Development of Ammonia Combustion Technology for NOx Reduction

Abstract

This study comprehensively reviewed the engineering theories and technologies required for using ammonia as a fuel. The slow reaction rate and high NOx emissions of ammonia remain challenging issues with existing combustion technologies. Accordingly, the causes of these problems with ammonia were analyzed and the results of research aimed at solving these issues and commercializing ammonia combustion were examined to explore future directions for the development of ammonia combustion technology. The equivalence ratio (ER) emerged as the most important factor, closely related to operational stability and NOx emissions. Various combustion technologies, such as staged combustion and flameless combustion, have been attempted, but NOx emissions remain high at overall ER < 1, necessitating post-treatment processes. The internal recirculation of combustion gases is a key technology that enhances the stability of ammonia combustion, and its extreme case, flameless combustion technology, is predicted to form stable ammonia combustion. This is related to supplying the radicals that are lacking in the pure ammonia combustion process through the recirculation of combustion gases. By utilizing this, if the stability of ammonia combustion is secured and staged ER control technology is established, it is believed that the commercialization of pure ammonia combustion technology will be possible in the future.

1. Introduction

Ammonia serves as a raw material in various industries, particularly in fertilizer production. When produced using renewable energy, ammonia is considered an alternative fuel and clean energy source, effectively reducing greenhouse gas (GHG) emissions [1]. One unique property of ammonia as a combustible is that it is a carbon-free fuel emitting no carbon dioxide, unlike conventional fossil fuels. This feature underscores its significance in transitioning toward a decarbonized economy and a cleaner future [2].

Ammonia has applications in gas turbines, internal combustion engines, stationary power plants, and transportation fuels, including heavy-duty and marine transport, either in pure or blended form. However, its utilization as a fuel comes with various challenges attributed to its properties, which differ from those of conventional hydrocarbon fuels [3]. These challenges encompass equipment size reductions, modifications in piping systems, and enhanced safety controls. Additionally, the toxic nature of ammonia poses a significant hurdle to its viability as a sustainable fuel, emphasizing the urgency in finding a resolution for these challenges [3].

The development of ammonia combustion technologies necessitates an understanding of combustion and heat transfer characteristics as well as identifying research gaps to overcome current challenges. The utilization of ammonia also requires adaptation of the hydrocarbon combustor system. This paper presents a literature review on ammonia utilization and provides a brief overview of its properties, combustion characteristics, and combustion technologies. It delves into various ammonia combustion technologies, with emphasis on oxidation reactions, flameless combustion, and NOx formation, alongside discussing NOx reduction methods.

1.1. Greenhouse Gas Emissions and Ammonia as a Future Energy

Energy production in the industry, transportation, and building sectors significantly contributes to greenhouse gas emissions. Currently, most energy (80%) is generated through the combustion of fossil fuels. Hence, substantial efforts are imperative to render combustion processes more environmentally friendly and safer than they have been historically [4].

Global CO₂ emissions from industrial processes show an annual increase. According to the IEA’s detailed regional analysis, CO₂ emissions have risen by 6% since 2020, reaching 36.3 gigatons (Gt). Specifically, in South Korea, CO₂ emissions increased from 566.1 million tons (Mton) in 2010 to 616.8 Mton in 2021, representing a growth rate of 1.1% per year [2]. Figure 1 illustrates CO₂ emissions within the context of energy consumption and industrialization processes, based on the latest official national data encompassing energy, economic, and meteorological information [3].

Consequently, sustainable energy sources such as ammonia and hydrogen are preferred as alternatives to fossil fuels due to their environmental compatibility [5]. Promoting clean hydrogen and hydrogen-based fuel technologies, as outlined in the 2021–2050 net-zero emissions scenario, is projected to reduce CO₂ emissions by 60 gigatons (Gt), constituting approximately 6% of the total cumulative emission reduction [4]. This necessitates the implementation of a demand generation strategy, intensified initiatives for innovative applications, the enhancement of hydrogen production infrastructure quality, the development of transitional energy process capacity, the mitigation of associated emissions, and the optimization of cost efficiency in hydrogen production [6].

Hydrogen emerges as a carbon-neutral solution to the imperative challenge of decarbonizing the economy amidst the pressing issue of climate change [4]. Gray hydrogen, constituting the most prevalent production pathway (96%), is derived from hydrocarbons and contributes to approximately 2% of total CO₂ emissions. Despite its prevalence, gray hydrogen stands as the most polluting form of hydrogen. However, green hydrogen remains two to three times costlier, and numerous factors to render green hydrogen significantly more appealing must be resolved. Also, akin to any nascent technology, challenges persist with green hydrogen, particularly concerning its industrial applications as an energy source [1].

The progression of hydrogen as a primary carbon-free energy source holds immense significance. Hydrogen has emerged as the optimal energy carrier for future energy systems, playing a pivotal role in the efforts to mitigate global warming (Figure 2). Hydrogen is produced through various methods, encompassing thermochemical, electrochemical, radiochemical, photochemical, biochemical, and integrated systems. A significant stride toward commercialization entails the exploration of cutting-edge technologies for hydrogen production, aiming to yield clean fuels and energy sources devoid of greenhouse gas (GHG) emissions [7].

Hydrogen production from ammonia typically involves processes such as ammonia decomposition and ammonia reforming. This process, known as ammonia cracking or thermolysis, is conducted at high temperatures (450–700 °C) in the presence of a catalyst. The products of this process include nitrogen and hydrogen gases [7].

1.2. Ammonia Production Industries

Ammonia is produced through a variety of pathways, ranging from biological or renewable feedstocks to carbon-based thermochemical processes such as the Haber–Bosch process. The latter is the most prevalent method, consuming over 30 gigajoules per ton of ammonia (GJ/tNH₃) and generating approximately 2.6 kg of carbon dioxide equivalent per kilogram of ammonia (kgCO₂eq/kgNH₃). The 176 million tons of ammonia produced annually (representing 90% of the global production of ammonia), emitting more than 420 million tons of CO₂ per year, from the steam methane reforming (SMR) system of the Haber–Bosch process accounts for 1.2% of anthropogenic global CO₂ emissions [8].

Ammonia production is categorized into gray, blue, and green ammonia based on the energy supply used in the production methods, as illustrated in Figure 3 [3]. Gray ammonia, associated with high carbon emissions, is synthesized through coal gasification, natural gas, naphtha reforming, pyrolysis, and autothermal reactors (ATR) [9]. The production of 200 million tons of gray ammonia annually results in the release of more than 1% of global CO₂ emissions [9]. Blue ammonia is produced through the reforming and gasification of fossil fuels, characterized by lower carbon content, which is facilitated by the carbon capture process. Green ammonia, devoid of carbon emissions, can be produced from air and water as renewable feedstocks and sustainable energy sources [10]. However, the production of green ammonia is costly, and its process requires further development [11].

