NiAl2O4 and NiO

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Ind. Eng. Chem. Res. 2009, 48, 7400–7405

High Reactivity and Mechanical Durability of NiO/NiAl2O4 and NiO/NiAl2O4/ MgAl2O4 Oxygen Carrier Particles Used for more than 1000 h in a 10 kW CLC Reactor Alexander Shulman,† Carl Linderholm,*,‡ Tobias Mattisson,‡ and Anders Lyngfelt‡ Department of Chemical and Biological Engineering and Department of Energy and EnVironment, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden

Chemical-looping combustion (CLC) is a promising technology for CO2 capture in the process of combustion of gaseous fossil fuels. Oxygen carrier materials based on Ni/NiO on NiAl2O4 and on NiAl2O4/MgAl2O4 support have previously shown high initial activity combined with high conversion of methane and low concentration of outgoing CO. The feasibility of the Ni/NiO system for CLC depends largely on the lifetime of the oxygen carrier particles in the reactor due to the high price of the material. Avoiding chemical and physical degradation of oxygen carriers is essential for long-term industrial-scale chemical-looping combustion operations, and the particles’ activity and mechanical durability have to remain high during long operation times. In this study, a series of oxygen carrier samples were collected from a 10 kW CLC combustor operated for a total of 1016 h using oxygen carrier materials based on NiO/NiAl2O4 (N-VITO) for 405 h and a mixture of used NiO/NiAl2O4 with fresh NiO/NiAl2O4/MgAl2O4 (N-VITOMg) for 611 h, respectively. These samples were collected after certain time intervals and analyzed in terms of reactivity with methane in a quartz batch reactor. Also, the structural and mechanical properties of these samples were investigated by means of powder XRD, BET surface area measurements, light and scanning electron microscopy, energy dispersive X-ray spectrometry, and crushing strength evaluation. It was shown that both N-VITO and N-VITO/N-VITOMg demonstrate high reactivity and mechanical durability after having been used for more than 1000 h in the CLC 10 kW reactor, which makes them excellent candidates for applications within the area of chemical-looping combustion. Introduction Chemical-Looping Combustion. Chemical-looping combustion (CLC) technology involves oxygen transfer from the combustion air to the fuel via oxygen carrier materials with no direct contact between air and fuel.1-3 This technology allows reducing the costs for CO2 capture since the energy demanding separation of CO2 and N2 is avoided. The process is carried out in a circulating system composed of two fluidized bed reactors: an air reactor and a fuel reactor (see Figure 1). The fuel, typically natural gas, syngas, or refinery gas, is introduced to the fuel reactor where it is oxidized by the oxygen carrier, producing CO2 and H2O, according to eq 1. (2n + m)MeyOx + CnH2m f (2n + m)MeyOx-1 + mH2O + nCO2 (1) The reduced oxygen carrier particles are thereafter transferred to the air reactor, where they are regenerated to their original form by reacting with oxygen from the air, as shown in eq 2. (2n + m)MeyOx-1 + (n + 1/2)O2 f (2n + m)MeyOx (2) The overall reaction for the process is presented in eq 3. Water can easily be removed from the gaseous product mixture by condensation, allowing for convenient recovery of pure CO2. CnH2m + (n + 1/2m)O2 f mH2O + nCO2

(3)

* To whom correspondence should be addressed. Fax: (+46) 31 772 14 43. E-mail: [email protected]. † Department of Chemical and Biological Engineering. ‡ Department of Energy and Environment.

