An experimental and kinetic modeling study of premixed NH3/CH4/O2/Ar flames at low pressure (original) (raw)
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An experimental and kinetic modeling study of premixed nitromethane flames at low pressure
Proceedings of The Combustion Institute, 2011
An experimental and modeling study of 11 premixed NH 3 /CH 4 /O 2 /Ar flames at low pressure (4.0 kPa) with the same equivalence ratio of 1.0 is reported. Combustion intermediates and products are identified using tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. Mole fraction profiles of the flame species including reactants, intermediates and products are determined by scanning burner position at some selected photon energies near ionization thresholds. Temperature profiles are measured by a Pt/Pt-13%Rh thermocouple. A comprehensive kinetic mechanism has been proposed. On the basis of the new observations, some intermediates are introduced. The flames with different mole ratios (R) of NH 3 /CH 4 (R0.0, R0.1, R0.5, R0.9 and R1.0) are modeled using an updated detailed reaction mechanism for oxidation of CH 4 /NH 3 mixtures. With R increasing, the reaction zone is widened, and the mole fractions of H 2 O, NO and N 2 increase while those of H 2 , CO, CO 2 and NO 2 have reverse tendencies. The structural features by the modeling results are in good agreement with experimental measurements. Sensitivity and flow rate analyses have been performed to determine the main reaction pathways of CH 4 and NH 3 oxidation and their mutual interaction.
Modeling of NH3/H2/O2/Ar flames at low pressure
Fifth European …, 2011
This study presents a new mechanism for ammonia combustion validated for the flame structure prediction of ammonia, hydrogen, oxygen, argon flames investigated at several low pressures and for various conditions of equivalence ratio and of initial hydrogen content [1]. This kinetic model is based on the one proposed by Konnov [2] and the comparison of the predictions of these mechanisms for the same ammonia flame is presented. The new mechanism contains 80 elementary reactions and 19 chemical species.
Structure and kinetics of CH4/N2O flames
Symposium (International) on Combustion, 1991
Laminar, premixed flat flames of CH 4 with 5120 have been stabilized and studied at 50 torr. This study represents the first nearly complete study of the structure and kinetics of CH4---N20 flames including stable and unstable species measurements and detailed chemical kinetic modeling. Three flames were investigated, with slightly fuel rich, near stoichiometric and lean mixtures. Stable species concentration profiles were measured using probe sampling with gas chromatographic sample analysis. Laser-Induced Fluorescence was used to measure the composition of the intermediate species CH, CN, Nil, NH~, and OH. Temperature was measured with coated, radiation corrected thermocouples and by the LIF rotational temperature of CN. The CH4----NaO flames have high N2 and H20 and low NO concentration in the products. The CO is oxidized to C02 through its reaction with OH. These flames were characterized by strong radical emission spectra, but no NH2 fluorescence signal was observed. A reaction mechanism was developed for the CH4--NzO flames and results are presented comparing the calculated and experimental profiles of species concentration for the flame with equivalence ratio 1.13. Comparison of the stable species profiles is very good; comparison of the unstable species profiles is qualitatively correct with some displacement of the concentration maxima calculated compared to the data. Sensitivity analysis was used to evaluate the effects of changing rate constants in the mechanism on the species concentration profiles. The flame modeling and sensitivity analysis suggest the major reaction paths responsible for the conversion of reactants to products and the formation and consumption of the intermediates.
