J. Agric. Food Chem. 2002, 50, 771−783
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Characterization of Char from the Pyrolysis of Tobacco RAMESH K. SHARMA, JAN B. WOOTEN, VICKI L. BALIGA, PAMELA A. MARTOGLIO-SMITH,† AND MOHAMMAD R. HAJALIGOL* Research Center, Philip Morris U.S.A., P.O. Box 26583, Richmond, Virginia 23261-6583
Pyrolysis of tobacco was studied in oxidative and nonoxidative (inert) environments at atmospheric pressure and temperatures ranging from 150 to 750 °C. The objective was to study the effect of pyrolysis conditions on the characteristics of the solid residue, i.e., char. The char was characterized using cross-polarization 13C nuclear magnetic resonance (CPMAS NMR), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), the Brunauer-Emmett-Teller (BET) surface area, and the elemental composition. The char yield from pyrolysis (i.e., nonoxidative) decreased sharply with an increase in temperature to ca. 22% (dry, ash-free basis) at high temperatures. In oxidative pyrolysis, i.e., in 5% oxygen, the char was completely oxidized above 600 °C. The gaseous product from pyrolysis at high temperatures contained a significant concentration of hydrogen. The surface area of the char was low, with a maximum of 8 m2/g at 400 °C. SEM analysis indicated that pyrolysis of the tobacco led to a gradual accumulation of inorganic crystals on the exposed surfaces, and some constituents also melted, resulting in the formation of vesicles by evolving gas. NMR analysis showed significant changes in pectin and sugar constituents of the tobacco and breaking of glycosidic bonds of cellulose at 300-500 °C before the char became predominantly aromatic at high temperatures. FTIR results showed a continuous decrease in the intensity of the OH stretch with temperature and the aromatic character to be at maximum at 550-650 °C. The H/C ratio of the char decreased continuously with temperature, while the O/C ratio became constant above 300 °C due to the presence of oxides and carbonates in the char. The results are consistent with the analysis of the evolved gases. KEYWORDS: Tobacco; char; characterization; pyrolysis
INTRODUCTION
Tobacco has been a widely used agricultural product at least since American colonial times. Currently, high-speed machines fabricate cigarettes by inserting a blend of tobaccos into a paper rod and attaching a filter tip. A cigarette is typically lit by a smoker with a match or other type of lighter and undergoes self-sustaining combustion. Many thousands of chemical substances are generated by combustion and pyrolysis reactions during the formation of cigarette smoke. The peak temperatures in the combustion region of the cigarette coal during a puff can reach 800-900 °C (1). Such high temperatures can lead to the formation of polycyclic aromatic hydrocarbons (PAHs). PAHs are ubiquitous products that form during the combustion of many forms of biomass. They can also be found, for example, in charred foods. A growing body of evidence suggests that PAHs arise from the high-temperature pyrolysis of the solid char that is formed at lower heating temperatures (2-6). Thus, an understanding of the chemistry of the char may lead to a better understanding of the mechanism of the formation of PAHs. * To whom correspondence should be addressed. † Present address: 3641 Braddock Dr., Lafayette, IN 47909.
The pyrolysis of tobacco has much in common with the pyrolysis of other forms of biomass. Pyrolysis of biomass is complex and leads to the formation of many volatile and semivolatile products and a solid residue, i.e., char (7-11). The relative distribution of the products is usually dependent on the pyrolysis conditions as well as on the type of biomass. Much of the literature on pyrolysis of biomass is focused on the analysis of the volatile components and the pyrolysis kinetics (12-15). Under severe pyrolysis conditions, it is the volatile products that are believed to undergo secondary reactions, resulting in the formation of PAHs (6, 16, 17). The extent of such reactions in the product char is not known but could be important under smoldering conditions (4, 5, 18-20). The extent of secondary reactions may be dependent on the characteristics of the char. There is a limited amount of information on the characterization of chars from the pyrolysis of different biomasses. Sekiguchi and Shafizadeh (19) and Mok et al. (21) found that chars from cellulose, formed at temperatures below 400 °C, had higher concentrations of aliphatic carbons and were more reactive compared to chars formed at higher temperatures. Boon et al. (2) also studied cellulose pyrolysis and suggested that the
10.1021/jf0107398 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/18/2002
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cellulose changed first into a furan/phenol polymer at ca. 270 °C. This was followed by disproportionation of the furandominated intermediate, which resulted in a completely aromatized product. Bonammy (3) studied the effect of pyrolysis conditions such as temperature and heating rate on transformations in petroleum pitches and other materials at temperatures up to 1000 °C. It was found that temperature or heating rates increase the production of volatiles (governed by chemical reactions) and also favor their retention (controlled by diffusion). Two distinct stages in the carbonization of organic materials were suggested. The first stage, solidification, is characterized by the completion of tar evolution, resulting in a loss of viscoelasticity. The second stage, carbonization, is the complete loss of gaseous products, leaving behind a brittle residue. Freitas et al. (22) performed NMR analysis on chars from peat and observed a preferential loss of carbohydrates and carboxylic acids before the chars became aromatic above 500 °C. Recent work with chars from chlorogenic acid and pectin (23) showed that the chars became increasingly aromatic with increasing temperature and showed virtually identical characteristics at high temperatures, even though chlorogenic acid is a phenolic compound and pectin is a carbohydrate. The effect of pyrolysis conditions such as temperature and reaction environment on the composition of char from a biomass such as tobacco has not been studied, and it is not clear how the complex makeup of the substrate would affect the char formation via interactions among its various constituents. An understanding of char formation from tobacco may provide an insight into the role of char in secondary reactions that occur during combustion, e.g., in a burning cigarette, as well as help understand the pyrolysis of other types of biomass. In this work, the effect of pyrolysis conditions on the composition of char from tobacco was studied. The char represents the solid residue that remains after pyrolysis. The pyrolysis experiments were done at atmospheric pressure and temperatures ranging from 150 to 750 °C. Both oxidative and nonoxidative atmospheres were used. The characterization was done using cross-polarization 13C nuclear magnetic resonance (CPMAS NMR), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), the BrunauerEmmett-Teller (BET) surface area, and the elemental composition. The volatile products from pyrolysis were analyzed by on-line mass spectrometry and by gas chromatography/mass spectrometry (GC/MS). The results of characterization are discussed in relation to the evolved gas analysis. MATERIALS AND METHODS The tobacco sample was a mixture of Bright, Burley, and Oriental tobaccos in a 0.3/0.3/0.3 ratio. The sample was used in ground form (
