An experimental and kinetic modeling study of premixed NH3/CH4/O2/Ar flames at low pressure

An experimental and modeling study of 11 premixed NH₃/CH₄/O₂/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 NH₃/CH₄ (R0.0, R0.1, R0.5, R0.9 and R1.0) are modeled using an updated detailed reaction mechanism for oxidation of CH₄/NH₃ mixtures. With R increasing, the reaction zone is widened, and the mole fractions of H₂O, NO and N₂ increase while those of H₂, CO, CO₂ and NO₂ 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 CH₄ and NH₃ oxidation and their mutual interaction.     

Experimental and numerical study of the role of NCN in prompt-NO formation in low-pressure CH4-O2-N2 and C2H2-O2-N2 flames

We report an experimental and modeling study on prompt-NO formation in low-pressure (5.3 kPa) premixed flames. Special emphasis is given to the quantitative detection (and prediction) of NCN, whose role in prompt-NO formation has recently been confirmed in alkane flames. Here a rich (Φ = 1.25) CH₄-O₂-N₂ flame and rich (Φ = 1.25) and stoichiometric C₂H₂-O₂-N₂ flames have been investigated. Absolute concentration profiles of CH and NCN radicals and NO species are obtained by combining laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRDS). Temperature profile is determined in each flame using OH and NO-LIF thermometry. Flame modeling is performed to determine the role of NCN in prompt-NO formation and to test the capacity of the present chemical mechanisms to predict some intermediate species involved in prompt-NO formation. The methane flame is modeled using GDFkin®3.0_NCN mechanism [El Bakali et al., Fuel 85 (2006), 896-909]. The acetylene flames are modeled using the Lindstedt and Skevis C/H/O mechanism [Lindstedt and Skevis, Proc. Combust. Inst. 28 (2000), 1801-1807], completed by the submechanism issued from GDFkin®3.0_NCN for nitrogen chemistry. This submechanism includes the initiation reaction CH + N₂ = NCN + H. Rate constants of NO-sensitive reactions of the submechanism are modified by taking into account the recent literature. In particular, the C₂O route could be explored thanks to a significant presence of C₂O in acetylene flames. Globally, the modified submechanism of nitrogen chemistry coupled with the two hydrocarbon mechanisms leads to a satisfying prediction of NCN and NO mole fraction profiles, even though refinements of rate constant determination is still required. The role of NCN in prompt-NO formation in acetylene flames is demonstrated.     

Studies of C4 and C10 methyl ester flames

The oxidation of three model biodiesel fuels, namely methyl butanoate (C₅H₁₀O₂, CAS No. 623-42-7), methyl crotonate (C₅H₈O₂, CAS No. 623-43-8), and methyl decanoate (C₁₁H₂₂O₂, CAS No. 110-42-9) was investigated in laminar premixed and non-premixed flames. The experiments were conducted in the counterflow configuration at atmospheric pressure, for a wide range of equivalence or inert-dilution ratios, and elevated reactant temperatures. Laminar flame speeds and local extinction strain rates were determined by measuring the flow velocities using digital particle image velocimetry. The experimental data were compared against those derived for flames of n-alkanes of similar carbon number, in order to assess the effects of saturation, the length of carbon chain, and the presence of the ester group. Several recent chemical kinetic models were tested against the experimental data, and major differences were identified and assessed. The accuracy of the Lennard–Jones potential parameters assigned to the methyl esters in the transport databases of the different models was evaluated and new values were estimated. Insight was provided into the high-temperature kinetic pathways of methyl esters in flame environments. Additionally, the reduced sooting propensity of methyl ester flames compared to n-alkane flames was investigated computationally.     

Structure of atmospheric-pressure fuel-rich premixed ethylene flame with and without ethanol

Effect of ethanol (EtOH) addition to unburnt gas mixture on the species pool in a fuel-rich flat, premixed, laminar ethylene flame at atmospheric pressure is studied experimentally and by chemical kinetic modeling. Mole fraction profiles as a function of height above burner of various stable and labile species including reactants, major products and intermediates (C₁–C₄ hydrocarbons) are measured using molecular beam mass spectrometry with electron ionization in C₂H₄/O₂/Ar and C₂H₄/EtOH/O₂/Ar flames. The experimental profiles are compared with those calculated using three different chemical kinetic mechanisms. Performances and deficiencies of the mechanisms are discussed. An analysis of the mechanisms is carried out in order to identify the reason of the ethanol effect on the mole fraction of propargyl, the main precursor of benzene. A modification of some mechanisms in order to improve their capability to predict acetylene and diacetylene mole fraction profiles is proposed.     

