ZrO2 with petroleum coke

Fuel 88 (2009) 683–690

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Chemical-looping with oxygen uncoupling using CuO/ZrO2 with petroleum coke Tobias Mattisson a,*, Henrik Leion b, Anders Lyngfelt a a b

Department of Energy and Environment, Division of Energy Technology, Chalmers University of Technology, S-412 96 Göteborg, Sweden Department of Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96 Göteborg, Sweden

a r t i c l e

i n f o

Article history: Received 10 April 2008 Received in revised form 17 September 2008 Accepted 17 September 2008 Available online 14 October 2008 Keywords: Chemical-looping with oxygen uncoupling (CLOU) CuO Petroleum coke Carbon capture CLC

a b s t r a c t Oxygen carrier particles of CuO/ZrO2 were reacted with petroleum coke using chemical-looping with oxygen uncoupling (CLOU). The fuel was burnt in gas-phase oxygen released from the oxygen carrier particles during the fuel oxidation. The particles were then regenerated in 5–21% oxygen. In this process, the carbon dioxide from the combustion is inherently separated from the rest of the flue gases without the need for an energy intensive air separation unit. Copper oxide has thermodynamic characteristics that make it suitable as an oxygen carrier in CLOU. Particles were prepared by freeze granulation and were exposed cyclically with petroleum coke and oxygen in a laboratory fluidized bed reactor of quartz. The reaction temperature and oxygen concentration during the oxidation were varied. The average conversion rate of petroleum coke was a function of temperature and varied between 0.5%/s and 5%/s in the set-point temperature interval 885–985 °C. The conversion rate is considerably higher than rates obtained with the same fuel using iron-based oxygen-carrier in chemical-looping combustion. As for the regeneration with oxygen, the reduced particles reacted at low oxygen concentrations, with a considerable part of the reaction occurring near the thermodynamic equilibrium. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Chemical-looping with oxygen uncoupling (CLOU) is a process for oxidation of a solid fuel [1]. The method is suitable to capture carbon dioxide during combustion, in order to reduce effects on climate, and may give substantially reduced costs for carbon dioxide capture. The method may be especially suitable for solid fuels, such as coal, petroleum coke or biofuels [1]. The combustion technique involves three steps in two reactors, as shown in Fig. 1. In the air reactor, an oxygen carrier captures oxygen from the combustion air (step 1)

O2 ðgÞ þ Mex Oy2 $ Mex Oy

ð1Þ

The oxygen carrier is transported to the fuel reactor, where it releases oxygen (step 2) according to reaction

Mex Oy $ Mex Oy2 þ O2 ðgÞ

ð2Þ

The released oxygen reacts with a fuel (step 3)

Cn H2m þ ðn þ m=2ÞO2 ðgÞ ! nCO2 þ mH2 O

2. Thermodynamic analysis

ð3Þ

The reduced oxygen carrier is then recirculated to the air reactor to be regenerated, i.e. step 1. Since reaction (1) and (2) cancel each other, the net reaction over the CLOU system is simply reaction (3), i.e. normal combustion. This means that the total heat * Corresponding author. Tel.: +46 31 772 1425; fax: +46 31 772 3592. E-mail address: [email protected] (T. Mattisson). 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.09.016

release over the fuel and air reactor is also the same as for conventional combustion. The advantage is that the CO2 and H2O are inherently separated from the nitrogen in the combustion air, and thus no energy is needed to obtain pure CO2. This is the same as in regular chemical-looping combustion (CLC) [2–4]. However, the combustion of solid fuel with normal CLC requires a slow gasification step in the fuel reactor, which is not required in CLOU [1,5,6]. This has the implication that much less oxygen carrier material is needed in the system, which will also reduce the reactor size and associated costs. This study will present the thermodynamic aspects of CLOU with CuO and will present experimental results from investigations of the reaction between an oxygen carrier of CuO/ZrO2 and petroleum coke using the CLOU mechanism. The experiments were carried out in a small quartz laboratory fluidized bed reactor using different temperatures and oxygen concentrations.

