Effective direct chemical looping coal combustion with bi-metallic Fe–Cu oxygen carriers studied using TG-MS techniques

Effective direct chemical looping coal combustion with bi-metallicFe–Cu oxygen carriers studied using TG-MS techniques

Ewelina Ksepko • Grzegorz Łabojko

Received: 1 October 2013 / Accepted: 28 January 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract This paper contains the results of research on a

promising combustion technology known as chemical

looping combustion (CLC). The noteworthy advantage of

CLC is that a concentrated CO2 stream can be obtained

after water condensation without any energy penalty for

CO2 separation. The objective of this work was to prepare

novel bi-metallic Fe–Cu oxygen carriers and to evaluate

the performance of these carriers for the CLC process with

hard coal/air. One-cycle CLC tests were conducted with

supported Fe–Cu oxygen carriers in thermogravimetric

analyzer (TG) utilizing hard coal as a fuel. The effects of

the oxygen carrier chemical composition, particle size, and

steam addition on the reaction rates were determined. The

fractional reduction, fractional oxidation, and the reaction

rates were calculated from the TG data. Notably, the sup-

port had a considerable effect on the reaction performance.

Moreover, bi-metallic Fe–Cu oxygen carriers exhibited

significantly improved reactivity compared with monome-

tallic Fe oxygen carriers. Furthermore, the addition of a

second reactive metal oxide stabilized the oxygen carrier

structure. The oxidation reaction was significantly faster

than the reduction reaction for all supported Fe–Cu oxygen

carriers. The TG data indicated that these oxygen carriers

had stable performances up to 900 �C and may be effec-

tively used for direct coal CLC reactions.

Keywords Chemical looping combustion � Bi-metallic

Fe–Cu based oxygen carriers � Coal combustion � TG

Introduction

Carbon dioxide is believed to be a greenhouse gas

responsible for possible global climate change. A vast

amount of CO2 is produced during fossil fuel combustion.

Current commercial CO2 separation technologies require

large amounts of energy; this requirement presents a sig-

nificant disadvantage. However, in a novel combustion

technology called chemical looping combustion (CLC), the

energy penalty problem may be solved [1]. Moreover, a

significant advantage of a CLC system is that a concen-

trated CO2 stream can be obtained from the combustion

gases after water condensation without requiring any

energy penalty for its separation with a significant nitrogen

oxide (NOx) reduction [2]. In CLC, direct contact between

air and the fuel is avoided because oxygen is delivered by a

solid oxygen carrier, typically MeO. Many primary simple

metal oxides, such as NiO, CuO, Mn2O3, Fe2O3, or Co3O4,

have previously been investigated as oxygen carriers [3–9].

The literature reports only limited work with mixed metal

oxides as oxygen carriers that contain more than two metal

oxides. Recently, promising results have been obtained for

bi-metallic oxygen carriers such as Co–Ni/Al2O3/YSZ

[10, 11], CeO2–Fe2O3 [12], Fe–Cu [13, 14], perovskite-

type as LaMnO3 substituted by Ca, or La0.8Sr0.2Co0.2-

Fe0.8O3–d [15, 16], Fe2O3–NiO/Al2O3 [17], and Fe2O3–

MnxOy/support [18–20], or Mn–Co [21] where synergy

effects have been observed. For CoO-NiO/YSZ, excellent

stable performance has been reported [12]. Ryden et al.

[18] concluded that combined oxides of Mn and Fe have

favorable thermodynamic properties and could potentially

be suitable for CLC applications. However, they reported

that the physical stability of the examined material was

low. Therefore, they suggested that stability could be

achieved via the addition of an inert material. Supported

E. Ksepko (&) � G. Łabojko

Institute for Chemical Processing of Coal, 1 Zamkowa,

41-803 Zabrze, Poland

e-mail: [email protected]

123

J Therm Anal Calorim

DOI 10.1007/s10973-014-3674-x

Fe–Mn oxides were demonstrated for their potential utili-

zation in the CLC of coal synthesis gas [21]. For Fe2O3–

NiO/Al2O3 [22], oxygen carriers increased reaction rates

and improved stability, and better fuel conversion was

reported. Adding Fe2O3 also decreased the amount of

harmful NiO and helped to assuage environmental con-

cerns. The CLC reactivity of CuFe2O4 with two types of

Chinese coals have been studied using thermogravimetric

analysis (TG) and Fourier transform infrared spectroscopy

(FTIR) [23]. At 300–600 �C, CuFe2O4 was reduced into Cu

and Fe3O4 initially via the transfer/combustion of the lat-

tice oxygen in CuFe2O4; Fe3O4 was then further reduced to

Fe2.962O4 at temperatures above 800 �C. CuFeO2 and Cu2O

were formed through the direct decomposition of CuFe2O4.

Wang et al. [24] also conducted bench-scale reactor tests of

CLC with an MgAl2O4-supported Fe3O4-CuO mixture and

coke oven gas. The optimized oxide composition contained

45 % Fe2O3 and 15 % CuO supported on 40 % MgAl2O4.

A higher temperature had a positive effect on reaction

performance. Particle breakage and CuO sintering were

observed on the used sample after 15-h fluidized-bed tests.

The maximum fuel conversion of the CuO–Fe2O3 mixture

was approximately 92 %.

Few studies have been published containing results

obtained using oxygen carriers for the combustion of solid

fuels including coal, as shown in [15, 21, 23–28]. Many

works illustrate the problems of gaseous fuels combustion

using the CLC method. Therefore, research on the com-

bustion/gasification of coal remains a major challenge.

