CO2 Capture in Coal Combustion by Chemical-Looping with Oxygen Uncoupling (CLOU) with a Cu-based

Oxygen-Carrier

Iñaki Adánez-Rubio, Pilar Gayán, Alberto Abad, Francisco García-Labiano, Luis F. de

Diego, Juan Adánez

Instituto de Carboquímica (C.S.I.C.), Dept. of Energy & Environment, Miguel Luesma

Castán, 4, Zaragoza, 50018, Spain

Abstract

The Chemical Looping with Oxygen Uncoupling (CLOU) process is a Chemical

Looping Combustion (CLC) technology that allows the combustion of solid fuels with

inherent CO2 separation. As in the CLC technology, in the CLOU process the oxygen

necessary for the fuel combustion is supplied by a solid oxygen-carrier, which contains

a metal oxide. The CLOU technology uses the property of some metal oxides which can

generate gaseous oxygen at high temperatures. The oxygen generated by the oxygen-

carrier reacts directly with the solid fuel, which is mixed with the oxygen-carrier in the

fuel-reactor. The reduced oxygen-carrier is transported to the air-reactor where it is

oxidized by air. Oxides of copper, manganese, and cobalt have been identified as

materials with suitable thermodynamic properties to be considered as oxygen-carrier for

the CLOU process. This work demonstrates the CLOU technology in a 1.5 kWth

continuously operated unit. The plant was basically composed of two interconnected

fluidized-bed reactors joined by a loop seal, a riser for solids transport from the air-

reactor to the fuel-reactor, a cyclone to recover the entrained solids, and a solids valve

to control the solids circulation flow rate between both reactors. Promising particles

prepared by spray drying containing 60 wt% CuO and MgAl2O4 as supporting material

were used as oxygen-carrier. A Colombian bituminous coal was used as fuel. Coal is

continuously fed to the fuel-reactor through a two steps screw feeder placed just above

the distributor plate. The effect of operating conditions, such as temperature of the fuel-

reactor and the rate of coal feeding, on the combustion efficiency were investigated. The

experiments were carried out at 900-960 ºC in the fuel-reactor. Fast rate of oxygen

generation was observed and full combustion of coal in the fuel-reactor was obtained at

the highest temperature. The results obtained are analyzed and discussed in order to be

useful for the scale-up of a CLOU process fuelled with coal.

1. Introduction

The technology may be especially suitable for solid fuels, such as coal, petroleum coke

or biomass. Solid fuels are considerably more abundant and less expensive than natural

gas, and it would be highly advantageous if the CLC process could be adapted for these

types of fuels. One way of performing this is to directly introduce the solid fuel to the

fuel reactor, where the oxygen-carrier directly reacts with the fuel. However the solid–

solid reaction between the char and the metal oxide is not very likely to occur at an

appreciable rate and the gasification via and a gasifying agent, e.g. H2O, has been

proposed as an option. However, the gasification reaction of coal is a slow step which

limits the conversion rate of a coal in the CLC. Figure 1 shows a CLOU system

schematic design. The Chemical-Looping with Oxygen Uncoupling (CLOU) process is

a Chemical Looping Combustion (CLC) technology that allows the combustion of solid

fuels with inherent CO2 separation using oxygen-carriers [1]. This technology has low

energy penalty and thus low CO2 capture costs. The CLOU technology takes advantage

of the property of some metal oxides which can generate gaseous oxygen at high

temperatures. The oxygen generated by the oxygen-carrier reacts directly with the solid

fuel, which is mixed with the oxygen-carrier in the fuel-reactor. Thus, the need of solid

fuel gasification, that it is a limiting step in conventional CLC process with solid fuels,

is avoided.

Figure 1: CLOU system schematic design.

The oxygen-carrier for CLOU needs to have special characteristics and needs to react

reversibly with oxygen at high temperatures. The oxygen-carriers for CLOU must have

the ability to react both with oxygen in the air-reactor and then also to release this

oxygen through reduction in the fuel-reactor. Thus CLOU utilizes the fact that some

metal oxides have suitable equilibrium oxygen partial pressure at temperatures of

interest for combustion, i.e. 800–1200ºC. Three such metal oxide systems have been

identified: CuO/Cu2O, Mn2O3/Mn3O4, and Co3O4/CoO [1]. After a thermodynamic

study, it was found that copper-based oxygen-carriers fulfill all the requirements of the

process.

