Increase of biogas production by incorporating residual streams from agriculture

Sammanfattning: To expand the biogas production in Region Västra Götaland (RVG), residual streams from the agricultural sector can function as a large-scale supplier of biomass. The production of biogas decreases the dependence of fossil fuels and the digestate that is formed can also be returned to the agricultural sector, creating a closed loop. The expansion was assessed in this thesis by the investigation of the biogas producers in RVG and the total potential of agricultural residues. An interview study was conducted for co-digestion and farm-based biogas plants, to investigate their capacity, interest and ability to introduce agricultural residues in their process. The utilisation of mechanical pretreatment was analysed to facilitate the implementation of fibrous residues. This study shows that the total theoretical potential of agricultural residual streams is equal to 1.31 TWh/year in RVG. The current biogas production in RVG is 300 GWh/year, where co-digestion plants account for 56 % and farm-based biogas plants for 5 %. The most used agricultural residue, with 91 % is slurry from cattle and pig. The interview study concludes that the biogas plants could increase the current capacity with 184 GWh/year by incorporating agricultural residues. Large co-digestion plants have the greatest opportunity of adapting its current facilities, but their main limitation is to receive a permit allowing them to expand the production and utilise other substrates. Out of all agricultural residues, manure is of highest interest for the biogas plants to include in its production, whereas fibrous residues are associated with challenges. The higher water content and receiving the manure-gas aid contributes to that manure is prioritised over other agricultural residues. The fibrous residues consisting of solid manure fractions, excess and discarded ley silage and straw account for around 68 % of the total theoretical potential and to include these substrates into the biogas production, pretreatment is needed. The present study shows mechanical pretreatment is currently the best choice of handling dry and fibrous substrates, with the hammer mill being the most suitable for large-scale operations and the mixer-wagon for small-scale operations. The energy spent on mechanical pretreatment is paid off if it increases the methane yield with 5 % and has an energy demand below 60 kWh/ton wet weight (ww) for manure, 150 kWh/ ton ww for ley silage, and 290 kWh/ton ww for straw. An economic evaluation concludes that an electricity price under 3 SEK/kWh with a biogas income of 0.8 SEK/kWh is needed for mechanical pretreatment to be profitable. The study concluded that further research and development is required to design a robust and reliable process to include straw and ley silage to a larger extent.