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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.
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