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Reaction Kinetics of Mixed CuO−Fe2O3 with Methane as Oxygen Carriers for Chemical Looping Combustion Esmail R. Monazam,†,‡ Ronald W. Breault,*,† Hanjing Tian,§ and Ranjani Siriwardane† †

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 § Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506, United States ABSTRACT: Reduction kinetics of alumina supported mixed Cu−ferrite oxide by methane was investigated for chemical looping combustion by the thermogravimetric analyzer (TGA) in the temperature range of 750−900 °C using continuous streams of 10%, 20%, and 30% CH4 concentrations balanced by helium. The rate of reduction was determined by weight change. The variations of activation energies and n values (JMA exponent) during the reduction conversion indicate that the methane combustion with CuO and Fe2O3 proceeds via a multistage reaction process. A kinetic model based on three parallel reactions was applied to the reduction data. The analysis of reduction showed that three reduction steps proceed simultaneously with the activation energies of 55.32 ± 3.11, 70.0 ± 2.0, and 165.0 ± 5.4 kJ/mol, respectively.

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INTRODUCTION Fossil fuel utilization to generate electricity produces approximately one-third of CO2 emissions to the atmosphere which may be responsible for global climate change.1 The chemical looping combustion (CLC) technology has been proposed as a new combustion process to reduce CO2 emission and improve fuel combustion efficiency with a low cost as compared to postcombustion CO2 capture technologies.2 Chemical looping combustion is one of promising technology that consists of separate fuel and air reactors, thereby avoiding direct contact between fuel and air. In the fuel reactor, the metal oxide, named as solid oxygen carrier, is reduced by the fuel that is converted to highly concentrated CO2 gas. In the air reactor; the reduced metal oxide is reoxidized to its original form by the O2 in the air.3 Selection of a metal oxide with low cost, high reactivity, and high durability and that is environmentally friendly is a key step in the development of CLC technology. A desirable property for solid oxygen carriers is that they should be capable of converting fuel to CO2 and H2O by >99% in addition to high and fast reactivity.4 Different metal oxides systems such as NiO, CuO, Mn2O3, and Fe2O3 supported on different inert materials, such as Al2O3, SiO2, bentonite, and TiO2 have been studied to be used in a CLC process. Cu-based oxygen carriers have been extensively studied due to their high reactivity and good catalytic performance for many reactions with fuel such as CH4 and H2 for steam re-forming and water gas shift reaction.5,6 However, the difficulty of homogeneous dispersion of Cu particles on supports, degradation of the composite catalyst due to the sintering of copper and carbon deposition, and poor thermal stability have been the major drawbacks for deployment of CuO oxide in chemical looping process.7,8 Different researchers have shown that iron-based oxygen carriers possess fair oxygen carrier capacity and have enough reactivity at atmospheric9,10 and pressurized conditions,11 high melting temperature and © 2015 American Chemical Society

mechanical strength, low cost and environmental impact. On the other hand, iron-based oxygen carriers such as Fe2O3 possess relatively less reactivity with fuel such as methane and complicated gas−solid reaction due to structural changes involved.12,13 Recently, mixed metal oxide such as ferrites with the formula MFe2O4 (M = Cu, Ni, Co, and Cr) belonging to the spinel structural type with high thermodynamic stability has attracted global attention for use as precursors and model catalysts for a variety of catalytic reactions.14 Spinels represent a large class of inorganic materials that possess many important properties not found in the more limited binary oxide mixtures.15 The main advantage of the spinel metal oxides such as CuFe2O4 is their high catalytic performance during oxygen reduction (fuel combustion) and the subsequent immiscible interaction between Cu and Fe in this example. The high catalytic performance is due to the formation of composite structures induced by reductive decomposition of the CuFe2O4 where Cu particles homogeneously dispersed within the porous Fe3O4.16 Also, reaction of CuO with methane is exothermic that releases heat during its reduction as opposed to Fe2O3 which is endothermic during its reduction with methane. Therefore, the combined use of CuO and Fe2O3 can limit the temperature drop in the fuel rector. Synergetic effects due to the presence of CuO with Fe2O3 in CLC have also been reported.17,18 Given the large natural gas (methane) boom in the U.S. and elsewhere with the introduction of hydrofracturing technology to shale formations, there is a significant quantity of low priced natural gas. Even though natural gas has a relatively low carbon emission as compared to coal, it makes sense to look at low Received: Revised: Accepted: Published: 11966

