The thermodynamic evaluation and process simulation of the chemical looping steam methane reforming of mixed iron oxides

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The thermodynamic evaluation and process

simulation of the chemical looping steam methane

Cite this: RSC Adv., 2021, 11, 684

reforming of mixed iron oxides

a

Virginia H. Collins-Martinez,

Jose F. Cazares-Marroquin,^abJesus M. Salinas-

´ ´

Gutierrez,^aJuan C. Pantoja-Espinoza,^aAlejandro Lopez-Ortiz

and Miguel J. Melendez-Zaragoza^a

a

*

Steam reforming chemical looping (CL-SMR) using mixed iron oxides has the potential as an alternative to

the current partial oxidation (POX) and steam reforming (SMR) processes. In this study, the use of FeMoO₄,

Fe₂ZnO₄and Fe₂MnO₄as oxygen carriers (OC) under the CL-SMR reaction scheme was proposed to

overcome the current disadvantages of methane POX and SMR processes. This research is aimed at

ﬁnding potential iron-based metal oxides for the production of syngas, which can be regenerated under

favorable conditions in steam, while producing H₂. Thermodynamic evaluation and process simulation of

the CL-SMR reaction scheme using mixed iron-oxides was performed. Results indicate that FeMoO₄,

Fe₂ZnO₄and Fe₂MnO₄generated syngas at 750 ^ꢀC, 730 ^ꢀC and 600 ^ꢀC, respectively. However, FeMoO₄

was not fully regenerated under favorable conditions, whereas Fe₂ZnO₄and Fe₂MnO₄were completely

regenerated at 440 ^ꢀC and 640 ^ꢀC, respectively. Fe₂MnO₄showed the most favorable operating

conditions among the studied OC towards the production of syngas. Preliminary experimental studies

involved the synthesis of Fe₂MnO₄through a solid-state method using Fe₂O₃and MnO as precursors,

which was characterized via XRD, while its redox performance was evaluated in a TGA CH₄–H₂O redox

cycle, with reduction using CH₄followed by the steam oxidation of OC. Results indicate that both

reduction with methane and oxidation with water vapor using Fe₂MnO₄present reasonable reduction–

oxidation rates to be used in the CL-SMR reaction scheme, verifying the feasibility of the theoretical

study performed in the present investigation.

Received 9th October 2020

Accepted 6th December 2020

DOI: 10.1039/d0ra08610j

rsc.li/rsc-advances

renewable energy can rely on hydrogen as an energy vector to

reduce the impact of the inherent intermittent nature of

1. Introduction

renewable energy.²

According to the University of Oxford,¹the energy demand in

2019 reects the fact that the world still relies on fossil fuels,

with 33% oil, 27% coal and 24% natural gas, while the

remaining 16% comes from nuclear and renewable sources.

This situation of fossil fuels consumption for energy demand

needs to be changed, not only due to environmental implica-

tions, but also to reduce the dependence on fossil fuels. To

achieve this goal, the use of renewable forms of energy must

increase signicantly, and consequently the consumption of

fossil fuels will monotonically be reduced. Hydrogen as an

energy vector may help in this transition as today it is produced

mainly through natural gas steam reforming (SMR), although

other hydrogen generation processes based on renewables are

being more frequently employed worldwide. Meanwhile,

Today, hydrogen as a raw material plays a key role in

diﬀerent chemical industries such as the metallurgical, phar-

maceutical, petrochemical and oil rening industries. However,

hydrogen is generally produced using fossil fuels rather than

renewable energy sources. Hydrocarbons originating from

fossils constitute the cheapest raw material to produce

hydrogen, where natural gas is the preferred feedstock in the

steam methane reforming (SMR, reaction (1)) process.^3–8

CH_4ðgÞþ H₂O ¼ CO_ðgÞþ 3H_2ðgÞ

;

DH_R^ꢀ¼ 49:2 kcal mol^ꢁ1

ðgÞ

(1)

Presently, large-capacity hydrogen SMR plants have been

established worldwide because of the large demand in the oil

rening industry. This demand is due to the recent environ-

mental regulations in many countries, which limit fuels to

a

´

Departamento de Ingenierıa y Quımica de Materiales, Centro de Investigacion en

Materiales Avanzados, S. C. Miguel de Cervantes 120, Chihuahua, 31136, Mexico.

a

very low sulfur content. Hence, processes such as

E-mail: alejandro.lopez@cimav.edu.mx

b

´

ESIQIE-Instituto Politecnico Nacional, Unidad Profesional Adolfo Lopez Mateos,

Edicio 8, Ciudad de Mexico, Mexico

684 | RSC Adv., 2021, 11, 684–699

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hydrodesulfurization (HDS), which require large amounts of a severe uneven temperature distribution within the catalyst

hydrogen, will continually increase the demand for H₂.⁸

The main benet of this process is its high hydrogen yield.

One of the key features of the SMR process is its capacity to

achieve energy eﬃciencies in the range of 13.1–14.6 GJ per 1000

Nm³, thus resulting in the yields of 2.4–2.7 mol H₂per mole of

CH₄feed.⁵

bed.¹⁰

2CH_4ðgÞþ O_2ðgÞ¼ 4H_2ðgÞþ 2CO

;

DH_R^ꢀ¼ ꢁ17:18 kcal mol^ꢁ1

ðgÞ

(2)

Recently, development in the process towards the produc-

tion of syngas through POX has been reported. This involves the

use the stored lattice oxygen in metal oxides as the oxidant

source for the POX reaction. This process is called chemical

looping partial oxidation (CLPO) or chemical looping steam

methane reforming (CL-SMR). This technology employs natural

gas and light hydrocarbons¹¹and was originally conceptualized

by Mattisson et al.¹²One of the main advantages of this process

is that besides the production of syngas, the generation of high

purity hydrogen is possible in a second reactor within the

process. Due to the fact that heat transfer can be exchanged

between the reducing gas and the oxygen carrier, it is possible to

reduce the size of the reactors, thus making this CL-SMR

technology more eﬃcient and presumably cheaper than

conventional methane partial oxidation.^11,13

The current oxygen plant in the POX process can avoid the

use of oxygen carriers as an oxygen source for reaction (2) to

occur. In the CL-SMR process, partial oxidation of the hydro-

carbon (methane) is achieved by reacting with a metal oxide

(MO) through reaction (3) in a fuel reactor, while generating

syngas (CO + H₂) with the consequent reduction of MO to M as

a solid product. Next, the reduced metal (M) is sent to a second

stage (oxidation reactor), where it reacts with steam to produce

high purity hydrogen through reaction (4) and a regenerated

solid metal oxide (MO), which is then recycled back to the fuel

reactor to complete a full redox cycle of the metal oxide, thus

forming a chemical loop. Fig. 1 shows a simple representation

of the CL-SMR process. In this gure, the gas products from the

fuel and oxidation reactors are syngas and pure hydrogen,

respectively.

However, the SMR process presents many drawbacks such as

a large energy demand because of its high endothermic reaction

ðDH_R^ꢀ¼ 49:2 kcal mol^ꢁ1Þ; very severe operating conditions, for

example a temperature of 900 ^ꢀC and pressure of around 10

atm, while releasing huge amounts of CO₂into the atmosphere

under normal operation. The gas released during this process

usually reaches a substantial amount of 25 tons of CO₂/1 MMscf

of H₂product. Thus, to alleviate this issue, other processes have

been developed, such as natural gas reforming using CO₂as an

oxidant (dry reforming) employing metal catalysts.^4–7This

process is based on the reaction of CO₂(a greenhouse gas) with

methane to produce a gaseous mixture of CO and H₂, which is

commonly called syngas, through the following reaction: CH₄+

CO₂¼ CO + H₂. One of the main diﬃculties during the opera-

tion of this process is related to the sintering of the dry

reforming catalysts due to the high temperature conditions and

the carbon deposition over the surface of the catalyst, causing

deactivation due to the active sites of the catalyst becoming

blocked, similar to the case of the nickel-based catalysts.⁶

Recent research in syngas and hydrogen production has

been focused on making use of renewables as raw materials

such as water electrolysis, biomass gasication and nuclear

energy. However, these technologies are still not economically

feasible. A possible bridge between the current hydrogen

production technologies and that based on renewables is to

continue to employ natural gas as a raw material, with the

option of CO₂capture during the process, thus making this

more environmentally friendly while other technologies

become more competitive and mature. Consequently, there is

a need to develop novel processes to produce hydrogen from

hydrocarbons with the aim to reduce production costs and CO₂

emissions with comparable process eﬃciencies to SMR.⁵

The partial oxidation of hydrocarbons (POX, reaction (2)) has

been found to exhibit higher conversions than SMR towards the

production of syngas. Specically, methane partial oxidation

shows many advantages over the SMR process such as being an

exothermic reaction, which implies a lower energy demand and

the possibility to take advantage of faster kinetics, while

employing short residence times, thus requiring smaller size

reactors compared to SMR. Methane partial oxidation for the

production of syngas has been studied employing doped and

undoped transition metals towards the development of cata-

lysts with high activity and selectivity.⁹Nevertheless, several

disadvantages still remain to be solved for this process to be

competitive with SMR, such as the need for an exclusively

devoted oxygen plant to provide the on-site oxidant for the

process, thus making the initial investment of this technology

extremely high, while relatively high operating temperatures in

the range of 700–900 ^ꢀC lead to problems associated with

Fig. 1 CL-SMR process scheme.

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CL applications and these CoFe₂O₄/Co/FeO and NiFe₂O₄/Ni/FeO

oxide couples show R_ovalues of 6.8 and 13.6, respectively, which

are higher than that of their single couple counterparts.

Furthermore, other iron-based perovskite materials²²have been

reported to possess enhanced R_odue to one signicant feature,

which deals with their ability to accommodate important non-

stoichiometric oxygen in their structure. Hence, binary metal

oxides have been reported to overcome these limitations,

exhibiting increased selectivity towards syngas production.

Aston et al.²³reported examples of these materials such as

mixed-metal ferrites. They observed an H₂yield of 99% with

complete reoxidation of the oxygen carrier. They also compared

the results between materials such as mixed-metal spinel oxides

and single iron oxides, observing greater reduction conversions

and H₂production by the spinel-type materials than single

Fe₂O₃under similar conditions.

CH_4(g)+ MO_(s)¼ 2H_2(g)+ CO_(g)+ M_(s)

H₂O_(g)+ M_(s)¼ MO_(s)+ H_2(g)

(3)

(4)

Some authors have called this process POX-MeO.¹⁴In this

process, one of the key components is the oxygen carrier, which

must be able to withstand a large number of reduction–oxida-

tion cycles without loss of its activity and physical deterioration,

while maintaining a high oxygen storage capacity and thermal

stability.¹⁵

Many oxygen carriers have been proposed and experimen-

tally tested under the chemical looping process scheme. To

date, more than 700 materials have been synthesized and

evaluated as oxygen carriers.¹⁶Initially, oxygen carriers based on

Fe₃O₄, CaSO₄, Co₃O₄, NiO and CuO were proposed; however,

sintering eﬀects resulted in the deactivation of these materials

with an increase in the number of redox cycles. Thus, the latest

developments in oxygen carrier materials have been focused on

providing thermal stability to these oxides.

