Dechlorination Mechanism Of Pyrolysis Of Medical Waste And The Control Of Dioxins By Two-stage Incineration

Pyrolysis incineration technology has become the main disposal method of medical waste due to its high degree of harmlessness,good reduction effect and high energy recovery efficiency.However,the polyvinyl chloride（PVC）content in medical waste is high,and its internal chlorine element has a large proportion.The HCl generated during the thermochemical treatment process will not only cause high-temperature corrosion of the treatment equipment,but also provide chlorine source for the generation of dioxins.Therefore,in-depth research on the generation and control of HCl and dioxins in the pyrolysis incineration process of medical waste will help to improve the treatment process,enhance the efficient clean treatment and the energy recovery efficiency of medical waste.In this study,based on the pyrolysis incineration process,the characteristics of pyrolysis products of different components of medical waste were studied,and the prepared calcium-based catalysts were investigated for the removal of HCl during the pyrolysis process and the improvement of the efficiency of combustible gas in the syngas.Then the formation and control mechanism of dioxins caused by the chlorination and oxidation of polycyclic aromatic hydrocarbons in the pyrolysis incineration process of medical waste were deeply studied.And focused on the further removal of dioxins by high temperature incineration of biomass micro-fuel.Finally,a comprehensive evaluation of the environmental impact,energy consumption,cost analysis and social impact of medical waste treatment was carried out using the life cycle assessment（LCA）method,and the specific results were as follows:（1）For the characteristics of pyrolysis products of multi-component medical wastes,they were classified firstly according to their component,and the characteristics of gas-phase,liquid-phase and solid-phase pyrolysis products of different types of medical wastes were studied.The results showed that the pyrolysis products of fiber samples were mainly ketones,aldehydes and acids.For woody samples,cellulose and hemicellulose were the main substances that constitute the volatile components,and the weight loss rate of the samples was as high as 73.10%in the temperature range of 270～410°C.The precipitation amount of HCl in the polymer samples increased with the increase of temperature,and the precipitation amount was as high as 333.76 mg/g at 700°C.（2）Novel mesoporous catalysts(Ca₁₂Al₁₄O₃₃@m Ca O and Ca₁₂Al₁₄O₃₃@m Cu/Ca O)were successfully prepared,and the mechanism of catalysts for the removal of HCl and the improvement of the efficiency of combustible gas in the pyrolysis syngas were studied.Under the optimal reaction conditions,H₂could reach the highest stable value within 10min,the corresponding H₂yields of Ca₁₂Al₁₄O₃₃@m Ca O and Ca₁₂Al₁₄O₃₃@m Cu/Ca O were 59.01μmol/g/min and 74.51μmol/g/min,respectively.And in the first five cycles,the H₂yield did not decrease significantly,and the HCl removal rate was always above60%.（3）For the occurrence and competition mechanism of chlorination and oxidation of polycyclic aromatic hydrocarbons in the PVC pyrolysis incineration process,the generation and transformation of chlorine source in the PVC pyrolysis incineration process under the condition of O₂participation were studied.During the pyrolysis process of PVC,HCl was converted into active chlorine atoms（Cl-）in the presence of O₂,and Cl^-easily reacted with polycyclic aromatic hydrocarbons to form PCDD/Fs precursors.When the incineration temperature was 800～900°C and the peroxygen concentration was5%～10%,the chloride content was about 10%,and the pyrolysis incineration process was dominated by the chlorination reaction.When the peroxygen concentration was 10%～20%,the chloride content in the product droped to about 1%,and the increase of O₂inhibited the occurrence of the chlorination reaction.Under the condition of high temperature and high peroxygen concentration,the products of PVC pyrolysis incineration were mainly oxidative cracking reactions.When the temperature was 900～1100°C and the peroxygen concentration was 10%～20%,the chloride content was lower than 0.2%,and the high temperature strong oxidation inhibited the occurrence of the chlorination reaction.（4）For the PCDD/Fs generated during the thermal treatment of medical waste,the co-processing method of catalyst and two-stage incineration was used to study the formation and control mechanism of PCDD/Fs.The results showed that the PCDD/Fs removal rate in the process with Ca₁₂Al₁₄O₃₃@m Ca O was stable at 75%-90%.Secondly,using biomass micro-fuel as the second-stage heat source could provide a stable high temperature of 1000°C,and the secondary removal rate of PCDD/Fs was greater than90%.（5）The 3E+S model was constructed by using the LCA method.Based on the entropy weight method and the AHP coupled calculation system,the environmental impact,energy consumption,economic analysis and social impact of the medical waste treatment process were comprehensively evaluated.Among the three treatment modes of traditional incineration（scheme 1）,pyrolysis incineration（scheme 2）and pyrolysis two-stage incineration（scheme 3）,The addition of calcium oxide in scheme 3 had a good inhibitory effect on the production of acid gases and PCDD/Fs,and its HTP value was reduced by 69.60%compared with scheme 1.Iin scheme 3,biomass was used as a substitute for partial energy supply,the power consumption was only 48.77%of that in Scheme 1.Schemes 2 and 3 achieved high-value conversion of energy.The above experimental results are helpful for the improvement of the thermochemical treatment technology of medical waste,and provided theoretical guidance and data support for further improving the efficient cleaning treatment of medical waste and the efficiency of energy recovery and utilization.