Bentonite

Article pubs.acs.org/EF

Effect of Carbon Deposition on the Oxidation Rate of Copper/ Bentonite in the Chemical Looping Process Esmail R. Monazam,†,‡ Ronald W. Breault,*,† Ranjani Siriwardane,† Hanjing Tian,†,§ Thomas Simonyi,†,§ and Stephen Carpenter†,§ †

National Energy Technology Laboratory, United States Department of Energy, 3610 Collins Ferry Road, Morgantown, West Virginia 26507-0880, United States ‡ REM Engineering Services, PLLC, 3537 Collins Ferry Road, Morgantown, West Virginia 26505, United States § URS Energy and Construction, Incorporated, 3610 Collins Ferry Road, Morgantown, West Virginia 26505, United States ABSTRACT: The presented work is part of the Industrial Carbon Management Initiative (ICMI) on the development of metal oxide oxygen carriers for use in the chemical looping combustion process. An oxygen carrier, CuO/bentonite (60:40%), was reacted with methane gas and then oxidized in air. The change in weight and reaction gas concentrations were measured using a thermogravimetric analyzer (TGA) equipped with a real-time gas analyzer. The reduction−oxidation cycle was conducted within the temperature range of 750−900 °C for 10 cycles, using 20, 50, and100% CH4 concentrations in N2 for the reduction segment and dry air for the oxidation segment. Several analysis methods were evaluated to fit the oxidation of reduced CuO (i.e., Cu) data over the complete conversion range with suitable rate expressions derived from existing models for oxidation, including the shrinking core model (diffusion and reaction control), first- and second-order reaction rates, parallel and series reaction mechanisms, and Johnson−Mehl−Avrami (JMA) rate. The best agreement between the experimental data and the models of the Cu oxidation was accomplished using the JMA model. The reactivity of the oxygen carrier during the oxidation reactions was affected by the CH4 concentration as well as the temperature. The rate of fractional uptake of oxygen onto the carrier decreased as the temperature increased, contrary to expectations and indicative that the mechanism is changing during the test. Analysis of the exit gas provided evidence of carbon deposition on the reduced sorbent particle and resulted in the CO2 product upon oxidation. The oxidation of this carbon releases significant heat that is capable of changing the particle morphology (Zhu, Y.; Mimura, K.; Isshiki, M. Oxid. Met. 2004, 62, 207−222). On the basis of experimental results, the overall reaction process in the fuel reactor may be considered to consist of the decomposition of CH4 into C and H2 and reduction of CuO/bentonite by the resulting H2 and the parallel reaction of CH4 with CuO/bentonite. The extent of carbon deposition in the carrier particle increased with an increasing temperature and CH4 concentration. This deposited carbon not only leads to CO2 release from the oxidation reactor but, more importantly, causes degredation of the carrier capacity and its reactivity.

■

INTRODUCTION Coal-fired power plants produce and release carbon dioxide (CO2) to the atmosphere, a major greenhouse gas contributing to global climate change. Given the growing global energy demand with fossil fuels as primary sources of energy, substantial measures are necessary to stabilize atmospheric CO2. To improve the combustion efficiency of fuels, the conception of chemical looping combustion (CLC) has been proposed as an alternative process for the combustion of fossil fuels, such as coal and natural gas, providing complete CO2 capture.1−4 The idea of CLC was first introduced in 1983 by Richter and Knoche.5 They suggested instead of oxidizing the fuel with oxygen from the combustion air, the fuel is oxidized by an oxygen carrier, i.e., an oxygen-containing compound, in a flameless combustion process. A CLC system consists of two reactors: a fuel and an air reactor, and an oxygen carrier transfers the oxygen from the air reactor to the fuel reactor, as shown in Figure 1. In this way, fuel and air are never mixed and the fuel oxidation products CO2 and water leave the system undiluted by excess air. Looking first at the air reactor (left), a reduced oxygen carrier, such as supported copper, is oxidized or burned to produce the oxygen carrier, supported copper oxide. The heat generated © 2012 American Chemical Society

Figure 1. Schematic presentation of CLC.

