Reactivity of Oxygen Carriers for Chemical-Looping Combustion in

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Reactivity of Oxygen Carriers for Chemical-Looping Combustion in Packed Bed Reactors under Pressurized Conditions H. P. Hamers,† F. Gallucci,† G. Williams,‡ P. D. Cobden,§ and M. van Sint Annaland*,† †

Chemical Process Intensification, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands ‡ Johnson Matthey Public Limited Company, Sonning Common, Reading, West Berkshire RG4 9NH, United Kingdom § Energy Research Centre of The Netherlands (ECN), P.O. Box 1, 1755ZG Petten, The Netherlands ABSTRACT: For the design, scale-up, and optimization of pressurized packed bed reactors for chemical-looping combustion (CLC), understanding of the effect of the pressure on the reactivity of the oxygen carriers is very important. In this work, the redox reactivity of CuO/Al2O3 and NiO/CaAl2O4 particles at elevated pressures have been measured in a pressurized hightemperature magnetic suspension balance. The experiments have demonstrated that the pressure has a negative influence on the reactivity and that this effect is kinetically controlled. The negative effect of the pressure might be caused by the decrease in the number of oxygen vacancies at higher pressures. Moreover, the reactant gas fraction has been demonstrated as an important parameter, probably related to competition between different species for adsorption on the oxygen carrier surface. These effects have been included in the kinetic model leading to a good description of the experimental results. The impact of these findings on packed bed CLC applications with larger oxygen carrier particles has been investigated with a particle model that considers diffusion limitations and kinetics. It has been shown that the impact of diffusion limitations decreases with increasing pressure, due to the decrease in reaction rates and the increase in diffusion fluxes caused by Knudsen diffusion. The results have been validated by experiments with 1.7 mm NiO/CaAl2O4 particles. These results corroborate that the selection of larger particles because of pressure drop considerations does not lead to a large decrease in effective reaction rates, which is beneficial for packed bed CLC applications.

1. INTRODUCTION If a conventional CO2 capture technology (for CCS) is added to a coal-fired power plant, an energy intensive separation step is included that can result in a dramatic decrease in the overall process efficiency.1 This energy intensive separation step is intrinsically avoided with chemical-looping combustion (CLC). In this case, the fuel is indirectly combusted with an oxygen carrier (MeO which is reduced to Me), which results in a stream that contains mainly CO2 and steam. After steam condensation, a rather pure CO2 stream is obtained that can be compressed and stored. The oxygen carrier is reoxidized with air (oxidation of Me), which is an exothermic reaction. Equation 1 shows an overview of the reactions that are involved with the CLC process with syngas as fuel. After summing up the reactions, a regular combustion reaction is reached, and this means that the same amount of heat is produced as with the regular combustion reaction.

production (via expansion in a gas turbine). The highest overall electrical efficiency is obtained if a combined cycle is applied (a gas turbine and a heat recovery steam generator). This means that the hot depleted air stream has to be produced at high temperature (at least 1200 °C) and elevated pressure (around 20 bar).1 For this reason, the CLC process has to be carried out at elevated pressure. In most power plant integration studies, it is assumed that the process is operated at elevated pressure, but at this moment, little information is known about the reactivity of the oxygen carriers at elevated pressures, which is essential to get an accurate prediction of the CLC process at elevated pressure. The goal of this paper is to study the influence of the pressure on the kinetics of the oxygen carriers. ́ Garcia-Labiano et al. have published the kinetics at elevated pressure for copper, nickel, and iron based oxygen carriers.2 The kinetics have been determined at 450−950 °C and a pressure between 1 and 30 bar. It appears that a high total pressure has a negative effect on the kinetics. They observed the same behavior with different particle sizes (0.09−0.25 mm), which indicates that they were measuring kinetics.2 The authors have indicated that the decline in reactivity cannot be caused by a decrease in surface area and pore volume. For other gas solid reactions, similar results have been obtained, for example, for the reactions between CaO and H2S or calcination of CaCO3.

nH 2 + n MeO → nH 2O + n Me mCO + m MeO → mCO2 + m Me n+m O2 + (n + m)Me → (n + m)MeO + 2 n+m nH 2 + mCO + O2 → nH 2O + mCO2 2

(1)

