CHARACTERIZATION OF OXYGEN CARRIERS FOR CHEMICAL-LOOPING COMBUSTION

Juan Adanez*, Francisco Garca-Labiano, Luis F. de Diego, Pilar Gayn, Javier Celaya, Alberto Abad

Instituto de Carboqumica (CSIC), Department of Energy and Environment. Miguel Luesma Castn 4, 50015 Zaragoza, Spain

ABSTRACT

Different oxygen carriers, based on copper, iron, manganese, and nickel and produced by three different preparation methods (mechanical mixing, impregnation, and freeze granulation), were tested during 100 successive oxidation-reduction cycles in a thermogravimetric analyzer (TGA) and in a fluidized bed (FB). The behavior of the different oxygen carriers with respect to selectivity towards complete oxidation products, durability in the cyclic reactions, and attrition and agglomeration during fluidized bed cyclic reactions were analyzed.

In the multicycle tests in TGA, it was observed that most of the oxygen carriers exhibited high reactivity and excellent chemical stability but the Cu- and Ni-based oxygen carriers prepared by mechanical mixing underwent a rapid degradation of their mechanical properties as the number of cycles increased. Based on the TGA results, five oxygen carriers were selected to be tested in a FB reactor: Cu-Si-M, Cu-Si-I, Fe-Al-M, Mn-Zr-M, and NiCUT-FG. In the FB tests, it was observed that Mn and Cu based carriers prepared by mechanical mixing (M) showed agglomeration, however, this problem was not observed with the Cu-based carriers prepared by impregnation (I). The Ni- and the Fe-based oxygen carriers did not agglomerate. The attrition rates of the carriers were usually high in the first cycles due to the rounding effects on the particles and because of the fines sticked to the particles during preparation. Later, the attrition rates due to the internal changes produced in the particles by the successive reduction and oxidation processes decreased, and all carriers showed low attrition rates. The product distribution during the oxidation of the fuel depended on the metal oxide used in the oxygen carrier. Complete conversion of CH4 to CO2 and H2O was obtained with the oxygen carrier Cu-Al-I. With the oxygen carrier Fe-Al-M the gas outlet composition was associated to the different reaction steps of the iron oxide. Finally, with the oxygen carrier NiCUT-FG prepared by freeze granulation (FG), a combination of CO2, H2O, CO, and H2 was formed almost immediately after introduction of CH4 into the reactor and after a short reaction time the process was mainly selective towards the formation of H2 and CO.

INTRODUCTION

It is generally accepted that a reduction of gas emissions promoting the greenhouse effect is a necessity in the developed industrial countries. Carbon dioxide coming from fossil fuel combustion is one of the most important greenhouse gases contributing to global warming. Until now, the main attention for decreasing CO2 emissions to atmosphere has been focused on the use of alternative energies, change of fuels, and the increase of the efficiency in the conversion and use of energy. However, because it is not clear if it is possible to reach the desired low levels in CO2 emissions only by these ways, it is nowadays increasing the interest in using CO2 capture and sequestration from the combustion of fossil fuels as an alternative process [1].

In a chemical-looping combustion (CLC) process, fuel gas (natural gas, syngas, etc) is burnt in two reactors. In the fuel reactor, a metallic oxide that is used as oxygen source is reduced by the feeding gas to a lower oxidation state, being CO2 and steam the reaction products. In the oxidation reactor, the reduced solid is regenerated with air to the fresh oxide, and the process can be repeated for many successive cycles. CO2 can be easily recovered from the outlet gas coming from the first reactor by simple steam condensation. Consequently, CLC is a clean process for the combustion of carbon containing gaseous fuels, preventing the CO2 emissions to atmosphere. The main drawback of the overall process is that the carriers are subjected to strong chemical and thermal stresses in every cycle and the performance and mechanical strength can decay down to unacceptable levels after enough number of cycles in use.

*Corresponding author: Tel.+34 976 733977, Fax.+34 976 733318, Email: jadanez@carbon.icb.csic.es

Different metal oxides have been proposed in the literature [2-4] as possible candidates for CLC process: CuO, CdO, NiO, Mn2O3, Fe2O3, and CoO. In general, these metal oxides are combined with an inert which acts as a porous support providing a higher surface area for reaction, as a binder for increasing the mechanical strength and attrition resistance, and, additionally, as an ion conductor enhancing the ion permeability in the solid [5-6]. An oxygen carrier in a CLC power plant must show high redox reactivity with high selectivity towards complete oxidation products, resistance against carbon deposition, sufficient durability in successive cycle reactions and high mechanical strength.

