Article

Ammonia and ammonia/hydrogen blends oxidation in a jet-stirred reactor: Experimental and numerical study

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Abstract

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.

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... Previous investigations on fundamental combustion characteristics of NH 3 co-firing with DME and H 2 focused on binary mixtures, i.e., NH 3 / DME mixtures [20][21][22][23][24] and NH 3 /H 2 mixtures [25][26][27][28][29][30][31][32][33][34][35][36][37][38]. The measurements mainly include global combustion parameters [20][21][22][23][25][26][27][28][29][30][31][32][33][34][35], such as laminar burning velocities (LBVs) and ignition delay times (IDTs), and speciation in various reactors and burners [24,[36][37][38]. ...
... Previous investigations on fundamental combustion characteristics of NH 3 co-firing with DME and H 2 focused on binary mixtures, i.e., NH 3 / DME mixtures [20][21][22][23][24] and NH 3 /H 2 mixtures [25][26][27][28][29][30][31][32][33][34][35][36][37][38]. The measurements mainly include global combustion parameters [20][21][22][23][25][26][27][28][29][30][31][32][33][34][35], such as laminar burning velocities (LBVs) and ignition delay times (IDTs), and speciation in various reactors and burners [24,[36][37][38]. For NH 3 /DME mixtures, Issayev et al. [21], Yin et al. [22], and Xiao and Li [23] measured their LBVs at 298-423 K, 1-5 atm, and varying fuel contents and equivalence ratios (ϕ) using the outwardly propagating spherical flame method. ...
... Similar improvement of fuel reactivity by shortening the IDTs can be noticed in previous NH 3 /H 2 co-firing studies using RCM [34,35]. Besides, speciation in JSR and laminar premixed flames of NH 3 /H 2 was also performed using Fourier transform infrared spectroscopy and mass spectroscopy [36][37][38]. These experimental progresses offer valuable validation targets for kinetic models of NH 3 /DME [20][21][22][23][24]39,40] and NH 3 /H 2 [27,29,30,36,41,42]. ...
Article
Combustion enhancement is essential for actualizing the practical application of ammonia (NH3) which belongs to low-calorific-value fuels. In this work, a new NH3 enhancement strategy is proposed by co-firing NH3 with dimethyl ether (DME)/hydrogen (H2) mixtures under methane-equivalent calorific value (MECV), resulting in fuel mixtures with almost half the contents contributed by DME and the rest contributed by NH3/H2. The laminar flame propagation is investigated at the initial temperature of 298 K and varying fuel contents, equivalence ratios, and initial pressures. A kinetic model of high-temperature NH3/DME/H2 combustion is constructed and validated against both the present laminar burning velocity (LBV) data and experimental data in the literature. The co-firing can effectively enhance the combustion intensity and laminar flame propagation of NH3. For fuel mixtures with less or more abundant H2, the DME or H2 chemistry plays the dominant role in the enhancement of fuel reactivity and laminar flame propagation, respectively. Pressure effects and fuel content effects are analyzed in detail, demonstrating apparent dependency on reaction circumstances. Compared with the NH3/H2 co-firing, co-firing NH3 with DME/H2 mixtures under MECV has much lower increments and generally smoother variations in normalized LBV with the decreasing NH3 content, which results from the over 80% contributions of DME to the fuel volumetric calorific values. Test experiments in a gas turbine model combustor are also performed to investigate the performance in gas turbine combustion regime. The measured swirl combustion characteristics indicate enhanced fuel reactivity and combustion stability with the decreasing NH3 content, which can be associated with the observed trends of LBV in this work. Both the LBV and swirl combustion experiments indicate that the 40.0%NH3/49.4%DME/10.6%H2 and 30.0%NH3/50.1%DME/19.9%H2 mixtures behave generally the best among the six fuel mixtures in mimicking the combustion characteristics of methane, demonstrating the potential of NH3/DME/H2 mixtures to be a novel carbon-neutral alternative of methane in practical applications without the need of great mechanical retrofits.
... The high-temperature and low-to-intermediate-temperature reactions of NH 3 oxidation and NO x formation and consumption reactions coupled with NH 3 oxidation are added to the model with their generalized scheme illustrated in Fig. 1. Since pure NH 3 is rather un-reactive as a fuel and its oxidation onset temperature is approximately 1200 K at atmospheric pressure [35][36][37]39], we define the low-to-intermediate-temperature range for NH 3 combustion to be 1000-1500 K and the high-temperature combustion regime to be at temperatures above 1500 K. The important reactions in these regimes and their rate constants are discussed here. ...
... The speciation data measured by Zhang et al. [37] and Osipova et al. [39] respectively in JSRs for NH 3 /H 2 mixtures, at the same pressure of 1 atm and at a residence time (t res ) of 1 s, at different equivalence ratios of 0.25-1.5 and H 2 fractions of 0-70%, are presented together with our model predictions in Fig. 17. The model captures the NH 3 , H 2 O, NO and N 2 O concentrations at φ = 0.25 and different H 2 ratios reasonably well for the fuel-lean data, although it slightly over-predicts fuel reactivity for H 2 ratios higher than 50%, Fig. 17(a). ...
... (c) NH 3 /H 2 (30 %) at φ = 0.6, 1.0 and 1.5 (in black, red and blue, respectively). Solid symbols are experimental data from Zhang et al. [37] while half-filled symbols are experimental data from Osipova et al. [39]. Lines are predictions of the model developed in this work. ...
... The aim of this work is to update and develop a chemical kinetic model that can predict IDTs, speciation and laminar burning velocities for pure ammonia and targeted ammonia blends including NH 3 /H 2 , NH 3 /CH 3 OH, and NH 3 /n-C 7 H 16 . The model is validated against available literature data [24,29,30,32,34,35,36] from fundamental combustion experiments of NH 3 and NH 3 /blends. The combustion chemistry of NH 3 at both high-and low-to-intermediate temperature regimes, and the effect of different blends will be discussed in this work. ...
... Figure 7 shows experimental and C3MechV3.4 model predicted species mole fraction profiles for reactants and the main products including NH 3 , O 2 , H 2 O, N 2 and H 2 , from the experiments performed by Osipova et al. [32]. As shown in Figure 7a for pure ammonia and at φ =1.0, the oxidation process for pure ammonia starts at a temperature of approximately 1200 K. ...
... According to Osipova et al. [32] H-atom abstraction from ammonia by ȮH radicals and Ö and Ḣ atoms are the main channels consuming the fuel. These include NH 3 + ȮH = ṄH 2 + H 2 O, NH 3 + Ö = ṄH 2 + ȮH and NH 3 + Ḣ = ṄH 2 + H 2 , with NH 3 + ȮH being predominant. ...
Conference Paper
Full-text available
div class="section abstract"> Ammonia is a promising carbon-free alternative fuel for use in combustion systems. The main associated challenges are its relatively low reactivity and high NOx emissions compared to conventional fuels. Therefore, the combustion behaviour of ammonia and ammonia blends still needs to be better understood over a wide range of conditions. To this end, a comprehensive chemical kinetic mechanism C3MechV3.4, which is an update of C3MechV3.3, has been developed for improved predictions of the combustion of ammonia and ammonia blends. C3MechV3.4 has been validated using a wide range of experimental results for pure ammonia and ammonia/hydrogen, ammonia/methanol and ammonia/ n -heptane blends. These validations target different data sets including ignition delay times, species profiles measured as a function of time, and/or temperature and laminar flame speeds over a wide range of conditions. The updated developed mechanism gives good predictions for pure ammonia and its blends with hydrogen, methanol and n -heptane. The most important reactions affecting predictions in different regimes for the various ammonia mixtures are discussed. </div
... This is because the central region exhibits higher combustion temperatures, which promotes thermal NO generation. NO is generated in ammoniahydrogen mixture through both the thermal pathway (extended Zeldovich pathway) and the ammonia oxidation pathway (NH 3 → NH 2 /NH → HNO → NO) [35]. Increasing the hydrogen proportion promotes the NO generation through both thermal and fuel pathways, driven by elevated combustion temperatures and increased OH concentration, respectively. ...
