeting and thus the number of papers which could be presented was limited.

A.Cluer K.F.Scott P.A.A.Scott

Institute of Petroleum Institution of Civil Engineers Royal Institute of Chemistry

Working Group Members

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THE WATT COMMITTEE ON ENERGY

The prospect of a biological-photochemical approach for the utilisation of solar energy

Professor D.O.Hall

Institute of Biology Tate & Lyle Ltd

Dr. J.Coombs

MAIN CONTRIBUTORS TO DISCUSSION Dr. D.C.Goodacre

University of Reading

Professor A.L.Titchener

University of Aukland, New Zealand

Note:

At the December 1978 meeting. Dr. K.Langley kindly presented the paper in the absence of the authors.

THE WATT COMMITTEE ON ENERGY

The prospect of a biological-photochemical approach for the utilisation of solar energy Introduction

about half of the world’s population relies mainly on wood for their cooking (four-fifths of total household energy use) and heating (ii) supply statistics of non-commercial energy can be out by factors of 10 or even 100 times.

There are not many people nowadays who need reminding that our fossil carbon reserves—whether for fuel or chemicals—are the products of past photosynthesis. Here we discuss the possibilities of present day products of photosynthesis meeting the ever increasing demands for energy and chemical feedstock as oil and related byproducts decrease in availability and increase in price. Photosynthesis is the key process in life and as developed by plants can be simply represented as:

A particularly worrying problem has been the increase in CO2 content of the atmosphere (25% over the last 125 years) due to burning of fossil fuel. The magnitude of the problem will increase if coal becomes the major energy source during the next decade or so. Recently more attention has been paid to such problems so that some of the factors involved are now understood, although much more information is needed before confident predictions can be made as to the effects on the world’s climatic patterns. However, there is no doubt that the adoption of biological (and photochemical) systems for fuel production will help stabilize the atmospheric CO2 concentration since increased biomass production will result in formation of a larger temporary carbon sink.

In addition to C, H and O, the plants also incorporate nitrogen and sulphur into the organic material via lightdependent reactions (this latter point is often not appreciated). In the past photosynthesis has given us coal, oil and gas, fuelwood, food, fibre and chemicals. The relative emphasis placed on the use of these fixed carbon sources has varied over the years and will undoubtedly change again in the near future. For the last fifty years or so, with oil abundant and cheap, the products of presentday photosynthesis were mainly evident to the developed world as food. Now we need to re-examine and, if possible, re-employ the previous systems which also gave us fuel and fibre. But, with today’s increased population and standard of living, we cannot revert to old technology but must develop new means of utilizing present-day photosynthesis more efficiently.

Fortunately plants are very adaptable and exist in great diversity—they could thus continue indefinitely to supply us with a wide range of products. This paper deals briefly with some of the possibilities. Some, such as the utilization of wood, biological and agricultural wastes, and energy farming, could be put into practice immediately, whereas others may never become practicable. Since plant systems are diverse and adaptable they can be tailored to suit an individual country taking into consideration total energy available from the sun, local food and fibre needs, ecological aspects, climate and land use.

Each year plant photosynthesis fixes about 2×1011 tonnes of carbon with an energy content of 3×1021 J; this is about ten times the world’s annual energy use and over two hundred times our food energy consumption. The efficiency of use of solar energy may be only 0.2 to 0.3% overall, whereas agriculture may be about 0.5% efficient. It should be realised that these efficiencies represent stored reusable energy and not just the initial conversion efficiencies as so often quoted for other solar energy systems. Photosynthesis is of particular importance in the recycling of carbon, hydrogen and oxygen. All the atmospheric CO2 is cycled through plants every 300 years, all the O2 every 2,000 years and all the H2O every 2 million years. This magnitude and role of photosynthesis is often unrecognised, in part because we utilize only a small fraction of the total carbon fixed each year and in part because of the length of time of each cycle. However, any interference in this latter role by generation of pollutants, or drastic changes in the world’s pattern of vegetation—the destruction of tropical forests in particular—could have serious and long-lasting, perhaps irreversible, consequences.

In the short term these biological systems will be of greater significance in the developing countries. The socalled “second energy crisis” has arisen as a result of the diminishing supplies of non-commercial or traditional fuels such as wood, dung or straw. This has led in turn to increasing deforestation, damage to ecosystems and rural poverty in many developing countries. The prospects of fuels-from-biomass programmes are being considered in many developed and developing countries. It seems that one of the biggest difficulties in implementing such programmes is that its simplicity belies its credibility. In addition to the more direct biological conversions of solar energy by growing intact plants there are significant longer-term prospects for photochemical/photobiological systems which will produce gases (such as hydrogen), fixed carbon compounds and electricity. Research in practical aspects of such proposals has recently been rejuvenated and is proceeding well. Such systems have significant advantages over whole plant systems in terms of conversion efficiencies. Hence, if the practical problems can be solved, in vitro systems will play an important part in “solar energy for fuel” devices of the future.

It is often not widely appreciated that one-sixth of the world’s annual fuel supplies are wood fuel and that about half of all the trees cut down are used for cooking and heating purposes. In the non-OPEC developing countries, which contain over 40% of the world’s population, noncommercial fuel often comprises up to 90% of their total energy use. This non-commercial fuel includes wood, dung and agricultural waste and because of its nature is seldom thoroughly considered, e.g. (i) total wood fuel consumption is probably three times that usually shown in statistics and

Available Light Energy The annual mean surface global irradiance on a horizontal plane as mapped by Budyko1 is shown in Figure 1 (taken from ref.2). The mean amount of energy available on a horizontal plane is greatest on the continental desert 2

THE PROSPECT OF A BIOLOGICAL-PHOTOCHEMICAL APPROACH FOR THE UTILISATION OF SOLAR ENERGY

Figure 1

Annual mean global irradiance on a horizontal plane at the surface of the earth. (W/m2 averaged over 24 hours)

areas around latitude 25° north and south, and falls off both towards the equator and towards the poles. The highest annual mean irradiance is 300 W/m2 in the Red Sea area. Cloud in the equatorial region reduces the mean global irradiance considerably; for example, the annual mean irradiance for Singapore is almost the same as that for Central France. However, equatorial regions usually have very little variation in mean horizontal surface global radiation from season to season, in contrast with high latitudes where the summer/winter ratios are large. The mean annual horizontal surface irradiance for Australia is around 200 W/m2, for the US around 185 W/m2, and for the UK around 105 W/m2. While the UK does not have a particularly favourable radiation climate, across-world differences in annual totals are far smaller than may be thought. The main problem in the UK is the low mean global irradiance in winter as compared with summer. While the winter climate is less cold than in the US, the winter/ summer solar radiation difference is much greater.

content of 0.47 MJ/mol of CO2 and the energy of a mole quantum (einstein) of red light at 680 nm (the least energetic light able to perform photosynthesis efficiently) is 0.176 MJ. Thus the minimum number of mole quanta of red

Energy Used By Plants Not all the solar energy which reaches the surface of the globe can be used by green plants. In fact plants use radiation between 400 and 700 nm, the so-called photosynthetically active radiation (PAR). This PAR has variously been estimated at between 40 and 50% of the total solar irradiance (differences arise due to variations in the amount of radiation assumed to be absorbed during passage through the atmosphere). A typical spectral distribution curve is shown in Figure 2 (from ref.3). The overall practical maximum efficiency of conversion of solar energy into fixed carbon compounds over a prolonged period of time is approximately 5–6%. This figure is derived from present knowledge of the process of CO2 fixation, and the physical and physiological losses involved. Fixed CO2 in the form of carbohydrate has an energy

Figure 2 3

Solar spectral irradiance distribution

THE WATT COMMITTEE ON ENERGY light needed to fix one mole of CO2 in theory is 0.47/ 0.176=2.7. However, the two photo-reaction Z scheme of photosynthesis (Figure 3) requires at least 8 (probably 10) quanta of light to transfer the four electrons from water to fix one CO2. The theoretical CO2 fixation efficiency of light is thus 2.7/8 or 2.7/10 or about 30% of the PAR. However, further losses result from reflection, inactive absorbtion and transmission by the leaves themselves. As a result about 10 to 12% of the total solar irradiance which strikes the plant may be used to fix CO2. However, once again this value does not equal the true conversion efficiency, since the plants will not in general present a completely closed canopy to intercept the light. In general the maximum leaf

Figure 3

area index (LAI, ratio of area of intercepting leaves to total ground area under consideration) will be about 0.8. Further losses occur due to respiration for growth and maintenance energy in non-green parts of the plants and during periods of darkness. As a result of all these figures the final overall photosynthetic efficiency for higher land plants is probably between 5 and 6%. Under optimum field conditions values of between 4 and 5% have been reported (Table 1). However, these values are for short-term growth periods. In practice photosynthetic conversion efficiencies in temperate areas are typically between 0.5 and 1.3%,2 while values for sub-tropical crops may rise to between 0.5 and 2.5%. The total yields of dry matter production which can be expected

The Z scheme of photosynthesis 4

THE PROSPECT OF A BIOLOGICAL-PHOTOCHEMICAL APPROACH FOR THE UTILISATION OF SOLAR ENERGY Table 1 Some hight, short-term dry weight yields of crops and their short-term photosynthetic efficiencies (see ref.2)

photosynthesis. It has been estimated that in 1850 the CO2 concentration in the air was about 268 ppm;4 i.e. so far about 1.8×109 tonnes of carbon per annum has been added since that date. Present day fossil fuel use is at a rate of about 5×109 tonnes per annum which would increase the CO2 content of the air by an amount of 2.4 ppm per annum if all of it accumulated in the atmosphere. This is where the problems lie in predicting what in fact will happen in the future, since the measured annual increase is only about 1 ppm, corresponding to an increase in CO2 of the air of 2.3×109 tonnes each year, or only half of what might be expected. Hence, there is a balance of 2.7×109 t of fossil fuel carbon per year which must be accounted for. It has been calculated4,5 that the oceans could absorb between 3×109 and 4.1×109 t carbon/yr. Thus the carbon balance from fossil fuels would be quantitatively correct if there was no net present CO2 release by the biosphere, through for example destruction of the forests and the oxidation of soil organic matter. However, it has been suggested7 that there is a net release from the biosphere of from 4 to 8×109 t carbon/yr, although lower figures of 2×109 t or less have been suggested4,6. If the higher figures are taken then either the capacity of the ocean to absorb CO2 has been underestimated, or there are other terrestrial sinks which have not been recognized. The forest biomass has been suggested as a major sink. However, recent evidence suggests that forests are being removed at an ever-increasing rate. If the rate of clearing of tropical forests was about 1% of the standing stock per annum this alone would release 4.5×109 t of carbon. Replacement of these primary forests by agriculture, or secondary forests results in a much lower productivity and less carbon storage as biomass. On these assumptions the forests would not appear to be a sink for CO2. Increases in atmospheric CO2 have been postulated to have serious consequences on the earth’s climate. Doubling the preindustrial CO2 concentration in the air to about 540–590 ppm would increase global temperature by about 3°C. However, this increase would not be uniform. The increase at the poles would be in the order of 6 or 7°C, but that at the equator only about 1°C. Such temperature changes would result in profound changes in the pattern of vegetation, desertification and agriculture. Increases in CO2 concentration, temperature and rainfall would all benefit

under various sunlight intensities at these various photosynthetic efficiencies are shown in Figure 4. Present Photosynthetic Production As shown in Table 2, the present net photosynthesis produces energy at about 3×1021 J per annum which is about 10 times the world’s present annual use of energy of about 0.3×1021 J. Furthermore, the energy content of the 8×1011 tonnes carbon presently stored in the world’s biomass is nearly 100 times the world’s total energy consumption. The amount of carbon in biomass is approximately equal to the carbon present as CO2 in the atmosphere or that present as CO2 in the ocean surface layers. These figures indicate the relative importance of carbon cycling in the biosphere due to photosynthesis, and the related problems of CO2 accumulation in the atmosphere (a 25% increase since 1850) due to burning of fossil fuels over the last century or so. An estimate of the present global carbon balance, showing the main carbon deposits, and annual exchange of carbon into and out of the atmosphere, is shown in Figure 5. Since availability of CO2 is of prime importance in determining photosynthetic yield, in the tropics in particular, these figures will be considered in more detail. CO2 in the Atmosphere

Figure 4

The present-day carbon content of the atmosphere (7×1011 t) represents a CO2 concentration of 334 ppm. There is a winter to summer oscillation of 5 to 15 ppm due to 5

Expected annual plant yields as a function of annual solar irradiance at various photosynthetic efficiencies

THE WATT COMMITTEE ON ENERGY Table 2 Fossil fuel reserves and resources, biomass production and CO2 balances (Data calculated from Grenon, Woodwell, Stuiver, Boardman and Pimentel—see ref.29 for original reference)

plant productivity to varying degrees, but whether such effects would be regional or global is unknown.

organic acid oxalo acetic (OAA). The third type of plants are known as CAM species. This abbreviation stands for crassulacean metabolism and CAM type photosynthesis is found mainly in desert plants. (Crassulaceae are succulent plants like cacti).

The amount of fossil fuel which could be burnt if the CO2 concentration of the atmosphere is not to be raised more than 50% above that of the preindustrial era has been estimated8 and would permit present rate of consumption until the end of this century with only a 1 to 2°C global temperature increase.

As indicated later, more work needs to be done on the mechanism of CO2 assimilation by plants under the influence of light, but a brief summary of existing knowledge is given in the Annex at the end of this paper.

CO 2 Assimilation Present and Potential Crop Productivity

A key factor which determines the amount of biomass formed over a given area of land is the ability of the crop grown to assimilate CO2 from the atmosphere. It is now well established that higher plants can be divided into three groups which differ in their mechanisms of carbon fixation. Two of these groups are named on the basis of the number of carbon atoms which occur in the first product to be formed from atmospheric CO2. These are the C3 plants which form the three-carbon organic acid phosphoglyceric (PGA) and the C4 plants which form the four-carbon

The potential productivity of a given system will depend upon first the agricultural inputs and secondly the climatic and geographical characteristics of the area under consideration. In the first area of consideration five levels of increasing yield can be recognised:— a) Productivity in a natural ecosystem with no human influence.

6

THE PROSPECT OF A BIOLOGICAL-PHOTOCHEMICAL APPROACH FOR THE UTILISATION OF SOLAR ENERGY b) Production of crops in a labour-orientated agricultural system with no inputs from sources external to the farm, such as artificial fertilizers, pesticides, insecticides, mechanised land preparation and/or mechanical irrigation. c) Present day average agriculture, with varying technical inputs, some mechanisation, and reasonable soil and water management. d) Present day intensive farming on present cultivated land. e) Photosynthetic maximum production on all suitable land.

increased, as is the yield per person employed. But, at the same time the yield per unit of energy input drops considerably. In other words an increase in productivity has been achieved at the cost of the consumption of considerable fossil fuel energy. Hence, it would appear that to reach level e) may require too great an input to be feasible. Some predicted costs of producing a variety of forms of bio mass in the United States are shown m Table 3 a. Comparison of these figures with those for the final product used as a direct energy source (Table 3 b) indicates that farm production costs are a significant part of the overall costs, which in general are higher than those for the same energyproducts derived from conventional sources.

Level b) is typified by subsistence agriculture as found in many tropical countries and underdeveloped regions. It is characterised by an absence of cash inputs, low crop yields, decreasing soil nutrient content, shifting cultivation, often with destruction of forest followed by destruction of the soil; where production is increased this results from an expansion of the area cultivated with yields per hectare static or falling. In contrast at levels c) and d) yields are substantially increased by inputs such as inorganic fertilizers, chemical control of pests, mechanical cultivation and harvesting, irrigation where and when necessary, and proper storage and process facilities. As a result yields per hectare are

Figure 5

Table 4 shows some data for average crop production, present optimum values and calculated values for maximum rates of production in the USA.20 Further data of a similar nature can be found in references 2,21,22,23. This table shows quite clearly the higher yields obtained from C4 plants and emphasises the need for a full understanding of the mechanism of C4 photosynthesis. Although C4 plants give the highest yield per hectare, they are restricted in both their habit, and the areas in which they flourish. In order to

The global carbon balance sheet showing the main carbon deposits and annual exchange rates (simplified from refs.4,7) 7

THE WATT COMMITTEE ON ENERGY fully utilise all types of plant material on a world-wide basis a number of different approaches will have to be taken. Crops can be generally recognised as falling into one of three groups: a) lignocellulose crops, i.e. trees; b) crops producing a large amount of fermentable carbohydrate (starch or sugar) e.g. sugar cane, sugar beet, maize, cassava or potatoes, etc.; c) aquatic plants, fast growing water weeds, or algae. Various zones can be recognised throughout the world in which these options might have a preferential chance of development in the near future. The factors affecting the choice of any particular system include considerations of agricultural capacity, environmental factors, population density, labour intensity in the agricultural sector, and the energy demand per capita. In a simple classification the following areas can be distinguished.

Table 4 Photosynthetic productivity of various potential biomass systems.2,20 Tonnes/ha/yr

a) Temperate Industrial Areas (North America, Western Europe and Japan) In these areas biomass will only produce a small fraction of energy demand—in the short term it is predicted that emphasis will be placed on the production of speciality chemicals from biomass.24 Process and agricultural wastes may be used within a given industry to power its own activities (e.g. lumber mills, sugar factories, maize processing). The use of wood as a direct source of fuel is also possible.25 b) Tropical/sub-tropical regions of good soil and high rainfall (e.g. Brazil, India (parts), Africa (parts), Indo-China, North Australia) Energy from biomass is a real possibility with fermentation to form ethanoi and anaerobic digestion (biogas) also of importance supplementing more efficient use of quickgrowing wood species such as Leucaena.26

biological systems are possible if artificial environments are constructed. This could include nutrient film techniques under selectively permeable plastic covers.27 d) Marine and other aqueous regions (lakes, rivers, etc.) using fast growing water weeds, seaweeds or microalgae. Theoretically high productivity is possible, though not yet proven. In the short term the most likely use will be as feedstock for anaerobic digestion.

c) Northern (polar) and arid regions Here high solar irradiance may favour non-biological systems. However, Table 3 a) Energy farm biomass predicted production costs— USA (1976 $) (106 BTU=109 J)

The Biomass Concept The general concept of harvesting the sun’s energy through photosynthesis is receiving considerable attention

Figure 6

8

The pathway of photosynthetic carbon fixation into various products

THE PROSPECT OF A BIOLOGICAL-PHOTOCHEMICAL APPROACH FOR THE UTILISATION OF SOLAR ENERGY worldwide.28 Recent programmes have been discussed in the UK, Ireland, France, Germany, Denmark, Sweden, USA, Canada, Mexico, Brazil, Australia, New Zealand, India, Philippines, Thailand, Israel and China. The following advantages of biomass have been identified; a) systems can store energy in a form usable at will; b) renewable; c) can be produced with available state-of-the-art technology; d) can be developed with present capital, manpower and material resources; e) ecologically inoffensive—in general, although problems of fire hazard and disease should not be minimized if large wide-spread monocultures are used; f) does not add to atmospheric CO2. The most obvious disadvantages or problems are a) competition for land use and food production; b) the large land areas required; c) supply uncertainties in the initial phases; d) fertilizer, soil and water requirements; e) the diffuse nature which results in high collection costs and f) in many regions its seasonal variation.

The major sources of plant material and possible conversion technologies are shown in Figure 8. This will not be discussed further since the various options are the subjects of following papers in this symposium. However, two points should be noted; a) the source of material may be either purpose grown or wastes resulting from agricultural or food industries; b) the treatment of the material and the nature of the product depends to a large extent on the water content of the original biomass. The estimated costs of producing fuels by some of these routes is shown in Table 5. These figures are discussed in more detail in references 25,29,30.

Future Possibilities

Algal systems The idea of using algae and bacteria in solar energy systems is not new but has received more serious attention over the last five or ten years.32 Many liquid and semi-solid wastes from domestic, industrial or farm sources are ideal for the growth of mixed cultures.33,34 Under good conditions rapid growth with about 3 to 5% solar conversion efficiency can be obtained. Although the algae may be harvested and fed to animals, fermented to produce methane, or burnt to produce electricity, at present most schemes are designed for waste disposal and/or water purification. It is estimated that such algal systems are half to three quarters as expensive as conventional waste disposal systems in California. A schematic drawing of an algal-bacterial pond system is shown in Figure 9.

Conversion Technology

The main economic problem is harvesting costs, but the development of new techniques and the use of easily harvested algae are decreasing these. Harvested biomass can be fermented to methane (equivalent to 11.6 MJ/kg algae) while the residues would contain virtually all the

Figure 7

The C4 cycle showing the four partial reactions

Figure 8

Sources of biomass, possible conversion technologies and fuel products 9

THE WATT COMMITTEE ON ENERGY Table 5 Estimated biomass and energy product costs (various countries)

fixation in C3, C4 and CAM plants; b) Factors controlling photo-respiration; c) Genetic engineering, using plant cell tissue cultures and protoplasts; d) Factors in bioproductivity; e) Multiple test analysis systems for field use; f) Plant selection and breeding to overcome stresses due to drought, temperature extremes and salinity; g) Regulation of production of specific end-products, e.g. fibre, starch, or protein; h) Selection of plants and algae with useful products or by-products, e.g. fats, oils, glycerol, waxes, pigments; i) The contribution of symbiotic and free living nitrogen fixers (blue green algae) to soil fertility.

Synthetic systems Many of the limiting factors which restrict photosynthetic yield are associated with the needs of maintaining an intact plant or contributing to its growth and reproduction. It should be possible to construct artificial systems which mimic certain parts of the photosynthetic process and so obtain a higher efficiency of solar energy conversion, perhaps approaching the 10–12% efficiency of the photochemical side. The unique reaction catalyzed by plants is the splitting of water by visible light to produce oxygen and protons (hydrogen). In the intact plant the protons are used in reactions mediated by the nonheme iron-sulphur protein ferredoxin, and NADP to reduce CO2. However, under some conditions two protons can be brought together to produce hydrogen gas. In order to discuss this in more detail, some aspects of the light reactions must be considered. The production of NADPH, ATP and O2 in the lamellae of the chloroplast involves the transfer of electrons through a chain of electron carriers as shown in Figure 3. This electron transfer requires that each of the carriers in return becomes reduced and then oxidized in order that the energy can be passed along the chain. The oxidationreduction (redox) potentials of biological electron carriers are expressed on a voltage scale at biological pH which gives the H2O:O2 couple a positive potential of +0.82 V, while the H+:H2 gas couple has a negative potential of -0.42 V. In the process of photosynthesis, electron transport takes place between these two extremes, light providing the energy which is required. The formulation shown in Figure 3, now generally accepted and known as the “Z” scheme, requires two distinct light reactions to raise the electrons from the level of water (+0.82 V) to the level of NADPH (0.34 V). In the intact chloroplast, photosystem I will produce electrons of considerably lower potential which are diverted to the iron sulphur protein ferredoxin, which in turn will reduce the pyridine nucleotide. Certain algae and bacteria contain an enzyme known as hydrogenase which will

phosphorus and nitrogen of the algal biomass, so providing good agricultural fertilizer. One unit of algal pond will provide the fertilizer required by 10 to 50 units of agriculture. By optimization of yields and including energy inputs and conversion losses a net production of 520 GJ/ ha.yr of methane appears feasible,25 at a 30° latitude this would represent a 1.5% annual photosynthetic conversion efficiency. The cost of methane so produced was calculated at US $ 2.6 to 3.9/GJ in 1976. Although these costs are high they do not take into account the benefit value of waste treatment and by-products. It has been estimated that complete municipal waste treatment could provide about 5% of the USA methane usage. In California33 average yields of algae in excess of 100 kg dry weight/ha.day are obtained, with a peak summer production of about three times these figures. Yields of 50– 60 tonnes dry material per year would produce 74000 kWh of electricity, furthermore 4 million ha of algal pond systems could produce all the US protein requirements, as compared to 121 million ha of agricultural land for the same protein production by conventional means.

Plant selection A particular task for the plant breeder is to try and select, or breed by genetic manipulation, plants which will give higher yields of biomass whether it be for fuel, fibre, chemicals or food within an agricultural system in which the input/output energy ratio is decreased. More attention needs to be paid to studies of whole plant physiology and biochemistry and in particular their interaction with environmental factors. In particular the following areas are in need of study: a) Mechanism of photosynthetic carbon

Figure 9

10

Production of algal biomass in an algal/ bacterial pond

THE PROSPECT OF A BIOLOGICAL-PHOTOCHEMICAL APPROACH FOR THE UTILISATION OF SOLAR ENERGY

Figure 10 The coupling of solar energy to hydrogen production using stabilized chloroplast membranes and hydrogenase accept electrons from ferredoxin and catalyse the formation of hydrogen gas from protons. Such hydrogen production has now been demonstrated in both algal and in vitro systems formed by combining chloroplasts, hydrogenase and ferredoxin isolated or purified from various organisms,35 as shown in Figure 10.

hydrogen production is specially separated from oxygen evolution. The great interest in biophotolysis systems derives from the facts that they are the only energy systems currently known to have the following characteristics: a) the substrate (water) is ubiquitous; b) the driving force is unlimited (the sun); c) the product (hydrogen) is storable and non-polluting. There are problems of stability in the living systems which would need to be overcome before any biologically based system could be practical. Progress has been made in identifying stable hydrogenase enzymes from photosynthetic bacteria. However, the ultimate aim of one line of research is to use a completely synthetic system which would mimic the algal or plant based system. In this case an Fe-S catalyst would replace the hydrogenase, a chlorophyll layer or vesicle would be used instead of the chloroplast, and a manganese catalyst would evolve O2 from water.

Hydrogen Production The term “biophotolysis” has been used to describe photosynthetic systems which split water to produce hydrogen gas. This term has been applied to both living systems, such as algae, and to in vitro systems derived from a combination of isolated chloroplast membranes and enzymes. It has been known for over 40 years that green algae have the ability to evolve H2 gas in the light. Bluegreen algae may also evolve H2 from the specially adapted heterocysts (which usually catalyse the reduction of nitrogen gas to ammonia) in the absence of N2. Research in this field was stimulated by the first observation36 of hydrogen production in an in vitro system, in which water was the electron donor. The ability of illuminated chloroplasts to evolve H2 gas in the presence of hydrogenase had been demonstrated much earlier.37 However, in these early experiments only photosystem I operated with cysteine used as the electron donor, coupled with the electron transport chain through a dye. When the initial results of the in vitro system capable of producing H2 from water were first reported there was some doubt as to the production of O2 gas. Since then a number of workers38,39 have shown that an active photosystem II is required and that O2 is evolved. It is now routinely possible to evolve H2 for many hours at 50 µmole H2/mg chlorophyll/ hr. One problem with such systems is that hydrogen and oxygen gas are both produced at the same time. It is now known that the intermediates of the photosynthetic electron chain are arranged across the chloroplast membrane as shown in Figure 11. Hence, there is the long-term possibility of orientating membranes in such a way that

Economics of Photochemical Systems

(See Review by Bolton & Hall3) There have been very few studies of the economics of photochemical solar energy conversion and storage, possibly because the field is quite new and also because as yet very few systems have been shown to work with reasonable efficiency and longevity. Harrigan has made a limited study and concludes that the photochemical production of fuels could be economic only in a hybrid system which also provided thermal energy. Mattux has examined some of the materials costs which are applicable to almost any kind of collector, but he does not consider photochemical systems specifically. It is possible to put some limits on the capital costs of a photochemical collector. Depending on the location, the average (365 day—24 hour average) solar irradiance on a south-facing collector at the optimum angle for the latitude will be between 200–400 Wm-2. If the energy conversion and storage efficiency is assumed to be 10%, then such a 11

THE WATT COMMITTEE ON ENERGY collector will store energy as a fuel at a rate of 20–40 Wm-2. If the fuel is hydrogen this corresponds to 60–120 m3 of hydrogen produced per m2 of collector per year. The 1978 price of hydrogen was approximately $80 per 1000 m3. Thus, the value of solar generated hydrogen which might be produced per m2 per year is $4.80 to $9.60. This income could perhaps justify a capital cost of $20–$40 per m2 of collector but not much more.

photogalvanic (photoelectrochemical) system is the key to all photosynthetic processes. It is possible that it could also be used for the direct production of electricity. Artificial chlorophyll-containing membrane bilayers, vesicles and plasticized particles have been investigated and some shown to produce both charge separations and electrical current, although at present efficiencies and stabilities are low.

