Scholarly article on topic 'Performance of a low Ni content oxygen carrier for fuel gas combustion in a continuous CLC unit using a CaO/Al2O3 system as support'

Performance of a low Ni content oxygen carrier for fuel gas combustion in a continuous CLC unit using a CaO/Al2O3 system as support Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — P. Gayán, A. Cabello, F. García-Labiano, A. Abad, L.F. de Diego, et al.

Abstract The behavior of a Ni-based oxygen carrier with low NiO content (11.8wt.% NiO) prepared by impregnation on CaAl2O4 has been studied in a continuous CLC unit (500Wth) using different gases as fuels (methane, H2, CO, syngas, ethane and propane). More than 90h of successfully operation at high temperature (1173K) have been carried out analyzing the effect of the oxygen carrier-to-fuel ratio and fuel gas composition regarding combustion efficiency and product gas distribution. Using syngas, pure CO or H2 as fuels, full combustion can be achieved working at oxygen carrier-to-fuel ratios, ϕ, higher than 1.2. Regarding methane, the maximum fuel combustion efficiency is reached in a narrow range of ϕ values close to 1 (1.0–1.2). An increase in the value of this parameter produces a decrease in the combustion efficiency. This behavior is different to that found using most of the Ni-based oxygen carriers present in literature, and can be attributed to the low global catalytic activity of the reduced oxygen carrier for reforming reactions. When light hydrocarbons are used as fuels, the oxygen carrier presents a similar behavior than in the case of methane combustion tests, reaching to the maximum fuel combustion efficiency at the same ϕ values. This fact also indicates that light hydrocarbons combustion mechanism is carried out through cracking reaction. The solids inventory needed to obtain a methane combustion efficiency of 99% is lower than 180kg/MWth, which corresponds to a metallic Ni inventory around 17kg/MWth. This value is the lowest referred in the literature for any kind of Ni-based oxygen carrier. This remarkable result is due to the low NiO content (11.8wt.%) and the very high reactivity of this oxygen carrier because all nickel in the particle is present as free NiO, since the formation of less reactive nickel compounds is avoided using CaAl2O4 as inert support.

Academic research paper on topic "Performance of a low Ni content oxygen carrier for fuel gas combustion in a continuous CLC unit using a CaO/Al2O3 system as support"

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International Journal of Greenhouse Gas Control

journal homepage www.elsevier.com/locate/ijggc

Performance of a low Ni content oxygen carrier for fuel gas combustion in a continuous CLC unit using a CaO/Al2O3 system as support

P. Gayän *, A. Cabello, F. Garcia-Labiano, A. Abad, L.F. de Diego, J. Adänez

Instituto de Carboquimica (ICB-CSIC), Department of Energy and Environment, Miguel Luesma Castän 4, Zaragoza 50018, Spain

ARTICLE INFO

Article history: Received 2 August 2012 Received in revised form 14 December 2012 Accepted 15 January 2013 Available online 14 February 2013

Keywords: CO2 capture

Chemical-looping combustion Nickel

Oxygen carrier Fuel gas

ABSTRACT

The behavior of a Ni-based oxygen carrier with low NiO content (11.8 wt.% NiO) prepared by impregnation on CaAl2 O4 has been studied in a continuous CLC unit (500 Wth) using different gases as fuels (methane, H2, CO, syngas, ethane and propane). More than 90 h of successfully operation at high temperature (1173 K) have been carried out analyzing the effect of the oxygen carrier-to-fuel ratio and fuel gas composition regarding combustion efficiency and product gas distribution.

Using syngas, pure CO or H2 as fuels, full combustion can be achieved working at oxygen carrier-to-fuel ratios, p, higher than 1.2. Regarding methane, the maximum fuel combustion efficiency is reached in a narrow range of p values close to 1 (1.0-1.2). An increase in the value of this parameter produces a decrease in the combustion efficiency. This behavior is different to that found using most of the Ni-based oxygen carriers present in literature, and can be attributed to the low global catalytic activity of the reduced oxygen carrier for reforming reactions. When light hydrocarbons are used as fuels, the oxygen carrier presents a similar behavior than in the case of methane combustion tests, reaching to the maximum fuel combustion efficiency at the same p values. This fact also indicates that light hydrocarbons combustion mechanism is carried out through cracking reaction.

The solids inventory needed to obtain a methane combustion efficiency of 99 % is lower than 180kg/MWth, which corresponds to a metallic Ni inventory around 17kg/MWth.This value is the lowest referred in the literature for any kind of Ni-based oxygen carrier. This remarkable result is due to the low NiO content (11.8 wt.%) and the very high reactivity of this oxygen carrier because all nickel in the particle is present as free NiO, since the formation of less reactive nickel compounds is avoided using CaAl2O4 as inert support.

© 2013 Elsevier Ltd. All rights reserved.

1. Introduction

CLC has been suggested as one of the most promising technologies for CO2 capture in power generation using a fuel gas because the CO2 separation is inherent to the process (Kerr, 2005). CLC technology is based on the transfer of oxygen from air to fuel by means of an active metal oxide (MexOy) as an oxygen carrier avoiding direct contact between fuel and air. The CLC concept has been proposed to be accomplished in different type of reactors and configurations: two interconnected moving or fluidized-bed reactors, fixed-bed reactors, or a rotating reactor. Nevertheless, all of the CLC plants existing worldwide at the moment use the configuration composed of two interconnected fluidized-bed reactors called as fuel and air reactor (Adanez et al., 2012). In the fuel reactor (FR), the oxygen carrier is reduced to a metal (Me) or a reduced form (MexOy-1) and oxidizes the fuel to H2O and CO2 which can be easily separated by

* Corresponding author. Tel.: +34 976 733 977; fax: +34 976 733 318. E-mail address: pgayan@icb.csic.es (P. Gayän).

condensation obtaining a high concentrated CO2 stream ready to be transported and stored.

(2n + m - p) MexOy + CnH2mOp ^ (2n + m - p) MexOy—

+ n CO2 + m H2O AHr

In the air reactor (AR), the metal or reduced metal oxide is oxidized with air so that the regenerated material is ready to start a new cycle. The gas obtained in this second step contains nitrogen and unreacted oxygen.

