Catalytic Activity of Ni-Based Oxygen-Carriers for ... - ACS Publications

Article pubs.acs.org/EF

Catalytic Activity of Ni-Based Oxygen-Carriers for Steam Methane Reforming in Chemical-Looping Processes María Ortiz, Luis F. de Diego,* Alberto Abad, Francisco García-Labiano, Pilar Gayán, and Juan Adánez Department of Energy and Environment, Instituto de Carboquímica (ICB-CSIC), Miguel Luesma Castán 4, 50018 Zaragoza, Spain ABSTRACT: Chemical-looping technology has been suggested as one of the most promising technologies for reducing the cost of CO2 capture using fossil fuels. This technology involves the use of an oxygen-carrier, which transfers oxygen from air to the fuel avoiding the direct contact between them. Oxygen carriers based on nickel have been the most extensively analyzed in the literature because of their good performance working at high temperatures (900−1100 °C). It is well-known that Ni catalyzes steam methane reforming (SMR) and water−gas shift (WGS) reactions, which are produced both in chemical-looping combustion (CLC) and chemical-looping reforming (CLR) processes. In this work, the catalytic activity of two Ni-based oxygencarriers prepared by impregnation, NiO18-αAl2O3 and NiO21-γAl2O3, with respect to SMR and WGS reactions, have been determined in a fixed-bed reactor at different operating conditions. The catalytic activity was similar for both oxygen-carriers but lower than that exhibited by conventional Ni catalysts. Nevertheless, the catalytic effect was of great relevance to achieve complete CH4 conversion in CLC and CLR systems. In addition, it was found that the catalytic activity of the oxygen-carriers for the SMR depended on the oxidation degree of the oxygen-carrier. This fact must be considered when the SMR takes place simultaneously to the reduction of NiO.

1. INTRODUCTION Greenhouse gas emissions, especially CO2 formed by combustion of fossil fuels, highly contribute to the global warming problem. Carbon dioxide capture and sequestration (CCS) has been identified as a potential option to reduce CO2 emissions from power plants still using fossil fuels.1 On the other hand, CO2 capture technology applied to the transport sector is more complex. One possible option to reduce the CO2 emissions in transport is the use of H2 as fuel. However, H2 is an energy carrier that must be produced from a primary energy source. If H2 is used as a free-CO2 energy carrier, it must be produced without a net release of CO2, for example, from renewable energy sources or from fossil fuels with CCS implementation. With the technologies available today, the most important economic and energetic cost for CCS is related to the CO2 capture process. Capture of CO2 means to separate CO2 from the processes where the fossil fuels are being used to obtain a CO2rich stream ready to be stored. In this sense, chemical-looping combustion (CLC) has been suggested as one of the most promising technologies for reducing the cost of CO2 capture using fossil fuels.2,3 CLC is a novel combustion technology with inherent separation of the greenhouse gas CO2 that involves the use of an oxygencarrier, which transfers oxygen from air to the fuel avoiding the direct contact between them. Figure 1a shows a schematic diagram of the CLC process. A CLC system is made of two interconnected reactors, designated as air and fuel-reactors. In the fuel-reactor, the fuel gas (CnH2m) is oxidized to CO2 and H2O by a metal oxide (MeO) that is reduced to a metal (Me) or a reduced form of MeO. The spent oxygen-carrier is further transferred into the airreactor where it is oxidized with air, and the material regenerated is ready to start a new cycle. The flue gas leaving the air-reactor contains N2 and unreacted O2. The exit gas from the fuel-reactor contains only CO2 and H2O. After water condensation, almost © 2011 American Chemical Society

Figure 1. Schematic diagram of (a) chemical-looping combustion and (b) chemical-looping reforming processes.

pure CO2 can be obtained with little energy lost for component separation. The total amount of heat evolved from reactions in the two reactors is the same as for normal combustion, where the oxygen is in direct contact with fuel. Autothermal chemical-looping reforming (CLRa or simply CLR) utilizes the same basic principles as CLC, the main difference being that the desired product in CLR is not heat but H2 and CO. Figure 1b shows a scheme of the process. In the CLR process, the air to fuel ratio is kept low to prevent the complete oxidation of the fuel to CO2 and H2O. When a water− gas shift (WGS) reactor is used downstream of the fuel reactor, the H2 yield in this process can reach 2.7 mol H2 per mol of CH44 but with the advantage that the CO2 capture is accomplished and no additional energy from an external source is needed. CLR, as described in Figure 1b, was initially proposed by Mattisson and Lyngfelt in 2001.5 The selection of a suitable oxygen-carrier is a key issue for the large-scale application of chemical-looping technologies. Received: September 9, 2011 Revised: December 20, 2011 Published: December 20, 2011 791

dx.doi.org/10.1021/ef2013612 | Energy Fuels 2012, 26, 791−800

Energy & Fuels

Article

Ni-based oxygen-carriers, and usually, it is considered to be at thermodynamic equilibrium at the operating conditions. This reaction changes the gas concentration profiles in the reactor, therefore changing the reaction rate of the oxygen-carrier. The reforming reactions of methane (reactions R8−R9) have been found to be relevant. Johansson et al.,16 Dueso et al.17 and Jerndal et al.18 carried out pulse and continuous experiments in a batch fluidized bed reactor with Ni-based oxygen-carriers using CH4 as fuel. They observed that during the early seconds of the oxygen-carrier reduction the product gas showed unconverted CH4, which disappeared as the reduction reaction progressed. This was due to the fact that there were no Ni sites to catalyze the CH4 reforming reactions at the beginning of the reaction. Kolbitsch et al.11 observed that, during CH4 combustion tests in a 140 kWth CLC pilot plant with two Nibased oxygen-carriers, the catalytic activity of the oxygen-carrier was not constant during the experiment, on the contrary, it depended on the reduction degree of the oxygen-carrier particles. As the oxygen-carrier was reduced (i.e., more metallic Ni was formed), the CH4 conversion, as well as the CO and H2 concentrations, was higher, which means that the CH4 was converted by reforming reactions catalyzed by the metallic Ni. The reduction of NiO in oxygen-carrier particles have usually been modeled by using a combination of noncatalytic (reactions R2−R6) and catalytic reactions (reactions R7−R9).19,20 Catalytic reactions change the pathway of CH4 reaction by increasing the formation of H2, CO, and CO2 by steam methane reforming (SMR) and the subsequent oxidation of H2 and CO by the oxygen-carrier. In previous works, a constant activity20 or a linear variation of the activity with the metallic Ni content in the oxygen carrier19 was assumed for SMR catalysis over all the reduction of NiO. As it was above-described, experimental work showed that the catalytic activity against reforming reaction is affected by the oxidation degree of the Ni in particles.11,16−18 Therefore, the evolution of the SMR catalytic activity of oxygencarrier particles during reduction must be known. The effect of the NiO and metallic Ni contents in the oxygen carrier on the catalytic activity of the oxygen-carrier has only been considered by Iliuta et al.,19 who modeled the CH4 reaction by using a combination of catalytic and noncatalytic reactions. The objective of this work was to determine the catalytic activity of two Ni-based oxygen-carriers (NiO18-αAl2O3 and NiO21γAl2O3) for the SMR and WGS reactions. The kinetic parameters of the catalytic SMR and WGS reactions as well as the effect of the oxidation degree of the oxygen-carrier on its catalytic activity were determined. These oxygen-carriers have been chosen because the NiO18-αAl2O3 oxygen-carrier had been successfully used in a continuous CLC plant with different fuels,10,21 even in the presence of sulfur22 and light hydrocarbons,23 and both NiO18αAl2O3 and NiO21-γAl2O3 particles have also been tested in CLR experiments in a continuous CLR plant24 and at pressurized conditions in a fluidized bed reactor,25 with good results in both cases.

