ARTICLE pubs.acs.org/EF
Redox Characteristics of FeNi/SiO2 Bimetallic Oxygen Carriers in CO under Conditions Pertinent to Chemical Looping Combustion Hui Song, Elham Doroodchi, and Behdad Moghtaderi* Priority Research Centre for Energy, Chemical Engineering, School of Engineering, Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan, New South Wales 2308, Australia ABSTRACT: This study is concerned with the reduction/oxidation (redox) behavior of bimetallic oxygen carries of Ni and Fe under conditions pertinent to chemical looping combustion (CLC). Bimetallic oxygen carriers of Ni- and Fe-based metal oxides with different mixture ratios were developed through the incipient wet impregnation (IWP) method by employing SiO2 as the support. The redox properties of metal oxide mixtures were determined in a magnetic balanced thermogravimetric analyzer (TGA) over a range of temperatures between 500 and 950 °C at atmospheric pressure under alternating reduction and oxidation half cycles. The reduction half cycle was carried out in the presence of CO, CO2, and N2 mixtures with different volume fractions of CO (typically 5, 10, and 70%), while the oxidation half cycle was conducted in normal air. Oxygen carriers exhibited temperaturedependent reactivity during both reduction and oxidation half cycles. The reactivity of monospecies Ni-based oxides on the silica support under the reducing environment of CO was found to be lower than that of Fe-based oxides. Furthermore, with the exception of 950 °C, reaction rates and the extent of reactions at high temperatures were generally found to be higher than those of low temperatures. The kinetic properties of all monospecies and bimetallic oxygen carriers were determined using the shrinking core model, from which the mixture kinetics of nickel and iron oxide on the SiO2 support were further correlated to the mixture ratio through a detailed theoretical analysis.
1. INTRODUCTION Fears of global warming and climate change as a result of CO2 emissions from fossil-fuel-based power plants (particularly, coalfired power plants) have given rise to a search for cleaner ways of power generation from fossil fuels.15 Among the emerging technology options, the chemical looping combustion (CLC) concept has attracted a growing interest partly because of its inherent ability for CO2 separation during the combustion process and partly because of its low energy penalty.133 Unlike conventional combustion, in the CLC process, the direct contact between the fuel and air is prevented and, instead, the oxygen required for the combustion process is provided by circulation of particles of an oxygen carrier between two interconnected reactors (see Figure 1), commonly known as the air reactor (AR) and the fuel reactor (FR). As a result, the outlet gas stream from the AR unit is primarily composed of unreacted N2 originated from the feed air, while the exhaust stream from the FR unit contains CO2 and steam only. Carbon dioxide can be readily captured from the exhaust stream of the FR unit after condensing the steam. As such, the CLC process does not require large quantities of energy for CO2 capture and would not have any NOx emission issues.5 However, the successful application of the CLC concept is largely underpinned by the redox characteristics of oxygen carriers, and for this reason, a large number of recent studies have focused on developing new oxygen carriers for CLC applications.334 Metal oxides, especially oxides of transitional metals, such as Ni, Fe, Cu, Mn, and Co, were found to be the most promising candidates for CLC applications. Ni-Based carriers have been studied in greater detail because they possess good redox, thermodynamic, and mechanical properties and relatively high chemical stability. However, Ni-based oxygen carriers are relatively expensive and quite r 2011 American Chemical Society
hazardous. Perhaps the most important drawback of Ni-based carriers is their thermodynamic limitation for full conversion of hydrocarbon fuels (e.g., CH4). The literature715 indicates that Fe-based carriers exhibit relatively modest redox properties, and of the multiple oxidation states of Fe, only the transition between Fe2O3 and Fe3O4 is thermodynamically favorable for full conversion of hydrocarbon fuel. However, because of their low cost and environmental compatibility, Fe-based oxygen carries are considered attractive options for CLC applications. Cu-Based oxygen carriers have shown that they can achieve higher reduction rates than Fe-based carriers, but copper oxides are expensive and quite susceptible to sintering and agglomerations at temperatures of interest in CLC applications. Co- and Mn-based oxides have generally showed limited conversion capability and are suitable for specialized applications, such as chemical looping air separation.22 Previous studies have revealed that pure metal oxides are unsuitable for extended use as oxygen carriers in CLC systems. Inert support materials, such as alumina (Al2O3), yttria-stabilized zirconia (YSZ), and to a lesser extent, various metal aluminates (e.g., NiAl2O4 and MgAl2O4), TiO2, and silica (SiO2), were employed as binders in the past to enhance the mechanical strength and reactivity of carrier particles by improving particle porosity and surface characteristics. While both alumina and YSZ Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 1, 2011 Revised: September 25, 2011 Published: September 28, 2011 75
dx.doi.org/10.1021/ef201152u | Energy Fuels 2012, 26, 75–84
Energy & Fuels
ARTICLE
Table 1. Properties of the Oxygen Carriers Prepared in This Study loading content (wt %) oxygen
support
carriers
material
NiO
NiO/SiO2
SiO2
26.37
NF24
SiO2
11.36
24.31
NiO, Fe2O3, SiO2, NiFe2O4a
NF28
SiO2
8.83
28.31
NiO, Fe2O3, SiO2,
Fe2O3 0
crystalline phase NiO, SiO2
NiFe2O4a
Figure 1. Schematic representation of the CLC concept.
