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 C8–C16 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-C8–C14 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-C8–C14 mechanisms on IP and on monohydroperoxides (ROOH) concentrations. Under pure O2 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 C8 to C12 and a plateau for higher chain length. Rate of Consumption (RoC) analyses of n-C8 and n-C12 mechanisms evidenced the main role of peroxy radicals in autoxidation through fuel consumption, and ROOH and polyhydroperoxides (R(OOH)2) formation.
Because of the recent changes in the formulation and handling of middle-distillate fuels, oxidation stability is becoming an increasingly important issue. However, liquid-phase oxidation kinetics of middle-distillate fuels remains poorly understood. The purpose of this study was to gain an in-depth understanding of the impact of fatty acid methyl ester (FAME) addition on autoxidation kinetics. A detailed kinetic mechanism for the autoxidation of a n-dodecane/methyl oleate (MO) surrogate mixture was generated and validated against original well-controlled accelerated oxidation experiments. Results emphasize the nonlinear oxidation promoting effect of MO on n-dodecane autoxidation. Pathway analyses reveal that HO 2 and OH propagation steps as well as the duration of initiation and propagation phases strongly affected sensitivity analysis by MO addition. On the basis of these analyses and the detailed mechanism, an analytical model was derived and validated against experiments on binary surrogate mixtures as well as blends of conventional commercial fuels and FAME. These results open up the use of bottom-up liquid-phase oxidation modeling strategies for the in silico formulation of alternative fuels and the design of innovative injection fuel systems.
Oxidation and thermal stability (OTS) are key concerns for the development of alternative jet fuels, as they imply complex physical and chemical phenomena such as autoxidation, pyrolysis, cooxidation reactions and transfer-limitation. The OTSof an alternative aviation fuel was characterized using PetroOxy test from 120-160°C and JFTOT test at 325°C. The alternative jet fuel is a Synthetic Paraffinic Keroseneproduced from Hydroprocessed Esters and Fatty Acids (HEFA-SPK). Results showed a high thermal stability of HEFA-SPK. However, a low oxidation stability was also observed. The oxidation stability of8model cyclicmolecules was evaluated. Results allowed to estimate the influence of the molecular structure of cyclic molecules on liquid phase reactivity involving the number and the hydrogenation of the aromatic rings and the number and chain-length of the aromatic alkyl groups.The addition of several alkylbenzenes increased almost linearly the induction period of HEFA-SPK. Tetralin and decalin acted as inhibitors of the radical chain mechanism at low concentration, although having inherently low oxidation stability. Besides offering a better oxidation stability, the addition of specific low fractions of several alkylbenzenes, tetralin and decalin to HEFA-SPK allowed to achieve a good thermal stability as well. These molecules represent good candidates to improve OTS of HEFA-SPK. This work opens the way for the development of future fit-for-purpose formulations of alternative jet fuels with an increased fraction of renewables.
Toluene is an important compound in the chemical industry as well as an often chosen simple surrogate compound for aromatic components in transport fuels. As a result, an improved understanding of the liquid phase oxidation of toluene is of interest to both the chemical industry and the transportation sector. In this work, a detailed autoxidation mechanism for the liquid phase oxidation of toluene is developed using an automated mechanism generation tool. The resultant mechanism is significantly improved using quantum chemistry calculations to update the thermodynamic parameters of key species in solution. Comparisons are made between the predicted and experimentally measured induction period and the obtained mechanism. The agreement between both is found to be within 1 order of magnitude. Rate of production analysis and sensitivity analysis are carried out to explain and understand the reactions paths present in the mechanism. The behavior of the mechanism is commented upon qualitatively; however, no quantitative data could be obtained with the selected test method.
Liquid phase stability is a major concern in the transportation and the energy field where fuels, lubricants and additives have to be stable from their production site to their application (engine, combustors). Although alkanes are major constituents of commercial fuels and well-documented solvents, their respective reactivities and selectivities in autoxidation are poorly understood. This experimental and modeling study aims at (i) enhancing the current knowledge on alkane autoxidation and (ii) reviewing and correcting the previously established structure reactivity relationships in alkane autoxidation. Experimentally, this study investigates the influence of branching [0-3] and temperature [373-433 K] on the autoxidation of alkanes using four octane isomers: n-octane (C8), 2-methylheptane (MH), 2,5-dimethylhexane (DMH) and the 2,2,4-trimethylpentane(TMP). Induction Period (IP) and qualitative species identification are used to characterize the autoxidation processes of alkanes. The present study also presents new detailed liquid-phase chemical mechanisms obtained with an automated reaction mechanism generator. Experimental results highlight a non-linear effect of the paraffins branching on IP according to compound structure and similar oxidation products for both normal and branched paraffins. The four iso-octanes mechanisms reproduce fairly well the temperature and the branching effects on IP within a factor of 4 for high temperature range (T>403 K). From rate-of-reaction and sensibility analyses, similarities in alkane autoxidation have been evidenced with notably the key role of peroxy radicals in both normal and branched alkane autoxidation. The origin of the structure-reactivity relations was confirmed from a kinetic point of view with the main role of the hydrogen type on the molecule. Finally, based on experimental results available in literature, an empirical relation involving simple descriptors (number of carbons, type of carbons, temperature) is proposed to estimate alkane stability.
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