... The development of clean renewable fuels is a significant issue currently due to pollutant emissions, climate change, and the lack of fossil resources [1][2][3]. Hydrogen, as a renewable and carbon-free fuel, will play a promising role in energy sources and decarbonizing transport in the future [4,5]. Generally, there are two main methods to use hydrogen energy: hydrogen engines (H2ICEs) [6] and hydrogen fuel cells [7]. ...
... According to their results, channel a) is faster than b) above 700 K at 1 atm. Neat NH 3 is a relatively unreactive fuel, and its oxidation starts at approximately 1200 K at atmospheric pressure [16,21,74,87,88]; therefore, in the investigated NH 3 /air flames, channel a) dominates over channel b). ...
Article
Full-text available
Testing detailed combustion mechanisms typically concludes that some mechanisms reproduce the experimental data well at most conditions but are inaccurate at other conditions. However, other mechanisms may perform well under these conditions. A better mechanism ("mosaic mechanism") may be obtained by identifying the overall best-performing mechanism and loaning the most important reaction steps and their rate parameters from another mechanism with good performance at the conditions where the overall best model is ill-performing. A new algorithm based on this approach is presented here, which is successfully applied using a comprehensive collection of NH3/air laminar burning velocity data (348 data points in 61 data series) and eight recent detailed NH3 combustion mechanisms. The suggested new mosaic mechanism is an improved version of the CEU-2022 mechanism and provides a better reproduction of the utilised data than the previously published mechanisms. The proposed algorithm can be applied to any chemical kinetics system and any other types of experiments. All data needed to apply the algorithm to various combustion systems are already available or can be generated with minimal human effort using the experimental data files, mechanisms, and codes available on the ReSpecTh (https://ReSpecTh.hu) website.
... NH 3 and H 2 O speciation profiles in the oxidation of NH 3 in ~99% Ar. φ =2.03.3.4 Osipova et al.12 ...
Article
Ammonia (NH3) is emerging as a promising fuel due to its high energy density, high hydrogen content, and zero carbon emissions from combustion. The study of chemical kinetics in NH3...
... Li mechanism realizes the transformation from NO to NH 2 through OH by R60 NH 2 +NO<=>NNH+OH, while the Ni mechanism through H by R36 NH+NO<=>N 2 O+H. Yet, both reactions were reported as important DeNOx reactions in previous studies [59,60]. The analysis based on the Okafor mechanism gets the same loop structure as that of the Ni mechanism, with minor differences in the dominant reaction in the NH 2 dehydrogenation-oxidation loop. ...
Article
Unstable combustion states are frequently observed in ammonia/hydrogen blended combustion systems. This study aims to explore the underlying thermokinetic feedback driving forces responsible for the occurrence of unstable combustion states based on the identified dominant reactions. To achieve this, the newly developed functional weight analysis method is employed and compared with the combination of heat-release/temperature sensitivity anaylsis. Analysis conducted within several popular mechanisms over a wide temperature and pressure range reveals that the important reactions identified in different mechanisms consistently exhibit loop-formed kinetics with similar mechanism functions.These loops include a hydrogen chain-branching loop, a NH 2 dehydrogenation-oxidation loop, and a NOx-DeNOx loop. All of these loops encompass a combination of exothermic and endothermic reactions, and they mutually promote each other due to their loop-formed kinetic. Consequently, when the heat release/heat absorption ratios within the loop kinetics meet specific quantitative criteria, they establish effective thermokinetic feedback. This feedback mechanism initiates and sustains non-constant heat release, serving as a monopole sound source that produces acoustic waves. It is worth noting that the discovery of NOx-DeNOx loop provides a compelling explaination for the frequently observed association between oscillatory combustion states and elevated NOx emissions.
... Ammonia (NH 3 ) is one of the most prominent carbon-free fuel contenders for the decarbonization of energy production by virtue of its intrinsic high energy density and the deep-rooted supply chain. Unfortunately, besides being corrosive and toxic, NH 3 suffers from unideal combustion properties, including low laminar burning velocity and radiation intensity, high ignition energy, and narrow flammability limits [1][2][3]. In addition, the propensity to emit noxious N-based pollutants such as nitric oxides (NO x ) and nitrous oxide (N 2 O) makes NH 3 's implementation for combustion applications not yet profitable and environmentally sustainable at industrial levels. ...
... Of note, the good performance of the Zhang-2021 model is anticipated because it incorporates the thermodynamic data for NH, NH 2 , NNH, NH 2 , as well as the reactions involved in the NH 3 sub-mechanism, DeNO x , and NO x formation. Moreover, the model takes into account the third-body collision efficiency of NH 3 in NH 2 + NH 2 (+M) ⇔ N 2 H 4 (+M) [39]. Accordingly, the Zhang-2021 model is effective in simulating the WSR combustion of NH 3 and hence can be used in the subsequent sections. ...
... Ammonia can act as a green fuel, as it does not contain carbon resulting in zero-carbon emissions, and is relatively hydrogen dense, possessing 17.8% hydrogen by weight. Importantly, ammonia can be liquified either at 8.5 atm at room temperature or at − 33 • C and atmospheric pressure, which makes it much easier for storage compared to hydrogen [22]. On the other hand, the requirement of ammonia storage is similar to another commercial: propane. ...
... Of note, the good performance of the Zhang-2021 model is anticipated because it incorporates the thermodynamic data for NH, NH 2 , NNH, NH 2 , as well as the reactions involved in the NH 3 sub-mechanism, DeNO x , and NO x formation. Moreover, the model takes into account the third-body collision efficiency of NH 3 in NH 2 + NH 2 (+M) ⇔ N 2 H 4 (+M) [39]. Accordingly, the Zhang-2021 model is effective in simulating the WSR combustion of NH 3 and hence can be used in the subsequent sections. ...
... 12 Especially for H 2 , with partial thermal or catalytical cracking, 13 mixtures of ammonia and hydrogen can be readily produced onboard to achieve desired flame speed enhancement. The combustion properties of ammonia and hydrogen blends have been extensively investigated in various reactor configurations, including laminar premixed flame, 14 jet flame, 15 shock tube, 16 rapid compression machine, 17 flow reactor, 18 jet stirred reactor, 19 and swirl combustors. 20 In addition, various detailed kinetic mechanisms have been established to model the combustion chemistry of ammonia, for example, Glarborg et al., 21 Nakamura et al., 22 Otomo et al., 23 Okafor et al., 5 and Mei et al. 24 With quantitative experimental measurement and kinetic modeling, in-depth understanding and reasonable prediction have been produced for ammonia combustion in well-controlled laboratory combustors. ...