(2n + m- p)MexOy-1 +(n + m/2- p/2)O2^ (2n + m- p)MexOy AHo

In this system it is important to point out that the total heat involved in both reactors is the same as for normal combustion where fuel is in direct contact with air.

CnH2mOp +(n + m/2- p/2)O2 ^nCO2 +mH2O AHC = AHr + AHo (3)

Many materials have been tested to assess if they are good candidates for CLC technology. A review of the status can be found in Adanez et al. (2012).

Nickel materials have received more attention than other candidates for CLC combustion due to its high reactivity and thermal

1750-5836/$ - see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijggc.2013.01.025

Table 1

Summary of Ni-based oxygen carriers tested in continuously operated CLC units for gaseous fuels.

Oxygen carrier3 Metal oxide (wt.%) Support material Power (kW) Reacting gasb Reference

Ni18-aAl:Hl 18 a-Al2O3 0.5 CH4 Adanez et al. (2009a)

0.5 H2 , CO, syngas Dueso etal. (2009)

0.5 C2 H6, C3 H8 Adanez et al. (2009b)

0.5 CH4 + H2S Garcia-Labiano et al. (2009)

Ni35-Al:COP 35 Al2O3 1 Syngas + H2 S Shenet al.(2010)

Ni60-NiAl:SF 60 NiAl2O4 10 n.g Linderholm et al. (2008)

Ni60-NiAl:FG 60 NiAl2O4 0.3 n.g Johansson et al. (2006a)

Ni40-NiAl:FG 40 NiAl2O4 10 n.g. Johansson et al. (2006b) and Lyngfelt and Thunman

(2005)

Ni40-NiAl:SD 40 NiAl2O4 H2 Ishida et al. (2002)

Ni40-NiAl:SD 40 NiAl2O4 10 n.g Linderholm et al. (2009)

65 H2,CO Kolbitsch et al. (2009a) and Kolbitsch et al. (2010)

140 n.g Bolhar-Nordenkampf et al. (2009), Kolbitsch et al.

(2009a,b) and Proll et al. (2009)

140 CH4 Kolbitsch et al. (2009a) and Kolbitsch et al. (2010)

Ni40-NiAl-Mg:SD 40 NiAl2O4-MgO 10 n.g Linderholm et al. (2009)

140 n.g Bolhar-Nordenkampf et al. (2009), Kolbitsch et al.

(2009a,b) and Proll et al. (2009)

Ni20-MgAl:FG 20 MgAl2O4 0.3 n.g Ryden etal. (2008)

Ni60-MgAl:FG 60 MgAl2O4 0.3 n.g Johansson et al. (2006a)

0.17 Syngas Johansson et al. (2006b) and Mattisson et al. (2007)

Ni60-B:MM 60 Bentonite 50 CH4 Ryu et al. (2004)

Syngas Ryu et al. (2010a)

n.g. Ryu et al. (2010a)

Ni60-B:MM 60 Bentonite 1.5 CH4 Son and Kim (2006)

Ni40-Zr-Mg:FG 40 ZrO2-MgO 0.3 n.g Ryden et al. (2009)

OCN 702-1100 n.a. 50 n.g Ryuet al. (2010b)

OCN 703-1100 n.a. 50 n.g Ryu et al. (2010a)

Syngas Ryu et al. (2010a)

a HI, hot incipient impregnation; COP, co-precipitation; SF, spin flash; FG, freeze granulation; SD, spray drying; MM, mechanical mixing. b n.g., natural gas.

stability at high temperatures (1173-1373 K). Adanez et al. (2012) reported that more than 2500 h of the 3500 h of total operational experience in continuous CLC and CLR plants for gaseous fuels have been carried out with Ni-based oxygen carriers. Furthermore, Ni appears as one of the most interesting metals to be used for CLR technology due to its strong catalytic properties (Adanez et al., 2012). However, this material presents some disadvantages such as thermodynamic equilibrium that cause a small presence of CO and H2 in the gas outlet of the FR, its higher cost compared to other metal oxides or the requirement of additional safety measures because of its toxicity.

The use of alumina-based compounds as support material for Ni-based oxygen carriers has been extensively investigated in the literature. In comparison with other metal oxides, most of Ni-oxygen carriers supported on Al2O3 compounds showed very high reactivity with all fuel gases, no agglomeration problems, low attrition rates during operation in fluidized beds and avoidance of carbon deposition at CLC conditions. However, reduction of NiO/Al2O3 particles is limited by the partial transformation of NiO into NiAl2O4 spinel compound (Copeland et al., 2001; Ryu et al., 2003), which has poor reactivity for CLC (Villa et al., 2003). Nevertheless, high reactivity and low NiAl2O4 formation was found in some cases using mechanical mixing and impregnation methods (Adanez et al., 2004; Gayan et al., 2008; Sedor et al., 2008a,b).

Table 1 summarizes the Ni-based oxygen carriers that have been tested in continuously operated CLC units for gaseous fuels by the moment. As it can be seen in this table, the content of NiO used in the particles of the different oxygen carriers can be high, up to 40-60 wt.%. High contents of NiO are used when oxygen carriers are prepared by spray drying or freeze granulation methods in order to increase the crushing strength of the particles. However, particles obtained by the impregnation method can be prepared with low contents of NiO. Due to the high cost and the special environmental-safety precautions that have to be taken with nickel-based materials, the current tendency in the development

of CLC technologies related to Ni-based oxygen carriers is directed to minimize the amount of NiO that contains the oxygen carrier particles.