The fertilizer industry consumes approximately 80% of the annual ammonia production [2]. Besides its role as a fertilizer component, ammonia serves as a clean energy source, playing a vital role in fostering a carbon-free economy and contributing significantly to the global trade in carbon-free fuel [12]. It holds a prominent position in the market as an ideal carrier for hydrogen, representing the largest production and sales market [4]. Ammonia is considered a versatile fuel and is being explored as a candidate for various applications, including transportation, power generation, distribution networks, heating, cooling support, and overall decarbonization efforts. Its potential for sustainable development is considerable [13].

1.3. Properties of Ammonia as a Carbon-Free Energy Source

Anhydrous ammonia is colorless. When diluted with water, it becomes non-flammable, dissolves with an alkaline reaction, and produces ammonium hydroxide. However, its toxicity and pungent odor pose challenges for its safe use as a fuel [14].

Ammonia serves not only as a carbon-free fuel capable of replacing fossil fuels but also as a promising candidate for storing excess energy from intermittent renewable sources. Its versatile applications extend to serving as a chemical platform molecule or being directly utilized as a fuel. Furthermore, it is being considered as a candidate fuel for fuel cells [15].

When utilized as a primary fuel, the energy content of ammonia is 5.17 MWh/ton, as determined by the lower heating value (LHV) of ammonia (Table 1) [16]. Conversely, when converted back to hydrogen, the energy content amounts to 33.33 MWh/ton, based on the LHV of hydrogen (Table 2) [11].

The specific energy and energy density characteristics of ammonia are compared with carbon-based fuels in Figure 4 and Figure 5 and Table 2 in terms of energy storage materials for mobile applications and energy conversion technology efficiency [11].

2. Theoretical Framework of Ammonia Combustion

In the realm of renewable energy development, ammonia is recognized as an alternative fuel and clean energy source conducive to achieving carbon neutrality. The energy content of ammonia can be harnessed through direct combustion [17].

2.1. Theoritical Ammonia Combustion

The combustion of ammonia can be described by the following equations.

Oxidizer oxygen: NH_3(g) + 3/4 O_2(g) →1/2 N_2(g) + 3/2 H₂O_(g)(1)

Oxidizer air: 2 NH_3(g) + 1.5 (O_2(g) + 3.76 N_2(g)) → 3 H₂O_(g) + 6.64 N₂(2)

The adiabatic flame temperature of ammonia combustion is also high, with a peak occurring at a stoichiometric ratio of φ = 1.1, reaching around 1900 °C, which is lower than that of other fuels. Figure 6 depicts the adiabatic flame temperature in relation to ERs of various fuels, i.e., ammonia, DME, hydrogen, methane, and syngas [18].

2.2. Ammonia Combustion Technologies

Ammonia combustion encounters efficiency and stability challenges stemming from various factors, including poor ignition quality (with an octane number of approximately 130), narrow flammability limits (16–28% by volume in air), low burning velocity, and lower energy density than hydrocarbon fuels, as well as lower heat flux and high NOx emissions [8]. Moreover, ammonia safety concerns encompass explosion hazards arising from high-temperature processes and its toxic gas properties, which pose obstacles to its utilization as a sustainable fuel. Overcoming these challenges is imperative for realizing its potential as a safe and environmentally friendly energy source.

Additional challenges that ammonia faces as a viable energy source in establishing an effective energy system include achieving carbon-free synthesis, enhancing power generation capabilities for various scales, securing public acceptance through safety regulations and community engagement, and ensuring economic viability for the integration of technologies and environmentally friendly ammonia production [14].

Various combustion technologies have been developed to address the challenges associated with ammonia, including premixed and diffusion flame combustion and flameless combustion, each with a distinct performance in energy efficiency [19]. The combustion process is significantly influenced by the design and configuration of the combustor as well as the geometry and orientation of the fuel and air nozzles.

In general, unlike premixed combustion, diffusion flame combustion features a wider reaction zone, wherein the composition changes and chemical reactions between ammonia and air occur [20]. This occurs through a combination of convection and diffusion, allowing for the mixing of the fuel and oxidizer before their reaction (Table 2) [11]. Continuous combustion ignition is sustained by the elevated chamber temperature, the presence of a highly reactive species (radicals), and a stable environment. In diffusion flame combustion, the burning rate is often constrained by the transport and mixing processes rather than chemical kinetics [20].

Flameless combustion processes, which used to be called MILD (moderate or intense low oxygen dilution) [21], HiTAC (high-temperature air combustion) [22], FLOX (flameless oxidation) [23], etc., represent a novel approach to achieving efficient and environmentally friendly fuel utilization. Due to the combination of a low peak temperature and highly diluted mixtures preheated to a high degree, flameless combustion has unique characteristics in terms of combustion, heat and mass transfer, and NOx emission compared to diffusion [24] and premixed combustion [21].

In flameless combustion, a big portion of hot exhaust gases recirculate within the combustion chamber alongside fresh fuel and air injections. This simultaneous dilution of fresh reactants by the recirculated exhaust gases controls furnace temperatures and reactions effectively and mitigates pollutant emissions while promoting complete fuel oxidation. Transitioning from traditional combustion to flameless conditions involves altering main system parameters, such as preheating and dilution in fuel and air feeding configurations [12]. Under non-flameless conditions, noticeable temperature gradients are evident, whereas flameless combustion conditions showcase a uniform temperature throughout the furnace with no observable flame zone, as shown in Figure 7 [24].

Various burners incorporate a combination of air-preheating and multi-step burner technologies to enhance efficiency and reduce emissions. For example, regenerative burners operate in flameless combustion mode, while recuperative burners facilitate the counterflow of burned gases with air in a heat exchanger to recover their enthalpy. These approaches aim to maximize energy efficiency and minimize waste heat by utilizing heat recovery mechanisms [15]. The efficiency of these burners is primarily determined by the temperature of the exhaust gases. The air-preheating rate of recuperative burners varies depending on the temperature ratio between the preheated air and the exhaust gases. Furthermore, flue gas recirculation is one of the most important and necessary phenomena, producing the effect of regeneration by directly mixing hot flue gas with incoming fresh air and fuel. The recirculation rate is expected to be higher than 2. Therefore, these two methods for recycling the waste heat of flue gas make flameless combustion appear to be the most stable and efficient combustion method.

Flameless combustion technology offers several advantages over conventional combustion methods. However, the reaction rate of flameless combustion, particularly in the context of pure ammonia, is not well defined [3].

2.3. Chemical Kinetics of Ammonia Combustion

A fundamental understanding of the ammonia oxidation process and the appropriate kinetic mechanisms is crucial for the development of its applications and NOx reduction strategies [25]. Although various studies have proposed oxidation models and conducted experiments under different conditions, predicting ammonia combustion accurately remains challenging [26].

