A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 4

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A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation Tay Yu Cong a,b, Abhijeet Raj a,*, Jirawan Chanaphet a,c, Shabin Mohammed a, Salisu Ibrahim a, Ahmed Al Shoaibi a a

Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, United Arab Emirates Department of Chemical Engineering, Universiti Teknologi Petronas, Perak, Malaysia c Department of Chemical Engineering, King Mongkut's University of Technology Thonburi, Thailand b

article info

abstract

Article history:

Hydrogen production from the thermolysis of undesirable hydrogen sulphide (H2S) rep-

Received 27 December 2015

resents a viable means of extracting energy from it while enhancing the efficiency of Claus

Received in revised form

process, which is a widely used technology for sulphur recovery from H2S laden streams of

5 March 2016

oil and gas industries. This paper examines the thermolysis of H2S for direct hydrogen and

Accepted 9 March 2016

sulphur production. A detailed reaction mechanism is proposed that captures the chem-

Available online xxx

istry involved in its high temperature decomposition. The simulation results obtained using the proposed mechanism is compared with a wide range of experimental data from

Keywords:

plug flow and stirred reactors, premixed laminar flames, and shock tubes, and a satisfac-

H2S

tory agreement between them is found. Significant improvements in model predictions are

Thermolysis

obtained with the proposed mechanism when compared to previously published mecha-

Reaction mechanism

nism. After its validation, the mechanism is then used to investigate the major reactions

H2 yield

involved in hydrogen production. It is shown through simulations under adiabatic condi-

Synergistic effect

tions that the addition of small amount of oxygen in the inlet H2S gas stream exhibits a synergistic effect in H2 yield, and can significantly enhance the production of hydrogen. The results reported herein provide design guidelines and viable means of seeking cost effective methods of hydrogen production from industrial waste streams containing H2S. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Hydrogen sulphide (H2S) is often produced as a by-product from various chemical processes such as amine extraction [1], hydrodesulphurisation of hydrocarbons [2], upgrading of heavy oils [3], bitumen [4], and other available methods for oil

and natural gas desulphurization [5]. It is highly toxic and corrosive [6,7], due to which it cannot be emitted to the environment in high concentrations. In chemical processes involving noble metal catalysts, the presence of H2S in the gas stream leads to catalyst poisoning [8]. Though it is a flammable gas, its combustion products are also harmful for living

* Corresponding author. Tel.: þ971 2 6075738. E-mail address: [email protected] (A. Raj). http://dx.doi.org/10.1016/j.ijhydene.2016.03.053 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Cong TY, et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.053

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beings, environment, and industrial equipment [9]. For example, H2S combustion results in SO2 production that is very corrosive, and causes acid rain [10,11]. Due to these harmful effects of H2S, the environmental enforcement agencies worldwide have imposed stringent emission regulations on H2S and other sulphurous compounds [12]. This makes it imperative to reduce the sulphur content of oil and gas streams to acceptable levels before further processing. The most common process used for H2S removal from natural gas is amine extraction [13e19]. The by-product of amine extraction process, consisting of H2S, CO2, mercaptans and aromatic compounds, is further processed to hinder its health and environmental hazards. Claus process is a widely used technology to process gas streams containing H2S to recover Sulphur [20e25]. In this process, H2S undergoes partial oxidation in air in the thermal reactor to form SO2 and H2O [5]. Elemental sulphur is produced by the pyrolysis of H2S and by the reaction between unreacted H2S with SO2. Thereafter, the unreacted H2S reacts with SO2 in the presence of catalyst to produce more sulphur [24,26e29]. The global reactions involved in this process are shown below as (R1)e(R3) [30,31]: H2 S þ 1:5O2 /SO2 þ H2 O

(R1)

2H2 S þ SO2 /3S þ 2H2 O

(R2)

Combining the two reactions: 2H2 S þ O2 /2S þ 2H2 O

(R3)

Though sulphur produced from Claus process is a marketable product, it does not allow full exploitation of H2S as its hydrogen content is wasted as low grade steam [32e34]. As the energy demand increases worldwide, an efficient utilization of available natural resources is needed [35]. Therefore, the direct decomposition of H2S is desired so as to obtain not only sulphur, but also valuable hydrogen, which is a desired energy source because of its promising characteristics such as cleanliness [36,37] and highest energy content and energy conversion efficiency [38]. It is also used in hydrotreating processes such as hydro-isomerisation to convert n-paraffins into iso-paraffins, and de-aromatisation to hydrogenate aromatics into cyclo-alkanes [39,40]. Various decomposition methods of H2S have been suggested in the literature, including thermochemical, electrochemical, photochemical and plasma methods [33,34,41e48]. Apparently, most of the hydrogen production processes from H2S have not been realized in large scale due to economic or technical feasibility [33]. In general, the simplest and direct method that can be used is catalytic or non-catalytic thermal decomposition, following the reaction given below [33,49]: 2H2 S/2H2 þ S2