Our research group at the Instituto de Carboquímica (ICB-CSIC) has carried out several studies using Ni-based oxygen carriers. Firstly, potential oxygen carriers were prepared using different supports (Adánez et al., 2004). Later, different oxygen carriers prepared by dry impregnation using ^-Al2O3 as support were tested in a thermogravimetric analyzer and in a batch fluidized bed to assess the reactivity, selectivity, attrition rate and agglomeration behavior during methane combustion (Gayán et al., 2008). It was concluded that modifications of the support via thermal treatment, to transform it into a-Al2O3, or via chemical deactivation with Mg or Ca oxides, to obtain MgAl2O4 or CaAl2O4 as supports, minimized the interaction of NiO with alumina. Very high reactivity, regenerability and high methane combustion selectivity to CO2 and H2O were achieved using those supports. The next step was to evaluate the behavior of an oxygen carrier containing 18 wt.% NiO impregnated on a-Al2O3 in a continuously operated 500 Wth CLC prototype using methane and syngas as fuels (Adánez et al., 2009a; Dueso et al., 2009). Regarding methane combustion, at 1153 K in the fuel reactor, an oxygen-carrier to fuel ratio, (, higher than 3 and a solid inventory in the fuel reactor of 600 kg per MWth were necessary to reach combustion efficiencies close to the maximum allowed by the thermodynamic equilibrium. On the other hand, when syngas was used as fuel, high combustion efficiencies were reached at temperatures as low as 1073 K for ( values above 5. In both cases, the oxygen carrier exhibited an adequate behavior regarding processes such as attrition, agglomeration and carbon deposition during 100 h with methane and 50 h with syngas of continuous operation. Moreover, Adánez et al. (2009b) investigated the effect of the presence of light hydrocarbons (C2H6 and C3H8) in the feeding gas of a CLC system using that Ni-based oxygen carrier (Adánez et al., 2009a; Dueso et al., 2009). They concluded that an oxygen carrier-to-fuel ratio higher than 3

and a temperature of 1153 K were needed to obtain high combustion efficiencies. According to these results, no special measures should be adopted due to the presence of light hydrocarbons in the fuel gas of a CLC plant using a Ni-based oxygen-carrier.

The formation of NiAl2O4 together with NiO during the oxidation process of this Ni-based oxygen carrier tested in the CLC unit at the ICB (Adanez et al., 2009a,b; Dueso et al., 2009) was the responsible for the elevated solids inventories needed to burnt properly the different gaseous fuels. NiAl2O4 spinel reacts with these gases, but the corresponding reaction rates are lower than the ones that free NiO presents. Therefore, a Ni-based oxygen carrier without interaction between NiO and the support was further developed.

The objective of this work was to test the behavior of a Ni-based oxygen carrier with low NiO content, NillCaAl, for the combustion of different fuels in a 500 Wth CLC plant under continuous operation at atmospheric pressure. This oxygen carrier was developed by the incipient wet impregnation method using CaAl2O4 as support in order to avoid its interaction with NiO. The influence of different operating conditions, such as the oxygen carrier-to-fuel ratio and the fuel gas concentration, were analyzed.

2. Experimental

2.1. Preparation of the oxygen carrier

The support, CaAl2O4, was obtained by mixing commercial 7-Al2O3 (Panreac, dp< 10 |im) with CaCO3 (Panreac, dp< 10 |im) and graphite in the following proportions: 33wt.% CaCO3, 58wt.% 7-Al2O3 and 9wt.% graphite. The mixture was then pelletized by pressure in a hydraulic pelletizer. Pellets were calcined at 1673 K during 18 h. After that, pellets were milled and sieved to obtain CaAl2O4 particles in the range +100-300 |im.

CaAl2O4 particles were impregnated with a saturated solution (293 K, 4.2M) of Ni(NO3)2-6H2O (>99.5% Panreac) corresponding to the total pore volume of the support particles. The aqueous solution was slowly added to the support particles, with thorough stirring at room temperature. Two successive impregnation steps were applied to obtain the desired active phase loading (11.8 wt.%). In the first impregnation, 0.2 mL of nitrate solution were added per gram of support. In the case of the second impregnation, the volume of solution added to the material was 0.15 mL/g. The material resulting from the first impregnation was calcined at 823 K in air atmosphere for 30 min to decompose the impregnated metal nitrate into the metal oxide. Finally, after the second impregnation, the oxygen carrier was sintered in a furnace at 1223 K for 1 h. The carrier was designated as Ni11CaAl.

2.2. Oxygen carrier characterization

Several techniques have been used to characterize physically and chemically the fresh oxygen carrier particles. The force needed to fracture a particle was determined using a Shimpo FGN-5X crushing strength apparatus. The mechanical strength was taken as the average value of at least 20 measurements. The pore volume was measured by Hg intrusion in a Quantachrome PoreMaster 33, whereas the surface area of the oxygen carrier was determined by the Brunauer-Emmett-Teller (BET) method by adsorption/desorption of nitrogen at 77 K in a Micromeritics ASAP-2020 (Micromeritics Instruments Inc.). The identification of crystalline chemical species was carried out by powder X-ray diffractometer Bruker AXS graphite monochromator. Finally, some oxygen particles were analyzed in a scanning electron microscope (SEM) ISIDS-130 coupled to an ultra thin window PGT Prism detector for energy-dispersive X-ray (EDX) analysis.

Table 2 shows the main physical properties of the Ni11CaAl oxygen carrier. The NiO active content for the CLC process was

Table 2

Properties of the oxygen carrier NillCaAl.

Units Values

NiO content wt.% 11.8

Oxygen transport capacity, Roc % 2.5

Particle size mm 0.1-0.3

Particle density kg/m3 1400

Crushing strength N 1.2

Porosity % 40.4

Specific surface area BET m2/g 2.1

XRD species NiO, CMI2O4, CMI4O7

determined by complete reduction of the sample with H2 in TGA at 1223 K. The oxygen transport capacity was defined as the percentage of mass fraction of oxygen that can be used in the oxygen transfer, calculated as Roc = [(mox -mred)/mox] x100, where mox and mred are the masses of the oxidized and reduced form of the oxygen carrier, respectively. The mechanical strength of the fresh particles is 1.2 N, which it is adequate since values lower than 1 N are believed to be too soft for long-time circulation (Johansson et al., 2004). A suitable particle density of 1400 kg/m3 for fluidiza-tion operation was measured considering the particle size of the material and the gas flows. The specific surface area is low. This value is associated with the extreme conditions adopted during the calcination process of the support (1673 K during 18 h). The porosity of particles is equal to 40.4%. Finally, XRD analysis shows that the only Ni-based crystalline phase present in the fresh particles of the oxygen carrier is NiO. NiAl2O4 formation is avoided due to the interaction of calcium with alumina to form two different calcium aluminates: CaAl2O4 and CaAl4O7.

