Methods, systems and units for liquefaction of combustible material are provided. After separating the combustible material from waste rock gravitationally in an aqueous salt solution selected to have a density which is intermediate between a density of the combustible material and a density of the waste rock and after heating and grinding the separated combustible material to yield a paste of purified combustible material, the paste is fluidizing and hydrogenated underground in a hydrogenation chamber including a Segner turbine. The described processes significantly reduce the energy consumption of the process, remove environmental hazards and result in more efficient liquefaction with respect to existing technologies.