| The impact of the emitted particulate matter during coal combustion,as one of the pillar energy supplements in our country,cannot be ignored.With the development of ultra-low emission technologies,particulate matter in the flue gas from coal-fired power plants has been significantly reduced.The particles produced by coal combustion are called total particulate matter(TPM),which include filterable particulate matter(FPM)and condensable particulate matter(CPM).Most of the attention is attracted by the emission of FPM.With the application of ultra-low emission technology,the concentration of condensable particles emitted by coal-fired power plants could be even higher than that of filterable particles.The study showed that both filterable particulate matter and condensable particulate matter contain mercury.In this study,the test methods of filterable particulate matter and condensable particulate matter were first evaluated.Secondly,the mercury contained in different sizes particles in flue gas from the power plant and entrained flow reactor was measured.Then,chemical forms of mercury were studied and verified through thermodynamic equilibrium calculations.This study solved two technical problems:(1)the influence of relative humidity on the accuracy of particle size distribution during sampling,and(2)analysis of the chemical forms of mercury contained in different sizes of particles.At the same time,two scientific issues were explored:(1)the influence of flue gas temperature and excess air coefficient on the particle size distribution,chemical form,and total mass of condensable particulate matter,and(2)the distribution and chemical morphology of mercury in different size FPM and CPM.After that,the issues mentioned above were analyzed in a 1000 MW power plant equipped with a modified fly ash injection mercury removal system.Electrical low-pressure impactor+(ELPI+)was used to study the particle size distribution under different relative humidity conditions.The results show that the total concentration of particulate matter increased with increased humidity.Heterogeneous condensation increased in particulate matter larger than 0.02 ?m and a decrease in particulate matter smaller than 0.02 ?m.Condensation of water vapor caused a significant increase in the amount of 0.1 ?m sized particles.Collision-coalescence of particulate matter resulted in a significant increase in the number of particles greater than 2 ?m in size.When relative humidity is below 33.4%,a Nafion drying tube can be used to reduce the humidity level and obtain approximately accurate particle sampling results.For relative humidity conditions exceeding 33.4%,a diluter and a Nafion drying tube must be used simultaneously for dehumidification to ensure accurate results.By analyzing the influence of relative humidity on FPM,it is found that there are three main mechanisms for the influence of relative humidity on the sampling results.Heterogeneous condensation,vaporization condensation,particle collision-coalescence.As the temperature and excess air coefficient increase,the concentration of CPM will also increase.Heterogeneous condensation will cause the increase of PM in the entire size range,while homogeneous condensation will only cause the increase of finer PM.In coal combustion,broken,condensation,evaporation,and condensation are the main formation mechanisms of particulate matter.The FPM and CPM generated by the above-mentioned mechanism are the main forms of primary particles emitted by power plants.This study investigated the effects of the sampling temperature and excess air coefficient(EAC)on the total mass,chemical speciation,and particle size distribution of CPM by integrating Electrical Low-Pressure Impactor+(ELPI+)sampling devices with the EPA Method 202(dry impinger method).The total mass of CPM increased with the sampling temperature and EAC.Specifically,the total concentration of CPM was 10.51-39.93 mg/m3,in which the mass fraction of organic species varied between 8.74 and 49.80°,and the organic components in CPM followed the ranking order of alkanes/alkenes(62.6-78.6°),oxygen-containing volatile organic compounds(OVOCs)(19.7-35.4°),and aromatics(5.6°).Compared with other inorganic species such as HCl and NOx,SO3 had a higher migration tendency from the flue gas to CPM.The particle size distribution suggested that heterogeneous condensation was responsible for the whole size range of particles in CPM,whereas the homogeneous condensation led to the increase of finer particles(smaller than 0.2 ?m).Accordingly,adjusting the emission temperature and EAC could help to control the emission of CPM.The occurrence of these species was also confirmed by a thermodynamic equilibrium calculation and temperature-programmed desorption(TPD)analysis.Particle-bound mercury discharged with fine particulate matter from coal-fired power plants causes atmospheric pollution that impacts human health.In this study,the speciation and size-specific distribution of particle-bound mercury in filterable particulate matter(FPM)from an ultra-low emission power plant and condensable particulate matter(CPM)from an entrained flow reactor were analyzed.Before and after Selective Catalytic Reduction(SCR),both HgCl2 and HgBr2 are detected,and HgBr2 is dominant.As the flue gas flows,the concentration of HgCl2 gradually increases and exceeds HgBr2 until the wet flue gas desulfurization(WFGD).It should be noted that the discharge of the electrostatic precipitator(ESP)can produce HgO,and this part of HgO is dissolved in the water and removed by the WFGD.The chemical form of particle-bound mercury in CPM was mainly HgCl2.The concentration of particle-bound mercury in CPM will decrease significantly as the flue gas temperature decreases,and the decrease at 100? to 50 ? is more obvious than that at 150? to 100?.Before ESP,the size-specific mass fraction of particle-bound mercury of fine particles(less than 0.01 ?m)was several magnitude orders higher than that of larger particles(greater than 1 ?m).After ESP,the mercury bound to particles is greatly removed.WFGD effectively removes the mercury bound to particles that are larger than 1 ?m and smaller than 0.1 ?m.Although it contributes to the concentration of particle-bound mercury for particles between 0.1 ?m and 1?m,the total concentration is still reduced.After installing the modified fly ash injection mercury removal system,the removal efficiency of mercury can reach 94.8%,and the impact of the system on the emission concentration of particulate matter can be neglected.Before the ESP,the major mercury species is HgCl2 and HgBr2.Specifically,an ESP removed most particle-bound mercury and introduced a small amount of HgO.Similarly,WFGD dramatically reduced particle-bound mercury for most particles,except those between 0.1 and 1 ?m.At the outlet of WFGD,mercury bound with FPM10(smaller than 10?m)is only 0.15%of the total mercury at the inlet of selective catalytic reduction(SCR).The mercury bound with PM10 before the ESP increased by 154.56%after modified fly ash injection,while the mercury bound with PM10 after the ESP increased by 102.42%after injection.The final mercury bound with PM10 emitted from the power plant increased by 55.23%after injection.The increase of particle-bound mercury is mainly concentrated in particles between 0.1 ?m and 1 ?m while the chemical composition of mercury has not changed significantly. |
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