Only a limited number of studies have delved into the reaction mechanisms of pure NH₃ and air, the flame structure, and the heat release characteristics of an ammonia flame. In comparison to experimental data, kinetic mechanism models for ammonia combustion generally show agreement with each other in terms of the flame speed, ignition delay, extinction strain rates, and the mole fractions of the product gases at ERs for both rich and lean conditions [27]. Some of the characteristics of ammonia combustion can be predicted by adjusting the constants of reactions involving NH₂, HNO, and N₂H₂ radicals [28]. A crucial component of NH₃ oxidation and H₂ production at intermediate temperatures is NH₂ + NH₂ = N₂H₂ + H₂ [29]. Furthermore, the rate constant for the reaction pathway from NH₂ to N₂H₂ differs widely among the models due to the uncertainty in its values [30]. The production of H₂ has been overestimated in models using larger rate constants for this reaction. These adjustments are particularly influential in determining the ignition delay time and laminar flame speed [31]. While there is insufficient information regarding the radical recombination of NH₂ and reaction modeling relating to N₂H_x species [32], the ignition and combustion of ammonia in flames is primarily influenced by the critical role of the O/H radical pool and the concentration of HO₂ in the active reaction pathway [31].

Some of the major ammonia oxidation reactions, which are listed in Table 3 [30], are related to NOx emissions and reduction [31], according to Glarborg [31], Tian [27], Konnov [30], and Nakamura [33]. Some notable studies in this field include those by Okafor et al. [34], Stagni et al. [31], and Otomo et al. [25].

The mass fraction and temperature parameters were derived from simulations using the chemical kinetics of the ammonia–methane mechanism based on the Okafor model, which includes 59 species and 356 reactions, across various ERs (φ = 0.8–1.2) [34]. Figure 8 illustrates the key species (NH, NO, and OH radicals) and temperature profiles for ammonia–air premixed flame combustion as a function of ER (φ = 0.8–1.2) along the flame axis. The NH and NO layers increase as ERs increase to φ = 1 [35]. Some of the major ammonia oxidation reaction mechanisms are illustrated in Table 3 [30].

3. Ammonia Combustion Characteristics

The characteristics of ammonia combustion encompass flame temperature, ignition temperature, laminar flame speed, and laminar burning velocity [36]. Additionally, factors such as heat transfer, ignition delay time, chemical kinetics, stability, extinction characteristics, and explosion behavior are also studied. Lift-off may occur before the full range of ammonia flames, and flashback may also be observed [37].

3.1. Propagation of Ammonia–Air Flames

Flame propagation speed is influenced by various factors, including temperature, pressure, fuel and air flow rates, ER, and turbulence levels. The laminar burning velocity (LBV) of combustion is a basic parameter, and it follows the power-law relationship (3) as follows [38]:

S_u = Su₀[(T_u/T₀)^α][(p/p₀)^β](3)

where S_u (m/s) is the LBV, Su₀ is the reference laminar flame speed, T_u is the unburned temperature (K) with T₀ at 298 K, p is the pressure (atm) with p₀ at 1 atm, and α and β are constants related to the fuel. The constant (α, β) and Su₀ parameters for the calculation of the laminar burning velocity (LBV) can be calculated using Equations (4)–(6), where ER is φ; A, B, and φ₀ are constants determined by the fuel type; Su₀ is a constant at the specific ER (φ₀). The parameters A and B can be found from the simulation and experimental measurement of the LBV and the polynomial expression Equation (6) [39]:

α = 2.18 − 0.8(φ − 1)(4)

β = − 0.16 + 0.22(φ − 1)(5)

Su₀ = A + B(φ − φ₀)²(6)

The heat of ammonia–air combustion per mass is approximately 40% of that of typical hydrocarbon fuels, and the maximum LBV of an ammonia flame is approximately 20% of that of hydrocarbon fuels [38]. Moreover, the LBV (S_u) is associated with characterization parameters such as ER, temperature, and pressure, as well as the reaction rate of the fuel and its kinematic properties across the flame zone [39].

The experiments and simulations of the ammonia laminar flame characteristics according to ammonia chemical reaction kinetic models, based on the ER at atmosphere, are illustrated in Figure 9a,b [40] and Figure 9c [30]. The highest flame propagation speed after igniting pure ammonia was observed at an equivalent ratio of 1.1. Following this, an ER of 1.0 exhibited the next fastest burning rate, while a ratio of 1.2 took third place [40]. However, the flame cannot be generated at an ER under 0.6 due to slow reaction kinetics, which is the challenging aspect to be overcome for ammonia energy utilization. This is because the radicals generated at ignition activity, such as NHx, OH, and HO₂, are not sufficiently populated to sustain the chain reaction to generate the flame. The maximum velocity of an ammonia flame is approximately 7 cm/s, which is significantly lower than that of hydrogen (approximately 290 cm/s) [41] and methane (approximately 40 cm/s) [42].

3.2. Fuel Mixing and Pressure

Mixtures of ammonia with other fuels can significantly increase the flame velocity. This increase in velocity is attributed to the enhanced reactivity of NH₃ combustion when mixed because radicals from other fuels induce the combustion of NH₃ [37]. The LBVs of NH₃/H₂ at various ERs (0.6–1.4) and pressures (3 and 5 bar) are shown in Figure 10 and Figure 11, which demonstrate that hydrogen mixing in the range of 40~50% makes the LBV similar to that of hydrocarbons. The variation in the LBV of the ammonia mixtures with hydrogen is a function of the hydrogen fraction X_H2 at different pressures. As the hydrogen fraction increases, (Figure 11a), the LBV exponentially increases [8]. A comparison of the experimental and numerical data is illustrated in Figure 10 based on the proportion of hydrogen contents in the mixture [42]. Also, the laminar burning velocity varies with the initial pressures (Figure 11b) at P₀ = 0.1 MPa and temperatures (T₀ = 360 K) for an ER of 1.0 [11]. Following this, adding hydrogen led to an increasing burning rate [8]. It is also observed that the laminar flame speed decreases with increasing pressure and mixture density [42].

The laminar flame speeds of NH₃, its mixture with H₂ and N₂, and CH₄ in air at various temperatures and elevated pressures over a wide range of ERs are shown in Figure 12a [43]. In Figure 12b, increasing the H₂ concentration in NH₃/H₂ mixtures increases the flame speed significantly, as it increases the reactivity of NH₃ combustion, where the rate of formation of important radicals such as OH, H, and O is enhanced [5]. The laminar burning velocity of the NH₃/CH₃OH/air mixture increases as the molar fraction of CH₃OH (X₍CH₃OH₎) rises, with a peak occurring around φ = 1.1 and a slight shift toward fuel-rich conditions with higher CH₃OH content (Figure 12c) [44].

3.3. Oxygen-Enriched Combustion

The effect of oxygen-enriched combustion on the laminar burning velocity (LBV) at different oxygen concentrations from 28% to 35% in ammonia premixed combustion at ERs ranging from 0.6 to 1.5 are illustrated in Figure 13 [44]. As oxygen concentrations increase from 28 to 35%, the LBV increases from 22 to 36.1 cm/s, respectively, and the operable ER expands. However, LBV is dependent on the equivalent ratio up to 1.1, and then it decreases [42].

The influence of oxygen concentrations from 15 to 40% in ammonia premixed combustion at ERs ranging from 0.9 to 1.15 is illustrated in Figure 14. When the oxygen concentration increases, the flame velocity of ammonia increases rapidly. When it reaches 35%, it approaches the flame velocity of methane [44].

