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 en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 0
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Design of systems for hydrogen production basedon the CueCl thermochemical water decompositioncycle: Configurations and performance
Mehmet F. Orhan, Ibrahim Dincer, Marc A. Rosen*
Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North,
Oshawa, Ontario L1H 7K4, Canada
a r t i c l e i n f o
Article history:
Received 31 August 2010
Received in revised form
4 February 2011
Accepted 7 February 2011
Available online 1 April 2011
Keywords:
Hydrogen production
Thermochemical water
decomposition
Nuclear
Renewable energy
Thermodynamic analysis
Copperechlorine cycle
* Corresponding author.E-mail addresses: [email protected]
0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.02.034
a b s t r a c t
In this study, we analyze several Cu-Cl cycles by examining various design schemes for an
overall system and its components, in order to identify potential performance improve-
ments. The factors that determine the number and effective grouping of steps for new
design schemes are analyzed. A thermodynamic analysis and several parametric studies
are presented for various configurations. The energy efficiency is found to be 44% for the
five-step thermochemical process, 43% for the four-step process and 41% for the three-step
process, based on the lower heating value of hydrogen. Also, conclusions regarding
implementation of these new configurations are discussed and the potential benefits
ascertained.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction All devices incur irreversibilities. Systems or devices
Concerns regarding global climate change are significant and
have resulted in extensive R&D on alternative, clean
energy sources.Whilemany of the available renewable energy
resources are limited due to their reliability, quality, quantity
and density, nuclear energy has the potential to contribute
a significant share of energy supply without contributing
significantly to climate change. Hydrogen production via
thermochemical water decomposition is a potential process
for direct utilization of nuclear thermal energy to increase
efficiency and thereby facilitate energy savings.
(M.F. Orhan), ibrahim.di2011, Hydrogen Energy P
designed to produce work via chemical reaction, such as
thermochemical cycles and power plants, invariably are also
subject to other losses associated with their operation.
Efficient use of energy is a significant contributor to any
sustainable plan for meeting growing energy demands. The
main objective of a plant designer is to determine the optimal
plant configuration and operating conditions to satisfy
requirements according to specified constraints (environ-
mental and otherwise). As one part of pre-commercialization
optimization, it is essential to evaluate alternative/developed
design schemes in terms of their energy and exergy
[email protected] (I. Dincer), [email protected] (M.A. Rosen).ublications, LLC. Published by Elsevier Ltd. All rights reserved.
Fig. 1 e The CueCl thermochemical cycle for hydrogen
production.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 011310
efficiencies. The overall efficiencies of alternative design
schemes depend on parameters like operating temperature,
conversion efficiencies of subprocesses, and complexity of the
plant and its subsystems.
Thermochemical water splitting with a copperechlorine
(CueCl) cycle is a promising process that could be linked with
nuclear reactors to decompose water into its constituents,
oxygen and hydrogen, through intermediate copper and chlo-
rine compounds. The CueCl cycle is a hypothetical process
that has not yet been constructed. However, many studies of
the CueCl cycle are available in the literature. For example, Al-
Dabbagh and Lu have studied the design and reliability of
control systems for a CueCl thermochemical hydrogen
productionplant [1]. Equilibriumconversion in theCueCl cycle
multiphase processes has been studied by Daggupati et al. [2].
They perform a thermodynamic equilibrium analysis of indi-
vidual steps within the cycle. Recently, they studied solid
particle decomposition and hydrolysis reaction kinetics in
CueCl [3], andasolid conversionprocessduringhydrolysis and
decomposition of cupric chloride in the CueCl cycle [4]. One of
themost challenging steps in the thermochemical CueCl cycle
is the hydrolysis of CuCl2 into Cu2OCl2 and HCl while avoiding
the need for excess water and the undesired thermolysis
reaction, which yields CuCl and Cl2. Argonne National Labo-
ratory has designed a spray reactorwhere an aqueous solution
of CuCl2 is atomized into a heated zone, into which steam/Ar
are injected in co- or counter-current flow [5]. Also, an experi-
mental studyusing a spray reactorwith anultrasonic atomizer
has been carried out [6]. Jaber et al. [7] have studied heat
recovery from molten CuCl in the CueCl cycle. In that study,
they examined the convective heat transfer between molten
CuCl droplets and air in a counter-current spray flow heat
exchanger [7]. Ceramic carbon electrode-based anodes for use
in the CueCl cycle have been studied by Ranganathan and
Easton [8,9]; CCE materials, prepared using 3-aminopropyl tri-
methoxysilane, are investigated in that study. Wang et al. [10]
have compared sulphureiodine and copperechlorine ther-
mochemical hydrogen production cycles from the perspec-
tives of heat quantity, heat grade, thermal efficiency, related
engineering challenges, and hydrogen production cost [10].