Article
With the growing trend of decarbonization in ground transportation, low and zero-carbon fuels have attracted extensive research interest. Liquid ammonia is a promising alternative fuel due to its relatively high volumetric energy density, mature production and distribution infrastructure, convenience of storage, and zero carbon emissions. However, ammonia combustion also suffers from low flame speed and weak chemical reactivity. In this work, we computationally investigate the suitable engine-relevant thermochemical conditions for auto-ignition of constant volume ammonia spray, as well as its spray dynamics, vaporization, flash boiling effects, and emissions. The simulation is first validated by comparing it against available experimental data from a vaporizing ammonia spray and is then extended to chemically reactive conditions. Results show that ammonia sprays under engine-relevant conditions (60 bar and 1200 K) can only successfully auto-ignite for cases with ambient hydrogen addition, through enhancement of thermal condition and chemical reactivity. A chemical flux analysis is conducted to further understand the important species and reactions that promote ammonia auto-ignition from hydrogen, which potentially can be introduced via H 2 solubility, exhaust gas recirculation, and onboard ammonia thermal decomposition. Furthermore, results have indicated that charge cooling effects can further decrease the temperature in the flow field and make auto-ignition more difficult. This study provided useful insights for the application of ammonia as a zero-carbon diesel fuel for ground transportation.
... The molecular compositions of the studied mixtures are listed in Table 1 , as well as their corresponding properties. A 36-species skeletal chemical mechanism of NH 3 /H 2 /air mixtures developed by Zhang et al. [41] was adopted, as it has been extensively validated with experiments under engine-relevant conditions for different levels of NH 3 /H 2 blending [42][43][44][45] . Due to its high accuracy in predicting the ignition delay time and flame speed, the chemical mechanism is considered suitable for the current study to capture the occurrence of the localized auto-ignition and to ascertain whether the generated flame front could be coupled with shock waves for detonation initiation [19,46] . ...
Article
Full-text available
This study numerically investigates the detonation development of carbon-free fuels, namely ammonia and hydrogen (NH3 and H2), using one-dimensional (1D) simulations under the end-gas autoignitive conditions relevant to internal combustion (IC) engines. Five stoichiometric NH3/H2/air mixtures with different NH3/H2 blending ratios are studied. A 1D hot spot with varied lengths and temperature gradients is used to induce different ignition modes. The detonation peninsulas are quantitatively identified by two non-dimensional parameters, namely the resonance parameter, ξ, and the reactivity parameter, ε. Increasing the H2 blending ratio up to 80% results in a unique horn-shaped detonation peninsula, i.e., the magnitude of the upper and lower ξ limits, ξu,l, near the leftmost boundaries of the detonation peninsula of the rich-H2 mixtures becomes larger by an order of magnitude as compared to those of the lean-H2 mixtures. Such behavior is attributed primarily to the large heat diffusion of hydrogen, leading to rapid heat dissipation of the hot spot and the significantly decreased transient ξ over time, thus promoting detonation development. The analysis reveals that the characterization of detonation propensity in the rich-H2 mixtures needs to account for the fast heat diffusion of the initial hot spot, in which the initial magnitude of ξ is not representative of its detonability. As such, a correction factor, β, weighted by the ignition Damköhler number, is proposed to resolve the discrepancy of the ξu,l limits between different NH3/H2/air mixtures. With this correction, the transient magnitudes of ξ, ξt, prior to the main ignition are well predicted such that a unified shape of the detonation peninsula for different NH3/H2/air mixture compositions is achieved.
... To further support the model development, numerous new studies have been reported in the past few years [47 -49] which provided huge data sets from speciation measurements for model refinement. Those data were measured in flames [47,48], flow reactors [49][50][51][52], jet stirred reactors [27,53 -56], and shock tubes [57 -62] via diagnostics such as molecular beam mass spectrometer (MBMS) [47,52,53,56], gas chromatography-mass spectrometer (GC/MS) [51,54,63], Fourier transform infrared spectrometer (FTIR) [27,55], tunable diode laser absorption spectroscopy (TDLAS) [57][58][59][60][61][62], and gas analyzers [49,51,55,63]. Temperature-resolved and time-resolved speciation data shed more light on the detailed reaction pathway and great improvement has been achieved from different groups. ...
Article
Full-text available
Ammonia has recently attracted numerous attentions from researchers and policy makers as promising energy and hydrogen carrier for mitigating the carbon footprints in the energy sector. The mature infrastructure for the production, storage, and transportation of ammonia allows a quick role of ammonia in energy systems. By applying green hydrogen and renewable energies, green ammonia can be produced, which makes ammonia even more attractive. Prior to the commercial use of ammonia for large-scale energy systems, e.g., internal combustion engines and stationary gas turbines, a fundamental understanding of the ignition and combustion behaviors of ammonia is essential. In the past decades, a lot of studies on ammonia combustion have been published. Those studies covered broad topics including experimental and numerical investigations which were either fundamental or practical oriented. To continuously follow state-of-the-art and to provide a brief overview of the most recent research on ammonia combustion so that others can easily identify the most relevant work, this review summarizes the recent progress on combustion characteristics of ammonia and ammonia fuel blends as well as their potential use in internal combustion engines. Combining the advantages and drawbacks identified in both fundamental and practical studies, a clearer road map for ammonia application is given.
... 22,29−32 Specifically, Alturaifi et al. 22 and He et al. 29 have performed shock tube experiments with varied NH 3 /H 2 mixture composition, employing laser diagnostics to measure NH 3 and H 2 O time histories in the former and temperature and NH 3 concentrations in the latter work, but time-resolved NO profiles at similar conditions have not been reported. Zhang et al., 30 Osipova et al., 31 and Manna et al. 32 have analyzed the reaction behavior of several NH 3 /H 2 mixtures in a jet-stirred reactor (JSR) at atmospheric pressure in the temperature range from 800 K to 1350 K, determining species with different techniques. All these authors have compared their experimental results to several available reaction models or developed their own mechanism, but still, none of the mechanisms can predict well over a wide range of conditions. ...
Article
Ammonia (NH3) is a promising fuel, because it is carbon-free and easier to store and transport than hydrogen (H2). However, an ignition enhancer such as H2 might be needed for technical applications, because of the rather poor ignition properties of NH3. The combustion of pure NH3 and H2 has been explored widely. However, for mixtures of both gases, mostly only global parameters such as ignition delay times or flame speeds were reported. Studies with extensive experimental species profiles are scarce. Therefore, we experimentally investigated the interactions in the oxidation of different NH3/H2 mixtures in the temperature range of 750-1173 K at 0.97 bar in a plug-flow reactor (PFR), as well as in the temperature range of 1615-2358 K with an average pressure of 3.16 bar in a shock tube. In the PFR, temperature-dependent mole fraction profiles of the main species were obtained via electron ionization molecular-beam mass spectrometry (EI-MBMS). Additionally, for the first time, tunable diode laser absorption spectroscopy (TDLAS) with a scanned-wavelength method was adapted to the PFR for the quantification of nitric oxide (NO). In the shock tube, time-resolved NO profiles were also measured by TDLAS using a fixed-wavelength approach. The experimental results both in PFR and shock tube reveal the reactivity enhancement by H2 on ammonia oxidation. The extensive sets of results were compared with predictions by four NH3-related reaction mechanisms. None of the mechanisms can well predict all experimental results, but the Stagni et al. [React. Chem. Eng. 2020, 5, 696-711] and Zhu et al. [Combust. Flame 2022, 246, 115389] mechanisms perform best for the PFR and shock tube conditions, respectively. Exploratory kinetic analysis was conducted to identify the effect of H2 addition on ammonia oxidation and NO formation, as well as sensitive reactions in different temperature regimes. The results presented in this study can provide valuable information for further model development and highlight relevant properties of H2-assisted NH3 combustion.
... In addition, little is known about the formation of NO x from ammonia impurities, and its impact on fuel oxidation, combustion stability, and emissions under high-pressure oxy-fuel sCO 2 cycle conditions. Most of the ammonia related work in the literature were performed at lower pressures or non-oxy-combustion conditions [29][30][31][32][33][34] and [35][36][37][38][39][40][41][42]. ...
