NiAl2O4 Particles as Oxygen Carriers for

Energy & Fuels 2009, 23, 665–676

665

Investigation of Different NiO/NiAl2O4 Particles as Oxygen Carriers for Chemical-Looping Combustion Erik Jerndal,*,† Tobias Mattisson,‡ and Anders Lyngfelt‡ Department of Chemical and Biological Engineering, DiVision of EnVironmental Inorganic Chemistry, and Department of Energy and EnVironment, DiVision of Energy Technology, Chalmers UniVersity of Technology, Go¨teborg S-412 96, Sweden ReceiVed August 13, 2008. ReVised Manuscript ReceiVed NoVember 14, 2008

Chemical-looping combustion is a combustion technology, where CO2 is separated from the rest of the flue gases without an energy-consuming gas-separation process. The combustion is performed in two reactors, with metal oxide particles circulating between them, transferring oxygen from the combustion air to the fuel. Particles of NiO, supported by NiAl2O4, have been reported earlier as excellent oxygen carriers for this process. The aim of the present investigation is to verify that commercially available raw materials can be used to produce oxygen carrier particles with properties suitable for the technology. A total of 36 oxygen carrier materials were prepared by freeze granulation and investigated with respect to parameters important for chemicallooping combustion. The reactivity of the particles was investigated in a small fluidized bed reactor by exposing them cyclically to CH4 and 5% O2 in N2, at 950 °C. Although defluidization occasionally occurred for some materials, it was clear that the gas conversion and the reactivity were generally high. An addition of Ca(OH)2 to the oxygen carriers increased the strength and thus reduces the risk of fragmentation and attrition in a chemical-looping combustion device. An addition of MgO enhanced the fuel conversion early in reduction, which seemed to be restricted because of the limited amounts of metallic Ni. An increased sintering temperature generally resulted in harder particles of higher density; however, the risk of defluidization seemed to increase for such particles. Carbon formation was only detected when the oxygen carriers were highly reduced and the fuel conversion was incomplete, i.e., at conditions not expected in a real chemical-looping combustion device. Two of the investigated particles, NOV1T1400 and NOV2T1400, displayed a combination of high reactivity and strength as well as excellent fluidization behavior and should be feasible for use in a chemical-looping combustion unit.

1. Introduction The global atmospheric concentration of CO2 has increased from a pre-industrial value of about 280 to 379 ppm in 2005.1 This value exceeds by far the natural values over the last 650 000 years, which have varied between 180 and 300 ppm, as determined from ice core analysis. The drastic increase in CO2 concentration is predominantly due to the intensified use of fossil fuels, which in 2004 was the source of 80% of the total primary energy supply for the world.2 It is widely accepted that the release of CO2 into the atmosphere enhances the natural greenhouse effect, resulting in an increased average temperature on earth. During the past hundred years, 1906-2005, the global average temperature has increased by 0.74 ( 0.18 °C,3 most likely because of the increased amounts of CO2 released into the atmosphere. To deal with this issue, a phase out of fossil fuels in favor of renewable energy sources is one option. However, this transition may not * To whom correspondence should be addressed. Telephone: +46-31772-28-86. E-mail: [email protected]. † Department of Chemical and Biological Engineering, Division of Environmental Inorganic Chemistry. ‡ Department of Energy and Environment, Division of Energy Technology. (1) Intergovernmental Panel on Climate Change (IPCC). Changes in atmospheric constituents and in radiative forcing. Climate Change: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. (2) Industrial Environmental Association (IEA). Key world energy statistics, 2006.

be accomplished rapidly enough, and an option to avoid the release of CO2, produced in power production, which has received an increased interest in recent years, is separation and sequestration of CO2.4-7 A combustion technique with inherent separation of CO2 is chemical-looping combustion, where oxygen is transferred by an oxygen carrier from the combustion air to the fuel, thus avoiding mixing between air and fuel. The main advantage with this technique is that, after condensation of water, CO2 is obtained in a separate stream with no extra energy needed for the separation. 1.1. Chemical-Looping Combustion. The basic ideas of chemical-looping combustion can be found in a patent from 1954, where it is presented as a technique to produce pure CO2 from fossil fuels.8 A similar concept was proposed as a power (3) Intergovernmental Panel on Climate Change (IPCC). Observations: Surface and atmospheric climate change. Climate Change: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. (4) Intergovernmental Panel on Climate Change (IPCC). Special report on carbon dioxide capture and storage, 2005. (5) Herzog, H. J.; Eliasson, B.; Kaarstad, O. Capturing greenhouse gases. Sci. Am. 2000, 282 (2), 72–79. (6) Benson, S. M.; Surles, T. Carbon dioxide capture and storage: An overview with emphasis on capture and storage in deep geological formations. Proc. IEEE 2006, 94 (10), 1795–1805. (7) Abu-Khader, M. Recent progress in CO2 capture/sequestration: A review. Energy Sources, Part A 2006, 28 (14), 1261–1279. (8) Lewis, W. K.; Gilliland, E. R. Production of pure carbon dioxide. U.S. Patent 2,665,972, 1954.

10.1021/ef8006596 CCC: $40.75  2009 American Chemical Society Published on Web 01/07/2009

666 Energy & Fuels, Vol. 23, 2009

Jerndal et al.

Figure 1. Schematic view of chemical-looping combustion: (1) air reactor and riser, (2) cyclone, and (3) fuel reactor.

production technique in 1983 by Richter and Knoche,9 and in 1994, Ishida and Jin suggested that chemical-looping combustion could be used as a technology to separate CO2 from the rest of the flue gases in a power plant.10 The separation is achieved by performing the combustion in two separate reactors, as shown in Figure 1, with an oxygen carrier, in the form of a metal oxide, circulating between them. In the air reactor, the reduced metal oxide is oxidized by air according to reaction 1, and in the fuel reactor, it is reduced back to its initial state by a gaseous fuel, forming CO2 and H2O according to reaction 2. O2 + 2MexOy-1 T 2MexOy

(1)

