Unraveling Pressure Effects in Laminar Flame Propagation of Ammonia: A Comparative Study with Hydrogen, Methane, and Ammonia/Hydrogen

As a promising zero-carbon fuel, ammonia (NH3) has attracted
great attention in combustion research. Understanding the pressure
effects in the laminar flame propagation of NH3 is crucial
for its practical applications in gas turbines, internal combustion
engines, boilers, and industrial furnaces. In combination with new
measurements in a high-pressure constant-volume cylindrical combustion
vessel and experimental data in the literature, the pressure effects
in the laminar flame propagation of NH3 were explored in
this work and compared to those of hydrogen (H2), methane
(CH4), and NH3/H2. A kinetic model
for the combustion of NH3, H2, CH4, and NH3/H2 was developed to simulate the
laminar burning velocities of these fuels and reproduce the pressure
effects in their laminar flame propagation. The pressure-dependent
coefficients of laminar burning velocities of fuels are found in the
general order of H2 < NH3 < CH4 ∼ NH3/H2 under the investigated conditions,
which reveals the reverse trend of the global reaction order in their
combustion. Modeling analysis was performed to provide mechanistic
explanations to the order of observed pressure effects. It is concluded
that, in the laminar flame propagation of NH3 and H2, the most important pressure-dependent reaction is H + O2 (+M) = HO2 (+M), while a chain-termination reaction
NH2 + HO2 = NH3 + O2 can
convert HO2 to O2 in the laminar flame propagation
of NH3 and leads to the enhanced pressure effects compared
to that of H2. In the laminar flame propagation of CH4, both H + O2 (+M) = HO2 (+M) and CH3 + H (+M) = CH4 (+M) contribute to the pressure
effects, which explains its greater pressure effects than those of
H2 and NH3, especially under rich conditions.
In the laminar flame propagation of NH3/H2,
a synergistic effect between NH3 and H2 occurs
at the reaction NH2 + HO2 = NH3 +
O2 as a result of the simultaneously abundant production
of NH2 and HO2, which further enhances the role
of H + O2 (+M) = HO2 (+M) in the laminar flame
propagation of NH3/H2. This explains the interesting
enhancement in its pressure effects, which can even become comparable
to those of CH4.

氨（NH3）作为一种极具前景的零碳燃料，在燃烧研究领域受到广泛关注。明晰氨在层流火焰传播（laminar flame propagation）过程中的压力效应，对于其在燃气轮机、内燃机、锅炉及工业炉中的实际应用至关重要。本研究结合高压定容圆柱燃烧器（constant-volume cylindrical combustion vessel）的新测量数据与文献公开实验数据，对氨的层流火焰传播中的压力效应展开探究，并将其与氢（H2）、甲烷（CH4）及NH3/H2混合燃料的压力效应进行对比。本研究构建了面向氨、氢、甲烷与NH3/H2混合燃料燃烧的动力学模型（kinetic model），用以模拟上述燃料的层流燃烧速度（laminar burning velocities），并复现其层流火焰传播过程中的压力效应。在本次研究的工况范围内，各燃料层流燃烧速度的压力相关系数大致遵循H2 < NH3 < CH4 ≈ NH3/H2的顺序，这与其燃烧过程中的总反应级数（global reaction order）呈现相反的变化趋势。本研究通过建模分析，为观测到的压力效应顺序提供了机理性解释。研究表明，在氨与氢的层流火焰传播过程中，最重要的压力相关反应为H + O2 (+M) = HO2 (+M)；而在氨的层流火焰传播中，链终止反应（chain-termination reaction）NH2 + HO2 = NH3 + O2可将HO2转化为O2，使其压力效应相较于氢更为显著。在甲烷的层流火焰传播过程中，H + O2 (+M) = HO2 (+M)与CH3 + H (+M) = CH4 (+M)这两个反应均对压力效应存在贡献，这也解释了为何甲烷的压力效应强于氢与氨，尤其是在富燃工况（rich conditions）下。在NH3/H2混合燃料的层流火焰传播过程中，由于NH2与HO2同时大量生成，二者在反应NH2 + HO2 = NH3 + O2处产生了协同效应（synergistic effect），进一步强化了H + O2 (+M) = HO2 (+M)在该混合燃料层流火焰传播中的作用。这也解释了其压力效应出现显著增强的原因——其压力效应甚至可与甲烷相媲美。