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.     

Syngas production by methane-rich combustion in a divergent burner of porous media

The superadiabatic combustion in porous media contributes to the efficient conversion of methane to syngas. In this paper, a divergent packed bed burner of two-layer was proposed to obtain the characteristics of methane partial oxidation. The divergent angle, interface location and pellet diameter were used to study the temperature and species distributions. Results indicate that the upper limit of velocity gradually decreased as the equivalence ratio increased and the limit of the divergent burner is obviously higher than that of the cylindrical one. The increasing of the divergent angle within a certain range enhances the methane conversion and the 15° shows the best among the selected five angles. The mole fractions of H₂ and CO gradually decrease when the interface locations move from the cylindrical region to the divergent one. As the equivalence ratio increased from 1.3 to 3.5, the yields of H₂ and CO and the energy conversion efficiency of syngas increase first and then decrease, and the maximum efficiency of 45.9% appears at the equivalence ratio of 2.0. The divergent region weakens the influence of inlet velocities and contributes to the stability of reforming reactions.     

Turbulent flame topology and the wrinkled structure characteristics of high pressure syngas flames up to 1.0 MPa

The turbulent flame topology characteristics of the model syngas with two different hydrogen ratios were statistically investigated, namely CO/H₂ ratio at 65/35 and 80/20, at equivalence ratio of 0.7. The combustion pressure was kept at 0.5 MPa and 1.0 MPa, to simulate the engine-like condition. The model syngas was diluted with CO₂ with a mole fraction of 0.3 which mimics the flue gas recycle in the turbulent combustion. CH₄/air flame with equivalence ratio of 1.0 was also tested for comparison. The flame was anchored on a premixed type Bunsen burner, which can generate a controllable turbulent flow. Flame front, which is represented by the sharp increased interface of the OH radical distribution, was measured with OH-PLIF technique. Flame front parameters were obtained through image processing to interpret the flame topology characteristics. Results showed that the turbulent flames possess a wrinkled character with smaller scale concave/convex structure superimposed on a larger scale convex structure under high pressure. The wrinkled structure of syngas flame is much finer and more corrugated than hydrocarbon fuel flames. The main reason is that scale of wrinkled structure is smaller for syngas flame, resulting from the unstable physics. Hydrogen in syngas can increase the intensity of the finer structure. Moreover, the model syngas flames have larger flame surface density than CH₄/air flame, and hydrogen ratio in syngas can increase flame surface density. This would be mainly attributed to the fact that the syngas flames have smaller flame intrinsic instability scale l_i than CH₄/air flame. S_T/S_L of the model syngas tested in this study is higher than CH₄/air flames for both pressures, due to the high diffusivity and fast burning property of H₂. This is mainly due to smaller L_M and l_i. V_f of the two model syngas is much smaller than CH₄/air flames, which suggests that syngas flame would lead to a larger possibility to occur combustion oscillation.     

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

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

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.     

Burning syngas in a high swirl burner: Effects of fuel composition

Flame characteristics of swirling non-premixed H₂/CO syngas fuel mixtures have been simulated using large eddy simulation and detailed chemistry. The selected combustor configuration is the TECFLAM burner which has been used for extensive experimental investigations for natural gas combustion. The large eddy simulation (LES) solves the governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modelling based on the localised dynamic Smagorinsky model and the steady laminar flamelet model respectively. The predictions for H₂-rich and CO-rich flames show considerable differences between them for velocity and scalar fields and this demonstrates the effects of fuel variability on the flame characteristics in swirling environment. In general, the higher diffusivity of hydrogen in H₂-rich fuel is largely responsible for forming a much thicker flame with a larger vortex breakdown bubble (VBB) in a swirling flame compare to the H₂-lean but CO-rich syngas flames.     

Stability characteristics of non-premixed turbulent jet flames of hydrogen and syngas blends with coaxial air

The stability characteristics of attached hydrogen (H₂) and syngas (H₂/CO) turbulent jet flames with coaxial air were studied experimentally. The flame stability was investigated by varying the fuel and air stream velocities. Effects of the coaxial nozzle diameter, fuel nozzle lip thickness and syngas fuel composition are addressed in detail. The detachment stability limit of the syngas single jet flame was found to decrease with increasing amount of carbon monoxide in the fuel. For jet flames with coaxial air, the critical coaxial air velocity leading to flame detachment first increases with increasing fuel jet velocity and subsequently decreases. This non-monotonic trend appears for all syngas composition herein investigated (50/50 → 100/0% H₂/CO). OH^∗ chemiluminescence imaging was performed to qualitatively identify the mechanisms responsible for the flame detachment. For all fuel compositions, local extinction close to the burner rim is observed at lower fuel velocities (ascending stability limit), while local flame extinction downstream of the burner rim is observed at higher fuel velocities (descending stability limit). Extrema of the non-monotonic trends appear to be identical when the nozzle fuel velocity is normalized by the critical fuel velocity obtained for the single jet cases.     

