High-resolution X-ray computed tomography observations of the thermal drying of lump-sized subbituminous coal

Abstract

Drying of low-rank coals affects: coal cleaning, combustion, comminution, gasification, liquefaction, and in-seam fluid-flow (water, coalbed methane, and carbon dioxide for sequestration/enhanced coalbed methane). To evaluate the extent of drying-induced transitions, 3 lump-sized (approximately 6 × 2 × 2 cm) Powder River Basin subbituminous coal samples were thermally dried in an air-drying coal oven at 50 °C over two weeks. A high-resolution industrial X-ray computed tomography scanner was utilized to generate (non-destructively) three-dimensional regional volumetric renderings, as-received and over 3-stages of drying. The lumps had cleats, both open and mineral filled, with a degree of fracture diversity along the longitudinal plane. Comparison of the virtual slice surfaces, at identifiable locations, allowed the induced cracking and shrinkage accompanying the transitions during 19% moisture loss to almost dry to be observed. Under these drying conditions, the heat transfer, and thus extent of drying, proceeded radially inward. With increased drying time the fractures extend and become larger in aperture as the coal shrinks. The major fractures mostly followed the existing cleat system. With additional drying, these cleats widened and the aperture increase propagated deeper into the coal extended into the butt cleats. New fractures were located mostly perpendicular to the cleat fracture surface. The external volume of the coal lumps had limited shrinkage. The axial extent of the shrinkage length (lump edge to lump edge) was on the order of 4–6%, the bulk of the shrinkage being accommodated by the internal shrinkage between cleats.