Ammonia Combustion Research Articles

Investigating NO emissions, stability, and flame structure in co-fired premixed NH3/CH4/air swirling flames

Ammonia combustion poses challenges due to low reactivity and high NOx emissions, requiring optimization of combustor designs and fueling strategies. This study examines NO emissions, flame stability, and structure in co-fired premixed NH3/CH4/air flames using a double-swirl burner. The inner swirl stream consists of NH3/CH4/air mixtures with varying ammonia mole fractions (xNH3: 0 to 1) and equivalence ratios (Φin: 0.4 to 1.4), while the outer stream contains CH4/air mixtures with Φout ranging from 0.5 to 0.8 and Reynolds numbers (Reout) of 4350, 5250, and 6000. NO emissions varied significantly with Reout, Φin, and Φout, prompting further investigation of flame structure using OH-NO PLIF and PIV diagnostics for three flame sets: FA (Φin=0.4), FB (Φin=0.8), and FC (Φin =1.4). Far-rich (FC) and far-lean (FA) flames exhibited an early conical OH layer followed by a V-shaped OH layer, while NO dispersed across the flame, forming a thin layer at the OH boundary with a V-shaped distribution downstream. Higher Reout facilitated V-OH/NO formation through enhanced mixing, increased recirculation, and more effective ammonia cracking in rich mixtures. At Reout=4350, the absence of a V-OH layer in FA resulted in reduced NO emissions. Flame FB showed a broader, positively correlated NO and OH structure along the central region of the flame, indicating enhanced NHi oxidation to NO. Overall, co-firing ammonia with methane in the outer stream was crucial for improving flame stability. To minimize NO emissions, it is important to lower Reout, increase Φout, and avoid premixing NH3/CH4 in the inner stream. At high Reout, limiting rich Φin to 1.2 or leaning it out, combined with increasing Φout, was the most effective strategy for reducing NO emissions.

Revealing the NO Formation Kinetics for NH3/CH4 Blends Under Dual-Flame and Premixed Swirl Flame Configurations

Ammonia stands out as a promising zero-carbon fuel and an efficient hydrogen carrier, offering great promise for industrial applications in gas turbines and boilers. However, different combustion modes significantly influence the flame structure and combustion characteristics of ammonia. In this study, two distinct fuel injection strategies were employed in a model combustor: ammonia and methane, under fully premixed and dual-flame combustion modes. Numerical simulations were performed to analyze the flame structure, velocity fields, and temperature distribution, complemented by planar flow field, flame OH* chemiluminescence, and NO emission measurements. Findings reveal that with an increasing NH3 ratio, the flame front becomes more elongated with more pronounced temperature fluctuations at the swirler exit. Particularly, at 50% NH3, a significant reduction in flame temperature is observed, notably at a height of 30 mm from the burner. For dual flames, the reaction NH2 + O ↔ HNO + H was less significant compared to its effect in premixed flames, whereas the H + O2 ↔ O + OH reaction demonstrated the highest sensitivity coefficient. An increase in the NH3 ratio correspondingly led to a reduction in NO consumption reaction rates, heightening the sensitivity coefficient for NO inhibition, and providing critical insights into ammonia combustion optimization.

Effects of boundary conditions on flash boiling ammonia sprays from a twin-fluid atomizer

Ammonia is a carbon-free fuel with a high hydrogen capacity (17.8 wt%) and well-established production and distribution infrastructures, making it a viable option for decarbonized and dispatchable power. In addition, ammonia combustion in its liquid phase is preferred for simplified storage, higher energy density, and compatibility with high-pressure industrial conditions. However, ammonia fuel has drawbacks that must be addressed. This work aims to investigate experimentally the effect of the boundary conditions on the dynamics of an ammonia spray injected by a commercial twin-fluid atomizer in a chamber at room pressure and temperature. A flash boiling analysis was conducted to assess the thermodynamic conditions that promote this phenomenon and the accuracy of existing correlations to predict this transition to flash boiling. The result showed that low injection pressures and preheated atomizing flow promote the formation of bubbles and the subsequent collapse inside the atomizer, creating an unstable spray. Such instabilities are significantly reduced when the injection pressure is increased. Additionally, increasing the atomizing air flow, injection pressure, and atomizing flow temperature improves the fuel breakup, widening the spray angle. However, the spray angle decreases at a certain point, indicating that atomization is more sensitive to thermodynamic conditions than a mechanical breakup.

