air flames under claus conditions

Applied Energy 109 (2013) 119–124

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Effect of oxygen enrichment on acid gas combustion in hydrogen/air flames under claus conditions H. Selim, S. Ibrahim, A. Al Shoaibi 1, A.K. Gupta ⇑ Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, United States

h i g h l i g h t s  Effect of oxygen enrichment on acid gas (H2S and CO2) combustion was examined.  Increase in the percentage of oxygen enrichment increased the rate of SO2 production.  CO2-laden acid gas had faster rates of reactants decomposition/products evolution.  Existence of CO in the reaction pool was evident on CO2 role as an oxidizer provider.  Presence of CO enhanced the rate of other sulfurous compounds formation, CS2 and COS.

a r t i c l e

i n f o

Article history: Received 3 January 2013 Received in revised form 28 February 2013 Accepted 10 March 2013 Available online 27 April 2013 Keywords: Hydrogen sulfide Claus process Sulfur chemistry Oxygen enrichment in claus process Chemical kinetics

a b s t r a c t Results are presented to examine the combustion of acid gas (H2S and CO2) in hydrogen-fueled flames using a mixture of oxygen and nitrogen under Claus conditions (U = 3). Specifically the effect of oxygen enrichment in the above flames is examined. The compositions of acid gas examined are100% H2S and 50% H2S/50% CO2 with different percentages of oxygen enrichment (0%, 19.3% and 69.3%) in the oxygen/nitrogen mixtures. The results revealed that combustion of acid gas formed SO2 wherein the mole fraction of SO2 increased to an asymptotic value at all the oxygen concentrations examined. In addition, increase in oxygen enrichment of the air resulted in increased amounts of SO2 rather than the formation of more desirable elemental sulfur. In case of 50% H2S/50% CO2 acid gas, carbon monoxide mole fraction increased with oxygen enrichment which is an indicator to the availability of additional amounts of oxygen into the reaction pool. This gas mixture resulted in the formation of other sulfurous–carbonaceous compounds (COS and CS2) due to the presence of carbon monoxide. The results showed that the rate of COS formation increased with oxygen enrichment due to the availability of higher amounts of CO while that of CS2 reduced. The global reactions responsible for this observed phenomenon are presented. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction Hydrogen sulfide is amongst the major common contaminant that is known to be present in almost all crude natural gas wells. Utilization of crude natural gas in any chemical to thermal energy transformation process, e.g. combustion, results in the formation of acid gases, such as, SO2 and SO3 which are subsequently transformed to sulfurous and sulfuric acid, if released to the atmosphere. Moreover, the presence of these gases lowers the dew point of the exhaust combustion gases from the furnace so that these gases must be released at higher temperatures to prevent the combustion equipment from corrosion. Consequently, hydrogen sulfide must be removed from natural gas prior to its utilization. Amine extraction process [1,2] is commonly used for the ⇑ Corresponding author. Tel.: +1 301 405 5276. 1

E-mail address: [email protected] (A.K. Gupta). The Petroleum Institute, Abu Dhabi, UAE.

removal of acidic gases, mainly H2S and CO2, from crude natural gas wherein alkaline-based organic compounds are used to absorb H2S from the fuel stream. Even though H2S concentration is very low in the gas stream, it is crucial that hydrogen sulfide undergoes treatment process to hinder its hazardous effects on human health, environment and industry and also preserve the aesthetics of buildings and structures. Claus process [3–5] is a commonly used process for hydrogen sulfide treatment wherein reaction between H2S and O2 occurs under fuel-rich conditions (at U = 3) to form elemental sulfur. In this chemical reaction one third of H2S is burned with oxygen (air) to form SO2. The reaction continues between SO2 and the unburned H2S to form elemental sulfur, which is then captured in liquid or solid form. In practice, the thermal process is divided into two main stages of thermal stage and catalytic stage. Both stages in the reactor have same chemical reactions. The use of catalyst enhances the reaction rate at low temperatures, which subsequently reduces the H2S concentration in the stack gases to very low concentration.