CnH2m + (2n + m)MexOy T nCO2 + mH2O + (2n + m)MexOy-1 (2) Reaction 1 is strongly exothermic, while reaction 2 could be either exothermic or endothermic depending upon the fuel used and the characteristics of the oxygen carrier. However, the sum of the energy released in these two reactions is always the same as the heat released in conventional combustion, where the air and the fuel are in direct contact; hence, no extra energy is needed in the CO2 separation process. The exiting gas stream from the air reactor contains mainly N2 and some unreacted O2 if an excess of air is used. From the fuel reactor, almost pure CO2 can be obtained for storing after condensation of the steam. Because the process requires a good contact between gases and solids as well as a flow of solid material supplying oxygen for the combustion process, the use of two interconnected fluidized beds should have advantages over alternative designs of a chemical-looping combustion system.11 1.2. Oxygen Carriers. In a comprehensive thermal study of the suitability of several different oxides, on the basis of transition-state metals, to serve as oxygen carriers in chemicallooping combustion devices, it was concluded that systems based on Cu, Mn, Fe, and Ni were the most promising with regard to their thermodynamic restrictions.12 (9) Richter, H. J.; Knoche, K. F. Reversibility of combustion processes, efficiency and costing, second law analysis of processes. ACS Symp. Ser. 1983, 235, 71–85. (10) Ishida, M.; Jin, H. A new advanced power-generation system using chemical-looping combustion. Energy 1994, 19 (4), 415–422. (11) 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. (12) Jerndal, E.; Mattisson, T.; Lyngfelt, A. Thermal analysis of chemical-looping combustion. Chem. Eng. Res. Des. 2006, 84 (A9), 795– 806.

To increase the performance of these oxygen carriers, they should preferably be supported by an inert material.13 This material contributes to improve some important properties of the oxygen carrier. It increases the porosity and hence the reactivity of the active material, and it also helps maintain the particle structure during the reactions. Previous studies have showed that oxygen carriers based on Ni generally have a high reaction rate with methane, in combination with a high melting temperature, and thus have an advantage over carriers based on other metals.14,15 However, drawbacks with Ni are the fact that there is a thermodynamic restraint in the fuel conversion to CO2 and H2O,16 the higher toxicity, and the higher cost of Ni, compared to other metals proposed as oxygen carriers. The NiO particles in this study are supported by NiAl2O4, and the mass ratio between the active NiO and the supporting NiAl2O4 is 4:6. This material combination was first tested as an oxygen carrier for chemical-looping combustion in 1999, when Jin et al. stated that it may play a vital role in developing chemical-looping combustion.17 NiO supported by NiAl2O4 at a ratio of 4:6 has also been successfully used for 100 h of combustion in a 10 kW prototype chemical-looping combustion unit,18,19 and Johansson et al. reported that no major physical or chemical changes occurred during this continuous operation.20 Linderholm et al. have obtained similar results with carriers based on these materials but with a higher fraction of active NiO in a 10 kW unit, accomplishing a fuel conversion of above 99%,21 and the reactions of NiO/NiAl2O4, with alternating methane and oxygen, have been studied in detail by Mattisson et al.22 Here, it was found that, although NiAl2O4 can supply oxygen to the fuel, this reaction is extremely slow compared to the reaction of NiO with CH4, hence NiAl2O4 is considered as an inert material. Further, there is a thermodynamic limitation of the NiAl2O4 in converting methane fully to CO2 and H2O. In the previous studies, the materials were prepared using extremely fine and pure materials or by wet-chemical methods. (13) Ishida, M.; Jin, H. A novel combustor based on chemical-looping reactions and its reaction kinetics. J. Chem. Eng. Jpn. 1994, 27 (3), 296– 301. (14) Johansson, M.; Mattisson, T.; Lyngfelt, A. Comparison of oxygen carriers for chemical-looping combustion of methane-rich fuels. Proceedings of the 19th Fluidized Bed Combustion (FBC) Conference, Vienna, Austria, May 21-24, 2006. (15) Abad, A.; Adanez, J.; Garcia-Labiano, F.; de Diego, L. F.; Gayan, P.; Celaya, J. Mapping of the range of operational conditions for Cu-, Fe-, and Ni-based oxygen carriers in chemical-looping combustion. Chem. Eng. Sci. 2007, 62 (1-2), 533–549. (16) Mattisson, T.; Johansson, M.; Lyngfelt, A. The use of NiO as an oxygen carrier in chemical-looping combustion. Fuel 2006, 85 (5-6), 736– 747. (17) Jin, H.; Okamoto, T.; Ishida, M. Development of a novel chemicallooping combustion: Synthesis of a solid looping material of NiO/NiAl2O4. Ind. Eng. Chem. Res. 1999, 38 (1), 126–132. (18) Lyngfelt, A.; Kronberger, B.; Ada´nez, J.; Morin, J.-X.; Hurst, P. The GRACE project. Development of oxygen carrier particles for chemicallooping combustion. Design and operation of a 10 kW chemical-looping combustor. Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies, Vancouver, British Columbia, Canada, Sept 59, 2004. (19) Lyngfelt, A.; Thunman, H. Construction and 100 h of operational experience of a 10-kW chemical-looping combustor. Carbon Dioxide Capture for Storage in Deep Geologic FormationssResults from the CO2 Capture Project, 2005; Vol. 1, pp 625-645. (20) Johansson, M.; Mattisson, T.; Lyngfelt, A. Use of NiO/NiAl2O4 particles in a 10 kW chemical-looping combustor. Ind. Eng. Chem. Res. 2006, 45 (17), 5911–5919. (21) Linderholm, C.; Abad, A.; Mattisson, T.; Lyngfelt, A. 160 h of chemical-looping combustion in a 10 kW reactor system with a NiO-based oxygen carrier. Int. J. Greenhouse Gas Control 2008, 2 (4), 520–530. (22) 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.

NiO/NiAl2O4 Particles as Oxygen Carriers

Energy & Fuels, Vol. 23, 2009 667

Table 1. Raw Materials Used in Oxygen Carrier Production density (kg/m3) Novamet refractory grade Novamet green grade F OMG standard grade OMG HSA grade Umicore Vogler brown grade

BET surfacea (m2/g)

Raw Material, NiO 6600 6810 6210 5790 6800 6100

3.5 3.6 46.5 130.0 58.5 1.3

Raw Material, R-Al2O3 3960 3960

7.0 1.0

MagChem30

Additive, MgO 3590

24.2

Nordkalk SL

Additive, Ca(OH)2 2330

16.0

Almatis CT3000SG Almatis CT800FG

a

Measured by Micromeritics, Flowsorb II 2300.