Several concepts have been pursued. One of the concepts

involves a process that can be performed via the direct

combustion and gasification of coal by directly mixing

oxygen with a carbon carrier. Another concept involves the

potential utilization of a different oxygen carrier that

releases oxygen at high temperatures and directs this

oxygen to another reactor where the gaseous O2 and carbon

react (CLOU- chemical looping oxygen uncoupling). The

primary mechanism in CLOU for the combustion of solid

fuels is their reaction with gaseous oxygen released by the

decomposition of a metal oxide, which differs from the

CLC of solid fuels in which the solid fuel must first be

gasified. The slower gasification reaction in CLC is sub-

sequently followed by the combustion of the fuel with a

circulating oxygen carrier. Moreover, the understanding of

the physical and chemical processes that occurring in coal

while heated up may be helpful for design and also for the

optimization of practical conversion systems. Some of the

previous works [29, 30] on the temperature-controlled

combustion characteristics of multiply coals of different

origin by using DSC and TG demonstrated distinct tran-

sitional stages in the entire coal samples. The combustion

process was suggested to be complex and many competing

processes may contribute to the thermal analysis curves.

There are many challenges associated with the direct

use of the carbon that enters with the reactions of the

oxygen carrier in the CLC. Limited oxygen carriers can

be applied because the oxygen carrier should have suffi-

cient reactivity (good reaction rate of coal combustion), a

sufficient ability to release oxygen to facilitate the inter-

action between the carbon and the oxygen carrier, stable

reactivity when conducting multiple cycles of reduction

and oxidation, low abrasion and low reactivity with coal

ash and other contaminants. The separation of coal ash

from the waste oxygen carrier is an important technical

aspect; therefore, special attention should be paid to the

oxygen carrier cost because some oxygen carriers could

be lost in the CLC process. Ksepko [28] showed potential

application of low-cost natural carrier for coal combus-

tion, since the sewage sludge ash was successfully used

for direct coal combustion. Also complete regeneration of

oxygen carrier was achieved. The XRD and SEM data

proved that the Fe-based carrier was coal ash-resistant.

Lyon and Cole performed several coal gasification tests in

fluidized-bed reactors using an oxygen carrier in the form

of Fe2O3. The observed amounts of CO and CO2 were

consistent with the amount of carbon that was added to

the reactor [31]. Some tests were also performed by Leion

et al. [32, 33] and Berguerand et al. [34], who showed the

potential of practical petroleum coke gasification with

Fe2O3, one of the cheapest oxides. Furthermore, others

showed the gasification of fly ash using Fe2O3 [33]. CLC

was also successfully demonstrated to be used for elec-

tricity generation [35]. Wolf et al. [36] demonstrated that

coal could be utilized effectively in chemical looping

reforming [chemical looping combustion reforming

acceptor (CLCAR)] at 788–815 �C, 3.2 MPa and

800–950 �C at atmospheric pressure. The optimal process

conditions and the best oxygen carrier have not yet been

defined. However, based on the literature survey, iron

oxide and process temperatures of up to 950 �C are likely

parameters.

The development of suitable oxygen carriers is crucial

for the successful operation of a chemical looping coal

combustion power plant. To improve the reactivity of

Fe2O3 and the physical stability of CuO, mixed metal oxide

carriers containing primarily Fe2O3 and CuO were evalu-

ated. The objective of this study was to prepare mixed

metal oxide oxygen carriers consisting of 60 % Fe2O3 by

mass and 20 % CuO by mass that were supported on

sepiolite, bentonite, TiO2, Al2O3, and SiO2 and to evaluate

their performance in the direct coal CLC process. This

paper discusses the advantages of using mixed metal oxi-

des and the support’s effect on the performance of the

mixed metal oxides. Janina coal was used in this study

because hard coal has the potential to be a viable com-

mercial process.

E. Ksepko, G. Łabojko

123

There are many advantages for using Fe and Cu oxides

as oxygen carriers. Both Fe and Cu oxides are readily

available and are less expensive than oxygen carriers that

are based on Ni or Co oxides. Furthermore, health and

environmental issues are minimal. Fe2O3-based oxygen

carriers are very attractive primarily due to their low cost

and low health concerns. However, Fe2O3 is less reactive

with fuel compared with CuO or NiO oxygen carriers. CuO

is a candidate for CLC reactions because of its reactivity

and favorable thermodynamics for complete gaseous fuels

conversion. Nevertheless, pure CuO has a low melting

point (approximately 1,085 �C), which may cause an

agglomeration tendency and surface sintering during cyclic

reactions.

Experimental

Preparation of Fe2O3–CuO supported on sepiolite,

bentonite, TiO2, Al2O3 and SiO2

Oxygen carriers with a composition of 60 % Fe2O3 by

mass and 20 % CuO by mass on various supports, such as

sepiolite, bentonite, TiO2, Al2O3, or SiO2, were prepared

using a mechanical mixing method. High-purity powders

(\99 %, Sigma Aldrich) of Fe2O3 and CuO were mixed

thoroughly with sepiolite (Mg4Si6[OH]2.6H2O), bentonite

(H2Al2O6Si), or other supports. Deionized water was then

added to create a paste, which was dried and calcined at

850 �C in air for 20 h. After cooling, the sample was

crushed and thoroughly mixed again, then calcined at

850 �C. Each time, 10 % graphite by mass was added to

the mixture. The graphite was oxidized to CO2 during

calcination at 850 �C in air, which contributes to an

increase in the surface area of the oxygen carriers. The

calcined sample was sieved to obtain a particle size of

180 lm, which was used for the TG experiments. For the

particle size effect study, additional samples were prepared

with particles of 600 and 40 lm. For the comparison of

reactivity and stability, additional samples containing sin-

gle metal oxide as 80 % Fe2O3 by mass and 20 % by mass

of various supports such as sepiolite, bentonite TiO2,

Al2O3, or SiO2 were also tested.

