NH measurements in low-pressure premixed
NH_3/O_2/N_2 flames

Nour El Baba, Pascale Desgroux, Nathalie Lamoureux

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Nour El Baba, Pascale Desgroux, Nathalie Lamoureux. NH measurements in low-pressure premixed NH_3/O_2/N_2 flames. 15th Asia-Pacific Conference on Combustion (ASPACC2025), May 2025, Singapour, Singapore. hal-05164872

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# NH measurements in low-pressure premixed NH₃/O₂/N₂ flames

...

Nour El Babaª, Pascale Desgrouxª, Nathalie Lamoureuxª,*,...

a Univ. Lille, CNRS, UMR 8522 - PC2A - Physicochimie des Processus de Combustion et de l'Atmosphère, F-59000 Lille, France

## Abstract

NH, an intermediate species that plays an important role in NO formation in ammonia oxidation, has been poorly investigated in previous works of the literature. In this work we aim to provide a reliable data set of quantitative NH measurements in three laminar low-pressure (10 kPa) premixed NH₃/O₂/N₂ flames stabilized on a porous burner, with equivalence ratio ranging from 0.87 to 1.33. The variation of NH as function of the height above the burner (HAB) was measured in situ using laser-induced fluorescence (LIF) by probing the R₁(6) transition at 333.9 nm. Fluorescence signal was collected along the Q-branch centered at 335.4 nm. Obtained relative population profiles were calibrated using single pass absorption targeting the R₁(6) transition in the lean flame at peak NH position. Obtained experimental NH profiles were compared to kinetic simulations of four of the most recent models in literature. All models well-predict the shape and the peak position of NH in the nearly stoichiometric and rich flames.

Keywords: NH₃ flames; NH profiles; Kinetic modelling;

<footnote>*Corresponding author.</footnote>
Information for Colloquium Chairs and Cochairs, Editors, and Reviewers

1) Novelty and Significance Statement

Modelers are eager for new data on the chemical structure of NH₃ (as sole fuel) flames in relation to NO formation. In situ quantitative NH mole fraction profiles obtained by laser-induced fluorescence quantified by absorption, will provide useful data for modeling assessment. Our work complements the only three datasets of the literature (two old papers from 1967, 1986 and a recent one from 2016). In such flames, NH profiles have been rarely measured although this intermediate species is expected to be a key element. Our work complements the dataset (OH and NO) recently performed in our laboratory.

2) Author Contributions

* NEB: Investigation, Data curation, Formal analysis, Methodology, Writing-review

* PD: Project administration, Validation, Supervision, Writing-review & editing

* NL: Conceptualization, Project administration, Funding acquisition, Supervision, Validation, Writing-original draft & editing

3) Manuscript length:

* Four-page paper ☒

* Eight-page paper ☐

This work is still a work-in progress because the modelling part still requires to be further developed. The experimental results are already important regarding the rare NH profiles found in similar conditions in the literature.

4) Colloquium selection:

* Colloquium topic 1: Emission Mitigation from Flames

* Colloquium topic 2: Diagnostics
## 1. Introduction

2

3 The continuing increase in energy demand is a
4 challenge to achieving the net-zero CO₂ emission
5 needed to overcome the global warming [1].

6 Ammonia is a promising alternative for achieving
7 carbon neutrality in combustion. However, there are
8 some issues using NH₃ as fuel in combustion
9 application since the NOₓ emissions and unburnt NH₃

10 release can be important under lean and rich
11 conditions, respectively [2]. Moreover, it is important
12 to note that N₂O is a greenhouse gas with a global
13 warming potential 273 times higher than CO₂.

14 Numerous kinetic models dedicated to NH₃
15 chemistry have been published in the literature in the
16 last 2-decades, see e.g. [3,4]. As reported in [3], these
17 models have been validated on different sets of
18 experimental data for pure NH₃ mixtures, or NH₃
19 blend with H₂ or hydrocarbon fuels. Most model
20 validations are performed on global parameters:

21 ignition delay time (IDT), or laminar burning velocity
22 (LBV). Speciation measurements obtained in shock

23 tubes, perfectly homogeneous reactors (Jet-Stirred
24 Reactor), or flow reactors (Plug-Flow Reactor) were
25 used to a lesser extent for model validation. Finally,
26 very few validations have been based on species
27 profiles measured in NH₃ laminar premixed flames
28 stabilized above a burner (Burner-Stabilized Flames,
29 BSF). As example, Table 3 in [3] reports only 3
30 models validated using speciation in BSF. Girhe et al.

31 [4] performed a systematic comparison of the
32 simulated and the experimental results using a
33 numerical approach developed in [5]. However,
34 speciation performed in BSF were not considered as
35 target. It is shown that none of the models tested is
36 able to predict satisfactorily the ammonia chemistry
37 over a wide range of operating conditions.

38 The measurement of the species in absolute
39 concentration is feasible either after gas probe
40 sampling with Molecular Beam Mass Spectrometry
41 (MBMS), or with spectroscopic laser diagnostics as
42 LIF and absorption that offer the possibility to
43 measure radical species in situ.

