Evaluation of the oxygen carrier particle lifetime in a Chemical Looping

Combustion process

P. Gayán, F. García-Labiano, A. Cabello, L. F. de Diego, A. Abad, J. Adánez

Instituto de Carboquímica (ICB-CSIC), Miguel Luesma Castán 4, 50018-Zaragoza, Spain

P. Gayan, 34976733977, pgayan@icb.csic.es

Abstract

Chemical Looping Combustion, CLC, is one of the most promising processes to capture CO2 at a low cost. It is based on the transfer of oxygen from air to the fuel by using a solid oxygen carrier that circulates between two interconnected fluidized-bed reactors: the fuel- and the air-reactor. The CO2 capture is inherent to this process, as air does not get mixed with fuel. The key issue in the system performance is the oxygen carrier material. The oxygen carrier must fulfil several characteristics such as high reactivity and good fluidization properties that will rely on their redox system and the support. Other important feature of an oxygen carrier is the cost. The cost of an oxygen carrier will be the sum of several factors, but its particle lifetime has an important impact on the final cost. The particle lifetime of an oxygen carrier is related to its attrition behaviour, an important characteristic for its use in fluidized bed reactors. For this purpose, several particle characteristics related with the attrition resistance have been analysed in this work for more than 25 oxygen carriers, both natural and synthetic, prepared by different methods and metal oxides. The particle crushing strength and the ASTM standard attrition index (AJI), commonly accepted for FCC technology, have been determined for the materials. Moreover, the attrition rate has also been determined during multi-cycle redox reactions in a batch fluidized bed or in a continuously operated CLC unit. A comparison of the different methods to determine the attrition behaviour was done. It was found a poor correlation among crushing strength, AJI and particle lifetime parameters corresponding to materials with low particle lifetimes (< 500 h) in the whole range of crushing strength or AJI values. However, the AJI value determined to particles subjected to long term CLC operation can be used as a standard and comparative tool to evaluate oxygen carrier attrition in order to give an indication of their potential use at an industrial scale.

1. Introduction

CO2 Capture and Storage (CCS) is a promising option to reduce net CO2 emissions into the

atmosphere [1]. Therefore, the development of CCS technologies is essential for the production of

clean energy from fossil fuels combustion both to ensure a continued role of these fuels, and to reduce CO2 global emissions [2]. Great efforts have been made during last years to develop new low-cost CCS technologies. Among them, the Chemical-Looping Combustion (CLC) process was suggested among the best alternatives to reduce the cost of CO2 capture [3]. The estimated cost of the capture per tonne of CO2 avoided ranged between 6 and 13 €. The main drawback attributed to CLC was a very low confidence level as a consequence of the lack of maturity of the technology. It must be considered that this is an emerging technology. The interest in CLC technologies is continuously increasing because of the promising results found. About 4000 hours of operational experience in continuous CLC plants of different size have been carried out with more than 40 different materials [4]. Considering that the experimental experience of this technology is less than 15 years old, the development of the process can be considered very successful.

The CLC process is based on the transfer of oxygen from air to the fuel by means of a solid oxygen carrier avoiding direct contact between fuel and air. Figure 1 shows a general scheme of this process. In a first step, the fuel is oxidized to CO2 and H2O by a metal oxide (MexOy) that is reduced to a metal (Me) or a reduced form MexOy-1. The gas produced in this first step contains primarily CO2 and H2O.

After steam condensation and purification, a highly concentrated stream of CO2 ready for transport and storage is achieved. This concept is the main advantage of the process in comparison with other CO2 capture technologies. In this sense, CLC is a combustion process with inherent CO2 separation, i.e. avoiding the need of CO2 separation units and without any energy penalty. The metal or reduced metal oxide is further oxidized with air in a second step, and the regenerated material is ready to start a new cycle. The flue gas obtained contains N2 and unreacted O2. The net chemical reaction over the two steps, and therefore the combustion enthalpy, is the same as in conventional combustion where the fuel is burned in direct contact with oxygen from air. Therefore, the total amount of heat evolved in the CLC process is the same as in conventional combustion. The majority of the CLC plants existing worldwide at the moment use the configuration composed of two interconnected fluidized-bed reactors, one of them being the fuel-reactor and the other the air-reactor. In addition, two loop-seal devices must be used in order to avoid gas leakage between reactors.

