Article pubs.acs.org/EF
Spinel Ni(Al,Fe)2O4 Solid Solution as an Oxygen Carrier for Chemical Looping Combustion Seugran Yang,† Kyeongsook Kim,*,† Jeom-In Baek,† Ji-Woong Kim,† Joong Beom Lee,† Chong Kul Ryu,† and Gaeho Lee*,‡ †
Technology Commercialization Office, Korea Electric Power Corporation (KEPCO) Research Institute, 65 Munji-Ro, Yuseong-gu, Daejeon 305-760, Republic of Korea ‡ Department of Chemistry, Chungnam National University, 99 Daehak-Ro, Yuseong-gu, Daejeon 305-764, Republic of Korea ABSTRACT: Our research staff has been able to perform long-term operation of a 200 kW gas-circulating combustion system successfully for over 100 h with NiO/Al2O3 oxygen carrier particles that were produced in large quantities using a pilot-plant spray dryer. After such a result, we are planning to scale-up the process to a 1−10 MW system. Iron oxide, which is relatively cheaper than expensive NiO, has been considered for this step, and nine different types of NiO/Fe2O3/Al2O3 mixed metal oxides were produced by adding iron oxide to NiO with a 22.5−47.5 wt % ratio, adding 30 wt % Al2O3 as support, and finally, forming material using the spray-drying method. Results from both physical property tests [tapping density, particle size analysis, attrition index (AI), porosity analysis, and Brunauer−Emmett−Teller (BET) surface area] and chemical property tests [scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA), and X-ray diffraction (XRD)] suggest that a mixed metal oxide of 22.5 wt % NiO, 47.5 wt % Fe2O3, and 30 wt % Al2O3 calcined at 1100 °C is suitable for the fluidized-bed process with excellent oxygen-transfer capability. Oxygen-transfer capability was higher than the theoretical value of N1F2 particles, and it can be explained as spinel Ni(Al,Fe)2O4 solid solution through XRD analysis. It is believed that mixed metal oxides produced are good candidates for media-circulating combustion of the fluidized-bed process for the large capacity of a 1−10 MW system for the future. Because oxygen-transfer capability of the mixed metal oxides was maintained without degradation after multiple cycles and despite the oxidation rate being relatively slow, the reduction rate was very fast.
1. INTRODUCTION The concerns over climate change as a result of carbon dioxide (CO2) emitted from the combustion of fossil fuels have been growing for decades. There is CO2 capture and storage (CCS) technology that can reduce CO2 emissions efficiently, but the disadvantages of this technology are low power generation efficiency and high CO2 capture costs. Chemical looping combustion (CLC, hereafter) is a technology that is drawing significant attention as a next-generation combustion and power generation technology because it shows the promise of overcoming those shortcomings. An oxygen carrier (OC, hereafter) is a medium that carries oxygen between an oxidation reactor and a reduction reactor in CLC. Thus, the development of the OC with superior performance is considered as a key success factor of CLC, because the overall performance of CLC relies quite heavily upon the oxygen-transfer capacity (OTC, hereafter) of OC. The requirements of the OC include superior reactivity with fuel and oxygen and a high OTC that is sufficient for the perfect combustion of fuel to CO2 and H2O. Also, it should have high mechanical strength and thermal stability that can withstand the fluidized-bed process. Its costs should be low, and it should be ecofriendly. There are several candidate OCs that meet these requirements, including NiO/Ni,1−9,12,21 Cu2O/ Cu, 5,10−12,16,21 Mn 3 O 4 /MnO, 5,11−13,16,19,21 and Fe 2 O 3 / Fe3O4,5,12,14−22 and more than 100 OC particles have been reported. However, since 2008, carriers that are based on NiO as the active substance and Al2O3 as the support substance have been seemingly a mainstream in CLC. Thus, NiO/Al2O3 has © 2012 American Chemical Society
