H2S Comb Distrib 092805

APPLIED CHEMISTRY
Hydrogen Sulfide Combustion: Relevant Issues under Claus
Furnace Conditions
Ivan A. Gargurevich
Combustion & Process Technologies, San Diego, California 92122
The major chemical paths for the combustion of hydrogen sulfide under conditions typical of
the Claus furnace (i.e., fuel-rich conditions) are presented. The manuscript begins with a brief
survey of recently published research that involves sulfur chemistry in high-temperature
environments, including the results of sensitivity analysis for some of the systems involved.
Recommended values for the heats of formation of sulfur species are included. The reaction
mechanism that is presented consists of more than 150 reactions. Issues such as the formation
and destruction of COS and CS2 are presented: new chemical paths for the formation of COS
and CS2 (not involving elementary carbon) are illustrated, on the basis of sound thermochemical
and kinetic considerations. The formation of COS and CS2 is of great importance in the design
of sulfur plants in industry. Possible reactions of COS and CS2 with SO2, and CO2 with H2S and
sulfur species, also are discussed, prompted by experimental observations in flow reactors. The
mechanism can explain the formation of hydrogen, which also is an important issue in sulfur
plant design and associated tail gas units. Species such as H2S2 seem to have an important role
during the combustion of hydrogen sulfide. Higher-molecular-weight linear H2Sx species are
also considered, and it is concluded that their role is possibly minor. The chemical steps leading
to the formation of Sx species by molecular growth are presented. The ring structure of some of
the Sx species is discussed, as well as intramolecular ring conversions for S8, S7, S6, and S5. The
possibility of H,OH radical recombination catalyzed by oxygenated sulfur species may explain
the delayed oxidation of hydrocarbon species in the Claus furnace that has been observed in
previous experiments by other authors. This could be an important design consideration for
Claus plants to minimize the coking of catalyst beds in the process. The most likely chemical
paths for the radical quench are presented and based on past observations. Controversy persists
in regard to the actual mechanism and the rate constants of the reactions involved in the radical
recombination, as well as the thermochemistry of some of the oxygenated sulfur species involved.
More studies are needed to resolve the issues. The study also reveals the lack of high-temperature
data for the kinetic coefficients of some of the reactions. Much rate data are based on atmospheric
studies, rather than high-temperature oxidation. Similarly, better thermodynamic data are
lacking for some important oxygenated sulfur species in the mechanism. This is most important
for temperature- and pressure-dependent reactions, such as unimolecular reactions and
chemically activated reactions. Studies that involve hydrogen sulfide flames under fuel-rich
conditions are lacking. Most of the studies have been limited to the impact of sulfur species on
the formation of other species, such as CO and NOx, in flames or reactors.
Introduction
This manuscript examines the gas-phase combustion
of hydrogen sulfide under reducing conditions such as
those found in the Claus process, for example. The main
chemical species resulting from the combustion process
are identified, and, most importantly, the main chemical
paths in the combustion are identified based on chemical
principles and thermodynamics (see Table 5 later in this
work). The manuscript does not attempt to develop any
chemical reaction rate coefficients for the reactions; this
is left for future work. Nevertheless, the work of other
authors is presented, introducing rate coefficients for
reactions that lead to major species such as SO, SO2,
S2, H2S2, H2, S2, COS, CS2, CO, and CO2.
It has been the finding of the author that not much
information is available, in either experimental or
computational quantum chemistry, concerning the rate
coefficients of many of the reactions in Table 5 at high
temperatures that are typical of flames.
Recent developments in computational chemistry and
the advent of faster computers have made it possible to
develop large chemical kinetic models that are composed
of hundreds of elementary reactions. The purpose is
often to predict the formation of trace species. These
species often have an important environmental impact,
e.g., the well-known formation of NOx in the hot region
of flames.1
* To whom correspondence should be addressed. Tel: (858)-
5696742. E-mail: ivan_gargurevich@yahoo.com.
7706 Ind. Eng. Chem. Res. 2005, 44, 7706-7729
10.1021/ie0492956 CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/23/2005

Despite the aforementioned discussion, the combus-
tion of hydrogen sulfide has not received much consid-
eration at the molecular level. A review of sulfur
chemistry by Johnsson and Glarborg2 indicates that
most of the chemistry has been concerned mainly with
the effect of sulfur on the emissions of other pollutants,
such as NOx and CO (there will be more discussion
about this point later in this paper). In this respect, the
work of Chernyshera et al.,3 which involved the mech-
anism of H2S oxidation at high temperatures, is an
exception.
Most of the work involving the industrial aspects of
H2S combustion that has been published only considers
the main overall reactions that occur during the high-
temperature oxidation of hydrogen sulfide.4-6 The work
of Monnery et al.6 also shows that empirical correlations
used to determine gas-phase composition (e.g., COS and
CS2 concentrations at the exit of the waste heat boiler
during the Claus process) are often inadequate.
One very important application of hydrogen sulfide
combustion is the Claus reaction. Other applications
such as the high-temperature decomposition of hydro-
gen sulfide to form hydrogen are also being considered.7
The thermodynamics of super-adiabatic partial oxida-
tion of hydrogen sulfide in an inert porous media has
also been studied by Slimane et al.8 The study consid-
ered various acid gas and oxidizer feeds, equivalence
ratios, interstitial gas velocity, and temperatures. Most
of the calculations involved temperatures well in excess
of 1000 K. The results of the equilibrium calculations
show favorable conversions to hydrogen. Thermody-
namic equilibrium modeling can be representative of
flame temperatures and product compositions, and this
is most significant in the case of fast chemical kinetics
during the process. Thermodynamic predictions are
usually less useful at low temperatures, because of
slower rates of the chemical reactions in the process.
Claus Reaction
Refinery fuel gas, as well as other refinery hydro-
carbon streams, will contain quantities of hydrogen
sulfide; this is the result of the distillation of crude oil
in the main crude distillation column or treatment of
the distillation cuts in hydrotreaters and other treat-
ment units. The resulting fuel gas is treated to remove
hydrogen sulfide in amine units, which is a dangerous
substance, resulting in a hydrogen sulfide-rich stream
to be treated in Claus plants.9,10
The Claus plant or sulfur recovery units make use of
the well-known Claus reaction:
To obtain the necessary SO2 for the reaction above, one-
third of the hydrogen sulfide is combusted in a high-
temperature furnace, or
The overall reaction is then
The temperature in the combustion furnace can be
as high as 2000 °F. The overall Claus plant is depicted
in Chart 1. Both acid gas and, in some cases, sour water
stripper gas are fed to the main furnace. After partial
oxidation of H2S in the furnace, the high-temperature
gas is cooled in a waste heat boiler; the gas then
proceeds to a condenser, where the gas is cooled to its
dew point. Low-pressure steam is generated for this
purpose.
The Claus plant then consists of various stages of gas
reheating, catalytic reaction, and condensation of sulfur.
In the catalytic reactor, Claus reaction 1 proceeds at
much lower temperature (450-610 °F), thanks to an
alumina-based catalyst. The gas is reheated in the
reheaters to bring it to reaction temperature. Care is
taken to reheat the gas to a sufficiently high tempera-
ture, so that the gas exiting the catalytic reactor that
follows is above the sulfur dew point. This way, plugging
of the reactor is avoided. After reheating, the gas then
proceeds to the catalytic reactor to form sulfur via the
Claus reaction. Finally, the gas flows to the sulfur
condenser, where the gas is cooled to its sulfur dew point
by producing low-pressure steam in a shell-and-tube
exchanger. The process described above is repeated
several times to increase conversion to sulfur. The flow
diagram in Chart 1 depicts three catalytic stages.
An important problem in modeling Claus plants is the
estimation of the gas composition as the gas flows from
the reaction furnace to the waste heat boiler. The gas
composition in the furnace is very close to equilibrium
(because of the high temperature and residence time).
Chart 1. Process Flow Diagram of the Overall Claus Plant.
2H2S + SO2 T
3
x
Sx + 2H2O (1)
H2S + 1.5O2 T SO2 + H2O (2)
H2S +
1
2
O2 T
1
x
Sx + H2O (3)
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7707

As the gas is cooled in the waste heat boiler, the gas
continues to react and follow the temperature drop to
some extent, depending on the reaction that is being
considered. The waste heat boiler exit temperature is
typically 700 °F.4,6
Most process simulators (SULFSIM, TSWEET) simu-
late the conditions at the waste heat boiler, based on
equilibrium considerations and/or estimated quench
temperatures for reactions of some species such as
hydrogen, CO, and CO2. A difficulty in the simulation
is the prediction of trace species such as COS, CS2, and
mercaptans, because no chemical kinetic mechanism is
featured in these software programs.
The results of equilibrium calculations indicate that
significant amounts of H2 and CO are produced in the
reaction furnace.6 The hydrogen is most likely produced
by the thermal decomposition of H2S. There is some
debate in regard to the mechanism of CO formation.
Plant samples taken after the waste heat boiler seem
to indicate the reassociation of H2 and S2 to form H2S.
Similarly, CO formed in the furnace seems to react in
the waste heat boiler to form COS.6
Plant samples taken after the waste heat boiler also
seem to indicate substantially higher concentrations of
COS and CS2 than what is predicted by equilibrium
calculations at furnace conditions.6 CS2 formation seems
to correlate well with the amount of hydrocarbon in the
feed gas. As previously indicated, an important problem
is that empirical correlations are often inadequate in
predicting gas composition.
The work of Clark and co-workers4,11 is also important
in this matter. They conducted studies using an exter-
nally heated tubular reactor to simulate Claus furnace
conditions with variable quenching of the hot gas. They
found that CO2/sulfur species do not result in CS2, but
hydrocarbons do react with sulfur to produce CS2. Under
the partial oxidation conditions of the furnace, they
found that H2S is destroyed more quickly than any
hydrocarbon in the feed gas (the author gives a possible
explanation for this in this manuscript, below). They
also studied new chemical pathways that involved the
reaction of CS2 and COS with major species such as SO2,
CO2, and H2. The destruction of COS and CS2 by
reaction with water occurs very rapidly. COS is also
known to react with hydrogen; CS2, on the other hand,
does not seem to react with hydrogen.4,6
The author does not know of any recent comprehen-
sive studies that examine the chemistry of H2S combus-
tion under reducing conditions that are typical of the
Claus process. The work of Kennedy12 and Zachariah
and Smith13 are important in this respect; however,
their kinetic mechanisms do not include the molecular
growth that leads to S8. Similarly, the chemistry of COS,
and CS2, is not considered. Their mechanisms include
the chemistry that leads to the formation of SO, SO2,
SO3, and S2, as well as other chemical paths for the
destruction of H2S. Another important source of chem-
istry and kinetics data that is more recent can be found
in the University of Leeds, U.K. Sulfur Mechanism
(which can be found on the Internet at www.chem-
.leeds.ac.uk/Combustion/Combustion.html).
Other considerations beyond the scope of this work
are fluid dynamics and residence time within the
reaction furnace of the Claus plant. Both are important
in determining the real approach to equilibrium within
the furnace.14,15 Computational methods, including tur-
bulent combustion, have been reviewed by Eaton et al.16
Reaction furnace design considerations are further
discussed by Hyne.5
Discussion
A first step in the assembly of the main chemical
paths is to consider all or some of the possible species,
radical or stable, that can partake in the destruction of
the initial mixture that contains hydrogen sulfide. These
are listed in Table 1.10,17,18 This table must include
species that lead to the formation of elemental sulfur
in the Claus furnace as well as important trace species
such as COS and CS2.
In addition to hydrogen sulfide, acid gas may contain
hydrocarbons such as methane and ethane. Further-
more, there are instances when sour water stripper gas
that contains ammonia must be treated in the Claus
plant;9 for this reason, ammonia is included in Table 1.
The oxidation of methane has been studied exten-
sively (see, for example, GRI Mechanism 3.0, which can
be found on the Internet at www.me.berkeley.edu/
gri_mech), as well as comprehensive discussions and
reaction compilations in dissertations by Gargurevich,17
and Wang19 (more below); the hydrocarbon species
considered in Table 1 are taken from these references.
It is important to assess the concentration level of
these species under typical reaction conditions in the
Claus furnace and waste heat boiler. For this reason,
equilibrium simulations were performed with ASPEN
Plus 10.1. The simulations consisted of isotherms at
different temperatures including the adiabatic temper-
ature. It must be noted that similar calculations have
been conducted by Meisen and Bennett.10 The results
of the calculations for this manuscript are shown in
Figures 1-7. The well-known fact that radicals can be
present in flames in excess of their equilibrium values
must be considered when producing the elementary
chemical steps of the combustion process.
Before proceeding, it is important to become familiar
with the molecular geometry of some of these species.
This is very relevant to the discussion of the reactions
that can occur during the combustion process. Table 2
shows the molecular structure for some of the sulfur
species in Table 1. Sulfur, as well as oxygen, has six
valence electrons and requires two more to satisfy the
octet rule. There is no indication that the sulfur in the
species SO2, SO3 shown in Table 2 makes use of d
orbitals.20 Both involve double-bonded resonance struc-
tures.
It is important to note that the oxygenated species
SO, SO2, and SO3 provide double bonds for radical
Table 1. Chemical Species under Consideration in
Equilibrium Calculation Hydrogen Sulfide
Combustion-Reducing Conditions (Claus Process)
Major Species
O2, N2, NH3, NO, H2O2, H2, H2O
CO, CO2, COS, CS2, HCN, CH4, C2H6, C2H2, C2H4
CH2O
SO, SO2, SO3, H2S2, H2S3, H2S4, H2S5, H2S6, H2S7, H2S8
S2O2, H2SO2, CH3SH, C2H5SH
S2, S5, S6, S7, S8
Radical Species
O, OH, HO2, H
CS, CN, CH3S, C2H5S, CH3, CH2, CH, C2H5, C2H, C2H3
CHO, CH3O
S2O, S, S3, S4, HSO, HSO2
HS2, HS3, HS4, HS5, HS6, HS7, HS8
7708 Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005

addition reactions to occur. These reactions are impor-
tant in the flame, e.g.,
It is well-known that S8 in the vapor phase forms a
puckered ring structure. There are several alleotropes
of solid sulfur, and the most common ones are the
rhombic and monoclinic crystal structures; the rhombic
form is the most stable of the two.
It is seldom discussed in the literature that species
such as S7, S6, and S5 can also form ring structures.21,22
The ring structutre of S5 is similar to that of cyclopen-
tane; similarly, the S6 ring structure is an hexagonal
chair that is similar to that of cyclohexane. S7 has also
been shown to have a chairlike structure. However, the
smaller species (S3, S4) seem to have a linear geometry.21
Yet, at the high temperatures of combustion, it should
be possible to open up the rings previously described to
produce the linear geometry. The energy required to
open the S8 ring is estimated to be 33.8 kcal/mol.23
Raghavadari et al.21 also gives energy estimates for
the following ring conversions:
Figure 1. Concentration plot of the CO, CO2, and H2 species in the furnace gas over a range of temperatures.
Figure 2. Concentration plot of the sulfur species (S1-S8) in the furnace gas over a range of temperatures.
SO2 + O T SO3 (4)
S8(c) T
8
5
S5(c) (29.1 kcal/mol) (5)
S8(c) T
8
6
S6(c) (9.2 kcal/mol) (6)
S8(c) T
8
7
S7(c) (5.2 kcal/mol) (7)