Different air-preheating conditions and oxygen concentrations manifest unique ammonia flame characteristics. At a constant air temperature, the flame size diminishes by 2% to 21% with an increase in oxygen concentration. The typical bright-yellow color observed in regular air transforms into a yellowish flame in ammonia combustion as both the temperature and oxygen concentration rise [45].

The ER significantly influences ammonia flame morphology. Based on ERs, there is a slight upward shift in the ammonia–air flame structure (Figure 15a) [16]. As the ER increases to 1.1, the laminar burning velocity (S_L) increases to a maximum of 7.06 cm/s and then decreases [46]. Indeed, the flame structure of lean NH₃/O₂ and N₂ mixtures can be predicted by different mechanisms that incorporate reactions (Figure 15a) [16], such as NH₂ + O, N₂H₃, and N₂H₄, it can also be optimized using a machine learning model, as predicted for pure ammonia (Figure 15b) under various ERs [46]. These reactions play crucial roles in determining the behavior and characteristics of the flames in these mixtures [46].

3.4. Flame Thickness

A conceptual illustration of a premixed combustion flame structure of NH₃–air at Φ = 1 and P = 0.1 MPa [8], flame zones, and the experimental results of the laminar flame thickness at different ERs (Φ) is represented in Figure 16. The flame structure generally includes a preheat zone, a thin inner reaction layer (with the sum of their thickness denoted as flame thickness δ), and a post-flame zone (Figure 16b). The post-flame zone is the region where hot combustion products and NOx reactions take place [35].

The laminar flame thickness (δ_L) is a function of the maximum temperature gradient, given by (7):

δ_L = (T_b − T_u)/ [max (dT/dx)](7)

where T_b is the burned temperature and T_u is the unburned temperature [42].

The unstretched laminar burning velocity (S_L) is obtained from (8)—mass conservation as the density ratio of the burned mixture (ρ_b) to the unburned mixture (ρ_u) across the thin flame front:

S_L = α/δ = Ss (ρ_b/ρ_u)(8)

where S_S is the unstretched flame speed, δ is the thermal diffusion flame thickness, α is thermal diffusivity, and (ρ_b/ρ_u) is the mixture density ratio between the burned gas and the unburned gas [40].

According to Figure 17, with different ammonia/hydrogen mixtures, as the initial ambient pressure increases, the flame thickness decreases proportionally to the hydrogen contents of the mixture [40] and is the inverse of pressure. The increase in the actual reaction rate is attributed to the increase in density resulting from the pressure increase, which can be observed from the decrease in flame thickness [40].

Figure 18 shows variations in ammonia flame thickness as a function of ERs and the different initial pressures, as determined by experimental and numerical laminar speed measurements [47]. In addition, there is a good correlation between the experimental flame thickness and the numerical laminar flame speed. The minimum ammonia flame thickness was 0.013 mm at an ER of 1.0 because the laminar flame speed is the fastest at this ER. As the ER increased from 1.0 to 2.0, the flame thickness increased from 0.02 mm to 0.1 mm at barometric pressure. As the ER decreased from 1.0 to 0.4, the flame thickness increased from 0.02 mm to 0.038 mm. As the initial pressure increased, the flame thickness decreased proportional to the inverse of the pressure [47].

3.5. Ignition Characteristics

The ignition characteristics of NH₃ are closely related to its flammability limits in air, which are narrower than those of H₂ [47]. Experiments on mixtures of NH₃/H₂/N₂ over various ERs at ambient temperature and pressure demonstrate that combustion under leaner and richer conditions requires higher minimum ignition energy, as shown in Figure 19a [5]. Furthermore, as the H₂ fraction increases, the minimum ignition energy of the mixture also shifts toward a leaner state [46]. Also, the flammable range defined by UFL (upper flammability limit) and LFL (low flammability limit) shows a critical relationship with hydrogen concentrations (Figure 19b) [5].

A comparison of the combustion properties of different fuels, such as volumetric energy content (kg/m³), flame speed (cm/s) and autoignition point (°C), are illustrated in Figure 20 [3]. Ammonia (both gas and liquid) has the highest ignition temperature (red arrows = 650 °C), a low flame speed (circle size = 7 cm/s), and a low energy density (5.2 kWh/kg) but does not have CO₂ intensity (green coolers) [3].

The ignition delay time for ammonia combustion shows its slow reaction kinetics, and an exponential relationship with the inverse of temperature and a proportional relationship with the inverse of pressure (Figure 21) [30].

3.6. The Lean Blow-Off Limit (LBO) of Ammonia Flames

The LBO is a crucial parameter in the design and optimization of combustion systems, as it defines the minimum fuel consumption rate and the operating range of the system. Understanding the LBO ER of a flame is essential for ensuring the safe and efficient operation of combustion systems, especially in industrial applications such as gas turbines, boilers, and furnaces. Under LBO conditions, the flame is no longer sustained due to insufficient fuel. This limit is characterized by the leanest fuel-to-air ratio at which the flame can be stabilized. In practical applications, the LBO is typically determined experimentally by gradually decreasing the fuel flow rate until the flame is extinguished [48].

Several factors can influence the LBO of a flame, including the type of fuel, the burner design, the mixing type of fuel and air, and the combustion environment. Typically, fuels with higher burning velocities and lower ignition temperatures have a lower LBO limit. Additionally, swirl burner designs that facilitate the efficient mixing of fuel and air can enhance the LBO by ensuring flue gas recirculation at the burner front to supply radicals to maintain the flame [48].

As shown in Figure 22a, mixing with hydrogen or methane can lower the LBO, with hydrogen showing a better performance than methane due to its high reactivity. The addition of 30% hydrogen can lower the LBO to 0.3. However, flashback (FB) can occur for hydrogen (or methane) fractions higher than 30%, so the mixing ratio should be carefully determined. The addition of more than 5% hydrogen in ammonia blends is comparable to 5% methane in terms of lean blow-off (LBO) limits. However, with a hydrogen fraction rate of more than 30%, the reactivity of binary blends changes abruptly with less than 30% methane [48].

The minimum inlet velocity (U) is limited by flashback (Figure 22b), with no clear transition observed below 3.8 m/s for the NH₃ flame due to its weak nature and significant heat loss. Due to the low burning velocity of ammonia, mixing with conventional fuel is a practical solution at combustion. Without additional fuel, ammonia has a high lean blowout (LBO) limit (0.6–0.8), making it difficult to stabilize the flame at high inlet velocities (U) or under lean conditions compared to the hydrocarbon fuel flame. When extinction begins, the entire flame is extinguished suddenly [49].

