Applied Energy 113 (2014) 1667–1674

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Applied Energy

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Continuous operation characteristics of chemical looping hydrogenproduction system

0306-2619/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apenergy.2013.08.078

⇑ Corresponding author. Fax: +82 42 860 3428.E-mail addresses: kimsd@kaist.ac.kr (S.D. Kim), cspark@kier.re.kr (C.S. Park).

1 Co-corresponding author. Fax: +82 42 350 3910.

Won Chul Cho a,b, Do Yeon Lee b, Myung Won Seo c, Sang Done Kim b,⇑, KyoungSoo Kang a,Ki Kwang Bae a, Change Hee Kim a, SeongUk Jeong a, Chu Sik Park a,1

a Hydrogen and Fuel Cell Department, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Koreab Department of Chemical and Biomolecular Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Koreac Clean Fuel Department, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea

h i g h l i g h t s

� 300 Wth TRCL unit was successfully operated for 13 h using oxygen carrier (OC).� 94% CH4 conversion was achieved in a moving bed operation of the fuel reactor.� High purity (>99.9%) H2 was generated in the steam reactor (SR) in TRCL operation.� Carbon-deposited OC generated additional H2, lowering purity of H2 in the SR.� The OC showed high durability and maintained its physical and chemical properties.

a r t i c l e i n f o

Article history:Received 18 March 2013Received in revised form 23 August 2013Accepted 26 August 2013

Keywords:Three-reactor chemical looping (TRCL)HydrogenCO2 separationIron oxide (Fe2O3)Redox reaction

a b s t r a c t

Three-reactor chemical looping (TRCL) is a hydrogen and electricity production technology with inherentseparation of carbon dioxide by use of the iron-based oxygen carrier particles. In this study, an integratedcontinuous operation test unit (300 Wth) using 20 wt% Fe2O3/ZrO2as an oxygen carrier was successfullyoperated over 13 h. The average CH4 conversion was 94.15% in a fuel reactor in a moving bed mode.Nearly pure hydrogen (99.95%) was obtained from a steam reactor (SR). The steam conversion to H2

was around 63% of the thermodynamic limit in the SR. The hydrogen containing CO was observed fromthe SR when the carbon deposited particles were introduced into the SR. During the continuous opera-tion, the oxygen carrier particles maintained its physical and chemical properties with high durabilityin the TRCL system.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Hydrogen is considered as an environmentally friendly energycarrier and an attractive substitute for fossil fuel because it simplychanges to water during consumption without emission of CO2.The use of H2 as an energy carrier is a long-term option for energysystem to reduce the risk of greenhouse effect. Despite its simplic-ity and abundance, H2 does not produced naturally as a gas on theEarth – it is always combined with other elements such as water,many organic compounds such as gasoline, natural gas, methanoland propane [1]. Today, most of H2 is produced from the hydrocar-bon fuel and 80–85 % of the world’s total H2 is provided by thesteam–methane reforming (SMR) process because of low cost,abundance of the natural gas, and proven technology. However,

the SMR process has its limitations of contamination of H2 by COdue to the water–gas-shift reaction equilibrium and undefinedcapital cost of heat integration at small scales of production. Even50 ppm of CO can cause significant reductions in fuel cell perfor-mance [2]. The SMR process is only economically viable whenthe cost of natural gas is inexpensive.

In contrast, cyclic operation of steam–iron process provides theprospect of generating very pure H2 viably from the small to largescale, allowing simultaneous capture of a pure steam of CO2. Thisprocess involves the use of iron oxide (Fe2O3) as an oxygen carrierto the following reactions:

12Fe2O3 þ CH4ðgÞ ¼ 8Fe3O4 þ CO2ðgÞ þ 2H2OðgÞDH

�

800 �C ¼ 164:09 kJ=mol ð1Þ

4Fe2O3 þ CH4ðgÞ ¼ 8FeOþ CO2ðgÞ þ 2H2OðgÞDH

�

800 �C ¼ 372:92 kJ=mol ð2Þ

Nomenclature

A cross sectional area of the reactor (m2)g acceleration of gravity (9.8 m/s2)gc conversion factor (1 kg m/N/s2)i.d. inner diameter (cm)m mass of oxygen carrier in the reactor (g)mFe2O3 mass of the oxygen carrier when fully oxidized (i.e.,

Fe2O3) (g)mFe mass of the oxygen carrier when fully reduced (i.e., iron)

(g)_ni molar flow rate of i component obtained on the wet gas

meter (mol/min)pi partial pressure of gas i in the reactor (kPa)

