The Technology and Economy of Farm-Scale, High-Solids Anaerobic Digestion of Plant Biomass
Abstract: Anaerobic digestion is a microbially mediated process occurring in nature in the absence of oxygen and other non-carbonaceous electron acceptors. The majority of the carbon of the organic matter degraded in the process is transformed into carbon dioxide and methane. Most of the energy potential of the degraded material is conserved in the methane, thus providing a renewable energy carrier, which can be converted into heat and/or electricity, or upgraded for use as a vehicle fuel. The remaining undigested, recalcitrant lignocellulosics and mineralised nutrients constitute an excellent biofertiliser. Compared to using undigested organic material as fertiliser, the nutrient content of the digestate is more readily taken up by the plants. In addition, the lower levels of easily degradable carbon sources and nitrogen in the soil lead to reduced emission of greenhouse gases and nutrients to the air and water. The lignocellulosics of the digestate maintain the humic content of the soil, which is crucial for the long-term productivity of the soil. Anaerobic digestion of the organic waste fractions and agricultural residuals is a sustainable way to control and direct all the flows of recycled nutrients in society without risking excessive losses. Anaerobic digestion would enable the energy potential of agricultural crop residues such as sugar beet tops and ley crops to be harnessed. Sweden is so sparsely populated that full utilisation of this potential (11 TWh/yr) by conventional centralised slurry-based technology is difficult. In addition, process disturbances in the form of crust formation make operation more costly and lower methane yields may result. It appears that simple but effective high-solids reactor systems have a better chance of being economically viable on farm-scale (50-500 kW). The first part of this study shows that using straw beds improves the process performance in high-solids, anaerobic, stratified bed digesters by shortening the start-up phase of sequential fed-batch operations, and by enhancing the rate and extent of anaerobic digestion. After a non-feeding stabilisation period of the straw bed, the priming straw bed functions both as a particle filter and a microbial carrier. Methane yields from the sugar beet tops fed to pilot- and laboratory-scale equipment ranged between 0.33 and 0.39 m3/kg volatile solids at average solids retention times of 11 to 39 days. In the second part of this study, calculations on the economy of farm-scale digestion of a mixture of wheat straw and sugar beet tops were performed, assuming that the biogas was converted into heat or combined heat and electrical power, or upgraded to vehicle fuel. Among the three different reactor designs tested, the stratified bed digester was found to be the most competitive. However, the scale and utilisation rate of the equipment were too low to achieve reference case unit costs that were comparable to those of commercially available energy carriers. By increasing the scale (from 51 kW to 67 and 201 kW), replacing wheat straw by ley crops, and increasing the degree of utilisation of the equipment, the reference case unit costs were lowered to such an extent that they were on a par with most of the commercially available energy carriers (5.3 ?ct/kWhheat, 8.1 ?ct/kWhvehicle fuel). Both studies showed the great importance of full utilisation of the energy carriers produced, which might prove difficult on the farm-scale. Vehicle fuel is probably the best alternative, but the distribution system of biogas is currently restricted to certain areas. The incentive for buying biogas vehicles is reduced by the high engine conversion cost. Expanding the market by implementing some kind of ambitious, long-term subsidy programme is necessary for the wider implementation of anaerobic digestion of plant biomass in Sweden. A prime mover could be the organic farmers with no livestock, who would be able to improve their nitrogen management by removing and anaerobically treating the presently mulched or ploughed-in green manure of ley crops and other crop residuals.
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