Modeling char surface area evolution during coal pyrolysis: Evolving characteristics with coal rank

The trend of changing char N₂ adsorption specific surface area (s_N2) during pyrolysis depends on coal rank significantly, and this is influenced by the arrangement of planar polycyclic aromatic structures in char. Two exponents, k_{mat order} and k_{crlmet order}, representing the order degrees of planar polycyclic aromatic structure arrangement in coal matrix and crosslinked metaplast, respectively, were introduced in the previous model, which could predict s_N2 of lignite coals and predict char CO₂ adsorption specific surface area (s_CO2) of lignite and high-volatile bituminous coals. A correlation between k_{crlmet order} of s_N2 and f_cross/f_v (the fraction ratio of the crosslinked metaplast to the volatile yield) was proposed. The previous model was extended to include predicting N₂ adsorption specific surface area of char generated from subbituminous and high-volatile bituminous coals at atmospheric pressure and was validated with 5 subbituminous coals, and 2 bituminous coals. The change of s_N2 with coal ranks was explained by the change of k_{crlmet order} with f_cross/f_v. When the crosslinked metaplast is adequate and the volatile yield is small, the planar polycyclic aromatic structures in metaplast overlap tightly and s_N2 is small. When the crosslinked metaplast content is small and the volatile yield is big, the metaplast is arranged disorderly, making s_N2 in crosslinked metaplast larger than that in the coal matrix and take a considerable proportion of total char surface area. The transition characteristics of subbituminous coal can be predicted by the improved model. When the temperatures and heating rates are high enough, the predicted s_N2 of subbituminous coals changed more like lignite coals, while at lower temperatures and heating rates, the predicted s_N2 changed like high-volatile bituminous coals.