Torrefaction of waste wood-based panels: More understanding from the combination of upgrading and denitrogenation properties

As a type of special biowaste, waste wood-based panels (WWPs) have a high nitrogen content and typical biomass defect properties, which limit their clean and efficient thermal utilization. An upgrading and denitrogenation procedure prior to thermal utilization is necessary, but the relevant mechanisms and characteristics have not been clearly elucidated. Torrefaction pretreatment of three typical WWPs (plywood, fiberboard, particleboard) was conducted to investigate the evolution of carbon and nitrogen functionalities in relation to the upgrading/denitrogenation procedure. The results demonstrated that the combined upgrading/denitrogenation of WWPs was possible. Under optimal operating conditions (300 degrees C and 10 min), the denitrogenation efficiency could reach 60-70 wt% with a slight energy loss of < 30% and a high HHV (20-22 MJ/kg) of the corresponding torrefied products. The carbon functionality evolution (upgrading) was believed to be the continuous change from low-energy -C-H/-C-O/-C=O to high-energy aromatic -C-C/-C=C, which was related to the reactions of lignin (demethoxylation, chain scission and polycondensation), hemicellulose (deacetylation and degradation) and cellulose (degradation). The nitrogen functionality evolution (denitrogenation) was regarded as a conversion of amide-N into more stable heterocyclic-N (pyrrolic-N and pyridinic-N) through cross-linking reactions, together with a significant release of nitrogen containing gases (NH3 and HCN).