Combustion and emission characteristics of ammonia–diesel and ammonia–PODE dual fuel engines with multi-time ignition mixed-mode combustion (MIMC) mode

Combustion and emission characteristics of ammonia–diesel and ammonia–PODE dual fuel engines with multi-time ignition mixed-mode combustion (MIMC) mode

Numerical study on the pretreatment of ammonia by nanosecond pulsed discharge for combustion enhancement: Effects of pretreatment uniformity

The nanosecond (NS) pulsed discharge for ammonia combustion assistance is numerically investigated with a special focus on the effects of discharge pretreatment uniformity. A method to mimic spatially non-uniform discharge pretreatment is established. Using NH3/O2/He and NH3/O2/N2 mixtures as examples, the ignition delay time (IDT), the laminar burning velocity (LBV), and the extinction strain rate (ESR) of mixtures pretreated by different discharge pretreatment methods were analyzed under a wide range of pretreatment uniformity. The results indicate that for the premixed NH3/O2/He (or N2) pretreated by NS discharge pulses, with the increase of the pretreatment non-uniformity, the IDT initially increases slightly, reaching a maximum and then decreases rapidly by 80 %, while the LBV and the ESR first decreases slowly, reaching a minimum and then increases rapidly by 10 %. The initial increment of IDT with the non-uniformity can be attributed to the deterioration of discharge-induced chemical effect but the rapid decrease of IDT as the discharge pretreatment becomes highly non-uniform is largely determined by the combustion-related chemical effect. The effects of pretreatment uniformity on LBV and ESR can be largely due to the thermal effect. When the oxidant is pretreated by the NS discharge, the minimal IDT, the maximal LBV and ESR can be observed under uniform conditions owing to the chemical effects and thus a uniform discharge pretreatment is preferred. Furthermore, a comparative analysis of the three pretreatment methods under identical energy input reveals that under uniform pretreatment, the direct pretreatment of pure NH3 is optimal, since it can generate more H2 to enhance the combustion. However, under highly non-uniform conditions, the discharge pretreatment of premixed fuel/oxidant mixtures is the most efficient for combustion enhancement.

Numerical investigation and optimization of the ammonia/diesel dual fuel engine combustion under high ammonia substitution ratio

This study investigated the effects of initial temperature, equivalence ratio, and diesel injection timing on engine combustion and emission characteristics at high ammonia substitution ratios. Increased compression temperature and pressure significantly reduce ignition delay, enhance combustion speed and efficiency, and decrease N2O and unburned NH3 emissions. A strong correlation exists between the amount of N2O produced and the mass of unburned NH3 when ammonia combustion efficiency is high. The N2O distribution is concentrated near the cylinder walls and the bottom surface of the piston platform, in areas with high concentrations of unburned NH3. As the equivalence ratio increases from 0.6 to 0.75, flame propagation speed and indicated thermal efficiency (ITE) increase, while NOx, N2O, and unburned NH3 emissions decrease. The combustion performance and emissions were optimized by advancing the diesel injection timing and increasing the equivalence ratio to accelerate the combustion speed. This adjustment increases ITE to 47.6% at an 80% ammonia energy ratio. Post-optimization results show a reduction in unburned NH3 emissions from 51.7 g/kW·h to 5.9 g/kW·h and a decrease in N2O emissions from 0.930 g/kW·h to 0.370 g/kW·h, culminating in a 60.4% reduction in greenhouse gas (GHG) emissions.

