Mineral transformation and morphological change during pyrolysis and gasification of victorian brown coals in an entrained flow reactor

Mineral transformations and morphological changes of Victorian brown coals were investigated in a two-stage process, coal pyrolysis followed by gasification of char in CO₂, using an entrained flow reactor. The mineral transformations during coal pyrolysis and char gasification were examined over a wide range of temperatures between 700 and 1400°C by X-ray diffraction and secondary scanning electron microscopy with energy-dispersive X-ray spectroscopy. In general, mineral transformations of Victorian brown coals were found to happen at high temperatures (1000-1400 °C), not at low temperatures. During coal pyrolysis, Fe₂O₃ from the oxidation of Fe₃O₄ was formed in Yallourn (YL) samples, but Fe₃O₄ from the reduction of Fe₂O₃ was formed in Maddingley (MD) samples. During char gasification, reduction of Fe₂O₃ by CO and decomposition of CaSO₄ by CO were found in YL and MD samples. Furthermore, CaO from CaSO₄ decomposition was transformed to Ca₂SiO₄ in YL and was transformed to CaMgSiO₄ in MD samples. YL and MD also showed similar morphological changes during gasification. Mineral constituents with high Fe content were first found at 1000°C because of reduction of Fe₂O₃. Melting of sulfates like MgSO₄ and ablite was then found at 1200°C. However, Loy Yang coal with a high percentage of SiO₂ seemed to be thermochemically stable in terms of the behavior of mineral matters. Mineral transformations during char gasification of YL and MD tended to decrease the ash fusion temperature and slag viscosity, and enhance the gasification rate in CO₂.