Torrefaction of Wood in a Quiescent Layer of Talc

摘要

Biomass torrefaction can be carried out in the unsealed reactor filled with a quiescent layer of mineral filler. Mineral filler limits the oxygen access toward the biomass from the environment and favors more uniform temperature distribution inside the reactor. This paper is devoted to the experimental analysis of the torrefaction of softwoods (pine and spruce) and hardwoods (trembling aspen, maple, willow, white birch, acacia, and apple tree) in a quiescent layer of talc. It is established that pine and spruce provide the maximum mass yield independent on the regime of torrefaction. The difference in the mass yield for different wood species can be 20% if other conditions are equal. At low temperatures, torrefaction proceeds in a kinetic regime, in which the rate of chemical reactions limits the macroscopic rate of hemicellulose decomposition. At elevated temperatures of torrefaction, a diffusive regime is realized. In this regime, the mass yield mainly depends on the intensity of oxygen and water vapor diffusion through the mineral layer. We propose a simple phenomenological model to substantiate the dependence of mass yield on the height of the mineral layer. Biomass combustion characteristics are studied on the base of thermogravimetry-differential scanning calorimetry analysis. It is shown that the intensity of the biofuel combustion on the initial stage decreases with the increase in torrefaction temperature. Hydrophobicity of the biofuel is assessed via measuring the contact angle. When all the torrefaction conditions are the same, pine appears to be more hydrophobic compared to birch.