Structure of a freely propagating rich CH4/air flame containing triphenylphosphine oxide and hexabromocyclododecane

The chemical and thermal structure of a premixed rich CH₄/air/N₂ flame (?=1.18±0.02) that contains either triphenylphosphine oxide [(C₆H₅)₃PO] or hexabromocyclododecane [C₁₂H₁₈Br₆] and that is stabilized on a Mache-Hebra burner was studied experimentally using molecular beam mass spectrometry (MBMS) and the microthermocouple technique. Compounds such as hexabromocyclododecane (HBCD) and triphenylphosphine oxide (TPPO) are representative flame-retardant additives that are added to polymers to reduce the flammability of the base polymer. Both compounds provide flame retardation in the gas phase by the production of active species that effectively scavenge key combustion radicals to shut down the combustion process. The MBMS method was used to determine the concentration profiles of stable and active species directly in the flame, which includes atoms as well as free radicals. Thin thermocouples were employed to determine temperature profiles in a flame stabilized on a Mache-Hebra burner at a pressure of 1 atm. A comparison of the experimental data and simulation results for the flame structure shows that MBMS is suitable for studying the structure of flames that are close to freely propagating conditions. The relative effectiveness of flame inhibition by the compounds tested was estimated from changes in the peak concentrations of H and OH radicals in the flame and from changes in the estimated flame velocity.     

The influence of ethanol addition on a rich premixed benzene flame at low pressure

The aim of the study is to analyze the effect of ethanol in rich benzene flame, to observe the influence of this oxygenated species and to understand the kinetics of ethanol in the benzene combustion. Two premixed rich benzene/oxygen/argon (11.5% C₆H₆, 43.2% O₂, 45.3% Ar) and benzene/ethanol/oxygen/argon (10.7% C₆H₆, 2.1% C₂H₅OH, 43.2% O₂, 44.0% Ar) flat flames are stabilized at low pressure (45 mbar) on a burner with the same equivalence ratio of 2.0. Identification and monitoring of signal intensity profiles of species within the flames are carried out using molecular beam mass spectrometry (M.B.M.S.). The substitution of some C₆H₆ by C₂H₅OH is responsible for a reduction of the maximum concentrations of main intermediate species such as C₂H₂, C₄H₂, C₄H₄ and C₅H₆. The UCL mechanism is extended to heavier hydrocarbons, tested against these flames to check its validity and used to underline the effect of ethanol on soot precursors formation. It contains 1028 elementary reactions and 184 chemical species.     

Effect of hydrocarbon addition on tip opening of hydrogen-air bunsen flames

The effect of hydrocarbon addition on tip opening of lean and stoichiometric hydrogen-air flames is studied computationally by performing two-dimensional numerical simulations. The numerical study reveals that the flame tip of the H₂-air burner stabilized flame is open at lean and stoichiometric mixture conditions. The flame tip closes upon hydrocarbon addition. The tip closing is mainly affected by preferential diffusion of the multi-component mixture and the stretch effects. In the addition of light hydrocarbon (CH₄), the tip closing starts at a higher percentage of hydrocarbon addition in H₂-air flames. Whereas, upon the addition of heavy hydrocarbons such as propane and butane in H₂-air flames, tip closing starts with a lesser amount of hydrocarbon addition. Temperature, OH mole fraction and heat release rate have been investigated, focusing on the flame structure at the tip. The tip opening regime diagram for H₂–CH₄-air, H₂–C₃H₈-air and H₂–C₄H₁₀-air mixtures are presented.     

