N2-diluted Flames Research Articles

Understanding of MILD Combustion Characteristics of NH3 Air Flames in N2 And H2O Steam Diluted Environment at Atmospheric Pressure

Ammonia is becoming increasingly popular as a carbon-neutral fuel with zero carbon dioxide emissions. However, a significant hurdle lies in its combustion, which leads to substantial emissions of NOx. The current research involves conducting a chemical kinetic investigation to examine the characteristics of Intense Low oxygen Dilution (MILD) or Moderate combustion in ammonia (NH3)/air flames. This study is carried out under specific conditions, such as oxygen concentrations ranging from 11to 23%, premixed reactant temperatures between 1300 and 1700 K, and a pressure of one atmosphere. The study focuses on investigating the combustion characteristics of MILD using dilution with H2O and N2.With the rise in the inlet temperature of the premixed reactant, the peak temperature of the flame also rises. Moreover, flames diluted with H2O exhibit lower peak temperatures compared to flames diluted with N2.Flames diluted with H2O result in lower NOx emissions compared to flames diluted with N2. Additionally, for N2diluted flames, the exit NOx emissions rise as the oxygen concentration increases.Despite this, NOx emissions from H2Odiluted flames demonstrate non-monotonic behaviour.This means that the exit NOx increases initially as the oxygen concentration reaches 21%, but then begins to decrease. In contrast to N2and H2Odiluted flames exhibits a wider regime of no-ignition.Moreover, the rise in peak temperature in H2Odiluted flames is less apparent than in N2diluted flames, corresponding to broader ranges MILD combustion ranges.Furthermore, to attain MILD combustion in H2O diluted flames at a specific O2 concentration, the temperature of reactant needs to be higher than that required for N2diluted flames.

Raman-corrected two-photon absorption laser induced fluorescence of atomic oxygen in premixed hydrogen, cellular tubular flames

Femtosecond, Two-photon Absorption Laser Induced Fluorescence (fs-TALIF) corrected for collisional quenching with Raman scattering is used to capture spatially resolved atomic oxygen profiles in lean premixed, hydrogen cellular tubular flames. This method has allowed comparisons of number density and O-atom concentration distributions in flames of variable stretch rates in a manner similar to that previously performed on the minor flame species H and OH. As stretch rate increases, the radii of peak O-atom in the cells decrease while O-atom concentrations remain relatively unaffected. This differs from non-cellular flame data where increasing stretch rate increases minor species number densities. Three chemical mechanisms are employed to perform direct numerical simulations of the O-atom profiles in the tubular flames and are found to be in close agreement with one another. For N2-diluted flames, the simulations predict O-atom number densities within the uncertainty of the data for the cellular region but over-predict the O-atom number densities in the dearth region of the 2D flames. Additionally, simulated O-atom concentrations contradict the trend of the data and increase with stretch rate. Changing the diluent from N2 to CO2 lowers the peak concentrations of atomic oxygen as CO2 becomes reactive at flame temperatures. This allows the CO+O(+M)⇌CO2(+M) reaction to consume atomic oxygen. Flames diluted with carbon dioxide caused the model to over-predict the O-atom concentration in these flames. This discrepancy is similar to past minor species measurements in cellular tubular flames though it does not occur in minor species profiles of non-cellular (1D), CO2-diluted tubular flames. The discrepancy could be caused by the simplifying relationships employed to convert the 3D geometry to 2D in the simulations.

Comparative analysis of the stability and structure of premixed C3H8/O2/CO2 and C3H8/O2/N2 flames for clean flexible energy production

The study compares the macrostructure and stabilization parameters of premixed oxy-propane (C3H8/O2/CO2) and oxygen-enriched air-propane (C3H8/O2/N2) flames at fixed inlet velocity of 5.2 m/s and under similar conditions of oxygen fraction (OF= 21%–70%) and equivalence ratio (ϕ=0.1−1.0) in a model dry-low-emissions (DLE) gas turbine combustor. The combustor stability maps were plotted within the ϕ-OF domain against background contours of constant adiabatic flame temperature (AFT), to quantify the blowout and flashback limits. It was found for both CO2- and N2-diluted flames that the blowout and flashback limits followed constant AFT contours on the stability maps. The blowout limit of both types of flames roughly followed the same AFT contour of about ∼1580 K, while the flashback limits followed the constant AFT contour of 2350 K. This implies that C3H8/O2/N2 flames blow out at leaner conditions compared to C3H8/O2/CO2 flames. The effect of diluent type on blowout limit diminishes at higher OF; above 55% both flames blow out at ϕ∼0.2. All flames of the same AFT exhibited considerably identical shapes despite having different OF and ϕ. Based on that, the design and operation of future oxy-fuel turbines is recommended to be AFT-based rather than OF or ϕ.

