Experimental and numerical analysis of the autoignition behavior of NH3 and NH3/H2 mixtures at high pressure

Experimental and numerical analysis of the autoignition behavior of NH3 and NH3/H2 mixtures at high pressure

Measurements of autoignition delay times of NH3 and NH3/H-2 mixtures in a rapid compression machine are reported at pressures from 20-75 bar and temperatures in the range 1040-1210 K. The equivalence ratio, using O-2/N-2 /Ar mixtures as oxidizer, varied for pure NH3 from 0.5 to 3.0; NH3/H-2 mixtures with H-2 fraction between 0 and 10% were examined at equivalence ratios 0.5 and 1.0. In contrast to many hydrocarbon fuels, the results indicate that, for the conditions studied, autoignition of NH3 becomes slower with increasing equivalence ratio. Hydrogen is seen to have a strong ignition-enhancing effect on NH3. The experimental data, which show similar trends to those observed previously by He et al. (2019) [28], were used to evaluate four NH3 oxidation mechanisms: a new version of the mechanism described by Glarborg et al. (2018) , with an updated rate constant for the formation of hydrazine, NH2 + NH2 (+M) = N2H4 (+M), and the literature mechanisms from Klippenstein et al. (2011) [30], Mathieu and Petersen (2015) [25], and Shrestha et al. (2018) [31]. In general, the mechanism from this study has the best performance, yielding satisfactory prediction of ignition delay times both of pure NH3 and NH3/H-2 mixtures at high pressures (40-60 bar). Kinetic analysis based on present mechanism indicates that the ignition enhancing effect of H-2 on NH3 is closely related to the formation and decomposition of H2O2 ; even modest hydrogen addition changes the identity of the major reactions from those involving NHx radicals to those that dominate the H-2/O-2 mechanism. Flux analysis shows that the oxidation path of NH3 is not influenced by H-2 addition. We also indicate the methodological importance of using a non-reactive mixture having the same heat capacity as the reactive mixture for determining the non-reactive volume trace for simulation purposes, as well as that of limiting the variation in temperature after compression, by limiting the uncertainty in the experimentally determined quantities that characterize the state of the mixture. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

本文报道了在快速压缩机中对氨气（NH₃）以及氨气/氢气（NH₃/H₂）混合物的自燃延迟时间的测量结果，测量压力范围为20 - 75巴，温度范围为1040 - 1210 K。以氧气/氮气/氩气（O₂/N₂/Ar）混合物作为氧化剂时，纯氨气的当量比在0.5到3.0之间变化；氢气含量在0到10%之间的氨气/氢气混合物在当量比为0.5和1.0的条件下进行了检测。与许多碳氢燃料不同的是，结果表明，在所研究的条件下，氨气的自燃随着当量比的增加而变慢。氢气对氨气有很强的助燃作用。实验数据与何等人（2019年）之前观察到的趋势相似，这些数据被用于评估四种氨气氧化机制：一种由格拉堡等人（2018年）描述的机制的新版本，其中联氨（NH₂ + NH₂ (+M) = N₂H₄ (+M)）形成的速率常数已更新，以及克利彭施泰因等人（2011年）、马蒂厄和彼得森（2015年）以及什雷斯塔等人（2018年）的文献机制。总体而言，本研究中的机制性能最佳，能在高压（40 - 60巴）下对纯氨气以及氨气/氢气混合物的自燃延迟时间做出令人满意的预测。基于当前机制的动力学分析表明，氢气对氨气的助燃作用与过氧化氢（H₂O₂）的形成和分解密切相关；即使少量添加氢气，主要反应的类型也会从涉及氨（NHₓ）自由基的反应转变为在氢气/氧气机制中占主导的反应。通量分析表明，氨气的氧化路径不受氢气添加的影响。我们还指出了使用与反应混合物具有相同热容量的非反应性混合物来确定用于模拟目的的非反应体积轨迹的方法学重要性，以及通过限制表征混合物状态的实验测定量的不确定性来限制压缩后温度变化的重要性。（C）2020燃烧学会。由爱思唯尔公司出版。保留所有权利。