Chemical structure and reactivity alterations of brown coals during thermal treatment with aromatic solvents

• De-oxygenation and extraction occurs during slurry-heating at 300 °C.
• Cross-linking occurs and decreases reactivity of coal during slurry-heating.
• Polar solvents are more tightly combined with coal than non-polar solvents.
• Hydrogen bonds and π–π conjugations potentially formed between coals and solvents.
• Hydrogen-donor solvent is superior to non-donor solvent for thermal treatment.

Thermal treatments of two brown coals (Chinese and Australian brown coals) -slurries with four different aromatic solvents (hydrogen donor and non-donor, polar and non-polar solvents) were conducted with bench-scale apparatus, and the treated samples (TSs) were separated and characterized in detail to investigate the structural alterations of the coals, as well as the interactions between the solvents and the coals. It is observed that the thermal treatments in aromatic solvents are effective in upgrading the brown coals, but the reactivity of brown coals towards pyrolysis is reduced due to the extraction of small molecules and the enhancement of cross-linking reactions. The solvents are tightly combined with the brown coals owning to the formation of new chemical forces (such as hydrogen bonds or π–π conjugations) as well as the physical disruptions such as the collapse of pores during slurry-heating. Different solvents behave variously with their unique properties, especially tetrahydroquinoline (THQ), which is more tightly combined with the brown coals than the other solvents, including tetralin (THN), 1-methylnaphthalene (1-MN), recycle solvent (RS) due to the electron-affluent nitrogen atom on its aromatic ring. Hydrogen-donor solvent is superior to non-donor solvent for thermal treatment of brown coals.