Ozcan2016 8
Ozcan2016 8
Ozcan2016 8
ScienceDirect
Article history: The magnesiumechlorine cycle is proposed to be one of the promising thermo-
Received 13 August 2015 electrochemical (hybrid) hydrogen production cycles. The main steps of the cycle include
Received in revised form hydrolysis of MgCl2, chlorination of MgO, and electrolysis of HCl. Based on the information
21 October 2015 from the high temperature hydrolysis of common salts (i.e., KCl, NaCl, CaCl2, CuCl2), a
Accepted 22 October 2015 higher steam/solid ratio is utilized for better conversion of the reactants. Thus, it is ex-
Available online 18 December 2015 pected that HCl would be in mixture with steam after the hydrolysis process, which yields
to a possible aqueous electrolytic process, resulting in a higher power consumption in the
Keywords: electrolyzer. This study aims to develop an optimum configuration of the MgeCl cycle in
MgeCl cycle order to decrease the electrical energy requirement of the electrolysis step, as well as to
Hydrogen production overcome the problems, such as oxygen evolution and chlorine solubility in water by
Efficiency substituting the aqueous electrolysis step to anhydrous electrolysis. Two configurations of
Energy the three-step cycle are comparatively assessed in terms of their efficiencies and energy
Exergy requirements. A novel fourth step is then introduced for anhydrous HCl production with an
additional decomposition step. Energy requirements of all individual cycles are presented
and comparatively discussed. The newly developed four-step cycle shows higher ther-
modynamic performances than those of three-step options with decreased electrical work
consumption. The calculated energy and exergy efficiencies of the four-step MgeCl cycle
are 43.7% and 52%, respectively, resulting in a lower electrical work consumption than that
of water electrolysis with a maximum temperature of 723 K.
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
* Corresponding author.
E-mail addresses: Hasan.Ozcan@uoit.ca (H. Ozcan), Ibrahim.Dincer@uoit.ca (I. Dincer).
http://dx.doi.org/10.1016/j.ijhydene.2015.10.078
0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
846 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 4 1 ( 2 0 1 6 ) 8 4 5 e8 5 6
Renewable energy resources, such as solar, hydro, wind, tidal, temperature manipulations through streams. The RGibbs
biomass, geothermal and nuclear power and process heat are reactor aims to minimize the Gibbs free energy of a reaction
used for sustainable hydrogen production [2]. which is a promising built-in reactor type to determine reaction
Thermochemical splitting of water is an alternative to conditions. However, majority of the reactions are already
electrolytic splitting of water requiring more than one chem- known with their existing kinetics, thus, these data are also
ical reaction and sums thermolysis of water at elevated tem- taken into consideration for the yields. Here, RYield type re-
peratures. Water splitting at high temperature (thermolysis) is actors are used for this case for known conversions such as
possible at very high temperatures (>2000 K) and results in chlorination reactor. There are several exothermic reactions
production of oxygen and hydrogen as one output which re- throughout the cycle. These type of reactors can be conveniently
quires a very challenging separation process [3]. Multi-step used to determine the thermodynamics of a reaction with
thermochemical cycles work at lower maximum tempera- known conversions at specific reactor conditions. These re-
tures than the one step thermolysis through a cyclic process of actions are assumed to be isothermal for the sake of preventing
splitting water. These steps vary from 2 to 6 steps in relation any interruption of the reaction rates. Thus, RYield type
with the chemistry of substances where a higher number of reactors are adapted considering existing experimental data
steps tends to work at lower maximum temperatures [3]. from the literature. Initially three options are evaluated with
While the majority of hybrid methods shows lower maximum respect to their heat requirements. The first option is taken to be
temperatures than pure thermochemical processes, the high temperature hydrolysis of MgCl2 into MgO and HCl gas
UeEueBr cycle shows 573 K maximum temperature and is a (MgeCl-A). The second option is relatively lower temperature
purely thermochemical cycle [4]. However, technical issues, hydrolysis of MgCl2 into MgOHCl and HCl gas, where stoichi-
such as very low reaction rates of exothermic reactions, are ometry of MgCl2 should be doubled in order to obtain same
required to be considered for a more feasible cycle. amount of HCl gas as in the first option (MgeCl-B). Finally, a
Hybrid thermochemical cycles can be arranged as thermo- novel fourth step is introduced in the third option with a
electrochemical, thermo-photo-electrochemical, and thermo- decomposition step of the MgOHCl into MgO and HCl gas
radiochemical cycles. Thermo-electrochemical cycles utilize (MgeCl-C). All configurations are designed to produce
electricity at least one of the steps through the cycle. Thermo- 3600 kmol/h of hydrogen, and the net heat and work re-
photo-electrochemical cycles need a photochemical reaction quirements are determined.
