Use of rheometry and micro-CT analysis to understand pore structure development in coke

Pore structure is known to play a role in the strength of coke. However, across cokes with similar porosities, strength can be variable, indicating that certain features of the pore structure are playing a role. An understanding of what those features are and how they form can pave the way for better coke strength prediction models and reveal new ways to manipulate the coking process to prevent or enable certain features forming. In this paper, rheometry has been used to characterise the viscoelastic properties and follow the pore structure development through normal force measurements as coal is heated at 3 degrees C/min. Coal samples were quenched in the rheometer at various temperatures corresponding to key events of bubble growth, bubble coalescence, minimum viscosity and resolidification and these quenched samples underwent micro-CT imaging, using the Imaging and Medical Beamline of the Australian Synchrotron to obtain 3D representations of the structure with approximately 10 mu m resolution. The number of isolated pores, the pore size distributions of the isolated and connected pores, and the overall porosity were determined. Pore connectivity was found to be established very early on in the softening phase and coincides with the bubble coalescence phenomenon. For the high fluidity coals, virtually all of the pore space in the semi-coke is connected. It is proposed that bubble coalescence and applied forces play key roles in the contraction process and pore structure properties, in particular the size of the pore openings. In addition, during resolidification, the porosity reached a limit, however, the size of the pore openings continued to change, with a reduction in smaller openings and corresponding increase in larger openings. This trend suggests that the pore space becomes more channel-like, which could be due to the gas flow behaviour or could be due to surface tension effects. Semi-fusinite appears to reduce expansion. The precise mechanism by which it reduces expansion is not known. It may act as a conduit for the release of volatiles. This research is part of a broad set of activities which include understanding behaviour in the sole heated oven (SHO) and studying the mechanisms of coke breakage. (C) 2016 Published by Elsevier B.V.