Application of NH₃ Fuel in Power Equipment and Its Impact on NO_x Emissions

Abstract

Due to high greenhouse gas emissions, countries worldwide are stepping up their emission reduction efforts, and the global demand for new, carbon-free fuels is growing. Ammonia (NH₃) fuels are popular due to their high production volume, high energy efficiency, ease of storage and transportation, and increased application in power equipment. However, their physical characteristics (e.g., unstable combustion, slow flame speed, and difficult ignition) limit their use in power equipment. Based on the structural properties of the power equipment, NH₃ fuel application and emissions characteristics were analyzed in detail. Combustion of NH₃ fuels and reduction measures for NO_x emissions (spark plug ignition, compression ignition, and gas turbines) were analyzed from various aspects of operating conditions (e.g., mixed fuel, fuel-to-exhaust ratio, and equivalence ratio), structure and strategy (e.g., number of spark plugs, compression ratio (CR), fuel injection, and ignition mode), and auxiliary combustion techniques (e.g., preheating, humidification, exhaust gas recirculation, and secondary air supply). The performance of various NH₃ fuel cell (FC) types was analyzed, with a focus on the maximum power achievable for different electrolyte systems. Additionally, the application and NO_x emissions of indirect NH₃ FCs were evaluated under flame and catalytic combustion conditions. The system efficiency of providing heat sources by burning pure NH₃, anode tail gas, and NH₃ decomposition gas was also compared. Based on a comprehensive literature review, the key factors influencing the performance and emissions of NH₃-powered equipment were identified. The challenges and limitations of NH₃-powered equipment were summarized, and potential strategies for improving efficiency and reducing emissions were proposed. These findings provide valuable insights for the future development and application of NH₃ FCs.

1. Introduction

The issue of global warming, which is brought about by the excessive release of greenhouse gases, has become a pressing concern that humans must urgently address [1,2,3]. Reducing emissions is the primary means of achieving the “two-carbon” goals [4]. Controlling global warming at 1.5 °C instead of 2 °C would bring significant advantages to humanity, encompassing aspects such as sea level rise, meteorological disaster, water scarcity, permafrost thawing, and more [5]. The largest obstacle we face in our generation is climate change. To meet the temperature criterion of 1.5 °C laid down by the Intergovernmental Panel on Climate Change and reduce global greenhouse gas emissions, it is imperative to identify technically reliable, commercially significant, environmentally friendly, and safe solutions [6,7]. The global response to greenhouse gases is shown in Figure 1.

With the pressing need to address climate change, vigorously developing renewable and carbon-free energy sources has become paramount [1,2,3]. Energy carriers are crucial in balancing intermittent renewable energy production with ever-increasing demand [8,9]. For the mass transportation of hydrogen-containing fuels, liquefied forms of energy storage are preferred [10]. Liquid fuels, including liquid hydrogen or hydrogen-containing carriers such as ammonia (NH₃), have gained traction in various applications [11,12]. Safe, reliable, and easily transportable energy is essential for achieving the “two-carbon” goals [13,14]. However, the lack of large-scale, reliable infrastructure hampers the efficient storage and transportation of hydrogen energy [15,16]. In contrast to hydrogen, NH₃ energy boasts a well-established global infrastructure, a complete industrial chain for production, transportation, storage, and terminal applications, and favorable physical properties such as higher energy density [17]. These advantages position NH₃ as a promising alternative to fossil fuels, with green NH₃ production technologies further enhancing its appeal as a sustainable decarbonization fuel [18,19,20]. Moreover, NH₃ exhibits superior safety compared to methane (CH₄), hydrogen, liquefied petroleum gas, methanol, and gasoline [17,21]. As evident from

Table 1. Physical property parameters of fuel [27].

Property	NH3	Gasoline	n-Heptane	Diesel	CH4	H2
Lower heating value (MJ/kg)	18.8	44.5	44.6	45	50	120
Density at 1 bar and 298 K (kJ/m3)	0.72	736	684	849	0.67	0.09
Vaporizing heat (kJ/kg)	1370	348.7	320	232.4	511	455
Autogenous ignition temperature (K)	930	503	477	527–558	859	773–850
Laminar burning velocity (m/s) (close to stoich)	0.07	0.58	0.37	0.86	0.38	3.51
Flammability limit (vol. %)	15–28	0.6–8	1.05–6.7	1–6	5–15	4.7–75
Air/fuel ratio by mass	6.05	15	15.4	14.5	17.3	34.6
Octane number	130	90–98	0	-	120	>100
Adiabatic flame temperature (K)	1800	2138	2294	2300	1950	2110

, NH₃ is emerging as a frontrunner for future renewable energy solutions [17,21,22,23]. Its cost-effectiveness, low carbon emissions, high efficiency, and safety make it a compelling choice to replace carbon-based energy sources. However, as NH₃ energy gains recognition, it is crucial to closely monitor pollutant emissions and the far-reaching impacts of the nitrogen cycle on the global environment throughout NH₃’s life cycle, from nitrogen synthesis to final decomposition [24,25,26]. NH₃ fuel applications and emissions as shown in Figure 2.

Utilizing alternative energy sources through minor modifications in carbonaceous energy-powered equipment presents a rapid and cost-effective approach to developing NH₃ energy [28]. However, NH₃ combustion faces challenges such as ignition difficulties, slow flame propagation, and instability. Additionally, a low heat release rate (HRR) and high NO_x emissions hinder the widespread use of NH₃ in power equipment. NH₃ also serves as an effective hydrogen carrier, enabling the decomposition of NH₃ to generate hydrogen as a green energy source. Simultaneously, NH₃ fuel can participate in FC reactions to produce a stream of electrons, generating electricity [8]. However, NH₃’s weak reaction kinetics and slow chemical reaction rate limit its power output and hinder its broader application in fuel cells (FCs) [17]. Mixing NH₃ with fuels with higher reactivity can effectively improve its combustion rate. Other measures, such as structural adjustments, strategic optimization, and auxiliary combustion technologies, have also yielded promising results in enhancing NH₃ combustion [17].

To advance the application of NH₃ power equipment, researchers have delved into NH₃ combustion, considering the advantages and limitations of green NH₃ synthesis. Studies have shown that pure NH₃ combustion presents challenges that must be addressed. Olabi et al. [20] explored the potential of NH₃ fuel in the energy sector and identified technical, economic, environmental, and regulatory hurdles that need to be overcome. Vries et al. [18] examined the current research status of various NH₃ applications as a green decarbonization fuel for heating, thermal power generation, and power equipment. Berwal et al. [19] reviewed the synthesis, chemical kinetics, and practical applications of NH₃ as a future combustion fuel, drawing insights from the combustion mechanism. Zamfirescu et al. [11] discussed and analyzed potential methods for using NH₃ as a sustainable fuel in FCs, evaluating efficiency and availability parameters based on the chemical reaction mechanism.

For power equipment that relies on thermal energy as the power source, the combustion chamber structure and combustion characteristics vary among different types of equipment, leading to distinct physical parameter requirements and pollutant emissions profiles for fuel. Similarly, for FCs that generate electricity through chemical reactions, each type of FC exhibits unique chemical reactions and optimal reaction temperatures, resulting in different pollutant emissions and corresponding mitigation strategies. Considering the diverse structures and operating principles of power equipment, this paper provides a comprehensive overview of NH₃ utilization as a decarbonization fuel and the associated pollutant emission reduction measures. It categorizes the output performance and pollutant emission characteristics of different power equipment, summarizes the challenges faced by NH₃ application, and presents an outlook on the future of NH₃ as a power source. The following sections delve into the NH₃ fuel application and NO_x emission characteristics of spark plug ignition (SI) engines, compression ignition (CI) engines, and gas turbines (GTs) from various aspects of operating conditions (e.g., mixed fuel, fuel-to-exhaust ratio, and equivalence ratio), structure and strategy (e.g., number of spark plugs, compression ratio (CR), fuel injection, and ignition mode), and auxiliary combustion techniques (e.g., preheating, humidification, exhaust gas recirculation, and secondary air supply). Meanwhile, the paper also reviews the output performance of different ammonia fuel cells under chemical combustion and flame combustion, especially the system efficiency and emission of indirect ammonia fuel cells based on supply from different heat sources (burning NH₃ combustion, anode tail gas combustion, and NH₃ decomposition gas). These findings will provide valuable insights for the development and application of NH₃ FCs.

2. Ammonia Used in Spark Ignition Engines

2.1. Mixing Fuels with Strong Activity

In SI engines, the fuel is compressed to the appropriate pressure and temperature for ignition through piston movement. High-energy spark plug injection ignites the fuel, causing it to expand and perform work, as illustrated in Figure 3. As a result, alternative fuels must have strong anti-knock properties and readily form a uniform mixture with air. NH₃ possesses excellent anti-knock properties, making it a promising carbon-free green energy source for SI engines. However, the development of NH₃ fuel in SI engines is hindered by its unfavorable physical properties. Several researchers, including Cornelius et al. [28], Starkman et al. [29,30], Cornelius et al. [22], Liu et al. [31], and Grannell et al. [32], have experimentally investigated NH₃ as a fuel for SI engines. Their findings indicate that pure NH₃ faces challenges such as inefficient flame propagation, difficult ignition, and unstable combustion, making it challenging to operate effectively in SI engines. Furthermore, NO_x and unburned NH₃ emissions are relatively high when using pure NH₃.

