Ammonia Combustion Research Articles

Effects of partial fuel cracking on the forced ignition and spherical flame propagation in ammonia/air mixtures

As a zero-carbon fuel, ammonia has promising applications and thereby has received great attention recently. However, compared to traditional hydrocarbon fuels, ammonia has very low laminar flame speed and extremely high minimum ignition energy (MIE). A feasible method to promote ammonia combustion is partial fuel cracking, during which ammonia undergoes decomposition into hydrogen and nitrogen. This work reports a computational study on forced ignition and spherical flame propagation in partially cracked ammonia/air mixtures. The objective is to assess the effects of fuel cracking on the MIE and flame propagation. It is found that even a slight amount of ammonia cracking can significantly reduce the MIE and promote ignition. In addition, the influence of positive stretch rate on the spherical flame propagation speed, which is quantified by the Markstein length, is shown to be greatly affected by ammonia cracking. The mechanism of ignition promotion and change in spherical flame propagation by ammonia cracking is interpreted and discussed.

Quantitative comparison of the benefits of cracking and nanosecond repetitive pulsed discharges on the lean blow-off, emissions, and topology of ammonia premixed swirl flames

In the last decade, ammonia has gained interest as an alternative fuel for the energy sector. The main advantages include its carbon neutrality and ease of storage at industrial scale. However, to consider ammonia as an alternative carbon-free fuel it is necessary to solve two main issues, namely flame stability and pollutant emission. In this work we compared two promising technologies used to enhance ammonia combustion, specifically partial cracking and plasma assisted combustion (PAC). The experiments were carried out for different blends of ammonia–hydrogen–nitrogen–air that reproduce the conditions of cracking as well as for pure ammonia assisted by nanosecond repetitive pulsed (NRP) discharges for the PAC. The cracking and PAC were compared for the same added power ratio at atmospheric conditions. Different added power ratios, from 0.1% to 4% of the flame’s total thermal power, were tested. The lean blow-off limits were measured for a range of bulk flow velocities between of 4 and 12 m/s. For the pollutant emissions, NOx, NH3, and N2O were considered. The effect of both strategies on the Laminar burning velocity (LBV) was explored by numerical simulations. For the same added power ratio, partial cracking extended the lean blow-off limits further than NRP discharges, even with a high pulse repetition frequency of 30 kHz. This result indicated that cracking was more effective in stabilizing the flame than PAC. Regarding NOx emissions, NRP discharges induced only a third of the increase observed with cracking. However, the reduction in N2O associated with cracking was twice as much as that for NRP discharges. Finally, cracking proved more efficient in reducing the flame height, which was supported by the simulations of LBV. These results suggest that PAC strategies optimized for hydrocarbon flames are less efficient on ammonia combustion, and that more work is needed to develop plasma actuators for NH3 combustion.Novelty and Significance statement This work marks the first one-to-one quantitative comparison between plasma actuation and thermal cracking on ammonia swirl flames. It serves as a pivotal exploration to assess and understand the economic aspects of these strategies, providing valuable insights into their efficiency and potential for practical implementation.

Assessment of combustion development and pollutant emissions of a spark ignition engine fueled by ammonia and ammonia-hydrogen blends

Ammonia is a promising carbon-free fuel for internal combustion engines, but its combustion properties are very poor if compared with conventional fuels. Thus, hydrogen enrichment can be the proper solution in order to overtake the issues related to neat ammonia fueling. In this paper, the authors use a 3D numerical model coupled with a chemical kinetic approach to assess the operation of a light-duty spark-ignition engine fueled by neat ammonia and ammonia-hydrogen blends (up to 15% of hydrogen by volume). The proposed model reproduces the analyzed experimental operating points with a good accuracy. As example, considering the in-cylinder pressure peak, the difference between the calculated maximum pressure and the mean measured one ranges between 1.6% and 6.8% for the examined cases. The 3D approach enables an in-depth analysis of flame development that also allows to detect the effect of hydrogen enrichment on both combustion development and emissions. Unburned ammonia trends are captured, reproducing the measured decrease of ammonia emissions for increasing hydrogen enrichment. The emissions of NOx are also well reproduced. Simulations also highlighted that dissociation of ammonia produces H2, which is detected in the combustion chamber also if neat ammonia is burned and, at least according to the chemical mechanism used, not all the produced hydrogen oxidizes. Only very small quantities of intermediates related to ammonia combustion are calculated at exhaust valve opening, decreasing for an increasing hydrogen content in the fuel mixture.