August 3, 2015 October 29, 2015 November 16, 2015 November 16, 2015 DOI: 10.1021/acs.iecr.5b02848 Ind. Eng. Chem. Res. 2015, 54, 11966−11974

Article

Industrial & Engineering Chemistry Research carbon emissions for utilizing this new resource. Therefore, the purpose of this study is to investigate the reduction kinetics and reaction mechanisms of the spinel mixed metal oxide, CuFe2O4, with methane.

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EXPERIMENTAL SECTION The copper/iron/alumina oxygen carrier was prepared at Nextech. The CuO (99%; 5 μm) and Fe2O3 (99%; 5 μm) powder was well mixed with alumina−sol (∼36% of Al dry base) to obtain a composition of 30 wt % CuO, 30 wt % Fe2O3, and 40 wt % alumina, resulting in 2 mol of CuO to 1 mol of Fe2O3. The mixed slurry was spary-dried to form spherical particles and then calcinated at 1100 °C in a muffle oven for 10 h. The particle size was 106−150 μm with an average size of 128 μm. The reductions of mixed CuO−iron oxide oxygen carriers were carried out using a thermogravimetric analyzer (TGA, TA model 2050) equipped with a mass spectrometer (MS, Pfeiffer Omnistar GSD-301). For a typical test, about 60 mg of mixed CuO−Fe2O3 sample was initially heated in a quartz pan at a ramp rate of 10 °C/min to the desired temperature under ultrahigh pure (UHP) helium. A gas flow rate of 45 sccm was selected to eliminate the diffusion effect on the reaction to promote chemically controlled reactions. When the desired temperatures (750−900 °C) were reached, the helium with variation of methane (10%, 20%, and 30%) was injected into the TGA under isothermal conditions. The reduction− oxidation was conducted for 10 cycles with reduction time of 10 min and oxidation reaction time of 45 min for all experiments. The system was flushed with UHP helium for 10 min between each reaction segment. The purge gas (UHP He) enters the reaction chamber at the top and keeps the balance in an inert environment. Air flow used for the oxidation cycle was obtained from Butler Gas Products Co. Inc. The concentrations of H2, H2O, CO, CO2, and O2 from the exit gas stream of the reactor were analyzed using a mass spectrometer. Typical TGA experimental data showing weight changes during reduction/oxidation cyclic tests at a given temperature are illustrated in Figure 1. In general, the performance becomes stable after about the fifth cycle, and we selected the data after the fifth cycle for our analysis, averaging the data in cycles 5− 10.

Figure 2. TGA experimental weight loss profiles obtained from the thermal reduction of CuFe2O4 at different temperatures using 20% CH4.

Figure 3. Time variations of the evolved gas product during reduction of CuO + CuFe2O4 with 20% CH4 and 800 °C.

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RESULTS AND DISCUSSION Figure 2 illustrates the weight changes of mixed Cu/Fe2O3 at different temperatures (750−900 °C) during the reaction with

Figure 4. Activation energy values as a function of X.

20% methane in helium. The data indicate that there is rapid weight loss of the CuO/Fe2O3 for the first 0.5−2 min, and then reduction slows down. Then there was a continuous weight increase at higher temperatures (800−900 °C). The decrease in mass can be attributed to the reduction of Cu/Fe2O3 by CH4. The subsequent increase in mass at higher temperature is

Figure 1. Typical mass and temperature measurements for the CuO− Fe2O3 particles using 20% CH4 for reduction and air for oxidation reactions. 11967

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accompanied by a decrease in the water content and is likely due to the steam carbon reaction. Particle Concept and Kinetic Parameter Determination. Upon the basis of the molar ratio of CuO and Fe2O3, the particle consists of free copper in the form of CuO (copper oxide) and bound copper in the form of CuFe2O4 (copper− iron spinel).17It is expected that each of these particles species will react with the methane by means of separate and different mechanisms. The copper oxide is expected to be reduced according to the two-step process

Table 1. Theoretical Weight Changes (%) of the Possible Stoichiometric Decomposition of CuO + CuFe2O4 reaction 3CuO 3CuO 3CuO 3CuO