to complete the cycle, and a radiochemical reaction is needed
for thermo-radiochemical cycles [3]. MgCl2eMgO (MgeCl-A) cycle
Most thermo-electrochemical cycles have been proposed
in order to decrease the Reverse Deacon Reaction temperature The Hybrid MgeCl cycle is initially proposed as a three-step
by using electrical work lower than the water electrolysis to thermo-electrochemical cycle utilizing heat and electrical
make the cycle a feasible one. Lower temperature cycles tend work. Main steps of the cycle are; hydrolysis of MgCl2, chlori-
to consume more electrical work to bring down the Gibbs free nation of MgO, and electrolysis of HCl gas. It can be run with two
energy to zero. Low maximum temperatures are one of the different options, namely MgCl2eMgO, and MgCl2eMgOHCl
most important advantages of hybrid cycles to utilize heat cycle as reported by Ref. [9]. Feasible chemical reactions,
from relatively low temperature heat sources such as nuclear mature electrolysis technology, and relatively low maximum
and solar energy. The chlorine family cycles as well as other temperature of the cycle are promising in terms of integrating
hybrid cycles under study can be listed as follows [5e8]: this cycle with nuclear and solar energy sources. Aspen flow-
sheet of this cycle is represented in Fig. 1. Argonne National
Hybrid Sulfur cycle (HyS) (Ispra Mark 11) Laboratory (ANL) studied the hydrolysis step of this cycle with
Hybrid Chlorine cycle (Hallet air products) additives and obtained promising results for yields of the re-
CueCl cycle (3, 4 and 5 steps) actants at desired temperatures [6]. Individual reactions of the
HBr cycle (ANL) three step MgeCl cycle has also been studied and adapted into
MgeCl cycle (ANL) integrated systems and can be found elsewhere [10e12].
aqueous HCl electrolysis is a mature process where 1.8 V per requirements, this option directly reduces the electrical
mole of hydrogen is required with several solubility issues of work consumption by 11.2% compared to three-step
oxygen and chlorine gas in water, which makes the electrol- configurations.
ysis step less advantageous than the conventional water The stoichiometry to produce same amount of hydrogen as
electrolysis even though the theoretical requirement is lower in the MgeCl-B option can be reduced from two to one, by
(0.99 V). The anhydrous (dry) HCl production is one of the capturing HCl gas in the decomposition step.
crucial issues throughout the cycle in order to consume less Produced MgO from the high temperature hydrolysis is less
electrical work than water electrolysis. Fig. 3 represents reactive with the chlorine gas and has less surface area for
practical voltage comparison of some electrolysis processes proper reaction than the hydrolysis of Mg(OH)2. However,
for hydrogen production [13e17]. experimental studies showed that fine MgO can be pro-
duced from decomposition of MgOHCl which would
Four step MgeCl (MgeCl-C) cycle enhance the reactivity of MgO with the chlorine gas [18].
