Creating a Synergy Effect by Using Mixed Oxides ... - ACS Publications

Energy & Fuels 2006, 20, 2399-2407

2399

Creating a Synergy Effect by Using Mixed Oxides of Iron- and Nickel Oxides in the Combustion of Methane in a Chemical-Looping Combustion Reactor Marcus Johansson,*,† Tobias Mattisson,‡ and Anders Lyngfelt‡ Department of Chemical and Biological Engineering, EnVironmental Inorganic Chemistry, and Department of Energy and EnVironment, Energy Technology, Chalmers UniVersity of Technology, S-412 96 Go¨teborg, Sweden ReceiVed February 16, 2006. ReVised Manuscript ReceiVed July 6, 2006

Chemical-looping combustion is a new technology that could help reconcile the contradictory requirements of an increasing energy demand and the reduction of greenhouse gas emissions. This technique involves combustion of fossil fuels with inherent CO2 capture by means of an oxygen carrier that is circulated between air and fuel reactors. In this article, synergy effects of the combustion of methane by using a small amount of nickel oxide in a bed of iron oxide are presented. Reactivity was investigated on particles 125-180 µm in a laboratory fluidized bed reactor of quartz. Reduction was performed in 50% CH4/50% H2O. The two oxygen carriers used are 60% NiO/40% MgAl2O4 sintered at 1400 °C and 60% Fe2O3/40% MgAl2O4 sintered at 1100 °C. It is shown that, at 950 °C, a bed consisting of the mixed system of 3 wt % nickel oxides with 97 wt % iron oxides produces almost two times as much carbon dioxide per time unit in comparison to the sum of carbon dioxide when the oxygen carriers were tested separately. This effect is likely due to the catalytic action of metallic Ni, which reforms the methane to CO and H2, which then reacts with the iron oxide at a considerably higher rate than methane. Furthermore, no carbon formation or defluidization occurred.

Introduction It is widely accepted today that CO2 emissions escalate the greenhouse effect and hence contribute to a higher average temperature on earth. About a third of the global CO2 emissions comes from the burning of fossil fuels in power production.1 One way to decrease CO2 emissions in the future is to change to renewable fuels, such as wind, solar, or biomass. Because it may not be possible to achieve a change to renewable fuels rapidly enough, another option to speed up the reduction of emissions is to use CO2 capture and storage. In this way, abundant sources of fossil fuels could still be used for power production. Some of the proposed options for CO2 storage are in gas fields, oil fields, at the bottom of the sea, or in aquifers. Today, 5-8 Mt of CO2 has already been injected 550-1500 below the sea floor in the Utsira aquifer formation in the North Sea outside Norway.2,3 Several techniques for CO2 capture have been proposed, including posttreatment, O2/CO2 firing, and CO shift. The problem with these options is the high costs and energy penalties caused by the technologies needed for the separation of gases.1 A way to avoid the cost and energy penalty of gas separation is to use unmixed combustion, as done in fuel cells or in chemical-looping combustion. * Corresponding author. Tel: 46-31-7722887. Fax: 46-31-7722853. E-mail: [email protected]. † Department of Chemical and Biological Engineering, Environmental Inorganic Chemistry, Chalmers University of Technology. ‡ Department of Energy and Environment, Energy Technology, Chalmers University of Technology. (1) Lyngfelt, A.; Leckner, B. Technologies for CO2 Separation. In Minisymposium on Carbon Dioxide Capture and Storage, Chalmers University of Technology and Go¨teborg University, Go¨teborg, Sweden, Oct 22, 1999; pp 25-35. (2) Chadwick, R. A.; Zweigel, P.; Gregersen, U.; Kirby, G. A.; Holloway, S.; Johannessen, P. N. Energy 2004, 29, 1371-1381. (3) Torp, T. A.; Gale, J. Energy 2004, 29, 1361-1369.

Chemical-Looping Combustion. Chemical-looping combustion (CLC) is a combustion technology that consists of two reactors: an air and a fuel reactor.4 Oxygen carriers, in the form of metal-oxide particles, are circulating between the reactors and transferring oxygen from the combustion air to the fuel. In this way, direct contact between air and fuel is avoided and thus nitrogen and residual oxygen from the combustion air will not be mixed with the combustion products from the fuel reactor. The air reactor could be a riser similar to that in a circulating fluidized bed combustor, whereas the fuel reactor could be a bubbling fluidizing bed reactor. A proposed design4 of a real reactor system can be seen in Figure 1. The fuel is introduced to the fuel reactor in a gaseous form, where it reacts with an oxygen carrier to CO2 and H2O. The reduced oxygen carrier is transported back to the air reactor, where it is reoxidized by air. The fuel could be syngas from coal gasification, natural gas, or refinery gas. The overall reactions in the reactors are given below.