Burning velocities and kinetics of H2/NF3/N2, CH4/NF3/N2, and C3H8/NF3/N2 flames

The combustion characteristics and reaction mechanism of mixtures containing nitrogen trifluoride (NF₃) were investigated. Burning velocities for H₂/NF₃/N₂, CH₄/NF₃/N₂, and C₃H₈/NF₃/N₂ flames were determined for the first time at various equivalence ratios and N₂ mole fractions. The burning velocities of the latter two flames were similar and showed peaks at equivalence ratios of ∼1.0, while those of the H₂/NF₃/N₂ flames had the pronounced peak at low equivalence ratios where the formation of the wrinkled flames was observed. A detailed kinetic model was constructed to simulate the laminar burning velocities of H₂/NF₃/N₂ and CH₄/NF₃/N₂ flames. The model accurately reproduced the experimental results. Analyses of the reaction mechanism revealed the major reaction pathways that involve the decomposition of NF₃, the oxidation and chain-fluoridation of H₂ and CH₄, and the formation of N₂.     

Shock tube and kinetic study of C2H6/H2/O2/Ar mixtures at elevated pressures

Using a high-pressure shock tube facility, the ignition delay times of stoichiometric C₂H₆/H₂/O₂ diluted in argon were obtained behind reflected shock wave at elevated pressures (p = 1.2, 4.0 and 16.0 atm) with ethane blending ratios from 0 to 100%. The measured ignition delay times were compared to the previous correlations, and the results show that the ignition delay times of ethane from different studies exhibit an obvious difference. Meanwhile, numerical studies were conducted with three generally accepted kinetic mechanisms and the results show that only NUIG Aramco Mech 1.3 agrees well with the measurements under all test conditions. Sensitivity analysis was made to interpret the poor prediction of the other two mechanisms. Furthermore, the effect of ethane blending ratio on the ignition delay times of the mixtures was analyzed and the results show that ethane blending ratio gives a non-linear effect on the auto-ignition of hydrogen. Finally, chemical interpretations on this non-linear effect were made from the reaction pathway analysis and normalized H radical consumption analysis.     

Experimental and numerical investigation of low-pressure laminar premixed synthetic natural gas/O2/N2 and natural gas/H2/O2/N2 flames

The oxidation of laminar premixed natural gas flames has been studied experimentally and computationally with variable mole fractions of hydrogen (0, 20, and 60%) present in the fuel mixture. All flames were operated at low pressure (0.079 atm) and at variable overall equivalence ratios (0.74<?<1.0) with constant cold gas velocity. At the same global equivalence ratio, there is no significant effect of the replacement of natural gas by 20% of H₂. The small differences recorded for the intermediate species and combustion products are directly due to the decrease of the amount of initial carbon. However, in 60% H₂ flame, the reduction of hydrocarbon species is due both to kinetic effects and to the decrease of initial carbon mole fraction. The investigation of natural gas and natural gas/hydrogen flames at similar C/O enabled identification of the real effects of hydrogen. It was shown that the presence of hydrogen under lean conditions activated the H-abstraction reactions with H atoms rather than OH and O, as is customary in rich flames of neat hydrocarbons. It was also demonstrated that the presence of H₂ favors CO formation.     

Quantitative HCN measurements in CH4/N2O/O2/N2 flames using mid-infrared polarization spectroscopy

Quantitative measurements of hydrogen cyanide (HCN) were nonintrusively performed using mid-infrared polarization spectroscopy (IRPS) in atmospheric pressure flames. The lifted flat, laminar, premixed CH₄/N₂O/O₂/N₂ flames were stabilized on a 7 cm diameter home-built McKenna-type burner with variable proportion of N₂O and O₂. The characteristic spectral structure of HCN molecules was identified in the rotational line-resolved IRPS spectra collected in flames at around 3248 cm⁻¹. The P20 line belonging to the CH stretching band was chosen for quantitative measurements and the line-integrated IRPS signal was recorded in a series of fuel-rich CH₄/O₂/N₂O/N₂ flames with equivalence ratios of 1.2, 1.4 and 1.6. Absolute mole fractions of HCN molecules in these flames were obtained through in situ calibration of the optical system with nonreactive gas flow of N₂ seeded with known amount of HCN on the same burner. Moreover, the experimental results were compared with calculations performed using the Konnov detailed C/H/N/O mechanism, which implements NCN prompt-NO reactions. Generally good agreement was found with some discrepancies indicating the need for further model improvement.     

