Study On Chlorine Migration And Control During Waste Pyrolysis And Gasification And Life Cycle Sustainability Assessment Methodology

As the amount of municipal solid waste?MSW?increases rapidly in China,the safe disposal of MSW faces serious challenges.MSW incineration technology,with features of harmlessness,large volume reduction and energy recovery,becomes a key part of waste management system.However,due to the high content of chlorine in MSW,HCl released in the operation of MSW incineration.The incineration plant mainly removes HCl by spraying the calcium-based absorbent into the deacidification tower,but it cannot avoid the high-temperature corrosion and dioxin formation caused by HCl.Through the"two-step"conversion technology,the syngas is produced from MSW pyrolysis and gasification and is sent to high-efficiency power generation equipment such as gas turbine or gas engine for efficient and clean MSW utilization.Thus,the calcium-based absorbent can be considered as a dechlorination additive in the furnace during MSW pyrolysis and gasification.In addition,sustainable development has become a requirement for various fields,which has requirements for environmental,economic and social performance.A comprehensive assessment method is needed to evaluate the sustainable performance of the waste treatment system.In view of these,this thesis studies the conversion characteristics of chlorine during the waste thermal conversion,proposes that the calcium-based additives are taken as in-furnace additives to remove HCl during waste pyrolysis and gasification,and study the effect of the calcium-based additives on the emission of chlorinated contaminants,providing a theoretical and technical basis for efficient and clean MSW utilization;and establishes a life cycle sustainability assessment method to comprehensively evaluate the waste treatment systems.Firstly,the conversion characteristics of chlorine in the thermal conversion process of different chlorine-containing components are studied.Result shows that the chlorine from NaCl exists mainly in the form of NaCl during the thermal conversion process,and the chlorine content in the gas phase product is less than 3%at 850 ^oC.The chlorine from PVC exists mainly in the form of HCl during the thermal conversion process,and more than 95%of the chlorine is transferred to the gas phase product at 400 ^oC.Therefore,the organic chlorine source PVC is the main source of HCl during the waste thermal conversion.The effect of calcium-based additives on HCl emission during waste pyrolysis is studied,the distribution of chlorine in product is analyzed,and the mechanism is discussed.Result shows that adding CaO in the furnace can decrease the HCl emission and the HCl removal efficiency is above85%.This thesis studies the effects of temperature,Ca/Cl molar ratio,particle size and types on the HCl removal efficiency.Results find the suitable parameters for adding calcium-based additives in the furnace are the temperature lower than 772 ^oC?CaCl₂ melting temperature?under normal pressure,the Ca/Cl molar ratio 2:1 and the particle size needs to be selected according to furnace types.Adding CaO,calcined dolomite,or Ni/CaO can increase the apparent activation energy of the de-HCl reaction during the PVC pyrolysis from 90.3 kJ/mol to 143.6 kJ/mol,155.3kJ/mol,155.8 kJ/mol,and the reaction is random scission model;the experimental results show that the HCl removal efficiency by adding calcined dolomite or Ni/CaO is higher than that by adding CaO and the HCl removal principle is obtained by thermodynamic simulation and bottom ash analysis.The effect of calcium-based additives on the chlorine distribution during simulated MSW pyrolysis,gasification and incineration is studied by the experiment in moving bed reactor with continuous feeding device.The dechlorination performance of calcium-based additives during pyrolysis and gasification was better than that during incineration,indicating the reducing atmosphere is good for the calcium-based additive to remove HCl.When the reaction material changed from PVC to simulated MSW,the chlorine content in tar from pyrolysis is increased.Calcium-based additives can effectively reduce the chlorine content in tar?efficiency above 67%?,and calcined dolomite and Ni/CaO perform better than CaO because their effect on tar cracking.The effect of calcium-based additives on the product during simulated MSW pyrolysis in moving bed reactor is studied.Result shows that adding calcium-based additives in the furnace can significantly increase the yield of pyrolysis gas and improve the quality of gas;the mass ratio of tar is reduced,and the proportion of heavy polycyclic aromatic hydrocarbons is reduced;calcined dolomite and Ni/CaO performs better than CaO.The chlorobenzene?CBz?in tar was also tested.The yield of TrCBz was