The Migration Of Heavy Metals During The Hydrothermal Carbonization And Combustion Of Waste PVC

Hydrothermal carbonization?HTC?technology is a novel upgrading technology for fuel quality.Hydrochar,solid phase product of hydrothermal carbonization,shows the advantages of high carbon content,high energy density,hydrophobicity,uniformity and brittleness,which is in favor of the combustion organization.Thus,HTC has a bright future in the field of waste incineration power generation.As the classification of MSW?Munuciple Solid Waste?in China is not yet popular,the composition of MSW is complex and variable,and the content of heavy metals is high.Heavy metals have extremely high physiological toxicity and can be enriched in the human body through the food chain,causing chronic poisoning,seriously threatens human health.After hydrothermal carbonization,heavy metals may dissolve in hydrothermal solution,and may also enrich in hydrochar.The direct discharge of hydrothermal solution which contains high concentrations of heavy metals or the combustion utilization of hydrochar which contains high concentrations of heavy metals may cause serious heavy metal contamination.Therefore,it is of great significance to find out the fate of heavy metal during HTC process and hydrochar combustion.First,PVC powder was hydrothermally carbonized at 200°C,230°C and 260°C using a high pressure reactor to study the efficiency of PVC hydrothermal carbonization and the distribution characteristics of heavy metals.The results show that after HTC process,the contents of volatile decrease,the fixed carbon contents and calorific values increase,O/C–H/C is close to that of sub-bituminous coal,which indicates the fuel quality is improved greatly.After HTC treatment,the contents of Cd,Pb and Zn in hydrochar decrease to some extent,but the contents of Cd,Cr,Ni and Pb are still far higher than the contents of corresponding heavy metals in typical MSW.Once the HTC temperature exceeds 230°C,the contents of Cd,Cr,Ni and Pb in HTC solution is relatively high,showing a risk of heavy metal pollution.Then,the thermogravimetric and product characteristics of PVC and its hydrochar were investigated using TGA-FTIR,and the activation energy required for the combustion was calculated by FWO?Flynn-Wall-Ozawa?method.The results show that the DTG curve of PVC combustion contains three peaks,and as HTC temperature increases,the peak number of DTG curve decreases.The combustion characteristic indexs of hydrochars are slightly lower than that of PVC,but still higher than typical lignite.The main combustion products of PVC and hydrochar are H₂O,HCl and CO₂.The absorption peak of HCl appears first,while the absorption peak of H₂O and CO₂ appear synchronously.The activation energies required for hdyrochars combustion are much lower than that of PVC,indicating that HTC pretreatment can greatly increase the reactivity of PVC.The lowest average activation energy is achieved during the combustion of HC200,which is 98.8 kJ/mol.Next,the combustion experiments of PVC and hydrochars were carried out at 600°C,700°C,800°C and 900°C under an O₂/N₂ atmosphere using an electric heating tubular furnace.The heavy metals in the bottom ash were determined for studying the migration of heavy metals during combustion.The results show that the combustion temperature has no significant effect on the volatilization rates of Cr,Cd,Ni,Pb and Zn during PVC combustion,while the volatilization rates of Cr,Ni and Zn increase with the increase of combustion temperature during hydrochars combustion.The effects of HTC treatment on the volatilization rates of Ni are greatest,followed by Zn,Cr,Cd and Pb.HTC temperature has little effect on the volatilization rate of Cr and Ni,whereas the effects of HTC temperature on the volatilization rate of Cd,Pb and Zn get greater with the increase of HTC temperature.At600°C,the volatilization rates of Cd,Pb and Zn gradually decrease with the increase of HTC temperature.Finally,the combustion experiments of PVC and hydrochars were carried out at 600°C,700°C,800°C and 900°C under an O₂/CO₂ atmosphere using an electric tubular furnace.The heavy metals in the bottom ash were determined.The results show that in O₂/CO₂ atmosphere,the sequence of sensitivity of heavy metal volatilization rate to combustion temperature is:Zn>Ni>Cr>Cd=Pb;the sequence of sensitivity to HTC treatment is:Ni>Zn>Cr>Cd>Pb.HTC temperature has significant effects on the volatilization rates of Cd,Pb and Zn,but has little effect on the volatilization rates of Cr and Ni.Compared with PVC,the volatilization rate of heavy metals in hydrochars is more affected by the combustion atmosphere,and the higher the HTC temperature and combustion temperature,the greater effects of the combustion atmosphere.