Journal of Environmental Sciences 26 (2014) 1062–1070

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Journal of Environmental Sciences

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Sulfur evolution in chemical looping combustion of coal with MnFe2O4 oxygencarrier

Baowen Wang1,2,∗, Chuchang Gao1, Weishu Wang1, Haibo Zhao2, Chuguang Zheng2

1. College of Electric Power, North China University of Water Conservancy and Hydroelectric Power, Zhengzhou 450045, China.E-mail: david-wn@163.com2. State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China

a r t i c l e i n f o

Article history:

Received 28 July 2013

revised 06 January 2014

accepted 10 January 2014

Keywords:CO2 capture

chemical looping combustion

sulfur evolution

MnFe2O4

DOI: 10.1016/S1001-0742(13)60546-X

a b s t r a c t

Chemical looping combustion (CLC) of coal has gained increasing attention as a novel combustion

technology for its advantages in CO2 capture. Sulfur evolution from coal causes great harm from either

the CLC operational or environmental perspective. In this research, a combined MnFe2O4 oxygen

carrier (OC) was synthesized and its reaction with a typical Chinese high sulfur coal, Liuzhi (LZ)

bituminous coal, was performed in a thermogravimetric analyzer (TGA)-Fourier transform infrared

(FT-IR) spectrometer. Evolution of sulfur species during reaction of LZ coal with MnFe2O4 OC was

systematically investigated through experimental means combined with thermodynamic simulation.

TGA-FTIR analysis of the LZ reaction with MnFe2O4 indicated MnFe2O4 exhibited the desired

superior reactivity compared to the single reference oxides Mn3O4 or Fe2O3, and SO2 produced was

mainly related to oxidization of H2S by MnFe2O4. Experimental analysis of the LZ coal reaction with

MnFe2O4, including X-ray diffraction and X-ray photoelectron spectroscopy analysis, verified that

the main reduced counterparts of MnFe2O4 were Fe3O4 and MnO, in good agreement with the related

thermodynamic simulation. The obtained MnO was beneficial to stabilize the reduced MnFe2O4 and

avoid serious sintering, although the oxygen in MnO was not fully utilized. Meanwhile, most sulfur

present in LZ coal was converted to solid MnS during LZ reaction with MnFe2O4, which was further

oxidized to MnSO4. Finally, the formation of both MnS and such manganese silicates as Mn2SiO4

and MnSiO3 should be addressed to ensure the full regeneration of the reduced MnFe2O4.

Introduction

The major contribution of coal to the national energy

supply in China has resulted in an abrupt increase in CO2

emission, which causes great public concern over global

warming (Wang et al., 2013). Therefore, it is of great

necessity to capture CO2 and mitigate its emission from

coal utilization. As compared to various existing tech-

nologies, such as post-combustion, oxy-fuel combustion

and pre-combustion for CO2 capture, chemical looping

combustion (CLC) is drawing great interest for its estab-

lished advantage in carbon capture (Yang et al., 2008).

In the CLC system, oxygen is transferred from the air

∗ Corresponding author. E-mail: david-wn@163.com

reactor (AR) to the fuel reactor by means of an oxygen

carrier (OC) to realize the full conversion of coal into

CO2 and steam. After condensation, the pure CO2 can be

obtained for subsequent sequestration without extra energy

consumption, while the reduced OC is transported back to

the AR and regenerated therein for initiation of the next

cycle of coal combustion.

Currently, combined OC have received great attention

from researchers for their better performance in reactivity

improvement, mechanical integrity, cost abatement and

benign environmental nature compared to single metal

oxide-based OC, whose disadvantages include the poten-

tial carcinogenic tendency of NiO, the low melting point

and poor resistance to serious sintering of CuO, and the

Journal of Environmental Sciences 26 (2014) 1062–1070 1063

low reactivity but high cost of CoO (Adanez et al., 2012).

Among all the potential combined OCs, Mn-Fe combined

OCs have found wide application in the CLC system,

but most of its reduction reactions have been confined to

gaseous fuels, such as CH4 (Lambert et al., 2009; Shulman

et al., 2009), natural gas (Ryden et al., 2011) and syngas

(Ksepko et al., 2012; Azimi et al., 2013). Considering that

coal is more abundant than other gaseous fuels and more

CO2 is generated from coal utilization, it would be more

meaningful to investigate the reaction characteristics of

Mn-Fe combined OC with coal, especially in China.

