Interaction between sewage sludge and biomass components for enhanced biofuel production via pyrolysis based on studies of the mild acid pre-treatment process

Sewage sludge (SS) and rice straw (RS) are being investigated as a potential valuable resource for the production of biofuel through pyrolysis in a fixed bed reactor. Sewage sludge and rice straw, along with their treated variations, were co-pyrolyzed in this study under fixed temperature. The product yields and characterization study were assessed in order to evaluate the impact of acid pre-treatment of SS and RS. The distribution and composition of the biofuel yields were affected differently by the co-pyrolysis of raw SS or treated sewage sludge (TSS) with either raw RS or treated rice straw (TRS). Individual pyrolysis of RS produced maximum pyrolysis oil of 47.4 wt% than the pyrolysis of SS. On the other hand, pyrolysis of TRS and TSS produced the maximum oil yield of 54.3 wt% and 40.6 wt%, respectively. Under co-pyrolysis operation, the combination of TRS + TSS produced the maximum pyrolysis oil, char, and gas yield of 47.7 wt%, 42.0 wt%, and 10.3 wt%, respectively. In this study, the degree of synergy on oil yield was lowest in SS + RS and maximum in TSS + TRS. Pre-treatment of SS outperformed the production of char yield, where it reduced the yield of char by up to 11.8 % and produced biochar with a maximum heating value of 16.02 MJ/kg. Compared to other combinations, the char obtained from TRS has a higher heating value of 22.77 MJ/kg. Additionally, pre-treatment and co-pyrolysis affected the contents of pyrolysis oils, raising the heating value to 18.01 MJ/kg. The chromatographic analysis of the pyrolysis oil results shows that the pre-treatment process increased the yield of phenols, furans, and other aromatic hydrocarbons. At last, the co-pyrolysis in combination with pre-treatment affected the pyrolysis gas profile, altering the yield of various gas fractions such as hydrogen (H₂), methane (CH₄), carbon monoxide (CO), carbon dioxide (CO₂), and ethane (C₂H₆).