Applied Energy 166 (2016) 84–95

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Applied Energy

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Sulfur behavior in chemical-looping combustion using a copper oreoxygen carrier

http://dx.doi.org/10.1016/j.apenergy.2016.01.0110306-2619/� 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +86 27 8754 4779x8208; fax: +86 27 8754 5526.E-mail address: klinsmannzhb@163.com (H. Zhao).

Kun Wang, Xin Tian, Haibo Zhao ⇑State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China

h i g h l i g h t s

� The sulfur fate of copper ore with H2S-containing synthesis gas was investigated.� Cu2S and FeS were the main sulfide products for copper ore reacted with H2S.� H2S is easier to react with copper oxides than iron oxides.� H2S led to a degradation of oxygen transport capacity and reactivity of the OC.

a r t i c l e i n f o

Article history:Received 3 June 2015Received in revised form 28 December 2015Accepted 10 January 2016

Keywords:CO2 captureChemical-looping combustion (CLC)Copper oreSulfur

a b s t r a c t

Chemical-looping combustion (CLC) is a promising technology that provides a novel route for CO2 capturewith low cost and energy penalty. Interaction between the oxygen carrier and sulfur contaminants in fuelis a significant concern in chemical looping systems, which will degrade the captured CO2 purity and evenaffect the reactivity of oxygen carrier. Experiments of a sulfur-containing synthesis gas (4000 ppm H2S,25 vol.% H2, 35 vol.% CO, and 39.6 vol.% CO2) as fuel and copper ore as oxygen carrier were performedby thermogravimetric analysis and Fourier transform infrared spectroscopy (TGA–FTIR). The effects ofreducing atmosphere, temperature and redox cycle number were studied. A weight gain was observedin all TGA experiments with 4000 ppm H2S synthesis gas as fuel, due to the sulfidation of the copperore oxygen carrier. For the reaction of copper ore with H2S-containing synthesis gas, the main metal sul-fide products were Cu2S and FeS, while the gaseous sulfur species were mainy SO2, COS, and CS2. H2S waseasier to react with copper oxides than iron oxides. Moreover, the sulfidation of copper ore was furtherinvestigated in a laboratory scale fluidized bed reactor at 900 �C, using copper ore as oxygen carrier andsynthesis gases with/without H2S as fuel. The results showed that the sulfidation of copper ore degradedits oxygen transport capicity and reactivity to some extent.

� 2016 Elsevier Ltd. All rights reserved.

1. Introduction

It is widely accepted that the increase of CO2 concentration inthe atmosphere has caused an increase of global temperature [1].The combustion of fossil fuels for power generation is a majorsource of CO2 emissions [2]. Chemical-looping combustion (CLC)is an emerging and highly promising technology that provides anovel route for inherent CO2 capture [3]. Two inter-connectedreactors, namely fuel reactor (FR) and air reactor (AR), areemployed in the CLC process. A kind of metal oxide, known as oxy-gen carrier (OC), circulates between FR and AR to provide latticeoxygen needed for fuel conversion. In such a way, the direct con-tact between air and fuel is avoided. Consequently, the flue gas

leaving FR mainly consists of CO2 and H2O, allowing high-purityCO2 to be acquired for storage or utilization after a simple steamcondensation process [4].

Selection of suitable OC is one of the key parameters to success-fully implement the CLC process. Plenty of metal oxides have beeninvestigated, including Ni-, Fe-, Cu-, Co- and Mn-based OCs [5–9].Adanez et al. [10] made a comprehensive review of OC develop-ment in CLC technology, in which more than 700 different materi-als have been systematically evaluated. Among these OCs, Cu-based OCs have received a great deal of attention due to the supe-rior reactivity. In the early years, investigations on Cu-based OCswere concentrated on gaseous fuel-derived CLC process [11,12].In 2009, an innovation of CLC, known as chemical looping withoxygen uncoupling (CLOU), was proposed by Mattisson et al.[13]. CLOU retains the merits of CLC but a kind of OC thatcan release gaseous oxygen at appropriate temperature

Nomenclature

CLC chemical looping combustionCLOU chemical looping with oxygen uncouplingFR fuel reactorAR air reactorOC oxygen carrierTGA thermogravimetric analysisDTG differential thermogravimetricFTIR Fourier transform infrared spectroscopyXRD X-ray diffraction analysisXRF X-ray fluorescence spectrometry

ESEM environment scanning electron microscopeEDX energy dispersive X-ray detectorBET Brunauer–Emmett–Tellerd attrition index of oxygen carrier particlesXt instantaneous weight ratio of the oxygen carrierRo oxygen transport capacity of the oxygen carriercCO2 CO2 yieldx mass-based conversion of the oxygen carrier

Table 1Copper ore compositional characteristics.

CuO CuFe2O4 SiO2 CaSO4 Al2O3

Content (wt.%) 21.04 70.05 5.53 2.29 1.08

Table 2Physical and chemical properties of the fresh copper ore OC.

Particle size (mm) 0.125–0.180Attrition index (%/h) 1.74Specific surface area, BET (m2/g) 0.2True density (kg/m3) 5353Crushing strength (N) 1.53XRD main phases CuO, CuFe2O4

K. Wang et al. / Applied Energy 166 (2016) 84–95 85

(800–1000 �C) is required. Cu-based OCs were found to be applica-ble for applying in the CLOU process due to the fast oxygen releaserate and suitable equilibrium partial pressure of oxygen at the tem-perature range of CLOU. Recently, Mei et al. [14] found that thechemical looping process for Cu-based OC with high concentrationgaseous fuel was usually a CLC process, while was a CLOU processwith low concentration gaseous fuel. Synthetic Cu-based OCs, likeCuO/ZrO2, CuO/MgAl2O4, CuO/Al2O3 and CuO/CuAl2O4, have gainedplenty of investigations due to the good performance in CLC orCLOU processes [15–20]. However, synthetic OCs generally havethe disadvantages of high cost, long preparation period and smallbatch preparation. On the contrary, natural copper ore was foundto be possible OC candidate with advantages of low cost and

Fig. 1. Equilibrium constants of reactions for (a) copper oxides and (b

abundant reserves, although it may exhibit comparatively lowreactivity due to the uncontrollable chemical composition andphysical structure. Recently, the CLOU performance of copper oreswith different copper contents has been investigated by Wen et al.[21] and Zhao et al. [22–24].

