Reduction and Oxidation Kinetics of a Copper-Based Oxygen Carrier

8168

Ind. Eng. Chem. Res. 2004, 43, 8168-8177

Reduction and Oxidation Kinetics of a Copper-Based Oxygen Carrier Prepared by Impregnation for Chemical-Looping Combustion F. Garcıá-Labiano,* L. F. de Diego, J. Ada´ nez, A. Abad, and P. Gaya´ n Instituto de Carboquı´mica (CSIC), Department of Energy and Environment, Miguel Luesma Casta´ n 4, 50015 Zaragoza, Spain

The kinetics of reduction with CH4, H2, and CO and oxidation with O2 of a Cu-based oxygen carrier prepared by impregnation on alumina to be used in a chemical-looping combustion (CLC) system have been determined in a thermogravimetric analyzer. The oxygen carrier exhibited high reactivity in both reduction and oxidation with times for complete conversion lower than 40 s at 1073 K and 5-70 vol % of the fuel gas and 5-21 vol % of O2. The analysis of the sample carried out by scanning electron microscopy using energy-dispersive X-ray and chemisorption showed that the CuO was well dispersed in the porous surface of the alumina matrix and a uniform thin layer on the porous surface was considered. The shrinking-core model for platelike geometry of the reacting surface was used for the kinetic determination, in which the chemical reaction controlled the global reaction rate. No effect of the gas products (H2O and CO2) on the reaction rate was detected. The reaction order depended on the fuel gas, and values of 0.4, 0.6, and 0.8 were found for CH4, H2, and CO, respectively. The order of the oxidation reaction was 1. The activation energies for the reduction and oxidation reactions varied between 14 and 60 kJ mol-1. The reactivity data together with the operating variables were used to calculate some design parameters for a CLC system. It was found that the total solid inventory and the recirculation rate are linked. To optimize both parameters, conversion variations of the oxygen carrier in the fuel and air reactors, ∆Xs, should be about 0.2-0.4. For a typical CLC operating condition, the total solids inventory for this Cu-based oxygen carrier (10 wt % CuO) was 133 kg/MWf if the fuel gas was CH4, 86 kg/MWf if the fuel gas was H2, and 104 kg/MWf if the fuel gas was CO, with recirculation rates of about 12 kg s-1 per MWf. The high reactivity of the material, in both reduction and oxidation, demonstrated the feasibility of this oxygen carrier to be used in a CLC system. Introduction Chemical-looping combustion (CLC) has been suggested as an energetically efficient method for the capture of carbon dioxide from fuel gas. CLC is a combustion technology with inherent separation of carbon dioxide, which involves the use of a metal, as an oxygen carrier, which transfers oxygen from the air to the fuel, avoiding the direct contact between fuel and air. The CLC process is composed of two reactors, air and fuel. The fuel in gaseous form (CH4 or CO + H2) is introduced into the fuel reactor, where it reacts with the metal oxide present in the oxygen carrier according to

(CH4)(CO)(H2) + MeO f (CO2 + H2O)(CO2)(H2O) + Me (1) where Me represents a metal or a reduced form of MeO. This metal or reduced oxide is further transferred into the air reactor, in which it is oxidized with air:

Me + O2 f MeO

(2)

and the regenerated carrier is ready to start a new cycle. * To whom corresondence should be addressed. E-mail: [email protected].

The flue gas leaving the air reactor contains N2 and some unreacted O2. The exit gases from the fuel reactor contain CO2 and H2O, which are kept separate from the rest of flue gas. After H2O condensation, almost pure CO2 is obtained without any energy lost for component separation. Different metal oxides have been proposed in the literature as possible candidates for CLC process: CuO, CdO, NiO, Mn2O3, Fe2O3, and CoO.1-4 To increase the reactivity and durability of the oxides, the particles have been doped with several inerts such as Al2O3, YSZ, TiO2, or MgO, which act as a porous support. Lyngfelt et al.5 showed a review of the literature data on oxygen carriers in CLC. Among the possible metal oxides, CuO is one of the cheapest materials and has the highest oxygen transport capacity. There are several investigations in the literature related with the use of Cu as an oxygen carrier for CLC. Copeland et al.6,7 at TDA Research developed Cu-based sorbents to be used in their sorbent energy transfer system, a type of CLC where the power cycle operates in a very conventional manner. The copper-containing oxygen carriers demonstrated excellent chemical stability; however, in refs 6 and 7, these materials were rejected by the agglomeration problems detected in the fluidized-bed tests. Mattisson et al.8 prepared, by dry impregnation, NiO-, CuO-, CoO-, and Mn3O4-based carriers on an alumina

