Variation of Char Reactivity during Catalytic Gasification with Steam: Comparison among Catalytic Gasification by Ion-Exchangeable Na, Ca, and Na/Ca Mixture

摘要

The reactivity profile of char during catalytic gasification is crucial for designing and optimizing the gasification process, and it is greatly affected by the types and changes in activity of the catalyst during gasification. The catalytic gasification of Na-Char, Ca-Char, and Na/Ca-Char mixtures with different concentrations of steam was conducted within the temperature range of 700-900 degrees C using a microfluidized bed reaction analyzer. The results indicate that the reactivity of Na-Char is always higher than that of Ca-Char during the initial gasification stage (0-X-i) and then lower than that of Ca-Char in the later stage. The observations are mainly attributed to the different deactivation paths for the Na and Ca catalysts. The drastic loss of Na during gasification corresponds well to the sharp decrease of Na-Char reactivity within the initial carbon conversion, and the gradual change of the Ca dispersion contributes to the deactivation of the Ca catalyst. It is also demonstrated that the catalysis of Ca species exhibits better persistence than that of Na species at relatively high gasification temperatures (800-900 degrees C). With an increasing steam concentration, the catalytic activity of Ca is substantially promoted, resulting in a higher reactivity of Ca-Char than that of Na-Char. In addition, a cooperative effect between Na and Ca on the gasification reactivity is revealed, and the Na-Char and Ca-Char in the 30%:70% mixed ratio is an optimum for promoting char reactivity under the current conditions. The good performance of the Ca catalyst while interacting with the mineral matter enhances both the Na/Ca-Char reactivity and the resistance of Na to deactivation. The Na additive not only increases the gasification reactivity but also ensures the persistence of the Ca catalyst to deactivation, due to the formation of a low-temperature melting eutectic catalyst. This work provides new insights into the high-efficiency gasification of low-rank coal with in situ ion-exchangeable Na or Ca catalysts.