The oxygen-carrier must have sufficient oxygen transport capacity, high reactivity under alternating reducing and oxidizing conditions, low tendency of carbon deposition, avoidance of agglomeration, high mechanical and chemical stability for successive cycles in a fluidized-bed system, and high CH4 conversion to CO2 and H2O in the case of CLC and to CO and H2 for CLR. Other requirements are high availability and low cost of the metal, as well as low environmental impact. Several transition state metals, such as Ni, Cu, Mn, Co, and Fe, have been proposed as the most suitable materials for oxygen-carriers in CLC and CLR. A selection of oxygen-carrier materials for natural gas reforming and combustion and for syngas combustion has been summarized by Lyngfelt et al.,6 Hossain and de Lasa,7 and Adánez et al.8 Ni-based oxygen-carriers have been the most extensively analyzed materials in the literature. Ni-based oxygen-carriers have shown very high reactivity and good performance working at high temperatures (900−1100 °C). In the CLC process, nearly complete CH4 conversion to CO2 and H2O was obtained, although thermodynamic restrictions result in a small presence of CO and H2 in the gas outlet of the fuel-reactor.9−12 For CLR process, the Ni-based oxygen-carriers show the highest selectivity toward H2 production, and in addition, they were the most promising because of their strong catalytic properties.13−15 In fact, metallic Ni is used as catalyst in most commercial steam reforming processes. However, the use of Ni-based oxygencarriers may require safety measures because of its toxicity, and in addition, nickel oxide is more expensive than other metal oxides, although this problem may be solved using particles with low nickel content, high reactivity, and low attrition rate. For the design, simulation, and optimization of CLC or CLR systems, it is necessary to know the kinetics of all the reactions taking place in the air- and fuel-reactors. The main reactions involved in CLC and CLR processes using Ni-based oxygencarriers are the following: Air-reactor

1 O2 → NiO 2 Fuel-reactor CH4 + 4NiO ↔ CO2 + 2H2O + 4Ni Ni +

(R1)

(R2)

CH4 + 2NiO ↔ CO2 + 2H2 + 2Ni

(R3)

CH4 + NiO ↔ CO + 2H2 + Ni

(R4)

H2 + NiO ↔ H2O + Ni

(R5)

CO + NiO ↔ CO2 + Ni

(R6)

Ni

CO + H2O ↔ CO2 + H2

(R7)

Ni

CH4 + H2O ↔ CO + 3H2

(R8)

2. EXPERIMENTAL SECTION

Ni

CH4 + 2H2O ↔ CO2 + 4H2

(R9)

2.1. Materials. The catalytic activity for the SMR reaction of two Ni-based oxygen-carriers, NiO21-γAl2O3 and NiO18-αAl2O3, prepared by impregnation at the ICB-CSIC has been analyzed in this study. NiO21-γAl2O3 was prepared by incipient wetness impregnation over commercial γ-Al2O3 (Puralox NWa-155, Sasol Germany GmbH) and NiO18-αAl2O3 was prepared by hot incipient wetness impregnation over α-Al2O3 (obtained by calcination of γ-Al2O3 at 1150 °C during 2 h). The details of the preparation of both oxygen-carriers

Together with the gas−solid reactions between the fuel gas and the oxygen-carrier (reactions R2−R6), side reactions can be relevant in the fuel-reactor. The WGS reaction is usually considered when CO2, H2O, CO, and H2 are present in the reacting gas (reaction R7). The WGS shift reaction is relatively fast at temperatures involved in CLC and CLR systems using 792


Energy & Fuels

Article

have been described elsewhere.26,27 The main characteristics of the two oxygen-carriers are listed in Table 1.

The section below the bed was filled with inert material (ceramic rings). The gas was fed in at the top of the reactor to avoid the entrainment of the particles and left the reactor at the bottom. For security, there was a hot filter downstream from the reactor. The gas outlet stream was drawn to online gas analyzers to obtain continuous data of H2O, CH4, CO, CO2, and H2 concentrations. Both gas feeding and gas exit lines were heated to avoid water condensation. H2O was measured by an infrared gas analyzer, via Fourier transform infrared (FTIR, Temet CX4000). After water condensation, CH4, CO, and CO2 were measured by a nondispersive infrared analyzer (NDIR, Maihak S710). A thermal conductivity detector analyzer (Maihak S710) was used for H2 concentration determination. The system operation was controlled by a computer interface, and all data were collected by means of a data logger connected to the computer. 2.3. Experimental Procedure. Two kind of experiments were carried out: (a) experiments starting with the oxygen-carrier completely reduced, to evaluate its catalytic activity for the SMR and the WGS reactions, and (b) experiments starting with the oxygen-carrier completely oxidized, to analyze the effect of the oxygen-carrier conversion on its catalytic activity. The experiments with the oxygen-carrier completely reduced were done following the procedure described by Xu and Froment.29 Thus, the kinetics of the SMR was obtained at the same time as the kinetics of the reverse WGS reaction. To carry out these experiments, a homogeneous mixture of 0.2 g (100−300 μm) of oxygen-carrier particles and 6 g of sand particles (300−500 μm) was introduced into the reactor. The amount of oxygen-carrier was selected because preliminary experiments ensuring high catalytic activity and avoiding reaching a chemical equilibrium. The sand was used as inert and diluting material allowing working at isothermal conditions. Once loaded in the reactor, the oxygen-carrier was heated to 900 °C in N2 atmosphere and reduced by hydrogen for 1 h. Then, the reactor was purged with N2, and the temperature was fitted to the desired reaction temperature. Upon reaching this temperature, the reactant gases were introduced into the reactor. Table 2 shows the experimental conditions used for the study of the SMR and the reverse WGS reactions with both oxygen-carriers.