NF29
SiO2
0.75
29.1
NiO, Fe2O3, SiO2, NiFe2O4a
could provide high mechanical strength and chemical stability, the pore volume of silica binders is typically much higher than those of other binders, and as such, silica-based binders would provide better support for chemical reactions. Moreover, silica is abundant in nature and low in cost (almost as cheap as alumina). Despite its potential, only a few investigations were conducted in the past on the use of SiO2 as a binder.7,8,2934 Zafar et al.,8 for example, investigated the use of Ni-, Fe-, Cu-, and Mn-based oxides on the SiO2 support in CLC of methane. They found that Ni- and Cu-based oxygen carriers exhibited the highest reactivity in methane CLC at reaction temperatures below 950 °C.8 Notable deteriorations in the reactivity of all metal oxides were observed at high temperatures after repeated redox cycles (e.g., T g 950 °C for Ni and T g 950 °C for Fe). These authors attributed the drop in the reactivity of the oxygen carriers to the irreversible silicate formation at high temperatures.7 However, their X-ray diffraction (XRD) measurements for Ni-based oxides did not confirm the existence of silicates, but silicate formations for Fe- and Mn-based oxides were observed. It is also not known whether the silicates are formed under reacting environments other than methane (e.g., CO or H2). Furthermore, Zafar and coworkers8 only examined monospecies oxygen carriers, where only a single species of metal oxide existed in the oxide/binder mixture. Recent studies on mixed metal oxides, however, have illustrated that many of the drawbacks associated with monospecies oxygen carriers can be resolved when two or more metal oxides are incorporated into the oxide/binder mixture.12,21,22,3032 Given the above background, the primary aim of the research reported in this paper is to examine the feasibility of using silica binders in mixed Fe- and Ni-based oxygen carriers under a reacting environment of CO. The choice of carbon monoxide as the reacting environment is based on several considerations concerning solid fuels. It is widely accepted that the global deployment of the CLC concept as a viable technology option hinges upon extending the concept to solid fuels, particularly, coal. This can be achieved in a number of alternative ways, but perhaps the simplest approach is an intermediate fuel gasification stage in the presence of steam followed by CLC of the product gas (i.e., synthesis gas, which is a mixture of CO and H2). Within the product gas mixture, the reactions associated with hydrogen are generally very fast, while those related to CO are typically slow. As such, CO-related reactions are rate-determining steps and should be examined in greater details.
Fe2O3/SiO2 a
SiO2
0
32
Fe2O3, SiO2
Minor phase.
impregnation method using commercial silica gel from Grace Davison. Before impregnation, the silica gel was dried overnight at a temperature of 110 °C to completely remove the moisture in the pores. Weighed iron nitrate nonahydrate and nickel nitrate hexahydrate were added to deionized water in proportions corresponding to the desired weight ratio of each metal oxide in the bimetallic oxygen carrier sample. The mixture was then stirred with a magnetic stirrer at the desired temperature until the iron and nickel nitrate were dissolved entirely to form a highly concentrated aqueous solution. The metal nitrate solution was then rapidly added to the dried silica at a volume of 2.35 cm3/g (the same as the pore volume of dried silica). The resultant sample was dried at 250 °C for 2 h and then calcined at 600 °C for 3 h in a muffle furnace at atmospheric pressure to ensure the decomposition of metal nitrates into metal oxides. The resulting fresh metal oxide samples on the silica support were sintered further at 950 °C for 6 h. All samples were then sieved to a uniform size of 90106 μm for reactivity tests. For cases where the loading content could not be attained by a single impregnation attempt, the impregnation was repeated until the desired loading was achieved. Monospecies iron oxide and nickel oxide on the SiO2 support were also prepared using the above method for comparison purposes. The qualities of both monospecies and bimetallic metal oxide samples were examined using a range of analytical techniques. The crystalline phases within the samples were indentified using the XRD method, while the loading content was confirmed by X-ray fluorescence (XRF). The properties of the samples prepared in our laboratory are summarized in Table 1. 2.2. Experimental Apparatus and Method. All reactivity tests were carried out in a Rubotherm magnetic suspension balanced thermogravimetric analyzer (TGA, Hp-TGA MSB-Aus-2004-00188) at ambient pressure. As Figure 2 shows, the Rubotherm TGA consists of a magnetic balance, a magnetic suspension coupling unit, a furnace, pressure controllers, and a gas switching unit. The sample crucible is dropped down to the furnace by a very thin platinum suspension wire that is coupled to the magnetic suspension part of the balance via a cage. With this arrangement, the suspension part would lift the wire and crucible assembly from the balance for accurate measurement of the weight change and could be unloaded for setting zero points in situ at adjustable intervals. All collected data (e.g., temperature, weight of the sample, zero point, flow rate, and pressure of the reactant gas) are continuously recorded using a data acquisition system connected to a central processing unit, i.e., computer. In addition, the heating of the furnace, gas switching, and pressure regulation are all managed by a series of programmable controllers linked with the computer. As far as the experimental procedure is concerned, each reactivity test consisted of several steps, as described below. About 20 mg of the sample
2. EXPERIMENTAL SECTION 2.1. Preparation of Oxygen Carriers. Fe- and Ni-based bimetallic oxygen carriers on the SiO2 support were prepared by the dry 76
dx.doi.org/10.1021/ef201152u |Energy Fuels 2012, 26, 75–84
Energy & Fuels
ARTICLE
Figure 2. Schematic of the Rubotherm TGA for reactivity tests. was first charged into a platinum crucible with an internal diameter of 10 mm and a height of 6 mm. The sample was then heated in the crucible to the desired reaction temperature (500950 °C) under a nitrogen atmosphere. Next, the flow of nitrogen was stopped, and the sample was swept with the relevant reacting gases; i.e., CO balanced by CO2 and N2 during the reduction half cycle and air during the oxidation half cycle. A 120 mL/min [standard temperature and pressure (STP)] flow of N2 was introduced between the reduction and oxidation half cycles for about 10 min to purge the remaining gases from the system after each half cycle. The steps were then repeated for five full cycles, and the sample weight loss was recorded throughout the test for determination of reaction properties. The volumetric concentration of CO was systematically varied during the experimental campaign with values typically regulated at 5, 10, and 70%. The addition of CO2 to the reducing gases was also a preventive measure to avoid the formation of carbon deposits on metal oxide samples.6 The flow of reacting gases was typically set to 120 mL/min (STP), over which almost no observable changes were found in the resultant data from TGA. Samples with weights greater than and less than the nominal 20 mg were also tested to find if the collected data is weight-dependent. No measurable difference was observed. Similarly, varying the particle size in the range of 90212 μm did not show any significant change in the data. These trials demonstrated that external and/or internal diffusional effects were not dominant under set conditions, and thus, the reactions were chemically controlled.