Air

O2/N2

CH4 (or CO+H2)

CO2

H2O

Me/MeO

Air

reactor

Fuel

reactor

Condenser

MeO/Me

Me + ½ O2 MeO CH4 + MeO CO2 + H2O + Me

Air

O2/N2

CH4 (or CO+H2)

CO2

H2O

Me/MeO

Air

reactor

Fuel

reactor

Condenser

MeO/Me

Me + ½ O2 MeO CH4 + MeO CO2 + H2O + Me

Figure 1. CLC concept

The key issue in the system performance is the oxygen carrier material. To be considered as a suitable oxygen carrier for CLC, the aforementioned material must show some characteristics: enough oxygen transport capacity; favourable thermodynamics regarding the fuel conversion to CO2 and H2O; high reactivity for reduction and oxidation reactions in order to reduce the solids inventory in the reactors; negligible carbon deposition to avoid the release of CO2 in the air reactor which would reduce CO2 capture efficiency; good fluidization properties (no presence of agglomeration); resistance to attrition to minimize losses of elutriated solids; limited cost and environmental friendly behaviour. The first two characteristics are intrinsically dependent on the redox system. The cost and the environmental characteristics are referred to the type of metal oxide used (Fe, Mn, Cu or Ni). The quality of the other required characteristics must be experimentally determined for each specific material. In this sense, the method used in the preparation of the materials strongly affects the properties of the oxygen carrier. The active materials are dispersed on inert support materials such as Al2O3, MgAl2O4 or ZrO2 to increase the reactivity. The distribution of the metal oxide on the support and the possible interaction between them can affect the oxygen carrier reactivity, as well as the strength and material stability during consecutive redox cycles. There are some methods of preparation in which powders of metal oxide and support are mixed (mechanical mixing and extrusion, freeze granulation, spray drying, or spin flash). In other methods, the solid compounds are generated by precipitation (co-precipitation, dissolution, sol-gel, solution combustion). Finally, there is the impregnation method where a solution containing the active metal is deposited on a resistant and porous solid support. At the moment, the methods of preparation planned for oxygen carrier at large-scale production are spray drying, and impregnation [4]. Natural ores and waste materials from industrial activities have also been proposed as oxygen carriers and are nowadays considered very interesting especially for solid fuels.

The attrition behaviour of the solids is an important characteristic for its use in fluidized-bed reactors. Attrition is influenced by bed material properties, reactor geometry, operating conditions and occurrence of chemical reactions [5]. Particle breakage may result in the formation of fine and/or coarse fragments. Thus, attrition influences in two important aspects of fluid bed operation, namely particle elutriation and particle size distribution. Elutriation of fine particles out of the system may lead to the loss of bed material from the reactors. On the other hand, the change of the particle size distribution of the bed material affects the operation of the FB unit. In fact, bed fluid-dynamics, heat and mass transfer coefficients, and heterogeneous reaction rates depend on the particle size distribution of the bed material [6].

The lifetime of an oxygen carrier can be defined as the mean time that a particle is under reaction (reduction or oxidation) in the system without any reactivity loss or without suffering high attrition/fragmentation processes that produce bed elutriation out of the system. Normally, loss of fines

is defined as the loss of particles smaller than 45 m [7]. It is assumed that particles of this size have a short residence time in a commercial unit and thus they are of little use in the process. The cost of the makeup stream of solids to replace loss of fines depends on the particles lifetime and on the cost of the oxygen carrier. The cost of an oxygen carrier will be the sum of several factors including the cost of the metal oxide, the inert material, and the manufacturing cost. When industrial methods are used, the manufacturing costs of the oxygen carrier are rather low and the final cost is mainly given by the price of the raw materials. Abad et al. [8] presented an evaluation of the cost of the metal oxide over the CO2 capture cost based on the reactivity and lifetime of the materials. The whole cost of the oxygen carrier in the process will depend on the lifetime of the particles. Therefore, a longer lifetime of the oxygen carrier is preferred to keep the cost increase low. In order to determine the particle lifetime accurately, it is necessary to operate in continuous CLC units during long time periods. However, there is currently little information available concerning long term operation with oxygen carriers [9-16] and industrial data are not available so far.