10.1021/ie0493311 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/25/2004

Ind. Eng. Chem. Res., Vol. 43, No. 26, 2004 8169

support and analyzed their reactivities in a thermogravimetric analyzer (TGA). They observed that Ni- and Cu-containing materials showed high reactivities at all temperatures and cycles tested, with reduction rates up to 100% min-1 for CuO and 45% min-1 for NiO and oxidation rates of up to 25% min-1 for the oxidation of both reduced metals. However, Mn- and Co-containing carriers showed rather poor reactivity. In a later work, Cho et al.9 compared the reactivities of several oxygen carriers, prepared by freeze granulation, in a fluidizedbed reactor. Oxygen carriers based on Ni, Cu, and Fe showed high reactivity, although they found agglomeration problems with the Cu-based materials. de Diego et al.10 analyzed the effects of the carrier composition and preparation method on the behavior of Cu-based oxygen carriers. They concluded that, to obtain Cu-based oxygen carriers with high reduction and oxidation reaction rates while maintaining their mechanical properties for a high number of successive reduction-oxidation cycles, the only valid preparation method was impregnation on a support. On the other hand, a key parameter for the design of a CLC system is the solids inventory in the fuel and air reactors as well as the recirculation rate of oxygen carriers between the reactors.11 Both parameters are linked and depend on the reactivity of the materials and on the oxygen transport capacity of the oxygen carrier. In some cases, as for the oxygen carriers based on Fe and Ni, the recirculation rate is governed not only by the amount of oxygen that needs to be recirculated but also by the overall heat balance. However, the Cu-based oxygen carriers are not as affected by the heat balance because both their oxidation and reduction reactions are exothermic. Lyngfelt et al.5 developed the design of a CLC process based on two interconnected fluidized beds, a highvelocity riser and a low-velocity bed. The design was based on conversion rate data, defined as percent per minute, which can be obtained in different laboratory reactors, including fixed and fluidized beds, and TGA. They concluded that the conversion rate data of the oxygen carriers available in the literature fulfilled the values needed for the proposed design. Later, Mattisson et al.8 determined the reactivity in a TGA of some metal oxides supported on alumina. They used 10 vol % of oxygen and fuel gas, which was obtained by assuming plug flow of gas and a reaction order of 1, to simulate the average concentration that the particles would be exposed to in a real CLC system for both oxidation and reduction reactors. There are very few works in the literature related to the determination of kinetic parameters of oxygen carriers. Ryu et al.12 and Ishida et al.13 used an unreacted-core model at the whole particle to interpret their experimental results in the oxidation and reduction reactions of Ni-based oxygen carriers. The reduction reaction was controlled by the chemical reaction resistance, and the oxidation reaction was controlled by product layer diffusion resistance12 or by an intermediate regime between chemical reaction control and internal particle diffusion control.13 These authors found an activation energy between 17 and 131 kJ mol-1 for the oxidation reaction and between 37 and 82 kJ mol-1 for the reduction reaction. To design a CLC system working with Cu-based oxygen carriers, it is necessary to determine the reactivity under different operating conditions of temperature

Table 1. Properties of the Oxygen Carrier Cu10Al-I (Aldrich Chemical Co.) active CuO content (wt %) particle size (mm) porosity BET specific surface area (m2 g-1) ASA (m2 g-1) apparent density (kg m-3) molar density of CuO (mol m-3) molar density of Cu (mol m-3) CuO layer thickness (m) Cu layer thickness (m)

10 0.1-0.3 0.57 41.3 39.6 1800 80 402 140 252 4.0 × 10-10 2.3 × 10-10

and gas concentration. It must be considered that the oxygen carriers will be found at different environments during their stage in the fluidized-bed fuel reactor ranging from 100% of fuel gas at the inlet to 0% at the outlet for complete conversion. Similarly, during the oxidation, the O2 concentration will vary from 21% at the inlet to about 4% at the outlet, depending on the excess air used for the reaction. Moreover, no data are available in the literature about the reaction order of the oxygen carriers with respect to either the oxidation reaction or the reduction reaction with the different fuel gases used in a CLC system. The objective of this work was to determine the kinetics of reduction with CH4, H2, and CO and oxidation with O2 of a Cu-based oxygen carrier prepared by impregnation in the range of operating conditions valid for CLC systems. The effect of temperature, gas concentration, and gas products of the reaction, CO2 and H2O, was examined. The kinetic parameters obtained were used to determine the solids inventory necessary for a CLC system working with this Cu-based oxygen carrier. Experimental Section Material. Particles of copper(II) oxide supported on alumina (Aldrich Chemical Co.) with a 10 wt % of active metal oxide were used as an oxygen carrier (Cu10Al-I). The particles (0.8-1.2 mm) were crushed and sieved to obtain the desired particle size for the kinetic determination (0.1-0.3 mm). Table 1 shows the main physical characteristics of the material. The analysis of the samples by scanning electron microscopy using energydispersive X-ray (SEM-EDX) showed that the CuO was well dispersed in the material. Figure 1a shows the internal surface of an original particle. EDX analysis showed that the CuO concentration was the same in the external surface and at different locations inside the particle. Because the agglomeration is the main problem related to the use of the Cu-based oxygen carriers, the material was first tested in a fluidized bed at 1073 K. The oxygen carrier did not show any agglomeration problems and maintained the same reactivity during 100 reduction/oxidation cycles. Moreover, the sample maintained the dispersion of CuO after the reaction. Figure 1b shows a SEM photograph using secondary electrons to give the topography of the internal part of the sample after reaction in the fluidized bed. The sample was covered by a wool-shaped CuO structure homogeneously distributed on the alumina support all around the sample. The high dispersion level of CuO on the Al2O3 matrix was also detected by other techniques. H2 chemisorption carried out in a Micromeritics Pulse Chemisorption 2700 of the fully reduced samples after 100 reduction/oxida-

8170

Ind. Eng. Chem. Res., Vol. 43, No. 26, 2004

Figure 1. (a) Internal surface of an original particle obtained by SEM-EDX. (b) SEM photograph using secondary electrons of the internal zone of the oxygen carrier particle after reaction in the fluidized bed.

