Mechanism And Numerical Simulation Of Char Gasification In Mixtures Of CO₂and H₂O

The gasification technology is one of the clean coal technology to meet the global energy demand. Governments and research institutions have given a lot of attention to the coal gasification technology. Most of published investigations studied reactions of coal chars at a single CO2or H2O gasification atomosphere. The surface reaction rates, especially the effect of gas partial pressures on surface reaction. have been successfully modeled using Langmuir-Hinshelwood equations. However, there is little research on char gasification mechanisms in the mixture of CO2and H2O. The main differences among these mechanisms are the relationship between the active sites for the char-CO2reaction and that for the char-H2O reaction, and the competition between the CO2-adsorption and the H2O-adsorption. The main factors affecting the active sites are pyrolysis conditions, coal types and catalyst, and the main factors affecting the competition between the CO2-adsorption and the H2O-adsorption are gas partial pressure and temperature.A method has been established to study the kinetics of the char gasification in a micro fluidized-bed reactor (MFBR) via rapid gas analysis. Both a thermo-gravimetric analyzer (TGA) and the MFBR were used to study the char gasification with mixtures of CO2and H2O. A new mechanism of char gasification with mixtures of CO2and H2O has been proposed in the paper. Moreover, the reaction mechanism has been successfully applied to the numerical simulation of a gasifier. The main contents include as following:The fluid dynamics between the emulsion and the bubble phase on the intrinsic reaction kinetics has been avoided in the MFBR and the char gasification was closely to the intrinsic reaction process. The kinetics parameters of char gasification in the MFBR were compared to those in the TGA. It was found that char gasification rate in the MFBR was higher than that in the TGA under the same conditions. The most probable reaction mechanism function in the MFBR was homogeneous model, while the most probable reaction mechanism function in the TGA was unreacted shrinking core model, because of more gas-solid coupling in the MFBR.Two kinds of char were prepared from a lignite by fast pyrolysis using a drop tube furnace or by slow pyrolysis using a fixed-bed furnace at the temperature of1273K. The chars were gasified with CO2, H2O and their mixtures in the TGA and MFBR to effect of pyrolysis conditions on the char gasification with mixtures of CO2and H2O. It was found that char gasification rate in the mixtures of CO2and H2O is obviously lower than the sum of the two rates of the char independently reacting with CO2and H2O but higher than the rate of each independent reaction, for both the fast-char gasification and slow-char gasification. Char-H2O reaction was independent on char-CO2reaction, while char-CO2reaction was inhibited by char-H2O reaction.Six chars, such as four coal chars of varying rank and two chars derived from acid-washed coals were prepared by fast pyrolysis using a drop tube furnace at1323K in order to study the effect of coal type on the char gasification with mixtures of CO2and H2O at the atmospheric condition at1273K. Moreover, one of the chars was used to measure gasification rates, at1173K,1223K and1273K, respectively, to study the effect of temperature on the char gasification with mixtures of CO2and H2O. The results showed that coal types, minerals and temperature had significant impact on the gasification rate, but had little impact on the competition between the CO2-adsorption and the H2O-adsorption. Char-H2O reaction was independent on char-CO2reaction, while char-CO2reaction was inhibited by char-H2O reaction. A new mechanism of char gasification with mixtures of CO2and H2O has been proposed, which is appropriate to predict the char-mixture reaction rates in both the thermo-gravimetric analyzer and the fluidized-bed reactor.A thermal equilibrium model was established to study the effect of O2/coal ratio and steam/coal ratio on the syngas compositions and temperature during coal gasification. And a three-dimensional numerical simulation model was established to obtain the temperature distribution, flow distribution, composition distribution and particle trajectories in the gasifier using commercial CFD software. Moreover, a simple one-dimensional model was developed, using the more comprehensive heterogeneous gas-solid reaction sub-model. The derived possible mechanism of char gasification with mixtures of CO2and H2O was applied successfully in the one-dimensional model. Comparison between the simulation and experimental data shows that most features of the gasification performance were identified accurately by the new gasification mechanism.