49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition
4 - 7 January 2011, Orlando, Florida
AIAA 2011-440
Examination of Emission Spectra from Hydrogen Sulfide
Flames
H. Selim1, A. Al Shoaibi2 and A. K. Gupta3
Department of Mechanical Engineering
University of Maryland
College Park, MD 20742
Spectroscopic examination of the emission spectra of excited species in hydrogen/air
flames both without and with H2S addition and in hydrogen sulfide/oxygen flame are
conducted. The baseline case of hydrogen/air flame showed one distinct global peak of
OH* at 309.13nm. However, higher resolution spectrum analysis showed the presence of
three major OH* peaks at 306.13, 309.09, and 312.9nm. The addition of hydrogen sulfide
to hydrogen/air flame resulted in the presence of a bluish cone located at inner regions of
the flame. The spectrum of the blue cone showed group of peaks in the 350-470nm
spectral range. The addition of H2S drastically reduced the peak value of OH* due to
extensive consumption of the hydroxyl group during H2S combustion. The group of
peaks in the blue cone spectrum can be divided into three major bands. The first band is
formed by SO* within 320-350nm, the second band is attributed to SO3* within 350400nm, and the third band is caused by H* within 400-470nm. However, the distinction
of SO3* band and H* band around 400nm is an issue that requires further examination.
Absorption bands of SH were observed at 324.03nm and 328.62nm. The effect of sulfur
dioxide on the spectrum was observed by neither emission bands nor absorption bands
because of its reaction with elemental oxygen to produce excited sulfur trioxide. Gas
chromatography analysis showed that combustion products did not contain any SO2. The
spectra of H2S/O2 flame have also been examined under lean conditions (at ĭ=0.5). In
contrast to H2/air/H2S flames, the spectra of H2S/O2 showed strong absorption bands of
SO2 within 280-310nm. Strong continuum was observed between 280-460nm with
distinct group of peaks superimposed in the spectra. The continuum is attributed to the
afterglow of singlet and triplet SO2. The superimposed peaks are attributed to SO3* and
H*.
Introduction
Hydrogen sulfide is a common contaminant during oil and gas extraction from the
wells and is one of the most important gases that require attention during the refinery
processes for several reasons, such as, being problematic toxic gas, corrosive, health
hazardous and detrimental to the environment if it is released untreated. The hazardous
effects of hydrogen sulfide posed on both environment and industry makes it regulated
for discharge into the atmosphere by the environmental regulations. Furthermore, there is
increased need for the hydrolysis and hydrogenation of other sulfur compounds present in
fossil fuel wells, such as, carbonyl sulfide and carbon disulfide1. These compounds are
also formed during energy and material recovery processes from hydrogen sulfide. With
1
Graduate Student, Student member AIAA
Assistant Professor, The Petroleum Institute, Abu Dhabi, UAE
3
Distinguished University Professor, Fellow AIAA
2
Copyright © 2011 by Authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
the shrinking reserves of fossil fuels we must place increased emphasis on extracting
energy from wells that contain higher amounts of sulfur compound. Hydrogenation and
hydrolysis of sulfurous compounds transforms all of them into hydrogen sulfide2-3.
COS + H2O Ù H2S+ CO2
(1)
CS2 + 2H2O Ù 2 H2S+CO2
(2)
SO2 + 2H2 Ù H2S + 2H2O
(3)
Sn + nH2 Ù nH2S
(4)
Reactions (1) and (2) represent the hydrolysis of carbonyl sulfide and carbon disulfide,
while reactions (3) and (4) describe the hydrogenation of sulfur dioxide and elemental
sulfur. The formed hydrogen sulfide is absorbed from pure fuel stream via efficient
absorbents such as zinc oxide (ZnO) or alkanolamines4. Treatment of the absorbed
hydrogen sulfide is carried out in Claus process5-10 wherein reaction between H2S and O2
takes place under rich conditions (ĭ =3) to form elemental sulfur. During this reaction
one third of H2S is burned to form SO2 (reaction 5); the reaction continues between SO2
and unburned H2S to form molecular sulfur (reaction 6) which can be captured in liquid
or solid form. In practice the process is divided into two main stages of thermal and
catalytic. Both stages incroporate the same chemical reaction, but catalysts are used for
the later stage to enhance H2S conversion efficiency, wherein its H2S concentrations are
considerably low. However, with high conversion efficiency of the thermal stage the
number and load on the catalytic stages can be reduced thus reducing the plant
operational costs and enhancing operational efficiency.
