Hydrogen Sulfide Combustion Chemistry

APPLIED CHEMISTRY

Hydrogen Sulfide Combustion: Relevant Issues under ClausFurnace Conditions

Ivan A. Gargurevich

Combustion & Process Technologies, San Diego, California 92122

The major chemical paths for the combustion of hydrogen sulfide under conditions typical ofthe Claus furnace (i.e., fuel-rich conditions) are presented. The manuscript begins with a briefsurvey of recently published research that involves sulfur chemistry in high-temperatureenvironments, 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 reactionmechanism that is presented consists of more than 150 reactions. Issues such as the formationand destruction of COS and CS2 are presented: new chemical paths for the formation of COSand CS2 (not involving elementary carbon) are illustrated, on the basis of sound thermochemicaland kinetic considerations. The formation of COS and CS2 is of great importance in the designof sulfur plants in industry. Possible reactions of COS and CS2 with SO2, and CO2 with H2S andsulfur species, also are discussed, prompted by experimental observations in flow reactors. Themechanism can explain the formation of hydrogen, which also is an important issue in sulfurplant design and associated tail gas units. Species such as H2S2 seem to have an important roleduring the combustion of hydrogen sulfide. Higher-molecular-weight linear H2Sx species arealso considered, and it is concluded that their role is possibly minor. The chemical steps leadingto the formation of Sx species by molecular growth are presented. The ring structure of some ofthe Sx species is discussed, as well as intramolecular ring conversions for S8, S7, S6, and S5. Thepossibility of H,OH radical recombination catalyzed by oxygenated sulfur species may explainthe delayed oxidation of hydrocarbon species in the Claus furnace that has been observed inprevious experiments by other authors. This could be an important design consideration forClaus plants to minimize the coking of catalyst beds in the process. The most likely chemicalpaths for the radical quench are presented and based on past observations. Controversy persistsin regard to the actual mechanism and the rate constants of the reactions involved in the radicalrecombination, 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-temperaturedata for the kinetic coefficients of some of the reactions. Much rate data are based on atmosphericstudies, rather than high-temperature oxidation. Similarly, better thermodynamic data arelacking for some important oxygenated sulfur species in the mechanism. This is most importantfor temperature- and pressure-dependent reactions, such as unimolecular reactions andchemically activated reactions. Studies that involve hydrogen sulfide flames under fuel-richconditions are lacking. Most of the studies have been limited to the impact of sulfur species onthe formation of other species, such as CO and NOx, in flames or reactors.

Introduction

This manuscript examines the gas-phase combustionof hydrogen sulfide under reducing conditions such asthose found in the Claus process, for example. The mainchemical species resulting from the combustion processare identified, and, most importantly, the main chemicalpaths in the combustion are identified based on chemicalprinciples and thermodynamics (see Table 5 later in thiswork). The manuscript does not attempt to develop anychemical reaction rate coefficients for the reactions; thisis left for future work. Nevertheless, the work of otherauthors 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 muchinformation is available, in either experimental orcomputational quantum chemistry, concerning the ratecoefficients of many of the reactions in Table 5 at hightemperatures that are typical of flames.

Recent developments in computational chemistry andthe advent of faster computers have made it possible todevelop large chemical kinetic models that are composedof hundreds of elementary reactions. The purpose isoften to predict the formation of trace species. Thesespecies often have an important environmental impact,e.g., the well-known formation of NOx in the hot regionof flames.1

* To whom correspondence should be addressed. Tel: (858)-5696742. E-mail: [email protected].

7706 Ind. Eng. Chem. Res. 2005, 44, 7706-7729

10.1021/ie0492956 CCC: $30.25 © 2005 American Chemical SocietyPublished 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 sulfurchemistry by Johnsson and Glarborg2 indicates thatmost of the chemistry has been concerned mainly withthe effect of sulfur on the emissions of other pollutants,such as NOx and CO (there will be more discussionabout this point later in this paper). In this respect, thework of Chernyshera et al.,3 which involved the mech-anism of H2S oxidation at high temperatures, is anexception.

Most of the work involving the industrial aspects ofH2S combustion that has been published only considersthe main overall reactions that occur during the high-temperature oxidation of hydrogen sulfide.4-6 The workof Monnery et al.6 also shows that empirical correlationsused to determine gas-phase composition (e.g., COS andCS2 concentrations at the exit of the waste heat boilerduring the Claus process) are often inadequate.

One very important application of hydrogen sulfidecombustion is the Claus reaction. Other applicationssuch as the high-temperature decomposition of hydro-gen sulfide to form hydrogen are also being considered.7The thermodynamics of super-adiabatic partial oxida-tion of hydrogen sulfide in an inert porous media hasalso been studied by Slimane et al.8 The study consid-ered various acid gas and oxidizer feeds, equivalenceratios, interstitial gas velocity, and temperatures. Mostof the calculations involved temperatures well in excessof 1000 K. The results of the equilibrium calculationsshow favorable conversions to hydrogen. Thermody-namic equilibrium modeling can be representative offlame temperatures and product compositions, and thisis most significant in the case of fast chemical kineticsduring the process. Thermodynamic predictions areusually less useful at low temperatures, because ofslower rates of the chemical reactions in the process.

Claus ReactionRefinery fuel gas, as well as other refinery hydro-

carbon streams, will contain quantities of hydrogensulfide; this is the result of the distillation of crude oilin the main crude distillation column or treatment ofthe distillation cuts in hydrotreaters and other treat-ment units. The resulting fuel gas is treated to removehydrogen sulfide in amine units, which is a dangeroussubstance, resulting in a hydrogen sulfide-rich streamto be treated in Claus plants.9,10

The Claus plant or sulfur recovery units make use ofthe 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 beas high as 2000 °F. The overall Claus plant is depictedin Chart 1. Both acid gas and, in some cases, sour waterstripper gas are fed to the main furnace. After partialoxidation of H2S in the furnace, the high-temperaturegas is cooled in a waste heat boiler; the gas thenproceeds to a condenser, where the gas is cooled to itsdew point. Low-pressure steam is generated for thispurpose.

The Claus plant then consists of various stages of gasreheating, catalytic reaction, and condensation of sulfur.In the catalytic reactor, Claus reaction 1 proceeds atmuch lower temperature (450-610 °F), thanks to analumina-based catalyst. The gas is reheated in thereheaters to bring it to reaction temperature. Care istaken to reheat the gas to a sufficiently high tempera-ture, so that the gas exiting the catalytic reactor thatfollows is above the sulfur dew point. This way, pluggingof the reactor is avoided. After reheating, the gas thenproceeds to the catalytic reactor to form sulfur via theClaus reaction. Finally, the gas flows to the sulfurcondenser, where the gas is cooled to its sulfur dew pointby producing low-pressure steam in a shell-and-tubeexchanger. The process described above is repeatedseveral times to increase conversion to sulfur. The flowdiagram in Chart 1 depicts three catalytic stages.

An important problem in modeling Claus plants is theestimation of the gas composition as the gas flows fromthe reaction furnace to the waste heat boiler. The gascomposition 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 T3xSx + 2H2O (1)

H2S + 1.5O2 T SO2 + H2O (2)

H2S + 12O2 T

1xSx + H2O (3)

Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7707

As the gas is cooled in the waste heat boiler, the gascontinues to react and follow the temperature drop tosome extent, depending on the reaction that is beingconsidered. The waste heat boiler exit temperature istypically 700 °F.4,6

Most process simulators (SULFSIM, TSWEET) simu-late the conditions at the waste heat boiler, based onequilibrium considerations and/or estimated quenchtemperatures for reactions of some species such ashydrogen, CO, and CO2. A difficulty in the simulationis the prediction of trace species such as COS, CS2, andmercaptans, because no chemical kinetic mechanism isfeatured in these software programs.

The results of equilibrium calculations indicate thatsignificant amounts of H2 and CO are produced in thereaction furnace.6 The hydrogen is most likely producedby the thermal decomposition of H2S. There is somedebate in regard to the mechanism of CO formation.Plant samples taken after the waste heat boiler seemto indicate the reassociation of H2 and S2 to form H2S.Similarly, CO formed in the furnace seems to react inthe waste heat boiler to form COS.6

Plant samples taken after the waste heat boiler alsoseem to indicate substantially higher concentrations ofCOS and CS2 than what is predicted by equilibriumcalculations at furnace conditions.6 CS2 formation seemsto correlate well with the amount of hydrocarbon in thefeed gas. As previously indicated, an important problemis that empirical correlations are often inadequate inpredicting gas composition.

