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