4. NOx in Ammonia Combustion

4.1. The Mechanism of NOx Formation and Reduction

Ammonia’s potential as a clean fuel is constrained by NOx emissions. The formation and reduction mechanisms of nitrogen oxides (NO, NO₂, N₂O) depend on the conditions within the surrounding gas phase [50]. The generation of NO in a stabilized premixed flame resulting from the incomplete combustion of ammonia in atmospheric air occurs as follows:

4NH₃ + 5O₂ → 4NO + 6H₂O(9)

2NO + O₂ → 2NO₂(10)

In the oxidation process, the primary pathway for fuel–NOx formation involves the reaction of NH₂, NH, and N radicals with O₂, O, OH, and HO₂ to generate NO or through intermediate species such as HNO and H₂NO. One crucial reaction leading to the formation of HNO is the following:

NH₂ + O → HNO + H(11)

which becomes less favorable at elevated temperatures. The pivotal reaction influencing nitric oxide (NO) production within the oxidation pathway is governed by molecular oxygen (O₂) [51]. The significant reaction contributing to NO production in the oxidation pathway is controlled by O₂ through a series of interconnected reactions, including the following:

NH+ O₂ → HNO + O(12)

HNO+ O₂ → NO + HO₂(13)

NH+ O₂ → NO + OH(14)

HNO + H → H₂ + NO(15)

OH  +  HNO → H₂O  +  NO(16)

NH + O → H +NO(17)

Another main responsible reaction of NO formation is the following:

H + O₂ → O + OH(18)

During combustion, fuel-derived NO forms through reactions involving NH₂ and O atoms. However, as the fuel-to-oxidant ratio (ER) increases, oxygen availability decreases. This diminishes the significance of NO formation reactions while promoting NO reduction by NH₂, resulting in reduced fuel NO emissions as the ER rises from 0.6 to 1.6 [52]:

NH₂ + O → HNO + H(19)

HNO + OH → NO + H₂O(20)

NH₂ + NO → N₂ + H₂O(21)

The importance of NHi radicals (i = 0, 1, 2) lies in their participation pathways in NH₃ combustion and their influence on the formation and decomposition of NO and N₂O, as detailed in Table 3 and affected by the ER. NHi radicals are formed through reactions of NHi₊₁ with OH and H radicals. Simultaneously, they contribute to the reduction in NO, resulting in the formation of N₂ [28].

The elementary reactions involving OH and N₂O radicals provide enhanced predictions for the generation of NH₂ radicals under high-pressure conditions. Additionally, these reactions contribute to the formation of NNH, NH₂, OH, HO₂, and H₂NO radicals in the NOx formation process [52].

However, the mechanisms are unable to accurately predict the profiles of NO and N₂O compared to NH₃ concentrations measured in flue gas from the combustor [33]. The Konnov reduced reaction mechanisms demonstrate satisfactory predictive capability for NOx emissions and laminar flame speed across a wide range of ERs, mixture compositions, and pressures. This prediction accuracy compares favorably to both the full mechanism and experimental data [30].

Figure 23 illustrates the reaction mechanism of the reduction in ammonia combustion and the formation of NOx [8]. An experimental investigation of a stoichiometric ammonia flame under high-pressure conditions reveals a decrease in the mole fraction of NO. This observation qualitatively aligns with the outcomes predicted by numerical simulations [53].

Recent advancements in combustion mechanisms highlight the crucial roles played by the low-temperature-favored H₂NO pathway and the high-pressure-favored NHi recombination reactions in predicting the ignition delay time and flame speed. However, these mechanisms still harbor undesirable uncertainties [35]. The composition of the fuel influences NO sensitivity, and reactions sensitive to the burning rate may impact NO production by affecting the enhancement and reduction of O/H radical pools. Increasing pressure levels diminish the O/H radical pool, thereby reducing NO emissions [54].

4.2. Effect of ERs and Fuel Mixing on NOx Emissions

Figure 24 illustrates the NOx emissions from a premixed flame compared to the non-premixed flame combustion of CH₄/NH₃ at different ERs [55,56]. NOx concentrations exhibit a close relationship with ERs in ammonia combustion. Mixing ammonia with methane can enhance combustion stability, and flameless combustion results indicate lower NO and CO emissions up to an ER of 0.8 [55,56]. However, there is a significant increase in CO emissions at ERs above 0.8 [50].

NO emissions in relation to the ERs of an ammonia–methane mixture combustion are shown in Figure 24a [11] and 24b [55]. Under mixed fuel conditions, the NOx emissions in Figure 24a are increased compared to pure ammonia and pure methane combustion, which means methane mixing can improve combustion stability; however, NOx emissions have become a problem. When comparing the 50% mixing of methane in Figure 24a,b, a slightly different trend is captured. Peak NOx emissions appear at ER = 0.6 in Figure 24a, and a peak at ER = 0.8~0.9 appears in Figure 24b. NO emissions from a pure ammonia/air premixed flame at various ERs are shown in Figure 24c and pressures in Figure 24d [50].

NOx emissions from an ammonia gas turbine combustor decrease with increasing ERs and can be negligible in fuel-rich regions. The combustion chemistry of ammonia shows significant differences between lean and rich conditions [57]. Figure 25 illustrates the ability of various reduced reaction mechanisms to predict NOx emissions satisfactorily across different ERs [16]. Among these mechanisms, Tian’s model shows the most satisfactory prediction [13], while GRI-Mech 3.0 and Konnov’s model show the least satisfactory predictions [16]. The NO concentration decreases sharply when ammonia is burned at an ER of 1 or higher, although its temperature is near the adiabatic flame temperature [57].

Under lean conditions, combustion generates a substantial pool of O and OH radicals, with HNO serving as an intermediate in the production of fuel NOx. In contrast, rich combustion results in the formation of abundant NHi radicals within the H radical pool, leading to NNH formation through NHi recombination reactions. The production of N₂ can occur via intermediate channels such as N₂O and NNH, facilitating thermal DeNOx processes [58].

Lean combustion at low ERs is expected to have low NOx emissions due to the mixing effect of air. Hence, as shown in Figure 26, methane fuel emits a low EINO at lower ERs under 1.0. However, the EINO of an ammonia–methane mixture does not follow this rule because the air supply has more radicals, such as HNO, which react with NHi radicals to generate NOx. When NH₃ exists, fuel NOx is the major component of total NOx, which is very different from pure CH₄, and the EINO increases dramatically at low ERs. Also, CH₄ shows a peak in the EINO near 1.0 ER at 1600 K (Figure 26), while NH₃-containing fuel shows the highest peak at the lowest ER. Meanwhile, as the temperature increases to 2000 K, thermal NOx generation is activated, so methane has similar NOx emissions as an ammonia mixture at low ERs [59]. To compare the NO emissions under different conditions, using the NO emission index (EINO), which can be calculated by dividing the NO formation rate by the fuel consumption rate, eliminates the dilution effect [60].

Under lean fuel conditions, the primary pathway contributing to NO production is the HNO pathway. This observation sheds light on the increased NO emissions observed during the co-firing of ammonia with hydrogen, as the combustion of hydrogen intensifies the presence of OH radicals. Consequently, while enhancing ammonia flame stabilization, the co-firing strategy also leads to an increase in NOx emissions, compared to pure ammonia combustion (Figure 27a). This increase in NOx emissions is particularly notable when the fuel contains only 10% hydrogen and reaches a maximum at 80% hydrogen, and then decreasing as NH3 content decrease. This trend is predicted by different mechanism, as shown in Figure 27b [61].

