jung_development of once-through hybrid sulfur process for nuclear h2 production

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Development of the once-through hybrid sulfur process

for nuclear hydrogen production

Yong Hun Jung, Yong Hoon Jeong*

KAIST (Korea Advanced Institute of Science and Technology), 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Republic of Korea

a r t i c l e i n f o

Article history:Received 24 March 2010

Received in revised form

28 July 2010

Accepted 30 July 2010

Available online 16 September 2010

Keywords:

Hybrid sulfur

Nuclear hydrogen

Sulfur

Sulfur combustion

Sulfur dioxide

Sulfuric acid

a b s t r a c t

The Once-through Hybrid Sulfur (Ot-HyS) process, proposed in this work, produceshydrogen using the same Sulfur dioxide Depolarized water Electrolysis (SDE) process found

in the original Hybrid Sulfur cycle (HyS). In the process proposed here, the Sulfuric Acid

Decomposition (SAD) process in the HyS procedure is replaced with the well-established

sulfur combustion process. First, a flow sheet for the Ot-HyS process was developed by

referring to existing facilities and to the work done by the Savannah River National

Laboratory (SRNL) under their reasonable assumptions. The process was then simulated

using Aspen Plus with appropriate thermodynamic models. It was demonstrated that the

Ot-HyS process has higher net thermal efficiency, as well as other advantages, over

competing benchmark processes. The net thermal efficiency of the Ot-HyS process is 47.1%

(based on LHV) and 55.7% (based on HHV) assuming 33.3% thermal-to-electric conversion

efficiency of a nuclear power plant with no consideration given to the work for the air

separation. Hydrogen produced through the Ot-HyS process would be used as off-peak

electricity storage, to relieve the burden of load-following and could help to expandapplications of nuclear energy, which is regarded as a ’sustainable development’

technology.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction

Humanity has been facing major energy challenges such as

increasingly severe climate change, threats to energy security,

and a global energy shortage that is especially dire in the

developing world. Growing awareness of global warming hasled to efforts to develop sustainable energy technologies

beneficial to the economy, social welfare, and the environ-

ment. Water-splitting nuclear hydrogen production is expec-

ted to help to resolve those challenges when high energy

efficiency and low cost hydrogen production become possible.

Among ‘sulfur-based’ water-splitting thermo-chemical cycles

which are recognized as high priority candidates for research

and development, the Hybrid Sulfur cycle (HyS), first proposed

by Westinghouse Electric Corp., has been researched by

Savannah River National Laboratory (SRNL) under the Nuclear

Hydrogen Initiative (NHI) established by the U.S. Department

of Energy’s Office of Nuclear Energy, Science and Technology

(DOE-NE). SRNL has recently developed conceptual designs of

HyS processes using sulfur dioxide depolarized electrolyzersnow being developed and that are powered by an advanced

nuclear reactor heat source such as Pebble Bed Modular

Reactor (PBMR) [1,2].

The HyS cycle, also known as the Westinghouse Process, is

a combined, all-fluids cycle of the electrolysis and thermo-

chemical processes with only two chemical reactions andonly

one additional element (sulfur) outside of hydrogen and

oxygen [3e7]. In the HyS cycle, hydrogen and sulfuric acid are

* Corresponding author. Tel.: þ82 42 350 3826; fax: þ82 350 3810.E-mail addresses: [email protected] (Y.H. Jung), [email protected] (Y.H. Jeong).

A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / h e

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 3 5 ( 2 0 1 0 ) 1 2 2 5 5 e1 2 2 6 7

0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijhydene.2010.07.168

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produced by sulfur dioxide depolarized water electrolysis, the

so-called SDE process. The presence of sulfur dioxide in

addition to the water in SDE reduces the required cell poten-

tial; in other words, the electric energy required for this

process is well below that required for conventional pure

water electrolysis [8,9]. In order to supply sulfur dioxide and

waterto theSDE processagain, the sulfuric acid productat the

anode is recycled to the high-temperature, catalytic, vaporphase thermal decomposition of sulfuric acid (SAD), which is

commonly found in all sulfur-based cycles as one of the most

challenging technical issues and as the largest energy

consumer in all sulfur-based cycles [10e12].

It is challenging to find materials to decompose sulfuric

acid that are both chemically and thermally stable in highly

corrosive environments. A SiC bayonet decomposition reactor

has been developed by SandiaNational Laboratories (SNL) and

experiments have been performed up to 900 C with no severe

corrosion issues [13,14]. Although corrosion problems have

been partly addressed, more emphasis is needed with respect

to the scaling up process for industrial application. The

availability and cost of the components for installations andplant maintenance may determine the overall success for

commercial applications. This is one of the leading motives

which have driven the development of the Ot-HyS process.

In the Once-through Hybrid Sulfur process proposed in this

work, however, this challenging issue, the high-temperature

SAD process, is replaced by the sulfur combustion process,

which is a well-established technology in manufacturing

sulfur dioxide as a final product or as an intermediate product

in manufacturing sulfuric acid. Although the concept of the

once-through process for hydrogen production has been

proposed and studied before, under the name of the “open-

loop” thermo-chemical cycle, there has been no detailed

analysis and there are many differences between the detailsof that processand the processdescribed in this study [15e17].

The purpose of this study is to demonstrate that the

proposed Ot-HyS process is an energy-efficient hydrogen

production process that can help to resolve ongoing energy

challenges. To achieve this purpose, the scope of the study

was established as the development of a detailed flow sheet

forthe simulation first and then, based on that, the estimation

of the net thermal efficiency of the Ot-HyS process through an

Aspen Plus simulation with appropriate thermodynamic

models. Finally, a study was conducted to compare the net

thermal efficiency of the proposed process with those of other

benchmark processes for hydrogen production.

2. Background

The Ot-HyS process proposed in this work is one of the water-

splitting nuclear hydrogen production processes and is based

on the same SDE process found in the HyS cycle used to

produce hydrogen. However, because the Ot-HyS processdoes

not include theSAD process, sulfuric acid produced in theSDE

process is just extracted from the whole process after it has

reached the desired acid concentration. Sulfur dioxide for

the continuous operation of the SDE process is prepared by

the sulfur combustion process in which sulfur is burned in the

presence of oxygen to produce sulfur dioxide. Therefore, to

produce 1 mol of hydrogen and sulfuric acid, the process

inputs are a certain amount of electric and thermal energy for

the process as well as 1 mol of sulfur and oxygen or corre-

sponding air and 2 mol of water. In the real process simula-

tion, 3.83093 mol of water should be supplied in compensation

for 1.79616 mol of water extracted together with 1 mol of

sulfuric acid assuming 75 wt% of the acid concentration, and

0.03477 mol of water extracted together with waste purge.Differences in the reaction steps of the HyS and Ot-HyS

processes are described in Fig. 1. Because the technically

challenging SAD process is replaced with the well-established

sulfur combustion process as stated above, the Ot-HyS pro-

cess bears less technical challenges than the original HyS

cycle does. Therefore, it is expected to advance the realization

of large-scale nuclear hydrogen production by feeding an

initial nuclear hydrogen stock.

