This work investigated the kinetics and mechanism of CO2 gasification of chars from 11 types of Mongolian lignites (ash contents of 7-27 wt % dry) and the corresponding acid-washed (catalyst-free) lignites. The catalytic gasification contributed to 83-99% of the initial rate of char conversion to gas. The non-catalytic gasification obeyed first-order kinetics with respect to the fraction of unconverted char, 1 - X, with little or no effect of the physical property of char, such as specific surface area and porosity. The proposed kinetic model, which considered progress in parallel of non-catalytic and catalytic gasification, assumed the presence of three to four different types of catalysts together with a type of precursor, if necessary. The optimized kinetic parameters enabled quantitative description of the measured changes with time of X over its near entire range, X = 0-0.9995. The kinetic analysis revealed that the most and least active catalysts in each char had initial activities differing by 2-3 orders of magnitude. The initial rate of catalytic gasification, which varied with the lignite type by a factor of 16, was correlated well and linearly with (Ca + Na)/Si or (Ca + Na + Fe)/Si molar ratio. It was suggested that more or less portions of catalytic species had lost their activities prior to the gasification by reaction with Si and Al species. The catalyst also underwent deactivation during the gasification. The rate constant for the deactivation was a function of neither Si nor Si + Al content in the char but depended upon the initial activity of the catalyst.
All Science Journal Classification (ASJC) codes
- General Chemical Engineering
- Fuel Technology
- Energy Engineering and Power Technology