The catalytic influence of low-cost natural minerals on sewage sludge gasification for hydrogen production

Sewage sludges were evaluated for hydrogen production by catalytic gasification in a fixed bed reactor under various gasifying agents, including CO₂, air, steam, steam-air and steam-CO₂ mixtures in order to improve the gasification efficiency. Catalytically active minerals, dolomite, olivine and limonite, have brought obvious improvements by a complete carbon conversion. The best results regarding the carbon conversion of 88% were obtained in steam-air gasification whereas carbon conversion exceeded 99% in the presence of catalytic minerals. The low calorific value of the char samples in 0.1–0.2 MJ/kg and high syngas yields of 15.4–17.0% in total upon catalytic gasification supported the improved efficiencies in comparison to the noncatalytic situation with 1.9 MJ/kg and 14.4%, respectively. CH₄ and C2–C5 compounds in the product gas have been reduced in the presence of catalytic minerals as an indication of the increased tar reforming activity favoring hydrogen production. GC-MS measurements of tar samples for steam-air gasification have confirmed the catalytic influence of these minerals on tar reduction via the ring-opening reactions or dealkylation reactions. All these effects were more pronounced in the presence of limonite as the optimal natural mineral for hydrogen production.