Ishida, Jin, and co-workers [5-11] at Tokyo Institute of Technology have investigated the effect of temperature, particle size, gas composition, pressure, and type of binder on the reduction and oxidation rates and on carbon deposition of Fe, Ni, and Co oxides in a thermogravimetric analyzer (TGA), using H2, CO, or CH4 as fuels and air as oxidizing gas. They concluded that the carbon deposition and the reaction rates and conversions, in addition to the operating conditions used, depended strongly on the chemical nature of the solid materials [9-10]. Mattisson, Lyngfelt, and co-workers [12-13] at Chalmers University of Technology have investigated Fe-based samples using CH4 and air in fixed-bed and fluidized-bed reactors. They found for synthetic samples higher reaction rates and lower particle breakage than the exhibited by natural samples. They [14-15] also prepared Fe, Ni, Cu, Co, and Mn based carriers on alumina and kaolin supports, and observed that oxygen carriers based on Ni and Cu showed high reactivity, however, oxygen carriers based on Mn and Co showed a rather poor reactivity. Copeland et al. at TDA Research Inc. [16] developed oxygen carriers, containing Cu, Fe, and Ni with a variety of binder materials and active metal oxide contents, to be used in their Sorbent Energy Transfer System (SETS). They eliminated Cu as a potential oxygen carrier by agglomeration problems in the fluidized bed, and obtained successfully results with Fe and Ni based carriers. Ryu and co-workers at Korea Institute of Energy Research [17-18] investigated the reactivity of NiO with bentonite as support using CH4 as fuel. Villa et al. [19] at Politecnico di Milano investigated the redox properties of Ni-Al-O and Ni-Mg-Al-O mixed oxides for CH4 combustion, indicating the poor selectivity towards CO2 and H2O, being CO and H2 the most abundant products.

In a previous work [20], based on crushing strength measurements of fresh particles and reactivity data obtained in a TGA, a preliminary screening of the most feasible oxygen carriers to be used in a CLC system was carried out. The aim of this work was to investigate the behavior of these most promising carriers with respect to selectivity towards complete oxidation products, durability in the cyclic reactions, and attrition and agglomeration during fluidized bed cyclic reactions.

EXPERIMENTAL

Preparation of Oxygen Carriers

The oxygen carriers were composed of a metal oxide as an oxygen source for the combustion process, and an inert as a binder for increasing the mechanical strength. In this work, carriers prepared by three different preparation methods were used. The carriers were designated with the chemical symbol referred to the active metal oxide, followed by the weight concentration of active phase used, the symbol for the binder used (Al = alumina, Si = silica, Ti = titania, and Zr = zirconia), and finally the preparation method used (M = mechanical mixing, I = impregnation, FG = freeze granulation).

Mechanical mixing: The oxygen carriers were prepared from commercial pure oxides as powders of particle size < 10 m, being CuO, Fe2O3, MnO2, NiO the active oxides and Al2O3, SiO2, TiO2, ZrO2 the inerts. In addition, graphite as a high-temperature pore forming additive enhancing chemical reaction was also added during preparation.

A powder mixture including the active metal oxide and the inert in the desired concentration, and 10 wt% of graphite, was converted by water addition into a paste of suitable viscosity to be extruded in a syringe, obtaining cylindrical extrudates of about 2 mm diameter. These extrudates were softly dried at 80 C overnight, cut at the desired length, and sintered at different temperatures between 950 and 1300 C for 6 h in a muffle oven. The extrudates were ground and sieved to obtain the desired particle size.

Impregnation: Commercial -Al2O3 (Puralox NWa-155, Sasol Germany GmbH) particles of 0.1-0.3 mm were impregnated with a saturated aqueous solution of Cu(NO3)2. The desired active phase loading was achieved by applying successive impregnations followed by calcination at 550 C to decompose the impregnated copper nitrate

into insoluble copper oxide.

Freeze granulation: The freeze granulated particles were prepared at Chalmers University of Technology. The preparation method was described elsewhere [15].

