Modeling kinetics-transport interactions during biomass torrefaction: The effects of temperature, particle size, and moisture content

Abstract

A comprehensive one-dimensional model accounting for the effects of heat and mass transfer, chemical kinetics, and drying was developed to describe the torrefaction of a single woody biomass particle. The thermochemical sub-models depend only on previously determined or measured characteristics, avoiding the use of fitting or tuning parameters and enabling a rigorous energy balance of the process. Moreover, a high temperature drying sub-model is introduced which overcomes the difficulties associated with existing approaches to give physically consistent results, smooth implementation, and numerical stability. The particle model was validated against experimental data from the literature for intraparticle temperature profiles, particle mass and energy yields over a range of particle sizes and reaction temperatures. The modeling results describe well the three distinct stages observed during the torrefaction of large particles including the heatup, drying, heat release due to exothermic reactions resulting in thermal overshoot, followed by thermal equilibrium where conversion is governed by mass loss kinetics. The nonlinear effects of particle size, temperature, moisture content, and residence time on the mass and energy yields are quantified and explained. Larger particles exhibit a significant internal temperature gradient and strong temperature overshoot especially at the centerline. The magnitude of the overshoot is a function of the conductivity, particle size, and average heat release rate. Because of the rise in the reaction rate, higher temperatures increase the sensitivity of the process to particle size. Due to the dependence of drying rate on heat transfer limitations, the sensitivity of torrefaction to initial moisture content increases strongly with particle size.