Model of an Oxygen Transport Membrane for Coal Fired Power Cycles With CO2 Capture

Abstract

Greenhouse gas emissions from power generation will increase in future if the demand for electrical energy does not subside. Therefore capture and storage of carbon dioxide (CO2) will become important technologies for lowering the rate of increase of global CO2 emissions, or even reducing them. A promising technology for coal fired power cycles is the integrated gasification combined cycle (IGCC), where CO2 is separated from the syngas coming from the gasifier before the syngas is combusted in a more or less conventional gas turbine. But oxygen is required for the gasification process to achieve a high carbon conversion rate. The energy demand for the cryogenic air separation unit (ASU) lowers the net power output of the IGCC cycle. An alternative way of producing the oxygen could eliminate this disadvantage of the IGCC cycle. Oxygen transport membranes (also known as mixed conducting membranes – MCM) show a high potential for such applications in power cycles. In this paper results of an investigation on an IGCC cycle with CO2 capture and an integrated oxygen transport membrane (OTM) reactor are reported. The operating conditions of the membrane reactor have been analyzed; the feed inlet temperature and the pressure differences between permeate and retentate sides of the membrane reactor have been varied. The impact on the overall IGCC cycle has been discussed. The most optimistic assumptions give an overall net efficiency close to the case without CO2 capture. In this case the net efficiency is reduced by only 3 percentage points compared to an IGCC process without CO2 capture. But these assumptions lead to very challenging conditions for the membrane reactor. A pressure difference of 14.5 bar is assumed. Less severe operating conditions for the OTM reactor, which seem closer to realization, show less promising results. For sweep stream pressures of 10 and 15 bar the net efficiency ranges from 36% to 39%. This is in the range of an IGCC process with cryogenic ASU which achieves a net efficiency of 37% to 38%. It can be concluded that the integration of an OTM reactor into the IGCC cycle is an option with good prospects if the membrane is capable of bearing the challenging operating conditions. Calculations of investment costs have not been investigated in the frame of this work. Both the total capital costs and the durability are very important aspects for the membrane technology to be realized in power cycles such as IGCC.