Experimental and numerical analyses of nitrogen oxides formation in a high ammonia-low hydrogen blend using a tangential swirl burner

Ammonia has been considered as a novel fuel for decarbonization purposes. However, emissions from combustion systems are still posing a problem. Therefore, experimental and numerical simulations have been conducted to study the concentration of exhaust emissions (Nitric oxide “NO”, Nitrous oxide “N2O”) from burning the ammonia/hydrogen (NH3/H2) blend 85/15 (vol%). The effects were measured at various thermal powers ranging 10 to 20 kW and with different Reynolds numbers from 20,000—40,000. The experimental points were numerically investigated in the Ansys CHEMKIN-Pro environment employing seven chemical kinetic mechanisms taken from the literature. All experiments have been undertaken at standard atmospheric conditions. The experimental results showed that both NO and N2O gradually increased when the Reynolds number increased from 20,000 to 40,000. Along with that, the concentration of NO emissions at the exhaust reported minimum level when the Re = 20,000 due to lower reactivity radical formation, all that led to a deterioration of the flame characteristics. Also, the integrated radical intensities of NO*, OH*, NH*, and NH2* demonstrate an increasing trend as Re increased from 20,000 to 40,000. In terms of thermal power, N2O suffered an abrupt decrease when the thermal power increased up to 15 kW, while the opposite occurs for NO. In addition, the radicals intensity of OH*, NH*and NH2* figures show an increase in their concentration when the thermal power increased up to 15 kW then decreased with increasing thermal intensity to reach 20 kW, reflecting into increased NO productions and decreased N2O levels. The numerical analysis showed that Stagni, Bertolino, and Bowen Mei were the most accurate mechanisms as these give a good prediction for NO and N2O. The study also showed that the chemical reaction (HNO + O2 ↔ NO + HO2) is the main source of NO formation. While the chemical reaction (NH + NO ↔ N2O + H) is responsible for the formation of N2O by consuming NO and when there will be abundance in NH radicals. Finally, dealing with a blended fuel of high ammonia concentration encourages ammonia chemistry to become more dominant in the flame. It decreases the flame temperature, hence lowering heat loss between the flame and the surrounding.


Introduction
Reducing carbon dioxide (CO 2 ) emissions is the primary goal towards the transition to an environmentally friendly future that does not depend on fossil fuels. Out of the potential sources that can replace fossils, ammonia is one of the most attractive candidates that has drawn considerable attention, leading to many studies that analyze the molecule as a promising free-carbon energy carrier [1,2]. The role of ammonia as a carbon-free fuel and its potential as a source to leverage excess energy from renewable energy has encouraged numerous studies in engines and gas turbines [3,4]. These studies have complemented essential combustion analyses to determine flame propagation and ignition delay time in pure and blended ammonia mixtures.
The combustion characteristics of ammonia (NH 3 ) when it is injected directly as pure ammonia into the combustion system differs from standard hydrocarbon fuels: the higher ignition delay time, higher heat of vaporization, higher minimum ignition energy, low burning velocity, higher auto-ignition temperature, lower heating value, restricted flammability range, low radiation intensity, lower adiabatic flame temperature, and slower chemical conversion rate, all are features that make ammonia a very distinctive fuel [5]. Therefore, a better understanding of ammonia combustion is essential to overcome the limitation of knowledge that bound the behavior of the molecule at various scales.
One of the detriments of ammonia combustion is the production of nitrogen oxides (NO X ). Even though ammonia is carbon-free and can be produced through renewable methods, the combustion products contain NO X , blend composition, residence time, equivalence ratio, and initial conditions [2,[6][7][8]. The term NO X refers to all nitrogen oxides that form from the combustion process, being nitric oxide (NO), nitrogen dioxide (NO 2 ), and nitrous oxide (N 2 O) typically produced in combustion systems. When ammonia is employed, the dominant component of the emitted NO X is NO. Similarly, NO 2 is the result of NO oxidation processes, being an emission that contributes to acid rain formation [9]. Further, N 2 O is considered a very potent greenhouse gas with up to 300 times the Global Warming Potential of carbon dioxide (CO 2 ) [9,10]. The formation of nitrogen oxides can be governed by several combustion mechanisms. Ammonia/hydrogen combustion mechanisms have two primary channels. The first includes the fixation process of molecular nitrogen contained in the combustion air (thermal NOx); the other process involves the oxidization of organic nitrogen that is bounded chemically in the fuel (fuel NOx). Figure 1 simply illustrates a layout of the most significant sources of nitrogen oxides in the combustion process.