Most of the heat is produced during the oxidation of Me with air. A hot depleted air stream is produced due to this exothermic reaction, and this stream is used for power © 2015 American Chemical Society

Received: December 14, 2014 Revised: March 16, 2015 Published: March 17, 2015 2656

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the original particles. Before the tests, the oxygen carrier was activated by exposure to two redox cycles with reductions with H2 at 900 °C. 2.2. High Pressure Magnetic Suspension Balance. The experiments have been carried out in a magnetic suspension balance (Rubotherm) that can operate between 200 and 1200 °C and 1−30 bar. An oxygen carrier sample of 100 mg is placed in a porous quartz glass sample holder. An electromagnet is attached to the bottom of the balance. It lifts a so-called suspension magnet, which consists of a permanent magnet, a sensor core, and a measuring load decoupling cage. The basket is placed on an Ir wire that is hanging on a permanent magnet. The electromagnet, which is attached to the underfloor weighing hook of the weighing balance, maintains a freely suspended state of the suspension magnet via an electronic control unit. The change in mass is transmitted from the suspension magnet to the permanent magnet and from this to the weighing balance. The reactant gases are supplied at the top of the reactor. The reactor is surrounded by a vessel that is maintained at lower temperature. Argon is supplied to this vessel to prevent reactant gases from entering and mixing in the insulation layer. The argon is mixed with the reactant mixture after the sample point to prevent argon from influencing the measurements. A schematic overview of the setup is provided in Figure 1.

Chauk et al. have found a decrease in the reactivity of CaO with H2S at higher pressures but have attributed this to a lower pore volume and surface area.3 Similar behavior was found for the calcination reaction where the reactivity was described by increasing the negative pressure effect in Fuller’s equation for molecular diffusion.4 From these studies, it can be concluded that the pressure could have a negative effect on the reactivity, but there is no general consensus on the cause for the observed negative effect. Some experimental work has been carried out for the application of CLC in pressurized circulating fluidized bed reactors. Such a system has been constructed by Xiao et al.5 Experiments have been carried out with Shenhua bituminous coal as fuel and iron ore as oxygen carrier at three different operating pressures (1, 3, and 5 bar). Stable operation has been reached. A higher combustion efficiency was observed at elevated pressure. However, the oxygen carrier fine production was also higher (caused by increased attrition at elevated temperatures), which is very detrimental for power production in downstream turbines. In our previous work, we demonstrated that the maximum efficiency of the process is mainly influenced by the generated temperature and pressure of the hot air stream and is not affected much by the type of reactor selected, as long as the reactor can work at these conditions (about 1200 °C and 20 bar).6 To better accommodate the CLC process at elevated pressure, packed bed reactors have been selected in this study. In packed bed reactors, larger oxygen carrier particles are required to maintain a low pressure drop, but the selection of larger particles may also imply that the role of diffusion limitations inside the particle may become more dominant. The effect of diffusion limitations has been described by Noorman et al. for atmospheric applications.7 This model considers molecular diffusion and Knudsen diffusion. From these studies, it was concluded that Knudsen diffusion was the ratedetermining step in the oxygen carrier particles considered. The objective for this paper is to measure the pressure effect on the kinetics and to evaluate this impact for packed bed CLC applications. For the oxygen carrier reaction rate measurements, CuO/Al2O3 and NiO/CaAl2O4 particles have been tested in a high pressure magnetic suspension balance. A kinetic expression is determined from these measurements, and this correlation is used to describe the behavior in packed bed reactors using an advanced particle model. The model is validated by experiments with larger particles.

Figure 1. Schematic overview of the magnetic suspension balance setup. The experimental procedure was as follows. Before a series of experiments was started, the system was pressurized and the reactor was set at the desired operating temperature. When this temperature had been reached and the system had been stabilized, several redox cycles were carried out. Each redox cycle consisted of a 20 min reduction (with CO or H2), a 10 min purge with N2, a 10 min oxidation (with air), and a 10 min purge with N2. After a certain number of cycles, the experimental setup was cooled down again. During the experiments, a total flow rate of 480 mLn/min is fed. Each experiment was also reproduced with a lower flow rate (320 mLn/min) to demonstrate that the measured reactivity is not influenced by external mass transfer limitations. Every experiment has been repeated at least two times to ensure the reproducibility of the results. The gas stream that is fed to the reactor can also influence the observed mass change, and these effects were quantified during blank experiments. Blank experiments were carried out with only a sample holder (and no oxygen carrier) and were subtracted from the data obtained with the oxygen carrier sample. 2.3. Particle Model. The conversion of the solid is described by a numerical particle model that assumes a spherical oxygen carrier particle with a uniform porosity, fixed particle diameter, and a uniform pore size.7 The conversion of a single particle is representative for a

