Exergy analysis of a gas-turbine combined-cycle power plant with precombustion CO2 capture

A concept for natural-gas (NG) fired power plants with CO2 capture was investigated using exergy analysis. NG was reformed in an auto-thermal reformer (ATR), and the CO2 was separated before the hydrogen-rich fuel was used in a conventional combined-cycle (CC) process. The main purpose of the study was to investigate the integration of the reforming process and the combined cycle. A corresponding conventional CC power plant with no CO2 capture was simulated for comparison. A base case with CO2 capture was specified with turbine-inlet temperature (TIT) of 1250 °C and an air-compressor outlet pressure of 15.6 bar. In this case, the net electric-power production was 48.9% of the lower heating value (LHV) of the NG or 46.9% of its chemical exergy. The captured and compressed CO2 (200 bar) represented 3.1% of the NG chemical exergy, while the NG, due to its pressure (50 bar), had a physical exergy equal to 1.0% of its chemical exergy. The effects of the changed NG composition and environmental temperature were investigated. Higher pressure in the gas turbine and reformer increased the combustion in the ATR and reduced the overall efficiency. Supplementary firing (SF) was investigated as an alternative means of heating the ATR. This also reduced the efficiency. Heating the feeds of the ATR with its product stream was shown to reduce the irreversibility and improve the efficiency of the plant. Both this, and the effect of increased TIT to 1450 °C were investigated. Combining both measures, the net electric-power production was increased to 53.3% of the NG LHV or 51.1% of the NG chemical exergy. On the other hand, both increased TIT and the ATR product-feed heat exchange reduced the conversion of hydrocarbons to CO2.

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