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.