The only endothermic reaction is the decomposition of
As the last alternative, a fourth step is introduced to decompose MgOHCl, and maximum temperature of this reaction can
MgOHCl in order to recover the remaining HCl in anhydrous be reduced to 723 K from ~800 K by using an inert gas to
form, and this step is introduced for the following reasons: remove HCl gas from the surface of MgO particles. This also
leads to a very fast decomposition of MgOHCl into desired
It is possible to obtain dry HCl gas by decomposing products. The lower maximum temperature provides the
MgOHCl, and considering the practical voltage option to link this cycle to low-temperature heat sources.
848 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 4 1 ( 2 0 1 6 ) 8 4 5 e8 5 6
Fig. 3 e Practical Voltage comparisons of some selected electrolysis methods for hydrogen production.
Both stability and decomposition kinetics of MgOHCl sub- and chlorination is assumed to be same, where an additional
stance have been studied by [18] and it was concluded that the heat exchanger is required to cool down MgCl2 from chlorina-
removal of HCl gas with Argon gas increases the rate of the tion step. Both hydrolysis and chlorination steps are
reaction significantly. This reaction occurs from 649 K, and a exothermic, and the only endothermic reaction is decomposi-
minimum of 806 K is needed to fully decompose into HCl and tion of MgOHCl. The chlorine gas from both electrolysis paths is
MgO. However lower reaction temperatures have not been mixed and heated to the chlorination reaction temperature.
considered with a higher inert gas flowrate. The Aspen flow
sheet for this configuration is represented in Fig. 4.
The hydrolysis step is set to 553 K to form MgOHCl where the Thermodynamic assessment
reactor temperature for decomposition step is set to 723 K. The
required steam for the hydrolysis step is same as the MgeCl-B The minimum electrical energy requirement of the electro-
cycle and it is set to 11:1 [9]. A preheating process is considered chemical step of all MgeCl cycle options can be calculated
to bring the MgOHCl temperature to the lower limit of decom- using the Nernst equation, depending on the change in the
position (649 K). Argon gas is used as the inert gas to remove the Gibbs free energy as follows:
HCl gas from MgO surface and adapted in the simulation with
varying flow rates. The produced HCl gas is utilized in the dry DG ¼ nFE0 ¼ Wel (1)
HCl electrolysis process, where the steam/HCl mixture from the 0
where F denotes Faraday's constant, E is the standard cell
hydrolysis step is cooled to the electrolysis temperature for potential and n is the number of moles of electrons exchanged
aqueous HCl electrolysis. The temperature of decomposition in the electrochemical reaction. DG for decomposition
n_H2 LHVH2
hen;MgCl ¼ (2)
Q_ heat þ W_ elec
_ H
Ex
hex;MgCl ¼ 2
(3)
_ heat þ W
Ex _ elec
up to 10% with full conversion, the changes are proportional heat and work requirements, and the main output of the
and slightly different at every 50 K increase. Electrolysis of the system is hydrogen at 298 K. Considering the excess steam
aqueous HCl has some major issues in terms of solubility of requirement of 17 for the conventional cycle (MgeCl-A), the
chlorine gas, and oxygen evolution. Thus, one should consider water boiling process for a large amount of water is the most
generating pure HCl gas in order to make the MgeCl cycle a energy consuming process. However, the potential of recov-
feasible one. A more practical approach depicts that the ering this energy by internal heat recovery, and low grade heat
electrical work saving can be up to 13%, which is less than the requirement of the water boiling process makes it easier to
theoretical ratio. The four-step MgeCl cycle leads to produce provide the internally recovered energy. Table 1b represents
half of the HCl in anhydrous form with the decomposition the energy requirement of the conventional MgeCl cycle.
step, where the other half is still in mixture with steam. For The calculation of the energy need is based on internal
the sake of less work requirement, a separation process at heat recovery from exothermic heat exchanging components
slightly elevated temperatures can be considered to increase with a heat exchanger effectiveness of 85%. Here, the chlori-
the amount of anhydrous HCl gas. nation reactor also shows an exothermic trend. The total heat
requirement of this cycle results in 310.85 MJ/kmol H2. Here, it
Performance assessment should also be noted that the heat requiring components at
high temperatures are only the hydrolysis reactor and the
The Aspen Plus results for the conventional MgeCl cycle are chlorine heating exchanger. Even if Hex-1 shows the highest
summarized in Table 1a and b. The results are given for heat requirement, this heat supply can be supplied around the
1 kmol/s hydrogen production from the electrolysis step. phase change temperature of water. The electrolysis power
Excess water from electrolysis step is fed back to the hydro- requirement is calculated based on 1.8 V requirement and
lysis reactor by mixing it with the external water. The main results in 347.35 MW for 1 kmol/s H2 production.