The work of Clark and co-workers4,11 is also importantin this matter. They conducted studies using an exter-nally heated tubular reactor to simulate Claus furnaceconditions with variable quenching of the hot gas. Theyfound that CO2/sulfur species do not result in CS2, buthydrocarbons do react with sulfur to produce CS2. Underthe partial oxidation conditions of the furnace, theyfound that H2S is destroyed more quickly than anyhydrocarbon in the feed gas (the author gives a possibleexplanation for this in this manuscript, below). Theyalso studied new chemical pathways that involved thereaction of CS2 and COS with major species such as SO2,CO2, and H2. The destruction of COS and CS2 byreaction with water occurs very rapidly. COS is alsoknown 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 theClaus process. The work of Kennedy12 and Zachariahand Smith13 are important in this respect; however,their kinetic mechanisms do not include the moleculargrowth that leads to S8. Similarly, the chemistry of COS,and CS2, is not considered. Their mechanisms includethe chemistry that leads to the formation of SO, SO2,SO3, and S2, as well as other chemical paths for thedestruction of H2S. Another important source of chem-istry and kinetics data that is more recent can be foundin 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 workare fluid dynamics and residence time within thereaction furnace of the Claus plant. Both are importantin determining the real approach to equilibrium withinthe furnace.14,15 Computational methods, including tur-bulent combustion, have been reviewed by Eaton et al.16

Reaction furnace design considerations are furtherdiscussed by Hyne.5

Discussion

A first step in the assembly of the main chemicalpaths is to consider all or some of the possible species,radical or stable, that can partake in the destruction ofthe initial mixture that contains hydrogen sulfide. Theseare listed in Table 1.10,17,18 This table must includespecies that lead to the formation of elemental sulfurin the Claus furnace as well as important trace speciessuch as COS and CS2.

In addition to hydrogen sulfide, acid gas may containhydrocarbons such as methane and ethane. Further-more, there are instances when sour water stripper gasthat contains ammonia must be treated in the Clausplant;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 canbe found on the Internet at www.me.berkeley.edu/gri_mech), as well as comprehensive discussions andreaction compilations in dissertations by Gargurevich,17

and Wang19 (more below); the hydrocarbon speciesconsidered in Table 1 are taken from these references.

It is important to assess the concentration level ofthese species under typical reaction conditions in theClaus furnace and waste heat boiler. For this reason,equilibrium simulations were performed with ASPENPlus 10.1. The simulations consisted of isotherms atdifferent temperatures including the adiabatic temper-ature. It must be noted that similar calculations havebeen conducted by Meisen and Bennett.10 The resultsof the calculations for this manuscript are shown inFigures 1-7. The well-known fact that radicals can bepresent in flames in excess of their equilibrium valuesmust be considered when producing the elementarychemical steps of the combustion process.

Before proceeding, it is important to become familiarwith the molecular geometry of some of these species.This is very relevant to the discussion of the reactionsthat can occur during the combustion process. Table 2shows the molecular structure for some of the sulfurspecies in Table 1. Sulfur, as well as oxygen, has sixvalence electrons and requires two more to satisfy theoctet rule. There is no indication that the sulfur in thespecies SO2, SO3 shown in Table 2 makes use of dorbitals.20 Both involve double-bonded resonance struc-tures.

It is important to note that the oxygenated speciesSO, SO2, and SO3 provide double bonds for radical

Table 1. Chemical Species under Consideration inEquilibrium Calculation Hydrogen SulfideCombustion-Reducing Conditions (Claus Process)

Major SpeciesO2, N2, NH3, NO, H2O2, H2, H2OCO, CO2, COS, CS2, HCN, CH4, C2H6, C2H2, C2H4CH2OSO, SO2, SO3, H2S2, H2S3, H2S4, H2S5, H2S6, H2S7, H2S8S2O2, H2SO2, CH3SH, C2H5SHS2, S5, S6, S7, S8

Radical SpeciesO, OH, HO2, HCS, CN, CH3S, C2H5S, CH3, CH2, CH, C2H5, C2H, C2H3CHO, CH3OS2O, S, S3, S4, HSO, HSO2HS2, 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 apuckered ring structure. There are several alleotropesof solid sulfur, and the most common ones are therhombic and monoclinic crystal structures; the rhombicform is the most stable of the two.

It is seldom discussed in the literature that speciessuch 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 hexagonalchair that is similar to that of cyclohexane. S7 has alsobeen 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 shouldbe possible to open up the rings previously described toproduce the linear geometry. The energy required toopen the S8 ring is estimated to be 33.8 kcal/mol.23

Raghavadari et al.21 also gives energy estimates forthe 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) T85S5(c) (29.1 kcal/mol) (5)

S8(c) T86S6(c) (9.2 kcal/mol) (6)

S8(c) T87S7(c) (5.2 kcal/mol) (7)


At the high combustion temperatures, these reactionsshould occur. No mechanism is given for the conversionsdescribed by Raghvadari et al.21

A. Hydrogen Sulfide Combustion: ChemicalEquilibium Calculations. As previously noted, theconcentration of chemical species under equilibriumconditions can only be considered as a guide to theirimportance in the combustion process. Measurementsof radicals in laminar flames with microprobes, forexample, have shown that these species can be foundin levels exceeding their equilibrium concentrationsduring the combustion process.

However, temperatures and residence times typicalof Claus furnace designs make it possible to achieve aclose approach to equilibrium, and the chemical com-

positions shown by the calculations in this section atthe higher temperatures should be viewed as a closerepresentation of the furnace products in typical ap-plications. Thus, the results presented here are mostrelevant in understanding the chemistry that occurs inthe furnace at high temperatures.

As stated previously, chemical equilibrium calcula-tions have been conducted by other authors10 for amixture of hydrogen sulfide and air under conditionstypical of the operation of Claus units. This authorperformed calculations at the adiabatic temperature andisotherms ranging from 600 °F to 2000 °F. Species forwhich concentration profiles were provided are shownin Table 3. The minimum concentration reported wason 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 speciesH, OH, and O reach concentrations at the ppm level onlyat the highest temperatures (2400-3100 °F). This iswhat is expected from what is known about combustionchemistry. The calculations show that, for temperaturesof >800 K, the most abundant species are S2, S3, S4,and HS, with S2 being the predominant molecule. Sulfurspecies such as S5, S6, S7, and S8 become abundant atlower temperatures (well below 1330 °F). Monatomicsulfur (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 dueto the lower bond energy in S-H (89 kcal/mol), ascompared 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 levelat temperatures above 1330 °F. These species arethought to involve reactions of CO2 and CO (more aboutthis observation will be presented later in this paper).The importance of CO, H2, and CO2 chemistry has beenpreviously discussed. The work of Meisen and Bennett10

showed that significant amounts of CO and H2 areformed above 620 °F. The concentrations of both speciescontinue to increase with increasing temperature.

They found almost insignificant amounts of ammoniathat was created from the feed nitrogen. At the highesttemperatures, 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 whenoptimization of the operation of the Claus process isattempted.24

As part of the work presented here, as well as toexpand on the previously given results, equilibriumcalculations were performed for a mixture of the fol-lowing composition for acid gas and sour gas: H2S, 87.31vol %; CO2, 3.82 vol %; NH3, 1.51 vol %; C1, 1.62 vol %;C2, 1.62 vol %; H2O, 4.12 vol %; total, 100.00 vol %. Thisgas composition would be the type that is treated in aClaus unit designed to handle acid gas and sour waterstripper gas that contains ammonia at 10 psig. For theequilibrium calculations, the gas was burned with airby the stoichiometry of eq 3, adiabatically and isother-mally, in the temperature range of 400-2200 °F. Theresults of the calculations are shown in Figures 1-7.Table 4 shows both stable and radical species exceedingthe ppmv concentration level.