Characterization of the coal samples

Hard coal from the Janina coal mine (Poland) was used in

the solid fuel CLC testing. Ultimate and proximate analy-

ses were conducted prior to testing. Technical analyses of

the Janina coal were conducted using LECO TG701, and

the elemental analyses were performed using LECO

TrueSpec CHN and LECO SC632 analyzers. The com-

bustion heating values were measured using a LECO

AC200. Tables 1, 2 present basic data on the quality of this

coal. The Janina coal is a typical power coal and exhibits

high reactivity because of its high volatile matter content

(33.5 %). Janina hard coal was selected because it is a

promising and prospective coal for use in coal gasification

in Poland. This carbon has a preferred low sulfur content

(1.1 %), low ash content (6.9 %) and high calorific value of

25.32 (MJ kg-1). Therefore, it appears to be an ideal

candidate for combustion processes using the CLC method.

Janina hard coal samples were prepared via grinding to a

particle size of \200 lm.

XRD analysis

X-ray diffraction patterns (XRD) of the bi-metallic oxygen

carriers were obtained using a PANalytical X’Pert Pro

X-ray diffractometer with Cu Ka radiation

(k = 1.54056 A). The diffraction patterns were collected

using a 2H scan from 10 to 80�. The data analysis was

Table 1 Proximate and ultimate analysis of coal sample

Proximate analysis/ad/mass% Ultimate analysis/ad/mass% Low heating value/MJ kg-1

Moisture Volatile Fixed carbon Ash C H Oa N S

8.8 33.5 50.8 6.9 65.2 4.00 12.9 1.01 1.11 25.32

ad air dried basisa Calculated by difference

Table 2 Analysis of ash from Janina coal

Ash analysis/%

SiO2 Al2O3 Fe2O3 CaO MgO P2O5 SO3 Mn3O4 TiO2 SrO Na2O K2O

46.48 28.43 9.59 3.33 2.76 0.15 1.46 0.03 1.11 0.06 3.49 2.46

Effective direct CLC with bi-metallic Fe–Cu oxygen carriers

123

performed using the HighScore Plus software package

supplied by PANalytical. The ICSD database was used to

identify the phases.

Thermogravimetric analysis (TG)

TG experiments were conducted in a thermal analyzer

(Netzsch STA 409 PG Luxx) coupled with a QMS (403C

Aeolos). The mass spectrometer used for the evolved gas

analysis could detect mass numbers between 1 and 300

amu in the SCAN or MID mode. During the TG exper-

iments, the mass change of the metal oxide oxygen car-

riers was measured isothermally as a function of time.

One reduction/oxidation cycle was conducted at atmo-

spheric pressure to determine the reactivity of the oxygen

carriers.

In this study, the one-cycle test was chosen, since from

technical point of view in TG, it is difficult to separate coal

ash from metal oxide after redox reaction and continue

cycling with new coal portion. In the real CLC systems,

there are some technical approaches of removing the coal

ash from oxygen carrier that are based on density differ-

ences [2, 11]. Moreover, the one- cycle redox test provides

all needed data on reactivity of oxygen carriers.

A mixture of fuel and oxygen carrier was prepared in the

stoichiometric ratio, which was calculated from the maxi-

mum amount of oxygen that can supply carrier to the fuel.

Hard coal was manually mixed with the oxygen carrier

sample. The mass ratio of metal oxide to coal was 16.3:1.

The sample consisted of * 10 mg of carbon and approx-

imately 200 mg of oxygen carrier. The mixture was placed

in a crucible (diameter 15 mm, height 25 mm) and heated

in the chamber to the desired measurement temperature

(900 �C) at a heating rate of 15 K min-1 under an inert Ar

atmosphere. Both the coal and oxygen carrier oxides were

dried in a balance drier at 115 �C to determine their water

content before thermogravimetric testing. After the set

temperature was reached, isothermal measurements were

performed for 120 min. Janina coal was used for reduction

of the oxygen carrier, and 20 % of the oxygen was bal-

anced by nitrogen and used for the oxidation segment. The

oxidizing condition was set at 15 min for a reaction gas

flow rate of 100 mL min-1. The reactivity test parameters

such as heating, gas flow rates, and mass of the sample

were determined based on the preliminary TG tests results.

The each measurement was also repeated to ensure the

quality of measurement.

The TG chamber was purged with Ar for 5 min after the

reaction with coal to remove the combustion gases. As a

result of the sample pretesting up to a temperature of

1,000 �C, it was concluded that the most favorable process

temperature for CLC was 900 �C. This temperature was

determined using the maximum reaction rate peaks for both

the pyrolysis and combustion products of coal that were

observed in the temperature range of 300–900 �C. A

measurement temperature of 900 �C was also selected

because it prevented the melting of copper oxide (CuO is

one of the oxygen carrier components). Copper oxide is

known to have an excellent reactivity, though a lower

melting point of approximately 1,085 �C.

In the TG analysis, different variants were tested. One

variant was used to study the effect of the chemical com-

position by comparing the reactivities of the mono- and bi-

metallic oxygen carriers. Therefore, the effect of a 20 %

CuO by mass addition to Fe2O3 was analyzed using a

12.3:1 mass ratio (oxygen carrier: coal) in the TG

measurements.

