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Chemical structure of premixed ammonia/hydrogen flames at elevated pressures

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Abstract

Because it is a carbon free fuel with high volumetric and gravimetric hydrogen density, ammonia is considered to be a promising hydrogen carrier molecule; its combustion chemistry, consisting of ammonia and ammonia/hydrogen blends, are of great importance in engine and gas turbine systems. This paper presents experimental data and kinetic modeling of the structure of NH3/H2/O2/Ar premixed flames at elevated pressures. Equivalence ratios were maintained at 0.8, 1.0 and 1.2, and the NH3/H2 ratio was 1:1 (molar ratio). Experiments were performed at pressures of 4 and 6 atm. Eight recently published chemical-kinetic mechanisms of ammonia combustion and oxidation were applied to numerically simulate flame structure. Experimental and numerical data showed that the main nitrogen containing compounds in the post flame zone were N2 and NO, while the concentrations of N2O and NO2 were negligible. In terms of NO emissions reduction, it was revealed that rich conditions were more effective. At the same time, pressure increases resulted in decreasing NO concentration in the post flame zone, as well as lower maximum concentration of NO and N2O. Numerical analysis showed that N2O and NO2 were formed mainly from NO. To improve the agreement between experimental and numerical data, rate kinetic parameters of these reactions should be refined.

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... This revised mechanism was derived from the NH 3 /CH 4 mechanism proposed by Okafor et al. [47]. During its development, interference with NO-related reactions was avoided, thereby preserving the extensively validated NO emission characteristics [48,49]. ...
... The mechanism was adjusted based on published experimental data on ammonia-hydrogen flame structure to enhance the prediction of N 2 O emissions. Osipova et al. [49] evaluated different ammonia combustion mechanisms and concluded that making appropriate changes to the reaction parameters for NO conversion to N 2 O would not significantly disturb the agreement between experiment and predictions. The formation of N 2 O is primarily governed by the NH + NO --N 2 O + H reaction. ...
... Fig. 2 depicts the numerical results of N 2 O peak values using the 1-D premixed laminar flames model under high pressure, and compared it with the experimental results of ammonia-hydrogen flame structure. The experimental details are available in Ref. [49]. At the pressures of 4 and 6 atm, the improved mechanism demonstrated better predictive capability for N 2 O peak compared to other commonly used ammonia combustion mechanisms [47,[50][51][52][53][54], thereby validating the accuracy of the improved mechanism. ...
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... Such data can be obtained from non-intrusive in-situ optical measurements such as laser induced fluorescence (LIF) [37,43,48,49], Raman scattering [50], and light absorption spectroscopy [51]. Probe-based intrusive methods with subsequent chemical analysis such as MBMS [52,53] and FTIR [47] are also possible. ...
... Tian et al. [58] (Duynslaegher et al. [53]) later performed chemical speciation in low-pressure NH3/CH4(H2)/O2/Ar flames using MBMS. More recently, Osipova et al. resolved the chemical structures of NH3-H2 flames at atmospheric [59] and at elevated pressures [52] with MBMS, with later work showing higher pressure conditions are beneficial for NO reduction. Spatially-resolved LIF NO measurements have also been performed in non-premixed flames. ...
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... Figs. 4-6 presents the comparison between the measured [52][53][54][55][56][57][58][59][60][61] and simulated species concentration for NH 3 , NH 3 /H 2 , NH 3 /CO and NH 3 /CH 4 blends in the BSF, JSR and PFR. Fig. 4 shows that both the detailed and simplified mechanisms can reasonably predict the mole fraction of various species in NH 3 , NH 3 /H 2 and NH 3 /CH 4 blends, with the simplified mechanism providing a more accurate prediction for NO formation. ...
... As a result, the thermal effect can be decoupled from chemical and transport effects. Fig. 8 displays LBV Fig. 4. Measured [52][53][54] and simulated mole fractions of (a) NH 3 , (b) NH 3 /H 2 , and (c) NH 3 /CH 4 blends in the BSF. ...
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... Three argon diluted flames with an equivalence ratio of ϕ=0.8 (fuellean), ϕ=1.0 (stoichiometric), and ϕ=1.2 (fuel-rich) were chosen in this work, as the similar flames were investigated by MBMS and numerically simulated in our group [12] recently to validate available kinetic models (neutral chemistry) for ammonia combustion. These flames have comparable concentrations of hydrogen and ammonia in the unburnt mixture, similarly to what is expected to be used in practice. ...
... To describe the chemical transformations of the neutral species in the flames we used a chemical kinetic model for combustion and oxidation of NH 3 and H 2 /NH 3 mixtures proposed by Zhang et al. [25] (263 reactions involving 38 neutral species). There is a variety of kinetic models for ammonia combustion in the literature, however, we have chosen this mechanism, as it was demonstrated [12] to provide a quite good predictive ability to spatial distribution of neutral components in the burner-stabilized premixed H 2 /NH 3 flames. This model was augmented with 34 reactions involving 7 charged species. ...
... Species concentrations of the combustion of hydrogen/ammonia blends were reported by Zhu et al., 54 Zhang et al., 19 Stagni et al., 14 Osipova et al., 55 Zhou et al., 56 and Alturaifi et al. 4 based on experiments in jet-stirred reactors, shock tubes, and flow reactors. Mass spectrometry and laser absorption spectroscopy were applied in Zhang et al. 19 and Alturaifi et al. 4 to determine the species concentrations. ...
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... Table 1 displays the composition of the unburnt gas blends. A comprehensive description of the experimental setup and data processing is available in our previous works [37], and thus only the crucial details are presented in the present paper. The flames were stabilized on a flat-flame burner (Botha-Spalding type [38]) made of a perforated brass disc (9.7 mm in diameter and 1.5 mm thick) incorporated in a brass housing. ...
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... Given the broad promise of zerocarbon ammonia-hydrogen engines, the engine community is actively researching and investing in these innovative technologies [10]. These endeavors will significantly contribute to achieving the global energy transition, mitigating the impacts of climate change, and establishing a solid foundation for future sustainable development [11]. Nitrogen oxides (NO X ) emissions from NH 3 eH 2 blend combustion are a significant concern [12]. ...
... Other techniques include the metal sorption of H 2 , forming metal hydrides [137] or in the form of ammonia [138]. Even if these strategies seem efficient, they have several environmental limitations, such as a large metal requirement or nitrogen oxide emission [139,140]. Physical sorption techniques over metalorganic frameworks [141] and carbon materials [142] have also been explored for H 2 storage. The hydrogen storage capacity of carbon materials depends on the temperature, pressure and structural properties like high specific surface area and pore volume [143]. ...
