Pressure effects on soot formation and maturity in propane–ammonia diffusion flames

This study examines the effects of ammonia addition on soot formation in pressurized propane co-flow diffusion flames at 1–6 atm. Spatially resolved soot volume fraction, soot temperature, and soot maturity were obtained using combined multi-wavelength line-of-sight attenuation and spectral soot emission diagnostics. Ammonia addition increased the visible flame height at all pressures and reduced both the maximum soot volume fraction and the soot yield, defined here as the soot mass flow rate divided by the fuel carbon mass flow rate, throughout the investigated pressure range. At 1 atm, the maximum soot volume fraction decreased from 2.3 ppm for neat propane to 1.2 ppm for the flame with 0.6 ammonia mole fraction, while at 6 atm it decreased from 31.5 to 12.7 ppm. Although the absolute soot reduction increased with pressure, the relative reduction in soot yield became weaker at higher pressure. This indicates that ammonia remains an effective soot suppressant under elevated pressure, but its chemical influence becomes less pronounced in percentage terms as pressure enhances soot-forming chemistry. Soot maturity showed a different pressure dependence. From 1 to 3 atm, ammonia generally produced less mature soot, whereas from 4 to 6 atm this trend weakened and in several cases reversed, indicating more mature soot despite lower soot loading. The results show that soot suppression by ammonia and soot maturation do not vary in parallel under pressure, which is important for interpreting soot behaviour in ammonia-doped hydrocarbon flames and for modelling soot formation in pressurized combustion systems. Novelty and significance statement: This work advances ammonia co-firing research by examining soot formation in pressurized propane diffusion flames, a well-characterized C 3 hydrocarbon system relevant to transitional ammonia utilization in practical combustors. The novelty lies in extending ammonia-soot measurements beyond atmospheric conditions and beyond bulk soot trends by combining spatially resolved soot volume fraction, soot temperature, and optically inferred soot maturity over 1–6 atm while maintaining a constant carbon flow rate across fuel mixtures. This provides a controlled basis for isolating ammonia-specific effects on soot formation under pressure. The work is significant because it shows that soot suppression by ammonia and soot maturation do not vary in parallel under pressure. More broadly, the study provides new experimental constraints on how pressure and ammonia addition jointly modify flame structure, soot growth, and particle evolution in propane-based flames, which are relevant to interpreting soot behaviour in ammonia-doped hydrocarbon flames and to improving high-pressure soot models.