Experimental exploration of the N-containing soot precursors in C2H4-NH3 co-flow diffusion flames

The chemical effect of ammonia can reduce the formation of soot precursors in hydrocarbon fuel flames. The nitrogen from ammonia can combine with hydrocarbon species to reduce polycyclic aromatic hydrocarbons (PAHs) while forming nitrogen-containing polycyclic aromatic hydrocarbons (NPAH). In this work, an in-depth experimental investigation was conducted to identify the chemical effect of ammonia on the changes of N-containing functional groups in soot surface and the NPAH formation in C₂H₄-NH₃ co-flow diffusion flames. The X-ray photoelectron spectroscopy (XPS) and microscopic imaging infrared spectrometer (MIR) analysis were carried out to investigate the chemical composition of soot particles and determine the types and structural characteristics of functional groups on the soot surface. The results showed that ammonia addition increased the proportion of nitrogen and oxygen in soot and enriched the nitrogen/oxygen-containing functional groups on the soot surface. The soot sampled in ethylene flames with and without ammonia addition has similar chemical composition and surface functional groups. In particular, the aromatic CN group was found in the soot from ethylene-ammonia diffusion flames. Also, the NPAH containing CN bond was further determined through gas chromatography-mass spectrometry (GC-MS) analysis. The observed NPAH are mainly cyano substituted-PAHs such as 1-Naphthalenecarbonitrile (A2CN, m/z=153), 5-acenaphthylenecarbonitrile (A2R5CN, m/z=177), etc. It indicated that the active sites on the aromatic surface facilitated the binding of HCN and CN bond to generate the cyano substituted-PAHs such as A2CN and A2R5CN. But the inhibitory effect of NPAH containing CN bond on the formation of large PAHs and soot is limited. This experimental study confirmed that ammonia promoted the formation of NPAH containing CN bond in soot.