Reduction in ash constituents by water washing, and also at specific pH values, was systematically studied for three low-rank coals. Acid treatment removed inorganic constituents if accompanied by efficient water washing; at elevated temperatures this can also reduce moisture. The ash components are present in the coal matrix and also as numerous aluminosilicates particles containing K, Na, Mg, Fe and Ca; acid treatment reduced the ash constituents to mainly quartz and clay particles. The ash chemistry of treated and untreated coals was studied over the temperature range 800 degrees C to 1400 degrees C. The ash from treated coals consisted mostly of a-quartz, and when heated at 800 degrees C to 1500 degrees C, was transformed into mainly amorphous silica and cristobalite, and melted at 1300-1500 degrees C. Ash from untreated coals melted at or below 1100 degrees C. The impact of low rank coals with lower moisture, and also coals with lower ash and moisture, was assessed for power generation using the GateCycle (TM) package. The results show: (i) moisture reduction lowers CO2/MWh by a small amount but often increases fouling, and (ii) reduction in ash and moisture would eliminate ash fouling and significantly reduce the CO2/MWh compared with current power plant. (c) 2007 Elsevier B.V. All rights reserved.