Sulfur (contained in fuel) evolution is a headache problem inboth CLC and CLOU processes, which should be carefullyaddressed. The effects of sulfur presence on a CLC or CLOU systemsmainly embody in two aspects. On one hand, the sulfur content inthe fuel may react with OC to formmetal sulfides or sulfates, whicheventually affect the OC reactivity to some extent. Moreover, thelow melting point of the metal sulfides or sulfates, e.g. 805 �C forCuSO4 and 1100 �C for Cu2S, may easily lead to agglomeration/sin-tering problem and affect the solid circulation pattern between theinterconnected fluidized-bed reactors. On the other hand, therelease of SO2 in FR and AR gas stream will degrade the purity ofthe captured CO2 and pollute the atmospheric environment,respectively [25].

Up to now, investigations on sulfur evolution in CLC or CLOUprocess are relatively rare. Thermodynamic calculations of Ni-,Cu-, Fe-, Mn-, Co-based OCs with H2S-containing synthesis gas asfuel were investigated by Wang et al. [26]. The interaction ofH2S-containing coal-derived synthesis gas with NiO and Fe–Mnoxides supported on different inert supports was investigated byKsepko et al. [27,28]. Forero et al. [29] and Garcia-Labiano et al.[30] studied the fate of sulfur in CLC process using Cu- and Ni-based OCs and natural gas as fuel. Shen and his coworkers[31,32] evaluated the effect of H2S on Ni-based and iron ore OCswith H2S-containing synthesis gas in CLC processes. Recently,Adanez-Rubio et al. [25] studied the fate of sulfur in a continuously

) iron oxides with H2, CO and H2S between 600 �C and 1000 �C.

Fig. 2. TG curves for the reduction and oxidation of the copper ore under differentreduction atmosphere at 900 �C. Fig. 3. (a) TG curve of the experiment reacting with 4000 ppm H2S synthesis gas for

about 80 s at 900 �C and (b) XRD result of the reduced sample.

86 K. Wang et al. / Applied Energy 166 (2016) 84–95

operated CLOU unit and its effect on the performance of a syntheticCu-based OC. Arjmand et al. [33] and Cabello et al. [34] investi-gated the effects of SO2 and H2S in CH4 on the performance ofperovskite-type OCs (CaxMn1�yMyO3�d (M = Mg, Ti)) in a batchfluidized-bed reactor and a continuous running CLC unit (500 Wth).

Up to date, no study has been carried out to investigate theeffect of sulfur presence in the fuel on the behavior of the copperore OC. The objective of this work was to investigate the sulfur fatein the CLC process using a copper ore as OC and synthesis gas asfuel. The effect of sulfur on copper ore OC was first studied by ther-mogravimetric analysis and Fourier transform infrared spec-troscopy (TGA–FTIR) using a H2S-containing synthesis gas as fuel.Then, the sulfidation process of copper ore was further investigatedin a laboratory scale fluidized bed reactor.

Fig. 4. (a) TG curve of the experiment reacting with 4000 ppm H2S synthesis gas forabout 900 s at 900 �C and (b) XRD result of the reduced sample.

2. Experimental section

2.1. Preparation of oxygen carrier

In this work, a refined copper ore from Zhongtiaoshan (China)was selected as raw OC material. In order to improve the crushingstrength and eliminate inherent sulfur of the ore (the original cop-per oxide ores with a low copper content usually undergo the sul-fidation reaction before the flotation process), it was calcined at500 �C for 5 h and then 1000 �C for 10 h in an air-atmosphere muf-fle furnace [24]. Finally, the particles after calcination were groundand sieved to 0.125–0.180 mm to obtain the fresh OC products. Thecomposition of fresh OC was determined by X-ray fluorescencespectrometry (XRF, EDAX EAGLE III) and X-ray diffraction analysis(XRD, X’PertPRO), as shown in Table 1. It can be seen that the cop-per ore was mainly composed of 21 wt.% CuO and 70 wt.% CuFe2O4.Moreover, it is worth noting that there was a few percent CaSO4

contained in the OC, which cannot be ignored in this work.Table 2 shows the physical and chemical properties of the fresh

OC particles. The BET surface area of OC particles was evaluated byN2-absorption method (Micromeritics, ASAP2020). The real densitywas measured by an automatic true density analyzer (AccuPyc1330). The crushing strength was determined by measuring theforce needed to fracture a particle using a crushing strength appa-ratus (Shimpo FGJ-5), taking the average value of 20 measure-ments. The attrition resistance of the OC particles was measuredwith an abrasion tester (DGM-100, made by Dalian intelligent test-ing machine factory) [6]. About 30 g OC was placed into a stainlesscylinder (length 14.5 cm, diameter 12.0 cm) with a 1.5 cm baffle,and rotated on a ball-mill roller for 50 min at a rotate speed of

10 rpm. Sieving and weighing the attrited OC particles (the sizeof the sieve is 0.125 mm), the attrition index was calculated by,

d ¼ ðm1 �m2Þ=m1 � 100% ð1Þwhere m1 is the mass of the OC before test, and m2 is the mass aftertest.