Reaction taking place in the fuel-reactor with copper oxide as oxygen-carrier are the

following:

2224 OOCuCuO +→

(1)

Coal → Volatile Matter + Char (2)

Volatile Matter2 2 2

O CO H O+ → + (3)

2 2 2Char O CO H O+ → + (4)

2 2 24 2CuO Coal Cu O CO H O+ → + + (5)

The oxygen release reaction (r.1) is endothermic but the global reaction (r.5) in fuel

reactor is exothermic due to the coal combustion (r.3 and r.4). This is an advantageous

characteristic of using this metal oxide, as there can be a temperature increase in the

fuel-reactor.

The reduced oxygen-carrier from the fuel-reactor is regenerated in the air-reactor:

CuOOOCu 4222

→+ (6)

Figure 2 shows the equilibrium concentration of O2 versus temperature for copper

oxide, calculated using the HSC Chemistry 6.1 software [2]. The oxygen concentration

Coal

CO2 + H2O

N2 (+O2)

MexOy

H2O(l)

CO2

Fuel

Reactor

MexOy-1

Condenser

Ash CO2

Air

Air

Reactor

CO2

at equilibrium conditions greatly depends on the temperature. Thus, an equilibrium

concentration is 1.5 vol% O2 can be reached in the fuel-reactor at 900 ºC, whereas the

equilibrium concentration increases up to 12.4 vol% at 1000 ºC. Nevertheless it could

be not needed to reach the equilibrium concentration in the fuel-reactor. Indeed it should

be desirable to get low concentration of oxygen from the fuel-reactor in order to obtain

a high purity CO2 stream. In the air-reactor, the metal oxide is stable below 950 ºC if the

maximum oxygen concentration from the air-reactor is higher than 4.5 vol%.

T (ºC)

850 900 950 1000 1050 1100

Oxygen

equil

ibri

um

(vol%

)

0

5

10

15

20

CuO

2Cu O

Figure 2: Equilibrium oxygen concentrations over the CuO/Cu2O system as a function

of temperature.

An analysis about the suitability of Cu-based materials was carried out previously at

ICB-CSIC [3]. Particles prepared by several methods with different supporting

materials and different metal oxide contents were tested. From this work, it was shown

that particles containing 60 wt% CuO and using MgAl2O4 as supporting material are

adequate for its use as oxygen-carrier for the CLOU process. On the whole, this material

shows adequate values of reactivity and oxygen transport capacity, high attrition

resistance and does not have tendency to agglomerate during operation in a fluidized-

bed reactor.

The aim of this work was to investigate the performance of a CLOU system using a Cu-

based oxygen-carrier. The combustion of coal was carried out in a continuously

operated 1.5 kWth CLOU unit. The effect of operating conditions –such as temperature

of the fuel-reactor, residence time and the rate of coal feeding – on the combustion

efficiency were investigated. The experiments were carried out at 900-960 ºC in the

fuel-reactor. The results obtained are analyzed and discussed in order to be useful for

the scale-up of a CLOU process fuelled with coal.

2. Experimental section

2.1. The Cu-based oxygen-carrier

The material used was a Cu-based oxygen-carrier prepared by spray drying. Oxygen

carrier particles were manufactured by VITO (Flemish Institute for Technological

Research, Belgium) using MgAl2O4 as inert material. The CuO content was 60 wt%.

Particles were calcined for 24 h at 1100ºC and sieved (100–200 µm). From now on the

oxygen-carrier was named as Cu60MgAl. Table 1 shows the main properties of this

oxygen-carrier. It has a low porosity and superficial area. The mechanical strength of

the particles after 24 h of calcination was adequate for its use in a fluidized bed. The

compounds found by XRD analysis were CuO and MgAl2O4.

Table 1. Properties of the oxygen-carrier Cu60MgAl (after 24 h of calcination).

