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 NH3/H2 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 . 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 N2O 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 N2O 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 N2O 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 NH2* 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 NH2* 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 NH3/H2 flames. In Fig. 7, the flame brush expands with increasing Re, giving an indication of an increase in the radical concentrations. The increase in NH2* radicals with increasing Re is due to OH* radical abundance and its role in the chemical reaction NH3 + OH ↔ H2O + NH2. OH* radicals are also responsible for NH* production through the reaction NH2 + OH ↔ NH + H2O and can be considered the largest source of NH production from NH2. Eventually, NH* radicals are consumed through the chemical reaction NH + NO ↔ N2O + H to form N2O by consuming NO. The considerable increase in N2O 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.
Effects of thermal power
Figures 8 and 9 illustrate the variation of NO and N2O 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 N2O. The figure illustrates a sharp decay in the concentration of N2O when the thermal power equals 15 kW. As power was increased to 20 kW, the N2O 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 N2O.
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 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, 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 NH2* radical concentrations, Fig. 11. The increased production of these radicals at 15 kW reflects into increased NO productions and decreased N2O 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 NH2 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 NH2 + NO ↔ NNH + OH and the chain-terminating reaction NH2 + NO ↔ H2O + N2 are known as the key chain reactions for NO consumptions .
As can be seen from the concentration profiles for both NO and N2O in the above figures, Han’s mechanism gave the highest over-prediction for NO and N2O. At the same time, Zhang’s mechanism predicts a low concentration of N2O. 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 N2O as well as the NOX 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 N2O 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 N2O, 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 + O2 ↔ O + OH leads to an increase in the possibility of the formation of HNO through the reactions NH2 + 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 + O2 ↔ NO + HO2 and HNO + H ↔ NO + H2. At the same time, the Bowen Mei mechanism illustrates that both reactions HNO + O2 ↔ NO + HO2 and HNO + OH ↔ NO + H2O are the most effective in producing NO from HNO. Most importantly, the chemical reaction NH + NO ↔ N2O + H is responsible for consuming the NO and transforming it to N2O by reacting with NH. All three mechanisms indicate that NH + NO ↔ N2O + 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 NH2, NH, and N radicals tend to react with NO to produce N2 but at different concentrations. Also, the substantial contribution of both chemical reactions, NH2 + NO ↔ NNH + OH and NH2 + NO ↔ N2 + H2O for converting NO to NNH and N2, 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 NH2 to produce NNH through the chemical reaction NH2 + NO ↔ NNH + OH, and with NH2, NH and N by NH2 + NO ↔ N2 + H2O, NH + NO ↔ N2 + OH and N + NO ↔ O + N2, compared to the other kinetic mechanisms considered here which demonstrate lower reacting concentrations of NO to produce NNH and N2.
[NO] Sensitivity analysis when Re = 40,000
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.
As can be seen from the pathway’s diagram N, NH, NH2, 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 N2H2 in the formation of NO.
Figure 15 illustrates NO sensitivity analysis at the flame zone when Reynolds number hit 40,000. The sensitivity level of the reaction H + O2 ↔ O + OH is high and gives a positive value to produce O and OH radicals. The abundance of O and OH radicals encourage the reactions NH + OH ↔ HNO + H and NH2 + O ↔ HNO + H in the formation of HNO which is the main source of NO formation as these reactions show high positive sensitivities of HNO formation.
Figure 16 shows NO production rates and illustrates that the dominant chemical reactions for the creation of NO are HNO + O2 ↔ NO + HO2 and N + O2 ↔ NO + O for both Stagni and Bertolino models. However, the Bowen Mei mechanism predicts that NO could be produced due to the reaction of HNO with O2 and OH through the reactions HNO + O2 ↔ NO + HO2, and HNO + OH ↔ NO + H2O, respectively, and can be considered the controlling reactions responsible for NO formation. In addition to that, the chemical reaction NH + NO ↔ N2O + 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 NH2, NH, and N radicals to react with NO. All three mechanisms give similar trends but with different concentrations since the chemical reactions NH2 + NO ↔ NNH + OH and NH2 + NO ↔ N2 + H2O are responsible for consuming NO to form NNH and N2. Further, the pathway’s diagram clearly shows the effect of Reynolds number on NH3, NH2, and NH compared to the thermal power pathway’s diagram (Fig. 14). It shows an increase in the reactive amount of NH3 with OH to produce NH2, a similar effect that occurs with NH2 and NH for all three models. The Bowen Mei mechanism clearly indicates the role of N2H2 in the formation of NO compared with Stagni and Bertolino, as the reaction is not included in their predicted pathway diagram.