Furthermore, the effect of the oxygen carrier particle

size on the reaction rate was studied by applying different

particle sizes within the ranges of 600, 180, and 40 lm

for the selected oxygen carrier that exhibited good per-

formance. Moreover, to determine the effects of steam

addition, two variants were tested, i.e., with and without

the use of steam. During the TG experiments with the

addition of steam, water vapor with a flow rate of

0.5 g h-1 was used. This process was aimed at predicting

the most optimal conditions for the combustion/gasifica-

tion of coal.

The fractional conversions (i.e., the fractional reduction

and the fractional oxidation) were calculated from the TG

data. The fractional conversion (X) is defined as follows:

Fractional Reduction Xð Þ ¼ Moxd�Mð Þ= Moxd�Mredð Þð1Þ

Fractional Oxidation Xð Þ ¼ M�Mredð Þ= Moxd�Mredð Þð2Þ

For the direct coal CLC measurements, M denotes the

instantaneous mass of the metal oxide/coal mixture, Moxd is

the mass of a completely oxidized oxide/coal mixture in

the TG experiment after the introduction of air, and Mred

denotes the mass of the reduced sample in the TG exper-

iment, i.e., the mass of the metal oxide/coal mixture, which

consists of the reduced metal, the ash and the unreacted

coal at the end of the reaction. The oxygen uptake and

combustion percentages were obtained using the TG mass

change data in the equations given below:

Percent oxygen consumption

¼ ðexperimental oxygen consumption=

� theoretical capacity of oxygen present

in the metal oxideÞ � 100

ð3Þ


123

The global reaction rates (dX/dt) at different fractional

conversions (X) were calculated by differentiating the data.

Results and discussion

Material characterization

Characterization of oxygen carrier particles via XRD

The phase composition of the oxygen carriers was previ-

ously analyzed and described elsewhere [13]. Well-crys-

tallized phases of (hematite) Fe2O3, CuFe2O4, and Fe2TiO5

for the TiO2-supported bi-metallic oxygen carrier were

observed, and silica (SiO2) and alumina (Al2O3) were

detected via the X-ray powder diffraction data. The phase

composition of the fresh mono-metallic oxygen carriers

showed primarily (hematite) Fe2O3 and peaks from the

inert materials, such as silica (SiO2) and alumina (Al2O3).

The natural sourced inert as sepiolite and bentonite, were

rather amorphous due to lack of the diffraction peaks in the

X-ray powder diffraction pattern.

CLC reaction performance of bi-metallic Fe–Cu oxygen

carriers using a hard coal

Direct coal combustion is an option in CLC; therefore, one-

cycle TG reduction/oxidation tests were performed in this

study. At high temperatures, the direct combustion of metal

oxide and carbon is thermodynamically favored: the com-

bustion reaction proceeded at 900 �C. The coal/metal oxide

mixtures were prepared as discussed previously in the

sample preparation section.

The study examined the reactivity by using combined

methods. In addition to the impact of water vapor, particle

size and temperature also affect the chemical composition

of the oxygen carrier for the combustion of coal. Therefore,

two options were considered as mono-and bi-metallic

oxides. Mono-metallic oxygen carriers include one active

oxide in the form of iron oxide and one inert material. In a

variation of a bi-metallic oxygen carrier composed of two

active oxides (actively participating in the redox reaction),

we included the iron oxide and copper oxide, as well as one

inert oxide.

The results of the thermogravimetric examinations of

the combustion/gasification of coal with oxygen that was

released from the structures of the mono-and bi-metallic

oxygen carriers are shown in Fig. 1a–e. The figures show

the mass changes versus time, as well as the calculated

reaction rates. Black indicates the results for the bi-metallic

oxygen carriers, and gray indicates those for their mono-

metallic counterparts. The continuous line is used to show

changes in mass (mass%), while the dotted line illustrates

the changes in the calculated reaction rates (% min-1). The

presented results of the analysis of the reactivity of solid

samples of oxygen carriers using hard coal solid fuel

(Janina) showed that all bi-metallic oxygen carriers reacted

with coal: a mass decrease within the reaction time was

observed.

Figure 1a–e shows the mass profile and the reaction rate

that were obtained by differentiating the mass data for the

combustion of coal with supported Fe–Cu oxygen carriers

in the TG experiments, while the maximal reaction rates

observed at given temperatures during the reduction and

oxidation reactions of bi- and mono-metallic oxygen car-

riers are listed in Table 3. The mass changes at given

temperature ranges during reaction of mono- and bi-

metallic oxygen carrier with hard coal are listed in Table 4.

The sharp decrease in mass at the beginning of the

reduction reaction corresponded to the water loss from coal

dehydration. The maximum dehydration rate occurred at

approximately 200 �C, which occurred within 4–21 min of

the reaction and agreed well with the MS profile for water

(Fig. 2).

The TG data showed other mass decreases numbered

1–3. Steps 1–2 corresponded to a hard coal volatilization

reaction that was initiated at 400 �C and for which the

maximum reaction rate occurred at range between 437

and 442 �C. The observed MS signal of CH4, H2, CO,

and CO2 confirmed that the hard coal pyrolysis took place

(Fig. 2). In general, the coal pyrolysis temperature does

not depend on the oxygen carrier composition. Step 3

corresponded to the coal combustion reaction, which was

initiated at approximately 600 �C and for which the

maximum reaction rate occurred at approximately 900 �C,

with the exception of Fe–Cu/TiO2 (approximately

600 �C). This exception could be explained by the release

of oxide from the CuFe2O4 structure and the reaction with

coal char, resulting in coal combustion. For the TiO2-

supported oxygen carrier (Fe–Cu/TiO2), an additional

peak was observed, perhaps due to the presence of the

Fe2TiO5 phase. Therefore, for the Fe–Cu/TiO2 : coal

system, a two-step oxygen release is observed. One step is

from CuFe2O4, as observed for other bi-metallic sup-

ported Fe–Cu oxygen carriers, and one additional smaller

mass decrease peak where oxygen was released from the

Fe2TiO5.