Much research has been conducted to improve or to find a better understanding of the concept of NO X formation mechanisms in the combustion of ammonia to develop unique strategies that enable controlling such an emission [11][12][13][14][15][16]. Recently, fuel NO X productions from ammonia have been improved with more accurate results for more complex designing applications [17][18][19][20], investigate the formation and the reduction of NO X by developing a unique NH 3 model that covers a wide range of experimental conditions. An oxidation mechanism has been published by [17] which used Shock Tube experiments as a reference. They optimised their work with nine different models to highlight the discrepancies between ammonia kinetic Fig. 1 Reaction pathway diagram illustrating the significant formation/Reburn steps of NO models. The laminar flame speed has also been investigated in vast research [21] used a cylinder combustion chamber to undertake some measurements related to laminar burning velocity. They used ammonia-air blends under elevated pressure. In this study, the data has been compared with five kinetic mechanisms that were obtained from the literature and that denoted the discrepancies between models and the lack of agreement between the nitrogen-based reactions occurring with these blends. Glarborg et al. [22] established a model that offers reliable diagnostic for the creation and destruction of NO under a wide range of conditions. Their selected model can predict and deliver data for a great variety of industrial processes. The model of [23] was utilized to study the reaction structure zone and reaction pathways of NO and NO 2 in non-premixed flames. This work found a high level of NO concentration in the high-temperature zone, particularly at stoichiometry. Also, the concentration of NO 2 dominated under lean conditions rather than under stoichiometry, where it was found that the level of NO 2 is quite small. The study indicated that the most influential reaction for NO 2 production is NO + HO 2 = NO 2 + H, hence reflecting the reason behind the low level of NO 2 at high temperatures. Klippenstein et al. [24] have been conducting their simulations at elevated pressure on the production of prompt NO for premixed flames. The study compared the modeling data with available experimental results and found that high pressure significantly impacts the prompt NO mechanism. Another study has been carried out by [25] to investigate the gas characteristics resulting from a premixed laminar blend of ammonia/ air with various equivalence ratios and elevated pressure experimentally and numerically. The study found that both equivalence ratio and pressure affect NO mole fraction.
In conclusion, the complete mechanism for oxidization of intermediates to NO X is narrow and not fully understood. Due to the lack of information from experimental work, there are still discrepancies in kinetic model predictions, which include uncertainties effect on the thermochemistry and the kinetic rate data. Even though the effort of several researchers by developing NH 3 -NO X kinetic model, a reliable model is not developed yet. Therefore, the present work focuses on studying the concentration of NO and N 2 O from the combustion of NH 3 /H 2 at 85/15 VOL% at various thermal powers and Reynolds numbers using experimental and numerical tools. This particular blend is unique, as it can potentially enable the use of some doping whilst ensuring reduced ammonia cracking to recover the doping agent (hydrogen). The work is done using a bespoke combustor, whilst the numerical analyses are conducted by employing seven chemical kinetic mechanisms.

Experimental work
A burner with a tangential swirl and a geometric swirl number of S g = 1.05 was used in this work at different Re and thermal powers, as illustrated in Fig. 2. A network of Bronkhorst mass flow controllers was utilized to feed the system with flows within 15-95% of the full capabilities with an accuracy of ± 0.5% Rd. The unit has been fed using a blended fuel of ammonia and hydrogen, as mentioned in Table 1. The ammonia and air were introduced via a mixing chamber, while hydrogen was fed through 6 holes with an equal slot. The holes are angled at 45• and positioned 4 cm below the burner exit. When the hydrogen is injected, it will go directly into the swirl for premixing with ammonia and air. A CH 4 continuous pilot flame was used for ignition and to ignite the flame and kept on to avoid flame extinctions during instability. Along with that, the pilot flame is located above the sampling point to avoid interference with the emissions data. Different conditions were carried out to assess and determine the influence of ammonia concentration on NO X formation at a constant equivalence ratio.
