The role of N2O and NNH in the formation of NO via HCN in hydrocarbon flames

A new way of forming HCN in flames via N₂O and NNH reacting with CH_i radicals is proposed and tested for rich and lean gaseous premixed flames of CH₄ and air and also of CH₄, N₂O and Ar. This new route is thermodynamically more probable than Fenimore’s direct reaction of N₂ with CH_i radicals. In fact, it is shown that the new mechanism is more important than Fenimore’s reaction in both rich and lean flames. Rate constants of the new reactions forming NO have been suggested on the basis of numerical modeling. It has been shown that the formation of NO through HCN is most effective as the result of reactions initiated by N₂O + CH₃ → CH₂NH + NO, followed by CH₂NH + H → H₂CN + H₂ and CH₂NH + O → H₂CN + OH. In flames of CH₄ and air, a substantial source of N₂O comes from the reverse of the reaction N₂O + CH₃ → CH₃O + N₂ in the reaction zone. A formula based on the steady state assumption and partial equilibrium limits the number of nitrogen conversion reactions to only 12; this was tested using a premixed flame of CH₄ and air.     

Turbulence and combustion interaction: High resolution local flame front structure visualization using simultaneous single-shot PLIF imaging of CH, OH, and CH2O in a piloted premixed jet flame

High resolution planar laser-induced fluorescence (PLIF) was applied to investigate the local flame front structures of turbulent premixed methane/air jet flames in order to reveal details about turbulence and flame interaction. The targeted turbulent flames were generated on a specially designed coaxial jet burner, in which low speed stoichiometric gas mixture was fed through the outer large tube to provide a laminar pilot flame for stabilization of the high speed jet flame issued through the small inner tube. By varying the inner tube flow speed and keeping the mixture composition as that of the outer tube, different flames were obtained covering both the laminar and turbulent flame regimes with different turbulent intensities. Simultaneous CH/CH₂O, and also OH PLIF images were recorded to characterize the influence of turbulence eddies on the reaction zone structure, with a spatial resolution of about 40 μm and temporal resolution of around 10 ns. Under all experimental conditions, the CH radicals were found to exist only in a thin layer; the CH₂O were found in the inner flame whereas the OH radicals were seen in the outer flame with the thin CH layer separating the OH and CH₂O layers. The outer OH layer is thick and it corresponds to the oxidation zone and post-flame zone; the CH₂O layer is thin in laminar flows; it becomes broad at high speed turbulent flow conditions. This phenomenon was analyzed using chemical kinetic calculations and eddy/flame interaction theory. It appears that under high turbulence intensity conditions, the small eddies in the preheat zone can transport species such as CH₂O from the reaction zones to the preheat zone. The CH₂O species are not consumed in the preheat zone due to the absence of H, O, and OH radicals by which CH₂O is to be oxidized. The CH radicals cannot exist in the preheat zone due to the rapid reactions of this species with O₂ and CO₂ in the inner-layer of the reaction zones. The local PLIF intensities were evaluated using an area integrated PLIF signal. Substantial increase of the CH₂O signal and decrease of CH signal was observed as the jet velocity increases. These observations raise new challenges to the current flamelet type models.     

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.     

Comparative study on the effect of CO2 and H2O dilution on laminar burning characteristics of CO/H2/air mixtures

A study on the effect of CO₂ and H₂O dilution on the laminar burning characteristics of CO/H₂/air mixtures was conducted at elevated pressures using spherically expanding flames and CHEMKIN package. Experimental conditions for the CO₂ and H₂O diluted CO/H₂/air/mixtures of hydrogen fraction in syngas from 0.2 to 0.8 are the pressures from 0.1 to 0.3 MPa, initial temperature of 373 K, with CO₂ or H₂O dilution ratios from 0 to 0.15. Laminar burning velocities of the CO₂ and H₂O diluted CO/H₂/air/mixtures were measured and calculated using the mechanism of Davis et al. and the mechanism of Li et al. Results show that the discrepancy exists between the measured values and the simulated ones using both Davis and Li mechanisms. The discrepancy shows different trends under CO₂ and H₂O dilution. Chemical kinetics analysis indicates that the elementary reaction corresponding to peak ROP of OH consumption for mixtures with CO/H₂ ratio of 20/80 changes from reaction R3 (OH + H₂ = H + H₂O) to R16 (HO₂+H = OH + OH) when CO₂ and H₂O are added. Sensitivity analysis was conducted to find out the dominant reaction when CO₂ and H₂O are added. Laminar burning velocities and kinetics analysis indicate that CO₂ has a stronger chemical effect than H₂O. The intrinsic flame instability is promoted at atmospheric pressure and is suppressed at elevated pressure for the CO₂ and H₂O diluted mixtures. This phenomenon was interpreted with the parameters of the effective Lewis number, thermal expansion ratio, flame thickness and linear theory.     