Fuel reactor: (2n + m)MyOx + CnH2m f (2n + m)MyOx-1 + mH2O + nCO2 (1) Air reactor: (2n + m)MyOx-1 + (n + 1/2m)O2 f (2n + m)MyOx (2) Net reaction: CnH2m + (n + 1/2m)O2 f mH2O + nCO2 The total amount of heat evolved from reaction 1 plus reaction 2 is the same as for normal combustion, where the oxygen is in direct contact with the fuel. However, the advantage with this (4) Lyngfelt, A.; Leckner, B.; Mattisson, T. Chem. Eng. Sci. 2001, 56 (10), 3101-3113.

10.1021/ef060068l CCC: $33.50 © 2006 American Chemical Society Published on Web 09/15/2006

2400 Energy & Fuels, Vol. 20, No. 6, 2006

system compared to normal combustion is that the CO2 and H2O are inherently separated from the rest of the flue gases, and no major energy is expended for this separation. Thus, compared to other technologies for the capture of CO2, CLC is potentially much cheaper, because no costly gas-separation equipment is necessary.1 Although a real process could be either pressurized or atmospheric, a first step is to investigate CLC under atmospheric conditions. The expected temperature range could be 800-1200 °C for the air and fuel reactors, although the temperature would need to be higher in the air reactor for cases where the reduction in the fuel reactor is endothermic. There are different criteria for the process that should be optimized to decrease the cost and increase the efficiency of a chemical-looping combustor. One of the most important is that there is a full conversion of the incoming fuel. This is dependent on the reactivity of the oxygen carriers and in the end determines the bed mass that has to be used in the fuel reactor. Furthermore, the products from the fuel reactor should contain as much CO2 and water as possible. Because pure CO2 is the desired product of the process, unconverted CH4 as well as any CO and H2 formed would need to be addressed. The easiest way could be to add some oxygen at the fuel reactor outlet to oxidize these compounds. Oxygen Carriers. In a thermodynamic analysis of different oxygen carriers, Jerndal et al. concluded that the metal-oxide pairs of Fe2O3/Fe3O4, Cu2O/Cu, Mn3O4/MnO, and NiO/Ni were feasible candidates to be used as oxygen carriers in CLC when using methane as a fuel.5 These oxygen carriers would preferably be supported by an inert material. The inert material should add positive properties, among which the most important are to maintain the pore structure inside the particle. Different research groups have in recent years performed experiments on oxygen carriers for chemical-looping combustion, e.g., on iron oxides6-20 and nickel oxides,6-16,18,20-32 which are the two types of metal oxides most investigated. Normally, nickel oxide is the most reactive carrier, but a drawback is that it is toxic and cannot thermodynamically convert the fuel to 100% CO2 and H2O.5,28 By this, it is meant that small amounts of CO and H2 (5) Jerndal, E.; Mattisson, T.; Lyngfelt, A. Chem. Eng. Res. Des. 2006, 85 (A9), 795-806. (6) Johansson, M.; Mattisson, T.; Lyngfelt, A. Therm. Sci. 2006, in press. (7) Johansson, M.; Mattisson, T.; Lyngfelt, A. Comparison of Oxygen Carriers for Chemical-Looping Combustion of Methane-Rich Fuels. 19th FBC Conference, Vienna, May 21-24, 2006. (8) Adanez, J.; de Diego, L. F.; Garcia-Labiano, F.; Gayan, P.; Abad, A.; Palacios, J. M. Energy Fuels 2004, 18 (2), 371-377. (9) Cho, P.; Mattisson, T.; Lyngfelt, A. Fuel 2004, 83 (9), 1215-1225. (10) Ishida, M.; Jin, H.; Okamoto, T. Energy Fuels 1998, 12 (2), 223229. (11) Jin, H.; Okamoto, T.; Ishida, M. Ind. Eng. Chem. Res. 1999, 38 (1), 126-132. (12) Lee, J.-B.; Park, C.-S.; Choi, S.-I.; Song, Y.-W.; Kim, Y.-H.; Yang, H.-S. J. Ind. Eng. Chem. (Seoul) 2005, 11 (1), 96-102. (13) Zafar, Q.; Mattisson, T.; Gevert, B. Ind. Eng. Chem. Res. 2005, 44 (10), 3485-3496. (14) Cho, P.; Mattisson, T.; Lyngfelt, A. Ind. Eng. Chem. Res. 2006, 45 (3), 968-977. (15) Lyngfelt, A.; Thunman, H. Construction and 100 h of Operational Experience of a 10 kW Chemical-Looping Combustor. In Carbon Dioxide Capture for Storage in Deep Geologic FormationssResults from the CO2 Capture Project; Thomas, D., Benson, S., Eds.; Elsevier: New York, 2005; Vol. 1, pp 625-645. (16) Cho, P.; Mattisson, T.; Lyngfelt, A. Ind. Eng. Chem. Res. 2005, 44 (4), 668-676. (17) Johansson, M.; Mattisson, T.; Lyngfelt, A. Ind. Engi. Chem. Res. 2004, 43 (22), 6978-6987. (18) Mattisson, T.; Jaerdnaes, A.; Lyngfelt, A. Energy Fuels 2003, 17 (3), 643-651. (19) Mattisson, T.; Johansson, M.; Lyngfelt, A. Energy Fuels 2004, 18 (3), 628-637. (20) Son, S. R.; Kim, S. D. Ind. Eng. Chem. Res. 2006, 45 (8), 26892696.

