Comparison of Ashes from Ca-rich and Ca-poor Vietnamese Coals

The thermal behaviors of ash components in three Vietnamese coals, anthracite from Quang Ninh (QN) prov-ince, bituminous coal from Thai Nguyen (TN) province, and brown coal from Lang Son (LS) province, were investigated using XRD, TG-DTA, and TEM measurements. QN and LS coals had very high SiO2 and Al2O3 contents and low CaO content. TN coal had high CaO content and low SiO2 and Al2O3 contents. The weight losses of QN, TN, and LS coals were 91, 84, and 77 % at 1200 degrees C under an air atmosphere, respectively. Kaolin-ite in SiO2 and Al2O3-rich QN and LS raw coals was transformed into mullite through illite after heat treatment under an air atmosphere. Quartz was found in all QN and LS ashes. Calcite in CaO-rich TN is considered to change to anhydrite and lime after heat treatment under an air atmosphere. Pyrite in TN and LS raw coals was oxidized to hematite after heat treatment. The needle-like crystals seen in QN and LS coals are likely to be illite, although those in TN may be calcite based on the XRD and TEM results.