| Along with urban construction development and improvement in standard of living, MSW generation is increasing yearly and represents an urgent problem that needs to be addressed. In China, the principal treatment of MSW is to landfill. However, in large cities and the eastern coastal areas where the economy is more developed with high population density there is limited land area available for landfills. Therefore, incineration will become a more suitable option to treat MSW in large cities.In recent years, MSW pyrolysis and combustion technology has become more and more popular. Compared with incineration, pyrolysis and combustion has the following advantages:1. High combustion efficiency, low emissions and compact. This is because MSW combustion is a gas-solid heterogeneous reaction with low diffusibility and miscibility. However, pyrolysis and combustion is a homogeneous process.2. Easily controlled combustion process. This is because MSW is a complicated mixture with many components. Furthermore, the characteristic of each component is very different from each other, with different rates of combustion that are hard to control. Consequently, pyrolysis and combustion is adopted to transform the multi-component MSW into a homogeneous fuel with good diffusibility. As a result, the process of combustion becomes more stable and easy to control.Thus far, worldwide there is not a deep understanding of the characteristics of MSW pyrolysis. This makes the design and operation of MSW pyrolysis furnace more difficult. This article chooses PE, PVC, wood, carbon copy paper, fabric and bicycle tires to simulate the four main components of MSW, which are plastic, biomass, fabric and rubber. In order to study the pyrolysis process within the pyrolysis and combustion technique, this study is divided into three different studies: mechanical research, basic research and application research. These three studies will provide a reference for the design and operation of MSW pyrolysis and combustion furnace.The first part: The mechanical research uses a thermal balance where the temperature increases linearly. The minimum sample mass (10 mg) is used for pyrolysis and the effects of heat and mass transfer can be ignored. The main conclusions are: 1. MSW has different components with different pyrolysis temperatures. The thermal balance temperature is controlled to be greater than the initial temperature of pyrolysis. The pyrolysis process does not occur continuously, but rather occurs within different temperature ranges. At a controlled temperature of 500℃, all of the MSW components will undergo pyrolysis transformation at levels of 70% or above. From 500℃till 700℃, a further 15% of the components will undergo pyrolysis. 2. As the different components combine during pyrolysis, individual components will affect the pyrolysis mixture. The combinations of plastic and biomass, plastic and rubber, biomass and water will be affected, but not significantly. 3. Either the integral method or the differential method is used to calculate the results, with the linear dependence relation used as an objective to maintain the dynamic parameter. The results of the two methods often do not agree. It is proposed to use the two methods together to find the activation energy number and to devise a function for the chemical reaction. 4. The mechanism of the MSW pyrolysis is usually limited by diffusion. That is to say, the diffusion of the volatile gases from within the MSW is the rate-limiting step in the MSW pyrolysis process. The diffusion process in the MSW pyrolysis industry is not limited by the diffusion of gas from the pyrolysis chamber to the combustion furnace, but is infact limited by the diffusion of gas from inside the MSW out into the pyrolysis chamber. Thus, for actual industrial operation, it is requested to break-up the MSW into small pieces and to continuously turn the material so that the gas can be transported from the pyrolysis chamber to the combustion furnace as soon as possible.The second part: For the basic pyrolysis research, the thermal balance is maintained at a constant temperature. A higher quantity of mass (～10 g) is used for pyrolysis and now the heat and mass transfer effects cannot be ignored. This situation is a closer approximation to real life pyrolysis conditions. The conclusions are: 1. The different MSW components and their mixtures will pyrolysize almost within the same time period, however the starting and ending time of pyrolysis will be different for each component. Amongst the components, PE is a special case. At the constant temperature of 500℃. it has the slowest start and end time for pyrolysis. 2. As the temperature of the thermal balance increases, the time needed for pyrolysis decreases, also the start time and end time of pyrolysis for the different components will be closer together. MSW pyrolysis usually occurs at temperatures lower than the thermal balance control temperature that was set. So,in situations where there is the different components, a temperature range of 500℃to 600℃is suggested. 3. When the different components in MSW are paired together, under most conditions the individual components will still affect the pairs. For research at 600℃, the mixed pyrolysis of biomass and fabric often exhibits retardation of pyrolysis start time or prolongation of the pyrolysis process. For mixed pyrolysis of plastic and biomass, the hydrocarbon in the plastic will decompose and react with the product of the biomass pyrolysis through oxidation which will accelerate the overall pyrolysis process. For the mixed pyrolysis of rubber and biomass, it will usually not be affected by the individual reactions of the components. The different pairings between PVC, rubber and fabric will also generally show retardation of pyrolysis start time or prolongation of the pyrolysis process. Besides, PVC obviously exhibits that when mixed with sawdust or when mixed with cotton/polyester blend fabric, that the pyrolysis process will be accelerated. 