Ammonia-Based Clean Energy Systems: A Review of Recent Progress and Key Challenges

Abstract

Ammonia is gaining increasing attention as a zero-carbon fuel and hydrogen carrier, offering high energy density, mature liquefaction infrastructure, and strong compatibility with existing energy systems. This review presents a comprehensive summary of the recent advances in ammonia-based clean energy systems. It covers the fuel’s physicochemical properties, green synthesis pathways, storage and transport technologies, combustion behavior, NO_X formation mechanisms, emission control strategies, and safety considerations. Co-firing approaches with hydrogen, methane, coal, and DME are evaluated to address ammonia’s low reactivity and narrow flammability limits. This paper further reviews engineering applications across power generation, maritime propulsion, and long-duration energy storage, drawing insights from current demonstration projects. Key technical barriers—including ignition delay, NO_X emissions, ammonia slip, and economic feasibility—are critically examined. Finally, future development trends are discussed, highlighting the importance of integrated system design, low-NO_X combustor development, solid-state storage materials, and supportive policy frameworks. Ammonia is expected to serve as a strategic energy vector bridging green hydrogen production with zero-carbon end-use, facilitating the transition to a sustainable, secure, and flexible energy future.

1. Introduction

In response to the escalating global imperative for decarbonization, energy systems are undergoing a profound transformation, marked by an accelerated transition toward low- and zero-carbon alternatives. Ammonia (NH₃) has attracted considerable attention as a promising carbon-free fuel and hydrogen carrier, owing to its high energy density, mature liquefaction and storage technologies, and strong compatibility with existing energy infrastructure [1]. Recently, ammonia has evolved beyond its conventional role as a chemical feedstock and fertilizer, emerging as a versatile energy vector with broad potential applications in power generation, maritime propulsion, and long-duration energy storage [2,3]. Compared to hydrogen, ammonia provides distinct advantages in volumetric energy density, transportability, and storage safety, rendering it especially attractive for maritime shipping and long-distance energy transmission [4]. Furthermore, utilizing renewable electricity to produce hydrogen via electrolysis—subsequently combined with nitrogen to form carbon-neutral “green ammonia”—offers a low-emission lifecycle and a feasible pathway toward an integrated hydrogen–ammonia energy system [5]. However, the practical engineering application of ammonia fuel still faces multiple key challenges, including low reaction activity, long ignition delay time, slow flame propagation speed, and narrow flammability range [6]. In addition, the emission control, safety leak monitoring, and toxicity prevention of NO_X and N₂O are also technical bottlenecks for the promotion of ammonia fuel [7].

In recent years, research efforts have increasingly concentrated on various aspects of ammonia utilization, including its synthesis processes, combustion enhancement techniques, co-combustion strategies with hydrogen or methane, and the development of integrated ammonia-based energy systems [8]. However, a comprehensive review that holistically addresses the ammonia energy pathway from a full energy system value-chain perspective remains lacking. This review therefore aims to systematically examine the full lifecycle trajectory of ammonia in clean energy systems, encompassing its physicochemical characteristics, green synthesis pathways, storage and transport infrastructures, combustion behavior, emission mitigation strategies, and representative applications. Furthermore, this study highlights the current technical bottlenecks and delineates future research priorities, with the aim of offering theoretical foundations to guide the system integration and engineering deployment of ammonia-based clean energy solutions.

2. Characteristics, Synthesis, and Storage and Transport of Ammonia Fuel

2.1. Physicochemical Properties of Ammonia

NH₃ is a colorless gas with a pungent odor, a molecular weight of 17.03 g/mol, and a density lower than that of air. It boils at −33.4 °C under standard atmospheric pressure. It readily liquefies under moderate pressure or low temperatures, enabling large-scale storage and transportation. Typically, ammonia liquefies at temperatures below −33.4 °C under atmospheric pressure or at pressures around 1.0 MPa at ambient temperature, which fall within the typical definitions of low temperature and moderate pressure, respectively. Among various prospective clean fuels, ammonia stands out for its combined advantages in volumetric energy density and manageable storage conditions. At ambient pressure, liquid ammonia exhibits an energy density of approximately 11.5 MJ/L, significantly exceeding that of gaseous hydrogen at standard conditions (~0.012 MJ/L) and even that of compressed liquid hydrogen (10.05 MJ/L), underscoring its superior volumetric energy storage potential [9]. Compared to other hydrogen carriers—such as methanol, metal hydrides, and liquid organic hydrogen carriers—ammonia allows for high-density hydrogen storage and transport under ambient or moderately elevated conditions, positioning it as a viable alternative in the green hydrogen economy. As shown in Figure 1, NH₃ achieves a relatively high volumetric hydrogen density under conditions of 1 MPa and 298 K, ranking it among the most promising liquid and solid hydrogen storage media. This property not only supports efficient storage and transportation at industrial scales but also underpins the integration of ammonia into advanced energy systems.

However, the relatively high auto-ignition temperature of ammonia (~651 °C), coupled with its low laminar flame speed and narrow flammability range in air (approximately 16–25% by volume), complicates its direct combustion and presents significant challenges for burner design and stable flame anchoring [10]. Moreover, ammonia’s relatively low adiabatic flame temperature compared to conventional hydrocarbon fuels is beneficial for suppressing peak NO_X formation, but it may also limit combustion efficiency. Ammonia is also chemically aggressive and may cause stress corrosion cracking in metals such as copper and zinc, requiring the adoption of ammonia-compatible materials and stringent containment strategies in storage and transport systems. Its inherent toxicity further necessitates the deployment of real-time leak detection and adequate ventilation systems in operational environments. Nevertheless, its pungent odor facilitates early leak detection, thereby contributing to intrinsic safety to some extent [11]. These physicochemical attributes not only highlight ammonia’s promise as a carbon-free fuel but also underscore the multifaceted technical complexities associated with its practical utilization.

2.2. Synthesis Pathways of Ammonia

Industrial ammonia synthesis has historically been dominated by the Haber–Bosch process, which continues to account for the vast majority of global ammonia production. The process involves reacting hydrogen and nitrogen under high temperature and pressure. Typically, the reaction is carried out at temperatures between 400 and 500 °C and pressures of 15–30 MPa, depending on catalyst type and process design. Despite its technological maturity, it is extremely energy-intensive and contributes substantially to greenhouse gas emissions. According to Krzysztof et al., the Haber–Bosch method consumes vast amounts of energy and emits approximately 1.6 tonnes of CO₂ for every tonne of ammonia produced. In pursuit of decarbonizing ammonia synthesis, current research efforts are increasingly centered on the development of “green ammonia”—produced by coupling renewable electricity-powered water electrolysis for hydrogen with nitrogen extracted from air [12]. As shown in Figure 2, this integrated pathway—combining water electrolysis, air separation, and ammonia synthesis—represents a core pillar of emerging zero-carbon energy infrastructures.

