# Numerical Simulation of Ammonia Combustion at Different Hydrogen Peroxide Concentrations

Authors: Guan-Bang Chen; Fang-Hsien Wu
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Ammonia (NH3) is a promising carbon-free fuel. However, modifying its high ignition temperature and low flame speed remains challenging. In this study, NH3 combustion was numerically investigated using hydrogen peroxide (H2O2) of different concentrations as the oxidizer to overcome these limitations. Adiabatic, free-propagating laminar burning velocities for NH3/H2O2/H2O mixtures were examined using a detailed chemical model. The maximum flame temperature between equivalence ratios is not considerably different at high concentrations of H2O2 (>80%), because the heat release is dominated by H2O2 decomposition. The equivalence ratio does not significantly affect the flame speed when the H2O2 concentration is less than 50%. The impact of H2O2 on the burning velocity at low concentrations is mainly attributed to the chemical effect. The chemical effect gradually decreases and the thermal effect increases because of the high heat release at high H2O2 concentrations. Moreover, the different concentrations of H2O2 consequently divide the flame into two sections. The first section is dominated by H2O2 reactions and the second by NH3 reactions. However, when the H2O2 concentration is 90 or 100%, flames do not appear in both sections. Finally, a quasilinear relationship between the flame speed and (NH2+OH)max mole fraction is identified.


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## Body

## 1. Introduction

Many countries have committed to developing low-carbon/hydrogen-rich fuels as alternatives to fossil fuels. Ammonia (NH3) is a carbon-free fuel with a higher hydrogen density than that of hydrogen [1, 2]. The hydrogen content of NH3 is 17.7%, which is higher than those of CH3OH, C2H5OH, and gasoline. Liquid NH3 has a higher volumetric energy density than liquid H2. The primary products of NH3 combustion are nitrogen and steam; therefore, it is an environmentally friendly fuel. In addition, the liquefaction temperature of NH3 (293.8 K) is higher than that of H2 (20 K), and the storage pressure of H2 is approximately 87.5 times that of NH3 [3]. Thus, the cost of transporting liquid NH3 is lower than that of liquid H2 [4, 5]. However, to suitably use NH3 in combustion applications, several parameters such as a high ignition temperature, slow flame velocity, and narrow flammability limit must be controlled [6, 7].Regarding studies on NH3/air combustion, Han et al. [8] analyzed the temperature dependence of laminar burning velocity by evaluating the overall activation energy, temperature, species distribution, and reaction pathways of NH3/air flames. They observed a unique flame structure on the fuel-rich side of adiabatic NH3/air flames, similar to the “overrich” phenomenon in hydrocarbon flames. In several studies, flame stabilization during NH3 combustion has been considered and investigated [9, 10]. The flammability of NH3 combustion has been extended by cocombusting NH3 with other fuels (such as H2 [11, 12], H2/NO/N2O [13], and CH4 [14, 15]). Han et al. and Ichikawa et al. [16, 17] found that the flame velocity of NH3/H2/air initially increased as the ER was increased to 1.05 and subsequently decreased. Kumar and Meyer [3] studied the flame speeds for H2-NH3-air mixtures in laminar jet flames using experiments and numerical simulations. They found that OH was the primary radical resulting in the decomposition of NH3. Li et al. [18] numerically studied the effects of the equivalence and H2 addition ratios on the heat release characteristics of NH3/air flames and found that the reaction was dominated by the chemical effect at low H2 addition, whereas it was dominated by the transport effect at high H2 addition. Colson et al. [19] used a nonpremixed jet burner to determine the effect of adding NH3 on the flame stability of NH3/CH4/air diffusion flames. CH∗ chemiluminescence was used to identify the flame tip position, and the flame stabilization mechanisms were observed. Han et al. and Xiao et al. [16, 20] found that the flame speed of NH3/CH4/air flames increased almost linearly with the CH4 content and enhancement applied from fuel-lean to fuel-rich conditions owing to the reactivity and relatively high burning velocity of CH4.In addition to cofiring with other fuels, the combustion of NH3 can be enhanced under oxygen-enriched, oxy-fuel, or different pressure conditions [21, 22]. Li et al. [23] studied the effects of oxygen content on NH3 combustion and found that increasing the O2 content positively affected the laminar flame speed, adiabatic flame temperature, and the NO emissions. Mendiara and Glarborg [24] investigated NH3/CH4 combustion in a flow reactor considering oxy-fuel (at high CO2 concentrations) from fuel-rich to fuel-lean conditions. The results showed that a high CO2 content promoted NO formation, which was facilitated by the increased OH/H ratio under reducing conditions, whereas NO was inhibited under stoichiometric and lean conditions.The application of NH3 to gas turbines or spark-ignition (SI) engines has been studied. Xiao et al. [25] used OpenFOAM to model NH3/H2 combustion in a swirl burner in a generic gas turbine. A large eddy simulation can typically provide detailed information on the flow field, flame structure, and emissions for lean premixed conditions. Duynslaegher et al. [26] numerically simulated the characteristics of NH3/air combustion under SI engine operation conditions. At an ER of 1.12, the laminar burning velocity reached its maximum value. Both the equivalence and compression ratios considerably influence the laminar burning velocity and adiabatic flame temperature. Moreover, only the ER significantly affects the formation of NO. Valera-Medina et al. [10] studied the flame stability and exhaust emissions of NH3/CH4/air in a swirl burner for gas turbine power generation using experimental and numerical methods. High power conditions could improve the response of NH3 with low CH4 addition, and the flame could only be successfully characterized at 61% NH3/39% CH4. In addition, Kurata et al. [27] used a 50 kW class micro gas turbine system for the first time to successfully perform NH3/air combustion. Additionally, experimentally evaluating the NH3/CH4/air combustion revealed that NO emissions increased with increasing NH3 fuel ratios. In addition, the NH3 combustion intensity was improved after adding CH4, and flame stabilization was enhanced in the NH3 micro gas turbine system.As mentioned above, oxygen-enriched or oxy-fuel conditions benefit NH3 combustion. According to our previous studies, hydrogen peroxide (H2O2) can adequately improve NH3 combustion [28] and CH4 combustion [29, 30]. H2O2 is a strong oxidizer that exists in a liquid state at normal temperature and pressure. Therefore, it facilitates storage and transportation. H2O2 undergoes self-decomposition at an appropriate temperature, which is an exothermic reaction. The decomposition products are O2 and steam; therefore, H2O2 can be used as an auxiliary agent for low calorific fuels. Owing to its high exothermicity and decomposition temperature, high concentrations of H2O2 are typically utilized as propellants [31, 32]. The decomposition reaction of H2O2 is defined as follows:
(1)H2O2⟶H2O+0.5O2+ΔHTable1 lists the properties of different types of fuels [33, 34]. H2O2 has a relatively high boiling point (150°C), and the other fuels are gaseous at room temperature. The calorific value of NH3 is lower than those of H2 and CH4; the combustion speed of NH3 is also low, and its adiabatic flame temperature approaches the adiabatic decomposition temperature of gaseous H2O2. The autoignition temperature of NH3 is higher than those of H2 and CH4. Moreover, H2O2 homogeneously decomposes at 427°C under normal pressure [34].Table 1
Properties of different fuels [33, 34].
FuelNH3H2CH4H2O2Boiling temperature at 1 atm (°C)−33.4−253−161150Condensation pressure at 25°C (atm)9.9N/AN/AN/ALower heating value, LHV (MJ/kg)18.612050N/AFlammability limit (equivalence ratio)0.63–1.40.10–7.10.50–1.7N/AAdiabatic flame temperature (°C)1800211019501814aMaximum laminar burning velocity (m/s)0.072.910.373.91bMinimum autoignition temperature (°C)650520630427caAdiabatic decomposition temperature of 150°C H2O2 vapor. bPredicted burning velocity of 150°C H2O2 vapor in this study. cHomogeneous decomposition temperature at 1 atm.Regarding the study of H2O2 for combustion applications, Golovitchev et al. and Golovitchev and Chomiak [35, 36] found that the impact of H2O2 addition on the self-ignition of CH4 was more evident than that of H2 addition. Chen et al. and Ting and Reader [30, 37] evaluate the effect of H2O2 on CH4/air combustion by numerical simulation. The adiabatic flame temperature and flame speed were considerably improved. With reference to exhaust emissions, adding H2O2 to the diesel combustion could reduce the NO emission in the exhaust gas and converted it to NO2 [38]. Martinez et al. [39] found that adding a small amount of H2O2 to a pilot-scale natural gas combustion chamber could reduce unburned hydrocarbon, CO, and NOx emissions.The combustion characteristics of NH3 with other fuels and under oxygen-enriched conditions have been extensively investigated. However, although the addition of H2O2 to NH3/air combustion has been examined and proved to significantly enhance reactions [28], the combustion of NH3/H2O2/H2O with H2O2 at different concentrations as the oxidizer has not yet been analyzed. Compared to pure H2O2, H2O2 at different concentrations has considerable enhancements while reducing the cost and is easier for storage. Therefore, in this study, the effects of adding different concentrations of H2O2 on the combustion characteristics of NH3-premixed flames, including adiabatic flame temperature, flame velocity, flame structure, and species distribution, were numerically investigated. The flame temperature and flame speed of NH3 mixtures under oxygen-enriched conditions were also calculated to evaluate the thermal and chemical effects of different concentrations of H2O2 on the burning velocity. In addition, a linear relationship between the flame speed and species concentration was identified.