Table 1. Properties of the NiO-Based Oxygen-Carriers As Prepared (Fresh) and after Operation in a CLR Unit (Used) NiO21-γAl2O3 properties NiO content (wt %) apparent density (g cm−3) BET surface area (m2 g−1) porosity (%) mechanical strength (N) XRD phases a

NiO18-αAl2O3

useda

fresh

useda

fresh

21 1.7

21 1.9

18 2.5

18 2.5

83.4

29.0

7.0

6.8

50.7 2.6

48.4 2.4

42.5 4.1

42 3.7

γ-Al2O3, γ-Al2O3, NiOb, α-Al2O3, NiO, α-Al2O3, NiO, NiAl2O4 NiAl2O4 NiAl2O4 NiAl2O4

particles used in this work. bminor phase.

The samples of the two oxygen-carriers used in this study were taken from batches of particles previously used in a continuous CLC pilot plant during 50 h of operation under CLR conditions.24 So, Ni2+ in the sample of the NiO18-αAl2O3 oxygen-carrier used in this work was shared in NiO (80%) and NiAl2O4 (20%), whereas the Ni2+ in the sample of the NiO21-γAl2O3 oxygen-carrier was almost completely forming NiAl2O4. Previous works showed the good behavior of these particles with low Ni content as oxygen-carrier for CLC10 and CLR24 processes. In addition, it was shown that low Ni content is preferable when the enthalpy balance, cost, and environmental impact are considered.4,28 2.2. Experimental Setup. The experiments were carried out in a fixed-bed reactor. The experimental setup (Figure 2) was composed by

Table 2. Experimental Conditions Used in the Fixed-Bed Reactor CH4 concn (%)

H2O/CH4/H2

temp (°C)

8 8 8 CO2 concn (%)

3:1:1 5:1:1 7:1:1 H2/CO2

700, 800, 900 700, 800, 900 700, 800, 900 temp (°C)

10 10 10

0.5 0.75 1

500, 600, 700 500, 600, 700 500, 600, 700

The SMR reaction was studied to determine the kinetic parameters under different operating conditions: reaction temperature, H2O/CH4 molar ratio, and space time (W/FCH4 (g h/mol CH4)). The reforming temperature was varied from 700 to 900 °C. The space time was in the range 0.2−0.4 g h/mol CH4. To analyze the effect of the H2O/CH4 molar ratio and to avoid the oxidation of the oxygen-carrier with H2O, hydrogen was added to the gas that was fed to the reactor during the experiments, and the H2O/CH4/H2 molar ratios used were 3:1:1, 5:1:1, and 7:1:1. Figure 3 shows the gas outlet concentrations measured as a function of time during a typical SMR experiment in the fixedbed reactor using the NiO21-γAl2O3 oxygen-carrier. Similar results were obtained with the other oxygen-carrier and other operation conditions. In the same way as in the SMR reaction, the reverse WGS reaction was studied under different operation conditions. In these tests, the H2/CO2 molar ratios used were 0.5, 0.75, and 1, the space time (W/FCO2 (g h/mol CO2)) was varied between 0.15 and 0.4, and the temperature varied between 500 and 700 °C. Temperatures higher than 700 °C were not used because the chemical equilibrium is reached very quickly, so it is not possible to obtain kinetic data. Figure 4 shows the

Figure 2. Experimental setup. a gas feeding system, a fixed-bed reactor of 27 mm i.d. and 745 mm length, and a gas analysis device. The gas feeding system had different specific mass flow controllers to obtain accurate flow rates of feeding gases. Water was fed with a liquid mass flow controller and then evaporated and mixed with other gases. The reactor made of Kanthal was heated by an external furnace. The sample was loaded over a layer of quartz wool, and the temperature was measured by a thermocouple that was located just above the sample in the tests carried out with the oxygen-carrier completely reduced and in the middle of the fixed-bed in the tests carried out with the oxygen-carrier completely oxidized. 793


Energy & Fuels

Article

equations:

XCH 4 =

Q inPCH 4,in − Q outPCH 4,out Q inPCH 4,in

(1)

XCO2 =

Q inPCO2,in − Q outPCO2,out Q inPCO2,in

(2)

3. EXPERIMENTAL RESULTS 3.1. Experimental Results with the Oxygen-Carrier Completely Reduced. Figure 5 shows the CH4 conversion Figure 3. Gas product distribution obtained during a typical SMR experiment. NiO21-γAl2O3; T = 700 °C; W/FCH4 = 0.4 g h/mol CH4; H2O/CH4/H2 = 5:1:1.

Figure 4. Gas product distribution obtained during a typical reverse water−gas shift experiment. NiO18-αAl2O3; T = 500 °C; W/FCO2 = 0.2 g h/mol CO2; H2/CO2 = 1. gas outlet concentrations measured as a function of time during a typical reverse WGS experiment in the fixed-bed for the NiO18-αAl2O3 oxygencarrier. Similar results were obtained with the other oxygen-carrier and other operation conditions. Experiments were also carried out with the oxygen-carrier completely oxidized to analyze the effect of the oxygen-carrier conversion on its catalytic activity for the SMR. In these experiments, in a way similar to the experiments carried out with the oxygen-carrier completely reduced, a homogeneous mixture of oxygen-carrier and sand particles was introduced into the reactor; however, the load of oxygencarrier used in these tests was higher than that used with the oxygencarrier completely reduced. NiO21-γAl2O3 oxygen-carrier (10 g) well mixed with sand (30 g) and NiO18-αAl2O3 oxygen-carrier (1 g) mixed with sand (8 g) were used. The amounts of each oxygen-carrier and inert were selected from preliminary tests where different amounts of oxygen-carrier and inert were used. The amount of oxygen carrier selected allowed us to observe the changes in gas concentrations, and so, allowed us to determine the evolution of the oxygen-carrier conversion with time, and at the same time avoided the complete conversion of CH4 in the first period of the reaction, that is, avoided reaching a chemical equilibrium. A higher amount of NiO21-γAl2O3 oxygen-carrier than NiO18-αAl2O3 oxygen-carrier was used because of its lower reduction reaction rate. The amount of inert was selected to maintain all the fixed-bed at isothermal conditions and to avoid axial dispersion. With both oxygen-carriers, experiments were carried out working with a H2O/CH4/H2 molar ratio of 3:1:1 and at a reaction temperature of 900 °C. The conversions of CH4 during the SMR experiments and the conversion of CO2 during the reverse water−gas shift experiments were calculated from the gas outlet concentrations measured by the following

