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Syngas combustion in a Chemical-Looping Combustion system using an impregnated Ni-based oxygen carrier
Cristina Dueso, Francisco García-Labiano *, Juan Adánez, Luis F. de Diego, Pilar
Gayán, Alberto Abad
Department of Energy and Environment, Instituto de Carboquímica (C.S.I.C.)
Miguel Luesma Castán 4, 50018 Zaragoza, Spain
Phone number: +34 976 733 977
Fax number: +34 976 733 318
E-mail:
RECEIVED DATE:
Corresponding Author. Tel.: +34-976-733977; fax: +34-976-733318; E-mail address:
glabiano@icb.csic.es (F. García-Labiano)
Abstract
Greenhouse gas emissions, especially CO2, formed by combustion of fossil fuels, highly
contribute to the global warming problem. Chemical-Looping Combustion (CLC) has
emerged as a promising option for CO2 capture because this gas is inherently separated
from the other flue gas components and thus no energy is expended for the separation.
This technology would have some advantages if it could be adapted for its use with coal
as fuel. In this sense, a process integrated by coal gasification and CLC could be used in
power plants with low energy penalty for CO2 capture. This work presents the
combustion results obtained with a Ni-based oxygen carrier prepared by impregnation
in a CLC plant under continuous operation using syngas as fuel. The effect on the
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oxygen carrier behaviour and the combustion efficiency of several operating conditions
was determined in a continuous CLC plant. High combustion efficiencies (~99%), close
to the values limited by thermodynamic, were reached at oxygen carrier-to-fuel ratios
higher than 5. The temperature in the FR has a significant influence, although high
efficiencies were obtained even at 1073 K. The syngas composition has small effect on
combustion, obtaining high and similar efficiencies with syngas fuels of different
composition, even in presence of high CO concentrations. The low reactivity of the
oxygen carrier with CO seems to indicate that the water gas shift reaction acts an
intermediate step in the global reaction of the syngas in a continuous CLC plant. No
agglomeration or carbon deposition problems were detected during 50 hours of
continuous operation in the prototype. The obtained results showed that the impregnated
Ni-based oxygen carrier could be used in a CLC plant for syngas combustion produced
in an Integrated Gasification Combined Cycle (IGCC).
Keywords: CO2 capture, chemical-looping combustion, nickel, oxygen carrier, syngas
1. Introduction
Carbon dioxide emissions, mainly formed by the combustion of fossil fuels for power
generation, transport and industry, contribute significantly to the global warming [1, 2].
This has led to a need to reduce CO2 emissions to the atmosphere in an attempt to
mitigate the increasing temperatures of the earth. It is widely accepted that CO2 Capture
and Storage (CCS) is one of the key solutions for combating climate change still using
fossil fuels while building a bridge to a truly sustainable energy system. CO2 separation
can be made by a number of different available or currently under development
techniques, i.e. pre-combustion, post-combustion and oxyfuel technologies [2].
Nevertheless, most of these processes have the disadvantage of the large amount of
energy required, reducing the overall efficiency of a power plant and increasing the cost
of the energy production.
Chemical-Looping Combustion (CLC) has been suggested as one of the most promising
technologies to reduce the cost of the CO2 capture using a fuel gas because the
separation is inherent to the process [3]. This new kind of combustion, initially
proposed by Ritcher and Knoche [4], also improves the thermal efficiency as much as
52-53% in a combined cycle CLC plant operating at 1450 K in the air reactor and 1.3
MPa [5]. Another important advantage for CLC is the absence of NOx [6], as a result of
the introduction of fuel and air into different reactors and the moderate AR operating
temperature.
CLC is a two-step gas combustion process that produces a pure CO2 stream, ready for
compression and sequestration. A solid oxygen carrier (OC), in the form of metal oxide
particles, transports the oxygen from the combustion air to the fuel. Since the fuel is not
mixed with air, the subsequent CO2 separation process is not necessary. Although other
designs are possible, the CLC system is usually composed of two reactors, the air and
the fuel reactor and a high velocity riser with the solid circulating between them [7].
The fuel gas is introduced to the FR, where it is oxidized by the OC following the
general reaction for syngas:
(n+m) MexOy + n CO + m H2 (n+m) MexOy-1 + n CO2 + m H2O (1)
where MexOy denotes a metal oxide and MexOy-1 its reduced compound. The exit gas
stream from the FR contains only CO2 and H2O, and almost pure CO2 is obtained after
H2O condensation.