demonstrated the best reactivity based on research results. However, NiO/Al2O3 presents several disadvantages, such as its tendency to produce NiAl2O4. NiAl2O4 does not completely reduce CH4 or H2. Although this problem can be improved using NiAl2O4 instead of Al2O3 as a support because NiAl2O4 is an expensive substance, it reduces the economic efficiency of CLC, which is one of the advantages of the CLC process among various carbon-capturing technologies.2−4,7,9 Some papers also reported toxicity of NiO.3,18,19 Lots of researchers are trying to scale-up the gas-circulating combustion system based on the success of the small-scale system. Cheaper oxides need to be developed because about 100 tons of OC particles are required for a 300 MW unit and the lifetime of OC particles is less than 1 year. Iron oxide, because it is known as the most widely available, cheapest substance, is one of the most attractive candidates for commercialization of CLC and, being a thermodynamically superior substance, has received major attention for a long time.14,18 Mattisson reported iron oxide as an OC for CLC in 2001. Natural hematite and iron ore were selected because they are relatively cheaper and environmentally safer than Ni or Co. It was reported that the reduction process of hematite with CH4 at 850 °C was processed in a phase order of Fe2O3−Fe3O4− FeO−Fe and only α-Fe2O3, a stable form of iron oxide, remains at the end of the reduction process. Although the cost of Received: April 27, 2012 Revised: June 22, 2012 Published: June 22, 2012 4617
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pressure), and the analysis conditions are as shown in Table 2. The crystal structure of OC was analyzed with XRD (Rigaku, D/Max).
natural hematite is low and it can be used as OC immediately, breakage of particles caused by the chemical reaction was observed from image analysis of particles that reacted. It was proposed to produce synthetic iron oxide by adding a matrix substance, such as Al2O3, to avoid particle fracturing in fast speed consecutive bed exchange.23 The Ishida group also observed that, after calcinating Fe2O3/ Al2O3 mixed particles consisting of 25 wt % Fe2O3 at 800− 1370 °C for 10 h, it was observed through X-ray diffraction (XRD) analysis that only solid solution for hematite was created at 1200 °C and solid solution for both hematite and corundum was created at 1320 °C.20 Son et al. produced single or mixed OC particles through a direct mixing method using NiO and Fe2O3 oxides of less than 10 μm with bentonite, TiO2, and Al2O3 as supports and conducted experiments using a circulating fluidized-bed reactor with a double-loop structure.24 The ratio of mixed metal oxide/ support substance was 3:2, while the ratio of NiO and Fe2O3 was adjusted to produce three different types of OC particles. The mixed OC particles of NiO/Fe2O3/bentonite had superior reactivity and a crystalline structure of NiFe2O4, NiO, and Fe2O3.24 Previously, we operated a 200 kW gas-circulating combustion system for more than 100 h using NiO/Al2O3 as OC particles. The OC particles were produced in large quantities using a pilot-plant spray dryer this year. For the next step, we are planning to scale-up the process to a 1−10 MW system. As preliminary work to scale-up, we are currently investigating the optimal conditions for mass production of NiO/Fe2O3/Al2O3 mixed OC particles using the spray-drying method. In this work, nine samples of the NiO/Fe2O3/Al2O3 OC for CLC were prepared by the spray-drying method. The effects of composition and calcination temperature used in its preparation were investigated through both OTC tests and physical property measurements. On the basis of these properties, the most promising NiO/Fe2O3/Al2O3 OC was selected.
