Numerical investigations on flashback dynamics of premixed methane-hydrogen-air laminar flames

Injecting hydrogen into the natural gas network to reduce CO₂ emissions in the EU residential sector is considered a critical element of the zero CO₂ emissions target for 2050. Burning natural gas and hydrogen mixtures has potential risks, the main one being the flame flashback phenomenon that could occur in home appliances using premixed laminar burners. In the present study, two-dimensional transient computations of laminar CH₄ + air and CH₄ + H₂ + air flames are performed with the open-source CFD code OpenFOAM. A finite rate chemistry based solver is used to compute reaction rates and the laminar reacting flow. Starting from a flame stabilized at the rim of a cylindrical tube burner, the inlet bulk velocity of the premixture is gradually reduced to observe flashback. The results of the present work concern the effects of wall temperature and hydrogen addition on the flashback propensity of laminar premixed methane-hydrogen-air flames. Complete sequences of flame dynamics with gradual increases of premixture velocity are investigated. At the flame flashback velocities, strong oscillations at the flame leading edge emerge, causing broken flame symmetry and finally flame flashback. The numerical results reveal that flashback tendency increase with increasing wall temperature and hydrogen addition rate.     

The effect of hydrogen containing fuel blends upon flashback in swirl burners

Lean premixed swirl combustion is widely used in gas turbines and many other combustion Processes due to the benefits of good flame stability and blow off limits coupled with low NO_x emissions. Although flashback is not generally a problem with natural gas combustion, there are some reports of flashback damage with existing gas turbines, whilst hydrogen enriched fuel blends, especially those derived from gasification of coal and/or biomass/industrial processes such as steel making, cause concerns in this area. Thus, this paper describes a practical experimental approach to study and reduce the effect of flashback in a compact design of generic swirl burner representative of many systems. A range of different fuel blends are investigated for flashback and blow off limits; these fuel mixes include methane, methane/hydrogen blends, pure hydrogen and coke oven gas. Swirl number effects are investigated by varying the number of inlets or the configuration of the inlets. The well known Lewis and von Elbe critical boundary velocity gradient expression is used to characterise flashback and enable comparison to be made with other available data.Two flashback phenomena are encountered here. The first one at lower swirl numbers involves flashback through the outer wall boundary layer where the crucial parameter is the critical boundary velocity gradient, G_f. Values of G_f are of similar magnitude to those reported by Lewis and von Elbe for laminar flow conditions, and it is recognised that under the turbulent flow conditions pertaining here actual gradients in the thin swirl flow boundary layer are much higher than occur under laminar flow conditions. At higher swirl numbers the central recirculation zone (CRZ) becomes enlarged and extends backwards over the fuel injector to the burner baseplate and causes flashback to occur earlier at higher velocities. This extension of the CRZ is complex, being governed by swirl number, equivalence ratio and Reynolds Number. Under these conditions flashback occurs when the cylindrical flame front surrounding the CRZ rapidly accelerates outwards to the tangential inlets and beyond, especially with hydrogen containing fuel mixes. Conversely at lower swirl numbers with a modified exhaust geometry, hence restricted CRZ, flashback occurs through the outer thin boundary layer at much lower flow rates when the hydrogen content of the fuel mix does not exceed 30%. The work demonstrates that it is possible to run premixed swirl burners with a wide range of hydrogen fuel blends so as to substantially minimise flashback behaviour, thus permitting wider used of the technology to reduce NOx emissions.     

Flame stabilization mechanisms and shape transitions in a 3D printed,hydrogen enriched,methane/air low-swirl burner

Flame shapes and their transitions of premixed hydrogen enriched methane flames in a 3D-printed low-swirl burner are studied using simultaneous OH×CH₂O planar laser induced fluorescence and stereoscopic particle image velocimetry. Three different flame shapes are observed, namely bowl-shape, W-shape, and crown-shape. The bowl-shaped flame has its base stabilized through flame-flow velocity balance and its sides stabilized in the inner shear layer. While the bulges of the W-shaped flame rely on a similar stabilization mechanism in the central flow, its outer edges are stabilized by large-scale eddies in the outer shear layer. The crown-shaped flame is also aerodynamically stabilized in the center, but its outer edges are anchored to the burner hardware. At a fixed equivalence ratio, the statistical transitions between flame shapes across test conditions are jointly dominated by hydrogen fraction and bulk velocity. Dynamically, W-to-crown transition is attributed to the upstream propagation and attachment of the flame outer edges.     

