Chemical kinetic modeling of high-pressure propane oxidation and comparison to experimental results

• Published in: Symposium, International, on Combustion Vol. 26; no. 1; pp. 633 - 640
• Main Authors: Koert, David N., Pit, William J., Boelli, Joseph W., Cernansky, Nicholas P.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 1996
• DOI: 10.1016/S0082-0784(96)80270-0
• ISSN: 0082-0784

A pressure-dependent kinetic mechanism for propane oxidation is developed and compared to experimental data from a high-pressure flow reactor. Experimental conditions range from 10–15 atm and 650–800 K and have a residence time of 198 ms for propane-air mixtures at an equivalence ratio of 0.4. The experimental results clearly indicate negative temperature coefficient (NTC) behavior. The chemistry describing this phenomenon is critical in understanding automotive engine knock and cool flame oscillations. Results of the numerical model are compared to a spectrum of stable species profiles sampled from the flow reactor.
Rate constants and product channels for the reaction of propyl radicals, hydroperoxy-propyl radicals, and important isomers (radicals) with O
2 were estimated using thermodynamic properties, with multifrequency quantum Kassel theory for
k(E) coupled with modified, strong collision analysis for falloff.
Results of the chemical kinetic model show an NTC region over nearly the same temperature regime as observed in the experiments. Sensitivity analysis identified the key reaction steps that control the rate of oxidation in the NTC region. The model reasonably simulates the profiles for many of the major and minor species observed in the experiments.
Numerical simulations show that some of the key reactions involving propylperoxy radicals are in partial equilibrium in this residence-time, temperature, and pressure regime. This indicates that their relative concentrations are controlled by a combination of thermochemistry and other rate-controlling reaction steps. Major reactions in partial equilibrium include C
3H
7+O
2=C
3H
7O
2, C
3H
6OOH=C
3H
6+HO
2, and C
3H
6OOH+O
2=O
2C
3H
6OOH. This behavior means that thermodynamic parameters of the oxygenated species, which govern partial equilibrium concentrations, are especially important. QRRK/falloff results also show that the reaction of propyl radical and hydroperoxy-propyl radicals with O
2 proceeds, primarily, through pressure-stabilized adducts rather than chemically activated channels; thus, dissociation and isomerization rates of these adducts are important.