Chemical kinetic study of gasoline surrogate with ammonia on combustion: Iso-octane modeling

Abstract

To facilitate the transition towards a carbon–neutral society, it is necessary to undertake comprehensive research on the utilization of ammonia in gasoline engine applications. In the present work, a new chemical kinetic mechanism for iso-octane/ammonia (iC8H18/NH3) was proposed and widely validated with the experiment data of ignition delay time (IDT) and laminar burning velocity (SL). The iC8H18 model of Cai [Combust. Flame, 162, 2015] and NH3 model of Zhang [Combust. Flame, 234, 2021] was chosen as the based model, and they connected through the chemical coupling between the iC8H18 and the NH2 chemistry. In addition, the influence of C-N interaction on the prediction ability of iC8H18/NH3 model was also analyzed and found that the H-abstraction reaction between NH2 and iC8H18 is the key reaction affecting the IDT and SL of mixture. Present work was also focused on the chemistry kinetic of iC8H18/NH3 mixture. The reaction pathway analysis shows that the addition of NH3 slightly enhanced the H-abstraction reaction of iC8H18 in tertiary site (cC8H17) and slightly inhibited the H-abstraction reaction of primary site (aC8H17, bC8H17, and dC8H17). In the end, the mole fraction of NO in iC8H18/NH3 flames is analyzed. The results show that NH3 addition increased the concentration of NO, but with the gradual increase of equivalence ratio, the increasing trend of NO concentration gradually decreases. A higher equivalence ratio can be selected to effectively control the increase of NOx concentration. Through the analysis of NO formation and reduction reaction pathway, it was found that NH3 addition increases the mole fraction of NH2 radical, the NO formation pathway was stronger than NO reduction pathway due to the sufficient OH radical, and finally leading to the increase of NO concentration. However, with the increase of equivalence ratio, the concentration of NH2 radical exceeds OH, and the number of OH is insufficient in satisfying the NH3-related NO reaction pathway, resulting in a decrease of the influence of NH3 addition on NO concentration.