5. NOx Reduction Methods

5.1. Staged Combustion

Ammonia is considered a means of decarbonizing economies in the future and ensuring sustainability, but NOx emissions need to be reduced through the development of combustor designs, advanced combustion techniques, and ER control. Thus, staged combustion to burn hydrogen produced by the cracking of NH₃ and the remaining NH₃ is the solution to control NOx emissions [62].

NOx emissions vary with the rich-burn stage ER of ammonia–methane combustion in a gas turbine depending on the degree of inter-stage mixing. Higher ERs at the rich-burn stage generally lead to increased NOx formation due to higher combustion temperatures (Figure 28a) [62], with the lowest NOx emissions at 21% oxygen content and an ER of 1.25 (Figure 28b) [63]. As the oxygen concentration increases, the minimum NOx emission occurs at higher ERs and at lower the value. Improved inter-stage mixing can help moderate NOx emissions by promoting better fuel–air distribution and optimizing subsequent lean-burn conditions [63].

The effects of residence time variation on NOx emissions were evaluated for the lean premixed swirling flames of ammonia–methane–air at elevated pressures. The residence time is modified by changing the chamber length and swirl intensity, which is altered by the recirculation zone’s size and speed. The residence time of swirling flames inside the combustion chamber can effectively increase when the chamber is extended or the swirl number is increased [64].

Figure 29 illustrates the effects of chamber length (L) and swirl number (S) variation on NOx emissions as a function of the ER (0.50 to 0.95) and ammonia fraction (0 to 100%). The chamber length ranges from 120 to 360 mm, and the swirl number ranges from 0.6 to 1.0, indicating that the volume fraction of ammonia varies according to the ER for the fuel blend [64]. The NO concentration increases rapidly until 40% of ammonia, but it decreases when the ammonia rate increases further (Figure 29c,f). There is a peak in NO emissions at ER = 0.8, and the emission decreases when the emission deviates from 0.8 on the leaner and richer sides (see Figure 29c).

The staged injection of NH₃ facilitates NO removal by reducing the agents and additives in the selective non-catalytic reduction (SNCR) process. The SNCR reaction includes the following equations [65]:

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O(22)

6NO₂ + 8NH₃ → 7N₂ + 12H₂O(23)

In a tangential injection combustor employing fuel staging, the NOx emission characteristics of the premixed and non-premixed combustion of ammonia–methane mixtures are influenced by a significant inner recirculation zone induced by intense swirling motion. This provides substantial operational flexibility to the ammonia tubular flame, especially under ambient temperature inlet conditions [66]. Figure 30 illustrates NO emissions as a function of ER at different bulk velocities of the premixed ammonia flame [66]. Ammonia–air combustion is implemented in a dry low-NOx system using a multi-stage fuel technology, challenging the established rich–quench–lean staging configuration paradigm for NOx reduction strategies. The bulk velocity varies NOx emissions due to swirling turbulence differences; however, the overall ER is the most important parameter for controlling NOx emissions [67].

5.2. Flameless Combustion

In contrast to a classic diffusion flame, a flameless combustion process involves the combustion of preheated air surrounded by one or more jets of fuel [68]. In flameless combustion, the fuel is mixed with burned gases to lower the concentration before being mixed with preheated air [69] and then generates a distributed slow reaction. Flameless combustion shows various characteristics depending on the type of burner and the combustion chamber; however, it has not yet been standardized and is still undergoing research and development [70].

Nevertheless, the excellent performance in some areas shows the potential of flameless combustion as a combustion technology using ammonia as a fuel in the future [71].

Reduced NOx emissions and improved burner efficiency are achieved by limited temperatures and decreased oxygen concentration in flameless combustion mode [72]. Flameless combustion offers lower peak temperatures, minimizing thermal NOx formation, while flue gas recirculation (Figure 31a,b) reintroduces gases rich in inert species like N₂, H₂O, and CO₂ [69]. Figure 31b explores advanced combustion techniques like flameless combustion with flue gas recirculation, which hold promise for reducing fuel NOx emissions [59,69].

Figure 32 illustrates the NOx emissions from a swirl flame compared to the flameless combustion of CH₄/NH₃ (99 and 1%) at different ERs (0.68–0.87). However, there is a significant increase in CO emissions at ERs above 0.8 [58].

Flameless combustion is characterized by a homogeneous reaction zone, diluted mixtures of reactants, and reduced temperature peaks, resulting in a uniform temperature and species distribution. In flameless combustion, three components are mixed in the combustion zone—fuel, air, and recirculated burned gas—which produces a gas above its autoignition temperature [72]. Auto-ignited combustion occurs in an oxygen-deficient environment, attributed to the dilution of the hot burned gas. The most distinctive characteristic of flameless combustion is its appearance without flame, which occurs when oxygen levels in the air are low, typically less than 2% [73].

Flameless combustion is based on mixing air with flue gas before reaction to slow down the reaction speed by a lower oxygen concentration [68]. Due to the low oxygen fraction, there are notable limitations in temperature increases. It is crucial to highlight that the temperature profile remains constant after the initial ignition stage, without exhibiting a distinct temperature peak. NOx emission levels are directly proportional to both the oxygen concentration in the incoming air and the maximum combustion air temperature [74]. According to Figure 33, the variation in NOx emissions over time for different oxygen concentrations is depicted at 1000 °C (Figure 33a) and 800 °C(Figure 33b), which illustrates that higher O₂ concentrations correspond to a higher concentration of NOx, with the peak observed at a concentration of 70% O₂ [68].

Flameless combustion occurs at high temperatures above the autoignition temperature with low oxygen levels, where reactions take place within a diffusion layer. This results in significant gradients in fuel and oxygen concentrations after injection, leading to a rapid decrease in their concentrations. Furthermore, the consistent temperature and low oxygen levels characteristic of flameless combustion help to reduce the formation of certain combustion byproducts, such as NOx and unburned ammonia [74].

The predominant approach to enhance flameless combustion involves either preheating the air or utilizing a heat exchanger to recuperate the enthalpy from exhaust gases [71]. The impact of the preheated air temperature on flameless combustor performance is noteworthy, with negligible effects on NOx emissions observed at flue gas recirculation ratios exceeding a certain threshold. By selecting an appropriate air nozzle diameter and combustor geometry, it is possible to achieve the recirculation ratio necessary for sustaining flameless combustion [75]. This unique process exhibits distinctive chemical and physical characteristics, including the macroscale uniformity of scalars associated with microscale distributed reacting regions, stability, and low NOx emissions, setting it apart from conventional flames. Through this process, the two factors—high oxygen concentrations and elevated temperatures—in the combustion chamber that contribute to NOx formation can be mitigated [75].

