Exergy Analysis of Gas-Turbine Combined Cycle With CO2 Capture Using Pre-Combustion Decarbonization of Natural Gas

Abstract

A concept for natural-gas fired power plants with CO2 capture has been investigated using exergy analysis. The present approach involves decarbonization of the natural gas by authothermal reforming prior to combustion, producing a hydrogen-rich fuel. An important aspect of this type of process is the integration between the combined cycle and the reforming process. The net electric power production was 47.7% of the Lower Heating Value (LHV) or 45.8% of the chemical exergy of the supplied natural-gas. In addition, the chemical exergy of the captured CO2 and the compression of this CO2 to 80 bar represented 2.1% and 2.7%, respectively, of the natural-gas chemical exergy. For a corresponding conventional combined cycle without CO2 capture, the net electric power production was 58.4% of the LHV or 56.1% of the fuel chemical exergy. A detailed breakdown of irreversibility is presented. In the decarbonized natural-gas power plant, the effect of varying supplementary firing (SF) for reformer-feed preheating was investigated. This showed that SF increased the total irreversibility and decreased the net output of the plant. Next, the effects of increased gas-turbine inlet temperature and of gas-turbine pressure ratio were studied. For the conventional plant, higher pressure led to increased efficiency for some cases. In the decarbonized natural-gas process, however, higher pressure ratio led to higher irreversibility and reduced thermal-plant efficiency.