The conversion of brown coal to a dense, dry, hard material

Abstract

A technology has been developed for transforming run-of-mine brown coals into a dense, dry, hard product. Raw coal is attritioned in batch or continuous kneaders to a mean particle size of approximately 10 μm. This size reduction releases the water naturally occurring within the porous coal structure, such that a paste-like material results. The paste can then be easily formed into products of the desired shape and size by extrusion devices. A hard, low-moisture content briquette-equivalent product is produced by the evaporative removal of water at or near ambient atmospheric drying conditions. The resultant product exhibits considerable strength, which may be further improved by the addition of pH modifiers in the case of suitable coals. Advantages of this densified brown coal process are that a hard, dense, fuel product can be manufactured from a raw brown coal of high initial water content without the need for, or prolonged, expensive drying conditions at elevated temperature. The resultant product is suitable either as a briquette equivalent fuel, or as feedstock for the manufacture of smokeless fuels, chars and activated carbons. The product retains the Volatile and Fixed Carbon values of the raw coal but possesses a Net Wet Specific Energy equivalent to that of a high rank coal. The densified coal has an advantage over a black coal in that dependant upon the coal stock selected the mineral content can be quite low. The process is more general and has been applied to other feedstocks — a peat and some lignites as well as brown coals have been found suitable. The chemical analyses indicate the involvement of phenolic residues in the coal as reactive centres. Reactions of these residues are initiated by the shearing-attritioning step in which fresh and reactive surfaces of the coal particles are exposed. The reactive centres are brought together by extrusion allowing cross-linking reactions to occur during drying.