For purpose of the calculations, the COMBUSTthermodynamic databank of the ASPEN package wasused. This is based on the JANAF ThermochemicalTables, which were published by Dow Chemical Co.,Midland, MI, in 1979. The databank contains the idealgas heat capacity, free energy of formation, and en-thalpy of formation for many species, and these valuesare accurate at the high temperatures that are typicalof combustion for more than 59 stable and radicalspecies.

Generally, the results are in agreement with the workof Meisen and Bennett.10 Figure 1 shows the concentra-tion of major species such as CO, CO2, and H2. As withthe work of Meisen and Bennett,10 the concentrationsof CO and H2 increase significantly at temperatures

above 620 °F. The concentrations of both CO and H2increase with temperature, reaching an equilibriummole fraction of ∼0.01 in both cases at the highesttemperature shown or 2400 °F.

Figure 2 shows the distribution of the sulfur speciesS, S2, S3, S4, S5, S6, S7, and S8. The smaller species, suchas S1, S2, and S3, are significant at the higher temper-atures and above 1000 °F. Elemental sulfur (S2) is thepredominant species at these temperatures. Moleculessuch as S5, S6, S7, and S8 become most significant atlower temperatures (<700 °F). S8 overtakes all the otherspecies such as S6 and S7 as the temperature approaches500 °F or lower.

Figure 3 shows that the formation of COS and CS2does not become significant until the temperaturereaches 1000 °F or above, with the COS mole fractionbeing higher by 2 orders of magnitude, or 100 ppmv.The simulations also show that the concentrations ofspecies such as H2S2 and H2S3 start becoming signifi-cant at temperatures higher than 600 °F (see Figures 4and 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 molefraction of 10 ppmv at 1000 °F.

The oxygenated species SO can reach a mole fractionof 1000 ppm levels at the higher temperatures shownor 2300 °F. In contrast, S2O is most important, even atlower temperatures; it peaks at 1000 °F with a molefraction as high as 1000 ppmv, or 3 orders of magnitudehigher than the concentration of SO at the sametemperature. The levels of species such as HSO andH2SO2 are not as significant as SO or S2O above (seeFigure 4). The mole fraction of HSO can reach ∼1 ppmv,at the higher temperature shown or 2300 °F, whereasH2SO2 remains well below 1 ppmv even at the highesttemperatures or above 2000 °F.

The concentrations of radical species such as HS andHS2 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 + 12O2 T

1xSx + H2O (8)


parison, HS3 is not as significant; its concentration islower by 3 orders of magnitude.

Species such as H2S2 and H2S3 could have an impor-tant role in the combustion chemistry of H2S. Theconcentration of H2S2 peaks at 1000 °F, reaching 1000ppmv (see Figure 4), only to decrease slightly at thehigher temperatures. H2S3 displays a similar behavior,but its highest concentration is only 10 ppmv (see Figure5). In contrast, a larger molecule, such as H2S4, reachesconsiderably lower concentrations than the aforemen-tioned H2S2 or H2S3 (<1 ppmv) (see Figure 5).

Equilibrium calculations show insignificant amountsof hydrogenated species such as H2S5, H2S6, H2S7, andH2S8. This could be due to hydrogen elimination reac-tions, such as

The reactions result in the formation of the ringstructures for the sulfur species. The heats of reactionare given parentheses and are based on average bondenergies. These reactions could easily occur at flametemperatures.

B. Chemical Reactions Found in Combustion(Illustrating the Chemistry Typical in Combus-tion). This section is a short tutorial in combustionchemistry fundamentals. Combustion involves radicalspecies and radical-chain mechanisms.17 The existenceof radical species such as H and OH in the gas phase ispossible because of the high temperature of combustion.

Chain mechanisms consist of initiation, propagation,and termination steps. The type of reactions have beendescribed 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 differenttypes:

and

where the Cl atom transfers to the second C atom inthe molecule.

(c) Termination reactions are radical-radical additionreactions or,

These result in a decrease of the radical species in thegas phase.

Table 2. Molecular Structure of Sulfur Species

chemical formula molecular structure

SO SdO

SO2

SO3

H2SO2 H-S-O-O- -HHSO2 H-O-O- -SS2O S-O- -SS2O2 S-O-O- -SS2 SdSCS C)SCS2 SdCdSCOS OdCdSS3 S-S- -SS4 S-S-S- -S

S5(c)

S6(c)

S7(c)

S8(c)

H2S8 H-S-S-S-S-S-S-S-S- -HH2S7 H-S-S-S-S-S-S-S- -HH2S6 H-S-S-S-S-S-S- -HH2S5 H-S-S-S-S-S- -HH2S4 H-S-S-S-S- -HH2S3 H-S-S-S- -HH2S2 H-S-S- -HCH3SH CH3- -SHC2H5SH CH3-CH2- -SH

H2S5 T S5(c) + H2 (16 kcal/mol, ESTIM) (9)




Table 3. Species under Consideration in the Modeling ofChemical Equilibrium Calculationsa

Species

stable radical

H2O, H2, O2 H, OH, ONO, NH3 CS, HSCO, CO2, COS, CS2 S, S3, S4S2, S5, S6, S7, S8 HS, SNSO2, SO, S2O, SO3H2S, H2S2

a Data from Meisen and Bennett.10

Table 4. Species Used in Chemical EquilibriumCalculations and Showing Concentrations in the Partsper Million by Volume (ppmv) Rangea