Thermophysical properties of copper compounds in the CueCl
cycle have been studied by Zamfirescu et al. [11]. Furthermore,
the kinetics of the copper/hydrochloric acid reaction in the
CueCl cycle have been studied [12].
Although technical studies of the CueCl cycle have been
reported, there is a need for further developments and
improved design schemes to facilitate eventual commerciali-
zation. At present, there are different variations of the CueCl
cycle, which consist of different intermediate steps. The
numbers of steps and the methods of grouping have influ-
ences on the scale-up challenges and overall cycle efficiencies.
Therefore, it is important to compare the advantages and
disadvantages of different configurations of CueCl cycles. In
this study, we analyze the CueCl cycle by examining various
design schemes for an overall system and its components, in
order to identify potential performance improvements.
Several variations of the CueCl cycles with different numbers
of steps andmethods of grouping them are compared, and the
factors that determine the more effective number and
grouping of steps are analyzed. The main characteristics of
the cycles with different steps and numbers of steps are dis-
cussed. Also, conclusions regarding implementation of these
new configurations are presented and the potential benefits
ascertained. Finally, a thermodynamic analysis and several
parametric studies are presented for various configurations.
2. Description of systems
The CueCl cycle (Fig. 1) consists of a set of reactions to achieve
the overall splitting of water into its constituents, hydrogen
and oxygen. The overall net reaction is H2O(g)/H2(g)þ 1/
2O2(g). The CueCl cycle decomposes water into hydrogen and
oxygen through intermediate copper and chloride
compounds. These chemical reactions form a closed internal
loop that recycles all chemicals on a continuous basis, without
emitting any greenhouse gases.
Many studies (e.g. [1e13]) of the CueCl cycle have shown
that it offers a potentially attractive option for generating
hydrogen from nuclear energy. Compared with other
hydrogen production options, the thermochemical CueCl
cycle is expected to have a higher efficiency, to produce
hydrogen at a lower cost, and to have a smaller impact on the
environment by reducing airborne emissions, solid wastes
and energy requirements.
3. Analysis of configurations
The concept of thermochemical production of hydrogen from
water was first studied thermodynamically in the 1960s. The
decomposition, by the use of thermal energy, is limited by the
Carnot’s theorem. At standard temperature and pressure the
free energy and enthalpy changes for the direct splitting of
water are DG¼ 56.7 kcal/gmol and DH¼ 68.3 kCal/gmol,
respectively. It is not until around 4400 �C that the DG for the
reaction becomes negative. At such an extreme temperature
there are great problems with materials and separations
rendering the direct decomposition infeasible. The work
required for the one-step process can be reduced by increasing
the operating temperature; however, if anupper temperature of
1100 �C is imposed the reduction is modest. In a multi-step
Table 1 e Reactions in the three-step CueCl cycle.
Step Name Reaction Temperature range
S-i Hydrolysis 2CuCl2(aq)þH2O(g)
/ 2CuCl(l)þ 2HCl(g)
þ 1/2O2(g)þH2O(g)
400e600 �C
S-ii Electrolysis 4CuCl(s)þH2O(l)
/ 2Cu(s)þ 2CuCl2(aq)
20e80 �C
S-iii H2
production
2Cu(s)þ 2HCl(g)
/ 2CuCl(l)þH2(g)
430e475 �C
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 en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 0 11311
process it is, in theory, possible to reduce thework requirement
to zero by operating reactions with positive entropy changes at
high temperatures and reactions with negative entropy
changes at low temperatures. Considering a two-step cycle it is
possible to calculate ideal values of DH and DS in that cycle.
Comparing thosevalueswith existing chemical products shows
that no two-step cycle is possible within an 1100 �C limit.
Consequently all possible cycles will have three or more steps.
Three different variations of the CueCl cycle are currently
under investigation: 3-step, 4-step and 5-step cycles [13].
3.1. Three-step CueCl cycle
The CueCl cycle illustrated in Fig. 2 consists of the threemajor
reactions shown inTable 1. It canbe found that step i is roughly
equivalent to the grouping of steps 1, 2 and 4 of the five-step
cycle, or the combinationof steps I and II of the four-step cycle.
