Characterization Of PM_2.5 Emissions From Conventional Coal Fired Power Plants During Flue Gas Cleaning Processes

As one of the important emission sources of fine particulate matter(PM2.5),coalfired power plants(CFPPs)have been widely equipped with flue gas desulfurization(FGD)and denitrification installations to reduce the emissions of SO2 and NOX,respectively,and the existing electrostatic precipitators(ESPs)will be retrofitted to further reduce PM2.5 emissions.However,changes in the chemical composition and physical properties of PM2.5 have been observed during the processes of these emission control facilities.Therefore,investigations on the PM2.5 emission characteristics of conventional CFPPs as well as the impact mechanism by the emission control facilities are of great importance for emission control from CFPPs and ambient air quality improvement.This dissertation focuses on the characterization of PM2.5 emissions from conventional CFPPs during flue gas cleaning processes.The field campaign was carried out in several conventional CFPPs equipped with novel emission control facilities.The results showed that SCR reduced NOX and PM2.5 emission.If ammonia slip occurs,SCR will introduce significant ammonium by-product,thus increasing PM1 emissions.A higher collection efficiency of PM2.5 was achieved by reducing flue gas temperature in ESPs,while the content of water-soluble ions in PM2.5 was increased due to the gas-toparticle convention.In addition,limestone-based FGD reduced SO2 and PM2.5 emission.When the mist eliminator was not operating properly,it introduced additional watersoluble ions in primary PM2.5,which increase the PM2.5 emission.The mechanisms of PM2.5 chemical composition change and primary PM2.5 emission enhance were also investigated.It's suggested that,ammonia slip indicates increase of SO42-and NH4+ emission in primary PM2.5,due to catalytic enhancement of SO2 oxidation to SO3 and introducing of NH3 as reducing agent.The gas-to-particle conversion rate increases as flue gas temperature decreases due to the low-low temperature ESPs.SO3 condensation improve the collection of PM2.5 in electrostatic precipitators associated with the lower electrical resistivity.Simultaneously,the composition of ionic compounds in PM2.5 has been changed.In addition to the entrainment of desulfurization slurry droplets by gas flow,the evaporation of volatile components in the desulfurization slurry under high flue gas temperature,and then condensational nucleation occur while the temperature decreases,might contribute to the additional primary PM2.5 emissions.Therefore,the unreacted ammonia from SCR was largely converted to NH4+ during FGD,with even higher contribution to primary PM2.5 emissions than SO42-.The enhanced NH4+ emission from CFPPs by ammonia slip may decrease the benefit of NOX control on ambient secondary PM2.5 concentration reduction,which should be considered in future air pollution control from CFPPs.