The reduced particles of the OC are transferred to the AR where they are regenerated by
taking up the oxygen from the air:
MexOy-1 + ½ O2 MexOy (2)
The exit gas stream from the AR is formed by N2 and some unreacted O2. The oxidized
particles are returned to the FR to start a new cycle. In this system, the total amount of
heat evolved from reactions in the two reactors is the same as for normal combustion,
where the oxygen is in direct contact with the fuel. It must be remarked that, for syngas,
both the reduction and the oxidation are exothermic for all the metal oxides investigated
in the literature.
Natural gas [7-9], refinery and industrial gases [10, 11], synthesis gas from coal
gasification [12-14], and coal [15, 16] have been considered to be used as fuels in a
CLC system. The use of syngas from coal gasification in a CLC process could be
advantageous still using solid fossil fuels [17]. Simulations made by Wolf et al. [18] and
Jin and Ishida [19] showed that an integrated IGCC-CLC process has the potential to
achieve efficiency 5-10% higher than a similar integrated gasification with combined
cycle (IGCC) process that uses conventional CO2 capture technology. To get these
benefits, the CLC system must operate at pressurized conditions and at high
temperatures, e.g. 1-2 MPa and 1400 K.
A key issue for the CLC technology development is the selection of an OC with suitable
properties, such as high oxygen transport capacity, high reactivity under alternating
reducing and oxidizing conditions, full conversion to CO2 and H2O, low tendency to
carbon deposition, avoidance of agglomeration and high mechanical and chemical
stability for thousands of cycles in a fluidized bed system. Fe-, Ni-, Cu-, Co- and Mn-
based OCs supported on different inert materials, such as Al2O3, SiO2, TiO2 or yttrium
stabilized zirconia (YSZ) have been studied to be used in a CLC process for syngas
combustion [13-14, 20-26]. Nickel materials have received more attention due to its
higher reactivity and thermal stability. Wang et al. [27] studied the carbon formation
and the effect of the presence of sulphur in the syngas, and they found that the
pressurized conditions led to more solid carbon and sulphur species deposited on the
OC. Also, some experimental data at pressurized conditions can be found. To
investigate the reactivity of potential OC particles, the materials are exposed alternately
to air and fuel, using H2, CO or syngas as fuel. Siriwardane et al. [28] studied the
behaviour of a Ni-based oxygen carrier (60 wt% NiO) supported on bentonite in a high-
pressure flow reactor at temperatures between 973 and 1173 K. García-Labiano et al.
[29] determined the effect of pressure on the reduction and oxidation reactions of three
oxygen carriers based on Fe, Ni and Cu in a pressurised thermogravimetric analyser
(TGA). Both works showed that the pressure had a positive effect on the reaction rates,
although the reactivity increase was not as high as expected.
The CLC process has been developed until now for the use with gaseous fuels and it has
been successfully demonstrated in two 10 kW units for 260 h with a Ni-based OC [30,
31] and during 200 h using a Cu-OC [32]. Moreover, Ryu et al. [33] presented the
results of 3.5 h of continuous run in a 50 kW Chemical-Looping combustor. In all cases
methane was used as fuel. However, only a few studies at lower scales have been
reported using syngas as fuel in continuous CLC systems. Johansson et al. [20] carried
out different tests using syngas and natural gas as fuel in a continuous 300 W CLC
system with a Ni-based OC, with very high conversion of the fuel and, therefore, high
combustion efficiencies. The same facility was used by Abad et al. [21, 22] to perform
experiments using a Mn-based and a Fe-based oxygen carrier and syngas as fuel for 70
hours with efficiencies higher than 0.999 at temperatures in the range 1073-1223 K. All
these works are limited at atmospheric pressure, due to the trouble working at high
pressures in continuous CLC systems at laboratory-scale.
The aim of this work was to test the behaviour of a Ni-based OC prepared by
impregnation on -Al2O3 when syngas is used as fuel in a CLC plant under continuous
operation at atmospheric pressure. The influence of several operating conditions, such
as the fuel reactor temperature, the oxygen carrier-to-fuel ratio and the CO/H2 ratio in
the syngas fuel, was analyzed. Tests on TGA and in a batch fluidized bed reactor were
also used to better understand the results obtained in the continuous plant.