Table 2. TGA Conditions items
Table 1. Type, Composition, and Calcination Temperature of OCs composition (wt %)
mixed metal oxide single metal oxide
N2F1 N1F1 N1F2 F70 N70
47.5 35 22.5
Fe2O3 Al2O3 22.5 35 47.5 70
30
950 5 CH4 (10 vol %) + CO2 (90 vol %) air (100 vol %) N2 (100 vol %) 10 30
3. RESULTS AND DISCUSSION 3.1. Physical Property Characterization. Table 3 shows physical properties of the mixed OC particles. The average particle size was measured in a range of 93−126 μm for all nine samples. As a result, all of them appeared to be suitable for the circulating fluidized-bed process to be used. The packing density was excellent as well, with its value over 1.04 g/mL. Attrition resistance based on the AI values did not affect the formation of particles, but a strong dependency upon the calcined temperature was observed. First, the proper level of attrition resistance suitable for the fluidized-bed process was observed for samples calcined at 1100 and 1300 °C because their values were in the range of 4.2−16.3. However, OC particles calcined at 1200 °C were not suitable for the fluidizedbed process because they were out of a range required by fluidized catalytic cracking (FCC) criteria (AI < 22.5%, and CAI < 18%). On the other hand, the range of porosity of particles calcined at a temperature between 1100 and 1200 °C appeared to be good for acting as OC particles because their porosity ranged from a minimum of 16.8% to a maximum of 33%, but samples calcined at 1300 °C may not be suitable for OC particles, because porosity was significantly lower, regardless of their mixed structures. Samples calcined at 1300 °C also showed significantly low BET surface area. 3.2. Surface Analysis. Figure 1 shows the SEM results of the mixed OC. The OC calcined at 1100 °C exhibited a good structure with a uniform distribution of fine particles. However, the one calcined at 1200 and 1300 °C showed the formation of relatively large grains. It was confirmed that some of the grains were melted and formed a strong bond, so that the air holes were blocked. Therefore, it can be concluded that the OC that
2.1. Preparation of NiO/Fe2O3/Al2O3 OC Particles. The type, composition, and calcination temperature of nine OCs are shown in Table 1. Fe2O3 (iron oxide red 3150 RM, Deqing United Pigment Co.),
NiO
30 300
Scanning electron microscopy (SEM, JEOL, JSM-6360) was used for surface image analysis, where a magnification of 10000 was applied to compare the characteristics of the surface. Elemental distribution for Ni, Fe, and Al was performed with electron probe microanalysis (EPMA, Shimadzu, EPMA 1600). A packing volume/tap density device (Quantachrome, single autotap) was used to measure the tapping density in accordance with American Society for Testing and Materials (ASTM) D4168. The average particle size was measured with a sieve and sieve shaker (Meinzer II) in accordance with ASTM E-11. Brunauer−Emmett− Teller (BET) surface area and porosity were measured with Autosorb2000 of Qusntachrome and the Auto Pore IV 9500 Hg porosity meter of Micromeritics. Attrition resistance was measured with the attrition tester manufactured in accordance with the specification of ASTM D5757-95, and the measurements are shown as attrition index (AI) and corrected attrition index (CAI).
2. EXPERIMENTAL SECTION
type
conditions
sample weight (mg) gas flow rate (standard cubic centimeters) isothermal temperature (°C) inert gas injection time (min) reduction gas oxidation gas inert gas oxidation time (min) reduction time (min)
calcination temperature (°C) 1100 1200 1300
70
NiO (nickel oxide green), and γ-Al2O3 (VGL-15, Union Showa K.K) were mixed thoroughly with the mass ratio as specified in Table 1. Deionized water was applied to the well-mixed metal oxide, and it was crushed with a ball mill (High Energy Ball Mill, FrymaKoruma, MS-2) to produce a uniform slurry. The slurry was formed into spherically shaped particles using a spray dryer. The particles were dried in a convection oven for 24 h and calcined at a temperature of 1100−1300 °C for over 4 h. 2.2. Apparatus. The OTC of OC was analyzed with thermogravimetric analysis (TGA, Thermocahn, Thermomax 500 high 4618
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Table 3. Physical Properties for the NiO/Fe2O3/Al2O3 OC Particle attrition index (%) particle name N2F1
N1F1
N1F2
calcination temperature (°C)
tapping density (g/cm3)
average particle diameter (μm)
AI
CAI
porosity (%)
BET surface area (m2/g)
1100 1200 1300 1100 1200 1300 1100 1200 1300
1.13 1.61 2.40 1.10 1.59 2.44 1.04 1.10 2.05
109 103 93 122 111 101 126 122 104
13.7 89.3 4.2 14.9 88.5 2.4 16.3 98.5 8.2
6.6 78.6 3.3 16.4 83.1 1.8 3.8 95.7 7.2
33.0 16.8 4.5 32.6 23.0 2.3 26.7 32.2 13.0
5.63 2.09 0.21 4.84 1.74 0.16 3.48 2.18 0.42
3.4. Crystal Structure Analysis. Figure 2 shows XRD patterns for the mixed OC calcined at 1100 °C. Peaks (2θ 37.00,
Figure 1. SEM images of the NiO/Fe2O3/Al2O3 OC particle.
