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Ind. Eng. Chem. Res. 2006, 45, 968-977
Defluidization Conditions for a Fluidized Bed of Iron Oxide-, Nickel Oxide-, and Manganese Oxide-Containing Oxygen Carriers for Chemical-Looping Combustion Paul Cho,† Tobias Mattisson,*,‡ and Anders Lyngfelt‡ Departments of EnVironmental Inorganic Chemistry and Energy and EnVironment, Chalmers UniVersity of Technology, S-412 96 Go¨teborg, Sweden
For combustion with CO2 capture, chemical-looping combustion with inherent separation of CO2 is a promising technology. Chemical-looping combustion uses oxygen carriers that are composed of metal oxide to transfer oxygen from the combustion air to the fuel. The defluidization of oxygen-carrier particles was investigated to improve the understanding of when particle agglomeration may occur. The study was made in a laboratory fluidized-bed reactor at 950 °C, simulating a chemical-looping combustion system by exposing the sample to reducing and oxidizing conditions in an alternating manner. The oxygen-carrier particles used were based on oxides of iron, nickel, and manganese and produced by freeze granulation. For iron oxide particles, there was no defluidization of the bed when the content of available oxygen in the particle was high. The defluidization occurred during the oxidation period after long reduction periods, in which a significant reduction of the magnetite to wustite occurred. This is an important observation, because the reduction to wustite is not expected in chemical-looping combustion with high fuel conversion. Thus, laboratory experiments with iron oxide performed with long reduction times may give an unduly exaggerated impression of the risks of agglomeration. For nickel oxide, the defluidization was dependent on the sintering temperature with no defluidization in experiments conducted with particles sintered at 1300 and 1400 °C. The nickel oxide particles that were sintered at 1500 °C only defluidized once in a total of 49 cycles, whereas the particles that were sintered at 1600 °C defluidized already in the first cycle. For the nickel oxide particles, it was not possible to see any effect of the length of the reducing period on the defluidization. There was no defluidization of the manganese oxide particles. The defluidization of the bed leads to agglomeration for the iron oxide particles, but not for the particles of nickel oxide, where the bed was still loosely packed. Carbon was formed on the particles based on nickel oxide and manganese oxide. 1. Introduction The concentration of CO2 in the atmosphere has risen to a value of ∼370 ppm today, from the preindustrial value of 280 ppm, and it is generally accepted that a reduction in emissions of greenhouse gases is necessary. This could be achieved by increasing the use of renewable energy sources, such as biofuels and wind power, as well as by increasing the efficiency of energy conversion and the use of energy. However, in the short term, fossil fuels are still the dominating energy source worldwide. An additional option for achieving reduced emissions is the capture and geological storage of CO2 released from the combustion of fossil fuels. Many known techniques can be used to perform this separation, but a major disadvantage with most of these techniques is the large amount of energy that is required to obtain the CO2 in pure form, which means that the efficiency of power plants decreases by 7-12 percentage units.1 Furthermore, the costs for this separation are significant. It is thus important to find better methods for the capture of CO2 from combustion. In chemical-looping combustion, a metal oxide is used as an oxygen carrier, which transfers oxygen from the combustion air to the fuel.2 The main advantage with chemical-looping combustion, compared to normal combustion, is that CO2 is inherently separated from the other flue gas * To whom correspondence should be addressed. Tel.: +46(0)317721425. E-mail:
[email protected]. † Department of Environmental Inorganic Chemistry. ‡ Department of Energy and Environment.
Figure 1. Schematic diagram of chemical-looping combustion.