Fresh particles of the Ni11CaAl oxygen carrier were analyzed by SEM to determine its microstructure. Moreover, some of these particles were cut, polished and analyzed by EDX to carry out an elemental microanalysis. An image of the cross section of a fresh particle is shown in Fig. 1. From that image it can be observed a high void volume inside the particle. The dark space inside the particle

Fig. 1. SEM-EDX images of a Ni11CaAl fresh particle. (a) BSE (back-scattering electrons) photograph of a cross section of a fresh particle. (b) EDX line profile of Ni in a cross section of a fresh particle.

corresponds to large size pores (20-40 |im) interconnected among them. On the other hand, EDX analysis confirms that Ni content was homogeneously distributed inside the particles.

2.3. ICB-CSIC-gl facility

Fig. 2 shows the 500 Wth CLC unit operating at atmospheric pressure, designated as ICB-CSIC-g1, used to determine the behavior of the oxygen carrier under continuous combustion of different fuel gases. The unit was composed of two interconnected fluidized-bed reactors, a riser for solid transport, a solid valve to control the solids circulation between reactors, a loop seal and a cyclone. This design allowed the control and measurement of the solid circulation flow rate between both reactors.

FR (1) consisted of a bubbling fluidized-bed (0.05 m i.d.) with a bed height of 0.1 m. The AR is 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. A further description of this CLC prototype is present elsewhere (Adanez et al., 2009a). The gas outlet streams of the FR and AR were drawn to respective on-line gas analyzers to get continuous data of gas composition. The outlet gas from the FR was composed by N2, CO2, H2O, CH4, H2, and CO. The composition of gases from the AR was N2 and unreacted O2. If carbon formation on particle occurs in the FR, also CO and CO2 could be form in the AR when is oxidized with air. CH4, CO2, CO and H2 concentrations in the gas outlet stream from the FR were measured after steam condensation, whereas O2, CO, and CO2 concentrations were measured at the gas outlet stream from the AR.

2.3.1. Testing conditions

Combustion tests under different conditions and fuel gases were conducted in the 500 Wth CLC unit with the Ni11CaAl oxygen carrier. The total solids inventory in the system was ^1.0 kg, of which 0.2 and 0.4 kg were in the FR and AR, respectively. The particle size of the oxygen carrier is 0.1-0.3 mm. A total operation of about 90 h at hot conditions, of which 50 h were at combustion conditions, was carried out using the same batch of oxygen carrier particles.

The inlet flow in the FR was varied from 170 to 255 l N/h which corresponds to an inlet gas velocity of 0.1-0.15 m/s, i.e., about 4-6 times the minimum fluidization velocity for particles with a size of 0.3 mm and a density of1400 kg/m3. Air was used as fluidizing gas in the AR, which was divided into the fluidizing gas in the bottom bed (7201 N/h) and the secondary air in the riser (1501 N/h). Nitrogen was used as a fluidizing gas in the particle loop seal (38 l N/h).

Table 3 shows a summary of the different operating conditions used in the tests. Different types of fuels were used during the experimental work: methane, syngas, CO, H2 and light hydrocarbons (C2H6 and C3H8).

The oxygen carrier-to-fuel ratio, p, was defined by Eq. (4), where FMeO is the molar flow rate of the metal oxide and fFuej is the inlet molar flow rate of the fuel in the FR. The parameter b is the stoichiometric coefficient of the fuel gas. A value of p = 1 corresponds to the stoichiometric MeO amount needed for maximum conversion of fuel to CO2 and H2O.

b ■ FFuel

Two different experimental test series were carried out when methane was used as fuel for the CLC unit. In test series M1-M6, the effect of the oxygen carrier/fuel ratio, p, on the combustion efficiency was analyzed by changing the fuel concentration whereas the gas velocity and solids circulation rates, fs, were maintained roughly constant. For these tests, methane concentrations ranging from 25 to 50 vol.%, corresponding to power input between 422 and 850 Wth, were used.

Two additional tests were also carried out to analyze the effect of the H2O concentration in the FR on the combustion efficiency (tests M7 and M8) to aid in the analysis of the obtained results. Test M8 was carried out feeding 20 vol.% of steam mixed with the gas fuel stream, at similar operating conditions as in test M7.

Regarding syngas combustion, two different CO/H2 ratios, 1 and 3, have been studied (S9-S16). These ratios correspond to typical gas compositions obtained in fluidized-bed and entrained-bed gasi-fiers, respectively. The resulting equilibrium compositions of these experiments are shown in Table 3 assuming the WGS equilibrium in the FR. Experiments using only CO (S1-S6) and H2 (S7-S8) as fuel gases were also done for comparison purposes. In addition, combustion tests of light hydrocarbons, i.e., C2H6 and C3H8, were conducted (Tests LHC1-LHC6). Two different gas compositions were used for each type of fuel (see Table 3). These compositions were calculated to have the same oxygen demand in both cases. In this sense, it must be considered that ethane and propane require more oxygen for combustion than methane. In all tests with C2H6 and C3H8, 30 vol.% steam was introduced to avoid carbon formation in the distributor plate of the FR.

The behavior of the OC with different fuel gases was assessed by means of the effect of the oxygen carrier-to-fuel ratio on the combustion efficiency. The combustion efficiency (rc) 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 rc gives an idea about how the CLC operation is close or far from the complete combustion of the fuel, i.e., rc = 100%.

(2xCO2 + xCO +xH2O)out ■Fout - (2xCO2 + xCO + xH2O)in ■fin

(XCO + XH2 + 4xch4 + 7XC2 H6 + 10xc3 H8 )in ■ Fn

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 i.

3. Results

3.1. Tests in the continuous CLC plant

Combustion tests under different operation conditions were carried out in the CLC plant using the Ni11CaAl as oxygen carrier. The oxygen carrier-to-fuel ratio and the fuel composition were studied as variables affecting to the combustion efficiency, rc. The gas product concentrations of the FR and AR were measured by online analyzers and the corresponding results were used to make carbon, hydrogen and oxygen mass balances over the whole system. Regarding the oxygen carrier behavior, neither deactivation nor agglomeration were detected during operation in the continuous unit.

Fig. 3 shows the effect of the p value on the combustion efficiency for combustion tests M1-M6 where CH4 fuel flow was varied. Regarding combustion tests M1-M6, very high CH4 combustion efficiencies were found operating with low oxygen carrier-to-fuel ratios (1.1-1.2). A sharp decrease in efficiency was found for values above 1.2. An important difference was found with respect to the results obtained with most of the previous oxygen carrier materials tested in the literature (Adanez et al., 2012), including the one developed by Adanez et al. (2009a), where an increase in the p value produces an increase in the combustion efficiency. These results are also shown in Fig. 3 for comparison purposes. However, Lyngfelt and Thunman (2005) observed a similar behavior using an oxygen carrier prepared by freeze granulation over Al2O3.

