Design of solid oxide fuel cell–molten carbonate fuel cell combined system for power generation and carbon dioxide emission reduction

Abstract

This research concentrates on the design and performance analysis of a solid oxide fuel cell (SOFC) and a molten carbonate fuel cell (MCFC) integrated system with using methane as fuel. Because SOFCs cannot completely use their fuel, there is remaining fuel leaving the system. In addition, the exhaust gas from an SOFC can be directly fed into an MCFC. The integrated fuel cell system shows an electrical efficiency of 55.22%, which is higher than a single fuel cell system. Fuel utilization of both fuel cells, SOFC temperature dramatically affect the performance of the integrated system. Four configurations were next proposed to be investigated and compared the performance in terms of power generation, CO2 utilization, heat duty and NiO formation to determine the suitable design of the integrated fuel cell system. The results showed that system (B) is suitable for power generation improvement consideration with no NiO formation possibility found.

However, the control strategy of such a system needs to be considered for the efficient operation. A control structure design is performed based on economic optimization to select manipulated variables, controlled variables and control loop configurations. The objective (cost) function includes a carbon tax to get an optimal trade-off between power generation and carbon dioxide emission, and constraints include safe operation. The relative gain array (RGA) is applied to select input-output pairings. PID controllers are implemented to control the integrated system.

As electricity demand can vary considerably and unpredictably, it is necessary to integrate energy storage with power generation systems. The gas turbine (GT) and advanced adiabatic compressed air energy storage (AA-CAES) system are implemented into the integrated fuel cell system to enhance the system flexibility. The results showed that the implementation of the GT and AA-CAES into the integrated fuel cell system allows the system to cope with the variations in power demand.