The crystalline phases of the copper ore were determined byXRD analysis. Finally, the surface morphologies of the fresh andused OC particles were examined by Environment Scanning Elec-tron Microscope (ESEM, FEI Quanta 200).

2.2. Experiment setup

Experiments were first conducted in a thermogravimentric ana-lyzer (WCT-1D, TGA) combined with Fourier transform infraredspectroscopy (TENSOR 27, FTIR). For each test, approximately30 mg OC sample was placed in a ceramic bowl and positioned intothe TGA reactor. A simulated coal-derived synthesis gas mixture of4000 ppm H2S, 25 vol.% H2, 35 vol.% CO, and 39.6 vol.% CO2 wasused as fuel in the reduction period, while air was used in the oxi-dation period. The gas flow rates in both reduction and oxidationperiods were controlled at 60 ml/min by mass flow meter/con-troller. In order to avoid the mixing of synthesis gas with air, the

Fig. 5. (a) TG curves and (b) DTG of the experiments reacting with 4000 ppm H2S synthesis gas at different temperatures.

Fig. 6. TG curves for the reduction–oxidation cyclic experiments with twosynthesis gases conducted at 900 �C.

Fig. 7. Evolution of the weight change between 0 s and 300 s in each reductioncycle of the two cyclic experiments as a function of cycle number.

K. Wang et al. / Applied Energy 166 (2016) 84–95 87

system was purged with nitrogen for at least 210 s before and aftereach reduction reaction. For comparison, experiments were alsoconducted with a synthesis gas composed of 25 vol.% H2, 35 vol.%CO, and 40 vol.% CO2 (without H2S). TGA tests were carried out atfour typical temperatures, i.e., 900 �C, 925 �C, 950 �C and 975 �C.Furthermore, 8 redox cycles were conducted at 900 �C to deter-mine the stability of the copper ore OC. The exhaust gas of TGAwas introduced to the FTIR spectrometer by a line heated at150 �C (to avoid the condensation of steam). Spectra were recordedwith a temporal resolution of 10 s. The infrared absorption band isin the range of 600–4000 cm�1 and the infrared peaks of CO2, CO,COS, CS2 and SO2 are at 2358, 2165, 2051, 1527, and 1371 cm�1,respectively. Because gas concentration is proportional to theabsorbance with a value <0.7, the absorbance can be used to char-acterize the concentration of the gas products. The sulfidaiton ofcopper ore by H2S-containing synthesis gas was also simulatedusing the HSC Chemistry software 5.0 to better understand theresults obtained in TGA experiments.

Then, the sulfidation process of copper ore OC was furtherinvestigated in a laboratory-scale fluidized bed reactor. The systemconsists of a gas feeding unit, a fluidized-bed reaction unit, and agas detection unit. The reaction tube has a length of 890 mm andan inner diameter of 26 mm, which is heated by an electrical fur-nace. A porous plate is placed in the tube at a height of 400 mm

from the bottom. Temperatures of the reactor are measured byK-type thermocouples located 10 mm above the porous plate. OCparticles were introduced to the reaction tube from the hopperlocated on the top of the reactor. Detailed description of the flu-idized bed system can be found in our recent publications[22,35,36]. The off gas from the reactor was led to on-line gas ana-lyzers (Gasboard Analyzer 3100) to detect the concentrations ofCO2, CO, CH4, H2, O2 and SO2. Additionally, 5 redox cycles were con-ducted in the fluidized bed at 900 �C using synthesis gas (with H2Sand without H2S) as fuel and copper ore as OC, aiming to investi-gate the effect of H2S in synthesis gas on the reactivity and struc-tural changes of copper ore OC. For each test, a total amount of15 g copper ore particles were first placed on the stainless porousplate, and air was introduced for 30 min at a set-point temperatureto ensure complete oxidation. Then, the fluidization gas wasswitched to N2 for about 180 s. When the O2 partial pressure wasclose to the equilibrium partial pressure of the OC at the set-point temperature, synthesis gas was introduced into the reactorfor about 240 s. Afterwards, the fluidization gas was switched toN2 again for about 10 min. Finally, the fluidization gas wasswitched to air and the reduced OC was re-oxidized. In each cycle,the inlet flow rates of the N2, synthesis gas and air were 600, 450and 800 ml/min, respectively, which were dertermined through apreliminary experiment to enure the best experimental effect.

Fig. 8. Measured gas concentrations for the cyclic experiments of product gas from the reduction (a–e) and oxidation (f) of copper ore by 4000 ppm H2S synthesis gas at900 �C.

Table 3Species considered in the HSC calculation for copper ore.a

Gas species Solid oxide species Solid sulfur species Solidcarbonspecies

Simplesubstancespecies

H2(g), H2O(g), CO(g), CO2(g), CH4(g), C(g), C2(g), C3(g),C4(g), C5(g), H2S(g), SO2(g), SO3(g), COS(g), CS2(g),O2(g), S(g), S2(g), S3(g), S4(g), S5(g), S6(g), S7(g), S8(g)

CaCO3, CaO, CaSO4, CuCO3, CuFeO2, CuFe2O4, CuO, Cu2O,CuO ⁄ CuSO4, CuSO4, Cu2SO4, CuSO4 ⁄ H2O, CuSO4 ⁄ 3H2O,CuSO4 ⁄ 5H2O, FeCO3, FeO, Fe2O3, Fe3O4, FeSO4, Fe2(SO4)3,FeSO4 ⁄ H2O, FeSO4 ⁄ 4H2O, FeSO4 ⁄ 7H2O

CaS, CuFeS2, CuS,Cu2S, Fe0.877S, FeS,FeS2, FeS2(M), Fe2S3,Fe7S8

Fe3C,Fe3C(B)

C, C(A), C(D), Cu, Fe,S

a g is the gaseous state; C(A) and C(D) are amorphous and diamond carbon, respectively; Fe3C(B) is b-iron carbide and FeS2(M) is marcasite.

