NO Mole Fraction Research Articles

Spatial structure and NO formation of a laminar methane–nitrogen jet in hot coflow under MILD conditions: A spontaneous Raman and LIF study

We report the spatial structure of the reaction zone from a diluted, preheated fuel jet in hot coflowing oxidizer in laminar flow, using an axisymmetric laminar-jet-in-hot-coflow (LJHC) burner. In the experiments, a preheated CH4/N2 (20/80) mixture at ∼1100K flows into the 1500K products of a flat, premixed burner-stabilized flame, having 3mol% residual oxygen. Measurements of major species and temperature profiles using spontaneous Raman scattering show a temperature rise in the reaction zone of less than 300K, indicating that the reaction zone was in the MILD combustion regime. Analysis of the data shows that the spatial development of the combustion has the characteristics of an “ordinary” coflow diffusion flame, but with changes in temperature and species fractions due to combustion that are nearly an order of magnitude smaller than when using undiluted reactants. Measurements of profiles of NO mole fraction using Laser Induced Fluorescence (LIF) show that the contribution of the MILD combustion to the NO fraction is less than 2ppm, in agreement with other studies on MILD combustion in turbulent flows. Analysis of the NO fraction as a function of mixture fraction indicates that the NO field is dominated by mixing of the NO formed in the coflow with the reaction products of the diluted fuel, with negligible NO formation from the fuel. Replacement of the nitrogen in the hot coflow with argon resulted in NO fractions below the detection limit of the measurement system.

Analysis of NOX Formation in an Axially Staged Combustion System at Elevated Pressure Conditions

The objective of this investigation was to study the effect of axially staged injection of methane in the vitiated air cross flow in a two stage combustion chamber on the formation of NOX for different momentum flux ratios. The primary cylindrical combustor equipped with a low swirl air blast nozzle operating with Jet-A liquid fuel generates vitiated air in the temperature range of 1473–1673 K at pressures of 5–8 bars. A methane injector was flush mounted to the inner surface of the secondary combustor at an angle of 30 deg. Oil cooled movable and static gas probes were used to collect the gas samples. The mole fractions of NO, NO2, CO, CO2, and O2 in the collected exhaust gas samples were measured using gas analyzers. For all the investigated operating conditions, the change in the mole fraction of NOX due to the injection of methane (ΔNOX) corrected to 15% O2 and measured in dry mode was less than 15 ppm. The mole fraction of ΔNOX increased with an increase in mass flow rate of methane and it was not affected by a change in the momentum flux ratio. The penetration depth of the methane jet was estimated from the profiles of mole fraction of O2 obtained from the samples collected using the movable gas probe. For the investigated momentum flux ratios, the penetration depth observed was 15 mm at 5 bars and 5 mm at 6.5 and 8 bars. The results obtained from the simulations of the secondary combustor using a RANS turbulence model were also presented. Reaction modeling of the jet flame present in a vitiated air cross flow posed a significant challenge as it was embedded in a high turbulent flow and burns in partial premixed mode. The applicability of two different reaction models has been investigated. The first approach employed a combination of the eddy dissipation and the finite rate chemistry models to determine the reaction rate, while the presumed JPDF model was used in the further investigations. Predictions were in closer agreement to the measurements while employing the presumed JPDF model. This model was also able to predict some key features of the flow such as the change of penetration depth with the pressure.

Nitrous Oxide Emission from Riparian Buffers in Relation to Vegetation and Flood Frequency

The nitrate (NO(3)(-)) removal capacity of riparian zones is well documented, but information is lacking with regard to N(2)O emission from riparian ecosystems and factors controlling temporal dynamics of this potent greenhouse gas. We monitored N(2)O fluxes (static chambers) and measured denitrification (C(2)H(2) block using soil cores) at six riparian sites along a fourth-order stretch of the White River (Indiana, USA) to assess the effect of flood regime, vegetation type, and forest maturity on these processes. The study sites included shrub/grass, aggrading (<15 yr-old), and mature (>80 yr) forests that were flooded either frequently (more than four to six times per year), occasionally (two to three times per year), or rarely (every 20 yr). While the effect of forest maturity and vegetation type (0.52 and 0.65 mg N(2)O-m(-2) d(-1) in adjacent grassed and forested sites) was not significant, analysis of variance (ANOVA) revealed a significant effect ( < 0.01) of flood regime on N(2)O emission. Among the mature forests, mean N(2)O flux was in this order: rarely flooded (0.33) < occasionally flooded (0.99) < frequently flooded (1.72). Large pulses of N(2)O emission (up to 80 mg N(2)O-m(-2) d(-1)) occurred after flood events, but the magnitude of the flux enhancement varied with flood event, being higher after short-duration than after long-duration floods. This pattern was consistent with the inverse relationship between soil moisture and mole fraction of N(2)O, and instances of N(2)O uptake near the river margin after flood events. These results highlight the complexity of N(2)O dynamics in riparian zones and suggest that detailed flood analysis (frequency and duration) is required to determine the contribution of riparian ecosystems to regional N(2)O budget.

