Operation Optimization Of CFB Unit Combustion And Pollutant Treatment System Based On Life Cycle Assessment

Coal-fired power plants are one of the major power suppliers in China.Combustion and pollutant treatment systems have become an important part of coal-fired power plants with the development of ultra-low emission transformation.However,in order to reduce greenhouse gas emissions and adapt to the large-scale grid connection of renewable energy,coal-fired power plants had to carry out deep peak-shaving operations.This kind of operation brought frequent changes in operating conditions,which triggered a series of interrelated problems,and made it difficult to evaluate their environmental and economic effects.Therefore,it is necessary to scientifically and comprehensively analyze the environmental and economic performance of each process unit of the combustion and pollutant treatment system under different loads.Based on this analysis,effective optimization strategies can be devised to enhance the eco-friendliness and cost-effectiveness of coal-fired power plants during operations with variable loads.This paper adopted the method of life cycle assessment（LCA）and process simulation to analyze the environmental and economic performance of combustion and pollutant treatment system of the circulating fluidized bed（CFB）unit at different loads.Firstly,through field investigation and data collection,process simulation was carried out for each unit process of the system,and digital software was developed to obtain its material consumption,energy consumption and pollutant emission data under different loads.Secondly,an LCA model was established using e Footprint system and global warming potential（GWP）,primary energy demand（PED）,acidification potential（AP）,abiotic depletion potential（ADP）,eutrophication potential（EP）,respiratory inorganics potential（RI）,ozone depletion potential（ODP）,photochemical ozone creation potential（POCP）and ecotoxicity potential（ET）were evaluated.The environmental impacts and economic costs of each process unit under different load conditions were quantified,and key items and significant environmental impact factors were identified.Finally,in order to improve the environmental and economic performance of the unit during load changes,an optimized scheme of furnace pressurization was proposed.This scheme involves shutting down the furnace and using waste heat to maintain a 5 MW load for 1 h when the load is below 50%,and then,the boiler is ignited and started until it is restored to 50%load and stable operation.The environmental impacts and economic costs were analyzed,and the results showed that the lower the load,the higher the coal consumption,environmental impact and operating cost per unit of electricity generation.When the load dropped from 90%to30%,the system’s comprehensive environmental impact increased by 40%,and the total cost increased by 308.52 yuan/MWh.In addition,the fuel combustion unit had the greatest impact on the environment and economy,followed by the in-furnace desulfurization unit and the limestone/gypsum wet flue gas desulfurization technology（WFGD）unit.Among them,the fuel combustion unit accounted for more than 50%of each environmental type’s impact,and its direct emission of CO₂from coal combustion accounted for 84.64%of the system’s total emissions;due to limestone powder and electricity consumption,in-furnace desulfurization unit and WFGD unit had some impact on AP,EP,ODP,POFP and RI,but none of them exceeded 25%.In contrast,the selective non-catalytic reduction（SNCR）unit,bag filter unit and wet electrostatic precipitator unit made smaller contributions to each environmental impact type.Sensitivity analysis showed that key items in system operation were raw coal,electric energy and CO₂of fuel combustion unit,limestone,electric energy and CO₂of in-furnace desulfurization unit,and electric energy of WFGD unit.The comparative analysis between the operation scenario of boiler banked fire and the 30%load scenario showed that all the listed results of the operation scenario of boiler banked fire were lower than those of the 30%load operation.The coal consumption,electricity consumption and CO₂generation of the whole system decreased by 19.61%,7.14%and 20.77%,respectively.In addition,the operation scenario of boiler banked fire also had lower environmental impact potential and economic cost.Compared with 30%load operation,the process boiler banked fire on GWP,PED,ADP,ODP and ET are obviously reduced by about 19%,while the remaining environmental impacts decreased by about 10%;the total economic cost decreased by 18.35%.Therefore,the operation of boiler banked fire is a feasible and effective way to optimize deep peak-shaving operation.