Study On CO₂ Capture Of Nature Gas Combined Cycle Power Plant And System Integration

The climate change caused by the excessive emission of CO2 and other greenhouse gases has become a major problem in the development of the human society.Fossil power plants are CO2 point emission sources with the largest intensity and they contribute more than 40%to the greenhouse effect.The CO2 emission of the natural gas power plant is about 60%of the coal-fired power plant.From a long-term point of view,the capture of CO2 in the natural gas power plant will become an important technical reserve to cope with the climate change.Post-combustion CO2 capture technology based on the chemical absorption method has higher maturity and better coupling with power plants.It has certain research,development and application prospects in the future CO2 emission reduction of the power industry.In this paper,the optimalintegration of large-scale post-combustion CO2 capture system and 450MW Nature Gas Combined Cycle(NGCC)plants is the research object.Based on the study of energy consumption parameters of the CO2 capture demonstration project in a certain NGCC power plant,a joint simulation platform combining NGCC and the CO2 capture system is established by the secondary development of the computation model with the chemical simulation software Aspen Plus and optimization with the power generation simulation software GT Pro.On the basis of system integration interface analysis and combined simulation operation,aNGCC-CO2 thermal analysis model is established.The thermal distribution characteristics and the heat transfer path of the carbon capture units are revealed by using the exergy analysis method of stream and energy flow in the model.Then the inside and outside optimizing integration paths of the NGCC-CO2 system are further studied:heat recovery steam generator(HRSG)supplementary combustion reduces energy consumption of reboiler in CO2 capture system;absorption heat pump reclaims flue gas waste heat and improves unit energy level.Besides,the economic feasibilities of different carbon capture scenarios are compared and analyzed.The main results are as follows:(1)Based on CO2 capture pilot project test of 450MW NGCC,the optimal operation conditions of typical MEA process are as follows:the best flue gas temperature is 38??40 ?,the absorber circulation flow is 20?22m3/tCO2,regeneration temperature is 112?,energy consumption is 4.5 GJ/tCO2,and the power consumption is less than 200 kWh/tCO2.(2)The influence of the large-scale CO2 capture system on the energy efficiency of NGCC power plants is forecast by joint simulating NGCC and the CO2 capture system.After integrating the CO2 capture system,the overall power generation efficiency of 450MW NGCC decreases by 8.63 percentage points from 56.88%to 48.25%under standard operating conditions(International Organization for Standardization,ISO).The efficiency loss of the summer condensing mode is 9.2%.while the winter condensing mode is 8.32%.The efficiency loss caused by CO2 capture integration can not be fundamentally solved by the regulation of the operation mode of the unit itself.(3)The thermal analysis model ofthe HRSG-steam turbine-CO2 capture system is established.Andall the streams and devices in the model are thermodynamically analyzed using the exergy analysis method.For 450MW NGCC,with the integration of the CO2 capture system,the exergy efficiency of the thermodynamic system decreases from 67.9%to 39.5%and the exergy loss increases from 69.10MW to 130.22MW.The exergy efficiency of the CO2 capture system is only 18.7%,which decreases the energy consumption of the entire NGCC-CO2 system.The exergy loss of the heat exchange process,such as the decrease of flue-gas temperature,low-pressure steam and the rich fluid heat transfer,is the main reason of the exergy efficiency decreasing in the CO2 capture system.(4)The compatibility of the stream in the heat exchanger networks of the CO2 capture system is analyzed using the pinch technology and the potential of the thermodynamic optimization in the NGCC-CO2system is excavated.The exergy efficiency of the system increases by 1.4 percentage points by recycling condensed water of the reboiler and replacing the thermal load of the reboiler by interstage CO2waste heat.The energy utilization level of the NGCC-CO2 system can not be effectively improved,only through the optimization of the internal heat exchanger network.(5)HRSG supplementary combustion can improve the energy level of NGCC-CO2 system.When the supplementary combustion temperature is 850?,the main steam pressure is controlled between 170bar and 180bar,the highest exergy efficiency is 42.79%,which increases by 3.16 percentage points before optimizing.For cogeneration units,the supplementary-fired HRSG can improve steam turbine capacity from 75.76 MWh to 135.84MWh,and the equivalent efficiency of cogeneration reaches to 50.2%,which increases by 1.95 percentage points before optimizing.The heat recovery steam generator and steam turbine need a certain transformation in the active NGCC unit,so it is more suitable in the design stage.(6)The energy consumption of the NGCC-CO2 system is increased by recycling the fuel gas afterheat using the absorption heat pump.The first type of the absorption heat pump can recycle 47.2MJ of the fuel gas afterheat and the exergy efficiency loss of the system decreases by 2.60 percentage points.The retrofit of the absorption heat pump to recover the waste heat of the flue gas is small and does not affect the layout of the thermal system of the original unit,so it is more suitable for the active NGCC power plant.(7)The economic performance of the 450MW NGCC-CO2 system is analyzed using the cash flow technology and Monte-Carlo Simulation.When adopting the first type of heat pumps to optimizing and integrating,the heating load of the units increases by 56.3%compared to that before optimization.The chance is 80%for FNPV to be within 41 million CNY to 1298 million CNY and the probability of profitability in the full lifecycle is 91%.