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International Journal of Hydrogen Energy 32 (2007) 3907 – 3914
www.elsevier.com/locate/ijhydene
Hydrogen production from a chemical cycle of H2S splitting
Hui Wang ∗
Department of Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A9
Received 22 August 2006; received in revised form 20 May 2007; accepted 23 May 2007
Available online 2 July 2007
Abstract
The sulphur–iodine thermochemical water-splitting cycle (S–I cycle) developed for hydrogen production from water is fundamentally based
on the following three chemical reactions:
H2 SO4 → H2 O + SO2 + 0.5O2 ,
2H2 O + SO2 + I2 → H2 SO4 + 2HI,
2HI → H2 + I2 .
This paper proposes to replace the H2 SO4 decomposition with a reaction between H2 S and H2 SO4 and the replacement gives rise to a
H2 S-splitting cycle that produces H2 and elemental S from H2 S, shown as follows:
H2 S + H2 SO4 → S + SO2 + 2H2 O,
2H2 O + I2 + SO2 → H2 SO4 + 2HI,
2HI → H2 + I2 .
Combined with the reactions such as
O2 + S → SO2 ,
SO2 + 0.5O2 → SO3 ,
SO3 + H2 O → H2 SO4 ,
this new cycle cannot only produce more H2 and extra H2 SO4 but also facilitate flexible H2 to H2 SO4 production ratio. Thermodynamic
analysis shows that the new cycle is more energy-efficient than the S–I cycle of water-splitting because a series of endothermic reactions in
H2 SO4 decomposition have been replaced with an exothermic reaction between H2 S and H2 SO4 . Technologies to be developed from this
chemistry will be able to convert H2 S from sour or acid gas into H2 and elemental S or H2 SO4 , which are more valuable than elemental S
only, the product of the Claus process. In upgrading and refinery, the H2 produced can be returned to use in hydrotreating; and in gas plants,
H2 from H2 S splitting is an alternative clean fuel. Environmentally, H2 production based on this H2 S-splitting cycle is carbon free.
䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Keywords: Hydrogen sulphide splitting; Hydrogen; S–I cycle; Sulphur removal; Elemental sulphur; Sulphuric acid
1. Introduction
Bitumen and heavy oil upgrading and refining requires vast
amount of hydrogen (H2 ) which is mainly converted into
∗ Tel.: +1 306 966 2685; fax: +1 306 966 4777.
E-mail address: [email protected].
hydrogen sulphide (H2 S) in a hydrotreater. H2 S is then turned
into elemental sulphur (S) and water (H2 O) in sulphur removal
and recovery units such as the Claus plant. On the other hand,
the hydrogen needed is produced from the reforming of hydrocarbons, mainly natural gas methane, where greenhouse gas
carbon dioxide (CO2 ) is emitted. In addition, gas plants also
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2007.05.030
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H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914
produce a tremendous amount of H2 S as a by-product, of which
the H element is now wasted in sulphur removal processes. This
paper proposes a chemical route that converts H2 S into H2 and
elemental S through the following chemical reactions:
H2 S + H2 SO4 → S + SO2 + 2H2 O,
(1)
2H2 O + I2 + SO2 → H2 SO4 + 2HI,
(2)
2HI → H2 + I2 .
(3)
Given all other substances H2 SO4 , SO2 , H2 O, I2 , and HI are
cycled in the system, the three reactions in fact form a novel
chemical reaction cycle that splits H2 S into H2 and S:
H2 S → H2 + S.
(4)
This cycle is denoted as the H2 S-splitting cycle. If elemental
sulphur from reaction (1) can be further oxidized into SO2 :
O2 + S → SO2 ,
(5)
the subsequent reactions (2) and (3) would occur in doubled
scale because there are two moles of SO2 for reaction (2). As
a result, there must be one mole of water input to the cycle.
With including reaction (5), the cycle, denoted as H2 S–H2 Osplitting cycle, eventually splits each mole of H2 S and H2 O,
giving rise to two moles of H2 and one mole of sulphuric acid.