3H2S +1.5O2 ĺ H2O+SO2+2H2S
2H2S +SO2ĺ 1.5 S2+H2O
ǻHr = -518 KJ/mol (5)
ǻHr = 47kJ/mol
(6)
The chemistry of H2S/O2 reaction has been investigated by several researchers to
elucidate and understand the detailed chemistry in the Claus process. Cerru and coworks11-12 reduced the detailed mechanism of hydrogen sulfide that consisted of 70
reversible reactions by using sensitivity analysis approach and the implementation of
steady state assumption for the minor species (HSO, HOSO, HOSO2, H2S2, and S). They
showed good agreement with other detailed mechanisms at different conditions. Selim
and co-workers13,14 provided a detailed strategy for the reduction of a detailed mechanism
of H2S/O2 reaction with negligible error on the major species formed. They reduced the
detailed mechanism of Leeds University15 which included 111 reactions down to only 19
reactions with maximum error of 16% using direction relation graph and error
propagation methodology coupled with direct elementary reaction error approach.
Frenklach et al.16 studied numerically and experimentally the ignition delay associated
with the oxidation of hydrogen sulfide using a reflected shock wave tube. Their
numerical modeling has been carried out using an adopted reaction mechanism that
consisted of 57 reactions. Bernez-Cambot et al.17 investigated experimentally the flame
structure of H2S/air diffusion flame under Claus conditions. According to their results,
they divided the flame into three distinct zones. First zone consisted of thermal and
chemical decomposition of H2S wherein hydrogen is the major product. Second zone
incorporated oxidation of both H2S and H2 formed in the first zone. Third zone included
partial consumption of hydrogen and to a lesser extent sulfur diffusing from the flame.
The decomposition of hydrogen sulfide was shown to be significant at high
temperatures18-19 where considerable amounts of hydrogen is formed especially under
Claus process conditions wherein the reaction takes place under fuel-rich conditions.
Hawbolt et al.20 studied the kinetics of H2S pyrolysis under Claus process conditions.
They determined an expression for the pyrolysis of H2S over temperature ranges of 8501150°C. These results have revealed that the rate of dissociation of H2S is minimal below
1000°C which suggests the need for high temperatures in Claus process.
Even though the aforementioned investigations have provided extensive erudition
about H2S/O2 reaction, the role of intermediate species is still controversial. The main
reason is that all experimental gas analyzers (gas chromatograph, FTIR and mass
spectrometer) can only provide information on stable combustion byproducts. The
deficiency of gas analyzers to detect intermediate species have triggered the need to use
optical techniques in H2S/O2 combustion in order to track the detailed behavior of
intermediate spices and to seek full interpretation of H2S/O2 chemistry in Claus process.
Muller et al.21 studied sulfur chemistry in fuel-rich H2/O2/N2 flames with 0.25, 0.5 and
1% of H2S. They measured concentrations of SH, S2, SO, SO2, and OH using quantitative
laser fluorescence measurements. With the help of the aforementioned radical
measurements they were able to determine kinetic parameters for various possible
intermediate chemical reactions associated with sulfur compounds. Azatyan et al.22
examined the behavior of hydrogen sulfide, carbon disulfide, and carbonyl sulfide
combustion at low-pressure using electron spin resonance technique along with gas
chromatography. The results revealed that first stage of H2S formation includes the
formation of H2, SO2 and SO and they cite that the presence of H2S is considered
inhibitor of H2 oxidation. The second stage includes hydrogen oxidation coupled with the
formation of hydroxyl group radical.
Absorption spectrometry has also been used to investigate absorption bands of
sulfur compounds. For instance, Fuwa et al.23 determined the presence of a strong
absorption band of sulfur dioxide within 200-220nm spectra. They also determined
another important absorption band within 250-300nm spectra but this was less strong as
compared to that in the 200-220nm spectra. Ding et al.24 investigated the absorption
spectrum of hydrogen sulfide between 1000nm and 1048.2nm. The strongest absorption
bands of H2S were found between 1015.7nm and 1021.97nm. Lewis et al.25 investigated
the absorption bands of SH wherein they formed SH radical from H2S by pulses of
radiofrequency current which is synchronized to immediately precede the flashlight to be
absorbed by SH. They were able to find only one band of SH absorption at 323.7nm.
