Novel Power Plant Cycles with CO2 Capture at Elevated Pressure
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This thesis provides a comprehensive overview of effective process modifications that can be made to the conventional natural gas-fired power cycle in order to reduce the high electrical efficiency penalty caused by CO2 capture based on chemical absorption. Four novel power cycles enabling increased CO2 concentration due to exhaust gas recirculation (EGR) have been simulated and evaluated mainly in terms of net plant efficiency and component sizing of separation equipment. Three of them have in combination with EGR operated with high-pressure CO2 absorption which has proved two major advantages; reduced heat requirement of the reboiler and reduced component sizing due to increased CO2 partial pressure and decreased volumetric flow rate. MEA and MDEA were used as solvents for the atmospheric capture and elevated capture processes, respectively. The first part of the thesis gives an introduction to the fundamentals of CO2 capture by absorption, power generation, and process integration and modification. Focus has been attached to the advantages and challenges that exhaust gas recirculation offer and the potential of high-pressure absorption. Based on the theory given in the first part, the last part of the thesis presents the process development, results and evaluation of the power cycles that were simulated in UniSim Design Suite/GT PRO. An inevitable consequence of EGR is the reduced power output of the gas turbine due to increased compressor inlet temperature, caused by mixing hot recirculated exhaust gas and ambient air, and increased specific heat capacity, due to increased CO2 concentration. At the same time the exhaust gas holds a higher temperature than for a conventional gas turbine due to changes in the gas properties. Since the energy in the exhaust gas is recovered in the steam cycle for a combined cycle power plant (CCPP), the exhaust gas temperature is not an important factor. The main concerns with EGR include, among other things, lower oxygen concentration in the combustion chamber. This may affect the combustion stability and completeness negatively. On the other hand, this may also have positive effects in terms of reduced NOx emission and amine degradation. Experimental results show that existing dry low NOx combustors can accept EGR ratios up to 35% without modifications. Through minor modifications it is predicted that EGR ratios beyond this can be achieved. The most promising power cycle concept in this report is the tail-end capture, where part of the exhaust gas coming from the HRSG is re-compressed before it enters the absorber. Further, the treated exhaust enters a recuperator before it is expanded in a low pressure turbine (LPT). Results show that the net plant efficiency with 90% CO2 capture can be increased from 50.7%LHV for a conventional natural gas-fired combined cycle with the state-of-the-art capture process, to 51.5%LHV by utilizing the tail-end capture power cycle configuration. This result is based on an EGR ratio of 50% and an absorber pressure of 2.2 bar, which gave a reboiler duty of 2.6 MJ/kg CO2. Including the CO2 capture to the process resulted in an efficiency penalty of 5.4 %-points. The required diameter of the absorber was found to be 8.0 meters, more than halved compared to base case (17.4 meters).Post-compression CO2 capture is another promising process similar to the tail-end capture. The exhaust gas enters the absorber at a much higher pressure, 17.2 bar, resulting in a reboiler duty of 1.5 MJ/kg CO2. The net plant efficiency with CO2 capture was 49.7%LHV. Due to the combination of high-pressure absorption and EGR the corresponding absorber diameter was only 2.8 meters. Post-expansion CO2 capture partially expands the exhaust gas before it enters the separation plant. It was the least promising power cycle as it did not fully take advantage of the two main effects of EGR; namely, increased CO2 concentration in the flue gas and reduced mass flow entering the absorber. The result was a net plant efficiency of 47.1%LHV. Nevertheless, the process showed the lowest CO2 emission; only 20.9g/kWhel compared to 40.7g/kWhel for base case.Also, a conventional NGCC with EGR was simulated in order to verify the two abovementioned advantages related to EGR. The result showed a net plant efficiency of 51.1%LHV and an absorber diameter of 12.0 meters. The corresponding reboiler duty was 3.7 MJ/kg CO2.Operating with EGR reduced the diameter of all processes by approximately 31%, except for the post-expansion process which had more or less constant absorber diameter. Through simulations it was found that the combination of EGR and high-pressure absorption is a potentially effective method to reduce the high capital and operating costs of today s state-of-the-art amine plant related to separation of highly diluted CO2 from large volumetric flow of exhaust gas.