Multicycle Tests in a Thermogravimetric Analyzer

Multicycle tests of the oxygen carriers were carried out in a TGA system, CI Electronics type, described elsewhere [20]. For the experiments, the oxygen carrier was loaded in the platinum basket and heated to the set operating temperature in air atmosphere. After stabilization, the experiment started by exposing the oxygen carrier to alternating reducing and oxidizing conditions.

The gases used were CH4 for reduction and air for oxidation. The reducing gas was saturated in water by bubbling it through a water containing saturator at the selected temperature to reach the desired water concentration. To avoid mixing of combustible gas and air, nitrogen was introduced for two minutes after each reducing and oxidizing period. The experiments were carried out at temperatures up to 950 C for the oxygen carriers based on Fe, Mn and Ni, and 800 C for the oxygen carriers based on Cu, because at higher operating temperatures CuO, although stable in air, decomposed in N2 atmosphere into Cu2O with the subsequent loss of oxygen transport capacity of the carrier.

Multicycle Tests in Fluidized Bed

Figure 1 shows the experimental set-up used for the oxygen carrier testing in a fluidized bed (FB). It consisted of a system for gas feeding, a fluidized bed reactor, a two ways system to recover the solids elutriated from the FB, and a gas analysis system. The gas feeding system had different mass flow controllers connected to an automatic three-way valve. This valve always forced to pass N2 between the reducing gas and the oxidation gas, to avoid explosions. The FB reactor of 54 mm D.I. and 500 mm height, with a preheating zone just under the distributor, was composed by about 200 g of oxygen carrier with a particle size of 0.1-0.3 mm. The entire system was inside an electrically heated furnace. Two hot filters located downstream from the FB recovered the solids elutriated from the bed during the successive reduction-oxidation cycles, which allowed to obtain elutriation data at different times or number of cycles. The gas composition at each time was continuously measured by different gas analyzers. The H2O, CO, CO2 and CH4 were determined in two infrared analyzers, the O2 in a paramagnetic analyzer, and the H2 by gas conductivity.

Thermocouple

P

Gas Analysis

Oxygencarrier

Filters

Stack

CH4 -N2 N2 O2 - Ar

Furnace

Thermocouple

PP

Gas Analysis

Oxygencarrier

Filters

Stack

CH4 -N2 N2 O2 - Ar

Furnace

Time (min)0 1

Con

vers

ion

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2

reduction oxidation

cycle5255075100.

Figure 1: Experimental set-up for multicycle testing in fluidized bed.

Figure 2: Effect of number of reduction-oxidation cycles on reactivity of sample Fe60Al-M in TGA.

The tests were carried out at 950 C for the oxygen carriers based on Fe, Mn and Ni, and 800 C for the oxygen carriers based on Cu. The inlet superficial gas velocity into the reactor was 10 cm/s. The composition of the gas was 25% CH4 in N2 during reduction and 8% O2 in Ar during oxidation. This O2 concentration was used instead of air to avoid a large temperature increase during the exothermic oxidation reaction because the reactor had not a cooling system. To avoid mixing of CH4 and O2, nitrogen was introduced for two minutes after each reducing and oxidizing period.

Data Evaluation

Reactivity data were obtained in TGA tests from the weight variations as a function of time during the reduction and oxidation cycles. These weight data were transformed into conversion data by using the following equations:

For reduction: redox

ox

mmmmX

=

For oxidation: redox

ox

mmmmX

= 1 (1)

where m is the actual mass of sample, mox is the mass of the sample fully oxidized and mred is the mass of the sample in the reduced form.

To convert weight data into carrier conversions a description of the involved chemical reactions was necessary. On the basis of thermodynamics the reactions involved for the different oxygen carriers were: CuO-Cu, NiO-Ni and Mn3O4-MnO. For Fe-based oxygen carriers prepared with Fe2O3 as active metal oxide, different transformations are possible, which correspond to the transformations Fe2O3-Fe3O4, Fe2O3-FeO, or Fe2O3-Fe. The oxygen transport capacity of the carriers is highly dependent on the reaction considered being for the reaction Fe2O3-Fe three times higher than for the reaction Fe2O3-FeO and for this reaction three times higher than for the reaction Fe2O3-Fe3O4. The stable iron species are dependent on the reducing gas composition and temperature. In this work, the weight variations observed in the tests were mainly associated with the transformation Fe2O3-FeO.