For environmental reasons, the sulfur feed for the SDE

processcan be supplied by a desulfurizationprocess to recover

sulfur during the processing of natural gas and petroleum

refining. This kind of sulfur recovered for environmental

SO2 + 2H2O→H2 + H2SO4

½O2 + SO2 + H2O←H2SO4

SO2 + H2OH2SO4(H2O)

INPUTS :

1. Water

2. Heat ( 800 )

3. Electricity

OUTPUTS :

1. Hydrogen

2. Oxygen

3. Waste Heat

½O2 Heat ( 800 )

Electricity H2

SO2 + 2H2O→H2 + H2SO4

SO2 ← S + O2

SO2

INPUTS :

1. Recovered S

2. Water & Oxygen

3. Electricity

4. Heat for Acid Concentration

OUTPUTS :

1. Hydrogen

2. Sulfuric Acid

Recovered S

Electricity H2

O2 (Air)

H2SO4(H2O)

Heat for Acid

Concentration

Hybrid sulfur cycle

Once-through hybrid sulfur process

H2O

2H2O

Fig. 1e

Reaction steps of the HyS and Ot-HyS processes.








reasons, the so-called recovered sulfur, is now the predomi-

nant source of sulfur worldwide and is expected to increase

significantly, even resulting in severe oversupply due to rising

environmental concerns, expanded oil and gas operation and

the development of oil sands [18,19]. Sulfur statistics are being

developed in favor of the Ot-HyS process.

In addition, the sulfur combustion itself is also favorable to

the process in terms of the process efficiency. The processefficiency can be enhanced by recovering the great amount of

high-temperature excess heat from the sulfur combustion

process to supply process electricity as well as heat duty,

because sulfur combustion is a highly exothermic chemical

reaction releasing 297 kJ/mol of combustion heat and results

in a large furnace temperature rise (generally above 1000 C).

This high grade excess heat from the sulfur combustion

process can make it possible to employ the advanced Power

Conversion System (PCS) which generates process electricity

at a high thermal-to-electric conversion efficiency.

The sulfuric acid product, which is another product of the

Ot-HyS process, is also favorable to the process in terms of

the sulfur statistics. It is reasonable to assume that nearly allthe environmentalsulfur is consumedafterit is first converted

intosulfuricacid andsulfuricacid is the leading sulfurend-use

in all forms. Although the long-term worldwide oversupply

situation is likely to continue, sulfuric acid consumption will

also continue to increase at a low rate [20,21]. Therefore,

sulfuric acid produced by the Ot-HyS process instead of by

a sulfuric acid manufacturing plant could meet this addition-

ally rising sulfuric acid demand by feeding the severely

increasing sulfur surplus recovered for environmental

reasons. The sulfuric acid product from the Ot-HyS process

could be supplied to the market or consumed directly near the

plant site. If the sulfuric acid product is directly provided to

neighboring consumers, it may be concentrated as necessaryfor individual customers. According to demand, the sulfuric

acid product may not be concentrated at all. In this case, the

process efficiency could be increased by as much as the

amount of reduced heat duty for the thermal sulfuric acid

concentration process.

In the sulfur furnace, a side reaction of sulfur dioxide with

oxygen forms sulfur trioxide. This is an exothermic equilib-

rium reaction with a very slow reaction rate and is the reverse

reaction of the high-temperature SAD (SO3 decomposition). In

the sulfuric acid manufacturing plant, this reaction, or so-

called ‘Conversion process,’ is favorable and is positioned in

the second stage after the first sulfur combustion stage

because sulfur trioxide is needed to produce sulfuric acid bybeing absorbed into water. Therefore, several catalyst beds,

generally vanadium pentoxide (V2O5), which is only effective

above its melting point of 400 C, are used to increase the

reaction rate. The product of each bed should be cooled

because sulfur trioxide is readily formed at temperatures of

about 400e700 C. Both a catalyst and cooling are needed for

reaction kinetics and equilibrium [20,21]. For the purpose of

sulfur dioxide production in this work, however, the forma-

tion of sulfur trioxide is undesirable and should be sup-

pressed. By maintaining a high sulfur dioxide concentration,

sulfur trioxide formation can be suppressed due to the

temperature dependency of the reaction equilibrium. If a high

sulfur dioxide concentration can be achieved, then the

furnace temperature becomes high enough to suppress the

formation of sulfurtrioxide. In this work, for the simplification

of the simulation, the formation of sulfur trioxide is dis-

regarded. This is because, under the given conditions of this

work, a high sulfur dioxide concentration and a high furnace

temperature without anycatalyst, only a negligiblequantity of

sulfur trioxide can be formed.

Temperatures, oxygen concentration, and residence timesin the typical “air” sulfur furnace make possible the formation

of thermal NOx [20,21]. The formation of NOx has its

maximum value at somewhere between 18e20% of sulfur

dioxide concentration. Although special staged-combustion

technology and gas recycling can be used to reduce NOx

formation, nitrogen, NOx and Ot-HyS process may not be

a good match. One of the advantages of the HyS cycle

including the Ot-HyS process is that it has a small number of

additional elements, chemical reactions, and separation

steps. The HyS cycle has only sulfur as the additional element

outside of hydrogen and oxygen. This advantage can be

deteriorated by the addition of undesirable nitrogen and NOx

if atmospheric air is directly used to burn the sulfur like inmost sulfur burning acid plants. Undesirable nitrogen and

NOx from sulfur combustion with the air eventually results in

the increase in chemical reactions, separation steps, as well as

the size of equipment because nitrogen makes up about 79%

by volume of air. For that reason, especially in the Ot-HyS

process, it is favorable that the oxygen used is as pure as

possible. Oxygen may be prepared by an air separation

system. In this work, for the simplification of the simulation,

sulfur is burned in the presence of the pure oxygen with no

consideration given to the work for the air separation. Addi-

tional work or cooling load for the separation process would

result in a few percent loss of the net thermal efficiency of the

overall Ot-HyS process.

3. Analysis

3.1. Flow sheet development

To estimate the net thermal efficiency of the proposed Ot-HyS

through Aspen Plus, which is a renowned chemical process

simulator, an appropriate flow sheet and thermodynamic

models are required.