Johansson et al.

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

will always be present in the exit of the fuel reactor. Iron oxide is much cheaper than nickel oxides; the market price between the metals differs by almost a factor of 300.33 Besides, iron oxide is not toxic; however, a disadvantage is its low reactivity with methane.6-8 A good iron-based oxygen carrier, 60% Fe2O3/ 40% MgAl2O4 sintered at 1100 °C (F6AM1100), was presented by Johansson et al.17 It had relatively high and constant reactivity with methane throughout the redox cycles and did not agglomerate. It was speculated that the reaction proceeded via CO and H2 but that the rate-limiting step was reforming of methane. A very reactive nickel oxide, 60% NiO/ 40% MgAl2O4 sintered at 1400 °C (N6AM1400), was presented by the same authors.6 This particle has also been tested successfully by Johansson et al. in a 300 W chemical-looping combustor.25,26 In recent studies by Mattisson and co-workers, it was found that nickel oxides could convert methane almost completely to CO2 and H2O, although very small bed masses were used.27,30 These investigations found that NiO also likely reacted via the intermediate CO and H2, but because the metallic nickel formed more on the particle surface enhanced the reforming of methane, the overall reaction rate was much greater in comparison to iron (21) Ryu, H. J.; Bae, D. H.; Jin, G. T. Korean J. Chem. Eng. 2003, 20 (5), 960-966. (22) Song, K. S.; Seo, Y. S.; Yoon, H. K.; Cho, S. J. Korean J. Chem. Eng. 2003, 20 (3), 471-475. (23) Villa, R.; Cristiani, C.; Groppi, G.; Lietti, L.; Forzatti, P.; Cornaro, U.; Rossini, S. J. Mol. Catal. A: Chem. 2003, 204, 637-646. (24) Corbella Beatriz, M.; de Diego Luis, F.; Garcia-Labiano, F.; Adanez, J.; Palacio Jose, M. EnViron. Sci. Technol. 2005, 39 (15), 5796-803. (25) Johansson, E.; Mattisson, T.; Lyngfelt, A.; Thunman, H. Chem. Eng. Res. Des. 2006, 85 (A9), 819-827. (26) Johansson, E.; Mattisson, T.; Lyngfelt, A.; Thunman, H. Fuel 2006, 85 (10-11), 1428-1438. (27) Mattisson, T.; Johansson, M.; Lyngfelt, A. Fuel 2006, submitted for publication. (28) Mattisson, T.; Johansson, M.; Lyngfelt, A. Fuel 2006, 85 (5-6), 736-747. (29) Lyngfelt, A.; Kronberger, B.; Ada´nez, J.; Morin, J.-X.; Hurst, P. The GRACE Project: Development of Oxygen Carrier Particles for Chemical-Looping Combustion. Design and Operation of a 10 kW Chemical-Looping Combustor. In Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies, Vancouver, BC, Sept 5-9, 2004. (30) Johansson, M.; Mattisson, T.; Lyngfelt, A. Ind. Eng. Chem. Res. 2006, 45 (17), 5911-5919. (31) Adanez, J.; Garcia-Labiano, F.; de Diego, L. F.; Gayan, P.; Celaya, J.; Abad, A. Ind. Eng. Chem. Res. 2006, 45 (8), 2617-2625. (32) Jin, H.; Okamoto, T.; Ishida, M. Energy Fuels 1998, 12 (6), 12721277. (33) http://minerals.usgs.gov/minerals/; U.S. Geological Survey: Reston, VA.

Use of Mixed Oxides in a Chemical-Looping Combustion Reactor

Energy & Fuels, Vol. 20, No. 6, 2006 2401

Table 1. Oxygen Carriers metal oxide

wt % metal oxide

inert

wt % inert

nomenclature

porosity

BET (m2/g)

sintering temperature (°C)