The effects of hydrogen addition on NO formation in atmospheric-pressure, fuel-rich-premixed, burner-stabilized methane, ethane and propane flames

The effects of hydrogen addition on NO formation in fuel-rich, burner-stabilized methane, ethane and propane flames are reported. Profiles of temperature and NO mole fraction were obtained using spontaneous Raman scattering and laser-induced fluorescence (LIF), respectively. Experiments were performed at equivalent ratio of 1.3, with 0 and 0.2 mole fraction of hydrogen in the fuel; and the mass flux through the burner was varied for each mixture. The addition of hydrogen only modestly affects the flame temperature and NO mole fraction. For the vast majority of the flames studied, the temperature and NO decrease by less than 40 K and 20% (relative), respectively, upon hydrogen addition. The decrease in NO fraction is more distinct in methane and propane flames, and more modest for ethane. The comparison of the experimental data obtained for a given fuel in near-adiabatic C nH 2n+2/H 2/O 2/N 2 and burner-stabilized C nH 2n+2/Air flames shows that the NO mole fraction at a given mass flux is practically independent of the composition of the oxidizer. Comparison of the experimental profiles with the predictions of one-dimensional flame calculations with detailed chemical mechanisms indicates that the decrease in the Fenimore NO formation with hydrogen addition arises from the concomitant decrease in CH fraction. Analysis of the computational results suggests that the reaction NCN + H → CH + N 2 returns a considerable fraction of NCN back to N 2.

Calibration of the Total Carbon Column Observing Network using aircraft profile data

Abstract. The Total Carbon Column Observing Network (TCCON) produces precise measurements of the column average dry-air mole fractions of CO2, CO, CH4, N2O and H2O at a variety of sites worldwide. These observations rely on spectroscopic parameters that are not known with sufficient accuracy to compute total columns that can be used in combination with in situ measurements. The TCCON must therefore be calibrated to World Meteorological Organization (WMO) in situ trace gas measurement scales. We present a calibration of TCCON data using WMO-scale instrumentation aboard aircraft that measured profiles over four TCCON stations during 2008 and 2009. These calibrations are compared with similar observations made in 2004 and 2006. The results indicate that a single, global calibration factor for each gas accurately captures the TCCON total column data within error.

Experimental and numerical study of the role of NCN in prompt-NO formation in low-pressure CH 4–O 2–N 2 and C 2H 2–O 2–N 2 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) CH 4–O 2–N 2 flame and rich ( Φ = 1.25) and stoichiometric C 2H 2–O 2–N 2 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 + N 2 = NCN + H. Rate constants of NO-sensitive reactions of the submechanism are modified by taking into account the recent literature. In particular, the C 2O route could be explored thanks to a significant presence of C 2O 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.

Measurements of greenhouse gases and related tracers at Bialystok tall tower station in Poland