DP pressure difference (Pa/min)DP1 differential pressure between the distributor and the

bed reactor below the solids inventory (kPa)DP2 differential pressure between the distributor and the

bed reactor above the solids inventory (kPa)Xs conversion of oxygen carrier or the degree of oxidationWth watt in thermal unitcCO2

gas yield of methane to carbon dioxide (�)u fuel-to-oxygen carrier ratio (�)

1668 W.C. Cho et al. / Applied Energy 113 (2014) 1667–1674

3Fe2O3þCOðgÞ¼2Fe3O4þCO2ðgÞ DH�

800 �C¼�40:87 kJ=mol ð3Þ

Fe3O4 þ COðgÞ ¼ 3FeOþ CO2ðgÞ DH�

800 �C ¼ 12:59 kJ=mol ð4Þ

3Fe2O3þH2ðgÞ¼2Fe3O4þH2OðgÞ DH�

800 �C¼�6:75 kJ=mol ð5Þ

Fe3O4 þH2ðgÞ ¼ 3FeOþH2OðgÞ DH�

800 �C ¼ 46:71 kJ=mol ð6Þ

3FeOþH2OðgÞ ¼ Fe3O4 þH2ðgÞ DH�

800 �C ¼ �46:71 kJ=mol ð7Þ

4Fe3O4 þ O2ðgÞ ¼ 6Fe2O3 DH�

800 �C ¼ �482:89 kJ=mol ð8Þ

Natural gas (mainly CH4) or syngas (CO:H2 = 1:2) is commonlyused as a reducing agent for iron oxide as of Eqs. (1)–(6). The gasesare indirectly combusted in a fuel reactor (FR) to produce CO2 andsteam. The reduced form of iron oxide (FeO or FeO/Fe mixture) issubsequently oxidized by steam to generate H2 (Eq. (7) in a steamreactor (SR). Finally, partially oxidized iron oxide (Fe3O4) returns toits original form (Fe2O3) by reacting with air (O2) in a riser (Eq. (8)),or an air reactor (AR). The reaction between Fe2O3 and feedstock(CH4 or syngas) is endothermic, and consequently there will be atemperature drop in the FR. However, heat is released over the en-tire system so that the residual heat from product gas steams isused to generate electricity in the system.

The development of oxygen carrier is the key issue in chemicallooping technology. Important characteristics of the oxygen carrierare sufficient rate of redox reaction, resistance to attrition, steamconversion to H2, and degree of carbon formation, etc. The reactionrates in combination with the oxygen transfer capacity determinethe amount of bed inventory in the reactors and needed circulationrate of particles [3,4]. The oxygen carriers becomes fragmented andagglomerated due to attrition while circulating the reactors. Theloss of particles and decrease in the reactivity of oxygen carrierdue to fragmentation and agglomeration influence the capital costand system efficiency. Steam conversion to H2has a significant ef-fect on the plant performance [5]. The carbon deposited particlescontaminate the hydrogen product with CO/CO2 gases by reactingwith steam. Many tests have been conducted in laboratory batchreactor, for instance in thermo-gravimetric analyzer (TGA) or labo-ratory moving-bed reactor to examine the reactivity of the pre-pared oxygen carriers [6–9]. The thermal efficiency evaluationshave been performed based on the mass and heat balance [10–13]. To sum up, numerous reports can be found in the scientific lit-erature, and these reports mainly focused on the oxygen carrierdevelopment, the reactor design, and the process synthesis.

However, there are limits to the lab-scale tests and simulationresults because they do not appropriately reflect non-ideal condi-tions including gas bypassing, limited heat and mass transfer,and mixing of gas and solid, which occurs in the real system. Thus,

the continuous operation of the chemical looping system is neededto demonstrate the design scheme and to verify performance of theprepared oxygen carrier particles. To date, many continuous oper-ations of chemical looping combustion (CLC) with two intercon-nected bubbling fluidized beds have been conducted using nickel,copper, iron, and manganese based oxygen carrier [15–23]. Onthe contrary, comparatively very limited amount of studies onthe continuous operation of chemical looping have been reportedfor generation of hydrogen using the iron oxide as an oxygen car-rier. In the Ohio State University, a 25 kWth sub-pilot SCL unit wasconstructed to demonstrate the feasibility of high purity H2 pro-duction system with simultaneous carbon capture [25,25]. Testruns employing gas solid counter-current flow were conductedusing 4.5 mm � 2.5–4.5 mm cylindrical oxygen carriers comprisedof 60 wt% iron oxide. They reported 100% syngas conversion andcontinuous H2 production with an average purity of 99.9% were ob-tained. In the Chalmers University of Technology, the continuousoperation of chemical looping in a two-compartment fluidized-bed reactor was also demonstrated [27]. The experiments wereperformed in a fluidized bed with transition from Fe3O4 to FeOusing freeze-granulated particles of 60 wt% Fe2O3 and 40 wt% inertMgAl2O4in the size range of 90–250 lm. They demonstrated COgas conversion of 60–80% with H2 production rate of 0.33–0.58 L/min depending on the gas (CO/steam) flow rate and the reactiontemperature.

