Mild-thermal pretreatment of agro-residues enhances biomethanation potential: a comparative study of napier grass and rice straw

Lignocellulosic agro-residues can be used as a low-cost raw material for the production of biomethane, which in turn can be used as a sustainable alternative for conventional fossil fuels. In the present study, the effect of mild-thermal pretreatment on the biomethanation potential of two different lignocellulosic substrates, rice straw and napier grass, was evaluated. The substrates were thermally pretreated at 80 °C and 120 °C for 1 h and further subjected to anaerobic digestion at 30 °C for 40 days. The thermally pretreated napier grass showed a methane yield enhancement of 14% (25.66 ml CH4/gsub) with respect to the control, whereas the thermally pretreated rice straw showed an enhancement of 56% (15.12 ml CH4/gsub) with respect to the control. The enhancement in the biomethane yield of the pretreated substrates may be attributed to the partial biodegradation of carbon present in the biomass, which can be validated by the COD reduction observed (up to 67%) after the anaerobic digestion process. FTIR analysis of the substrates indicated the degradation and/or transformation of the organic compounds after thermal pretreatment. In conclusion, the study revealed that mild-thermal pretreatment can be employed as an effective strategy to enhance the biomethanation potential of lignocellulosic substrates.