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.