Richard R. Baker

The pyrolysis of tobacco ingredients

Relationships between tobacco components and smoke products are complex and often difficult to unravel. Pyrolysis experiments have commonly been used to establish such relationships. However, unless they are performed under dynamic conditions that are relevant to those that occur during tobacco burning, results can be obtained which have little resemblance to those obtained during cigarette smoking. The relevance of pyrolysis experiments to the behaviour of tobacco ingredients in a burning cigarette is considered. Based on the temperature, heating rate, oxygen levels and gas flow conditions that occur inside the burning zone of a cigarette, together with a review of relevant pyrolysis and smoking experiments, a set of pyrolysis conditions has been developed that approximates those occurring in the pyrolysis region of the burning cigarette. The conditions include heating the sample at 30 ◦ Cs −1 from 300 to 900 ◦ C under a flow of 9% oxygen in nitrogen. Experiments on the pyrolytic behaviour of eleven relatively volatile substances under these conditions give results that are in good agreement with results from thirteen published studies in which cigarettes incorporating labelled versions of the substances were smoked. Subsequently, 291 single-compound tobacco ingredients have been pyrolysed under this set of conditions, most of which are relatively volatile. This enables the behaviour of these ingredients in a burning cigarette to be estimated in terms of intact transfer to mainstream smoke versus pyrolytic decomposition. It is predicted that almost a third of the substances would transfer to mainstream smoke at least 99% intact, and almost two-thirds would transfer 95% intact. Where pyrolytic decomposition does occur, the products are listed together with an estimate of the levels in smoke that would arise from the ingredient.

The Retention of Tobacco Smoke Constituents in the Human Respiratory Tract

Measurements on the retention of cigarette smoke constituents in the human respiratory tract have been undertake for more than 100 years. The first studies on nicotine retention were begun by Lehmann in Germany in 1903 and published in 1908. The first studies on the retention of smoke particulate matter were published by Baumbereger in the United States in 1923. Since those early publications, many studies have been undertaken, more or less continuously. This article is a review of the work that has been done over the last 100 years, including a large number of unpublished studies undertaken by British American Tobacco in Southampton, UK. The techniques used have evolved over the years and there is a certain amount of variation in the data. However, the general trends in the results are reassuringly consistent. The bulk of the studies indicate that, on average, 60 to 80% of the mainstream smoke particulate matter is retained in the lungs after inhalation. For nicotine, carbon monoxide, nitric oxide, and aldehydes the total retentions are of the order of 90–100, 55–65, 100, and approximately 90%, respectively, during cigarette smoke inhalation. For most smoke constituents the retentions in the mouth only are considerably smaller than in the whole respiratory tract. The lung retention values for smoke particulate matter are dependent on the depth of inhalation, hold time in the lungs, exhalation volume, and other factors. However, the degree of nicotine retention following inhalation is not markedly influenced by changes in respiratory parameters. Furthermore, the percentage retentions for smoke particulate matter and nicotine are smaller for nonsmoking subjects exposed to environmental tobacco smoke than with active smoking. The smoke retentions are related to properties of the smoke aerosol particles and gases and their behavior as they travel through the respiratory tract. This includes particle growth in the respiratory tract and evaporation of gases out of the particles, and relevant aspects of these processes are also reviewed.