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

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

Flame front structure of turbulent premixed flames of syngas oxyfuel mixtures

In order to investigate oxyfuel combustion characteristics of typical composition of coal gasification syngas connected to CCS systems. Instantaneous flame front structure of turbulent premixed flames of CO/H₂/O₂/CO₂ mixtures which represent syngas oxyfuel combustion was quantitatively studied comparing with CH₄/air and syngas/air flames by using a nozzle-type Bunsen burner. Hot-wire anemometer and OH-PLIF were used to measure the turbulent flow and detect the instantaneous flame front structure, respectively. Image processing and statistical analyzing were performed using the Matlab Software. Flame surface density, mean progress variable, local curvature radius, mean flame volume, and flame thickness, were obtained. Results show that turbulent premixed flames of syngas possess wrinkled flame front structure which is a general feature of turbulent premixed flames. Flame surface density for the CO/H₂/O₂/CO₂ flame is much larger than that of CO/H₂/O₂/air and CH₄/air flames. This is mainly caused by the smaller flame intrinsic instability scale, which would lead to smaller scales and less flame passivity response to turbulence presented by Markstain length, which reduce the local flame stretch against turbulence vortex. Peak value of Possibility Density Function (PDF) distribution of local curvature radius, R, for CO/H₂/O₂/CO₂ flames is larger than those of CO/H₂/O₂/air and CH₄/air flames at both positive and negative side and the corresponding R of absolute peak PDF is the smallest. This demonstrates that the most frequent scale is the smallest for CO/H₂/O₂/CO₂ flames. Mean flame volume of CO/H₂/O₂/CO₂ flame is smaller than that of CH₄/air flame even smaller than that of CO/H₂/O₂/air flame. This would be due to the lower flame height and smaller flame wrinkles.     

A pore level study of syngas production in two-layer burner formed by staggered arrangement of particles

A 2D model of simplified porous burner is developed to analyze the syngas production from fuel-rich CO₂/CH₄ partial oxidation with detailed chemical kinetics GRI-Mech 1.2. The geometry simulates a two-layer burner, which is composed of 2.5 mm particles in the upstream and 7.5 mm particles in the downstream. A 2D packed bed of connected particles with staggered arrangement is developed. The discrete ordinates (DO) model is used to compute the surface to surface radiation and gas radiation. The solid conduction between the neighboring particles is taken into account by bridge approach. The predicted results are compared with experiment and good agreement between the predictions and experiment is observed. The effect of CO₂ injection in the system is examined. It is demonstrated that 2D pore level simulations by simplified geometry with detailed chemical kinetics can capture the features of syngas compositions and temperature profiles. Local information of species, temperature and velocity distributions within pores in the burner is presented and analyzed. Results show that thermal nonequilibrium in the same particles exists in the entire burner, and that chemical nonequilibrium for the main syngas of H₂, CO and CO₂ is observed from exothermic zone to the burner outlet.     

The inhibition/promotion effect of C6F12O added to a lithium-ion cell syngas premixed flame

The objective of this study is to investigate the impact of different fractions (0–0.05) of C₆F₁₂O addition on laminar flame speed of hydrocarbon syngas by varying the fuel/oxidizer equivalence ratio (0.6–1.2) using Bunsen burner method. The determination of the syngas composition comes from the venting gas of lithium-ion cell during thermal runaway. It is found that C₆F₁₂O is significantly more effective at stoichiometric and fuel-rich conditions compared to lean flames regardless of fuel species, which implies more suitable for syngas/air flame inhibition than CH₄. The laminar flame speeds of syngas/air increased with lower concentration (<0.01) of C₆F₁₂O when equivalence ratio less than 0.67, while it decreased with arbitrary concentration of C₆F₁₂O at the condition of equivalence ratio not less than 0.67 due to the increased heat release rate by exothermic reaction involving C₆F₁₂O. The laminar flame speed was more sensitive to C₆F₁₂O addition at stoichiometric and fuel-rich conditions due to the inhibitory effect of substances containing fluorine. Comparison between experimental and numerical results shows a better agreement under fuel-lean conditions with lower C₆F₁₂O additions using a modified mechanism derived from USC Mesh II. Thermodynamic equilibrium calculations and sensitivity analyses are showed separately that the variation of flame radical concentrations is consistent with laminar flame speeds and the lean flames are more sensitive to the reactions containing fluorine compared to rich for syngas/air flame with C₆F₁₂O addition.     