Sulfur evolution from coal is of great concern from both

the CLC operational and environmental perspective and

therefore more attention should be given in this direction

(Shen et al., 2010a; Adanez et al., 2012). On the one

hand, in the FR, different solid sulfur species could form

through interaction between the evolved sulfur from coal

and OC, which not only deactivate OC but also potentially

cause defluidization of the CLC system (Garcıa-Labiano

et al., 2009). Meanwhile, the gaseous sulfur species emit-

ted in the FR flue gas will negatively influence CO2

compression, transport and storage (Pipitone and Bolland,

2009). On the other hand, in the AR, the entrained solid

sulfur compounds will be oxidized and released as SO2 to

the atmosphere without desulfurization measures (Garcıa-

Labiano et al., 2009). Overall, the presence of sulfur

jeopardizes the whole process of CLC with coal as fuel,

and it is necessary to deeply explore sulfur evolution in

coal for CLC application.

In this research, MnFe2O4 was prepared as a Mn-Fe

combined OC using a novel sol-gel combustion synthesis

(SGCS) method, and its reaction with a Chinese high

sulfur coal, Liuzhi (LZ) coal, was investigated through

TGA analysis. Furthermore, sulfur evolution during re-

action of LZ coal with MnFe2O4 OC was the focus

of this research. The evolved gases were in situ detect-

ed by Fourier transform infrared (FT-IR) spectroscopy

coupled with TGA. Solid products from the reaction

of LZ coal with MnFe2O4 OC were systematically in-

vestigated through such experimental means as field

scanning microscopy/energy-dispersive X-ray spectrom-

etry (FSEM-EDX), X-ray diffraction (XRD) and X-ray

photoelectron spectroscopy (XPS). Finally, a thermody-

namic simulation of the reduction of MnFe2O4 with this

selected coal was conducted to gain comprehensive infor-

mation on the sulfur evolution from coal during its reaction

with MnFe2O4.

1 Experimental section

1.1 Materials and characterization

The OC used in this work, including MnFe2O4 and the

reference oxides Fe2O3 and Mn3O4, were synthesized by

the SGCS method with hydrated metal nitrates and urea as

the precursors. The detailed preparation procedure can be

found in our previous research (Wang et al., 2011a). After

grinding and sieving, the as-prepared MnFe2O4 with 63–

106 μm particle size was collected for use.

Apart from OC, a typical high sulfur coal was col-

lected from Liuzhi, Guizhou Province to investigate the

sulfur evolution and designated as LZ below. After drying,

grinding and sieving, the LZ sample with 63–106 μm

particle size was collected and used. The sieved coal

sample was further characterized using proximate and ulti-

mate analyses according to the Chinese National Standard

GB/T215-2003 as well as ash composition analysis with

X-ray fluorescence (PW 2400, Philips, the Netherlands).

The analysis results are provided in Table 1, where sulfur

content present in LZ is as high as 5.23%. Contents

of pyrite, sulfate and organic sulfur were determined as

3.44%, 0.307%, and 1.48%, respectively and pyrite was

dominant.

Finally, both the synthesized MnFe2O4 and the as-

prepared coal sample were evenly mixed in a lab mortar at

the designated mass ratio with the developed mass balance

method, as described in our previous research (Wang et al.,

2011b, 2012). Based on the properties of LZ coal shown

in Table 1, the relative chemical formula of 1 kg LZ coal

can be represented as C19.9H9.9N0.35S0.95O2.97. When the

MnFe2O4 oxygen excess number Φ was equal to 1, i.e.,

the MnFe2O4 theoretically just satisfied full conversion of

LZ coal, the mass ratio of MnFe2O4 to LZ was determined

to be 7.54. In addition, mass ratios of the reference oxides

Mn3O4 and Fe2O3 to LZ were equal to 9.98 and 20.89,

Table 1 Properties of Liuzhi (LZ) bituminous coal

Proximate analysisa (%) Ultimate analysis (%, dry basis) Lower heating value (MJ/kg)

Moisture content Volatile matters Ash content Fixed carbon C H N S Oa

0.54 16.82 41.88 8.67 41.07 1.78 0.85 5.23 8.67 23.354

Ash analysis of LZ (%)

SiO2 Al2O3 Fe2O3 SO3 CaO TiO2 Co3O4 K2O MgO Na2O

41.30 24.07 17.00 4.60 3.12 2.80 2.02 1.41 1.58 0.63

a: the O content was determined by difference.

1064 Journal of Environmental Sciences 26 (2014) 1062–1070

respectively.

1.2 Experimental methods

The reaction characteristics of the synthesized MnFe2O4

with LZ at the oxygen excess number Φ = 1 were in-

vestigated using the simultaneous thermal analyzer (STA

409C, Netzsch Corp., Germany). The mixture of LZ with

MnFe2O4 was heated from ambient to 900◦C at 25◦C/min,

remaining at this temperature for 20 min to ensure the

sufficient conversion of coal. N2 atmosphere was used. The

flow rate of N2 and the total mass for the mixture of coal

and OC were set at 80 mL/min and 15 mg, respectively, to

ensure the reproducibility of experimental results.

