Polygeneration of SNG, heat and power based on biomass gasification and water electrolysis—concepts and their assessment

The gasification of biomass and the subsequent conversion to electric power, heat, or synthetic natural gas (SNG) offers the possibility to produce different products in one process under minimizing potential losses. In a so called polygeneration approach, the operation of such a process may be linked to changing energy market situations to improve the economic viability. The integration of water electrolysis for hydrogen production enables the process to provide positive as well as negative controlling power; i.e. it can provide system service and contribute to the stability to the electricity grid. Against this background, the aim of this paper is the technical, economic and environmental analysis of a polygeneration approach based on biomass gasification processes for the provision of SNG, heat and power taking the current electricity spot market into consideration. Therefore, several processes schemes are developed and analysed in terms of process efficiency (exergetic efficiency), economic viability (SNG production costs) as well as environmental performance (specific greenhouse gas emissions). For comparison, a biogas-based polygeneration approach for the production of biomethane, heat and power is analysed as well. In case of an operating mode optimised for SNG production, the exergetic efficiency results between 65.4 and 67.2 % for the gasification-based processes and 55.6 to 63.3 % for the biogas-based processes. If primarily electric power is provided in case of high spot market prices, the operation mode is optimised for power production; this results in significantly lower exergetic efficiencies of 48.0 to 44.9 % for the gasification-based processes and 38.0 to 41.2 % for the biogas-based processes. The production costs are calculated depending on the full load operating hours for maximizing SNG production and maximised power production, respectively. The results show a clear dependency of SNG production costs from the operating hours for all processes. The GHG emissions for maximum SNG production range between 43.4 and 95.1 gCO₂-eq/kWh_Methane. If more power is produced, the emissions rise to values between 55.4 and 127.6 gCO₂-eq/kWh_Methane. Lower emissions can be achieved by SNG production via gasification than via biogas.