Article
Full-text available
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To achieve zero carbon emissions, replacing conventional fuels in combustion engines and gas turbines with carbon-free fuel is of utmost important. While hydrogen is effective in mitigating climate change, the cost challenges in liquefaction and transportation persist. Ammonia (NH3), a carbon-free fuel with its higher volumetric energy density and cost advantages, emerges as a potential substitute. This review reports the most recent studies on NH3 as a fuel for micro gas turbine (MGT), highlighting both advantages and limitations. The performance and emissions in gas turbines are discussed. The main obstacles to a widespread usage of NH3 blends as fuel for MGT power generation are addressed, along with the current stage of commercialization. The review explores all the numerical and experimental works on NH3 blend in combustion system of MGT, and further presents ways to overcome the limitations associated with the combustion, such as high NOx emissions and low burning velocity.
Chapter
The use of ammonia as a fuel has been recognized as a promising way for various energy and power devices toward carbon neutrality. However, significantly different combustion characteristics of ammonia from conventional fossil fuels pose great challenges to its applications. Among them, the extremely low combustion reactivity of ammonia can lead to poor flame stability and low combustion efficiency, which consequently hinders its direct utilization in existing combustion devices. Therefore, this chapter focuses on the challenge and reviews the emerging attempts in which enhancement strategies have been proposed and tested in laboratory burners or real-scale combustors. Details of the combustion enhancement strategies, such as reactive fuel-cofiring, fuel-cracking, pre-heating, oxygen-enrichment, and combustion auxiliary technologies have been introduced from theoretical, diagnostic, numerical, and technical aspects. These state-of-the-art progresses greatly enable a deeper mechanistic understanding of ammonia combustion enhancement strategies and further development of clean and efficient ammonia combustion technologies, thus facilitating applications of ammonia in practical combustion devices, such as internal combustion engines, aero-engines, land-based gas turbines, furnaces, and boilers.
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Ammonia (NH3) is a carbon-free fuel. Therefore, many researchers have proposed detailed mechanisms for NH3, yet the existing mechanisms have widely varied combinations of reactions and were validated using different experimental datasets. Thus, this study suggests developing a NH3 mechanism by exploring the vast reaction pool of the five existing mechanisms. Several reaction combinations from the reaction pool were tested to validate the experimental datasets. The developed mechanism showed the lowest mean squared error (MSE) among the referenced mechanisms for predicting the ignition delay times (IDT). Furthermore, the MSE of the laminar burning velocity (LBV) prediction was less than those of the three referenced mechanisms. Although the mechanism was developed by employing only NH3 fuel mixtures, the estimated IDT and LBV of NH3/H2 fuel mixtures showed low MSE. Analyses of many tested sample mechanisms revealed that the rate coefficients of NH3 combustion reactions should be further elucidated for enhanced prediction.
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Ammonia (NH3) has been suggested as a fuel to attain zero carbon emissions. However, dealing with ammonia needs careful studies to reveal its limits as a suitable and promising fuel for broad applications within large power requirements. Chemical reaction mechanisms, widely employed in the modeling of these applications, are still under development. Therefore, this review is aimed to shed light on the current mechanisms available in the literature, highlighting modeling parameters that directly affect reaction rates which in turn govern the performance of each reaction mechanism. The key findings denote that most of the reaction mechanisms have poor performance when predicting combustion characteristics of ammonia flames such as laminar flame speed, ignition delay time, and nitrogen oxide emissions (NOx). In addition, none of the mechanisms have been optimised efficiently to predict properly experimental measurements for all these combustion characteristics. For example, Duynslaegher's mechanism perfectly predicted the laminar flame speed at lean and stoichiometric conditions, while Nakamura's reaction mechanism worked properly at rich conditions for the estimation of laminar flame speed. Although the aforementioned mechanisms achieved good estimation in terms of laminar flame speed, they showed poor performance against NO mole fractions. Similarly, Glarborg's (2018) mechanism properly estimated NO mole fractions at lean and stoichiometric flames while Wang's mechanism performed well in rich conditions for such emissions. Other examples are presented in this manuscript. Finally, the prediction performance of the assessed mechanisms varies based on operating conditions, mixing ratios, and equivalence ratios. Most mechanisms dealing with blended NH3 combinations gave good predictions when the concentration of hydrogen was low, while deteriorating with increasing hydrogen concentrations; a result of the shift in reactions that require more research.
Poster
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Introduction Ammonia (NH3) is a promising carbon-free fuel because it can be used in a sustainable and recyclable loop for energy production. It is often blended with other fuels in practical applications. One of the most often used co-fuels is hydrogen (H2). Accurate chemical kinetic models are needed that can describe the combustion of fuel mixtures containing NH3 under typical conditions of industrial applications. In this work, we compare the performance of 18 reaction mechanisms against a large amount of experimental data on the combustion of neat NH3 and NH3/H2 fuel mixtures. This work is an extension of our previous work on the same combustion system (Szanthoffer et al., Appl. Energ. Combust. Sci. 14 (2023) 100127.).
Article
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A possible solution to improve the combustion properties of ammonia is to blend it with other fuels. Two of the most usually used co-fuels are hydrogen and syngas (H2/CO). To investigate the chemistry of the co-combustion with these fuels, a large amount of indirect experimental data for the combustion of neat NH3, and NH3/H2 and NH3/syngas fuel mixtures were collected from the literature including ignition delay times measured in shock tubes, concentration measurements in jet stirred and flow reactors, and laminar burning velocity measurements. Altogether, 4898 data points (in 472 data series) were recorded which cover wide ranges of equivalence ratio, temperature, and pressure. These experimental data are available in data files in the ReSpecTh site (http:// respecth.hu). The performances of 18 recently published detailed reaction mechanisms were quantitatively assessed using the collected experiments. There are significant differences between the performances of the models, and the performance of a mechanism may also vary significantly with the different types of experiments. The best-performing mechanisms are POLIMI-2020, Han-2020, and KAUST-2021 for NH3/H2 fuel mixtures, and Shrestha-2021, Mei-2021, and Mei-2020 for NH3/syngas systems. The results indicate that further mechanism development is needed to reproduce the measurements more accurately. Local sensitivity analysis was carried out on the kinetic and thermodynamic parameters of the best-performing mechanisms. Even though the inves tigated models have different parameter sets, the most important reactions and thermodynamic properties are similar. The most important reactions are not the same for the different types of experiments but most of them include the NH3, NH2, and/or NNH species. Among the thermodynamic parameters, model outputs are most sensitive to the data of NH3 and NH2.
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With the global need to achieve carbon neutrality and to stop global warming by reducing greenhouse gas emissions, ammonia is being growingly recognized as a promising renewable-energy-sourced energy carrier to replace conventional fossil fuels in propulsion and power generation systems. Unfortunately, unlike conventional fuels, ammonia combustion features some unfavorable characteristics like high nitrogen oxide (NOx) emissions, significantly hindering its practical industrial utilization. The present work overviews the state-of-the-art advances in ammonia combustion, that summarize the reaction mechanisms, NOx generation mechanism and mitigation strategies, and how ammonia addition affects soot formation. Fundamental kinetic studies of ammonia pyrolysis and oxidation mechanisms are first reviewed, followed by a discussion of different effective strategies for abating nitrogen oxides. Then, ammonia addition to soot formation in hydrocarbon fuels-based combustion is discussed, and the underpinning mechanisms are described. Finally, it is concluded with a discussion of technical challenges and future research prospects on ammonia combustion with lower NOx emissions.