88 K. Wang et al. / Applied Energy 166 (2016) 84–95

2.3. Data evaluation

In TGA experiments, the weight change of the OC versus reac-tion time t, Xt, was defined as the ratio of the instantaneous weightof the OC sample,mt, to the weight of OC at fully oxidized state,mo,

Xt ¼ mt

mo� 100% ð2Þ

The oxygen transport capacity of copper ore OC, Ro, was definedas:

Ro ¼ mo �mr

mo� 100% ð3Þ

where mr represents the weight of OC at fully reduced state.In the fluidized bed experiments, the OC reactivity was evalu-

ated by the gas yield of CO2, cCO2, defined as:

cCO2¼ yCO2

yCO2þ yCO

ð4Þ

where yCO and yCO2 are the molar fraction of CO and CO2 at each sec-ond in the flue gas, respectively. It is noteworthy that the CO2 gasyield is in the range of 0.53–1.00, due to the inherent CO2 in synthe-sis gas.

The mass-based conversion of the OC as a function of time wascalculated as:

xi ¼ xi�1 �Z ti

ti�1

Mo

monH2O þ nCO2 ;new þ 2nO2

� �dt ð5Þ

where MO is the molar mass of oxygen; nH2O, nO2 and nout are themolar flow rates of H2O, O2 and flue gas stream after steam conden-sation; nCO2,new is the molar flow rate of the CO2 from the conver-

sion of CO. Moreover, nH2O ¼ nout2575 yCO2

þ yCO� �

� yH2

� �,

nCO2 ;new ¼ nout3575 yCO2

þ yCO� �

� yCO� �

and nO2 ¼ nout � yO2, respec-

tively. yO2 and yH2 are the molar fractions of O2 and H2 in fluegas, respectively.

3. Results and discussion

3.1. Theoretical background

When a sulfur-containing fuel is used in a CLC or CLOU process,sulfur species are usually oxidized by the OC to form SO2, H2S, andother sulfur-containing gases. While as known, some oxides likeFe2O3, NiO, CuO and Mn2O3 were often used in some occasions toremove the hydrogen sulfide (H2S) at a medium temperature of400–700 �C [37], by forming metal sulfides. Therefore, in a CLC orCLOU process, metal sulfides can be formed in the FR whensulfur-containing fuel is employed, which may affect the reactivityand performance of the OC.

Fig. 9. Main sulfur species and reduction productds for copper ore reacting with (a) and (c) 4000 ppm H2S synthesis gas and (b) and (d) 0 ppm H2S synthesis gas as a functionof fuel to OC ratio at 900 �C.

K. Wang et al. / Applied Energy 166 (2016) 84–95 89

In this work, the copper ore mainly consists of CuO and CuFe2-O4. For the thermodynamic study, the CuFe2O4 was considered tobe separated CuO and Fe2O3 with the same Cu and Fe valencestates. From this point, the possible reactions of copper oxides withCO and H2 are as follows:

4CuO $ 2Cu2Oþ O2 ðR1Þ

2CuOþH2 $ Cu2OþH2O ðR2Þ

Cu2OþH2 $ 2CuþH2O ðR3Þ

2CuOþ CO $ Cu2Oþ CO2 ðR4Þ

Cu2Oþ CO $ 2Cuþ CO2 ðR5ÞThe sulfidation of copper oxides by H2S may occur via different

pathways, as shown below [29,38]:

6CuOþ 4H2S $ 3Cu2Sþ 4H2Oþ SO2 ðR6Þ

2CuOþH2S $ 2Cuþ SO2 þH2 ðR7Þ

2CuOþ 2H2S $ Cu2Sþ 2H2Oþ S ðR8Þ

Cu2OþH2S $ Cu2SþH2O ðR9Þ

2CuþH2S $ Cu2SþH2 ðR10ÞNoted that, both Cu2S and SO2 are generated by direct reaction

between CuO and H2S as (R6), while (R8)–(R10) only produce Cu2S.Thus, the sulfidation of CuO could be originated from these reac-

tions, i.e., (R6)–(R10), depending on the volume ratio of reducinggases (H2 and CO) versus H2S. Fig. 1a shows the thermodynamicfeasibility of (R2)–(R10) within 600–1000 �C. A value of logK > 0implies that the corresponding reaction proceeds in the positivedirection and the value of which reflects the reaction extent inthermodynamics. As can be seen, all of these reactions have equi-librium constants larger than 0, i.e., thermodynamically feasiblebetween 600 and 1000 �C. To be noted, if reducing gases (such asCO or H2) are present together with H2S, CuO favors thermodynam-ically to react with CO/H2 first, rather than with H2S via reactions(R6)–(R8) [38]. From this perspective, the reactions of (R6)–(R8)may be limited by the presence of reducing gases. The equilibriumconstants of (R6), (R9) and (R10) have a decreasing trend with tem-perature, which means that these reactions are less thermodynam-ically feasible at a higher temperature, while reactions (R7) and(R8) are just the opposite.

Then, the reduction process for iron oxides with CO and H2 mayoccur through following reactions:

3Fe2O3 þ CO $ 2Fe3O4 þ CO2 ðR11Þ

Fe3O4 þ CO $ 3FeOþ CO2 ðR12Þ

FeOþ CO $ Feþ CO2 ðR13Þ

3Fe2O3 þH2 $ 2Fe3O4 þH2O ðR14Þ

Fe3O4 þH2 $ 3FeOþH2O ðR15Þ

FeOþH2 $ FeþH2O ðR16Þ

Fig. 10. Evolution of the gas composition for copper ore reacting with (a) 0 ppm H2S synthesis gas and (b) 4000 ppm H2S synthesis gas at 900 �C.