At the high combustion temperatures, these reactions
should occur. No mechanism is given for the conversions
described by Raghvadari et al.21
A. Hydrogen Sulfide Combustion: Chemical
Equilibium Calculations. As previously noted, the
concentration of chemical species under equilibrium
conditions can only be considered as a guide to their
importance in the combustion process. Measurements
of radicals in laminar flames with microprobes, for
example, have shown that these species can be found
in levels exceeding their equilibrium concentrations
during the combustion process.
However, temperatures and residence times typical
of Claus furnace designs make it possible to achieve a
close approach to equilibrium, and the chemical com-
positions shown by the calculations in this section at
the higher temperatures should be viewed as a close
representation of the furnace products in typical ap-
plications. Thus, the results presented here are most
relevant in understanding the chemistry that occurs in
the furnace at high temperatures.
As stated previously, chemical equilibrium calcula-
tions have been conducted by other authors10 for a
mixture of hydrogen sulfide and air under conditions
typical of the operation of Claus units. This author
performed calculations at the adiabatic temperature and
isotherms ranging from 600 °F to 2000 °F. Species for
which concentration profiles were provided are shown
in Table 3. The minimum concentration reported was
on the order of 1 ppmv (parts per million by volume).
Figure 3. Concentration plot of the COS and CS2 species in the furnace gas over a range of temperatures.
Figure 4. Concentration plot of the H2S2, HSO, S2O, SO, and H2SO2 species in the furnace gas over a range of temperatures.

They found that the amounts of the radical species
H, OH, and O reach concentrations at the ppm level only
at the highest temperatures (2400-3100 °F). This is
what is expected from what is known about combustion
chemistry. The calculations show that, for temperatures
of >800 K, the most abundant species are S2, S3, S4,
and HS, with S2 being the predominant molecule. Sulfur
species such as S5, S6, S7, and S8 become abundant at
lower temperatures (well below 1330 °F). Monatomic
sulfur (S) does not become significant until tempera-
tures above 1700 °F are reached. The relative abun-
dance of S and HS from H2S decomposition could be due
to the lower bond energy in S-H (89 kcal/mol), as
compared to the C-H bond energy in CH4, for example
(104 kcal/mol).
Significant amounts of COS are formed at tempera-
tures above 970 °F. CS2 formation is at the ppmv level
at temperatures above 1330 °F. These species are
thought to involve reactions of CO2 and CO (more about
this observation will be presented later in this paper).
The importance of CO, H2, and CO2 chemistry has been
previously discussed. The work of Meisen and Bennett10
showed that significant amounts of CO and H2 are
formed above 620 °F. The concentrations of both species
continue to increase with increasing temperature.
They found almost insignificant amounts of ammonia
that was created from the feed nitrogen. At the highest
temperatures, the amount of SO2 exceeds that of H2S,
which suggests that elemental sulfur competes success-
fully for oxygen.
Figure 5. Concentration plot of the H2S3, H2S4, and NH3 species in the furnace gas over a range of temperatures.
Figure 6. Concentration plot of the H, HO, HS, HS2, and HS3 species in the furnace gas over a range of temperatures.

The maximum in sulfur yield under adiabatic condi-
tions occurs under conditions where the oxygen con-
sumption is given by the overall reaction
This is a well-known fact to Claus plant operators when
optimization of the operation of the Claus process is
attempted.24
As part of the work presented here, as well as to
expand on the previously given results, equilibrium
calculations were performed for a mixture of the fol-
lowing composition for acid gas and sour gas: H2S, 87.31
vol %; CO2, 3.82 vol %; NH3, 1.51 vol %; C1, 1.62 vol %;
C2, 1.62 vol %; H2O, 4.12 vol %; total, 100.00 vol %. This
gas composition would be the type that is treated in a
Claus unit designed to handle acid gas and sour water
stripper gas that contains ammonia at 10 psig. For the
equilibrium calculations, the gas was burned with air
by the stoichiometry of eq 3, adiabatically and isother-
mally, in the temperature range of 400-2200 °F. The
results of the calculations are shown in Figures 1-7.
Table 4 shows both stable and radical species exceeding
the ppmv concentration level.
For purpose of the calculations, the COMBUST
thermodynamic databank of the ASPEN package was
used. This is based on the JANAF Thermochemical
Tables, which were published by Dow Chemical Co.,
Midland, MI, in 1979. The databank contains the ideal
gas heat capacity, free energy of formation, and en-
thalpy of formation for many species, and these values
are accurate at the high temperatures that are typical
of combustion for more than 59 stable and radical
species.
Generally, the results are in agreement with the work
of Meisen and Bennett.10 Figure 1 shows the concentra-
tion of major species such as CO, CO2, and H2. As with
the work of Meisen and Bennett,10 the concentrations
of CO and H2 increase significantly at temperatures
above 620 °F. The concentrations of both CO and H2
increase with temperature, reaching an equilibrium
mole fraction of ∼0.01 in both cases at the highest
temperature shown or 2400 °F.
Figure 2 shows the distribution of the sulfur species
S, S2, S3, S4, S5, S6, S7, and S8. The smaller species, such
as S1, S2, and S3, are significant at the higher temper-
atures and above 1000 °F. Elemental sulfur (S2) is the
predominant species at these temperatures. Molecules
such as S5, S6, S7, and S8 become most significant at
lower temperatures (<700 °F). S8 overtakes all the other
species such as S6 and S7 as the temperature approaches
500 °F or lower.
Figure 3 shows that the formation of COS and CS2
does not become significant until the temperature
reaches 1000 °F or above, with the COS mole fraction
being higher by 2 orders of magnitude, or 100 ppmv.
The simulations also show that the concentrations of
species such as H2S2 and H2S3 start becoming signifi-
cant at temperatures higher than 600 °F (see Figures 4
and 5). The mole fraction of H2S2 peaks at 1000 ppmv,
only to decrease slightly at temperatures above 1000
°F. H2S3 displays the same behavior peaking at a mole
fraction of 10 ppmv at 1000 °F.
The oxygenated species SO can reach a mole fraction
of 1000 ppm levels at the higher temperatures shown
or 2300 °F. In contrast, S2O is most important, even at
lower temperatures; it peaks at 1000 °F with a mole
fraction as high as 1000 ppmv, or 3 orders of magnitude
higher than the concentration of SO at the same
temperature. The levels of species such as HSO and
H2SO2 are not as significant as SO or S2O above (see
Figure 4). The mole fraction of HSO can reach ∼1 ppmv,
at the higher temperature shown or 2300 °F, whereas
H2SO2 remains well below 1 ppmv even at the highest
temperatures or above 2000 °F.
The concentrations of radical species such as HS and
HS2 reach levels as high as 1000 ppmv (for HS radical),
at temperatures of ∼2000 °F (see Figure 6). In com-
Figure 7. Concentration plot of the H2O, H2S, and SO2 species in the furnace gas over a range of temperatures.
H2S +
1
2
O2 T
1
x
Sx + H2O (8)

parison, HS3 is not as significant; its concentration is
lower by 3 orders of magnitude.
Species such as H2S2 and H2S3 could have an impor-
tant role in the combustion chemistry of H2S. The
concentration of H2S2 peaks at 1000 °F, reaching 1000
ppmv (see Figure 4), only to decrease slightly at the
higher temperatures. H2S3 displays a similar behavior,
but its highest concentration is only 10 ppmv (see Figure
5). In contrast, a larger molecule, such as H2S4, reaches
considerably lower concentrations than the aforemen-
tioned H2S2 or H2S3 (<1 ppmv) (see Figure 5).
Equilibrium calculations show insignificant amounts
of hydrogenated species such as H2S5, H2S6, H2S7, and
H2S8. This could be due to hydrogen elimination reac-
tions, such as
The reactions result in the formation of the ring
structures for the sulfur species. The heats of reaction
are given parentheses and are based on average bond
energies. These reactions could easily occur at flame
temperatures.
B. Chemical Reactions Found in Combustion
(Illustrating the Chemistry Typical in Combus-
tion). This section is a short tutorial in combustion
chemistry fundamentals. Combustion involves radical
species and radical-chain mechanisms.17 The existence
of radical species such as H and OH in the gas phase is
possible because of the high temperature of combustion.
Chain mechanisms consist of initiation, propagation,
and termination steps. The type of reactions have been
described by Pryor:23
(a) Initiation reactions involving molecular cleavage,
producing the pool of radical species that start the chain,
e.g.,
(b) Propagation reactions can be of four different
types:
and
where the Cl atom transfers to the second C atom in
the molecule.
(c) Termination reactions are radical-radical addition
reactions or,
These result in a decrease of the radical species in the
gas phase.
Table 2. Molecular Structure of Sulfur Species
chemical formula molecular structure
SO SdO
SO2
SO3
H2SO2 H-S-O-O- -H
HSO2 H-O-O- -S
S2O S-O- -S
S2O2 S-O-O- -S
S2 SdS
CS C)S
CS2 SdCdS
COS OdCdS
S3 S-S- -S
S4 S-S-S- -S
S5(c)
S6(c)
S7(c)
S8(c)
H2S8 H-S-S-S-S-S-S-S-S- -H
H2S7 H-S-S-S-S-S-S-S- -H
H2S6 H-S-S-S-S-S-S- -H
H2S5 H-S-S-S-S-S- -H
H2S4 H-S-S-S-S- -H
H2S3 H-S-S-S- -H
H2S2 H-S-S- -H
CH3SH CH3- -SH
C2H5SH CH3-CH2- -SH
H2S5 T S5(c) + H2 (16 kcal/mol, ESTIM) (9)
Table 3. Species under Consideration in the Modeling of
Chemical Equilibrium Calculationsa
Species
stable radical
H2O, H2, O2 H, OH, O
NO, NH3 CS, HS
CO, CO2, COS, CS2 S, S3, S4
S2, S5, S6, S7, S8 HS, SN
SO2, SO, S2O, SO3
H2S, H2S2
a Data from Meisen and Bennett.10
Table 4. Species Used in Chemical Equilibrium
Calculations and Showing Concentrations in the Parts
per Million by Volume (ppmv) Rangea
Species
stable radical
CO, CO2 H, OH
H2, H2O HS, HS2, HS3
COS, CS2 S, S3, S4
NH3 HSO
H2S2, H2S3, H2S4
SO2, S2O, SO, H2SO2
S2, S5, S6, S7, S8
a Data from this work.
Cl2 T 2Cl (13)
atom transfer, such as hydrogen abstractions, e.g.,:
R′ + RH T R′H + R (14)
addition reactions, e.g.;
Cl + RCHdCH2 T RCH-CH2Cl (15)
fragmentation reactions, e.g.;
RCH2-CH2 T R + CH2dCH2 (16)
radical rearrangement reactions, e.g.;
CH3-C(H)-CH2Cl T CH3-C(HCl)-CH2 (17)
R + Cl T RCl (18)