One strategy is to add an inert material to the main metal

oxide to alleviate sintering of the oxygen carriers at high

temperature. Accordingly, Fe, Ni, Cu, Mn, and Co have been

studied as main metal oxides, while Al₂O₃, TiO₂and SiO₂have

been proposed as inert materials. However, the addition of

these inert materials directly hinders the oxygen storage

capacity of the oxygen carrier.¹¹Furthermore, several

researchers have developed oxygen carriers for the chemical

looping (CL) process. Svoboda et al.¹⁷proposed the use of Ni-

and Co-based carriers for the production of high purity H₂,

while Diego et al.¹⁸employed a uidized bed reactor to test their

NiO–Al₂O₃oxygen carrier, and Kang et al.¹⁹proposed the use of

The major challenges for oxygen carriers under the CL-SMR

scheme are thermodynamic limitations for metal reoxidation

under steam, slow oxidation kinetics since steam is a weaker

oxidant than air, insuﬃcient high-temperature stability of many

carrier materials, and particle attrition in the circulating uid-

ized bed conguration of the typical CL-SMR process.²⁴This

process mode (steam oxidation) is restricted to only a few metal

oxides, which once reduced, can be thermodynamically regen-

erated with H₂O such as Fe, Co, Ce, W and Zn. In addition, this

operating mode of chemical looping has not been extensively

studied due to the abovementioned thermodynamic limitations

of oxygen carrier regeneration under steam. Single oxide oxygen

carriers, such as that based on Ni cannot be regenerated under

steam, while iron oxides are well known to form dense oxide

overlayers under a steam atmosphere, which severely reduce the

reaction kinetics and limit accessibility to bulk iron,²⁴and

consequently iron oxygen carriers possess low reactivity with

CH₄and poor selectivity towards synthesis gas. However, iron

oxides are commonly applied for chemical looping partial

oxidation of methane due to their low cost and natural abun-

dance.^25,26Recent research has been focused on attempts to

solve these issues. Because of their high activity and thermal

stability, La-based perovskites with Fe, Mn or Co as B-site

cations have been recently investigated for CL-methane POX

and CL-SMR,²⁷and other perovskite materials based on Co, Fe,

Cr, Mn and Ce metals have been examined.^28,29In addition, the

advantages of mixed-based iron oxides have not been fully

explored for CL-SMR until recently.³⁰One of the limitations of

these current studied materials is their sluggish kinetics^31,32

during their regeneration with steam and propensity to sinter

under steam atmospheres at high temperature.³³However,

some materials have been proven to perform fairly well under

these conditions such as Fe–Ce,^34,35Ba–Mn,³⁶NiFe₂O₄,³¹

CoFeAlOx,³⁷Ca₂Fe₂O₅,³⁸and Fe–xM/Al₂O₃(M ¼ Ca or Ce).³⁹

Nevertheless, the development of these materials is still in its

infancy for practical applications, but research focused on the

compositional tailoring and structures of these oxygen carriers

opens extensive possibilities for future improvements of the CL-

RMS chemical looping process.

a

three-reactor chemical looping conguration (TRCL).

However, one of the most relevant studies in this area was re-

ported by Fan et al.,²⁰who evaluated oxygen carriers of Fe, Ni,

and Ce. Their results showed a syngas (CO + H₂) purity of

greater than 90%. However, under the conditions employed in

their studies, the formation of carbon over the surface of these

materials occurred, and carbides (Fe₃C) were found. As

mentioned before, one of the main obstacles in the CLPO

oxygen carrier (OC) systems is the possible formation of carbon

during the operation of the fuel reactor under certain condi-

tions, and consequently diﬀusional limitations arise, hindering

the gas–solid reaction. Accordingly, promoters have been

proposed to avoid carbon deposition during the cyclic redox

operation, but with the drawback of limiting methane conver-

sion and lowering the production of syngas.

Some disadvantages of single metal oxides as oxygen carriers

are related to their thermodynamic, kinetic, stability and

durability limitations. Furthermore, another important limita-

tion of single oxygen carriers towards their application in the

chemical looping eld is related to their oxygen carrier capacity

(R_o), as dened by Lyngfelt et al.²¹The R_ois relatively small for

single oxide redox couples such as Fe₃O₄/FeO, which has an R_o

of 6.9%, while Co₃O₄/CoO and CuO/Cu₂O show an R_oof 6.6 and

10.1, respectively. As an alternative to this limitation, Svoboda

et al.¹⁷proposed NiFe₂O₄and CoFe₂O₄as potential materials for

With respect to iron-based mixed oxides, Lambert et al.⁴⁰

studied Fe–Mn oxygen carriers for the chemical looping

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combustion process and found that fast reduction of the H₂O. Within the fuel reactor, other possible reactions may

(Mn,Fe)₂O₃oxygen carrier to MnO and Fe was possible. occur. For example, two unwanted reactions are related to

However, their research was only devoted to synthesizing this methane decomposition (7a) and Boudard's reaction (7b),

oxygen carrier and testing it under a reducing atmosphere (fuel) which both form carbon as a byproduct. Furthermore, reaction

to generate indirect heat (chemical looping combustion), while (8) describes the regeneration of the reduced metal species by

its regeneration was performed with air at very high tempera- oxidation with steam to produce high purity hydrogen and

tures (900 ^ꢀC). Additionally, another interesting Fe-based mixed Fe_xMO₄. Simultaneously, any carbon formed from reactions (7a)

oxide that can be used as an oxygen carrier is Fe₂ZnO₄. This and (7b) can be gasied through reactions (9a) and (9b) with

spinel has been proposed as an oxygen carrier for the produc- steam, respectively, while producing additional syngas, H₂and

tion of H₂through solar-thermochemical water splitting appli- CO₂during the regeneration stage. Ni and Co ferrites were

cations.⁴¹Also, FeMoO₄has been reported to present high experimentally studied by Aston et al.²³as oxygen carriers for the

electron transfer mobility of its lattice oxygen in redox reactions production of syngas.

during the partial oxidation of propylene to acrolein.^42,43All

Moreover, in the chemical looping eld, it is common

these Fe-based binary oxides present very advantageous features practice to employ thermodynamic and process simulations to

for CL applications such as high mechanical strength,⁴⁴envi- assess the viability and potential of proposed oxygen carriers

ronmentally friendly nature, low cost, easily re-oxidized, and and processes.^47–49Previous studies employed the Aspen Plus

high theoretical oxygen carrier capacities, with R_ovalues of 20.8, thermodynamic module to perform simulations and the results

26.5 and 29.7 for Fe₂MnO₄, FeMoO₄and Fe₂ZnO₄, respectively. were compared to experimental data, which showed very close

Furthermore, these Fe-based binary oxides have not been agreement. Accordingly, Li et al.⁵⁰performed syngas chemical

previously reported for CL-SMR applications towards the looping gasication process simulation and reactor studies

production of syngas and H₂, even though they may have using the Aspen Plus simulator in the temperature range of

ꢀ

potential to be used for this process technology since their 750–900 C, where methane and an iron-based oxygen carrier

reduced species are susceptible to oxidation under a steam were used to produce syngas in the reducer reactor. They

atmosphere, and their oxidation kinetics have not been evalu- employed the RGIBBS module to determine equilibrium

ated to date. Therefore, to screen prospective materials (iron- conditions and key parameters such as physical and thermo-

based mixed oxides), rstly, from a theoretical point of view dynamic properties. They used the Peng-Robinson (PR) and

(thermodynamic analysis) and later experimentally, and based Redlich-Kwong (RW) equations of state for the solid and

on the relevant performances reported in the literature,^28,45,46gaseous phases, respectively. They reported that the simulation

Fe₂MnO₄, FeMoO₄and Fe₂ZnO₄were selected as good candi- calculations showed good consistency with both the thermo-

dates to be applied towards a possible CL-SMR reaction scheme. dynamic analysis and the experimental results, and concluded

Consequently, the present research aimed to perform a theo- that the feasibility of the syngas chemical looping concept was

retical thermodynamic study of Fe binary oxides (Fe₂MnO₄, validated with both the bench-scale tests and simulations. In

Fe₂ZnO₄and FeMoO₄), and consequently, select a suitable another study, Luo et al.⁵¹examined the shale gas-to-syngas

oxygen carrier under a methane CL-SMR process to determine (STS) chemical looping process to obtain high purity syngas.

the optimum operating conditions through a process simula- They performed process simulation using Aspen Plus to

tion to ensure high methane conversions towards syngas compare the energy conversion eﬃciencies of the STS process

production and generation of pure H₂during oxygen carrier with the conventional natural gas reforming process. They used

regeneration. This process is based on the following reactions: the PR method as the physical property method and compared

their results with experimental tests using a xed bed bench-

4CH_4(g)+ FeMO_4(s)¼ 8H_2(g)+ 4CO_(g)+ Fe_(s)+ M_(s)

CH_4(g)+ FeMO_4(s)¼ 2H₂O_(g)+ CO_2(g)+ Fe_(s)+ M_(s)

CH_4(g)¼ C_(s)+ 2H_2(g)

(5)

scale reactor and a sub-pilot scale moving bed system in the

temperature range of 990–1060 ^ꢀC. They found a high degree of

agreement between their simulation and test results and

conrmed the viability of the STS process for the conversion of

shale gas to high purity syngas. Furthermore, Tijani et al.⁵²

carried out a process simulation and thermodynamic analysis

of a chemical looping combustion system using methane as the

fuel and NiO as the oxygen carrier in a moving-bed reactor using

Aspen Plus. They also experimentally studied this system by

(6)

(7a)

(7b)

(8)

2CO_(g)¼ C_(s)+ CO_2(g)

4H₂O_(g)+ Fe_(s)+ M_(g)¼ FeMO_4(s)+ 4H_2(g)

C_(s)+ H₂O_(g)¼ H_2(g)+ CO_(g)

(9a) varying the fuel reactor temperature from 700 ^ꢀC to 1200 ^ꢀC.

