Effects of ammonia addition on soot formation in ethylene laminar diffusion flames

In light of the increasingly stringent pollutant emission regulation and energy crisis, ammonia (NH3) has been regarded as a highly competitive alternative fuel. Previous studies mainly focused on the fundamental combustion characteristics and NOx emission of co-firing NH3 with hydrocarbons; however, soot emission was rarely studied. In this study, effects of NH3 addition with varying ratios (between 0% and 60% by mole fraction while the remainder is argon) on soot formation in ethylene (40% by mole fraction) laminar diffusion flames were experimentally and numerically investigated. Two-color laser induced incandescence and planar laser induced fluorescence method were used to measure the spatial distributions of soot volume fraction, polycyclic aromatic hydrocarbons (PAHs), and OH radical. Temperature profiles along the flame centerline were obtained through rapid thermocouple insertion technique. Results show that with NH3 addition, the flame height increases and the temperature at the same height decreases. Peak concentration and total loading show that the inhibitory effect of NH3 addition on PAHs and soot volume fraction (SVF) is in the order: A1 < A2&A3 < A4 < SVF. The primary particle size obtained by transmission electron microscope substantially decreases with the increasing NH3. In addition, the signal intensity of OH radical was diminished, indicating that instead of promoting the soot oxidation, NH3 reduces soot formation by inhibiting PAH growth, soot inception, and surface growth. Chemical kinetic analysis suggests that NH3 addition reduces the concentrations of H, C2H2, and C3H3, thus markedly inhibiting the rate-limiting step of A1 formation and PAH growth.