3.2. Four-step CueCl cycle
A four-step CueCl cycle is shown in Fig. 3, with the main
reactions described in Table 4. It can be found from Fig. 3 that
P1
S-
S-
S-iii
HE
HE7
H2O
INPUT
HCl (g)
CuCl(l)
CuCl(s)
Cu(s)
P2
H2 (20ºC )
OUT
CuCl(l)
430ºC -475ºC
430ºC -475ºC
400ºC
HE1
HCI(g) production,Fluidized bed
Steam (4
HE6
Heat recoveryheat
H2
H2 production
Fig. 2 e Conceptual flow chart
step I is roughly equivalent to the combination of steps 1 and 4
in the five-step cycle. As shown below, in the case of 5-step
cycle, aqueous cupric chloride is first dried into a solid product
of cupric chloride particles, and then fed to the hydrolysis
reactor to produce copper oxychloride. The 4-step cycle
combines these processes by supplying aqueous cupric chlo-
ride into the hydrolysis chamber, such as spraying the
solution with co-flowing steam to produce the same copper
oxychloride product (see Table 2). The 4-step process has
an advantage of reducing complexity by eliminating
solids handling and thus requiring less equipment. However,
i
ii
3
water
CuCl2+ Water
O2 (20ºC )
PUT
400ºC
20ºC
20ºC20ºC
HE2
00ºC )O2
HE4
400ºC
500ºC
heat
Cu production
HE: Heat Exchanger
S: Step
P: Pump
HE5Heat recovery
400ºC
20ºC
of three-step CueCl cycle.
P1
S-I
HE3
S-II
S-III
S-IV
HE4
HE8
H2O
INPUT
Cu2OCl2(s)
HCl (g)
CuCl(l)
CuCl(s)
water
CuCl2+ Water
Cu(s)
P2
H2 (20ºC ) O2 (20ºC )
OUTPUT
500ºC
400ºC
500ºC
20ºC
20ºC
CuCl(s)
430ºC -475ºC
20ºC
430ºC -475ºC
400ºC
HE1
HE2
HCI(g) production,Fluidized bed
Steam (400ºC )
O2 production
HE5
400ºC
HE7
500ºC
Heat recoveryheat
heat
H2
Cu production
H2 production
HE: Heat Exchanger
S: Step
P: Pump
HE6Heat recovery
400ºC
20ºC
Fig. 3 e Conceptual flow chart of four-step CueCl cycle.
Table 2 e Reactions in the four-step CueCl cycle.
Step Name Reaction Conditions
S-I Hydrolysis 2CuCl2(aq)þ 2H2O(g)
/Cu2OCl2(s)
þ 2HCl(g)þH2O(g)
375e400 �C
S-II O2 production Cu2OCl2(s)/ 1/2O2(g)
þ 2CuCl(l)
500e530 �C
S-III Electrolyzer 4CuCl(s)þH2O(l)
/ 2Cu(s)þ 2CuCl2(aq)
20e80 �C
S-IV H2 production 2Cu(s)þ 2HCl(g)
/ 2CuCl(l)þH2(g)
430e475 �C
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 011312
the 5-step process may be advantageous from the viewpoint
of energy and exergy efficiencies, because lower grade heat
can be used to remove the water in the drying process, rather
than high temperature heat in the hydrolysis reactor for the
unnecessary latent heat of vaporization of water.
3.3. Five-step CueCl cycle
A conceptual layout of a five-step CueCl cycle is illustrated in
Fig. 4. This case includes three thermochemical reactions and
one electrochemical reaction as shown in Table 3.
The cycle involves five steps:
1. HCl(g) production, using such equipment as a fluidized bed,
2. oxygen production,
3. copper (Cu) production,
4. drying, and
5. hydrogen production.
A chemical reaction takes place in each step, except drying
(see Table 3).