The oxygen carriers for CLOU must have the ability to react with both oxygen in the air reactor and then also release this oxygen as O2 in the fuel reactor. Thus oxygen carriers for CLOU need to have special thermodynamic characteristics in comparison to oxygen carriers for normal CLC where the fuel, or gasified fuel, reacts directly with the oxygen carrier without any release of O2. Mattisson et al. [1] made a thermodynamic analysis of three possible metal oxide systems: CuO/Cu2O, Mn2O3/Mn3O4 and Co3O4/CoO [1]. The first

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Nomenclature Ft M mox mred _ ox m _ oc m _o m Ro rav to t1

molar flow rate of gas after condensation of steam, mol/s mass of oxygen carrier, kg mass of fully oxidized oxygen carrier, kg mass of fully reduced oxygen carrier, kg circulation rate of fully oxidized particles, kg s1 MW1 fuel

t2

circulation rate of oxygen carrier particles, kg s1 MW1 fuel mass flow of oxygen needed for complete combustion, 1 1 kg s MWfuel oxygen ratio, defined in Eq. (9) average conversion rate of the fuel, %/s time when fuel is added to the reactor, s time elapsed since the start of the cycle, s

X Xo

two systems had overall exothermic reactions in the fuel reactor when reacting with carbon. This will thus result in a temperature increase in the fuel reactor, which is an advantage because higher equilibrium partial pressures of oxygen are then possible in the fuel reactor, in comparison to the partial pressure at the outlet of the air reactor, which is generally related to the air ratio. Below follows a discussion on the thermodynamic aspects of using an oxygen carrier of CuO together with ZrO2 as inert material. The details of the mass and heat balance can be found in Mattisson et al. [1]. In order to maintain a high power plant efficiency it is important to keep the outlet partial pressure of O2 from the air reactor as low as possible, i.e. a low air ratio. Thus, in the calculations below it was assumed that the temperature in the air reactor was 913 °C, which corresponds to a concentration of O2 of 2% as calculated from equilibrium data from HSC Chemistry 6.1 [7]. The actual concentration from the air reactor will depend upon oxygen carrier reactivity and oxygen carrier inventory, but will likely always be higher than the equilibrium concentration. Here it is assumed that the solids inventory is large enough so O2 concentrations near equilibrium can be reached at the outlet from the air reactor. Assuming that the fuel is carbon, C, the oxygen carrier particles enter the fuel reactor and react according to reactions (2) and (3), giving the overall exothermic reaction

4CuO þ C ) 2Cu2 O þ CO2

DH950 ¼ 133:5 kJ=mol

ð4Þ

which should result in a temperature increase in the fuel reactor and subsequently also an increase in the partial pressure of O2. This is

N2, O2

Air Reactor

CO2, H2O

Me xOy

Me x O y ↔ Mex O y − 2 + O2 ( g )

Me x O y − 2 + O2 ( g ) ↔ Me x O y

MexOy-2

Air

Fuel reactor

C + O2 ( g ) → CO 2

u umf xi

DX yCuO

s

time when all of the carbon in the fuel has been converted, s superficial velocity, m/s minimum fluidization velocity, m/s volume fraction of species i at the outlet of the reactor after condensation of steam conversion of the solid oxygen carrier MexOy conversion of the solid oxygen carrier MexOy at the outlet of the air reactor difference in conversion of MexOy between the air and fuel reactor mass fraction of CuO in the fully oxidized oxygen carrier residence time of fuel in the fuel reactor, s

illustrated in Fig. 2a, which shows the O2 partial pressure as a function of the temperature increase in the fuel reactor with respect to the temperature chosen in the air reactor. The actual temperature rise is given by the change in the conversion of the solid particles, as is seen in Fig. 2b. Here, the degree of solids conversion, X, is defined as

X¼

m  mred mox  mred

ð5Þ

where m is the actual mass of the oxygen carrier, mox the mass of the fully oxidized oxygen carrier, i.e. CuO/ZrO2, and mred the mass of the fully reduced oxygen carrier, i.e. Cu2O/ZrO2. Even at rather low change in conversion, the equilibrium partial pressure of O2 increases significantly. For example, at a 20% conversion of the solid oxygen carrier, the partial pressure of O2 is between 4.5% and 12.4% depending upon the amount of inert material, see Fig. 2b. 3. Experimental 3.1. Materials The oxygen carrier particle was prepared by freeze granulation and was composed of 40 wt% active material of CuO and 60% ZrO2. A water-based slurry was prepared by mixing CuO (Panreac No. 141269) and ZrO2 (Sigma–Aldrich No. 24.403-1). This mixture was ball milled for 24 h. A small amount of dispergent was also added to this mixture in order to improve slurry characteristics. After milling, an organic binder was added to the slurry as a binder to keep the particles intact during later stages in the production process, i.e. freeze-drying and sintering. Spherical particles were produced by freeze granulation, i.e. the slurry is pumped to a spray nozzle where passing atomising-air produce drops, which are sprayed into liquid nitrogen where they freeze instantaneously. The frozen water in the resulting particles is then removed by sublimation in a freeze-drier operating at a pressure that corresponds to the vapour pressure over ice at 10 °C. After drying, the particles were sintered at 950 °C for 6 h using a heating rate of 5 °C/min. The characteristics of the oxygen carrier can be found in Table 1. The petroleum coke was from the Cadereyta refinery in Mexico and it was crushed and sieved to obtain particles of diameter 180– 250 lm. Table 2 shows the analysis of the petroleum coke and Fig. 3 shows SEM images of the fresh oxygen carrier particles.

