Effects of Fuel Diluents on Flame Characteristics of Laminar Methane-Oxygen Inverse Diffusion Flames

Inverse co-flow diffusion flames (IDF) are the fundamental flame configuration in which autothermal reforming (ATR) of natural gas is based, a technology for clean hydrogen production. However, soot formation is unavoidable for IDFs because of fuel-rich conditions. This study assessed the effects of various diluents, including carbon dioxide (CO2), nitrogen (N2), argon (Ar), and helium (He), introduced into the fuel stream on the properties of oxy-fuel laminar IDFs, with the aim of improving the understanding of soot formation in IDFs at atmospheric pressure. The flame structure, temperature, syngas (H2+CO), polycyclic aromatic hydrocarbons (PAHs), and soot formation in methane IDFs were investigated using laser-based diagnostic techniques and numerical simulations. Pure oxygen (O2) was used as an oxidizer to mimic the ATR process. Results show that diluent addition reduces the peak flame temperature and shifts the flame structure axially downstream, increasing the flame height due to buoyancy-induced acceleration and slower diffusion. OH-PLIF measurements reveal that CO₂-diluted flames exhibit the longest flame lengths, linked to Peclet number (Pe) trends and suppressed buoyancy-driven radial convection. PAH formation follows the order: He > Ar > N2 > CO2, with CO2 reducing PAH levels by promoting oxidation of key intermediates via increased OH production. Soot spatial distribution is shifted downstream, with the peak soot volume fraction (SVF) following Ar > N2 > He > CO2, correlating with flame temperature and residence time. CO2 had the strongest soot suppression effect, acting through both thermal and chemical mechanisms. Numerical results indicate that temperature and OH mole fraction govern the syngas composition. CO2 dilution resulted in higher CO and lower H₂ production, as reaction pathway analysis showed that CO2 enhances OH and CO formation while reducing H radicals, limiting H₂ generation. These findings provide insights into the role of diluents in controlling soot and syngas formation in IDFs.反向同轴扩散火焰（Inverse co-flow diffusion flames, IDF）是天然气自热重整（autothermal reforming, ATR）清洁制氢技术所依托的基础火焰构型。然而，受富燃料工况影响，反向同轴扩散火焰的炭烟生成难以避免。本研究评估了引入燃料流的各类稀释剂——包括二氧化碳（CO₂）、氮气（N₂）、氩气（Ar）与氦气（He）——对富氧层流反向同轴扩散火焰特性的影响，旨在加深对常压下反向同轴扩散火焰中炭烟生成过程的理解。研究采用激光诊断技术与数值模拟手段，探究了甲烷基反向同轴扩散火焰的火焰结构、温度、合成气（H₂+CO）、多环芳烃（PAHs）以及炭烟生成特性。实验以纯氧（O₂）作为氧化剂以模拟自热重整工艺。研究结果表明，添加稀释剂会降低火焰峰值温度，并使火焰结构沿轴向向下游偏移；同时，受浮力诱导加速与扩散速率减缓的影响，火焰高度随之升高。OH平面激光诱导荧光（OH-PLIF）测量结果显示，经二氧化碳稀释的火焰拥有最长的火焰长度，这一现象与佩克莱特数（Peclet number, Pe）的变化趋势以及被抑制的浮力驱动径向对流相关。多环芳烃生成量遵循如下排序：氦气>氩气>氮气>二氧化碳，其中二氧化碳通过提升羟基（OH）生成量以促进关键中间体的氧化，从而降低多环芳烃的生成水平。炭烟的空间分布向下游偏移，峰值炭烟体积分数（soot volume fraction, SVF）的排序为氩气>氮气>氦气>二氧化碳，该排序与火焰温度及停留时间具有相关性。二氧化碳展现出最强的炭烟抑制效果，其作用同时涵盖热机制与化学机制。数值模拟结果表明，火焰温度与羟基摩尔分数决定了合成气的组分比例。二氧化碳稀释会提升合成气中一氧化碳的占比并降低氢气的生成量，反应路径分析显示，二氧化碳会促进羟基与一氧化碳的生成，同时减少氢自由基的数量，进而限制氢气的生成。本研究的发现为理解稀释剂在调控反向同轴扩散火焰中炭烟与合成气生成过程中的作用提供了理论参考。