As illustrated in Fig. 4, only water and nuclear-derived heat
enter the cycle and only H2 and O2 are produced (with no
greenhouse gas emissions). Liquid water at ambient temper-
ature enters the cycle and passes through several heat
exchangers where it evaporates and increases in temperature
to 400 �C. Heat for this process is obtained from cooling the
hydrogen and oxygen gases before they exit the cycle. Steam
P1
S1
HE3
S2
S3
S5
HE4
HE8
H2O
S4
HE6
INPUT
Cu2OCl2(s)
HCl (g)
CuCl(l)
CuCl(s)
waterCuCl2
+ Water
Cu(s)
P2
CuCl2(s)
CuCl2(s) 400ºC
H2 (20ºC ) O2 (20ºC )
OUTPUT
500ºC
400ºC
500ºC
20ºC
20ºC
150ºC
CuCl(s)
430ºC -475ºC
20ºC
430ºC -475ºC
150ºC
400ºC
HE1
HE2
HCI(g) production,Fluidized bed
Steam (400ºC )
O2 production
HE5
150ºC
Flash dryer
HE7
500ºC
Heat recoveryheat
heat
heat
H2
Cu production
H2 production
HE: Heat Exchanger
S: Step
P: Pump
Fig. 4 e Conceptual flow chart of five-step CueCl cycle.
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 en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 0 11313
at 400 �C and solid copper chloride (CuCl2) at 400 �C from the
dryer enter the fluidized bed (S1), where a chemical reaction
occurs. The chemical formulation for this reaction is
2CuCl2(s)þH2O(g)/CuO * CuCl2(s)þ 2HCl(g) (1)
Table 3 e The five steps in the CueCl cycle with theircorresponding reactions.
Step Name Reaction Temperaturerange
S1 Hydrolysis 2CuCl2(s)þH2O(g)/Cu2OCl2(s)
þ 2HCl(g)
375e400 �C
S2 O2
production
Cu2OCl2(s)
/ 2CuCl(l)þ 1/2O2(g)
500e530 �C
S3 Electrolyzer 4CuCl(s)þH2O(l)
/ 2CuCl2(aq)þ 2Cu(s)
20e80 �C
S4 Dryer CuCl2(aq)/CuCl2(s)þH2O >100 �CS5 H2
production
2Cu(s)þ 2HCl(g)
/ 2CuCl(l)þH2(g)
430e475 �C
This reaction is endothermic and yields hydrochloric acid
gas (HCl) and Cu2OCl2. Hydrochloric acid gas is compressed
and Cu2OCl2 is transferred to another process step after its
temperature is increased to the oxygen production reaction
temperature of 500 �C.In the second (oxygen production) step (S2) an endo-
thermic chemical reaction takes place:
CuO * CuCl2(s)/ 2CuCl(l)þ 1/2O2(g) (2)
in which Cu2OCl2 is heated and O2 and copper monochloride
(CuCl) are produced. Liquid copper monochloride is solidified
by cooling it to 20 �C, after which it enters the third (copper
production) step (S3) together with the solid copper mono-
chloride from the fifth step. In the third process step solid
copper monochloride and water react endothermically at
20 �C as follows:
4CuCl(s)þH2O/ 2CuCl2(aq)þ 2Cu(s) (3)
S1 HE1 S2 S3 S5HE2 HE3
H2O(g)
S4HE4
Cu2OCl2(s)
HCl (g)
CuCl(l)
O2
(500ºC )
CuCl(s)
water
CuCl2
+ Water
Cu(s)Cu2OCl2(s)
P1
HCl (g)
CuCl2(s)
H2
(430ºC -475ºC )
O2
OUTPUT
400ºC 400ºC 500ºC
400ºC
500ºC 20ºC
20ºC
150ºC
20ºC
CuCl(s)
430ºC -475
20ºC
430ºC -475ºC
150ºC400ºC
OUTPUT
Cu-Cl Cycle
Fig. 5 e Variation of the five-step CueCl cycle for effective heat recovery within the cycle.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 011314
However in this reactionwateractsasacatalyst, anddoesnot
react with the other elements or compounds. Another specifi-
cation for this third reaction that differentiates this step from
others and makes it the most expensive, based on the price of
electricity, is that electrolysis occurs. In this reaction, solid
copper and a copper chlorideewater solution are produced. A
mixture of copper chloride andwater is transferred to thedryer
(S4), and solid copper enters the fifth step after its temperature
is increased to that step’s operating temperature. A physical
reaction takes place in the dryer as follows:
CuCl2(aq)/CuCl2(s)þH2O(l) (4)
In the fifth (hydrogen production) step (S5), hydrochloric gas
and copper enter, and are converted to gaseous hydrogen (H2)
and solid copper monochloride (CuCl). The reaction takes
place at 450 �C at steady state as follows:
2Cu(s)þ 2HCl(g)/ 2CuCl(l)þH2(g) (5)
An alternative layout of a five-step CueCl cycle is given in
Fig. 5. This layout has been designed for effective heat
recovery within the cycle. As explained above, there are five
main steps in the cycle; a chemical reaction takes place
in each step at various and high temperatures. These chem-
ical reactions form a closed internal loop that recycles all
chemicals on a continuous basis. The product of one step is
a reactant for another. However, since each step is at different
temperature, the product of a step needs to be cooled or
heated to next step’s temperature before entering it. Thus,
Table 4 e Reaction energy of the CueCl cycle steps and their aa pressure of 101 kPa.