components (i.e., N2 and unused O2), and, thus, costly equipment and efficiency losses for the separation of CO2 are avoided.3,4 1.1. The Chemical-Looping Combustion System. The chemical-looping combustion (CLC) system is composed of an air reactor and a fuel reactor, as depicted in Figure 1. Lyngfelt et al. proposed a design where these reactors were interconnected fluidized beds.5 The gaseous fuel is introduced to the fuel reactor, which contains an oxygen carrier, which is reduced according to
(2n + m)MexOy + CnH2m f (2n + m)MexOy-1 + nCO2 + mH2O (1) where MexOy is the fully oxidized oxygen carrier and MexOy-1 is the reduced oxygen carrier. The flue gas stream leaving the fuel reactor contains only CO2 and H2O if full oxidation of the fuel has been achieved, which means that pure CO2 can be obtained by cooling the gas and removing the water condensate. The reduced oxygen carrier, MexOy-1, is then transported to
10.1021/ie050484d CCC: $33.50 © 2006 American Chemical Society Published on Web 12/24/2005
Ind. Eng. Chem. Res., Vol. 45, No. 3, 2006 969 Table 1. Oxygen Carriers Active Metal Oxide oxygen carrier
Support
Sintering Conditions
material
mass fraction (%)
F6A
Fe2O3a
60
Al2O3b
40
N6ANYc
NiOa
60
NiAl2O4
40
M6Z
Mg3O4
40
Mg-ZrO2d
60
a
material
mass fraction (%)
heating rate schedule 2 °C/min to 200 °C, 1 °C/min to 450 °C, 10 °C/min to 1300 °C 2 °C/min to 200 °C, 1 °C/min to 450 °C, 10 °C/min to Yc 2 °C/min to 1150 °C
temperature (°C)
time (h)
1300
4
Yc
4
1150
6
From Johnson Matthey Gmbh. From Alcoa. Y ) 1300, 1400, 1500, or 1600 °C. Magnesium-stabilized ZrO2 (Mandoval MSZ-8). b
c
d
the air reactor and oxidized by oxygen in the air, according to
2MexOy-1 + O2 f 2MexOy
(2)
The flue gas stream leaving the air reactor contains N2 and some remaining unused O2. The oxidized oxygen carrier, MexOy, is then transported back to the fuel reactor for another cycle. As a result, CO2 can be inherently separated in this combustion process, thus potentially avoiding costs and energy consumption associated with a gas separation process. The total amount of heat released in reactions 1 and 2 is equal to the heat released from normal combustion. The process has been successfully demonstrated in a 10 kW unit.6,7 1.2. Defluidization. Fluidized beds that are operated at high temperature are frequently used in industry as very efficient chemical reactors. One difficulty that may occur in some cases is the formation of particle agglomerates, which may also lead to defluidization of the bed. Defluidization phenomena can be caused by various mechanisms. Reichhold et al.8 have described that the tendency to defluidize increases with temperature, and with decreasing softening temperature. The tested bed material was converter dust and fluid catalytic cracking powder. Defluidization becomes less likely with increasing gas velocity, and mean particle diameter of the bed material. Hayashi and Iguchi reported defluidization of bed during the reduction of different iron ores with N2-H2 as the reduction gas at 900 °C. They found that iron ore with a low content of Al2O3 agglomerated when metallic iron was formed. Here, the surface contained short iron whiskers and/or porous iron. Iron ores that were rich in Al2O3 showed no agglomeration, despite the presence of metallic iron on the surface.9 The reduction of iron ore in fludized beds has been studied extensively. However, this process reduces Fe2O3 to elemental iron, whereas, in the case of CLC, the hematite is reduced to Fe3O4. Furthermore, CLC also involves the subsequent oxidation of the Fe3O4. Thus, experimental results from iron ore reduction are not directly applicable for CLC. 1.3. Objective. In CLC with fluidized beds, a continuous circulation of oxygen-carrier particles is necessary to transport the oxygen from the air to the fuel. Thus, it is important to avoid agglomeration of the bed particles, which could lead to technical difficulties. Mattisson et al.10,11 reported the tendency of agglomeration with iron-based oxygen carriers and nickel oxide with titanium oxide as the support. Lyngfelt and Thunman,7 and Johansson et al.12 reported the tendency of agglomeration with iron-based oxygen carriers. In previous works, the authors have reported the agglomeration tendencies of some of the iron oxide-based oxygen carriers13 and showed that measurement of the pressure drop over the reactor gives good indication whether the particles bed was fluidized or not.14 The latter work also indicated that studies of defluidization could be useful to obtain a better understanding of factors that influence agglomeration. Moreover, the work also suggested
