Oxygen Carrier for Chemical-Looping Comb - American Chemical

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Ind. Eng. Chem. Res. 2010, 49, 9720–9728

Experimental Investigation of a CuO/Al2O3 Oxygen Carrier for Chemical-Looping Combustion Sander Noorman, Fausto Gallucci, Martin van Sint Annaland,*,† and Hans J. A. M. Kuipers Chemical Process Intensification, Multiphase Reactors, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands

Chemical-looping combustion (CLC) has emerged as an interesting alternative for conventional power production technologies, intrinsically combining power production and CO2 capture. The performance of the oxygen carrier particles used in this technology is of vital importance for the overall technical and economical feasibility of CLC technology, and therefore, the behavior of a selected oxygen carrier (CuO/Al2O3) has been investigated in more detail using thermogravimetry. The experimental study focused on the reactivity during the oxidation and reduction cycles and the stability of the particles over multiple alternating sequences of these cycles. Particles of relatively large size were used, to investigate the possibility of using the material in packed-bed CLC. Interpretation of the experimental results on a quantitative level was achieved through comparison with numerical simulations using a detailed particle model in which the effects of both reaction kinetics and intraparticle mass-transfer limitations were fully taken into account. It was found that the experimentally determined conversion rate of the particles during oxidation could be very well described by the particle model that accounts for changes in the particle morphology. The average pore size needed in the simulations to reproduce the experimental results matched well with the most common pore size found by nitrogen adsorption-desorption experiments using the BET and BJH methods. For the reduction cycles using hydrogen as the reducing agent, it was concluded that the conversion characteristics could be described reasonably well for moderate conversions, but for higher conversions, the discrepancies were larger, especially at rather low operating temperatures. When reduction cycles were carried out with methane, carbon deposition was observed. The generated data and insights help in assessing the optimal reactor configuration for CLC and its feasibility. Introduction Chemical-looping combustion (CLC) has emerged as an interesting option to intrinsically combine power production and CO2 capture, thus facilitating CO2 sequestration at lower energy penalties in comparison with state-of-the-art technologies such as postcombustion CO2 capture using amine scrubbing. The principle of chemical-looping combustion is based on the reasoning that separation processes aimed at acquiring a highpurity carbon dioxide stream can be circumvented when direct contact between air and fuel is avoided, so that a stream consisting of carbon dioxide and steam only can be generated. Through condensation, the water can easily be separated, after which the CO2 stream can be processed. To achieve this end, in chemical-looping combustion, an intermediate oxygen carrier (a metal/metal oxide of the form MexOy) is used that is alternately reduced and oxidized (see Figure 1). The oxidation of the oxygen carrier is strongly exothermic, which is used to heat an air stream to very high temperatures. During the (mostly endothermic) regeneration with a hydrocarbon fuel, carbon dioxide and steam are formed. In addition to the production of pure CO2 at a very high CO2 capture potential, this process also ensures that no NOx will be present in the flue gas because of the absence of the very high temperatures occurring when a flame is used in the combustion of hydrocarbons. Since the first introduction of chemical-looping combustion as an alternative for traditional power production, the fact that * To whom correspondence should be addressed. E-mail: [email protected]. † Current affiliation: Chemical Process Intensification, Multiphase Reactors, Eindhoven University of Technology, Den Dolech 2, 5612 AZ Postbus 513, Helix, STW 0.39, 5600 MB Eindhoven, The Netherlands.

the quality of the oxygen carrier is essential for the viability of this technology has been beyond question. Consequently, in the open literature, a considerable number of articles can be found that deal with the selection and development of suitable oxygen carrier materials for chemical-looping combustion. From these studies, a number of requirements have been identified for the oxygen carrier to be of interest for application in an industrialscale CLC process.1,2 The list of demands includes chemical, mechanical, and thermal stability over a large number of reduction and oxidation cycles at representative process conditions; high reactivity during both reactive processes; good selectivity to the desired products, carbon dioxide and steam; high resistance to carbon deposition; and a high oxygen capacity. As the amount of oxygen carrier needed to operate an industrialscale CLC process will be substantial, the materials used should be inexpensive, with respect to both material costs and synthesis and preparation expenses. Finally, of course, low environmental impact of the particles is highly preferable, especially when the stability of the material is limited, which would imply a regular replacement of the oxygen carrier.

