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Ignition delay times of NH3 /DME blends at high pressure and low DME fraction: RCM experiments and simulations
Abstract
Autoignition delay times of ammonia/dimethyl ether (NH3/DME) mixtures were measured in a rapid compression machine with DME fractions of 0, 2 and 5 and 100% in the fuel. The measurements were performed at equivalence ratios φ=0.5, 1.0 and 2.0 and pressures in the range 10–70 bar; depending on the fuel composition, the temperatures after compression varied from 610 K to 1180 K. Admixture of DME is seen to have a dramatic effect on the ignition delay time, effectively shifting the curves of ignition delay vs. temperature to lower temperatures, up to ~250 K compared to pure ammonia. Two-stage ignition is observed at φ=1.0 and 2.0 with 2% and 5% DME in the fuel, despite the pressure being higher than that at which pure DME shows two-stage ignition. At φ=0.5, a reproducible pre-ignition pressure rise is observed for both DME fractions, which is not observed in the pure fuel components. A novel NH3/DME mechanism was developed, including modifications in the NH3 subset and addition of the NH2+CH3OCH3 reaction, with rate coefficients calculated from ab initio theory. Simulations faithfully reproduce the observed pre-ignition pressure rise. While the mechanism also exhibits two-stage ignition for NH3/DME mixtures, both qualitative and quantitative improvement is recommended. The overall ignition delay times for ammonia/DME mixtures are predicted well, generally being within 50% of the experimental values, although reduced performance is observed for pure ammonia at φ=2.0. Simulating the ignition process, we observe that the DME is oxidized much more rapidly than ammonia. Analysis of the mechanism indicates that this ‘early DME oxidation’ generates reactive species that initiate the oxidation of ammonia, which in turn begins heat release that raises the temperature and accelerates the oxidation process towards ignition. The reaction path analysis shows that the low-temperature chain-branching reactions of DME are important in the early oxidation of the fuel, while the sensitivity analysis indicates that several reactions in the oxidation of DME, including cross reactions between DME and NH3 species presented here, are critical to ignition, even at fractions of 2% DME in the fuel.
... Although the ignition reaction studies have been conducted, there is still much left to understand about the ignition process of DMAZ. Density functional theory can simulate molecular reactions, analyze the mechanism of the reaction, and provide guidance for optimizing the reaction process [15][16][17]. It has developed rapidly and is widely used in the field of molecular reaction mechanism. ...
... Paths 11,12,14, and 15 are the secondary reactions. Paths 13,16,18,19, and 20 have small reaction rate constants, indicating that they hardly occur. The three-parameter Arrhenius formula was obtained by fitting the reaction rate constants of DMAZ and NO 2 in the liquid phase at 258 ~ 378 K, and the results are shown in Table 8 . ...
This study focuses on the application of N,N-dimethylazidoethylamine (DMAZ) as an alternative fuel to the traditional hydrazine fuel. Shortening ignition delay time is the key factor for the DMAZ application. In order to explore the ignition mechanism of DMAZ and NTO, the reaction mechanism between DMAZ and NO2/NTO was studied based on density functional theory (DFT). The results showed that under the conditions of the gas phase and NTO liquid phase, the main path of the reaction is that NO2 attacks the secondary hydrogen atom of DMAZ. The gas-phase reaction enthalpy is higher than that in the NTO liquid phase, indicating that the gas-phase reaction absorbs more energy and is not easy to proceed, while the NTO liquid-phase reaction is easier. The combustion mechanism of DMAZ and NTO was preliminarily obtained. It is speculated that under actual working conditions, DMAZ and NTO mainly undergo the liquid-phase reaction.
... Around 176 million tons of ammonia are produced annually worldwide, mostly using the Haber-Bosch process, which uses the steam reforming of methane to create hydrogen Despite its advantages, NH 3 poses challenges, including its corrosiveness and toxicity at certain concentrations (>150 ppm) [6]. Its narrow flammability limit, high octane number and lower laminar flame speed (Table 1) compared to other fuels further complicate combustion stability [7,8]. However, research [1,9] indicates that blending NH 3 with highly reactive fuels like H 2 , methane (CH 4 ) and natural gas can enhance its combustion characteristics by about 10-20% [10]. ...
Ammonia (NH3) is gaining recognition as a viable “green” transportation fuel due to its zero-carbon characteristic, its high energy density and its widespread availability. However, NH3 has a high auto-ignition temperature, resulting in potential emissions of NOx and unburned NH3. Addressing combustion challenges requires innovative solutions, such as the application of combustion promoters to enhance NH3 combustibility. This review article focuses on the compatibility of NH3 as a fuel for spark-ignition (SI) engines, examining its combustion under various modes including pure NH3 combustion, gasoline blends, NH3/hydrogen (H2) blends, and NH3/natural gas blends in single or dual-fuel configurations. The formation of nitrogen oxides (NOx) and slip-NH3 is explored to understand emissions species such as NO and N2O. Additionally, the article highlights the limitations of NH3 as a fuel for SI combustion. The comprehensive discussion provided in this review aims to fill a critical gap in the literature regarding NH3’s feasibility as a zero-carbon fuel for SI engines, particularly in the maritime sector. By offering insights into NH3 combustion characteristics and emissions profiles, the article seeks to provide a roadmap for leveraging NH3 as a suitable non-carbon fuel to decarbonize the marine sector and advance global sustainability goals.
... Blending ammonia with a highly reactive fuel is a commonly employed technique to enhance the combustibility of ammonia. Previous studies have successfully blended ammonia with gasoline [19] and other fuels [20]- [23]. Moreover, several previous studies have also succeeded in improving ammonia combustibility by implementing a sub-chamber in a spark ignition engine and combustion chamber [19], [24], [25]. ...
Article Info Due to the problem with carbon dioxide (CO2) emissions, alternative fuels such as ammonia (NH3) have been garnering a lot of attention lately. This is due to its carbon-free molecular structure, ease of transport, and high energy density. Unfortunately, ammonia is not without flaws since it is considered a difficult fuel to burn in conventional internal combustion engines. To further investigate the burning characteristics of ammonia, this study is conducted for ammonia/gasoline co-combustion using a modified engine equipped with a sub-chamber. The engine ran at 1000 RPM and had a 17.7 compression ratio with two injection timings of-55° and 10° crank angle (CA) after the top dead center (ATDC), while the ammonia energy ratios were adjusted across a range from 40% to 70%. The results show that the earlier injection timing allowed better premixing between the air and fuel mixture, thus enhancing the overall combustion characteristics. For the later injection timing, the nitrogen oxides (NOx) emissions decrease at the higher ammonia energy ratio due to the denitrification of nitrogen oxides (DeNOX) process. Overall, the earlier injection timing appears to be optimal for 40% to 70% ammonia energy ratio under the present condition.
... Conversely, as ϕ increases from 1.0 to 2.0, the IDT exhibits a pronounced increase. This outcome aligns with the observations reported by Shu et al. [51] and Dai et al. [52]. When the equivalence ratio is 2.0, the proportion of oxygen is relatively low, the proportion of the NH 3 /CH 3 OH mixture is high, and the concentration of reactants is high. ...
