In this work, Norway spruce stem wood, stump and bark were torrefied in a bench scale tubular reactor at 225, 275 and 300 °C with two residence times (30 and 60 minutes). The effects of torrefaction process conditions and feedstock type on the physical properties, chemical composition and grindability of torrefied biomass samples were investigated. Furthermore, information was also obtained by conducting scanning electron microscopy (SEM) analysis to gain insights into changes of microstructure and morphology of biomass samples upon torrefaction at different conditions. Higher heating value and fixed carbon content of torrefied biomass samples increased with increased torrefaction severity. Torrefaction caused decrease of hydrogen-to-carbon (H/C) and oxygen-tocarbon (O/C) atomic ratios with increasing temperature and residence time, which results in increase of energy density of torrefied biomass samples. Chemical compositions of torrefied biomass samples considerably changed with increase of torrefaction severity.For the stem wood and stump, the relative hemicellulose content significantly decreased from 42.3% and 29.8% to less than 1% after torrefaction at 300 °C for 60 minutes, *Manuscript Click here to view linked References respectively. The hemicellulose content of untreated bark decreased from 27.5% to 0.14% after torrefaction at the same conditions. Additionally, the cellulose content of the torrefied bark drastically decreased already to half the initial value at a torrefaction temperature of 275 °C, with only trace amounts left in the 300 °C torrefied products. The grindability of stem wood and stump were substantially improved after torrefaction treatment. The energy required for grinding stem wood and stump torrefied at 225 °C decreased to respectively 87 and 70 kwh/ton, which are less than 50% of the energy needed for grinding the untreated samples. For raw bark, much less grinding energy is required compared to those for raw stem wood and stump, and torrefaction has minor effects on the grindability of bark. The ground torrefied biomass samples have much smaller particles than those of the untreated ones. The improvement of grindability of torrefied biomass samples can be coupled to the weakening of the fibre bonds indicated by change in chemical compositions. SEM analysis results show that particles from ground torrefied samples lose their fibrous structure with decrease of length-to-diameter ratios, compared to untreated biomass samples. It explains the shift in particle size distribution curves towards smaller particles as obtained from the sieving tests.

Wet torrefaction of typical Norwegian biomass fuels was studied within the temperature window of 175–225 °C, using a benchtop autoclave reactor of 250 mL in volume from Parr Instrument. Two types of local biomass fuels were employed as feedstock, Norway spruce (softwood) and birch (hardwood). Effects of process parameters including pressure, reaction temperature, holding time, and feedstock particle size on the yield and properties of the solid products were investigated. It appears that birch wood is more reactive and produces less solid products than spruce wood in the same wet torrefaction conditions. Increasing pressure above the saturated vapor pressure of water enhances the torrefaction rate. Both reaction temperature and holding time have significant effects on solid product yield and fuel properties of wet torrefied biomass. The yield of solid products is slightly reduced with decreasing feedstock particle size. The ash content of biomass fuel is significantly reduced by wet torrefaction. In addition, a comparison between wet and dry torrefaction supported by regression analyses and numerical predictions shows that wet torrefaction can produce solid fuels with greater heating values at much lower temperatures and shorter holding times.

ABSTRACT. The pyrolysis kinetics of Norwegian spruce and birch wood was studied to obtain information on the kinetics of torrefaction. Thermogravimetry (TGA) was employed with nine different heating programs, including linear, stepwise, modulated and constant reaction rate (CRR) experiments. The 18 experiments on the two feedstocks were evaluated simultaneously by the 2 method of least squares. Part of the kinetic parameters could be assumed common for both woods without a considerable worsening of the fit quality. This process results in better defined parameters and emphasizes the similarities between the woods. Three pseudocomponents were assumed. Two of them were described by distributed activation energy models (DAEM), while the decomposition of the cellulose pseudocomponent was described by a self-accelerating kinetics. In another approach all the three pseudocomponents were described by n-order reactions. Both approaches resulted in nearly the same fit quality but the physical meaning of the model based on three n-order reactions was found to be problematic. The reliability of the models was tested by checking how well the experiments with higher heating rates can be described by the kinetic parameters obtained from the evaluation of a narrower subset of 10 experiments with slower heating. A table of data was calculated that may provide guidance about the extent of devolatilization at various temperatureresidence time values during wood torrefaction.

This work aims to analyze the torrefaction process with Norwegian birch and spruce as feedstocks. Torrefaction experiments were performed in a macro-TGA (thermogravimetric analysis) reactor with provisions for continuous volatile measurements through micro-GC (gas chromatography) and FTIR (Fourier transform infrared spectroscopy). The process temperature (225 and 275 °C), holdup time (30 and 60 min), and sample size (10 and 40 mm cubes) were included as variations in the experimental matrix. Fuel characterizations, DTG (derivative thermogravimetric) curves, product yields, hydrophobicity tests, grinding energies, and particle-size distributions are discussed. The raw fuels were used as a reference for the comparisons. It was found that the birch has a higher devolatilization rate than the spruce under all tested conditions, resulting in a larger percentage increase in its carbon content. An increase in the temperature has the strongest effect on the properties of the torrefied product among all of the studied parameters. At 275 °C, the solid yield decreased to 63% and 75% for the torrefied birch and spruce, respectively. In terms of torrefied product properties, the torrefied samples absorbed approximately one-third of the moisture compared to the raw fuels. The total grinding energy decreased up to 40−88% for the torrefied samples of both feedstocks. An increased percentage of fine particles (<180 μm) was found in the particle-size distributions of most of the torrefied samples. Overall, considerable improvements were observed in the properties of the torrefied products for both feedstocks. Results obtained from this study form the basis of a torrefaction feasibility study in Norway.

Torrefaction has been recognized as a promising strategy to improve handling and storage properties of wood-based pellets, thus producing a uniform-quality commodity with high energy density and hydrophobicity. In this work, pellets produced from spruce stem wood, bark, and forest residues were torrefied in a bench-scale tubular reactor at 225 and 275 °C with two residence times (30 and 60 min). The effects of torrefaction on general properties, grindability, mechanical properties, hydrophobicity, and microstructure of the studied pellets were investigated. The increase of torrefaction severity reduced mass yields, but the heating values and the fixed carbon content of the torrefied pellets considerably increased. The grindability of raw pellets was substantially improved after torrefaction treatment. The energy required for grinding torrefied pellets is less than 50% of the energy needed for grinding the untreated pellets. In comparison to untreated pellets, the particles from ground torrefied pellets have clearly smaller sizes in a narrower size range. The increase of torrefaction severity improved hydrophobicity of the pellets, which have high resistance to water uptake and maintain their integrity after immersion testing. Upon torrefaction treatment, the durability and tensile strength of the pellets slightly decreased. Scanning electron microscopy analysis results show that particles from wood pellets torrefied at 275 °C lost their fibrous structure with an evident decrease of length/diameter ratios compared to untreated wood pellets. The particles from ground torrefied pellets are more uniform in terms of shape and size. Torrefaction can considerably improve grindability and uniformity of wood-based pellets and make them more acceptable in pulverized fuel applications.