The non-intrusive investigation of excited species chemiluminescence in flames
was proved to be an efficient technique for qualitative detection of radicals. Toyoda et
al.26 studied the emission spectra of carbon disulfide and hydrogen sulfide along with
other non-sulfurous compounds. The species of interest were excited using controlled
electron beam. A heated tungsten filament was used as electron source. Emission
spectrum of carbon disulfide showed the most intense bands at 282nm and 285nm.
Hydrogen sulfide emission spectrum showed strong bands at 486, 434, 410, 397, and
389nm which are attributed to the hydrogen Balmer series27-28. Folwer et al.29 studied the
spectrum of carbon disulfide flame and found that CS2 flame emissions extend from
ultraviolet to blue wavelength. They attributed the formation of bands primarily to the
presence of S2 and SO. Sulfur bands were found to be absorption bands, but they were
obtained as emitting bands when a stream of oxygen was directed into the flame. Sulfur
monoxide emission bands were feebly obtained as compared to S2 bands. Gaydon et al.30
studied the spectra and characteristics of hydrocarbon flames containing small amounts
of SO2 and SO3. The results showed band systems for S2, CS, SO, and SH and they
discussed the mechanisms of the formation of these radicals. A strong ultraviolet
emission band was also observed and they attributed this to the reaction between SO2 and
atomic oxygen to form SO3. In this paper we investigate the emission spectra of
hydrogen/air flame both without and with addition of trace amount of hydrogen sulfide
into the flame. Gas chromatography has been used for gas analysis at the same spatial
location used to measure the excited species emission. In addition, the emission spectrum
of H2S/O2 flame has been examined under lean conditions. The results provide improved
understanding of the various sulfur species formed.
Experimental
A schematic diagram of the experimental facility is shown in Figure 1. The
facility consists of a quartz tube reactor of 7.5 inch length and 1.6 inch inner diameter. A
double-concentric tubular burner was designed and used for all our experiments wherein
the oxidizer (air or oxygen) is injected into the outer annulus of the burner while the fuel
(hydrogen or hydrogen sulfide) is injected into the central tube of the burner. The burner
has a bluff body stabilizer at the exit to stabilize the flame immediately downstream from
the burner exit. The gases from the burner are allowed to flow into the quartz tube
reactor. Sonic throat quartz sampling probe is used for gas sampling. Throat diameter of
the sampling probe is of the order of few microns so that the flow is chocked at its throat.
The rapid expansion of the gases after the throat section of the quartz sampling probe
rapidly quenches the gases to freeze the gas composition. The inner and outer diameters
of the sampling probe were 3 and 4 millimeters, respectively. A suction pump is
connected to the sampling line to introduce the sampled gas into gas chromatograph (GC)
for gas analysis. The sampled gas is split into two streams inside the GC. First stream is
injected into thermal conductivity detector which is responsible for the analysis of nonsulfur compounds. Second stream is injected into flame photometric detector which is
responsible for gas sampling of stable sulfur compounds. A spectrometer coupled with an
ICCD camera was used for the detection of chemiluminescence signal from the excited
species. The spectrometer slit was set to 10 microns. The signal from the flame region
was passed to the spectrometer through a fiber optic cable. Two gratings were used for
different resolutions of the spectrum. The coarse grating was used to obtain coarse
resolution of the spectrum (~270nm) while fine resolution of the spectrum (~70nm) was
obtained with the second grating. A mercury lamp was used for the recalibration of
spectrometer after changing the grating or in case of change of spectrum of interest.
Computer controlled traversing mechanisms were used to move the sampling probe/fiber
optic cable in axial and radial directions. The resolution of traversing mechanism
movement was 25 microns in the axial direction and 1.5 microns in the radial direction.
The whole experimental setup was placed inside a fume hood. The fume hood was
connected to an exhaust duct where a fan is used to induce air into the fume hood for
safety purposes.
Experimental Conditions
Experiments were conducted to investigate the emission spectra associated with
the combustion of hydrogen sulfide for two different cases. In the first case we
investigate the emission spectra of hydrogen/air flame without/with hydrogen sulfide
injection, while in the second case emission spectra of H2S/O2 flame are examined. Gas
chromatography was used for gas sampling in the first case in order to support the
explanation of the emission spectra. The experimental conditions for the two different
cases are given below.
