Life cycle assessment of power-to-methane and renewable methane production technologies

The role of renewable energy, such as biogas or renewable methane is important for decarbonisation of the energy sector. This study analyses environmental impacts of renewable methane production from seven different technologies including wind- and solar-powered electrolysis and methanation with direct air-captured CO2, anaerobic digestion of sewage sludge, pig and dairy manure, and food waste, as well as landfill gas upgrade. Life cycle impact assessment using ReCiPe 2016 method found that sewage sludge system exhibited the highest global warming impact due to electricity consumption during digestate dewatering by centrifuge, whereas anaerobic digestion of pig manure in a covered lagoon with digestate dewatering in Geotube® demonstrated the lowest impact. When centrifuge and Geotube® were compared for the same production system, Geotube® had 78 % lower global warming impact than dewatering with centrifuge. Overall, less energy and resource requirements, and environmental credits assigned to avoided products, such as mineral fertiliser and air-captured CO2 led to lower impacts, where seven out of 11 production systems presented lower global warming impacts than natural gas, indicating potential reduction of emissions by substituting natural gas with renewable methane. Increasing efficiency of wind and solar electricity appeared to be more sensitive than increasing efficiency of electrolyser, while transport distance of the landfill gas had significant effect on the global warming impacts. Production of renewable methane from waste can be promoted by various supports, such as carbon credits, amendment of national gas standard, and waste management policies for energy recovery.