NH3/C2H6 and NH3/C2H5OH oxidation in a shock tube: Multi-speciation measurement, uncertainty analysis, and kinetic modeling

This study examines NH3, NO, and CO profiles during NH3/C2H6 and NH3/C2H5OH oxidation in a shock tube using laser absorption spectroscopy at 1317–1957 K and ∼2.8 bar, with 5–20% C2H6 or C2H5OH, equivalence ratios of 0.5/1.0/1.5, and a 0.9 argon dilution ratio. Metrological uncertainty analysis on mole fractions have been performed. A novel dynamic uncertainty methodology is proposed to time-resolved speciation profiles. From the experimental measurements, the promotional effect on ammonia ignition by C2H6 or C2H5OH is identical under investigated conditions. The multi-speciation profiles demonstrate reasonable relations among the consumption of NH3 and the formation of NO and CO, which show strong temperature and equivalence ratio dependence. An updated PTB-NH3/C2 1.1 mechanism accurately predicts ignition delay times and speciation profiles. Sensitivity analysis reveals NH3 chemistry’s significant control, with NH3+O2/H/OH and N2H2+M reactions being critical at >1300 K, and HO2 interactions being pivotal at 800–1200 K. The reactions related to N2H2 and N-C chemistries dominate for fuel-rich mixtures at high temperatures, resulting in a reverse equivalence ratio dependence on IDTs, namely fuel-rich mixtures exhibit the lowest reactivity. The N-C chemistries, particularly the pronounced reaction pathway of HCN→CN→NCO→HNCO→CO play a crucial role in maintaining the CO level constant after reaching its peak position under fuel-rich conditions·NH3 consumption pathways shift from NH2→H2NO→HNO→NO→NO2→N2O→N2 at intermediate temperatures to NH3→NH2→NH→N(HNO)→NO→(N2O)N2 at high temperatures. The C2-additives participate in the oxidation process by introducing additional OH/H/CH3 radicals, thereby facilitating the reactive radical pool and developing the N-C intersystem-crossing pathways. In summary, the extensive multi-speciation data, metrological uncertainty analysis, mechanism validation and development, together with comprehensive kinetic modelling can be valuable resources for further studies of ammonia combustion.

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

Abstract The power generation sector has been recently moving towards 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. 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). 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. The detailed chemical kinetic mechanism was reduced to a smaller version (32 species mechanism) without significant loss in accuracy using the direct 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 CFD simulations that will enable the development of gas turbines for zero-carbon power generation.

Numerical Study on Optimization of Combustion Cycle Parameters and Exhaust Gas Emissions in Marine Dual-Fuel Engines by Adjusting Ammonia Injection Phases

Decarbonizing maritime transport hinges on transitioning oil-fueled ships (98.4% of the fleet) to renewable and low-carbon fuel types. This shift is crucial for meeting the greenhouse gas (GHG) reduction targets set by the IMO and the EU, with the aim of achieving climate neutrality by 2050. Ammonia, which does not contain carbon atoms that generate CO2, is considered one of the effective solutions for decarbonization in the medium and long term. However, the concurrent increase in nitrogen oxide (NOx) emissions during the ammonia combustion cycle, subject to strict regulation by the MARPOL 73/78 convention, necessitates implementing solutions to reduce them through optimizing the combustion cycle. This publication presents a numerical study on the optimization of diesel and ammonia injection phases in a ship’s medium-speed engine, Wartsila 6L46. The study investigates the exhaust gas emissions and combustion cycle parameters through a high-pressure injection strategy. At an identified 7° CAD injection phase distance between diesel and ammonia, along with an optimal dual-fuel start of injection 10° CAD before TDC, a reduction of 47% in greenhouse gas emissions (GHG = CO2 + CH4 + N2O) was achieved compared to the diesel combustion cycle. This result aligns with the GHG reduction target set by both the IMO and the EU for 2030. Additionally, during the investigation of the thermodynamic combustion characteristics of the cycle, a comparative reduction in NOx of 4.6% was realized. This reduction is linked to the DeNOx process, where the decrease in NOx is offset by an increase in N2O. However, the optimized ammonia combustion cycle results in significant emissions of unburnt NH3, reaching 1.5 g/kWh. In summary, optimizing the combustion cycle of dual ammonia and diesel fuel is essential for achieving efficient and reliable engine performance. Balancing combustion efficiency with emission levels of greenhouse gases, unburned NH3, and NOx is crucial. For the Wartsila 6L46 marine diesel engine, the recommended injection phasing is A710/D717, with a 7° CAD between injection phases.