往复流多孔介质燃烧器的二维数值模拟与结构改进

对往复式惰性多孔介质燃烧器进行了二维数值模拟,模型的有效性通过实验数据进行验证.在燃烧器中分别填充4孔/cm泡沫陶瓷或小球,研究其内部的燃烧温度和压力损失.结果表明,由相同材料制成但结构不同的多孔介质对燃烧器内的高温区域和压力损失有显著的影响.孔隙率较大的泡沫陶瓷适合于布置在燃烧区,而孔隙率较小的小球适合于布置在热交换区域.改进燃烧器结构,即在燃烧器的中间布置泡沫陶瓷,而在两端布置小球.对于当量比为0.1的甲烷与空气混合气,得到了更为宽广的高温区域和适度的压力降.     

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.     

Fuel‐lean VOCs combustion in a porous burner stacked with alumina balls: A case for ethylene combustion

A porous burner stacked in turn with 3‐ and 9‐mm alumina pellets was established to perform C₂H₄ combustion experiments by acquiring the flammable limits, temperature variation characteristics, combustion wave velocity, pollutant emissions, and treatment efficiency. The burner operated well at equivalence ratios within 0.3 to 0.7. Larger alumina pellets widened the burner's lower flammable limit. As the flame propagated downstream, the higher premixed gas flow velocity and larger alumina pellets, the higher combustion wave velocity, whereas the circumstances were opposite as the flame spread upstream. The combustion temperature increased with the equivalence ratio and premixed gas flow velocity. In response to the effect of the alumina pellet dimension, 3‐mm alumina pellets corresponded to higher combustion temperatures, lower CO emissions, and higher treatment efficiency than those less than 9‐mm conditions.     

Laminar burning velocity with oxygen-enriched air of syngas produced from biomass gasification

Several studies on the laminar burning velocity of syngas mixtures have been conducted by various researchers. However, in most of these studies, dry air was used as the oxidizer, whereas very few studies have been conducted on syngas combustion in oxygen – enriched air. In this work, a numerical and experimental study on the laminar burning velocity of a mixture of H₂, CO and N₂ (20:20:60 vol%) was performed using air enriched with oxygen as the oxidizer, varying the oxygen content from 21% up to 35% for different equivalence ratios. Numerical calculations were conducted using three detailed reaction mechanisms and transport properties. Flames were generated using contoured slot-type nozzle burners, and Schlieren images were used to determine the laminar burning velocity with the angle method. The experiments were performed under the conditions of Medellin (1550 m.a.s.l.), 0.838 atm and 298 K. The laminar burning velocity increases with the concentration of the oxygen in the mixture due to the increase of the reaction rate; for a stoichiometric mixture, the laminar burning velocity increases by almost 25% with an increment of 4% of oxygen in the oxidant. However, the flammability limits also increase, allowing stable flames to exist in a wider range of equivalence ratios.     

Effect of radiation emission and reabsorption on flame temperature and NO formation in H2/CO/air counterflow diffusion flames

The radiation effect on flame temperature and NO emission of H₂-lean (0.2H₂ + 0.8CO) and H₂-rich (0.8H₂ + 0.2CO) syngas/air counterflow diffusion flames was numerically investigated using OPPDIF code incorporated with the optical thin model, statistical narrow band model and adiabatic condition. Firstly, the coupled effect of strain rate and radiation was studied. Disparate tendencies of NO emission with an increasing strain rate between H₂-lean and H₂-rich syngas flames were found at very small strain rate, and the effect of radiation reabsorption on NO formation can be neglected when the strain rate was greater than 100 s^?1 for both H₂-lean and H₂-rich syngas flames. Because the radiation effect is vital to flames with small strain rate, its impact on flame temperature and NO emission was investigated in detail at a strain rate of 10 s^?1. The results indicated that NO formation is more sensitive to radiation reabsorption than flame temperature, especially for the H₂-rich syngas flame. The underlying mechanism was discovered by using reaction pathway analysis. Furthermore, the radiation effect under CO₂ dilution of the syngas fuel was examined. It was demonstrated that the radiation effect on flame temperature became more prominent with the increase of CO₂ concentration for both H₂-lean and H₂-rich syngas. The radiation effect on NO emission increased first and then decreased with an increasing CO₂ content for H₂-lean syngas, whereas for H₂-rich syngas the radiation effect is monotonic.     

