Evaluation of CuAl2O4 as an Oxygen Carrier in Chemical-Looping

Article pubs.acs.org/IECR

Evaluation of CuAl2O4 as an Oxygen Carrier in Chemical-Looping Combustion Mehdi Arjmand,*,† Abdul-Majeed Azad,‡ Henrik Leion,† Tobias Mattisson,§ and Anders Lyngfelt§ †

Department of Chemical and Biological Engineering, Division of Environmental Inorganic Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden ‡ Department of Chemical Engineering, The University of Toledo, Toledo, Ohio 43606-3390, United States § Department of Energy and Environment, Division of Energy Technology, Chalmers University of Technology, SE-412 96 Göteborg, Sweden ABSTRACT: The chemical-looping combustion (CLC) process is a novel solution for efficient combustion with intrinsic separation of carbon dioxide. The process uses a metal oxide as an oxygen carrier to transfer oxygen from an air to a fuel reactor where the fuel, or gasification products of the fuel, reacts with the solid oxygen carrier. In this work, copper(II) aluminate (CuAl2O4) was assessed as a potential oxygen carrier using methane as fuel. The carrier particles were produced by freeze− granulation and calcined at 1050 °C for a duration of 6 h. The chemical-looping characteristics were evaluated in a laboratoryscale fluidized-bed reactor in the temperature range of 900−950 °C during 45 alternating redox cycles. The oxygen carrier exhibited reproducible and stable reactivity behavior in this temperature range. Neither agglomeration nor defluidization was noticed in any of the cycles carried out at 900−925 °C. However, after reactivity tests at 950 °C, soft agglomeration and particle fragmentation were observed. Systematic phase analysis of the Cu−Al−O system during the redox cycle was carried out as a function of duration of reduction and oxygen concentration during the oxidation period. It was found that the CuAl2O4 is reduced to copper(I) aluminate (CuAlO2; delafossite), Cu2O, and elemental Cu. The CuAlO2 phase is characterized by slow kinetics for oxidation into CuO and CuAl2O4. Despite this kinetic limitation, complete conversion of methane with reproducible reactivity of the oxygen carrier is achieved. Thus, CuAl2O4 could be a potential oxygen carrier for chemical-looping combustion.

1. INTRODUCTION As suggested by the intergovernmental panel on climate control (IPCC), a 50−85% reduction in total CO2 emission by 2050 is necessary to limit the anticipated global temperature rise of 2 °C.1 A number of alternative technologies have been proposed to mitigate the rising levels of carbon dioxide in the atmosphere. Among these, carbon capture and storage (CCS) is considered promising. The chemical-looping combustion (CLC) process allows intrinsic separation of pure CO2 from hydrocarbon combustion. In a CLC system, two reactors, a fuel reactor and an air reactor, are interconnected.2−4 When fuel and air are introduced into the respective reactor, the following reactions occur (2n + m)MexOy + CnH 2m → (2n + m)MexOy − 1 + nCO2 + mH 2O

2MexOy − 1 + O2 → 2MexOy

Figure 1. Schematic of the chemical-looping combustion (CLC) process.

(1) (2)

relative heats of combustion and carrier reduction.5 However, the sum of the heat from reactions 1 and 2 is the same as for conventional combustion. Thus, the CLC process does not entail any direct cost or energy penalty for CO2 separation. CLC has been successfully demonstrated in a number of units of sizes up to 120 kW.6 Overviews of current achievements in

Here, MexOy and MexOy−1 are the fully oxidized and reduced forms of an oxygen carrier. The scheme of the process is shown in Figure 1. In case of complete conversion of the fuel, the exhaust stream from the fuel reactor consists of only CO2 and H2O, from which pure CO2 could be obtained after condensation of water. The reduced form of the oxygen carrier, MexOy−1, is then transferred to the air reactor where it is reoxidized by air making it ready for the next cycle. The oxidation reaction is always exothermic while the reduction reaction can be exothermic or endothermic depending on the © 2012 American Chemical Society

Received: Revised: Accepted: Published: 13924

February 17, 2012 September 17, 2012 October 3, 2012 October 3, 2012 dx.doi.org/10.1021/ie300427w | Ind. Eng. Chem. Res. 2012, 51, 13924−13934

Industrial & Engineering Chemistry Research

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CLC are given by Lyngfelt,6,7 Hossain and de Lasa,8 and Adanez et al.9 The reactivity of the oxygen carrier during oxidation and reduction and the ability to fully convert the fuel are among the most sought-after criteria. In addition, thermal stability, mechanical strength, fluidizability, and resistance to attrition and agglomeration are important. In order to achieve this, the active phase (i.e., the reactive metal oxide) is often mixed with an inert support such as TiO2, SiO2, ZrO2, Al2O3, or MgAl2O4.8 The CLC process can be used with gaseous, liquid, or solid fuels. In the case of solid fuels, the char remaining after devolatilization is gasified in the presence of steam, producing CO and H2 which can then react with the oxygen carrier. In lieu of CLC for solid fuels using gasification of the char, chemicallooping with oxygen uncoupling (CLOU),10 in which the char reacts directly with gaseous oxygen released from the oxygen carrier, can be used. In comparison to CLC where the reduction of oxygen carrier and oxidation of the gaseous fuel generally occurs in a single step, an additional step is needed in CLOU for the release of gaseous oxygen from the carrier prior to conversion of the fuel according to MexOy → MexOy − 2 + O2

