A techno-economic feasibility analysis of solutions to cover the thermal and electrical demands of anaerobic digesters

The use of biogas from anaerobic digestion (AD) has been switching in recent years from electricity to biomethane (BM) production. This choice leads to a higher energy efficiency compared to the use of biogas for combined heat and power (CHP) production, but results in a higher electricity demand for the upgrading and in the necessity of a heat supply for the digesters. This study analyses possible technical solutions, such as heat pumps or wood chip boilers to cover the thermal needs of the digester and photovoltaic panels (PV) to supply electricity. Three feedstocks were considered with high (organic waste), low (livestock manure), and intermediate (sewage sludge) biochemical methane potential (BMP), resulting in different electricity and heating needs. Results show that heat pumps (HP) are an economically viable solution for medium/low BMP feedstocks, for which relative payback times are in the range 1.5-5.4 years; in these cases, a reduction of 47-83 % of greenhouse gases (GHG) emissions and of 59-68 % of non-renewable primary energy is also achieved. For high BMP feedstocks, economic benefits are possible only with low electricity cost (below 130/MWh(el)), and a reduction of GHG emissions is possible only using low-carbon electrical energy (below 202 kgCO(2eq)/kWh).