tion cycles showed an active surface area (ASA) of 39.6 m2 g-1, very similar to that obtained by N2 physisorption by the Brunauer-Emmett-Teller (BET) method (41.3 m2 g-1) in a Micromeritics ASAP 2000. Therefore, it can be assumed that the Cu was covering all of the porous surface of the inert support with a thin layer of about 0.23 nm of thickness. Considering that the reacting surface is the same both in the reduction and in the oxidation because of the geometry considered, the thickness of the CuO layer for the fully oxidized sample was 0.4 nm. Experimental Setup. The kinetics of the reduction and oxidation reactions of the Cu-based oxygen carrier have been carried out through thermogravimetric analysis on a thermobalance (CI Electronics Ltd.). The reactor consists of two concentric quartz tubes (24 mm i.d. and 10 mm i.d.) placed in an oven. The sample holder was a wire mesh platinum basket (14 mm diameter and 8 mm height). The reacting gas mixture (6 cm3 s-1 STP) was measured and controlled by electronic mass flow controllers, and it was introduced at the upper part of the reaction tube. The gas was heated at the desired temperature flowing down through the external annulus of the reactor before contacting with the sample, which it was located at the bottom of the reactor. The gas left the reactor through an internal quartz tube after mixing with the gas coming from the head of the balance. Procedure. For reactivity experiments, the oxygen carriers were loaded into the platinum basket. The sample weight used for the experiments was about 15 mg. To avoid contact between the particles and to eliminate the interparticle mass-transfer resistance, the particles were loaded between layers of quartz wool. The oxygen carriers were heated to the desired temperature in an air atmosphere. Once the set temperature was reached, the experiment was started by exposing the oxygen carrier to alternating reducing and oxidizing conditions. To avoid the mixing of combustible gas and air, nitrogen was introduced for 2 min after each reducing and oxidizing period. The composition of the gas used to determine the kinetics of the reduction reaction was varied to cover the great majority of the gas concentrations present in the fluidized-bed fuel reactor of a CLC system (fuel, 5-70 vol %; H2O, 0-48 vol %; CO2, 0-40 vol %). For the oxidation reaction, oxygen concentrations from 5 to 21 vol % were used. Five cycles of reduction and oxidation were normally carried out for each experimental condition. The sample normally stabilized after the first reduction cycle, which was slower compared to the following cycles. The experiments were carried out at temperatures of up to

1073 K to avoid the decomposition of CuO to Cu2O in a N2 atmosphere during the inert period between the oxidation and reduction. To determine the reduction kinetics over the same reacting compound, the sample was always oxidized in the previous step at 1073 K in air. For the oxidation kinetic determination, the reduction in the previous step was always carried out at 1073 K in an atmosphere composed of 40 vol % of fuel gas and 30 vol % of the gas product (CO2 or H2O). Data Evaluation. The reduction of CuO by the different fuel gases and the oxidation of Cu are given by the following equations:

4CuO + CH4 f 4Cu + CO2 + 2H2O

(3)

CuO + CO f Cu + CO2

(4)

CuO + H2 f Cu + H2O

(5)

2Cu + O2 f 2CuO

(6)

The conversion level of the oxygen carrier was calculated for the reduction and oxidation reactions as

Xs,r )

mox - m moxR0xCuO

Xs,o ) 1 -

mox - m moxR0xCuO

(7)

(8)

The oxygen transport capacity, R0, defined as the oxygen content ratio in the reduced and oxidized forms was 0.2 for the transformation CuO/Cu. Kinetic Model There are several resistances that can affect the reaction rate of the oxygen carrier with the fuel gas or the oxidation with air. Previous calculations14 showed that external and internal mass-transfer resistances were not important in the reactions involved in CLC systems. Moreover, several experiments showed that the particle size did not affect the reaction rates. On the other hand, the analysis of the sample carried out by SEM-EDX and chemisorption showed that CuO was well dispersed in the porous surface of the alumina matrix in a thin layer of about 0.4 nm of thickness. In this case, the shrinking-core model for platelike geometry in the porous surface of the particle was used for the kinetic determination in Cu-based oxygen carriers prepared by impregnation. Considering the chemical reaction as the main resistance to the global reaction,


Figure 2. Effect of the H2 concentration on the reduction reaction of the oxygen carrier Cu10Al-I (1073 K). H2 (vol %): 0, 5; O, 10; ], 40; 4, 70. (s) Model predictions.

Figure 3. Plot of ln Cg vs ln(Fm,iLi/bjτj) to obtain the reaction order of the reduction and oxidation reactions for different reacting gases (1073 K): 0, CH4; O, CO; ], H2; 2, O2.

the equations that describe this model for plates are the following:15

Table 2. Kinetic Parameters for the Reduction and Oxidation Reactions of the Cu10Al-I Oxygen Carrier

t/τj ) Xs,j

(9)

τj ) Fm,iLi/bjkjCgn

(10)

The thickness of the layer, Li, over the Al2O3 support was determined considering the ASA obtained by H2 chemisorption and the weight fraction of active CuO in the oxygen carrier particles. Results Reduction Reaction. In the fuel reactor of a CLC system, the oxygen carrier is reduced by the fuel gas (CH4, CO, or H2) under different environments at different locations of the fluidized bed. At the bottom, CuO will be in contact with pure fuel gas. At the top of the fluidized bed, the gas stream will be composed mainly of CO2 and/or H2O. To determine the effect of the fuel gas concentration on the reduction reaction rate, several experiments were carried out at 1073 K with CH4, CO, and H2. As an example, Figure 2 shows the results obtained with the oxygen carrier as a function of the H2 concentration. The experimental data are represented by symbols, and the model predictions with the kinetic parameters finally obtained will be represented in all of the figures as continuous lines. The conversion of CuO was complete in all of the cases, showing an oxygen transport capacity of 2%. The reaction rate of the oxygen carrier with all fuel gases was very quick, with values of τr lower than 40 s for fuel concentrations above 5 vol %. According to eq 9, the values of the time for complete conversion, τr, at different operating conditions were obtained from the slope of the line representing values of X versus time. When eq 10 was rewritten and logarithms were taken, eq 11 was obtained.