Role of CO2 in the CH4 oxidation and H2 formation during fuel-rich combustion in O2/CO2 environments

The effect of CO₂ reactivity on CH₄ oxidation and H₂ formation in fuel-rich O₂/CO₂ combustion where the concentrations of reactants were high was studied by a CH₄ flat flame experiment, detailed chemical analysis, and a pulverized coal combustion experiment. In the CH₄ flat flame experiment, the residual CH₄ and formed H₂ in fuel-rich O₂/CO₂ combustion were significantly lower than those formed in air combustion, whereas the amount of CO formed in fuel-rich O₂/CO₂ combustion was noticeably higher than that in air. In addition to this experiment, calculations were performed using CHEMKIN-PRO. They generally agreed with the experimental results and showed that CO₂ reactivity, mainly expressed by the reaction CO₂ + H → CO + OH (R1), caused the differences between air and O₂/CO₂ combustion under fuel-rich condition. R1 was able to advance without oxygen. And, OH radicals were more active than H radicals in the hydrocarbon oxidation in the specific temperature range. It was shown that the role of CO₂ was to advance CH₄ oxidation during fuel-rich O₂/CO₂ combustion. Under fuel-rich combustion, H₂ was mainly produced when the hydrocarbon reacted with H radicals. However, the hydrocarbon also reacted with the OH radicals, leading to H₂O production. In fact, these hydrocarbon reactions were competitive. With increasing H/OH ratio, H₂ formed more easily; however, CO₂ reactivity reduced the H/OH ratio by converting H to OH. Moreover, the OH radicals reacted with H₂, whereas the H radicals did not reduce H₂. It was shown that OH radicals formed by CO₂ reactivity were not suitable for H₂ formation. As for pulverized coal combustion, the tendencies of CH₄, CO, and H₂ formation in pulverized coal combustion were almost the same as those in the CH₄ flat flame.     

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.     

NOx formation and reduction mechanisms in staged O2/CO2 combustion

The purpose of this study was to investigate the NO_x formation and reduction mechanisms in staged O₂/CO₂ combustion and in air combustion. A flat CH₄ flame doped with NH₃ for fuel-N was formed over the honeycomb, and NO_x formation characteristics were investigated. In addition, chemiluminescence of OH^* distribution was measured, and CHEMKIN-PRO was used to investigate the detailed NO_x reduction mechanism. In general, the NO_x conversion ratio decreases with decreasing primary O₂/CH₄ ratio, whereas NH₃ and HCN, which are easily converted to NO_x in the presence of O₂, increases rapidly. Therefore, a suitable primary O₂/CH₄ ratio exists in the staged combustion. Our experiments showed the primary O₂/CH₄ ratio, which gave the minimum fixed nitrogen compounds in O₂/CO₂ combustion, was lower than in air combustion. The NO_x conversion ratio in O₂/CO₂ combustion was lower than in air combustion by 40% in suitable staged combustion. This could be explained by high CO₂ concentrations in the O₂/CO₂ combustion. It was shown that abundant OH radicals were formed in O₂/CO₂ combustion through the CO₂ + H → CO + OH, experimentally and numerically. OH radicals produced H and O radicals through H₂ + OH → H + H₂O and O₂ + H → OH + O, because a mass of hydrogen source exists in the CH₄ flame. O and OH radicals formed in the fuel-rich region enhanced the oxidation of NH₃ and HCN. NO_x formed by the oxidation of NH₃ and HCN was converted to N₂ because the oxidation occurred in the fuel-rich region where the NO_x reduction effect was high. In fact, the oxidation of NH₃ and HCN in the fuel-rich region was preferable to remaining NH₃ and HCN before secondary O₂ injection in the staged combustion. A significant reduction in NO_x emission could be achieved by staged combustion in O₂/CO₂ combustion.     