All the experiments reported here were carried out at atmospheric pressure (~ 1.1 bar) and temperature (~ 288 K) conditions. A pair of LaVision intensified CCD Cameras was employed to examine chemiluminescence traces of several species. The devices were activated simultaneously at 10 Hz and with a gain of 90%. Several Edmond filters were employed for different species, namely, OH* (309 nm) [26], NH* (336 nm) [26][27][28], and NH2* (630 nm) [26][27][28][29][30]. Five hundred frames per flame have been obtained by using Lavision Davis V10. The frames were post-processed employing a bespoke MatLab script [31], which was designed to undertake Abel Deconvolution after obtaining 3 × 3-pixel median filters and temporary averaging of the 500 images. Five thermocouples have been employed for determining the temperature at five certain points, as illustrated in Fig. 3. The thermocouples with 3 mm diameter were k-type and used in locations 1,4, and 5, while the other thermocouples, 5.60 mm and 10.23 mm diameters were Ceramic shielded R type, and have positioned in locations 3 and 4, respectively. Location 2 was chosen to monitor the flame temperature at the centerline of the flame at 3.9 cm from the quartz exit, while locations 3 and 4 were selected at the same height to monitor the temperature difference at the liner, thus calculating estimated heat loss. Location 1 and 5 were chosen to monitor temperature at the quartz exit liner and burner nozzle wall, respectively. The thermocouples were directly plugged into a RS data logger and linked to a computer. The calibration process was done previously for the thermocouples, which showed an average error of 3%. The thermocouples measurements have been corrected, taking into consideration the surrounding convective and radiative heat transfer of the thermocouple by implementing Eq. (1), [32]. (1) where T g is the gas temperature; T tc is the thermocouple bead temperature; T w is the characteristic radiant surrounding temperature (wall temperature); K is the gas thermal conductivity; ε tc is the emissivity of the thermocouple and Nu is the Nusselt number. A bespoke Emerson CT5100 quantum Cascade laser analyzer was used to measure several species of interest produced from the combustion process, such as NO, N 2 O, NO 2 , NH 3 , H 2 O and O 2 . the measurement was undertaken at a frequency of 1 Hz, linearity of  0.999, ± 1% repeatability and 190 °C of sampling temperature. The dilution process was carried out via adding N 2 within the sample with ± 10% repeatability, which was conducted when the wet reading crossed the range of the analyzer detection. A period of 120 s was allocated to do the recording of emission data for each points and to calculate the average for each condition. The flame stability was monitored by using a webcam type Logitech C270, which was installed 5 m away from the burner. Finally, A UV/visible-capable optical fiber head (Stellernet Inc DLENS with F600 fiber optic cable) was installed 3 cm above the burner's exit and 10 cm away from its central axis. The other end of the optical fiber was connected to a UV/visible-capable spectrometer (Stellernet Inc BLUE-Wave) featuring a 100-mm focal length and a 25-µm wide entry slit. The spectrometer was equipped with a 600-grooves/mm grating and a Si-CCD detector (Sony ILX511b) featuring 2048 effective pixels of size 14 × 200 µm 2 , yielding a spectral resolution of 0.5 nm. The detector's exposure time was set to 1 s and 20 scans were averaged to improve the signal-to-noise ratio (SNR). The Y-axis of the spectrometer was calibrated using a standard light source (SL1 Tungsten Halogen). In this study, contributions from different species, or specific spectral ranges, were quantified by integration of the chemiluminescence spectra over some specified wavelength ranges. These ranges are 221-261 nm for NO*, 302-326 nm for OH*, 335-346 nm for NH*, and 622-642 nm for NH 2 *.

Chemical kinetic modeling
In the present study, Ansys CHEMKIN-Pro [33] was applied for all numerical simulations to calculate the concentration of NOX emissions. Based on the previous studies [31,[34][35][36], a chemical reactor network (CRN) was improved for better predictions of emissions from the combustion zone, as illustrated in Fig. 3.