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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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To investigate the oxidation of ammonia (NH3)/hydrogen (H2) mixtures at intermediate temperatures, this work has implemented jet-stirred reactor (JSR) oxidation experiments of NH3/H2 mixtures at atmospheric pressure and over 800-1280 K. The H2 content in the NH3/H2 mixtures is varied from zero to 70 vol% at equivalence ratios of 0.25 and 1.0. Species identification and quantification are achieved by using Fourier-transform infrared (FTIR) spectroscopy. A kinetic model for pure NH3 and NH3/H2 mixtures is also developed for this research, and validated against the present experimental data for pure NH3 and NH3/H2 mixtures, as well as those for pure NH3, H2/NO, H2/N2O, NH3/NO, NH3/NO2 and NH3/H2 mixtures in literature. The model basically captures the experimental data obtained here, as well as in literature. Both measured and predicted results from this work show that H2 blending enhances the oxidation reactivity of NH3. Based on the model analysis, under the present experimental conditions, NH3 + H = NH2 + H2 proceeds in its reverse direction with increasing H2 content. The H atom produced is able to combine with O2 to produce either O and OH via a chain-branching reaction, or to yield HO2 through a chain-propagation reaction. HO2 is an important radical under the present intermediate-temperature conditions, which can convert NH2 to OH via NH2 + HO2 = H2NO + OH; H2NO is then able to convert H to NH2 and OH. In this reaction sequence, NH2 and H2NO are chain carriers, converting HO2 and H to two OH radicals. Since the OH radical is the dominant radical to consume NH3 under the present conditions, the enhanced OH yield via H + O2 = O + OH, NH2 + HO2 = H2NO + OH and H2NO + H = NH2 +OH, with increasing H2 content, promotes the consumption of NH3. For NOx formation, non-monotonous trends are observed by increasing the content of H2 at the 99% conversion of NH3. These trends are determined by the competition between the dilution effects and the chemical effects of H2 addition. Nitrogen related radicals, such as NH2, NH and N, decrease as H2 increases, and this dilution effect reduces NOx formation. For chemical effects, the yields of oxygenated radicals, such as O, OH and HO2, are enhanced with increasing H2 content, which results in enhancing effects on NO formation. For N2O formation, the enhanced oxygenated radicals (O, OH and HO2) suppress its formation, while the enhanced NO promotes its formation.
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The influence of the addition of ammonia on the oxidation of methane was investigated both experimentally and numerically. Experiments were carried out at atmospheric pressure, using a fused silica jet-stirred reactor, and a recrystallized alumina tubular reactor designed on purpose to reach temperatures as high as ∼2000 K. A temperature range of 600–1200 K was investigated in the jet-stirred reactor at a residence time of 1.5 s, while experiments in the flow reactor were carried out between 1200 and 2000 K, for a fixed residence time of about 25 ms in the reactive zone. A methane/ammonia mixture, diluted in helium, was used in both reactors with equivalence ratios varied between 0.5 and 2 in the first reactor, while stoichiometric conditions were investigated in the second one. The measurements indicate that CH4 reactivity was promoted by NH3 addition below 1200 K, but not so much influenced above. These results were interpreted and explained using a comprehensive kinetic model, previously validated over a wider range of operating conditions. The mechanism allowed to shed light on the underlying causes of the anticipated methane reactivity at low temperature, and of the major role played by NOx in it. This effect was shown to become less significant at higher temperatures, where the reactivity is mainly governed by H-abstractions on both fuels.
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Mixing ammonia with syngas can be a promising way to overcome the low reactivity of ammonia, allowing it to find usage in IGCC (Integrated Gasification Combined Cycle) systems and gas turbines for power generation. However, fundamental experimental data on laminar burning velocity of NH 3 /syngas/air are rather scarce, especially at elevated pressures. This information is critical for the development and validation of reaction mechanisms and advances in combustor design. In the present work, measurements of the laminar burning velocities (S L) of NH 3 /syngas/air, NH 3 /CO/air, and NH 3 /H 2 /air premixed flames were performed by the heat flux method at pressures up to 5 atm, equivalence ratios ranging from 0.7 to 1.6, ammonia mole fractions in the fuel mixture from 0.2 to 1.0 in the NH 3 /syngas/air mixtures and 0.03-1.0 in the NH 3 /CO/air mixtures. Several recently published ammonia oxidation mechanisms were tested against the present experimental data. The measurements and predictions of S L exhibit discrepancies especially for NH 3 /H 2 /air flames at elevated pressures. The pressure exponent factors, β, characterizing burning velocity at elevated pressure via empirical power-law correlation S L / S L0 = (P/P 0) β are extracted from the measured S L and compared with the numerical results. The thermal, diffusion, and chemical effects of blending syngas with ammonia on S L of the mixtures are distinguished, and the dominant role of the adiabatic flame temperature on the variation of the pressure exponent β is discussed. Kinetic model-ing and sensitivity analyses showed that reactions of NH i to N 2 H i (i = 0-4) species affect the predicted S L under rich conditions. At elevated pressures, these reactions also affect the NO formation via third-body collision reactions and NH i + NO reactions. Even for rich flames, the ammonia consumption is favored with the addition of syngas which also promotes NO formation by enriching the H and OH radical pools and increasing the flame temperature. The addition of hydrogen or carbon monoxide has equally promoting effect on the ammonia decomposition and NO x formation although their flame speed differs a lot.
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A complete understanding of the mechanism of ammonia pyrolysis and oxidation in the full range of operating conditions displayed by industrial applications is one of the challenges of modern combustion kinetics. In this work, a wide-range investigation of the oxidation mechanism of ammonia was performed. Experimental campaigns were carried out in a jet-stirred reactor and a flow reactor under lean conditions (0.01 ≤ Φ ≤ 0.375), such to cover the full range of operating temperatures (500 K ≤ T ≤ 2000 K). Ammonia conversion and the formation of products and intermediates were analyzed. At the same time, the ammonia decomposition reaction, H-abstractions and the decomposition of the HNO intermediate were evaluated ab initio, and the related rates were included in a comprehensive kinetic model, developed according to a first-principles approach. Low-temperature reactor experiments highlighted a delayed reactivity of ammonia, in spite of the high amount of oxygen. A very slow increase in NH3 consumption rate with temperature was observed, and a full reactant consumption was possible only ∼150–200 K after the reactivity onset. The use of flux analysis and sensitivity analysis allowed explaining this effect with the terminating effect of the H-abstraction on NH3 by O2, acting in the reverse direction because of the high amounts of HO2. The central role of H2NO was observed at low temperatures (T < 1200 K), and H-abstractions from it by HO2, NO2 and NH2 were found to control reactivity, especially at higher pressures. On the other side, the formation of HNO intermediate via NH2 + O = HNO + H and its decomposition were found to be crucial at higher temperatures, affecting both NO/N2 ratio and flame propagation.
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Ammonia is attracting more and more attentions due to its role as both a carbon-free fuel for gas turbines and an effective H2 carrier. Only a limit number of investigations on the laminar flame propagation and laminar burning velocity of ammonia have been performed on elevated pressures, which were focused on ammonia/air mixtures and suffered strong buoyancy effect. In this work, laminar flame propagation of ammonia/O2/N2 mixtures covering wide ranges of equivalence ratios, oxygen contents and initial pressures was investigated in a high-pressure constant-volume cylindrical combustion vessel. The oxygen enrichment speeds up the spherically expanding flames and consequently reduces buoyancy effect on the laminar flame propagation of ammonia. The laminar burning velocity was observed to increase with the increasing oxygen content, but decrease with the increasing initial pressure. A kinetic model of ammonia combustion consisting 38 species and 265 reactions was constructed from previous models with updated rate constants of important reactions. The present model can reasonably reproduce the laminar burning velocity data in this work and literature, as well as the ignition delay time and speciation data in literature. Based on the model analysis, effects of oxygen enrichment, equivalence ratio and initial pressure on laminar burning velocities of ammonia were analyzed in detail. It is revealed that the enhanced flame propagation with oxygen enrichment is mainly due to the increase of adiabatic flame temperature which in turn leads to higher concentrations of key radicals like H, OH and NH2. For NH3 and its major decomposition products like NH2 and NH, reactions with oxygenated species such as OH, O, O2 and NO are generally more important in the lean flames, while the role of reactions with H, NH and NH2 becomes crucial in the rich flames. The calculated pressure dependent coefficient indicates that NH3/O2/N2 flames exhibit clear pressure dependence, while this pressure dependence is weaker than those of the hydrocarbon and biofuel flames.