Oxygen Carriers. The feasibility of oxygen carrier materials for time-extended industrial-scale chemical-looping combustion operations is dictated by the long-term reactivity of the particles with the fuel. Additionally, the oxygen carrier particles must be resistant to mechanical and chemical degradation by attrition, agglomeration, fragmentation, composition changes, etc. During the past few years, a lot of work has been focused on the development of oxygen carrier materials suitable for CLC. It has been shown that metal oxide systems of NiO/Ni, Mn3O4/ MnO, Fe2O3/Fe3O4, Cu2O/Cu, and CoO/Co are the most feasible for use in chemical-looping technologies.4,5 However, the performance of these materials has not been evaluated yet during long-standing CLC operations, and it is utterly crucial to obtain more knowledge of the oxygen carrier particles’ behavior during extended operation periods. The aim of this study is to investigate the viability of oxygen carrier particles based on NiO/Ni system for industrial-scale application within chemicallooping combustion by analyzing the effects of long-term CLC operation on reactivity, chemical composition, and mechanical strength of these oxygen carriers. The oxygen carrier particles of NiO/NiAl2O4 (N-VITO), containing 40 wt % NiO and 60 wt % NiAl2O4, were produced by spray-drying from a water-based slurry containing NiO, Al2O3, organic binder, and dispersants. The spray-dried material was sieved to obtain the right particle-size distribution and sintered at 1450 °C. NiO/NiAl2O4/MgAl2O4 (N-VITOMg) oxygen carrier, containing 40 wt % NiO, 42 wt % NiAl2O4, and 18 wt % MgAl2O4, was prepared in a similar fashion with 5 wt % MgO added to the initial slurry and sintering of the spray-dried material at 1400 °C. The overall reactions for the oxygen carriers based on NiO/ Ni system in the fuel and air reactors are as follows:

10.1021/ie900342f CCC: $40.75  2009 American Chemical Society Published on Web 06/12/2009

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Fuel reactor: 4NiO + CH4 f 4Ni + 2H2O + CO2

(4)

Air reactor: 4Ni + 2O2 f 4NiO

(5)

Net reaction: CH4 + 2O2 f 2H2O + CO2

(6)

In a recent investigation in a 10 kW prototype chemicallooping combustor,6 N-VITO oxygen carrier particles were used as oxygen carriers for 405 h of combustion. The CLC combustor was then emptied, and the used N-VITO particles were sieved. Thereupon a mixture containing equal amounts of used N-VITO oxygen carrier particles and fresh N-VITOMg particles was added to the bed, and the operation with the mixture was analyzed for additional 611 h. Addition of magnesium containing oxygen carrier particles was aimed at improvement of fuel conversion and combustion efficiency.6 Several samples were collected from the air filter of the reactor after various time intervals and sieved to yield particles with a diameter of 125-180 µm. Reactivity of these samples with methane was analyzed in a quartz batch reactor at 850 and 950 °C. Moreover, the analysis of the possible chemical and structural transformations that occurred in the samples during the 10 kW reactor experiments is performed using light microscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), crushing strength evaluation, and Brunauer-Emmett-Teller (BET) surface area analysis by means of nitrogen sorption. Several types of agglomerated particles, microagglomerates, and wall deposits were collected from various parts of the 10 kW CLC combustor. Microagglomerates, i.e., particles that appear to have melted together, forming lumps with the size of 0.2-0.3 mm, were collected from both the air reactor and the fuel reactor. Large but frail chunks of agglomerates were collected from the fuel reactor. Additionally, samples from deposits constituted by layers of particles that have fused together were collected from the walls of the fuel reactor. The structures and compositions of the agglomerates and wall deposits were analyzed using powder X-ray diffraction, SEM, and EDX. Experimental Section Oxygen Carriers. Table 1 summarizes the oxygen carrier particles analyzed in this study. Laboratory Fluidized Experiments. The reactivity of the particles was analyzed in a laboratory batch fluidized-bed reactor. Investigations in the laboratory fluidized bed involved introduction of 15 g of oxygen carrier to the quartz reactor and exposure of the particles to several consecutive cycles of oxidizing and reducing periods at temperatures of 850 and 950 °C. Prior to the experiments, the particles were heated to the

Figure 1. Schematic view of chemical-looping combustion (CLC).

