Unraveling the Reaction Mechanism for FAAE Formation during Fast Pyrolysis of Mixed Food Waste

This paper presents the detailed reaction mechanism for the formation of biodiesel-range compounds, i.e., fatty acid alkyl esters (FAAE), via a novel route involving fast pyrolysis. Our recent studies have shown that large fractions of FAAE can be obtained in bio-oil if fast pyrolysis of extractive-rich biomass is conducted under a particular combination of operating conditions, following extensive experimental work. The extractives must contain large fractions of lipids, and operating conditions must have an optimal value for volatile residence time, temperature, and heating rate for FAAE to form. Together, these factors lead to FAAE formation reactions, which require detailed mechanistic investigation for the advancement of this novel route to biodiesel production. Therefore, this paper presents an unravelling of the detailed reaction mechanism involving various possible routes having several intermediate steps. Overall, the FAAE formation reaction can be summarized as the reaction between the free fatty acids (FFA) generated via decomposition of lipids in the biomass and in situ alcohols (IA) generated via decomposition of the lignocellulosic part of the biomass. Formation of these intermediates was confirmed by the corresponding experimental results. Moreover, fatty acids and alcohols are necessary for FAAE formation, thus involving intermediate steps for their formation. The formation of FFA and IA, and their subsequent esterification to produce FAAE, is postulated to take place in the vapor phase inside the pyrolyzer. This postulation was based on variation in the bio-oil composition, especially the fraction of FAAE, FFA, and IA as the operating conditions changed. A higher fraction of FFA in bio-oil indicates that alcohols got consumed before the esterification reaction was completed. Conversely, in other cases, alcohol fractions were higher, thus hinting at the complete conversion of FAAE.