Temperature dependence of aspen torrefaction kinetics

• Three-stage chemical kinetic reaction model based on gas-phase product evolution.
• Product evolution is dominated by chemically formed water at early times.
• Larger processing times or severity is required to remove undesired organics.
• Arrhenius behavior with activation energies 1.05E2, 1.27E2, and 1.72E2 kJ/mol.
• Pre-exponential factors were found to be 3.32E9, 1.43E11, and 2.08E14 min−1.

Torrefied biomass provides opportunities for an alternative, renewable fuel in the energy market. Much work on torrefaction is based on weight loss transients without much insight into the evolution of volatile species. A previous model from our group has been developed that predicts the degradation of raw biomass into specific chemical species during torrefaction (water, acetic acid, carbon monoxide, carbon dioxide, formic acid and furfural) in three consecutive reaction steps. The goal of this work is to understand how the kinetics of torrefaction change with temperature. Kinetic parameters for aspen wood were obtained for the first 90 min of 260–300 °C (1000 °C/s) torrefaction. The pre-exponential factors were 3.32E9, 1.43E11, and 2.08E14 min−1, and the activation energies 1.05E2, 1.27E2, and 1.72E2 kJ/mol for the three reactions. Kinetic parameters found correspond well to similar values from global weight loss studies, and suggest that increased torrefaction severity leads to progressively more recalcitrant forms of torrefied biomass. These continuous production traces for the volatile organic species yield insight into torrefaction, and require careful consideration of torrefaction time and temperature to design the desired solid fuel product.