investigated the use of CuO oxygen carriers in CLC/CLOU for solid fuels.10,13,14,22,32−38 Despite these attractive features, use of CuO as an oxygen carrier is not without limitations. For example, copper-oxide suffers from tendency toward agglomeration upon full reduction,15 due to formation of elemental copper which has a rather low melting temperature (1085 °C). Thus, with a few exceptions,15,17,21,23 most of the CLC studies with CuO are limited to lower operating temperatures (800−850 °C) to avoid agglomeration. However, a higher temperature in the fuel and/or the air reactor(s) may be an advantage with respect to kinetics. In order to achieve this, the CuO oxygen carrier must be able to resist agglomeration or defluidization at higher temperatures while providing stable reactivity. Recent investigations21,23 have expanded the frontier of high temperature CLC application of CuO oxygen carriers to 900 °C in continuous operation, without considerable particle attrition or operational difficulties. In the case of Al2O3 as support, a difficulty arises due to the facile interaction between CuO and Al2O3 either during synthesis or during operation, resulting in partial loss of CuO by formation of copper(II) aluminate (CuAl2O4) and copper(I) aluminate (CuAlO2; delafossite).17,21,23 Nevertheless, since the copper-aluminate phases are highly reducible,16,17,21,23,29,39 the interaction between the support and the active phase does not cause any problem with respect to CLC application. For CLOU, however, this interaction needs to be avoided in order to maintain CuO as active phase. Since the interaction of CuO with Al2O3 seems to be difficult to avoid, other supports such as TiO2, ZrO2, SiO2, or MgAl2O4 likely need to be employed for CLOU.13,14,17,21−23,28,29,34 Nonetheless, the use of Al2O3 as support for CuO oxygen carriers in CLC is favored due to its abundance and lower cost (e.g., compared to ZrO2). Recently, Chuang et al.16 observed that the existence of CuAl2O4 in Al2O3-supported CuO oxygen carriers prevented agglomeration. The use of CuAl2O4 as an oxygen carrier for CLC was suggested previously17 in view of its high reactivity and agglomeration resistance at 900 and 925 °C, albeit with only a few cycles carried out. In the present work, a more comprehensive investigation of CuAl2O4 as an oxygen carrier is carried out, with redox experiments performed in a fluidizedbed batch reactor over a larger number of cycles. The multicycle reactivity test in the temperature range of 900− 950 °C was intended to investigate the suitability of the proposed oxygen carrier in this high temperature interval. In addition, analysis of the solid phase composition in the Cu− Al−O system during the redox cycle is carried out through stepwise sequential reduction and oxidation in different oxygen concentrations. This was performed in order to analyze the reversibility of the phases during the redox process and to better understand the reducing and oxidizing pathways, when using CuAl2O4 as an oxygen carrier.

(3)

This is followed by the normal combustion of the fuel via ⎛ m⎞ CnH 2m + ⎜n + ⎟O2 → nCO2 + mH 2O ⎝ 2⎠

(4)

The reduced oxygen carrier is transferred to the air reactor for reoxidation. The net heat of reaction for the CLOU processes is the same as CLC, and only the mechanism by which oxygen is accessed by the fuel differs. However, when using solid fuels like coal, the CLOU process avoids the slow gasification of the solid fuel needed to produce syngas as a prerequisite for the reaction with the oxygen carrier.10 The oxygen carrier in CLOU must be able to release O2 and oxidize at temperatures suitable for the process, i.e., 800 to 1200 °C. Conventional circulating fluidized bed (CFB) boilers often operate at an air ratio of 1.2,2 which means that the oxygen concentration at the outlet of the air reactor in a realistic CLC unit will be close to 5%. Therefore, oxide systems with an equilibrium partial pressure low enough for oxidation, i.e., below 5% O2, are required. The oxide should however be able to release a significant concentration of oxygen in the fuel reactor. Thus, such thermodynamic and kinetic requirements limits the choice of oxygen carriers for the CLOU process. Oxides of transition metals (Mn, Fe, Co, Ni, and Cu), their mixtures, and a number of natural ores have been used as oxygen carriers in CLC.6−9 Copper-oxide has received a great deal of attention as an efficient oxygen carrier, owing to its high reactivity, high oxygen transport capacity, and absence of thermodynamic limitation for complete combustion of the fuel. Research has been conducted using copper-oxide as oxygen carrier in fluidized-bed11−18 and fixed-bed19,20 batch reactors, continuous operation,21−25 and thermogravimetric studies,13,14,26−31 with and without supports. Among various supports for CuO oxygen carriers, Al2O3 has received considerable attention.12,15−17,21,23,24,27,30−33 In addition, CuO−Cu2O is one of the prominent oxide pairs suitable for the CLOU process.10,34 CuO decomposes to Cu2O when the actual concentration of oxygen is lower than the equilibrium concentration. As a result, oxygen is released, thereby making CLOU possible. Some studies have also