CuO content (wt.%) 60

Oxygen transport capacity, Ro (wt.%) 6

Crushing strength (N) 2.4

Real density (g/cm3) 4.6

Porosity (%) 16.1

Specific surface area, BET (m2/g) < 0.5

XRD main phases CuO, MgAl2O4

2.2. Coal “El Cerrejón”

The fuel used was a bituminous Colombian coal “El Cerrejón”. The properties of this

coal are gathered in Table 2. The coal particle size used for this study was +200–300

µm. In order to avoid coal swelling and bed agglomeration, the coal was subjected to a

thermal pre-treatment for pre-oxidation. Coal was heated at 180 ºC in air atmosphere for

28 hours, placed in trays so that the height of the coal layer did not exceed 3mm.

Table 2. Properties of fresh and pre-treated “El Cerrejón” coal.

Pre-treated Colombian coal

C 65.8 % Moisture 2.3 %

H 3.3 % Volatile matter 33.0 %

N 1.6 % Fixed carbon 55.9 %

S 0.6 % Ash 8.8 %

O 17.6 %

Low Heating Value: 21899 kJ/kg

2.3. Experimental set-up

A schematic view of the CLOU unit is shown in Figure 3. The set-up was basically

composed of two interconnected fluidized-bed reactors –the air- and fuel-reactors–

joined by a loop seal, a riser for solids transport from the air-reactor to the fuel-reactor,

a cyclone and a solids valve to control the solids circulation flow rate in the system. A

diverting solids valve located below the cyclone allowed for the measurement of the

solids flow rates at any time. Therefore, this design allowed us to control and measure

the solids circulation flow rate between both reactors.

Figure 3. Schematic view of the 1.5 kWth CLC rig fuelled with coal.

Because of heat losses, the system is not auto-thermal and it is heated up by means of

various independent ovens to get independent temperature control of the air-reactor,

fuel-reactor, and freeboard above the bed in the fuel-reactor, which is actually kept

constant at about 900 ºC in all the experiments. During operation, temperatures in the

bed and freeboard of the fuel-reactor, air-reactor bed and riser were monitored as well

as the pressure drops in important locations of the system, such as the fuel-reactor bed,

the air-reactor bed and the loop seal.

The fuel-reactor consisted of a bubbling fluidized bed with 50 mm of inner diameter and

200 mm bed height. N2 was used as fluidizing gas. The gas flow was 186 LN/h. Coal is

fed by a screw feeder at the bottom of the bed right above the fuel-reactor distributor

plate in order to maximize the time that the fuel and volatile matter are in contact with

the bed material.

The oxygen-carrier is reduced in the fuel-reactor, evolving gaseous oxygen to the

surroundings. The oxygen burns the volatiles and char proceeding from coal pyrolysis

in the fuel-reactor. Reduced oxygen-carrier particles overflowed into the air-reactor

through a U-shaped fluidized bed loop seal with 5 cm of inner diameter, to avoid gas

mixing between fuel and air. A N2 flow of 60 LN/h was introduced in the loop seal.

The oxidation of the carrier took place in the air-reactor, consisting of a bubbling

fluidized bed with 8 cm of inner diameter and 10 cm bed height, and followed by a

riser. The air flow was 1740 LN/h. In addition, a secondary air flow (240 LN/h) was

introduced at the top of the bubbling bed to help particle entrainment through the riser.

N2 and unreacted O2 left the air-reactor passing through a high-efficiency cyclone and a

filter before the stack. The oxidized solid particles recovered by the cyclone were sent

to a solids reservoir, setting the oxygen-carrier ready to start a new cycle. In addition,

these particles avoid the leakage of gas between the fuel-reactor and the riser. The

regenerated oxygen-carrier particles returned to the fuel-reactor by gravity from the

solids reservoir through a solids valve which controlled the flow rates of solids entering

the fuel-reactor. A diverting solids valve located below the cyclone allowed the

H2O N2Air

Sec.Air

AR

FR

Product Gas

Flue Gas

Coal

N2 H2O N2Air

Sec.Air

AR

FR

Product Gas

Flue Gas

Coal

N2

Filter

Gas analysis

Gas analysis7

82

4

5

6

10

3

1

9

1110

1.- Fuel Reactor, FR 7.- Solids control valve2.- Loop Seal 8.- Coal3.- Air Reactor, AR 9.- Screw feeders4.- Riser 10.- Furnaces5.- Cyclone 11.- Vaporizer6.- Diverting solids valve H2O N2Air

