Study On The Emission Mechanism And Control Of Volatile Organic Compounds In Coal-Fired Power Plant

China's energy will continue to be dominated by coal for a long time,and more and more attentions have been paied on the organic pollutant emissions from coal combustion.This study mainly investigated the emission of organic pollutants generated during the coal combustion,which included the coal pyrolysis evolved gas,factors affecting the volatile organic compounds in the evolved gas,effect of different heating rate combustion on the release of volatile organic compounds,and the study of organic pollution distribution and the control of organic pollutants in coal-fired power plants based on the above research.The simultaneous use of thermogravimetric(TG)analysis,Fourier-transform infrared(FTIR)spectroscopy,and gas chromatography/mass spectrometry(GC/MS)can provide a unique method for studying the coal pyrolysis.About 124 volatile organic compounds(VOCs)were identified by TG-FTIR-GC/MS during the coal pyrolysis based on the characteristic fragments or Kovats index.These compounds included alkanes,?-olefins,benzene,toluene,ethylbenzene,xylenes,and phenols.n-Alkanes from C5 to C24 with regular one-carbon retention intervals in the GC/MS system were used to calculate the Kovats indexes of all compounds.The results showed that the temperature was the key factor affecting VOCs release.The VOCs released below 400? represented only 23.1%of the total VOCs released during the pyrolysis of bituminous(coal BA).Thus,the use of thermal desorption(TD)with GC/MS to analyze coal pyrolysis could only provide VOCs release information before 400?.It would underestimate the VOCs release because of the temperature limitation on the TD technique.The heating rate of the thermogravimetric analyzer had a great influence on the release of VOCs,and the high heating rate could delay the release temperature of VOCs.The different particle sizes at a fixed heating rate had little effects on the release temperature of VOCs in the pyrolysis evolved gas.This might be due to the small effect of particle sizes on the temperature conduction at the low heating rate.Different coal banks had a greater impact on the thermal release of VOCs.The yields of phenolic hydrocarbons from lignite were higher than those from bituminous coal.This could be the result of a higher level of oxygen cross-linking in lignite than in bituminous coal,which was proven by the results of curve fitting and GC/MS.The evolved gas under different atmospheres was analyzed by the combination of FTIR and GC/MS.The composition,relative contents,and changing trends of VOCs evolved from coal pyrolysis were studied.The results showed that during the evolved gas from bituminous coal pyrolysis reacted with oxygen,phenolic substances were the first to be consumed,followed by olefins,alkanes,and aromatics.Even in an oxygen atmosphere,a small amount of VOCs was released.This results could provide the basis support for the VOCs emissions and control in the power plant.The release of VOCs in coal under combustion conditions could be simulated under different atmospheres,but it was not suitable to simulate real combustion at the low heating rate.Therefore,in a further study,the emission of VOCs from coal combustion at different heating rates in a fix bed(low than 100?·min-1),a drop tube furnace(low than 2000?·s-1),and a power plant boiler(about 1050C·s-1)were compared.TD-GC/MS and online FTIR were used to measure VOCs concentrations during the coal combustion.This study showed that the combustion temperature had a distinct effect on VOCs composition.Larger VOCs concentrations were detected in the flue gas collected at higher furnace temperatures in a drop tube furnace.Higher furnace temperatures led to a faster heating rate and a shorter residence time for the flue gas in the furnace,resulting in increased VOCs generation.The distributions of organic compounds such as methane,non-methane hydrocarbon(NMHC),and polyaromatic hydrocarbons(PAHs)in the effluent from an ultra-low emission power plant were investigated.The methane and NMHC in the flue gas were analyzed using a modified portable volatile organic hydrocarbon analyzer according to the EPA method 25A,and a lower boiler load was found to increase the NMHC concentration.The empirical parameter E was used to represent the amount of organic compounds in the flue gas as a percentage of the organics obtained from coal pyrolysis.The E values based on methane and NMHC were below 0.2%at the boiler outlet and less than 0.1%at the stack inlet.The total VOCs emission concentration was below 1.5mg·m-3.These results confirmed that the high combustion efficiency obtained from ultra-supercritical pulverized coal-fired boilers incorporating ultra-low emission air pollutants control devices that eliminated organic compounds through oxidation,condensation,and water absorption.The PAHs contents in solid samples were analyzed by the solvent extraction and the high-rank coal was found to have a higher concentration.Mass balance calculations determined that the methane and NMHC levels in the flue gas were less than 0.01%and 0.07%of the amounts obtained from the coal pyrolysis.The methane,NMHC,and PAHs concentrations in fly ash were 0.01%,0.02%,and 0.06%,respectively,while the slag contained less than 0.01%of each.The distributions and reductions of VOCs(alkanes,aromatics,and halogenated hydrocarbons)emitted from two coal-fired power plants were compared.The partial removal of VOCs by the air pollution control devices was assessed.The data showed that VOCs levels in flue gas were reduced by 5%to 35%after the selective catalytic reduction,0%to 40%after the air preheating and electrostatic precipitation,and 10%to 20%after the flue gas desulfurization and wet electrostatic precipitation.This decrease in emissions was attributed to oxidation,deposition,condensation,and water absorption mechanisms.Our group has applied the modified fly ash for removing mercury pollutants.In this project,the removal effect of modified fly ash on VOCs was studied without increasing the cost.Spraying modified fly ash was found to further reduce the effluent concentration of VOCs by 10%to 20%,resulting in an overall VOCs reduction of 40%to 80%.Both physical and chemical mechanisms were involved in the absorption removal.