inputs to the system are water, various grades of heat, and The energy efficiency of the MgeCl-A cycle is calculated to be
electrical work for the electrolysis process. In the efficiency 36.8% where it is drastically lower than that of calculations made
assessment, energy content of water input and oxygen output theoretically under stoichiometric conditions. Note that the
are not included due to their negligible contents compared to previous studies reported up to 52% for the ideal cycle [10e12].
Table 1 e State point information (a) and energy requirement (b) of the MgeCl-A cycle.
(a)
State Stream n_ T h s exph extot _
Ex
kmol/s ( C) kJ/kmol kJ/kmolK kJ/kmol kJ/kmol MW
1 H2O 1 25.00 285,820 163.14 241,790 3120 3.09
2 H2O 17 67.36 282,640 153.15 238,610 3120 55.91
3 H2O 17 537.00 223,430 8.78 223,430 11,710 331.78
4 MgCl2 1 537.00 601120 88.55 840,490 151,860 168.92
5 Mix 19 537.00 226,700 3.81 e
6 H2O/HCl 18 537.00 207,180 0.51 e 388.63
7 H2O/HCl 18 70.00 259,290 128.19 289.47
8 Mix 18 70.00 249,290 129.34 e
9 H2/H2O 17 70.00 264,620 139.88
10 H2O 16 70.00 282,440 152.57 238,410 3120 53.05
11 H2 1 70.00 1302 4.17 240,698 238,490 238.55
12 H2 1 25.00 1 0.11 241,999 238,490 238.46
13 MgO 1 537.00 577,990 63.35 695,680 59,170 69.26
14 Cl2 1 70.00 1485 4.82 160,225 117,520 117.60
15 Cl2 1 537.00 18,473 35.94 143,237 117,520 125.31
16 Mix 1.5 537.00 395350 48.60 e
17 O2 0.5 537.00 16,173 31.29 16,173 3970 5.41
18 O2 0.5 25.00 8 0.09 8 3970 1.97
(b)
Component Process Temperature DH W_
C MJ/kmol H2 MW
Hydrolysis MgCl2 hydrolysis 537 92.21 e
Chlorination MgO Chlorination 537 33.5 e
Electrolysis HCl(aq) electrolysis 70 e 347.35
Hex-1 Water superheating 537 1006.4 e
Hex-2 HCl cooling 70 937.9 e
Hex-3 Cl2 heating 537 16.98 e
Hex-4 H2 cooling 25 1.3 e
Hex-5 O2 cooling 25 8.1 e
Total 310.85 347.35
852 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 4 1 ( 2 0 1 6 ) 8 4 5 e8 5 6
Use of excess water and practical voltage for the electrolysis cell The main difference between the option A and B are the hy-
are main efficiency reducing factors. Exergy efficiency of the drolysis step of the cycle. The hydrolysis reaction for option B
cycle is also calculated by considering the exergy rate of state 12, requires lower temperatures than that of direct MgCl2 hydro-
in Table 1a. Here, the exergy contents of water input and oxygen lysis as in option A, and this reaction is exothermic.