Transport budgets in turbulent lifted flames of methane autoigniting in a vitiated co-flow

Autoignition of hydrocarbon fuels is an outstanding research problem of significant practical relevance in engines and gas turbine applications. This paper presents a numerical study of the autoignition of methane, the simplest in the hydrocarbon family. The model burner used here produces a simple, yet representative lifted jet flame issuing in a vitiated surrounding. The calculations employ a composition probability density function (PDF) approach coupled to the commercial CFD package, FLUENT. The in situ adaptive tabulation (ISAT) method is used to implement detailed chemical kinetics. An analysis of species concentrations and transport budgets of convection, turbulent diffusion, and chemical reaction terms is performed with respect to selected species at the base of the lifted turbulent flames. This analysis provides a clearer understanding of the mechanism and the dominant species that control autoignition. Calculations are also performed for test cases that clearly distinguish autoignition from premixed flame propagation, as these are the two most plausible mechanisms for flame stabilization for the turbulent lifted flames under investigation. It is revealed that a radical pool of precursors containing minor species such as CH₃, CH₂O, C₂H₂, C₂H₄, C₂H₆, HO₂, and H₂O₂ builds up prior to autoignition. The transport budgets show a clear convective-reactive balance when autoignition occurs. This is in contrast to the reactive-diffusive balance that occurs in the reaction zone of premixed flames. The buildup of a pool of radical species and the convective-reactive balance of their transport budgets are deemed to be good indicators of the occurrence of autoignition.     

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.     

An experimental and modeling study of methyl propanoate pyrolysis at low pressure

Methyl propanoate (MP) pyrolysis in a laminar flow reactor was studied at low pressure (30 Torr) within the temperature range from 1000 to 1500 K. About 30 products were detected and identified in the pyrolysis process using the photoionization mass spectrometry, including H₂, CO, CO₂, CH₃OH, CH₂O, CH₂CO, C1 to C4 hydrocarbons and radicals (such as CH₃, C₂H₅ and C₃H₃). Their mole fraction profiles versus temperature were also measured. For the unimolecular dissociation reactions, the rate constants were calculated by high precision theoretical calculations. Based on the theoretical calculations and measured mole fraction profiles of pyrolysis species, a kinetic model of MP pyrolysis containing 98 species and 493 reactions was developed. The model simulates the primary decomposition process well with the calculated rate constants. According to the rate of production analysis, the decomposition pathways of MP and the formation channels of both oxygenated and hydrocarbon products were discussed. It is concluded that the main decomposition pathway is MP → CH₂COOCH₃ → CH₃CO + CH₂O → CO.     

Conversion of natural gas jet flame burners to hydrogen

In a study of conversion from CH₄ to H₂, jet flame characteristics of these gases and their blends are compared on a burner diameter scale of mm. Low velocity H₂ and CH₄ jets, burned on pipes of different diameters, indicate higher blow-off limits for H₂, but lower heat release rates, a consequence of its lower specific energy. Compensation for this might be obtained through increased H₂ flow velocity, or a small increase in pipe diameter. Blended CH₄/H₂ flames have lower heat release rates than CH₄ alone, yet small proportions of H_2, with CH₄ might still be burned, on a CH₄ burner. Throughout, fundamental understanding is enhanced through two dimensionless groups: laminar flame thickness normalised by burner diameter, δ_k/D, and the dimensionless flow number, U1. These suggest an optimal role for H₂ combustion, utilizing its high acoustic and blow-off velocities, in high intensity, subsonic, combustors, at low δ_k/D, and high U1.     

The structure of coal-air-ch4 laminar flames in a low-pressure burner: cars measurements and modeling studies

An experimental study is described of the structure of a flat, premixed, laminar, coal-air flame, with some methane added for flame stability. A low-pressure burner, at a combustion pressure of 30.4 kPa, was employed, in order to extend the reaction zone. Gas temperatures were measured by the CARS technique and the C₂ emissions observed with the laser diagnostics were found to depend upon the laser power. Concentration profiles of permanent species also were measured over a range of equivalence ratios. Measured values are compared with those predicted by a mathematical model, which assumes that CH₄ and HCN devolatilize from the coal and react in the gas phase. Allowance also is made for reactions of char and radiative heat transfer. The model gives good predictions of the temperature and oxygen concentration profiles, while predictions of NO are somewhat higher than those measured. Formation of NO is favored by OH and removal of it by NH₂ and NH. The sensitivity of the modeled results to various activation energies and pre-Arrhenius constants is examined and optimal values of these are in line with other values in the literature. The principal limitation in the model is the overprediction of CO concentration. An explanation of this lies in the formation, neglected in the model, of tarry structures of high molecular mass, followed by the generation of soot. This interpretation is supported by the measured profiles of C₂ emission intensity and their dependence upon the laser power, in contrast to the weaker emissions from rich CH₄-air flames, which show no such dependence and are less persistent.     