The evolved gases from the reaction of LZ with

MnFe2O4 in the TGA were detected in situ by an FT-

IR spectrometer (EQUINOX 55, Bruker Corp., Germany).

The IR scanning range was 4000–500 cm−1, and the

resolution and sensitivity were pre-set at 4 cm−1 and 1,

respectively.

The morphology and elemental composition of the solid

products from the reaction of LZ with MnFe2O4 were stud-

ied using FSEM (Siron 200, FEI Company, Netherlands)

coupled with an EDX (GENESIS, EDAX Corp., USA) at

a magnification 5000 and an accelerating voltage of 20 kV.

Furthermore, the formed phases were identified by XRD

(X’Pert PRO, PANalytical Corp., Netherlands) with 40 kV

40 mA CuKα (λ = 0.154 Å) radiation and the step-scanned

range of 10–90◦.Finally, the chemical states and surface compositions

of the solid products were analyzed by an XPS spectra

(VG MultiLab 2000, Thermo Electron Corp., USA) with

a monochromatic Mg Ka source and a charge neutralizer.

All of the binding energies were referenced to the C 1s

peak at 284.6 eV of the surface adventitious carbon.

1.3 Thermodynamic simulation of the reaction ofMnFe2O4 OC with LZ

Although thermodynamic analysis has some limitations

without consideration of kinetic constraints (Adanez et

al., 2012), including turbulent mixing and temperature

gradients, the thermodynamic simulation of LZ reaction

with MnFe2O4 would enable a better understanding of the

transfer of oxygen involved in MnFe2O4 and transforma-

tion of LZ coal. Based on minimization of the total Gibbs

free energy, reaction of MnFe2O4 with LZ was further

investigated using the HSC-Chemistry software 4.1.

According to the properties of LZ coal in Table 1,

including its proximate and ultimate analysis as well as

ash analysis, a complex reaction system was established

for LZ coal, which was considered to consist of the main

matrix elements (such as C, H, N, S and O) as well as

seven categories of potential minerals and their possible

intermediates, similar the representation of coal in our

previous study (Wang et al., 2011b).

2 Results and discussion

2.1 TGA investigation of the reaction of MnFe2O4 withLZ coal

Reaction of LZ coal with the synthesized MnFe2O4 at Φ

= 1 under N2 atmosphere was performed in the TGA at

25◦C/min. The results of TG and DTG analyses are shown

in Fig. 1 for the reaction of LZ with MnFe2O4. Meanwhile,

LZ pyrolysis and its reaction with the single oxides Mn3O4

and Fe2O3 are included as well for reference.

0 10 20 30 40 50 6085

90

95

100

200

400

600

800

20 30 40 50

0.10

0.20

20 30 40 50

0.12

0.24

10 2010 30 40 50 600.00

0.06

0.12

0.1889.04%

86.35%

85.52%

93.97%

DTGTG

Time (min) Time (min)

Weig

ht

loss

rate

(w

t%/m

in)

Weig

ht

loss

(w

t%)

538.5oC

341.1

750.2

296.8505.3

583.5

597.2

846.1

252.8

889.8

573.6

oC

oC

oC

oC

oC

oC

oC

oC

oC

oC

LZ+MnFe2O4

LZ+MnFe2O4

LZ+Fe2O3

LZ+Fe2O3

LZ-N2

LZ-N2

LZ+Mn3O

LZ-Mn3O4

LZ-Mn3O4 (Fe2O3)

Tem

pera

ture

( oC

)

Fig. 1 TG and DTG curves of the reaction of LZ coal with MnFe2O4.

Journal of Environmental Sciences 26 (2014) 1062–1070 1065

First, as the baseline, LZ pyrolysis under N2 atmosphere

was conducted as shown in Fig. 1. According to these

two figures, after dehydration below 200◦C, LZ pyrolysis

experienced two distinct stages with the characteristic

temperatures (i.e. the peak temperature relative to the DTG

peak) occurring around 341.1 and 538.5◦C, mainly due to

the breaking of weak bonds in LZ (Yang et al., 2007).

Being different from LZ pyrolysis, as shown in Fig. 1,

the final weight losses from the LZ reaction with Fe2O3

and Mn3O4 reached 6.03% and 10.96%, much lower than

that of LZ reaction with MnFe2O4 of 13.65%, which meant

that more weight loss was obtained for LZ with MnFe2O4,

and implied that MnFe2O4 exhibited better reactivity in

the reaction with LZ coal. Therefore, MnFe2O4 held great

promise for realistic CLC application due to its high

reactivity, which is consistent with previous experimental

observations (Shulman et al., 2009; Ksepko et al., 2012),

although different fuels were used.