2. MATERIALS AND METHODS 2.1. Oxygen Carriers. The CuO/Al2O3 particles were obtained from Sigma-Aldrich with an active weight content of 13 wt % and a particle size of 1.1 mm. For kinetic experiments, the particles were crushed and sieved to a size of 110−150 μm. The NiO/CaAl2O4 particles used in this work are a Johnson Matthey product, HiFUEL R110 (Ni based catalyst supported on CaAl2O4 for steam reforming of natural gas), available in pelleted form from Alfa Aesar. The particles were received in the form of shaped pellets, comprised of 4 holes and 4 flute domed cylinders. For this study, the pellets were crushed and sieved to particles with average sizes of 1.7 mm and 0.15 mm. Separate TGA experiments proved that the mass change (and thus the active weight content) is not the same for each sample. The experiments with CO and O2 were carried out with a sample with an active weight content of 18.5 wt % (assuming full oxidation and reduction), while an active weight content of 17 wt % was measured for the experiments with H2, which is attributed to heterogeneities in the weight distribution of the active component in 2657

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Energy & Fuels Table 1. Equations of the Particle Model7

100 mg basket with particles, because it was verified that the experiments were not influenced by external mass transfer limitations, which means that all the particles were exposed to the same gas composition. The model describes the gas transport inside the particle from the moment that the particle is exposed to a reactant at certain operating conditions. For the gas transport, the reaction kinetics and molecular and Knudsen diffusion (internal mass transfer limitations) and external mass transfer limitations are taken into account. Because all these effects were included, particles with a variety of sizes can be simulated. During a simulation, the solid conversion is simulated as a function of time and these data are compared with TGA results. The equations applied in the model are listed in Table 1. The kinetics for CuO are ́ the same as those used by Hamers et al.8 (which are based on GarciaLabiano2,9), and the kinetics for the NiO/CaAl2O4 are taken from Medrano et al.10 The particle properties are based on pycnometer (Quantachrome Micro-ultrapyc1200) and Brunauer-Emmett-Teller (Thermscientific Surfer) measurements and are listed in Table 2.

3. RESULTS AND DISCUSSION 3.1. Pressure Effect on Kinetics. Experiments were carried out varying the total pressure (1−20 bar), while the partial pressure of the reactant was kept constant at 1 bar. In this way, the reactant gas concentration and the temperature were fixed, so that solely the influence of the total pressure is measured. The reactants were diluted with N2 (and some CO2 in the case of CO as reactant) at elevated pressures to provide a mixture with a 1 bar reactant pressure. The pressure influence can also be determined by fixing the reactant gas fraction and varying the pressure, but this experimental series will be discussed at the end of this section. Redox cycles have been measured with CuO/Al2O3 and NiO/CaAl2O4 as oxygen carriers at 600 and 800 °C. In the case of CO as reactant, a CO2/CO ratio of 1 and 3 was used at 800 and 600 °C, respectively. The results plotted as solid conversion (defined in eq 2) as a function of time for the experiments at constant temperature and different total pressures are shown in Figures 2 and 3. During the reduction cycles, full solid conversion was not reached. The maximum solid conversion depends on the reduction temperature and the type of the support material.12,13 The support material could be present as an inert layer in the solid structure, which might influence the accessibility of the oxygen and thus the degree of reduction. The maximum degree of reduction depends on the

Table 2. Particle Properties for the NiO/CaAl2O4 Particle11 oxygen carrier particle diameter, mm particle porosity, m3gas/m3particle average pore size, Å

17−18.5 wt % NiO on CaAl2O4

TGA experiments

1.7

sieved

0.55

derived from combination dry and liquid pycnometer BET porosimetry

130

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Figure 2. Effect of pressure on the redox kinetics of NiO/CaAl2O4 at 800 °C with H2, CO, and air. The markers show the experimental data, and the lines show the model predictions.