Beyond modifications to the conventional process, novel ammonia synthesis routes have become prominent research frontiers. One promising approach is electrochemical ammonia synthesis, which relies on the electrochemical or photoelectrochemical reduction of N₂ to NH₃. This approach envisions transitioning from large-scale, high-temperature, high-pressure facilities to compact, distributed devices operating under ambient conditions—positioning it as a highly attractive alternative. Prakash Kumar et al. [14] investigated electrochemical green ammonia synthesis powered by renewable electricity, demonstrating significant reductions in CO₂ emissions. Mostafa El-Shafie et al. highlighted that catalyst selection and long-term stability remain the key bottlenecks in electrochemical ammonia synthesis. Although a range of advanced electrocatalysts—based on transition metal nitrides, single-atom active sites, and ionic liquids—have been explored, their current ammonia yield rates remain well below the threshold for industrial viability. Another emerging route is photocatalytic ammonia synthesis, in which light energy drives semiconductor catalysts to convert nitrogen and water directly into ammonia [15]. However, the low activity and selectivity in most photocatalytic systems remain barriers to practical application. Recently, Xiong et al. developed a Zn-based coordination polymer (NJUZ-1) that not only photocatalytically converts external N₂ to NH₃ but also utilizes its own lattice dinitrogen anions as internal nitrogen sources, thus enabling self-sustained catalytic cycles under ambient conditions [16]. Besides traditional N₂ feedstocks, some studies have explored the possibility of using nitrate (NO₃⁻) as a nitrogen source due to its higher solubility and reactivity under ambient conditions. For instance, Jiang et al. synthesized 3D porous Cu@Cu₂O microspheres that achieved a nitrate-to-ammonia yield of 327.6 mmol h⁻¹ g⁻¹ and a Faradaic efficiency of 80.57% under ambient conditions, significantly outperforming conventional N₂-based electrochemical systems [17]. More recently, Cu/Cu₂O nanosheet catalysts with engineered oxygen vacancies and heterojunctions achieved 91.1% Faradaic efficiency and high yield in nitrate electroreduction, emphasizing the critical role of defect engineering in optimizing ammonia production from NO₃⁻ [18]. Moreover, Jiang et al. proposed a pyrolysis-free strategy for synthesizing quasi-phthalocyanine-based covalent organic frameworks (COFs) embedded with Ti–N₄ active centers, achieving an NH₃ yield of 26.89 μg h⁻¹ mg⁻¹ and a Faradaic efficiency of 34.62% under ambient conditions, which offers a promising decentralized route for green ammonia production without relying on high-temperature and high-pressure systems [19]. In addition, compared with the conventional Haber–Bosch process, electrochemical nitrogen reduction (ENR) is an emerging green ammonia synthesis pathway that operates under ambient temperature and pressure, making it particularly suitable for small-scale and distributed production scenarios. However, the current ammonia yield and energy efficiency of ENR remain far below the thresholds required for industrial applications, with its scalability limited by critical bottlenecks such as insufficient catalyst selectivity, poor electrode stability, and competing side reactions [14,15]. By contrast, the Haber–Bosch process can achieve continuous large-scale production but suffers from high energy consumption and significant carbon emissions [20]. Plasma-catalytic methods, while offering promising advantages in reaction controllability and intermediate activation, are still at the laboratory or early pilot stage and have not yet demonstrated industrial-scale stability [15]. Therefore, ENR is currently not competitive with traditional routes for large-scale ammonia synthesis, but it holds significant promise as a decentralized green ammonia production method in future energy systems.

In summary, the Haber–Bosch process is likely to remain the cornerstone of global ammonia fuel supply in the foreseeable future [20]. In the near term, ammonia production-related carbon emissions may be mitigated via energy system transitions—such as the use of green hydrogen—and process innovations, including efficiency enhancements and carbon capture integration. Over the long term, breakthroughs in disruptive technologies such as electrocatalysis may facilitate small-scale, decentralized ammonia synthesis, fundamentally reshaping production and supply paradigms [11]. For instance, recent studies on amorphous Ru-based single-atom catalysts have demonstrated excellent electrochemical ammonia synthesis performance under ambient conditions, achieving high selectivity and yield, thus reinforcing the feasibility of decentralized green ammonia production pathways [21]. In addition, recent systematic studies have reviewed the catalytic behavior of thermocatalytic ammonia synthesis catalysts, highlighting the role of electronic structure modulation and support interaction in enhancing activity and selectivity under green synthesis conditions [22]. Recent advances in high-temperature protonic ceramic electrolysis cells (PCECs) have demonstrated significant progress in coupling nitrogen reduction with oxygen evolution under carbon-free conditions, offering a scalable electrochemical pathway for green ammonia synthesis [23].

2.3. Storage and Transportation Strategies for Ammonia

2.3.1. Ammonia Storage Technologies

Ammonia storage technologies represent a pivotal link in enabling its large-scale deployment as an energy carrier. Currently, ammonia is predominantly stored in three industrial forms: high-pressure storage, cryogenic storage, and semi-refrigerated storage. These methods rely on liquefied ammonia and are chosen based on factors such as required storage capacity, operational context, and economic feasibility.

Under ambient-temperature, high-pressure storage, ammonia is liquefied by compressing it to 8.58 bar (around 1.0 MPa), making it suitable for small-scale or decentralized industrial scenarios. According to Rouwenhorst et al., this storage mode offers deployment flexibility but demands high standards for tank structural integrity and leak-tightness [24]. Cryogenic storage cools ammonia to its boiling point (−33 °C) at atmospheric pressure and is widely adopted in large port facilities and chemical distribution terminals. Khaksar et al. emphasized that cryogenic storage offers superior volumetric energy density and reduced pressure demands, yielding considerable economic advantages [25]. Semi-refrigerated storage adopts intermediate conditions (approximately −10 °C and 0.2 MPa), striking a balance between safety and cost, and is commonly used in medium-sized industrial plants. Ghavam et al. reported that semi-refrigerated storage ensures stable operation and low maintenance requirements, making it one of the most widely implemented ammonia storage solutions [26]. In addition, novel solid-state ammonia storage technologies—such as metal ammine complexes, metal–organic frameworks (MOFs), and zeolites—have garnered growing academic attention in recent years. These materials offer safe, low-pressure storage and transportation capabilities and hold promise for decentralized storage applications in the future. A recent review provided a comprehensive overview of emerging materials such as MOFs, metal ammines, and ionic liquids for solid-phase ammonia storage and reversible conversion systems [27].

2.3.2. Transport Technologies for Ammonia

Ammonia transport technologies are relatively well-established and typically involve long-distance shipment in liquefied form. The primary modes of ammonia transport include maritime shipping, pipeline transmission, rail freight, and road haulage. Maritime shipping remains the dominant mode for global ammonia trade, playing a critical role in large-volume transactions. According to Dimitriou and Javaid, around 20 million tonnes of liquefied ammonia are shipped annually across regions via maritime routes. These transport systems are largely adapted from liquefied petroleum gas (LPG) carrier designs, ensuring high safety standards and well-established operational protocols [3].