Moreover, the thermogravimetric data were also sup-

ported by the mass spectrometry data; the intensive signals

of CO2 and H2O during the coal combustion reaction were

observed (Fig. 2). During the regeneration reaction, all bi-

metallic oxygen carriers showed a decrease in O2 con-

centration during the regeneration reaction (Fig. 3).

For the Fe–Cu/Al2O3 carrier (Fig. 1e), a sharp mass

decrease at the beginning of the reaction was observed,


123

which corresponded to water content. The maximum rate

of dehydration was observed at *150 �C. The other mass

decrease (Table 4) was due to the hard coal volatilization

reaction that was initiated at 300 �C, with a maximum rate

observed at 608 �C (Table 3) and a coal combustion

reaction that was initiated at 713 �C with a maximum rate

observed at 880 �C.

The calculated expected mass decreases, while the

entire reduction of the bi-metallic oxygen carrier is equal

to a 22.06 % by mass decrease. For that reduction reac-

tion, the Cu2? and Fe3? would be reduced to metallic

forms of Cu0 and Fe0. For the partial reduction of bi-

metallic oxygen carriers, the 10.3 % by mass decrease

would be expected while being reduced to the FeO and

0 25 50 75 100 125 150

85

90

95

100

0

200

400

600

800

1000

DT

G/%

min

–1

Tem

p./°

C

DT

G/%

min

–1

Tem

p./°

C

TG

/mas

s%

Time/min

60 % Fe2O3, 20 % CuO, 20 % bentonite

80 % Fe2O3, 20 % bentonite

(a)

–1

0

1

2

0 25 50 75 100 125 15080

85

90

95

100

0

200

400

600

800

1000

DT

G/%

min

–1

Tem

p/°C

TG

/mas

s%

Time/min

(b)

60 % Fe2O3, 20 % CuO, 20 % sepiolite

80 % Fe2O3, 20 % sepiolite

–1

0

1

2

0 25 50 75 100 125 15080

85

90

95

100

0

200

400

600

800

1000

TG

/mas

s%

Time/min

(c)

60 % Fe2O3, 20 % CuO, 20 % TiO280 % Fe2O3, 20 % TiO2

–1

0

1

2

0 25 50 75 100 125 150

80

85

90

95

100

0

200

400

600

800

1000

DT

G/%

min

–1

Tem

p./°

C

TG

/mas

s%

Time/min

(d)

60 % Fe2O3, 20 % CuO, 20 % SiO280 % Fe2O3, 20 % SiO2

–1

0

1

0 25 50 75 100 125 150

85

90

95

100

0

200

400

600

800

1000D

TG

/%m

in–1

Tem

p./°

C

TG

/mas

s%

Time/min

(e)

60 % Fe 2O3, 20 % CuO, 20 % Al2O380 % Fe2O3, 20 % Al2O3

–1

0

1

2

Fig. 1 Coal combustion with oxygen released from, a Bi-metallic

Fe–Cu/bentonite and the mono-metallic Fe/bentonite, b Bi-metallic

Fe–Cu/sepiolite and the mono-metallic Fe/sepiolite, c Bi-metallic Fe–

Cu/SiO2 and the mono-metallic Fe/SiO2, d Bi-metallic Fe–Cu/TiO2

and the mono-metallic Fe/TiO2, e Bi-metallic Fe–Cu/Al2O3 and the

mono-metallic Fe/Al2O3


123

Cu0 forms. The other partial reduction had a maximum

extent of reduction of 6.03 % by mass because Cu2? and

Fe3? were reduced to Fe3O4 and Cu0. The observed mass

decreases showed that the oxygen carriers were reduced

to FeO and Cu0, and all coal was combusted (Table 4).

This was also demonstrated for the bi-metallic oxygen

carriers in the MS data.

Effect of the chemical composition of the oxygen carrier

The use of 20 % by mass copper oxide added to 60 % by

mass of iron oxide was evaluated. Therefore, the mono-

metallic oxygen carriers containing only one active oxide

(Fe2O3) were analyzed in the study by applying the same

reaction conditions as those that are shown in Fig. 1a–e.

The results of the TG testing for the supported Fe oxides

are listed in Table 3, where both maximal reaction rates

observed at given temperatures during the reduction by

coal and oxidation by air reaction of the mono-metallic

oxygen carriers are given. The mass changes at given

temperature ranges during reaction of mono- and bi-

metallic oxygen carrier with hard coal are listed in Table 4.

