The prospects for biogas—A European point of view

Biomass 3 (1983) 1-42

The Prospects for Biogas- A European Point of View E i l c c n S. P a n k h u r s t London Research Station, British Gas Corporation, Michael Road, Eondon SW6 2AD, UK (Received: 23 June, 1982)

ABSTRA CT In many parts o f the world, alternative energy sources are already needed or will be needed to augment fossil fuel supplies. 'Biogas', methanol, ethanol and hydrogen are all potentially useful in this respect, as they can be obtained (biologically or thermochemically) from renewable resources like biomass (trees, crops, etc.) and organic wastes (sewage, domestic refuse, animal manure). Some aspects o f biofuel research and development in Member States o f the European Community are highlighted, and estimates o f the contributions that these fuels might make to current and future energy demands are reviewed Some o f the potential uses o f biogases and liquid biofuels in a gas industry context are described. In the author's opinion, production o f biogases and other biofuels will ultimately play a significant but relatively small role in conserving supplies o f natural gas and other fossil fuels, and in alleviating or preventing environmental pollution. Key words: biogas, biomass, ethanol, methanol, hydrogen, wastes, pollution,

Europe.

INTRODUCTION Most deposits o f coal, oil, and gas are derived from terrestrial or aquatic plants and animals buried millions of years ago and then subjected to high temperatures and pressures. These fossil fuels are part of the same 1 Biomass 0144-4565/83/0003-0001/$03.00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain

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E.S. Pankhurst

carbon cycle as the fuels that can be produced currently from crops and carbon-containing wastes. A common link between them is the vitally important process of photosynthesis, which enables green plants to utilise energy from the sun to form carbohydrates from carbon dioxide and water. The potential energy contained by these fuels, whether fossil or contemporary, is liberated by burning, and carbon dioxide and water are formed once more. The temporary shortage of oil in Europe in 1973, and consequential price rises, probably did more to focus world attention on energy than any other single event. The fact that supplies of fossil fuels cannot last for ever has become more widely recognised, and terms like 'alternative sources of energy' and 'renewable energy resources' have acquired a special significance for engineers and politicians alike. Inevitably, the contribution that biogas and other fuels derived from renewable resources (biomass and wastes) might make towards satisfying current and future energy demands is a subject for debate and research. Many of the earlier publications l-s on biomass and wastes in an energy context dealt with potential resources in the USA and Canada, and in Brazil. Data specific to European countries have been sparse until comparatively recently, but the intrinsic differences between Europe and America (in terms of land areas, climate, population density, availability of indigenous fuels, etc.) are reflected in differences of emphasis in the various programmes. In this paper, some of the European information is reviewed, and aspects that are particularly relevant to the interests of the gas industry are considered. The author reminds readers that many of the opinions and most of the numerical data given herein are derived from the references cited. She recognises that some of the forecasts are disputed, and that in some cases the figures given by individual research groups do not agree.

1. SOME BASIC FACTS AND DEFINITIONS 1.1. Solar energy and

photosynthesis

Energy from the sun, via photosynthesis, is the driving force behind the carbon cycle and hence behind the formation of fossil and contemporary

The prospects for biogas - a European point o f view

3

fuels. However, o f the 3 X 1024 J o f solar radiation that reaches the earth's surface each year, only about 0.1% or 3 × 1021 J (equivalent to about 2 X 1 0 l i t carbon) finishes up in the form of green plants. 6-8 Nevertheless, this figure is still approximately l0 times the world's annual use of energy, and about 70 times the current consumption o f primary energy products and equivalents by the countries of the European Community. Green plants can make use of about 40-50% of incident solar radiation (the proportion with wavelengths between 400 and 700 nm), but comparatively little of the energy they trap is used. Under ideal conditions, whole plants are believed capable o f utilising approximately 13.3% of the total incoming radiant energy, but in practice a net efficiency of about 5.6% is thought to be the maximum achievable. 6 In global terms, the average efficiency of photosynthesis (on an annual basis) is about 0.1-0.3%; plants like maize and sugar can exhibit high efficiencies (2-3%) during periods o f maximum growth, and microalgae have an efficiency approaching 5%. Under laboratory conditions, an efficiency of 18% has been claimed recently for micro-algae, 9 but this figure is disputed, and there is still considerable controversy over the theoretical mechanisms proposed for photosynthesis. Although the efficiency o f photosynthetic conversion of solar energy is not very high, it is still sufficient to ensure 7'8 that all atmospheric carbon dioxide is cycled through plants once every 300 years, all the oxygen every 2000 years and all the water every two million years. The carbon at present stored in the world's terrestrial and aquatic plants (90% in trees, and only about 2% in ocean biomass) is estimated to be about 8 X 1011 t. v Similar quantities of carbon (ca. 7 X 1011 t) occur as carbon dioxide in the atmosphere, and as carbon dioxide in the surface layers of the ocean. The concentration of atmospheric carbon dioxide is about 334 ppm by volume, with a winter to summer oscillation of between 5 and 15 ppm caused by photosynthesis. ~ The 25% increase in carbon dioxide that has occurred in the last 125 years has been attributed to the burning of fossil fuels, although calculations 8 suggest that an equivalent amount to that derived every year from fossil fuels (4-8 x l0 9 t carbon year -1) is released during the destruction of forests and by oxidation of organic matter in the soil. These carbon balances are, of course, only approximations. They illustrate, however, why the extent of plant photosynthesis, the rate o f

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E.S. Pankhu~t

fossil fuel utilisation and the rate of deforestation are so fundamental to the arguments about rising global temperatures - the so-called greenhouse effect. 1.2 Biomass, biogas, biofuels The term 'biomass' in its widest sense covers all material of biological origin, whether plant or animal, aquatic or terrestrial, living or dead. However, the term has also been used more narrowly to describe 'energy crops' that have been grown specifically for conversion into fuels. The word 'biogas' was originally adopted as an abbreviation for the mixture of methane and carbon dioxide formed during the anaerobic digestion of sewage and animal manures, but now embraces the similar mixture of gases produced during anaerobic digestion of virtually any natural or man-made materials containing a high proportion of organic carbon (e.g. energy crops, agricultural wastes, domestic refuse, industrial wastes). In this paper, the broadest of definitions are used. Thus 'biogas' covers substitute natural gas (SNG) produced directly or indirectly from 'biomass' and wastes, and the term 'biofuels' includes all the gaseous and liquid fuels that can be produced from these materials, whether b y biological or thermal processes. 1.3 Potential starting materials for biofuel production The wide range of biomass and wastes that can provide feedstocks for biofuels is summarised in Table 1. In practice the range for any one country is much narrower when the various geographic, climatic, economic and other factors listed in Table 2 (not necessarily in order of importance) are taken into account. Some o f the constraints are obviously virtually unchangeable, but others, often interrelated, may alter or can be influenced in some way. For example, a change in government can lead to rapid and complete reversals of policies, so that the standing of particular options (especially those affecting energy and agriculture) can alter dramatically.

Energy crops

Land-based Cereal crops (wheat, barley, maize, oats) Sugar beet Potatoes Grasses Lucerne, clover Catch crops (grown between main crops) Forestry energy plantations (coppiced woods, shortrotation forestry, etc.) Natural vegetation (weeds, grasses, bracken, heather, etc.) Water-based Micro-algae Macro-algae (seaweeds) Marshland plants (reeds, etc.)

Wastes

Agricultural Straw Other crop residues Animal manures and wastes Forestry Wood residues Sawdust, etc. Paper wastes Industrial Food and beverage residues Canning wastes Miscellaneous Municipal sewage Domestic refuse

TABLE 1 Materials with Potential for Biofuel Production in Europe

2

r~

I

t~

Land use - competition, etc. Agricultural/forestry policies

Available coastline

Territorial waters

Assessment o f scale potential

Costs o f biofuel production

A t t i t u d e s - towards research and development, budgets, facilities

Wastes - collection facilities

Environmental - preservation o f fauna and flora, etc.

S o c i a l - population growth, public awareness

Energy policy

Freshwater area - lakes, etc.

Indigenous and fossil fuels - costs and availability

General government policy

Land area

Established economy - rural/industrial

C l i m a t e - temperature, rainfall, sunshine, etc.

Changeable

Geography - longitude, latitude

Fixed

TABLE 2 Factors that Affect Biofuel Potential

The prospects for biogas - a European point o f view

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TABLE 3

The Energy Contents of Some Fuels, Wastes and Biomass Typical gross calorific value in MJ kg -1 (but see footnoteJ Solid, gaseous or liquid fuels and closely related chemicals

Hard coal (10) 29.3 Hard coal coke (10) 28.4 Peat, dry ash-free (11) 21-0 Peat, air-dried (25% H20) (12) 16.3 Hard brown coal (10) 14.7 Lignite, commercial fuel (11) 26.8 Crude oil (10) 42.3 Heavy fuel oil (10) 41.0 Motor gasoline (10) 43.5 Naphtha 45-6-47.8 LPG (10) 45-9 Refinery gas (10) 41.0 Natural gas (10) a 31.7-39-8 Coke oven gas (10) a 16-1 Hydrogen (13) a 12-1 Methane (13) a 37.7 Biogas (60% CI-I4) (14) a 22.3 Methanol (15) 22.4 Ethanol 95% (15) 28-2 Ethanol 100% (15) 29.7 Isopropanol (15) 33.0 n-Propanol (15) 33.6 n-Butanol (15) 36-2 a

Gross calorific values in MJ m -a. Numbers in parentheses refer to references.

