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Distribution of sulfur species in the burnt gas of fuel-rich propane-air flames
Combustion
and Flame
Distribution of Sulfur Species in the Burnt Gas of Fuel-E&h Rropane-Air Flames Many industrial flames are of the diffusion type With fuet-rich zones. Surprisingly. the distribution of sulfur species in such James has received little attention, despite the current intense interest in the combustion chemistry of suIfur. Relevant information is available for hydrogen flames [I -4-j ; however. as these give rise to burnt gases differing fundamentally from
those produced
by fueI-rich
hydrocarbon
flames. it could be misleading to infer similar behavior in the two systems. In this communication the computed equilibrium distributions of sulfur species in the burnt gas of fuel-rich propane-ajr flames as a function of fuel-air ratio are compared with the measured relative concentrations >f three sulfur-containing species, nameIy, S02. H,S, and COS. Measured concentration profiles (along the vertical axis of the flame) cf these three species are alsc compared with the calcufated profiles based on measured reversaltemperature profiles. In the experiments to be described, a watercooled burner was used. similar to that described by Padley and Sugden [5], which provided shielded, Iaminar, premixed, propaneair flames. The fuel was a liquefied petroleum gas containing more than 85 % propane, the remainder being propylene except for traces of other hydrocarbons. The fuel was analyzed
at frequent intervals to check for compo:,itio:~ changes due to fractionation, and allowance was made for these in the stoichiometry caIculations. Samples
of the flame gases were withdrawn
by using an uncooled quartz microprobe similar in design lo that of Frrstrom and Westenberg [6]. Gas chromatography and mass spectrometry were used to analyze these sampies for S02- HIS, and COS. For gas chromatography the sampled gases were passed through a trap kept ai: -2WC to remove water, then frozen i:l a trap immersed in liquid nitrogen, and finally stored for later analysis. Under these conditions no reaction between ti2S and SO2 in the sampling systetn was expected [2, 31. but some H2S could have been formed from l-IS in the sampling system [2]+ The chromalographic analyses were carried out ~4th helilrm carrier gasI using a l-meter (2-mm4.d.) column containing Porapak Q* held at 5O’C and a thermal-conductivity detector. The SO2 peak heights obtained from sampling the burnt gas were found to b? linearly related to the SOI concentration in the unburnt gas. The linearity between peak hei_eht and concentration was assumed in the case of the much smaller H2S *A
porous. aromatic
Associales. hc.,
polymer
Frsminghrn,
manuhztured
Mass.. U.S.A.
__-hg Wat@rs
313 I_
G. M Johnson, C. J. Matthews,
burnt-gas concentrations. Corrections were made for the effects of temperature and viscosity on the sampling vattune. In some instances the sample was supplied directly from the flame to a quadrupoie mass spxtrometer (Atlas AMP31 for analysis. The. concentrations of species were estimated directly from the heights of the parent mass peaks. Ftamt: temperatures were measured by the sodium D-line rcvcrsal method [7]. A computer program was developed to calcrtfate the burnt-gas eqni1ibriun-i compositions ;rt fhlc thcorctical adiabatic flame temperature rtnu at any other specified temperature. The ~ourx of’ the ~hcrmodynarnic data was the and OX
M. Y. Smith,
and I). I. ‘willi8trnr
JANAF tables f8]. The equilibrium cuncentrations were obtained by m subprogram based nn the free-energy minimimlicm tnclbd [9jV tnnd this was alticd to a hrut&rrl;inec tiubprogr;rtla which enabled flame tempentures to be G& culated by an iterative process. For ptopancpropylene-air flames without the presence of sulfur compounds, ten species (CO, C02, 0,. H,, l-I@, H, OH. 0, NO, and N,) wcrc used in the calculations [73. Other spcdcs, such a8 CH, CH2, C,H, HCU, C. Cl. C,, CN. and N. have been included in athcr calculations (see, e.g., Ref. lo), but their concentrations have proved to be negligible. When suIf’ur species were added to the flames, an additionaf ten species were included in the calculations. These were the eight species suggested as important by Deme~&rche and Sugden [I] (SOz. SO. SO,, S, S,, HS, H,S, and CS), as well as COS and CS2. The distribution of sulfur s~cics (for I “,;,SO2 in the unburnt gas) in propane-air flames, catculared at the adiabatic flame tcmperaturti* over a wide range of fuel-air ratios, is shown in Fig. I. Simiiar results were obtained when H,S was the source of the sulfur species. The conccn~ration of CS, was found to be ncgligihlc. Figure 1 ah gives the expcrimentatly dcrermined concentrations of SO?, H,S, and COS. scaled to coincid:: with thecalculated valse at L = 0.60. where /. is the ratio of the flow rate of air supplLx1, tn the flow rale of air rcyuircd 10 combu:;t the fuel completely to CO2 and I-1,0. The
