Catalytic conversion of bio-oil in supercritical water: Influence of RuO2/γ-Al2O3 catalysts on gasification efficiencies and bio-methane production

•High concentrations of bio-oil completely converted under supercritical water conditions.•Increased ruthenium loading in the catalyst led to dramatic increase in carbon gasification.•Increased ruthenium loading led in increased bio-methane production up to 29 wt%.•CH4/CO2 molar yield ratio exceeded unity at high ruthenium loadings.•High ruthenium loading in the catalyst showed higher catalytic activity and stability.

Catalytic supercritical water gasification of heavy (dewatered) bio-oil has been investigated in a batch reactor in the presence of ruthenium catalysts in the form of RuO2 on γ-Al2O3 support. The reactions were carried out at temperatures of 400 °C, 450 °C and 500 °C and reaction times of up to 60 min using 15 wt% of bio-oil feed. Increased ruthenium oxide loading led to increased carbon gasification efficiencies (CGE) and bio-methane production. Hence, using the 20 wt% RuO2/γ-Al2O3 catalyst, CGE was 97.4 wt% at 500 °C and methane yield reached nearly 30 wt% of the bio-oil feed, which gave a CH4/CO2 molar ratio of 1.28. There was evidence that the RuO2 was involved in the initial conversion of the bio-oil to carbon oxides and hydrogen as well as the reduction of the CO2 to methane via CO methanation. However, competition for CO consumption via the water-gas shift reaction was also possible due to the large presence of water as the reaction medium. This work therefore demonstrates that high concentrations of heavy fraction of bio-oil can be catalytically converted to a methane-rich gas product under hydrothermal conditions at moderate temperatures. The calorific values of the gas product reached up to 54 MJ kg−1, which is nearly 3 times the HHV of the bio-oil feed.

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