Two-Stage Gasification of Sewage Sludge for Enhanced Hydrogen Production: Alkaline Pyrolysis Coupled with Catalytic Reforming Using Waste-Supported Ni Catalysts

Abstract

Sewage sludge is a potential source of H2-rich fuel via a catalytic thermo-chemical reaction, which can facilitate the economical disposal of this hazardous waste. In this study, dry sludge is subject to a two-stage gasification process, i.e., the alkaline pyrolysis (P-stage) and the catalytic reforming (R-stage). The evolved volatiles from the P-stage are reformed in the R-stage with significant enhancement in both yield and concentration of H2. The Ni-doped catalysts are supported on dental wastes (DWs), with both significant environmental and cost benefits. The optimized H2 production (29.03 mmol g–1 sludge, dry basis) is observed when the R-stage temperature is 800 °C with the sludge/catalyst mass ratio at 1:1 (Ni loading of 10 wt % in the catalyst, specific surface area of 9.1 m2 g–1). The synergy between P- and R-stages is investigated through GC-MS analysis, indicating that the presence of NaOH in the P-stage eliminates the fatty acids in the tar with production of intermediates (e.g., alkane) which most likely contributes to easier R-stage reactions and enrichment of the H2 fraction.