Signatures that differentiate thermal degradation and heterogeneous combustion of tobacco products and their respective emissions

Characterization of the onset of combustion reactions within organic materials, such as the tobacco in tobacco products, is very complex. Such materials are commonly used as a porous matrix and the combustion reactions, if present, are generally heterogeneous in nature and compete with drying, vaporization, endothermic and exothermic pyrolysis, as well as assisted exothermal degradation. Fundamental approaches towards the characterization of the onset of combustion reactions are not well established and therefore the literature only provides information that can be used as signatures that aim to reflect the presence or absence of a combustion reaction. These signatures many times link characteristic emissions to specific temperature bands in the organic material assuming local thermal equilibrium between solid and gas phases. This paper presents a review of the literature that reports on the characterization of ignition of heterogeneous combustion within porous combustible materials and the processes commonly associated with thermal degradation and combustion. The review covers emissions and other signatures that aim to establish the onset of heterogeneous combustion, and while the discussion remains general, this review focuses on the evidence that serves to differentiate thermal degradation from the onset of combustion of tobacco products. The literature shows consistency in that below a characteristic temperature of 400 oC, thermal degradation of the tobacco in tobacco products is net endothermic, the formation of products that can be related to oxidation is limited, yields of polycyclic aromatic hydrocarbons (PAHs) are negligible and atmospheric oxygen has very limited influence on the degradation chemistry. Furthermore, the reactions leading to the conversion of PAHs into soot do not occur until much higher temperatures. Therefore, the characteristic signatures associated to the effluent released below a characteristic temperature of 400 oC consist of gaseous compounds and liquid droplets. These liquid-based aerosols are fundamentally different from the carbonaceous soot particles that represent a characteristic signature of combustion. Moreover, an extensive review of the effluent of heated tobacco products during operation establishes that the effluent exhibits the signatures of liquid-based aerosols and does not show the characteristic signatures of combustion processes.