Thermodynamical Analysis Of The Gasification Process Of Biomass And Coal

Gasification is a promising way of utilization and conversion of coal and biomass, and is the key technology and important part of poly-generation system. The components and composition of the production gas affect the continuity and stabilization of the poly-generation system. As for a poly-generation system, the gasified material may be coal, petrolic coke, all kinds of organic matter or biomass, and each material has its own ultimate composition and volatile content. On the other hand, in order to ensure that the poly-generation system works continuously and stably, the components and composition of the crude production gas must be kept within a certain range. But due to the complexity of the materials and the gasification process, knowledge about the process of gasification and the effects of operating parameters on the fuel gas composition are far to be sufficient. In order to ensure the stabilization of the composition of the fuel gas, we need to adjust the operating parameters pertinently. And this will rely on the deep understanding of the gasification process of coal and biomass and the effects of operating parameters on the fuel gas composition, so that we can determine the optimal condition through the equations among operating parameters and the fuel gas composition.Been considered as the most effective way of investigating complex heterogeneous reaction system, thermodynamical equilibrium analysis could be applied investigate the effects of operating parameters, such as temperature, pressure and the amount of gasified agent added, on the fuel gas composition, and would be significant in both theoretic and realistic.Therefore, two typical materials (Jincheng Coal and Jiaocheng Corn Straw powder) with great differences in both ultimate composition and volatile content were chosen to be research objects. After a deep and comprehensive analysis of gasification processes of biomass and coal, this paper calculated biomass and coal gasification system based on the Gibbs-Free-Energy-Minimization method, using CHEMKIN software platform. And discussed the features of biomass gasification system and coal gasification system respectively, and then examined the gasification parameters, such as Temperature (T), Pressure (P), Equivalence Ratio (ER) and Steam/Material Ratio (S/M), on the equilibrium composition of gasification products. In view of maximum utilization of energy and elements, this paper mostly discussed optimization of gasification parameters. All these will help to better understanding of biomass and coal gasification process, and will help to adjusting gasification conditions relevantly to achieve a maximum utilization of energy and elements, and minimum emission of wastes and pollutants.Through thermodynamical calculation and analysis, the following conclusions can be educed:1 The main ingredients in both biomass and coal gasificationequilibrium system are CH₄,CO,CO₂,H₂,H₂O,H₂O(l) and C(s), and may contain some of N₂ when air is introduced as gasify agent. Within coal gasification system, the main existing form of N species is N₂, followed by NH₃ and HCN. And the main existing form of S species is H₂S, followed by COS and S. Because coal is rich in Carbon and lack in Hydrogen and Oxygen, more air (/oxygen) and steam is needed for coal gasification compared with biomass gasification. But the gasification process, the equilibrium compositions, and the influences of the gasification parameters on the equilibrium compositions of coal gasification system is exact similar to biomass gasification system.2 Temperature, Pressure, Equivalence Ratio and Steam/Material Ratio are all-important parameters for biomass and coal gasification system. Because the gasification process is dominated by endothermic reaction and the volume of the gasification system increased during gasification, enhanced temperature and depressed pressure are profitable to gasification process. Air(/oxygen) and steam both act as gasify agents, their total amounts is roughly fixed. And the proportions of CO and H₂ can be well controlled by adjusted the amount of steam used.3 As biomass and coal gasification system is a complex multiphase reaction system with quite a number of species, factors that may influence the equilibrium composition of it are massive. Furthermore, all the operating parameters have some certain relationships with each other. For example, temperature and pressure has opposite influences on equilibrium composition of gasification system. By enhance temperature properly, the problems of decrease in fuel gas fraction and low carbon conversion ratio result from raised pressure.4 By using nonlinear regression analysis, the relationship between equilibrium temperature (T_E) and pressure (P) was obtained: T_E/K =1162.98+61.61×1n(p/MPa-0.00138) . This equation can help to determine the optimal temperature for biomass gasification under different pressures accurately and rapidly.Equivalence Ratio (ER) has inverse ratio linearity with Optimal Steam/Material Ratio (S/M_O) under different temperature and pressure. And the regression curves of ER and S/M_O are almost parallel, the slopes of the regression curves are similar. Therefore, we can make the intercepts of the curves independent variables of temperature or pressure: by average the slopes of the curves. Then can we obtain the function of S/M_O to ER and p. as well as S/M_O to ER and T. Take biomass air-steam gasification as an example, when T=1173 K, the function of S/M_O to ER and p is: S/B_O = -1.48×ER-7.88E-03×p² + 9.10E-02×p + 0.31, and the function of S/M_Oto ER and T under atmospheric pressure is: S/B_O= -1.54×ER+1.29E03×exp(-T/74.10) + 0.31.These two equations can be applied to determine the amount of air (or oxygen) and relevant steam needed for biomass gasification under different temperatures or pressures. Compared to groping by a mass of expenriments, thermodynamical analysis based on Gibbs free energy minimization can provide a convenient method to calculate the equilibrium composition of biomass and coal gasification system.