that there could be a connection between agglomeration and the length of the reducing period, i.e., the change in conversion over a cycle. A possible consequence would be that the relevance of laboratory experiments for judging risks of agglomeration may be strongly dependent on the experimental conditions. The aim of the present work is to assess if the change in conversion of the oxygen carrier has any effect on the tendency to defluidize. Particles based on the oxides of iron, nickel, and manganese were selected for the study. The support material for iron oxide and nickel oxide was Al2O3, and for manganese oxide, it was magnesia-stabilized ZrO2 (Mg-ZrO2). These combinations of active and inert materials have been shown to be good, with respect to parameters that are important for CLC, such as reactivity under many cycles of reducing and oxidizing conditions and strength. 2. Experimental Section 2.1. Oxygen Carrier Preparation. Oxygen-carrier particles based on the oxides of iron, nickel, and manganese were prepared using freeze granulation (see Table 1). In this process, a commercial metal oxide powder and an inert material (i.e., Al2O3 or Mg-ZrO2) were mixed with distilled water and a small amount of dispersion agent. The mixture was then ground in a ball mill and the resulting slurry was converted to frozen spherical particles by spraying the slurry through a nozzle into liquid nitrogen. The water in the frozen particles was then removed by freeze drying. The particles were then heated to remove organic material and sintered at the desired temperature for 4-6 h. The heating rate, sintering temperature, and time of sintering are presented in Table 1. The oxygen carriers were then sieved to a size range of 125-180 µm. The mass ratio of Fe2O3/Al2O3 was 6/4 and of Mn3O4/Mg-ZrO2 was 4/6. Metal oxides with an oxidation number of +2 for the metal, such as NiO and CuO, form metal aluminum spinel compounds (metal aluminates, MeAl2O4, where Me ) Ni, Cu) via solid-state reaction with Al2O3. Because these compounds are inert or react slowly with methane or oxygen, the active metal oxide was added in excess to obtain a NiO/NiAl2O4 mass ratio of 6/4, i.e., a mass fraction of 60% “free” NiO. The nomenclature used to identify the particles includes information about the active metal oxide, the mass fraction of active metal oxide, and the inert material; for example, for F6A, “F” denotes the iron oxide (Fe2O3), “6” represents the mass fraction (60%), and “A” indicates that the inert material is Al2O3. See Table 1 for the denomination of the oxygen-carrier particles. 2.2. Experimental Setup and Procedure. The experimental setup is shown in Figure 2. Two smaller types of batch fluidizedbed reactors were used in the experiments. Although it is not possible to simulate the conditions of a real CLC system exactly, these reactors provide a means of testing a smaller amount of oxygen carriers under well-defined conditions. The Type IV quartz reactor had a length of 820 mm, with a porous quartz
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Ind. Eng. Chem. Res., Vol. 45, No. 3, 2006
Figure 2. Sketch showing the experimental setup.
plate 22 mm in diameter that was placed 400 mm from the bottom of the quartz reactor. The inner diameter of the bottom section was 10 mm, and the top section was 22 mm. The Type V quartz reactor also had a length of 820 mm, but it had a porous quartz plate 10 mm in diameter that was placed 400 mm from the bottom of the reactor. The section above the porous plate was conically formed, and the diameter increases from the quartz plate and 20 mm above the plate, where the full diameter (30 mm) is attained. Thus, for the same volumetric inlet flow of gas, the gas velocity in the bottom part will be much higher for the Type V reactor, because of the smaller diameter of the quartz plate. In this work, the fluidization velocity, with respect to the minimum fluidization velocity (u/umf), was 3.0-5.4 for the reducing period and 9.8-17.2 for the oxidizing period using the Type IV reactor. For the conically shaped reactor (Type V), the u/umf value was 14.7-25.9 in the narrow bottom section for the reducing period and 47.3-83.3 for the oxidizing period. Note that, because of volume expansion during the reducing period, the velocities in the bed are actually higher. The temperature of the gas entering the bed was measured 5 mm below the porous quartz plate using a 10% Pt/Rh thermocouple that was enclosed in a quartz shell. The temperature of the fluidized bed was measured 25 mm above the porous quarts