The Effect of Mixture Variation and Initial Temperature On the NH2* Thickness of Spherically Propagating Laminar Ammonia Flames

Abstract Interest in ammonia (NH3) in combustion has increased recently as a carbon-free fuel alternative. Understanding its combustion characteristics is crucial. One way to increase knowledge of ammonia combustion is by investigating the flame zone of a laminar flame. Using a high-spatial-resolution flame zone measurement technique developed by the current research group, the flame zone of different NH3-containing mixtures was measured experimentally. These measurements were achieved by investigating spherically propagating flames using a chemiluminescence imaging diagnostic with a focus on NH2* profiles. The effect of the fuel mixture on the profile shape was examined by investigating two different mixtures: an oxy-ammonia mixture consisting of NH3 + oxygen-enriched oxidizer with varying O2 concentrations from 25% to 40%, and a blend of NH3-H2 with NH3 concentrations (XNH3) varied from 0.5 to 0.8. Additionally, the effect of the initial temperature was investigated by varying it from 293 to 373 K. In all investigated mixtures, the initial pressure was fixed at 1 atm and the equivalence ratio at 1.0. The study revealed that increasing the O2 concentration in the oxy-ammonia mixture produced thinner flames, while increasing XNH3 in the NH3-H2 blend produced slightly thicker flames. Varying the initial temperature had different effects: it thinned the flame in the oxy-ammonia mixture and NH3-H2 blend, but slightly thickened the flame in the H2-NH3-N2 blend. The predicted NH2* profile thicknesses from chemical kinetics agreed with measurements, except for the H2-NH3-N2 blend.

Numerical study on laminar burning velocity and ignition delay time of ammonia/methanol mixtures

As a carbon-free hydrogen-carrying fuel, ammonia can effectively alleviate the problem of CO2 emissions and has great application potential in marine engines. The low laminar burning velocity and calorific value, and the narrow flammability limit range make the combustion of pure ammonia difficult. Ammonia/methanol mixed combustion is a feasible method to improve ammonia combustion and mitigate CO2 emissions. It is difficult to accurately predict the laminar burning velocity (LBV) and ignition delay time (IDT) of ammonia/methanol combustion simultaneously by the existing chemical kinetic mechanisms. This study aims to develop a detailed chemical kinetic mechanism that can simultaneously predict the LBV and IDT. The new-developed mechanism was fully validated based on experimental data. Furthermore, it was employed to investigate the effects of methanol blending ratios and equivalence ratios on the LBV and IDT in NH3/CH3OH mixed flames. Besides, chemical kinetic analyses were conducted to elucidate the influence of methanol mixing ratios and equivalence ratios on LBV and IDT. Reaction pathway analysis revealed that the NH3 and CH3OH chemistry interacted by the sharing of H, OH and O radicals in fuels’ decomposition, and chemical interactions of N-containing radicals with the C1 species. The reactions involving C-containing species can notably improve the LBV of NH3/CH3OH mixed combustion. At low equivalence ratio, LBV increases in proportion to the fuel proportion. Conversely, at high equivalence ratio, the effect of oxygen concentration on LBV becomes the dominant factor.

Theoretical kinetics study of key reactions between ammonia and fuel molecules, part II: H-atom abstraction from alcohols and ethers by ṄH2 radicals

