Torrefaction kinetics of almond and walnut shells

The effects of inorganic species on the torrefaction kinetics of almond and walnut shells between 240 and 300 °C were determined using thermogravimetric analysis. Raw shells were leached with water at 80 °C for 2 h to remove 80 and 86% of potassium from raw almond and walnut shells, respectively. During each isothermal kinetics run, the leached almond shells had higher mass yields than the raw shells, even at 240 °C. In comparison, the raw and leached walnut shells had comparable mass yields at 240 °C with the mass yields becoming different at higher temperatures. This was due to the higher concentration of 0.89% (w/w) potassium in almond shells compared to 0.08% (w/w) in walnut shells, since potassium had been shown to catalyze decomposition of biomass. The two consecutive parallel reactions kinetic model exhibited the best fit to experimental data. However, the one-step reaction model gave the best predictions of mass yields for torrefaction of raw almond shells in a fixed-bed reactor.