2.1.1. Combustion and Emission of NH₃/Gasoline SI Engines

Gaseous ammonia is pre-mixed with high-pressure ejected gasoline for 5 h, then mixed with a certain amount of air and injected into the cylinder through a direct ejector [33]. Supplementing NH₃ with reactive fuels is considered a crucial step in improving combustion stability and reducing the coefficient of variation (COV) of NH₃ combustion flame cycles [34]. For example, the laminar flame speed of NH₃ is limited to only 0.07 m/s [35], whereas that of gasoline is nearly 0.58 m/s. Therefore, incorporating gasoline into the mixture enhances flame propagation within the combustion chamber [25]. As mentioned earlier, SI engines rely on high-energy spark plug injection after compression to ignite the fuel. Consequently, the fuel should not self-ignite during the compression process, as this can lead to knocking and reduced SI output performance. NH₃’s anti-knock properties enable NH₃/gasoline SI engines to operate at higher pressures, resulting in improved work efficiency [33]. While a high CR can significantly enhance the combustion characteristics of NH₃ fuel, a CR of 12:1 or higher is not recommended due to the intersection of the minimum advance of the optimal torque detonation limit and rough limit crossover [36]. The schematic diagram of the ammonia and gasoline mixed-combustion test process as shown in Figure 4. On the other hand, mixing NH₃ with gasoline requires an increase in the ignition advance angle of the SI engine. A significant portion of the spark advance adjustment occurs when NH₃ contributes approximately 10% of the overall fuel energy input, but further increases in NH₃ content do not have any noticeable impact on flame speed [37].

Blending NH₃ leads to a deterioration in NO_x emissions due to fuel–NO_x formation. However, the sensitivity of NO_x emissions to the NH₃ blending ratio is generally moderate, and the trend of NO_x emissions with the ignition advance angle is opposite to that of pure gasoline, indicating that fuel–NO_x emission is strongly correlated with in-cylinder pressure [37]. Under high CR test conditions, Westlye et al. [38] found that NO_x was primarily produced during gap volume combustion and observed that NO emission levels were similar to those observed in gasoline measurements when the excess air coefficient (λ) is 1, mainly due to a lower flame temperature. The simulation involving the mechanism model revealed that the reaction between NH₂ and NO₂, as well as NH and NO, significantly influences the magnitude of this reaction, which is strongly influenced by the timing of ignition.

2.1.2. Combustion and Emission of NH₃/CH₄ SI Engines

CH₄, with its higher auto-ignition temperature and ability to form a more uniform mixture with air, is widely used in gasoline SI engines, requiring only minor modifications to the existing structure to meet operational requirements [27,28]. The physical properties of CH₄, such as density at room temperature, adiabatic combustion temperature, volumetric energy density, and octane number, closely resemble those of NH₃. Additionally, the laminar burning velocity of CH₄ combustion is over five times greater than that of NH₃ as a fuel. Therefore, combining NH₃ and CH₄ in combustion holds promising prospects for enhancing the combustion properties of NH₃. Compared with the pure NG engine, increasing X_NH3 decreased the peak pressure (P_max) and HRR while increasing the best thermal efficiency (BTE) by 1%. NO_x emissions, originating from NH₃ at 873 K, initially increased with increasing X_NH3 but then decreased [39]. The schematic diagram of the ammonia and natural gas mixed-combustion test process as shown in Figure 5.

Under high CR operating conditions, Zhang et al. [40] compared the performance and combustion characteristics of NH₃ fuel and CH₄ engines using a single-cylinder SI engine and verified the decrease in engine performance caused by physical properties such as flame propagation speed and the ignition difficulty of NH₃ fuel. Thermal imaging of combustion flame propagation indicated that the primary reason for the decrease in performance is the increase in the stagnant time of the NH₃ combustion flame during the initial flame development process. While the flame propagation of CH₄ is highly sensitive to temperature, NH₃ flame propagation is primarily influenced by turbulence [41].

Kurien et al. [42] investigated the effects of X_NH3 and workload on the combustion stability of SI engines fueled by NH₃/CH₄ blends. At a low engine load of 8 Nm, increasing X_NH3 from 0 to 60% reduced the laminar burning velocity and flame propagation speed of the fuel mixture, causing the COV of indicated mean effective pressure (IMEP) to increase from 1.36% to 14.9%. Conversely, at a higher engine load of 16 Nm, increasing X_NH3 from 0 to 60% narrowed the COV of IMEP from 14.9% to 4.3% [42]. The addition of NH₃ reduces the combustion rate of the fuel mixture due to the lower combustion activity of NH₃, resulting in decreased peak pressure (P_max) and HRR [43]. High-speed camera images of flame propagation revealed that increasing X_NH3 leads to a less wrinkled flame structure due to the addition of NH₃. This weakens the combustion reaction rate, lowers the flame speed, and ultimately reduces the intensity of turbulent combustion. The decrease in NO_x emissions is attributed to the lower combustion temperature caused by the addition of NH₃ [43].

Simulation studies have investigated the attenuation mechanism of CH₄ combustion induced by adding NH₃. Adding NH₃ fuel reduces the quantity of important radicals such as H and OH, leading to a decrease in the reaction rate of the fuel mixture. This causes a delay in the start of ignition (SOI) and prolongs the crank angles (CA) of 0~10° and 10~50° as X_NH3 increases [43]. Increasing NH₃ also increases NH₃ slip, but it is beneficial that the slipped NH₃ effectively promotes NO reduction. Oh et al. [44] introduced fuel comprising a mixture of NH₃ and CH₄ into a modified six-cylinder SI and found that as X_NH3 continued to increase, the amount of NH₃ slip increased and NO showed a trend of increasing first and then gradually reaching saturation. With the assistance of selective noncatalytic reduction (SNCR), NO emissions can be reduced to less than 10 ppm [44,45].

2.1.3. Combustion and Emission of NH₃/H₂ SI Engines

According to the data presented in Table 1, the flame propagation speed of H₂ is 3.5 m/s, approximately 50 times faster than NH₃‘s flame propagation speed. Despite NH₃ possessing good anti-knock properties, the octane number of H₂ is comparable to that of gasoline. Consequently, X_H2 should not be excessively high. In the pursuit of determining the optimal X_H2 threshold suitable for gasoline direct injection (GDI) engines, researchers [46] injected NH₃ and H₂ into the cooperative fuel research (CFR) engine manifold and investigated the impact of X_NH3 on engine performance. Compared to traditional gasoline SI engines, efficiency and power witnessed a significant increase due to the potential for a higher CR. The findings also indicated that a fuel with X_H2 = 10% exhibited the highest efficiency and power performance. When X_H2 is lower than 60%, the deflagration tendency remains consistent at the boundary, but it changes significantly as X_H2 increases continuously [46]. The optimal X_H2 threshold of the NH₃/H₂ mixture is influenced by load and equivalence ratio. In the initial stages of combustion, H₂ demonstrated a particularly noticeable accelerating effect, serving as an ignition promoter. Blending H₂ with X_H2 = 20% improved cyclic stability, prevented misfires, and resulted in optimal power output and indicated efficiencies comparable to stoichiometry. Increasing workload enhanced output performance and expanded operational limits concerning mixture composition [47].

Under high CRs of up to 13, the impact of adding H₂ on the combustion characteristics of NH₃ in a single-cylinder SI engine was analyzed. Additional H₂ was found to worsen indicated thermal efficiency, as the high-temperature combustion of H₂ resulted in significant heat transfer loss [48]. The visualization of combustion further revealed a critical threshold for the energy ratio between H₂ and NH₃. High-speed flame images depicting different H₂/NH₃ mixing ratios are shown in Figure 6. The addition of H₂ to the air–NH₃ mixture enhanced combustion speed, with optimal ratios being more dependent on engine load rather than speed. The NH₃/H₂ mixture was introduced into the intake manifold of a two-cylinder, four-stroke, low-speed marine engine. Experimental results [49] revealed that a minimum energy ratio of approximately X_H2 = 7% was necessary for the engine to operate reliably at full load, while a ratio of 11% was required at half load. A proposed method involving the ignition of NH₃ with an H₂ jet flame successfully achieved full-load power output for the SI engine [49].

Similarly, in the quest for the optimal X_NH3 threshold in NH₃/H₂ mixed fuel, when X_NH3 exceeded 80%, it could induce flame instability and even misfire. Under lean combustion conditions, the engine’s combustion performance is less sensitive to NH₃ fractions [50]. The detailed combustion and emission characteristics of NH₃-fueled SI mixed with other fuels under different operating conditions are outlined in

Table 2. Combustion and emissions of NH₃-fueled SI with mixed fuel.