Figure 1. Schematic overview of chemical-looping combustion.

10.1021/ie100869t  2010 American Chemical Society Published on Web 09/17/2010

Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010

Most research efforts in this field have been aimed at transition metals. Iron and copper oxides have gained much attention in view of their abundant availability and relatively low costs.2 With nickel oxide, very good reactivity was observed, which compensates for the somewhat higher material costs. The use of pure metal oxides has generally been rejected based on poor reactivity over multiple cycles, which was attributed to agglomeration of the material.3-5 The use of a support material was found to be crucial in improving the reactivity and stability of the oxygen carrier and diluting the active material. Moreover, fluidization behavior was found to improve with the application of supports. Most often, alumina is suggested as the most interesting support material, even though interaction of the support with the metal oxides was observed in several cases. In particular, the formation of nickel aluminate (NiAl2O4) was often reported, so that several studies were actually aimed at using this complex as the support material.6,7 Other support materials considered include (yttrium-stabilized) zirconia, titania, and silica, with which varying results were obtained. For a more detailed overview of the work performed on the development and preparation of oxygen carriers (including combinations of metal oxides), the interested reader is referred to the review of Hossain et al.2 In a previous work, it was proposed that CLC be performed using packed-bed reactor technology.8 The main advantage of this approach over a circulating fluidized-bed reactor system, which is generally envisaged to be applied for CLC,9 is that fines production under harsh process conditions can be avoided, which is of particular relevance because of the downstream gas turbine. In packed-bed CLC, however, larger particles must be used in view of pressure drop considerations. With the application of larger particles, the effect of intraparticle mass- (and heat-) transfer limitations will increase. For this reason, in this work, attention is directed not so much at developing a suitable oxygen carrier material, but rather at obtaining, for a selected oxygen carrier, an improved understanding of the effect of masstransfer limitations, which is needed to arrive at the assessment of the suitability of packed-bed technology as a reactor concept for CLC. In this work, the performance of a copper oxide (CuO) on alumina oxygen carrier with a relatively large particle size was investigated, especially with respect to stability and reactivity, using thermogravimetry. In the remainder of this article, the reactor setup and experimental settings are first discussed briefly. Then, experimental results obtained on the stability and reactivity of the oxygen carrier are presented, after which these results are compared with numerical simulations using a particle model that is described elsewhere in more detail.10 Setup and Settings Thermogravimetric analysis (TGA) is a very commonly used technique for studying reactive gas-solid systems and was also used in this work to investigate the behavior of oxygen carriers. In TGA (for a schematic overview of the system, see Figure 2), the mass change of a sample that is exposed to a reactive gas flow is tracked in time. With this approach, the conversion characteristics of the oxygen carrier particles during oxidation and reduction cycles can be determined. Moreover, the stability of the materials over multiple reaction cycles can be investigated, in terms of both the oxygen capacity of the material and the observed reaction rates. In the setup used (Sartorius), a sample (typically 100-200 mg was used to achieve sufficient accuracy during the measurements) was placed in a porous quartz glass

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Figure 2. Schematic representation of the TGA setup (where CW ) counterweight).