Figure 5. CH4 conversion vs space time curves working at different H2O/CH4/H2 molar ratios and temperatures. Full symbols: NiO21γAl2O3. Empty symbols: NiO18-αAl2O3.

obtained during SMR, as a function of space time working with the NiO21-γAl2O3 oxygen-carrier at different H2O/CH4/H2 molar ratios and different reaction temperatures. As can be observed, the CH4 conversion increased when the space time and the temperature increased and when the H2O/CH4 molar ratio decreased. The increase of CH4 conversion with the space time and temperature was due to the increase of the contact time between the CH4 and the oxygen carrier and the increase in the reforming reaction rate, respectively. The increase in CH4 conversion with decreasing H2O/CH4 molar ratio was attributed by Hou and Hughes30 to the high steam concentration that hinders CH4 from adsorbing in catalyst surface, particularly at high temperatures, because high temperature is favorable for water vapor adsorption with dissociation on the catalyst surface. Figure 6 shows the CO2 conversion vs space time curves obtained in the reverse WGS reaction for the NiO18-αAl2O3 oxygen-carrier working at different H2/CO2 molar ratios and different temperatures. It was found that an increase either in the H2/CO2 molar ratio or in the reaction temperature produced an increase in the CO2 conversion. It must be remarked, as it was shown in Figure 4, that methane was not found in the outlet gas 794


Energy & Fuels

Article

During the SMR process, several reactions can occur simultaneously, through which the CH4 can disappear or the CO2 can be formed. Table 3 shows the possible reactions that can occur Table 3. Possible Reactions in the Steam Methane Reforming Process N°

reaction

ΔH298 (kJ mol−1)

1 2 3 4 5 6 7 8 9 10 11

CH4 + H2O ⇔ CO + 3H2 CO + H2O ⇔ CO2 + H2 CH4 + 2H2O ⇔ CO2 + 4H2 CH4 + CO2 ⇔ 2CO + 2H2 CH4 + 3CO2 ⇔ 4CO + 2H2O CH4 ⇔ C + 2H2 2CO ⇔ C + 2CO2 CO + H2 ⇔ C + H2O CO2 + 2H2 ⇔ C + 2H2O CH4 + 2CO ⇔ 3C + 2H2O CH4 + CO2 ⇔ 2C + 2H2O

206.1 −41.15 165.0 247.3 330.0 74.82 −173.3 −131.3 −90.13 −187.6 −15.3

during the SMR process. However, according to the literature,29,32 only the first three reactions occur significantly, while the rates of the other reactions (reactions 4−11) are negligible. Therefore, the CH4 disappearance and CO2 formation rates can be expressed as follows: Figure 6. CO2 conversion vs space time curves working at different H2/CO2 molar ratios and temperatures. Full symbols: NiO21-γAl2O3. Empty symbols: NiO18-αAl2O3.

− rCH4 = r1 + r3 rCO2 = r2 + r3

XCO2

⎛ W ⎞ ⎛ W ⎞2 ⎛ W ⎞3 ⎜ ⎟ ⎜ ⎟ ⎟⎟ = b0 + b1⎜ ⎟ + b2 ⎜ F ⎟ + b3⎜⎜ F ⎝ FCO2 ⎠ ⎝ CO2 ⎠ ⎝ CO2 ⎠

⎛ ⎞ ⎛ ⎞ W ⎟ W ⎟ = a1 + 2a2⎜⎜ + 3a3⎜⎜ ⎟ ⎟ d(W /FCH4) ⎝ FCH4 ⎠ ⎝ FCH4 ⎠ dXCH4

(11)

pH pCO ⎞ k2 ⎛ 2 2 ⎜pCO pH O − ⎟ /(DEN )2 2 pH ⎝ K2 ⎠

(12)

2

⎛ pH4 pCO ⎞ k3 ⎜ 2 2 2⎟ r3 = 3.5 pCH pH O − /(DEN )2 ⎜ ⎟ 4 2 K3 pH ⎝ ⎠ 2

(3)

(4)

+ K H2OpH O /pH 2

dXCO2

(6)

Taking into account that the CH4 and CO2 conversions are zero for a W/F = 0, the initial rates of disappearance of CH4 and formation of CO2 for each operation condition studied were obtained: (7)

rCO2 = b1 for W /FCO2 = 0

(8)

2

where pi is the partial pressure of gas i, kj and Kj are the kinetic constant and the equilibrium constant of reaction j, respectively, and Ki is the adsorption constant of gas i. These constants follow an Arrhenius type dependence with temperature:

(5)

− rCH4 = a1 for W /FCH4 = 0

(13)

DEN = 1 + K COpCO + K H2pH + K CH4pCH 2 4

2

⎛ ⎞ ⎛ ⎞2 W ⎟ W ⎟ ⎜ ⎜ rCO2 = = b1 + 2b2⎜ ⎟ + 3b3⎜ F ⎟ d(W /FCO2) ⎝ FCO2 ⎠ ⎝ CO2 ⎠

⎛ pH3 pCO ⎞ k1 ⎜ 2 ⎟ /(DEN )2 r1 = 2.5 pCH pH O − ⎜ 4 2 K1 ⎟ pH ⎝ ⎠ 2

r2 =

By differentiating eqs 3 and 4 with respect to space time, the CH4 disappearance and CO2 formation rates can be expressed as − rCH4 =

(10)