As a carbon-free hydrogen-carrying fuel, ammonia can effectively alleviate the problem of CO 2 emissions and has great application potential in marine engines. The low laminar burning velocity and calorific value, and the narrow flammability limit range make the combustion of pure ammonia difficult. Ammonia/methanol mixed combustion is a feasible method to improve ammonia combustion and mitigate CO 2 emissions. It is difficult to accurately predict the laminar burning velocity (LBV) and ignition delay time (IDT) of ammonia/methanol combustion simultaneously by the existing chemical kinetic mechanisms. This study aims to develop a detailed chemical kinetic mechanism that can simultaneously predict the LBV and IDT. The new-developed mechanism was fully validated based on experimental data. Furthermore, it was employed to investigate the effects of methanol blending ratios and equivalence ratios on the LBV and IDT in NH 3 /CH 3 OH mixed flames. Besides, chemical kinetic analyses were conducted to elucidate the influence of methanol mixing ratios and equivalence ratios on LBV and IDT. Reaction pathway analysis revealed that the NH 3 and CH 3 OH chemistry interacted by the sharing of H, OH and O radicals in fuels' decomposition, and chemical interactions of N-containing radicals with the C 1 species. The reactions involving C-containing species can notably improve the LBV of NH 3 /CH 3 OH mixed combustion. At low equivalence ratio, LBV increases in proportion to the fuel proportion. Conversely, at high equivalence ratio, the effect of oxygen concentration on LBV becomes the dominant factor.
... Sin embargo, existen algunos problemas en la combustión del NH 3 como son su dificultad de ignición, baja velocidad de combustión y posibles emisiones de NO. En este punto, el uso de mezclas de amoniaco, con dimetil éter (DME), entre otros, puede ser una solución para mejorar sus características de combustión [2]. ...
Con el fin de mitigar las emisiones globales de CO2 y de buscar combustibles alternativos, en el presente trabajo se ha investigado la combustión de amoniaco (NH3) con dimetil éter (DME) a alta presión. Los resultados experimentales han sido simulados con un mecanismo cinético detallado de reacción.
Ammonia (NH3) has gained considerable attention as a promising carbon-free hydrogen carrier fuel for internal combustion engines, but its direct use in compression-ignition engines presents challenges, often requiring high-reactivity fuels to ignite the premixed NH3/air mixture and initiate combustion. This study focuses on the ignition process of binary NH3 and dimethyl ether (DME) mixtures, as DME is a carbon-neutral, high-reactivity fuel. A key novelty of this paper is the comparison of the ignition processes of DME and NH3/DME mixtures from a temporal, process-oriented perspective, analyzing chemical kinetics across distinct ignition phases rather than focusing solely on instantaneous reactions at discrete time points. The stage-wise analysis reveals that NH3 has minimal impact on the control mechanism governing the two-stage ignition process of DME. Specifically, DME still largely depends on OH radical proliferation during low-temperature oxidation (LTO), which releases heat as the reaction progresses. As the temperature increases, LTO branching pathways gradually shift to chain-propagation pathways, reducing overall reaction activity. The reactivity and temperature rise rate of the system are then governed by the H2O2 loop mechanism before thermal ignition. However, ammonia significantly extends the ignition delay of DME by competing with OH radicals, which are critical for DME oxidation, thus inhibiting ignition. As the ignition reaction proceeds, ammonia kinetics become more involved. For example, nitrogen-containing species from NH3 oxidation, such as NO, NO2, and NH2, react with CH3OCH2 to form CH3OCHO, reducing the flux through the LTO pathway of DME. While ammonia reaction pathways also produce OH radicals, this occurs at the expense of HO2 and H radicals, leading to H2O2 formation. Overall, these findings demonstrate the substantial impact of ammonia addition on DME ignition, highlighting the need for further research to better understand NH3/DME binary fuel ignition and to optimize the design and operation of NH3/DME dual-fuel engines for improved efficiency and reliability.
This study examines NH3, NO, and CO profiles during NH3/C2H6 and NH3/C2H5OH oxidation in a shock tube using laser absorption spectroscopy at 1317-1957 K and ~2.8 bar, with 5-20% C2H6 or C2H5OH, equivalence ratios of 0.5/1.0/1.5, and a 0.9 argon dilution ratio. Metrological uncertainty analysis on mole fractions have been performed. A novel dynamic uncertainty methodology is proposed to time-resolved speciation profiles. From the experimental measurements, the promotional effect on ammonia ignition by C2H6 or C2H5OH is identical under investigated conditions. The multi-speciation profiles demonstrate reasonable relations among the consumption of NH3 and the formation of NO and CO, which show strong temperature and equivalence ratio dependence. An updated PTB-NH3/C2 1.1 mechanism accurately predicts ignition delay times and speciation profiles. Sensitivity analysis reveals NH3 chemistry's significant control, with NH3+O2/H/OH and N2H2+M reactions being critical at >1300 K, and HO2 interactions being pivotal at 800-1200 K. The reactions related to N2H2 and N-C chemistries dominate for fuel-rich mixtures at high temperatures, resulting in a reverse equivalence ratio dependence on IDTs, namely fuel-rich mixtures exhibit the lowest reactivity. The N-C chemistries, particularly the pronounced reaction pathway of HCN→CN→NCO→HNCO→CO play a crucial role in maintaining the CO level constant after reaching its peak position under fuel-rich conditions. NH3 consumption pathways shift from NH2→H2NO→HNO→NO→NO2→N2O→N2 at intermediate temperatures to NH3→NH2→NH→N(HNO)→NO→(N2O)N2 at high temperatures. The C2-additives participate in the oxidation process by introducing additional OH/H/CH3 radicals, thereby facilitating the reactive radical pool and developing the N-C intersystem-crossing pathways. In summary, the extensive multi-speciation data, metrological uncertainty analysis, mechanism validation and development, together with comprehensive kinetic modelling can be valuable resources for further studies of ammonia combustion.
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.
Ammonia (NH3) is a promising carbon-free fuel, but its slow burning velocity and narrow flammability limit hinder its use in combustion systems. Blending NH3 with additives like nitrous oxide (N2O) can improve its combustion characteristics. This study investigated the oxidation of NH3/N2O mixtures using a newly updated experimental method, which combined a rapid compression machine (RCM) with time-resolved molecular beam mass spectrometry. The pyrolysis of N2O and the oxidation of NH3/O2 were also studied for comparison. The species concentration profiles were recorded at temperatures from 1208 to 1383 K and pressures from 10 to 20 bar. At lower temperatures (~1220 K), minimal N2O decomposition was observed, while more noticeable pyrolysis of N2O occurred at higher temperatures (~1370 K). The kinetic model, with parameters updated from prior studies, effectively predicted decomposition profiles, demonstrating the robustness of the developed experimental method. In contrast to the pyrolysis of N2O or the oxidation of an NH3/O2 mixture, the NH3/N2O mixture exhibited substantial reactivity at 1220 K. Significant decreases in NH3 and N2O concentrations, coupled with increased N2 production, indicated a synergistic oxidation phenomenon for NH3 and N2O. Further exploration through reaction flux analysis provided insights into the synergistic oxidation mechanism. In the presence of N2O, not only did the first step of hydrogen abstraction of NH3 get initiated by radicals released by N2O, but the further oxidation of the NH2 radical was also promoted by its direct reaction with N2O. The consumption of N2O was also accelerated during its interaction with NH3 compared to its sole decomposition pathway.
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.
ion from dimethyl ether (DME) by nitric dioxide (NO2) has been considered to be very sensitive to the DME ignition behavior. Thermal rate coefficients for the reaction of DME with NO2 are computed by employing the multi-structural canonical variational transition-state theory with multidimensional tunneling (MS-CVT/MT) at temperatures from 200 to 3000 K. The B2PLYP/TZVP theoretical level has been verified in the first part of this study against experiment for geometries of stationary points and against accurate CCSD(T)/CBS and CCSD(T)/6-311+G(2df,2p) computations for the energetics related to the title reaction, and then the compound CCSD(T)/6-311+G(2df,2p)//B2PLYP/TZVP computations are performed here to obtain the properties of all stationary points and the geometries, energies, gradients, and Hessians of non-stationary points along each reaction minimum-energy pathway. It has been proposed that the internal rotations in the transition structures are anharmonic and strongly coupled to each other to produce multiple conformations whose contributions are included in their partition function approximations. The previous evaluations for these rate coefficients based on transition-state theory with one-dimensional tunneling corrections and one-dimensional hindered rotor treatments of torsions are essentially distinct from our MS-CVT/SCT results where the critical effects of corner cutting due to small reaction-path curvature are seriously treated via multidimensional small-curvature tunneling (SCT) treatment and torsions are explicitly considered by multistructural treatments, and thus the MS-CVT/SCT rate coefficients are proposed particularly to refine the current kinetic model for DME combustion.
