Assessing the cost variability of emissions abatement in small-scale on-farm anaerobic digestion

Agriculture is considered a hard to abate sector in which 2050 net-zero greenhouse gas (GHG) emissions targets will be challenging. Anaerobic digestion is a technology that can reduce agricultural emissions whilst producing renewable energy and a biofertiliser. In Ireland this technology was previously evaluated to be a high cost of abatement solution. However, it is not clear if the potential variations in anaerobic digestion systems were accounted for in these analyses; scale, plant design, technology, feedstock and biogas end-use differ between systems and can directly impact abatement costs. This study assesses different biogas end-use options (generation of heat, electricity and biomethane) and varying farm sizes for on-farm anaerobic digestion systems digesting grass silage and cattle slurry feedstocks. To evaluate and compare each biogas end-use for the varying farm sizes, the abatement cost and potential was obtained based on the net present value (NPV) and the total discounted GHG emissions for each system configuration. The abatement cost of the on-farm anaerobic digestion systems assessed varied between −7 €/tCO_2eq to 816 €/tCO_2eq. For a farm with 185 dairy cows, the integration of anaerobic digestion with the use of a boiler to produce heat sold through a district heating network was found to be a financially viable option. Biogas upgrading to produce biomethane was not financially viable due to the high operational and capital costs of small-scale upgrading systems. A key result of the analysis shows that if a single input variable is changed within the system boundary, the financial and environmental performance of a system can be significantly changed. For example, an increase in tariffs and biomethane sale prices can substantially improve the financial viability from +72 €/tCO_2eq to −227 €/tCO_2eq. Similarly, a higher cost for grass silage feedstock (€43 per tonne) will unfavourably impact the abatement cost; raising it from −7 €/tCO_2eq to 492 €/tCO_2eq.