Table 5. Chemical Paths for the Combustion of H2S-Reducing Conditionsa
Heat of Reaction Data
reaction heat of reaction (kcal/mol) notes reaction rate coefficient data
H2S + H ) H2 + HS -13.9 JANAF Kennedy,12 UNIV LEEDS
H2S + OH ) H2O + HS -28.91 JANAF Kennedy,12 UNIV LEEDS
H2S + O ) OH + S 20.91 JANAF Kennedy,12 UNIV LEEDS
H2S + O ) SO + H2 -54.65 JANAF Kennedy,12 UNIV LEEDS
H2S + S ) HS + HS 5.25 JANAF Kennedy,12 UNIV LEEDS
HS + HS + M ) H2S2 + M -66.6 JANAF Kennedy,12 UNIV LEEDS
HS + HS ) H2S + S -5.25 JANAF Kennedy,12 UNIV LEEDS
HS + HS ) S2 + H2 -35.89 JANAF Kennedy,12 UNIV LEEDS
HS + OH ) S + H2O -15.53 JANAF Kennedy,12 UNIV LEEDS
HS + H ) S + H2 -19.16 JANAF Kennedy,12 UNIV LEEDS
H2S2 + H ) HS2 + H2 -49.46 JANAF UNIV LEEDS
H2S2 + OH ) HS2 + H2O -44.92 JANAF UNIV LEEDS
H2S2 + O ) HS2 + OH -28.05 JANAF UNIV LEEDS
H2S2 + S ) HS + HS2 -29.37 JANAF UNIV LEEDS
H2S2 + M ) 2HS + M 64.48 Hynes and Wine28 Sendt et al.32
H2S2 + H ) H2S + SH -26.65 Hynes and Wine28 Sendt et al.32
H2S2 + HS ) H2S + HS2 -36.42 Hynes and Wine28 Sendt et al.32
HS2 + M ) HS + S + M 77.36 JANAF UNIV LEEDS
HS2 + OH ) S2 + H2O -58.6 JANAF UNIV LEEDS
HS + S2 ) HS2 + S 47 ESTIM, BE Kennedy12
HS2 + M ) 2HS + M 64.48 Hynes and Wine28 Sendt et al.32
HS2 + S2 ) HS3 + S 47 ESTIM, BE Kennedy12
HS2 + HS ) H2S + S2 -14.78 Hynes and Wine28 Sendt et al.32
HS2 + H ) 2SH 9.77 Hynes and Wine28 Sendt et al.32
HS2 + H ) H2 + S2 -14.93 Hynes and Wine28 Sendt et al.32
HS2 + H ) H2S + S 2.72 Hynes and Wine28 Sendt et al.32
H2S + S ) S2 + HS -7.7 Hynes and Wine28 Sendt et al.32
HS2 + HS2 ) H2S2 + S2 21.64 Hynes and Wine28 Sendt et al.32
HS + O2 ) SO + OH -22.48 JANAF Kennedy12
HS + O2 ) S + HO2 33.45 JANAF Kennedy12
HS + O2 ) HSO + O -0.4 JANAF Kennedy12
HS + O2 ) SO2 + H -52.14 JANAF Kennedy12
S3 + H2 ) HS3 + H 23 ESTIM, BE
HS3 + H2 ) H2S3 + H 23 ESTIM, BE
HS3 + H2S ) H2S3 + HS 0 ESTIM, BE
HS3 + S ) HS + S3 0 ESTIM, BE
HS3 + HS ) H2S + S3 0 ESTM. BE
HS3 + OH ) S3 + H2O -30 ESTIM, BE
H2S3 + OH ) HS3 + H2O -30 ESTIM, BE
H2S3 + S ) HS3 + HS -8 ESTIM, BE
H2S3 + HS ) H2S + HS3 -8 ESTIM, BE
S4 + H2 ) HS4 + H 23 ESTIM, BE
H2S4 + OH ) HS4 + H2O -30 ESTIM, BE
H2S4 + S ) HS4 + HS -8 ESTIM, BE
H2S4 + HS ) HS4 + H2S -8 ESTIM, BE
HS4 + H2 ) H2S4 + H 23 ESTIM, BE
HS4 + H2S ) H2S4 + HS 0 ESTIM, BE
HS4 + S ) HS + S4 0 ESTIM, BE
HS4 + HS ) H2S + S4 0 ESTIM, BE
HS4 + OH ) S4 + H2O -30 ESTIM, BE
HS3 + M ) HS2 + S + M 54 ESTIM, BE
HS4 + M ) HS3 + S 54 ESTIM, BE
S2 + O ) SO + S -22.5 JANAF Kennedy,12 UNIV LEEDS
S + O + M ) SO + M -124.29 JANAF Kennedy,12 UNIV LEEDS
S + O2 ) SO + O -5.19 JANAF Kennedy,12 UNIV LEEDS
SO2 + O + M ) SO3 + M -83.14 JANAF Kennedy,12 UNIV LEEDS
SO + O + M ) SO2 + M -131.75 JANAF Kennedy,12 UNIV LEEDS
SO + S + M ) S2O + M -66.24 JANAF Kennedy,12 UNIV LEEDS
S + S + M ) S2 + M -101.78 JANAF NIST
S2 + S + M ) S3 + M -7 Hynes and Wine28
S3 + S + M ) S4 + M -54 Hynes and Wine28
S4 + S + M ) S5(c) + M -96 Hynes and Wine28
S2 + S3 + M ) S5(c) + M -49 Hynes and Wine28
S3 + S ) S2 + S2 -94 Hynes and Wine28

Table 5 (Continued)
S6 + S ) S5(c) + S2 -89 Hynes and Wine28
S5(c) ) M ) S5 + M 30 Hynes and Wine28
S6 (c) + M ) S6 + M 32 Hynes and Wine28
S7(c) + M ) S7 + M 33 Hynes and Wine28
S8(c) + M ) S8 + M 33 Pryor23
S3 + S5 ) + M S8(c) + M -107 Hynes and Wine28
S4 + S4 ) + M S8 (c) + M -107 Hynes and Wine28
S8(c) T 8/5S5(c) 29.1 Raghavachari et al.21
HS2 + S3 ) S5(c) + H -15 ESTIM, BE
HS2 + S2 ) S4 + H 74 ESTIM, BE
HS2 + S4 ) S6(c) + H -21.7 ESTIM, BE
HS2 + S5 ) S7(c) + H -23.8 ESTIM, BE
HS2 + S6 ) S8(c) + H -28.1 ESTIM, BE
HS2 + HS2 ) S4(c) + H2 -50 ESTIM, BE
HS3 + S2 ) S5(c) + H 32 ESTIM, BE
HS3 + S2 ) HS4 + S 47 ESTIM, BE
HS3 + HS3 ) S6(c) + H2 -44.7 ESTIM, BE
HS3 + HS2 ) S5(c) + H2 -38 ESTIM, BE
HS4 + S2 ) HS5 + S 47 ESTIM, BE
HS4 + S2 ) S6(c) + H 25.3 ESTIM, BE
HS4 + HS4 ) S8(c) + H2 -51.1 ESTIM, BE
HS4 + HS3 ) H2 + S7(c) -46.8 ESTIM, BE
HS + S4 ) S5(c)+ H -15 ESTIM, BE
HS + S5 ) S6(c) + H -21.7 ESTIM, BE
HS + S6 ) S6(c) + H -23.8 ESTIM, BE
HS + S7 ) S8(c) + H -28.1 ESTIM, BE
CO + S + M ) COS + M -72.91 JANAF UNIV OF LEEDS
COS + H ) CO + HS -11.91 JANAF NIST, UNIV LEEDS
COS + OH ) CO2 + HS -37 JANAF NIST, UNIV LEEDS
COS + O ) CO2 + S -56.5 JANAF NIST, UNIV LEEDS
COS + O ) CO + SO -52.9 JANAF NIST, UNIV LEEDS
COS + S ) CO + S2 NIST, UNIV LEEDS
C + S + M ) CS + M -170.54 JANAF UNIV OF LEEDS
CS + S + M ) CS2 + M -105.29 JANAF UNIV OF LEEDS
C + S2 ) CS + S -68.4 JANAF UNIV OF LEEDS
CS + S2 ) CS2 + S -3.51 JANAF UNIV OF LEEDS
CS2 + O ) COS + S -54.3 JANAF NIST
CH3+ O2 ) CH2O + OH -53.2 JANAF Gargurevich,17 GRI MECH
CH3O + M ) CH2O + H + M 20.50 JANAF Gargurevich,17 GRI MECH
CH2O + HS ) CHO + H2S -0.1 JANAF
S2 + CHO ) COS + HS -76.4 JANAF
CH3 + S2 + M ) CH3-S-S + M 47.5 Hynes and Wine28
CH3-S-S + H2S ) HS + CH3-S-SH 7 Hynes and Wine28
CH3-S-SH + M ) CH3S + HS + M 54 Hynes and Wine28
CH3S + HS ) H2S + CH2dS -63 Hynes and Wine28
CH2dS + HS ) H2S + CHdS 10 Hynes and Wine28
CHdS + S2 + M ) S-S-CHdS + M -18 Hynes and Wine28
S-S-CHdS + H2S ) S ) CH-S-SH + HS 7 Hynes and Wine28
SdCH-S-SH + M ) SdCH-S + HS 54 Hynes and Wine28
SdCH-S + HS ) H2S + CS2 -63 Hynes and Wine28
S + C2H2 T HCS + CH 92 JANAF
CHS + M T H + CS + M 51 JANAF
CH3SH + H ) CH3 + H2S -16.72 JANAF NIST
C2H5SH + H ) C2H5 + H2S -41.52 JANAF NIST
CH3SH + HS ) CH3 + H2S2 7.01 JANAF
C2H5SH + HS ) C2H5 + H2S2 -17.8 JANAF
CH3SH + S ) CH3 + HS2 -3.75 JANAF
C2H5SH + S ) C2H5 + HS2 -28.6 JANAF
CH3 + S2 ) CH2 ) S + HS -3.9 Hynes and Wine28
CH2dS + HS ) CHdS + H2S 3.9 Hynes and Wine28
CHdS + S2 ) CS2 + H2S --39.10 Hynes and Wine28
CH3 + HS ) CH3SH -74.44 Hynes and Wine28 Petherbridge et al.36
CH3 + HS ) H2 + CH2S -41.50 Hynes and Wine28 Petherbridge et al.36
CH3SH + H ) CH3 + H2S -16.70 Hynes and Wine28 Petherbridge et al.36
CH3SH + H ) CH3S + H2 -16.77 Hynes and Wine28 Petherbridge et al.36
CH3S + H ) CH2S + H2 -54.47 Hynes and Wine28 Petherbridge et al.36
CH2S + H) HCS + H2 -9.1 Hynes and Wine28 Petherbridge et al.36
HCS + H ) H2 + CS -55.67 Hynes and Wine28 Petherbridge et al.36
HS + CS ) H + CS2 -21.0 Hynes and Wine28 Petherbridge et al.36

Another important process that is common in com-
bustion systems is the process of chemical activation.
A reaction involving addition, such as a radical addition
to a double bond, leads to the formation of a chemically
activated adduct that possesses excess energy due to the
bond formation. This adduct can further react, leading
to products. One such reaction leading to the formation
of formaldehyde is the addition of a methyl radical to
the double bond in oxygen, or
The chemically activated adductsin this case, CH3-O-
O* has excess energy and leads to the formation of
products. The reader can find more-complete discussions
of chemical activation in ref 25.
Warnatz has published an interesting manuscript
that examined the issues of hydrocarbon oxidation and
high-temperature chemistry.26 It describes the main
chemical paths in the combustion of hydrocarbons that
are common to most molecules.
Hydrogen sulfide combusts then via chemical paths
that are similar those previously mentioned. A summary
of the elementary chemical reactions considered in this
study is given in Table 5. The table includes the heat of
reaction for each reaction. One must recall that, because
of the concentration factor in the rate of a chemical
reaction, typically, radical-radical reactions should not
be as important as radical-stable-species reactions.
C. A Brief Discussion of Recently Published
Sulfur Chemistry. It is not the objective of this section
to present a comprehensive review of published chem-
istry; this has been done by other authors who will be
mentioned below. This part of the manuscript will
attempt to describe the main results of previous studies,
as well as show some very relevant and important
conclusions: (i) there is a lack of high-temperature
kinetic data; (ii) accurate thermodynamic data for some
important sulfur species are also lacking; (iii) most
studies examine H2S combustion at high temperature
only indirectly, and their aim is to observe the effect of
the sulfur species on the formation of pollutants such
as CO and NOx; and (d) the effect of combustion
conditions on the formation of SO3 is examined.
A comprehensive review of the combustion of gaseous
sulfur compounds was conducted by Cullis and Mulcahy
in 1972.18 The review examines the chemistry of sulfur
compounds that either undergo combustion themselves
or may be present in other gaseous combustion systems.
Their study is based on low-temperature photolysis
experiments and flame studies. In some ways, this is
the starting point for later works on sulfur chemistry
and oxidation. They identify the final and intermediate
products of combustion: SO2 is always the main prod-
uct, with small amounts of SO3, depending on the
stoichiometric conditions. Other sulfur oxides of interest
are SO and S2O, which are intermediates. Other prod-
ucts of combustion under substoichiometric conditions
are H2S, COS, and elemental sulfur. Cullis and
Mulcahy18 continued by identifying some of the elemen-
tary reactions of interest. These are listed as follows.
H Atoms.
O Atoms.
They agree that the main reaction for formation of SO3
is
Table 5 (Continued)
CO2 + S ) SO + CO 2.9 JANAF
CO2 + HS ) HSO + CO 7.69 JANAF
H + SO2 + M T HSO2 + M 6.0 Hynes and Wine28 Zachariah and Smith13
H + HSO2 T SO2 + H2 -110.0 Hynes and Wine28 Zachariah and Smith13
OH + SO + M T HSO2 + M -23.49 Hynes and Wine28 Zachariah and Smith13
OH + HSO2 T SO2 + H2O -80.2 Hynes and Wine28 Zachariah and Smith13
H + SO + M T HSO + M -52.3 Hynes and Wine28 Zachariah and Smith13
HSO + H T H2 + SO -54.2 Hynes and Wine28 Zachariah and Smith13
HSO + H T H2S + O -5.0 Hynes and Wine28 Zachariah and Smith13
HSO + H T SH + OH -7.6 Hynes and Wine28 Zachariah and Smith13
HSO + OH T H2O + SO -13.0 Hynes and Wine28 Zachariah and Smith13
HSO + O T SO + OH -47.10 Hynes and Wine28 Zachariah and Smith13
HSO + O T H + SO2 -77.50 Hynes and Wine28 Zachariah and Smith13
HSO + O T HS + O2 -23.28 Hynes and Wine28 Zachariah and Smith13
HSO + O2 T SO + HO2 7.15 Hynes and Wine28 Zachariah and Smith13
SH + HSO T H2S + SO -35.73 Hynes and Wine28 Zachariah and Smith13
a ”UNIV LEEDS” refers to the University of Leeds, U.K., Sulfur Mechanism (http://garfield.chem.elte.hu/Combustion/Combustion.html).
“GRI Mech” refers to the GRI Mechanism 3.0 for Methane Combustion (www.me.berkeley.edu/gri_mech). “NIST” refers to the kinetic
database provided by the National Institute of Standards (www.nist.gov).
CH3 + O2 T [CH3-O-O]* T CH2dO +OH (19)
H + H2S T HS + H2 (20)
H + HS T H2 + S (21)
H + CH3SH T H2 + CH3S (22)
H + SO2 + M T HSO2 + M (23)
S2 + O T SO + S (24)
H2S + O T SO + H2 (25)
O + H2S T OH + SH (26)
O + SH T SO + H (27)
COS + O T SO + CO (28)
COS + O T CO2 + S (29)
CS2 + O T CS + SO (30)
O + CS T CO + S (31)
S + O2 T SO + O (32)
SO + O + M T SO2 + M (33)

