Torrefaction for biofuel enhancement and solid biofuels: Process optimisation and characterisation

Torrefaction is increasingly recognised as a promising thermochemical pre-treatment for upgrading low-grade biomass into coal-like solid biofuels with improved energy density, hydrophobicity, grindability, and storage stability. This review focuses on recent advances in torrefaction processes, fuel-property enhancement, process optimisation, and scale-up. Conventional dry torrefaction remains the most established route; at the same time, emerging methods such as hydrothermal carbonisation, oxidative torrefaction and superheated steam offer opportunities to improve feedstock flexibility, heat transfer, reaction kinetics, and product quality. Torrefaction generally increases higher heating value, energy densification, and fixed carbon content, and reduces moisture and volatile matter; however, these benefits are often accompanied by reduced mass yield and ash concentration. Recent optimisation studies, using response surface methodology and analysis of variance, have improved understanding of parameter interactions and feedstock-specific operating windows. Despite this progress, large-scale deployment remains limited by feedstock variability, non-uniform heat transfer, atmosphere control, and continuous feeding challenges.