This analysis agrees with one carried out by Pearlstein in 1974 when an inflation factor is built in. This crude economic analysis points out that the system must be simple and should operate as a single unit producing H2 and O2 separately. A photolysis system can be envisaged to meet these requirements (whereas the photovoltaic or concentration system plus electrolysis cannot) but the stability problem of photolysis has yet to be overcome.

Carbon Reduction

In vitro systems which emulate the plant’s ability to reduce CO2 to the level of carbohydrate are also of great interest. Recently both the formation of keto-acids and amino acids using an alkyl-mercaptan and Fe-S protein analogue with an inorganic reductant, and the photochemical reduction of CO2 to formic acid, formaldehyde and methanol has been reported.

If thermal collection is combined with fuel production, then a capital cost of perhaps $200–$300 per square metre might be permitted. This emphasizes the importance of considering hybrid systems. It is very difficult to make economic assessments in the absence of systems which work with reasonable efficiency in the laboratory. Clearly, the first priority must be to establish scientific feasibility for photochemical conversion and storage systems and then assess critically the economic questions.

Conclusion In conclusion it should be remembered that photosynthesis is the key process in the living world and will continue to be so for the continuation of life as we know it. The development of photobiological energy conversion systems has long-term implications from points of view of food, energy, fibre and chemicals. Present systems may be more applicable in the short term to tropical and sub-tropical areas and countries. However, in general these countries lack the abilities to develop such systems. Whatever systems are developed in the temperate climate zones will work that much better in the tropics. Thus the temperate countries may be able to become more self-sufficient, but

Chlorophyll Systems The chlorophyll containing membranes of all photosynthetic organisms are able to separate positive and negative charges on either side of the membrane under the influence of light as shown in Figure 11. This basic

Figure 11 The distribution of the photosynthetic electron transport chain within the chloroplast membrane 12

THE PROSPECT OF A BIOLOGICAL-PHOTOCHEMICAL APPROACH FOR THE UTILISATION OF SOLAR ENERGY perhaps of greater importance should be able to assist the development of less developed countries by not competing unnecessarily for their fuel, food and raw materials.

Both the C4 plants and CAM species have a second mechanism for assimilating CO2 from the atmosphere based on the carboxylation of a second acceptor molecule phosphoenol pyruvate (PEP). Carbon is fixed in the cytoplasm13 and transported to the chloroplasts in the form of a 4C organic acid. In the chloroplasts of CAM plants or the bundle sheath of C4 plants18 the fixed carbon is released as CO2 and then reassimilated through the normal PCR cycle.

ANNEX Mechanism of CO2 Assimulation by Plants

The literature concerning C4 photosynthesis has been complicated by the fact that a number of different variations exist.14,15,16 Early attempts to put all results obtained from biochemical investigations into a single scheme have led to a proliferation of complex diagrams which have tended to obscure the simplicity of the actual C4 cycle17 which is outlined in Figure 7. This pathway is similar to the PCR cycle in that it consists of a carboxylation step followed by a reduction (both driven in the direction of net synthesis using energy from the light reactions of photosynthesis). However, there is no net gain of CO2. In other words the C4 cycle does not act as a photosynthetic pathway in the true sense, but rather functions as a pump, moving CO2 from the atmosphere to the chloroplasts. As a result the CO2 concentration at the carboxylation site of the PCR cycle is increased, hence the inhibitory effect of oxygen is decreased.

The process of photosynthesis in which CO2 is converted to carbohydrates (sugars) is the reverse of respiration; the same cells in the growing leaves will carry out both processes. In order to allow this to occur in an efficient manner, the competing reactions are separated in specific compartments or organelles. Those organelles in which photosynthesis takes place are called chloroplasts and those in which respiration occurs are known as mitochondria. Both organelles possess an outer envelope and an inner system of membranes. The inner membrane system is particularly complicated in the chloroplast, providing a large surface area for the light-absorbing chlorophyll molecules. The components of the electron transport chain are located within these membranes, whereas most of the reactions of carbon metabolism are catalysed by water-soluble protein enzymes located between the membranes. In addition, other enzymes such as those responsible for synthesis of sucrose itself from simpler sugars, are located as soluble protein in the matrix of the cell known as the cyfoplasm, i.e. not enclosed in membranes or organelles.

As a result of the addition of the C4 cycle the C4 plants are more efficient at assimilating CO2 and thus produce higher yields of dry matter per land area than do C3 plants. In addition they are more efficient at conserving water. For instance, maize may only lose (transpire) 370g of water from the leaves per g dry matter produced whereas temperate C3 cereals may require over 500g of water per g dry matter produced.19

It is now well established that all known green plants and algae which are capable of splitting water possess the photosynthetic carbon reduction (PCR) cycle9 as their major route of net carbon assimilation. In this pathway the initial reaction is the carboxylation of ribulose-bis-phosphate (RBP), resulting in the formation of two molecules of phosphoglyceric acid (PGA). Energy and a reducing compound formed by the light reactions of photosynthesis are used to reduce this acid to a sugar from which a variety of cell components can then be formed (Figure 6). Those plants which have only the primary PCR cycle are known as C3 plants.

CAM plants also operate a similar secondary carboxylation reaction leading to formation of C4 acids. However, in these desert plants this additional reaction serves as an adaptation for water conservation. These plants fix CO2 in the dark into acids. In the light the stomata (pores in the leaf surface through which gas exchange takes place) close, thus preventing excessive water loss. The acids are then broken down releasing CO2 within the leaf which is then refixed by the PCR cycle.

The ability of C3 plants to assimilate and retain atmospheric CO2 is limited by the fact that carbon fixation may be inhibited by oxygen.10 This inhibition is associated with a loss of some of the already fixed carbon as CO2 formed during the process of photorespiration.11 Oxygen inhibits photosynthesis by competing with CO2 at the binding site on the enzyme which catalyzes the initial carboxylation reaction so that either of the following reactions may occur:12

References 1. Budyko, M.I. The heat balance of the earth’s surface. Eng. Trans. N.Stepanova. US Dept of Commerce Weather Bureau, Washington, D.C. (1958) 2. Anon. Solar Energy—a UK assessment. UKISES (1976) 3. Bolton, J.R. Science, 202, 705 (1978); Bolton, J.R., Hall, D.O. Ann Rev of Energy. 4 (1979) 4. Stuiver M., Science, 199, 253 (1978)

In the photorespiratory pathway two molecules of the twocarbon compound (glycollate) are used to form one molecule of PGA (thus conserving some at least of the fixed carbon) and the fourth carbon is lost as CCb. It can be estimated that at the present levels of CO2 in the atmosphere about 40% of the carbon fixed flows through the photorespiratory pathway resulting in a 10 to 15% loss in photosynthetic efficiency. Photorespiration is favoured by high light intensities, high partial pressures of O2, low CO2 concentrations and high temperatures. Hence, although one might assume that crop yields would increase as one moved towards the equator this may not be true for C3 plants.

5. Wong, C.S. Science, 200, 197 (1978) 6. Bolin B., Science, 196, 613 (1977) 7. Woodwell, G.M. Science, 199, 141 (1978) 8. Siegenthaler, U., Oeschger, H. Science, 199, 388 (1978) 9. Bassham, J.A., Calvin, M. The path of carbon in photosynthesis. Prentice-Hall, Englewood Cliffs, NJ. (1957) 10. Tolbert, N.E. Ann Rev Plant Physiol. 22, 45 (1971) 13

THE WATT COMMITTEE ON ENERGY tropics. National Academy of Sciences, Washington D.C. (1977)

11. Coombs, J., Whittingham, C.P. Proc. Roy. Soc. B. 164, 511 (1966)

27. Bassham, J.A. In International Workshop on Biosaline Research . San Pietro (ed). NSF report under grant INT 09541 to Indiana University (1978)

12. Siegelman, H.W., Hind, G. Photosynthetic Carbon Assimilation. Plenum Press, NY and London (1978) 13. Coombs, J. Proc. Roy. Soc. B. 79, 221 (1971)

28. Hall, D.O. Nature, 278, 151 (1979)

14. Coombs, J. In Barber, J. (ed). The Intact Chloroplast. Amsterdam, NY, Oxford (1976)

29. Hall, D.O. Solar Energy, 22, 14 (1979) 30. Hall, D.O. in ref 31

15. Gutierrez, M., Gracen, V.E., Edwards, G.E. Planta, 119, 279 (1974) 16. Hatch, M.D., Kagawa, T., Craig, S. Aust J. Plant Physiol. 2, 111 (1975)

31. St Pierre, L.E. (ed). Future sources of organic raw materials. Proc. CHEMRAWN conference, Toronto 1978. Pergamon (1979)

17. Coombs, J., Baldry, C.W., Bucke, C. in Perspectives in Experimental Biology Vol 2 Botany (Sunderland, N. ed), p 177. Pergamon, Oxford (1976)

32. White, J.W., McGrew, W. (eds). Clean fuels from biomass, sewage, urban refuse and agricultural wastes. Inst. Gas Tech., Chicago, III. (1976)

18. Hatch, M.D., Slack, C.R. Ann Rev Plant Physiol. 21, 141 (1970)

33. Beneman, J.R., Weismann, J.C. Koopman, B.L., Oswald, W.T. Nature, 268, 19 (1977)

19. Woolhouse, H.W. Endevour, 2, 35 (1978)

34. Shelef, G., Moraine, R., Bener, T., Levi, A., Oran, G. In Photosynthesis 77. (Hall, D.O., Coombs, J., Goodwin, T.W. eds). Biochem Soc., London (1978)

20. Bassham, J.A. Science, 197, 630 (1977) 21. Cooper, J.P. (ed). Photosynthesis and Productivity in different environments. Cambridge Press (1975)

35. Bolton, J.R., Hall, D.O. Ann Rev Energy, 4 (1979) 36. Benemann, J.R., Berenson, J.A., Kaplan, N.O., Kamen, M.D. Proc. Natl. Acad Sci. USA. 70, 2312 (1973)

22. Warren-Wilson, J. Maximum Yield Potential. In Transition from extensive to intensive agriculture with fertilizers. IPI, Berne (1971) 23. Westlake, D.F. Biol. Rev. 38, 385 (1963)

37. Arnon, D.I., Mitsui, A., Panaque, A. Science, 134, 1425 (1961)

24. Coombs, J., Righelato, R.C., Khan, R. and Vlitos, A.J. in ref 31

38. Rao, K.K., Rosa, L., Hall, D.O. Bchem. Bphys. Res Comm. 68, 21 (1976)

25. Hall, D.O. Fuel, 57, 322 (1978)

39. Fry, I., Papageorgiou, G., Tel Or, E., Packer, L. Nature, 32, 116 (1977)

26. Anon. Leucaena. Promising forage and tree crop for the

EXTRACTS FROM DISCUSSION

Dr. D.C.Goodacre

Theoretically, photosynthesis should trap some 6% of solar energy into carbon compounds. However, total yields are usually less than 1% with harvests approaching 0.1%. Factors depressing yields, particularly in the tropics, are poor control of agriculture, pests, diseases, and losses. Hence, research in agricultural and process practice is more important than that of photosynthesis per se. On the other hand, in the long term use of plant based photochemical systems could lead to use of hydrogen gas as a fuel. However, this represents a distant prospect.

At present, it seems that three biomass systems approach practical applications:—a) growth of trees (wood)—followed by combustion, pyrolysis or hydrogenation; b) growth of C4 panacoid grasses (cane, maize, sorghum) for ethanol production by fermentation; and c) anaerobic digestion for methanol production from wastes or algae. These could provide electricity, liquid or gaseous fuel. However, their contribution to the energy used is limited in particular by considerations of land area per capita, length of growing season and energy requirement. Hence several different areas can be recognised, i.e. developed high population (Europe), developed large land area (USA), developing—with no water problem (tropical) or developing—with water problems (deserts).

Chairman It is known that various plants display differences in efficiency of converting solar energy into biomass. Sugar cane is regarded as one of the most efficient plants in this respect. Might it be possible to find other species which display an even higher efficiency and thus give a higher yield of biomass per unit land area. Could there for example be some obscure grass, yet to be discovered, which has this characteristic?

Obviously, the key factor is energy trapping per area cultivated, which in turn relates directly to the process of photosynthesis. It is only in the tropical countries with a high irradiance and low per capita energy requirement that biomass energy could contribute a significant percentage of the energy used. Even here, the most favourable systemsugar cane to ethanol—gives only a slight net energy gain. In the developed countries such as the USA, the maximum contribution of biomass might be 4% of the total energy use, and in Europe even less. In these regions biomass may be used for production of speciality chemicals.

Dr. D.C.Goodacre It is possible but not very likely that such plants could be discovered, or even produced by techniques such as genetic engineering. However, in the short term, greater 14

THE PROSPECT OF A BIOLOGICAL-PHOTOCHEMICAL APPROACH FOR THE UTILISATION OF SOLAR ENERGY improvement can be made in the yield of useful energy per unit land area in many countries by improving the agricultural and process practices using established crops.

the atmosphere has increased appreciably over the last 80–100 years. The CO2 balance sheet is therefore not a static set of figures, and there is considerable ongoing, but as yet inconclusive argument about possible long term climate effects that may arise as a result of changes in the CO2 level in the air.

Professor A.L.Titchener

Professor D.O.Hall

Dr. Goodacre correctly emphasized the importance of land area in relation to population in the pursuit of energy in large quantities from biomass. To fuel continuously all the petrolengined road vehicles in New Zealand with a liquid fuel made solely from biomass would require about 20% of the total forest area of New Zealand. New Zealand has a population of 3 million, and a total area rather greater than that of Great Britain and Ireland. It has more, but not many more, road vehicles per head of population than the UK. The possibility of supplying a large fraction of the United Kingdom’s energy needs from biomass therefore appears remote.

Most of the points raised by Professor Titchener are answered in the complete manuscript. We would agree with his comments on the use of power alcohol for fuel in the UK if the assumption is made that this would come from UK biomass. However, if the EEC becomes self-sufficient for sugar production from beet, tropical countries which at present export sugar to Europe could instead export power alcohol. As far as the CO2 cycle is concerned, it is well established that the level of CO2 in the atmosphere is increasing. However, at present the exact nature of the equilibrium between production and use by photosynthesis and deposition in non-biological sinks is not understood. Neither is it clear exactly what effects increased CO2 concentration will have on the weather. In addition to the references given in the text, this has been discussed in Nature (News & Views) 278, 121, 1979.

The paper, in referring to the carbon dioxide cycle, does not appear to take into account the possibly significant “production” of CO2 from geothermal action, nor the permanent storage of CO2 as carbonates (fossil). Also the amount of CO2 stored, dissolved, in the ocean is variable. It is related to the CO2 content of the atmosphere, although changes in it lag the changes in the air. The CO2 content of

15

THE WATT COMMITTEE ON ENERGY

Fuel crops—an assessment of the UK potential

Professor C.R.W.Spedding Mrs. D.M.Bather Dr. J.M.Walsingham

Institute of Biology University of Reading Institute of Biology

MAIN CONTRIBUTORS TO DISCUSSION Dr. W.O.Binns

Institution of Foresters of Great Britain

Mr. S.R.G.Taylor

Institution of Mechanical Engineers

Dr. P.Hobson

Rowett Institute

Dr. P.J.Agius

Royal Institute of Chemistry

Mr. P.H.W.Everitt Mr. D.W.Gee Mr. J.Hawthorn

Institution of Production Engineers Chartered Institute of Patent Agents Institute of Food Science & Technology

THE WATT COMMITTEE ON ENERGY

Fuel crops—an assessment of the UK potential Introduction

iv) There is a stable market for the products.

Crops have been used as a source of fuel, chiefly in the form of wood (and charcoal), for a very long time and, in many parts of the world, crop by-products, such as millet straw, are the main source of domestic fuel.

The potential for fuel cropping in the UK can therefore be considered in terms of three time periods, according to the degree of change required in current land use systems, and the competitiveness of the system with conventional fuels and land uses.

In the UK, however, crops have not made a significant contribution to the nation’s fuel supplies since wood became a relatively unimportant source, and it is only since the ‘energy crisis’ that it has seemed worthwhile to examine the possibility that deliberate use could be made of plant material (biomass) to produce a range of usable fuels.

i) Short-term: The use of plant material currently grown in agriculture and forestry but regarded as residues may have potential for fuels in the short-term. Existing residues could be used almost immediately, if collected. Management of the system to increase the output of the fuel component, in response to increasing demand and more stable markets, may be possible at a later stage. Such integrated systems would therefore produce a fuel, as well as a food, animal feed, timber or pulp component. Changes in the structure of agricultural and forestry systems would be very small at first, and there should be no reduction in output of traditional products.

Since plant material varies greatly in its moisture content, from quite dry wood and fibrous stems to very wet, even succulent leafy vegetation, a variety of conversion methods may be necessary. The relative efficiencies of these methods, the degree of preprocessing that is required and the monetary and energy costs, including those needed for transport, storage and distribution, will all influence the contribution that fuel crops can make.

ii) Medium-term: Within UK agriculture there is land that is currently unused for all or part of the year. The development of stable outputs for fuel crops may encourage the planting of these with fuel crops, as part of integrated fuel and food/ animal feed systems. There would be no major changes in the structure or the food/animal feed output of agricultural systems.

Strictly speaking, a fuel crop may be regarded as one specifically grown for that purpose. It may not be grown for that purpose alone, however, and indeed, it looks increasingly likely that we may need to grow large amounts of biomass—in order to make good use of solar radiation— and then consider how best to fractionate the crop for purposes of food, animal feed, raw materials and fuel.

iii) Long-term: In the much longer term, land currently used for other purposes may be transferred to fuel production in the event of stable outputs and financial incentives. Such fuel cropping systems might initially be integrated with current land use systems (e.g. as break crops in agricultural crop rotations). In the longer term larger plantations, either purely for fuel, or consisting of dual purpose crops (producing a fuel as well as a traditional agricultural or forestry component) might be planted. Such systems would involve major changes in the structure and output of UK agriculture and forestry.

Furthermore, there is no reason why natural vegetation (terrestrial or aquatic) should not be cropped (i.e. harvested and even cultivated to some extent) for some of these purposes. The following brief assessment, therefore, deals with the main categories of land use for plant production and examines the potential of each for contributing to UK fuel supplies. Current Major Land Uses in the UK—Agriculture, Natural Vegetation and Forestry

Green plants are able, through photosynthesis, to trap and store solar energy, which is a renewable source of energy. Fuel cropping systems can, therefore, operate as renewable energy sources, providing that they:

Nearly half of the UK land area is covered with agricultural crops (arable and permanent grassland). A further 27% is classed as rough grazing, semi-natural vegetation grazed extensively by sheep and cattle, which also contributes to the UK food supply (Table 1). However, although UK agriculture uses 75% of the UK land area, it provided only 68% of UK temperate food requirements (54% of UK total food requirements) in 1975. Forestry occupies nearly 9% of the UK land area and supplied 8% of the timber used in the UK in 1975.

Table 1 Distribution of land use in the United Kingdom

As 90% of the UK land area is currently urban, agricultural or afforested, a large proportion of UK land cannot be devoted to fuel crops without some effect on agriculture and forestry. Fuel crops will not be grown in the UK unless: i) They can be converted to desirable fuels, financially competitive with conventional fuels; ii) Fuel cropping systems are financially competitive with current land uses; iii) The use of a fuel crop for fuel production is financially competitive with the use of that crop for other purposes; 18

FUEL CROPS—AN ASSESSMENT OF THE UK POTENTIAL i) Have a net output of fuel energy (fuel energy inputs to production must not exceed that contained in the output);

Agriculture Potential fuel cropping systems within agriculture are considered in terms of the three time-scales mentioned above.

ii) Are not wholly dependent on large inputs of nonrenewable resources (e.g. fossil fuels, phosphorus).

Short-term: The Use of Crop Residues The choice of the most suitable species for all the fuel cropping systems discussed below would have to be made with regard to the need for both financial competitiveness and efficiency of use of non-renewable resources. The essential characteristics of a fuel crop can be summarised as:

The major agricultural crop residue is cereal straw. Cereals cover about 75% of the total UK tillage area but the gross energy content (at the farm gate) of all UK cereal straw is equivalent to less than 2% of total UK primary energy consumption (Table 2). The energy contained in other crop residues (e.g. sugar beet tops, potato haulm, vegetable wastes) make up an even smaller proportion of UK requirements.

i) High yield of plant dry matter per hectare over the time period available; ii) Low inputs of fossil fuels and other non-renewable resources and low financial inputs;

Management of agricultural systems to increase the output of straw, such as lowering the combine cutter-bar or breeding for varieties exhibiting higher straw yields are unlikely to lead to vast improvements in potential contribution. There are, too, competing uses for straw (e.g. animal feed and bedding). Agricultural crop by-products are unlikely, therefore, to make large contributions to the UK fuel supply. However, they may be of value on individual farms; for example, supplying the fuel for grain drying.

iii) A composition suited to efficient conversion to a desirable fuel. The type of fuel desired would be an important factor in the choice of species. This paper deals with the production, rather than the conversion, aspects of fuel cropping, however, so no attempt has been made to select the most appropriate fuels, and crops suited to all the main conversion systems have been considered.

Medium-term: The Use of Currently Unused Land Areas of land within arable rotations are sometimes left

Table 2 Potential contribution of various agricultural cropping systems to UK fuel supplies

Current barley area Current wheat area 3 Current oats area 4 Current tillage 1

5

2

6 7

Current arable+permanent pasture Current rough grazing Current arable

Notes to Table 2 1. The area required to grow the seed for the fuel crop is not included. 2. Fossil fuel energy inputs to cultivation and harvesting have not been subtracted from the output. 3. There will be further inputs of fossil fuel energy and losses of crop energy during processing. 4. UK primary energy consumption is assumed to be 9×1018 J per year. 5. The suitable area is the maximum available, in practice it would be less because of rotational restrictions and current uses of the land.

19

THE WATT COMMITTEE ON ENERGY Table 3 Agricultural crops selected for long term fuel cropping systems

cropped for some months, usually between the summer harvesting of vegetables or cereals and planting of the next crop the following spring. In the past a “catch” crop was often grown during such periods and used for either winter animal feed or green manure. The disappearance of animal enterprises from cereal farms and the advent of artificial manures resulted in more of these areas of land being left totally bare. It may be possible to use such land for fuel crops. The use of machinery and labour already available on the farm would reduce the fixed costs of the fuel crop and might even lead to more efficient use of the farm’s fixed resources. It is also possible that the fuel “catch” crop would have a beneficial effect on the succeeding food/animal feed crop by improving soil structure or increasing soil fertility. The species chosen for a fuel crop would depend on the time available for growth; it must give a harvestable yield without disrupting harvesting and planting of the main crops. Fodder beet, if planted in June, would yield nearly 14 tonne dry matter per hectare, but a September planting would require a faster growing plant such as fodder radish, yielding about 4 tonne dry matter per hectare at a November harvest. There may also be potential in crops currently considered as weeds, because many of these exhibit autumn germination and rapid growth (e.g. chickweed, common hemp nettle). All such crops, being fast growing, would be wet and green, and thus most suited to anaerobic digestion to methane.

but also by the area considered to be available. Table 2 gives contributions for some possible long-term fuel cropping systems, estimated assuming that all the land environmentally suited to each crop is so used. The highest possible contribution (41% of consumption) is from Perennial Ryegrass, because most of the UK land area can be used to grow this crop. It is possible that increased fuel prices will lead to future reductions in UK energy consumption, in which case fuel crops could make a greater proportional contribution to requirements.

It is difficult to estimate accurately the land area that might be available for such fuel crops, but a crude estimate can be made. On this basis potential gross energy yield for the UK, before conversion losses, is equivalent to 4% of consumption (Table 2). The potential output of such fuel crops and their suitability for anaerobic digestion, which can be successfully operated on a small scale, make it likely that the resulting fuel would be used at the site of production, (i.e. on the farm).

It is highly unlikely, however, that large proportions of the agricultural area will be transferred to pure fuel crop production, as assumed in Table 2, because this would mean a great reduction in agricultural output in a country which is not self-sufficient in temperate foods. The only potential for pure fuel crop plantations would be:

Another form of unused land within agriculture is that occurring between rows of agricultural crops. The most obvious examples are orchards, where it might be possible to insert a fuel crop between the trees. The potential contribution of these systems is likely to be small, however.

i) If future population changes, increases in agricultural productivity, or increased imports from the EEC, released land currently used in agriculture;

Long-term: Transfer of Agricultural Land to Fuel Crops On land currently used for food or animal feed production, the first development of fuel crops could be the growing of fuel “break” crops in conventional arable rotations. Such fuel crops may prevent the build-up of weeds, pests and diseases in cereal and root crops and eventually be more profitable than current “break” crops. In the longer term, fuel crop plantations might be established, consisting of either crops purely for fuel, or of dual purpose crops which produce a fuel as well as a food or animal feed component.

ii) On land currently used only extensively for agriculture, (e.g. rough grazing), where a transfer to fuel cropping would lead to only small reductions in agricultural output; iii) If there were substantial changes in the nation’s diet. 70% of the crop output of the land used for agriculture is fed to animals. A reduction in the consumption of animal products could release land for fuel crops.

A systematic survey of species has identified the crops considered to have the most potential for these fuel cropping systems (Table 3). Further selection for an individual situation would need to take into account environmental conditions, fuel type required and the need to maintain a regular supply of material to the conversion plant. Species other than those listed in Table 3, including wild species, may be more suitable for situations with extreme environmental conditions (for example, Jerusalem artichokes may be a better root crop to grow on poor land).

The growth of dual purpose crops on land currently used for agriculture would have less effect on agricultural output, but might mean a substantial change in the nature of the crops grown. Natural Vegetation If the use of current agricultural land for fuel crops involved substantial reductions in UK food output, is there potential in the use of land not used in agriculture? Such land includes, apart from areas of forestry, areas of natural vegetation (i.e. coastal areas, mountains, salt marshes, etc.).

The potential contribution of fuel crops to UK energy consumption is determined, not only by yield per unit area, 20

FUEL CROPS—AN ASSESSMENT OF THE UK POTENTIAL and allowing it to regrow from the stump. A small number of broad-leaved species, including poplar, coppice well in the UK.

Rough grazing land is semi-natural vegetation, although used to some extent by agriculture. These areas often cannot be sown with traditional agricultural crops, due to physical limitations to mechanisation. The natural vegetation may, anyway, outyield introduced species because it will be more suited to the local environment. Indeed, annual yields of more than 10 tonnes dry matter per hectare have been quoted for bracken and reed swamps. Such material may be unpalatable and poorly digested by animals, but it may be of value as a fuel, either for direct combustion, if dry, or anaerobic digestion to methane if wet.

Average annual total dry matter yields for clear felling range from 2 to 16 tonne per hectare; coppice yields range from 1 to 20 tonnes. Yields vary with species and environment, high coppice yields requiring fertile lowland. As with natural vegetation, the harvesting of forest material for fuel would require specialised machinery, preferably chopping the material at the site.

Shorter-term: Harvesting Natural Vegetation Short-term: The Use of Forest Residues and Unmanaged Woodland

In the shorter term, harvesting the material as it stands, without any other attempt to manage the system for fuel crop production, may have potential. Unlike agricultural fuel cropping systems which, initially at least, can make use of traditional agricultural machinery, the harvesting of natural vegetation under unfavourable conditions, such as steep slopes, would require the development of special machines.

