| Coal-fired power plants are one of China’s important sources of air pollution,which is the largest coal consumption industry in China.According to the National Bureau of Statistics,coal consumed by coal-fired power plants accounted for 50%of China’s total coal consumption in 2016.With the gradual tightening of air pollutant emission requirements,coal-fired power plants have implemented ultra-low emissions policies through large-scale installations and upgrades of air pollution control devices（APCDs）,which have reduced the emission of air pollutants and have had a greater impact on the the physical and chemical emissions of particulate matter from coal-fired power plants.The high-resolution emission inventory and source profile are the basis for analyzing the source of atmospheric particulate matter using the atmospheric diffusion model and receptor model,and their accuracy and precision affect the accuracy and reliability of the simulation results.At present,the local emission inventory and source profile of coal-fired power plants are lacking and with highly uncertain,which has affected the estimation of the level of emissions from ultra-low emission power plants.Therefore,understanding the emission characteristics of atmospheric pollutants and their transformation under the influence of ultra-low retrofit and then building a high-resolution emission inventory in coal-fired power plants are of great significance,which can provide a scientific basis for the next improvement of air quality.In this study,a continuous emission monitoring system（CEMS）was carried out for online monitoring in 33 units of 13 coal-fired power plants in a large city in eastern China.Meanwhile,field measurements were carried out on several ultra-low emission power plants equipped with typical APCDs,namely selective catalytic reduction（SCR）for NOx reduction,low-low temperature electrostatic precipitator（LLT-ESP）for PM reduction and wet desulfurization device（WFGD）for SO2 reduction.The emission factors and high time-resolution emissions inventory of sulfur dioxide（SO2）,nitrogen oxides（NOx）,and particulate matter（PM）for coal-fired power plants were established,which based on CEMS data for the year of 2017and combined with the high-resolution activity level data of the power plant.To understand the physical and chemical characteristics of particulate matter emitted from ultra-low emission power plants,the field measurement was carried out through the dilution channel sampling system and particle size-fractionated sampling device.Besides,the migration and transformation of particulate components（elements,ions,carbon）in different purification processes were discussed.Based on the source emission process,the emission inventory for PM2.5 components from ultra-low emission power plants was constructed.The main conclusions of this study are:（1）Emission factors of PM,SO2,and NOx for coal-fired power plants have been established based on the CEMS data and detailed survey data.The average concentrations of PM,SO2,and NOx after ultra-low emission retrofit have been reduced to 1.7 mg/m3（0.5～3.9 mg/m3）,10.5 mg/m3（0.5～17.8 mg/m3）and 24.0 mg/m3（12.1～42.6 mg/m3）,respectively,which decreased by 72%,34%and 52%compared with those before the ultra-low emission retrofit,respectively.The emission factor calculated based on the CEMS measurement method is significantly lower than the calculation results of the material balance algorithm and the recommended parameters,which better reflects the current emissions of coal-fired power plants.Compared with those before the ultra-low emission retrofit,PM,SO2 and NOx emission factors of the coal-fired power plants after the ultra-low-emission retrofit were reduced from 0.06 g/kg to 0.01 g/kg,0.15 g/kg to 0.08g/kg,0.50 g/kg to 0.17 g/kg,respectively,which decreased by 83%,47%and 66%,respectively.The reduction ratio of PM emission factor is the largest,indicating that the installation of a low-temperature economizer before traditional electrostatic precipitator has a better removal effect on PM.（2）Field test of several ultra-low emission power plants equipped with typical APCDs show that the average PM2.5 concentration of the power plant after the ultra-low emission retrofit is less than 1 mg/m3,and the emission factor of PM2.5 is 1 to 2 orders of magnitude lower than the value before the ultra-low emission retrofit.Carbon components（OC and EC）are the most abundant components in PM2.5,and the total proportion of carbon components in PM2.5 reaches 17.3%～61.0%;the proportion of water-soluble ions in PM2.5reaches 2.8%～38.0%;element proportion is relatively low（0.9%～11.4%）.Compared with the emission characteristics of coal-fired power plants before ultra-low emissions retrofit,the proportion of OC,EC,and NH4+in PM2.5 of ultra-low emissions power plants is relatively high,the proportion of geological elements is relatively low,and the OC/EC ratio range approaches 5.（3）Before the APCDs in coal-fired power plants,the particle size of the particulate matter showed a bimodal distribution.The main components in PM2.5were crustal elements,and the ions were mainly SO42-and Cl-.In the SCR outlet,PM mass concentration presents a trimodal distribution,and the fine modal peak near 0.1μm,the coarse modal peak near 1.8μm,and the intermediate modal peak appears at 0.56μm.The element concentration and proportion of PM2.5 decrease in the SCR outlet.LLT-ESP had a good removal effect on particulate matter,and the removal efficiency of coarse modal particle size particles is higher than that of fine modal particles.（4）After the ultra-low emissions retrofit,the improvement of coal combustion quality and the APCDs modification will affect the PM2.5emission characteristics from power plants.The proportion of NH4+,OC,EC in PM2.5 increased,and the proportion of geological element such as Al decreased.The ash content and sulfur content in coal combustion were significantly reduced,from 28.5%～33.5%,1.71%～1.72%to 11.6%～11.8%,0.37～0.49%,respectively.The lower ash content and sulfur content helped reduce the amount of geological elements and SO2 generated during combustion and in turn reduced the mass concentration of SO42-in PM2.5.The excessively injected NH3 in SCR process may enter the WFGD process,causing the increasing proportion of NH4+and SO42-in PM2.5.LLT-ESP reduces the temperature at the entrance of traditional ESP,which reduces the viscosity,resistivity and flow rate of flue gas,promotes the condensation of volatile elements from the gas phase into particles,and has better removal efficiency of geological elements.However,the efficiency of capturing carbon particles in ESP is relatively low,resulting in a relatively increased proportion of OC and EC.And compared with dry ESP,the smoke in the WFGD outlet presents high humidity and higher chemical reaction intensity,which greatly increases the concentration of secondary organic carbon and makes the OC/EC ratio increase.（5）APCDs in coal-fired power plant can remove about 15%mass concentration of VOCs in flue gas;the VOCs concentrations at the SCR and WFGD outlet reduced by 2%and 83%,respectively;the VOCs concentrations at the ESP outlet increased by 423%.There are mainly three different mechanisms for controlling VOCs in the APCDs of ultra-low emission power plants,including the oxidation of VOCs by Ti O2 and V2O5catalysts in SCR systems,the deposition and condensation of VOCs after the smoke temperature is reduced,and the scour effect of water mist in WFGD.（6）According to the CEMS,the SO2,NOx and PM emissions of typical large-scale coal-fired power plants were 2895.0 tons,5348.3 tons,and 503.8tons in 2017,and the annual SO2,NOx,and PM emissions of a single unit were 0.9 to 348.3 tons,6.1 to 438.6 tons,1.2 to 78.4 tons.Compared with the emissions in 2005,the SO2,NOx and PM emissions in 2017 decreased by 1 to 2 orders of magnitude.Besides,the emission factors and emission inventory of particulate matter components for coal-fired power plants in typical megacity in 2017 were constructed.Compared with the results of the2010 study,the emissions amounts of heavy metals reduced by 1-2 orders of magnitude,and the carbon components reduced by 50%～70%.The emission inventory and emission factors of ultra-low emission power plants updated in this study can better reflect the time-varying characteristics of air pollutant emissions from ultra-low emission power plants,and reduce the uncertainty of emission inventories caused by overestimation of emission factors after technological improvements. |
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