Pipeline transport is primarily employed for long-distance and continuous ammonia supply chains. Nezam Khaksar et al. investigated the NuStar and Magellan ammonia pipeline systems in the United States and concluded that pipeline-based transport can substantially enhance logistics efficiency while reducing per-unit transportation costs [25]. In addition, rail and road transport are generally adopted for medium- and small-scale or geographically dispersed ammonia distribution scenarios. Although rail transport is widely employed in industrial ammonia logistics, it often results in higher costs owing to complex handling procedures and stringent safety regulations [25].

2.4. Section Summary

As a promising clean energy carrier, ammonia’s value chain encompasses four core stages—green synthesis, efficient storage, multimodal transport, and clean utilization. Figure 3 provides a visual overview of the full ammonia value chain, from green synthesis to end-use, emphasizing the interconnectedness of storage, transport, and utilization technologies. Ammonia synthesis is undergoing a gradual transition from fossil-fuel-based pathways to renewable-powered green ammonia production. Once produced, liquid ammonia can be stored using high-pressure, cryogenic, or semi-refrigerated approaches, depending on storage needs, and offers favorable energy density and economic performance. In terms of distribution, ammonia is transported via pipelines, maritime vessels, and rail systems, enabling efficient global deployment. Notably, it holds significant promise as a hydrogen carrier for long-distance energy transmission. Overall, as an enabler of the green hydrogen economy, the integrated development of ammonia’s synthesis, storage, and transport infrastructure is poised to become a cornerstone of future zero-carbon energy architectures.

3. Combustion Mechanisms and Engineering Adaptability of Ammonia Fuel

3.1. Combustion Characteristics of Ammonia Fuel

Due to its unique chemical properties, ammonia exhibits combustion behavior that differs markedly from that of conventional hydrocarbon fuels. Under atmospheric conditions, ammonia exhibits an auto-ignition temperature of approximately 651 °C—significantly higher than that of gasoline or hydrogen—leading to a notably prolonged ignition delay. Moreover, ammonia’s laminar flame speed ranges from only 7 to 12 cm/s—substantially lower than that of typical hydrocarbon fuels—making it difficult to maintain stable combustion in conventional burner systems [2,10].

Figure 4 presents a comparative analysis of the fundamental combustion characteristics of ammonia and four representative fuels—DME, hydrogen, methane, and syngas—under both boiler and gas turbine operating conditions. As illustrated in Figure 4a, ammonia exhibits a maximum flame propagation speed of approximately 20 cm/s—only around 40% or less than that of DME (~50 cm/s), hydrogen (~260 cm/s), and methane (~35 cm/s). Moreover, its narrow variation range and weak sensitivity to parameter fluctuations suggest intrinsically low combustion reactivity. As shown in Figure 4b, ammonia consistently demonstrates longer ignition delays than the other fuels across the entire equivalence ratio range, with this disparity being especially pronounced under gas turbine conditions. These findings clearly illustrate the practical engineering challenges associated with ammonia combustion, particularly in terms of ignition difficulty and flame stabilization.

To mitigate the aforementioned challenges, researchers have proposed a series of systematic enhancement strategies from multiple technical perspectives. For instance, increasing the compression ratio raises the in-cylinder temperature, accelerates the generation of reactive radicals, and shortens the ignition delay. Song et al. [29] experimentally demonstrated that high compression ratio configurations in ammonia/n-heptane dual-fuel systems can significantly enhance reaction activity. Moreover, adjusting the equivalence ratio and premixing conditions has been shown to significantly affect combustion efficiency. Kim et al. [30] conducted experiments using a swirl combustor fueled with ammonia–hydrogen/air mixtures and found that high premixing levels and elevated pressure significantly increased flame propagation speed and OH radical concentrations, thereby enhancing flame stability. In terms of unconventional ignition strategies, Taneja et al. [31] employed nanosecond pulsed plasma to activate ammonia–air mixtures, which not only expanded the lean flammability limit but also markedly reduced ignition delay. Mong et al. further demonstrated through numerical simulation that appropriately configuring auxiliary fuels (e.g., methane, biogas) and diluents (e.g., CO₂), combined with flow field optimization, can effectively improve flame stability in ammonia combustion [32].

In summary, the ongoing advancement of multi-dimensional technological strategies is progressively mitigating these critical limitations and establishing a viable foundation for the practical application of ammonia in clean energy systems.

Figure 4. Fundamental combustion performance comparison of ammonia, DME, hydrogen, methane, and syngas under boiler and gas turbine conditions. The x-axis in both subplots represents the equivalence ratio (ER). (a) Laminar flame speed; (b) ignition delay time [33].

Figure 4. Fundamental combustion performance comparison of ammonia, DME, hydrogen, methane, and syngas under boiler and gas turbine conditions. The x-axis in both subplots represents the equivalence ratio (ER). (a) Laminar flame speed; (b) ignition delay time [33].

3.2. NOₓ Formation Mechanisms and Mitigation Strategies

Owing to its nitrogen-rich molecular structure, ammonia generates significant amounts of nitrogen oxides (NO_X) during high-temperature combustion, representing a major obstacle to its clean utilization. The main NO_X formation mechanisms include thermal NO, fuel NO, and prompt NO, with fuel NO being the most significant pathway, as it arises directly from the oxidation of nitrogen atoms in the NH₃ molecule. Mathieu and Petersen [34], based on shock tube experiments and kinetic analysis, demonstrated that NO is formed via a radical chain reaction pathway: NH₃→NH₂→NH→N→NO. The OH radical acts as a key catalyst in this reaction pathway; as the gas temperature increases in the shock tube environment, OH concentration rises, thereby accelerating the NO formation rate [34]. To illustrate the detailed chemical pathway of NO formation during ammonia oxidation, Figure 5 presents the radical chain mechanism, highlighting the key role of OH radicals in promoting NO generation. To verify the universality of this mechanism, Zhang et al. [35] developed a detailed NO_X kinetic model applicable to ammonia, hydrogen, and syngas fuel blends. The simulation results exhibited strong agreement with experimental data under high-temperature conditions, especially in capturing the concentration profiles of NH_X intermediates, thereby validating the proposed reaction pathway. In practical combustion systems, hydrogen is frequently co-fired to improve ignition characteristics and flame propagation. However, Pochet et al. [36] pointed out that while hydrogen addition reduces ignition delay, it also raises flame temperature, thereby enhancing thermal NO formation. Therefore, both the hydrogen blending ratio and the equivalence ratio require careful optimization to minimize overall NO emissions. Joo et al. [37] investigated premixed ammonia/hydrogen flames under varying equivalence ratios and observed that lean combustion (low φ), characterized by excess oxygen, inhibits NH₃ decomposition and NH_X formation, thereby reducing NO production. Under fuel-rich conditions, the enhanced reducing atmosphere facilitates the partial reduction of NO to N₂, providing a secondary suppression mechanism for NO emissions. This study underscores that precise control of the air–fuel ratio represents an effective strategy for regulating NO emission levels.

