Effect of hydrothermal treatment on the structure and pyrolysis product distribution of Xiaolongtan lignite

•Hydrothermal treatment is divided into three stages with the temperature increasing.•Hydrothermal treatment promotes the cleavage of weak bonds by hydrolysis.•Hydrogen transfer takes place between water and lignite.•Pyrolysis tar yields increase 22% and pyrolysis water decreases after hydrothermal treatment at 513 K.•The char yields increase and the gas yields decrease for the treated lignite pyrolysis.

Hydrothermal treatment of Xiaolongtan lignite (XLT) was carried out in a lab autoclave and the pyrolysis was studied in a tube reactor at 873 K for 15 min. The structure of the lignite was characterized by solid state 13C nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The results show that the lignite structure and pyrolysis product distribution have a significant change after hydrothermal treatment. Hydrothermal treatment is divided into three stages with the temperature increasing. The oxygen functional groups decompose and substituted by hydrogen in the first stage (<493 K). The pyrolysis tar yields increase from 7.97 wt.% of XLT to 9.60 wt.% of XLT-493 and the pyrolysis water decreases. In the second stage (493–533 K), hydrothermal treatment disrupts the weak covalent bond such as ether, ester and side-chain substituent by hydrolysis. Meanwhile, hydrogen transfer takes place between water and lignite or among the intra-molecular structure lead to increment of the CH2/CH3 ratio. The pyrolysis tar yields increase to 9.76 wt.% of XLT-513 and the pyrolysis water decreases sequentially. Then, the pyrolysis water increases and pyrolysis tar decreases because of the obvious increment of the oxygen-linked aromatic carbon and little increase of the ratio of CH2/CH3. The pyrolysis water and tar decrease subsequently in the third stage (>533 K) as the result of the cleavage of covalent bonds.