Hydrodynamic analysis of full-scale in-situ biogas upgrading in manure digesters

The addition of hydrogen to anaerobic digesters is an emerging technique for the sustainable upgrading of biogas to biomethane with renewable electricity. However, it is critically dependent on the effective gas-liquid transfer of hydrogen, which is a sparingly soluble gas. Very little is known about the impact of liquid and gas flow and bubble size on gas-liquid transfer during H₂ injection in full-scale anaerobic digesters. A computational fluid dynamic model was developed using a two-fluid approach for non-Newtonian liquid in the open-source computational fluid dynamics (CFD) platform, OpenFOAM. The newly developed model was validated against published experimental data-sets of a gas-mixed, laboratory-scale anaerobic digester, with good agreement between the numerical and experimental velocity fields. The hydrodynamics of the full-scale in-situ biomethanation system using venturi ejectors for H₂ injection was then simulated to investigate gas-liquid dynamics, including gas-liquid mass transfer, at different operational conditions. Gas-liquid mixing is mainly controlled by the gas-plumes interaction, which promotes turbulence at the interaction zone, resulting in increasing gas bubbles mixing with the liquid and the gas-liquid interfacial area. However, beyond the plume interaction zone, the digester had flow short-circuiting and inactive zones. It was found that, due to this short-circuiting behaviour, an increase in gas flow-rate may not be an effective option in reducing inactive zones, although it can increase the gas-liquid interfacial area. Comparative analysis of the impact of gas flow and bubble size indicated that gas flow had a linear effect on both kLa and gas holdup, but that bubble size had a non-linear impact, with higher kLa values achieved at bubble sizes less than 2 mm. Comparison against measured data in the same system indicated the predicted kLa values were at the same level as measured kLa, at a bubble size of 2 mm.