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A detailed chemical kinetic mechanism has been developed to describe the pyrolysis and oxidation of the hydrogen/NOx and syngas/NOx systems. The thermodynamic data of nitrogenous compounds have been updated based on the study of Bugler et al. (2016). The rate constants of individual elementary reactions associated with the Zeldovich mechanism, the N/O sub-mechanism (NO2, N2O and NO3), the H/N/O sub-mechanism (HNO/HON, HNO2/HONO and HONO2) and the NH3 mechanism (NNH and NH2OH) have been selected through a synthetic comparison of the data available in the literature and the adoption of the latest available published rate constant data. The proposed mechanism has been validated against a large number of experimental data including pyrolysis histories, ignition delay time data, species profile versus time and temperature and flame speed measurements over a wide range of initial combustion conditions and various experimental devices including shock tubes, flow reactors, jet-stirred reactors and spherical combustion bombs. The simulations of the proposed model have also been compared to those from five recently published kinetic models available in the literature. It was found that although these mechanisms generally reproduced well the data for which they were validated, they did not globally capture the combustion characteristics of all of the hydrogen/NOx and syngas/NOx systems. Finally, the proposed model has been used to simulate the formation of NO at practical gas-turbine relevant conditions. A detailed flux analysis has been performed to kinetically explore the NO formation mechanism under various combustion conditions.
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This paper evaluates the potential of hydrogen (H2) and ammonia (NH3) as carbon-free fuels. The combustion characteristics and NOx formation in the combustion of H2 and NH3 at different air-fuel equivalence ratios and initial H2 concentrations in the fuel gas were experimentally studied. NH3 burning velocity improved because of increased amounts of H2 atom in flame with the addition of H2. NH3 burning velocity could be moderately improved and could be applied to the commercial gas engine together with H2 as fuels. H2 has an accelerant role in H2–NH3–air combustion, whereas NH3 has a major effect on the maximum burning velocity of H2–NH3–air. In addition, fuel-NOx has a dominant role and thermal-NOx has a negligible role in H2–NH3–air com- bustion. Thermal-NOx decreases in H2–NH3–air combustion compared with pure H2–air combustion. NOx concentration reaches its maximum at stoichiometric combustion. Furthermore, H2 is detected at an air-fuel equivalence ratio of 1.00 for the decompo- sition of NH3 in flame. Hence, the stoichiometric combustion of H2 and NH3 should be carefully considered in the practical utilization of H2 and NH3 as fuels. H2 as fuel for improving burning performance with moderate burning velocity and NOx emission enables the utilization of H2 and NH3 as promising fuels.
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The kinetics of the reactions H2NO + O2(³Σg−) → HNO(X˜1A′) + HO2 and NH2 + HO2 → NH3 + O2(³Σg−), which are, respectively, very sensitive chain-propagation and chain-termination reactions in ammonia kinetic models, have been revisited by means of high-level electronic structure and variational transition state theory calculations with the goal of improving former predictions and the performance of ammonia kinetic models. In addition, the rate constants of the reactions H2NO + O2(³Σg−) → HNO(a˜3A″) + HO2, NH2 + HO2 → H2NO + OH, and NH2 + HO2 → NH3 + O2(¹Δg), which take place on excited-state potential energy surfaces and/or yield the electronically excited species HNO(a˜3A″) and O2(¹Δg), have been also calculated for the first time in order to assess their importance in ammonia oxidation. We observed that spin contamination and multi-reference character are pronounced in many of the investigated reactions, and these features were handled by performing post-CCSD(T) electronic structure calculations with the W3X-L composite method as well as restricted open shell coupled cluster calculations. Branching ratios were also analyzed, and indicate that the contribution of the electronically excited species HNO(a˜3A″) and O2(¹Δg) are of little importance even at very high temperatures; however, we do not preclude an effect of those species at certain conditions that contribute to their yield. The calculated rate constants were implemented in two recent kinetic models to perform jet stirred reactor, rapid compression machine, and flow reactor simulations, concluding that the model predictions are very sensitive to the reactions H2NO + O2(³Σg−) → HNO(X˜1A′) + HO2 and NH2 + HO2 → NH3 + O2(³Σg−) .
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The present work is focused on the analysis of the ammonia oxidation process and the formation of main nitrogen oxides (NO, NO2 and N2O) over a wide range of temperatures and O2 reaction environments. Experiments are performed at atmospheric pressure in a laboratory quartz tubular flow reactor, covering the temperature range of 875 to 1450 K and for different air excess ratios (from pyrolysis to very oxidizing conditions). The experimental results are simulated and interpreted in terms of a detailed chemical-kinetic mechanism. Reaction path and sensitivity analyses are used to delineate the NH3 oxidation scheme.
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In low-temperature flash photolysis of NH3/O2/N2 mixtures, the NH2 consumption rate and the product distribution is controlled by the reactions NH2 + HO2 → products (R1), NH2 + H (+M) → NH3 (+M) (R2), and NH2 + NH2 (+M) → N2H4 (+M) (R3). In the present work, published flash photolysis experiments by, among others, Cheskis and co-workers, are re-interpreted using recent direct measurements of NH2 + H (+N2) and NH2 + NH2 (+N2) from Altinay and Macdonald. To facilitate analysis of the FP data, relative third-body collision efficiencies compared to N2 for R2 and R3 were calculated for O2 and NH3 as well as for other selected molecules. Results were in good agreement with the limited experimental data. Based on reported NH2 decay rates in flash photolysis of NH3/O2/N2, a rate constant for NH2 + HO2 → NH3 + O2 (R1a) of k1a = 1.5(±0.5) × 1014 cm3 mol-1 s-1 at 295 K was derived. This value is higher than earlier determinations based on the FP results but in good agreement with recent theoretical work. Kinetic modeling of reported N2O yields indicates that NH2 + HO2 → H2NO + O (R1c) is competing with R1a, but perturbation experiments with addition of CH4 indicate that it is not a dominating channel. Measured HNO profiles indicate that this component is formed directly by NH2 + HO2 → HNO + H2O (R1b), but theoretical work indicates that R1b is only a minor channel. Based on this analysis, we estimate k1c = 2.5 × 1013 cm3 mol-1 s-1 and k1b = 2.5 × 1012 cm3 mol-1 s-1 at 295 K, with significant uncertainty margins.
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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.
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To understand the effect of hydrogen addition on the auto-ignition of ammonia at high temperatures, ignition delay times of stoichiometric ammonia/hydrogen blends were measured in a shock tube at temperatures from 1020 to 1945 K, pressures of 1.2 and 10 atm, and hydrogen fractions from 0% to 70%. The measured ignition delay times were compared with seven available kinetic models. Chemical kinetic analyses were performed using both the Glarborg Model and Otomo Model to interpret the interactions between ammonia and hydrogen during the high temperature auto-ignition. Experimental results show that ammonia ignites slower than hydrogen, and hydrogen addition can reduce the ignition delay time nonlinearly. The numerical analysis identifies that 5% hydrogen addition does not significantly affect the reaction flux of ammonia at 20% fuel consumption, however, it makes the fuel H-atom abstraction reaction NH3 + H <=> NH2 + H2 proceed in the reversed way at the initial stage of ignition, therefore generate active H radical for further chain branching and promote the reactivity.
