Anaerobic digestion of crops and farm wastes in the United Kingdom
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Agriculture, Ecosystems and Environment, 30 (1990) 89-95 89 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Anaerobic Digestion of Crops and Farm Wastes in the United Kingdom
RALPH E.H. SIMS 1 and KEITH M. RICHARDS 2
1Agricultural Machinery, Massey University, Palmerston North (New Zealand) 2Biofuels, Energy Technology Support Unit, HarweU, Oxfordshire (Gt. Britain)
(Accepted for publication 4 April 1989 )
Sims, R.E.H. and Richards, K.M., 1990. Anaerobic digestion of crops and farm wastes in the United Kingdom. Agric. Ecosystem Environ., 30: 89-95.
The research and development programme undertaken since 1979 by the Energy Technology Support Unit (E.T.S.U.) on behalf of the Department of Energy is reviewed. It has resolved many of the original uncertainties concerning on-farm production of biogas. It has also demonstrated that in a number of pilot and commercial farm-scale plants the digestion of animal wastes and other feedstocks from plant material is technically feasible.
However, the economic viability of a typical farm installation (based on either the traditional stirred-tank digester or more advanced designs) is likely to be poor, resulting in long or even infinite payback periods. The national benefit from agricultural anaerobic-digestion systems is therefore likely to be insignificant unless credit can be given for resulting environmental improve- ments.
In some circumstances, an individual farmer may well be advised to invest in an anaerobic- digestion plant if he has a specific environmental problem (rather than as the major objective of producing an on-farm energy supply). However, the biogas produced could be used to offset the costs involved in controlling the problem.
Because the current British environmental regulations are unlikely to be made more stringent in the foreseeable future, there appears to be little need for further research and development into on-farm biogas. Consequently the E.T.S.U. programme has been concluded, but a watching brief will be maintained should future developments occur which might make biogas production more attractive.
Since 1979, the Energy Technology Support Unit (E.T.S.U.), on behalf of the British Department of Energy, has conducted a research and development programme to investigate the potential of biogas production on farms. Anaer- obic digestion of animal wastes and other feedstocks from plant materials were studied in a series of laboratory, pilot-scale and commercial-scale installations
0167-8809/90/$03.50 1990 Elsevier Science Publishers B.V.
90 R.E.H. SIMS AND K.M. RICHARDS
which have been described in detail in a summary report (Sims and Richards, 1986).
This paper reviews the 19 projects conducted within the programme and places the conclusions alongside those from other studies undertaken else- where around the world.
Within the U.K., approximately 2.5 Mt coal equivalent year-1 of animal wastes, possibly up to 4.5 of green plant material from catch crops, and a fur- ther 27 from native crops such as bracken, have been identified as being "tech- nically feasible" digestion feedstocks (Larkin et al; 1981). However only a small fraction of these large supplies can be regarded as a useful resource, since an economic demand for the biogas produced needs to be shown, and this is not often the case.
The main problem with the anaerobic digestion of animal wastes has been shown to be the need to increase and maintain the total solids concentration in order to provide consistent feedstock quality at the digester (Hobson et al., 1983 ). The ideal case would be where the slurry handling and storage system of the livestock unit can be fully integrated with a digester plant at the con- struction stage. For existing livestock units this would not be possible without expensive modifications to the buildings.
The use of crops as digester feedstock is technically feasible but still at the development stage with regard to masceration and handling techniques (Staf- ford and Etheridge, 1983 ).
PRODUCTION OF BIOGAS
Anaerobic digestion of on-farm feedstock is technically a practical proposi- tion. Current digester designs are adequate and second generation designs show future promise (Franco, 1981). However, the ancillary equipment has been shown to be insufficiently robust for the rigorous, low-maintenance environ- ment found on typical British farms (Demuynck and Nyns, 1984).