Autoignition and flame lift-off behavior of a fuel jet mixing with turbulent hot air coflow

For advanced lean premixed gas turbine combustors that have high inlet air temperatures, autoignition that occurs during the fuel/air mixing process is becoming an issue, which can cause flame-holding inside the premixing device and burn the hardware. This paper reports an experimental study on the autoignition and flame lift-off behavior of a fuel jet injected into hot air coflow that mimics the fuel/air mixing process of lean-premixed combustors. Autoignition occurred when a fuel jet (diluted with N2 or CO2) was injected concentrically into the hot turbulent air coflow. Three distinctive types of autoignition behavior were observed at increasingly higher jet velocity: (1) attached flame; (2) lifted flame followed by intermittent autoignition spots; (3) lifted flame. Lift-off height of the flame was measured at various fuel jet velocities and dilution ratios. The liftoff height increases monotonically with fuel jet velocity and dilution ratio. The CO2-diluted flame has higher lift-off height than the N2-diluted flame. The lift-off height data were fitted to various models to determine the flame stabilization mechanism. A modified large-scale mixing model can capture the effects of both turbulence mixing and autoignition kinetics on lift-off height, underscoring the importance of both turbulent mixing and autoignition kinetics in flame stabilization.

Dilution effects of N2 and CO2 on flame structure and reaction characteristics in CH4/O2 flames

The excited radical chemiluminescence spectra of flames contain a variety of combustion characteristic information such as flame structure and temperature field, and are therefore widely used in the flame spectral diagnosis. In this work, the two-dimensional OH* distribution in the methane/oxygen co-flow diffusion flames was obtained by an UV imaging system, and an Abel inversion algorithm was performed as well. Based on the variation of OH* distribution characteristics, the effects of the addition of N2 and CO2 as diluent on the flame reaction zone and structural characteristics were investigated, and the differences in the flame combustion characteristics between the N2-diluted flame and the CO2-diluted flame were further compared. As the diluent concentration increases in the oxidant, the flame becomes elongated. At the same diluent concentration, the CO2-diluted flame has a narrower flame front and the chemical reaction intensity is significantly lower than the N2-diluted flame. The continuous addition of the diluent leads to a transition in the position of flame core reaction zone, which gradually close to the burner exit. The transition is affected by the flow field, and the reaction mechanism is also one of the influencing factors. The dominant reaction to generate OH* gradually changes from R2 (H + O + M → OH* + M) to R1 (CH + O2 → CO + OH*) with the dilution level increases. At a low dilution level, the CO2-diluted flame shows a thinner and longer flame shape. However, with the dilution level increasing, the height and reaction zone thickness in the N2-diluted flame become greater, compared with the flame of CO2 dilution.

Effect of N2 Replacement by CO2 in Coaxial-Flow on the Combustion and Emission of a Diffusion Flame

In this study, a double concentric burner burning methane with an annular coaxially-flowing oxidizer was adopted to operate the diffusion flame in lifted flame regime. The effects of coaxial-flow velocity, coaxial-flow composition variation through total and partial replacement of N2, and coaxial-flow oxygen enrichment were experimentally investigated in terms of the resultant changes in the flame stability, and thermal and emission characteristics. Consistent with the triple flame theory, the current stability tests show a linear increase in flame lift height with increasing coaxial-flow velocity and the blowout of lifted flames occurred at constant flame tip height. Replacement of N2 by CO2 in the coaxial-flow deteriorated the flame stability by significantly reducing the threshold coaxial-flow velocity. Due to combustion enhancement that is caused by oxygen enrichment, the threshold coaxial-flow velocity increased and this increase is more significant for the N2-diluted flame than CO2-diluted. Two of the most important NOx formation mechanisms, Zeldovich and Fenimore, were analyzed under the relatively low temperature flame conditions, generally below 1300 °C in this study. Results show that NOx is principally produced via the Fenimore mechanism for both N2- and CO2-diluted flames. NOx productions can be significantly affected by coaxial-flow composition and coaxial-flow velocity. An increase in the velocity of N2-diluted coaxial-flow increases NOx emissions, while a reverse trend occurred, as N2 in the coaxial-flow was replaced or partially replaced by CO2, which is ascribed to the strong combustion-resisting behavior of CO2. For all cases, CO emissions vary in the opposite direction of NOx emissions. Due to the strong thermal and chemical effects of CO2 on combustion in comparison to N2, total or partial replacement of N2 by CO2 results in a steep increase in CO emissions.

