Study On Chlorine Migration And Transformation During Chemical Looping Co-gasification Of Sludge And Waste Plastics

With the rapid growth of China’s population,the production and use of plastics are more and more frequent,resulting in a huge output of waste plastics.At the same time,due to the development of industry,the output of sludge is also increasing."White pollution"and"sludge besieged city"have brought serious threats to the environment.Based on the criteria of recycling and harmless treatment of waste plastics and sludge,Chemical Looping Gasification（CLG）is adopted for treatment.Iron ore oxygen carrier is selected as the basic oxygen carrier,and the three alkali（earth）metal ions（Ca,K,Na）are used to load the carrier,which can not only enhance the activity of the oxygen carrier,but also be used to remove HCl gas generated in the chemical looping gasification process of PVC plastics;because the sludge contains a lot of alkali（earth）metal elements,this paper uses oxygen carriers loaded with sludge and alkali（earth）metal to remove the HCl gas produced in the chemical looping gasification process of PVC plastics,and then inhibit the formation of dioxin.At present,the research on chemical looping gasification of waste plastics and sludge and chlorine containing pollutants is relatively less.In this paper,the mechanism of chlorine conversion and removal in chemical looping gasification of waste plastics and sludge will be studied.Firstly,HSC thermodynamics software was used to screen oxygen carriers and alkali（earth）metal ions.According to the thermodynamic simulation,it can be seen that chlorine will mainly exist in the form of HCl gas in the temperature range of 600～1000℃.It was found that iron ore oxygen carriers as basic oxygen carriers will not consume HCl in the process of experiment,and oxygen carriers loaded with Ca,K and Na ions can react with HCl gas to produce stable chloride.At the same time,according to the thermodynamic equilibrium constant,the reaction between the oxide formed by loading alkali metal ions and HCl will be more difficult with the increase of temperature.Secondly,in order to verify the results of thermodynamic simulation and the feasibility of sludge dechlorination,the gas products of waste PVC plastics and sludge pyrolysis gasification process were analyzed in TG-FTIR-MS,and the production and removal of HCl under different conditions were studied.Firstly,the pyrolysis of sludge and PVC plastics was analyzed.It was found that the main product of PVC plastics in the pyrolysis process was HCl gas,and the sludge did not produce HCl gas in the pyrolysis process.Three kinds of oxygen carriers were used to gasify PVC plastics.It was found that oxygen carriers provided lattice oxygen during the gasification process,which made PVC plastics pyrolyze to generate CO₂.At the same time,it was also found that HCl gas generated from PVC pyrolysis was completely removed,which verified the thermodynamic simulation results.Then,the dechlorination effect of sludge ash was discussed in TG-FTIR-MS and fixed bed,and it was found that the dechlorination of sludge ash was feasible when the sludge ash/PVC mass ratio reached 20:1,the dechlorination efficiency reaches 98.3%.The XRD analysis of sludge ash before and after the reaction shows that the dechlorination of sludge ash is mainly caused by alkali（earth）metal compounds,which confirms the previous conjecture.Finally,the mechanism of chlorine conversion and removal during chemical looping gasification of waste plastics and sludge was studied in a fixed bed.It was found that the dechlorination capacity of oxygen carriers decreased with the increase of temperature;the dechlorination efficiency of oxygen carriers increased with the increase of alkali（earth）metal ions loading;sludge also had a certain dechlorination capacity;in the process of chemical looping gasification,in the process of chemical looping gasification,the order of dechlorination efficiency of three oxygen carriers is Na>K>Ca.When the mass of oxygen carrier is 0.5g,The dechlorination efficiency of the oxygen carrier loaded with 15%Na ion is98.4%;when the mass of the oxygen carrier is 2g,the dechlorination efficiency of the three oxygen carriers is almost the same,which is more than 95%.At the same time,it is also found that the iron ore oxygen carrier can improve the carbon conversion of raw materials and promote the decomposition of tar and semi coke.After modification with alkali（earth）metal ions,not only the carbon conversion of raw materials can be further improved,but also the removal of HCl gas has a good effect.The experiment confirmed that the sludge has a certain dechlorination ability.The main reason is that the alkali（earth）metal compounds contained in the sludge react with HCl gas to form stable chloride to realize dechlorination.According to the formation principle of dioxin,the inhibition mechanism of dioxin by sludge and oxygen carrier is deduced.