Analysis of Convection and Secondary Reaction Effects Within Porous Solid Fuels Undergoing Pyrolysis

Abstract

A mathematical model of transport phenomena (heat, momentum and mass transfer) and chemical processes (primary and secondary reactions) of the thermal degradation of wood is presented. Implicit finite difference equations for energy, momentum and chemical species mass balances are formulated according to an operator-splitting technique and are numerically solved. The progress of the pyrolysis process along a wooden slab, radiatively heated on one side, is characterized by the following main processes: 1( a virgin wood region, crossed by a slow flow of pyrolysis products, where temperature and pressure values decrease as the non-irradiated boundary is approached; 2( a primary pyrolysis region where, due to the relatively low temperatures, secondary reactions are not active and 3( a char layer where volatile products of primary pyrolysis mainly flow and, temperature being higher, undergo secondary reactions. For low medium permeabilities, a peak in the gas overpressure is observed, separating the virgin wood and the pyrolysis region and two velocity distributions, directed towards the virgin wood and the char layer. Time and space evolution of main variables and reaction product distribution, as internal flow convection varies on dependence of wood and char permeabilities, are simulated. The effects of variations in the kinetic data and energetics of primary and secondary reactions, with special emphasis on the coupling between flow convection and extent of secondary reactions, are also analyzed.

Key words:

• modeling
• wood
• pyrolysis
• heat transfer