Sorption behaviour of torrefied wood and charcoal determined by dynamic vapour sorption

The most important variable of biomass-based fuels is moisture, because it affects the entire logistic chain by creating problems related to transport, handling, storage, and combustion. Recently, there has been a growing interest in thermal pre-treatment of biomass-based fuels by torrefaction. Torrefaction is intended to overcome the moisture-related problems by significantly reducing hygroscopicity and thus the logistics of solid biofuels could be improved. In order to gain a deeper insight on the changes induced by torrefaction, the sorption properties of Finnish birch and spruce wood were investigated. The sorption isotherms, hysteresis, accessibility, and surface area were investigated with dynamic vapour sorption. Also the particle size distributions and the clustering behaviour of water molecules were examined. As a result of the thermal pre-treatment, accessibility, adsorption of water vapour, and hysteresis were reduced. Particle size distribution was shifted towards smaller particles and the surface area measured with the BET method was reduced. Hysteresis was also reduced, which may be linked to the overall reduction in material's hygroscopicity, as well as changes in porosity. The particle size affects sorption by increasing the surface area, but has a dual effect on accessibility, as it also blocks access to pores through agglomeration. In practice, the most important result is the increased hydrophobicity, but also the increased porosity and reduced particle size are to be considered as they can affect the handling and storage properties of torrefied and charred material negatively.