First case: Hydrogen/air flame without and with H2S injection
Hydrogen flow rate: 3 lit/min
Air flow rate: 7.2 lit/min
H2S flow rate: 4 cm3/min
Second case: Hydrogen sulfide/oxygen flame
H2S flow rate: 0.3 lit/min
Oxygen flow rate: 0.9 lit/min
Results and Discussion
The emission spectrum of hydrogen/air flame is examined first under the
abovementioned experimental conditions. The effect of injecting hydrogen sulfide on the
emission spectrum is then presented. The results obtained on the spectrum of H2/air flame
with H2S addition will be corroborated with the results obtained form gas
chromatography on the combustion products in order to obtain better understanding of
combustion kinetics. The emission spectrum of H2S/O2 will then be presented and
discussed. Both fine and coarse gratings have been used in the spectrometer to elucidate
key features associated with the spectra.
i) H2/air flame without/with H2S injection
Figures 2 and 3 present the spectrum of hydrogen/air flame using both coarse and
fine gratings, respectively. Figure 2 shows a wide spectrum of hydrogen/air flame
between 230nm and 500nm. The spectrum shows one global peak at 309.13nm which is
attributed to OH* radical. Figure 3 elaborates the global peak of OH* using the fine
grating between 250nm and 315nm. Three major peaks of OH* radical were observed.
The first peak is at 306.13nm, the second peak (strongest amongst the OH*peaks) is at
309.09 nm, and the third major peak is at 312.9nm. These results agree with the general
findings reported in the literature on the OH* by several investigators31-32. However, the
exact wave length of the strongest peak of OH* is still controversial. For example, Smith
et al.33 have reported the strongest OH* peak at 306 nm, Walsh et al.34 have reported
OH* to peak at 307.8 nm, Harber et al.35 have reported OH* maximum peak at 308 nm,
and Gaydon32 have reported OH* to peak at 310 nm. This clearly shows that there are
wide variations in the observed peak wavelength value of OH* from the flames. Our
results support the close proximity of the observed OH* peak by Gaydon32.
Computer controller
traverse mechanism
Gas sample into gas chromatograph
Sonic-throat
sampling probe
8.94
3.58
9.94
Quartz tube
reactor
Burner
H2S
Excited species
emission
Fiber optic cable
Air+CH4
[All dimensions in millimeters]
ICCD Spectrometer
camer
Figure 1. A schematic diagram of the experimental setup
Hydrogen/air flame has a very faint color (almost colorless), see figure 4a.
However, with the addition of only a trace amount of H2S a strong bluish inner cone was
formed at the flame base, see figure 4b. Increased amounts of H2S to the H2/air flame
resulted in bluish white color flame, see Figure 4c. In this paper we focus our attention to
examine the characteristics of blue cone with trace amount of H2S addition to H2/air
flame. In order to provide an understanding of the distinct change in the formation of a
blue cone with trace amounts of H2S flame spectroscopy was used at various locations in
the flame. Figure 5 shows the spectrum of this blue cone obtained on longitudinal axis of
the flame at L= Z/Djet=1.773 (where, Z is the distance in axial direction measured from
the burner tip and Djet is diameter of the inner injection tube of the burner) using a coarse
grating in the spectrometer. Strong series of peaks can be observed between 320 and
470nm and this can be grouped into 3 bands. The first band is caused by SO* within 320350nm, the second band is attributed to SO3* within 350-400nm, and the third band is
due to H* within 400-470nm.
It is important to note that the previously-observed OH* peaks are suppressed
significantly with only trace amounts of H2S addition into the H2/air flame. Only the
strongest peak observed for OH* at 309.09nm could be detected. This shows a direct role
of H2S additions on the OH radicals present in the flame. The reaction of hydrogen
sulfide is discussed in detail in one of our papers36 where H2S consumes OH during the
reaction to form primarily SO which is then transformed to SO2 as an end product. This
supports the deterioration of the observed OH* peaks obtained here.
Emission spectrum of the inner blue cone does not change within its core.
However, the spectrum of the inner blue cone starts to fade near its tip (at L=5.32) and
OH* peak becomes the dominant at or immediately downstream of the blue cone tip.