Figure 3. Effect of the temperature on mass variations at the fifth cycle for NiO/SiO2 with 5 vol % CO applied in reduction. T = ( 3 3 3 ) 500 °C, (- 3 -) 700 °C, (- - -) 875 °C, and (—) 950 °C. Reduction and oxidation time = 20 min. Time for purging by N2 = 10 min.
m is the instantaneous weight during a given test. The difference between mox and mred is the maximum amount of active oxygen that can be transported under reducing conditions. A typical fifth cycle experimental data set is shown in Figure 3 for an experiment conducted on NiO/SiO2 under a reducing environment of 5 vol % CO and an oxidation environment of air at different temperatures (i.e., 500, 700, 875, and 950 °C). As can be seen, after each period of nitrogen purging there are short delays before the reduction or oxidation reactions commence. Also, it is evident from Figure 3 that higher temperatures generally lead to a greater sample weight loss and, thus, higher metal oxide conversions. Panels a and b of Figure 4 illustrate the conversion of monospecies NiO/SiO2 samples under reduction and oxidation half cycles, respectively. The conversion results calculated from eqs 1 and 2 have been plotted against time in Figure 4 for the first 150 s of the reduction half cycle and the first 50 s of the oxidation half cycle. These conversion results were obtained from experiments carried out with a reducing gas mixture of 5 vol % CO at temperatures ranging from 500 to 950 °C. Clearly, both reduction and oxidation conversions are strongly dependent upon the reaction
3. RESULTS AND DISCUSSION As noted earlier, typically five redox cycles were conducted during reactivity tests. However, only the data corresponding to the fifth cycle were used for reactivity analysis because the redox behavior of oxygen carriers in the first few cycles is not usually stable. The transition of metal oxides from Fe2O3 to Fe3O4 and NiO to Ni was considered as an applicable transformation pair for delivering oxygen. The conversion of oxygen carriers was calculated using the following equations: mox m ð1Þ Xred ¼ mox mred Xod ¼ 1
mox m mox mred
ð2Þ
where mox represents the sample weight at the fully oxidized state, mred donates the entirely reduced weight of the carriers, and 77
dx.doi.org/10.1021/ef201152u |Energy Fuels 2012, 26, 75–84
Energy & Fuels
ARTICLE
Figure 5. Mass variations in five consecutive cycles for the experiment carried out on NiO/SiO2 at 950 °C with 5 vol % CO applied in reduction.
Figure 4. Reactivity as a function of time for NiO/SiO2 at temperatures of (9) 500 °C, (2) 700 °C, (1) 875 °C, and (O) 950 °C: (a) reduction (5 vol % CO) and (b) oxidation.
associated with 950 °C does not follow the general trend outlined earlier of the increasing degree of conversion with the reaction temperature. More specifically, the reduction and oxidation conversions for the 950 °C case are lower than those of 875 °C. While this inconsistency might be due to the formation of nickel silicates, our XRD measurement could not conclusively show the existence of relevant crystalline entities, perhaps because silicates might be in the amorphous phase. Zafar and co-workers,7,8 however, reported that their thermodynamic analysis on NiO/SiO2 oxygen for methane CLC did confirm the existence of stable nickel silicates at high temperatures, but similar to our findings, their XRD measurements showed no sign of silicates. Furthermore, as shown in Figure 5 under stable operating conditions (essentially after the first cycle), our results show no major cycle-to-cycle variations of weight losses or weight gains during repeated redox cycles. This finding, which somewhat contradicts that by Zafar and co-workers7,8 for methane CLC, is another indication that, at least for cases where reduction is carried out under CO, nickel silicates may not be entirely responsible for the observed drop in the conversion rate of monospecies NiO/SiO2 oxygen carriers at 950 °C. Figures 68 show the results of reactivity tests on monospecies samples of Fe-based metal oxides on the silica support. Similar to the previous case for Ni-based oxygen carriers, the results shown in Figures 68 correspond to reactivity measurements carried out at temperatures ranging from 500 to 950 °C. Again, the reduction environment for experimental cases summarized in Figures 68 was 5 vol % CO, while the oxidation environment was normal air. Figure 6 illustrates a typical fifth cycle experimental data set for reduction and oxidation of hematite (Fe2O3) on the silica support. Because iron has multiple oxidation states, during the reduction half cycle, hematite can be reduced to both magnetite (Fe3O4) and wustite (FeO). For the case under investigation, the boundaries between the two lower oxidation state iron oxides have been marked in Figure 6 as two horizontal dashed lines: one at a weight of 21.87 mg corresponding to magnetite and the other at a weight of 21.37 mg corresponding to wustite. The zone between the magnetite and wustite boundaries represents an oxidation state between those of Fe3O4 and FeO (or a mixture of the two oxidation states). It can be observed from Figure 6 that, under experimental conditions pertinent to Figure 3 (e.g., reduction under 5 vol % CO), hematite is reduced to an oxidation state between those of magnetite and wustite at