In summary, NO_X formation during ammonia combustion is governed by the interplay of multiple factors, including radical reaction pathways, flame temperature, and fuel blending ratios. Looking ahead, optimized strategies such as hydrogen blending, staged combustion, and real-time equivalence ratio control offer promising routes to simultaneously enhance combustion efficiency and suppress NOₓ emissions.

3.3. Safety Management for Ammonia Utilization as a Fuel

As a promising carbon-free fuel, ammonia has garnered significant attention; nevertheless, its toxic and corrosive nature presents critical safety concerns in practical applications. Ammonia becomes noticeable in ambient air at concentrations of several tens of ppm, with a strong, irritating odor that causes discomfort. The short-term permissible exposure limit (PEL, 15 min) is only 35 ppm. Exposure above this threshold leads to significant eye and throat irritation, while concentrations over 300 ppm are classified as immediately dangerous to life and health (IDLH) [38]. Additionally, ammonia acts as a chemical asphyxiant; inhalation at high concentrations may result in respiratory distress or acute pulmonary edema. To prevent personnel injury from ammonia leakage, industrial systems must be equipped with reliable detection and ventilation systems. Traditional leak detection involves igniting a sulfur stick near suspected leakage zones, which reacts with ammonia to form visible white ammonium sulfate fumes. However, modern fuel systems should be equipped with electronic ammonia sensors that enable real-time monitoring and rapid response, fundamentally improving operational safety.

In addition to toxicological risks and personnel protection, ammonia presents notable challenges in terms of combustion stability and material compatibility. Li et al. [10] performed numerical simulations to assess the stability of ammonia combustion and its influence on pressure fluctuations under varying oxygen concentrations. The results revealed that oxygen enrichment enhances ammonia’s combustion rate but also generates stronger pressure waves, thereby imposing higher structural requirements on enclosed systems and introducing new challenges for emission control and flame regulation. Additionally, ammonia exhibits corrosive behavior toward metallic materials during storage and transportation. Kobayashi et al. [2] proposed the use of 316 L stainless steel, nickel-based alloys, and ceramic-lined materials for pipeline and storage components to effectively mitigate corrosion risk and reduce leakage probability, thereby improving the safety and reliability of the overall system. In conclusion, while ammonia possesses strong potential as a clean fuel, its large-scale application necessitates comprehensive safety measures—including leak monitoring, combustion optimization, and corrosion-resistant material integration—to enable its sustainable adoption in industrial energy infrastructures.

3.4. Blended Combustion Strategies Involving Ammonia and Alternative Fuels

Owing to its long ignition delay, low laminar flame speed, and limited heating value, ammonia often suffers from flame blowout and incomplete combustion in conventional burner configurations. Consequently, fuel blending is widely regarded as a key strategy to improve the feasibility and stability of ammonia utilization in combustion systems. The addition of highly reactive fuels—such as hydrogen, methane, dimethyl ether (DME), or diesel—to ammonia can not only compensate for its poor ignition and flame propagation characteristics but also enable the coordinated control of flame temperature and NO_X emissions.

3.4.1. Ammonia–Hydrogen Blended Combustion

With the growing emphasis on decarbonization in energy systems, ammonia–hydrogen blends have attracted increasing research interest, owing to their dual advantages of carbon-free combustion and high chemical reactivity. Ammonia, when used alone, exhibits a high ignition temperature and low laminar flame speed, which leads to poor flame stability. In contrast, hydrogen features exceptionally high reactivity and rapid flame propagation, making it an ideal supplement to address ammonia’s combustion limitations. Hydrogen addition to ammonia significantly enhances ignition behavior and overall combustion performance, thereby broadening its application potential in gas turbines, internal combustion engines, and industrial furnaces [39,40]. In practical energy systems, it is also noteworthy that hydrogen can be produced via on-site ammonia cracking, enabling flexible co-firing configurations without the need for separate hydrogen supply chains.

Yovino et al.’s experimental results further support this conclusion: under varying hydrogen blending ratios, the laminar flame speed of ammonia fuel exhibits a clear upward trend with an increasing equivalence ratio. The addition of 30%, 50%, and 70% hydrogen significantly enhances the overall flame propagation speed, particularly peaking near the stoichiometric equivalence ratio (ϕ ≈ 1.0). Figure 6 compares the laminar flame speeds of ammonia–hydrogen mixtures under different hydrogen blending conditions, visually illustrating the pronounced improvements in flame propagation and combustion performance brought about by hydrogen enrichment. This enhancement not only improves flame stability but also confirms the critical role of hydrogen blending in expanding the practical operating window of ammonia-based fuels [41].

Recent studies have shown that appropriately blending hydrogen can significantly enhance the stable combustion characteristics and thermal efficiency of ammonia-based fuels. For instance, Meng et al. [42] used optical diagnostics and numerical simulations to show that, under lean combustion conditions, ammonia–hydrogen blends with 30% H₂ can extend the lower equivalence ratio limit to 0.4, while sustaining a reasonable flame speed and maintaining low NO_X emissions. This highlights the pivotal role of hydrogen in expanding the operational window for practical ammonia combustion. Zhao et al. [43] further validated these findings using a one-dimensional engine simulation model. Their results indicated that increasing hydrogen blending advanced combustion phasing and improved thermal efficiency across a range of engine loads and speeds. At high engine speeds in particular, hydrogen enrichment proved essential for achieving reliable ignition.

In addition to enhancing power output, hydrogen blending in ammonia combustion introduces complex effects on emission characteristics. On one hand, hydrogen enrichment raises the flame temperature, which enhances NO_X formation and poses additional challenges for emission control. For example, Bayramoğlu et al. [44] reported that increasing the hydrogen blending ratio from 5% to 15% caused a near-linear increase in NO_X emissions and a rearward shift in the temperature field, highlighting the need to optimize blending ratios during combustor design. To further illustrate how the equivalence ratio influences nitrogen-based pollutant formation, Figure 7 presents the axial distributions of key nitrogen-containing species (NO, N₂O, and NO₂) in premixed ammonia–hydrogen flames. These profiles reveal significant variations across different equivalence ratios, reflecting the thermal NO formation peak in the high-temperature reaction zone near the leading edge of the flame, as well as secondary N₂O/NO₂ pathways that dominate under fuel-rich conditions. NO is primarily formed in the high-temperature reaction zone near the leading edge of the flame, where thermal NO mechanisms dominate. Its mole fraction increases significantly with the equivalence ratio, consistent with enhanced NO formation under fuel-rich and high-temperature conditions. In contrast, the peak concentrations of N₂O and NO₂ decrease as the equivalence ratio increases, likely due to the suppression of side reaction pathways under fuel-rich and high-temperature conditions. These observations indicate that although hydrogen addition enhances ignition performance and flame stability, it also intensifies local high-temperature regions and promotes NO_X by-product formation, thereby requiring careful trade-offs in system design. On the other hand, Chai et al. [40] highlighted in their review that with proper mixture control under high-pressure conditions, ammonia–hydrogen blends can achieve NO_X emissions below 5 ppm, thereby providing theoretical support for the future development of low-carbon, high-efficiency energy systems.

