Some chemical kinetics issues in reburning: The branching fraction of the HCCO+NO reaction

Abstract

We have determined theoretically some critical kinetic parameters in the mechanism of NO_x reburning under flow-reactor conditions. Specifically, using a variety of electronic-structure methods to investigate the potential energy surfaces and the maximum free-energy method of Quack and Troe to determine the resulting rate coefficients, we have deduced the values of k₂ and k₃ for the reactions HCNO+O a3HCO+NO (R2) and HCNO+OH a3HCOH+NO (R3) to be k₂≈7×10¹³ cm³/mole s and k₃≈2×10¹³ cm³/mole s independent of temperature for 300 K<T<2700 K. With such fast reactions converting HCNO to NO, a critical parameter in the reburn mechanism is α(T)=k_1b(T)/k₁(T), the branching fraction of the HCCO+NO reaction, HCCO+NO a3HCNO+CO (R1a) -a3HCN+CO₂ (R1b) -a3NONC+CO (R1c) Again using PES information from a variety of electronic-structure methods (including the QCISD barrier heights of Nguyen et al.), we have used the statistical-theoretical methodology of Miller, Parrish, and Brown to determine α(T)=0.985 exp (−T/1748), valid for 300 K<T<2000 K. Using a value of k₁=k_1a+k_1b+k_1c=2.4×10¹³ cm³/mole s independent of temperature (consistent with experiment), we have determined modified Arrhenius expressions for k_1a and k_1b, k_1a=1.17×10¹¹ T^0.65 cm³/mole s and k_1b=1.45×10¹⁶ T^−0.968 exp(−648/RT) cm³/mole s for 300 K<T<2000 K. Reaction (R1c) never contributes as much as 1% to the total rate coefficient. Our predictions for α(T) disagree with an experimental determination at T=700 K, but they are only slightly smaller than those used in modeling for 1000 K<T<1400 K.

The theoretical analyses and the reburn mechanism are discussed in detail.