OH Radicals.
Other Reactions. Other reactions that will be sig-
nificant in our work include
The presence of methyl radicals from fossil fuel would
accelerate the decomposition of H2S.
S Atoms.
A reaction of high interest, because it could lead to CS2
during the combustion of H2S, when in the presence of
hydrocarbons, is
Also,
Molecular growth occurs via the reactions
Their summary mechanism for the combustion of H2S
under fuel-lean conditions consists of the main reactions
Unfortunately, the early work of Cullis and Mulcahy18
lacks information on the thermodynamics and kinetic
data for much of the information presented, and, as
noted previously, much of the referenced experimental
data have been obtained at temperatures much lower
than the combustion temperatures. However, it is a good
starting point for any sulfur compound combustion or
decomposition mechanism.
Johnsson and Glarborg2 presented developments in
the sulfur chemistry of combustion. The point is made
that there are studies for the purpose of kinetic model-
ing in shock tubes, flow reactors, and flames. They
stated that earlier models suffered from a lack of
accurate thermodynamic and kinetic data. Rate con-
stants for important reactions involving SO2 and SO3
are presented. Sulfur dioxide catalyzes the recombina-
tion of the main chain carriers in the flame (this will
be discussed further below) and it impacts the concen-
tration of CO and NOx in flames. The reaction mecha-
nism of Glarborg et al.27 is recommended, because of
its completeness in thermodynamic data.
Hynes and Wine28 expanded on the work of Cullis and
Mulcahy18 in an attempt to update the species thermo-
dynamics (see Table 6) and rate coefficients. They note
that kinetic studies have focused on low-temperature
chemistry, as required to obtain rate coefficient data for
O + SO2 + M T SO3 + M (34)
OH + SO T SO2 + H (35)
CH3 + H2S T CH4 + HS (36)
S + H2 T H + HS (37)
H + COS T CO + SH (38)
S + CH4 T CH3 + SH (39)
S + C2H2 T HCS + CH (40)
HCS T H + CS (41)
S + COS T S2 + CO (42)
S + O2 T SO + O (43)
S + S + M T S2 + M (44)
S + S2 + M T S3 + M (45)
S3 + S T 2S2 (46)
H2S + O2 T HO2 + HS (47)
H2S + M T H + HS + M (48)
HS + O2 T OH + SO (49)
OH + H2S T H2O + HS (50)
SO + O2 T SO2 + O (51)
O + H2S T H2 + SO (52)
O + H2S T OH + HS (53)
Table 6. Sulfur Species Heat of Formationa
species ∆f H°298 (kJ/mol) species ∆f H°298 (kJ/mol)
S 277.0 ( 0.3 S2 128.6 ( 0.3
S3 142 ( 8 S4 146 ( 8
S5 109 ( 8 S6 102 ( 8
S7 114 ( 8 S8 100.4 ( 0.6
SO 5.0 ( 1.3 S2O -56 ( 34
SO2 -296.8 ( 0.2 SO3 -395.8 ( 0.7
SH 143 ( 3 HS2 27 ( [20]
H2S -20.5 ( 0.8 HSSH 16 ( 15
HSO -4 ( 3 HOS 18 ( [15]
HSO2 -54 ( 15 HOSO -188 ( 15
HOSO2 -385 ( [10] H2SO4(g) -735.1 ( 8.4
H2SO4(l) -814.0 ( 0.7
CS 280 ( 25 CS2 117 ( 1
COS -138.4 ( 0.5
HCS 295 ( [10] H2CS 115 ( [10]
CH3S 125 ( 2 CH3SH 214 ( 9
CH3SH -22.9 ( 0.6 C2H5SH -46.3 + 0.6
C6H5SH 112.4 ( 0.8 H2CdCdS 165 ( [15]
CH3SCH3 -37.5 ( 0.5 CH3SCH2 135 ( 3
CH3SSCH3 -24.2 ( 1.0 CH3SSSCH3 11 ( [10]
CH3SS 72 ( 5 CH3SSS 86 ( 5
c-CH2CH2S 82.1 ( 1.2 CH3SC2H5 -59.6 ( 1.1
(C2H5)2S -84 ( 1 (C6H5)2S 231 ( 3
C4H4S 115.0 ( 0.4 C4H8S -34.1 ( 0.9
CH3SO -62 ( [15] CH3SOO 76 ( 4
CH3SO2 -238 ( [15] CH3SO3 -350 ( [5]
(CH3)2SO -151.3 ( 0.8 (CH3)2SO2 -373 ( 3
CH2CH2SO -30 ( [15] (C2H5)2SO -205.6 ( 1.5
(C6H5)2SO 107 ( 3 (CH3O)2SO -483 ( 2
CH3C(O)SH -175 ( 8 CH3SOH -90 ( [25]
CH3S(O)OH -360 ( [25] CH3SO3H -567 ( [25]-
SCSOH 110 ( [10] (CH3)2SOH 60 ( [10]
HSNO 94 ( [20] NS 263 ( 105
HNCS 126 ( 115] CH3NCS 131 ( [15]
CH3SCN 160 ( [15] (SCN)2 350 ( 6
(NH2)2CS -25 ( [15] CH3C(S)NH2 10 ( 1
FS 13.0 ( 6.3 SF2 -297 ( 17
CIS 156 ( 17 S2C1 78.6 ( 8.4
Cl2S 17.6 ( 3.3 Br2S 21 ( 17
SF6 -1220.5 ( 0.8 SF5C1 -1039 ( 11
SF4Cl2 -858 ( 13 SF5 -908 ( 15
SF4Cl -741 ( [20] SF4 -763 ( 21
SF3 -503 ( 34 HSI 42 ( 3
F2CS -350 ( [15] Cl2CS -27 ( [15]
CH3SCl -28 ( 6 CH3SI 30 ( 3
C6H5SCl 106 ( 6 CH3SCH2Cl -90 ( [5]
CH3SCH2Cl 26 ( 5
FSSF -335 ( 42 ClSSCl -16.7 ( 4
BrSSBr 35 ( [10] CH3SSCl -21 ( 6
C6H5SSCl 113 ( 6
F2SO -544 ( 21 Cl2SO -213 ( [20]
Br2SO -107 ( [20] F2SO2 -758.6 ( 8.4
Cl2SO2 -354.8 ( 2.1 FClSO2 -557 ( 21
a Data from Hynes and Wine.28

atmospheric modeling studies. Most rate data would
seem to apply to a temperature in the proximity of 1000
K, and extrapolations are required for flame tempera-
tures. The rate coefficients and chemistry of ∼48
elementary reactions are presented, with the applicable
temperature being as high as 2500 K. The reactions
involve species such as SO2, SO3, H2S, HS, S, S2, HSO2,
CH3SH, CH3SCH3, CS2, COS, and CS.
The work of Alzueta et al.29 has some important
observations concerning the inhibition of moist CO
oxidation by SO2 in flow reactors. Their main objective
is to re-examine the interaction of SO2 with the radical
pool under different conditions of temperature, SO2
concentration, and stoichiometry (ranging from very
lean to rich). The reactor temperature ranged from 800
K to 1500 K at a pressure of 1.05 bar. For this purpose,
they assembled a reaction mechanism that was com-
prised of 82 reactions and sulfur species such as SO,
SO2, SO3, HSO, HOSO, HSO2, HOSO2, S, SH, S2, HS2,
and H2S2. The mechanism used in their modeling is
essentially that reported by Glarborg et al.27
They found that the extent of CO inhibition is
dependent on the stoichiometry and the amount of SO2.
Under very lean conditions, SO2 inhibits CO oxidation
via the following reaction that captures O radicals:
However, at near-stoichiometric conditions, the promo-
tion of CO oxidation occurs, because of the increase in
the radical pool by the reactions
and
The overall reaction results in radical chain branching:
On the other hand, under fuel-rich conditions, SO2
inhibits the oxidation of CO:
They note that, to match the experimental data with
their mechanism, they had to modify the heat of
formation of HOSO to a value of -236.3 kJ/mol (versus
-188 kJ/mol in Table 6 from Hynes and Wine28).
They determined that, in the flame experiment of
Zachariah and Smith,13 the H-atom recombination by
the sulfur species could also be explained (using their
mechanism) by the reaction sequence
This is in contrast to the reaction originally proposed
by Zachariah and Smith13 to produce HOSO:
As this work illustrates, both the thermodynamic prop-
erties of species such as HOSO, as well as the issue of
H-radical recombination in flames, requires further
investigation to elucidate the real chemistry that occurs
in SO2 inhibition.
Schofield’s study46 is interesting because it questions
previous studies by other authors that have involved
flames and sulfur chemistry. It presents equilibrium
calculations and kinetic modeling of flames of H2, CH3-
OH, and C3H8 in air at several equivalence ratios (fuel-
rich) and isotherms and doped with amounts of SO2
(0.3%-0.9%). Their work seems to substantiate a partial
equilibrium assumption involving the reactions of HS,
S2, S, SO2, SO, H, and OH:
For kinetic modeling, they seem to prefer Zachariah and
Smith,13 because of its validation against experimental
data, and measurements of OH, H, and O radicals in
their flames. They present a system of 16 reactions that
involves the formation of COS from CS and the destruc-
tion of COS in fossil-fueled flames. One interest point
is that nonequilibrium has a tendency to move the
sulfur speciation in the direction of SO2, SO, and S. At
the temperatures considered, species such as HSO,
HSO2, HSO3, H2S2, S2O, S3, etc. contribute little to the
overall sulfur balance.
The report by Glarborg et al.27 is, without a doubt,
one of the most quoted papers tha tinvolves reaction
mechanisms for sulfur species oxidation. They studied
the impact of SO2 and NO on CO oxidation using flow
reactors. They note that previous studies of flames,
shocks, and flow reactors provided some understanding
of sulfur chemistry, although the modeling efforts lacked
accurate thermodynamic data and rate data. Their
experiments determined that SO2 inhibits CO oxidation,
and it is most pronounced at high O-atom concentra-
tions. The addition of NO significantly reduces the
impact of SO2. They revised the thermochemistry for
the H/S/O system, based on recent experimental and
theoretical results. Their work included revised ther-
modynamic properties for the HxSOy species, as well as
QRRK treatment for reactions involving these species.
The mechanism consists of 67 reactions and more than
15 species.
The work of Frenklach et al.30 also is often quoted.
Their experimental and modeling work directly involved
H2S oxidation in shock tubes (induction times). The
experimental conditions were 4%-22% H2S in air, with
2%-13% H2O. The experiments were conducted at a
pressure of 1 atm and a temperature of 950-1200 K.
The kinetic model consisted of 17 species and 57
reactions. The agreement between experiment and
model is reported to be satisfactory. Frenklach et al.30
conducted a rate and sensitivity analysis for their
mechanism. The main reactions are listed below, be-
SO2 + O + M T SO3 + M (54)
SO2 + H T SO + OH (55)
SO + O2 T SO2 + O (56)
H + O2 T O + OH (57)
H + SO2 + M T HOSO + M (58)
HOSO + O2 T SO2 + HO2 (59)
HOSO + H T SO2 + H2 (60)
H + SO + M T HSO + M (61)
H + S2 + M T HS2 + M (62)
SO2 + H + M T HOSO + M (63)
HS + H2 T H2S + H (64)
H + S2 T HS + S (65)
S + H2 T HS + H (66)
H + SO2 T SO + OH (67)
S + OH T SO + H (68)
S2 + OH T S2O + H (69)
H2 + OH T H2O + H (70)

