Performance Evaluation of an Organic Rankine Cycle Fed by Waste Heat Recovered from CO2 Capture Section

Natural gas-fueled combined cycle (NGCC) allows to reach the best performance among power plants fed by fossil fuels, but causes considerable CO2 emissions. With the aim of reducing greenhouse gases impact, NGCC could be integrated with post-combustion CO2 removal systems, typically based on chemical solvents like amines, that cause very large net efficiency penalties (about 9-12 percentage points at 90% overall CO2 capture). To reduce these high capture penalties, exhaust gas recirculation (EGR) has been studied. To further enhance the overall plant efficiency, the recovery of available low temperature heat from the solvent-based CO2 removal systems could be also performed. Low temperature heat is available in flue gas coolers (80-100°C), in the amine reboiler water cooling (130-140°C) and in the splitter condenser (100-130 °C). This waste thermal energy could be recovered by means of an Organic Rankine Cycle (ORC) that is able to convert heat into electricity efficiently even at comparably low temperatures. N-Butane was found to be as the most promising organic working fluid for the cycle operating temperatures and pressures. ORC produces additional electrical power improving the global performance of the power plant, for example, up to 1-1.5 percentage points in efficiency.