Furthermore, three reaction stages were observed for LZ

with the reference oxides Mn3O4, Fe2O3 and MnFe2O4

in Fig. 1. The first reaction stages for LZ with Mn3O4 or

Fe2O3 at the characteristic temperatures 296.8 and 505.3◦Cshould be mainly attributed to pyrolysis of LZ in its mix-

tures with different OCs, similar to the first reaction stage

of LZ with MnFe2O4 at the characteristic temperature

252.8◦C, as evidenced by FT-IR analysis of the gaseous

distributions (Fig. 2). But with reaction temperature in-

creased, the following two reaction stages of LZ with

Mn3O4, Fe2O3 and MnFe2O4 occurred, with characteristic

temperatures at around 570–600◦C and 750–900◦C, re-

spectively. FT-IR analysis of the gaseous products (mainly

CO2 and H2O, as referenced in Fig. 2) confirmed that

reaction of LZ with MnFe2O4, Mn3O4 and Fe2O3 initiated

at 573.6, 583.5 and 597.2◦C, respectively, which was much

lower than the temperatures reported by Siriwardane et

al. (2009), mainly due to the higher reactivity of the

SGCS-synthesized OC in our research than the materials

Siriwardane ordered from commercial sources.

1500 2000 2500 3000 3500

0.075

0.150

0.225

0.300

1500 2000 2500 3000 3500

0.02

0.04

0.06

1500 2000 2500 3000 3500

0.08

0.16

0.24

1000 1500 2000 2500 3000 3500

0.00

0.08

0.16

CO

CH4

Abso

rbance (

cps)

LZ pyro-341°C

CH4

CH4

CO

SO2

SO2

SO2

CO2

CO2

CO2

CO2

LZ+MnFe2O4 573oC

LZ+MnFe2O4 252oC

LZ pyro-538°C

Wavenumber (cm-1

)

Fig. 2 FT-IR spectra of gas products from the reaction of LZ coal with MnFe2O4.

1066 Journal of Environmental Sciences 26 (2014) 1062–1070

2.2 FT-IR analysis of gaseous sulfur evolution duringLZ reaction with MnFe2O4 OC

After the TG investigation on the characteristics of the

reaction of LZ with MnFe2O4, the fate of gaseous sul-

fur evolution was studied through FT-IR analysis. FT-IR

spectra of gaseous distributions from LZ pyrolysis and its

reaction with MnFe2O4 OC are presented in Fig. 2.

First, from Fig. 2a–b, according to the FT-IR spectra of

various gaseous distributions for LZ pyrolysis under N2,

except for the main gaseous species (CO, CO2 and CH4),

only the gaseous sulfur species SO2 was detected for LZ

pyrolysis at the characteristic temperatures of two different

pyrolysis stages, even though such gaseous sulfur species

as H2S, COS, SO2 and CS2 have been reported to evolve

from coal pyrolysis (Bassilakis et al., 1993). The far higher

of IR absorbance of SO2 at the characteristic temperature

341◦C indicated that SO2 mainly resulted from LZ pyroly-

sis at this low temperature pyrolysis stage and was closely

related to the decomposition of iron sulfate (Ibarra et

al.,1994), oxidization of sulfide with chemisorbed oxygen

in the porous coal, or pyrolysis of organic sulfones present

in coal (Calkins et al.,1987). In contrast to SO2, H2S

was not identified throughout LZ pyrolysis, even though it

should be dominant, mainly due to its weak IR absorbance

(Solomon et al., 1990). In addition, the fairly weak and

noisy COS and CS2 spectra indicated their concentrations

formed were quite low (Zhang et al., 2011).

Furthermore, the fate of sulfur in LZ during its reaction

with MnFe2O4 was studied (Fig. 2c–d). SO2 was the only

dominant gaseous sulfur species detectable through FT-IR

analysis. The first SO2 yielded at the low characteristic

temperature 252◦C should arise from LZ pyrolysis, but

the ensuing SO2 formed at the characteristic temperature

573◦C should be ascribed to oxidization of H2S evolved

from LZ coal by MnFe2O4 (Ibarra et al.,1994).

2.3 FSEM-EDX and XRD analysis of chemistry andmicrostructure of solid products

To clarify the reaction mechanisms and better understand

the evolution of sulfur distribution during LZ reaction with

MnFe2O4, the morphologies and elemental compositions

of the solid residues from pyrolysis of LZ and its reaction

with MnFe2O4 were characterized using FSEM-EDX,

as shown in Fig. 3. The elemental compositions of LZ

pyrolysis and its reaction with MnFe2O4 at the optionally

selected spots on the FSEM figure were calculated using

the ZAF correction method and listed in Table 2. The

detailed phases involved were further identified through

XRD analysis, as presented in Fig. 4.