With previous Ni-based oxygen carriers developed to be tested in CLC continuous units, it was necessary to operate at higher p values (3-3.5) to reach near complete combustion efficiency

Fig. 2. Schematic diagram of the ICB-CSIC-g1 facility.

(Adanez et al., 2009a; Linderholm et al., 2008; Lyngfelt and Thunman, 2005). However, with this material the combustion efficiency reached the maximum value allowed by the thermodynamic equilibrium at p values around 1.1-1.2. A further increase

£ c a ô it a c o

■Q 70 E o o

\ J3- D

w- Ni18-aAl:HI

(Adanez et al. 2009a)

Ni11CaAl \

Fig. 3. Effect of the oxygen carrier/fuel ratio (p) on the combustion efficiency (rc) for CH4 combustion with the Ni11CaAl oxygen-carrier. T =1173 K. Tests M1-M6 (• ). Tests M7-M8 (O, with H20/4 , without H20). For comparison reasons, tests using Ni18-aAl:HI oxygen-carrier have been added.

of p presented a negative effect on the combustion efficiency. Decreasing the oxygen carrier/fuel ratio below 1.0, combustion efficiency decreases as less oxygen is available for full oxidation of the fuel.

In addition, two new tests (M7-M8) were carried out to study the effect of steam addition in the inlet flow to FR and similar results were obtained in both cases.

Fig. 4 shows the effect of the p value on the combustion efficiency for syngas tests (S1-S16). Complete combustion is achieved working at oxygen carrier-to-fuel ratios higher than 1.2 for all fuel gas compositions, including pure CO, which is the gas that presents the worst behavior when it reacts with a Ni-based oxygen carrier. Note that in this case combustion efficiency does not decrease at high values of p-

From the experimental data obtained in the tests LHC1-LHC6, the combustion efficiency, rc, was calculated according to Eq. (5). Fig. 5 illustrates the effect of p on the combustion efficiency for both ethane and propane. The maximum fuel combustion efficiencies were reached at same p values than methane. Furthermore, better efficiencies were obtained for ethane than for propane as fuel gas. In the case of ethane, a combustion efficiency of 99% was obtained working at an oxygen carrier-to-fuel ratio of 1.1.

In order to explain the remarkable results obtained in the combustion tests carried out with the Ni11CaAl oxygen carrier, the gas composition at the outlet of the FR is analyzed for each type of gas combustion tests.

Table 3

Operating conditions and main data for the experiments carried out in the ICB-CSIC-gl facility.

Test Tfr (K) Qfr (l/h) Fuel

M1 1173 170 50

M2 1173 170 45

M3 1173 170 40

M4 1173 170 35

M5 1173 170 30

M6 1173 170 25

M7 1173 170 25

M8a 1173 170 25

Syngas

S1 1153 255 80

S2 1153 213 80

S3 1153 170 80

S4 1153 170 80

S5 1153 170 60

S6 1153 170 40

S7 1153 170 60

S8 1153 170 40

Syngas CO/H2 = 1

S9 1153 255 80

S10 1153 213 80

S11 1153 170 80

S12 1153 170 40

Syngas CO/H2=3

S13 1153 255 80

S14 1153 213 80

S15 1153 170 80

S16 1153 170 40

Light hydrocarbons

LHC1b 1173 170 17.2

LHC2b 1173 170 14.3

LHC3b 1173 170 11.4

LHC4b 1173 170 12

LHC5b 1173 170 10

LHC6b 1173 170 8

Bold numbers refer to the changed variable.

a 20 vol.% H2O added.

b 30 vol.% H2O added.

Equilibrium concentration (vol.%)

fs (kg/h)

Power (Wth)

Solid inventory in FR(kg/MWth)

CO H2O CO2 H2

9.4 0.9 846 154

9.4 1.0 761 172

9.4 1.1 677 193

9.4 1.3 593 221

9.4 1.5 507 258

9.4 1.8 422 310

12 2.2 422 310

12 2.2 422 310

6.6 0.9 716 183

6.6 1.1 594 221

6.6 1.4 516 254

9.2 2.0 477 275

9.2 2.6 358 366

9.2 4.0 238 550

9.2 2.6 306 428

9.2 4.0 204 642

40 10 8 40 6.6 0.9 651 201

40 10 8 40 6.6 1.1 542 242

40 10 8 40 6.6 1.4 434 302

20 5 4 20 6.6 2.9 217 604

60 6 14 20 6.6 0.9 682 192

60 6 14 20 6.6 1.1 568 231

60 6 14 20 6.6 1.4 455 288

30 3 7 10 6.6 2.9 227 577

6.8 0.9 517 253

6.8 1.1 431 317

6.8 1.4 345 380

6.8 0.9 517 253

6.8 1.1 431 317

3.1.1. Methane combustion

Fig. 6 shows the gas composition obtained at the outlet of the FR as a function of 0. As it was mentioned above, methane was fully converted only at 0 values around 1.1-1.2. At lower

values, partial oxidation of fuel takes place producing CO + H2 and methane does not appear at FR outlet. At higher values, methane is the only unburnt product, indicating that the partial oxidation/reforming reactions are not occurring at considerable amount

a ö iE

3 ■Q

Fig. 4. Effect of the oxygen carrier/fuel ratio (0) on the combustion efficiency (%) for syngas combustion with the NillCaAl oxygen-carrier. T =1153 K.

Fig. 5. Effect of the oxygen carrier/fuel ratio (0) on the combustion efficiency (-qc) for light hydrocarbons combustion with the NillCaAl oxygen-carrier. T =1173 K.

£ 8 o

H2 T A A

CO \ \ Aw,

--A—J

1.5 ♦

Fig. 6. Concentration of CO, H2 and CH4 measured at the outlet of the FR in the 500 Wth unit for combustion tests carried out with methane. TFr = 1173K.

Tests M1-M7: CO (• ), H2 (■ ), and CH4 (A ). Test M8: CO (O ), H2 (□ ), and

CH4 (A).

in these conditions. These facts indicate that reaction occurs by different pathways depending on the p value.