Clear felling of trees for conventional uses discards the branches and roots, which have a value as fuels and would require only the additional costs of collection. The branch wood discarded in thinnings and areas of currently unmanaged woodland may also have potential as fuels in the short term.

Long-term: Management of Natural Vegetation Systems for Fuel Crop Production

Medium-term: The Use of Currently Unused Land

It could be argued that, in the longer term, some management of natural vegetation systems (e.g. fertilising, encouragement of desired species) would result in even higher yields of fuel crops.

The development of a market for fuel crops may lead to the planting of small areas of land currently unused (e.g. corners of farm fields) with trees for fuel. Shorter rotation clear felling would allow a quicker return on investment in such areas, but wet lowland sites may be more suited to coppicing. These fuel crops could provide fuel for on-farm use as well as an additional source of income.

The potential output of areas of natural vegetation, even if they amounted to 2.4 million hectares (Table 1) and yielded 10 tonnes dry matter per hectare, would be about 400×1015 J per year (about 4.4% of UK energy consumption). A further 1170×1015 J (13%) could be obtained from rough grazing lands at the expense of agricultural output.

Long-term: The Development of Tree Plantations for Fuel Production

However, when considering natural vegetation the following points should be remembered:

In the long term large plantations of trees (either single stem or coppiced) for fuel might be envisaged. It is unlikely that land currently used for timber and pulp would be transferred to fuel production as the UK produces only 8% of its timber requirements. However, if cropping of land currently under natural vegetation or rough grazing for fuels was considered, trees might be chosen in particular situations for their higher yields or suitability for direct combustion.

i) Although not used for agriculture or forestry, such land often has recreational or amenity value, which might be affected by its use for fuel crops; ii) The fact that much of the vegetation is poorly digested by animals implies that efficiency of conversion to methane in anaerobic digestion will also be poor; iii) Managing natural vegetation would change the characteristics of the eco-system. Reported yields may not be sustainable under continued harvesting, fertilising and other forms of interference.

Wastelands The areas of land which could be most easily released for the production of energy (fuel) crops are primarily those which are ‘so damaged by industrial or other development that they are incapable of beneficial use without treatment’ (derelict) (DoE, 1974) and, secondly, those which, for a variety of reasons are not producing either food or timber. The total area of derelict land is about 74,000 ha in Great Britain of which a high proportion is in sites of less than 3 hectares in area. It includes spoil heaps, excavation pits, abandoned railway land and abandoned military areas. If all the areas which are disused or vacant, together with neglected farmland, woodland, allotments and other unused areas in the urban fringe, were included the total would be very much higher than 74,000 ha. Almost all the sites require some form of restoration before plants can be grown and most commonly they are returned to amenity or woodland use and to some extent to grazing. It is unlikely that all the areas could be used for fuel crop production but if, bearing in mind the errors in estimating the total area, we assume that 70,000 could be restored and used for fuel

There may be some potential, however, in natural vegetation systems, particularly in bracken and heather (the latter is already burnt off at regular intervals), and more information on yields is required for a detailed assessment. Forestry Tree crops are considered to be comparatively high yielding, particularly in less favourable environments. They also act as a convenient store of solar energy, which can be harvested as required, and, being dry, can be either burnt or pyrolised. There are two main systems of harvesting tree crops: i)

Clear felling (single stem trees)-harvesting the whole tree after up to 60 years and then replanting. Spruces, pines, larches, oaks and beech are often harvested in this way;

ii)

Coppicing-cutting the tree at about 5-yearly intervals 21

THE WATT COMMITTEE ON ENERGY Table 4 Areas1 of inland water2

cropping, then the yield of energy can be estimated. There are a number of constraints, like the physical characteristics, low nutrient status and toxic materials of disturbed sites but even so there are virtually no measures of productivity for such sites. Tree species are most frequently used but losses may be considerable and it is not clear whether plants like bracken, heather or gorse could be established more easily. If so, they would give a more rapid plant cover and perhaps reduce erosion problems in the early years. Harvesting machinery is being developed for these plants in upland Britain, so that they could be collected for fuel, but whether the large number of small sites converted from derelict land would restrict harvesting machinery, on cost and energy input grounds, remains to be seen. If all the 70,000 ha could be planted with alder, birch and poplar producing at an average level then the amount of energy produced would represent some 0.0002% of current national consumption of primary fuels.

1 2

Central Statistical Office (1977) Annual Abstract of Statistics includes all areas of inland water greater than 0.1 acre (0.04 ha) in area.

inland waterways although macrophytes in chalk streams have been found to produce 5.8 t dry matter per hectare in southern England. If one assumed that 300000 ha of water might be available to produce 4 t dry matter per hectare then the enerqy content of the harvested material would be 18×1015 J or about 0.2% of the primary energy consumed each year in the UK. Less than this amount of energy could be obtained by harvesting seaweeds from coastal areas or cultivating marshland, or by the growth of algae in sewage wastes but all of these require some energy inputs and their contribution to the nation’s energy requirements seem to be small.

Areas which are not producing food or timber include the transport lanes and hedgerows. The possibilities of using transport lanes depends on the mode. There are 200,000 ha alongside railway lines but some of it would probably be unsuitable for planting; even where planting was possible harvesting might present problems where tracks are in constant use unless some kind of “harvesting train” could be devised to collect the material. Motorway verges (6000 ha) are perhaps a more realistic possibility although at present they are not put to any economic use. The production of trees for fuel would overcome many of the problems associated with timber production on such areas and if coppicing were carried out then clear-felling and replanting would be unnecessary. The banks and slopes of motorway verges may represent difficult planting grounds but, where there is the necessary width, the coppiced trees would be harvested before they reached a dangerous size. This type of production could be expected to yield a little more than 0.00001% of the nation’s energy.

Conclusions In the UK the needs for food and forest products must receive high priority in the use of land: greater potential for fuel cropping exists in countries with low density populations. It is possible that the role of fuel crops in this country will consist of many small-scale contributions in different forms and waste and derelict land and freshwater areas may be able to contribute in this way. Forestry, agriculture, natural vegetation and marine areas currently cover a greater area and could make larger contributions, but the transfer of land from agriculture and forestry to fuel production would lead to reductions in food, timber and pulp production.

Aquatic Vegetation Production of energy from aquatic vegetation will almost certainly depend on conversion processes that can utilise wet materials. In some waters existing plants could be harvested for fuel as in the case of seaweed in coastal areas and the many aquatic weeds in inland waterways. However, maximum production could only be achieved by the selection of appropriate plants for given situations together with optimum production conditions. The areas of water worth considering for use are shown in Table 4 but it should be noted that many of them have alternative uses which may not be compatible with their use for fuel production.

Fuel could be obtained from land remaining in forestry and agricultural production, however, by using waste and by-products, and by growing fuel crops when food crops are not occupying the land. A change in the nation’s diet, away from animal products, could possibly release large areas of grassland for fuel cropping. Otherwise it seems probable that fuel crops would be converted locally, and perhaps used on the farms that produced them.

There are few estimates of the potential productivity of

22

FUEL CROPS—AN ASSESSMENT OF THE UK POTENTIAL

EXTRACTS FROM DISCUSSION

Dr. W.O.Binns

other for the whole world—as both assume major contributions from biomass.

Table 1 below gives estimates of production for 2 and 4 million hectares of coniferous forest. The total biomass (including roots) would only produce about 2½% of present energy consumption. Furthermore, 4 million hectares, when in full production, woutd only provide 25%–30% of the present demand for wood and wood products in Great Britain. The total land area of Great Britain, 24 million hectares, could only provide about 13% of present energy consumption (PEC=9000×106 GJ).

The first is “Solar Sweden” by Thomas B.Johansson and Peter Steen of the Secretariat for Future Studies, Fäck, S103 10, Stockholm, Sweden. This foresees 100% solar and other renewable energy supply by 2015 for a demand that is some 37% greater than 1975, of which some 62% would be from biomass. The second is “The Solar Energy Timetable” by Denis Hayes of Worldwatch Institute, 1776, Massachusetts Avenue, N.W., Washington, D.C., 20036 USA. This foresees 5/6 ths solar and other renewable energy supply by 2025 for a demand that is about double that in 1975, of which some 44% would be from biomass.

Table 1 Whole crops (all figures in millions)

Perhaps such a study could now be done for the UK.

Dr. P.Hobson The present high yields of crops in countries like Britain is at the expense of high fertilizer inputs. Calculations we made on biogas production from grass continuously cropped showed that fertilizer (given at present rates) was the biggest energy input into the system. Biogas production gives a residue with the nitrogen, phosphorus and potassium of the input conserved. If this residue is used as fertilizer then some 90 odd percent of the fertilizer chemical input can be provided and so, overall, more energy is produced by the process.

Table 2 below shows the small contribution which forest residues can make to the energy supplies of the country. Because transport costs will be high, due to the scattered location of the forests, local use of residues seems the most likely possibility. It must however be borne in mind that removal of leaves or needles adds greatly to the nutrient drain on the site; and that branches left on the ground can improve trafficability and help to prevent soil damage on heavy soils in high rainfall areas. Maximum use of residues (including stumps and roots) on 4 million hectares could provide less than 1½% of present energy consumption.

Fertilizer input must be considered in the overall energy balances of fuel crop production.

Table 2 Forest residues (all figures in millions)

Mrs. D.M.Bather Fertilizer inputs to non-legumes do dominate both energy and financial costs. The use of residues, including that from anaerobic digestion, would reduce fuel crop costs, and would be particularly effective in the case of ‘catch’ crops, where fertilizer energy is equivalent to a greater proportion of the energy contained in the crop.

Dr. P.J.Agius Lowland hardwood coppice systems, while highly productive, must be limited because of the relatively good land needed. Their use for energy production seems possible only on a small scale for local consumption, possibly for a single factory or small processing plant.

It has been said that in the foreseeable future on a world basis, there will not be enough food for the forecasted growth in population, and hence the need for birth control.

Present work on forest crops for energy is largely restricted to desk studies. If there are to be realistic trials on the ground, then there must be a modest R & D effort, designed by biologists as well as engineers.

Mrs. D.M.Bather

How then can there be land available for fuel crops, i.e. either you eat or freeze!

In many cases agricultural land is not realising its food production potential, and improvements in productivity could increase food supplies. Alternatively, an additional crop for fuel, or a crop producing a fuel component as well as a food/animal feed component, could be grown in many areas, resulting in more efficient use of the land. Such ‘catch’ fuel crops or dual purpose crops, as discussed in the paper, would provide plant material for fuels without displacing food production systems.

Mr. S.R.G.Taylor I am very favourably impressed by the magnitude of the biomass resources indicated for the UK. They would appear to be potentially capable of making a significant contribution to UK energy supply. I should like to draw your attention to two recent studies for renewable energy scenarios—one for Sweden and the

Land requirements for food production also depend on 23

THE WATT COMMITTEE ON ENERGY the nature of the diet. A reduction in the consumption of animal products would release areas of land, possibly for the growth of fuel crops.

production than on selected food components of it. It is often argued that green crops have been selected for ‘digestibility’ as an animal feed and that use for fuel production would allow selection for higher dry matter yields. However, ‘digestibility’ is also important in fuel production because it influences the efficiency with which the crop is converted to methane or ethanol. Only small improvements in energy availability, if any, will be possible through plant breeding, therefore.

Mr. P.H.W.Everitt I would like to add a comment to the last speaker’s statements regarding growth of food production world wide relative to population. In the USA one farmer feeds 58 people, whereas in USSR only 10—see Time Magazine, November 1978. Also population growth in Mexico is 3.5% but agriculture growth only 2%. To match population requirements with agricultural growth, more energy has to be used.

There is more potential for plant breeding in crops currently regarded as weed species, but which might have potential as fuel crops. Selection of high yielding genotypes and growth of the species under agricultural conditions (which include fertilising and weed, pest and disease control) could improve yields by 50% over perhaps 15 years. One of the possible advantages of current wild species over conventional crops is that despite their generally lower yields, input requirements are lower. The development of high yielding varieties could, as in the case of the ‘Green Revolution’, lead to increased costs and increased energy inputs, thus reducing the advantage.

Mrs. D.M.Bather It is true that to match population requirements with agricultural growth more energy has to be used but this does not necessarily have to be fossil fuel energy. Renewable energy sources (such as wind, solar energy) might supply some of the additional requirements. We ought not to be concerned with how many people one farmer can feed: the number of people that can be fed from one hectare of land is more important.

Mr. J.Hawthorn

Mr. D.W.Gee

In discussing fuel crops, another questioner pointed out that the rapidly growing world population was increasing the demand for food and that agricultural land was running out. Did the tenor of the lecture infer that we stay warm and starve, or eat and be cold?

The land area required for energy from the biomass competes with that required for agriculture. We are aware of the significant improvements (the “Green Revolution”) being made by plant geneticists in food production from existing areas; based on those improvements can we assume similar improvements in plants bred for their “energy” potential?; what percentage improvement (in energy availability from existing areas) is likely by the end of the century if a similar effort was made to that being given to the Green Revolution?; and what institutions are currently working on this aspect of plant breeding?

I would point out that despite a world population growth rate of 70 millions per annum, the best figures available show that food production per capita has actually increased significantly between 1962 and 1975. There is still plenty of capacity to further expand food production. There is therefore a trade-off position in which we can choose between food and fuel. However, if we are to substantially increase fuel crops, in the long run annual production, which is an extravagant way to use land, would probably be curtailed.

Mrs. D.M.Bather Plant breeding will make less impact on ‘biomass’

24

THE WATT COMMITTEE ON ENERGY

Energy from biomass by anaerobic digestion

Dr. D.A.Stafford

University College Cardiff

Mr. R.Norton

Polytechnic of Wales

Mr. D.Hawkes

Polytechnic of Wales

MAIN CONTRIBUTORS TO DISCUSSION Mr. H.Brown Mr. R.Pegg Dr. G.B.R.Feilden

Institution of Plant Engineers Institute of Petroleum British Standards Institute

Mr. A.N.Emery

Institution of Chemical Engineers

Dr. R.H.Taylor

Central Electricity Generating Board

THE WATT COMMITTEE ON ENERGY

Energy from biomass by anaerobic digestion Introduction

compared with aerobic bacteria, and some of them have been shown to require the addition of certain ‘growth factors’ to promote growth. Although many strains of methane bacteria are capable of growth on simple sources of carbon, nitrogen and sulphur, the presence of complex organic molecules appears to promote their development and some of them probably depend on these organic sources in anaerobic digesters.6 The complex ecosystem provides a ready supply of compounds for growth and energy. Other bacteria are also present which specifically degrade particular polymers such as celluloses, fats and proteins.7 However the ‘designed’ end product of the process is methane and this is produced by the reduction of carbon dioxide using hydrogen. Acetate can also be reductively cleaved directly to produce methane and carbon dioxide. The basic reactions are biochemical oxidation/ reduction reactions where a number of compounds are oxidised by the removal of hydrogen. The carbon dioxide provides an oxidant for the methane bacteria and is thus reduced to methane. A vigorous fermentation is thus described whereby gases are readily produced from organic substrates, with the ‘key’ intermediates being the volatile fatty acids. Acetate is the dominant fatty acid in normally operated systems, being responsible for about 70% of the fatty acids present. The relative proportions of fatty acids produced will depend on the nature of the digester raw feed, but wastes high in fats would produce a

Annual radiation from the sun failing on the UK contains enough energy to supply our industrial and domestic demands ten thousand times over. Vast areas of solar collectors in the form of green plant matter utilize this energy and supply us with our food either directly, or via other animal converters. Extracting energy for fuel from this biomass can be done in a number of ways (see Figure 1), and the continuing rise in prices of conventional fuels has encouraged a closer investigation into the feasibility of these processes. One method which is particularly well suited to dealing with most vegetable and animal wastes with appreciable water content is anaerobic digestion. With the exception of wood, plant material normally contains a high proportion of moisture so that wet methods of utilizing the stored energy are to be preferred. Anaerobic digestion appears to be one of the most promising methods of using biomass of a high moisture content (greater than about 45%). For the past few decades most of the research into this method of energy extraction has been directed towards third world applications. This is mainly because the level of digester technology can be low, and therefore suitable for rural conditions and also that often the need for even small amounts of energy generated locally is great. In the past few years, however, other more industrialised countries have shown an increasing interest in the process of anaerobic digestion and much of the research currently being undertaken outside the UK is financed by Governments. It is now generally considered that the advantages inherent in the process mean that it deserves a closer investigation than hitherto.

Microbiology and Biochemistry Anaerobic digestion is a process whereby certain strains of bacteria are mixed with waste material in the absence of air to degrade organic polymers to small liquid and gaseous molecules. This dissimilatory process has been observed for centuries with gas production being documented and known as ‘inflammable air’.3 A wide range of biochemical activities have been found in anaerobic systems and this is closely linked with a diverse microbial population. Substrates used by the mixed microbial consortium include lignins, celluloses, fats, proteins, long and short chain fatty acids, sugars, alcohols, ketones and amino acids.4 The methane producing bacteria are known as methanogens which utilize acetate, hydrogen and carbon dioxide, products of the acidogenic and other bacteria. The methane bacteria are among the most strictly anaerobic organisms known, and they occur naturally in the rumen of cows, in marshes as well as in sewage works digesters.5 They are capable of being able to manufacture all of their cell structure from simple carbon compounds. Generally the methane bacteria are slow growers when

Figure 1

26

Ways of extracting energy for fuel from biomass

ENERGY FROM BIOMASS BY ANAEROBIC DIGESTION higher proportion of fatty acids than those high in protein or carbohydrates. The relative proportions of propionic and butyric acids also influence methane production especially when related to acetic acid concentrations. In some systems the longer chain fatty acids may stimulate the rate of gas production. In others, as in our laboratory, the addition of acetate in high concentrations will, for a time after dosing, double the rate of gas production. Other parameters such as the pH buffering capacity of the system and the balance of specific metal ions, influences gas production. For example, magnesium and calcium salts of acetate are stimulatory to gas production, whereas potassium and sodium salts are inhibitory.8 For the maximum activity of the microbial process it is important to retain the organisms within the digester for as long as possible. Many digesters remove these ‘active’ bacteria when loading the digester tank, and with a hydraulic retention time of 10 days the organisms will remain for about 10 days in the system. It has been shown that with solids retention or recycle the performance of the digestioq process improves because maximum activity is obtained from the higher concentration of ‘active’ organisms; there is also little danger of ‘washout’ of organisms essential to efficient methane production. Similarly adsorbed essential growth factors ‘may’ also remain on the anaerobic sludge floe which will also stimulate methanogenesis. It would be expected from basic kinetics that performance should be increased by ‘total’ solids retention, especially if some form of continuous feeding could be incorporated to minimise ‘shock’ loadings. The overcoming of the ‘rate limiting’ steps by carefully controlling feeding rates also is another development essential to improve digester performances.

chlorinated hydrocarbons used in some pesticides may inhibit methane production at a concentration of less than 0.1 mg/l. Detergents and some heavy metals have similarly been shown to inhibit the process. However, many microorganisms have adaptive enzymes and resistance to some inhibitors has been shown. Much more research requires to be done in this area. It has been reported in Finland that chlorinated phenols can be degraded by anaerobic systems10 and in our own laboratories phenol up to 1,000 mg/l has been successfully degraded in admixture with sewage sludge.

Digester performance parameters

Types of Digesters

Digester performances are measured by determining the gas yields and relating them to the amount of organic material fed to the digesters. The organic matter can be measured in a number of ways: (i) Biological Oxygen Demand (BOD) which is determined by observing the consumption of oxygen by a microbial culture in the presence of the waste. This parameter is useful for some liquid effluents; (ii) Volatile Solids are determined by substracting the weight of a solid or liquid waste after volatilizing at 500°C from the dry weight or total solids. Thus gas yields may then be expressed in terms of m3/Kg of VS added or destroyed during the process.

Digester desxigns vary considerably and construction depends on many factors. Each needs to be carefully evaluated so that an optimum is arrived at to suit a particular set of circumstances. The general arrangement of a plant to suit local conditions and labour in say rural India would not be suitable for application in the United Kingdom. A typical digester for that country is shown in Figure 2; these have been developed for treating cow dung.

Figure 2

Typical digester, India

The Indian Agricultural Research Institute produced a design as early as 1939. The type is fairly typical of the kind often used in rural areas and is widespread throughout India. They have been constructed since the mid fifties in increasing numbers and by 1973 the Khadi and Village industries Commission had set up over 6,000 plants. By 1975 the number had reached 12,000 with a target of installations of 100,000 units.11

Digesters are usually fed with waste material in a batch wise process, i.e. once or twice a day a given quantity of organic material is introduced into the digester tank. The volume of tank divided by the feed volume per unit time gives the retention time. With a continuously loaded digester the retention time may be then calculated on a daily basis but the feed is continuous and this minimises any minor shock loadings to the microbial population.

In rural India there is a shortage of fertilizer and supplies of the traditionally inexpensive fuel, firewood, are severely limited. The only source of fuel that can be afforded to many is cowdung which when dried in the sun burns well. However, only about 9% of the heat energy is available for use and virtually all the nutrients are lost. The widespread introduction of small biogas plants means that the fertilizer content of the cowdung is retained, and the energy available in this organic material is more efficiently extracted.

Performance figures therefore can be based upon gas yields per amount of organic material presented, operated at a particular hydraulic retention time and this allows comparisons to be made between different digester systems. Merely determining gas yield per unit volume of digester ignores the amount of material presented to the system, and also the operational retention time.

The design is a vertical displacement type, where the cattle dung is mixed with water in a 4:5 proportion and introduced down the inlet pipe (usually at daily intervals)

Many malfunctions of digesters can occur as a result of the introduction of toxic components.9 For example, 27

THE WATT COMMITTEE ON ENERGY

Figure 3

Typical European digester, circa 1940

Design and Operation of Digesters

into the digester. This in turn displaces an equal amount of contents into the drying bed. Sizes and construction vary, but usually the fermenter is a brick lined cylindrical pit between 3.5 metres and 6 metres deep with diameters varying between 1.6 and 6 metres, and 3.6 metres deep and would be suitable for accepting the waste from, say, five cows, or 40 to 50 kilos of dung/day. The output could be in the order of 3 m3 of biogas per day of about 55% methane, which is sufficient to cater for the simple needs of a family in that particular part of the world. This very simple conception has variations and adaptations for many different countries with somewhat similar lifestyles.

If digesters0 are to be used in the future as an economic and efficient way of producing energy, then more attention must be paid to their design and operation. Any digester produces energy through the microbial breakdown of the organic matter and consumes energy in heating the contents and in operating the necessary pumps and motors. It is the role of the microbiologist and biochemist to determine the optimum conditions for the microbial activity and to understand the consequences of deviating from this ideal. It is the role of the engineer to design and produce equipment best able to achieve these optimum conditions at minimum costs.

In Europe, during World War II, many hundreds of digesters were installed, these were somewhat different in design from the Indian conception and were basically as shown in Figure 3. The organic material, dung, straw, vegetable waste, etc., were put into the series of pits. As most versions were unheated, retention times were long, especially in winter. Clearly this configuration was very labour intensive and loading and unloading must have been very unpleasant.

For a given volume and percentage solids of influent a reduced retention time means loading at a higher rate which allows use of a smaller digester; this in turn means a less costly unit. However, in order to achieve a higher total solids percentage and hence less influent and a smaller digester, it may be necessary to install extra thickening equipment. The exercise then becomes one of the cost benefit analysis and each situation has to be evaluated separately. Often there is an advantage in thickening prior to digestion because of the savings in heating costs. A 2% solids sludge contains 98% water which has to be heated to the operating temperature, usually about 35°C. An increased amount of water adds to the running costs, unlike the solid portion which is the source of the combustible gas. The effect of altering the percentage total solids on the net energy available from a series of digesters is shown in Figure 9. Here the original volume of a 5.0 m3/day at 2.0% T.S. is thickened to give reduced volumes and smaller digesters at a constant retention time of ten days. In a practical situation unlike a paper study or even a laboratory

Probably the longest experience of relevant digester design and operation for UK application has been obtained from the sewage treatment field. Figure 4 shows a photograph of a modern sewage digester in Germany. Although the object in their case is not principally to generate gas but to treat a polluting waste, some sewage works, Mogden in London, for example, have been producing methane for many years. Electricity is generated on site by this application, the cooling water from the gas engines being circulated back through the digester to retain them at their optimum working temperature. Excess gas is also bottled and sold on a commercial basis in some plants. 28

ENERGY FROM BIOMASS BY ANAEROBIC DIGESTION evaluation, one usually has a fixed volume of sludge per day to treat and may have constraints as to the solid content. In that case the cost of the digester, both in terms of capital and running costs, can be reduced at the design stage by planning to operate at a higher loading rate that will produce a shorter retention time.

problems, however, associated with feeding waste of a high solids content. Details like the shape of the tank are very important because of the necessity for adequate mixing, and clearly the shape of the tank and the method chosen for mixing are related. For batch type and plug flow digesters mixing is not essential although some type of agitation is often included in the former. Mixing by gas recirculation appears to offer most advantages,12,13,14,15 although the explanation for its improvement is not clear and several theories have been put forward.16,17,18

One other way to reduce costs and sizes is to separate the solid and liquid part of the sludge after digestion, returning to the digester the solid portion containing the bacteria and the substrate particles to which they are attached. By this means a much shorter hydraulic retention time can be achieved, whilst the solid retention time can be extended to improve the breakdown as to maintain the previous loading rate. For a non-recycle system the hydraulic retention time and the solids retention time are of course synonymous.

Reducing the complexity of the process, but at the same time ensuring a high rate efficient and reliable performance, presents many difficulties, but is necessary for an efficient digester plant.

Where recycle is practised then

The material available for digestion The biomass suitable for digestion can be considered as one of two types. Either grown specifically or mainly as an energy source, or waste material which can be treated using anaerobic digestion giving an energy gain as a by-product. The potential for energy crops is presently being investigated by a number of countries, notably the USA and New Zealand.19 It has been calculated that it would be possible to supply all of the energy needs of New Zealand from maize

There is an advantage in feeding a digester with a high solids contents input within the range of loading rates that can be tolerated. The effect of a higher total solids can be seen in Figure 10. There are practical engineering

Figure 4

Modern sewage digester, Germany 29

THE WATT COMMITTEE ON ENERGY

Figure 5

Energy balance for digesters with input of 3.8 m3/day

Figure 7

Loading rate v retention time for various percentage solids 30

Figure 6

Heat losses for an insulated and uninsulated digester

Figure 8

Loading rate v sludge concentration for various retention times

ENERGY FROM BIOMASS BY ANAEROBIC DIGESTION grown for digestion on just over 10% of the country’s presently farmed land. Australia would be self sufficient on only 2% of its agricultural land, whilst the densely populated UK on the other hand would require 13 times the farming land to meet its energy requirements.19 Nevertheless there are still vast areas of waste land in the UK on which suitable energy crops such as grasses and clovers could perhaps be grown to supply a proportion of our energy.

consider their breakdown by anaerobic digestion. However, only a proportion of the refuse is organic matter, as is shown by Table 2.

Table 1 UK annual waste 1977

Algae can also be grown for the purpose of extracting the photosynthetically converted solar energy using either unicellular or multicellular types in fresh or sea water. This is also the subject of research, particularly in the USA. Although the amount of gas that can be extracted from these materials by anaerobic digestion is known, and that it can be done has been proved in the laboratory, it is not yet certain how much energy could realistically be produced in the UK from these energy crops. It could be very significant. Table 2 Range of composition of London Refuse

The other source of biomass of which the supply is assured is waste organic matter. This is mainly domestic refuse, cattle manure, poultry waste, pig waste and sewage sludge, together with industrial process effluents such as breweries, food processing, etc. Table 1 shows the annual waste arising for the UK for 1977 for some of these. Municipal Refuse Since the organic portion of municipal refuse comprises essentially carbohydrates, fats and proteins, it is logical to

Figure 9

Figure 10 Net energy available for varying solids content and volume at 10 day retention time 73% volatile solids

The effect of thickening the contents to reduce sludge volume, on net energy produced 31

THE WATT COMMITTEE ON ENERGY digestion of pig wastes.32,33 A retention period of 10 days could be considered normal for high rate digesters, with good methane proportions of around 70%. Much of the available research work on this animal waste again shows some variation on likely gas generation. Nevertheless, the gas output is somewhat similar to the previous range, that is between 0.1 to 0.8 m3/Kg VS added.32,34 This would indicate a likely energy generation of 1.2×104 TJ per annum as being theoretically possible.