Fuel (+ H2O/CO2) 3.2. Experimental procedure

Fig. 1. Principal layout of chemical-looping oxygen uncoupling. The oxygen carrier is denoted by MexOy and MexOy2, where MexOy is a metal oxide and MexOy2 is a metal or a metal oxide with lower oxygen content compared to MexOy. The fuel is here carbon (C). The fuel reactor could be fluidized using recirculated CO2 or steam when burning solid fuel.

The experiments were conducted with a batch fluidized bed reactor of quartz. The cylindrical shaped reactor had a total length of 870 mm and an inner diameter of 22 mm with a porous quartz

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T. Mattisson et al. / Fuel 88 (2009) 683–690 Table 1 Experimental data

Temperature (˚C) 960

920

a

1000

Fuel and oxygen carrier Oxygen carrier Oxygen ratio, Ro Apparent density, particles (kg/m3) BET of oxygen carrier (m2/g) Size interval of particles (lm) Crushing strength (N) Mass of oxygen carrier (g) Fuel Mass of fuel (g) Particle size of fuel (lm)

Partial pressure of O2

0.2

0.1 0.08 0.06

Reactor data

0.04

0.02

0.01 0

80

40

120

Δ T(˚C)

b

40 wt% CuO/60wt% ZrO2 0.04 2140 1.4 125–180 0.8 15 Petroleum coke 0.1 180–250

Height bed (mm) Number of cycles in experiment Temperature reactor (°C) Pressure (bar) Oxidizing gas Inert gas Flow in reduction (nitrogen) (mlN/min) Flow in oxidation (mlN/min) u/umf (reduction)* u/umf (oxidation)* *

30 15 880–985 1 5, 10 and 21% O2 in N2 100% N2 900 900 27 27

u/umf is calculated with respect to the inlet flows.

0.6

Change in conversion (ΔX)

Table 2 Analysis of petroleum coke Fuel analysis Hi (MJ/kg) (as received)

0.4

30.9

0.2

0

0

80

40

120

Δ T(˚C) Fig. 2. (a) The partial pressure of oxygen as a function of the change in temperature of the fuel reactor with respect to the temperature in the air reactor and (b) the change in conversion for CuO/ZrO2 as a function of the temperature change in the fuel reactor. It is assumed that the oxygen carrier particles enter the fuel reactor at 913 °C and that the fluidizing gas is CO2 which is preheated to 400 °C. Fraction of ZrO2 in the oxygen carrier: 0 wt% (–),40 wt% (- - -), 60 wt% (— —). All thermodynamic calculations were carried out using data from HSC Chemistry 6.1 [7].

plate placed 370 mm from the bottom of the reactor. The temperature was measured 5 mm under and 10 mm above the porous quartz plate, using 10% Pt/Rh thermocouples enclosed in quartz shells. The temperatures presented in this study are the temperatures measured above the porous quartz plate in the bed of oxygen carriers. A sample of 15 g of oxygen carrier particles of size 125–180 lm was placed on the porous plate giving a bed height of 30 mm when the bed was not fluidized. The sample was initially heated in an oxidizing atmosphere to the reaction temperature. The particles were then alternatingly exposed to O2 and the fuel, thus simulating the cyclic conditions of a CLOU-system with particles being circulated between the air and fuel reactor. During the reducing period, the fluidizing gas was nitrogen, which was introduced from the bottom of the reactor. At the same time as the fluidizing gas of

Proximate (wt%, as received)

(wt% d.a.f)

Ultimate (wt%, d.a.f.)