The oxygen transport capacity, R0, for the respective active metal oxide can be defined by the oxygen content ratio in the reduced, mred, and oxidized, mox, forms through the expression:

ox

redox

mmmR =0 (2)

The maximum oxygen transport capacity corresponded to the oxygen carriers based on Cu (R0=0.201) and Ni (R0=0.214) being lower for Mn (R0=0.07) and Fe (R0=0.1 in Fe2O3-FeO). However, the transport capacity of the carrier obviously decreased due to the presence of the inert.

In the FB reactor tests, the oxygen carrier conversion as a function of the time was calculated from the outlet gas concentrations by

( ) ++= 10

222

0

t

tO,outHCO,out,outCO

tot

out dtPPPPM

nX (3)

and for the oxidation period by

( ) = 10

220

2t

t,outOout,inOin

totdtPnPn

PMX (4)

where X is the conversion of oxygen carrier, M0 are the moles of oxygen which can be removed from fully oxidized oxygen carrier, nin is the molar flow of the gas coming into the reactor, nout is the molar flow of the gas leaving the reactor, Ptot is the total pressure, Pi,in is the partial pressure of gas i going to the reactor, Pi,out is the partial pressure of gas i exiting the reactor, and t is the time.

RESULTS AND DISCUSSION

In a previous work [20], 240 samples of potential oxygen carriers composed of 40-80 % of Cu, Fe, Mn or Ni oxides on Al2O3, sepiolite, SiO2, TiO2 or ZrO2 were prepared by mechanical mixing as cylindrical extrudates. The

samples were sintered at four temperatures between 950 and 1300 C. The effects of the chemical nature and composition of the carrier and the sintering temperature were investigated by analyzing the mechanical strength of the carrier and the reactivity in a TGA using CH4 as fuel. It was observed that the crushing strength was highly dependent on the type of active metal oxide and its concentration, the inert used as a binder, and the sintering temperature. In general, a higher sintering temperature increased the crushing strength of the oxygen carriers. However, high sintering temperatures for some carriers were limited by decomposition or melting of the involved compounds. On the other hand, there was not a clear correlation between crushing strength and active metal oxide content. The reactivity of the carriers varied considerably as a function of the conversion and among the different oxygen carriers. In general, an increase in the sintering temperature makes to decrease the reaction rate. Based on the reactivity tests in TGA and the crushing strength measurements with the oxygen carriers prepared by mechanical mixing, it was concluded that carriers prepared with SiO2 or TiO2 as inerts and sintered at 950C were the best Cu-based oxygen carriers. Among the Fe-based oxygen carriers, those prepared with Al2O3 and ZrO2 as inerts showed the best behavior. ZrO2 was the best inert for preparing Mn-based oxygen carriers, and TiO2 for preparing Ni-based oxygen carriers.

However, in order to find the best carriers, it is necessary to analyze the behavior of these carriers with respect to durability in the cyclic reactions, attrition and agglomeration during fluidized bed cyclic reactions, and selectivity towards complete products.

Multicycle Testing in TGA

In every cycle, the carrier undergoes important chemical and structural changes at high operating temperature and, consequently, it is expected substantial changes in performance of the carriers with the number of cycles. TGA experiments allowed to analyze the reactivity of the oxygen carriers under well-defined conditions, and in the absence of complex fluidizing factors such as those derived from particle attrition and interphase mass transfer processes.

The oxygen carriers that exhibited acceptable crushing strengths and high conversions and reactivities were selected for 100-cycles testing in successive oxidation-reduction tests in TGA. As an example, Figure 2 shows the reactivity of the sample Fe60Al-M in several selected cycles for reduction and oxidation. The conversion-time curves were almost coincident revealing that the carrier reactivity was not affected substantially by the number of cycles in use. Very similar results were observed with the other carriers prepared by mechanical mixing (excepting with the Mn-Si-M carriers in which the reactivity and maximum conversion decreased -until 40% of the initial after 100 cycles- with increasing the number of cycles). However, in the carriers Cu-Si-M, Cu-Ti-M, Ni-Ti-M, and those prepared with a MeO:inert ratio of 80:20, the original cylindrical shape of the fresh extrudates was completely converted in an amorphous powder pile after reaction, which indicated that the mechanical strength of these carriers was severely affected.