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Molecular beam mass-spectrometry was used to study the structure of a premixed H2/O2/Ar (0.26/0.13/0.61) flame with and without additives (0.1–1.1% DMMP) stabilized on a flat-flame burner at 43–80 Torr(burner temperature 95°C). The behavior of DMMP in the flame has been studied. Mass spectra of samples taken from flames, and intensity profiles of peaks 18 (H20), 32(O2),40(Ar),47(PO), 63(PO2), 647(HOPO), 80,94, 110, 124(DMMP) amu have been measured as a function of the distance from the burner surface to the sampling probe using a quadrupole mass-spectrometer and electron impact ionization at 12.1–21.6 eV with a spread of electron energy ±0.25 eV. Intensity profiles of masses 110,94,80 pass through a maximum. This shows that the species responsible for these masses are intermediates. PO(CH3)(OCH)(OH), P2(OCH3) are possible intermediates. The profile of the temperature in the flame has been determined by using a Pt-PtRh(1O%) thermocouple covered by Ceramobond 569. The effect of promotion on the H2/O2//Ar flame by the additive DMMP has been observed. The possible detailed chemical mechanism of the destruction of DMMP is presented.
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This paper presents experimental data on the flame structure of laminar premixed ammonia and ammonia/hydrogen flames at different equivalence ratios (φ = 0.8, 1.0 and 1.2) and the laminar flame speed of ammonia/hydrogen flames (φ = 0.7–1.5) at 1 atm. Experimental data were compared with modeling results obtained using four detailed chemical-kinetic mechanisms of ammonia oxidation. In general, all models adequately predict the flame structure. However, for the laminar burning velocity, this is not so. The main nitrogen-containing species present in the post-flame zone in significant concentrations are N2 and NO. Experimental data and numerical simulations show that the transition to slightly rich conditions enables to reduce NO concentration. Numerical simulation indicate that increasing the pressure rise also results into reduction of NO formation. However, when using ammonia as a fuel, additional technologies should be employed to reduce NO formation.
Article
One of the most important problems of modern energy industry is the transition to carbon free fuels, which can mitigate the negative environmental effects. This paper presents experimental data on ammonia and ammonia/hydrogen blends oxidation in an isothermal jet-stirred reactor over the temperature of range 800–1300 K. Experiments were performed under atmospheric pressure, residence time of 1 s, various equivalence ratios, and with argon dilution at ≈0.99. It was revealed that hydrogen addition shifts the onset temperature of ammonia oxidation by about 250 K towards the lower region. A detailed chemical kinetic model which showed the best predictive capability was used to understand the effect of hydrogen addition on ammonia reactivity. It was shown that hydrogen presence results into higher concentrations of H, O and OH radicals. Moreover, these radicals start to form at lower temperatures when hydrogen is present. However, the change of the equivalence ratio has only slight effect on the temperature range of ammonia conversion.
Article
The application of ammonia (NH3) blended with hydrogen (H2) as a fuel in combustion systems is a practical approach to decarbonise the energy sector, and the combustion of the fuel at rich conditions is relevant in emissions control through rich-lean combustion. However, the chemistry of rich NH3/H2 flames at high pressure, and the interaction between NH3 and H2 still need to be clarified. Therefore, the present study focuses on the chemical kinetics of NH3/H2/air flames at rich conditions and elevated pressures. To validate chemical kinetics in the literature, the laminar burning velocity of NH3/H2/Air premixed flames were measured at 0.1 and 0.5 MPa and equivalence ratios up to 1.8. The results show that the seven kinetics mechanisms studied could not satisfactorily predict the measurements at fuel-rich conditions and elevated pressure. The kinetics mechanism by Han et al. was optimized, leading to a new detailed kinetics, which can be reduced to 26 species and 119 reactions and satisfactorily predicts the present measurements and those in the literature. Analysis of the chemistry of NH3/H2 flames using the new mechanism shows NH3 and H2 kinetics are strongly coupled through a H2 decomposition/recovery mechanism, here named H2 recovery mechanism, which is important in modelling the burning velocity of the flame at fuel-rich conditions. The burned gas Markstein length was also extracted from the measured flame speed and its behaviour was studied using theoretical correlations.
Article
The low combustion intensity of ammonia (NH3) raises great research needs of combustion enhancement strategies for its practical applications. Considering the high proportion of hydrogen in cracked gas of NH3, partial fuel cracking is a feasible strategy to enhance NH3 combustion. This work reports an experimental and kinetic modeling study on the laminar flame propagation of partially cracked NH3/air mixtures (NH3/H2/N2/air mixtures) up to 10 atm. Laminar burning velocities (LBVs) of partially cracked NH3/air mixtures are measured at various cracking ratios and equivalence ratios using a high-pressure constant-volume cylindrical combustion vessel. Our recently reported NH3/syngas model is updated to simulate the experimental results, which shows satisfactory performance on predicting the partially cracked NH3/air LBVs in this work, as well as the LBV and speciation data of NH3 and NH3/H2 combustion in literature. modeling analysis is performed to provide insight into effects of equivalence ratio, cracking ratio and pressure on laminar flame propagation of partially cracked NH3/air mixtures. The modified fictitious diluent gas method reported in our recent work is adopted to separate thermal and other effects in enhancement of NH3 combustion. The analysis results indicate that thermal effect only plays a minor role in the enhancement of laminar flame propagation of NH3 in partial fuel cracking strategy, while chemical effect should be significant for the enhanced laminar flame propagation. Due to the presence of H2 in cracked gas, reaction H + O2 (+M) = HO2 (+M) shows enhanced importance and makes the laminar flame propagation of partially cracked NH3/air mixtures more pressure-dependent. Furthermore, NO formation characteristics with increasing cracking ratio is also numerically investigated, which shows that fuel NO is the major NOx formation source. The results reveal a dramatic non-monotonic behavior of NO formation as the cracking ratio increases, which originates from the transition of NH3 chemistry to cracked gas chemistry.
Article
Ammonia is considered a key energy carrier with potential applications for low carbon energy storage, transportation and power generation. This carbon-free molecule offers several advantages, including high energy density and a well-established production and distribution infrastructure that have been optimized for over a century. In this perspective, we analyze the potential roles of ammonia as an energy carrier, and summarize research areas requiring further development for the implementation of ammonia as a building block in the global low-carbon energy landscape. Ammonia technologies are reviewed with an emphasis on current limitations and recent advances. Focus is placed on available technologies for ammonia synthesis, decomposition into COx-free hydrogen and direct use of ammonia for power generation and transportation.
