Fast pyrolysis of agricultural residues: Reaction mechanisms and effects of feedstock properties and microwave operating conditions on the yield and product composition

Fundamental understanding of the pyrolysis process plays an indispensable role in valorization of wastes and the development of novel sustainable technologies. This study introduces a novel approach by investigating the reaction mechanisms involve in Microwave-Assisted Fast Pyrolysis (MAFP) to unveil the thermal decomposition of agricultural residues: pecan nutshell (NS), sugarcane bagasse (SB), and orange seed (OS) biomasses. The holistic understanding of the pyrolysis process for these biomasses was analyzed based on the final chemical compositions and yields of bio-oil, biochar and biogas and correlated to the microwave processing conditions and feedstock's chemical composition. The findings revealed that the bio-oil is enhanced at moderated microwave energy (<5 GJ/t) as result of endothermic reactions such as heterolytic fragmentation, Maccoll elimination, Friedel-Craft acylation, Piancatelli rearrangement and methoxylation. The maximum yield of bio-oil for protein-rich biomass was due to selective heating (Paal-Knorr pyrrole synthesis, Baeyer-Villiger oxidation, Maillard reaction, and ring conversion of gamma-butyrolactone). The formation of biochar and biogas is attributed to the repolymerization of aromatic aldehydes, hydrocarbons, amines, and ethers, as well as dehydroxymethylation and dealkylation processes. This study provides a comprehensive understanding of the reaction mechanisms for several wastes using microwave pyrolysis, to establish the bases for effective valorization and agricultural waste management.