Figure 6 shows the spectrum at the tip of the blue cone where OH* peak becomes
dominant and the blue cone bands diminish. This conjectures that hydrogen sulfide
combustion has almost completed and this point will be verified later as supported from
the results obtained with the GC. The spectrum of the blue cone is now examined in
further detail using a fine grating in the spectrometer that helped to provide improved
resolution. The fine grating in the spectrometer helped us to judicially identify the
wavelength of each peak and to assist in better understanding of the various chemical
species responsible for the emission spectra. This also assists one to determine the
chemical species responsible for the major reaction pathways in H2S combustion.
Figures 7, 8, and 9 present in details the spectrum of the inner blue cone formed in
hydrogen/air flame with the injection of trace amount of hydrogen sulfide. No significant
bands were observed below 306nm. The peaks observed beyond 320nm has been
suggested by several investigators to be attributed to SO*37, SO2*38, 39, or SO3*30, 40. The
spectrum of SO* was found to be extended from 244.2nm to 394.1nm with more than 40
peaks distributed along this band of wavelengths. However, these bands are not likely to
be occurring under one condition and they are more common in rich flames. In the
current study, it is unlikely to have all these bands since the concentration of sulfur
species is very minimal and the mixture is slightly lean. However, some of the observed
bands can be attributed to SO* which are the relatively weak bands within 320-340nm,
(fig. 7). The findings by Gaydon et al.30 support this explanation. The likelihood of SO2*
to be the cause of the blue cone bands beyond 340nm is very minimal as the GC analysis
of the combustion products inside the blue cone did not show any measurable amounts of
SO2 (as shown later in figs. 10, 11 and 12). Moreover, the reported spectra of SO2* in the
literature revealed the presence of a group of distinct peaks superimposed on a continuum
band which is attributed to SO2 afterglow38-41. Sulfur dioxide afterglow causes increase in
the background signal of the spectrum between 250nm and 500nm which was not
observed in the blue cone spectrum (details of SO2 afterglow will be discussed in case ii).
The explanation of SO3* responsible for the peaks beyond 340nm is the most acceptable
one. The findings by Gaydon et al.30,42 and Dooley et al.43 suggested that the peaks
beyond 350nm are attributed to the presence of SO3*. Sulfur trioxide can be formed in
the reaction zone due to the reaction between sulfur dioxide and atomic oxygen or
hydroxyl group as follows30:
SO2 + O Ù SO3 +81Kcal
(7)
SO2 + OH Ù SO3 + H _ 20Kcal
(8)
Reactions (7) and (8) create a strong channel for the formation of SO3* which is
responsible for the continuous band of peaks beyond 350nm. On the other hand, excited
hydrogen radical possesses an important role in H2S combustion. Therefore, peaks
beyond 400nm are likely to be attributed to the Balmer series27-28 formed by H*. Sulfur
trioxide is responsible for most of the peaks within 350-400nm while H* is responsible
for most of the peaks above 400nm. However, the interference between SO3* peaks and
H* peaks is still an issue that needs further investigations. Hydrogen radical is formed
through the group of reactions as follows36:
SO2 + OH Ù SO3 + H
(9)
S + OH Ù SO + H
(10)
SO + OH Ù SO2 + H
(11)
Reactions (8) through (11) also elucidate the suppression of observed OH* peaks with the
injection of H2S.
One of the most important radicals that did not show any noticeable band in the
spectrum presented above is SH which has been consistently proven by several
investigators13-15 to provide a significant role as an intermediate species in H2S
combustion. The bands of SH* emission are difficult to distinguish in the spectrum
because it is mostly masked by SO* peaks30. However, absorption bands of SH are the
proper method to distinguish the presence of SH. Absorption bands of SH are reported to
be at 323.5nm and 327.8nm25,30. This relatively agrees with the results presented here
wherein the light intensity shows its lowest magnitudes at 324.03nm and 328.62nm.
Table 1 shows the wavelength of all peaks observed in the spectrum of the inner blue
cone in the flame.
