Mineral Transformation in Char and Its Effect on Coal Char Gasification Reactivity at High Temperatures, Part 1: Mineral Transformation in Char

The mineral transformation in the ash, slow pyrolysis char, and partially gasified char at high temperatures were investigated in this part by XRD and SEM-EDX. All the mineral transformations possibly affecting gasification were considered. The results show that the mineral transformation in slow pyrolysis char greatly differs from that in high temperature ash (950-1500 degrees C). The predominant minerals in slow pyrolysis char and high temperature ash are oldhamite and gehlenite, respectively. The crystalline mineral disappears in ash above 1300 degrees C while it still exists in the char even at 1500 degrees C. In the char, anhydrite is reduced by carbon to oldhamite at high temperature and increasing pyrolysis temperature is beneficial for the formation of oldhamite. It is verified that the presence of char impedes the formation of aluminosilicates and thus the catalytic minerals remain in char even at 1500 degrees C. However, the oldhamite in char also transforms to gehlenite during char gasification with the presence of CO2. The carbon thermal reaction is proved to be feasible only under inert atmosphere rather than reducing atmosphere in this study. Besides, morphology of minerals in char at high temperature is characterized. Considering the interaction between minerals and coal matrix, it is not adequate to evaluate the mineral transformation during gasification by only using ash.