rcsuks
for
each compour~tl
wr32 satIcd
separately. The experimental me;rsurerncnts were made at a point in the name where, from !!re leveling-off of the profile concentrations. it appeared that equilibrium had &en e~tabhshed. This &stance was between I5 and 30 mm above the tops of the primary cones. From the agreement in the trends illustrated in Fig. 1 it would appear that the ca~culatcd equilibria involving the 20 species rc~rc~ic~,t ir good model of the burnt gas. The divcr~,tincc between caicufated anJ measured CC)S L’O~I-
t
down of SO2 to other
species, such as IiS, H2S, and S2. However, since H and CW concsntraticxx do not depart significantly from equilibrium values in fuel-rich hydrucarbon frames [ill, it. would seem that a different mechanism (possibly one involving carboncontaining species) must be responsible for establishing the high SO? concentrations near the reaction zone of these flames. With leaner propane flames (i > 0.80. Figs. Za and 2b,l, SO, is the predominant sulfur species and is at or close to its equilibrium concentration throughout the burnt gas. Il’rJ slrolrld likr to thmk Rcdm’i’h [VJt’~,
.Jdriscw!.
CormlittcT
tlw Nirtium~l jiwjim7ciaI
Cod/ sup-
and Flame
Distribution of Sulfur Species in the Burnt Gas of Fuel-E&h Rropane-Air Flames Many industrial flames are of the diffusion type With fuet-rich zones. Surprisingly. the distribution of sulfur species in such James has received little attention, despite the current intense interest in the combustion chemistry of suIfur. Relevant information is available for hydrogen flames [I -4-j ; however. as these give rise to burnt gases differing fundamentally from
those produced
by fueI-rich
hydrocarbon
flames. it could be misleading to infer similar behavior in the two systems. In this communication the computed equilibrium distributions of sulfur species in the burnt gas of fuel-rich propane-ajr flames as a function of fuel-air ratio are compared with the measured relative concentrations >f three sulfur-containing species, nameIy, S02. H,S, and COS. Measured concentration profiles (along the vertical axis of the flame) cf these three species are alsc compared with the calcufated profiles based on measured reversaltemperature profiles. In the experiments to be described, a watercooled burner was used. similar to that described by Padley and Sugden [5], which provided shielded, Iaminar, premixed, propaneair flames. The fuel was a liquefied petroleum gas containing more than 85 % propane, the remainder being propylene except for traces of other hydrocarbons. The fuel was analyzed
at frequent intervals to check for compo:,itio:~ changes due to fractionation, and allowance was made for these in the stoichiometry caIculations. Samples
of the flame gases were withdrawn
by using an uncooled quartz microprobe similar in design lo that of Frrstrom and Westenberg [6]. Gas chromatography and mass spectrometry were used to analyze these sampies for S02- HIS, and COS. For gas chromatography the sampled gases were passed through a trap kept ai: -2WC to remove water, then frozen i:l a trap immersed in liquid nitrogen, and finally stored for later analysis. Under these conditions no reaction between ti2S and SO2 in the sampling systetn was expected [2, 31. but some H2S could have been formed from l-IS in the sampling system [2]+ The chromalographic analyses were carried out ~4th helilrm carrier gasI using a l-meter (2-mm4.d.) column containing Porapak Q* held at 5O’C and a thermal-conductivity detector. The SO2 peak heights obtained from sampling the burnt gas were found to b? linearly related to the SOI concentration in the unburnt gas. The linearity between peak hei_eht and concentration was assumed in the case of the much smaller H2S *A
porous. aromatic
Associales. hc.,
polymer
Frsminghrn,
manuhztured
Mass.. U.S.A.