In the context of practical applications, Fąfara et al. [46] performed computational fluid dynamics (CFD) simulations on a micro gas turbine combustor to systematically examine the temperature and flow field distributions of ammonia–hydrogen fuel blends at varying hydrogen ratios. Their results showed that at a hydrogen blending ratio of 48%, the temperature field closely matched that of methane combustion, while simultaneously ensuring flame stability, enhancing thermal efficiency, and reducing the risk of NO_X formation. This demonstrates the strong compatibility of ammonia–hydrogen fuels with micro gas turbine systems, offering practical engineering support for their real-world implementation. Novella et al. [47] further investigated the thermal efficiency behavior of ammonia–hydrogen fuel blends under various compression ratios and intake temperatures using a virtual engine modeling framework. Their findings suggested that higher compression ratios allow for sustained thermal efficiency without inducing knock, reinforcing the feasibility of ammonia–hydrogen operation in advanced engine platforms.

Notably, the choice of experimental techniques—such as constant-volume combustion chambers (CVCC), flame visualization, and high-resolution CFD modeling—as well as variations in chemical kinetic mechanisms, can significantly affect the assessment of ammonia–hydrogen combustion characteristics. To enhance the comparability of findings, Tang et al. [48] proposed using standardized flame structure metrics—such as unstretched laminar flame speed, combustion duration, and flame front visualization—to facilitate integration and validation between experimental and simulation studies.

Despite substantial research supporting the beneficial effects of hydrogen blending on ammonia combustion performance, several challenges still persist. First, there is currently no universally applicable approach for determining the optimal hydrogen blending ratio under varying operational conditions—especially under dynamic engine loads and ultra-lean combustion regimes. Second, there remains a fundamental trade-off between NO_X emission reduction and the high-temperature conditions needed for flame stability, which calls for integrated control strategies—such as optimized combustor design, exhaust gas recirculation (EGR), or selective catalytic reduction (SCR) systems [49]. Third, certain studies suggest that ammonia and hydrogen exhibit only partial chemical synergy, with some decoupling effects observed—especially in DeNO_X mechanisms, where the introduction of hydrogen may complicate NO formation pathways [42].

Overall, ammonia–hydrogen blended combustion represents a promising zero-carbon fuel pathway with significant potential for deployment in internal combustion engines, gas turbines, and distributed energy systems. Future research should continue to emphasize optimizing adaptability across diverse operating conditions, developing integrated low-NO_X mitigation strategies, and refining fundamental kinetic models to accelerate the transition of ammonia–hydrogen fuels from laboratory studies to industrial deployment.

3.4.2. Ammonia–Methane Blended Combustion

As the primary component of natural gas, methane offers excellent infrastructure compatibility when blended with ammonia, along with improved flame stability and controllability. In a combined experimental and numerical study of NH₃/CH₄ premixed flames, Xiao et al. [50] observed a near-linear increase in laminar flame speed with rising methane content, with the most pronounced enhancement occurring near the stoichiometric equivalence ratio (ϕ ≈ 1.0). The study also revealed that at higher ammonia concentrations, the OH radical concentration in the high-temperature reaction zone near the leading edge of the flame significantly decreases, which is a key factor contributing to slower flame propagation.

Rocha et al. [51] further conducted a systematic comparison of laminar flame speeds across various NH₃/CH₄ blending ratios, pressures, and equivalence ratios using both experiments and direct numerical simulations (DNS). As shown in Figure 8, methane addition significantly increased flame speed near the stoichiometric condition (ϕ ≈ 1.0), while higher pressures and ammonia-rich mixtures led to substantial reductions in propagation speed, indicating strong sensitivity to operating conditions. These findings further confirm the critical role of methane in enhancing the reactivity and stability of ammonia-based combustion systems.

Ariemma et al. [52] focused on NO_X emission behavior in NH₃/CH₄ co-combustion and revealed a nonlinear influence of the ammonia blending ratio on the dominant emission pathways. They observed that at low-to-moderate ammonia blending ratios, increasing fuel-bound nitrogen content leads to NO formation primarily via thermal and fuel N pathways, resulting in a gradual rise in total emissions. However, at higher ammonia concentrations, residual NH₃ in the post-flame region can reduce NO to N₂, leading to a secondary de-NO_X effect and suppressing NO accumulation. As shown in Figure 9, NO_X emissions exhibit a rise-then-fall trend across all equivalence ratio conditions. Specifically, under stoichiometric and slightly fuel-rich conditions (ϕ = 1.0–1.2), peak NO_X emissions occur at 30–40% ammonia blending. Beyond this point, further NH₃ enrichment reduces emissions, supporting the dominance of reductive de-NO_X reactions in fuel-rich regimes. This synergistic mechanism enables NH₃/CH₄ combustion to offer not only improved flame tunability but also emission control potential, thus supporting the theoretical and practical transition of natural gas systems toward low-carbon alternatives.

3.4.3. Ammonia–Coal Blended Combustion

With the ongoing transformation of the global energy structure and the tightening of carbon reduction policies, ammonia is emerging as a promising zero-carbon fuel for coal-fired power systems. Owing to its favorable storage and transport properties, as well as high compatibility with existing infrastructure, ammonia–coal co-firing is widely regarded as an effective transitional solution for decarbonizing coal-fired plants—particularly in scenarios that do not require extensive system retrofitting [53,54]. In recent years, extensive studies have been conducted on the combustion stability, pollutant formation mechanisms, and system compatibility of ammonia–coal co-firing, demonstrating its technical feasibility in improving energy efficiency and mitigating pollutant emissions.

In terms of combustion characteristics, ammonia itself has problems with high ignition temperature and slow flame propagation speed. If burned alone, it can easily lead to ignition difficulties and unstable flameout. However, by co-firing with coal powder, the combustion process of ammonia can be significantly improved by utilizing the high-temperature zone formed by the release of volatile matter in coal, thereby enhancing overall ignition performance and flame stability [55,56]. Lin et al. [53] conducted a real furnace ammonia blending test in a 300 MW boiler, and the results showed that the stability of the boiler operation remained basically unchanged when the calorific value blending ratio of ammonia was less than 20%. In addition, Wu et al. [55] pointed out that under the condition of ammonia addition, NH₃ can be released in advance to participate in gas-phase reactions, promote the combustion of coal powder volatiles, and indirectly improve the burnout rate and temperature uniformity.