Received: August 23, 2012 Revised: September 26, 2012 Published: October 9, 2012 6576

dx.doi.org/10.1021/ef301389h | Energy Fuels 2012, 26, 6576−6583

Energy & Fuels

Article

reaction that causes the problem of CO2 release from the air reactor. Reaction 3 can offset this, but some solid carbon may still be retained on the oxygen carrier particle. Excess steam can be added with the fuel to drive reaction 3 to completion but it may contribute to additional expense. The engineering questions suggested are how much carbon can be passed to the air reactor and what are the effects of the carbon on the carrier particles. This paper addresses some of the issues related to this latter question. The reduced oxygen carrier is then transported to the air oxidizer, where it is reconstituted to its original oxidation state according to following reaction: 1 Cu + O2 → CuO (6) 2 Unfortunately, the carbon that becomes deposited on the oxygen carrier particles is converted to CO2, as noted above. It is likely that the existence of this carbon in the carrier particle and/or the heat released from this reaction (approximately 2.5 times that released in reaction 6) changes the particle structure, giving the observed decrease in the reaction rate with increases in the temperature.

from this oxidation process heats the effluent noted as depleted air and the oxygen carrier. The hot oxygenated solid is transferred to the fuel reactor, where it is reduced with the fuel. Thus, flue gases from the fuel reactor do not contain N2 from the air, which is the major component of flue gases from the classical combustion techniques. Theoretically, pure CO2 is produced after condensation of water vapor and removal of the liquid water.6 The flue gases leaving the air reactor contain N2 and the unreacted O2 of air. The reactions in the air reactor are exothermic, and the reactions in the fuel reactor are either exothermic or endothermic depending upon the fuel and oxygen carriers. Therefore, the CLC process does not require an expensive CO2 separation process. The success of the CLC system is dependent upon finding suitable oxygen carriers that have high reaction rates and good selectivity toward the production of carbon dioxide over undesired byproducts, such as hydrogen, carbon monoxide, and solid carbon, but should also exhibit high chemical, thermal, and mechanical stability over a large number of reduction and oxidation cycles. Different metal oxides have been proposed as a possible candidate for the CLC process:5 the reactivity of the four most studied supported oxygen carriers is in descending order NiO > CuO > Mn2O3 > Fe2O3. Various support materials, namely, Al2O3, MgAl2O4, ZrO2, TiO2, and bentonite, are used to enhance some of the desirable characteristics of oxygen carrier particles.7 Among the metal oxides suited for oxygen carriers, Cu-based oxygen carriers have several advantages: CuO is not a very expensive material compared to nickel and has a very high oxygen transport capacity, and CuO reduction is favored thermodynamically to reach complete conversion of gaseous hydrocarbon fuels into CO2 and H2O.8 Methane, which is the most cited fuel in the literature, is consumed in the fuel reactor according to the following reactions: CH4 ↔ C + 2H 2

(1)

4CuO + CH4 ↔ 4Cu + CO2 + 2H 2O

(2)

C + H 2O ↔ CO + H 2

(3)

CO + CuO ↔ CO2 + Cu

(4)

H 2 + CuO ↔ H 2O + Cu

(5)

C + O2 → CO2

(7)

In summary, from decomposition of methane (eq 1), carbon deposits are formed on the solid (oxidized or reduced state), so that the subsequent gases release from the air reactor would contain carbon species as unaccepted impurities. In this work, the extent of carbon deposition in the carrier particle and its effect on the reactivity of the oxygen carrier are examined.

■

EXPERIMENTAL SECTION

The experiments were carried out using a TA model 2050 thermogravimetric analyzer (TGA) with an on-line mass spectrometer gas analyzer, as shown in Figure 3. The advantage of using a mass

Thermodynamic calculations determine that the decomposition of methane is favored above 590 °C (Figure 2). It is this first

Figure 3. Sketch of the TGA setup. spectrometer is the capability of measuring all relevant gas components continuously, including H2 and H2O. There are some minor drawbacks. First, it is difficult to distinguish between N2 and CO because these two gases have the same molecular mass. Second, there are also minor interferences because of molecules briefly converted into ions during analysis. The reactor was initially heated to the reaction temperature under nitrogen; about 20 mg of the CuO/ bentonite (60:40%) sample was then exposed to reduction−oxidation cycles with a duration of 45 min for reduction and 60 min for oxidation for each cycle. A typical test experiment was carried out for 10 cycles. The fuel used for reduction was 100, 50, and 20% methane in nitrogen, and reoxidation was carried out in air. Between each phase