## 2. Numerical Method

Laminar burning velocity is generally the most important parameter for investigating and describing the premixed combustion process. It is typically used to characterize the combustion of various fuel-oxidizer combinations and to determine the flammability limit of the mixture. This is a single feature of a gas mixture with a fixed composition, initial temperature, and pressure. This parameter expresses the essential characteristics of the fuel/oxidizer mixtures in terms of thermal diffusivity, heat release, and reactivity.In this study, Chemkin-Pro 2020 R1 software was used to predict adiabatic, freely propagating laminar burning velocities for NH3/H2O2/H2O combustion. Nakamura and Shindo [40] reported that radiation heat loss affects simulated flame speeds at all equivalence ratios. The relative difference between adiabatic and radiative flame speeds is greater than 3% even near the stoichiometric condition. However, the heat release and radical enhancement of hydrogen oxide could significantly weaken the effect of radiation heat loss even at lower concentrations. Preliminary simulation results show that the relative difference in flame speeds is always smaller than 1% for all equivalence ratios (0.8~1.2). Therefore, radiation heat loss is not considered in this study.The computational domain for this study was 4.0 cm for accurately predicting the laminar burning rates. Otherwise, the resulting low mass flow would result in partial heat loss through the cold boundary. Therefore, the initial guess for the inlet mass flow rate was set to 0.09 g/cm2·s−1 for flame speed calculation, and the actual mass flow rate was determined as part of the flame speed solution. Additionally, the quality of the adaptive mesh was controlled by the mesh control values in accordance with the solution gradient and curvature.The Chemkin-Pro 2020 R1 software was also used to calculate the adiabatic flame temperature of NH3/H2O2/H2O. The initial species moles, temperature, and pressure of the reactant mixture were specified with the constraints of a constant pressure and enthalpy. For the chemical kinetics, the GRI-Mech 3.0 mechanism [41] was adopted, which is composed of 53 species and 325 elementary chemical reactions, and each reaction is presumed to proceed according to the modified Arrhenius equation. For NH3/H2O2/H2O combustion, the relevant species include major species such as NH3, H2O2, O2, H2O, and N2, free radicals such as OH, O, H, and HO2, and N-related intermediate species of NH2, NH, NNH, N, NO, NO2, and N2O. GRI-Mech 3.0 has been used in some studies to simulate H2O2 reactions [42], NH3/air combustion at various pressures [43], and NH3/O2 combustion [44]. It well predicted laminar flame velocities of NH3/air combustion. In the study of Okafor et al., GRI-Mech 3.0 predicted the burning velocity of methane-ammonia-air flames closely; however, it overpredicted NO emission for mixtures with higher ammonia concentrations [45]. Since this study mainly focuses on the burning velocity and flame pattern of NH3/H2O2/H2O mixture, the influence on NO emissions is not included in this study. It is adequate to predict the NH3/H2O2/H2O combustion characteristics by the numerical method and GRI-Mech 3.0 mechanism in the study. The initial temperature for the reactants was set at 150°C (the boiling temperature of H2O2), and atmospheric conditions were set as the initial pressure. All reactants were in the gaseous phase at this initial temperature.The simulation was performed using different concentrations of H2O2 as the oxidizer. The NH3/O2 ratio was fixed to sustain a constant ER, and O2 was obtained when H2O2 decomposed. Because the oxidizer consists entirely of H2O2, it does not have N2 in the composition. The chemical reaction of NH3/H2O2/H2O under stoichiometric conditions is described as follows:
(2)2NH3+3H2O2+αH2O⟶6+3αH2O+N2,where α is related to the concentration of H2O2. The concentration of H2O2 is defined as
(3)%H2O2=xH2O2xH2O2+xH2O×100%,where x denotes the number of moles used. Under oxygen-enriched conditions, the O2 concentration in the oxidizer is between 21 and 40%. The overall reaction is expressed as follows, where β is the O2 concentration in the oxidizer.
(4)4NH3+3O2+1−ββN2⟶6H2O+1+ββN2.In addition, for both conditions, the equivalence ratio (ER) is defined as(5)ER=NH3/H2O2NH3/H2O2st=NH3/O2NH3/O2st,where NH3/H2O2 denotes the actual volume ratio of ammonia and hydrogen peroxide and NH3/O2 denotes the actual volume ratio of ammonia and oxygen. NH3/H2O2st and NH3/O2st denote the complete combustion by the overall reactions (2) and (4), respectively.The simulation was initiated with a coarse mesh (20 grids). After obtaining an initial solution, additional grids were added to the regions with rapid solution changes or gradients. The initial guess for finer grids was obtained from the interpolation of the solution for the coarse grids. This process was continued until a denser grid of points was not required to achieve the specified degree of convergence.


## 3. Results and Discussion

First, the predicted laminar burning velocities for adiabatic, free-propagating premixed NH3/air combustion are compared with the experimental results of Pfahl et al. [46] and Takizawa et al. [47], as shown in Figure 1. Pfahl et al. performed experiments in a constant-volume vessel, and they measured the horizontal growth of the flame front to minimize the effect of buoyancy. Takizawa et al. measured the burning velocity of an NH3/air mixture using the spherical vessel method. They also observed flame propagation using the schlieren photography method.Figure 1
Comparison of NH3/air laminar burning velocities.From the experimental results of Pfahl et al. and Takizawa et al., the ER was considered between 0.85 and 1.25, and the flame speed was between 4 and 7.2 cm/s. The predicted speeds were in the range of 3–6.13 cm/s, and the ER was in the range of 0.8–1.2. The predicted results were consistent with the experimental values, particularly in the fuel-lean to stoichiometric conditions, and the maximum velocity value was obtained atER=1.1. Moreover, a maximum burning velocity of 7.2 cm was obtained at an ER of 1.13 by Takizawa et al. The simulation results successfully predicted the variation of flame speed and confirmed the appropriate settings for the model in this study.