0306-2619/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.03.026

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Several researchers have investigated the flame structure of H2S/air under Claus conditions, both numerically and experimentally. Frenklach et al. [6] studied numerically and experimentally the ignition delay of hydrogen sulfide oxidation using a reflected shock wave tube. Their numerical modeling was carried out using an adopted reaction mechanism that consisted of 57 reactions. Bernez-Cambot et al. [7] investigated experimentally the flame structure of H2S/air diffusion flame under Claus conditions. In their results, they divided the flame into three distinct zones. First zone consisted of thermal and chemical decomposition of H2S wherein hydrogen was the major product. Second zone was the oxidation of both H2S and H2 formed in the first zone. Third zone incorporated partial consumption of hydrogen and to a lesser extent sulfur diffusing from the flame. Muller III et al. [8] studied sulfur chemistry in fuel-rich H2/O2/N2 flames with 0.25%, 0.5% and 1% of H2S in the mixture. They measured concentrations of SH, S2, SO, SO2, and OH using quantitative laser fluorescence measurements. With the help of the aforementioned radical measurements they were able to provide kinetics parameters for various possible intermediate chemical reactions of sulfur compounds. Azatyan et al. [9] examined the behavior of hydrogen sulfide, carbon disulfide, and carbonyl sulfide combustion at low-pressure using electron spin resonance technique along with gas chromatography. Their results revealed that first stage of H2S formation includes the formation of H2, SO2 and SO and they suggested that the presence of H2S is considered inhibitor of H2 oxidation. The second stage includes hydrogen oxidation coupled with the formation of hydroxyl group radical. These results agree with the findings of Bernez-Cambot et al. [7], but the major contradiction was the lack of OH presence in the first stage of the reaction. Selim and co-workers investigated the chemical kinetics of hydrogen sulfide combustion under wide range of experimental conditions. In one of their papers [10] they investigated hydrogen sulfide combustion in methane/air flame under different equivalence ratios. The results revealed that the presence of oxygen H2S combustion tends to form SO2. However, in case of oxygen depletion the net reaction tends to form sulfur instead of sulfur dioxide. In another paper by Selim et al. [11] they studied experimentally the effect of acid gas composition on sulfur recovery. Their study

showed that the presence of carbon dioxide lessens the probability of sulfur recovery where most of H2S is transformed to SO2. On the other hand, nitrogen did not impose the same effect wherein it behaved mainly as an inert medium. Moreover, they examined the combustion of different acid gas compositions (H2S and CO2) in H2/air flame [12]. They found that hydrogen sulfide prohibits H2 oxidation in the first reaction zone. In addition, they showed that presence of CO2 enhances the oxidizing medium of the reaction pool as well as the formation of carbonaceous–sulfurous compounds such as COS and CS2. Formation of such compounds deteriorates the performance of Claus process. Oxygen enrichment has been commonly used in the Claus process [13,14] in order to destruct several other the undesired compounds and contaminates present in natural gas wells, e.g., mercaptans, benzene, toluene and xylene (BTX) [15]. In this paper we investigate the effect of oxygen enrichment on acid gas (mixture of H2S and CO2) combustion in hydrogen/air flames under Claus conditions on the formation of major combustion products. 2. Experimental facility A schematic diagram of the experimental setup is shown in Fig. 1. The facility consisted of a quartz tube reactor of 19 cm length and 4 cm diameter. A double concentric tubular burner was used for all the experiments wherein oxygen was premixed with nitrogen (in desired mixture composition) and injected into the annulus of the burner. Hydrogen, hydrogen sulfide and carbon dioxide were premixed and injected into the central tube of the burner. Bluff body was used to stabilize the flame immediately downstream of the burner exit. Gases from the burner were allowed to flow into the subsequent quartz tube reactor. Sonicthroat quartz sampling probe was used for gas sampling. Throat diameter of the sampling probe was of the order of microns so that the flow is choked at its throat. The rapid expansion of the gases after the throat section quenches the gases to freeze the gas composition. Computer controlled traversing mechanism was used to move the sampling probe axially at any desired position along the reactor. The inner and outer diameters of the sampling probe were 3 and 4 mm, respectively. A suction pump was connected