New rapid compression machine (RCM) ignition delay data for dimethyl ether (DME), methanol (MeOH), and their blends are acquired at engine-relevant conditions (T = 600 K–890 K, P = 15 bar and 30 bar, and equivalence ratios of ϕ = 0.5, 1.0, and 2.0 in synthetic dry air). The data are then used to validate a detailed DME/MeOH model in conjunction with literature RCM and shock tube data for DME and MeOH. This detailed DME/MeOH model, constructed by systematically merging literature models for the combustion of the individual fuel constituents, is capable of accurately predicting the experimental ignition delay data at a wide range of temperatures and pressures. The experiments and simulations both show a non-linear promoting effect of DME addition on MeOH autoignition. Additional analyses are performed using the merged DME/MeOH model to gain deeper insight into the binary fuel blend autoignition, especially the promoting effect of DME on MeOH. It is found that the unimolecular decomposition of HO2CH2OCHO plays an essential role in low temperature DME/MeOH blend autoignition. The accumulation of HO2CH2OCHO before the first-stage ignition and later quick consumption not only triggers the first-stage ignition, but also causes the non-linear promoting effect by accumulating to higher levels at higher DME blending ratios. These analyses suggest the rate parameters of HO2CH2OCHO unimolecular decomposition are critical to accurately predict the first-stage and overall ignition delay times as well as the first-stage heat release profile for low temperature DME/MeOH oxidation.
This work presents a detailed investigation into the isomerization and decomposition of \ce{HONO} and \ce{HNO2}. State-of-the-art electronic structure theory is used to compute the \ce{HNO2} potential energy surface. Temperature and pressure dependent rate coefficients are computed using microcanonical rate theory and the master equation. The electronic structure theory properties are optimized against the relevant experimental data. A novel strategy was developed to incorporate uncertainty in the minimum energy pathway into the optimized mechanism. The new mechanism is in excellent agreement with all available experimental data for H + \ce{NO2} OH + NO and OH + NO HONO. The calculations identify OH + NO as the dominant products for \ce{HNO2}, which were neglected from all previous mechanisms in the literature.
This paper focuses on the potential use of ammonia as a carbon-free fuel, and covers recent advances in the development of ammonia combustion technology and its underlying chemistry. Fulfilling the COP21 Paris Agreement requires the de-carbonization of energy generation, through utilization of carbon-neutral and overall carbon-free fuels produced from renewable sources. Hydrogen is one of such fuels, which is a potential energy carrier for reducing greenhouse-gas emissions. However, its shipment for long distances and storage for long times present challenges. Ammonia on the other hand, comprises 17.8% of hydrogen by mass and can be produced from renewable hydrogen and nitrogen separated from air. Furthermore, thermal properties of ammonia are similar to those of propane in terms of boiling temperature and condensation pressure, making it attractive as a hydrogen and energy carrier. Ammonia has been produced and utilized for the past 100 years as a fertilizer, chemical raw material, and refrigerant. Ammonia can be used as a fuel but there are several challenges in ammonia combustion, such as low flammability, high NOx emission, and low radiation intensity. Overcoming these challenges requires further research into ammonia flame dynamics and chemistry. This paper discusses recent successful applications of ammonia fuel, in gas turbines, co-fired with pulverize coal, and in industrial furnaces. These applications have been implemented under the Japanese ‘Cross-ministerial Strategic Innovation Promotion Program (SIP): Energy Carriers’. In addition, fundamental aspects of ammonia combustion are discussed including characteristics of laminar premixed flames, counterflow twin-flames, and turbulent premixed flames stabilized by a nozzle burner at high pressure. Furthermore, this paper discusses details of the chemistry of ammonia combustion related to NOx production, processes for reducing NOx, and validation of several ammonia oxidation kinetics models. Finally, LES results for a gas-turbine-like swirl-burner are presented, for the purpose of developing low-NOx single-fuelled ammonia gas turbine combustors.
For long-term storage, part of the excess renewable energy can be stored into various fuels, among which ammonia and hydrogen show a high potential. To improve the power-to-fuel-to-power overall efficiency and reduce NOx emissions, the intrinsic properties of Low Temperature Combustion (LTC) engines could be used to convert these carbon-free fuels back into electricity and heat. Yet, ignition delay times for ammonia are not available at relevant LTC conditions. This lack of fundamental kinetic knowledge leads to uncertain ignition delay predictions by the existing ammonia kinetic mechanisms and prevents from determining optimal LTC running conditions. Using a Rapid Compression Machine (RCM), we have studied the ignition delay of ammonia with hydrogen addition (0%, 10%, and 25%vol.) under LTC conditions: low equivalence ratios (0.2, 0.35, 0.5), high pressures (43 bar and 65 bar) and low temperatures (1000 K - 1100 K). This paper presents the comparison of the experimental data with simulation results obtained with five kinetic mechanisms found in the literature. It then provides a sensitivity analysis to highlight the most influencing reactions on the ignition of the ammonia-hydrogen blends. The obtained range of ignition delays for pure ammonia and for the ammonia-hydrogen blends prove their suitability for LTC engines. Still the hydrogen addition must be greater than 10%vol. to produce a significant promotion of the ignition delay. The two best performing mechanisms still predict too long ignition delays for pure ammonia, while the delays become too short for ammonia-hydrogen blends. A third mechanism captures correctly the relative influence of hydrogen addition, but is globally over-reactive. Through a sensitivity analysis, H2NO has been identified as the main cause for the under-reactive pure ammonia kinetics and N2Hx has been identified as the main cause for globally over-reactive ammonia-hydrogen mechanisms.
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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.
This work presents an updated experimental and kinetic modeling study of n-heptane oxidation. In the experiments, ignition delay times of stoichiometric n-heptane/air mixtures have been measured in two different high-pressure shock tubes in the temperature range of 726–1412 K and at elevated pressures (15, 20 and 38 bar). Meanwhile, concentration versus time profiles of species have been measured in a jet-stirred reactor at atmospheric pressure, in the temperature range of 500–1100 K at φ = 0.25, 2.0 and 4.0. These experimental results are consistent with those from the literature at similar conditions and extend the current data base describing n-heptane oxidation.
Based on our experimental observations and previous modeling work, a detailed kinetic model has been developed to describe n-heptane oxidation. This kinetic model has adopted reaction rate rules consistent with those recently developed for the pentane isomers and for n-hexane. The model has been validated against data sets from both the current work and the literature using ignition delay times, speciation profiles measured in a jet-stirred reactor and laminar flame speeds over a wide range of conditions. Good agreement is observed between the model predictions and the experimental data. The model has also been compared with several recently published kinetic models of n-heptane and shows an overall better performance. This model may contribute to the development of kinetic mechanisms of other fuels, as n-heptane is a widely used primary reference fuel. Since the sub-mechanisms of n-pentane, n-hexane and n-heptane have adopted consistent reaction rate rules, the model is more likely to accurately simulate the oxidation of mixtures of these fuels. In addition, the successful implementation of these rate rules have indicated the possibility of their application for the development of mechanisms for larger hydrocarbon fuels, which are of great significance for practical combustion devices.