Biomass and wastes, and some constituent compounds (dry unless stated)

' Typical' biomass (16) Bacteria (17) Californian kelp (18) Micro-algae (18) Straw (15% H20) (19) Straw, dry (18) Herbaceous material (I 8) Natural vegetation (20) Green wood (wet) (18) Wood, dry ash-free (11) UK crude refuse (21) Refuse-derived fuel (21) Cattle manure (18) Pig wastes (18) Monosaccharides (22) Polysaccharides (22) Lignin (22) Protein (22) Fat (22) Carbohydrates (22) Crude fibre (22)

14.0 22.7 10.0-15.0 20-0 14-2-15.5 17.0 17-5 18.3 16.0 18.7 9.0-11.0 14.7-15.6 21-4 20.0 15-5 17-6 31.1 23.9 31-4 16-7-17.6 18.8-19.7

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E.S. Pankhurst

In addition, the type of farming and forestry (and hence biomass production) is profoundly affected by the monetary value that society puts on the end-product; the greater the reward, the greater the incentive. Thus the emphasis may switch from arable farming to dairying, or from dairying to beef cattle rearing. Also important are soil fertility, fertiliser requirements, availability of water for irrigation, pest control costs, on-farm fuel needs and transport costs. Contrary, perhaps, to the impression often given by the popular press, no single biofuel option suits all economies, and it may be unwise to opt wholeheartedly for one particular alternative, especially if a major change in land use is involved. The much publicised alcohol production programme in Brazil may help to compensate for lack of indigenous oil, but because the farmer tends to be paid more for 'alcohol' crops, less food tends to be produced. The energy that biomass and wastes contain (see Table 3) can be reclaimed in various ways. For many materials, the simplest and most traditional method is by burning and utilising the heat released. It is a sobering thought that about 40% of the world's population still obtains the bulk of its energy in this way from wood. Other methods (outlined in Fig. 1) include fermentation to yield ethyl alcohol, anaerobic digestion to give a gas rich in methane, or the comparatively modern techniques of pyrolysis, gasification and hydrogenation which yield liquid and/or gaseous fuels. Details of these different techniques have been admirably described, 22-29 and results for laboratory and pilot scale operation of

..aeroOi____¢¢ q,j__.tio. _-: C., (--____.__S.G)I BIOMASS AND/OR WASTES

biological methods (10-60 *C, pressures ~ , fermentation near atmospheric ) ~"~ biophotoly sis thermochemical methods (high temperatures, often l high pressures)

pyrolysis ..-• gasification

I (steam/air/O2)

Fig. 1. Methods for obtaining biofuels.

ID C2H50 H

[

• H2 m

gaseous& liquid fuels eg. methanol

~

H2 /CO/CH 4 ( D methanol) ,,,0

The prospects for biogas- a European point o f view

9

many thermal, as well as biological, conversion processes can be found in the proceedings of symposia held in various parts of the world within the last five or six years. 1'2'4,3°'31 Some processes are operated commercially, for example, ethyl alcohol production in Brazil, SNG production from manure from cattle feedlots in the USA, and production of synthesis gas from wood in Canada. For Europe, as in other parts of the world, availability of potential feedstocks is the key factor, followed closely by cost. In the latter respect, wastes have an advantage because they are usually 'free', although unless they can be used on the site where they arise, some collection and transportation costs are inevitable.

2. BIOFUEL RESEARCH AND DEVELOPMENT IN THE EUROPEAN COMMUNITY Some of the statistics that affect the agricultural and energy policies of the Member States of the European Community (EC) are summarised in Tables 4-6. Most European countries are assessing the impact that various forms of alternative energy can have on their energy resources. The scope of each individual programme, its emphasis and the money devoted to it vary from country to country, according to the constraints listed in Table 2. These differences are also reflected in the Solar Energy Research and Development projects that are being carried out under the auspices of the Commission of the European Communities. 2.1. Expenditure and forecasts The Commission awards contracts to industry, universities and research institutes, providing up to 50% of the costs of approved proposals. The purpose of the first programme (1975-79), successfully achieved, was to initiate, stimulate and strengthen solar energy R & D activities in the Community; the budget 32'a3 was 17.32 million European Units of Account (EUA) ($22.3 million), of which about $2-3 million was spent on biomass studies. The second four year programme (1979-83)began on 1 July, 1979, with a total budget of 46 million EUA ($64.4 million).

23 499 281 059

840 104 327 277 361 311 714 358 13 942 55 902

9 5 61 53 9 3 56

Population in 1978 ('000)

2 23

323 118 247 98 71 47 188 138 338 229

Population density (per k m 2)

670.00 4 288-00

14-47 29.27 131-76 321.87 92-65 . 178.74 1.30 20.54 184-56

Total

93.78 0.58 8.20 69.32 436.84 1 873.86

.

7.46 26-42 75-01 174-01 40.12

Arable land a

.

233.16 2 414-14

6.84 2-68 51.55 129.78 52.53 . 51.67 0.70 11-89 113-78

Permanent grassland

-

32.69 0.01 0.38 0.69

0.16 O. 14 2.04 15.51 -

Land under permanent crops b

Agricultural used area ('000 k m 2)

4 431.08 3 071.01

6.16 4-93 72-07 147.65 25.12 3-59 63.45 0.82 3-09 20-64

Wooded area c ('000 k m 2)

Derived from Basic Statistics of the Community: 18th Issue.128 a Cereals, rice, dried pulses, root crops, industrial crops, vegetables, flowers and ornamental plants, fodder from green arable land (1978 figures). b Fruits, vineyards, olive growing, etc. c 1977 figures.

9 922.3 9 363-1

30.5 43.1 248-6 544.0 132-0 70-3 301.3 2-6 41.2 244-1

Belgium Denmark FR Germany France Greece Ireland Italy Luxembourg Netherlands United Kingdom

North America Canada USA

Area ('000 k m 2)

EC member state

TABLE 4 Population, Land Area and Land Use for EC Countries, and, for Comparison, for Canada and the USA

"~

42 879 259 681

1 821 6 924 21 562 39 102 3 725 1 654 15 307 109 1 207 15 746 1 043 24 270

5 021 3 206 19 436 24 768 2 920 1 439 12 515 0 6 275 6 596 43 1 461

0 977 6 421 425 6 656 15 <1

Cereals Sugar Wine average beet 1975-77 1976-78 1975-77 ('O00t) ('O00t) ('O00t)

Production

12 877 116 625

2 870 3 052 15 007 23 507 1 035 6 130 8 724 215 4 797 13 507

Cattle ('000)

6 714 56 584

992 184 641 765 936 1 148 8 922 91 9 301 7 964

4 9 22 10

Pigs ('000)

418 13 742

100 56 1 172 12 721 12 493 2 418 9 953 5 548 21 740

Sheep and goats ('000)

Livestock populations, 1978

Derived from Basic Statistics o f the Community: 18th Issue.128 n.a. = not available. a In pure nutrient content.

Canada USA

North America

Belgium Denmark F R Germany France Greece Ireland Italy Luxembourg Netherlands United Kingdom

EC member state

Production

-

177 374 1 325 1 832 230 812 14 447 1 177

2 146 12 433

584 12 1 989 2 034 287 n.a. 1 170 2 166 n.a.

13 529 53 397

1 673 4 154

1 453 1 199 1 342 9 339

812 221 7 571 4 963 262 88 4 612

Wood pulp, paper and board 1978 ('O00t) 651 117 1 305 1 470 280 120 1 029

Nitro- Synthetic Nitrogenous ammonia genous fertiliser a N2 fertilisers 1977/78 1977 N ( 'O00t) ('O00t) 1977/78 ('O00t)

Consumptio n

TABLE 5 Selected Agricultural and Forestry Data for EC Countries, and, for Comparison, for Canada and the USA

7.

r~

I

~"

t~

219.2 1 857.3

46-3 19.6 267-9 184.3 15-3 a 7.5 135.7 4.5 64-1 208.1

Total energy (Mtoe)

90.2 888.4

25.0 15.4 139-3 113.1 11.3 5.8 92.6 1-4 27.5 91.6

Crude petroleum (Mtoe)

43-5 464.7

8.5 . 41.4 18.8 . 0 22-4 0.5 32.0 37.1 .

.

1 965.0 21 448-6

396-6 . 1 926-8 876-5 . 0.4 1 044.5 21.1 1 489-0 1 725-2

Natural gas (Mtoe) ('000 TJ [GCVJ] a

Gross inland consumption o f some primary energy products and equivalents for 19 78

720.0 307-1

1.3

2 914.0 20 594-7

0.4 526-0 3 120-2 1 517-6

.

.

75.7 565-7

0 13-7 93.7 38.5

19.8 8.1

0

Production ('000 TJJ b (109m3) c

Natural gas (19 78)

Derived from Tables 50 and 59 in Basic Statistics o f the Community: 18th issue. 128 a Variations o f stocks accounted for. b Colliery methane included. c Volume under standard conditions. d In most cases, the addition o f probable and possible reserves would increase these figures considerably.

Canada USA

North America

Belgium Denma rk F R Germany France Greece Ireland Italy Luxembourg Netherlands United Kingdom

EC Member State

TABLE 6 Selected Energy Data for EC Countries, and, for Comparison, for Canada and the USA

1 670 5 670

1 650 706

155 250 70 113 30 180

m

Proven reserves d (109 m 3)

~V

to

T h e p r o s p e c t s f o r biogas - a E u r o p e a n p o i n t o f view

13

For comparison, the USA's expenditure on biomass research for the year 1980 was reputed to be about $60 million. The importance of ethanol in the American programme was confirmed by an autumn 1981 announcement that the Department of Energy had picked 11 companies to receive conditional loan guarantees totalling $706 million for the production of ethanol. 34 In the current EC programme, there are eight projects, two of which (Projects D and E) are of direct interest in terms of biofuel production. Project D (Photochemical, Photoelectrochemical and Photobiological Processes) has a budget of 2.6 million EUA ($3.6 million), and covers fundamental work on the photochemical production of fuels, improvement of hydrogen production via living cells, and construction of artificial systems based on photosynthesis. 32 Project E (Energy from Biomass) receives funding of 7.4 million EUA ($10-4 million). Its aims have been summarised, 3s and include the recovery of several million tons of oil equivalent (Mtoe) from animal wastes and from forestry and agricultural residues by 1985, and the introduction of energy crops for biofuel production by the year 2000. In assessing the available resources, two main themes for Project E are 'the analysis of the obstacles which could explain the low utilisation of agricultural residues as energy sources on a local basis', and 'the harvest of forestry remnants and of the unused natural coppice, abundant in France and Italy'. 36 Subjects that have attracted funding include the following: continuous anaerobic digestion of relatively solid biomass such as manure, and of catch crops (i.e. crops growing between the harvesting and sowing of main crops); the feasibility of thermophilic anaerobic digestion; thermal processing of materials like straw; identification of high potential energy crops, including catch crops, short rotation forestry, and growth of algae; conversion of biomass (particularly wood) to methanol; hydrolysis of lignocellulosic material; and the potential uses of hydrocarbons produced and stored by plants. The sources and energy potential of agricultural wastes and biomass in Europe have been reviewed in detail 37 in connection with Project E. Some of the findings are summarised in Table 7, although, taken in isolation and without reference to the qualifications described in the book, these figures can be misleading. The many factors that influence whether or not these potential energy resources can be fully utilised have been mentioned already (Table 2), but in addition, competition for a particular waste or energy crop may be especially strong, as is the

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E. S. Pankhurst

TABLE 7

Potential Contributions a from Agricultural Wastes and Energy Crops to the Energy Requirements of the European Community Categories o f wastes and crops

Gross energy content (Mtoe year -1)

Potential contribution in the form o f b iogas (Mto e y ear - 1)

2.87 34.6 1.98

1.5 11.5 1.13

33-47 7.31 2.7 16.3

4.09 4.38 (solid fuel) (solid fuel)

99.23

22.60

63-9

38.3

30.2 0-27 1.0

18.12 0.13 0.5

95-37

57.05

Agricultural wastes

Accessible animal wastes Pigs Cattle Poultry Crop residues Grain crops Green plant matter Woody residues Forest and wood waste Total Energy crops b

Annual catch crops Perennial plantation crops c on 'unused land' Seaweed Micro-algae Total

a Derived from Tables 11.1 and 11.2 in the general conclusions and recommendations given in Energy from Biomass in Europe. 18. Competition for feedstocks is not taken into account. b Energy crops and plantation schemes not involving radical changes of farming practice or land use apart from intensification and the take-up of unused land. c Includes short rotation forestry.