### 3.1. Adiabatic Temperature and Laminar Burning Velocity

Owing to its high heat release during decomposition, H2O2 can promote and accelerate the combustion reaction, especially for fuels such as NH3 with a low calorific value and flame speed. In this section, the flame temperature and burning velocity of NH3 are initially compared under oxygen-enriched conditions or using H2O2 at various concentrations as an oxidizer. Subsequently, the thermal and chemical effects of various concentrations of H2O2 on the burning velocity are discussed.Figure2 shows the adiabatic flame temperatures of NH3/O2/N2 for different oxygen-enriched conditions and NH3/H2O2/H2O for different H2O2 concentrations. The oxygen-enriched conditions are considered up to 40% O2, and the H2O2 concentration was from 30% to 100%. The results showed that increasing the concentration of O2 or H2O2 increases the adiabatic flame temperature, and relatively high values are obtained using H2O2. Under oxygen-enriched conditions, the highest flame temperatures are obtained at an ER of 1.0, followed by ER=1.2 and 0.8. A similar trend is observed for different H2O2 concentrations; however, the difference between different ERs is small, especially at high concentrations (>80%). The maximum temperature difference is less than 30 K. This is because the heat is mainly released when H2O2 decomposes at high concentrations. In addition, when ER=0.8, the adiabatic flame temperature at a 40% H2O2 concentration approaches that at 25% O2. The adiabatic flame temperature at 65% H2O2 is similar to that at 40% O2. When the concentration of H2O2 is 70%, the adiabatic flame temperature exceeds that at 40% O2 for an ER of 0.8 and is approximately equal to that at 40% O2 for an ER of 1.2. When the H2O2 concentration is 80%, the adiabatic flame temperature approaches that at 40% O2 under stoichiometric conditions.Figure 2
Adiabatic flame temperatures and laminar burning velocities for (a) NH3/air with various O2 content in air and (b) NH3/H2O2 at various H2O2 concentrations.
(a)(b)Figure2 also shows the laminar burning velocities of NH3/O2/N2 for different oxygen-enriched conditions and NH3/H2O2/H2O for different H2O2 concentrations. For oxygen-enriched conditions (Figure 2(a)), the burning velocity is slightly different between the fuel-rich (ER=1.2) and stoichiometric (ER=1.0) conditions when the O2 content is less than 27%. However, the fuel-lean condition (ER=0.8) exhibits a significantly low burning velocity. When the O2 concentration exceeds 27%, the stoichiometric condition exhibits the highest burning velocity among the ERs. For different H2O2 concentrations (Figure 2(b)), when the H2O2 concentration is below 50%, the burning velocity is similar for ERs of 0.8, 1.0, or 1.2. When the H2O2 concentration is above 50%, the burning velocities begin to differ. Burning velocities in the order of high to low are observed at ERs of 0.8, 1.0, and 1.2.The maximum burning velocity occurs atER=0.8 and increases from 6.8 cm/s for air to 106.6 cm/s for 70% H2O2 and 146 cm/s for 90% H2O2, which indicates an increase of 15.7 and 21.5 times, respectively. This increasing trend is primarily observed because OH is considerably formed when H2O2 is decomposed; OH is the dominant species for laminar burning velocity. In Figure 2, under similar adiabatic flame temperatures, when ER=0.8, the burning velocity at 40% H2O2 concentration is 2.95 times that at 25% O2 concentration. Under stoichiometric conditions, the burning velocity at 45% H2O2 concentration is 2.75 times that at 25% O2, and the burning velocity at 55% H2O2 concentration is 2.55 times that at 30% O2. In contrast to the oxygen-enriched condition, different H2O2 concentrations demonstrate distinct chemical effects on the flame burning velocity.To determine the effects of O2 enrichment and H2O2 at different concentrations as oxidizers on the NH3 combustion reactions, a sensitivity analysis of the laminar flame speed was presented in Figure 3. The six main reactions that dominate the flame speed are (6), (7), (8), (9), (10), and (22), which are numbered R38, R212, R214, R183, R89, and R203 in the GRI-Mech 3.0 mechanism, for the NH3/air condition.
(6)O2+H↔O+OH(7)H+NO+M↔HNO+M(8)HNO+H↔H2+NO(9)N2O+H↔N2+OH(10)OH+H2O2↔HO2+H2O(11)NH2+OH↔NH+H2OFigure 3
Sensitivity analysis of laminar flame speed for NH3/air combustion and NH3/H2O2 combustion at various H2O2 concentrations (ER=1.0).Under oxygen-enriched conditions (30% O2+70% N2), the dominant reactions are in the sequence: (6), (8), (7), (9), (11), and (10), which are numbered R38, R214, R212, R183, R203, and R89 in the GRI-Mech 3.0 mechanism. Reactions (8) and (22), numbered R214, and R203 in the GRI-Mech 3.0 mechanism, are enhanced to take precedence over (7) and (10), numbered R212, and R89 in the GRI-Mech 3.0 mechanism, respectively. When the oxidizer is H2O2 with different concentrations, the dominant reactions are (32), (33), (6), (11), (7), and (8), which are numbered R85, R89, R38, R203, R212, and R214 in the GRI-Mech 3.0 mechanism. The primary reaction that positively affects the flame speed is (32), numbered R85 in the GRI-Mech 3.0 mechanism, whereas (33), numbered R89 in the GRI-Mech 3.0 mechanism, is mainly responsible for the negative effect.
(12)2OH+M↔H2O2+MH2O2 promotes the generation of OH and HO2 radicals and the reaction of NH2 and OH (11), numbered R203 in the GRI-Mech 3.0 mechanism, thereby enhancing its positive effect on the flame speed.The influence of H2O2 on the burning velocity mainly includes thermal and chemical effects. The thermal effect is mainly from the heat released by the decomposition of H2O2, while the chemical effect is from the free radicals generated by the H2O2 decomposition reactions. Figure 4 shows the thermal and chemical effects of H2O2 on the laminar burning velocity of NH3/H2O2 combustion at various H2O2 concentrations. These two effects can be defined as follows [48]:
(13)ΔVtherm=VTn−Vn=0Vn−Vn=0,(14)ΔVchem=1−ΔVtherm,where ΔVtherm and ΔVchem represent the thermal and chemical effects of H2O2 on NH3 laminar burning velocity, respectively. Vn=0 is the laminar burning velocity of NH3/air flames. Vn is the burning velocity of NH3/H2O2/H2O flames under condition n.Figure 4
Thermal and chemical effects of H2O2 on laminar burning velocity for NH3/H2O2 combustion at various H2O2 concentrations.The difference betweenVn and Vn=0 is derived from the thermal and chemical effects of using H2O2 as the oxidizer. To determine the thermal effect of H2O2, pure NH3/air flames are adjusted by replacing a fraction of N2 with O2 to equate the adiabatic flame temperature with that of the NH3/H2O2/H2O flames; the corresponding flame is denoted by VTn. The difference between VTn and Vn=0 is considered a consequence of the thermal effect, and the chemical effect is obtained from Equation (14).In addition, as shown in Figure2, the adiabatic flame temperature at 30% H2O2 is 174 K lower than that in air at an ER of 1.0; however, the laminar flame speed is higher than that in air. This indicates that the increase in the flame speed at 30% H2O2 is mainly attributed to chemical effects. Because the net exothermic rate for the decomposition of H2O2 is considerably higher than that for NH3/air combustion, it is significantly dominated by the high-concentration H2O2 decomposition. Therefore, as shown in Figure 4, irrespective of the ER, the thermal effect is gradually increased, and the chemical effect is relatively decreased with gradually increasing H2O2 concentrations. When the H2O2 concentration is 100%, the thermal effect is similar to the chemical effect, both of which are approximately 50%.


