# Transfer Functions of Ammonia and Partly Cracked Ammonia Swirl Flames

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^{2}

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## Abstract

**:**

## 1. Introduction

_{3}) and hydrogen (H

_{2}) are considered alternative solutions to hydrocarbon fuels in combustion-based energy conversion systems [1,2,3]. Despite a low higher heating value of 22.5 MJ/kg, NH

_{3}has the advantage of a high density at ambient temperature and relatively low pressures, as it liquefies at 8.88 bar for a temperature of 294 K. Therefore, storage and transportation of ammonia are key advantages compared to other carbon-free fuels. However, the combustion of ammonia has numerous challenges, such as a narrow range of flammability [4], a low burning velocity, and large pollutant emissions such as nitrogen oxides (NOx), nitrous oxide (N

_{2}O), and unburned NH

_{3}[1,5,6,7]. To increase the flammability range and accelerate the burning velocity, previous studies have demonstrated that cracking a fraction of ammonia into H

_{2}and nitrogen (N

_{2}) prior to combustion could be very efficient [4,5,6,8,9,10,11].

_{3}and H

_{2}in blends of these fuels could have a dramatic impact on the NO

_{2}concentration in the burned gases. The best blend for lean combustion was found to be 95% of NH

_{3}and 5% of H

_{2}(in volume). Another strategy studied for the reduction of NOx emissions was to burn ammonia in a two-stage, rich-lean combustor [18,19]. Note that premixed flames of hydrogen and blends of hydrogens with other fuels are known to be susceptible to thermo-diffusive instabilities [20,21,22].

## 2. Materials and Methods

#### 2.1. Experimental Setup

_{3}, H

_{2}, and N

_{2}, mimicking the thermal cracking of ammonia upstream of the burner. The percentage of cracking, α, is defined as:

_{3}, H

_{2}, and N

_{2}studied in this work correspond to 10% cracking, 20% cracking, and 28% cracking. This maximal cracking of 28% was chosen because, at this condition, the flammability limits approach those of methane–air flames [8]. Table 1 summarizes the mass flow rates of the gases for each of the conditions studied during the investigation of the flame response to acoustic modulation of the flow. For all conditions, the equivalence ratio was 0.95, the bulk flow velocity was 7 m/s, and the flame thermal power (calculated for adiabatic combustion) was about 3.4 kW. The corresponding Reynolds number was around 4500.

#### 2.2. Diagnostics

_{2}in the burned gases, as well as chemiluminescence imaging of the flames were performed.

_{2}, and O

_{2}sensors. The uncertainty in the concentration measured was about ±5 ppm for measured values between 0 and 99 ppm, ±5% for measured values between 100 and 1999 ppm, and ±10% for measured values between 2000 and 4000 ppm for NO; ±5 ppm for measured values between 0 and 99 ppm and ±5% of measured value between 100 and 500 ppm for NO

_{2}, and ±0.8% for O

_{2}.

#### 2.3. Methods

_{2}was measured in the quartz tube, 4 cm below its upper end. For these measurements, the length of the quartz tube was 40 cm to ensure good mixing of the burned gases at the location of sampling. A scan over the diameter of the tube (70 mm) was performed, and the results obtained were averaged. For a given combustion condition, the root mean square (RMS) error of the concentrations obtained during a scan was negligible compared to the measurement uncertainty. Therefore, the error bars in the figure correspond to the measurement uncertainties.

## 3. Results

_{2}in the burned gases. Finally, the FTFs of the flames presented in Figure 1 are reported (Supplementary).

#### 3.1. Effect of Ammonia Cracking on the Overall Burning Velocity

#### 3.2. Lean Blow-Off Limits

_{3}–H

_{2}–N

_{2}–air blends considered in this study.

