Study On The Migration And Distribution Of Heavy Metals In The Flue Gas Of Municipal Solid Waste Incineration Power Plants

Municipal solid waste incineration(MSWI)power generation is one of the most effective methods to solve the current problem of"municipal solid waste(MSW)siege",but its heavy metal emission pollution problem has received widespread attention.The study of the migration and transformation of heavy metals during the incineration process is beneficial for the development of heavy metal pollution control strategies.At present,a majority of researches only focus on elements which have a very strong or weakly volatility such as Hg and Cu;However,there are few studies on heavy metals such as arsenic(As)and selenium(Se),which have a relatively strong volatility and pose a significant threat to human health.Further more,the environmental risks of these heavy metals with different speciation forms require further investigations.In this study,the flue gas,bottom ash and fly ash samples from a MSWI power plant located in north China were collected to investigate the migration and distribution characteristics of As and Se during incineration;further,fabric filter(FF)fly ash,low temperature economizer(LTE)ash,flue gas deacidification(FGD)ash and bottom ash samples from five MSWI power plants in China were analyzed to reveal the enrichment and mass distribution characteristics of 10 heavy metals including As and Se in the solid by-products.The speciation characteristics and bioavailability of As and Se,and leaching characteristics of 10 heavy metals in solid by-products were investigated to assess their environmental risks.The results showed that As and Se in the flue gas of MSWI existed in the particulate and gaseous forms.And the particulate form was predominant.The total concentrations of As and Se in the inlet of air pollutant control devices(APCDs)ranged from 346.76 to 391.27μg/Nm~3 and 52.20 to 60.44μg/Nm~3,respectively,of which the percentages of particulate As and Se were 98.24%and 79.02%,respectively.After the efficient synergistic removal by APCDs,the total concentrations of As and Se in the outlet decreased to between 5.26-5.67μg/Nm~3 and 1.38-3.78μg/Nm~3,respectively.The total removal efficiencies of APCDs were 98.49%and 96.04%,respectively,with the FF achieving a high synergistic removal efficiency of 99.02%and 99.08%for particulate As and Se in the flue gas,while the removal efficiencies for gaseous As and Se in the flue gas were slightly different,68.32%and 85.03%,respectively.As and Se were mainly migrated to the bottom ash and fly ash of MSWI power plants,where the mass percentages of As and Se in the bottom ash were 70.07%and 51.23%,respectively;the migration of relatively high volatile Se into the fly ash was significantly higher than that of As,and the percentages of As and Se in the fly ash were 29.60%and 47.12%,respectively;the percentages of As and Se in flue gas were both very low,only 0.33%and 1.65%.There were obvious differences in the content of heavy metals in the solid by-products of MSWI power plants,such as bottom ash,LTE ash,FGD ash and fly ash,but the content of heavy metals in the solid by-products of each power plant was roughly in the same order from high to low,which was Zn>Cu>Pb>Sb>Cr>As>Cd>Ni>Co>Se.The regularity of enrichment and distribution of heavy metals were found in bottom ash,LTE ash,FGD ash and fly ash.The average contents of As,Se,Cd and Sb in bottom ash were the lowest among all solid wastes,with 7.60 mg/kg,0.40 mg/kg,13.67 mg/kg and 21.55 mg/kg,respectively;while the average contents of Cu and Co in bottom ash were 553.34 mg/kg and 17.48 mg/kg,which were significantly higher than their contents in other by-products.The contents of Cr and Ni in the bottom ash(116.19 mg/kg and 71.56 mg/kg),on the other hand,were second only to their contents in LTE ash(182.46 mg/kg and 60.77 mg/kg),but significantly higher than their contents in fly ash and FGD ash,indicating that heavy metal volatility is an important factor affecting the enrichment characteristics of heavy metals.The heavy metals Cu,Cr,Ni and Co in municipal solid waste(MSW)were primarily distributed in the bottom ash after incineration and flue gas cleaning,with the average percentages as high as 96.64%,95.76%,97.51%and 97.41%,respectively;most Zn and Pb in MSW were also distributed in the bottom ash,with the average percentages of 10.26%and 26.47%in the fly ash,respectively.As,Se,Sb and Cd in fly ash were significantly higher than the other six heavy metals,with average percentages of 46.19%,56.61%,32.27%and 38.44%,respectively.The amorphous and poorly-crystalline hydrous oxides of Fe and Al phase was the main speciation forms of As and Se in the solid by-products of MSWI,indicating that unstable Fe and Al were the principal adsorption carriers of As and Se.Non-specifically sorbed As and Se,however,were largely found in LTE ash(11.86%and 21.23%on average)while specifically-sorbed As and Se were relatively widespread in the solid by-products with the highest average percentage of specifically-sorbed As(23.39%)in the bottom ash and the highest percentage of specifically-sorbed Se(35.50%)in FGD ash.The percentages of residual phase As and Se in the bottom ash were significantly higher than those of other solid by-products,especially the average percentage of residual phase Se in the bottom ash was as high as 84.30%.In addition,As in the solid by-products of MSWI had high bioavailability,with the highest bioavailability(76.07%on average)in LTE ash.On the perspective of Se,the highest bioavailability was found in FGD ash(average 83.67%).Compared to As,Se in the bottom ash was very stable,and there was only 11.16%of Se bioavailable in the bottom ash on average.Besides,the leaching concentrations of heavy metals in fly ash were the highest,followed by LTE ash and FGD ash;the leaching concentrations of heavy metals Cd and Pb exceeded the relevant limits and needed to be stabilized before landfill.The leaching concentrations of heavy minerals in the bottom ash were all the lowest,and all of them were lower than the relevant limits.Therefore,more attention should be paid to the leaching behaviors of Cd and Pb in the process of landfill disposal and resource utilization of solid by-products of MSWI.