Dechlorination And Its Effect On Dioxins Mitigation During MSW Pyro-gasification And Exergetic Life Cycle Assessment

With the rapid growth in municipal solid waste?MSW?generation and the leading energy,environmental,and social consequences,MSW treatment has become one of the most severe challenges of morden society.Thermal treatment can achieve effectively mass and volume reduction of MSW and at the same time energy recovery.However,due to the presence of chlorine in MSW,the formation of HCl during thermal conversion causes severe problems,such as corrosion,toxic organic contaminants formation,and acidification.Currently in MSW incineration plants,HCl is removed together with other acid gases at temperature lower than 250^oC,which means under such condition the role of HCl as a corrosive agent and Cl source for dioxins formation has already been performed.Therefore,it is of great importance to remove HCl at high temperature conditions.Novel MSW thermal treatment methods on basis of pyrolysis and gasification technologies offer the possibility to achieve high temperature HCl removal,energy efficient and environmental-firendly conversion due to the“two-step”conversion configuration.In view of the facts,the goal of the present study was to investigate the effect of in-furnace CaO addition on MSW conversion characteristics and pollutants?especially HCl and dioxins?mitigation.Besides,life cycle assessment?LCA?and exergetic LCA of MSW incineration-and gasification-based systems were thoroughly evaluated to compare the environemental impact,energy conversion efficiency and thermodynamic perfection of the systems.The obtained results can be served as theoretical basis and practical experience to facilitate the development of clean and efficient MSW management systems.In terms of thermochemical conversion characteristics during MSW pyrolysis and gasification,effects of reaction temperature,equivalence air ratio?ER?and in-furnace CaO addition were experimentally investigated in a fixed bed reactor and a fluidized bed furnace.Results reveal that combustible gases and H₂ yield are significantly increased by increasing the reaction temperature.Increasing ER promotes the yield of syngas;but at high ER conditions the quality of syngas is declined due to excessive oxidation.The in-furnace addition of CaO accelerates the catalytic thermal cracking of tar compounds,thus increase combustible syngas yield.Meanwhile,CaO can be served as a CO₂ absorber in the furnace and shifts the water-gas shift reaction to promote H₂ yield and enhance the quality of generated syngas.In terms of pollutants control during pyrolysis and gasification,the effect of in-furnace CaO addition of HCl mitigation was firstly studied.Kinetic analysis show that the addition of CaO decrease the apparent activation energy of PVC pyrolysis reaction from 197.3 kJ/mol to 148.9kJ/mol;however,when focusing on the separated de-HCl reaction,the apparent activation energy is increased from 152.9 kJ/mol to 162.6 kJ/mol,indicating that in-furnace addition of CaO inhibited the release of HCl.Increasing the amount of CaO and decreasing its particle size can enhance the mitigation efficiency of HCl during pyrolysis.HCl mitigation efficiency by CaO additive is significantly inhibited when increasing the reaction temperature.When the reaction temperature is increased from 550°C to 850°C,HCl mitigation efficiency decreases continuously from 92.4%to 72.4%.It is mainly attributed to the shift of the chemical reaction equilibrium between CaO and HCl at higher temperature;and meanwhile,CaO additive and the de-HCl products on CaO surface is sintered at high temperature,causing the reduction of specific surface area and pore volume and influence the mitigation efficiency.The optimum dechlorination temperature window obtained under the experimental conditions is at temperature lower than 750°C,and the Ca/Cl molar ratioat 2:1.The formation characteristics of dioxins during MSW pyrolysis and incineration were investigated.Results reveal that the total amount of dioxins and corresponding toxicity equivalent generated during pyrolysis is significantly lower than that of incineration.The formation of dioxins is significantly inhibited at reducing atmosphere.The reasons are mainly attributed to two folds:one the one hand,anaerobic environment during pyrolysis inhibits the de novo synthesis process of dioxins;on the other hand,the H₂-rich atmosphere can inhibit the formation of C-Cl bonds in pyrolytic products and at the same time,the generated C-Cl bonds is destroyed by H₂ to form C-Cl bonds,which significantly reduces Cl-containing precursor for dioxin formation process.Effect of in-furnace CaO dechlorination on the formation of dioxins during pyrolysis is also considered.The in-furnace addition of CaO during MSW pyrolysis reduces the total amount of dioxins and the corresponding toxicity equivalent.Under the experimental conditions,mass concentration reduction efficiency and toxic equivalent reduction efficiency of dioxins caused by in-furnace CaO are in the range of 22.29%-34.93%and 20.48%-37.74%,respectively.Compared to the working condition of incineration at 850°C,the mass concentration reduction efficiency and toxic equivalent reduction efficiency of dioxins caused by in-furnace CaO addition are as high as 95%,which indicates that the use of in-furnace CaO can significantly reduce the formation of dioxins during MSW pyro-gasification.In order to comprehensively and systematically evaluate the overall operation performance of MSW management systems and to figure out the optimization methods,life cycle assessment?LCA?and exergetic LCA of incineration-and gasification-based MSW thermal treatemtn systems were also evaluated.Results from LCA show that compared to MSW direct incineration,gasification-based technologies reduce the environmental impacts.The reasons are mainly due to the higher energy conversion efficiency?which means higher electricity generation?and lower pollutants emission form gasification-based“two-steps”conversion systems.With respect to exergetic LCA,results reveal that gasification-based hot syngas cleaning system and in-furnace HCl mitigation system lead to higher energetic efficiencies.