Pilot impact on turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor

The impact of pilot flame operation on the combustion of pure methane and hydrogen-enriched methane (H₂/CH₄: 50/50 in vol%) fuels was investigated in a gas turbine model combustor under atmospheric conditions. The burner assembly was designed to mimic the geometry of an industrial burner, the Siemens DLE Burner, in which a concentric annular ring equipped with pilot flame burners is implemented in the dome of the combustor. Two pilot burner configurations have been investigated: a non-premixed and a partially premixed pilot arrangement. The performance of the pilot burners was evaluated for varying Reynolds number (Re) and H₂ enrichment. High-speed OH1 chemiluminescence imaging, as well as simultaneous planar laser-induced fluorescence measurements of the OH radicals and formaldehyde (CH₂O) were used for evaluating the dynamics and structures of the flames for different conditions. Furthermore, emission measurements were carried out to determine the influence of hydrogen dilution on the NO_x and CO emission levels. The main findings are (a) the effect of the pilot flame is sensitive to the Reynolds number of the main flame and the type of the pilot flame, (b) the stability range becomes narrower with increasing hydrogen ratio, due to the tendency to flashback, (c) non-premixed pilot flames lower the NO_x and increase the CO emissions, albeit rather small differences in the emissions have been detected, and (d) the NO_x and CO emissions become significantly lower with increasing hydrogen ratio.     

Experimental investigation of the stability and emission characteristics of premixed formic acid-methane-air flames in a swirl combustor

Formic acid (FA) is a potential hydrogen energy carrier and low-carbon fuel by reversing the decomposition products, CO₂ and H_2, back to restore FA without additional carbon release. However, FA-air mixtures feature high ignition energy and low flame speed; hence stabilizing FA-air flames in combustion devices is challenging. This study experimentally investigates the flame stability and emission of swirl flames fueled with pre-vaporized formic acid-methane blends over a wide range of formic acid fuel fractions. Results show that by using a swirl combustor, the premixed formic acid-methane-air flames could be stabilized over a wide range of FA fuel fractions, Reynolds numbers, and swirl numbers. The addition of formic acid increases the equivalence ratios at which the flashback and lean blowout occur. When Reynolds number increases, the equivalence ratio at the flashback limit increases, but that decreases at the lean blowout limit. Increasing the swirl number has a non-monotonic effect on stability limits variation because increasing the swirl number changes the axial velocity on the centerline of the burner throat non-monotonically. In addition, emission characteristics were investigated using a gas analyzer. The CO and NO concentrations were below 20 ppm for all tested conditions, which is comparable to that seen with traditional hydrocarbon fuels, which is in favor of future practical applications with formic acid.     

Hydrogen substitution of natural-gas in premixed burners and implications for blow-off and flashback limits

Two multi-perforated premixed burners, designed for natural gas, are fueled with increasing hydrogen content to assess the limits of H₂ substitution and investigate potential risks associated to it. The burners feature a different design, which affects flame stabilization and heat exchange between the fresh mixture and the hot burner walls. First, results are presented by means of stability maps that were collected at constant power and over a wide range of equivalence ratio, from pure methane-air to pure hydrogen-air mixtures. The impact of hydrogen addition on blow-off and flashback limits is then analyzed. On one side, it is observed that hydrogen addition increases blow off resistance, extending the operating range towards ultra-lean conditions. On the other side, hydrogen raises the thermal load on the burner favoring flashback. It is shown that the competition between the bulk velocity at the burner outlet and the laminar burning velocity is not a reliable parameter to predict flashback occurrence, while the thermal state of the burner represents a determining factor. An analysis of the thermal transient reveals a strict correspondence between the onset of flashback for a given mixture composition and the burner surface temperature. Results highlight the challenges linked to the design of fuel-flexible systems, pointing out practical limits of H₂ substitution in burners designed for operation with natural gas.     