Study of the Chemical Structure of Laminar Premixed H2/CH4/C3H8/O2/Ar Flames at 1–5 atm
Energy & Fuels, 2017
The paper presents an experimental and modeling study of the chemical structure of laminar premixed stoichiometric H 2 /CH 4 /C 3 H 8 /O 2 /Ar flames stabilized on a flat burner at 1, 3 and 5 atm. The flames structure was simulated using four different detailed chemical kinetic mechanisms proposed in the literature for oxidation of small hydrocarbons. The width of the zone of consumption of the fuel components was shown to differ appreciably at the three pressures. Hydrogen was shown to have the largest consumption zone, while propane has the smallest one. The kinetic analysis provided an explanation for the observed phenomenon, which assumes the formation of additional pathways for hydrogen and methane production in the flames of ternary fuel mixtures. Comparison of the measured and simulated flame structures shows that all the mechanisms satisfactorily predict the mole fraction profiles of the reactants, products and some intermediates at atmospheric and elevated pressures. It is noteworthy that the mechanisms adequately predict the spatial variations in the mole fractions of free radicals, including the H, OH and CH 3 radicals, within the pressure range. However, some drawbacks of the mechanisms used have been identified. The mechanisms were shown to overpredict the mole fractions of some unsaturated hydrocarbons, including ethylene and acetylene, at elevated pressures. Therefore, the rate constants of the crucial reactions responsible for production/consumption of these species, as well as their pressure dependences, should be specified, and the mechanisms should be refined. To provide a deeper insight into the combustion chemistry of ternary fuel mixtures, one should focus on the structure of rich flames.
Flame-ion probe of the reaction zone in a CH 4 –O 2 –Ar flame with added HCN, NH 3 and NO
Canadian Journal of Chemistry, 1981
The addition of 0.3% of the fuel-nitrogen (fuel-N) compounds HCN, NH3, or NO to a premixed, fuel-rich, CH4–O2–Ar flame burning at atmospheric pressure demonstrated the rapid interconversion of nitrogenous intermediates in the reaction zone. The nitrogenous species (HCN/CN, HNCO/NCO, NH3, NH2, NH, NO, NO2) were observed as ions (CN−, H2CN+, NCO−, H2NCO+, NH4+, NH3+, NH2+, NO+, NO2−, and hydrate ions) formed in chemical ionization processes discussed previously (1). The ions were sampled directly into a flame-ion mass spectrometer which had sufficient spatial resolution for the measurement of ion concentration profiles through the reaction zone. The study bears on Fenimore's suggestion for the formation of "prompt NO" in fuel-rich hydrocarbon flames. These additive results were compared with previous results involving nitrogenous species present in a similar CH4–O2 flame doped with 10% N2. The increased sensitivity of the additive approach confirmed many of the mass assi...
Kinetics and mechanism of chemical reactions in the H2/O2/N2 flame at atmospheric pressure
Kinetics and Catalysis, 2009
The kinetics and mechanism of chemical reactions in the H 2 /O 2 /N 2 flame were studied experimentally and by simulating the structure of premixed laminar flat atmospheric H 2 /O 2 /N 2 flames of different initial compositions. The concentration profiles for stable compounds (H 2 , O 2 , and H 2 O), H atoms, and OH • radicals in flames were measured by molecular-beam sampling mass spectrometry using soft electron-impact ionization. The experimental data thus obtained are in good agreement with the results of simulations in terms of three familiar kinetic mechanisms, suggesting that these mechanisms are applicable to the description of the flame structure in hydrogen-oxygen mixtures at atmospheric pressure.
Kinetics of the overall reaction CH 3 + OH (1) were studied close to the high-pressure limit using the laser flash photolysis/transient UV absorption method (LFP/TAS) and in the fall-off regime with discharge flow/far infrared laser magnetic resonance (DF/LMR) at 298 K and 473 K, respectively. The product channel 1 CH 2 + H 2 O (1.1) was also studied with the DF/LMR method. The following rate constants and branching ratio were determined (in He): k 1 (1463 mbar, 298 K) ≥ 6.2 10 13 cm 3 mol-1 s-1 , k 1 (1.16 mbar, 473 K) ≥ 5.2 10 13 cm 3 mol-1 s-1 and k 1.1 / k 1 > 0.7 (1.16 mbar, 473 K). Flame velocity for a standard CH 4-air flame was calculated in relation to the kinetics results.
