Microscopic observations of the swelling of a high-volatile bituminous coal in response to organic solvent

Abstract

The swelling behaviour of individual coal macerals in response to solvents is thought to be important since it represents an early stage in the disintegration of the coal, which is, in turn, of fundamental importance to coal hydrogenation at elevated temperatures. Therefore, a detailed incident-light microscope study of solvent-treated, polished coal surfaces, involving a new edge-on mode of observation, was undertaken in order to observe the way in which solvents interact with the different coal macerals. This study was carried out on blocks of Bayswater Seam coal, using tetralin, pyridine, and 1:3 maleic anhydride-xylene solutions at temperatures near their respective boiling points. The degree of swelling of the polished coal surface depends on the nature of the organic solvent and the individual coal macerals. Vitrinite and, in some instances, low-reflectance semifusinite exhibit varying degrees of swelling in response to the solvent treatment; whereas exinite and the majority of the inertinite macerals appear to be unaffected by the solvents. Within the vitrinite group the lower-reflectance sub-macerals exhibit the greater degree of swelling. At temperatures of less than 200 °C the ability of the solvents to produce swelling in the coal decreases in the order pyridine → 1:3 maleic anhydride-xylene → tetralin.