Catalytic hydrothermal gasification of activated sludge

In this paper, Raney nickel is used as catalyst to gasify activated sludge in near-critical water (NCW) at a temperature range of 320–410 °C and a corresponding pressure range of 11.3–28.1 MPa in a batch reactor. At 380 °C and with 15 min reaction time, carbon gasification ratios (CGR) increased with increasing catalyst loading and reached 69% at 1.8 g catalyst/g dry sludge. Methane yield also linearly increased with catalyst loading while hydrogen production exhibited a maximum at about 1.5 g catalyst/g dry sludge. The gaseous product in this case consisted of 46% hydrogen, 25% methane and 29% carbon dioxide with trace amounts of carbon monoxide. The amount of generated gas as well as its composition did not change significantly after 30 min; however, increasing the temperature had a positive effect on the total yield up to 380 °C. The addition of sulfur to activated sludge resulted in a significant reduction of the carbon gasification ratio. CGR dropped by about 25% as the sulfur content of the feed increased from 0.8% to 3.0%. The deactivation of the catalyst due to hydrothermal sintering was also examined. Raney nickel significantly lost its activity upon pre-exposure to near-critical water at 380 °C for 8 min. This loss in activity had a major impact on methane production where methane yield dropped by six folds to a level similar to the catalyst-free gasification reaction and hydrogen yield was decreased by 50%. The decrease in catalytic activity of Raney nickel was likely due to an increase in average nickel crystallite size caused by hydrothermal sintering.

Graphical abstractNear-critical water gasification of municipal activated sludge at 380 °C.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Raney nickel exhibited good activity to gasify municipal activated sludge in near-critical water. ► At 380 °C and with 1.8 g catalyst/g dry sludge, carbon and hydrogen gasification ratios were 69% and 113%; respectively, after 15 min reaction time. ► Raney nickel lost half of its activity after 8 min exposure to NCW at 380 °C.