The flow sheet for the Ot-HyS process is shown in the

Figs. 2 and 3. Stream conditions and compositions are

detailed in Table 1 and process net inflow and outflow aresummarized in Table 2. The main flow sheet and thermo-

dynamic model are based on SRNL’s recent work. All the

assumptions which were used in SRNL’s work are also

applied in this study [1,2]. PEM-type SDE is treated as a black

box operated at 100 C and 20 bar under the development

performance targets of the SRNL, such as a cell potential of

0.6 V versus a current density of 500 mA/cm2 and a sulfur

dioxide conversion of 40%. The acid concentration of the SDE

spent anolyte and the final sulfuric acid product after the

concentration process are set to be 50 wt% and 75 wt%,

respectively. A nine-equilibrium staged vacuum distillation

column including a partial condenser and a kettle reboiler is

used to concentrate the spent anolyte using thermal energy.

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 3 5 ( 2 0 1 0 ) 1 2 2 5 5 e1 2 2 6 7 12257







In addition to these elements, the main flow sheet consists of

an eight-equilibrium staged sulfur dioxide absorber, a three-

staged sulfur dioxide recycle compressor, and an anolyte

preparation tank. It also includes several knock-out drums,

pumps, valves, and two vacuum ejectors. Many water coolers

are required throughout the whole flow sheet under theassumption that the process cooling water is able to cool

down the process streams to about 40 C. Two recuperative

preheaters are also required. They are positioned in a row

before the vacuum distillation column. The first recuperative

preheater transfers heat from all the streams which have too

high a temperature to just release into the cooling water. The

main flow sheet was simulated using the OLI-MSE model,

which has been found to work very well in the H2SO4e

H2Osystem over the entire concentration range and to tempera-

tures below 500 C [22].

Fig. 2 e Flow sheet for the Ot-HyS process.

Fig. 3e

Flow sheet for the SCHRS including PCS.








Table 1 e Ot-HyS flow sheet stream table.

ID Phase Component mole flow [kmol/sec] T [C] P [bar]

H2O H2SO4 SO2 O2 H2 S Total

C1 L þ V 9 0 0 0 1.00229 0 10.00229 100.00 20.00

C2 L þ V 9 0 0 0 1.00229 0 10.00229 76.96 20.00

C3 V 0.02185 0 0 0 1 0 1.02185 76.96 20.00

C4 V 0 0 0 0 1 0 1 40.00 20.00

C5 L 8.97815 0 0 0 0.00229 0 8.98044 76.96 20.00

C6 L 0.02185 0 0 0 0 0 0.02185 40.00 20.00

C7 L 1 0 0 0 0 0 1 40.00 20.00

C8 L 10 0 0 0 0.00229 0 10.00229 73.19 20.00

C9 L 10 0 0 0 0.00229 0 10.00229 73.20 21.00

A1 L 26.4794 4.88405 1.5 0.00353 0 0 37.75103 100.00 20.00

A2 L 21.05779 3.88405 1.19288 0.0028 0 0 30.02158 100.00 20.00

A3 L 21.05779 3.88405 1.19288 0.0028 0 0 30.02158 80.00 20.00

A4 V 0.09086 0 0.28671 0.00072 0 0 0.37829 83.87 1.01

A5 L þ V 0.09086 0 0.28671 0.00072 0 0 0.37829 40.00 1.01

A6 V 0.03731 0 0.01769 0 0 0 0.055 80.18 0.30

A7 L þ V 0.03731 0 0.01769 0 0 0 0.055 40.00 0.30

A8 V 0.00267 0 0.00083 0 0 0 0.0035 38.66 0.09

A9 V 0.0035 0 0 0 0 0 0.0035 169.99 7.91

A10 V 0.00617 0 0.00083 0 0 0 0.007 94.01 0.30

A11 L 0.00591 0 0.00002 0 0 0 0.00593 40.00 0.30

A12 L 0.00591 0 0.00002 0 0 0 0.00593 40.04 1.01

A13 V 0.00026 0 0.00081 0 0 0 0.00107 40.00 0.30

A14 L 0.03162 0 0.00012 0 0 0 0.03174 40.00 0.30

A15 L 0.03162 0 0.00012 0 0 0 0.03174 41.24 21.00

A16 V 0.00595 0 0.01838 0 0 0 0.02433 40.00 0.30

A17 V 0.02433 0 0 0 0 0 0.02433 169.99 7.91

A18 V 0.03028 0 0.01838 0 0 0 0.04866 93.41 1.01

A19 L 0.02886 0 0.00041 0 0 0 0.02927 40.00 1.01

A20 L 0.03477 0 0.00043 0 0 0 0.0352 40.02 1.01

A21 V 0.00142 0 0.01797 0 0 0 0.01939 40.00 1.01

A22 L 0.06825 0 0.00096 0 0 0 0.06921 40.00 1.01

A23 L 0.06825 0 0.00096 0 0 0 0.06921 41.20 21.00

A24 V 0.02403 0 0.30371 0.00072 0 0 0.32846 40.00 1.01

A25 L 0.01562 0 0.00065 0 0 0 0.01627 40.00 2.78

A26 L 0.01562 0 0.00065 0 0 0 0.01627 41.11 21.00

A27 L 0.0084 0 0.29972 0.00001 0 0 0.30813 40.00 7.65

A28 L 0.0084 0 0.29972 0.00001 0 0 0.30813 41.25 21.00

A29 L þ V 0.00001 0 0.00334 0.00071 0 0 0.00406 40.00 21.00

A30 L 0.00001 0 0.00296 0 0 0 0.00297 40.00 21.00

A31 L 0.00841 0 0.30268 0.00001 0 0 0.3111 41.24 21.00

A32 V 0 0 0.00038 0.00071 0 0 0.00109 40.00 21.00

A33 V 0.00116 0 0.00852 0.25797 0 0 0.26766 42.44 21.00

A34 V 0 0 0 0.25797 0 0 0.25797 40.00 21.00

A35 L 0.00116 0 0.00852 0 0 0 0.00968 42.44 21.00

A36 L 2.8031 0 0 0 0 0 2.8031 40.00 21.00

A37 L 6.39641 0 0.99527 0.00071 0 0 7.3924 75.12 21.00

A38 e 0 0 0 0 0 0 0

A39 L 27.4794 3.88405 2.5 0.00353 0 0 37.75103 84.12 21.00A40 L 27.4794 3.88405 2.5 0.00353 0 0 37.75103 78.77 21.00

B1 L 5.42161 1 0.30712 0.00072 0 0 7.72945 100.00 20.00

B2 L þ V 5.42161 1 0.30712 0.00072 0 0 7.72945 83.87 1.01

B3 L 5.33075 1 0.02042 0 0 0 7.35116 83.87 1.01

B4 L þ V 5.33075 1 0.02042 0 0 0 7.35116 80.18 0.30

B5 L 5.29344 1 0.00272 0 0 0 7.29616 80.18 0.30

B6 L þ V 5.29344 1 0.00272 0 0 0 7.29616 66.92 0.11

B7 L þ V 5.29344 1 0.00272 0 0 0 7.29616 79.72 0.11

B8 L þ V 5.29344 1 0.00272 0 0 0 7.29616 94.41 0.11

B9 L 1.79616 1 0 0 0 0 3.79616 123.62 0.13

B10 L 3.4946 0 0.0019 0 0 0 3.4965 38.66 0.09

B11 L 3.4946 0 0.0019 0 0 0 3.4965 38.86 21.00

S10 V 0 0 1.00043 0.25797 0 0 1.25841 75.00 21.00

(continued on next page)








