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Review

A Review of Ammonia Combustion Reaction Mechanism and Emission Reduction Strategies

by
Xiqing Zhang
1,2,3,*,
Shiwei Zhao
1,2,3,
Qisheng Zhang
2,3,4,
Yaojie Wang
1,2,3 and
Jian Zhang
1,2,3
1
School of Vehicle and Traffic Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China
2
Shanxi Engineering Research Center of Internal Combustion Engine Power Technology, Taiyuan 030024, China
3
Advanced Technology Innovation Center of Zero Carbon Power Special Vehicle, Taiyuan 030024, China
4
High-end Heavy Machinery Equipment Research Institute, Taiyuan University of Science and Technology, Taiyuan 030024, China
*
Author to whom correspondence should be addressed.
Energies 2025, 18(7), 1707; https://doi.org/10.3390/en18071707
Submission received: 9 March 2025 / Revised: 23 March 2025 / Accepted: 26 March 2025 / Published: 28 March 2025
(This article belongs to the Special Issue Advanced Combustion Technologies: Challenges and Solutions for a Low-Carbon Future)
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Abstract

:
Combustion is a key method for converting energy, historically relying on fossil fuels like coal and oil, which have significant drawbacks for sustainable development. Ammonia (NH3) is highlighted as a viable hydrogen carrier with high hydrogen content, easy liquefaction, and better transportation characteristics compared to hydrogen. Despite its potential, ammonia combustion faces challenges such as NOx emissions and combustion performance, necessitating further research into its combustion dynamics. This systematic review is geared towards encapsulating the latest advancements in the research and development initiatives pertaining to ammonia fuel combustion, with a particular emphasis on elucidating the chemical kinetics and strategies for controlling nitrogen oxide emissions, and delineates the technical hurdles and prospective research avenues associated with ammonia combustion.

1. Introduction

Energy serves as the fundamental material basis for the sustained operation of human society. Typically, energy must be transformed into a form that is usable by humans. Among various methods of energy conversion, combustion plays a pivotal role. Historically, fossil fuels, including coal and oil, have been predominantly employed as primary energy sources owing to their high energy density, efficient conversion processes, and ease of use, which have significantly supported various social activities. Prior to the Industrial Revolution, the utilization of fossil fuels was relatively limited in scale and scope. However, by the late 19th century, the advent of the internal combustion engine and the subsequent growth of the automobile industry led to oil emerging as a significant energy source, ultimately becoming the predominant energy form throughout the 20th century. The early 20th century also saw the commercial development and utilization of natural gas, which gradually established itself as a vital fossil energy source. Nevertheless, the excessive consumption of fossil fuels has resulted in considerable adverse effects on the sustainable development of human society due to their inherent drawbacks.
In the realm of practical engineering applications, the combustion of fossil fuels remains the predominant method for energy supply. Nonetheless, this process generates significant quantities of greenhouse gases (GHGs), which contribute to global climate change. In pursuit of carbon neutrality and the mitigation of pollutant emissions, there is a growing emphasis on the investigation of low-carbon and no-carbon renewable fuels, as well as the development of innovative combustion technologies [1,2,3].
From the perspective of fuel composition, an ideal fuel would produce combustion products devoid of greenhouse gas emissions or harmful pollutants, a characteristic that hydrogen fuel possesses. However, the volumetric energy density of hydrogen is exceedingly low, and its application is considerably hindered by the intricate challenges and expenses related to its storage and transportation [4,5]. Prior to the large-scale substitution of fossil fuels with hydrogen fuel, several challenges, including storage technology, safety, and cost, must be addressed. Consequently, the exploration of fuels with a composition similar to that of hydrogen represents a pragmatic approach to implementing low-carbon strategies at the present stage. Ammonia (NH3) is deemed a highly promising substitute as a fuel source.
Ammonia serves as a potent hydrogen energy carrier, boasting a substantial hydrogen content of 17.7% by weight, and is devoid of carbon, thereby eliminating carbon dioxide emissions. It can be easily liquefied at ambient temperature under a pressure of roughly 10 bars. When compared to hydrogen and methane, ammonia exhibits a significantly higher fuel density for pipeline transportation, making it more suitable for long-distance storage and distribution. Ammonia can be produced in substantial quantities through the established Haber–Bosch process, which contributes 1.2% of the total global carbon dioxide emissions produced by humans [6,7], as well as through renewable energy sources and direct electrochemical nitrogen reduction reactions to yield green ammonia. The energy consumption associated with this process varies between 27.4 and 31.8 GJ/t, with a potential enhancement in overall energy efficiency of up to 65%.
The decarbonization of global transportation can be facilitated through the utilization of ammonia, particularly in internal combustion engines (ICEs). In the forthcoming years, ammonia is poised to emerge as a significant renewable fuel for gas turbines, internal combustion engines, and fuel cells, as it generates no carbon emissions. Gas turbines are identified as the optimal system for large-scale ammonia application, while ammonia can also be employed directly in both spark-ignition and compression-ignition engines. Lifecycle analysis forecasts indicate that vehicles powered by ammonia, as an alternative to conventional gasoline, could potentially decrease greenhouse gas emissions by a factor of three. Although the energy density of compressed ammonia is comparatively lower than that of gasoline and diesel, it remains substantially higher than that of compressed natural gas or liquid hydrogen. It is important to note that research into the application of ammonia in internal combustion engines is still in its nascent stages [8,9,10].
In essence, ammonia is garnering widespread acknowledgment as a crucial energy source for the future, owing to its extensive suitability for high-energy output uses, distributed energy solutions, and industrial locations that lie beyond the reach of the conventional power grid [11]. Table 1 shows some of the properties of the fuels.
Although ammonia holds great promise as a sustainable option for the future’s low-carbon energy sector, its widespread implementation is currently hampered by various factors, including NOx emissions and suboptimal combustion performance. Continuing studies into the intricacies of ammonia combustion have uncovered a multitude of elements that sway the chemical reaction trajectories of ammonia-derived fuels, including the equivalence ratio, the composition of the fuel mixture, the ambient pressure, and the temperature. Consequently, an in-depth examination of the reaction dynamics and chemical kinetic models pertaining to ammonia and its gas blends is essential to advance our comprehension of ammonia’s combustion properties, improving its combustion efficiency and stability, advancing clean energy technologies, and fostering innovation in internal combustion engine design.
Prior research has examined the decomposition of ammonia; however, the combustion reaction process of ammonia requires additional elaboration and clarification to enhance its application as a clean energy source and to advance sustainable energy development.
This review is geared towards encapsulating the latest advancements in the research and development initiatives pertaining to ammonia fuel combustion, with a particular emphasis on elucidating the chemical kinetics and strategies for controlling nitrogen oxide emissions. Section 2 introduces the research methods. Section 3 reviews foundational research on NH3 combustion chemistry, which can inform the study of its combustion characteristics. Section 4 delves into the characteristics of emissions during the combustion of ammonia, highlighting the efficacious technologies for mitigating NOx emissions. In conclusion, Section 5 delineates the technical hurdles and prospective research avenues associated with ammonia combustion.