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Table 1. Oxygen Carriers Investigated in This Paper sample

oxygen carrier (%)

total time in the 10 kW reactor (h)

S1 S2 S3 S4

N-VITO (100%) N-VITO (100%) N-VITO (100%) N-VITO (50%) N-VITOMg (50%) N-VITO (50%) N-VITOMg (50%) N-VITO (50%) N-VITOMg (50%) N-VITO (50%) N-VITOMg (50%)

0 170 405 405 0 475 70 710 305 1016 611

S5 S6 S7

temperature of interest in a flow of nitrogen gas, hence achieving fluidization of the bed. As the required thermal conditions were reached, the particles were fully oxidized in a flow of 5% O2 in N2. The reduction of the oxygen carriers was accomplished by subjecting the particles to the flow of pure CH4. Purging of the reactor from reactive gases and gaseous products of the reaction was achieved by introduction of N2 for a certain period of time, thus separating the oxidation and the reduction steps by an inert step. After removal of water vapor from the gaseous product mixture by condensation, the outgoing dry gas concentration was recorded using a Rosemount NGA 2000 multicomponent gas analyzer. The outlet H2 concentration was not measured online but instead was assumed to be related to the outlet partial pressures of CO and CO2, as previously described in the literature.7,8 The pressure drop over the bed was recorded using a Honeywell pressure transducer at a frequency of 20 Hz. Reactivity Evaluation. The degree of oxidation, X, describes the extent to which the oxygen carriers are oxidized and is defined as X)

m - mred mox - mred

(7)

where m is the actual mass of the sample, mox is the mass of the fully oxidized sample, and mred is the mass of the sample in its fully reduced metallic form. The degree of oxidation of oxygen carriers during the reduction period as a function of time is calculated from the outlet gas concentrations using eq 8. Xi ) Xi-1 -

∫

t1

t0

1 n˙ (4p + 3pCO,out - pH2,out) dt M0Ptot out CO2,out (8)

Correspondingly, the degree of conversion for the oxidation period is determined using the relationship Xi ) Xi-1 +

∫

t1

t0

1 (n˙ p - n˙out pO2,out) dt M0Ptot in O2,in

(9)

where Xi is the conversion as a function of time for a period i, Xi-1 is the degree of conversion after the foregoing period; t0 and t1 are the times for the start and the finish of the period, respectively; M0 is the number of moles of active oxygen in the unreacted oxygen carrier; nin and nout are the molar flows of dry gas entering and exiting the reactor, respectively; Ptot is the total pressure; pCO2,out, pH2,out, and pCO,out are the outlet partial pressures of CO2, H2, and CO after removal of water vapor, respectively; and pO2,in and pO2,out are the partial pressures of incoming and exiting oxygen, respectively. For analysis of fuel conversion, the gas yield of CO2, γred, and fraction of CH4, γmethane, in the outlet gas flow were calculated as follows:

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γred ) γCO2 )

γmethane )

pCO2 pCH4 + pCO2 + pCO pCH4

pCH4 + pCO2 + pCO

(10)

(11)