For this system to be viable for use in a power plant, it is important that the raw materials are both commercial and can be obtained in large quantities at a reasonable cost. Thus, the aim of the present work is to verify that commercially available raw or semi-finished materials can be used to produce oxygen carriers of NiO/NiAl2O4, with the same or similar properties and reactivity as previously used materials. The NiO materials used in the granulation process, in the present investigation, were obtained from various commercial suppliers, and the prices varied in a range approximately 15-120% above the world market price of Ni as obtained from the London Metal Exchange. The R-Al2O3 materials used for particle production were also commercially available, with a market price only a fraction of that for highly pure and fine R-Al2O3, which has been used in earlier studies.16 2. Experimental Section 2.1. Preparation of Oxygen Carriers. To obtain spherical particles, the preparation of oxygen carriers was performed using the freeze-granulation technique. A water-based slurry of a fine powder of the raw material NiO and R-Al2O3 and a small amount of poly(acrylic acid) as a dispersant were prepared by ball milling for about 24 h. After milling, some polyethylene glycol was added to the slurry as a binder to keep the particles intact during freezedrying and sintering. Spherical particles were produced by pumping the slurry to a spray nozzle where passing atomizing-air produced drops, which were sprayed into liquid nitrogen where they froze instantaneously. The frozen water in the resulting particles was then removed by sublimation in a freeze drier operating at a pressure that corresponds to the vapor pressure over ice at -10 °C. To achieve a better size selectivity, the particles were sieved at this stage and small and large particles were removed and, in some cases, reused as raw material. After drying, the particles were sintered at three different temperatures, 1400, 1500, and 1600 °C for 6 h, using the following heating ramp: 2 °C/min to 200 °C, 1 °C/min to 450 °C, and 5 °C/min to the desired temperature. During heat treatment, the R-Al2O3 reacted completely with part of the NiO, forming the supporting spinel NiAl2O4. An excess of NiO was used in the preparation to achieve a mass ratio between NiO and NiAl2O4 of 4:6. After sintering, the particles were cooled by 10 °C/min to 1000 °C and hereafter at no specific rate. Finally, the particles were sieved to obtain particles in the size range of 125-180 µm. In an initial screening, a total of 30 oxygen carriers were prepared by combining the different raw materials from various suppliers, shown in Table 1, using different sintering temperatures. These particles are presented in Table 2, where the sintering temperature is given in the abbreviation for each carrier. Of the oxygen carriers prepared, seven failed in the production phase; i.e., no spherical

particles were obtained, and six formed too soft granules to be measureable in the crushing strength device. With the cheaper and coarser R-Al2O3 material, i.e., Almatis CT800FG, these problems occurred to a larger extent. The reason why preparation failed exclusively with this R-Al2O3 is likely the larger size of the particles in the raw material, which limits the contact area for reaction with the nickel oxides. For this reason, some material combinations were not prepared using Almatis CT800FG. The influence of small additions of MgO and Ca(OH)2 to the oxygen carriers were also investigated. The inclusion of MgO has earlier been found to have beneficial effects on the methane conversion,16,23 and Ca(OH)2 is a well-known additive to increase the mechanical strength of materials, such as cement.24 Further, Johansson et al. have reported an increased strength when CaO was added to NiO particles supported by NiAl2O4.25 Therefore, six oxygen carriers using OMG NiO and R-Al2O3 were prepared, still with a ratio of 4:6 between the active NiO and the supporting material but with an addition of 1% MgO, 5% MgO, and 1% Ca(OH)2, respectively, based on weight, as shown in Table 3. These materials were added to the water-based slurry of raw materials prior to ball milling during the preparation stage. 2.2. Characterization of Oxygen Carriers. To confirm the chemical composition and investigate possible phase transitions of the oxygen carriers, after both the sintering process and the reactivity tests, the particles were analyzed using X-ray powder diffraction by a Siemens D5000 powder diffractometer using Cu KR radiation. The X-ray analysis was performed on oxidized fresh samples and reduced samples after the reactivity tests. The shape and morphology of the oxygen carriers, before and after the reactivity investigation, were studied using a FEI, Quanta 200 environmental scanning electron microscope FEG. The force needed to fracture the particles, i.e., the crushing strength, was measured using a Shimpo FGN-5 device, and the value obtained is an average of 30 fractured particles sized 180-250 µm. The crushing strength test gives an indication of how resistant the oxygen carrier would be toward fragmentation and attrition in a real chemical-looping combustion application. Also, the apparent density of all of the fresh particles, sized 125-180 µm, was measured. Here, a void factor of 0.37 was assumed, because this is the theoretic voidage of a normally packed bed with uniformly sized spherical particles.26 2.3. Reactivity Investigation of Oxygen Carriers. The reactivity investigations were conducted in a fluidized bed reactor of quartz, with a length of 870 mm and an inner diameter of 22 mm. The reactor had a porous quartz plate placed 370 mm from the bottom, and the temperature was measured 5 mm under and 25 mm above this plate, using Pentronic CrAl/NiAl thermocouples enclosed in inconel-600 alloys, inside quartz shells. The pressure drop over the bed was measured by Honeywell pressure transducers at a frequency of 20 Hz, and from the pressure fluctuations obtained, it was possible to establish if the particles fluidized during the experiment. Bed material sized 125-180 µm was placed on the porous plate and heated in an inert atmosphere of N2 to a desired reaction temperature of 950 °C. All oxygen carriers were tested using a bed mass of 15 g, while some oxygen carriers were also tested using a 1 g bed mass to facilitate comparison by obtaining an incomplete fuel conversion. In the latter case, the oxygen carrier particles were diluted in inert quartz, to obtain a total bed mass of 15 g. When the experimental temperature was reached, the particles were exposed alternatingly to 5% O2 in N2 and 100% CH4, thus (23) Johansson, M.; Mattisson, T.; Lyngfelt, A.; Abad, A. Using continuous and pulse experiments to compare two promising nickel-based oxygen carriers for use in chemical-looping technologies. Fuel 2008, 87, 988–1001. (24) Kitamura, M.; Kamitani, M. Rapid hardening of cement by the addition of a mechanically activated Al(OH)3-Ca(OH)2 mixture. J. Am. Ceram. Soc. 2000, 83 (3), 523–528. (25) Johansson, M.; Mattisson, T.; Lyngfelt, A. Comparison of oxygen carriers for chemical-looping combustion. Therm. Sci. 2006, 10 (3), 93– 107. (26) Kunii, D.; Levenspiel, O. Fluidization Engineering; ButterworthHeinman: Oxford, U.K., 1991.


