Heavy Metal Migration Law And Environmental Impact Assessment During Co-pyrolysis Of Biomass And Sludge

The amount of sludge produced by domestic sewage treatment plants is increasing.Co-pyrolysis of sludge and biomass is an effective means to achieve sludge reduction,harmlessness and resource utilization.The migration of heavy metals during the co-pyrolysis process is an urgent problem.In this paper,the migration of heavy metals in the process of co-pyrolysis of sludge and biomass is studied.By analyzing the distribution of heavy metals in different pyrolysis products,the conditions for controlling the solidification of heavy metals in solid products are found.This paper also uses the Simapro software and adopts the ecological index method to carry out a life cycle assessment of the sludge pyrolysis process under different conditions and different amounts of wood chips added,and analyzes its impact on the environment.The experimental results show that when the sludge is pyrolyzed separately,heavy metals are enriched in the pyrolysis semi-coke.As the temperature rises,the yield of pyrolysis gas increases,and at the same time the heavy metals volatilize at their melting points.The masses of the five heavy metals in the pyrolysis gas generally increase.The best pyrolysis conditions for inhibiting heavy metals from entering the pyrolysis gas are: the sludge moisture content is 20wt%,and the pyrolysis temperature is 600℃.The masses of five heavy metals Cu,Zn,Mn,Ni,and Cr in the pyrolysis gas accounted for 0.78%,4.45%,0.6%,3.11%,and 5.42% of the total mass of the raw materials.When the sludge with a moisture content of 20wt% is co-pyrolyzed,the addition of sawdust promotes the generation of pyrolysis syngas,and the excessive addition of sawdust causes the heavy metals transferred to the pyrolysis gas to increase.The best process conditions for inhibiting excessive transfer of heavy metals to pyrolysis gas are: pyrolysis temperature of 600℃,and the proportion of wood chips added is 10wt%.At this time,the proportions of the heavy metals Cu,Zn,Mn,Ni,and Cr in the pyrolysis gas to the total mass in the raw materials were 0.36%,1.77%,0.4%,2.25%,and 5.66%,and the proportion of heavy metals in the pyrolysis semicoke was 74.66%,90.82%,78.26%,76.19% and 77.19%.The results of the life cycle assessment showed that the increase in temperature increased the relative load of environmental damage during the pyrolysis stage.By comparison,when the temperature is 600℃.and the proportion of wood chips is 50%by weight,the damage to the environment is minimal.At this time,the relative load of the damage is 8.2421Pt/t.Combined with the analysis results of heavy metalmigration laws,in order to reduce the impact of the process on the environment,the pyrolysis temperature should be reduced as much as possible within a certain range to increase the amount of biomass added.