Bentonite by Methane (CH4) during

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Kinetics of the Reduction of CuO/Bentonite by Methane (CH4) during Chemical Looping Combustion Esmail R. Monazam,‡,§ Ranjani Siriwardane,† Ronald W. Breault,*,† Hanjing Tian,§ Lawrence J. Shadle,† George Richards,† and Stephen Carpenter§ †

National Energy Technology Laboratory, U.S. 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 & Construction, Inc., 3610 Collins Ferry Road, Morgantown, West Virginia 26505, United States ABSTRACT: Chemical looping combustion (CLC) is a process that uses an oxygen-carrier metal, instead of air or pure oxygen, to provide oxygen for combustion. The products of CLC of methane are CO2 and H2O. After condensation of H2O, a concentrated CO2 gas stream is produced and ready for sequestration. An important issue for the CLC process is the selection of metal oxide as an oxygen carrier, because it must retain its reactivity through many cycles. In this study, isothermal thermogravimetric analysis is used to evaluate the rates of reduction of CuO impregnated in bentonite with methane (CH4) over the range 1023−1173 K for 20%, 50%, and 100% CH4 over 10 reduction cycles. The mechanism and reactivity of the CuO oxygen carrier were evaluated by 10 different rate models. The results indicate that the transformation kinetics described by the Johnson−Mehl−Avrami (JMA) model was the best fit. The Avrami exponent n ranges from 1.55 to 2.16. The average value of 1.77 indicates that the crystallization mechanism is mainly two-dimensional diffusion-controlled. The activation energy was estimated to be 37.3 ± 1.3 kJ/mol. No deactivation was observed over 10 cycles at any CH4 concentration. In the first 10 reaction cycles, the reaction rates increased slightly with the increasing number of cycles. Moreover, the rate−time and rate− conversion curves for all the temperatures show that the maximum rate occurred at t > 0. This was confirmed by the outlet gas measurements. The experimental results suggested that the CuO/bentonite oxygen carrier is a promising candidate for the CLC system burning methane.

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INTRODUCTION Carbon dioxide from fossil fuel burning is a major contributor to greenhouse gases. Approximately 83% of the greenhouse gas emissions in the U.S. are produced from the combustion and other uses of fossil fuels.1 The U.S. Department of Energy (DOE) estimates that the consumption of fossil fuels (coal, petroleum, and natural gas) will increase by 27% over the next 20 years, thereby increasing U.S. CO2 emissions from the current 6000 million tons per year to 8000 million tons per year by 2030.2 To decrease the emissions of CO2 from fossil fuelled power plants, it is necessary to separate the CO2 from the other exhaust gases and conduct sequestration. 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.3−6 The CLC concept was first introduced in 1983 by Richeter and Knoche,7 who suggested oxidizing the fuel by an oxygen carrier, i.e. an oxygen-containing compound, in a flameless combustion process instead of oxidizing the fuel with oxygen from the combustion air. The reduced oxygen carrier is then reoxidized by air in a second reactor and recirculated to the first reactor. In this way, fuel and air are never mixed and the fuel oxidation products CO 2 and water leave the system undiluted by excess air. Pure CO2 is produced after condensation of water vapor and removal of the liquid water.8 Thus, the CLC process does not require an expensive CO2 separation process. Different metal oxides have been proposed as possible candidate for the CLC process;7 The four most studied © 2012 American Chemical Society

supported oxygen carriers in descending order by reactivity are NiO > CuO > Mn2O3 > Fe2O3. In general, these metal oxides are combined with an inert material, which acts as a porous support providing higher surface area for the reaction and also acts as a binder to provide higher mechanical strength and attrition resistance for cyclic use. The development of oxygen carrier particles has been investigated by several research groups such as the Korea Institute of Energy Research,9,10 Tokyo Institute of Technology,11−16 TDA Inc.,17,18 and U.S. Department of Energy (DOE).18 Ryu et al.9 at Korea Institute of Energy Research investigated NiO on bentonite support as an oxygen carrier with methane as the fuel in a thermogravimetric analyzer (TGA), showing its promising activity and stability under repeated redox cycles. Ishida and co-workers at Tokyo Institute of Technology have investigated oxides of Fe, Ni, and Co with Al2O3 as a support with H2 as the primary fuel11−15 using thermogravimetric analysis (TGA) in a fixed-bed reactor.16 Copeland et al.17,18 at TDA Research Inc. investigated copper-, iron-, and nickel-based oxygen carriers with H2 and syngas as fuel. The copper containing oxygen carriers demonstrated excellent chemical stability. Researchers at the U.S. Department of Energy19 studied the CuO/bentonite and CuO-BHA nanocomposites as oxygen carriers in CLC of simulated synthesis gas. Overall, Both types Received: January 13, 2012 Revised: March 8, 2012 Published: March 8, 2012 2779

