Electrochemical Carbon Separation in a SOFC-MCFC Poly-Generation Plant With Near-Zero Emissions

High temperature fuel cells have been studied as a suitable solution for Carbon Capture and Storage (CCS) purposes at a large scale (>100 MW). However, their modularity and high efficiency at small-scale make them an interesting solution for Carbon Capture and Utilisation at the distributed generation scale when coupled to appropriate use of CO₂ (i.e., for industrial uses, local production of chemicals etc.). These systems could be used within low carbon micro-grids to power small communities in which multiple power generating units of diverse nature supply multiple products such as electricity, cooling, heating and chemicals (i.e., hydrogen and CO₂).

The present work explores fully electrochemical power systems capable of producing a highly pure CO₂ stream and hydrogen. In particular, the proposed system is based upon integrating a Solid Oxide Fuel Cell (SOFC) with a Molten Carbonate Fuel Cell (MCFC). The use of these high temperature fuel cells has already been separately applied in the past for CCS applications. However, their combined use is yet unexplored. Moreover, both industry and US national laboratories have expressed their interest in this solution.

The reference configuration proposed envisions the direct supply of the SOFC anode outlet to a burner which, using the cathode depleted air outlet, completes the oxidation of the unconverted species. The outlet of the burner is then fed to the MCFC cathode inlet which separates the CO₂ from the stream. Both the SOFC and MCFC anode inlets are supplied with pre-reformed and desulfurized natural gas. The MCFC anode outlet, which is characterised by a high concentration of CO₂, is fed to a CO₂ separation line in which a two-stage Water Gas Shift (WGS) reactor and a PSA/membrane system respectively convert the remaining CO into H₂ and remove the H₂ from the exhaust stream. This has the significant advantage of achieving the required CO₂ purity for liquefaction and long-range transportation without requiring the need of cryogenic or distillation plants. Moreover, the highly pure H₂ stream can either be sold as transportation fuel or a valuable chemical.

Furthermore, different configurations are considered with the final aim of increasing the Carbon Capture Ratio (CCR) and maximising the electrical efficiency. Moreover, the optimal power ratio between SOFC and MCFC stacks is also explored.

Complete simulation results are presented, discussing the proposed plant mass and energy balances and showing the most attractive configurations from the point of view of total efficiency and CCR.