Characterization of Oxygen Carriers and Agglomerated Particles. The analysis of the phase compositions of the oxygen carrier particles was performed on a Siemens D5000 powder X-ray diffractometer (Cu, KR1, λ ) 1.54056 Å). The structure and the surface composition of the particles as well as the fragments and agglomerates produced in the course of experiments in the 10 kW prototype reactor were analyzed in an JEOL JSM 6301F analytical scanning electron microscope. The BET surface areas of the particles were determined using nitrogen adsorption and desorption isotherms recorded at -196 °C using a Micrometrics Tristar equipment. Crushing strength of the oxygen carriers, that is, the force required to fracture the particles, was investigated using a Shimpo FGN-5 crushing strength apparatus. Each sample is represented by 30 different particles collected from the sample. All the characterization experiments above were conducted on the samples prior to reactivity investigations in the fluidized-bed batch reactor. Results and Discussion Reactivity Evaluation. The results of the reactivity evaluation of the oxygen carrier particles at 850 and 950 °C are presented in Figures 2 and 3. As is evident from Figure 2, there is a small fraction of methane released at high values of oxidation. This has been seen for most Ni-based particles in earlier studies and is related to the lack of Ni-sites on the particles at high degrees of particle oxidation.7,8 For the N-VITO particles (Figure 2), the gas yield, γred, appears to increase for oxygen carriers that have spent more time in the 10 kW reactor, i.e., S3 has higher γred compared to S1 both at 850 °C and at 950 °C. Moreover, the fraction of CH4, γmethane, decreases for the particles that have been used for a longer period of time in the 10 kW reactor. Figure 2 demonstrates that γmethane is lower for S3 compared to both S2 and S1. The reason for increasing methane conversion is conceivably the accumulation of metallic Ni on the surface of craters and openings in the used N-VITO particles and increased porosity of the particles, as described below. The metallic Ni sites contribute to partial oxidation of methane at early stages of the reduction period, when the oxidation degree of the oxygen carriers is close to unity. Additionally, as the porosity increases, the availability of active sites for oxidation of the fuel increases as well, thus enhancing the methane conversion. The use of the mixture of N-VITO and N-VITOMg particles (Figure 3) shows a similar behavior to the single oxygen carrier system, i.e., N-VITO, with the notable exception that the methane fraction is considerably smaller. This is likely due to the presence of Mg in the particles, which previously has been shown to have an enhancing effect on the activity of Ni-based catalysts.9-11 The mixed particle system exhibits a similar trend in the reactivity change both at 850 and 950 °C as for the N-VITO particle. As shown in Figure 3, the CO2 yield, γred, initially being lower for the S4 sample, reaches its maximum, already succeeding 70 h of exposure of the sample to the chemicallooping process in the 10 kW reactor (sample S5), whereupon it remains high and virtually unchanged for samples S6 and S7. Very low methane fraction, γmethane, for sample S4, combined

Figure 2. Gas yield of CO2 and fraction of CH4 as a function of conversion for the third reduction period for oxygen carrier particles S1-S3 at (a) 850 °C and (b) 950 °C.

Figure 3. Gas yield of CO2 and fraction of CH4 as a function of conversion for the third reduction period for oxygen carrier particles S4-S7 at (a) 850 °C and (b) 950 °C.

Figure 4. X-ray powder diffractograms of (a) samples S1-S3 and (b) samples S4-S7. Peaks marked by “*” represent NiAl2O4; peaks marked by “´” represent MgAl2O4; peaks marked by “+” represent NiO.

with the somewhat lower gas yield, γred, for the same sample at both 850 and 950 °C implies that production of CO is relatively high for this oxygen carrier mixture, a fact also observed in the course of the experiment. This occurs presumably due to addition of fresh N-VITOMg particles to N-VITO particles used for 405 h in the 10 kW reactor. However, as the particles’ operation time increases, i.e., for samples S5-S7, the formation of CO drops to low levels. Characterization of Oxygen Carriers. Powder XRD. X-ray powder diffractograms of samples S1-S3 and S4-S7, presented in Figure 4, demonstrate that the oxygen carrier particles have undergone no phase changes during the 10 kW reactor experiments. Pronounced peaks assigned to NiO and NiAl2O4 for N-VITO particles (Figure 4a) and to NiO, NiAl2O4, and MgAl2O4 for the mixture of N-VITO and N-VITOMg particles (Figure 4b) are clearly visible in all the diffraction patterns. Less distinct halos in the 2θ interval between 12-18°, typical for every diffraction pattern presented in Figure 4, may be an evidence of minute amounts of amorphous phases present in the samples. X-ray powder diffractograms presented in Figure 4 confirm that oxygen carrier particles of N-VITO and N-VITOMg are

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Figure 5. Photographs of (a) S1; (b) S3; and (c) S7.