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With the growing attention on ammonia (NH3) combustion, understanding NH3 and nitric oxide (NO) interaction at temperatures higher than DeNOx temperature region or even flame temperature becomes a new research need. In this work, the outwardly propagation spherical flame method was used to investigate the laminar flame propagation of NH3/NO/N2 mixtures and constrain the uncertainties of the specific kinetics. The present experiments were conducted at initial pressure of 1 atm, temperature of 298 K and equivalence ratios from 1.1 to 1.9. A kinetic model of NH3/NO combustion was updated from our previous work. Compared with several previous models, the present model can reasonably reproduce the laminar burning velocity data measured in this work and speciation data in literature. Based on model analyses, the interaction of NH3 and NO was thoroughly investigated. As both the oxidizer and a carrier of nitrogen element, NO frequently reacts with different decomposition products of NH3 including NH2, NH and NNH, and converts nitrogen element to the final product N2. It is found that the laminar burning velocity experiment of NH3/NO/N2 mixtures using the outwardly propagating spherical flame method can provide highly sensitive validation targets for the kinetics in NH3 and NO interaction.
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Ammonia (NH3) is considered as a promising carbon free energy carrier for energy and transportation systems. However, its low flammability and high NOx emission potential inhibit the implementation of pure NH3 in these systems. On the other hand, methane is a favorable low emission fuel that can be used as a co-firing fuel in ammonia combustion to promote the reactivity and control the emission levels. However, knowledge of the ignition properties of NH3/CH4 mixtures at intermediate temperatures and elevated pressures is still scarce. This study reports ignition delay times of NH3/CH4/O2 mixtures diluted in Ar or Ar/N2 over a temperature range of 900–1100 K, pressures of 20 and 40 bar, and equivalence ratios of 0.5, 1.0, and 2.0. The results demonstrate that a higher CH4 mole fraction in the fuel mixture increases its reactivity, and that the reactivity decreases with increasing the fuel-oxygen equivalence ratio. The most recent mechanisms of Glarborg et al. (2018) and Li et al. (2019) were compared against the experimental data for validation purposes. Both mechanisms can predict the measurements fairly well, and key elementary reactions applied in both mechanisms were compared. A modified mechanism is provided, which can reproduce the measurements with smaller discrepancies in most cases. Detailed modeling for emissions indicated that adding CH4 to the fuel mixture increases the emission of NOx.
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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.
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As an indirect storage medium of hydrogen, ammonia (NH3) has drawn significant attention from academia and industry. Understanding nitrogen combustion chemistry is a major challenge in applying ammonia for converting chemical energy to thermal energy. Diazene (N2H2), diazenyl radical (NNH), amidogen radical (NH2), hydrogen cyanide (HCN) and isocyanic acid (HNCO) are the crucial intermediate species in the combustion of NH3 or its mixtures with other hydrocarbons. In light of that, this study provides advanced theoretical treatment of 14 important reactions in the oxidation of these intermediates, including isomerization, dissociation and abstraction reactions. The rate constants of all these reactions, and the temperature-dependent thermochemistry of the species involved in the reactions, were calculated utilizing high level quantum chemical methods. Ro-vibrational properties of the reactants, products and stationary points were determined at the M06–2X/6–311++G (d,p) level of theory. Coupled cluster (CCSD(T)) methods were employed, with two large basis sets (cc-pVTZ and cc-pVQZ), and complete basis set of extrapolation techniques to compute the energies of the resulting geometries. All calculated results were compared with experimental and theoretical results in the literature. Finally, the implications of this work for combustion modeling were investigated, and the simulated species’ profiles of HCN and HNCO demonstrated the influence of the updated rate coefficients on kinetic model predictions.
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Alternative fuels are essential to enable the transition to a sustainable and environmentally friendly energy supply. Synthetic fuels derived from renewable energies can act as energy storage media, thus mitigating the effects of fossil fuels on environment and health. Their economic viability, environmental impact, and compatibility with current infrastructure and technologies are fuel and power source specific. Nitrogen-based fuels pose one possible synthetic fuel pathway. In this review, we discuss the progress and current research on utilization of nitrogen-based fuels in power applications, covering the complete fuel cycle. We cover the production, distribution, and storage of nitrogen-based fuels. We assess much of the existing literature on the reactions involved in the ammonia to nitrogen atom pathway in nitrogen-based fuel combustion. Furthermore, we discuss nitrogen-based fuel applications ranging from combustion engines to gas turbines, as well as their exploitation by suggested end-uses. Thereby, we evaluate the potential opportunities and challenges of expanding the role of nitrogen-based molecules in the energy sector, outlining their use as energy carriers in relevant fields.
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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.
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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.
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Chemical‐kinetic combustion mechanisms for hydrogen‐oxygen‐nitrogen systems, motivated originally by concerns about NOx emissions during hydrogen burning, have recently acquired renewed interest as a result of the possibility of employing ammonia‐hydrogen mixtures in gas turbines and reciprocating engines as drop‐in fuel to replace the use of natural gas. Specifically, this is of relevance to the implementation of engineering approaches for economical power generation with carbon sequestration or to large‐scale energy‐storage schemes, based on hydrogen or efficient hydrogen carriers such as ammonia. Because computational investigations are facilitated by short mechanisms (since the use of large mechanisms is often prohibitively expensive in reactive flow simulations), in response to the original concerns, a short nitrogen mechanism was developed in San Diego in the 1990s (not updated since 2004), without consideration of ammonia combustion. In view of the renewed interest in this topic, that mechanism has now been expanded to encompass 60 elementary steps among 19 reactive chemical species, including ammonia burning and NOx production, as reported herein, greatly improving predictions. With particular attention to high reactant temperatures and high‐pressure conditions, relevant to industrial applications, it is shown that the present short mechanism retains satisfactory accuracy, exhibiting deviations that in most cases are within acceptable bounds (±20%). The revisions maintain the shortness of the original mechanism, adding only one more reactive species and six more elementary steps (while updating values of rate parameters of nine other steps, on the basis of newly available information). In addition, the short mechanism is applied herein to the analysis of fundamental combustion properties of ammonia/hydrogen/nitrogen‐air laminar premixed flames, at unstrained and strained conditions, for comparison with methane‐air flames as a reference gas‐turbine fuel. It is found by comparing carbon‐free and hydrocarbon laminar flames that these reactive mixtures, even if characterized by nearly identical adiabatic flame temperature and laminar flame speeds, nevertheless exhibit substantially different resistance to strain, with the ammonia/hydrogen flames exceeding the strain limit of methane flames by a factor of 5. Develop a short mechanism for NH3/H2 combustion over a wide range of initial conditions Good agreement with experimental data for ignition delay time, laminar burning velocity, extinction strain rate and NOx formation Provide quantitative information of the relationship between NH3‐derived fuel and methane combustion behaviors
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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
The selective non-catalytic reduction of NO by ammonia (SNCR) has been extensively studied but no activation of ammonia oxidation by nitric oxide had been reported. Experiments performed in a jet-stirred reactor (JSR) at atmospheric pressure for various equivalence ratios (0.1–2) and initial concentrations of NH3 (500 to 1000 ppm) and NO (0 to 1000 ppm) revealed kinetic interactions similar to the so-called mutual sensitization of the oxidation of hydrocarbons and NO. The experiments were performed at fixed residence times of 100 and 200 ms, and variable temperature ranging from 1100 to 1450 K. Kinetic reaction mechanisms were used to simulate these experiments and ammonia oxidation. The most reliable model from the literature was updated (NH2 + H → NH+H2, HNO+H → NO+H2) to better predict ammonia-air burning velocities. It showed the mutual sensitization of the oxidation of ammonia and nitric oxide proceeds through several reaction pathways leading to OH production which is mainly responsible for ammonia oxidation in the current conditions: NH2 + NO → NNH + OH, NNH → N2 + H, NNH + O2 → N2 + HO2, H + O2 → OH + O, H + O2 + M → HO2 + M, and NO + HO2 → NO2 + OH.