sample holder (with an outer diameter of 8 mm, a height of 16 mm, and a wall thickness of 1.7 mm) that was connected to the balance with a platinum wire. A permanent nitrogen purge stream was used to avoid exposure of the electronics in the balance to reactive species. The sample was located in a cylindrical quartz glass tube (15mm inner diameter), surrounded with a heating wire and insulation material. The reactor temperature was measured just below the sample and was used to control the heating system to allow for experimentation at constant temperature. The system was operated at atmospheric conditions. A four-way valve was used to switch between an inert flow (nitrogen) and either oxidizing or reducing conditions. In the experiments, a total gas flow of 10 mLn · s-1 was used during the reaction cycles. The concentrations of the reactive species were varied by dilution with nitrogen. It was ensured that changing of the gas flow rate did not affect the behavior of the system, implying that external mass-transfer limitations to the sample holder were absent and that the sample was surrounded by a constant gas composition. Results and Discussion Copper Oxide on Alumina Oxygen Carrier. A detailed understanding of the reactivity and stability of oxygen carrier systems is of great relevance for the assessment of the suitability of materials for application in an industrial-scale CLC system. In this work, copper oxide on alumina was studied. The oxygen carrier was used as received from Sigma-Aldrich without pretreatments. The nominal composition of the particles was 13 wt % CuO/Al2O3; however, the actual composition of the material used in the experiments, which was determined using X-ray fluorescence (XRF), can be found in Table 1. From this analysis, it was concluded that no significant reduction of the copper oxide content in the oxygen carrier occurred as a consequence of the chemical reactions in the system. Consequently, the active weight content of the oxygen carrier particles ox was assumed to remain constant during the experiments (ωs,act ) 0.125). Furthermore, apart from the expected presence of copper oxide and alumina, traces of cobalt, nickel, iron, and manganese were observed. The extent to which these species are involved in the conversion processes is unknown. In the analysis to follow, only the reaction of copper oxide is taken into consideration. The average diameter of the particles used

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Table 1. Composition (wt %) of Cu/CuO on Al2O3 Oxygen Carriera

a

composition (wt %)

fresh sample 1

fresh sample 2

used sample 1

used sample 2

CuO Al2O3 Ni Mn Co Fe

12.8 86.3 0.37 0.21 0.16 0.11

12.4 86.8 0.35 0.21 0.16 0.07

12.5 86.7 0.35 0.21 0.16 0.06

12.3 87.0 0.34 0.21 0.16 0.05

Data obtained from XRF analysis of four samples.

Figure 3. Stability and reactivity of copper oxide on alumina over multiple reduction/oxidation cycles at different temperatures: (a) observed mass change and temperature settings and (b) reaction time τ* and maximum reaction rate observed with the oxidation cycles. Reductions were carried out with 50/50 vol % hydrogen/nitrogen; oxidations with air. The sample mass was 170 mg.

was 1.1 mm and was found not to change significantly with the number of cycles. Stability over Many Oxidation-Reduction Cycles. The stability of oxygen carriers in an industrial CLC process is of great importance, as regular replacement of the particles is undesired and the predictable behavior of the materials is of great relevance to achieve stable operation in the long run. Figure 3 presents the results obtained with an experiment using 170 mg of CuO/Al2O3 that was reduced and oxidized for almost 80 cycles. All reduction cycles were performed using a 50/50 vol % hydrogen/nitrogen mixture, so that uncertainties in the interpretation and material degradation due to carbon deposition could be eradicated. The reduction cycles were executed until a virtually constant sample mass was attained, so that the

maximum conversion of the oxygen carrier would be reached. The oxidation was performed with air. The mass in the completely oxidized state was found to remain the same, even after a large number of alternate cycles had been performed. To facilitate a proper comparison between the experiments, a reaction time τ* was introduced. This time was defined as the period between 5% and 95% of the total conversion. This measure was introduced to eliminate effects of gas dispersion at the beginning of the oxidation cycle and somewhat lower accuracy at the end of the reaction. In the experiment, the reaction temperature was varied as indicated in Figure 3a. Heating and cooling between cycles was done under oxidizing conditions at a rate of 10 °C · min-1. When the desired temperature was reached, the system was allowed to rest for typically 15 minutes. It can be seen that there is a significant influence of both the reaction temperature and the number of cycles performed (at the same temperature) on the oxidation characteristics. During the first 10 cycles, which were executed at a temperature of 700 °C, it was found that the average reaction rate increased strongly, resulting in a lower characteristic reaction time τ*. Other researchers, too, reported that the first cycles performed with a sample (although prepared in a different way) lacked consistent behavior.11 Moreover, with an increasing number of cycles, a decrease in the apparent conversion of the material was observed (it must be noted that the expected mass change based on the composition of the oxygen carrier was (2.5%), whereas the maximum reaction rate increased slightly. From this behavior, it can be concluded that, in this phase, important changes in the morphology of the material occurred that resulted in the diminishing effect of diffusion limitations, although the accessibility of the active material remained the same. The effective active weight content in the oxygen carrier was approximately 10%, based on the first 10 cycles performed in this experiment. This corresponds well with the conversions reported in the literature.12 Subsequently, the reaction temperature was decreased to 500 °C. Surprisingly, it can be seen clearly that this strongly affected the behavior of the oxygen carrier. First, the strong decrease in the apparent capacity of the material stands out. It seems that the temperature adaptation affected the particle in such a way that the availability of copper oxide decreased. Whether this was due to the accessibility of the active material, which could be affected by the morphology of the particle, or to the chemical stability of copper oxide (interaction between the active material and the support material through the formation of CuAl2O4) cannot be determined with sufficient certainty. It can be seen that, at this temperature, the oxygen capacity of the material increased with increasing number of cycles, but it remained at a lower level than for the reaction temperature of 700 °C. As a consequence of the increasing capacity, the reaction time also increased when more cycles were executed. When the reaction temperature was raised again, an increase of the material accessibility, the maximum observed reaction