Article
Laminar flame speeds of ammonia with oxygen-enriched air (oxygen content varying from 21 to 30 vol.%) and ammonia-hydrogen-air mixtures (fuel hydrogen content varying from 0 to 30 vol.%) at elevated pressure (1–10 bar) and temperature (298–473 K) were determined experimentally using a constant volume combustion chamber. Moreover, ammonia laminar flame speeds with helium as an inert were measured for the first time. Using these experimental data along with published ones, we have developed a newly compiled kinetic model for the prediction of the oxidation of ammonia and ammonia-hydrogen blends in freely propagating and burner stabilized premixed flames, as well as in shock tubes, rapid compression machines and a jet-stirred reactor. The reaction mechanism also considers the formation of nitrogen oxides, as well as the reduction of nitrogen oxides depending on the conditions of the surrounding gas phase. The experimental results from the present work and the literature are interpreted with the help of the kinetic model derived here. The experiments show that increasing the initial temperature, fuel hydrogen content, or oxidizer oxygen content causes the laminar flame speed to increase, while it decreases when increasing the initial pressure. The proposed kinetic model predicts the same trends than experiments and a good agreement is found with measurements for a wide range of conditions. The model suggests that under rich conditions the N2H2 formation path is favored compared to stoichiometric condition. The most important reactions under rich conditions are: NH2+NH=N2H2+H, NH2+NH2=N2H2+H2, N2H2+H=NNH+H2 and N2H2+M=NNH+H+M. These reactions were also found to be among the most sensitive reactions for predicting the laminar flame speed for all the cases investigated.
Article
Reactivity enhancement is crucial for the potential applications of ammonia (NH3) as a gas turbine fuel. Doping more reactive fuels like H2, methane and syngas is a widely adopted strategy in this field, however fundamental combustion studies of NH3/reactive fuel mixtures under gas turbine-relevant high pressure conditions are still very limited. This work reports an effort to study laminar flame propagation of NH3/syngas mixtures up to 10 atm. Laminar burning velocities (LBVs) of NH3/syngas/air mixtures were measured at 298 K, various syngas contents in fuel mixtures (α) and H2 contents in syngas (β), and equivalence ratios of 0.7–1.5 in a high-pressure constant-volume cylindrical combustion vessel. A kinetic model was developed for NH3/syngas combustion based on our recent NH3 model and a recent syngas model in literature. It shows reasonable predictions on the present NH3/syngas LBVs at 1–10 atm, as well as previous data including NH3/syngas LBVs at 1 atm, NH3/syngas ignition delay times at various pressures and pure NH3 LBVs at various pressures. Modeling analysis including the sensitivity analysis and rate of production analysis provides kinetic interpretation for the effects of fuel composition (α and β), equivalence ratio and pressure on NH3/syngas LBVs. It is found that the addition of syngas shifts the chemistry from NH3 sub-mechanism to syngas sub-mechanism. A modified fictitious diluent gas method was proposed to separate the chemical and thermal effects of syngas addition, which shows that the chemical effect is more responsible for the enhanced laminar flame propagation. NH3/syngas/air flames have similar reaction networks but different preferred pathways under lean and rich conditions. Compared with pure NH3 flames, the addition of syngas also improves the importance of H + O2 (+ M) = HO2 (+ M) and consequently leads to strong pressure dependency of NH3/syngas/air flames.
Article
Measurements of autoignition delay times of NH3 and NH3/H2 mixtures in a rapid compression machine are reported at pressures from 20–75 bar and temperatures in the range 1040–1210 K. The equivalence ratio, using O2/N2/Ar mixtures as oxidizer, varied for pure NH3 from 0.5 to 3.0; NH3/H2 mixtures with H2 fraction between 0 and 10% were examined at equivalence ratios 0.5 and 1.0. In contrast to many hydrocarbon fuels, the results indicate that, for the conditions studied, autoignition of NH3 becomes slower with increasing equivalence ratio. Hydrogen is seen to have a strong ignition-enhancing effect on NH3. The experimental data, which show similar trends to those observed previously by He et al. (2019) [28], were used to evaluate four NH3 oxidation mechanisms: a new version of the mechanism described by Glarborg et al. (2018) [29], with an updated rate constant for the formation of hydrazine, NH2 + NH2 (+M) = N2H4 (+M), and the literature mechanisms from Klippenstein et al. (2011) [30], Mathieu and Petersen (2015) [25], and Shrestha et al. (2018) [31]. In general, the mechanism from this study has the best performance, yielding satisfactory prediction of ignition delay times both of pure NH3 and NH3/H2 mixtures at high pressures (40–60 bar). Kinetic analysis based on present mechanism indicates that the ignition enhancing effect of H2 on NH3 is closely related to the formation and decomposition of H2O2; even modest hydrogen addition changes the identity of the major reactions from those involving NHx radicals to those that dominate the H2/O2 mechanism. Flux analysis shows that the oxidation path of NH3 is not influenced by H2 addition. We also indicate the methodological importance of using a non-reactive mixture having the same heat capacity as the reactive mixture for determining the non-reactive volume trace for simulation purposes, as well as that of limiting the variation in temperature after compression, by limiting the uncertainty in the experimentally determined quantities that characterize the state of the mixture.
Article
To achieve a reduced chemical model for comprehensive prediction of ammonia/hydrogen/methane mixture combustion, a detailed chemical mechanism with 128 species and 957 reactions was first assembled using models from literature. Directed relation graph with error propagation (DRGEP) with sensitivity analysis reduction method was then used to obtain compact reaction models. The studied reduction conditions cover ɸ = 0.5–2.0, temperature 1000–2000 K, and pressure 0.1–5 MPa. Finally, two reduced models have been obtained: 28 species and 213 reactions for ammonia/hydrogen and 51 species and 420 reactions for ammonia/hydrogen/methane. Ignition delay times and laminar burning velocities for single component and fuel mixtures predicted using the detailed and reduced mechanisms were compared with available experiments. Results showed that both detailed and reduced mechanisms performed fairly well for ignition delays, while over-predicted laminar burning velocity at fuel-rich conditions for single ammonia fuel and mixtures. The 51 species reduced mechanism was also tested in non-premixed coflow hydrogen/methane jet flames, while 1%–50% mole ammonia were added to the fuel stream. Modelling results showed that this 51-species mechanism was suitable for CFD modelling, and the speedup factor was over 5 when using the reduced mechanism with different codes. The flame structure, as well as NO and NO2 formation was studied. High NO concentrations were found in high-temperature region near the stoichiometric zone, while NO2 was dominant in the lean flame zone. Reaction flux analysis was performed to better understand NH3 oxidation and NOx emissions at low- and high-temperature conditions.