They found that their tests and simulation results were in good

(9b)

C

_(s)+ 2H₂O_(g)¼ 2H_2(g)+ CO_2(g)

agreement with their experiments and the results reported in

the literature. Therefore, it is interesting to explore the feasi-

In reaction (5), the Fe_xMO₄oxygen carrier (M ¼ Mn, Mo, and

Zn) and methane produce syngas (H₂+ CO) together with

reduced metal species (Fe and M). Furthermore, complete

oxidation may occur according to reaction (6), where FeMO₄

and methane produce the undesirable products of CO₂and

bility of the use of Fe₂MnO₄, FeMoO₄and Fe₂ZnO₄mixed metal

oxides in the CL-SMR reaction conguration through thermo-

dynamic and process simulation studies. Hence, the present

research aimed to perform a thermodynamic evaluation of

Fe₂MnO₄, FeMoO₄and Fe₂ZnO₄as oxygen carriers in the CL-

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SMR process conguration for the production of syngas and reoxidation, respectively. This test was carried out as a standard

hydrogen. Thermodynamic analyses were performed to select procedure followed in our laboratory to prepare the sample for

the most suitable oxygen carrier to be further studied using the next reduction and oxidation cycle using methane and

process simulation to evaluate its potential as an oxygen carrier steam, respectively. The goal was to measure the redox perfor-

in CL-SMR technology. Furthermore, to select the most suitable mance under these gases and to generate preliminary data for

oxygen carrier among the proposed materials, thermodynamic the oxygen carrier kinetics using the previously determined

calculations (using Aspen Plus sensitivity analyses) were per- reaction conditions in this study. The second consecutive redox

formed to determine the most feasible chemical looping oper- cycle was performed at a constant temperature of 775 ^ꢀC using

ating conditions, with the criteria of fuel reactor operating 12 vol% CH₄/Ar and 2.2 vol% H₂O/Ar for reduction and reox-

temperature, CH₄/FeMO₄feed molar ratio, no carbon forma- idation, respectively. Before switching from reduction to

tion, full oxygen carrier regeneration, oxidation reactor oxidation atmospheres and vice versa, the remaining reactive

temperature and H₂O/Fe–M feed molar ratio. Based on the gases were removed by an argon ush ow for 5 min. The

thermodynamic results, the oxygen carrier presenting the best duration of the reduction step was determined to achieve the

performance was used to perform a process simulation to reduction of Fe₂MnO₄to Fe and MnO with a theoretical weight

determine the optimal process operating conditions based on loss of 20.8%. For the reoxidation of the sample, mixtures of

this oxygen carrier. The process simulations were performed water vapor and argon were supplied by water saturation of an

using an Aspen Plus© process simulator together with sensi- Ar ow at room temperature, at a concentration of 2.2 vol%

tivity analyses to determine the material and energy balance H₂O/Ar. The duration of this step was determined to complete

and optimal operating conditions of the CL-SMR process. the reoxidation of Fe and MnO back to the approximate initial

Subsequently, the simulation results were used to calculate the weight of the Fe₂MnO₄sample (until no mass change could be

thermal eﬃciencies of the process, and the H₂and syngas yields detected). Then the oxidized sample was cooled down at room

were compared with similar current CL-SMR processes reported temperature, placed in a vial and further characterized via XRD

in the literature to assess the potential of the proposed oxygen analysis.

carrier and chemical looping technology. Finally, the most

promising mixed iron oxide was synthesized, characterized via

XRD, and its redox performance evaluated in two TGA consec-

utive redox cycles to experimentally explore its reduction and

oxidation kinetics under the CL-SMR reaction scheme proposed

in the present investigation.

2.4. Thermodynamic analysis

The Gibbs free energy minimization technique was employed to

obtain the equilibrium compositions for each gaseous and solid

species within each reaction system employing Fe₂MnO₄,

Fe₂ZnO₄and FeMoO₄as oxygen carriers as a function of

temperature at atmospheric pressure conditions. For this, the

RGIBBS reactor model in the Aspen Plus© simulator was

employed. This technique considers all the possible reactions

that occur within the thermodynamic reaction system

2. Materials and methods

2.1. Synthesis of oxygen carrier

The Fe₂MnO₄oxygen carrier was synthesized through a solid-

state method by mixing Fe₂O₃and MnO (99.5% pure J.T.

Baker) powder in an agate mortar in an Fe/Mn molar ratio of

2 : 1 at room temperature. Su_ꢀbsequently, they were calcined in

an alumina crucible at 1000 C for 4 h and allowed to cool at

room temperature.

composed by the following gaseous species: CH_4(g), CO_(g)

,

CO_2(g), H_2(g), Zn_(g)and H₂O_(g). The solid species included in the

systems were MnO₂, MnO, Mn, ZnO, Zn, MoO₃, MoO₂, MoO,

Mo, Fe, C, Fe₂MnO₄, Fe₂ZnO₄and FeMoO₄. As stated before,

this thermodynamic analysis was aimed at evaluating the

viability of each binary metal oxide under the CL-SMR scheme

according to the process owsheet presented in Fig. 2.

2.2. Oxygen carrier characterization

The feed to the POX-MeO reactor, as shown in Fig. 2, con-

sisted of 4 kmol h^ꢁ1of CH₄, which was xed (stoichiometric

value) according to eqn (5), and the amount of oxygen carrier

(Fe₂MnO₄, Fe₂ZnO₄and FeMoO₄) was varied from 1.0 to 3.0

kmol h^ꢁ1. In addition, the temperature of the POX-MeO reactor

The calcined sample was characterized to study its crystalline

structure via X-ray diﬀraction analysis (XRD) on a Panalytical

XpertPRO diﬀractometer (Malvern, UK), equipped with KCua

radiation (l ¼ 0.15405 nm).

2.3. Thermogravimetric redox test

The redox behavior of the synthesized Fe₂MnO₄powder was

investigated via the conventional thermogravimetric analysis

(TGA) system, monitoring the weight change (wt%) signal with

respect to time. The redox experiment was carried out at

atmospheric pressure, the total reactive gas ow rate was 100

mL min^ꢁ1and the amount of Fe₂MnO₄sample used was 20 mg,

which was placed in a platinum sample holder. The rst redox

cycle was conducted at a constant temperature of 775 ^ꢀC using

5

vol% H₂/Ar and 2.2 vol% H₂O/Ar for reduction and Fig. 2 Process ﬂowsheet employed for the thermodynamic analysis.

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was changed from 100 ^ꢀC to 1000 ^ꢀC. The gas products from the

POX-MeO reactor were separated in a cyclone (CYCLON1) and

the solid products were sent to the regeneration reactor, where

they were combined with steam to become the feed to the

REGEN reactor (Fig. 2). The steam feed was varied from 4

(according to the stoichiometric value from reaction (8)) to 9

kmol h^ꢁ1, while the temperature in this reactor was changed

from 25 ^ꢀC to 900 ^ꢀC. Then, the solid and gas products from the

REGEN reactor were determined. Both reactors shown in Fig. 2

employed the RGIBBS reactor model within the Aspen Plus

simulator, which makes use of the Gibbs free energy minimi-

zation technique to determine the thermodynamic equilibrium

compositions as the products from each reactor.

2.6. Syngas yield and thermal eﬃciency

Based on the rst law of thermodynamics, Smith⁵⁴employed

a denition of the thermal eﬃciency in chemical processes,

which consists of the ratio of the energy generated as syngas (H₂

+ CO) to the energy supplied to the process (CH₄) and calculated

according to the following expression:

ꢂ

m_iꢃ LHV_i

m_iꢃ LHV_iþ W_iþ q_i

h ¼

(10)

ꢂ

_

where m_iand LHV_irepresent the mass ow rate and the lower

heating value of the “i” compound, respectively. Furthermore, q_i

and W_iare dened as the heat and mechanical work (based on

the rst law) demand of the process, which may include,

pumps, valves, compressors, and steam generators. The

mechanical work by itself was not considered in this eﬃciency

calculation since this requires detailed equipment specica-

tions that are out of the scope of the present study.

2.5. Process simulation

The material and energy balance under steady state conditions

were determined in each reactor (fuel and regeneration) using

the Aspen Plus© simulation engine, while the optimal operating

conditions were determined by employing sensitivity analyses

according to the process variables. In the fuel reactor, the

temperature was varied from 100 ^ꢀC to 1000 ^ꢀC under atmo-

spheric conditions (1 atm), while the feed iron mixed oxide

molar ow rate (Fe₂MnO₄, Fe₂ZnO₄and FeMoO₄) was varied

from 1 to 3 kmol h^ꢁ1. In contrast, in the regeneration (oxida-

tion) reactor, the steam feed molar ow rate was varied from 2

to 9 kmol_ꢀh^ꢁ1and the temperature range employed was from

100–1000 C at 1 atm of pressure.

The thermodynamic equations of state (EOS) employed in

each system were the Redlich-Kwong-Aspen and Peng-Robinson

equations for the gas and solid phases, respectively. These were

used to estimate physicochemical properties of chemical

species present in the reaction system and those that were not

found in the Aspen property database. The EOS used are

commonly employed in reaction systems that involve hydro-

carbons (methane partial oxidation product compounds) and

their mixtures with polar components (H₂O) at low and medium

pressures and in solids.⁵³In the fuel reactor a xed methane

feed molar ow rate of 4 kmol h^ꢁ1was used with each Fe-based

mixed oxide simulation, while the conditions for carbon-free

operation were investigated.

The optimal operating conditions were examined based on

each iron-based mixed oxide (Fe₂MnO₄, Fe₂ZnO₄and FeMoO₄)

as the oxygen carrier via sensitivity analyses. The fuel reactor

target optimal conditions involved establishing the operating

temperature and the CH₄/Fe_xMO₄feed molar ratio to obtain the

highest possible yield towards syngas production in the fuel

reactor, while simultaneously avoiding carbon formation,

which eventually may block methane gas on the surface of the

oxygen carrier (lattice oxygen), and consequently stop the gas–

sold reaction, thus deactivating the process. Furthermore, in

the second reactor (oxidation), the optimal operating condi-

tions were explored to ensure complete regeneration of the

previously reduced oxygen carrier, determine the reactor oper-

ating temperature and the H₂O/ROC (ROC, reduced oxygen

carrier) feed molar ratio in order to achieve the highest possible

yield towards the production of high purity hydrogen.

On the other hand, energy requirements were established

based on the heat duty demand of each process step and this

was readily calculated during each simulation run and reported

in a table. Therefore, the eﬃciency was estimated based on eqn

(11) as follows:

ꢂ

h ¼ m_Hꢃ LHV_Hþ m_COꢃ LHV_CO

2

(11)

ꢂ

m_CHꢃ LHV_CHþ q_i

4

Table 1 presents the low and high caloric values, LHV and

HHHV, respectively, of hydrogen, carbon monoxide and

methane employed in the calculations of the thermal eﬃciency

of the CL-SMR process.