Test Fuel	Baseline	Power Equipment	Operating Conditions	Performances	Emissions	Ref.
Gasoline/ NH3	Gasoline	CFR engine 1-cylinder	CR (7, 8, 9, 10) 1600 rpm IMEP (550 kPa)	Combustion duration ↓ 10 °CA, 40% gasoline power; best CR = 10.	-	[32]
NH3/H2	Gasoline	CFR engine	CR (7~15) 1000 rpm λ (1~1.4) Full throttle XNH3 = 80%	-	NO ↓ 1500 ppm as SOI = 30 °CA BTDC. NO2 ↑ 100 ppm, N2O ↓ 50 ppm as SOI ↑ 40 °CA.	[38]
NG/NH3	NG	6-cylinders 4-stroke turbocharged premixed 4-valve	CR = 11.5 XNH3 (0~60%) λ (1.1~1.5)	XNH3 ↑, CP ↓, combustion duration ↑; λ = 1.3 and XNH3 = 30%, optimum BTE; λ = 1.1, XNH3 = 50%, max HRR.	XNH3 ↑, NH3 and N2O ↑; XNH3 = 30%, λ = 1.2, NOx (6000 ppm).	[39]
NG/NH3	NG	NG engines 6-cylinder premixed turbocharged	CR = 10.5 1~100 rpm Max load XNH3 (0~50%) λ (0.9~1.6)	λ ↑, BTE ↑; XNH3 ↓ BTE ↑; Max BTE = 41.7%, λ = 1.5.	XNH3 ↑, NOx →; NH3 slip ↓, NO ↑.	[44]
H2/NH3	H2	4-stroke 4-cylinder	1500 rpm CR = 10.7 XNH3 (10~80%) λ (1, 1.4)	XNH3 ↑, BMEP ↑, BTE ↑; max BTE XNH3 (60~70%); limit XNH3 = 80%.	XNH3 = 20%, max NOx = 8000 ppm.	[50]
H2/NH3	Gasoline	4-stroke 2-cylinder	CR = 10.7 2500~5000 rpm λ = 1 Full load H2/NH3 = 6~8%	BTE ↓ 2.5% regardless of engine speed; BP ↓ 3 kW (3500 rpm); NH3 has certain demerits as a fuel load.	Full load NOx ↓ 1000 ppm.	[49]
H2/NH3	NH3	1-cylinder 4-stroke 4-valve	CR = 10.5 1500 rpm H2 share (0~60%) λ (0.6~1.2) Engine loads (CP = 0.1 MPa, 0.12 MPa); Direct injection	λ ↑, PMEP ↑, combustion duration ↓, best XH2 = 20%; engine load ↑, operating boundaries ↑.	XH2 ↑, NOx ↑, NH3 slip ↓.	[51]
H2/NH3/CH4 (any two)	H2/CH4	4-stroke single-cylinder premixed	CR = 10.5 1500 rpm XH2 (0~15%), XCH4 (5~15%)	NH3/15%H2, max peak HRR = 31.2; 100% CH4, min peak HRR = 10.5.	-	[52]
H2/NH3	H2/NH3	1-cylinder 4-stroke (modified from a CI engine)	CR (8~15) XH2 (5~21%) 1400/1800 rpm	XH2 from 5% to 21%, BP ↑ 16.89%, BTE ↑ 33%, volumetric efficiency ↓ 13.64%.	XH2 ↑, peak temperature ↑, NOx ↑.	[53]

↓, ↑ and → are down, up, unchanged respectively.

.

2.2. Structure Adjustment and Auxiliary Combustion Measures

2.2.1. Structure Adjustment and Strategy

The enhancement of combustion stability, flame propagation speed, and reduction of NO_x emissions can be further achieved with the implementation of operation and control strategies [24]. The injection strategy of pilot fuel proves effective in reducing the minimum ignition energy of NH₃ mixed fuel. This aids in establishing conditions for the formation of H and OH radicals, thereby shortening the initial stagnant time of NH₃ combustion [54]. The vaporization of liquid NH₃ demands a substantial amount of energy, leading to a significant decrease in the temperature of the air introduced into the cylinder. This, in turn, hinders the subsequent propagation of the turbulent flame, resulting in a decline in combustion efficiency or even potential misfire. Preheating proves beneficial by increasing the saturated vapor pressure, raising the initial temperature, and accelerating the reaction rate of fuel combustion [55]. Preheating also impacts NO_x emissions, as illustrated in Figure 7.

Implementing a direct injection system, which mixes fuels and injects the mixture directly into the cylinder, eliminates the volume of the intake manifold and improves volumetric efficiency. Ryu et al. introduced a direct injection system in a CFR engine operating at 1800 rpm [56]. The engine output ranged from 1.25 to 2.75 kW, slightly lower than when using gasoline alone. Consequently, the higher levels of NH₃ observed in the exhaust did not indicate a decline in combustion efficiency but rather reflected a greater supply of NH₃. The key factor in improving combustion performance lies in the regular injection of the mixed fuel into the cylinder.

Pandey [57] compared the impact of ignition time on the combustion performance of an engine using a NH₃/H₂ mixture and pure H₂ as fuel. This study utilized a 0.661 L, four-stroke, one-cylinder SI engine. The findings revealed that increasing the delayed ignition angle decreased the in-cylinder pressure of the NH₃/H₂ mixture and delayed P_max. The sensitivity of in-cylinder pressure to the ignition angle was weak when H₂ was burned. The peak of the HRR for the NH₃/H₂ mixed fuel occurred behind the top dead center (TDC), while the combustion of H₂ mainly occurred in front of the TDC. The delay angle tended to result in a delayed peak value of HRR, typically elevating the in-cylinder pressure of pure H₂. Whether it was an NH₃/H₂ mixture or pure H₂, thermal efficiency was closely related to NO_x emissions [57], as verified by [46].

Xin et al. [58] investigated the effect of variable valve timing (VVT) on a water-cooled, four-stroke, four-cylinder SI engine provided by the direct injection system under the Miller cycle. Advancing the intake valve timing and maintaining a larger valve overlap angle led to a decrease in combustion stability, an elongated flame development period (0–10 °CA), an increase in COV, an exceedance of MAX_COVP beyond 25%, and a rapid drop in BTE from 32.8% to 30%. However, retarding the intake valve timing by 25 °C increased IMEP and BMEP by 8% and 16%, respectively. Based on this research, advancing the intake valve timing is unsuitable for the Miller cycle H₂-enriched NH₃ engine [50,58,59].

In their comparison of control strategies on NH₃-fueled SI engines, Xin et al. [50] evaluated three load control strategies: throttle, NH₃/H₂ mixing ratio, and equivalence ratio. Experimental research revealed that controlling the throttle broadened the engine’s output power threshold, enhanced BMEP, and maintained thermal efficiency above 33%. The NH₃/H₂ mixing ratio strategy, due to the fast propagation speed of the H₂ flame, proves suitable for cold start conditions. The strategy of controlling the equivalence ratio can achieve higher thermal efficiency [59]. Hong et al. [60] investigated the effects of various load control strategies on the performance of NH₃-fueled SI engines. Qualitative control of VVT combined with quantitative control enabled the engine to meet a wide range of BMEP regulations while maintaining BTE above 37% in most cases. As the air/fuel ratio increased from 1.0 to 2.0, BMEP and P_max gradually decreased due to reduced fuel flow in the cycle. After increasing the air/fuel ratio, BTE initially increased and then decreased rapidly, reaching its maximum value of 39% when the air/fuel ratio was 1.4 [60].

Improving the CR is also a crucial measure for enhancing engine performance [37,61,62]. According to [63], the study found that increasing the CR from 9.41 to 11.51 increased P_max by 59%. Additionally, under supercharged conditions, introducing 10% H₂ into the mixture improved indicated efficiency by 37%. As the CR increased from 8 to 15, flame development accelerated, shortening the combustion period. Increasing X_H2 from 5% to 21% improved brake power and increased BTE by 16.89% and 33%, respectively, at 1400 and 1800 rpm, but decreased volumetric efficiency by 13.64%. However, with increasing X_H2 and CR, the exhaust temperature decreased, and the influence of X_H2 on the exhaust temperature diminished [64]. Despite the relatively small amount of thermal NO_x generated by fuel combustion, where fuel NO_x was the primary component, an increase in X_H2 led to a continuous rise in NO_x due to the heightened peak temperature resulting from the addition of H₂ [53].

The addition of mixed H₂-rich reforming gas serves to increase the concentration of H and OH radicals, consequently elevating in-cylinder pressure and shortening the combustion duration. The introduction of 10% H₂-rich reforming gas improves combustion efficiency by 96.3% and thermal efficiency by 43.6%. However, when the mixing ratio exceeds 12.5%, the negative impact becomes more pronounced [64]. Increasing the CR yields higher in-cylinder pressure and temperature, enhancing NH₃ combustion and engine output performance while creating conditions conducive to NO_x generation. Thus, striking a balance between engine output performance and NO_x emissions is crucial. Auxiliary combustion technologies, such as exhaust gas recirculation (EGR), prove effective in improving engine output performance while maintaining NO_x emissions within a controllable threshold [65,66,67].

2.2.2. Auxiliary Combustion Measures

Developing the application of NH₃ fuel with minimal mechanical modifications to a given engine is a key focus [56,68], as are auxiliary combustion measures. Upon examining the impact of EGR on combustion performance by varying EGR rates from 0 to 25, it was found that pure H₂ exacerbated combustion instability compared to the NH₃/H₂ mixture at higher EGR rates [57]. Implementing an initial EGR rate enhances the mean gas temperature by heating the intake with recirculated gases, thereby improving fuel combustion, boosting flame velocity, and enhancing the conversion of NH₃ to N₂. In summary, with the assistance of EGR (above 15%), the combustion stability advantage of NH₃/H₂ mixed fuel is significant [57]. This was attributed to the fast combustion rate of pure H₂ and its shorter stagnant time compared to the NH₃/H₂ mixture, which explains the significant reduction in NO_x emissions in the NH₃/H₂ mixture under highly delayed ignition conditions.

Increasing the number of spark plugs effectively reduces NO_x emissions while enhancing flame propagation speed. Uddeen et al. [69] modified the existing engine ignition system to equip it with four spark plugs, which showed that multi-spark-plug ignition promotes flame propagation, shortens the initial flame retention time, and improves the pure NH₃ combustion rate. However, it increases in-cylinder pressure and temperature, creating an unfavorable environment for NO_x formation. To enhance combustion performance, Liu et al. [70] employed pre-chamber turbulent jet ignition (TJI) technology on a 0.5 L, one-cylinder, four-stroke SI engine with a CR of 14, as shown in Figure 8. TJI generates turbulent jet flames, enabling multi-point ignition and stable combustion for low-reactivity fuel mixtures in the combustion chamber [70]. Adopting the TJI mode shortens combustion time and delays ignition time owing to an increase in ignition energy, which is beneficial to improving the combustion stability of the NH₃ SI and increasing the combustion rate. Regardless of the ignition method, the NO_x and NH₃ emissions of NH₃ SI engines are higher [70].