Table 3 Maximal reaction rates observed at given temperatures

during reduction and oxidation reaction of bi- and mono-metallic

oxygen carriers

Fe–Cu/bentonite Fe/bentonite

Reduction Regeneration

Peak No 1 2 3 4 1

Temp./�C) 437 – – 901 900

436 563 648 904 900

Reaction rate/% min-1 0.431 – – 0.517 1.945

0.139 0.173 0.169 0.387 0.996

Fe–Cu/sepiolite Fe/sepiolite


Peak No 1 2 3 4 1

Temp./�C 442 – – 903 900

439 551 672 903 900


0.133 0.145 0.165 0.392 1.482

Fe–Cu/TiO2 Fe/TiO2


Peak No 1 2 3 4 1

Temp./�C 439 608 – 901 900

439 654 – 899 900

Reaction rate/% min-1 0.258 0.135 – 0.651 1.807

0.145 0.194 – 0.527 1.218

Fe–Cu/SiO2 Fe/SiO2


Peak No 1 2 3 4 1

Temp./�C 439 – – 900 901

458 751 901 905 901


0.124 0.113 0.364 0.338 1.287

Fe–Cu/Al2O3 Fe/Al2O3


Peak No 1 2 3 4 1

Temp./�C 440 – – 901 901

434 608 880 903 901


0.172 0.203 0.329 0.417 1.835

Table 4 Mass losses observed during reaction of bi- and mono-

metallic oxygen carriers with coal at given temperature ranges

Fe–Cu/bentonite Fe/bentonite

Peak No 1 2 3 4

Maximum temp./�C 437 – – 901

436 563 648 904

Mass losses/mass% 3.41 5.30 3.37

2.48 2.80 3.72

Fe–Cu/sepiolite Fe/sepiolite

Peak No 1 2 3 4


439 551 672 903


5.52 2.93 2.62

Fe–Cu/TiO2 Fe/TiO2

Peak No 1 2 3 4

Maximum temp./�C 439 608 – 901

439 654 – 899


2.44 5.19 3.14

Fe–Cu/SiO2 Fe/SiO2

Peak No 1 2 3 4


458 751 901 905


2.29 2.86 2.03

Fe–Cu/Al2O3 Fe/Al2O3

Peak No 1 2 3 4


434 608 880 903


2.35 2.94 2.38


123

For comparison purposes, particularly useful in respect

to the reactivity evaluation of the carrier may be the mass

differentiation with respect to time (the reaction rates).

This demonstrates the various stages of the processes.

By comparing the bi-metallic oxygen carrier TG curves

with that for the mono-metallic oxygen carrier, additional

decreases of the masses were observed (Fig. 1a–e).

Moreover, the calculated reaction rates indicated that the

addition of CuO improved the reactivity of the oxygen

carrier in the direct coal combustion reaction. The reaction

rates increased significantly. For the pyrolysis reaction, the

reaction rate increased by a factor of 2.36 for the material

containing Al2O3 and by 3.25-fold for the material con-

taining TiO2. The combustion reaction rate also increased

from 1.22 to 2.12-fold for Fe–Cu/bentonite and Fe–Cu/

TiO2, respectively. The least impact on the reduction rate

was observed for the materials that consisted of inert

additives in the form of natural minerals such as bentonite

or sepiolite. Moreover, a similar beneficial effect of a 20 %

CuO by mass addition to a mono-metallic Fe2O3 was

observed for the regeneration reaction. CuO significantly

contributed to the increase in the reactivity of oxygen

carriers.

In Ref. [25], a variety of metal oxides, including CuO,

NiO, Fe2O3, Mn2O3, and Co3O4, for use in the combustion/

gasification of coal were analyzed. Based on the TG

curves, the authors demonstrated that CuO had the best

characteristics for the combustion of coal. They also found

that CuO had the lowest combustion temperature and the

fastest combustion reaction, as well as the greatest degree

of fuel conversion (100 % carbon conversion). For the

mixture of carbon and Mn2O3, they observed that the

combustion reaction occurred at a higher temperature

(978 �C), but this reaction also showed the lowest reaction

rate compared with the other four oxides. However, a

positive aspect of the use of this oxide was that the com-

bustion reaction was slightly exothermic, and the percent-

age of coal combustion was approximately 71 %. For a

mixture of Fe2O3-carbon, the maximum reaction rate was

obtained at approximately 977 �C, which was higher than

that observed for CuO (780 �C). In addition, they obtained

a high degree of fuel conversion (92 %). Because the

reduction reaction of Fe2O3-carbon is endothermic, heat

must be supplied to the reduction reactor. Moreover, the

reduction reaction rates of iron oxide were lower than those

obtained for copper oxide; however, the iron oxide

regeneration reactions took place fourfold faster than the

regeneration of the copper oxides. For Co3O4, the highest

combustion rate was observed (based on the TG curve) at

781 �C, although a lower fuel conversion (83 %) was

obtained.

Interestingly, the group of mono-metallic-type oxygen

carriers based on Fe/inert exhibited comparable behavior.

Four peaks were observed that reflected a reduction of the

carrier due to the pyrolysis and combustion of the fuel.

Only the position of the maximum, in which processes

achieved the highest value of reaction rate, was variable.

This maximum usually depends on the chemical compo-

sition of the material. Generally, the first mass loss takes

place in the peak at approximately 440 �C, where coal

pyrolysis occurs.

Peaks with the highest reaction rates in the range of

551–751 �C reflect the partial reduction of the oxygen

carrier from Fe2O3 to Fe3O4/FeO and the partial combus-

tion of coal. The peaks observed within the range of

648–901 �C reflect complete fuel combustion and the

reduction of the oxygen carrier from Fe3O4/FeO to Fe/FeO.

140700

Ion

curr

ent/a

.u.

Time/min

H2O

CO2

COCH

4

H2

Fig. 2 MS data for coal combustion reaction with oxygen released

from bi-metallic Fe–Cu/Al2O3

140 145 150 155

Ion

curr

ent/a

.u.

