Enhanced solid biofuels from metal-oxide-assisted torrefaction of waste wood-based panels: Composition, structure and combustion behaviors

Waste wood-based panels (WWPs), which consist of wood fibers and adhesives, are a potentially valuable source of biowastes. However, their high nitrogen content and low energy density pose considerable challenges for energy recovery. This study proposed a novel strategy, namely metal-oxide-assisted torrefaction (MOT) integrated with combustion, to improve the waste-to-energy conversion of WWPs. The effects of metal oxides (CaO, Fe2O3, and MgO) on the composition, structure, and combustion behavior of the resulting MOT-derived biofuels were investigated using proximate and ultimate analyses, XPS, XRD, FTIR, and TG-MS. Under optimal torrefaction conditions (5 wt%, 300 °C, 30 min), metal oxides were found to improve both fuel properties and combustion performance. Specifically, they enhanced fuel upgradation (energy density ratio: 1.14-1.17, energy recovery efficiency: 81.9-86.7%) and denitrogenation (nitrogen density ratio: 0.60-0.65, nitrogen removal efficiency: 53.3-56.3%) by facilitating the formation of high-energy carbon bonds and stable nitrogen functionalities such as pyrrole-N and pyridine-N during torrefaction. Furthermore, compared to raw or conventionally torrefied feedstock, all MOT products exhibited better combustion behavior, characterized by reduced emissions of inert and nitrogenous gases, increased reaction stability, and lower activation energy. These improvements were most pronounced with CaO, owing to the promoting effects of alkali salts (e.g., CaCO3 and CaCxNy) formed during the torrefaction process. Therefore, integrating MOT with combustion offers a viable pathway for converting WWPs into clean and high-quality solid biofuels, providing important insights for optimizing the thermal valorization of such waste materials.