Jerndal et al. Table 2. Physical Properties of Particles Produceda R-Al2O3 Almatis CT3000SG

NiO Novamet refractory grade Novamet green grade F OMG standard grade OMG HSA grade Umicore Vogler brown grade

oxygen carrier NOV1T1400 NOV1T1500 NOV1T1600 NOV2T1400 NOV2T1500 NOV2T1600 OMG1T1400 OMG1T1500 OMG1T1600 OMG3T1400 OMG3T1500 OMG3T1600 UMI1T1400 UMI1T1500 UMI1T1600 VOG1T1400 VOG1T1500 VOG1T1600

Almatis CT800FG

crushing apparent BET surfaceb crushing apparent BET surfaceb strength (N) density (kg/m3) oxygen carrier strength (N) density (kg/m3) (m2/g) (m2/g) 0.91 0.52 0.04 1.85 0.97 0.18 1.43 0.55 1.28 0.48 2.38 1.53 0.47 0.48

1.4 4.1 11.7 1.6 3.8 9.2 Sc 0.4 1.3 S 0.5 1.6 0.2 0.4 1.2 S S 0.6

a Oxygen carriers in italic defluidized during reactivity testing. crushing strength.

2400 3400 4460 2490 3180 4380 1770 2060 2810 1440 1790 2920 1470 1940 3140 1290 1690 2410 b

was not prepared

OMG2T1400 OMG2T1500 OMG2T1600 OMG4T1400 OMG4T1500 OMG4T1600 UMI2T1400 UMI2T1500 UMI2T1600 VOG2T1400 VOG2T1500 VOG2T1600

no spherical particles no spherical particles 1.14 0.61 0.41


0.2 0.4 0.7 no spherical particles S S c

2070 2290 2410 1350 1550

S ) too soft to measure the

Table 3. Physical Properties of Particles Produced with Additivesa NiO/R-Al2O3 OMG HSA grade/Almatis CT3000SG additive 1% MgO MagChem30 5% MgO MagChem30 1% Ca(OH)2 Nordkalk SL

oxygen carrier

BET surfaceb (m2/g)

OMG5T1500 OMG5T1600 OMG6T1500 OMG6T1600 OMG7T1500 OMG7T1600

a Oxygen carriers in italic defluidized during reactivity testing. crushing strength.

0.69 0.37 0.19 0.056 b

crushing strength (N)

apparent density (kg/m3)

Sc

1680 2380 1770 2780 3730 4330

0.6 S 0.9 4.2 7.0


simulating the cyclic conditions of a chemical-looping combustion system. The low O2 concentration was used to avoid a large temperature increase, because of the heat produced in the highly exothermic oxidation reaction. CH4 was used during reduction because it is the major constituent in natural gas and simplifies the experimental evaluation compared to using natural gas. Inert N2 was introduced for 180 s, to flush the system between the oxidizing and reducing periods and, hence, avoid reactions between oxygen and methane. The gas from the reactor was led to an electric cooler, where the water produced in the reduction was condensed and removed, and then to a Rosemount NGA-2000 gas analyzer, where the concentrations of CO2, CO, CH4, and O2, as well as the flow, were measured. The experimental setup of the reactivity analysis equipment is presented in Figure 2. The inlet gas flows used were 450 mLN/min during reduction and inert periods and 1000 mLN/min during oxidation periods, where the flows are normalized to 1 bar and 0 °C. These flows correspond to 8-28umf for the oxidation period and 3-9umf for the reduction period, where umf is the minimum fluidization velocity, i.e., the gas velocity were the oxygen carriers would start to fluidize theoretically. The reason why a lower flow was used during reduction periods was the gas expansion when methane reacts with the oxygen carrier, forming 3 mol of product gas for each mole of reacted methane, which increases the gas volume and thus the flow. An example of the outlet gas concentrations, after water condensation and removal, and flow profiles for a reduction and an oxidation period is given in Figure 3. To obtain a more accurate concentration profile of the gases leaving the reactor, where residence times and back-mixing in the system are considered, a method of deconvolution was used to

c

S ) too soft to measure the

Figure 2. Experimental setup.

correct the CH4, CO2, and CO concentrations during reduction.27,28 Figure 4 shows the corrected gas concentrations, when dispersion in the system has been taken into account. The calculations show that most of the dispersion takes place in the electric cooler after the reactor. Because steam is condensed before reaching the gas



Figure 4. Concentration profiles during reduction of NOV1T1400 after correction for dispersion in the system. Vertical dashed lines indicate the end of the reduction period and transition to inert gas.

where Xi is the conversion as a function of time for period i, Xi-1 is the conversion after the preceding period, t0 and t1 are the times for the start and finish of the period, respectively, no is the number of moles of active oxygen in the unreacted oxygen carrier, n˙in and n˙out are the molar flows of the gas going into and leaving the reactor after water has been condensed and removed, respectively, Ptot is the total pressure, and pO2,in, pO2,out, pCO2,out, pCO,out, and pH2,out are the partial pressures of incoming and outlet O2 and outlet partial pressure of CO2, CO, and H2 after H2O removal. pH2,out was not measured online but assumed to be related to the outlet partial pressure of CO and CO2 through a relation based on the equilibrium of the gas shift reaction Figure 3. Outlet gas concentrations and flow profiles during (a) reducing and (b) oxidizing conditions of NOV1T1400. Vertical dashed lines indicate the end of the reacting period and transition to inert gas.

analyzer, the H2O concentration is calculated by estimating equilibrium between the reacting gases. The major H2 peak in the end of the reduction period is not measured in the gas analyzer but estimated to make up a total gas concentration of 100%. From measured outlet concentrations of CH4, CO2, CO, and O2, together with flow measurements, the reactivity of the particles could be calculated. The degree of oxidation of the oxygen carriers or the conversion was defined as

X)

m - mred mox - mred

(3)

where m is the actual mass of the sample, mox is the mass of the sample when fully oxidized, i.e., NiO, and mred is the mass of the sample in the fully reduced form, i.e., Ni. The degree of conversion was calculated as a function of time for the reducing period as

Xi ) Xi-1 -

∫

t1

t0

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

The degree of conversion was also calculated during the oxidation period to verify that the oxygen carriers were completely oxidized

Xi ) Xi-1 +

∫

t1

t0

2 (n˙ p - n˙outpO2,out)dt noPtot in O2,in

(5)

CO + H2O T CO2 + H2

(6)

A gas yield for the reduction period was defined as the degree of incoming methane that had been completely converted to CO2

pCO2

γred )

pCH4 + pCO2 + pCO

(7)

where pi is the partial pressure of component i in the outlet gas stream. To quantify the amount of unreacted methane leaving the reactor, a ratio of methane fraction in the outlet gas stream was defined as

frCH4 )

pCH4 pCH4 + pCO2 + pCO

(8)

To facilitate a comparison between different oxygen carriers, a methane index was introduced, defined as the fraction of methane leaving the reactor when X was between 0.95 and 0.99. These values of conversion, X, were chosen because an initial methane peak appeared somewhere in this interval for all oxygen carriers studied. The methane index was defined as

CH4, index )

∫

X)0.99

X)0.95

∫

X)0.99

X)0.95

(pCH4)dX

(pCH4 + pCO2 + pCO)dX

(9)


Jerndal et al.