According to the kinetic model developed by Xu and Froment29 for the SMR, the rate expressions for each reaction (r1−r3) are

stream in these experiments. Methane could be produced either from direct reaction of carbon dioxide and hydrogen or from indirect reaction of carbon monoxide and hydrogen due to the steam produced in the reaction is low, and so, the SMR reaction is unimportant. The relationships between CH4 and CO2 conversion and space time were obtained from the conversion vs space time curves using third degree polynomial regressions (continuous lines in Figures 5 and 6), in the same way as other authors:30,31 ⎛ ⎞ ⎛ ⎞2 ⎛ ⎞3 W ⎟ W ⎟ W ⎟ ⎜ ⎜ ⎜ XCH4 = a0 + a1⎜ ⎟ + a 2⎜ F ⎟ + a3⎜ F ⎟ ⎝ FCH4 ⎠ ⎝ CH4 ⎠ ⎝ CH4 ⎠

(9)

⎛ Ej ⎞ kj = k 0, j exp⎜ − ⎟ ⎝ RT ⎠

(14)

⎛ ΔHi ⎞ ⎟ K i = K 0, i exp⎜ − ⎝ RT ⎠

(15)

The experimental initial CH4 disappearance and CO2 formation rates obtained for each oxygen-carrier and each operation condition (a1, b1), and eqs 11−13, were substituted in eqs 9 795


Energy & Fuels

Article

and 10 to obtain two new equations for each operation condition. To obtain the kinetic parameters for the SMR and WGS reactions, these equations were solved by means of a multivariable regression using the program XLSTAT.33 Figure 7 shows the

Figure 8. Comparison between the initial reaction rates of CH4 disappearance determined experimentally and the reaction rates predicted by the model. Dotted lines: ± 10% error. Figure 7. Temperature dependence of kinetic and adsorption constants. NiO18-αAl2O3.

temperature dependence of the kinetic and adsorption constants. The parameter estimations obtained for each oxygen-carrier are showed in Table 4. Figure 8 shows a comparison between the initial reaction rate values predicted by the model and those determined experimentally for each oxygen-carrier. It can be observed that the values predicted and the values measured experimentally are in good agreement. The adsorption constants KCO, KH2 and KCH4 are omitted in Table 4 because it was observed, during the regression, that their influence was negligible compared to that of the adsorption constant KH2O. The activation energies of the reforming reactions (r1 and r3) obtained for both oxygen-carriers were quite similar to the activation energies found by other authors working with commercial catalysts.29,31 Figure 9 shows a comparison between the initial CH4 disappearance rate obtained with the oxygen-carriers studied in this work and the obtained by other authors working with conventional Ni catalyst. It can be observed that the catalytic activity of the oxygen-carriers studied in this work was lower than that of the conventional Ni catalyst. However, it must be taken into account that the oxygen-carriers are materials with low specific area and metallic dispersion, whereas conventional catalyst are materials with high metallic dispersion due to the fact that they are prepared and tested at lower temperatures than the temperatures used in CLC or CLR processes. 3.2. Experimental Results with the Oxygen-Carrier Completely Oxidized. In the CLC or CLR systems, the oxygen-carrier is not completely reduced, but it is reduced as it reacts with the fuel gas. So, it is needed to analyze the effect of the oxygen-carrier conversion on its catalytic activity. For that, experiments were carried out in the fixed-bed reactor, as described above, with the oxygen-carrier completely oxidized. Figure 10 shows the gas composition (solid lines) and the reduction oxygen-carrier conversion obtained experimentally

Figure 9. Initial disappearance rate of CH4 obtained with the oxygencarriers studied in this work and the obtained by other authors working with conventional Ni catalyst.

for both oxygen-carriers. It can be observed that, for the same operation conditions, the conversion reached by the NiO18αAl2O3 oxygen-carrier due to reduction reactions was higher than that reached by the NiO21-γAl2O3 oxygen-carrier. It can be also observed that, at the beginning of the reaction, the CH4 conversion was mainly to CO2 and H2O due to the oxygen excess in the system when the oxygen-carrier is completely oxidized. However, it was found that the CH4 conversion was not complete in the first period of the reaction because there is not enough Ni metallic formed to catalyze the reforming reactions. This fact means that the CH4 reforming reactions are needed to achieve complete CH4 conversion, even when there is oxygen excess in the system, as in CLC operation conditions. To analyze these results, a mathematical model was developed to describe the reaction of CH4 and to obtain the evolution with time of the gases formed with the oxygen-carriers in the fixed-bed reactor. In the model, a homogeneous and isothermal

Table 4. Kinetic Parameters Obtained for Each Oxygen-Carrier oxygen-carrier

k0,1 (mol kg−1 s−1)

E1 (kJ mol−1)

k0,2 (mol kg−1 s−1)

E2 (kJ mol−1)

k0,3 (mol kg−1 s−1)

E3 (kJ mol−1)

K0,H2O

ΔHH2O (kJ mol−1)

NiO21-γAl2O3 NiO18-αAl2O3

2.0 × 1015 1.4 × 1016

275 272

3.3 × 105 2.0 × 102

86 50

7.2 × 108 3.1 × 1012

144 207

1.6 × 104 1.2 × 106

62 90

796


Energy & Fuels

Article

carbon formation was not observed because H2O was fed with the CH4 to the reactor. Thus, carbon formation through CH4 decomposition was not taken into account in the model. The mass balance for each gas present in the reactor on a differential cross section of the bed yields:

∂CCH4 ∂t

=−

ug ∂CCH4 1 − εfb − f ρ̅ εfb ∂z εfb w,s sol

(1/4(−rOC1) + r1 + r3)

∂C H2 ∂t

=−

ug ∂C H2 εfb ∂z

+

(16)

1 − εfb f ρ̅ εfb w,s sol

(3r1 + 4r3 + r2 − ( − rOC2))

(17)

ug ∂CCO ∂CCO 1 − εfb =− + f ρ̅ ∂t εfb ∂z εfb w,s sol (r1 − r2 − ( − rOC3)) ∂CCO2 ∂t

=−

ug ∂CCO2 εfb

∂z

+

(18)

1 − εfb f εfb w,s

ρsol ̅ (( − rOC1) + r3 + r2 + ( − rOC3))

∂C H2O

Figure 10. Gas product composition at the exit of the fixed-bed reactor. Continuous lines: experimental results. Dotted lines: values predicted by the model assuming the catalytic activity of the oxygencarrier fully reduced. (a) NiO18-αAl2O3 and (b) NiO21-γAl2O3. T = 900 °C.

