KINETICS OF H ATOM ATTACK ON UNSATURATED HYDROCARBONS USING SPECTRAL UNCERTAINTY PROPAGATION AND MINIMIZATION TECHNIQUES

__David A. Sheen*__, Claudette M. Rosado-Reyes and Wing Tsang

Unsaturated hydrocarbons are an important component of
hydrocarbon fuels and intermediates in their oxidation. Under rich conditions,
H atom attack is one of the principle pathways of the decomposition of these
unsaturated compounds. Consequently, it is critical to understand the H atom
attack mechanisms as part of chemical model development. Previous studies have
examined the kinetics of H atom attack on various unsaturated hydrocarbons in
single pulse shock-tubes. These studies have noted that there are multiple
pathways by which H atom attack can proceed, so it is straightforward to
measure relative rates but absolute rates are more difficult to estimate. In
addition, there is a confounding influence from secondary chemistry. A
multiparameter optimization and uncertainty minimization technique is used to
constrain a chemical model for the oxidation of H_{2}/CO/C_{1}-C_{4} hydrocarbons
against a range of measurements of the H atom attack process on toluene,
trimethylbenzene (TMB), propyne, and propene. The recommended rate constant
expressions are as follows:

*k*(CH_{4}+H↔CH_{3}+H_{2}) = 10^{8.82}^{±}^{0.21} s cm^{3} mol^{-1} K^{-1.6 }*T*^{1.6}exp(-5400±500 K/*T*)

*k*(C_{2}H_{2}+CH_{3}↔*p-*C_{3}H_{4}+H)
= 10^{11.40}^{±}^{0.18} s cm^{3}
mol^{-1} K^{-0.6 }*T*^{0.6}exp(-7200±400 K/*T*)

*k*(*p-*C_{3}H_{4}+H↔*a-*C_{3}H_{4}+H)
= 10^{18.29}^{±}^{0.21} s cm^{3}
mol^{-1} K *T*^{-1}exp(-6100±500 K/*T*)

*k*(C_{3}H_{6}+H↔C_{2}H_{4}+CH_{3})
= 10^{24.52}^{±}^{0.20} s cm^{3} mol^{-1} K^{3 }*T*^{-3}exp(-7400±700 K/*T*)

*k*(C_{3}H_{6}+H↔*a-*C_{3}H_{5}+H_{2})
= 10^{5.24}^{±}^{0.19} s cm^{3}
mol^{-1} K^{-2.5 }*T*^{2.5}exp(-1300±200 K/*T*)

*k*(C_{6}H_{5}CH_{3}+H↔C_{6}H_{6}+CH_{3})
= 10^{6.29}^{±}^{0.19} s cm^{3}
mol^{-1} K^{-2.2 }*T*^{2.2}exp(-2000±400 K/*T*)

*k*(C_{6}H_{5}CH_{3}+H↔C_{6}H_{5}CH_{2}+
H_{2}) = 10^{14.10}^{±}^{0.25} s cm^{3} mol^{-1} exp(-4500±500 K/*T*)

*k*(TMB+H↔*
m*-xylene +CH_{3}) = 10^{13.83}^{±}^{0.18} s cm^{3} mol^{-1} exp(-3900±400 K/*T*)

*k*(TMB+H↔(CH_{3})_{2}CH_{2}C_{6}H_{3}+H_{2})
= 10^{4.57}^{±}^{0.25} s cm^{3}
mol^{-1} exp(-4300±600 K/*T*)

In addition, we quantify the effect of secondary chemistry on these rate estimates and the contribution to their uncertainty. Furthermore, we demonstrate how the detailed measurements constrain the model's predictions of global properties such as ignition delay time in propene oxidation.