# ON AMMONIA KINETICS OF OXIDATION UNDER HIGHLY TURBULENT CONDITIONS AND IMPACT OF NITRIC OXIDE

Philippe Dagaut

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Philippe Dagaut. ON AMMONIA KINETICS OF OXIDATION UNDER HIGHLY TURBULENT CONDITIONS AND IMPACT OF NITRIC OXIDE. 2nd Symposium on Ammonia Energy, University of Orléans, Jul 2023, ORLEANS, France. hal-04165877

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2nd Symposium on Ammonia Energy

ON AMMONIA KINETICS OF OXIDATION UNDER HIGHLY
TURBULENT CONDITIONS AND IMPACT OF NITRIC OXIDE

Dagaut, Philippeᵃ*

ªCNRS 1C Avenue de la Recherche Scientifique, Orléans, France

* Corresponding author.
e-mail address: philippe.dagaut@cnrs-orleans.fr

**Topics: Combustion Fundamental, Reaction Modelling**

**Introduction**

Ammonia has been used extensively for the selective non-catalytic reduction (SNCR) of nitric oxide (NO). This
process efficiently reduces NO over a limited temperature range centred around 1200 to 1250 K. Experimental
and kinetic modelling studies of SNCR are numerous[1-5].

Nowadays, the interest for ammonia as energy carrier or fuel is rising. Indeed, ammonia is regarded as a zero-
carbon alternative fuel usable in power stations[6] including gas turbines [7]. However, its combustion needs
further studies [6] and kinetic interpretation of experimental results needs further investigations since it has been
reported that kinetic reaction mechanisms have weaknesses and occasionally fail to simulate the kinetics of
oxidation of ammonia [6]. Furthermore, under practical conditions, turbulence and recirculation favour the
interaction between ammonia and nitrogen oxides and may influence ammonia overall kinetics of oxidation.
We report experimental results obtained in a jet-stirred reactor (JSR) at 1 bar for several equivalence ratios in
the range 0.1 to 2, for several initial mole fractions of ammonia and nitric oxide, at fixed mean residence times
and over a range of temperature. Kinetic modelling of JSR experiments and burning velocities was performed
and reaction pathway analyses were performed to interpret the results.

**Materials and Methods**

Oxidation experiments were completed using a small volume (27.5 cm³) fused-silica JSR introduced previously
[8]. It consists of a 40 mm o.d. sphere equipped with 4 injectors (nozzles of 1 mm i.d.), for the admission of the
gases which achieve the stirring. The JSR is located inside a temperature-controlled electric oven (1500 W),
isolated by ceramic wood. In the experiments, high dilution by nitrogen was used. Ammonia, nitric oxide, N₂,
and O₂ flows were delivered using thermal mass-flow meters. The JSR operated under macro-mixing conditions
which allows using of a perfectly-stirred reactor model. A good thermal homogeneity along the vertical axis of
the reactor was observed in the experiments using a Pt/Pt-Rh10% thermocouple of 0.1 mm diameter located
inside a thin-wall fused-silica tube (<0.5mm), which avoids catalytic reactions on the thermocouple wires.

Thanks to a high degree of dilution the temperature rise in the JSR was small (<20 K). Samples of the reacting
mixtures (30–40 Torr) were collected via a sonic quartz probe for off-line gas chromatography (GC) analyses.

To increase the detection of samples components by GC, they were pressurized at 1 bar before injection into the
GC column, using an in-house glass piston. Quantification of permanent gases was done using a GC with
thermal conductivity detector. The mole fractions of H₂O, NO, NO₂, N₂O, and NH₃ were measured on-line by
Fourier transform infrared spectroscopy (FTIR). The uncertainties on mole fractions were determine to be in the
range ±10 to 15% for the reactants and products.