The origin of this novel chemical route of H2 S splitting is
based on the inspiration of a well-known sulphur–iodine thermochemical water-splitting cycle (S–I cycle). Although most
of the hydrogen is currently produced from the reforming of
natural gas and other hydrocarbons, people never stop pursuing hydrogen production from a water-splitting reaction, which
is more environmentally friendly. Therefore, water electrolysis, thermal decomposition, and thermochemical water-splitting
are intensively studied. It turns out that the chemical cycles of
water-splitting are more technically viable and more energyefficient [1,2]. Among those cycles, one that attracts the most
research interest is the S–I cycle that consists of also three reactions:
H2 SO4 → H2 O + SO2 + 0.5O2 ,
(6)
2H2 O + SO2 + I2 → H2 SO4 + 2HI,
(2)
2HI → H2 + I2 .
(3)
The overall reaction is
H2 O → H2 + 0.5O2 .
(7)
Comparatively, the new cycle of H2 S splitting (reactions 1–3)
is formed by replacing reaction (6) in the S–I cycle with
reaction (1).
This paper will first briefly review the research and development work that has been done for the S–I cycle of water-splitting
and H2 production. Then it will present the framework of the
new thermochemical cycle of H2 S splitting and the alternative
route layouts. The economical and environmental benefits of
H2 production from H2 S splitting will be discussed. Finally,
a plan of the research and development of technologies based
on this novel H2 S-splitting cycle will be proposed.
2. Brief review of the S–I cycle
The S–I cycle has been considered the one of the most
promising routes for hydrogen production from water splitting
on a large scale [3,4]. Therefore, tremendous research has been
done with this cycle, including experiments, process modelling,
efficiency estimation, energy coupling, and chemical modification, etc. The S–I cycle was originally investigated by General
Atomics Co. (GA) in the 1970s and the involved reactions with
phase specification and reaction temperatures were presented
in [3,4] as follows:
(9I2 )l + (SO2 )g + (16H2 O)l →
(2HI + 10H2 O + 8I2 )l−2
+ (H2 SO4 + 4H2 O)l−1
(393 K),
(8)
L − 2 = (2HI + 10H2 O + 8I2 )l−2 →
(2HI)g + (10H2 O + 8I2 )l
(2HI)g → H2 + (I2 )l
(500 K),
(600 K),
(9)
(10)
L − 1 = (H2 SO4 + 4H2 O)l−1 →
(H2 SO4 )l + 4(H2 O)l
(570 K),
(11)
(H2 SO4 )l → (H2 SO4 )g
(630 K),
(12)
(H2 SO4 )g → (SO3 )g + (H2 O)g
(SO3 ) → (SO2 )g + 0.5O2
(670 K),
(1140 K).
(13)
(14)
According to GA’s nomenclature, reaction (8), known as the
Bunsen reaction, is Section 1 of the cycle. Compared with reaction (2) in Introduction, an excess of both H2 O and I2 allows
the product to form two immiscible phases after this exothermic reaction occurring at a mild temperature. A light H2 SO4
solution phase and a heavy iodine/iodide–water phase enable
an easy phase separation. In the subsequent step, Section 3 including reactions (9) and (10), the separation of HI from L − 2,
the heavier iodine/iodide–water phase, is the most critical scenario of the cycle [4] and believed to be the most expensive
and energy-consuming step [5]. After establishing the thermodynamic correlation of phase equilibrium for the quaternary
mixture of H2 O/HI/I2 /H2 , Roth and Knoche [5] designed a reactive distillation column in which reactions (9) and (10) could
be performed simultaneously under elevated pressure and temperature. And Section 2 is the H2 SO4 concentration and decomposition, also an energy-intensive phase in the cycle due
to the heavy duty of water vaporization and endothermic reactions. Distillation was proposed to perform the concentration
of dilute sulphuric acid (57 wt%) from the Bunsen reaction (reaction (8)) and several distillation flow sheets were simulated
[4], including a series of flash evaporators [6]. The decomposition of H2 SO4 usually consists of two steps, reactions (13) and
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H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914
(14), both requiring high-temperature heat, either from nuclear
reactors or from solar concentrators, to facilitate [6,7]. Suppiah
et al. [8] developed a catalytic method allowing 100% conversion of H2 SO4 into SO3 (thermal decomposition at 773 K) and
then from SO3 into SO2 (catalytic decomposition at 1173 K).
