(272f) High temperature electrolysis of water for hydrogen production and simultaneous hydrotreating of dissolved organics in HTL-aqueous phase – Thermodynamic analysis

After hydrothermal liquefaction (HTL) of biomass (algae, food waste, lignocellulose etc.), the more hydrophobic organic compound partition into the oil phase while the more hydrophilic organic compounds partition into the aqueous phase. The chemical oxygen demand (COD) of organics lost to the aqueous phase are in the range of 25,000-75,000 ppm depending on the feedstock. The presence of organics in the aqueous phase, represent two major problems; environmental problems in terms of meeting discharge limits and energy loss that would have been added to the energy recovery of the entire HTL process. Electrochemical reduction of HTL-aqueous phase (HTL-AP) is gaining potential to becoming a viable solution for valorizing the organics in HTL-AP and producing hydrogen gas.

Electrolysis of water is a process that is employed to split water into its various hydrogen and oxygen components. Oxygen evolves at the anode, while hydrogen is reduced at cathode side of the electrolytic cell. The electrochemical reduction of organics intends to utilize the protons released during the water splitting reaction. Increasing the temperature will provide extra heat advantage that can shift the equilibrium toward the production of more oxygen and protons, higher concentration of protons will increase the rate of organic reduction reactions and hydrogen gas production.

Amongst the numerous compounds that were detected in the HTL-AP, about nine of the most abundant compounds were analyzed in this thermodynamic study. Acetic acid was reduced to methane or ethane or ethanol, propanoic acid was reduced to propane, butanol to butane, acetone to propane, methanol to methane, glycerol to propane, ethanol to ethane, pyridine to pentane, furfural to pentane. The results show that while it is rather difficult to reduce some HTL-AP organics in thermal reactor, it is possible to reduce a spectrum of HTL-AP organics in an electrolytic cell with a minimum energy requirement of -1.23 V. Based on the thermodynamic reduction potentials, the reaction mechanism also proved that the organics would be reduced in most cases before hydrogen evolution. The increase in temperature will increase product concentrations significantly and reduce energy requirements.