dx.doi.org/10.1021/ef300072d | Energy Fuels 2012, 26, 2779−2785

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The mesh sizes used in the present study were 60−100 mesh (250− 150 μm). The sample was oxidized again prior to the test. TGA Testing. To examine the fundamental kinetics of the CuO conversions and the rate of CH4 uptake information using CuO/ bentonite sorbent, experiments were carried out using a TA Model 2050 thermogravimetric analyzer. For a typical test, about 20 mg of the CuO/bentonite sample was heated in a quartz bowl to the reaction temperature. The reduction−oxidation cycle was conducted within the temperature range 750−900 °C for 10 cycles, using 20%, 50%, and 100% CH4 concentrations in N2 for the reduction segment and in dry air for the oxidation segment. Reaction gas flow rates were set at 45 sccm, reduction reaction times were set at 10 min, and oxidation reaction times were 60 min for all experiments. Note, that prior to running the experiments to determine the kinetics, scanning tests were run to ensure that the gas flow rate of 45 sccm was sufficient to eliminate mass transfer effects. To avoid the mixing of reduction gas mixture and air, the system was flushed with nitrogen for 5 min before and after each reaction phase. The concentrations of O2, CO2, CH4, and H2O from the exit gas stream of the reactor were analyzed using a mass spectrometer. A typical experimental observation for temperature and changes in weight during TGA experiments is illustrated by Figure 1.

of CuO carriers exhibited excellent reaction performance and thermal stability for the CLC process at 700−900 °C. The CLC process could also be achieved through a modification known as chemical looping with oxygen uncoupling (CLOU) and was first proposed by Mattison et al.20,21 CLOU provides a novel method of the avoiding the gasification step in the fuel oxidation process when using solid fuels like coal, hence increasing the reaction rates dramatically. The advantage of CLOU over regular CLC is the elimination of the slow gasification step when employing solid fuels, so less oxygen, carrier material is needed in the system, which also reduce the reactor size and associated costs.22,23 Among the metal oxides suited as oxygen carriers, Cu-based oxygen carriers have several advantages: CuO is not a very expensive material and has very high oxygen transport capacity, and CuO reduction is favored thermodynamically to reach complete conversion of gaseous hydrocarbon fuels into CO2 and H2O.24 In addition, both reduction and oxidation reactions are exothermic, avoiding the need for a heat supply in the reduction reactor.25 However, few studies in the literature were related to the determination of kinetic parameters of CuO based oxygen carriers. The knowledge of the kinetics data is of great importance in the design of a CLC process, because it determines the solid inventory necessary in the fuel and air reactors, as well as the recirculation rate of oxygen carriers bet́ ween the reactors.26 Garcia-Labiano et al.24 used shrinking coreplate-like geometry to interpret their TGA data for the reduction and oxidation of Cu-based oxygen carriers, using CH4 and air. They concluded that the activation energy for the reduction reactions was 60 kJ/mole for the reduction reactions. Adánez et al.27 used a grain model to explain the reactivity of the CuO/ Al2O3 oxygen carrier. The gases used were CH4 for reduction and air for oxidation. The reducing gas was saturated in water to avoid carbon formation. The reduction reaction was control by chemical reaction resistance, and the oxidation reaction was controlled by the intermediate regime between chemical reaction control and internal diffusion control. The primary difference of this work from the works discussed above24−27 is that the current work investigates the reaction of CH4 with CuO at lower temperature where the addition of steam is not needed to control carbon deposition. Thus, the kinetics presented here are not confounded with the parallel reaction of steam reformation of methane and steam gasification of the deposited carbon. At these lower temperatures, steam, whether fed or produced, is nothing but a diluent and will not contribute to the steam reforming process, which is typically catalyzed by nickel oxide.28 The addition of steam will decrease the reactivity because it is a product formed during the reaction, so the effect of addition of new steam to the inlet gas will be different from the effect steam produced during reaction. The objective of this paper is to determine the kinetics parameters of the reduction reactions of Cu-based oxygen carriers, using CH4. The effects of temperature and methane concentration were evaluated.

