含氨燃料预混火焰的层流火焰速度及NO排放特性

将甲烷或氢气与氨气共燃可以克服NH₃火焰的点火能量高、燃烧速度慢的缺点。为了解NH₃作为燃料的燃烧特性，对含NH₃燃料进行一维层流预混火焰数值模拟，研究其层流火焰速度及NO排放特性。采用文献中5个简化反应机理进行数值计算，发现Okafor机理模拟NH₃/CH₄/air火焰精度更高；Xiao机理模拟NH₃/H₂/air、NH₃/air精度适中，计算时间较短。此外，开展了当量比、燃料混合物组分比例、压力等参数对含NH₃燃料燃烧时烟气中NO浓度影响的研究。研究发现：含NH₃燃料燃烧时NO主要通过OH、H、O自由基和O₂分子的消耗而生成，主要通过与NH_i（i=0， 1， 2）自由基反应消耗；含NH₃燃料在富燃状态下燃烧可有效减少NO排放，但富燃燃烧效率低，可采用富燃-贫燃分级燃烧技术来提高燃烧效率，同时保持NO的低排放；掺有较多NH₃的含NH₃燃料在中高压下燃烧时可有效减少NO排放。

Laminar flame speed and NO emission characteristics of premixed flames with different ammonia-containing fuels

Co-combustion of ammonia with methane or hydrogen can overcome the disadvantages of high ignition energy and slow combustion speed of NH₃ flame. To understand the combustion characteristics of NH₃ as a fuel, a one-dimensional laminar premixed flame numerical simulation was carried out on NH₃-containing fuel, and its laminar flame speed and NO emission characteristics were studied. Five reduced reaction mechanisms proposed in the literature were employed for numerical calculation. The results show that the mechanism of Okafor predicts NH₃/CH₄/air flame with higher accuracy, and the mechanism developed by Xiao simulates NH₃/H₂/air, NH₃/air flame with a moderate accuracy and shorter computational time. Additionally, the effect of equivalence ratio, composition of the fuel mixture and pressure on the concentration of NO in flue gas were studied. The analysis of NO production rate demonstrates that NO is mainly produced by the consumption of OH, H, O radicals and O₂ molecule and the NO consumption mainly takes place through the reaction with NH_i (i=0, 1, 2) radicals for the combustion of NH₃-containing fuel. NH₃-containing fuel burning in rich condition can effectively reduce the NO emission, but the combustion efficiency of rich combustion is low. This can be tackled with the “rich burn - lean burn” staged combustion concept while maintaining the low NO emission at the same time. It is also found that combustion of NH₃-containing fuel with more NH₃ under medium and high pressure is more preferable for reducing NO emissions.