The instability of NH₃ combustion poses challenges for cold-starting NH₃ SI engines. Koike et al. [71] pioneered the use of H₂-rich reforming gas (resulting from the online thermal decomposition of NH₃) for cold-starting an SI engine. Achieving a fast cold start was possible when the NH₃/H₂ mixing ratio was 1:2, and stable operation was maintained under no-load conditions. To address the emissions of unburned NH₃ during the cold start and transition to stable operation, it is crucial to implement post-treatment measures for emissions. Test results indicated that with the assistance of a three-way catalyst, NH₃ emissions were close to zero [71]. The detailed combustion and emission characteristics of NH₃ SI engines, assisted by changes in structure and strategy, are presented in

Table 3. Combustion and emission of NH₃-fueled SI engines assisted by changing the structure and strategy.

Test Fuel	Baseline	Power Equipment	Operating Conditions	Performance	Emissions	Ref.
NH3/CH4	CH4	1-cylinder 4-stroke 2-spark plug	CR = 10 1500 rpm XNH3 (0~60%) load (8 Nm to 16 Nm) λ = 1	XNH3 ↑, engine performance ↓; XNH3 ↑ (0 to 60%), COV of IMEP ↑ (1.36% to 14.9%); operating load ↑, COVIMEP ↓ (14.9~4.3%), combustion stability ↑.	NH3 ↓ by the SCR.	[42]
NG/NH3	NG/NH3	6-cylinder turbocharged premixed	840 rpm λ (1.2~1.5) split ratios (0~50%)	λ is limited to 1.5; λ and split ratio of NH3 ↑, burn duration ↓.	fuel-NOx dominated the total emission; λ and split ratio of NH3 ↑, NOx ↑.	[45]
H2/NH3	Gasoline	CFR GDI	1200 rpm λ > 1 NH3/H2 share: 90/10	ITE ↑ by 0.5% as CR ↑ by 2.6.	NOx emission was 5500 ppm (CR = 8.9, λ = 1.3~1.4, H2/NH3 volume ratio = 70/30).	[46]
H2/NH3	H2	1-cylinder 4-stroke premixed	CR = 15 Ignition (−24 °CA to ~12 °CA) EGR rates (0~25%)	EGR rate helps to improve HRR, CP; best EGR rate = 7.5.	EGR ↑, NOx has a parabolic descent.	[57]
H2/NH3	H2/NH3	4-cylinder 4-stroke	CR = 10.7 1500 rpm λ = 1 XNH3 = 70% 60% throttle opening	Intake valve timing ↑, combustion stability ↓, °CA 0–10 ↑, MAXCOVP ↑ 25%, BTE ↓ (32.8~30%); Intake valve timing ↓ 25 °CA, IMEP ↑ 8% and BMEP ↑ 16%.	-	[58]
H2/NH3	H2/NH3	4-cylinder 4-stroke 4-load control strategies	CR = 10.7 1500 rpm NH3/H2 ratio (10–70%) λ = (1~1.7) VVT (−20 to −25) °CA	Air/fuel ratio: BTE ↑ to 40%; Throttle: the widest power range.	-	[59]
H2/NH3	H2	4-cylinder GDI	CR = 10.7 1500 rpm NH3 share (0–70%) λ (1~2)	BTE ↑ 37%; MAXCOVP is raised to nearly 9% when XNH3 is 70%; Peak BTE close to 39%, MAXCOVP below 4% when λ is 1.4.	NOx above 8000 ppm.	[60]
NH3	NH3	1-cylinder 4-spark plug GDI	CR = 10.5 Spark timing (−35, −30, −25, −20)	CP ↑; Combustion duration ↓; Combustion stability ↑; Combustion rate ↑; Engine power ↑.	T ↑, NOx ↑.	[69]
NH3	NH3	1-cylinder 4-stroke TJI	CR = 14 Intake temperature (300 K, 343 K)	Combustion stability ↑, in-cylinder pressure ↓, and the peak of HRR ↑.	Lower NO by TJI.	[70]
Gasoline/H2/NH3	Gasoline/ NH3	1-cylinder CFR engine 4-stroke GDI	CR = 10 1800 rpm mgasoline = 27.5 g/min mNH3 = 3.75~13.85 g/min	BP ↑~0.2 kW Brake Specific Energy Consumption ↓ 5 MJ/kWh.	NOx ↓ 25 g/kWh (mNH3 = 7.2 g/min) NH3 slip ↓ 83%. (mNH3 = 7.2 g/min).	[72]
H2/NH3	NH3	2-cylinder 4-stroke 2-valve	CR = 10.7 2500~3500 rpm Full throttle λ = 1 mH2 = 2 g/min mNH3 = 88.5–93.6 Nm3/h	BTE ↓ 3% (2500 rpm); BTE ↑ 1.5% (3500 rpm).	NOx ↓ 1000 ppm. High load NOx > half load NOx by 200 ppm.	[73]

↓ and ↑ are down and up, respectively.

.

In summary, the pathways to enhance combustion and reduce emissions in NH₃ SI engines are illustrated in Figure 9. The addition of other fuels results in a significant increase in flame propagation speed, a reduction in the flame stagnation time during the initial stage of combustion, and a notable improvement in BTE. However, this comes at the cost of a substantial increase in fuel–NO_x emissions. Altering the structure of an SI engine or adjusting fuel injection or ignition timing can effectively raise pressure and temperature in the cylinder, leading to a significant upward trend in engine BTE. Nevertheless, NO_x emissions and NH₃ slip are not satisfactorily addressed. The incorporation of auxiliary combustion technology proves effective in enhancing NH₃ combustion efficiency and consequently reducing NO_x emissions.

2.3. Exhaust Gas Aftertreatment to Minimize NO_x Emissions from SI Engines

For traditional fuel SI engines, NO_x emissions can meet relevant emission standards through some structural improvements and some operational strategy optimizations. For NH₃ fuel engines, fuel–NO_x is a major component of NO_x emissions. It is necessary to adopt after-treatment of emissions similar to SCR systems to make NO_x emissions meet emission standards [74]. Ammonia is a common reducing agent for NO_x, so the presence of unburned ammonia fuel in the exhaust is beneficial to reducing NO_x emissions. Studies show that rich conditions can lead to elevated ammonia levels, while nitrous oxide generation remains low in all circumstances [75]. In addition, in order to reduce the problem of low fuel utilization caused by rich combustion, Vitaly et al. used a lean SI engine to install an ammonia device that can store upstream generated ammonia on the original supporting aftertreatment system platform, thereby achieving the effect of reducing NO_x emissions [76].

When pure NH₃ or NH₃ mixed with other active fuels is burned, NO_x emissions increase significantly, but the advantage of ammonia as a nitrogen oxide reducing agent increases. Oh et al. [44] used the aftertreatment system of the original SI engine, including a selective catalytic reduction system and an ammonia oxidation catalyst, to study the characteristics of nitrogen oxides and residual ammonia before and after the exhaust flow through SCR under different air/fuel ratios and fuel mixing ratios, and found that as the ammonia energy fraction increased, the temperature during combustion decreased significantly. Although the fuel–NO_x emissions increased, the NO_x emissions after passing through the SCR system tended to be saturated. At the same time, it was found that the remaining ammonia can reduce the NO in the exhaust gas before passing through the SCR at a suitable temperature. When passing through the SCR system, the remaining ammonia will reduce the remaining NO again, and the final NO emission can be controlled within 10 ppm. Other NO_x emissions are less than hundreds of ppm. After the remaining ammonia acts as a NO_x-reducing agent repeatedly, the residual ammonia is within 10 ppm.

3. Ammonia Used in Compression Ignition Engines

The air injected into the cylinder undergoes compression to reach a specific temperature and pressure due to piston movement. Subsequently, fuels are introduced into the cylinder through atomization, ignition, and expansion [77]. For CI engines, the fuel must possess good auto-ignition performance and a short ignition delay period [78]. The challenging ignition characteristics of NH₃ make its normal operation in CI engines more difficult than in SI engines.

NH₃ has the most significant impact on improving the ignition delay period of diesel, while it has the smallest effect on improving P_max and efficiency. Several potential strategies for using NH₃ as a CI fuel include (a) blended fuel combustion [79], (b) high CR [80], and (c) injection strategies [81,82]. Overcoming the combustion challenges of NH₃ fuel often requires a combination of these measures. An effective strategy to enhance the combustion performance of NH₃ is to blend it with a more reactive fuel. Similar to SI engines, if the auto-ignition temperature of the fuel is higher than the cylinder temperature at the end of compression, it can be mixed with NH₃ fuel and injected into the combustion chamber of the CI engine to mitigate the defects in the physical characteristics of NH₃ fuel. The engine’s performance during combustion heavily depends on the cetane number (CN) of the mixed fuel, where a higher CN results in optimal performance with shorter ignition delays and optimum combustion efficiencies.

3.1. Mixing Fuels with Strong Activity

3.1.1. Combustion and Emission of NH₃/Diesel CI Engines

Regardless of whether NH₃ fuel is supplied to the engine in a liquid or gaseous state, it can meet typical engine load and thermal efficiency requirements through a blended fuel premix mode ignited by the pilot fuel [83]. The release of heat is faster in the case of liquid NH₃, leading to higher cylinder pressure. Compared to liquid NH₃, the HRR decreases by 50% and 25% and the pressure reduces by 6.5% and 11.5% under the energy supply of gaseous NH₃ at 100% and 80%, respectively. Unfortunately, NO_x emissions far exceed Tier III limits [83]. For achieving the same output power as diesel in CI engines, the intake manifold of a four-cylinder CI engine is slightly modified to be suitable for the direct NH₃/diesel injection strategy. As the X_NH3 increased, the CA corresponding to the P_max in the cylinder also increased while the peak value gradually decreased (from 9.5 to 7.5 MPa). Additionally, the HRR showed a significant increase (from 0.04 to 0.1), and the emission of NO under blended-fuel operation (X_NH3 < 40%) decreased significantly, resulting in lower thermal NO emissions but higher fuel–NO emissions (X_NH3 > 60%) [84]. The performance of a CI engine fueled by a NH₃–diesel blend as shown in Figure 10. By adopting a split diesel injection strategy (SODI), combustion thermal efficiency has been increased to 39.72% but also a resulting 83.5% decrease in unburned NH₃ emissions, surpassing that of pure diesel [78]. Leveraging the advantages of the split fuel injection strategy, optimizations such as SODI-pre and SODI-main can be implemented [85]. By reasonably controlling the injection time of diesel and NH₃, the NH₃ combustion speed could be accelerated, maintaining liquid NH₃ in a stable state throughout the entire combustion stage [86].