A computational study of combustion and extinction of opposed-jet syngas diffusion flames

This paper reports a numerical study on the combustion and extinction characteristics of opposed-jet syngas diffusion flames. A model of one-dimensional counterflow syngas diffusion flames was constructed with constant strain rate formulations, which used detailed chemical kinetics and thermal and transport properties with flame radiation calculated by statistic narrowband radiation model. Detailed flame structures, species production rates and net reaction rates of key chemical reaction steps were analyzed. The effects of syngas compositions, dilution gases and pressures on the flame structures and extinction limits of H₂/CO synthetic mixture flames were discussed. Results indicate the flame structures and flame extinction are impacted by the compositions of syngas mixture significantly. From H₂-enriched syngas to CO-enriched syngas fuels, the dominant chain reactions are shifting from OH + H₂→H + H₂O for H₂O production to OH + CO→H + CO₂ for CO₂ production through the key chain-branching reaction of H + O₂→O + OH. Flame temperature increases with increasing hydrogen content and pressure, but the flame thickness is decreased with pressure. Besides, the study of the dilution effects from CO₂, N₂, and H₂O, showed the maximum flame temperature is decreased the most with CO₂ as the dilution gas, while CO-enriched syngas flames with H₂O dilution has highest maximum flame temperature when extinction occurs due to the competitions of chemical effect and radiation effect. Finally, extinction limits were obtained with minimum hydrogen percentage as the index at different pressures, which provides a fundamental understanding of syngas combustion and applications.     

Effect of hydrogen addition on NOx formation in high-pressure counter-flow premixed CH4/air flames

A laboratory-scale laminar counterflow burner was used to investigate NO formation in high pressure premixed CH₄/H₂/air flames. New experimental results on NO measurements by LIF were obtained at high pressure in CH₄/H₂/air flames with H₂ content fixed at 20% in the fuel at pressures ranging from 0.1 to 0.7 MPa and an equivalence ratio progressively decreased from 0.74 to 0.6. The effects of hydrogen addition, equivalence ratio and pressure are discussed. These results are satisfactorily compared to the simulations using two detailed mechanisms: GDFkin®3.0_NOmecha2.0 and the mechanism from Klippenstein et al., which are the most recent high-pressure NOx formation mechanisms available in the literature. A kinetic analysis based on Rate of Production/Rate of Consumption and sensitivity analyses of NO is then presented to identify the main pathways that lead to the formation and consumption of NO. In addition, the effect of hydrogen addition on NO formation pathways is described and analysed.     

Two-color, two-photon laser-induced polarization spectroscopy (LIPS) measurements of atomic hydrogen in near-adiabatic, atmospheric pressure hydrogen/air flames

Two-color, two-photon laser-induced polarization spectroscopy (LIPS) of atomic hydrogen has been demonstrated and applied in atmospheric pressure hydrogen/air flames. Fundamental and frequency-doubled beams from a single 486-nm dye laser were used in the experiments. The 243-nm pump beam in the measurements was tuned to the two-photon n=1→n=2 resonance of the hydrogen atom. The 486-nm probe beam was tuned to the single-photon n=2→n=4 resonance of the hydrogen atom. Measurements were performed in an atmospheric pressure H₂/air flame stabilized on a near-adiabatic, flat-flame calibration burner (the Hencken burner). For the range of pump beam intensities used, the LIPS signal was found to be nearly proportional to the square of the pump beam intensity over a wide range of flame equivalence ratios. Spectral lineshapes were recorded at flame equivalence ratios ranging from 0.85 to 2.10. Vertical H-atom number density distribution profiles were measured in the Hencken burner. The vertical H-atom number density profiles measured along the burner centerline for various flame equivalence ratios were compared with the results of a numerical flame calculation using the UNICORN (Unsteady Ignition and Combustion with Reactions) code. Good agreement between theory and experiment was obtained for stoichiometric and rich flame conditions. For flames with equivalence ratios greater than 1.5, the H-atom concentration was substantially above the adiabatic equilibrium value, even at 50 mm above the burner surface. The slow approach to the adiabatic equilibrium H-atom concentration value can be explained by assuming partial equilibrium in the postflame gases; the H-atom concentration is proportional to the O₂ concentration which requires significant residence time to decrease to its very low equilibrium concentration. These results suggest that the use of the Hencken burner as a radical measurement technique calibration source may be of questionable value for equivalence ratios greater than 1.5 and less than 0.8.