M

A

Combustibles

VM

C

H

N

S

O

8.0

0.5

91.5

10.9

88.8

3.1

1.0

6.6

0.5

the reducing cycle entered the bottom of the reactor, 0.1 g of solid fuel was inserted in the top of the reactor, falling down into the fluidized bed. The diameter of the fuel particles was between 180– 250 lm, and was somewhat larger than that of the oxide particles. The gas from the reactor was led to an electric cooler, where the water was condensed and removed, and then to a gas analyzer (Rosemount NGA-2000) where the concentrations of CO2, CO, CH4, and O2 were measured in addition to the gas flow. All experiments were conducted with a fluidizing gas flow of 900 mLn/min. Using measurements of the pressure drop over the bed at 20 Hz it was possible to see if the bed was fluidized or not, c.f. Cho et al. [8]. The main experimental parameters are summarized in Table 1 and the experimental setup is also shown in Fig. 4. 3.3. Data evaluation The reaction of the petroleum coke with the oxygen carrier was monitored by measuring the concentrations of CO, CO2 and CH4 as a function of time. The average conversion rate of the fuel was established by integration of these concentrations as a function of time and calculated from,

R t1 rav ¼

F t ðxCO2 þ xCO þ xCH4 Þdt R t2 F t ðxCO2 þ xCO þ xCH4 Þdt t0

t0

t1

ð6Þ

where t0 is the time when fuel was added to the reactor, t1 the time elapsed since the start of the cycle and t2 the time when all added carbon has been converted. Ft is the total molar flow of gas from the outlet of the reactor, which was measured in connection with the gas analysis and xi is the concentration of species i after condensation of steam. In this work the average rate was calculated for the period needed to reach 95% conversion of the carbon added.

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Fig. 3. SEM images of a fresh CuO/ZrO2 particle. The white bars in the figures corresponds to (a) 150 lm, (b) 45 lm and (c) 10 lm.

Fig. 4. Experimental set-up.

4. Results 4.1. Redox behaviour Fig. 5 shows the outlet gas concentrations after condensation of water as a function of time for a reducing and oxidizing period. The set-point temperature, here defined as the temperature as measured in the bed prior to the reduction period, is 885 °C. The oxygen concentration is 21% before the reducing period, since air is

Regeneration 920

0.2

910

O2 0.15

900

Temperature 0.1

890

O2 CO 2

0.05

0

0

200

Temperature (˚C)

Volume fraction (-)

Reduction with pet coke 0.25

880

400

600

870

Time (s) Fig. 5. Concentration profile for conversion of 0.1 g of petroleum coke with 15 g CuO/ZrO2. The O2 concentration during regeneration of the oxygen carrier is 21%. The set-point temperature is 885 °C and the fluidizing gas is pure nitrogen. Also included is the equilibrium concentration of O2, as determined from the temperature measurement in the bed (+).

the fluidizing gas. As the fluidizing gas is switched to nitrogen the concentration of oxygen falls to near the equilibrium partial pressure which for this temperature is somewhat below 1%. This is accompanied by a small temperature decrease, due to the endothermic nature of reaction (2). As the fuel is added a small peak of CH4 is seen in the beginning of the reduction period, which is due the devolatilization of the fuel. At the same time the combustion starts and the CO2 concentration increases, to reach a maximum of 6%. The oxygen concentration is more or less constant at the equilibrium concentration during the reduction, except for an initial dip during devolatilization. This means that more oxygen is released from CuO than is needed for the combustion. Thus, the release of oxygen from the oxygen carrier is fast, and mainly limited by thermodynamics. There is a temperature increase during the reduction due to the combustion, giving a maximum of 895 °C as measured in the bed of material. No CO was detected during the reducing period indicating that there is full selectivity to CO2, with exception for the initial devolatilization. When no gaseous carbon species, i.e. CO2, CO or CH4, are detected in the outlet gas from the reactor, the burnout of the fuel is assumed to be complete. After burnout of the fuel, there is still oxygen released from the oxygen carrier particles and thus the oxygen carrier particles are not fully reduced to Cu2O. As only 0.1 g of petroleum coke was added to the bed, only a part of the active CuO should be reduced corresponding to a DX of approximately 0.4 as determined from the analysis of the petroleum coke in Table 2. Here it is assumed that the combustibles in the petroleum coke, i.e. carbon, hydrogen and sulphur react completely to oxidized products. At approximately 520 s the oxygen carrier particles are exposed to air to regenerate the Cu2O to CuO. The initial part of this regeneration is characterized by a relatively large temperature increase due to the exothermic nature of this reaction. The initial outlet O2 concentration is low, and also near equilibrium, thus indicating

