Co-gasification of municipal sludge and black liquor in supercritical water for hydrogen production

This study investigated co-gasification of municipal sludge (MS) and black liquor (BL) in supercritical water for hydrogen production and pollution handling. The influence of the blending ratio (0–100 %), alkali content of BL (10 %, 20 %), reaction time (5–40 min), reaction temperature (400–600 °C) and total concentration (5-20 wt%) was investigated through thermodynamic analysis and experimental study. Thermodynamic modelling showed that MS addition increased the hydrogen fraction, but reduced the yield of other gas components. A synergistic enhancement was observed in MS/BL co-gasification, and the gasification efficiency had the highest improvement with MS/BL ratio of 40:60. Co-gasification also reduced N-containing compounds and alkylphenols in the liquid residue but increased phenol content. Increasing the alkali content of BL to 20 % boosted the synergistic effect, and shifted the optimal ratio to 50:50. The extension of reaction time from 5 to 20 min improved the gasification efficiency and hydrogen production, but further extension beyond 20min showed little impact. Increasing temperature favored the gasification, and the highest gasification efficiency (118.57 %), hydrogen yield (18.73 mol/kg) and COD removal rate (97.97 %) were obtained at 600 °C. These parameters showed different impact on the cold gas efficiency, H₂ selectivity and heating value of the gas product, where temperature exhibited highest influence.