| With the rapid economy development and accelerated urbanization, more and more municipal solid waste (MSW) is produced in China. However, because of the low disposal capacity and poor management policy, a large amount of construction waste, electronic waste and household waste are stacked in the surrounding of many cities, not only taking up a lot of land, but also releasing many smelly and toxic gases into atmosphere. Moreover, some solid waste contains much content of heavy metals, such as mercury, lead, zinc et al., some of which can be leached by rainwater and immerged into soil and groundwater, causing serious potential hazards for local residents. With the strengthening of people’s environmental protection consciousness, the treatment and disposal of MSW has increasingly become a serious problem. As is well-known, combustible components occupy a large part of MSW, for example sewage sludge, industrial leather residues, sawdust and the main components of household wastes including paper, plastic and wood. These combustible solid wastes are produced in a large amount and have a high heating value, therefore show a great potential in energy utilization. At present, incineration is an important approach in waste to energy, but the existing incineration technology can’t adapt to the Chinese combustible solid waste well, characterized with complex components and high content of moisture, and expose a series of problems such as low heat efficiency and serious secondary pollution of dioxin and heavy metals. Therefore, it is the significant step for implementing the clean and high-efficiency utilization of combustible solid waste that further strengthening the basic research on its thermochemical reaction properties and putting forward the suitable thermal treatment technology for the combustible solid waste in China.Based on the above analysis, this article firstly studied the thermal reaction properties of three kinds of RDF, which was made of local municipal solid wastes different on components, under inert and oxidizing atmospheres using thermoanalysis approach. The influences of component in RDF and reaction ambience on devolatilizing rate, reaction heat released or absorbed and characteristic parameter was studied, meanwhile reaction kinetics parameters under inert atmosphere were obtained through dynamics analysis method. In this paper, the linear weighted sum method was employed to retrieve mass fractions of key components in different RDFs through thermogravimetric (TG) analysis. A new Gaussian-fitting-based adjusting model is proposed to assess the effect of the interaction on the decomposition of individual components based on differential thermogravimetric (DTG) analysis, and using this model the changes of activation energy and reaction time of key components after mixed in RDF were quantitatively analyzed.In terms of thermochemical reaction mechanism, we studied the thermal conversion process of wood pellets under oxygen-controlled atmosphere in a tube-type fixed bed, focusing on revealing the law of heating rate and pore structure inside the wood pellet as well as mass loss rate. By capturing and processing the shape of single wood sphere under different pyrolysis environment instantaneously, the evolution of sample geometry was obtained and a shrinkage model was proposed in this paper. The proposed shrinkage model was coupled with one-dimensional unsteady wood pyrolysis model to predict the temperature profiles and mass variation as well as products distribution within wood spheres. One-step drying mechanism and three parallel primary decomposition reactions as well as three secondary cracking reactions were used to describe the entire pyrolysis process. By comparing with experimental results, the improved one-dimensional pyrolysis model coupled with the proposed shrinkage model could precisely predict the pyrolysis process of wood sphere, and the predicted temperature evolution and mass loss curves were in good agreement with experimental results. Moreover, the effects of volume shrinkage on temperature, weight loss as well as products distribution were analyzed and discussed.The influences of operation conditions on thermal conversion of combustible solid waste were also studied in a U-type gasifying/catalytic reaction system which consisted of a downdraft gasifier, a cyclone separator and a catalytic tar cracking bed. The two kinds of representative combustible solid wastes, artificial leather residue and municipal household waste, were used as the fuel of gasification. The influences of air/fuel equivalence ratio (ER), gasifying agent, catalyst, and reaction temperature on hydrogen yield, lower heat value (LHV) of produced gas, carbon and energy conversion efficiencies were experimentally studied. Results show that lower value of ER, using air/steam and calcined dolomite as gasifying agent and catalyst respectively can greatly enhance the energy conversion efficiency. For the artificial leather residue investigated in this study, the highest energy conversion efficiency 84.4% is obtained at the gasification temperature of 700℃ and the ER of 0.2 when air/steam and calcined dolomite are used as gasifying agent and catalyst. Experiment results for household waste show that ER plays an important role on produced gas components and LHV, and carbon and energy conversion efficiencies. Besides, the leaching behavior of heavy metals for bottom ash is greatly influenced by ER. By comparing the results of two wastes we find that the disposal approach of waste is determined according to its quality in some extent. High quality fuel is suitable for producing high quality syngas, which can be used as chemical materials or fuel gas for gas turbine/engine with a higher energy utilization efficiency, and low quality fuel can be disposed by direct gasification-combustion to provide thermal power.According to the obtained basic theory and reaction mechanism, a two-stage moving grate pilot system, consisting of a reciprocating grate and a rotating grate, was designed for thermal conversion of combustible solid waste in oxygen-controlled atmosphere. The thermal conversion method of waste fuel in furnace is a compound processes including primary gasification under oxygen-controlled atmosphere and secondary combustion. In this paper, a numerical simulation method based on zoning gasification model was firstly presented for the optimization of the reactor. This method coupled the software of chemical reaction kinetics and hydromechanics to predict the process of gasification in moving bed and combustion in gas space. In order to verify the simulated results and the obtained basic theory, experimental study on the thermal conversion process of two kinds of municipal household waste under oxygen-controlled atmosphere was carried out using this reactor. The law of produced gas from gasification and leaching behavior of bottom ash mostly agree with the results from the U-type fixed bed. For the low quality waste, furnace exit temperature is relative low and the dioxin concentration before flue gas cleaning system is high, therefore conventional flue gas purification device is still needed. |
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