Because the fuel conversion was incomplete in the tests with 1 g of oxygen carriers diluted in quartz, the reactivity was not limited by the amount of CH4 supplied. Therefore, these experiments were used to determine the reactivity of the oxygen carriers by calculating a rate index, using the same method as described in detail by Cho29 and Mattisson et al.30 The rate index, expressed in %/min, was calculated as

rate index ) 60 · 100 ·

( dωdt )

norm

(10)

where ω is the mass-based conversion defined as

ω)

m mox

(11)

and (dω/dt)norm is the normalized average rate expressed in s-1 and calculated from

dω dt

( )

norm

) keffpref

(12)

where pref is a reference partial pressure of methane, here equal to 0.15, which would approximately correspond to an average concentration of methane in the fuel reactor, assuming complete gas conversion. This statement is valid if the mass-transfer resistance between the bubble and emulsion phases in the fluidized bed reactor is small and if the reaction between incoming methane and the oxygen carriers is first-order with respect to methane. The exposure of particles to methane can then be represented by a log-mean partial pressure defined in terms of the inlet and outlet methane partial pressure

pm )

pin - pout pin ln pout

( )

(13)

For calculations of the rate index and the normalized rate of reaction, an average rate constant was calculated in the interval of ∆ω ) 0.01, where the reactivity was the fastest, defined as

keff )

1 dω pm dt

(14)

the amount of carbon formed during the reduction period over the total amount of carbon introduced during the reduction period and is defined as

Cdep ) Cadded

∫

t3

t1

∫

t3

t0

n˙out(pCO2,out + pCO,out)dt

n˙out(pCH4,out + pCO2,out + pCO,out)dt

(16)

where t0 is the time when the reduction started, t1 is the time when the inert period started, and t3 is a point of time in the inert period when no more carbon dioxide or carbon monoxide was registered. Because of the time delay caused by gas residence time in the system, t1 is defined as the point of time when no more CH4 is detected in the outlet flow.

3. Results 3.1. Characterization of Oxygen Carriers. A total of 36 different oxygen carriers were prepared, and of these, 29 formed spherical particles, where 21 were hard enough for crushing strength measurements. Figure 5 shows the apparent density as a function of the sintering temperature for the 23 spherical particles prepared without additives, and Figure 6 shows the six particles prepared with additives along with the three OMG3 materials, which are included for comparison. Figure 7 displays the crushing strength for the 21 oxygen carriers, where such a value could be obtained, as a function of the sintering temperature. Both the apparent density and the crushing strength, of the oxygen carriers, increased with an increased sintering temperature and were highly dependent upon the starting material. Particles with low apparent density have generally also low crushing strength and run the risk of fragmentation or attrition, when exposed to circulation in a large-scale application. However, as seen in Figures 6 and 7, the apparent density and the crushing strength can be increased considerably by a small addition of Ca(OH)2 to the raw material. The Brunauer-Emmett-Teller (BET) surface area was measured on all materials that were hard enough for crushing strength measurements. All materials had a BET surface area of below 2.5 m2/g, and from Figure 8, it is evident that the

From the normalized conversion rate, the needed bed mass in the fuel reactor can be calculated from the relationship

mbed )

ωm ˙0 dω dt norm

( )

(15)

where ω is the average mass conversion in the reactor and m ˙ 0 is the stoichiometric mass flow of oxygen transferred between the reactors. In this type of batch experiments, the particles were sometimes reduced to relatively low degrees of conversion. Here, there was formation of carbon on the particles. The carbon formation during the reduction period was calculated by integrating the total amounts of carbon dioxide and carbon monoxide produced during the subsequent inert period. The carbon formation ratio, Cdep/Cadded, is (27) Levenspiel, O. Chemical Reaction Engineering; John Willey and Sons: New York, 1981. (28) Abad, A.; Mattisson, T.; Lyngfelt, A.; Johansson, M. The use of iron oxide as oxygen carrier in a chemical-looping reactor. Fuel 2007, 86 (7-8), 1021–1035. (29) Cho, P. Development and characterisation of oxygen-carrier materials for chemical-looping combustion. Ph.D. Thesis, Chalmers University of Technology, Go¨teborg, Sweden, 2005. (30) Mattisson, T.; Johansson, M.; Lyngfelt, A. Multicycle reduction and oxidation of different types of iron oxide particlessApplication to chemical-looping combustion. Energy Fuels 2004, 18 (3), 628–637.

Figure 5. Apparent density as a function of the sintering temperature for NOV1 (b), NOV2 (O), OMG1 (9), OMG3 (0), UMI1 ([), UMI2 (]), VOG1 (2), and VOG2 (∆).


Figure 6. Apparent density as a function of the sintering temperature for OMG3 (0), OMG5 (0 with + inside), OMG6 (0 with × inside), and OMG7 (!). Dashed lines indicate particles prepared with additives.

Figure 7. Crushing strength as a function of the sintering temperature for NOV1 (b), NOV2 (O), OMG1 (9), OMG3 (0), UMI1 ([), UMI2 (]), VOG1 (2), OMG5 (0 with + inside), OMG6 (0 with × inside), and OMG7 (!). Dashed line indicates particles prepared with additives.

