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

•Viscoelasticity of coal increases upon oxidation to a level where bubble coalescence is inhibited.•The increase coincides with an increase in oven wall pressure.•This work shows viscoelasticity to be a dominant factor in the mechanism for oven wall pressure.•Inert components also appear to suppress oven wall pressure. 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