was calcined at 1100 °C is most suitable for use in CLC, because it had the most suitable physical properties for the fluidized-bed process and the fine grains were uniformly distributed on the surface. 3.3. TGA. TGA was used to measure the OTC. The OTC of OC calcined at 1100 °C, where promising physical properties were achieved, was measured, and Table 4 summarizes both the Figure 2. XRD patterns of NiO/Fe2O3/Al2O3 OC particles calcined at 1100 °C.
Table 4. OTC of the Mixed OC Particle Calcined at 1100 °C OTC (wt %) OC particle
measured value
theoretical value
N2F1 N1F1 N1F2
10.14 8.61 8.90
10.65 8.40 6.15
45.009, 59.632, and 65.545) corresponding to NiAl2O4 as a result of the reaction between NiO and Al2O3 were observed, as shown in eq 1, for N70, and a high intensity of NiO peaks (2θ 37.256, 43.288, 62.882, 75.418, and 79.412) were observed because they were of a higher amount than Al2O3. On the other hand, FeAl2O4, which is expected from eq 2, was not observed for F70, and only a low intensity of Fe2O3 peaks (2θ 24.133, 33.146, 35.598, 40.832, 49.428, 62.385, and 63.936) were observed. This result is consistent with a spinel metal aluminate formation sequence of FeAl2O4 < NiAl2O4 < CoAl2O4 < CuAl2O4 in the MeOx/Al2O (Me = Ni, Co, Cu, and Fe) system, which was mentioned by Bolt et al.7 They mentioned that hematite was a thermodynamically stable iron oxide at 1 atm O2 and 1000 °C, could dissolve Al2O3, but did not react to form of FeAl2O4 (hercynite). However, our test has verified no
theoretical and measured values. OTC is defined as the maximum mass ratio of the oxygen that can be carried by a unit mass of OC under given reaction conditions. The theoretical value of OTC is calculated from the OTC of NiO and Fe2O3 while also considering the composition ratio. The theoretical and measured values were similar in the cases of N2F1 and N1F1, but the OTC of N1F2 was surprisingly 145% of the theoretical value. Crystal structure analysis of the OC particle was performed to try to explain this result. 4619
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N1F2 > N1F1 > N2F1, suggesting that the aforementioned N1F2 is a spinel Ni(Al,Fe)2O4 solid solution close to NiFe2O4. The content of Ni was in the order of N2F1 > N1F1 > N1F2, and N1F2 showed a uniform concentration. However, a highconcentration NiO cluster was found in N2F1 and N1F1. This result matches well with the NiO peak in the XRD analysis, and the existence of NiO that did not participate in a thermal solid−solid interaction was confirmed. The contents of Al2O3 were 30 wt % in all cases, but the distribution was not uniform for two materials, except N1F1. Many of the high-concentration Al clusters were found in N1F2, and this result suggested that the presence of Al2O3 did not participate in the reaction. 3.6. OTC. Figures 4 and 5 show the conversion rate for a given weight as a result of the reaction time during reduction
