Free radical reaction mechanism on improving tar yield and quality derived from lignite after hydrothermal treatment

Coal pyrolysis process is accompanied by free radical generation, stabilization and condensation. Free radical concentration plays a crucial role in pyrolysis product distribution. Hydrothermal treatment of lignites is used as a pretreatment method to regulate pyrolysis product distribution analyzed by free radical reaction mechanism. The free radical concentration in lignite and char was monitored by electron paramagnetic resonance (EPR). The results show that free radical concentration in lignite increases during hydrothermal treatment, Hydrolysis of oxygen functional group and dissociation of the small molecules in the lignite interlayer stacking structure generate free radicals with medium molecular weight during hydrothermal treatment. Meanwhile, the increase of hydrogen radical donated from water through the interaction of water and lignite is utilized to stabilization of free radical with medium molecular weight to hinder the second cleavage and cross-linking of free radical during pyrolysis. The increasing rate of free radical concentration decreases to a minimum at a turning point temperature. The turning point temperature of hydrothermal treatment of lignites is different because of the difference in their structure. The pyrolysis tar yield increases around 20% after hydrothermally treated at the turning point temperature. The light tar (hexane soluble in tar) yield from lignite pyrolysis increases also. According to analysis of free radical in char, it shows that the pyrolysis weight loss numerically correlated to variation of free radical concentration during pyrolysis after hydrothermal treatment. The results can provide the data for the application of hydrothermal treatment on regulating gas and liquid fuel quality from carbon-based material for energy supply and environment protection.