Abstract. Quasi-continuous, in-situ measurements of atmospheric CO2, O2/N2, CH4, CO, N2O, and SF6 have been performed since August 2005 at the tall tower station near Bialystok, in Eastern Poland, from five heights up to 300 m. Besides the in-situ measurements, flask samples are filled approximately weekly and measured at Max-Planck Institute for Biogeochemistry for the same species and, in addition, for H2, Ar/N2 and the stable isotopes 13C and 18O in CO2. The in-situ measurement system was built based on commercially available analysers: a LiCor 7000 for CO2, a Sable Systems "Oxzilla" FC-2 for O2, and an Agilent 6890 gas chromatograph for CH4, CO, N2O and SF6. The system was optimized to run continuously with very little maintenance and to fulfill the precision requirements of the CHIOTTO project. The O2/N2 measurements in particular required special attention in terms of technical setup and quality assurance. The evaluation of the performance after more than three years of operation gave overall satisfactory results, proving that this setup is suitable for long term remote operation with little maintenance. The precision achieved for all species is within or close to the project requirements. The comparison between the in-situ and flask sample results, used to verify the accuracy of the in-situ measurements, showed no significant difference for CO2, O2/N2, CH4 and N2O, and a very small difference for SF6. The same comparison however revealed a statistically significant difference for CO, of about 6.5 ppb, for which the cause could not be fully explained. From more than three years of data, the main features at Bialystok have been characterized in terms of variability, trends, and seasonal and diurnal variations. CO2 and O2/N2 show large short term variability, and large diurnal signals during the warm seasons, which attenuate with the increase of sampling height. The trends calculated from this dataset, over the period August 2005 to December 2008, are 2.02±0.46 ppm/year for CO2 and −23.2±2.5 per meg/year for O2/N2. CH4, CO and N2O show also higher variability at the lower sampling levels, which in the case of CO is strongly seasonal. Diurnal variations in CH4, CO and N2O mole fractions can be observed during the warm season, due to the periodicity of vertical mixing combined with the diurnal cycle of anthropogenic emissions. We calculated increase rates of 10.1±4.4 ppb/year for CH4, (−8.3)±5.3 ppb/year for CO and 0.67±0.08 ppb/year for N2O. SF6 shows only few events, and generally no vertical gradients, which suggests that there are no significant local sources. A weak SF6 seasonal cycle has been detected, which most probably is due to the seasonality of atmospheric circulation. SF6 increased during the time of our measurement at an average rate of 0.29±0.01 ppt/year.

Suggestion on the Simultaneous Reduction Method of CO and NOx in Premixed Flames for a Compact Heat Exchanger

This study focused on the design of a domestic gas-condensing boiler, in which the burner and the heat exchanger were contained within a small combustion chamber. A method was suggested for the simultaneous reduction of CO and NOx by optimum placement of the heat exchangers in the postflame region of the CH4/air premixed flame. In this study, the effect of the location of heat exchangers on NO and CO emission characteristics was numerically simulated using a quasi-one-dimensional model. To validate this effect, the distance (L1) between the burner exit and the first heat exchanger and the distance (L2) between the first and second heat exchangers were varied. The radiative heat loss and the effective heat-loss coefficients (heff,exch) for the conductive and convective heat losses were also considered. In general, the NO mole fraction decreased as the distance L1 decreased. That is to say, when the burner exit was closer to the flame, it reduced the peak temperature, which decreased the formation of thermal NO. However, the CO mole fraction increased, as opposed to the NO mole fraction, as the distance L1 decreased, because the temperature was already below 1200 K. The amount of CO production continuously decreased because of the increased oxidation of CO as the distance L2 increased, whereas the amount of NO production showed little change. Consequently, the location of heat exchangers directly affected the amount of NO and CO production. The NO production was mainly controlled by the distance L1, whereas the CO production was controlled by the distance L2. The distance L1 should be set up close to the flame to reduce NO production, and the distance L2 should be long enough to guarantee the sufficient conversion time from CO to CO2 for the purpose of reducing CO emissions.

Numerical study of the effect of water addition on gas explosion

Through amending the SENKIN code of CHEMKIN III chemical kinetics package, a computational model of gas explosion in a constant volume bomb was built, and the detailed reaction mechanism (GRI-Mech 3.0) was adopted. The mole fraction profiles of reactants, some selected free radicals and catastrophic gases in the process of gas explosion were analyzed by this model. Furthermore, through the sensitivity analysis of the reaction mechanism of gas explosion, the dominant reactions that affect gas explosion and the formation of catastrophic gases were found out. At the same time, the inhibition mechanisms of water on gas explosion and the formation of catastrophic gases were analyzed. The results show that the induced explosion time is prolonged, and the mole fractions of reactant species such as CH 4, O 2 and catastrophic gases such as CO, CO 2 and NO are decreased as water is added to the mixed gas. With the water fraction in the mixed gas increasing, the sensitivities of the dominant reactions contributing to CH 4, CO 2 are decreased and the sensitivity coefficients of CH 4, CO and NO mole fractions are also decreased. The inhibition of gas explosion with water addition can be ascribed to the significant decrease of H, O and OH in the process of gas explosion due to the water presence.

The effects of hydrogen addition on Fenimore NO formation in low-pressure, fuel-rich-premixed, burner-stabilized CH 4/O 2/N 2 flames

We investigate the effects of hydrogen addition on Fenimore NO formation in fuel-rich, low-pressure burner-stabilized CH 4/O 2/N 2 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 CH 4/O 2/N 2 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.

