Pressure effects on ammonia driven chemical soot suppression in methane diffusion flames

Due to the growing interest in ammonia-hydrocarbon co-firing, ammonia’s role in soot formation has gained importance in recent years. However, our understanding of the effect of pressure on ammonia’s soot suppression remains limited, especially for methane flames. Therefore, this study investigates the effects of ammonia and nitrogen dilution on soot formation in methane–air co-flow diffusion flames at elevated pressures from 2 to 8 atm. Methane flames were diluted by mole fractions of 0.1, 0.2, and 0.3 ( X diluent / X total ). Soot volume fraction (SVF) and soot temperature were measured using Line-of-Sight Attenuation and Spectral Soot Emission techniques, respectively. Results showed that both nitrogen and ammonia suppress soot, with the latter introducing a strong chemical effect that dominates over physical dilution. Ammonia addition lifted the flames and delayed soot formation, shifting the peak soot volume fraction downstream at all pressures. The soot suppression effect of ammonia remained nearly constant with pressure, decreasing the maximum soot yield by 32% ( ± 4%) for each X NH 3 = 0.1 dilution. In contrast, the soot suppression effect of nitrogen decreased with increasing pressure. The pressure-independence of ammonia’s soot suppression is attributed to the competing influences between pressure-enhanced soot formation and ammonia-induced chemical inhibition, while the weakening effect of nitrogen reflects the inability of physical dilution to counter increasingly dominant soot formation mechanisms at elevated pressures. Furthermore, it was found that methane flames are more sensitive to chemical soot suppression through ammonia addition compared to ethylene and n-heptane flames, consistent with observations reported at ambient pressure.