The prospects for biogas - a European point o f view

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case for wood, for example. The EC currently produces 80 × 10 6 m 3 of wood, and its net imports (as roundwood) amount to 120 × 10 6 m3; only for oil products is there a higher commercial deficit than this. 3a Until recently it was thought that energy from biomass could make only a marginal contribution to the total energy requirements of the EC. Now, however, it is suggested as that the contribution could be as much as 75 Mtoe by the year 2000 (8% of the 1978 total energy consumption - see Table 6). 2.2. Some highlights o f individual programmes 2.1.1. B e l g i u m

A two stage anerobic digestion process for dealing with biomass has been proposed by the University of Louvain. 39 It has been used successfully to produce methane from algae grown in lagoons containing cooling water from the nuclear power station at Tihange; the methane production rate is reputed to be 3-4 times higher than the rate achieved with a single stage digester. Algal growth is increased by 50-200% because the water is warm, and yields of 12-15 t dried algae ha -1 year -1 have been obtained. 4° It is suggested that growing algae in this way, followed by anaerobic digestion, is an excellent way of recovering from cooling waters low grade heat that would otherwise be wasted. 2. 2. 2. D e n m a r k

A Ministry of Energy report 41 suggested that one-third of Denmark, not already supplied with natural gas or district heating, could be heated by renewable energy. It was estimated that 40% of the heat demand and some of the electricity demand could be provided in this area by installing about half a million plants of different types (straw furnaces, wood furnaces, biogas plants, windmills, solar collectors, district heating plant). This would cost 28-5 × 109 DKr ($5-1 × 109), and the energy provided would be equivalent to 6% of the gross energy consumption in Denmark. Even so, it was concluded that investment in renewable energy (in 1980) would not be profitable, although steep rises in oil prices would alter the situation. Estimates4~ showed that 1 × 109 m a year-~ biogas (about 0-6 Mtoe, or 4% of fuel oil consumption) could be obtained by anaerobically digesting all the animal manure in Denmark. However, although three experimental biogas plants erected by the State operated satisfactorily for

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E. S. Pankhurst

two years, less gas was produced than was expected, and from an economic viewpoint the results were disappointing. With funds provided b y the County of Aarhus and the Ministry o f Education, a new type o f anaerobic digester (the bio-funnel) is being developed specifically to deal with a mixture o f pig manure and straw with a low water content. 42 Project Godrum is designed to demonstrate that energy derived from agricultural wastes, combined with solar and wind energy, could provide surplus energy for use in rural communities. The aim is to satisfy the total heating requirements (400 MWh year -I) and most o f the electricity demand (100 MWh year -1) of a State farm, using wastes produced by a residential hall, 100 ha o f wheat, and 600 pigs. The energy system includes a windmill, a straw furnace, a digester and a biogas engine and generator set. Straw production in Denmark is approximately 6.5 X 10 6 t year -1, of which about 2 X 106 t are burned, 1.4 X 106 t are used for feed, 2.8 × 106 t for bedding, 0.1 X 106 t for cellulose and the rest is treated with alkali or fired. As a result o f EC-funded studies by the Institute o f Agricultural Engineering and the Royal Veterinary and Agricultural University, it was concluded that anaerobic digestion o f straw would not be profitable.19 At prevailing costs o f gathering straw and at current oil prices, it was considered better to burn the straw in a furnace. Each o f the 20 000 straw furnaces in Denmark has an average capacity of 10-20 t straw per year; 41 if they were all fully utilised, they could save 0.050- 1 Mtoe year -1 (about 1% of the annual fuel oil import). Waste wood, according to the Danish Forest Research Institute, could provide energy equivalent to 0.26-0-32 × l 0 6 t heavy fuel oil. 4x Release o f easily fermented constituents b y treating cellulose with anhydrous hydrogen fluoride is being investigated at the Technical University of Denmark, in conjunction with French researchers. 4s

2. 2. 3. Federal Republic o f Germany In the current Energy Research and Technologies Programme o f the F D R an annual budget of DM60 million (I;33 million) has been allocated for solar research and development. 44 Gasification processes for biomass and wastes are being studied, and particular interest is being shown in utilisation of biomass for cogeneration o f heat and power. 4s One o f the largest potential energy sources is straw, disposal of which is a problem in wheat growing areas where there is little livestock farming. Production in 1974 was about 23 X 106 t, almost 25% o f the

The prospects for biogas - a European point o f view

17

total production in the EC; anticipated production for 1985 is 30 X 106 t. In EC-funded studies by the Technical University in Munich, ~9 the amount of surplus straw available for energy production was assessed at 5 X 10 6 t, possibly rising to 7-9 X 106 t by 1985. Optimum combustion systems were considered to be either simple 'through-combustion boilers' for domestic heating and small drying installations, or big 'bale boilers' or 'hot-air' generators (fed continuously with straw)for high power requirements. The activities of the various groups working on biogas projects have been reviewed,46 and practical applications described. 4~-49 Investigations include biogas production from slaughter-house wastes, s° and anaerobic digestion of municipal sewage sludge at thermophilic temperatures, sl The Federal Research Centre of Agriculture in Braunschweig concluded that only a small proportion of the potential energy in animal wastes could be used for biogas production, given the existing structure of German agriculture, s2 Thus installation of digesters would only be economic for 5% of the total number of pig farms in the FDR. About 90% of the Republic's domestic refuse is landfilled, s3 The problems associated with methane production from tip areas have emphasised the potential benefits of gas reclamation systems. One scheme for utilising landfill gas is already operational at Pforzheim, and two more (at Am Lemberg and Braunschweig) are at the planning stage. The Pforzheim landfill site receives 120 000 t refuse per year. The gas that forms contains about 60% methane, and sufficient is produced (approximately 120 m 3 h -1) to generate at least 200 kW of electricity. 2.2.4. France In 1980, the French Energy Commission s4 recommended that research into new energy sources should be directed mainly towards solar energy, and towards techniques for using biomass (including methanol production, gasification, and fermentation of animal faeces). It was suggested that between 1980 and 1990, biomass (especially wood) would make a significant contribution (about 8 M t o e y e a r -1) to energy supplies; this would be about 4-3% of the 1978 gross inland c o n s u m p t i o n - see Table 6. Future objectives would include making technical progress to reduce costs and to facilitate energy production on an industrial scale. Development of more effective techniques for converting biomass into energy, and the progressive use of energy crops, form part of a 'Green Energy' policy, ss France exploits (chiefly for the paper and

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E, S. Pankhurst

wood industry) only about two-thirds of what is the largest forested area in Europe (15 million ha, over 25% of her territory). Plans are being made to utilise abandoned coppices, to recover felling residues, and to collect wastes like bark and sawdust. There are also schemes for growing trees like poplars in plantations designed exclusively for energy production. Frost-resistant E u c a l y p t u s hybrids are being bred experimentally, s6 as these fast-growing trees would be ideal for short rotation coppices. Government grants of more than $8 million have been given for research into energy plantations, and for studies on production of algae, etc. s7 Experiments on rapid pyrolysis of wood s8,s9 are being carried out (with EC support), and in 1980 construction began on a 1 t day -1 pilot plant that will produce a medium-MJ gas. Preliminary laboratory-scale studies were carried out with sawdust from beech or Douglas fir. Pyrolysis of dry wood at 1000°C produced more than 60% mass yield o f gases, containing about 50% carbon monoxide and 25% hydrogen. In the first phase of EC-funded studies by the National Institute of Agronomic Research, unutilised straw in France was assessed at 3 Mtoe year-1.19 The distribution o f straw production was mapped, and energy balances calculated. It was concluded that for use in large burners (e.g. 1200 t straw year -1 for a maize-drying unit), straw was less expensive than domestic solid fuel but more expensive than heavy fuel and natural gas. However, burning straw to dry other crops is not the only way of utilising the energy it contains. Incomplete combustion will result i n the formation of synthesis gas, from which methanol can be produced. Research at the University o f Poitiers 6°'61 includes development of catalysts for low pressure ( < 5 0 b a r s ) production o f methanol from synthesis gas prepared from plant material like straw. Biogas production from wet wastes, especially animal manures, is receiving attention. An unusual feature of one particular scheme on a farm with 2 2 0 0 0 pigs is that the methane is being converted to methanol. 62 It is estimated ss that by-products and wastes could provide 5 or 6 M t o e by 1985, rising to over 10Mtoe in the year 2000. F o r energy crops, in addition to work on tree plantations, an experimental area (291 ha) for growing Donax reed has been established in Southern France. In 1981 the government approved a $26 million programme for developing gasohol, 63 with the hope that b y 1990 alcohol/petrol