In this work, since a low NiO content oxygen carrier was used, the relevance of the reduction degree needed to achieve full methane conversion was pointed out as responsible of this behavior. Only working at very low oxygen carrier to fuel ratios (1.0-1.2), the full conversion of methane was achieved.

The effect of water concentration on the steam reforming reaction was analyzed through the methane conversion obtained using H2O concentrations in the FR up to 20vol.%. In the experiments using CH4 as fuel gas, water was not fed into the system because previous works in the same CLC facility using a Ni-based oxygen carrier showed that carbon formation did not happen at relevant extension (Adanez et al., 2009a,b), anywhere in the FR. Furthermore, during methane combustion tests, CO and CO2 were never detected at the outlet of the AR, indicating no carbon formation in the FR. As it can be seen in Figs. 3 and 6, similar combustion efficiencies and gas product distributions were obtained in tests M7 and M8, indicating that methane reforming reaction, at high oxygen carrier-to-fuel ratio values, is negligible using this material even in the case when a steam concentration of 20 vol.% was added.

3.1.2. Syngas combustion

Several syngas combustion tests under continuous operation were carried out in the CLC plant. Table 3 shows the different fuel gas compositions that were used. Fig. 4 illustrates that the combustion efficiency is very high for all syngas tests where p >1.2, close to the value that corresponds with the thermodynamic equilibrium. Furthermore, for fixed p values, similar combustion efficiencies were obtained independently of the fuel gas composition.

Dueso et al. (2009) found that this high combustion efficiency achieved when CO is used as fuel was partially due to the increase of CO consumption through the faster disappearance of H2. This fast disappearance of H2 shifts the WGS equilibrium toward the formation of more H2 and CO2, which implies a higher CO conversion.

In order to highlight the high reactivity obtained with the Ni11CaAl oxygen carrier when syngas is used as fuel, a comparison with other Ni-based oxygen carrier developed by impregnation using a-Al2O3 as support is pointed out. Dueso et al. (2009) found that with the Ni18-aAl2O3 oxygen carrier, p values higher than 5 were needed to obtain high combustion efficiencies (98.3%) of syngas when the temperature in the FR reached 1153 K. At the

vt re ■C

Fuel gas

C2H6 C3H8

CO o •

H2 □ ■

CH4 A A

.¡= 2

1.2 ♦

Fig. 7. Concentration of CO, H2 and CH4 measured at the outlet of the FR in the 500 Wth unit for combustion tests carried out with light hydrocarbons. TFr = 1173 K.

same testing conditions of fuel composition and temperature, maximum combustion according to thermodynamic equilibrium was achieved with the Ni11CaAl oxygen carrier working at oxygen-to-fuel ratios only higher than 1.2 for all fuel gas compositions. The high reactivity of Ni11CaAl oxygen carrier with both gases i.e., CO and H2, is mainly due to the fast reaction of free NiO with such gases. As in this case the formation of less reactive nickel compounds, such as NiAl2O4, is avoided using CaAl2O4 as inert support, the behavior of this oxygen carrier is largely improved when syngas is used as fuel. On the contrary, as Dueso et al. (2009) used alumina as support, NiAl2O4 was formed during Ni oxidation and very higher p values were needed to burn the syngas completely.

3.1.3. Light hydrocarbons (LHC) combustion

Combustion tests of light hydrocarbons, i.e., ethane and propane, were conducted in the 500 Wth CLC facility described above. Two different gas compositions were used for each type of fuel (see Table 3).

Fig. 7 illustrates that the amounts of unreacted of CO, H2 and CH4 at the outlet of the FR are higher for propane than for ethane and thus higher rc values were measured for ethane. At p ^ 1, the majority of unburnt products are CO and H2 due to the catalytic activity of the oxygen carrier working at very low oxygen-to-fuel ratios. However, at higher values of p, which correspond to lower values of variation of the solids conversion in the reactor, AXS, the oxygen carrier does not present that catalytic activity and methane appears as the main unburnt product. In this sense, the presence of methane in the gas stream at the outlet of the FR indicates that the mechanism of light hydrocarbons combustion is carried out through the cracking reaction. These results are different from the ones obtained by Adanez et al. (2009b) using a Ni-based oxygen carrier supported on alumina, who never detected methane at the outlet of the FR when light hydrocarbons were burnt.

Carbon formation process was also analyzed during the continuous LHC combustion tests carried out on the CLC unit. The carbon formed on the Ni11CaAl particles in the FR should be transferred to the AR and burned giving CO and/or CO2. CO and CO2 were never detected at the outlet of the AR, indicating no carbon formation in the FR and, consequently, no losses in CO2 capture by carbon transfer to the AR.

Finally, from these results it must be remarked that the Ni11CaAl oxygen carrier presents very high combustion efficiencies when light hydrocarbons are used as fuels in the CLC prototype operating

at low oxygen carrier-to-fuel ratio values ^0.99 in the case of ethane for a 0 value of 1.1). However, other Ni-based oxygen carriers using Al2O3 as support required oxygen carrier-to-fuel ratio values higher than 3 to achieve similar combustion efficiencies.

4. Discussion

In order to carry out a further analysis related to combustion efficiency and gas product distribution results obtained for the different combustion tests presented in the previous section of this work, the possible reactions involved in a CLC process using a nickel-based oxygen carrier during combustion tests with methane, ethane, propane, H2 and CO, must be taken into account.