Figure 2. Variation of the equilibrium constant and Gibbs free energy for decomposition of CH4 with temperature. 6577

dx.doi.org/10.1021/ef301389h | Energy Fuels 2012, 26, 6576−6583

Energy & Fuels

Article

and final mass after a full cycle are the same. During the N2 purge there is a small amount of copper oxide dissociated to cuprous oxide. When the reduction gas is added, all CuO are reduced to metallic copper. For more analysis on the reduction process, see the study by Monazam et al.9 The oxidation analysis discussed in this paper explores and explains the data for conversion of the metallic copper to copper oxide.

of reduction or oxidation, 100% nitrogen was fed to the reactor for 10 min to purge the reactants. The experiments were carried out with a gas flow of 45 mL/min [standard temperature and pressure (STP)] and the temperature range of 750−900 °C. The preparation of the sample is described in detail by Monazam et al.9 The Brunauer− Emmett−Teller (BET) surface area of the particles is 4 m2/g. Typical experimental observations for temperature and changes in weight during TGA experiments are illustrated by panels a and b of Figure 4. The initial loss in weight from a value of about 43 mg to 41 mg is due to the release of moisture from the carrier as it is stored at ambient conditions.

■

RESULTS AND DISCUSSION Figure 6 illustrate the outlet gas composition as a function of time during the fifth reduction cycle for a temperature of 900

Figure 6. Outlet gas concentrations in terms of the ionic count during a reduction of CuO/bentonite with 100% CH4 concentration and 900 °C.

°C and 100% CH4 concentration. The traces are essentially identical for each of the last 6 of the 10 cycles comprising the experiment. Note that the concentration is presented in terms of the ionic count in this figure and is directly related to more conventional concentration units of moles per liter. As shown by Figure 6, CO2 and H2 were formed immediately after the introduction of CH4 into the reactor, indicating that both reactions 1 and 2 proceed from the beginning. After 1 min, copper oxide reached near complete conversion, as indicated by both the constant CH4 concentration in Figure 6 and the TGA data presented in Figure 5, while a certain amount of H2O and H2 was observed. The CO2 concentration increases immediately and then decreases steadily until it reaches zero after about 1−2 min, indicating that CuO is completely reduced. However, additional reactions continue to occur, as indicated by the changes to the H2O and H2 concentrations. The H2O and H2 concentrations increase to a maximum and then drop to constant values. This phenomenon is explained in more detail in the following paragraph. It was hypothesized that the oxygen had to be coming from bentonite. To understand this phenomenon, experiments were conducted with CH4 and H2 as the reactive gases passing over a pure bentonite sample and by heating bentonite in an inert gas. These results are shown in Figures 7−9. Looking first at Figure 7, it can be seen that, during the reduction cycle, H2, H2O, and CO2 are all formed with the reaction of CH4 and bentonite. Bentonite is an aluminum silicate with a common chemical composition of Al2O34(SiO2)H2O for its hydrated condition at ambient conditions; however, at the reaction temperatures of 700−900 °C, it exists only as Al2O34(SiO2). It is likely that the oxidation state for silicon changes from +4 to +2 during this reduction, freeing up oxygen to react with the carbon to form the CO2 peak and hydrogen to form the H2O peak. The results show the same thing in Figure 8, as hydrogen reacts with the oxygen from the reducing bentonite to form H2O. Figure 9 confirms this, showing a weight loss for bentonite when

Figure 4. (a and b) Typical mass and temperature measurement for the CuO/bentonite particle of 200 μm and 100% CH4 for reduction and air for oxidation reactions. Figure 5 shows the TGA reduction and oxidation data for cycle 6 at two temperatures, 800 and 900 °C. It should be noted that the initial

Figure 5. TGA reduction and oxidation data for cycle 6 at two temperatures, 800 and 900 °C. 6578

dx.doi.org/10.1021/ef301389h | Energy Fuels 2012, 26, 6576−6583

Energy & Fuels

Article

Figure 10. Effect of the temperature on outlet gas concentrations during a oxidation of reduced CuO/bentonite (i.e., Cu/bentonite) with air.

