Co-carbonization of selected American coals with commercial additives

Because of the short supply of high-quality coking coals in certain areas of the world, many methods of improving the coking characteristics of poorly coking or noncoking coals have been examined as alternatives to importing more expensive, better quality coals. Co-carbonization, or the addition of coal-derived or petroleum-derived materials to the coal charge prior to carbonization, has been used on a commercial basis in the Japanese coking industry. These additives have been used in both solid and liquid form as binders in coal briquettes or as direct additions to the coal blend.In this study three different coal lithotypes were sampled from each of three United States bituminous coal seams: (1) a marginally coking high-volatile B-rank Illinois No. 6 Seam; (2) a highly fluid, good coking quality, high-volatile A-rank Pittsburgh Seam; and (3) a strongly coking low-volatile Blue Creek Seam. Each lithotype sample was carbonized in small-scale (50 g) charges with each of three additives at 0, 2, 5, and 10% additive by weight. The additives included ASP, an asphalt pitch; KRP, a petroleum residue pitch; and SRC, a solvent-refined coal product. The different lithotypes were sampled to examine the effects of coal type as well as rank. A micro-tumbler test was used to give at least a relative coke-strength value for the cokes produced. In addition, all the cokes produced were examined microscopically to determine the effects of co-carbonization on the coke structure.The Illinois No. 6, Pittsburgh, and Blue Creek Seam coals all showed substantial strength increases when co-carbonized with 2, 5, and 10% of each of the three additives, particularly at the 5 and 10% levels. The SRC appears to be the best additive overall for the three ranks of coal, as judged by its ability to combine with the coal to make a higher strength coke. There appear to be no conclusive coke-strength differences among lithotype samples for any of the three coals, probably because of the small scale of the tests and the relatively small differences in inert maceral content among the lithotypes. Five percent by weight of additive appears to be sufficient, if properly blended with the coal charge, to produce higher strength cokes. This is also probably the maximum economically viable level, particularly in the United States coking industry. Two percent is probably the minimum additive level for adequate mixing on a commercial scale.