The gas–solid contact mode in chemical looping process for H2

production is different from that in the CLC process which is com-posed of two interconnected bubbling fluidized beds in which solidconversions are constrained. The extent of solid conversion is be-tween Fe2O3 and Fe3O4 assuming the iron oxide as an oxygen car-rier, while three-reactor chemical looping (TRCL) process iscomposed of three reactors in a moving bed or multi-staged fluid-ized bed operation. The iron oxide should be at least reduced toFeO in the FR because reduced form of FeO is thermodynamicallyfavored to produce H2 by decomposing steam in the SR. The multioxidation state of (Fe2O3–Fe3O4–FeO) together with gas conversionof CH4 or syngas to CO2 and H2O can be achieved by employingmoving bed or multi-staged fluidized bed operation, which maxi-mizes both solid and gas conversions [28].

An integrated test unit (300 Wth) was constructed to generatehigh-purity H2 and CO2 using iron oxide with methane as a feed-stock. All together the unit was operated for 27 h includes startup(6.6 h) period, steady state (13 h) operation and carbon formation(8.0 h) operation.

The reactors were operated according to the followingreactions.

FR : 4Fe2O3 þ CH4 ¼ 8FeOþ CO2 þ 2H2O

DH�

800 �C ¼ 306:642 kJ=mol ð9Þ

W.C. Cho et al. / Applied Energy 113 (2014) 1667–1674 1669

SR : 8FeOþ 8=3H2O ¼ 8=3Fe3O4 þ 8=3H2

DH�

800 �C ¼ �124:547 kJ=mol ð10Þ

AR : 8=3Fe3O4þ2=3O2¼4Fe2O3 DH�

800 �C¼�321:931 kJ=mol

ð11Þ

Net : 2=3H2Oþ 2=3O2 þ CH4 ¼ CO2 þ 8=3H2

DH�

800 �C ¼ �139:836 kJ=mol ð12Þ

The objective of this study was to demonstrate the designscheme and to verify performance of the prepared oxygen carrierparticles. The FR was operated in a moving bed to determine suit-ability of the selected gas–solid contact mode. The hydrogen pro-duction rate was compared with the theoretical limit todetermined reactivity of the synthesized oxygen carrier. The effectof carbon deposition on the purity and the amount of H2 was deter-mined. The oxygen carrier was analyzed before and after theexperiments using SEM, BET surface area, and particle size distri-bution of the prepared particles.

2. Experimental

2.1. Experimental setup

The schematic diagram and photograph of the TRLC test unit isshown in Fig. 1. The apparatus consists of an air preheater, a riser, acyclone, AR, FR, SR, and loop-seals. The FR and SR both operate atthe moving bed state with similar inventory and operating temper-ature. Inner diameter of the reactor was 0.14 m (5.5 in.) havingseveral measuring taps for temperature, pressure, and gas

Fig. 1. (a) Drawing of test unit system. (1) Riser, (2) cyclone, (3) air reactor (AR), (4) AR loand (9) distributor. (b) Photograph of TRCL test unit.

sampling. The reactor was placed inside an electrically heated fur-nace for the desired isothermal conditions. The baffle plates wereinstalled in the reactors to avoid gas bypassing which leads to poorgas–solid contacting. Non-mechanical valve configuration (U-shaped fluidized loop seal) was used between the reactors to trans-fer the oxygen carrier particles. U-shaped fluidized loop seal had ani.d. of 0.108 m and bed height of 0.3 m [29]. Nitrogen gas was in-jected into the loop-seal at two locations; the supply chamberand recycle chamber. The supply chamber secures fluidity of thedownward particles and delivers them to the recycle chamber.The recycle chamber enables to transport particles to the nextreactor by crossing over the threshold [30]. The loop seal was flu-idized with N2 gas for stable operation. Compressed air was fedinto the air preheater through a filter, pressure regulators, and aflow meter. Then it is introduced into the riser after it was heatedto 550–600 �C in the preheater. The riser operates as an entrainedflow reactor to oxidize the oxygen carrier particles and conveythem through the AR. To ensure full oxidation of the oxygen car-rier, the AR (i.d. = 8.89 cm) acts as a combustor to oxidize the par-ticles and to transfer them into the FR making a closed circulationas the process scheme. A mass flow controller (Brooks, 5850E) wasinstalled on each gas inlet to regulate the gas flow rate. Prior to thegas analysis, fine solids were collected by a cyclone and steam wascondensed from the sample to protect the gas analyzer (Fuji Elec-tric System, ZAF3). A cyclone was installed on the back of eachreactor for fines collection. CaCl2 was placed in the sampling lineto remove water and a suction pump and a needle valve were usedto control the gas flow rate to the analyzer. All the operating con-ditions (temperatures, pressures) and compositions of outlet gaswere recorded every 1.0 s interval using a data logger (Graphtec,GL-820).