Computer controller traverse mechanism

Gas sample to gas chromatograph

8.94

Sonic-throat sampling probe

3.58 H2 + H2S + CO2

9.94

Quartz tube reactor

Burner

Dimensions are in millimeters

Air

Fig. 1. A schematic diagram of the experimental setup.

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2000 1800

Temperature (K)

to the sampling line to introduce the sampled gas to a gas chromatograph (GC) for online gas analysis. Facility also existed for storing the gas samples in bottles in case of less sampling time between samples was required. The sampled gas was split inside the GC into two streams. First stream was injected into the thermal conductivity detector, which was responsible for the detection and analysis of carbon monoxide and hydrogen. Second stream was injected into flame photometric detector, which was responsible for gas sampling of stable sulfur compounds (H2S, SO2, COS, and CS2). Mean temperatures were measured using K-type thermocouple using a traverse mechanism and online data acquisition system. The whole experimental setup was placed inside a closed environment of fume hood. The fume hood was connected to an exhaust duct wherein an internal blower fan was used to induce air into the fume hood for safety purposes.

1600 1400 1200 1000

H2S in 69.3% Oxygen Enriched Flame

800 600

H2S/CO2 in 69.3% Oxygen Enriched Flame 0

5

10

15

20

25

30

35

40

45

50

W Fig. 2. Temperature profile of H2/O2–N2 flame with 100% H2S and 50% H2S/50% CO2 addition at U = 3.0.

3. Experimental conditions

4. Results and discussion Temperature profile along the reactor centerline is presented first. Secondly, species distribution along the centerline of the reactor is presented in case of 100% hydrogen sulfide acid gas addition. Finally, products analysis along reactor centerline for the 50% H2S/ 50% CO2 acid gas is addressed. Dimensionless axial distance (W) was used for all the results presented here. Inner jet diameter of the burner was used to transform the linear distances into dimensionless parameters, i.e., W = axial distance/Djet. 4.1. Mean temperatures Mean temperature of the reactor was measured along the reactor centerline using K-type thermocouple. A traverse mechanism was used to move the thermocouple incrementally along the reac-

1800 1600

Temperature (K)

Experiments were conducted to examine the effects of oxygen enrichment of air on acid gas (H2S and CO2) addition in H2/O2–N2 flames under Claus condition. Table 1 depicts the test matrix for all test conditions reported in this paper. Baseline case was investigated first (H2/O2–N2 flame). Effect of acid gas was examined for two cases. The first case represented acid gas consisting of 100% hydrogen sulfide. The second case represented (50% CO2 and 50% H2S) acid gas stream. In order to achieve the required experimental conditions H2/O2–N2 mixture was combusted under slightly fuellean conditions. According to the flow rate of the excess oxygen, hydrogen sulfide and carbon dioxide were injected to achieve the targeted equivalence ratio (U = 3.0) for the Claus condition for all the experiments reported here. The equivalence ratio was calculated based on the combustion of hydrogen sulfide. It was assumed that the addition of CO2 did not impact the equivalence ratio of the resulting flame. Gas sampling and analysis of local species were conducted along the longitudinal axis of the reactor. Table 1 presents the experimental test matrix.