plate with the same type of thermocouple for both reactors. The amount of oxygen carrier used in experiments was 15 g, and the size of the oxygen-carrier particles was 0.125-0.180 mm. In this work, all the experiments with oxygen carriers have been conducted at a temperature of 950 °C. The particles were preheated in an inert atmosphere (N2, 900 mL/min at 0 °C and 1 bar) to the desired experimental temperature and then exposed to 5% O2 in N2 to ensure that the particles were fully oxidized before the reduction period. The flow of the oxygen-containing gas was 1000 mL/min. The reason for using a low concentration of oxygen (i.e., 5%) is to avoid large temperature increases during the highly exothermic oxidation reaction. In the experimental setup that has been applied, it is not possible to cool the reactor, as would be the case of a real CLC air reactor. After all periods of oxidation and reduction, inert gas (N2) was introduced for 200 s to avoid air and methane mixing during the shift between the reduction and oxidation periods. The
particles were then exposed to 100% CH4. The flow of the CH4 was 450 mL/min. The reduction period varied between 30 s and 400 s. After a subsequent period of inert gas, a new cycle was started again by exposing the oxygen carriers to 5% O2 in N2. The number of the oxidation-inert-reduction-inert cycles varied over a range of 6-40. The fluidization regimes for all particles under both oxidation and reduction should fall within the fluidization of Geldart Type B particles. The gas flows were led through mass flow controllers (Brooks, model 5850E) to preprogrammed magnetic three-way valves, which supplied either of the gases CH4, N2, or O2/N2 to the reactor while the remaining two gases were led to a fumehood. The flue gas from the reactor was led to an electric cooler (M&C Products, model ECP 1000) where the condensed water was removed, and then led to a gas analyzer (Rosemount model NGA-2000) where the concentrations of CO2, CO, CH4, and O2 were measured. The high-purity gases were delivered by AGA Gas. The pressure difference between the inlet and outlet gases from the reactor was measured at 20 Hz with Honeywell pressure transducers. Information about the number of cycles, as well as the length of the reducing period for the tests, is shown in Table 2. 2.3. Data Evaluation. The gas yield (γ) of methane (CH4) to carbon dioxide (CO2) was defined as
γ)
pCO2,out pCH4,out + pCO2,out + pCO,out
(3)
The conversion of oxygen carrier, or the degree of oxidation, was defined as
X)
m - mred mox - mred
(4)
The reduced form could be either the metal or a metal oxide with a lower oxidation number than the oxidation number of the fully oxidized metal oxide. Thus, the difference between mox and mred in eq 4 is the maximum theoretical amount of oxygen in the oxygen carrier that can be removed through oxidation of the fuel (here, methane). The conversion of the oxygen carrier (X), as a function of time during the reduction period, can be calculated from the outlet gas concentrations and the molar gas flow after the condensation of water, n˘ out:
X)1-
∫t t
1
0
n˘ out (4p + 3pCO,out - pH2,out) dt (5) nOptot CO2,out
Here, it is assumed that the oxygen carrier is fully oxidized after each oxidation cycle (i.e., X ) 1). The outgoing gas flow was measured in connection with the gas analysis and was confirmed by calculations using the inlet flow, together with a mass balance of gaseous species over the reactor. The partial pressure of H2 was estimated assuming an equilibrium of H2, H2O, CO, and CO2 in the shift reaction described in reaction 6,
CO + H2O f CO2 + H2
(6)
using the relation described in eq 7:
pH2,out )
2KpCO,out(pCO2,out + pCO,out) pCO2,out + KpCO,out
(7)
where K, which is the equilibrium constant for reaction 6, is
Ind. Eng. Chem. Res., Vol. 45, No. 3, 2006 971 Table 2. Length of Reduction Periods for Different Cases case
total number of cycles, Ntot
F6A F6A F6A F6A F6A
FA FB FC FD FEa
14 19 31 40 15
40, 2 × 50, 54, 58, 62, 66, 70, 74, 78, 82, 86, 86, 90, 94 3 × 50, 60, 15 × 70 20 × 40, 50, 5 × 60, 70, 80, 90, 100, 110 40 × 40 3 × 40, 4 × 50, 70, 80, 90, 100, 110, 150, 200, 300
N6AN1300 N6AN1400 N6AN1500 N6AN1500 N6AN1500 N6AN1500 N6AN1600 N6AN1600
NA NB NC NF NG NH ND NEa
12 12 8 8 24 9 1 1
60, 6 × 70, 100, 120, 2 × 160, 400 60, 6 × 70, 100, 120, 140, 160, 400 60, 6 × 70, 100 5 × 50, 100, 200, 400 2 × 40, 20 × 50, 100, 150 2 × 50, 60, 70, 80, 100, 150, 300, 400 60 30
M4Z
MA
20
40, 6 × 50, 60, 70, 80, 90, 100, 120, 140, 160, 200, 300, 400, 2 × 50
oxygen-carrier particle
a
reduction period, tred (s)
With Type V reactor.