op-seal, (5) fuel reactor (FR), (6) FR loop-seal, (7) steam reactor (SR), (8) SR loop-seal,

1670 W.C. Cho et al. / Applied Energy 113 (2014) 1667–1674

2.2. Oxygen carriers

The carrier, Fe2O3 (20 wt%)/ZrO2, particle was prepared by theco-precipitation method. First, 0.27 mol of Fe(NO3)3�9H2O (Kanto,99%) and 0.72 mol of ZrO(Cl)2�8H2O (Dongsan Chemical Co.,>97%) were dissolved in 1.0 liter of deionized water. The solutionwas thoroughly mixed at 313 K under continuous stirring. To facil-itate the precipitation, 25% of ammonium hydroxide (OCI ChemicalCo.) was added to adjust pH of 8. After stirring for 1 h, the precip-itate was filtered and washed with water. The precipitate was thendried at 373 K for 24 h and calcined at 1673 K for 6 h in air atmo-sphere. The calcined sample was pulverized in a planetary ball milland sieved to a particle size of 125–300 lm. The measured mini-mum fluidized velocity (Umf) above 800 �C was 0.03 m/s. TheSEM analysis (Hitachi S-4700) was conducted with the preparedand processed samples. The BET surface area of the prepared sam-ple was measured with BELSORP-MINI instrument (BEL, Japan).The SEM, density and BET surface area of the particles were deter-mined before and after the experiments.

0

100

200

300

400

500

600

700

800

900

1000

Tem

pera

ture

(°C

)

Time (min)

Riser bottom Riser top AR Loop seal FR Loop seal SR Loop seal

0 50 100 150 200 250 300 350 400

Fig. 2. Temperature profile in the test unit during startup.

2.3. Data analysis

The conversion of the oxygen carrier or the degree of solid oxi-dation is defined as

Xs ¼m�mFe

mFe2O3 �mFe� 100% ð13Þ

The bed height of the reactor (H) was determined by the differ-ential pressures in the bed according to the following equation:

H ¼ DP2

DP1=Lð14Þ

where DP1 (kPa) is differential pressure between the distributor andthe bed reactor below the solids inventory and L (m) is the length ofDP1. DP2 is the differential pressure between the distributor and thebed reactor above the solids inventory.The CH4 conversion, gas yield(c) of CH4 to CO2 is defined as

cCO2¼

_nCO2

_nCH4

ð15Þ

where _nx is the molar flow rate of the components (CO2, CO, andCH4).

The solid circulation rate was determined by monitoring pres-sure of each reactor and obtained as follows:

Solid circulation rate ðg=minÞ ¼ 1000DPAgc

gð16Þ

where DP is pressure difference (Pa/min), A is cross sectional area ofthe reactor (m2), g is acceleration of gravity (9.8 m/s2), and gc is theconversion factor (1 kg m/N/s2).

When carbon is deposited on the surface of the oxygen carrierduring the reduction in the FR, deposited carbon will lower purityof H2 from the SR generating CO and CO2 simultaneously. Theamount of deposited carbon is estimated by measuring the CO/CO2 concentration of the product gas from the SR. Contaminatesof CO and CO2 can be defined as:

CO=CO2contaminates ð%Þ ¼pCO þ pCO2

pCO þ pCO2þ pH2

� 100 ð17Þ

The fluidized bed reactor in the chemical looping system involvestwo phases (gas and solid), multi components (CH4, CO2, CO, H2,Fe2O3, FeO, etc.), and multi reactions that does not reach completeequilibrium. Therefore, the equilibrium module in HSC Chemistry

5.0 was used to calculate the chemical equilibrium composition ina fluidized bed reactor because it enables to calculate multi-compo-nents equilibrium composition in heterogeneous system.