Autoignition delay times of n-butane and iso-butane have been measured in a Rapid Compression Machine in the temperature range 660–1010 K, at pressures varying from 14 to 36 bar and at equivalence ratios φ = 1.0 and φ = 0.5. Both butane isomers exhibit a negative-temperature-coefficient (NTC) region and, at low temperatures, two-stage ignition. At temperatures below ∼900 K, the delay times for iso-butane are longer than those for the normal isomer, while above this temperature both butanes give essentially the same results. At temperatures above ∼720 K the delay times of the lean mixtures are twice those for stoichiometric compositions; at T < 720 K, the equivalence ratio is seen to have little influence on the ignition behavior. Increasing the pressure from 15 bar to 30 bar decreases the amplitude of the NTC region, and reduces the ignition delay time for both isomers by roughly a factor of 3. In the region in which two-stage ignition is observed, 680–825 K, the duration of the first ignition stage decreases sharply in the range 680–770 K, but is essentially flat above 770 K. Good quantitative agreement is found between the measurements and calculations for n-butane using a comprehensive model for butane ignition, including both delay times in the two-stage region, with substantial differences being observed for iso-butane, particularly in the NTC region.
Cantera is an open-source collection of object-oriented software tools for problems involving chemical kinetics, thermodynamics, and transport processes.
A detailed chemical kinetic model for oxidation of methylamine has been developed, based on theoretical work and a critical evaluation of data from the literature. The rate coefficients for the reactions of CHNH + O CHNH / CHNH + HO, CHNH + H CH + NH, CHNH CHNH, and CHNH + O CHNH + HO were calculated from ab initio theory. The mechanism was validated against experimental results from batch reactors, flow reactors, shock tubes, and premixed flames. The model predicts satisfactorily explosion limits for CHNH and its oxidation in a flow reactor. However, oxidation in the presence of nitric oxide, which strongly promotes reaction at lower temperatures, is only described qualitatively. Furthermore, calculated flame speeds are higher than reported experimental values; the model does not capture the inhibiting effect of the NH group in CHNH compared to CH. More work is desirable to confirm the products of the CHNH + NO reaction and to look into possible pathways to NH in methylamine oxidation.
Laminar flame speeds of dimethyl ether and dimethoxymethane at pressures from 1 to 5 bar and initial temperatures from 298 to 373 K were determined experimentally using a constant volume spherical vessel and a heat flux burner setup. This study is the first to report dimethoxymethane laminar flame speeds at a pressure higher than 1 bar. Using these experimental data along with data available in the literature, a new kinetic model for the prediction of the oxidation behavior of dimethyl ether and dimethoxymethane in freely propagating and burner stabilized premixed flames, in shock tubes, rapid compression machines, flow reactors, and a jet-stirred reactor has been developed. The experimental results from the present work and literature are interpreted with the help of the derived kinetic model. This newly developed reaction mechanism considers the redox chemistry of NOx to accommodate the influence of the oxygen level on the onset of fuel conversion and interconversion of NO and NO2. The current model suggests that an increased O2 level promotes the HO 2 production, which in turn leads to the formation of OH radicals, which promotes the combustion of the fuel/air mixture under lean conditions. The increase of OH radical concentrations is mainly via the NO/NO2 interconversion reaction channel, NO + HO2 = NO2 + OH, NO2 + H = NO + OH, CH3OCH3 + NO2 = CH3OCH2 + HONO , followed by the thermal decomposition of HONO. This work extends the kinetic database and helps to improve the understanding of dimethyl ether and dimethoxymethane combustion behavior. The kinetic model presented in this work can serve as a base model for hydrocarbons and oxygenated fuels higher than C2 .
Autoignition delay times of NH3/CH4 mixtures with CH4 fractions of 0%, 5%, 10% and 50% were measured in a rapid compression machine at equivalence ratio φ = 0.5, pressures from 20 to 70 bar and temperatures from 930 to 1140 K. In addition, measurements were performed for NH3 mixtures with 10% CH4 at φ = 1.0 and 2.0. Methane shows a strong ignition-enhancing effect on NH3, which levels off at higher CH4 fractions, as the ignition delay time approaches that of pure methane. Autoignition delay times at 10% CH4 at φ = 0.5 and 1.0 are indistinguishable, while an increase of ignition delay times by factor of 1.5 was observed upon increasing φ to 2.0. The experimental data were used to evaluate six NH3 oxidation mechanisms capable of simulating NH3/CH4 mixtures. The mechanism previously used by the authors shows the best performance: generally, it predicts the measured ignition delay times to better than 30% for all conditions, except for 50% CH4 addition for which the differences increase up to 50% at the highest temperature. Sensitivity analysis based on the mechanism used indicates that under lean conditions the reaction CH4 + NH2 = CH3 + NH3 significantly promotes ignition for modest CH4 addition (5% and 10%), but becomes modestly ignition-inhibiting at 50% CH4. Sensitivity and rate-of-production analyses indicate that the ignition-enhancing effect of 50% CH4 addition is closely related to the formation and decomposition of H2O2. Flux analysis for NH3/CH4 mixtures indicates that CH4 + NH2 = CH3 + NH3 contributes substantially to the decomposition of methane early in the oxidation process, while CH3 + NO2 (+M) = CH3NO2 (+M) is a significant reservoir of NO2 at low temperature. Additionally, an anomalous pre-ignition pressure rise phenomenon, which is not reproduced by the simulations, was observed with high reproducibility for the NH3 mixture with 50% CH4 addition.
Ammonia (NH3) is receiving increasing attention as an alternative engine fuel due to its carbon-free nature. However, its fundamental combustion characteristics, such as higher autoignition temperature and lower burning velocity compared to conventional hydrocarbons, limit its direct use in traditional engines. To confront this dilemma, hydrocarbon/NH3 blending fuels are seen as one of the most appropriate ways to exploit its advantage and offset its weaknesses. Using n-heptane as a representative of hydrocarbons, this study investigated the effect of NH3 addition on the low-temperature autoignition of n-heptane. The ignition delay times of five n-heptane/NH3 mixtures with NH3 fractions of 0%, 20%, and 40% were measured in a rapid compression machine at temperatures of 635–945 K, pressures of 10 and 15 bar, and equivalence ratios of 1.0 and 2.0. Experimental results show that the n-heptane/NH3 blending fuels exhibit pronounced low-temperature reactivity, and both the total and the first-stage ignition delay times increase with the increase of NH3 fraction. A blending mechanism of n-heptane/NH3 was compiled based on the existing n-heptane mechanism and NH3 mechanism. It is found that the blending mechanism is capable to predict the inhibition effect of NH3 addition on n-heptane autoignition and qualitatively capture the dependence of ignition delay time on equivalence ratio and oxygen mole fraction. Nevertheless, there are significant discrepancies between the experiments and the simulation. Furthermore, kinetic analyses, including species evolution, rate of production and sensitivity of OH radical were conducted sequentially to reveal the autoignition kinetics of NH3/n-heptane blends and the interaction between n-heptane and NH3. Suggestions are provided for the further development of the blending mechanism.
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.
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.