2. Experimental section
2.1. Oxygen carrier
A nickel-based oxygen carrier was prepared by the hot incipient wet impregnation
(HIWI) method on -Al2O3 to be used in this work [34]. Previous studies showed that
the highest reactivity of the Ni-based OC using Al2O3 as support is achieved when this
inert phase is in the form of -Al2O3 [35]. Commercial -Al2O3 (Puralox NWa-155,
Sasol Germany GmbH) particles of 100-300 m were calcined during 2 hours at 1423 K
to obtain -Al2O3 ( = 1900 kg/m3, = 48.5%). The HIWI method involves the addition
of a volume of a saturated solution (6 M) of Ni(NO3)2·6H2O (> 99.5% Panreac) at 333-
353 K over hot -Al2O3 particles (353 K), which corresponds to the total pore volume
of the particles. A planetary mixer was used to stir thoroughly the aqueous solution and
the solid. Two successive impregnation steps were applied to obtain the desired active
phase loading (18 wt%). The resulting material was calcined at 823 K in air atmosphere
for 30 minutes to decompose the impregnated metal nitrate into the metal oxide. Finally,
the oxygen carrier was sintered in a furnace at 1223 K for 1 hour. The main
characteristics of this material are shown in Table 1. Here, the oxygen transport capacity
was defined as the mass fraction of oxygen that can be used in the oxygen transfer,
calculated as ROC = (mox – mred) / mox, where mox and mred are the masses of the
oxidized and reduced form of the oxygen carrier, respectively.
2.2. Batch fluidized bed reactor
Reduction-oxidation multi-cycles in a batch fluidized bed (FB) reactor allow to
determine the gas product distribution obtained for a given OC at different operating
conditions. These tests also provide information about other processes such as attrition,
agglomeration tendency and carbon deposition.
The experimental set-up, shown elsewhere [36], consisted of a system for gas feeding, a
fluidized bed reactor (FB), 54 mm D.I. and 500 mm height , two filters to recover the
solids elutriated from the FB, and the gas analysis system. The total solid inventory in
the reactor was 300 g. The gas feeding system had different mass flow controllers
connected to an automatic three-way valve. This allowed the feeding of the syngas
during the reduction period and air for the oxidation of the OC. An inert period with
nitrogen was used between the two stages to avoid the mixing of the fuel and the
oxygen. The entire system was inside an electrically heated furnace. The reactor had
two connected pressure taps in order to measure the differential pressure drop in the
bed. Agglomeration problems could be detected by a sharp decrease in the bed pressure
drop during operation, causing defluidization of the bed. Carbon deposition can be
detected through the presence of CO and CO2 in the flue gases during the oxidation
period.
Different on-line gas analyzers measured continuously the gas composition. CO and
CO2 were determined in a non-dispersive infrared (NDIR) analyzer (Maihak S710 /
UNOR), the O2 in a paramagnetic analyzer (Maihak S710 / OXOR-P), and the H2 by a
gas conductivity detector (Maihak S710 / THERMOR). All data were collected by
means of a data logger connected to a computer. To improve data analysis, the gas flow
dispersion through the sampling line was corrected for all the measured gas
concentrations.
2.3. Continuous CLC plant
To simulate the behaviour of the oxygen carrier in a chemical-looping system similar to
the designs proposed for full-scale systems [7], a continuous CLC plant was used (Fig.
1). The plant was composed of two interconnected fluidized-bed reactors, a riser for
solid transport, a solid valve to control the solids fed to the reactor, a loop seal and a
cyclone. This design allowed the variation and control of the solid circulation flow rate
between both reactors. The FR (1) consisted in a bubbling fluidized bed (0.05 m i.d.)
with a bed height of 0.1 m. In this reactor the fuel gas reacts with the oxygen carrier to
form CO2 and H2O. Reduced oxygen carrier particles overflowed into the AR (3),
through a U-shaped fluidized loop seal (2), which avoid gas mixing between fuel and
air. The oxidation of the carrier took place at the AR, a bubbling fluidized bed (0.05 m
i.d.) with a bed height of 0.1 m. It was followed by a riser (4) of 0.02 m i.d. and 1 m
height. The regeneration of the oxygen carrier happened in the dense bed part of the AR
allowing residence times high enough for the complete oxidation of the reduced carrier.