From these multicycle tests in TGA, it was concluded that the oxygen carriers prepared by mechanical mixing exhibited high reactivity and excellent chemical stability but some of them (specially Cu and Ni-based oxygen carriers) poor mechanical strength. Consequently, the method of preparation of the Cu and Ni-based oxygen carriers must be improved to decrease the unacceptable rapid degradation of their mechanical properties as the number of cycles increased.

The effects of the accumulative chemical and thermal stresses in every cycle could be minimized if MeO as active phase is retained by impregnation within the porous texture of an inert support with high mechanical strength. In a previous work [21], samples of titania and silica impregnated with CuO were prepared and analyzed. These carriers showed good chemical stabilities and high reactivities, similar to or even higher than those prepared by mechanical mixing. In addition, crushing strength and scanning electron microscopy measurements revealed that the mechanical properties of the fresh carriers were preserved after reaction in multicycle tests. These results suggested that oxygen carriers prepared by impregnation on rigid and porous supports were potential candidates for CLC process. In this work, a Cu-Al-I oxygen carrier was prepared by impregnation on commercial -Al2O3 particles. This carrier exhibited very high reactivity and excellent chemical stability during the multicycle tests in the TGA.

Multicycle Testing in FB

To improve the screening it is necessary to know the behavior of the oxygen carriers during successive reduction-

oxidation cycles in a FB. The structural changes produced in this reactor because of the chemical reaction, the attrition phenomena existing in fluidized beds, as well as the possible agglomeration of the solids are taken into account. After the tests in the TGA, four oxygen carriers were selected and prepared to be tested in the FB reactor: Cu-Si-M, Cu-Si-I, Fe-Al-M, and Mn-Zr-M. In addition, a Ni-based oxygen carrier prepared at Chalmers University of Technology by freeze granulation, NiCUT-FG, which had showed a good chemical stability and mechanical strength and a very high reactivity in the multicycle testing in TGA was also used.

Agglomeration: Cu-based oxygen carriers prepared by mechanical mixing (Cu-Si-M) showed agglomeration in the FB tests, however, this problem was not observed with the Cu-based oxygen carriers prepared by impregnation (Cu-Al-I). It was concluded that the use of Cu-based oxygen carriers in a CLC system was restricted to the particles prepared by impregnation. The Mn-Zr-M carriers showed agglomeration and most of the particles were sticked to the reactor wall. Finally, the Ni-based oxygen carriers (NiCUT-FG) and the Fe-based oxygen carriers (Fe-Al-M) did not agglomerate.

Attrition: Figure 3 shows the attrition rates measured with the oxygen carriers selected because of their good behavior in the TGA tests and that did not agglomerate. For all of these carriers the attrition rates were usually high in the first cycles due to the rounding effects on the particles and because of the fines sticked to the particles during preparation (grinding+sieving). Later, the attrition rates due to the internal changes produced in the particles by the successive reduction and oxidation cycles decreased, and all carriers showed very low attrition rates.

Oxidation products: An important difference among the oxygen carriers based on Cu, Fe and Ni was the product distribution generated during the fuel oxidation. Figures 4-6 show the product concentration (dry basis) profiles at the outlet stream as a function of the reaction time. In these plots, the delay time caused by the residence time in the gas paths has been considered. However, the gas concentrations versus time profiles have not been corrected for gas dispersion in the sampling line and analyzers.

In the case of the Cu-Al-I oxygen carrier (Figure 4), after introduction of CH4 to the reactor, CO2 and H2O were formed and no CH4, CO and H2 were observed during most of the carrier reduction time, thus indicating that methane conversion was complete. Only at the end of the reduction period, when the oxygen carrier conversion was higher than 95%, started the formation of CO and H2. During most of the carrier oxidation period the O2 outlet concentration was zero (the oxygen carrier oxidation was limited by the supply of O2 to the reactor) followed by a rapid increase to 8%. However, upon O2 introduction to the reactor, a small concentration of CO and CO2 immediately appeared at the reactor outlet stream. This was attributed to the oxidation of the small amount of carbon deposits formed in the previous reduction step.

