Effect of SiO2 on loss of catalysis of inherent metallic species in CO2 gasification of coke from lignite

Abstract

Inherent metallic species retained by coal char or coke, such as Na and Ca, behave as catalysts in gasification. The char/coke normally contains inherent SiO2, which can react with the inherent catalysts to form silicates, resulting in catalyst deactivation over the range of pyrolysis, carbonization and gasification, and thereby reducing the char/coke reactivity. The present authors simulated the inherent catalyst deactivation experimentally by blending a Victorian lignite with SiO2, briquetting the SiO2/lignite blend, carbonizing the briquette, and then gasifying the coke with CO2. The kinetic analysis of the gasification employed a comprehensive model, which assumed progress in parallel of non-catalytic and catalytic gasification. The model quantitatively described the measured kinetics of the coke gasification with different SiO2 contents over a range of coke conversion up to 99.9%. The kinetic analysis revealed that the SiO2 deactivated substantial and entire portions of the most active catalyst and its precursor, respectively, before the gasification (i.e., during the carbonization). The catalyst deactivation also occurred during the gasification, but mainly following a self-deactivation mechanism that involved no silicates formation.