output are also included in the calculations, where it is calcu- The chlorination reaction for MgOHCl chlorination is
lated as 44.1%. Compared to exergy efficiency values in the highly endothermic, and results in steam production at high
previous studies for stoichiometry (68%), almost 35% reduction temperature in mixture with oxygen. The separation process
is observed for the cycle for practical assumptions. Another of these substances might be very challenging and can pre-
important issue to be pointed out is that the energy consumption vent the reuse of steam in the cycle. Thus, an additional
of this cycle is higher than that of conventional water electrol- drying process for water separation from oxygen would be
ysis. Since most thermo-electrochemical cycles are proposed to required, in order to recirculate the water content after the
be better performing and less power consuming alternatives chlorination. For the reaction chain inside the reactor, one can
than the water electrolysis, MgCl-A cycle is not a feasible alter- predict that a possible MgOHCl decomposition is initially
native considering practical applications. required to liberate HCl gas first. This may cause mixture of
The literature studies show that another configuration for HCl in the outlet stream. However, previous information for
the MgeCl cycle can be simulated by using a low temperature reaction kinetics of MgO and HCl shows that this reaction is
hydrolysis reaction. However, for the same amount of more feasible and faster than MgO chlorination, with a very
hydrogen production, stoichiometry should be doubled, where good conversion of MgO into MgCl2 [21].
the required steam has to be doubled as well. Simulation re- The energy calculations of the cycle shows relatively
sults of the MgeCl-B cycle are represented in Table 2a and b. higher energy load mainly due to energy intensive phase
Table 2 e State point information (a) and energy requirement (b) of the MgeCl-B cycle.
(a)
State Stream n_ T h s exph extot _
Ex
kmol/s C kJ/kmol kJ/kmolK kJ/kmol kJ/kmol MW
1 H2O 1 25 285,820 163.142 33.773 3086.227 3.086
2 H2O 22 67.8 282,600 153.04 175.890 3295.89 72.509
3 H2O 22 280 233,040 22.9991 2433.74 14143.74 311.162
4 MgCl2 2 280 621940 119.425 5438.501 157298.5 314.597
5 Mix 23 280 268,400 31.5061
6 H2O/HCl 21 280 218,920 15.5103 14322.06 300.763
7 H2O/HCl 21 70 262,630 131.78 13810.47 290.019
8 Mix 21 70 254,090 132.826
9 H2/H2O 20 70 267,290 141.779
10 H2O 19 70 282,440 152.569 195.502 3315.502 62.994
11 H2 1 70 1301.857 4.171019 57.3860 238547.4 238.547
12 H2 1 25 0.987862 0.107765 32.633 238457.4 238.457
13 Cl2 1 70 1484.522 4.822405 569247.4 628417.4 628.417
14 Cl2 1 537 18473.2 35.93761 7792.224 125312.2 125.312
15 MgOHCl 2 280 787,930 199.462 3209.706 148329.7 296.659
16 MgOHCl 2 537 776,180 182.004 9757.192 154877.2 309.754
17 Mix 3.5 537 405,020 46.3679
18 O2 0.5 537 16173.08 31.29298 16174.96 20144.96 10.072
19 O2 0.5 25 7.95734 0.088981 32.592 3937.408 1.968
20 Mix 3 537 475,220 61.9581
21 MgCl2 2 537 601,120 88.5468 17056.94 168916.9 337.833
22 H2O 1 537 223,430 8.7809 7806.704 19516.7 19.5167
(b)
Component Process Temperature DH W_
C MJ/kmol H2 MW
Hydrolysis MgCl2 hydrolysis 280 35.6 e
Chlorination MgOHCl Chlorination 537 116.3 e
Electrolysis HCl(aq) electrolysis 70 e 347.35
Hex-1 Water superheating 280 1040.8 e
Hex-2 HCl cooling 70 817.8 e
Hex-3 Cl2 heating 537 16.98 e
Hex-4 H2 cooling 25 1.3 e
Hex-5 O2 cooling 25 8.1 e
Hex-6 MgOHCl heating 537 23.5
Hex-7 MgCl2 cooling 280 41.7
Total 459.02 347.35
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 4 1 ( 2 0 1 6 ) 8 4 5 e8 5 6 853
change process for large amount of required water. The investigations can be made to enhance the cycle performance
endothermic reactor of this cycle also requires 20.5% more and reduce the energy requirements by capturing HCl in dry
heat than the hydrolysis reaction of MgeC-A at elevated form, developing a less steam requiring hydrolysis reactor
temperatures. Energy and exergy efficiency of this option is configuration, and investigating the temperature dependence
30% and 37.7%, respectively. Considering the electrolysis and reaction characteristics of the MgOHCl chlorination.
