Assessment of carbon capture thermodynamic limitation on coal-fired power plant efficiency

Abstract

The most mature CO2 carbon capture process comes with a 12–10% pts efficiency loss when coupled with a coal-fired power plant and for both post and oxy-combustion. Pre-combustion induces less efficiency loss but at the cost of a more complicated overall process. Since the last decade numerous improvements have been made and this work tries to evaluate the thermodynamic minimum impact of CO2 capture processes on such coal power plants. After detailing the calculation hypothesis, the purely thermodynamic impact has been assessed: they are 3.2% pt for post-combustion divided into 40% for separation and 60% for compression, 4.2% for pre-combustion especially due to CO-shift (60%), the rest evenly divided between separation and compression and 2.9% for oxy-combustion divided into one third for O2 production and two thirds for compression and with a small efficiency gain compared to aero-combustion due to the reduction in flue gas volume. In the second part of this work, the realistic minimum energy consumption is assessed with some assumptions about compressor and pump efficiency, temperature pinch and overall process conditions. This study shows a 6.8% pt loss of efficiency for MEA absorption post-combustion process combined with classical compression train, 5.8% pt for a cryogenic post-combustion process, 6.6% pt for a MDEA absorption pre-combustion process with classical compression train, 5.4% pt for the cryogenic ASU oxy-combustion with standard cryogenic CPU.These realistic minimal impacts of CO2 capture on supercritical coal power plants highlight the difficulty in designing a process with less than 5% pt of efficiency loss which is the main technological challenge of the CO2 capture field at the moment. The better performance achievable seems around 6.0% pt loss of efficiency with good integrated low regeneration duty solvent for post-combustion or highly efficient ASU and CPU for oxy-combustion. However, a power plant practical efficiency of more than 40% is readily achievable with a well designed IGCC plant and, in near future, with USCPC power plant.