Theoretical Investigations Into Low Temperature Oxidation Kinetics Of Typical Surrogated Fuels

In the face of the current environment and energy issues, alternative fuel strategy is of great significance and concern. Combustion is one of the main energy supply methods, and combustion reaction kinetics is the theoretical basis to achieve efficient and cleaning combustion. In order to develop low temperature combustion techniques,such as homogeneous charge compression ignition (HCCI), low temperature oxidation reaction kinetics of the typical alternative fuels is urgently needed. As a promising combustion technology for internal combustion engine, HCCI engine not only can ensure a high thermal efficiency, but also it can greatly reduce the emissions of smoke and nitrogen oxides (NOx). Therefore, exploring the low temperature combustion reaction mechanism of alternative fuels is one of the most important aspects for efficient and cleaning engine design process. The primary research goal of this paper is to select some typical alternative fuels and to explore its low temperature oxidation reaction kinetics.The low temperature oxidation kinetics has been widely investigated for chain alkanes, otherwise for cycloalkanes. Cycloalkane is an important component of petroleum fuels, and also is essential components of alternative fuels. In previous studies of cycloalkane kinetic model, most of the kinetics data were obtained by analogy with chain alkanes. Although the low temperature oxidation kinetics of chain alkanes has been well verified, the research on the kinetics of low temperature oxidation of cyclic alkanes is still controversial. Theoretical calculations can provide a powerful tool for exploring the relationship between cycloalkane structures and low temperature oxidation reactivity. In this paper, we first take methylcyclohexane as a representative case, e.g. oxygen molecule attacking different methylcycloalkane radical sites. These different structures include the side chain radical site of methylcyclohexane (cy-C6H11CH2*), the tertiary carbon radical site on the methylcyclohexane ring (tcy-C6H10(*))CH3) and the secondary carbon radical site on the methylcyclohexane ring(ortho-cy-C6H10(*)CH3). The reaction pathways of different structures have been investigated by high level quantum chemical calculations. The reaction kinetics was studied by the ab initio transition state theory based on master equation methodology.Based on these kinetics data, we discussed the competition relationship between different channels such as chain branching, chain propagation and chain termination,and analyzed the relationship between the reaction structure and the reactivity.In the development and improvement process of the low temperature oxidation mechanism, the kinetics data of these key intermediates are critical, such as carbonyl hydroperoxides which have been detected by more and more experiments. Carbonyl hydroperoxides are a class of typical key intermediates, and they are prone to induce dissociation reactions due to the presence of weak O-OH bond, which is a decisive step for the chain branching reactions. Thus the thermodynamic and kinetic data of this intermediate could determine the overall reactivity, products distribution etc. For the dimethyl ether (DME) reaction mechanisms, only the estimated data for carbonyl hydroperoxides was used by most of models, which will lead to a significant errors for the developing of DME model. To have a more comprehensive understanding of the mechanism and reactivity of the DME low-temperature oxidation, it is necessary to carry out detailed chemical kinetics research on this key intermediate. In this paper, we select the carbonyl hydroperoxide (HOOCH2OCHO) during the low temperature oxidation of DME to carry out a series of high level theoretical calculations. Firstly, the decomposition pathways of HOOCH2OCHO were calculated at QCISD(T)/CBS//B3LYP/6-311++G(d,p) level. Then, the temperature- and pressure-dependent rate constants were computed using microcanonical variational transition state theory coupled with the RRKM/master equation calculations. The new competitive relationship will have an effect on the prediction of DME low temperature oxidation reactivity.As described above, oxygenates with carbonyl and hydroperoxy functional groups are important intermediates that are generated during the autoignition of transport fuels,particularly for the performance of advanced internal combustion engines. At the same time, it is also an important intermediate which is generated during the autoxidation of organic compounds in the atmosphere. A key fate of the carbonyl hydroperoxides is the reaction with OH radicals, for which kinetics data are experimentally unavailable. Here,we study 4-hydroperoxy-2-pentanone (CH3C(=O)CH2CH(OOH)CH3) as a model compound to clarify the kinetics of OH reactions with carbonyl hydroperoxides,including H-atom abstraction and OH addition reactions. With a combination of electronic structure calculations, we determine previously missing thermochemical data,and with multipath variational transition state theory (MP-VTST), a multidimensional tunneling (MT) approximation, multiple-structure anharmonicity, and torsional potential anharmonicity we obtained much more accurate rate constants. The roles of these various factors in determining the rates are elucidated. The pressure-dependent rate constants for the addition reaction are computed using system-specific quantum RRK theory. The accurate thermodynamic and kinetics data determined in this work are indispensable in the detailed understanding and prediction of ignition properties of hydrocarbons and alternative fuels.Through these above studied cases, we found that the role of theoretical calculations for chemical reaction kinetics becomes more and more important,especially for the unavailable experimental conditions. It is necessary to understand the uncertainty and the uncertainty propagation process in the theoretical calculation process for the future accurate theoretical calculations. However the error bars of computed rate constants are rarely evaluated rigorously. In this work, global uncertainty and sensitivity analysis is applied to the propagation of the uncertainties in the input parameters (e.g. barrier heights, frequencies and collisional energy transfer parameters et al.) to those in the rate constants computed by the RRKM/master equation method for the decomposition of ethanol. This case study provides a systematic exploration of the effect of temperature and pressure on the parametric uncertainties in RRKM/master equation calculations for a prototypical single-well multiple-channel dissociation.According to the sensitivity analysis, we analyzed the parameters with high sensitivity coefficient, and discussed the sensitivity coefficients varing with temperature and pressure. Based on the uncertainty analysis, the uncertainty factors of the rate constants of the two reaction channels are quantitatively evaluated, and the variations of the uncertainty factors with the temperature under the high pressure limit and pressure dependence are given. Also detailed explanations for these phenomenon are provided.Although ethanol was chose as an example, the conclusion we obtained is not only limited to this case. For the more general reaction system in the RRKM/master equation,the direction of the parameterization uncertainty was specified by our work. The present study illustrates the value of detailed qualitative and quantitative studies of the uncertainties in theoretical kinetics predictions.