Effects of oxidative degradation on the single-droplet burning behavior of refined palm and Jatropha curcas oil

Abstract

To clarify the effect of oxidative degradation of vegetable oils on their droplet combustion behaviors, a single-droplet combustion experiment was carried out. Refined palm oil (RPO) and Jatropha curcas oil (JCO) were used as representative vegetable oils. First, these vegetable oils were oxidatively degraded for 12 h by the Rancimat method to produce degraded RPO (RPO12) and JCO (JCO12), respectively. The effects of degradation on fuel properties were evaluated in terms of the kinematic viscosity and fatty acid composition, which were measured with an Ubbelohde viscometer and GC–MS, respectively. The results revealed that JCO was more susceptible to oxidative degradation and its kinematic viscosity significantly increased after degradation. This was due to its high content of unsaturated fatty acids, such as oleic and linoleic acids. Then, single-droplet combustion experiments were performed at 0.1 MPa and 1023 K. A droplet with a diameter of approximately 0.5 mm was inserted into an electric furnace and spontaneously ignited. Puffings were observed under all conditions, regardless of fresh or degraded RPO and JCO. The puffing periods of RPO12 and JCO12 were longer than those of RPO and JCO due to the longer time from bubble expansion to droplet disruption. Furthermore, the change in the droplet diameter caused by droplet disruption was suppressed for JCO12 and RPO12, which was probably attributed to the increased kinematic viscosity. Finally, the diameter of secondary droplet released by the droplet disruption was explored. The average diameters of secondary droplet of RPO12 and JCO12 were larger than those of RPO and JCO, and this was probably because of the increase in surface tension. Novelty and Significance StatementThe novelty of this study is that a series of experiment, i.e., production of oxidatively degraded vegetable oils, fuel property analysis (viscosity measurement and fatty acid composition analysis), and droplet combustion experiment, were simultaneously carried out. Thus, effects of oxidative degradation on combustion behavior were systematically clarified for the first time. This study is significant in that it provides the knowledge on treating vegetable oils in internal combustion engines, which is necessary for realizing a carbon neutral society.