Figure 6. SEM micrographs of (a) S1; (b) S2; (c) S3; (d) S4; (e) S5; (f) S6; and (g) S7.

not prone to chemical degradation during CLC operation lasting for 1016 h for the former and 611 h for the latter. • Microscopy (Light, SEM/EDX). Visual images of samples S1, S3, and S7 are shown in Figure 5. These images demonstrate that the oxygen carrier particles in most cases maintain their structural integrities and original shapes. This fact is confirmed by the SEM micrographs presented in Figure 6. The EDX spectra collected from the surfaces of such particles reveal the presence of Ni, Al, and O (see Figure 7b). In some cases, crater-shaped openings appear on the surface of the particles, or the particles attain donut shape (see, e.g., Figure 6c, d, and g). These structural peculiarities were previously attributed to the spray-drying process used for preparing the oxygen carrier particles on the industrial scale. Attrition or migration of Ni may also contribute to the appearance of channels and craters in the oxygen carrier particles. However, the exact mechanisms of formation of these structural defects are unidentified. The EDX spectra collected from such particles reveal that pure metallic Ni is accumulated on the surface of such openings (see, e.g., Figure 7c) and no signals due to Al or O are observed. This implies that the surfaces of openings and craters are not oxidized. BET Surface Area. Figure 8 shows the change in the BET surface area of the samples with the time the oxygen carriers spent in the CLC 10 kW reactor. The accuracy of the measurements is ∼0.5%. Clearly, there is a relatively large increase between S1 and S2, i.e., after 170 h of combustion. But after this, there does not seem to be any significant increase, and the surface area for the sample taken after 405 h, S3, is similar to that for S2. A similar type of behavior can be observed for the mixture of

N-VITO and N-VITOMg. The increase of the surface area of the oxygen carriers is likely caused by formation of channels and craters in the particles, as described above. Crushing Strength. The results of the investigation of the crushing strengths of the samples of N-VITO (S1-S3) and a mixture of N-VITO and N-VITOMg (S4-S7) are presented in Figure 9. The mechanical strength of the particles decreases slightly with the time they have spent in the 10 kW reactor, apparently due to increased porosity of the particles. The values obtained in the course of crushing strength evaluation differ from ones presented by Linderhom et al.6 due to different particle sizes. Linderhom et al.6 evaluated crushing strength of oxygen carrier particles in the size range of 180-212 µm, whereas particles in samples S1-S7, analyzed in this work, have diameters of 125-180 µm, as mentioned above. Agglomerates. As mentioned above, a variety of agglomerated particles and wall deposits have been collected from the air and fuel reactors of the 10 kW CLC unit. Photographs of microagglomerates collected in the air and fuel reactors, as well as of the wall deposits and the large chunks of agglomerated particles, both collected in the fuel reactor, are presented in Figure 10. The powder X-ray diffractograms for these samples are presented in Figure 11, where they are compared to the XRD diffractogram of pure N-VITO. The microagglomerate collected from the air reactor bears close resemblance to the pure N-VITO and N-VITOMg particles, exhibiting all the peaks typical for NiO on NiAl2O4 and NiAl2O4/MgAl2O4 support. X-ray diffractograms of microagglomerated particles and wall deposits collected in the fuel reactor show no traces of NiO. Instead a peak assigned to pure Ni is present (see Figure 11c and d). These results are confirmed by SEM images and EDX

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Figure 9. Crushing strength for (a) samples S1-S3 and (b) S4-S7. The black rectangles represent the spread of the median 10 values, whereas the solid lines represent the observed distribution and the extremes of all 30 values.

Figure 7. (a) SEM micrograph of S4 (N-VITO 405 h; N-VITOMg, fresh); (b) EDX spectrum collected from Site 1; (c) EDX spectrum collected from site 2. Peaks marked by “b” represent O; peaks marked by “2” represent Al; peaks marked by “9” represent Ni.

