Continuous hydrogen production from biodiesel wastewater treatment via supercritical water gasification

Abstract

The supercritical water gasification (ScWG) of wastewater samples generated at a local biodiesel industry was evaluated using a continuous plug flow reactor. The production of combustible gases, especially hydrogen, and the simultaneous degradation of the organic matter found in the wastewater samples were investigated. Firstly, the ScWG experiments focused on H2 production were performed. Under optimized conditions — temperature of 600 °C, feed flow rate of 10 mL/min, pressure of 25 MPa, and undiluted biodiesel industry wastewater — a carbon conversion of 74.74%, an H2 yield of 2.18 mol/mol C, and a total gas flow rate of 2864.6 N mL/min were achieved. Then, the effects of the addition of nickel-based catalyst, supported in cordierite, were assessed. The Ni/CRD catalyst improved carbon conversion to 88.74% and achieved a total gas flow rate to 3100.0 N mL/min. Secondly, the efficiency of wastewater treatment by the proposed ScWG process was evaluated. A carbon conversion of 98.8%, indicated by the reduction in the total organic carbon concentration, was achieved at a temperature of 700 °C, feed flow rate of 5 mL/min, pressure of 25 MPa without the addition of catalyst or oxidizing agents. The liquid phase generated in the process was thoroughly characterized (chemical oxygen demand, biochemical oxygen demand, oils and greases, pH, nitrogen compounds, metal concentrations, gas chromatography-mass spectrometry analysis) and phytotoxicity assays with cucumber seeds. The results indicated a significant reduction in toxicity, suggesting its potential use as fit-for-purpose water.