2. EXPERIMENTAL SECTION 2.1. Preparation and Manufacturing of the Oxygen Carrier. The oxygen carrier used in this investigation and its physical properties are summarized in Table 1. The particles were manufactured by freeze−granulation. Here, a water-based slurry of CuO and α-Al2O3 (A16SG, Alcoa) powders in the ratio of 40/60 wt % (46/54 mol %) along with small amount of dispersant (Dolapix PC21) is prepared. The mixture is then ball milled for 24 h, and a binder (polyvinyl alcohol) is added prior to granulation. The slurry is pumped through a spray nozzle 13925

dx.doi.org/10.1021/ie300427w | Ind. Eng. Chem. Res. 2012, 51, 13924−13934


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Table 1. Physical Properties and Characteristics of the Oxygen Carrier Used in This Work active phase support phase theoretical content of active phase (wt %) theoretical oxygen transport capacity, Ro effective density (g/cm3) BET specific surface area (m2/g) crystalline phases identified by XRD crushing strength (N) a

CuAl2O4 α-Al2O3 90 0.08 1.28 4.2 CuAl2O4, α-Al2O3, CuOa 0.5

Minor phase.

and into liquid nitrogen to form spherical particles upon instantaneous freezing. The material is then freeze−dried at a temperature corresponding to the vapor pressure of water at −10 °C. This was followed by calcination at 1050 °C for 6 h at a ramp rate of 5 °C/min. At this temperature, CuO reacts with Al2O3 to form CuAl2O4,17 although as shown in Table 1, a small amount of CuO remained unreacted. The calcined material was sieved through stainless steel screens to yield particles in the range of 125−180 μm. 2.2. Characterization of the Oxygen Carrier. The oxygen carrier was analyzed before and after the experiments using powder X-ray diffraction (Siemens, D5000 Diffractometer) with Cu Kα radiation. The morphological investigation was carried out with an environmental scanning electron microscope (ESEM) fitted with a field emission gun (FEI, Quanta 200). The Brunauer−Emmett−Teller (BET) surface area of the particles was evaluated by N2 absorption (Micromeritics, TriStar 3000). The particle size distribution (PSD) was determined using a light microscope (Nikon SMZ800) and using ImageJ software40 to measure the area of an ellipse fitted to a large number of particles. The effective density of the particles, sized 125−180 μm, was measured assuming a theoretical void fraction of 0.37 of a packed-bed with uniform spherical particles. The crushing strength of the particles was measured as the strength needed to fracture a single particle sized within 180−250 μm. An average of 30 tests per sample was obtained using a digital force gauge (Shimpo, FGN-5). The crushing strength was found to be less than 0.5 N. As will be shown later in Section 3, the low crushing strength did not cause any problem with respect to agglomeration or fragmentation of particles during the reactivity test at 900 and 925 °C. Nonetheless, for use in a full scale plant, the crushing strength would need to be increased, e.g., by optimizing the manufacturing technique. 2.3. Experimental Setup and Procedure. Experiments were carried out in a quartz fluidized-bed reactor, 870 mm long and 22 mm in inner diameter. A porous quartz plate was placed at a height of 370 mm from the bottom, and the reactor temperature was measured with chromel-alumel (type K) thermocouples sheathed in inconel-600 located about 5 mm below and 25 mm above the plate. Honeywell pressure transducers with a frequency of 20 Hz were used to measure pressure drop over the bed in order to determine if the bed was fluidized or not. The exit gas stream from the reactor was led into a condenser to remove the water. The composition and flow rate of the dry gas was determined on a volumetric base by a Rosemount NGA-2000 analyzer which measured the concentrations of O2, CO2, CO, CH4, and H2. The schematic of the experimental setup used in this investigation is shown in Figure 2.

Figure 2. Scheme of the experimental setup used in this investigation.