NiO Fe2O3

60 60

MgAl2O4 MgAl2O4

40 40

N6AM1400 F6AM 1100

0.42 0.52

1.24 8.31

1400 1100

oxide. Nickel-oxide particles have been successfully used in a 10 kW chemical-looping combustor, using natural gas as fuel.15,29 Only very small amounts, near the thermodynamically equilibrium, of CO and H2 were present in the exit of the fuel reactor, and the operation was conducted without problems for 100 h. A shorter test was made in the same reactor with the same iron-based oxygen carriers as those used in this paper.15 For these, the conversion of CH4 to CO2 was unsatisfactory, with too high amounts of CH4 and CO leaving the fuel reactor. The indication that nickel catalyzes methane conversion to CO and H2, and the fact that iron oxide reacts fast with these, suggests that a combination of both types of oxides may show synergy effects with an increased overall rate of reaction with respect to iron. This may have great implication in terms of the cost and safety of oxygen carriers. The investigation of oxygen carriers based on mixed metal oxides has been performed by several authors with different types of synergy effects desired. The investigations were performed either by mixing different metal oxides into each particle or mixing different metal-oxide particles in a bed. Jin et al. prepared a CoO-NiO particle on yttria-stabilized zirconia.32 In this way, a synergetic effect was achieved where the reactivity of the oxygen carrier was maintained at a high level, but without formation of solid carbon on the particles. Son and Kim mixed NiO and Fe2O3 of different ratios on bentonite support.20 The reason for this was mainly to use iron to increase the strength of highly reactive nickel-based carriers. Ada´nez et al. mixed CuO-NiO on an alumina support as well as investigated the mixing of particles based on CuO and NiO in a bed.31 The desired synergetic effect in this case was to use copper-based carriers to achieve full conversion, because the oxidation of fuel that occurs with nickel-based oxygen carriers is rapid but thermodynamically incomplete. In this study, the effect of using small amounts of N6AM1400 in a bed of F6AM1100 was investigated to see if the nickel or nickel oxide could significantly increase the reactivity of the iron bed with methane. Different ratios of iron oxides and nickel oxides and different reaction temperatures are investigated. No focus is put here on the oxidation, because it was shown earlier to be fast for both investigated carriers.6,17 Experimental Section Preparation of Oxygen Carriers. Spherical particles in the size range 125-180 µm composed of 60% NiO/ 40% MgAl2O4 (N6AM1400) and 60% Fe2O3/40% MgAl2O4 (F6AM1100) were prepared by freeze granulation. The details of this procedure have been described previously.6,17 The particles are presented in Table 1. Reactivity Investigation. The experiments were conducted in a fluidized bed reactor of quartz. The reactor had a length of 820 mm with a porous quartz plate 30 mm in diameter placed 370 mm from the bottom. The inner diameters of the bottom and top sections were 19 and 30 mm. The temperature was measured 5 mm under and 38 mm above the porous quartz plate, using 10% Pt/Rh thermocouples enclosed in quartz shells. The pressure drop over the bed was measured by means of Honeywell pressure transducers and measured at a frequency of 20 Hz. From the pressure fluctuations, it was possible to establish if the particles were fluidized during the oxidizing period. In the reducing period, this type of analysis was not possible because of flow variations. A sample of 15 g of oxygen carrier particles, in the size range 125-

180 µm, was initially heated in an inert atmosphere to 950 °C. The particles were then exposed alternately to 5% O2 and 50% CH4/ 50% H2O, thus simulating the cyclic conditions of a CLC system. To avoid air and methane mixing during the shifts between reduction and oxidation, we introduced nitrogen gas for 180 s after each period. The particles were tested in this manner for 7-12 cycles. The gas from the reactor was led to an electric condenser, where the water was removed, and then to a gas analyzer, where the concentrations of CO2, CO, CH4, and O2 were measured in addition to the gas flow. Because of the heat produced during the oxidation period, a gas mixture with 5% O2 in N2 was used instead of air. Thus, large temperature increases were avoided. The experiments were conducted with an inlet gas flow of 900 mLn/ min for the reducing gas. For the oxidizing gas, the gas flow was 1000 mLn/min, and for the inert gas, flows of either 600 or 900 mLn/min were used. The degree of oxidation, or conversion, is defined as X)

m - mred mox - mred

(3)

where m is the actual mass of sample, mox is the mass of the sample when fully oxidized, and mred the mass of the sample in the fully reduced form. The degree of conversion can be calculated as a function of time for the reducing period through Xi ) Xi-1 -

∫

t1

to

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

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 moles of active oxygen in the unreacted oxygen carrier, and n˘ out is the molar flow of the gas leaving the reactor after the water has been removed; Ptot is the total pressure, and pCO2,out, pH2,out, and pCO,out are the outlet partial pressures of CO2, H2, and CO after the removal of H2O. pH2,out was not measured online but assumed to be related to the outlet partial pressure of CO and CO2 through an empirical relation based on the equilibrium of the gas-shift reaction. The mass-based degree of conversion is defined as ω)

m ) 1 + Ro(X - 1) mox

(5)

where Ro is the oxygen ratio, defined as Ro ) (mox - mred)/mox

(6)

The oxygen ratio is the theoretical maximum mass fraction of oxygen that can be used in the oxygen transfer and is dependent on the metal oxide used as the oxygen carrier as well as the amount of inert in the particles. To see the conversion of methane to carbon dioxide, we used the gas yield, defined as γred )

pCO2,out (pCH4,out + pCO2,out + pCO,out)

(7)

The CO ratio is defined as γCO ) COout/(COout + CH4(out)) The outlet fraction of methane in the bed is

(8)

2402 Energy & Fuels, Vol. 20, No. 6, 2006 γCH4 ) CH4(out)/CH4(in)

Johansson et al. (9)