2. Methods

This research employs a systematic literature review methodology to advance knowledge in the domain of ammonia combustion. The investigation primarily utilizes the Web of Science (WoS) database, recognized for its comprehensive coverage and rigorous indexing standards. The initial search of the literature produced a variety of sources, including original research articles, technical reviews, and conference proceedings. To maintain the integrity and relevance of the literature, duplicate entries were identified and eliminated. Following this, the abstracts of the remaining publications underwent a meticulous screening process to assess their pertinence. This evaluative step ensured that only the most relevant studies were incorporated into the final review. Subsequently, a comprehensive systematic evaluation and analysis of the selected studies was performed. The studies were categorized based on their nature (experimental or numerical) and their focus (ammonia combustion or emissions), with a thorough examination of the research findings. This analysis encompassed technical challenges, advancements in the field, and prospective research trajectories. The overall methodology of this study is illustrated in Figure 1a. This structured approach guarantees a comprehensive and systematic review of the existing literature, upholding high standards of quality and relevance to the research topic.
The data collection sources are limited to English, primarily covering the time period from 2010 to 2024. The research process and specific studies included/excluded at each stage are shown in Figure 1a,b.

3. Basic Research on Ammonia (NH3) Combustion

The development of precise and comprehensive kinetic mechanisms, along with their modeling, is crucial for accurately predicting combustion processes and analyzing their outcomes [12]. This segment provides an in-depth explanation of the oxidation and pyrolysis phenomena integral to the combustion chemistry of ammonia, followed by a concise compilation of the chemical reaction mechanisms that are essential for forecasting laminar flame speeds, determining ignition delay periods, and estimating nitrogen oxide (NOx) emissions.

3.1. Oxidation and Pyrolysis of NH3-Based Combustion Chemistry

A comprehensive grasp of the combustion mechanism is crucial for the numerical simulation and examination of real-world scenarios where ammonia serves as a fuel. In-depth research has been carried out into the gas-phase oxidation and thermal decomposition of ammonia, as well as its various blends. Early studies by Miyama [13] focused on ammonia oxidation, where ignition delay times were measured using reflected shock waves, with UV absorption records of hydroxyl (OH) and ammonia (NH) radicals. Subsequent to this, numerous research teams initiated kinetic investigations to pinpoint pivotal reactions, simultaneously exploring the function of ammonia in NOx chemistry and the denitrification mechanisms [8,9,10,11,12,13,14]. Lindstedt and associates carried out meticulous examinations of premixed flames, involving combinations of ammonia/hydrogen/oxygen and ammonia/oxygen, emphasizing the crucial role of interactions between NO and NH, as well as N radicals, in the synthesis of N2 and the routes for NO transformation [15,16].
The resurgence of ammonia as a prospective energy carrier has spurred the development of advanced experimental methodologies, which are now meticulously employed to elucidate and quantify the critical reaction stages and their respective rate constants in the realm of ammonia combustion chemistry. Frequently utilized methods encompass flow reactors, shock tubes, and jet-stirred flow reactors. Stagni [17] and colleagues carried out an exhaustive investigation, merging experimental, theoretical, and kinetic modeling strategies to explore the breakdown of ammonia, as well as the resultant products and intermediates. This study employed both stirred flow reactors and flow reactors, in conjunction with a master equation approach based on ab initio transition state theory. Their findings indicated that formation of HNO from the reaction NH2 + O = HNO + H, along with its decomposition at elevated temperatures, significantly influences flame propagation characteristics and the NO/N2 ratio. In the demanding environment of elevated pressure and reduced temperature, H2NO assumes a crucial function in influencing ignition latency. Extensive experimental studies and kinetic evaluations of ammonia oxidation processes were carried out within diverse gas compositions, such as NH3/H2/CO/CH4/N2 and NH3/CH4/O2/CO2/N2, utilizing flow reactors [18,19]. These investigations highlight the indispensable role of the NH2-NO reaction in the synthesis of N2 and the mitigation of NO levels.
Apart from delving into the oxidation dynamics of ammonia combustion, a thorough exploration of the pyrolysis mechanism has garnered significant attention, owing to its pivotal role in deciphering the mechanisms behind flame spread and ignition characteristics. This comprehensive understanding has been accomplished through an integrated approach, merging experimental findings with computational simulations and theoretical insights. Cohen [20,21,22,23,24,25] reviewed the reaction O + NH3 = NH2 + OH, emphasizing the potential influence of secondary reactions on the rate coefficients estimated via transition state theory. The research team led by Dean [26] delved into the kinetics of environments with high ammonia content under atmospheric pressure by employing laser absorption spectroscopy, identifying significant reactions involving NHi (i = 1, 2) and calculating their rate constants using unimolecular decomposition theory. An enhanced mechanism for ammonia pyrolysis has been formulated by fine-tuning the rate constants of pivotal reactions producing NH and NH2, specifically within the temperature bracket of 2200 to 2800 K, employing narrow-linewidth laser absorption technology [27]. These investigations contribute to a nuanced understanding of ammonia chemistry.