The modeling process of Mixing zones, Flame zones, central recirculation zone (CRZ), and external recirculation zone (ERZ) were imitated and modeled using a perfectly stirred reactor (PSR). Residence times were obtained from results obtained during previous numerical validated campaigns [37], delivering values close to 0.05 s at the central recirculation zone and 0.005 s at the shearing flow, with an estimated recirculation of species derived from the previous experimental campaigns [31,38] as shown in Fig. 3, within the constant volume of the combustion chamber. Although it is emphasized that these numbers are condition dependent (i.e., power, flow, temperature and pressure), their use in previous analyses have delivered acceptable results for the study of various ammonia blends [34]. Thus, they have been also used in this study. As displayed in Fig. 3, The numbers 30% and 70% stand for the volumetric split ratio of the gas supplied to the mixer and flame zone, respectively. While 5%, 25%, and 70% refer to the gas percentages in the external recirculation zone (ERZ), central recirculation zone (CRZ), and post flame zone after the gas crossing the flame zone. Depending on the thermocouple measurements, the approximation of heat loss data has been derived experimentally. The plug flow reactor (PFR) was employed for modeling the post-flame zone with a one-dimensional length of 15 cm, Table 2. The objective of kinetic modelling is to understand the discrepancy between models and identify the most sensitive reactions to the production of NO and N 2 O.  Table 2 Grid properties adopted in the current work

Number of grid points 2000
Adaptive grid control of solution gradient 0.02 Adaptive grid control of solution curvature 0.02 Starting axial position 0 cm

The investigated kinetic models
Seven kinetic reaction mechanisms, published in the past two years including NO x combustion chemistry suited, were selected to compare the numerical data with experimental measurements for the combustion of the NH 3 -H 2 blend. In the following, the kinetic models will be pointed to by a short name representing the first author followed by the year of publication. The kinetic mechanisms are detailed in Table 3 in terms of the number of reactions, the number of species, and the fuel mixture used in each model, as well as the target factors and describe the effects of parameters in the combustion characteristics of each model. The mentioned chemical kinetic mechanisms were selected for sensitivity analysis of reactions and to check the accuracy of the NO X sub-mechanisms that were newly added to their database depending on previous numerical and experimental campaigns and further validation using a chemical reaction network. Bertolino et al. [39] adopted a method with a hierarchical and systematic procedure to improve the nominal model of [43] by considering all the target datasets and uncertain parameters. a cumulative impact function (CIF) and Evolutionary Algorithm have been introduced to select the reactions with major impact and to optimize the solution. Their methodology has been supported by using a database of 635 experimental data points that cover ignition delay times, speciation, and laminar burning velocity of ammonia combustion.
Mei et al. [40] introduced a kinetic model for combustion of NH 3 /NO at atmospheric conditions (P = 1 atm and T = 298 K) with various equivalence ratios. the model has been improved based on the model reported from [18]. the rate constants of key chemical reactions have been updated, especially those which are directly or indirectly implicated in the interactivity between NH 3 and NO such as NH 2 + NO = NNH + OH, NNH = N 2 + H, NNH + NO = N 2 + HNO and NH 2 + NO = N 2 + H 2 O. The kinetic model can offer accurate targets of sensitivity validation for the kinetics of NH 3 and NO interaction.
Han et al. [41] investigated the effect of N 2 O enrichment in fuel on the combustion characteristics of NH 3 . The impact of rate constants for the most sensitive reactions in ammonia self-ignition and flame propagation of NH 3 + O 2 and H 2 + N 2 O blends were considered in the study. Their detailed model has been improved based on [46,47]. The thermodynamic data of nitrogen-based species have been selected from the database of Burcat and Rusicc [48]. while the rate constant of the kinetic reaction N 2 + O = NO + N was considered based on the experimental and kinetic modeling investigation of NO formation in CH 4 + O 2 + N 2 premixed flames [48]. Their model has been validated experimentally using the heat flux method at 1 atm and 298 K and the entire range of equivalence ratio.