Figure 3 presents SEM images of the products of LZ

pyrolysis and its reaction with MnFe2O4. From Fig. 3a,

the solid pyrolysis residues of LZ coal were observed to

mainly consist of relatively dark bulk in spot 1 and the

fragmentary discrete particles in spot 2 and spot 3, respec-

1

2

3

1

2

a LZ pyrolysis b LZ–MnFe2O4 (Red)

Fig. 3 FSEM images of LZ coal pyrolysis (a), LZ coal reaction with MnFe2O4 (b).

Table 2 Elemental analysis (%) of Liuzhi reaction with MnFe2O4 OC by FSEM-EDX, applying ZAF correction method

Sample C O Si Fe Al S Ca Mn

LZ Spot 1 92.14 2.20 1.63 0.44 1.25 1.63 0.07 0.00

pyrolysis Spot 2 71.07 11.93 3.61 7.28 2.86 1.46 0.44 0.00

Spot 3 68.28 14.83 8.14 0.70 5.85 0.95 0.03 0.00

LZ-MnFe2O4 Spot 1 5.89 49.16 0.16 26.91 0.21 0.44 0.51 16.26

(Red) Spot 2 4.16 48.12 0.81 27.10 0.63 0.49 0.52 15.53

Journal of Environmental Sciences 26 (2014) 1062–1070 1067

20 30 40 50 60 70 80

900

1200

1500

1800

20 30 40 50 60 70 80

300

450

600

10 20 30 40 50 60 70 80 90

500

750

1000

1250

1 1

1111

1

1

1

34343

6

33

53

647

2

3

65

3

745

Inte

nsi

tyθ(

cps)

Inte

nsi

tyθ(

cps)

Inte

nsi

tyθ(

cps)

988 8110

88

198

8

8

8

8

51

8

1

1

8

51

8

51

MnFe2O4θ(SGCS)

LZ+MnFe2O4θ(Red)

LZ+MnFe2O4θ(Ox)

2θθ(degree)

a

b

c

Fig. 4 XRD patterns of SGCS-made MnFe2O4 (a), reduction of MnFe2O4 with LZ coal (b), oxidization of the reduced MnFe2O4 with LZ coal (c).

Peaks 1: MnFe2O4; 2: MnO; 3: Fe3O4; 4: MnS; 5: Mn2SiO4; 6: MnSiO3; 7: SiO2; 8: Fe2O3; 9: MnSO4; 10: MnO2.

tively. From the results of EDX analysis shown in Table 2,

the atomic fraction of C in spot 1 was 92.14%, much higher

than in spot 2 and spot 3, possibly resulting from the main

carbon matrix not being completely disintegrated during

LZ pyrolysis, while spot 2 and spot 3 were identified as

minerals dispersed within the carbon matrix, mainly com-

posed of Si, Al and Fe. But for LZ reaction with MnFe2O4,

as shown in Fig. 3b, the residues were evenly distributed

without discernible agglomeration or sintering. The atomic

fractions of Mn, Fe and O in spots 1 or 2 were uniformly

distributed around 17%, 27% and 50%, respectively. The C

atomic fraction was below 10%, far less than the C fraction

after LZ pyrolysis. XRD analysis of the solid residues after

reaction of MnFe2O4 with LZ in Fig. 4b indicated that

MnFe2O4 was mainly reduced to Fe3O4 and MnO, which

agreed with the previous observation on the reduction of

Mn-based oxides in a reducing gas environment (Tabata et

al., 1993; Ben-Slimane et al., 1994). Although MnO could

not be further reduced to Mn and the oxygen in MnO was

not fully utilized for coal conversion, the presence of MnO

offered better stability at high temperatures (Alonso et al.,

2000).

In addition, some side products formed during the LZ

coal reaction with MnFe2O4 should be well noted. As

shown in Fig. 4, some sulfur species evolved from LZ

coal were found to react with the reduced MnO to form

MnS (Tian et al., 2009), which could be easily reoxidized

with air to MnSO4 in the AR (Alonso et al., 2000), as

observed in Fig. 4. Meanwhile, such silicates as Mn2SiO4

and MnSiO3 were produced, as observed in Fig. 4. Fur-

thermore, as shown in Fig. 4 for the reoxidization of the

reduced MnFe2O4, in addition to the desired MnFe2O4,

some solid side products also occurred, such as Mn2SiO4

and MnSO4. Based on the XRD diffraction intensities of

these species, though much lower amounts of the side

products were formed than the main regenerated coun-

terpart, MnFe2O4, the detrimental effect was significant

and complete regeneration of the reduced MnFe2O4 could

not be achieved. Therefore, effective separation of solid

side products from the reduced MnFe2O4 should be fully

addressed in the future to ensure successful operation of

the CLC system.