temperature and residence time (i.e., the period that the sample is held in the reaction zone). For a given time, more weight loss can be generally attained at high temperatures than at low temperatures (950 °C, which is discussed below, is an exception). Similarly, the longer the residence time, the greater the conversion, although the variation in conversion plateaus at relatively high values of residence time. As such, the extent of conversion for low-temperature and short-residence-time experiments is very limited. At 500 °C and t = 60 s, for example, the reduction conversion is 30%, while at 875 °C and t = 60 s, the reduction conversion is about 60%. This implies that, after 60 s, the extent of reduction at 875 °C is approximately 2 times greater than that at 500 °C. If the same comparison is made at t = 150 s, the extent of conversion corresponding to 875 °C would be 2.2 times greater than that at 500 °C. The impact of the temperature on the oxidation conversion for monospecies NiO/SiO2 samples appears to be less dramatic than that on the conversion under reducing conditions. Moreover, none of the samples show zero oxidation conversion at the beginning of the oxidation half cycle, as shown in Figure 4b. This is an interesting difference between the theory and practice. Theoretically, oxidation should commence from 0 provided that 100% conversion has been achieved in the reduction half cycle. In practice, however, the conversion during the reduction half cycle is not complete (i.e., less that 100%), as can be seen, for example, from Figure 3 for NiO/SiO2, particularly, at low temperatures. This inevitably implies that the oxidation conversion at the beginning of the oxidation half cycle is not equal to 0 as theory suggests (note that Xod = 1 Xred). As discussed earlier, the samples lose more weight during reduction at high temperatures, and therefore, the oxidation conversion corresponding to low temperatures was greater than that of high temperatures at the beginning. Regardless of the reaction temperature, reduced samples seem to be almost completely oxidized back to their initial weight at the conclusion of the oxidation process. In contrast, the impact of the residence time on the oxidation conversion emerges to be as powerful as that corresponding to reduction conversion (compare panels a and b of Figure 4); i.e., higher oxidation conversion can be obtained at a longer residence time, unless full oxidation is achieved. One of the striking features of Figure 4 is that, for reduction of monospecies NiO/SiO2 oxygen carriers, the conversion plot 78
dx.doi.org/10.1021/ef201152u |Energy Fuels 2012, 26, 75–84
Energy & Fuels
ARTICLE
Figure 8. Reactivity as a function of time for Fe2O3/SiO2 at temperatures of (9) 500 °C, (2) 700 °C, (1) 875 °C, and (O) 950 °C: (a) reduction (5 vol % CO) and (b) oxidation.
Figure 6. Effect of the temperature on mass variations and final products at the fifth cycle for Fe2O3/SiO2 with 5 vol % CO applied in reduction. T = ( 3 3 3 ) 500 °C, (- 3 -) 700 °C, (- - -) 875 °C, and (—) 950 °C. Reduction and oxidation time = 20 min. Time for purging by N2 = 10 min.
variations of sample weight are exhibited by iron oxides under stable operating conditions. Figure 8 shows the effects of the reaction temperature on the reactivity of Fe-based metal oxides during reduction and oxidation half cycles. In comparison to the results presented earlier in Figure 4a for Ni-based oxides, the reactivity (i.e., rate of conversion) of Fe-based oxides for both reduction and oxidation half cycles appears to be much higher than that of their Ni-based counterparts (see Figures 4a and 8a). This is quite consistent with the results reported by Abad and co-workers6 for the reduction of alumina-supported Ni- and Fe-based oxygen carriers under a reacting environment of CO. It is generally understood that, regardless of the binder/support material, Ni-based carriers tend to perform better than Fe-based oxides when the reacting environment is a hydrocarbon (e.g., CH4) or hydrogen, rather than CO.6,9,12,21,26 Despite the differences, Figure 8 has striking similarities to Figure 4 for Ni-based oxygen carriers. In particular, the conversion rate during the reduction half cycle increases in both figures when the reaction temperature is raised from 500 to 875 °C. Also, very much like the Ni-based metal oxides (see Figure 4a), for experiments conducted at 950 °C, a dramatic decrease in the rate of reduction conversion is observed in Figure 8a, to the extent that the rates of reduction conversion associated with 950 °C are even lower than those associated with 500 °C. As far as the oxidation conversion of monospecies Fe-based oxides is concerned, the deterioration in conversion rates becomes evident from 875 °C, whereas as noted earlier for Ni-based oxides (see Figure 4b), the drop-off in performance becomes noticeable at reaction temperatures greater than or equal to 950 °C. As can be seen from Figure 8b, at high temperatures, i.e., 875 and 950 °C, the reduction conversion rate approaches a value of approximately 65%, while values of 85 and 90% are reached at reaction temperatures of 700 and 500 °C, respectively. Although the reasons behind such performance degradations are not fully understood, as noted before, the slow oxidation of iron silicate35 formed during redox reactions might be partly responsible. Furthermore, it should be highlighted that, while the conversion rate is generally faster for oxidation than reduction at the early stages of any given redox reaction, the time for complete oxidation is usually longer than that of reduction. For example, full reduction of hematite at 500 °C would take approximately 90 s compared to around 15 min for complete oxidation of the magnetite/wustite mixture to hematite. Given that Figure 8b only shows the first
Figure 7. Mass variations in five consecutive cycles for the experiment carried out on Fe2O3/SiO2 at 950 °C with 5 vol % CO applied in reduction.