In general, this cycle is an energy-intensive process. During
the past two or three decades, a lot of research work focuses
on coupling this cycle with possible energy sources, improving
energy efficiency for separations and reaction processes, and
modifying the cycle by involving more chemical species so as to
lower the energy demand. Sadhankar [9] reported the work that
the Atomic Energy of Canada Limited (AECL) engaged in for
the development of a novel technology that integrates thermochemical cycles of hydrogen production with its Generation IV
reactor system. Vitart et al. [3] suggested that the S–I cycle be
coupled with a VHTR (very high-temperature reactor) in Commissariat à l’Energie Atomique (CEA) in France. Solar energy
is another heat source. Giaconia and co-workers [10] developed
a method of H2 /methanol production by the S–I cycle with the
power of solar/fossil energy. In addition to the experimental
work on individual reactions in the laboratory scale with batch
reactors [3,8,11], mainly done by GA, the Japan Atomic Energy
Agency (JAEA) has experimentally demonstrated the continuous hydrogen production in 32 L/h for 20 h in a closed-loop
facility made of glass and fluorine [12]. In fact, in early 1990s
in Japan, Mizuta and Kumagai built a continuous flow system
of a modified Mg–S–I cycle which successfully demonstrated
for 33 h a nearly constant generation of H2 and O2 in 0.5 and
0.25 L/h, respectively [13]. On the other hand, process modelling and simulation allows one to understand the phase equilibria and thermodynamic properties involved in the S–I cycle,
and to conduct energy-efficiency analysis, process integration
and design, and equipment sizing and cost analysis, either for
individual sections or for the cycle as a whole [4–7,11,12,14].
All the attempts that have been made are to improve the technical viability and the energy efficiency of the cycle and push forward this conceptual chemical route to a commercial technology. However, all the work is still in research and development
scenario.
3. Presentation of the H2 S-splitting cycle
As discussed, the novel H2 S-splitting cycle that this paper
proposes consists of three chemical reactions:
H2 S + H2 SO4 → S + SO2 + 2H2 O,
(1)
2H2 O + I2 + SO2 → H2 SO4 + 2HI,
(2)
2HI → H2 + I2 .
(3)
The concept of this cycle is schematically represented in
Fig. 1, which takes acid gas, a mixture of H2 S and CO2
from amine treatment train in a gas plant, as a feedstock. In
principle, any H2 S-containing gas can be used as a feed to
this cycle. However, the impact of the other components than
H2 S, such as CO2 in acid gas or CH4 and other gases in sour
natural gas, on the reactions and relevant separations must be
taken into consideration. Compared with the illustration of
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the S–I cycle in a publication by Vitart et al. [3], the involvement of such “other components” adds more complexity to the
H2 S-splitting cycle. To make this new cycle work, a process
flow diagram is also proposed in this paper, shown in Fig. 2.
The process can be seen as an integration of three sections,
each including one chemical reaction and relevant separation
processes: the oxidation of H2 S, or reaction between H2 S
and concentrated sulphuric acid (Section 1), the Bunsen reaction, or the formation of hydrogen iodide solution (Section
2), and hydrogen iodide vaporization and decomposition, or
hydrogen generation (Section 3). Although H2 SO4 , SO2 , I2 ,
and HI are working agents that are recycled within the cycle, separations or purifications are necessary to have them
ready from products of one reaction to reactants of the other
reaction.
3.1. Section 1—H2 S oxidation
In this section, reaction (1) takes place between the H2 Scontaining gas and concentrated H2 SO4 solution, where H2 S
is oxidized. In addition to reaction (1), another reaction that
would occur simultaneously is [15]
2H2 S + SO2 → 3S + 2H2 O.
(15)
Reactions (1) and (15) occur in concentrated H2 SO4 solution
(94–98 wt%) but the latter is much slower [16,17]. Overall, they
give rise to SO2 in gas phase, less concentrated H2 SO4 aqueous solution due to the production of water, and elemental S in
solid or immiscible liquid phase depending on the temperature
of the reaction system. SO2 , rather than elemental S, is desired
for the subsequent reaction, the Bunsen reaction; thus, reaction
(15) becomes an undesired one that can be suppressed by using
concentrated H2 SO4 (98–100 wt%) and high-reaction temperatures (∼ 150 ◦ C). Except H2 S, most components in sour gas
streams in gas plants or oil upgrading and refinery plants, for
example, CO2 and saturated low hydrocarbons, are inert to concentrated H2 SO4 [18]. Thus, these reactions can be conducted
by passing the sour gas stream through concentrated H2 SO4
solution in a packed-bed column reactor. Necessary separations
should follow so as to give relatively pure sweetened gas, elemental S, gas SO2 , and H2 SO4 solution. If more H2 is anticipated, then reaction (5) is carried out to convert elemental S
generated in reaction (1) into SO2 . This reaction has been commonly used in sulphuric acid production and the technology
to perform this reaction in engineering is mature. In this case,
the following two reactions, (2) and (3), will be conducted in
a twice larger scale.