Figure 7. Effect of the reaction of bentonite with methane (20%) on the outlet gas compositions at 900 °C.

concentration rapidly reached a maximum and then started to decrease over time. The appearance of the CO2 concentration is indicative of carbon deposits in the preceding reduction phase. The area under the CO2 curve is proportional to the amount of carbon being deposited during the reduction cycle. During this initial period, there was no indication of the oxygen concentration at the outlet of the reactor, indicating the complete consumption of O2 in the reaction by reactions 5 and 6. The oxygen concentration increases rapidly during the period that the CO2 concentration is reducing, after which the rise gradually slows. Cho et al.10 mentioned two possible mechanisms of carbon formation during reduction of CuO: methane decomposition and the Boudouard reaction (redox of CO to CO2) given by Figure 8. Effect of temperatures on the outlet gas compositions for bentonite and 10% H2 reaction.

2CO ↔ CO2 + C

(8)

For the temperature range used during the present experiments, methane decomposition is favored over the Boudouard reaction. In other words, the Boudouard reaction is dominant at low temperatures and the methane decomposition dominates at high temperatures.11 Kinetically, methane decomposition is slow in the absence of a catalyst. However, metals, such as Cu, could act as a catalyst.12 Carbon deposition may have started when there was sufficient Cu present, i.e., toward the end of the reduction phase. These results imply that there is competition between the reaction of CH4 with CuO and cracking of CH4 in the presence of Cu metal acting as a catalyst for CH4 decomposition and, hence, carbon formation on the outer surface of the particles.12 The CH4 cracking at high temperatures in the presence of Cu metal could have led to high concentrations of H2 and H2O. Snoeck et al.13 studied carbon deposition on a nickel-based catalyst [for steam− methane reforming (SMR)], and they observed carbon filament formation. These filaments allow for access to the nickel particles and, thus, prevent the carrier from deactivating in a reduced environment. Carbon grows from nickel and develops on one side of the particle. This kind of morphology was named “moss-like” carbon by Matsukata et al.14 The carrier was black at the end of the experiments, and no sign of deactivation was observed. These filaments were easily burnt in air at high temperatures, however releasing carbon from the air reactor. The effect of the temperature on the outlet CO2 and O2 concentrations during oxidation of reduced CuO (using 100% CH4 concentration) is also presented in Figure 10 for temperatures ranging from 750 to 900 °C. Figure 10 shows

Figure 9. Effect of temperatures on the weight loss for the reaction of bentonite with methane (20%).

exposed to an inert gas at a temperature, basically showing a decomposition from Al2O34(SiO2) to Al2O34(SiO). Note that, in Figure 7, carbon is deposited on bentonite during the reduction cycle and is liberated in the form of CO2 during the oxidation cycle. Figure 10 shows the outlet gas composition during oxidation to produce CuO using synthetic air at different temperatures for reduction under a 100% CH4 concentration. During the oxidation phase, after air was introduced into the reactor, a peak of CO 2 appeared in the outlet of the reactor. This 6579

dx.doi.org/10.1021/ef301389h | Energy Fuels 2012, 26, 6576−6583

Energy & Fuels

Article

that the CO2 peak decreases as the temperature was decreased, which is indicative that there is less carbon deposited at lower temperatures. This temperature effect is shown in Figure 11 for a CH4 concentration of 100%.

Figure 13. Variation of the Gibbs free energy for the oxidation reaction of Cu and oxygen with the temperature.

temperature ranges of 750−900 °C is illustrated in Figure 14. In Figure 14, the thick lines represent the experimental data, Figure 11. Apparent solid carbon on the reduced carrier from integrated CO2 versus time data.

The effect of the CH4 concentration in the reduction process on oxidation of reduced CuO is presented in Figure 12 for

Figure 14. Effect of the reaction temperature (thin line) and curve fitting of the JMA rate equation to oxidation conversion for the reduced CuO/bentonite particle with air reaction.

while the thin lines represent model results. The fractional conversion of Cu with air, X, is defined as Figure 12. Effect of the CH4 concentration during reduction of CuO/ bentonite on the outlet gas composition of oxidation of reduced CuO/ bentonite (i.e., Cu/bentonite) with air for 850 °C.