Ammonia, as a carbon-free fuel, is expected to be a carbon-neutral alternative fuel to control pollution emissions. However, with lower reactivity, pure ammonia needs to be blended with highly reactive biofuels including alcohols and ethers, for a better combustion performance. Alcohols and ethers can be produced from various sources, including renewable materials and fossil fuels. Therefore, the combination of ammonia with alcohols/ethers is a promising choice for developing carbon-neutral energy solutions. As an important intermediate, ṄH2 radicals can easily be formed during the pyrolysis or oxidation processes for ammonia combustion. The H-atom abstraction reaction between ṄH2 radicals and fuel molecules is vital in determining the blending fuel reactivity. High-level ab initio calculations have been carried out in this study to perform a systematic theoretical chemical kinetic investigation of H-atom abstraction by ṄH2 radicals from C1–C5 alcohols, 2,3-dimethyl-2-butanol, C1–C4 ethers and methyl isobutyl ether with a total of 61 H-atom abstraction reaction channels. The QCISD(T)/CBS//M06–2X/6–311++G(d, p) level of theory was employed to investigate the potential energy surfaces of the title reactions involved. Electronic geometry optimization, frequency analysis, and zero-point energy correction were performed for reactants, transition states and products related to 61 reaction channels at the M06–2X/6–311++G(d, p) level of theory. Single-point energy calculations were performed at the QCISD(T)/cc-pVXZ (X = D, T) and MP2/cc-pVYZ (Y = D, T, Q) level of theory and then extrapolated to the complete basis set. Conventional transition state theory was used to calculate the rate constants for different channels in the temperature range from 500 to 2000 K. The calculated individual and average rate constants for H-atom abstraction from different sites of alcohols and ethers were also obtained to explore the kinetic influence from the functional group. This is the first systematic study providing the rate constants for the title reactions which can be used to develop the combustion kinetic models for ammonia/alcohols and ammonia/ethers blends.

Study on the synergistic control of nitrogenous emissions and greenhouse gas of ammonia/diesel dual direct injection two-stroke engine

This study developed a numerical model for an ammonia/diesel dual direct-injection two-stroke engine and validated it based on engine bench test results. The engine combustion and emission characteristics were discussed under the cases of different injection parameters, intake temperatures, and exhaust gas re-circulation (EGR) rates. The findings indicate that altering the injection angle of ammonia and diesel sprays impacts the in-cylinder swirl and the ignition timing of the ammonia spray, consequently affecting the heat release phase of ammonia combustion. A suitable arrangement spray plumes can effectively reduce the emissions of unburned ammonia and greenhouse gas (GHG), while increase the emissions of NOx. On the other hand, higher intake temperatures can increase the average in-cylinder temperature, reducing unburned ammonia, N2O and GHG emissions. However, it also results in greater NO emissions, which can affect fuel economy negatively. Furthermore, the use of EGR technology can reduce the combustion temperature, thereby lowering the emission of nitrogen-based pollutants, but it also reduces ammonia combustion efficiency. By simultaneously applying the three aforementioned technologies within a certain range of conditions, it is possible to effectively synergize and control nitrogenous emissions and GHG while maintaining high thermal efficiency of ammonia/diesel dual direct injection engines.

Combustion characteristics and flame development of ammonia in an optical spark-ignition engine

To meet the ever-increasing serious challenges of global energy shortages and environmental pollution, it is urgent to apply alternative carbon-free fuels in the transportation field to reduce global carbon dioxide emissions. Ammonia is considered a prospective carbon-free fuel for engines due to its advantages in production, storage and transportation. The combustion and flame development characteristics of ammonia at different excess air ratios (λ) were researched in an optical spark ignition (SI) engine employing high-speed natural flame luminosity (NFL) imaging, OH* chemiluminescence imaging and flame spectra measurement. The combustion performance and flame development of ammonia were compared with methane. Results indicate that ammonia has the optimal combustion performance at λ = 0.9, with the most concentrated heat release, the highest power output and the best combustion stability. The peak natural flame luminosity intensity is highest at λ = 0.9, and the flame speed of 13.7 m/s is the fastest among testing cases, which results in the shortest ignition delay and combustion duration. OH* and NH2* spectra intensity of the ammonia flame were quantified and analyzed under different λ. The peak OH* intensity is highest at λ = 0.9, while the largest intensity of NH2* is obtained at the fuel-rich case (λ = 0.8). The maximum overall OH* chemiluminescence intensity at the same crank angles is obtained at λ = 0.9. Finally, despite the delayed spark timing, the combustion performance of methane presents higher indicated mean effective pressure (IMEP) and better combustion stability as well as faster flame propagation speed than ammonia at the same λ. In summary, ammonia shows the best combustion and flame development characteristics at λ = 0.9. The combustion performance of ammonia in SI mode is significantly worse than that of methane.