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Figure 1. Typical mass and temperature measurement for CuO/ bentonite particle of 200 μm and 100% CH4 for reduction and air for oxidation reactions.

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RESULTS AND DISCUSSION The reduction of CuO by CH4 is given as 4CuO + CH 4 → 4Cu + CO2 + 2H2O [ΔG = −590 kJ/mol, ΔH = − 217 kJ/mol]

(1)

The fractional conversion and outlet gas analysis of CH4 and CO2 for the reduction of copper oxide (CuO) with methane (100% CH4) using a mean particle size of 200 μm and reaction temperature of 900 °C is illustrated in Figure 2. Data in Figure 2 also shows that the CH4 concentration at the outlet increases rapidly and then gradually slows down. The CO2 concentration peak that appears when CH4 concentration increases is due to the reaction rate at its maximum. The fraction of CuO conversion, X, is defined as

EXPERIMENT

Materials and Preparation. A 60% CuO/bentonite oxygen carrier was prepared by the mechanical mixing method. Pure Cu2O (Aldrich, >99.95%) and bentonite (Fisher, laboratory grade) were mixed thoroughly with deionized water added to the powder mixture to obtain a paste. The paste was dried at 105 °C for 24 h. The dry material was then calcined at 900 °C in air for 6 h. The calcined sample was crushed into smaller particles of the desired mesh size.

X=1−

m (t ) − m r mo − mr

(2)

where m(t) is the instantaneous weight of the solid during the exposure to CH4. Parameters mo and mr are initial and final weight of the sorbent, respectively. For conversion, X, there was a slight delay period, followed by a steady increase in CuO 2780


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sigmoid shape if the reaction proceeds via the nucleation model. Attempts were made to fit the reduction of CuO data over the complete conversion range with suitable rate expressions derived from existing models for reduction, including the shrinking core model (diffusion and reaction control), linear, parabolic, and cubic rate laws; first- and second-order reaction rates; parallel and series reaction mechanisms; and Johnson− Mehl−Avrami (JMA) rate. These 10 different rate equations were examined to fit the TGA experimental data for the CuObentonite/methane reduction reaction. 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.31−33 For example, a parallel mechanism was tested for CuO and Cu2O going to Cu, and the results were not satisfactory compared with the nucleation model analysis, as presented in the manuscript, nor was it even as good as the series model of CuO going to Cu2O going to Cu, which was rejected as the kinetics because step 1 and step 2 were equal within 0.5%indicating a one step process. Note the good agreement of the JMA model with the experimental data in Figure 3 for the reaction of CH4 with CuO at 850 °C.

Figure 2. Conversion, X, and outlet gas analysis of CH4 and CO2 for the reduction of copper oxide (CuO) with methane (100% CH4) using a mean particle size of 200 μm and reaction temperature of 900 °C.