+ + + +

3CuFe2O4 3CuFe2O4 3CuFe2O4 3CuFe2O4

→ → → →

1.5Cu2O + 3CuFe2O4 3Cu + 3CuFe2O4 6Cu + 2Fe3O4 6Cu + 6Fe

theoretical weight change (%)

conversion

2.5 5 11.667 25

0.1 0.2 0.4666 1

3CuO → 1.5Cu 2O → 3Cu

(1)

giving up the oxygen to react with the methane and possible carbon monoxide and hydrogen that are likely to be there. Monazam et al.20 showed that copper oxide dissociation to cupric oxide (Cu2O) was instantaneous when compared to the reaction of the solid with the gaseous reductant. This later step occurs with JMA kinetics with an n value of approximately 2 and activation energy (E/R) of approximately 4365. Therefore, it is expected that only one reaction step associated with the free copper oxide will be required to model this part of the particle. A question arises as to how the copper iron spinel (CuFe2O4) will react with the methane. It is likely to dissociate into CuO and Fe2O3 with the CuO reducing to Cu and the Fe2O3 reducing to Fe3O4. Knowing that the reaction of Fe2O3 to Fe3O4 is relatively slow12,13,21−23 as compared to the reduction of CuO,20 it is expected that the spinel dissociation and formation of Cu occur too quickly to distinguish in the data due to the data collection frequency. Therefore, it is assumed that reaction of the spinel with methane occurs as one reaction step:

Figure 5. Local n values with reduction conversion.

3CuFe2O4 → 3Cu + 2Fe3O4

(2)

with the Fe3O4 likely being an equimolar mixture of FeO and Fe2O3. Finally, there is the reaction of theFe3O4 (FeO/Fe2O3) to form Fe as shown in the step below. Fe3O4 → 3Fe

(3)

Therefore, we can expect that the data when analyzed will give us three reaction processes with JMA kinetics likely for each of these reactions. However, to be complete, the authors will explore the validity of various other models including the shrinking core model as applied by Monazam et al.24 Many recent articles12,13,21−24 have been devoted to the kinetic analysis of experimental data using the following equation:

Figure 6. Effect of reaction temperature on conversion of CuFe2O4 to Cu−Fe using 20% CH4 indication of the conversion limits corresponding to CuFe3O4 and CuFeO.

attributed to carbon deposition.19 Figure 2 also shows that the reduction of Cu/Fe2O3 ceased at about 7, 5, and 3 min at temperatures 800, 850, and 900 °C, respectively. The product gas compositions measured as mass spectrometer ion current values as a function of time are shown in Figure 3. There was an initial peak corresponding to CO, which was followed by CO2. The CO2 signal continued to decrease after optimum value while CO continued to increase contributing to secondary peak. When the secondary CO peak was at a maximum, CO2 signal was near zero. The conversion of the CH4 to these combustion products continues until oxygen is depleted from the oxygen carrier and the CH4 subsequently cracks to form solid carbon and H2 on the reduced oxygen carrier. This is supported by the lack of CO2 at the longer times. The increase in H2 and CO formation at about 4.5 min is

⎛ E ⎞ dX ⎟ f (X ) = A exp⎜ − ⎝ RT ⎠ dt

(4)

In which dX/dt represents the kinetic rate, t is the time, T is the temperature, X is the extent of conversion, A is pre-exponential factor, E corresponds to apparent activation energy, and f(X) is a mathematical function that depends on the kinetic model used. Equation 4 has been usually used to describe the reaction rate of single step kinetics. However, Figures 2 and 3 suggest that the reduction behavior of Cu/Fe2O3 with CH4 may belong to multistage kinetics rates. Hence, in order to distinguish multistage kinetics from single-step kinetics, isoconversional methods were applied to the experimental data. Isoconversional methods allow kinetic evaluation without making any 11968