Step Name Reaction heat (kJ/kmol H2) El
1 Fluidized bed 105,266
2 O2 production step 110,523
3 Cu production step e
4 Dryer 18,346
5 H2 production step �55,493
there are many heat recovery opportunities within the cycle.
The recovered energy as well as the energy that is released
from the exothermic reactions could be re-used in the cycle,
as illustrated in Fig. 5.
4. Performance analysis
The heat transfer for a chemical process, involving no work
interaction W, is determined from the energy balance_Ein � _Eout ¼ D _Esystem as applied to the system with W¼ 0. For
a steady state reaction process, the energy balance reduces to
Q ¼ Hp � HR ¼X
nP
�ho
f þ h� ho�P�X
nR
�ho
f þ h� ho�R
(6)
Here, the amount of product obtained in a chemical reaction is
called the yield, which is usually expressed in terms of grams
ormoles (molar yield). The effectiveness of a chemical process
can be measured by its fractional yield yf, calculated by
dividing the actual amount of the product obtained to the
stoichiometric (theoretical) amount. That is,
fractional yield�yf
�¼ actual yield
stoichiometric yield(7)
or in percentage form
percentage yield�yp
�¼ actual yield
stoichiometric yield100% (8)
The ideal or theoretical yieldof a chemical reaction is 100%, since
it is calculated based on the stoichiometry of the reaction.
However, 100% yield is impossible to achieve in actual cases
ssociated exergy values at the specified temperature and
ectricity (kJ/kmol H2) Exergy (kJ/kmol H2) Temp. (�C)
e 58,654 400
e 67,915 500
140,557 140,557 25
e 5421 150
e �32,620 450
Fig. 6 e Variation of reaction heat with reaction
temperature for the hydrolysis step in the CueCl cycle. Fig. 8 e Variation of reaction heat with reaction
temperature for the Cu production step in the CueCl cycle.
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 en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 0 11315
due to practical limitations. For example, in actual cases, the
reactions do not always take place stoichiometrically. Gener-
ally, one or more reactants are used in excess. Yields around
100% are called quantitative, above about 90% are called excel-
lent, above about 80% very good, above about 70% are called
good, below about 50% are called fair, and below about 40% are
called poor [14].
The overall energy efficiency of the CueCl cycle, he, can be
described as the fraction of energy supplied that is converted
to the energy content of H2 based on its lower heating value:
he ¼LHVH2
Q in þ Q loss
(9)
where LHVH2is the lower heating value, Q loss is the total heat
loss from theCueCl cycle andQ in is the total energyusedby the
process to produce a unit amount of product hydrogen, all per
kmol of hydrogen. The total energy demand of the CueCl cycle
is the sumof the reactionheats of thefivemain steps described
above. Note that, in this summation, the exothermic reaction
heat (i.e., the fifth step) is taken as negative, assuming this heat
canbeused for other endothermic reactions. The lowerheating
value of hydrogen is 240,000 kJ/kmolH2.
Fig. 7 e Variation of reaction heat with reaction
temperature for the O2 production step in the CueCl cycle.
Asmentioned earlier, we analyze a conceptual CueCl plant
which has not yet been built. Furthermore, many parameters
and specific details on quantity, capacity and material of
equipment (pumps, heat exchangers, compressors, fluidized
bed, evaporator, etc.) that are needed for such analyses are
unknown. Therefore, for its simplified analysis for overall
efficiency calculations we consider only the five main steps of
the CueCl cycle and assume that there are no heat losses in
these steps, individually. However, overall we assume that the
total heat losses (Qloss) from the CueCl cycle cover
a percentage of the total heat input (Qin) supplied to the cycle.
Theoverall exergyefficiencyof theCueCl cycle expressedas
hex ¼ exout
exin(10)
Using the exergy balance for the system, the exergy efficiency
can be written alternatively as
hex ¼ 1� exdestroyed
exin(11)
Fig. 9 e Variation of dryer inlet heat duty with drying
temperature.