conversion. It is significant that the mass spectrometry detection of CH4, and CO2 were observed simultaneously with CuO reduction, indicating of fast reaction. The observation of carbon dioxide (CO2) indicates that the reactive gas (CH4) initiates copper oxide reduction. Hossain and de Lasa29 reviewed the kinetics for the reduction of a metal−supported oxygen carrier and notice that mainly two general types of models have been considered: (i) an unreacted shrinking core model and (ii) a nucleation and nuclei growth model. According to the unreacted shrinking core model, the reacting surface front separates the solid reactant core from the outer (product) layer. The thickness of the product layer increases with time, leading to the shrinking unreacted solid core. Therefore, in addition to the adsorption and desorption of the gaseous products, the heterogeneous reaction proceeds via three steps: (i) external mass transfer, (ii) internal mass transfer, and (iii) chemical reaction. According to the nucleation and nuclei growth model,30 the gas−solid reactions proceed by nucleation (nuclei formation) and subsequent nuclei growth. Before nucleation, there is a delay or induction period for the activation of the solid phase to form nuclei. The length of the induction period primarily depends on the gas−solid system and reaction temperature. Nucleation is a dynamic process, which is assumed to occur in a manner analogous to droplet formation from a supersaturated vapor mixture. That is, CH4 diffuses into the particle at a location where, at some point and time, the local concentration is greater than that which equilibrium can support, and a “droplet” of Cu is formed from the CuO, with gaseous species H2O and CO2 being released to bring the local site back into equilibrium state . The progress of the reaction then continues with nucleation and growth of the already formed nuclei. The nuclei growth occurs as a result of the overlap of the nuclei and/or ingestion of a nuclei site. The overall conversion of the reaction is determined by the relative rate of nucleation, nuclei growth, and the concentration of the potential nucleus-forming sites (also called germ nuclei). For a particular gas−solid reaction, the nuclei growth rate is constant at a given temperature as well as at a given composition of the gas phase. Either nucleation, nuclei growth, or their combination can be the ratedetermining step of the overall reaction, and the energetics of the reaction process is primarily dependent upon the rate-determining step. Generally, the experimental X−t (conversion−time) profiles of a gas−solid reaction show a

Figure 3. Curve fitting of experimental reduction data using shrinking core model (diffusion and reaction control) and JMA rates for 100% methane and 850 °C.

The JMA’s model of nucleation was fitted to the measurement of reduction of CuO, using the form X = 1 − exp−(kt )n

(3)

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

(4)

where A is the frequency factor, T is the absolute temperature, and E is activation energy for the transformation process with R the gas constant. Note that eq 2 describes the isothermal processes, so k(T) is a constant at a given temperature. For a given temperature, values of n and k were determined by curve fitting the rate data of Figure 2 with the parameters in eq 2 using 2781


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TABLECURVE available from Statistical Package for the Social Sciences. The comparison of the experimental CuO conversion, X, data to the JMA model as presented in eq 3 is illustrated in Figure 4 for different temperatures. The model data and

Figure 6. Effect of cycles on k of JMA rate (CuO/CH4).

2.5 indicates the growth of nuclei occurs with decreasing nucleation rate, n = 2.5 reflects growth of nuclei with constant nucleation rate, and n > 2.5 corresponds to the growth of small nuclei with an increasing nucleation rate. Therefore, CuObentoine/methane reduction with an n value of 1.9 should have growth in the number of nuclei with a decreasing nucleation rate. The value of n is shown to be a function of the methane concentration, as shown in Figure 7. The data points at each

Figure 4. Effect of reaction temperature and Curve fitting of JMA rate equation to reduction conversion for CuO/bentonite particle and CH4 reaction.

experimental data agree over the entire conversion time with overall variance (R2) greater than 99%. Also 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.34 The data in Figure 4 also shows that the rate of reduction of CuO increases as the reaction temperature is increased. Figures 5 and 6 illustrate the variation of n and k with increases in the cycles at different temperatures for the reaction

Figure 7. Effect of CH4 concentration on n values.

concentration are the average values of the sixth through 10th cycle values for n for each concentration averaged over the temperature range. A linear fit to the functionality gave an R2 of 0.8. Equation 4 can be rewritten as ln k = ln A − Figure 5. Effect of cycles on n of JMA rate (CuO/CH4).

E RT

(5)

According to eq 5, the plot of ln k vs 1/T should result in a straight line with a slope of −E/R and intercept of ln A. A plot of ln k vs 1/T for reduction of CuO/bentonite with particle size of 200 μm is shown in Figure 8 at different reaction temperatures and at various inlet CH4 concentrations (20, 50, and 100%). The pre-exponential factor, A, and activation energy, E, were obtained from the intercept and slope of the straight line in Figure 8. The value of A increased with increasing CH4 inlet

with 100% CH4. There is a slight increase (∼10%) in n with increasing cycles of up to 10 at 750, 800, and 850 °C (n = 1.7− 1.93) and about 30% increase at T = 900 °C (1.6−2.1). The average of n is about 1.9 at all temperatures. According to the diffusion-controlled growth theory,35 n = 1.5 means that growth of nuclei occurs with a nucleation rate close to zero; 1.5 < n < 2782