The biggest difference between the Ot-HyS and HyS flow

sheet comes from the replacement of the SAD process with

the sulfur combustion process. A detailed flow sheet for the

sulfur combustion process and the Sulfur Combustion Heat

Recovery System (SCHRS), including the PCS (in this case, the

Rankine cycle) was developed by referring to existing facili-ties. The processwas simulated using an ideal model, which is

generally used for the sulfur combustion system at high-

temperature gas phase, and the STEAMNBS model, which is

the recommended property model for the steam cycles

including compressors and turbines.

The pure oxygen sulfur combustion process was developed

by referring to the liquid sulfur dioxide manufacturing plant

developed by Quimetal in Chile [20]. In this configuration,

molten sulfur fed to the furnace is burned in the presence of

a gas mixture containing oxygen and sulfur dioxide. This gas

mixture consists of the following elements: first, an internally

recycled stream to maintain the moderate operating temper-

ature of the sulfur furnace (1079 C); second, an externallyrecycled oxygen stream from the sulfur dioxide absorber; and

last, an additional feed stream of dried pure oxygen to make

up for the deficient oxygen feed. A small amount of sulfur

dioxide loss purged through the ejector blow-downs is

compensated in the sulfur combustion process by feeding

more molten sulfur and oxygen of the same amount. The

resulting combustion gas is regulated to 79.5% of sulfur

dioxide and 20.5% of oxygen. The furnace temperature of the

sulfur combustion with pure oxygen at this level of high gas

strength will become very high and therefore cause some

problems. However, the sulfur furnace can be maintained at

a moderate operating temperature just by recycling a large

amount of cold combustion gas to the sulfur furnace again.

The resulting combustion gas is cooled in the SCHRS inside

the sulfur furnace, for the process heat duty of the vacuum

distillation column’s reboiler, and for the electricity genera-

tion in the PCS. After that, it really leaves the sulfur furnace.

As a PCS, a simple steam Rankinecycle was used to convert

the recovered heat of sulfur combustion for electricity. Themaximum steam pressure and temperature of the Rankine

cycle were set to be 160 bar and 540 C, respectively, with

reference to the 200 MW units at Western Power’s Kwinana

Power Station [23]. Steam is superheated and reheated once

each and is expanded twice in the high pressure turbine and

low pressure turbine which have 90% of the isentropic effi-

ciencies. The back pressure of the low pressure turbine was

set as 0.2 bar for the condensation pressure and temperature

limitations under the assumption that the overall process

streams can be cooled to 40 C with process cooling water. The

back pressure of the high pressure turbine was set as 40 bar

for the vapor fraction limitation of the low pressure turbine

downstream. At this pressure, the vapor fraction leaving thelow pressure turbine is high enough (about 95%).

After pressurization to 21 barusing a fresh SO2 compressor,

the combustion gas leaving the sulfur furnace is cooled to

100 C for theprocess heat duty of the second preheater, and is

then cooled again to 75 C, which is slightly above its boiling

point, using processcooling water. Because energy transferred

to the stream bythe fresh SO2 compressor shouldbe recovered

as much as possible, the process heat duty for the second

preheater is recovered after the fresh SO2 compressor.

After that, the majority of the combustion gas is recycled to

the sulfur furnace to avoid a high furnace operating temper-

ature. It does not affect the gas concentration because it is the

same concentration as the gas existing in the sulfur furnace.

Table 1 ( continued )



A34 V 0 0 0 0.25797 0 0 0.25797 40.00 21.00

S0 L 0 0 0 0 0 1.00043 1.00043 132.22 10.34

S1 V 0 0 0 1.00043 0 0 1.00043 40.00 21.00

S11D V 0 0 4.00172 1.0319 0 0 5.03362 75.00 21.00

S2 V 0 0 4.00172 2.2903 0 0 6.29202 67.59 21.00

S3 V 0 0 5.00215 1.28987 0 0 6.29202 1078.99 10.34

S3D V 0 0 5.00215 1.28987 0 0 6.29202 1078.99 10.34

S3 V 0 0 5.00215 1.28987 0 0 6.29202 1078.99 10.34

S4 V 0 0 5.00215 1.28987 0 0 6.29202 895.44 10.34

S5 V 0 0 5.00215 1.28987 0 0 6.29202 807.90 10.34

S6 V 0 0 5.00215 1.28987 0 0 6.29202 189.65 10.34

S7 V 0 0 5.00215 1.28987 0 0 6.29202 280.10 21.00

S8 V 0 0 5.00215 1.28987 0 0 6.29202 100.00 21.00

S9 V 0 0 5.00215 1.28987 0 0 6.29202 75.00 21.00

R1 L 3.22867 0 0 0 0 0 3.22867 41.78 160.00

R2 V 3.22867 0 0 0 0 0 3.22867 540.00 160.00

R3 V 3.22881 0 0 0 0 0 3.22881 333.58 40.00

R4 V 3.22881 0 0 0 0 0 3.22881 540.00 40.00

R5 M 3.22867 0 0 0 0 0 3.22867 60.07 0.20

R6 L 3.22867 0 0 0 0 0 3.22867 40.00 0.20

S9D V 0 0 5.00215 1.28987 0 0 6.29202 75.00 21.00

S9 V 0 0 5.00215 1.28987 0 0 6.29202 75.00 21.00

S10 V 0 0 1.00043 0.25797 0 0 1.2584 75.00 21.00

S11 V 0 0 4.00172 1.0319 0 0 5.03362 75.00 21.00








The remaining amount of the combustion gas consists of

79.5% sulfur dioxide and 20.5% oxygen and is finally fed to the

bottom stage of the SO2 absorber. After being separated from

the sulfur dioxide in the SO2 absorber, the oxygen is dried and

then recycled to the sulfur combustion process again.Together with the recovered anolyte and water, dissolved

sulfur dioxide from the combustion gas makes the fresh

anolyte. It is supplied to the SDE to produce hydrogen.