3. Results and discussion

3.1. Startup operation

Before the experiment, the carrier particles (20 wt% Fe2O3/ZrO2)were loaded into the top reducer. Pressurized He gas was injectedinto the test unit, which was sealed at all the outlet streams. Theleak test was successful after 3 bars of pressure maintained forover 1 h. After the leak test, the reactors were gradually heatedby the electric heaters under N2 purge gas for heat transfer be-tween the particles and the reactor walls. The solid circulation rate(Eq. (16)) was calibrated by N2 gas in the loop-seals. Preheated(500 �C) N2 gas was introduced into the loop-seal to circulate theoxygen carrier (20 wt% Fe2O3/ZrO2) at a rate of 65–75 g/min, whichcorresponds to the operating conditions of Eqs. (4)–(6). Tempera-tures at different location of the reactor are shown in Fig. 2. Asthe particles began to move between the reactors, the reactor tem-perature also increased at t = 50–200 min, as shown in Fig. 2 due togradual heating of the particles and the reactors. After 300 min,stable solid transport can be achieved at high temperature. It tookabout 300 min to heat up the particles and reactors to the desiredtemperature before injecting the reacting gases.

Once steady solid flow was attained, N2 gas was graduallyswitched to the reducing gas (CH4) into the FR. Meanwhile, steamwas fed to the SR at a rate of 0.14 mol/min (2.5 cc/min), correspond-ing to 5.67 of the stoichiometric ratio of CH4 to H2O because thesteam conversion to H2 at 800 �C was 47.1% thermodynamically.When the CH4/N2 gas mixture (CH4 = 100 cc/min and N2 = 2 L/min) was introduced at t = 100 min, temperature of the FR droppedabruptly from 900 �C to 820 �C as can be seen in Fig. 3, which showstemperature and CO2 gas concentration in the FR as a function oftime. This behavior resulted from the strongly endothermic reac-tion of Eq. (4). The flow rate of CH4 gradually increased to drivethe complete combustion of CH4 into CO2. The initial temperaturedrop by CH4 injection was recovered because the FR temperaturewas controlled by PID controller. As CH4 flow rate was slowly in-creased, CO2 concentration also increased and temperature in theFR increased to 850 �C for t > 100 min in Fig. 3.

3.2. Steady state operation

After all the reactors temperature stabilized, there was no solidscirculation problem due to the formation of particle agglomeration.

Tem

pera

ture

(°C

)

0 50 100 150 200 250 300 350 400800

820

840

860

880

900

920

940

960

980

CO

2 Concentration (%

)

Time (min)

0

2

4

6

8

CH4: 100 cc/min+ N2: 2 L/min

CH4: 120 cc/min + N2: 1.4 L/min

CH4: 150 cc/min + N2: 1.6 L/min

CH4: 350 cc/min + N2: 1 L/min

Fig. 3. Temperature and CO2 concentration profiles in the FR during startup.

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Time (h)

0

10

20

30

40

50

60

70

80

90

100

110

Gas

con

cent

ratio

n, N

2 fre

e ba

sis

(%)

H2

CO2

CO CH4

Fig. 5. Gas concentration (N2 free basis) from SR under steady state with 4 cc/minof H2O at 800 �C.

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Stoichiometric ratio H2O/CH4

H2 p

rodu

ctio

n ra

te (L

/min

)

Steam flow rate (mol/min)

Theoretical Experimental

0 2 4 6 8 10 12 14

Fig. 6. Effect of steam flow rate on hydrogen production rate in the bench unit at800 �C.

W.C. Cho et al. / Applied Energy 113 (2014) 1667–1674 1671

Continuous solid transport was successfully achieved by the loop-seal system at high temperature. The methane flow rate increasedat a rate of 550 cc/min into the moving bed. After steady-stateoperation was reached, the gas from the FR had an average CO2

yield of 94.15% for 13 h as shown in Fig. 4. The average bed heightof FR was determined to be 250 mm from the gas distributorsbased on Eq. (14). Most of the product gas is CO2, and the rest isCH4. The unreacted CH4 is thought to be due to the imperfectgas–solid contact or insufficient inventory. High gas conversion isdemonstrating the high contacting efficiency in the countercurrentmoving bed.

Fig. 5 shows the dry-gas concentration (N2 free basis) from theSR at 810–830 �C under steady state condition. The average hydro-gen purity generated from the SR is 99.95%. Purity of hydrogen pro-duced from the continuous operation depends on purity of steamused, gas leakage between the reactors, and carbon deposition.High-purity water was used to feed a steam generator to the SR.The system was operated in a stable manner, and methane wasnot detected from the SR. This result indicated that gas leakage be-tween the two interconnected beds is negligible. Minor CO concen-tration was detected from the SR, which was thought to be due tothe carbon deposition.

The effect of steam flow rate on H2 product rate from the SR isshown in Fig. 6. Eqs. (9)–(12) show that 8/3 mol of H2 can be pro-duced by 1 mol of CH4 and 8/3 mol of H2O in the present operatingcondition. However, the steam conversion to H2 in Eq. (10) is ther-modynamically limited to 47% because the equilibrium constant

0 1 2 3 4 5 6 7 8 9 10 11 12 130

10

20

30

40

50

60

70

80

90

100

Gas

con

cent

ratio

n, N

2 fre

e ba

sis

(%)

Time (h)

CO CO2

CH4

Fig. 4. Gas concentration (N2 free basis) from FR under steady state with 0.55 L/minof CH4 at 900 �C.