all temperatures. As a result, two reaction phases can be identified in the reduction period. The reduction of hematite to magnetite primarily occurs in the first phase, followed by the second phase, in which magnetite formed during phase one is further reduced to wustite at a relatively slower rate. The extent of the reduction is clearly a function of the temperature; that is, at high temperatures, more Fe3O4 remaining in the sample can be converted to FeO compared to that at low temperatures. Although yet again (i.e., similar to the nickel case), the data set associated with the 950 °C reaction temperature does not follow this general trend, showing a lower reaction rate and extent of reduction than 875 °C. This discrepancy is very likely due to the formation of iron silicates, as also observed and reported by Zafar et al.,8 as part of their experiments on Fe-based metal oxides on the silica support. While our XRD measurements could not accurately identify the silicate entities because of the minute quantities of samples used in each test and the amorphous nature of silicates formed, from a close examination of the cycle-to-cycle weight loss variation, we were able to conclude that iron silicates are the most likely culprits. As shown in Figure 7 and in contrast to what was shown earlier in Figure 5 for nickel-based oxides, slight cycle-to-cycle 79
dx.doi.org/10.1021/ef201152u |Energy Fuels 2012, 26, 75–84
Energy & Fuels
ARTICLE
Figure 9. Reactivity versus time at a temperature of 800 °C for (9) NiO/SiO2, (2) NF24, (b) NF28, and (1) Fe2O3/SiO2: (a) reduction (5 vol % CO) and (b) oxidation.
50 s of the oxidation process, the differences between the lowand high-temperature results are more dramatic in this figure. Indeed, our data indicate that, for the case under investigation, the differences between the low- and high-temperature results diminish toward the end of the oxidation cycle (∼20 min). Apart from monospecies Fe- and Ni-based metal oxides, reactivity experiments were also conducted systematically on their bimetallic mixtures. Experiments were conducted on NF24 and NF28 samples (see Table 1) at different temperatures and various volumetric concentrations of CO during reduction. As an example, consider Figure 9, which shows plots of the reduction and oxidation conversions versus time for NF24 and NF28 samples at a reaction temperature of 800 °C. Similar plots for NiO/SiO2 and Fe2 O3 /SiO 2 monospecies samples have also been included for comparison. It is observed from Figure 9 that the reactivity of bimetallic oxygen carriers, either reduction or oxidation, is related to the loading content of NiO and Fe2 O3 in the bimetallic mixture. As expected, a higher loading of NiO or lower content of Fe2O3 would lead to low conversion rates during reduction and oxidation. Because the NF24 sample contains higher quantities of NiO (11.36%, as opposed to 8.83% in NF28) and NiO/SiO2 is less active compared to Fe2O3/SiO 2, NF24 exhibits a lower reactivity than NF28 in reduction and oxidation, as shown in panels a and b of Figure 9. It should be noted at the same time that NF24 attains a slightly higher conversion (∼5%) than NF28 at oxidation conversion levels greater than 80%, while over the conversion range between 0 and 80%, NF28 consistently exhibits much higher conversion than NF24. Given the importance of the 080% conversion range in practical applications, 6,26 overall, NF28 can be considered more reactive than NF24. The time to achieve full oxidation is also reduced through adding NiO to Fe2O3/SiO2 , although it slightly lowers the oxidation rate of the resulting bimetallic mixture. As seen in Figure 9b, the oxidation conversion at a residence time of 50 s is raised from 87 to approximately 92% for NF28 and to about 97% for NF24. The reduction kinetic properties of bimetallic oxygen carriers were determined from the experimental data (e.g., weight loss and conversion) using the method described in one of our earlier studies,21 which is based on the well-known shrinking core model (SCM).36 The spherical grains geometry was considered for all oxygen carrier particles in this work, and as such, the conversion
was expressed by the following equations: t ¼ 1 ð1 Xred Þ1=3 τ τ¼
Fm r bkCn
ð3Þ ð4Þ
where τ is the time require for complete conversion, r denotes the grain radius of the oxygen carrier particles, Fm is the mole density of active metal oxides, b is the stoichiometric factor, k is the reaction rate constant, n is the reaction order, and C is the concentration of carbon monoxide. When the constants Fm, r, b, and k were incorporated into a new parameter K, τ can be rewritten as τ¼
1 KCn
ð5Þ
When eqs 3 and 5 are combined, then the conversion can be described as 1 ð1 Xred Þ1=3 ¼ KCn t
ð6Þ
On the other hand, the reaction rate constant can be expressed by kinetic parameters, such as the activation energy (E) and the preexponential factor (k0), according to an Arrhenius-type rate expression. k ¼ k0 eðE=RTÞ