The prospects for biogas - a European point o f view

19

mixtures will provide 25-50% of the nation's transport fuel requirements. Initially methanol will be obtained conventionally from fossil fuels, but ethanol production from wood, straw and Jerusalem artichokes is planned. A process for fermenting glucose syrup by means of immobilised cells, and simultaneously removing the ethanol by liquid-liquid extraction is being investigated at the National Institute of Applied Science. 64 The extraction technique overcomes the problem of build-up of ethanol, which inhibits further fermentation. The feasibility of large-scale hydrocarbon production from algae which accumulate 15-75% of hydrocarbons within themselves (on a dry weight basis) is also attracting research effort. 6s

2.2.5. Republic of Ireland According to recent calculations, 66 Ireland's 1-2 × 10 6 ha of peat bogs will be exhausted by the year 2020. Thus, much importance is attached to identifying future uses for cutaway peat bogs, and to finding alternative fuels to replace the peat. Short-rotation forestry is potentially an ideal solution to both problems. Ireland has a small population relative to her land area, an increasing area of low-grade land unsuited to conventional agriculture, and a climate sufficiently mild for tree growth to be three times faster than in Scandinavian countries. 66 With EC support, the coppicing of hardwood trees grown in former peat bogs is being investigated by the Agricultural Research Institute and the State Department of Forestry and Fisheries. The ultimate aim is to obtain sufficient fuel in the form of wood chips to replace 10% of the oil currently used, with particular emphasis on fuel for steam turbine generating stations. Forty per cent of the £5.5 million ($12.8 million) needed for a 405 ha demonstration project is being provided by the EC; in addition to coppiced hardwood trees, conifers (which have to be replanted after harvesting) are also being grown. 67 Taking into account the results obtained so far, land availability and economic and market criteria, biomass from short-rotation forestry could provide a significant proportion of Ireland's energy requirements. 68 Whether the proportion could be as high as 10%, as has been suggested, remains to be seen, but there is little doubt that tree plantations offer considerable scope for a country that currently imports over 80% of its energy needs.

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The main uses of w o o d y biomass are seen as electricity generation, and domestic and industrial heating. Conversion to alcohols for use as transport fuel is seen as a distinct possibility for the future. The importance of making an early commitment to establishing large energy plantations has been stressed. 68 Data collected by the Agricultural Research Institute over an 1 1-year period have confirmed ~9 that growth yields for annual crops are highly sensitive to the number of days in a year when growth is possible, which is highly dependent on climate. Many annual crops have a growth cycle of 120-150 days, but genetic improvements in ryegrass and sugarbeet have extended the growing season to more than 200 days year -1, so that higher yields can be achieved. 2.2.6. Italy Coppiced forests cover an area o f over 3-6 × 106ha, about 57% o f Italy's afforested land. 69 The potential for more efficient utilisation o f coppices is being investigated by the National Agency for Cellulose and Paper, with financial assistance from the EC. The quantities o f w o o d y biomass that could be used either by industry or for energy purposes will be evaluated, and the suitability o f such biomass for different enduses assessed. As the paper industry finds it increasingly difficult to obtain adequate supplies of raw material, a careful evaluation o f potential indigenous resources is important. Conflicting interests in biomass utilisation may well arise, as in other West European countries. As a wine-producing and olive oil-producing country, Italy has considerable quantities o f cellulose-containing wastes which could be utilised for alcohol production. Appropriate methods for hydrolysing and fermenting such wastes are being investigated at the University of Naples, 7° with EC support. Promising results have been obtained with enzymatic hydrolysis and continuous fermentation in a combined reactor system. Ultra-filtration membrane reactors have been used for hydrolysis o f agricultural wastes, 71 and ethanol has been produced by immobilised yeast cells. 2.2. 7. Netherlands Methane production by anaerobic digestion of wet wastes and biomass has been studied at the Agricultural University in Wageningen, for energy reclamation and as a means o f preventing pollution. 23 Anaerobic waste water treatment is suggested as an alternative to the conventional aerobic activated sludge process, and two new processes are described:

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21

an anaerobic filter process, and an upflow anaerobic sludge blanket process. Each is simple and cheap to operate for liquid wastes, and can be used as a second step in the digestion of complex solid materials after they have been liquefied. A novel batch reactor which is said to make anaerobic digestion o f solid organic matter much easier has been developed during EC-funded research at the Institute for Storage and Processing of Agricultural Produce in Wageningen. ~2 Water is continuously percolated through the decaying solid mass, and effluent from the reactor is treated in a conventional continuous anaerobic digester, from which biogas is evolved. In a comprehensive review of thermochemical conversion techniques, 2s gasification o f biomass was rated as attractive for small-scale p o w e r generation, and for combined heat and power. However, a need was identified for processes that require minimum feedstock preparation and preferentially produce gas with a calorific value o f 9 - 1 5 MJ m -a without the use of oxygen. In 1974, an increasing demand for wood (for conventional use, and as a potential source o f energy) was the stimulus for initiation o f research into short-rotation forestry at the Institute for the Promotion o f Industrial Wood Production. Particular attention is being paid to fast-growing poplars, and to short-rotation periods of 5-6 years. 7a

2.2.8. United Kingdom Studies carried out on behalf o f the Department o f Energy suggest that about 8-13% (18-30 Mtoe year -a) of the UK's energy demand could be met by utilising biomass and wastes. 74' 7s The geographical distribution of farm animal wastes and crop residues for England and Wales has been mapped on a 5 × 5 km grid square basis. 76 From data obtained in this and similar ways, it has been estimated that about 17 × 106 dry t agricultural wastes, with an energy content of 299 PJ (6-67 Mtoe, i.e. 17 MJ kg-a), are produced annually. The UK surplus o f straw is about 6 X 1 0 6 t y e a r -x. Process flow studies, 77 calculations o f mass and energy balances, and capital and processing costs suggest that steam gasification is the most promising route for energy reclamation, and methanol the most economically promising fuel. The use o f forest biomass for energy purposes is considered potentially viable. 7a In an assessment carded out b y Aberdeen University, forest residues and early thinnings were identified as existing sources o f

22

E S. P a n k h u ~ t

wood that could be used either directly as solid fuel, or after conversion as gaseous or liquid fuels. A third potential source o f forest biomass was tree plantations. The energy equivalents likely to be available in the year 2000 were estimated at 0.7 Mtoe year -1 for recoverable forest residues, 0.3 Mtoe year -1 for early thinnings, and 4-4 Mtoe year -1 from energy plantations - a possible total o f 5.4 Mtoe year -1. Using 10 tree species with good potential biomass yields, experimental energy plantations are planned on three different types of land in four geographical regions of Britain. The area of land regarded as suitable for plantations amounts to 1.5 X 10 6 ha, and it is suggested that much of the 0-33 × l 0 6 ha of derelict land, and other waste land in the UK, could be used for forest energy crops. As in Italy, however, largescale use of wood for energy could cause conflict. The present forestry area of approximately 2 × 10 6 ha is said to produce only about 1.2 × 10 6 dry t year -1 of wood, and it has been claimed that even twice this area under full production would not yield more than about 25-30% of the demand for w o o d and wood products. 79 Northern Ireland has good potential for growing coppice willow for biomass, as promising yields can be obtained on poor quality soils. The end use o f the willow was originally seen as pulp wood, but now production of pellets for cattle feed is favoured, and cattle acceptance trials are in progress, a° In studies funded by the EC and the Department of Energy, the performances o f three different gasifiers for producing a methanol synthesis gas from biomass are being compared. 8~ Quantitative results for pine wood have been obtained, and theoretical methanol yields predicted for a proprietary methanol loop programme. Two main operating modes are feasible: low temperature gasification with steam to produce a methane-rich gas, and high temperature gasification with steam and oxygen to give a hydrogen-rich product. A recent study b y the Institute of Terrestrial Ecology s2 suggested that 40% of rural Britain not used for agriculture or silviculture (woodland) could be devoted to energy crops other than trees. Natural vegetation often gives higher yields than agricultural and forestry crops, and could be used as an opportunity crop without altering land use, although fertiliser requirements would have to be considered. Yields o f plants like heather, bracken, and marsh grass have been determined, aa It is estimated a4 that bracken in Scotland could yield 2.2 X 106 dry t year -x (39.7 PJ or 0-89 Mtoe), which is more than 5% o f the primary

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23

energy consumption o f that country. Fresh bracken can be anaerobically digested, 1 t yielding 2 7 m 3 o f biogas (60% methane); old bracken can be gasified or combusted directly after pelletising. The productivity and potential of catch crops for fuel is also being studied, 8s as is the feasibility o f growing kelp in the North Sea and elseelsewhere in UK territorial waters, for conversion to methane by anaerobic digestion. 86 In a pilot project landfill gas is being fed to a large brickworks. 87 The gas (50-55% methane) from five wells in a 73 ha site is fed to a small kiln, replacing the 2-5 t o f coal per week needed to produce 50 000 bricks. The site may contain enough gas to meet the energy needs (0-15 X 106 t coal year -1) o f the entire brickworks for the next 25 years, as well as generating power for a nearby housing estate. 88 In another scheme, the Greater London Council and the National Coal Board hope to reclaim methane emanating from a 28 ha landfill site. 89

3. SOME POTENTIAL LONG-TERM AND SHORT-TERM IMPLICATIONS F O R GAS INDUSTRIES Biogas is still too often thought of only in a narrow sense, as gas produced b y anaerobic digestion o f farm wastes and sewage sludge. However, biogas can be produced from biomass and wastes by thermal conversion processes, and liquid biofuels can be used for SNG production. Additionally, future advances in our knowledge o f photosynthesis may lead to the ability to mimic biophotolysis, and ultimately perhaps to the production o f commercial quantities o f hydrogen. The value and possible impact o f biofuels on the gas industry obviously varies from country to country, and is not easy to define. In addition to the constraints identified in Table 2, much depends on the size and general organisation o f a particular industry, on the volume, pressure and type o f gas distributed, and on the characteristics o f the distribution system. The first step is for an individual gas industry to decide on the minimum quantity o f gaseous or liquid fuel that would be o f interest to it. The second step would be to use the information already obtained for many individual countries of the European C o m m u n i t y 37 to determine whether or not appropriate amounts o f wastes or biomass exist, or could be made available (see Table 7), for conversion to this amount of fuel.

