Biogenic methane production from lignite in cube: Comparison of the inner and outer part of coal

•A significant biogenic methane was produced from lignite in cube.•Microbes can degrade organics and generate methane inside the coal matrix.•Element contents and aromatic functional groups decreased and even more in exterior.•SSA and PV increased in the exterior but decreased in the interior.•Degradation and pore development are better in the exterior than interior.