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The rate−time curves for all the temperatures show that the maximum rate of process is obtained at times greater than 0. This is in direct contrast to a kinetic controlled reaction in which the maximum rate occurs at time equal to 0, or in other words, n equal to 1 in the JMA equation. Data in Figure 9 show that the value of maximum rate increased with the increasing temperature. The rate of reaction as a function of conversion is also presented in Figure 10 at different temperatures (750−900 °C). The

Figure 8. Temperature and CH4 concentration dependence of the reaction rate, k (s−1).

concentration. The fact that A varies or is a function of the CH4 concentration is proof that the reaction proceeds initially with a local build up in the concentration of CH4. The temperature dependence, apparent E, was fairly constant for different CH4 concentration, as evidenced by the parallel lines, of ln k vs 1/T. Combining all the values of k for different CH4 concentrations, the following equation is obtained; 0.654 exp( − 4482.34/T ) k (min−1) = 91yCH 4

Figure 10. Effect of temperature on reaction rate as a function of frational conversion using 100% methane.

rate−conversion curves for all the temperatures also show that the maximum rate of process is obtained at times greater than 0. Figure 11 shows the experimental conversion and rate versus time for the isothermal operating temperature of 900 °C, along

(6)

where yCH4 is the mole fraction of methane. The Avrami exponent n ranges from 1.55 to 2.16. The average value, around 1.77 ± 0.13, indicates that the crystallization mechanism is mainly two-dimensional diffusion-controlled.35 An expression for the reaction rate, dX/dt; can be derived by differentiating eq 3 with respect to t, at constant temperature, as shown in eq 7: as follows: dX = kn(1 − X )[− ln(1 − X )]1 − 1/ n dt

(7)

The rate can be computed as a function of reduction conversions of the CuO at a given temperature. The experimental rate−time (dX/dt vs t) data obtained at different temperatures (750−900 °C) are shown in Figure 9.

Figure 11. Similarity in the shape of the CO2 curve with time as compared to the conversion rate data (numerical derivative) of the TGA reduction data.

with the CO2 concentration, as measured with a mass spectrometer at the exit of the reactor (TGA). Note the similarity in the shape of the CO2 curve with time as compared to the conversion rate data (numerical derivative) of the TGA reduction data. This confirms the appropriateness of the JMA model. Thus, the results from Figures 4, 9, and 10 designated the sigmoid group of kinetic models (this group has dX/dtmax at t > 0). The data suggest that the reduction process can be explained by the mechanism of nucleation and growth of copper. A plot of ln(dX/dt) as a function of 1/T for different conversions with 100% CH4 inlet concentration is shown in Figure 12.

Figure 9. Effect of reaction temperature on the rate reduction of CuO/bentonite particle and CH4 reaction. 2783


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The activation energies and Arrhenius constant, A, were also determined by the values obtained from the slope and intercept of the straight line of Figure 12. The similarities of these values also suggest that particles are completely penetrated by the reactive gas without any diffusion resistance; resulting in a uniform reactive gas concentration inside the particle.

CONCLUSIONS

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AUTHOR INFORMATION

Chemical looping combustion is a promising combustion technique for the separation of CO2. Particles of CuO were prepared on a bentonite support by impregnation. The rates of reduction with methane were investigated in a cyclic manner, using TGA. The kinetics of reduction of coprecipitated mixtures of copper oxides and bentonite by methane (CH4) was faster in the temperature range of 750−900 °C. At these temperatures, the reaction between CH4 and CuO followed the transformation kinetics described by the Johnson−Mehl− Avrami (JMA) model. According to the JMA model, The results indicated that reduction of CuO using methane follows a series of steps: (i) an induction period that could be associated with the initial reduction of CuO and the appearance of Cu metal nuclei or clusters; (ii) acceleration of the reduction rate as the size of the nuclei growth; and (iii) the reaction process in which CuO disappeared and Cu appears until reduction slowed at a fractional conversion of about 0.8. The rate of reduction of the oxygen carriers during the 10-cycle test slightly increased with the number of cycles.

Corresponding Author

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

The authors declare no competing financial interest.

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REFERENCES

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Figure 12. Arrhenius plot for different conversion.

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ACKNOWLEDGMENTS

The authors acknowledge the Department of Energy for funding the research through the office of Fossil Energy’s Gasification Technology and Advanced Research funding programs. 2784


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Bentonite by Methane (CH4) during

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