Steam-oxygen gasification of sewage sludge: Reduction of tar, H2S and COS with limestone as bed additive

Abstract

Biogenic residue conversion to fuel and chemicals via fluidized bed gasification and synthesis is a promising pathway allowing biogenic residues to replace fossil carbon sources. This study focuses on sewage sludge gasification with the application of limestone as bed additive for tar, H2S and COS reduction. Experiments were conducted in a 20 kW fuel input bubbling fluidized bed facility with steam and oxygen as gasification agents. The syngas composition, considering the concentrations of H2, CO, CO2, CH4, CxHy, H2S, COS, NH3 and tar species, was determined for different limestone to fuel ratios. The results show that the produced syngas has high H2 and CO concentrations of up to 0.37 m3 m−3 and 0.18 m3 m−3 respectively and is thus suitable for synthesis. Tar species from sewage sludge gasification are characterized by various methods and differ from those from wood gasification. It was found that a small additive ratio of ~0.06 kg kg−1 (limestone to fuel ratio) is sufficient to reduce heavy tar species up to 75%, and with higher additive ratios, light tar species can also be partially reduced. Moreover, H2S and COS capture is demonstrated, with H2S and COS concentrations decreasing by 40%–65% with increasing limestone additive ratio. The ammonia (NH3) concentration was not affected by the additive. This study concludes that limestone addition is an effective and low cost measure to mitigate tar related facility problems and to reduce the downstream tar and sulfur cleaning efforts.