Fig. 11. Evolution of the SO2 concentration for copper ore reacting with 4000 ppmH2S synthesis gas at 900 �C.

Fig. 12. The gas yield of CO2, cCO2, for the two redox fluidized bed experiments as afunction of mass-based OC conversion at 900 �C.

90 K. Wang et al. / Applied Energy 166 (2016) 84–95

The Fe2O3 may reduce to Fe3O4, FeO or even Fe, depending onthe amount of synthesis gas. When H2S is introduced, the sulfida-tion of iron oxides by H2S may occur via these reactions [32]:

9Fe2O3 þH2S $ 6Fe3O4 þ SO2 þH2O ðR17Þ

3Fe3O4 þH2S $ 9FeOþ SO2 þH2O ðR18Þ

3Fe3 þH2S $ H2Oþ SO2 þ 3Fe ðR19Þ

FeOþH2S $ FeSþH2O ðR20Þ

Fe2O3 þ 3H2S $ Fe2S3 þ 3H2O ðR21Þ

FeþH2S $ FeSþH2 ðR22ÞFig. 1b shows the equilibrium constants of (R11)–(R22)

between 600 and 1000 �C. Except for (R18) and (R19), all of thereactions have a value of logK larger than 0 or close to 0, whichmeans that these reactions are thermodynamically feasible. At900 �C, the equilibrium constant of (R17) is larger than that of(R11) and (R14). This indicates that (R17) is more thermodynami-cally favorable than (R11) and (R14). Moreover, the equilibriumconstants of (R20)–(R22) decrease with the increase of tempera-ture and are always lower than that of (R17), which means that

higher temperatures are thermodynamically feasible to generateSO2, while lower temperatures are beneficial to generate ironsulfides.

In addition, H2S can be oxidized by copper and iron oxides togenerate S2. Then the S2 and H2S can react with CO to produceCOS and CS2 [30]. These reactions and the oxidation reactions ofCOS and CS2 are not listed here. In the AR, the reactions for copperand iron oxides as well as metal sulfides are much simpler, asfollowing:

CuxOy�1 þ ðx� yþ 1Þ=2O2 $ xCuO ðR23Þ

CuxSy þ ðxþ 2yÞ=2O2 $ xCuOþ ySO2 ðR24Þ

FexOy�1 þ ð3x� 2yþ 2Þ=4O2 $ x=2Fe2O3 ðR25Þ

FexSy þ ð3xþ 4yÞ=4O2 $ x=2Fe2O3 þ ySO2 ðR26Þ

3.2. TGA–FTIR experiments

3.2.1. Effect of different reducing atmosphereThree TGA experiments with different reducing atmospheres

were first conducted at 900 �C. The weight loss curves as a functionof time are shown in Fig. 2. The three reducing atmospheres were

Fig. 13. XRD analysis results for the reduced OC samples after at the 5th cycle offluidized bed experiments with (a) 4000 ppm H2S synthesis gas and (b) 0 ppm H2Ssynthesis gas.

K. Wang et al. / Applied Energy 166 (2016) 84–95 91

pure N2, synthesis gas with 4000 ppm H2S and without H2S,respectively. Fig. 2 also shows the theoretical oxygen transportcapacities for copper ore at different reduction states. It can beseen that the copper ore can release oxygen in an inert atmo-sphere, due to the decomposition of CuO and CuFe2O4. The weightfor copper ore reached 95.85%, which is close to the theoreticalvalue 95.56% for the reduced states of Cu2O and CuFeO2. Whenthe experiments were carried out with synthesis gas (at 210 s),the weight loss first increased slowly until 210 s due to the oxygenrelease under the N2 atmosphere. Then, the weight loss increasedrapidly to about 13.7% corresponding to the reduced states of Cuand FeO, which indacates that the main reactions in this stage werethe gas–solid reduction reactions of copper ore with H2 and CO.With time proceeding, the weight loss for the experiment withoutH2S increased gradually to a value of 17.56%, which might beattributed to the deeper reduction reaction of FeO by CO and H2.However, for the experiment with 4000 ppm H2S, the weight lossfirst increased to a peak value of 17.09% and then decreased lin-early, which is ascribed to the sulfidation of copper ore. A fasterweight loss rate for experiment with 4000 ppm H2S compared tothe experiment without H2S between 400 s and 1200 s wasobserved. The possible reasons are: (1) FeO reacts directly withH2S (R19) to produce Fe and SO2, which leads to a weight loss;(2) (R10) and (R22) occur to generate H2, leading to a relativelyhigher H2 concentration around the particles. Thus, higher reactionrate for (R16) is attained.

With regard to the oxidation process, the weight for the exper-iment without H2S increased very rapidly when air was introducedinto the reactor and finally reached a weight of 97.54%. The reasonfor the weight not reaching 100% was that CaSO4 and CuFe2O4 incopper ore react with CO and H2, which cannot be fully regener-ated with air in such short reaction time. While for the experimentwith 4000 ppm H2S, there was a rapid weight gain first, mainly dueto the oxidation of reduced copper or iron oxides. A weight losswas then observed due to the oxidation of sulfides in the sample((R24) and (R26)). Simultaneously, sulfate formation might alsooccur, which contributed to a transient weight loss between5200 and 5250 s. At last, the weight reached 99.99%, indicatingthat the sulfidation reaction of copper ore was helpful for the oxi-dation of the reduced copper ore to its initial state.

