Study On Pyrolysis And Combustion Characteristics Of MSW And Incineration Experiment With Co-current Shaft Furnace

With the speeding-up of industrialization and urbanization process of the human society, the quantity of municipal solid waste (MSW) has increased significantly and the quanlity has become complex increasingly. MSW has been one of the envirornmental hazards in every country and an urgent envirornmental protection issues to be resolved. Currently, the MSW disposals widely adopted include landfill, compost and incineration, among which, incineration has become one of the main disposal because of its advantages of large volume reduction and resource utilization. Due to the imperfect reactor design and process operation, however, secondary pollutions, such as dioxin and heavy metals, are generated during incineration. Then the application of incineration technique is restricted. Therefore, based on deep understanding of MSW pyrolysis and combustion mechanism, research on new MSW technologies have been the subject concerned in the environmental technologydevement all over the world.The characteristics of MSW and the problems in its incineration were anlysized comprehensively in this work. The application and development on direct gasification and melting incineration technology of MSW were discussed and the characteristics of MSW pyrolysis and combustion were summaried. The experiment technique of TG-DSC thermal analysis combined with mass spectrometry (MS) were used to investigate the mass loss, thermal effect and gas evolution from pyrolysis and combustion of MSW. The combination of thremogravimetry kinetics and calorific kinetics was used to study the pyrolysis and combustion mechanism and reaction kinetics. A new direct gasification and melting incineration process with co-current shaft furnace was proposed and simulation experiment of the co-current shaft furnace had been conducted, arriving at the following results.1. The pyrolysis mechanism and reaction kinetics. There is an endothermic softening stage before the pyrolysis of high-polymer organic MSW such as plastic and rubber. The pyrolysis process of such substances occurs in one stage. The evolved gas generated from pyrolysis comprises mainly alkane and olefin. There is an endothermic effect in plastic pyrolysis and an exothermic effect for rubber. For the biomass substances of MSW, including wood, leaves, vegetable and cotton, the pyrolysis process occurs in three stages. During the first stage, there is a minute mass loss due to water vapour evolution from the sample. After that, there is a rapid mass loss owing to the decompositin of cellulose, and the gas product comprises mainly H2O, CO2, C2H2, C2H4and CH4. Finally, there is a slow mass loss due to the lignin transformatio into char, and the gas product is mainly CO2. There are endothermic effects for cotton and paper and exothermic effects for woods, leaves and vegetable. The magnitude order of pyrolysis activation energy from big to small is wood, leaves, rubber, paper, vegetable, cotton, and plastic.The relationship between pyrolysis activation energy and thermal effect obeys the Evans-Palanyi formula. The activation energy of exothermic reaction is usually smaller than that of endothermic reaction.2. The combustion mechanism and reactions kinetics. The mechanism and pathway of combustion reaction were examined comprehensively through analyzing the curves of TG, DSC and MS. The calorific kinetics was applied to resolve combustion kinetic parameters of MSW samples. For the high polymer organic compounds such as plastic and rubber, the ignition temperature is high and the combustion is rapid. For the biomass MSW such as wood, leaves, vegetable, cotton and paper, the combustion reaction proceeds in three stages as dehydration, rapid combustion and slow combustion. During the rapid combustion, the samples are pyrolyzed and evolve volatile gases, then the volatile gases are oxidaed continuously. Subsequently, slow combustion occurs, and its burning rate is lower than the rapid combustion. The slow combustion is the oxidation of intermediate products and char, and thereby a lot of heat is released. The index P for combustion characterics of cotton and plastic is higher than the others, so they have better combustion properties.The combustion properties of wood is the worst in all samples. For the high polymer organic compounds, the activation energy of plastic is higher than that of rubber. For the biomass MSW, the activation energy of cotton is higher than those of wood, leaves, vegetable and paper. There are some discrepancies between thremogravimetry kinetic parameters and calorific kinetic parameters,and these two approaches can be used in complements. 3. The new direct gasification and melting incineration process route with co-current shaft furnace. In this process, MSW and combustion air are in co-current from top to bottom in a shaft furnace. Combustible gas from pyrolysis burns completely in the bottom in order to offer energy for slag melting. The aim of the technology is to reduce or even eliminate the ues of supplementary fuel so as to realize self-heating incineration and simplify the process and equipment. For the direct gasification and melting incineration process with co-current shaft furnace, both mass balances and heat balance equations are established, and critical heat value and adiabatic flame temperature meeting the processs requirement are calculated. In the same condition, comparative study on traditional count-current shaft furnace shows that MSW critical moisture of count-current shaft furnace restricts moisture severely. MSW critical heat value of co-current shaft furnace is much lower than that of count-current shaft furnace. The difference between them is about5800kJ-kg-1. The simulation experiment of the co-current shaft furnace has been carried out on an experimental furnace. The results of simulation show that the temperature under the condition of co-current is much higher than that of countercurrent in the reaction tube. In the countcurrent process, there are tar, more CO and combustible hydrocarbon gas discharged from reaction tube. In the co-current experiment, the combustible gases and tar from pyrolysis and gasification are burned completely and consequently more CO2are generated.The co-current shaft furnace can realize drying, pyrolysis, gasification and combustion of fuel gas and char to take place within the same reactor. So the co-current shaft furnace is more suitable for direct gasification and melting incineration process. There may be some advantages of co-current shaft furnace process to disposal MSW of lower heat value, and the process is more suitable to disposal domestic MSW. This may provide theoretical and technological support for the innovation of MSW incineration technologies.