Experimental investigation and kinetic analysis of ammonia addition on the laminar flame speed of toluene and α-methylnaphthalene

This study explores the effects of ammonia (NH3) on the laminar flame speed (SL) and kinetic interactions with toluene (C7H8) and alpha-methylnaphthalene (AMN), two predominant aromatic hydrocarbons in transportation fuels. Utilizing the spherical flame technique, we measured the SL of both C7H8/NH3 and AMN/NH3 mixtures at an initial temperature (Ti) of 484 K, NH3 concentrations (XNH3) up to 70%, and equivalence ratios (phi) ranging from 0.8 to 1.3 under atmospheric pressure and elevated pressures. A chemical kinetic model was developed to integrate NH3 with these hydrocarbons, based on the advanced CRECK model, and includes cross C-N reaction pathways involving amine (NH2) with C7H8, AMN, and benzene (C6H6). The model effectively replicated the experimental SL and ignition delay time data for AMN/NH3 mixtures. Through sensitivity and reaction pathway analyses, the study identified critical reaction types: such as small molecule chain branching reactions (e.g., H+O2=O+OH and CO+OH=CO2+H) and H-abstraction from both the methyl and ring sides, as pivotal in influencing SL. Furthermore, the study examines the formation of NOx and soot, revealing that NH3 addition both increased the mole fraction of NO in C7H8/NH3 and AMN/NH3 mixtures, but the mole fraction of NO in C7H8/ NH3 mixture is higher than AMN/NH3 mixture, attributed to the higher HNO/NH2/N radicals in C7H8/NH3 mixture, and finally leading to the promoted effect on the reaction pathways of NO production. The addition of NH3 also inhibits soot formation by reducing the production of soot precursors and C2H2, while increasing the production of HCN and blocking the formation of larger PAHs.