Results Reactivity of Beds of Oxygen Carriers with Different Nickel/Iron Mass Ratios. Experiments with mixed beds of 1, 3, 5, 10, and 50 wt % N6AM1400 in a bed with F6AM1100 were performed in the laboratory fluidized bed. Two unmixed reference experiments with solely F6AM1100 or N6AM1400 were also conducted for comparison. The amount of oxygen that the oxygen carriers can transfer, characterized by the oxygen ratio, Ro, differs significantly for nickel and iron oxides. The system of NiO-Ni contains relatively large amounts of oxygen available for redox reactions in comparison to iron oxide. For iron, the system Fe2O3-Fe3O4 is considered, because iron oxides of lower oxidation states are excluded for thermodynamic reasons, i.e., their unacceptably low conversion of fuel to CO2 and H2O.5 Thus, in the experiments conducted with varying masses of nickel oxide, the theoretical limits of the mass-based conversion range, ω, will be different, as seen in Table 2. An ω of 0.98 means that 2% of the total bed mass has removed due to reduction. The concentration profiles for the reduction of the two nonmixed reference experiments and 3% N6AM1400 in a bed of F6AM1100 are presented in Figure 2a-c. The graph starts when the reducing gas was introduced and is followed by a delay of approximately 20-30 s before the gas reaches the analyzers. The vertical dashed lines indicate when the inert period is switched on. This change of gas is also followed by a delay of approximately 20-30 s before nitrogen reaches the analyzer. The gas flow is measured in connection with the gas analysis; flow variations upstream of the analyzers will rapidly affect the measured flow, whereas changes in gas concentrations have to reach the analyzers before being recorded. Hence, the flow and concentration profiles in Figure 2 correspond to the gas analyzer and not the concentration/flow behavior in the bed. The mass balance in eq 4 is determined on the basis of the correlations of flow/concentrations that actually reach the measuring devices. The reason eq 4 is based on outgoing and not ingoing flow is due to all the different side reactions that can take place, especially at low degrees of oxidations, which complicate the flow behavior. Some of these different side reactions are described in the following sections. The flow behavior seen in Figure 2a can be explained as follows. Steam is added to the methane by using a temperaturecontrolled humidifier set to give 50% steam, and the first decrease in flow seen is most likely associated with difficulties keeping a stable concentration of steam in connection with the switching of flows. The subsequent increase in flow is explained by the oxidation of CH4, whereby each molecule of CH4 gives one CO2 and two H2O, and the latter decrease is when the steam added with the incoming methane, as well as that produced by the reaction, reaches the condenser. After this decrease follows a period of stable flow of approximately 450 Ln/min, when the flow of CO2, CO, and CH4 coming out is almost equal to that of methane coming in. There is a strong increase when the gas is switched back to N2, and the transient is caused by gradually reducing the steam concentration in the gas reaching the condenser, as well as increasing the hydrogen concentration. The latter effect is a flow increase attributable to incomplete oxidation of CH4, giving one CO and two H2. This flow increase also explains why the flow coming out is significantly higher than the incoming flow for a period. The behavior seen in Figure 2c is similar, except that the flow increase associated with hydrogen formation starts much earlier.

Table 2. Limits of ω () 1 - Ro) for the Formation of Ni and Fe3O4 wt % N6AM1400

wt % F6AM1100

ω at formation of Ni and Fe3O4

0 1 3 5 10 50 100

100 99 97 95 90 50 0

0.980 0.979 0.977 0.975 0.969 0.926 0.872

The flow measurement device was calibrated in pure nitrogen and although it is not perfectly linear, gas molecules of similar or higher molecular weights (as CO2, CO, O2) give correct flows. There is a negligible underestimation of methane flows, whereas the small hydrogen molecule clearly gives an underestimated flow. Therefore, a slight error in the mass balances occurs when there are very large proportions of hydrogen passing through the reactor, as can take place in the later stages of reduction when the degree of oxidation is low and reforming and net carbon formation can take place. This can be verified when comparing mass balances of ingoing methane and accumulated carbon sources out of the reactor; if the gas yield is high, the mass balances are correct, with an error of only a few percent. The major difference between the beds of nickel oxide and iron oxide is clearly seen when comparing panels a and b of Figure 2. With the exception of a small methane peak initially for the nickel-based oxygen carrier (not noticeable in the figure), the conversion of methane is complete initially for both carriers; however, at longer reaction times, the methane concentration differs. For the bed of nickel oxide in Figure 2a, no methane leaves the reactor, as it is reformed to the reactants CO and H2 (not shown) by Ni0 created during the reduction. The formation of CO and H2 could occur through steam reforming

CH4 + H2O f CO + 3H2

(10)

and/or methane pyrolysis

CH4 f C + 2H2

(11)

C + H2O f CO + H2

(12)

followed by

Another possible reaction mechanism for the formation of CO and possibly CO2 that has been reported in earlier publications is the solid-solid phase reaction between carbon formed and lattice oxygen in the particles14,16,28

C + MyOx f CO/CO2 + MyOx-1/2

(13)

The products will be the same, independent of whether the reaction path is via reaction 10 or reactions 11 and 12. As Ni0 is a known catalyst for both reactions 10 and 11, the full conversion of methane is not surprising. For the bed of iron oxide, in Figure 2b, it is also possible that the reaction proceeds via the formation of CO and H2, reaction 10, as suggested by Johansson et al.17 However, unlike the bed with nickel oxide, there is a release of unconverted methane throughout the period, in addition to CO2 and CO. For the experiment of 3% N6AM1400 in F6AM1100 in Figure 2c, it is shown that the small amount of added nickeloxide carrier is sufficient to completely convert all incoming methane after a small peak of the latter. This small initial methane peak exists most likely at a stage where insufficient amounts of Ni0 exist in the bed to fully catalyze methane according to reactions 10 and/or 11. Results from the seven



