Experimental Study On The Cooperative Control Of Dioxins And NO_x During MSW Gasification Gas Combustion Process

Dioxins will be produced in the process of municipal solid wastes?MSW?incineration and the cost of investment and operation for flue gas cleaning equipment is expensive.Fly ash from incineration also needs to be treated as hazardous waste for landfill.In order to control dioxins emission,an updraft gasification–ash melting–syngas homogenous conversion–clean combustion system is suggested in this paper based on the prophase research by our group.The system has some characteristics as follows:1.Ash could be converted to glass slag by melting technology,at the same time the heat energy could be supplied for gasification process.2.The bed of updraft gasifier has the effect to filter particles so that heterogeneous synthesis of dioxins can be controlled.3.Homogenous conversion can transform chlorinated hydrocarbons and‘active chlorine'into normal hydrocarbons and HCl so that gas phase synthesis of dioxins can be controlled.4.Combustion of gasification gas is a homogenous process which leads to better mixing and burnout than solid combustion.Based on this system,this research focuses on the formation of dioxins in the combustion process of gasification gas.Moreover,NO_x emission also needs to be considered so that the combustion technology which can control both dioxins and NO_x will be studied.The formation of dioxins and NO_x is related to the gasification gas components during gasification gas combustion process.In order to confirm the tar components,chlorine containing components and nitrogen containing components,experiments were carried out on an updraft gasifier–homogenous conversion reactor and a TG-MS analyzer by using different ratio of MSW model.The results showed that benzene and phenolic substances were the main tar components and HCl was the main chlorine containing component in gasification gas.The formation and conversion of NO_x were also considered.Finally,benzene and phenol were chosen as tar models as well as HCl and NH₃ were chosen as chlorine containing model and nitrogen containing model for further research.In the formation of C-Cl bonds,the key structure in dioxin molecule,‘active chlorine'plays an important role.In the process of gasification gas combustion,HCl will convert to‘active chlorine'by reacting with oxygen.In order to know the reaction conditions which can control the formation of‘active chlorine',experimental studies were conducted by using a homogenous tube flow reactor.The change rules of HCl converting to‘active chlorine'at different temperature and oxygen concentration was obtained.Oxygen can react with hydrocarbons so that oxidation reactions could compete with chlorination reactions.In order to explore the reaction conditions which can promote the oxidation reactions and control the chlorination reactions,in this research benzene and phenol were respectively used as tar model.Chlorine was used as‘active chlorine'.Experimental studies were conducted by using a homogenous tube flow reactor.FTIR and GC-MS were used to detect the products.The formation regulation of C-Cl bonds was obtained.The competition mechanism between oxidation reactions and chlorination reactions was revealed.The conversion routes of benzene reacting with oxygen and chlorine was given.As for the conversion of benzene,there are three main routes:chlorination reactions,oxidation reactions and polymerization reactions.Under low temperature and low oxygen concentration conditions,chlorination reactions dominate and C-Cl bonds would be easily formed.Under low temperature and high oxygen concentration conditions,oxidation cracking reactions would happen and small molecule chlorinated hydrocarbons would be formed.Under high temperature and low oxygen concentration conditions,C-Cl bonds would be broken and polymerization reactions would happen.Under high temperature and high oxygen concentration conditions,hydrocarbons would be oxidized to CO₂ and C-Cl bonds could be controlled.According to the research results above,in order to realize the combination control of both dioxins and NO_x,air staged combustion was suggested to control both dioxins and NO_x at the same time in this paper.On the one hand,using the lean oxygen zone in the air staged combustion process can control NO_x formation.On the other hand,using the peroxide oxygen zone in the air staged combustion process can control dioxins formation.In order to ensure the reaction conditions for the combination control of both dioxins and NO_x,an air staged reactor was established and mixture gas was used to carry out the experiments.The influence rules of NH₃ and tar model on NO formation under different temperature and air distribution was obtained.Considering the results above,model compounds of municipal solid wastes was used as fuels in this research.An updraft gasification–homogenous conversion–air staged combustion reactor was established to carry out the experiments.The influence rules of air distribution,temperature,residence time and homogenous conversion on the formation of dioxins were obtained.The effect of air staged combustion on the cooperative control of both dioxins and NO_x has been confirmed and it provides reference for engineering application.