Article
The present study introduces new laminar burning velocity data for ammonia/hydrogen/air mixtures measured by means of the outwardly propagating spherical flame method at atmospheric pressure, for previously unseen unburned gas temperatures ranging from 298 to 473 K, hydrogen fractions ranging from 0 vol% to 60 vol% in the fuel and equivalence ratios in the range [0.8–1.4]. Results show increasing velocities with increasing hydrogen fraction and temperature, with maximum values obtained for rich mixtures near stoichiometry. The new experimental dataset is compared to dedicated laminar burning velocity correlations from the literature and to simulations using detailed kinetic mechanisms. The ammonia/air correlation presents a good agreement with measurements over the whole range of experimental conditions. The ammonia/hydrogen/air correlation captures the effect of the initial temperature satisfactorily for equivalence ratios below 1.3 and hydrogen fractions below 50 vol% in the fuel, but discrepancies are observed in other conditions. The effect of hydrogen addition is reproduced satisfactorily for hydrogen fractions between 20 and 40 vol% in the fuel, but discrepancies are observed for rich mixtures below 20 vol% hydrogen and for all mixtures containing 50 vol% hydrogen and more. An optimization of both correlations is proposed thanks to the experimental data obtained, but only with partial improvement of the ammonia/hydrogen/air correlation. State-of-the-art detailed kinetic reaction mechanisms yield values in close agreement with the present experiments. They could thus be used along with additional experimental data from different techniques to develop more accurate correlations for time-effective laminar burning velocity estimates of NH3/H2/air mixtures.
Article
Ammonia (NH3) is a promising energy carrier to store and transport renewable hydrogen (H2) that can be generated using, e.g., wind and solar energy. Direct combustion of NH3 is one of the possible methods to utilize the energy by the end users. To understand the combustion characteristics of NH3 as a fuel, the laminar burning velocities of NH3/air, NH3/H2/air, NH3/CO/air and NH3/CH4/air premixed flames were investigated experimentally using the heat flux method. Measurements are reported for a wide range of equivalence ratios and blending ratios. Kinetic modeling was also performed using available chemical kinetic mechanisms, namely the GRI-Mech 3.0, the Okafor et al. and the San Diego mechanisms. The experimental results for NH3/air flames agree well with the literature data and it is found that blending NH3 with H2 is the most effective manner to increase the burning velocity of NH3 based fuel mixtures. None of the kinetic mechanisms used can accurately predict most of the measured data. Sensitivity and reaction path analyses indicate that the oxidation of NH3 blended with the additive fuels considered can be understood as the parallel oxidation processes of the individual fuels, and that the source of discrepancy between the experimental and modeling results is related to the inaccuracy of the rate parameters of the N-containing reactions. In this regard, the present detailed and reliable experimental data is of special value for model development and validation.
Article
Hydrogen storage technology is essentially necessary to promote renewable energy. Many kinds of hydrogen storage materials, which are hydrogen storage alloys, inorganic chemical hydrides, carbon materials and liquid hydrides have been studied. In those materials, ammonia (NH3) is easily liquefied by compression at 1 MPa and 298 K, and has a highest volumetric hydrogen density of 10.7 kg H2/100 L. It also has a high gravimetric hydrogen density of 17.8 wt%. The theoretical hydrogen conversion efficiency is about 90%. NH3 is burnable without emission of CO2 and has advantages as hydrogen and energy carriers.
Article
International shipping currently accounts for about 3% of total global greenhouse gas (GHG) emissions, but would continue to rise as transport capacity expands. If the shipping industry aims at delivering its proportionate contribution to curbing global warming under the Paris agreement, the sector has to, inevitably, promote energy conservation and emission reduction. A rapidly growing oceangoing fleet size and correspondingly rising GHG emissions on a global scale raise an interesting research question: could a certain relationship between the two be characterized as a function so that further emissions can be forecast based on the model? The paper adopts an allometric approach based on biological scaling laws to explore the potential relationship between the fleet size and corresponding GHG emissions from shipping. The results show that both the slowdown of the navigation speed and the current implementation of the Energy Efficiency Design Index (EEDI) and Energy Efficiency Operation Index (EEOI) are effective on the whole. By employing the model, the development trends of GHG emissions from shipping in the future can be better understood. Through model applications and result analysis, numerical results validate the effectiveness of this method. The paper not only studies the development of GHG emissions from shipping in the past, but aslo evaluates its specific emission quantities in the future which is in line with the GHG emission reduction targets proposed by IMO on the 72nd IMO meeting, which will be helpful for policy decisions on the quota of GHG emissions to the International Maritime Organization (IMO) and port administrators.
Article
This work reports on a study of chemical kinetic modelling of ammonia/hydrogen/air ignition, premixed flame propagation and NO emission. A survey of chemical mechanisms available in the literature was first conducted, and the performance of 10 mechanisms was analysed in terms of the prediction of shock tube ignition delay times, laminar flame speeds and NOx concentrations for NH3/air and NH3/H2/air flames for a wide range of operating conditions. Then, three of these mechanisms were reduced and their performance compared against the behaviour of the original mechanisms. The results confirm that pure NH3 flames have high ignition delay times and rather low flame speeds, and that the addition of H2 to NH3 flames increases exponentially the flame speed, and significantly the NOx emission. The currently available chemical kinetic mechanisms predict rather scattered ignition delay times, laminar flame speeds, and NOx concentrations in NH3 flames, indicating that improvements in the sub-mechanisms of NH3 and NH3/H2 oxidation are still needed. Sensitivity analysis for NO formation indicates that NO formation in NH3 flames is mainly produced through the NH3/O2 chemical process, and sensitivity analysis for flame speed reveals that the differences among mechanisms are due to the relative importance of the reactions of the NNH and HNO sub-mechanisms. The reduced mechanisms show high fidelity when compared with the original ones, despite some discrepancies at high pressures.
