Thermodynamic analysis of aqueous phase reforming of three model compounds in bio-oil for hydrogen production

Thermodynamic analysis with Gibbs free energy minimization was performed for aqueous phase reforming of methanol, acetic acid, and ethylene glycol as model compounds for hydrogen production from bio-oil. The effects of the temperature (340-660 K) and pressure ratio ^Psys/PH2_O (0.1-2.0) on the selectivity of H₂ and CH₄, formation of solid carbon, and conversion of model compounds were analyzed. The influences of CaO and O₂ addition on the formation of H₂, CH ₄, and CO₂ in the gas phase and solid phase carbon, CaCO₃, and Ca(OH)₂ were also investigated. With methanation and carbon formation, the conversion of the model compounds was >99.99% with no carbon formation, and methanation was thermodynamically favored over hydrogen production. H₂ selectivity was greatly improved when methanation was suppressed, but most of the inlet model compounds formed solid carbon. After suppressing both methanation and carbon formation, aqueous phase reforming of methanol, acetic acid and ethylene glycol at 500 K and with ^Psys/PH2_O = 1.1 gave H₂ selectivity of 74.98%, 66.64% and 71.38%, respectively. These were similar to the maximum stoichiometric hydrogen selectivity of 75.00% (methanol), 66.67% (acetic acid), and 71.43% (ethylene glycol). At 500 K and 2.90 MPa, as the molar ratio of CaO/BMCs increased, the normalized variation in H₂ increased and that for CH₄ decreased. Formation of solid carbon was effectively suppressed by addition of O₂, but this was at the expense of H ₂ formation. With the O₂/BMCs molar ratio regulated at 1.0, oxidation and CO₂ capture increased the normalized variation in H₂ to 33.33% (methanol), 50.00% (acetic acid), and 60.00% (ethylene glycol), and the formation of solid carbon decreased to zero.