Influence of thermoplastic properties on coking pressure generation: Part IV – Further evidence of the role of bubble coalescence in the mechanism for pressure generation

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The fundamental mechanism(s) for high oven-wall pressure are still not completely understood. The hypothesis put forward in this series of papers is that bubble-growth combined with a lack of bubble-coalescence in the plastic-layer is the primary reason for high oven-wall pressure, and that a lack of bubble-coalescence occurs when the minimum viscosity and elasticity are above a certain threshold. Because it is known that coal oxidation decreases its fluidity, an examination of the changes to both viscoelastic properties of the plastic-layer and oven-wall pressure during the coking of oxidised coals was considered to be a promising way to test the hypothesis, as most of the other properties of the coal, such as volatile matter, remain unaltered.Viscoelastic properties were measured using high-temperature oscillatory shear rheometry. For the fresh coals studied, the viscoelastic properties were such that bubble-growth and bubble-coalescence occurred and the oven-wall pressure was low. After subjecting the coal to enhanced oxidation, the minimum viscosity and elasticity increased to a level whereby bubble-growth occurred but bubble-coalescence did not. A large increase in OWP was found to coincide with this change. It is envisaged that bubble-coalescence enables channels to form in the plastic-layer and that the continuous release of volatiles keeps the channels open. Therefore, bubble-coalescence is considered to strongly influence gas permeability. When gas permeability through the semi-coke is severely restricted, volatiles would be forced to move to the centre of the charge, condense, and then revolatilise as the temperature rises, leading to high late OWP peaks. If oxidation was allowed to progress further, it is anticipated that viscosity and elasticity would increase to a level whereby bubble-growth would be restricted and OWP would not be high, but the coke would be highly non-fused and very weak. These results may elucidate why variable OWP results are found for different coals before and after oxidation; it depends on the initial and final viscoelastic properties.For another coal, OWP remained low at <6kPa. Although the viscoelastic properties might have suggested that the coal undergo very little bubble-coalescence, this coal was found to expand to only a limited extent (as evidenced by the expansion profile and axial force measurements). The limited expansion suggested that gas was able to escape. It is proposed that for this coal volatiles are escaping via the solid-like inertinite and semifusinite components within the coal, which is a mechanism that has been previously proposed.This work reinforces the proposed coking pressure mechanism and indicates that coking pressure could be controlled by manipulating the viscosity and elasticity to provide a significant region of bubble-coalescence or by adding inert solids. It follows that the phase angle – complex viscosity mapping plots, in conjunction with the ΔL and/or axial force profiles, could be used to identify whether a coal or coal blend is likely to generate a high pressure. Considerable refinement of the tests is thought to be necessary to enable more accurate and reliable predictions.