Mapping of tobacco conversion characteristics in electrically heated systems: Effect of air and temperatures on the onset of combustion and formation of volatile species

The understanding of tobacco pyrolysis and oxidation mechanisms is an important topic for tobacco science in order to reduce the emissions of toxic species and to better control the conversion of tobacco in electrically heated tobacco products (EHTPs) by avoiding combustion. In this work, we have instrumented experimental Tobacco Heating Devices (expTHD) with a micro-positioning system of a thin thermocouple. Each expTHD has a pre-programmed set temperature (between 250 and 550°C) to be able to investigate the influence of temperature on tobacco conversion characteristics and emissions. Puffing cycles were conducted under air or N₂ in order to understand the effect of oxygen on tobacco conversion and the resulting emissions. Electrically Heated Tobacco Products (EHTPs) were heated by the expTHD to the different final set temperatures, and puffs were drawn according to the specified puffing cycles. The conversion of tobacco becomes clearly exothermic with air from 400°C after 8 puffs, indicating that combustion was triggered. Below a set temperature of 400°C, the tobacco conversion is always net endothermic, which includes the tobacco temperatures of commercially available Tobacco Heating Systems (THS) during operation (typically lower than 325°C). Furthermore, the volatiles emitted during the puffing cycles were sampled in cold impingers. PAHs (naphthalene, phenanthrene, pyrene, and benzo[a]pyrene) were quantified by HPLC-UV Fluorescence. Benzene and toluene were quantified by GC/MS. Gases (CO, CO₂, CH₄, H₂) were quantified by FTIR and µGC. The mass yields of all these species are presented as a function of the final set temperature of the expTHD and of the carrier gas used during the puffs (air or N₂). CO₂ yields are higher for air than for N₂ even at 250°C, highlighting some low temperature oxidation reactions, but they did not lead to a detectable exothermic regime. A jump in CO formation is observed from 400°C under air, indicating combustion of the tobacco. Benzene and PAHs are promoted by air (compared to N₂) from 400°C (heater set temperature). Therefore, air does not promote the formation of these species during the operation of the commercially available THS.