Figure 3. Effect of pressure on the redox kinetics with CuO/Al2O3 with H2, CO, and air. The markers show the experimental data, and the lines show the model predictions.

operating temperature. The lowest conversion is reached for the H2 experiments with CuO at 600 °C. For demonstration of the pressure effect, the curve has been zoomed in on the first 60 s, but after that moment, the particle keeps on reacting and a conversion of 80% is reached after longer times.8 From the experiments, it can be concluded that the maximum solid conversion does not depend on the operating pressure but only on the temperature.

reactivity with the pressure is observed for all reactants with both oxygen carriers. At higher pressures, more fluctuations in the experimental results can be seen which is related to limitations of the experimental setup. These fluctuations could in principle be decreased by reducing the total flow rate, but in that case, external mass transfer limitations could occur. Despite these fluctuations, a clear trend can still be discerned from the results at 20 bar. External mass transfer limitations cannot be the cause for the decrease in the reaction rate, because the same conversion curves were obtained from experiments with a lower gas flow rate (320 mLn/min instead of 480 mLn/min). This experiment demonstrates that the reactant flow rate was sufficiently high to refresh the gas around the sample and to supply a sufficient amount of reactants for the gas/solid reactions. This has been validated for all the operating conditions investigated. Moreover, experiments have been carried out with different particle sizes, and again, the same conversion curves have been

solid conversion =

observed mass change maximum mass change for the assumed active weight content

(2)

As can be observed from Figures 2 and 3, where the partial pressure of the reactant was fixed, but the total pressure was varied, the reaction rate decreases with increasing total pressure. It has to be noted that, in the experiments with a higher total pressure, the reactant was thus more diluted, because the partial pressure of the reactant was kept constant. The decreased 2659

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with the reactant and the active material. Since the reactant and the active material was the same, the discrepancy with our experiments can be ascribed to an effect of the support. Furthermore, the quality of the fitting is different especially at high pressures. Also a different series of experiments has been carried out, in which the gas fraction was fixed at 20% and the pressure was varied. Also in that case, a small decrease in the reactivity was observed with the pressure. These experiments have been carried out for both the Ni and the Cu based material and CO, H2, and O2, and the same trend was observed in all these cases. Therefore, only one example of this experimental series is shown in Figure 5. This figure shows the temporal evolution of the solid conversion of NiO during reduction with 20% CO in 20% CO2 and 60% N2 at 1, 5, and 20 bar.

obtained. This is illustrated in Figure 4, where the oxidation of Cu/Al2O3 is displayed at 20 bar (with pO2 = 4 and 1 bar) and

Figure 4. Particle size effect on the solid reaction rate for the oxidation of Cu/Al2O3 at 800 °C and 20 bar. The reactivity of 1.1 mm particles is represented by markers and that of the 0.15 mm particles by lines.

the particle size was varied between 0.15 mm (lines) and 1.1 mm (markers). The same trends can be observed, and for that reason, internal mass transfer limitations can also be ruled out as cause for the observed decrease in reactivity at elevated pressures. This is also supported by model simulations. Thus, the decrease in reactivity with increasing pressure has to be kinetically controlled. An expression for the reaction rates including a pressure correction factor has been introduced by ́ Garcia-Labiano et al.2 and the kinetic term is demonstrated in eq 3. r=

⎡ −E ⎤ k0 exp⎢ act ⎥C n ptot q ⎦ ⎣

( ) 105

RT

Figure 5. Influence of the pressure (from 1 to 20 bar) on the reduction reactivity of NiO/CaAl2O4 with CO, while the CO fraction is fixed at 20%.

3.2. Discussion. In the previously described experiments, the reactant partial pressure was kept constant, while the total pressure was varied. The partial pressure was fixed by increasing the dilution at higher pressures. During both the oxidation and the reduction reactions, the reaction rates decrease with increasing pressure, which might to a large extent have been caused by the dilution of the reactant gas. The following reaction mechanism has been proposed in the literature for redox reactions.14,15 First, the reactant adsorbs on the oxygen carrier, and subsequently, an oxygen atom is transferred from the adsorbed gas to the oxygen carrier or vice versa.14 CO2 or H2O is formed during reduction, and this molecule is desorbed from the oxygen carrier. In principle, not only the reactant could adsorb to the oxygen carrier surface but also the other gases that are considered to be inert in the reaction (like N2). In the case of competitive adsorption, the reactant gas fraction is a relevant parameter for the kinetics. Furthermore, the surface is not expected to be flat, also because the metal (for example, Ni has an atomic radius of 125 pm16) has a different atomic radius than the oxide (atomic radius of 66 pm16). Due to this difference, cavities could be present on the solid surface. The gas molecule that is present in the cavities could be inert or reactive with the solid. If the space of a cavity is occupied by an inert gas, it blocks the pathway of the reactive gas. Therefore, a reactive spot on the solid remains unoccupied and this results in a lower reaction rate. These diffusion limitations are not dependent on the particle size, because the gases are distributed in the particle by pores that are much bigger, so that the gas close to the solid surface still has the feed composition.