Secondary air could be introduced at the top of the bubbling bed to help particle
entrainment. N2 and unreacted O2 left the AR passing through a high-efficiency cyclone
(5) and a filter (9) before the stack. The recovered solid particles by the cyclone were
sent to a solid reservoir setting the oxygen carrier ready to start a new cycle and
avoiding the mixing of fuel and air out of the riser. The regenerated oxygen carrier
particles returned to the FR by gravity from the solid reservoir located above a solids
valve (7) which controlled the flow rates of solid entering the FR. A diverting solids
valve (6) located below the cyclone allowed the measurement of the solid flow rates at
any time. Fine particles produced by fragmentation/attrition in the plant were recovered
in the cyclones and the filters that were placed downstream of the FR and AR.
The prototype had several tools of measurement and system control. Thermocouples
and pressure drop transducers located at different points of the plant showed the current
operating conditions at any time. Specific mass flow controllers gave accurate flow
rates of feeding gases. The gas outlet streams of the FR and AR were drawn to
respective on-line gas analyzers to get continuous data of the gas composition. CO, H2,
and CO2 concentrations in the gas outlet stream from the FR were measured after steam
condensation. O2, CO, and CO2 concentrations were obtained at the gas outlet stream
from the AR. The gas analysis system was the same as the one showed for the batch FB
experiments.
The two parameters that allow knowing the behaviour of the OC in the continuous CLC
plant are the combustion efficiency and the oxygen carrier-to fuel ratio. The combustion
efficiency (c) has been defined as the ratio of oxygen consumed by the gas leaving the
FR to that consumed by the gas when the fuel is completely burnt to CO2 and H2O. So,
the c gives an idea about how the CLC operation is close or far from the full
combustion of the fuel, i.e. c = 100%.
F)xx(F)xx2(F)xxx2(
inHCO
inCOCOoutOHCOCO
c
2
222
(3)
where Fin is the molar flow of the inlet gas stream, Fout is the molar flow of the outlet gas
stream, and xi is the molar fraction of the gas j.
The oxygen carrier-to fuel ratio () was defined by equation 4, where FMeO is the molar
flow rate of the metal oxide and FFuel is the inlet molar flow rate of the fuel in the FR. A
value of = 1 corresponds to the stoichiometric MeO amount needed for a full
conversion of the fuel to CO2 and H2O:
F
F
Fuel
MeO (4)
2.4. Reactivity tests in TGA
Reactivity tests of solid samples taken after operation in the continuous CLC plant were
carried out in a TGA. Detailed information about this instrument and the operating
procedure were described elsewhere [37]. The sample was exposed to alternating
reducing and oxidizing conditions during five cycles. The OC reactivity corresponding
to the first cycle was used for comparison purposes. The reactivity of the oxygen carrier
with H2, CO and two different mixtures CO/H2 simulating syngas was determined at
1223 K. Table 2 shows the composition of the reacting gas used in each experiment.
CO2 was fed together with the CO to avoid the carbon formation through the Boudouard
reaction.
The composition of the gases for the different CO/H2 ratio mixtures was selected to
fulfil the Water Gas Shift (WGS) equilibrium at the operation temperature.
CO + H2O CO2 + H2 (5)
3. Results and Discussion
3.1. Tests in the Batch Fluidized Bed Reactor
Reduction-oxidation multicycles in a batch FB reactor were carried out to determine the
gas product distribution during the reaction of the oxygen carrier with syngas in similar
operating conditions to a CLC system. This kind of experiments also allows analyzing
the fluidization behaviour of the OC with respect to the attrition, agglomeration and
carbon formation.
H2, CO, and syngas with different CO/H2 ratio have been used as fuels in the FB
reactor, with a total number of cycles over 50 with the same oxygen carrier batch. The
experiments were performed at 1223 K with an inlet superficial gas velocity of 0.1 m/s.
The reduction time was fixed at 900 seconds in all cases, and the oxidation time
necessary for complete oxidation varied between 1000 and 1200 seconds. The total
solids inventory in the reactor was 300 g.