4. Results and discussion

4.1. Energy requirement and efficiency

The specifications on the SCHRS, including the Rankine cycle,

are presented in Table 3. In the HXC-RB inside the sulfur

furnace, 60.1 MWth /kmol-H2 heat of sulfur combustion is

recovered for the heat duty of the vacuum distillation

column’s reboiler. After the fresh SO2 compressor, in the HXC-

02 outside the sulfur furnace, 51.2 MWth /kmol-H2 heat of

sulfur combustion is recovered for the heat duty of the second

preheater. Of the 215.7 MWth /kmol-H2 heat of sulfur

combustion supplied from the sulfur furnace through steamgenerators RSG-01and RSG-02 to the Rankine cycle, 37.3% is

converted to the work which is produced in the high and low

pressure turbines (RTB-HP, RTB-LP) minus the work required

for the condensate/feed pump (RPP). Under the assumption

that there is no conversion loss from work to electricity, this

corresponds to 80.5 MWe /kmol-H2 of electric power and is

provided to the main flow sheet. The rest of the heat supplied

to the Rankine cycle, 135.2 MWth /kmol-H2, is thus rejected

from the process by means of process cooling water.

There are a total of 22 heat exchanger blocks in the whole

flow sheet. Among them, 2 are steam generators for the

Rankine cycle and1 is a water coolerused forthe Rankine cycle

condenser. There are also another 11 water coolers using

Table 2 e Ot-HyS flow sheet net inflow and outflow.



1. SO2 depolarized electrolysis

Cathode inflow C9 L 10 0 0 0 0.00229 0 10.00229 73.20 21.00

Cathode outflow C1 M 9 0 0 0 1.00229 0 10.00229 100.00 20.00

Cathode reaction Diff. À1 0 0 0 1 0 0 26.8 À1

Anode inflow A40 L 27.4794 3.88405 2.5 0.00353 0 0 37.75103 78.77 21.00

Anode outflow A1 L 26.4794 4.88405 1.5 0.00353 0 0 37.75103 100.00 20.00

Anode reaction Diff. À1 1 À1 0 0 0 0 21.23 À1

SDE net reaction (Outflow-Inflow) Diff. À2 1 À1 0 1 0

2. Ot-HyS except sulfur combustion

Hydrogen production C4 V 0 0 0 0 1 0 1 40.00 20.00

Sulfuric acid production B9 L 1.79616 1 0 0 0 0 3.79616 123.62 0.13

Oxygen separation A34 V 0 0 0 0.25797 0 0 0.25797 40.00 21.00

Waste purge A20 L 0.03477 0 0.00043 0 0 0 0.0352 40.02 1.01

Total outflow 1.83093 1 0.00043 0.25797 1 0 5.08933

Additional water for SDE C7 L 1 0 0 0 0 0 1 40.00 20.00

Sulfur combustion gas S10 V 0 0 1.00043 0.25797 0 0 1.25841 75.00 21.00

Steam for vacuum ejector #1 A17 V 0.02433 0 0 0 0 0 0.02433 169.99 7.91


Make-up anolyte A36 L 2.8031 0 0 0 0 0 2.8031 40.00 21.00

Total inflow 3.83093 0 1.00043 0.25797 0 0 5.08934

Net flow (Outflow-Inflow) Diff. À2 1 À1 0 1 0

3. Ot-HyS including sulfur combustion

Hydrogen production C4 V 0 0 0 0 1 0 1 40.00 20.00

Sulfuric acid production B9 L 1.79616 1 0 0 0 0 3.79616 123.62 0.13

Waste purge A20 L 0.03477 0 0.00043 0 0 0 0.0352 40.02 1.01

Total outflow 1.83093 1 0.00043 0 1 0 4.83136 e e

Sulfur feed for sulfur combustion S0 L 0 0 0 0 0 1.00043 1.00043 132.22 10.34

Oxygen feed for sulfur combustion S1 L 0 0 0 1.00043 0 0 1.00043 40.00 21.00

Additional water feed for SDE C7 L 1 0 0 0 0 0 1 40.00 20.00



Make-up anolyte A36 L 2.8031 0 0 0 0 0 2.8031 40.00 21.00

Total inflow 3.83093 0 0 1.00043 0 1.00043 5.83179 e e

Ot-HyS net reaction (Outflow-Inflow) Diff. À2 1 0.00043 À1.00043 1 À1.00043








process cooling water. The remaining 8 heat exchangers are

divided into hot sides and cold sides as if there are actually 3

heat exchangers. However, the enthalpy differenceof the each

streammightnotbedirectlyusedfortheheattransfer.Instead,the hot side streams might be cooled with water, which might

then be used to heat the cold side streams as an intermediate

heat transfer medium. The first preheater, HX-01, consists of

HXC-01A, HXC-01B, HXC-01C, which are its hot sides being

cooledand HXH-01whichis itscold side being heated. As inthe

case of HX-01, the second preheater, HX-02, and the reboiler of

the vacuumdistillation columnalso consistof HXC-02, HXH-02

and HXC-RB, HXH-RB, respectively.

Process heat duties and cooling loads are detailed in

Table 4. All the heat duties are required only for the acid

concentration process and correspond to 190.7 MWth /kmol-

H2. The heat duty for the first and second preheaters (HX-01,

HX-02) accounts for 68.5% (130.6 MWth /kmol-H2) of the heatduties, while that of the vacuum distillation column’s reboiler

accounts for the remaining 31.5% (60.1 MWth /kmol-H2). Only

41.7% (79.5 MWth /kmol-H2) for HX-01 comes from process

internal heat recuperation of the main flow sheet while 58.3%

(111.2 MWth /kmol-H2) for HX-02 and the reboiler comes from

the heat of the sulfur combustion.

There are a total of 12 water coolers using process cooling

water in the whole flow sheet except for a condenser for the

Rankine cycle, as mentioned above. The amount of waste heat

rejected by cooling water is 194.4 MWth /kmol-H2. The cooling

load for the condenser of vacuum distillation column

accounts for 86.4% (À167.9 MWth /kmol-H2). If the cooling load

for the Rankine cycle condenser (À135.2 MWth /kmol-H2) isconsidered, then the total amount of waste heat becomes

329.6 MWth /kmol-H2. In this case, the cooling load for the

vacuum distillation column’s condenser accounts for 50.9%,

while that for the Rankine cycle condenser accounts for 41.0%

in total.

