3.3. Structure changes of lignite
Fig. 10 illustrated the weight loss curve of lignite at the heating rate
of 15 °C/min over a temperature fromambient to 600 °C under nitrogen
conditions. As showed in Fig. 10, an evident distinction between the
dried coal with and without acoustic excitation still existed. With the
introduction of acoustic waves, the weight loss rate at the stage of
0 – 600 °C decreased. The decrease in the weight loss indicated an
increase of the water and volatile matter, which can be attributed to
the reduction of oxygen functional groups and to changes in pore
structure.
Fig. 11 showed the FTIR spectrum of raw lignite and dried lignite
at T = 120 °C, t = 15 min, and ug = 1.25 m/s with and without
acoustic excitation. The presence of a band at 3410 cm−1 was due
to vibrational stretching of hydroxyl (OH) groups involved in
hydrogen bonding, possibly due to adsorbed water. The band at
3000–2800 cm−1 corresponds to C–H vibrational bands in methyl
and methylene groups. The band at 1850–1500 cm−1 could be
attributed to carboxyl (C_O) groups, aromatic (C_C) groups
(around 1610 cm−1) and carboxylate (COO−) groups. Remarkable
changes occur in the regions 3410 cm−1 (hydroxyl adsorption
region) and 1850–1500 cm−1 (carbonyl adsorption region). Investigation
of the spectra obtained for all the samples at different
drying temperatures revealed that Carbonyl (C_O) decreased progressively
with the assistance of acoustic field while carboxyl
(COOH) and hydroxyl (OH) increased and decreased thereafter.