2.4 XPS identification of surface solid species formedfrom LZ reaction with MnFe2O4

Although EDX and XRD analysis above provided useful

information on the solid products formed from reaction of

LZ coal with MnFe2O4, some solid species could not be

identified due to their low contents or weak crystallinity

1068 Journal of Environmental Sciences 26 (2014) 1062–1070

635 640 645 650 655 660 6650

2000

4000

6000

8000

10000Mn species distribution

Inte

nsi

ty (

cps)

Bindering energy (eV)

705 710 715 720 725 7300

2000

4000

6000

8000

10000

Bindering energy (eV)

Inte

nsi

ty (

cps)

Fe species distribution

284.6

(Mn 2P1/2)

284.6 (Mn 2P1/2)

284.6 (Mn 2P3/2)

284.6

(Mn satellite)

710.7 (Fe 2P3/2):Fe3O4

719.8 (Fe 2P3/2 satellite):Fe3O4

713.6 (Fe 2P1/2):FeS724.5 (Fe 2P1/2):Fe3O4

Fig. 5 XPS patterns of solid sulfur species formed during LZ coal reaction with MnFe2O4.

(Wang et al., 2012). Therefore, in order to obtain a more

accurate insight into the reduced MnFe2O4 and solid sulfur

species associated, Mn 2p and Fe 2p XPS spectra were

analyzed by deconvolution and are presented in Fig. 5.

From Fig. 5, XPS analysis of the Mn 2p region of the

reduced MnFe2O4 revealed that the binding energy (BE)

values for Mn 2p3/2 and Mn 2p1/2 were 640.7 and 652.5 eV

with the splitting BE difference of 11.8 eV, indicating the

presence of Mn in its oxidized states. The special satellite

peak of Mn 2p3/2 observed at 647.1 eV further verified that

the Mn oxide was MnO (Tian et al., 2009), which was

in accordance with the XRD analysis above. Apart from

MnO, MnS was also observed with its Mn 2p1/2 BE value

at around 643.2 eV.

Similar to the XPS analysis of the Mn 2p region, as

shown in Fig. 5 for the Fe 2p region, both the splitting BE

difference between Fe 2p3/2 and Fe 2p1/2 and the special

satellite peak of Fe 2p3/2 around 719 eV indicated that

Fe3O4 was produced after MnFe2O4 reduction with LZ

coal (Daou et al., 2006), which was also in accordance with

the XRD analysis above. In addition, the presence of FeS

with the Fe 2p1/2 BE value of 713.6 eV mainly resulted

from reduction of pyrite (FeS2) with reducing volatiles

evolved from LZ coal (Calkins et al., 1987; Zhang et al.,

2011).

2.5 Thermodynamic investigation of LZ coal reactionwith MnFe2O4

Finally, in order to overcome the limitations of FT-IR

analysis in H2S identification and comprehensively under-

stand LZ coal conversion, evolution of sulfur and minerals

involved in LZ coal better, thermodynamic simulation

of the LZ reaction with MnFe2O4 was conducted and

presented in Fig. 6.

From Fig. 6, it can be observed that C involved in

LZ coal was mainly converted to CO2 and the enhanced

reaction temperature was beneficial to convert more LZ

coal into CO2, which agreed with the findings of other

researchers (Shen et al., 2010b). But some CO was found

to occur over 500◦C, which meant that the conversion of

LZ coal was incomplete, mainly due to the side compounds

formed between the reduced MnFe2O4 with sulfur and

minerals involved in LZ coal, as discussed below.

Furthermore, from Fig. 6b–c, the reduced counterparts

of MnFe2O4 were MnO and Fe3O4, consistent with the

XRD analysis in Fig. 4 and XPS analysis in Fig. 5 above.

While from Fig. 6d, although the sulfur content was high

enough in LZ coal, after reaction with MnFe2O4 most of it

was converted into solid MnS at the temperature of interest

for CLC applications (f.g. 800–1000◦C). Meanwhile, such

manganese silicates as Mn2SiO4 and MnSiO3 were also

verified to exist, as shown in Fig. 5, and all of these

side products should be responsible for the incomplete

conversion of LZ coal, as per our observation above.

3 Conclusions

Reaction of MnFe2O4 with a typical high sulfur coal was

performed in TGA coupled with FT-IR, and then sys-

tematically investigated using both experimental means,

including FSEM-EDX, XRD and XPS, and thermodynam-

ic simulation. The following conclusions were reached.

(1) TGA analysis of the MnFe2O4 reaction with LZ coal

demonstrated the desired superior reactivity of MnFe2O4

compared to the single reference oxides Fe2O3 and Mn3O4.