Soot surface oxidation in hydrocarbon/air diffusion flames at atmospheric pressure

Soot surface oxidation was studied experimentally in laminar hydrocarbon/air diffusion flames at atmospheric pressure. Measurements were carried out along the axes of round fuel jets burning in co-flowing dry air considering acetylene-nitrogen, ethylene, propylene-nitrogen, propane and acetylene-benzene-nitrogen in the fuel stream. Measurements were limited to the initial stages of soot oxidation (carbon consumption less than 70%) where soot oxidation occurs at the surface of primary soot particles. The following properties were measured as a function of distance above the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using Transmission Electron Microscopy (TEM), concentrations of major stable gas species (N₂, H₂O, H₂, O₂, CO, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, and C₆H₆) by sampling and gas chromatography, concentrations of some radical species (H, OH, O) by deconvoluted Li/LiOH atomic absorption and flow velocities by laser velocimetry. For present test conditions, it was found that soot surface oxidation rates were not affected by fuel type, that direct rates of soot surface oxidation by O₂ estimated from Nagle and Strickland-Constable (1962) were small compared to observed soot surface oxidation rates because soot surface oxidation was completed near the flame sheet where O₂ concentrations were less than 3% by volume, and that soot surface oxidation rates were described by the OH soot surface oxidation mechanism with a collision efficiency of 0.14 and an uncertainty (95% confidence) of ±0.04 when allowing for direct soot surface oxidation by O₂, which is in reasonably good agreement with earlier observations of soot surface oxidation rates in both premixed and diffusion flames at atmospheric pressure.     

Extinction and near-extinction instability of non-premixed tubular flames

Tubular non-premixed flames are formed by an opposed tubular burner, a new tool to study the effects of curvature on extinction and flame instability of non-premixed flames. Extinction of the opposed tubular flames generated by burning diluted H₂, CH₄ or C₃H₈ with air is investigated for both concave and convex curvature. To examine the effects of curvature on extinction, the critical fuel dilution ratios at extinction are measured at various stretch rates, initial mixture strengths and flame curvature for fuels diluted in N₂, He, Ar or CO₂. In addition, the onset conditions of the cellular instability are mapped as a function of stretch rates, initial mixture strengths, and flame curvature. For fuel mixtures with Lewis numbers much less than unity, such as H₂/N₂, concave flame curvature towards the fuel suppresses cellular instabilities.     

Measurements and modeling of SiCl4 combustion in a low-pressure H2/O2 flame

Laser-induced fluorescence (LIF) was used to measure temperature and OH concentration profiles as a function of distance from a McKenna-style burner in premixed one-dimensional low-pressure H₂/O₂/Ar SiCl₄-doped flames. The addition of SiCl₄ was shown to affect the flame temperature and OH concentration profiles. A gas-phase chemical kinetics mechanism for the combustion of SiCl₄ in an H₂/O₂/Ar flame was proposed, and experimental results were compared with predictions for a premixed one-dimensional laminar flame model based on CHEMKIN. The low-pressure flame data are sensitive to the overall kinetics of the mechanism. In order to obtain the best fit to the observed data for all flame configurations, we had to modify six different rates from our original estimates. None of the modified rates are well known for the temperature regime of our flame. Particle formation and surface chemistry were not taken into account.     

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.     

