Mineral Transformation and Morphological Change during Pyrolysis and Gasification of Victorian Brown Coals in an Entrained Flow Reactor

摘要

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 degrees 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 degrees 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 degrees C because of reduction of Fe2O3. Melting of sulfates like MgSO4 and ablite was then found at 1200 degrees 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.