CHARACTERISTICS OF AMMONIA COMBUSTION AND MODERN GORENJE STABILIZATION STUDIES

In the light of current problems such as difficulties with ammonia ignition, low combustion stability and high NOx emissions, the features of basic ammonia combustion are considered and the available stabilization technologies are summarized. Gorenje gorenje. Among these technologies, the mixed combustion of ammonia, hydrogen and methane stands out, as well as methods of enhancing heat transfer, such as high oxygen pressure, and external assistive technologies, including plasma. Gorenje The aspects of ammonia combustion stability in various types of engines such as internal combustion engines, gas turbines and boilers are discussed. Gorenje The need for future research in the field of ammonia fuels, including the development of gorenje gorenje stabilization technologies, mixed combustion of ammonia with hydrogen, ammonia with solid fuels, and others, is emphasized. The importance of adapting various gorenje stabilization technologies gorenje depending on specific application areas to optimize the ammonia combustion process is noted. The prospects of ammonia-hydrogen mixed combustion systems are discussed, and the need for further research and development in this area is emphasized. Gorenje It is also noted that there is limited experimental data on the mixed combustion of ammonia with solid fuels, which provides potential for research in this direction. Gorenje Achieving a carbon peak and carbon neutrality is an essential strategic course in the context of countering global warming and is becoming an internal necessity to stimulate high-quality economic development. This prospect will necessarily entail significant transformations in the energy structure of Russia.

Microscopic insights into the ammonia-methanol blended combustion at high temperatures

Ammonia and methanol have a broad application prospect in the future as green renewable liquid fuels. Methanol has high reactivity and good solubility with ammonia, which is conducive to solving the dilemma of ammonia combustion. However, there are few studies on ammonia-methanol blended combustion, and the effect of blended combustion on the reaction pathways needs to be further investigated. Therefore, this study used the reaction molecular dynamics simulations to reveal the role of methanol on the ammonia combustion and emission process at the microscopic level, focusing on the effects of different fuel blending ratios and oxygen concentration in the air under stoichiometric conditions. The variation of oxidation pathways from ammonia to NO and methanol to CO under different conditions was analyzed in detail. It was found that the methanol accelerated ammonia combustion mainly through enhancing OH production, but methanol and its intermediates inhibited the initial oxidation of ammonia. When the fuel blending ratio was 0.75, the system had lower fuel-type NO production alongside a higher ammonia consumption rate under stoichiometric conditions. The crucial role of NH in the HNO formation at high-temperature ammonia oxidation conditions was revealed. The pathway of HNO + OH → NO + H2O dominated the conversion of HNO to NO in this study. Increasing the methanol content promoted the conversion of NH to HNO and the pyrolysis of HNO while inhibiting the reaction between HNO and O2. In addition, the finding indicated that the high O2 level in air can enhance the reactions between ammonia and OH radicals but inhibit the pyrolysis of CH2OH/CH3O significantly.

Climate and air quality impact of using ammonia as an alternative shipping fuel

As carbon-free fuel, ammonia has been proposed as an alternative fuel to facilitate maritime decarbonization. Deployment of ammonia-powered ships is proposed as soon as 2024. However, NO x , NH3 and N2O from ammonia combustion could impact air quality and climate. In this study, we assess whether and under what conditions switching to ammonia fuel might affect climate and air quality. We use a bottom–up approach combining ammonia engine experiment results and ship track data to estimate global tailpipe NO x , NH3 and N2O emissions from ammonia-powered ships with two possible engine technologies (NH3–H2 (high NO x , low NH3 emissions) vs pure NH3 (low NO x , very high NH3 emissions) combustion) under three emission regulation scenarios (with corresponding assumptions in emission control technologies), and simulate their air quality impacts using GEOS–Chem high performance global chemical transport model. We find that the tailpipe N2O emissions from ammonia-powered ships have climate impacts equivalent to 5.8% of current shipping CO2 emissions. Globally, switching to NH3–H2 engines avoids 16 900 mortalities from PM2.5 and 16 200 mortalities from O3 annually, while the unburnt NH3 emissions (82.0 Tg NH3 yr−1) from pure NH3 engines could lead to 668 100 additional mortalities from PM2.5 annually under current legislation. Requiring NH3 scrubbing within current emission control areas leads to smaller improvements in PM2.5-related mortalities (22 100 avoided mortalities for NH3–H2 and 623 900 additional mortalities for pure NH3 annually), while extending both Tier III NO x standard and NH3 scrubbing requirements globally leads to larger improvement in PM2.5-related mortalities associated with a switch to ammonia-powered ships (66 500 avoided mortalities for NH3–H2 and 1200 additional mortalities for pure NH3 annually). Our findings suggest that while switching to ammonia fuel would reduce tailpipe greenhouse gas emissions from shipping, stringent ammonia emission control is required to mitigate the potential adverse effects on air quality.