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If the resources are sufficient, it would then be necessary to establish that they are conveniently located and reliable, and that the costs o f conversion are acceptable. The final step would be to make a bid for the resource, bearing in mind the fact that competition could be strong for materials which after conversion into liquid fuels could be used as feedstocks for the chemical industry, or as transport fuels. The potential contribution that biogas and/or biofuels can make towards future SNG requirements in countries with well-developed gas industries is often overestimated. Well-meaning enthusiasts sometimes have little or no conception o f the rates at which natural gas reserves are being depleted, nor o f the quantities o f feedstock that are needed to produce equivalent amounts of SNG. In the UK, for example, the average demand for natural gas is about 130-140 × 10 6 m 3 day -1, and on a peak day, demand may double. The daily coal requirement for a plant producing 7 × 106 m 3 day -1 of SNG would be about 15 000 tonnes. Four plants of this size, operating at a high load factor, could supply about 20% o f the present demand for gas in the UK, and would require about 20 × 106 t coal year -1. 90 Biomass and wastes, particularly any single resource, cannot match a requirement of this size, but might be useful in small gasification plants with outputs o f 0.03-0-15 × 10 6 m a d a y -1. A slagging gasifier might produce only 1.4 × 106 m 3 day -1 SNG, and a pilot plant could have an o u t p u t as small as 0.3 × 10 6 m 3. Existing Catalytic Rich Gas (CRG) plants have a capacity o f about 0.33 × 10 6 m 3 day -1 although new ones would probably be about ten times as large. But although the contribution that biomass and wastes might make in practice is small, it is not insignificant. Small quantities of gas are useful in terms of conservation of fossil fuels, and may be very welcome in some rural areas to augment gas supplies. 3.1. Methane 3.1.1. Anaerobic digestion The microbiological breakdown o f organic matter in the absence o f air is an important part of the carbon cycle. It occurs at ambient temperatures and pressures, and in nature leads to the formation o f 'will o' the wisp' gas o r 'marsh gas', and methane production in the stomachs of ruminants like cows and sheep. All these sources contribute to the

The prospects f o r biogas - a European

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little-known fact that the methane concentration in the atmosphere is often about 2 ppm v/v. Anaerobic digestion is the only realistic way o f producing methane from crops, crop residues, animal wastes and sewage that contain more than about 35% water. For small-scale applications, the only equipment needed is simple and inexpensive, and can be operated by relatively unskilled people with a minimum of training. In India and China, groups of people in rural areas have operated digesters on mixtures of human, animal and plant wastes for m a n y years. In India, original home of the 'gobar' (cattle dung) digesters, there are said to be around 80 000 units, each yielding 1.7-10 m s gas day-l. 91 China is said to have 7 million biogas plants, mostly built since 1972;92 a typical one for a family of five produces about 1-2 m 3 biogas day -1,9a although it is suggested 93 that more than half have now fallen into a state of disrepair. There are financial and environmental incentives for 'on-the-farm' biogas production from wet agricultural wastes. Anaerobic digestion o f the wastes where they arise is a logical way of providing energy for local use. With little or no treatment, the gas produced can be burned directly to provide heat for livestock houses, etc., used as fuel in converted tractors and other vehicles, or used to generate electricity for a variety of purposes. Anaerobic digestion also helps to overcome what is frequently a severe disposal problem. Farm wastes are m u c h stronger than sewage, and the quantities produced, especially from intensive cattle or poultry farming, are too large to be returned to adjacent land without causing pollution. After digestion, the residual sludge makes an excellent nitrogenous fertiliser, or it can be fed back to the cattle. Biogas produced by anaerobic digestion has specific disadvantages from a gas industry viewpoint: its volume is usually comparatively small, it is produced at close to atmospheric pressure, contains 30-50% carbon dioxide, and is sometimes contaminated with small amounts o f hydrogen sulphide and ammonia. Although large commercial-scale digesters are perfectly feasible, they are not necessarily so easy to operate as small units. The process is still essentially a batch one, and it is often unpredictable, because the various micro-organisms involved are easily 'upset', so that methane production slows down or stops. Recent improvements in digester design, and progress with continuous modes o f operation, may resolve some o f these problems. Sewage collected from large cities and towns in Europe is potentially a source of m u c h larger volumes of biogas. For example, the sewage o f

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Munich yields 12.5 × 106 m3 gas year -1, with an energy content o f 23-27 MJ m-a, 4s and about 92 × 106 m 3 year -1 is produced from London's sewage, 94 from a contributing population of 7.5 million people. The biogas is usually fully utilised on-site, contributing to self-sufficiency and helping to offset the costs of electricity and fuel supplies from outside sources. Upgrading is unnecessary, although in some large sewage works, carbon dioxide is removed and the methane is pressurised. Biogas derived from sewage sludge is undoubtedly valuable in an energy conservation sense, but few sewage works produce more gas than is required for their own use. In many heavily populated countries, however, anaerobic digestion o f a mixture o f sewage and sorted domestic refuse could give rise to far more significant quantities of gas. This concept of combined treatment of wet and solid wastes is the basis of the 'IGT BIOGAS Process'. 95 Well-organised systems already exist for collecting sewage and domestic refuse from most large conurbations in Europe, and sufficient gas might be produced to warrant the expense o f purification, pressurisation and incorporation of the pipeline-quality gas into gas industry distribution systems. Implementation within EC countries might be difficult, however, simply because many large cities already deal with sewage and domestic refuse separately; the treatment facilities are generally adequate but usually incompatible, and reorganisation would be costly. Most domestic and industrial wastes are currently disposed of by controlled tipping on to land set aside for the purpose, although small quantities are incinerated (the heat released is sometimes used for steam generation). Landfill sites (refuse tips) are themselves a source o f potentially useful quantities of biogas. When waste materials containing organic carbon are mixed with soil, microbial growth occurs, and anaerobic decomposition is inevitable. Combustible gas emanating from landfill sites has been regarded as an embarrassment, particularly when explosions or fires occur on land being reclaimed for building or for recreation. At some sites, primitive systems have been designed to vent the gas, or to collect it and burn it, in order to alleviate potential h a z a r d s . 96 N o w , however, there are schemes for abstracting the gas for use in a more positive way. Landfill biogas is not pure methane, of course. Even from 'mature' sites it is often mixed with more than 50% carbon dioxide, and frequently contains hydrogen sulphide and traces of other undesirable gases. It is very wet, produced at pressures only slightly higher than

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atmospheric, and is often generated for years after tipping has finished. The first landfill site to be commercially exploited was probably the one on the Palos Verdes Peninsula in California, where development started in the mid-1970s. 97 Biogas from this site is upgraded to pipeline quality, and about 16 000 m 3 day -1 of 37 MJ m - 3 gas is fed into the distribution system of the Southern California Gas Company. In 1981, six full-scale methane recovery schemes were reported to be operating in the USA, with 17 more at the testing or construction stage. 98 Whether or not these schems are really cost-effective is not clear from the published data. There is also scope for biogas production from landfill areas in Europe (see Sections 2.2.3 and 2.2.8), although the present author suggests that such gas is more likely to be used in an unpurified form by local industry than upgraded for gas industry use. In the F D R and in the UK, landfilling accounts for 85-90% o f solid wastes, and although the emphasis has tended to be on prevention of gas migration and associated hazards, schemes for utilisation of low-MJ gas are already operating.S3, 87 The maximum yield o f biogas from wet refuse is said to be about 0 - 4 5 m 3 kg -1, and rates of 0-006-0.038 m 3 kg -a year -a are claimed for landfill sites during the most active period of gas production. 98'99 Temperature, water content and refuse density have all been shown to affect productivity; ~°°'l°a moisture movement through decomposing waste appears to be particularly important, and can increase the rate of methane generation by 25-50%. 98 Techniques proposed for stabilising and enhancing methane production include the use of clay and other lining materials as barriers, and the addition of appropriate microorganisms to the landfill to encourage more rapid decomposition. ~°2 3.1.2. Thermochemical conversions An excellent review of the characteristics, applications and potential o f different processes and reactor types for thermal gasification of biomass was published in 1980. 2s Its author concluded that the introduction o f biomass gasification on a large or intermediate scale for the production of power, SNG, methanol, etc., would depend on developments in the field of gasification of coal and municipal solid wastes, and on the price o f various forms o f biomass. Pyrolysis, gasification and hydrogenation are generally uneconomic if the feedstock contains more than about 35% water. For relatively dry

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materials, however, the advantages o f these techniques are that chemical reactions occur rapidly at the high temperatures and pressures usually employed, and large quantities of feedstock can be converted into gaseous and/or liquid fuels far more quickly than is the case for anaerobic digestion. Compared with anaerobic digestion, the disadvantages are that capital investment costs are high, reactor vessels are often complex and expensive to manufacture, and operating costs can be high, especially when large quantities of steam, oxygen or hydrogen are needed. One of the drawbacks in using biomass or wastes as feedstock for a thermochemical process is that adequate supplies would be needed on a regular basis for operation at a high load factor. Supplies o f domestic refuse and wood might be adequate, and straw is also available in sufficiently large amounts, although its production is seasonal. The answer may be to design gasifiers capable of accepting a wide range o f feedstocks, or alternatively to blend small quantities of any available and suitable biomass or wastes with coal feedstocks. In some instances, introduction o f such a procedure might even have favourable side effects, by preventing caking or swelling o f coal, for example. Biomass contains relatively little sulphur, so that it could be a useful 'diluent' for coals with a high sulphur content. 3.2, Ethanol The fermentation of sugary materials to produce ethyl alcohol is a very ancient art to which scientific principles have been applied only comparatively recently, as far as fuel production is concerned. Materials containing starch and cellulose can also be fermented, but only after hydrolysis by acids or enzymes to release constituent sugars. The theoretical yield o f ethanol from glucose on a weight basis is 51%, but because the yeasts multiply during the fermentation and other products are formed, in practice the yield is about 47%. The fact that fermentation occurs only in aqueous solutions causes problems, as most yeasts are inactivated b y ethanol concentrations above 10-12%. Alcohol tolerance has been increased experimentally, however, so that for short periods of time some yeasts can withstand a concentration of 25%; 1°3 progress has been made also with techniques for continuously extracting the ethanol as it is formed, 1°4,1°s and with the use of immobilised yeast cells that have high alcohol tolerance.