rate, and a decrease in τ* were observed, until, at a reaction temperature of 900 °C, the conversion of the material was more than 90% of the maximum conversion based on the composition of the oxygen carrier particles (see Table 1). The steady decrease of the total reaction time suggests that the particle morphology gradually changed with the number of cycles and that, with every temperature shift, more of the active content became available to be converted. Finally, when the temperature was again decreased from 900 °C to lower temperatures, it was found that the capacity of the material dropped to a level slightly below that of previous cases in which the same temperature level was used. This can especially be concluded from the experiments performed at a temperature of 700 °C (starting at cycle numbers 1, 37, and 71). Structural changes in the material that might be due to sintering, however, especially resulted in a significant decrease in the effective reaction time. It is also noted that, when the temperature was decreased (e.g., from 700 to 500 °C or from 800 to 700 °C), it was necessary to perform more cycles to obtain stable operation in terms of the observed mass change than when the temperature was increased. A proper explanation for this effect can unfortunately not be given. In conclusion, it was observed that the stability of this material is probably insufficient when operation is pursued with large variations in the temperatures to which the oxygen carrier particles are exposed. It appears that this is the case in both packed-bed CLC and fluidized-bed CLC, in which the temperature in the oxidation reactor will typically be considerably higher than the temperature in the reduction reactor. In both cases, sufficient solids mixing [also with packed-bed CLC, intermittent fluidization can be applied to level (solids) concentration and temperature profiles8] will ensure that constant behavior over the particle bed can be realized. Given that the total oxygen capacity of the material did not seem to change strongly during the experiments and that the reaction time observed essentially only became shorter with the number of cycles executed, the material as such seems, in principle, to be suitable for long-term operation, especially considering that this oxygen carrier was not specifically designed for application in chemical-looping combustion. However, only detailed experimental studies in a suitable reactor setup can provide sufficiently accurate predictions on the long-term stability of the materials in industrial operation. Comparison with Numerical Simulations with the Particle Model. In this section, a comparison between experimental results with the oxidation cycles and simulations using a particle model described elsewhere10 is presented both to assess the accuracy of the particle model and to quantitatively interpret the experimental results obtained with the copper oxide on alumina oxygen carrier. An advanced numerical model was developed in which the effects of reaction kinetics and internal and external mass- and heat-transfer processes were incorporated in a systematic and complete manner.10 In this model, the particle morphology is taken into account using a single average pore size. From theoretical analysis, diffusion limitations were found to be especially affected by Knudsen diffusion. In the model description, kinetics data for the oxidation of copper were obtained from the literature.13 The component mass and energy balances, along with the initial and boundary conditions, are summarized in Tables 2 and 3, respectively. (An explanation of the symbols can be found in the Nomenclature section.) Oxidation. First, two experiments are discussed in which samples of approximately the same mass (∼170 mg) were

