Thermodynamic and related analysis of natural gas combined cycle power plants with and without carbon sequestration

Thermodynamic and related ‘exergoeconomic’ performance criteria have been used to evaluate natural gas combined cycle (NGCC) power generation systems, with and without carbon dioxide (CO2) removal technologies. These plants were previously studied by the US National Energy Technology Laboratory employing conventional energy and mass balance results, and have now been evaluated using detailed energy, exergy and exergoeconomic analyses. The plant consisted of a gas turbine together with a steam cycle having three pressure levels. Such NGCC plants show the least exergetic improvement potential amongst competing fossil fuel generators going forward, because they are already enhanced by use of a thermodynamic ‘topping’ cycle. Carbon capture was simulated on the basis Of CO2 recovery from the flue gas stream that leaves the heat recovery steam generator via a commercial amine process. Ninety per cent of the CO2 was captured in this way, and then compressed into a high-pressure liquid. This was achieved with significant power penalty (some 21 %) and increase in generating cost per MWh (44%). Combustion and heat transfer processes are the main sources of exergy degradation within power cycles. Fuel combustion accounted for some 32% of exergy destruction. Even with CO2 sequestration, the NGCC system is still a thermodynamically attractive option compared with modern fossil fuel alternatives. Overall, the exergoeconomic results indicate that significant improvements can be achieved by considering the power generation systems as a whole, rather than concentrating on the improvement in performance of individual components (which is a common practice in exergoeconomic optimization studies).