[N2O] Sensitivity analysis
One of the most important drawbacks of ammonia combustion could be N2O, especially under lean conditions. The gas has a much greater Global Warming Potential than CO2. 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 N2O in the ammonia combustion process in two conditions previously discussed.
[N2O] Sensitivity analysis for 20 kW of thermal power
Figures 18 and 19 show the sensitivity analysis and rate of production of N2O using the three kinetic mechanisms when the thermal power is equal to 20 kW. It has been noticed that the chemical reaction NH + NO ↔ N2O + H has a considerable effect on the formation of N2O. Along with that, NH2 + OH ↔ NH + H2O and H + O2 ↔ O + OH also participate in the creation of N2O. All three mechanisms give nearly the same trend for N2O concentration. In addition, the Bowen Mei mechanism shows that the N2O formation rate is higher than that predicted by Stagni and Bertolino kinetic models. The chemical reaction N2O + H ↔ N2 + OH gives a pessimistic sensitivity prediction for N2O, which can be observed for all three kinetic models. In Fig. 19, the third body reaction N2O(+ M) ↔ N2 + O(+ M) showed a negative production rate for N2O in all three models.
The pathways diagram from Fig. 17 indicates that both NH and NO are the major sources for the formation of N2O. NO reacts with NH radicals through the chemical reaction NH + NO ↔ N2O + H to produce N2O. At the same time, Both Stagni and Bertolino’s chemical models predict the consumption path of N2O to N2 via the chemical reactions N2O + H ↔ N2 + OH and the decomposition path of N2O through the chemical reaction N2O + NH2 ↔ N2H2 + NO. In comparison, the Bowen Mei mechanism indicates that N2O + NH ↔ HNO + N2 is also responsible for consuming N2O to produce HNO, and this reaction does not appear in Stagni and Bertolino models. All three models indicate that the reaction N2O + H ↔ N2 + OH is substantial in consuming N2O to N2.
[N2O] Sensitivity analysis when Re = 40,000
Figures 20 and 21 refer to the sensitivity analysis and rate of production of N2O at operational conditions at Re = 40,000. Figure 20 shows that both reactions NH + NO ↔ N2O + H and NH2 + OH ↔ NH + H2O have a positive sensitivity for all three models. Along with that, the Bowen Mei model gives higher predictions to both previous chemical reactions compared with Stagni and Bertolino models. In addition, NH2 + NO ↔ NNH + OH and N2O + H ↔ N2 + OH give negative sensitivity of N2O for all three models, whilst the Bowen Mei mechanism also predicts a high negative sensitivity level of NH2 + NO ↔ NNH + OH in comparison to Stagni and Bertolino kinetic mechanisms.
Figure 21 indicates that NH + NO ↔ N2O + H is one of the dominant reactions for the formation of the pollutant. Further, the third body reaction N2O(+ M) ↔ N2 + O(+ M) and N2O + H ↔ N2 + OH present negative rates of production that lead to a decrease in the concentration of N2O. Also, all three mechanisms indicate that the chemical reaction N2O + H ↔ N2 + OH is mainly responsible for the consumption of N2O.
As can be seen from Fig. 17, Stagni and Bertolino’s models predict the formation of N2O due to the chemical reaction NH + NO ↔ N2O + H. Stagni and Bertolin models identify small amounts of N2O consumptions via the reaction with N2 and N2H2 with the radicals H and NO, respectively. However, the Bowen Mei mechanism denotes NH radical’s role in producing HNO through the reaction NH + N2O ↔ N2 + 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 N2O compared to thermal power. The Reynolds number effect extends to the consumption side of N2O; all the models demonstrate an increase in the consumption rate of N2O, thus producing N2 and N2H2. Even though there are differences in the final concentrations of N2O, all the kinetic mechanisms gave similar trends.