| Incineration is one of the common disposal means of municipal sludge,but sludge has low calorific value and incineration is unstable.Biomass has high calorific value and low sulfur/nitrogen content,and can be made into a fuel mixture with sludge for energy utilization.To this end,this paper makes a fuel mixture using sludge and typical biomass（mahogany,corn cob,peanut shell）,and investigates the combustion characteristics of the fuel and the migration and transformation patterns of heavy metals.The research contents and results are as follows.Firstly,the industrial analysis,element analysis and calorific value determination of sludge and biomass were carried out.Proportion of biomass（20%～50%）of single biomass and sludge,fuel particle size and biomass type on fuel calorific value were studied by orthogonal test.The results show that the proportion of biomass has the most significant effect on the calorific value of fuel,and the fuel particle size is the weakest.Next,the combustion characteristics of single sludge and biomass were analyzed separately using thermogravimetric tests,and the effects of biomass percentage and heating rate（5-30%/min）on the mixing process were investigated.It was obtained that.（1）The thermogravimetric（TG）and micro-commercial thermogravimetric（DTG）curves of sludge and biomass can be divided into four stages:water evaporation stage,volatile analysis out and its combustion stage,fixed carbon combustion stage and combustion exhaustion stage.Compared with biomass,sludge has a lower weight loss（36.5%）,ignition temperature（234°C）and higher burnout temperature（840°C）.（2）In the range of 20%to 50%of biomass,the fuel ignition temperature,flammability index and overall combustion characteristics index increased with the increase of the ratio,while the combustion temperature decreased.The best combustion performance under the test conditions was obtained when the biomass proportion was 50%:the maximum combustion rate was 4.34～7.64%/min,the flammability index was 6.13×10-5～8.43×10-5,and the overall combustion characteristics index was 0.99×10-7～1.77×10-7.（3）In the range of heating rate from 5 to 30%/min,the fuel ignition temperature,flammability index,burnout discrimination index and comprehensive combustion characteristics index increased with the increase of heating rate,while the burnout temperature increased and then decreased.The best combustion performance was achieved at a heating rate of 30%/min:the maximum burning rate was 8.65～12.10%/min,the flammability index was11.71×10-5～13.27×10-5,the burnout discrimination index was2.15×10-4～4.96×10-4,comprehensive combustion characteristics index is 3.04×10-7～5.82×10-7.Among them,sludge and mahogany mixed firing performance is the most prominent,followed by peanut shells.Finally,the migration and transformation laws of heavy metals（Zn,Cu,Cr,Pb,Ni）in incineration ash were investigated by the biomass percentage and incineration temperature.It was obtained that.（1）When mixed fuels were burned,the volatilization rate of most heavy metals showed a gradual decrease with the increase of the biomass percentage.When the proportion of mahogany increased from 20%to 80%,the volatilization rates of Zn,Cu,Cr,and Pb decreased from 42.5%,23.1%,36.6%,and 67.6%to 32.6%,13.1%,17.2%,and 48.3%,respectively,and the volatilization rate of Ni had even decreased to 0;when the proportion of corn cob increased from 20%to 80%,the volatilization rates of Zn,Cu,Cr,and Pb decreased from 44.7%,21.6%,26.8%,and 50.2%to 3.0%,6.0%,2.1%,and 30.3%,respectively,while Ni was all left in the ash residue;when the proportion of peanut shells increased from 20%to 80%,the volatilization rates of Zn,Cu,Cr,and Ni showed a decreasing trend.Among them,the volatilization rates of Zn,Cr,and Ni converged to 0 when the ratio exceeded 60%,indicating that the increase of biomass proportion in a certain range is beneficial to reduce the risk of pollution of Zn,Cu,Cr,and Pb.In addition,the percentage of biomass has almost no effect on the heavy metal form in the incineration ash.（2）In the range of 450°C to 900°C,the volatility rate of heavy metals increased to different degrees with the increase of incineration temperature,and the increase was 11.8%to85.3%,and the increase of incineration temperature increased the volatile pollution of heavy metals.Compared with sludge incineration,the volatilization rate decreased by 9.8%～28.4%,and the incorporation of biomass reduced the volatile pollution of heavy metals during sludge incineration.The bio-available state（acid soluble state+reducible state）of heavy metals in ash gradually decreased and the residue state gradually increased with the increase of incineration temperature,and the increase of incineration temperature was beneficial to reduce the bioeffectiveness and toxicity of heavy metals in ash.Compared with the pre-incineration period,the bioeffective state of heavy metals decreased by 7.7%～32.9%,and the residue state increased by 10.7%～48.9%,indicating that incineration is beneficial to enhance sludge stability.（3）In the range of 450℃～900℃,with the increase of incineration temperature,the migration and transformation pattern of heavy metals in the co-burning of sludge,corn cob and peanut shell（30%of corn cob and peanut shell respectively）is similar to that in the co-burning of sludge/corn cob and sludge/peanut shell,and the volatilization rate of heavy metals and the proportion of residue state are between them,so from the perspective of reducing heavy metal pollution,it is desirable to incinerate sludge with corn cob as fuel.From the perspective of reducing heavy metal pollution,sludge should be burned as fuel with corn cob.（4）According to the results of potential ecological risk evaluation of heavy metals,the bioavailability and ecotoxicity,ground accumulation index,Nemero index,and potential ecological risk index of heavy metals in sludge mixed with biomass ash were significantly lower than those in sludge,and the degree of heavy metal pollution was reduced.The risk levels of Zn,Cr,Pb,and Ni in sludge are low risk,and Cu is very high risk level,and the high ecological risk of sludge and ash residue is mainly caused by CuTaking into account factors such as combustion performance,prevention and control of heavy metal risks and treatment costs,sludge is preferable to be incinerated with corn cobs as fuel. |
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