Importance of catalyst dispersion in the gasification of lignite chars

Lignite chars were prepared in N₂ under widely varying conditions of pyrolysis heating rate, temperature, and residence time. Their reactivities were measured by isothermal thermogravimetric analysis in 0.1 MPa air. A major decrease in char reactivity was observed with increasing severity of heat-treatment conditions. The relatively high gasification reactivity of lignite chars, compared to those obtained from higher rank coals, is due to the catalytic effect of the initially very highly dispersed CaO on the char surface. Char deactivation is caused primarily by CaO crystallite growth, measured by X-ray diffraction line broadening. When the reactivities of the various chars are expressed as turnover frequencies, i.e., per unit catalyst site, differences in observed rates of about 200 times are reduced to within 1 order of magnitude. Thus, it has been shown that the commonly observed and heretofore empirically treated lignite char deactivation with increasing severity of pyrolysis conditions can be correlated with a decrease in a measurable fundamental property of the chars: catalyst dispersion.