Achieving a high injection pressure is crucial to ensuring effective atomization of the fuel mixture [87]. Zhou et al. [88] developed a combustion model for a CI engine under the high-pressure direct fuel injection mode (HPDF). Compared to the commercial low-pressure, blended-fueled direct fuel injection mode (LPDF), this mode can reduce NO_x emissions by approximately 47% while maintaining the same thermal efficiency. Utilizing the same spray device, Zhu et al. [89] determined that the NH₃ addition threshold is unsuitable for exceeding 40%, considering the impact of NH₃ addition on ignition delay, premixing efficiency, and NO_x emissions [88]. CI ignited by the pilot fuels under two modes as shown in Figure 11.

Numerical simulation results have successfully validated the feasibility of the new high-temperature cylinder gas recirculation (HTCGR) technology [88]. The new HTCGR technology as shown in Figure 12. Consequently, it is crucial to investigate the impact of mixing combustion-generated water with NH₃ on engine combustion and emissions. Frost et al. [90] conducted combined numerical analysis and experimental research to explore the effects of different concentrations of NH₄OH (ammonium hydroxide) on CI combustion under various workloads. At an IMEP of 4 bar, although the ignition delay time is longer than that of pure diesel, NH₃ can be ignited and contribute 25% of the engine load. Increasing NH₄OH concentration increases NO_x emissions, as the elevated cylinder temperature during this stage is closely related to NO_x emissions. After continuously increasing NH₄OH concentration and contributing 20% to the engine load, NO_x levels decrease. Excessive NH₃ concentration increases combustion instability and lowers the in-cylinder temperature [91]. These findings align with results in the literature [92], where the addition of a NH₃ solution reduces combustion pressure and HRR, increases engine diameter and ignition delay, reduces engine performance, and significantly decreases NO_x emissions.

In addition to the NH₃ energy ratio, factors such as engine speed and load also influence engine combustion and emissions. Shahin et al. [93] studied the impact of NH₃ energy ratio, speed, and load on combustion and emissions. In the medium load range, total fuel consumption, as measured by specific fuel consumption, decreased with increasing load. Additionally, the presence of air fuel (NH₃ injection in the air) resulted in an increase in total fuel consumption under slightly higher load conditions, with a similar trend observed for NO_x emissions. Optimal outcomes regarding engine performance and exhaust emissions were observed for NH₃ fumigation at 2600 rpm. Under 8 Nm and 2600 rpm, the maximum effective efficiency increased by 15.05% with an air/fuel ratio of 8.5% [93]. A mixture of 25% NH₃ and 75% water solution was used and injected into the intake through a carburetor with an adjustable main nozzle. It was proven that NH₃ can reduce NO_x by more than 70% in the parameters analyzed, while the effect on brake-specific consumption is negligible. Compared to pure diesel CI, the mixture (with 10% H₂ content) increased peak pressure by 5.3% and maximum temperature by 5.7% [94]. Combustion and emission of a NH₃ CI engine with mixed fuel, as shown in

Table 4. Combustion and emission of a NH₃ CI engine with mixed fuel.

Test Fuel	Baseline	Power Equipment	Operating Conditions	Performance	Emissions	Ref.
NH3/diesel	diesel	4-cylinder Turbocharge direct injection	XNH3 (20~80%) constant power output operation	Combustion efficiency range of 91~97%; power output is same; HRR ↑ (0.04~0.1).	NH3/diesel (60%/40%), NH3 = 3200 ppm; NO ↓.	[84]
aqueous NH3/diesel	diesel	4-stroke 1-cylinder direct injection	CR = 15.5 100% load XNH3OH (40%, 50%, and 60%)	pressure ↓ (16.4%, 20.2%, 22.7%), peak value of HHR ↓ (29.8%, 45.6%, 64.9%), brake power ↓ (36.3%, 45.3%, 54.5%) at XNH3 ↓ (40%, 50%, 60%).	NOx ↓ 25%, 52.3%, and 61.8%, respectively.	[91]
aqueous NH3/diesel	diesel	1-cylinder 4-stroke direct injection	CR = 20 2200, 2600 and 3000 rpm XNH3 (2, 4, 6, 8, 10%) Engine loads of (2, 4, 6, and 8) Nm	Effective efficiency ↑ 15.05% (8.5% AF ratio, 8 Nm, 2600 rpm).	NOx emissions are about 1.5 times that of 4 Nm and 2.5 times that of 2 Nm.	[93]
NH3/DME	DME	1-cylinder 4-stroke Direct injection	CR = 20 XDME (100%, 80%, 60%, 40%) CAD (10~15, 10~20, 15~30)	P (XNH3 = 40%) is higher; the peaks of HRR increase with XNH3.	NOx of mixture (20% NH3, 80% DME) is higher.	[95]
NH3/DME	DME	1-cylinder 4-stroke High-pressure common rail direct injection	CR = 20 XDME (100%, 80%, 60%, 40%)	XNH3 ↑, injection timing ↑; engine performance decreases as XNH3 ↑.	NH3 concentration ↑, NOx emissions ↑, NH3 emissions ↑	[96]
NH3/H2(HCCI)	H2	1-cylinder 4-stroke	CR = 21.9 XNH3 (0 to 94%) ER (0.1 to 0.6) T (50 to 240 °C) EGR	XNH3 (0~94%), IMEP ↑ 67%; Pmax ↑ 10 bar/CAD	NOx ↑ significantly with the T from 1400 K to 1800 K.	[97]
NH3/H2	H2	4-cylinder 4-stroke direct injection	CR = 15.3 XNH3 (0~40%)	IMEP of mixture with NH3 reached 3.1 bar, 1.2 times pure H2.	NOx (750 and 2000 ppm), N2O ↑ under 1400 K.	[98]

↓ and ↑ are down and up respectively.

.

3.1.2. Combustion and Emission of NH₃/H₂ CI Engines

The higher CR and intake pressure work together to assist NH₃ combustion, reducing the required intake temperature, which is closely related to NO_x emissions, thereby maximizing IMEP and efficiency. However, strategies are needed to recover the loss of combustion efficiency and to avoid N₂O, which is prone to production below 1400 K [98]. To understand the feasibility of clean and efficient NH₃ combustion, the CR of the HCCI test stand was modified to 22:1, increasing IMEP by 67% with varying levels of X_NH3 from 0 to 95%. However, when the P_max exceeds 10 bar/CAD, heat and friction losses sharply rise, ultimately limiting IMEP, indicating that NH₃ inhibits the overall combustion rate [97]. By adjusting the CR and injection time of the NH₃/H₂-fueled CI engines, NO_x emissions can be brought into compliance with Tier III emission standards while ensuring that the engine maintains sufficient power [61]. The best injection timing combination in the engine not only has optimum power and economy but also allows the engine’s NO_x emissions to meet Tier II emission standards [99].

Building on these research results [61,99], Wang et al. [100,101] conducted a more detailed study on the influence of the NH₃/H₂ energy ratio on the pressure in the engine cylinder, power, and emissions. Mixing H₂ from 0 to 30% increased the cylinder explosion pressure by 10.7% and power by 1.8% while reducing the fuel consumption rate and NH₃ escape by 0.3% and 99.1%, respectively. Although NO_x emissions increased by 58.8%, the comprehensive performance of the CI engine after H₂ mixing combustion was improved. With the help of a small amount of diesel ignition, the engine adopted a direct injection method, and when using a mixture of NH₃ and H₂ as fuel, the optimal X_H2 was found to be 30% [101]. Pathways to promote combustion and emission reduction in NH₃ CI engines as shown in Figure 13.

Due to the advantage of faster transmission speed that NH₃/H₂ combustion possesses and considering the convenience of energy transportation, NH₃ decomposition mixed with H₂ is an important measure to reduce the difficulty of H₂ storage and transportation. Gill et al. [103] conducted experiments to study the combustion and emission properties of various fuel types, including diesel, pure NH₃, pure H₂, NH₃/H₂/N₂ (1%, 75%, 24%), and NH₃/H₂/N₂ (2%, 75%, 23%). The results showed that incorporating more H₂ and NH₃ had a negative impact on BTE at both loads compared to pure diesel. At high loads, BTE with pure NH₃ was superior to BTE with added decomposition H₂, which was similar at low loads. Upon reducing the cylinder pressure from 5 bar to 3 bar, the total COV decreased by approximately 50%, BTE decreased by around 26%, and emissions of NO_x increased by about 27% [103].

3.1.3. Combustion and Emission of NH₃/DME CI Engines

Dimethyl ether (DME) shares physical properties with liquefied petroleum gas, featuring a high CN and low ignition temperature [104]. When mixed with other fuels, DME can enhance the combustion of low-activity fuels [105]. The addition of DME increases the reactivity of NH₃ and reduces ignition delay times, although this influence decreases with increasing DME, temperature, and pressure. This promotional effect is attributed to the increased formation rate and concentration of free radicals, especially OH [95].

Despite the inevitable increase in NO_x emissions due to the nitrogen bound in NH₃, improvements can be achieved under the same operating conditions by increasing the injection pressure. Using X_NH3 (20% and 40%) results in an extended ignition delay time due to the elevated auto-ignition temperature and sluggish flame propagation speed, limiting the engine’s load output. Higher NH₃ concentrations, coupled with higher injection pressures, can enhance the fuel–air mixture for better combustion and achieve a higher engine load output [106]. Given the high auto-ignition temperature of NH₃, the optimal injection time needs to be advanced with the increase in X_NH3. The suitable injection timing range for a DME/NH₃ blend (40%/60%) was determined to be 90 to 340 °CA before top dead center (BTDC). This advanced injection timing resulted in the engine displaying HCCI combustion characteristics [96]. Combustion and emission of NH₃ CI engines assisted by changing the structure and strategy as shown in

Table 5. Combustion and emission of NH₃ CI engines assisted by changing the structure and strategy.