BET surface is clearly correlated with sintering temperature, with very low surface areas for the materials sintered at high temperatures. The X-ray powder diffraction investigation revealed that no unexpected phase transition occurred within any of the oxygen carriers during the reactivity investigation and that no differences in phase composition were seen between materials sintered at different temperatures. However, it is well-known that all materials used contain minor amounts of impurities, although these amounts are too small to be detected by X-ray powder diffraction. As specified by each NiO supplier, most materials contain impurities of primarily Co, Cu, and Fe. The amounts of these are highest for the nickel oxides Novamet refractory


Figure 8. BET surface as a function of the crushing strength for NOV1 (b), NOV2 (O), OMG1 (9), OMG3 (0), UMI1 ([), UMI2 (]), VOG1 (2), OMG5 (0 with + inside), OMG6 (0 with × inside), and OMG7 (!). Dashed line indicates particles prepared with additives.

grade and Vogler brown grade, where the fraction of Co is up to 1.5 wt %, while the fractions of Cu and Fe are somewhat lower. All of the investigated particles were studied using scanning electron microscopy (SEM), which confirmed that 29 of the 36 particles were spherical, although they displayed some variation in surface structure. This is illustrated in Figures 9 and 10, which show some particles prior to and after reactivity testing. It can be clearly seen from the SEM images that particles with high BET surface, low apparent density, and crushing strength, e.g., UMI1T1400, have a more rough surface structure of fine granules, while particles with low BET surface, high apparent density, and crushing strength, e.g., NOV1T1600, have a smoother surface made up of coarser granules. After the reactivity testing, the soft particles have obtained a somewhat smoother surface structure, while the harder particles show major changes with respect to surface structure, which may explain the defluidization of these particles. 3.2. Reactivity of Oxygen Carriers. The gas concentrations and flow profiles during the reactivity testing had some common characteristics, regardless of oxygen carrier material and length of reduction period, and an example of this can be found in Figures 3 and 4. The conversion to CO2 was generally high, with some unreacted methane released early in the reduction when the oxygen carriers were highly oxidized and some released late in reduction when the carriers were fully reduced. The thermodynamic limitation of using nickel as an oxygen carrier can be seen from the small amount of CO always present during reduction. The CO released from the reactor had two peaks. The first is connected with the decrease of CO2 and is caused by the lack of oxygen to fully convert the fuel. The second is caused by a solid-solid reaction of carbon formed with some remainders of oxygen in the oxygen carriers and will be discussed in the Carbon Formation section. These two peaks are clearly seen in Figure 4. The gas flow variations observed during reduction is caused by gas expansion during reaction and contraction during steam condensation. Flow variations in the reactor will rapidly affect the measured flow, while changes in gas concentration have to reach the analyzer before being observed. This fact can be seen


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Figure 9. SEM images of oxidized particles and their surfaces prior to reactivity testing: (a and d) UMI1T1400, (b and e) NOV1T1400, and (c and f) NOV1T1600.

Figure 10. SEM images of reduced particles and their surfaces after reactivity testing: (a and d) UMI1T1400, (b and e) NOV1T1400, and (c and f) NOV1T1600.

in Figure 3a, where the flow increases almost instantaneously at the start of the reduction, while the gas concentrations increase about 10 s later. Apart from this, the flow behavior is dependent upon the time delay when changing incoming gas to the reactor, because of the gas residence time between the mass flow control unit and the gas analyzer. This time delay varies with the flow, as well as with back-mixing in the system, which can be seen in the transient changes in the gas concentrations.

From Figure 3b, it is clear that the oxidation rate is very fast and initially limited by the oxygen supplied, meaning that all oxygen provided for the first 1100 s reacts with the oxygen carrier. The following rapid oxygen concentration increase is explained by the fact that the oxygen carrier is nearly completely oxidized and added oxygen starts to pass unreacted through the bed, which is also shown by the slightly increased flow. Because of the simple nature of the oxidation reaction, most of the focus


Figure 11. Gas yield, γred, and fraction unreacted CH4, as a function of the conversion, X, for NOV1T1400 (b), NOV2T1400 (9), and OMG3T1500 ([). The sample amount is 15 g.

of the present investigation will be on the reduction of the oxygen carriers with CH4. During the reactivity testing, which was performed on all particles produced regardless of strength and apparent density, defluidization occasionally occurred, as indicated in Table 2. Defluidization always came about late in a reduction period or early in a subsequent inert period, and to avoid this, the inert flow was increased to make fluidization recommence. However, defluidization then re-occurred during one of the subsequent reduction periods or its following inert period until it was irrevocable. The length of the reduction periods was gradually increased during the reactivity testing until full conversion of the particles was reached. This means that the gas velocity in the end of the reductions gradually decreased, because less gas was formed by the reaction, leading to an increased risk of defluidization. After defluidization, the beds were still relatively loosely packed and no or little force was needed to separate the particles. Of the 36 oxygen carriers tested in reactivity experiments, 12 showed excellent fluidization behavior with no tendencies of agglomeration, regardless of time in reduction. These materials showed high reactivity in both reduction and oxidation and high fuel conversion in reduction. Figures 11-14 show the gas yield and methane fraction as a function of oxygen carrier conversion for some of the materials with good fluidization behavior. In general, the reactivity is very high for all of these materials. In fact, the gas yield approached 95% for all materials with just 1 g of oxygen carrier, except for the materials prepared with an addition of 5% MgO. These small bed masses correspond to only 4 kg/MW fuel. Note that the scale on the y axis differs between the tests with 15 g of material, i.e., Figures 11 and 13, and the tests with 1 g of active material, i.e., Figures 12 and 14. The amount of methane observed at high degrees of oxidation was highly dependent upon the material used, as seen from the methane index in Table 4. The reason for the unconverted methane from the outlet at high degrees of oxidation is likely associated with the fact that there are limited sites of metallic Ni available for catalytic reactions with methane when the oxygen carriers are highly oxidized. Such sites seem to be needed to obtain complete fuel conversion. The gas yield during reduction when the bed mass used was 15 g was close to one for all of the oxygen carriers that fluidized, with exceptions when the oxygen carriers were highly oxidized


Figure 12. Gas yield, γred, and fraction unreacted CH4, as a function of the conversion, X, for NOV1T1400 (b), NOV2T1400 (9), and OMG3T1500 ([). The sample amount is 1 g.