(R4)

In this method, sufficient heat is supplied to break down H2S into H2 and S2. The reaction is highly endothermic and, therefore, more favourable at high temperatures. Galuszka et al. [34] predicted a conversion of 20% at 1273 K and 38% at 1473 K, based on thermodynamic equilibrium calculations. They reported that temperatures above 1648 K are required to obtain H2S conversion above 50%. During this process, the

product gases (H2 and S2) must be passed through quenching zone to prevent the recombination reactions. The requirement of high temperatures for non-catalytic decomposition of H2S has motivated several experimental studies in the search of catalysts for its low-temperature decomposition with high H2 yield. In Refs. [50e53], the use of MoS2 was suggested to achieve about 95% conversion of H2S to H2. In Ref. [54], metal catalysts were proposed for the lowtemperature decomposition of H2S, where about 15% conversion of H2S to H2 and S2 was achieved at room temperature. The conversion, however, decreased with increasing temperature. The use of metal oxides as catalysts was suggested in Ref. [55], where H2S decomposition was studied in the temperature range of 500e900  C. The production of H2 through the partial oxidation of H2S over alumina catalyst was studied by Clark et al. [56], where H2S conversion of 64.6% was achieved at 400  C, and less than 0.5% of H2S was converted to SO2. In Ref. [57], the use of cobalt sulphide as a catalyst for H2S decomposition was demonstrated through a kinetics study. A recent study [58] discusses the application of perovskite catalysts for H2S decomposition, where about 15% of H2S was shown to decompose at 800  C. The thermal decomposition often also includes catalytic H2S oxidation process [59,60]. For instance, sulphur is formed from H2S oxidation in the presence of stoichiometric amount of oxygen over the TiO2 catalyst in the mobile direct oxidation process [61,62]. Kalinkin et al. investigated the kinetics of H2S oxidation with V2O5 as catalyst at 423e523 K with initial H2S concentrations up to 3 vol% and O2:H2S ratio above 4 [63]. Zhou et al. also studied the oxidation of H2S in a flow reactor under atmospheric condition at a temperature range of 950e1150 K in fuel-lean conditions, catalysed by silica surface although the silica's catalytic effect is suppressed by a coating of B2O3 [64]. There is still a need to explore efficient means of enhancing H2 production from H2S thermal (non-catalytic) decomposition due to lower capital cost requirements as compared to catalytic methods. This requires a reliable kinetic model based on a detailed reaction mechanism that can be used to seek optimum reactor conditions to enhance H2 production from H2S with minimal environmental burden. Several studies have examined the thermal decomposition mechanism of H2S, and proposed global reaction rate expressions for it. Monnery et al. [30] experimentally investigated the pyrolysis of H2S, including the reactions of H2 and S2 re-association. It was observed that the decomposition of H2S was insignificant at temperatures below 1273 K, while at temperatures above 1273 K, the conversion rate reached 68%. It was conjectured that temperatures greater than 1323 K and residence time greater than 0.5 s are favourable operating conditions for H2S thermolysis. A new rate of reaction was proposed to predict H2S cracking and re-association as shown below: 45:0 RT

r ¼ 5260e

PH2 S P0:5 S2  14e

23:6 RT

PH2 S PS2

Kaloidaset al. [65] studied the kinetics of thermal decomposition of H2S in a non-isothermal flow reactor over a temperature range of 873e1133 K and pressures of 1.3e3 atm with specific flow rates of 3.4e3.6 mol/m2s. Their results showed

Please cite this article in press as: Cong TY, et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.053

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 4

that the conversion of H2S decreases with increasing molar flow rate. A reaction mechanism, as shown below, was also developed to explain the experimental observations and trends. H2 S/SH þ H

(R5)

H2 S þ H/H2 þ SH

(R6)

SH þ SH/H2 S þ S

(R7)

S þ S/S2

(R8)

