A two dimensional model for torrefaction of large biomass particles

•A 2D phenomenological model coupled to two step kinetic model was developed.•Kinetic model considers formation of secondary char from volatiles.•Conversions, temperatures, gas velocity, internal pressure can be predict.•2D contours of properties inside the particle can be obtained.•Optimal operational conditions for different particle sizes are possible to find.

Torrefaction is defined as a thermal pre-treatment process performed within a temperature range of 200–300 °C, at low-heating rates (<20 °C/min) and for residence times between 15–60 min in inert environments. A phenomenological model of the torrefaction process of large biomass particles is developed in this work. Mass and energy balance coupled to a kinetic model take into account two steps of the biomass decomposition. First of the two steps, considers simultaneous production of vapor and solids from raw biomass. The vapor phase comprises a mixture of condensable and non-condensable gases, while the solid phase consists of torrefied biomass. The second step involves decomposition of volatiles into gases and secondary char. The model analyzes torrefaction behavior of both large and small biomass particles, predicting their final solid and gas yields, temperatures distribution, internal pressure and velocity of the gas phase within the particles. The model also predicts maximum conversions for given particle sizes and temperatures during the process. For given set of conditions small particles showed higher (∼77%) than that (∼52%) for large particles. Maximum interstitial gas velocities inside the large particle (25 mm in diameter and 65 mm in length) was about 1.2 mm/s and pressure gradients of about 2000 kPa and it occurred after 20 min in the process.