The pyrolysis and oxidation of dimethyl ether (DME) and its mixture with methane were investigated at high pressure (50 and 100 bar) and intermediate temperature (450–900 K). Mixtures highly diluted in nitrogen with different fuel–air equivalence ratios (Φ=∞, 20, 1, 0.06) were studied in a laminar flow reactor. At 50 bar, the DME pyrolysis started at 825 K and the major products were CH4, CH2O, and CO. For the DME oxidation at 50 bar, the onset temperature of reaction was 525 K, independent of fuel–air equivalence ratio. The DME oxidation was characterized by a negative temperature coefficient (NTC) zone which was found sensitive to changes in the mixture stoichiometry but always occurring at temperatures of 575–625 K. The oxidation of methane doped by DME was studied in the flow reactor at 100 bar. The fuel–air equivalence ratio (Φ) was varied from 0.06 to 20, and the DME to CH4 ratio changed over 1.8–3.6%. Addition of DME had a considerable promoting effect on methane ignition as the onset of reaction shifted to lower temperatures by 25–150 K. A detailed chemical kinetic model was developed by adding a DME reaction subset to a model developed in previous high-pressure work. The model was evaluated against the present data as well as data from literature. Additional work is required to reconcile experimental and theoretical work on reactions on the CH3OCH2OO PES with ignition delay measurements in the NTC region for DME.
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.
Unimolecular β-scission of the methoxymethyl (CH3OCH2) radical has been considered to be the crucial chain-propagating step in both oxidation and pyrolysis of dimethyl ether. The present work employs hybrid density functionals M06-2X, BB1K, B3LYP, and MPW1K with the MG3S basis set as well as double-hybrid density functional B2PLYP and Moller-Plesset perturbation theory MP2 with the TZVP basis set to study the detailed mechanism of unimolecular decomposition of CH3OCH2. Energies of all stationary points are refined with the CCSD(T), QCISD(T), CBS-QB3, and G4 calculations. The minimum energy path was computed at the CCSD(T)/aug-cc-PVTZ//M062X/MG3S level. Kinetic calculations are performed by means of high-pressure multi-structural canonical variational transition state (MS-CVT) theory and pressure-dependent Rice–Ramsperger–Kassel–Marcus (RRKM) theory to clarify the available experimental observations and previous theoretical results. A kinetic model for the low and the high-pressure limiting, and falloff region was extracted. For high pressure limit, k∞ = 2.08 × 10¹² (T/300)1.002 exp(–11097.64/T) s⁻¹ at temperatures of 200–2600 K based on the MS-CVT/SCT method. Furthermore, the intermediate falloff curve was found to be best represented by k/k∞=[x/(1+x)]Fcent1/[1+(a+logx)2/(N±ΔN)2] with x = k0/k∞, a = 0.263, N = 1.208, ΔN = 0.096, (+ΔN for (a + logx) < 0 and –ΔN for (a + logx) > 0), and Fcent(DME) = 0.348 independent of temperature. The low and high pressure limiting rate constants have been extracted by extrapolation of the fall-off curves: k0 = [DME] 2.49 × 10¹⁶ (T/300)0.053 exp(–9067.58/T) cm³ mol–1 s–1 and k∞ = 1.88 × 10¹² (T/300)1.05 exp(–11061.79/T) s⁻¹ at temperatures of 450–800 K, which agree well with the reported experimental low and high pressure limit results.
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.
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.
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.
The rapid compression machine (RCM) is a great tool for investigating fuel properties under engine relevant conditions (high-pressures, low temperatures). The most common diagnostics is measuring the pressure over time and determining the ignition delay time (IDT). In this study, for the first time, the OH* luminescence of ethanol/air mixture is measured within an RCM experiment at 15 and 20 bar for Φ = 0.5. Combining the common pressure measurements with the simultaneously recorded high-speed images (up to 74.5 kHz framerate) gives a first insight into understanding the ignition modes and the corresponding pressure traces. At 74.5 kHz, in contrast to findings in literature, the ethanol ignition did not show to be purely homogeneous. Four different propagating fronts of OH* luminescence have been recorded. Besides a flame kernel and a detonation-like ignition front two further fronts prior to main ignition have been observed. The propagating speeds of the fronts have been determined and depend on the overall IDT.
Dimethyl ether (DME)/n-butane mixtures are ideal fuel for homogeneous charge compression ignition (HCCI) engine. Fundamental ignition delay data under engine-related conditions are essential to better understand HCCI combustion and to validate DME/n-butane models. However, there is a significant lack of ignition delay data of DME/n-butane mixtures under high-pressure and low-temperature conditions. Therefore, ignition delays of DME/n-butane mixtures at equivalence ratio of 0.5 were measured at different DME blending ratios (0, 20, 40, 60, 80 and 100%), pressures (12, 16, 24 and 30 bar), temperatures (622–929 K) and fuel concentrations (1.6 and 2.2%) using a rapid compression machine (RCM). A numerical analysis was performed using the NUI Aramco Mech 2.0, and a good agreement to experimental data was obtained. The results well indicate the two-stage ignition and negative temperature coefficient (NTC) behavior of DME/n-butane mixtures. With DME addition, system reactivity is enhanced and air-fuel mixtures can be ignited at lower temperatures. Meanwhile, DME addition promotes both the first-stage and total ignition, and the total ignition delay as a function of DME blending ratio decreases nonlinearly, especially at low pressures. In addition, the DME/n-butane mixtures ignite faster at higher fuel concentration and pressure conditions, which is more prominent in the NTC region. Kinetic analysis indicates that compared with slower internal H-atom isomerization rate of n-butane, DME has a more active chain branching path, which promotes the low-temperature oxidation of n-butane and overall fuel, thus faster development of total radical pool, increased concentration of radicals and decreased ignition delay. The addition of 20% DME results in a significant increase of free radicals and heat release rate while the DME chemistry becomes the most ignition-sensitive one in DME/n-butane mixtures when the DME blending ratio reaches 60%.
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.
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.
Cantera is a suite of object-oriented software tools for problems involving chemical kinetics, thermodynamics, and/or transport processes. Cantera provides types (or classes) of objects representing phases of matter, interfaces between these phases, reaction managers, time-dependent reactor networks, and steady one-dimensional reacting flows. Cantera is currently used for applications including combustion, detonations, electrochemical energy conversion and storage, fuel cells, batteries, aqueous electrolyte solutions, plasmas, and thin film deposition.
For the first time, NH3–air combustion power generation has been successfully realized using a 50 kW class micro gas turbine system at the National Institute of Advanced Industrial Science and Technology (AIST), Japan. Based on the global demand for carbon-free power generation as well as recent advances involving gas-turbine technologies, such as heat-regenerative cycles, rapid fuel mixing using strong swirling flows, and NOx reduction using selective catalytic reduction (SCR), allow us to realize NH3–air combustion gas-turbine system, which was abandoned in the 1960′s. In the present system, the combustor adopted gaseous NH3 fuel and diffusion combustion to enhance flame stability. The NH3 pre-cracking apparatus for combustion enhancement using generated H2 was not employed. The NH3–air combustion gas-turbine power generation system can be operated over a wide range of power and rotational speeds, i.e., 18.4 kW to 44.4 kW and 70,000 rpm to 80,000 rpm, respectively. The combustion efficiency of the NH3–air gas turbine ranged from 89% to 96% at 80,000 rpm. The emission of NO and unburnt NH3 depends on the combustor inlet temperature. Emission data indicates that there are NH3 fuel-rich and fuel-lean regions in the primary combustion zone. It is presumed that unburnt NH3 is released from the fuel-rich region, while NO is released from the fuel-lean region. When diluted air enters the secondary combustion zone, unburnt NH3 is expected to react with NO through selective non-catalytic reduction (SNCR). NH3[single bond]CH4–air combustion operation tests were also performed and the results show that the increase of the NH3 fuel ratio significantly increases the NO emission, whereas it decreases the NO conversion ratio. To achieve low NOx combustion in NH3–air combustion gas turbines, it is suggested to burn large quantities of NH3 fuel and produce both rich and lean fuel mixtures in the primary combustion zone.
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.