Flow characterization and dilution effects of N2 and CO2 on premixed CH4/air flames in a swirl-stabilized combustor

Numerically-aided experimental studies are conducted on a swirl-stabilized combustor to investigate the dilution effects on flame stability, flame structure, and pollutant emissions of premixed CH4/air flames. Our goal is to provide a systematic assessment on combustion characteristics in diluted regimes for its application to environmentally-friendly approaches such as biogas combustion and exhaust-gas recirculation technology. Two main diluting species, N2 and CO2, are tested at various dilution rates. The results obtained by means of optical diagnostics show that five main flame regimes can be observed for N2-diluted flames by changing excess air and dilution rate. CO2-diluted flames follow the same pattern evolution except that all the domains are shifted to lower excess air. Both N2 and CO2 dilution affect the lean blowout (LBO) limits negatively. This behavior can be counter-balanced by reactant preheating which is able to broaden the flammability domain of the diluted flames. Flame reactivity is degraded by increasing dilution rate. Meanwhile, flames are thickened in the presence of both diluting species. NOx emissions are significantly reduced with dilution and proved to be relevant to flame stability diagrams: slight augmentation in NOx emission profiles is related to transitional flame states where instability occurs. Although dilution results in increase in CO emissions at certain levels, optimal dilution rates can still be proposed to achieve an ideal compromise.

Experimental studies of flame extinction in a swirl-stabilized oxy-fuel burner

We report on an experimental investigation of oxy-fuel combustion carried out in a co-axial, horizontally-fired swirl-stabilized burner. Retrofitting possibilities and flame extinction have been investigated with methane or liquefied petroleum gas (LPG) as fuel and mixtures of oxygen (O2)–carbondioxide (CO2) or air as oxidant. Temperature profile measurements show that a higher oxygen concentration of around 33–34% by volume in the oxidizer is needed in oxy-fuel combustion to get temperature profiles similar to those for normal air combustion. Oxy-fuel flames have been found to be stable even for O2 concentrations less than 30% by volume, though at reduced peak flame temperatures. The results of extinction studies with CO2 or nitrogen (N2) dilution indicate that CO2-diluted flames are more prone to extinction by dilution than N2-diluted flames. The former get extinguished at an oxygen concentration of 21% by volume whereas the latter can be sustained at oxygen concentrations down to 15% by volume.

산소부화를 통한 화염온도 변화에 따른 연소합성된 TiO2나노입자의 결정구조 변화

In this work, TiO₂ nanoparticles were synthesized using N₂-diluted and Oxygen-enriched co-flow hydrogen diffusion flames. The effect of flame temperature on the crystalline structure of the formed TiO₂ nanoparticles was investigated. The measured maximum centerline temperature of the flame ranged from 2,103K for oxygen-enriched flame to 1,339K for N2-diluted flame. The visible flame length and the height of the main reaction zone were characterized by direct photographs. The crystalline structures of TiO₂ nanoparticles were analyzed by XRD. From the XRD analysis, it was evident that the crystalline structures of the formed nanoparticles were divided into two sorts. In the higher temperature region, over the 1,700K, the fraction of formed TiO₂ nanoparticles having anatase-phase crystalline structure increased with increasing the flame temperature. On the contrary, in the lower temperature region, below the 1,600K, the fraction of anatase-phase nanoparticles increased with decreasing the flame temperature.

수소 확산화염에서 화염온도가 TiO2나노입자의 합성에 미치는 영향

In this work, TiO₂ nanoparticles were synthesized using a N₂-diluted hydrogen coflow diffusion flame. The effects of flame temperature on the crystalline structure and the size of formed nanoparticles were investigated. The maximum centerline temperature of the flame ranged from 1,920K for H₂-only flame to 863K for 81% N₂-diluted flame. The morphology and the crystal structure of TiO₂nanoparticles were analyzed by a TEM and a XRD, respectively. The particle size distribution was also measured by using a scanning mobility particle sizer (SMPS). The mean particle diameter was calculated from the TEM images depended on the flame temperature, having minimum at about 1,200K. Based on the SMPS measurements, the mean particle diameter of TiO₂nanoparticles at flame temperatures > 1,300K was smaller than that at flame temperatures < 1,300K. From the XRD analysis, it was evident that the anatase fraction increased with decreasing the flame temperature. The portion of anatase phase in TiO₂ nanoparticles might be greater than 80% when the flame temperature was lower than 1,000K.

Determination of rates of collision-induced vibrational and intramultiplet transitions for YO(A2II) molecules in Ar- and N2-diluted flames

Abstract Rate contants for single-step collisional transitions between specified vibrational and electronic-doublet levels of YO(A 2 II) molecules were determined under multiple-collision conditions at constant total gas pressure. We recorded simultaneously the thermal emission spectrum and the fluorescence spectra induced by a cw dye laser tuned successively at various vibronic transitions. We used premixed N 2 -diluted or Ar-diluted H 2 -O 2 flames at 1 atm and = 2350 K, containing YO vapour. Relative rate constants (normalized to the quenching plus radiative de-excitation rate constants) were obtained by solving a set of simultaneous master equations involving the stationary laser-induced population increments. Absolute rate constants were derived therefrom by determining indirectly the fluorescence efficiency. Exoergic doublet-mixing collisions appeared to have greater probabilities than Δ v = −1 or −2 transitions and were about equally probable as quenching collisions. Ar and N 2 were about equally efficient as collision partner. The rate constants of each process and of its reverse were compared with detailed-balance. The rotational levels in the pumped band appeared to preserve the e/f symmetry index partially.