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T. Mattisson et al. / Fuel 88 (2009) 683–690

based particles with oxygen, several cycles were conducted with smaller concentrations of oxygen, i.e. 5% and 10%. One such experiment, using 10% O2, is shown in Fig. 7. The set-point temperature is 955 °C prior to the reduction period. In general both the fuel oxidation and the oxygen carrier regeneration follow similar patterns as those shown in the previous two figures. However, the temperature increase during the oxygen carrier regeneration is less here. This is not surprising since the heat release is less as a function of time due to the lower inlet oxygen concentration. Still the characteristic step-type of regeneration of the particles is also seen here. 4.2. Rate of fuel conversion Fig. 8 shows the time needed to reach 95% conversion of the fuel for different set-point temperatures and Fig. 9 shows the average rate of reaction for a 95% conversion of the fuel. The conversion rate is highly dependent on the reaction temperature. At the highest set-point temperature, 985 °C, only 20 s is needed to convert 95% of the fuel. The rate of conversion increases with the temper-

Reduction

Regeneration

0.4

970

Temperature

950 0.2

CO2 O2

0.1

0

940

O2

Temperature (˚C)

960

0.3

Volume fraction (-)

a high rate of reaction as the particles react with most of the added oxygen. It should be noted that in these tests there is a time delay between the temperature measurements, and the resulting equilibrium oxygen partial pressure, on one hand and the measured gas concentrations on the other hand, which is caused by the time needed for the gases to reach the gas analyser. In Fig. 5a correction for this delay was made so that temperature and measured concentrations would be reasonably well in line with each other during the reduction period. Unfortunately the delay is flow dependent, and as the flow is smaller during the regeneration there is still a delay in the data shown in Fig. 5 during regeneration. If it were not for this delay, the measured O2 would follow very close to the equilibrium curve during the first 30 s of the regeneration. At approximately 600 s the oxygen concentration increases rapidly to reach a partial pressure of 0.15, where there is a temporary stabilization, after which the oxygen partial pressure again increases and approaches the inlet concentration. Most oxidation periods were characterized by this type of behaviour. Fig. 6 shows the outlet gas concentrations after condensation of water as a function of time for a reducing period with a set-point temperature of 985 °C. The gas concentration and temperature profiles are qualitatively similar to those for the experiment at the lower temperature seen in Fig. 5. The CO2 concentration increases to a maximum of about 51% during the reduction. The petroleum coke has reacted completely within 30 s, and the absence of CO or CO2 when the air is introduced, indicates that all of the fuel has been converted. In contrast to the experiment at the lower temperature, Fig. 5, the oxygen concentration decreased during the combustion of the petroleum coke even though the equilibrium partial pressure increases. However, the reaction is never limited by access to oxygen since the concentration does not decrease to zero but reaches a minimum of about 2%. As in the previous experiment, there was no CO detected at the outlet, again indicating full conversion of the fuel. Just as in Fig. 5 above, there is a time delay between the curves of measured and equilibrium O2 during regeneration. As mentioned previously, in order for CLOU to be a feasible power production concept, it is necessary for the oxygen-carrier particles to be able to react with the combustion air and at the same time reach low outlet concentrations of oxygen in the air reactor. To further investigate the reaction phenomena of the Cu-

930

0

200

400

600

800

920 1000

Time (s) Fig. 7. Concentration profile for conversion of 0.1 g of petroleum coke with 15 g CuO/ZrO2. The O2 concentration during regeneration of the oxygen carrier is 10%. The set-point temperature is 955 °C and the fluidizing gas is pure nitrogen.

Reduction

Regeneration

0.6

1020 160

O2 980

CO2

0.2

0

O2

CH4 0

100

200

960

940 300

Time for 95% conversion (sec)

1000 0.4

Temperature (˚C)

Volume fraction (-)

Temperature

120

80

40

Time (s) Fig. 6. Concentration profile for conversion of 0.1 g of petroleum coke with 15 g CuO/ZrO2. The O2 concentration during regeneration of the oxygen carrier is 21%. The set-point temperature is 985 °C and the fluidizing gas is pure nitrogen. Also included is the equilibrium concentration of O2, as determined from the temperature measurement in the bed (+).

880

900

920

940

960

980

1000

Temperature (˚C) Fig. 8. Time for 95% conversion of 0.1 g of petroleum coke with 15 g CuO/ZrO2. The fluidizing gas is pure nitrogen.