Clearly it would be economically impractical to digest all the waste, and most schemes of this kind include various kinds of screens and filters which recover the glass and metal content, and in so doing would allow only the putrescible fraction to enter the digester. Thus the amount of waste remaining to be digested could be around 20% of the total refuse collected, and as this could yield about 0.3 m3 of gas per Kg dry solids,21 the total energy content of the gas would be around 1×104 TJ. Current developments indicate that a good method of extracting gas from domestic refuse in warm climates is by batch digestion. In the United States, firms have already begun to exploit their vast sanitary landfills for the recovery of methane. Some 56,600 m3 per day (28,300 m3 of almost pure methane) are being delivered directly to the Southern California Gas Company for distribution by one such company.22 There could be 13,000 to 20,000 waste disposal sites existing in that country,23 and it has been estimated that many hundreds have commercial potential for methane recovery.

Animal manures are of course presently widely distributed so that collection becomes a major problem. Many intensive farms and most sewage sludge however are in a position where anaerobic digestion could be seriously considered. A comparison between the amount of primary energy consumed in the UK and the energy available from the sources of waste described can be seen in Tables 3 and 4. Of course these figures are over-optimistic since the process itself requires energy to maintain the temperature and to feed and mix the digester. Transportation from the site of origin to the plant also requires energy, although, of course, much of this is necessary with whatever method of disposal is used already. Similarly other treatment processes require energy to run them without producing any surplus, Table 3, showing a total energy production of about 8.4×104 TJ is computed from mean figures and it should be possible, with further research, to easily reach the maximum yields that have been obtained already. This would give a total of at least 12×104 TJ that is about 7% of this country’s current consumption of natural gas. Allowing for further developments in digester technology, which is in its infancy, these figures could well be net production figures for the near future.

Cattle Waste Cattle waste is readily digestible although the yield from milking cows is somewhat lower due to methogenic bacteria having already been active in the cows stomach. Digesters operate well at the conventional temperature of 35°C, but retention times as low as 3 days have been achieved at 60°C.24 In the United States great interest is being shown in this method of treating cow manure, modern beef production uses feedlots that contain as many as 50,000 head of cattle. In Bartow, Florida, digesters designed and constructed by Hamilton Standard Inc. are currently being commissioned to operate on a cattle feedlot, the residue from the plant will be fed back to the cattle as a source of protein and nutrient. The effluent is circulated around a system of lagoons, the final pond containing talapia fish which feed on algae produced by the effluent contents. The fish are harvested annually, their bones being ground up to supplement the cattle’s diet. The result of this recycling process is a final effluent containing less impurities than the influent water supply into the feedlot. The gas produced by the digesters will be utilized in the cattlelot’s meat packing plant. Any excess gas will be piped into the local mains for domestic consumption by the nearby township.25

Table 3 Possible energy production by anaerobic digestion from already occurring organic wastes

Gas yields from digesting cattle manure appear to vary from 0.1 to 1 m3/Kg VS added.26,27,28 From this it would appear that total gas generation from UK cattle arising could be over 3×104 TJ per annum.

Table 4 UK consumption of primary fuels for energy

Poultry wastes The daily production of poultry manure is about 180 g/hen having a solids content of about 25% with about 70% as volatile solids.29 In other poultry situations, broilers for example, the yield varies according to the age and size of the bird. It is possible to obtain good yields of gas per available VS in digestion by controlling feeding rates and mixing, and it has been shown to be amenable to anaerobic digestion at fairly low retention times.30 The range of likely gas yields again shows a fairly wide distribution of between 0.1 to 0.9 m3/Kg VS added.26,31 Total gas yield from available chicken manure could then be in the order of 1.8×104 TJ per annum.

Such a comparison serves to illustrate that the potential energy generation by anaerobic digestion must not be dismissed as totally insignificant, especially since this energy is in a readily available form and that these figures do not take into account energy crops or industrial wastes, which could well boost the proportion of gas from anaerobic digestion to over 20% of the current natural gas usage.

Pig wastes A great deal of practical information is available for the

It must be remembered that for an individual producer of 32

ENERGY FROM BIOMASS BY ANAEROBIC DIGESTION organic waste, for example a farm, dairy factory, brewery, the gas that could be obtained by this method could be a very large proportion of his total energy needs.

14. Sawyer, C.H. and Ray, H.K. A laboratory evaluation of high rate sludge digestion. Sew. and Ind. Wastes, 27, 12, 1356–1363, Dec. 1955

The Cost of this Energy

15. Estrada, A.A. Design and cost considerations in high rate sludge digestion. Jul. San. Eng. Div. ASCE, SA3, 111–127, May 1960

One of the important requirements of energy for the future is not only that it will be available, but that it will be cheap enough to use. Anaerobic digestion can produce methane gas, a clean premium fuel, at a cost not dissimilar to that of natural gas today.

16. Finney, C.D. and Evans, R.S. Anaerobic digestion—the rate limiting process and the nature of inhibition . Science, 190, 4219, 1088–1089, Sept. 1975 17. Graef, S.P. Dynamics and control strategy for the anaerobic digester. Ph.D. Thesis, Dept. of Env. System Engrs., Clemson Univ., May 1972

For example, it should be quite possible, with a well designed system, to manufacture digesters of, say, 100 m3, at a present day cost of £20,000 with a working life of 15 years. Such a digester, with even current gas production efficiencies, should give a net gas surplus of around 30,000 m3 per year. The energy content of the biogas is about 26 MJ/m3 and the cost would thus work out at about £1.8/GJ, the current cost of its energy from North Sea gas.

18. Konstadt, H.G. and Roediger, H. Engineering operation and economics of methane gas production. Microbial Energy Conversion. Ed. Schlegel, H.G., Barnea, J. Pergamon Press, 1977 19. Stewart, D.J. The possible role of agriculture in supplying New Zealand’s future energy demands. Min. of Agric. and Fisheries, Mosgiel, 1976

As other sources of non-renewable energy are depleted, methane from biomass is likely to become very much more important than it is today.

20. Douglas, E. and Jackson, D.V. Waste as a raw material. Jul. Soc. Env. Engrs., 10, 1972 21. Le Roux, N.W. and Wakerley, D.S. The microbial production of methane from the putrescible fractions of sorted household waste. Proc. 1st World Recycling Congress, Basle 6–8th March 1978

References 1. “Methane production from waste organic matter” (to be published by CRC Press Inc., 1979) 2. Long, G. Solar energy—its potential contribution within the UK. HMSO, London 1976

22. Hekimian, K.K., Lockman, W.J. and Hirt, J.H. Methane as recovery from sanitary landfills. Waste Age, 2nd Dec. 1976

3. Pine, M.J. The methane fermentations. In Adv. in Chem. Series, 105, 1, 1971

23. Greo, J.R. Land disposal of wastes, a necessary technology for environmental management. Nat. Solid Waste Manmt. Assoc., Tech. Bulletin No. 6, 10, Nov. 1976

4. Stafford, D.A. Methane production from waste. Effluent and Water Treatment J., 14, 73, 1974

24. Bryant, M.P., Varel, V.H., Frobish, R.A. and Isaacson, H.R. Biological potential of thermophilic methogenisis from cattle wastes in Microbial Energy Conversion. Ed. Schlege, H.G. and Barnea, J. Pergamon Press 1977

5. Zeikus, J.G. The biology of methane bacteria. Bacterial ., 41, 514, 1977

25. Personal visit. June 1978

6. Wolfe, R.S. Microbial formation of methane. Adv. in Microbiol. Physiol., 6, 107, 1971 7. Hungate, R.E. The anaerobic mesophilic cellulolytic bacteria. Bacterial Revs., 14, 1, 1950

26. Gramms, L.C., Polkovski, L.B. and Witzel, S.A. Anaerobic digestion of farm animal wastes. Trans. Amer. Soc. Agric. Engr., 14, 7, 1971

8. McCarty, P.L. and McKinney, R.E. Salt toxicity in anaerobic digestion. J. Wat. Pollut. Contr. Fed., 33, 399, 1961

27. Conversa, J.C. Dairy manure degradation under mesophilic and thermophilic temperatures. Winter Meeting, Amer. Soc. Agric. Engr., Chicago, III, 1975

9. Hobson, P.N. and Shaw, B.G. The anaerobic digestion of waste from an intensive pig unit. Water Research, 8, 437, 1973

28. Pfeffer, J.T. and Quindy, G.E. Biological conversion of biomass to methane. Beeflot manure studies. Report No. UILU ENG-78-2011, May 1978. US D. of Energy Contract No. EY-76-S-02-2917

10. Salomen, M. (personal communication). University of Helsinki

29. Taigandies, E.P. Anaerobic digestion of poultry manure. Nat. Symp. on Poultry Ind. Waste Mgmt, Lincoln, Nebraska 1963

11. Kashkari, C. Energy resources, demand and conservation. McGraw Hill, 1975

30. Hawkes, D.L., Horton, R. and Stafford, D.A. The application of anaerobic digestion to producing methane gas and fertilizer from farm wastes. Process Biochemistry, 32, 1976

12. Morgan, P.F. Neuspiel, P.J. Environmental control of anaerobic digestion with gas diffusion. Biological Treatment of Sew. & Ind. Wastes, Vol. 11. Ed. McCabe, J. and Eckenfelder, W.W.Reinhold NT 1957

31. Hart, S.A. Digestion test of livestock wastes. Wat. Pol. Cont. Fed.,35, 748, 1963

13. Sen, B.P. and Bkaskoran T.R. Anaerobic digestion of liquid molasses distillery wastes. Ind. Water Poll. Cont. Fed., 34, 10, 1014–1025, Oct. 1962

32. Hobson, P.N. and Shaw, B.G. The anaerobic digestion 33

THE WATT COMMITTEE ON ENERGY of waste from an intensive pig unit. Water Research, 7, 437, 1973

characteristics and anaerobic digestion. Animal waste Mgmt. Conf., 50, Cornell Univ., 1969 35. Digest of United Kingdom Energy Statistics 1978. Dept. of Energy. HMSO, London 1978

33. Fry, J.L. Practical building of methane power plants. Standard Printing, Santa Barbara, California, 1974 34. Schmid, L.A. and Lipper, R.I. Swine wastes

EXTRACTS FROM DISCUSSION Mr. H.Brown

their units, appreciating research and development difficulties and how their problems are being overcome. We at the Polytechnic of Wales, for example, could prepare a complete dossier of advantages and disadvantages of their variety of methods, written by designers for designers as it were. If this information could also be fed into our computer model, this would certainly yield invaluable information for any intended United Kingdom application.

The papers have been based on anaerobic digestion. Have the authors studied the possibilities of aerobic digestion where blown air is being used within 30 miles of London? They operate boilers on methane to show considerable cost savings as against treatment of industrial waste at municipal sewage works in terms of capital and recurring expenditure.

Dr. G.B.R.Feilden

Dr. D.A.Stafford

Re the ‘Biological Power Station’ and the economical aeration of sewage for floculation of solids prior to anaerobic digestion. Is there scope for a major design advance in the aerodynamics and hydrodynamics of the aeration process?

Yes, we have studied aerobic digestion, but this is much more costly than anaerobic digestion since oxygen has to be introduced into the system. Anaerobic digestion produces a usable fuel which can be enough to run sewage treatment plants, although the economics of doing this will depend largely on seasons and mode of operation of the digester plant.

Dr. D.A.Stafford The control of flocculation in aerobic processes is a complex process, but flocculation is usually more easily obtained by aerobic sludges than by anaerobic sludges. Advances are going to be made perhaps in the field of bioflocculation of anaerobically digested sludges and by shape and configuration of the settling banks.

Mr. R.Pegg a) Do you see anaerobic digestion more applicable to the local scene than for a solution to our national energy problem? If the latter, then we must consider gas clearup, and how does this affect the situation?

Mr. A.N.Emery Referring to Figure 5 in the paper, the net energy production appears to be some 20–30% of the gross gas energy produced. Does this figure allow for energy usage in gas clean-up and (more particularly) treatment of the remaining sludge (pressing or other methods) and liquors (aerobic biological treatment)? If these are taken into account, what in fact is the net energy production likely to be?

b) Are not most of the plants being built in the US intended as demonstration plants and not really intended to be a commercial success?

Mr. R.Horton Yes. At this moment we see anaerobic digestion being applied very much to the local scene, although the situation could change. For example, countries like New Zealand are considering energy crops as a significant contribution to their energy demand in the future. However, our research is very much concerned with making the process economically viable in small scale situations like farms and industries producing organic waste, such as breweries, distilleries and food manufacturing plants, etc. If our designs are feasible on this small scale, dealing with a wide variety of different wastes, then clearly, extrapolating this up in size to cater for relatively large amounts of waste is a straightforward exercise. The economic viability of the process rapidly improves with increase in size of plant.

Mr. D.Hawkes Figure 5 in our paper is an example of the situation in the winter (0°C) for a particular sewage sludge. It is drawn from results obtained on our computer simulation for this low total solids sludge. It does illustrate the need to appreciate the facts concerning digestion design and operation before predicting performance figures. This set of figures is for one particular situation, i.e. a fixed volume of sewage sludge per day with 2.5% Total Solids, 86% Volatile Solids and with our own design of digester. If any of the parameters are changed, for example the ‘U’ factors for insulation, the ambient temperature % TS, etc., then the whole picture changes.

In answer to your second point, I would agree that the US is building many of their digesters for demonstration purposes. But some of their projects are being handled by eminent engineering companies, who see much commercial potential in the venture. Incidentally, so many different designs are being built and developed there at this time that it appears to me that a tremendous opportunity is now presenting itself in learning so much about digester technology merely by watching. A person well acquainted with digester design and technology could acquire immense knowledge of the various ways and means by investigating

For pig slurry, for instance, which our design of digester is also suitable for, the % TS is about 7.5 with 70% VS and at a ten day retention time. Even in winter conditions there is a net energy production for anything over about a 3m diameter digester—a very healthy situation indeed in energy terms.

Dr. R.H.Taylor You showed slides in your presentation of digesters 34

ENERGY FROM BIOMASS BY ANAEROBIC DIGESTION working in other parts of the world, including American ones working on domestic refuse and landfill sites. Is the domestic refuse treated in any way before digestion, such as the separation of infill wastes in American studies using urban wastes

the whole of the refuse is used and the major criterion determining gas production rates, apart of course from temperature, is moisture content. In the Pompano Beach facility, which my colleague, Rex Norton visited, only the putrescible fraction is digested and that is mixed with some sewage. The refuse there is first sorted and re-usable materials, such as metals and glass, are extracted.

Also, could the presenters comment on the economics of the various processes.

Dr. D.A.Stafford and Dr. D.Hawkes

Domestic refuse is not the best feed material for an anaerobic digester and in our own opinion it is unlikely that the Pompano Beach digester will ever be a net energy producer.

Some landfill sites separate metals prior to disposal, but this is only done to some extent where solid wastes are to be used in controlled anaerobic digesters.

However, the gas from one landfill site in California is piped to 12,000 homes and the economics depend upon gas purification costs, pipework and the charges made for disposal of waste. Those operating such plants consider gas collection in this way to be profitable.

When we visited the USA earlier in the year to look at a number of working digesters, we spoke to scientists at, for example, Illinois University, where studies on simulated landfills have been going on for a number of years. There

35

THE WATT COMMITTEE ON ENERGY

Biogas production— agricultural wastes

Dr. P.N.Hobson

Rowett Research Institute

MAIN CONTRIBUTORS TO DISCUSSION Mr. J.C.Hawkins

Institution of Agricultural Engineers

Mr. A.N.Emery

Institution of Chemical Engineers

Dr. D.E.Brown

Institution of Mechanical Engineers

THE WATT COMMITTEE ON ENERGY

Biogas production—agricultural wastes towards factors which have to be considered in discussing the production of biogas from agricultural wastes and primarily animal wastes.

Introduction Biogas, a mixture of methane and carbon dioxide generated by the breakdown of organic material by a complex mixture of bacterial species, has been used over the past 50–60 years (since pioneering experiments at Birmingham sewage works) for production of power in town sewage works. The organic material used here is a sludge of about 4 to 6% solids formed from the larger particulate matter (faeces, papers, etc.) settled from the sewage water entering the works, plus any excess sludge from the aerobic treatment plants.

Since animal wastes, like sewage sludges, are generally about 90% water, the energy production per unit volume is not too high and if the waste has to be collected from a large area, economic and energy costs of this collection may begin to outweigh energy production, although at the moment calculations show that it is economic considerations rather than energy itself which dictate the collection area, particularly if road tankers are used. However, collection over a large area is not very feasible and we must look to use of wastes which are concentrated in a small area. In the case of animal wastes, this means the intensive farming unit, whether it uses indoor housing or outdoor ‘feedlot’ housing. However, because of the difficulties of handling sludges and the need for automated working and in cold countries for digester heating there are advantages of scale and below a minimum size gas production does not outweigh economic and energy costs of constructing and running the digester plant. Because of these factors, figures quoted for wastes produced per country are meaningless, we need to know sizes of animal production units before we can begin to decide how much of a country’s total animal population is usable in terms of energy production.

In this case the process, anaerobic digestion, has the primary aim of converting a highly polluting sludge into a stabilised, virtually odour-free, sludge which can be safely land-spread as fertilizer or dumped on land or at sea. The sludge can be dried leaving an innocuous, ‘gritty’ material which can be bagged for use as powder fertilizer. The biogas production is a secondary aspect of the process. The gas may be used by direct burning in boilers only to keep the digesters heated to mesophilic temperatures, but in the bigger installations it is used for powering gas engines or turbines; the waste heat from these is sufficient to heat the digesters. There are some points to note from the domestic sewage systems which are relevant to further discussion. First, although the power output from the digester system of a big sewage works is large it is obtained from the sewage from a large population spread over some square miles. Since this sewage is water-borne its collection to the works does not involve great energy expenditure (unless local topography dictates much pumping from different levels), but, importantly, since the sewers have been in existence for years usually and are in any case a separate charge on rates, the capital costs of this collection network for the digester feedstock are not directly involved in the economics of gas production. Secondly, although the power output is large it is all or nearly all used in the sewage works. Admittedly, the largest proportion of the power generated goes to running the aerobic treatment plants; if this wastewater treatment were not needed then more power would be available for other uses. But even so, the power produced from sewage sludge digestion is only a fraction of the needs of the big town which provides the feedstock. Thirdly, digestion is only economic, in terms of capital and running costs of digesters against energy and sludge stabilisation obtained, in the larger sewage works. In Britain Swanwick3 noted that there were about 350 sewage works, out of some 5000 altogether, that had anaerobic digesters and these served a population of some 25 million. Some of the other works could usefully employ digesters; the majority were too small. The total gas produced is about 7.08×105 m3/day with 65% methane, compared with a daily natural gas (100% methane) usage in Britain of around 1.1× 108 m3

The use of power on site in the sewage works also suggests that even the largest biogas plant will produce power only for its immediate surroundings. The future of biogas plants, I suggest, is as producers of power for the larger farm, its surrounding houses, perhaps a village or small town. I do not suggest each village would have its biogas plant, but that where these plants are possible they should be considered on the lines of replacing the old local gas works or electricity generating station, not replacing the huge oil or atomicpowered generating plant now in use in the grid system. One further point to be considered is that the domestic sewage digester is primarily for pollution control purposes, the gas is a useful by-product. At present in Britain, Europe and other countries, anaerobic digestion of agricultural, and particularly animal, wastes is being looked at as a method of reducing pollution, particularly odour, and providing a safe fertilizer. In this respect it is in many ways more successful than aerobic plants. Pollution control is expensive no matter what method is used and if the digester gas offsets the cost of this control, or makes a profit, as against other methods which have no monetary return, then this is a reason for putting in a digester plant. Also aerobic or other treatment plants require a high energy input. The anaerobic plant requires less energy per unit of pollution control and it can provide this energy, or its equivalent, from its own running. In the economics of present farm digester plants, then, some allowance must be made for the pollution control aspect; but this is difficult to quantify unless the farmer is threatened with closure of his animal unit until there is effective control of pollution. This is beginning to happen in some cases.

In summary one can say that sewage-sludge digestion can save energy required for a necessary process. Without digestion this energy has to be provided from some other source, the oil-fired power stations of the national grid. It does not, overall, provide energy which can be used outside the sewage works.

The economic return on capital and running costs provided by the gas from an agricultural digester has changed over the years, and will continue to change. As the price of conventional fuels has increased, so the value of digester gas

But this introductory summary gives some pointers 38

BIOGAS PRODUCTION—AGRICULTURAL WASTES simple in principle and operation, it can be used on any scale, and it is carried out at temperatures not above that of ‘hot’ water and at low pressures.

has increased. At the moment in Britain the price of natural gas is low compared with that of electricity or petroleum gas and fuel oils. The return in terms of using the gas to generate electricity to replace mains electricity is greater than that of using the gas in burners to replace natural gas.

The microbiological processes occurring in anaerobic digestion have been considered in detail elsewhere (Hobson, Bousfield & Summers2) and will not be described here. These processes lead to a step-wise conversion of various constituents of the waste to a mixture of methane and carbon dioxide, and the energy generated by the bacteria is used to form microbial cells. Theoretically the final product should be the gas and bacterial cells. In practice with animal or plant wastes the liquid always contains a small amount of the intermediate product acetic acid. Also because much of the material is unavailable to the bacteria there is residual lignified plant fibre and ammonia not used for cell production and various mineral salts and inorganic matter such as grit and stones. From the point of view of pollution control this solid material is relatively inert, but we shall not consider pollution control further in this paper.

Obviously, as said before, these economic aspects will change in the future, but they have a bearing on present research and development in the building of digesters for agricultural wastes. The pollution load generated by even a medium-sized pig unit (say 5000 pigs) is equivalent to that of a small town, as one pig is equivalent to about four humans in excretion, and far less numbers of cattle or the birds contained in a few poultry houses are similarly polluting. However, while the costs of town sewage works are measured in hundreds of thousands of pounds, economics dictate that the farm ‘sewage works’ of equivalent loading must be measured in a few tens of thousands. Even at this price the farmer is looking for a return from the biogas, so capital costs must be such as to provide this return and amortize plant over a few years. The building of comparatively cheap, yet reliable and simple to run, plants is, thus, the aim of present work. Because of this aspect of cheapness, and because in general the size of plant which would be used on the majority of British or Continental farms is about the 400 m3 or less volume which is smaller than the size usually used in domestic sewage works, little of the domestic sewage engineers technology can be transferred to farm plants. The sludges used also differ in properties from domestic sludges. So while the basic principles are the same, farm digester design has had to start almost from scratch.

From the energy-generation point of view this solid material represents a loss of potential gas. Our figures show that in pig waste, for instance, there is gasification of some 41% of the cellulose, 48% of the hemicellulose and 53% of the fat. As breakdown follows an assymptotic course some little increase could be obtained by increasing solids detention time, but this would not be worthwhile in overall plant costs or complexity, at the present time. Increased digestion could probably be obtained by, for instance, alkali treatment of the waste, but again this adds complexity and costs. For farm use the single-stage, high-rate digester seems to be the best design. It is the type most suitable for thick sludges and is relatively simple in design and construction. This latter is important if one is considering a plant to be run with a minimum of attention by unskilled men. And from a practical standpoint agricultural sludges are very difficult to pump and pipe. Problems now occurring with agricultural digesters are entirely due to the physical properties of the sludges, not the biological side of the process. The sludges contain plant fibres up to some inches long, cereal grains and husks, animal hairs, slimy materials, as well as grit, and these easily clog pipe bends and pumps and are extremely abrasive. The less long pipe runs, bends, constrictions, solids separation systems, etc., the better.

Amounts of Gas Produced from Agricultural Wastes Agricultural wastes, whether of animal or vegetable origin, almost all contain large amounts of water. Dry straw has perhaps the lowest water content, but green vegetable matter is some 80% or more water. Faeces plus urine slurries from cattle are at least about 85% water and from pigs about 90% when fresh from the animals. In practice because of leaks from animal watering systems the water content is usually higher. With animals on liquid feeding systems water content is 95% or more. Poultry droppings look dry but have a water content of 70% or so, although the water content of droppings plus litter from ‘deep-litter’ houses may be less.

The digesters now in use are being run at mesophilic temperatures and in this kind of single-stage, mesophilic digester the following results have been obtained in our own experiments with pilot-plant and small-farm scale digesters. Where similar experiments have been done by others the results have been confirmed. The results are averages from ‘steady-state’ running of digesters over many months at the various loading rates and temperatures. The results show only gas production and this is given as gas per unit weight of Total Solids, as Total Solids are an easy parameter to measure for planning and controlling farm digesters. Much of our work has been on pollution control, but this, including experiments on secondary treatments of sludges, etc., is not relevant here.

Because of the water content, pyrolysis to produce gas and oil, which needs dry feedstock, does not seem really feasible, as the energy needed to dry wet wastes to a suitable state for pyrolysis is of the same order as the energy produced by the process. For instance, the energy cost of production of a ton of dried grass from green grass is about 3800 kWh. The total energy production as gas, oil and char from a ton of dried cow manure was from smallscale experiments calculated as about 3750 kWh and from a ton of rice straw about 2850 kWh (Crenz1). The gas consists of hydrogen, carbon monoxide and methane and needs special burners. Hydrogenation processes can, on the other hand, be carried out with wet wastes to give oil and gas. However, both these types of process need plant which can withstand high temperatures and in some processes high pressures. Such plant is costly and could not be thought of for use on a comparatively small-scale farm and by other than fully-trained personnel.

PIGGERY WASTES Faeces-urine-water slurry with some sawdust from pens and grit, cement dust, etc. Organic matter (Volatile Solids) about 70% of Total Solids (TS). All gas 65–70% (usually ca. 70%) methane 30% carbon dioxide, traces of other gases.

Detention time at 35° C

The advantage of microbiological production of gas is that it can be carried out with very wet feedstock, it is

10 days and over, gas 300 l/kg TS added to digester 39

THE WATT COMMITTEE ON ENERGY

A 10 day detention time was used for all the following experiments.

tested. It is of interest to compare these gas productions with coal gasification. The old town gas process produced about 75 Therms of gas of 500 Btu/ft3 from a ton of coal and left coke. The Lurgi plant converted more of the coal into gas and produced about 176 Therms of gas of 300 Btu/ft3 per ton of coal. The gas produced by digestion of a ton of pig-slurry solids is about 70 Therms of gas at about 650 Btu/ft3.

Solids required

Farm-Scale Digesters

Below 2% TS feedstock the digester bacteria begin to washout. From 2% to 8% TS gas production per kg TS added is the same.

The 100–150 I digesters used in obtaining the data in the previous section have been operating for periods up to 7 years. Microbiologically the digesters have been completely stable in spite of the wide variations in loading to which they have been subjected. All have been subject to short periods of no loading and the pig-waste digester has been subjected to periods of no loading, with or without heating of up to about 6 weeks. The digester contents have also been exposed to air and entirely removed and replaced. Digestion has always restarted immediately operating conditions have been attained.

7 days, gas 284 l/kg TS 5 days, gas 240 l/kg TS 3 days, gas 170 l/kg TS and CH4 production failing due to washout of methanogenic bacteria.

