Emission Characteristics And Control Of PCDD/Fs And Heavy Metals From Small-scale Thermal Treatment Furnace For Disposing Rural Domestic Solid Waste

At present,it is the primary solution to treat pollution of rural domestic solid waste for improving rural environmental quality.In recent years,small-scale thermal furnaces for disposing rural domestic solid waste have been wildly used in mountainous rural area in China due to advantages of reducing transport cost,high reduction effect in volume and mass,and easy site selection.However,the toxic emission levels of dioxins(PCDD/Fs)and heavy metals are not clear during the thermal treatment process of solid waste so far.In this work,the emission levels,migration and transformation mechanisms and control technologies of PCDD/Fs and heavy metals in small-scale thermal treatment furnaces were studied in practical engineering.The results are of great significance to know the emission environmental risks of PCDD/Fs as well as heavy metals and improve the emission control technology of PCDD/Fs and heavy metals for small-scale thermal treatment furnaces.First of all,twelve typical small-scale thermal treatment furnaces for disposing rural domestic solid waste including the incinerator,gasification incineration furnace and gasifier located in Guangxi,Yunnan,Guangdong provinces in China were surveyed.These small furnaces were found equipped with simple flue gas cleaning systems and in poor management state,resulting high emission concentration of dioxin in flue gas,and the difference was obvious.The emission contents of PCDD/Fs in flue gas from incinerators,gasification incineration furnaces and gasifiers were0.70~24.88?3.14~4.84?0.02~0.48 ngI-TEQ/Nm~3,respectively.The contents of PCDD/Fs in bottom ashes from three types furnaces were 26.50~231.43,21.55~161.69,293.70~1062.22 ngI-TEQ/kg,respectively,which were higher than those of common large-scale municipal solid waste incineration(MSWI).The emission contents of heavy metals(Cd,Cr,Pb and Hg)in flue gases from twelve furnaces,except Cd content from an incinerator,were very low,and Pb content in the flue gases from all furnaces was higher than Cd?Cr?Hg.Secondly,in view of the heavy metals in bottom ash easily getting into the soil and water environment by wind or rain during open-air stacking of bottom ash from small-scale thermal treatment furnaces,leaching toxicity concentrations and morphological distribution of heavy metals(Cu,Zn,Cr,Pb,Cd,As and Hg)in bottom ash from three types of furnaces were analyzed.The results showed that the contents of heavy metals(Cu?Zn?Cr?Pb?Cd)in bottom ash from twelve furnaces,except As and Hg,were much higher than Grade II limited values of the Soil Environmental Quality Standard(GB15618-1995)for upland soil.Speciation analysis exhibited similar speciation distributions of heavy metals in bottom ashes from three types of furnaces.Zn,Cr,Pb,Cd,As and Hg existed mainly in the form of residue,and Cu was mainly in the form of organic bound and residue.The content of available Cd in bottom ash from the incinerator and gasification incineration furnace was high with the great ecological toxicity.Heavy metals in bottom ash were leached by using the simulated different p H acid rain.Results showed that the leaching concentrations of Pb,Cd and Cr exceed the grade V standard of Environmental Quality Standard for Surface Water when the pH of acid rain decreased to 4.5.Therefore,the bottom ash from small-scale thermal treatment furnaces would lead to environmental pollution of heavy metals while they were leached by acid rain or randomly brought into the soil environment.Toxicity leaching concentrations of heavy metals in bottom ash revealed that these bottom ashes from these three types of furnaces did not belong to hazardous waste.Thirdly,a typical small-scale updraft fixed bed gasifier for disposing rural domestic solid waste was studied.The results demonstrated that the emission pollutants in the flue gas,except CO,could meet the standard limits of the Standard for Pollution Control on the Municipal Solid Waste Incineration(GB18485-2014)after cleaning of wet washing combining with electrostatic precipitator.The distribution characteristics of PCDD/Fs congeners in all gasification products revealed that PCDD/Fs were mainly produced in the gasifier chamber instead of the flue gas purification devices.Mass balance distribution of PCDD/Fs showed that the total output of PCDD/Fs in gasification process was 50.04 ngI-TEQ/t.The proportion of PCDD/Fs in bottom ash,tar and flue gas was found 88.06%,11.80%,0.07%,respectively.Mass balance distributions of heavy metals showed that 94.00%Pb,80.45%Cu,75.93%Cd,77.29%Cr and 75.79%As in raw solid waste were remained in the bottom ash;however,64%Hg was discharged with the flue gas.High temperature heating(700~900?)can effectively remove great amount of CO in the flue gas,but may increase 4.24 times of PCDD/Fs.Finally,in order to decrease the PCDD/Fs emission during the gasification incineration of solid waste,the bag filter(BF)and activated carbon injection coupled with bag filter(ACI+BF)were applied to remove PCDD/Fs in the flue gas.Results showed that the emission contents of PCDD/Fs cannot meet the standard value of(GB18485-2014)without using the bag filter,however use of BF only would increase the content of PCDD/Fs from 4.83I-TEQng/Nm~3 to 13.69 ngI-TEQ/Nm~3,while the ACI coupled with BF could decrease the PCDD/Fs in flue gas to 0.042 ngI-TEQ/Nm~3.The removal efficiencies for 17 kinds of PCDD/Fs congeners were 95.79%~99.12%by the ACI+BF,and for 10 PCDFs congeners the removal efficiency decreased with increasing chlorination degree,indicating that the ACI coupled with BF had a good synergistic effect for removing PCDD/Fs in the flue gas.The total emission factor of PCDD/Fs decreased from 21.415?g I-TEQ/t to10.889?g I-TEQ/t after ACI+BF,among which the emission factor proportion of PCDD/Fs in bottom ash,fly ash and flue gas was 97.48%?1.63%?0.89%,respectively,suggesting the ACI+BF can decrease the synthesis of PCDD/Fs in the process of flue gas purification.