With the Fe-Al-M oxygen carrier, (Figure 5) during the first part of the reduction period, absence of CO and H2 indicates that all the reacted CH4 was converted to CO2 and H2O. As the reaction progressed the outlet concentration of CO2 decreased, whereas the concentrations of CO and H2 increased. As commented before, for the carriers prepared with iron oxide as active metal oxide, different reactions are possible, which correspond to the transformations Fe2O3-Fe3O4, Fe2O3-FeO, or Fe2O3-Fe. The only reduction reaction in the iron oxide system that is able to convert CH4 completely to CO2 and H2O at high temperatures is the transformation Fe2O3-Fe3O4, which is produced during the first part of the reduction period. For the reduction to a lower iron oxide, such as FeO, it is not possible to have full CH4 conversion in the fuel reactor [4]. The increase of the CO and H2 concentrations observed in the experiments were produced when the carrier conversion was higher than 30% assuming the transformation Fe2O3-FeO, that is, when the transformation Fe2O3-Fe3O4 was complete. During the oxidation period, the results were similar to those observed with the Cu-Al-I carrier.

With the NiCUT-FG oxygen carrier, (Figure 6) almost immediately after introduction of CH4 into the reactor, CO2, H2O, CO and H2 were formed. During a very short period of time the CH4 was almost completely converted to CO2 and H2O. A small amount of CO and H2, which is associated with the thermodynamic limitation of NiO to convert CH4 fully to CO2 and H2O at this temperature [4], was observed. After this short time the concentration of CO2 decreased, whereas the concentrations of CH4, CO, and H2 increased, indicating that the reduction process was mainly selective towards the formation of H2 and CO. The same results were observed by Villa et al. [19] working in a TGA. These authors suggested that the change in the product selectivity during the reduction step can be likely associated to changes in the catalyst degree of oxidation. At the beginning, the catalyst fully oxidized favors the total oxidation of CH4. As the sample was reduced the selectivity of product formation changes from CO2 and H2O to CO and H2, possibly assisted by the occurrence of the CH4 reforming reaction, catalyzed by reduced Ni active sites. It is

necessary to indicate that the oxygen carrier (NiO) also oxidizes CO and H2 [5, 8]. However, it seems that the CO and H2 generation rates were higher than the oxidation rates in the main part of the reduction reaction period. Only in the initial stages of the oxygen carrier reduction, which will correspond to high solid circulation rates between the oxidation and fuel reactors, the CH4 was almost completely converted to CO2 and H2O. This will explain to good results obtained by Lyngfelt and Thunman [22] with this carrier in a CLC pilot plant. During the oxidation period, after O2 introduction to the reactor, CO and CO2 immediately appeared at the reactor outlet stream, indicating that the carbon formation during reduction was also important with this carrier. However, the oxidation period where the O2 outlet concentration was zero was shorter with this carrier than with the Cu and Fe carriers, and the oxygen increase was slower. This may be due to lower conversion reached during the reduction reaction.

Number of cycles0 20 40 60 80 100

wei

ght l

oss

(%)/

cycl

e

0.00

0.05

0.10

0.15

0.20

0.25Cu-Al-IFe-Al-MNiCUT-FG

time (s)0 250 500 1500 2000

Con

cent

ratio

n (v

ol %

, dry

bas

is)

0

5

10

15

20

25

30

CH4

CO2

H2

CO2

O2

CO

CO

N2Reduction Oxidation N2

Figure 3: Attrition rates of the oxygen carriers in the

fluidized bed. Figure 4: Product gas distribution in a cycle for the

oxygen carrier Cu-Al-I during multicycle testing in FB.

time (s)

0 200 400 1000 1100

Con

cent

ratio

n (v

ol %

, dry

bas

is)

0

5

10

15

20

25

30

CH4

CO2

H2

CO2

O2

CO

N2Reduct. Oxidation N2

CO

time (s)0 500 1000 2000 2500

Con

cent

ratio

n (v

ol %

, dry

bas

is)

0

5

10

15

20

25

30

CH4

CO2H2

CO2

O2

CO

CO

N2Red. Oxidation N2

Figure 5: Product gas distribution in a cycle for the

oxygen carrier Fe-Al-M during multicycle testing in FB.

Figure 6: Product gas distribution in a cycle for the oxygen carrier NiCUT-FG during multicycle testing in FB.

In all of the carriers tested in the FB, the product distribution profile at the outlet stream as a function of the

reaction time was hardly affected by the number of cycles, indicating the good chemical stability of these oxygen carriers (Cu-Al-I, Fe-Al-M, NiCUT-FG) and that the oxygen carrier reactivity was not affected substantially by the number of cycles in use.

