The brown-coal/water system: Part 3. Thermal dewatering of brown coal

Abstract

The mechanism of a process for removal of water as liquid from soft brown coals has been investigated. Coals were heated in water under pressure to temperatures of 150 to 300 °C, and the yields of dry coal, water, organic functional groups and inorganics in the product were measured. The earlier conclusions by coalification chemists on dehydration and decarboxylation under these conditions have been confirmed by the functional group analyses, and it was shown that the removal of liquid water is initiated principally by a disruption of the coal/water interactions caused by the thermal destruction of functional groups. The process is then completed by expulsion of water by the carbon dioxide evolved, and by changes in the surface wettability and shrinkage of the coal gel. At temperatures of 250–300 °C approximately three quarters of the water originally present in the coal is removed. If the coal is allowed to cool in the water after treatment at 150 °C complete reabsorption of the water occurs, but as the treatment temperature is raised progressively less reabsorption occurs. Alkali and alkaline-earth metals, chlorine and sulphur are also partly removed by the process, whereas iron, aluminium and silicon are little affected.