Terahertz time-domain spectroscopy of high-pressure flames

Laser spectroscopy in the visible and near infrared is widely used as a diagnostic tool for combustion devices, but this approach is difficult at high pressures within a sooty flame itself. High soot concentrations render flames opaque to visible light, but they remain transparent to far-infrared or terahertz (THz) radiation. The first far-infrared absorption spectra, to the best of our knowledge, of sooty, non-premixed, ethylene high-pressure flames covering the region of 0.2–2.5 THz is presented. A specially designed high-pressure burner which is optically accessible to THz radiation has been built allowing flame transmission measurements up to pressures of 1.6 MPa. Calculations of the theoretical combustion species absorption spectra in the 0.2–3 THz range have shown that almost all the observable features arise from H₂O. A few OH (1.84 and 2.51 THz), CH (2.58 THz), and NH₃ (1.77 and 2.95 THz) absorption lines are also observable in principle. A large number of H₂O absorption lines are observed in the ground vibrational in a laminar non-premixed, sooty flame (ethylene) at pressures up to 1.6 MPa.     

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.     

Cu2O/Cu/TiO2 nanotube Ohmic heterojunction arrays with enhanced photocatalytic hydrogen production activity

Cu₂O/Cu/TiO₂ nanotube heterojunction arrays were prepared by assembling Cu@Cu₂O core-shell nanoparticles on TiO₂ nanotube arrays (NTAs) using a facile impregnation-reduction method. SEM and TEM results show that Cu@Cu₂O plate-like nanoparticles with tens of nanometers in size are confined inside TiO₂ NTAs. Only the outmost several nanometers of the nanoparticles are Cu₂O and the predominant inner of the nanoparticles are Cu metals. Cu L₃VV Auger spectra of Cu₂O/Cu/TiO₂ NTAs suggest that Cu metals are enveloped by at least several nanometers Cu₂O on the surface, which further confirms the Cu@Cu₂O core shell structure of Cu nanoparticles. The ability of light absorption of Cu₂O/Cu/TiO₂ NTAs is enhanced. The range of absorption wavelengths changes from 400 to 700 nm due to the surface plasmon response of Cu metals core and Cu₂O nanoparticles shell. The photocatalytic hydrogen production rate of Cu₂O/Cu/TiO₂ heterojunction arrays is enhanced when compared with those of Cu₂O/TiO₂ NTAs and TiO₂ NTAs under UV light. Moreover, a stable H₂ generation property was obtained under visible light (λ gt; 400 nm). The Cu metal core is believed to play a key role in the enhancement of photocatalytic properties of Cu₂O/Cu/TiO₂ nanotube heterojunction arrays.     

Chemical effects of added CO2 on the extinction characteristics of H2/CO/CO2 syngas diffusion flames

Chemical effects of added CO₂ on flame extinction characteristics are numerically studied in H₂/CO syngas diffusion flames diluted with CO₂. The two representative syngas flames of 80% H₂ + 20% CO and 20% H₂ + 80% CO are inspected according to the composition of fuel mixture diluted with CO₂ and global strain rate. Particular concerns are focused on impact of chemical effects of added CO₂ on flame extinction characteristics through the comparison of the flame characteristics between well-burning flames far from extinction limit and flames at extinction. It is seen that chemical effects of added CO₂ reduce critical CO₂ mole fraction at flame extinction and thus extinguish the flame at higher flame temperature irrespective of global strain rate. This is attributed by the suppression of the reaction rate of the principal chain branching reaction through the augmented consumption of H-atom from the reaction CO₂ + H→CO + OH. As a result the overall reaction rate decreases. These chemical effects of added CO₂ are similar in both well-burning flames far from extinction limit and flames at extinction. There is a mismatching in the behaviors between critical CO₂ mole fraction and maximum flame temperature at extinction. This anomalous phenomenon is also discussed in detail.     

