1

Reactivity of oxygen carriers for CLC in packed bed reactors under

pressurized conditions

H.P. Hamersa, F. Gallucci

a, G. Williams

b, P.D. Cobden

c, M. van Sint Annaland

a,1

a Chemical Process Intensification, Department of Chemical Engineering and Chemistry,

Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands.

b Johnson Matthey Public Limited Company, UK

c Energy research Centre of the Netherlands (ECN), P.O. Box 1, 1755ZG Petten, The

Netherlands.

Abstract

For the design, scale-up and optimization of pressurized packed bed reactors for chemical-

looping combustion, understanding of the effect of the pressure on the reactivity of the

oxygen carriers is very important. In this work, the redox reactivity of CuO/Al2O3 and

NiO/CaAl2O4 particles at elevated pressures have been measured in a pressurized high-

temperature magnetic suspension balance. The experiments have demonstrated that the

pressure has a negative influence on the reactivity and that this effect is kinetically controlled.

The negative effect of the pressure might be caused by the decrease in the number of oxygen

vacancies at higher pressures. Moreover, the reactant gas fraction has been demonstrated as

an important parameter, probably related to competition between different species for

adsorption on the oxygen carrier surface. These effects have been included in the kinetic

model leading to a good description of the experimental results. The impact of these findings

on packed-bed CLC applications with larger oxygen carrier particles has been investigated

with a particle model that considers diffusion limitations and kinetics. It has been shown that

the impact of diffusion limitations decrease with increasing pressure, due to the decrease in

reaction rates and the increase in diffusion fluxes caused by Knudsen diffusion. The results

have been validated by experiments with 1.7 mm NiO/CaAl2O4 particles. These results

corroborate that the selection of larger particles because of pressure drop considerations, does

not lead to a large decrease in effective reaction rates, which is beneficial for packed-bed

CLC applications.

Keywords: chemical-looping combustion, packed beds, CO2 capture, NiO/CaAl2O4,

CuO/Al2O3

1 Corresponding author. Tel.: +31 40 247 2241; fax: +31 40 247 5833.

E-mail address: M.v.SintAnnaland@tue.nl (M. van Sint Annaland).

2

1. Introduction

With chemical-looping combustion (CLC), fossil fuels can be converted in power, while no

additional separation step is required to obtain a relatively pure CO2 stream, thus the carbon

capture is integrated in the power production process. During the CLC process, hot air is

produced and converted into power; the highest electrical efficiency is obtained if a combined

cycle is applied, which means that the hot air stream has to be produced at high temperature

(at least 1200 °C) and elevated pressure (around 20 bar). In most power plant integration

studies it is assumed that the process is operated at elevated pressure, but at this moment little

information is known about the reactivity of the oxygen carriers at elevated pressures.

García-Labiano et al. have published the kinetics at elevated pressure for copper, nickel and

iron based oxygen carriers 1. The kinetics have been determined at 450-950 °C and a pressure

between 1 and 30 bar. It appears that a high total pressure has a negative effect on the

kinetics. They observed the same behavior with different particle sizes (0.09-0.25 mm),

which indicates that they were measuring kinetics 1. The authors have indicated that the

decline in reactivity cannot be caused by a decrease in surface area and pore volume. For

other gas solid reactions, similar results have been obtained, for example for the reactions

between CaO and H2S or calcination of CaCO3. Chauk et al. have found a decrease in the

reactivity of CaO with H2S at higher pressures, but have attributed this to a lower pore

volume and surface area 2. Similar behavior was found for the calcination reaction where the

reactivity was described by increasing the negative pressure effect in Fuller’s equation for

molecular diffusion 3. From these studies, it can be concluded that the pressure could have a

negative effect on the reactivity, but there is no general consensus on the cause for the

observed negative effect.

Some experimental work has been carried out to the application of CLC in pressurized

circulating fluidized bed reactors. Such a system has been constructed by Xiao et al. 4.