1400 1200 1000 H2S in 0% Oxygen Enriched Flame 800

H2S/CO2 in 0% Oxygen Enriched Flame

600 0

5

10

15

20

25

30

35

40

45

50

W Fig. 3. Temperature profile of H2/O2–N2 flame with 100% H2S and 50% H2S/50% CO2addition at U = 3.0.

tor centerline. Two cases were considered; 0% and 69.3% oxygen enrichment in the air. Fig. 2 shows temperature profile for 0% oxygen enrichment in H2/O2–N2 flame with 100%H2S and 50% H2S/50% CO2 acid gas addition to the burner. Fig. 3 shows temperature profile for 69.3% oxygen enriched H2/O2–N2 flame with 100% H2S and 50% H2S/50% CO2 acid gas case. Similar trends were observed in both cases as evident in both figures with major difference being the magnitude of temperatures. Under all conditions, temperature near the burner exit was found to be lower than that in the further downstream flame zone. This is attributed to the fact that hydrogen is injected into the central tube at room temperature. Decrease in temperature beyond W  15 is attributed to the heat loss to reactor walls. Addition of carbon dioxide caused flame temperature to decrease immediately upon injection. This is attributed to injection of relatively cold flow of carbon dioxide. Further downstream the reactor, temperature increase was observed, possibly

Table 1 The test matrix. Flow rate (cm3/min) O2 Enrichment in air (%)

H2

O2

N2

H2S

CO2

100% H2S acid gas

0 19.5 69.3

2000 2000 1600

1165 1165 932

4380 3476 1690

330 330 264

0 0 0

50% H2S and 50% CO2 acid gas

0 19.5 69.3

2000 2000 1600

1165 1165 932

4380 3476 1690

330 330 264

330 330 264

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due to the fact that carbon dioxide help assist in enhancement of the oxidizing medium.

Addition of 100% H2S acid gas resulted in the formation of several sulfurous compounds. Fig. 4 shows H2S mole fraction along the reactor centerline at different percentages of oxygen enrichment in the air. Results showed that rate of decay of H2S increases with the percentage increase of oxygen enrichment in air. This is attributed to the increase in temperature. Reactions (1)–(5) describe H2S oxidation [9,10].

H2 S þ M () S þ H2 þ M

ð1Þ

H2 S þ H () SH þ H2

ð2Þ

H2 S þ S () 2SH

ð3Þ

SH þ S () S2 þ HÞ

ð4Þ

SH þ H () S þ H2

ð5Þ

Fig. 5 shows the evolution of sulfur dioxide along centerline of the reactor for the different percentage of oxygen enriched air examined here. The results showed that SO2 increases monotonically until it reaches an asymptotic value. An increase in oxygen enrichment in air increases the rate of sulfur dioxide formation and the magnitude of asymptotic value. Asymptotic value of SO2 increases because of the higher rate of H2S oxidation at higher temperatures. However, reaction between H2S and SO2 to form sulfur was not observed to occur. This is due to the tendency of H2S to form

H2S Mole fraction (%)

16 14

69.3% oxygen enrichment in air

12

19.5% Oxygen enrichment in air

10

0% Oxygen enrichment in air

8 6 4 2 0

0

5

10

15

20

25

30

35

40

45

50

W Fig. 4. Hydrogen sulfide mole fraction. Flame conditions: H2/O2–N2 with 100% H2S addition at U = 3.0.

H2 Mole fraction (%)

4.2. Case 1 addition of 100% H2S acid gas

100

0% oxygen enrichment in air

40 20

0

5

10

15

20

25

30

35

40

45

50

W Fig. 6. Hydrogen mole fraction. Flame conditions: H2/O2–N2 with 100% H2S addition at U = 3.0.