Figure 13. Gas yield, γred, and fraction unreacted CH4, as a function of the conversion, X, for OMG5T1500 (b), OMG6T1500 (9), and OMG6T1600 ([). The sample amount is 15 g.

or highly reduced. The reactivity of three of the materials, on the basis of NiO/NiAl2O4 only, is displayed in Figure 11. The reason for the incomplete fuel conversion early in reduction is the initial methane released from the reactor. Further, the reason why the gas yield is below one throughout the reduction is due to the thermodynamic restraint, which limits the gas yield. From Figure 12, it is seen that a high gas yield of well above 90% is obtained with a bed of oxygen carriers as small as 1 g. There is methane released from the bed for almost the entire conversion span, although the amount decreases as a function of time. All materials with good fluidization behavior, on the basis of NiO/NiAl2O4 only, showed a similar appearance as those presented in Figures 11 and 12, although the fraction of unconverted CH4 and, hence, the gas yield varied to some extent. As mentioned earlier, the use of MgO in this type of Nibased oxygen carrier has been shown to have positive effects on methane conversion in earlier investigations. The oxygen carriers prepared with an addition of 1 and 5% MgO, respectively, OMG5 and OMG6, showed an increase in methane conversion early in reduction, compared to other materials,


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Figure 14. Gas yield, γred, and fraction unreacted CH4, as a function of the conversion, X, for OMG5T1500 (b), OMG6T1500 (9), and OMG6T1600 ([). The sample amount is 1 g. Table 4. Methane Index and Rate Index of Particles Produced CH4 index (%)

rate index (%/min)

OMG1T1400 OMG1T1500 OMG3T1400 OMG3T1500 NOV1T1400 NOV2T1400 VOG1T1400 VOG1T1500 UMI1T1400 UMI2T1500

Oxygen Carrier 7.3 4.6 3.1 3.8 6.4 2.6 17.8 8.4 2.8 2.8

78 87 74 88 72 82 83 84 77 85

OMG5T1500 OMG6T1500 OMG6T1600

Oxygen Carrier with Additives 2.8 0.3 0.6

84 78 62

which can be seen in Figure 13 and more clearly in Table 4. However, from Figure 13, it is also seen that the gas yields for the materials prepared with an addition of 5% MgO decreases well before these particles are completely reduced. Here, the oxidation of methane is only partial, and a large amount of CO is produced. Further, a clear difference can be seen in the fuel conversion between OMG6 produced at 1500 and 1600 °C, with a higher conversion for the material produced at the lower temperature. The differences in reactivity between these Mg-doped materials are even more obvious in Figure 14. With an addition of 1% MgO, the gas yield is well above 90%, but with 5% MgO added, the gas yield is considerably lower, although the fraction of unreacted methane is somewhat less. For the OMG6 sintered at 1600 °C, the gas yield starts at about 75% early in reduction and is then decreased rapidly because the fuel conversion is only partial and CO is the main carbon-containing product gas. For the OMG7 materials, where 1% Ca(OH)2 was added, defluidization occurred during the reactivity investigation; hence, no gas conversion data are presented for these. From the methane index presented in Table 4, it is clearly seen that the materials based on Vogler NiO have the lowest fuel conversion, while materials based on Novamet green grade F and Umicore NiO have the highest, when comparing the materials prepared without additives. When considering the particles prepared with an addition of MgO, the methane conversion is improved. This fuel conversion increase is higher

Figure 15. Carbon formation ratio, Cdep/Cadded, as a function of the conversion, X, for OMG1T1400 (b), OMG1T1500 (9), OMG3T1400 ([), OMG3T1500 (+), NOV1T1400 (O), NOV2T1400 (0), VOG1T1400 (]), VOG1T1500 (×), UMI1T1400 (f), UMI1T1500 (g), OMG5T1500 (4), OMG6T1500 (3), and OMG6T1600 (2).

for the carrier prepared with an addition of 5% MgO, where the methane index decreases by almost a factor of 10 compared to the same particle without MgO addition. The rate index calculations showed that the reactivity was similar for most materials tested, with a rate index varying between 72 and 88%/min for all materials, except OMG6T1600, which had a lower reactivity and a rate index of 62%/min, as seen in Table 4. Further, it was seen that the reactivity was fairly constant regardless of the reduction number, with exceptions for the very first reduction, where the reactivity was generally lower. 3.3. Carbon Formation. During the reduction period, carbon might form on the oxygen carrier particles, most likely at low gas yields.31 In these types of batch experiments, carbon deposition is registered during the inert and oxidizing period following the reduction through the formation of CO and CO2. On the basis of a thermodynamic limitation, carbon formation is strongly dependent upon the availability of oxygen. When CH4 is used as fuel, carbon formation should not be expected at 950 °C, as long as at least one-fourth of the oxygen needed for complete methane conversion is supplied.31 One possible way to suppress carbon formation is therefore to add steam to the CH4 during oxygen carrier reduction and thereby meet the oxygen demand even at low degrees of oxygen carrier oxidation.32 To study the onset of carbon formation and determine possible differences with respect to the different oxygen carriers analyzed, the time in reduction was increased for each cycle in the reactivity tests. The carbon formed during reduction and its correlation with the oxygen carrier conversion are shown in Figure 15. In general, carbon formation was only detected at low degrees of conversion, where the gas yield was low. As seen in Figure 15, carbon formation was not detected until only about 5% of (31) Cho, P.; Mattisson, T.; Lyngfelt, A. Carbon formation on nickel and iron oxide-containing oxygen carriers for chemical-looping combustion. Ind. Eng. Chem. Res. 2005, 44 (4), 668–676. (32) Ishida, M.; Jin, H.; Okamoto, T. Kinetic behavior of solid particle in chemical-looping combustion: Suppressing carbon deposition in reduction. Energy Fuels 1998, 12 (2), 223–229.


Figure 16. Carbon formation ratio, Cdep/Cadded, as a function of the gas yield at the break point, γred, i.e., at the end of the reduction period, for OMG1T1400 (b), OMG1T1500 (9), OMG3T1400 ([), OMG3T1500 (+), NOV1T1400 (O), NOV2T1400 (0), VOG1T1400 (]), VOG1T1500 (×), UMI1T1400 (f), UMI1T1500 (g), OMG5T1500 (4), OMG6T1500 (3), and OMG6T1600 (2).

the oxygen in the particles remained for all materials, with small differences seen between the different oxygen carriers. Figure 16 shows the carbon formation correlated with the gas-phase composition, i.e., the carbon formation ratio as a function of the gas yield at the break point. This gas yield is defined as the fuel conversion in the middle of the transient in the end of a reduction, i.e., where the CH4 concentration has decreased by 50%. As is evident, the onset of carbon formation is clearly correlated with a low fuel conversion. This indicates that significant carbon formation only occurs when the particles are too reduced to convert the fuel completely. 4. Discussion Oxygen carriers for chemical-looping combustion based on commercially available NiO and R-Al2O3 are generally very promising, with high reactivity for most materials independent of the supplier and sintering temperature. However, there were clear defluidization tendencies for some materials in the batch reactor. Oxygen carriers with higher density, sintered at higher temperature, generally showed increased defluidization tendencies. The reason for this is that dense particles need higher gas velocities to be fluidized. Because all tests were performed with the same gas velocities, dense particles were tested at conditions closer to their minimum fluidization velocity. Defluidization of the oxygen carriers always came about late in a reduction period or early in a subsequent inert period, when the oxygen carriers were highly reduced, and it is therefore believed that the risk of defluidization can be decreased by terminating the reduction period before the oxygen carriers are fully reduced. This was not performed in the experiments because it would counteract some of the aims of the study, i.e., to study the fuel conversion for highly reduced oxygen carriers and the onset of carbon formation, which occur when oxygen carriers are reduced to a large extent. In all cases where defluidization occurred, only soft and easily breakable agglomerates were formed and the oxygen carriers remained spherical. Therefore, it is not likely that the defluidization is caused by the formation of any liquid phases, for example, melting of the Cu impurities, which might