Furthermore, based on the hydrogen and carbon monoxide

production results of each simulation run, the mean syngas

yield of the process was calculated as follows:

Y

þ Y_CO

H₂

Y_syngas

¼

ꢃ 100

(12)

2

where Y_H¼ mH₂/mH_2max, Y_CO¼ mCO/mCO_max, m_iis the ob-

2

tained molar content in kmol h^ꢁ1of species i in the product gas

and m_i,max¼ kmol h^ꢁ1is the molar content obtained to

maximum reactant conversion.⁵⁵Additionally, a Pinch analysis,

which is a simulation technique employed to minimize the

energy consumption in a chemical process, was employed. This

was accomplished by calculating the thermodynamically

achievable energy targets (for minimum energy consumption)

by optimizing the heat recovery systems, energy supply methods

and process operating conditions. The Pinch analysis is an

available subroutine within the Aspen Plus© simulator and was

Table 1 HHV and LHV of process chemical species in thermal process

Chemical Species

HHV (J kg^ꢁ1)10⁶

LHV (J kg^ꢁ1)10⁶

H_2(g)

CO_(g)

CH_4(g)

142.2

10.1

55.5

121.2

10.1

50.0

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Fig. 3 Process simulation ﬂowsheet.

used to recover heat coming from the temperature gradients carbon the formation (C kmol h^ꢁ1) in Fig. 4(a). The amount of

involving the reactant and product streams in the CL-SMR regenerated oxygen carrier produced (Fe₂MO₄in kmol h^ꢁ1) is

process.

a function of the amount of steam feed (H₂O kmol h^ꢁ1), the

Fig. 3 shows the process simulation owsheet of CL-SMR xed amount of reduced metals (Fe and M ¼ Mo, Zn or Mn) and

employed in the present study. For the Pinch analysis, heat the regeneration reactor temperature, as shown in Fig. 4(b).

exchangers (HEATEX1 and HEATEX2) were located directly in Therefore, according to the results presented in Fig. 4, it can be

the product steams (GAS-1 and GAS-2) to exchange heat with the concluded that the FeMoO₄oxygen carrier can be dismissed as

fuel methane stream (CH₄) to preheat it and for the generation a candidate for the proposed CL-SMR process since it cannot be

of steam (STEAM).

completely oxidized with steam under the present studied

conditions. Even though, Fe₂ZnO₄exhibited the largest free

carbon formation operating area, its regeneration area (green in

Fig. 4(b)) was reduced to limited conditions, which include

relatively low temperatures, thus inhibiting conditions in the

3. Results and discussion

3.1. Thermodynamic analysis

ꢀ

The results of the thermodynamic analysis performed on the

Fe₂MnO₄, Fe₂ZnO₄and FeMoO₄oxygen carriers are presented

in Fig. 4. The carbon-free operating conditions in the fuel

reactor are presented in Fig. 4(a), while full oxygen carrier

regeneration with steam conditions is shown in Fig. 4(b).

According to the results presented in Fig. 4(a), the greatest

area for carbon-free operating conditions belongs to Fe₂ZnO₄

(green area), followed by FeMoO₄(red area) and Fe₂MnO₄(blue

area). In contrast, for the full oxygen carrier steam oxidation

conditions shown in Fig. 4(b), the largest area is exhibited by

Fe₂MnO₄(blue area), followed by Fe₂ZnO₄(green area), while

the complete oxidation for FeMoO₄was not a combination of

a specic oxygen carrier feed (kmol h^ꢁ1of FeMO₄, M ¼ Mo, Zn

and Mn) per 4 kmol h^ꢁ1of methane fed to the POX-MeO reactor

with respect to a specic temperature range for the case of the

regeneration reactor to temperatures below 400 C and steam

feed ow rates (H₂O) of less than 9 kmol h^ꢁ1. Also, temperatures

ꢀ

below 400 C are very low to produce reasonably fast reaction

kinetics, while a large amount of steam would be needed to fully

regenerate this oxygen carrier, requiring a higher energy

demand, which may increase the process operating costs. In

contrast, Fe₂MnO₄presented a reasonable large free-carbon

operating region with feed Fe₂MnO₄ow rates greater than

1.4 kmol h^ꢁ1and temperatures higher than 620 C, which will

ꢀ

presumably favor fast reduction kinetics in the fuel reactor

towards syngas production. In contrast, in the regeneration step

(oxidation reactor), a wide range of temperatures and steam

molar ow rates are available to ensure full oxygen carrier

oxidation. Hence, for the Fe₂MnO₄oxygen carrier in the fuel and

in the regeneration reactors, both operating conditions are ex-

pected to favor carbon-free operation and full carrier regener-

ation, while promoting conditions for reasonable reaction

kinetics in throughout the CL-SMR process.

From a thermodynamic point of view and the results pre-

sented above, it can be concluded that Fe₂MnO₄is the best

oxygen carrier that ensures the process operating requirements

for the methane CL-SMR and high purity hydrogen production

targets of this research. Therefore, this oxygen carrier was

selected to be further studied via process simulation.

3.2. Process simulation

The process simulation study was divided in two sections. In the

Fig. 4 (a) Fuel reactor carbon-free operating conditions and (b)

regeneration reactor full oxygen carrier steam oxidation conditions.

rst section, sensitivity analyses were performed to determine

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the optimal process operating conditions using the Fe₂MnO₄

oxygen carrier. Consequently, the material balance of the hole

CL-SMR process was obtained. In the second section, a Pinch

analysis was employed to determine the optimal operating

conditions based on the energy balance. This procedure was

performed in an iterative fashion since the optimal conditions

in the rst and second sections had to be continuously recal-

culated to establish proper operating conditions, resulting in

convergence to the optimal results and in strict agreement with

the process owsheet diagram of Fig. 3. Again, carbon-free

operation, full oxygen carrier regeneration and lowest possible

operating temperature and fuel consumption constituted the

main optimization criteria in this simulation study.

Fig. 6 Process simulation sensitivity analysis results of H₂production

as a function of temperature and Fe₂MnO₄feed ﬂow rate in the fuel

reactor.

Fig. 5 presents the carbon generation in kmol h^ꢁ1as

a function of Fe₂MnO₄oxygen carrier per 4 kmol h^ꢁ1of

methane fed to the POX-MeO reactor as result of the sensitivity

analysis performed in this reactor.

ꢀ

operation since temperatures close to or greater than 1000 C

cause sintering of the oxygen carrier, in addition to the engi-

neering challenge of uidized bed reactors operating under

these conditions, which are unpractical from the construction

materials point of view. Therefore, lower operating tempera-

tures are expected for optimal syngas production in the fuel

reactor (POX-MeO).

In Fig. 5, the free carbon operating region that was previously

observed in the thermodynamic analysis section can be

conrmed with results presented in this gure, where carbon-

free formation can be achieved in temperature range of 600 to

ꢀ

1000 C and Fe₂MnO₄feed ow rate of approximately 1.5 to 3

Fig. 7 shows CO and CO₂production with respect to the

Fe₂MnO₄molar feed ow rate and fuel reactor temperature. In

this gure, comparable CO and H₂production behavior can be

seen (Fig. 7a and 6, respectively). The CO production results

compared with H₂generation can be seen in terms of the H₂/CO

ratio, which is an important feature of syngas. A temperature of

650 ^ꢀC and molar Fe₂MnO₄owrates of 1.4 kmol h^ꢁ1and

greater result in H₂/CO ratios of 2.0 and above. These process

conditions are a very signicant result since an H₂/CO ratio of 2

is suitable as a feedstock for the Fischer–Tropsch (FT) process

for the production of liquid hydrocarbons (gasoline, kerosene,

diesel and lubricants) from synthesis gas. Masters⁵⁶claims that

on a commercial scale, lower H₂/CO ratios will produce serious

diﬃculties during normal operation of FT reactors to achieve

optimal syngas processing.

kmol h^ꢁ1. Furthermore, the maximum carbon formation (1.8

kmol h^ꢁ1) can be observed in the temperature range of 526 ^ꢀC to

621 ^ꢀC at all Fe₂MnO₄feed ow rates studied in this sensitivity

analysis (1–3 kmol h^ꢁ1). Also, as shown in Fig. 5, a small carbon-

free operation region in the range of 100–200 ^ꢀC at most of the

Fe₂MnO₄feed range studied (1–3 kmol h^ꢁ1) can be seen.

Even though this region predicts no carbon is formed, its

generation is likely unfavorable presumably due to the slow

kinetics at these low temperatures, which eventually may

prevent any carbon formation. Besides, at these temperatures,

there is no carbon formation, but the production of syngas is

negligible, and therefore are of no process operating interest.

Fig. 6 presents the sensitivity analysis results of hydrogen

production from the fuel reactor (POX-MeO). In this gure, an

increase in H₂generation is evident at temperatures greater

than 600 ^ꢀC combined with Fe₂MnO₄feed ow rates in the

range of 1–1.5 kmol h^ꢁ1. Furthermore, the process conditions_ꢁi₁n

Therefore, lower operating temperatures are expected for

optimal syngas production in the fuel reactor (POX-MeO).

Fig. 7 shows the CO and CO₂production with respect to

Fe₂MnO₄molar feed ow rate and fuel reactor temperature.

These gures show comparable CO and H₂production behavior

(Fig. 7a and 6, respectively).

the POX-MeO reactor, T ¼ 1000 ^ꢀC and Fe₂MnO₄¼ 1 kmol h

,

.

will lead to a maximum hydrogen production of 7.89 kmol h^ꢁ1

However, this high temperature is not suitable for CL-SMR

Fig.

5 Process simulation sensitivity analysis results for carbon Fig. 7 Process simulation sensitivity analysis results of Fe (a) and MnO

generation as a function of temperature and Fe₂MnO₄feed ﬂow rate (b) formation in the fuel reactor as a function of temperature and

(kmol h^ꢁ1) in the fuel reactor.

Fe₂MnO₄feed ﬂow rate.

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The CO production results compared with H₂generation can the main products from the fuel (POX-MeO) reactor are shown

be seen in terms of the H₂/CO ratio, which is an important as a function of reactor temperature (650 ^ꢀC, 700 ^ꢀC, 750 ^ꢀC,

feature of a syngas. These process conditions are a very signif- 800 ^ꢀC, 850 ^ꢀC and 900 ^ꢀC), and the H₂/CO ratio is presented. All

icant result since an H₂/CO ratio of 2 is suitable as a feedstock the results were generated at an optimal molar feed ow rate of

for the Fischer–Tropsch (FT) process for the production of Fe₂MnO₄of 1.63 kmol h^ꢁ1, where a carbon-free operating

liquid hydrocarbons (gasoline, kerosene, diesel and lubricants) conditions prevail. According to the results in Table 2, it is

from synthesis gas. Masters⁵⁶claims that on the commercial evident that temperatures of 800 ^ꢀC and greater will generate no

scale, lower H₂/CO ratios will produce serious diﬃculties during signicant increase in syngas production (H₂+ CO) with only

normal operation of FT reactors to achieve optimal syngas a 2.4% increase from 800 ^ꢀC to 900 ^ꢀC. These very high

processing. Furthermore, he indicates that a typical H₂/CO ratio temperatures (T > 800 ^ꢀC) may represent high energy, and

of 2.15 is the optimal value for the correct operation of the consequently increased operational costs within the process,

cobalt-based catalyst used in the FT process. Additionally, and thus does not justify their application.

Fig. 7(b) presents results of the CO₂generation. Here, it can be

Also, by taking a closer look at the H₂/CO ratio, temperatures

seen that at a temperature of around 620 ^ꢀC and Fe₂MnO₄molar of 750 ^ꢀC and greater will generate ratios of 2.0 and higher,

ow rate of approximately 1.4 kmol h^ꢁ1, the CO₂generation which are suitable to be employed as a feedstock for the Fisher–

increases; however, higher temperatures (z700 C) eventually Tropsch process.⁵⁶Furthermore, the temperature range of 750–

ꢀ

result in a reduction in the generation of CO₂, thus favoring the 800 C reects the combined results of the carbon-free opera-

production of syngas. Thus, the abovementioned indicated tion, limited CO₂production, full Fe and MnO formation, and

conditions will promote the complete oxidation of methane optimal syngas generation.

(reaction (6)), and the production of CO₂, and consequently,

these operating conditions are to be avoided.