__-hg Wat@rs
313 I_
G. M Johnson, C. J. Matthews,
burnt-gas concentrations. Corrections were made for the effects of temperature and viscosity on the sampling vattune. In some instances the sample was supplied directly from the flame to a quadrupoie mass spxtrometer (Atlas AMP31 for analysis. The. concentrations of species were estimated directly from the heights of the parent mass peaks. Ftamt: temperatures were measured by the sodium D-line rcvcrsal method [7]. A computer program was developed to calcrtfate the burnt-gas eqni1ibriun-i compositions ;rt fhlc thcorctical adiabatic flame temperature rtnu at any other specified temperature. The ~ourx of’ the ~hcrmodynarnic data was the and OX
M. Y. Smith,
and I). I. ‘willi8trnr
JANAF tables f8]. The equilibrium cuncentrations were obtained by m subprogram based nn the free-energy minimimlicm tnclbd [9jV tnnd this was alticd to a hrut&rrl;inec tiubprogr;rtla which enabled flame tempentures to be G& culated by an iterative process. For ptopancpropylene-air flames without the presence of sulfur compounds, ten species (CO, C02, 0,. H,, l-I@, H, OH. 0, NO, and N,) wcrc used in the calculations [73. Other spcdcs, such a8 CH, CH2, C,H, HCU, C. Cl. C,, CN. and N. have been included in athcr calculations (see, e.g., Ref. lo), but their concentrations have proved to be negligible. When suIf’ur species were added to the flames, an additionaf ten species were included in the calculations. These were the eight species suggested as important by Deme~&rche and Sugden [I] (SOz. SO. SO,, S, S,, HS, H,S, and CS), as well as COS and CS2. The distribution of sulfur s~cics (for I “,;,SO2 in the unburnt gas) in propane-air flames, catculared at the adiabatic flame tcmperaturti* over a wide range of fuel-air ratios, is shown in Fig. I. Simiiar results were obtained when H,S was the source of the sulfur species. The conccn~ration of CS, was found to be ncgligihlc. Figure 1 ah gives the expcrimentatly dcrermined concentrations of SO?, H,S, and COS. scaled to coincid:: with thecalculated valse at L = 0.60. where /. is the ratio of the flow rate of air supplLx1, tn the flow rale of air rcyuircd 10 combu:;t the fuel completely to CO2 and I-1,0. The
rcsuks
for
each compour~tl
wr32 satIcd
separately. The experimental me;rsurerncnts were made at a point in the name where, from !!re leveling-off of the profile concentrations. it appeared that equilibrium had &en e~tabhshed. This &stance was between I5 and 30 mm above the tops of the primary cones. From the agreement in the trends illustrated in Fig. 1 it would appear that the ca~culatcd equilibria involving the 20 species rc~rc~ic~,t ir good model of the burnt gas. The divcr~,tincc between caicufated anJ measured CC)S L’O~I-
t
down of SO2 to other
species, such as IiS, H2S, and S2. However, since H and CW concsntraticxx do not depart significantly from equilibrium values in fuel-rich hydrucarbon frames [ill, it. would seem that a different mechanism (possibly one involving carboncontaining species) must be responsible for establishing the high SO? concentrations near the reaction zone of these flames. With leaner propane flames (i > 0.80. Figs. Za and 2b,l, SO, is the predominant sulfur species and is at or close to its equilibrium concentration throughout the burnt gas. Il’rJ slrolrld likr to thmk Rcdm’i’h [VJt’~,
.Jdriscw!.
CormlittcT
tlw Nirtium~l jiwjim7ciaI
Cod/ sup-