Understanding the impact of flow rate and recycle on the conversion of a complex biorefinery stream using a flow-through microbial electrolysis cell

The effect of flow rate and recycle on the conversion of a biomass-derived pyrolysis aqueous phase in a microbial electrolysis cell (MEC) were investigated to demonstrate production of renewable hydrogen in biorefinery. A continuous MEC operation was investigated under one-pass and recycle conditions using the complex, biomass-derived, fermentable, mixed substrate feed at a constant concentration of 0.026 g/L, while testing flow rates ranging from 0.19 to 3.6 mL/min. This corresponds to an organic loading rate (OLR) of 0.54–10 g/L-day. Mass transfer issues observed at low flow rates were alleviated using high flow rates. Increasing the flow rate to 3.6 mL/min (3.7 min HRT) during one-pass operation increased the hydrogen productivity 3-fold, but anode conversion efficiency (ACE) decreased from 57.9% to 9.9%. Recycle of the anode liquid helped to alleviate kinetic limitations and the ACE increased by 1.8-fold and the hydrogen productivity by 1.2-fold compared to the one-pass condition at the flow rate of 3.6 mL/min (10 g/L-d OLR). High COD removal was also achieved under recycle conditions, reaching 74.2 ± 1.1%, with hydrogen production rate of 2.92 ± 0.51 L/L-day. This study demonstrates the advantages of combining faster flow rates with a recycle process to improve rate of hydrogen production from a switchgrass-derived stream in the biorefinery.