Figure 2. Outlet gas concentrations (dry basis) for the reduction of (a) 100% N6AM1400, (b) 100% F6AM1100, and (c) 3% N6AM1400 with 97% F6AM1100. The ratio of bed mass to fuel flow was 15 g/900 mL for all experiments. The x axis starts when fuel is introduced into the reactor. Vertical dashed lines indicate when the gases are switched to inert gas. The time needed for the gas to reach the analyzer is approximately 20-30 s, depending on the flow and reactivity.

experiments with different ratios of iron oxide and nickel oxide are presented in Figure 3a-d. In Figure 3a, the gas yield is presented as a function of the mass-based conversion range. For the reference experiment with only nickel oxide, the yield is almost complete for the entire conversion range presented in the graph. Thermodynamically, full conversion to CO2 cannot occur for the combustion of methane with nickel oxides; at 800 and 1000 °C, the theoretical maximum conversion to CO2 is 0.9949 and 0.9883, respectively.5 The experiment with 50% N6AM1400 also exhibits an almost-full conversion for a large degree of mass-based conversion, although here there was also CO and H2 present from the outlet at high gas conversions. For the reference experiment with only iron oxide, the yield falls continually as the degree of the mass-based conversion gets smaller. To a smaller extent, this is also the case for the experiment with 1% N6AM1400, whereas the other three experiments (3, 5, and 10% N6AM1400) show a different behavior. This includes a continual increase in the gas yield for the first 2% of the conversion range, followed by a decrease. These results indicate that the conversion of methane to CO2 is greatly favored by the formation of Ni0 that catalyzes the formation of the reactants of CO and H2. These reactants then reduce the oxygen carriers according to

CO + MyOx f CO2 + MyOx-1

(14)

H2 + MyOx f H2O + MyOx-1

(15)

The reason the gas yield does fall eventually is likely associated with the formation of Ni0 and iron of lower oxidation degrees than Fe3O4, cf. with the ω limits in Table 2. When reducing iron oxides to lower oxidation states than magnetite, Fe3O4, full conversion of any of the reactants according to reactions 1, 14, and 15 is not thermodynamically achievable.5 However, even though the ratio of produced CO2 is decreasing, there is still a very high conversion of methane for the samples with an N6AM1400 ratio of at least 3%, as seen in panels b and c of Figure 3, where the CO ratio and the remaining fraction of methane are presented as a function of the mass-based conversion range. This is not the case for experiments with 1% N6AM1400 and the reference experiment with only F6AM1100. In these latter cases, there is not enough, or any, Ni0 available to fully convert all the incoming methane. In Figure 3d, the outlet fraction of methane as a function of the mass-based conversion range is presented for a shorter initial range of the reduction. As seen, the outlet CH4 and the conversion range of this CH4 peak are inversely proportional to the amount of N6AM1400 present in the bed. The only exception to this is that a smaller CH4 peak is present for the sample of 50% N6AM1400 than for 100% N6AM1400. Note that the unconverted methane in Figure 3 could be eliminated by using a higher ratio of bed mass/MWfuel for the different experiments. Synergy Effect of a Mixed Bed. It is clear from Figure 3


Johansson et al.

Figure 3. Experiments performed with different ratios of N6AM1400 with F6AM1100. (a) Gas yield, γred, as a function of the mass-based conversion rate, ω. (b) Ratio of CO/(CO + CH4) out as a function of ω. (c) Remaining fraction of CH4 as a function of ω. (d) Remaining fraction of CH4 as a function of ω presented for a narrower range. 0% N6AM1400 (b), 1% N6AM1400 (×), 3% N6AM1400 (2), 5% N6AM1400 (4), 10% N6AM1400 (9), 50% N6AM1400 (0), and 100% N6AM1400 (+).

Figure 4. (a) Outlet gas concentrations (dry basis) for the reduction of 3% N6AM1400 with 97% quartz. (b) Accumulated volume of CO2 as a function of time in reduction. 100% F6AM1100 (14.55 g) (b), 3% N6AM1400 (0.45 g) with 97% F6AM1100 (14.55 g) (2), 3% N6AM1400 (0.45 g) with quartz ([), sum of 100% F6AM1100 and 3% N6AM1400 with quartz (- - -). The x axis in (b) starts when gases reach the analyzer. Vertical dashed lines in (b) represent the ω limit to Fe3O4 and Ni.

that the addition of small amounts of nickel oxide to beds with iron oxide has beneficial effects with respect to gas conversion. However, as the oxygen ratio of the nickel-oxide particles is much higher than that of the iron oxide, it is important to establish that the seemingly enhanced effect is not due to the reaction of nickel oxide alone. Thus, to verify the positive effects of a bed of mixed oxides in comparison to unmixed beds separately, we performed a supplementary experiment of 3% (0.45 g) N6AM1400 with 97% (14.55 g) quartz (125-180 µm). The inert quartz was added to obtain a proper fluidizing bed.