Flame stabilization and pollutant emissions of turbulent ammonia and blended ammonia flames: A review of the recent experimental and numerical advances

Compared to traditional hydrocarbon fuels, ammonia presents significant challenges as a fuel, including high ignition energy, low reactivity, slow flame propagation, and high NOx emissions, which hinder its use as a renewable fuel. Blending ammonia with fossil fuels like natural gas improves its combustion reactivity and helps mitigate CO2 emissions. However, there is still much to understand about the complex dynamics of ammonia and its blends with hydrocarbons. Key areas such as reaction kinetics mechanisms, ignition properties, flame propagation behaviors, and methods for controlling combustion performance under various conditions require further elucidation. This paper reviews recent advancements in experiments and numerical simulations aimed at developing stable, and low-emission combustors for ammonia-fired power generation. Recent burner and flame configurations, including non-swirling jets, single-stage swirl burners, two-stage burners, and newly developed double-swirl burners are analyzed for their flame stability and pollutant emission potential when firing ammonia and ammonia blends. Chemical kinetic modeling of ammonia and its blends plays a crucial role in understanding combustion behavior and pollutant emissions, particularly for NOx. However, there are challenges in predicting NOx emissions accurately, with significant disparities among different models. High-fidelity numerical simulations using detailed and skeletal mechanisms, direct numerical simulation, and large eddy simulation, have helped uncover crucial operational conditions affecting combustion and pollutant emissions, such as combustor pressure, air dilution, wall cooling, fuel/air mixing, and fuel blending. Nonetheless, the accuracy of chemical kinetic models and their integration into turbulent flow simulations remain critical limitations for numerical simulations of ammonia combustion.

Chemical reaction network analysis on N2O emissions control strategies of ammonia/ methanol Co-combustion

As a kind of substitute fuel of hydrocarbon fuel, ammonia has gained increasing attention for its potential to reduce carbon emissions. Blending with carbon-neutral methanol is expected to deal with the slow flame speed of ammonia. Nevertheless, the emission of N2O, a byproduct of ammonia combustion, would undermine the carbon reduction benefits due to its high global warming potential. This study employed Chemkin to construct a Chemical Reaction Network (CRN) model to analyze the reaction kinetics of ammonia combustion when blended with a small amount of methanol. It finds that the increases in the methanol blending ratio, pressure, and temperature can reduce greenhouse gas emissions effectively. Further analysis indicates that pressure and temperature have distinct effects on the emission mechanisms of N2O and NO. Increasing the combustion pressure weakens the chain reactions, thereby reducing the formation of both NO and N2O; the increase in initial temperature raises NO emissions, but also promotes the decomposition of N2O due to the abundance of H in the post-combustion phase. Optimizing post-combustion efficiency and controlling the formation of NO are crucial for N2O emission control. Thus, a staged methanol injection strategy is proposed, where early injection reduces NO emissions and late injection facilitates the decomposition of N2O.

Direct numerical simulations of pure and partially cracked ammonia/air turbulent premixed jet flames