ANAEROBIC DIGESTION OF CROPS AND WASTES 91
Integration of the biogas system within the existing farm-management plan is difficult to achieve in practice as it requires a high level of technical exper- tise. The person responsible for daily operation of the plant needs to be trained and enthusiastic and should act as the link between all the farm operations which could affect the daily running of the plant.
High levels of total solids (around 6% for pig slurry) with good digestibility (i.e. over 70% of total solids being volatile of which 40% is biodegradable dur- ing the digestion process ) are required to give acceptable gas yields of 1 m 3 m- 3 digester volume day-1. Such yields proved difficult to obtain in commercial livestock units. Monitored full-scale digesters have only achieved 60-65% of their design projections which could not be sustained over long periods largely because of the variations in feedstock quality (Larking et al., 1981; Hamworthy Engineering Ltd., 1984).
Because high gas yields have rarely been sustained there has been little in- centive to place emphasis on maximising biogas utilization. Matching the gas supply with demand so that it can be utilized 100% throughout the year and
Fig. 1. An 800 m s biogas digester on a large pig farm in southern England. The gas is utilised successfully to run a co-generation plant, the power from which is used to run the commercial- scale lucerne drying and processing plant shown in the left background. Surplus electricity is sold to the national grid.
92 R.E.H. SIMS AND K.M. RICHARDS
Fig. 2. An experimental compression ignition engine converted to run on scrubbed biogas. Loca- tion of gas tanks and supplying sufficient range are l imitations of the system.
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Fig. 3. Biogas powered Ford diesel engines converted to spark ignition and used to power electric generators. Corrosion from the raw biogas resulted in the heat exchangers requiring replacements as well as the engine small end bearings.
ANAEROBIC DIGESTION OF CROPS AND WASTES 93
within a few hours of production is difficult to achieve, as energy demands on farms vary both seasonally and daily. This inherent mismatch will make high gas-utilisation rates even more difficult to attain should higher levels of gas production become feasible.
Even at the lower gas yields typically obtained at present, full utilisation would be unlikely, unless a continuous high energy-consuming processing plant (such as a gas-scrubbing and vehicle-refuelling station, green feed-crop drying facility, or brick firing kiln) is associated with the farm and located nearby (Fig. 1. ). The common mismatching problem usually found with traditional livestock enterprises can have a significant negative implication on the antic- ipated national exploitation of the resource.
The use of the gas as a vehicle fuel (Fig. 2) demands prior scrubbing (Meynell and Coombs, 1985) which is also recommended for stationary engine applications (Fig. 3 ). Pressure water scrubbers to remove carbon dioxide (no energy value) and iron oxide beds to remove hydrogen sulphide (corrosive) were considered to be the most practical options.
Anaerobic digestion of farm waste offers considerable potential environ- mental benefits. Compared with spreading of untreated animal manures on to the land, both groundwater pollution and odours can be significantly reduced without lowering the nutrient contents of the effluent (Etheridge et al., 1983). This may be particularly important for livestock units situated near urban areas and/or where insufficient land area is associated with the enterprise. Current legal requirements and deterrents are too weak to entice most farmers to undertake methods of environmental control voluntarily, these being usu- ally more expensive than spreading manure directly on to the land.
It is thought unlikely that more rigid environmental control regulations for the disposal of animal wastes will be introduced into the U.K. in the near fu- ture. Partly for this reason the Ministry of Agriculture has recently wound down its own parallel studies of on-farm digestion systems.
Should stricter legislation eventually be applied, then anaerobic digestion will be one of several possible methods of controlling water pollution and odours. It would have the added advantage that the biogas produced could be seen as a useful by-product to offset the costs of treatment but may still be more ex- pensive than other options such as aerobic treatments.
Only a very few plants in Europe and the U.K. have achieved economic suc- cess and these have often depended on good design, a considerable do-it-your- self input, and an enthusiastic approach to produce the level of dedicated man- agement necessary to make the process work (Demuynck and Nyns, 1984).