A review of pyrolysis studies to unravel reaction steps in burning tobacco

Abstract The chemical constituents of tobacco smoke are generated in the burning zone of the cigarette where the processes of combustion, pyrolysis, distillation and aerosol formation occur. Tobacco itself consists of many different chemical components and consequently a large number of reactions occur in parallel as the cigarette burns. Relationships between tobacco components and smoke products are complex and difficult to unravel. Pyrolysis experiments have commonly been used to establish such relationships. However, unless they are performed under dynamic conditions that are relevant to those that occur during tobacco burning, results can be obtained which have little resemblance to those obtained during cigarette smoking. Bearing in mind this limitation, a variety of pyrolysis studies are reviewed which give insights into the mechanisms and reaction pathways occurring in the cigarette. The oxides of carbon are formed by thermal decomposition and combustion of tobacco constituents, and carbon dioxide is further converted to carbon monoxide by carbonaceous reduction. Pyrolysis studies used to elucidate these three processes are discussed. Pyrolysis results are presented which indicate that the majority of the so-called semi-volatile components of cigarette smoke are formed from tobacco at temperatures below 600° C. A literature survey of the tobacco component-semi-volatile product routes has been summarised. It indicates that their formation is complex and only partially understood. A few components (e.g. nicotine and other alkaloids) are transferred directlyfrom the tobacco; most are formed principally as a result of pyrolytic decomposition of many tobacco components in parallel.

Product formation mechanisms inside a burning cigarette

Abstract Inside a burning cigarette a large variety of chemical and physical processes are occurring in an oxygen-deficient, hydroge-rich environment, with temperatures up to 950°C. The physical processes occurring during combustion are rationalised and a picture is presented of the major mechanistic regions inside the cigarette during puffing and natural smouldering. There are two major regions inside the burning zone of the cigarette where products are released: a heat producing combustion zone, and a pyrolysis/distillation zone just downstream of the combustion zone. The vast majority of organic smoke products are formed in the pyrolysis/distillation region. The location of these zones has been determined by mapping internal gas concentrations, and by measuring internal density and temperature changes. Product formation routes have traditionally been unravelled using isolated pyrolysis experiments. Recent work with radioactively labelled nicotine has shown that the results from such experiments can be very different to those obtained from the dynamic conditions inside the cigarette. However, attempts have been made in recent years to design pyrolysis experiments which simulate more closely the real conditions inside the burning zone of the cigarette. The oxides of carbon are formed by both combustion and thermal decomposition of tobacco constituents. The major features of these two mechanisms are discussed. Many studies have also deduced that a significant proportion of carbon monoxide is formed by the carbonaceous reduction of carbon dioxide. The occurrence of this reaction is used to interpret the effect of ventilation on the ratio of carbon monoxide/carbon dioxide yields, and the relative values of this ratio in mainstream and sidestream smoke. The generation of mainstream and sidestream smoke is reviewed, together with factors that contribute to the relative deliveries for particular smoke components. The most critical factor is the mechanistic origins of the component. Finally, recent mathematical models of some of the processes contributing to smoke formation are discussed. It is likely that this technique will be used increasingly in the future to test our ideas on smoke formation mechanisms.

Combustion and thermal decomposition regions inside a burning cigarette

Abstract Cigarettes have been smoked under continous draw conditions in an atmosphere containing oxygen-18, in order to locate the regions where carbon monoxide and dioxide are formed by thermal decomposition and combustion of tobacco. The carbon oxides found inside the burning zone (combustion coal) at temperatures above 500°C are formed largely by combustion of tobacco. Those found behind the coal at temperatures below 500°C are produced mainly by thermal decomposition of tobacco. The gas concentration at a given position inside the cigarette is dependent on the net chemical production of the gas, and the net rates of diffusive and convective flow through that region. Similarly, the temperature at any point inside the cigarette is determined by the net heat released by local chemical reaction, as well as heat transfer by conduction, convection, and radiation. The net rates of formation of heat and gases by chemical reaction at various positions inside the cigarette have been calculated from the empirical temperature and gas distributions, using heat and mass transfer equations. There is a strong exothermic region inside the coal where oxygen is consumed and the carbon oxides are formed by combustion. There is a strong endothermic region behind the coal where the carbon oxides are formed by thermal decomposition of tobacco.

