Ammonia is a compelling carbon-free fuel, and co-firing ammonia with conventional hydrocarbon fuels receives growing interest as a feasible solution to mitigate CO₂ emission. This study experimentally and numerically investigated the soot formation characteristics of n-heptane co-flow laminar diffusion flames with different contents of ammonia (up to 40% by volume, balanced using argon). The soot volume fraction (SVF) in the flames was quantitatively measured via laser-induced incandescence technique; PAHs and OH radical were qualitatively determined via laser-induced fluorescence technique. Results showed that ammonia addition reduced the SVF, i.e., a peak SVF decrease of 35% at 40% ammonia. Meanwhile, PAH content also decreased with ammonia addition, demonstrating that ammonia would effectively inhibit the formation of PAHs. Thus, the inception and growth of soot decreased. Interestingly, OH radical was reduced with ammonia addition, revealing that ammonia also chemically inhibited soot oxidation. The results confirmed that ammonia reduced the soot loading in the flames due to inhibitory effect on the inception and growth of soot rather than the enhancement on oxidation. The established mechanism effectively captured the variation of different PAHs in the experiments. Numerical analysis showed that the formation of the first aromatic ring benzene was reduced by ammonia as a result of the reduction of C₄H₅ and R203:C₄H₅+C₂H₂ reaction. In addition, the reduced hydrogen abstraction due to decrease in H, coupled with the reduced small PAH, led to the decrease in PAH growth. Nevertheless, a more comprehensive mechanism including the reactions between aromatics and nitrogen species, such as NH, CN, and NOx, is still required in future studies.

氨是一种引人注目的无碳燃料，将氨与传统碳氢燃料混合燃烧作为缓解 CO ₂的可行解决方案受到越来越多的关注排放。本研究通过实验和数值研究了不同氨含量（高达 40% 体积，使用氩气平衡）的正庚烷共流层流扩散火焰的烟尘形成特性。火焰中的烟尘体积分数 (SVF) 通过激光诱导白炽技术定量测量；PAHs和OH自由基通过激光诱导荧光技术定性测定。结果显示氨添加降低了SVF，即在40%氨时SVF峰值降低35%。同时，PAH 含量也随着氨的加入而降低，表明氨能有效抑制 PAHs 的形成。因此，烟灰的产生和生长减少了。有趣的是，OH 自由基随着氨的加入而减少，这表明氨也化学抑制了烟尘氧化。结果证实，氨降低了火焰中的烟尘负荷，因为它抑制了烟尘的产生和生长，而不是增强了氧化作用。建立的机制有效地捕捉了实验中不同多环芳烃的变化。数值分析表明，由于C的还原，第一芳环苯的形成被氨还原。₄ H ₅和R2O3:C ₄ H ₅ +C ₂ H ₂反应。此外，由于 H 减少导致的取氢减少，加上小 PAH 的减少，导致 PAH 生长减少。然而，在未来的研究中仍需要更全面的机制，包括芳烃与氮物种（如 NH、CN 和 NOx）之间的反应。