Study On Modeling Of Coal Pyrolysis And Gasification Process

In our country, coal is the main primary energy, which is difficult to change in a short time. Efficient and clean coal conversion technologies are the inevitable choices to meet our energy needs, reduce environmental pollution and realize the energy saving and emission reduction targets in China. Gasification is an important development direction of clean coal technologies and pulverized coal pressurized dense transport bed gasifier is one kind of gasification technologies that applicable for coal of low rank and large-scale direct gasification. Experiments and modeling were carried out for analyzing pyrolysis and gasification processes in the gasifier, which can provide some references for experimental data analysis and design of gasifier.In this paper, researches on pyrolysis characteristics of Zhundong coal, Shenmu coal, Datong coal and used oak segments were carried out using Thermax500thermal gravimetric analyzer. The effects of heating rate, coal type, reaction atmosphere and pressure on coal pyrolysis reaction were analyzed. Transport bed gasification block models were established, analyzing the energy balance during coal pyrolysis and the impact of design and operation parameters on carbon conversion and gas components. The main contents and conclusions of the paper are as follows:1. Research on coal pyrolysis and kineticsTGA experiments of Zhundong coal, Shenmu coal, Datong coal, and used oak segments under different reaction conditions were conducted. Using distributed activation energy model, the main kinetic parameters of pyrolysis process were solved. Energy balance during pyrolysis process was discussed by the pyrolysis products calculation model. The results show that, the pyrolysis process of three kinds of coal and used oak segments can be divided into transition stage, depolymerization and decomposition reaction stage and polycondensation reaction stage. CO atmosphere has a role in inhibiting pyrolysis reaction when temperature is greater than700℃, whereas H2atmosphere has a role in promoting pyrolysis reaction when temperature is greater than550℃. Below450℃, increasing pressure can promote cleavage reaction. Above550℃, increasing pressure can inhibit precipitation of volatile and secondary cleavage reaction. The activation energy distribution of different coals is inconsistent within the range of200～400kJ/mol, which is slightly higher than used oak segments. The endothermic pyrolysis process of coal is mainly to provide sensible heat of the reactants or products.2. Simplified mathematical model of transport bed gasifierCoal gasification was divided into three parts, including coal pyrolysis, char combustion and char gasification. The gasifier reactor model was established combining fluid dynamics model and chemical reaction kinetics equation. The model parameters were checked using experimental data.3. Study on influences of transport bed gasification process operating parametersUsing the above model, the influences of gasifier operating parameters were analyzed. The results show that, under different thermal conditions of the gasifier, in order to maintain the same operating temperature, the air flowrate increases with the increase of the amount of heat release. As the excess air ratio increases from0.089up to0.445, carbon conversion rate is improved by31.80%. When the amount of heat release is constant, carbon conversion rate is improved with the increase of air flow rate or air temperature. The carbon conversion rate of gasifier can be improved by increasing the amount of steam and the riser height. But when the riser height is up to a certain extent, it has little effect on the carbon conversion.