Effect of temperature and acid zeolites on gas yield and composition during pyrolysis of municipal sewage sludge

Abstract

Pyrolysis gas derived from municipal sewage sludge can be employed for numerous applications, from energy recovery through combustion to fermentation toward biofuels and value-added materials production. Nevertheless, municipal sewage sludge-derived pyrolysis gas is generally subjected to cleaning treatments prior to valorization because of the harmful effect that components such as H₂S may have on technical equipment during combustion or on biological fermentation processes. To avoid these cleaning treatments, the quality of pyrolysis gas can be improved by adding catalysts during the pyrolysis process. In the present study, municipal sewage sludge pyrolysis was conducted at variable temperatures (350–800 °C) in a bench-scale batch pyrolyzer. In addition, the effect of different types of acid zeolitic catalysts (i.e., H-Mordenite and H-ZSM5), at variable SiO₂/Al₂O₃ ratios on product yields, pyrolysis gas composition, H₂S production, and S migration during pyrolysis at 500 °C was assessed. Zeolites addition resulted in an up to 55% increase in pyrolysis gas yield and an up to 24% increase in bio-oil yield. While H-ZSM5 (SiO₂/Al₂O₃ ratio = 1880) addition resulted in a 46% reduction of the H₂S concentration in the pyrogas, mainly through dilution in higher pyrogas yield, H-Mordenite (SiO₂/Al₂O₃ ratio = 220) resulted in a 42% reduction of the H₂S concentration, through the direct suppression of its formation. The results obtained suggest different selectivity of the zeolites toward the retention of H₂S and other S compounds, enlightening new perspectives on using zeolites in the catalytic pyrolysis of municipal sewage sludge.