Emission Spectra of Ammonia Laminar Flames in Spherically Propagating Flames and a Modified McKenna Burner

ABSTRACT

Ammonia (NH₃) has received significant attention recently as a promising carbon-free, low-cost, and safe candidate as a chemical energy storage element and/or fuel alternative. As for any new fuel, fundamental aspects of the combustion process, such as the detailed chemical kinetic pathways of NH₃ oxidation, flame propagation and stability, and emissions, are to be understood to predict its combustion behavior in practical systems. One aspect of improving our understanding of the combustion behavior of NH₃ is to explore the emission spectra of spherically expanding and/or Bunsen-type flames under controlled laboratory conditions. In this study, flame spectra from NH₃-containing mixtures were obtained utilizing two different experimental flame configurations, namely a spherically propagating flame and a modified Mckenna burner flame. Two different fuel mixtures were investigated. The first was an oxy-ammonia mixture consisting of NH₃ + (25% O₂ +75% N₂) with the goal of studying neat ammonia. The second mixture consisted of H₂:NH₃:N₂ with a ratio of 45:40:15 by volume. Several excited-state species within the UV-visible spectral region were identified. For instance, NO* emission features were found between 220 and 280 nm. In addition, OH* emission was detected around 310 nm, and an NH* feature was seen around 337 nm. Moreover, a broadband intensity was reported over the visible range (400 ~ 800 nm), where several NH₂* features were also detected. The effect of the mixture on the flame spectra was investigated by examining the two different mixtures utilizing the same experimental apparatus (spherical flame in this case), and it was shown that the mixture has an effect on the presence of NH₂* features, which was also backed by chemical kinetics predictions for the NH₂* mole fraction profiles. It was shown (from kinetics) that the oxy-ammonia mixture produced more NH₂* than the H₂:NH₃:N₂ mixture by up to 113%. Additionally, the effects of the flame configuration were also investigated by studying the same mixture (the H₂:NH₃:N₂ mixture in this case), and it was revealed that the spectra produced using the modified Mckenna burner had less noise, and all features were easily identified. Lastly, laminar flame speeds of the oxy-ammonia mixture were measured using the spherical flame apparatus. The laminar flame speed followed the expected pattern, peaking near a 1.1 equivalence ratio with a measured flame speed of 22.7 cm/s.

KEYWORDS:

• Ammonia
• emission spectra
• spherical flames
• piloted-ammonia flames
• laminar flames
• chemiluminescence

Acknowledgements

Scholarship support for the primary author was provided by King Saud University (KSU) through the Saudi Arabian Cultural Mission (SACM).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available from the corresponding author, [YA], upon reasonable request.