SO2 in presence of oxygen. Fig. 6 depicts hydrogen mole fraction under the investigated conditions. Hydrogen mole fraction decreases monotonically until it reaches an asymptotic value. Mole fraction of H2 does not reach zero value due to several reasons. Firstly, the fuel-rich mixture makes it difficult for all reactants to be completely oxidized. Secondly, hydrogen sulfide combustion produces an additional amount of hydrogen as shown in reactions (2) and (5). Moreover, the rate of hydrogen oxidation is slightly higher in case of 0% oxygen enrichment as compared to 19.5% and 69.3% cases. This supports the hypothesis that additional amount of hydrogen is formed from hydrogen sulfide oxidation which could be significant at higher temperatures. And lastly, hydrogen sulfide is considered an inhibitor for hydrogen oxidation [9,10]. 4.3. Case 2 addition of 50% H2S/50% CO2 acid gas Figs. 7–9 describe the mole fractions of hydrogen sulfide, sulfur dioxide, and hydrogen, respectively, along the centerline of the reactor at different percentages of oxygen enrichment. Similar trends were observed for case 2. However, the rate of decay in case of hydrogen sulfide and hydrogen were faster in case 2 as compared to case 1. Similarly, rate of sulfur dioxide production was slightly faster in case 2 as compared to case 1. The faster rates of production/decomposition are attributed to the fact that carbon dioxide releases oxidizer into the reaction pool. Fig. 10 depicts mole fraction of carbon monoxide along the reactor centerline. Presence of carbon dioxide triggered the formation of carbon monoxide. Mole fraction of carbon monoxide increases at higher percentage of oxygen enrichment in air because of higher temperature. Formation of carbon monoxide can occur through CO2 thermal or chemical decomposition, reactions (6) and (7) [11,16], respectively.

14

H2S Mole fraction (%)

SO2 Mole Fraction (%)

19.5% oxygen enrichment in air

60

0

0.8 0.7 0.6 0.5 0.4

69.3% Oxygen enrichment in air

0.3 19.5% oxygen enrichment in air

0.2

0% oxygen enrichment in air

0.1 0

69.3% oxygen enrichment in air

80

12 69.3% oxygen enrichment

10

5

10

15

20

25

30

35

40

45

50

W Fig. 5. Sulfur dioxide mole fraction. Flame conditions: H2/O2–N2 with 100% H2S addition at U = 3.0.

0% oxygen enrichment

6 4 2 0

0

19.3% oxygen enrichment

8

0

5

10

15

20

25

30

35

40

45

50

W Fig. 7. Hydrogen sulfide mole fraction. Flame conditions: H2/O2–N2 with 50% H2S/ 50% CO2 acid gas at U = 3.0.

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0.035

CS2 Mole Fraction

SO2 mole Fraction (%)

1 0.8 0.6 0.4

69.3% oxygen enrichment in air 19.5% oxygen enrichment in air

0.2 0

0% oxygen enrichment in air 0

5

10

15

20

25

30

35

40

0.03 0.025 0.02 0.015 0.01

69.3% oxygen enrichment in air 19.5% oxygen enrichment in air 0% oxygen enrichment in air

0.005 45

0

50

0

5

10

15

20

W

H2 mole fraction (%)

80 69.3% oxygen enrichment in air

35

40

45

50

Fig. 11. Carbondisulfide mole fraction. Flame conditions: H2/O2–N2 with 50% H2S/ 50% CO2 acid gas at U = 3.0.

SO2 þ CO () CO2 þ SO

ð8Þ

SO þ CO () CO2 þ S

ð9Þ

19.5% oxygen enrichment in air 0% oxygen enrichment in air

CO þ H2 O () CO2 þ H2

ð10Þ

CO þ O2 () CO2 þ O

ð11Þ

40

20

0

0

5

10

15

20

25

30

35

40

45

Fig. 11 shows the behavior of carbon disulfide mole fraction under the investigated conditions. Formation of carbon disulfide is dependent on the presence of carbon monoxide in the reaction pool as shown in reactions (12)–(17) [16–21].

50

W Fig. 9. Hydrogen mole fraction. Flame conditions: H2/O2–N2 with 50% H2S/50% CO2 acid gas at U = 3.0.

2.5

CO Mole Fraction (%)

30

W

Fig. 8. Sulfurdioxide mole fraction. Flame conditions: H2/O2–N2 with 50% H2S/50% CO2 acid gas at U = 3.0.

