Biogas from energy crops - Optimal pre-treatments and storage, co-digestion and energy balance in boreal conditions

The objective of this research was to evaluate the biogas production from crops in boreal conditions, focusing on the optimal pre-treatment and storage methods, co-digestion and energy balance of farm-scale crop based biogas plants. Alkaline treatments offered some potential for improving the methane yield from grass and sugar beet tops. The results show that the CH₄ yield of energy crops can be maintained by appropriate ensiling conditions for even after 11 months in ambient conditions. The CH₄ yield was best preserved with wet grass mixture without additives. Co-digestion of manure and crops was shown to be feasible with feedstock volatile solids (VS) containing up to 40% of crops. The highest specific methane yields of 268, 229 and 213ICH ₄kg^-1 VS_added in co-digestion of cow manure with grass, sugar beet tops and straw, respectively, were obtained during feeding with 30% of crop in the feedstock, corresponding to 85-105% of the total methane potential in the substrates as determined by batch assays. The energy output:input ratio of farm-scale grass silage based biogas plant varied significantly (3.5-8.2) with different assumptions and system boundaries being lowest when using only inorganic fertilizers and highest when half of the heat demand of the system could be covered by metabolic heat.