LCA of anaerobic digestion: Emission allocation for energy and digestate

Abstract Anaerobic digestion produces renewable energy and recycled fertilisers, and thereby may improve energy and nutrient self-sufficiency and decrease greenhouse gas (GHG) emissions from energy production, agriculture and waste management. Finland aims to increase bioenergy production and nutrient recycling from agricultural biomass, via digestion of non-food crops and side streams from food production. According to the national climate and energy strategy, the life cycle carbon balance of bioenergy must be accounted for to ensure its sustainability. However, all emissions and abatement potential are often allocated only for energy and not for digestate production and use. This emphasises climate impacts for energy, and fails to consider digestate-related emissions and credits. This study used different allocation methods to assess the climate emissions of the entire anaerobic digestion chain from feedstock procurement to both energy and digestate use in comparison to fossil energy and mineral fertiliser. Three feedstock scenarios with pig slurry and different co-feedstocks (separated solid fraction of slurry or grass from uncultivated fields or food industry side streams) were assessed. The climate emissions of energy were decreasing and the climate emissions of digestate were increasing when allocating all emissions from anaerobic digestion to energy and moving towards using economic allocation method and finally mass allocation method. With all allocation methods the three scenarios with different co-feedstocks led to lower climate emissions compared to the production of fossil fuels and mineral fertilisers. The climate emissions of anaerobic digestion were highest for the scenario that made use of grass as co-feedstock and lowest with food industry side streams as co-feedstock. The largest climate emissions of anaerobic digestion were from combined heat and power plant, and manure storage. When looking further the digestate storage, transportation and utilization as fertiliser on field the emissions were higher compared to mineral fertiliser utilization on field. However, when combining emissions of anaerobic digestion allocated for digestate and emissions of the digestate utilization on field, they were lower compared to mineral fertiliser production and use on field. The largest climate emissions of digestate were from digestate field use.