Test Fuel	Baseline	Power Equipment	Operating Conditions	Performance	Emissions	Ref.
NH3/Algal biodiesel	Algal biodiesel	1-cylinder 4-stroke	CR = 17.5 XNH3 (20%, 30%, 40%, and 50%) load (20%, 40%, 60%, 80%, and 100%)	BTE were 34.1%, 35.4%, 36.7%, 33%, and 30.9% with different AEF.	NOx ↓ 36.19%, 42.65%, 50.05%, and 52.64% with different AEF.	[54]
NH3 (HTCGR)	diesel	6-cylinder 2-stroke	CR (20.5, 25, 50, 75) load (100%)	Thermal efficiency can approach 50%, heat transfer loss is 3.4 times lower than in the original mode.	T ↑, P ↑, NOx ↑	[81]
NH3/H2	NH3/H2	1-cylinder 4-stroke High-pressure common direct injection	CR (13.5 to 16.5) injection timing from 12 to 24 (°CA BTDC) NH3/H2 (7:3)	Delayed the injection timing, the ignition delay period ↓. CR increases by 1, hot zone appears 2 °CA ahead of time, and the area ↑.	At 12 °CA BTDC and CR = 13.5, the NOx emissions suit Tier III.	[61]
NH3/diesel (liquid)	NH3/diesel (gaseous)	2-stroke premixed combustion	CR = 12 124 rpm Pilot injection timing = −8 °CA ATDC	HRR ↑, P ↑; HRR ↑ 33%, 25%, P ↑ 11%, 18%, at 100% and 80% NH3 energy supply, respectively.	NOx emissions suit Tier II but not Tier III.	[83]
NH3/diesel	diesel	1-cylinder direct injection 4-stroke diesel pilot fuel	CR = 17.7 XNH3 (35%) DEG = 33 °CA	Total efficiency ↓ 3.4%; 1.6 kW.	NOx ↑ 7 times.	[107]
NH3/H2	NH3/H2	2-stroke pre-chambers dual fuel injection	CR = 12 H2 injection time (346 to 355 °CA) spark ignition time (348.5 to 354.5 °CA)	Achieving the full-load power output.	NOx emissions suit Tier III.	[108]
NH3/DME	DME	1-cylinder direct injection 4-stroke high-pressure common rail	CR = 20 engine speed (2200 to 3600 rpm)	Better combustion and higher workloads.	NOx emissions at ppm level.	[106]
NH3/H2/diesel	NH3/H2	1-cylinder 4-stroke high-pressure common rail	CR = 13.5 1800 rpm load (100%) the NH3/H2 mixing ratio is 7:3	P ↓ 5.71 MPa, engine power ↓ 2.59% under D708/A/H708.	NOx emissions suit Tier III.	[99]

↓ and ↑ are down and up respectively.

.

3.2. Exhaust Gas Aftertreatment to Minimize NO_x Emissions from CI Engines

To address the NO_x emission issue in NH₃-fueled CI engines, the use of NH₃ for the selective catalytic reduction (SCR) of NO_x is prevalent. For safety considerations, vehicle aqueous urea was injected into the aftertreatment device as an NH₃ carrier. However, the uneven distribution of aqueous urea can impact catalytic efficiency. In this context, researchers compared the impact of pressure-driven ejectors and air-assisted ejectors on the uneven distribution of aqueous urea [109]. The results indicate that air-assisted injection has clear advantages in both droplet size and injection speed. However, under the influence of airflow, aqueous urea is prone to solidify. In response, researchers designed I-type and L-type ejectors, influencing the amount and uniformity of NH₃ injection, respectively [110].

To enhance the mixing ratio of vehicle urea and exhaust gas and improve volume utilization, the influence of mixing chambers and swirl mixers with NO_x conversion was compared [111]. This comparison found that the NO_x conversion efficiency can be increased by 61.3%, 76.2%, and 86.3%, respectively, by using the mixing chamber, the swirl mixer, and a combination of the two [111]. Ko et al. [112] observed that NH₃ slip is inevitable in the process of NO_x conversion, especially during engine start, where it is challenging to eliminate NH₃ slip. However, Kuta et al. [92] discovered that NH₃ slip provides the possibility to re-activate the SCR system and improve its effectiveness under different engine operating conditions. The key factor affecting effectiveness is temperature, which can be easily controlled by modifying the structure of the exhaust device. SCR systems as shown in Figure 14.

4. Ammonia Used in Gas Turbines

The gas turbine (GT) is an internal combustion power machine that utilizes high-pressure air compressed by the compressor and fuel injected into the combustion chamber. The mixture of air and fuel ignites, creating high-temperature gas. Subsequently, this gas expands, driving the turbine to work externally. Combustion stability is achieved when the flame propagation speed matches the mixture flow velocity. The GT imposes strict requirements on fuel performance due to its high inlet flow rate, which is essential for maintaining stable combustion.

The combustion chamber of the GT exhibits distinctive features. These include high temperatures, elevated airflow velocity (leading to unstable flames that are prone to extinguishing, making combustion completion challenging), heightened combustion intensity (requiring a large fuel supply per unit volume and high combustion speed, necessitating a uniform outlet temperature field), and a high excess coefficient. The latter is particularly notable in areas of poor combustion, where working conditions are variable. These characteristics are illustrated in Figure 15.

Verkamp et al. [114] experimentally studied the performance of the burner, minimum ignition energy, quenching distance, flame stability limit, and other physical properties of a NH₃/air mixture in a gas turbine burner. According to the existing combustion characteristics, it is predicted that pure NH₃ cannot be used as an alternative fuel for fueling GT power devices [115] unless the energy of the ignition system is increased and the injection strategy is optimized. To understand the NH₃ spray characteristics more comprehensively, Li et al. [115] established the relationship between superheat, fuel viscosity, temperature, ambient density, and NH₃ spray characteristics in a constant volume chamber. These experimental data will provide basic information for understanding NH₃ combustion. Based on the existing models of NH₃ sprays, An et al. [116] compared the simulation results with the recently published experimental data of NH₃ flash evaporation spray vaporization and discussed the effectiveness of the liquid NH₃ phase transition model concerning penetration length, diameter, and spray morphology. Through model evaluation, it was found that combining and optimizing multiple evaporation models to match the actual evaporation characteristics of liquid NH₃ would be an effective pathway.

4.1. Mixing Fuels with Strong Activity

4.1.1. Mixing NH₃ with CH₄

The low heat value and flame propagation speed of CH₄ are higher than those of NH₃. Adding CH₄ proves advantageous in enhancing the combustion efficiency of NH₃ [117]. Zhang et al. [118] discovered that incorporating CH₄ can broaden the ignition threshold of NH₃ flames. Additionally, the latent heat of CH₄ evaporation is lower than that of NH₃. The introduction of CH₄ helps mitigate the inhomogeneity of fuel droplets, reducing heat transfer and minimizing NO_x emissions and NH₃ slip [119]. Consequently, increasing the inlet temperature of CH₄/NH₃ mixed fuel and minimizing heat loss from the combustion chamber wall can compensate for fuel droplet spray-induced heat transfer losses. This improves droplet spray combustion characteristics and further reduces pollutant emissions [120]. The GT operated stably until X_NH3 reached 63% [121]. Beyond 22% X_NH3, a noticeable drop in combustion efficiency and thermal efficiency occurred. A comparison between the NH₃ flame and the mixed fuel flame containing 10% CH₄, with different equivalence ratios, is illustrated in Figure 16.

For the multi-stage swirl combustor, an equivalence ratio of 1.2 is optimal for controlling minimum NO and NH₃ emissions [122]. However, for a single-stage swirl combustor, the optimal equivalence ratio differs. Considering the relationship between NO_x emissions and unburned fuel, an equivalence ratio of 1.06 is optimal when X_NH3 is 70% [123]. Irrespective of the NH₃ fraction, an equivalence ratio slightly greater than 1.05 is optimal for lower NO [124,125].

Figure 16. Comparison of the NH₃ flame and the mixed fuel flame containing 10% CH₄ with different equivalence ratios [126].

Figure 16. Comparison of the NH₃ flame and the mixed fuel flame containing 10% CH₄ with different equivalence ratios [126].

4.1.2. Mixing NH₃ with H₂

It has been demonstrated that the combustion intensity of NH₃/air can be enhanced by incorporating active fuels such as CH₄ and H₂. When a GT was fueled by NH₃/H₂/air and CH₄/air, it became evident that the GT fueled by NH₃/H₂/air exhibited superior performance, with a 37% higher efficiency. However, this improvement comes at the cost of unsatisfactory NO_x emissions. Zhang et al. [126] delved into the impact of CH₄ and H₂ on the emission characteristics of an NH₃-fueled GT burner. At an equivalence ratio of 0.1, whether it was NH₃/H₂, NH₃/CH₄, or NH₃ alone, the variations in NO and NO₂ were similar [126]. As the mixing ratio exceeded 0.1, owing to the higher combustion temperature, NH₃/CH₄ demonstrated notable performance in NO_x emissions. Utilizing the dynamic thickened flame model, it was verified that NH₃ co-fired with 10% H₂ is a promising mixture capable of enhancing combustion stability and maintaining NO_x emissions within a controllable threshold [127]. However, focusing on NO_x reduction alone results in lower efficiencies than CH₄, emphasizing the urgent need for new injection and ignition technologies to simultaneously reduce nitrogen oxides and unburned NH₃ emissions while maintaining power outputs [128]. Combustion techniques and combustor design for efficient combustion and low NO_x emission from an ammonia MGT swirl combustor are proposed, NO_x emission of 42 ppm was achieved [129]. Basic research parameters of a GT as shown in Figure 17.