### 3.2. NH3/H2O2/H2O Combustion

The combustion characteristics of premixed NH3/air and NH3/40% H2O2/60% H2O at ER=1.0 are demonstrated for comparison. Figure 5 shows the temperature and concentration distribution of major species, which include NH3, O2, H2O, and H2O2 at 40% H2O2/60% H2O. The temperature of NH3 burning with air increases to the flame temperature in a single stage. The reactants and products exhibit decreasing trends during the combustion process. However, when NH3 is burned using 40% H2O2 as the oxidizer, its combustion process is divided into two stages. Additionally, NH3 and H2O exhibit a two-stage change as the temperature increases. O2 tends to be initially produced in the first stage and then consumed in the second stage. These two-stage reactions mainly occur because of the decomposition of H2O2, which releases heat to increase temperature and generate O2 and steam. At this stage, NH3 is partially consumed owing to the high temperature, whereas H2O2 is completely consumed.Figure 5
Temperature and major species concentration profiles for NH3 combustion at ER=1.0 (real line: 100%air; dash line: 40%H2O2/60%H2O).The second stage mainly comprises the combustion of NH3, and the exothermic combustion further increases the temperature. The concentration of steam (the major product) is further increased. NH3 and O2 are completely consumed during this stage. This two-stage phenomenon is also apparent in the heat release rate (not shown here); the maximum exothermic rate of the first stage at 40% H2O2 concentration is 2.72×1011J/m3·s. For the second stage, the maximum exothermic rate is 3.79×1010J/m3·s, both of which are higher than the maximum exothermic rate of pure NH3/air combustion (1.88×108J/m3·s).Figure6 shows the minor species concentration profiles of NH3 in air or at a concentration of 40% H2O2 under stoichiometric conditions. At 40% H2O2, except for HO2 and NO, all other species display two-stage generation. The first-stage generation results from the decomposition of H2O2 and NH3 and occurs further upstream compared to NH3/air combustion. HO2 is mainly derived from the reactions of H2O2 ((33), R116, (36), (35), R186, and R287, which are numbered R89, R116, R88, R87, R186, and R287 in the GRI-Mech 3.0 mechanism.). The yields of HO2 and OH are considerably enhanced compared with those under the NH3/air combustion conditions. The second-stage generation originates from the combustion of NH3. The species of NH2, OH, NH, HNO, NO, NO2, N, and O are produced further upstream.Figure 6
Minor species concentration profiles for NH3 combustion at ER=1.0.The temperature distribution can be used to estimate flame thickness (Figure7). The flame thickness is determined from the interval betweente the cold boundary temperatureTu and equilibrium temperature Tf. In this region, the unburned gas is preheated by heat transfer from the reaction zone. The unburned gas expands and accelerates because of the increasing temperature, and the temperature curve is concave upward. On reaching the ignition temperature Ti, which is the inflection point of the temperature profile, the resultant reactions release heat, modifying the temperature curve to concave downward. The temperature continues to increase until the equilibrium value Tf is attained. The tangent lines formed by Tu, Ti, Tf, and the temperature curve are then used to determine the flame thickness [49, 50]. When the increase in the temperature is divided into two sections, the flame is dominated by different mechanisms. The flame thickness τ consists of two parts, τ1 and τ2, which are obtained from Tu, Ti1, and Tr and Tf, Ti2, and Tr, respectively. Tr is the intersection of the tangents of Ti1 and Ti2.Figure 7
Computed flame thickness for stoichiometric NH3/air combustion and NH3/H2O2 combustion at various H2O2 concentrations.Figure7 also shows the flame thickness under different conditions. In the case of air combustion, the flame is created in only one section, and when the oxidizer is changed from air to H2O2 at different concentrations, the flame is divided into two sections. The first section is dominated by the reactions of H2O2, which is the source of O2. The second section is dominated by the NH3 reactions. The total flame thickness is the sum of the two sections. For the two-section flames, the flame thickness of the first section is thinner than that of the second section because the heat release rate of H2O2 is greater than that of NH3 combustion. The total flame thickness at 30% H2O2 concentration is greater than that in air. The high water content at 30% H2O2 partially absorbs heat and reduces the reaction, thus increasing the flame thickness. A lower concentration of H2O2 will reduce the exothermic rate. For instance, the average heat release rate for the case of 40%H2O2 is 6.186×108J/m3·s while it is obviously reduced to 7.988×107J/m3·s for 30%H2O2. However, the flame in the first section (0.095 cm) is considerably thinner than that in air (0.21 cm). When the concentration of H2O2 is increased, the decomposition reactions are enhanced. This subsequently increases the reactions of NH3, and the flame thickness in the second section is gradually decreased. When the H2O2 concentration is increased to 90 or 100%, the flame is no longer divided into two sections. The decomposition of H2O2 and combustion of NH3 occur almost simultaneously, and the flame thickness is considerably small (5–6 mm).