(Kp) is only 0.89 at 800 �C. Thus, 5.67 mol of H2O is required toachieve the thermodynamic limit of H2 production. The unreactedsteam from the SR in Eq. (10) and O2 depleted air gas from the ARin Eq. (11) are used to generate the electricity. The calculation wascarried out using the thermodynamic data of the HSC Chemistry5.0 program. The flow rate of methane was set at 550 cc/min sothat the maximum flow rate of hydrogen was 1466 cc/min, corre-sponding to the mole ratio H2O/CH4 = 5.67. Thus, the productionrate of H2 is constant after a mole ratio of H2O/CH4 = 5.67 as shownin Fig. 6. The experimental production rate was approximately be-low 63% of the theoretical H2 production rate when H2O/CH4 molarratio was from 1 to 5.67, in which SR was in a moving bed state. Incontrast, the experimental H2 production rate approached approx-imately 64% of the theoretical one when the stoichiometric ratio ofCH4 to H2O was in the range of 5.67 to 13, in which SR was at a flu-idizing state. The gap between the thermodynamic limit and theexperimental data is mainly attributed to the reactivity of the oxy-gen carrier and the gas bypassing. The lower steam conversion ofthe particles than the thermodynamic limit was verified in a previ-ous batch experiment [5]. The gas bypassing, or non-uniform flowpattern due to the channeling might lead to low steam conversioneven though the baffles were installed. It seems that using oxygencarrier with 125–300 lm (Geldart B) was responsible for the gas–solid contacts of axial mixing in this study.

The fuel-to-oxygen carrier ratio (u) was defined by Eq. (18),where _nFe2O3 is the molar flow rate of Fe2O3 and _nCH4 is the inlet

0.4 0.6 0.8 1.0 1.2 1.450

60

70

80

90

100

CH

4 con

vers

ion

(%)

fuel to oxygen carrier ratio, φ (-)

Experimental results Theoretical limit

1Umf

Fig. 7. Effect of fuel-to-oxygen carrier ratio on CH4 conversion in the FR at 900 �C.

1672 W.C. Cho et al. / Applied Energy 113 (2014) 1667–1674

molar flow rate of CH4 in the fuel reactor (a value of = 1.0 corre-sponds to the stoichiometric Fe2O3amount needed for a full con-version of CH4 to CO2 and H2O):

u ¼ 4 _nCH4

_nFe2O3

ð18Þ

Fig. 7 shows the effect of fuel-to-oxygen carrier ratio on CH4

conversion in the FR. The solid line represents the experimental re-sults in a moving bed operation. The dashed line is thermodynamiclimit of CH4 conversion in a fluidized bed condition using the equi-librium module in HSC Chemistry 5.0. The FR was operated in amoving bed at the value of u < 1.1 corresponding to below 1.0Umf considering 3 times (CO2 and 2H2O) volumetric expansion ofCH4in Eq. (9). When the CH4 was fed to the FR in a sub-stoichiom-etric ratio (u < 0.8), nearly 100% of CH4 was converted to CO2. Andthe increase in the inlet flow rate of CH4 (1.0 < u < 1.1) slightlylowered CH4 conversion (>90%) and only CO2 and unreacted CH4

were measured and H2 or CO was not measured. UnconvertedCH4 is possibly due to gas bypassing, insufficient heat/mass trans-fer, etc. On the contrary when the inlet flow rate of CH4 increasedto the value of u = 1.3 corresponding to the fluidized bed regime,CH4 conversion dropped to �68% and not only CO2/unreactedCH4 but also CO/H2 were measured. The CH4 conversion was higherthan the thermodynamic limit (dashed line). It seems that the baf-fles plates inside the reactor increases CH4 conversion reducingbubbling gas in the FR. The FR is needed to be operated in a movingbed condition to achieve the complete conversion. This findingmatches with the previous simulation and experimental results[3,25].