The kinetics of tobacco pyrolysis

Abstract When tobacco is pyrolysed under non-isothermal flow conditions in an inert atmosphere, variation of the inert gas or its space velocity has only a minor effect on the profiles of formation rate versus temperature for seven product gases. Thus, mass transfer processes between the tobacco surface and the gas phase are very rapid, and the products are formed at an overall rate which is determined entirely by that of the chemical reactions. The effect of radical chain inhibitors (nitrogen oxides) on the pyrolysis is complex because of the resultant oxidation. Nevertheless, no evidence was found for the occurrence of radical chain reactions in the gas phase. A small proportion (less than 10%) of all the gases monitored are formed by homogeneous decomposition of volatile and semi-volatile intermediate products, in the furnace used. At temperatures above about 600°C the reduction of carbon dioxide to carbon monoxide by the carbonaceous tobacco residue becomes increasingly important. However, when tobacco is pyrolysed in an inert atmosphere, only a small amount of carbon dioxide is produced above 600°C and consequently its reduction to carbon monoxide contributes only a small proportion to the total carbon monoxide formed above that temperature. The rate of the tobacco/carbon dioxide reaction is controlled by chemical kinetic rather than mass transfer effects. Carbon monoxide reacts with tobacco to a small extent. When the tobacco is pyrolysed in an atmosphere containing oxygen (9–21% v/v), some oxidation occurs at 200°C. At 250°C the combustion rate is controlled jointly by both kinetic and mass transfer processes, but mass transfer of oxygen in the gas phase becomes increasingly important as the temperature is increased, and it is dominant above 400°C. About 8% of the total carbon monoxide formed by combustion is lost by its further oxidation. The results imply that inside the combustion coal of a burning cigarette the actual reactions occurring are of secondary importance, the rate of supply of oxygen being the dominant factor in determining the combustion rate and heat generation. In contrast, in the region immediately behind the coal, where a large proportion of the products which enter mainstream smoke are formed by thermal decomposition of tobacco constituents, the chemistry of the tobacco substrate is critical, since the decomposition kinetics are controlled by chemical rather than mass transfer effects. tobacco substrate is critical. In addition, the heat release or absorption due to the pyrolytic reactions occurring behind the coal will depend on the chemical composition of the substrate. Thus, together with the differing thermal properties of the tobacco, the temperature gradient behind the coal should depend on the nature of the tobacco.

Kinetic parameters from the non-isothermal decomposition of a multi-component solid

Abstract The literature on the application of isothermal homogeneous gas or liquid phase kinetic equations to the non-isothermal decomposition of solids is briefly reviewed. It is concluded that the deriving of kinetic parameters for solid-state decomposition reactions in terms of Arrhenius pre-exponential factor, activation energy and reaction order is empirically useful, but the theoretical significance that these parameters have for gas and liquid phase reactions cannot be extended to the solid phase. A method is presented whereby these kinetic parameters can be derived when a multi-component solid decomposes non-isothermally into several products. The formations of hydrogen, methane, and ethane from the thermal decomposition of tobacco are best described by a mechanism in which each product is formed from a different solid component. The high correlation coefficients obtained show that the homogeneous kinetic and Arrhenius equations are very good empirical descriptions of the reactions.

Electron spin resonance and spin trap investigation of free radicals in cigarette smoke: development of a quantification procedure

Abstract It has been shown in previous work that free radicals are generated in the mainstream smoke of cigarettes. The most direct method for the detection and quantification of these radicals is electron spin resonance (ESR) spectroscopy in conjunction with the spin trapping method. However, the nature of the spin adduct spectrum and the concentration of the radicals trapped in solution, will vary markedly depending on the experimental conditions employed. In order to apply ESR–spin trapping for analytical experiments in the quantification of free radicals in cigarette smoke, a rigorous set of experimental protocols must be developed. In the current paper, experiments were conducted in order to determine the optimal conditions for maximum signal intensities and reproducibility of results. A set of experimental protocols is therefore described for free radical quantification. These tests were optimised using the University of Kentucky IR4F reference cigarette and also applied to a set of commercial cigarette samples. The results show that radical concentrations in smoke vary amongst cigarettes in both the gas phase and particulate phases. Using the series of commercial cigarettes, where many parameters change from cigarette to cigarette, no statistically significant correlations were found between radical levels and total particulate matter in smoke. However, a weak correlation was found between the gas phase radical levels and total particulate matter levels in smoke. There may also be a complex effect of tobacco type on radical levels in smoke.