Sec.Air

AR

FR

Product Gas

Flue Gas

Coal

N2 H2O N2Air

Sec.Air

AR

FR

Product Gas

Flue Gas

Coal

N2

Filter

Gas analysis

Gas analysis7

82

4

5

6

10

3

1

9

1110

1.- Fuel Reactor, FR 7.- Solids control valve2.- Loop Seal 8.- Coal3.- Air Reactor, AR 9.- Screw feeders4.- Riser 10.- Furnaces5.- Cyclone 11.- Vaporizer6.- Diverting solids valve

AR Exhaust gas

FR Exhaust gas

H2O N2Air

Sec.Air

AR

FR

Product Gas

Flue Gas

Coal

N2 H2O N2Air

Sec.Air

AR

FR

Product Gas

Flue Gas

Coal

N2

Filter

Gas analysis

Gas analysis7

82

4

5

6

10

3

1

9

1110

1.- Fuel Reactor, FR 7.- Solids control valve2.- Loop Seal 8.- Coal3.- Air Reactor, AR 9.- Screw feeders4.- Riser 10.- Furnaces5.- Cyclone 11.- Vaporizer6.- Diverting solids valve H2O N2Air

Sec.Air

AR

FR

Product Gas

Flue Gas

Coal

N2 H2O N2Air

Sec.Air

AR

FR

Product Gas

Flue Gas

Coal

N2

Filter

Gas analysis

Gas analysis7

82

4

5

6

10

3

1

9

1110

1.- Fuel Reactor, FR 7.- Solids control valve2.- Loop Seal 8.- Coal3.- Air Reactor, AR 9.- Screw feeders4.- Riser 10.- Furnaces5.- Cyclone 11.- Vaporizer6.- Diverting solids valve

AR Exhaust gas

FR Exhaust gas

measurement of the solids flow rates at any time. The total oxygen-carrier inventory in

the system was 2.0 kg, being about 0.4 kg in the fuel-reactor.

CO, CO2, H2, CH4, and O2 were analyzed in the outlet stream from fuel-reactor,

whereas CO2, CO and O2 were analyzed from the flue gases of the air-reactor.

2.4. Experimental planning

Table 3 shows a compilation of the main variables used in each test. Totally, the same

batch of oxygen-carrier particles were used during 40 h of hot fluidization conditions,

whereof 15 h with coal combustion.

Table 3. Main Data for Experimental Tests in the CLOU Prototype

Test TFR

(ºC)

φ Fg,inFR

(LN/h)

ug,inFR

(cm/s)

smɺ

(kg/h)

coalmɺ

(g/h)

Power

(W)

mFR

(g)

*

FRm

(kg/MWth)

B01 903 1.2 186 11.3 4.2 112 681 412 605

B02 917 1.2 186 11.5 4.2 112 681 412 605

B03 941 1.2 186 11.7 4.2 112 681 373 547

B04 958 1.2 186 11.9 4.2 112 681 393 577

B05 924 4.3 186 11.5 9.0 67 408 471 1156

B06 929 3.2 186 11.6 9.0 89 541 452 835

B07 917 2.6 186 11.5 9.0 112 681 412 605

B08 920 2.1 186 11.5 9.0 135 821 373 454

B09 925 1.1 186 11.5 9.0 256 1557 368 236

Experimental test series B01-B09, pre-treated “El Cerrejón” coal was fed to the fuel-

reactor, thus analyzing the combustion efficiency of coal in the CLOU system. Here, the

fuel-reactor temperature (B01-B04) or the coal feeding rates (B06-B09) were varied.