3.2. Section 2—Bunsen reaction that may play role of SO2
scrubbing
Depending on the feedstock of H2 S-containing gas for Section 1, the SO2 for the Bunsen reaction may be in a mixture
with CO2 or CH4 and other low hydrocarbons. Whether this
SO2 -containing mixture can be directly used for the Bunsen
reaction is determined by whether there are reactions between
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H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914
S
Acid gas
(CO2 + H2S)
H2
Reaction (1)
Reaction (3)
SO2
H2O
CO2
H2SO4
H2SO4
Concentration
I2
HI
HI Vaporization
Reaction (2)
H2SO4+H2O
HI+H2O+I2
Purified CO2
Fig. 1. Schematic representation of the H2 S-splitting cycle for hydrogen production.
H2O
I2
Reaction (2)
Reactor
Acid Gas
H2S + CO2
Reaction (1)
Reactor
SO2+ CO2
H2SO4
HI
Buffer
CO2 out
SO2
H2 out
Gas-Liquid
Separator
Reaction (3)
Reactor
HI, I2
HIX Solution
Distillation
HIX
HI Phase
Purification
Liquid Phase
Separator
S out
Liquid Phase
Separator
H2SO4, S, SO2
H2SO4
H2SO4
H2SO4
Upgrading
HI
HI, I2
Fig. 2. Process flow diagram of H2 S-splitting cycle for hydrogen production.
the species involved in the Bunsen reaction such as iodine and
iodide on one hand and the components such as CO2 , CH4
and other hydrocarbons on the other. That is, if there are no
such reactions, the SO2 mixture can be fed for the Bunsen
reaction where SO2 is reacted and SO2 -free gas is released.
Thus, the Bunsen reaction can play a role of SO2 scrubbing.
If the species such as iodine or iodide would react with CO2
or hydrocarbons, for example, the addition of HI and olefin
if the unsaturated hydrocarbons present, SO2 has to be sep-
arated from the gas stream before it undergoes the Bunsen
reaction.
According to Vitart et al. [3], an excess of both water and
iodine is provided in reactants such that the products of this
reaction will form two immiscible liquid phases (reaction (8)),
the light phase of H2 SO4 solution and the heavy phase of HI,
I2 and water solution. The advantage of doing so is to allow
easier phase separation to separate the two products, H2 SO4
and HI; the disadvantage is that it leads to less concentrated
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H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914
H2 SO4 solution and HI solution in products, which desire further concentration or purification before they are sent for the
reactions they subsequently participate in.
3.3. Section 3—decomposition of HI to produce H2
Compared with the other two sections, Section 3 in the H2 Ssplitting cycle is exactly the same as it is in the S–I cycle of
water-splitting. Any species that are brought into the system by
sour natural gas or acid gas after amine treatment will not come
to this section. Therefore, they will not have any deleterious
effect on this section and its related separations. If the heavier
aqueous solution of HI and I2 is used as a feedstock, reactions
(9) and (10) will be conducted. The vaporization of HI from
hydroiodic acid solution (reaction (9)) can also be expressed
as the formation of gaseous molecular HI from ions of H+ and
I− or I3− in the aqueous solution:
+
−
H + I = HI(g)
(16)
or
H+ + I3− = HI(g) + I2 .
(17)
Reaction (17) is likely the most reasonable because of the presence of I2 in excess. This section is known as the most costly
and energy intensive step in the S–I H2 O-splitting cycle [5,11].
It is also the most expensive section in the H2 S-splitting cycles. The technical challenges that have been identified in the
S–I cycle, for example, the separation and concentration of HI
from the hydroiodic acid solution, the recycle of the unreacted
HI and the separation of H2 , must be solved in the new cycles
as well.