Temperature 30°C, gas 300 l/kg TS added; 35°C, gas 300 l/kg TS; 40°C, gas 360 l/kg TS; 44°C, gas 420 l/kg TS. At 25°C and above 45°C gas production is falling off. CATTLE WASTES Faeces-urine slurry from fattening cattle on mixed rations on slatted housing with no bedding. (Pure dairy-cattle waste is now being tested). Organic matter again about 70% of TS. Gas 55–60% CH4, remainder CO2.

Similar stability has been found with the small farm-scale digester (see below), where the contents have actually been held in open tanks for some weeks while mechanical alterations to the digester were made. No digesters have ever been reinoculated after initial commissioning.

Detention time at 35° C and about 4.5 to 5% TS slurry (see below)

Apart from the pilot-plant we have operated a ‘small farmscale’ digester of 13 m3 (3000 gal) capacity on piggery waste for some six years. In the two years or so after initial start up some different stirring methods were tested and so running was somewhat intermittent. For the last three or four years running has been continuous, except for a short period when the contents were removed to change a stirrer bearing.

20 days and over, gas 215 l/kg TS added 10 days, gas 195 l/kg TS added. Less than 10 days lower gas production.

Solids required at 35°C, 20 day detention 5% TS slurry, gas 189 l/kg TS added 8% TS slurry, gas 258 l/kg TS 10% TS slurry, gas 264 l/kg TS.

The detention time has been kept at 10 days in general. However, the solids content of the feed slurry has varied considerably and for some periods recently loading has been rather intermittent because of alterations to the farm slurry system, and at times mixtures of pig, cattle and poultry wastes have been digested (at 10 day detention time). At one period last year waste grass juice was added with the pig waste, with a large increase in gas production. Results with pig waste have entirely duplicated pilot-plant results.

The increase in specific gas production with increasing TS is, we think, due to the increasing ammonia concentration in the slurry; the cattle slurry of 5% TS is of lower ammonia concentration than similar TS pig or poultry slurries. POULTRY WASTES

While periods of more than a year of running on the same pig waste have enabled good average results to be calculated, the occasional ‘interruptions’, especially in the last year, have shown that periods of low loading or no loading such as might obtain on a farm for various reasons, and admixture of other wastes on occasion can be coped with perfectly adequately by a digester running primarily on one waste. Although, of course, various measurements are done for experimental purposes, the man-hours needed for routine running of the digester are minimal, noting of gas production and a quick inspection of pumps, etc., for any blockages being all the general daily maintenance required. A simple test for solids concentration in the input slurry can also be done if required.

Solid excreta from caged layers, no litter, slurried with water to required TS. Organic matter about 70% TS.

Detention time at 35° C and about 6% TS slurry (see below) 20 days or over, gas 380 l/kg TS added 15 days, gas 362 l/kg TS added.

Solids required at 35° C, 20 day detention 4% TS slurry, gas 480 l/kg TS added 6% TS slurry, gas 379 l/kg TS added 12% TS slurry, gas 291 l/kg TS added The decrease in specific gas production with increasing TS is, almost certainly, due to very high ammonia concentrations in the slurries of high TS.

These results are for mesophilic digestion. Thermophilic digestion has not been looked at extensively. The group in Illinois4 have done laboratory-scale experiments on thermophilic digestion of cattle wastes, but it is difficult to make definite recommendations from their work as longterm experiments have not been done. The process and the extent of degradation of the wastes seems to be the same as in the mesophilic digestion. It is probable that some decrease in detention time would be possible with thermophilic digestions, but the three-day detention limit

Rotten potatoes or silage liquid digested well with pig slurry at 10 day detention to give increased gas production. Dry straw plus pig slurry required a 20 day detention and then gave extra gas production equivalent to about 35% digestion of the straw. These results show the gas production to be expected from various wastes. Green vegetable matter has not yet been 40

BIOGAS PRODUCTION—AGRICULTURAL WASTES (except one which was for a specialised unit) planned for farms of above the 2000 pig, 75–100 cattle size and are dealing with more Jike 5000 or more pigs, 250–300 cattle or mixed stocking farms. Such digesters are in the 300 m3 range of volume (75000 gal). They are designed to take slurries of animal wastes drawn from under slatted-floor housings and from tanks at the ends of scraper-cleared dung channels. Some poultry wastes are collected by a water-flushing slurry system, but most are collected dry and would have to be slurried with water. So far no commercial digesters are being built for poultry units, although some enquiries have been made by farmers.

noted above seems to apply to the thermophilic digestion as well. The carbon dioxide concentration in the gas coming off may be higher than in the mesophilic digester because of solubility changes at the higher temperature. The concentration of solids in the input slurry which can be used without digester failure is about the same as for mesophilic digestion, but could possibly be higher. However, if this latter is true it is largely academic as the limit to solids is set by pumping problems, not biological ones. Although it might be possible to run thermophilic digestions on a factory basis, it seems to my own group that thermophilic digestion is not practicable on a farm scale. From the point of view of pollution control (not dealt with by the Illinois group) thermophilic digestion of domestic wastes is said to be poor. But also, 60–65°C is uncomfortably hot for a farm process (from the operator’s point of view) and temperature control would have to be very accurate. Unless a very efficient output-input heat exchanger (see later) were used, then the extra gas energy involved in heating the input and the digester would off-set to a large extent any increased gas production over the mesophilic unit. Calculations can quite easily show the extent of this ‘loss’ which is quite considerable.

Digester tank design The cylindrical tank has many advantages in circulation of digester contents and surface area to volume aspects. Our own digesters are above-ground and fabricated in mild steel which has shown no corrosion. This fabrication has advantages for our experimental work, but could be too expensive on a large scale. However, a cylindrical tank means that standard slurrystore, glass-lined tanks can be used and these are cheap and easily available, and tops are made to a standard pattern. Some commercial digesters now made or being made use these tanks with a suitable layer of plastic insulation. Concrete is used for domestic sewage digesters, and a small (12 m3) commercial farm unit is built in concrete partially buried in earth for extra insulation. For unknown reasons this concrete required some paint sealing to make it impervious to gas and water.

In the same way the extra gas production noted from pig waste at 44°C would be offset by increased heating requirements and again temperature control would have to be more exact than at 35°C. The 13 m3 digester in our own unit has probably been running longer than any other fairly large scale experimental unit using farm wastes, and has shown the feasibility of digestion using the single-stage process. Other digesters on broadly similar lines are running or have run. The batchloading systems now being tested on the continent do not seem satisfactory for a farm system. Gas production from a batch digestion is variable with time and the gas may not be of constant composition, obviously a series of batch digestions running out of phase is needed to get reasonably constant gas yield. Some of the ideas tried or suggested for loading and unloading seem to involve too much equipment and too many man-hours.

A commercial farm digester now on the point of commissioning is a rectangular concrete tank sunk below ground level. Time will show any problems in this design. The top of the digester can be made to different designs and this will be dealt with under ‘gas storage’.

Input and output A ‘continuous culture’ such as a digester works optimally when truly continuously loaded. This is not always possible, and a close approximation with intermittent loading is usually used. Such loading, because of the need to keep daily input volumes constant, is generally done by pump. The rectangular digester mentioned above will use gravity loading, but apart from any difficulties that may arise with gravity flow of thick, unhomogeneous slurries, the relative heights of digester and slurry system usually preclude gravity feed.

Some big digesters are being built and run in Britain and we are able to keep contact with progress here, so this will be the basis of the following discussion. Reports on a few larger digesters in the USA and Europe show that the same problems are being encountered. Many projects heard of turn out to be only in the planning stage or have not yet begun to run properly; the data on plants that have run for any length of time are small.

Piggery wastes are much the same from all intensive pig systems and pigs are mostly housed on partly slatted floors without bedding or with only a little sawdust. Although difficulties have arisen with pipe blockages due to accumulation of pig hairs, barley husks, etc., these can be overcome by suitable pipe-line design. However, cattle slurries are presenting more problems as cattle are often bedded on straw and straw fibres tend to float and form scums and block pipes and pumps. Faeces from cattle fed long fibre feeds also contain fibrous residues as well as spilled food. Some form of ‘chopper’ either separate or attached to the feed pump is needed to handle these slurries, or straw and long fibres must be separated from the slurry before it reaches the digester pump. Work on these aspects is proceeding. Similar problems and solutions apply to the feathers in poultry wastes, and to fibrous vegetable wastes.

However, what is needed now is data on running large digesters and the overcoming of developmental problems. Negotiations are under way at the time of writing to provide a 300 m3 experimental unit for the author’s group. This would be invaluable for further development. While some of the designs now being considered or used by other groups may eventually turn out to be impracticable all large-scale work will help towards a final solution. Assuming, then, that the single-stage mesophilic automated digester is the one of choice the developmental problems occur in the following areas. These are all being investigated either with a second digester in use in our own unit, or in commercial units now being built in Britain.

Since the larger solids in most slurries tend to settle on standing and as at least a small ‘ballast’ tank must be provided for the digester feed pumps, some form of stirring in the feed tank is necessary to provide a uniform feed for

As said before, there is a lower limit to viable digester size from an energy generation point of view. The commercial digesters now built or being built here are 41

THE WATT COMMITTEE ON ENERGY uniform digester running and to prevent pump blockages. This is comparatively easy to arrange and is of low energy input if stirrer running is timed with input pump running.

stabilisation system must be interposed before the gas can be piped to its place of use. Also a pressurised digester makes input and output more complex.

In our own experience the scroll and flexible-starter type of pump is most suitable for the comparatively low volume pumping required for digester running. These pumps will push the input into the middle of the digester contents for optimum mixing.

Design and fabrication of floating top digesters or water gas-holders is not, however, a problem.

Digester heating Our own digesters are heated by sludge circulation through an external heat exchanger containing hot water from a digester-gas-fired boiler. Such an external heat exchanger (using water from a gas boiler or cooling water from a gas engine) has some advantages, but has the disadvantage of requiring a large sludge pump to circulate digester contents. This is costly in money and energy even though circulation occurs only when digester heating is required. A much smaller water pump is required when the hot water heat exchangers are placed in the digester and this is the system being used on the commercial digesters now being built. However, care must be taken to ensure that heat exchangers are not too hot and that sludge circulation over the exchangers is adequate to prevent sludge deposits on the exchangers, overheating of sludge in the vicinity of the exchangers and ensure distribution of the heat throughout the digester contents. Research and development on heat-exchanger design, positioning with regard to stirring, and combinations of stirring and heating using heated draught tubes, etc., is proceeding.

The output from a digester can be pumped, but for simplicity and saving in energy our experimental digesters and most farm digesters now built have a gravity overflow of some kind arranged as a ‘weir’ to conserve a slight positive pressure in the digester (see later). Suitable size and good design should obviate most troubles with blockages.

Digester stirring This is an area where much developmental work needs to be done. The mechanical stirrer (working 3 min/h) on our first digester has been very satisfactory, but mechanical instability would prevent a much larger version being used in a larger digester. The same argument applies to a number of the mechanical stirrers used in experimental digesters. Mechanical stirrers for big digesters can be made and we have done some experiments, but cost and complexity and short life of some parts in animal sludges tend to rule these out.

The biggest heat input to a digester is not to compensate for digester cooling but, particularly if the slurry is drawn from tanks external to the animal housing, to heat the input to digester temperature. If heat from the output could be transferred to the input then a large energy saving could be made. This, however, is not a matter of building a simple heat exchanger. The system must not need extra, large pumps or savings in heat energy are offset by increased power consumption, and apart from physical difficulties in flow of slurries, with a weir output system the output occurs after the input has moved (all, or nearly all, digesters will run on an intermittent loading system). A heat exchanger on an American experimental digester ran into such problems of blockage and so on that it was abandoned.5 One of our 13 m3 digesters has an output-input heat exchanger, this works but by no means near theoretical efficiency. Further research and development is in hand.

Intermittent gas recirculation stirring offers many advantages in cost, energy use and lack of moving parts in the sludge. Gas stirring is used in some domestic digesters, but the agricultural sludges differ from domestic sludges and, again, cost considerations come in. ‘Pure’ cattle slurries seem more difficult to stir than pig slurries, apart from any problems with straw bedding. A commercial digester now being developed on a farm has had to have an intermittent mechanical stirrer added to the gas system to help reduce problems caused by straw in the cattle slurry which is mixed with the pig slurry input. Research and development work on gas recirculation stirring is proceeding with our own group and commercial firms. Again, pig slurry is not so much the problem as cattle slurries or mixed animal slurries. Vegetable slurries will probably add more problems. However, solutions will be found.

Gas storage

Gas usage

In a farm situation storage of gas under high pressure is not recommended from the points of view of cost and especially safety. Digesters should ideally be run at low pressure and the gas stored at low pressure (a few inches WG). Since a working digester will produce 1% or more digester volumes of gas a day, storing more than a day or so’s production of gas under low pressure is not practicable. The systems now being built envisage a constant use for the gas, so only a fraction of a day’s production will be stored as a ballast for the gas usage. Gas can be stored at low pressure in a floating top digester or a water-sealed gas holder. Only one of the digesters now being built here has a floating top, this is the rectangular one which has a top of unique construction. Other digesters, including our own, have a fixed top and a separate water-sealed gas holder. There are points for and against both types of storage and on the whole little to choose in costs. One farm digester has a fabric-butyl-rubber top which inflates under pressure. There are many points against this, not least that the pressure varies and some form of pressure reduction and

The type of intensive farming unit which will provide large amounts of slurry has a large demand for energy. The use envisaged for the present generation of digesters is for farm energy. Much larger schemes whereby very large feedlots would provide energy for a nearby town are mainly in the theoretical stage, although one system is now built and being tested. Intensive farm units need, according to the farm’s purpose, energy for heating young stock, ventilation of houses, heating houses, mixing and conveying feed, milking machinery, excreta removal, etc. Heating could be done by hot water from a digester gas boiler, direct gas burners, or by electricity. Electricity is in many ways the most convenient form of energy as it can be used in many ways and demand of one sort and another is almost continuous during the 24 hours. Mains electricity is also of high price and so substitution presents a good monetary return. On one new farm now installing a digester the consultants have programmed the various farm operations 42

BIOGAS PRODUCTION—AGRICULTURAL WASTES to, so far as possible, even out the electricity demand, so that a gas engine generator can be used to maximum efficiency. Other farms are planning to use gas-generated electricity to take over a specific part of the load. Isolation circuitry such that part of the farm circuit can be run from a generator is relatively easy to install. Electricity Boards so far seem to have no objections to part of a farm load being taken by a gas generator, provided some mains electricity is taken to justify the retention of mains connections and wiring. Indeed one Electricity Board is collaborating in the installation of a digester and generator on a farm.

burned in a proper flare-off burner if necessary. In general, provided a few simple sensible precautions are taken near the digester and ‘engine house’, and the latter, where leaking gas might accumulate, is well ventilated, there should be no danger. Use of gas in burners, etc., is no more dangerous than use of town gas. If the gas came into general use in domestic buildings then it might be possible that levels of hydrogen sulphide might be laid down. The hydrogen sulphide content is not enough for the gas to require scrubbing for engines and the authorities are only concerned with sulphide levels for domestic use of gas.

With efficient heat-exchangers on cylinders and exhaust a gas engine using all the digester gas generated will provide more than enough heat for a mesophilic digester in a cool climate.

Conclusion This has been a rather brief survey of research and development in the field of farm-waste digesters. Work in Britain has been mainly quoted, work elsewhere is with a few minor exceptions, perhaps, no further advanced. However, research funds seem to be financing more experimental medium to large digesters on the Continent. In Britain the large digesters are now being built by private funds and it is only through our own efforts and help we have given to the various firms and farmers that we can keep informed of these developments and extend our research to the large scale.

The 12 m3 digester previously mentioned is providing gas-heated water which heats the digester and in conjunction with a tail-gas heat exchanger on an incinerator is heating experimental animal buildings and offices, with a large saving in fuel on the conventional system. There are no difficulties in using digester gas directly in natural-gas heating appliances with a slightly enlarged jet, or in dual-fuel or spark ignition engines. Research is going on at Leicester6 into development of a carburettor for a small engine which can inject gas and paraffin, the paraffin ratio being increased if digester gas supply should decrease. This system will be tested on some of the farm digesters now being built. Gas-engine generators in the 50–100 KVA range are installed or being considered for most digesters at the moment.

There is more work to be done on obtaining basic parameters for digestion of different wastes, but a lot of the work now lies in bioengineering research and development. The work described here is from a joint project between The Microbiology Department, Rowett Research Institute and The Engineering Division, North of Scotland College of Agriculture and the principal workers involved with the author are Sheila Bousfield, P.J.Mills and R.Summers.

There seem to be no practical difficulties in using digester gas providing that a more or less continuous use can be found to obviate problems of storing large volumes of the gas. There has been some discussion of the running life of small gas-engines as opposed to the very large, low speed gas engines used in large sewage works, but only experience will provide the answer.

References

Use of gas for vehicle engines is not to be considered.

1.

Crentz, 1973. Proc. Int. Biomass Energy Conf. X, 1–26

2.

Hobson, P.N., Bousfield, S. and Summers, R. (1974). Crit. Rev. Environment. Cont. 4, 131–191

3.

Swanwick, D.J. (1975) in Methane, Int. Tech. Publications, pp. 5–7

4.

Project on biological conversion of biomass to methane. Dept. of Civil Engineering, Univ. of Illinois

5.

Ecotope Group, Seattle, a joint project of US Dept. of Energy. Washington State Department of Ecology and Washington Department of Social and Health Services

6.

Picken, D.J., Leicester Polytechnic, Leicester

Safety Safety aspects are obviously being thought about. A low pressure digester has built-in safety devices in weirs and water seals which act as safety valves against pressure build up. The pressure should prevent ingress of air through any small hole in a seam, etc. If large amounts of air are pumped in, say through a leaking sludge-suction pipe, the digester bacteria will use up the oxygen leaving nitrogen to dilute the digester gas. Flame traps can be fitted to prevent any possible blow-back to gas holders. Excess gas can be

EXTRACTS FROM DISCUSSION Mr. J.C.Hawkins

digestion does not materially reduce their value as a fertilizer. On a small farm with, say, 50 cows, 50 sows and their progeny, this can amount to over £3000 a year. Farmers will, therefore, wish to retain their manure on the farm to replace purchased inorganic fertilizers, which are increasing rapidly in cost, because of the high energy content of nitrogenous fertilizers in particular.

Firstly, from a recent EEC Seminar on farm wastes, it was clear that Britain, and Dr. Hobson’s team in particular, are probably world leaders in the field of methane production from animal manures. Any manufacturer interested in the design of anaerobic digesters for farm wastes would do well to consult them before becoming too involved.

Dr. P.N.Hobson

Secondly, small-scale plants are required for the treatment of animal manures because it has to be an onfarm process. This is dictated by the problems of transporting farm wastes and by the fact that anaerobic

Modesty forbids comment on your first observation! The second is agreed, and also because in the British context it is unlikely that the gas (or electricity) generated would be 43

THE WATT COMMITTEE ON ENERGY

Dr. D.E.Brown

great enough to supply more than the farm and its immediate surroundings. This is because none of our farms are anything like the scale of the 100,000 animal American feedlot systems from which ‘mains’ gas is proposed and where the digester system is really a commercial power station.

I think that the detailed discussion of anaerobic digestion is leading us away from the general problem of energy production. The previous two papers are suggesting that small units are feasible although figures have shown that cost/unit volume falls as the size rises. Is the interest shown by farmers at parochial levels really caused by the enormous increases in effluent charges? If producing methane was an economic exercise would the sewage works not encourage delivery of sludges by reducing charges?

Mr. A.N.Emery Given that the net energy production is likely to be reduced significantly in cold weather, it would seem that dual fuelling at least would be necessary. Is in fact such a system (a) feasible, and (b) economic in the farming context?

Dr. P.N.Hobson

Dr. P.N.Hobson

1. It is true that cost/unit volume increases as size decreases, so in the context of ‘Western’ mechanical farming in a cold country, there is a lower limit of size for an economic digester. In a warm country where needs for the gas are simple and manpower is not expensive, a very much smaller unit is economically possible.

With an insulated digester and particularly if the waste is taken from underground tanks or channels under animal houses, or if some form of heat recovery on the digester outlet is used, then energy needed for digester heating should not vary greatly between summer and winter in a temperate climate. Even without heat recovery a digester running on a slurry of reasonable solids content should not need extra fuel in winter. It depends on so many factors, solids in waste (i.e. gas production per unit volume of waste), method of using the gas energy (engine or gas heating), siting and design of digester to some extent, etc. In very cold weather there could be some change in net energy production, so that the usable energy output could change and some additional boost might be needed for, say, piggery heating normally covered by digester gas.

2. I am not sure what is meant. Some farmers can run waste to the sewers. The charges are high and have gone up considerably lately, but most farmers cannot put their waste into sewers and pollution controls can face them with closure on complaints of nuisance, or fighting a case can bankrupt them. Part of the interest shown by farmers is due to anti-pollution legislation, not an increase in ‘effluent charges’—they had no ‘charges’ before because they did nothing to the waste; now they must do something. 3. There is very much of a limit to how much farm waste can be put into sewers, supposing these are available. One pig is equivalent to tour humans in waste—it does not take many pigs to completely overload a small town sewage system (which is the system nearest to most farms). The farm waste is strong and so means extra power inputs to the sewage works and so it is charged for if it is taken.

No general answer can be given, really, but the digester itself should not need extra heating. With an intensive farm unit, input of waste to the digester is pretty constant all the year, or all winter (in the case of some cattle units) so gas production is constant. A dual-fuel system for a digester is quite feasible. In fact, our own digesters have gas and oil boilers, the oil boiler being used for starting up before the digester gases properly, or it will cut in with a switch from the gas-holder if gas volume is too much reduced due to experimental running conditions or, say, a loading pump failure, etc.

The only feasible way to get farm waste to a sewage works is via the sewers and this overloads the aerobic plant. Producing methane is an economic exercise if the waste is at the digester site. It can still be economic if the waste is collected from a small radius. But when one begins to think of road tankers obviously there is a limit to the range of a 5000 gallon road tanker carrying waste of 90–95% water at which the tanker uses more fuel and costs more to run than the value of the gas obtainable from the wastes.

In a farm context a similar set-up could be used to take over either digester or building heating if necessary. Economics—the question is really unanswerable. If the animal houses have been heated, ventilated or generally powered by electricity or gas or LPG, then if the digester can take this load on all but perhaps a few days at the coldest part of the year (when additional heating might have been needed in the original set-up), then obviously the digester can be economic. If the digester takes any large part of the farm energy load it can be economic.

I should think most sewage works now tend to be overloaded, without encouraging more waste input, whatever the theoretical economies of a particular situation. But, see also Mr. Hawkin’s comments. The digested sludge is valuable as fertilizer. If the waste were sent to a domestic sewage works, the digested sludge would have to be returned to the farm; another energy input.

44

THE WATT COMMITTEE ON ENERGY

Fuel alcohol production from biomass

Mr. A.N.Emery

Institution of Chemical Engineers

Dr. C.A.Kent

Institution of Chemical Engineers

MAIN CONTRIBUTORS TO DISCUSSION Mr. R.Wheeler

Ricardo Consulting Engineers

Mr. F.W.Hayes Mr. J.Hawthorn Professor A.L.Titchener

Institute of Food Science & Technology University of Auckland, New Zealand

THE WATT COMMITTEE ON ENERGY

Fuel alcohol production from biomass Apart from these newly developing programmes it must not be forgotten that commercial industrial alcohol production by fermentation is carried out in many parts of the world. The total production for fuel and industrial use is insignificant (only India, South Africa, Brazil and perhaps Cuba would be considered major users in this respect) but considerable tonnages are produced for potable and other ‘reserved’ uses. Such production is of course carried out within the constraints of having to produce or eliminate certain flavour/ odour components, as well as having to use traditional raw materials and production methods, so that experience with this aspect of alcohol production may not always be applicable when one considers fuel alcohol production.

Introduction “The use of alcohol as a fuel for internal combustion engines is by no means a new development”. So starts the preface to ‘Power Alcohol’, written by Monier-Williams1 in 1921. That there is little that is new in the world is demonstrated a few pages later:— “Within the last few years the scarcity and rising price of motor spirit has become a serious problem and has directed attention to the whole question of the future development of mechanical road transport”. The current scarcity of coal-fired steam lorries suggests that Monier-Williams was perhaps over-pessimistic. Even nearly 60 years later it is hardly the scarcity of oil that has led to the rapid rise in prices in recent years. Yet, all over the world, for one reason or another, countries face the motorised future with the same apprehension as was reflected in ‘Power Alcohol’ in 1921.

Alcohol as a fuel component It may seem strange to review the end use of the product before discussing its production, but it is useful in that it establishes the principal advantages of alcohol as a fuel— which may subsequently be offset by difficulties in its production.

Then ethyl alcohol was almost exclusively produced by fermentation and was important not only as a fuel but as a major raw material in the organic chemical industry. Subsequently the production of industrial alcohol by fermentation has virtually disappeared, at least in industrialised Europe and North America, as has the use of alcohol as a fuel. The 2nd World War saw the last major boom in production, in Germany, Japan, Scandinavia, North and South America, and elsewhere, but shortly thereafter fermentation processes were rapidly eclipsed by synthetic processes using hydrocarbon feedstocks and alcohol was again restricted to its more traditional role in beverages, pharmaceuticals and toiletries. Consequently technical development can be seen to have slowed down abruptly in the 1940s except in relation to these uses. Those manufacturers who are still in operation use the technology, by and large, of the 1940s or before.

The production of alcohol by fermentation is effectively the only method by which a predominantly single component transportable liquid fuel can be produced from biomass with currently available technology. Absolute alcohol (~99.5% alcohol) can be blended in all proportions to form stable mixtures with petrol and it has been suggested that the problems of phase separation that occur with blends of petrol with impure alcohol (the 95% azeotrope with water) can be overcome through the use of additives.2 When blended with petrol in proportions of up to 25% (by volume) of alcohol, the advantages of greater volumetric efficiency and octane number improvement offset the lower calorific value of alcohol (26.6 mJ/kg) versus petrol (44.0 mJ/ kg). Furthermore the low volatility and higher flash point gives increased safety and may improve fuel economy. The lower engine temperature obtained with alcohol, due to its higher latent heat of vaporisation, results in lowered emission of nitric oxides, less carbon deposit and a lower carbon monoxide content in the exhaust. All this may be achieved without significant engine modification and at no cost penalty. Petrol with a low octane rating may be used for blending, the alcohol achieving the knock rating improvement usually obtained with, for example, TEL.

The fermentation of agricultural products and residues to produce alcohol is a short cut method for the conversion of the sun’s energy to a transportable, clean-burping fuel source and this factor has renewed its appeal in the energyconscious 1970s. Particularly has this been so in locations more sensitive to the rapidly rising cost of petroleum-based fuels and where, for one reason or another, there are potential raw materials available or which can be made available for conversion to alcohol. Probably the furthest advanced in commercial development programmes is that in Brazil initiated in November 1975 with the aim of replacing up to 25% of that country’s oil imports with alcohol derived from sugar cane, manioc (Cassava) and sweet sorghum. A total annual production of some 2½ million tonnes of alcohol is envisaged in 145 distilleries. Also based on sugar cane and molasses are developments in Australia, India and South Africa, while in New Zealand there are tentative programmes based on forest wood and on farm crops. In the USA the Department of Energy ‘Fuels from Biomass’ (FFB) programme has an alcohol plan within which are sponsored approximately 12 experimental and assessment projects, and which envisages a commercial demonstration plant in operation in 1988. In the UK the Solar Biological Programme of the Energy Technology Support Unit has included the production of alcohol from biomass within its remit.