Wavelength
(nm)
408.43
415.86
425.71
430.0
439.6
443.7
449.09
453.8
458.8
465.2
Species
SO3 and H* bands
SO3 and H* bands
SO*
band
SO* band
Wavelength
(nm)
309.09
322.21
324.03
326.62
328.62
333.92
336.99
338.97
341.61
343.15
344.7
Table 1: Blue cone emissions peaks
Species
Wavelength Species
(nm)
OH*
346.59
SO*
347.83
SH (absorption band)
349.85
SO*
355.24
SH (absorption band)
358.77
364.75
368.42
373.42
383.16
393.71
404.57
Gas chromatograph was used to provide gas analysis at different locations of the blue
cone. Due to symmetry of the flame, the gas sampling was carried out on only one half of
the blue cone. Figures 10, 11, and 12 show the mole fraction of stable combustion
products of H2S, H2, and O2, respectively in the flame. Dimensionless axial and radial
distances are used with respect to the jet diameter, i.e., W= R/Djet, where, R is the
distance in radial direction measured from the burner centerline, and Djet is the diameter
of the inner injection tube of the burner. These results show that H2S is totally consumed
at the tip of the inner blue cone which supports the previous conclusions that were based
on the light emission spectroscopy. On the other hand, the chromatograms did not contain
any SO2 mole fraction, which supports the abovementioned interpretation about the
formation of SO3.
ii) H2S/O2 flame
Figures 13, 14, 15, and 16 show emission spectrum of H2S/O2 under lean
conditions (at ĭ=0.5) using a fine grating in the spectrometer. It was not possible to use
the coarse grating because of the wide variation in peaks intensity which hindered
showing all the spectrum details in one figure. Unlike the previous case, this spectrum
shows a large continuum band within 280-460nm which is attributed to SO2 afterglow.
However, the spectrum also shows groups of distinct peaks superimposed on the
continuum band between 280-310nm, and 360-480nm. The first group of peaks is
attributed to the absorption bands of SO241,44 (in the range of 280-315nm). Although SO
is considered an important intermediate species for SO2 production, the presence of
distinct SO* peaks is uncertain because they are masked by the strong continuum of SO2
afterglow, if indeed they do exist. Similar to the previous case, the second group of peaks
is attributed to SO3* and H*. Sulfur dioxide afterglow is a strong chemiluminescence
continuum band which is reported in the literature to be between 250-500nm38,40,41. The
obtained continuum starts at ~280nm reaches its maximum at ~365nm, then it decays
until 460nm. The spectrum of this study matches well with the SO2 afterglow spectrum
reported by Gaydon41. On the other hand, Mulcahy et al.38 suggested that the afterglow
continuum is formed by excited singlet and triplet states of SO2*. He suggested that the
singlet emission of SO2* is in the region around 350nm while the triplet emission of
SO2* is around 425nm. This agrees with the findings presented here where the continuum
peaks at ~365 is due to singlet SO2* emission and the continuum beyond 400nm is much
weaker which is attributed to triplet emission of SO2*. The group of chemiluminescent
reactions responsible for this continuum is as follows19,38,45:
SO + O Ù SO2*
(12)
SO2 + O + O Ù O2 + SO2*
(13)
Reaction (12) contributes significantly in SO2 afterglow in case of high O concentrations
(which is our case) and it was found to produce triplet SO2 emission45. Reaction (13) is
assumed to occur in two steps38 as follows:
SO2 + O + M Ù SO3* + M
1
(14)
SO3* + O Ù SO2* + O2
(15a)
SO3* + O Ù 3SO2* + O2
(15b)
In case of oxygen depletion chemiluminescent reactions will not progress to reaction 15
which interprets the absence of SO2 afterglow in case (i). However, in case (ii) reaction
(15) occurs in both its forms (15a and 15b) to produce singlet and triplet SO2 afterglow
emissions, respectively. Reaction (14) was found to produce triplet excited SO3*. Table 2
shows the wavelength of all peaks observed in the spectrum of H2S/O2 flame.
SO2 afterglow
Species
SO3 and H* bands
SO2 afterglow
SO2 absorption
bands
Table 2: H2S/O2 flame spectrum
Wavelength
Species
Wavelength (nm)
(nm)
300.22
384.68
302.24
386.56
304.29
394.74
306.26
397.23
308.75
405.11
311.05
411.58
313.67
417.03
360.07
421.61
366.09
427.99
369.68
432.68
375.7
459.17
SO3 and
H*
bands
SO2 afterglow
Species
SO2 absorption bands
Wavelength
(nm)
281.65
283.04
284.75
286.45
287.94
289.64
291.23
292.94
294.65
296.49
298.35
Conclusions
Spectra of excited species of hydrogen sulfide flames have been examined in
detail using hydrogen/air flame with the addition of trace amounts of H2S. The excited
species formed have also been obtained from a hydrogen sulfide/oxygen flame.