In spite of the relevance of the particle attrition, a critical parameter for scale-up, the determination of this parameter for oxygen carriers has received little attention in the CLC community [17-20]. In fact, the majority of these studies have developed their own devices and attrition indexes which make very difficult to extrapolate and compare the obtained results. However, in the practical industry, a standard ASTM method exists for catalysts used in the Fluid Catalytic Cracking process (FCC). In this process, particles circulate between reactors in a similar way to in the CLC process, as shown in Figure 1. The ASTM standard method D5757-00 has been primarily designed for catalysts, but can be effectively applied to spherically or irregularly shaped particles. In this sense, this method can also be a useful tool to assess oxygen carrier attrition. Therefore, the attrition value determined with this test can be compared to FCC catalysts giving an indication to rank the mechanical resistance and to evaluate the potential use at an industrial scale. It should be noted that in the catalyst industry, materials with an AJI of 5 % or below are generally accepted as suitable for use in a transport reactor [21].

The objective of this work was to evaluate the ASTM method as a reference of the attrition behaviour of an oxygen carrier in a CLC process. Furthermore, several particle characteristics related with the attrition resistance have been also analysed for different oxygen carriers, both natural and synthetic, prepared by different methods and metal oxides. In this sense, the particle crushing strength and the ASTM standard attrition index have been determined. However, it is necessary to consider that thermal and chemical stresses due to redox reactions are present together with physical attrition effects in a CLC process. Thus, the attrition behaviour obtained during multi-cycle redox reactions at high temperature in a CLC unit has also been determined. A comparison of the different methods to determine the attrition behaviour was done.

2. Experimental section

2.1 Oxygen carrier materials

In this work 26 different oxygen carriers were evaluated. These materials are based on manganese, nickel, copper and iron. Most of them were prepared by the incipient impregnation method, except the Mn-based oxygen carriers which are pervoskites prepared by spray drying and three of the Fe-based oxygen carriers, which are natural ores or a residue obtained from the bauxite process. Different

supports were used for the oxygen carriers prepared by the impregnation method such as -Al2O3 and α-Al2O3 from several commercial suppliers, CaAl2O4, MgAl2O4 and ZrO2. The particle size of the Ni-

and Fe-based oxygen carriers ranged between 100 and 300 m, whereas particles in the size ranges

+90-200 m and +300-500 m were prepared for the Mn-based and Cu-based oxygen carriers, respectively. These differences in the particle size of the oxygen carriers were mainly due to the different values of density and preparation methods used.

2.2 Oxygen carrier characterization

The mechanical strength was measured using a Shimpo FGN–5X apparatus, see Figure 2 (a). This parameter was obtained as the average value of at least 20 measurements. On the other hand, the attrition resistance was determined using a three-hole air jet attrition tester, ATTRI-AS (Ma.Tec.

Materials Technologies Snc) configured according to ASTM-D-5757-00 [22], see Figure 2 (b). As specified in the ASTM method, 50 g of material were tested under an air flow of 10 l/min. The weight loss of fines was recorded after 1 h and 5 h of operation, respectively. The percentage of fines after a 5 h test is defined as the Air Jet Attrition Index (AJI). According to the ASTM method, particles with a

size lower than 20 m are considered as fines. AJI values were determined by duplicate both for fresh and for used samples subjected to redox cycles at high temperature in a CLC unit. Moreover, the particle size distribution of the samples was analysed via a laser diffraction technique, according to ISO13320 with a LS13320 Beckman Coulter equipment.

(a) (b)

Figure 2. Photographs of (a) Shimpo FGN–5X crushing strength apparatus and (b) ATTRI-AS

three-hole air jet attrition tester.

