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NUMERICAL MODELLING OF WOOD PARTICLE PYROLYSIS

DOI: 10.1615/ICHMT.2009.CONV.1260
17 pages

Ulf Sand
Department of Public Technology, Fluid Dynamics Research Group Mälardalen University, Box 883, SE - 721 23, Västerås, Sweden

Rebei Bel Fdhila
Department of Public Technology, Fluid Dynamics Research Group Mälardalen University, and ABB AB, Corporate Research, Västerås, Sweden

Résumé

This article provides a description of the modelling and simulation of the pyrolysis process of a small wood cylinder of 5.4mm height and diameter in a heating chamber containing nitrogen gas. The model considers forced and free convection inside and outside the particle. Convective, conductive and radiative heat transfer modes are included in this approach. The conversion from wood to char, tar and pyrolysis gas is modelled using a two-step reaction scheme.
This approach offers a novel analysis advantage by involving simultaneously the particle and the surrounding atmosphere. This allows for a realistic prediction of the strong coupling between the inside and the outside of the particle. Earlier numerical works on pyrolysis of wood particles are generally considering the surrounding gas through the boundary conditions of the particle.
Numerical results are successfully compared with measurements of the particle surface temperature profiles and the mass depletion rate at quiescent external flow. The influence of free and forced convection (0 ≤ ReD ≤ 135) on the overall pyrolysis time is investigated and a Nusselt number correlation is proposed. Based upon the results of the present investigation, a Nusselt number correlation accounting for the pyrolysis effects is proposed.
The pyrolysis gas plumes can reach 0.1 m s−1. This is the same magnitude as the wood particles slip velocity in industrial applications. This reveals that in complex process situations as gasification or fluidized beds, the gas released by many small wood particles can strongly influence the dispersion characteristics. However, this study shows that the rocket force magnitude is significantly smaller than the drag and buoyancy forces.

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