Above 25% concentration of alcohol, the reduced calorific value of alcohol has a predominant effect and power output and fuel economy are adversely affected. Modified engines may even use 100% alcohol (even the 95% azeotrope) and a recent estimate of the cost penalty per vehicle for modification for such unblended fueling has been given as £75.3 A serious disadvantage with such a system is that a completely separate retail distribution system is required. Alcohol cannot be blended with diesel fuel and in any case cetane numbers would be seriously reduced. Work is however in progress in West Germany4 and in India5 on the use of alcohol fuels in compression ignition engines, using dual fuel systems. Although this paper deals primarily with fuel use of alcohol, at least passing mention must be made of the industrial use of alcohol. As well as its general solvent use, 46

FUEL ALCOHOL PRODUCTION FROM BIOMASS alcohol can be further processed to yield the following groups of products:—6 1. Dehydration products—principally ethylene and its products. 2. Oxidation and dehydrogenation products—acetaldehyde, acetic acid and their derivatives. 3. Modification products—butadiene, ethyl chlorides, amines, ether, etc. Of these, the first is undoubtedly the most important— ethylene derivatives represent approximately 50% of petrochemical manufacture—but the factors that determine whether alcohol can replace petroleum sources are complex, including non-economic considerations relating to overall strategies in petroleum product manufacturing, marketing and distribution as well as the obvious economic questions. At current prices, alcohol would need to be reduced in cost by 70% for it to be competitive. Alcohol production methods—present and potential Alcohol is produced from biomass by extracting hexoses or polymers of hexoses, depolymerising the polymers by hydrolysis and fermenting the hexoses together with other nutrients. The fermentation is carried out by yeasts of the same species, Saccharomyces cerevisiae, as the familiar brewers, winemakers, and bakers yeasts. The stoichiometric equation may be expressed as:—

The ideal yield should therefore be 51.1% alcohol. Because other products, including more yeast, are formed, the optimum yield is 94.5% of this and in practice figures much in excess of 92% are not often achieved. Thus each kg of hexose in practice should yield approximately 470g of ethyl alcohol. The activity of the yeast is severely reduced as the alcohol concentration increases and fermentation rates, even for specially selected yeasts, are slowed drastically at concentrations much above 10–12% w/v. Some wine yeasts are known that will ferment up to 20% w/v. This restriction creates the principal problems (in terms both of economy and energy) that face the alcohol producer. 1. The alcohol must be separated from the aqueous solution in which it is formed. Figure 1

2. The aqueous residue, representing 10 times or more the volume of alcohol produced, must be disposed of. With technology presently available the only method available for 1. is distillation and this is highly energy intensive—to the point that the energy requirement for this and for disposal may, and for many manufacturers does, exceed the energy value of the product. The problem is exemplified in Figure 1 which represents the total fuel energy bill compared with the energy produced as alcohol product as quoted by a grain-alcohol manufacturer in the USA.7 In attempting to improve the net energy production of the conversion, broadly one may look for the following:

Energy balance for commercial alcohol production from grain.7 Basis: 1 US gallon absolute alcohol

waste (or by-product) processing. 3. Using by-products of the conversion process to provide energy for the processing, i.e. by combustion of solid residues, or perhaps by anaerobic digestion and gas production. Let us consider each point in turn. 1. Increasing alcohol concentration

1. Increasing the alcohol concentration in the fermentation section, and hence reducing both the energy demand on the distillation section and the volume of aqueous effluent (the stillage) that must be treated or disposed of.

Energy demand in the distillation process is primarily dependent on the concentration of alcohol in the feed to the still (the beer). Table 1 shows the energy demand per unit of alcohol produced as it is related to the beer alcohol concentration for a typical 3-column process for the

2. Improving the thermal efficiencies of the distillation and 47

THE WATT COMMITTEE ON ENERGY To a greater or lesser extent all these are feasible but all are achieved only at the expense both of increased complexity, hence higher capital cost and higher risk, and of a loss of flexibility in terms of the ability to deal with fluctuating feed flow rates and compositions. Given freedom from such restrictions it is reasonable to assume that substantial savings could be made in energy needs for distillation, (a 50% saving of typical current energy requirements might not be unreasonable).

production of 95% alcohol (rectified spirit) and for a process (the Melle 5th technique) for the direct production of absolute alcohol.8 The latter process incorporates an azeotropic distillation within a 3-column process rather than requiring the production of rectified spirit followed by a subsequent water removing distillation. Table 1 Distillation steam requirement for the production of rectified spirit and absolute alcohol as related to the alcohol concentration in the beer

As seen from Figure 1, a further major source of energy loss is in the treatment of residues, particularly stillages. The figure is particularly high in the case quoted since the residue is a valuable product (as an animal feed component) when evaporated and dried and consequently the energy expenditure is worthwhile. One cannot avoid considering some possible energy expenditure in the disposal of stillage, given the volume of material that must be disposed of, even if one only considers conventional water treatment as a means of disposal. Alternatively it is possible that membrane filtration techniques might be applicable to the concentration of stillage (and consequent recovery of water) but there has been no demonstration of the feasibility of this as yet.

After ref. 8

As mentioned above the higher alcohol concentrations are indeed attainable. Indeed even higher concentrations are attainable though at the expense of more extended residence times—thus affecting the economics rather than the energetics of the process. The benefits in distillation energetics become increasingly marginal as the concentration rises but of course the benefits in terms of the reduced effluent problem are not to be ignored.

It is difficult to estimate the energy needs for stillage disposal for many potential substrates, due to the lack of information on the composition and characteristics of the beers, and consequently stillage, likely to be produced. Data are only available for processes based on grain and molasses, and for the former the availability of established markets for the dried products of stillage has meant that a large energy expenditure has been accepted by the producers as a necessity. For molasses, disposal is more difficult due to the stillage composition, particularly the high level of inorganic salts which limit its usefulness as a feed component. Stillage disposal on land is commonly practised but there are many disadvantages in terms of cost, distribution and seasonality and it is reported that after numerous applications the soil becomes acidic, with odour problems and deleterious results in crop growth. Biomass production and methane generation by anaerobic digestion have been suggested but both these processes still produce effluents of considerable strength and volume, and solid products of unproven value. Swedish and Finnish companies have however recently co-operated in a design10 for stillage evaporation and incineration which, it is claimed, gives a net surplus of usable energy, a high-value lowvolume ash product and a very low pollution load in the effluent. It may well be possible to apply such techniques to the stillage arising from other raw materials than molasses but information is not yet available.

As we shall see in subsequent sections the limitations on beer alcohol concentration may not always be in the fermentation itself, i.e. due to the micro-organisms. In many cases the limitation is in the concentration that can be achieved in the feed to the fermenter, bearing in mind that the hexose concentration in this feed must be approximately twice the final desired alcohol concentration (w/v). There is as yet no practical demonstration of the processing of cellulose/ hydrolysates of such concentration as would be required to achieve 10% w/v concentrations of alcohol—indeed no demonstration of the fermentability of such hydrolysates. The pre-fermentation processing may necessarily produce much lower concentrations, unless (again energy-intensive) preconcentration is used. These strictures do not however generally apply to starchy materials—which can be processed at high concentrations. 2. Distillation efficiency General moves towards energy saving in distillation have only attracted widespread attention in recent years, though, as Freshwater pointed out as far back as 1951,9 much of the early work on energy saving was carried out, largely by the French, in the context of alcohol production. The thermodynamic efficiency of alcohol distillation, defined as:—

3. Energy from by-products We shall consider this in greater detail in the sections below dealing with alternative raw materials, but for the moment it may be stated (obviously) that the availability of energy from by-products is primarily dependent on the raw material to be converted, and further; the higher the hexose content as a proportion of the total dry weight of the raw material, then the lower the likelihood of recovery of sufficient energy to meet the processing need. It is also essential that the solid residues are simply dewaterable and combustible. Bagasse may be separated readily and when burned can raise sufficient steam for the complete sugar juice extraction, fermentation and distillation of cane-sugar alcohol, and have some to spare.11 Sugar beet pulp is on the

is typically extremely low (much less than 5%) and this suggests that tremendous scope for improvement exists. In broad terms the methods available for energy saving, are (still as they were in 1951!):— a) Multiple effect methods b) Vapour recompression or Heat Pump methods c) Indirect methods. 48

FUEL ALCOHOL PRODUCTION FROM BIOMASS exploited is the lactose from cheese whey (a commercial plant for potable alcohol is in operation in the Irish Republic). This process is probably only applicable in a very limited number of situations where the milk processing facilities are of sufficient scale and concentration. Even then the total potential contribution to energy availability is limited.

other hand particularly difficult to dewater and the net energy yield from its dewatering and incineration would be small. The raw materials for alcohol production These may be characterised as follows:— a) Those in which the major sources of fermentable sugars are mono-and di-saccharides. b) Starch-containing materials.

b) Starchy raw materials

c) Cellulose-and hemi-cellulose-containing materials.

Starch is available in grain and also in the tubers of potatoes and cassava. As with molasses the technology of its processing is well-developed, though generally only within the constraints of producing neutral spirit for potable use. The raw materials, in some cases chopped or milled, must be cooked to release and gelatinise the starch which must then be hydrolysed to fermentable sugars, using enzymes or possibly acids. Traditionally the source of liquifying and saccharifying enzymes, at least in the west, was barley malt, but nowadays the use of concentrated purified microbial amylases is gaining increasing importance. Concurrent saccharification and fermentation is being occasionally applied, and it is usual, at least in the case of grains, for the complete mash, spent grains and yeast to go forward to the distillation unit.

a) Mono-and di-saccharides Cane sugar is the main raw material of the Brazilian National Alcohol Programme. The sucrose-containing waste of sugar extraction, “blackstrap” molasses, is also widely used for alcohol production, including one plant in the UK. The principal advantage of sucrose is of course that it is directly fermentable without further hydrolysis. Direct use of the extracted juice is generally only possible during a limited part of the year due to the rapid deterioration of the harvested cane or beet. Storage of juice to allow yearround processing can only be considered after concentration to higher than 50–60% sucrose, so-called “hightest” molasses, and there are consequent penalties, energetically in terms of the needs for concentration, and economically in terms of the storage capacity required. The techniques of molasses fermentation, beer distillation and stillage disposal are well developed and are immediately commercially available. As indicated previously the energy recovery prospects for cane processing are considerably better than for beet processing, and there is also considerable promise in the use of Sweet Sorghum. For this, and for cane, new processing techniques are under development aimed at improving the separation of juice, pith and fibre, as well as yielding fibres of a quality high enough for paper making.12 The problems of stillage disposal have been mentioned above.

Without the constraint of production for potable use, it is doubtful that the technology of fuel alcohol production would follow along the lines currently employed for neutral spirit production, which are essentially traditional in nature. Nevertheless this side of the industry has much to offer in terms of technical ability and innovation. Particularly one must mention new methods of rapid fermentation, both batch and continuous, that have been developed in the brewing industry, and in which the UK has technical leads. Energetically, alcohol production from grains and tubers is unattractive since (a) the amount of combustible residue is limited and (b) in any case, under present conditions, this residue has a high market value.

Table 2 quotes the energy balances for alcohol production, as claimed in Brazilian conditions,11 using sugar-cane, sweet sorghum and cassava. It should be noted though that the Brazilian figures do not include any energy component for the disposal of stillage. This may be justified if effective evaporation/incineration techniques can indeed be applied.

c) Cellulosic raw materials Much effort was expended in attempts to produce alcohol from wood and wood-processing residues during the 1940s with plants operational in Germany, Switzerland, Scandinavia, North America and Eastern Europe.13,14 However, none of the processes then developed could now be considered economic either in terms of cost or energy.

Another source of di-saccharides that has been

Table 2 Energy balance for alcohol production in Brazil (Ref.11) Basis. Production from 1 hectare in 1 year

(1) (2) (3) (4)

No utilisation of stems for steam raising Stems used for steam raising Plant crop Ratoon crop 49

THE WATT COMMITTEE ON ENERGY The yields of alcohol were low and the energy requirements high due to the very low concentrations of alcohol produced in the fermentation (1–2% was typical).

Sulphuric acid as potential solvents and a flow sheet of a potential process using the latter has been made.17 This flow sheet has been further adapted by us to apply to a number of cellulosic substrates, including wood, cereal straw, ryegrass, newsprint and sorted refuse, so that the potential yields may be assessed19 (see below).

The problem with cellulose is that it has been designed to form firm structures responsible for the strength and rigidity of plants, and then secured in a glue-like matrix with hemicellulose and lignin.15 There are therefore two aspects of its utilisation; first its release from the matrix to make it accessible for hydrolysis and secondly its hydrolysis to fermentable sugars. The hemicelluloses may be either predominantly xylan, whose hydrolysis products are not fermentable by alcohol-forming yeasts, or glucomannan (more commonly in softwoods) which can be hydrolysed to hexoses. Lignin and its products are not fermentable by yeasts.

In the past two years considerable improvement has been made in the production of cellulolytic enzymes, at the US Army Natick Laboratories and at Rutgers University. Enzyme yields and productivities have increased 7–10 fold and progress made in reducing the economic handicap of requiring expensive inducers for the enzyme production.20 It is claimed that a considerable potential for further increases (4-fold is quoted)21 may be realised in the short term. Similarly, an increased knowledge of the kinetics of acid hydrolysis has led to the specification of operating conditions and flow sheets for new processes that differ radically from those operated in the 1940s.16,17

In recent years cellulose hydrolysis has received renewed and intensive attention, particularly in the USA, and the results of this work suggest that at least the concept of cellulose hydrolysis is approaching potential implementation. Nonetheless, it must be emphasised that much of this work is as yet incomplete and prospective designs must at present incorporate a number of assumptions that await experimental verification, even in the laboratory, let alone commercial plant. Thus there is no demonstration yet that recently developed processes can act on solids concentrations higher than 10%. Thus concentration of the hydrolysate would be necessary. There is, however, no fundamental reason why the use of higher concentrations should not be possible and some designs (on paper) have been published assuming concentrations as high as 30%.16 Furthermore, the alcohol fermentation of cellulose hydrolysates has not been realistically demonstrated and the characteristics of the beer, and consequently stillage, are therefore completely unknown. The scale of the assumptions required in overall designs can therefore be appreciated.

We have now completed analyses of model material and energy flow sheets based on these proposed processes for various appropriate feedstocks from which have been calculated the total alcohol production rates and net energy production from the feedstocks.19 The results for two processes, one of solvent extraction and acid hydrolysis,17 the other of physical pretreatment and enzymic hydrolysis,22 each being followed by conventional fermentation and distillation processes, are shown for various cellulosic feedstocks in Table 3. The basis for these calculations was a throughput of 100 dry tonnes feed/day. The scale of the assumptions made in drawing up these tables cannot be overemphasised—there is no practical data to apply as yetbut nonetheless it is felt that at least comparisons between feedstocks have some validity. Obviously only sorted refuse, and to a lesser extent wood, show much real potential as feedstocks—largely due to the availability and combustibility of the non-hydrolysable residues. It is certainly to be hoped that the solvent extraction processes can be proved practically applicable because calculations made using acid hydrolysis processes without such extraction steps indicate worse energy balances. The picture is in fact less satisfactory even than this table shows, since no account has been taken in this analysis of the energy requirements for the production of the large quantity of chemicals used in these processes.

Recent developments have been in three principle areas:— i) Extraction of cellulose and hemicellulose fractions from natural raw materials. ii) Enzyme hydrolysis of cellulose extracts, iii) Acid hydrolysis. The solvent extraction of hydrolysable cellulose appears to offer a number of advantages over other pre-treatment processes. Tsao17,18 has investigated Cadoxen (trisethylenediamine-Cadmium hydroxide), CMCS and

No account of energy demand or supply in the disposal of stillage has been taken in any of the above calculations. In

Table 3 Comparison of alcohol yield and energy balance from various cellulosic materials: Based on adaptation of processes described for cellulose extraction by sulphuric acid, and acid hydrolysis,17 and physical pretreatment followed by enzymic hydrolysis22 HCV—Total higher calorific value of alcohol produced NEP—Net energy production. This represents the gross energy produced as alcohol fuel less those processing energy requirements which cannot be met from combustion of residues. Where negative it represents a requirement for imported energy to the process in excess of that available from the product. BASIS: 1 tonne dry feed material.

50

FUEL ALCOHOL PRODUCTION FROM BIOMASS the absence of knowledge of the stillage characteristics the applicability of evaporation/combustion methods cannot be confirmed. Of course a practical result of improving the efficiencies of extraction, hydrolysis and fermentation is that the residual stillage is much reduced in its content of organic carbon—to the point where combustion is impossible (given that the ash content is unlikely to be reduced) and other methods, perhaps more energy-intensive, may have to be adopted. It is possible, for example, that membrane filtration techniques may be applicable.

development, and also the possibilities raised by modern developments in the genetic improvement of microorganisms. The consideration of processes based on cellulose feedstocks is of course as subject to the lack of knowledge, and hence assumptions required, as the energy balances already mentioned. In the USA a large number of paper analyses has been published22,23,24,25 with widely varying results. Estimated alcohol production costs have ranged from 80.60/US gallon to $6.00/US gallon, though there is an apparent consensus more recently that a production cost comparable to that of grain-based alcohol can be achieved.

The present picture for enzymic hydrolysis of cellulosic substrates does not look very promising. There is, however, considerable potential for improvement. In particular, the flow sheets used in the calculations quoted above incorporate considerable mechanical and thermal energy inputs for feedstock pretreatment and a large thermal requirement for distillation due to the low concentrations of alcohol produced. As earlier indicated, significant reductions in these inputs should be achievable as a result of present development. It has also been claimed that solvent extraction processes may be used to prepare cellulose for enzymic hydrolysis with considerable advantage, particularly as regards the energy balance. Enzyme production improvements may also have significant effects on enzyme hydrolysis, though rather on cost than the energy balance.

We have made our own calculations based on figures produced by two US groups,22,16 one using enzymic hydrolysis, the other using acid hydrolysis, but translating the analysis to a UK situation. Tables 4 and 5 show the relationship between capital and operating costs and plant size for processes involving, (Table 4) enzymic hydrolysis of urban waste and (Table 5) acid hydrolysis of newsprint. Figures 2 and 3 show the results of applying similar analyses to, respectively, enzymic and acid hydrolysis processes using other cellulosic wastes. No by-product credits have been claimed in any of these analyses. The effects of applying by-product credits (or raw material costs) to the analysis is exemplified in Figure 4, based on acid hydrolysis. The credits and penalties in this figure are expressed in terms of £/tonne of alcohol produced.

The costs of ethanol production from biomass In Europe and the USA, the price of alcohol produced by fermentation of grain, molasses, surplus wine, etc., is comparable to that of synthetic alcohol (from ethylene). The USA position is more useful to study since it is less subject to regulation. The price of alcohol, from either source, is approximately $1.30/US gallon (£220/tonne). The current price of motor spirit—ex-refinery—is $0.36/US gallon (£627 tonne). Discounting statutory regulation it can be seen that oil based fuel prices must rise considerably before there is apparent competitiveness as a fuel, bearing in mind that such price rises would also be reflected in the cost of producing fuels from biomass.

Conclusions As a guide to discussion, an idea is required of the scale of the operations needed to make a credible impact on energy availability. If one takes as an aim 1% of the UK energy demand, incidentally approximately 7% of the transport fuel demand, then an annual production of approximately 3.4× 106 tonnes of alcohol is required. Table 6 shows how this may be translated into feedstock requirements, and hence demand for land and other resources. Clearly few if any of these demands could be approached without a vast revolution in land use, farming, and food supply patterns. Given this, and also the poor prospects for energy efficiency in alcohol production from agricultural and forestry products we are doubtful of the applicability of alcohol production from ‘energy crops’ to countries in Western Europe, and probably in North America, though here there is superficially at least a greater land area potentially available. Brazil and some other South American, African and Australasian countries show high biological productivity potential and large land area availability-and in these circumstances the fuel use advantages of alcohol marginally outweigh the disadvantages apparent elsewhere.

Looking at current processes, the cost of raw materials represents 50% or more of the current total cost of alcohol produced. The remaining costs may be (very approximately) split equally between fermentation, distillation and handling/ disposal of residues. For fermentation the major cost is the capital related charge (associated in conventional processes with the very large capacity required). Although productivity may be improved by an order of magnitude using high rate fermenters, or perhaps vacuum fermentation in which the alcohol concentration is continuously maintained at a low level in the fermenter by boiling off the alcohol as it is formed, a proportionately reduced cost cannot be assumed due to the increased complexity of the processes and equipment. Likewise, costs arising through fuel for distillation use may be substantially reduced, but only at the expense of increased complexity (and hence capital cost) and loss of flexibility.

With one major exception the use of residues does not appear to represent a credible alternative source of raw material for alcohol production. Most agricultural residues are too wet, too scattered and too seasonal to make a credible contribution (note the figures in Table 6 for cereal straw). Corn stover has been under consideration by a number of US groups but those that have considered collection and storage have found that costs escalated rapidly to untenable levels. The residues of forestry and timber processing are sufficiently large that they could be considered as a credible potential raw material. However, it has been suggested,26 that these industries could

The costs of both fermentation and distillation are, as already suggested, greatly dependent on the alcohol concentration that can be achieved in the beer going forward to distillation, without loss of productivity. Increasing this to 15–18% w/v would not only give marked reductions in the size of plant involved, it would reduce energy requirements and the problems of stillage disposal. Such improvements do seem to be possible within the next decade, given sufficient incentive to technical research and 51

THE WATT COMMITTEE ON ENERGY Figure 2

Ethanol production cost as related to plant size. Enzymic hydrolysis22 (US Army Natick Laboratories)

Table 4 Capital and operating costs as related to plant size for the enzymic hydrolysis of urban waste and subsequent production of fuel alcohol Based on processes and costs given in Ref. 22, with modification to a UK context. (Costs at end 1977)

52

FUEL ALCOHOL PRODUCTION FROM BIOMASS Figure 3

Ethanol production costes as related to plant size. Acid hydrolysis16 (Grethlein)

Table 5 Capital and operating costs as related to plant size for the acid hydrolysis of newsprint and subsequent production of fuel alcohol Based on processes and costs given in Ref. 16, with modification to a UK context. (Costs at end 1977)

53

THE WATT COMMITTEE ON ENERGY Figure 4

Ethanol production cost as a function of plant capacity, showing effect of by-production credits and raw material costs

Table 6 Raw material needs for fuel alcohol production from various raw materials in the UK, showing also the number of plants required to meet 1% of UK energy demand (equivalent to 3.36×106 tonnes alcohol/annum) and the appropriate area of land required to be dedicated to raw material supply for each such plant

54

FUEL ALCOHOL PRODUCTION FROM BIOMASS themselves directly use all of their own residues for fuel replacement, leaving no surplus available for fuel alcohol production. Furthermore, alcohol production from wood residues appears to be energetically inefficient-at best only marginally yielding positive net energy.

9. Freshwater, D.C. Trans I. Chem. E., 29, 149, 1951 10. Alfa-Laval Co. Ltd. Involvement in Molasses Distilleries, Company Literature, 1978 11. da Silva, J.G., Serra, G.E., Moreira, J.R, Goncalves, J.C., Goldemberg, J. Cultural Energy Balance for Ethyl Alcohol Production. Inst. de Fisica, Universidade de Sao Paulo, Brazil, November 1977

The one exception is refuse, domestic and industrial. As seen in Table 3 and Figures 2 and 3, the use of sorted refuse appears to be energetically and economically feasible. Furthermore, the raw material is apparently present in credible quantities, and an established collection system exists. To the domestic refuse listed in Table 6 must be added large quantities of industrial paper and board wastes. Nevertheless, it must be emphasised that these conclusions are as yet based on extremely incomplete data, this being a process in which the source of fermentable hexoses is cellulose. It is a strange anomaly that although the production of alcohol by fermentation has been exploited for thousands of years and indeed was the basis of the modern organic chemical industry, there is now so little hard information on which one may reliably assess its position in our industrial future.

12. Atchison, J.E., referred to by Lipinsky, E.S., Science, 19, 644, 1978 13. Underkofler, L.D., Mickey, R.J. (Eds). Industrial Fermentations, Vol. 1, Chem. Pub. Co., New York, 1954 14. Rieche, A. Outline of Industrial Organic Chemistry, 3rd Edition, Butterworth and Co., 1968 15. Linko, M. Adv. in Biochem. Engng. 5, 25, 1977 16. Grethlein, H.E. Biotechnol. Bioengng. 20, 503, 1978 17. Tsao. G.T. Fuels from Biomass Fermentation Newsletter. Ed. K.Bungay, RPI, Troy, New York, April 1978 18. Tsao, G.T. Paper 30, Proceedings 2nd Ann. Fuels from Biomass Symposium, RPI, Troy, New York, June 20th 1978

References 1. Monier-Williams, G.W. Power Alcohol. Its production and utilisation. Oxford Technical Publications, 1922

19. Emery, A.N., Kent, C.A. Unpublished report

2. Anon: Ethanol. A Chemical Marketing Reporter Supplement, November 23rd 1975

20. Mandels, M., Dorval, S., Medeiros, J. Paper 34, Proc. 2nd Ann. Fuels from Biomass Symposium, RPI, Troy, New York, June 20th 1978

3. Lee, W., Menrad, H., Konig, A., Bernhardt, W. Paper 1.5, Intrn’l Symp. on Alcohol Fuel Technology, Wolfsburg, November 1977

21. Montenecourt, B.S., Eveleigh, D.E. Paper 33, Ibid 22. Spano, L. Ibid

4. Bandel,W. Paper 2.3. Ibid

23. Cysewski, G.R., Wilke, C.R. Biotechnol. Bioengng., 18, 1297 and 1315, 1976

5. Pachapakesan, N.R., Gopalakrishnan, K.V., Murthy,B.S. Paper 2.2. Ibid

24. Lipinsky, E.S. Presented at ASSCT Meeting, Fort Walton Beach, Florida, June 23rd 1977

6. Thampy, R.T. Ethyl Alcohol Production Techniques. Noyes Development Corp., Pearl River, N.Y., 1964

25. SRI International, Menlo Park, California. Final Report to US Dept. of Energy, January 1978

7. Cray, C.L., Jr. Gasohol Seminar, Rio de Janeiro, Brazil, September 26th 1977

26. Grantham, J.B. in Fuels and Energy from Renewable Resources. Ed. D.A. Tillman, K.V.Sarkanen, L.A. Anderson. Academic Press, NY, 1977

8. Paturau, J.M. By-products of the Cane Sugar Industry. Elsevier, 1969

EXTRACTS FROM DISCUSSION Mr. R.Wheeler

same time, and this coupled with improved petroleum refining processes eliminated ethanol from the scene, except for a brief spell before World War II, as a minor addition to petrol in Cleveland ‘Discol’.

This conference is oriented towards the production of energy from biomass and not how the energy might be used, although the paper we have just heard does include a section on ethyl alcohol as a fuel component. However, since The Watt Committee includes many institutions whose members would be more particularly concerned with the usage of alcohols in engines rather than its production, I have been asked to enlarge on this topic in the discussion, mainly from the viewpoint of the engine and vehicle designers.

Methanol is also a good high octane fuel produced by pyrolysis of wood or by partial oxidation of methane, but more probably it will be produced from synthesis gas (carbon monoxide and hydrogen), which can be made from coal or general biomass. Both alcohols can thus be made quite easily, and both can be used as petrol extenders with only a few engine modifications of a very minor character. Ethanol from fermentation of surplus grain in Nebraska, USA will be used to extend petrol, and as shortages of petroleum increase this move will spread but has severe limits. These are described in the US Department of Energy Report* on their

Back in 1924, Sir Harry Ricardo in the famous Empire Motor Fuels Committee Report, showed the superiority of ethanol to the petrol of the day in knock rating. Only toluene was net to it in terms of work done per BTU. However, Midgely in the USA discovered tetra-ethyl lead at about the 55

THE WATT COMMITTEE ON ENERGY alcohol fuels programme, e.g. in studying the economic impacts of using biomass for alcohol production, an important issue to be considered is “the effects of controlling production at relatively low levels to stabilise farm commodity prices, thereby reducing or eliminating expensive federal price support and set-aside programs; and the effect of substantially higher production levels on food prices and inflation”.