Species

stable radical

CO, CO2 H, OHH2, H2O HS, HS2, HS3COS, CS2 S, S3, S4NH3 HSOH2S2, H2S3, H2S4SO2, S2O, SO, H2SO2S2, 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 LEEDSH2S + OH ) H2O + HS -28.91 JANAF Kennedy,12 UNIV LEEDSH2S + O ) OH + S 20.91 JANAF Kennedy,12 UNIV LEEDSH2S + O ) SO + H2 -54.65 JANAF Kennedy,12 UNIV LEEDSH2S + S ) HS + HS 5.25 JANAF Kennedy,12 UNIV LEEDSHS + HS + M ) H2S2 + M -66.6 JANAF Kennedy,12 UNIV LEEDSHS + HS ) H2S + S -5.25 JANAF Kennedy,12 UNIV LEEDSHS + HS ) S2 + H2 -35.89 JANAF Kennedy,12 UNIV LEEDSHS + OH ) S + H2O -15.53 JANAF Kennedy,12 UNIV LEEDSHS + H ) S + H2 -19.16 JANAF Kennedy,12 UNIV LEEDSH2S2 + H ) HS2 + H2 -49.46 JANAF UNIV LEEDSH2S2 + OH ) HS2 + H2O -44.92 JANAF UNIV LEEDSH2S2 + O ) HS2 + OH -28.05 JANAF UNIV LEEDSH2S2 + S ) HS + HS2 -29.37 JANAF UNIV LEEDSH2S2 + 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 LEEDSHS2 + OH ) S2 + H2O -58.6 JANAF UNIV LEEDSHS + 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, BEHS3 + H2 ) H2S3 + H 23 ESTIM, BEHS3 + H2S ) H2S3 + HS 0 ESTIM, BEHS3 + S ) HS + S3 0 ESTIM, BEHS3 + HS ) H2S + S3 0 ESTM. BEHS3 + OH ) S3 + H2O -30 ESTIM, BEH2S3 + OH ) HS3 + H2O -30 ESTIM, BEH2S3 + S ) HS3 + HS -8 ESTIM, BEH2S3 + HS ) H2S + HS3 -8 ESTIM, BES4 + H2 ) HS4 + H 23 ESTIM, BEH2S4 + OH ) HS4 + H2O -30 ESTIM, BEH2S4 + S ) HS4 + HS -8 ESTIM, BEH2S4 + HS ) HS4 + H2S -8 ESTIM, BEHS4 + H2 ) H2S4 + H 23 ESTIM, BEHS4 + H2S ) H2S4 + HS 0 ESTIM, BEHS4 + S ) HS + S4 0 ESTIM, BEHS4 + HS ) H2S + S4 0 ESTIM, BEHS4 + OH ) S4 + H2O -30 ESTIM, BEHS3 + M ) HS2 + S + M 54 ESTIM, BEHS4 + M ) HS3 + S 54 ESTIM, BES2 + O ) SO + S -22.5 JANAF Kennedy,12 UNIV LEEDSS + O + M ) SO + M -124.29 JANAF Kennedy,12 UNIV LEEDSS + O2 ) SO + O -5.19 JANAF Kennedy,12 UNIV LEEDSSO2 + O + M ) SO3 + M -83.14 JANAF Kennedy,12 UNIV LEEDSSO + O + M ) SO2 + M -131.75 JANAF Kennedy,12 UNIV LEEDSSO + S + M ) S2O + M -66.24 JANAF Kennedy,12 UNIV LEEDSS + S + M ) S2 + M -101.78 JANAF NISTS2 + 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, BEHS2 + S2 ) S4 + H 74 ESTIM, BEHS2 + S4 ) S6(c) + H -21.7 ESTIM, BEHS2 + S5 ) S7(c) + H -23.8 ESTIM, BEHS2 + S6 ) S8(c) + H -28.1 ESTIM, BEHS2 + HS2 ) S4(c) + H2 -50 ESTIM, BEHS3 + S2 ) S5(c) + H 32 ESTIM, BEHS3 + S2 ) HS4 + S 47 ESTIM, BEHS3 + HS3 ) S6(c) + H2 -44.7 ESTIM, BEHS3 + HS2 ) S5(c) + H2 -38 ESTIM, BEHS4 + S2 ) HS5 + S 47 ESTIM, BEHS4 + S2 ) S6(c) + H 25.3 ESTIM, BEHS4 + HS4 ) S8(c) + H2 -51.1 ESTIM, BEHS4 + HS3 ) H2 + S7(c) -46.8 ESTIM, BEHS + S4 ) S5(c)+ H -15 ESTIM, BEHS + S5 ) S6(c) + H -21.7 ESTIM, BEHS + S6 ) S6(c) + H -23.8 ESTIM, BEHS + S7 ) S8(c) + H -28.1 ESTIM, BECO + S + M ) COS + M -72.91 JANAF UNIV OF LEEDSCOS + H ) CO + HS -11.91 JANAF NIST, UNIV LEEDSCOS + OH ) CO2 + HS -37 JANAF NIST, UNIV LEEDSCOS + O ) CO2 + S -56.5 JANAF NIST, UNIV LEEDSCOS + O ) CO + SO -52.9 JANAF NIST, UNIV LEEDSCOS + S ) CO + S2 NIST, UNIV LEEDSC + S + M ) CS + M -170.54 JANAF UNIV OF LEEDSCS + S + M ) CS2 + M -105.29 JANAF UNIV OF LEEDSC + S2 ) CS + S -68.4 JANAF UNIV OF LEEDSCS + S2 ) CS2 + S -3.51 JANAF UNIV OF LEEDSCS2 + O ) COS + S -54.3 JANAF NISTCH3+ O2 ) CH2O + OH -53.2 JANAF Gargurevich,17 GRI MECHCH3O + M ) CH2O + H + M 20.50 JANAF Gargurevich,17 GRI MECHCH2O + HS ) CHO + H2S -0.1 JANAFS2 + CHO ) COS + HS -76.4 JANAFCH3 + 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 JANAFCHS + M T H + CS + M 51 JANAFCH3SH + H ) CH3 + H2S -16.72 JANAF NISTC2H5SH + H ) C2H5 + H2S -41.52 JANAF NISTCH3SH + HS ) CH3 + H2S2 7.01 JANAFC2H5SH + HS ) C2H5 + H2S2 -17.8 JANAFCH3SH + S ) CH3 + HS2 -3.75 JANAFC2H5SH + S ) C2H5 + HS2 -28.6 JANAFCH3 + 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 additionto a double bond, leads to the formation of a chemicallyactivated adduct that possesses excess energy due to thebond formation. This adduct can further react, leadingto products. One such reaction leading to the formationof formaldehyde is the addition of a methyl radical tothe double bond in oxygen, or

The chemically activated adductsin this case, CH3-O-O* has excess energy and leads to the formation ofproducts. The reader can find more-complete discussionsof chemical activation in ref 25.

Warnatz has published an interesting manuscriptthat examined the issues of hydrocarbon oxidation andhigh-temperature chemistry.26 It describes the mainchemical paths in the combustion of hydrocarbons thatare common to most molecules.

Hydrogen sulfide combusts then via chemical pathsthat are similar those previously mentioned. A summaryof the elementary chemical reactions considered in thisstudy is given in Table 5. The table includes the heat ofreaction for each reaction. One must recall that, becauseof the concentration factor in the rate of a chemicalreaction, typically, radical-radical reactions should notbe as important as radical-stable-species reactions.

C. A Brief Discussion of Recently PublishedSulfur Chemistry. It is not the objective of this sectionto present a comprehensive review of published chem-istry; this has been done by other authors who will bementioned below. This part of the manuscript willattempt to describe the main results of previous studies,as well as show some very relevant and importantconclusions: (i) there is a lack of high-temperaturekinetic data; (ii) accurate thermodynamic data for someimportant sulfur species are also lacking; (iii) moststudies examine H2S combustion at high temperatureonly indirectly, and their aim is to observe the effect ofthe sulfur species on the formation of pollutants suchas CO and NOx; and (d) the effect of combustionconditions on the formation of SO3 is examined.

A comprehensive review of the combustion of gaseoussulfur compounds was conducted by Cullis and Mulcahy

in 1972.18 The review examines the chemistry of sulfurcompounds that either undergo combustion themselvesor may be present in other gaseous combustion systems.Their study is based on low-temperature photolysisexperiments and flame studies. In some ways, this isthe starting point for later works on sulfur chemistryand oxidation. They identify the final and intermediateproducts of combustion: SO2 is always the main prod-uct, with small amounts of SO3, depending on thestoichiometric conditions. Other sulfur oxides of interestare SO and S2O, which are intermediates. Other prod-ucts of combustion under substoichiometric conditionsare H2S, COS, and elemental sulfur. Cullis andMulcahy18 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 SO3is

Table 5 (Continued)



CO2 + S ) SO + CO 2.9 JANAFCO2 + HS ) HSO + CO 7.69 JANAFH + 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 kineticdatabase 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 wouldaccelerate the decomposition of H2S.

S Atoms.

A reaction of high interest, because it could lead to CS2during the combustion of H2S, when in the presence ofhydrocarbons, is

Also,

Molecular growth occurs via the reactions

Their summary mechanism for the combustion of H2Sunder fuel-lean conditions consists of the main reactions

Unfortunately, the early work of Cullis and Mulcahy18

lacks information on the thermodynamics and kineticdata for much of the information presented, and, asnoted previously, much of the referenced experimentaldata have been obtained at temperatures much lowerthan the combustion temperatures. However, it is a goodstarting point for any sulfur compound combustion ordecomposition mechanism.

Johnsson and Glarborg2 presented developments inthe sulfur chemistry of combustion. The point is madethat there are studies for the purpose of kinetic model-ing in shock tubes, flow reactors, and flames. Theystated that earlier models suffered from a lack of

accurate thermodynamic and kinetic data. Rate con-stants for important reactions involving SO2 and SO3are presented. Sulfur dioxide catalyzes the recombina-tion of the main chain carriers in the flame (this willbe 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 ofits completeness in thermodynamic data.