3.2. Chemical Kinetic Mechanisms

Contrary to the extensively validated chemical reaction pathways for hydrogen and methane, the rate constants for the critical steps in ammonia combustion are either insufficiently recorded or altogether unidentified, complicating the conduct of precise numerical simulations [28,29,30]. The complexity of these simulations is further exacerbated by the intricate interactions among flow dynamics, combustion processes, heat transfer, and chemical kinetics. Miller and Bowman [31] were among the pioneers in simulating ammonia reaction mechanisms. Recent empirical and kinetic modeling research has been centered on refining ammonia reaction mechanisms, with a specific emphasis on aspects such as laminar flame speed, ignition delay, nitrogen oxide production, flammability boundaries, extinction strain rates, and the characteristics of its flammable composition [32,33,34,35,36].
The ignition delay time is a pivotal factor that significantly impacts the performance of reciprocating internal combustion engines, encompassing both spark-ignition and compression-ignition varieties [37,38,39]. Various methodologies, including rapid compressors, shock tubes, and flow reactors, have been employed to explore the ignition properties of ammonia [40,41,42]. Dai and colleagues [43] have developed a reaction mechanism for the mixture of ammonia, dimethyl ether, and air, which precisely forecasts the ignition delay times across a variety of fuel compositions, spanning from lean to rich blends, under high-pressure conditions. Dagaut et al. [44] introduced a relatively novel ammonia oxidation mechanism capable of predicting NH3 ignition under diverse conditions. Mathieu and Petersen [45] expanded upon this work by proposing a predigested mechanism and validating cognition delay times across high temperature ranges (1560–2455 K) and varying fuel conditions using an enhanced shock tube. This sophisticated mechanism includes a total of 35 distinct species and encompasses 159 fundamental steps. Following the development of this reaction mechanism by Mathieu and Petersen, Shu [46] and colleagues applied it to delve into the autoignition dynamics and the formation of nitrogen oxides in ammonia/air blends within the intermediate temperature range of 1100 to 1600 K. Their study yielded a commendable alignment, with the observed ignition delay times exhibiting a notable consistency with the values predicted by the mechanism. Under moderately cool conditions, the reaction between NH2 and NO, producing N2 and H2O, serves to reduce the formation of NOx. Concurrently, the process of autoignition is profoundly affected by the chemical transformation NH2 + NO = NNH + H2O. Subsequent studies expanded the scope of autoignition observations to include premixed mixtures of NH3/O2 and NH3/H2/O2 within the temperature bracket of 950 to 1150 K, highlighting the pivotal role of reactions involving NH2 with NO and the transformation H2NO + O2 = HNO + HO2 in achieving precise predictive outcomes.
A crucial factor in assessing chemical mechanisms is the laminar flame speed, a key determinant in the propagation and stability of premixed blends under given operational circumstances. An extensive array of laminar flame velocities for different ammonia fuel mixture compositions has been established through experimental research under a variety of operational conditions [47,48,49]. The research team led by Nakamura [50] conducted simulations on the dynamics of ammonia-air lean mixtures with a temperature-regulated micro burner, highlighting the pivotal role of N2Hx chemistry in the reaction zone at relatively low temperatures, around 1300 K. Okafor [51] and associates’ experimental and kinetic modeling work has shown that the impact of C-N reactions on the rate of methane-ammonia blends is minimal. Meanwhile, Xiao and co-workers [52,53] have streamlined the reaction mechanisms for ammonia-hydrogen and ammonia-methane systems, tailored to the conditions pertinent to gas turbine operations. Overall, the scholarly works demonstrate a marked association between the forecasts yielded by streamlined mechanisms and empirical observations. Wang et al. [54,55] carried out an exhaustive series of experimental and kinetic modeling investigations involving ammonia mixed with hydrogen, methane, syngas, and methanol, confirming the measured laminar burning velocities. Recently, Yin et al. [56] reported favorable agreement between experimental results and their laminar velocity model for ammonia/dimethyl ether/air under elevated inlet temperature and pressure conditions. The research team led by Da Rocha [57] evaluated the efficacy of ten varied reaction mechanisms in forecasting the laminar combustion velocities and ignition delays for ammonia/air and ammonia/hydrogen/air blends, highlighting that the precision of these mechanisms in depicting combustion characteristics varies with the specific operational conditions.
Current methodologies have predominantly been confirmed in relation to factors such as laminar combustion velocity, ignition delay duration, flame configuration, and the production of nitrogen oxides. During the initial studies into the chemical kinetics of ammonia, the viability of the suggested mechanisms was largely assessed in accordance with the flame structure. In the past few years, the swift progress in both invasive and non-invasive experimental measurement methods has enabled the adoption of more accurate assessment criteria, including laminar burning velocity and ignition delay time, to deepen our comprehension of the chemical and physical principles governing combustion processes. In order to gauge the environmental consequences of ammonia combustion, there is a pressing need to intensify efforts toward the development of accurate mechanisms for nitrogen oxide formation.

