Structural Evolution and Formation Mechanisms of Caking Components of Modified Lignite in Subcritical H_2O-CO Systems

摘要

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 C-13-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 degrees 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 degrees 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.