CO_2 Sorption on Coals: Contribution of Minerals and Influence of Supercritical CO_2 Pre-exposure

摘要

The carbon dioxide (CO_2) sorption isotherms of coals from different ranks and coals treated by HC1, HC1 and HF sequentially by supercritical carbon dioxide (SCCO_2) alone and with H_2O (SCCO_2/H_2O), and by N_2 with H_2O (N_2/H_2O) are compared to study the influence of minerals in coals and the influence of SCCO_2. 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 CO_2 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, HC1 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 SCCO_2/H_2O-treated and SCCO_2-treated coal samples, as well as the untreated and N_2/H_2O-treated coal samples for QS and DT coals. A higher sorption capacity of the former pair indicates that the pre-exposure of coal in SCCO_2 alone or with H_2O leads to a similar increase in CO_2 sorption capacity. H_2O in pretreating does not make a significant difference in CO_2 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 SCCO_2 or SCCO_2 with H_2O 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.