44 It is well known that during its oxidation, NH₃
45 quickly forms NH₂ and NH radicals. Using MBMS,
46 NH₂ radicals were measured in different flame
47 conditions, mostly in premixed NH₃/H₂/O₂/Ar flames
48 [6,7]. However, NH radicals were reported only in
49 NH₃/O₂ low-pressure flames using MBMS [8], and
50 NH absorption [9]. More recently, Brackmann et al.
51 [10] reported absolute NH radicals mole fraction
52 profiles in NH₃/Air BSF using saturated LIF
53 measurements. Probing the R₂(8) line in the A³Π-X³Σ⁻
54 (0,0) band, NH profiles were obtained in 3
55 atmospheric premixed NH₃-Air flames with an
56 equivalence ratio, φ, ranging from 0.9 to 1.2.

57

58 In the present work, and complementary to our
59 previous paper [11], NH profiles were measured in 3
60 premixed NH₃/O₂/N₂ flames stabilized at 10 kPa. The
61 experimental results are compared to 4 selected

62 models [12–15]. These models showed a good ability

63 to predict ammonia oxidation [4] and for OH and NO

64 profiles obtained in [11].

65

## 2. Experimental setup

67

### 2.1 Burner setup

69

70 Experiments have been carried out in three laminar
71 NH₃/O₂/N₂ premixed flames stabilized above a 6-cm
72 diameter stainless-steel water-cooled McKenna
73 burner (Holthuis & Associates) [11,16]. Pressure is
74 maintained at 10 kPa using a vacuum pump.
75 Volumetric flow rates of the gas mixture are regulated
76 by using Mass Flow Controller (MFC), see Table 1.
77 The burner is movable in the vertical direction. The
78 height above the burner (HAB) is defined by the
79 distance between the laser axis and the burner surface.

80

81 Table 1

82 Flame conditions in volumetric flow rate in slpm unit (at
83 273 K and 101.3 kPa).

<table><thead><tr><th>Flame</th><th>NH<sub>3</sub></th><th>O<sub>2</sub></th><th>N<sub>2</sub></th><th>φ</th></tr></thead><tbody><tr><td>F1</td><td>1.07</td><td>0.92</td><td>0.64</td><td>0.87</td></tr><tr><td>F2</td><td>1.23</td><td>0.84</td><td>0.64</td><td>1.10</td></tr><tr><td>F3</td><td>1.29</td><td>0.73</td><td>0.64</td><td>1.33</td></tr></tbody></table>

84

85

### 3.1 Laser diagnostic

87

88 For the LIF diagnostics, NH radicals were probed
89 along the line R₁(6) of the A³Π-X³Σ⁻ (0,0) band at
90 333.9 nm, following the procedure previously
91 performed in [17]. The pulsed laser system consists of
92 a frequency-doubled Nd:YAG laser (DLS8000,
93 Continuum) pumping a dye laser (ND6000,
94 Continuum). The laser beam was introduced
95 unfocused parallel to the burner surface and shaped
96 using a horizontal slit of 300 µm height and 2 mm
97 width in front of the burner. The LIF signal was
98 collected by a two-lens system (f=300 and 200 mm)
99 and focused on the entrance slit of a 0.300-m
100 monochromator (Princeton SP300, f/3.9, 1200 gr/mm,
101 blazed at 300 nm). The entrance slit (200-µm width,
102 10-mm height) was parallel to the laser axis. The
103 collection volume is estimated to be equal to 2 mm³.
104 The output slit was adjusted to obtain the desired
105 collection bandwidth of 2.4 nm centered at 335.4 nm,
106 corresponding to the Q-branch. To ensure to be in the
107 linear regime of LIF, the energy laser was maintained
108 at 70 µJ/cm²/pulse. LIF signals were collected using a
109 photomultiplier (R2059, Hamamatsu, rise-time 1.3
110 ns).

111

### 4.1 Methodology

113

114 The relative rotational population profiles of NH
115 are obtained from LIF measurements (accumulation
116 of 300 laser shots at each height above the burner,
117 HAB), performed at the temporal peak of the LIF
1 signal (prompt-LIF). The use of prompt-LIF detection

2 in low pressure flames overcomes the quenching

3 correction because the LIF decay-time is sufficiently

4 longer than the laser pulse [16]. Each rotational

5 profile is obtained independently in the 3 flames from

6 an average of at least 3 profiles, and is normalized at

7 its peak value. Then, LIF measurements are

8 performed alternatively between the 3 flames at their

9 peak value location, in order to get the relative

10 rotational profiles.

11 Integrated spectral absorptivity was measured

12 around the line R₁(6) of NH in the lean flame (F1) at

13 HAB = 4 mm. For that, 2 photodiodes were located

14 before and after the burner enclosure similarly to [16].