Increasing the proportion of ammonia in the fuel blend markedly alters the flame morphology and temperature field distribution. Liu et al. [56], using CFD simulations of a low-NO_X swirl combustor, demonstrated that increasing the ammonia content from 10% to 50% led to a progressive transition in flame structure—from a swirling form to an elongated shape, and ultimately to a “+25 candle-like” profile—substantially influencing both the NO formation pathways and spatial concentration distributions. Figure 10 illustrates CFD-derived ammonia reaction rate fields for three injection strategies—central (CA), partial (PA), and staged injection (SA)—across nozzle configurations R8 to R10, as reported by Liu et al. Under the CA condition, the reaction region forms a compact, column-shaped zone. In contrast, the SA configuration results in a bifurcated flame structure with a downstream-extended reaction region, favorably repositioning the high-temperature zone and suppressing NO generation.

Meanwhile, the inherently high nitrogen content of ammonia presents a significant challenge for controlling NO_X emissions. Numerous studies have demonstrated that NO formation in ammonia–coal co-firing is influenced not only by the direct oxidation of ammonia but also by variables such as ambient oxygen concentration, ammonia injection location, and the implementation of air-staging techniques [57,58]. Figure 11 illustrates the axial distribution of NO emissions for various ammonia blending ratios under both unstaged and air-staged combustion conditions. The results indicate that under unstaged conditions, NO emissions rise sharply with increasing ammonia content, with a dramatic surge observed at 100% NH₃. In contrast, the implementation of burnout air staging leads to a significant reduction in NO levels. When the ammonia blending ratio is maintained below 30%, emissions become comparable to those from pure coal combustion, highlighting the crucial role of air staging in mitigating NO from nitrogen-rich fuels. Chen et al. [57] emphasized that the ammonia injection method has a pronounced impact on NO formation. Compared to premixed injection, staged injection more effectively suppresses thermal oxidation, achieving a distinct “NO valley” effect at approximately 20% ammonia blending. Zha et al. [58] further demonstrated through comparative experiments under MILD (Moderate or Intense Low-oxygen Dilution) combustion that moderate incorporation of ultrafine coal particles enhances flame structural uniformity, thereby indirectly reducing NO formation.

In terms of system integration, the operational adaptability of ammonia–coal co-firing across varying load conditions has drawn considerable attention. Jin et al. [60] performed numerical simulations on a 1050 MW ultra-supercritical boiler and demonstrated that even at a 30% ultra-low load, the furnace temperature field remained stable when the ammonia blending ratio was maintained below 40%. Moreover, CO₂ emissions were reduced to nearly 20% of the levels observed under baseline coal-fired conditions. These findings suggest that ammonia–coal co-firing offers robust adaptability in large-scale engineering systems, providing a realistic solution for achieving simultaneous decarbonization and operational stability.

In addition to modifying flame behavior and gas-phase product formation, ammonia–coal co-firing may also influence fly ash particle characteristics and the volatilization behavior of alkali metals, thereby affecting slagging propensity and secondary pollutant emissions. Pu et al. [54] found in a 4 MW pilot-scale boiler that moderate ammonia blending (20%) effectively suppressed PM₁ particle emissions without significantly altering the ash composition, while also inhibiting the vapor-phase release of sodium (Na) and potassium (K). Figure 12 presents the variations in sodium concentration, flame temperature, and radiative heat transfer characteristics under various ammonia blending ratios and combustion strategies. The experimental data revealed that sodium vapor concentrations decreased progressively beyond 40% ammonia blending and significantly declined under staged combustion, suggesting that ammonia addition may modulate alkali metal release through thermal and structural flame adjustments, ultimately alleviating high-temperature corrosion and secondary particulate formation. Wu et al. [55] further reported through kinetic simulations that NH₃ undergoes conversion to N₂ in high-temperature regions, facilitating in situ NO reduction in the burnout zone and thereby indirectly suppressing NO slip.

Despite its promising engineering potential, ammonia–coal co-firing still faces several critical challenges that must be resolved. First, a fundamental trade-off exists between flame stability and NO_X mitigation. High ammonia blending ratios often lead to flame displacement, increased CO emissions, and sharp NO surges [56,60]. Second, prevailing CFD models struggle with accurately coupling turbulence, combustion, and pollutant chemistry, highlighting the need for model calibration using experimental validation data [55,56]. In addition, ammonia slip and its associated secondary reactivity are still insufficiently understood and controlled, necessitating the integration of precise injection techniques and optimized air-staging strategies.

In summary, ammonia–coal co-firing serves as a promising transitional solution for low-carbon energy, demonstrating significant advantages in operational stability, emission control, and system integration. Future efforts should prioritize elucidating the combustion mechanism, mapping NO formation dynamics, and constructing multi-objective optimization frameworks for representative ammonia–coal blends, thereby facilitating deployment in large-scale power plants and supporting carbon peak and neutrality targets.

3.4.4. Ammonia Blending with DME and Diesel

In addition to hydrogen, methane, and coal, researchers have increasingly explored ammonia co-firing with alternative fuels such as dimethyl ether (DME) and diesel, aiming to broaden its applicability across various thermal energy systems. As a highly reactive, oxygenated fuel, DME has emerged as a particularly promising candidate for synergistic co-combustion with ammonia. Cai and Zhao [61] conducted a systematic experimental investigation on NH₃/DME/air premixed flames and reported that adding as little as 10% DME significantly increased the laminar flame speed and broadened the flammable equivalence ratio range. Notably, enhanced flame stability was achieved under lean combustion conditions. Furthermore, DME addition was shown to reduce ignition delay time due to its rapid thermal decomposition at high temperatures, which releases active radicals that trigger ammonia reaction pathways and enhance overall combustion reactivity. Regarding NO_X emissions, although DME’s high reactivity may elevate flame temperatures, its oxygen-rich molecular structure facilitates more complete oxidation, thereby limiting NO formation from incomplete C–N reactions and leading to only moderate increases in total NO output [62].

In the area of ammonia–diesel co-combustion, Reiter and Kong examined dual-fuel compression ignition in which diesel acted as an ignition promoter for ammonia. Their results demonstrated that atomized diesel injection effectively initiated ammonia combustion, achieving substitution ratios as high as 70% without sacrificing overall thermal efficiency. Subsequent research indicated that fine-tuning injection timing and in-cylinder EGR parameters can significantly mitigate combustion inhomogeneity and ammonia slip [6].

These findings suggest that fuel blending provides a highly adaptable pathway for practical ammonia utilization, allowing for the integrated enhancement of ignition performance, flame stability, and NO_X mitigation. In future engineering practice, it is crucial to advance the integrated design of blending ratios, fuel delivery schemes, and emission mitigation strategies, tailored to the operational characteristics of diverse energy systems. Such efforts will facilitate the advancement of high-efficiency, low-carbon ammonia-based clean fuel systems.