To achieve high efficiency, fuel steam is directly introduced into the combustion chamber. Combined with other energy-saving technologies, this approach can achieve a total efficiency of 59%, while emissions still comply with European NO_x thresholds [130]. Lean premixed combustion technology, along with swirl-induced flame stabilization, represents one method for reducing NO_x emissions in traditional GTs. This method demonstrates that NO_x emissions remain below 30 ppm even when X_NH3 reaches 40%, using air-staged combustion fueled by the NH₃/CH₄ mixture [131].

In the pursuit of understanding the NO_x emission mechanism, researchers conducted studies on chemical kinetics models, incorporating actual NO_x emission tests to validate and refine the model parameters, ensuring the reliability of model simulation. Xiao et al. [132] compared five different chemical kinetics models to evaluate their accuracy in representing reaction dynamics in real GT burner conditions. These chemical kinetic models can simulate and predict the chemical pathway of CH₄ as an NH₃ combustion enhancer and the key reactions leading to NO_x emissions. Through the analysis of the chemical reaction pathway, it was observed that the addition of CH₄ promoted the formation of OH radicals, subsequently enhancing NH₃ conversion. Even in mixtures with lower NH₃ content, free radicals generated in the CH₄ oxidation pathway contributed to the formation of NO_x [133].

Various combustion mechanisms suggest that during NH₃/H₂ premixed combustion, NO is primarily reduced to NNH [134]. When burning the NH₃/H₂ mixture mixed with steam, the decomposition of H₂O promotes significant H, O, and OH radicals. While this hinders the formation of NO, the increased presence of OH radicals promotes the formation of NH₂, subsequently leading to the formation of NO [135]. These findings have been confirmed in [128] on NH₃/H₂ lean premixed combustion in a swirl GT combustor. The change in NO_x emissions aligns with the concentration pattern of OH radicals in the combustion chamber, underscoring the significance of OH radicals in fuel NO_x [136]. The increase in pressure enhances the reaction of NH + NO = N₂O + H, with higher pressure resulting in a lower NO fraction [137].

With a 40% increase in NH₃ addition, the OH concentration consistently follows the temperature trend [138]. Analyzing the NO reaction pathway influenced by temperature reveals that the main factor affecting NO formation in the flame zone is HNO free radicals, while the primary factor influencing NO reduction in the post-flame zone is NHi free radicals [138]. The addition of H₂ increases the concentrations of H, NH, and NH₂ at the flame root, leading to increased NO consumption [127].

4.2. Structure Adjustment and Auxiliary Combustion Measures

Due to the significant advantages of plasma in enhancing the NH₃ decomposition rate and laminar combustion rate, as illustrated in Figure 18, plasma-assisted combustion strategies are extensively employed in GT ignition systems [139]. Scientists utilize nanosecond plasma discharge to increase pulse energy density and have observed that the flame combustion rate (X_H2 = 12%) remains consistent with conventional fuel [133].

To effectively reduce NO_x emissions, it is recommended to employ the non-premixed injection mode and maintain an equivalence ratio of 0.9 to 1.2 when operating with a higher proportion of NH₃ in the fuel mixture [140]. Kurata et al. [113] successfully developed a non-premixed combustion chamber suitable for NH₃ combustion with low NO_x emissions, capable of achieving a power output of up to 40 kW under pure NH₃ combustion conditions, effectively demonstrating the potential of NH₃ combustion to achieve dual carbon targets [141]. The lower swirl combustion burner exhibited a wider uniform reaction area, fewer hot spots in the reaction zone, and a more uniform temperature distribution. The maximum NO_x concentration in the reaction zone with the higher swirl combustion burner was twice that of the lower swirl combustion burner [142].

Although a larger equivalence ratio contributes to lower NO_x emissions, it also compromises the overall efficiency of the GT. Okafor et al. [121] determined that the optimal primary combustion zone for low NO_x emissions corresponds to an equivalence ratio ranging from 1.30 to 1.35, depending on the ammonia mole fraction (X_NH3). Lean-rich combustion of CH₄/NH₃ mixtures yielded lower NO_x emissions compared to pure NH₃ combustion due to the faster flame speed of CH₄/NH₃ mixtures, which reduced NO_x formation in the secondary combustion zone [121]. Somarathne et al. [119] conducted a comparative analysis of combustion and emission characteristics of pure NH₃ and CH₄ in a combustor designed for a rich-lean GT. NO concentration was primarily influenced by local temperature. While cooling the combustor wall considerably impacted NH₃ oxidation and NH₃ slip, it also reduced NO emissions by lowering the concentration of OH radicals in the flame zone due to wall heat losses [119]. Lean premixed flames, a promising technique for hydrocarbon combustion, effectively comply with strict legislative regulations on emissions of combustion pollutants in GT combustion but lead to combustion instability. Lean-rich combustion systems can achieve minimal NO levels while producing zero unburnt NH₃ emissions [113]. NO_x emissions are significantly influenced by the mixture stoichiometry [143]. Moreover, with increasing X_NH3, the decrease in NO_x emissions becomes more pronounced as the equivalence ratio increases [129]. In summary, Figure 19 illustrates the technical approach for improving the combustion and emission performance of NH₃-fueled GTs. NO_x emissions and NH₃ slip can be effectively reduced by optimizing the structure and strategy and incorporating auxiliary combustion measures. While blending highly active fuels can improve thermal efficiency, it is relatively easy to increase NO_x emissions.

5. NH₃ Fuel Cells

5.1. Application of NH₃ in FCs

The breakthrough in the research and development of NH₃ FCs can be attributed to the large-scale production of fuel, application processes that significantly reduce carbon emissions, and other advantages [10,144]. NH₃ FCs are classified based on different types of fuel supply and electrolytes [145], as illustrated in Figure 20. In the pursuit of higher power density, scholars have conducted studies on factors influencing the performance of NH₃ FCs, including reaction temperature, electrolyte thickness, electrolyte type, and electrode type. The research results on the performance of NH₃ FCs are combined, and different power segments are depicted, in Figure 21 [146].

According to Figure 21, the solid oxide fuel cell (SOFC) fueled with NH₃ demonstrates superior power output compared to the alkaline electrolyte fuel cell (AEFC). The highest power density reported for AEFC in [142] is only 467 mW/cm², and the volume power density is a mere 8 kW/m³ [147].

Indirect ammonia FCs (IA-FCs) utilize H₂ derived from the thermal decomposition of NH₃ as fuel, offering a wider range of applications. Researchers have conducted preliminary studies on the performance of IA-FCs [147]. Although indirect ammonia–proton exchange membrane fuel cells (IA-PEMFC) exhibit slightly lower efficiency, they boast remarkable power density, with a volume power density approximately 15.5 times that of IA-AEFCs and 12.2 times that of SOFCs fueled with NH₃ [146].

5.2. Research Routes of IA-PEMFCs

Research on IA-PEMFCs is progressively deepening, with three main research routes. Firstly, the issue of heat supply through electric heating emerges as a stable and easily adjustable heating technique [148]. However, the use of the end product, electricity, may inevitably reduce the overall power generation efficiency for FCs [149]. This method is primarily employed to provide a stable heat source for verifying the effectiveness of catalysts (such as zeolite-based [150], Mo2N [151], Ru-Al monoliths [152], nickel-based [153], and Ce-doped [154]) and other improvement measures for FCs [153]. Catalytic combustion, a flameless combustion mode, offers advantages such as high combustion efficiency, a stable and controllable process, and relatively uniform and stable combustion. The catalytic combustion heating mode can decompose NH₃ to form an ideal exothermic and endothermic coupling system with high volume utilization efficiency. Currently, NH₃ catalytic combustion is primarily utilized in power equipment with small power and strict volume requirements. Ongoing research focuses on catalysts to enhance catalytic combustion efficiency (including Mn/Bi/Al catalysts [155], platinum [156], Ru/Al₂O₃ [157], and Pt gauzes/oxide monolithic layers [158]) and reactor structure [159]. Flame combustion, on the other hand, is a simple, direct, and rough heating method with high combustion temperature but low combustion efficiency. Current research on NH₃ flame combustion primarily concentrates on the stability of NH₃ combustion (including pure NH₃ combustion [55,160,161], mixed combustion [162,163,164], and auxiliary combustion [31,165]) and reducing NO_x emissions [166,167,168,169]. The chemical reaction process in direct NH₃ FCs and IA-PEMFCs is illustrated in Figure 22.

The second research route involves purifying the gas resulting from NH₃ decomposition. In an ideal state, the gas would be a 1:3 mixture of N₂ and H₂. However, achieving a 100% NH₃ decomposition rate is highly unlikely, and the gas will inevitably contain some undecomposed NH₃. Additionally, NH₃ decomposition gas contains 25% N₂, and the presence of N₂ is ineffective, further reducing the mass transfer effect [170,171]. However, PEMFCs are highly sensitive to NH₃ content, which needs to be below 0.1 ppm for optimal performance [172]. Therefore, to improve the service life and power generation efficiency of subsequent FCs, the purification of NH₃ decomposition gas is of great significance [173]. Currently, the purification of NH₃ decomposition gas is primarily studied in H₂-selective membranes [174], membrane reactors [175,176,177], and the liquefaction of N₂ [178].

