Influence mechanisms of ScCO2 action on the organic composition and structure of different rank coals

ZHANG Xiaodong; JI Haijun; KANG Hongdong; ZHANG Shuo; PAN Yuliang

doi:10.13247/j.cnki.jcumt.20240429

Abstract：In order to find out the influence of supercritical CO2(ScCO2)on the organic composition and structure of coal,experimental studies were carried out on the ScCO2 extraction of different rank coals.Then,through comparing the differences in the main functional groups and macromolecular structure parameters of coal before and after extraction,the influence mechanism of ScCO2 on the low molecular compounds and organic structure of coal was discussed.The results show that:1)There are four types of benzene ring substitutions in the aromatic structure of coal,among which benzene trisubstitution accounts for the highest proportion,and the proportion of this substitution shows a decreasing trend with the increase of coal rank.ScCO2 has the best effect on the benzene trisubstitution structure of coal,so the proportion of benzene trisubstitution structure in low-rank coal decreases under the influence of ScCO2 fluid,while the proportion of benzene disubstituted,benzene tetrasubstituted and benzene pentasubstituted in the aromatic structure relatively increase.However,the benzene pentasubstitution in the aromatic structure of middle-and high-rank coals increase,with the proportion of benzene trisubstituents decreasing.2)The relative content of oxygen functional groups(I2)shows a decreasing trend as the coal ranks increases.After ScCO2 extraction oxygen functional groups in middle-rank and low-rank coals decreases,while oxygen functional groups in high-rank coals increase.ScCO2 extraction causes a reduction in aliphatic chains(I3)of different rank coals,and the hydrogen-rich index(I1)and the aliphatic-carbon ratio(fal)reach the maximum values in 1/3 coking coals,with a trend of increasing first and then decreasing.3)Under the influence of ScCO2 fluids,some low molecular weight compounds are dissolved,thus causing changes in the content of various functional groups,but with the increase of the coal rank,the type of main functional groups does not change much.The extraction of ScCO2 can break the hydrogen bonds among organic molecules of the coal which contribute to the disorder and relaxation of organic macromolecule structure.It was concluded that the changes of the main functional groups in coal after the action of ScCO2 were the result of the combined effects of the solubilisation of various functional groups with ScCO2,as well as chemisorption and chemical reactions.