Hygroscopic transformation of woody biomass torrefaction for carbon storage

Biochar is a potential medium for carbon storage, so its production and storage have been considered as is a crucial route to effectively achieve negative CO₂ emissions. Meanwhile, torrefaction is a thermochemical conversion process for producing biochar. Biochar is featured by its hydrophobicity, which makes it different from its parent biomass with hygroscopicity and is conducive to material storage. To evaluate the hygroscopic transformation of biomass from torrefaction, two woody biomass materials of poplar (hardwood) and fir (softwood) are torrefied at temperatures of 200–230 °C, and the variations of color, equilibrium moisture content, and contact angle of raw and torrefied samples are examined. The results indicate that the total color difference of torrefied woods increases linearly with increasing mass loss. The hygroscopicity reduction extent in torrefied fir is higher than in torrefied poplar, and can be increased by up to 57.39% at 230 °C. The tests of the contact angle suggest that the hygroscopicity of the raw woods is evidently exhibited, whereas the angles of the torrefied woods are in the range of 94–113°, showing their hydrophobic surfaces (>90°). The decarbonization, dehydrogenation, and deoxygenation phenomena of the biomass during torrefaction are also analyzed. It is found that the three indexes can be correlated well by the total color difference and hygroscopicity reduction extent. A comprehensive study on the improvement of hydrophobicity of produced biochar has been performed, which clearly shows the potential of carbon storage and negative CO₂ emissions by biochar.