CONCLUSIONS

Different oxygen carriers, based on copper, iron, manganese and nickel were tested during 100 successive oxidation-reduction cycles in a TGA and in a FB to analyze their behavior with respect to durability in the cyclic reactions, attrition and agglomeration during fluidized bed cyclic reactions, and selectivity towards complete oxidation products. The oxygen carriers exhibited high reactivity and excellent chemical stability during multicycle tests in TGA, but the mechanical properties of Cu- and Ni-based carriers prepared by mechanical mixing were severely affected. To minimize the effects of the accumulative chemical and thermal stresses, other preparation methods were used. A Cu-based oxygen carrier was prepared by impregnation of commercial -Al2O3. This carrier exhibited very high reactivity and conversion, maintained the chemical and mechanical properties during FB multicycle tests, and did not undergo agglomeration. A Ni-based carrier was prepared at Chalmers University of Technology by freeze-granulation. This carrier showed high reactivity, good chemical stability and low attrition rates in the FB multicycle tests.

Cu and Mn based carriers (Cu-Si-M, Mn-Zr-M) prepared by mechanical mixing showed agglomeration. However, this problem was not observed with the Cu-based carriers prepared by impregnation (Cu-Al-I). The Ni-based oxygen carriers (NiCUT-FG) and the Fe-based oxygen carriers (Fe-Al-M) did not agglomerate. The attrition rates of all of the carriers were usually high in the first cycles due to the rounding effects on the particles and because of the fines sticked to the particles during preparation. Later, the attrition rates due to the internal changes produced in the particles by the successive reduction-oxidation cycles decreased, and all carriers showed low attrition rates.

An important difference among the carriers based on Cu, Fe and Ni was the selectivity towards complete oxidation products. With the Cu-Al-I, the conversion of CH4 was complete to CO2 and H2O, and only at the end of the reduction period, when the carrier conversion was higher than 95%, started the formation of CO and H2. With the Fe-Al-M oxygen carrier, the gas outlet composition during reduction was associated to the different reactions of the iron oxide. During the transformation Fe2O3-Fe3O4 all the reacted CH4 was converted to CO2 and H2O. As the reaction proceeded to FeO the outlet concentration of CO2 and H2O decreased, whereas the concentrations of CO and H2 increased. With the NiCUT-FG carrier, almost immediately after introduction of CH4 into the reactor, CO2, H2O, CO and H2 were formed, and only during a short period of time the CH4 was almost completely converted to CO2 and H2O. After this short time the concentration of CO2 decreased, whereas the concentrations of CH4, CO, and H2 increased, indicating that the reduction process was mainly selective towards the formation of H2 and CO. During the oxidation period, upon O2 introduction to the reactor, a small concentration of CO and CO2 immediately appeared at the reactor outlet stream, indicating that carbon deposits were formed in the previous reduction step with all the carriers tested.

The chemical stability and reactivity of the oxygen carriers Cu-Al-I, Fe-Al-M, and NiCUT-FG was not affected substantially by the number of cycles in use in a fluidized bed.

ACKNOWLEDGEMENTS

This paper is based on the work performed in the frame of the GRACE (Grangemouth Advanced CO2 Capture) Project, coordinated by BP and funded by the EU (ENK5-CT-2001-00571) and by the CCP (CO2 Capture Project), a partnership of BP, Chevron Texaco, EnCana, Eni, Norsk Hydro, Shell, Suncor, and Statoil. Thanks to A. Lyngfelt and T. Mattisson for the preparation of the freeze granulated particles.

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18. Ryu, H. J., D. H. Bae, G. T. Jin. 2003. Effect of temperature on reduction reactivity of oxygen carrier particles in a fixed bed chemical-looping combustor. Korean J. of Chem. Eng. 20: 960-966.

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22. Lyngfelt, A., H. Thunman. 2004. Chapter 31. Chemical-looping combustion: Design, construction and 100 h of operational experience of a 10 kW prototype. In The CO2 Capture and Storage Project (CCP) for Carbon Dioxide Storage in Deep Geologic Formations for Climate Change Mitigation. Volume 1- Capture and Separation of Carbon Dioxide from Combustion Sources. Ed: Thomas, D., Elsevier Science, London.