7706
Ind. Eng. Chem. Res. 2005, 44, 7706-7729
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 * To whom correspondence should be addressed. Tel: (858)5696742. E-mail:
[email protected].
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
10.1021/ie0492956 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/23/2005
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7707 Chart 1. Process Flow Diagram of the Overall Claus Plant.
Despite the aforementioned discussion, the combustion of hydrogen sulfide has not received much consideration 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 mechanism 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 hightemperature 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 hydrogen sulfide to form hydrogen are also being considered.7 The thermodynamics of super-adiabatic partial oxidation of hydrogen sulfide in an inert porous media has also been studied by Slimane et al.8 The study considered 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. Thermodynamic 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 hydrocarbon 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 treatment 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:
3 2H2S + SO2 T Sx + 2H2O x
(1)
To obtain the necessary SO2 for the reaction above, onethird of the hydrogen sulfide is combusted in a hightemperature furnace, or
H2S + 1.5O2 T SO2 + H2O
(2)
The overall reaction is then
1 1 H2S + O2 T Sx + H2O 2 x
(3)
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 temperature, 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).
7708
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005
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) simulate 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 externally 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 comprehensive studies that examine the chemistry of H2S combustion 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 chemistry 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 turbulent combustion, have been reviewed by Eaton et al.16
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
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. Furthermore, 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 extensively (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 temperature. 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 structures. It is important to note that the oxygenated species SO, SO2, and SO3 provide double bonds for radical
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7709
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.
addition reactions to occur. These reactions are important in the flame, e.g.,
SO2 + O T SO3
(4)
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 cyclopentane; 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:
8 S8(c) T S5(c) 5
(29.1 kcal/mol)
(5)
8 S8(c) T S6(c) 6
(9.2 kcal/mol)
(6)
8 S8(c) T S7(c) 7
(5.2 kcal/mol)
(7)
7710
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005
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.
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 applications. 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 calculations 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).
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7711
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.
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 temperatures above 1700 °F are reached. The relative abundance 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 temperatures 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 successfully for oxygen.
7712
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005
Figure 7. Concentration plot of the H2O, H2S, and SO2 species in the furnace gas over a range of temperatures.
The maximum in sulfur yield under adiabatic conditions occurs under conditions where the oxygen consumption is given by the overall reaction
1 1 H2S + O2 T Sx + H2O 2 x
(8)
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 following 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 isothermally, 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 enthalpy 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 concentration 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 temperatures 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 (
