Rich methane premixed laminar flames doped with light unsaturated hydrocarbonsI. Allene and propyne

Abstract

The structure of three laminar premixed rich flames have 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 equivalent ratio of 1.25 for the pure methane flame. In both doped flames, 2.49 % of C 3 H 4 was added, corresponding to a ratio C 3 H 4 / CH 4 of 12 % and an equivalent 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, iso-butene, 1-butyne, vinylacetylene and benzene. The temperature was measured using a thermocouple in PtRh (6%)-PtRh (30%) settled inside the enclosure and ranged from 700K close to the burner up to 1850K. In order to model these new results, some improvements have been made to a mechanism previously developed in our laboratory for the reactions of C 3 -C 4 unsaturated hydrocarbons. The main reaction pathways of consumption of allene and propyne and of formation of C 6 aromatic species have been derived from flow rate analyses.Keywords : Premixed laminar flame, methane, allene, propyne, modeling. 3 INTRODUCTIONSoots and polyaromatic hydrocarbons (PAH), which are present in the exhaust gas of diesel engine, represent a high part of the urban pollution. Many efforts have then been focused on reducing the emissions of these compounds. The formation of soot precursors and PAH in combustion involves small unsaturated hydrocarbons, the chemistry of which is still very uncertain. Different reaction pathways have been proposed for the formation and the oxidation of the first aromatic compounds, involving the reactions of C 2 (acetylene), C 3 or C 4 unsaturated species [1][2][3][4][5].As the determinant role of propargyl radicals in forming benzene, the first aromatic ring, is now well accepted, it is important to better understand their reactions. With that purpose, the oxidation of allene (propadiene) and propyne has been already studied in several oxidation conditions: shock tubes [6][7][8], flow reactor [9,10], jet-stirred reactor [7,11] and premixed flames [10,11]. Previous works in flames include studies of the influence of the addition of allene on The purpose of the present paper is to experimentally investigate the structures of two premixed laminar methane flames containing propadiene and propyne, respectively, and to compare them with that of a pure methane flame containing the same mole fractions of methane and oxygen. The use of a methane flame will allow us to have a reactive mixture rich in methyl radicals and to be more representative of combustion mixtures containing larger hydrocarbons than hydrogen or ...

Rich methane premixed laminar flames doped by light unsaturated hydrocarbons

“…The maximum of the peak of benzene is obtained around 0.4 cm above the burner and that of toluene around 0.6 cm. The maximum value reached by the mole fraction of benzene is around three times that measured in the flame doped by 1,3-butadiene [10] and ten times that measured in the flame doped with allene [9]. The amount of toluene is more than fifty times higher than in the flame doped by 1,3-butadiene.…”

“…As we have used the same apparatus, the same method to measure temperature and the same analytical techniques as what is extensively described in the part I of this paper [9], they are not presented again here. A single improvement was made to allow a liquid reactant to be injected in the gaseous mixture feeding the flame.…”

“…The mechanism proposed here to model the oxidation of cyclopentene includes the previous mechanisms that were built to model the oxidation of C 3 -C 4 unsaturated hydrocarbons [9,10,15,16], benzene [17] and toluene [18]. Thermochemical data were estimated by the software THERGAS developed in our laboratory [19], which is based on the additivity methods proposed by Benson [20].…”

“…These linear radical can isomerize to give the resonance stabilized 1-penten-3-yl radicals (reaction 52), the reactions of which were considered in the C 3 -C 4 reaction base [9], decompose to yield we have considered, as proposed by Lifshitz et al [8], the isomerization between both radicals (reaction 80) and the formation of cyclohexadienyl radicals, which are precursors of benzene, from cyclopentadienyl methylene radicals (reaction 81). We have not considered the formation of cyclohexadienyl radicals from the resonance stabilized methyl cyclopentadienyl radicals, as proposed by Marinov et al [5], but their recombination with H-atoms (reaction 82) was also added.…”

“…Nevertheless, some papers suggest a link between C 6 and C 5 cyclic species [1,7,8]. In a first part of this paper [9], we have investigated the reactions of allene and propyne, as they are precursors of propargyl radicals, which have an important role in forming benzene. In a second part [10], we have analyzed the reaction of 1,3-butadiene in a methane flame and putted in evidence an important production of benzene due to reactions of C 4 compounds under these particular conditions.…”