Production of Fuel Gas through the Hydrothermal Gasification of Wastewater Using Highly Active Carbon-Base Catalyst

Abstract

Treatments of wastewaters containing various organic compounds in small concentrations are causing serious problems elsewhere in the world, therefore, the development of efficient process for treating wastewaters has been desired. In this study a novel Ni-supported carbon catalyst that we developed was used for gasifying the organic compounds dissolving in an industrial wastewater under hydrothermal conditions. The organic compounds were almost completely gasified above 275°C under the conditions examined. Main gaseous products were CH4, H2, and CO2, having the lower heating value of around 20 MJ/m3, indicating that the product gas can be used as fuel gas. The product gas composition was highly dependent on pressure: H2 and CO2 were dominant components under 5 MPa, and CH4 and CO2 were dominant ones under 20 MPa when the wastewater was gasified at 300°C. Equilibrium calculation for the reactions clarified that these results reflected the state of water, vapor or liquid, in the reactor. A simple calculation for mass and energy balance of the proposed process showed that 46% of the lower heating value of the organic compound (aqueous isopropyl alcohol solution of 3.33 wt% in concentration) can be recovered as fuel gas under the conditions of Tg = 350°C, P = 20 MPa even when this process is operated as a stand-alone one. This result shows that the proposed process can be not only a wastewater treatment process but an energy production process if certain conditions were met.