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a

Table 2. Governing Equations in the Particle Model continuity equation

∂(εgFg) 1 ∂ ) - 2 r2ntot - m ˙ gfs,tot ∂t r ∂r

gas-phase components

∂(εgFgωg,i) 1 ∂ ) 2 r2ni - m ˙ gfs,i ∂t r ∂r where Ng-1

ni ) jt + ωg,tntot ) -Fg

∑D

tk

k)1

∂ωg,k + ∂r ωg,tntot

solid-phase components

∂(εsFsωs,j) ) ∂t

Ng

∑ m˙

gfs,i

i)1

whereb ox ωs,MeO ) ωs,act (1 - ωs,Me) ox ωs,Me(1 - ωs,act )

energy balance

(εgFgCp,g + εsFsCp,s)

1 ∂ ∂T ∂T ) 2 r2λeff ∂t ∂r r ∂r Ng

∑ i)1

reaction rate

νsMMeO MMe

m ˙ gfs,i (-∆HR,i) Mi

m ˙ gfs,i ) νεsFskrCg,inCs,jmMi where

( )

kr ) kr,0 exp -

Eact RgT

a Boundary conditions presented in Table 3. MeO/inert system.

b

In the case of the Me/

oxidized and reduced at a constant temperature (T ) 600 and 800 °C) for 25 cycles, using the settings that were also used for the experiments reported in the previous section. Results obtained from these experiments are provided in Figure 4. Again, a significant difference in the oxygen capacity between the two experiments was found, but the mass changes observed in the experiments were more or less constant over the number of cycles and comparable to those observed at these temperatures in Figure 3. The reaction time for the experiment performed at T ) 600 °C did not change significantly with an increasing number of cycles, but for the experiment performed at T ) 800 °C, a clear increase in the average reaction rate was observed. In Figure 5a, the reaction rate derived from the 20th cycle of both experiments is depicted. It is concluded that there was a rather significant effect of gas dispersion during the first seconds of the reaction, so that the reaction rate increased where it would have been expected to be constant. After this period, the reaction rate monotonically decreased, suggesting that there was a significant effect of intraparticle diffusion limitations in the experiments. Using the particle model discussed before, numerical simulations were performed in which the initial conditions for the oxidation cycle were derived from the experimentally obtained mass change, where the porosity of the particles was estimated

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Table 3. Initial and Boundary Conditions Used in the Particle Model condition

component balance (gas phase)

energy balance

ωg,N2 ) 1.0

T ) Tbulk

t)0 r ) 0 (center)

r ) R (surface)

∂ωg,i ∂r Ng-1

∑D

i,k

k)1

film model

∂ωik ) ∂r

|

∂T ∂r

)0 r)0

Ng-1

∑k

g,ik(ωg,k

- ωbulk,k)

λeff

|

)0 r)0

∂T ) R(T - Tbulk) ∂r

k)1

0 kg,ik ) Ξikkg,ik

where Φik and Ξik ) exp(Φik) - 1

from the apparent density of the material and the densities of the components comprising the oxygen carrier (from which it followed that εg,p ≈ 0.6) and where an average pore size was used to represent the particle morphology. In Figure 5c,d, a comparison between the TGA experiments and simulation results is presented. It can be seen that, for both of the experiments, both the shape of the experimental curves and the total conversion time can be well described by the simulations using a pore size of ∼100 Å for the experiment at 600 °C and ∼200 Å for the experiment at 800 °C. Consistent with the findings described before, for the higher-temperature experiment, a larger pore size was needed to correctly describe the particle behavior. In Figure 5b, the evolution of the effectiveness factor for the two cases is shown. From the maximum values of η, it can be concluded that there was a very strong effect of internal diffusion limitations. Using this approach, the particle morphology was essentially used as a fitting parameter for the experiments, and it is of great interest to investigate whether the obtained values are realistic. For this reason, nitrogen adsorption-desorption experiments using Brunauer-Emmett-Teller (BET) and Barrett-JoynerHalenda (BJH) analyses were carried out.14,15 In Figure 6, the differential pore volumes and areas of four copper oxide samples are depicted. Apart from the samples discussed in this section, a fresh sample and the sample discussed in the previous section were analyzed. From Figure 6, it can be concluded that, with increasingly harsh handling of the material, the pore area strongly decreased, whereas the total pore volume varied much less. Consequently,