formation of the crystalline structure, although the calcination temperature was increased to 1300 °C. On the other hand, Shaheen published the how formation temperature affected the solid−solid interaction and physical/ chemical characteristics of NiO/Fe2O3 through XRD analysis.23 The majority of divalent metal (MO) formed MFe2O4-type ferrite through its reaction with Fe2O3, in which metals were Co, Ni, Cu, Zn, and Mn. They created a crystalline NiFe2O4 phase from 700 °C through the solid−solid interaction between NiO and Fe2O3 and reported that the created ferrite is largely affected by prehistory, the ratio of constituents in the parent solid, and the addition of a small amount of foreign oxides. The report also stated that, because there was no interaction between NiO−Fe2O3 when the formation temperature was lower than 700 °C, long-term heating might be required at a temperature over 1000 °C for complete conversion to NiFe2O4. However, the crystalline structure of NiFe2O4 was not verified in our test conditions, as implied in eq 3. NiO + Al 2O3 → NiAl 2O4
(1)
Fe2O3 + Al 2O3 → FeAl 2O4
(2)
NiO + Fe2O3 → NiFe2O4
(3)
NiO + Fe2O3 + Al 2O3 → Ni(Al, Fe)2 O4
(4)
The peaks of Ni(Al,Fe)2O4 were observed from all of the mixed OCs, as shown in eq 4, and the reason for the higher OTC only for N1F2 is because of the different composition of NiO/ Fe2O3/Al2O3. The spinel Ni(Al,Fe)2O4 is a solid solution with Ni in the Td (tetrahedral) position and Al and Fe mixed uniformly in the Oh (octahedral) position. Ni(Al,Fe)2O4 will have a similar structure as NiFe2O4 in N1F2, because the composition ratio of Fe2O3 becomes higher. The measured value of the OTC for N1F2 was higher than the theoretical value because the OTC of NiFe2O4 is about 28 wt % higher than that of NiO (21 wt %).25 3.5. EPMA. Figure 3 shows the results of the EPMA for the mixed OC calcined at 1100 °C. Fe was uniformly distributed in all OC particles, but the content of each was in the order of
Figure 4. Mass-based conversion as a function of the temperature for reduction (fuel reactor, left) and oxidation (air reactor, right) of the OCs calcined at 1100 °C.
Figure 5. Mass-based conversion as a function of the temperature for reduction (fuel reactor, left) and oxidation (air reactor, right) of the OCs calcined at 1300 °C.
and oxidation reactions at 950 °C for mixed oxides, which were calcined at 1100 and 1300 °C, respectively. As shown in Figure 4, the reduction reaction time of OC calcined at 1100 °C for N2F1 was completed within 5 min, which was similar to N70 (produced with 70% NiO and 30% Al2O3) and Fe70 (produced with 70% Fe2O3 and 30% Al2O3), while the reduction reaction
Figure 3. EPMA images of the NiO/Fe2O3/Al2O3 OC particle calcined at 1100 °C. 4620
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time for N1F2 and N1F1 was around 30 min. Unlike such a reduction process, the oxidation process was completed within a few minutes for all samples. On the other hand, in the case of OC calcined at 1300 °C, as shown in Figure 5, the reduction reaction time for N1F2 was around 30 min, while samples with other composition ratios had reduction and oxidation reaction times with almost a similar reaction rate as reference substances N70 and F70. Here, it is observable that the conversion ratio of samples calcined at 1100 °C in Figure 4 is much higher than that of samples calcined at 1300 °C in Figure 5. Figure 6 shows OTCs as oxidation/reduction cycles increase. It is shown that consistent OTC of all three types of mixed Figure 7. Assessment of the economic efficiency for OCs calcined at 1100 °C (the price was shown in KRW; 1 USD = ∼1150 KRW).