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T. Mattisson et al. / Fuel 88 (2009) 683–690

0.1

Partial pressure of O2 (bar)

Average rate, rav, (% / s)

6

4

2

0 880

0.08

0.06

0.04

0.02

0 900

920

940

960

980

0

1000

Temperature (˚C) Fig. 9. Average rate of reaction to reach 95% conversion of 0.1 g of petroleum coke with 15 g CuO/ZrO2. The fluidizing gas is pure nitrogen.

ature and reduces the oxygen concentration. The increased difference between actual and equilibrium concentration promotes the release which is also greatly enhanced by the increase in equilibrium pressure at higher temperature. In earlier studies of the reactivity of the same fuel with both an oxygen carrier of Fe2O3/ MgAl2O4 prepared by freeze granulation and the natural mineral ilmenite, it took approximately 15 min for both materials to convert 95% of the petroleum coke at 970–980 °C [5,6]. This is done in about 20 s with CuO/ZrO2 at the same temperature. This means an improvement by a factor 45 in time of conversion. The large difference in conversion time is due to the different mechanisms of reaction as described earlier. 4.3. Regeneration with oxygen In normal CLC the oxidation of the oxygen carriers in the air reactor can take place at any oxygen concentration. In contrast, for CLOU, the concentration of oxygen at the outlet of the air reactor is restricted to the thermodynamic concentration. Thus in order for the particles to react with air, the partial pressure of oxygen during oxidation has to be higher in comparison to the equilibrium partial pressure. If the partial pressure needed to regenerate the particles is significantly higher than the equilibrium concentration, higher air ratios are needed, which would mean lower efficiencies of the power process. The oxidation reaction is exothermic, and there is a temperature increase, clearly seen in Figs. 5–7. Directly following the switch to oxidizing conditions, the outlet concentration of oxygen is, for a short time, close to equilibrium. After this, the concentration increases and stabilizes somewhat before returning to the inlet concentration as the regeneration reaction reaches completion. To investigate the effect of the oxygen concentration on the regeneration reaction, a number of cycles were performed in which a sample of 15 g of CuO/ZrO2 was reduced with 0.1 g of petroleum coke and regenerated at a set-point temperature of 905 °C or 955 °C, with 5% and 10% oxygen in the fluidizing gas. The oxidation part of the experiments is presented in Fig. 10 for a set-point temperature of 955 °C with 10% O2 in the oxidizing flow, and in Fig. 11a and b when using 5% and 10% oxygen at 905 °C set-point temperature. Here the exiting concentration of O2 is seen as a function of the cumulative fraction of oxygen reacted during the regeneration period. The equilibrium concentration is included in the graphs, as determined from the measured

0.2

0.4

0.6

0.8

1

Cumulative fraction of oxygen reacted Fig. 10. The partial pressure of O2 in the outlet from the reactor as a function of the cumulative fraction of oxygen reacted The reacting gas has an O2 concentration of 10% and the set-point temperature is 955 °C. The solid and dashed lines indicate different experiments at the same conditions. Included for comparison is the equilibrium partial pressure (+) as determined from temperature measurements in the bed.

temperature in the bed. Clearly, a large part of the oxidation occurs near the equilibrium concentration. In fact, the outlet concentration is actually below equilibrium, although this difference could be due to experimental error, e.g. if the measured temperature deviates somewhat from the actual temperature of the particles in the bed. As the reaction rate slows down, the O2 concentration increases and approaches the inlet. These experiments clearly indicate that oxidation of the particles should be possible in the CLOU-process, meaning that the oxygen partial pressure from the air reactor can be chosen reasonably close to the equilibrium partial pressure. 4.4. Solids recirculation and inventory The oxygen carrier from the air reactor needs to transport a sufficient amount of oxygen carrier to the fuel reactor in order to obtain complete combustion. If the reactions in the fuel reactor are endothermic it is also necessary for the oxygen carrier to supply enough heat so that the resulting temperature drop is not too high for the reactions to occur. Considering the use of CLOU with CuO, the latter requirement does not need to be fulfilled since the reactions in the fuel reactor are exothermic. The circulation rate of oxygen carrier, expressed per MW of fuel, is thus given by

_ OX _ oc ¼ ð1 þ RO yCuO ðX O  1ÞÞm m

ð7Þ

where mox is the circulation rate of fully oxidized oxygen carrier, calculated as

_ o =ðyCuO DXÞ _ ox ¼ m m

ð8Þ

and Ro is the oxygen ratio of the oxygen carrier, defined as

R0 ¼

ðmox  mred Þ mox

ð9Þ

where mox and mred are the masses of fully oxidized and reduced oxygen carrier, respectively. Further, yCuO is the mass fraction of _ o is the mass flow rate of oxygen needed CuO in the particles, m for complete combustion and Xo the conversion of the oxygen carriers from the outlet of the air reactor. Fig. 12 shows the calculated recirculation rates of oxygen carrier particles as a function of the change in conversion of the oxygen carrier. The recirculation rate

