Pyrolysis Characteristics Study Of Coal-calcium Pellets

Coal is the main primary energy resource in China,and its reserves are abundant? However,most of them are low-rank coal.Clean and efficient utilization of low-rank coal is of great significance to the sustainable development of energy in China.As an innovative technology,“Regenerative Calcium Carbide Production Process” was proposed for clean and efficient utilization of low-rank coal.This technology takes pulverized medium to low rank raw coal and raw lime as raw materials that can form pellets.Then the pellets were sent into sealed calcium carbide furnace after pyrolysis in preheating furnace to obtain calcium carbide and pyrolysis oil and gas at the same time.The pyrolysis process of pellets is the key step of the technology.A comprehensive analysis of the influence of lime addition,pellet properties and operation conditions on pellet pyrolysis process plays an important role in comprehensively understanding pellet pyrolysis process and optimizing this new technology.Firstly,the thermogravimetric experiments and basic experiments of pellet pyrolysis in fixed bed were carried out.The effects of calcium oxide,heating rate,pyrolysis temperature and pyrolysis time on the pyrolysis process of pulverized coal were investigated.The experimental data were provided for the establishment of pyrolysis kinetics and the comprehensive mathematical model of pellet pyrolysis process.Then,based on the thermogravimetric experiments data,the applicability of Kissinger's kinetic model,Coast-Redfern model and distributed activation energy model(DAEM)to pellet pyrolysis process was compared with the experimental data from thermogravimetric analysis.The double distributed activation energy model based on genetic algorithm is finally chosed to establish the pyrolysis kinetics.The results show that the double distributed activation energy model based on genetic algorithm can accurately describe the dynamic process of pellet pyrolysis.By comparing the pyrolysis kinetics,it is found that the addition of calcium oxide can reduce the activation energy of pulverized coal pyrolysis and increase the reaction rate of coal pyrolysis.Finally,a comprehensive mathematical model of pellet pyrolysis process was established by coupling the flow,heat transfer and mass transfer models and the pyrolysis kinetics for a single pellet with bituminous coal.The model was validated by the experimental data from the literature.Then based on basic experiments of pellet pyrolysis in fixed bed for coal-calcium pellets,a mathematical model that is suitable for the pyrolysis process of coal-calcium pellets was established.The model was further applied to investigate the pellet properties(including pellet size,pellet shape and pellet porosity)and pyrolysis conditions(including pyrolysis temperature and nitrogen purge rate)on the devolatilization process of coal-calcium pellets.It was found that the effect of nitrogen purge rate on the devolatilization process of coal and quicklime mixed pellets is greater than that of pyrolysis temperature.With respect to pellet properties,it was found that the pellet shape has largest influence on the devolatilization process,followed by pellet size and pellet porosity.The optimum size and shape of the pellet was determined to be " a = 40?b = 29?c = 20 mm" of ellipsoidal pellet.