Large eddy simulations and experimental studies of the influence of equivalence ratio on the combustion characteristics of turbulent NH3–CH4 premixed flames

As one of the most promising zero-carbon fuels, ammonia has attracted widespread attention. However, ammonia combustion faces problems such as high nitrogen oxide emissions. This work aims to investigate the combustion characteristics of fuel gas mixture of 60% NH3 and 40% CH4 by volume, under five different equivalence ratios, i.e., 0.7, 0.85, 1.0, 1.15, and 1.3, respectively. Both large eddy simulation (LES) and experimental test are conducted. The results reveal that flame temperature and nitric oxide (NO) emissions exhibit an initial increase, followed by a decrease with rising equivalence ratios. Notably, the highest temperature is observed at ϕ = 1.0, while peak NO emission is found at ϕ = 0.85. As the equivalence ratio changes, the variation of turbulent flow fields and mass recirculation rates is not significant. On the contrary, NO and OH radicals exhibit distinct shifts in relation to the equivalence ratio. The NO emissions predicted by LES agree well with the experimental results. A chemical reaction network (CRN) analysis is also conducted, which effectively predicts NO variation trends and clarifies NO generation pathways and key mechanisms. The CRN analysis highlights variations in the sensitivities of crucial constituents, such as NH3, OH, and NO, to variations in the equivalence ratio.

Economic and Environmental Assessment of Ammonia as a Sustainable Fuel for Tugboat Operations at the Port of Texas

This study assesses the feasibility of ammonia as a sustainable alternative fuel for tugboat operations in the Port of Texas, a major U.S. maritime, oil and gas hub. Tugboats, while critical to port logistics, contribute significantly to localized air pollution and greenhouse gas emissions, impacting both environmental quality and public health in surrounding communities. Given the International Maritime Organization’s (IMO) goals to reduce carbon emissions by 40% by 2030 and 70% by 2050, exploring alternative fuels has become imperative. Ammonia, a carbon-free fuel, presents both opportunities and challenges. While ammonia combustion produces no direct CO₂ emissions, making it attractive from a decarbonization standpoint, its lower energy density results in higher fuel volumes and increased operational costs compared to marine fuel oil (MFO). This paper presents a comparative analysis based on real-world data, mathematical modeling, and emission trade-offs, examining the economic implications, environmental impact, and safety concerns of adopting ammonia as a tugboat fuel. Results indicate that while ammonia could enable significant CO₂ reductions, challenges related to fuel storage, cost, and infrastructure must be addressed. The findings suggest that with appropriate policy support and investment in handling infrastructure, ammonia can become a viable fuel option for sustainable maritime operations.

Combustion characteristics of Ammonia/Air mixture ignited by flame jet with and without hydrogen injection in Pre-Chamber

Ammonia is one of the important fuels that can promote the achievement of carbon neutrality, but its combustion characteristics are not conducive to its application in engines. Injecting hydrogen in the pre-chamber to form a flame jet to ignite the ammonia/air mixture is a method to improve the combustion of ammonia. This ignition method was investigated with the constant volume combustion chamber, high-speed video camera observation and the main-chamber pressure analysis. The flame behavior and combustion characteristics were compared with those ignited by the ammonia flame jet. Hydrogen flame jet significantly enhanced the combustion of the mixture and extended the lean flammability limit. Under conditions of hydrogen injection, the pressure rise delay and combustion duration became shorter, the average heat release rate became higher, and the combustion process was more stable. The hydrogen flame jet rapidly penetrated the main-chamber and impinged on the lower surface, generating intense turbulence. As a result, the combustion pressure took less time to rise from 10% to 50% than it did to go from 50% to 90%. This was different from the situation without hydrogen injection, where time required for both was similar. The hydrogen flame jet was shuttle-shaped when touching the lower surface owing to the rapid combustion speed of hydrogen, while the ammonia flame jet was spindle-shaped with the flame kernel in the center. The combustion process of the flame jet igniting the mixture exhibited two peaks in the heat release rate, reflecting two combustion stages dominated by the flame jet and flame propagation.

