Observed Kinetic Parameters during the Torrefaction of Red Oak (Quercus rubra) in a Pilot Rotary Kiln Reactor

Abstract

The torrefaction of red oak (Quercus rubra) was performed in a pilot rotary kiln reactor, and the apparent kinetic results were compared with the results of torrefaction performed in a bench-scale fluidized reactor. Mass loss, gross calorific analyses, ultimate analyses, and proximate analyses were applied to the final torrefied material. The experimental torrefaction temperatures were 250, 275, 300, and 325 °C, and the experimental total torrefaction times were 20, 35, 50, and 80 min. A significant variation of the energy content occurred in the range of temperature between 275 and 300 °C, with the energy yield changing from 97.5% to 83.6%, respectively. The molar ratios H:C:O for the torrefied red oak presented a behavior independent of the experimental equipment when the temperature ranged between 250 and 325 °C. For the torrefaction process of red oak in the pilot rotary kiln reactor, a first-order reaction and one-step kinetic model were fitted with a maximum error of about 7.5% at 325 °C. The observed reaction rate constant (k) for the rotary reactor was 0.072 min−1 at 300 °C, which was 71% lower than the reaction rate constant for torrefied red oak in a bench-scale fluidized reactor. Arrhenius analysis determined an activation energy of 20.4 kJ/mol and a frequency factor of 5.22 min−1. The results suggest significant external heat and mass-transfer resistances in the rotary system.