equal to 0.67 at a temperature of 950 °C. The conversion of oxygen carrier, X, as a function of time during the oxidation period, was defined as
Table 3. Oxygen Ratio (RO) for Pure Iron-, Nickel-, and Manganese-Oxide Systems and for The Oxygen Carriers Used in This Work MexOy/MexOy-1
X ) X0 +
∫t
t4
2
2 (n˘ p - n˘ outpO2,out) dt nOptot in O2,in
(8)
where X0 is the conversion at the start of the oxidation period, t2 the time when the oxidation started, and t4 the point in time when the oxidation period ended. Because different oxygen carriers can transfer different amounts of oxygen per mass unit, it is an advantage to be able to compare them using a massbased conversion. In this work, a mass conversion was defined as
ω)
m ) 1 - RO(1 - X) mox
a
Fe2O3/Fe3O4 NiO/Ni Mn3O4/MnO
RO
oxygen carrier
RO
0.03 0.21 0.07
F6A N6AN M4Z
0.02 0.16 0.028
a R value for Fe O /Fe is 0.30 and the corresponding value for the iron O 2 3 oxide-based oxygen carrier is 0.18.
(9)
where RO is the oxygen ratio and is defined as
RO )
(mox - mred) mox
(10)
The oxygen ratio shows the maximum mass flow of oxygen that can be transferred between the air and fuel reactor in chemical-looping combustion for a given mass flow of circulating oxygen-carrier particles. The RO values for both pure iron oxide and nickel oxide systems and for the oxygen carriers used in this work are shown in Table 3. The carbon formation during the reduction period is calculated by integrating the total amounts of CO2 and CO produced during the subsequent inert and oxidation period. The carbon formation ratio (C/Ctot) is the amount of carbon formed during the reduction period, relative to the total amount of carbon introduced during the reduction period; this parameter is defined as
∫t t n˘ out(pCO ,out + pCO,out) dt 3
C ) Ctot
2
1
∫t n˘ out(pCH ,out + pCO ,out + pCO,out) dt t3
0
4
(11)
2
where t0 is the time when the reduction started, t1 the point in time when the inert period started, and t3 a point of time in the oxidation period when no more CO2 or CO was registered. 2.4. Oxygen Carrier Characterization. The oxygen carriers were characterized before and after the experiments using powder X-ray diffraction (XRD) (Siemens, model D5000
Figure 3. Concentrations of CO2, CO, CH4, and O2 as a function of time for F6A particles during (a and b) a short cycle and (c and d) a long cycle (reduction-inert-oxidation-inert). Time periods are shown in Table 4. The ticks between the upper and lower diagram show when the three-way valve switches to the different gas streams: reducing period (R), oxidation period (O), and inert period (I).
powder diffractometer, utilizing Cu KR radiation). The force needed to crush the oxygen carriers (crushing strength) was measured with a digital force gauge (Shimpo, model FGN-5). The density of oxygen carrier particles was calculated assuming that the void was 0.37, corresponding to particles of spherical form and a loosely packed bed. 3. Results Outlet gas concentrations of CO2, CO, CH4, and O2 are shown as a function of time for the reduction and oxidation of F6A oxygen-carrier particles in Figure 3, for N6AN1400 oxygencarrier particles in Figure 4, and for M4Z oxygen-carrier particles in Figure 5. The upper graphs (panels a and b) in
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Ind. Eng. Chem. Res., Vol. 45, No. 3, 2006 Table 5. Physical Properties of Investigated Oxygen Carriers Crushing Strength (N)
Figure 4. Concentrations of CO2, CO, CH4, and O2 as a function of time for N6AN1400 particles during (a and b) short cycles and (c and d) long reduction-oxidation cycles (reduction-inert-oxidation-inert).
