Characteristics Of PCDD/Fs Related To Chlorobenzenes In Municipal Solid Waste Incineration

With the implementation of the new standard for pollution control on the municipal solid waste incineration(MSWI)in China,the government and publics has paid more and more attention to the environmental problems.The monitoring and controlling of polychlorinated dibenzo-p-dioxins and dibenzofurans(PCDD/Fs)meets great challenges and strict requirements.In order to get further studies on the formation mechanism of PCDD/Fs and succeed to monitor PCDD/Fs near-on line,this paper has studied the characteristics of PCDD/Fs related chlorobenzenes(CBzs)comprehensively in different types of incinerators,in different positions and with different air pollution control devices.Furthermore,this paper carried out a series of experimental study on formation of PCDD/Fs and environmental persistent free radicals(EPFRs)from oxidation of 1,2,3-trichlorobenzene over an CuO-silica surface.The main results and conclusions include:(1)In the paper,we have researched the emissions and relationships of PCDD/Fs and CBzs in the stack gas from typical circulating fluidized bed(CFB)incinerator and grate-type incinerators in detail.The concentrations of CBzs are always 100-1000 times higher than that of PCDD/Fs in the stack gas from all incinerators.And the gaseous phase dominates in both PCDD/Fs and CBzs.Teta-to hexa-chlorinated PCDD/Fs are primary pollutants in the stack gas of CFB incinerators with high emission levels.And hepta-to octa-chlorinated PCDD/Fs are obvious in CFB and grate-type incinerators with low emission levels.But trichlorobenzenes always have the highest emission concentrations.PCDD/Fs and CBzs have the similar variation tendencies in the same incinerators.Compared with chlorophenols(CPhs)and polycyclic aromatic hydrocarbons(PAHs),CBzs have excellent correlations with PCDD/Fs,especially PCDFs,in the changeable operation conditions.Different trichlorobenzenes have been found as the good indicators of PCDD/Fs correlation models in different ranges of emission levels.1,2,3-trichlorobenzene could be the best indicator of toxic equivalent quantity(TEQ)with linear correlation models(R2=0.94)when the PCDD/Fs concentrations in CFB incinerators are high(>1 ng I-TEQ/Nm3).1,2,4-trichlorobenzene could be the best one when the PCDD/Fs concentrations are nearly 0.1 ng I-TEQ/Nm3.In the range of 0.01-0.1 ng I-TEQ/Nm3 in the stack gas of stokers,a logarithmic model with trichlorobenzenes could be established.Another logarithmic model with trichlorobenzenes for TEQ from both CFB incinerators and stokers could be also established and proved to work well.(2)This paper studied the formation of PCDD/Fs and CBzs in the MSWI system.Temperature plays an important role.PCDD/Fs and CBzs are primarily generated in low-temperature areas(200-500?).The generated concentrations of CBzs are higher but the increment ratios of PCDD/Fs are higher.The concentrations of PCDD/Fs and CBzs from the flue gas of the furnace outlet and high-temperature superheater outlet are nearly the same in the gaseous phase and solid phase.With the decrease of temperature in the flue gas,amounts of PCDD/Fs are absorbed in particulate matters and thus concentrations in the solid phase become much higher,with only 14%of the gaseous PCDD/Fs.In contrast,CBzs still dominate in the gaseous phase(88%).The concentration of CBzs with higher levels of chlorination are much lower.The concentrations of TrCBzs are the highest.1,2,4-TrCBz is rather obvious in the flue gas of the furnace outlet and high-temperature superheater outlet with lower than 0.3 ng/Nm3 of PCDD/Fs.Linear correlations(R2>0.8)between 1,2,3-TrCBz and gaseous PCDD/Fs before and after the APCD could be found.Quadratic correlations(R2>0.9)between 1,2,3-TrCBz and solid-phase PCDD/Fs before the APCD could also be obtained.1,2,4-TrCBz in the stack gas has a high negative correlation with PCDD/Fs in the bag ash,with a correlation coefficient of 0.891.TrCBzs could be the indicators of PCDD/Fs in the flue gas and fly ash.It could help achieve to monitor PCDD/Fs comprehensively in the whole MSWI.(3)After the treatment of the APCD in three investigated MSWIs,the concentrations of PCDD/Fs all have a marked decline.But CBzs are different.The removal efficiency for PCDD/Fs in the system of semi-dry process and bag filters with activated carbon,is in the range of 87.0%-98.2%.The removal efficiency for TrCBzs is much low.And thus,the concentrations of TrCBzs are much higher than that of other CBzs.In the actual flue gas,the removal efficiency for CBzs of the selective catalytic reduction(SCR)reactor is affected both by the destruction of themselves and CBz generation from other organic pollutants,decomposition.The efficiency is only 17.9%,much lower than that of PCDD/Fs.With the increase of chlorination,the removal efficiency of PCDD/Fs and CBzs decrease.There are "memory effects" found in the wet scrubbers,resulting in the major increase of penta-,hexa-and hepta-chlorinated congeners among PCDD/Fs.TrCBzs are also influenced obviously,with increasing concentrations.The system of semi-dry process and bag filters with activated carbon rarely affects the correlation between PCDD/Fs and CBzs,but the SCR reactor and wet scrubber have very complex effects on their correlation models established.(4)Temperature,atmosphere and metal catalyst compositions will have different effects on the formation of PCDD/Fs,dibenzofuran(DF)and EPFRs.High temperatures could promote the formation of PCDD/Fs,especially PCDFs from 1,2,3-TrCBz exponentially.The formation of DF will increase first and decline then twice with the increase of temperature.Chlorination will be promoted.PCDFs could be generated rapidly at the key temperature of 400-450?.PCDD/Fs and DF could still be generated from 1,2,3-TrCBz on the surface of CuO-silica in inert atmosphere.The oxygen content could help chlorination.The concentration of DF decreases and the concentrations of PCDD/Fs increase.The oxygen content also has selectivity for PCDD and PCDF.Low oxygen atmosphere could promote PCDD formation,especially TCDD.The formation of PCDF,especially TCDF,will advanced in the high oxygen atmosphere.Iron oxides are better than copper oxides to catalyze the formation of PCDD/Fs,especially PCDF,from TrCBzs.The dechlorination effect of iron oxides results in more DF and low chlorinated dioxins.With 10%of chlorophenols added in the reactants,PCDD/Fs concentrations increased by 6.9 times.When CBzs are more than CPhs among the reactants,the major production is PCDF.In contrast it becomes PCDD.The fingerprint characteristics of dioxins will change significantly.Furthermore,oxygen-center EPFRs,the phenoxyl group with g values of 2.0044-2.0047,and EPR signals with low g values of 2.0014-2.0021(carbon-center EPFRs or F-center)could be found through the catalytic formation of dioxins.The species of EPFRs will increase with the rise of temperature.O-semiquinone with higher g value,and EPR signals with lower g value will appear or be enhanced.The oxygen content enhances oxygen-center EPFRs.High contents of iron oxides reduce the spins concentrations of oxygen-center radicals but increase the spins concentrations of signals with low g values.It is found that EPR signals with low g values could help promote the formation of PCDD/Fs rapidly.