A Model for the Combustion of Large Particles of Densified Wood

Abstract

A mathematical model describing the thermal degradation of densified wood particles is presented. The influence of structural changes on the heat transfer properties of wood in its different stages is studied. The model uses a specific discretization scheme and combines intraparticle processes such as drying, pyrolysis and char heterogeneous reactions with extraparticle processes such as heat and species diffusional and convective transport. The model also accounts for particle shrinkage. The model is used to compare the predicted data to data on the mass loss dynamic and internal temperature of several particles from the relevant literature, and a good agreement is found.

An overview of processes and considerations in the modelling of fixed-bed biomass combustion

“…However, this model was not validated by experimental measurements and more work should be carried out to develop a comprehensive model that takes into account mixing effects. It should be noted the correlations of Bryden et al [103], Blasi [144], Yang et al [65] as well as Porteiro et al [119] are useful for wooden biomass, whereas for straw a different correlation appears more relevant [93]. Table 16 represents these correlations.…”

“…Pyrolysis models with competing parallel reactions are used in the investigations [68,119] and it is believed that these models have sufficient flexibility and accuracy to describe pyrolysis process in biomass combustion. Wood decomposition into gas, tar and char is modelled through three parallel first order reactions by Shafizadeh and Chin [120].…”

“…Different types of kinetic constants proposed in addition to the consideration of feedstock types derived using various experimental temperature ranges that summarized by di Blasi [10]. Pyrolysis models with competing parallel reactions are used in the investigations [68,119] and it is believed that these models have sufficient flexibility and accuracy to describe pyrolysis process in biomass combustion. The literature covers a wide spectrum of reaction rates from very fast by Thurner and Mann [124], fast by Nunn et al [125], medium rate by Wagenaar et al [126], slow rate by di Blasi [10], Thurner and Mann [124] and Font et al [127], and very slow rate by Samolada and Vasalos [128], Ward and Brashlaw [129].…”

“…This volume shrinkage occurs during all sub-processes of thermal conversion. Porteiro et al [119] studied a model for the combustion of a large and densified wood taking into account particle shrinkage. They assumed that shrinkage varies linearly during drying and pyrolysis processes and defined a shrinkage factor as the ratio of actual volume to initial volume of the solid particle considering the pore size and specific inner surface area.…”

Ignition Behavior of Bio-Coke (Highly Densified Biomass Fuel) in High-Temperature Air Flows

“…Concerning Biot numbers, the BIC in the present study is classified in the "thermally thick" regime, at which Bi of the BIC is calculated around 2.5 by using the thermal conductivity derived from the literature [22]. Although some researchers [11,[23][24] have investigated the pyrolysis and combustion behavior of a large biomass block which is in the thermally thick regime, the ignition process was not taken into account in their studies. Since they did not focus on the ignition behavior of "thermally thick" biomass blocks, the two modes of ignition which is found in the present study have not been pointed out.…”