Hynes and Wine28 expanded on the work of Cullis andMulcahy18 in an attempt to update the species thermo-dynamics (see Table 6) and rate coefficients. They notethat kinetic studies have focused on low-temperaturechemistry, 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 ( 0.3S3 142 ( 8 S4 146 ( 8S5 109 ( 8 S6 102 ( 8S7 114 ( 8 S8 100.4 ( 0.6

SO 5.0 ( 1.3 S2O -56 ( 34SO2 -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 ( 15HOSO2 -385 ( [10] H2SO4(g) -735.1 ( 8.4H2SO4(l) -814.0 ( 0.7

CS 280 ( 25 CS2 117 ( 1COS -138.4 ( 0.5

HCS 295 ( [10] H2CS 115 ( [10]CH3S 125 ( 2 CH3SH 214 ( 9CH3SH -22.9 ( 0.6 C2H5SH -46.3 + 0.6C6H5SH 112.4 ( 0.8 H2CdCdS 165 ( [15]CH3SCH3 -37.5 ( 0.5 CH3SCH2 135 ( 3CH3SSCH3 -24.2 ( 1.0 CH3SSSCH3 11 ( [10]CH3SS 72 ( 5 CH3SSS 86 ( 5c-CH2CH2S 82.1 ( 1.2 CH3SC2H5 -59.6 ( 1.1(C2H5)2S -84 ( 1 (C6H5)2S 231 ( 3C4H4S 115.0 ( 0.4 C4H8S -34.1 ( 0.9

CH3SO -62 ( [15] CH3SOO 76 ( 4CH3SO2 -238 ( [15] CH3SO3 -350 ( [5](CH3)2SO -151.3 ( 0.8 (CH3)2SO2 -373 ( 3CH2CH2SO -30 ( [15] (C2H5)2SO -205.6 ( 1.5(C6H5)2SO 107 ( 3 (CH3O)2SO -483 ( 2CH3C(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 ( 105HNCS 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 ( 8.4Cl2S 17.6 ( 3.3 Br2S 21 ( 17

SF6 -1220.5 ( 0.8 SF5C1 -1039 ( 11

SF4Cl2 -858 ( 13 SF5 -908 ( 15SF4Cl -741 ( [20] SF4 -763 ( 21SF3 -503 ( 34 HSI 42 ( 3F2CS -350 ( [15] Cl2CS -27 ( [15]CH3SCl -28 ( 6 CH3SI 30 ( 3C6H5SCl 106 ( 6 CH3SCH2Cl -90 ( [5]CH3SCH2Cl 26 ( 5

FSSF -335 ( 42 ClSSCl -16.7 ( 4BrSSBr 35 ( [10] CH3SSCl -21 ( 6C6H5SSCl 113 ( 6

F2SO -544 ( 21 Cl2SO -213 ( [20]

Br2SO -107 ( [20] F2SO2 -758.6 ( 8.4Cl2SO2 -354.8 ( 2.1 FClSO2 -557 ( 21

a Data from Hynes and Wine.28


atmospheric modeling studies. Most rate data wouldseem to apply to a temperature in the proximity of 1000K, and extrapolations are required for flame tempera-tures. The rate coefficients and chemistry of ∼48elementary reactions are presented, with the applicabletemperature being as high as 2500 K. The reactionsinvolve 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 importantobservations concerning the inhibition of moist COoxidation by SO2 in flow reactors. Their main objectiveis to re-examine the interaction of SO2 with the radicalpool under different conditions of temperature, SO2concentration, and stoichiometry (ranging from verylean to rich). The reactor temperature ranged from 800K 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 isessentially that reported by Glarborg et al.27

They found that the extent of CO inhibition isdependent on the stoichiometry and the amount of SO2.Under very lean conditions, SO2 inhibits CO oxidationvia the following reaction that captures O radicals:

However, at near-stoichiometric conditions, the promo-tion of CO oxidation occurs, because of the increase inthe radical pool by the reactions

and

The overall reaction results in radical chain branching:

On the other hand, under fuel-rich conditions, SO2inhibits the oxidation of CO:

They note that, to match the experimental data withtheir mechanism, they had to modify the heat offormation 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 ofZachariah and Smith,13 the H-atom recombination bythe sulfur species could also be explained (using theirmechanism) by the reaction sequence

This is in contrast to the reaction originally proposedby 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 ofH-radical recombination in flames, requires furtherinvestigation to elucidate the real chemistry that occursin SO2 inhibition.

Schofield’s study46 is interesting because it questionsprevious studies by other authors that have involvedflames and sulfur chemistry. It presents equilibriumcalculations 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 partialequilibrium assumption involving the reactions of HS,S2, S, SO2, SO, H, and OH:

For kinetic modeling, they seem to prefer Zachariah andSmith,13 because of its validation against experimentaldata, and measurements of OH, H, and O radicals intheir flames. They present a system of 16 reactions thatinvolves the formation of COS from CS and the destruc-tion of COS in fossil-fueled flames. One interest pointis that nonequilibrium has a tendency to move thesulfur speciation in the direction of SO2, SO, and S. Atthe temperatures considered, species such as HSO,HSO2, HSO3, H2S2, S2O, S3, etc. contribute little to theoverall sulfur balance.

The report by Glarborg et al.27 is, without a doubt,one of the most quoted papers tha tinvolves reactionmechanisms for sulfur species oxidation. They studiedthe impact of SO2 and NO on CO oxidation using flowreactors. They note that previous studies of flames,shocks, and flow reactors provided some understandingof sulfur chemistry, although the modeling efforts lackedaccurate thermodynamic data and rate data. Theirexperiments determined that SO2 inhibits CO oxidation,and it is most pronounced at high O-atom concentra-tions. The addition of NO significantly reduces theimpact of SO2. They revised the thermochemistry forthe H/S/O system, based on recent experimental andtheoretical results. Their work included revised ther-modynamic properties for the HxSOy species, as well asQRRK treatment for reactions involving these species.The mechanism consists of 67 reactions and more than15 species.

The work of Frenklach et al.30 also is often quoted.Their experimental and modeling work directly involvedH2S oxidation in shock tubes (induction times). Theexperimental conditions were 4%-22% H2S in air, with2%-13% H2O. The experiments were conducted at apressure of 1 atm and a temperature of 950-1200 K.The kinetic model consisted of 17 species and 57reactions. The agreement between experiment andmodel is reported to be satisfactory. Frenklach et al.30

conducted a rate and sensitivity analysis for theirmechanism. 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 importantreactions for oxidation of H2S at high temperature:

The work of Chernysheva and co-workers,3,31 whichexamined mechanistic issues in the gas-phase oxidationof hydrogen sulfide and carbon disulfide, is also animportant step in the understanding of the most rel-evant chemistry in both processes. The H2S oxidationmodel3 consists of 201 reactions and 23 species, and itdescribes experimental data in a wide range of temper-ature, although the data available were limited toignition delay and the concentration of the major species(no intermediates are reported). The model includesspecies such as S, S2, HS, HS2, H2S2, SO, S2O, HSO,HOS, HSO2, HOSO2, (HSO)2, SO2, and SO3. They notedthat the mechanism should still be considered to be anapproximation for the high-temperature oxidation ofH2S. 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, thekinetic data are estimates for the reactions in themechanism. As with the model for H2S, the predictionsfor ignition delay and the concentration of major speciesin flames are well-correlated (no intermediates arereported). Their comment is that, generally, the mech-anism will need refinements (reaction channels, ratecoefficients). The most important reactions in CS2oxidation are identified as (fuel-lean conditions)

For the oxidation of COS (fuel-lean):

The work of Sendt et al.32 is also important, becauseit 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 wasvalidated against a diverse collection of published datafor flow reactors (residence time of 0.2-1800 s, temper-ature of 873-1423 K, pressure of 0.04-3 bar, H2S molefractions 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 showthat the most important reactions are as follows (shownwith 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 beimportant.

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 thereactions shown below were obtained from the JANAFThemochemical Tables. For some of the reactions in-volving stable and radical sulfur species, the recent dataof Hynes and Wine,28 as presented in Table 6, was usedas indicated. The errors associated with the heats offormation 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 asubject of active research. The recommended heats offormation in Table 6, which contains over 100 sulfurcompounds, show that the values for many key oxygen-ated sulfur species remain unacceptably high. Some ofthese species are HOS, HSO2, HOSO, HOSO2, H2SO4(g), CH3SO, CH3SO2, CH3SO3, CH3SOH, CH3S(O)OH,and CH3SO3H. Many these species have a role in themechanism of radical recombination by SO2, as will bediscussed below.

Prior to the discussion, some important considerationsthat have a significant role in considering chemicalpaths must be noted. The rate of reaction is dependenton the concentration of the chemical species and themagnitude of the rate constant. In the chemical pathsdepicted below, radical-molecule reactions would beconsidered typically more important than radical-radical reactions, because of concentration effects. Reac-tions that involve radical-radical additions, as well aschemically activated reactions, such as a radical addi-tion to a double bond, usually have favorable activationenergies. Reactions that involve atomic abstraction byO, OH, and H radicals in the combustion process alsoinvolve, typically, relatively low activation energies. Itis 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 arefundamental to high-temperature combustion phenom-ena,34 because they participate in the radical-chainmechanism.

