Integration of anaerobic digestion with thermal gasification and pressurized solid oxide electrolysis cells for high efficiency bio-SNG production

The integration of anaerobic digestion and thermal gasification can ensure a high biomass utilization, as the unconverted biomass from digesters can be converted by thermal gasification. When integrating steam electrolysis or co-electrolysis, all the CO and CO₂ in the biogas and gasification gas can be upgraded to synthetic natural gas (SNG), achieving a very high bio-SNG yield per biomass input. In this paper, a highly integrated system combining anaerobic digestion, thermal gasification, and pressurized solid oxide cells for bio-SNG production from manure is presented and analyzed by thermodynamic modeling. The system is compared to a similar system without anaerobic digestion. The analysis finds that the energy yield of bio-SNG can reach 138% in relation to the manure input (LHV-dry), while the yield drops to 107% without anaerobic digestion. The total energy efficiency from manure and electricity to bio-SNG can reach 79% with anaerobic digestion and 64% without it. By combining thermal gasification and anaerobic digestion, it is thereby almost possible to reach the same efficiency as a thermal gasification system operating on wood (up to 84%).