For reactivity experiments, 15 g of the sample was placed on the porous plate inside the fluidized-bed reactor and the bed was exposed to alternating oxidizing and reducing conditions. Prior to the test, the reactor was heated to 900 °C in a stream of 10% O2 to ensure that the oxygen carrier was in a fully oxidized state. In a previous investigation,17 a 5% O2 stream was used during the oxidation phase in order to examine whether the oxygen carrier can be oxidized in an oxygen deficient condition similar to the outlet of the air reactor. The rationale of using 10% O2 stream instead in this work was to accelerate the oxidation process so that the carrier could be tested over a larger number of cycles to assess its suitability. However, it has been previously demonstrated that the oxidation of the reduced form of the CuAl2O4 oxygen carrier (i.e., Cu and Al2O3) can also be achieved with 5% O2,17 which showed that this oxygen carrier could also be oxidized in a real CLC system. To achieve complete reduction, the oxygen carrier was exposed to a stream of 100% methane for 4 min. The time to achieve complete oxidation of the reduced carrier was approximately 20 min. Occasionally, steam is also injected to avoid carbon deposition when using methane as fuel;11 however, this was not the case in this work. Thus, since the reduction time exceeded the oxygen capacity of the carrier, some carbon was deposited on the particles. This was evident from the carbon burnoff in the subsequent oxidation cycle, shown by the CO and/or CO2 concentrations. Nitrogen was used as an inert purge for 60 s in between oxidation and reduction to avoid the mixing of gases. Inlet flow rates of 450, 900, and 600 mLN/min were used during reduction, oxidation, and inert, respectively. These flow rates were chosen to achieve incoming gas velocities ranging between 14 and 15 times umf during oxidation, 9 and 10 times umf during inert, and 4 and 5 times umf during reduction, where umf is the minimum fluidization velocity.41 However, it should be noted that, due to gas expansion during reduction, the actual velocity in the bed is higher, as CH4 is converted to CO2 and H2O. The reactivity experiments were carried out for 15 cycles at each of the three temperatures: 900, 925, and 950 °C. Thus, the performance of the carrier was evaluated in a total of 45 cycles corresponding to approximately 20 h of operation in the hot condition in the fluidized-bed reactor. At the end of the test at each temperature, about 1 g of the oxidized sample was extracted for characterization and analysis. In addition, approximately 0.3−0.4 g of material elutriated as fines or was lost during the sample extraction from the reactor. Thus, about 13926



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Figure 3. Time-dependent concentration and temperature profiles for CuAl2O4 during three typical redox cycles at 925 °C.

2.4. Data Analysis. The reactivity of a given oxygen carrier is quantified in terms of gas yield or conversion efficiency, γ, and is defined as the fraction of fully oxidized fuel divided by the carbon containing gases in outlet stream, in this work CO2, CO, and CH4. yCO 2 γ= (5) yCO + yCH + yCO

1.3−1.4 g of fresh sample was mixed with the used mass, and the experiment was resumed at the next temperature. This was done in order to maintain the ratio of fuel flow to bed mass constant for the experiment at the next operating temperature and to examine the integrity of the particles by imposing an even more severe experimental condition on the carrier particles. In order to follow the phase evolution during the reduction period, additional experiments were conducted. In these tests, 10 g of the fresh oxygen carrier was used and the reactor was heated to 900 °C in a stream of 10% O2. The bed was subjected to stepwise reduction using methane with different durations. After every reduction step, the reactor was cooled in N2 and the sample was taken out for X-ray diffraction (XRD) and was returned again to the reactor. The experiment was resumed by reheating the system in N2 to 900 °C for the next reduction step. This procedure was continued for a total reduction time of 60 s. The experiment was concluded by subjecting the depleted oxygen carrier to oxidation in 10% O2. XRD was also used to examine the phase evolution during the oxidation period, although with a slightly different sample. Initially, 15 g of the oxygen carrier was exposed to 10 cycles of alternating reduction and oxidation similar to the reactivity test at 900 °C albeit with shorter reduction span (20 s). This was done in order to obtain a considerable fraction of delafossite (CuAlO2) in the sample, which is known to have slow kinetics for oxidation into CuO and CuAl2O417 and, in addition, to confirm the stable reactivity of the oxygen carrier with shorter reduction time. Thus, 1 g of the used sample (oxidized) from this reactivity test was placed inside the reactor and was fluidized in N2 to 900 °C. Subsequently, stepwise reoxidation was carried out in 5, 10, 15, and 21% O2 environments with the same amount (1 g) of the used sample. The reoxidation time was limited to 30 min for every reoxidation step, followed by cooling of the reactor in the respective oxygen concentration. Following that, the XRD pattern was collected and the sample was put back in the reactor for another reoxidation attempt at the next oxygen concentration. At the end of the experiment, the oxygen carrier was reoxidized in air (21% O2) for an additional 1 h. Thus, in total, the sample had been exposed to 3 h at varying oxygen concentrations.

2

4

Here, γi denotes the concentration (vol %) of the respective gas, obtained from the gas analyzer. The theoretical oxygen capacity of the oxygen carrier is defined in terms of oxygen ratio, RO, as the maximum mass change of oxygen in the oxygen carrier: m − mred R O = ox (6) mox where mox and mred are the mass of the oxygen carrier in fully oxidized and reduced state, respectively. The mass-based conversion of the oxygen carrier, ω, is defined as: m ω= (7) mox where m is the actual mass of the oxygen carrier during the experiments. Equation 8 is employed for calculating ω as a function of time during the reduction period from the measured concentrations of various gaseous species in the gas analyzer: ωi = ωi − 1 −

∫t

t1 0

nout ̇ MO (4yCO + 3yCO + 2yO − yH ) dt 2 2 2 mox

(8)

where ωi is the instantaneous mass-based conversion at time i, ωi−1 is the mass-based conversion in the preceding instant, t0 and t1 are the initial and final time of measurement, MO is the molar mass of oxygen, and ṅout is the molar flow rate of dry gas at the reactor outlet.