60

25

69.3% oxygen enrichment in air 19.5% oxygen enrichment in air 0% oxygen enrichment in air

2 1.5

0.5

0

5

10

15

20

25

30

35

40

45

ð12Þ

CO þ SO () CS þ O2

ð13Þ

CO þ S2 () CS2 þ O

ð14Þ

CS þ SO () CS2 þ O

ð15Þ

CS þ S2 () CS2 þ S

ð16Þ

CS þ SO2 () CS2 þ O2

ð17Þ

Fig. 12 shows the distribution of carbonyl sulfide mole fraction along reactor centerline at different percentages of oxygen enrichment. Reactions (18)–(21) depict the possible channels for COS formation [18,19]. However, increase in mole fraction of COS with oxygen enrichment in air is attributed to the higher amounts of CO present and higher reactor temperature. Moreover, release of oxygen, from increased activity of CO2, at higher temperature substantiates the role of reactions (19)–(21).

1

0

CO þ S () CS þ O

50

Axial Distance (Inches) Fig. 10. Carbon monoxide mole fraction. Flame conditions: H2/O2–N2 with 50% H2S/ 50% CO2 acid gas at U = 3.0.

ð6Þ

CO2 þ H () CO þ OH

ð7Þ

At 0% oxygen enrichment in air, reaction (7) is expected to be more dominant since CO2 thermal oxidation occurs at significantly high temperature. Moreover, the significant dominance of hydrogen radical in H2/O2–N2 combustion enhances the rate of reaction (7). However, at higher percentage of oxygen enrichment, contribution from reaction (6) is expected to be significant due to higher temperatures. This is evident in the noticeable increase of carbon monoxide mole fraction with the increase in percentage of oxygen enrichment. On the other hand, carbon monoxide mole fraction decreases downstream due to the possible recombination according to the following reactions (8)–(11) [11,16,17].

COS Mole Fraction (%)

0.025

CO2 þ M () CO þ O þ M

0.02 0.015 0.01 69.3% oxygen enrichment in air 19.5% oxygen enrichment in air 0% oxygen enrichment in air

0.005 0 0

5

10

15

20

25

30

35

40

45

50

W Fig. 12. Carbonylsulfide mole fraction. Flame conditions: H2/O2–N2 with 50% H2S/ 50% CO2 acid gas at U = 3.0.

