NO formation/reduction mechanisms of ammonia/air premixed flames at various equivalence ratios and pressures

Abstract

Ammonia is a carbon-free fuel and its application to internal combustion engines is expected. However, few studies on ammonia flames, especially at high pressures, have been carried out because ammonia has not been considered to be a fuel owing to its lower combustion intensity. Most of NOx, which is formed by ammonia combustion, is considered to be the fuel NOx. The objectives of this study were to investigate the fundamental characteristics of NOx experimentally, such as NO emission and chemiluminescence of ammonia/air flames not only at the atmospheric pressure but also under high pressures and to explore NO formation/reduction mechanisms using numerical simulation. Experiments were carried out using a nozzle-type burner. NH2 ammonia  band spectra were observed, and it was clarified that the color of ammonia flame is mainly determined by the NH2 ammonia  band and H2O spectra. Burned gas was sampled from ammonia flame stabilized at the burner. The mole fraction of NO decreased with the increase in equivalence ratio at atmospheric pressure. Reaction flow analysis was performed, and it was clarified that the decrease in the mole fraction of NO for rich mixtures was caused by NHi (i = 2, 1, 0). High pressure experiments were performed using a high pressure combustion test facility for stoichiometric ammonia flame. Consequently, the decrease in the mole fraction of NO was experimentally observed and its tendency was found to qualitatively agree with the results of the numerical simulation. It was clarified that the third body reaction of OH + H + M  H2O + M plays an important role in the reduction of the mole fraction of NO at high pressure.

Extinction characteristics of ammonia/air counterflow premixed flames at various pressures

“…This reduction in the production of HNO is consistent with the reduction of NO emission observed by Hayakawa (2015). The two main contributors to heat release, which are NH3+OH⇋NH2+H2O and NH2+OH⇋NH+H2O, are only weakly affected by a rise in pressure, which is consistent with a lower decrease in NH3 flame burning velocity with pressure, P -0.20 , than for CH4.…”

“…Results are plotted in Colson, Hayakawa, Kudo and Kobayashi, Journal of Thermal Science and Technology, Vol.11, No.3 (2016) It is initially noted that the extinction stretch rate of NH3 flame (100 s -1 ) is around 20 times lower than that of CH4 flame (2000 s -1 ) at 1 atm. This difference might be understood given the large difference in their laminar burning velocities, around 7 cm/s for NH3 flame and around 37 cm/s for CH4 flame for stoichiometric conditions (Hayakawa et al, 2015). From Fig.…”

<i>Operando</i> XAFS studies of supported copper oxides for catalytic ammonia combustion

“…NH 3 has recently been regarded as a renewable and carbon-free energy source due to its high energy density, 13) but it has problems including high ignition temperature and N 2 O/NO x production, in comparison with fossil fuels. Previously, we proposed novel catalytic NH 3 combustion systems that enable low ignition temperatures as well as negligible N 2 O/NO x emissions, 14)16) and reported that the NH 3 combustion activity of metal oxides increases with a decrease in their metaloxygen bond energy.…”

Study on flame stability in oxygen-enriched ammonia/N₂/O₂ laminar diffusion flame

“…近年，欧米をはじめとする諸外国を中心にアンモニアを代替燃料とするための研究が行われている (Zamfirescu and Dincer, 2008, Takizawa et al, 2008, Ronny, 1988, Kumar and Meyer, 2013, Duynslaegher et al, 2009, Duynslaegher et al, 2010, Pfahl et al, 2000, Li et al, 2014, Hayakawa et al, 2015a, Hayakawa et al, 2015b, Ichikawa et al, 2015, 武石他, 2015, 武石他, 2016, 小林, 早川, 2016, 小池他, 2016, 武石他, 2017 (水谷, 矢野, 1978, Chung and Lee, 1991, Takahashi and Schmoll, 1990, 小谷他, 2004, Otakeyama et al, 2009 …”