Upgrading of low rank coal by hydrothermal treatment: Coal tar yield during pyrolysis

Hydrothermal treatment of coal was carried out in a lab autoclave at 533 K for 30 min and coal pyrolysis was completed in a tube reactor at 873 K for 15 min with the heating rate of 5 K/min. The thermal analysis of the raw and treated coal was compared by thermogravimetric analyzer. The distribution of carbon in coal was detected by solid state 13C nuclear magnetic resonance. The free radical concentrations in coal were determined by electron paramagnetic resonance spectrometer. The results show that hydrothermal treatment is an effective method for upgrading and deoxygenation of low rank coal. The pyrolysis water yields decrease from 9.21 wt.% of the raw brown coal to 7.40 wt.% of the treated brown coal. The pyrolysis tar raises about 18% for the treated brown coal and 5% for the treated sub-bituminous. After hydrothermal treatment, the treated coal has lower contents of oxygen functional group and higher percentage of alkyl carbons in comparison with the raw coal, the free radical concentrations in treated coal increase, especially for brown coal, which is the result in the increment of tar yield.