cause of its power to illustrate the most important
reactions for oxidation of H2S at high temperature:
The work of Chernysheva and co-workers,3,31 which
examined mechanistic issues in the gas-phase oxidation
of hydrogen sulfide and carbon disulfide, is also an
important step in the understanding of the most rel-
evant chemistry in both processes. The H2S oxidation
model3 consists of 201 reactions and 23 species, and it
describes experimental data in a wide range of temper-
ature, although the data available were limited to
ignition delay and the concentration of the major species
(no intermediates are reported). The model includes
species such as S, S2, HS, HS2, H2S2, SO, S2O, HSO,
HOS, HSO2, HOSO2, (HSO)2, SO2, and SO3. They noted
that the mechanism should still be considered to be an
approximation for the high-temperature oxidation of
H2S. The ignition studies reveal some important reac-
tions under stoichiometric conditions:
The carbon disulfide mechanism consists of 70 elemen-
tary reactions.31 It includes species such as O, S, S2, SO,
S2O, CS, COS, SO2, and SO3. For the most part, the
kinetic data are estimates for the reactions in the
mechanism. As with the model for H2S, the predictions
for ignition delay and the concentration of major species
in flames are well-correlated (no intermediates are
reported). Their comment is that, generally, the mech-
anism will need refinements (reaction channels, rate
coefficients). The most important reactions in CS2
oxidation are identified as (fuel-lean conditions)
For the oxidation of COS (fuel-lean):
The work of Sendt et al.32 is also important, because
it elucidates the importance of the chemical species H2S2
(HSSH) in the thermolysis of H2S and H2 sulfidation.
Their objective is to validate a chemical kinetic mech-
anism that consists of 21 reactions and the species H2S,
S2, H2, HSSH, HSS, SH, S, and H. The mechanism was
validated against a diverse collection of published data
for flow reactors (residence time of 0.2-1800 s, temper-
ature of 873-1423 K, pressure of 0.04-3 bar, H2S mole
fractions of 0.02-1.0). To estimate the rate constants,
computational methods were often used, such as transi-
tion-state theory, QRRK methods, and quantum chem-
istry estimates of energy barriers.
A sensitivity analysis of their mechanism results show
that the most important reactions are as follows (shown
with the heat of reaction):
For the species HSS, the decomposition reaction is
The HSSH species decomposes mainly by
The channel HSSH T H + SSH is not considered to be
important.
H + O2 T OH + O (71)
H + O2 + M T HO2 + M (72)
HO2 + H T OH + OH (73)
HO2 + HO2 T H2O2 + O2 (74)
H2O2 + M T OH + OH + M (75)
S + O2 T SO + O (76)
SO + O2 T SO2 + O (77)
SO + O2 + M T SO3 + M (78)
H + H2S T H2 + HS (79)
HS + HS T H2S + S (80)
HS + H T H2 + S (81)
H2S + O T OH + HS (82)
H2S + O T SO + H2 (83)
H2S + O T HSO + H (84)
H2S + OH T H2O + HS (85)
HS + O2 T SO + OH (86)
HS + O2 T SO2 + H (87)
HS + HO2 T H2S + O2 (88)
HS + O T SO + H (89)
H + SO + M T HSO + M (90)
HSO + O2 T HO2 + SO (91)
HO2 + SO2 T SO3 + OH (92)
H2S + O2 T HS + HO2 (93)
HS + O2 T OH + SO (94)
HS + O2 T SO2 + H (95)
HS + O2 + M T HSO2 + M (96)
SO + O2 T SO2 + O (97)
H2S + O T HS + OH (98)
H2S + OH T HS + H2O (99)
CS2 + O T CS + SO (100)
SO + O2 T SO2 + O (101)
CS2 + O2 T COS + SO (102)
S + O2 T SO + O (103)
CS + O T CO + S (104)
COS + O T CO + SO (105)
S + O2 T SO + O (106)
CO + SO T CO2 + S (107)
COS + O T CO2 + S (108)
SO + O T S + O2 (109)
CO2 + S T CO + SO (110)
H2S + M T H2 + S + M (71.05 kcal/mol) (111)
H + HSS T 2SH (9.77 kcal/mol) (112)
HSSH + M T 2SH + M (64.48 kcal/mol) (113)
HSSH + H T H2S + SH (-26.65 kcal/mol) (114)
HSS + M T S2 + H + M (76.35 kcal/mol) (115)
HSSH + M T 2HS + M (64.48 kcal/mol) (116)

HSS will react according to the sequence
Finally, HSSH reactions include
D. Results of this Study: H2S Combustion Chemi-
cal Paths. The heats of reaction calculated for the
reactions shown below were obtained from the JANAF
Themochemical Tables. For some of the reactions in-
volving stable and radical sulfur species, the recent data
of Hynes and Wine,28 as presented in Table 6, was used
as indicated. The errors associated with the heats of
formation are indicated in Table 6. If data was lacking,
in this work estimates were made using bond energies.
As Hynes and Wine28 have noted, the thermochem-
istry of sulfur species in combustion phenomena is a
subject of active research. The recommended heats of
formation in Table 6, which contains over 100 sulfur
compounds, show that the values for many key oxygen-
ated sulfur species remain unacceptably high. Some of
these species are HOS, HSO2, HOSO, HOSO2, H2SO4
(g), CH3SO, CH3SO2, CH3SO3, CH3SOH, CH3S(O)OH,
and CH3SO3H. Many these species have a role in the
mechanism of radical recombination by SO2, as will be
discussed below.
Prior to the discussion, some important considerations
that have a significant role in considering chemical
paths must be noted. The rate of reaction is dependent
on the concentration of the chemical species and the
magnitude of the rate constant. In the chemical paths
depicted below, radical-molecule reactions would be
considered typically more important than radical-
radical reactions, because of concentration effects. Reac-
tions that involve radical-radical additions, as well as
chemically activated reactions, such as a radical addi-
tion to a double bond, usually have favorable activation
energies. Reactions that involve atomic abstraction by
O, OH, and H radicals in the combustion process also
involve, typically, relatively low activation energies. It
is useful to realize that the energy barrier for endother-
mic reactions is at least equal to the heat of reaction.
Chemicals paths that involve O, OH, and H radicals are
fundamental to high-temperature combustion phenom-
ena,34 because they participate in the radical-chain
mechanism.
D.1. Formation of Oxygenated Species: SO, SO2,
SO3, and S2O. The formation of oxygenated sulfur
species occurs via radical addition reactions.12 Consid-
eration must be given to the reducing conditions under
which the Claus process occurs; thus, H and OH radicals
should be more abundant than O.
The addition of S and HS to oxygen leads to the
formation of SO:
Because of the lower O radical concentration, the
addition to elemental sulfur should be secondary:
The bond between sulfur and oxygen in SO is a double
bond.
The formation of SO2, and SO3, occurs via similar
chemical paths:
Both SO2 and SO3 consist of resonance structures in
which the oxygen is double-bonded to sulfur. In this
work, because of the reducing conditions, no significant
amounts of SO3 were calculated at equilibrium. The
previously described mechanism would seem to confirm
this, because its formation involves O radicals, which
are less abundant in fuel-rich flames.
Finally, the formation of S2O would seem to follow
the path
D.2. Formation of Sulfur Vapor (S2). The thermal
decomposition of H2S leads to an abundance of S and
HS radicals in the Claus furnace. Elemental sulfur
vapor (S2) is formed via the following chemical paths:
This reaction leads to the formation of hydrogen, which
is a species with important design considerations for the
Claus Plant.
D.3. Destruction of Hydrogen Sulfide. The initial
decomposition of hydrogen sulfide is initiated by its
unimolecular decomposition at high temperature, or
The decomposition then follows paths that involve
radical-molecule reactions12 typical of combustion, or
HSS + SH T H2S + S2 (-14.78 kcal/mol) (117)
H + HSS T H2 + S2 (-14.93 kcal/mol) (118)
H + HSS T H2S + S (2.72 kcal/mol) (119)
S + HSS T S2 + SH (-7.70 kcal/mol) (120)
HSS + HSS T HSSH + S2 (21.64 kcal/mol)
(121)
HSSH + H T HSS + H2 (-49.46 kcal/mol)
(122)
HSSH + H T H2S + SH (-26.65 kcal/mol)
(123)
HSSH + SH T H2S + HSS (-36.42 kcal/mol)
(124)
HSSH + S T HSS + SH (-29.37 kcal/mol)
(125)
S + O2 T SO + O (-5.19 kcal/mol) (126)
HS + O2 T SO + OH (-22.48 kcal/mol) (127)
S2 + O T SO + S (-22.50 kcal/mol) (128)
HS + O2 T SO2 + H (-52.14 kcal/mol) (129)
SO + O + M T SO2 + M (-132.0 kcal/mol)
(130)
SO2 + O + M T SO3 + M (-83.14 kcal/mol)
(131)
SO + S + M T S2O + M (-66.24 kcal/mol) (132)
HS + HS T S2 + H2 (-35.89 kcal/mol) (133)
M + S + S T S2 + M (-101.78 kcal/mol) (134)
M + H2S T H + HS + M (90.30 kcal/mol) (135)
H2S + H T H2 + HS (-13.90 kcal/mol) (136)

This reaction leads to the formation of hydrogen, which
is an important species in Claus plant design.
This continues to form sulfur (S):
These are radical-radical reactions; however, the
concentration of HS radicals was shown to be high in
our equilibrium calculations. Reactions that involve HS,
S, and O can lead to SO and SO2, as described previ-
ously.
D.4. Molecular Growth of Sulfur in the Gas
Phase. The equilibrium calculations show that concen-
trations of S, HS, S2, H2S2, and HS2 are relatively high,
and molecular growth to S8 could involve all of these
species. To a lesser extent, species such as H2S3 and
H2S4 can also participate.
The first step would be the formation of S3, or
which is followed by
The growth continues, or
where, for thermodynamic reasons, the ring or cyclic
structure of S5(c) would be formed. Depending on
temperature, an equilibrium would be formed between
the open linear structure and its ring counterpart, or
The growth continues to form S6 and S7 species:
Finally, S8 is formed:
H2S + OH T HS + H2O (-28.91 kcal/mol) (137)
H2S + S T HS + HS (5.25 kcal/mol) (138)
HS + OH T S + H2O (-15.53 kcal/mol) (139)
HS + H T S + H2 (-19.16 kcal/mol) (140)
HS + HS T S + H2S (-5.25 kcal/mol) (141)
S2 + S + M T S3 + M
(-9 kcal/mol; from Hynes and Wine28
) (142)
S3 + S + M T S4 + M
(-65.25 kcal/mol; from Hynes and Wine28
)
(143)
S2 + HS2 T S4 + H
(49.8 kcal/mol; from Hynes and Wine28
) (144)
S4 + S + M T S5(c) + M
) (145)
S3 + HS2 T S5(c) + H
) (146)
S3 + S2 + M T S5(c) + M
) (147)
HS3 + S2 T S5(c) + H
) (148)
HS + S4 T S5 (c) + H
)
(149)
S5(c) + M T S5 + M (30 kcal/mol, ESTIM) (150)
S5 + S + M T S6(c) + M
)
(151)
S4 + HS2 T S6(c) + H
)
(152)
S4 + S2 + M T S6(c) + M
) (153)
HS4 + S2 T S6(c) + H
) (154)
HS + S5 T S6(c) + H
) (155)
S6 + S + M T S7(c) + M
)
(156)
S5 + HS2 T S7(c) + H
) (157)
S5 + S2 + M T S7(c) + M
) (158)
HS + S6 T S7(c) + H
) (159)
S7 + S + M T S8(c) + M
)
(162)
S6 + HS2 T S8(c) + H
) (163)
S6 + S2 + M T S8(c) + M
)
(164)
HS + S7 T S8(c) + H
)
(165)
S8(c) + M T S8 + M (33 kcal/mol; from Pryor23
)
(166)

Other reactions that can also occur, but at lower
temperatures (because of concentration effects), are
There is the question concerning the magnitude of the
activation energies for the reactions responsible for the
formation of cyclic sulfur species up to S8. Radical
addition reactions typically have relatively low activa-
tion energies, which is a well-known fact. The ring-
opening reactions are endothermic in nature, and the
activation energy should be at least equal to the heat
of reaction. The work of Huang et al.33 in a similar
reaction that involved C atoms and examined the
decyclization of the phenyl radical to C6H5 would seem
to indicate that, for the larger molecules (such as S6 and
S8), the energy barrier may be close to the heat of
reaction. For smaller molecules (such as S5), semiem-
pirical quantum chemistry calculations by Gargurev-
ich,17 which involved five carbon species, would indicate
that the decyclization energy could be somewhat higher
than the heat of reaction. The same studies would seem
to indicate that the magnitude of the energy barriers
for cyclization of linear sulfur species to its ring struc-
ture should be fairly low.
D.5. Formation of Trace Species CS2 and COS.
The chemical paths leading to the formation of carbon
disulfide (CS2) and carbonyl sulfide (COS) also involve
radical-addition reactions. The most important path
leading to the formation of COS is the addition of the S
radical to the triple bond of CO, or
Studies indicate that there is a good correlation between
the formation of COS and the presence of CO and
sulfur.6
The formation of formaldehyde and CH3O are well-
known chemical paths during the oxidation of hydro-
carbons.17 COS could very well form following pathways
that involve these two species. First, CH3 and SO2 are
abundant species during the combustion of H2S laden
with hydrocarbon species. Thus, the addition of CH3 to
SO2 leads to a chemically activated adduct that results
in the formation of CH3O and SO:
The NIST Database shows a rate coefficient of 1.1 ×
10-13 exp[(-1.50 kcal/mol)/(RT)] (in units of cm3 mole-
cule-1 s-1) for the formation of the stabilized adduct
CH3SO2 at 298 K. Other data show a rate coefficient of
2.92 × 10-13 cm3 molecule-1 s-1 for the formation of
products at 298 K. The nature of the products is not
reported.
This is similar reaction to the addition of CH3 to O2
(for the validity of this type of approach to the develop-
ment of kinetic models, see ref 34, with a low energy
barrier of 8.9 kcal/mol):
In this reaction, a chemically activated adduct is
produced in which a bridge can be formed between the
last O atom, and a hydrogen attached to carbon, which
results in the elimination of OH. The net reaction
has an activation energy of 8.94 kcal/mol.17 For the
theoretical treatment of O2 addition reactions, see, for
example, the work of Sheng et al.;35 that paper in-
volves the more-complex addition of C2H5 to oxygen,
leading to a multiplicity of products. The formation of
CH3 and SO is followed by the formation of formalde-
hyde:
Formaldehyde then reacts with sulfur species, leading
to COS:
The comparable reaction CH2O + O ) CHO + OH has
an activation energy of 2.772 kcal/mol.17
This reaction involves a chemical adduct, which
leads to HS elimination and COS formation. The
comparable addition of CHO to O2 has a low activation
energy.
A very simple mechanism for the formation of CS2
involves C and S radical species:
This is not very likely, because Claus furnaces should
not be operated in a manner that leads to coke forma-
tion; it would lead to coking of the catalytic reactors.
The formation of CS2 seems to correlate well with the
presence of hydrocarbons in the Claus furnace.6 A
mechanism for the formation of CS2 is given by Clark
S3 + S3 + M T S6(c) + M
)
(167)
S3 + S4+ M T S7(c) + M
)
(168)
S3 + S5 + M T S8(c) + M
) (169)
S4 + S4 + M T S8(c) + M
)
(170)
M + CO + S T COS + M (-72.91 kcal/mol)
(171)
CH3 + SO2 T [CH3-O-S-O]* T CH3O + SO
(-15.5 kcal/mol) (172)
CH3 + O2 T CH2O + OH
(∆Hf ) -53.20 kcal/mol, Ea ) 8.9 kcal/mol)
(173)
CH3O + M T CH2O + H + M (20.50 kcal/mol)
(174)
CH2O + HS T CHO + H2S (-0.10 kcal/mol)
(175)
S2 + CHO T [S-SCHdO]* T CSO + HS
(-76.4 kcal/mol) (176)
C + S + M T CS + M (-170.54 kcal/mol) (177)
CS + S + M T CS2 + M (-105.29 kcal/mol)
(178)