(3)

The same method has been followed here. By fitting the experimental data, a number for the parameter q was determined. In most of the cases, a good description has been obtained by this method, except for the reduction of NiO with H2. The determined numbers for q are displayed in Table 3 together with the data from the literature for comparison. In ́ general, the same trend is observed as by Garcia-Labiano et al.2, but somewhat different values for q have been found. There may be different reasons for the observed discrepancies; first, the experiments reported in the literature may have been carried out with different support materials, that might interact Table 3. Determined Values for q (in Eq 2) from the Experimental Data and Comparison with Values Found in the Literature CuO/Al2O3

gaseous reactant

q from ́ GarciaLabiano 2 et al.

H2 CO O2

0.53 0.83 0.68

NiO/CaAl2O4

q from this reaction work order, n9 1.0 1.2 1.3

0.6 0.8 1

q from ́ GarciaLabiano 2 et al. 0.47 0.93 0.46

q from this reaction work order, n10 0.75 0.85 1.05

0.6 0.65 0.9 2660

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Energy & Fuels Other experiments have been carried out to exclude some effects and to prove what could be the reason for the observed behavior in the experiments. Reductions have been carried out with varying CO2/CO ratios at different pressures, and no significant differences on the kinetics was observed. Therefore, it seems that the desorption of gaseous products is not the limiting step. Furthermore, oxidations have been carried out with air (25% air) that is diluted by either N2 or CO2 (so a 25% air/75% N2 vs a 25% air/75% CO2 mixture). It is expected that CO2 adsorbs on the solid surface, and therefore, competitive adsorption with O2 is expected. However, the experiments showed that the reaction rates did not change significantly, when air was diluted with CO2. This means that either N2 adsorbs on the surface by physisorption with the same impact as CO2 or that the adsorption is not a rate limiting step. In the latter case, the blocking of the reactant gases in the cavities could explain the decrease in reactivity when the mixture is more diluted (but this remains a speculative conclusion). In any case, the reactant gas fraction has a large effect on the observed kinetics, but unfortunately, no explanation can be proven at this stage. Another point to be taken into account is that, during oxidation, not all the metal is available at the surface. In fact, the metal does not form a monolayer between the support material and the pore. According to the Wagner oxidation theory, metal ions and electrons migrate to the surface of the metal grain, while oxygen ions move to the bulk. This transport is carried out by vacancies in the metal oxide structure.17 If the solid structure contains more metal than oxygen according to the stoichiometry (a so-called oxygen deficient situation), the formation of oxygen vacancies is possibly the rate limiting step. These vacancies are formed according to eq 4. In this equation, VO is an oxygen vacancy, O(s) is an oxygen atom in the solid matrix, and O2(g) is a gaseous oxygen molecule. O(s) ⇄ VO + 1/2O2 (g)

r∼

Cn ptot

q

( ) 105

=

1 ptot

q

( ) 105

pn ∼ pn ptot−q = x nptot n − q = x qpn − q RnT n (5)