significantly higher than that of TeCBz,PCBz and HCBz.After adding CaO,calcined dolomite and Ni/CaO,the CBz yield was decreased by 39.3%,59.7%,and 66.4%,respectively.The characteristics of dioxin formation during simulated MSW pyrolysis,gasification,and incineration in moving bed reactor with continuous feeding device is studied,the dioxins removal by different calcium-based additives is analyzed,and the reaction mechanism and path of dioxins degradation at high temperature by quantum chemical calculation method is obtained.Without calcium-based additives,the dioxins yield under pyrolysis or gasification conditions is reduced by 79.0%and 82.1%compared to incineration conditions,and total toxicity is reduced by 76.7%and 77.1%,indicating that the reducing atmosphere can effectively inhibit dioxins generation.The addition of calcium-based additives in the furnace can effectively control the generation of dioxins;after adding CaO,the dioxins yield and total toxicity are reduced by 63.4%and 66.2%under incineration condition,43.4%and 36.7%under pyrolysis condition,48.6%and 37.7%under gasification condition respectively,especially the inhibition effect on dioxins formation in gas phase products and oil phase products;calcined dolomite and Ni/CaO have better inhibition effect on dioxins generation than CaO;it is mainly because the calcium-based additive effectively absorbs HCl gas,promotes the benzene ring breaking,reduces the generation of dioxin precursors,and promotes the generation of H₂,thereby effectively reducing the generation of dioxins.Thus,adding calcium-based additive in the furnace during MSW pyrolysis and gasification can effectively reduce the amount of dioxin emissions by 88.1%.Besides,quantum chemical calculation is used to analyze the reaction mechanism and path of the dioxins degradation under the reducing atmosphere at high temperature;as for the 2,3,7,8-T₄CDD molecule,the C-O bond and C-Cl bond are easier to break than the C-H bond;based on the transition state analysis of the high-temperature degradation reaction of 2,3,7,8-T₄CDD by H₂,the Gibbs Free Energy changes of the four reaction paths that H₂ attacking the C-Cl bond or the C-O bond are negative,and H₂attacking the C-Cl bond needs lower activation energy than attacking the C-O bond;H₂ attacking the C-O bond generally destroys at least two C-O bonds,in which the formation of two monobenzene ring products is easier than the formation of diphenyl ether products.The environmental performance evolution of Hangzhou's integrated MSW management system from 2007 to 2016 is dynamically analyzed by life cycle assessment?LCA?,and the effect of MSW management strategies on the environment is reflected.Because the environmental performance of MSW incineration is better than landfill with energy recovery,the environmental performance of integrated MSW management system increases with the incineration rate;the implementation of source-separated collection can improve the environmental performance of MSW incineration;therefore,The MSW management system in 2010 had best environmental performance.Anaerobic digestion for food waste has excellent environmental performance,but its capacity is too small;improving the source-separated collection efficiency and expanding the scale of anaerobic digestion can effectively optimize the environmental performance of the existing MSW management,and provide a reference for decision-makers in the future.A novel 3E+S model from a life cycle perspective is established for sustainability assessment.The model includes environmental,energy,economic,and social aspects;the results of the four aspects are analyzed by multi-criteria decision making;their weight factors are calculated by the integration of analytic hierarchy process and entropy weight method,and the final ranking is determined by Technique for Order Preference by Similarity to an Ideal Solution;sensitivity analysis is adopted to demonstrates strong robustness of the model.Based on the previous 3E model,social factors are added to reflect the social impacts of MSW treatment systems,and the calculation of weight factors is optimized to reduce man-made disturbances and have stronger robustness.The 3E+S model is used to evaluate the environmental performance,energy consumption,economic burden,social impacts,and sustainability performance of four waste treatment scenarios.Results show that incineration with fluidized bed furnace and incineration with moving grate furnace ranks the top two with a slight gap and perform much better than landfill scenarios.The application of the 3E+S model in the waste management system not only illustrates the practicability and scientificity of the model,but also evaluates the sustainable performance of different waste treatment technologies,providing a scientific and sustainable solution to decision-makers.