Thus, based on all the features discussed above, the

temperature range of 750–800 ^ꢀC is very suitable for CL-SMR

Therefore, the optimal operating conditions must be chosen using Fe₂MnO₄as an oxygen carrier in the current process

to prevent (as much as possible) the promotion of reaction (6) simulation research. Also, _ꢀby close examin_ꢀation of this

(complete oxidation) over reaction (5) (partial oxidation).

temperature range (750–800 C), a value of 775 C can be sug-

The process simulation results from the sensitivity analyses gested to be appropriate for this CL-SMR process, while at this

performed on the fuel reactor (POX-MeO) are presented in temperature an H₂/CO molar ratio of 2.03 is produced and

Fig. 8. In this gure, the generation of Fe and MnO is shown as Fe₂MnO₄is fully reduced to Fe and MnO, in agreement with

a function of reactor temperature and Fe₂MnO₄feed molar ow reaction (5).

rate. Fe generation is promoted at temperatures greater than

Moreover, the methane conversion in the fuel reactor (POX-

620 ^ꢀC and the feed ow rate of MnO (Fig. 8(b)) is almost MeO) as a function of Fe₂MnO₄molar ow rate (kmol h^ꢁ1) and

a mirror image of the Fe plot, which behaves similarly as temperature is presented as a contour plot in Fig. 9.

described above. It is important to mention that the regions

According to the determined optimal fuel reactor conditions

that favor Fe and MnO production agree with the same process of 750 ^ꢀC, 1.63 kmol h^ꢁ1of Fe₂MnO₄and 4 kmol h^ꢁ1of CH₄feed

conditions where no carbon formation is thermodynamically ow rate, a methane conversion of 95.9% can be achieved with

predicted, which means that at these conditions, POX reaction no carbon formation, while generating 10.52 kmol h^ꢁ1of syngas

(5) is favored over reactions (7a) and (7b). Also, it needs to be (CO + H₂) and an H₂/CO molar ratio of 2.03. Furthermore, as

addressed that under conditions studied in this research, the indicated above, temperatures greater than 800 ^ꢀC and

reduction of MnO to Mn was not feasible due to thermodynamic Fe₂MnO₄feed molar ow rates equal to or higher than 2.0 will

limitations, which is reected in the solid products of the fuel lead to a methane conversion of 99% and higher. However, this

reactor being only Fe and MnO. only represents a diﬀerence of only 3.1% increase from the

Table 2 presents the results of the comparison among optimal reacting conditions determined above.

diﬀerent sensitivity optimization analyses, where several oper-

Moreover, Fig. 10 show the results of the sensitivity optimi-

ating scenarios are shown. In this table, Fe, MnO, H₂and CO as zation analyses for the oxygen carrier regeneration reactor

(REGEN).

Table 2 Optimization sensitivity analysis results of main product

Temperature H₂

CO

Fe

MnO

H₂/CO

) ratio

(^ꢀC)

(kmol h^ꢁ1

)

(kmol h^ꢁ1

)

(kmol h^ꢁ1

)

(kmol h^ꢁ1

650

700

750

800

850

900

5.31

6.17

6.84

7.21

7.36

7.39

1.61

2.54

3.33

3.62

3.69

3.75

3.2

1.6

3.3

2.4

2.1

2.0

1.9

Fig. 8 Process simulation sensitivity analysis results of CO (a) and CO₂

(b) production in the fuel reactor as a function of temperature and

Fe₂MnO₄feed ﬂowrate.

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rate of 4.5 kmol h^ꢁ1at a temperature of 100 ^ꢀC and the

formation Fe₂MnO₄as products from the reaction between

steam and Fe + MnO. However, these reaction conditions in

terms of temperature are too low (100 ^ꢀC) to favor suitable

reaction kinetics (reaction (8)). Voldsund et al.⁵⁷reported that

a temperature of at least 400 ^ꢀC is needed to achieve reasonable

reaction kinetics for oxygen carriers with low regeneration heat

demand, and they claimed that these higher temperatures

would also help to alleviate the need for large cooling and

reheating systems within the process.

Moreover, the simulation results for the whole CL-SMR

process according to the owsheet shown in Fig. 3 are pre-

sented in Table 3. In this table, the full material and energy

balance is presented. It is important to highlight that additional

fuel (methane) was employed to provide the necessary heat duty

for each reactor. Therefore, methane (CH₄) was preheated (CH₄-

PRH) and split in two at separator S1, with one stream deliv-

ering methane for the burners (CH4-BRN) and the other stream

to the fuel reactor feed (CH4-RXN). Also, stream CH₄-BRN was

further divided in splitter S2 into two additional streams, one

for each fuel burner (B1 and B2). Furthermore, in Table 3, it can

be seen that methane streams B1 and B2 were combined with

the required amount of air for each burner (AIR1 and AIR2) to

provide the corresponding heats Q1 and Q2 for the fuel (POX-

MeO) and regeneration (REGEN) reactors, respectively.

Fig. 9 Methane conversion in the fuel reactor (POX-MeO) as a func-

tion of Fe₂MnO₄molar ﬂow rate (kmol h^ꢁ1) and temperature.

The values reported in Table 3 were found by sensitivity

analyses, and therefore these constitute the nal optimal

operating conditions for the whole CLPO process. Also,

according to the reported values in Table 3, a heat duty of 987.73

MJ h^ꢁ1(Q1) is needed to maintain the fuel reactor (POX-MeO)_ꢁa₁t

Fig. 10 Optimization sensitivity analysis results of H₂(a) and Fe₂MnO₄

(b) formation in REGEN reactor.

ꢀ

a temperature of 775 C, whereas a heat duty of 192.35 MJ h

The production of H₂(Fig. 10(a)) and the amount of regen-

erated Fe₂MnO₄(Fig. 10(b)) as a function of steam (H₂O) molar

ow rate (kmol h^ꢁ1) and reactor temperature are shown. These

results were used for the determination of the optimal reactor

operating conditions during the oxidation of Fe and MnO with

steam to produce high purity H₂.

According to Fig. 10(b), the results of the optimization

sensitivity analysis for the hydrogen production coming out

from the regeneration reactor lead to a maximum molar ow

(Q2) is required to be supplied to the regeneration reactor

(REGEN).

The other important results found in Table 3 include the

maximum hydrogen production of 7.05 kmol h^ꢁ1coming from

the fuel reactor (POX-MeO), while carbon oxides CO and CO₂

were 3.47 kmol h^ꢁ1and 0.38 kmol h^ꢁ1, respectively, with only

a small amount of water (H₂O, 0.65 kmol h^ꢁ1). Therefore, the

main product from the rst reactor was constituted by the

syngas, which corresponds to 10.52 kmol h^ꢁ1(H₂+ CO) with an

Table 3 CLPO-Fe₂MnO₄process simulation results to produce syngas

Mole ow rate

STREAM

T (^ꢀC)

(kmol h^ꢁ1

)

CH₄

CO

CO₂

H₂

H₂O

MnO

Fe

Fe₂MnO₄

CH₄

25.0

750.0

501.2

775.6

191.7

775.6

25.0

501.2

102.4

501.2

288.0

5.40

4.00

1.63

16.59

11.70

4.89

6.80

8.43

6.80

5.40

4.00

0.00

0.15

—

3.47

—

0.38

—

7.05

—

4.89

—

0.65

—

6.80

1.91

6.80

1.91

—

1.63

—

1.63

—

3.26

—

3.26

—

1.63

—

1.63

—

CH₄-PRH

CH₄-RXN

MEO

POX-MEO

GAS-1

SYNGAS

ME

WATER

GAS-2

STEAM

REGEN

H₂

—

4.89

—

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H₂/CO ratio of 2.03, with only a small amount of unreacted produced during the reduction step. These results in terms of

methane (0.15 kmol h^ꢁ1). The solids coming out of this reactor syngas are around 13% lower than that shown in Table 3 of the

were Fe and MnO with 1.63 and 3.2 kmol h^ꢁ1, respectively, present research, while pure hydrogen in the CL-SMR-Fe₂MnO₄

which represent the reduction of the Fe₂MnO₄oxygen carrier.

process was 0.7 times that produced in the fuel reactor. This

Also, the results in Table 3 indicate the full regeneration of diﬀerence can be attributed to two factors, the rst deals with

the Fe and MnO materials back to Fe₂MnO₄at the exit_ꢀof the the fact that in their experiments they could achieve metallic Cu

regeneration reactor (REGEN) at a temperature of 501 C with during the reduction step, while in the case of results from

a molar ow rate of 1.63 kmol h^ꢁ1, which was sent back to the Table 3, only Fe was completely reduced together with MnO as

fuel reactor (MEO stream), while the main gas product of this solid products from the fuel reactor. The other factor is related

reactor (GAS-2) resulted in a generation of 4.89 kmol h^ꢁ1of to the greater CH₄conversion found in the CL-SMR-Fe₂MnO₄

hydrogen accompanied with an unreacted amount of steam process (95.9%), which is 10.9% higher than the conversion

(1.91 kmol h^ꢁ1).

found by Lee et al.⁶³

The results from the CL-SMR simulation can be compared to

Furthermore, this higher methane conversion is reected in

a process where syngas is today commercially produced, which the H₂/CO ratio, which is 2.0 for the present this research,

is the steam methane reforming process (SMR). In this process, compared to 1.75 reported by Lee et al. In another comparable

an H₂/CO ratio of 6.25 is produced, while typically H₂, CO, CO₂study, the CL-SMR system was studied using Ce_1ꢁxFe_xO_2ꢁdas

and unreacted CH₄are reported to be 75%, 12%, 6% and 7%, the oxygen carrier, which was reduced with methane and

respectively.⁵⁸

reoxidized with steam, as reported by Zhu et al.⁶⁴They found

In contrast, the CL-SMR process based on the Fe₂MnO₄a dry-gas syngas composition of 62% H₂, 32% CO, 5% CO₂and

oxygen carrier theoretically is capable of producing concentra- 1% CH₄, an H₂/CO ratio of 1.94 and a CH₄conversion of 95%

tions of H₂, CO, H₂O, CO₂and unreacted CH₄of 60%, 30, 6%, during the reduction step, while aer the oxidation of the

3% and 1%, respectively, accompanied with an H₂/CO ratio of reduced oxygen carrier with steam they found 0.4 times the

2.05. Although the CLPO-Fe₂MnO₄process produces a lower amount of H₂produced in the fuel reactor with reduction and

amount of H₂with respect to SMR, this last process is targeted oxidation of the oxygen carrier being performed at 850 ^ꢀC. These

to the production of H₂, while the rst is aimed at the produc- results are very close to that reported in Table 3 for the CL-SMR-

tion of syngas. Furthermore, the CLPO-Fe₂MnO₄process results Fe₂MnO₄process, with the only diﬀerence being the amount of

in a greater methane conversion (95.9% for CLPO vs. 90% SMR) H₂produced in the regeneration step, where our work achieved

and a smaller number of byproducts (unreacted CH₄and CO₂) 1.75 times the amount of H₂than that reported by Zhu et al. at

than the SMR process.