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T. Mattisson et al. / Fuel 88 (2009) 683–690

20

0.1

Recirculation rate (kg s-1 MWfuel-1)

a

Partial pressure of O2 (bar)

0.08

0.06

0.04

0.02

16

12

8

4

0 0

0 0

0.2

0.4

0.6

0.8

1

Cumulative fraction of oxygen reacted

b

0.2

0.4

0.6

0.8

1

Change in conversion (Δ X) Fig. 12. Calculated recirculation rates of CuO/ZrO2 oxygen carriers between the air and fuel reactor as a function of the change in conversion of the oxygen carrier between the air and the fuel reactor. Inert fraction: 0 wt% (–), 40 wt% (– –), 60 wt% (- - -).

0.05

Partial pressure of O2 (bar)

0.04 high circulation of oxygen carrier will mean a large solids inventory and thus large reactor sizes. Assuming that a residence time, s, of 30 s is needed for complete burnout, cf. Fig. 8 and a DX of 0.3–0.5 for the oxygen carrier particles, the inventory would be between 120 and 200 kg/MWfuel in the fuel reactor for particles composed of 40wt% CuO. The relatively small amount of bed material needed is indicative of a significant advantage of CLOU in comparison to normal CLC for solid fuels. For instance, Leion et al. [5] estimated that 2000 kg oxygen carrier would be needed per MW of fuel in the fuel reactor of a CLC system using petroleum coke as fuel.

0.03

0.02

0.01

0 0

0.2

0.4

0.6

0.8

5. Discussion

1

Cumulative fraction of oxygen reacted Fig. 11. The partial pressure of O2 in the outlet from the reactor as a function of the cumulative fraction of oxygen reacted at a set-point temperature of 905 °C for an inlet O2 concentration of (a) 10% and (b) 5%. The solid and dashed lines indicate different experiments at the same conditions. Included for comparison is the equilibrium partial pressure (+) as determined from temperature measurements in the bed.

decreases as the change in conversion increases and as less inert material is included in the oxygen carrier particles. The maximum recirculation rate of approximately 20 kg s MW1 fuel should be feasible, considering a system of reactors similar to circulating fluidized bed boilers [9]. However, it is likely preferable to operate the reactors at lower recirculation rates, and hence obtain higher temperatures and partial pressure of O2 in the fuel reactor, see Fig. 2. This will also mean a lower solids inventory, as will be discussed next. The solids inventory needed in the CLOU system is the sum of the oxygen carrier particles in the fuel and air reactors. In the air reactor, the inventory needed is inversely proportional to the rate of regeneration with air. In the fuel reactor it is determined by the residence time and the rate of oxygen carrier circulation

_ oc s moc;fr ¼ m

ð10Þ

where s is the residence time needed for reasonably complete burnout of the solid fuel, which is dependent upon the actual conversion rate of all individual fuel particles in the reactor as well as the reactor flow arrangement. A low reactivity in combination with

The concept of chemical-looping with oxygen uncoupling, or CLOU, has been demonstrated using a Cu-based oxygen carrier together with a low-volatile petroleum coke. The rate of conversion of the fuel increased as a function of temperature and the average rate to reach 95% fuel conversion was roughly 5%/s at a set-point temperature of 985 °C, which is considerably faster than application of regular CLC with the same fuel, and should thus result in much smaller reactor sizes. The reason for the difference is of course that in normal CLC with solid fuels, the overall reaction rate is limited by the slow gasification rates of the fuel with H2O and/or CO2 [5]. This gasification step is avoided in CLOU where the overall reaction rate is governed by the release of oxygen from the metal oxide particles and the combustion rate of the fuel. To understand the limitations the following two extreme cases can be considered:  Consumption of oxygen by fuel is slow and release of oxygen from particles is rapid, which would give an oxygen concentration close to equilibrium,  Consumption of oxygen by fuel is rapid and release of oxygen from particles is slow, which should result in an oxygen concentration close to zero during combustion. From Figs. 5–7 it appears as though combustion is more rate limiting at the lower temperatures, e.g. Fig. 5, and release of oxygen more limiting at higher temperature, see Fig. 6. In earlier work using CuO for regular CLC, there have been agglomeration problems when using Cu-based oxygen carriers [10]. This has been

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attributed to the low melting temperature of metallic Cu, which has a melting temperature of 1089 °C. However, the work of Adanez et al. concerning Cu-based oxygen carriers suggests that it may be possible to manufacture Cu-based oxygen carriers which do not suffer from agglomeration [11–14]. In this work the pressure drop over the bed was measured continuously, and although there were some defluidization phenomena during some parts of the experiments, no permanent agglomerations were detected. It is also important to mention that the oxygen carrier is not reduced to Cu in the CLOU process, and both CuO and Cu2O have higher melting temperatures compared to metallic Cu, 1446 °C and 1235 °C respectively, which should have positive effects with respect to avoiding agglomeration.

