Catalytic hydrothermal gasification of activated sludge

• Published in: Applied catalysis. B, Environmental Environmental, 2011-06, Vol.105 (1-2), p.136-143
• Main Authors: Afif, Elie, Azadi, Pooya, Farnood, Ramin
• Format: Article
• Language: English
• Subjects: Activated sludge; Catalysis; Catalytic gasification; Chemistry; Exact sciences and technology; General and physical chemistry; Hydrogen; Hydrothermal; Raney nickel; Supercritical water; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0926-3373; 1873-3883
• DOI: 10.1016/j.apcatb.2011.04.003

Near-critical water gasification of municipal activated sludge at 380°C. [Display omitted] ► Raney nickel exhibited good activity to gasify municipal activated sludge in near-critical water. ► At 380°C and with 1.8g catalyst/g dry sludge, carbon and hydrogen gasification ratios were 69% and 113%; respectively, after 15min reaction time. ► Raney nickel lost half of its activity after 8min exposure to NCW at 380°C. 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 11.3–28.1MPa in a batch reactor. At 380°C and with 15min reaction time, carbon gasification ratios (CGR) increased with increasing catalyst loading and reached 69% at 1.8g 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 30min; 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 8min. 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.