3.3. Construction of a Comprehensive Chemical Kinetic Model

The development of a chemical kinetic model for the combustion of ammonia markedly boosts the intrinsic comprehension of its reactive system, while also aiding in the design and fine-tuning of practical application devices [58]. Throughout the years, a multitude of unique chemical kinetic models tailored specifically for ammonia, distinguished from those applicable to other fuels, have been introduced. The majority of these models focus on distinct combustion scenarios and have been corroborated against a narrow spectrum of experimental objectives. These models, which are robust chemical kinetic frameworks, draw upon empirical data to encapsulate the dynamics of chemical reactions under diverse temperatures, pressures, and equivalence ratios within varied reactor types [59,60]. They stand out as more dependable tools for unraveling the intrinsic combustion chemistry and its practical implications in engine technology.
The primary motivation behind the creation of chemical kinetic models for the combustion of ammonia stemmed from the need to clarify the pivotal kinetics governing the thermal de-nitrification process, with a specific focus on selective non-catalytic reduction (SNCR) [61]. In 1981, Miller [62] crafted a kinetic model aimed at elucidating the decisive attributes of the interaction between ammonia and nitrogen monoxide, meticulously describing the intrinsic steps that dictate DeNOx properties and pinpointing the key reactions involving amino (NH2) radicals and nitrogen monoxide. The researchers inferred that the amalgamation of NH2 and NO predominantly results in chain branching (NNH + OH) and chain termination (N2 + H2O) processes. Following this, Miller and Glarborg suggested that the ratio of branching for NH2 + NO in these pathways is dependent on the temperature and pressure conditions [63].
Among the pioneering efforts in the realm of ammonia combustion studies, Miller and colleagues introduced a seminal model in 1983 [64]. This model was meticulously corroborated through its application to both stable and spontaneously spreading NH3/O2 and NH3/H2/O2 flames. The researchers delved into the distinct reaction mechanisms that result in NO formation, examining conditions ranging from fuel-lean to fuel-rich environments. Subsequently, Miller and Bowman introduced a dynamic kinetic model to elucidate the interactional mechanisms involving NH3, HCN, and various nitrogen-rich compounds with C1-C2 hydrocarbons or free radicals. Their scholarly work presented the thermodynamic data and rate constants for the intrinsic reactions within the ammonia model, establishing the foundational knowledge necessary for comprehending ammonia’s chemical characteristics and the processes behind the formation and breakdown of nitrogen oxides [65].
In 1995, Lindstedt et al. [66,67] meticulously crafted comprehensive models for NH3, NH3/H2, NH3/NO, NH3/CO, NH3/CH4, and NH3/C2H6, through an extensive review and evaluation of the kinetic data found in the existing literature. These models were subsequently validated by comparing them with empirical data obtained from flow reactors, burner-stabilized furnaces, and counterflow diffusion burners. The study concluded that the generation and degradation of NO are chiefly controlled by NHi radicals, which include NH2, NH, and N. The prominence of these NHi radicals in the diverse pathways of NO formation varies depending on the specific conditions, with NH2 holding a crucial position in the reduction of NO.
Glarborg and Klippenstein [68] conducted theoretical studies on key reactions related to NNH and established detailed models based on their calculations. The model’s efficacy was confirmed through the utilization of shock tube and DeNOx experimental results obtained from jet-stirred and flow reactors, encompassing a temperature spectrum from 900 to 2160 K and a pressure range of 1 to 5 atm.
In 2009, Duynslaegher et al. [69] noted during their investigation of stable flames in ammonia–hydrogen mixtures that the Konnov mechanism excessively predicted higher molar fraction distributions for amino radicals (NH2), whereas it significantly underestimated the molar fraction distributions of N2O; moreover, the model exhibited limited measurements under low-pressure conditions, lacking sufficient data to justify corrections to the rate constants. Then, Duynslaegher et al. [70] proposed an improved model comprising 19 species and 80 elementary reactions, which enhanced the reactions of N2O and NH2 radicals in 2012. They adjusted the rate constants for four fundamental reactions, NH + NO = N2O + H, N2O + H = N2 + OH, NH2 + H = NH + H2, and NH2 + NH2 = N2H2 + H2, resulting in improved concordance between computed results and experimental data.
In recent years, Mei [71], along with her team, meticulously crafted an elaborate ammonia model designed to anticipate the laminar flame speed metrics and the high-temperature scenarios documented in the literature, encompassing phenomena like burner-stabilized flames and the ignition delay times. While this method significantly improved the model’s dependability under high-temperature conditions, its efficacy at low and moderate temperatures still harbors ambiguity, thereby lacking comprehensive validation.
In 2017, the Nakamura [72] model was crafted, building upon the foundational work of Miller’s modeling efforts. These chemical properties of N2H4, N2H3, and N2H2 were updated in accordance with the Konnov model [73]. Following a literature review and evaluation, the thermodynamic data for key reactions and multiple kinetic rate parameters have been meticulously refined. The comprehensive model integrates 38 distinct species and comprises 232 individual reactions, having been meticulously validated against empirical data on species formation, ignition delay times, and laminar flame speeds, all obtained from microflow reactors. Moreover, the study incorporated and verified sub-mechanisms for ammonia, hydrazine, the H2/N2O system, and the H2/NO2 system within the model framework.
In 2018, Glarborg and colleagues [74] conducted an extensive review of nitrogen combustion chemistry, culminating in the proposition of a refined kinetic model that elucidates the interactions between NH3, HCN, and hydrocarbons/nitrogen. This model is grounded in their cumulative 30 years of experience in modeling research. They showcased the validation outcomes of their DeNOx model through experiments involving NH3/C1-C2 hydrocarbons in flow reactors and also provided data from jet-stirred reactors along with the process of ammonia oxidation, detailing the stable flame morphology distribution and the ignition delay times. Their model, having been robustly validated through the analysis of ignition delay times and laminar flame speeds under the conditions of a rapid compression machine, has been expanded to enhance its predictive capabilities for processes such as ammonia pyrolysis, DeNOx reactions, oxidation of NH3/methanol, C2H2/NOx interactions, CH3NH2 oxidation, and the oxidation of CH3CN.
The research team led by Shrestha [75] has crafted a kinetic model to analyze the combustion dynamics of both pure NH3 and NH3/H2 mixtures. To augment the prognostic precision of the experimental findings, they meticulously recalibrated multiple kinetic variables and introduced the application of diverse computational models to corroborate the experimental objectives for H2/CO/NO/NO2/N2O/NH3 in an array of reactors. This encompassed the integration of morphological data from jet-stirred reactors, flow reactors, and burner-stabilized furnaces, in conjunction with overarching parameters such as ignition delay times and laminar flame velocities.
In 2020, building upon the foundational Polimi model and the contributions of Song et al., Stagni and associates crafted an elaborate ammonia model [76,77,78]. Utilizing sophisticated theoretical approaches, they have determined the rate constants for pivotal reactions involved in ammonia’s pyrolysis and oxidation. Their model, having undergone rigorous examination, was successfully verified using data sourced from both jet-stirred reactors and flow reactors, in addition to the extensive experimental results documented in the scientific literature. The validation process was conducted across a broad range of conditions, including temperatures spanning from 500 to 2800 K, pressures ranging from 0.026 to 100 atm, and pyrolysis equivalence ratios of 0.01. Building upon this robust foundation, the model was subsequently utilized as a critical framework for the simulation of NH3/H2, NOx/CH4, and NH3/CH4 combustion processes.
Jiang and his team [79] have refined the San Diego model to integrate the complexities of NH3 combustion processes and the formation of NOx. This enhanced model has been meticulously verified against established benchmarks for species production, derived from burner-stabilized flames and counterflow premixed double flames, across a broad range of temperatures (900 to 2500 K), pressures (0.05 to 40 atm), and equivalence ratios (0.5 to 2.0). Additionally, its performance aligns with key global parameters, including ignition delay times, laminar flame speeds, and extinction rates. The model’s abridged mechanism has been successfully employed in direct numerical simulations, producing results that align with expectations.
In the year 2021, X. Zhang and his team [80] developed an NH3 kinetic model (KAUST model) tailored for the pyrolysis and oxidation processes of both pure NH3 and NH3/H2 blends. This sophisticated model was meticulously crafted upon an extensive review of existing literature, previous modeling endeavors, and novel theoretical computations derived from their own empirical research. Its efficacy was confirmed through meticulous comparisons with the species formation data across diverse reactors, as well as the assessment of global parameters—including ignition delay times and laminar flame speeds—across a broad range of conditions spanning 450–2800 K, 0.05–100 atm, and equivalence ratios that fluctuated from 0.0016 to infinity. Drawing upon the foundation of this model, X. Zhang and associates subsequently crafted the NH3/C1 model to delineate the chemical interactions between ammonia and C1 fuels, including syngas, CH3OH, and CH4 [81].
The Thomas/Shrestha model [82] from 2023 has been expanded to forecast the extinction of NH3/H2 flames, followed by the subsequent generation of NO. The 2023 Glarborg model [83] updated the key steps for NH2 ignition and N2O generation, validated with new experimental data from batch and flow reactors. The 2023 Stagni model [84] has been enhanced to incorporate an NH3/H2 oxidation module, specifically designed for low and medium temperature ranges, providing thorough validation of the influence of hydrogen on the oxidation of ammonia under diverse experimental settings.
A detailed chemical kinetic model (NUIG model) for NH3/H2 mixtures was developed under a wide range of engine-relevant conditions by Y. Zhu et al. [85]. It has been comprehensively validated to describe the combustion of NH3/H2 mixtures using available experimental data from the literature, including ignition delay times, laminar flame speeds, and species concentration profiles. The new model can represent the combustion properties of pure ammonia and NH3/H2 mixtures well under most conditions. Through sensitivity and reaction path flux analyses, the key reactions that control the fuel reactivity in the high (≥1500 K) and low to intermediate (1000 ≤ T ≤ 1500 K) temperature ranges have been identified. Moreover, the formation and consumption pathways of nitrogen oxides (NOx) during the combustion of NH3/H2 mixtures under different conditions have also been investigated. It is found that these pathways are highly coupled with the underlying chemical reactions determining the fuel reactivity.
The CM score serves as a quantitative measure to evaluate the degree of consistency between experimental data and its corresponding computed data. For those interested in a more detailed mathematical exposition, the work of Ramalli et al. [80] is recommended. The CM score employs a normalized scale, in the range of (0, 1], to assess the level of similarity, with a score of 1 denoting perfect similarity. Figure 2 shows the CM scores of different models under different conditions.
It is essential to recognize that various classification methodologies may produce differing rankings. Given that no single model consistently achieves the highest performance across all categories, it is evident that there is no universally superior model. However, the NUIG_2023 and KAUST_2023 models appear to excel in terms of overall performance. Furthermore, the Polimi_2023 model demonstrates commendable scores across all categories, suggesting that it has benefited from enhancements made in its prior iterations. The Mei_2020 model and the Mei_2021 model attained the highest scores in the DeNOx (thermal denitrification) and low heating value (LBV) datasets, respectively. Each model encounters distinct challenges and possesses unique advantages, underscoring the intricacies involved in accurately capturing various combustion characteristics.