6. Conclusion Chemical-looping with oxygen uncoupling (CLOU) was used to react oxygen carrier particles of CuO/ZrO2 with petroleum coke. Here the fuel is burnt in gas-phase oxygen without the need for an energy intensive air separation unit. In CLOU, the carbon dioxide from the combustion is inherently separated from the rest of the flue gases. CuO-based oxygen carriers were prepared by freeze granulation and were alternatingly reacted with petroleum coke and air in a laboratory fluidized bed reactor of quartz. The reaction temperature and oxygen concentration during the regeneration were varied. The average reaction rate of petroleum coke was a function of temperature and varied between 0.5%/s and 5%/s at a set-point temperature from 895 °C to 985 °C. These conversion rates of petroleum coke are considerably higher than rates obtained with the same fuel using iron-based oxygen-carriers which do not release oxygen in the fuel reactor. The reduced particles were rapidly regenerated at low oxygen concentrations. The recirculation rate and solids inventory of oxygen carrier particles needed in a CLOU system have been calculated and are a function of the change in conversion of the solids between the reactors. Neither defluidization nor agglomeration was seen during the current experiments in the batch fluidized bed reactor.

Acknowledgement This work was partly financed by the EU research project Enhanced Capture of CO2 (ENCAP), SES6-2004-502666, in addition to the Swedish Energy Agency. References [1] Mattisson T, Lyngfelt A, Leion H. Chemical-looping oxygen uncoupling for combustion of solid fuels. Int J Greenhouse Gas Control, in press. [2] Adanez J, de Diego LF, Garcia-Labiano F, Gayan P, Abad A, Palacios JM. Selection of oxygen carriers for chemical-looping combustion. Energ Fuel 2004;18(2):371–7. [3] Ishida M, Jin H. A novel combustor based on chemical-looping reactions and its reaction kinetics. J Chem Eng Jpn 1994;27:296–301. [4] Lyngfelt A, Leckner B, Mattisson T. A fluidized-bed combustion process with inherent CO2 separation; application of chemical-looping combustion. Chem Eng Sci 2001;56:3101–13. [5] Leion H, Mattisson T, Lyngfelt A. The use of petroleum coke as fuel in chemicallooping combustion. Fuel 2007;86:1947–58. [6] Leion H, Mattisson T, Lyngfelt A. Solid fuels in chemical-looping combustion. Int J Greenhouse Gas Control 2008;2:180–93. [7] HSC Chemistry 6.1, Chemical Reaction and Equilibrium Software with extensive thermochemical database. (Outokumpu, 2007). [8] Cho P, Mattisson T, Lyngfelt A. Defluidization conditions for fluidized-bed of iron, nickel and manganese oxide containing oxygen-carriers for chemicallooping combustion. Industrial Eng Chem Res 2006;45:968–77. [9] Abad A, Adanez J, Garcia-Labiano F, De Diego LF, 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;62:533–49. [10] Cho P, Mattisson T, Lyngfelt A. Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion. Fuel 2004;83:1215–25. [11] Adánez J, Gayán P, Celaya J, de diego L, Garcia-Labiano F, Abad A. Behavior of a CuO–Al2O3 Oxygen carrier in a 10 kW chemical-looping combustión plant. In: 19th international conference on fluidized bed combustion, Vienna, May 21– 24, 2006. [12] Adánez J, Gayán P, Celaya J, de diego L, Garcia-Labiano F, Abad A. Chemical looping combustion in a 10 kWth prototype using a CuO/Al2O3 oxygen carrier: Effect of operating conditions on methane combustion. Industrial Eng Chem Res 2006;45:6075–80. [13] de Diego LF, García-Labiano F, Adánez J, Gayán P, Abad A, Corbella B, et al. Development of Cu-based oxygen carriers for chemical-looping combustion. Fuel 2004;83:1749–57. [14] de Diego LF, Gayán P, García-Labiano F, Celaya J, Abad A, Adánez J. Impregnated CuO/Al2O3 oxygen carriers for chemical-looping combustion: Avoiding fluidized bed agglomeration. Energ Fuel 2005;19:1850–6.