Formation of carbon oxides during tobacco combustion: Pyrolysis studies in the presence of isotopic gases to elucidate reaction sequence

During the combustion of tobacco, carbon monoxide is formed by the thermal decomposition of tobacco with primary products such as carbon dioxide and water. These three processes occur in parallel and are interdependent. The temperature ranges over which each process occurs, and their relative importance have been assessed by pyrolysing tobacco in the presence of various isotopically labelled gases. Non-isothermal pyrolyses were conducted at a heating rate of 1.6 K s−1 up to 1000°C, with the products analysed by mass spectrometer. Pyrolysis in the presence of oxygen-18 indicates that combustion of tobacco starts at 180°C. Carbon dioxide and water are formed by combustion at 180°C, while carbon monoxide is not formed as a combustion product until 460°C. The quantities of carbon monoxide and dioxide formed by thermal decomposition of tobacco above 400°C are significantly reduced by the occurrence of combustion. Pyrolysis in the presence of carbon-13 dioxide or carbon dioxide-18 shows that its major reaction, endothermic reduction to form carbon monoxide begins at 450°C. Pyrolysis in an oxygen-18/carbon-13 dioxide atmosphere has shown that this endothermic reduction of carbon dioxide occurs in parallel with the strongly exothermic oxidising reactions. 30% of the total carbon monoxide formed was produced by thermal decomposition of the tobacco. 36% was produced by combustion of the tobacco, and at least 23% was produced via carbon dioxide. The remainder was produced by an interaction of the carbon dioxide reduction and the oxidation. Similar proportion would be expected inside the reaction zone of a burning cigarette. Pyrolysis in the presence of heavy water has shown that the major reaction of the water is to quantitatively produce carbon monoxide and hydrogen above 600°C. Considerable isotopic exchange reactions also occur. Pyrolysis in the presence of carbon monoxide-18 has shown that carbon monoxide reacts with tobacco to a small extent at temperatures above 220°C mainly to abstract oxygen combined in the tobacco and produce carbon dioxide. A sequence of general chemical steps for the production of the carbon oxides and water during tobacco combustion has been deduced. This is based on the present work together with considerations of previously published studies on graphite and coal reactions.

Kinetic mechanism of the thermal decomposition of tobacco

Equations have been derived to describe the chemical kinetic factors that affect the rate of formation of products when a mixture of solid components (tobacco) decomposes on heating. Using these equations, a computer model of tobacco pyrolysis has been constructed which can calculate the gas formation rate/temperature profile from a given set of reaction parameters. By comparing the predictions of the model with experimental results at heating rates between 0.8 and 25 deg C s−1, a generalised kinetic mechanism for the thermal decomposition of tobacco has been developed. For carbon monoxide and other low molecular weight gases, the mechanism is an independent formation of each gas from one solid tobacco component in each temperature region. Pyrolysis of some individual tobacco components in other studies suggests that each gas is actually produced from many components in each temperature region. This more complex mechanism is kinetically equivalent to the deduced mechanism of independent formation from one component. The region in which a given decomposition reaction takes place moves to higher temperatures as the heating rate increases. The amounts of gases formed over any temperature region from 200 to 900°C can be calculated for a given heating rate using the mechanism and the kinetic constants. The present results imply that 75–90% of the carbon monoxide produced by tobacco decomposition at temperatures up to 900°C during a puff on a cigarette corresponds to that formed in the “low temperature region” (200–450°C) defined for pyrolysis experiments at the lower heating rates of 1–10 deg C s−1.