Figure 10. (a) Photograph of microagglomerate collected in the air reactor; (b) photograph of microagglomerate collected in the fuel reactor; (c) photograph of wall deposits collected in the fuel reactor; (d) photograph of large fragile chunks of agglomerated particles collected in the fuel reactor.

Figure 8. BET surface area for (a) samples S1-S3 and (b) samples S4-S7.

spectra of these species, shown in Figure 12. The EDX spectra imply accumulation of Ni on the surface of the particles (Figure 12b and c), and in the bridges that connect the particles of the microagglomerate (Figure 12d), which has also been previously reported in literature.12 The large fragile chunks of agglomerated particles collected in the fuel reactor, however, demonstrate no difference compared to the particles of N-VITO and N-VITOMg (see Figure 11e). The nature of the interactions between the particles that make them stick together is yet not fully understood. Conclusion The ability of N-VITO oxygen carrier particles and of the mixture of N-VITO particles with N-VITOMg oxygen carriers

Figure 11. X-ray powder diffractograms of (a) pure N-VITO (sample S1) compared to samples of (b) microagglomerate collected in the air reactor; (c) microagglomerate collected in the fuel reactor; (d) wall deposit collected in the fuel reactor; (e) large frail agglomerate chunks collected in the fuel reactor. Peaks marked by “*” represent NiAl2O4; peaks marked by “´” represent MgAl2O4; peaks marked by “+” represent NiO; peaks marked by “)” represent Ni.

to exhibit very high methane conversion after a long-term CLC operation (over 1000 h for the former and over 600 h for the latter) is demonstrated in this study. Photographs, SEM images, and XRD powder diffractograms of the particles show that the structural integrity and chemical composition of the major fraction of the particles is maintained after long-term operation in a 10 kW CLC reactor. A minute fraction of the oxygen carrier particles form agglomerates and wall deposits. Metallic Ni

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t0 ) time for the start of the period t1 ) time for the finish of the period M0 ) number of moles of active oxygen in the unreacted oxygen carrier nin ) molar flow of dry gas entering the reactor nout ) molar flow of dry gas exiting the reactor Ptot ) total pressure pCO2,out ) outlet partial pressures of CO2 after removal of water vapor pH2,out ) outlet partial pressures of H2 after removal of water vapor pCO,out ) outlet partial pressures of CO after removal of water vapor pO2,in ) partial pressures of incoming oxygen pO2,out ) partial pressures of exiting oxygen γred ) gas yield of CO2 γmethane ) fraction of CH4 Acknowledgment Figure 12. (a) SEM micrograph of N-VITO and N-VITOMg agglomerate particles from FR, d > 212 µm; (b) EDX spectrum collected from Site 1; (c) EDX spectrum collected from Site 2; (d) EDX spectrum collected from Site 3. Peaks marked by “b” represent O; peaks marked by “2” represent Al; peaks marked by “9” represent Ni.

accumulates on the surface of the agglomerates collected in the fuel reactor and in the “bridges” holding the agglomerated particles together. The mechanical strength of the oxygen carrier particles of N-VITO and N-VITOMg decreases slightly with the operation time, whereas the BET surface area increases. These structural alterations might arise due to formation of channels and craters in the particles, thus increasing their total surface area. Metallic Ni accumulates on the surface of these craters, plausibly facilitating methane conversion at high degrees of oxygen carrier oxidation, X. The reactivity of the predominantly larger fraction of N-VITO and N-VITOMg oxygen carriers in terms of high methane conversion and low CO formation increases with the operation time and remains very high even subsequent to more than 1000 h in the 10 kW prototype CLC reactor. Being the first commercially manufactured oxygen carriers to exhibit such properties in combination with high mechanical durability, the systems containing NiO/NiAl2O4 and NiO/NiAl2O4/MgAl2O4 open new possibilities for the application of oxygen carrier materials in long-term industrial-scale CLC processes. List of Symbols and Acronyms CLC ) chemical-looping combustion N-VITO ) oxygen carrier particles containing 40 wt % NiO supported on 60 wt % NiAl2O4 N-VITOMg ) carrier particles containing 40 wt % NiO supported on 42 wt % NiAl2O4 and 18 wt % MgAl2O4 MeyOx ) oxygen carrier in the fully oxidized form MeyOx-1 ) oxygen carrier in the fully reduced form CnH2m ) generic hydrocarbon fuel X ) degree of conversion Xi ) conversion as a function of time for a period i Xi-1 ) degree of conversion after the foregoing period m ) actual mass of the sample mox ) mass of the fully oxidized sample mred ) mass of the sample in its fully reduced form