Electric power requirements are listed in Table 6. A total of

142.4 MWe /kmol-H2 is required in the process. 81.3%

(115.8 MWe /kmol-H2) of it is consumed by SDE and 16.7%

(23.8 MWe /kmol-H2) by fresh SO2 compressor COeSO2. The

approximate remainder of about 2.0% comes from SO2 recycle

compressor CO-01 and other pumps. It should be noted that

the actual pumping power will be higher due to frictional

losses ignored during the simulation. Of the total 142.4 MWe /

kmol-H2 electricity required for the process, 56.5% (80.5 MWe /

kmol-H2) is supplied by the Rankine cycle in SCHRS while only

61.9 MWe /kmol-H2, or about 43.5%, is supplied by nuclear

energy. If a thermal-to-electric conversion efficiency of the

nuclear power plant of 33.3% is assumed, the electric power

supplied by nuclear energy corresponds to a heat input of

185.7 MWth /kmol-H2. Considering 215.7 MWth /kmol-H2 of

sulfur combustion heat recovered by the Rankine cycle, thetotal equivalent heat requirement corresponding to the total

electric power requirement is 401.4 MWth /kmol-H2.

A small quantity of steam is also required for the two-stage

vacuum ejector. Stream A17 for the first-stage steam ejector

and stream A9 for the second-stage steam ejector carry

a small amount of steam under the one-to-one molar

entrainment ratio. The total amount of steam is about

0.02783 kmol/s, which corresponds to a 1.31 MWth /kmol-H2

heat duty. The heat duty for the steam was estimated by the

enthalpy difference between the boiler feed water at 40 C and

the 7.91 bar steam, as shown in Table 5. Estimated heat forthis

steam generation is less than one-percent. Therefore, it hardly

affects the efficiency estimation.As shown in the Table 7, the total heat and equivalent heat

supplied to the Ot-HyS by the SCHRS as well as by the nuclear

power plant is 513.9 MWth /kmol-H2. Therefore, the net

thermal efficiency is 47.1% based on the lower heating value

(LHV) of hydrogen, at 242 MWth /kmol-H2 and 55.7% based on

the higher heating value (HHV) of hydrogen, at 286 MWth /

kmol-H2 with no consideration given to the work for the air

separation. Schematic configuration and major simulation

results of the Ot-HyS process are shown in Fig. 4.

Table 3 e Thermal-to-electric conversion efficiency of theRankine cycle in the SCHRS.

Steam Rankine cycle -PCS(Power Conversion System)

Heat or work [MWth]or [MWe]

Heat input to Rankine cycle 215.7

RSG-01 28.3

RSG-02 187.4Work output from Rankine cycle 81.8

RTB-HP 21.0

RTB-LP 60.8

Work input to Rankine cycle (RPP) 1.2

Net work output from Rankine cycle 80.5

Heat rejected from Rankine cycle (RCD) 135.2

Thermal-to-electric conversion efficiency 37.33%

Table 4 e Required process heat in the Ot-HyS process.

Block ID Heat[MWth]

Temperature [C] Heat exchangedwith :

Inlet Outlet

CL-01 À3.6 83.87 40.00 Cooling Water




CO-01/Stage 1

Cooler

À2.1 137.99 40.00 Cooling Water

CO-01/Stage 2

Cooler


CO-01/Stage 3

Cooler


DR-01 À2.0 76.96 40.00 Cooling Water

DR-02 À0.2 42.44 40.00 Cooling Water

HXC-01A À17.7 100.00 76.96 HXH-01

HXC-01B À15.5 84.12 78.77 HXH-01

HXC-01C À46.2 100.00 80.00 HXH-01

HXC-02 À51.2 280.10 100.00 HXH-02

HXC-RB À60.1 1079.82 896.29 HXC-RB

HXH-01 79.5 66.92 79.72 HXC-01A, B, C

HXH-02 51.2 79.72 94.41 HXC-02

HXH-RB (TO-01

Reboiler)

60.1 e 123.62 HXC-RB

CL-CD (TO-01

Condenser)

À167.9 e 38.66 Cooling Water

CL-SO2À

7.1 100.00 75.00 Cooling Water








4.2. Discussion

Using Aspen Plus with appropriate thermodynamic models,

simulation of the Ot-HyS process successfully demonstrates

that it is an energy-efficient hydrogen production process. The

process produces 1 kmol/s of hydrogen and 75 wt% dilute

sulfuric acid containing 1 kmol/s of sulfuric acid while

consuming 1 kmol/s of sulfur and oxygen with 3.83 kmol/s of

water. It also consumes 513.9 MWth /kmol-H2 of the total heat

and equivalent heat supplied by the SCHRS (including the

Rankine cycle) as well as by the nuclear power plant with its

thermal-to-electric conversion efficiency of 33.3%, resulting in

a process net thermal efficiency of 47.1% (based on LHV) and

55.7% (based on HHV). If a nuclear power plant provides

electricity with a higher thermal-to-electric conversion effi-

ciency of about 42.0%, as can a PBMR, then the process net

thermal efficiency would be 50.9% (based on LHV) and 60.1%

(based on HHV).

To investigate how much more efficient the Ot-HyS

process is, conventional water electrolysis coupled to nuclear

power plant and SRNL’s PBMR/HyS cycle were chosen as the

benchmarks, because the Ot-HyS process is based on water

electrolysis technology and the HyS cycle. Alkaline electrol-

ysis and PEM electrolysis can produce hydrogen at high

pressure using 387 MWe /kmol-H2 (4.8 kWh/Nm3) [24] and

355 MWe /kmol-H2 (4.4 kWh/Nm3) [25]. Combining these with

a conventional nuclear power plant with a thermal-to-electric

conversion efficiency of 33.3% brings 24.6% and 26.9% of net

thermal efficiencies based on HHV. Their total equivalent heat

requirements are 1161.1 MWth /kmol-H2 and 1065.1 MWth /

kmol-H2, respectively. If they are combined with a PBMR

nuclear power plant that has a thermal-to-electric conversion

efficiency of 42.0%, then their HHV basis net thermal effi-

ciencies are 31.0% and 33.8%, with total equivalent heat

requirements of 921.4 MWth /kmol-H2 and 845.2 MWth /kmol-

H2, respectively. Based on SRNL’s work, its PBMR/HyS cycle

has HHV basis net thermal efficiencies of 40.6%, excluding the

power needed for the helium circulators. That of the reference

500-MWth PBMR/HyS cycle, including the electric power

requirements of the helium circulator, NHSS, PGS and BOP, is

even lower (36.7%) Net thermal efficiency benchmarking

results are summarized in Table 8.