The coal feed was varied from 67 to 256 g/h, which corresponded to a thermal power of

408 to 1560 Wth. The oxygen carrier to fuel ratio (φ) was defined by the following

equation:

CuO

coal coal

0.25F

mφ =

Ω ɺ (7)

FCuO being the molar flow rate of CuO and coalmɺ the mass-based flow of coal fed to the

reactor. Ωcoal is the stoichiometric mols of oxygen as O2 to convert 1 kg of coal to CO2

and H2O. This value was calculated from the proximate and ultimate analysis of the

coal, see Table 2, taking a value of Ωcoal = 59 mol O2 per 1 kg of coal.

A value of φ = 1 corresponds to the stoichiometric flow of CuO to fully convert coal to

CO2 and H2O throughout reaction (1-4).

The temperature in the air-reactor was maintained at around 950 ºC. Air flow into the

air-reactor was maintained constant for all tests, always remaining in excess over the

stoichiometric oxygen demanded by the fuel.

3. Results and discussion

To determine the behaviour of a CLOU system using the Cu60MgAl oxygen-carrier,

several tests under continuous operation were carried out in the experimental 1.5 kWth

rig using coal as fuel. The gas composition at the exit gases of fuel- and air-reactors was

determined. N2 instead CO2 was used as fluidizing gas in order to improve the accuracy

for calculation of C burnt in the fuel-reactor. In a previous work it was determined that

the fluidization agent does not have any influence on the oxygen-carrier behaviour [3].

Two series of experiments were carried out. The effect of the fuel-reactor temperature

(tests B01-B04) and the coal feeding flow rate (tests B05-B09) on the combustion

efficiency was evaluated.

As example, Figure 4 shows the concentration of gases (dry basis) measured as a

function of the operating time for test series B01-B04 where the fuel-reactor

temperature was varied from 903 ºC to 958 ºC. Pre-treated “El Cerrejón” coal was used

as fuel. The solids circulation rate was maintained at a mean value of 4.2 kg/h, whereas

the coal feeding rate was 0.112 kg/h. The oxygen-carrier to fuel ratio, φ, was 1.2, as

defined by equation (7). At steady state, the gas outlet concentration and temperature

were maintained uniform during the whole combustion time. Each experimental was

maintained at steady state at least for 30 min. When temperature was varied, a transition

period appeared and stable combustion was reached usually in less than 10 min.

In all cases, CH4, CO or H2 were not detected in the gases exiting from the fuel-reactor.

The possible presence of tars or light hydrocarbons was also analyzed. For one

experiment with conditions kept constant for longer times that two hours, tar

measurements in the fuel-reactor were done using tar protocol. The results showed that

there were not tars in the fuel-reactor outlet flow, that is, no hydrocarbons heavier than

C5. In addition, in some selected experiments, gas from the outlet stream was collected

in bags and analysed with a gas chromatograph (GC). The analysis proved that there

were not C2-C4 hydrocarbons in the gases. Thus, CO2, H2O and O2 were the only gases,

together N2 introduced as fluidizing gas. Also, small fractions of SO2 and NO were

present in the gases coming from sulphur and nitrogen present in the coal. However,

these components were not evaluated in this work.

Tem

pera

ture

(ºC

)

860

880

900

920

940

960

980

CO

2 or

O2

(vol

.% )

0

10

20

30

40

50

time (min)0 50 100 150 200 250 300

860

880

900

920

940

0

10

20

FR

AR

CO2

O2

T

O2

CO2

T

Figure 4. Evolution of the gas composition in the air- and fuel-reactor as temperature in

the fuel-reactor was varied. Experimental tests B01-B04. smɺ = 4.2 kg/h;

coalmɺ = 0.112

kg/h.

Therefore, volatiles were fully converted into CO2 and H2O in the fuel-reactor by

reaction with the oxygen released from the CuO decomposition. In addition, the oxygen

release rate was high enough to supply an excess of gaseous oxygen (O2) exiting

together the combustion gases.

Figure 5(a) and (b) show the O2 and CO2 concentration (dry basis) exiting from the air-

and fuel-reactors as a function of the fuel-reactor temperature and the coal feeding rate,

respectively.

The effect of the fuel-reactor temperature on the CO2 and O2 concentration is Fig. 5(a).