The result from this experiment can be seen in Figure 4a. After an initial CO2 peak, both CO and CH4 exit the reactor. There is clearly carbon formation during the experiment, as seen by the CO during the inert period. Here, carbon formed during the reduction is oxidized by the lattice oxygen in the particle, i.e., via reaction 13. Thus, 3% N6AM1400 with quartz does not lead to full conversion of the methane, as opposed to the case of 3% N6AM1400 with F6AM1100. The synergy effect of a mixed bed is clearly shown in Figure 4b, where the accumulated amount of CO2 is displayed as a function of time for these two



Figure 5. Results from experiments of 3% N6AM1400 with 97% F6AM1100 (s) and 100% F6AM1100 (- - -) at different reaction temperatures. (a) Gas yield, γred, as a function of the mass-based conversion, ω. (b) Ratio of CO/(CO + CH4) out as a function of ω. (c) Outlet fraction of CH4 as a function of ω. 950 (+), 850 (]), 750 (O), and 650 °C (4).

experiments, together with the experiment with F6AM1100 without any nickel particles. Note that the accumulated amount of CO2 from the experiment with 100% F6AM1100 (15.00 g) is weighted by a factor of 97/100 to correspond to the mass of the 97% iron oxides (14.55 g). The dashed line in Figure 4b corresponds to the sum of accumulated CO2 from the two experiments run separately on each oxide. It is clearly seen that this amount is much lower than the accumulated CO2 from the mixed-oxide experiment with 3% N6AM1400 and 97% F6AM1100. Further, the theoretical mass-based conversion limits to Fe3O4 and Ni, cf. Table 2, are indicated as vertical dashed lines that intersect with the accumulated CO2 curves for the mixed-oxide experiment and the experiment with only iron carriers. As seen, the beneficial effect of mixed oxides is present long before the undesired FeO region is reached. This clearly indicates the synergy effect of mixing the two oxides, using the catalytic effect of Ni to convert CH4 to CO and H2, which subsequently react with the iron oxide. It could be argued that the amount of CO2 formed in the experiment with only 3% N6AM1400 with quartz may not be representative because of carbon formation, as seen in Figure 4a, or the incomplete mass balances when too much hydrogen is present in the exit gases, as explained earlier. However, even in the case of full conversion to CO2, the accumulated volume of CO2 produced by the nickel oxide would not exceed approximately 0.02 L. As can be seen in Figure 3b, this amount would increase the level of the dashed line somewhat, but would not affect the conclusions of a strong synergy effect. Effect of Reaction Temperature. To see the effect of the reaction temperature, experiments of 3% N6AM1400 with

F6AM1100 were performed at 950, 850, 750, and 650 °C. Results from experiments with 100% F6AM1100 at different temperatures from an earlier study17 are included for comparison. The results can be seen in Figure 5a-c. In this figure, a strong temperature dependence is seen; it can also be observed that the experiments with mixed oxides show higher conversions than the ones with only iron oxide. However, to verify a possible synergy effect, we made the same type of comparison as that in Figure 4b. The experiment with 3% N6AM1400 in quartz was made only at 950 °C. Nevertheless, the accumulated amount of CO2 from that experiment is used here to represent the 3% N6AM1400 in quartz also for these lower temperatures. This means that the accumulated amount of CO2 will be somewhat overpredicted, because the conversion of methane to CO2 increases with higher temperatures when using N6AM1400.25,26 The accumulated amount of CO2 for mixed oxides compared to the oxides investigated separately at 850 and 750 °C are presented in panels a and b of Figure 6. From this figure, it is clear that the synergy effect is present at 850 °C but not at 750 °C. At 750 °C, the sum of the two unmixed experiments initially results in even more CO2 than in the mixed-oxide experiment, which could be an effect of the overestimation of CO2 from the experiment with 3% N6AM1400 in quartz. There is also another possible explanation: If there is no synergy effect at all, the CO2 production should be somewhat lower for the mixed oxides compared to sum of the two unmixed experiments. This is because in the mixed experiments, the total flow of methane to convert is lower, 450 mLn/min, compared to that of the two


Johansson et al.