### 3.3. Species Analysis of NH3/H2O2/H2O Combustion and their Relation to the Burning Velocity

Because using H2O2 at various concentrations as the oxidizer changes the initial NH3 concentration, the concentrations of N-related species in the reaction zones are affected and it will obscure the actual variation resulting from H2O2-enhanced reactions. Therefore, the mass fraction or production rate of the N-related species should be normalized [28].Figure8 shows the normalized NH3 mass fractions under different combustion conditions. As the H2O2 concentration increases, the reaction rate increases, and NH3 is consumed further upstream. The consumption of NH3 was divided into two stages at 40 and 60% H2O2 concentrations. The first stage represents the decomposition of H2O2; thus, NH3 is partially consumed. The second stage involves the combustion of the remaining NH3. For 80% H2O2 concentration, in contrast to the temperature distribution, the fractional consumption of NH3 is not apparent. Figure 9 shows the profiles of normalized NH3 reaction rates under different combustion conditions. In air, the dominant reactions related to the production and consumption of NH3, which are numbered R277, R278, R279, and R277 in the GRI-Mech 3.0 mechanism, are as follows:
(15)Production:NH2+H2↔NH3+H(16)Consumption:NH3+OH↔NH2+H2O(17)NH3+O↔NH2+OH(18)NH3+H↔NH2+H2Figure 8
Normalized NH3 mass fraction profiles for the combustion of NH3/air and NH3/H2O2 at various H2O2 concentrations.Figure 9
Normalized NH3 consumption rate profiles for the combustion of NH3/air and NH3/H2O2 at various H2O2 concentrations.When the oxidizer is changed to H2O2, the yields of OH, O, and H radicals increase. This considerably enhances the reaction of NH3 and moves it upstream. (15), numbered R277 in the GRI-Mech 3.0 mechanism, initially produced and then consumed NH3. These reaction zones are distinctly narrowed with increasing H2O2 concentration. Additionally, the consumption reactions are divided into two stages.The normalized mass fractions of NH2 for different combustion conditions are shown in Figure 10. In air, NH2 is produced further downstream with a low concentration. With increasing H2O2 concentration, the NH2 is produced further upstream, and its concentration is considerably increased. As shown in Figure 6, the NH2 concentration exhibits two peaks; the first peak is higher than the second peak. The H2O2 decomposition upstream promotes the reaction of NH3 to yield NH2. The combustion of the remaining NH3 downstream yields the second NH2 peak. For 80% H2O2 concentration, the two-peak phenomenon is less evident.Figure 10
Normalized NH2 mass fraction profiles for NH3/H2O2 combustion at various H2O2 concentrations.Figure11 shows the normalized NH2 production rates under different combustion conditions. In air, the production of NH2 is dominated by reactions (16), (17), and (15), which are numbered R278, R279, and R277 in the GRI-Mech 3.0 mechanism, whereas its consumption is dominated by reactions (11), (23), (24), and (15), which are numbered R203, R202, R201,and R277 in the GRI-Mech 3.0 mechanism.
(19)Production:NH3+OH↔NH2+H2O(20)NH3+O↔NH2+OH(21)NH3+H↔NH2+H2(22)Consumption:NH2+OH↔NH+H2O(23)NH2+H↔NH+H2(24)NH2+O↔H+HNO(25)NH2+H2↔NH3+H(25), numbered R277 in the GRI-Mech 3.0 mechanism, is divided into two stages, namely, the consumption of NH2 and the generation of NH2. Moreover, the overall production rate of NH2 shows that it is initially produced and then consumed. When the oxidizer is changed to different concentrations of H2O2, the dominant reactions related to NH2 production and consumption are not significantly different; however, their intensity increases with increasing H2O2 concentration. The reaction zones become narrower with increasing H2O2 concentration. In addition, most of the reactions display two peaks; the first peak is attributed to the large amounts of OH, O, and H produced by the H2O2 reactions, which enhances the relevant reactions (such as (16), (15), and (11)), numbered R278, R277, and R203 in the GRI-Mech 3.0 mechanism).Figure 11
Normalized NH2 consumption rate profiles for the combustion of NH3/air and NH3/H2O2 at various H2O2 concentrations.Figure12 depicts the OH concentration profiles under different combustion conditions. Compared with air, increasing concentrations of H2O2 considerably increase the OH concentration. The OH is also gradually yielded upstream owing to the addition of H2O2. The OH concentration exhibits two peaks under the conditions of H2O2. As shown in Figure 3, the burning rate increases with increasing H2O2 concentration. As shown in Figure 4, (32) and (33), numbered R85 and R89 in the GRI-Mech 3.0 mechanism, are the most critical reactions that dominate the laminar burning velocity of the NH3/H2O2/H2O flame. Both reactions are related to OH. The order of burning velocity follows: ER=0.8>1.0>1.2; the OH concentration for the H2O2 condition presents a similar order, wherein the OH mass fractions are 0.0417, 0.0369, and 0.0271 at ERs of 0.8, 1.0, and 1.2, respectively. When ER=1.0, the OH concentration at 100% H2O2 is approximately 31 times that in air.Figure 12
OH mass fraction profiles for the combustion of NH3/air and NH3/H2O2 at various H2O2 concentrations.Figure13 illustrates the profiles of OH production rates under different combustion conditions. In air, the main reactions related to OH, which are numbered R86, R38, R279, R278, R84, and R203 in the GRI-Mech 3.0 mechanism, are as follows:
(26)Production:O+H2O↔2OH(27)H+O2↔O+OH(28)NH3+O↔NH2+OH(29)Consumption:NH3+OH↔NH2+H2O(30)OH+H2↔H+H2O(31)NH2+OH↔NH+H2OFigure 13
OH consumption rate profiles for the combustion of NH3/air and NH3/H2O2 at various H2O2 concentrations.When the oxidizer is changed to 40% H2O2, (32), numbered R85 in the GRI-Mech 3.0 mechanism, becomes the main reaction for OH formation, and the main consumption reactions are (33), (34), (35), (36), and (37), numbered R89, R278, R87, R88, and R203 in the GRI-Mech 3.0 mechanism.
(32)Production:H2O2+M↔2OH+M(33)Consumption:OH+H2O2↔HO2+H2O(34)NH3+OH↔NH2+H2O(35)OH+HO2↔O2+H2O(36)OH+H2O2↔HO2+H2O(37)NH2+OH↔NH+H2OThe most critical reactions are related to H2O2. When the concentration of H2O2 is increased to 60 or 80%, the dominant reaction for producing OH is still (32), numbered R85 in the GRI-Mech 3.0 mechanism. However, R46 (H+HO2↔2OH) and (26) (O+H2O↔2OH), numbered R86 in the GRI-Mech 3.0 mechanism, also significantly contribute to its production. The most important reaction for consuming OH is (33), numbered R89 in the GRI-Mech 3.0 mechanism. In addition, the OH reaction zone under the condition of H2O2 is divided into two stages; the H2O2 reactions that dominate the first stage are considerably stronger than those in the second stage. As the concentration of H2O2 is increased, the OH reactions are enhanced, and the reaction zone is narrowed.Finally, a quasilinear relationship between burning velocity and species concentration was established in this study. The laminar flame speeds of hydrocarbon fuels have quasilinear relationships with (OH+H)max in the reaction zone [51]. Li et al. [23] investigated the NH3 combustion under oxygen-enriched conditions and found a quasilinear relationship between the laminar burning velocity (cm/s) and (NH2)max at various O2 concentrations in air. However, in this study, when H2O2 was used as the oxidizer, the reaction was enhanced, and the laminar flame speed was not dominated by NH2. When comparing the burning velocity with the concentrations of various species, such as NH2, OH, O, and H, the burning velocity reveals a quasilinear relationship with the maximum value of the (NH2+OH) mole fraction in the flame. Figure 14 shows the laminar burning velocity of NH3 as a function of the (NH2+OH)max mole fraction when replacing a fraction of the air by H2O2 (10–100%) or using different concentrations of H2O2 (30–100%) as the oxidizer. These two conditions are expressed as follows:Figure 14
Laminar burning velocity of NH3 as a function of the maximum (NH2+OH) radical in the reaction zone for (a) different percentages of air replaced with H2O2 and (b) various H2O2 concentrations.
(a)(b)For air partially replaced by H2O2,
(38)Vbcms=4094.8×NH2+OHmax+1.8.For different H2O2 concentrations and (NH2+OH)max mole fraction≤0.025,
(39)Vbcms=5677.6×NH2+OHmax−8.5.For different H2O2 concentrations and (NH2+OH)max mole fraction>0.025,
(40)Vbcms=2461×NH2+OHmax+52.5.However, when the replacement percentage of H2O2 or the concentration of H2O2 is sufficiently high (>60% for ER=0.8 and 1.0, and > 70% for ER=1.2), the maximum concentration of OH is markedly higher than that of NH2. Thus, (NH2+OH)max is dominated by the OH concentration, and (OH)max is approximately equal to (NH2+OH)max. Therefore, under this condition, the burning velocity is considered to have a quasilinear relationship with (OH)max. Overall, when H2O2 is used to partially replace air or when different concentrations of H2O2 are used as the oxidizer, with an increase in the replacement percentage or the concentration of H2O2, the linear relationship of burning velocity with (NH2)max is changed to (NH2+OH)max and finally to (OH)max.