H2/Air 비예혼합화염의 화염신장율에 따른 NO 생성경로의 상세해석

Detailed analysis of NO formation routes and its contributions with strain rate in hydrogen/air flames were numerically investigated. LiG detailed reaction mechanism has been used for calculation, which is compared with experimental data in literature. It shows good agreement with experiment for both temperature and NO mole fraction. Three routes have been found important for NO formation in hydrogen flames. These are the Thermal route, NNH route and N2O route. Strain rate were varied to discuss the EINO reduction trend in hydrogen nonpremixed flames, which are analyzed by each NO formation routes. As a result , as the strain rate increase, EINO decrease sharply until strain rate 100s -1 and decrease slowly until strain rate 310s -1 again, after that EINO keeps nearly constant. It can be identified that EINO trend with the strain rate is well explained by a combination of variation of production rate of above Thermal, NNH and N2O route. Also result of Thermal-Mech. that includes only thermal NO reaction is compared with those of Full-Mech. . As a result, It can be identified that there was difference between the two results of calculation. It is attributed to result that Thermal-mech did not consider contributions of NNH and N2O route. From these result, we can conclude that NOx emission characteristics of hydrogen nonpremixed flames should consider contributions of above three routes simultaneously.

Numerical investigation of spray combustion in jet mixing type combustor for low NO x emission

The present paper describes a numerical investigation of spray combustion in a jet mixing type combustor. In this combustor, kerosene spray was injected with a pressure atomizer, and high speed combustion air was introduced towards the spray flow through some inlet air nozzles to improve mixing of the spray and the air. In the numerical simulation, the conservative equations of mass, momentum and energy in the turbulent flow field were solved in conjunction with the k– ε two equation turbulence model. The effects of the diameter and the number of air inlet nozzles on the combustion behavior and NO emission were numerically investigated. When the diameter of the inlet air nozzle decreased from 8 to 4 mm, the calculated NO mole fraction in the exhaust gas was drastically decreased by about 80%. An increase in the inlet velocity resulted in improvement of the mixing of the spray and the air, and hence, the high temperature region where thermal NO was formed became narrow. As a result, the exhaust NO mole fraction decreased. Furthermore, a decrease in exhaust NO mole fraction was explained by a decrease in the residence time in the high temperature region above 1800 K.

Pressure dependence of NO formation in laminar fuel-rich premixed CH 4/air flames

Effects of pressure on NO formation in CH 4/air flames at a fixed equivalence ratio of 1.3 are investigated. The axial profiles of temperature, OH, CH, and NO mole fractions are measured using laser-induced fluorescence and compared with one-dimensional flame calculations. The measured and calculated temperature, CH, and NO profiles in free flames are observed to vary upon increasing the pressure from 40 to 75 Torr, following a scaling law derived for a chemical mechanism containing only second-order reactions. At pressures 300–760 Torr, the measurements and calculations in burner-stabilized flames show increasing flame temperature and NO mole fractions when the mass flux is increased linearly with pressure, while the CH profiles remain unchanged. The observed deviation from the scaling law in the temperature profiles arises from the increasing contribution of three-body reactions to the flame front propagation velocity, leading to a decrease in the degree of burner stabilization. The deviation from the pressure scaling law for the NO mole fractions is due to the temperature dependence of the rate coefficient for the reaction between CH and N 2 and the fact that the temperature profiles themselves do not scale. In contrast, the surprisingly good scaling of the CH mole fractions with pressure indicates the dominant role of two-body reactions participating in the chain of chemical reactions leading to CH formation. The calculations using GRI-Mech 3.0 substantially overpredict (up to 50%) the measured nitric oxide concentrations for all pressures studied. The observed differences in the NO mole fraction may be addressed by improving the CH prediction.

Prompt-NO formation in methane/oxygen/nitrogen flames seeded with oxygenated volatile organic compounds: Methyl ethyl ketone or ethyl acetate

