Off-design Analysis And Exergoenvironmental Assessment Of A Carbon Capture In Solar Assisted Coal-fired Power Plant

For coal-fired carbon capture systems,there is a large amount of waste heat,and the recovery of waste heat can improve energy utilization efficiency.While the introduction of clean energy into the system can also compensate for the thermal performance of the system.Therefore,a 660MW solar-coal-fired power generation system integrated ORC（Organic Rankine Cycle）and CCS（Carbon Capture and Storage）is constructed.The system recovers the waste heat of the CO₂ compression process and the reboiler condensate,and the solar collector field provides energy for the reboiler.Through the Ebsilon software,the thermal performance and exergy performance of the integrated system under off-design conditions are analyzed,and the changes of parameters in the system and the influence of solar operating characteristics on the thermal performance of the system are explored.The research shows that:under off-design conditions,with the increase ofλ（ratio of IP steam extraction to the steam flowrate entering a reboiler）,the thermal performance of the integrated system decreases,and the exergy efficiency of the steam turbine increases;as the load decreases,the exergy efficiency of the steam turbine increases,and the exergy efficiency of the boiler decreases.When the reboiler condensate is recovered by ORC and returns to the H9 low-pressure heater,the thermal efficiency of the ORC is higher,and the net power of the system is higher.At the same direct normal irradiance（DNI）when the load is lower,theλis lower,which greatly improves the thermal performance of the system;at the spring equinox,summer solstice and autumn equinox,stable DNI leads to the system operation at a lowerλ,and the thermal performance is improved greatly;at winter solstice,the apparent fluctuation of DNI causes the unit to operate at a highλ,and the improvement of thermal performance is relatively limited.When the energy consumption of the reboiler,and the carbon capture rate increase,the energy required by the reboiler will increase;when the carbon capture rate increases,the power consumption of the CO₂ compression process increases,and the energy recovered from the intercooler increases,which can slightly compensate the thermal performance of the unit.Based on Sima Pro software,Eco-indicator 99 method is used to conduct a complete life cycle assessment（LCA）of each input flows and components of the integrated system,and the exergoenvironmental assessment of system is carried out to explore the impact of each auxiliary system on the environment,and the environmental impact of the original secondary reheat unit and three scenarios of coupled carbon capture system is compared.The results show that the specific environmental impact of electricity of the 660MW double reheat coal-fired unit is 18.01m Pts/MJ;the introduction of solar energy could reduce the specific environmental impact of the electricity,and when the solar energy replaces the NO.1 high-pressure heater,the specific environmental impact of the electricity will be reduced most significantly,which is 8.8%.When the energy of reboiler is provided by solar heating condensate water,the specific environmental impact of electricity is reduced by 1.6%;when the energy is supplied by the IP steam extraction,the specific environmental impact of electricity increases by 2.23%;when the system has desuperheating and decompression devices,the impact will be transmitted to the environment in the form of exergy loss.The specific environmental impact of electricity of the scenario IV is 18.68m Pts/MJ,and the waste heat utilization system can reduce the environmental impact and directly discharge it to the environment.The specific environmental impact of electricity of the four scenarios is:scenario IV>scenario II>scenario I>scenario III.However,considering the power generation,the environmental impact of producing 1k Wh of electricity is scenario II>scenario IV>scenario III>scenario I.From the perspective of technical economy and environment,scenario IV is the most reasonable.According to the analysis of exergy parameters,only the boiler produces pollutants,and the coupling of the carbon capture system does not change the combustion of the boiler,the generation of pollutants plays a leading role in the environmental impact,and for other components,the exergy loss plays a leading role.It is pointed out that r in H6-H10 and condensate pump in scenario 4 are relatively large.Therefore,these components can be further optimized to improve the environmental performance of the system.