Mineral Transformation in Char and Its Effect on Coal Char Gasification Reactivity at High Temperatures, Part 2: Char Gasification

The effect of mineral transformation on the gasification reaction of char with CO2 was investigated from 950 to 1500 degrees C by comparing the gasification rate and gasification reactivity of raw char, demineralized char, and demineralized char blended with different minerals. The gasification experiments were performed by thermogravimetry (TG) using an isothermal method. Differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) were used to investigate the melting process of minerals during char gasification at around the ash melting temperature and to evaluate the effect of melting of minerals on the char gasification at high temperatures. The results show that the minerals in the char could obviously catalyze the gasification reaction below the deformation temperature of coal ash. The catalytic minerals are anhydrite (CaSO4), oldhamite (CaS), hematite (Fe2O3), and magnetite (Fe3O4). No remarkable catalytic effect on the gasification by the vitreous minerals themselves was found, but they could transform to crystalline minerals, which exhibit a catalytic effect during char gasification. The melting process of minerals during coal gasification could be divided into three stages according to the DSC curve. It is found that the hindering effect on the char gasification by mineral melting is also influenced by the carbon content in char. This process was also revealed by SEM analysis of some partially gasified chars.