2.3 ICB-CSIC-g1 CLC continuous facility

An attrition rate under relevant CLC conditions (high temperatures and cyclic reduction and oxidation reactions) was also measured during continuous CLC combustion tests. The combustion and attrition behaviour of the majority of the oxygen carriers was analysed in the ICB-CSIC-g1 facility, see Figure 3. This continuous CLC pilot plant (500 Wth) consists of two bubbling fluidized bed reactors, the fuel reactor and the air reactor, interconnected by a loop seal, a riser for solids transport, a cyclone, and a solids valve to control de circulation rate of the oxygen carrier, which can be measured by means of a diverting solids valve. Furthermore, this facility has two small deposits to take samples from both reactors to carry out subsequent analyses, and two filters which allow the collection of the fines elutriated during the continuous CLC process. This unit was described in detail in a previous work [9]. Different gases such as CH4, CO, H2, syngas, light hydrocarbons (C2-C3) or PSA-off gas were used as fuels. More than 1000 hours of continuous operation were carried out with the different oxygen carriers tested in this unit. The fuel reactor temperature ranged between 800 ºC and 900 ºC, whereas the air reactor temperature was varied between 800 ºC and 950 ºC depending on the metal oxide of the oxygen carrier. The total solids inventory in the system ranged between 1.0 and 1.7 kg depending on the density of the solid material. Regarding the solids circulation rate, this parameter took a value between 4 and 20 kg/h depending on the oxygen transport capacity of the material. The inlet flow in the FR was varied between 170 and 255 Nl/h, corresponding to gas velocities from 0.1 to 0.15 m/s. Air was used as fluidizing gas in the AR, which was divided into the fluidizing gas in the bottom bed (720 Nl/h) and the secondary air in the riser (150 Nl/ h). Nitrogen was used as a fluidizing agent in the particle loop seal (37.5 Nl/h).

Full conversion of fuels was achieved with most of the oxygen carriers analysed at specific operating conditions, with the exception of the Ni-based oxygen carriers due to thermodynamic restrictions, and the natural Fe-based materials which present a poor CH4 conversion.

An experimental attrition rate as a function of the operation time can be determined by collecting and weighing elutriated particles from the reactors. The particles collected in the filters with a size higher

than 45 m were returned again into the system. Thus, the attrition rate, A (%/h), is defined through the following equation:

3600100

tm

mA

t

f (1)

where mf is the mass of elutriated particles with a particle size lower than 45 m during a particular

period of time, t, and mt is the total mass of solids inventory in the CLC plant. The attrition rates measured were usually high during the first hours of operation due to rounding effects, but progressively decreased until a constant value for many of the materials. These stabilized values were taken to estimate the particle lifetime. The particle lifetime of an oxygen carrier in a CLC process can be estimated from the attrition rate through equation:

AlifetimeParticle

100 (2)

H2O N2 CH4 CO CO2 C2H2n+2 H2N2Air

SecondaryAir

Gas analysis

O2, CO, CO2

7

H2O

stack

8

Gas analysis

CH4, CO2, CO, H2

stack

1

2

3

4

5

6

1.- Fuel reactor

2.- Loop seal

3.- Air reactor

4.- Riser

5.- Cyclone

6.- Diverting solids valve

7.- Solids valve

8.- Water condenser

9.- Filter

10.- Furnace

11.- Sample extraction

12.- Evaporator

10

10

9

9

11

12

Figure 3. Schematic diagram of the continuous 500 Wth CLC ICB-CSIC-g1 facility.

3. Results

Table 1 shows the mechanical strength and the AJI measured for fresh oxygen carriers selected in this work. Some of these materials were also evaluated during a high number of redox cycles at high temperature in continuous or batch fluidized bed CLC facilities. Experimental conditions of these tests are summarized in Table 1. In these cases, the stabilized attrition rate and the corresponding particle lifetime were also shown in this Table. Moreover, the AJI values corresponding to used samples subjected to long-term operation CLC combustion tests were determined. The different materials have been divided in 4 groups based on the active metal oxide present in the oxygen carrier. Specifically, the impregnated oxygen carriers were designated with the chemical symbol referred to the metal oxide followed by the weight content and with the inert used as support. Finally, letter “I” means that the material was prepared by impregnation. As an example, ‘‘Ni11CaAl_I” indicates an oxygen carrier with 11 wt.% of NiO prepared by impregnation on CaAl2O4. Regarding the test conditions, it must be point out that the temperature values showed in Table 1 refer to the operating temperature used in the fuel reactor of the continuous CLC unit.

Table 1. Attrition measurements and CLC test conditions of the different oxygen carriers.