Investigation of NO Production and Flame Structures in Ammonia-Hydrogen Flames
2023
Ammonia/hydrogen fuel blends have recently emerged as a promising solution to the de-carbonization of the energy and transport sectors. However, concerns over performance and, more importantly, NOx emissions have impeded their progress so far. Before effective NOx mitigation strategies can be developed, the fundamental chemical mechanisms involved in NOx production in NH3/H2 flames must be well understood. Although NOx formation in hydrocarbons and the oxidative processes involved in NH3 combustion have been well studied, there is a significant lack of such information for NH3/H2 flames. Key insights on NO formation mechanisms and flame structures for NH3/H2 mixtures are required to develop and improve chemical kinetic models. In this work, laminar Bunsen NH3/H2 flames with 65/35 NH3/H2 volume fraction were tested at two equivalence ratios (rich and lean). For all test conditions, the adiabatic flame temperature was kept constant. Measurements of simultaneous OH/NO-PLIF and OH-PLIF/chemiluminescence (of NH*, NH2* and OH*) were conducted and compared to computational results of four reaction mechanisms (available in literature) applicable to NH3/H2 flames. The maximum OH-PLIF signal gradient was used as a spatial reference point for each simultaneous measurement and one-dimensional line profiles were determined for each species of interest. The simultaneous OH/NO-PLIF images show that the NO signal intensity in the NH3/H2 flames were up to 100 times more than a CH4 reference flame. OH* and NH* chemiluminescence results showed a good spatial correlation with the maximum OH-PLIF signal gradient for both test conditions. NH* also showed a positive correlation with the computed HRR values from lean to rich, indicating it is a promising candidate for direct HRR measurement but warrants further investigation over a wide range of equivalence and mixture ratios. The results also indicate that all the mechanisms underestimate the profile widths of the measured species. Similarly, the experimental results showed a much higher relative increase in NH2 production from lean to rich compared to the computed profiles. The data analysis approach employed in this paper, based on simultaneous measurements, could be further used for optimizing chemical kinetics mechanisms for NH3/H2 flames.
Chemical Engineering Research & Design, 2018
The flame propagation in nitrogen-diluted CH 4-N 2 O mixtures was monitored by pressure measurements during explosions in a spherical vessel with central ignition. The burning velocities were obtained from experimental measurements of pressure variation during closed vessel explosions and from the detailed modelling of free laminar premixed flames. Lean-and stoichiometric methane-nitrous oxide mixtures (equivalence ratios: 0.8 and 1.0) diluted by various amounts of nitrogen between 40 and 60 vol% were studied at various initial pressures between 0.3 and 1.8 bar and ambient initial temperature. Nitrogen addition to each CH 4-N 2 O mixture results in the decrease of laminar burning velocity and flame temperature, along with the increase of flame width. Examination of burning velocity variation against average flame temperature in experiments at constant initial pressure and various inert concentrations allowed the determination of the overall activation energy; examination of burning velocity dependence on pressure, at constant inert concentration, allowed the determination of the overall reaction orders. For all CH 4-N 2 ON 2 mixtures, the temperature, volumetric rate of heat release and reactive species concentration profiles across the flame front were examined versus similar data characteristic to stoichiometric methane-air mixtures. The most important elementary reactions in CH 4-N 2 ON 2 and CH 4-air mixtures were identified by means of sensitivity analysis.
Effect of O2(a1Δg) on the low-temperature mechanism of CH4 oxidation
Combustion and Flame, 2013
The effect of electronically excited oxygen O 2 (a 1 D g ) on the combustion of methane in air at low temperatures is investigated. Sensitivity and rate of production analysis indicated that reactions CH 3 + O 2 (a 1 D g ) M CH 3 O 2 (A 0 ); CH 2 O + OH; CH 3 O + O are important for low-temperature methane oxidation. The potential energy surface of the reactions was investigated at the multi-reference configuration interaction level of theory. It was found that the association reaction corresponding to the channel of excited CH 3 O 2 (A 0 ) formation has a threshold with an energy of 8.5 kcal/mole. The rate parameter of the reaction CH 3 + O 2 (a 1 D g ) ? CH 3 O 2 (A 0 ) was refined and makes k ¼ 2:76 Â 10 À7 Â T À2:71 Â exp À 8:61 kcal=mole