When some coals are carbonised in slot-type ovens, pressures can build up to levels that could damage oven walls. The search for the fundamental mechanism(s) that govern this behaviour has been the focus of many research programmes for decades. Predictive models which use coal properties tend to have a large empirical component, and have limited reliability when applied over a wider range of coals than used in developing the models. Predicting the pressure that a given coal or blend of coals might generate relies on pilot-scale oven trials. If the mechanisms for coking pressure were better understood, a more reliable predictive model could be developed.

Small-amplitude, oscillatory-shear, high-temperature rheometry is providing a unique fundamental understanding of the foaming process as coal is heated to 600°C in the absence of oxygen. Relating the viscoelastic properties to normal force measurements has revealed the complex viscosity (η*) and phase angle (δ) required for bubble nucleation, growth and coalescence. It has been found that for coals that generate high oven wall pressures, considerable bubble growth occurs but very little coalescence prior to resolidification (curing) and/or the bubble growth region is considerable with respect to time prior to coalescence. It is proposed that the bubble growth region represents the major barrier to the passage of volatiles. This paper will present the theory for oven wall pressure generation.