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The price o f raw material for fermentation is obviously important, but so too is the cost o f removing water associated with the alcohol, as for most end uses (with the notable exception o f alcoholic beverages) virtually anhydrous alcohol is required. Distillation is still the best way of removing the water, 1°6 but accounts for about one-third of commercial-scale production costs, excluding the price o f the raw carbohydrate, l°a The energy consumption in distilleries is often said to be approximately equal to the fuel value of the ethanol produced. This is why sugar cane has a marked advantage over other fermentable materials, as fibrous waste (bagasse) from the cane provides fuel for the distillation. 1°7 If vacuum stripping and membrane separation techniques can be developed for industrial use, fermentation economics m a y alter. ~03 In energy terms, ethanol is nearly always thought o f as a transport fuel. The Brazilian, the American and the French alcohol programmes all have a similar a i m - either to produce enough ethanol to replace conventional m o t o r fuel altogether (the ultimate intention in Brazil), or to produce a petrol/alcohol blend that will help to conserve oil supplies and stabilise prices. Four refineries for producing ethanol from biomass are planned in Austria; 9 the first plant was commissioned in 1981 and will ultimately produce 40 × 1061itres ethanol year -1 from 1 1 0 0 0 0 t wheat or maize, with sugar beet or cellulose as alternative feedstocks. But there are other potential uses for ethanol. For example, it is a versatile starting material for organic syntheses in the chemical industry, and it is also of interest to the gas industry. As one might expect, it is easily gasified catalytically: 2C2HsOH

) 3CH4 + CO2

At low temperatures (300°C, for example), no carbon deposition occurs; although no water is needed stoichiometrically, steam is added during gasification. Ethanol is sulphur-free, and because it is a liquid at atmospheric temperatures and pressures, it is easy to transport and store. Possibly its only disadvantage is its price. In 1980, the cost o f producing ethanol from sugar cane was £220-250 ($512-582) per tonne. For comparison, some other 1980 prices per tonne in the UK were: £32 ($75) for coal, including delivery; £162 ($377) for naphtha, including delivery; £30 ($70) for sugar beet, price paid to growers; and in 1981, £ 170 ($396) for raw sugar, before refining.

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If oil products become virtually unobtainable, ethanol could be used as a peak load feedstock, and a higher price could be justified; LNG based on SNG from coal might be no cheaper. In the UK, storage capacity for LNG constitutes a considerable investment in peak shaving, but represents less than 1% of the annual demand for natural gas. In this context, 2 0 0 0 0 0 t ethanol (which could be produced from approximately 1 × 106 t straw or domestic refuse) might make a useful and economically worthwhile contribution to future peak shaving requirements. In the production of town gas from naphtha by high pressure steam reforming, it was possible to increase the production o f lean gas by injecting methanol into the water gas shift reaction vessel. It might be possible to use a similar technique by injecting ethanol at the appropriate stage in a coal gasification process. Injection straight into the methanators might be attractive. For all these potential gas industry applications, if the ethanol could be produced near where it was to be used, there would be no need for a distillation stage to remove water. The cost o f the ethanol would then be significantly less.

3.3. Methanol Although methanol (wood alcohol) can be obtained by pyrolysing wood, it is now usually prepared catalytically from synthesis gas derived from natural gas, although it can be obtained by gasifying various feedstocks, including coal, naphtha, biomass or wastes. Like ethanol, methanol is usually regarded as a transport fuel. In fact, it is sometimes preferred to ethanol as a petroleum additive, and at least one car manufacturer has designed vehicles to run on methanol/petrol blends and on pure methanol, x°8 In a recent review, x°T it was suggested that with w o o d as the most likely starting material, gasification processes leading to methanol were more likely to become commercial in Europe than ethanol fermentation processes. H o w widely this view is shared remains to be seen, although certainly gasification o f waste straw, followed by methanol synthesis, is claimed to be economically promising in the UK. 77 As a fuel or as a feedstock, methanol has the convenience o f being a clean liquid at ambient temperatures, and therefore easy to transport. It has been suggested that the vast natural gas reserves in the remoter parts of the world could be converted to methanol and brought to areas

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where natural gas is needed. The methanol would then be converted to SNG. Although this method is claimed to be economic only for exceptionally long distances, it has the distinct advantage that the methanol could be transported in conventional tankers, which cost far less than the expensive, specially constructed cryogenic tankers that are currently used for transporting LNG. Similarly, synthesis gas derived from biomass grown in tropical areas, for example, could be converted into methanol and exported in liquid form. Potential gas industry uses for methanol are very much like those for ethanol. It can be used instead of, or as well as, LPG for peak shaving. Although the technology for production of SNG from methanol has not yet been demonstrated commercially, no serious problems are envisaged, and overall conversion efficiencies and operating costs are likely to be similar to those of producing SNG from naphtha. 3.4. Hydrogen Methane can be produced from coal by the two main process routes of methanation or direct hydrogenation. The overall reaction is the same for both routes. 2C + 2H:O

~ CH4 + CO2

Each route requires a source of hydrogen and this is usually produced through one of the process steps. As the above equation indicates, this results in half the carbon in the coal being rejected as carbon dioxide. If a cheap and plentiful supply of hydrogen were to become available from an external source, then a much greater proportion of the coal could be hydrogenated to methane 1°9 via the reaction: C + 2H2

~ CH4

If the hydrogen were to be produced by photolysis of water, an additional advantage for coal conversion processes would be that oxygen would be available as well. 110 Hydrogen can be produced biologically in various ways. For example, it is produced during the fermentative breakdown of organic matter by anaerobic bacteria, and is also evolved by photosynthetic bacteria, and by blue-green and green algae during light dependent reactions. 111'1a2 In nature, however, hydrogen rarely accumulates. It disappears quickly because it is readily utilised by other micro-organisms, including the

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methane producers and organisms known as 'Knallgas' (hydrogen) bacteria. Although there is considerable interest in all the microbial pathways of hydrogen production, the most intensive research effort in this field is probably directed towards understanding photosynthesis. It is during the first stage of this process that energy from the sun is utilised by green plants to split water into its constituent elements. This photochemical dissociation is known as biophotolysis, and it has already been duplicated in the laboratory, using irradiated systems containing plant chloroplasts (the parts o f the plant cell that contain the pigment chlorophyll) and hydrogenase enzymes from bacterial cells. 113'1x4 Hydrogen has been evolved from such systems continuously for several hours, at rates o f up to 94 micromoles hydrogen mg -1 chlorophyll h-l. us Scaled up, this would be roughly equivalent to 50 × l0 a m 3 t -1 dry chlorophyll day -1, but at present, an extrapolation o f this magnitude is totally unjustified. For large scale SNG production by hydrogenation of coal, the amount of hydrogen required would be about 7 × 10 6 m 3 day-1.116. It is most unlikely that biophotolysis could ever be used to produce quantities of hydrogen o f this magnitude. The same applies to the related research being carried out with artificial catalysts, 117-122 the aim of which is to enable scientists to imitate or improve upon the natural process. Biophotolysis is attractive because water is abundant and sunlight free, but the problem is that, whether a natural or an artificial system is envisaged, the scale for hydrogen production is limited by the area that would be needed to collect sufficient solar energy. The mean insolation in the UK, averaged over the year, is 9 MJ m -2 day -1. At this energy density, the minimum land area required to generate 7 × 10 6 m 3 day -1 o f hydrogen via solar radiation, assuming 100% efficiency, would be 9.9 X 10 6 m 2, or using a more realistic figure of 30% conversion efficiency (for all wavelengths), 33 × 10 6 m2.116 For small quantities of hydrogen, and in areas of the world where insolation is twice as intense (e.g. in Australia), the areas of land involved might be acceptable. But collecting the hydrogen, and separating it from the oxygen, would be a problem, as the hydrogen would be produced at atmospheric pressure, and less than 0 . 2 m 3 would be generated per square metre o f surface per day. 116 It is suggested that various novel ways of concentrating solar radiation will be discovered. For example, it might be possible to use satel-

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lites in some way. However, for the time being, green plants still offer the most complete energy service. Not only do they trap solar energy (admittedly with efficiencies rarely exceeding 4-5%, and on average usually about 0.1-0.2%), but they accomplish biophotolysis, and by means of photosynthesis, they store hydrogen and carbon in a form that makes energy reclamation possible at any time. Whether this stored energy is used as food to fuel the human body, or as fuel to keep that body warm and for other purposes, is for us to choose. 3.5. Miscellaneous Various plants store liquid hydrocarbons or waxes within themselves or within their seeds. Some o f these plants are already cropped, usually as sources of vegetable oils. Sunflower oil, soya bean oil, rape seed oil, ground-nut oil and olive oil are all used for nutritional and sometimes pharmaceutical purposes. Many have been shown to be potentially useful as transport fuels, although impurities can be a problem. Most of these crops are or can be grown under European conditions. It may well be worth evaluating crude extracts o f plant oils as possible feedstocks for CRG plants, for example. Rubber trees provide latex for the natural rubber industry, and there is now interest in other latex or oil-producing plants. Many bush species of E u p h o r b i a , which grow in arid or semi-arid areas, produce interesting quantities of oil; yields equivalent to 25 barrels oil ha -1 year -1 might be obtainable, compared with Malaysian rubber latex yields equivalent to 62 barrels oil ha -1 year-1.123 In Brazil, there is a small pilot plant for extracting light oils from 20 t day -~ o f a bush known as the black quince, ~24 and for a time there were hopes that it would be possible to tap a heavy oil from a tree called the C o p a i b a . 12s It has even been suggested that by implanting gall-causing micro-organisms into certain trees, the trees could be induced to produce hydrogen, which could then be tapped. Hydrocarbons that accumulate in algae are attracting attention. ~26 For example, glycerol is produced in large quantities by a red alga which can be grown in shallow ponds containing saline water; the best way of separating and purifying the glycol from the algal biomass is being investigated in Israel. ~27 Apart from ethanol, other end-products o f bacterial fermentations include butanol and acetone, which have been produced on a commercial scale at various times. None of these

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E. S. Pankhurst

sources can separately provide large quantities o f hydrocarbon feedstock for gas industry use, but nevertheless they could be considered as small contributors towards energy requirements.