#### 3.3. NO and NO_{2} Emissions

_{2}, in the burned gases of the four flames presented in Figure 1, i.e., for a percentage of ammonia cracking, $\alpha $, varying from 0 to 28%. The equivalence ratio was 0.95, and the bulk flow velocity was 7 m/s. Each data point corresponds to an average of four measurements. The repeatability of the measurements was good compared to the accuracy of the gas analyzer; therefore, the error bars correspond to the uncertainty of the measurements (see Section 2.2). For pure ammonia, the average concentration of NOx of about 2900 ppm agrees with what was reported in other studies for an equivalence ratio close to stoichiometry [4]. At 10% of ammonia cracking, the effect on the NOx concentration was negligible, with an increase of less than 2%. However, for a larger percentage of ammonia cracking, the concentration of NOx in the burned gases increased significantly, up to 6700 ppm for $\alpha =28\%$.

_{2}, as a function of the equivalence ratio, Φ, for pure ammonia (black squares), ammonia at 10% cracking (green circles), ammonia at 20% cracking (blue triangles), and ammonia at 28% cracking (red stars) flames. A correction was performed to avoid underestimating the amount of NOx produced per unit of fuel burned in lean cases due to dilution by air. The corrected concentration [NOx]

_{corr}, was obtained from Equation (7) [35]:

_{corr}dropped below 500 ppm at Φ ≈ 0.8. For 20% cracking, [NOx]

_{corr}= 500 ppm was achieved for Φ ≈ 0.67, and for 28% cracking, Φ ≈ 0.6 should be reached to obtain this corrected concentration. Again, these results agree well with what was found in previous studies [4], and this will be one of the main challenges of the ammonia cracking strategy in industrial systems subjected to regulations on NOx emissions. Note that strategies such as a two-stage, rich-lean combustor [18,19] have shown promising results in addressing this issue. Furthermore, a study on the effect of non-equilibrium plasma produced by nanosecond repetitively-pulsed discharges showed that non-equilibrium plasma could be used to decrease the NOx production of ammonia flames [36]. The development of plasma actuators could be a potential solution for industrial systems such as gas turbine engines.

#### 3.4. Flame Transfer Functions

## 4. Discussion

#### 4.1. Stabilization Mechanisms

#### 4.2. Flame Dynamics

## 5. Conclusions

- For a swirl flame of about 3.4 kW, with an equivalence ratio of 0.95 and a bulk flow velocity of 7 m/s, cracking of ammonia in the range of 0–28% in mass linearly increased the overall burning velocity.
- For pure ammonia and ammonia at 10% cracking, the equivalence ratio at blow-off did not follow a monotonic trend when the bulk flow velocity was increased from 4 to 13 m/s. A step at about 8 m/s was observed, corresponding to a change in the stabilization mechanism.
- While cracking of ammonia was very efficient in enhancing the lean stability limit, the concentration of NOx in the burned gases was increased by about a factor of two for 28% cracking. In parallel, the response of the flame to acoustic modulation was also enhanced for a large range of frequencies, making these flames more prone to thermoacoustic problems than pure ammonia flames.
- The strong response of ammonia–hydrogen–nitrogen–air flames to acoustic perturbation of the incoming flow can be explained by the strong fluctuation of the top of the flame, i.e., the flame vortex roll-up, which can be explained by the strong resistance of hydrogen flames to strain rate.

## Supplementary Materials

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Direct visualization of ammonia, ammonia at 10% cracking, ammonia at 20% cracking, and ammonia at 28% cracking flames, with a thermal power of 3.4 kW, a bulk flow velocity of 7 m/s, an equivalence ratio of 0.95, and a swirl number of 1.67.

**Figure 3.**Output voltage of the hot-wire anemometer as a function of the flow velocity for the four mixtures used in this study. Error bars correspond to the maximal discrepancy between two measurements at the same condition.

**Figure 4.**Overall flame length and overall burning velocity of the four NH

_{3}–H

_{2}–N

_{2}–air blends with a global equivalence ratio of $\mathsf{\Phi}=0.95$, a bulk flow velocity of 7 m/s, and a corresponding flame thermal power of about 3.4 kW. Error bars represent uncertainties in the flame dimension measurements.

