Structural transformation of fluid phase extracted from coal matrix during thermoplastic stage of coal pyrolysis

Abstract

Two-stage solvent extraction, involving acetone (light extraction) and tetrahydrofuran (heavy extraction), was used to obtain materials from the fluid component of the coal matrix during the thermoplastic stage of pyrolysis. These extracted materials were characterized using Laser desorption ionization time of flight mass spectrometry, Gas chromatography-mass spectrometer, 1H nuclear magnetic resonance and Fourier transform infrared spectroscopy. After identifying the different chemical compounds using the type difference technique, their structural transformation behaviour was considered. Results show that the molecular weight distribution of extracted materials was within 576 Da. Fluid phase obtained from the extract contained aromatic (benzene with alkyl) and aliphatic (long-chain unbranched alkanes) components. The quantity of these two materials increased as pyrolysis temperature increased from 400 to ∼450 °C, and then decreased with further increases in temperature from 450 to 500 °C. The overall structural transformation of the fluid phase in thermoplastic stage could be divided into two stages: (a) formation and stabilization of the fluid phase, and (b) the cross-linking and re-attaching of the fluid phase to coal char. In the temperature range of 400 to ∼450 °C, the level of both light and heavy fluid phases increased because of the combination of free radicals with hydrogen formed from the cracking of the coal matrix – resulting in shortening of the aliphatic chain length and improving generation of hydrocarbon in the extract. At higher temperatures, the cross-linking and preferential re-attaching of materials to the coal char caused a decrease in the fluid phase level. In turn, this resulted in the lengthening of the aliphatic chains, which weakened their hydrocarbon-generating potential. These two stages explain the transient behaviour of the fluid phase generated in the thermoplastic stage, and contribute toward a better understanding of the softening and solidification behavior of coal during coking.