Oxygen carrier Crushing strength

AJI fresh AJI used Attrition rate

Particle lifetime

Test conditions

facility operation time fuel TFR

(N) (%) (%) (%/h) (h) (h) (ºC)

Mn_1 CaMn0.9Mg0.1O3 1.1 14.1 9.9 0.03 3900 CLC500Wth 71 CH4 (+H2S) 900

Mn_2 CaMn0.875Ti0.125O3 1.4 29.2 --- --- --- --- --- ---

Ni_1 Ni21Al_1_I 2.6 4.8 5.3 0.04 2500 BFB 60 CH4 800 – 950 Ni_2 Ni18Al_2_I 4.1 5.0 4.1 0.01 10000 CLC500Wth 100 CH4 800 – 880 Ni_3 Ni11CaAl_I 1.2 14 29.2 0.25 400 CLC500Wth 90 CH4, syngas,

HCs 900

Cu_1 Cu16Al_3_I 1.1 1.5 --- --- --- --- Cu_2 Cu15Al_2_I 4.6 2.2 21.3 > 0.4 < 250 CLC500Wth 30 CH4 900 Cu_3 Cu14Al_1_I 2.9 4.3 --- 0.04 2500 *

CLC10kWth 100 CH4 800

Cu_4 Cu14Al_1_I 2.9 4.3 10.9 0.09 1100 CLC500Wth 63 CH4 900

Cu_5 Cu12.8Ni3Al_1_I 3.1 4.6 8.0 0.04 2700 CLC500Wth 67 CH4 900 Cu_6 Cu12MgAl_I 2.1 1.6 13.2 0.20 500 CLC500Wth 50 CH4 900 Cu_7 Cu14Zr_I 1.4 10.1 --- --- --- --- --- --- ---

Fe_1 Fe24Al _4_I 1.3 5.6 15.6 0.25 420 CLC500Wth 51 CH4 900 Fe_2 Fe20Al_1_I 1.5 4.7 12.8 0.09 1100 CLC500Wth 75 CH4 880 Fe_3 Fe20Al_5_I 0.5 40.0 79.3 0.32 310 CLC500Wth 52 CH4 900 Fe_4 Fe20Al_6_I 0.7 6.1 17.1 0.09 1050 CLC500Wth 50 CH4 900 Fe_5 Fe20Al_6_Ia 1.1 1.2 --- --- --- --- --- --- --- Fe_6 Fe20Al_7_I 1.0 2.3 45.0 0.09 1125 CLC500Wth 52 CH4 900 Fe_7 Fe20Al_8_I 0.4 38.0 --- --- --- --- --- --- --- Fe_8 Fe20Al_9_I 0.4 33.3 --- --- --- --- --- --- --- Fe_9 Fe20Al_10_I 0.6 5.5 --- --- --- --- --- --- --- Fe_10 Fe18Al_3_I 0.7 1.7 18.8 0.09 1150 CLC500Wth 77 CH4 900 Fe_11 Fe17Al_11 _I 1.1 7.6 10.9 0.14 700 CLC500Wth 19 CH4 900 Fe_12 Ilmenite 4.0 0.6 2.4 0.06 1700 BFB 56 CH4, syngas 900 Fe_13 Iron waste 2.9 3.8 --- 0.20 500 CLC500Wth 100 CH4, syngas,

PSA-off gas 830-880

Fe_14 Iron ore 4.6 6.3 4.7 0.05 2000 CLC500Wth 50 CH4, PSA-off gas 830-930

* de Diego et al. [12];

Al_1: -Al2O3 SASOL NWa-155; Al_2: α-Al2O3 ICB-CSIC; Al_3: -Al2O3 Saint Gobain; Al_4: α-Al2O3 Johnson Matthey; Al_5: -Al2O3 Chinese supplier 1; Al_6: -Al2O3 SASOL

SCCa; Al_6_Ia: -Al2O3 SASOL SCCa 12h calc.; Al_7: α-Al2O3 Saint Gobain; SASOL NWa-155; Al_8: -Al2O3 Chinese supplier 2; Al_9: -Al2O3 Chinese supplier 3; Al_10: α-

Al2O3 PIDC; Al_11: -Al2O3 Johnson Matthey.

CLC = continuous combustion unit; BFB = batch fluidized bed.

