| Domestic sewage sludge,as the final product of urban domestic sewage treatment,contains a large number of toxic and harmful substances such as bacterial cells,pathogenic microorganisms and heavy metals etc,which are seriously damaging to the ecological environment and human health.As the most thorough and promising method for treating sludge,incineration has received extensive attention from researchers at home and abroad.Due to high moisture content,high ash content and very low heating value,co-incineration of sludge with other fuels such as coal is more advantageous than sludge mono-incineration.O2/CO2 combustion is a CO2 emission reduction technology with great development and application prospects.In this paper,experiments on the migration characteristics of semi-volatile heavy metals(SVHMs)(such as Zn、Pb、Cd、Cu、Ni and Cr)with domestic sewage sludge and bituminous coal during incineration under O2/CO2 atmosphere were carried out.The mechanism of migration and transformation of SVHMs during incineration was discussed.The main work and conclusions are presented as follows:Taking sewage sludges as the object,the effect of sludge chemical composition on the speciation and release fractions of SVHMs during sludge mono-incineration under O2/CO2 atmosphere was studied in the fixed-bed incinerator.The result indicated that the six kinds of sludge all contain plenty of ash and volatile matter,all have high heating value and the content of C and O are relatively high.N and S content in sludge is comparable to bituminous coal.However,the content of Si,Ca,Fe and A1 in the six kinds of sludge are great different.The speciation of Zn5 Pb and Cd in bottom ash,fly ash and flue gas was significantly different in the separate incineration process of six kinds of sludge.The speciation of Zn?Pb and Cd in bottom ash and fly ash was significantly higher than that in flue gas.The speciation of Cu,Ni and Cr has less difference,mainly distributed in the bottom ash,which are more than 80%.The release fractions of Zn,Pb and Cd were higher in the separate incineration process of the six types of sludge,ranging from 35.28%to 56.24%,21.21%to 66.46%,and 27.70%to 42.80%,respectively.On the contrary,the release fractions of Cu,Ni and Cr were lesser,ranging from 5.78%-17.89%,1.89%-13.25%and 2.92%-l 1.43%,respectively.By using Orthogonal experimental method and taking sewage sludge and bituminous coal as the object,the influence of incineration temperature(7),O2 concentration(cpO2),mass ratio of sludge to coal,incineration time,moisture content and chlorine content on the migration characteristics of SVHMs during co-incineration of sludge and coal has been researched and the optimal operating conditions of co-incineration of sludge and coal has been determined.The results showed that among the six factors,incineration temperature,mass ratio of sludge to coal and moisture content had a great influence on the release fractions of SVHMs.When Tr=700-1100℃,increasing the incineration temperature is conducive to promoting the release of SVHMs.When φ02=10%-50%,with the increase of O2 concentration,the release fractions of Pb and Cd decreased first,then increased and then decreased again.The release fractions of Pb and Cd reached the minimum when the O2 concentration is 30%.With the increase of O2 concentration,the release fractions of Zn and Ni decreased first and then gradually increased,while that of Cu gradually increased.The increase of mass ratio of sludge to coal(0-100%)is beneficial to inhibit the release of Zn,Pb,Cd,Cu and Cr.When the moisture content changes within the range of 0-60%,the release fraction of Zn first increased and then decreased,and the release fractions of Pb and Cd both showed a trend of first increased,then decreased and then increased.In the incineration time of 3-40 min,the extension of incineration time promoted the release of Pb,Cd,Cu,Ni and Cr.With the increase of chlorine content from 0 to 10%,the release fraction of Pb increased significantly,while the release fractions of Zn and Cd increased slowly.The optimal operating conditions of co-incineration of sludge and coal under O2/CO2 atmosphere are as follows:incineration temperature:900-1000℃;O2 concentration:30%in the O2/CO2 atmosphere;moisture content in the mixed sample:40%.The condition will become better with larger mass ratio of sludge to coal and shorter incineration time.The effects of incineration temperature,the type of atmosphere,O2 concentration and kaoline content on the migration characteristics of SVHMs during co-incineration of sludge and coal were studied in the fixed-bed incinerator with sewage sludge and bituminous coal.The results showed that the release fractions of the six SVHMs in the atmosphere of 21%O2/79%CO2 were lower than those in the atmosphere of 21%O2/79%N2 under the same incineration temperature.The type of atmosphere and O2 concentration did not change the overall trend that the release fractions of SVHMs increased with the increase of incineration temperature.With the increase of incineration temperature,the influence of the type of atmosphere and O2 concentration on the migration characteristics of SVHMs is weakened.When kaoline content was 4%,all the six heavy metals were well controlled,and the release fractions of them ranged from 6.03%to 37.61%. |
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