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Industrial & Engineering Chemistry Research Table 2. Parameters of the Parallel Kinetics Model for Different Temperatures and CH4 Concentrations 3CuO + 3CuFe2O4 → 3Cu + 3CuFe2O4 T (°C)

yCH4

3Cu + 3CuFe2O4 → 6Cu + 2Fe3O4

6Cu + 2Fe3O4 → 6Cu + 6Fe

X∞

wAB

aAB

nAB

kAB

wBC

aBC

nBC

kBC

wCD

aCD

nCD

kCD

1.323 1.479 1.537 1.234

0.647 0.684 0.696 0.622

0.058 0.076 0.102 0.201

0.720 0.877 1.035 1.184

0.019 0.053 0.110 0.258

0.201 0.180 0.173 0.216

3.40 11.24 18.27 30.93

2.66 2.76 2.35 2.32

1.58 2.41 3.44 4.39

0.151 0.135 0.130 0.162

6.49 28.40 47.85 17.48

1.18 1.63 1.27 1.15

4.86 8.11 9.64 12.14

1.16 1.12 0.93 0.96

0.606 0.583 0.500 0.514

0.075 0.064 0.112 0.248

0.784 1.117 1.462 1.452

0.037 0.085 0.224 0.383

0.225 0.238 0.286 0.277

1.880 4.662 8.915 15.233

2.655 2.451 2.297 2.115

1.268 1.874 2.592 3.624

0.169 0.179 0.214 0.208

6.141 14.175 40.103 48.741

1.106 1.254 1.525 1.558

5.16 8.29 11.25 12.12

1.700 2.367 3.188 1.743

0.726 0.803 0.854 0.732

0.037 0.021 0.024 0.047

0.585 0.783 0.831 1.029

0.004 0.007 0.011 0.051

0.157 0.113 0.084 0.153

0.472 0.787 2.453 3.036

3.106 2.253 1.479 1.522

0.785 0.899 1.834 2.074

0.118 0.084 0.063 0.115

6.182 22.997 26.081 46.019

1.120 1.529 1.383 1.586

5.09 7.78 10.58 11.19

30% 750 800 850 900 20% 750 800 850 900 10% 750 800 850 900

These methods yield the activation energy as a function of the extent of conversion.26−28 Since estimating this sole dependence is sufficient for both making kinetic predictions and drawing mechanistic conclusions, the use of the isoconversional methods gives rise to completely model-free kinetic analysis.25 The main disadvantage of isoconversional methods is that they do not allow direct evaluation of either the pre-exponential factor or reaction model. The variation of the activation energy as a function of X under isothermal conditions can be obtained by taking the logrithm and rearranging eq 4 as ⎛ ln t = ⎜ −ln A + ln ⎝

∫0

X

E dX ⎞ ⎟+ f (X ) ⎠ RT

(5)

By plotting ln(t) versus 1/T according to eq 5, the activation energies were found for a given X value from the slope of a regression line. Figure 4 provides the activation energy for reduction at the temperature range of 750−900 °C with 20% CH4. With increasing X (0.05 < X < 0.1), the activation energy initially decreased from 70 kJ/mol to 62 kJ/mol, then increased to 72 kJ/mol (0.1 < X < 0.20) followed by a decrease to 54 kJ/ mol (0.25 < X < 0.46) and again increasing to above 100. kJ/ mol. It is interesting to note that these transitions occur at approximately the conversion levels associated with the following reactions (eqs 6−8).

Figure 7. Comparison of the experimental CuO + CuFe2O4 reduction, X, data (symbol) to the multistep reaction scheme (750−900 °C) model (solid line) using 20% CH4.

3CuO + 3CuFe2O4 → 1.5Cu 2O + 3CuFe2O4

(6)

3CuO + 3CuFe2O4 → 3Cu + 3CuFe2O4

(7)

3CuO + 3CuFe2O4 → 6Cu + 2Fe3O4

(8)

The full reduction to Cu and Fe would occur according to eq 9 shown below. This would be near the point (1,100) in Figure 4 and beyond the conversion of the experiments in this study. 3CuO + 3CuFe2O4 → 6Cu + 6Fe

Figure 8. Three predicted curves of conversion as a function of time during isothermal reaction of CuO + CuFe2O4 with 20% CH4. Curve with symbol points is experimental data.