a lower temperature of 775 ^ꢀC. Here, it is important to note that

Here, it is important to indicate the diﬀerences between when the methane conversion is greater or equal than 95%, the

several technologies that have been developed based on the theoretical (thermodynamic) predicted CL-SMR gas product

chemical looping concept. When the main product of the distribution and the H₂/CO ratio match the experimental values

process is energy (indirect heat), as a result of the complete reported in the literature, which represents a way to in principle

reduction of the oxygen carrier with a gaseous fuel, followed by validate our process simulation results under the so-called CL-

the oxidation of the reduced oxygen carrier with air to generate SMR or in our case the CL-SMR-Fe₂MnO₄process.

heat, this process is called chemical looping combustion (CLC),

Moreover, it is remarkable that a mixed iron oxide such as

which is the case of the Ca₂Fe₂O₅oxygen carrier as reported by Fe₂MnO₄can achieve greater methane conversion (95.9%) than

Ismail et al.⁵⁹When the fuel is a solid such as carbon or the simple iron oxide redox pair (Fe₃O₄/FeO). Thus, the inu-

biomass, it is called chemical looping gasication (CLG).^60,61ence of MnO on Fe₂MnO₄results in more favorable thermody-

Furthermore, when the main product is syngas and the oxygen namic equilibrium than the simple Fe₃O₄species. Presumably,

carrier is reduced with a gaseous fuel such as methane and is re- this is due to the inuence that MnO has within the Fe₂MnO₄

oxidized with air, this process is named chemical looping structure, enhancing the reducibility of this oxygen carrier

21

´

reforming (CLR). Finally, when syngas is the principal product under CH₄, as proposed by Ryden et al. However, methane is

gas due to the oxygen carrier reduction with methane, and in not thermodynamically feasible to reduce MnO to Mn, which is

the second step of the process, this carrier is reoxidized with the only disadvantage that this CL-SMR-Fe₂MnO₄process may

steam (H₂O), generating a high purity H₂stream, this process is have.

called chemical looping steam methane reforming (CL-SMR).⁶²

Concerning the regeneration stage, it is important to

Another notable comparison is related to the study reported consider that experimental studies related to the reactions of

by Lee et al.⁶³associated with the use of Cu–ferrite metals with steam to produce hydrogen have been reported to

(Cu_0.67Fe_2.33O₄) supported on yttria-stabilized-zirconia (YSZ) as present relatively high conversions (86%) as in the case of In to

an oxygen carrier in CL-SMR with steam regeneration of the In₂O₃at 400 C, as reported by Otzuka et al.⁶⁵compared to Fe

ꢀ

oxygen carrier. They reduced this material with methane at (75%) at 600 ^ꢀC. However, In₂O₃has a lower oxygen-carrying

900 ^ꢀC, while regeneration was performed at 700 ^ꢀC with steam capacity per weight (17%) compared to Fe₂O₃(30%). Other

(H₂O), and they observed a methane conversion of 85% with authors suggest the use of promoters to enhance the reaction

a dry-gas syngas composition of 51.4% H₂, 27.1% CO, 14.3% rates of iron regeneration, as reported by Kodama et al.⁶⁶who

CO₂and 7.2% CH₄, with a H₂/CO ratio of 1.75, while generating added some In₂O₃to Fe₃O₄for this purpose. Other research

1.6 times more H₂during the reoxidation step than that related to the use of mixed iron oxygen carriers reported the use

694 | RSC Adv., 2021, 11, 684–699

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ꢀ

of Ni(II)-ferrite both in reduction at temperatures above 700 C was 88.7% and 92.8% using the LHV and HHV values, respec-

and in oxidation above 500 ^ꢀC.⁶⁷These previous studies do not tively. Furthermore, Table 4 presents a comparison of the ob-

diﬀer very much to the optimal process conditions that were tained CL-SMR-Fe₂MnO₄yields with the current process

found in the present research.

eﬃciencies among the commercially established syngas

production processes reported in the literature based on

diﬀerent features such as H₂/fuel molar ratio, syngas yield

(Y_syngas), and H₂/CO ratio. According to this table, it can be seen

that the CL-SMR-Fe₂MnO₄process produced twice as much H₂

than SMR⁵⁸per mol of methane fed to the fuel reactor. This can

be explained in terms of the extra hydrogen that is produced

during the regeneration of the oxygen carrier with steam

(reaction (8)). Also, the CL-SMR-Fe₂MnO₄process from a ther-

modynamic (theoretical) point of view is capable of achieving an

H₂/fuel molar ratio of 2.98 and syngas yield of 87.4%, which

represent an increase by 0.48% and 20% than that reported by

Diego et al.¹⁸using the NiO–Al₂O₃oxygen carrier. Furthermore,

the H₂/fuel ratio obtained for the CL-SMR-Fe₂MnO₄process in

this work is very similar to that reported by De Souza et al.⁶⁸in

their methane autothermal process (ATR), with only a diﬀerence

of 0.07; however, the CL-SMR-Fe₂MnO₄process achieved

a greater conversion of methane. It is worth noting that the ATR

process has the disadvantage of employing a pure source of O₂

in the process, thus needing an exclusively devoted oxygen plant

as a side facility, making this process very expensive. Another

advantage of the CL-SMR-Fe₂MnO₄process is its comparable

syngas yield to the SMR process, which is lower for other

processes such as ATR and even the CLR-NiO process. Never-

theless, it is noteworthy that this yield represents only a theo-

retical value based on process simulation and sensitivity

analyses, while the reported values in Table 4 are based on

experimentation. Therefore, it is important to validate the

present simulation research reported herein with suitable

experimental results to evaluate the real potential of Fe₂MnO₄

as an oxygen carrier towards the CL-SMR process scheme.

3.3. Thermal eﬃciency and process yield

According to eqn (10), the process thermal eﬃciency was

calculated based on the material balance results from Table 3

and the LHV and HHV values of Table 1, which resulted in

theoretical thermal eﬃciencies of 88.7% and 92.8% using the

LHV and HHV heating values, respectively, which are shown in

Table 4. Also, this table presents a comparison between CL-

SMR-Fe₂MnO₄and SMR in terms of process eﬃciency, where

SMR typically presents eﬃciencies in the order of 70–85%,

whereas CL-SMR-Fe₂MnO₄shows 3.7–22.8% higher eﬃciencies

than SMR. Furthermore, according to values presented in this

table, it can be seen that CL-SMR-Fe₂MnO₄shows greater

theoretical eﬃciencies than SMR,⁵⁸autothermal reforming

(ATR)⁶⁸and chemical looping reforming using NiO–Al₂O₃as the

oxygen carrier (regeneration with air).¹⁸These results can be

explained in terms of the extra hydrogen production obtained

during the regeneration of the oxygen carrier with steam, which

is translated to a higher H₂/CO molar ratio of 3.44 in the overall

process according to the values shown in Table 3, therefore

generating superior thermal eﬃciency.

These results can be explained in terms of the extra hydrogen

production obtained during the regeneration of the oxygen

carrier with steam, which is translated in a higher H₂/CO molar

ratio of the overall process that according to values of Table 3 is

3.44 and therefore, generating a superior thermal eﬃciency.

This is a convenient feature of the CL-SMR-Fe₂MnO₄process

since, as mentioned before, the product of the fuel reactor

generates an H₂/CO ratio of about 2.0. However, if other

processes require the use of higher H₂/CO ratios such as the

methanol, oxo-synthesis and carbonylation processes, which

are oriented to the production of ne chemicals,⁶⁹this extra

hydrogen production from the regeneration of the oxygen

carrier represents an advantage of the CL-SMR-Fe₂MnO₄

process over current industrial syngas processes. This H₂/CO

can be conveniently tuned to the desired target ratio by adding

the surplus H₂produced during the regeneration stage of the

oxygen carrier, making this process highly exible.

3.4. XRD characterization results

Fig. 11 shows the results of the XRD characterization of the

synthesized FeMn-1 sample. In this diﬀractogram, it can be

clearly seen that there are only two crystallographic phases,

which match the results from the sample diﬀraction pattern,

showing manganese oxide as Mn₂O₃(ICDD: 00-10-0069) and

hematite as Fe₂O₃(ICDD: 00-87-1165). Both of these materials

exhibit high crystallinity since their reection peaks are narrow

Moreover, the CL-SMR-Fe₂MnO₄process yield was calculated

from eqn (12) and the process simulation results previously

reported in Table 3. The theoretically calculated syngas yield

Table 4 CLPO-Fe₂MnO₄process comparison with other industrial

and similar CL processes

Thermal

eﬃciency

H₂/fuel

ratio

Process

Y_Syngas(%)

H₂/CO ratio

SMR

ATR

CLR-NiO

CLPO-Fe₂MnO₄

70–85%

60–75%

89–93%

1.47

2.91

2.50

2.98

86.7

61.0

67.0

87.4

3.0–5.0

1.6–2.6

2.6

2.03–3.44

Fig. 11 XRD results of the as-synthesized FeMn-1 sample.

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and well dened. The XRD results presented in Fig. 11 are

consistent with that reported by Larring et al.,⁷⁰who found that

calcination in an oxygen atmosphere at 1000 ^ꢀC led to the

production of hematite and bixbyite phases, which are also

consistent with the phase diagram of the Fe–Mn–O system re-

ported by Katsutoshi et al.,⁷¹predicting the formation of

a mixture of Mn₂O₃and Fe₃O₄. Therefore, this was employed as

the starting material for the TGA test.

Once the sample was exposed to a redox cycle, rst with

methane (12 vol% CH₄/Ar) for reduction, followed by oxidation

with water vapor (2.2% H₂O/Ar), this sample (FeMn-1) was

characterized via XRD and the results of this analysis are pre-

sented in Fig. 12.

The results in Fig. 12 indicate that aer the oxidation with

steam, the sample (FeMn-1) presented two crystalline phases,

where the rst is associated with Fe₂MnO₄(iron manganese

oxide, jacobsite ICDD: 00-10-0069) and the second is the

hematite phase of Fe₂O₃(ICDD: 00-87-1165). Therefore, it can

be concluded that aer steam regeneration of the reduced

oxygen carrier, the Fe₂MnO₄species was obtained together with

some unreacted Fe₂O₃.

Fig. 13 TGA results of two consecutive redox cycles with H₂and CH₄

and oxidation with steam.

Fig. 11 is a mixture of hematite (Fe₂O₃) and bixbyite (Mn₂O₃)

and its reduction with hydrogen caused a weight loss of 21.11%,

which compared to the theoretical weight loss of the metallic

iron (Fe) and MnO species of 20.1% is very close, and thus it can

be considered that these are the species obtained at the end of

the reduction step with hydrogen. On the other hand, the

oxidation of these species with water vapor caused an increase

of 18.90%, which presumably corresponds to a mixture of

jacobsite (Fe₂MnO₄) and hematite (Fe₂O₃) phases according to

the XRD results presented in Fig. 12 aer the oxidation stage.