In summary, the basic H2 S-splitting cycle includes three reactions:
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3.4. Other options in H2 SO4 concentration
The mass concentration of the (H2 SO4 + 4H2 O) phase from
reaction (8) is 57.65 wt%. For concentrating one mole H2 SO4
in this solution, either 3.8 mol of H2 O should be removed or
3.5 mol of SO3 should be added, so as to upgrade it to 98 wt%,
the necessary concentration of the salable sulphuric acid or the
acid for reaction (1). If sulphuric acid is more desired than H2 ,
it is suggested that part of the SO2 from reactions (1) or (5) be
converted into SO3 :
SO2 + 0.5O2 → SO3 .
(21)
Then the less concentrated H2 SO4 can be upgraded through the
reaction
SO3 + H2 O → H2 SO4
(22)
another mature technology in sulphuric acid plants. Calculation using HSC Chemistry 5.11 shows that the heat of reaction
(22) is −106.741 kJ at 298.15 K. Due to its exothermicity, to
concentrate the dilute sulphuric acid, adding SO3 to the less
concentrated H2 SO4 solution is more energy-efficient than removing H2 O utilizing either distillation or vaporization. Another advantage of involving reactions (21) and (22) is to allow
the system to adjust the production ratio of H2 to H2 SO4 based
on market demand. The other option is to import S to this cycle
to generate SO3 for H2 SO4 concentration without compensating the H2 production, thus leading to more H2 SO4 production.
This option is prompted by the fact that the production of sulphur from Canada’s gas and oil industries is excess to the world
sulphur market that Canada can access. This market imbalance
will persist well into the foreseen future so that elemental S
is almost a waste and long-term sulphur storage methods are
sought [19].
H2 S + H2 SO4 → S + SO2 + 2H2 O,
(1)
4. Thermodynamic considerations
2H2 O + SO2 + I2 → H2 SO4 + 2HI,
(2)
2HI → H2 + I2 ,
(3)
It is understood that there is a lot of work to be done to make
a chemically feasible route become an engineering process. In
other words, an engineering process is more complicated than
just a few reactors where the involved reactions are conducted.
However, there has not been a satisfied design for the S–I water
splitting process though some attempts have been made [11].
Therefore, it is expected that the thermodynamic analysis on
both the novel H2 S-splitting cycle and the S–I water-splitting
cycle can give a rough comparison between their energy
efficiency.
Let H , S and G be the changes in enthalpy, entropy and
Gibbs free energy of a chemical reaction at a given temperature,
and K be the equilibrium constant of the reaction, respectively.
That is, for a chemical reaction,
which overall lead to the splitting of one mole of H2 S into one
mole of H2 and one mole of S. If this S is turned into SO2
via reaction (5), the cycle will be performed following another
stoichiometry:
H2 S + H2 SO4 → S + SO2 + 2H2 O,
(1)
O2 + S → SO2 ,
(5)
4H2 O + 2I2 + 2SO2 → 2H2 SO4 + 4HI,
(18)
4HI → 2H2 + 2I2 .
(19)
aA + bB = cC + dD,
(23)
The overall reaction then appears to produce two moles of
hydrogen and one mole of sulphuric acid from a feed of one
mole of hydrogen sulphide, one mole of oxygen and two moles
of water:
H = cH C + dH D − aH A + bH B ,
(24)
H2 S + O2 + 2H2 O → H2 SO4 + 2H2 .
