Use of Methanation for Optimization of a Hybrid Plant Combining Two-Stage Biomass Gasification, SOFCs and a Micro Gas Turbine

A hybrid plant producing combined heat and power (CHP) from biomass by use of the two-stage gasification concept, solid oxide fuel cells (SOFCs) and a micro gas turbine (MGT) was considered for optimization. The hybrid plant is a sustainable and efficient alternative to conventional decentralized CHP plants.

The demonstrated two-stage gasifier produces a clean product gas, thus ensuring the need for only simple gas conditioning prior to the SOFCs. Focus in this optimization study was on SOFC cooling and the investigation was conducted by system-level modelling combining zerodimensional component models in the simulation tool DNA.

By introducing an adiabatic methanation reactor prior to the SOFCs, the excess air flow for SOFC cooling could be reduced due to additional endothermic reforming reactions internally in the SOFCs, thus lowering the air compressor work. Installing an adiabatic methanator reduced the mass flow of cathode air by 27% and increased the turbine inlet temperature by 17% resulting in an electrical efficiency gain from 48.6 to 50.4% based on lower heating value (LHV).

Furthermore, the size of several components could be reduced due to the lower air flow. The study also showed that combining alternative product gas preheating and adiabatic methanation made the traditional anode in/out heat exchanger redundant and an electrical efficiency of 52.5% (LHV) was achieved.