occur at elevated temperatures. Kuusik et al. have found that defluidization for particles based on the same raw materials as NOV1 in the present investigation is caused by adhesion of particles by bridges containing pure Ni.33 These soft agglomerates will likely break when exposed to the higher gas velocities in a large-scale circulating chemical-looping combustion system. Adding MgO to the raw material in the particle production phase will lead to the formation of the spinel MgAl2O4. Earlier studies have shown that, by using MgAl2O4 as supporting material, for nickel-based oxygen carriers, the methane conversion is enhanced compared to using NiAl2O4.16,23,34 This methane conversion improvement has been explained to be caused by the improved dispersion of active nickel, which increases the catalytic performance when CH4 is partially oxidized to CO and H2.35 An addition of Ca(OH)2 during particle preparation will most likely lead to the formation of the spinel CaAl2O4. However, the amount of Ca(OH)2 added was too small to confirm this formation by X-ray analysis, although the assumption of CaAl2O4 formation was supported by thermodynamic equilibrium calculations. The rate index was calculated as a normalized conversion rate, expressed in terms of percentage mass change per minute and based on a few general assumptions. These assumptions were that the reaction is first-order, with an almost complete fuel conversion, corresponding to a pm value of 0.15, and that the mass-transfer resistance between the bubble phase and dense phase is small. The reaction order for particles with the same chemical composition and production method as the oxygen carriers in the present investigation has been determined by Abad et al.15 to 0.8. Therefore, the assumption of a first-order reaction seems reasonable. The assumption of a small mass-transfer resistance is supported by the fact that all incoming oxygen was consumed in the initial part of the oxidation period. Because the rate index is not based on detailed kinetics, it should be regarded as an indicative number used to compare different oxygen carriers and to estimate a needed mass of solids and not as a definite value to determine the solids inventory in a chemical-looping combustion unit. The high rate indexes correspond to the mass of solids in the fuel reactor of less than 10 kg/MW, which is of course consistent with the fact that up to 95% conversion is attained in the 1 g tests, i.e., with a bed mass of 4 kg/MW. Of the 30 oxygen carriers prepared without additives, 10 showed excellent fluidization behavior and, hence, rate indexes and fuel conversion could be obtained for these. All of them showed high reactivity, although the fraction of unreacted methane varied considerably. Of these 10 materials, only 2 had a crushing strength exceeding 1 N; hence, the others run an increased risk of fragmentation and attrition in a large-scale circulating system. However, the harder particles that defluidized should not be entirely excluded because the higher gas velocities that they are exposed to in a large-scale application might improve their fluidization behavior. The two most promising oxygen carriers are NOV1T1400 and NOV2T1400, because they displayed excellent fluidization behavior in combination with (33) Kuusik, R.; Trikkel, A.; Lyngfelt, A.; Mattisson, T. High temperature behavior of NiO-based oxygen carriers for chemical looping combustion. Presented at the 9th International Conference on Greenhouse Gas Control Technologies, Washington, D.C., November 16–20, 2008. (34) Johansson, E.; Mattisson, T.; Lyngfelt, A.; Thunman, H. Combustion of syngas and natural gas in a 300 W chemical-looping combustor. Chem. Eng. Res. Des. 2006, 84 (A9), 819–827. (35) Qiu, Y.-J.; Chen, J.-X.; Zhang, J.-Y. Effects of MgO promoter on properties of Ni/Al2O3 catalysts for partial oxidation of methane to syngas. J. Fuel Chem. Technol. 2006, 34 (4), 450–455.


comparably high strength. However, the particles of lower strength might be suitable if a small amount of Ca(OH)2 is added. Commercially available raw materials can clearly be used to produce high performing oxygen carriers for chemical-looping combustion. To reduce the production cost of these oxygen carriers even further, they should be prepared by a production technique suitable for large scale, e.g., spray drying. It has to be investigated if oxygen carriers with the same or similar properties can be prepared in this manner, instead of with the freeze-granulation production method, used in the present investigation, which is suitable for small-scale production but would be associated with high costs in the larger scale. 5. Conclusions A total of 36 oxygen carriers based on NiO and R-Al2O3 were prepared and investigated with respect to parameters important for chemical-looping combustion. The oxygen carriers were based on six commercially available NiO materials, which were combined with two different commercially available supporting R-Al2O3 materials and prepared using freeze granulation. Each material was sintered at three different temperatures. Although some materials failed during preparation, all of the materials that were tested with respect to reactivity showed high reactivity during the reduction with methane. Of the 36 particles made, 6 were produced with small amounts of additives to

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increase their performance. It was clear that an addition of Ca(OH)2 increases the strength and thus reduces the risk of fragmentation and attrition in a chemical-looping combustion device. An addition of MgO to the oxygen carriers can be used to increase the fuel conversion early in reduction, which seems to be restricted because of the limited amounts of metallic Ni. An increased sintering temperature generally gives harder particles of higher density; however, the risk of defluidization seems to increase for such particles. Carbon formation was only detected when the oxygen carriers were highly reduced and the fuel conversion was incomplete. The implication of this is that carbon formation should not be expected in a circulating system, where the fuel conversion is high and the particles are recycled to the air reactor well before they are fully reduced. Twooftheinvestigatedparticles,NOV1T1400andNOV2T1400, displayed a combination of high reactivity and strength as well as excellent fluidization behavior and should be feasible for use as oxygen carriers in a large-scale chemical-looping combustion process. Acknowledgment. This work was part of the EU financed project Chemical-Looping Combustion CO2-Ready Gas Power (CLC Gas Power, EU contract 019800) led by Chalmers University of Technology. The project is also part of phase II of the CO2-Capture Project (CCP) via Shell. EF8006596

NiAl2O4 Particles as Oxygen Carriers for

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