In the present work, CH and NO profiles are determined using laser-induced fluorescence (LIF) measurements in eight low-pressure laminar flames of CH 4/O 2/N 2 containing various amounts of methyl ethyl ketone or ethyl acetate with respect to the equivalence ratio. Relative CH LIF signals are calibrated using cavity ring-down spectroscopy (CRDS), while NO LIF calibration is performed in the burned gases of NO seeded flames. Temperature measurements are obtained using a coated Pt/Rh thermocouple and serve as temperature profiles input for Chemkin modeling. Volatile organic compound (VOC) submechanisms, previously validated upon major and intermediate species profiles measured using sampling techniques, are incorporated into the GDF-Kin 3.0_NCN for the CH 4 oxidation. This mechanism is adjusted and validated to predict the effect of VOCs seeding on CH and NO formation. It takes into account not only the CH and NO profiles but those previously measured. When methane is replaced by either MEK or EA, the NO mole fraction in the burned gases and the CH peak value are found to jointly decrease, indicating that NO is mainly formed according to the prompt-NO mechanism. According to the kinetic analysis, the VOC impact on NO formation is demonstrated. It is shown that CH radical formation, through the C1 sequence, mainly involves the CH 3 radical, which is formed with the same propensity by one molecule of methane, MEK, or EA. Due to the methane replacement by VOC while the equivalence ratio is maintained constant, we found that the CH peak value decrease follows the total fuel volumetric flow rate decrease, yielding, an NO decrease in the burned gases.

A numerical and experimental study of counterflow syngas flames at different pressures

Synthesis gas or “Syngas” is being recognized as a viable energy source worldwide, particularly for stationary power generation due to its wide availability as a product of bio and fossil fuel gasification. There are, however, gaps in the fundamental understanding of syngas combustion and emissions characteristics, especially at elevated pressures that are relevant to practical combustors. This paper presents a numerical and experimental investigation of the combustion and NO x characteristics of syngas fuel with varying composition, pressure and strain rate. Experiments were performed at atmospheric conditions, while the simulations considered different pressures. Both experiments and simulations indicate that stable non-premixed and partially premixed counterflow flames (PPFs) can be established for a wide range of syngas compositions and strain rates. Three chemical kinetic models, GRI 3.0, Davis et al., and Mueller et al. are examined. The Davis et al. mechanism is found to agree best with the experimental data, and hence used to simulate the PPF structure at different pressure and fuel composition. For the pressure range investigated, results indicate a typical double flame structure with a rich premixed reaction zone (RPZ) on the fuel side and a non-premixed reaction zone (NPZ) on the oxidizer side, with RPZ characterized by H 2 oxidation, and NPZ by both H 2 and CO oxidation. While thermal NO is found to be the dominant route for NO production, a reburn route, which consumes NO through NO + O + M→ NO 2 + M and H + NO + M → HNO + M reactions, becomes increasingly important at high pressures. The amount of NO formed in syngas PPFs first increases rapidly with pressure, but then levels off at higher pressures. At a given pressure, the peak NO mole fraction exhibits a non-monotonic variation with syngas composition, first decreasing to a minimum value, and then increasing as the amount of CO in syngas is increased. This implies the existence of an optimum syngas composition that yields the lowest amount of NO production in syngas PPFs, and can be attributed to the combined effects of thermal and reburn mechanisms.

Spray combustion simulation including soot and NO formation

Abstract A numerical simulation of turbulent spray combustion was performed to predict the combustion behavior in a jet burner that generates a high speed, high temperature jet of exhaust gas. The conservation equations of mass, momentum and energy in the turbulent flow field were solved in conjunction with the k – e turbulence model. The soot distribution in the combustor was calculated by solving a transport equation for the soot mass fraction using simple expressions for the soot formation and oxidation rates. A post-processing NO formation model was used to predict the concentration of thermal and prompt NO. Calculated results are compared with experimental data to demonstrate the validity of the numerical model. The effect of heat radiation from soot on the combustion characteristics was also examined. The calculated temperature without soot radiation was higher than that of the experimental data. However, in the case with heat radiation from the soot, the differences between the calculated and experimental results became small. This is because the soot greatly affects the radiative heat transfer in the jet burner. In addition, the calculated exhaust NO mole fraction with soot is in good agreement with the experimental one, although that without soot is almost 10 times larger than the experimental one. Moreover, to improve the combustion behavior in the jet burner, a numerical simulation considering a baffle plate that causes recirculation flows was also performed. When a suitable baffle plate is applied to the combustor, the NO mole fraction in the exhaust gas decreased by about 40%.

In situ measurements of nitric oxide in coal-combustion exhaust using a sensor based on a widely tunable external-cavity GaN diode laser

A diode-laser-based sensor has been developed to measure nitric oxide mole fractions using absorption spectroscopy. The sensor is based on sum-frequency mixing of a 395 nm external-cavity diode laser (ECDL) and a 532 nm laser in a beta-barium-borate crystal. Using a new tuning scheme, the GaN ECDL wavelength was modulated over 90 GHz without mode hops. The sensor was applied for measurements of the NO mole fraction in the exhaust of a laboratory-scale, 30 kW(t) coal-fired boiler burner. Absorption measurements were successfully performed despite severe attenuation by scattering from ash particles in the exhaust stream and on the exhaust-section windows. A detection limit (1sigma) of 4.5 ppm m/(square root)Hz at 700 K was demonstrated in coal- combustion exhaust at a maximum detection rate of 5 Hz.