Experimental investigation on ammonia combustion ignited by methanol-enriched active pre-chamber in an optical engine

To enhance the ignitability and burning rate of ammonia-fueled spark ignition engines, it is an effective way to adopt active pre-chamber (PC) ignition with high-reactivity fuel. Therefore, in this paper, the ammonia combustion characteristics using methanol-enriched active PC ignition were investigated, based on optical engine experiments with simultaneous in-cylinder flame imaging and pressure measurement. Results show that the overall combustion process is characterized by a three-stage heat release: (I) jet flame controlled combustion; (II) ammonia flame propagation; (III) post jet induced combustion. In stage I, methanol active PC jets with high-momentum and high-reactivity are discharged to ignite MC ammonia mixture, contributing to the first peak of heat release rate. In stage II, ammonia turbulent flame propagation dominates the combustion. In stage III, post jets induce the third peak of heat release rate. In this way, with a minimal methanol energy ratio (ER) of lower than 10 %, stable and fast ammonia combustion is achieved under stoichiometric (λMC = 1) and lean-burn (λMC = 1.2) condition, with cyclic variation of indicated mean effective pressure (IMEP) lower than 5 %, flame probability higher than 80 % and maximum global flame speed of about 4 m/s. A highest IEMP of 0.72 MPa and a peak indicated thermal efficiency (ηit) of 30 % is obtained at λMC = 1 and λMC = 1.2, respectively. However, at the rich limit of λMC = 0.9 and lean limit of λMC = 1.4, the quenching of jet flames leads to weak ignition and unstable combustion, which is identified by reduced flame probability distribution and increased cyclic variation of IMEP. In addition, to ensure the cyclic combustion stability, there is also an operating range of methanol ER. At λMC = 1.2, unstable ignition and deteriorated combustion occur below the lower limit of ER = 6 % and above the upper limit of ER = 17 %. This paper can provide data support and operation strategy for the combustion improvements of ammonia-methanol active jet ignition engines.

Low-temperature lean combustion and emissions characteristics of pilot diesel-ignited premixed ammonia in a constant volume chamber

In this study, an optical experiment was performed in a constant volume chamber to investigate the effects of ambient temperature (700–800 K), ambient pressure (1.2–2.2 MPa), and ammonia energy fraction (30–90%) on low-temperature lean combustion and emissions characteristics of pilot diesel-ignited premixed ammonia. As the temperature was increased from 700 K to 800 K, a transition occurred from diesel-dominated diffusion combustion to ammonia-dominated premixed combustion, marked by a shift from a single to a double-peaked heat release rate (HRR). Notably, 750 K appears to be a demarcation point between these two combustion regimes. The increase in temperature also expands the spatial distribution of the NO group and results in its earlier formation. Elevated ambient pressure leads to a shorter ignition delay, a higher spatial integrated natural luminosity (SINL, a soot indicator), and a larger percent flame area (PFA). Moreover, peak HRR and NO emissions are accelerated, bringing the combustion process closer to diesel-like characteristics. High ammonia energy fractions have a substantial negative impact on combustion, but significantly reduce NO production in the later stages of combustion. Compared to a 30% ammonia energy fraction, a 90% ammonia energy fraction results in a 78.6% reduction in peak HRR and a 277.5% increase in ignition delay. Meanwhile, both SINL and PFA are significantly reduced. A 50% ammonia energy fraction can significantly increase the ammonia content of blended fuels without notably affecting combustion and emissions characteristics.

Laminar burning characteristics of ammonia and hydrogen blends at elevated initial pressures up to 2.5 MPa