X=

m (t ) − m r mo − mr

(9)

where m(t) is the instantaneous weight of the solid during the exposure to air. Parameters mo and mr are the fully oxidized and reduced weight of the sorbent, respectively. For conversion, X, there is a delay period, followed by a steady increase in Cu conversion. This delay period is related to the temperature because higher temperatures have longer delay periods. The fractional conversion and outlet gas analysis of CO2 are also illustrated in Figure 15 for reaction temperature ranges of 750−900 °C. Figure 15, again, shows that the delay period increases as the temperature increases. It is significant that the mass spectrometry detection of CO2 was observed immediately after air introduction, indicating that the delay period could depend upon the extent of combustion of deposited carbon with oxygen. A small peak of CO2 at the beginning of the oxidation phase was observed at lower temperatures, indicating a small amount of carbon deposition in the bed during the preceding reduction phase. Both Figures 14 and 15 show that the rate of the reaction decreased as the temperature increased. Note again that the area under the CO2 data curves is proportional to the amount of carbon that deposited onto the carrier during the reduction step and that the higher temperature reduction process yielded significantly more

reduction with 100 and 50% CH4. This figure also shows that the CO2 peak has a longer residence time for a higher CH4 concentration, which results in a higher carbon deposited on the sorbent. That indicates that the area under the curves is proportional to the total carbon deposited on the carrier during the reduction step. This is completely consistent with the data presented in Figure 11, where at 850 °C, the mass spectral signal corresponding to carbon deposition was higher when reduction was done at 100% CH4 than that with 50%. To determine the causes of the results of carbon deposition on the reactivity of the sorbent during oxidation of reduced CuO, a more detailed investigation of the fractional conversion was conducted, the oxidation of reduced CuO by oxygen, given by eq 8. Figure 13 presents the Gibbs free energy of formation as a function of the temperature for reaction 6, as well as for other possible oxidation paths. This figure also shows that all of the reactions are more favored at lower temperatures because of more negative values of Gibbs free energy. The fractional conversion for the oxidation of reduced CuO as a function of time for 200 μm particles and reaction 6580

dx.doi.org/10.1021/ef301389h | Energy Fuels 2012, 26, 6576−6583

Energy & Fuels

Article

Figure 15. Effect of the reaction temperature on oxidation conversion and outlet CO2 concentration for the reduced CuO/bentonite particle with air reaction. Figure 17. Curve fitting of experimental oxidation data using first- and second-order reaction rates, series reaction, and JMA rates for 100% methane reduction and 850 °C.

deposited carbon than the low temperature processes. Carbon formation may have blocked the reaction between oxygen and Cu in the inner core of the particles. That could be one of the reasons why the rate of the reaction decreased as the temperature increased, giving the apparent negative activation energy. It should also be noted that, during oxidation at 900 °C, the mass gain increases normally at the beginning time; however, the increase changed its slope at about 20 min, indicating that there is a phase change (Figure 14). As stated earlier, the reaction atmosphere in the present work was 1 atm pressure air. It is important to know the oxygen concentration at the Cu2O− CuO interface associated with the equilibrium oxygen pressure over Cu2O to CuO.15 The equilibrium oxygen pressure over Cu2O and CuO from 600 to 1050 °C was calculated using HSC Chemistry software and plotted in Figure 16. It can be seen that

good agreement of the JMA model with the experimental data in Figure 14 for the reaction of O2 with Cu at different temperatures (750−900 °C). The JMA model of nucleation was fitted to the measurement of oxidation of Cu, using the form n

X = 1 − e−(kt )

(10)

where X is the oxidation conversion at time t, n is the kinetic exponent, which depends upon the mechanism of growth and the dimensionality of the nuclei, and k (min−1) is the overall nucleation rate constant. The temperature dependence is generally expressed by the Arrhenius equation k(T ) = A e−E / RT