Article
This work introduces a newly developed reaction mechanism for the oxidation of ammonia in freely propagating and burner-stabilized premixed flames as well as shock-tube, jet-stirred reactor, and plug-flow reactor experiments. The paper mainly focuses on pure ammonia and ammonia–hydrogen fuel blends. The reaction mechanism also considers the formation of nitrogen oxides as well as the reduction of nitrogen oxides depending upon the conditions of the surrounding gas phase. Doping of the fuel blend with NO2 can result in acceleration of H2 autoignition via the reaction NO2 + HO2 ⇋ HONO + O2, followed by the thermal decomposition of HONO, or deceleration of H2 oxidation via NO2 + OH ⇋ NO + HO2. The concentration of HO2 is decisive for the active reaction pathway. The formation of NO in burner-stabilized premixed flames is shown to demonstrate the capability of the mechanism to be integrated into a mechanism for hydrocarbon oxidation.
Article
Ammonia (NH3) has been considered as a promising alternative energy carrier for automobile engines and gas turbines due to its production from renewable sources using concepts such as power-to-gas. Knowledge of the combustion characteristics of NH3/air and the formation of pollutants, especially NOx and unburned NH3, at intermediate temperatures is crucially important to investigate. Detailed understanding of ammonia reaction mechanism is still lacking. The present study reports ignition delay times of NH3/air mixtures over a temperature range of 1100–1600 K, pressures of 20 and 40 bar, and equivalence ratios of 0.5, 1.0, and 2.0. The experimental results are compared to the literature mechanism of Mathieu and Petersen (2015) and reasonable agreement is observed. Detailed modeling for ammonia emissions is performed, and the NH3/air combustion is found to be potentially free from NOx and unburned NH3 at fuel-rich conditions.
Article
Renewable energy sources, such as wind and solar electricity, are the fastest growing energy sources in the world. But the sunniest and windiest spots on Earth are often far from cities where the power is needed, and there are few effective ways to ship electricity over long distances. That leaves much of the world's renewable energy potential untapped. Now, researchers are looking to use renewable electricity to make ammonia, which is already produced on an industrial scale as the main ingredient in fertilizer, and shipped worldwide. Today, ammonia is made primarily by stripping hydrogen molecules from natural gas and combining them with nitrogen purified from air, a process that generates about 1% of all humanmade carbon dioxide. In the future, ammonia could be produced by combining that nitrogen with hydrogen generated by splitting water. Devices called reverse fuel cells could also make it directly by using electricity and catalysts to split water and combine the hydrogen with nitrogen. These advances could enable ammonia to be produced economically on a small scale, allowing farmers to make their own fertilizer and renewable energy producers to store any excess power they may have and ship it where it's needed. 2017
Article
Understanding of the chemical processes that govern formation and destruction of nitrogen oxides (NOx) in combustion processes continues to be a challenge. Even though this area has been the subject of extensive research over the last four decades, there are still unresolved issues that may limit the accuracy of engineering calculations and thereby the potential of primary measures for NOx control. In this review our current understanding of the mechanisms that are responsible for combustion-generated nitrogen-containing air pollutants is discussed. The thermochemistry of the relevant nitrogen compounds is updated, using the Active Thermochemical Tables (ATcT) approach. Rate parameters for the key gas-phase reactions of the nitrogen species are surveyed, based on available information from experiments and high-level theory. The mechanisms for thermal and prompt-NO, for fuel-NO, and NO formation via NNH or N2O are discussed, along with the chemistry of NO removal processes such as reburning and Selective Non-Catalytic Reduction of NO. Each subset of the mechanism is evaluated against experimental data and the accuracy of modeling predictions is discussed.
Article
The structure of premixed ethyl butanoate/O2/Ar flames stabilized on a flat burner at atmospheric pressure was studied by molecular beam mass spectrometry. Mole fraction profiles of the reactants, stable products, and major intermediates and temperature profiles were obtained in flames of stoichiometric (φ = 1) and rich (φ = 1.5) combustible mixtures. Experimental data are analyzed and compared with previously obtained experimental and numerical data for methyl pentanoate flames. The structure of ethyl butanoate flames is simulated using a detailed literature chemical-kinetic mechanism for the oxidation of fatty acid esters. The experimental profiles are compared with the simulated ones, and the conversion pathways of ethyl butanoate are analyzed. Based on a comparative analysis of experimental and simulated data, the main shortcomings of the model presented in the literature are identified and possible ways are proposed to improve the model. The decomposition of ethyl butanoate and methyl pentanoate are discussed based on an analysis of their conversion pathways; similarities and characteristic differences between their oxidation processes due to the structural differences of the molecules of the fuels are outlined.
Article
To achieve comprehensive prediction of ammonia combustion in terms of flame speed and ignition delay time, an improved mechanism of ammonia oxidation was proposed in this work. The present model (UT-LCS) was based on a previous work [Song et al., 2016] and improved by relevant elementary reactions including NH2, HNO, and N2H2. The model clearly explained reported values of laminar flame speed and ignition delay time in wide ranges of equivalence ratio and pressure. This suggests that NH2, HNO, and N2H2 reactivities play a key role to improve the reaction mechanism of ammonia oxidation in the present model. The model was also applied to demonstrate NH3/H2/air combustion. The present model also appropriately predicted the laminar flame speed of NH3/H2/air combustion as a function of equivalence ratio. Using the model, we discussed the reduction of NO concentration downstream and H2 formation via NH3 decomposition in NH3/H2 fuel-rich combustion. The results provide suggestions for effective combustion of NH3 for future applications.
Article
With the renewed interest in ammonia as a carbon-neutral fuel, mixtures of ammonia and methane are also being considered as fuel. In order to develop gas turbine combustors for the fuels, development of reaction mechanisms that accurately model the burning velocity and emissions from the flames is important. In this study, the laminar burning velocity of premixed methane–ammonia–air mixtures were studied experimentally and numerically over a wide range of equivalence ratios and ammonia concentrations. Ammonia concentration in the fuel, expressed in terms of the heat fraction of NH3 in the fuel, was varied from 0 to 0.3 while the equivalence ratio was varied from 0.8 to 1.3. The experiments were conducted using a constant volume chamber, at 298 K and 0.10 MPa. The burning velocity decreased with an increase in ammonia concentration. The numerical results showed that the kinetic mechanism by Tian et al. largely underestimates the unstretched laminar burning velocity owing mainly to the dominance of HCO (+H, OH, O2) = CO (+H2, H2O, HO2) over HCO = CO + H in the conversion of HCO to CO. GRI Mech 3.0 predicts the burning velocity of the mixture closely however some reactions relevant to the burning velocity and NO reduction in methane–ammonia flames are missing in the mechanism. A detailed reaction mechanism was developed based on GRI Mech 3.0 and the mechanism by Tian et al. and validated with the experimental results. The temperature and species profiles computed with the present model agree with that of GRI Mech 3.0 for methane–air flames. On the other hand, the NO profile computed with the present model agrees with Tian et al.’s mechanism for methane–ammonia flames with high ammonia concentration. Furthermore, the burned gas Markstein length was measured and was found to increase with equivalence ratio and ammonia concentration.
