Structural changes and sodium species redistribution of a typical sodium-rich coal during thermal dissolution with aromatic solvents

Abstract

Sodium-rich coals are widely distributed all over the world and they are considered as suitable materials for thermal dissolution of coals. In this work, to better understand structural changes of sodium-rich coals during thermal dissolution process, TG-FTIR was used to characterize weight loss and evolution of gases released from pyrolysis of a typical sodium-rich coal and extraction residues. Additionally, the occurrence mode of sodium species was determined by sequential chemical extraction method in order to examine sodium species redistribution. The results show that the organic oxygen content of coal was reduced by thermal treatment, which implied that thermal dissolution tended to remove oxygen and upgrade coal. TG-FTIR analysis also revealed that the CO2 content released from pyrolysis of extraction residues was on a lower level than that released from pyrolysis of raw coal. During thermal dissolution, the chemical structures of raw coal got more compact. It suggests that coal reactivity toward further thermal conversion would be weakened. In pyrolysis process, weight loss of samples treated by tetralin was higher compared with those treated by 1-methylnaphthalene, especially at 300 °C. This difference proves that raw coal had been pre-hydrogenated by tetralin even as low as 200–300 °C. Furthermore, tar produced in pyrolysis of samples treated by tetralin was deoxygenated by the residual tetralin and it leaded to decrease of tar-O structures, which influenced release of CO. Besides, it was observed that most sodium species were reserved in the extraction residue, as retention ratios were significantly high. At the same temperature, more exchangeable sodium species were promoted to be transformed into water-soluble ones when tetralin was used, which demonstrated that tetralin had better promotive effects on decomposition of –COO−. Along with decomposition of –COO−, exchangeable sodium species were detached from coal matrix and transformed into other chemical forms.