4. Engineering Applications of Ammonia Fuel in Representative Energy Systems

4.1. Applications and Demonstration Projects of Ammonia Fuel in Power Generation Systems

Under the dual-carbon targets of carbon peaking and carbon neutrality, ammonia has emerged as a promising alternative energy carrier for power generation, owing to its carbon-free combustion, high energy density, and ease of liquefaction and storage. Current research efforts have primarily centered on co-firing ammonia in existing coal-fired power plants and developing dedicated pure-ammonia combustion systems for electricity generation. Recent comprehensive reviews have highlighted the system-level challenges and development opportunities for ammonia-based energy storage and utilization, emphasizing its integration into grid-scale generation, fuel cells, and renewable energy networks [63].

Ammonia co-firing in coal-fired power units is widely considered a technically feasible and incremental pathway toward decarbonization. Japan’s IHI Corporation has successfully developed ammonia–coal co-firing technology, demonstrating in a 10 MW pilot-scale furnace that a 20% low heating value (LHV) ammonia blend can achieve NO_X emissions below 200 ppm and ammonia slip as low as 1 ppm [64]. The system maintained stable pulverized coal combustion and ensured high thermal efficiency by incorporating an optimized burner design and a two-stage staged air supply strategy to suppress NO_X formation (see Figure 13).

Secondly, pure ammonia combustion is gaining traction as a critical breakthrough for medium- and long-term clean power generation, progressively moving from laboratory-scale studies toward real-world deployment. Xu et al. [65] noted that directly substituting ammonia in gas turbines is limited by its high ignition energy and low flame propagation speed. However, hydrogen co-firing significantly enhances flame stability and reaction kinetics, thereby extending burner operability and improving NO_X emission control. Meanwhile, Morlanés et al. [66] emphasized that integrating ammonia turbines with renewable hydrogen production and a co-located Haber–Bosch synthesis system can enable round-the-clock operation and enhanced system flexibility, positioning it as a promising zero-carbon technology for future peak-load balancing.

In summary, ammonia fuel is playing an increasingly versatile role in power systems, spanning transitional coal–ammonia co-firing to long-term ammonia–hydrogen–electricity integration strategies. As illustrated in Figure 14, ammonia-based power systems encompass several application pathways, including coal co-firing, ammonia-fueled gas turbines, and distributed generation. The schematic highlights ammonia’s multifaceted role across the “production–storage–utilization” value chain, including renewable hydrogen-driven ammonia synthesis, liquefaction and transportation, and terminal applications such as ammonia gas turbines, coal co-firing, and solid oxide fuel cells (SOFCs).

4.2. Demonstration and Pilot Projects of Ammonia Fuel in Marine Propulsion Systems

The maritime sector represents a significant source of global greenhouse gas (GHG) emissions. The International Maritime Organization (IMO) has committed to a 50% reduction in carbon emissions by 2050, thereby accelerating the transition toward zero-carbon marine fuels. Owing to its carbon-free combustion and compatibility with current port infrastructure, ammonia is increasingly regarded as a leading candidate for next-generation marine propulsion fuels. Cheliotis et al. [67] conducted a comprehensive review of the suitability of ammonia-based fuel cells for maritime propulsion, emphasizing that safety assessment protocols, leak management strategies, and redundancy design are critical prerequisites for large-scale deployment. Braun et al. [68] demonstrated the practical viability of ammonia fuel for small vessels using inland waterway tests. By integrating ammonia cracking with hydrogen–ammonia co-combustion in a high-speed diesel engine, they achieved nearly 40% thermal efficiency, offering a real-world case for ammonia-based propulsion in small marine platforms.

4.3. Ammonia as an Energy Carrier in Storage Systems: Chemical Storage and Fuel Cell Pathways

As a hydrogen energy carrier, ammonia is playing an increasingly prominent role in long-duration energy storage systems. Compared with liquid hydrogen, liquid ammonia provides a higher volumetric energy density and more stable storage and transport characteristics, making it particularly suitable for seasonal load shifting and the long-distance electrochemical energy redistribution of renewables. Research indicates that ammonia–fuel cell systems, coupled with water electrolysis units, can enable integrated “generation–storage–utilization” energy pathways in scenarios such as renewable power curtailment, islanded microgrids, and data center backup systems [69]. In addition, future research may explore innovative storage paradigms that integrate ammonia with solid-state hydrogen storage media to further improve system safety and energy density.

Moreover, the integration of ammonia as a fuel in solid oxide fuel cells (SOFCs) and direct ammonia fuel cells (DAFCs) is gaining traction, offering a promising route toward low-carbon electricity generation. SOFCs are particularly suited for ammonia applications, given their high operating temperatures (typically 700–1000 °C) and tolerance for various fuels, which help overcome the ignition limitations of ammonia. Maffei et al. reported that in ammonia-fueled SOFCs employing YSZ (yttria-stabilized zirconia) electrolytes, the endothermic nature of ammonia cracking improves thermal efficiency and reduces the demand for external cooling air. In 2020, Shy et al. [70] conducted the first trial of direct ammonia feeding in pressurized SOFCs, demonstrating operational stability at high temperatures with minimal degradation. In contrast, DAFCs offer strong potential for decentralized applications such as distributed energy and auxiliary power units, owing to their ambient-temperature startup and compact system design. Zhao et al. [69] developed a high-performance DAFC prototype, achieving a peak power density of 135 mW/cm² at 80 °C, significantly exceeding the performance of previously reported systems.

4.4. Section Summary

Overall, ammonia fuel is transitioning from a laboratory concept to large-scale engineering deployment. In power generation, SOFCs and DAFCs enable efficient, low-carbon electricity production; in the maritime sector, ammonia offers a viable route toward decarbonization; and in energy storage, it provides dual value as both a storage medium and transport carrier. However, combustion stability, leakage detection, system-level integration, and regulatory alignment remain critical challenges hindering widespread commercialization. Future efforts should focus on cross-scenario validation and the co-optimization of system architectures across multiple technological routes.

5. Future Prospects and Engineering Challenges of Ammonia-Based Clean Energy Systems

Ammonia fuel systems are at a pivotal stage of transition from laboratory validation to engineering-scale deployment. Ammonia’s carbon-free combustion, high energy density, and ease of liquefaction and transport make it highly compatible with various energy applications. Against the backdrop of global energy transition and carbon neutrality goals, ammonia’s strategic importance is becoming increasingly evident. Compared with hydrogen, ammonia offers higher volumetric energy density and superior compatibility with existing infrastructure, making it more suitable for long-distance energy dispatch and maritime shipping. Moreover, ammonia can be synthesized from renewable hydrogen and atmospheric nitrogen, enabling a closed-loop green cycle that enhances its role within the integrated “green hydrogen–green ammonia” ecosystem. Multiple nations and energy companies have initiated ammonia-based demonstration projects—ranging from gas turbine retrofits to SOFC integration and marine propulsion adaptation—highlighting its transition from an alternative fuel to a system-level energy carrier [1].