B212 標準添加LIF法によるジェット燃料火災中の―酸化窒素濃度・温度計測

An LIF imaging spectroscopy was applied to the quantitative measurements of NO concentration and temperature profiles in a jet fuel flame. The NO mole fractions were determined comparing the LIF signals of NO-seeded flame and non-seeded flame. A two-line thermometry technique was used for obtaining the temperature distributions. A narrow-band pass filter was used to sufficiently remove noise lights, and two-dimensional distributions of concentration and temperature of NO were obtained. The flame was stabilized inserting a honeycomb ceramic tube on the burner exit of preheated kerosene-air mixture. The result shows that NO is produced via the thermal NO mechanisms and that the NO concentration and temperature were higher in comparison with a methane-air premixed flame in a standard Bunsen burner.

The effects of burner stabilization on Fenimore NO formation in low-pressure, fuel-rich premixed CH 4/O 2/N 2 flames

We investigate the effects of varying the degree of burner stabilization on Fenimore NO formation in fuel-rich low-pressure flat CH 4/O 2/N 2 flames. Towards this end, axial profiles of flame temperature and OH, NO and CH mole fractions are measured using laser-induced fluorescence (LIF). The experiments are performed at equivalence ratios between 1.3 and 1.5. The flame temperature is seen to decrease by 200–300 K, with a concomitant decrease in OH mole fraction, upon reducing the total flow rate from 5 to 3 L/min, thus increasing stabilization. At equivalence ratios between 1.3 and 1.5, this decrease in flow rate lowers the maximum CH mole fraction by a factor of 2, and the NO mole fraction by ∼40% in all flames studied. Integrating the reaction rate for CH + N 2 to estimate Fenimore NO formation, using the rate coefficient in GRI-Mech 3.0, and the measured temperatures and CH profiles show very good agreement with the measured NO mole fraction for ϕ = 1.3 and 1.4, supporting the current choice for this rate. This agreement also shows that the increase in residence time caused by increased stabilization is an important factor in the ultimate impact of the changes in CH mole fraction on NO formation. The results at ϕ = 1.5 suggest that substantial quantities of fixed nitrogen species, e.g., HCN, are only slowly oxidized in the post-flame zone under these conditions, leading to a significant discrepancy between the measured NO mole fraction and that obtained by integrating over the CH profile. Detailed calculations using GRI-Mech 3.0 predict the experimental results at ϕ = 1.3 nearly quantitatively, but show increasing differences with the measurements for both CH and NO profiles with increasing equivalence ratio.

Influence of C 2 and C 3 compounds of natural gas on NO formation: an experimental study based on LIF/CRDS coupling

This study reports CH and NO mole fraction profiles measured in 12 premixed flames representative of natural gas combustion. Four mixtures CH 4/O 2/N 2, CH 4/C 2H 6/O 2/N 2, CH 4/C 3H 8/O 2/N 2, and CH 4/C 2H 6/C 3H 8/O 2/N 2 have been investigated at three equivalence ratios (0.7, 1, and 1.25). The aim of this work was to study the influence of C 2 and C 3 compounds of natural gas on NO formation in flames. Flames were stabilised on a flat flame burner at low pressure (33 torr). The experimental approach was exclusively based on non-intrusive techniques: laser induced fluorescence (LIF) for OH-LIF thermometry and species concentration profiles determination, and cavity ring-down spectroscopy (CRDS) for absolute calibration. Thus, an extended database including a large dynamic of combined CH and NO mole fraction profiles (ranging, respectively, from 0.7 to 10 ppm and from 2.6 to 24 ppm) has been obtained and will serve for further improvement of chemical mechanisms of NO formation in natural gas flames. It is shown that, for a given equivalence ratio, NO quantity remains unchanged when substituting methane by ethane and/or propane, while CH mole fraction exhibits a slight decrease. Predominance of prompt-NO mechanism is shown to be sensitive to the equivalence ratio. These new results have been modelled by using the GRI3.0 mechanism, and a reaction path analysis has been performed in the stoichiometric methane flame to highlight the different routes of NO formation in the different zones of the flame, i.e., the preheat zone, the reaction zone, and the burned gases.