As an example, Fig. 2 shows the gas product distribution obtained in the experiment in
the batch FB reactor when the fuel gas was composed by 15 % H2, 10 % CO and 10 %
CO2. Full combustion of the combustible gases was obtained initially. For the first 200
seconds of the reduction, the oxygen carrier was selective to the formation of CO2 and
H2O. CO and H2 concentrations were those corresponding to the thermodynamic
equilibrium at the reaction temperature. When the amount of available oxygen in the
system decreased, the concentration of CO and H2 started increasing. This corresponded
to a solid conversion about 40 %.
During the period of full combustion, no evidences of carbon formation were observed
from mass balances to carbon and hydrogen atoms. Nevertheless, CO2 was detected
during the oxidation period by the combustion of the carbon generated in the previous
reduction period. This carbon was produced when the OC started to loose its reactivity,
and the CO and H2 concentrations increased.
The length of the selective period to the formation of CO2 and H2O will be important in
the later operation in a continuous CLC system. If the residence time in the FR of the
CLC plant is within this range, the combustion of the fuel will be complete and carbon
deposition would be avoided. It must be remarked that agglomeration problems were
never detected during operation in the batch FB, even in those cases with high carbon
formation.
The attrition rate of the OC is another important parameter to take into account as a
criterion for using a specific OC in a FB reactor, because a high attrition rate will
decrease the lifetime of the particles increasing the cost of the process. Attrition data
obtained during these tests were similar to that obtained by Gayán et al. [34] for an
impregnated OC using CH4 as fuel gas. After 20 cycles, the attrition rates decreased and
were almost constant, with a weight loss about 0.01% per cycle.
3.2. Tests in the continuous CLC plant
To determine the behaviour of the Ni-based oxygen carrier during the syngas
combustion, several tests under continuous operation were carried out in the CLC plant,
using different temperatures and fuel gas compositions.
Table 3 shows the gas compositions used during the experimental tests. The solid
circulation flow rate, fs, was varied between 3 and 14 kg/h. The steady-state for each
operating condition was maintained at least for 60 minutes. The oxygen carrier particles
were fluidized in hot conditions with fuel fed for approximately 50 hours. No
agglomeration or any other type of operational problems was detected during this
experimental time.
As an example, Fig. 3 shows the temperature profiles and the gas product distribution in
the FR and AR using syngas as fuel (CO/H2= 1). The temperatures in the FR and AR
were 1153 and 1223 K, respectively. The outlet gas concentrations and the temperatures
were maintained uniform during the whole combustion time. The CO2 concentration
was a bit lower than the theoretical values for the full conversion of the syngas as a
consequence of the dilution with the N2 coming from the loop seal and the small
concentrations of CO and H2 at the outlet. The combustion efficiency was high, 98.3%,
quite close to the equilibrium value, 99.3%. It must be considered that thermodynamical
restrictions using Ni-based oxygen carriers prevent the full conversion of the fuel to
CO2 and H2O.
Carbon formation was evaluated by measuring the CO and CO2 concentrations in the
outlet of the AR. These gases were never detected in the AR which indicated the
absence of carbon deposition in the FR. Thus, no losses in CO2 capture were produced
by carbon transfer to the AR.
3.2.1. Effect of temperature
Fig. 4 shows the effect of the oxygen carrier-to-fuel ratio, on the combustion
efficiency at two FR temperatures (1073 and 1153 K) and a syngas composed by a
CO/H2 ratio equals to 1. High combustion efficiencies, close to the limit given by
thermodynamic, were obtained at both temperatures working at values above 5.
However, the decrease of efficiency at lower values was more significant at 1073 K
than at 1153 K. This is due to the lower reaction rate obtained at lower temperatures as a
consequence of the dependence of the kinetic constants with temperature. Activation
energies values ranging from 14 to 33 kJ/mol have been reported by Abad et al [14] for
the reduction reactions with H2 and CO of several Cu-, Ni-, and Fe-based oxygen
carriers.
3.2.2. Effect of syngas composition
The syngas composition (CO, H2, CO2, and H2O) depends on the gasifier type. In this
work, the effect of the composition of the syngas on the combustion efficiency has been
studied for two different CO/H2 ratios, 1 and 3, corresponding to typical gas
compositions obtained in fluidized-bed and entrained bed gasifiers, respectively. For
comparison purposes, experiments using only H2 or CO as fuel gas were also done. The
gas compositions used in the experiments are shown in Table 3. The oxygen demand in
the FR to get complete combustion of fuel gas was the same in all tests. CO2 was
introduced to avoid carbon formation in the FR through the Boudouard reaction, and the
WGS equilibrium was assumed to be reached in the FR.
