Clean combined-cycle SOFC power plant — cell modelling and process analysis

The design principle of a specially adapted solid-oxide fuel cell power plant for the production of electricity from hydrocarbons without the emission of greenhouse gases is described. To achieve CO2 separation in the exhaust stream, it is necessary to burn the unused fuel without directly mixing it with air, which would introduce nitrogen. Therefore, the spent fuel is passed over a bank of oxygen ion conducting tubes very similar in configuration to the electrochemical tubes in the main stack of the fuel cell. In such an SOFC system, pure CO2 is produced without the need for a special CO2 separation process. After liquefaction, CO2 can be re-injected into an underground reservoir. A plant simulation model consists of four main parts, that is, turbo-expansion of natural gas, fuel cell stack, periphery of the stack, and CO2 recompression. A tubular SOFC concept is preferred. The spent fuel leaving the cell tube bundle is burned with pure oxygen instead of air. The oxygen is separated from the air in an additional small tube bundle of oxygen separation tubes. In this process, mixing of CO2 and N2 is avoided, so that liquefaction of CO2 becomes feasible. As a design tool, a computer model for tubular cells with an air feed tube has been developed based on an existing planar model. Plant simulation indicates the main contributors to power production (tubular SOFC, exhaust air expander) and power consumption (air compressor, oxygen separation).