Wide Range Experimental and Kinetic Modeling Study of Chain Length Impact on n-Alkanes Autoxidation

The control of deposit precursors formation resulting from the oxidative degradation of alternative fuels relies strongly on the understanding of the underlying chemical pathways. Although C-8-C-16 n-alkanes are major constituents of commercial fuels and well-documented solvents, their respective reactivities and selectivities in autoxidation are poorly understood. This study experimentally investigates the influence of chain length, temperature (393-433 K), purity, and blending on n-alkanes autoxidation kinetics under concentrated oxygen conditions, using both Induction Period (IP) and speciation analysis. It also numerically constructs new detailed liquid-phase chemical mechanisms for n-C-8-C-14 obtained with an automated mechanism generator. Macroscopic reactivity descriptors such as IP, combined to microscopic ones, obtained from GC-MS analyses, are herein used to emphasize similarities and discrepancies in n-alkanes autoxidation processes. Experimental results highlight a nonlinear IP evolution with n-alkanes chain length, a linear IP variation for two component paraffinic blends, and similarities among oxidation product families. Experimental data from the present study and from the literature are used to evaluate n-C-8-C-14 mechanisms on IP and on monohydroperoxides (ROOH) concentrations. Under pure O-2 conditions, mechanisms generally predict IPs within a factor of 3 for intermediate and high temperature and even lower when air is used instead of pure oxygen. In addition, the chain length impact is also well reproduced, with a reactivity increase from C-8 to C-12 and a plateau for higher chain length. Rate of Consumption (RoC) analyses of n-C-8 and n-C-12 mechanisms evidenced the main role of peroxy radicals in autoxidation through fuel consumption, and ROOH and polyhydropermddes (R(OOH)(2)) formation.