Co-pyrolysis of sewage sludge and lignocellulosic biomass: Synergistic effects on products characteristics and kinetics

Abstract

Co-pyrolysis of sewage sludge (SS) and corn stalk (CS) was conducted, followed by various online and offline analyses of pyrolysis products and reaction kinetics. Significant synergistic effects were found with the addition of CS to SS: the char yield decreased while the bio-oil yield increased significantly, and CO and CH4 yields increased. The sequential response order of functional groups in bio-oil to CS percentage is: alcohols → ketones → alkanes. The interaction of SS and CS significantly promoted the production of phenols and furans while inhibited the production of oxygen heterocycles and nitrogen compounds, which was beneficial for the subsequent utilization of bio-oil. The addition of CS to SS significantly increased the carbon content while reduced the sulfur content in the char. Activation energy (Ea) of the mixture was assessed by two iso-conversional methods, and Flynn-Wall-Ozawa (FWO) always had a higher correlation to the reaction than Kissinger-Akahira-Sunose (KAS). Average Ea of SS (604.42 and 631.76 kJ/mol) was higher than that of CS (251.18 and 258.05 kJ/mol), which implied that the addition of biomass was beneficial for SS pyrolysis. The pre-exponential factor was ≥1010 s−1, suggesting that the reactions occurring in co-pyrolysis were the loose junctional complex (simple complex) in nature. The co-pyrolysis of SS and CS followed the power law model reaction mechanism (P3). The synergistic effects were mainly caused by the radicals (e.g., H, HO, CH3, OCH3) formed from the devolatilization of CS and the metallic elements (e.g., Al, Fe, Ca, Mg, K) present in SS. The research findings provided practical and new insights into a cleaner and more efficient co-pyrolysis strategy.