First of all, a comparison between AJI and mechanical strength values of fresh materials is shown in Figure 4, a poor correlation between crushing strength and AJI for fresh particles was observed. It must be highlighted that the needed, and generally accepted, values of both parameters for the CLC process have been marked in all Figures, i.e., 5 % and 1N , respectively. The crushing strength well-

known value of 1N is accepted for oxygen carriers in the size range used in this work (+90-500 m). In

case of AJI, the accepted value of 5 % is suitable for particle sizes from 10 m to 180 m. In this sense, it must be taken into consideration that it was found that the crushing strength usually increases when particle size increases. However, when the effect of the particle size on the AJI value

was analysed (in the range +90-500 m), a poor effect was measured.

These parameters were also correlated with the particle lifetime determined during CLC tests, see Figures 5 and 6. In these cases, the correlations were also scarce. A value of 2000 h for the particle lifetime has been indicated as a suitable value for oxygen carriers. Low particle lifetimes (< 500 h) were found in the whole range of crushing strength or AJI values. These results were likely expected since the thermal and chemical stresses that a particle suffers during operation in a CLC process can affect in a great extent over its mechanical behaviour and they were not taken into account during crushing strength or AJI measurements for fresh materials. However, some general comments can be stated regarding the active metal oxide present in the oxygen carriers.

The Mn_1 material, a spray dried perovskite with a high particle lifetime of 3900 h, presented a low crushing strength and a slightly high fresh AJI value. This result can be explained since the manganese and the calcium present in the perovskites are soft materials, whereas the addition of H2S in the fuel gas during continuous CLC combustion tests reduced the attrition rate of the particles up to 0.03 wt %/h.

Regarding the Ni-based oxygen carriers, it was found a linear correlation between the crushing strength and the AJI for the fresh particles. Furthermore, the materials that presented low AJI values reached high particle lifetimes in the continuous CLC facility (Ni_1 and Ni_2).

In the case of the Cu-based oxygen carriers, it was concluded that the main parameters that should be analysed in order to know if a material presents a good attrition behaviour in a continuous CLC unit were not the AJI or the crushing strength, but other variables such the operating temperature (Cu_3) or the support used (Cu_2, Cu_5 and Cu_6) since their effects over the particle lifetime in the CLC unit seemed to be more important.

Finally, regarding the Fe-based oxygen carriers it can be stated that the materials prepared by the impregnation method were generally soft. Thus, low particle lifetime values were usually obtained. Analysing the results corresponding to the effect of the AJI values for fresh particles over the particle lifetime, it was not possible to find a definite conclusion since several Fe-based materials with low AJI values did not present a good attrition behaviour in the continuous CLC unit. Moreover, the attrition behaviour of the Fe-based oxygen carriers prepared by impregnation on alumina from several suppliers (Fe_1 to Fe_11 materials) was very different with a high dispersion in the crushing strength, AJI and particle lifetime values. These results reveal the importance of selecting a support with good physicochemical properties when impregnated materials are considered to be used. On the other hand, in the cases of the fresh iron waste and the iron ore materials, the crushing strength, not the AJI, was the parameter that showed a better correlation with the particle lifetime values obtained in the ICB-CSIC-g1 CLC continuous facility.

Based on the results obtained from Figures 4-6, it can be stated that a high AJI value for fresh oxygen carriers implies that the aforementioned materials present a bad performance in a continuous CLC unit in terms of attrition resistance and, consequently, these materials should be rejected as acceptable oxygen carriers for their use in a CLC process. However, the fact that an oxygen carrier presents a low AJI value does not mean that this material shows a good behaviour in a continuous CLC unit in terms of attrition resistance (including thermal, chemical, and physical stresses). Therefore, the AJI parameter for fresh oxygen carrier particles cannot be proposed as a suitable method to predict the mechanical behaviour in a CLC process.

With the aim of finding a suitable parameter that could provide reliable information regarding the attrition behaviour of an oxygen carrier for the scale-up of the CLC process, AJI tests were carried out to samples of oxygen carriers after being subjected to continuous CLC combustion tests. Figure 7 shows the correlation between the AJI values obtained for used oxygen carriers and the

corresponding particle lifetime values. In this case, the lowest AJI values (~ 5%) for used particles are related to the highest particle lifetimes determined in the CLC units. On the other hand, the AJI values corresponding to used particles of some materials increased after being subjected to redox cycles. This result indicates that the mechanical resistance of the material changed due to thermal and chemical stresses suffered during CLC operation. Thus, those oxygen carriers with AJI values higher than 15-20 % after being tested in CLC conditions should improve their attrition performance before being selected for scale-up.