Φik )

ntot 0 Fgkg,ik

the pore size in the system increased. From the graphs shown, the average pore sizes for the different samples were estimated to be approximately 70, 100, 220, and 380 Å. As can be seen from the comparison of experiments and simulations in Figure 5, the estimates from Figure 6b,c correspond very well with the estimates based on the particle simulations. When the particles were exposed to higher temperatures during the experiments, it appears that an increasingly large fraction of the surface area was represented by very small pores, whereas most of the pore volume was found at increasingly large pore sizes. This suggests that the presence of micropores (dpore < 20 Å), which might have formed as a result of sintering of the material, increased during the experiments. However, apparently, the amount of active content present in these micropores was very small, and the overall reactivity was especially determined by the increasingly large pores that made up the majority of the pore volume. Overall, it can be concluded that mass-transfer limitations became less relevant when the material was exposed to more extreme conditions, as increases in the pore size apparently did not occur at the expense of the material accessibility. The surface areas obtained from the analysis, which were measured using the BET technique, were 188, 144, 77, and 43 m2 · g-1 for Figure 6a-d, respectively. Finally, now that it is known that numerical simulations with the particle model can be used to describe the experiments performed with TGA, an estimate of the evolution of the average pore size with the number of cycles in the experiment presented in the previous section (see Figure 5) can be made. In this case,

Figure 4. Experimental results for the oxidation cycle of CuO/Al2O3: (a) relative mass change and (b) characteristic reaction time as a function of the number of cycles at two different temperatures.


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Figure 5. Comparison of experimental results and model predictions for the oxidation cycle of CuO/Al2O3: (a) temporal evolution of the reaction rate for the 20th cycle for the experimental runs at 600 and 800 °C, (b) variation of the effectiveness factor in time from the simulations (dpore,600 °C ) 100 Å, dpore,800 °C ) 200 Å), and evolution of conversion with time in experiments and simulations using different pore sizes at (c) 600 and (d) 800 °C.

Figure 6. Pore size distributions obtained from nitrogen adsorption-desorption experiments (BET/BJH). Shown are adsorption data. The CuO/Al2O3 samples analyzed had previously been treated in the following ways: (a) no treatment (i.e., fresh sample), (b) 25 cycles at T ) 600 °C, (c) 25 cycles at T ) 800 °C, and (d) 80 cycles at different temperatures (see Figure 3).

for a selected number of cycles, the pore size was determined that provided the best correspondence between theory and experiment. In this way, the (apparent) evolution of the particle morphology could be followed. As can be seen in Figure 7a, showing the results of these calculations, it was found that a lower pore size was typically needed to describe the conversion

characteristics when the temperature was decreased and vice versa. However, this process was not completely reversible. In Figure 7b, two cycles are displayed, and good correspondence between the experiments and the numerical simulations can be seen. The similarity seems to have improved with increasing number of cycles (especially in terms of the shape

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Figure 7. (a) Evolution of the particle morphology (average pore size) with the number of cycles and temperature. (b) Conversion characteristics at two specific oxidation cycles. Lines indicate the simulations; symbols, the experiments.

of the curves), which can be attributed to a diminished effect of the pore size distribution. Of course, accounting for the particle morphology with only the average pore size is a simplification of the actual system, but it turns out to give reasonable accuracy. Reduction. From the investigation of the oxidation cycle, it was concluded that significant diffusion limitations affected the conversion characteristics of the oxygen carrier and that, by accounting for the particle morphology with an average pore size, a good correspondence between experiments and model results could be attained. This shows that both the model description and assumptions and the kinetic data used for the description of this process are trustworthy. For the reduction cycles, however, both the reliability of the kinetic data and the model assumptions are more questionable. In the work by Garcıá-Labiano et al.,13 from which the reaction kinetics used here were obtained, it was found that constant reactivity of the oxygen carrier over the entire reduction cycle, which would indicate that chemical reaction kinetics fully dominate the conversion rate of the particles, was obtained only at rather high temperatures (T > 800 °C). Below such temperatures, a strong decline of the reduction rates with time and signs of incomplete conversion of the active material were observed. As the particle size used in their experiments was rather low (∼100 µm), the possibility of intraparticle diffusion limitations could be eradicated. Apparently, there is another process that has not yet been taken into account but that plays an important role during the reduction cycles, especially at lower temperatures. This could, for example, be related to the activation of adsorption and desorption of reactant and product gases or to the migration of oxygen in copper oxide multilayers.16 Either way, an additional relevant resistance affects the reaction rates at high conversion and would need to be taken into account in a particle model. More details on the microscopic behavior of the material at these conditions would be needed to improve the model description. For the reduction with hydrogen, a reasonably accurate prediction of the processes occurring on particle scale could still be obtained; see Figure 8. Only at high conversion of the oxygen carrier was a strong decrease in the reaction rate observed that was not predicted in the numerical simulations for the same system. In the numerical simulations, the total mass change during the reduction cycle was assumed to be the same as that observed in the experiments. For this reaction, it was concluded that, at lower temperatures, the reaction is fully controlled by the reaction kinetics. At high temperatures (T ) 900 °C), still only a rather small effect of intraparticle diffusion limitations could be observed: the maximum effectiveness factor

