A two dimensional model for torrefaction of large biomass particles

Abstract

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 15–60min 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 (25mm in diameter and 65mm in length) was about 1.2mm/s and pressure gradients of about 2000kPa and it occurred after 20min in the process.