Modelling and Simulation of Moist Wood Drying and Pyrolysis

Abstract

A transient, one-dimensional model which can simulate drying and pyrolysis of moist wood is presented. The porous wood is divided into four phases: solid, bound water, liquid water and gas phases. Conservation equations for energy and mass together with Darcy’s law for velocity and algebraic equations for the transport properties and physical properties are presented. The drying model is based on equilibrium between water vapor and bound or liquid water in the porous wood. For the thermal degradation process, two reaction schemes including a single one-step global and a multiple competitive reaction model which have been proposed in the literature, are included. A comparison between the two pyrolysis models has been made on dry wood and the results reveal that for the multiple reaction model, the heating rate and pyrolysis time have great influence on the ultimate char, tar and gas yields calculated. Simultaneously drying and pyrolysis of large wood particles are simulated. The effect of moisture content on the pyrolysis time is presented together with characteristic profiles for temperature, pressure and moisture distribution inside the particle. A temperature plateau can be observed at about 100°C where evaporation and condensation of liquid water takes place. The simulations show that the in-depth moisture content increases and exceeds the initial moisture content before evaporation. The reason for this increase is that when water evaporates in the front, some of it will be transported by convection and diffusion into the colder region and condensed.