3. RESULTS 3.1. Reactivity of the CuAl2O4 Oxygen Carrier. The multicycle test assists in evaluating the reactivity and stability of the selected oxygen carrier. Figure 3 shows the concentration 13927



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that the oxidation reaction is limited by the flow of the oxygen containing gas. Thus, the zero O2 concentration indicates that the oxidation is very fast. Furthermore, the low O 2 concentration also reveals that there cannot be much CLOU effect for this material. Also, it appears that the deposited carbon fraction is small. The 10% oxygen consumed results in less than 1% CO2, which means that less than 10% of oxygen added is used for oxidizing carbon. Figure 4 shows the gas conversion, γ, as a function of massbased conversion of the oxygen carrier, ω, for the final cycle of

profile during three typical reduction, inert, and oxidation cycles at 925 °C. The concentrations have been corrected for time-lag of the gases reaching the analyzer; however, the data is not deconvoluted. Similar trends in concentration profiles were observed at 900 and 950 °C, except for slight variations in peak concentrations. It was previously17 shown that CuAl2O4 is incapable of releasing oxygen when subjected to long inert-N2 at 900 °C. Thus, it is likely that the low concentration of oxygen during the first 60 s prior to fuel injection, as shown in Figure 3, is due to the presence of the small amount of CuO in the oxygen carrier (Table 1) which decomposes into Cu2O above 850 °C.10 The decrease in oxygen concentration to zero in the early phase of reduction is also likely connected with the free CuO in the oxygen carrier being exhausted. As will be shown in Section 3.3, the reduction of CuAl2O4 by methane is followed by the formation of CuAlO2, Cu2O, and Cu. Thus, the heat of reaction of corresponding phases in the Cu−Al−O system becomes relevant. Table 2 shows the Table 2. Standard Heat of Reaction (ΔHor ) of Relevant Phases in the Cu−Al−O System with Methane at 15 °C42 reduction system

ΔHor (kJ/mol CH4)

8CuO + CH4 → 4Cu2O + CO2 + 2H2O 4Cu2O + CH4 → 8Cu + CO2 + 2H2O 4CuO + CH4 → 4Cu + CO2 + 2H2O 8CuAlO2 + CH4 → 8Cu + 4Al2O3 + CO2 + 2H2O 8CuAl2O4 + CH4 → 8CuAlO2 + 4Al2O3 + CO2 + 2H2O 8CuAl2O4 + CH4 → 4Cu2O + 8Al2O3 + CO2 + 2H2O 4CuAl2O4 + CH4 → 4Cu + 4Al2O3 + CO2 + 2H2O

−236.9 −119.7 −178.3 −24.4 −479.7 −384.4 −252.0

Figure 4. Gas yield, γ, as a function of oxygen carrier mass-based conversion, ω, for CuAl2O4 reduced by CH4 for the 15th, 30th, and 45th cycles at 900, 925, and 950 °C, respectively. The ratio of bed mass to fuel flow was 57 kg/MW.

standard heat of reaction, ΔHro, of relevant phases with methane. The thermochemical database of the Cu−Al−O system by Knacke et al.42 was found to be more reliable for calculation of the reaction enthalpies.43 These values were also found to be in reasonable agreement with those reported by Jacob and Alcock44 based on the corresponding standard Gibbs free energy, ΔG0. It can be seen in Table 2 that the overall reduction of CuAl2O4 with methane should be exothermic. This is in agreement with Figure 3 where the temperature rises during the initial stage of the reduction phase (t = 1−2 min.). It is also worth noting that the overall reduction of CuAl2O4 is more exothermic than that of CuO (Table 2). This is corroborated in Figure 3, where the temperature increases to 950 °C. Interestingly, in an earlier work17 with an oxygen carrier of 40 wt % CuO supported on Al2O3, the temperature was found to increase to 945 °C upon full reduction of the carrier with methane with a set temperature of 925 °C. A similar temperature increase was observed in experiments conducted at 900 and 950 °C. Complete conversion of methane is obtained during the initial period of reduction phase (t = 1−2 min). Beyond this, unconverted CH4, CO, and H2 are observed in the exit stream as the carrier is being depleted of oxygen. Thus, the temperature decreases to below 920 °C (t = 2−3.5 min.), due to the endothermic nature of the carbon formation on oxygen carrier particles.45 The deposition of carbon may also be inferred from the increase in H2 concentration to approximately 50% due to decomposition of methane during reduction and carbon burnoff during the following oxidation phase. As shown in Figure 3, the oxygen concentration is zero for most of the oxidation period, i.e., more than 10 min, indicating