In this paper, a binary fuel model for dimethyl ether (DME) and propane is developed, with a focus on engine-relevant conditions (10–50 atm and 550–2000 K). New rapid compression machine (RCM) data are obtained for the purpose of further validating the binary fuel model, identifying reactions important to low-temperature propane and DME oxidation, and understanding the ignition-promoting effect of DME on propane. It is found that the simulated RCM data for DME/propane mixtures is very sensitive to the rates of C3H8 + OH, which acts as a radical sink relative to DME oxidation, especially at high relative DME concentrations. New rate evaluations are conducted for the reactions of C3H8 + OH = products as well as the self-reaction of methoxymethyl peroxy (in competition with RO2 = QOOH isomerization) of 2CH3OCH2O2 = products. Accurate phenomenological rate constants, k(T, P), are computed by RRKM/ME methods (with energies obtained at the CCSD(T)-F12a/cc-pVTZ-F12 level of theory) for several radical intermediates relevant to DME. The model developed in this paper (120 species and 711 reactions) performs well against the experimental targets tested here and is suitable for use over a wide range of conditions. In addition, the reaction mechanism generator software RMG is used to explore cross-reactions between propane and DME radical intermediates. These cross-reactions did not have a significant effect on simulations of the conditions modeled in this paper, suggesting that kinetic models for high- and low-reactivity binary fuel mixtures may be assembled from addition of their corresponding submodels and a small molecule foundation model.
Combustion and emissions characteristics of a compression-ignition engine using mixtures of ammonia and dimethyl ether (DME) are investigated. Ammonia can be regarded as a carbon-free fuel and is one of the world's most synthesized chemicals. Its infrastructure is well established and it can be produced from renewable resources, making it an attractive energy carrier. In this study, a high-pressure mixing system is developed for blending liquid ammonia with DME, the latter serving to initiate combustion. The present engine uses a modified injection system without fuel return to prevent fuel mixture from vaporizing. Results using different mixtures of ammonia and DME show that ammonia causes longer ignition delays and limits the engine load conditions due to its high autoignition temperature and low combustion rate (e. g., flame speed). The inclusion of ammonia in the fuel mixture also decreases combustion pressure and temperature, resulting in higher CO and HC emissions. NOx emissions also increase due to the formation of fuel NOx when ammonia is used. However, improvements for the same operating conditions can be achieved by increasing the injection pressure. Exhaust ammonia emissions is on the order of hundreds of ppm under most of the conditions studied. Soot emissions are extremely low for all cases studied. Double injection schemes are also employed and their effects on the exhaust emissions vary with operating conditions.
Combustion and emissions characteristics of a compression-ignition engine using ammonia (NH3) and dimethyl ether (DME) mixtures were investigated in this study. The experiments were conducted using three different mixtures, including 100%DME, 60%DME-40%NH3, and 40%DME-60%NH3 (by weight). The injection pressure was maintained at approximately 20.6 MPa and engine combustion and exhaust emissions were measured in order to analyze and compare the performance of different mixture compositions. Results show that engine performance decreases as ammonia concentration in the fuel mixture increases. Significant cycle-to-cycle variations are observed when 40%DME-60%NH3 is used. The injection timing for best torque needs to be advanced with increased ammonia concentration in the fuel mixture due to the high resistance to autoignition of ammonia. Moreover, with the increase in ammonia concentration, both engine speed and engine power exhibit limitations relative to 100%DME cases. For 40%DME-60%NH3, the appropriate injection timing was found to range from 90 to 340 BTDC and the engine exhibits homogeneous charge compression ignition (HCCI) combustion characteristics due to the highly advanced injection timing. 40%DME-60%NH3 conditions also results in higher CO and HC emissions due to the low combustion temperature of ammonia. Soot emissions for 40%DME-60%NH3 remain extremely low. When ammonia is used, NOx emissions are increased due to the formation of fuel NOx. Exhaust ammonia emissions also increase as ammonia concentration in the fuel mixture increases from 40% to 60%. Overall, in this study appropriate strategies are developed to enable the use of ammonia in direct-injection compression-ignition engines and the corresponding engine performance is evaluated.
Ammonia oxidation and ignition delay time measurements for pressures above 10 atm are scarce. In addition, NH3 is known to adsorb on stainless steel, so measurement results could be in question if wall passivation is not employed for apparatuses utilizing steel. To overcome these measurement difficulties and overall lack of high-pressure data for ammonia, new and methodical ignition delay time measurements have been performed behind reflected shock waves over a wide range of temperatures (1560–2455 K), pressures (around 1.4, 11, and 30 atm) and equivalence ratios (0.5, 1.0, and 2.0) for mixtures of ammonia highly diluted in Ar (98–99%). The new set of data from the present study was compared to several models from the literature. It was found that a large majority of the models do not predict the ignition delay times with accuracy, and there is a surprisingly wide variation amongst the predictions. One satisfactory model, from Dagaut et al. (2008), was selected and extended to compounds other than NH3 using H2/O2/CO, N2O, NO2, and NNH sub-mechanisms from the literature. The resulting comprehensive mechanism predicts well the ammonia ignition delay time data from the present study along with other NH3, NO2, and N2O data from the authors as well as from the literature with high accuracy. In addition to the new ammonia oxidation data and related model comparisons, the present paper documents a state-of-the-art NOx sub-mechanism that can be used for a wide range of combustion calculations when added to, for example, baseline mechanisms involving hydrogen and hydrocarbon kinetics.
The development of accurate chemical kinetic models capable of predicting the combustion of methane
and dimethyl ether in common combustion environments such as compression ignition engines and gas
turbines is important as it provides valuable data and understanding of these fuels under conditions that
are difficult and expensive to study in the real combustors. In this work, both experimental and chemical
kinetic model-predicted ignition delay time data are provided covering a range of conditions relevant to
gas turbine environments (T = 600–1600 K, p = 7–41 atm, / = 0.3, 0.5, 1.0, and 2.0 in ‘air’ mixtures). The
detailed chemical kinetic model (Mech_56.54) is capable of accurately predicting this wide range of data,
and it is the first mechanism to incorporate high-level rate constant measurements and calculations
where available for the reactions of DME. This mechanism is also the first to apply a pressure-dependent
treatment to the low-temperature reactions of DME. It has been validated using available literature data
including flow reactor, jet-stirred reactor, shock-tube ignition delay times, shock-tube speciation, flame
speed, and flame speciation data. New ignition delay time measurements are presented for methane,
dimethyl ether, and their mixtures; these data were obtained using three different shock tubes and a
rapid compression machine. In addition to the DME/CH4 blends, high-pressure data for pure DME and
pure methane were also obtained. Where possible, the new data were compared with existing data from
the literature, with good agreement.
Autoignition experiments of stoichiometric mixtures of s-, t-, and i-butanol in air have been performed using a heated rapid compression machine (RCM). At compressed pressures of 15 and 30 bar and for compressed temperatures in the range 715–910 K, no evidence of a negative temperature coefficient region in terms of ignition delay response is found. The present experimental results are also compared with previously reported RCM data of n-butanol in air. The order of reactivity of the butanols is n-butanol > s-butanol ≈ i-butanol > t-butanol at the lower pressure but changes to n-butanol > t-butanol > s-butanol > i-butanol at higher pressure. In addition, t-butanol shows preignition heat release behavior, which is especially evident at higher pressures. To help identify the controlling chemistry leading to this preignition heat release, off-stoichiometric experiments are further performed at 30 bar compressed pressure, for t-butanol at = 0.5 and = 2.0 in air. For these experiments, higher fuel loading (i.e., = 2.0) causes greater preignition heat release (as indicated by greater pressure rise) than the stoichiometric or = 0.5 cases. Comparison of the experimental ignition delays with the simulated results using two literature kinetic mechanisms shows generally good agreement, and one mechanism is further used to explore and compare the fuel decomposition pathways of butanol isomers. Using this mechanism, the importance of peroxy chemistry in the autoignition of the butanol isomers is highlighted and discussed.
