Hydrogen production and phosphorus recovery via supercritical water gasification of sewage sludge in a batch reactor

Abstract

In this study, gasification of sewage sludge in supercritical water using a batch reactor was investigated. The effects of temperature, retention time, and the oxidation coefficient on gas composition, gas yield, total organic carbon removal efficiency (X_TOC), gasification efficiency (GE), carbon gasification efficiency (CE), and phosphorus release rate (X_p) were investigated. The experimental results indicated that the yields for hydrogen, methane, and carbon dioxide increased with the increase in temperature from 380 °C to 460 °C. A maximum hydrogen molar fraction of 55.72% and a yield of 19.86 mol/kg were obtained at 460 °C and 27 MPa after 6 min. The GE, CE, X_TOC, and X_p also increased with the increase in temperature. An extension of the retention time promoted the gasification of sludge, thereby resulting in an increase in the hydrogen and methane molar fraction, yield, GE, CE, X_TOC, and X_p. Under the conditions of 420 °C and 27 MPa after 6 min, with an increase in the oxidation coefficient from 1.5 to 2.5, the oxidation reaction became dominant in the supercritical water. Hydrogen and methane were converted to carbon dioxide and water with an excess of hydrogen peroxide, which resulted in a lower hydrogen yield. However, the decomposition of organic compounds in the sludge was promoted with the addition of hydrogen peroxide, thereby resulting in an increase in the GE, CE, X_TOC, and X_p. When the oxidation coefficient reached 2.5, a maximum GE of 131.6% and X_p of 98.74% were obtained.

Keywords: Gasification; Hydrogen production; Phosphorus recovery; Sewage sludge; Supercritical water.