Controlling the thermo-fluid pattern in the combustion chamber using air and fuel nozzles is one of the most critical design parameters of flameless combustion systems. Due to the high-speed air jet creating a robust recirculating flue gas, flameless combustion exhibits enhanced convective heat transfer, which results in a uniform temperature distribution. A notable feature of RAI flameless combustion is its unique configuration, where fuel is injected at the rear end of a potent air jet, distinguishing it from conventional combustor designs [76].

The unique features of MILD combustion, a branch of flameless combustion technology, include its simultaneous capability to reduce emissions and enhance efficiency, especially for low-reactive fuels such as ammonia. The numerical simulation of MILD combustion has been carried out to evaluate the reaction mechanism of ammonia–air, the flame structure, and the heat release, comparing it with CH₄–air and H₂–air [77]. The mixture of ammonia and methane under MILD combustion produces higher NO_x emissions than their pure combustion, which is similar to an ammonia–hydrogen mixture. The MILD combustion of ammonia and methane mixtures demonstrates NOx emissions at less than 3% of those generated by traditional combustion, highlighting distinct NOx pathways between MILD and conventional combustion processes.

For a 70/30 NH₃/H₂ blend, NOx emissions reach their minimum at 450–654 ppm under lean conditions (ϕ = 0.5–0.8) and 344–211 ppm under rich conditions (ϕ = 1.0–1.2) [78]. Unburned NH₃ and H₂ emissions remain minimal in lean conditions. Both lean and rich regimes exhibit similar or improved emission performance in flameless combustion mode compared to conventional combustion [78].

6. Conclusions

The combustion characteristics of ammonia are distinctly different from those of conventional fuels, necessitating significant modifications to burner design theories. Above all, the limited types of radicals and their low concentrations lead to unstable chain reactions, which narrow the operational range of the burner. Additionally, the nitrogen content inherent in the fuel contributes to higher NOx emissions due to fuel NOx generation, surpassing thermal NOx emissions.

Considering these characteristics of ammonia combustion, various technologies such as blending conventional fuels, high oxygen concentration, staged combustion, and flameless combustion have been researched and established. While blending conventional fuels and supplying high oxygen concentrations improves ammonia flame stability, they also lead to increased NOx emissions, making them impractical solutions. From the foundational research results, the most influential factor in NOx emissions was identified as the equivalence ratio (ER), and staged combustion, which involves excess air combustion following fuel-rich combustion, has become essential for ammonia combustion. Nevertheless, unstable combustion requires sufficient radicals to be supplied in the combustion zone, which necessitates the internal recirculation of combustion gases as a solution.

Internal recirculation technology, which circulates the radicals produced during the combustion process within the combustion chamber to react with incoming fuel, can stabilize ammonia combustion similarly to the methods of fuel blending and high oxygen concentration oxidizers. Furthermore, if staged combustion technology is effectively integrated, it is believed that stable ammonia combustion technology along with low NOx emissions can be developed. This is because ammonia itself can be utilized as a NOx reducing agent, as evidenced by existing research showing a significant NOx reduction effect around ERs of 1.25. Therefore, ammonia combustion technology that reduces environmental burdens through combustion gas recirculation and ER control in staged combustion could be commercialized in the future. In this context, flameless combustion, which maximizes combustion gas recirculation, has shown the lowest NOx emissions at ERs above 1, and introducing the concept of staged combustion could lead to the most practical ammonia combustion technology.

Conflicts of Interest

The authors declare no conflicts of interest nor any personal circumstances or interests that may be perceived as inappropriately influencing the representation or interpretation of the reported research results. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Footnotes

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Figures and Tables

Figure 1. The comparison of greenhouse gas emissions from fossil fuels and industry, 1900–2020: (a) worldwide; (b) South Korea [3].

Figure 2. An overview of the movement from fossil fuel toward sustainable hydrogen energy. Reprinted with permission [7].

Figure 3. Ammonia production methods based on the energy supply for production [3].

Figure 4. Comparison of carbon-free and carbon-based fuels in terms of their energy density [1].

Figure 5. Volumetric and gravimetric density of fuels: (a) hydrogen storage capacity [8], (b) energy density [14].

Figure 6. Comparison of the theoretical adiabatic temperature of various fuels—ammonia, DME, hydrogen, methane, and syngas—as a function of ER (0.1–2.5) [18].

Figure 7. A comparison of temperature distribution: (a) conventional diffusion combustion, (b) flameless combustion (BG: burned gas, F: fuel, A: air) [3].

Figure 8. The variation in the species concentrations and temperatures of ammonia premixed combustion at equivalent ratios of (a) 0.8, (b) 1.0, and (c) 1.2 based on the distance from the nozzle on the flame axis [35].

Figure 9. The comparison of experiments (a) [30] and simulations (b,c) results of ammonia laminar flame characteristics according to an ER at ambient atmosphere [40].

Figure 10. The comparison of numerical and experimental results of the LBV of ammonia–hydrogen mixtures (NH3/H2/air) (a) at different hydrogen mixing levels; (b) at different pressures based on ERs with the selected mechanisms [42].

Figure 11. Laminar burning velocity of an ammonia/hydrogen mixture (NH3/H2) combustion change as (a) a function of the volumetric hydrogen fraction at different pressures [8], (b) a function of pressure [40].

Figure 12. Comparison of laminar flame speeds (SL) of (a) ammonia and syngas [43]; (b) pure ammonia, H2, CH4, and NH3/H2 mixtures with air [5]; (c) NH3/CH3OH with air at 298 K in premixed flame combustion at different ERs [44].

Figure 13. Comparison of the LBV of ammonia oxygen-enriched combustion, based on equivalent ratios, at 1 atm [44].

Figure 14. The flame velocity of the oxygen-enriched combustion of premixed ammonia [44].

Figure 15. (a) The laminar burning velocity (SL) of the ammonia–hydrogen/nitrogen mixture [16], and (b) optimized machine learning model predictions of the pure ammonia under various ERs at 298 K and P = 0.1 MPa [46].

Figure 16. (a) The flame structure of a premixed combustion of NH3–air at Φ = 1 and P = 0.1 MPa [8], (b) flame zones, and (c) laminar flame thickness at different ERs (Φ) [35].

Figure 17. Flame thickness of the different ammonia/hydrogen mixtures based on the initial pressures at ER ϕ = 1.0 and T0 at 360 K [40].

Figure 18. Comparison of the experimental and numerical ammonia flame thicknesses based on (a) ERs and (b) initial pressure [47].

Figure 19. Comparison of the minimum ignition energy of NH3 and H2 (a) over various ERs and (b) at different volume percentages of H2 and N2. The symbols are ◻: H2; ○: NH3; ♦: NH3/H2/N2 (8 vol% H2, 2.8 vol% N2); △: NH3/H2/N2 (18.5 vol% H2, 6.1 vol% N2); •: NH3/H2/N2 (32.8 vol% H2, 11 vol% N2) [5].

Figure 20. Autoignition temperature based on the energy density of ammonia compared to other fuels. In this context, circle size represents flame speed, meaning larger circles indicate higher flame propagation rates. Color intensity denotes carbon content, where deeper or more intense colors signify higher carbon concentrations [3].