Time/min

O2

Fig. 3 MS data for regeneration reaction for bi-metallic Fe–Cu/

Al2O3


123

As shown in Fig. 1e, for the bi-metallic material com-

posed of 60 % Fe2O3 by mass, 20 % CuO by mass, 20 %

Al2O3 by mass and a mono-metallic carrier with a com-

position of 80 % Fe2O3 by mass and 20 % Al2O3 by mass,

a systematic decrease in the mass mixture of coal: carrier

was observed. It should be noted that the heating rate was

15 K min-1, and the desired temperature of the process

was achieved in approximately 60 min. The measurement

was then performed isothermally, which reflects the tem-

perature curve. A steady mass loss of mixture was observed

in approximately 150 min. The curve of mass loss for a

mixture of mono-metallic Fe2O3/Al2O3 oxygen carrier with

coal is shown in Fig. 1e. The four main stages of the

processes were observed as shown in Fig. 1e. The initial

mass loss was observed at a temperature of 434 �C with a

reaction rate of 0.1715 % min-1. This mass loss can be

attributed to the pyrolysis process. Subsequently, a second

peak at 608 �C was observed with a maximum rate of

0.20316 % min-1, a third peak was observed with a

maximum reaction rate of 0.32981 % min-1 (880 �C), and

a fourth peak was observed with a maximum reaction rate

of 0.41688 % min-1 (903 �C). The observed mass losses

with the specified rates indicated that the process of Janina

coal combustion with oxygen slowly released from the

structure of the Fe2O3/Al2O3 oxygen carrier. However, the

coal combustion process was more than twofold faster than

the pyrolysis process. The observed gradual decreases in

mass may be related to the partial overlapping of the iron

III oxide reduction stages with those of other oxide forms.

Therefore, oxygen is released from the structure of Fe2O3,

which is reduced to Fe3O4 and then turned into FeO. This

hypothesis can be confirmed on the basis of the individual

mass losses. Under certain conditions, the reduction reac-

tion to metallic Fe may also occur. The regeneration

reaction appears to be fast with an estimated rate of

1.8355 % min-1 compared with other reduction reaction

rates.

In summary, in the group of the mono-metallic Fe2O3/

inert, the most reactive are Fe/Al2O3 and Fe/TiO2 due to

the greatest degree of reaction of the coal: oxide mixture

(approximately 13 % mass loss) and because of the

greatest reaction rate of combustion (0.41688 % min-1 at

903 �C and 0.329 % min-1 at 880 �C for Fe/TiO2 and Fe/

Al2O3, respectively).

The mixture of coal with bi-metallic Fe–Cu/SiO2 oxide

(Fig. 1d) shows only two mass decreases. Comparing its

reactivity with the reactivity of the mono-metallic coun-

terpart Fe/SiO2, a significant improvement in reactivity was

observed. First, the mass loss of the mixture was higher for

the bi-metallic compared with mono-metallic carrier. This

demonstrates the preferred release of oxygen from the

structure of an oxygen carrier in the fuel combustion

reaction. Furthermore, the addition of CuO significantly

increases the reaction rates by a factor of 3.4. For the bi-

metallic oxygen carrier, a large mass loss is observed

compared with its mono-metallic counterpart, with a

maximum temperature of 902 �C and a reaction rate of

0.62541 % min-1. Based on these observations, it may be

concluded that that is a reasonable time for a Janina coal

combustion reaction with oxygen that is released from a

Fe–Cu oxide structure.

The reactivity study observations via TG for both the

reduction and oxidation reactions (summarized in Table 3)

demonstrated that a process of pyrolysis is followed by the

combustion of the fuel with oxygen released from the

structure of an oxygen carrier. This has been observed for

all the indicated materials. Both good reactivity with fuel

and air was observed in the TG curves. Moreover, the

regeneration reaction rates were improved for the bi-

metallic oxygen carriers compared with the reaction rates

obtained for the mono-metallic Fe oxides. The oxidation

reaction was significantly faster than the reduction reaction

for all supported Fe–Cu oxygen carriers.

The bi-metallic materials showed better reactivity per-

formance than their mono-metallic counterparts. As the TG

data demonstrated, the redox reactions had higher reaction

rates of fuel combustion. This provides an opportunity to

combust more fuel in less time, thus allowing for the

conservation of considerable processing time. The TG

results indicated that better reactivity was observed for the

bi-metallic oxygen carriers than that for mono-metallic

carriers due to the synergy effect of two reactive oxides

(Fe2O3 and CuO). Moreover, an excellent oxygen transport

capacity was also observed. Therefore, the addition of a

second reactive metal oxide stabilized the oxygen carrier

structure.

Effect of particle size of the oxygen carrier

The effect of particle size of the oxygen carrier was also

studied to determine the most optimal operating parame-

ters. The influence of the oxygen carrier particles on the

reaction rate of gasification/combustion of hard coal was

studied for two selected bi-metallic oxygen carriers (Fe–

Cu/Al2O3 and Fe–Cu/SiO2). For that reason, samples with

different grain sizes (600, 180 and 40 lm) were prepared.

The reactivity results for these carriers showed no signifi-

cant effect of oxygen carrier grain size on the rate of the

reaction gasification/combustion of coal, as indicated by

the data shown in Table 5. The rate for the selected particle

size differs only subtly in favor of smaller grains. In the

pyrolysis step, no difference in rates was observed, while

that for the combustion reaction was slightly faster. Thus,

for example, the ratio of the reaction rate for the particle

size of 40/180 microns was barely 1.01–1.26-fold. A sim-

ilar negligible increase in the reaction rate was also


123

observed for the regeneration reaction. The influence of

particle size on the regeneration reaction rate was negli-

gible; the increase in the rate was 1.01–1.23. This was

better for the material supported with SiO2 than that sup-

ported with Al2O3.