## 3.1. Adiabatic Temperature and Laminar Burning Velocity

Owing to its high heat release during decomposition, H2O2 can promote and accelerate the combustion reaction, especially for fuels such as NH3 with a low calorific value and flame speed. In this section, the flame temperature and burning velocity of NH3 are initially compared under oxygen-enriched conditions or using H2O2 at various concentrations as an oxidizer. Subsequently, the thermal and chemical effects of various concentrations of H2O2 on the burning velocity are discussed.Figure2 shows the adiabatic flame temperatures of NH3/O2/N2 for different oxygen-enriched conditions and NH3/H2O2/H2O for different H2O2 concentrations. The oxygen-enriched conditions are considered up to 40% O2, and the H2O2 concentration was from 30% to 100%. The results showed that increasing the concentration of O2 or H2O2 increases the adiabatic flame temperature, and relatively high values are obtained using H2O2. Under oxygen-enriched conditions, the highest flame temperatures are obtained at an ER of 1.0, followed by ER=1.2 and 0.8. A similar trend is observed for different H2O2 concentrations; however, the difference between different ERs is small, especially at high concentrations (>80%). The maximum temperature difference is less than 30 K. This is because the heat is mainly released when H2O2 decomposes at high concentrations. In addition, when ER=0.8, the adiabatic flame temperature at a 40% H2O2 concentration approaches that at 25% O2. The adiabatic flame temperature at 65% H2O2 is similar to that at 40% O2. When the concentration of H2O2 is 70%, the adiabatic flame temperature exceeds that at 40% O2 for an ER of 0.8 and is approximately equal to that at 40% O2 for an ER of 1.2. When the H2O2 concentration is 80%, the adiabatic flame temperature approaches that at 40% O2 under stoichiometric conditions.Figure 2
Adiabatic flame temperatures and laminar burning velocities for (a) NH3/air with various O2 content in air and (b) NH3/H2O2 at various H2O2 concentrations.
(a)(b)Figure2 also shows the laminar burning velocities of NH3/O2/N2 for different oxygen-enriched conditions and NH3/H2O2/H2O for different H2O2 concentrations. For oxygen-enriched conditions (Figure 2(a)), the burning velocity is slightly different between the fuel-rich (ER=1.2) and stoichiometric (ER=1.0) conditions when the O2 content is less than 27%. However, the fuel-lean condition (ER=0.8) exhibits a significantly low burning velocity. When the O2 concentration exceeds 27%, the stoichiometric condition exhibits the highest burning velocity among the ERs. For different H2O2 concentrations (Figure 2(b)), when the H2O2 concentration is below 50%, the burning velocity is similar for ERs of 0.8, 1.0, or 1.2. When the H2O2 concentration is above 50%, the burning velocities begin to differ. Burning velocities in the order of high to low are observed at ERs of 0.8, 1.0, and 1.2.The maximum burning velocity occurs atER=0.8 and increases from 6.8 cm/s for air to 106.6 cm/s for 70% H2O2 and 146 cm/s for 90% H2O2, which indicates an increase of 15.7 and 21.5 times, respectively. This increasing trend is primarily observed because OH is considerably formed when H2O2 is decomposed; OH is the dominant species for laminar burning velocity. In Figure 2, under similar adiabatic flame temperatures, when ER=0.8, the burning velocity at 40% H2O2 concentration is 2.95 times that at 25% O2 concentration. Under stoichiometric conditions, the burning velocity at 45% H2O2 concentration is 2.75 times that at 25% O2, and the burning velocity at 55% H2O2 concentration is 2.55 times that at 30% O2. In contrast to the oxygen-enriched condition, different H2O2 concentrations demonstrate distinct chemical effects on the flame burning velocity.To determine the effects of O2 enrichment and H2O2 at different concentrations as oxidizers on the NH3 combustion reactions, a sensitivity analysis of the laminar flame speed was presented in Figure 3. The six main reactions that dominate the flame speed are (6), (7), (8), (9), (10), and (22), which are numbered R38, R212, R214, R183, R89, and R203 in the GRI-Mech 3.0 mechanism, for the NH3/air condition.
(6)O2+H↔O+OH(7)H+NO+M↔HNO+M(8)HNO+H↔H2+NO(9)N2O+H↔N2+OH(10)OH+H2O2↔HO2+H2O(11)NH2+OH↔NH+H2OFigure 3
Sensitivity analysis of laminar flame speed for NH3/air combustion and NH3/H2O2 combustion at various H2O2 concentrations (ER=1.0).Under oxygen-enriched conditions (30% O2+70% N2), the dominant reactions are in the sequence: (6), (8), (7), (9), (11), and (10), which are numbered R38, R214, R212, R183, R203, and R89 in the GRI-Mech 3.0 mechanism. Reactions (8) and (22), numbered R214, and R203 in the GRI-Mech 3.0 mechanism, are enhanced to take precedence over (7) and (10), numbered R212, and R89 in the GRI-Mech 3.0 mechanism, respectively. When the oxidizer is H2O2 with different concentrations, the dominant reactions are (32), (33), (6), (11), (7), and (8), which are numbered R85, R89, R38, R203, R212, and R214 in the GRI-Mech 3.0 mechanism. The primary reaction that positively affects the flame speed is (32), numbered R85 in the GRI-Mech 3.0 mechanism, whereas (33), numbered R89 in the GRI-Mech 3.0 mechanism, is mainly responsible for the negative effect.
(12)2OH+M↔H2O2+MH2O2 promotes the generation of OH and HO2 radicals and the reaction of NH2 and OH (11), numbered R203 in the GRI-Mech 3.0 mechanism, thereby enhancing its positive effect on the flame speed.The influence of H2O2 on the burning velocity mainly includes thermal and chemical effects. The thermal effect is mainly from the heat released by the decomposition of H2O2, while the chemical effect is from the free radicals generated by the H2O2 decomposition reactions. Figure 4 shows the thermal and chemical effects of H2O2 on the laminar burning velocity of NH3/H2O2 combustion at various H2O2 concentrations. These two effects can be defined as follows [48]:
(13)ΔVtherm=VTn−Vn=0Vn−Vn=0,(14)ΔVchem=1−ΔVtherm,where ΔVtherm and ΔVchem represent the thermal and chemical effects of H2O2 on NH3 laminar burning velocity, respectively. Vn=0 is the laminar burning velocity of NH3/air flames. Vn is the burning velocity of NH3/H2O2/H2O flames under condition n.Figure 4
Thermal and chemical effects of H2O2 on laminar burning velocity for NH3/H2O2 combustion at various H2O2 concentrations.The difference betweenVn and Vn=0 is derived from the thermal and chemical effects of using H2O2 as the oxidizer. To determine the thermal effect of H2O2, pure NH3/air flames are adjusted by replacing a fraction of N2 with O2 to equate the adiabatic flame temperature with that of the NH3/H2O2/H2O flames; the corresponding flame is denoted by VTn. The difference between VTn and Vn=0 is considered a consequence of the thermal effect, and the chemical effect is obtained from Equation (14).In addition, as shown in Figure2, the adiabatic flame temperature at 30% H2O2 is 174 K lower than that in air at an ER of 1.0; however, the laminar flame speed is higher than that in air. This indicates that the increase in the flame speed at 30% H2O2 is mainly attributed to chemical effects. Because the net exothermic rate for the decomposition of H2O2 is considerably higher than that for NH3/air combustion, it is significantly dominated by the high-concentration H2O2 decomposition. Therefore, as shown in Figure 4, irrespective of the ER, the thermal effect is gradually increased, and the chemical effect is relatively decreased with gradually increasing H2O2 concentrations. When the H2O2 concentration is 100%, the thermal effect is similar to the chemical effect, both of which are approximately 50%.