Article
The paper presents an experimental and modeling study of the chemical structure of laminar premixed stoichiometric H2/CH4/C3H8/O2/Ar flames stabilized on a flat burner at 1, 3 and 5 atm. The flames structure was simulated using four different detailed chemical kinetic mechanisms proposed in the literature for oxidation of small hydrocarbons. The width of the zone of consumption of the fuel components was shown to differ appreciably at the three pressures. Hydrogen was shown to have the largest consumption zone, while propane has the smallest one. The kinetic analysis provided an explanation for the observed phenomenon, which assumes the formation of additional pathways for hydrogen and methane production in the flames of ternary fuel mixtures. Comparison of the measured and simulated flame structures shows that all the mechanisms satisfactorily predict the mole fraction profiles of the reactants, products and some intermediates at atmospheric and elevated pressures. It is noteworthy that the mechanisms adequately predict the spatial variations in the mole fractions of free radicals, including the H, OH and CH3 radicals, within the pressure range. However, some drawbacks of the mechanisms used have been identified. The mechanisms were shown to overpredict the mole fractions of some unsaturated hydrocarbons, including ethylene and acetylene, at elevated pressures. Therefore, the rate constants of the crucial reactions responsible for production/consumption of these species, as well as their pressure dependences, should be specified, and the mechanisms should be refined. To provide a deeper insight into the combustion chemistry of ternary fuel mixtures, one should focus on the structure of rich flames.
Article
A lack of available experimental data for spatial distributions of the species mole fractions in the flames of hydrogen at the pressures higher than atmospheric, which are required for developing and validating reliable kinetic models for hydrogen combustion, motivated this study. Stoichiometric laminar premixed H2/O2/Ar flames stabilized on flat burners at pressures 1, 3 and 5 atm were examined in this work by molecular beam mass spectrometry. Mole fraction profiles of all flame species (H2, O2, H2O, H2O2, H, O, OH, HO2) were measured. A decrease in the peak mole fractions of H, O, OH radicals and an increase in the peak mole fractions of HO2 and H2O2 with pressure was observed. Two detailed kinetic mechanisms proposed recently by Konnov (2008) and Burke et al. (2012) for hydrogen combustion were validated against new experimental data reported in this work. Both mechanisms reproduced well the mole fraction profiles of H2, O2, H2O and H, O, OH radicals in the flames. However, the mechanism of Burke et al. was found to be more adequate in predicting the mole fraction profiles of peroxy species in the flames. The observed changes in the flame structure with pressure were explained on the basis of a kinetic analysis of the model developed by Burke et al. The experimental data reported in this work can help in further development and improvement of the future kinetic models of hydrogen combustion at elevated pressures.
Article
Ammonia oxidation experiments were conducted at high pressure (30 bar and 100 bar) under oxidizing and stoichiometric conditions, respectively, and temperatures ranging from 450 to 925 K. The oxidation of ammonia was slow under stoichiometric conditions in the temperature range investigated. Under oxidizing conditions the onset temperature for reaction was 850–875 K at 30 bar, while at 100 bar it was about 800 K, with complete consumption of NH3 at 875 K. The products of reaction were N2 and N2O, while NO and NO2 concentrations were below the detection limit even under oxidizing conditions. The data were interpreted in terms of a detailed chemical kinetic model. The rate constant for the reaction of the important intermediate H2NO with O2 was determined from ab initio calculations to be 2.3 × 102 T2.994 exp(−9510 K/T) cm3 mol−1 s−1. The agreement between experimental results and model work was satisfactory. The main oxidation path for NH3 at high pressure under oxidizing conditions is NH3 NH2 H2NO HNO NO N2. The modeling predictions are most sensitive to the reactions NH2 + NO = NNH + OH and NH2 + HO2 = H2NO + OH, which promote the ammonia consumption by forming OH radicals, and to NH2 + NO = N2 + H2O and NH2 + NO2 = N2O + H2O, which are the main chain-terminating steps.
Article
The structure of premixed ammonia + air flames, burning at atmospheric pressure under strain-stabilized conditions on a porous-plug burner, has been investigated using laser-diagnostic methods. Profiles of OH, NH, and NO were acquired by laser-induced fluorescence (LIF) and quantitative concentrations of OH and NO were retrieved using a concept for calibration versus absorption utilizing the LIF-signal itself, whereas NH concentrations were evaluated employing a saturated fluorescence signal. In addition, temperatures and relative oxygen concentrations were measured by rotational Coherent Anti-stokes Raman Spectroscopy (CARS). The new experimental data for flames with equivalence ratios of 0.9, 1.0, and 1.2 were used to validate and rank the performance of four contemporary detailed kinetic models. Simulations were carried out using experimental temperature profiles as well as by solving the energy equation. Two models of the same origin, developed by Mendiara and Glarborg (2009) and by Klippenstein et al. (2011), in most cases showed good agreement in terms of radical concentrations, however, the model of Mendiara and Glarborg had better prediction of temperatures and flame front positions. The model by Shmakov et al. (2010) had comparable performance concerning radical species, but significant discrepancies appeared in the prediction of flame front positions. The model of Duynslaegher et al. (2012), in addition to the flame front positions, deviated from experiments or other models in terms of NH and NO concentrations. A sensitivity analysis for the Mendiara-Glarborg mechanism indicated that remaining uncertainties of the rate constants implemented in the recent H/N/O models are difficult to scrutinize unambiguously due to experimental uncertainties.