旋流预混燃烧室燃烧特性及排放特性的数值模拟研究

为了综合考察燃气轮机燃烧室在高稳定性、低排放以及燃料适应性等方面的新要求,基于旋流预混燃烧技术,通过三维数值模拟方法开展了甲烷/空气、丙烷/空气预混燃烧特性及排放特性研究。结果表明：在一定的预混气进气质量流量条件下,当量比增大易引发回火,燃烧温度更高,同时NO_x排放指数增大,增加预混气质量流量,可在一定程度上提高回/熄火极限;当量比固定,增加预混气进气质量流量可避免潜在的回火现象,且NO_x排放指数线性降低;旋流器的旋流数增大能形成强旋流,稳定火焰,降低NO_x排放指数,但过大的旋流强度会引发回火现象;相比于甲烷/空气预混燃烧,丙烷/空气预混燃烧温度偏高,NO_x排放指数较大,但回熄火边界更宽,对应更广阔的稳定燃烧区间。     

Influence of precessing vortex core on flame flashback in swirling hydrogen flames

This study examines the influence of vortex core precession on flame flashback of swirl-stabilised hydrogen flames. Theoretical considerations suggest that the angular velocity of a swirling flow is reduced as vortex precession causes it to acquire an eccentric motion around the central axis of the burner. The eccentric motion of the vortex generates a secondary flow, which is thought to reduce the angular velocity and tangential momentum available to the primary flow, and thereby reduce the flashback propensity at the centre of the vortex core. Experiments measuring the influence of the eccentric motion of the flame tip on flame flashback behaviour were conducted using high-speed sequences of OH*-chemiluminescence images. Temporal analysis of a large sample of images revealed the existence of a systematic rotational frequency of the flame tip around the central axis of the burner. Analysis of the radial position of the flame tip in relation to its axial propagation velocity showed that flashback velocity increased as the flame tip eccentricity approached zero and flashback velocity decreased as the eccentricity amplitude of the flame tip reached larger values. This suggested that flame eccentricity caused by vortex core precession may be detrimental to upstream flame propagation and may be effective in inhibiting flame flashback in swirl-stabilised flames.     

Combustion performances of premixed ammonia/hydrogen/air laminar and swirling flames for a wide range of equivalence ratios

Ammonia, a carbon-free source of hydrogen has recently gained considerable attention as energy solution towards a green future. Previous works have shown that adding 30_VOL.% hydrogen with ammonia can eradicate the drawbacks of pure ammonia combustion but no study in the literature has investigated this blend across a wide range of equivalence ratios. The present work investigates 70/30_VOL.% NH₃/H₂ blend from 0.55 ≤ Φ ≤ 1.4 for both premixed laminar spherically expanding flames and turbulent swirling flames at atmospheric conditions. A detailed chemistry analysis has been conducted in Ansys CHEMKIN-PRO platform using a chemical reactor network (CRN) model to simulate the swirling turbulent flames. NO and NO₂ emissions have followed similar bell-shaped trends, peaking at around Φ = 0.8, while N₂O emission rises at lean conditions (Φ ≤ 0.7). The results indicate that Φ = 1.2 is the optimum equivalence ratio with reduced NO_X emissions and some ammonia slip.     

An experimental study of mild flameless combustion of methane/hydrogen mixtures

This paper presents an experimental study of mild flameless combustion regime applied to methane/hydrogen mixtures in a laboratory-scale pilot furnace with or without air preheating. Results show that mild flameless combustion regime is achieved from pure methane to pure hydrogen whatever the CH₄/H₂ proportion. The main reaction zone remains lifted from the burner exit, in the mixing layer of fuel and air jets ensuring a large dilution correlated to low NOx emissions whereas CO₂ concentrations obviously decrease with hydrogen proportion. A decrease of NOx emissions is measured for larger quantity of hydrogen due mainly to the decrease of prompt NO formation. Without air preheating, a slight increase of the excess air ratio is required to control CO emissions. For pure hydrogen fuel without air preheating, mild flameless combustion regime leads to operating conditions close to a "zero emission furnace", with ultra-low NOx emissions and without any carbonated species emissions.     

Effects of swirl on the stability of jet diffusion flames

An experimental study was made, using a double-swirl burner, of the stability of swirling-fuel-jet diffusion flames in swirling air streams. The fuels were hydrogen and methane. The primary variables studied were swirl intensities of the fuel jet and the air stream. It was found that the stability of flame depended on the swirl intensity of both the fuel jet and the air stream. The application of swirl to the fuel jet decreased the rim stability of the flame, but increased the blowout stability of the lifted diffusion flame. For low swirl intensity of the air stream, the effect was similar to that of the fuel jet. At higher swirl intensities of the air stream, above a critical value, the flame stability increased noticeably because of the formation of a recirculation zone near the injector exit. Even in strongly swirling air streams, the favorable effect of fuel swirl on stability of the lifted flame was evident, particularly for the methane flame.     