Despite its carbon-neutral profile and high energy density, ammonia faces several critical engineering barriers that limit its large-scale implementation. Combustion stability remains a primary bottleneck. The high ignition temperature and low flame propagation speed of ammonia often lead to unstable flames and incomplete combustion, limiting its direct use in internal combustion engines and gas turbines [1]. NO_X mitigation is another pressing challenge. Ammonia combustion tends to generate significant amounts of NO_X and N₂O, and without effective denitrification systems, its environmental advantages may be undermined [30]. Additionally, ammonia’s toxicity and corrosiveness pose stringent safety requirements for storage and transport infrastructure. This is especially critical under high-pressure and long-distance conditions, where material compatibility and leak detection technologies require further advancement [34]. More critically, the cost of producing green ammonia remains economically uncompetitive. Its commercial viability still depends on policy incentives and carbon pricing mechanisms to bridge the cost gap with fossil fuels [2]. These challenges suggest that the large-scale deployment of ammonia fuel systems requires not only technological breakthroughs but also cross-disciplinary and system-level optimization.

At present, ammonia combustion mechanisms and kinetic models require further refinement under multi-component fuel conditions. Key parameters—such as ignition delay and radical reaction pathways—still show discrepancies between experimental results and numerical simulations. In parallel, low-NO_X combustion strategies—such as hydrogen enrichment, air–fuel ratio tuning, and plasma-assisted ignition—are under systematic evaluation to enhance flame reactivity and reduce emissions [48]. From a storage and transport perspective, the development of advanced materials is essential for improving system safety and energy efficiency. Solid-state ammonia storage systems—such as metal ammine complexes and metal–organic frameworks (MOFs)—demonstrate excellent thermal stability and storage capacity under low-pressure conditions [15]. The integrated advancement of combustion system design, ammonia storage technologies, and safety monitoring systems will constitute a major focus of future research efforts.

Moreover, the systemic deployment of ammonia fuel is progressing from isolated pilot trials to integrated multi-scenario applications, spanning power generation, energy storage, transportation, and industrial by-product utilization. Within power systems, ammonia can act as a carbon-free peaking fuel when co-fired with gas turbines or solid oxide fuel cells (SOFCs), enabling the formation of highly flexible generation modules [65]. In maritime transport, ammonia exhibits high compatibility with existing LNG infrastructure, making it a promising option for medium-range and inland waterway applications. Furthermore, ammonia can be integrated with industrial by-product systems—such as pulp and chemical manufacturing—to recover low-grade heat for reuse in ammonia synthesis loops. In remote or islanded regions, ammonia can operate synergistically with microgrid systems to develop distributed energy architectures characterized by high adaptability and operational safety [71]. Achieving such multi-scenario integration necessitates holistic system-level co-design and end-to-end optimization, encompassing ammonia production, transport, and utilization and enabling policy frameworks.

As a pivotal enabler in the development of future clean energy systems, ammonia fuel is progressing rapidly from fundamental research to demonstration-scale implementation. However, realizing its large-scale role in the global energy transition requires addressing several interrelated engineering challenges. These issues—including combustion kinetics, NO_X mitigation, safety in storage and transport, and economic viability—are deeply interlinked and require integrated, system-level optimization. Future research should prioritize the refinement of kinetic models, the design of high-performance combustors, the development of solid-state ammonia storage materials, and the integration of intelligent management platforms to enable multi-scenario deployment. More importantly, the development of ammonia fuel has moved beyond individual technologies and now demands systemic integration across the energy, power, transportation, and industrial sectors. With the support of coherent policies, regulatory standards, and industrial collaboration, ammonia is well-positioned to serve as both a strategic enabler and a systems integrator in the path toward carbon neutrality and energy system transformation.

6. Summary

Under the impetus of carbon neutrality and peak carbon policies, ammonia fuel is gaining significant traction as a core component in clean energy system design, owing to its zero carbon emissions, high energy density, and favorable storage and transport characteristics. This review systematically examines the latest progress in utilizing ammonia as an energy vector, covering its physicochemical attributes, green synthesis pathways, storage and transport methods, combustion behavior, NO_X mitigation, safety protocols, and hybrid combustion strategies. Furthermore, it highlights engineering practices involving ammonia fuel across power generation, marine propulsion, and long-duration energy storage applications. Cumulative research findings suggest that ammonia is not only a feasible transitional fuel from fossil-based systems to carbon-neutral energy but also a highly compatible and reconfigurable component within future energy infrastructures.

Ammonia fuel development has transitioned from fundamental scientific inquiry to a stage of parallel, multi-pathway validation. Nevertheless, several constraints continue to hinder its large-scale commercial deployment. On one hand, inherent combustion issues—such as high ignition temperatures and slow flame propagation—remain unresolved. On the other hand, high-temperature NO_X mitigation, ammonia leakage prevention, and the cost-effectiveness of green synthesis remain critical engineering bottlenecks. Furthermore, varying application scenarios—ranging from gas turbines and retrofitted coal-fired plants to solid oxide fuel cell (SOFC) systems—impose diversified requirements on ammonia adaptability, thereby increasing the complexity of system-level integration. The advancement of ammonia as a fuel has evolved into a multidisciplinary systems challenge—encompassing combustion kinetics, thermodynamic system design, material safety, emission mitigation, and regulatory frameworks.

Looking ahead, the development of ammonia-based clean energy systems should prioritize the following research and implementation directions: First, at the level of fundamental combustion kinetics, efforts should be directed toward in situ diagnostics and multi-scale modeling to gain deeper insight into radical pathways and the coupled mechanisms of NO formation and reduction. Second, at the applied system level, the development of low-emission, high-efficiency combustors, advanced storage and transport materials, and dynamic safety response systems should be accelerated to enhance device-level integration. Third, within integrated energy systems, future studies should promote the synergistic coupling of ammonia with hydrogen, electricity, and heat and establish holistic models that span the entire chain, from production, storage, and conversion to end-use. Fourth, on the policy and economic front, leveraging carbon pricing mechanisms and green finance tools is essential to improve the early-stage feasibility of green ammonia initiatives and to foster coordinated action across the industrial value chain.

Overall, ammonia fuel stands as a vital pillar in the pursuit of a global energy transition, characterized by both immense strategic potential and significant implementation challenges. With synergistic advances in technology, policy frameworks, and industrial ecosystems, ammonia is poised to become a central energy nexus—bridging green hydrogen production, renewable energy integration, and end-use decarbonization. This evolution will accelerate the realization of a clean, efficient, safe, and sustainable energy landscape for the future.