Flash photolysis of ammonia in the presence of oxygen was studied using the intracavity laser spectroscopy technique. The reaction NH2 + O2 was demonstrated to be of no importance and a value of k6 < 1.5 × 10−17 cm3/molecule s was estimate Accelerating NH2 decay with oxygen addition is due to the reaction NH2 + HO2 and its rate constant was found to be k7 = 2.5 × 10−11 cm3/molecules.
In modeling rapid compression machine (RCM) experiments, zero-dimensional approach is commonly used along with an associated heat loss model. The adequacy of such approach has not been validated for hydrocarbon fuels. The existence of multi-dimensional effects inside an RCM due to the boundary layer, roll-up vortex, non-uniform heat release, and piston crevice could result in deviation from the zero-dimensional assumption, particularly for hydrocarbons exhibiting two-stage ignition and strong thermokinetic interactions. The objective of this investigation is to assess the adequacy of zero-dimensional approach in modeling RCM experiments under conditions of two-stage ignition and negative temperature coefficient (NTC) response. Computational fluid dynamics simulations are conducted for n-heptane ignition in an RCM and the validity of zero-dimensional approach is assessed through comparisons over the entire NTC region. Results show that the zero-dimensional model based on the approach of 'adiabatic volume expansion' performs very well in adequately predicting the first-stage ignition delays, although quantitative discrepancy for the prediction of the total ignition delays and pressure rise in the first-stage ignition is noted even when the roll-up vortex is suppressed and a well-defined homogeneous core is retained within an RCM. Furthermore, the discrepancy is pressure dependent and decreases as compressed pressure is increased. Also, as ignition response becomes single-stage at higher compressed temperatures, discrepancy from the zero-dimensional simulations reduces. Despite of some quantitative discrepancy, the zero-dimensional modeling approach is deemed satisfactory from the viewpoint of the ignition delay simulation. (author)
One of the remaining issues in our understanding of nitrogen chemistry in combustion is the chemistry of NNH. This species is known as a key intermediate in Thermal DeNOx, where NH3 is used as a reducing agent for selective non-catalytic reduction of NO. In addition, NNH has been proposed to facilitate formation of NO from thermal fixation of molecular nitrogen through the so-called NNH mechanism. The importance of NNH for formation and reduction of NO depends on its thermal stability and its major consumption channels. In the present work, we study reactions on the NNH+O, NNH+O2, and NH2+O2 potential energy surfaces using methods previously developed by Miller, Klippenstein, Harding, and their co-workers. Their impact on Thermal DeNOx and the NNH mechanism for NO formation is investigated in detail.
Autoignition delay times of stoichiometric methane, ethane and methane/ethane mixtures doped with 100 and 270ppm of NO2 have been measured in a RCM in the temperature range 900–1050K and pressures from 25 to 50bar. The measurements show that addition of NO2 to CH4/O2/N2/Ar and CH4/C2H6/O2/N2/Ar mixtures results in a significant reduction in the autoignition delay time and that the ignition-promoting effect of NO2 increases substantially with increasing temperature, from ∼20% to more than a factor of two over the range of temperature studied. Addition of NO2 to C2H6/O2/N2/Ar mixtures results in only a modest reduction in ignition delay time over the range of pressure and temperature measured. Computations with an updated chemical mechanism show good agreement with the measurements for undoped methane, but overpredict the delay times for undoped ethane and underestimate the effects of replacing 10% methane by ethane. For NO2-containing mixtures, the model predicts the observed trend in decreasing delay time with increasing NO2 fraction. However, the computations tend to overestimate the effect of NO2 addition on ignition, particularly for C2H6 mixtures. Analysis of the reaction mechanism for the effects of NO2 addition to methane mixtures indicates that the ignition-promoting effect of NO2 is related to the appearance of new conversion channels for CH3 and CH3OO, i.e., NO2+CH3→NO+CH3O and NO+CH3OO→NO2+CH3O, generation of chain-initiating OH radicals through NO/NO2 interconversion, i.e., NO2+H→NO+OH and NO+HO2→NO2+OH, and to the direct initiation step CH4+NO2→CH3+HNO2. Analyses further show that the formation of CH3NO2 via CH3+NO2(+M)↔CH3NO2(+M) essentially inactivates NO2. This reaction limits the promoting effect of NO2 at lower temperatures and higher pressures, where stabilization of CH3NO2 is favored, explaining the experimentally observed trends.
The hydrogen abstraction reaction CH2O + NH2 → CHO + NH3 has been studied using direct ab initio dynamics method. All of the information along the minimum energy path (MEP) was calculated at the UMP2/6-311+G(d, p) level of theory. Energetic data along the MEP were further refined using the scheme G2 with the UMP2/6-311+G(d, p) optimized geometries. The barrier heights for the forward and reverse reactions were obtained as 5.89 and 24.44 kcal/mol, respectively. Reaction rate constants and activation energies were calculated for the temperature range 250−2500 K by the improved canonical variation transition state theory (ICVT) incorporating a small-curvature tunneling correction (SCT). The rate constant at the room temperature was predicted to be 5.25 × 10-17 cm3 molecule-1 s-1, which is about 2 orders of magnitude smaller than that of the hydrogen abstraction reaction of acetaldehyde with aminogen.
Systematic ab initio calculations of potential energy surfaces for the reactions of NH2 with various alkanes (CH4, C2H6, C3H8, and i-C4H10) which involve abstraction of a hydrogen atom from primary, secondary, and tertiary C−H bonds have been performed using the G2M method. The calculated activation barrier for the NH2 + CH4 reaction, 15.2 kcal/mol, is higher than those for the H-abstraction from a primary C−H bond in C2H6, C3H8, and i-C4H10, 11−12 kcal/mol. The barrier height decreases to 8.4 and 8.3 kcal/mol for the abstraction from a secondary C−H bond in C3H8 and a tertiary C−H bond in i-C4H10, respectively, in line with the weakening strength of the C−H bond and the increase of the reaction exothermicity. The G2M energies and the molecular and transition-state parameters are used to compute thermal reaction rate constants within the transition-state theory formalism with tunneling corrections. A good agreement of the theoretical rate constants with the experimental is found if the computed barriers are adjusted by 0.5−2 kcal/mol, which is within the accuracy of the G2M method. The H-abstraction from the tertiary C−H bond is shown to be faster than the other considered reactions at T ≤ 2000 K, while the secondary H-abstraction is the second fastest reaction at T ≤ 1600 K. The rate of the primary H-abstraction decreases with the increase of the alkane size, from ethane to propane and to isobutane. The calculated rate constants for the H-abstraction by NH2 from primary, secondary, and tertiary C−H bonds can serve as models for the reactions of the amino radical with various saturated hydrocarbons.
The major source of error in most ab initio calculations of molecular energies is the truncation of the one‐electron basis set. Extrapolation to the complete basis set second‐order (CBS2) limit using the N−1 asymptotic convergence of N‐configuration pair natural orbital (PNO) expansions can be combined with the use of relatively small basis sets for the higher‐order (i.e., MP3, MP4, and QCI) correlation energy to develop cost effective computational models. Following this strategy, three new computational models denoted CBS‐4, CBS‐q, and CBS‐Q, are introduced. The mean absolute deviations (MAD) from experiment for the 125 energies of the G2 test set are 2.0, 1.7, and 1.0 kcal/mol, respectively. These results compare favorably with the MAD for the more costly G2(MP2), G2, and CBS‐QCI/APNO models (1.6, 1.2, and 0.5 kcal/mol, respectively). The error distributions over the G2 test set are indistinguishable from Gaussian distribution functions for all six models, indicating that the rms errors can be interpreted in the same way that experimental uncertainties are used to assess reliability.