During the experiments with a fixed reactant partial pressure, the gas fraction and the total pressure were varied, which can be fitted by q. The obtained values for q are in general a factor of 0.2−0.4 larger than the reaction order, n. This results in a negative number for n − q with an order of magnitude of p−0.2 to p−0.4. It should be noted that the same trend is observed in the number of vacancies at different pressures.17 This indicates that a similar trend is observed in the above-mentioned diffusion flux inside a solid matrix. This indicates that the decrease in reactivity with increasing pressure might be related with the reduced rate in the formation of oxygen vacancies at higher pressures. ́ Concluding, the lumped expression given by Garcia-Labiano et al.2 can capture the effect of observed phenomena, but a more detailed study preferably with in situ analysis should be carried out to elucidate the phenomena prevailing at elevated pressures in more detail. The experimentally determined pressure effect will be used in the next section to investigate its implications for pressurized packed bed CLC applications with relatively large particles. 3.3. Pressure Effect in Large Particles for Packed Bed Applications. An increase in pressure results in a decrease in the reaction rates by about a factor of 3 at 20 bar relative to atmospheric pressure. In this section, the effect of reduced reaction rates and the extent of internal mass transfer limitations for the relatively large particles used in packed bed reactors will be studied in more detail. For packed bed applications, larger particles have to be used to avoid an excessive pressure drop over the reactor, which would reduce the overall process efficiency. When the particle size is increased, the influence of internal diffusion limitations could increase, which could result in a decrease of the effective reaction rates. Experiments have been carried out with different particle sizes to evaluate the impact on the operation of packed bed reactors. The NiO/CaAl2O4 particles were available with a larger particle size and have been used in this part of the investigation. A particle model has been developed to describe the effective reaction rates inside oxygen carrier particles considering reaction kinetics, molecular diffusion, and Knudsen diffusion through the pores. In the previous section, it has been shown that the redox kinetics have a negative pressure dependency. The molecular diffusion coefficient is inversely dependent on the pressure, whereas the Knudsen diffusion coefficient is independent of pressure. The overall diffusion coefficient is multiplied by the gas density that linearly increases with the pressure. Hence, no pressure dependency is expected in the molecular diffusion limited regime, and a positive pressure dependency is expected if Knudsen diffusion is the limiting step. At atmospheric pressure, the Knudsen diffusion is by far the most important limitation for gas/solid reactions with oxygen carriers.7 Thus, the pressure might reduce the diffusion limitations in the particles and increase the effective reaction rates. The measured solid conversion as a function of time during the reduction with CO and the oxidation of a NiO/CaAl2O4 particle with a relatively large average particle size of 1.7 mm is displayed in Figure 6 by the markers. The operating conditions were at 800 °C, and the reactant partial pressure was fixed,

(4)

Equation 4 describes that, if an oxygen gas molecule reacts with the metal, two oxygen vacancies in the solid matrix are replaced by oxygen atoms. If the oxygen pressure is increased, the equilibrium shifts to the left and the number of vacancies can decrease. Because of the decrease in oxygen vacancies, the diffusion through the solid matrix decreases, when the transport via oxygen vacancies is rate determining. In the some cases with oxygen deficient materials, the dependency of the diffusion flux inside the solid matrix is with pO2−1/6. In such a situation, a decrease in kinetics can be observed when the pressure is increased, while the reactant gas fraction is fixed. During reduction, the oxygen has to be transported in the opposite direction. If the reactant gas fraction and the oxygen vacancies in the oxygen carrier have influence on the kinetics, the Arrhenius approach might not be the right approach to describe these ́ gas/solid reactions at different pressures. Garcia-Labiano et al.2 proposed to include an additional term in the equation that is dependent on the total pressure with which the experiments could be described, eq 3. This can be rewritten using Dalton’s law to an expression that has a dependency on the reactant gas fraction, x, and the partial pressure of the reactant as reported in eq 5. 2661

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Figure 7. Simulated influence of the particle size on the effective reaction rates for a reduction of a NiO/CaAl2O4 particle with a 50% CO/CO2 mixture at 800 °C and 20 bar.

chemical-looping combustion has been experimentally investigated using a pressurized magnetic suspension balance. Experiments with a fixed reactant partial pressure (1 bar) showed a dramatic decrease in the kinetics, while it was proven that they were not influenced by mass transfer limitations. During this experimental series, the gas reactant is diluted more to keep the partial pressure at a constant level, which affects the results. This may have been caused by competitive adsorption (also of inert species) or blockage of cavities at the reactive sites. These effects were excluded in experiments with varying pressure and a fixed reactant gas fraction, but in this case, a small decrease in the kinetics was observed with the pressure, which might be related with a reduced number of oxygen vacancies. From the experimental results, it was concluded that the pressure decreases the solid reaction rate and it is proven that this decrease is kinetically controlled. The experimental results have been described well with a numerical particle model, if a pressure factor with a negative exponent is included in the kinetic description. This was also the case for large particles (required for packed bed applications), in which internal diffusion limitations occur. In this case, increasing the pressure also has a positive effect on the overall reaction rates, because the internal diffusion limitations (controlled by Knudsen diffusion flux) decrease with increasing pressure. For that reason, still reasonably high reaction rates can be obtained when using relatively large oxygen carrier particles. In the end, it can be concluded that the pressure has a negative effect on the oxygen carrier kinetics and a description has been found for this effect.