Figure 8. Conversion characteristics during the reduction of copper oxide with 20 and 50 vol % hydrogen in nitrogen at T ) 800 °C: comparison between experimental results and simulations. In the simulations, a pore size of 200 Å was used.

was 0.8. With lower concentrations of the reactant gas, the effect of diffusion limitations increases, which is caused by the fact that the reaction order is below unity. For the reduction with methane, the effect of carbon deposition also became apparent at higher temperatures. Because of this phenomenon, it was not possible to discern the rate of oxidation and reduction from the weight change. In Figure 9, results for a few reduction cycles with methane are given, and comparisons with simulations are presented. For the simulations, it was assumed that all active material available in the oxygen carrier could actually be converted, even though this is clearly not a completely correct assumption. Carbon deposition was completely ignored in this description, because a proper description of the kinetics of this process is lacking. In the plots, it can be seen that the reactions proceed very rapidly at high temperatures and moderate concentrations of the reactant gas, which is of great interest for the final application. Because of the activation energy of the reduction with methane, which is rather high compared to that of the oxidation reaction, a strong decrease in the reaction rate was found in the simulations. This decrease seemed to be even stronger than that observed in the experiments, so that it could be concluded that the activation energy of this reaction was probably overestimated. Finally, it can be seen that the effect of changes in the reactant gas concentration was captured rather well. As was the case with the reduction with hydrogen, this process was also mostly controlled by the gas-solid reaction kinetics. Only at high temperatures did the effect of diffusion limitations start to become more pronounced, but clearly not as relevant as for the oxidation cycle.


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Figure 9. Reduction of copper oxide with methane: comparison of the temporal evolution of the relative mass change between experimental results and simulations with respect to the effects of temperature and methane concentration. In the simulations, it was assumed that all active material present in the oxygen carrier could be converted, so that the maximum mass change would be 2.5%. The particle morphology was taken into account by using an average pore size of 200 Å. In a and b, yCH4 ) 0.50 and T ) 600, 700, 800, and 900 °C; in c and d, T ) 800 °C and yCH4 ) 0.15, 0.20, 0.30, and 0.40.

It is concluded that a good description of the oxygen carrier, both qualitatively and quantitatively, was obtained when the oxidation cycles were considered, despite issues regarding the stability of the material and the variation of the material behavior with process conditions. The oxidation cycle was significantly affected by both the oxidation kinetics and intraparticle mass-transfer limitations, more specifically, by Knudsen diffusion. For the reduction cycles, it is concluded that, at high temperatures, a good quantitative correspondence between the experiments performed and the numerical simulations was obtained. This process was mostly controlled by the reaction kinetics for smaller conversions, although the effect of diffusion limitations became more important when lower concentrations of the reactant gases and higher reaction temperatures were used. At higher conversions, an additional resistance became apparent that can probably be described only when more information on the processes occurring on a molecular scale becomes available. For the reduction of copper oxide with methane, carbon deposition on the particle surface would need to be included to obtain a complete description of the relevant phenomena. Again, this would highly complicate the system and is beyond the scope of this work. Rather than being able to fully account for the effect of carbon deposition in numerical simulations, suppression of this effect through steam addition or flue gas recirculation should be attempted. Conclusions In this study, the performance of a CuO/Al2O3 oxygen carrier for chemical-looping combustion was investigated. It is beyond doubt that the reactivity and stability of the particles used in a CLC process are essential for the success of the technology as a whole. Using thermogravimetric analysis, the performance of particles with a relatively large