S = cS C + dS D − aS A + bS B ,
(25)
(20)
H , S, G and K are defined as follows [20]:
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H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914
Table 1
Thermodynamic data for involved reactions at 298.15 K
Reactions
H (kJ)
S(J K−1 )
The S–I H2 O-splitting cycles
H2 SO4 (ia) = H2 SO4 (g)
H2 SO4 (g) = SO3 (g) + H2 O(g)
SO3 (g) = SO2 (g) + 0.5O2 (g)
3I2 + SO2 (g) + 2H2 O = 2H(+a) + 2I3 (−a) + H2 SO4 (ia)
2H(+a) + 2I3 (−a) = 2HI(g) + 2I2
2HI(g) = H2 (g) + I2
For producing one mole of H2 from one mole of H2 O,
174.264
97.538
98.952
−143.846
155.645
−52.718
329.772
278.721
146.800
94.022
−237.804
166.807
−166.360
91.163
53.770
70.919
−72.945
105.911
−3.118
1.065 × 10−16
3.793 × 10−10
3.752 × 10−12
6.035 × 1012
2.775 × 10−19
3.518
The H2 S-splitting cycle
H2 SO4 (ia) = H2 SO4 (l)
H2 S(g) + H2 SO4 (l) = S + SO2 (g) + 2H2 O
3I2 + SO2 (g) + 2H2 O = 2H(+a) + 2I3 (−a) + H2 SO4 (ia)
2H(+a) + 2I3 (−a) = 2HI(g) + 2I2
2HI(g) = H2 (g) + I2
For producing one mole of H2 from one mole of H2 S,
121.057
−59.636
−143.846
155.645
−52.718
20.439
249.947
−55.593
−237.804
166.807
−166.360
46.535
−43.061
−72.945
105.911
−3.118
7.023 × 10−9
3.505 × 107
6.035 × 1012
2.775 × 10−19
3.518
The H2 S .H2 O-splitting cycle
H2 SO4 (ia) = H2 SO4 (l)
H2 S(g) + H2 SO4 (l) = S + SO2 (g) + 2H2 O
S + O2 = SO2 (g)
6I2 + 2SO2 (g) + 4H2 O = 4H(+a) + 4I3 (−a) + 2H2 SO4 (ia)
4H(+a) + 4I3 (−a) = 4HI(g) + 4I2
4HI(g) = 2H2 (g) + 2I2
For producing two mole of H2 from each mole of H2 S and H2 O,
121.057
−59.636
−296.813
−287.692
311.290
−105.437
−317.231
249.947
−55.593
11.001
−475.608
333.614
−322.721
46.535
−43.061
−300.093
−145.889
211.823
−6.236
7.023 × 10−9
3.505 × 107
3.797 × 1052
3.642 × 1025
7.699 × 10−38
1.238
G(kJ)
K
Table 2
Thermodynamic data at the temperature at which G = 0 for those reactions with positive G at 298.15 K
Reactions
H (kJ)
S (J K−1 )
Reactions in both H2 S- and H2 O-splitting cycles
H(+a) + I3 (−a) = HI(g) + I2
155.888
241.934
643.88
94.630
97.267
141.132
92.268
670.51
1054.18
Reactions in S–I H2 O-splitting cycle
H2 SO4 (g) = SO3 (g) + H2 O(g)
SO3 (g) = SO2 (g) + 0.5O2 (g)
G = cGC + dGD − aGA + bGB ,
(26)
ln K = G/(−RT ),
(27)
where, a, b, c, and d are the stoichiometric coefficients of
substances A, B, C and D, respectively; H, S and G are the
enthalpy, entropy and Gibbs energy, respectively; R is gas constant (8.314 J K −1 mol−1 ); and T is temperature in K. The
values of these thermodynamic properties of the involved reactions at 298.15 K were calculated using HSC Chemistry v5.11.
It should be mentioned that HSC Chemistry requires specifying
the physical states of the substances that are involved in a reaction. For a reaction, specifying different states of a substance
will result in different calculation results. For instance, dilute
sulphuric acid, H2 SO4 (ia), liquid sulphuric acid, H2 SO4 (l) and
gas sulphuric acid, H2 SO4 (g) will give different values for a
reaction that H2 SO4 is involved. Therefore, the reactions with
the specified physical states of their reactants and products, together with values of the corresponding thermodynamic properties, are listed in Table 1. The reactions are grouped based on
T (K)
the three cycles they belong to, even though some of them may
be presented in more than one cycles taking the same or different stoichiometric coefficients. First of all, comparison of the
overall reaction heat of the cycles at 298.15 K indicates that the
H2 S-splitting cycle needs much less energy than the S–I watersplitting cycle and that the H2 S.H2 O-splitting cycle becomes
exothermic in overall. This is because the new H2 S-splitting
cycles do not include the highly endothermic H2 SO4 decomposition but are joined with the exothermic reactions such as
the oxidation of H2 S and elemental S. Secondly, comparison
on G values shows that the endothermic, G-positive H2 SO4
decomposition reactions (13) and (14) in the original cycle have
been replaced by the exothermic, G-negative reactions such
as reaction (1) or reactions (1) and (5). Table 2 shows the values of H and S at the temperature at which G equals 0 for
those reactions with a positive G at 298.15 K. Only one of
such reactions is involved in the H2 S-splitting cycles, which is
the concentration of HI from the hydroiodic acid solution. The
reaction temperature has to be at least higher than 643.88 K
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H. Wang / International Journal of Hydrogen Energy 32 (2007) 3907 – 3914
such that the reaction occurs spontaneously. Nonetheless, the
new H2 S-splitting cycles do not need high-temperature heat to
heat a reactor at least to 1054 K to facilitate the SO3 decomposition for SO2 .