Fig. 5 shows the combustion efficiency obtained in the continuous plant for the different
fuel gases above mentioned. The highest efficiencies were obtained for H2 and the
lowest for the CO. However, it is remarkable the small differences obtained for the two
syngas compositions, even considering the high CO concentration existent in the fuel
gas with CO/H2 ratio of 3.
To determine the causes of the results obtained in the CLC pilot plant, some deep
investigations were carried out. Solid samples taken after experiments in the plant were
used in the TGA to determine their reactivity with respect different fuel gases. Fig. 6
shows the reactivity obtained during the first reduction cycle using H2, CO and two
syngas mixtures (see Table 2) as fuel. Two different zones can be distinguished. The
first stage corresponds to the fast reduction of the free NiO phase and the second one is
the slow reduction of NiAl2O4 [38]. For the fresh OC particles, the relative amount of
free NiO with respect to the total Ni in the particle was about 65 %. However, this
percentage decreases after continuous operation because a part of the reduced Ni from
the FR is converted into NiAl2O4 during the solid oxidation in the AR. Adánez et al.
[38] determined that about 75 % of reduced Ni is oxidized into free NiO, whereas the
rest is transformed into NiAl2O4, being both compounds active to transfer oxygen in a
continuous CLC system, but with very different reactivity. The NiO/NiAl2O4 fraction
obtained depends on the oxidation temperature and the NiO content of the OC.
The reactivity of the OC during the first stage of the reduction, i.e. the reaction with the
free NiO phase, was high with all the fuels although some differences depending on the
gas were observed. These results agree with the obtained by Abad et al. [14], who found
the highest reactivites working with H2 and the lowest for CO. Nevertheless, during the
second stage of the reduction, corresponding to the NiAl2O4 reaction with the syngas,
important differences depending on the fuel gas was observed. The highest reactivities
were observed for H2, and this decreased as CO content increased. Therefore, CO has an
inhibiting effect on the OC reactivity and the conversion variation was almost negligible
when CO was the unique fuel gas. Therefore, the low combustion efficiencies for the
CO in the CLC pilot plant can be explained by the low reactivity of the OC with this
gas, especially during the reaction of the NiAl2O4.
The slow reaction of the OC with the CO detected in the TGA seems to indicate that
other mechanism is the responsible of the CO reaction in a continuous CLC plant using
syngas as fuel. Fig. 7 shows the CO and H2 concentrations obtained at the outlet of the
FR for the experiments carried out with syngas. It was observed that H2 concentration
was higher than CO for the experiments carried out with CO/H2 ratio of 1. On the
contrary, the CO concentration was higher than H2 for the experiments carried out with
CO/H2 ratio of 3. In all cases, the flue gases almost fulfilled the WGS equilibrium at the
operating temperature. Previous works have proved that, for a Ni-based oxygen carrier,
the reaction rate of H2-CO mixtures corresponds to that of the gas reacting faster, that is
the H2 [14]. The faster disappearance of the H2 by the reaction with the OC shifts the
WGS equilibrium towards the formation of more H2 and CO2, also increasing the CO
consumption. This suggests that this gas phase reaction acts as an intermediate step in
the whole reaction of the OC with the syngas fuel.
On the other hand, the linked action of gas-solid and WGS reactions could explain the
similar efficiencies obtained during syngas combustion even for very different gas
compositions and in presence of high concentrations of the low reactive CO.
The results obtained in this work in the continuous CLC plant showed that the Ni-based
material prepared by impregnation could be and adequate oxygen carrier to be used in a
CLC plant for syngas combustion.
4. Conclusions
The behaviour of a Ni-based oxygen carrier (18 wt% NiO) prepared by impregnation on
-Al2O3 has been studied in a continuous CLC pilot plant using syngas as fuel.
Combustion tests were performed to analyze the effect of the main operating conditions,
such as the fuel reactor temperature, the oxygen carrier-to-fuel ratio and the syngas
composition.
The fuel reactor temperature and the oxygen carrier-to-fuel ratio, , had a great
influence on the combustion efficiency of the process. High combustion efficiencies,
~99%, were reached for values above 5 even at temperatures as low as 1073 K.
