Effects of Preheating of Coal on Coke Strength

Abstract

Prior to researches on semi-industrial scale at the Coal Mining Research Center, fundamental studies on the effects of preheating were carried out at the Resources Research Institute. In this paper, the author summarized the results of the studies.According to the experimental results, preheating makes it possible to blend indigenous non-or feebly caking coal to some extent with normal oven charge without reducing coke strength. Non caking coal can not be blended so much as feebly caking one. Permissible amount of these coals also depends on the composition of oven charge itself. Medium volatile coking coal and high volatile coal high in fluidity have the capacity to hold non caking coal to a greater extent.Coals are divided into three groups from the view point of preheating effects. For the first group which contains feebly caking and weakly caking coal of low fluidity, the adequate temperatures of preheating and hot charging are up to 300°C, and the lower the rank of coal is, the higher the adequate temperature becomes. Properties of coals preheated to this temperature show little change, except significant decrease in moisture holding capacity measured at 20°C, relative humidity of 76 per cent. The effects of hot charging on these coals, may therefore, be mainly due to increases in bulk density of the charge and in heating rate.Weakly caking coal of high fluidity shows quite different behaviours. Strength of its coke reaches a maximum at the preheating temperature of 400°C. In addition, it increoses even if preheated charge is coked after being cooled. Furthermore, effects of preheating are accelerated in the presence of oxygen. The cause of these phenomena is evidently due to chemical changes occured in preheated coal. Excess amount of caking constituents such as pyridine and chloroform extracts are reduced, thus fluidity is properly regulated. As the volatile matter content is decreased, the rate of devolatilization at a solidification point decreases. Fissure formation in coke will therefore, be reduced. DTA-and DTG curves also become similor to those of strongly coking coals.The third group consisting of strongly coking coals show only negative preheating effects on the strength of coke.