Zhang et al. [42] constructed a kinetic model which deals with NH 3 and NH 3 -H 2 combustion. The model was  [18]. The kinetic model has been revisited for the reactions related to NH 3 sub-mechanism. The thermodynamic data of nitrogen-related radicals, especially NH, NH 2 , NNH, and N 2 H 2 , were updated based on the work published by [49]. Also, the rate kinetics of the most important reactions, particularly those related to NO X formation, have been evaluated based on some experimental [50,51] and theoretical [43,49,[52][53][54][55] studies. Along with that, the numerical data from model prediction have been validated with JSR experimental measurements of oxidation on NH 3 -H 2 blended flames at atmospheric pressure, low -moderate temperatures (800-1280 K), different H 2 contents, 0-70VOL% in the fuel, and various equivalence ratios. Stagni et al. [43] developed a kinetic model that deals with ammonia oxidation in a wide range based on various studies, including NO X sub-mechanism from [64], updating the HONO/HNO 2 chemistry depending on the work reported by [56], and updating the thermodynamic database of all species depending on [48]. Also, the NH 2 OH chemistry and dissociation reaction were considered, particularly for the reactions involving NH and NH 2 radicals from work published by [57]. The capability of the kinetic model has been tested in various configuration systems (Shock tube, Rapid compression machine, Flow reactor, Laminar flame speed, and burner-stabilized flame) and assessed in a wide range of operation conditions.
Han et al. [44] reported a kinetic model that estimates NO X emissions characteristics for ammonia oxidation. The study of Varga et al. [58] was the starting point for developing their kinetic model. The study included updating the database of constant rate parameters, collision factors, and pressure-dependent coefficients for the most critical reactions in ammonia oxidations, considering the individual investigation option for each reaction and its effect on laminar burning velocity. The role of HONO, HNOH, HON, HNO 2 , HONO 2 , and NO 3 reactions on the combustion characteristics has also been reported. In terms of validation, the model was tested and compared with several experimental measurements from literature with various operation conditions and equivalence ratios.
De Persis et al. [45] developed a kinetic mechanism for predicting nitrogen oxide formation. The effect of both equivalence ratio and pressure on NO mole fractions has been considered. The NO X sub mechanism has been developed and revised based on the investigation work done by Lamoureux et al. [59]. The contribution of NO major pathways (prompt, thermal, NNH, and N 2 O) has been reported and compared with the study from [24]. The mechanism has been validated based on experimental measurements of NO mole fraction profile using the LIF method in laminar counterflow configuration system at various equivalence ratios and pressure ranging from 1 -7 atm.
As shown in Table 3, all the mechanisms improved the NH 3 sub mechanisms in their database with corresponding thermochemistry data. Most of these models directly describe the combustion of NH 3 except de Persis model that deals with CH 4 flames, but it already has a NO X formation mechanism in its database and improved to some limits to forecasting acceptable levels of NO X .
The kinetic mechanisms containing nitrogen chemistry in its database would describe the combustion of ammonia. Along with that, the weak performance of these models does not mean the model is not well applicable for the conditions of interest, which it was originally improved for.

Results and discussions
In this section, the experimental results are compared with predicted data obtained via detailed kinetic models described in Sect. 2.1. Both experimental and numerical data were carried out in the same conditions of pressure and temperature (NTP). The equivalence ratio for the lean mixture of NH 3 /H 2 fuel was kept constant for all cases (∅ = 0.65).