The effect of the syngas composition was analyzed for two different CO/H2 ratios, 1 and
3, corresponding to typical gas compositions obtained in fluidized-bed and entrained
bed gasifiers, respectively. For comparison purposes, experiments using only H2 or CO
as fuel gas were also done. The highest efficiencies were obtained for H2 and the lowest
for the CO due to the low reactivity of the oxygen carrier with this gas, especially
during the reaction of the NiAl2O4 present in the oxygen carrier. However, high and
similar combustion efficiencies, close to the obtained with the very reactive H2, were
obtained for the two different syngas compositions used. This result can be considered
surprising considering the differences on the CO content of the two syngas
compositions. Deeper investigations showed that the water gas shift reaction plays an
important role in the combustion of the CO present in the syngas, acting as an
intermediate step in the global combustion reaction.
The oxygen carrier prepared by impregnation exhibited an adequate behaviour
regarding processes such as attrition, agglomeration and carbon deposition during 50
hours of continuous operation in the prototype. The results showed that this oxygen
carrier could be used with high efficiency in a CLC plant for syngas combustion
produced in an Integrated Gasification Combined Cycle (IGCC).
Acknowledgements
This research was conducted with financial support from the Spanish Ministry of
Education and Science (Projects No. CTQ2004-04034 and CTQ2007-64400). C. Dueso
thanks MEC for a F.P.I. fellowship.
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Table 1. Properties of the oxygen carrier Ni18-Al
NiO content (wt %) 18
Oxygen transport capacity, ROC 0.0386
Particle size, m 100-300
Porosity 0.42
Solid density (kg·m-3) 4290
Particle density (kg·m-3) 2470
Specific surface area BET (m2·g-1) 7.0
Crushing strength (N) 4.1
XRD phases -Al2O3, NiO, NiAl2O4
Table 2. Composition of the gas used during reduction in the TGA experiments.
CO CO2 H2 H2O N2 CO/H2 Ratio
15 85
15 10 15 15 45 1
15 20 5 10 50 3
15 20 65
Table 3. Composition of the gas fuel used in the experiments carried out in the
continuous CLC plant.
TFR (K) Gas fed (vol %) Equilibrium conditions (vol.%)
CO CO2 H2 N2 CO CO2 H2 H2OCO/H2
ratio
1073 15 10.5 25 49.5 20 5.5 20 5 1
1073 27 13 13 47 30 10 10 3 3
1153 40 60
1153 15 9 25 51 20 4 20 5 1
1153 27 10 13 50 30 7 10 3 3
1153 40 15 45
Fig. 1. Schematic diagram of the continuous Chemical-Looping Combustion plant.
Air
SecondaryAir
Gas analysisO2, CO, CO2
7
H2O
stack
8
Gas analysisCO2, CO, H2
stack
1
2
3
4
5
6
1.- Fuel reactor2.- Loop seal3.- Air reactor4.- Riser5.- Cyclone6.- Diverting solids valve7.- Solids valve8.- Water condenser9.- Filter10.- Furnace
10
10
9
9
H2H2 CO N2CO N2N2N2Air
SecondaryAir
Gas analysisO2, CO, CO2
7
H2O
stack
8
Gas analysisCO2, CO, H2
stack
1
2
3
4
5
6
1.- Fuel reactor2.- Loop seal3.- Air reactor4.- Riser5.- Cyclone6.- Diverting solids valve7.- Solids valve8.- Water condenser9.- Filter10.- Furnace
10
10
99
99
CO2
Air
SecondaryAir
Gas analysisO2, CO, CO2
7
H2O
stack
8
Gas analysisCO2, CO, H2
stack
1
2
3
4
5
6
1.- Fuel reactor2.- Loop seal3.- Air reactor4.- Riser5.- Cyclone6.- Diverting solids valve7.- Solids valve8.- Water condenser9.- Filter10.- Furnace
10
10
9
9
H2H2H2H2 CO N2CO N2CO N2CO N2N2N2N2N2Air
SecondaryAir
Gas analysisO2, CO, CO2
7
H2O
stack
8
Gas analysisCO2, CO, H2
stack
1
2
3
4
5
6
1.- Fuel reactor2.- Loop seal3.- Air reactor4.- Riser5.- Cyclone6.- Diverting solids valve7.- Solids valve8.- Water condenser9.- Filter10.- Furnace
10
10
99
99
CO2
CO2
Fig. 2. Gas product distribution obtained in the batch fluidized bed reactor at 1223 K.
