Structural Response of Coal to Drying and Pentane Sorption

Abstract

We performed experiments that probed the response of coal toward drying and subsequent sorption and desorption of pentane. The weight loss measured after evacuating Argonne Premium coal in ultrahigh vacuum at room temperature matched the handbook values for moisture. All powder coal samples shrink upon drying, and some shrinkage may be associated with better packing of the coal particles induced by shrinkage of the coal matrix upon drying. The change in porosity due to drying was calculated from the difference between the expected volume change associated with H2O loss and the observed volume change. For low rank coals, a slight increase (∼0.04 cm3/g) in porosity was found. For coals >85 wt % carbon a slight decrease (∼0.04 cm3/g) in porosity occurred. Pentane sorption experiments were conducted for up to 365 days using coals dried at room temperature. A significant amount of pentane could not be removed upon evacuation for 100 min at room temperature, conditions suitable for removal of this sorbate from interconnected macro-, meso-, and micropores. However, much of this pentane could be removed upon extended evacuation (6500 min). This result is consistent with a situation where pentane removal is limited by its diffusion across the glassy space of the coal matrix constrictions isolating coal pores. Therefore, this pentane is referred to as “slow reversible” pentane. Negligible swelling of coal accompanies pentane sorption despite its considerable presence. This indicates that pentane adsorbs on surfaces of open pore space. The quantity of “slow reversible” pentane was compared to the quantity of CO2 determined from BET experiments. From this analysis, the degree of connectivity of the pore system of coal was estimated from the ratio of “slow reversible” pentane surface area to CO2 BET surface area. A ratio of 1.0 indicates a completely unconnected pore network, while a ratio of 0.0 indicates a completely interconnected pore network. The range of this ratio was 0.46−0.73 for coal dried under high vacuum at room temperature. These findings indicate that interconnection among pores is incomplete for these coals. For most coals, drying at 150 °C significantly affects the quantity of “slow reversible” pentane, and this is associated with alteration in the connectivity of the network pore structure of coal.