Laminar burning velocity and interchangeability analysis of biogas/C3H8/H2 with normal and oxygen-enriched air

Numerical and experimental measurements of the laminar burning velocities of biogas (66% CH₄ – 34% CO₂) and a biogas/propane/hydrogen mixture (50% biogas – 40% C₃H₈ – 10% H₂) were made with normal and oxygen-enriched air while varying the air/fuel ratio. GRI-Mech 3.0 and C₁–C₃ reaction mechanisms were used to perform numerical simulations. Schlieren images of laminar premixed flames were used to determine laminar burning velocities at 25 °C and 849 mbar. The mixture's laminar burning velocity was found to be higher to that of pure biogas due to the addition of propane and hydrogen. An increase in the laminar burning velocities of both fuels is reported by enriching air with oxygen, a phenomenon that is explained by the increased reactivity of the mixture. Additionally, an analysis of interchangeability based on both the Wobbe Index and the laminar burning velocity between methane and a biogas/propane/hydrogen mixture is presented in order to consider this mixture as a substitute for natural gas. It was found that the variations of these properties between the fuels did not exceed 10%, enabling interchangeability.     

Onset of cellular instability in adiabatic H2/O2/N2 premixed flames anchored to a flat-flame heat-flux burner

The onset of cellular instability in adiabatic H₂/O₂/N₂ premixed flames anchored to a heat-flux burner is investigated numerically. Both hydrodynamic instability and diffusional-thermal instability are shown to play an important role in the onset of cellular flames. The burner can effectively suppress cellular instability when the flames are close to the burner, otherwise the burner can suppress the instabilities only at large wavenumbers. Because of differential diffusion, local extinction can occur in lean H₂/O₂/N₂ flames. When the flames develop to take on cellular shapes, the surface length, the overall heat release rate and the mean burning velocity are all increased. For near stoichiometric fuel-rich flames the mean burning velocity can increase by as much as 20%–30%. For lean flames with an equivalence ratio of 0.56, the mean burning velocity can be 2–3 times of the burning velocity of the corresponding planar flame.     

The effects of hydrogen addition on Fenimore NO formation in low-pressure,fuel-rich-premixed,burner-stabilized CH4/O2/N2 flames

We investigate the effects of hydrogen addition on Fenimore NO formation in fuel-rich, low-pressure burner-stabilized CH₄/O₂/N₂ flames. Towards this end, axial profiles of temperature and mole fractions of CH and NO are measured using laser-induced fluorescence (LIF). The experiments are performed at equivalent ratios of 1.3 and 1.5, using 0.25 mole fraction of hydrogen in the fuel, while varying the mass flux through the burner. The results are compared with those reported previously for burner-stabilized CH₄/O₂/N₂ flames. The increased burning velocity caused by hydrogen addition is seen to result in a lower flame temperature as compared to methane flame stabilized at the same mass flux. This increase in burner stabilization upon hydrogen addition results in significantly lower CH mole fractions at φ = 1.3, but appears to have little effect on the CH profile at φ = 1.5. In addition, the results show that not only the maximum flame temperature is reduced upon hydrogen addition, but the local gas temperature in the region of the CH profile is lowered as well. The measured NO mole fractions are seen to decrease substantially for both equivalence ratios. Analysis of the factors responsible for Fenimore NO formation shows the reduction in temperature in the flame front to be the major factor in the decrease in NO mole fraction, with a significant contribution from the decrease in CH mole fraction at φ = 1.3. At φ = 1.5, the results suggest that the lower flame temperature upon hydrogen addition further retards the conversion of residual fixed-nitrogen species to NO under these rich conditions as compared to the equivalent methane flames.     

The influence of ethanol addition on a rich premixed benzene flame at low pressure

The aim of the study is to analyze the effect of ethanol in rich benzene flame, to observe the influence of this oxygenated species and to understand the kinetics of ethanol in the benzene combustion. Two premixed rich benzene/oxygen/argon (11.5% C₆H₆, 43.2% O₂, 45.3% Ar) and benzene/ethanol/oxygen/argon (10.7% C₆H₆, 2.1% C₂H₅OH, 43.2% O₂, 44.0% Ar) flat flames are stabilized at low pressure (45 mbar) on a burner with the same equivalence ratio of 2.0. Identification and monitoring of signal intensity profiles of species within the flames are carried out using molecular beam mass spectrometry (M.B.M.S.). The substitution of some C₆H₆ by C₂H₅OH is responsible for a reduction of the maximum concentrations of main intermediate species such as C₂H₂, C₄H₂, C₄H₄ and C₅H₆. The UCL mechanism is extended to heavier hydrocarbons, tested against these flames to check its validity and used to underline the effect of ethanol on soot precursors formation. It contains 1028 elementary reactions and 184 chemical species.     