On the one hand, both O2 and CO2 concentrations from the fuel-reactor increased with

the temperature. However, the oxygen concentration was slightly lower than the

equilibrium concentration, likely due to O2 reaction in the freeboard with char particles

or a little fraction of un-burnt gases. This fact was in contrast to the equilibrium

concentration observed when coal was not fed to the reactor. On the other hand, CO2

concentration in the air-reactor decreased as the fuel-reactor temperature increased. This

fact indicates that the amount of char passed to the air-reactor decreased with the fuel-

reactor temperature because of a higher rate of char combustion in this reactor.

However, the oxygen concentration in the air-reactor was maintained roughly constant.

This fact is evident if it is considered that the oxygen reacted in the CLOU system is

used to burn the coal, either in the fuel-reactor or air-reactor, and the oxygen demanded

by coal is constant.

Temperature (ºC)

900 920 940 960

O2

and

CO

2 (v

ol.%

dry

)

0

10

20

30

40

50

60

70

Coal feeding rate (g/h)

0 50 100 150 200 250

(a) (b)

Figure 5. CO2 and O2 concentration from the fuel- and air-reactor obtained at (a)

different fuel-reactor temperatures; and (b) coal feeding rates. Fuel-reactor: CO2 ( )

and O2 ( ). Air-reactor: CO2 ( ) and O2 ( ). Oxygen concentration at

equilibrium in the fuel-reactor ( ).

The effect of the coal feeding rate is evident on the concentration of gases, see Figure

5(b). In this case it is observed that CO2 concentration increases in both air- and fuel-

reactors with the coal feeding rate, and the oxygen concentration in the air-reactor

decreases correspondingly to the increase in coal feeding. The oxygen concentration in

the fuel-reactor slightly decreases as the coal feeding rate increases. This effect is not

due to that the oxygen-carrier was constrained to supply the oxygen needed, which

should be observed as a sharply decrease in the oxygen concentration. Likely, the slight

decrease in the oxygen concentration was due to a higher amount of released oxygen

reacting in the freeboard with char or a little fraction of un-burnt gases.

To analyze the confidence of the results, a mass balance to oxygen and carbon was

carried out using the measurements of the analyzers from the air- and fuel- reactors. The

dry basis product gas flow in the fuel-reactor, FoutFR, was calculated as

2

inFRoutFR

CO ,outFR

FF

1 y=

− (8)

2CO ,outFRy being the CO2 fraction in dry basis exiting from the fuel-reactor. Notice that

CO2 and H2O were the only combustion products in the gas stream and gas

concentration were dry basis.

The outlet air-reactor gas flow, FoutAR, was calculated through the introduced N2, 2

N ,ARF .

( )2

2 2

N ,AR

outAR

O ,outAR CO ,outAR

FF

1 y y=

− + (9)

Thus, the exiting flows of O2 and CO2 from the air- and fuel-reactors can be easily

calculated using the actual concentration of every gas i.

i,out i,out outF y F= (10)

Notice that nitrogen is used as fluidizing agent in fuel-reactor, thus CO2 comes uniquely

from the coal combustion. The mass balances to O2 and CO2 were found to be accurate

by using the measurements of the analyzers from the air- and fuel- reactors.

Figures 6(a) and (b) show the molar flow of CO2 exiting from the air- and fuel-reactor

as a function of the fuel-reactor temperature and coal feeding, respectively. The

variation of solids conversion is also shown in theses Figures. Figure 6(a) shows an

increase in the fuel-reactor temperature produced an increase in the CO2 and O2 flow at

the outlet of the fuel-reactor. In that order, the CO2 flow from the air-reactor decreased

with the fuel-reactor temperature because more char is burning in the fuel-reactor. There

is a lower char concentration in fuel-reactor due to the higher char combustion rates and

a lower amount of coal is transferred to the air-reactor. Moreover, as more char is

burning in the fuel-reactor, the variation of solids conversion increases slightly.

