Examination of Change in Coal Gel Structure Due to Solvent Swelling by Size Exclusion Chromatography

An inverse liquid chromatography (ILC) technique was applied to evaluating the porous structure of coals. Swollen Morwell coal (C = 65 wt %, daf) or Pocahontas coal (C = 91 wt %, daf) particles were packed in a ILC column as the stationary phase, and -hexane (HEX) or tetrahydrofuran (THF) was used as the mobile phase. For any combinations of coals and carrier solvents, n-alkanes of C₅-C₂₂ that were used as the molecular probes eluted in the size-exclusion region. In the Pocahontas-HEX system, all probes eluted nearly at the size-exclusion limit. This indicates that there were few pores of 1-10 nm in that coal. In the Morwell-HEX and Morwell-THF systems, however, the elution volume was varied by the chain length of probes. Under the assumption that the elution was controlled by the steric-exclusion mechanism alone, the volume of pores into which pentane could penetrate was 0.04 and 0.83 cm³/g of coal for the Morwell-HEX and Morwell-THF systems, respectively. The swelling of Morwell coal by THF (swelling ratio = 2.15) created micro- and mesopores accessible to n-alkane molecules.