Karan et al. [66] studied the reaction kinetics for thermal decomposition of H2S using a quartz tubular reactor at different temperatures and initial H2S concentrations. The results showed that, below 1473 K, the decomposition is highly dependent on temperature, while H2S conversions are close to equilibrium above 1473 K, even within a short residence time of 200 ms. The data also showed that H2S conversion was independent of the change in the initial concentration of H2S for all the studied cases. Through regression analysis, the decomposition of H2S was found to be first-order with respect to H2S concentration with a rate constant of k ¼ ð1:68±0:86Þ  1011 expðð28940±840Þ=TÞ. However, the above rate expression did not fit experimental data under a wide temperature range. Therefore, they proposed another rate expression, k ¼ ð1:12±0:11Þ  1011 expðð28360±200Þ=TÞ that matched the experimental data under low and high temperature condition (1073e3373 K). Adesina et al. [41] examined H2S decomposition over a temperature range of 1040e1084 K in a tubular flow reactor, and also found the overall reaction rate to be first-order with respect to H2S partial pressures. This agrees with the conclusions of Reymont [67,68]. However, Darwent and Roberts [69] conducted kinetic experiments in static reactor within a temperature range of 773e923 K, and reported a second-order kinetics with respect to H2S concentration for its pyrolysis. Tesner et al. [69,70] assumed that H2S decomposition was first-order with respect to H2S concentration, and interpreted their experimental data based on this assumption, even though they could not to verify this assumption through a systematic study. It is important to note that Darwent and Roberts measured the rate of H2 formation, while Tesner et al. measured the rate of S2 formation. The H2S concentrations were not directly measured in either of these studies, even though the rate constant for the H2S decomposition reaction was reported. As evident, these studies have failed to provide unified description of the reaction mechanism of H2S thermal decomposition. In the light of these conflicting results on rate expressions and reaction order, a detailed reaction mechanism for H2S pyrolysis was proposed by Binoist and co-workers [71]. They also conducted kinetic experiments at temperatures of 1073e1323 K and residence times of 0.4e1.6 s in a perfectly mixed quartz reactor. They proposed a detailed (22 reactions) and a reduced (9 reactions) reaction mechanism, whose rate constants were obtained by fitting the simulation results through their experimental data. Their kinetic model did not match the experimental data very well at high temperatures

3

(above 1223 K). Manneti et al. [32] then proposed a revised mechanism of 20 reactions for H2S pyrolysis. It showed 10e20% improvement over Binoist et al.'s mechanism [71], depending on the operating conditions when compared quantitatively. Despite this improvement, the revised mechanism predicted poorly the experimental data at temperatures above 1223 K. Moreover, both Binoist and Manneti did not validate their mechanism over different experimental conditions. Therefore, modifications to the existing kinetic models are required to improve model predictions under high temperature conditions such as those encountered in Claus process. It is also desirable to validate kinetic mechanisms with experimental data obtained under different sets of conditions and reactor types for its reliability. The objective of this paper is to develop a detailed and reliable reaction mechanism for the high temperature pyrolysis of H2S to form H2 and S2 under different operating conditions. The developed mechanism is then validated using a wide range of experimental data available in the literature. The effect of oxygen addition to H2S feed on the production of H2 is also examined through the profiles of important species involved in H2S thermolysis under different reactor operating conditions.

Reaction mechanism development In order to develop a complete and detailed mechanism for the decomposition of H2S, the elementary reactions involved in it were derived from recent works [32,64,72e74]. This section provides the details on the base mechanisms that were selected from the literature for mechanism development. The mechanism is categorized according to the species, H2S, H, H2, S, S2, SH, HSS, HSSH, HS2, H2S2. The reactions of HSS and HSSH are derived from Refs. [32,72] with an exception of the reaction, HSSH þ M ¼ 2SH þ M, which is extracted from the work of Zhou [64]. Some reactions take place through their collision with a molecule present in the gas phase. The term “M” represents any molecule present in the gas phase. In the rate expressions, the mixture concentration is used to represent its concentration [75]. The reactions involving HS2 and H2S2 are adopted from Ref. [73]. The thermodynamic properties of the species were adopted from Refs. [74,76]. The resulting reaction mechanism for the thermal decomposition of H2S is presented in Table 1. To investigate the role of O2 in H2 production from H2S, the H2S oxidation reactions were also included. The complete mechanism consists of 432 reactions and 89 species, and is provided in the Supporting Information.

Results and discussion This section presents the validation of the developed reaction mechanism and the effects of O2 addition to H2S feed stream on H2 production. The reaction pathways involved in the thermolysis of H2S in the presence and absence of O2 are also discussed. All the simulations and reaction path analyses reported in this study have been conducted using LOGEsoft software [77]. This software provides a sophisticated numerical tool to simulate zero and one-dimensional reactors

Please cite this article in press as: Cong TY, et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.053

