New Exploration Of The Study On The Kinetics And Mechanism Model Of Char Gasification

Coal/biomass gasification technology is one of the key technologies to promote the clean and efficient utilization of coal and improve the utilization of renewable clean energy.The coal/biomass char-CO2 reaction is rate-determining step in the whole gasification process of coal/biomass gasification,and the research on coal/biomass char-CO2 gasification kinetics is the basis of gasifier design and optimization.Thermogravimetric analyzer(TGA)is the most commonly used thermal analysis instrument to investigate the kinetics of char-CO2 gasification.However,the mass coupled transfer and counter diffusion effects co-exist when the conventional TGA technique is used,which results in uncertainties with respect to practical application of the kinetic results.The Ca-based catalysts have become an excellent catalyst for coal/biomass char-CO2 catalytic gasification due to its low cost and high catalytic activity.The activity of the loaded or inherent Ca-based catalysts in coal/biomass char-CO2 catalytic gasification is the decisive factor for its catalytic efficiency.However,there is a lack of reasonable explanation on the specific reasons for Ca deactivation and the affect of gasification conditions on Ca deactivation.The gasification characteristics of different types of coal and biomass are very complex.It is difficult to obtain model parameters of the random pore model(RPM)and the more complex modified random pore model(MRPM),and there is a lack of effective modeling procedures and methods.In this thesis,a variety of coal and biomass are used as raw materials.The key issues in the kinetics of the coal/biomass char-CO2 gasification,such as the mass transfer limitations,Ca deactivation and kinetic mechanism modeling,are studied in detail.The main research contents and results are as follows:(1)The diffusion behavior of reaction gas in the gas stagnation layer and sample bed has been deeply studied.The results show that the bed diffusion caused by the mass of sample and the external diffusion caused by the existence of gas stagnant layer will reduce the overall reaction rate.The bed diffusion can be eliminated by laying the sample particles in the form of a near-monolayer configuration.The corresponding CO2 partial pressure in the gas stagnation layer inside the crucible can be measured based on the new method proposed in this paper.There is an obvious CO2 pressure gradient in the gas stagnant layer.The method of eliminating the external diffusion limitation by increasing the gas flow rate is not applicable.In this paper,the flat alumina disc(FAD)is obtained by optimizing the structure of the TGA standard crucible,which achieves the purpose of eliminating external diffusion in the reaction.By adjusting the experimental conditions and optimizing the structure of the TGA standard crucible,the intrinsic kinetics experimental method is established.(2)The catalytic effects of minerals such as alkali metals and alkaline earth metals(AAEMS)in coal and biomass on gasification reaction are studied,focusing on the deactivation characteristics and mechanism of calcium-based catalysts in coal and biomass char-C02 gasification.The results show that the AAEMS in ash can effectively catalyze the coal and biomass-CO2 gasification,and the catalytic effect has a certain correlation with the alkalinity index.CaO carbonation is a direct cause of the loaded or inherent Ca-based catalysts deactivation in the coal and biomass char-CO2 gasification.which follows the equilibrium regularity of CO2 over CaCO3 at different temperatures.(3)The influence of external diffusion,bed diffusion and Ca deactivation on the measurement of the reaction order of char-CO2 gasification is further investigated.The results show that the intrinsic reaction orders are almost identical during the whole reaction process,and the reaction order was not influenced by the external diffusion.In the reaction with bed diffusion,the reaction order at the initial stage of the reaction is always equivalent to the intrinsic order.With the progress of the reaction,the reaction order increases with the increase of bed diffusion limitation.The deactivation of the loaded or inherent Ca-based catalysts in char-CO2 catalytic gasification will increase the uncertainty of reaction order.The effect of Ca deactivation on the reaction order can be eliminated by appropriately adjusting CO2 pressure range at various temperatures based on the equilibrium regularity of CO2 over CaCO3.The change regular of the reaction order with temperature is related to the type of sample,and the reaction order of different types of sample shows temperature dependence.(4)The calculation methods of activation energy are compared and summarized,and the influence of external diffusion,bed diffusion and Ca deactivation on the measurement of the activation energy of char-CO2 gasification is further investigated.The results show that the activation energy calculation methods based on the assumption of PCO2n=constant are unreasonable due to the temperature-dependence of the reaction order.The normalized activation energy calculation method is not only suitable for PCO2n≠ constant,but also can normalize the activation energy at different CO2 partial pressures.Ca deactivation would lead to an increase in activation energy,which could be eliminated by appropriately adjusting CO2 pressure range at various temperatures based on the equilibrium regularity of CO2 over CaCO3.The intrinsic activation energy of the char-CO2 reaction is almost identical during the whole reaction process.The existence of the external diffusions would lead to the decrease of the activation energy,and the effect of the external diffusion on reducing activation energy is always the same in the whole reaction process.In the reaction with bed diffusion,the higher diffusion limitation leads to a lower value of the activation energy.There is no kinetic compensation effect during the reaction process in the intrinsic experiments,and the kinetic compensation effect is the consequence of diffusional limitations.(5)A simple and standard modeling process and model parameter solving method are proposed.The results show that the new method proposed in this paper can accurately solve the related model parameters in RPM and MRPM,and establish an accurate kinetic mechanism model.The model parameters obtained based on the intrinsic reaction are also applicable to the reaction with only external diffusion.The kinetic mechanism model established based on the intrinsic reaction is not applicable to the reaction with bed diffusion and Ca deactivation,and the unified kinetic mechanism model cannot be established if bed diffusion or Ca deactivation exist.