particle size was investigated, envisaging the possibility of applying these materials in packed-bed reactor technology. Particularly, the reactivity of the materials under both oxidizing and reducing conditions and the stability of the particles over multiple cycles were studied. A good qualitative and quantitative description of the behavior of oxygen carrier particles could be obtained. By using a single average pore size, the particle morphology was represented in the simulations, and values used were found to correspond reasonably with those found by nitrogen adsorption-desorption experiments (BET/BJH). The oxidation rate was found to be strongly affected by internal mass-transfer limitations, especially as a consequence of Knudsen diffusion. With the reduction cycle, the conversion rate was mostly dominated by gas-solid reaction kinetics. With the depletion of oxygen in the material, an additional resistance that had not been taken into account in the model description was found to become relevant. The stability of copper oxide on alumina was quite good, considering that, over a large number of reactive cycles, only decreases in the time needed to fully convert the material were observed. The main disadvantage of the material as an oxygen carrier for an industrial-scale CLC system, however, is related to the low melting temperature of the material and the inevitable efficiency loss associated with operating this process at a relatively low temperature. Therefore, this oxygen carrier might especially be of interest when an application that operates at rather moderate temperatures can be developed. Finally, with this study, increased understanding of the relevance of masstransfer limitations in oxygen carriers for chemical-looping combustion has been obtained, which is necessary for arriving at a well-founded technical and economic assessment of the suitability of packed-bed CLC for power production with inherent CO2 capture.

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Acknowledgment TNO is gratefully acknowledged for financial support. Nomenclature Roman Letters Cp ) heat capacity of the gas or solid phase (J · kg-1 · K-1) dp ) particle size (m) dpore ) average pore size (Å) Di,k ) element of the diffusion matrix (m2 · s-1) Hi ) enthalpy (J · mol-1) ∆HR ) reaction enthalpy (J · mol-1) ji ) diffusive mass flux of component i (kg · m-2 · s-1) k0 ) pre-exponential factor (s-1) keff ) effective reaction constant (s-1) kg,ik ) element of the mass-transfer coefficient at the actual masstransfer rate (m · s-1) k0g,ik ) element of the mass-transfer coefficient at a low mass-transfer rate (m · s-1) m ) reaction order in the solid phase ∆m ) mass change (kg) m0 ) sample mass in the oxidized state (kg) m ˙ gfs ) mass-transfer rate (kg · m-3 · s-1) Mi ) molecular weight of component i (kg · mol-1) n ) reaction order in the gas phase Ng ) number of components in the gas phase ni ) mass flux of component i (kg · m-2 · s-1) ntot ) drift flux (kg · m-2 · s-1) r ) radial coordinate (m) rmax ) maximum observed reaction rate (% · s-1) t ) time (s) T ) temperature (K) X* ) (normalized) conversion y ) mole fraction Greek Letters R ) heat-transfer coefficient (W · m-2 · K-1) ε ) porosity η ) effectiveness factor λeff ) effective heat dispersion (W · m-1 · K-1) ν ) stoichiometric factor (mol · mol-1 · g) Ξik ) element of correction factor matrix F ) density (kg · m-3) τ ) reaction time (s) Φk ) rate factor ωg,i ) weight fraction of gas material ox ωs,act ) active weight content in fully oxidized state ωs,j ) weight fraction of solid material Sub- and Superscripts bulk ) gas bulk g ) gas phase

i, j, k ) component in the gas or solid phase Me ) metal MeO ) metal oxide ox ) oxidative state red ) reductive state s ) solid phase

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ReceiVed for reView April 13, 2010 ReVised manuscript receiVed August 19, 2010 Accepted September 1, 2010 IE100869T

Oxygen Carrier for Chemical-Looping Comb - American Chemical

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