ð7Þ
Thus, the parameter K introduced in eq 5 can be related to kinetic properties through eq 7 as follows: K ¼
bk0 ðE=RTÞ e Fm r
ð8Þ
Finally, if the term bk0/Fmr is taken as k*, K can be expressed by K ¼ k eðE=RTÞ
ð9Þ
Equations 6 and 9 were used in this work to determine the kinetic parameters associated with the reduction of bimetallic NiFe/ SiO2 metal oxide oxygen carriers. For this purpose, a series of experiments was conducted under a range of reducing environments with different CO volume fractions (i.e., 5, 10, and 70%) in a temperature range from 500 to 950 °C. Because of the anomalies associated with the 950 °C data, the experimental data for this temperature were excluded from the kinetic analysis. The kinetic properties were determined 80
dx.doi.org/10.1021/ef201152u |Energy Fuels 2012, 26, 75–84
Energy & Fuels
ARTICLE
following the procedure outlined in one of our previous papers.21 This procedure is briefly described here for the benefit of the reader. Essentially, three plots need to be developed for determination of the kinetic properties. These are (1) plot of 1 (1 Xred)1/3 versus time t according to eq 6, (2) plot of ln(KCn) versus ln C, and (3) plot of ln K versus 1/T related to eq 9. From the first plot, values of KCn at each temperature and CO volume fraction are determined. These values are then used to draw the second plot, from which the values of ln K and n are calculated (note that, in the second plot, ln K represents the intercept with the vertical axis, while n represents the slope). In the third plot, ln K (from the second plot) is drawn against the inverse of the reaction temperature. The slope of the lines in the third plot would be the activation energy divided by the universal
gas constant, while the intercept would be (ln k*), from which the value of k* can be determined (see eq 9). The kinetic properties determined from experimental data using the above method have been summarized in Table 2. Reduction conversions predicted by the above method together with the actual experimental data are presented in Figures 10 and 11. In both figures, the model predictions are shown by solid lines, whereas experimental data are shown by symbols. Figure 10 summarizes the conversion of monospecies oxygen carriers of Fe2O3/SiO2 and NiO/SiO2 as a function of the residence time, reaction temperature, and CO volume fraction. Figure 11 on the other hand illustrates the conversion of bimetallic NF24 and NF28 samples along with monospecies samples at 875 °C and different CO volume fractions. Evidently, the model predictions agree very well with the experimental data for all oxygen carrier samples. As seen from Figure 10, the predictions fit the reduction results for NiO/SiO2 anywhere from 0 to 80% conversion, although the agreement between the predictions and experimental data worsens at conversion levels beyond 80%. Moreover, reduction conversions under high CO concentrations can be fitted better than their low concentration counterparts, as see from Figure 10b. The same observations can be made about Fe2O3/SiO2 carriers in Figure 10, although the agreement between the predictions and experimental data is limited to conversions below 70% in this case. The agreement between predictions and experimental data for bimetallic NF24 and NF28 is also good, as indicated in Figure 11. However, again, the range of accuracy of the model appears to be between 0 and 80% conversion under reduction half cycles.
Table 2. Summary of Reduction Kinetic Properties of Oxygen Carriers Used in the Reactivity Testa oxygen
activation energy,
carriers
E (kJ/mol)
reaction order, n
k* (moln m3n s1)
NiO/SiO2
24.2
0.85
0.095
NF24 NF28
19.9 19.4
0.911 0.916
0.116 0.119
NF29
18.1
0.934
0.126
Fe2O3/SiO2
18
0.94
0.127
a NF24, NF28, and NF29 referred to 11.36% NiO24.31% Fe2O3/ SiO2, 8.83% NiO28.31% Fe2O3/SiO2, and 0.75% NiO29.1% Fe2O3/SiO2, respectively.
Figure 10. Reduction conversion versus time for Fe2O3/SiO2 and NiO/SiO2: (a) Fe2O3/SiO2, CO = 5 vol %, and T = (9) 500 °C, (1) 600 °C, (b) 700 °C, and (2) 875 °C; (b) NiO/SiO2, T = 875 °C, and CO = (9) 5 vol %, (b) 10 vol %, and (2) 70 vol %. Model predictions are shown as continuous lines (—).
Figure 11. Model prediction compared to experiment results under reduction at a temperature of 875 °C with (a) 5 vol % CO and (b) 10 vol % CO for (9) NiO/SiO2, (2) NF24, (b) NF28, and (1) Fe2O3/SiO2. Model predictions are shown as continuous lines (—). 81
dx.doi.org/10.1021/ef201152u |Energy Fuels 2012, 26, 75–84
Energy & Fuels
ARTICLE
Figure 12. Reduction kinetic parameters versus the weight percentage of Fe2O3: (a) (b) reaction order and (b) (2) activation energy and (O) k*. Model predictions using eqs 1214 are shown as continuous lines (—).
Figure 13. Plots of the reaction rate and ROT against the temperature for ( 3 3 3 ) 75% Fe2O325% NiO/Al2O3 and (- 3 -) 50% Fe2O350% NiO/Al2O3 from our previous work,21 (—) NF28, and (- - -) NF24: (a) reaction rate and (b) ROT.