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Table 1 e Rate constants of reactions involved in H2S pyrolysis, expressed in the form of k ¼ ATnexp (¡E/RT). The rate coefficient format is detailed in Ref. [86]. The units are in cal, K, mol, cm and s. The low-pressure limit rate constants and Troe falloff parameters [87] are also provided for some reactions. Reactions H2S ¼ SH þ SH H2S þ M ¼ SH þ H þ M H2S þ M ¼ H2 þ S þ M N2/1.5/SO2/10/H2O/10/ (Enhanced collision efficiencies [86]) H2S þ S ¼ SH þ SH H2S þ S ¼ H2 þ S2 H2S þ S ¼ HS2 þ S2 H þ H þ M ¼ H2 þ M H2 þM ¼ H þ H þ M H2O/12/H2/2.5/Ar/0/He/0/ (Enhanced collision efficiencies [86]) H2 þ Ar ¼ H þ H þ Ar H2 þ He ¼ H þ H þ He S þ H þ M ¼ SH þ M S þ H2 ¼ SH þ H S þ S þ M ¼ S2 þ M S2 þ M ¼ 2S þ M S2 þ H þ M ¼ HSS þ M S2 þ H þ M ¼ HS2 þ M N2/1.5/SO2/10/H2O/10/ (Enhanced collision efficiencies [86]) SH þ S ¼ S2 þ H SH þ SH ¼ S2 þH2 SH þ SH ¼ H2S þ S SH þ SH þ M ¼ HSSH þ M SH þ SH (þM) ¼ HSSH (þM) Low pressure limit rate constant: Troe falloff parameters [86,87]: HSS þ H ¼ SH þ SH HSS þ H ¼ H2 þ S2 HSS þ H ¼ H2S þ S HSS þ S ¼ S2 þ SH HSS þ SH ¼ H2S þ S2 HSS þ HSS ¼ HSSH þ S2 HSSH þ M ¼ SH þ SH þ M HSSH þ M ¼ 2SH þ M HSSH þ H ¼ HSS þ H2 HSSH þ H ¼ H2S þ SH HSSH þ S ¼ HSS þ SH HSSH þ SH ¼ H2S þ HSS HS2 þ H ¼ S2 þ H2 HS2 þ S ¼ S2 þ SH HS2 þ H þ M ¼ H2S2 þ M N2/1.5/S02/10/H2O/10/ (Enhanced collision efficiencies [86]) H2S2 þ H ¼ HS2 þ H2 SH þ NH ¼ SN þ H2 N þ SH ¼ SN þ H N þ SN ¼ N2 þ S S þ NH ¼ SH þ N N2 þ M ¼ N þ N þ M N/5/O/2.2/ (Enhanced collision efficiencies [86])

A

n

E

References

7.63Eþ14 1.76Eþ14 2.00Eþ14

0 0 0

82155 64000 66000

[32,88] [83] [89]

8.30Eþ13 6.02Eþ12 2.00Eþ13 1.87Eþ18 4.58Eþ19

0 0 0 1 1.4

2052.689 4968.03 7400.06 0 104380

[32,72e74,90e92] [73] [73,74,92] [73,90] [64]

5.84Eþ18 5.84Eþ18 6.20Eþ16 1.40Eþ14 1.20Eþ17 4.80Eþ13 1.15Eþ25 1.00Eþ16

1.1 1.1 0.6 0 1 0 2.84 0

104380 104380 0 19275.97 0 77103.87 1665 0

[64] [64] [32,72,90] [32,72e74,90e93] [32,88] [32,72e74,90e92] [72] [73,74,90e92]

3.32Eþ12 3.01Eþ10 1.00Eþ14 8.70Eþ15 3.46Eþ12 2.33Eþ31 1.00Eþ00 9.72Eþ07 4.19Eþ08 4.41Eþ13 4.17Eþ06 6.27Eþ03 9.56Eþ00 1.40Eþ15 2.31Eþ14 4.99Eþ07 3.66Eþ08 2.85Eþ06 6.40Eþ03 1.20Eþ07 8.30Eþ13 1.00Eþ16

0.5 0 0 0.76 0.2 4.94 254 1.62 1.6 0 2.2 3.05 3.37 1 1 1.93 1.72 2.31 2.98 2.1 0 0

29 0 430 0 1432 1990/ 2373/ 1030 472 6326 600 1105 1672 57030 57030 1408 467 1204 1480 700.33 7352.69 0

[64] [88] [89] [90] [64]

[32,72,90] [64] [32,72,90] [32,72,90] [32,72,90] [32,72,90] [32] [72] [32,72,90] [72] [32,72,90] [32,72,90] [73,74,91,92] [73,74,91,92] [73,74,91,92]

1.20Eþ07 1.00Eþ14 6.31Eþ11 6.30Eþ11 1.00Eþ13 1.00Eþ28

2.1 0 0.5 0.5 0 3.3

715.4 0 8009.56 0 0 225000

[73,74,91,92] [74] [74] [74] [74] [94]

(homogeneous reactors, flames, engine models, and catalytic converters) with detailed reaction mechanism in a reasonable computational time. It also allows post-processing of the simulation results to obtain the reaction pathways that lead to major product formation through the analysis of the rate of production of chemical species.

