Producing pipeline-quality biomethane via anaerobic digestion of sludge amended with corn stover biochar with in-situ CO₂ removal

This study presents a novel process for producing pipeline-quality biomethane by anaerobic digestion (AD) of sludge with in-situ biogas cleanup and upgrading using corn stover biochar. The biochar has high surface area (105m²/g), high ash content (45.2% dry weight) and high concentrations of potassium, calcium and magnesium (14.2% K₂O, 3.9% CaO and 4.2% MgO of the ash content, respectively). The biochar-amended digesters produced near pipeline-quality biomethane (>90% CH₄ and <5ppb H₂S), facilitated CO₂ removal by up to 86.3%, boosted average CH₄ content in biogas by up to 42.4% compared to the control digester, close to fungibility of natural gas. The biochar addition enhanced the methane yield, biomethanation rate constant and maximum methane production rate by up to 7.0%, 8.1% and 27.6%, respectively. The biochar addition also increased alkalinity and mitigated ammonia inhibition, providing sustainable process stability for thermophilic sludge AD. The biochar-amended digestate is enriched with nutrients such as potassium, nitrogen and phosphorus, and therefore has great potential for soil applications.