Sewage Sludge And Municipal Solid Waste Co-pyrolysis/Co-incineration And Products Utilization:Migration Characteristics Of Mercury

With the social and economic development of China,sewage sludge(SS)and municipal solid waste(MSW)have been gradually increasing and are facing severe disposal problem.It has been reported that co-pyrolysis and co-incineration could economically and efficiently treat and utilize SS and MSW.However,mercury(Hg)emission risk during this process is also attracting growing attention.Accordingly,this work researched the migration characteristics of Hg during co-pyrolysis/co-incineration and products utilization of SS and MSW.The main study contents and related conclusions are as follows:(1)Influential parameters including blending ratio,pyrolysis temperature,heating rate and residence time towards Hg distribution in co-pyrolysis products were studied.The results indicated that blending ratio of SS and MSW had significant effect on the distribution of Hg in pyrolysis tar,gas and char.There was no significant synergetic effect among the four factors.(2)Fixation optimization and ecological risk evaluation of Hg in co-pyrolysis char was conducted.The results indicated that with blending ratio of 87.5 wt%,heating rate of 7?/min,pyrolysis temperature of 300?,and residence time of 10 min,the Hg fixing ratio could reach 25.68 wt%in co-pyrolysis char.Under these optimal conditions,the environmental risk index of Hg in co-pyrolysis char was less than 40,which was within the range of low environmental risk.(3)Chemical form of Hg during combustion of co-pyrolysis tar and gas was studied.The results indicated that during the combustion of co-pyrolysis tar and gas,more Hg existed in Hg⁰ phase.With the increment of combustion temperature,more Hg²⁺was reduced into Hg⁰.HCl,Cl₂,SO₂ and CH₄ components had significant impact on Hg⁰ formation,and Cl was the most important element which affected the transformation of Hg.(4)Comparison of Hg migration characteristics between co-incineration and co-pyrolysis was conducted.The results indicated that the optimal fixing rate of Hg towards solid product during co-pyrolysis(25.68wt%)was much higher than that during co-incineration(7.57wt%),making Hg easier to control during co-pyrolysis process.Besides,Hg in co-pyrolysis char existed in chemical forms with lower environmental risk.Thus the environmental risk factor of Hg in co-pyrolysis char(<40)was much lower than that in co-incineration(>320).As a result,the synergetic effect of the two feedstocks on the distribution and chemical form of Hg during co-pyrolysis was revealed,the ecological risk of different forms of Hg in co-pyrolysis products was demonstrated,and the difference of Hg distribution,chemical form and ecological risk between co-incineration and co-pyrolysis was discussed.The results and discussions in this work provide fundamental knowledge about emitting and controlling mechanisms of Hg during this process,and eventually promote the cleaner disposal and utilization of SS and MSW.