Study on stretch extinction limits of CH4/CO2 versus high temperature O2/CO2 counterflow non-premixed flames

Extinction limits of counterflow non-premixed flames with normal and high temperature oxidizers were studied experimentally and numerically for development of new-type oxygen-enriched mild combustion furnace. Extinction stretch rates of CH₄/CO₂ (at 300 K) versus O₂/CO₂ flames at oxygen mole fractions of 0.35 and 0.40 and oxidizer temperatures of 300 K, 500 K, 700 K and 1000 K were obtained. Investigation was also conducted for CH₄/N₂ (at 300 K) versus air (O₂/N₂) flames at the same oxidizer temperatures. An effect of radiative heat loss on stretch extinction limits of oxygen-enriched flames and air flames was investigated by computations with optical thin model (OTM) and adiabatic flame model (ADI). The results show influence of radiative heat loss on stretch extinction limits was not significant in relative high fuel mole fraction regions. The extinction curve of the oxygen-enriched flames with oxygen mole fraction of 0.35 was close to that of the air flames at the oxidizer temperature of 300 K. However, the extinction curve of air flames with high temperature oxidizer was comparable with that of oxygen-enriched flames with oxygen mole fraction of 0.40. Scaling analysis based on asymptotic solution of stretch extinction was applied and it was found that stretch extinction limits can be expressed by two terms. The first term is total enthalpy flux of fuel stream based on thermo-physical parameters. The second term is a kinetic term which reflects an effect of the chemical reaction rate on stretch extinction limits. OH radicals which play important roles in chain propagating and main endothermic reactions were used to represent the kinetic term of both oxygen-enriched and air flames. The global rates of OH formation in these two cases were compared to understand the contribution of kinetic term to stretch extinction limits. Variation of extinction curves of oxygen-enriched flames and air flames was well explained by the present scaling analysis. This offers an effective approach to estimate stretch extinction limits of oxygen-enriched flames based on those of air flames at the same oxidizer temperature.     

Measurement of propagation speeds in adiabatic flat and cellular premixed flames of C2H6+O2+CO2

Adiabatic burning velocities of premixed flat flames and propagation speeds of adiabatic cellular flames of mixtures of ethane+oxygen+carbon dioxide are reported. The oxygen content O₂/(O₂+CO₂) in the artificial air was varied from 26 to 35%. Nonstretched flames were stabilized on a perforated plate burner at 1 atm. A heat flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Under specific experimental conditions the flames become cellular; this leads to significant modification of the flame propagation speed. Measurements in cellular flames are presented and compared with those for laminar flat flames and also with qualitative predictions of a theoretical model. The onset of cellularity was observed throughout the stoichiometric range of the mixtures studied. Cellularity disappears when the flames become only slightly subadiabatic.     

Numerical study on CH4 laminar premixed flames for combustion characteristics in the oxidant atmospheres of N2/CO2/H2O/Ar-O2

The freely-propagating laminar premixed flames of CH₄–N₂/CO₂/H₂O/Ar-O₂ mixtures were conducted with the PREMIX code. The effects of the equivalence ratio and various oxidant atmospheres on the basic combustion characteristics were analyzed with the initial pressure and temperature of 1 atm and 398 K, respectively, O₂ content in the oxidant of 21%. The chemical reaction mechanism GRI-Mech 3.0 was chosen to determine the effects of the oxidant atmospheres of N₂/O₂, CO₂/O₂, H₂O/O₂, and Ar/O₂ on the adiabatic flame temperature, laminar burning velocity, flame structure, free radicals, intermediate species, net heat release rate and specific heat of the fuel/oxidant mixtures. The numerical results show that the maximum adiabatic flame temperatures and laminar burning velocities are at Ar/O₂ atmosphere. The mole fractions of CO and H₂ increased fastest at CO₂/O₂ atmosphere and H₂O/O₂, respectively. The mole fractions of CH₃ and H follow the order Ar/O₂> N₂/O₂>H₂O/O₂>CO₂/O₂. In addition, for 4 oxidant atmospheres, the peak mole fraction of C₂H₂ is following the order H₂O/O₂>Ar/O₂>N₂/O₂>CO₂/O₂ and the net heat release rate is following the order Ar/O₂>N₂/O₂>H₂O/O₂>CO₂/O₂ for all equivalence ratios.     

