Kinetics mechanism of inert and oxidative torrefaction of biomass

Torrefaction of biomass is a promising pre-treatment process capable of substantially improving the properties of raw biomass for its use as a solid biofuel, among many other potential applications. Oxygen-lean torrefaction can effectively reduce the cost and complexity of inert torrefaction. In this work, torrefaction tests of crushed olive stones were conducted in a thermogravimetric analyzer in various oxygen concentrations, under both non-isothermal and isothermal conditions. Non-isothermal torrefaction measurements were used to gain fundamental knowledge on the inert and oxidative torrefaction processes by applying a model-fitting kinetics method. Analysis of these measurements showed that an accurate description of inert torrefaction based on a two-step reaction mechanism is possible, whereas a three-step reaction mechanism is necessary for oxidative torrefaction. The new three-step mechanism was found to be accurate for describing the global mass loss during isothermal torrefaction, obtaining average root mean squared errors between the model predictions and the TGA measurements below 2.0 % for inert and oxidative torrefaction of olive stones. Furthermore, predictions using the extended mechanism were in good agreement with the more fundamental torrefaction reactions derived from the kinetics analysis of the non-isothermal torrefaction measurements.