Article
Ammonia shows promise not only as a hydrogen-energy carrier but also as a carbon-free fuel. However, combustion intensity of ammonia must be improved to enable its application to practical combustors. In order to achieve this, hydrogen-added ammonia/air flames were experimentally and numerically investigated at elevated pressures up to 0.5 MPa. The hydrogen ratio, which is defined as the hydrogen concentration in the fuel mixture, was varied from 0 to 1.0. The unstretched laminar burning velocity and Markstein length of spherically propagating laminar flames were experimentally evaluated. The results showed that, unstretched laminar burning velocity increases non-linearly with an increase in the hydrogen ratio. The Markstein length varies non-monotonically with an increase in the hydrogen ratio. The unstretched laminar burning velocity, and the Markstein length decrease with an increase in the initial mixture pressure. Although the decrease in the Markstein length is larger when the initial mixture pressure increases from 0.1 to 0.3 MPa, the values of Markstein lengths at 0.5 MPa are almost the same as those at 0.3 MPa.
Article
Experimental data are reported on the structure of laminar premixed methane/oxygen/argon flames stabilized over a flat burner at 1, 3, and 5 atm with different equivalence ratios φ (0.8–1.2). Mole fraction profiles of the reactants (CH 4 , O 2), major stable products (CO 2 , H 2 O, H 2 , CO) and intermediates such as H, OH, CH 3 radicals, as well as ethylene and acetylene, were measured by molecular-beam mass spectrometry. The temperature profiles in the flames were measured by thermocouples in the presence of a sampling probe to take into account the flame cooling effect due to the probe. The structures of stoichiometric flames at 1, 3 and 5 atm were compared to elucidate the effect of pressure on the mole fractions of the flame species. Fuel-lean (φ = 0.8) and fuel-rich (φ = 1.2) flames at 5 atm were also investigated in this work. All the experimental data were compared with the numerical simulations using the Premix code and three detailed chemical kinetic mechanisms for methane combustion available in the literature: the GRI-Mech 3.0, AramcoMech 1.3 and USC Mech II. The absolute mole fractions of CH 4 , O 2 , H 2 O, CO, CO 2 , H 2 , H, OH, CH 3 in the flames and their dependences on pressure were captured by both mechanisms reasonably well. An analysis of the reaction mechanisms was performed to gain insights into the kinetics of methane combustion in stoichiometric conditions in the range of pressures from 1 to 5 atm and to explain the observed pressure effects on peak mole fractions of flame radicals. The decrease of peak mole fractions of acetylene and ethylene with pressure increase, which was observed in the experiments, was not reproduced by the mechanisms. Both mechanisms predicted the increase in their peak mole fractions with pressure (in the range from 1 to 3 atm). The kinetic analysis indicated the need to revise the pressure-dependent chemistry of acetylene and ethylene formation in the mechanisms.
Article
Ammonia is expected to be useful not only as a hydrogen-energy carrier but also as a carbon-free fuel. In order to design an ammonia fueled combustor, fundamental flame characteristics of ammonia must be understood. However, knowledge of the characteristics of ammonia/air flames, especially at the high pressures, has been insufficient. In this study, the unstretched laminar burning velocity and the Markstein length of ammonia/air premixed flames at various pressures up to 0.5 MPa were experimentally clarified for the first time. Spherically propagating premixed flames, which propagate in a constant volume combustion chamber, were observed using high-speed schlieren photography. Results indicate that the maximum value of unstretched laminar burning velocities is less than 7 cm/s within the examined conditions and is lower than those of hydrocarbon flames. The unstretched laminar burning velocity decreases with the increase in the initial mixture pressure, tendency being the same as that of hydrocarbon flames. The burned gas Markstein length increases with the increase in the equivalence ratio, the tendency being the same as that of hydrogen/air flames and methane/air flames. The burned gas Markstein lengths at 0.1 MPa are higher than those at 0.3 MPa and 0.5 MPa. However, the values of burned gas Markstein length at 0.3 MPa and 0.5 MPa are almost the same. In addition, numerical simulations using CHEMKIN-PRO with five detailed reaction mechanisms which are presently applicable for the ammonia/air combustion were also conducted. However, qualitative predictions of unstretched laminar burning velocity using those reaction mechanisms are inaccurate. Thus, further improvements of reaction mechanisms are essential for application of ammonia/air premixed flames.
Article
An extensive set offlat laminar premixedNH3/H2/O2,NH3,/NO/H2/O2 and NH3O2 flames have been investigated by detailed chemical kinetic modelling to facilitate the construction of a reaction mechanism capable of satisfactory predictions for a wide range of flames. Available information for the rate coefficients of all the reactions in the detailed mechanism has been reviewed. An extensive sensitivity analysis has been performed to distinguish the reactions of greatest importance to the formation and destruction of nitric oxide. The relative significance of the different NO formation channels is found to depend entirely on the flame conditions: (i) for all flames the reaction of NH2 with the O radical is found to be significant; (ii) in pure ammonia flames the reaction of the NH radical with OH becomes important; (iii) in hydrogen flames with ammonia and ammonia with nitric oxide dopants the Zel'dovich mechanism becomes increasingly significant with increasing fuel concentrations. The conversion of NO to N2 is dominated by reactions involving the NH2 and N radicals with NH providing a secondary path. In pure ammonia and doped lean hydrogen flames the reaction of NO with NH2 becomes the major NO conversion path. In doped stoichiometric and rich hydrogen flames the reaction of NO with N is dominant.
Article
Ammonia oxidation and ignition delay time measurements for pressures above 10 atm are scarce. In addition, NH3 is known to adsorb on stainless steel, so measurement results could be in question if wall passivation is not employed for apparatuses utilizing steel. To overcome these measurement difficulties and overall lack of high-pressure data for ammonia, new and methodical ignition delay time measurements have been performed behind reflected shock waves over a wide range of temperatures (1560–2455 K), pressures (around 1.4, 11, and 30 atm) and equivalence ratios (0.5, 1.0, and 2.0) for mixtures of ammonia highly diluted in Ar (98–99%). The new set of data from the present study was compared to several models from the literature. It was found that a large majority of the models do not predict the ignition delay times with accuracy, and there is a surprisingly wide variation amongst the predictions. One satisfactory model, from Dagaut et al. (2008), was selected and extended to compounds other than NH3 using H2/O2/CO, N2O, NO2, and NNH sub-mechanisms from the literature. The resulting comprehensive mechanism predicts well the ammonia ignition delay time data from the present study along with other NH3, NO2, and N2O data from the authors as well as from the literature with high accuracy. In addition to the new ammonia oxidation data and related model comparisons, the present paper documents a state-of-the-art NOx sub-mechanism that can be used for a wide range of combustion calculations when added to, for example, baseline mechanisms involving hydrogen and hydrocarbon kinetics.
