Anoxic thermal degradation of kānuka and tobacco between 180°C and 390°C : a comparative study : a thesis presented in partial fulfilment of the requirements for the degree of Masters of Engineering in Chemical and Bioprocess Engineering at Massey University, Manawatu, New Zealand
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2021
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Massey University
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Abstract
Heat-not-burn (HnB) devices are a recent innovation used to heat tobacco and other herbal material to temperatures up to 350°C, which are significantly below those typical of cigarettes. Similarly, wood smoke for food smoking is made at temperatures between 280-350°C in friction smokers, to far higher in smouldering combustion systems where local temperatures are similar to cigarettes. The drive towards lower temperature smoke generation is to allay concerns about harmful compounds that are produced at higher temperatures. The most well-known of these are polycyclic aromatic hydrocarbons (PAHs). Most research in tobacco smoking has naturally focussed on smouldering combustion up to 950°C. This research reported here is aimed at low temperatures, 180°C to 390°C, and in anoxic conditions. It compares two types of tobacco - loose leaf tobacco for roll-your-own cigarettes and HEETS which are a Phillip Morris cigarette used in the Philip Morris iQOS device - and kānuka wood, to understand the similarities and differences in energetics and compound formation. The analytical techniques used were thermogravimetric analysis (TGA), simultaneous thermal analysis using thermogravimetry and differential scanning calorimetry (STA-TG/DSC), evolved gas analysis mass spectrometry (EGA/MS), pyrolysis gas chromatography mass spectrometry (Py-GC/MS) and scanning electron microscopy (SEM). HEETS and tobacco have a weight loss event prior to pyrolysis, related to the evaporation of low molecular mass compounds, the most prominent being nicotine, but also the additive glycol. The degradation of hemicellulose, cellulose and lignin can be seen in the thermogravimetric data. Kānuka degradation occurs at higher temperatures than the tobaccos, likely due to greater cellulose crystallinity. The heat of pyrolysis trends from endothermic for very small samples to more exothermic for larger sample masses, due to the greater tortuous path distance that the escaping volatiles must travel leading to more secondary reactions. Tobacco and HEETS become exothermic with increasing sample size more quickly than kānuka. They have ten times the ash, which will catalyse reactions. The EGA/MS and Py-GC/MS identified the expected degradation compounds like acetic acid, furfural, mequinol, syringol, levoglucosan and isoeugenol. As a function of temperature, few degradation compounds were present at 180°C, but more appeared with temperature. PAHs were identified. Small molecule PAHs appear at 180°C and increase with temperature. The largest PAHs generally did not appear at 390°C. Specific nitrogen compounds are seen in the tobacco vapours, arising from nicotine (C₁₀H₁₄N₂) and tobacco specific nitrosamines. This research has shown that compounds, like highly carcinogenic benzo(a)pyrene, are less prominent at temperatures below 350°C. Tobacco and kānuka are both plant biomass and have similar components, although the high ash content of tobacco and the large cellulose crystalline structure of kānuka lead to compounds being released from kānuka at higher temperatures. If heat-not-burn technology was applied to wood smoke a higher set point temperature would be required. Overall, this research has shown the dynamics of low temperature heating although more research has been done to quantify health risks. Knowing how composition is affected by temperature is important for improving operational safety and minimizing chemical hazard risk of low temperature devices.