(11)

where A is the frequency factor, T is the absolute temperature, and E is the activation energy for the transformation process, with R being the gas constant. For a given temperature, values of n and k were determined by curve fitting the rate data of Figure 14 with the parameters in eq 10 using TABLECURVE available from SPSS. The comparison of the experimental Cu conversion, X data to the JMA model as presented in eq 10, is illustrated in Figure 14 for different temperatures. An agreement between the model data and experimental data is excellent over the entire conversion time, with overall variance (R2) greater than 99%. It is also important to note that JMA kinetics are global or overall kinetics for nucleation and growth. This means that we cannot, in principle, obtain any information about individual nucleation and growth processes from JMA parameters alone.19 Even though the observed rate decreases with an increasing temperature, contrary to expectations and indicative that particle morphology is likely not constant because of possible carbon blocking, as noted above, the effect of the temperature is being analyzed in a typical manner. Equation 11 can be rewritten as

Figure 16. Equilibrium oxygen pressure over Cu2O and CuO.

the thermodynamic driving force for Cu2O oxidation is very large (dependent upon the partial pressure of oxygen) at lower temperatures. However, at a higher temperature around 900 °C, the driving force becomes quite small. Therefore, the phase change is due to where Cu2O−CuO equilibrium is occurring. Attempts were made to fit the oxidation of reduced CuO (i.e., Cu) data over the complete conversion range with suitable rate expressions derived from existing models for oxidation, including the shrinking core model (diffusion and reaction control), first- and second-order reaction rates, parallel and series reaction mechanisms, and Johnson−Mehl−Avrami (JMA) rate (Figure 17). None of the models tested gave satisfactory results over the range of temperatures from 750 to 900 °C, with the exception of the JMA equation.16−18 Note the

ln k = ln A −

E RT

(12)

According to eq 12, the plot of ln k versus 1/T should result in a straight line with a slope of −E/R and an intercept of ln A. A plot of ln(k) versus 1/T for oxidation of Cu/bentonite with a particle size of 200 μm is shown in Figure 18 at different 6581

dx.doi.org/10.1021/ef301389h | Energy Fuels 2012, 26, 6576−6583

Energy & Fuels

Article

Figure 20. Effect of the CH4 concentration on n values of the oxidation rate.

Figure 18. Temperature and CH4 concentration dependence of the oxidation reaction rate.

reaction temperatures and at various inlet CH4 concentrations (20, 50, and 100%) during the reduction process. The preexponential factor, A, and activation energy, E, were obtained from the intercept and slope of straight line in Figure 18. The value of the pre-exponential factor, A, decreased with an increasing CH4 concentration of reduction (Figure 19). The

Figure 21. Effect of the temperature on the oxidation rate using 100% methane in reduction.

Figure 22 for reaction temperatures between 750 and 900 °C. Thus, the results from Figures 21 and 22 designated the sigmoid group of kinetic models (this group has dX/dtmax at t > 0). Figure 19. Effect of the CH4 concentration on A and E/R values of the oxidation rate.

temperature dependence, apparent E, is a linear function of the CH4 concentration of reduction. The activation energy decreased as the CH4 concentration decreased (Figure 19). It should also be noted that, because the rate of reaction for oxidation of Cu/bentonite decreased as the temperature increased, it resulted in negative apparent activation energies. Reactions with negative activation energies, although rare, have been noted for the oxidation of Cu2O to CuO.14 Zhu et al.14 reported a negative activation energy of −45 kJ/mol in the temperature range of 950−1100 °C. They proposed an explanation in terms of the presence of a thin CuO layer over Cu2O. The value of n in eq 10 is obtained to be a function of the methane concentration, as shown in Figure 20. The data points at each concentration are the average values from the 6th to 10th cycle values for n for each concentration averaged over the temperature range. A linear fit to the functionality gave a R2 of greater than 92%. The calculated rate−time (dX/dt versus t) data obtained at different temperatures (Figure 21) show the maximum rate of process obtained at t > 0. Figure 21 also shows that the maximum rate decreased with the increasing temperature. The rate of reaction as a function of Cu conversion is presented in

Figure 22. Effect of the temperature on the reaction rate as a function of fractional conversion using 100% methane in reduction.

