Application of Coal Petrography in the Liquefaction of Subbituminous Coals and Lignites

Alberta's deposits of subbituminous coal represent a large potential source of liquid fuels. Surface mineable coal in the Plains Region of Alberta is the most attractive feedstock for future liquefaction plants. The Energy Resources Conservation Board (1981) estimates 6.7 gigatonnes of strippable coal reserves (table1). If oil prices continue to increase and an efficient conversion technology is developed, coal could compete economically with oil sands as a source of synthetic liquids.

The chemical composition of Alberta Plains coals is well known; the petrographic characteristics are not. Coal is an extremely heterogenous mixture of various mineral phases and macerals. The application of coal petrography to technical processes such as coal liquefaction, carbonization and combustion could be aimed at developing, improving or controlling such processes. It is not known what characteristics (petrographic or chemical) are important in determining coal conversion reactivity.

This work systematically scans Alberta Plains coals from the major coal regions for their petrographic composition and develops methods for petrographic analysis of solid residues from liquefaction. The ultimate aim of these studies is to predict the yield of liquid and gaseous products, starting from the given rank of the coal and a given maceral composition.

Until recently, no systematic research was undertaken to study the morphology and the nature of liquefied coal solid residues or to make comparisons with the petrography of the feed coal. Petrographers from different countries have just started an intensive research program, observing changes in the nature and composition of residues produced during small-scale hydrogenation at progressively increased temperatures, pressures or residence time in the reaction (Mitchell et al., 1977; Shibaoka, 1978; Shibaoka and Ueda, 1978; Wakeley, et al., 1979; Diessel, 1979; Barras et al., 1979). Preliminary investigations indicate that in high volatile coals used for liquefaction, macerals of vitrinite and liptinite groups are regarded as more or less reactive, whereas inertinite macerals are to be subdivided into low reflecting types (R = < 1.5="" percent)="" -="" reactive="" to="" a="" certain="" extent,="" and="" high="" reflecting="" types="" (r=""> 1.5 percent), which are always inert.