## 3.2. NH3/H2O2/H2O Combustion

The combustion characteristics of premixed NH3/air and NH3/40% H2O2/60% H2O at ER=1.0 are demonstrated for comparison. Figure 5 shows the temperature and concentration distribution of major species, which include NH3, O2, H2O, and H2O2 at 40% H2O2/60% H2O. The temperature of NH3 burning with air increases to the flame temperature in a single stage. The reactants and products exhibit decreasing trends during the combustion process. However, when NH3 is burned using 40% H2O2 as the oxidizer, its combustion process is divided into two stages. Additionally, NH3 and H2O exhibit a two-stage change as the temperature increases. O2 tends to be initially produced in the first stage and then consumed in the second stage. These two-stage reactions mainly occur because of the decomposition of H2O2, which releases heat to increase temperature and generate O2 and steam. At this stage, NH3 is partially consumed owing to the high temperature, whereas H2O2 is completely consumed.Figure 5
Temperature and major species concentration profiles for NH3 combustion at ER=1.0 (real line: 100%air; dash line: 40%H2O2/60%H2O).The second stage mainly comprises the combustion of NH3, and the exothermic combustion further increases the temperature. The concentration of steam (the major product) is further increased. NH3 and O2 are completely consumed during this stage. This two-stage phenomenon is also apparent in the heat release rate (not shown here); the maximum exothermic rate of the first stage at 40% H2O2 concentration is 2.72×1011J/m3·s. For the second stage, the maximum exothermic rate is 3.79×1010J/m3·s, both of which are higher than the maximum exothermic rate of pure NH3/air combustion (1.88×108J/m3·s).Figure6 shows the minor species concentration profiles of NH3 in air or at a concentration of 40% H2O2 under stoichiometric conditions. At 40% H2O2, except for HO2 and NO, all other species display two-stage generation. The first-stage generation results from the decomposition of H2O2 and NH3 and occurs further upstream compared to NH3/air combustion. HO2 is mainly derived from the reactions of H2O2 ((33), R116, (36), (35), R186, and R287, which are numbered R89, R116, R88, R87, R186, and R287 in the GRI-Mech 3.0 mechanism.). The yields of HO2 and OH are considerably enhanced compared with those under the NH3/air combustion conditions. The second-stage generation originates from the combustion of NH3. The species of NH2, OH, NH, HNO, NO, NO2, N, and O are produced further upstream.Figure 6
Minor species concentration profiles for NH3 combustion at ER=1.0.The temperature distribution can be used to estimate flame thickness (Figure7). The flame thickness is determined from the interval betweente the cold boundary temperatureTu and equilibrium temperature Tf. In this region, the unburned gas is preheated by heat transfer from the reaction zone. The unburned gas expands and accelerates because of the increasing temperature, and the temperature curve is concave upward. On reaching the ignition temperature Ti, which is the inflection point of the temperature profile, the resultant reactions release heat, modifying the temperature curve to concave downward. The temperature continues to increase until the equilibrium value Tf is attained. The tangent lines formed by Tu, Ti, Tf, and the temperature curve are then used to determine the flame thickness [49, 50]. When the increase in the temperature is divided into two sections, the flame is dominated by different mechanisms. The flame thickness τ consists of two parts, τ1 and τ2, which are obtained from Tu, Ti1, and Tr and Tf, Ti2, and Tr, respectively. Tr is the intersection of the tangents of Ti1 and Ti2.Figure 7
Computed flame thickness for stoichiometric NH3/air combustion and NH3/H2O2 combustion at various H2O2 concentrations.Figure7 also shows the flame thickness under different conditions. In the case of air combustion, the flame is created in only one section, and when the oxidizer is changed from air to H2O2 at different concentrations, the flame is divided into two sections. The first section is dominated by the reactions of H2O2, which is the source of O2. The second section is dominated by the NH3 reactions. The total flame thickness is the sum of the two sections. For the two-section flames, the flame thickness of the first section is thinner than that of the second section because the heat release rate of H2O2 is greater than that of NH3 combustion. The total flame thickness at 30% H2O2 concentration is greater than that in air. The high water content at 30% H2O2 partially absorbs heat and reduces the reaction, thus increasing the flame thickness. A lower concentration of H2O2 will reduce the exothermic rate. For instance, the average heat release rate for the case of 40%H2O2 is 6.186×108J/m3·s while it is obviously reduced to 7.988×107J/m3·s for 30%H2O2. However, the flame in the first section (0.095 cm) is considerably thinner than that in air (0.21 cm). When the concentration of H2O2 is increased, the decomposition reactions are enhanced. This subsequently increases the reactions of NH3, and the flame thickness in the second section is gradually decreased. When the H2O2 concentration is increased to 90 or 100%, the flame is no longer divided into two sections. The decomposition of H2O2 and combustion of NH3 occur almost simultaneously, and the flame thickness is considerably small (5–6 mm).