every temperature set (i.e., cycles 15, 30, and 45). The theoretical decrease in the mass-based conversion, Δω, for the investigated oxygen carrier is 8% for full reduction of CuAl2O4 to Cu and Al2O3, corresponding to ω going from 1 to 0.92 which is also observed in the experiments as shown in Figure 4. Moreover, there is complete gas conversion at every temperature (i.e., 900, 925, and 950 °C) and a constant value of γ = 1 while ω decreases from 1 to 0.92 after 15, 30, and 45 cycles, suggesting that the content of the active phase (CuAl2O4) remained practically constant over the entire duration. It is likely that the carbon deposition and burnoff in the subsequent oxidation phase did not affect the oxygen carrier performance. As shown in Figure 4, one would expect poor methane yield for the latter cycles, which was not the case. In a realistic application, carbon formation should not be a problem, because the process would be run under conditions of high yields of the fuel and not complete reduction of the oxygen carrier.45 Neither agglomeration nor defluidization (detected by the change in the pressure profile over the reactor) was encountered in any of the cycles carried out at 900 and 925 °C. This indicates that the carrier was also thermally stable in this temperature range. At 950 °C, however, some fluidization anomalies were observed. After the reactivity test at 950 °C, some of the particles had agglomerated. However, the lumps formed fell apart easily when handled, thus indicating soft agglomeration. 3.2. Physical Aspects of the CuAl2O4 Oxygen Carrier under Redox Cycle. The CuAl2O4 spinel used as oxygen carrier in this work was characterized at the end of every 13928



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there is no sign of agglomeration in either case. Particles tested at 950 °C undergo drastic change in morphology showing a higher degree of fines formation with some evidence of agglomeration. This can be readily seen from the image shown in Figure 6d. Energy-dispersive X-ray (EDX) analysis of the particles surface showed even distribution of the Cu, Al, and O elements with only minor changes in composition after the reactivity test at different temperatures. 3.3. Phase Analysis in the Cu−Al−O System during Redox Cycle. The relationship among various phases in the pseudobinary CuO−Al2O3 system is of great relevance to the use of Al2O3-supported CuO as oxygen carrier in the CLC process. A detailed analysis of the CuO−Al2O3 system has been recently reported,17 which showed that, during synthesis, CuAl2O4 spinel is formed at high temperatures (around 900− 1000 °C)46−49 as per the reaction:

isothermal reactivity test, which ended with oxidation of the sample after the 15th cycle at 900 °C, after the 30th at 925 °C, and after the 45th cycle at 950 °C. The XRD analyses showed that, in all cases, it was possible to oxidize the reduced samples to a composition similar to the fresh oxygen carrier (i.e., CuAl2O4 and Al2O3 with small concentration of CuO). After each set of experiments (15 cycles), it was found that the bed height increased, which could be ascribed to the decrease in the apparent density from 1.28 g/cm3 in the fresh to 0.76, 0.7, and 0.63 g/cm3 for the used particles, respectively, after the 15th, 30th, and 45th cycles. This may indicate that most of the decrease in the apparent density had occurred in the initial cycles. The BET specific surface areas of the used samples increased slightly from 4.2 m2/g in the fresh to 4.8 m2/g for the used particles after the reactivity test at 950 °C, although the observed small change could be ascribed to measurement error. Moreover, the decreasing trend in the apparent density is adverse to the general consensus that materials become densified upon heat treatment. However, it is worth noting that the conditions in a redox operation are different from those during calcination. Forrero et al.23 also reported a decrease in the apparent density for CuO/γ-Al2O3 oxygen carrier after operation at 900 °C in a continuous 500 Wth CLC unit. The particle size distribution (PSD) of the oxygen carrier before and after the reactivity test is shown in Figure 5. After

CuO + Al 2O3 → CuAl 2O4

(9)

In the present work, CuAl2O4 was formed as the major phase upon calcination of the 40 wt % CuO−60 wt % Al2O3 mixture at 1050 °C for 6 h (Table 1). As a result of chemical-looping process, the phase field in the Cu−Al−O system is dependent on the oxygen partial pressure during the redox cycle in the temperature range of relevance. However, considerable disagreement exists as to the accuracy of the thermodynamic stability of various phases in the Cu−Al−O system.44,50,51 The standard Gibbs energy data of the Cu−Al− O system was determined by Gadalla and White50 using thermogravimetry by measuring the dissociation temperatures of the compounds in the CuO−Al2O3 system, under different oxygen partial pressure ranging from 0.21 to 1 atm. Later, Jacob and Alcock44 investigated the Gibbs energy of formation of two ternary oxides (CuAl2O4 and CuAlO2) in the range of 700−900 °C using a solid state galvanic cell. Figure 7 shows the equilibrium phase relationships at 1 atm. total pressure using the Gibbs energy data reported by Jacob and Alcock.44 In the context of the envisaged application of CuAl2O4 as an oxygen carrier in continuous operation, other reactions in addition to those depicted in Figure 7 should be also considered. Consequently, Figure 8 shows a series of pO2dependent reactions in the temperature range of interest to CLC at a total pressure of 1 atm. The equilibrium lines shown in Figure 8 were based on the Gibbs energy data of Jacob and Alcock.44 Among these, of particular interest is the following reaction:

Figure 5. Particle size distributions (PSD) for the CuAl2O4 oxygen carrier after 15, 30, and 45 cycles, respectively, at 900, 925, and 950 °C.