Effects of Reynolds number
Constant Reynolds number is one of several essential parameters in investigating the performance of many practical systems, especially power generation and comparison aims between combustion system configurations [60]. To analyze the influence of turbulence factors on the combustion characteristics such as flame and emissions, three values of Reynolds number (20,000, 30,000, and 40,000) will be considered at a fixed equivalence ratio of 0.65. The figures below compare the present NO and N 2 O measurements with data estimated from kinetic models and for different Reynolds numbers in the range 20,000-40,000 at combustor exhaust. The mole fractions of NO and N 2 O in Figs. 4 and 5 are presented in units of ppmv. As shown in Fig. 4, the mole fraction of NO increases with the Reynolds number. The NO mole fraction resulting via experiments was close to data obtained by Stagni's model. Based on the comparison between the present study results and the modelling data from the literature, both Bertolino's and Stagni's mechanisms have a closer, reasonable agreement with the experimental data. Figure 5 shows the variation of the mole fraction of N 2 O in ppmv when the Reynolds number increases from 20,000 to 40,000 at zone exhaust. Bertolino's, Stagni's, and Bowen Mei Mechanisms gave better estimation to these experimental results than other models. Figures 6 and 7 demonstrate the radical spectroscopic spectrum and the chemiluminescence data, respectively, of different radicals of interest at a constant equivalence ratio of 0.65 and under different Reynolds numbers at the flame zone. The chemiluminescence images of OH*, NH*, and NH 2 * in Fig. 7 were normalized to image dataset max to display radicals distributions in the flame. Table 4 shows the integrated intensities of different radicals from the spectrum shown in Fig. 6. As can be seen, the concentration of NO emissions at the exhaust reported minimum level when the Re = 20,000. The reason behind that is the deterioration of the flame characteristics due to lower reactivity and radical formation at Re = 20,000. This behavior can be seen clearly in Fig. 7, the distribution of radicals expands with increasing Re. The integrated radical intensities of NO*, OH*, NH*, and NH 2 * in Table 4 shows an increasing trend as Re increased from 20,000 to 40,000 which is also obvious in Fig. 7. It was interesting to notice the increase in radical intensities is much lower when Re increased from 30,000 to 40,000, which reflects into exhaust emissions as well, thereby signifying the importance of these radicals in NH 3 /H 2 flames. In Fig. 7, the flame brush expands with increasing Re, giving an indication of an increase in the radical concentrations. The increase in NH 2 * radicals with increasing Re is due to OH* radical abundance and its role in the chemical reaction NH 3 + OH ↔ H 2 O + NH 2 . OH* radicals are also responsible for NH* production through the reaction NH 2 + OH ↔ NH + H 2 O and  can be considered the largest source of NH production from NH 2 . Eventually, NH* radicals are consumed through the chemical reaction NH + NO ↔ N 2 O + H to form N 2 O by consuming NO. The considerable increase in N 2 O when Re = 40,000 can be attributed to increased flame temperature, thus higher heat loss through the quartz liner. This phenomenon will be analyzed further in the latter part of the study. Figures 8 and 9 illustrate the variation of NO and N 2 O concentration in ppmv in terms of thermal power at the exhaust. The figures also include data from seven literature models run under the same operating condition. As can be seen, the mole fractions of NO peaked at 15 kW thermal power. NO mole fraction decreased with the decrease or increase in thermal power from 15 kW. All models predicted similar trends. However, the NO concentration resulting from experiments was close to those predicted by Stagni's, Betrolino's, and Zhang's mechanisms. Figure 9 shows the concentration of N 2 O. The figure illustrates a sharp decay in the concentration of N 2 O when the thermal power equals 15 kW. As power was increased to 20 kW, the N 2 O mole fraction increased and nearly gave the same value to that of a thermal power equal to 10 kW. Stagni's, Betrolino's, and Bowen Mei's mechanisms achieved a good agreement with the   experimental results of the present study. In contrast, the other mechanisms gave the same trend but with a difference in the concentration of N 2 O. The radical spectroscopic spectrum at different thermal power with a constant equivalence ratio of 0.65 at the flame zone is presented in Fig. 10, while Fig. 11 denotes the chemiluminescence imprints of this blend at various powers. Similar to previous discussions, colormaps were normalized to image dataset maximum to show the changes in radicals distributions as thermal power changes. Table 5 provides the integrated intensities of different radicals from Fig. 10. OH*, NH*and NH 2 * figures show an increase in their concentration when the thermal power increased up to 15 kW then decreased with increasing thermal intensity to reach 20 kW, Table 5. Simultaneously, this can be appeared clearly, especially when the thermal intensity increases from 10 to 15 kW, the flame brush expands due to increasing the OH*, NH*, and NH 2 * radical concentrations, Fig. 11. The increased production of these radicals at 15 kW reflects into increased NO productions and decreased N 2 O productions. This is further confirmed by the maximum radical intensity of NO* at 15 kW, Table 5. It is believed that NO production, in this case, is due to the conversion of NH 2 to NH through reactions with OH radicals, then combining NH and OH to form HNO, which is known to be the main source of NO formation in most mechanisms. Simultaneously, the chain branching reaction NH 2 + NO ↔ NNH + OH and the chain-terminating

Sensitivity analyses
As can be seen from the concentration profiles for both NO and N 2 O in the above figures, Han's mechanism gave the highest over-prediction for NO and N 2 O. At the same time, Zhang's mechanism predicts a low concentration of N 2 O. Stagni, Betrolino, and Bowen Mei mechanisms are the most accurate models since they predict data near the range of experimental combustion results. Therefore, sensitivity analyses and rate of production for NO and N 2 O as well as the NO X formation/consumption pathways at the flame zone are presented using Stagni, A.Betrolino, and Bowen Mei's kinetic models. As the highest exhaust emissions were observed at high thermal power, 20 kW, and high Reynolds number, 40,000, these conditions have been selected for analyzing the sensitivity and rate of production of both NO and N 2 O species in this part of the study.