process, the practical voltage requirement is at the same The newly developed four-step MgeCl cycle is named as
range of the MgeCl-A cycle and highly energy intensive. MgeCl-C and the main idea behind the four-step option is to
Considering thermodynamic point of view, the MgeCl-B cycle capture HCl in dry form with an additional step to decompose
does not show any promising performance either for heat MgOHCl into solid and gas substances at elevated tempera-
requirement and power consumption. However, further tures. Table 3a and b represent the state point information
Table 3 e State point information (a) and energy requirement (b) of the MgeCl-C cycle.
(a)
State Stream n_ T h s exph extot _
Ex
kmol/s C kJ/kmol kJ/kmol K kJ/kmol kJ/kmol MW
1 H2O 1 25 285,820 163.142 33.773 3086.227 3.086
2 H2O 11 65.9 282,750 153.471 154.298 3274.298 36.017
3 H2O 11 280 233,040 22.9991 2433.744 14143.74 155.581
4 MgCl2 1 280 621,940 119.425 5438.501 157298.5 157.298
5 Mix 12 280 266,930 31.1178
6 H2O/HCl 11 280 219,570 15.8138 13762.5 151.387
7 H2O/HCl 11 70 263,540 132.758 13192 145.112
8 Mix 11 70 255,400 133.775
9 H2/H2O 10.5 70 268,010 142.293
10 H2O 10 70 282,440 152.569 195.502 3315.502 33.155
11 H2 0.5 70 1301.857 4.171019 57.386 238547.4 119.273
12 H2 0.5 25 0.987862 0.107765 32.633 238457.4 119.228
13 MgOHCl 1 280 787,930 199.462 3209.706 148329.7 148.329
14 MgOHCl 1 450 780,160 187.201 7326.017 152446 152.446
15 Mix 2 450 331,050 16.4064
16 HCl 1 450 79812.6 36.14458 4729.485 90679.49 90.679
17 HCl 1 70 91027.6 14.17917 60.197 86010.2 86.01
18 Mix 1 70 1406.357 10.28291
19 Cl2 0.5 70 1484.522 4.822405 75.877 117595.9 58.797
20 H2 0.5 70 1301.857 4.171019 57.386 238547.4 119.273
21 H2 0.5 25 0.987862 0.107765 32.633 238458.4 119.229
22 Cl2 0.5 70 1484.522 4.822405 75.877 117595.9 58.797
23 Cl2 1 70 1484.522 4.822405 75.877 117595.9 117.595
24 MgO 1 450 582,290 68.9573 7459.278 66629.28 66.629
25 Cl2 1 500 17097.69 34.19978 6934.588 124454.6 124.454
26 Mix 1.5 500 397,800 51.694
27 MgCl2 1 500 604,170 92.4003 15155.3 167015.3 167.015
28 O2 0.5 500 14926.99 29.71871 6072.696 10042.7 5.021
29 O2 0.5 25 7.95734 0.088981 32.592 3937.408 1.968
(b)
Component Process Temperature DH W_
C MJ/kmol H2 MW
Hydrolysis MgCl2 hydrolysis 280 17.82 e
Chlorination MgO Chlorination 500 31.51 e
Decomposition MgOHCl Decomposition 450 118.1 e
Electrolysis(aq) HCl(aq) electrolysis 70 e 173.67
Electrolysis(dry) HCl(dry) electrolysis 70 e 135.08
Hex-1 Water superheating 280 546.8 e
Hex-2 MgOHCl heating 450 7.8 e
Hex-3 HCl(aq) cooling 70 483.8 e
Hex-4 HCl(dry) cooling 70 11.2 e
Hex-5 H2 cooling 25 0.65 e
Hex-6 H2 cooling 25 0.65 e
Hex-7 Cl2 heating 500 15.61 e
Hex-8 O2 cooling 25 7.46 e
Hex-9 MgCl2 cooling 280 17.8 e
Total 244.98 308.75
854 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 4 1 ( 2 0 1 6 ) 8 4 5 e8 5 6
Fig. 9 e Energy load comparison of MgeCl cycle options (th: Fig. 11 e Effect of cycle maximum temperature and
theoretical calculations with Aspen yields). ambient temperature on exergy efficiency of cycles.