In order to further investigate the sulfidation reaction of copperore, two additional experiments with different exposure times tosynthesis gas were carried out at 900 �C. Fig. 3 shows the TG curveof the experiment reacting with 4000 ppm H2S synthesis gas for

about 80 s. It can be seen from Fig. 3a that the weight loss wasabout 13.2% when switching gas stream from synthesis gas to N2,which was just after the rapid weight loss stage (see Fig. 2 for ref-erence). From the XRD pattern of the reduced sample (shown inFig. 3b), it can be seen that the main reduction products at thistime were Cu and FeO, which agreed with the analysis results inFig. 2. In addition, some unreacted Fe3O4 and small amounts ofCu2S were found in Fig. 3b. These results suggested that the mainreductions in the period of 80 s were (R2)–(R5), (R11)–(R12) and(R14)–(R15). As iron sulfides were not detected, the main sulfida-tion reactions may be (R6) and (R8)–(R10). This indicates thatthe reactivity of copper oxides was higher than that of iron oxideswhen reacting with H2S.

Fig. 4 shows the TG curve of the experiment reacting with4000 ppm H2S synthesis gas for about 900 s. From the XRD patternof the reduced sample (shown in Fig. 4b), the main reduction prod-ucts at this time were Cu, Fe, and FeO. As a large amount of CO2

was present in synthesis gas, (R13) could be restrained [32]. Thus,the dominant reduction of FeO was (R16). In addition, smallamounts of Cu2S and FeS were also found in Fig. 4b, which mightbe caused by (R10), (R20) and (R22). The XRD intensity of Cu2Swas larger than FeS, showing that the reactivity of copper oxideswas higher than that of iron oxides when reacting with H2S onceagain.

3.2.2. Effect of temperatureThe effect of temperature on the sulfidation of copper ore was

also investigated. Four TGA experiments were carried out with4000 ppm H2S synthesis gas at 900–975 �C. Fig. 5 shows the TGcurves and DTG (differential thermogravimetric, formulated bydXt/dt) as a function of time. Obviously, the oxygen release rateincreased with temperature and a higher temperature contributedto a larger weight loss due to oxygen release. When synthesis gaswas introduced into the reactor, the weight loss for all the fourtests increased rapidly untill 300 s, after which a nealy identicalDTG was attained, showing that the effect of temperature on thisstage was small. The reason might be that the copper ore has extre-mely high reactivity at all these temperatures that there was noenough reducing gas around the particle in the TGA reactor. How-ever, the degree of weight loss decreased with temperature, as wellas the DTG between 300 and 900 s. This might be because that theincrease in the sulfidation rate of Cu was larger than the increase inthe reduction rate of FeO.

3.2.3. Cyclic behaviorIn order to investigate the effect of H2S on the stability of copper

ore, 8 redox cycles were conducted for synthesis gas with4000 ppm H2S at 900 �C and 10 redox cycles for synthesis gas with-out H2S were performed. It can be seen from Fig. 6a that the reduc-tion procsess between 300 s and 900 s in each cycle differed, i.e.,the extent of weight loss decreased with the cycle numbers. Thereason for this phenomenon might be ascribed to the structuralchange of the particles, which led to a faster sulfidation rate ofCu and a slower reduction rate of FeO. With regard to the weightof oxidation state, the change in weight loss was very small, show-ing that the chemical composition was stable after multiple redoxtests with 4000 ppm H2S synthesis gas. Furthermore, the oxygentransport capacity during the first 300 s of the reduction stageremained about 13.5% (the weight change from the beginning tothe end of fast decreasing stage), as shown in Fig. 7. However,when using synthesis gas without H2S (Fig. 6b), it can be seen thatthe mass of both reduced and oxidized OC decreased with cyclenumbers until the 8th cycle. The reason might be: (1) the reductiondegree increased with the cycle numbers, leading to a decreasingmass of reduced OC. Then, the deep reduction of iron oxides cannotbe regenerated to CuFe2O4 completely, leading to a decreasing

(b) 4000ppm H2S(a) 0ppm H2S

Fig. 14. ESEM images and EDX analyses of copper ore after experiments with (a) 0 ppm H2S synthesis gas and (b) 4000 ppm H2S synthesis gas.

92 K. Wang et al. / Applied Energy 166 (2016) 84–95

mass of oxidized OC. (2) the CaSO4 content in copper ore is reducedto CaO and it cannot be regenerated back to CaSO4. The CaSO4

decomposition was restrained by the presence of H2S in test of4000 ppm H2S synthesis gas (see thermodynamic analysis in Sec-tion 3.3). In Fig. 7, the weight loss decreased from about 14% incycle 1 to 11.5% in cycle 8, after which it remained relatively stable.It was noted that the weight change during the reduction period(almost no sulfidation reaction) for 0 ppm H2S synthesis gas waslarger than that for 4000 ppm H2S synthesis gas after the secondcycle. Therefore, it can be concluded that the sulfidation of copperore may be helpful for the oxidation of the OC to its initial state.

Fig. 8 illustrates the gas products from the reduction of copperore by 4000 ppm H2S synthesis gas. Time zero represents the pointwhen synthesis gas was introduced to the reactor. As shown inFig. 8a–e, the gaseous sulfur species mainly consisted of COS, CS2and SO2. The concentrations of CO2 and CO are also presented here.For all the four cycles, the CO2 concentration peak was found atabout 40 s, which was attributed to the oxidation of CO by copperore in the fast decreasing stage. The CO2 concentration thendecreased gradually and reached a constant at 200 s. This result

also indicated that the reduction of FeO was mainly caused byH2. In Fig. 8e, a peak of SO2 at 40 s was also observed, which wasbecause of the oxidation of H2S by gaseous oxygen released fromcopper ore. Then, the SO2 concentration immediately decreased.The SO2 concentration did not decrease to 0 after 300 s in the firstcycle, while it reached 0 in the remaining cycles. The reason maybe that oxidation of H2S by (R19) was significant in cycle 1 whilenegligible in the other cycles. This might also correspond to the dif-ference in the reduction procsess of between 300 s and 900 s ineach cycle (as shown in Fig. 6a). The peak value of cycle 1 waslower than that of other cycles, indicating that the oxidation ofH2S for cycle 1 was weaker than that for the following cycles.

