Effect of ammonia on soot volume fraction and morphology in laminar flames: modeling the impact of NH2 radicals

There is a growing scientific and industrial interest in ammonia as a zero-carbon fuel. This study examines the impact of ammonia on the reduction of soot formation in ethylene laminar flames using a comprehensive kinetic model. The study examines the influence of the NH2 radical by incorporating its interaction with gas-phase aromatic species and soot particles into the adopted model. In this context, reference reaction rates are proposed and discussed. The model is compared to a set of experimental measurements of different target quantities, including soot volume fraction, fv , in two distinct sets of counterflow flames. The model has been demonstrated to effectively predict pivotal morphological soot characteristics, including the progression of the average particle size ( D63 ) in counterflow flames and the particle size distribution (PSD) in premixed burner-stabilized stagnation flames at a height above the burner, Hp , equals to 5 mm for diverse NH3 concentrations. Nevertheless, it has been demonstrated that the model poorly predicts the bimodal distribution of the PSD when ammonia is introduced together with ethylene at Hp = 10 mm. Kinetic analyses are conducted to identify the primary competing reactions between N-containing species and hydrocarbons, which influence the observed and simulated reduction in 4-ring PAH in the presence of NH3 relatively to pure ethylene flames. It is essential that future experimental studies be conducted to quantify N-containing hydrocarbons in laminar flames. This will serve to validate the findings of the kinetic modeling study and refine the understanding of pathways controlling soot chemistry in ammonia-doped flames.