(9)

17,18

Our previous studies indicated the presence of CuO and CuFe2O4 and absence of Fe2O3 in the samples after calcination. Therefore, CuO and CuFe2O4 were used in the reactions. According to reactions 6−9), mixed CuO−Fe2O3 is reduced to Cu2O and CuFe2O4 and then to Cu metal and Fe3O4 where Fe3O4 was reduced to Fe via FeO as discussed above in justifying a three-reaction process.

assumptions about the analytical form of the reaction model.25This approach assumes that the reaction rate at a constant conversion is only a function of temperature and that the reaction mechanism is not dependent on temperature. 11969

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Figure 10. Arrhenius plot for reactions AB, BC, and CD in temperature range 750−900 °C.

These variations suggest that the process involves multistage reactions with different activation energies. Table 1 shows the weight change and conversion of the possible stoichiometric decomposition reactions for mixed CuO/Fe2O3. The n value of JMA nucleation and growth model also can give detailed information on the nucleation and growth during the reduction process. The local n(X) can be calculated using eq 10:

Figure 9. Effect of reaction temperature on conversion of AB, BC, and CD using 20% CH4 at temperature range 750−900 °C.

A model that described a series reaction did not fit the current data well.13 Presence of reduced copper oxide has also been found to enhance the deeper reduction of iron oxide.18 Iron oxide located in the vicinity of reduced CuO may be reacting differently from that and is closer to the unreduced copper oxide leading to various parallel reactions.

n(X ) = 11970

∂ ln( −ln(1 − X )) ∂ ln(t )

(10) DOI: 10.1021/acs.iecr.5b02848 Ind. Eng. Chem. Res. 2015, 54, 11966−11974

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Industrial & Engineering Chemistry Research

Figure 11. Effect of CH4 concentration on reaction rate for temperature range 750−900 °C.

Figure 5 shows the average n value at different temperatures as reduction conversion progressed. It is obvious that local n values are not constant during the reduction of mixed CuO/ Fe2O3 with CH4, usually exhibiting a decrease at the beginning (X < 0.2) and a slight increase before decreasing again (0.2 < X < 0.5) and a final increase (X > 0.5). Again, these conversion levels correspond to the theoretical conversion levels associated with the reactions 4−7. Hence, the n-value variations during the reduction process also suggest the possibility of a multistage reaction mechanism. For the purpose of representation and data analysis, the recorded weight changes from the thermogravimetric system were normalized according to the following equation, X=

mo − m(t ) mo − mf

(11)

where m(t) is the instantaneous weight of the solid during the exposure to CH4. Parameters mo and mf are initial and final weights, respectively, of the sample tested. In this study, the initial weight was considered as the weight of mixed CuO− Fe2O3 and final weight as the weight of Cu−Fe (corresponds to weight decrease of 25 wt %). By use of eq 11, the weight changes of mixed CuO−Fe2O3 at different temperatures (750−900 °C) with 20% methane were used to compute conversion−time data as illustrated by Figure 6. It is obvious that the reduction is favored by high temperature, which confirms the basic principle of chemical thermodynamics and kinetics. Due to the multistage nature of reaction as evidence by Figures 4 and 5, the introduction of a supplementary model usually proves to be more advantageous than using single model analysis because it improves the quality of fit considerably.29 Although the above analysis strongly suggests that there are four distinct reactions, there is insufficient time resolution of the data series to extract the oxygen uncoupling in the generation of the Cu2O. Therefore, in this study, it is assumed that at all temperatures there are three reaction fronts (eqs 6−8), each linked to one of the single reactions. The progress of XAD of the three reactions can be expressed as a function of the partial progresses of XAB, XBC, and XCD of the single reactions and can be written as30 XAD = wABXAB + wBCXBC + wCDXCD

Figure 12. Effect of temperature on the rate of reductions for each reactant using 30% CH4.

conversion from reaction 8 to 9. The wAB, wBC, and wCD are weight fractions depending on the oxygen loss of each single reaction with wAB + wBC + wCD = 1. In this study, the values of XAB and XBC are limited to the stoichiometric values of 0.2 and 0.2666, respectively. The progress of the three reactions is assumed to be in parallel and expressed as12,13,20−24

(12)

where XAB is conversion to reaction 7, XBC is difference in conversion from reaction 7 to 8, and XCD is difference in 11971