Article
A detailed chemical kinetic mechanism has been developed to describe the oxidation of small hydrocarbon and oxygenated hydrocarbon species. The reactivity of these small fuels and intermediates is of critical importance in understanding and accurately describing the combustion characteristics, such as ignition delay time, flame speed, and emissions of practical fuels. The chosen rate expressions have been assembled through critical evaluation of the literature, with minimum optimization performed. The mechanism has been validated over a wide range of initial conditions and experimental devices, including flow reactor, shock tube, jet-stirred reactor, and flame studies. The current mechanism contains accurate kinetic descriptions for saturated and unsaturated hydrocarbons, namely methane, ethane, ethylene, and acetylene, and oxygenated species; formaldehyde, methanol, acetaldehyde, and ethanol.
Article
Hydrogen oxidation at 50 bar and temperatures of 700–900 K was investigated in a high pressure laminar flow reactor under highly diluted conditions. The experiments provided information about H2 oxidation at pressures above the third explosion limit. The fuel–air equivalence ratio of the reactants was varied from very oxidizing to strongly reducing conditions. The results supplement high-pressure data from RCM (900–1100 K) and shock tubes (900–2200 K). At the reducing conditions (Φ = 12), oxidation started at 748–775 K while it was shifted to 798–823 K for stoichiometric and oxidizing conditions (Φ = 1.03 and 0.05). At very oxidizing conditions (O2 atmosphere, Φ = 0.0009), the temperature for onset of reaction was reduced to 775–798 K. The data were interpreted in terms of a detailed chemical kinetic model, drawn mostly from work of Burke and coworkers. In the present study, the rate constants for the reactions HO2 + OH, OH + OH, and HO2 + HO2 were updated based on recent determinations. The modeling predictions were in good agreement with the measurements in the flow reactor. The predicted H2 oxidation rate was sensitive to the rate of the HO2 + OH reaction, particularly at lean conditions, and the present data support recent values for the rate constant. In addition to the current experiments, the mechanism was evaluated against ignition delay time measurements from rapid compression machines and shock tubes. The model was used to analyze the complex dependence of the ignition delay for H2 on temperature and pressure.
Article
Using potential energy surface information from BAC-MP4 calculations and statistical-dynamical methods, we have calculated the branching fraction for the NH+NO reaction, NH+NO→N2+OH (1) →N2O+H. (2) We find that reaction (2) dominates over the entire temperature range considered, 300 K<T<3500 K, with f=k1/(k1+k2) varying from about 0.19 at room temperature to about 0.30 at 3500 K. In addition, we have calculated rate coefficients for the two-channel process, NH+O2→HNO+O (3) →NO+OH. (4) In this case we find that reaction (4) dominates at low temperature, reaction (3) at high temperature. Between 300 K and 3300 K these rate coefficients can be expressed as (cm3/mole-sec) k3=4.61×105T2.0 exp (−6500/RT) and k4=1.28×106T1.5 exp (−100/RT). All these results are in good agreement with available experimental data.
Article
Effect of ethanol (EtOH) addition to unburnt gas mixture on the species pool in a fuel-rich flat, premixed, laminar ethylene flame at atmospheric pressure is studied experimentally and by chemical kinetic modeling. Mole fraction profiles as a function of height above burner of various stable and labile species including reactants, major products and intermediates (C1–C4 hydrocarbons) are measured using molecular beam mass spectrometry with electron ionization in C2H4/O2/Ar and C2H4/EtOH/O2/Ar flames. The experimental profiles are compared with those calculated using three different chemical kinetic mechanisms. Performances and deficiencies of the mechanisms are discussed. An analysis of the mechanisms is carried out in order to identify the reason of the ethanol effect on the mole fraction of propargyl, the main precursor of benzene. A modification of some mechanisms in order to improve their capability to predict acetylene and diacetylene mole fraction profiles is proposed.
Article
The oxidation of NHâ during oxy-fuel combustion of methane, i.e., at high [COâ], has been studied in a flow reactor. The experiments covered stoichiometries ranging from fuel rich to very fuel lean and temperatures from 973 to 1773 K. The results have been interpreted in terms of an updated detailed chemical kinetic model. A high COâ level enhanced formation of NO under reducing conditions while it inhibited NO under stoichiometric and lean conditions. The detailed chemical kinetic model captured fairly well all the experimental trends. According to the present study, the enhanced CO concentrations and alteration in the amount and partitioning of O/H radicals, rather than direct reactions between N-radicals and COâ, are responsible for the effect of a high COâ concentration on ammonia conversion. When COâ is present as a bulk gas, formation of NO is facilitated by the increased OH/H ratio. Besides, the high CO levels enhance HNCO formation through NHâ+CO. However, reactions NHâ+ O to form HNO and NHâ+H to form NH are inhibited due to the reduced concentration of O and H radicals. Instead reactions of NHâ with species from the hydrocarbon/methylamine pool preserve reactive nitrogen as reduced species. These reactions reduce the NHâ availability to form NO by other pathways like via HNO or NH and increase the probability of forming Nâ instead of NO. (author)
Article
One of the remaining issues in our understanding of nitrogen chemistry in combustion is the chemistry of NNH. This species is known as a key intermediate in Thermal DeNOx, where NH3 is used as a reducing agent for selective non-catalytic reduction of NO. In addition, NNH has been proposed to facilitate formation of NO from thermal fixation of molecular nitrogen through the so-called NNH mechanism. The importance of NNH for formation and reduction of NO depends on its thermal stability and its major consumption channels. In the present work, we study reactions on the NNH+O, NNH+O2, and NH2+O2 potential energy surfaces using methods previously developed by Miller, Klippenstein, Harding, and their co-workers. Their impact on Thermal DeNOx and the NNH mechanism for NO formation is investigated in detail.
Article
Structures of diluted ammonia–hydrogen–oxygen–argon flame have been investigated at low pressure by molecular beam mass spectrometry to study the effect of the initial hydrogen content, the equivalence ratio and the working pressure on ammonia combustion. Excluding a slight effect on the NO formation, the initial hydrogen content does not have a significant impact on the composition of the burned gases. Around stoichiometric conditions, a decrease of the equivalence ratio strongly increases the NO formation. Furthermore, at high temperature, the NO formation is favoured as it has been noticed in flames where, due to different heat transfer, the maximum temperature increases with a decrease of pressure.The best agreement of the numerical simulation with the experimental results has been obtained by using Konnov’s mechanism excepted for NH2 and N2O. The impact of the main individual reaction pathways is very similar in all investigated flames.