This work was carried out within the EU-financed researchproject CLCGP (Chemical-Looping Combustion CO2-Ready Gas Power), Contract Number 019800. Literature Cited (1) Ishida, M.; Zheng, D.; Akehata, T. Evaluation of a Chemical-Looping Combustion Power-Generation System by Graphic Exergy Analysis. Energy 1987, 12 (2), 147–154. (2) Lyngfelt, A.; Leckner, B.; Mattisson, T. A Fluidized-Bed Combustion Process with Inherent CO2 separation; Application of Chemical-Looping Combustion. Chem. Eng. Sci. 2001, 56 (10), 3101–3113. (3) Anheden, M.; Svedberg, G. Exergy Analysis of Chemical-Looping Combustion Systems. Energy ConVers. Manage. 1998, 39 (16-18), 1967– 1980. (4) Johansson, M.; Mattisson, T.; Lyngfelt, A. Comparison of Oxygen Carriers for Chemical-Looping Combustion. Therm. Sci. 2006, 10 (3), 93– 107. (5) Adanez, J.; Diego, L. F. d.; Garcia-Labiano, F.; Gayan, P.; Abad, A.; Palacios, J. M. Selection of Oxygen Carriers for Chemical-Looping Combustion. Energy Fuels 2004, 18, 371–377. (6) Linderholm, C.; Mattisson, T.; Lyngfelt, A., Long-term integrity testing of spray-dried particles in a 10 kW chemical-looping combustor using natural gas as fuel. Fuel 2009, published online. (7) Mattisson, T.; Johansson, M.; Lyngfelt, A. The Use of NiO as an Oxygen Carrier in Chemical-Looping Combustion. Fuel 2006, 85, 736– 747. (8) Mattisson, T.; Johansson, M.; Jerndal, E.; Lyngfelt, A. The Reaction of NiO/NiAl2O4 Particles with Alternating Methane and Oxygen. Can. J. Chem. Eng. 2008, 86 (4), 756–767. (9) Tang, S.; Lin, J.; Tan, K. L. Partial oxidation of methane to syngas over Ni/MgO, Ni/CaO and Ni/CeO2. Catal. Lett. 1998, 51 (3-4), 169– 175. (10) Jerndal, E.; Mattisson, T.; Thijs, I.; Snijkers, F.; Lyngdelt, A. NiO particles with Ca and Mg based additives produced by spray drying as oxygen carriers for chemical-looping combustion, 9th International Conference on Greenhouse Gas Control Technologies, Washington, DC, November 16-20, 2008. (11) Villa, R.; Cristiani, C.; Groppi, G.; Lietti, L.; Forzatti, P.; Comaro, U.; Rossini, S. Ni based mixed oxide materials for CH4 oxidation under redox cycle conditions. J. Mol. Catal. A: Chem. 2003, 204-205, 637–646. (12) Kuusik, R.; Trikkel, A.; Lyngfelt, A.; Mattisson, T. High temperature behavior of NiO-based oxygen carriers for Chemical Looping Combustion, 9th International Conference on Greenhouse Gas Control Technologies, Washington, DC, November 16-20, 2008.

ReceiVed for reView March 2, 2009 ReVised manuscript receiVed May 22, 2009 Accepted May 27, 2009 IE900342F