Figure 5. Concentrations of CO2, CO, CH4, and O2 as a function of time for M4Z particles during (a and b) a short reduction-oxidation cycle and (c and d) a long reduction-oxidation cycle (reduction-inert-oxidationinert). Table 4. Length of Reduction (R) and Oxidation (O) Periods in Figures 3-5
Figure 3 Figure 4 Figure 5
panel a (R)
panel b (O)
panel c (R)
panel d (O)
40 s 70 s 50 s
700 s 1000 s 1400 s
100 s 160 s 400 s
1000 s 2500 s 1600 s
Figures 3-5 show a short reduction period and the subsequent oxidation period, whereas the lower graphs (panels c and d) show a long reduction period and subsequent oxidation period. The length of the reduction and oxidation periods in Figures 3-5 are shown in Table 4. The length of the inert periods between the reduction and oxidation periods is 200 s. Because of the residence time between the three-way valve and the gas analyzer, the outlet gas signal is delayed 15-20 s, depending on the flow rate of the gas. Generally, as the length of the reduction period is increased, the length of the subsequent oxidation period is increased, because more of the oxygen is removed from the particles during the reduction.
oxygen carrier
density (g/mL)
nominal value
standard deviation
F6A M4Z N6AN1300 N6AN1400 N6AN1500 N6AN1600
3.2 2.1 2.1 2.4 3.1 3.7
5.6 0.7 0.4 0.7 1.5 2.7
1.6 0.2 0.1 0.2 0.2 0.5
In the beginning of the reduction periods in Figures 3-5, a large portion of the CH4 is converted to CO2 and H2O for ironand manganese-based oxygen carriers, and for nickel oxidebased oxygen carriers, almost full conversion of CH4 was achieved. As the reactions proceed, the oxygen in the particles is depleted, and the outlet concentration of CO2 decreases, whereas the concentrations of CO and CH4 increase. In all cases shown in Figures 3-5, the outlet concentration of oxygen was zero during a large portion of the oxidation period, followed by a rapid increase to the inlet concentration. This indicates that all of the O2 going into the reactor was rapidly consumed and the rate of the oxidation was limited by the supply of O2 to the oxygen-carrier particles. The lengths of the oxidation periods were chosen to ensure that full oxidation of the oxygen-carrier particles was attained. 3.1. Physical Characteristics of Oxygen Carrier. The density and crushing strength of the oxygen-carrier particles are presented in Table 5. The strength is given as a mean value of 20 measurements of crushing strength, i.e., the force needed to crush a particle, where the size interval of the particles measured was 180-250 µm. The mean value of the crushing strength of the oxygen carriers varied over a range of 0.4-5.6 N and the standard deviation was 0.2-1.6 N. For comparison, the crushing strength of quartz sand particles is 9.2 N. The crushing strength of the nickel oxidebased oxygen carriers was correlated to the density and the sintering temperature. 3.2. Carbon Formation. For the N6AN1400 oxygen carrier, there was carbon formation during the reducing period, as observed by the subsequent slow decrease of CO concentration in the inert period and peaks of CO and CO2 in the beginning of the oxidation period (see Figure 4c and d). The C/Ctot value (eq 11), in this case, was 24%. In contrast, for the F6A oxygen carrier in Figure 3a and b, almost no carbon was formed during the reduction period, as observed from the low CO and CO2 concentrations throughout the inert and oxidation periods. As shown in a previous paper,14 carbon is generally formed when the availability of oxygen in the oxygen carrier is low, as well as the gas yield to CO2. The C/Ctot values for a long period of reduction for the iron oxide-based particle (see Figure 3c and d), and for a short period of reduction for a nickel oxide-based particle (see Figure 4a and b) were both 1%. The C/Ctot value for M4Z was 2% for the short reduction period (see Figure 5a and b) and 1% for the long reduction period (see Figure 5c and d). For the long period, the formation of carbon was significantly larger, but the ratio was, nevertheless, lower, because eight times more fuel was added. During the 20 cycles with varying length of reduction for the M4Z particle (case MA), there was limited formation of carbon (typically 1%-3.5%), depending on the length of the reduction period, except for one short period of reduction (40 s) and the last two reduction periods (50 s), when there was no carbon formation. There is a distinct difference in the carbon oxidation behavior of nickel, in comparison to that of iron and manganese. Carbon
Ind. Eng. Chem. Res., Vol. 45, No. 3, 2006 973
Figure 6. Pressure drop over the reactor in (a) oxidation to the following oxidation period in case FC when the beds is fluidized, (b) oxidation to the following oxidation period in case FC when the bed is defluidized, and (c) oxidation to the following oxidation period in case NC when the bed is defluidized. Panel d shows an enlargement of parts of oxidation periods from panel b when the bed is fluidized and defluidized.