## 3.3. Species Analysis of NH3/H2O2/H2O Combustion and their Relation to the Burning Velocity

Because using H2O2 at various concentrations as the oxidizer changes the initial NH3 concentration, the concentrations of N-related species in the reaction zones are affected and it will obscure the actual variation resulting from H2O2-enhanced reactions. Therefore, the mass fraction or production rate of the N-related species should be normalized [28].Figure8 shows the normalized NH3 mass fractions under different combustion conditions. As the H2O2 concentration increases, the reaction rate increases, and NH3 is consumed further upstream. The consumption of NH3 was divided into two stages at 40 and 60% H2O2 concentrations. The first stage represents the decomposition of H2O2; thus, NH3 is partially consumed. The second stage involves the combustion of the remaining NH3. For 80% H2O2 concentration, in contrast to the temperature distribution, the fractional consumption of NH3 is not apparent. Figure 9 shows the profiles of normalized NH3 reaction rates under different combustion conditions. In air, the dominant reactions related to the production and consumption of NH3, which are numbered R277, R278, R279, and R277 in the GRI-Mech 3.0 mechanism, are as follows:
(15)Production:NH2+H2↔NH3+H(16)Consumption:NH3+OH↔NH2+H2O(17)NH3+O↔NH2+OH(18)NH3+H↔NH2+H2Figure 8
Normalized NH3 mass fraction profiles for the combustion of NH3/air and NH3/H2O2 at various H2O2 concentrations.Figure 9
Normalized NH3 consumption rate profiles for the combustion of NH3/air and NH3/H2O2 at various H2O2 concentrations.When the oxidizer is changed to H2O2, the yields of OH, O, and H radicals increase. This considerably enhances the reaction of NH3 and moves it upstream. (15), numbered R277 in the GRI-Mech 3.0 mechanism, initially produced and then consumed NH3. These reaction zones are distinctly narrowed with increasing H2O2 concentration. Additionally, the consumption reactions are divided into two stages.The normalized mass fractions of NH2 for different combustion conditions are shown in Figure 10. In air, NH2 is produced further downstream with a low concentration. With increasing H2O2 concentration, the NH2 is produced further upstream, and its concentration is considerably increased. As shown in Figure 6, the NH2 concentration exhibits two peaks; the first peak is higher than the second peak. The H2O2 decomposition upstream promotes the reaction of NH3 to yield NH2. The combustion of the remaining NH3 downstream yields the second NH2 peak. For 80% H2O2 concentration, the two-peak phenomenon is less evident.Figure 10
Normalized NH2 mass fraction profiles for NH3/H2O2 combustion at various H2O2 concentrations.Figure11 shows the normalized NH2 production rates under different combustion conditions. In air, the production of NH2 is dominated by reactions (16), (17), and (15), which are numbered R278, R279, and R277 in the GRI-Mech 3.0 mechanism, whereas its consumption is dominated by reactions (11), (23), (24), and (15), which are numbered R203, R202, R201,and R277 in the GRI-Mech 3.0 mechanism.
(19)Production:NH3+OH↔NH2+H2O(20)NH3+O↔NH2+OH(21)NH3+H↔NH2+H2(22)Consumption:NH2+OH↔NH+H2O(23)NH2+H↔NH+H2(24)NH2+O↔H+HNO(25)NH2+H2↔NH3+H(25), numbered R277 in the GRI-Mech 3.0 mechanism, is divided into two stages, namely, the consumption of NH2 and the generation of NH2. Moreover, the overall production rate of NH2 shows that it is initially produced and then consumed. When the oxidizer is changed to different concentrations of H2O2, the dominant reactions related to NH2 production and consumption are not significantly different; however, their intensity increases with increasing H2O2 concentration. The reaction zones become narrower with increasing H2O2 concentration. In addition, most of the reactions display two peaks; the first peak is attributed to the large amounts of OH, O, and H produced by the H2O2 reactions, which enhances the relevant reactions (such as (16), (15), and (11)), numbered R278, R277, and R203 in the GRI-Mech 3.0 mechanism).Figure 11
Normalized NH2 consumption rate profiles for the combustion of NH3/air and NH3/H2O2 at various H2O2 concentrations.Figure12 depicts the OH concentration profiles under different combustion conditions. Compared with air, increasing concentrations of H2O2 considerably increase the OH concentration. The OH is also gradually yielded upstream owing to the addition of H2O2. The OH concentration exhibits two peaks under the conditions of H2O2. As shown in Figure 3, the burning rate increases with increasing H2O2 concentration. As shown in Figure 4, (32) and (33), numbered R85 and R89 in the GRI-Mech 3.0 mechanism, are the most critical reactions that dominate the laminar burning velocity of the NH3/H2O2/H2O flame. Both reactions are related to OH. The order of burning velocity follows: ER=0.8>1.0>1.2; the OH concentration for the H2O2 condition presents a similar order, wherein the OH mass fractions are 0.0417, 0.0369, and 0.0271 at ERs of 0.8, 1.0, and 1.2, respectively. When ER=1.0, the OH concentration at 100% H2O2 is approximately 31 times that in air.Figure 12
OH mass fraction profiles for the combustion of NH3/air and NH3/H2O2 at various H2O2 concentrations.Figure13 illustrates the profiles of OH production rates under different combustion conditions. In air, the main reactions related to OH, which are numbered R86, R38, R279, R278, R84, and R203 in the GRI-Mech 3.0 mechanism, are as follows:
(26)Production:O+H2O↔2OH(27)H+O2↔O+OH(28)NH3+O↔NH2+OH(29)Consumption:NH3+OH↔NH2+H2O(30)OH+H2↔H+H2O(31)NH2+OH↔NH+H2OFigure 13
OH consumption rate profiles for the combustion of NH3/air and NH3/H2O2 at various H2O2 concentrations.When the oxidizer is changed to 40% H2O2, (32), numbered R85 in the GRI-Mech 3.0 mechanism, becomes the main reaction for OH formation, and the main consumption reactions are (33), (34), (35), (36), and (37), numbered R89, R278, R87, R88, and R203 in the GRI-Mech 3.0 mechanism.
(32)Production:H2O2+M↔2OH+M(33)Consumption:OH+H2O2↔HO2+H2O(34)NH3+OH↔NH2+H2O(35)OH+HO2↔O2+H2O(36)OH+H2O2↔HO2+H2O(37)NH2+OH↔NH+H2OThe most critical reactions are related to H2O2. When the concentration of H2O2 is increased to 60 or 80%, the dominant reaction for producing OH is still (32), numbered R85 in the GRI-Mech 3.0 mechanism. However, R46 (H+HO2↔2OH) and (26) (O+H2O↔2OH), numbered R86 in the GRI-Mech 3.0 mechanism, also significantly contribute to its production. The most important reaction for consuming OH is (33), numbered R89 in the GRI-Mech 3.0 mechanism. In addition, the OH reaction zone under the condition of H2O2 is divided into two stages; the H2O2 reactions that dominate the first stage are considerably stronger than those in the second stage. As the concentration of H2O2 is increased, the OH reactions are enhanced, and the reaction zone is narrowed.Finally, a quasilinear relationship between burning velocity and species concentration was established in this study. The laminar flame speeds of hydrocarbon fuels have quasilinear relationships with (OH+H)max in the reaction zone [51]. Li et al. [23] investigated the NH3 combustion under oxygen-enriched conditions and found a quasilinear relationship between the laminar burning velocity (cm/s) and (NH2)max at various O2 concentrations in air. However, in this study, when H2O2 was used as the oxidizer, the reaction was enhanced, and the laminar flame speed was not dominated by NH2. When comparing the burning velocity with the concentrations of various species, such as NH2, OH, O, and H, the burning velocity reveals a quasilinear relationship with the maximum value of the (NH2+OH) mole fraction in the flame. Figure 14 shows the laminar burning velocity of NH3 as a function of the (NH2+OH)max mole fraction when replacing a fraction of the air by H2O2 (10–100%) or using different concentrations of H2O2 (30–100%) as the oxidizer. These two conditions are expressed as follows:Figure 14
Laminar burning velocity of NH3 as a function of the maximum (NH2+OH) radical in the reaction zone for (a) different percentages of air replaced with H2O2 and (b) various H2O2 concentrations.
(a)(b)For air partially replaced by H2O2,
(38)Vbcms=4094.8×NH2+OHmax+1.8.For different H2O2 concentrations and (NH2+OH)max mole fraction≤0.025,
(39)Vbcms=5677.6×NH2+OHmax−8.5.For different H2O2 concentrations and (NH2+OH)max mole fraction>0.025,
(40)Vbcms=2461×NH2+OHmax+52.5.However, when the replacement percentage of H2O2 or the concentration of H2O2 is sufficiently high (>60% for ER=0.8 and 1.0, and > 70% for ER=1.2), the maximum concentration of OH is markedly higher than that of NH2. Thus, (NH2+OH)max is dominated by the OH concentration, and (OH)max is approximately equal to (NH2+OH)max. Therefore, under this condition, the burning velocity is considered to have a quasilinear relationship with (OH)max. Overall, when H2O2 is used to partially replace air or when different concentrations of H2O2 are used as the oxidizer, with an increase in the replacement percentage or the concentration of H2O2, the linear relationship of burning velocity with (NH2)max is changed to (NH2+OH)max and finally to (OH)max.


## 4. Conclusions

In this study, a numerical simulation was performed to explore the effects of different concentrations of H2O2 as the oxidizer on NH3 combustion. Combustion characteristics such as the adiabatic flame temperature, laminar burning velocity, flame thickness, and species yield were evaluated. Moreover, the adiabatic flame temperature and flame speed under oxygen-enriched conditions were also calculated. The main conclusions are as follows:
(1)
At high concentrations of H2O2 (>80%), the maximum temperature difference between ERs is less than 30 K, because the heat release is dominated by H2O2 decomposition. When the H2O2 concentration is 80%, the adiabatic flame temperature approaches that at 40% O2 under stoichiometric conditions(2)
When the H2O2 concentration is less than 50%, the flame speed is not significantly affected by the ER. When the concentration is greater than 50%, the flame speeds follow a descending order for ERs of 0.8, 1.0, and 1.2. This trend is primarily observed because OH is formed from the H2O2 reactions(3)
The influence of H2O2 on the burning velocity includes thermal and chemical effects. The influence of H2O2 on the burning velocity at low concentrations is mainly attributed to the chemical effect. As the H2O2 concentration is gradually increased, the chemical effect is gradually decreased, and the thermal effect is increased. When the H2O2 concentration is 100%, the thermal effect is similar to the chemical effect, both of which are approximately 50%.(4)
When air is replaced with different concentrations of H2O2 as the oxidizer, the flame is divided into two sections. The first section is dominated by H2O2 reactions, and the second is dominated by NH3 reactions. The flame thickness of the first section is considerably thinner than that observed during combustion in air. When the H2O2 concentration is increased to 90 or 100%, the flame is no longer divided into two sections(5)
The burning velocity exhibits a quasilinear relationship with the maximum value of the (NH2+OH) mole fraction when replacing a fraction of the air with H2O2. For using H2O2 with different concentrations as the oxidizer, it exhibits two linear relationships for different ranges of the maximum value of the (NH2+OH) radical. Overall, the linear relationship between the burning velocity and (NH2)max in air is changed to (NH2+OH)max and finally to (OH)max



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*Source: 3371078-2023-02-04.xml*