Minor steps with major impact on biomass thermochemical conversion through drying and torrefaction

Biomass is a promising renewable energy resource, but its high moisture content, low density, and poor physicochemical stability hinder its full potential. Pretreatment techniques such as drying and torrefaction help in mitigating these challenges. Biomass drying is an essential preliminary step, typically carried out at temperatures around 110 °C, to remove both free and bound moisture without altering its chemical composition. Drying helps in the torrefaction process by providing a uniform feed, ensuring constant physicochemical properties. In contrast, torrefaction is a mild thermochemical process used to improve the physicochemical properties, such as energy and mass density, hydrophobicity, storage, and grindability, of lignocellulosic feedstocks. The majority of volatiles come from the hemicellulose part of the biomass, which breaks down at 200–300 °C under an inert atmosphere, resulting in solid biochar. Solid biochar is an upgraded form of raw biomass with better fuel quality and can be utilized in various sectors, such as energy, metallurgy, chemical synthesis, and agriculture. Recent developments in reactor design, kinetic modeling, severity optimization, and integration with gasification and other thermochemical systems have broadened the technological relevance of torrefaction. In addition, environmental evaluations, such as life-cycle assessment and emission analysis, are increasingly important for assessing the overall sustainability of torrefied biomass. This review focuses on recent trends in drying and torrefaction processes and explores the lab-to-scale-up challenges and opportunities.