Fig. 10 e Variation of dryer heat duty with inlet
temperature of the copper chloride solution.
Fig. 12 e Effect of yield on the overall efficiencies of the
CueCl cycles.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 011316
where exdestroyed is the total specific exergy destruction
including exergy losses and exin and exout are the input and
output specific exergies, all on amolar basis. Here, total exergy
needed to produce a unit amount of hydrogen is taken as
input and only exergy value of hydrogen is taken as output,
since it is the desired output. Detailed analyses of how the
input, output and destroyed exergy are obtained for each step
are given elsewhere [15e19]. For the overall cycle, we obtain
the total input and destroyed exergy of the cycle by adding the
input and destroyed exergy values of the individual steps.
Again, in the summation of input exergy, the exothermic
reaction (i.e., the fifth step) is taken as negative, assuming this
energy can be used for other endothermic reactions.
5. Results and discussion
The main steps of the CueCl cycle are evaluated individually
to determine how each process is influenced by varying key
operating and design parameters. A sensitivity analysis is also
performed in which one or more flowsheet parameters are
varied and the resulting effect on other variables studied, so as
to achieve a simple process optimization.
Fig. 11 e Variation of reaction heat of the H2 production
step with its temperature.
As discussed previously, three configurations currently
exist for the CueCl cycle (i.e., three-, four- and five-step
cycles). However, all of these combine one ormore steps in the
five-step CueCl cycle. Therefore, since the five-step CueCl
cycle is main design schematic, its five main reactors are
analyzed individually. The reaction energy of the main steps
in the CueCl cycle and their associated exergy values at
specified states are given in Table 4. The expected values of
the temperatures of themain cycle steps are also listed.When
we vary a step temperature in the analysis, the temperatures
for the other steps remain constant, at the value in Table 4. For
example, to investigate the relation between T1 (reaction
temperature of Step 1) and efficiencies (or some other vari-
ables), T2, T3, T4 and T5 (reaction temperature of Steps 2, 3, 4
and 5, respectively) are fixed at the values in Table 4.
The variation of the reaction heat (heat load) of HCl
production (Step 1) with reaction temperature is illustrated in
Fig. 6, for percentage yields of yp¼ 100% (complete reaction)
and yp¼ 80%. As explained earlier, an endothermic reaction
occurs in the fluidized bed. As reaction temperature increases,
the reaction heat for the HCl production step decreases, in
a nearly linear manner.
Fig. 13 e Variation of efficiencies of the CueCl cycle with
the reaction temperature of Step 1 (Hydrolysis Reactor).
Fig. 14 e Variation of efficiencies of the CueCl cycle with
the reaction temperature of Step 2 (Oxy-decomposition
Reactor).
Fig. 16 e Variation of efficiencies of the CueCl cycle with
the reaction temperature of Step 4 (Dryer).
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 en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 0 11317
The variation of the reaction heat of the oxy-decomposi-
tion reactor with reaction temperature is illustrated in Fig. 7.
An endothermic reaction takes place in this step. As reaction
temperature increases, the reaction heat for the O2 production
step increases.
The variation of the reaction heat for the Cu production
step with reaction temperature is illustrated in Fig. 8. An
endothermic reaction takes place in this step. As reaction
temperature increases, the reaction heat for the Cu produc-
tion step decreases.
The effects of varying the dryer temperature Tdryer and
inlet temperature of the CuC2/H2O mixture on the process
heat of the dryer are illustrated in Figs. 9 and 10, respectively.
The heat needed to evaporate water is seen to be approxi-
mately directly proportional to the evaporator temperature,
but to decrease approximately linearly as inlet temperature
increases. The evaporator inlet temperature is determined in
the previous step (copper production), where the reaction
temperature varies between 25 �C and 80 �C, respectively.
Fig. 15 e Variation of efficiencies of the CueCl cycle with
the reaction temperature of Step 3 (Electrolysis Reactor).
Although the amount of heat required for the drying step is
much higher than other steps in the cycle, it occurs at a lower
quality (a relatively low temperature, below 100 �C) and
therefore with heat that is more readily available. Thus, the
required heat can be obtained from low-grade “waste” or
recovered heat to improve the cycle efficiency. Spray drying is
an efficient method of water removal due to the relatively
large surface area available for heat and mass transfer,
provided the liquid atomizes into sufficiently small droplets
(on the order of a few hundredmicrons). Spray drying is a well
established industrial process that allows powder character-
istics and properties to be controlled and maintained nearly
constant throughout a drying operation.