Ammonia is a highly promising alternative fuel, and blending it with hydrogen can mitigate its poor combustion performance. However, the combustion mechanism of ammonia-hydrogen mixtures requires further validation through measurements of laminar burning velocity, particularly under high-pressure conditions relevant to practical combustors. Experimental data at high initial pressures (>1.0 MPa) are notably scarce. In this study, experiments were conducted using an outwardly propagating spherical flame in a high-pressure, high-temperature, constant-volume combustion chamber to determine the laminar burning velocities of ammonia-hydrogen blends at elevated initial pressures. The effects of different equivalence ratios (0.6 ∼ 1.4), volumetric hydrogen ratios (0.1 ∼ 0.6), and initial pressures (0.1 ∼ 2.5 MPa) were evaluated. Chemical kinetics of ammonia and hydrogen combustion was investigated at high pressures. The experimental results were compared with the simulation results under a wide range of conditions using several existing mechanisms to evaluate their applicability. The results show that the laminar burning velocity increases first and then decreases as the equivalence ratio (ϕ) increases and reaches a maximum value around ϕ of 1.1. The laminar burning velocity increases non-linearly with the volumetric hydrogen ratio, with more pronounced acceleration at higher hydrogen ratios. In contrast, the laminar burning velocity decreases non-linearly with the increasing initial pressure, and this reduction becomes progressively slower as the initial pressure rises. The predictive performance of the mechanisms by Okafor et al. and Gotama et al. is relatively satisfactory under certain conditions, but further optimization based on experimental data is necessary. Additionally, reaction pathway analysis of ammonia-hydrogen combustion indicates that introducing hydrogen increases the concentration of active radicals such as OH, O, and H, thereby enhancing ammonia combustion. Sensitivity analysis of the laminar burning velocity identified the critical elementary reactions across varying pressures, providing strong support for optimizing ammonia-hydrogen chemical kinetics models and developing simulation models for practical combustion systems.

Development of Detailed and Reduced Chemical Kinetic Models for Ammonia Gas Turbines

Abstract The power generation sector has been recently moving toward decarbonization, and there is an increased interest in replacing conventional fossil fuels with fuels that produce reduced/zero carbon emissions. One such fuel is ammonia (NH3). However, ammonia is hard to ignite, has a low flame speed, and produces a significantly large amount of nitrogen oxide (NOx) emissions. Hence, using 100% ammonia as fuel in gas turbines requires significant modifications and the development of novel combustors. Blending hydrogen with ammonia, however, helps in having better control over the combustion properties. For example, a 70%/30% mixture of NH3/H2 mixture has a flame speed comparable to natural gas. Before utilizing hydrogen-blended ammonia in an actual gas turbine combustor, thorough simulation studies are required to evaluate its performance, possible hazards, and emissions. The literature lacks well-validated chemical kinetic models for the combustion of hydrogen-blended ammonia for undiluted mixtures at gas turbine-relevant conditions (∼20 bar). Most models available in the literature have been developed for ammonia extremely diluted in diluents such as argon or nitrogen. Hence, in this work, we develop a detailed chemical kinetic model for hydrogen-blended ammonia combustion and validate it with a wide range of experimental data for both dilute and undiluted mixtures relevant to gas turbine operating conditions. We outline the strengths and weaknesses of the current mechanism to aid future users of our chemical kinetic mechanism. The detailed chemical kinetic mechanism was reduced to a smaller version (32 species mechanism) without significant loss in accuracy using the directed relation graph with error propagation (DRGEP) and full species sensitivity analysis. The resultant mechanism can predict a wide range of experimental results with the least cumulative error and will be a valuable tool in computational fluid dynamics (CFD) simulations that will enable the development of gas turbines for zero-carbon power generation.

Laser-induced plasma analysis of ammonia-oxygen and ammonia-oxygen-enriched-air flames at elevated pressures

This study employed Laser-induced breakdown spectroscopy (LIBS) to measure the fuel-oxidizer ratio (FOR) of ammonia combustion with oxygen-enriched air and pure oxygen flames at elevated pressures (100 - 300 kPa). The correlations between the spectral line intensity ratios of nitrogen (N), hydrogen (H), oxygen (O), and equivalence ratio were used to quantify the FOR of flames at various pressures. The effect of pressure on the stability and precision of the calibration profiles for the elemental intensity ratios in flames was investigated. It was observed that the H/O correlation decreases with pressure increase for both ammonia flames. N/O correlations decrease with elevated pressure for the ammonia-oxygen flame. Furthermore, the nitrogen (NII) spectral emission lines at 568 nm and 595 nm were used to estimate the plasma temperature, while the hydrogen (Hα) line at 656 nm was used for electron number density measurements.