Kinetic modelling was performed using the PSR software [9]. Flame speeds modelling was carried out using the PREMIX computer code [10]. Several published kinetic model were used: a SNCR-SOₓ scheme [4], a HCN-NOₓ model named DGA mechanism [11], Konnov mechanism [12], and the kinetic reaction mechanisms of Song et al. [13], and Otomo et al. [14].
## Results and Discussions

Experiments were completed at 1 bar for the equivalence ratios φ equal to 0.1, 0.5, 1, and 2. Here, we define φ = (NH₃% / O₂%) / (NH₃% / O₂%)ₐt and ammonia global oxidation by NH₃ + 1.25 O₂ = NO + 1.5 H₂O. We considered two initial mole fractions of ammonia (500 and 1000 ppm) and three initial mole fractions of nitric oxide (0, 500, and 1000 ppm). The experiments were carried out at two residence times, i.e., τ = 0.1 and 0.2 s. The JSR temperature was increased stepwise from 1100 to 1450 K. The mole fractions of the reactants O₂, NH₃, and NO, and of the products NO, NO₂, N₂O, and H₂O were obtained.

Our experimental results showed a significant effect of nitric oxide on ammonia conversion, not described in the literature. Indeed, the presence of 500–1000 ppm of nitric oxide reduces fuel-lean and fuel-rich NH₃ ignition temperature by ~120 K.

<figure><img src="image_1.png" /><figcaption>Figure 1. Impact of nitric oxide on ammonia conversion in a jet-stirred reactor at 1 bar: 0.1% NH₃, 1.25% of O₂, τ=100 ms; φ=0.1; no NO (open symbols), 1000 ppm of NO (solid symbols).</figcaption></figure>

At the same time, the conversion of NO to NO₂ was favoured by the presence of ammonia. Several published kinetic reaction schemes were used to model our JSR experiments. We observed that the fastest simulated oxidation of NH₃ was obtained by the scheme of Konnov [12], which is in line with results of previous modelling efforts [6, 15]. The schemes of Song et al. [13] and Otomo et al. [14] also overestimates the rate of oxidation of. We also tested the previously proposed DGA model [11] because it was reported by Kobayashi et al. [6] that it could predict well ammonia ignition in a shock-tube [16] and ammonia burning velocities. Indeed, we found that the DGA scheme performs better than the other kinetic schemes tested here. Nevertheless, the DGA scheme slightly over predicts ammonia-air burning velocities. Hence, sensitivity analyses were conducted to identify possible ways of improving the predictions of ammonia-air burning velocities and keeping good predictions of JSR data. It was found that the kinetics of NH₂ + H ⇌ NH + H₂ and HNO + H ⇌ NO + H₂ were significantly influencing the simulation results for flame speeds but not those of JSR or shock-tube. We updated the DGA scheme using the kinetic parameters proposed by Otomo et al. [14] for these two reactions. The updated kinetic reaction scheme could well predict flame speeds, JSR and shock tube data. Sensitivity analyses were also carried out to identify the reactions influencing the most predictions of published reaction mechanisms and rationalise discrepancies observed between simulations and JSR data.

## Conclusions

Experiments performed in a JSR under turbulent conditions at 1 bar for several equivalence ratios (0.1 to 2), initial mole fractions of ammonia (500 to 1000 ppm) and NO (0 to 1000 ppm) revealed significant acceleration of ammonia ignition and oxidation by nitric oxide and conversion of NO to NO₂.

A previously proposed kinetic model [11] was updated to improve the modelling of ammonia-air flame speeds. It could be used to well predict the JSR data, ammonia ignition, and ammonia-air flame speeds. According to computation using that updated model, the mutual sensitization of the oxidation of NH₃ and NO is due to a sequence of reactions producing (i) OH and (ii) HO₂ radicals, principally involved in the oxidation of ammonia: (i) NH₂ + NO → NNH + OH, NNH → N₂ + H, H + O₂ → OH + O, and (ii) NNH + O₂ → N₂ + HO₂ and H + O₂ + M → HO₂ + M, NO + HO₂ → NO₂ + OH.
## Acknowledgements

Support from the CAPRYSSES project (ANR- 11-LABX-006-01) funded by ANR through the PIA (Programme d'Investissement d'Avenir) is gratefully acknowledged.

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