1 Department of Chemical and Biochemical Engineering, Technical University of Denmark2 CHEC Research Centre, Department of Chemical and Biochemical Engineering, Technical University of Denmark3 Risø National Laboratory for Sustainable Energy, Technical University of Denmark4 Department of Mechanical Engineering, Technical University of Denmark
Torrefaction is a mild thermal (200 – 300 ÛC) treatment in an inert atmosphere, which is known to increase the energy density of biomass by evaporating water and a proportion of volatiles. In this work, the influence of torrefaction on the chemical and mechanical properties (grindability and hygroscopicity) of wood chips, wood pellets and wheat straw was investigated and compared. The mass loss during torrefaction was found to be a useful indicator for determining the degree of torrefaction. For all three biomass, higher torrefaction temperature or longer residence time resulted in higher mass loss, higher heating value, better grindability, and less moisture absorption. However, severe torrefaction conditions were found not necessary in order to save energy during grinding, since strain energy and grinding energy decreased tremendously in the first 5 - 25% anhydrous weight loss. By correlating the heating value and mass loss, it was found that wheat straw contained less heating value on mass basis than the other two fuels, but the fraction of energy retained in the torrefied sample as a function of mass loss was very similar for all three biomass. Gas products formed during torrefaction of three biomass were detected in situ by coupling mass spectrometer with a thermogravimetric analyzer (TGA). The main components were water, carbon monoxide, formic acid, formaldehyde, methanol, acetic acid, carbon dioxide, and methyl chloride. The cumulative releases of gas products from three biomass fuels at 300 ÛC for 1 h were compared, and water was found to be the dominant product during torrefaction. The degradation kinetics of wheat straw was studied in TGA by applying a two-step reaction in series model and taking the mass loss during the initial heating period into account. The model and parameters were proven to be able to predict the residual mass of wheat straw in a batch scale torrefaction reactor with different heating rates well. It means the mass yield of solids in the real torrefaction facility can be predicted by simply knowing the temperature curve of the sample. The pellets pressed from torrefied spruce increased significantly in length after pelletization, which indicates worse quality of inter-particle bonding with correlation to higher torrefaction temperatures. Pine pellets that are torrefied subsequent to pelletization exhibited better durability, no spring back effect or disintegration was observed. A good correlation was found among compression strength of single pellet, durability of the whole batch pellets, and the energy use during grinding. The pellet durability can thus be estimated based on compression strength data of about 25 pellets.