The H2S conversion and H2 and S2 yields are calculated using the following formula: H2 S conversion ð%Þ ¼

moles of H2 S at inlet  moles of H2 S at outlet moles of H2 S at inlet *100%

Please cite this article in press as: Cong TY, et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.053

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 4

60

(a)

1123K

1213K

1243K

1273K

1323K

H2S Conversion (%)

50

1323 K

40

1273 K

30

1243 K

20

1213 K

10 1123 K

0 0.0

H2 mol fraction (mol%)

(b)

0.3

0.6 Time (s) 0.9

1.2

2.5

1323 K

2.0

1273 K

1.5

1243 K

1.0

1213 K

0.5 1123 K

0.0 0.0 (c)

1.5

0.3

0.6 Time (s) 0.9

1.2

1.5

S2 mol fraction (mol%)

1.4 1.2

1323 K

1.0 1273 K

0.8

1243 K

0.6

1213 K

0.4 0.2

1123 K

0.0 0.0

0.3

0.6 Time (s) 0.9

1.2

1.5

Fig. 1 e Predicted and experimentally observed (a) H2S conversion, (b) H2 mol fraction, and (c) S2 mol fraction for feed containing 5 mol% H2S in Ar at 1 atm pressure and different temperatures. Points are experimental data from Ref. [71], dotted lines represent computed profiles from Binoist et al. model [71], and solid lines are computed profiles using our mechanism.

Perfectly stirred reactor moles of H2 produced *100% H2 yieldð%Þ ¼ moles of H2 S at inlet S2 yieldð%Þ ¼

moles of S2 produced *100% moles of H2 S at inlet

Mechanism validation The experimental data from the literature for different reactor configurations and operating conditions were used to validate the developed mechanism to ensure its reliability.

In Ref. [71], Binoist et al. conducted experiments on the pyrolysis of H2S in isothermal perfectly stirred reactor for residence times of 0.4e1.6 s in the temperature range of 1073e1373 K at 1 atm pressure. The feed stream with a composition of 5 mol% H2S and 95 mol% Ar was used. Fig. 1 provides a comparison of the computed profiles with the experimental observations on H2S conversion, and H2 and S2 yields. The computed profiles using the mechanism proposed by Binoist et al. are also presented in the figure. As evident, the profiles predicted by the proposed mechanism are in reasonable agreement with the experimental data at all the temperatures. The mechanism showed remarkable improvements over the previously published kinetic model by

Please cite this article in press as: Cong TY, et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.053

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 4

Fig. 2 e Reaction pathways for H2 formation from H2S.

Binoist et al. [71]. It is noteworthy that the data from H2S pyrolysis at temperatures above 1273 K could not be adequately captured by the Binoist kinetic mechanism (and the modified version of their mechanism proposed by Manenti et al. [32], which is not shown in this figure), but our proposed kinetic mechanism could satisfactorily capture the complex chemistry involved in the high temperature thermolysis of H2S. With increasing temperature and residence time in the reactor, the conversion of H2S and the yields of H2 and S2 increased due to the endodermic nature of the reactions. At 1323 K, the predicted and observed maximum H2S conversion were around 50%, which amounted to the mole fractions of approximately 2.44 mol% and 1.2 mol% (and yields of 48.8% and 24%) for H2 and S2, respectively. Ideally, for 50% conversion, the yields of H2 and S2 should be 50% and 25%, respectively. The marginally lower yields for H2 and S2 than their ideal values are due to the formation of minor species such as HS2, HSSH and H2S2 from H2S. The reaction path analysis was conducted to find the reactions responsible for the formation of H2 and S2 from H2S that are shown in Figs. 2 and 3. The intermediate species responsible for H2 and S2 formation were SH, HS2, H and S. Out of all the reactions presented in these figures, the most significant reactions involved in the formation and consumption of H2 and S2 are listed below as (R9)e(R14). H2 S þ S4H2 þ S2

(R9)

H2 S þ H4SH þ H2

(R10)

H2 S þ S þ M4HSSH þ M

(R11)

H2 S þ SH4HSSH þ H

(R12)

SH þ SH4H2 S þ S

(R13)

SH þ SH4H2 þ S2

(R14)

The production of H2 mainly occurred by the direct decomposition of H2S through (R9) and (R10), and through the recombination of SH radicals (R14). Sulphur production mainly occurred through the reactions (R13) and (R14). The reactions (R11) and (R12) were responsible for HSSH formation that decomposed to form SH radicals (involved in (R13) and (R14)).

Plug flow reactor In Ref. [78], Hawboldt et al. conducted experiments in an isothermal plug flow reactor under Claus process condition on H2S thermal decomposition. The reactor was operated within a temperature range of 1123e1473 K and residence times of 0.05e1.5 s. The feed stream consisted of 97.5 mol% N2 and 2.5 mol% H2S at 1 atm pressure. Fig. 4 presents the comparison between the model predictions and the experimental data on H2S conversion at different temperatures and residence times.