In conclusion, the Ot-HyS process is a significantly

energy-efficient hydrogen production process. For a simple

comparison, based on HHV and a nuclear power plant with

a thermal-to-electric conversion efficiency of 42.0%, the net

thermal efficiency of the Ot-HyS process is about 25.0e30.0%

higher than that of conventional water electrolysis and

about 20.0% higher than that of SRNL’s PBMR/HyS cycle.

Although we already knew that the Ot-HyS process is more

Table 5 e Steam generation and feed for the vacuum ejector.

Total Flow [kmol/sec] Pressure [bar] Temperature [C] Enthalpy [kJ/sec]

Steam Steam Water Steam Water Steam Water

1st-stage vacuum ejector 0.0035 7.91 7.91 169.99 40 À832.09 À996.395

2nd-stage vacuum ejector 0.02433 7.91 7.91 169.99 40 À5783.68 À6925.75

Total 0.02783 7.91 7.91 169.99 40 À6615.77 À7922.15

Enthalpy difference between steam and boiler feed water, MWth /kmol-H2 1.31

Table 6 e Required process electricity in the Ot-HySprocess.

Block ID Work [MWe]

PEM SDE 115.8

CO-SO2 23.8

CO-01/Stage 1 1.3

CO-01/Stage 2 1.2

CO-01/Stage 3 1.5.E-02

PP-01 2.2.E-02

PP-02 0.2

PP-03 2.6.E-05

PP-04 4.0.E-03

PP-05 8.6.E-03

PP-06 1.9.E-03

PP-07 2.6.E-02

Total 142.4

Table 7e

Net thermal efficiency of the Ot-HyS process.Once-through hybridsulfur process

Heat or work [MWth]or [MWe]

Total cooling load À385.1

Recuperative heat 79.5

Net cooling load À305.6

Supplied by sulfur combustion 111.2

Supplied by cooling water 194.4

Total heat requirement 190.7

Recuperative heat À79.5

Net heat requirement 111.2


Supplied by nuclear energy 0.0

Heat load for vacuum ejector

steam generation

1.3

Total electricity requirement 142.4


Efficiency 37.33%

Equivalent heat requirement 215.7

Supplied by nuclear energy 61.9

Efficiency 33.33%

Equivalent heat requirement 185.7

Total equivalent heat requirement 401.4

LHV of H2 242

HHV of H2 286

Net thermal efficiency

(based on LHV of H2)

47.09%

Net thermal efficiency

(based on HHV of H2)

55.65%








energy-efficient than benchmark processes because of its

characteristics, the focus of this benchmarking is on the

quantification of how much more efficient the Ot-HyS

process is.

Hydrogen produced from off-peak electricity by the Ot-HyS

process can be stored and reconverted into electricity again

during peak-load time by a fuel cell. Using heat from the sulfur

combustion that includes 80.5 MWe /kmol-H2 of electric

energy, 61.9 MWe /kmol-H2 of nuclear electricity generation is

first converted into 1 kmol/s of hydrogen production as energy

storage. Then, based on the LHV of hydrogen, this 242 MWth /kmol-H2 of heat is reconverted to electricity corresponding to

72.6 MWe /kmol-H2, 121 MWe /kmol-H2 and 169.4 MWe /kmol-

H2 under fuel cell efficiencies of 30.0%, 50.0% and 70.0%,

respectively (85.8 MWe /kmol-H2, 143.0 MWe /kmol-H2 and

200.2 MWe /kmol-H2, based on HHV). Nuclear power plants,

which have high capital cost, and low operating cost are

generally operated continuously generating the base load

because it is the most economic mode. However, it is also

technically the simplest way, since nuclear power plants

cannot alter power output as readily as other power plants.

Although some plants being built today have load-following

capacity, this is still a complex problem. With hydrogen

produced through the Ot-HyS process, nuclear energy, which

is a sustainable development technology, can be applied to

a greater range of applications. If 69,000 thousand metric tons

of all the sulfur annually produced in all forms worldwide

were supplied to the Ot-HyS process, about 4313 thousand

metric tons of hydrogen could be produced annually from the

Ot-HyS process based on one-to-one molar ratio. If electricity

were produced from the stored hydrogen 24 h a day, 365 days

a year, the produced electricity would be equivalent to

11.6 GWe (LHV basis, 70% of fuel cell efficiency is assumed). If

the stored hydrogen were used just for 3 h of peak-load time,the produced electricity would be equivalent to 92.7 GWe. The

electricity produced in 2007 was about 16.42 trillion kWh. This

is equivalent to 1900.5 GWe. Considering 92.7 GWe of supply

from stored hydrogen, 4.9% of electricity can be supplied from

hydrogen reservoir.

The effect of the thermal-to-electric conversion efficiency

ofthe PCSin SCHRS on netthermal efficiencyis shownthrough

graphs in Fig.5. Obviously, the netthermal efficiencyincreases

as the thermal-to-electric conversion efficiency of the PCS in

SCHRS increases. If it increases up to 42% using more efficient

PCS inSCHRS,andif thatof thenuclearpowerplantis also42%,

thenthe net thermalefficiency of the Ot-HyS processwouldbe

Fig. 4 e Schematic configuration of the Ot-HyS process with its major simulation results.

Table 8 e Net thermal efficiency benchmarking.

Benchmark (based on 1 kmol/s-H2) Ot-HyS SRNL’s PBMR/HyS [1,2] Conventional water electrolysis

Alkalineelectrolysis [24]

PEMelectrolysis [25]

Heat requirement [MWth] 112.5 417.1 0.0 0.0

Electricity requirement [Mwe] 142.4 120.9 387.0 355.0

Thermal-to-electric conversion efficiency of NPP 33.33% 42.00% 42.00% 33.33% 42.00% 33.33% 42.00%

Equivalent heat requirement [MWth] 401.4 363.0 287.9 1161.1 921.4 1065.1 845.2

Total heat requirement [MWth] 513.9 475.6 705.0 1161.1 921.4 1065.1 845.2

Net thermal efficiency (LHV basis) 47.09% 50.88% 34.33% 20.84% 26.26% 22.72% 28.63%

Net thermal efficiency (HHV basis) 55.65% 60.14% 40.57% 24.63% 31.04% 26.85% 33.84%








53.7% (LHV basis) and 63.5% (HHV basis). Because of the great

amount of high-temperature excess heat from the sulfur

combustion process, there is much room for the use of more

efficient PCS such as the supercritical CO2 Brayton cycle rather

than the simple Rankine steam cycle used in this work.