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Figure 8 illustrates that in the initial portion of the reaction, the time dependence of total reaction, curves (XAB + XBC + XCD), is dominated by reactant AB. At long times, the total reactant (XAB + XBC + XCD) is dominated by reactant CD. Hence, the reactant CD will have little influence on the initial part of the reduction process. The effects of reaction temperature on the conversion of each reaction (AB, BC, and CD) during the 10 min reduction are shown in Figure 9. The data indicate that the reaction rates of all reactions increase with increasing temperature. The reduction rate constant for all three reactants (AB, BC, and CD) was obtained as k(min−1) = a1/ n = A e−E /(RT )

Plots of ln k versus 1/T for reduction of CuO/Fe2O3 for all three reactant fronts are illustrated in Figure 10 for inlet CH4 concentrations of 30%, 20%, and 10% vol. The apparent activation energies for three reactant fronts AB, BC, and CD were estimated to be 55.32 ± 3.11, 70. ± 2.0, and 165 ± 5.4 kJ/ mol, respectively. The value of 55.32 ± 3.11 obtained in this paper is near the value of 37.3 ± 1.3 kJ/mol obtained by Monazam et al.24 for CuO/bentonite mixture. The higher magnitude of the activation energy obtained in this study may be due to the change in the mechanism from nucleation and growth24,31,32 to first order rate in this work. It is also interesting to note that the activation energy of 165 ± 5.4 is in the range associated with oxygen vacancy diffusion (154−203 kJ/mol)33 indicating that this last step is one associated with oxygen migration to the surface through the lattice structure. When all of the values of k for different CH4 concentrations are combined (Figure 10), the following equations are obtained for each reactant:

Figure 13. Enhancement achieved with the CuFe2O4 over Fe2O3 using 20% methane. nAB nBC XAD = wAB(1 − e−aABt ) + wBC(1 − e−aBCt ) X∞ nCD

+ wCD(1 − e−aCDt )

(14)

(13)

The values of nAB, nBC, and nCD define the type of reaction mechanism for the process. For a given temperature, values of X∞, wAB, wBC, aAB, aBC, aCD, nAB, nBC, and nCD were determined by curve fitting the rate data of Figure 6 with the parameters in eq 11. The kinetic parameters obtained from the TGA isothermal data are summarized in Table 2. The values of exponent “n” for reactant AB (nAB) range from 0.6 to 1.5, for reactant BC (nBC) range from 1.8 to 3.4, and for reactant CD (nCD) range from 1.1 to 1.63 for all the temperatures and all the CH4 concentration. The average values of nAB, nBC, and nCD were 1.0 ± 0.28, 2.33 ± 0.47, and 1.36 ± 0.2 (95% CL), respectively. The observed value of nAB = 1 suggests pseudo-first-order rate expression. According to diffusion controlled nucleation and growth theory, the magnitude of the shape factor, the “n” terms, reflects how the nucleation rate changes under isothermal conditions: 1.5 is a zero nucleation rate, 1.5−2.5 is a decreasing nucleation rate, 2.5 is a constant nucleation rate, and n values are greater than 2.5 for increasing nucleation rates.30 The comparison of the experimental mixed CuO−Fe2O3 conversion data (X) and the conversion based on parallel model as presented in eq 11 is illustrated in Figure 7 for different temperatures. The model data and experimental data agree over the entire conversion time with overall variance (R2) greater than 99.9%. The parallel reactions, eq 11, based on model and experimental data are illustrated in Figure 8. The reaction AB curve, which is first order as its nAB is close to 1, and reactant BC curve, which is sigmoidal, representing nucleation and growth, and CD, which is between first order and nucleation and growth, represent three processes of reactions occurring simultaneously but with different time dependence. The curve XAB which represents reactant AB has a faster time response than reactants BC and CD, and reactant BC has a faster time response than reactant CD. As time progresses, reactants AB, BC, and CD reach steady state at different degrees of reduction. Note that the curves denoted XAB and (XAB + XBC) are limited to the asmyptopic value equivalent to the conversion of mixed CuO/Fe2O3 to Cu + CuFe2O4 and Cu + Fe3O4, respectively.