4CuAl 2O4 ↔ 4CuAlO2 + 2Al 2O3 + O2

(10)

Reaction 10 indicates that CuAl2O4 could potentially be a CLOU material by releasing O2, albeit at lower equilibrium partial pressure compared to that of CuO−Cu2O at temperatures above 900 °C. Tsuboi et al.52 reported that, at 1050 °C, CuAl2O4 undergoes reduction into CuAlO2 in the case of thin copper films deposited on alumina substrate in a highly oxygen depleted vacuum. However, it was found earlier17 that CuAl2O4 subjected to N2-purge for 4 h at 900 °C was incapable of releasing oxygen; no CuAlO2 could be observed either. On the other hand, it is well-established that CuAl2O4 could be reduced by methane or syngas.16,17,21,23,29,39 Thermogravimetric and temperature programmed reduction (TPR) studies29,53−55 on bulk samples also corroborate this. Thus, the observation of Tsuboi et al.52 could perhaps be explained on the basis of the higher temperature and the ease with which thin film samples

reactivity test at 900 and 925 °C, the fraction of particles sized between 125 and 180 μm remained almost constant. However, after testing at 950 °C, the particles size shift slightly to a higher size range, possibly due to agglomeration of some of the particles. The ESEM images of the fresh and postreaction CuAl2O4 oxygen carrier are shown in Figures 6a−d; for the purpose of clarity, images at higher magnifications are also included. The surface morphology of the particles tested at 900 and 925 °C does not appear to have changed noticeably: the particles have retained the smooth and uniform granular surface, akin to that in the fresh sample. After reactivity test at 900 °C, no fragmentation of particles was observed, but after testing at 925 °C, fine particles in a small fraction begin to emerge. However, 13929



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Figure 6. ESEM images of fresh (a) and post-reaction CuAl2O4 carrier tested at (b) 900 °C after 15 cycles, (c) 925 °C after 30 cycles, and (d) 950 °C after 45 cycles. The micrometer bars in the inset and outset figures are 20 μm, 200 μm, and 2 mm, respectively.

Figure 7. Equilibrium phase relationships in the ternary Cu−Al−O system in a 1:1 molar mixture of CuO and Al2O3 at 1 atm total pressure based on the Gibbs energy data reported by Jacob and Alcock.44

Figure 8. Possible pO2-dependent phase equilibria in the Cu−Al−O system at 1 atm. total pressure based on the Gibbs energy data reported by Jacob and Alcock.44

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proceeds by the simultaneous formation of CuAlO2 and Cu2O via reactions 10 and 11, respectively.

undergo dissociation, compared to bulk samples such as those investigated here. Figure 9 shows the XRD pattern of CuAl2O4 subjected to isothermal stepwise reduction by methane at 900 °C in a

4CuAl 2O4 ↔ 2Cu 2O + 4Al 2O3 + O2

(11)

The formation and increase of CuAlO2 and Cu2O via reactions 10 and 11, respectively, after 16 and 24 s in Figure 9 is also in agreement with the pO2 dependency shown in Figure 8, given the highly reducing environment created by methane. In the sample reduced for 24 s, the onset of formation of elemental Cu is clearly detected. The copper concentration increases with time of reduction, as is evident from all the diffraction patterns at t ≥ 24 s. In case of reduction carried out for more than 36 s, the XRD pattern contains peaks for alumina and elemental copper with traces of delafossite (CuAlO2). In samples reduced for 60 s, only metallic copper and alumina are present. This again is in complete conformity with Figure 8 in regard to the ultimate reduction of CuAl2O4 to Cu and Al2O3 as also reported elsewhere.51,55 Thus, the oxygen carrier is completely reduced by methane in 60 s. The intensity of the α-Al2O3 should increase during the consecutive reduction steps as it is released from the CuAl2O4; however, this is not observed in Figure 8. A possible explanation for this may be that the alumina is still covered with the copper layer. The original phases (CuAl2O4 and Al2O3) are fully restored upon oxidation in 10% O2 as seen from the topmost XRD pattern in Figure 9. From the foregoing discussion and the reactivity test presented in Section 3.1, it is apparent that the CuAl2O4 spinel functions well as an efficient oxygen carrier. However, in a realistic CLC unit, the oxygen carrier is only partially reduced in order to avoid unconverted CH4, CO, and H2. Recently, Kumekawa et al. 56 showed that CuAlO2 is thermodynamically stable in air above 900−1000 °C, which agrees reasonably with Figures 7 and 8. However, below this temperature range, CuAlO2 undergoes the following kinetically hindered reactions33,36,39 4CuAlO2 + O2 → 2CuO + 2CuAl 2O4

(12)