3.5. Thermogravimetric redox test results

The TGA tests were performed according to a modied proce-

dure described by Larring et al.⁷⁰to obtain the jacobsite phase

(Fe₂MnO₄). They exposed a solid mixture of Mn₂O₃and Fe₃O₄to

a 5% H₂/Ar atmosphere for initial reduction followed by

oxidation with 4% O₂/Ar to obtain Fe₂MnO₄, whereas in the

present study, the oxidation was performed using 2.2% H₂O/Ar.

In our test, aer oxidation with steam, reduction with methane

(12 v% CH₄/Ar) was achieved. Aer reduction with methane, the

sample was reoxidized again using steam to evaluate the redox

behavior of the Fe₂MnO₄oxygen carrier in a CH₄–H₂O redox

cycle. Fig. 13 shows the results of the TGA evaluation of the

oxygen carrier exposed to two consecutive redox cycles at

a constant temperature of 775 ^ꢀC. The rst cycle used a mixture

of 5% H₂/Ar. Once this material was reduced, it was subjected to

oxidation using 2% H₂O/Ar. Once the oxidation of the material

was completed in a second consecutive cycle, it was subjected to

a reducing atmosphere of 10% CH₄/Ar, while nally it was

reoxidized again in a water vapor atmosphere of (2% H₂O/Ar).

Here, in this gure, it can be seen that the initial starting

material according to the X-ray diﬀraction results shown in

3.6. Thermogravimetric redox test results

Fig. 13 shows the results of the TGA evaluation of the oxygen

carrier exposed to two consecutive redox cycles at a constant

temperature of 775 ^ꢀC. The rst cycle used a mixture of 5% H₂/

Ar. Once this material was reduced, it was subjected to oxida-

tion using 2% H₂O/Ar. In the second reduction using methane,

the sample was reduced again to approximately the same weight

of the mixture between metallic iron (Fe) and MnO in

a consecutive way, showing a weight reduction of 19.7%, but

now with a slower rate compared to the reduction rate exhibited

in the initial cycle with hydrogen. Meanwhile, the reduced

sample was re-oxidized in a water vapor atmosphere and the

nal weight of the sample increased by 19.5%. Once again, the

oxidation products according to the X-ray diﬀraction results in

Fig. 12, are the crystallographic phases of Jacobsite (Fe₂MnO₄)

and hematite (Fe₂O₃). Furthermore, these TGA results show that

it is possible to reduce the sample with methane and oxidize it

with water vapor, which allow the reduction and oxidation rates

to be determined. The reduction with methane lasted approxi-

ꢀ

mately 53 min under the conditions of 10% CH₄and 775 C.

These values can be compared with that reported by Kwak

et al.,⁷²where Fe₂MnO₄was also reduced with CH₄to 9.51 wt%

at a concentration of 15 vol% and 850 ^ꢀC, reporting a reduction

time of 25 minutes (r_red¼ 0.38 wt% min^ꢁ1).

Therefore, it was observed that the reduction rate of our

sample is approximately double that reported by these

Fig. 12 XRD results of FeMn-1 sample after two redox cycles and

reoxidation with steam.

696 | RSC Adv., 2021, 11, 684–699

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researchers, with the premise that their tests were carried out at (r_oxi¼ 0.28 wt% min^ꢁ1), it can be concluded that this oxygen

a higher temperature and CH₄concentration.

carrier shows superior behavior in terms of reducibility, oxygen

On the other hand, concerning the oxidation with water storage capacity and reduction–oxidation kinetics.

vapor, our sample required an oxidation time of 73 min at

a temperature of 775 ^ꢀC, while the work of Kwak et al.⁷²only

4. Conclusions

needed 20 minutes (r_oxi¼ 0.47 wt% min^ꢁ1), considering that

the latter was carried out in air atmosphere and at a higher Thermodynamic and sensitivity process simulations were per-

ꢀ

temperature of 850 C. Furthermore, one way to explain these formed on FeMoO₄, Fe₂ZnO₄, and Fe₂MnO₄oxygen carriers to

results can be based on the fact that presumably the inclusion evaluate their potential feasibility to produce syngas and high

of Mn in the crystal lattice of hematite (Fe₂O₃) inhibits the purity hydrogen employing the Aspen Plus® process simulator.

formation of the magnetite phase of iron oxide (Fe₃O₄) during The thermodynamic analysis results indicate that Fe₂MnO₄is

its oxidation with water vapor, and simultaneously favors the the oxygen carrier that presents the best available process

oxidation of the phase ferrous oxide (FeO) phase to hematite conditions to achieve the highest syngas production and full

(Fe₂O₃), which consequently is the nal phase found together Fe₂MnO₄regeneration with steam without carbon formation in

with jacobsite (Fe₂MnO₄) according to the X-ray diﬀraction the fuel reactor of the chemical looping partial oxidation (CLPO)

results reported in Fig. 12.

scheme. The results of the process simulation under the CL-

Therefore, it can be concluded that both the reduction with SMR-Fe₂MnO₄owsheet indicated that the optimal operating

methane and the oxidation with steam of the Fe₂MnO₄oxygen conditions of the fuel reactor are 775 ^ꢀC, 1.63 kmol h^ꢁ1of

carrier present reasonable reduction–oxidation rates to be used Fe₂MnO₄and 4 kmol h^ꢁ1of CH₄feed ow rate, thus obtaining

under the chemical looping (CLPO) mode proposed in the present a methane conversion of 95.9% with no carbon formation,

investigation since they are comparable with the results reported while generating 10.52 kmol h^ꢁ1of syngas (CO + H₂) and an H₂/

in the literature (reduction with CH₄and oxidation by air).

CO molar ratio of 2.03. In contrast, the optimal reaction

Yang et al.²⁷studied La–Mn–Fe–O oxygen carriers for the conditions for the oxidation reactor were determined to be

chemical looping steam reforming of methane. They reported a temperature of 501 ^ꢀC and steam molar ow rate of 6.8 kmol

that the La_0.85MnFe_0.15O₃perovskite was reduced under h^ꢁ1, producing pure H₂at a rate of 5.19 kmol h^ꢁ1and a fully

methane and oxidized with steam at temperatures in the range regenerated Fe₂MnO₄oxygen carrier, making this process

ꢀ

of 700–850 C. In their TGA tests they found only a reduction– highly exible towards a desired target H₂/CO ratio. The CL-

oxidation of 3 wt%, thus reecting the limited oxygen storage SMR-Fe₂MnO₄process from a thermodynamic (theoretical)

capacity of this oxygen carrier material, while crucial informa- point of view is capable of achieving an H₂/fuel molar ratio,

tion related to the reduction and oxidation kinetics was not syngas yield and thermal eﬃciency greater than current syngas

reported. Huang et al.³¹studied the NiFe₂O₄oxygen carrier production processes and CL-based processes reported in the

under a water-splitting chemical looping reaction scheme. They literature. The Fe₂MnO₄oxygen carrier was synthesized through

subjected this material to CO–H₂O redox cycles and found that a solid-state method using Fe₂O₃and MnO and characterized by

the reducibility of NiFe₂O₄varied from 19.61 to 23.11 wt% at XRD, while its redox performance was evaluated in a CH₄–H₂O

790 ^ꢀC, while its reduction time lasted 60 min (r_red

¼

TGA redox cycle. The reduction was performed using CH₄fol-

0.36 wt% min^ꢁ1); however, oxidation with steam needed lowed by steam oxidation of the oxygen carrier. The results

130 min (r_oxi¼ 0.15 wt% min^ꢁ1) to be achieved. Furthermore, indicate that both the reduction with methane and oxidation

Ismail et al.³⁸proposed Ca₂Fe₂O₅as an oxygen carrier material with water vapor of Fe₂MnO₄present superior reduction–

for CLR applications. They reported their experiments using oxidation rates to be used in the CL-SMR mode proposed in the

CO–CO₂redox cycles, and to avoid the complication with present investigation.

feeding steam, they oxidized the reduced oxygen carrier using

CO₂, which they claimed has an oxidizing potential similar to

that of steam at 850 ^ꢀC. Their TGA results only showed

Conﬂicts of interest

a reduction of 6.74–6.95 wt%, while the redox kinetics consisted There are no conicts to declare.

of 80 min reduction with H₂(r_red¼ 0.08 wt% min^ꢁ1) and 40 min

oxidation with CO₂(r_oxi¼ 0.17 wt% min^ꢁ1). Finally, Zeng et al.³⁷

Acknowledgements

described a CoFeAlOx spinel oxygen carrier, which was exposed

ꢀ

´

to CO–H₂O redox cycles at 800 C. They found around 15 wt% Authors are grateful to Dr Pedro Piza Ruiz for the XRD analysis

change during the reduction–oxidation cycle. However, their and interpretation of powder samples.

reduction lasted for 90 min (r_red¼ 0.16 wt% min^ꢁ1), while

oxidation was achieved aer 60 min (r_oxi¼ 0.25 wt% min^ꢁ1),

References

which reected a fair kinetic behavior. By comparing the TGA

results for the Fe₂MnO₄oxygen carrier with the above results

reported in the literature, it can be seen that according to the

results presented in Fig. 13, consisting of a TGA redox weight

change of 19.5–19.7 wt% at 775 ^ꢀC, a reduction time of 50 min

(r_red¼ 0.39 wt% min^ꢁ1) and oxidation under steam of 70 min

1 H. Ritchie and M. Roser, Energy consumption by source,

https://ourworldindata.org/grapher/energy-consumption-by-

source-and-region, accessed August 22, 2020.

2 D. R. Brown, Worldwide renery hydrogen production

capacities

by

country,

https://h2tools.org/sites/de

RSC Adv., 2021, 11, 684–699 | 697

View Article Online

RSC Advances

Paper

fault/les/Worldwide_renery_hydrogen

28 L. Protasova and F. Snijkers, Fuel, 2016, 181, 75–93.

_production_capacities_by_country_Jan%202017.xlsx.

3 S. M. Abdel Moneim, T. A. Gad-Allah, M. F. El-Shahat,

29 D. Li, R. Xu, X. Li, Z. Li, X. Zhu and K. Li, Energy Fuels, 2020,

34, 5381–5413.

¨

A. M. Ashmawy and H. S. Ibrahim, J. Environ. Chem. Eng., 30 X. Zhu, Q. Imtiaz, F. Donat, C. R. Muller and F. Li, Energy

2016, 4, 4165–4172. Environ. Sci., 2020, 13, 772–804.

4 K. Tao, L. Shi, Q. Ma, D. wang, C. Zeng, C. Kong, M. Wu, 31 Z. Huang, Z. Deng, D. Chen, G. Wei, F. He, K. Zhao, A. Zheng,

L. Chen, S. Zhou, Y. Hu and N. Tsubaki, Chem. Eng. J.,

2013, 221, 25–31.

Z. Zhao and H. Li, ACS Sustainable Chem. Eng., 2019, 7,

11621–11632.

5 B. V. Ayodele, M. R. Khan, S. S. Lam and C. K. Cheng, Int. J. 32 J. Wu, L. Bai, H. Tian, J. Riley, R. Siriwardane, Z. Wang, T. He,

Hydrogen Energy, 2016, 41, 4603–4615.