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 underwhich the Claus process occurs; thus, H and OH radicalsshould be more abundant than O.

The addition of S and HS to oxygen leads to theformation of SO:

Because of the lower O radical concentration, theaddition to elemental sulfur should be secondary:

The bond between sulfur and oxygen in SO is a doublebond.

The formation of SO2, and SO3, occurs via similarchemical paths:

Both SO2 and SO3 consist of resonance structures inwhich the oxygen is double-bonded to sulfur. In thiswork, because of the reducing conditions, no significantamounts of SO3 were calculated at equilibrium. Thepreviously described mechanism would seem to confirmthis, because its formation involves O radicals, whichare less abundant in fuel-rich flames.

Finally, the formation of S2O would seem to followthe path

D.2. Formation of Sulfur Vapor (S2). The thermaldecomposition of H2S leads to an abundance of S andHS radicals in the Claus furnace. Elemental sulfurvapor (S2) is formed via the following chemical paths:

This reaction leads to the formation of hydrogen, whichis a species with important design considerations for theClaus Plant.

D.3. Destruction of Hydrogen Sulfide. The initialdecomposition of hydrogen sulfide is initiated by itsunimolecular decomposition at high temperature, or

The decomposition then follows paths that involveradical-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, whichis an important species in Claus plant design.

This continues to form sulfur (S):

These are radical-radical reactions; however, theconcentration of HS radicals was shown to be high inour 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 GasPhase. 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 thesespecies. To a lesser extent, species such as H2S3 andH2S4 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 cyclicstructure of S5(c) would be formed. Depending ontemperature, an equilibrium would be formed betweenthe 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


S3 + HS2 T S5(c) + H

(-16.3 kcal/mol; from Hynes and Wine28) (146)

S3 + S2 + M T S5(c) + M


HS3 + S2 T S5(c) + H


HS + S4 T S5 (c) + H


S5(c) + M T S5 + M (30 kcal/mol, ESTIM) (150)

S5 + S + M T S6(c) + M


S4 + HS2 T S6(c) + H


S4 + S2 + M T S6(c) + M


HS4 + S2 T S6(c) + H


HS + S5 T S6(c) + H


S6 + S + M T S7(c) + M


S5 + HS2 T S7(c) + H


S5 + S2 + M T S7(c) + M


HS + S6 T S7(c) + H




S7 + S + M T S8(c) + M


S6 + HS2 T S8(c) + H


S6 + S2 + M T S8(c) + M


HS + S7 T S8(c) + H


S8(c) + M T S8 + M (33 kcal/mol; from Pryor23)(166)


Other reactions that can also occur, but at lowertemperatures (because of concentration effects), are

There is the question concerning the magnitude of theactivation energies for the reactions responsible for theformation of cyclic sulfur species up to S8. Radicaladdition reactions typically have relatively low activa-tion energies, which is a well-known fact. The ring-opening reactions are endothermic in nature, and theactivation energy should be at least equal to the heatof reaction. The work of Huang et al.33 in a similarreaction that involved C atoms and examined thedecyclization of the phenyl radical to C6H5 would seemto indicate that, for the larger molecules (such as S6 andS8), the energy barrier may be close to the heat ofreaction. For smaller molecules (such as S5), semiem-pirical quantum chemistry calculations by Gargurev-ich,17 which involved five carbon species, would indicatethat the decyclization energy could be somewhat higherthan the heat of reaction. The same studies would seemto indicate that the magnitude of the energy barriersfor 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 carbondisulfide (CS2) and carbonyl sulfide (COS) also involveradical-addition reactions. The most important pathleading to the formation of COS is the addition of the Sradical to the triple bond of CO, or

Studies indicate that there is a good correlation betweenthe formation of COS and the presence of CO andsulfur.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 pathwaysthat involve these two species. First, CH3 and SO2 areabundant species during the combustion of H2S ladenwith hydrocarbon species. Thus, the addition of CH3 toSO2 leads to a chemically activated adduct that resultsin 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 adductCH3SO2 at 298 K. Other data show a rate coefficient of2.92 × 10-13 cm3 molecule-1 s-1 for the formation ofproducts at 298 K. The nature of the products is notreported.

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 energybarrier of 8.9 kcal/mol):

In this reaction, a chemically activated adduct isproduced in which a bridge can be formed between thelast O atom, and a hydrogen attached to carbon, whichresults in the elimination of OH. The net reactionhas an activation energy of 8.94 kcal/mol.17 For thetheoretical treatment of O2 addition reactions, see, forexample, 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 ofCH3 and SO is followed by the formation of formalde-hyde:

Formaldehyde then reacts with sulfur species, leadingto COS:

The comparable reaction CH2O + O ) CHO + OH hasan activation energy of 2.772 kcal/mol.17

This reaction involves a chemical adduct, whichleads to HS elimination and COS formation. Thecomparable addition of CHO to O2 has a low activationenergy.

A very simple mechanism for the formation of CS2involves C and S radical species:

This is not very likely, because Claus furnaces shouldnot 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 thepresence of hydrocarbons in the Claus furnace.6 Amechanism for the formation of CS2 is given by Clark

S3 + S3 + M T S6(c) + M


S3 + S4+ M T S7(c) + M


S3 + S5 + M T S8(c) + M


S4 + S4 + M T S8(c) + M


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 withthe reaction of methyl radicals with sulfur:

Their work does not provide rate coefficients for theaforementioned reactions.

Petherbridge et al.36 presented a sequence of reactionsleading to the formation of CS2. Their work involved thesimulation of gas-phase reactions that were occurringin a representative gas-phase environment used to growsulfur-doped diamond films via chemical vapor deposi-tion (CVD) for use in electronic devices. For the C/Hsystem, the GRI-Mech 3.0 was used, and the chemistryof the sulfur species was gathered from literature orestimated. Some estimates of the rate coefficientsinvolving the sulfur species were also made; mixturesof H2S/CH4/H2 and CS2/H2 were studied. The sequenceleading 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 ofspecies such as CH4, C2H2, CH3, H2S, CS2, and CS, usingmolecular beam mass spectroscopy in microwave-activated gas mixtures. Their model agrees fairly wellwith the experimental data, even after experimentalerrors in the species concentrations have been takeninto consideration.

A mechanism for the formation of CS2 that is proposedhere again uses a reaction similar to the formation offormaldehyde from CH3, or, as above,

Thus, the first step in the formation of CS2 would be

In this reaction, a chemically activated adduct is alsoproduced, and, in this case, a transition state is formed,in which a bridge is formed between the last S atomand a hydrogen attached to carbon, leading to HSelimination. The work of Petherbridge et al.36 wouldindicate that the net reaction would have a very lowactivation energy.

The reaction that follows is then

The estimate for the activation energy would be on theorder of 3.0 kcal/mol for the net reaction.36

Finally,

The magnitude of the activation energy can beestimated from the work of Petherbridge et al.36 as 1kcal/mol for the overall reaction.

This is a much more simple and elegant mechanismthat leads to CS2. It is very favorable thermodynami-cally, and the order of magnitude of the estimatedactivation energies is relatively low.