[NO] Sensitivity analysis
NO is one of the most critical ammonia combustion products since it has a dangerous effect on the ecosystem with the production of acid rain and other environmental impacts. To study NO's chemistry and investigate the contribution of NO in the formation of N 2 O, sensitivity analyses have been considered for two previously explained conditions in this part of the study.

[NO] Sensitivity analysis for 20 kW of thermal power
Figures 12, 13 and 14 depict the sensitivity analysis of NO, rate of production, and NO formation/decomposition pathways, respectively, at the combustion flame zone when thermal power is equal to 20 kW using three kinetic models. As can be seen from the pathway's diagram, HNO shows a great tendency to form NO at the flame zone and can be considered the primary source of NO production. From Fig. 12, all the kinetic models show high positive sensitivity for the reaction NH + OH ↔ HNO + H, and this reaction is considered the most influential reaction in producing HNO species, among other reactions. Also, the high positive sensitivity of the reaction H + O 2 ↔ O + OH leads to an increase in the possibility of the formation of HNO through the reactions NH 2 + O ↔ HNO + H and NH + OH ↔ HNO + H due to the abundance of O and OH radicals.
As can be seen from Fig. 13, Stagni and Bertolino's models indicate that the most prominent chemical reactions responsible for NO production from HNO are HNO + O 2 ↔ NO + HO 2 and HNO + H ↔ NO + H 2 . At the same time, the Bowen Mei mechanism illustrates that both reactions HNO + O 2 ↔ NO + HO 2 and HNO + OH ↔ NO + H 2 O are the most effective in producing NO from HNO. Most importantly, the chemical reaction NH + NO ↔ N 2 O + H is responsible for consuming the NO and transforming it to N 2 O by reacting with NH. All three mechanisms indicate that NH + NO ↔ N 2 O + H is the dominant reaction in consuming NO, among other reactions.   As shown from Fig. 14, the pathway diagrams predicted from all three kinetic models indicate that NH 2 , NH, and N radicals tend to react with NO to produce N 2 but at different concentrations. Also, the substantial contribution of both chemical reactions, NH 2 + NO ↔ NNH + OH and NH 2 + NO ↔ N 2 + H 2 O for converting NO to NNH and N 2 , respectively, can be noticed in all three kinetic mechanisms. Along with that, the Bowen Mei mechanism shows a considerable increase in the amount of NO reacting with NH 2 to produce NNH through the chemical reaction NH 2 + NO ↔ NNH + OH, and with NH 2 , NH and N by NH 2 + NO ↔ N 2 + H 2 O, NH + NO ↔ N 2 + OH and N + NO ↔ O + N 2 , compared to the other kinetic mechanisms considered here which demonstrate lower reacting concentrations of NO to produce NNH and N 2 . Figures 15,16 and 17 show the sensitivity analysis, the rate of production, and formation/destruction pathways for NO, respectively, using the three kinetic models when Reynolds number equals 40,000.