4. CONCLUSIONS 4.1. For most m e m b e r countries o f the European Community, biofuels (including biogas) have a significant role to play in the conservation o f fossil fuels. 4.2. Biomass and wastes are likely to contribute at least 75 Mtoe to the overall energy requirements of the European C o m m u n i t y by the year 2000 (i.e. about 7-5% o f the current energy demand). 4.3. Biogas produced by anaerobic digestion of agricultural wastes is a worthwhile but comparatively small source o f energy, best used locally, on farms for example; removal o f carbon dioxide is unnecessary. 4.4. The larger volumes o f biogas produced by anaerobic digestion of sewage sludge are already fully utilised in many European cities. There is probably scope for combined treatment of sewage and domestic refuse to yield even more gas, providing suitable collection facilities for the starting materials exist and can be harmonised. In some instances, enough gas might be obtained to warrant purification and pressurisation to pipeline quality, followed by incorporation into a gas industry distribution system. Upgrading is unlikely to be economic unless the volume produced exceeds about 28 X 103 m 3 day -1. 4.5. Old refuse tips (sanitary landfill areas) are likely to become the source of small but significant volumes of biogas that can be used locally. If quantities over 28 X 103 m 3 day -1 become available, upgrading to pipeline quality and incorporation into a gas industry distribution system might be appropriate. 4.6. Biomass and wastes with moisture contents of less than about 35% should be considered as potential supplementary feedstocks for conventional coal gasification units. 4.7. Ethanol produced by fermentation o f biomass and wastes should not be considered only as a transport fuel. It has potential as a peakload feedstock for gas industry use, and should be competitive with LNG produced via SNG from coal. 4.8. Methanol produced via synthesis gas from biomass and wastes also has considerable potential for gas industry use.

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4.9. The production o f energy crops (including trees)specifically for conversion into liquid or gaseous biofuels is likely to increase, in spite of the fact that competition for land use will continue to be intense. 4.10. In the future, scientific research may enable nitrogen-fixing ability to be induced in a wide range o f crops. This would have beneficial repercussions in a gas industry context, as it would no longer be necessary to produce large quantities o f ammonia and nitrogenous fertilisers from natural gas, and considerable savings in gas consumption would ensue. 4.11. It will be very many years, if ever, before abundant supplies o f cheap hydrogen become a reality. However, considerable progress has already been made in understanding biophotolysis, and the possibility that artificial water-splitting catalysts can be constructed is by no means remote. 4.12. A more critical appraisal o f many o f the figures that have been suggested for potential energy contributions is needed, as in the course of this review the author has identified some apparent discrepancies.

5. ACKNOWLEDGEMENTS This review is published with the permission o f the British Gas Corporation and the International Gas Union (62 rue de Courcelles, 75008 Paris, France). It was presented at the 15th World Gas Conference, Lausanne, 1982. Some of the views expressed are the author's own and do not necessarily reflect British Gas policy. The author particularly wishes to thank the friends and colleagues who have given her so m u c h help and advice, and gratefully acknowledges the tolerance and skill shown by Jean Morris and Gladys King in typing the manuscript.

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4. Institute of Gas Technology (1978). Energy from Biomass and Wastes. Chicago, Ill. 867 pp. 5. Klass, D. L. (1979). Wastes and biomass as energy resources. IGU (Paris) B8-79. 14th World Gas Conference, Toronto. 14 pp. 6. Bolton, J. & Hall, D. O. (1979). Photochemical conversion and storage of solar energy. Ann. Rev. Energy, 4, 335-401. 7. Hall, D. O. (1979). Solar energy use through biology-past, present and future. Solar Energy, 22, 307-28. 8. Hall, D. O. & Coombs, J. (1979). The prospect of a biological-photochemical approach for the utilisation of solar energy. In: Report No. 5, Watt Committee on Energy, London, pp. 2-15. 9. Pirt, S. J., Lee, Y.-K., Richmond, A. & Pirt, M. W. (1980). The photosynthetic efficiency of Chlorella biomass growth with reference to solar energy utilisation. J. Chem. Teck Biotechnol., 30, 25-34. 10. CEFIC (1980). Consumption of energy sources of the European chemical industry for feedstock and fuel. Vol. 1. Consumption 1974-1979. In: Energy Statistics, EEC, Brussels. 11. Rose, J. W. & Cooper, J. R., eds (1977). Technical Data on Fuel. British National Committee, World Energy Conference, London. 7th Edition. 12. O'Donnell, S. (1974). Ireland turns to peat. New Scientist, 63, 18-19. 13. Wrobel, J. & Wright, P. (1978). Calorific values and relative densities. Calculation from Composition. IGE/TM/1. Communication 1080. The Institution of Gas Engineers, London. 13 pp. 14. Goodrich, P. R., Gustafson, R. J. &Haver, K. L. (1979). Farm-scale generation of bio-gas. Energy, 4, 249-61. 15. Keenan, J. D. (1977). Fuels via bioconversion. Energy Conversion, 16, 95-103. 16. Goddard, K. (1979). Liquid fuels from biomass. Chartered Mech. Eng., 26, 33-5. 17. Keenan, J. D. (1979). Review of biomass to fuels. Process Biochem., 14, 9-12, 14-15, 30. 18. Palz, W. & Chartier, P., eds (1980). In: Energy from Biomass in Europe. London, Applied Science Publishers, pp. 125, 128, 65, 83, 94, 47, 45. 19. European Communities- Commission (1980). EUR 6959 - Solar energy programme o f the Commission o f the European Communities. Luxembourg: Office for Official Publications of the European Communities. 167 pp. 20. Lawson, G. J. & CaUaghan, T. V. (1981). Natural vegetation as a renewable energy resource in Great Britain. In: Ref. 31, pp. 83-9. 21. Porteous, A. (1981). Refuse Derived Fuels. London, Applied Science Publishers. 137 pp. 22. Keenan, J. D. (1977). Bioconversion of solar energy to methane. Energy, 2, 365-73.

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23. Lettinga, G. (1981). Anaerobic digestion for energy saving and production. In: Ref. 31, pp. 264-78. 24. Coombs, J. (1981). Ethanol- the process and the technology for production of liquid transport fuel. Ibid., pp. 279-91. 25. Van Swaaij, W. P. M. (1981). Gasification- the process and the technology. Ibid., pp. 485-95. 26. Reed, T. B. (1981). The combustion, pyrolysis, gasification and liquefaction of biomass. Ibid., pp. 496-508. 27. Miller, I. J. & Fellows, S. K. (1981). Liquefaction of biomass as a source of fuels or chemicals. Nature, 289,398-9. 28. Slessor, M. & Lewis, C. (1979). Biological Energy Resources. London, E. & F. N. Spon. 192 pp. 29. Ladisch, M. R., Flickinger, M. C. & Tsao, G. T. (1979). Fuels and chemicals from biomass. Energy, 4, 263-75. 30. Vogt, F. (1981). Energy Conservation and Use of Renewable Energies in the Bio-lndustries. Oxford, Pergamon Press. 574pp. 31. Palz, W., Chartier, P. & Hall, D. O. (1981). Energy from Biomass. 1st EC Conference. London, Applied Science Publishers. 982 pp. 32. European Communities-Commission (1980). Solar Europe. Newsletter of the Solar Energy Programme of the European Communities. Brussels. Issued by the Directorate-General XII for Research, Science and Education. October. 33. Financial Times (1980). World Solar Markets. A monthly review of political, financial and technological developments. April. 34. Anon. (1981). DOE selects eleven firms for ethanol aid. Oil & GasJ., 79 (34), Aug., p. 55. 35. Chartier, P. (1981). Prospects for energy from biomass in the European Community. In: Ref. 31, pp. 22-33. 36. Commission of the European Communities (1980). Energy from Biomass. Solar Energy Research and Development. Leaflet. 37. Palz, W. & Chartier, P., eds (1980). Energy from Biomass in Europe. London, Applied Science Publishers. 234 pp. 38. Bouvarel, P. (1981). The outlook for energy forestry in France and in the European Economic Community. In: Ref. 31, pp. 172--80. 39. Di San Marzano, C., Naveau, H. P. & Nyns, E.-J. (1981). Production of methane from freshwater macro-algae by an anerobic two step digestion system. Ibid., pp. 392-7. 40. Piron-Fraipont, C., Dujardin, E. & Sironval, C. (1981). Increasing biomass for fuel production by using luke-warm water from industries. Ibid., pp. 703-8. 41. Rexen, F. P. (1981). Straw and animal residues available for energy. Ibid., pp. 50-8. 42. Goldberg, T., Bjergbaek, B., Djernaes, E. & Mouritzen, A. (1981). An anaero-

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43. 44.

45. 46. 47. 48. 49. 50.

51. 52. 53. 54.

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E. S. Pankhurst bic digester for continual high solid loading: the biofunnel (a radially expanding, overflow digester). Gas Research Conference, Los Angeles. US Dept. of Energy, p. 451. Defaye, J. & Gadelle, A. (1981). Degradation of cellulose with hydrogen fluoride. In: Ref. 31, pp. 319-23. Friedrich, F. J. & Klein, H. (1982). Solar energy projects in Germany. Abstract. In: Solar Worm Forum. D. O. Hall & J. Morton, eds. Pergamon Press, Oxford, pp. 3206-10. Titl, A. J. (1981). Utilisation ofbiomass for cogeneration of heat and power. Gas Research Conference, Los Angeles. US Dept. of Energy, p. 533. Meier, K. (1980). Biogas-Aktivit~iten in Deutschland. Sonnenenergie & W~rmepumpe, 5 (1), pp. 10-12. Stutzer, D. (1980). Praktische Erfahrung in Benediktbeuren. Ibid., pp. 32-5. Aichele, F-X. (1980). Anlage in Langen ohne maschineUe Einbauten. Ibid., pp. 46-7. Urbanek, A. (1980). Kliiranlage Regensburg erzeugt aus Faulgas Strom. Ibid., p. 56. Lindauer, G. D. & Sixt, T. (1981). Application of recent developments in anaerobic waste water treatment to slaughterhouse wastes. In: Ref. 31, pp. 440-7. Kandler, O., Winter, J. & Temper, U. (1981). Methane fermentation in the thermophilic range. Ibid., pp. 472-7. Kleinhanss, W. (1981). Economic aspects of biogas- production from animal wastes. Ibid., pp. 466-71. Stegmann, R. (1981). Landfill gas problems-summary of West German experience. Paper 6. Landfill Gas Symposium, ETSU, Harwell, UK. Commissariat G6n6ral du Plan (1980). R6sum6 du rapport de la Commission de l'6nergie et des mati~res premi6res raise en place pour la pr6paration du VIIIe Plan. La Relev~ve du P6trole. Paris. Revue de l'~nergie. Commissariat ~ l'l~nergie Solaire (1980). Biomass. France's green energy. Leaflet. Paris, Service de Diffusion de rlnformation sur l'Energie Solaire (COMES, 208 rue Losserend, Paris 14). Marien, J. N. (1981). Selection d'Eucalyptus adapt6s aux conditions franqoises en rue d'instaUation de TaiUis ~ courtes rotations. In: Ref. 31, p. 257. Da Silva, E. J. (1981). Microbial biotechnology: a global pursuit. Process Biochem., 16, 38-44. Deglise, X., Richard, C., Rolin, A. & Franqois, H. (1981). Fast pyrolysis/ gasification of lignocellulosic materials at short residence time. In: Ref. 31, pp. 548-53. Deglise, X., Morli~re, P: & Schlicklin, P. (1981). Mass and energy balances for a two fluidised-bed pilot plant which operates on wood fast pyrolysis. Ibid., pp. 569-73.

