Some aspects of the generation of coking pressure during coal carbonization

Abstract

The relations between the gas pressure developed during carbonization in coke ovens, the changes in the coal macromolecular structure, thermoplastic properties, devolatilization with respect to temperature and coke porosity were investigated. The macromolecular structure, i.e. the apparent cross-link density, was characterized by the solvent swelling technique using pyridine as solvent for coals carbonized in the temperature range 300–600°C. The thermoplastic properties were assessed using constant-shear-rate and Gieseler plastometry and dilatometry. The thermal decomposition of the coal macromolecular structure starts prior to the softening of coal and the development of thermoplasticity. The temperature at which the apparent cross-link density reaches a minimum is close to the plastometry caking temperature, at which the maximum particle cohesion occurs. After the cross-link density reaches a minimum, primary decomposition of the coal macromolecular structure occurs, resulting in the development of thermoplasticity and the formation of new cross-links. The changes in microporosity with temperature correlate with the changes in the macromolecular structure, thermoplasticity and devolatilization. Initially, the microporosity increases as the cross-link density decreases, due to the opening up of the structure caused by the cleavage of cross-links. As carbonization proceeds, the microporosity decreases due to the blocking of the micropores by plastic material and the formation of new cross-links, until a minimum is reached in the thermoplastic phase. Thereafter it increases again due to the opening up of microporosity associated with devolatilization and the development of the semicoke microporous structure. The gas pressure in the plastic layer 100 mm from the oven wall (halfway between the oven wall and the oven centre) starts to rise at the caking temperature and reaches a maximum close to the resolidification temperature of maximum weight loss rate. In contrast, the gas pressure in the oven centre reaches a maximum at the caking temperature. This difference is discussed from the viewpoint of the existence of the zones of low gas permeability and the migration of the plastic layer. For dangerously swelling coals, the maximum rate of weight loss with respect to temperature occurs in a temperature range where the cross-link density of the macromolecular structure is high. A qualitative model based on the comparison of the rate of release of volatiles in relation to the cross-link density of the macromolecular structure is proposed.