Applied Catalysis B: Environmental, Vol.105, No.1-2, 136-143, 2011
Catalytic hydrothermal gasification of activated sludge
In this paper, Raney nickel is used as catalyst to gasify activated sludge in near-critical water (NON) at a temperature range of 320-410 degrees C and a corresponding pressure range of 11.3-28.1 MPa in a batch reactor. At 380 degrees 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 1.5g 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 degrees 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 degrees 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. (C) 2011 Elsevier B.V. All rights reserved.
Keywords:Supercritical water;Hydrothermal;Catalytic gasification;Activated sludge;Hydrogen;Raney nickel