Effect of hydrogen on H2/CH4 flame structure of MILD combustion using the LES method

Large eddy simulation (LES) method is employed to investigate the effect of the hydrogen content of fuel on the H₂/CH₄ flame structure under the moderate or intense low-oxygen dilution (MILD) condition. The turbulence–chemistry interaction of the numerically unresolved scales is modelled using the PaSR method, where the full mechanism of GRI-2.11 represents the chemical reactions. The influence of hydrogen concentration on the flame structure is studied using the profiles of temperature, CH₂O and OH mass fractions and the diffusion profiles of un-burnt fuel through the flame front. Furthermore, more details are investigated by contours of OH, HCO and CH₂O radicals in an area near the nozzle exit zone. Results show that increasing the hydrogen content of fuel reinforces the MILD combustion zone and increases the peak value of the flame temperature and OH mass fraction. This increment also increases the flame thickness and reduces the OH oscillations and diffusion of the un-burnt fuel through the flame front.     

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.     

Absorption coefficient of bulk and thin film Cu2O

The optical absorption coefficient of thin film and bulk Cu₂O at room temperature is obtained from an accurate analysis of their transmittance and reflectance spectra. These absorption spectra are modeled, together with the low temperature data reported in the literature, using an analytical expression to assess and quantify the role of the different absorption mechanisms. The results suggest that direct forbidden transitions and indirect transitions play an almost equally relevant role. A table of the optical constants of Cu₂O single crystal is given for reference.     

Computed flammability limits of opposed-jet H2/CO syngas diffusion flames

Extensive computations were made to determine the flammability limits of opposed-jet H₂/CO syngas diffusion flames from high stretched blowoff to low stretched quenching. Results from the U-shape extinction boundaries indicate the minimum hydrogen concentrations for H₂/CO syngas to be combustible are larger towards both ends of high strain and low strain rates. The most flammable strain rate is near one s⁻¹ where syngas diffusion flames exist with minimum 0.002% hydrogen content. The critical oxygen percentage (or limiting oxygen index) below which no diffusion flames could exist for any strain rate was found to be 4.7% for the equal-molar syngas fuels (H₂/CO = 1), and the critical oxygen percentage is lower for syngas mixture with higher hydrogen content. The flammability maps were also constructed with strain rates and pressures or dilution gases percentages as the coordinates. By adding dilution gases such as CO₂, H₂O, and N₂ to make the syngas non-flammable, besides the inert effect from the diluents, the chemical effect of H₂O contributes to higher flame temperature, while the radiation effect of H₂O and CO₂ plays an important role in the flame extinction at low strain rates.     

Investigation of photoelectrochemical characteristics of n-type Cu2O films

n-Cu₂O photoelectrodes are obtained by immersing a copper plate in a CuSO₄(10⁻³ M) and HCl (10⁻³ M) solutions. Samples are characterized with XRD and SEM measurements. It is found that Cu²⁺ ions and the copper substrate are essential to obtain n-type photoresponses from Cu₂O. Photocurrent action spectra are investigated with various Cu₂O amounts formed on the copper substrate, made from two different methods. Photoluminescence (PL) measurements of the samples prepared by the two methods are examined. Time development of the photocurrent is investigated in KI(10⁻² M)+I₂ (10⁻⁴ M) and trisodium citrate solutions.     

CdO/Cu2O solar cells by chemical deposition

CdO and Cu₂O thin films have been grown on glass substrates by chemical deposition method. Optical transmittances of the CdO and Cu₂O thin films have been measured as 60–70% and 3–8%, respectively in 400–900 nm range at room temperature. Bandgaps of the CdO and Cu₂O thin films were calculated as 2.3 and 2.1 eV respectively from the optical transmission curves. The X-ray diffraction spectra showed that films are polycrystalline. Their resistivity, as measured by Van der Pauw method yielded 10⁻²–10⁻³ Ω cm for CdO and approximately 10³ Ω cm for Cu₂O. CdO/Cu₂O solar cells were made by using CdO and Cu₂O thin films. Open circuit voltages and short circuit currents of these solar cells were measured by silver paste contacts and were found to be between 1–8 mV and 1–4 μA.     