Figure 21. The ignition delay time of ammonia combustion (NH3/O2/Ar) at different ERs (a,b) and different temperatures [30].

Figure 22. (a) Comparison of the lean blow-off (LBO) limits of ammonia combustion premixed with methane and hydrogen [48], and (b) flame transition ERs (ϕt) with blow-off ERs (ϕb) in a swirl burner with S = 0.73 at different ERs [49].

Figure 23. The reaction mechanism for NOx formation and reduction in ammonia combustion [8].

Figure 24. Profile of NOx emissions from the combustion of (a) ammonia mixed with methane [11] (b) comparison of combustion methods [55] and pure ammonia (c) as a function of ERs and (d) pressure [50].

Figure 25. Comparison of satisfactory predictions of different reaction mechanisms of NOx emissions with respect to ERs [16].

Figure 26. Effects of different ERs on NO formation at reaction temperatures of (a) 1273, (b) 1600, and (c) 2000 K [59].

Figure 27. Comparison of NOx emissions from combustion of NH3/air (a) and NH3/H2/air (b) with respect to hydrogen fractions [61].

Figure 28. NOx emissions of rich-burn stage ERs of ammonia combustion in a gas turbine for (a) different inter-stage mixing (ηl) [62] and (b) for different oxygen content [63].

Figure 29. Comparison effect of chamber length (L) and swirl number (S) on NOx emissions as a function of the ER (ϕ) and ammonia fraction (αNH3) (solid symbols: L = 120, S = 0.6 (L120S06); half-full symbols: L = 360, S = 0.6 (L360S06); and open symbols: L = 360, S = 1.0 (L360S10) [64]. (a) shows that, the O₂ mole fraction decreases as ER ϕ approaches stoichiometric conditions but does not change for higher ammonia fraction (αNH3) (d). The CO₂ mole fraction (b) increases as ER ϕ approaches stoichiometric conditions but (b) but decreases when αₙₕ₃ increases as shown in (e). In (c), the NO concentration decreases when ER ϕ deviates from approximately 0.7–0.8 on both the lean and rich sides. Additionally, the NOx concentration initially rises sharply as αₙₕ₃ increases up to 30–40% (c) but then declines with further increases in αₙₕ₃, as shown in (f).

Figure 30. NOx reduction performance (A) at different bulk velocities of the premixed ammonia flame through fuel staging (B) based on global ERs [66]: (a) Schematic of a multi-stage combustor. (b) cross section of burner. (A) NOx emission at different bulk velocities.

Figure 31. (a) Ammonia combustion methods based on flue gas recirculation (a,b) [69], (c) NOx reduction through transition from a swirl flame to flameless combustion [59]. Simplify of a gas turbine combustor (b) recirculation of fuel, air, and burned gas recirculation: 1—Fresh air, 2—recirculation zone; 3—combustion zone, 4—injected fuel; 5—mixing zone.

Figure 32. Comparison of NOx emissions: (a) swirl flame and (b) flameless combustion at different ERs [59].

Figure 33. NOx emissions as a function of O2 concentrations during flameless combustion over time at (a) 1000 °C and (b) 800 °C [68].

Table 1

The fundamental properties of ammonia [14].

Characteristics	Value
Odor threshold	5–50 ppm
Molecular weight	17.03 g/mol
Specific volume	1.3 m3/kg
Latent heat of fusion	331.37 kJ/kg
Boiling point	−33.5 °C
Latent heat of vaporization	1371.2 kJ/kg
Vapor pressure	8.88 bar (at 21 °C)
Critical point	112.8 bar (132.4 °C)
Gas density	0.86 kg/m3, 21 °C, 0.73 kg/m3 at boiling point, 1.013 bar
Specific gravity	0.597(1.013 bar at 21 °C) (air = 1)
Specific volume	1.411 m3/kg (1.013 bar at 21 °C)
Specific heat at constant pressure (cp)	0.037 kJ/(mol·K) (1.013 bar at 15 °C)
Specific heat at constant volume (cv)	0.028 kJ/(mol·K) (1.013 bar at 15 °C)
Ratio of specific heats (cp/cv)	1.309623(1.013 bar at 15 °C)
Dynamic viscosity	0.000098 Poise (1.013 bar at 0 °C)
Thermal conductivity	22.19 mW/(m·K) (1.013 bar at 0 °C)
Autoignition temperature	630 °C
Ignition temperature	651 °C
Upper flame limits	25–28%
Lower flame limits	15–16%
Lower heating value (LHV)	18.6(MJ/kg-fuel) = 5.167 kWh/kg fuel
Lower heating value stoichiometric * LHVst	2.62(MJ/kg-stoichiometric)
Higher heating value (HHV)	22.5(MJ/kg-fuel)
Volumetric energy density	11.2(MJ/L)

* The lower heating value (LHV) is based on the unit mass of a stoichiometric air–fuel mixture, defined as LHVst = LHV/(1 + (AFR_st)), where (AFRst) is the stoichiometric air-to-fuel ratio. Variations in the air flow rate for different fuels are considered in this calculation [14].

Table 2

Ammonia thermodynamic characteristics compared to other fuels [11].

Fuel	NH3	H2	CH4	C3H8
Minimum autoignition temperature (°C)	650	520	630	450
Lower heating value, LHV (MJ/kg)	18.6	120	50.0	46.4
LHVst (MJ/kg-stoichiometric)	2.62	3.39	2.75	2.79
Adiabatic flame temperature (°C)	1800	2110	1950	2000
Maximum laminar burning velocity (m/s)	0.07	2.91	0.37	0.43
Calorific value (MJ/kg)	18.6	120	50	46.4
Flammability limit (ER)	0.63–1.4	0.10–7.1	0.50–1.7	0.51–2.5

Table 3

Summary of the major ammonia oxidation reactions according to mechanisms provided by Glarborg [31], Tian [27], Konnov [30], and Nakamura [33].

Major Reactions		* A	* n	* Ea
NH3 + OH	⇋NH2 + H2O	2.040 × 106	2.04	5.660 × 102
NH2 + NO	⇋H2 + N2O	1.000 × 1013	0.00	3.370 × 104
NH2 + NO2	⇋N2O + H2O	1.600 × 1016	−1.40	2.680 × 102
NH2 + NO	⇋N2 + H2O	2.800 × 1020	−2.70	1.258 × 103
NO + HO2NO2 + HNO2 + HO2N2O + H	⇋NO2 + OH⇋NO + OH⇋HONO + O2 ⇋N2 + OH	2.100 × 10122.500 × 10141.910 × 1002.530 × 1010	0.000.003.320.00	−4.800 × 1026.760 × 1023.044 × 1034.550 × 103
HNO + HOH + HNO	⇋NO + H2⇋H2O + NO	4.400 × 10113.600 × 1013	0.720.00	6.500 × 1020.000

* A is the pre-exponential factor, Ea is the molar activation energy for the reaction, and n is a temperature exponent of the Arrhenius equation of k = ATⁿ exp (−Ea/RT).

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