Experimental Study On Emission Characteristics Of Condensable Particulate Matter And Typical Organic Pollutants In Coal-fired Flue Gas

Coal plays a major role in the energy consumption structure in China,which is the largest coal-consuming country in the world.Coal-fired power plants have been considered to release large amounts of pollutants,such as PM_2.5,heavy metals,NO_x,SO₂,and polycyclic aromatic hydrocarbons?PAHs?.Particulate matter?PM?emitted from coal-fired power plants include filterable particulate matter?FPM?and condensable particulate matter?CPM?.CPM is the predominant contributor to the total particulate matter emissions of coal-fired power plants.Currently,most control methods and related researches of PM are related to FPM.However,there is still a lack of research on the formation mechanism,emission characteristics,and corresponding control technologies for CPM emitted from coal-fired power plants.Meanwhile,Coal-fired power plants also emit large amounts organic pollutants,such as PAHs.PAHs are also important precursors and key toxic components of PM_2.5.There is correlation between PM_2.5 and PAHs emission from coal-fired power plants.This thesis is based on the field research and laboratory analysis,and comprehensively studies the emissioncharacteristics of PM_2.5 and PAHs in coal-fired flue gas.The contents and conclusions are listed as below:?1?The coagulation characteristics of the conversion of PAHs to condensable particles were initially studied.The simulated containing CPM flue gas generating device had good efficiency and stability for 16 PAHs.As the ring number of PAHs increase,the coagulation rate of gas phase PAHs fluctuatingly increases.The condensation rates of the 5-and 6-ring PAHs are relatively highest,and the condensation rates of the 3-rings PAHs such as Acpy,Acp,and Flu are relatively low.?2?Research on the emission and control characteristics of CPM and FPM in coal-fired power plants.The emission characteristics and influencing factors of PM_2.5?including FPM and CPM?of 6 coal-fired units?involving 660MW,1000MW and 1030MW boiler loads?were studied.The concentrations of FPM from the ultra-low emission coal-fired unit range from 0.51mg/Nm³ to 3.6 mg/Nm³,and PM_2.5 is the main component;The concentrations of CPM from the ultra-low emission coal-fired unit range from 7.9mg/Nm³ to 32.0 mg/Nm³.Condensable particulate matter actually accounts for the main part of the total PM emissions.S content in the burning coal affects the emission concentration of CPM.The higher S content leads to the higher concentration of CPM,especially for inorganic components.The use of high quality coal can reduce the emission of CPM.?3?The control characteristics of the low-low temperature electrostatic precipitator?LLT-ESP?,the rotary type electrostatic precipitator?RT-ESP?,the wet desulfurization device?WFGD?,the wet electrostatic precipitator?WESP?for PM_2.5.The removal efficiency of the LLT-ESP for FPM_2.5 is 90%,and 70%-90%for CPM.The lower operation temperature of the LLT-ESP led to good removal efficiency for FPM and CPM.Because there is no obvious cooling process of the flue gas,the removal efficiency of the RT-ESP for CPM is slightly lower than that of the LLT-ESP.The removal efficiency of the inorganic fraction of CPM is lower than that of the organic fraction.In this study,the removal efficiency of the WFGD in the non-ultra-low emission coal-fired power polant is 36.6%for CPM,and the removal efficiency of the WFGD in the ultra-low emission coal-fired power plant is 69.7%for CPM.The removal efficiency for the inorganic fraction of CPM was higher than of the organic fraction for WFGD.In the two research,the removal efficiency of the WESP for total condensable particles is 22.5%and 24.4%,respectively.The low removal efficiency maybe due to the removal mechanism of the WESP for CPM,which need to be researched further.The WESP has a good removal efficiency for FPM.After the flue gas passes through the WESP,the concentration of FPM,especially PM_2.5,decreases significantly.?4?Preliminary research on the composition of the organic and inorganic fraction of CPM.The organic fraction contains hundreds of organic compounds,with esters?36.8%on average?and alkanes?29.0%on average?being the main components.In the inorganic fraction,SO₄^2-occupies the main part.Cl^-occupied a certain proportion as well.The proportions of F-and NO₃^-are small.Na⁺and Ca²⁺are the main metal ion components in inorganic fraction,and the proprotions of which are significantly higher than those of Fe³⁺,Mg²⁺,and Al³⁺.?5?The control characteristics of the LLT-ESP,the RT-ESP,the WFGD,and the WESP for PAHs.The mass concentrations of gas phase PAHs and particulate phase PAHs in inlet flue gas of the LLT-ESP were 27.52?g/Nm³ and 14.35?g/Nm³,respectively.The mass concentrations of gas phase PAHs and particulate phase PAHs in outlet flue gas of the LLT-ESP were 3.38?g/Nm³ and 0.34?g/Nm³,respectively.The cooling process of the flue gas attibuteds to the good removal effect of the LLT-ESP for the total PAHs.The discharge process the electric field may cause the regeneration of some 4-rings and 5-rings PAHs in the flue gas.The gas phase PAHs in the inlet and outlet of WFGD accounted for the main part of the total PAHs.The proportions of 2-rings and 3-rings PAHs in the gas phase PAHs were relatively high,and the proportions of 5-rings and 6-rings PAHs components in the particulate phase PAHs were relatively high.The removal efficiency of gas phase PAHs of the WFGD is about 16%,the removal efficiency of particulate phase PAHs is about 27%,and the WFGD has selective removal effect for PAHs.The WESP could reduce the emission concentration of the total PAHs to a low value,the concentration of PAHs in the gas and particulate phase at the outlet is 0.423?g/Nm³ and 0.447?g/Nm³,respectively.The 5-ring PAHs contributed the most to the toxic equivalents of the total emissions PAHs.There might be a threshold value of the PAH removal efficiency for the WESP.?6?Preliminary research on the correlation between PAHs and PM_2.5.The mass concentration of PAHs in CPM emitted from the studied coal-fired power plants is approximately 50?g/g.The mass concentration of PAHs in FPM_2.5 at the outlet of the LLT-ESP is higher than that in PM_2.5 in fly ash?about 4?g/g?.The mean concentration of PAHs in PM_2.5.5 is1.86 times of that in PM_2.5-10 in fly ash.3-rings and 4-rings PAHs account for the main parts in PM2.5 in fly ash.3-rings and 4-rings PAHs also occupied the main part of the total PAHs in CPM and FPM_2.5.A positive correlation between the concentration of PAHs and CPM could be observed.The coefficients of determination?COD,R²?between the PAH contents and the concentration of CPM were about 0.9.However,the correlation between the concentration of the high rings PAHs and CPM is low,based on the results of different unit tests.This may be due to the relative emission concentrations of high rings PAHs in CPM.The R² of FPM_2.5&PAHs is lower than that of CPM&PAHs.The concentration distribution of PAHs in FPM_2.5 is not only correlated with the concentration of FPM_2.5,but also affected by the boiler load,coal,temperature change of the flue gas,the discharge process of electric field,etc.