A DNS study of hydrogen/air swirling premixed flames with different equivalence ratios

Hydrogen/air swirling premixed flames with different equivalence ratios are studied using direct numerical simulation. A fourth-order explicit Runge–Kutta method for time integration and an eighth-order central differencing scheme for spatial discretization are used to solve the full Navier–Stokes (N–S) equation system. A 9 species 19-step reduced mechanism for hydrogen/air combustion is adopted. The flames are stabilized with the help of a recirculation zone characterizing a high swirling flow. The vortex structures of the swirling premixed flames are presented. The flame structures are investigated in terms of the flame front curvature and tangential strain rate probability density functions (pdfs). The local flamelet temperature profiles are also extracted randomly along the flame front and compared with the corresponding laminar flame temperature profile. In order to study preferential diffusion effects, direct numerical simulation of two additional freely propagating planar flames in isotropic turbulence is conducted. Preferential diffusion effects observed in the planar flames are suppressed in the swirling flames. Further analysis confirms that the coherent small-scale eddies play important roles in the interactions between turbulence and the flame front. They are able to change the dynamic properties of the flame font and lead to enhanced burning intensity in the flame front with negative curvature for both stoichiometric and fuel-lean flames.     

Stability limits and NO emissions of premixed swirl ammonia-air flames enriched with hydrogen or methane at elevated pressures

This study reports measurements of stability limits and exhaust NO mole fractions of technically-premixed swirl ammonia-air flames enriched with either methane or hydrogen. Experiments were conducted at different pressures from atmospheric to 5 bar, representative of commercial micro gas turbines. The full range of ammonia fractions in the fuel blend, x_NH3, was considered, from 0 (pure methane or hydrogen) to 1 (pure ammonia), covering very lean (φ = 0.25) to rich (φ = 1.60) equivalence ratios. Results show that increasing pressure widens the range of stable equivalence ratios for pure ammonia-air flames. Regardless of pressure, there is a critical ammonia fraction above which the range of stable equivalence ratios suddenly widens. This is because flashback does not occur anymore when the equivalence ratio is progressively increased towards stoichiometric and rich blowout occurs instead. This critical ammonia fraction increases with pressure and is larger for ammonia-hydrogen than for ammonia-methane. Provided that enough hydrogen is blended with ammonia (x_NH3 < 0.9), flames with very lean equivalence ratios (φ < 0.7) can be stabilized and these yield competitively low NO emissions (<200 ppm), regardless of pressure. For this reason, very lean swirl ammonia-hydrogen-air flames are promising candidates for micro gas turbines. However, N₂O emissions have the potential to be unacceptably large for these operating conditions if heat loss is too large or residence time is too short. As a consequence, the post flame region must be considered carefully. Due to the lower reactivity of methane compared to that of hydrogen, very lean swirl ammonia-methane-air flames could not be stabilized and good NO performance is limited to rich equivalence ratios for ammonia-methane fuel blends. The equivalence ratio above which good NO performance depends on pressure and bulk velocity.     

Investigating the effect of local addition of hydrogen to acoustically excited ethylene and methane flames

While lean combustion in gas turbines is known to reduce NOx, it makes combustors more prone to thermo-acoustic instabilities, which can lead to deterioration in engine performance. The work presented in this study investigates the effectiveness of secondary injection of hydrogen to imperfectly premixed methane and ethylene flames in reducing heat release oscillations. Both acoustically forced and unforced flames were studied, and simultaneous OH and H atom PLIF (planar laser induced fluorescence) was conducted. The tests were carried out on a laboratory scale bluff-body combustor with a central V-shaped bluff body. Two-microphone method was used to estimate velocity perturbations from pressure measurements, flame boundary images were captured using high speed Mie scattering, while global heat release fluctuations were determined from OH* chemiluminescence.The results showed that hydrogen addition considerably reduced heat release oscillations for both methane and ethylene flames at all the forcing frequencies tested, with the exception of methane flames forced at 315 Hz, where oscillations increased with hydrogen addition. The addition of hydrogen reduced the extent of flame roll-up for both methane and ethylene flames, however, this reduction was larger for methane flames. NOx exhaust emissions were observed to increase with hydrogen addition for both methane and ethylene flames, with absolute NOx concentrations higher for ethylene flames, due to higher flame temperatures.     

