Model for the Evolution of Pore Structure in a Lignite Particle during Pyrolysis. 2. Influence of Cross-Linking Reactions, Molten Metaplast, and Molten Ash on Particle Surface Area

A model for the evolution of the pore structure in a lignite particle during pyrolysis was established previously based on the chemical percolation devolatilization (CPD) model, using coal polymer network parameters to calculate the surface area and porosity of the particle. In this paper, to get the accurate surface area of coal particle at high pyrolysis temperature, the previous model was improved by considering the effects of cross-linking reactions, molten Metaplast, and ash. The good agreement of the predicted surface area with experiments at temperature below 1200 K in the previous model is maintained, and model accuracy is improved at temperatures above 1200 K. A correlation between cross-links and cleaving of side chains was established to describe the increasing amount of cross-links during coal pyrolysis and was introduced to modify the amount of bridges calculated by the CPD model. Higher temperatures can provide more energy for cleaving of side chains, and therefore the cross-linking reactions have a greater influence on the change of the surface area in a lignite particle during pyrolysis at a higher temperature. The influence of Metaplast on the surface area of a lignite particle is limited. The molten ash reduces the particle surface area at high temperature, and this influence is larger on the CO₂ surface area than on the N₂ surface area.