Figure 6. Effective reaction rates of NiO/CaAl2O4 for reductions with CO (a) and oxidations (b) at 800 °C varying the operating pressure and constant reactant partial pressure (particle size = 1.7 mm).

while the total pressure was varied. The same experiments were simulated with the particle model (that also takes internal mass transfer limitations into account), and the model results are shown by the lines in Figure 6. The experiments are described quite reasonably by the model. The effectiveness factor that is obtained from the model increases with increasing pressure, meaning that the reaction becomes more kinetically controlled. This is caused by the decrease in the redox kinetics with increasing pressure. The good agreement between the model and the experiments with 1.7 mm particles indicates that the model can be used for the prediction of the behavior of particles in packed bed reactors. For the chemical-looping combustion process in packed bed reactors, an operating pressure of about 20 bar is expected to be optimal.6 As an example, the modeled solid conversion as a function of time is displayed in Figure 7 for a reduction cycle with 50% CO at 800 °C and 20 bar for different particle diameters. It is shown that the particle size does not have a large effect on the effective reaction rates when smaller than about 5 mm, because the extent of internal diffusion limitations decreases with increasing pressure. Hence, the fact that larger particles have to be used to reduce the pressure drop in a packed bed reactor does not have a negative overall impact on the process performance.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +31 40 247 2241. Fax: +31 40 247 5833. E-mail: M.v. [email protected].

4. CONCLUSIONS The pressure effect on the oxygen carrier kinetics is important for the description of the CLC process at elevated pressures (at which the highest process efficiency can be reached). In this work, the effect of the pressure on the reaction rates of CuO/ Al2O3 and NiO/CaAl2O4 particles as oxygen carriers for

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by the CATO-2 program under the project number WP1.3F2. 2662

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NOMENCLATURE b = gas solid reactant stoichiometric factor, mol solid/mol gas [B] = inversed diffusion matrix C = concentration, mol/m3 Cp = heat capacity, J/kg/K Di,k = diffusivity, m2/s D0 = pre-exponential factor for diffusion term, m2/s [D] = diffusion matrix EA = activation energy, J/mol ED = activation energy for the diffusive component, J/mol k0 = pre-exponential factor, mol1−ns−1 for CuO and mol1−nm3n−3s−1 for NiO kx = conversion dependent factor in reaction rate j = diffusive mass flux, kg/m2/s M = molar mass, kg/mol N = number of components n = reaction order related to gas phase ni = gas flux of component i, kg/m2/s p = pressure, Pa r = particle radius, m T = temperature, K t = time, s R = gas constant, J/mol/K X = solid conversion x = gas fraction

Greek Symbols

ΔHR = reaction enthalpy, J/mol ε = porosity, m3/m3 λ = heat dispersion, W/m/K ν = reaction stoichiometric factor (negative for reactants) νi = diffusion volume of component i, m3/mol νs = stoichiometric factor for solids, mol MeO/mol Me ρ = density, kg/m3 τ = tortuosity ω = mass fraction, kg/kg Subscripts

act = active Bin = binary eff = effective g = gas i = gas component number j = solid component number k = gas component number Kn = Knudsen MeO = metal oxide Me = metal mol = molecular n = number of gas components p = particle s = solid tot = total Superscripts

ox = in oxidized state

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REFERENCES

(1) Spallina, V.; Romano, M. C.; Chiesa, P.; Gallucci, F.; Van Sint Annaland, M.; Lozza, G. Int. J. Greenhouse Gas Control 2014, 27, 28− 41. (2) García-Labiano, F.; Adánez, J.; de Diego, L. F.; Gayán, P.; Abad, A. Energy Fuels 2006, 20 (1), 26−33. 2663

DOI: 10.1021/ef5027899 Energy Fuels 2015, 29, 2656−2663