et al.4 The mechanism is given below and it starts with
the reaction of methyl radicals with sulfur:
Their work does not provide rate coefficients for the
aforementioned reactions.
Petherbridge et al.36 presented a sequence of reactions
leading to the formation of CS2. Their work involved the
simulation of gas-phase reactions that were occurring
in a representative gas-phase environment used to grow
sulfur-doped diamond films via chemical vapor deposi-
tion (CVD) for use in electronic devices. For the C/H
system, the GRI-Mech 3.0 was used, and the chemistry
of the sulfur species was gathered from literature or
estimated. Some estimates of the rate coefficients
involving the sulfur species were also made; mixtures
of H2S/CH4/H2 and CS2/H2 were studied. The sequence
leading to CS2 used in their mechanism is as follows:
The heat of reaction is taken from Hynes and Wine,28
as is the case for the reactions below.
They experimentally determined the concentration of
species such as CH4, C2H2, CH3, H2S, CS2, and CS, using
molecular beam mass spectroscopy in microwave-
activated gas mixtures. Their model agrees fairly well
with the experimental data, even after experimental
errors in the species concentrations have been taken
into consideration.
A mechanism for the formation of CS2 that is proposed
here again uses a reaction similar to the formation of
formaldehyde from CH3, or, as above,
Thus, the first step in the formation of CS2 would be
In this reaction, a chemically activated adduct is also
produced, and, in this case, a transition state is formed,
in which a bridge is formed between the last S atom
and a hydrogen attached to carbon, leading to HS
elimination. The work of Petherbridge et al.36 would
indicate that the net reaction would have a very low
activation energy.
The reaction that follows is then
The estimate for the activation energy would be on the
order of 3.0 kcal/mol for the net reaction.36
Finally,
The magnitude of the activation energy can be
estimated from the work of Petherbridge et al.36 as 1
kcal/mol for the overall reaction.
This is a much more simple and elegant mechanism
that leads to CS2. It is very favorable thermodynami-
cally, and the order of magnitude of the estimated
activation energies is relatively low.
Another path for the formation of CS2 has been
outlined in the work of Cullis and Mulcahy18 and was
discussed previously; it involves acetylene, which is an
CH3 + S2 + M T CH3-S-S + M
) (179)
CH3-S-S + H2S T HS + CH3-S-SH
) (180)
CH3-S-SH + M T CH3S + HS + M
) (181)
CH3S + HS T H2S + CH2dS
) (182)
CH2dS + HS T H2S + CHdS
) (183)
CHdS + S2 + M T S-S-CHdS + M
(-18 kcal/mol; ESTIM) (184)
S-S-CHdS + H2S T SdCH-S-SH + HS
(7 kcal/mol; ESTIM) (185)
SdCH-S-SH + M T SdCH-S + HS
(54 kcal/mol; ESTIM) (186)
S)CH-S + HS T H2S + CS2
(-63 kcal/mol; ESTIM) (187)
SH + CH3 T CH3SH
(-74.44 kcal/mol; Ea ) 0 kcal/mol) (188)
SH + CH3 T H2 + CH2S
H + CH3SH T CH3 + H2S
(-16.70 kcal/mol; Ea ) 1.66 kcal/mol) (190)
H + CH3SH T CH3S + H2
H + CH3S T CH2S + H2
H + CH2S T HCS + H2
H + HCS T H2 + CS
SH + CS T H + CS2
CH3 + O2 T [CH3-O-O]* T CH2dO + OH
CH3 + S2 T [CH3-S-S]* T CH2dS + HS
) (197)
CH2dS + HS T CHdS + H2S
) (198)
CHdS + S2 T [S-S-CHdS]* T CS2 + HS
)
(199)

intermediate that is formed during the fuel-rich com-
bustion of methane:
This is also a much simpler path to carbon disulfide.
D.6. Destruction of COS and CS2 by Water or
Hydrogen. The overall reactions that seem to explain
the destruction of COS or CS2 best are as follows:
The hydrogenation of COS should follow the following
chemical paths involving an activated adduct:
The proposed mechanism for the destruction of COS
by water is as follows:
A chemically activated adduct is formed. The sulfur and
hydrogen (attached to oxygen in the proximity) form a
bridge that leads to the elimination of HS. The forma-
tion of CO2 as a major product is in agreement with the
experiments of Clark et al.11 The aforementioned hy-
drogenation of COS can then be followed by the well-
known reaction
The hydrolysis of CS2 involves the following reaction:
Again, a chemically activated adduct is formed, which
can lead to HS elimination and formation of COS. The
COS that is formed decomposes via the previously
described mechanisms (hydrolysis and hydrogenation).
The formation of COS as an intermediate during the
reaction of CS2 with water is in agreement with the
work of Clark et al.11
D.7. Reactions of CS2 and COS with SO2. Both
CS2 and COS have been observed to undergo reactions
in the presence of SO2 in tubular reactors4,11 to form
sulfur, CO2, and CO. The mechanism can involve the
thermal decomposition of the SO2 reactant, which would
provide O radicals:
This would be followed by the oxidation of COS and CS2
by O radicals, involving chemical activation:
For CS2, the chemical paths are
This is followed by the oxidation of the COS, as
previously discussed.
D.8. The Oxidation of Methane. Issues relevant to
the oxidation of hydrocarbons at high temperature have
been well-discussed by Warnatz.26 The overall scheme
for the combustion of methane is well-known, and it
involves, depending on the conditions, species such as
CH3, CH2O, CHO, CO, CO2, C2H6, C2H4, C2H2, CH2, CH,
C3H3, and C6H6. The two-carbon molecules become more
important under fuel-rich conditions. In this respect, the
work of Westbrook and Dryer37 is also very revealing:
methane oxidation is a hierarchical process that consists
of CO, H2, and C2 sub-mechanisms. It is not the purpose
of this study to dwell on these issues, except to refer
the reader to the proper sources for further study. Our
main intention here is to show the main chemical paths
for methane oxidation in the presence of sulfur species.
Simmie38 provided an excellent review of the recent
developments in the kinetic modeling of hydrocarbons
(methane, as well as heavier molecules). He discussed
several methane oxidation mechanisms: GRI-Mech 3.0
(325 reactions, 53 species, 1000-2500 K), the University
of Leeds, UK Mechanism (351 reactions, 37 species), and
the mechanism that has been attributed to Alexander
Konnov (1200 reactions, 127 species) (found at
http://homepages/vub.ac.be/∼akonnov/), among others.
The discussion here will rely heavily on the extensive
work by Wang19 and Gargurevich,17 because these are
comprehensive discussions on the issues of methane
combustion, rather than reviews or summaries. The
features described here have much in common with the
work of the previously mentioned authors. In the
absence of hydrogen sulfide, methane combustion, under
fuel-rich conditions, is initiated by
However, under the conditions prevalent in the Claus
process, S and HS radicals are also abundant and the
following reactions would be expected:
The destruction of CH3 can then proceed as follows:
S + C2H2 T HCS + CH (92 kcal/mol) (200)
HCS + M T H + CS + M (51 kcal/mol) (201)
COS + H2O T H2S + CO2 (-8.08 kcal/mol)
(202)
COS + H2 T CO + H2S (1.76 kcal/mol) (203)
CS2 + 2H2O T CO2 + 2H2S (-16.21 kcal/mol)
(204)
COS + H T [H-S-CdO]* T CO + HS
(-11.91 kcal/mol) (205)
COS + OH T [S-C(OH)dO]* T CO2 + HS
(-37 kcal/mol) (206)
CO + OH T CO2 + H (-24.85 kcal/mol) (207)
CS2 + OH T [S-C(OH)dS]* T COS + HS
(-37 kcal/mol) (208)
SO2 + M T SO + O + M (131.75 kcal/mol) (209)
SO + M T S + O + M (124.29 kcal/mol) (210)
COS + O T [S-C(O)dO]* T CO2 + S
(-56.5 kcal/mol) (211)
COS + O T [O-S-CdO]* T CO + SO
(-52.9 kcal/mol) (212)
CS2 + O T [S-C(O)dS]* T COS + S
(-54.3 kcal/mol) (213)
CH4 + H T CH3 + H2 (0.61 kcal/mol) (214)
CH4 + OH T CH3 + H2O (-14.40 kcal/mol)
(215)
CH4 + S T CH3 + HS (19.78 kcal/mol) (216)
CH4 + HS T CH3 + H2S (14.51 kcal/mol) (217)
CH3 + HO2 T CH3O + OH (-77.67 kcal/mol)
(218)

Formaldehyde is also formed according to
followed by
The following reaction can also be important if the
amount of hydrocarbons is high in the initial mixture.
CHO radicals decompose according to
Also important is
Carbon dioxide forms via the well-known reaction
D.9. Reactions of CO2 with H2S and S2. Monnery
et al.6 noted that plant data seems to indicate the
formation of CO via reactions of the type
The reactions involve the CO2 constituent in the Claus
feed gas only. The major products of reaction are H2,
CO, COS, and S2.11 The second reaction is highly
endothermic, in comparison to the first reaction, and is
not very likely. The mechanism for these reactions
should involve S and HS radicals that have been
produced from the decomposition of H2S, or
They are both chemically activated reactions that lead
to CO formation. The formation of COS follows:
Sulfur could then be formed by the reaction
This same mechanism would explain the formation of
COS and SO2 in the reaction of CO2 with S2.11 SO2
would form according to
D.10. Ethane Oxidation. The chemical reactions for
the combustion of ethane under fuel-rich conditions
have been discussed fully by Wang19 and Gargurevich.17
The presence of radicals such as S and HS introduce
additional paths for the decomposition.
The initial decomposition occurs according to
The ethyl radical continues to react, according to the
following reactions:
Ethylene (C2H4) continues to react:
Then,
The equilibrium calculations did not result in any
significant amounts of C2H2. This component reacts to
form CO:
D.11. Mercaptans. The equilibrium calculations did
not result in any significant amounts of mercaptans.
Perhaps this is due to the lower bond energy of the C-S
bond. Methyl and ethyl mercaptan react as follows:
CH3 + O2 T CH2O + OH (-53.20 kcal/mol)
(219)
CH3O + M T CH2O + H + M (20.50 kcal/mol)
(220)
CH2O + H, S, HS T CHO + H2, HS, H2S
(14.00 kcal/mol, 4.60, -0.10) (221)
CH2O + OH T CHO + H2O (-29.01 kcal/mol)
(222)
CH2O + CH3 T CHO + CH4 (-14.61 kcal/mol)
(223)
CHO + M T H + CO + M (15.29 kcal/mol)
(224)
CHO + O2 T HO2 + CO (-36.31 kcal/mol) (225)
CO + OH T CO2 + H (-24.85 kcal/mol) (226)
CO2 + H2S T CO + H2O +
1
2
S2 (-0.61 kcal/mol)
(227)
2CO2 + H2S T 2CO + H2 + SO2 (69.23 kcal/mol)
(228)
CO2 + S T SO + CO (2.90 kcal/mol) (229)
HS + CO2 T HSO + CO (7.69 kcal/mol) (230)
CO + S + M T COS + M (-72.91 kcal/mol)
(231)
SO + S T S2 + O (22.5 kcal/mol) (232)
SO + O + M T SO2 + M (-131.75 kcal/mol)
(233)
C2H6 + H, OH, S, HS T C2H5 + H2, H2O, HS, H2S
(-52.07, -67.08, -32.91, -38.17 kcal/mol)
(234)
C2H5 + O2 T C2H4 + HO2 (33.26 kcal/mol) (235)
C2H5 + H, S, HS T C2H4 + H2, HS, H2S
(-19.34, -0.18, 5.44 kcal/mol) (236)
C2H4 + H, S, HS T C2H3 + H2, HS, H2S
(-52.92, -33.77, -39.02 kcal/mol) (237)
C2H4 + OH T C2H3 + H2O (-67.93 kcal/mol)
(238)
C2H3 + O2 T CH2O + CHO (-29.02 kcal/mol)
(239)
C2H3 + H T C2H2 + H2 (-9.63 kcal/mol) (240)
C2H3 + S, HS T C2H2 + HS, H2S
(9.52, 4.27 kcal/mol) (241)
C2H2 + O T CH2 + CO (-47.81 kcal/mol) (242)
CH3SH + H T CH3 + H2S (-16.72 kcal/mol)
(243)
C2H5SH + H T C2H5 + H2S (-41.52 kcal/mol)
(244)
CH3SH + HS T CH3 + H2S2 (7.01 kcal/mol)
(245)
C2H5SH + HS T C2H5 + H2S2 (-17.80 kcal/mol)
(246)
CH3SH + S T CH3 + HS2 (-3.75 kcal/mol)
(247)
C2H5SH + S T C2H5 + HS2 (-28.60 kcal/mol)
(248)

