Impact of organically bonded alkali metals on torrefaction: Part 2. Modeling

In torrefaction, the mass yield depends on the biomass type, size of the biomass, torrefaction temperature, and residence time. Mass yield curves vs. residence time are usually modeled based on biomass type at different torrefaction temperature. This work is the second part of a study on the effect of alkali metals on torrefaction in which the K content affects the degradation of biomass during torrefaction.In this part of the study, the mass loss of spruce wood with different content of K was modeled using a two-step reaction model based on four kinetic rate constants. The results show that it is possible to model the mass loss of spruce wood doped with different levels of K using the same activation energies but different pre-exponential factors for the rate constants. Three of the pre-exponential factors increased linearly with increasing K content, while one of the pre-exponential factors decreased with increasing K content. Therefore, a new torrefaction model was formulated using the hemicellulose and cellulose content and K content. The new torrefaction model was validated against the mass loss during the torrefaction of aspen, miscanthus, straw and bark. There was good agreement between the model and the experimental data for the other biomasses, except bark. For bark, the mass loss of acetone extractable material was also needed to be taken into account. The new model can describe the kinetics of mass loss during torrefaction of different types of biomass. This is important for considering fuel flexibility in torrefaction plants.