An accelerated approach of biogas production through a two-stage BioH2/CH4 continuous anaerobic digestion system from Napier grass

•Hydrogen-rich biogas 20% was produced in the first reactor of the two-stage system.•The two-stage biohythane production system was achieved high efficiency.•The energy yields of single and two-stage systems were 13.14 and 10.10 kJ/g vS respectively.•The total bioenergy recovery in two-stage system was 30% higher than the single-stage system.