Physical and chemical characteristics of densified low-rank coals

Abstract

The conversion of Victorian brown coal into a hard densified product is described. Densified brown coal (DBC) is readily moulded or extruded into a convenient form and is characterized by reduced moisture content, greater bulk density and increased strength. Removal of water is initiated by a kneading process and continues during drying under ambient conditions. The loss of water is also accompanied by pellet shrinkage and an increase in crush strength. The development of crush strength is attributed to an increase in pellet density arising from greater coal compaction through the formation of a particulate gel network. The extent of compaction and the strength of the carbonaceous gel network depends on the pH and the nature of the acidic oxygenated functional groups in the coal. Derivatization of acidic functionalities, by O-methylation, results in a collapse of the coal network structure to give a product with inferior physical properties owing to the blockage of potential bonding sites. Acidic coals produce DBC of moderate density and strength, whereas base-treated coals produce very dense and stronger products. The improved physical properties of the base-treated DBCs is attributed in part to the presence of ionic associations between the coal particles to give a stronger three-dimensional network.