Effect of the equivalence ratio on the concentration of CH4/NO2/O2 combustion products

A chemical kinetic model for determining the mole fractions of stable and intermediate species for CH₄/NO₂/O₂ flames is developed. The model involves 30 different species in 101 chemical elementary reactions. The mole fractions of the species are plotted as a function of the distance from the surface of the burner. The effects of the equivalence ratio on the concentrations of CO, CO₂, N₂, NH₂, OH, H₂O, NO and NO₂ for lean CH₄/NO₂/O₂ flames in the post flame zone at 50 Torr are obtained. The flames are flat, laminar, one dimensional and premixed. The calculated concentration profiles as a function of the equivalence ratio and distance from the surface of the burner are compared with the experimental data. The comparison indicates that the kinetics of the flames are reasonably described by the developed model. The mole fraction of N₂, NH₂, OH, H₂O, CO₂ and CO increase while the mole fractions of NO and NO₂ decrease by increasing the equivalence ratio for lean flames. Copyright © 1999 John Wiley & Sons, Ltd.     

Effects of hydrogen addition on soot formation and oxidation in laminar premixed C2H2/air flames

In an effort to elucidate the influence of hydrogen addition on soot formation and oxidation, a series of numerical investigations was performed for fuel rich laminar C₂H₂/air premixed flat flames using a modified CHEMKIN-II PREMIX code with a detailed soot chemistry mechanism. To clarify the influence of hydrogen addition, the hydrogen content (in volume %) in the fuel mixture was gradually increased from 10 to 50%. The hydrogen addition was found to slow the oxidation of C₂H₂ near the burner surface. The lowered rate of C₂H₂ oxidation coupled with lower C₂H₂ concentration near the burner surface impedes the formation of benzene. However, the formation of benzene was enhanced with the hydrogen addition as the height above burner (HAB) was increased. This was due to the increased reverse rate of the H abstraction reaction that prevents the radical formation process. Through the identical mechanism, the hydrogen addition slows further growth of benzene to larger polycyclic aromatic hydrocarbons (PAHs), eventually lowering the rate of particle inception. Numerical results also indicated that reductions in the soot emissions were mainly attributed to a significant reduction in the mass growth of soot particles. The abundance of hydrogen in the flames deactivated the surface site of soot particles covered with C-H bonds, lowering the surface growth rate (which leads to reductions in the mass growth of soot particles).     

Explosion behavior of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures

In this work, the explosion behavior of stoichiometric CH₄/O₂/N₂/CO₂ and H₂/O₂/N₂/CO₂ mixtures has been studied both experimentally and theoretically at different CO₂ contents and oxygen air enrichment factors. Peak pressure, maximum rate of pressure rise and laminar burning velocity were measured from pressure time records of explosions occurring in a closed cylindrical vessel. The laminar burning velocity was also computed through CHEMKIN–PREMIX simulations.     

Vibrational spectra of Ti:C2H4(nH2) and Ti:C2H4(nD2) (n = 1-5) complexes and the equilibrium isotope effect: Calculations and experiment

Calculations of the ability of titanium-ethylene complexes of the type, Ti:C₂H₄, to absorb molecular hydrogen have been performed using density functional theory. A maximum of 5H₂ molecules can be adsorbed on Ti:C₂H₄ thereby giving an uptake capacity of 11.72 wt%, in excellent agreement with previous experimental results reported by two of us (Phys. Rev. Lett., 100, 105505, 2008). Calculations of the vibrational frequencies in such complexes with both H₂ and D₂, Ti:C₂H₄(nH₂) and Ti:C₂H₄(nD₂), n = 1-5, have also been performed and the values obtained used to find the Equilibrium Isotope Effect (EIE). Measurements of the EIE are also reported and these are in excellent agreement with the EIE calculated for 5H₂ molecules adsorbed in the complex.     

Production of H2 and C2 hydrocarbons from methane in a proton conducting solid electrolyte cell using a Au–5Ce–5Na2WO4/SiO2 anode

A novel approach of upgrading methane towards the simultaneous production/separation of H₂ and C₂ hydrocarbons (ethane and ethylene), is developed. The reaction system was studied in a solid state proton (H⁺) conducting cell. Mixtures of methane, steam (and oxygen) were introduced over the anode, while an inert gas flowed over the cathode. Under open-circuit, the reacting mixture produced H₂, C₂H₆, C₂H₄, CO and CO₂. Under closed-circuit and when protons (H⁺) were electrochemically “pumped” from the anode to the cathode, a considerable increase in the production of H₂ was observed while the production of C₂ compounds remained essentially unaffected.