Study On Oxy-fuel Combustion Characteristics And Heavy Metals Migration And Transformation Behaviors Of Sludge

With the rapid development of economy,the production of municipal sludge continues to grow rapidly.Municipal sludge with high content of organic matter is easy to corrupt and also contains pathogenic bacteria,heavy metals and other toxic and harmful substances.It is a very important issue about how to dispose sludge properly and make better use of the resources in sludge.Oxy-fuel combustion,as a new combustion mode,has unique technical advantages,which is beneficial for carbon emission reduction and has important significance to control greenhouse gas emissions and mitigate global warming.Therefore,it is necessary to explore the application of oxy-fuel combustion technology in sludge treatment.The combustion characteristics,ash morphology,heavy metal migration and transformation behavior of sludge under oxy-fuel conditions were studied by thermogravimetric analyzer,tube furnace,icnductively coupled plasma optical emission spectrometer and other equipment,which is helpful for the application and popularization of oxy-fuel combustion of sludge and provides a new method for sludge volume reduction,harmlessness and recycle.The oxy-fuel combustion characteristics of sludge were studied by thermal analysis technique,and the kinetic parameters were calculated.The results showed that the sludge combustion process mainly went through two stages of weightlessness.The first stage was between 180⁴00?,and the second stage was between 400⁶60?.At the same oxygen concentration,replacing N₂ with CO₂ was not conducive to the precipitation and combustion of volatiles in the sludge in the first stage,but promoted the weight loss in the second stage.As the oxygen concentration increased,the sludge was more likely to ignite,the burnout time is shortened,the weight loss rate and comprehensive combustion characteristic parameters increased,and the overall combustion property was improved.The average activation energy of sludge in simulated air atmosphere was 164.2 kJ/mol,less than that in 80%CO₂/20%O₂atmosphere.The combustion experiments of sludge at different temperatures and atmospheres were carried out in a tubular furnace.The sludge and ash sample were analyzed by SEM-EDX.The results showed that the sludge was irregular particles with different sizes and the surface of the particles was rough.During the combustion process,the agglomeration occurred and particle size increased.With the increase of temperature,more pores appeared in the particles and the porosity increased.The content of O element in ash was the highest,followed by Al and Si.The ash composition was complex,for aluminosilicate and other mineral salts.Acid digestion method was adopted to study heavy metals amount in sludge and ash by ICP-OES.The results showed that the contents of heavy metals in sludge were in the order:Zn>Cr>Cu>Pb>Ni>Cd.The concentrations of Zn,Cr,Cu and Cd exceeded the environmental quality standard for acid soils.After combustion,the concentrations of heavy metals in ash was about 1.5² times of that of raw sludge.The residual rates of Ni and Cr are high,about 90%or more.The residual rates of Pb and Zn were relatively low,ranging from70%to 90%.As the temperature increased,the residual rates of heavy metals decreased,especially for Cu and Cd.The residual rates of heavy metals showed different trend under different atmosphere.The residual rates of Ni,Pb,Zn and Cd in 80%CO₂/20%O₂ atmosphere were higher than those in 80%N2/20%O₂ atmosphere,but Cu and Cr were the opposite.With the increase of O₂ concentration,the content of Ni in ash decreased,while the content of Cu and Pb increased.When O2 concentration was 30%,the concentrationsof Zn was the smallest,the concentrations of Cr and Cd content was the highest.Temperature had a significant effect on the residual rates of all heavy metals,while atmosphere only had a significant effect on Cd.The chemical speciation of heavy metals in sludge and ash was investigated by improved BCR sequential extraction method.The results showed that a great amount of heavy metals in sludge distributed in mobile fractions.Compare with MS,the residual fraction of the heavy metals in the ash increased significantly,while the proportion of acid-soluble fraction and reducible fraction decreased.The mobile fractions were transferred to the stable fractions and the ecological risk of heavy metals were relieved.With the increase of temperature,the residual fraction of heavy metal in ash increased.Substituting CO₂ for N₂ at the same O₂concentration reduced the bioavailable fraction of Ni,Pb,Zn and Cu.However,Cd showed opposite change rule.The effect of O₂ concentration on the transformation behavior of these heavy metals was more obvious under low temperature combustion conditions.In conclusion,the combustion performance of sludge was improved with increasing oxygen concentration.After combustion,heavy metals enriched in the ash.Temperature has a significant effect on the migration and transformation of heavy metals.Atmosphere showed different influence on different heavy metals.