The third research route involves developing a FC structure suitable for the mixture of N₂ and H₂. Zhao et al. [179] demonstrated through experiments that it was challenging for an air-cooled H₂–air PEMFC to function properly when an N₂ and H₂ mixture was used as the anode fuel. Subsequent research by Zhao et al. [180] indicated that shortening the flow channel length could enhance the stability of PEMFCs using a N₂ and H₂ mixture as fuel, as revealed through simulation analysis. Building on this work, Hu et al. [181] further investigated the water–heat–gas distribution of NH₃/H₂ PEMFCs under parallel flow channels. They found that fuel dead zones tended to appear at the tail of long parallel flow channels, impacting the normal operation of the FC. Considering the mass transfer characteristics of the N₂/H₂ mixture, Hu et al. [147] employed biomimicry to construct a hedge-type triangular flow channel, optimizing it for N₂/H₂ mixture compatibility and determining size parameters for optimal output performance. These studies demonstrate that, by employing flow channel optimization and other measures, NH₃ decomposition gas can be directly injected into PEMFCs after removing NH₃, resulting in higher power density than conventional FCs.

Hunter et al. [182] explored the system efficiency of IA-PEMFCs by integrating an NH₃ cracking system, a purification unit, and a small PEMFC. The integration of a complete system for NH₃ decomposition and NH₃ decomposition gas purification, based on light metal amide imide with a 100 W PEMFC, is a significant milestone for the promotion and application of indirect NH₃ FCs. Ye et al. [148] used the heating method of NH₃/H₂ mixed flame combustion to heat NH₃ decomposition gas and added a NO_x emission reduction treatment device to analyze the fuel utilization efficiency of the indirect NH₃ FC system. Compared with conventional fuel-oil engines, the fuel utilization efficiency of the IA-PEMFC system is higher than that of water taxis but lower than cargo ships [148]. Lin et al. [183] categorized the heat sources needed for NH₃ decomposition in the IA-PEMFC system into three heating modes: electric heating, N₂/H₂ flame combustion, and PEMFC anode tail gas combustion. They then compared and analyzed the system efficiency under each mode. Among them, the IA-PEMFC system, where the combustion of NH₃ decomposition gas provided heat for NH₃ decomposition, had the highest efficiency, accounting for 56.6%. Building on the IA-PEMFC system, Zhao et al. [184] established a complete system that burned the anode tail gas of PEMFC to supply the heat required by the decomposer. When the heat from firing the anode tail gas is insufficient, fueling the NH₃ gas to supply the required heat to the decomposer becomes a compensatory strategy. The system’s energy efficiency is found to be 45% through simulation analysis.

The optimization results of system efficiency in the IA-PEMFC system above are inevitably different, as shown in Figure 23, due to the varying catalyst activity, catalyst dose, regenerator efficiency, and other parameters used in the experiment and simulation. However, the successful integration of the IA-PEMFC system above fully demonstrates the feasibility of the system. This paves the way for the future advancement of IA-PEMFCs.

5.3. NO_x Emission and NH₃ Slip in IA-PEMFCs

NH₃, while not classified as a greenhouse gas, is considered toxic. Therefore, NH₃ slip serves as an indicator of harmful emissions. NH₃ slip primarily occurs in direct NH₃ FCs and can be mitigated by factors such as purge strategy optimization of the flow channel structure, catalyst selection, or operating conditions. Additionally, water generation at the anode aids in NH₃ absorption. Indirect NH₃ FCs primarily emit NO_x, mainly due to intermediate products generated by catalytic and flame combustion. Unlike GTs, which have a turbulent swirl parameter design, SI engines, which involve ignition time and flame diffusion combustion, or CI engines, which involve ignition time and premix combustion, NH₃ flame combustion in IA-PEMFCs focuses primarily on flame stability. The relatively stable combustion environment and simple, controllable flame combustion in IA-PEMFCs necessitate NO_x emission reduction measures distinct from those employed in other power equipment. Typically, slightly richer combustion conditions [185] or higher initial pressure [186] can effectively suppress NO_x production in NH₃-fueled combustion systems. Adding H₂ can significantly enhance NH₃ fuel combustion speed and improve the HRR to meet the heat requirement for NH₃ decomposition [187]. However, the addition of H₂ also leads to increased NO_x emissions. Therefore, further research is warranted to investigate the interaction between NH₃ and NO in the reduction region under humidification conditions [188]. The reaction of O + H₂O = 2 OH reduces the concentration of O free radicals, limiting the reaction rate of N₂ + O = NO + N [189]. Plasma ignition can effectively control NO_x emissions while enhancing combustion intensity. This is attributed to the ability of plasma ignition to generate OH radicals, reduce flame stagnant time, hinder the NO_x generation pathway, and accelerate the rate of NH₃ combustion, directly generating N₂ [190].

Under ideal conditions, the products of NH₃ catalytic combustion are nitrogen and water. However, other reaction pathways may occur in the actual catalytic oxidation process, leading to the generation of NO_x and NH₃ slip. Wickham et al. [191] developed a selective catalyst combined with surface oxygen atom adsorption treatment technology, promoting the efficient and selective decomposition of NH₃ into N₂ and water. Noskov et al. [155] found that the Mn/Bi/Al catalyst exhibited a high selectivity to NO_x, reaching up to 88% in a fluidized bed reactor test. In an oxygen/steam environment, Warner et al. [156] investigated 95% Pt/5% Rh gauze. Based on the influence of temperature and equivalence ratio on NO_x formation, a selective formation mechanism model of NO_x in the catalytic oxidation process of NH₃ was established. This model provides a pathway for the selective formation of NO_x in later catalytic oxidation processes [156,158]. To reduce NO_x emissions, lowering the reactor temperature, utilizing N₂ selective catalysts (such as Pt/Al₂O₃), or employing SCR technology are considered effective measures [192].

6. NO_x Generation Mechanism Related to Ammonia Combustion

The mechanism of NO_x formation during ammonia oxidation under low- and high-temperature conditions is shown in Figure 24 (left). Compared to low-temperature conditions, under high-temperature conditions, NH₂ generates a new product, NH, under the action of OH radicals, which are reduced to NO through O₂. At the same time, the new intermediate product H₂NH increases the pathway of NO₂ generation [193]. The changes in ammonia oxidation under different air/fuel ratios at the same temperature are shown in Figure 24 (right). The air/fuel ratio only affects the conversion rate of various products and does not affect the types of products produced during the ammonia oxidation process. From Figure 24 (right)., the air/fuel ratio is increased to 0.9, and the most significant factors affecting the conversion rate are NNH→HNO, NH₂→HNO, and NO→NO₂, which in turn affect the amount of NO_x generated.

Figure 25 shows the simulation results of the ammonia oxidation process, producing NO through two commonly used reaction mechanisms, Mei and CEU-NH₃, under different combustion pressures. It can be seen from the figure that different reaction mechanism models correspond to different ammonia oxidation processes. In addition, all reactions involved in the oxidation of ammonia to NO gradually strengthen with the increase in combustion pressure, and the effect is significant.

When burning an ammonia–hydrogen mixture, the mechanism of NO_x generation with different energy ratios is shown in Figure 26. Changing the energy ratio of the mixed fuel not only affects the types of intermediate products but also affects the reaction pathways of different reactants. Among them, the newly added reactions are R178, R127, R173, R103, R123, R51, R50, and R105, and the more sensitive reactions were R20, R29, R53, and R49. When methane, DME, and traditional fuel are mixed with ammonia, their own products do not participate in the ammonia oxidation process. Therefore, when these fuels are mixed with ammonia, the actual influence is the ammonia oxidation reaction temperature and combustion chamber pressure, air/fuel ratio, energy ratio, etc. The influence of these factors on ammonia oxidation has been described above.

7. Conclusions

Based on the difference in the structure and working principle of different power equipment, this paper delves into the combustion and NO_x emission characteristics of NH₃ fuel in various power equipment, including SI, CI, and GT, from the perspectives of mixed fuel types and modes, structural design and operating strategies, and auxiliary combustion techniques. It also summarizes the current status of NH₃-FC applications and elaborates on the different heat sources and NO_x emissions associated with IA-PEMFCs.

With the comprehensive combination of mixed fuel, structural adjustment, auxiliary combustion, and other technologies, NH₃ fuel can burn stably in ICE, and the power and NO_x emissions are within the acceptable range. In addition to NH₃/H₂, NH₃/CH₄ and its mixture with gasoline can also be used in SI, NH₃/DME and its mixture with diesel can also be used in CI, and NH₃/CH₄ and its mixture with kerosene can also be used in GT. Compared with CI, when SI uses a NH₃/H₂ mixture for combustion, due to the controllable CR and ignition energy, the amount of NH₃ can be added in a wide range, while when NH₃/H₂ is used in GTs for combustion, the amount of NH₃ added is not more than 55%. In addition, beyond 22% X_NH3, a noticeable drop in combustion efficiency and thermal efficiency occurred, while the GT operated stably until X_NH3 reached 63%.

Due to the high autogenous ignition temperature of NH₃ fuel and the structural differences between SI and CI, the allowable CR of SI and CI are not more than 15 and 21.9, respectively. In addition, the combustion characteristics of NH₃ fuel in SI can be improved by supercharging, increasing CR, prolonging ignition time, increasing the number of spark plugs, and other structure adjustment technologies, as well as the use of EGR, preheating, NH₃ cracking, and other auxiliary combustion technologies. CI uses multiple fuel strategies and split diesel injection, while GT generally adopts structural strategies such as optimizing cyclones, non-premixed combustion, plasma discharge devices, and auxiliary combustion technologies such as rich lean combustion and secondary air supply.

Compared with DAFC systems, IAFC is the best option for train powertrains when considering storage space and fuel costs, but the high technology requirements of the purifier pose a challenge. Benefiting from the operating high temperature of DAFC, reducing the NH₃ cracker and purifier of DAFC systems is still at the lab level.

Emission reduction technologies are mainly divided into SCR technology, staged combustion technology, and medium–low oxygen combustion technology. The research on SCR Technology is that the chemical adsorption of functional groups on the surface of carbon-based materials should be further studied due to the influence of other pollutants.