.535 kAB(min−1) = 10926 yCH4 exp( −6653.7/T )

(15)

0.312 kBC(min−1) = 7321.9 yCH4 exp( −8419.4/T )

(16)

,

and 2.12 k CD(min−1) = 1.39 E + 8 yCH4 exp(− 19922.7/T )

(17)

where yCH4 is the mole fraction of CH4. The orders of reactions with respect to the gaseous reactant (CH4) are also shown in eqs 15−17 and Figure 11. An expression for the reaction rate, dX/dt, can be derived by differentiating eq 8 with respect to t, at constant temperature, as follows: (1 − 1/ ni) ⎛ ⎛ dX i X i ⎞⎞ ⎜ ⎟ = niai1/ ni(wX − − − X ) ln 1 ⎜ ⎟⎟ i ∞ i ⎜ dt wX ⎝ ⎝ i ∞ ⎠⎠

(18)

where i = AB, BC, and CD. Therefore, the total rate is dX t dXBC dX CD dXAB = + + dt dt dt dt

(19)

The rate−time (dX/dt versus t) data obtained at different temperatures (750−900 °C) using eq 18 are also shown in Figure 12 for reactant AB, BC, and CD. As shown in Figure 12, the rate−time curves for reactant AB show the maximum rate when time is equal to zero at all temperatures. This is consistent with the rate data of a kinetically controlled reaction 11972

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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.

in which the maximum rate occurs when time is equal to 0, or when n is equal to 1 in the Avarami equation. However, as shown in Figure 12, the rate−time curves for BC and CD show the maximum rate at a time greater than 0 which indicates that reactants BC and CD are controlled by nucleation and growth. Figure 12 also show that the value of maximum rate increases with the increasing temperature for all three reactants. Figure 13 illustrates the enhancement achieved with the mixed CuO/Fe2O3 by observing the differences in the degree of reduction achieved by using CuO/Fe2O3 as compared with that achieved by hematite (Fe2O3), with 20% methane at 900 °C. As shown in Figure 13, at reducing temperature of 900 °C, the pure CuO was reduced to the Cu metal within 2 min while pure Fe2O3 could partially (45%) transform to Fe at about 45 min. Therefore, it is obvious that hematite (Fe2O3) has low reduction rate as compare to CuO (Figure 13). When CuO and Fe2O3 are mixed, the mixture is reduced to Cu and Fe within 3 min. This indicates that Cu promotes the reduction rate of Fe2O3. It is interesting to note that CuO/Fe2O3, which has extra CuO, has the same conversion initially as CuO. Once the CuO is consumed, the conversion of copper ferrite initiates. The conversion of copper ferrite is lower than that with CuO but is significantly higher than that with Fe2O3. The data indicate that it is beneficial to combine Fe2O3 with CuO to enhance the reactivity of Fe2O3.

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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. Special thanks go to Duane D. Miller and Thomas Simonyi of URS Energy & Construction, Inc. for their assistance with experimental work and data.

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SUMMARY The reduction kinetics of the mixed Cu−ferrite with CH4 (10%, 20%, and 30%) was evaluated in the temperature range 750−900 °C using TGA and mass spectrometry. Three reactions kinetic rate model best defined the reduction process which included one chemical rate control and two nucleation/ growth processes. Reduction of mixed Cu−ferrite to metal Cu with CH4 is characterized as chemically controlled with apparent activation energy of 55.32 ± 3.11. The activation energy of the reduction of mixed Cu−ferrite to metal Cu and Fe3O4 and of mixed Cu−ferrite to metal Cu and iron is 70 ± 2 and 165 ± 5.4 kJ/mol, respectively. Mathematical modeling of experimental data suggests that the reaction rate for the other two was well represented by a nuclei growth model with increasing and decreasing nucleation. The mixed material has unique features: copper appears to promote the reduction of iron oxide. The kinetic model for the mixed oxide is different from those with the single metal oxides. Therefore, this investigation shows that the kinetic data for single metal oxides cannot be combined to achieve the kinetic data for the copper ferrite mixture.

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REFERENCES

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

The authors declare no competing financial interest. The U.S. Department of Energy, NETL, and REM contributions to this paper were 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 11973

DOI: 10.1021/acs.iecr.5b02848 Ind. Eng. Chem. Res. 2015, 54, 11966−11974

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DOI: 10.1021/acs.iecr.5b02848 Ind. Eng. Chem. Res. 2015, 54, 11966−11974