(2) Experimental investigation by FSEM-EDX, XRD and

XPS analysis indicated that the main reduced counterparts

of MnFe2O4 were Fe3O4 and MnO, which was also veri-

fied by the thermodynamic simulation. (3) FT-IR analysis

of gaseous sulfur species after reaction of MnFe2O4 with

LZ coal indicated that SO2 was dominant, but XRD and

XPS analysis revealed that the main solid sulfur species

was MnS. (4) Finally, the production of side products

should be noted, including MnS and Mn2SiO4, which were

not only detrimental to the reactivity of MnFe2O4 OC,

Journal of Environmental Sciences 26 (2014) 1062–1070 1069

200 400 600 800 1000 1200

0

20

40

60

80

100

CO

C

Temperature (oC)

C-c

onta

inin

g s

pecie

s (%

)

Various C-containing species

300 600 900 1200

0

20

40

60

80

300 600 900 1200

0.0

0.5

1.0

1.5

2.0

2.5

MnO

Vari

ous

Mn-c

onta

inin

g s

pecie

s (%

)

Temperature (oC)

Vari

ous

Fe-c

onta

inin

g s

pecie

s (%

)

Various Mn-containing species

MnS

200 400 600 800 1000 1200

0

10

20

30

40

50

60

70

80

90 Various Fe-containing species

Temperature (oC)

Fe-c

onta

inin

g s

pecie

s (%

)

300 600 900 1200

0

20

40

60

80

100

300 600 900 1200

0

10

20

30

40

FeS

MnS

Temperature (oC)

COS

Soli

d s

ulf

ur

specie

s (%

)

Gase

ous

sulf

ur

specie

s (%

)

Various sulfur species

CO2

SO2

H2S

MnCO3

Mn(OH)2

MnFe2O4

MnAl2O4

FeAl2O4

Mn2SiO4

Mn2SiO4

MnSiO3

MnFe2O4

Fe3O4

Fe2O3

Fig. 6 Equilibrium distribution of various species for LZ coal reaction with MnFe2O4.

but also made the reduced MnFe2O4 incapable of full

regeneration.

Acknowledgments

This work was supported by the National Natural Sci-

ence Foundation of China (No. 51276210, 50906030,

50936001), the financial grant of North China University

of Water Conservancy and Electric Power (No. 201012)

and partial funding from the National Basic Research

Program (973) of China (No. 2011CB707301). Mean-

while, the staffs from the Analytical and Testing Center,

Huazhong University of Science & Technology, were

appreciated for the related experimental analysis.

r e f e r e n c e s

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

Diego, L.F., 2012. Progress in chemical-looping combustion and

reforming technologies. Prog. Energy Combust. Sci. 38(2), 215–

282.

Alonso, L., Palacios, J.M., Garca, E., Moliner, R., 2000. Characterization

of Mn and Cu oxides as regenerable sorbents for hot coal gas

desulfurization. Fuel. Process. Technol. 62(1), 31–44.

Azimi, G., Leion, H., Ryden, M., Mattisson, T., Lyngfelt, A., 2013.

Investigation of different Mn-Fe oxides as oxygen carrier for

chemical-looping with oxygen uncoupling (CLOU). Energy Fuels

27(1), 367–377.

Bassilakis, R., Zhao, Y., Solomon, P. R., Serio, M. A., 1993. Sulfur and

nitrogen evolution in the Argonne coals: Experiment and modeling.

Energy Fuels 7(6): 710–720.

Ben-Slimane, R., Hepworth, M. T., 1994. Desulfurization of hot coal-

derived fuel gases with manganese-baed regenerable sorbents. 1.

Loading (sulfidation) tests. Energy Fuels 8(6): 1175–1183.

Calkins, W. H., 1987. Investigation of organic sulfur-containing structures

in coal by flash pyrolysis experiments. Energy Fuels 1(1): 59–64.

Daou, T. J., Pourrroy, G., Begin-Colin, S., Greneche, J. M., Ulhaq-

Bouille, C., Legare, P. et al., 2006. Hydrothermal synthesis

of monodisperse magnetite nanoparticles. Chem. Mater. 18(18),

4399–4404.

Garcıa-Labiano, F., de Diego, L. F., Gayan, P., Adanez, J., Abad, A.,

Dueso, C., 2009. Effect of fuel gas composition in chemical-

looping combustion with Ni-based oxygen carriers. 1. Fate of

sulfur. Ind. Eng. Chem. Res. 48(5), 2499–2508.

Ibarra, J. V., Bonet, A. J., Moliner, R., 1994. Release of volatile sulfur

compounds during low temperature pyrolysis of coal. Fuel 73(6),

933–939.

Ksepko, E., Siriwardane, R. V., Tian, H. J., Simonyi, T., Sciazko, M.,

2012. Effect of H2S on chemical looping combustion of coal-

derived synthesis gas over Fe-Mn oxides supported sepiolite, ZrO2,

and Al2O3. Energy Fuels 26(4), 2461–2472.

