Biofuel production from olive mill wastewater through its Ni/Al₂O₃ and Ru/Al₂O₃ catalyzed supercritical water gasification

This article presents the findings of our research regarding the supercritical water gasification of a real biomass, olive mill wastewater, with Ni/Al₂O₃ and Ru/Al₂O₃ catalysts. The experiments were performed at five reaction temperatures between 400 and 600 °C, for five reaction times ranging from 30 to 150 s and at a constant pressure of 25 MPa. It was seen that the employment of catalysts enhanced the gasification and biofuel production yield, which mainly involved methane and hydrogen. Ni/Al₂O₃ was selective for hydrogen formation, whereas Ru/Al₂O₃ favored the formation of methane. The highest calorific value of the produced biofuel was obtained as 56123 kJ/m³ with Ru/Al₂O₃ at 600 °C and 60 s. Apart from biofuel production, the simultaneous treatment of the wastewater was also accomplished. At elevated temperatures and times, treatment efficiencies greater than 97% based on total organic carbon conversion were reached. Moreover, the kinetic evaluation of the catalytic supercritical water gasification of olive mill wastewater was performed based on first order reaction kinetics.