Hydrogen production from low-rank coal by a two-step method combined subcritical depolymerization with supercritical gasification

•The swelling and depolymerization of low rank lignite in alkaline subcritical water were observed in situ.•The degree of swelling and depolymerization of lignite in alkaline subcritical water were quantitatively estimated.•Based on the depolymerization phenomenon, the near complete gasification of lignite in supercritical water was realized at mild temperature.