The variation of the reaction heat for the H2 production
step, with reaction temperature, is illustrated in Fig. 11. An
exothermic reaction takes place in the H2 production step. As
reaction temperature increases, the reaction heat for the H2
production step decreases in a nearly linear manner. As the
percentage yield ( yp) decreases from 100% (complete reaction)
to 80%, less heat is released from the reaction.
Fig. 17 e Variation of efficiencies of the CueCl cycle with
the reaction temperature of Step 5 (H2 Production).
0 10 20 30 40 500
5
10
15
20
25
30
35
40
45
T0 (°C)
η((%
)
ηeηe
ηexηex
Fig. 18 e Variation of efficiencies of the CueCl cycle with
reference-environment temperature.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 011318
Most previous theoretical studies on the CueCl cycle
reported in the literature have assumed complete reaction for
each step in the cycle [15e25]. To obtain more realistic values,
we carry out an assessment inwhich a parametric study of the
impact on percentage yield ( yp) is determined for each step.
Fig. 12 illustrates the variation of overall efficiencies of the
CueCl cycle with percentage yield of the steps. As can be
observed from this figure, the energy efficiency of the cycle
approaches 45% and the exergy efficiency 10%, when all
reactions become fully stoichiometric. Based on the para-
metric study included in this figure, we assume a percentage
yield of 80% for the steps in the CueCl cycle involving
a chemical reaction and a 75% heat exchanger effectiveness
for heat management and handling within the cycle. In the
sensitivity analysis, the 80% percentage yield for each step is
an assumption based on expected values (typical for this kind
of chemical reaction). This value is mainly used to determine
the effect of reaction yield on the overall cycle efficiency, for
example, by howmuch the overall cycle efficiency decreases if
the reaction yield is 80% instead of 100%.
Figs. 13e17 show the variations of energy (he) and exergy
(hex) efficiencies of the CueCl cycle with the temperatures of
main reactions in the cycle. These results appear to be lower
than the values obtained in previous theoretical studies. This
difference is likely attributable to the fact that we assume
a complete reaction for each step in the previous calculations.
The overall energy efficiency of the cycle varies from 42% to
44% and exergy efficiency from 6% to 8% (see Figs. 13e17).
Table 5 e Grouping of different steps of the CueCl cycle.
Cu-ClCu-Cl
Five-step cycle Five-step cycle Four-step cyFour-step cy
S1:S1:
S4:S4:
S2:S2: S-II:S-II:
S-I:S-I:
S3:S3: S-III:S-III:
S5:S5: S-IV:S-IV:
The overall energy and exergy efficiencies of the CueCl
cycle vary with the reaction temperature of each step as well
as with the reference-environment temperature. The varia-
tion of energy and exergy efficiencies of the cycle with refer-
ence-environment temperature (T0) is illustrated in Fig. 18.
The exergy efficiency decreases with increasing T0, while the
energy efficiency remains constant.
Table 5 and 6 summarize a number of grouping methods,
and the corresponding major advantages and disadvantages
of different copper-chlorine cycle variations. The main
advantage resulting from reducing the number of steps is that
the challenges arising from processing of solid particles are
dramatically reduced. The mixing and handling of a homoge-
nous liquid or gas is usually easier than of a solid. Also, the
heat and mass transfer for solid phases (e.g. solid-solid or
fluid-solid) is more complex and unpredictable. For example,
any non-ideal mixing may result in an incomplete hydrolysis
reaction in the Cu-Cl cycle, whichmay further add difficulty to
the separation of product particles from reactant particles
[13]. Feeding of solid particles into a reactor, caking prevention
of solid deposition on the reactor wall and clogging of pipes or
channels are some other challenges that may be faced during
processing of solid particles. These challenges can be signifi-
cantly reduced in the four-step and three-step Cu-Cl cycles,
where the feed is liquid only.