4. Nitrogen Oxide Emission Reduction Strategies

During the combustion of ammonia, the formation of nitrogen oxides (NOx) primarily transpires through the oxidation reactions involving the free radicals NH2, NH, and N. These radicals can interact with molecular oxygen (O2), atomic oxygen (O), hydroxyl radicals (OH), and hydroperoxyl radicals (HO2) to yield nitrogen monoxide (NO) or oxygen-containing intermediates such as H2NO and HNO, which can subsequently be transformed into NO. Specifically, NH2 can react with O to produce HNO and NH through the following reactions: NH2+ O → HNO + H and NH2 + O → NH + OH, respectively.
Recognizing the substantial production of nitrogen oxides during combustion is crucial. The reduction or elimination of ammonia emissions is of considerable importance prior to the implementation of this renewable fuel in practical combustion systems. Various viable and straightforward strategies have been proposed and assessed through theoretical, experimental, and numerical methodologies [82]. These approaches entail refined adjustments to burner architecture or operational protocols, incorporating advancements such as staged combustion, the utilization of external energy sources, and modifications to operational parameters including enhanced fuel-oxidizer blending methods, humidification of reactants, and the implementation of high-pressure environments. These methods each have their own advantages and disadvantages when it comes to dealing with the formation of nitrogen oxides. Table 2 briefly summarizes the advantages and disadvantages of different emission reduction strategies [86,87,88,89].

