Appendix D
Chemical Kinetic Reaction Mechanisms

Chemical kinetic reaction mechanisms consist of two parts: (1) thermochemical data for chemical species that have been assembled into a systematic group to describe the combustion of a particular fuel, and (2) reaction-rate coefficients for the elementary chemical reactions in which those species participate. The most common format for such reaction-rate data is the modified Arrhenius form for the rate k:

k = ATn exp(−Ea/RT)

where the coefficients A, n, and Ea are tabulated for every reaction involved in the mechanism, T is the temperature of the burning system, and R is the universal gas constant. Recent mechanisms for practical diesel and biodiesel fuels can include nearly 3,000 chemical species and 11,000 elementary reactions (Westbrook et al., 2010; Naik et al., 2010). Different researchers develop detailed chemical kinetic reaction mechanisms of their own, using practices that can be called idiosyncratic but understandable. The most sensitive reaction rates for all of hydrocarbon combustion are those dealing with the smallest molecules, and while these rates have received an enormous amount of attention, there are still differences of opinion concerning the best ways to capture their rates into Arrhenius parameters.

The rates for the smallest molecules form the core of more complex kinetics mechanisms. Kinetic reaction mechanisms are best visualized as being “hierarchical” in structure (Westbrook and Dryer, 1984), based on a first level containing the kinetics for the smallest components, such as



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Appendix D Chemical Kinetic Reaction Mechanisms Chemical kinetic reaction mechanisms consist of two parts: (1) ther- mochemical data for chemical species that have been assembled into a systematic group to describe the combustion of a particular fuel, and (2) reaction-rate coefficients for the elementary chemical reactions in which those species participate. The most common format for such reaction-rate data is the modified Arrhenius form for the rate k: k = ATn exp(–Ea/RT) where the coefficients A, n, and Ea are tabulated for every reaction involved in the mechanism, T is the temperature of the burning system, and R is the universal gas constant. Recent mechanisms for practical diesel and biodie- sel fuels can include nearly 3,000 chemical species and 11,000 elementary reactions (Westbrook et al., 2010; Naik et al., 2010). Different research- ers develop detailed chemical kinetic reaction mechanisms of their own, using practices that can be called idiosyncratic but understandable. The most sensitive reaction rates for all of hydrocarbon combustion are those dealing with the smallest molecules, and while these rates have received an enormous amount of attention, there are still differences of opinion concerning the best ways to capture their rates into Arrhenius parameters. The rates for the smallest molecules form the core of more complex kinetics mechanisms. Kinetic reaction mechanisms are best visualized as being “hierarchical” in structure (Westbrook and Dryer, 1984), based on a first level containing the kinetics for the smallest components, such as 88

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89 APPENDIX D H2, O2, CO, and CO2, which are common to all hydrocarbon fuels. Sub- mechanisms are then added for small hydrocarbons, such as methane (CH4), formaldehyde (CH2O), and methanol (CH3OH), followed by fuels with two C atoms, and then by those with three C atoms. This is contin- ued until the model size reaches that of the fuel being studied. Recent fuel mechanisms have been produced for fuels characteristic of gasoline, diesel fuels, and biofuels with as many as 16 to 18 carbon atoms (e.g., primary reference fuels, n-alkane, and methyl stearate, respectively). The numbers of chemical species and elementary reactions in a kinetic model increase rapidly with increased fuel molecule size: • A model for hydrogen oxidation includes about 10 chemical spe- cies and 30 elementary reactions, • A model for methane requires about 30 chemical species and 300 reactions, and • A mechanism for n-cetane (n-C16H34) includes more than 1,200 spe- cies and 7,000 chemical reactions (Westbrook et al., 2009). Starting with these core reactions and building on additional reactions for more complex fuels, each kinetic-model developer builds his or her own “house of cards” (Frenklach, 2007) that is similar to, but still not the same as, every other mechanism for the same fuels. For reaction rates with much less sensitivity than the above reactions, rates can vary much more widely from model to model. There is no real value in these differences from one model to another, but there is no motivation for convergence either. In the meantime, these multiple reaction mechanisms continue to exist, slowing the overall rate of progress in the direction of developing realistic reaction mechanisms for new, larger, and more practical fuel components. In addition, nonexperts who need usable and reliable kinetic mechanisms are confused and find it difficult to make choices from these multiple sources of models. Experience with GRIMech (see Appendix A in this report) illustrates that it is possible for the combustion community to accept and thrive in a common, highly validated, and thorough reaction mechanism environ- ment, but experience also has shown that when financial support for that kinetic model ended, the concept of a combustion community collective kinetic model disappeared. It should be noted that, like many other features of a cyberinfrastruc - ture, the chemical kinetic part would consist of a fairly complicated task in data management, visualization, and computer science in general, as well as in combustion kinetics. Large archives of experimental results would be needed as sources of model-validation data, and it would be necessary to provide such data in some sort of common data structure to

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90 TRANSFORMING COMBUSTION RESEARCH THROUGH CYBERINFRASTRUCTURE enable models to access the data many thousands of times in validation studies. Common evaluations of relative confidence in those experimental data would have to be established, and software would be needed to carry out the various types of validation and other studies leading to consensus recommendations. Teams of kinetics experts would be needed to develop analysis tools to arrive at common conclusions and recommendations, and optimization and evaluation tools would be required. Some of these were developed in the production of the GRIMech mechanisms, but the effort required to extend the approach to hydrocarbons in general would be an order-of- magnitude increase in necessary resources. REFERENCES Frenklach, M. 2007. “Transforming Data into Knowledge—Process Informatics for Combus - tion Chemistry.” Proceedings of the Combustion Institute, Vol. 31, pp. 125-140. Naik, C.V., C.K. Westbrook, O. Herbinet, W.J. Pitz, and M. Mehl. 2010. “Detailed Chemical Kinetic Reaction Mechanism for Biodiesel Components Methyl Stearate and Methyl Oleate.” Proceedings of the Combustion Institute, Vol. 33, doi: 10.1016/j.proci.2010.05.007. Westbrook, C.K., and F.L. Dryer. 1984. “Chemical Kinetics Modeling of Hydrocarbon Combustion.” Progress in Energy Combustion Science, Vol. 10, pp. 1-57. Westbrook, C.K.,W.J. Pitz, O. Herbinet, H.J. Curran, and E.J. Silke. 2009. “ A Comprehensive Detailed Chemical Kinetic Mechanism for Combustion of n-Alkane Hydrocarbons from n-Octane to n-Hexadecane.” Combustion and Flame 156(1):181-199. Westbrook, C.K., W.J. Pitz, M. Mehl, and H.J. Curran. 2010. “Detailed Chemical Kinetic Reaction Mechanisms for Primary Reference Fuels for Diesel Cetane Number and Spark-Ignition Octane Number.” Proceedings of the Combustion Institute, Vol. 33, doi:10.1016/j.proci.20.05.087.