Ammonia has been identified as a promising fuel to diminish greenhouse gas emission. However, ammonia combustion presents certain challenges including low reactivity and high NO emission. In the present study, three-dimensional direct numerical simulations (DNS) of ammonia/air premixed slot jet flames with varying Karlovitz numbers (Ka) and cracking ratios were performed. Three cases were considered, including two pure ammonia/air flames with different turbulence intensities and one partially cracked ammonia/air flame with high turbulence intensity. The effects of turbulence intensity and partial ammonia cracking on turbulence–flame interactions and NO emission characteristics of the flames were investigated. It was shown that the turbulent flame speed is higher for the flames with high turbulence intensity. In general, the flame displacement speed is negatively correlated with curvature in negative curvature regions, while the correlation is weak in the positive curvature regions for highly turbulent flames. Most flame area is consumed in negatively curved regions and produced in positively curved regions. It was found that the NO mass fraction is higher in the flame with partial ammonia cracking compared to the pure ammonia/air flames. The NO pathway analysis shows that the NH → NO pathway is enhanced, while the NO consumption pathway is suppressed in the partially cracked ammonia/air flame. The NO mass fraction is higher in regions of negative curvature than positive curvature. Interestingly, the NO mass fraction is found to be negatively correlated with the local equivalence ratio, which is consistent in both the DNS and the corresponding laminar premixed flames.

Development of a Global Mechanism for Ammonia Combustion Assisted by Machine Learning

Development of a Global Mechanism for Ammonia Combustion Assisted by Machine Learning

Fundamental study on the factors influencing the stability of ammonia combustion and the effects of pre-chamber on the combustion characteristics

Ammonia is gaining attention as a carbon-free alternative fuel for internal combustion engines. However, its poor combustion characteristics hinder its development, especially the combustion instability at low equivalence ratios. At present, most literature has not studied the factors affecting ammonia flame propagation stability under high ambient temperature and pressure. Therefore, this paper focuses on this issue, as well as the promotion of jet ignition combined with blending methane, and the prediction method of the flame propagation state under different conditions is proposed. The experimental results demonstrate that the main reason for the difficulty of stable propagation of ammonia flame at low equivalence ratios in top direct ignition mode is the buoyancy effect. The relationship between the flame propagation state and the corresponding Froude number with the flame radius of 20 mm (Fr20) is established. The flame propagation of the mixture can be stable in top ignition mode when its Fr20 is larger than 0.89, and the flame propagation of the mixture is unable in top ignition mode when its Fr20 is less than 0.50. The Fr20 increases with temperature, decreases with pressure, and first increases and then decreases with the increase of the equivalence ratio. Among the three parameters, the equivalence ratio has the greatest correlation with the Fr20. The prediction results show that under engine conditions, blending 20 % methane can expand the stable combustion limit of ammonia, but the effect is less compared with increasing the equivalence ratio to 0.55 and 0.6. In jet ignition mode, a higher equivalence ratio of the mixture in the pre-chamber can expand the low equivalence ratio limit of stable combustion of the mixture in the main chamber due to the acceleration of the longitudinal propagation of the flame by the jet. The combustion characteristics of the mixture in the main chamber are closely related to its properties, so at a too-low equivalence ratio, the jet has a limited promotion, in addition, the effect of blending 10 % methane is also little.

Enhancing ammonia combustion performance using hydrogen peroxide-enriched air: A computational fluid dynamics analysis

The effect of H2O2 addition to the air on ammonia combustion was investigated in the current research using a computational fluid dynamics approach. The numerical simulation was conducted in a 10-kW laboratory-scale furnace. The oxidizer and fuel were injected into the furnace in a non-premixed mode. Furthermore, a kinetic study was carried out to analyze the sensitivity of NO production during combustion and to determine reaction pathways under various conditions. The findings indicate that the addition of H2O2 to the mixture increases flame temperature and NO levels, while decreasing N2O levels. Moreover, the study demonstrates that, with a maximum concentration of only 10 ppm, the amount of NO2 is very low under various percentages of H2O2 and different operating conditions. Furthermore, it is concluded that during ammonia/air combustion, lowering the oxidizer inlet temperature and increasing wall heat extraction may cause the combustion to become unstable. Under pure air oxidizer conditions, where Tin = 973 K and Twall = 1273 K, the combustion is stable. However, instability occurs when Twall falls to 1173 K. In this instance, adding merely 5 % H2O2 to the oxidizer is sufficient to provide self-sustaining and stable burning. More H2O2 must be introduced into the furnace to maintain stable combustion as Tin and Twall continue to decline. Interestingly, while adding H2O2 raises NO levels, decreasing the inlet and wall temperatures at higher H2O2 concentrations can help regulate NO emissions. These findings clearly indicate that introducing H2O2 into the fuel mixture could be a promising strategy for reducing the inlet temperature and enhancing heat extraction, which will both reduce energy consumption and increase system efficiency.