ln(Fm,CuOLCuO/brτr) ) ln kr + n ln Cg

(11)

Figure 3 shows a plot of ln(Fm,CuOLCuO/brτr) vs ln Cg for the different fuel gases. The values of the reaction order, n, for the different reactions were obtained from the slope of the plots, which are shown in Table 2. The effect of the temperature on the reaction rate of the oxygen carrier was later investigated. To increase the validity of the data, several concentrations (5, 10, and 40 vol %) were used for each gas. Figure 4 shows

CH4

H2

CO

O2

k0 (mol1-n m3n-2 s-1) 4.5 × 10-4 1.0 × 10-4 5.9 × 10-6 4.7 × 10-6 E (kJ mol-1) 60 ( 3 33 ( 1 14 ( 1 15 ( 2 n 0.4 0.6 0.8 1.0

an example of the conversion versus time data obtained at temperatures from 773 to 1073 K for the three fuel gases investigated. H2O (30 vol %) was also added to avoid carbon formation during reduction with CH4. During reduction with CO, 10 vol % CO2 was added to avoid carbon formation by the Boudouard reaction. Experiments at higher temperatures were not carried out in the TGA because of decomposition of CuO to Cu2O observed during the inert period between oxidation and reduction. It is remarkable that the initial reaction rates of the oxygen carrier were high even at low temperatures. The reduction reaction was normally composed of two different steps at temperatures below 973 K. The initial stage was characterized by a high reaction rate, which was followed by a sharp decrease in the reactivity. These steps would be associated with different resistances to the global reaction, that is, the chemical reaction and the product layer diffusion, respectively. The decrease in the reactivity was reached at different conversion levels depending on the fuel gas and temperature. To corroborate the existence of different resistances, several experiments were done. As can be observed in Figure 5, the oxygen carrier was reduced with 5 vol % CO at 723 K up to 20 min, where the reaction rate was very low because the global reaction was controlled by the product layer diffusion. At this stage, the fuel gas was stopped, the sample was heated in N2 up to 1073 K, and then the fuel gas flow was restored. In this case, the reaction rate was the same as that obtained in the experiments carried out at this temperature from the beginning. It was concluded that there were two resistances affecting the reduction reaction of the oxygen carrier. At low temperatures, the product layer diffusion resistance controlled the reaction from a certain conversion level. However, the higher activation energy of this process with respect to the chemical reaction made this effect negligible at high temperatures. Thus, this sharp decrease in the reaction rate did not take place at temperatures typically used in CLC systems (>1023 K), and the full conversion was reached at a nearly constant reaction rate. In these cases, it can be considered that the chemical reaction controls the reduction of the

8172


Figure 5. Effect of the temperature on the product layer diffusion resistance during the reduction reaction (5 vol % CO): 0, first 20 min at 723 K and later at 1073 K; ], 1073 K (displaced in the time scale to compare).

Figure 6. Arrhenius plot for the chemical reaction rate constant of the reduction and oxidation reactions for different gases: 0, CH4; O, CO; ], H2; 2, O2.

Figure 4. Effect of the temperature on the reduction reaction of the oxygen carrier with different fuel gases. Temperature (K): 9, 723; b, 773; [, 823; 2, 873; 0, 923; O, 973; ], 1023; 4, 1073. (s) Model predictions.

oxygen carrier. Therefore, only the parameters corresponding to this resistance were determined in this study. Figure 6 shows the plot used to obtain the values of the kinetic parameters assuming an Arrhenius-type dependence with the temperature for the kinetic constant. Table 2 shows the values of the preexponential factor and activation energy determined for the different reactions. The low activation energies obtained with this oxygen carrier are noticeable, although these values are similar to those found by other authors for chemical reactions with different oxygen carriers.12,13 In the fuel reactor of a CLC system, the oxygen carrier is also in contact with the H2O and/or CO2 produced during the reaction. Several experiments carried out with different gas product concentrations showed that these gases did not affect the reaction rate of CuO impregnated on alumina with CH4, CO, or H2. Oxidation. The CLC system is composed of two interconnected reactors. The reduced oxygen carrier from the fuel reactor is regenerated in the air reactor.

Figure 7. Effect of the O2 concentration on the oxidation reaction of oxygen carrier Cu10Al-I (1073 K). O2 (vol %): 0, 5; O, 10; ], 15; 4, 21. (s) Model predictions.

Studies carried out in the TGA showed that the oxidation rate was the same independently of the gas previously used for the reduction. To determine the reaction order of the oxidation reaction, several experiments were carried out at 1073 K as a function of the O2 concentration (see Figure 7). The reaction order of the oxidation reaction obtained from the slope of the plot of ln(Fm,CuLCu/b0k0) versus ln CO2 was 1, as can be seen in Figure 3. To determine the kinetic parameters of the oxidation reaction of the Cu-based oxygen carrier, several experiments were carried out at different temperatures from 773 to 1073 K with a constant oxygen concentration of


Figure 8. Effect of the temperature on the oxidation reaction of the oxygen carrier. Temperature (K): b, 773; [, 823; 2, 873; 0, 923; O, 973; ], 1023; 4, 1073. (s) Model predictions.