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 4 1 ( 2 0 1 6 ) 8 4 5 e8 5 6 855
configurations of the four-step cycle. The four-step cycle requirement than the conventional water electrolysis for the
carries the potential to be more than 55% efficient with a same current density. Therefore, a four-step MgeCl cycle is
complete HCl electrolysis at the desired voltage values. Since proposed to generate dry HCl gas for the sake of lower elec-
one of the major energy consuming devices is the aqueous trical work requirement. The four-step cycle can work at 723 K
electrolysis step, a suitable dry HCl capturing process can be a maximum temperature with ~3% lower electrical energy
promising option for reduced electrical work consumption of consumption than the water electrolysis. The present studies
the cycle. The HCl capturing process can be another energy show a separation process can enhance this saving up to 13%.
intensive process which may contribute to the total thermal The present four-step cycle shows 43.7% energy efficiency and
energy requirement of the cycle. up to 52% exergy efficiency. A small scale experiment is under
Exergy efficiency values for all configurations are calcu- study in Clean Energy Research Lab (CERL) of UOIT to capture
lated based on thermal exergy input from total heat input. HCl in dry form, and with a successful HCl capture, the four-
Therefore, the heat exchanger effectiveness and the step MgeCl cycle can be a good candidate amongst other
maximum cycle temperature are two main parameters influ- hybrid thermochemical cycles with feasible reactions
encing cycle efficiencies. The thermal exergy input to the cy- throughout the cycle and mature electrolysis technology.
cles is strongly influenced by the sink and source
temperatures. The higher reference and the lower maximum Nomenclature
temperatures result in better exergetic efficiency values.
However, manipulation of both parameters is not suggested ex specific exergy, kJ kg1
due to their uncontrollable nature. Chemical reactions occur ex molar specific exergy, kJ mol1
at a specific temperature range for the best possible yield, and F Faraday's constant, 96,485 C mol1
changing this parameter would only vary the thermal exergy G Gibbs free energy, kJ kmol1
input, but changes in energy content of the streams cannot be h specific enthalpy, kJ kg1
kept constant. Variation of these parameters and their effect h molar specific enthalpy, kJ mol1
on exergy efficiency is represented in Fig. 11. Ke equilibrium constant
Since all calculations are made based on a heat exchanger LHV lower heating value, kJ mol1
effectiveness assumption, effect of this parameter is also N number of moles, mole, kmol
represented in Fig. 12. As an expected outcome, increase of R universal gas constant, kJ mol1 K1
this factor has a significant effect on cycle efficiencies. In a s specific entropy, kJ kg1 K1
perfect case with full internal heat recovery within all cycles, s molar specific entropy, kJ mol1 K1
up to 36% increase in energy efficiency and 25% increase in
Greek letters
exergy efficiency can be observed. MgeCl-B cycle is the one
h efficiency ()
effected the most from heat exchanger effectiveness factor,
which is due to relatively higher heat requirement of this Superscripts
configuration. 0 reference
Subscripts
end endothermic
Conclusion eq equilibrium
ext exothermic
The three configurations of the MgeCl cycle are comparatively F formation
studied by considering the individual reaction behaviors and
practical electrical energy and steam requirements. The direct
electrolysis of aqueous HCl results in a higher electrical work references
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polymer electrolyte membrane electrolyzers used to recycle