Temperature (ºC)

900 920 940 960

CO

2 flo

w fr

om F

R (

mol

/h)

0

5

10

15

Coal feeding rate (g/h)

0 50 100 150 200 250

Sol

id C

onve

rsio

n

0.0

0.2

0.4

0.6

0.8

1.0(a) (b)

Figure 6. Flow of CO2 exiting from the fuel-reactor ( ) and variation of the solids

conversion ( ) at (a) different fuel-reactor temperatures; and (b) coal feeding rates.

In the second series, it was studied the effect of the coal feeding rate on the process

performance, which was changed from 0.067 and 0.256 kg/h (408-1557 Wth). Figure

6(b) shows the CO2 molar flow from the fuel-reactor and the oxygen-carrier conversion.

In all cases the ratio of oxygen-carrier to fuel is above the stoichiometric value. At these

conditions, there was an excess of oxygen in the circulating solids and the coal

combustion was not limited by the availability of reactant. As expected, the CO2 flow

from fuel-reactor increased with the coal feeding rate because more fuel is burnt. Also,

more oxygen is transferred between the two reactors, as is indicated by the increase of

the variation of solids conversion.

The CO2 flow from the air-reactor also increases because a higher amount of char is

transferred to the air-reactor when the load of coal is increased. In addition, the oxygen

flow from the fuel-reactor slightly decreases with the coal feeding rate. The lower

values calculated for the oxygen flow at higher coal feeding rates in our unit are related

to the relatively higher amounts of char present into the freeboard. It is presumable that

there was a partial oxidation of the char present in the freeboard, where lesser fraction

of the denser oxygen-carrier particles can be thrown. Nevertheless, the elutriation of

char particles from the fuel-reactor was negligible regarding the carbon balance in the

system.

The highest coal feeding rate tested was 0.256 kg/h (test B09). The solids inventory in

the fuel-reactor at this condition was 236 kg/MWth. However, this experiment must be

stopped only few minutes after it started. At this condition, oxygen-carrier particles

were elutriated from the fuel-reactor because the high amount of gases generated in the

bed. Nevertheless, during the short time of operation full conversion of coal to CO2 was

observed, unburnt gases were not detected and there was O2 present. Unfortunately, the

growth up of gas velocity in the bed prevented to work with larger coal feeding rates,

although the Cu60MgAl oxygen-carrier would be able to supply the oxygen demanded

by coal. Thus, a lower inventory of solids would be used and full conversion of fuel be

still obtained. A possible solution to this problem could be to design the fuel-reactor as

a circulating fluidized bed itself.

Figure 7 shows the rate of oxygen transferred by Cu60MgAl oxygen-carrier as a

function of the coal feeding rate, corresponding to the flows of CO2, H2O and O2 exiting

from the fuel-reactor. It can be seen that the oxygen transferred increases proportionally

to the coal feeding rate increase. This confirms the statement made that in this system

the oxygen evolved in the fuel-reactor is not limited by the reaction rate of oxygen-

carrier, but for the demand of oxygen by coal.

Coal feeding rate (g/h)

50 100 150 200 250 300

(-r O

2)(kg

O2/h

per

kg

of O

C)

200

400

600

800

1000

1200

1400

Figure 7. Rate of oxygen transferred by Cu60MgAl oxygen-carrier as a function of the

coal feeding rate.

As it was mentioned above, the fuel was fully converted into CO2 and H2O either in the

fuel-reactor or the air-reactor. However, the efficiency of char combustion in the fuel-

reactor has influence on the efficiency of the carbon capture in a CLOU system. From

the results shown in Figure 6, the conversion of char in the fuel-reactor, Xchar, can be

calculated as:

2C CO ,outFR C,vol

char

C,fix coal

M F mX

w m

−=

ɺ

ɺ (11)

wC,fix being the carbon fraction as fixed carbon in the coal, and C,volmɺ the mass flow of

carbon containing the volatile matter calculated as

( )C,vol C,coal C,fix coalm w w m= −ɺ ɺ (12)

wC,coal being the carbon fraction in coal. In addition, the carbon capture efficiency, 2

COη ,

was defined as the fraction of carbon initially present in the coal fed in which is actually

at the outlet of fuel-reactor as CO2. This is the actual CO2 captured in the CLOU

system, the rest is exiting together nitrogen from the air-reactor.