Figure 10. Recirculation rate as a function of the oxygen carrier conversion (oxygen carrier Cu10Al-I; 1 MWf of fuel gas).

at higher concentrations because of the different reaction orders of the reactions. However, if we consider the reaction rates with respect to the fuel gas, (-rg)r, the reactivities with CO and H2 were the same than those with respect to the solid, whereas there was a big decrease for CH4. This was because the stoichiometry of the reactions was 1:1 for CO and H2, whereas it was 1:4 for CH4 (see eqs 3-5). In this case, the reaction rate followed the order H2 > CO > CH4. Application of Reactivity to Design Criteria

Figure 9. Solid and gas reaction rates for the reduction reaction as a function of the gas concentration (1073 K).

21 vol % and following the procedure described in the Experimental Section. The oxidation reaction was always complete and controlled by the chemical reaction in the entire range of temperatures tested. Figure 8 shows the results obtained, and Figure 6 shows the Arrhenius plot corresponding to these data. The oxidation reaction rate was very quick, with total reaction times lower than 15 s even at the lowest temperatures tested, 773 K, for an oxygen concentration of 21 vol %. As a consequence of these data, a low activation energy was obtained for the oxidation reaction (14 kJ mol-1; see Table 2). Comparison of Reactivities. The CLC system is valid for the use of different fuel gases, both natural gas (CH4) and syngas (CO and H2). The study carried out about reactivity showed the different behavior obtained with the three fuel gases. To better compare the reactivity of the Cu-based oxygen carrier with these gases, Figure 9 shows the reaction rate obtained as a function of the gas fuel concentration. The reaction rate was calculated for the reacting solid and the gas as

dXs,r dt

(12)

Fm,CuO dXs,r br dt

(13)

(-rs)r ) Fm,CuO (-rg)r )

The highest reaction rate for the fuel gas was always obtained with H2. Considering the reaction rate with respect to the solid, (-rs)r, CO was faster than CH4 at low concentrations, whereas the effect was the opposite

Recirculation Rate. The CLC concept is based in the transport of oxygen from the air to the fuel reactor by means of a carrier. The recirculation rate of the oxygen carrier can be calculated from a mass balance in the fuel reactor, and it mainly depends on the conversion variation obtained in the oxygen carrier in the fuel and air reactors. The recirculation rate, expressed as the mass of oxygen carrier totally oxidized, was calculated as

m ˘ ox ) br MCuOFf ∆Xf /xCuO∆Xs

(14)

The real recirculation rate, m ˘ , depends on the solid conversion and can be obtained from the following equation:8

m ˘ /m ˘ ox ) 1 + R0xCuO(Xs,o - 1)

(15)

Figure 10 shows the recirculation rate as a function of the solid conversion variation to obtain complete conversion of the fuel gas (∆Xf ) 1) and taking as reference a power plant of 1 MWf. To obtain 1 MWf, 1.25 mol of CH4 s-1, 3.53 mol of CO s-1, or 4.14 mol of H2 s-1 is necessary as a consequence of their different heats of combustion (∆Hc,CH4 ) -802 kJ mol-1, ∆Hc,CO ) -283 kJ mol-1, and ∆Hc,H2 ) -242 kJ mol-1). It must be also considered that, to obtain 1 MWf from the use of CH4, H2, or CO, the transport of 5, 4.14, or 3.53 mol of O s-1, respectively, is necessary. Obviously, the solid flow was higher with increasing the transport of oxygen needed for the reaction, although the differences between the three fuel gases were small. The solids flow per MWf of fuel gas varied between 28 and 40 kg s-1 when ∆Xs was 0.1 and between 3 and 4 kg s-1 when ∆Xs was 1. It must be considered that for the Cu-based oxygen carriers the recirculation rate is not limited by the heat balance, as happens with the Fe- and Ni-based oxygen carriers because both the reduction and oxidation reac-

8174


tions are exothermic. In this case, the recirculation rate will be a compromise between the transport capacity of the riser and the conversion reached in the air reactor. Solids Inventory. The solids inventory in the CLC system for any fuel conversion can be obtained from a mass balance to the solid and gas in the fuel and air reactors. For preliminary estimations, some assumptions about gas and solid flows in a fluidized bed were used. Assuming perfect mixing of the solid and gas plug flows, the following equations can be obtained for the fuel and air reactors, respectively,15

VCuO ∆Xs ) FCuO,0 (-rjCuO)r

(16)

∆Xs VCu ) FCu,0 (-rjCu)o

(17)

FCuObr Ff

(18)

xCuO(-rjCuO)r

MCuO FCubr Ff mox,AR ) MCu xCuO(-rjCu)o

(19)

The average reaction rates of CuO and Cu per volume unit of reacting solid, taking into account the residence time distribution of the carrier particles in the bed, can be calculated with the following equations:

dXs,r ) Fm,CuO (-rjCuO)r ) Fm,CuO dt dXs,o (-rjCu)o ) Fm,Cu ) Fm,Cu dt

trdXs,r EFR(t) dt (20) 0 dt

∫

to dXs,o

∫0

Fm,CuOLCuO (1 - X h r,inFR) br kr C h fn

to )

Fm,CuLCu bokoC h O2n

(24)

(1 - X h o,inAR)

(25)

The reacting time in the bed necessary to reach complete conversion will be tj, with this value being the upper limit of the integration in eqs 20 and 21. On the other hand, according to the kinetics obtained in this work, the solid reaction rate, dXs,j/dt, does not depend on the conversion or the reacting time, and it can be calculated from eq 9:

dXs,j 1 ) dt τ

(26)

j

Considering the mass balances for complete fuel gas conversion and the stoichiometry of reactions 3-6, the mass of the oxygen carrier in each reactor, expressed as the mass of the oxygen carrier totally oxidized, can be calculated as a function of the molar flow of fuel gas, Ff, with the following equations:

mox,FR )

tr )

dt

EAR(t) dt

(21)