It should be pointed out that the main purpose of this paper is
to propose a novel H2 production route from a H2 S-splitting cycle with which the abundant H2 S resources in oil and gas industries can be made use of not only for the “cheap gold” (sulphur)
but also for hydrogen; the latter being more economically and
environmentally valuable. Additionally, the H2 S.H2 O-splitting
cycle does split one mole of water, which means that simultaneous splitting of H2 S and H2 O would release the energy burden
that the S–I water-splitting cycle is bearing.
5. Case study of H2 S-splitting cycle application in
bitumen upgrading and refinery
A bitumen (oil sand) upgrading plant with 120 kbbl/d (kilobarrels per day) capacity [21] will produce H2 S in 425 kta (kilotonnes per annum), that is 12.5 million kmol per year. Based
on the stoichoimetry of the H2 S.H2 O-splitting cycle:
H2 S + O2 + 2H2 O → H2 SO4 + 2H2 ,
(20)
H2 production will be 25 million kmoles, i.e., 560 mcma
(million cubic meters per annum) at the standard temperature
and pressure (STP), and the H2 SO4 production will be 12.5
million kmoles, i.e., 1225 kta. For bitumen upgrading of such
a capacity, the H2 demand for hydrotreating processes such as
hydrocracking, hydrodesulphurization, hydrodenitrogenation,
and hydrodearomatization is 250 mcma [21]. Thus, the H2 surplus will be 310 mcma. This surplus can be used as H2 energy,
for instance, the feedstock for fuel cells.
6. Conclusions—plans for R and D
It can be seen that the H2 S-splitting cycle to produce H2 is
chemically feasible and thermodynamically more efficient than
the S–I cycle. The cycle that is able to convert H2 S into H2
and elemental S or H2 SO4 has great market potential in gas
and oil industry, especially oil sand bitumen upgrading and refinery to fuels and petrochemicals. Environmentally, H2 from
H2 S splitting is greenhouse gas free. And its use will reduce
the hydrogen reliance on natural gas reforming, and thus reduce CO2 emission from this sector. However, to make this
economically and environmentally beneficial chemistry a real
chemical engineering process, there are tremendous challenges
that include old ones that have been identified in the S–I cycle
research and the new ones that the particular reactions in the
new H2 S-splitting cycle bring about. In addition to the to-beimproved areas that Goldstein et al. [4] recently identified, further R and D work in the H2 S-splitting cycle will be presented as
follows:
The separation of elemental sulphur from downgraded (diluted) sulphuric acid after the reaction between H2 S and
H2 SO4 solution: Reaction (1) between H2 S-containing gas
and concentrated sulphuric acid in liquid phase can be carried
out in a trickle-bed reactor [18], giving rise to SO2 -containing
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gas and solid sulphur or liquid sulphur along with downgraded
sulphuric acid. Whether the elemental sulphur is liquid or
solid depends on whether the temperature is above the melting
point of elemental sulphur (119 ◦ C). And how much the acid
concentration is downgraded in one pass depends on the ratio
of H2 S to H2 SO4 in feed. Moreover, the dissolution of SO2 in
the acid solution is significant [22]. Therefore, the sulphuric
acid solution has to be upgraded by removing elemental S,
releasing SO2 , and concentrating. To develop engineering processes to conduct these separations constitutes the main tasks
in R&D scenario.
The use of I2 and water solution as solvent to absorb SO2
from gas stream: SO2 from the above section will come together
with the other gaseous components that are present in the acid
gas stream or sour natural gas but remain unchanged when
undergoing the contact with sulphuric acid. Whether the SO2
should be separated from this gas stream before it is sent for
the Bunsen reaction depends on whether there are interactions
between these “other components” in the gas stream and the
reactant and product species of the Bunsen reaction. This is
another particular issue that needs to be studied to develop the
H2 S-splitting process for H2 production.
It is hoped that the research on both the H2 S-splitting cycle
for H2 production from H2 S and the S–I cycle for H2 production
from water be able to benefit each other’s achievements and
speed up the commercialization of these technologies.
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