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60. Masson, C., Bourreau, A., Kabbava, N., Souil, F. & Goudea, J. C. (1981). Methanol synthesis by conversion of combustion gases from vegetable matter. Abstract. In: Solar Norm Forum. Congress and Exhibition, Brighton. International Solar Energy Society, 19 Albemarle Street, London Wl, p. 106. 61. Masson, C., Bourreau, A., LaUemand, M., Souil, F. & Goudeau, J. C. (1981). Methanol catalytical synthesis from carbon monoxide and hydrogen obtained from combustion of cellulose waste. In: Ref. 31, pp. 588-93. 62. Anon. (1979). Energy adventurers look for a green E1 Dorado. The Economist, 272 (7094), August, pp. 83-6. 63. Anon. (1981). Straws and Jerusalem artichokes. Pipeline & Gas J., 208 (7), p. 7. 64. Minier, M. & Goma, G. (1981). Ethanol production by extractive fermentation. In: Ref. 31, pp. 298-305. 65. Largeau, C., Casadevall, E., Dif. D. & Baillez, C. (1982). Renewable hydrocarbon production from the alga Botryococcus braunii. Effects of culture conditions and cell immobilisation on productivity. See Ref. 44, pp. 1263-7. 66. Neenan, M. & CahiU-O'Brien, T. (1982). Short-rotation forest-partial solution to EEC energy needs. Ibid., pp. 1258-65. 67. O'Connor, R. (1981). How Ireland is finding fresh use for its peat bogs. The Times, 9 Jan. 68. Neenan, M. & Lyons, G. (1981). Short rotation forestry as a source of energy. In: Ref. 31, pp. 232-8. 69. Scaramuzzi, G., Cianco, O. & Eccher, A. (1981). Coppice forests in Italy: their potential for energy. Ibid., pp. 222-7. 70. Alfani, F., Cantarella, M. & Scardi, V. (1981). Experimental study on continuous digestion of cellulose waste in a combined reactor system. Ibid., pp. 144-9. 71. Gianfreda, L. & Greco, G. (1981). Agricultural waste treatment by means of ultrafiltration membrane enzymatic reactors. Ibid., pp. 306-11. 72. Rijkens, B. A. (1981). A novel process for the anaerobic digestion of solid wastes leading to biogas and a compost-like material. Ibid., pp. 435-9. 73. Van der Meiden, H. A. & Kolster, H. W. (1981). Biomass production with poplar. Ibid., pp. 193-7. 74. Energy Technology Support Unit, Harwell (1980). Provisional assessment of the sources of biofuels. As reported in The Times, 5 Nov. 75. King, G. H. (1981). The UK Department of Energy Biofuels R&D Programme. Abstract. In: Solar World Forum. See Ref. 60. p. 424. 76. Larkin, S. B. C., Morris, R. M., Noble, D. H. & Radley, R. W. (1982). Mapping the distribution of agricultural wastes in England and Wales. See Ref. 44, pp. 1234-40. 77. Ader, G., Bridgwater, A. V. & Hatt, B. W. (1981). Techno-economic evaluation of thermal routes for processing biomass to methanol, methane and

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liquid hydrocarbons. In: Ref. 31, pp. 598-606. 78. Mitchell, C. P. & Matthews, J. D. (1981). Forest biomass as a source of energy in the UK; the potential and the practice. Ibid., pp. 239-43. 79. Spedding, C. R. W., Bather, D. M. & Walsingham, J. M. (1979). Fuel c r o p s an assessment of the UK potential. In: Report No. 5, Watt Committee on Energy, London, pp. 17-24. 80. Stott, K. G. (1981). Coppice willow for biomass in the UK. In: Ref. 31, pp. 198-204. 81. Gibbins, J. R. & Wilson, H. T. (1982). Some theoretical considerations related to production of methanol from biomass. See Ref. 44, pp. 1300-5. 82. Lawson, G. J. & Callaghan, T. V. (1981). Natural vegetation as a renewable energy resource in Great Britain. In: Ref. 31, pp. 83-9. 83. Callaghan, T. V., Scott, R., Whittaker, H. A. & Lawson, G. J. (1981). Selected natural and alien plant species as renewable sources of energy in Great Britain - experimental assessment. Ibid., pp. 90-6. 84. Callaghan, T. V., Lawson, G. J. & Scott, R. (1982). Bracken as an energy crop? See Ref. 44, pp. 1239-44. 85. Carruthers, S. P. (1981). The productivity of catch crops grown for fuel. In: Ref. 31, pp. 97-102. 86. Anon. (1981). Belfast don plans kelp-into-methane research. J. Instn Gas Eng., 21,264. 87. Cheyney, A. C. & Moss, H. D. T. (1981). Landfill gas as an energy source. Paper 9. Landfill Gas Symposium, ETSU, HarweU, UK. May. 88. Anon. (1981). Waste-tip gas fires UK brick kiln. J. Instn Gas Eng., 21,264. 89. Marchant, A. J. (1981). Practical aspects of landfill management of landfill g a s - a local authority view. Paper 7. Landfill Gas Symposium, ETSU, Harwell, UK, May. 90. Dean, F. E. & Goalby, B. B. (1979). Environmental effects of producing substitute natural gas. Paper presented at: Conference on 'The Environmental Effects of Utilising More Coal'. London, Council for Environmental Science and Engineering. December. 91. Hall, D. O., Barnard, G. W. & Moss, P. A. (1982). Biomass for Energy in the Developing Countries. Current Role. Potential. Problems. Prospects. Oxford, Pergamon Press. 197 pp. 92. Chen Ruchen (1981). The development of biogas utilization in China. Biomass, 1, 39-46. 93. Rao, R. (1981). Pearls before swine. Nature, 293, 179. 94. Summers, R. & Bousfield, S. (1976). Practical aspects of anaerobic digestion. Process Biochem., 11 (5), June, p. 3. 95. Ghosh, S. & Klass, D. (1976). SNG from refuse and sewage sludge by the BIOGAS Process. In: Clean Fuels from Biomass, Sewage, Urban Refuse, Agricultural Wastes. Institute of Gas Technology, Chicago, I11.,pp. 123-81.

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96. Shen, T. T. (1981). Control techniques for gas emissions from hazardous waste landfills. J. AirPoll. ControlAssoc., 31,132-5. 97. Rice, F. C. (1978). Commercial production of pipeline quality gas at the Palos Verdes Landfill. In: Energy from Biomass and Wastes. Institute of Gas Technology, Chicago, II1., pp. 345-52. 98. Klink, R. E. & Ham, R. K. (1981). Effects of moisture movement on methane production in solid waste landfills. Gas Research Conference, Los Angeles. US Dept. of Energy, p. 441. 99. Ham, R. K., Kekiman, K. K., Katten, S. L., Lockman, W. J., Lofy, R. J., McFaddin, D. E. & Daley, E. J. (1979). Recovering, processing and utilization o f gas from sanitary landfills. EPA-600/2-79-O01, Washington, D.C. February. 134 pp. 100. Rees, J. F. (1980). Optimisation of methane production and refuse decomposition in landfills by temperature control. J. Chem. Tech. Biotechnol., 30, 458-65. 101. Rees, J. F. (1981). Major factors affecting methane production in landfills. Paper 2. Landfill Gas Symposium, ETSU, Harwell, UK, May. 102. Institute of Gas Technology (1980). Fuel from biomass and wastes. IGT Gascope, Chicago, I11.No. 51. 103. Parker, K. J. & Righelato, R. C. (1979). Chemicals from carbohydrates. In: Processes for Chemicals from some Renewable Raw Materials. London, Instn Chem. Engs & Soc. Chem. Ind., pp. 9-21. 104. Minier, M. & Goma, G. (1981). Ethanol production by extractive fermentatation. In: Ref. 31, pp. 298-305. 105. Grout, H. J. & English, M. (1981). A low energy system for the production of biomass ethanol. Ibid., pp. 360-5. 106. Emery, A. N. & Kent, C. A. (1979). Fuel alcohol production from biomass. In: Report No. 5, Watt Committee on Energy, London, pp. 45-59. 107. Coombs, J. (1981). Ethanol- the process and the technology for production of liquid transport fuel. In: Ref. 31, pp. 279-91. 108. Bernardt, W. (1981). Alternative fuels from biomass and their use in transport. Ibid., pp. 815-25. 109. Gibson, G. H. (1978). Engineering economics applied to supply and demand in the gas industry. J. Instn Gas Eng., 18, 330-41. 110. Harrison, J. S., Merrick, D., Smith, M. & Rasmussen, G. (1981). The use of non-fossil derived hydrogen; a possible role for coal conversion. Int. J. Hydrogen Energy, 6, 363-75. 111. Weaver, P., Lien, S. & Siebert, M. (1979). Photobiologicalproduction o f hydrogen - a solar energy conversion option. Golden, Colorado, Solar Energy Research Institute. Report SER1/TR-33-122. 112. Schlegel, H. G. & Barnea, J., eds (1977). Microbial Energy Conversion. Gottingen, Pergamon Press. 643 pp. Hydrogen formation, pp. 201-80.

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