The residence time distribution in the fuel, EFR(t), and air, EAR(t), reactors can be expressed as

EFR(t) ) e-t/thFR/thFR

(22)

EAR(t) ) e-t/thAR/thAR

(23)

where htFR and htAR are the mean residence times of the oxygen carrier particles in the fuel and air reactors, respectively, which are dependent on the solid recirculation rate and on the reactor size. Equations 20 and 21 have been expressed to consider that the oxygen carrier is introduced into the fuel and air reactors with a mean solid conversion, X h r,inFR or X h o,inAR, higher than 0. The values of tr and to were defined as the reacting times of an oxygen carrier particle from zero conversion until the maximum variah o,inAR, tion in solid conversion, i.e., 1 - X h r,inFR and 1 - X respectively, calculated with an average gas concentration in the bed, C h g. These values can be obtained from eqs 9 and 10 as

where τjj represents the times for complete solid conversion of the reduction or oxidation reactions obtained at an average gas concentration, C h g. Assuming gas plug flow in the reactors and no resistance to the gas exchange between bubble and emulsion phases in the fluidized bed, the average concentrations of reacting gas, C h g, in their respective reactors can be obtained from the following equation:

C h gn )

∆XgCg,0n Xg,out 1 + gXg Xg,in 1 - Xg

∫

[

]

(27)

n

dXg

The parameter g in eq 27 considers the volume variation as a consequence of the reaction, and it was calculated as

g )

Vg,Xg)1 - Vg,Xg)0

(28)

Vg,Xg)0

The value of g was 2 for the reduction of CuO with CH4, 0 for the reduction of H2 and CO, and -0.21 for the oxidation reaction of Cu. Considering 100 vol % of fuel gas at the fuel reactor inlet and complete fuel gas conversion, the average concentration was 13 vol % for CH4, 24 vol % for H2, and 18 vol % for CO. On the other hand, considering an air excess of 20%, the average O2 concentration in the air reactor was 11 vol %. The mass of the oxygen carrier in each reactor was obtained by integration of eqs 20 and 21 and replacement of the results obtained for (-rjCuO)r and (-rjCu)o in the eqs 18 and 19 as

[ [

] ]

mox,FR )

τjr MCuObr Ff xCuO 1 - e-tr/thFR

mox,AR )

MCuObr Ff τjo xCuO 1 - e-to/thAR

(29)

(30)

The times tr and to depend on the solid conversion at the inlet to each reactor, and the mean residence time, ht, depends on the recirculation rate, i.e., the variation of the solid conversion (see Figure 10). Therefore, the solids inventory for a given oxygen carrier depends on the recirculation rate and on the solid conversion at the


Figure 11. Total solids inventory of Cu10Al-I oxygen carrier in the fuel and air reactors for the combustion of 1 MWf of CH4 as a function of the oxidation conversion in the air reactor, X h o,AR, and on the variation of the conversion between both reactors: (gray shading) zone with the same solids inventory; (- -) minimum solids inventory at a fixed ∆Xs. Table 3. Solids Inventory Data of the Cu10Al-I Oxygen Carrier per MWf minimum at ∆Xs ) 0.3

minimum mox,FR (kg) mox,AR (kg)

CH4

H2

CO

CH4

H2

CO

52 60

23 49

45 42

62 71

29 57

54 50

inlet to each reactor, as well as on their metal oxide content and reactivity. Figure 11 shows the total inventory of the oxygen carrier “Cu10Al-I” per MWf of CH4 in the CLC system as a function of the oxidation conversion in the air reactor, X h o,AR, and the variation of the solid conversion between the two reactors, ∆Xs. Obviously, X h o,ARis the same as the solid conversion at the inlet of the fuel reactor, X h o,inFR. The solid conversion at the inlet of the air reactor, X h o,inAR, can be calculated directly from X h o,inFR and ∆Xs. The level curves indicate the conditions with the same amount of the oxygen carrier in the CLC system. Similar diagrams can be made for H2 and CO as fuel gases. In Figure 11, it can be observed that, for a given ∆Xs, the total solids inventory presents a minimum, with X h o,AR due to the effect of tr and to on the solids inventory of each reactor (see eqs 29 and 30). On the other hand, for a given X h o,AR, the solids inventory in each reactor, and therefore in the whole CLC system, increases as ∆Xs increases. At values of ∆Xs lower than 0.1, there is a zone where it is verified that 1 . e-to/thAR and 1 . e-tr/thFR. Inside this zone there are different pairs of values (X h o,AR and ∆Xs) that fulfill the minimum solids inventory in the system (112 kg/MWf). Table 3 shows the minimum solids inventory in the fuel and air reactors per MWf of CH4, H2, and CO. Although the oxidation rate is the same independently of the gas used for the reduction, the differences observed among gas fuels are due to the different oxygen flows necessary to fulfill the mass balance in the system. However, low values of ∆Xs gave high recirculation rates. Oppositely, high values of ∆Xs