Syngas composition: 15% H2, 10% CO, 10% CO2.
Time (s)
0 200 400 600 800 1000 1200 1400
Con
cen
trat
ion
(vo
l %, d
ry b
asis
)
0
5
10
15
20
25
30
Sol
id c
onve
rsio
n
0.0
0.2
0.4
0.6
0.8
1.0
CO2
Reduction
CO
N2 Oxidation
COCO2H2
H2O
Fig. 3. Temperature and gas product distribution obtained at the outlet of AR and FR
during a typical experiment. TFR = 1153 K; TAR = 1223 K; CO/H2 = 1; fs = 7.5 kg/h; =
5.
Time (min)
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Gas
con
c. (
% v
ol.,
dry
bas
is)
0
5
10
15
20
25
O2
CO, CO2 (0%)
Tem
per
atur
e (º
C)
700
800
900
1000
Gas
con
c. (
% v
ol. d
ry b
asis
)
0
10
20
30
40
CO2
CO (0.4%)H2 (0.5-0.6%)
Syngas fed
FR
AR
Riser
FR
AR
Fig. 4. Effect of the FR temperature on the combustion efficiency. , TFR = 1153K. ,
TFR = 1073 K. TAR = 1223 K. CO/H2 = 1.
Oxygen Carrier to Fuel Ratio
0 1 2 3 4 5 6 7
Com
bu
stio
n E
ffic
ien
cy
80
85
90
95
100
Equilibrium
Stoichiometric
Oxygen Carrier to Fuel Ratio
0 1 2 3 4 5 6 7
Com
bu
stio
n E
ffic
ien
cy
80
85
90
95
100
Equilibrium
Stoichiometric
Fig. 5. Effect of the oxygen carrier-to-fuel ratio and fuel gas composition on the
combustion efficiency in the CLC prototype. Syngas CO/H2 = 1; Syngas CO/H2 = 3;
H2; CO. TFR = 1153 K. TAR = 1223 K.
Oxygen Carrier to Fuel Ratio0 2 4 6 8 10 12
Com
bu
stio
n E
ffic
ienc
y
70
75
80
85
90
95
100
Stoichiometric
Equilibrium
Oxygen Carrier to Fuel Ratio0 2 4 6 8 10 12
Com
bu
stio
n E
ffic
ienc
y
70
75
80
85
90
95
100
Stoichiometric
Equilibrium
Fig. 6. Conversion vs. time curves obtained in the TGA for the particles taken after
operation in the continuous CLC plant. ( ___ ) H2; ( _ _ _ ) CO; ( . . . . . ) Syngas CO/H2 = 1;
( _ . _ . _ ); Syngas CO/H2 = 3.
Time (min)
0 1 2 3 4 5 6 7 8 9 10 11 12
Con
vers
ion
0.0
0.2
0.4
0.6
0.8
1.0
0.0 0.2 0.40.0
0.1
0.2
0.3
Fig. 7. CO and H2 concentrations obtained at the outlet of the FR during syngas
combustion in the continuous CLC plant. TFR= 1153 K . TAR = 1223 K. CO ; H2 .
Concentrations obtained for fuel gas with 40% H2 or 40% CO were also showed for
comparison purposes ( H2, CO )
Oxygen Carrier to Fuel Ratio
0 2 4 6 8 10 12
Com
bu
stio
n E
ffic
ien
cy
0
5
10
15
20Stoichiometric
CO/H2 = 1
Oxygen Carrier to Fuel Ratio
0 2 4 6 8 10 12
Com
bu
stio
n E
ffic
ien
cy
0
5
10
15
20Stoichiometric
CO/H2 = 3
Oxygen Carrier to Fuel Ratio
0 2 4 6 8 10 12
Com
bu
stio
n E
ffic
ien
cy
0
5
10
15
20Stoichiometric
CO/H2 = 1
Oxygen Carrier to Fuel Ratio
0 2 4 6 8 10 12
Com
bu
stio
n E
ffic
ien
cy
0
5
10
15
20Stoichiometric
CO/H2 = 3
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