Occurrence and transformation behaviour of silicon-bearing minerals during high silicon coal combustion from Yunnan Province, China

Abstract

To understand the mineralogical characteristics and transformation behaviours of silicon-bearing minerals during coal combustion, seven typical high silicon (high-Si) coals and two high-Si fly ashes generated from high-Si coal-fired power plants were chosen. The coals were first treated by a method of low temperature ashing and the float-sink test for heavy minerals separation. Subsequently, low temperature ashes (LTA), light mineral fractions (LM), heavy mineral fractions (HM), and fly ashes from coal-fired power plants were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) combined with energy spectrum analysis (EDX), X-ray fluorescence spectrometry (XRF), and simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC).The results show that The mineral compositions in the LTA and LM are almost the same and majorly composed of quartz, kaolinite, calcite, dolomite, daphnite, chlorite-serpentine, pyrite, and anatase. The mineral compositions identified in the HM are clearly different from those in the LTA and LM. Pyrite, fluorapatite, anatase, siderite, dolomite and a plenty of silicate minerals including quartz, clinochlore, daphnite, chlorite, and chamosite are found in the HM. Chlorite is a quite common mineral in the HM with different shapes, including schistose, long tetragonal prism, cylindrical, block-shaped, and near-spherical, can be identified. The minerals identified in the fly ashes from the high-Si coal-fired power plants include quartz, mullite, and hematite. The mullite in the fly ashes is derived from the transformations of quartz, kaolinite, and chlorite in the coals. The quartz in the fly ash is mainly derived from the unchanged quartz in coal and a secondary origin from the SiO2-Al2O3 phase transformation. The high-Si fly ash with the size fraction of 38.5–74 μm contains the highest SiO2 content.