Structural Evolution and Formation Mechanisms of Caking Components of Modified Lignite in Subcritical H₂O–CO Systems

Abstract

A new method for the alkali catalytic modification of lignite in subcritical H2O–CO systems is presented, which can greatly improve its caking properties (caking index, >90). Soxhlet extraction technology coupled with GPC (gel permeation chromatography), FTIR (Fourier transform infrared spectroscopy), and 13C-NMR (carbon-13 nuclear magnetic resonance) analysis methods were used to investigate the structural evolution and formation mechanisms of solutes of n-hexane, benzene, and tetrahydrofuran in modified coal (denoted as NS, BS, and TS, respectively), which were the main caking components in the modified lignite. The results showed that the solutes were composed of four kinds of compounds with weight-average molecular weights in the range of 1170–2900, 300–550, 190–300, and 100–150, respectively. Oxygen-containing functional groups appeared in many forms, such as phenolic OH, aliphatic OH, ether bonds, and carbonyl and carboxyl groups, but mainly existed as C–O. When the temperature was <320 °C, NS consisted of aliphatic ring compounds, while BS and TS contained some aromatics, with an average of two aromatic rings per cluster in both. The appearance of monocyclic aromatic structures in the solutes occurred in the 320–330 °C temperature range, with one to three aromatic rings in the BS clusters, while polycyclic aromatic structures with more than five rings appeared in TS. The statistical structural models of the solutes all contained polycyclic aliphatic/aromatic structures as their main bodies, connected with substituents such as monocyclic aromatic structures, aliphatic side chains, and oxygen-containing functional groups. They are consistent with the structure of caking components in caking coal. The formation of caking components in modified lignite therefore underwent four distinct stages: separation of primary solutes, formation of pyrolysis solutes, formation of hydrogenated solutes, and formation of polycondensation solutes. This study provides valuable insights into the caking transition of modified coal and a basis for the high-added value utilization of lignite.

Role of FeS Catalyst in the Hydromodification of Lignite in a Subcritical Water–CO System

“…Previous research found that the solutes of Soxhlet extraction were the major caking components in modified coal. 33 Hence, modified coals were separated by Soxhlet extraction, and the extractible solutes were analyzed by 1 H NMR. As shown in Figure 5, the yield of each extractible solute in the modified coals was improved compared with that in raw coal.…”

“…This is also one of the main reasons for the improvement in the the caking index of M 6 . 3,33 Figure 6 presents the 1 H NMR spectra of extractible constituents in raw coal and modified coal with/without the FeS catalyst. For all of the curves, aliphatic hydrogens H al (6.0−0.5 ppm) were dominant, whereas the aromatic hydrogens H ar (9.0−6.0 ppm) were weak, indicating that the hydrogen in extractible constituents was mainly associated with the aliphatic carbons or substituents.…”

“…In fact, the improvement of the caking characteristic was not only related to the hydrogen content in coal but was also simultaneously influenced by the positions where the active hydrogen was introduced into the caking components. Previous research found that the solutes of Soxhlet extraction were the major caking components in modified coal . Hence, modified coals were separated by Soxhlet extraction, and the extractible solutes were analyzed by 1 H NMR.…”

Isotope Labeling to Study the Hydrogen Transfer Route during Lignite Modification in a Subcritical D2O–CO System

“…Thus, the S D value of BS-D and TS-D should also increase. However, previous work showed that BS-D and TS-D both contained polycyclic aromatic structures as their main structures and were attached with substituents such as monocyclic aromatic structures, aliphatic side chains, and oxygen-containing functional groups . Therefore, they undergo polycondensation reactions and produce small molecular byproducts with low aromaticities .…”

“…In the stage of 120–150 min, TS-D and BS-D continue to undergo polycondensation reactions and produce small-molecular byproducts with deuterium . Therefore, the deuterium content in NS-D continues to increase and the deuterium content in TS-D gradually decreases.…”