Extinction of premixed H2/air flames: Chemical kinetics and molecular diffusion effects

Laminar flame speed has traditionally been used for the partial validation of flame kinetics. In most cases, however, its accurate determination requires extensive data processing and/or extrapolations, thus rendering the measurement of this fundamental flame property indirect. Additionally, the presence of flame front instabilities does not conform to the definition of laminar flame speed. This is the case for Le<1 flames, with the most notable example being ultralean H₂/air flames, which develop cellular structures at low strain rates so that determination of laminar flame speeds for such mixtures is not possible. Thus, this low-temperature regime of H₂ oxidation has not been validated systematically in flames. In the present investigation, an alternative/supplemental approach is proposed that includes the experimental determination of extinction strain rates for these flames, and these rates are compared with the predictions of direct numerical simulations. This approach is meaningful for two reasons: (1) Extinction strain rates can be measured directly, as opposed to laminar flame speeds, and (2) while the unstretched lean H₂/air flames are cellular, the stretched ones are not, thus making comparisons between experiment and simulations meaningful. Such comparisons revealed serious discrepancies between experiments and simulations for ultralean H₂/air flames by using four kinetic mechanisms. Additional studies were conducted for lean and near-stoichiometric H₂/air flames diluted with various amounts of N₂. Similarly to the ultralean flames, significant discrepancies between experimental and predicted extinction strain rates were also found. To identify the possible sources of such discrepancies, the effect of uncertainties on the diffusion coefficients was assessed and an improved treatment of diffusion coefficients was advanced and implemented. Under the conditions considered in this study, the sensitivity of diffusion coefficients to the extinction response was found to be significant and, for certain species, greater than that of the kinetic rate constants.     

An improved H2/O2 mechanism based on recent shock tube/laser absorption measurements

An updated H₂/O₂ reaction mechanism is presented that incorporates recent reaction rate determinations in shock tubes from our laboratory. These experiments used UV and IR laser absorption to monitor species time-histories and have resulted in improved high-temperature rate constants for the following reactions: H+O₂=OH+OH₂O₂(+M)=2OH(+M)OH+H₂O₂=HO₂+H₂OO₂+H₂O=OH+HO₂ The updated mechanism also takes advantage of the results of other recent rate coefficient studies, and incorporates the most current thermochemical data for OH and HO₂. The mechanism is tested (and its performance compared to that of other H₂/O₂ mechanisms) against recently reported OH and H₂O concentration time-histories in various H₂/O₂ systems, such as H₂ oxidation, H₂O₂ decomposition, and shock-heated H₂O/O₂ mixtures. In addition, the mechanism is validated against a wide range of standard H₂/O₂ kinetic targets, including ignition delay times, flow reactor species time-histories, laminar flame speeds, and burner-stabilized flame structures. This validation indicates that the updated mechanism should perform reliably over a range of reactant concentrations, stoichiometries, pressures, and temperatures from 950 to greater than 3000 K.     

The effects of composition on burning velocity and nitric oxide formation in laminar premixed flames of CH4 + H2 + O2 + N2

Experimental measurements of adiabatic burning velocity and NO formation in (CH₄ + H₂) + (O₂ + N₂) flames are presented. The hydrogen content in the fuel was varied from 0 to 35% and the oxygen content in the air from 20.9 to 16%. Nonstretched flames were stabilized on a perforated plate burner at 1 atm. The heat flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane + hydrogen + nitrogen + oxygen mixtures were found in satisfactory agreement with the modeling. The NO concentrations in these flames were measured in the burnt gases at a fixed distance from the burner using probe sampling. In lean flames, enrichment by hydrogen has little effect on [NO], while in rich flames, the concentration of nitric oxide decreases significantly. Dilution by nitrogen decreases [NO] at any equivalence ratio. Numerical predictions and trends were found in good agreement with the experiments. Different responses of stretched and nonstretched flames to enrichment by hydrogen are demonstrated and discussed.