Table 1 summarizes the operating conditions and the obtaineddata of the integrated continuous operation of chemical loopingprocess for H2 production using iron-based oxygen carrier. Magnusand Mehdi [27] produced 330–580 cc/min of hydrogen from the SRwith 60.5–81.0% syngas conversion in the FR in a two-compart-ment fluidized-bed reactor. Both the FR and SR were operated in

Table 1Summary of the operating conditions in the continuous operation of chemical looping pro

Particlecomposition

Particlesize

Gas–solid contactpattern

Reducgas

Magnus andMehdi [27]

Fe3O4/MgAl2O4 90–250 lm

Fluidized bed Synga

Tong et al. [26] Fe2O3/ceramicmaterial

2 or4.5 mm

Counter currentmoving bed

Synga

This study Fe2O3/ZrO2 125–300 lm

Counter currentmoving bed

CH4

fluidized beds due to the redox pair of Fe3O4/FeO. They suggestthe possibility to realize the continuous three-reactor system bydemonstrating the two-reactor system. Tong et al. [26] successfullygenerated highly concentrated discrete streams of H2 and CO2 in a25 kWth sub-pilot SCL process. They conducted several test condi-tions by varying the solid flow rates, particle sizes, and the valveconfigurations. Nearly 100% syngas conversion and highly concen-trated stream of H2 were obtained in the moving bed reactors withthe non-mechanical valve configuration.

3.3. Carbon formation

Theoretically, the TRCL process has an advantage of producingpure H2 without the post purification process such as pressureswing adsorption. However, carbon can be deposited on surfaceof the particles during the reduction due to carbonous sources ofCH4 or CO gases in the FR. The carbon deposited particles can con-tribute to producing CO or CO2 with H2, lowering purity of H2 in theSR. The Boudouard reaction of Eq. (19) is known as carbon deposi-tion reaction and it is thermodynamically favored at a low temper-ature and high pCO/pCO2.

2COðgÞ ! Cþ CO2ðgÞ ð19Þ

The extent of solid conversion in the FR is also responsible forthe degree of carbon formation on the surface of the particles.When the solid was reduced to FeO, very pure H2 was produced.On the contrary, CO/CO2 contaminants were detected with H2

when the solids were deeply reduced in the batch experiments[4,5]. Cho et al. [30] also reported that the amount of carbon depo-sition increased rapidly after 80% of the available oxygen of NiO/NiAl2O4 was consumed. Thus, the carbon deposition depends onthe temperature, pCO/pCO2, and the extent of solid conversion inthe FR.

This experiment was carried out to demonstrate the carbon for-mation due to deeply reduced particles during the continuousoperation. The production rate of H2 gradually increased from 0to 1 L/min for 0 h < t < 4 h, when the process was operated toachieve steady state. The solids circulate rate through the reactorsat a flow rate of 65–75 g/min and CH4 flow rate to the FR was 0.55L/min. The iron oxide was reduced to FeO in the FR and nearly 100%H2 was produced from the SR at a rate of �1 L/min at 800 �C.

The bed inventory in the FR increased to reduce the iron oxideto Fe by reducing the solid flow rate from the FR to the SR for4 h < t < 5 h, as shown in Fig. 8c. After ensuring the carbon forma-tion of the particles in the FR, they was transferred to the SR att � 5 h. Carbon monoxide was detected and H2 production rate in-creased from 1 to 2.5 L/min for 5.5 h < t < 6 h as shown in Fig. 8aand b, respectively. These phenomena implied that the steam re-acted with deeply reduced and carbon-deposited particles. TheCO/CO2 mixture could be emitted from these as follows:

H2OðgÞ þ C ¼ COðgÞ þH2ðgÞ ð20Þ

H2OðgÞ þ COðgÞ ¼ CO2ðgÞ þH2ðgÞ ð21Þ

cess for H2 production using iron-based oxygen carriers.

ing Redoxpair

Gaseous fuel conversion to CO2

from the FRH2 concentration fromthe SR

s Fe3O4–FeO

60.5–81.0% Not available

s Fe2O3–Fe/FeO

Nearly 100% 99.99%

Fe2O3–Fe/FeO

94.15% 99.15%

0

20

40

60

80

100

c

bG

as c

once

ntra

tion

from

the

SR (%

)

H2

CO CO2

a

0

1

2

3 H

2 pro

duct

ion

rate

(L/m

in)

from

the

SR

0 1 2 3 4 5 6 7 8

0 1 2 3 4 5 6 7 8

0 1 2 3 4 5 6 7 8

2.4

2.6

2.8

3.0

3.2

Stat

ic p

ress

ure

(kPa

)of

the

FR

Time (h)

Fig. 8. (a) Gas concentration (N2 free basis) from the SR (b) H2 production rate from the SR and (c) static pressure of FR.

0 100 200 300 400 5000

1

2

3

4

5

6

7

8

Freq

uenc

y (%

)

Particle diameter (μm)

Fresh Used

Fig. 9. Particle size distribution for fresh and used Fe2O3/ZrO2 oxygen carrier.