Table 7. Simple Combustion Mechanism for H2S and SO/SO2/SO3/S2 Formationa
From Kennedy et al.12 From Zachariah and Smith13
reaction A n E (cal/mol) A n E (cal/mol)
H + O2 T OH + O 3.52 × 1016 -0.7 17070 1.20 × 1017 -0.91 16422
O + H2 T OH + H 5.06 × 104 2.67 6290 1.50 × 107 2 7542
2OH T O + H2O 6.00 × 108 1.3 0 1.50 × 109 1.14 0
OH + H2 T H2O + H 1.17 × 109 1.3 3626 1.00 × 108 1.6 3295
H + O2 + M T HO2 + M 6.76 × 1019 -1.42 0 2.00 × 1018 -0.8 0
H2O/12./H2/2.5/
H + HO2 T 2OH 1.70 × 1014 0 874 1.50 × 1014 0 1003
H + HO2 T H2 + O2 4.28 × 1013 0 1411 2.50 × 1013 0 692
OH + HO2 T H2O + O2 2.89 × 1013 0 -497 2.00 × 1013 0 0
H + H + M T H2 + M 1.80 × 1018 -1 0 2.00 × 1018 -1 0
H2O/6.5/O2/0.4/H2/1/N2/0.4/
H + OH + M T H2O + M 2.20 × 1022 -2 0 2.20 × 1021 -2 0
H2O/12./H2/2.5/
HO2 + HO2 T H2O2 + O2 3.02 × 1012 0 1390
H2O2 + M T OH + OH + M 1.20 × 1017 0 45500
H2O/15./H2/2.5/
H2O2 + OH T H2O + HO2 7.08 × 1012 0 1430
O + HO2 T O2 + OH 2.00 × 1013 0 0 2.00 × 1013 0 0
H + HO2 T O + H2O 3.10 × 1013 0 1720
H + O + M T OH + M 6.20 × 1016 -0.6 0
H2O/12./H2/2.5/
O + O + M T O2 + M 6.17 × 1015 -0.5 0
H2O/12./H2/2.5/
H2O2 + H T H2O + OH 1.00 × 1013 0 3590
H2O2 + H T HO2 + H2 4.79 × 1013 0 7950
O + OH + M T HO2 + M 1.00 × 1016 0 0
H2 + O2 T 2OH 1.70 × 1013 0 47780
S and SOx Section
S + O2 T SO + O 2.00 × 106 1.93 -1400 6.30 × 1011 0.5 0
O + S2 T SO + S 3.98 × 1012 0 0 6.30 × 101 0.5 0
SO + O2 T SO2 + O 6.20 × 103 2.42 3050 1.80 × 1011 0 0
SO + SO T SO2 + S 2.00 × 1012 0 4000 3.30 × 1011 0 2250
SO + O + M T SO2 + M 1.10 × 1022 -1.84 0 1.20 × 1022 -1.8 0
SO2 + O + M T SO3 + M 4.00 × 1028 -4 5250
SO + O2 + M T SO3 + M 1.00 × 1015 0 0
SO3 + O T SO2 + O2 1.30 × 1012 0 6100
SO3 + SO T SO2 + SO2 1.00 × 1012 0 4000
SO + S + M T SO2 + M 1.20 × 1022 -1.8 0
2S + M T S2 + M 1.00 × 1018 -1 0
S2 + H T S2O + H 1.80 × 1013 0 0
S and H Section
H2S + M T S + H2 + M 2.00 × 1014 0 66000
H + H2S T H2 + SH 1.20 × 107 2.1 700 1.20 × 1013 0 1710
SH + SH T H2S + S 1.00 × 1014 0 1430
H2S + S T SH + SH 4.00 × 1014 0 15100
H2 + S T SH + H 6.00 × 1014 0 24000 2.00 × 1014 0 76600
HS + H T H2 + S 5.16 × 1014 0 21000
SH + S T H + S2 2.69 × 1013 0 0 1.40 × 1013 0 478
S, H, and O Section
H2S + O T OH + SH 6.40 × 107 1.78 2840 4.36 × 1012 0 322
H2S + OH T H2O + SH 2.70 × 1012 0 0 1.40 × 1013 0 886
SH + H2O2 T HO2 + H2S 1.00 × 1011 0 0
SH + O2 T SO + OH 1.10 × 1011 0 5400
SH + O2 T SO2 + H 2.00 × 1011 0 5400
SH + HO2 T H2O2 + S 1.00 × 1011 0 0
SH + HO2 T H2S + O2 6.00 × 1012 0 0
SH + O T OH + S 2.29 × 1011 0.67 1920 6.31 × 101 0.5 8060
SH + O T SO + H 1.00 × 1014 0 0 3.56 × 1014 0 642
SH + OH T H2O + S 1.00 × 1013 0 0
OH + SO T SO2 + H 5.20 × 1013 0 0 1.80 × 1013 0 0
OH + S T SO + H 4.00 × 1013 0 0 7.20 × 1013 0 642
HO2 + S T SH + O2 1.00 × 1012 0 0
SO3 + H T SO2 + OH 1.00 × 1012 0 0
HSO2, HSO Reactions
H + SO2 + M T HSO2 + M 7.30 × 1016 0 0
H + HSO2 T SO2 + H2 1.60 × 1012 0 0
OH + SO + M T HSO2 + M 7.00 × 1016 0 0
OH + HSO2 T SO2 + H2O 6.80 × 1013 0 1772
OH + HSO2 T SO2 + H2O
H + SO + M T HSO + M 1.00 × 1015 0 0 1.60 × 1020 -1.5 0
HSO + H T H2 + SO 5.00 × 1011 0.5 2222
HSO + H T H2S + O 5.00 × 1010 0.5 4520
HSO + H T SH + OH 5.00 × 1011 0.5 4520
HSO + OH T H2O + SO 5.00 × 1013 0 1006

D.12. The Effect of SO2 on Radical Chemistry.
Zachariah and Smith,13 as well as Tseregounis and
Smith,39 have noted the effect of small amounts of SO2
on the radical chemistry, i.e., reactions that involve H,
O, and OH in H2/O2/Ar flames under fuel-rich condi-
tions. This study has been selected because of the
existence of experimental data, including measurements
of the H, OH, O radicals. They have noted that SO2,
and HSO2 (see Table 7), lead to radical recombination
reactions of the type
for H, Y ) H, OH. For example,
Similarly, for the O radical (from Smith et al.40),
Tseregounis and Smith determined, for example, that
the addition of SO2 leads to a substantial depletion of
atomic hydrogen. As the previously discussed reaction
between H and HSO2 shows, the presence of SO2
catalyzes the formation of H2 from H atoms. Zachariah
and Smith13 also have noted the role of HSO2 as a
channel for radical recombination. Smith et al.40 have
highlighted the role of SO2 as a catalyst for radical
recombinaton in CO/O2/Ar fuel-lean flames, as shown
by the last set of reactions previously presented involv-
ing atomic oxygen radicals.They noted the importance
of HSO2 to be able to better model flames that have been
doped with SO2.
This could be an important consideration during the
combustion of H2S in the Claus process. Clark et al.4
noted in their study of Claus chemistry that H2S
combusts more quickly than the hydrocarbons that were
present in the initial gas mixture. The aforementioned
discussion would seem to indicate that this could be the
result of the formation of SO2 in large quantities in the
Claus process and the resultant H and OH radical
recombination, which would then slow the hydrocarbon
decomposition. This should be an area of further study.
The equilibrium calculations in this study did not
result in any significant quantities of HSO2. This is in
agreement with the study of Smith et al.40 on CO/O2/
Ar flames under fuel-lean conditions. However, as
mentioned earlier, H2SO2 molecules seem to form at the
higher temperatures or above 2000 °F. This could be
the end product of HSO2 formation in the combustion
process.
The work of Alzueta et al.29 on the effects of SO2 on
the radical pool has been presented earlier. This is a
study conducted at a later time than the Zachariah and
Smith experiments and somehow is in contradiction
with what is presented here. The arguments presented
by Alzueta et al. seem to indicate that the effect of SO2
on the radical pool, as well as the mechanism for radical
recombination in flames, require further study. The
problem seems to involve a lack of better kinetic and
thermodynamic data.
D.13. Reaction Rate Coefficients. This work will
not address issues relating to methods for the construc-
tion of any detailed chemical kinetic model composed
of many reactions, as shown in Table 5, and the
estimation of chemical kinetic coefficients. Several
sources can be found that address the subject, including
the work of Wang19 and Gargurevich.17 These are also
good sources for the treatment of chemically activated
reactions, some of which have been presented in this
manuscript
The treatment of chemically activated reactions can
also be found in the work of Westmoreland et al.,25
Deanet al.,41 and Kazakov et al.42
It is important to note here the conclusion reached
by Hynes and Wine28 in their review of thermochemical
and kinetic data of sulfur reactions: “The paucity of
high-temperature kinetic data on elementary reactions
of sulfur is a substantial roadblock to understanding
sulfur combustion. More high-temperature studies are
needed for almost all of the reactions of sulfur species.”
“Modeling studies of sulfur chemistry under combustion
conditions have been handicapped by the lack of a
regularly upgraded, evaluated high-temperature data-
base similar to the NASA or CODATA compilations
used for atmospheric modeling”.
This consideration is most important for temperature-
and pressure-dependent reactions (such as unimolecular
reactions) and chemically activated reactions, which
have a fundamental role in combustion. The dependence
of the rate coefficient on reaction conditions must be
taken into account.
The author has not found a comprehensive study of
H2S combustion in the fuel-rich flames and experimen-
tal studies of the reactions presented in this manuscript.
Table 7 illustrates the kinetic models of Kennedy12 and
Zachariah and Smith.13 These models only examine the
formation of simple sulfur species (such as SO, SO2, SO3,
and S2) and are fairly similar. They are a good starting
point for kinetic data. Kennedy’s work12 is representa-
tive of a high-temperature model under reducing condi-
tions. The work of Zachariah and Smith,13 as noted
previously, provide both a model and the experimental
Table 7 (Continued)
From Kennedy et al.12 From Zachariah and Smith13
reaction A n E (cal/mol) A n E (cal/mol)
HSO2, HSO Reactions
HSO + O T SO + OH 5.00 × 101 0.5 2222
HSO + O T H + SO2 1.00 × 1014 0 26200
HSO + O T HS + O2 1.00 × 1012 0 0 5.00 × 1011 0.5 4520
HSO + O2 T SO + HO2 5.00 × 1011 0.5 2222
SH + HSO T H2S + SO 1.00 × 1012 0 0
a Values determined using the rate equation in Arrhenius form: k ) ATn exp[-E/(RT)]. Units involved include cm, mol, and s.
X + SO2 + M T XSO2 + M (249)
XSO2 + Y T XY + SO2 (250)
H + SO2 + M T HSO2 + M (251)
H + HSO2 T H2 + SO2 (252)
OH + HSO2 T H2O + SO2 (253)
O + SO2 + M T SO3 + M (254)
SO3 + O T SO2 + O2 (255)