A severe restriction on alcohol fuels has been their poor performance in diesel engines due to their low compressionignition quality. In a national emergency caused by petroleum shortages it would be the diesels (commercial vehicles) which must be kept going at all costs, and so some ways to run diesels on at least some alcohol fuel must be found. The Swedish Volvo company have pioneered a system of dual injection which allows about 90% of the fuel to be alcohol (to be described at the 3rd International Symposium on Alcohol Fuels, University of Santa Clara, California, May 29–31 1979).

The advantages and disadvantages of alcohol fuels in internal combustion engines may be summarised as follows:—

The only national economies imaginable running on 100% alcohol fuel would be a tropical (Brazilian) ethanol/ methanol economy with fermentation of ethanol being a semi “cottage industry” and larger more central plant for producing methanol from wood and general biomass. In the temperate zones the only possible economy is of methanol from coal, with a few percent of ethanol from fermentation.

Advantages 1. Clean fuel, free from sulphur and lead. No soot production and low emissions of nitrogen oxides. 2. Fuel economy (expressed as miles per BTU) slightly better than petrol. 3. High octane ratings (approximately 100) allow high compression-ratio engines. 4. Wide inflammability limits for very lean (and therefore efficient) operation. 5. Fires can be extinguished by water. 6. Can be locally produced in emergencies.

The poor volume energy content of methanol makes its nationwide distribution costly and its (100%) vehicle operation would need many costly changes to all cars and diesels, e.g. anti-corrosive fuel pumps and carburetters, much larger fuel tanks with consequent body redesign, cold start devices such as intake air heaters, perhaps a second fuel pump on diesel trucks and also new paintwork.

Disadvantages 1. Low volumetric energy content (methanol 0.45×petrol, ethanol 0.6×petrol). 2. High latent heat of vaporisation causes bad starting (petrol ca.80 cal/g, methanol 260 cal/g, ethanol 204 cal/g). 3. Exhaust emissions may contain aldehydes (odorous, require catalysts in silencers to eliminate them). 4. Poor resistance to pre-ignition (i.e. hot spot ignition prior to the spark, as distinct from true ‘knock’). 5. Methanol is toxic. 6. Problem with potability of ethanol (what denaturant)? 7. Corrosivity to metals, e.g. fuel tanks. 8. Degrade plastics and body finish. 9. Poor performance in diesel engines (low compressionignition quality) when carburetted. 10. Invisible flame hinders fire fighting.

However, in 1976 Mobil researchers described their processt for converting methanol to high octane petrol, then at the stage of a 100 barrels per day pilot plant, which could be commercialised during the 1980s. Figure 1 shows this process incorporated into a scheme whereby a coal and condensed biomass feed will readily produce with known technology all the fuels and feedstocks needed by the existing transportation and petrochemical industry. In this scheme, the methanol is an intermediate product only and would attract none of the disadvantages mentioned earlier. This economy would be in competition with more direct coal liquefaction processes which would produce another vital product—metallurgical coke. In the next 50 years, at least in the Western World, biomass will only contribute a few percent of energy needs. As long as coal remains, we are spared the vast labour of collecting biomass; it has already been done for us.

Many oil company people have opposed the use of alcohol blends on technical grounds, feeling that the problems of operation and distribution were not worth the slight energy saving. Some say it would be better to use any alcohol to fire boilers or gas turbines, thus releasing petroleum for petrol and diesel fuel. They cite the poor volume heat content of alcohols as making for costly logistics, so that it is better to use it near the source and not ship it.

Mr. A.N.Emery Broadly we are in complete agreement with Mr. Wheeler as to the likely impact of alcohol. We cannot agree with those who say that it would be better to use any alcohol in boilers and gas turbines for two reasons:

Despite these arguments, alcohols make some sense in limited situations such as methanol in New Zealand, where petroleum supply is bad and local natural gas can be converted to methanol for use in transport. Also in Brazil, with its favourable photosynthesis situation making a sugar fermentation industry an all the year round option. Abundance of cheap labour for collecting and transporting the crops is a vital factor.

a) The cost of producing alcohol can only be justified by its use as a premium fuel (or its replacement of a premium fuel-which boiler fuel is not), and b) The use of alcohol as a boiler fuel does not apparently have any positive advantage whereas as an automotive fuel in a mixture with petrol, the advantages listed by Mr. Wheeler are positive (for example, the elimination of lead from exhaust emissions). One must, we believe, be careful to distinguish in the list of disadvantages those which apply specifically to the use of ethyl alcohol in small proportions with petrol. Reports of recent automotive testing of such fuels have not revealed any serious operational disadvantages.

* Alcohol Fuels Program Plan. US Department of Energy UC-61, 90, 96 March 1978 † Mobil Process for the conversion of methanol to gasoline. J.J.Wise and A.J.Silvestri, 3rd Annual International Conference on Coal Gasification and Liquefaction, Pittsburgh, August 3–5, 1976

56

FUEL ALCOHOL PRODUCTION FROM BIOMASS

Figure 1A total fuel system

Mr. F.W.Hayes

average yield of sugar cane. Even so, such average yields for cane can be very high, e.g. in Malawi 130 tonnes per hectare of cane cut after 11.7 months growth; but almost double this figure has been obtained on experimental plots under irrigation.

Rather than giving my present “affiliation”, it would be more relevant in the present discussion if I should give my past “associations”. I was formerly Technical Director of National Chemical Products in South Africa and then worked with Distillers Company in the UK, and was onetime President of the S.A. Sugar Technologists’ Association.

We must remember that today’s cultivated canes are the result of long years of painstaking and expensive work on the part of the plant breeder; all with the object of obtaining the maximum yield of SUCROSE per hectare per year. Now, in the concept of an “ENERGY” crop, sucrose is of no consequence; it is the TOTAL fermentable substance that counts and this, of course, can include the reducing sugars AND the fibre. For maximum sucrose content, cane can be cut during a maximum of about 5 months in the year: this stricture does not hold in the total energy concept. Varieties exist which are discarded for cane sugar production because of their low sucrose and high fibre content, and some of these are extremely vigorous growers.

In the remarks I have to make I should like it understood that I am in basic agreement with the arguments and conclusions in the paper by Mr. Emery and Dr. Kent, but I feel that, in these discussions on “Energy from Agricultural Crops”, certain aspects are not always viewed in correct perspective. Figures on yields are often very misleading. The only yardstick on which to judge performance of cane, beet, cassava, kenaf, trees, or what-have-you is—“TOTAL FERMENTABLE CARBOHYDRATE PER HECTARE PER YEAR”. It is meaningless to quote figures which take no account of the time taken for growth to the cropping stage.

In many of the discussions which are now rife on the possibilities of ethanol production by fermentation on a large scale, the attempt is made to solve all the problems in one fell swoop: should we not rather be condensing the possibilities and mapping out sequential developments? After all, alleviating a serious fuel shortage by some 10– 15% is still a step in the right direction.

Sugar cane is simply one of many types of GRASS. On the figures we have available to us now, cane shows substantially higher yields than any other practical crop. Very frequently one sees yields of small experimental plots of, say, cassava, compared with a district, or even a country 57

THE WATT COMMITTEE ON ENERGY progressive improvements in, for example, the thermal efficiency of distribution.

In South Africa, “Union Spirit”, which is a 50/50 blend of petroleum spirit and absolute alcohol, is still a viable and commercially successful operation after 25 years of practical experience in this field.

The pentoses recovered from hemicelluloses will not be valuable until there are paying customers for them in the quantities that we have available. Two solutions are at hand: one is to develop new technologies based on xylose as a chemical or fermentation feedstock to produce bulk materials (the current development of xylitol sweeteners is of relevance to this); the other is to use genetic development methods to produce yeasts capable of using pentoses in the alcohol fermentation as well as hexoses. At present, there seems to be little apparent effort in the UK to either of these ends.

I have said that sugar cane is a grass—the conventional feed for raising good cattle—and it is as well that we should bear in mind that a method has been evolved for removing the rind from cane, to yield a product for animal feed. This de-rinded cane has a mean of 30% (27–31%) dry matter, and the d.m. has a mean of 18% (15–20%) of cellulose. To such a feedstuff the green cane tops could also be added, and when we consider the composition of the whole crop we have-cane tops 30%; pith and sugars 55%; rind 15%. To these figures we must also add the concept of the cow as a ‘walking fermenter’, bearing in mind that the efficiency of its fermentation process is dependent on the microflora in its rumen. Then reflect on the fact (established by long and strictly commercial experience) that the residues of industrial fermentations of sugar industry products (such as molasses) are almost magical in their improvement to this same rumen microflora if incorporated in cattle feeds.

Mr. J.Hawthorne Arising from the apparently unfavourable energy balance for Fuel Alcohol Production from biornass, I would like to point out that conventional distillation processes are merely devices for pushing heat into a boiler and throwing most of it away in the cooling water. If this waste heat were used to produce greenhouse crops or to heat fish farm ponds (as two Scottish whisky distillers are at present doing), the unfavourable balance suggested in the paper would be transformed.

I mention this because of the vital necessity to allow for the utilisation of the spent wash (“slop” or “dunder”) from industrial fermentations which will no longer be able to afford the luxury of “an effluent”.

Dr. C.A.Kent The unfavourable energy balance, so far as the production of alcohol is concerned, would remain no matter what you did with the waste heat. One would simply have to answer in the circumstances pertaining at the time, the question, “Are fish farming or greenhouse horticulture appropriate processes for dedicating so much energy to?” Even economically, such schemes must rely on a quantifiable benefit being assessed AND PAID FOR, by the user of the waste heat.

This last consideration leads me to talk of the danger of using current figures for fermentation production costs and then extrapolating for the purpose of evaluating long-term and large-scale future developments. The technological efficiencies must also be extrapolated. It is absurd to use the pitiful figures of steam and energy consumption of the traditional fermentation industry in visualising a technological advance over the next 25 years. The “heat and power” balance in the industry, generally speaking, is notoriously bad. Whatever the selected fuel source for the supply of steam and power, that material must be combusted efficiently, steam generated in modern steam producers at high pressures and temperatures, and the heat drop through back-pressure or pass-out turbines used to provide the low-pressure process steam. The “new” fermentation plant can only allow 100% recovery of the latent heat of vapourisation. Efficient heat exchange throughout the plant and a correctly balanced “heat and power” usage are mandatory for commercial success.

Chairman I understand that the price of petrol is likely to increase eventually to five times its present level, rather than 2–3 times. We should, therefore, not be too depressed by the price comparison with alcohol.

Prof essor A.L.Titchener I should like to comment briefly on the matter of the economics of various fuels. The cost of alcohol in the USA, as Mr. Emery has said, is about $1.20 per US gallon, whatever its source. A modest tax concession can make a vast difference to the “economics” of using this alcohol as a fuel. For the State of Nebraska a tax reduction of 5c./gal is available on “synthetic” fuels for the motor vehicle. A mixture of alcohol and petrol counts as “synthetic”. A “Gasohol” of 10% (grain) ethanol and 90% petrol sells competitively with petrol in this state, to the extent that the seller proposes to increase the number of filling stations selling this mixture. (A 5c./gal tax rebate on the mixture represents, of course, a 50c./gal subsidy on the alcohol). Pursuing this question of economics one can say, I believe, that what will in the end determine whether ethanol or methanol will be a sensible gasoline extender or substitute will be local factors. In Brazil it is low world sugar prices and little indigenous petroleum, which produces a crippling overseas oil bill, that makes their policy decision sensible. In New Zealand it is also a crippling overseas oil bill plus farm protection policies of the EEC which have led the Government of the country to decide recently to head towards a 15% methanol/85%

So, in talking of the future—our large-scale fermentations will certainly be continuous; they will be run at high temperatures using genetically engineered thermophilic organisms; the metabolic products of fermentation will be removed from the fermenting mash as formed; the spent wash from the fermentation will have outlets in the rumen feed business or in new, specially adapted processes of anaerobic digestion. Finally, if we are considering a grass like sugar cane, the process flowsheet will take due steps for the treatment of the hemicelluloses and the recovery of valuable pentoses as well as developed means for cellulose hydrolysis.

Mr. A.N.Emery and Dr. C.A.Kent The figures for fermentation and distillation efficiencies in our assessment studies do reflect the extrapolation of technological efficiencies referred to by Mr. Wynn Hayes. We have taken figures from the best of current design practice and in additional exercises looked at the effects of

58

FUEL ALCOHOL PRODUCTION FROM BIOMASS petrol mixture, the methanol to be obtained from a large undersea gas field about to come on stream.

Mr. A.N.Emery I fully agree. It follows though, that such monetary decisions will have to be demonstrated to be politically acceptable, at least in democracies. Already there is a danger that such measures may be seen to be meeting only sectional interests in the community. After all, where is Nebraska going to recoup its tax loss from—so that corn farmers’ incomes may be raised?

59

THE WATT COMMITTEE ON ENERGY

Conversion of biomass to fuels by thermal process

Dr. B.W.Hatt Dr. A.V.Bridgwater

Royal Institute of Chemistry Institution of Chemical Engineers

MAIN CONTRIBUTORS TO DISCUSSION Mr. A.N.Emery Mr. S.R.G.Taylor

Institution of Chemical Engineers Institution of Mechanical Engineers

Dr. R.E.Pegg

Institute of Petroleum

Mr. C.Martin

Institution of Public Health Engineers

THE WATT COMMITTEE ON ENERGY

Conversion of biomass to fuels by thermal process Why thermally process biomass?

ways of converting biomass to fuels efficiently and various routes have been investigated.

The chemical energy of biomass can be converted directly into heat energy by combustion with very high efficiencies; therefore any thermal processing of biomass into a secondary fuel must be justified on bases that offset the loss of energy necessarily involved in such a step. There are two major reasons for converting biomass into another fuel. Firstly, one may aim at improving the “quality” of the fuel which could be to increase its calorific value per unit volume or unit weight in order to make storage and transport easier or more economical. Alternatively the fuel produced might be compatible with existing combustion equipment (burners, boilers, etc.) or with existing patterns of energy use, whereas the biomass would require new methods of use to be adopted. Also the new fuel might give rise to fewer problems associated with pollution than would direct combustion of the biomass. The second reason for processing biomass is to produce a supply of fuel with consistent characteristics from a range of feedstocks of various quality.

The various thermal routes for converting biomass to fuels Thermal processes is a generic term for a group of processes that are based on the high temperature degradation of organic material, and they are given at the top of Figure 1. Each is named after the predominant chemical reactions that take place but, as will be apparent below, the same reactions occur in most of the processes and therefore distinction between the various routes is not always clear cut. Also the terms are sometimes used in slightly different ways by different authors. The easiest way to relate the processes to each other is to look at the chemical reactions that are taking place in each case.

Pyrolysis is fundamental to thermal processes, and is the degradation of molecules by the action of heat in the absence of oxygen. The complex organic molecules present in biomass begin to break down at temperatures around 250°C and at about 500°C pyrolysis occurs at a significant rate. Smaller, less complex molecules are formed which, being in the vapour phase at that temperature, tend to leave the pyrolysis reactor. However, in so doing they may degrade further, or react with one another, or react with fresh feed giving still more products. If the vapours are cooled upon leaving the reactor, both

The thermal processing of biomass has been practised for centuries. Wood has been converted into charcoal which has a higher calorific value per unit weight, gives little or no smoke as it burns, has an easily controlled combustion and can give higher temperatures than wood. In more recent times coal has been pyrolysed on a large scale to give coke and town gas, both of which have superior characteristics as fuels. Much work has been devoted over the years to

Figure 1

Outline of possible thermal routes for the conversion of biomass to fuels 62

CONVERSION OF BIOMASS TO FUELS BY THERMAL PROCESS steam. A closely-related process is hydrogenation where the biomass is reacted with hydrogen under less severe conditions to give a range of hydrocarbon liquids and gases.

liquids and gases are obtained. The process is outlined below:

Thermal process products as fuels The gas, liquid and solid products from most of the thermal processes are potential fuels. Whether it is worth producing and marketing a particular product depends on its various characteristics, and the properties of the various products are now summarized.

When biomass burns, this preliminary pyrolysis is followed by reaction of the hot vapours and the char with oxygen to give mainly carbon dioxide and water, and heat that causes further pyrolysis. Thus combustion of biomass is closely related to the pyrolysis of biomass to give vapours and char, followed by the subsequent combustion of these products.

Gas product The calorific value per unit volume of a gas is an important parameter in determining its value as a fuel. Gases are normally transported by pipeline (the present natural gas distribution system used pressures up to 70 bar in its main grid) and as compression of a gas requires energy, such a step will only be economical if the gas has a sufficiently high calorific value. Liquefaction is economical with the higher hydrocarbon gases such as propane and butane, but the gases produced by most thermal processes consist mainly of carbon monoxide, hydrogen and methane, and their liquefaction is unlikely to be economically attractive.

The non-condensable pyrolysis gas is predominantly carbon monoxide and hydrogen with some carbon dioxide and methane. There may also be present small amounts of higher hydrocarbons and other gases. The pyroligneous liquor separates into a non-aqueous phase consisting of tars and oils, and an aqueous phase that is a dilute solution of oxygenated organic compounds such as acetic acid, acetone and methanol. The amount of char, composed of carbon with inorganic impurities, will depend on the temperature, but also to some extent on the water content of the biomass as it can react to give carbon monoxide and hydrogen.

For convenience, gases can be grouped according to their calorific value:

Pyrolysis reactors or retorts can be heated externally but heat transfer is not good and an alternative is to pass a hot inert fluid through the bed to give intimate mixing. In both these methods the heat is generated outside the bed of feed.

At the present, it is probably uneconomic to transport the low and medium calorific value gases over distances of more than a few kilometers. However, these gases can be and are used by plant close to the point of production and several types of gasifier operate by burning the hot pyrolysis gas in a boiler immediately adjacent to the gasifier. Thus the gasifier and boiler are functioning as a two-stage incinerator. Such a process often has advantages in making it easier to avoid the polluting emissions that occur with normal incinerators or boilers.

In gasification or starved air combustion heat is generated within the bed by, in effect, burning part of the feed. A less-than-stoichiometric amount of oxygen is admitted to the hot bed and the products of the subsequent pyrolysis and oxidation are mainly gaseous. If air is used rather than pure oxygen the products will contain a considerable amount of nitrogen. One finds a lower concentration of combustion products than might be expected as they react further with the char and other pyrolysis products. Ideally the solid product from a gasifier should contain little carbon. If the gasifier is run at very high temperature (about 1650°C) the ash melts in the reactor and a slag is produced.

Gases are also characterised by their Weaver flame speed factor and their Wobbe Number (calorific value/square root of the specific gravity) and these relate to the type of appliance in which the gas may be used. Carbon monoxide is toxic and thus it is unlikely at present to be supplied to the domestic consumer who is accustomed to non-toxic natural gas. The gas products containing carbon monoxide, however, are suitable for industrial users situated close by.

When water is present in the pyrolyser, steam reforming reactions tend to give a gaseous product consisting almost entirely of carbon monoxide and hydrogen with few organic compounds in the distillate and little carbon in the char, and these reactions predominate in a steam reforming process for biomass. The steam can be added separately or it may be present due to the use of a wet feed. Sometimes steam is added to a gasifier to give autothermal operation whilst ensuring complete gasification.

Liquid product The oxygenated organic compounds in the aqueous phase, the pyroligneous acid, will have calorific values in the range 10–30 MJ/kg, but the value of this product will be considerably decreased by the water which is present. The water and organics can be separated by conventional chemical engineering processes but may require considerable energy and could well make the process unattractive. The tars and oils in the non-aqueous fraction

Hydrogasification reactions occur when pyrolysis takes place in the presence of hydrogen to produce mainly methane. The hydrogen can be added or generated in the reactor by the shift reaction between carbon monoxide and 63

THE WATT COMMITTEE ON ENERGY

Synthesis of higher alcohols

have a higher calorific value, but the yield of these is often not very high when a biomass feedstock is used, and they are difficult to handle. The pyroligneous liquor can be used as a fuel if the water content is sufficiently low but it requires special storage and piping as its acidity makes it corrosive. It is more difficult to atomize than normal fuel oils and, whilst it may be blended with hydrocarbon oils in a burner, the two fuels are not fully miscible. The Garrett Process, operated by Occidental for the pyrolysis of municipal waste, gives a “Garboil” with a calorific value of about 23 M J/kg, but the plastics in such waste may be expected to give higher values for the liquid product than obtained from biomass.

Synthesis of hydrogen (Shift Reaction)

In addition, Mobil Research and Development Corp. has recently developed a process for the direct catalytic conversion of methanol to high-octane gasoline with high specificity, and this route is an alternative to the direct Fischer-Tropsch process for the synthesis of mixed hydrocarbons.

The liquid products from hydrogenation are hydrocarbon oils that have high calorific values and could easily be used to supplement or replace existing oil supplies. These are conventional oils, unlike Garboil.

The pyroligneous liquor from pyrolysis could possibly be upgraded as a fuel by chemical treatment if removal of the water is viable; one might, for example, esterify the acids to give a more tractable product. However, it may be better either to recycle the liquor so that it is further degraded, or to use it as a fuel for the pyrolysis process, or it may be steam reformed to synthesis gas.

Solid product Biomass has a relatively low ash content and so the char, where it is a major product, will have a calorific value approaching that of carbon (32.8 M J/kg). This makes it a good fuel from the point of view of its calorific value, but while charcoal is widely used in some developing countries, in the United Kingdom charcoal is not used extensively either domestically or industrially. The yield of charcoal is rarely in excess of 25–30%, based on dry weight of biomass.

The char, too, can be converted into a gaseous product by gasification and water gas reactions which can then be used in the subsequent synthetic processes, or it can be burnt to provide the heat required by the pyrolysis process. Effect of feed, reactor variables and reactor type on the products .of thermal processing

The products of thermal processes that are most immediately attractive are those from hydrogasification and hydrogenation. Unfortunately these processes are the most sophisticated, requiring high pressures for effective operatior and so far neither has been developed commercially for biomass feedstocks. Thus if one does not wish to use the fuel products from pyrolysis, gasification and steam reforming directly, it is necessary to consider processing them further to give more acceptable fuels, and it is here that thermal processing shows its flexibility.

Feed All organic compounds are susceptible to pyrolysis and thus the thermal processes can accept a very wide range of feedstocks. Biomass contains substantial amounts of cellulose and thus cellulose has often been used in research projects to simulate biomass, although woody materials also contain lignins. The reactions that occur in pyrolysis are extremely complex being determined not just by the composition of the feed, but also by reactor design and other process variables, as are discussed below. There is little general evidence so far to permit a good correlation of product composition with the nature of the biomass feeds, or indeed with the reaction variables.

Conversion of the products of thermal processing to high quality fuels The gas from pyrolysis, gasification and steam reforming processes may be regarded as a crude synthesis gas as it contains considerable quantities of carbon monoxide and hydrogen. This mixture is a feedstock for several chemical processes extensively used in this century to produce, for example, methane, methanol, higher alcohols, hydrocarbon liquids and hydrogen, all of which can be used as fuels. The raw gas will usually require various purification steps before further processing, and the ratio of carbon monoxide to hydrogen can be adjusted, if necessary, by the shift reaction of carbon monoxide with steam. Some of the various uses for synthesis gas are here summarized:

Much of the recent work on thermal processing has used municipal refuse as feed, for this approach sterilizes the rubbish and greatly reduces its volume as well as producing fuels and energy. A large percentage of the organic content of refuse is cellulose and thus it approximates to biomass, but the small percentage of plastics pyrolyse to give hydrocarbon gases and oils. Such feeds also contain large amounts of ash, glass and metals which are essentially inert in thermal processes, although they represent an energy requirement as they also have to be heated to the process temperature unless they are removed beforehand. The char from processes using raw refuse will have a low calorific value and a commonly adopted alternative is to separate the inert and organic materials before processing.

Reactor variables One of the most important process variables in pyrolysis is temperature and the general trend in pyrolysis is for the yield of liquids to decrease with increasing temperature, and for the gas yield to increase. A typical graph is shown in Figure 2, but the actual percentage composition at any temperature will 64

CONVERSION OF BIOMASS TO FUELS BY THERMAL PROCESS environment may consist predominantly of pyrolysis gases, as in simple pyrolysis, or it may contain steam (either from moisture in the feed or from added water), carbon dioxide (particularly if combustion gases are used to heat the feed), hydrogen from the shift and water gas reactions, carbon monoxide, air or oxygen if a gasifier type of process is used, or an inert gas such as nitrogen. This again shows how closely related are the various thermal processes and distinction between them is based mainly upon which reactions predominate in a particular reactor, and this in turn is to a large extent determined by the gas composition in the reaction zone. Pressure also has a considerable effect on which reactions are favoured thermodynamically and would also be expected to affect the rates of reactions.

Figure 2

The rates of many of the reactions can be affected by catalysts which can be added in some way to the reaction zone or they may be present naturally in the feed—an example of the latter is the catalytic effect that sodium carbonate, formed during wood pyrolysis, has on gasification reactions. With municipal refuse many unexpected catalytic reactions can occur, perhaps at irregular intervals, due to the wide and varying range of chemicals present.

A typical graph of effect of pyrolysis temperature on yield of products

So far in this section the effect of reactor variables have been discussed mainly with respect to pyrolysis as it is a common factor in thermal processing. The conditions for gasification and steam reforming may also be varied within limits, but are usually chosen to give an optimum yield of gas and little char or liquid products. Hydrogasification is done at high pressure in the presence of catalysts to give good yields of methane. The hydrogenation process is somewhat atypical among the thermal process in that it is normally done with an aqueous or organic phase present, high pressures again being applied.

also depend on other variables. For example, the rate of heating has a strong influence on the products; a slow rate of heating enables thermodynamic control of pyrolytic processes to be dominant and so the products that are energetically more stable tend to be formed. Thus the charcoal yield from wood is highest with low heating rates. At high rates of heating kinetic control of the molecular degradation is important and products are formed that are not necessarily the most stable at that temperature. The yield of pyroligneous liquor is highest if the feed is heated rapidly to a relatively low temperature (about 500°C). In general:

Reactor type Many sorts of reactor can be and have been used in thermal processes and may be used for classification purposes. Whilst for a particular reactor several variables are used to alter the yields of products being produced, each type of reactor has certain inherent characteristics that cannot be changed easily, if at all, and which determine the range of product compositions that are possible. Thus, for example, one type of reactor may only be able to give a relatively slow heating rate, whilst another type is particularly well suited to encouraging reaction of the intermediate products with the char. Some of the types of reactor that have been used are:

* Liquid products are favoured by rapid heating rate to low temperature * Char products are favoured by slow heating rate to low temperature * Gas products are favoured by high temperatures and heating rate is less important. High heating rates are favoured by the feed having a small particle size, but comminution can require large amounts of energy that have to be included in the energy balance calculations. Finely ground solids are often more convenient to handle and for reactors such asfluidized beds such feeds may be necessary.

i) vertical shaft reactor—the feed falls slowly down a vertical cylinder which may be heated externally or by an upward (or downward) flow of hot gas. Air or oxygen blown gasifiers are usually of this type.

The space velocity of gases (and solids) in the reactor will determine the rate at which the products leave the hot reaction zone. High gas velocities will favour the more unstable products such as liquids as their further degradation will be halted. The direction of flow of the gases relative to the solids in the reactor will determine with what the initial products of pyrolysis can react. For example, in a vertical shaft reactor where the feed falls under gravity, an upward or countercurrent flow of gas will allow the initial products to mix and react with fresh feed, whereas a downward or co-current flow will enable these vapours to react further with the char. A cross-flow of gas will quickly remove the products from the reaction zone.

ii) rotary kiln—feed passes slowly down a rotating cylinder that is slightly inclined from the horizontal with either external or internal heating by gasification reactions or by a heat transfer medium. iii) cross-flow reactor—this is related to the vertical shaft reactor in that the feed falls vertically, but it is heated by hot gases flowing across a section of the reactor containing grids in the wall. iv) multiple-hearth reactor—this consists of series of vertically-mounted hearths that are continually raked to transfer the feed across each hearth and then to the hearth below, against an upward flow of hot gas.

