Direct Laser Absorption Measurements of NH ₃ -Relevant Pyrolysis Reactions

The pyrolysis kinetics of NH 3 and NH 3 -relevant radicals are of importance for the implementation of NH 3 as a fuel. The pyrolysis of NH 2 radicals was therefore studied behind reflected shock waves using 7–243 ppm of N 2 H 4 to generate NH 2 in bath gases of Ar and N 2 . Post-shock conditions spanned 2900–4600 K and 0.41–0.94 atm. Sensitive laser absorption diagnostics allowed NH and NH 2 concentrations to be monitored simultaneously under highly dilute conditions. Sensitivities of measured time-histories to secondary reactions were therefore suppressed, enabling low-uncertainty measurements of reaction rates for NH 2 + M ↔ NH + H + M (R1), NH + M ↔ N + H + M (R2), and the first direct, high-temperature measurements of NH + H ↔ N + H 2 (R3) in the written direction. The first chaperon efficiency measurements for N 2 relative to Ar (η N 2 ) were obtained for R1 and R2. Rate constant expressions and their associated temperature ranges as well as η N 2 values are summarized below: k NH 2 + Ar ↔ NH + H + Ar ( T ) = 10 14.6 ± 0.4 ⁡ exp ⁡ ⁡ ( − ( 73 ± 6 ) / ( R T ) ) cm 3 mol − 1 s − 1 ( 2900 − 4600 K, η N 2 = 4.4 ± 0.7 ) k NH + Ar ↔ N + H + Ar ( T ) = 10 14.0 ± 0.2 ⁡ exp ⁡ ⁡ ( − ( 71 ± 3 ) / ( R T ) ) cm 3 mol − 1 s − 1 ( 2500 − 4600 K, η N 2 ≈ 4 ± 2 ) k NH + H ↔ N + H 2 ( T ) = 10 15.1 ± 0.3 exp ⁡ ⁡ ( − ( 17 ± 4 ) / ( R T ) ) cm 3 mol − 1 s − 1 ( 1650 − 4600 K )