Similar observations for the mono-metallic CuO with

grain sizes of 5 and 63 lm have also been made [25]. A

negligible increase in the reaction rate for the combustion

of coal was observed; the reaction rate increased slightly

from 0.79 to 0.83 % min-1 with a maximum observed at a

temperature of 708 �C instead of 780 �C.

Given that a negligible effect of oxygen carrier particle

size on reaction rate was observed, carrier size is expected

to play no significant role in different coal combustion

reactions.

The effect of steam addition

The addition of 0.5 g h-1 water vapor was used to estimate

the effects of water vapor on the reaction rate of oxygen

with the fuel carriers. The data for both the mono- and bi-

metallic carriers is shown in Tables 6, 7.

The application of water vapor contributed to an

increase in the reaction rate of gasification/combustion of

the fuel for both the mono- and bi-metallic carriers. Most

notably, the addition of water vapor affected the gasifica-

tion reactions/Janina coal combustion via oxygen released

from the structures of bi-metallic oxides (such as Fe–Cu/

bentonite and Fe–Cu/TiO2), which precipitated an increase

in the reaction rate by a factor of 1.62–2.12. The effect of

water vapor on the gasification/combustion of the coal was

significantly positive. This was particularly marked for the

mono-metallic oxygen carriers. In addition, the Fe oxides

reduction proceeded two to threefold faster (2.44-fold for

Fe/bentonite and 2.75-fold for Fe/sepiolite). Moreover, this

positive effect was also observed for the regeneration

reaction. The most beneficial effect caused an increase in

the reaction rate by a factor of 1.79 among the bi-metallic

carriers; this effect was observed for Fe–Cu/Al2O3. Among

the mono-metallic carriers, the increase in reaction rate for

Fe/TiO2 was 1.45. We conclude that the development of a

porous structure perhaps created by the steam caused the

activation of the oxygen carrier. Therefore, an increase in

the reactivity of both the mono-and bi-metallic oxygen

carriers was observed. In practice, steam could be used to

stimulate the carrier in a CLC process, as shown by our

observations.

Conclusions

In summary, for the purposes of direct coal combustion by

CLC, new bi-metallic iron-copper oxygen carriers were

examined via TG. The addition of the second reactive

metal oxide to iron oxide and its effect on the reactivity of

oxygen carriers with the fuel was also examined. The

influence was positive; an increase in the reaction rate was

observed. The obtained results also allowed us to determine

the negligible impact of the size of the carrier grain on the

coal combustion rate. The optimal temperature for the

combustion of coal based on TG data curves was estimated

to be 850–900 �C. Moreover, this study also confirmed the

option of the gasification/combustion of coal with oxygen

released from the structure of solid oxygen carriers at lower

temperatures and a favorable fuel conversion degree in

relation to the oxygen carriers that was previously pro-

posed in the literature (such as Mn2O3 or NiO). The bi-

metallic oxygen carriers exhibited excellent oxygen trans-

port capacity, which was estimated to be approximately

Table 5 Particle size effect study for selected bi-metallic oxygen

carriers

Oxygen

carrier

Reduction

reaction rate

ratios for given

DTG peaks

Oxidation reaction

rate ratios

Particle size

ratio/lm

Fe–Cu/

Al2O3

1.06 1.07 0.63 0.91 180/600

1.03 0.89 1.01 1.01 40/180

Fe–Cu/SiO2 1.05 3.84 1.19 0.81 180/600

1.02 0.89 1.27 1.23 40/180

Table 6 The calculated ratio of reaction rates with steam/without

steam for bi-metallic oxygen carriers

Samples Ratios for reduction

reactions for given DTG

peaks

Ratios for

oxidation

reaction

Fe–Cu/bentonite 0.86 1.63 x 1.36

Fe–Cu/sepiolite 0.65 1.35 x 1.37

Fe–Cu/TiO2 1.71 1.96 x 1.61

Fe–Cu/Al2O3 1.10 1.53 x 1.79

Fe–Cu/SiO2 0.84 0.43 0.52 1.55

Table 7 The calculated ratio of reaction rates with steam/without

steam for mono-metallic oxygen carriers

Samples Ratios for reduction

reactions for given DTG

peaks

Ratios for

oxidation

reaction

Fe/bentonite 0.97 1.17 2.44 2.10 –

Fe/sepiolite 0.93 1.41 0.95 2.75 2.29

Fe/TiO2 0.83 1.45 1.02 1.50 –

Fe/Al2O3 0.79 0.98 0.81 1.73 1.49

Fe/SiO2 1.12 1.31 1.65 1.41 1.17


123

13 % mass. Moreover, support had a large effect on reac-

tion performance. The best support for the mixed metal

oxide was alumina and titania, concluded based on the TG

curves. Fe–Cu oxides are excellent candidates for appli-

cation to the direct coal combustion process because only

minimal interaction with Janina coal ash was observed.

This indicates that bi-metallic oxygen carriers are more

ash-resistant compared with recently utilized oxygen

carriers.

Bi-metallic materials are preferred over mono-metallic

counterparts; the TG data revealed that reactions with fuel

occur significantly faster with these materials. Moreover,

more fuel can be combusted in a shorter amount of time.

This reactivity improvement could facilitate a time-saving

process due to the bi-metallic oxygen utilization. In addi-

tion, our study showed that TG-MS techniques could be

valuable tools for power engineering research.

Acknowledgements We gratefully acknowledge financial support

from a project by the Polish Ministry of Higher Education and Sci-

ence (No. 685/N-USA/2010/0).

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Documents

Effective direct chemical looping coal combustion with bi-metallic Fe–Cu oxygen carriers studied using TG-MS techniques