Two-regime global kinetics of cellulose pyrolysis: The role of tar evaporation

Abstract

Pyrolysis is a key step in the flaming combustion of most organic solids. Thus, pyrolysis kinetics have been studied extensively for many materials, including the widely used cellulose-containing materials. The global mass loss kinetics of cellulose, pyrolysis were studied earlier in this laboratory under various heating rate conditions, and it was suggested that two different kinetic regimes exist. At high heating rates, the apparent activation energy appeared to be near 140 kJ/mol, as opposed to the more commonly reported low heating rate values of more than 200kJ/mol. This is a controversial suggestion. Heat transfer limitations do not appear to be the basis for the observed behavior. In this study, mass transport limitations have been examined under typical pyrolysis conditions. It is hypothesized that evaporative escape of tars, the main product of cellulose pyrolysis on a mass basis, might be the process that is transport limited. Evidence is presented showing tar mass transport limitations both within particles and external to particles (and even in the absence of particles). The vapor pressures of the tars have been measured and reveal a material of limited volatility. The volatility of tar is similar to that of a widely accepted major component of the tar, levoglucosan. The latent heat of vaporization of the tars (approximately 141 kJ/mol) is suggested to be the origin of the temperature dependence of mass loss kinetics at high heating rates.