CO2 Sorption on Coals: Contribution of Minerals and Influence of Supercritical CO2 Pre-exposure

The carbon dioxide (CO₂) sorption isotherms of coals from different ranks and coals treated by HCl, HCl and HF sequentially by supercritical carbon dioxide (SCCO₂) alone and with H₂O (SCCO₂/H₂O), and by N₂ with H₂O (N₂/H₂O) are compared to study the influence of minerals in coals and the influence of SCCO₂. The sorption isotherms and the maximum adsorbed amounts from Langmuir plots for different samples reveal that the contribution from the sorption of minerals is relatively small but that the minerals can act as an obstacle, preventing CO₂ from adsorbing on the carbonaceous matrix. Generally, HCl-soluble minerals account for about 5–24% of the mineral content of test coals, while HCl-insoluble/HF-soluble minerals account for about 74–94%. Although the content of HCl-insoluble/HF-soluble minerals is much higher than that of HCl-soluble minerals, HCl leaching causes more change than HF leaching. It may be related to the distribution of the minerals in coal. The small variation of the Langmuir constant between parent and treated samples indicates only a small contribution from the minerals or the acid-leaching process. Similar sorption behaviors are observed for the SCCO₂/H₂O-treated and SCCO₂-treated coal samples, as well as the untreated and N₂/H₂O-treated coal samples for QS and DT coals. A higher sorption capacity of the former pair indicates that the pre-exposure of coal in SCCO₂ alone or with H₂O leads to a similar increase in CO₂ sorption capacity. H₂O in pretreating does not make a significant difference in CO₂ sorption capacity. No clear change on the magnitude of the Langmuir constant can be observed among the untreated and treated coal samples. It seems that the pre-exposure of coal to SCCO₂ or SCCO₂ with H₂O induces a change on the sorption capacity but no change on the sorption equilibrium constant. The pretreating may cause a change on the sorption sites but not on the basic properties of the carbonaceous moiety.