The model accuracy is slightly improved when the CO concentration is raised from 5 to 10 vol %. Apart from the analyses shown in Figures 10 and 11, as well as those used to determine the kinetic parameters presented in Table 2, a series of additional analyses was carried out to better understand the characteristics of bimetallic NiFe oxides on the silica support. In particular, we were interested in examining the relationship between the properties of bimetallic mixtures and their parent metal oxides. We found in one of our earlier studies21 that the time required for complete reduction of bimetallic oxygen carriers on the Al2O3 support is the weighted geometric average of the reduction time of their parent oxides. This relationship can be mathematically expressed by τ m ¼ τ 1 β1 τ 2 β2
ð13Þ
km ¼ k1 β1 k2 β2
ð14Þ
Although eqs 1214 have been validated against kinetic data for physically mixed bimetallic oxygen carriers of Cu-, Fe-, and Nibased alumina Al2O3,21 we found in this study that the kinetic data predicted from these equations also fit those determined experimentally for FeNi/SiO2 bimetallic oxygen carriers. Figure 12 shows the kinetic parameters from both experiments and predictions, including the reaction order n, activation energy E, and k*, versus the weight percentage of Fe2O3 in the bimetallic mixture. Although k* is derived and shown in this figure rather than k0, it could be regarded as an expanded pre-exponential factor according to eq 9. The strong agreements between experiments and predictions for n, E, and k* are clearly illustrated in this figure, highlighting that, for FeNi/SiO2 bimetallic oxygen carriers, the expanded pre-exponential factor is the weighted geometric average of the expanded pre-exponential factor of iron and nickel oxides and that the activation energy and reaction order are weighted arithmetic averages of the activation energy and reaction order of the two parent metal oxides. The reactivity of bimetallic NiFe-based oxygen carriers on the silica support was further assessed by comparing their reactivity (r0 ) and the rate of oxygen transport (ROT) to those of NiFe-based oxygen carriers on the alumina support. The reactivity was determined from the following relationship for an average CO concentration of 5 vol % and a conversion of X = 30%, which are considered to be typical operating values in
ð10Þ
where τ1, τ2, and τm are the times of full conversion for metal oxide species 1, species 2, and their binary mixture, respectively. Parameters β1 and β2 are also the weight fractions of metal oxides 1 and 2 in the bimetallic oxygen carrier mixture, respectively, and their sum is equal to 1. When eq 10 is combined with eqs 5 and 9, the following expression can be obtained: km eðEm =RTÞ Cnm ¼ k1 β1 k2 β2 eðβ1 E1 þ β2 E2 =RTÞ Cβ1 n1 þ β2 n2 ð11Þ One can then show that nm ¼ β 1 n1 þ β 2 n2
Em ¼ β1 E1 þ β2 E2
ð12Þ 82
dx.doi.org/10.1021/ef201152u |Energy Fuels 2012, 26, 75–84
Energy & Fuels
ARTICLE
relevant CLC systems:6 r0 ¼
dX dt
A comparison of the reactivity and ROT for silica- and aluminasupported bimetallic mixtures revealed that, while silica-supported oxides exhibit higher reactivity, their overall ROT is lower than their alumina-supported counterparts. This impediment must be resolved, perhaps through the development of new preparation methods, before silica-supported oxides can be considered as viable candidates for CLC applications.
ð15Þ
Similarly, the ROT was determined from eq 16 ROT ¼ ROC
dX dt
ð16Þ
’ AUTHOR INFORMATION
where ROC denotes the oxygen transport capacities (ROC) of the oxides defined by mox mred ROC ¼ ð17Þ mox
Corresponding Author
*Telephone: +61 (2) 4985-4411. Fax: +61 (2) 4921-6893. E-mail:
[email protected].
in which mox and mred are the initial and final weights of a given bimetallic oxygen carrier sample, respectively. Figure 13 shows the comparison of reduction reactivity and ROT for alumina-21 and silica- (this study) supported NiFebased metal oxides under 5 vol % CO. The data corresponding to the alumina support were taken from one of our earlier works for bimetallic NiFe oxides with blending ratios of 25:75 and 50:50. The 25:75 NiFe/Al2O3 sample, in particular, is very similar to the NF28 sample, in which the weight ratio of NiO/Fe2O3 is 24:76. As Figure 13a indicates, the reactivity levels of silica-supported bimetallic oxides are generally higher than those of mixed NiFe oxides on the alumina support. This is most likely due to the relatively large pore volume of silica compared to alumina, which enables the redox reactions to proceed at faster rates. In contrast, the ROT for silica-supported bimetallic oxides appears to be much lower than that for alumina-supported bimetallic oxides. The reason is that the oxygen transport capacity, ROC, for silicasupported oxides is far less than that of oxides on the alumina support. This is partly due to the fact that the bimetallic NF24 and NF28 samples studied here were prepared chemically, whereas the alumina-supported bimetallic oxides shown in Figure 13 were prepared by physical mixing. Nevertheless, the lower values of ROT for silica-supported bimetallic oxides imply that much larger solid inventories are required in CLC systems if metal oxides on the silica support are used.