gave high solids inventories. The optimum values of ∆Xs to get low recirculation rates and low solids inventories should be about 0.2-0.4. Table 3 shows the minimum solids inventories in the fuel and air reactors at ∆Xs equal to 0.3 for different fuel gases. Estimations of the total inventory in a CLC system with different Cu-based oxygen carriers and CH4 as the fuel gas can be found in the literature.8,9 For their calculations, they did not consider the solids residence time distribution and they implicitly assumed that 1 . e-to/thAR and 1 . e-tr/thFR. Cho et al.9 obtained that the bed mass in the fuel reactor was 200 kg/MWf with a 60 wt % CuO oxygen carrier produced by freeze granulation. Mattisson et al.8 estimated that the amount of oxygen carrier was 70 kg/MWf in the fuel reactor, and 390 kg/ MWf in the air reactor, using a 33 wt % CuO oxygen carrier prepared by dry impregnation. The amounts of oxygen carrier in the fuel and air reactors shown by these authors were higher than those obtained in the present work with a 10 wt % CuO oxygen carrier (see Table 3) because of the lower reactivity of their materials. However, it must be remembered that all of these values correspond to preliminary data, where the resistance to the gas exchange between the bubble and emulsion phases, which can be important in a fluidized bed, has been ignored, as in the cited papers.8,9 It can be concluded that there are Cu-based oxygen carriers with high reactivity in both the reduction and oxidation reactions and that can validate their use in a CLC system with low solids inventory. Conclusions The reactivities of reduction and oxidation reactions of a Cu-based oxygen carrier (10 wt % active CuO) to be used in CLC have been analyzed. This oxygen carrier has been prepared by impregnation on alumina and did not show reactivity loss or agglomeration problems during 100 reduction/oxidation cycles in fluidized-bed tests. CuO was well dispersed in the porous surface of the alumina matrix, and a uniform thin layer on the porous surface was considered. In this case, the shrinking-core model for platelike geometry in the porous surface of the particle was used for the kinetic determination. The oxygen carrier exhibited high reactivity in both reduction and oxidation with times for complete conversion lower than 40 s at 1073 K and 5-70 vol % of fuel gas and 5-21 vol % of O2. The highest reaction rate for the fuel gas was obtained with H2, followed by CO and CH4. It was found that the chemical reaction controlled the global reaction rate at temperatures above 1023 K and the oxygen carrier reached complete conversion in both the reduction and oxidation reactions. The reaction order depended on the fuel gas, and values of 0.4, 0.6, and 0.8 were found for CH4, H2, and CO, respectively. The order of the oxidation reaction was 1. The activation energies for the reduction and oxidation reactions varied between 14 and 60 kJ mol-1, and these quantities are similar to those described in the literature. No effect of the gas products (H2O and CO2) on the reduction reaction rate was detected. The reactivity of the Cu-based oxygen carrier previously determined was later used as a design criterion for a CLC system. It was found that the total solids inventory in a CLC system is linked to the recirculation rate and the inlet conversion at the fuel and air reactors and they depend on the reactivity and transport capac-

8176


ity of the oxygen carrier. With the oxygen carrier used in this work, the conversion variations between both reactors should be about 0.2-0.4 to avoid high recirculation rates or high total solids inventories. For a typical CLC operating condition, the total solids inventory for this Cu-based oxygen carrier (10 wt % CuO) was 133 kg/MWf if the fuel gas was CH4, 86 kg/MWf if the fuel gas was H2, and 104 kg/MWf if the fuel gas was CO, with recirculation rates of about 12 kg s-1 per MWf. Acknowledgment This work was carried out in the frame of the ECSC (European Coal and Steel Community) project (Contract 7220-PR/125) and the GRACE (Grangemouth Advanced CO2 Capture) project. The latter was coordinated by BP and funded by EU (ENK5-CT-2001-00571) and by CCP (CO2 Capture Project), a partnership of BP, Chevron Texaco, EnCana, Eni, Norsk Hydro, Shell, Suncor, and Statoil. Nomenclature bj ) stoichiometric factor in the reaction j, mol of solid reacting (mol of gas)-1 Cg ) gas concentration, mol m-3 Cg,0 ) gas concentration at Xg ) 0, mol m-3 C h f ) average fuel concentration, mol m-3 C h g ) average gas concentration, mol m-3 C h O2 ) average oxygen concentration, mol m-3 E ) activation energy, J mol-1 Ff ) molar flow of the fuel gas, mol s-1 Fi,0 ) molar flow of reacting solid i at Xs ) 0, mol s-1 kj ) chemical reaction rate constant of the reaction j, mol1-n m3n-2 s-1 k0,j ) preexponential factor of the chemical reaction rate constant, mol1-n m3n-2 s-1 Li ) layer thickness of the reacting solid i, m m ) mass of the sample, g mox ) mass of the fully oxidized oxygen carrier, g m ˘ ) recirculation rate, g s-1 m ˘ ox ) recirculation rate of fully oxidized oxygen carrier, kg s-1 Mi ) molecular weight of the material i, g mol-1 n ) reaction order (-rg)j ) gas reaction rate of the reaction j, mol of gas (m3 of solid)-1 s-1 (-rs)j ) solid reaction rate of the reaction j, mol of solid (m3 of solid)-1 s-1 (-rjCu)o ) average reaction rate of the oxidation of Cu, mol of Cu (m3 of Cu)-1 s-1 (-rjCuO)r ) average reaction rate of the reduction of CuO, mol of CuO (m3 of CuO)-1 s-1 R0 ) oxygen transport capacity of the active metal oxide S ) specific surface area of the reacting solid, m2 g-1 t ) time, s tj ) reaction time from solid conversion 0 until the maximum variation in the solid conversion, s htAR ) mean residence time of oxygen carrier particles in the air reactor, s htFR ) mean residence time of oxygen carrier particles in the fuel reactor, s T ) temperature, K Vi ) volume of the reacting solid i, m3 Vg,Xg)0 ) volume of the gas mixture at Xg ) 0, m3 Vg,Xg)1 ) volume of the gas mixture at Xg ) 1, m3 xCuO ) mass fraction of CuO in the fully oxidized sample Xg ) gas conversion Xg,in ) gas conversion at the reactor inlet Xg,out ) gas conversion at the reactor outlet