6 G. Zhang, L. Hao, Y. Jia, Y. du and Y. Zhang, Int. J. Hydrogen

Energy, 2015, 40, 12868–12879.

7 R. Amin, B. Liu, Z. B. Huang and Y. C. Zhao, Int. J. Hydrogen

Energy, 2016, 41, 807–819.

J. Li, J. Zhang and J. Wu, Int. J. Greenhouse Gas Controla,

2020, 93, 102897.

33 G. Wei, H. Zhou, Z. Huang, A. Zheng, K. Zhao, Y. Lin,

G. Chang, Z. Zhao, H. Li and Y. Fang, Energy, 2021, 215,

119044.

8 T. Rostrup-Nielsen, Catal. Today, 2005, 106, 293–296.

9 Y. Zhu, S. Zhang, J. Shan, L. Nguyen, S. Zhan, X. Gu and

F. Tao, ACS Catal., 2013, 3, 2627–2639.

34 X. Zhu, Y. Wei, H. Wang and K. Li, Int. J. Hydrogen Energy,

2013, 38, 4492–4501.

35 J. R. Fosheim, B. J. Hathaway and J. H. Davidson, Energy,

2019, 169, 597–612.

10 W. Z. Weng, Q. G. Yan, C. R. Lou, Y. Y. Liao, M. S. Chen and

H. L. Wan, in Natural Gas Conversion VI, ed. E. Iglesia, J. J. 36 V. P. Haribal, F. He, A. Mishra and F. Li, ChemSusChem,

Spivey and T. H. Fleisch, Elsevier, 2001, vol. 136, pp. 233–

238.

11 H. R. Forutan, E. Karimi, A. Hazi, M. R. Rahimpour and

P. Keshavarz, J. Ind. Eng. Chem., 2015, 21, 900–911.

12 T. Mattisson, A. Lyngfelt and H. Leion, Int. J. Greenhouse Gas

Controla, 2009, 3, 11–19.

2017, 10, 3402–3408.

37 D. Zeng, Y. Qiu, S. Peng, C. Chen, J. Zeng, S. Zhang and

R. Xiao, J. Mater. Chem. A, 2018, 6, 11306–11316.

38 M. Ismail, W. Liu, M. T. Dunstan and S. A. Scott, Int. J.

Hydrogen Energy, 2016, 41, 4073–4084.

39 X. Zhu, H. Wang, Y. Wei, K. Li and X. Cheng, J. Nat. Gas

Chem., 2011, 20, 281–286.

´

13 A. Lopez-Ortiz, P. E. Gonzalez-Vargas, M. J. Melendez-

´

Zaragoza and V. Collins-Martınez, Int. J. Hydrogen Energy, 40 A. Lambert, C. Delquie, I. Clemeneçon, E. Comte,

2017, 42, 30223–30236.

14 T. D. los Rios, D. L. Gutierrez, V. C. Martınez and A. L. Ortiz,

V. Lefebvre, J. Rousseau and B. Durand, Energy Procedia,

2009, 1, 375–381.

´

Int. J. Chem. React. Eng., 2007, 5(1), A30.

15 Y. Chen, N. Galinsky, Z. Wang and F. Li, Fuel, 2014, 134, 521–

530.

16 J. Adanez, A. Abad, F. Garcia-Labiano, P. Gayan and L. F. de

Diego, Prog. Energy Combust. Sci., 2012, 38, 215–282.

17 K. Svoboda, A. Siewiorek, D. Baxter, J. Rogut and

41 A. Lewandowski, C. Perkins, A. Weimer, N. Siegal, R. Diver,

K. Kwan, Y. Chen, H.-T. Hsieh, B. Earl, A. Vermillion,

B. E. Russ, T. Cramer, R. Tolman, G. E. Besenbruch,

L. C. Brown and B. W. McQuillan, HIGH EFFICIENCY

GENERATION OF HYDROGEN FUELS USING SOLAR

THERMAL-CHEMICAL SPLITTING OF WATER (SOLAR

THERMO-CHEMICAL SPLITTING FOR H2), General Atomics,

Las Vegas, Nevada, 2010.

ˇ

M. Pohorel´y, Energy Convers. Manage., 2008, 49, 221–231.

´

18 L. F. de Diego, M. Ortiz, F. Garcıa-Labiano, J. Adanez, A. Abad

´

and P. Gayan, J. Power Sources, 2009, 192, 27–34.

42 S. A. S. Wibawanta, Masters of Philosophy, Department of

Chemical Engineering, Curtin University of Technology,

2011.

19 H. W. Kang, S. N. Lim, S. B. Park and A. H. A. Park, Int. J.

Hydrogen Energy, 2013, 38, 6323–6334.

20 L.-S. Fan, Chemical looping systems for fossil energy 43 A. A. Firsova, Yu. V. Maksimov, V. Yu. Bychkov, O. V. Isaev,

conversions, Wiley-AIChE, Hoboken, NJ, 2010.

I. P. Suzdalev and V. N. Korchak, Kinet. Catal., 2000, 41,

116–121.

´

21 M. Ryden, H. Leion, T. Mattisson and A. Lyngfelt, Appl.

Energy, 2014, 113, 1924–1932.

22 A. Murugan, A. Thurseld and I. S. Metcalfe, Energy Environ.

Sci., 2011, 4, 4639–4649.

44 X. Zhao, H. Zhou, V. S. Sikarwar, M. Zhao, A.-H. A. Park,

P. S. Fennell, L. Shen and L.-S. Fan, Energy Environ. Sci.,

2017, 10, 1885–1910.

´

23 V. J. Aston, B. W. Evanko and A. W. Weimer, Int. J. Hydrogen 45 A. Lambert, C. Delquie, I. Clemeneçon, E. Comte,

Energy, 2013, 38, 9085–9096.

24 R. D. Solunke and G. Veser, Ind. Eng. Chem. Res., 2010, 49,

11037–11044.

25 L. Qin, Z. Cheng, M. Guo, M. Xu, J. A. Fan and L.-S. Fan, ACS

Energy Lett., 2017, 2, 70–74.

V. Lefebvre, J. Rousseau and B. Durand, Energy Procedia,

2009, 1, 375–381.

46 X. Zhao, H. Zhou, V. S. Sikarwar, M. Zhao, A.-H. A. Park,

P. S. Fennell, L. Shen and L.-S. Fan, Energy Environ. Sci.,

2017, 10, 1885–1910.

26 Z. Cheng, L. Qin, M. Guo, J. A. Fan, D. Xu and L.-S. Fan, Phys. 47 V. Shah, R. Joshi and L.-S. Fan, Ind. Eng. Chem. Res., 2020, 59,

Chem. Chem. Phys., 2016, 18, 16423–16435. 15531–15541.

27 Z. Yang, Y. Zheng, K. Li, Y. Wang, Y. Wang, H. Wang, 48 F. Kong, J. Swi, Q. Zhang, L.-S. Fan and A. Tong, Fuel, 2020,

Y. Wang, L. Jiang, X. Zhu and Y. Wei, Chem. Eng. Sci.,

2021, 229, 116085.

279, 118479.

698 | RSC Adv., 2021, 11, 684–699

View Article Online

Paper

RSC Advances

49 Y. Yan, D. Thanganadar, P. T. Clough, S. Mukherjee, 61 Z. Huang, F. He, K. Zhao, Y. Feng, A. Zheng, S. Chang,

K. Patchigolla, V. Manovic and E. J. Anthony, Energy

Convers. Manage., 2020, 222, 113144.

Z. Zhao and H. Li, J. Therm. Anal. Calorim., 2014, 116,

1315–1324.

50 F. Li, L. Zeng, L. G. Velazquez-Vargas, Z. Yoscovits and 62 X. Zhu, K. Li, Y. Wei, H. Wang and L. Sun, Energy Fuels, 2014,

L.-S. Fan, AIChE J., 2010, 56, 2186–2199. 28, 754–760.

51 S. Luo, L. Zeng, D. Xu, M. Kathe, E. Chung, N. Deshpande, 63 D.-H. Lee, K.-S. Cha, H.-S. Kim, C.-S. Park and Y.-H. Kim, Int.

L. Qin, A. Majumder, T.-L. Hsieh, A. Tong, Z. Sun and

L.-S. Fan, Energy Environ. Sci., 2014, 7, 4104–4117.

52 M. M. Tijani, E. Mostafavi and N. Mahinpey, Chem. Eng.

Process., 2019, 144, 107636.

53 P. M. Mathias, Ind. Eng. Chem. Process Des. Dev., 1983, 22,

385–391.

J. Energy Res., 2014, 38, 1522–1530.

64 X. Zhu, Y. Wei, H. Wang and K. Li, Int. J. Hydrogen Energy,

2013, 38, 4492–4501.

65 K. Otsuka, A. Mito, S. Takenaka and I. Yamanaka, Int. J.

Hydrogen Energy, 2001, 26, 191–194.

66 T. Kodama, A. Aoki, T. Shimizu, Y. Kitayama and

S. Komarneni, Energy Fuels, 1998, 12, 775–781.

54 J. M. Smith, J. Chem. Educ., 1950, 27, 584.

´

55 A. E. A. M. de Souza, L. J. L. Maciel, N. M. de L. Filho and 67 T. Kodama, Y. Watanabe, S. Miura, M. Sato and Y. Kitayama,

C. A. M. de Abreu, Catal. Today, 2010, 149, 413–417. Energy, 1996, 21, 1147–1156.

56 C. Masters, in Catalysis and Organic Syntheses, ed. F. G. A. 68 A. E. A. M. de Souza, L. J. L. Maciel, N. M. de Lima Filho and

Stone and R. West, Academic Press, 1979, vol. 17, pp. 61–103. C. A. M. de Abreu, Catal. Today, 2010, 149, 413–417.

57 M. Voldsund, K. Jordal and R. Anantharaman, Int. J. 69 X. Song and Z. Guo, Energy Convers. Manage., 2006, 47, 560–

Hydrogen Energy, 2016, 41, 4969–4992. 569.

58 A. Sunny, P. A. Solomon and K. Aparna, J. Nat. Gas Sci. Eng., 70 Y. Larring, C. Braley, M. Pishahang, K. A. Andreassen and

2016, 30, 176–181. R. Bredesen, Energy Fuels, 2015, 29, 3438–3445.

59 M. Ismail, W. Liu, M. T. Dunstan and S. A. Scott, Int. J. 71 K. Ono, T. Ueda, T. Ozaki, Y. Ueda, A. Yamaguchi and

Hydrogen Energy, 2016, 41, 4073–4084. J. Moriyama, J. Jpn. Inst. Met., 1971, 35, 757–763.

60 L. F. de Diego, M. Ortiz, J. Adanez, F. Garcıa-Labiano, A. Abad 72 B. S. Kwak, N.-K. Park, J.-I. Baek, H.-J. Ryu and M. Kang, J.

´

and P. Gayan, Chem. Eng. J., 2008, 144, 289–298.

Nanosci. Nanotechnol., 2018, 18, 6378–6384.

RSC Adv., 2021, 11, 684–699 | 699

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The thermodynamic evaluation and process simulation of the chemical looping steam methane reforming of mixed iron oxides

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