Figure 6. Accumulated volume of CO2 as a function of time in reduction at (a) 850 and (b) 750 °C. 100% F6AM1100 (14.55 g) (b), 3% N6AM1400 (0.45 g) with 97% F6AM1100 (14.55 g) (2), 3% N6AM1400 (0.45 g) with quartz (at 950 °C) ([), sum of 100% F6AM1100 and 3% N6AM1400 with quartz (- - -).

unmixed experiments, 450 + 450 mLn/min. This means that the methane concentration is lower, which slows down the CO2 formation. Fluidization Behavior and Carbon Deposition. For several iron oxides and nickel oxides tested in the same laboratory equipment as that used here, defluidization and in some cases agglomeration have been noticed. Defluidization can be noticed as a decrease in the pressure drop, and its fluctuations, over the bed. Cho et al. recently investigated defluidization for iron and nickel oxides and determined the parameters that mainly influenced this.14 For nickel oxides, the defluidization seems mainly to be correlated to the sintering temperature used when particles were produced, which in turn strongly affects other properties of the particles, such as porosity, density, and reactivity. For iron particles, it was clearly shown that the defluidization was associated with the initiation of the phase change from magnetite (Fe3O4) to wustite (FeO). Although substantial wustite initiation has clearly occurred in the experiments conducted in this paper, cf. Table 2 and Figure 3a, no sign of defluidization occurred. Carbon formation is a side reaction that could affect CO2 capture efficiency. If carbon follows the particles to the air reactor and is oxidized, this will give emissions of CO2. Several papers inform that carbon formation has occurred for nickelbased oxygen carriers.8,9,13,16,28 Carbon formation can occur through any of the Ni0-catalyzed reactions of methane pyrolysis (reaction 11) or the Boudouard reaction

2CO f C + CO2

(16)

Cho et al. have proposed that carbon formation is an intermediate in the reduction of nickel oxides, where the carbon gets oxidized to CO (reaction 12) which then acts as a reactant (reaction 14).16 They reported that no or negligible amounts of net carbon out was detected as long as sufficient oxygen was present in the particles to oxidize the C, i.e., when the degree of oxidation was still quite high. In the experiments performed in this paper, all beds were subject to reduction beyond the point where Ni0 and Fe3O4 exist (cf. Table 2 and Figure 3a). Therefore, some carbon formation would be expected in the mixed-bed experiments. An example of carbon formation for a nickel oxide similar to that used here can be found in an earlier paper.28 However, in none of the mixed-bed experiments was carbon detected in the form of CO2 and/or CO leaving the reactor in the inert period (after the inevitable dead time of 2030 s) or in the oxidation. Hence, using a bed of iron oxides

with small amounts of nickel oxides does not only take advantage of the catalytic reactivity of nickel oxide, but also seems to suppress carbon formation. Discussion Many of the studies in the last couple of years have focused on the use of nickel oxide as an oxygen carrier for chemicallooping combustion. This is mainly due to the fact that it has a much higher reactivity than, for example, iron oxide and manganese oxide when using methane as fuel. However, as nickel oxide is toxic and expensive, it would be an advantage if the amounts of nickel oxide used could be reduced. This study suggests that an alternative is to use a bed of mixed nickeland iron-oxide particles. It has here been demonstrated that the reactivity of iron oxide with methane can be increased if only a small amount of nickel oxide is introduced. This is because created Ni0 catalyzes side reactions that form CO and H2, which react faster than methane. In these experiments, a bed containing only 3 wt % nickel-oxide particles showed a large increase in the reactivity of the iron-oxide particles. The active area of the nickel-oxide particles was low, 1.24 m2/g. As it is probable that the catalytic action of the nickel particles is proportional to the available surface area, it is likely that even less nickel oxide would be needed to get the beneficial effect for a high-surfacearea particle, e.g., prepared by impregnation. However, it should be noted that the nickel-oxide/nickel particles not only act as a catalyst but also as an oxygen carrier, providing bulk oxygen for the reactions with methane. It is interesting to note that there was always CO and H2 released from the outlet of the reactor when using the mixed oxides, and thus the thermodynamic limitations of NiO to convert the methane fully seems to still have an effect. For the system investigated in this paper, i.e., methane as fuel with iron- and/or nickel oxides, the oxidation is exothermic and the reduction endothermic. This means that the particles leaving the air rector must carry sufficient sensible heat to achieve a high enough temperature in the fuel reactor. In other words, the circulation rate of particles needs to be sufficient, which will also reduce the maximum possible change in conversion, ∆ω, in each cycle. Thus, ∆ω values of less than 2% are needed in order to keep the temperature decrease, relative to that in the air reactor, below 60-80 °C.19 Conclusion The use of a mixed-oxide system as the oxygen carrier for chemical-looping combustion has been investigated. The metal


oxides used are a nickel oxide, 60% NiO/40% MgAl2O4 sintered at 1400 °C, and an iron oxide, 60% Fe2O3/40% MgAl2O4 sintered at 1100 °C. A bed of iron oxides with only 3% nickel oxides was found to be sufficient to give a very high conversion of the incoming methane. It was shown that the mixed-oxide system of 3% nickel oxides in 97% iron oxides produced significantly more CO2 than the sum of the metal oxides run separately, thus giving evidence of the synergy in using nickel oxide together with iron oxide. A strong temperature dependency was seen for the mixed-oxide system, although the synergy


effect was also seen at 850 °C. Furthermore, no carbon formation or defluidization occurred in any of the experiments of mixed oxides. The implication of this study is that much less bed material would be needed of a bed of mixed oxides in comparison to only iron oxide. Acknowledgment. This work was financed by the Swedish Energy Agency. EF060068L

Creating a Synergy Effect by Using Mixed Oxides ... - ACS Publications

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