A quantitative evaluation of active site number for CO2 gasification of various coal chars by temperature-programmed desorption method.

Abstract

A temperature-programmed desorption (TPD) method was applied to the characterization of CO2 gasification of coal chars obtained by pyrolysis of coals of various ranks ranging from brown to sub-bituminous coals. Gasification was carried out at temperatures of 1073 and 1273 K and pressures from 0.4 to 1.6 MPa, using a high-pressure TGA apparatus. For all chars employed, the rate constant based on the initial char weight, kv, decreased with the progress of gasification. This variation of kv was unexplained in terms of the change in the BET surface area of the chars obtained at different conversions; in general, the rate constant based upon the surface area, ks, decreased with char conversion lower than about 0.2 and then remained constant before decreasing again at the conversion of 0.8 towards the completion of the reaction. The active site number, NCO, was evaluated on the basis of the amount of CO desorbed in TPD runs after CO2 was adsorbed. Thus, the rate constant per unit number of active sites, ka, was estimated on the basis of the NCO and the Kv values for chars having different conversion levels. The values of ka obtained at 1073 and 1173 K were found to be invariable for conversions up to about 0.8 to 0.9 and to be of the same order of the magnitude for all chars at a given temperature. However, in all cases they decreased with conversion in the range higher than 0.9. This decrease in char reactivity was attributed to variation in the carbon structure of the char. X-ray diffraction spectra indicated an appreciable growth of the graphite peak at the final stage of gasification.