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CO þ SO () COS þ O

ð18Þ

References

CO þ S () COS

ð19Þ

CS þ O2 () COS þ O

ð20Þ

CS2 þ O () COS þ S

ð21Þ

[1] Jensen AB, Webb C. Treatment of H2S-containing gases: a review of microbiological Alternatives. Enzyme Microb Technol 1995;17(1):2–10. [2] Selim H, Gupta AK, Sassi M. Acid gas composition effects on the reactor temperature in claus reactor. In: 6th AIAA international energy conversion engineering conference (IECEC), Cleveland, OH; July 28–30; 2008 [AIAA 2008– 5797]. [3] Larraz R. Influence of fractal pore structure in claus catalyst performance. Chem Eng J 2002;86(3):309–17. [4] Mora RL. Sulphur condensation influence in claus catalyst performance. J Hazard Mater 2000;79(1–2):103–15. [5] El-Bishtawi R, Haimour N. Claus recycle with double combustion process. Fuel Process Technol 2004;86(3):245–60. [6] Frenklach M, Lee JH, White JN, Gardnier JR, WC. Oxidation of hydrogen sulfide. Combust Flame 1981;41:1–16. [7] Bernez-Cambot J, Vovelle C, Delbourgo R. Flame structure of H2S–air diffusion flame. In: 18th Symposium (International) on combustion, The combustion institute; 1981. p. 777–83. [8] Muller III CH, Schofield K, Steinberg M, Brodia HP. Sulfur chemistry in flames. In: 17th Symposium (International) on combustion, The combustion institute; 1979. p. 867–79. [9] Azatyan VV, Gershenson UM, Sarkissyan EN, Sachyan GA, Nalbandyan AB. Investigation of low-pressure flames of a number of compounds containing sulfur by the ESR method. In: 12th Symposium (International) on combustion, The combustion institute; 1969. p. 989–94. [10] Selim H, Shoaibi A Al, Gupta AK. Effect of H2S in methane/air flames on sulfur chemistry and products speciation. J Appl Energy 2011;88(8):2593–600. [11] Selim H, Shoaibi A Al, Gupta AK. Effect of CO2 and N2 concentration in acid gas stream on H2S combustion. J Appl Energy 2012;98:53–8. [12] Selim H, Ibrahim S, Al Shoaibi A, Gupta AK. Investigation of sulfur chemistry with acid gas addition in hydrogen/air flames. The Journal of Applied Energy 2012. [13] McIntyre G, Lyddon L. Claus sulphur recovery options bryan research and engineering, Inc., petroleum technology quarterly. Springer; 1997. p. 57–61. [14] Rameshni M. How to process high-ammonia acid gas in a sulphur recovery unit. Worley Parsonsm resources & energy, Monrovia, CA. [15] Sharipov AKh. Mercaptans from gas condensates and crude oils. Chem Technol Fuel Oils 2002;38(4):280–5. [16] Park J, Lee K, Lee E. Effect of CO2 addition on flame structure in counterflow diffusion flame of H2/CO2/N2 fuel. Int J Energy Res 2001;25(6):469–85. [17] Wooldridge MS, Hanson RK, Bowman CT. A shock tube study of the CO + OH = CO2 + H reaction. Int J Chem Kinetics 1994;26(4):389–401. [18] Murakami Y, Kosugi M, Susa K, Kobayashi T, Fujii N. Kinetics and mechanism for the oxidation of CS2 and COS at high temperature. Bull Chem Soc Japan 2001;74(7):1233–40. [19] Cullis CF, Mulcahy MFR. The Kinetics of Combustion of gaseous sulphur compounds. Combust Flame 1972;18:225–92. [20] Hawboldt KA, Monnery WD, Svrcek WY. New experimental data and kinetic rate expression for H2S pyrolysis and Re-association. Chem Energy Sci 2000;55(5):957–66. [21] Khudenko BM, Gitman GM, Wechsler EP. Oxygen based claus process for recovery of sulfur from H2S gases. J Environ Eng 1993;119(6):1233–51.

The above hypothesis is supported by the fact that CS2 mole fraction decreased with oxygen enrichment in air. 5. Conclusions Acid gas (H2S and CO2) combustion was examined in H2/O2–N2 flame at different percentages of oxygen enrichment to the combustion air. Acid gases of 100% H2S and 50% H2S/50% CO2 composition are presented here to provide the role of CO2 in the acid gas stream. Three different percentages of oxygen enrichment of air are reported (0%, 19.5% and 69.3%). The rate of oxidation of hydrogen was found to be faster in case of 50% H2S/50% CO2 than 100% H2S due to higher amounts of oxidizer released from the carbon dioxide present in the reaction pool. Oxygen enrichment in air decreased the rate of hydrogen oxidation. Hydrogen sulfide reacted with oxygen to form SO2 rather than more favorable S2. This supports the increase in SO2mole fraction until it reached to an asymptotic value. Increase in oxygen enrichment in air increased the rate of SO2 production. Same trends were observed on H2S, SO2, and H2 with the addition of 50% H2S/50% CO2 acid gas. However, the rates of reactants decomposition and products formation were found to be faster as compared to the 100% H2S acid gas case. Carbon dioxide proved to contribute as an oxidizer provider into the reaction pool, in particular at higher temperatures. Presence of carbon monoxide also triggered the formation of other sulfurous–carbonaceous compounds, such as COS and CS2. With oxygen enrichment, rate of COS production increased while that of CS2decreased. This is attributed to increase in the mole fraction of CO and amount of oxygen released into the reaction pool. Acknowledgments The authors gratefully acknowledge the research support provided by The Petroleum Institute, ADNOC, and GASCO, Abu Dhabi, UAE.