Smoldering Combustion Studies Using TA/MS

Bruce Waymack & Diane Kellogg

Research, Development & Engineering Department

Philip Morris, Inc., PO Box 26583, Richmond, VA. 23261 USA

The purpose of this presentation is to give an overview of the processes and mechanisms involved in smoldering combustion of biomass materials such as paper, fabric, wood or tobacco. While flaming combustion has received more attention, the study of smoldering combustion is important, because it is the combustion mechanism when no flame exists. Smoldering often precedes flaming conditions with solid materials. With tobacco products, all smoke components arise from the smoldering process, therefore fundamental studies on the subject are needed to be able to make desirable changes.

Smoldering combustion proceeds by a process of pyrolysis, producing volatile products and a solid char; then the solid char in turn oxidizes with diffusing oxygen to produce the heat that keeps the whole process self-sustaining. The pyrolysis is a complex series of events producing basically all of the volatile products with the exception of carbon oxides and water. To gain understanding of any of the many classes of products formed, the pyrolysis step(s) need to be fully investigated. To gain understanding of how the process sustains itself, the oxidation step(s) need to be thoroughly studied.

Thermal analysis using TG/DSC/MS is a very useful technique for studying the pyrolysis events, performed in an inert atmosphere such as helium, and the oxidation events performed in an oxygen atmosphere. Small samples and controlled heating minimize the effects of heat and mass transfer and allow better elucidation of the principal chemical reactions. Pyrolysis starts with a vaporization of volatile materials, then a primary decomposition of non-volatile components producing a solid char. Depending on the availability of oxygen, the char may undergo secondary higher temperature pyrolysis before or concurrent with oxidation. The primary pyrolytic decomposition produces a large number of tars and gases. The secondary high temperature pyrolysis of the solid char is a source of many of the hydrocarbons and aromatic compounds found in the final volatile products. The amount and reactivity of the char formed determines how well the smoldering process will continue or spread. Above 300 C, pyrolysis and oxidation can occur simultaneously. Most of the pyrolytic reactions are endothermic and on the order of 100 J/g; whereas the oxidation processes are exothermic and on the order of 10,000 J/g. Data will be shown for tobacco, paper and a model biomass component, xylan. Some kinetic data will also be presented.