W.C. Cho et al. / Applied Energy 113 (2014) 1667–1674 1673

Cþ CO2ðgÞ ¼ 2COðgÞ ð22Þ

The increase in H2 production rate can be explained thermody-namically as

4H2OðgÞ þ 3Fe ¼ Fe3O4 þ 4H2ðgÞ Kp800 �C ¼ 6:05 ð23Þ

H2OðgÞ þ 3FeO ¼ Fe3O4 þH2ðgÞ Kp800 �C ¼ 0:89 ð24Þ

The capacity for producing H2 over Fe to Fe3O4 transition isapproximately 1.8 times greater than that over the transition fromFeO to Fe3O4, based on the equal moles of H2O according to Eqs.(23) and (24). Hydrogen exceeding the thermodynamic limit ofEq. (23) (�1300 cc/min) was produced as can be seen in Fig. 8b.Thus, this suggests that carbon on the surface of the particles is apossible reaction intermediate and contributes to increasing H2

production. Steam can react not only with reduced solid from theSR but also with deposited carbon and CO, producing H2 in theSR according to Eqs. (20)–(22). CO and H2 from Eqs. (20)–(22)canbe subsequently oxidized as follows:

H2ðgÞ þ FeO ¼ H2OðgÞ þ Fe ð25Þ

COðgÞ þ FeO ¼ CO2ðgÞ þ Fe ð26Þ

Therefore, deposited carbon acts as the reducing agent to steamand oxidized particles, generating additional H2 (Fig. 8b) whilelowering purity of H2 (Fig. 8a) in the SR. The reducing iron oxidenear FeO or FeO/Fe mixture would be favorable for obtaininghigh-purity H2 in the TRCL system.

3.4. Comparison of the particles

After the experiments, the particles were recovered from thereactor and agglomerated particles were not detected. Fig. 9 showsthe particle size distribution (PSD) of the fresh and used oxygencarriers. The analysis shows that the used particle size is nearlysame as that of the fresh ones. The loss of fine solids due to attri-tion was negligible. In contrast, it was reported that the particlesize decreased after the experiments in a circulating

fluidized-bed reactor [27]. In this study, the reactor was operatedin a moving bed state such that the solid particles were exposedto less attrition than that in a fluidized bed. The bulk densities ofthe fresh and used particles are 2461 kg/m3and 2405 kg/m3,respectively. The BET specific surface areas of the fresh and usedparticles are 0.135 m2/g and 0.12 m2/g, respectively. The changein bulk densities and BET surface areas were negligible.

The SEM images of Fe2O3/ZrO2 oxygen carrier before and afterthe experiments are shown in Fig. 10. The sample before the exper-iment has a relatively smooth surface due to the thermal sinteringof the oxygen carrier at 1673 K. The sample after the experimentsexhibits the small surface pores. The morphology changes could beexplained by the cyclic shrinkage and expansion via the redoxreaction. However, particles showed irregular shapes and the usedparticles were seen to be similar to the fresh ones in general. Thesignificant changes in the surface texture and solid structure ofthe oxygen carrier were not observed after the operation. Also,the oxygen carrier particles did not show agglomeration. These

Fig. 10. SEM images of (a) fresh and (b) used Fe2O3/ZrO2 oxygen carrier.

1674 W.C. Cho et al. / Applied Energy 113 (2014) 1667–1674

results suggest that these oxygen carriers could have a high dura-bility and thus it would be suitable for use in the TRCL system.

4. Conclusion

In the continuous operation of a 300 Wth TRCL test unit, ironoxide was reduced to FeO in the FR using CH4 as a reducing gas.The reduced particles were oxidized via the steam–iron reactionin the SR. The pressure balance for the three beds and loop sealswas controlled with minimal instability. Under steady state condi-tion, 94.15% of CH4 was converted to CO2 and 99.95% of H2 wasproduced in this system.

In the moving bed operation of the FR, CH4 conversion exceededover 90% while it decreased to 68% in a fluidized bed condition. Thehydrogen production rate was 63% of the theoretical limit possiblydue to less gas–solid mixing, reactivity, and the particles size. Thecarbon was easily deposited when the iron oxide was deeply re-duced near to Fe. The carbon on the surface of the particles con-taminated hydrogen product by CO/CO2 gas and it is a possiblereaction intermediate and contributes to increasing H2 productionin the SR. The significant changes in the surface texture and solidstructure of the oxygen carrier were not observed after the opera-tion. The employed oxygen carrier exhibits high durability andthus it is suitable for use in the TRCL system.

Acknowledgments

This work was supported by the New & Renewable Energy R&Dprogram (2009T100100424) of the Institute of Energy TechnologyEvaluation and Planning (KETEP) grant funded by the Korean gov-ernment Ministry of Knowledge Economy.

This work was conducted under the framework of Research andDevelopment Program of the Korea Institute of Energy Research(KIER) (B3-2413-03).

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