data to support it, but with reservations on the ther-
modynamic and kinetic data, which would need to be
updated.
The work of Radi et al.43 uses a novel spectroscopic
technique (multiplex spectroscopy) and it is applied to
measure S2 and OH concentration profiles in H2/Air/
SO2 flames obtained stabilized on a flat flame burner
at atmospheric pressure. The sulfur chemistry used in
their mechanism is taken from Zachariah and Smith13
for low-pressure H2/O2/SO2 flames. The experimental
results for the OH and S2 concentration profiles are in
good qualitative agreement with the mechanism results.
Discrepancies in the S2 concentration profile are at-
tributed to the fact that the mechanism of Zachariah
and Smith13 was developed for low-pressure flames. To
improve their mechanism, corrections must be made to
pressure-dependent reactions for the higher experimen-
tal pressure in their flame experiments.
Table 5 shows, for each reaction, the most recent
sources of chemical kinetic rate coefficient data. A good
source is also the NIST Database (which can be found
via the Internet at www.nist.gov). The sulfur mecha-
nism by the University of Leeds, U.K. presents kinetic
data that are based on the work of Alzueta et al.,29
including the most recent data for the HxSOy system of
reactions; it can be found at http://garfield.chem.elte.hu/
Combustion/Combustion.html. The compilation of Hynes
and Wine28 consists of 48 elementary reactions that
involve sulfur-containing species and it is also an
updated resource. For the combustion of simple hydro-
carbons, the extensive compilations by Baulch and co-
workers,44,45 Wang,19 and Gargurevich17 are good re-
sources.
Experimental rate coefficients (at high temperatures)
for reactions leading to sulfur S8(c), and the novel
reactions leading to COS and CS2, as presented in this
study, have not been found by the author.
Because experimental data is lacking, further studies
will be necessary to improve the rate coefficients and
thermodynamics of the reactions in Table 5 by the
methods described by Gargurevich17 and Senkan.34
Computational quantum chemistry can also be used for
the estimation of activation energies and the heats of
formation of the molecular species.
Experimental studies would also need to be pursued,
to validate the model depicted in Table 5, after the
reaction rate coefficients have been estimated.
Conclusions
The main objective of this study has been to present
the relevant chemical reactions that occur in the com-
bustion of hydrogen sulfide under Claus furnace (i.e.,
fuel-rich) conditions. As a result of a survey of literature,
fundamental chemical laws, and radical reactions fun-
damental to combustion phenomena, a chemical reac-
tion mechanism consisting of over 150 elementary
reactions is presented.
The mechanism is able to explain the high-tempera-
ture oxidation of H2S. The formation of hydrogen, which
is an important consideration in Claus plant design, can
be explained by the mechanism. Hydrogen generation
impacts the design of the Claus plant tail gas treating
units. Most importantly, novel chemical paths for the
formation of COS and CS2 are presented, based on
fundamental chemical laws. The heats of reaction and
activation energies for the reactions are estimated and
seem to indicate that the chemical paths presented could
have an important role. These are trace species with
important environmental impact, which must be con-
sidered in the design of sulfur treatment plants in
modern refineries.
Existing process simulators such as TSWEET and
SULFSIM rely on empirical correlations or an approach
to equilibrium to determine the amounts of H2, COS,
and CS2 produced in the Claus plant as the gas is cooled
to the sulfur dew point. Unfortunately, many of these
empirical correlations cannot represent all of the condi-
tions that may occur in the design of Claus plants.
The study introduces a plausible explanation for the
observed reduced rate of hydrocarbon oxidation in the
Claus furnace, which is due to radical recombination
catalyzed by SO2. This should be an important consid-
eration in the design of the combustion furnace of Claus
plants to minimize carbon deposition in the catalytic
reactors downstream of the furnace.
The molecular growth to S8, starting with S2, is also
included in the mechanism. Evidence is provided for the
ring structure of S5, S6, and S7; in contrast, S4 is a linear
molecule. Intramolecular reactions involving S8 and
leading to S7 and S6 are introduced. Heats of reaction
and activation energy estimates for the cyclization
reactions of Sx species are estimated. The energy bar-
riers for cyclization are favorable at the high tempera-
tures that are typical of the Claus process.
Reactions of COS and CS2 with SO2 are investigated
prompted by observations in flow reactor experiments.
For similar reasons, reactions of CO2 with H2S and S2
are considered.
The study highlights the consensus, in regard to the
lack of high-temperature kinetic data, as well as studies
of fuel-rich H2S flames. These are needed for a valida-
tion of the elementary reaction chemistry presented in
the manuscript. Similarly, experimental studies at high
temperature are needed to better elucidate the phe-
nomena of radical recombination by SO2.
Literature Cited
(1) Bowman, C. T. Kinetics of Pollutant Formation and De-
struction in Combustion. Prog. Energy Combust. Sci. 1975, 1, 33-
45.
(2) Johnsson, J. E.; Glarborg, P. Sulphur Chemistry in Com-
bustion I. Sulphur in Fuels and Combustion Chemistry. In
Pollutants from Combustion: Formation and Impact on Atmo-
spheric Chemistry; Vovelle, C., Ed.; Kluwer Academic Publishers:
Dordrecht, The Netherlands, 2000; p 263.
(3) Chernysheva, A. V. et al. Mechanism of Gas-Phase Oxida-
tion of Hydrogen Sulfide at High Temperatures; N. N. Semenov
Institute for Chemical Physics, Academy of Sciences of the
USSR: Moscow. (Translated from Izv. Akad. Nauk SSSR, Ser.
Khim. 1990, (9), 1956-1966.)
(4) Clark, P. D.; Dowling, N. I.; Huang, M. Chemistry of the
Claus Front-End Reaction Furnace: Hydrocarbon Reactions and
the Formation and Destruction of CS2. In ASR Quarterly Bulletin,
Vol. XXXIII, No. 4, Jan.-Mar. 1997; Alberta Sulphur Research:
Calgary, Alberta, Canada, 1997; pp 1-49.
(5) Hyne, J. B. Optimum Furnace Configuration for Sulphur
Recovery Units. Sulphur 1988, 198 (September-October), 24-
39.
(6) Monnery, W. D.; Svreck, W. Y.; Behie, L. A. Modelling the
Modified Claus Process Reaction Furnace and the Implications on
Plant Design and Recovery. Can. J. Chem. Eng. 1993, 71, 711.
(7) Slimane, R. B.; Lau, F. S.; Abbasian, J. Hydrogen Production
by Superadiabatic Combustion of Hydrogen Sulfide. In Proceedings
of the 2000 Hydrogen Program Review, Paper No. NREL/CP-570-
28890.
(8) Slimane, R. B.; Lau, F. S.; Khinkis, M.; Bingue, J. P.;
Saveliev, A. V.; Kennedy, L. A. Conversion of Hydrogen Sulfide

to Hydrogen by Superadiabatic Partial Oxidation: Thermo-
dynamic Consideration. Int. J. Hydrogen Energy 2004, 29, 1471-
1477.
(9) Chute, A. E. Tailor Sulfur Plants to Unusual Conditions.
Hydrocarbon Process. 1977, (April), 119.
(10) Meisen, A.; Bennett, H. A. Consider All Claus Reactions.
Hydrocarbon Process. 1974, (November), 171.
(11) Clark, P. D.; Dowling, N. I.; Huang, M.; Svreck, W. Y.;
Monnery, W. D. Mechanisms of CO and COS Formation in the
Claus Furnace. Ind. Eng. Chem. Res. 2001, 40, 497-508.
(12) Kennedy, L. A. Private communication regarding the
paper, Chemical Mechanism of Partial Oxidation of Hydrogen
Sulfide. (This mechanism was part of the work of Slimane (ref 7,
above) with the Superadiabatic Combustion of H2S, College of
Engineering, University of Illinois at Chicago, Chicago, IL, Janu-
ary 2002.)
(13) Zachariah, M. R.; Smith, O. I. Experimental Studies of
Sulfur Chemistry in H2/O2/SO2 Flames. Combust. Flame 1987, 69,
125-139.
(14) Bakker, A.; Haidari, A. H.; Marshall, E. M. Design Reac-
tors via CFD. Chem. Eng. Prog. 2001, (December), 30-39.
(15) Cusack, R. W. Reaction Engineering: Optimize Design and
Operation. Chem. Eng. 2000, (February 01).
(16) Eaton, A. M. Components Formulations, Solutions, Evalu-
ation, and Application of Comprehensive Combustion Models.
Prog. Energy Combust. Sci. 1999, 25, 387-426.
(17) Gargurevich, I. A. Combustion Models and the Prediction
of Policyclyc Aromatic Hydrocarbons (PAH) in Laminar Flames
of Simple Hydrocarbons: Methane, Methyl Chloride, and 1,1-
Dichloroethane, Ph.D. Dissertation, UCLA Chemical Engineering
Department, University of California-Los Angeles (UCLA), Los
Angeles, CA, 1997.
(18) Cullis, C. F.; Mulcahy, M. F. R. The Kinetics of Combustion
of Gaseous Sulphur Compunds. Combust. Flame 1972, 18, 225-
292.
(19) Wang, H. Detailed Chemical Kinetic Modeling of Soot
Particle Formation in Laminar Premixed Hydrocarbon Flames,
Ph.D. Thesis, Pennsylvania State University, University Park, PA,
August 1992.
(20) Cunningham, T. P.; Cooper, D. L.; Gerratt, J.; Karadakov,
P. B.; Raimondi, M. Chemical Bonding in Oxofluorides of Hyper-
coordinate Sulfur. J. Chem. Soc., Faraday Trans. 1997, 93, 2247.
(21) Raghavachari, K.; Rohlfing, C. M.; Binkley, J. S. Structures
and Stabilities of Sulfur Clusters. J. Chem. Phys. 1990, 93 (8),
5862-5874.
(22) Xie, Y. et al. Sulfur Clusters: Infrared and Raman Spectra
of Cyclo-S6 and Comparison with the Hypotherical Cyclo-O6
Molecule. Mol. Phys. 1992, 76 (3), 537.
(23) Pryor, W. A. Free Radicals; McGraw-Hill Series in
Advanced Chemistry; McGraw-Hill: New York, 1966.
(24) Guillemois, A.; Rossatti, F. Optimize Sulfur Recovery
Operations with Advanced Controls. Hydrocarbon Process. 2000,
79 (7).
(25) Westmoreland, P. R.; Howard, J. B.; Longwell, J. P. AIChE
J. 1986, 32, 1971.
(26) Warnatz, J. Hydrocarbon Oxidation High-Temperature
Chemistry. Pure Appl. Chem. 2000, 72 (11), 2101-2110.
(27) Glarborg, P.; Chiang, H.; Bozzelli, J. W. Impact of SO2 and
NO on CO Oxidation under Post-Flame Conditions. Int. J. Chem.
Kinet. 1996, 28, 773-790.
(28) Hynes, A. J.; Wine, P. H. Kinetics and Mechanisms of the
Oxidation of Gaseous Sulfur Compounds. In Gas-Phase Combus-
tion Chemistry; Gardiner, W. C., Ed.; Springer-Verlag: New York,
2000; pp 343-383.
(29) Alzueta, M. U.; Bilbao, R.; Glarborg, P. Inhibition and
Sensitization of Fuel Oxidation by SO2. Combust. Flame 2001, 127,
2234-2251.
(30) Frenklach, M.; Lee, J. H.; White, J. N.; Gardiner, W. C.,
Jr. Oxidation of Hydrogen Sulfide. Combust. Flame 1981, 41,
1-16.
(31) Chernysheva, A. V. et al. Mechanism of Gas-Phase Oxida-
tion of Carbon Disulfide at Elevated Temperatures. N. N. Semenov
Institute for Chemical Physics, Russian Academy of Sciences,
Moscow 117977. (Translated from Izv. Akad. Nauk, Ser. Khim.
1992, (4), 812-822.)
(32) Sendt, K.; Jazbec, M.; Haynes, B. S. Chemical Kinetic
Modeling of the H/S System: H2S Thermolysis and H2 Sulfidation.
Proc. Combust. Inst. 2002, 29, 2439-2446.
(33) Huang, J.-H.; Han, K.-L.; Deng, W.-Q.; He, G.-Z. Theoreti-
cal Studies of the Decyclization/Cyclization of the C6H5 Radical.
Chem. Phys. Lett. 1997, 273, 205-210.
(34) Senkan, S. M. Detailed Chemical Kinetic Modeling: Chemi-
cal Reaction Engineering of the Future. Adv. Chem. Eng. 1992,
18, 95-15.
(35) Sheng, C. Y.; Bozzelli, J. W.; Dean, A. M.; Chang, A. Y.
Detailed Kinetics and Thermochemistry of C2H5 + O2: Reaction
Kinetics of the Chemically-Activated and Stabilized CH3CH2OO•
Adduct. J. Phys. Chem. A 2002, 106, 7276-7293.
(36) Petherbridge, J. R.; May, P. W.; Shallcross, D. E.; Harvey,
J. N.; Fuge, G. M.; Rosser, K. N.; Ashfold, M. N. R. Simulation of
H-C-S Containing Gas Mixtures Relevant to Diamond Chemical
Vapour Deposition. Diamond Relat. Mater. 2003, 12, 2178-2185.
(37) Westbrook, C. K.; Dryer, F. L. Chemical Kinetic Modelling
of Hydrocarbon Combustion. Prog. Energy Combust. Sci. 1984, 10,
1-57.
(38) Simmie, J. M. Detailed Chemical Kinetic Models for the
Combustion of Hydrocarbon Fuels. Prog. Energy Combust. Sci.
2003, 29, 599-634.
(39) Tseregounis, S. I.; Smith, O. I. An Experimental Investiga-
tion of Fuel Sulfur-Fuel Nitrogen Interactions in Low-Pressure
Premixed Flames. In Proceedings of the Twentieth Symposium
(International) on Combustion; The Combustion Institute: Pitts-
burgh, PA, 1984; pp 761-768.
(40) Smith, O. I.; Wang, S.; Tseregounis, S. The Sulfur Cata-
lyzed Recombination of Atomic Oxygen in a CO/O2/Ar Flame.
Combust. Sci. Technol. 1983, 30, 241-271.
(41) Dean, A. M.; Bozzelli, J. W.; Ritter, E. R. CHEMCAT: A
Computer Code to Estimate Rate Gas Constants for Chemically
Activated Reactions. Combust. Sci. Technol. 1991, 80 (1-3), 63-
85.
(42) Kazakov, A.; Wang, H.; Frenklach, M. J. Phys. Chem. 1994,
98, 10598.
(43) Radi, P. P.; Mischler, B.; Schlegel, A.; Tzannis, A.-P.;
Beaud, P.; Gerber, T. Absolute Concentration Measurements Using
DFWM and Modeling of OH and S2 in a Fuel-Rich H2/Air/SO2
Flame. Combust. Flame 1999, 118, 301-307.
(44) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.; Frank,
P.; Just, Th.; Kerr, J. A.; Pilling, M. J.; Troe, J.; Walker, R. W.;
Warnatz, J. Evaluated Kinetic Data for Combustion Modelling.
J. Phys. Chem. Ref. Data 1992, 21, 411-734.
(45) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Frank, P.; Hayman,
G.; Just, Th.; Kerr, J. A.; Murrells, T.; Pilling, M. J.; Troe, J.;
Walker, R. W.; Warnatz, J. Evaluated Kinetic Data for Combustion
Modelling (Supplement 1). J. Phys. Chem. Ref. Data 1994, 23,
847-1033.
(46) Schofield, K. The Kinetic Nature of Sulfur’s Chemistry in
Flames. Combust. Flame 2001, 124, 137-155.
Received for review August 4, 2004
Revised manuscript received June 25, 2005
Accepted July 25, 2005
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