Application of fourier transform infrared (FTIR) spectroscopy to investigate the degradation of cooking oils during fire suppression

Introduction Auto-ignition of cooking oils causes a significant proportion of household fires [1,2]. In addition, the majority of the 50% of all accidental fires in hotels, restaurants and fast food outlets that start in kitchens involve liquid cooking oil or fat as the primary ignition source [3,4]. The potential risk of loss of life and property damage due to cooking oils fires is high. Cooking oils and fats have flash points in the broad range of 160 to 335°C, and high auto-ignition temperature varying from 315-445°C. The flash point and auto-ignition temperature values reported in the literature can differ substantially for the same type of oil [2,5]. Cooking oil fires are difficult to extinguish because the temperature of the burning oil can quickly reach 400-600°C, where a substantial pyrolysis to volatile lower molecular weight species occurs, and it is taxing to cool large amount of such hot oil to below its auto-ignition temperature. In addition cooking oil can change its auto-ignition temperature during heating and fire suppression. The new auto-ignition temperature may be as much as 60 o C below the original auto-ignition temperature. Such significant lowering of the auto-ignition temperature was noticed during fire suppression tests carried out at the Fire Risk Management Program (FRMP) when a vat of oil that had been overheated to auto-ignition was extinguished with water mist [6]. This observation suggested that thermal stress followed by exposure to water discharge causes deep changes in oil composition that lower the auto-ignition temperature and create fuel more dangerous than fresh oil. This new, low auto-ignition temperature (~300°C), further increases complexity of extinguishing cooking oil fires. To date, however, there is no study or understanding of how this depression of cooking oil's auto-ignition temperature occurs. During the recent full-scale water-mist suppression tests of cooking oil fires conducted by the Fire Risk Management Program (FRMP) at NRC, Fourier Transform Infrared (FTIR) spectroscopy was used to investigate the degradation of the oil. In this paper the qualitative differences between the IR spectral features of the fresh, heated, and exposed-to-water oil are presented. The breakdown products generated during the oxidative and hydrolytic degradation of the cooking oil are described and the main degradation pathways of the oil are explained. The role of the flash-hydrolysis products in the lowering of the auto-ignition temperature of the oil is also discussed. A commercial deep fat fryer with a capacity of 20 L of oil …