4. Between the temperature range of 500℃-600℃, for the same quantity of water the time when vaporization begins does not differ greatly. However at 850℃, vaporization of water to steam occurs much earlier. From an industrial perspective, the technology of rotary kiln and fluidized bed is used together, which will help the vaporization process as opposed to only using the fluidized bed for the high moisture content MSW. As a result the combustion process becomes more stable. 5. The mixed pyrolysis of different MSW components with water will be affected by interactions with the individual components. In particular, for the mixed pyrolysis of plastic with water or rubber with water, the time required for the complete pyrolysis will be the same as the time required for the pyrolysis or vaporization of the individual components. However, for the mixed pyrolysis of biomass and water or fabric and water, the time required for the complete pyrolysis will be longer than the time required for the pyrolysis or vaporization of the individual components. 6. The thermogravimetric analysis under conditions of slow linear increase in temperature and under constant temperature are complementary within the research of MSW pyrolysis and are irreplaceable for every aspect of this research field. From the study of slow linear increase in temperature, it can be observed at what temperature ranges the pyrolysis transformation will begin and end. For example, what is the starting temperature of pyrolysis for different MSW components? At what temperature ranges will heat diffusion occur? What is the speed of pyrolysis? This study encompasses microscopic and mechanical aspects. It is one kind of scientific experimental method and the conditions of this method do not exist in actual industrial operation. For the study under constant temperature, the duration of pyrolysis can be observed. As well the constant temperature study is closer to the actual pyrolysis conditions. By performing the first study of slow linear increase in temperature, the temperature range required for the second study of constant temperature can be decided, reducing uncertainty. The second study complements the first study by reducing the limitations with regards to the time aspect. At the same time, the experimental results will be beneficial to actual operations.The third part: The application research uses a continuously fed rotary kiln furnace for pyrolysis research. A much greater sample mass was used for pyrolysis (1kg/h) which is greatly affected by heat and mass transfer. The experimental research pyrolysis in this third part is similar to conditions of actual pyrolysis. The main conclusions are: 1. The products of pyrolysis are differentiated into solid, liquid and gas. With the exception of PE pyrolysis at 500℃, the other different components will have similar pyrolysis transformation process to the products (gas and tar) when the same quantity of sample is pyrolysized between 500℃to 700℃. For small sized material (for example flake film) there will be a high amount gaseous product from pyrolysis. On the other hand, for larger sized material, the amount of tar produced will be greater. 2. When comparing the heat value before and after pyrolysis: for plastic and rubber under conditions of no oxygen or oxygen lean, an endothermic reaction will occur during the pyrolysis process resulting in an increase in heating value after pyrolysis (except for PE at 500℃). The reason for the increase in heating value is because the heat is transformed into the pyrolysis products. The oxygen content in biomass and fabric is very high, in the pyrolysis process an exothermic reaction will occur, resulting in a lower total heating value after pyrolysis. The total heat value of the pyrolysis products minus the initial heat value of the feed material is the reaction heat of pyrolysis. 3. Energy consumption for pyrolysis: The energy consumption for the pyrolysis of MSW includes the heat loss from the pyrolysis apparatus, the reaction heat of pyrolysis and the heat of vaporization. When the reaction heat of pyrolysis is positive, the reaction heat will indeed be one part of the overall energy consumption. When the reaction heat of pyrolysis is negative, it is an exothermic reaction. In this situation, the reaction heat will be the only internal resource to supply the energy for pyrolysis. Under the condition when it is supposed that the individual MSW components do not chemically react with each other, a calculation model was built to calculate the energy consumption of pyrolysis. This model was designed based on an externally heated rotary kiln with length to diameter ratio of 4.36 and with a pyrolysis duration of 31.58 min. This model will be a good reference for the design of rotary kiln furnace for actual industrial application. In actual practice, in order to reduce the energy consumption of MSW pyrolysis, material with low heat capacity and good heat insulation will be used, the moisture content of MSW should be reduced and increase the proportion of high oxygen content biomass in the MSW.
|
PDF Full Text Request