Strategies to improve ammonia combustion in a dual fuel marine engine by using CFD

Maritime transportation is currently responsible for delivering most of the world trade, and it still plays a major role in the global economy and pollutant emissions. Due to their heavy loads and long distances, full electrification is generally assumed as an unsuitable pathway, leaving combustion engines as the dominant technology in the marine sector. For a long time, these engines have benefited from the low costs of heavy oils produced from oil refining, but these fuels present a difficult combustion and are associated with significant pollutant emissions and greenhouse gases. Today, carbon-free fuels such as hydrogen and ammonia are gradually getting a foothold in the shipping market.In the present paper, an investigation on several strategies to improve the oxidation of ammonia is carried out for a dual fuel (DF) medium speed marine engine. A CFD analysis using ANSYS FORTE® code is performed on an improved and more realistic bowl geometry by varying the premixed fuel according to methods validated in previous studies and introducing adequate kinetic mechanisms from the literature. Results show that the sole use of ammonia to replace natural gas, as premixed fuel, leads to incomplete combustion; the increase of diesel fuel can only mitigate this condition, but it cannot be considered as viable solution. Therefore, several blends of ammonia/hydrogen are tested to examine combustion development and pollutant emissions. The addition of hydrogen above 10% to the premixed charge composition demonstrates to promote the oxidation of ammonia and to effectively reduce CO2 emissions of 84% without a dramatic increase in nitrogen oxides. On the contrary, hydrogen percentages lower than 10% feature a worsening of the combustion of the premixed charge. Finally, the use of a fuel containing nitrogen promote a slight formation of NOx emissions.

Ammonia diffusion combustion and emission formation characteristics in a single cylinder two stroke engine

Liquid ammonia diffusion combustion (LADC) is an effective approach to achieving high ammonia energy ratio (AER) and combustion efficiency in engines. However, there is a lack of simulation studies of LADC mode in ammonia/diesel dual direct injection engines. To address this gap, the study develops a numerical model based on an independently modified marine two-stroke engine. This model analyzes the combustion performance of ammonia in LADC mode and the pollutant generation mechanisms in LADC mode across various AER conditions. The findings indicate that in the LADC mode, the liquid phase ammonia spray exhibits a concentrated distribution area, facilitating thorough combustion of ammonia fuel. The diesel mixture and flame are entrained into the liquid phase ammonia spray, facilitating multi point ignition. This process notably enhances combustion speed, thereby reducing overall combustion duration. In comparison to pure diesel combustion, soot emissions are reduced by 99 % under the LADC mode, and CA90 advanced by 11.8 °C A. In addition, when the AER range is from 50 % to 70 %, the engine exhibits favorable combustion and emission characteristics. By appropriately adjusting the liquid ammonia injection quantity, the indicated thermal efficiency improves by 10.7 %, the equivalent indication specific fuel consumption decreases by 13.9 %, and the emission such as NOx and soot can be also reduced effectively.

Numerical Study on Flame Geometric Characteristics of Ammonia-Fueled Small Boiler

Amidst global efforts to cut carbon emissions, the carbon-neutral fuel sector is at a crossroads of opportunity and challenge. The pursuit of clean, sustainable energy alternatives is critical. Ammonia, with its zero-carbon and hydrogen-rich profile, is emerging as a promising clean fuel, yet its combustion behavior in industrial boilers demands further study. This paper presents a numerical analysis of the flame geometry characteristics of ammonia combustion at an equivalence ratio of 1. The study delineates the dimensions of the flame, specifically its diameter and length, offering valuable insights that can inform the design of industrial boilers. This research contributes to the broader understanding of ammonia's role in the transition towards more sustainable energy solutions in industrial settings.

Perspectives on NO X Emissions and Impacts from Ammonia Combustion Processes.