The concentration of CO increased sharply after synthesis gaswas introduced and then increased relatively slow at 40–200 s,which was because of the slower consumption rate by reactingwith copper ore. Finally, it remained almost unchanged after200 s. For COS concentration, a trough was observed between40 s and 300 s, which was due to the reaction of COS with copperore. The formation of COS was from the reactions of SO2 and H2Swith CO. In addition, a higher concentration of COS was found in

K. Wang et al. / Applied Energy 166 (2016) 84–95 93

cycle 1, which might due to the higher concentration of SO2 after200 s. The CS2 concentration also increased sharply at first andthen increased slowly with time. The measured concentration ofCS2 decreased with cycle numbers, and was much less than COSand SO2, with orders of magnitude lower than CO or CO2.

In the oxidation process, the only sulfur species was SO2, whichwas much larger than that in the reduction process. Once air wasintroduced, a large amount of SO2 was observed, as shown inFig. 8f. Then, SO2 concentration decreased sharply to 0 after200 s. In addition, the peak value of SO2 concentration decreasedslightly with the cycle numbers, which was also observed in thefollowing fluidized bed experiments.

3.3. Thermodynamic investigations of the reaction of copper ore withsynthesis gas

To better understand the reactions in TGA experiments in Sec-tion 3.2, the reduction reactions of copper ore with synthesis gaswere investigated using HSC Chemistry software 5.0, which wasbased on the principle of Gibbs free energy minimization. The pre-dominant equilibrium compositions in the reactor were simulatedat a temperature of 900 �C, with focus on the sulfur species andreduction products of copper ore at different fuel to OC ratios.Here, a value of 1 of the fuel to OC ratio was defined as the amountof synthesis gas needed to reduce the CuO, CuFe2O4 and CaSO4 toCu, Fe and CaS, respectively. All the chemical species of copperore considered in this calculation system are shown in Table 3,mainly including the metal oxides, gas species and sulfur species.

In each simulation, 1 mol of copper ore was used, i.e., 0.39 molCuO, 0.433 mol CuFe2O4 and 0.025 mol CaSO4 were regarded as theactive substance. Fig. 9 shows the mole amounts of the mainreduction products of copper ore with different synthesis gasesas a function of fuel to OC ratio at 900 �C. For both simulationresults, it can be seen that CuFeO2 and Cu2O were generated bythe decomposition of CuFe2O4 and CuO when the fuel to OC ratiowas very small. Then, the contents of CuFeO2, CuO, Cu2O andFe2O3 decreased sharply to 0 when the fuel to OC ratio was about0.6, as seen in Fig. 9c and d. This is because of the reduction reac-tions with CO and H2 in the synthesis gas. Simultaneously, thereduction products of Cu, Fe, FeO and Fe3O4 increased quickly withthe increase of the fuel to OC ratio. The mole amounts of FeO andFe3O4 reached a peak value and then decreased gradually with theincrease of fuel to OC ratio. In Fig. 9b, the mole amounts of Cu andFe increased gradually, in contrast to a decreasing trend in Fig. 9a,especially for Cu, which was because of the reactions of (R10) and(R22), i.e. the sulfidation of Cu and Fe. In addition, it can be seenfrom Fig. 9a that the mole amount of Cu2S was much higher thanthose of FeS and Fe0.877S when the fuel to OC ratio was smaller than10, indicating that the copper sulfides is much easier to be gener-ated than iron sulfides under equilibrium conditions. With a higherfuel to OC ratio, more metal sulfides were produced. It is noted thatthe main species of Ca is CaS in Fig. 9a, while it is CaO in Fig. 9b.This indicates that the presence of H2S in synthesis gas can restrainthe formation of CaO from the reduction of CaSO4. As CaO cannotregenerate back to CaSO4, a weight decrease of copper ore in theoxidation state was found, as shown in Fig. 6b.

3.4. Experiments in fluidized bed reactor

To further evaluate the effect of sulfidation on the performanceof copper ore OC, two redox experiments of 5 cycles were con-ducted in a fluidized bed at 900 �C. In the experiment, 15 g copperore and about 240 s synthesis gas with a flow rate of 450 mL/minwere used, which corresponded to a fuel to oxygen carrier rationof about 1.2 (a value of 1 represents the copper ore to be fully

reduced to Cu2O and CuFeO2). The reduced samples at the 5th cyclewere retained for XRD and ESEM analyses.

Fig. 10 shows the outlet gas composition of the two fluidizedbed experiments as a function of time. The dash lines representthe dividing point of reduction and oxidation processes. For bothexperiments, when the O2 concentration decreased to about 2.4%,the fluidization agent was switched from N2 to synthesis. Then,the CO2 concentration increased sharply to above 92% and O2

concentration increased to about 4.8% (which is because of theincrease of the local temperature (about 40 �C) in the reactor).When the synthesis gas input was stopped, the CO2 concentrationdropped drastically to about 5% and then decreased slowly to 0.As seen in Fig. 10a, nearly no H2 or CO (<0.4%) was found. How-ever, in Fig. 10b, a peak value of 1.6% for CO concentration wasdetected at the same time. This result indicates that the presenceof H2S in the syntheisis gas degraded the reactivity of copper oreto some extent. The reason may be that the active component ofcopper ore (CuO and CuFe2O4) reacted with H2S to form metalsulfides, leading to a degradation of the oxygen transport capacityof OC.

