Feeding syngas to anaerobic digestion to enhance methane content

Abstract

High energy consumption and demand have significantly raised concerns regarding the need for sustainable energy production and effective waste management. This issue has intensified the global demand for renewable energy sources and the need to mitigate greenhouse gas emissions, which have led to a growing interest in biomethanation as a means of upgrading the traditional biogas production process. The potential for biomethanation has become particularly relevant with growing interest in syngas (synthetic gas) as feedstock for enhancing methane production. Syngas, a mixture primarily composed of carbon monoxide (CO), hydrogen (H₂), and carbon dioxide (CO₂), can be introduced into a biogas system, offering a promising method for increasing methane content and energy output. The syngas feeding process in the AD system has gained attention for its ability to address inefficiencies in traditional anaerobic digestion (AD) processes, and this process is seen as a more sustainable and cost-effective approach, especially when applied in-situ. However, the challenge lies in optimising conditions for syngas injection into AD reactors to maximize methane production while minimizing inhibitory effects. Thus, understanding the optimal flow rate of syngas could unlock the full potential of upgrading biogas efficiently.

The objective of this study was to enhance methane content in the AD process by introducing syngas through an in-situ feeding method. Various syngas flow rates were evaluated to optimise the conversion of syngas components. Three identical continuous stirred-tank reactors (CSTRs) with a working volume of 4.8 liters were designed to operate under thermophilic conditions. Cow manure was used as the substrate for the AD process in all reactors. Reactor 1 served as the control, operating as a standard anaerobic digester. Syngas was directly introduced into Reactor 2 at varying flow rates (0.5 mL/min, 1 mL/min, and 2 mL/min), while Reactor 3 employed gas recirculation in combination with syngas feeding at different flow rates.

Feeding syngas into the AD process did not result in an increase in methane content of biogas compared to the conventional AD process using the proposed in-situ method. However, a higher volumetric methane production was observed in reactor R2 (5.3 L/d) at a high syngas flow rate (2 mL/min), compared to the control reactor (2.5 L/d). The highest conversion efficiencies of CO (97%) and H₂ (99%) were achieved in R2 at a syngas flow rate of 0.5 mL/min. Syngas flow rates exceeding 1 mL/min were found to be suboptimal for maintaining favourable CO content stoichiometry in the gas recirculation reactor R3.