94 R.E.H. SIMS AND K.M. RICHARDS
Few current farm waste-digester installations have been shown to be eco- nomic in the U.K. with little progress made in improving full-scale digestion efficiencies, in spite of considerable funds spent on research, there appears little likelihood of commercial farm digestion becoming more economically vi- able in the short term.
The capital investment costs of the plant need to be significantly reduced and a major improvement in the performance efficiency would be necessary to improve the cost effectiveness of a planned installation. New digester designs offer some hope but in several cases the added investment costs would not be offset by the extra gas produced.
The cost of producing and harvesting crops to be used as feedstock for diges- tion is likely to be uneconomic, even assuming optimistically high gas yields could be produced. Only in a rare situation, where the feedstock is of good consistent quality, a credit can be given for environmental benefits, and the gas supply can be fully and profitably utilised on the farm, will an acceptable rate of return be possible. Based on energy prices for 1 GJ of electricity 10.39, natural gas 3.24, fuel oil 3.39, LPG 4.80 and coal 2.35 and interest rates of 12% before tax.
Continued research into anaerobic digestion of ligno-cellulose could be of some value in improving gas yields from on-farm digestion. Currently, less than a third of the carbon present in the feedstock is converted to methane by digestion. Gas yields could be greatly increased if the ligno-cellulosic material could also be converted.
The main beneficiaries of anaerobic-digestion research have not been the agricultural sectors so much as the municipal sewage-treatment plants, indus- tries which need to treat their effluent (e.g. abattoirs, food and drink process- ing companies) and potentially, but to a lesser degree, the treatment of landfill leachate. Such applications have enabled digester manufacturers to exploit these markets. The anticipated agricultural demand for digestion systems has not materialised and interest in manufacturing on-farm plants has waned.
Although technically feasible, on-farm biogas production and utilisation will remain economically unattractive as a process for widespread use on British farms in the foreseeable future or until environmental considerations assume greater importance. The programme conducted by E.T.S.U. has therefore been curtailed and only a watching brief will be maintained to cover any develop- ments in digester design or environmental regulations which might occur. Other related programmes (on landfill gas and industrial applications of anaerobic
ANAEROBIC DIGESTION OF CROPS AND WASTES 95
digest ion) are cont inuing, and should any s igni f icant deve lopments arise which might create renewed interest in on - fa rm biogas, the programme will be in a pos i t ion to respond accordingly.
Demuynck, M. and Nyns, E.J., 1984. Compendium, biogas plants in Europe - A practical hand- book. Solar Energy Research and Development in the European community, publication EUR 9096.
Etheridge, S.P., Hughes, D.A. and Stafford, D.A., 1983. The use of anaerobic digester effluent as a fertiliser. Final report to E.T.S.U., unpublished.
Franco, C., 1981. Limits to process intensity in methane (biogas/generator) systems. Final report to E.T.S.U., unpublished.
Hamworthy Engineering Ltd., 1984. Research and Development in anaerobic digestion of organic wastes and plant matter. Final report to E.T.S.U., unpublished.
Hobson, P.N., Summers, R., Harries, C., Feilden, N.E.H., Thomson, J. and Auld, I., 1983. A large farm scale digester at Pittrichie Pig Unit, Aberdeenshire. Final Report to E.T.S.U., unpublished.
Larkin, S.B.C., Morris, R.M., Noble, D.M. and Radley, R.W., 1981. Resource mapping of agricul- tural wastes and residues E.T.S.U., report B1055.
Meynell, P.J. and Coombs, J., 1985. A survey of existing methods of biogas scrubbing and utilis- ation. BABA - trade association for British Biomass Industries. Final report to E.T.S.U., unpublished.
Stafford, D.A. and Etheridge, S.P., 1983. The anaerobic digestion of crop materials. Final report to E.T.S.U., unpublished.
Sims, R.E.H. and Richards, K.M., 1986. The potential for biogas on farms in the U. K. Energy Technology Support Unit, Harwell, U.K., Report R-41.50 pp.