Reducing the number of steps in the Cu-Cl cycle can also
yield some disadvantages. The main disadvantages are the
higher grade heat requirement, as shown in Fig. 19, and
a reduction in the yield of desirable products (and increased
yield of undesirable products), as shown in Fig. 20). These
disadvantages are more significant in the three-step Cu-Cl
cycles since they need higher heat grade than the four-step or
five-step cycles. Also, changing the five-step cycle to a three-
step cycle increases the heat intensity more significantly than
reducing the five-step cycle to a four-step cycle, because the
heat load of three reactions in the five-step cycle are combined
and added to a single reactor in the three-step cycle. This
phenomenon can make the selection of reactor material very
difficult from a practical engineering perspective. Also,
combining two or more reactions causes more complex
reactions with additional engineering challenges for efficient
separation of products. These disadvantages taken together
result in a reduction of the overall efficiency of the cycle.
The variation of energy and exergy efficiencies of the cycle
with number of steps is illustrated in Fig. 21. The energy effi-
ciency decreases as thenumber of steps in the cycle decreases,
while the exergy efficiency increases. The exergy destruction
cycle cycle
clecle Three-step cycleThree-step cycle
S-i:S-i:
S-ii:S-ii:
S-iii:S-iii:
0
5
10
15
320 340 360 380 400 420 440 460
Reaction temperature (°C)
Und
esir
able
pro
duct
s co
nver
sion
(%
)
Fig. 20 e Dependence of by-products on reaction
temperature (adapted from [13]).
400
500
600
700
800
900
1000
1100
1200
Five-step Four-step Three-step Two-step
Configuration
Max
imum
cyc
le te
mpe
ratu
re (
°C)
Fig. 19 e Variation of the maximum operation temperature
with number of steps for various CueCl cycles.
0
5
10
15
20
25
30
35
40
Three-step Four-step Five-step
Cu-Cl cycle
Eff
icie
ncy
(%)
Energy
Exergy
Fig. 21 e Variation of efficiencies of various CueCl cycles
assuming an overall heat loss of 30% of the total input
energy to the cycle and an 80% reaction efficiency for each
step.
Table 6 e Main advantages and disadvantages ofdecreasing the number of steps in the CueCl cycle.
Configuration Advantages Disadvantages
Five-step cycle Less challenges to process solid particles Less steps and hence equipment Reduced complexity Better reaction kinetics Homogenous reaction mixture
Higher heat grade and intensityMore equipment material challenges More undesirable side productsLower efficiencies
Four-step cycle
Three-step cycle
•
••••
••
••
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 en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 0 11319
for a chemical reaction in any step of the cycle is very signifi-
cant becauseof thedifference betweenchemical exergy values
of chemical compounds. Thus, fewer steps and hence fewer
chemical reactions appear to yield higher exergy efficiencies.
However the opposite behaviour is observed for the energy
efficiencies. Since the quantity and grade of energy needed for
theproductionof aunit ofhydrogen increasesas thenumberof
cycle steps decreases, the energy efficiency decreases.
6. Conclusions
Further developments and improved design schemes of the
CueCl thermochemical water decomposition cycle for
hydrogen production are needed that utilize the results of the
various technical studies that have been reported previously.
In this study, various design schemes for the overall CueCl
cycle and its components are examined in order to identify
potential performance improvements. The implications of
implementing these new configurations are discussed and
the potential benefits ascertained. Also, a thermodynamic
analysis and several parametric studies are presented for
various configurations. This information should assist
ongoing efforts to understand the thermodynamic losses in
the cycle, to improve efficiency and to facilitate eventual
commercialization. Future work is expected to focus on
detailed analyses and comparisons of these new configura-
tions of the CueCl cycle using advanced simulation and
optimization tools.
Acknowledgements
The authors gratefully acknowledge the support provided by
the Ontario Research Excellence Fund and the Natural
Sciences and Engineering Research Council of Canada.
Nomenclature
e molar energy, kJ/kmol
E energy, kJ
Ex exergy, kJ
h specific enthalpy, kJ/kg
h molar enthalpy, kJ/kmol
h0
molar enthalpy at reference state, kJ/kmol
h0
f molar enthalpy of formation, kJ/kmol
LHV lower heating value, kJ/kg fuel
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 1 3 0 9e1 1 3 2 011320
n number of moles per mole hydrogen produced,
kmol/kmol H2
Q heat, kJ_Q heat transfer rate, kW
T temperature, K
yf fractional yield
yp percentage yield, %
Greek Letters
h efficiency
Subscripts
aq aqueous solution
e energy
ex exergy
f fractional
g gas
l liquid
p percentage
s solid
0 reference state
Superscripts
�(over bar) quantity per unit mole
_ (over dot) quantity per unit time
0 standard reference state
r e f e r e n c e s
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