[NO] Sensitivity analysis when Re = 40,000
As can be seen from the pathway's diagram N, NH, NH 2 , and HNO are the species responsible for NO formation at the flame zone. As seen earlier, HNO has a significant effect on the formation of NO and can be considered the dominant source of NO production among other species. In addition, the effect of Reynolds number was considerably noticed as the concentration of NO from HNO increased noticeably compared to NO-pathways from Fig. 14 for Stagni and Bertolino. At the same time, the Bowen Mei mechanism demonstrates a different effect, as the NO concentration is reduced and shows the role of the N 2 H 2 in the formation of NO.   the reaction of HNO with O 2 and OH through the reactions HNO + O 2 ↔ NO + HO 2 , and HNO + OH ↔ NO + H 2 O, respectively, and can be considered the controlling reactions responsible for NO formation. In addition to that, the chemical reaction NH + NO ↔ N 2 O + H gives a negative rate of NO production for all three kinetic mechanisms.
As can be noticed from Fig. 17, the pathway's layout indicates the consumption of NO is dependent on the availability of NH 2 , NH, and N radicals to react with NO. All three mechanisms give similar trends but with different concentrations since the chemical reactions NH 2 + NO ↔ NNH + OH and NH 2 + NO ↔ N 2 + H 2 O are responsible for consuming NO to form NNH and N 2 . Further, the pathway's diagram clearly shows the effect of Reynolds number on NH 3 , NH 2 , and NH compared to the thermal power pathway's diagram (Fig. 14). It shows an increase in the reactive amount of NH 3 with OH to produce NH 2 , a similar effect that occurs with NH 2 and NH for all three models. The Bowen Mei mechanism clearly indicates the role of N 2 H 2 in the formation of NO compared with Stagni and Bertolino, as the reaction is not included in their predicted pathway diagram.

[N 2 O] Sensitivity analysis
One of the most important drawbacks of ammonia combustion could be N 2 O, especially under lean conditions. The gas has a much greater Global Warming Potential than CO 2 . Hence, it is important to determine its sensitivity analyses, rates of production of various species and to create a pathway's diagram to examine the role of N 2 O in the ammonia combustion process in two conditions previously discussed.  for all three kinetic models. In Fig. 19  denotes NH radical's role in producing HNO through the reaction NH + N 2 O ↔ N 2 + HNO along with other reactions mentioned in the other models for the consumption of the species. From an operational condition perspective, the pathways diagram for both cases shows that Reynolds number has an obvious effect in increasing the concentration of N 2 O compared to thermal power. The Reynolds number effect extends to the consumption side of N 2 O; all the models demonstrate an increase in the consumption rate of N 2 O, thus producing N 2 and N 2 H 2 . Even though there are differences in the final concentrations of N 2 O, all the kinetic mechanisms gave similar trends.

Conclusions
The present work has investigated the concentrations of NO and N 2 O at 85/15 (%vol) of NH 3 /H 2 blend using both experimental and numerical analyses. Experiments at various Reynolds numbers and thermal powers have been carried out at standard atmospheric conditions (1.1 bar and 288 K). The experimental results have been used to validate the numerical data and check the accuracy of kinetic models adopted in this study. Seven recently published kinetic models for ammonia combustion have been used and showed different predictions. The kinetic mechanisms of Stagni et al. and Bertolino et al. have given the best agreement in the prediction of NO and N 2 O over various thermal powers and Reynolds numbers. The main conclusions of this study can be summarized as follows.
• Measurements in terms of thermal power: The maximum concentration of NO is when the thermal intensity reaches 15 kW and then decreases for other cases, while the N 2 O mole fraction reaches a peak when the thermal power is equal to 20 kW, and its concentration decreases to record a minimum value at 15 kW. The impact of Reynolds number and thermal intensity conditions on the N 2 O and NO emissions level has been clearly shown. Therefore, the intensity of produced radicals such as NH*, NH 2 *, and NO* increased with varying Reynolds conditions from 20,000 to 40,000, whereas peaks of the radicals were noticed at 15 kW thermal power. Finally, a considerable amount of NO is consumed by reactions with NH to produce N 2 O through the chemical reaction NH+NO↔N 2 O+H, a behavior that intensifies when Re =40,000.