Turning palm oil mill effluent into boiler fuel through biogas cofiring with palm kernel shells

Palm oil mill effluent (POME) is a high-strength wastewater stream that can be converted through anaerobic digestion into biogas for thermal energy recovery. This study provides a field-scale assessment of POME-derived biogas cofiring with palm kernel shells (PKS) in an industrial palm oil mill boiler, focusing on direct thermal utilization under real operating conditions. Biogas from a continuous stirred tank reactor was injected into a PKS-fired boiler at valve openings of 0–100%, and the analysis linked measured biogas thermal input with steam-side operating stability, energy-equivalent PKS substitution, operating-cost benefit, and methane-related greenhouse gas mitigation. The biogas contained 60.3% CH₄, corresponding to an estimated lower heating value of 5162 kcal Nm⁻³. At full valve opening, the biogas flow rate reached 109.83 Nm³ h⁻¹ and supplied 5.67 × 10⁵ kcal h⁻¹ of thermal energy. Using an estimated PKS lower heating value of 4358 kcal kg⁻¹, derived from proximate and ultimate analysis, this corresponded to an energy-equivalent PKS substitution potential of 130.10 kg h⁻¹. The operating-cost assessment showed a positive net operating benefit of IDR 226.36 million y⁻¹ at maximum biogas utilization. Methane capture and combustion also provided substantial greenhouse gas mitigation, with net avoided emissions estimated at 9579 t-CO₂-eq y⁻¹. These findings show that direct boiler cofiring can serve as a practical thermal-use pathway for POME-derived biogas in palm oil mills. The study contributes field-scale evidence by distinguishing energy-equivalent substitution potential from direct solid-fuel mass reduction and by linking biogas utilization with steam-side operation, operating-cost benefit, and methane mitigation.