Please cite this article in press as: Cong TY, et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.053

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 4

Fig. 3 e Reaction pathways for S2 formation from H2S.

Overall, the mechanism predicted the experimental data reasonably well except for the reactor temperature of 1423 K at low residence times. Below 1223 K, the maximum amount of H2S conversion is below 20%, but as the reactor temperature increases to 1423 K, the maximum conversion of H2S increases to nearly 67%. Also, at a high temperature of 1423 K, the model predicts a fast decomposition of H2S, where H2S conversion reaches its steady state value after a residence time of 0.3 s. In another study [30], Monnery et al. reported experimental data from a plug flow reactor under isothermal condition and maintained at 1 atm pressure with feed containing0.3e4 mol% SO2 and 0.25e3 mol% H2S in N2. The H2S/SO2 ratio was kept near 2 for most of the test conditions, the reactor temperature was varied from 850 to 1150 K, and the residence time was maintained between 0.05 and 1.2 s. Fig. 5 compares the

simulation results and the experimental data on H2S conversion for a feed containing 1.5 mol% SO2, 3 mol% H2S and 95.5 mol% N2. The model predictions agree very well with the experimental values for most of the temperatures tested, where the maximum difference between the two is found to be about 10%. The proposed mechanism was also used to predict the experimental data reported by Karan et al. [66], where the thermal decomposition of H2S using a coiled quartz tubular reactor with diameter of 0.005 m and lengths of 3.2 m, 6.4 m and 16 m was studied for different residence times in between 0.2 and 2.0 s under isothermal condition. The feed stream consisted of different concentrations of H2S in diluted nitrogen over a temperature range of 1073e1523 K. The inlet pressure varied from 110 kPa to 165 kPa. The effects of reactor lengths and initial H2S concentration on H2S decomposition

80

Sim 1123K Sim 1223K Sim 1273K Sim 1323K Sim 1423K Exp 1123K Exp 1223K Exp 1273K Exp 1323K Exp 1423K

H2S conversion (%)

70 60 50 40 30 20 10 0 0.00

0.20

0.40

0.60

0.80

1.00

1.20

Time(s) Fig. 4 e Comparison between experimental data (exp) [78] and simulation results (sim) on H2S conversion for a feed containing 2.5 mol% H2S and 97.5 mol% N2 at pressure of 1 atm. Please cite this article in press as: Cong TY, et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.053

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100

850K

H2S Conversion (%)

90

950K

1050K

1150K

80 70 60 50 40 30 20 10 0 0

0.2

0.4

0.6

0.8

Residence time (s)

1

1.2

1.4

Fig. 5 e H2S conversion vs residence time at different temperatures at atmospheric pressure with inlet streams of 1.5% SO2, 3% H2S and remaining N2. Lines are simulation results and points are experimental data from Ref. [30].

100 90 80 70 60 50 40 30 20 10 0

60

H2S conversion (%)

H2S conversion (%)

Sim (3.2m) Exp (3.2m) Sim (6.4m) Exp (6.4m) Sim (16m) Exp (16m)

50 40

Sim 900 C (16m) Exp 900 C (16m)

30

Sim 1000 C (16m) Exp 1000 C (16m)

20 10 0

800

900

1000

1100

1200

Temperature (oC) (a)

0

0.5

1

1.5

2

H2S concentration (mol %) (b)

Fig. 6 e (a) Comparison between experimental data (exp) [66] and the simulation results (sim) on H2S conversion at different reactor lengths (3.2 m, 6.4 m, and 16 m) at temperatures in between 800 and 1200  C and at a pressure of 1 atm with feed containing 1 mol% H2S and 99 mol% N2. (b) Comparison between experimental data [66] and simulation results on the effect of initial H2S concentration (in N2) on H2S conversion at 900 and 1000  C and at a pressure of 1 atm.

were illustrated, as shown in Fig. 6. The model predicted the experimental data satisfactorily, especially for the reactor lengths of 6.4 m and 16 m, though small differences between the simulated and experimental data could be seen for the data from 3.2 m long reactor. Smaller reactors had shorter residence times that resulted in lower H2S conversions for a given temperature. The experimental trend showing nearindependence of H2S conversion on the initial H2S concentration at temperatures of 900 and 1000  C was successfully captured with the mechanism.

