Hydrothermal liquefaction aqueous phase treatment and hydrogen production using electro-oxidation

Highlights: • Electrochemical oxidation can reduce 99% of chemical oxygen demand of HTL aqueous phase. • Complex organics are predominantly degraded to organic acids and eventually CO_2. • Hydrogen gas is produced simultaneously, potentially available for biocrude upgrading. • The energy consumption using electro oxidation for HTL water cleaning is significant. Post-hydrothermal liquefaction (HTL) aqueous phase treatment is an emerging issue and must be addressed for the commercial application of the HTL process. This study investigates the simultaneous production of hydrogen and reduction of chemical oxygen demand by applying electro-oxidation using boron doped diamond electrodes. Two types of hydrothermal liquefaction aqueous phases produced from wheat straw and sewage sludge are investigated and the largest chemical oxygen demand removal (82 and 99%) and hydrogen production rates (2.0 and 1.8 NL/h) were attained at the highest current density. However, a positive energy return on investment of an integrated HTL-electro oxidation process can only be achieved at the lowest current density where the removal of organics and hydrogen production is lower. Analysis via GC–MS and GC-FID shows that complex aromatic compounds are degraded first through the formation of organic acids that are oxidized only with longer residence times and more severe current densities.