’ ACKNOWLEDGMENT The authors acknowledge the financial support of the University of Newcastle (Australia), Moits Pty Ltd., NSW Clean Coal Council, and the Australian Research Council through the ARC-Linkage Grant LP100200871 (20102012). ’ REFERENCES (1) Energy Information Administration (EIA). International Energy Outlook 2010; U.S. Deparment of Energy: Washington, D.C., 2010. (2) Intergovernmental Panal on Climate Change (IPCC). IPCC Fourth Assessment Report: Climate Change 2007 (AR4); IPCC: Geneva, Switzerland, 2007. (3) Abu-Khader, M. M. Energy Sources, Part A 2006, 28 (14), 1261–1279. (4) Hossain, M. M.; de Lasa, H. I. Chem. Eng. Sci. 2008, 63 (18), 4433–4451. (5) Ishida, M.; Jin, H. Ind. Eng. Chem. Res. 1996, 35 (7), 2469–2472. (6) Abad, A.; García-Labiano, F.; de Diego, L. F.; Gayan, P.; Adanez, J. Energy Fuels 2007, 21 (4), 1843–1853. (7) Zafar, Q.; Mattisson, T.; Gevert, B. Ind. Eng. Chem. Res. 2005, 44 (10), 3485–3496. (8) Zafar, Q.; Mattisson, T.; Gevert, B. Energy Fuels 2006, 20 (1), 34–44. (9) Ishida, M.; Jin, H. G.; Okamoto, T. Energy Fuels 1998, 12 (2), 223–229. (10) Mattisson, T.; Johansson, M.; Lyngfelt, A. Energy Fuels 2004, 18 (3), 628–637. (11) Corbella, B. M.; Palacios, J. M. Fuel 2007, 86 (12), 113–122. (12) Son, S. R.; Kim, S. D. Ind. Eng. Chem. Res. 2006, 45 (8), 2689–2696. (13) Ryden, M.; Cleverstam, E.; Johansson, M.; Lyngfelt, A.; Mattisson, T. AIChE J. 2010, 56 (8), 2211–2220. (14) Ishida, M.; Jin, H. Energy Convers. Manage. 1997, 38, S187–S192. (15) Galvita, V.; Sundmacher, K. J. Mater. Sci. 2007, 42 (22), 9300–9307. (16) Abad, A.; Mattisson, T.; Lyngfelt, A.; Johansson, M. Fuel 2007, 86 (78), 1021–1035. (17) Cho, P.; Mattisson, T.; Lyngfelt, A. Fuel 2004, 83 (9), 1215–1225. (18) García-Labiano, F.; Adanez, J.; de Diego, L. F.; Gayan, P.; Abad, A. Energy Fuels 2006, 20 (1), 26–33. (19) Bohn, C. D.; Cleeton, J. P.; M€uller, C. R.; Davidson, J. F.; Hayhurst, A. N.; Scott, S. A.; Dennis, J. S. AIChE J. 2010, 56 (4), 1016–1029. (20) Adanez, J.; de Diego, L. F.; García-Labiano, F.; Gayan, P.; Abad, A.; Palacios, J. M. Energy Fuels 2004, 18 (2), 371–377. (21) Moghtaderi, B.; Song, H. Energy Fuels 2010, 24 (10), 5359–5368. (22) Moghtaderi, B. Energy Fuels 2010, 24, 190–198. (23) He, F.; Wang, H.; Dai, Y. N. J. Nat. Gas Chem. 2007, 16 (2), 155–161. (24) Johansson, M.; Mattisson, T.; Lyngfelt, A. Ind. Eng. Chem. Res. 2004, 43 (22), 6978–6987. (25) Wang, S.; Wang, G.; Jiang, F.; Luo, M.; Li, H. Energy Environ. Sci. 2010, 3 (9), 1353–1360.
4. CONCLUSION The redox properties of Fe- and Ni-based bimetallic metal oxide supported on SiO2 were determined through thermogravimetric analysis in a Rubotherm magnetic suspension balanced TGA at temperatures from 500 to 950 °C under a range of different reacting environments. A clear decline of the reduction conversion rate was observed in all cases at 950 °C most likely because of the formation of metal silicates. The conversion rate of Fe2O3/SiO2 was found to be generally higher than that of NiO/ SiO2 during both reduction and oxidation half cycles. However, the time for complete oxidation was quite short compared to NiO/SiO2. The properties of bimetallic oxygen carriers were found to be a function of the metal oxide loading content; that is, the higher the metal oxide loading, the higher the reactivity they may inherit from their parent metal oxides. In addition, the reduction process was theoretically analyzed on the basis of a shrinking core model. Kinetic parameters, including the expanded pre-exponential factor, activation energy, and reaction order, were also determined. It was found that the kinetic parameters of monospecies iron oxide and nickel oxide could be used to predict the kinetic parameters for their bimetallic oxygen carrier mixtures. 83
dx.doi.org/10.1021/ef201152u |Energy Fuels 2012, 26, 75–84
Energy & Fuels
ARTICLE
(26) Abad, A.; Adanez, J.; García-Labiano, F.; de Diego, L. F.; Gayan, P.; Celaya, J. Chem. Eng. Sci. 2007, 62 (12), 533–549. (27) Shen, L. H.; Wu, J. H.; Gao, Z. P.; Xiao, J. Combust. Flame 2009, 156 (7), 1377–1385. (28) Mattisson, T.; Johansson, M.; Lyngfelt, A. Fuel 2006, 85 (56), 736–747. (29) Bolhar-Nordenkampf, J.; Pr€oll, T.; Kolbitsch, P.; Hofbauer, H. Energy Procedia 2009, 1 (1), 19–25. (30) Linderholm, C.; Abad, A.; Mattisson, T.; Lynyfelt, A. Int. J. Greenhouse Gas Control 2008, 2 (4), 520–530. (31) Jin, H.; Okamoto, T.; Ishida, M. Energy Fuels 1998, 12 (6), 1272–1277. (32) Hossain, M. M.; de Lasa, H. I. AIChE J. 2007, 53 (7), 1817–1829. (33) Adanez, J.; García-Labiano, F.; de Diego, L. F.; Gayan, P.; Celaya, J.; Abad, A. Ind. Eng. Chem. Res. 2006, 45 (8), 2617–2625. (34) Park, J.-N.; Zhang, P.; Hu, Y.-S.; McFarland, E. W. Nanotechnology 2010, 21 (22), No. 225708. (35) Mackwell, S. J. Phys. Chem. Miner. 1992, 19 (4), 220–228. (36) Levenspiel, O. Chemical Reaction Engineering; John Wiley and Sons, Inc.: New York, 1981.
84
dx.doi.org/10.1021/ef201152u |Energy Fuels 2012, 26, 75–84