∂t

+ ( − rOC2))

reaction rate −rOC1 r1 r3 r2 −rOC2 −rOC3

(20)

The z-coordinate indicates the axial position in the bed, with z = 0 at the gas inlet point, that is, the upper limit of the bed. This equation was solved with the following initial and boundary conditions, respectively:

Table 5. Main Reactions Happening in the Fuel-Reactor of a CLC or CLR Process Using CH4 as Fuel reaction

ug ∂C H2O 1 − εfb + f εfb ∂z εfb w,s

ρsol ̅ (1/2( − rOC1) − r1 − r2 − 2r3

bed and plug flow conditions for the gas phase, with no axial and radial dispersions, were assumed. The mass balance in the reactor should be included in the reactor model according to the mechanism proposed. The reaction mechanism proposed for the Ni-based oxygen-carriers was based on the work of several authors19,34,35 and included the reactions that can take place in the fuel-reactor (see Table 5).

CH4 + 4NiO ↔ 4Ni + CO2 + 2H2O CH4 + H2O ↔ CO + 3H2 CH4 + 2H2O ↔ CO2 + 4H2 CO + H2O ↔ CO2 + H2 H2 + NiO ↔ H2O + Ni CO + NiO ↔ CO2 + Ni

=−

(19)

Ci|z = 0 = Ci ,in and Ci|z > 0 = 0 at t = 0

(21)

Ci|z = 0 = Ci ,in at t > 0

(22)

Two different nickel compounds, NiO and NiAl2O4, were present in both oxygen-carriers, as was shown in Table 1, and it was found in a previous work by Dueso et al.17,36 that both phases are active for oxygen transfer although with very different reactivities. Dueso et al.36 determined the kinetic parameters for the reduction of both nickel compounds in the NiO21γAl2O3 and NiO18-αAl2O3 oxygen-carriers, with CH4, CO, and H2 as fuels in a TGA. These kinetic parameters are shown in Tables 6 and 7 and have been used in this work to determine the

The kinetic parameters of the reduction of the oxygen-carriers with CH4, H2, and CO used in the model were determined in a previous work.36 According to this model, at the beginning of the reaction, the oxygen-carrier is completely oxidized, favoring the total oxidation of CH4 to form CO2 and H2O. As the reaction proceeds, the oxygen-carrier is partially reduced and the metallic Ni formed catalyzes the reforming reactions of CH4 to form CO, CO2, and H2. The CO and H2 formed can react with the oxygen-carrier to form CO2 and H2O, respectively. The CO also reacts with H2O through WGS reaction. Experimentally,

Table 6. Kinetic Parameters for the Reduction Reaction of the NiO Present in the Oxygen-Carriers n k0 (mol1−n m3n−2 s−1) Ea (kJ mol−1)

CH4

H2

CO

0.2 1.9 × 10−1 5

0.42 1.4 × 10−1 5

0.65 5.5 × 10−2 4

reaction rates rOC1, rOC2, rOC3. The NiO reduction in both oxygen carriers was described using an empirical linear model described by eq 23: 797


Energy & Fuels

Article

The differential mass balances in the reactor were solved to obtain the evolution with time of the longitudinal profiles of gas concentration and solid conversion through the bed. The model was solved using the finite differences method, considering differential elements of height Δz, and starting from the gas inlet point. The model solution gave the evolution with time of the CH4 conversion and concentration of gases at each differential element. Figure 10 shows the comparison between the gas composition obtained experimentally and the gas composition predicted by the model at the outlet of the reactor for both oxygen-carriers. It can be observed that the gas compositions given by the model do not agree with the experimental data in the first period of the reaction. In this period, the model predicted a higher CH4 conversion than that observed experimentally. This was because in the simulations carried out with the model, it was considered that the reforming reactions took place since the beginning of the reaction, which seems to be that it was not realistic because there was not, or there was not enough, metallic Ni formed to catalyze the reactions. For a better prediction of the experimental results, a factor of the catalytic activity (αcat) of the oxygen-carrier for the rates of reforming reactions (r1 and r3) was included, which increased with the reduction degree of the oxygen-carrier. This factor is 1 when the oxygen-carrier is completely reduced, and it is calculated as a function of the oxygen-carrier conversion when the oxygen-carrier is partially oxidized using the following expression:

Table 7. Kinetic Parameters for the Reduction Reaction of NiAl2O4 Present in the Oxygen-Carriers CH4

H2

NiO18-αAl2O3 Chemical Reaction n 1.7 0.6 k0 (mol1−n m3n−2 s−1) 2.8 × 1010 8.3 × 104 Ea (kJ mol−1) 400 235 NiO21-γAl2O4 Chemical Reaction n 1 0.6 k0 (mol1−n m3n−2 s−1) 1.5 × 1010 1.5 × 105 Ea (kJ mol−1) 375 235 Diffusion on the Product Layer Ds,0 (m2 s−1) 3.5 × 10−5 4.2 × 102 −1 EDs (kJ mol ) 200 280

XNiO =

CO

0.7 2.5 × 10−3 82

1 1.5 × 10−3 89

t τNiO

(23)

and

dXNiO 1 = dt τNiO

(24)

The reduction of the NiAl2O4 present in the oxygen-carriers was described with the changing grain size model. In this model, the particles are assumed to be formed by small spherical grains, each one following a shrinking core model during the reaction. For the NiO18-αAl2O3, it was assumed that the reaction rate was controlled by the chemical reaction, and for the NiO21-γAl2O3, it was assumed that the reaction rate was controlled by the chemical reaction and the diffusion through the product layer. The equations that describe this model under chemical reaction rate control are

t = 1 − (1 − XNiAl2O4)1/3 τch τch =

2 αcat = a + bXOC + cXOC

Taking into account this factor, the reaction rates for reforming reactions, r1 and r3, were calculated as a function of the oxygen-carrier conversion as follows:

r1(XOC) = αcat(XOC) r1(XOC = 1)

(25)

2 4

(26)

When the reaction rate was controlled by the chemical reaction and the diffusion through the product layer,

τ = τch + τdif

Table 8. Parameters of the Factor of Catalytic Activity

(27) a b c

being ⎡ t = 3⎢1 − (1 − XNiAl2O4)2/3 ⎢ τdif ⎣ +

τdif =

1 − [Z + (1 − Z)(1 − XNiAl2O4)2/3 ] ⎤ ⎥ ⎥ Z−1 ⎦

(28)

2 4

(29)

From these results, the reaction rates rOC1, rOC2, rOC3 were calculated using the following expression:

− rOC, i = R 0

dXOC 1 dt M O

NiO18-αAl2O3

NiO21-γAl2O3

0 1 0

0 0.19 0.81

parameters of the factor of catalytic activity obtained for each oxygen-carrier. The αcat values calculated with these parameters for the NiO21-γAl2O3 oxygen-carrier were lower than the obtained for the NiO18-αAl2O3 oxygen-carrier for the main part of the conversion range. However, as it can be seen in Figure 11, the disappearance rate of CH4 for the NiO21-γAl2O3 was higher than that of the NiO18-αAl2O3 oxygen-carrier, because of its higher reactivity when fully reduced (eqs 32 and 33). Figure 12 shows a comparison between the gas composition obtained experimentally and the gas composition predicted by the model at the outlet of the fixed-bed reactor, taking into account the factor of the catalytic activity. It can be observed that the gas composition predicted by the model and the gas composition obtained experimentally are in good agreement during the reaction period. So, on the basis of these results, it

2 ρm,NiAl O rg,NiAl 2O4

6bDsCi

(32)

r3(XOC) = αcat(XOC) r3(XOC = 1) (33) The experimental CH4 conversion versus time data shown in Figure 10 were fitted using the Nelder and Mead37 method to determine the relationship between the catalytic activity of the oxygen-carrier and its conversion, that is, to determine the values of the parameters a, b, and c in eq 31. Table 8 shows the

ρm,NiAl O rg,NiAl2O4 bkCin

(31)

(30) 798


Energy & Fuels

Article

WGS reactions. However, experimental results shown by several authors showed that the catalytic activity depended on the degree of oxidation of the oxygen carrier, but this fact has never been studied previously. In this work, the catalytic activity for the SMR and WGS reactions of two Ni-based oxygen-carriers, NiO18-αAl2O3 and NiO21-γAl2O3, used for CLC and CLR has been determined in a fixed-bed reactor. The kinetic parameters of the catalytic SMR and WGS reactions, as well as the effect of the oxidation degree of the oxygen-carrier on its catalytic activity, have been determined. It has been found that the catalytic activity for SMR and WGS reactions was similar for the two oxygen-carriers used but lower than the catalytic activity of other conventional Ni-based catalyst. This catalytic activity of the oxygen-carriers depended on their oxidation/reduction degree, increasing the catalytic activity with increasing the reduction conversion of the oxygencarrier. It was concluded that the methane reforming reactions are of great relevance to achieve a high CH4 conversion in a CLC or CLR system; thus, the knowledge of the kinetics of these reactions is essential for the subsequent design of CLC and CLR systems.

Figure 11. CH4 disappearance rate as a function of the reduction oxygen-carrier conversion. T = 900 °C.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +34-976-733977. Fax: +34-976-733318. E-mail: ldediego@ icb.csic.es.

■

ACKNOWLEDGMENTS This work was partially supported by the Spanish Ministry of Science and Innovation (MICINN) (CTQ2007-64400) and the European Commission, under the 6th Framework Program (CACHET Project, Contract No. 019972), and from the CCP2 (CO2 Capture Project), a partnership of BP, Chevron, ConocoPhillips, Eni Technology, Norsk Hydro, Shell, Suncor, and Petrobras. M. Ortiz thanks Diputación General de Aragón for the FPI fellowship.

■

NOMENCLATURE b stoichiometric factor in the reduction reaction of metal oxide (moles of MeO per mole of fuel gas). Ci concentration of gas i (mol m−3). Ci,in concentration of gas i coming to the reactor (mol m−3). Ds diffusion coefficient through the product layer (m2 s−1). Ds,0 preexponential factor of Ds (m2 s−1). Ej activation energy of reaction j (kJ mol−1). Ea activation energy (kJ mol−1). activation energy of Ds (kJ mol−1). EDs Fi molar flow of gas i fed to the fixed-bed reactor (mol s−1). fw,s weight fraction of solid in the bed. k chemical reaction rate constant (mol 1−n m3n−2 s−1). Ki adsorption constant of the gas i. kj kinetics constant of the reaction j (mol kg−1 s−1). Kj equilibrium constant of reaction j. k0 preexponential factor of the chemical reaction rate constant (mol 1−n m3n−2 s−1). K0,i preexponential factor of the adsorption constant. k0,j preexponential factor of the kinetics constant of the reaction j (mol kg−1 s−1). MO molecular weight of oxygen (kg mol−1). n reaction order. pi partial pressure of gas i (Pa).

Figure 12. Gas product composition at the exit of the fixed-bed reactor. Continuous lines: experimental results. Dotted lines: values predicted by the model assuming that the catalytic activity of the oxygen-carrier depends on its oxidation degree. (a) NiO18-αAl2O3 and (b) NiO21-γAl2O3. T = 900 °C.

can be concluded that the catalytic activity of the oxygencarriers depends on their reduction/oxidation degree and that the reforming reactions are of great relevance to achieve a complete CH4 conversion either in a CLR or CLC system.

4. CONCLUSIONS The main function of the oxygen-carriers in CLC and CLR processes is to transfer oxygen from the air-reactor to the fuelreactor. However, it has been observed and analyzed by some authors that oxygen-carriers, especially those based on Ni, present catalytic activity for some reactions happening in the fuelreactor of the CLC or CLR systems, such as the SMR and 799


Energy & Fuels Pi,in Pi,out Qin Qout R r rg,Al2O4 RO t T ug W Xi XNiAl2O4 XNiO XOC z Z ΔHi