■

CONCLUSION Cu-based oxygen carriers for CLC were investigated in a TGA for the cycles of reduction and oxidation with methane and air, respectively, at different temperatures (750−900 °C) and methane concentrations (20−100%). In-line mass spectroscopy was also used to analyze the outlet gas during reduction with methane (i.e., CH4, CO2, H2, H2O, and CO). The composition 6582

dx.doi.org/10.1021/ef301389h | Energy Fuels 2012, 26, 6576−6583

Energy & Fuels

Article

(8) García-Labiano, F.; de Diego, L. F.; Adánez, J.; Abad, A.; Gayán, P. Ind. Eng. Chem. Res. 2004, 43, 8168−8177. (9) Monazam, E. R.; Siriwardane, R.; Breault, R. W.; Tian, H.; Shadle, L. J.; Richards, G.; Carpenter, S. Energy Fuels 2012, 26, 2779−2785. (10) Cho, P.; Mattisson, T.; Lyngfelt, A. Ind. Eng. Chem. Res. 2005, 44 (4), 668−676. (11) Linderholm, C.; Jerndal, E.; Mattisson, T.; Lyngfelt, A. Chem. Eng. Res. Des. 2010, 88, 661−672. (12) Chandel, M. K.; Hoteit, A.; Delebarre, A. Fuel 2009, 88, 898− 908. (13) Snoeck, J. W.; Froment, G. F.; Fowles, M. J. Catal. 1997, 169, 240−249. (14) Matsukata, M.; Matsushita, T.; Ueyama, K. Chem. Eng. Sci. 1996, 51, 2769−2774. (15) Zhu, Y.; Mimura, K.; Isshiki, M. Oxid. Met. 2004, 62, 207−222. (16) Avrami, M. J. Chem. Phys. 1939, 7, 1103−1112. (17) Avrami, M. J. Chem. Phys. 1940, 8, 212−224. (18) Avrami, M. J. Chem. Phys. 1941, 9, 177−184. (19) Ruitenberg, G.; Woldt, E.; Petfor-Long, A. K. Thermochim. Acta 2001, 378, 97−101.

of the outlet gas during reoxidation was also measured (i.e., CO2, O2, CO, and N2). The kinetics of reoxidation of Cu/bentonite by air was slower at higher temperatures in the temperature range of 750− 900 °C, resulting in negative activation energy. At these temperatures, the reaction between air and Cu followed the transformation kinetics described by the JMA model. The reactivity of the oxygen carrier during the oxidation reactions was affected by the CH4 concentration of the reduction process as well as the temperature. The apparent activation energy of the reoxidation process was linearly proportional to the CH4 concentration of the reduction process; a higher CH 4 concentration resulted in higher activation energy. Analysis of exit gas provided evidence of carbon deposition on the reduced sorbent particle and resulted in a CO2 product upon oxidation. The calculated rate−time (dX/dt versus t) data obtained at different temperatures show that the maximum rate decreased with the increasing temperature.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: 304-285-4486. Fax: 304-285-4403. E-mail: ronald. [email protected]. Notes

Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the Department of Energy for funding the research through the Fossil Energy’s Gasification Technology and Advanced Research funding programs.

■

REFERENCES

(1) Mattisson, T.; Järdnäs, A.; Lyngfelt, A. Energy Fuels 2003, 17 (3), 643−651. (2) Mattisson, T.; Johansson, M.; Lyngfelt, A. Energy Fuels 2004, 18 (3), 628−837. (3) Corbella, B. M.; de Diego, L. F.; Garcia-Labiano, F.; Adanez, J.; Palacios, J. M. Energy Fuels 2005, 19 (2), 433−441. (4) Corbella, B. M.; de Diego, L. F.; Gracía-Labiano, F.; Adanez, J.; Palacios, J. M. Energy Fuels 2006, 20 (1), 148−154. (5) Ritcher, H. J.; Knoche, K. F. Reversibility of combustion process. In Efficiency and Costing, Second Law Analysis of Process; Gaggioli, R. A., Ed.; American Chemical Society (ACS): Washington, D.C., 1983; ACS Symposium Series, Vol. 235, Chapter 3, pp 71−85. (6) Anheden, M.; Svedberg, G. Energy Convers. Manage. 1998, 39 (16−18), 1967−1980. (7) Hossain, M. M.; de Lasa, I. H. Chem. Eng. Sci. 2008, 63, 4433− 4451. 6583

dx.doi.org/10.1021/ef301389h | Energy Fuels 2012, 26, 6576−6583