The Influence of Bedding-Plane Orientation on the Degradation Characteristics of a South African Waterberg Coal

ABSTRACT

A tomographic study of coal microstructure (coal bedding planes, pre-existing cracks, microlithotype boundaries, and mineral boundaries) was undertaken to explain its influence on coal degradation due to impact. A number of samples were cut from a large block of Waterberg run-of-mine coal. It was characterized using microfocus x-ray computed tomography before and after undergoing impact breakage in a single particle drop shatter test. The tomograms generated before and after impact were compared and the breakage characteristics determined. It was found that the internal structure of the coal samples influenced the breakage of coal, with new cracks initiating, terminating, or propagating along pre-existing cracks, microlithotype-, and mineral grain boundaries. The contribution of the physical structures to the breakage characteristics could not be predicted for individual particles, but the overall effect on a population of particles was fitted using a Rosin-Rammler distribution. The orientation of the bedding planes in relation to the impact surface also influenced the propagation of cracks through the samples. Newly formed cracks propagated deeper into a particle if the impact force was applied along the bedding planes.

KEYWORDS:

• Coal
• degradation
• micro-focus x-ray tomography
• single particle impact

Notes

¹ The outlier is due to a large mineral inclusion present in the particle. This inclusion greatly affected the quality of the tomogram, reducing the dynamic range of the tomogram and introducing serious beam-hardening and edge artifacts. The errors caused by the inclusion justify the exclusion of the particle from the observation above.