Fig. 11 shows the SO2 concentration (corresponding to Fig. 10b)as a function of time. It can be seen that a concentration of above100 ppm SO2 was found in the reduction periods, which is mainlycaused by the reactions of (R6), (R7) and (R17). Then, a narrow SO2

peakwas found in theoxidationperiods, showing thatmetal sulfidesindeed formed in the reduction stages. This eventually contributedto a degradation of the oxygen transport capacity of copper ore,which agreed with the result from Fig. 10. In addition, the SO2 con-centration in the oxidation periods showed a decreasing trend withthe cycle numbers, indicating that the amount ofmetal sulfides pro-duced in the reduction periods decreased as well. This may bebecause of the structural change caused by sintering on the surfaceof copper ore.

Fig. 12 shows the gas yield of CO2, cCO2, for the two fluidizedbed experiments as a function of the mass-based OC conversion.The yield of CO2 for mass-based OC conversion of 1 to 0.998 isnot shown here, which is caused by the oxygen release of OCin the initial stage. In both experiments the OC showed high reac-tivity, in which cCO2 of higher than 0.996 between the mass-based OC conversion of 0.995 and 0.97 was attained. When with-iout the presence of H2S (Fig. 12a), the cCO2 was maintainedhigher than 0.996 before the mass-based OC conversion of0.956, while it was only 0.97 when 4000 ppm H2S was present(Fig. 12b). This result indicates that the presence of H2S in thefuel degraded the reactivity of the copper ore, due to the sulfida-tion of copper ore. Therefore, it can be concluded that the pres-ence of H2S in fuel gas degrades the oxygen transport capacityand reactivity of copper ore in a CLC process. Nevertheless, it alsocan be found that if the OC is excess, the reactivity of copper orewith H2S-containing synthesis gas could maintain well, whichagrees well with the results obtained by Forero et al. [29]. Noobvious variation of the OC reactivity was observed within the5 redox cycles. However, much more redox experiments are stillneeded for further investigation.

Fig. 13 shows the XRD analysis results of the reduced samples atthe 5th cycle for the two fluidized bed experiments. As the fuel wasexcess for copper ore to be reduced to Cu2O and CuFeO2, the mainreduction products for the two experiments were CuFeO2, Cu2O, Cuand Fe3O4. Note that, for the OC sample of the experiment withH2S-containing sythesis gas (Fig. 13a), small amounts of Cu2S andFeS were found in the XRD pattern, showing that the sulfidationof copper ore indeed happened.

Fig. 14 shows ESEM images and EDX analyses of the OC parti-cles. With respect to the particle surface, the OC particles afterthe experiment without H2S displayed a porous structure. Whilefor particles after experiment with 4000 ppm H2S, the porous

94 K. Wang et al. / Applied Energy 166 (2016) 84–95

structure on the surface became denser. This may be caused bythe sulfidation and oxidation of the metal oxides on the surface.As the melting points of the metal sulfides were generally lowerthan those of metal oxides, the sulfidation of the particles mightresult in sintering and agglomeration at high temperatures.Therefore, the formation of metal sulfides should be avoided dur-ing the CLC processes when sulfur-containing fuels are used. Add-ing CaO into the OC particles could alleviate the sulfidation of OC[32]. EDX analysis of the particles is shown in Fig. 14, in whichCu, Fe, Ca, Si, Al, O and S were detected in both of the partices.It can be seen that more S content was found in the sample afterreacting with 4000 ppm H2S. This is because of the presence ofthe metal sulfides and CaS on the surface of the particles. It isnoteworthy that the difference of calcium and silicon contentsfor the two particles may be because of the uneven distributionof CaSO4 and SiO2 on the surface of particles.

4. Conclusions

In this work, the effect of H2S on the reactivity of copper ore OCwas first investigated in thermogravimetric analyzer and Fouriertransform infrared spectroscopy (TGA–FTIR). Synthesis gas of25 vol.% H2, 35 vol.% CO, 40 vol.% CO2 and with or without4000 ppm H2S was used as fuel. The effects of reducing atmo-sphere, temperature and cycle numbers were first studied. Then,based on the principle of Gibbs free energy minimization, thereduction reaction of copper ore with synthesis gas was simulatedusing HSC Chemistry software 5.0. Finally, sulfur evolution behav-ior of copper ore with H2S-containing synthesis gas was furtherinvestigated in a laboratory-scale fluidized bed reactor.

A weight gain due to the sulfidation of the copper ore OC wasobserved at all TGA tests with 4000 ppm H2S synthesis gas. Themain sulfide products were Cu2S and FeS, and H2S was easier toreact with copper oxides than iron oxides. The presence of H2S,which constrained the decomposition of CaSO4 and deep reductionof FeO to a certain extent, was helpful for re-oxidation of thereduced sample to its initial weight. A higher temperature led toan increase of sulfidation rate of copper ore. The cycle numbersseemed to have certain effect on the reduction and sulfidationrates of copper ore with H2S-containing synthesis gas. SO2, COS,and CS2 were the main gaseous sulfur species observed duringthe reduction of copper ore with 4000 ppm H2S synthesis gas. Notethat, the concentrations of gaseous sulfur species kept relativelystable value after the first cycle.

The fluidized bed reactor experiment results showed that thepresence of H2S in fuel gas degraded both the oxygen transportcapacity and reactivity of the oxygen carrier, which was due tothe sulfidation of copper ore. XRD patterns showed that the mainsulfide products were Cu2S and FeS. ESEM images showed thatthe sulfidation of copper ore indeed affected the surface morphol-ogy of the particles. From this perspective, efforts on the elimina-tion of sulfur species within the CLC or CLOU process aredeserved when using copper ore as OC.

Acknowledgements

This work was presented and benefited from discussions at‘‘CLC2014” conference (3rd International Conference on ChemicalLooping in Chalmers, Sweden) and received the best paper award.These authors were supported by ‘‘National Natural Science ofChina (51522603 and 51561125001)”. The staff of the Analyticaland Testing Center, Huazhong University of Science and Technol-ogy, were also appreciated for the related experimental analyses.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.apenergy.2016.01.011.

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