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
Auto-ignition properties of NH3/O2 and NH3/H2/O2 mixtures have been studied in a rapid compression machine at pressures from 20 to 60 bar, temperatures from 950 to 1150 K, and equivalence ratios from 0.5 to 2. The effect of the ammonia/hydrogen ratio in the fuel mixture has been also investigated. The experiments demonstrate that a higher H2 mole fraction in the fuel mixture increases its reactivity, while the equivalence ratio shows different influence as follows. When the fuel mixture contains 20% H2, the fuel-richer mixtures have shorter ignition delay times, while for mixtures containing 1% H2 in fuel the equivalence ratio dependence is opposite. With 5% H2 in fuel, the stoichiometric mixture presents the shortest ignition delay time. In mixtures without hydrogen, i.e., pure NH3, leaner mixtures show higher reactivity. In addition, numerical simulations were performed based on the literature mechanisms of Glarborg et al. (2018), Mathieu and Petersen (2015), and Klippenstein et al. (2011). While these models can predict well the ignition delay time of NH3/O2 mixtures, none of the models can predict the behavior of NH3/H2/O2 mixtures satisfactorily. The predictions are most sensitive to the branching reactions NH2 + NO and to the reaction H2NO + O2 = HNO + HO2. Hydrogen addition enriches the O/H radical pool consuming NH3 and NH2, but it has small effect on NOx emissions.
Article
The laminar flame speeds of CH 4 /NH 3 mixtures during oxy-fuel combustion conditions were measured under variable NH 3 /CH 4 ratios (0.1–0.2), O 2 mole fractions (35%–40%), and CO 2 mole fractions (45%–65%) in a counterflow flame configuration (set at atmospheric pressure and unburnt mixture temperature (T u = 300 K)). These experimental results were compared to the numerical results obtained through three detailed chemical kinetic mechanisms: the Okafor, Mendiara and HUST (Huazhong University of Science and Technology) mechanisms. The comparisons showed that the results obtained through the HUST Mechanism were in good agreement with the experimental results. The experimental results showed that the laminar flame speeds increased linearly with decreasing CO 2 or increasing O 2 concentrations under the conditions considered, while the slopes were irrelevant for the equivalence ratio. Nevertheless, the effects of NH 3 concentration depended on the equivalence ratio: the sensitivity and pathway analyses of NH 3 oxidation revealed that, among the N-containing reactions in the fuel-lean region, NO oxidation and reduction (NO + HO 2 = NO 2 +OH, NH 2 +NO = NNH + OH, NO 2 +H = NO + OH, and CH 3 +NO 2 = CH 3 O + NO) had the largest impact on the laminar flame speeds. In stoichiometric region, the NO reduction pathway (NH 2 +NO = N 2 +H 2 O, NH 2 +NO = NNH + OH, NH + NO = N 2 O + H, and NH + NO = N 2 +OH) greatly contributed to flame propagation. In fuel-rich region, N + NO = N 2 +O and N + OH = NO + H had the biggest impact over laminar flame speeds.
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
The reversible reaction NH3 + H ⇌ H2 + NH2, which plays an important role in NH3 fuel combustion, is studied with a theoretical approach that combines the high‐accuracy extrapolated ab initio thermochemistry (HEAT) protocol with semiclassical transition state theory (SCTST). The calculated forward reaction is endothermic by 11.8 ± 1 kJ/mol, in nearly perfect agreement with the active thermochemical tables (ATcT) value of 11.5 ± 0.2 kJ/mol. Using this improved thermochemistry yields better rate constants, especially at low temperatures. Experimental rate constants available from 400 to 2000 K for the forward and reverse reaction pathways can be reproduced (within 20%) by the calculations from first principles.
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
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
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
Termolecular association reactions involve ephemeral collision complexes—formed from the collision of two molecules—that collide with a third and chemically inert ‘bath gas’ molecule that simply transfers energy to/from the complex. These collision complexes are generally not thought to react chemically on collision with a third molecule in the gas-phase systems of combustion and planetary atmospheres. Such ‘chemically termolecular’ reactions, in which all three molecules are involved in bond making and/or breaking, were hypothesized long ago in studies establishing radical chain branching mechanisms, but were later concluded to be unimportant. Here, with data from ab initio master equation and kinetic-transport simulations, we reveal that reactions of H + O2 collision complexes with other radicals constitute major kinetic pathways under common combustion situations. These reactions are also found to influence flame propagation speeds, a common measure of global reactivity. Analogous chemically termolecular reactions mediated by ephemeral collision complexes are probably of significance in various combustion and planetary environments.
Article
Ammonia is considered to be one of the promising energy carriers in the future and reliable chemical kinetics to accurately predict ignition characteristics of ammonia/air mixtures is necessary for developing ammonia combustors. However, ignition characteristics of ammonia/air mixtures at low temperatures have not been well studied. The present study employed weak flames in a micro flow reactor with a controlled temperature profile, which have been extensively employed to examine ignition characteristics of hydrocarbons, to investigate ignition characteristics of ammonia/air mixtures at low temperatures. Species measurements for weak flames of ammonia/air mixtures at atmospheric pressure and equivalence ratios of 0.8, 1.0 and 1.2 under a maximum wall temperature of 1400 K were conducted using a mass spectrometer and profiles of the NH3, O2, H2O NO, and N2O mole fractions were obtained. Chemical kinetic modeling was conducted with extensive updates mainly for the N2Hx chemistry because N2Hx species were expected to be produced from the NH2 + NH2 reactions at low temperatures. The mechanism developed in the present study well predicted species profiles of NH3, O2 and H2O for weak flames measured in experiments. The present mechanism also well predicted the final values of the NO and N2O mole fractions behind the reaction zone of weak flames but overestimated these mole fractions in the reaction zone of weak flames. To confirm the existence of N2Hx species in the reaction zone of weak flames, signals from N2H4 were distinguished from measured signals. Sensitivity analysis and reaction flux analysis were conducted and the importance of the N2Hx chemistry in the reaction zone of weak flames at low temperatures was identified. Validation of the present mechanism with literature data on ignition delays and flame speeds were conducted and reasonable agreements with literature data were confirmed. For N2O and NO in the reaction zone of weak flames, however, there was discrepancy between measured and computational mole fractions and further improvements of chemical kinetics related to ammonia ignition are still necessary.
Article
In this study, measurements were performed to assess the overall mixing in jet-stirred reactors (JSRs) passively agitated by feed nozzles. The reactor diameter, nozzle shape, and nozzle diameter were varied to determine the effects of these geometrical parameters on mixing. The mixing was studied at ambient conditions using laser absorption spectroscopy to follow the exit concentration of a tracer gas, carbon dioxide, after a step change in its input flow. The results indicate that the use of a JSR of diameter D = 40 mm, having inclined or crossed nozzles of diameter d = 1 mm is recommended for low residence times up to 0.4 sec, while at moderate/high residence times 0.5-5 sec the use of a JSR of D = 56 mm and d = 0.3 mm having crossed nozzles is suggested.
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.