Comparisons of the impact of reformer gas and hydrogen enrichment on flame stability and pollutant emissions for a kerosene/air swirled flame with an aeronautical fuel injector

Experimental studies were conducted on an actual aeronautical fuel injector, at conditions close as possible of the idle regime of the aircraft (P = 0.3 MPa and T = 500 K), to characterize the flame stability and pollutant emissions of two-phase kerosene/air flames. The objective was to investigate the effects of H₂ and reformer gases (RG containing H₂) enrichment of kerosene at constant power for the adaptation to an aircraft engine. Two different gas injection configurations have been tested (partially premixed, PP, and fully premixed, FP) to evaluate the consequences of the fuel injection mode on gas enrichment. We demonstrate the main interest and the benefits of RGs for aeronautical gas turbines. Through chemical mechanisms, they increase the flame stability and strongly reduce CO emissions without dramatically increasing NO_x emissions, in comparison with the injection of pure hydrogen. Their overall behavior is independent from the injection configuration.     

Rich methane premixed laminar flames doped with light unsaturated hydrocarbons: I. Allene and propyne

The structure of three laminar premixed rich flames has been investigated: a pure methane flame and two methane flames doped by allene and propyne, respectively. The gases of the three flames contain 20.9% (molar) of methane and 33.4% of oxygen, corresponding to an equivalence ratio of 1.25 for the pure methane flame. In both doped flames, 2.49% of C₃H₄ was added, corresponding to a ratio C₃H₄/CH₄ of 12% and an equivalence ratio of 1.55. The three flames have been stabilized on a burner at a pressure of 6.7 kPa using argon as dilutant, with a gas velocity at the burner of 36 cm/s at 333 K. The concentration profiles of stable species were measured by gas chromatography after sampling with a quartz microprobe. Quantified species included carbon monoxide and dioxide, methane, oxygen, hydrogen, ethane, ethylene, acetylene, propyne, allene, propene, propane, 1,2-butadiene, 1,3-butadiene, 1-butene, isobutene, 1-butyne, vinylacetylene, and benzene. The temperature was measured using a PtRh (6%)-PtRh (30%) thermocouple settled inside the enclosure and ranged from 700 K close to the burner up to 1850 K. In order to model these new results, some improvements have been made to a mechanism previously developed in our laboratory for the reactions of C₃-C₄ unsaturated hydrocarbons. The main reaction pathways of consumption of allene and propyne and of formation of C₆ aromatic species have been derived from flow rate analyses.     

Tubular premixed and diffusion flames: Effect of stretch and curvature

Tubular flames are ideal for the study of stretch and curvature effects on flame structure, extinction, and instabilities. Tubular flames have uniform stretch and curvature and each parameter can be varied independently. Curvature strengthens or weakens preferential diffusion effects on the tubular flame and the strengthening or weakening is proportional to the ratio of the flame thickness to the flame radius. Premixed flames can be studied in the standard tubular burner where a single premixed gas stream flows radially inward to the cylindrical flame surface and products exit as opposed jets. Premixed, diffusion and partially premixed flames can be studied in the opposed tubular flame where opposed radial flows meet at a cylindrical stagnation surface and products exit as opposed jets. The tubular flame flow configurations can be mathematically reduced to a two-point boundary value solution along the single radial coordinate. Non-intrusive measurements of temperature and major species concentrations have been made with laser-induced Raman scattering in an optically accessible tubular burner for both premixed and diffusion flames. The laser measurements of the flame structure are in good agreement with numerical simulations of the tubular flame. Due to the strong enhancement of preferential diffusion effects in tubular flames, the theory-data comparison can be very sensitive to the molecular transport model and the chemical kinetic mechanism. The strengthening or weakening of the tubular flame with curvature can increase or decrease the extinction strain rate of tubular flames. For lean H₂-air mixtures, the tubular flame can have an extinction strain rate many times higher than the corresponding opposed jet flame. More complex cellular tubular flames with highly curved flame cells surrounded by local extinction can be formed under both premixed and non-premixed conditions. In the hydrogen fueled premixed tubular flames, thermal-diffusive flame instabilities result in the formation of a uniform symmetric petal flames far from extinction. In opposed-flow tubular diffusion flames, thermal-diffusive flame instabilities result in cellular flames very close to extinction. Both of these flames are candidates for further study of flame curvature and extinction.     