Premixed laminar flames Levy et al. [79e81] have conducted several experimental studies on the oxidation of H2S by O2 in premixed laminar flames at atmospheric and sub-atmospheric pressures, and

have reported concentration profiles for several species such as H2S, O2, SO2, H2 and H2O at different heights above the burner. The species profiles were measured at different equivalence ratios using a mass spectrometric-flame sampling technique, while the temperature profiles were measured using thermocouples. To validate the oxidation chemistry of H2S in the proposed mechanism, the experimentally observed species profiles in a premixed laminar H2S flame were compared to the simulated profiles, as shown in Fig. 7. The figure shows a good agreement between the experimental and the predicted profiles. The further validation of the H2S oxidation chemistry is provided in our previous work [82]. Such a validation was necessary to ensure that the role of O2 in the decomposition of H2S to form H2, as demonstrated in the following section, is reliably predicted.

Please cite this article in press as: Cong TY, et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.053

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 4

Fig. 7 e Experimental and computed profiles of major species in a premixed H2S flame [79].

Shock tubes The ignition delay times for H2S have been studied at different temperatures, pressures and H2S concentrations in the literature using shock tubes [83,84] The advantage associated with shock tubes is the possibility of performing high-temperature reaction kinetics experiments without wall surface catalytic effects that are often observed with silica in flow reactors [84]. These ignition delay measurements provide crucial data to validate kinetic models [84]. While most of the shock tube

9

Fig. 8 e Experimental and computed ignition delay times for H2S at different pressures and concentrations. Experimental data in (a) is from Ref. [95], and in (b) is from Ref. [83].

studies have focused on low H2S concentrations ([84] and refs. therein), Frenklach et al. [83] have carried out ignition delay measurements at relatively higher concentrations of up to 22 vol% H2S. Shock tube can be modelled as a homogeneous and isobaric zero-dimensional reactor. Fig. 8 provides a comparison between the experimental and the computed ignition delay times for H2S. A satisfactory match between the two can be seen in this figure. A slight over-prediction of ignition delay at low temperatures can be observed. This is expected because the modelling assumption of constant pressure in shock tubes

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becomes less valid at low temperatures [85], and the experimental data on pressure variation is generally not available.

Role of O2 in H2 production from H2S As presented earlier, a significant amount of work on the effects of residence time, temperature and initial concentration of H2S on H2 yield during thermal decomposition of H2S is available in the literature. In this study, in an attempt to find optimum conditions for H2S decomposition to maximize H2 production, simulations have been carried out in an adiabatic plug flow reactor with feed streams containing H2S with different amount of O2 and Ar. The purpose of these simulations was to study the effect of O2 on H2 production from H2S. The inlet gas pressure was 1 atm, while the inlet gas temperature was varied from 873 to 1673 K to assess the effect of initial temperature on H2 production. A residence time of 1.5 s

(a)

was maintained in the reactor. The typical residence time for H2S-rich feed in Claus furnace is about 1.5e2 s. This is why, many literature-based studies [30,66,71] have presented their experiments or simulation results on H2S decomposition at residence times near or below 2.0 s. Fig. 9 presents the effects of inlet temperature (873e1673 K), initial H2S concentration (10e40 mol%), and initial O2 concentration (0e50 mol%) on H2 yield at the reactor outlet. A synergistic effect can be observed in all the subfigures, where H2 yield increased with increasing O2 concentration, and after reaching a maximum value, it decreased with further O2 addition. For example, with 20% H2S in the feed diluted in Ar (Fig. 9b) at inlet temperature of 873 K, the H2 yield increased to a maximum value with the addition of about 20% O2 into the H2S inlet stream, but above this amount of O2, H2 yield decreased. The important reactions responsible for the observed trends were examined. At low O2

60

873K 1073K 1220K 1373K 1573K

H2 yield (%)

50 1673 K

40

Increasing T

30 20 10

873 K

0 0 (b)

973K 1173K 1273K 1473K 1673K

50

10

20

30

40

50

30

40

50

30

40

50

30

40

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1673 K Increasing T

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40 30 20 10

873 K

0 0 (c)

50

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1673 K Increasing T

H2 yield (%)

40 30 20 10 873 K

0 0 (d)

20

1673 K Increasing T

50

H2 yield (%)

10

40 30 20 10

873 K

0

0

10

20

Initial O2 concentration (mol%)

Fig. 9 e H2 yield vs initial O2 concentration (mol%) at different initial gas temperatures with a residence time of 1.5 s at atmospheric pressure under adiabatic condition with feed containing(a) 10% H2S in Ar, (b) 20% H2S in Ar, (c) 30% H2S in Ar, and (d) 40% H2S in Ar. Please cite this article in press as: Cong TY, et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.053

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Concentration (mol %)

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0.00 1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 1.E-11

0.02

Time (s)

0.04 H

0.06

S

0.08 SH

0.10 HS2

Fig. 10 e Concentration (mol %) vs time (s) for species involved in H2formation at 1 atm pressure under adiabatic condition for feed containing 20% H2S and 80% Ar at 1073 K.