1070 Journal of Environmental Sciences 26 (2014) 1062–1070

Lambert, A., Delquieb C., Clemencon, I., Comte, E., Lefebvre, V.,

Rousseau, J. et al., 2009. Synthesis and characterization of

bimetallic Fe/Mn oxides for chemical looping combustion. Energy

Procedia 1(1), 375–381.

Pipitone, G., Bolland, O., 2009. Power generation with CO2 capture:

Technology for CO2 purification. Int. J. Greenhouse. Gas. Control.

3(5), 528–534.

Ryden, M., Lyngfelt, A., Mattisson, T., 2011. Combined manganese/iron

oxides as oxygen carrier for chemical looping combustion with

oxygen uncoupling (CLOU) in a circulating fluidized bed reactor

system. Energy Procedia 4, 341–348.

Shen, L.H., Gao, Z.P., Wu, J.H., Xiao, J., 2010a. Sulfur behavior in

chemical looping combustion with NiO/Al2O3 oxygen carrier.

Combust. Flame 157(5), 853–863.

Shen, L.H., Wu, J.H., Gao, Z.P., Xiao, J., 2010b. Characterization

of chemical looping combustion of coal in a 1 kWth reactor

with a nickel-based oxygen carrier. Combust. Flame. 157(5),

934–942.

Shulman, A., Cleverstam, E., Mattisson, T., Lyngfelt, A., 2009. Man-

ganese/iron, manganese/nickel, and manganese/silicon oxide used

in chemical looping with oxygen uncoupling (CLOU) for combus-

tion of methane. Energy Fuels 23(10), 5269–5275.

Siriwardane, R., Tian, H.J., Richards, G., Simonyi, T., Poston, J., 2009.

Chemical looping combustion of coal with metal oxide oxygen

carrier and coal as fuel. Energy Fuels 23(8), 3885–3892.

Solomon, P. R., Serio, M. A., Carangelo, R. M., Bassilakis, R., Gravel,

D., Baillargeon, M. et al., 1990. Analysis of the Argonne premium

coal samples by thermogravimetric Fourier transform infrared

spectroscopy. Energy Fuels 4(3), 319–333.

Tabata, M., Nishida, Y., Kodama, T., Mimori, K., Yoshida, T., Tamaura,

Y., 1993. CO2 decomposition with oxygen-deficient Mn(II) ferrite.

J. Mater. Sci. 28(4), 971–974.

Tian, H. J., Simonyi, T., Poston, J., Siriwardane, R., 2009. Effect of

hydrogen sulfide on chemical looping combustion of coal-derived

synthesis gas over bentonite-supported metal-oxide oxygen carri-

ers. Ind. Eng. Chem. Res. 48(18), 8418–8430.

Wang, B.W., Zhao, H.B., Zheng, Y., Liu, Z.H., Yan, R., Zheng, C.G.,

2012. Chemical looping combustion of a Chinese anthracite with

Fe2O3-based and CuO-based oxygen carriers. Fuel Process. Tech-

nol. 96(1): 104–115.

Wang, B.W., Yan, R., Lee, D.H., Zheng, Y., Zhao, H.B., Zheng, C.G.,

2011a. Characterization and evaluation of Fe2O3/Al2O3 oxygen

carrier prepared by sol-gel combustion synthesis. J. Anal. Appl.

Pyroly. 91(1), 105–113.

Wang, B.W., Yan, R., Zhao, H.B., Zheng, Y., Zheng, C.G., 2011b. In-

vestigation of chemical looping combustion of coal with CuFe2O4

oxygen carrier. Energy Fuels 25(7), 3344–3354.

Wang, J.T., Chen, H.C., Zhou, H.H., Liu, X.J., Qiao, W.M., Long, D. H.

et al., 2013. Carbon dioxide capture using polyethylenimne-loaded

mesoporous carbons. J. Environ. Sci. 25(1), 124–132.

Yang, H. P., Chen, H. P., Ju, F. D., Yan, R., Zhang, S. H., 2007. Influence

of pressure on coal pyrolysis and char gasification. Energy Fuels

21(6), 3165–3170.

Yang, H.Q., Xu, Z.H., Fan, M.H., Gupta, R., Slimane, R.B., Bland, A.E.

et al., 2008. Progress in carbon dioxide separation and capture: A

review. J. Environ. Sci. 20(1), 14–27.

Zhang, C., Li, T. T., Xia, J., Chen, G., 2011. Experimental research on the

releasing characteristics of sulfur-containing gases in high-sulfur

coal under different atmospheres during mild thermal upgrading.

Proc. Chin. Soc. Electr. Eng. 31(14), 24–31.