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Gasification kinetics of western Canadian coals Yang, Yongxin


The gasification reactivities of eight types of Western Canadian coals ranking from bituminous to lignite were investigated. The chars derived from these coals under the same pyrolysis conditions were gasified with steam in a stirred-bed reactor, and the product gas was analysied on a gas chromatograph. The partial pressure of steam was 0.3 atm for all runs; temperature varied from 870-930°C; and particle diameter from 1.13-1.53 mm. BET surface areas of the chars were measured using N₂. Scanning electron microscopy was employed to observe the surface structure of the char. The carbon conversion versus time curves are generally S-Shaped except for the least reactive coal chars. It is evident that the reactivity of a char increases as the rank of its parent coal decreases. This relationship was indicated by correlations of the time to reach 50% conversion with the carbon contents and fuel ratios of the original coals. The influence of catalytic species, such as CaO, MgO etc. in the mineral matter of the coal on reactivity of was also shown. The maximum rate for each run was observed to lie between 10-25% carbon conversion. The random pore and random capillary models were found to be not suitable to represent the experimental data of this work, whereas the modified volumetric model (MVM) was capable to do so up to X = 0.78, especially for results of less reactive coal chars. The rate constant k evaluated from the MVM was closely related to BET surface area and moisture holding capacity of the char. The temperature dependence was not investigated in detail, but appeared similar for the four chars tested, which suggests that the gasification mechanism for each coal are similar. The fitting of Fung and Kim's correlation to the experimental data gave a good result with the correlation coefficient of 0.87-0.88. A correlation proposed by Sakata et al. was proved to predict the reactivities of these chars reasonably well. However, the rate constants measured by Sakata are 2.7-5.5 times as high as the values obtained in this work. By normalizing time with the time to reach fifty percent conversion a unified conversion function was obtained for all the present results up to about 70% percent conversion.

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