In the absence of a detailed model for its phenomenon, SDE

has been treated as a black box using the development

performance targets of SRNL. The SDE is assumed to operateat a fixed cell potential of 600 mV versus current density of

500 mA/cm2, with a fixed SO2 conversion of 40%, and at a fixed

50 wt% H2SO4 product. Among these, the cell potential is

directly related to the net thermal efficiency of the Ot-HyS

process. Electric energy required for the SDE, which is the only

remaining technical challenge of the Ot-HyS process,

accounts for the greatest portion of the total required energy.

In other words, it has the most dominant effect on the net

thermal efficiency. Based on the target energy demand of

150% for the SDE, the net thermal efficiency will be signifi-

cantly reduced to 35.2% (LHV basis) and 41.6% (HHV basis), as

shown in Fig. 6. In the past year, the SDE research project was

focused on demonstration of the SDE operation without sulfurlayer build-up inside of the MEA, which is a phenomenon that

reduces performance as well as limits operating lifetime. In

the past year, SRNL has demonstrated successful long time

operation of the single cell SDE without sulfur build-up limi-

tations using a new operating method. The test using the new

sulfur-limiting operating method was conducted with

a Nafion membrane, which has a 54 cm2 active cell area, at

80 C and 172 kPa of the cell operating conditions. The cell

potential was stable at approximately 760 mV, which is

beyond the development performance target of 600 mV, while

the current density was maintained at the development

performance target of 500 mA/cm2. However, if this method is

applied for more advanced membranes under highertemperature and pressure in the future configuration, the

target cell potential of 600 mV can be achieved.

Although a detailed heat transfer analysis has not yet been

conducted, temperature differences betweenthe hotsides and

cold sides were set high enough to assume that all heat

transfers can be achieved as designed for the purpose of the

process net thermal efficiency estimation. An additional

pumping power will be required to compensate the ignored

pressure dropsthrough the process. Additionalenergy willalso

berequiredtopumpprocesscoolingwaterthatallowsatotalof

329.6 MWth /kmol-H2 waste heat rejection to and from the

cooling tower. The amount of additionally required electric

energy also will not be significant and will cause no significant

Fig. 5 e The effect of the PCS efficiency on the net thermal efficiency of the Ot-HyS process.

Fig. 6 e The effect of the SDE electricity requirement on the

net thermal efficiency of the Ot-HyS process.








loss of net thermal efficiency. However, its ability to cool down

the process stream to about 40 C should be available year-

round by maintaining the temperature of process cooling

waterat 30 C orlower.Otherwise, some ofthe coolers, like the

partial condenser of the vacuum distillation column, would

need to be refrigerated condensers.

In the SRNL’s PBMR/HyS report, hydrogen price from the

PBMR/HyS plant was calculated as $6.18/kg H2 based on Shaw’sestimation and $5.34/kg H2 based on SRNL/Icarus’s estimation

[2]. In the CEA’s recent work, hydrogen price from its HyS cycle

configuration was calculated as V6.6/kg H2 [26]. Although no

detailed economic feasibility study was conducted here,

economics of the Ot-HyS plant has some positive and negative

economic potentials that can be easily found when compared

with that of the PBMR/HyS plant. In terms of O&M cost, there is

no need for nuclear fuels, but the Ot-HyS plant needs sulfur as

well as about 3.8timesmore wateras the process feed, than the

PBMR/HyS plant does. The Ot-HyS plant lost the oxygen by-

productcredit,but nowcan take 75 wt%sulfuric acidby-product

credit, instead. In addition to that, the Ot-HyS plant needs less

net electric power from grid than the PBMR/HyS plant does.From the view point of the capital cost, Ot-HySplant could save

costs for the SAD andPBMR systems which are more expensive

than the sulfur combustion system.

5. Conclusion

This study was conducted for the purpose of demonstrating

that the proposed Once-through Hybrid Sulfur (Ot-HyS)

process, which is competitive due to thefactthat it is basedon

technology of the SDE of the Hybrid Sulfur cycle (HyS) and on

the well-established sulfur combustion process, is an energy-

efficient nuclear hydrogen production process that can help toresolve the major energy challenges we are facing.

In order to demonstrate that, a flow sheet for the sulfur

combustion process, including its heat recovery system, was

developed by referring to existing facilities. By combining this

with the main flow sheet developed for the HyS cycle by SRNL

with small modifications, detailed flow sheet for the Ot-HyS

process was completed. After that, the flow sheet developed

for the Ot-HyS process was simulated using Aspen Plus with

appropriate thermodynamic models. Finally, using the simu-

lation result, the net thermal efficiency of the Ot-HyS process

was benchmarked with those of others and discussed.

It was found that the net thermal efficiency was 47.1%

(based on LHV) and 55.7% (based on HHV) assuming 33.3%thermal-to-electric conversion efficiency of the nuclear power

plant, and also 50.9% (based on LHV) and 60.1% (based on

HHV) assuming 42.0% thermal-to-electric conversion effi-

ciency of the nuclear power plant. The Rankine cycle in the

SCHRS (Sulfur Combustion Heat Recovery System) was

applied with 37.3% thermal-to-electric conversion efficiency.

This efficiency was also significantly higher than those of the

benchmarking hydrogen production processes such as

nuclear-powered conventional water electrolysis and SRNL’s

PBMR/HyS cycle.

There is much room for the use of a more efficient PCS

(Power Conversion System). If both the nuclear power plant

and PCS in the SCHRS have 42% thermal-to-electric conversion

efficiency, the net thermal efficiency would be 53.7% (LHV

basis) and 63.5% (HHV basis). The progress of development for

the SDE cell potential has a dominant effect on the net thermal

efficiency. The SDEis the only remaining technicalchallenge of

the Ot-HySprocess. In orderfor the Ot-HySprocess to maintain

its competitiveness, the development performance targets of

SRNL should be achieved as anticipated. Hydrogen produced

through the energy-efficient Ot-HyS process would be used asthe off-peak electric energy storage, to relieve the burden of

load-following and would help to expand applications of

nuclear energy.

Because of its competitive advantages, such as a higher net

thermal efficiency, less technical challenge and favorable

sulfur statistics, the Ot-HyS process could play an important

role in securing a bridge to the sustainable energy future

during the short-term transitional period. Although it was

demonstrated that the Ot-HyS process is an energy-efficient

hydrogen production process, it is the unit cost of H2

production ($/kg-H2) that will ultimately determine whether

the Ot-HyS processis feasible. If hydrogen produced by the Ot-

HyS process is also economically favorable, then thecompetitive edge of the Ot-HyS process could be further

secured. A detailed economic feasibility study will be the main

topic of our future work.

Acknowledgement

This research was supported by the WCU (World Class

University) program through the National Research Founda-

tion of Korea funded by the Ministry of Education, Science and

Technology (R33-2008-000-10047-0).

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