The nature of secondary reactions will also be determined by the reactor environment, that is, the composition of the gases in the reactor. This gaseous 65

THE WATT COMMITTEE ON ENERGY v)

fluidized bed reactor-finely divided sand, or similarly inert material, is fluidized by an upward flow of hot gas, and feed is added to the bed. Alternatively the feed and/or the char product can be fluidized on their own.

earlier. Energy efficiencies are really only useful in comparing processes that give products of the same composition or products that are regarded by other criteria to be of equal value. There is still considerable confusion in this area and there is a need for agreement on standard ways of evaluating efficiencies.

vi) transport reactor or entrained bed reactor—here the feed, together perhaps with char or other particles, is carried vertically by hot gases moving at a high velocity.

The state-of-the-art in thermal processing

An alternative way of classifying thermal process reactors is based on the method of heating the feed rather than on their physical form. If the heat is generated at a point remote from the reaction zone, it may be known as indirect heating, in contrast with direct heating where the heat is generated actually in the bed of feed, as occurs in gasifiers. Indirect heating requires a way of transferring the heat to the reactor and this may be done by taking hot gases, liquids or solids and mixing these with the feed in some manner. Several gases can be used, such as pyrolysis gases, carbon dioxide, steam, nitrogen and carbon monoxide, and that which is used will strongly effect the nature of the reactions taking place as most of them are chemically reactive. Where liquids and solids are used, they are normally unreactive and are recirculated between the reactor and the heater. Molten salts and metals have been successfully used as liquid heat transfer fluids, and among the solids used have been fluidized sand and ceramic or metal balls.

The last decade has seen a resurgence of interest in the thermal processing of biomass and related feedstocks, particularly in the USA, although work has also been done in Japan, West Germany, Great Britain, Denmark and other countries. The scale of the work has ranged from the conceptual or theoretical process to the demonstration and commercial plant. Municipal refuse has been a widely used feedstock as it has to be collected and disposed of anyway and any return in the form of thermal energy or fuel is attractive. It is unfortunate that refuse is a particularly intractable material, being of variable composition and highly inhomogenous. Several processes use sophisticated “front-end” treatments such as shredding and sorting which not only give a more easily handled feed with consistent characteristics but also improves operability and throughput. Such stages are energetically and financially costly but viable under certain circumstances. There have been several slagging vertical shaft reactors on a demonstration scale that accept untreated refuse and gasify it with air to give a low calorific value gas, e.g. Andco-Torrax. The Union Carbide “Purox” process is similar except that it uses prepared refuse and oxygen rather than air and gives a higher quality product that is suitable for purification to a synthesis gas. Pollution Control in Denmark have marketed an externally heated, vertical shaft reactor to pyrolyse municipal refuse, although the problems of heat transfer in such a system must make scale-up difficult and possibly uneconomic.

Efficiency of thermal processes An important way of assessing and comparing thermal processes is to examine how well each one converts the chemical energy in the feed to the chemical energy of the products. Every process requires an input of mechanical or thermal energy, which may be provided by burning part of the feed, some of the products, or by using an external energy source such as oil or electricity. The energy produced by a thermal process, including the chemical energy in the products and the heat energy lost by the processing, must equal the energy put into the system, and this includes not just the energy in the feed but also any process energy inputs. Efficiencies are usually quoted for the processes reported in the literature, but too often the basis for the calculation is not given. This has led to several ambiguities with different values being given for the efficiency of the same process.

Monsanto developed the “Landgard” process to deal with the refuse of the City of Baltimore, and it is based on a commercial-sized rotary kiln acting partly as a gasifier, although an oil burner is also introduced into the drum in several of the reported operations. The hot fuel gases are immediately burnt in a separate chamber to raise steam, as in many of the other commercial-scale plants, thus acting as a two-stages incinerator as discussed above. Other firms have heated steel rotary kilns in a fire box, giving higher quality, pyrolysis gases; the tumbling action would be expected to give improved heat transfer over the externallyheated vertical shaft. An alternative that has been operated by the Oil Shale Corp. is to pre-heat ceramic balls and then add them to the rotary kiln where they mix intimately with the feed and cause pyrolysis.

Values for process efficiency are only useful where they express the energy of the products relative to the total energy input to the system, and after allowing for any use of some of the products in the process to provide heat. But even this is not straightforward for there is a problem in equating energy in different forms. This is particularly so with electrical energy as it takes about three units of chemical fuel energy to produce one unit of electrical energy, and so it is arguable that all electrical energy inputs should be multiplied by three in calculating the energy efficiency.

Multiple hearths have been used by many firms to treat wastes such as sewage sludge and, provided the feed is not too wet, gasification to a fuel gas can be achieved. Runs with municipal refuse on a demonstration-scale have proved technically feasible, and more recently Garrett Energy Research and Engineering have conducted pilot scale studies on pyrolysis of manure in a multiple hearth.

Most thermal processes do not consume any other chemical than the feed, but an exception is where oxygen is used in a gasifier. The production of oxygen requires some input of energy and this too should be included in any energy accounting. However, the disadvantage in energy terms may well be offset by the process giving a more desirable fuel, and this again shows that efficiency in energy conversion to a fuel is not the sole criterion for evaluating processes-after all straightforward incineration or combustion can be regarded as 100% efficient but is not always the most suitable process for reasons discussed

Fluidized bed reactors have attracted considerable interest from R and D organizations as this type has very good heat and mass transfer characteristics and so should be capable of relatively high throughputs. Most of the work has aimed at gas production although tfie production of liquid products might also be readily achieved. 66

CONVERSION OF BIOMASS TO FUELS BY THERMAL PROCESS Although all this work and much else besides has been done (over 160 different centres have been identified where work on thermal processing has been or is being carried out), it is still not clear which processes and even which reactors are most suitable for the thermal processing of biomass. Surprisingly little comparative study and theoretical evaluation of the processes has been reported and the fact that a particular process has been employed on a large scale does not necessarily mean that it is the most suitable from an energy viewpoint; other factors often influence the choices made. The chemical reactions are very complex and the effect of process variables need more investigation to provide a sound data base upon which to design reactors and processes. At present some of the information appears to be conflicting due to the effect of variables that have not been controlled as their importance was not realised.

Conclusions Thermal processes offer a relatively straightforward and flexible way of converting a wide range of organic feedstocks into several products which are both useful fuels and valuable feedstocks for the chemical industry. Conceptual studies around the world have suggested that such processes can be economically and technically attractive. Acknowledgements This work was supported in part by the Energy Technology Support Unit of the Department of Energy, but the views expressed are solely those of the authors. They would also like to thank Dr. George Ader for his encouragement.

EXTRACTS FROM DISCUSSION Mr. A.N.Emery

attractive—domestic and commercial waste; a feedstock which contains the necessary carbon.

What is the methanol production from a feed rate of 500 tonnes/day?

However, we in the UK must, in future, save not only energy but materials. It follows therefore that we must avoid dustbin waste (prevention is better than cure). If we do become more socially conscious and responsive to this problem then there will be less carbon in the dustbin. If we have to collect from a wider area then the net energy balance may indeed be negative.

Dr. B.W.Hatt It is difficult to specify exactly the likely methanol production rate but it is likely to be in the range of 50–200 tonnes per day from feed of 500 tonnes per day dry biomass. The likely yield will depend on the type of thermal process and the conditions of degradation.

A second factor is that of communication technology. If we have the often promised revolution in electronic communications then again we may well see a large reduction in the calorific waste in the dustbin.

Mr. S.R.G.Taylor I should like to respond to the statement that we would not use wood to fuel a car directly. On the contrary, I believe that it is an attractive possibility with an external combustion-for example a Stirling—engine burning densified wood of specific gravity up to 1.25. This has a mass energy density about half and a volumetric energy density of the same order as those of gasoline: This means that the vehicle range can easily match that of a petrol engined car-unlike a battery electric car.

I would, whilst standing, also like to comment on the saving of energy by compressed wood burning. In principle this is fine but we have to consider the poor energy density and perhaps even more the rate of change of technology. To produce significant numbers of cars, engines, etc. having radically different technologies is a slow process as we have seen in the development by Detroit on emission controls for the automobile engine. One final comment—we tended to get bogged down with fuel. It is my opinion that we should be looking at biomass not only for fuel but as a chemical feedstock . This may be an easier target and could result in a reduction of energy (conventional fuel) consumption rather than trying the impossible viz. competing with gasoline.

Direct use of such “densified biomass fuel”—for which preparation plant is already available—avoids the need for expensive alcohol conversion plant with its major associated energy losses. Furthermore the Stirling engine has already demonstrated high thermal efficiency, low noise and low emissions in automotive use as well as multi-fuel capability. The quantitive information was taken from “Densified Biomass Fuel” by Dr. Tom Reed of the Solar Energy Research Institute, 1536 Cole Boulevard, Golden, Colorado, 80401, USA. He also provided the sample shown at the meeting.

Dr. A.V.Bridgwater The concept of an external combustion engine is of course quite possible. It does seem unlikely however that such a radical departure from accepted practices will be realised particularly with the associated pollution problems. This view seems to be supported by comments from Dr. Pegg.

Dr. R.E.Pegg I would like to congratulate the authors on their excellent review. They have clearly shown that thermal processes may offer an attractive method for the conversion of a wide range of organic feedstocks into products which are useful as fuels and, in my opinion, desirable chemical feedstocks.

The recent long range forecast has suggested that the most likely substitutes for natural petroleum are stored electricity and chemical fuels derived from coal and similar non-oil sources.

Conceptual studies carried out indicate that such processes may become economically and technically attractive. However, I would like to comment on one factor concerning feedstock which today has been suggested as

We do largely agree with what Dr. Pegg is saying in that thermal processing of biomass does offer the possibility of 67

THE WATT COMMITTEE ON ENERGY producing not only fuels but chemical feedstocks, and it is likely therefore that any fuels produced in such a way would have to compete directly with alternative uses. On the subject of refuse, the energy potential from the whole of the refuse in the UK, if realised, would only satisfy about 5% of current demand. There is, therefore, limited potential here, but potential that should not be wasted. As far as direct use of wood is concerned, one argument to be forwarded is that wood should be recycled as much as possible rather than being burned directly, i.e. wood might first be converted to primary paper, then recycled to secondary board before being relegated to the dustbin where it might be turned to chemicals. This can be done almost as efficiently as direct conversion with our existing collection and recycling system and therefore is not unattractive.

future fuel costs are going to distort rapidly as shortages develop. Surely research should concentrate on the maximum energy gain from processes.

Dr. A.V.Bridgwater

Mr. C.Martin

An industrialised western society is largely run on the basis of financial viability and therefore there is little likelihood of the considerable capital investments being put up if any processes appear to be hopelessly uneconomical. The only way this might be done is by some form of fiscal incentive. However, the time will come when conventional fuels will suffer an increase in price considerably above the price of the materials, i.e. above the inflation rate, and there will be a point at which fuels from non-oil sources will become cheaper as well as more attractive than from oil sources.

I am very concerned about the emphasis on economic and commercial viability. These relate to the present day cost of other fuels. We should look at the energy economics of these processes-they are designed for the future and in

We do agree that any processes to derive fuels and chemicals from biomass should be optimized but it does seem wrong at this stage to specify whether the optimization should be technical, economic or energetic.

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THE WATT COMMITTEE ON ENERGY

Department of Energy: solar biological programme: biofuels

Dr. G.H.King

CLOSING COMMENTS

Prof. Sir Hermann Bondi, KCB The Royal Society

THE WATT COMMITTEE ON ENERGY

Department of Energy: solar biological programme: biofuels Introduction

The assessment programme

1 Each year in the UK approximately 90 million tonnes (dry weight) of organic residues are produced on farms, in forests, in industry, and in the home. These residues contain energy equivalent to 60 million tonnes of coal each year and can be converted by various techniques to solid, liquid and gaseous fuels. At least in principle, waste therefore represents a significant source of useful fuels for the UK: fuels which are indigenous and renewable and which might be particularly valuable in the future when most of our supplies will have to come from coal. Under suitable circumstances the resource could be augmented by growing plants specifically as feedstocks for conversion to fuels (‘energy crops’). In general, the term biofuels can be used to describe fuels produced directly from plant material or indirectly via organic residues.

4 The programme was established in October 1977, and is expected to run for up to three years. It has the following objectives: — to determine the contribution that biofuels might make to the UK’s energy supply during the next 50 years; — to simplify the existing complex set of possible fuel production routes; — to define the research, development and demonstration (RD&D) required to bring the more promising routes to commercial viability. 5 The first year of this programme concentrated on technical assessments at an overall national level. It attempted to establish the technically feasible potential for biofuels in the UK, identify the more promising sources of raw materials, and evaluate technologies for converting them into fuels. The findings of these overview studies will lead us into the second year in which we will perform economic analyses on complete fuel production routes (ie, raw material plus conversion technology), taking into account regional factors. Because the evidence suggests that local conditions generally, and local arisings of residues in particular, are of critical importance to costs, it will be the regional studies which will be most closely associated with economic evaluations.

2 In common with other renewable energy sources, the technical and economic viability of the biofuel technologies is not certain. Some of the technologies are proven, but neglected, eg, fermentation by yeasts to produce alcohol; others have never been more than development projects, eg, ‘roasting’ organic material in the absence of air (pyrolysis) to produce gases and liquids; others are completely novel, eg, the concept of energy crops from plantations. Some, although not fully developed, are already nearly competitive for they are effectively subsidised by waste disposal (eg, refuse-derived solid fuel); many are presently too expensive. Even the existing resource, organic wastes, could be reduced by competition with non-energy uses, while energy crops might have to compete for land against food and feed crops. Other countries have gone ahead, committed themselves to one or more technologies, and are developing them into full scale production routes. Brazil, for example, has a large programme to produce alcohol from cane sugar and cassava, while the USA is working on a wide front, but is concentrating particularly on gasification of wood residues.

6 In the national studies ETSU set up projects with the Forestry Commission, the Institute of Terrestrial Ecology, University Departments of Forestry, Engineering, Agriculture and Botany; and with a firm of industrial planning consultants. Each contractor was responsible for assessing a well-defined subject area and for interlocking his own work with the activities of others. The programme maintains close contact with the Ministry of Agriculture, Fisheries and Foods, Department of the Environment, and Department of Industry on areas of common interest.

3 ETSU, on behalf of the Department of Energy, is looking closely at biofuels to establish a realistic estimate of their potential contribution and to define a research and development (R&D) programme which will prove whether or not the more promising biofuel production routes can be brought to commercial viability. These broad aims are similar to those for the Department’s other R&D programmes on the renewable sources of energy. In common with the other renewable sources of energy, an important uncertainty in biofuels is that the size of the resource is not known. At present energy plantations do not exist, and it is not easy to predict even the ultimate contribution they might make. For residues we have a fair idea of total quantities, but a reasonable estimate of the quantities and types which might actually be available for biofuel production requires a knowledge of their geographical distribution and data on likely non-energy uses now and in the future. Technical and economic limitations might further restrict the resource. Another major complicating factor is that a wide range of raw materials (especially residues) can be converted to an almost equally diverse range of fuel products, each production route having its own characteristics. Because of these uncertainties, before embarking on an R&D programme in biofuels, we have initiated a programme of detailed assessment studies.

The projects embraced three broad technical areas; residues, energy crops, and conversion technologies.

Residues The studies considered all forms of organic residues, such as wastes arising from farming, forestry and woodworking, solid and liquid industrial wastes, and urban refuse. Current arisings have been established and likely future trends evaluated. The more important types of residues were then selected for more detailed evaluation of their potential for recovery and use as an energy source. This included an analysis of the main cost factors in current methods of utilisation or disposal, and in utilisation as an energy source.

Energy crops 7 There is a wide range of possible routes for producing plant material specifically for fuel, all of which need careful evaluation. Examples of the types of system considered include: — in forestry, harvesting a greater proportion of the tree; at present up to 50% of each tree is left in the forest as branches, stumps, and roots; 70

DEPARTMENT OF ENERGY: SOLAR BIOLOGICAL PROGRAMME—BIOFUELS pyrolysis and various chemical methods. The assessments were seeking to establish for all these technologies the important processing parameters, the characteristics required of the input materials, any modifications that might be necessary to existing plant to optimise fuel production, and the potential of each process for improvement and development. An understanding of these parameters will form the basis of complete techno-economie feasibility studies.

— coppicing trees on short rotations, giving a regular output of wood only a few years after planting; — harvesting natural and semi-natural vegetation on a renewable basis, while still preserving its semi-natural character; — growth of crops for energy within the present agricultural system, either by using the poorer ground on farms or by planting an energy crop at the times when the land would otherwise be unused.

Findings of the first year assessment

Important parameters for these studies include land availability, soil quality, nutrient requirements, dry-weight yield, cultivation, and harvesting techniques.

10 In all, 13 projects (see Figure 1) have been set-up to perform the technical assessments. They were concerned with obtaining a national view of:

8 For agriculture, only digestible varieties of plants have been selected and bred to maximise their yields as food. There are, however, wild plants that are not suitable for food but which may possess the attributes required for an ‘energy plant’, in particular a tendency towards high yields in terms of dry weight. In the longer term it may be necessary to develop energy plants by breeding from these wild prototypes.

— the potential for producing feedstock by energy cropping using agricultural systems (Project 6), forestry (Projects 4 and 5), natural vegetation (Project 7), agriculture (Project 8) and derelict land (Project 3); — the total of residues arising in agriculture, forestry, industry, and in the home (Project 1); — methods for conversion to useful fuels (Projects 9 and 10).

Conversion technologies Additionally, one project (No. 11) was concerned with matching the feedstock resource to the technologies for conversion whilst another. Project 2, was a conceptual study concerned with the total productivity of the land in the UK.

9 Because plant material or residues, if sufficiently dry, can be burnt, solid-fuel preparation and combustion was one of the technologies investigated. The other technologies considered produce fluid fuels. These are the microbiological methods of fermentation to ethanol and related compounds, and anaerobic digestion; and thermal processes, including

Figure 1

11 Each project worked to a similar study brief (for an example, see Appendix 1), in which the first section was a

Relationships between the projects in the first year of the assessment programme 71

THE WATT COMMITTEE ON ENERGY

Figure 2

Assessment of the feedstock resource 72

DEPARTMENT OF ENERGY: SOLAR BIOLOGICAL PROGRAMME—BIOFUELS systematic survey to identify interesting possibilities, followed by more detailed evaluation of selected systems. During the investigation, the scope for practical work was noted and first estimates of likely potential contribution were made. Some indication of the breadth of possibility is given in Figures 2 and 3, along with an indication of the technically feasible potential for each one. The figures also show the first cost estimates. These are very approximate, especially for the feedstocks where only ‘national average’ figures for production costs are shown. In practice, the cost of feedstock production is likely to show a marked geographical variation.

of the tillage area in the UK, about 16 M tee per annum of feedstock could be produced at a cost of about £14/tce.

Summary of findings of assessment studies 13 The following are some of the most important findings from the first year’s work: — Organic wastes having no economic value are equivalent to about 24 Mtce per annum or 7% of the UK’s current primary energy requirement. — In addition to allocating land specifically for energy plantations, there is scope for augmenting the feedstock resource by growing opportunity crops within the present pattern of agriculture. As green crops at the farm gate these might cost as little as £14/tce (energy content in the plant).

12 A potentially important route for producing feedstock, which was identified by the survey, is that which we have loosely called opportunity cropping. The major attractions to his method are:

— Green wood chips obtained by coppicing hardwoods on unused agricultural land could cost as little as £15/tce.

— that the fuel production takes advantage of the existing farm and forest management, and so incurs only the marginal costs;

— First cost estimates suggest methanol (by pyrolysis) and methane (by anaerobic digestion) could be produced at competitive prices now in large scale conversion plants.

— growing the fuel crop does not displace food/feed/timber production.

— Although the main contribution to fuel production cost arises during conversion, it is in these technologies that RD and D can do most to reduce costs.

For example, when ripe cereals are harvested, the ground is often left unused until the following spring. By planting a rapidly germinating ‘catch’ crop in late summer, it is possible to obtain a yield of about 5 dry tonne/hectare of feedstock before the ground is required for the next season’s food production. If this practice were extended to the whole

Figure 3

— Use of some biofuels is already economic, since their production costs are effectively subsidised by waste disposal.

Assessment of conversion technologies 73

THE WATT COMMITTEE ON ENERGY

Mapping—Since the volume and type of feedstock is strongly dependent on location it is clearly important that the feedstock resource (wastes and energy crops) be mapped for the whole UK at an appropriate level of detail. Understanding geographical variation in greater depth would lead to firmer estimates of feedstock costs.

— A market exploration for solid fuels produced from refuse suggests that demand might be 1 Mtce in 1985, given appropriate R&D effort. Research, Development and Demonstration (RD&D) programme

Crops—Most of the crop plantation systems, and all the opportunity crops are likely to require further technical evaluation.

14 During the assessment programme it is expected that promising lines of RD&D will be identified and appropriate experimental work started. In these studies all projects will work towards well-defined aims. The nature of these aims, however, will depend on the present state of development of the production route concerned and the likely timescale before it is expected to be commercially viable. At this early stage of the assessments it is too early to be certain about the current states of many of the routes or their prospects. Early findings suggest, however, that tentative relations set out in the following table can be drawn between the routes, timescales, and the aims of the RD&D that may be justified.

Model systems—for example, following the first year’s work, it might be necessary to mount a study centred around anaerobic digestion technology. This study would take into account all the important elements in the fuel production system: production and procurement of feedstocks, transport, conversion to fuel, markets for fuel and by-products. Such evaluations would help to define the type and size of conversion plants offering the best opportunities for economic viability. Practical work 17 The practical work which might be mounted could include studies both on the feedstock resource and on conversion methods.

Feedstock resource—Most of the systems for growing plantation crops and energy crops require basic information on yields of plants under a variety of environmental and management regimes. Some practical work in this area could prove essential to the completion of the assessment studies. Conversion methods—The various laboratory and pilot plants for anaerobic digestion and thermal processes might need to be evaluated against the range of possible feedstocks. For the thermal processes basic work on parameter optimisation for the conversion process itself could be valuable. Besides assisting in the completion of the assessments, these studies could form the foundation of a fuller R&D programme should the latter prove to be desirable.

Energy conservation 18 We have already seen in paragraph 14 that some of the biofuel technologies, specifically, direct combustion of wastes as solid fuels, are very well developed already and on the point of being commercially attractive. Although burning of wastes does not lead, strictly, to reduction in energy consumption, it could lead to conservation of fossil fuels. Accordingly, novel, commercially viable schemes for burning of wastes are being worked into proposais for funding under the Government’s programme on energy conservation.

The future

19 The programme of demonstration projects was announced last December and was expected to cost £20M over the succeeding four years. The general aim of the programme is to encourage the more widespread adoption of energy conservation technologies, by demonstrating their technical and economic effectiveness in working conditions in,for example, industry, transport, agriculture, and commerce. Any potential user of energy who has a scheme for conserving energy, where either the conservation equipment or its application is novel, can apply for a cash grant. This will represent up to 25% of the capital and installation costs of any equipment. Since the Government’s interest is in publicising the results of the demonstrations, leading to replication of the application, the

15 At the time of writing neither the findings of the first year’s assessment nor the proposals for continuation work have been presented to the Department’s Chief Scientist for his comment and approval. Thus the nature of the next year’s work is somewhat tentative at this stage. It is likely, however, that more detailed assessment studies will be mounted, complemented where appropriate by practical work.

Assessments 16 — mapping the feedstock sources and their density; — further evaluation of energy crop vegetation systems; — detailed studies of complete model fuel production systems 74

DEPARTMENT OF ENERGY: SOLAR BIOLOGICAL PROGRAMME—BIOFUELS grant includes up to 100% of any necessary expenses incurred directly as a result of the demonstration (eg, the cost of monitoring energy savings, etc.). As far as biofuels are concerned a substantial number of projects is likely to be funded under this scheme during the next two years.

APPENDIX 1

GENERAL STUDY BRIEF FOR ENERGY CROPS IN THE NATIONAL PHASE OF THE PROGRAMME

Concluding remarks 20 The first document published by the Department of Energy on energy R&D in the UK (Energy Paper 11), included for all scenarios of the energy future, a contribution of around 4.5 Mtce per annum for fuels from wastes and plant material. At the time, 1975–76, this figure was not well substantiated but 3 years later and after one year of the assessment programme, there seems little justification to change it. We are, however, more certain that there is a real basis for the projection, and indeed, the schemes being presently assembled into demonstration projects are tangible evidence that biofuels can make a significant contribution to the UK’s energy supplies.

CLOSING COMMENTS Sir Hermann Bondi

kWh compared with 3p per kWh for the ordinary household supply.

Commenting on the day’s proceedings in relation to energy futures, Sir Hermann envisaged two kinds of energy shortages that needed to be examined: a shortage of energy in its widest sense; and a shortage of energy in particular forms, with the impact of the latter predominating.

In using biomass as a source of energy, the important factor was to use the energy reasonably close to the area of biomass production. Whether it was wood for external combustion engines or liquid fuel from pyrolysis processes or fermentation was immaterial, so long as there was a nearby outlet. One could then envisage individual communities or industries which generated organic wastes using this waste for their own local transport purposes. Modern bureaucracies could thus make good use of their large volumes of waste paper!

People were prepared to pay very high prices for particular forms of energy which were indispensable to their way of life, e.g. electrical energy from small batteries for calculators or hearing aids worked out at about £2.00 per 75

THE WATT COMMITTEE ON ENERGY

General Objectives The Watt Committee on Energy, being a Committee representing professional people interested in energy topics through their various institutions, has the following general objectives:— 1.

To make the maximum practical use of the skills and knowledge available in the member institutions to assist in the solution of both present and future energy problems, concentrating on the UK aspects of winning, conversion, transmission and utilisation of energy and recognising also overseas implications.

2.

To contribute by all possible means to the formulation of national energy policies.

3.

To prepare statements from time to time on the energy situation for publication as an official view of The Watt Committee on Energy in the journals of all the participating institutions. These statements would also form the basis for representation to the general public, commerce, industry and local and central government.

4.

To identify those areas in the field of energy in which co-operation between the various professional institutions could be really useful. To tackle particular problems as they arise and publish the results of investigations carried out if suitable. There would also, wherever possible, be a follow-up which might well involve work on actual plant.

5.

To review existing research into energy problems and recommend, in collaboration with others, areas needing further investigation, research and development.

6.

To co-ordinate future conferences, courses and the like being organised by the participating institutions both to avoid overlapping and to maximise co-operation and impact on the general public.

THE WATT COMMITTEE ON ENERGY EXECUTIVE MAY 1979

Dr. J.H.Chesters, OBE, F Eng, FRS (Chairman)

Mr. R.J.Godson, Society of Business Economists

Dr. B.C.Lindley (Deputy Chairman), Institution of Electrical Engineers

Mr. R.S.Hackett, Institution of Gas Engineers

Mr. C.W.Banyard (Treasurer), Institute of Cost & Management Accountants

Miss Wendy Matthews, Association of Home

Mr. J.R.Harrison, British Nuclear Energy Society

Economists

Prof. J.E.Allen, Royal Aeronautical Society Mr. N.S.Billington, OBE, Chartered Institution of building Services

Mr. H.D.Peake, Institution of Municipal Engineers

Mr. H.Brown, Institution of Plant Engineers

Dr. P.A.A.Scott, Royal Institute of Chemistry

Mr. R.Burton, Royal Institute of British Architects

Mr. J.M.Solbett, Institution of Chemical Engineers

Prof. I.C.Cheeseman, Chartered Institute of Transport

Prof. J.Swithenbank, Institute of Energy

Dr. A.F.Pexton, Institution of Mechanical Engineers

Mr. F.Walley, CB, Institution of Civil Engineers

Mr. A.Cluer, Institute of Petroleum

Prof. F.J.Weinberg, Institute of Physics

76