2

2

C CO ,outFR

CO

C,coal coal

M Fη

w m=

ɺ (13)

Figures 8(a) and (b) show the char conversion and the carbon capture efficiency as a

function of the fuel-reactor temperature and the coal feeding, respectively.

Temperature (ºC)

900 920 940 960

Effi

cien

cy o

f car

bon

capt

ure

0.90

0.92

0.94

0.96

0.98

1.00

Coal feeding rate (g/h)

0 50 100 150 200 250

Cha

r co

nver

sion

0.90

0.92

0.94

0.96

0.98

1.00(a) (b)

Figure 8. Char conversion ( ) and carbon capture efficiency ( ) at (a) different

fuel-reactor temperatures; and (b) coal feeding rates.

It can be seen that high values of char conversion and efficiency of CO2 capture were

obtained in all cases. It is noteworthy the positive effect of fuel-reactor temperature on

the CO2 capture efficiency, see Figure 8(a). Thus, when the fuel-reactor temperature

was 960 ºC, 99 % of carbon in coal is captured, i.e. only 1 % of carbon is exiting

together the nitrogen from the air-reactor.

The fuel load had not relevant effect on the CO2 capture efficiency, see Figure 8(b).

Since the circulation rate and the temperature are kept constant and there is oxygen

excess in all cases, the resulting CO2 capture efficiency for different coal feeding rates

does not change substantially. Thus, at the conditions used in the CLOU system, the

oxygen generated in the fuel-reactor was not limited by the reactivity of this oxygen-

carrier, i.e. the more oxygen is demanded, more oxygen is supplied.

4. Conclusions

Combustion of coal in the CLOU system was carried out during 15 h. In all cases,

unburnt compounds were not present in the fuel-reactor outlet, CO2, H2O and O2 being

the only products of reactions. Thus, complete combustion of coal was observed,

happening either in the fuel-reactor or in the air-reactor. The carbon capture efficiency

depended on the amount of char passed to the air-reactor, but in most of cases was

higher than 97 %.

The oxygen concentration from the fuel-reactor increased with the temperature. Thus, at

higher fuel-reactor temperatures, the capability of Cu60MgAl particles to release

oxygen is enhanced because the increase of the oxygen concentration at equilibrium

conditions. In addition, the char combustion is improved at higher temperatures,

reaching a conversion of 99 % at 960 ºC. At this condition, the carbon capture

efficiency showed a value as high as 99.3 %, and the presence of a carbon separation

system, e.g. the carbon stripper, could be avoided.

The maximum capability of the oxygen-carrier to produce oxygen was not reached

during the experimental tests. A maximum coal feeding rate of 256 g/h was reached,

corresponding to a power of 1560 W and a solids inventory of 236 kg/MWth in the fuel-

reactor. At this condition, CO2 and O2 remained as the only products from the fuel-

reactor, thus indicating that the oxygen-carrier could burn a higher supply of coal and at

lower solid inventories.

The results obtained in this work showed that the use of the Cu60MgAl oxygen-carrier

is suitable for the coal combustion by CLOU process.

5. REFERENCES

[1] Mattisson T, Lyngfelt A, Leion H. Chemical-looping oxygen uncoupling for

combustion of solid fuels. Int J Greenhouse Gas Control 2009;3:11-9.

[2] HSC Chemistry 6.1 2008. Chemical Reaction and Equilibrium Software with

Thermochemical Database and Simulation Module. Oututec Research Oy.

[3] Adánez-Rubio, I.; Gayán, P.; García-Labiano, F.; de Diego, L.F.; Adánez, J.; Abad,

A. Development of CuO-based oxygen-carrier materials suitable for Chemical-Looping

with Oxygen Uncoupling (CLOU) process. Energy Procedia 2010, in press.

Acknowledgement

This work was partially supported by the European Commission, under the RFCS

program (ECLAIR Project, Contract RFCP-CT-2008-0008), ALSTOM Power Boilers

(France) and by the Spanish Ministry of Science and Innovation (PN, ENE2010-19550).

I. Adánez-Rubio thanks CSIC for the JAE fellowship.