The effect of inherent minerals in coal on gasification in H2O and CO2 mixtures was investigated, and the synergistic effect was explained. First, low-temperature ash that has the same state as the inherent minerals in raw coal was prepared by low-temperature ashing with an oxygen plasma at 150 degrees C. The minerals were identified by X-ray diffraction. Then, the coal was successively demineralized with deionized water, HCl, HF, and HCl/HF. Char gasification reactivities of the successively demineralized coals with different mixtures of H2O and CO2 were investigated using a thermogravimetric analyzer at 800 degrees C. The gasification reactivity closely correlated with the Ca content in successively demineralized coals. The calcite present in coal catalyzes the gasification reaction, causing the synergistic effect. (C) 2015 Elsevier B.V. All rights reserved.