4. CONCLUSION On the basis of the physical and chemical analyses and OTCs of nine mixed OCs, we can make the following conclusions: (1) Average particle sizes of all nine mixed oxides that are produced using a spray-drying method were in a range of 93−126 μm, which is suitable for a fluidized-bed process. However, considering various physical properties, such as attrition resistance, porosity, and BET surface area, the best formation temperature appeared to be 1100 °C, which was confirmed by observation with SEM and EPMA. (2) Measured OTCs of N1F1 and N2F1 particles match well with theoretical values, but measured OTC of N1F2 was 145% of the theoretical value. It is believed that such a result happened because spinel Ni(Al,Fe)2O4 solid solution was formed by the thermal solid− solid interaction while forming NiO/Fe2O3/Al2O3 mixed OC particles and also because spinel Ni(Al,Fe)2O4 solid solution, which is close to NiFe2O4, is created at higher contents of Fe2O3. (3) All three types of mixed oxides produced in this research had good characteristics in that they maintained a constant OTC even after multiple cycles. However, although the oxidation rate is similar or faster at 950 °C compared to single OC particles, the reduction rate is still slow. Therefore, more research for process optimization is needed. (4) Simple assessment on the manufacturing cost for making 1 kg of OC particles for producing 1 wt % OTC suggests that N1F2 was the most cost-effective OC particle, which is 42% N70. In conclusion, it was confirmed that N1F2 (22.5 wt % NiO, 47.5 wt % Fe2O3, and 30 wt % Al2O3), which was formed at 1100 °C with the spray-drying method, of all the other mixed OC particles was a more cost-effective and better candidate substance than the one at media-circulating combustion of a fluidized bed for a 1−10 MW class that is currently under plan. It is our future plan that we will perform process optimization, including reaction temperature and reaction time, for mediacirculating combustion.
Figure 6. OTC of the OC calcined at 1100 °C as a function of the number of cycles.
metal oxide is maintained even after repetition of multiple cycles, which is similar to single metal oxide. 3.7. Assessment of Economic Efficiency. The manufacturing cost of 1 kg of OC was calculated. To simplify the cost evaluation, the raw material NiO [140 000 South Korean won (KRW)/kg] and Fe2O3 (1750 KRW/kg) were considered but Al2O3 was not included because the ratio of it was fixed at 30 wt %, as seen in Table 1. The simplified manufacturing costs were shown in Table 5, and Figure 7 shows the economic efficiency Table 5. Manufacturing Cost of 1 kg of OC and 1% OTC
a
sample name
OTC (measured value)
cost for 1 kg of OC (KRW)a
cost for 1% OTC (KRW/wt %)
N1F2 N1F1 N2F1 N70
8.9 8.61 10.14 15.51
32331 49613 66894 98000
3633 5762 6597 6319
1 USD = ∼1150 KRW.
assessment in manufacturing 1 kg of OC to produce 1 wt % OTC. For example, the manufacturing cost of 1 kg and cost for 1% OTC of N1F2 was calculated as follows. (1) Manufacturing cost of 1 kg of OC: 0.475 kg (47.5 wt %) × 1750 KRW/kg + 0.225 kg (22.5 wt %) × 140 000 KRW/kg = 32 331 KRW/kg. (2) Manufacturing cost for 1% OTC: 32 331 KRW ÷ 8.9 wt % = 3633 KRW/wt %. The manufacturing costs can be easily estimated for other OCs. The cheapest OC to produce 1 wt % OTC was N1F2, equivalent to 42% N70.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 82-42-865-5239 (K.K.); 82-42-821-6553 (G.L.). Fax: 82-42-865-5202 (K.K.); 82-42-821-8896 (G.L.). E-mail:
[email protected] (K.K.);
[email protected] (G.L.). Notes
The authors declare no competing financial interest. 4621
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ACKNOWLEDGMENTS This work was supported by the Power Generation and Electricity Delivery Research and Development Program (2009191919994C) under the Ministry of Knowledge Economy.
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