Xs,j ) solid conversion for the reaction j X h o,AR ) average solid oxidation conversion in the air reactor X h o,inAR ) average solid oxidation conversion at the inlet of the air reactor X h o,inFR ) average solid oxidation conversion at the inlet of the fuel reactor X h r,inFR ) average solid reduction conversion at the inlet of the fuel reactor Greek Letters ∆H0c,f ) standard heat of combustion of the gas fuel f, kJ mol-1 ∆Xf ) variation of the fuel conversion ∆Xg ) variation of the gas conversion ∆Xs ) variation of the solid conversion 0 ) initial particle porosity g ) coefficient of expansion of the gas mixture Fi ) real density of the reacting material i, g m-3 Fm,i ) molar density of the reacting material i, mol m-3 τj ) time for complete solid conversion for the reaction j, s τjj ) time for complete solid conversion for the reaction j at C h g, s Subscripts i ) reacting solid (CuO and Cu) j ) reaction (o, oxidation; r, reduction) g ) reacting gas (CH4, CO, H2, O2) f ) reacting fuel (CH4, CO, H2)

Literature Cited (1) Ritcher, H. J.; Knoche, K. F. Reversibility of Combustion Process. In Efficiency and Costing, Second Law Analysis of Process; Gaggioli, R. A., Ed.; ACS Symposium Series 235; American Chemical Society: Washington, DC, 1983; p 71. (2) Ishida, M.; Zheng, D.; Akehata, T. Evaluation of a ChemicalLooping-Combustion Power-Generation System by Graphic Exergy Analysis. Energy 1987, 12, 147. (3) Mattisson, T.; Lyngfelt, A. Capture of CO2 Using ChemicalLooping Combustion; Scandinavian-Nordic Section of Combustion Institute: Go¨teborg, Sweden, 2001. (4) Adańez, J.; de Diego, L. F.; Garcıá-Labiano, F.; Gayań, P.; Abad, A. Selection of Oxygen Carriers for Chemical-Looping Combustion. Energy Fuels 2004, 18, 371. (5) Lyngfelt, A.; Leckner, B.; Mattison, T. A Fluidized-Bed Combustion Process with Inherent CO2 Separation; Application of Chemical-Looping Combustion. Chem. Eng. Sci. 2001, 56, 3101. (6) Copeland, R. J.; Alptekin, G.; Cesario, M.; Gershanovich, Y. Sorbent Energy Transfer System (SETS) for CO2 Separation with High Efficiency. 27th International Technical Conference on Coal Utilization & Fuel Systems, CTA: Clearwater, FL, 2002; p 719. (7) Copeland, R. J.; Alptekin, G.; Cesario, M.; Gebhard, S.; Gershanovich, Y. A Novel CO2 Separation System. 8th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, Honolulu, Hawaii, 2000. (8) Mattisson, T.; Ja¨rdna¨s, A.; Lyngfelt, A. Reactivity of Some Metal Oxides Supported on Alumina with Alternating Methane and Oxygen-Application for Chemical-Looping Combustion. Energy Fuels 2003, 17, 643. (9) Cho, P.; Mattisson, T.; Lyngfelt, A. Comparison of Iron-, Nickel-, Copper-, and Manganese-Based Oxygen Carriers for Chemical-Looping Combustion. Fuel 2004, 83, 1215. (10) de Diego, L. F.; Garcıá-Labiano, F.; Adańez, J.; Gayań, P.; Abad, A.; Corbella, B. M.; Palacios, J. M. Development of Cu-based Oxygen Carriers for Chemical-Looping Combustion. Fuel 2004, 83, 1749. (11) Mattison, T.; Lyngfelt, A.; Cho, P. The Use of Iron Oxide as an Oxygen Carrier in Chemical-Looping Combustion of Methane with Inherent Separation of CO2. Fuel 2001, 80, 1953. (12) Ryu, H.-J.; Bae, D. H.; Han, K.-H.; Lee, S.-Y.; Jin, G.-T.; Choi, J.-H. Oxidation and Reduction Characteristics of Oxygen Carrier Particles and Reaction Kinetics by Unreacted Core Model. Korean J. Chem. Eng. 2001, 18, 831.

Ind. Eng. Chem. Res., Vol. 43, No. 26, 2004 8177 (13) Ishida, M.; Jin, H.; Okamoto, T. A Fundamental Study of a New Kind of Medium Material for Chemical-Looping Combustion. Energy Fuels 1996, 10, 958. (14) Garcıá-Labiano, F.; de Diego, L. F.; Adańez, J.; Abad, A.; Gayań, P. Temperature Variations in the Oxygen Carrier Particles during their Reduction and Oxidation in a Chemical-Looping Combustion System. Chem. Eng. Sci. 2004, Web Release Date: November 18, 2004.

(15) Levenspiel, O. Chemical Reaction Engineering; John Wiley and Sons: New York, 1981.

Received for review July 28, 2004 Revised manuscript received October 6, 2004 Accepted October 6, 2004 IE0493311

Reduction and Oxidation Kinetics of a Copper-Based Oxygen Carrier

Recommend Documents