A possible new route for NO formation in hydrogen combustion is explored. The reaction sequence that converts molecular nitrogen into nitrogen oxides involves sequential recombination of N2 with H atoms: N2→NNH→N2H2→N2H3. N-N bond cleavage occurs in the reaction of N2H3 with H2 forming NH3 and NH2; These last species are oxidized mainly in the sequence NH3→NH2→NH→N→NO. Key reactions of the N2H3 formation and consumption as well as other important reactions revealed by sensitivity analysis and reaction path analysis are examined and discussed. Kinetic modeling of hydrogen combustion in stirred reactors demonstrates that this mechanism can be of importance in rich mixtures at relatively low temperatures (below about 1500 K) when other routes of NO formation are suppressed. Available measurements of NO formation in hydrogen combustion in stirred reactors have been modeled and analyzed. They neither confirm nor contradict the proposed route forming NO via N2H3, because these experiments have been conducted outside the range of conditions where this route is manifested.
The effect of SO2 addition on the reduction of NO by ammonia in simulated thermal de-NOx conditions was studied in a JSR at atmospheric pressure for various equivalence ratios (0.1–2) and initial concentrations of NO, NH3, and SO2 (0–1000 ppm). The experiments were performed at fixed residence times of 100 and 200 ms, and variable temperature, ranging from 1100 to 1450 K. It was demonstrated that the addition of SO2 inhibits the reduction of NO by ammonia from fuel-lean to fuel-rich conditions. The effect of SO2 on the extent of NO reduction was controversial in the literature. This study confirms that SO2 does not reduce the efficiency of the thermal de-NOx process but shifts the optimal temperature to higher values. A kinetic reaction mechanism was used to simulate these experiments. According to the model, the inhibiting effect of SO2 is due to chain terminating processes (H + H + M = H2 + M) and radical pool reduction (H + O2 + M = HO2 + M) via the sequence of reactions SO2 + H + M ⇒ HOSO + M, HOSO + O2 ⇒ HO2 + SO2, and HOSO + H ⇒ SO2 + H2. The present results confirm the validity of the previously proposed kinetic reaction sub-mechanisms for the reduction of nitric oxide by ammonia and for the inhibition of the oxidation of fuels by SO2.
For the title reaction, the pressure dependent rate coefficients were studied at 298 K using two complementary experimental techniques. Pulse radiolysis combined with UV absorption was employed at pressures between 500 and 1000 mbar, while a fast-flow system with a quadrupole mass spectrometer (at low ionization energies) was applied at pressures in the range of 0.7–5.1 mbar. The fall-off curve was constructed in terms of Troe's analysis, and the following high- and low-pressure limiting rate coefficients were derived: krec,∞ = (1.3 ± 0.3) × 10−10 × (T/300)0.42 cm3 molecule−1 s−1 and krec,0/[He] = (1.8 ± 0.5) × 10−25 × (T/300)−3.85 cm6 molecule−2 s−1.
This study demonstrated the feasibility of ammonia combustion in compression-ignition diesel engines. Ammonia combustion does not produce carbon dioxide, a known greenhouse gas that contributes to global warming. Using this idea, a method was developed to introduce ammonia into the intake manifold and to inject diesel fuel or biodiesel directly into the cylinder to ignite the mixture. This dual-fuel approach was chosen because ammonia has a high resistance to autoignition. This approach was proven successful in a multicylinder, turbocharged diesel engine. The system developed required only a slight modification of the intake to implement the ammonia fuel line. The existing diesel fuel injection system remained unchanged. A liquid ammonia tank was used for fuel storage, and a high pressure relief valve regulated the ammonia flow rate. Engine combustion phasing (e.g., ignition) was controlled by diesel fuel injection. Both experiments and chemical kinetic studies were carried out for different diesel/ammonia ratios at various engine speeds and loads. Ammonia was used as an energy replacement for diesel fuel. The results showed that the peak engine torque could be achieved by using different combinations of diesel fuel and ammonia. During testing, a maximum energy replacement of 95% was measured. It should be noted that if more ammonia is added, a higher than rated power can be achieved depending on engine load conditions. This would be similar in practice to adding nitrous oxide to gasoline engines. It was also shown that CO2 emissions were reduced monotonically for the same engine torque output as the amount of the ammonia in the fuel mixture increased. Additionally, burning ammonia in engines does not necessarily increase NOx emissions despite the fuel-bound nitrogen. Lower levels of NOx emissions were obtained as long as energy substitution by ammonia did not exceed 60%. This is thought to occur because of the lower combustion temperature of ammonia. This study also showed that the engine could be operated at different load conditions by using a small quantity of diesel fuel with the appropriate amounts of ammonia to achieve desirable loads. Biodiesel was also used with ammonia at different ratios resulting in successful engine operation. Results of using biodiesel−ammonia were similar to those of using diesel fuel−ammonia.
A detailed chemical kinetic model has been used to study dimethyl ether (DME) oxidation over a wide range of conditions. Experimental results obtained in a jet-stirred reactor (JSR) at 1 and 10 atm, 0.2≤ϕ≤2.5, and 800≤T≤1300 K were modeled, in addition to those generated in a shock tube at 13 and 40 bar, ϕ=1.0 and 650≤T≤1300 K. The JSR results are particularly valuable as they include concentration profiles of reactants, intermediates, and products pertinent to the oxidation of DME. These data test the kinetic model severely, as it must be able to predict the correct distribution and concentrations of intermediate and final products formed in the oxidation process. Additionally, the shock-tube results are very useful, as they were taken at low temperatures and at high pressures, and thus undergo negative temperature dependence (NTC) behavior. This behavior is characteristic of the oxidation of saturated hydrocarbon fuels, (e.g., the primary reference fuels, n-heptane and iso-octane) under similar conditions. The numerical model consists of 78 chemical species and 336 chemical reactions. The thermodynamic properties of unknown species pertaining to DME oxidation were calculated using THERM. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 229–241, 1998.
Optimized scale factors for calculating vibrational harmonic and fundamental frequencies and zero-point energies have been determined for 145 electronic model chemistries, including 119 based on approximate functionals depending on occupied orbitals, 19 based on single-level wave function theory, three based on the neglect-of-diatomic-differential-overlap, two based on doubly hybrid density functional theory, and two based on multicoefficient correlation methods. Forty of the scale factors are obtained from large databases, which are also used to derive two universal scale factor ratios that can be used to interconvert between scale factors optimized for various properties, enabling the derivation of three key scale factors at the effort of optimizing only one of them. A reduced scale factor optimization model is formulated in order to further reduce the cost of optimizing scale factors, and the reduced model is illustrated by using it to obtain 105 additional scale factors. Using root-mean-square errors from the values in the large databases, we find that scaling reduces errors in zero-point energies by a factor of 2.3 and errors in fundamental vibrational frequencies by a factor of 3.0, but it reduces errors in harmonic vibrational frequencies by only a factor of 1.3. It is shown that, upon scaling, the balanced multicoefficient correlation method based on coupled cluster theory with single and double excitations (BMC-CCSD) can lead to very accurate predictions of vibrational frequencies. With a polarized, minimally augmented basis set, the density functionals with zero-point energy scale factors closest to unity are MPWLYP1M (1.009), τHCTHhyb (0.989), BB95 (1.012), BLYP (1.013), BP86 (1.014), B3LYP (0.986), MPW3LYP (0.986), and VSXC (0.986).