4.1. Rich Combustion at High Inlet Pressure

Rich combustion is hailed as a highly effective strategy for reducing nitrogen oxide emissions, largely attributed to the paucity of O and OH radicals. However, the method may lead to ammonia leakage, which can compromise combustion efficiency. Generally, a slightly rich combustion regime in ammonia fuel systems is advantageous for reducing nitrogen oxide emissions while sustaining relatively high efficiency. An alternative method to reduce the formation of NO in actual combustion systems entails increasing the starting pressure [83]. Nonetheless, the escalation in pressure leads to a diminished laminar burning velocity, which in turn impacts the combustion system’s stability. Figure 3 depicts the distribution of NH and NO radicals, as computed via direct numerical simulation (DNS), under two different inlet pressure conditions, alongside the corresponding planar laser-induced fluorescence (PLIF) signals [84]. In ammonia gas, NH2 radical serves as an indicator for NO destruction. The elevation of inlet pressure constricts the regions of elevated NH and NO concentrations, suggesting a reduction in NOx formation. This reduction may be attributed to the enhanced third-body effect under high-pressure conditions in NH3/H2/air and NH3/CH4/air mixtures, which suppresses OH production and consequently limits the reaction rate of HNO + OH = NO + H2O. Both rich combustion and elevated inlet pressure represent practical and efficient strategies, as they obviate the need for alterations to the burner. However, they possess certain limitations that can be mitigated by integrating them with some of the methods discussed subsequently.

4.2. Dual-Fuel Combustion

The dual-fuel combustion approach offers a viable resolution to the dilemmas linked with the combustion of pure ammonia, including the need for high ignition temperatures and slower flame propagation velocities. Within dual-fuel systems, the proportion of each fuel constituent markedly impacts the characteristics of the emissions profile. The research team led by Rocha undertook a comprehensive study, combining experimental and numerical methods, to assess NOx emissions resulting from the combustion of NH3/CH4/air and NH3/H2/air mixtures within a porous burner, with particular attention to the variation in ammonia concentration [85]. The data indicate that for the NH3/CH4/air blend, NOx emissions increase initially before decreasing as the ammonia concentration is adjusted, reaching a maximum at around XNH3 ≈ 0.5. This trend has been corroborated by previous studies. Conversely, for NH3/H2/air, emissions peak at XNH3 values of 0.5 and 0.8. This inconsistency might originate from the distinct nature of the NH3/CH4 and NH3/H2 phases, where the creation and degradation of NO are profoundly influenced by the oxidation of CH4 in the former, which utilizes oxygen radicals, and in the latter, the production of NO is less governed by H2 chemistry, with the oxidation of NH and NH2 becoming more dominant. These patterns are typically illuminated by the kinetic theories postulated, although they have a tendency to overpredict the emissions of nitrogen oxides, suggesting a requirement for further enhancement of the chemical models to correspond with empirical findings. On the whole, the production of nitrogen oxides within dual-fuel blends is orchestrated by the delicate balance between dilution and chemical influences.

4.3. Reactant Humidity

Another practical method to reduce the formation of nitrogen oxides entails the humidification of the reactants. The decrease in emissions stems from a combination of physical and chemical influences. The addition of water serves to reduce the temperature of the flue gas, thus enhancing the scope of the thermal de-NOx reaction. Additionally, water has the capability to boost the consumption of oxygen by facilitating the reaction O + H2O = OH + OH, thereby constraining the reaction rate of N2 + O = NO + N. Under specific conditions, the addition of water can reduce nitrogen oxides by an order of magnitude. The recent experimental assessments conducted by Ariemma and colleagues [90] have meticulously evaluated the efficacy of water augmentation and contrasted the influences of distinct fuel approaches on nitrogen oxide emissions during mildly or significantly diluted combustion processes involving ammonia (Figure 4). The temperature of combustion rises uniformly with the addition of water, whereas the emissions of nitrogen oxides vary in response to the increments of water introduced. Specifically, under fuel-lean conditions (ϕ = 0.7), the effect of water addition is more pronounced, followed by stoichiometric conditions. However, under fuel-rich conditions (ϕ = 1.1), the addition of water has a negligible impact on nitrogen oxide formation. This can be ascribed to the presence of a considerable number of O radicals under fuel-lean conditions, which interact with H2O, thereby suppressing NO formation. Furthermore, at ϕ = 1.1, nitrogen oxide emissions from premixed combustion exceed those from non-premixed combustion, underscoring the importance of optimizing the internal flow field of the burner and the reactant injection strategy.