The following reactions can take place in the system:

Fuel reactor: Combustion

CnH2n+2 + (3n +1)NiO ^ (3n + 1)Ni + nCO2 + (n + 1)H2O (6) H2 +NiO ^ Ni + H2 (7)

CO + NiO ^ Ni + CO2 (8)

Partial oxidation

CnH2n+2 + nNiO ^nNi + nCO + (n + 1)H2 (9)

Catalytic reforming

CnH2n+2 + nH2O ^nCO + (2n + 1)H2 (10)

CnH2n+2 + n CO2 ^ 2n CO + (n + 1)H2 (11)

Water gas shift

CO + H2O ^ CO2 +H2 (12)

Methanation

CO + 3H2 ^ CH4 + H2 (13)

Dehydrogenation

CnH2n+2 ^ CnH2n +H2 (14)

Cracking

CnH2n+2 ^ Cn-1H2(n-1) + CH4 (15)

Coke formation

CnH2n+2 ^n C + (n + 1)H2 (16)

CO + H2 ^ C + H2O (17)

Carbon gasification

C + CO2 ^ 2CO (18)

Air reactor:

Oxygen carrier oxidation

Ni + (1 /2) O2 ^ NiO (19)

Carbon combustion

C + O2 ^ CO2 (20)

C + (1/2) O2 ^ CO (21)

Regarding methane combustion, there is evidence that the partial oxidation/reforming reactions (9)-(11) can be decisive in methane conversion when Ni-based oxygen carriers are used. For Ni-based oxygen carriers the unconverted products were H2 and CO when the temperature was low (Jin and Ishida, 2002) or the oxygen in the particles was depleted (Adanez et al., 2006; Chandel et al., 2009; Dueso et al., 2010). These facts have been related to

that H2 and CO are produced as intermediate products by the steam reforming or partial oxidation of methane, and then they react to H2O and CO2 following reactions (7) and (8). Therefore, the mechanism for methane conversion via partial oxidation/steam reforming can be of higher relevance than the direct conversion of methane to CO2 and H2O under certain conditions. Moreover, the degree of the reduction of the NiO particles has shown a very strong influence on the catalytic activity for methane reforming. A high degree of oxidation results in an almost complete deactivation of Ni sites for methane adsorption, decreasing the catalytic activity of the material (Dewaele and Froment, 1999). It has been suggested (Abad et al., 2010a; Kolbitsch et al., 2009b) that the steam reforming of methane was catalyzed by metallic nickel formed during reduction.

Ortiz et al. (2012) found that the catalytic activity of the impregnated Ni-based oxygen-carriers depended on their oxidation/reduction degree, increasing the catalytic activity when the reduction conversion of the oxygen carrier was increased. Therefore, it can be said that the methane reforming reactions are of great relevance to achieve a high methane conversion in a CLC system. Furthermore, it must be pointed out that the 0 value is related to the variation of the solid conversion in the reactor, AXS, and the conversion of the oxygen carrier in the FR, Xr, as follows:

AXS =Xr = ^ (22)

In this work, since a low NiO content was used in the oxygen carrier, the relevance of the reduction degree needed to achieve full methane conversion was more important. Only working at very low oxygen to fuel ratios above stoichiometric conditions (1.0-1.2), which correspond to high reduction conversion values (AXS = 0.8-0.9), the full conversion of methane was achieved due to the catalytic activity of the oxygen carrier in these testing conditions.

On other hand, the high combustion efficiencies obtained when syngas combustion tests were carried out using the Ni11CaAl material can be explained in accordance with the following reasons. Firstly, the use of CaAl2O4 as inert support avoids the formation of a less reactive nickel compound during previous Ni oxidation, such as NiAl2O4, that reacts with CO and H2. This fact allows fast reactions of free NiO with such gases through reactions (7) and (8), particularly with H2. However, CO disappearance is also improved due to the increase of CO consumption through the faster disappearance of H2 by the WGS reaction (12) with the oxygen carrier that shifts the WGS equilibrium towards the formation of more H2 and CO2, which implies a higher CO conversion.

Within the mechanism of LHC combustion, methane formation mainly takes place through cracking reaction (Eq. (15)). In a further step, mechanism for methane conversion via partial oxidation/steam reforming can be of higher relevance than the direct conversion of methane to CO2 and H2O as it was discussed for the methane combustion case. In this sense, if 0 is fixed at a high value, consequently the degree of the reduction of NiO particles is low and the catalytic activity of the oxygen carrier for methane reforming decreases which gives an increase of unre-acted methane as a result. This could be clearly observed in Fig. 7 where the amount of unreacted methane is increased when the 0 value is varied from 0.9 to 1.5. Adanez et al. (2009b) did not find this phenomenon when they carried out light hydrocarbon combustion tests because as they used alumina as support, during Ni oxidation NiAl2O4 was also formed as a Ni-phase together with free NiO. The amount of nickel that came from the spinel compound, NiAl2 O4, presented catalytic activity for methane reforming, which permitted full methane combustion even in the situation

Table 4

Solids inventories needed to obtain a combustion efficiency of 99% using different Ni-based oxygen carrier particles and CLC prototypes.

Oxygen carrier Power (kW) 0 Solid inventory in FR (kg/MWth) Ni inventory in FR (kg/MWth) Reference

NillCaAl 0.5 1.0-1.2 180 17 This work

Ni18-aAl:Hl 0.5 3.1 600 85 Adanez etal. (2009a)

Ni40-NiAl:FG 10 5 335 170 Lyngfelt and Thunman (2005)

Ni60-NiAl:SF 10 5 335 200 Linderholm et al. (2008)

Ni60-MgAl:FG 0.3 20 630 300 Johansson et al. (2006a)

Ni60-NiAl:FG 0.3 10 400 240 Johansson et al. (2006b)

Ni60-B:MMa 1.5 >1.9 >760 >360 Son and Kim (2006)

Ni60-B:MM 50 - 1250 590 Ryuet al. (2010a)

a Vc <99%.

when the degree of reduction of NiO particles was low (Ortiz et al., 2012).

4.1. Considerations about design criteria

From the results shown in this work, the solids inventory needed to obtain a combustion efficiency of 99 % using this oxygen carrier for burning different gaseous fuels has been calculated. For methane combustion, a total solids inventory of about 180kg/MWth is obtained in the 500 Wth CLC plant. This solid inventory is much lower than other values also found for highly reactive impregnated Ni-based materials as it can be observed in Table 4. For example, 600 kg/MWth and 335 kg/MWth were needed for the Ni18-aAl (Adanez et al., 2009a) and Ni40-NiAl (Lyngfelt and Thunman, 2005) oxygen carriers, respectively. This remarkable result was due to the low NiO content and the very high reactivity of the Ni11CaAl oxygen carrier, since the formation of less reactive nickel compounds, such as NiAl2O4, was avoided using CaAl2O4 as inert support. Furthermore, the differences in the amount of Ni metal from these oxygen carriers are also noteworthy. On the basis of the amount of Ni metal per MWth, the nickel inventory for Ni11CaAl is around 17 kg/MWth. This extremely low value is due to the small amount of NiO (11.8 wt.%) that the oxygen carrier particles contain, which is almost completely used (AXS = 0.8-0.9). It must be pointed out that although this solid inventory was determined operating in a bubbling fluidized bed reactor, it is the lowest value found in the literature (Adanez et al., 2012) for any kind of Ni-based oxygen carriers. Nevertheless, these numbers just gives an estimate of the solids inventories that could be needed. The solids inventories that appear in Table 4 are not directly comparable among them because differences in the fluidization conditions at different CLC prototypes may have important effects on the mass-transfer resistance between gas and solids.

