Research

Novel hybrid electricity storage system producing synthetic natural gas by integrating biomass gasification with pressurized solid oxide cells

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Abstract

A future fossil-free energy system based on intermittent renewable energy sources will require efficient and flexible large-scale electricity storage systems, as well as alternative solutions for the heating and transport sector. Electricity storage systems using reversible solid oxide cells (SOCs) have shown to be promising solutions for large-scale electricity storage, in terms of cost and efficiency. The combination of pressurized SOCs and catalytic reactors is a particularly interesting option, enabling the production of synthetic natural gas (SNG) during electrolysis mode and allowing the use of the existing natural gas infrastructures for SNG storage. In fuel cell operating mode, the natural gas is reconverted to electricity and the produced CO2 is stored in an underground cavern for later use in electrolysis mode. Previous analysis indicate this solution can achieve roundtrip efficiencies of up to 80%. In this work, a hybrid electricity storage system using pressurized solid oxide cells and a biomass gasifier is analyzed by thermodynamic modelling. By using the syngas produced by a gasifier in both fuel cell mode and electrolysis mode, the system has the ability to produce extra SNG, which is not reused in fuel cell mode, and can be used instead for transportation or industrial processes. This also means that the operational time in electrolysis mode will be greater than the operational time in fuel cell mode, and by adjusting the size of the gasifier, the system can match the need of the electricity grid – e.g. 80% of the time in electrolysis mode and 20% of the time in fuel cell mode. Finally, it is expected that the system will operate non-stop, as it can choose to operate in part-load based on only converting the syngas to either SNG or power. Compared with the pure electricity storage system, the system design is simplified, as a pre-reformer is not needed in fuel cell mode as syngas is mixed with the SNG input. The SOC operating conditions are also improved. The results show that the overall syngas to SNG upgrading efficiency of the hybrid system is 87-88 % for different input syngas mass flows, which is higher than for syngas upgrading by hydrogen addition and methanation to SNG, which has a maximum upgrading efficiency of 78 %.