Reactivity of Biomass Char - Oxidation, Hydrocarbons and Inorganics
Bruce Waymack Philip Morris USA, PO Box 26583, Richmond, VA, USA
Smoldering combustion can be defined as self-sustaining combustion without the presence of a flame. This can occur in biomass materials such as paper, wood, fabrics and tobacco. These cellulosic materials form a solid carbonaceous char upon heating and release of volatiles, which combines with oxygen to produce the heat necessary to drive the whole process. The solid residue formed from the initial decomposition (usually 200 - 400 °C) is referred to as char. Study of the char is important for several reasons. First the reactivity of the char towards oxygen is critical to whether the process will sustain itself or happen at all. Secondly, we believe that some of the trace, less desirable, volatile products are formed during pyrolysis of this solid char at temperatures above 400 °C, especially in the presence of low oxygen concentrations experienced in the glowing coal zone of a smoldering combustion process. As well, we believe there are other gas-solid reactions than can occur within the solid char matrix during smoldering combustion that are important. Since most naturally occurring biomass materials contain significant inorganic constituents, we also need to study the effect of inorganics on char formation, gasification and reactivity.
This presentation will center on chars from cellulose, pectin, and tobacco. The general nature of char oxidation will be discussed for tobacco. Tobacco char undergoes at least two distinct oxidation processes due to different chemical functionalities in the char. The effects of inorganics will be shown with cellulose and pectin chars. The presence of alkali metals, in general, enhance the oxidation of cellulosic chars. However, in the case of pectin, where the metal starts out as a salt of the acid functionality of the carbohydrate, the metal complex has a profound retarding effect on oxidation.
The evolution of most aliphatic and aromatic hydrocarbons in smoldering combustion (such as in a cigarette) probably come from pyrolysis of the char in the range of 400 - 600 °C. This is also the same general temperature range over which oxidation of the char can occur as well. We will present data showing the rate of evolution of selected hydrocarbons and compare that with the rate of oxidation of tobacco chars. Effects of some inorganics on char pyrolysis will also be shown.
The thermal cracking reactions producing hydrocarbons from the char could be a gas-solid process, where the gas species react with the solid as they are leaving the solid matrix. We will also present data on reactivity of tobacco chars with gas phase species such as propene. The propene data shows two distinct reactions producing aromatic hydrocarbons; a lower temperature one in the solid and a higher temperature one in the gas phase.
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