Characterization of confined hydrogen-air jet flame in a crossflow configuration using design of experiments

Hydrogen combustion has many industrial applications and development of new hydrogen burners is required to fulfil new demands. A novel configuration of hydrogen burner utilizing crossflow injection of fuel jets into swirling combustion air is characterized empirically in this work. It is intended as a first step in the development of new burner technologies having reduced emission levels and improved efficiency. Experiments were designed using the full factorial design method. Operating parameters were varied simultaneously and the NO_X emissions from the flame stabilized on the burner were measured. Statistical analysis of the experimental data showed that overall equivalence ratio is the dominant factor and lower NO_X emissions are observed at low equivalence ratios, irrespective of the burner power level. The analysis yielded an empirical relationship among NO_X emission, overall equivalence ratio, and power level that is useful in the design activity for a future combustion system based on the proposed configuration.     

Development and characterization of a low-NOx partially premixed hydrogen burner using numerical simulation and flame diagnostics

The utilization of hydrogen as a fuel in free jet burners faces particular challenges due to its special combustion properties. The high laminar and turbulent flame velocities may lead to issues in flame stability and operational safety in premixed and partially premixed burners. Additionally, a high adiabatic combustion temperature favors the formation of thermal nitric oxides (NO). This study presents the development and optimization of a partially premixed hydrogen burner with low emissions of nitric oxides. The single-nozzle burner features a very short premixing duct and a simple geometric design. In a first development step, the design of the burner is optimized by numerical investigation (Star CCM+) of mixture formation, which is improved by geometric changes of the nozzle. The impact of geometric optimization and of humidification of the combustion air on NO_x emissions is then investigated experimentally. The hydrogen flame is detected with an infrared camera to evaluate the flame stability for different burner configurations. The improved mixture formation by geometric optimization avoids temperature peaks and leads to a noticeable reduction in NO_x emissions for equivalence ratios below 0.85. The experimental investigations also show that NO_x emissions decrease with increasing relative humidity of combustion air. This single-nozzle forms the basis for multi-nozzle burners, where the desired output power can flexibly be adjusted by the number of single nozzles.     

Reactive structures and NOx emissions of methane/hydrogen mixtures in flameless combustion

Methane/hydrogen combustion represents a concrete solution for the energy scenario to come. Indeed, the addition of hydrogen into the natural gas pipeline is one of the solutions foreseen to reduce CO₂ emissions. Nevertheless, the replacement of methane by hydrogen will enhance the reactivity of the system, increasing NOx emissions. To overcome this issue, non-conventional combustion technologies, such as flameless combustion represent an attractive solution. This study aims to improve our understanding of the behaviour of methane/hydrogen blends under flameless conditions by means of experiments and simulations. Several experimental campaigns were conducted to test fuel flexibility for different methane/hydrogen blends, varying the injector geometries, equivalence ratio and dilution degree. It was found that a progressive addition of hydrogen in methane enhanced the combustion features, reducing the ignition delay time and loosing progressively the flameless behaviour of the furnace. Reducing the air injector diameter or increasing the fuel lance length were found to be efficient techniques to reduce the maximum temperature of the system and NOx emissions in the exhausts, reaching values below 30 ppm for pure hydrogen. MILD conditions were achieved up to 75%H₂ in molar fraction, with no visible flame structures. Additionally, RANS-based simulations were also conducted to shed further light on the effect of adding hydrogen into the fuel blend. A sensitivity study was conducted for three different fuel blends: pure methane, an equimolar blend and pure hydrogen. The effect of chemistry detail, mixing models, radiation modeling and turbulence models on in-flame temperatures and NOx emissions was also studied. In particular, it was found that the usage of detailed chemistry for NOx, coupled with an adjustment of the PaSR model, filled the gap between experiments and predictions. Finally, a brute-force sensitivity revealed that NNH is the most important route for NOx production.