Modeling char surface area evolution during coal pyrolysis: Effect of swelling and gasification at high pressures

In this paper, a previous model for char surface area change during atmospheric coal pyrolysis was modified to include the effect of particle swelling and gasification to predict the N₂ adsorption specific surface area of high-volatile bituminous char generated at high pressures with or without gasification. A correlation between the change of char surface area and P₀ΔV_P/m_cm (the ratio of the expansion work for particle swelling to the mass of metaplast cross-linked with coal matrix) was developed and analyzed. The number of the defect regions generated by gasification was considered in calculating adsorption sites quantitatively. Particle swelling opens (at P₀ΔV_P/m_cm<1350 J/kg) and then compresses (at 1350 J/kg <P₀ΔV_P/m_cm<10,000 J/kg) the space between metaplast clusters, making the N₂ adsorption specific surface area of char increase first and then decrease. After the gaps between metaplast clusters are filled, the specific surface area changes are minimal. Gasification generates new defect regions in clusters and reduces the clusters in char, making the specific surface area of char first increase and then decrease.