Sustainable management of tropical timber residues for greenhouse gas mitigation: enhanced biogas recovery via hydrothermal-alkaline pretreatment.

Forest residues generated by tropical timber industries represent an abundant yet underutilized biowaste resource whose uncontrolled disposal contributes to greenhouse gas emissions and environmental degradation. This research assesses the potential of four major Cameroonian timber residues, Iroko (Milicia excelsa), Sapelli (Entandrophragma cylindricum), Movingui (Distemonanthus benthamianus), and Bilinga (Nauclea diderrichii), for sustainable biomethane production through hydrothermal-alkaline pretreatment followed by thermophilic anaerobic digestion. The objective was to improve the biodegradability of highly recalcitrant lignocellulosic biomass while assessing its contribution to renewable energy generation and GHG mitigation. Comprehensive physicochemical and structural characterizations of raw and pretreated substrates were performed using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX) to elucidate modifications in crystallinity, surface chemistry, and biomass morphology induced by pretreatment. Methane yields increased from 29.10 ± 2.11 to 230.34 ± 2.32 NmL gVS⁻1 for Iroko, from 96.93 ± 3.24 to 220.21 ± 3.64 NmL gVS⁻1 for Sapelli, from 17.11 ± 3.43 to 150.64 ± 3.24 NmL gVS⁻1 for Movingui, and from 10.53 ± 4.52 to 120.43 ± 4.43 NmL gVS⁻1 for Bilinga. Kinetic analysis confirmed enhanced methane production rates and improved substrate conversion under thermophilic conditions following pretreatment. Among the investigated species, Sapelli exhibited the highest valorization potential, with an estimated biomethane production capacity of 8.35 million m3, equivalent to 91,868 MWh yr-1. Overall, the combined residues from the four timber species could generate approximately 189,076 MWh yr-1, highlighting a substantial opportunity for decentralized renewable energy production in rural and forest-dependent regions of Cameroon. Based on the experimentally determined biomethane potential, the theoretical utilization of the recovered biomethane corresponds to a greenhouse gas emission equivalent of approximately 33.83 × 106 kg CO2-eq yr⁻1, providing a quantitative indicator for evaluating its potential contribution to renewable energy systems. This study supports the integration of forest-residue biomethanation into sustainable waste management practices and low-carbon energy transition strategies in tropical forest regions.