Climate change and global warming necessitate the shift toward low-emission, carbon-free fuels. Although hydrogen boasts zero carbon content and high performance, its utilization is impeded by the complexities and costs involved in liquefaction, preservation, and transportation. Ammonia has emerged as a viable alternative that offers potential as a renewable energy storage medium and supports the global economy's decarbonization. With its broader applicability in large power output applications, decentralized energy sources, and industrial locations off the grid, ammonia is increasingly regarded as an essential fuel for the future. Although ammonia provides a sustainable solution for future low-carbon energy fields, its wide-scale adoption is limited by NO X emissions and poor combustion performance under certain conditions. As research on ammonia combustion expands, recent findings reveal factors impacting the chemical reaction pathways of ammonia-based fuels, including the equivalence ratio, fuel mixture, pressure, and temperature. Investigations into ammonia combustion and NO X emissions, at both laboratory and industrial scales, have identified NO X production peaks at equivalence ratios of 0.8-0.9 for ammonia/hydrogen blends. The latest studies about the NO X emissions of the ammonia flame at different conditions and their generating pathways are reviewed in this work. Effective reduction in NO production from ammonia-based flames can be achieved with richer blends, which generate more NH i radicals. Other advanced NO X mitigation techniques such as plasma-assisted combustion have been also explored. Further research is required to address these challenges, reduce emissions, and improve efficiencies of ammonia-based fuel blends. Finally, the extinction limit of ammonia turbulent flame, its influential factors, and different strategies to promote the ammonia flame stability were discussed. The present review contributes to disseminating the latest advancements in the field of ammonia combustion and highlights the importance of refining reaction mechanisms, computational models, and understanding fundamental phenomena for practical implications.

Ammonia Sprays for Combustion: A Review

Ammonia is a globally transported chemical used for a variety of applications, most notably, the production of fertiliser. Over the past decade, attention has been afforded to the use of ammonia as an energy carrier, coupling global supply of renewable energy to demand regions. Ammonia’s advantages as an energy carrier include its ease of liquefaction and established international transportation routes; overcoming its low reactivity, excessive production of nitrogen oxides and its toxicity remain as challenges. For energy applications, fuel delivery is a critical aspect of effective combustion in boilers, burners and engines. Due to its adaptable phase change characteristics, ammonia fuel may be injected as a liquid or vapour, each with respective advantages or disadvantages. The focus of this review concerns the characterisation of liquid ammonia fuel injection for combustion, including recent research findings from experimental and simulation studies. Liquid ammonia injection can result in the highly dynamic so-called ‘flashing’ or ‘flash boiling’ phenomena. Research findings have been drawn from other related applications such as accidental hazardous releases. Bespoke optical experimental rigs together with diagnostic techniques and two-phase computational fluid dynamics (CFD) simulations have enabled studies of the flashing jets under various initial or final conditions, with recent work also examining ammonia spray combustion. The review concludes with an insight into future trends and requirements for liquid ammonia combustion. Reciprocating engines for marine propulsion are cited as potential early adopters of ammonia energy.

A detailed chemical kinetic model for methanol and ammonia co-oxidation under hydrothermal flames: Reaction mechanism and flame characteristics

A detailed chemical kinetic model for ammonia and methanol co-oxidation under hydrothermal flames was established with pressure and thermodynamic corrections. Simulation model was validated by comparing with experimental temperature rise, and methanol and ammonia removal efficiencies. Species evolutions, reaction paths, and reaction sensitivities for pure ammonia combustion, and ammonia and methanol co-oxidation under hydrothermal flames were analyzed. Ammonia concentration begins to decrease only when methanol is consumed in a certain amount in the ammonia and methanol co-oxidation under hydrothermal flames, indicating the addition of methanol promotes the degradation of ammonia. Moreover, reaction heat is more conductive to ignition and the conversion of ammonia to nitrogen than active free radicals provided from methanol. The ignition delay time presents a minimum as a function of ammonia/methanol concentration ratio, with the minimum values of ignition delay time present at approximately ωNH3/ωCH3OH = 1 for different methanol concentrations. Higher preheating temperatures favor more NOX but less N2O formation, while higher ammonia concentrations favor both NOX and N2O formations. The presence of ammonia increases the laminar flame speed and permits a lower extinction temperature, indicating the mixture of methanol and ammonia can improve the stability of hydrothermal flames.