15 The absorbance (A(ν̄) = (πe²/mₑc²)fJ''J, NJ''-lS)

16 peaks at 6%, with fJ'' = 2.653×10⁻³ [17]. Following

17 the procedure described in [11] for OH, the radial

18 distribution of NH was measured with an ICCD

19 camera (PIMAX-III, Princeton) in order to measure

20 the effective absorption pathlength, lS. Knowing the

21 temperature (T = 2231 K), the Boltzmann factor was

22 calculated to convert the absolute rotational

23 population of NH in absolute mole fraction, equal to

24 287±21 ppm at 4 mm in flame F1. Finally, the relative

25 rotational LIF profiles were converted in absolute

26 mole fraction for each flame. Applying Taylor series

27 expansion to the LIF signal, the experimental

28 uncertainties were estimated to be 15% in absolute

29 mole fraction, but the relative mole fraction between

30 the 3 flames is 4% only.

31
32 **3. Results and Discussion**

33

34 Experimental NH mole fraction profiles are

35 reported in Fig. 1. With the increase of φ, the inlet NH₃

36 mole fractions increase from 40% to 48%, the peak

37 value slightly decreases from 290 to 210 ppm. This

38 clearly indicates that the conversion from NH₃ to NH

39 is favored in lean condition as shown previously in

40 [11]. Also, the profiles expand further towards the

41 burned gases. In the rich flame F3, there is still 20 ppm

42 of NH at HAB = 2.5 cm. In the lean flame F1, the NH

43 is restricted within few mm centered at HAB = 4 mm.

44 In this flame, the NO profile peaks at 6.1 mm [11]

45 while almost NH are consumed. In F2 and F3 flames,

46 the NH profiles peak at 5, and 7 mm, respectively. The

47 NO profiles peak 1 mm downstream.

48 Simulated NH profiles were calculated using the

49 1D BurnerFlame object in Cantera 3.0.0 [18] with 4

50 detailed kinetic models; Mei, Zhu, Stagni, and Zhang.

51 Experimental temperature profiles and species mass

52 flow rates were used as input. Species transport

53 properties were calculated using the multi-component

54 transport model. The computational domain was

55 refined using the following grid refinement

56 parameters: ratio = 3, slope = 0.05, and curve = 0.2.

57 For clarity, and in the continuation of our previous

58 paper [11], only the profiles obtained with the Mei

59 model are reported in Fig. 1. Note that the 3 other

60 selected models tend to overpredict by 50% the

61 experimental peak mole fractions. Mei's predictions

62 are found in good agreement with the experimental

63 values, being 15-20% higher. The simulated profiles

64 fit correctly the shape of the experimental ones,

65 except in the case of the lean flame, where the

66 simulated NH profile peaks 1 mm upstream the

67 experimental one.

68 In the present work at low pressure, existence of

69 NH in the burned gases of the rich flame participates

70 as expected in the NO consumption through the

71 reaction of NH + NO yielding N₂+OH, N₂O+H, and

72 NNH+O. In [11], the reaction pathway showed that

73 the first 2 reactions prevail compared to the latter one.

74 In F1 flame, the NH radicals are quickly consumed

75 through reactions NH + OH and NH + O₂ yielding

76 HNO.

<figure><img src="image_2.png" /><figcaption>Fig. 1. NH mole fraction profiles, comparison with simulated profiles (dotted lines) using Mei model [13].</figcaption></figure>

4. Conclusions

83 NH species mole fraction profiles were obtained in

84 three NH₃/O₂/N₂ low-pressure flames. The present

85 paper was performed in the continuation of the on-

86 going work performed in our laboratory [11,16]. NH

87 mole fraction was obtained from LIF measurements,

88 and calibrated in absolute mole fractions using

89 spectral absorptivity along the line R₁(6) of the (0,0)

90 band of NH.

91 The experimental results were compared to

92 simulated NH profiles using Cantera software [18]

93 and the detailed kinetic model of Mei et al. [13]. It is

94 shown that both simulated and experimental results

95 are found in very good agreement for the flames at an

96 equivalence ratio of 1.10 and 1.33. However, the

97 simulated profile of the lean flame (φ = 0.87) peaks 1

98 mm upstream the experimental one. Further analysis

99 is still required to investigate if modelling predictions

100 can be improved.

101
102 Declaration of competing interest

104 The authors declare that they have no known

105 competing financial interests or personal relationships
1 that could have appeared to influence the work

2 reported in this paper.

3

4 Acknowledgements

5

6 This work is a contribution to ANR SIAC (ANR-

7 22-CE50-0022), MéOL platform of University of

8 Lille, and LabEx CaPPA (ANR-11-LABX-0005-01)

9 funded by french National Research Agency and to

10 CPER ECRIN funded by French Ministère de

11 l'Enseignement Supérieur et de la Recherche,

12 Regional Council “Hauts-de-France” and “European

13 Regional Development Fund”. Authors thank the

14 French Agency for Ecological Transition (ADEME)

15 for NEB PhD funding.

16

17 Supplementary material

18

19 No supplementary material submitted.

20

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