is oxidized by lattice oxygen for nickel during the inert period (see Figure 4c), whereas the oxidation of carbon for manganese and iron only occurs in the oxidation period (see Figures 3 and 5).14 A similar type of behavior has also been observed for nickel, manganese and iron supported on SiO2.15 3.3. Defluidization of the Bed. In this work, pressure drop fluctuations were used to indicate when the bed defluidized. Examples of measurements of the pressure drop at 20 Hz over the reactor are shown in Figure 6. The total pressure drop is a combination of the pressure drop over the quarz plate as well as the pressure drop necessary to fluidize the particles. As can be seen in the figure, a fluidized bed is characterized by high pressure fluctuations (typically 0.1 kPa or higher). On the other hand, a defluidized bed shows almost no fluctuations in the pressure drop (see Figure 6d). Figure 6a shows the pressure drop over the reactor during oxidation period 28, inert, reduction period 28, inert, and oxidation period 29 for the F6A oxygen carrier (case FC). Here, no indication of defluidization is observed. The large difference in the average pressure drop of the oxidizing, inert, and reducing periods is mainly due to the different gas flows used (e.g., 450 mL/min during reduction and 1000 mL/min during oxidation, which means that the pressure drop over the quartz plate will be higher during the oxidation). In contrast, a typical defluidization is shown in Figure 6b, showing the pressure difference from oxidation period 31 to oxidation period 32 for case FC. Here, the bed is defluidized early in oxidation period 32. The small decrease in the pressure drop that was observed when the bed defluidized could be explained by channel formation in the defluidized bed.
The pressure fluctuations are shown in more detail in Figure 6d (for a fluidized bed, oxidation period number 28; for a defluidized bed, oxidation period 32). Figure 6c shows when the bed is defluidized at the end of reduction period 8 for the N6AN1500 oxygen carrier (case NC). All the cases of defluidization of the bed with nickel oxidebased particles occurred at the end of the reduction period or the beginning of the subsequent inert period (cases NC, ND, and NE). In contrast, for the iron oxide-based particles, defluidization occurred in the beginning of the oxidation period (cases FA, FB, FC), with the exception of case FE, where the bed defluidized during the end of the reduction. There was no defluidization of the bed for case FD, where the length of reduction was kept at 40 s for 40 reduction-oxidation cycles. A major difference between the defluidized beds obtained for iron oxide- and nickel oxide-based particles was that the iron oxide-based particles formed a rather hard agglomeration, whereas the nickel oxide-based particles were still loosely packed. In the latter case, the bed could be poured out of the reactor after the test. 3.3.1. Iron Oxide Particles. The mass conversions after the reduction (ω) and the length of the reduction period (tred) are shown versus the cycle number in Figure 7a and b, respectively, for the F6A oxygen-carrier particle (cases FA-FE). In cases FA-FC, the length of the reduction period was gradually increased until the bed was defluidized. In almost all cycles, the change in mass conversion, ∆ω, was 2% or more, corresponding to ω values of