concentrations (before the maximum H2yield is attained), it was found that the direct decomposition of H2S and recombination of SH radicals contributed mostly to the formation of H2 through reactions (R9), (R10) and (R14). As the O2 concentration in feed is increased, its presence enhanced the radical pool, and stimulated recombination reactions to promote H2 production. It was observed that the increase in H2 yield to a maximum value was attributed to the occurrence of additional reactions (R15) and (R16). However, at higher concentrations of O2, the decrease in H2 yield was mainly due to more favourable oxidation reactions of H2S to form oxygenated radicals and species such as reactions (R17) and (R18), and the oxidation of H2 to form H2O. HS2 þ H4H2 þ S2

(R15)

SH þ H4H2 þ S

(R16)

H2 S þ O4HSO þ H

(R17)

H2 S þ OH4SH þ H2 O

(R18)

Concentration (mol %)

0.00 1.E+00

0.02

0.04

Fig. 9 also reveals the effect of varying the concentrations of H2S in the inlet feed stream in the range of 10e40%. At all the inlet temperatures, it was observed that a higher amounts of O2 is required to reach the maximum yield of H2, as the concentration of H2S in the inlet feed stream is increased. To further elucidate the effect of O2 addition on the concentration profiles of the radical species responsible for H2 formation (that are H, S, SH and HS2), simulations results with inlet streams containing 20% H2S/80% Ar and 20% H2S/10% O2/ 70% Ar at 1073 K in an adiabatic flow reactor were analysed. The H2 yields from these two feed streams can be seen in Fig. 9b, where O2 addition increased H2 yield by about 25%. Figs. 10 and 11 present the profiles of the important radical species for the two feed conditions. In the absence of O2, HS2 and SH were the main radicals species formed in the reactor, while H and S concentrations were very low. In the presence of O2 in the feed stream, at low residence times, when H2S oxidation by O2 has not taken place, the profiles of all the four radicals (H, S, SH and HS2) were found to be similar to the previous (anaerobic) case. However, as the H2S oxidation takes

Time (s)

0.06

0.08

0.10

1.E-01 1.E-02 1.E-03 1.E-04 1.E-05

H S SH HS2

1.E-06 1.E-07

Fig. 11 e Concentration (mol %) vs time (s) for species involved in H2 formation at 1 atm pressure under adiabatic condition for feed containing 20% H2S, 10% O2 and 70%Arat 1073 K. Please cite this article in press as: Cong TY, et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.053

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place at about 0.025 s, a high amount of these radicals are formed through the thermal and chemical decomposition of H2S. The combustion of H2S increased the gas temperature from its initial value of 1073 K to 1980 K that enhanced the radical pool in the reactor to form H2 through the chemical reactions listed before. In this manner, partial H2S combustion assists in increasing H2 yield. In these figures, the effect of residence times on radical concentrations (and the concentration of H2 that forms from them) can be seen. The species profiles were found to vary minimally after 0.04 s, which is indicative of thermodynamic equilibrium in the reactor. While the extent of the synergistic effect exhibited by H2 yield upon O2 addition to H2S feed stream needs to be verified through controlled experiments in the future, the results presented here provide an insight into viable means of enhancing H2 production while minimizing environmental burden arising from releasing H2S-containing waste streams of oil and gas refineries in air.

Conclusions To examine the production of H2 through H2S thermolysis, a detailed reaction mechanism that captures the complex chemistry of high temperature decomposition and oxidation of H2S was proposed. The reaction mechanism was validated by comparing the experimental data on H2S conversion, H2 and S2 yields, and species concentrations, obtained from different types of reactors under varied operating conditions, with the simulations results. A satisfactory agreement between the simulated and experimental profiles was obtained for the tested reactor conditions. The reaction pathways for the decomposition of H2S to form H2 and S2 were also determined through reaction path analysis. The detailed mechanism was then used to investigate the role of O2 on the formation of H2 from H2S. It was observed that an addition of small amount of O2 to H2S feed increased the H2production, while higher O2 concentrations caused the oxidation of the produced H2, thus reducing its yield at the reactor outlet. The role of the radicals H, S, SH, and HS2, in the production of H2 was highlighted. It was observed that the addition of O2 significantly enhanced the formation of S, H, SH and HS2, which improved H2 production. The well-validated H2S mechanism presented in this study is expected to facilitate the design and optimization of cost effective reactors for enhanced energy, H2 and S2 recovery from H2S.

Acknowledgements The authors acknowledge the financial support from the Petroleum Institute Research Centre, Abu Dhabi, UAE.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.03.053.

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Please cite this article in press as: Cong TY, et al., A detailed reaction mechanism for hydrogen production via hydrogen sulphide (H2S) thermolysis and oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.053