Another path for the formation of CS2 has beenoutlined in the work of Cullis and Mulcahy18 and wasdiscussed previously; it involves acetylene, which is an

CH3 + S2 + M T CH3-S-S + M


CH3-S-S + H2S T HS + CH3-S-SH


CH3-S-SH + M T CH3S + HS + M


CH3S + HS T H2S + CH2dS


CH2dS + HS T H2S + CHdS


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(-41.50 kcal/mol; Ea ) 0 kcal/mol) (189)

H + CH3SH T CH3 + H2S(-16.70 kcal/mol; Ea ) 1.66 kcal/mol) (190)

H + CH3SH T CH3S + H2

(-16.77 kcal/mol; Ea ) 2.59 kcal/mol) (191)

H + CH3S T CH2S + H2

(-54.47 kcal/mol; Ea ) 0 kcal/mol) (192)

H + CH2S T HCS + H2


H + HCS T H2 + CS(-55.67 kcal/mol; Ea ) 0 kcal/mol) (194)

SH + CS T H + CS2


CH3 + O2 T [CH3-O-O]* T CH2dO + OH(-53.2 kcal/mol; Ea ) 8.94 kcal/mol) (196)

CH3 + S2 T [CH3-S-S]* T CH2dS + HS


CH2dS + HS T CHdS + H2S


CHdS + S2 T [S-S-CHdS]* T CS2 + HS



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 explainthe destruction of COS or CS2 best are as follows:

The hydrogenation of COS should follow the followingchemical paths involving an activated adduct:

The proposed mechanism for the destruction of COSby water is as follows:

A chemically activated adduct is formed. The sulfur andhydrogen (attached to oxygen in the proximity) form abridge that leads to the elimination of HS. The forma-tion of CO2 as a major product is in agreement with theexperiments 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, whichcan lead to HS elimination and formation of COS. TheCOS that is formed decomposes via the previouslydescribed mechanisms (hydrolysis and hydrogenation).The formation of COS as an intermediate during thereaction of CS2 with water is in agreement with thework of Clark et al.11

D.7. Reactions of CS2 and COS with SO2. BothCS2 and COS have been observed to undergo reactionsin the presence of SO2 in tubular reactors4,11 to formsulfur, CO2, and CO. The mechanism can involve thethermal decomposition of the SO2 reactant, which wouldprovide O radicals:

This would be followed by the oxidation of COS and CS2by O radicals, involving chemical activation:

For CS2, the chemical paths are

This is followed by the oxidation of the COS, aspreviously discussed.

D.8. The Oxidation of Methane. Issues relevant tothe oxidation of hydrocarbons at high temperature havebeen well-discussed by Warnatz.26 The overall schemefor the combustion of methane is well-known, and itinvolves, depending on the conditions, species such asCH3, CH2O, CHO, CO, CO2, C2H6, C2H4, C2H2, CH2, CH,C3H3, and C6H6. The two-carbon molecules become moreimportant under fuel-rich conditions. In this respect, thework of Westbrook and Dryer37 is also very revealing:methane oxidation is a hierarchical process that consistsof CO, H2, and C2 sub-mechanisms. It is not the purposeof this study to dwell on these issues, except to referthe reader to the proper sources for further study. Ourmain intention here is to show the main chemical pathsfor methane oxidation in the presence of sulfur species.

Simmie38 provided an excellent review of the recentdevelopments in the kinetic modeling of hydrocarbons(methane, as well as heavier molecules). He discussedseveral methane oxidation mechanisms: GRI-Mech 3.0(325 reactions, 53 species, 1000-2500 K), the Universityof Leeds, UK Mechanism (351 reactions, 37 species), andthe mechanism that has been attributed to AlexanderKonnov (1200 reactions, 127 species) (found athttp://homepages/vub.ac.be/∼akonnov/), among others.

The discussion here will rely heavily on the extensivework by Wang19 and Gargurevich,17 because these arecomprehensive discussions on the issues of methanecombustion, rather than reviews or summaries. Thefeatures described here have much in common with thework of the previously mentioned authors. In theabsence of hydrogen sulfide, methane combustion, underfuel-rich conditions, is initiated by

However, under the conditions prevalent in the Clausprocess, S and HS radicals are also abundant and thefollowing 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 theamount 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. Monneryet al.6 noted that plant data seems to indicate theformation of CO via reactions of the type

The reactions involve the CO2 constituent in the Clausfeed gas only. The major products of reaction are H2,CO, COS, and S2.11 The second reaction is highlyendothermic, in comparison to the first reaction, and isnot very likely. The mechanism for these reactionsshould involve S and HS radicals that have beenproduced from the decomposition of H2S, or

They are both chemically activated reactions that leadto 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 SO2would form according to

D.10. Ethane Oxidation. The chemical reactions forthe combustion of ethane under fuel-rich conditionshave been discussed fully by Wang19 and Gargurevich.17

The presence of radicals such as S and HS introduceadditional paths for the decomposition.

The initial decomposition occurs according to

The ethyl radical continues to react, according to thefollowing reactions:

Ethylene (C2H4) continues to react:

Then,

The equilibrium calculations did not result in anysignificant amounts of C2H2. This component reacts toform CO:

D.11. Mercaptans. The equilibrium calculations didnot result in any significant amounts of mercaptans.Perhaps this is due to the lower bond energy of the C-Sbond. 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 + 12S2 (-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 16422O + H2 T OH + H 5.06 × 104 2.67 6290 1.50 × 107 2 75422OH T O + H2O 6.00 × 108 1.3 0 1.50 × 109 1.14 0OH + H2 T H2O + H 1.17 × 109 1.3 3626 1.00 × 108 1.6 3295H + O2 + M T HO2 + M 6.76 × 1019 -1.42 0 2.00 × 1018 -0.8 0H2O/12./H2/2.5/

H + HO2 T 2OH 1.70 × 1014 0 874 1.50 × 1014 0 1003H + HO2 T H2 + O2 4.28 × 1013 0 1411 2.50 × 1013 0 692OH + HO2 T H2O + O2 2.89 × 1013 0 -497 2.00 × 1013 0 0H + 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 1390H2O2 + M T OH + OH + M 1.20 × 1017 0 45500

H2O/15./H2/2.5/H2O2 + OH T H2O + HO2 7.08 × 1012 0 1430O + HO2 T O2 + OH 2.00 × 1013 0 0 2.00 × 1013 0 0H + HO2 T O + H2O 3.10 × 1013 0 1720H + 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 3590H2O2 + H T HO2 + H2 4.79 × 1013 0 7950O + OH + M T HO2 + M 1.00 × 1016 0 0H2 + O2 T 2OH 1.70 × 1013 0 47780

S and SOx SectionS + O2 T SO + O 2.00 × 106 1.93 -1400 6.30 × 1011 0.5 0O + S2 T SO + S 3.98 × 1012 0 0 6.30 × 101 0.5 0SO + O2 T SO2 + O 6.20 × 103 2.42 3050 1.80 × 1011 0 0SO + SO T SO2 + S 2.00 × 1012 0 4000 3.30 × 1011 0 2250SO + O + M T SO2 + M 1.10 × 1022 -1.84 0 1.20 × 1022 -1.8 0SO2 + O + M T SO3 + M 4.00 × 1028 -4 5250SO + O2 + M T SO3 + M 1.00 × 1015 0 0SO3 + O T SO2 + O2 1.30 × 1012 0 6100SO3 + SO T SO2 + SO2 1.00 × 1012 0 4000SO + S + M T SO2 + M 1.20 × 1022 -1.8 02S + M T S2 + M 1.00 × 1018 -1 0S2 + H T S2O + H 1.80 × 1013 0 0

S and H SectionH2S + M T S + H2 + M 2.00 × 1014 0 66000H + H2S T H2 + SH 1.20 × 107 2.1 700 1.20 × 1013 0 1710SH + SH T H2S + S 1.00 × 1014 0 1430H2S + S T SH + SH 4.00 × 1014 0 15100H2 + S T SH + H 6.00 × 1014 0 24000 2.00 × 1014 0 76600HS + H T H2 + S 5.16 × 1014 0 21000SH + S T H + S2 2.69 × 1013 0 0 1.40 × 1013 0 478

S, H, and O SectionH2S + O T OH + SH 6.40 × 107 1.78 2840 4.36 × 1012 0 322H2S + OH T H2O + SH 2.70 × 1012 0 0 1.40 × 1013 0 886SH + H2O2 T HO2 + H2S 1.00 × 1011 0 0SH + O2 T SO + OH 1.10 × 1011 0 5400SH + O2 T SO2 + H 2.00 × 1011 0 5400SH + HO2 T H2O2 + S 1.00 × 1011 0 0SH + HO2 T H2S + O2 6.00 × 1012 0 0SH + O T OH + S 2.29 × 1011 0.67 1920 6.31 × 101 0.5 8060SH + O T SO + H 1.00 × 1014 0 0 3.56 × 1014 0 642SH + OH T H2O + S 1.00 × 1013 0 0OH + SO T SO2 + H 5.20 × 1013 0 0 1.80 × 1013 0 0OH + S T SO + H 4.00 × 1013 0 0 7.20 × 1013 0 642HO2 + S T SH + O2 1.00 × 1012 0 0SO3 + H T SO2 + OH 1.00 × 1012 0 0

