Modification of a Sub-bituminous Coal by Hydrothermal Treatment with the Addition of CaO: Extraction and Caking Properties

Abstract

A Chinese sub-bituminous coal, i.e., Shengfu (SF) coal, was hydrothermally treated with and without CaO addition at different temperatures, and the extraction yields and caking indexes (G RI) of the treated coals were measured. The action of CaO during hydrothermal treatment was probed in this study. The results show that hydrothermal treatment can obviously increase the extraction yield and G RI of SF coal, and CaO can further promote this effect of hydrothermal treatment. The removal of oxygen functional groups, especially the hydroxyl group, dissociating the aggregated structure of coal, is responsible for the modifying results. CaO can supply a basic environment for the hydrothermal treatment, which is beneficial for the removal of acidic oxygen functional groups by acid–base interactions and promotes the effect of hydrothermal treatment for SF coal, resulting in additional increases in the extraction yield and G RI.

Significantly enhances caking index of long‐flame coal under tetralin‐glycerol/H2 system

“…This relation may be the reason for the increased solvent extraction yield of modified coal. The amount of soluble constituents has a considerable effect on the caking properties of coal, and the G RI values of the modified coals are clearly improved when the extraction yield is enhanced . Previously, some researchers suggested that the content of single‐component AS or PS has a positive effect on the caking properties of coal; therefore, to investigate the mechanism of SF coal catalytic hydro‐plasticization, the modified coal M 4 was extracted by Soxhlet solvent extraction with n‐hexane, benzene, and THF in turn.…”

“…However, at the range of 115°C to 520°C, the weight loss of the M 3.5 coal was larger than that for SF coal, suggesting more light fragments exist in the modified coal. Specifically, a new active component (such as PA or AS) with a medium molecular weight is produced during the hydro‐plasticization process, and it easy to decompose this component into small molecules during pyrolysis. According to the results of FTIR analysis, noncovalent bonds and parts of oxygen‐containing functional groups are removed during hydro‐plasticization, which indicates that modified coal has a less aggregated structure and greater molecular mobility than SF coal.…”

“…The decrease in the intensity of these oxygen‐containing functional groups suggests that some oxygen functional groups in SF coal are removed during hydro‐plasticization. The removal of oxygen‐containing functional groups, especially the hydroxyl group, is advantageous for macromolecular structure depolymerization in SF coal and promotes the transformation of the three‐dimensional structure connected by oxygen bridges in SF coal to the two‐dimensional structure of coking coal . Thus, this process has a positive effect on the caking properties of coal.…”

“…Converting noncoking coal into coking coal is an important process for the efficient use of low‐rank coal and has attracted the interest of researchers . Hydro‐plasticization involves the catalytic hydrogenation of low‐rank coal at high temperatures and pressures using molecular hydrogen and a hydrogen donor solvent to simultaneously supply hydrogen .…”

“…Second, the degree to which the aromatic ring is condensed increases in the presence of catalysts, leading to an ordered structure in the coal. Shui et al performed a hydrothermal treatment of Shenfu (SF) subbituminous coal and found that the caking properties of hydrothermally treated coal were positively correlated with the solvent extraction . Zhang et al performed the hydro‐plasticization of lignite in a CO‐subcritical water system and found that the caking properties of the modified coal depended on the extraction yield of preasphaltene (PS) .…”