Dueso (2010) developed a simplified model to calculate the most suitable operating conditions, in terms of solid circulation rate and inventories, for the CLC process when a Ni-based oxygen carrier was prepared using alumina as support. She proved that NiAl2O4 has a negative effect on the solid inventories in a CLC system when a Ni-based material was used as oxygen carrier. She calculated a reduction in the minimum solid inventory in the FR from 37 kg Ni/MW to 2 kg Ni/MW for methane combustion when the formation of the NiAl2O4 spinel is completely avoided working with a variation of solid conversion of AXS = 0.2. In this work, for the Ni11CaAl oxygen carrier, the optimum operation conditions, and complete oxidation in the AR, are obtained working at AXS = 0.8-0.9. In these conditions, the solid inventory in the FR predicted by the model of Dueso (2010) was 5.7 kg Ni/MWth using a Ni-based oxygen carrier without interaction NiO-support. In this point, it must be highlighted that the solids inventories were calculated using a simplified method developed by Abad et al. (2007) considering the reactivity of the particles but without taking into consideration

the effects of the resistance to exchange of gas between the bubbles and the emulsion in the fluidized bed. Therefore, higher solids inventories would be necessary in a real CLC system. In this sense, Abad et al. (2010b) calculated solid inventories 2-10 times higher when the resistance to the exchange of gas between bubble and emulsion phase is considered in the model. As a result, it can be stated that within this work it has been experimentally demonstrated that avoiding the formation of the spinel phase, NiAl2O4, lower solid inventories are needed to obtain complete gas conversion in a CLC continuous unit using a Ni-based material as oxygen carrier.

Additionally, the solid circulation in a CLC unit must be fixed to fulfill the mass and energy balances in the system. The solids circulation is limited by the transport capacity of the riser. The limit of the circulation rate in a CLC unit is not clear, but a value of 16 kg s-1 per MWth of methane can be taken as the maximum circulation rate feasible in a CLC plant without increased costs and with commercial experience (Abad et al., 2007). Assuming this value, oxygen carriers with oxygen transport capacity values lower than 0.4% could not be used for CLC because it would not be possible to transfer the required oxygen to fully convert the fuel to CO2 and H2O (Abad et al., 2010a). For Ni11CaAl oxygen carrier, an amount of 11.8 wt.% of NiO in the material is enough due to its high transport capacity (2.5%).

Once it has been demonstrated that the Ni11CaAl oxygen carrier can transfer enough oxygen to convert different fuels to CO2 and H2O, it must be proved that this material can also transfer enough enthalpy to avoid a large temperature difference in the FR when particles are circulating inside the CLC unit. In this sense, thermal integration among the AR and FR has consequences on the system operation. The energy balance must be especially considered when the reaction in the FR is endothermic, as in the case of the reduction of NiO by methane. In this case, the FR is heated by the circulating solids coming from the AR at higher temperatures and there is a temperature drop in the FR. To avoid a large temperature drop in the FR, limitations in the variation of solids conversion can be established to maintain a relatively high temperature. For Ni-based materials either the NiO content or the conversion of NiO should be low to avoid a large difference on temperature between both reactors. For the oxygen carrier considered in this work (11 wt.% NiO) in a CLC system using methane as fuel gas, a value for p of about 1.0 must be attained to maintain a temperature difference of 50°C between the AR and FR (Abad et al., 2007). However, when syngas is used as fuel, the reduction reaction is exothermic so that low temperature differences between AR and FR for every value of variation of solids conversion and NiO content are observed and for that reason no limitations have to be established.

From methane and syngas combustion sections (3.1.1 and 3.1.2), it could be concluded that oxygen carrier-to-fuel ratio values of 1.0-1.2, and higherthan 1.2, were necessary to obtain high methane and syngas combustion efficiencies, respectively. These oxygen carrier-to-fuel ratio ranges also fulfill the mass and heat balances

needed in the CLC system according to considerations mentioned above.

5. Conclusions

The behavior of a Ni-based oxygen carrier with a low NiO content (11.8 wt.% NiO) prepared by impregnation on CaAl2O4 has been studied in a continuous CLC 500 Wth unit using different gases (methane, H2, CO, syngas, ethane and propane) as fuels.

Using methane as fuel gas, a very high combustion efficiency is reached at 0 values close to 1 (1.1-1.2). A further increase in the oxygen carrier-to-fuel ratio produces a decrease in the combustion efficiency. This behavior is different to that found using most of the Ni-based oxygen carriers and it can be attributed to the low global catalytic activity of the oxygen carrier for methane reforming reactions. When light hydrocarbons are used as fuels, the oxygen carrier presents a similar behavior than in the case of methane combustion tests, reaching to the maximum fuel combustion efficiency at the same 0 values. This fact also indicates that light hydrocarbons combustion mechanism is carried out through cracking reaction. Finally, using syngas, pure CO or H2 as fuel gases, high combustion efficiencies can be achieved working at oxygen carrier to fuel ratios higher than 1.2.

Solids inventory needed to obtain a methane combustion efficiency of 99% was 180 kg/MWth, which correspond to a metallic Ni inventory around 17 kg/MWth. This value is the lowest referred in the literature for any other Ni-based oxygen carrier.

During more than 90 h of continuous operation in the CLC pilot plant the oxygen carrier also exhibited an adequate behavior in terms of agglomeration, deactivation and carbon deposition. Finally, it can be stated that this highly reactive oxygen carrier could be used with high efficiency in a CLC plant for combustion of a wide range of gaseous fuels.

Acknowledgements

This paper is based on the work performed in the frame of the INNOCUOUS (Innovative Oxygen Carriers Uplifting Chemical-Looping Combustion) Project, funded by the European Commission under the seventh Framework Programme (Contract 241401). P. Gayan thanks to CSIC for the financial support of the project 201180E102.

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