Hydrogen/air flame showed one distinct global peak of OH* at 309.13nm. However, a
higher resolution of the spectrum showed that OH* is responsible for three major peaks at
306.13, 309.09, and 312.9nm. Addition of a trace amount of hydrogen sulfide into
hydrogen/air flame caused the formation of a strong bluish inner cone located near to the
flame base. The spectrum of the blue cone showed very strong group of peaks within
320-470nm. Addition of H2S significantly weakened the OH* peaks which is attributed
to the extensive consumption OH during H2S combustion. The group of peaks formed
inside the blue cone can be grouped into three major bands. The first band is caused by
SO* within 320-350nm, the second band is attributed to SO3* within 350-400nm, and the
third band is due to H* within 400-470nm. However, distinction of SO3* band and H*
band around 400nm still requires future investigation. Absorption bands of SH were
observed at 324.03nm and 328.62nm. Sulfur dioxide was not shown to have significant
bands in this spectrum because it reacts with elemental oxygen to produce excited sulfur
trioxide. Gas analysis results obtained using gas chromatography showed absence of SO2
in the combustion products.
The spectra of H2S/O2 flame under lean conditions (ĭ=0.5) showed strong
absorption bands of SO2 within 280-310nm. Strong continuum was observed between
280-460nm wherein distinct group of peaks were found to be superimposed on it. The
continuum is attributed to singlet and triplet SO2 afterglow. Singlet excited SO2 afterglow
is around 365nm region while the triplet excited SO2 afterglow is beyond 400nm. The
superimposed peaks are attributed to SO3* and H*. The results of this study support
many of the previous findings and are aimed to provide better sulfur mechanism for the
improved sulfur capture in thermal stage of the Claus process.
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Figure 2. Emission spectrum of hydrogen/air flame (230nm-500nm)
Figure 3. Emission spectrum of hydrogen/air flame (250nm-318nm)
Blue cone
(a)
(b)
(c)
Figure 4. Flame photograph with (a) Hydrogen/air flame, (b) Hydrogen/air flame with trace amount
of H2S added to H2/air flame, (c) Hydrogen/air flame with increased amounts of H2S addition to
H2/air flame
Figure 5. Emission spectrum of hydrogen/air flame, with trace amounts of H2S addition in the range
230nm-500nm
Figure 6. Emission spectrum of hydrogen/air flame with the addition of H2S at the tip of the inner
cone (230nm-500nm)
Figure 7. Emission spectrum of hydrogen/air flame with addition of trace amount of H2S (308372nm)
Figure 8. Emission spectrum of hydrogen/air flame with addition of trace amount of H2S (364426nm)
Figure 9. Emission spectrum of hydrogen/air flame with addition of trace amount of H2S (420470nm)
L=0.0
L=1.773
L=3.54
L=5.32
H2S Mole Fraction (%)
0.025
0.02
0.015
0.01
0.005
-0.887
-0.710
-0.532
-0.355
-0.177
0
0.000
W
Figure 10. Axial and radial distribution of H2S mole fraction within the inner blue cone
80
60
L=0.0 in
L=1.773
L=3.54
L=5.32
40
20
H2 Mole Fraction (%)
100
0
-0.887
-0.710
-0.532
-0.355
-0.177
0.000
W
Figure 11. Axial and radial distribution of H2 mole fraction within the inner blue cone
L=0.0
L=1.773
L=3.54
L=5.32
3
2.5
2
1.5
1
0.5
0
-0.887
-0.710
-0.532
-0.355
-0.177
O2 Mole Fraction (%)
3.5
0.000
W
Figure 12. Axial and radial distribution of O2 mole fraction within the inner blue cone
Figure 13. Emission spectrum of H2S/O2 flame (250-314nm)
Figure 14. Emission spectrum of H2S/O2 flame (308-374nm)
Figure 15. Emission spectrum of H2S/O2 flame (362-426nm)
Figure 16. Emission spectrum of H2S/O2 flame (408-460nm)