Rich methane premixed laminar flames doped by light unsaturated hydrocarbons: III. Cyclopentene

In line with the studies presented in Parts I (methane flame seeded with allene and propyne) and II (methane flame seeded with 1,3-butadiene) of this paper, the structure of a laminar rich premixed methane flame doped with cyclopentene has been investigated. The gases of this flame contain 15.3% (molar) of methane, 26.7% of oxygen, and 2.4% cyclopentene, corresponding to an overall equivalence ratio of 1.79 and a C₅H₈/CH₄ ratio of 15.7%. The flame has 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 measured temperature ranged from 627 K close to the burner up to 2027 K. Species quantified by gas chromatography included the usual methane C₀–C₂ combustion products, but also propyne, allene, propene, propane, 1-butene, 1,3-butadiene, 1,2-butadiene, vinylacetylene, diacetylene, cyclopentadiene, 1,3-pentadiene, benzene, and toluene. A new mechanism for the oxidation of cyclopentene has been developed and added to the former model for the oxidation of small unsaturated hydrocarbons, benzene, and toluene described in Parts I and II. The whole mechanism involved 175 species in 1134 reactions. The main reaction pathways of consumption of cyclopentene and of formation of benzene and toluene are presented and discussed from flow rate analyses.     

Experimental investigation of thermoacoustic coupling using blended hydrogen–methane fuels in a low swirl burner

In this study, experimental testing and analysis were performed to examine the combustion instability characteristics of hydrogen–methane blended fuels for a low-swirl lean premixed burner. The aim of this study is to determine the effect of hydrogen addition on combustion instability, and this is assessed by examining the flame response to a range of constant amplitude, single frequency chamber acoustic modes. Three different blends of hydrogen and methane (93% CH₄–7% H₂, 80% CH₄–20% H₂ and 70% CH₄–30% H₂ by volume) were employed as fuel at an equivalence ratio of 0.5, and with four different acoustic excitation frequencies (85, 125, 222 and 399 Hz). Planar laser induced fluorescence of the hydroxyl radical (OH-PLIF) was employed to measure the OH concentration at different phases of acoustic excitation and a Rayleigh Index was then calculated to determine the degree of thermoacoustic coupling. It was found, as has been previously reported, that the combustion characteristics are very sensitive to the fraction of hydrogen in the fuel mixture. The flame shows significant increases in flame base coupling and flame compaction with increasing hydrogen concentration for all conditions. While this effect enhances the flame response at non-resonant frequencies, it induces only minimal compaction and appears to decreases the coupling intensity at the resonant frequency.     

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

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

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.     

Hydrogen production from rich combustion in porous media

This paper examines rich combustion of methanol, methane, octane and automotive-grade petrol inside inert porous media in an effort to examine the suitability of the concept for hydrogen production. Species concentrations were measured and operating limits were tested of steady rich flames stabilized inside a two-layer alumina foam burner and a two-layer alumina bead burner. Using a conversion efficiency based on lower heating values, up to 56% of the methanol was converted to syngas (H₂, CO) inside the alumina foam burner and 66% inside the alumina bead burner. Using the same efficiency definition, 45% percent of the methane and 36% of the octane and petrol was converted to syngas with a significant portion of the energy remaining trapped in CH₄, C₂H₂ and C₂H₄. For methanol, the highest equivalence ratio observed for stable combustion was 6.1 inside the foam burner and 9.3 inside the bead burner which are higher than the conventional upper flammability limit (UFL) of 4.1. Methane's UFL was increased to 1.9 and, at a minimum, the conventional upper flammability limits of iso-octane and petrol were attained. A wide operating envelope was observed, which allowed for large turndown ratios up to 20:1. The composition of the products of the methanol flames examined here were close to equilibrium for relatively low equivalence ratios, while those of hydrocarbon flames differed significantly from equilibrium for all φ suggesting that finite rate kinetics are important. The high conversion efficiencies, quick startup times, compact size, and the absence of a catalyst make the present burner suitable for consideration as part of a reformer in a fuel cell powered automobile.     

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

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