A Theoretical Study of NH2 Radical Reactions with Propane and Its Kinetic Implications in NH3-Propane Blends’ Oxidation

Abstract

The reaction of NH2 radicals with C3H8 is crucial for understanding the combustion behavior of NH3/C3H8 blends. In this study, we investigated the temperature dependence of the rate coefficients for the hydrogen abstraction reactions of C3H8 by NH2 radicals using high-level theoretical approaches. The potential energy surface was constructed at the CCSD(T)/cc-pV(T, Q)//M06-2X/aug-cc-pVTZ level of theory, and the rate coefficients were computed using conventional transition state theory, incorporating the corrections for quantum tunneling and hindered internal rotors (HIR). The computed rate coefficients showed a strong curvature in the Arrhenius behavior, capturing the experimental literature data well at low temperatures. However, at T > 1500 K, the theory severely overpredicted the experimental data. The available theoretical studies did not align with the experiment at high temperatures, and the possible reasons for this discrepancy are discussed. At 300 K, the reaction of NH2 with C3H8 predominantly occurs at the secondary C-H site, which accounts for approximately 95% of the total reaction flux. However, the hydrogen abstraction reaction at the primary C-H site becomes the dominant reaction above 1700 K. A composite kinetic model was built, which incorporated the computed rate coefficients for NH2 + C3H8 reactions. The importance of NH2 + C3H8 reactions in predicting the combustion behavior of NH3/C3H8 blends was demonstrated by kinetic modeling.

Theoretical Investigation on the H Atom Abstraction Reaction from C1–C4 Alkanes and Alkenes by ṄH2 Radicals

“…For the H atom abstraction from the primary carbon site on propane, namely C 3 H 8 + ṄH 2 ⇌ p -Ċ 3 H 7 + NH 3 , the rate constant from Siddique et al is slightly higher than our data from 400 to 1200 K, while beyond this temperature range an opposite phenomenon occurs. A similar phenomenon is also observed when comparing the rate constants for C 3 H 8 + ṄH 2 ⇌ s-Ċ 3 H 7 + NH 3 , because of the fact that the data in Siddique et al are still higher than our data but only in the temperature range of 400–1400 K. Rate constants for both channels in Giri et al are slightly lower than our data but show similar curvature as ours when the temperature rises.…”

“…In addition, the energy barriers in this paper are the same as those of Sun et al because of similar calculation methods, but Sun et al did not provide energy barriers for ṄH 2 +alkenes abstraction reaction channels. For ṄH 2 +propane channels, the barriers in this work are a bit lower than the data in ref . by 0.6–0.8 kcal·mol –1 .…”

“…Figure 11. Rate constant comparison between this work and literature for the (a) C 3 H 8 + N ̇H2 ⇌ C ̇3H 7 + NH 3 and (b) C 3 H 8 + N ̇H2 ⇌ p-C ̇3H 7 + NH 3 and C 3 H 8 + N ̇H2 ⇌ s-C ̇3H 7 + NH 3 channels 23,27,[31][32][33]56. …”

“…30 The comparison between methane and ethane indicated that the calculated rate constant of the N ̇H2 / CH 4 system was 1 order of magnitude lower than that of the N ̇H2 /C 2 H 6 system. Giri et al 31 used the CCSD(T)/cc-pV(T,Q)Z//M06-2X/aug-cc-pVTZ level of theory to obtain the rate constants of N ̇H2 +C 3 H 8 ⇌NH 3 +C ̇3H 7 , which showed improved prediction ability when adding to blending mechanism. Mebel and Lin 23 utilized the G2M method to construct potential energy surfaces and obtain rate constants for H atom abstraction reactions by N ̇H2 radicals from CH 4 , C 2 H 6 , C 3 H 8 , and i-C 4 H 10 .…”