4.4. Plasma-Assisted Combustion

In addition to boosting ignition and optimizing combustion procedures, plasma technology has proven to be highly effective in managing emissions. Experimental studies on plasma-assisted combustion for ammonia emissions have been documented in the literature. The deployment of plasma markedly diminishes nitrogen oxide emissions by promoting the creation of OH radicals and hastening the utilization of NH3 during the plasma discharge phase. This process culminates in the synthesis of N2, while simultaneously restricting the presence of NH3 and NH2 radicals, thereby obstructing the production of NOx during the combustion phase. The power output and electrical potential of the discharge play pivotal roles in determining the degree of emission mitigation. It has been observed in the recent studies conducted by Tang et al. [91] that at lower levels of discharge power and voltage, the influence of plasma-assisted combustion on the emissions of ammonia is considerably limited. Specifically, the impact of alternating current (AC)-driven sliding arc discharge on nitrogen oxide emissions becomes insignificant when the diameter ratio ϕ exceeds 0.75. Nevertheless, owing to the thermal de-nitrification process, it is feasible to attain exceptionally low nitrogen oxide emissions, reaching as low as 100 ppm, under conditions of reduced ϕ values. Consequently, a considerable plasma intensity is indispensable to efficiently mitigate nitrogen oxide emissions within the context of ammonia gas.

4.5. Staged Combustion

Staged combustion technology represents another promising approach to maintaining nitrogen oxide emissions from ammonia combustion at acceptable levels. This method entails the creation of two separate combustion regions, thereby generating both affluent and deficient fuel conditions. Moreover, post-combustion mitigation techniques, including selective non-catalytic reduction, can be incorporated seamlessly. Experimental and numerical investigations have demonstrated the effectiveness of staged combustion technology in reducing nitrogen oxide emissions.
In pursuit of eradicating nitrogen oxide emissions during ammonia combustion, Kurata and colleagues have innovatively enhanced a traditional gas turbine engine design. This was achieved by incorporating a sleeve friction gap zone, removing the primary dilution holes, and diminishing the swirler’s surface area. The assessment of staged combustion’s influence on the emission characteristics within an NH3/air-fueled gas turbine has been conducted, uncovering the correlation between the NO and NH3 emissions and the combustion chamber’s inlet temperature. The empirical data substantiate that augmenting the angle of the fuel nozzle leads to a decrease in NO emissions. The adoption of the rich-lean combustion strategy is particularly beneficial in curbing NH3 leakage, likely a consequence of the heightened flame temperatures. Moreover, the generation of NO in relation to the combustion chamber’s inlet temperature is significantly affected by the primary equivalence ratio and the orientation of the fuel nozzle. Okafor and colleagues performed both experimental and computational assessments on the nitrogen oxide emissions during single-stage and two-stage premixed methane–ammonia combustion processes. They utilized advanced laser diagnostic methods, including PIV and PLIF, in synergy with large eddy simulation techniques. Figure 5 delineates the numerical temperature and NO concentration profiles in relation to the primary and overall equivalence ratios. It is evident that both the temperature and NO emissions are significantly influenced by the distribution of primary and secondary equivalence ratios. In particular, the temperature exhibits a complex pattern, fluctuating rather than following a straightforward trend in correlation with the primary equivalence ratio. Concurrently, the formation of NOX experiences a decline, chiefly attributed to the restricted presence of O and OH radicals. As the mole fraction of ammonia drops from 0.3 to 0.1, the emissions of nitrogen oxides reach their minimum when the primary equivalence ratio of NH3 to CH4 is within the range of 1.30 to 1.35. By utilizing advanced staged combustion technology under increased inlet pressure conditions, it is possible to diminish nitrogen oxide emissions to a mere 49 ppmv, while achieving nearly zero ammonia leakage and a combustion efficiency as high as 99.8% [92].

5. Conclusions

The shift towards carbon-free fuels is essential for alleviating the detrimental impacts of greenhouse gas emissions resulting from combustion processes. Although hydrogen (H2) is recognized as a clean fuel, its broader implementation is impeded by the significant energy requirements and expenses linked to its liquefaction and transportation. Conversely, ammonia (NH3) possesses a greater hydrogen content and benefits from established production technologies, rendering it a more feasible alternative. This paper presents a thorough review of the pertinent research concerning ammonia combustion reactions. This review offers an exhaustive examination of the recent advancements in ammonia-fueled combustion, highlighting the intricacies of the chemical reaction processes and the methodologies aimed at mitigating the formation of nitrogen oxides. Its objective is to confront the hurdles linked with the utilization of ammonia as a sustainable energy source.
The intricate combustion chemistry of ammonia necessitates a thorough and precise characterization of the reaction pathways associated with oxidation and pyrolysis processes, the generation of nitrogen oxides, and the mechanisms for the abatement of these nitrogen oxides. Although considerable advancements have been made in the comprehension of ammonia chemistry, challenges and uncertainties persist. Reactions involving NH3, NH2, and NH radicals have been the subject of extensive investigation; however, certain reactions remain devoid of experimental data, resulting in ambiguities within the models. The NH2 and HO2 radicals play a crucial role under flame conditions and significantly influence the oxidation of ammonia. This review highlights the imperative to enhance the precision of laminar flame speed measurements of ammonia, particularly under elevated temperature and pressure conditions. Concerns regarding the inaccuracies introduced by radiation in these measurements have been raised, and the implications for combustion performance warrant further exploration. Future research should prioritize experimental investigations conducted under authentic high-temperature and high-pressure conditions, as well as the measurement of intermediates in ammonia combustion, including transient radicals.
It is essential to acknowledge the substantial production of nitrogen oxides (NOx) that occurs during the combustion of ammonia. The formation of NOx predominantly arises from the oxidation of NH2 and NH free radicals, with critical reactions involving the interaction of these free radicals with oxygen-containing species. However, due to a scarcity of data, particularly under combustion conditions, the rate constants for these reactions remain ambiguous, underscoring the necessity for additional experimental investigations to enhance the accuracy of predictive models. The generation and interaction of intermediates such as H2NO and HNO pose similar challenges, thereby necessitating both experimental and theoretical research endeavors. To improve the precision of models concerning ammonia pyrolysis and oxidation kinetics, it is crucial to address the uncertainties surrounding reaction rate constants through empirical measurements and theoretical analyses. Various viable strategies have been proposed and validated through theoretical, experimental, and numerical approaches, including high-pressure rich combustion and staged combustion, each presenting distinct advantages and disadvantages.
In conclusion, this systematic review underscores the promise of NH3 as a carbon-neutral fuel for diverse combustion applications. It underscores the imperative for additional research into its combustion properties to tackle existing technical challenges and enhance combustion conditions. Future investigations should focus on advancing experimental methodologies and the development of relevant technologies.