8CuAlO2 + O2 → 2Cu 2O + 4CuAl 2O4

(13)

To investigate the kinetic aspect of reactions 12 and 13, oxygen carrier samples from a reactivity test consisting of 10 cycles of alternating reduction (with methane) and oxidation at 900 °C, albeit with shorter reduction span (20 s), were used. Here, 1 g of the used sample (oxidized), which contained a considerable fraction of CuAlO2, was reoxidized for 30 min in streams containing 5, 10, 15, and 21% O2 at 900 °C. Figure 10 shows the phase evolution in each of the reoxidation cases. To begin with, the oxidized sample from the reactivity test with shorter reduction span (20 s; methane) contains a 3-phase mixture of CuAl2O4, CuAlO2, and Al2O3. As is evident from the figure, CuAlO2 persists under several reoxidation attempts, indicating that the oxidation of CuAlO2 according to reactions 12 and/or 13 is kinetically hindered. This is further exemplified by the fact that complete conversion of the delafossite phase into the parent spinel (CuAl2O4) is realized only after extended oxidation in air (pO2 = 21%) at 900 °C for an additional 1 h, i.e., a total of 3 h in varying oxygen concentrations. The increase of CuO amount in the fully reoxidized sample (topmost XRD pattern in Figure 10), compared to the fresh oxygen carrier (bottommost XRD pattern in Figure 9), is in agreement with reaction 12. It should be noted that, in an actual CLC unit, air is

Figure 9. Systematic phase evolution in CuAl2O4 oxygen carrier reduced by methane at 900 °C for 60 s; the topmost XRD pattern is for the oxygen carrier reduced for 60 s, after being oxidized in a 10% O2 stream.

sequence of varying durations. In the sample reduced for the shortest duration (10 s), peaks belonging to Cu2O appear. Longer (16 and 24 s) reduction is followed by the decrease in intensity (hence amount) of CuAl2O4 and increase in that of CuAlO2 and Cu2O. Even though some of the prominent diffraction peaks of Cu2O and CuAlO2 coincide with each other (e.g., at 2θ = 32° and 42°), the two compounds are distinguishable in terms of other peaks. It should be pointed out that, in a previous investigation,17 Cu2O was also seen during annealing in an inert environment, which was ascribed to the presence of a small amount of unreacted CuO in the fresh sample. A similar explanation is valid for the carrier used in this work. However, the increase in the amount of Cu2O seen in the diffraction patterns at 16 and 24 s in Figure 9 suggests that Cu2O is not only formed from remnant CuO in the CuAl2O4 oxygen carrier. Using TPR, Patrick et al.54,55 showed that the reduction of CuAl2O4 catalyst 13931



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results in a slight decrease in the oxygen ratio of the carrier as the solid phase obtained upon oxidation is mainly CuAl2O4 and CuAlO2. However, this may be compensated by a larger solids inventory. Therefore, there is technical potential in using CuAl2O4 as an oxygen carrier for chemical-looping combustion.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +46-31-772-2822. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge Vattenfall and Chalmers University of Technology via the Energy Area of Advance for the financial support of this work. One of the authors (A.M.A.) wishes to thank the United States Council for International Exchange of Scholars, for the 2010/11 Fulbright Distinguished Chair Award in Alternative Energy Technology at Chalmers. Special thanks also go to Professor Vratislav Langer (Chalmers University of Technology) for fruitful discussions concerning XRD measurements.

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REFERENCES

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Figure 10. Systematic phase evolution in CuAl2O4 oxygen carrier during reoxidation for 30 min in pO2 = 5, 10, 15, and 21% O2 streams at 900 °C; the bottommost XRD pattern is for the oxidized sample from the reactivity test with shorter reduction span (20 s; methane); the topmost XRD pattern is for the sample reoxidized in air (21% O2) for an additional 1 h.

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4. CONCLUSION CuAl2O4 prepared by freeze granulation was investigated as an oxygen carrier for CLC applications in a fluidized-bed reactor. The experiments were carried out in the temperature range of 900−950 °C in 45 cycles of reduction by 100% methane and oxidation in a stream containing 10% O2. The oxygen carrier exhibited reproducible and stable reactivity during both reducing and oxidizing periods in this temperature range. The time for complete reduction of the oxygen carrier was approximately 1 min, and the time for oxidation of the carrier was 15 min. Agglomeration and/or defluidization were not encountered in any of the cycles when the experiments were carried out at 900 and 925 °C. However, after reactivity testing at 950 °C, soft agglomeration and particle fragmentation were observed. Systematic phase analysis of the Cu−Al−O system during the redox cycle as a function of duration for reduction and oxygen concentration was carried out. The analysis reveals that the reduction of the spinel (CuAl2O4) involves the formation of CuAlO2, Cu2O, and elemental Cu. In addition, the oxidation of CuAlO2 to CuO and CuAl2O4 shows slow kinetics. This likely 13932



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Evaluation of CuAl2O4 as an Oxygen Carrier in Chemical-Looping

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