**Figure 5.**(

**a**) Equivalence ratio at blow-off as a function of the bulk flow velocity for pure ammonia flames (black squares), 10% cracking (green circles), 20% cracking (blue triangles), and 28% cracking (red stars). Each condition was repeated four times. (

**b**) Equivalence ratio at blow-off as a function of ammonia cracking for two bulk flow velocities, 7 m/s and 12 m/s. Error bars correspond to uncertainties in the measurements.

**Figure 6.**Concentration of NOx (i.e., NO + NO

_{2}) as a function of ammonia cracking. For all flames, the equivalence ratio was 0.95, and the bulk flow velocity was 7 m/s.

**Figure 7.**Corrected concentration of NOx (NO + NO

_{2}), [NOx]

_{corr}, as a function of the equivalence ratio, Φ, for pure ammonia, ammonia at 10% cracking, ammonia at 20% cracking, and ammonia at 28% cracking flames.

**Figure 8.**Transfer functions of a pure ammonia flame (black squares) and flames of ammonia at 10% cracking (green circles), 20% cracking (blue triangles), and 28% cracking (red stars). For all flames, the equivalence ratio was 0.95, and the bulk flow velocity was 7 m/s.

**Figure 9.**Direct visualization of (

**a**) a pure ammonia flame at $\mathsf{\Phi}=0.78$ and a bulk velocity of 6.8 m/s, (

**b**) a pure ammonia flame at $\mathsf{\Phi}=0.70$ and a bulk velocity of 8.5 m/s, (

**c**) a 10% ammonia cracking flame at $\mathsf{\Phi}=0.62$ and a bulk velocity of 6.8 m/s, and (

**d**) a 10% ammonia cracking flame at $\mathsf{\Phi}=0.55$ and a bulk velocity of 8.5 m/s.

**Figure 10.**Phase-locked Abel-deconvoluted images of the OH* chemiluminescence for the four flames of Figure 1 ((

**a**) pure ammonia flame, (

**b**) flame of ammonia at 10% cracking, (

**c**) flame of ammonia at 20% cracking, and (

**d**) flame of ammonia at 28% cracking), subjected to acoustic forcing at 192 Hz, with a forcing amplitude of 10%. The black isolines correspond to 25% intensity for the pure ammonia flame and 50% intensity for the partly cracked ammonia flames.

**Table 1.**Mass flow rates for the four fuel–air blends utilized for the measurements of the flame transfer functions. The equivalence ratio was 0.95, the bulk flow velocity was 7 m/s, and the corresponding thermal power released during adiabatic combustion was about 3.4 kW.

Condition | Ammonia (SLPM) | Hydrogen (SLPM) | Nitrogen (SLPM) | Air (SLPM) |
---|---|---|---|---|

Pure NH_{3} | 15.53 | 0 | 0 | 58.36 |

10% cracking | 13.69 | 2.28 | 0.76 | 57.15 |

20% cracking | 11.92 | 4.47 | 1.49 | 56.01 |

28% cracking | 10.56 | 6.16 | 2.05 | 55.12 |

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**MDPI and ACS Style**

Shohdy, N.N.; Alicherif, M.; Lacoste, D.A.
Transfer Functions of Ammonia and Partly Cracked Ammonia Swirl Flames. *Energies* **2023**, *16*, 1323.
https://doi.org/10.3390/en16031323

**AMA Style**

Shohdy NN, Alicherif M, Lacoste DA.
Transfer Functions of Ammonia and Partly Cracked Ammonia Swirl Flames. *Energies*. 2023; 16(3):1323.
https://doi.org/10.3390/en16031323

**Chicago/Turabian Style**

Shohdy, Nader N., Mhedine Alicherif, and Deanna A. Lacoste.
2023. "Transfer Functions of Ammonia and Partly Cracked Ammonia Swirl Flames" *Energies* 16, no. 3: 1323.
https://doi.org/10.3390/en16031323