Fe_12

Crushing strength (N)

0 1 2 3 4 5

AJI

fresh

(%

)

0

5

10

15

20

25

30

35

40

45

Mn

Ni

Cu

Fe

Fe_1

Fe_2

Fe_3

Fe_4

Fe_6

Fe_7

Fe_8

Fe_5

Fe_9

Fe_10Fe_12Fe_13

Fe_11

Mn_1

Mn_2

Ni_1

Ni_2

Ni_3

Fe_14

Cu_1 Cu_2

Cu_3

Cu_4

Cu_5

Cu_6

Cu_7

Fe_12

Crushing strength (N)

0 1 2 3 4 5P

art

icle

lif

eti

me (

h)

0

500

1000

1500

2000

2500

3000

3500

4000

9000

10000

11000

Mn

Ni

Cu

Fe

Fe_1

Fe_2

Fe_3

Fe_4

Fe_6

Fe_10

Fe_11

Fe_12

Fe_13

Fe_14

Mn_1

Ni_1

Ni_2

Ni_3

Cu_2

Cu_3

Cu_4

Cu_5

Cu_6

Figure 4. Attrition jet index (AJI) of fresh particles versus crushing strength values of

fresh oxygen carrier particles.

Figure 5. Particle lifetime determined in the ICB-CSIC-g1 facility versus crushing strength

values of fresh oxygen carrier particles.

Fe_12

AJI fresh (%)

0 5 10 15 20 35 40 45

Part

icle

lif

eti

me (

h)

0

500

1000

1500

2000

2500

3000

3500

4000

9000

10000

11000

Mn

Ni

Cu

Fe

Fe_1

Fe_2

Fe_3

Fe_4Fe_6

Fe_10

Fe_11

Fe_12

Fe_13

Fe_14

Mn_1

Ni_1

Ni_2

Ni_3Cu_2

Cu_3

Cu_4

Cu_5

Cu_6

AJI used (%)

0 10 20 30 40 50 60 70 80 90

Part

icle

lif

eti

me (

h)

0

500

1000

1500

2000

2500

3000

3500

4000

9000

10000

11000

Fe

Cu

Ni

Mn

Fe_2

Fe_3

Fe_4

Fe_10

Fe_1

Fe_6

Fe_11

Cu_4

Ni_2

Mn_1

Ni_1

Ni_3

Cu_5

Cu_2

Fe_14

Fe_12

Cu_6

Figure 6. Particle lifetime determined in the ICB-CSIC-g1 facility versus Attrition Jet Index (AJI) values of fresh oxygen carrier particles.

Figure 7. Particle lifetime determined in the ICB-CSIC-g1 facility versus Attrition Jet Index

(AJI) values of after-used oxygen carrier particles in the CLC unit.

4. Conclusions

The attrition behavior of more than 25 oxygen carriers has been evaluated through the analysis of several particle characteristics such as crushing strength, ASTM standard attrition index (AJI) and attrition rate during multi-cycle redox reactions in a batch fluidized bed or in a continuously operated CLC unit.

The crushing strength and the AJI for fresh oxygen carrier particles cannot be proposed as suitable parameters to predict the mechanical behaviour of materials in continuous CLC processes. In contrast, it can be concluded that the AJI test for particles subjected to long-term CLC operation can

be used as a standard and comparative tool to evaluate the attrition behaviour of oxygen carriers giving an indication of their potential use at an industrial scale. Consequently, this ASTM procedure could be fully suitable to CLC technology, if the effect of temperature and/or chemical reaction during the AJI test were included.

Acknowlegments

This paper is based on the work carried out within the framework of the SUCCESS project, funded by the European Commission under the Seventh Framework Programme (Contract 608571), by the Spanish Ministry of Science and Innovation (MICINN Project: ENE2011-26354) and by FEDER. A. Abad and A. Cabello thank CSIC for the financial support given to the project 201480E101.

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