Article

(15) Zafar, Q.; Mattison, T.; Gevert, B. Ind. Eng. Chem. Res. 2005, 44, 3485−3498. (16) Johansson, M.; Mattisson, T.; Lyngfelt, A.; Abad, A. Fuel 2008, 87, 988−1001. (17) Dueso, C.; Abad, A.; García-Labiano, F.; de Diego, L. F.; Gayán, P.; Adánez, J.; Lyngfelt, A. Fuel 2010, 89, 3399−3409. (18) Jerndal, E.; Mattisson, T.; Thijs, I.; Snijkers, F.; Lyngfelt, A. Int. J. Greenhouse Gas Control 2010, 4, 23−35. (19) Iliuta, I.; Tahoces, R.; Patience, G. S.; Rifflart, S.; Luck, F. AIChE J. 2010, 56, 1063−1079. (20) Azadi, P.; Otomo, J.; Hatano, H.; Oshima, Y.; Farnood, R. Chem. Eng. Sci. 2011, 66, 4196−4202. (21) Dueso, C.; García-Labiano, F.; Adánez, J.; de Diego, L. F.; Gayán, P.; Abad, A. Fuel 2009, 89, 3399−3409. (22) García-Labiano, F.; de Diego, L. F.; Gayán, P.; Adánez, J.; Abad, A.; Dueso, C. Ind. Eng. Chem. Res. 2009, 48, 2499−2508. (23) Adánez, J.; Dueso, C.; de Diego, L. F.; García-Labiano, F.; Gayán, P.; Abad, A. Ind. Eng. Chem. Res. 2009, 48, 2509−2518. (24) de Diego, L. F.; Ortiz, M.; García-Labiano, F.; Adánez, J.; Abad, A.; Gayán, P. J. Power Sources 2009, 192, 27−34. (25) Ortiz, M.; de Diego, L. F.; Abad, A.; García-Labiano, F.; Gayán, P.; Adánez, J. Int. J. Hydrogen Energy 2010, 35, 151−160. (26) Gayán, P.; de Diego, L. F.; García-Labiano, F.; Adánez, J.; Abad, A.; Dueso, C. Fuel 2008, 87, 2641−2650. (27) Gayán, P.; Dueso, C.; Abad, A.; Adánez, J.; de Diego, L. F.; García-Labiano, F. Fuel 2009, 88, 1016−1023. (28) Abad, A; Adanez, J.; Garcia-Labiano, F.; de Diego, L. F.; Gayan, P.; Celaya, J. Chem. Eng. Sci. 2007, 62, 533−549. (29) Xu, J.; Froment, G. F. AIChE J. 1989, 35, 88−96. (30) Hou, K.; Hughes, R. Chem. Eng. J. 2001, 82, 311−328. (31) Hoang, D. L.; Chan, S. H.; Ding, O. L. Chem. Eng. J. 2005, 112, 1−11. (32) Soliman, M. A.; Adris, M. A.; Al-Ubaid, A. S.; El-Nashai, S. S. E. H. J. Chem. Technol. Biotechnol. 1992, 55, 131−138. (33) XLSTAT; Addinsoft: Paris, France, 2009. (34) Dewaele, O.; Froment, G. F. J. Catal. 1999, 184, 499−513. (35) Abad, A.; Adánez, J.; García-Labiano, F.; de Diego, L. F.; Gayán, P.; Kolbitsch, P.; Pröll, T. Proceedings of the 1st International Conference on Chemical Looping, Lyon, France, 2010. (36) Dueso, C.; Ortiz, M.; Abad, A.; García-Labiano, F.; de Diego, L. F.; Gayán, P.; Adánez, J. Chem. Eng J.. Submitted for publication. (37) Nelder, J. A.; Mead, R. Comput. J. 1964, 7, 308−313.

partial pressure of gas i coming to the reactor (Pa). partial pressure of gas i leaving the reactor (Pa). molar flow of the gas coming into the reactor (mol m−3 s−1). molar flow of the gas leaving the reactor (mol m−3 s−1). universal gas constant (kJ mol−1 K−1). reaction rate (mol kg−1 s−1). grain radius of the NiAl2O4 (m). oxygen transport capacity of the oxygen carrier (kg O kg OC−1) time (s). temperature (K). gas velocity (m s−1). mass of oxygen-carrier (kg). conversion of gas i. nickel aluminate conversion. nickel oxide conversion. oxygen-carrier conversion. axial coordinate in the bed (m). expansion ratio. enthalpy change of adsorption of gas i (kJ mol−1).

Greek Letters

αcat factor of catalytic activity. εfb void fraction of the fixed-bed. ρm,NiAl2O4 molar density of NiAl2O4 in the solid (mol m−3 solid). ρ̅sol mean solid density in the bed (kg m−3). τ time for complete conversion (s). τch time for complete conversion when reaction rate is controlled by chemical reaction (s). τdif time for complete conversion when reaction rate is controlled by diffusion in the product layer (s).

■

REFERENCES

(1) IPCC. IPCC Special Report on Carbon Dioxide Capture and Storage; Prepared by Working Group III of the Intergovernmental Panel on Climate Change. Metz, B., Davidson, O., de Coninck, H. C., Loos, M., Meyer, L. A., Eds.; Cambridge University Press: Cambridge and New York, 2005. (2) Ishida, M.; Jin, H. Ind. Eng. Chem. Res. 1996, 35, 2469−2472. (3) Lyngfelt, A.; Leckner, B.; Mattisson, T. Chem. Eng. Sci. 2001, 56, 3101−3113. (4) Ortiz, M.; Abad, A.; de Diego, L. F.; García-Labiano, F.; Gayán, P.; Adánez, J. Int. J. Hydrogen Energy 2011, 36, 9663−9672. (5) Mattison, T.; Lyngfelt, A. Proceedings of the 2nd Nordic Minisymposium on Carbon Dioxide Capture and Storage, Göteborg, Sweden, 2001. (6) Lyngfelt, A.; Johansson, M.; Mattisson, T. 9th International Conference on Circulating Fluidized Beds (CFB-9), Hamburg, Germany, 2008. (7) Hossain, M. M.; de Lasa, H. I. Chem. Eng. Sci. 2008, 63, 4433− 4451. (8) Adánez, J.; Abad, A.; García-Labiano, F.; Gayán, P.; de Diego, L. F. Prog. Energy Combust. Sci. 2012, 38, 215−282. (9) Linderholm, C.; Abad, A.; Mattisson, T.; Lyngfelt, A. Int. J. Greenhouse Gas Control 2008, 2, 520−530. (10) Adánez, J.; Dueso, C.; de Diego, L. F.; García-Labiano, F.; Gayán, P.; Abad, A. Energy Fuels 2009, 23, 130−142. (11) Kolbitsch, P.; Bolhàr-Nordenkampf, J.; Pröll, T.; Hofbauer, H. Ind. Eng. Chem. Res. 2009, 48, 5542−5547. (12) Ryu, H. J.; Jo, S. H.; Park, Y.; Bae, D. H.; Kim, S. Proceedings of the 1st International Conference on Chemical Looping, Lyon, France, 2010. (13) Zafar, Q.; Mattisson, T.; Gevert, B. Energy Fuels 2006, 20, 34−44. (14) Rydén, M.; Lyngfelt, A.; Mattisson, T.; Chen, D.; Holmen, A.; Bjørgum, E. Int. J. Greenhouse Gas Control 2008, 2, 21−36. 800


Catalytic Activity of Ni-Based Oxygen-Carriers for ... - ACS Publications

Recommend Documents