Author Contributions

Writing—review and editing, writing—original draft, conceptualization, X.Z.; writing—review and editing, writing—original draft, conceptualization, S.Z.; review and editing, conceptualization, Q.Z., Y.W. and J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Shanxi Provincial Basic Research Program (202203021221160); Shanxi Provincial Demonstration Base for Joint Training of Graduate Students in the Integration of Industry and Education in Intelligent Transportation (2024JD10); and the first batch of “Unveiling the Leader” projects of Taiyuan City’s “Double Hundred Key Projects” (2024TYJB0121).

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Research methodology of systematic literature review. (a) Research process; (b) The specific studies included/excluded at each stage.
Figure 1. Research methodology of systematic literature review. (a) Research process; (b) The specific studies included/excluded at each stage.
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Figure 2. Compare the CM scores of the considered kinetic models across different situations. The color scale ranges from green to red and is normalized within each category. A more reddish color indicates a lower score. These models are sorted according to the overall mean score [81]. Copyright 2025, Elsevier.
Figure 2. Compare the CM scores of the considered kinetic models across different situations. The color scale ranges from green to red and is normalized within each category. A more reddish color indicates a lower score. These models are sorted according to the overall mean score [81]. Copyright 2025, Elsevier.
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Figure 3. The distribution of NH and NO radicals, as determined through DNS-based computations and LIF-based measurements, is analyzed in relation to the increase in inlet pressure from 1 to 3 bar. The accompanying color bar illustrates the mass fractions of the predicted values. Reprinted from [84]. Copyright 2025, ACS. (left) mass fractions from DNS and (right) PLIF signal intensity at (a,c) p = 1 atm and (b,d) p = 3 atm.
Figure 3. The distribution of NH and NO radicals, as determined through DNS-based computations and LIF-based measurements, is analyzed in relation to the increase in inlet pressure from 1 to 3 bar. The accompanying color bar illustrates the mass fractions of the predicted values. Reprinted from [84]. Copyright 2025, ACS. (left) mass fractions from DNS and (right) PLIF signal intensity at (a,c) p = 1 atm and (b,d) p = 3 atm.
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Figure 4. The fluctuations in NOx emissions and operational temperature in relation to water content are analyzed for (a) premixed and (b) non-premixed NH3/air/H2O flames. Reprinted from [90]. Copyright 2025, Elsevier.
Figure 4. The fluctuations in NOx emissions and operational temperature in relation to water content are analyzed for (a) premixed and (b) non-premixed NH3/air/H2O flames. Reprinted from [90]. Copyright 2025, Elsevier.
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Figure 5. Utilizing LES, the computed distributions of temperature and NO were analyzed in the context of two-stage non-premixed combustion of methane–ammonia/air flames. Reprinted from [92]. Copyright 2025, Elsevier.
Figure 5. Utilizing LES, the computed distributions of temperature and NO were analyzed in the context of two-stage non-premixed combustion of methane–ammonia/air flames. Reprinted from [92]. Copyright 2025, Elsevier.
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Table 1. Thermal properties and fundamental combustion characteristics of ammonia and hydrocarbon fuels.
Table 1. Thermal properties and fundamental combustion characteristics of ammonia and hydrocarbon fuels.
PropertiesNH3H2CH4
Boiling temperature at 1 atm (°C)−33.4−253−161
Condensation pressure at 25 °C (atm)9.90N/AN/A
Flammability limit (equivalence ratio)0.63–1.400.10–7.100.50–1.70
Lower heating value (MJ/kg)18.612050.0
Adiabatic flame temperature (°C)180021101950
Maximum laminar burning velocity (m/s)0.072.910.37
Minimum auto ignition temperature (°C)650520630
Table 2. Common characteristics of nitrogen oxides (NOx) reduction strategies.
Table 2. Common characteristics of nitrogen oxides (NOx) reduction strategies.
StrategiesAdvantagesDisadvantages
Rich combustion at high inlet pressuresEasy to implement, with a wide range of applicationsIncomplete combustion, ammonia leakage
Dual-fuel combustionEasy to implementGenerate greenhouse gases
Reactant humidificationLow flame temperatureEmission problem
Plasma-assisted combustionIgnition and flame enhancementThe scope of application is limited, making it difficult to apply on a large scale
Staged combustionHigh fuel utilization rateThe related equipment is complex
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Zhang, X.; Zhao, S.; Zhang, Q.; Wang, Y.; Zhang, J. A Review of Ammonia Combustion Reaction Mechanism and Emission Reduction Strategies. Energies 2025, 18, 1707. https://doi.org/10.3390/en18071707

AMA Style

Zhang X, Zhao S, Zhang Q, Wang Y, Zhang J. A Review of Ammonia Combustion Reaction Mechanism and Emission Reduction Strategies. Energies. 2025; 18(7):1707. https://doi.org/10.3390/en18071707

Chicago/Turabian Style

Zhang, Xiqing, Shiwei Zhao, Qisheng Zhang, Yaojie Wang, and Jian Zhang. 2025. "A Review of Ammonia Combustion Reaction Mechanism and Emission Reduction Strategies" Energies 18, no. 7: 1707. https://doi.org/10.3390/en18071707

APA Style

Zhang, X., Zhao, S., Zhang, Q., Wang, Y., & Zhang, J. (2025). A Review of Ammonia Combustion Reaction Mechanism and Emission Reduction Strategies. Energies, 18(7), 1707. https://doi.org/10.3390/en18071707

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