HSO2, HSO ReactionsH + SO2 + M T HSO2 + M 7.30 × 1016 0 0H + HSO2 T SO2 + H2 1.60 × 1012 0 0OH + SO + M T HSO2 + M 7.00 × 1016 0 0OH + HSO2 T SO2 + H2O 6.80 × 1013 0 1772OH + HSO2 T SO2 + H2OH + SO + M T HSO + M 1.00 × 1015 0 0 1.60 × 1020 -1.5 0HSO + H T H2 + SO 5.00 × 1011 0.5 2222HSO + H T H2S + O 5.00 × 1010 0.5 4520HSO + H T SH + OH 5.00 × 1011 0.5 4520HSO + 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 andSmith,39 have noted the effect of small amounts of SO2on 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 theexistence of experimental data, including measurementsof the H, OH, O radicals. They have noted that SO2,and HSO2 (see Table 7), lead to radical recombinationreactions 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, thatthe addition of SO2 leads to a substantial depletion ofatomic hydrogen. As the previously discussed reactionbetween H and HSO2 shows, the presence of SO2catalyzes the formation of H2 from H atoms. Zachariahand Smith13 also have noted the role of HSO2 as achannel for radical recombination. Smith et al.40 havehighlighted the role of SO2 as a catalyst for radicalrecombinaton in CO/O2/Ar fuel-lean flames, as shownby the last set of reactions previously presented involv-ing atomic oxygen radicals.They noted the importanceof HSO2 to be able to better model flames that have beendoped with SO2.

This could be an important consideration during thecombustion of H2S in the Claus process. Clark et al.4noted in their study of Claus chemistry that H2Scombusts more quickly than the hydrocarbons that werepresent in the initial gas mixture. The aforementioneddiscussion would seem to indicate that this could be theresult of the formation of SO2 in large quantities in theClaus process and the resultant H and OH radicalrecombination, which would then slow the hydrocarbondecomposition. This should be an area of further study.

The equilibrium calculations in this study did notresult in any significant quantities of HSO2. This is inagreement with the study of Smith et al.40 on CO/O2/Ar flames under fuel-lean conditions. However, asmentioned earlier, H2SO2 molecules seem to form at the

higher temperatures or above 2000 °F. This could bethe end product of HSO2 formation in the combustionprocess.

The work of Alzueta et al.29 on the effects of SO2 onthe radical pool has been presented earlier. This is astudy conducted at a later time than the Zachariah andSmith experiments and somehow is in contradictionwith what is presented here. The arguments presentedby Alzueta et al. seem to indicate that the effect of SO2on the radical pool, as well as the mechanism for radicalrecombination in flames, require further study. Theproblem seems to involve a lack of better kinetic andthermodynamic data.

D.13. Reaction Rate Coefficients. This work willnot address issues relating to methods for the construc-tion of any detailed chemical kinetic model composedof many reactions, as shown in Table 5, and theestimation of chemical kinetic coefficients. Severalsources can be found that address the subject, includingthe work of Wang19 and Gargurevich.17 These are alsogood sources for the treatment of chemically activatedreactions, some of which have been presented in thismanuscript

The treatment of chemically activated reactions canalso 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 reachedby Hynes and Wine28 in their review of thermochemicaland kinetic data of sulfur reactions: “The paucity ofhigh-temperature kinetic data on elementary reactionsof sulfur is a substantial roadblock to understandingsulfur combustion. More high-temperature studies areneeded for almost all of the reactions of sulfur species.”“Modeling studies of sulfur chemistry under combustionconditions have been handicapped by the lack of aregularly upgraded, evaluated high-temperature data-base similar to the NASA or CODATA compilationsused for atmospheric modeling”.

This consideration is most important for temperature-and pressure-dependent reactions (such as unimolecularreactions) and chemically activated reactions, whichhave a fundamental role in combustion. The dependenceof the rate coefficient on reaction conditions must betaken into account.

The author has not found a comprehensive study ofH2S 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 andZachariah and Smith.13 These models only examine theformation of simple sulfur species (such as SO, SO2, SO3,and S2) and are fairly similar. They are a good startingpoint 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 notedpreviously, 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 ReactionsHSO + O T SO + OH 5.00 × 101 0.5 2222HSO + O T H + SO2 1.00 × 1014 0 26200HSO + O T HS + O2 1.00 × 1012 0 0 5.00 × 1011 0.5 4520HSO + O2 T SO + HO2 5.00 × 1011 0.5 2222SH + 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 beupdated.

The work of Radi et al.43 uses a novel spectroscopictechnique (multiplex spectroscopy) and it is applied tomeasure S2 and OH concentration profiles in H2/Air/SO2 flames obtained stabilized on a flat flame burnerat atmospheric pressure. The sulfur chemistry used intheir mechanism is taken from Zachariah and Smith13

for low-pressure H2/O2/SO2 flames. The experimentalresults for the OH and S2 concentration profiles are ingood qualitative agreement with the mechanism results.Discrepancies in the S2 concentration profile are at-tributed to the fact that the mechanism of Zachariahand Smith13 was developed for low-pressure flames. Toimprove their mechanism, corrections must be made topressure-dependent reactions for the higher experimen-tal pressure in their flame experiments.

Table 5 shows, for each reaction, the most recentsources of chemical kinetic rate coefficient data. A goodsource is also the NIST Database (which can be foundvia the Internet at www.nist.gov). The sulfur mecha-nism by the University of Leeds, U.K. presents kineticdata that are based on the work of Alzueta et al.,29

including the most recent data for the HxSOy system ofreactions; it can be found at http://garfield.chem.elte.hu/Combustion/Combustion.html. The compilation of Hynesand Wine28 consists of 48 elementary reactions thatinvolve sulfur-containing species and it is also anupdated 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 novelreactions leading to COS and CS2, as presented in thisstudy, have not been found by the author.

Because experimental data is lacking, further studieswill be necessary to improve the rate coefficients andthermodynamics of the reactions in Table 5 by themethods described by Gargurevich17 and Senkan.34

Computational quantum chemistry can also be used forthe estimation of activation energies and the heats offormation of the molecular species.

Experimental studies would also need to be pursued,to validate the model depicted in Table 5, after thereaction rate coefficients have been estimated.

Conclusions

The main objective of this study has been to presentthe 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 elementaryreactions is presented.

The mechanism is able to explain the high-tempera-ture oxidation of H2S. The formation of hydrogen, whichis an important consideration in Claus plant design, canbe explained by the mechanism. Hydrogen generationimpacts the design of the Claus plant tail gas treatingunits. Most importantly, novel chemical paths for theformation of COS and CS2 are presented, based onfundamental chemical laws. The heats of reaction andactivation energies for the reactions are estimated andseem to indicate that the chemical paths presented could

have an important role. These are trace species withimportant environmental impact, which must be con-sidered in the design of sulfur treatment plants inmodern refineries.

Existing process simulators such as TSWEET andSULFSIM rely on empirical correlations or an approachto equilibrium to determine the amounts of H2, COS,and CS2 produced in the Claus plant as the gas is cooledto the sulfur dew point. Unfortunately, many of theseempirical correlations cannot represent all of the condi-tions that may occur in the design of Claus plants.

The study introduces a plausible explanation for theobserved reduced rate of hydrocarbon oxidation in theClaus furnace, which is due to radical recombinationcatalyzed by SO2. This should be an important consid-eration in the design of the combustion furnace of Clausplants to minimize carbon deposition in the catalyticreactors downstream of the furnace.

The molecular growth to S8, starting with S2, is alsoincluded in the mechanism. Evidence is provided for thering structure of S5, S6, and S7; in contrast, S4 is a linearmolecule. Intramolecular reactions involving S8 andleading to S7 and S6 are introduced. Heats of reactionand activation energy estimates for the cyclizationreactions 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 investigatedprompted by observations in flow reactor experiments.For similar reasons, reactions of CO2 with H2S and S2are considered.

The study highlights the consensus, in regard to thelack of high-temperature kinetic data, as well as studiesof fuel-rich H2S flames. These are needed for a valida-tion of the elementary reaction chemistry presented inthe manuscript. Similarly, experimental studies at hightemperature are needed to better elucidate the phe-nomena of radical recombination by SO2.

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Received for review August 4, 2004Revised manuscript received June 25, 2005

Accepted July 25, 2005

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