Experiments have been carried out with Shenhua bituminous coal as fuel and iron ore as

oxygen carrier at three different operating pressures (1, 3 and 5 bar). Stable operation has

been reached. A higher combustion efficiency was observed at elevated pressure. However,

the oxygen carrier fine production was also higher which is very detrimental for power

production in downstream turbines. In our previous work we demonstrated that the maximum

efficiency of the process will be mainly influenced by the generated temperature and pressure

of the hot air stream and is not much affected by the type of reactor selected, as long as the

reactor can work at these conditions 5.

To better accommodate the CLC process at elevated pressure, packed-bed reactors have been

selected in this study. In packed bed reactors, larger oxygen carrier particles are required to

maintain a low pressure drop. But the selection of larger particles may also imply that the role

of diffusion limitations inside the particle may become more dominant. The effect of

diffusion limitations has been described by Noorman et al. for atmospheric applications 6.

This model considers molecular diffusion and Knudsen diffusion. From these studies, it was

concluded that Knudsen diffusion was the rate determining step in the oxygen carrier

particles considered.

3

The objective for this paper is to measure the pressure effect on the kinetics and to evaluate

this impact for packed bed CLC applications. For the oxygen carrier reaction rate

measurements, CuO/Al2O3 and NiO/CaAl2O4 particles have been tested in a high pressure

magnetic suspension balance. An kinetic expression is determined from these measurements

and this correlation is used to describe the behavior in packed-bed reactors using an advanced

particle model. The model is validated by experiments with larger particles.

2. Materials and methods

2.1.Oxygen carriers

The CuO/Al2O3 particles were obtained from Sigma-Aldrich with an active weight content of

13wt% and a particle size of 1.1 mm. For kinetic experiments, the particles were crushed and

sieved to a size of 110-150 μm.

The NiO/CaAl2O4–particles used in this work is a Johnson Matthey product, HiFUEL®

R110

(Ni based catalyst supported on CaAl2O4 for steam reforming of natural gas), available in

pelleted form from Alfa Aesar. The particles were received in the form of shaped pellets,

comprising of 4 holes, 4 flute domed cylinders. For this study, the pellets were crushed and

sieved to particle sizes of 1.7 mm and 0.15 mm. Separate TGA experiments proved that the

mass change (and thus the active weight content) is not the same for each particle. The

experiments with CO and O2 were carried out with a particle with an active weight content of

18.5wt%, while an active weight content was measured of 17wt% for the experiments with

H2. Before the tests, the oxygen carrier was activated by exposure to two redox cycles with

reductions with H2 at 900 °C.

2.2.High pressure magnetic suspension balance

The experiments have been carried out in a magnetic suspension balance (Rubotherm) that

can operate between 200-1200 °C and 1-30 bar. An oxygen carrier sample of 100 mg is

placed in a porous quartz glass sample holder. The basket is placed on an Ir wire that is

hanging on a permanent magnet. The mass is determined by the strength of the magnet.

The reactant gases are supplied at the top of the reactor. The reactor is surrounded by a vessel

that is maintained at lower temperature. Argon is supplied to this vessel to prevent that

reactant gases can enter and mix in the insulation layer. A schematic overview of the setup is

provided in Figure 1.

Before a series of experiments is started, the system is pressurized and the reactor is set at the

desired operating temperature. When this temperature is reached and the system has been

stabilized, redox cycles are carried out that consist of a 20 min reduction, a 10 min purge, a

10 min oxidation and a 10 min purge. During experiments, a total flow rate of 480 mLn/min.

is fed. Some experiments have been carried out with a lower flow rate (320 mLn/min.) to

demonstrate that the measured reactivity is not influenced by external mass transfer

limitations. Every experiment has been repeated at least two times to assure the

reproducibility of the results.

4

Blank experiments have been carried out with only a sample holder (and no oxygen carrier)

so that the influence of the flow change on the measured mass change can be determined and

corrected for. The blank experiment data are subtracted from the data obtained with the

oxygen carrier sample.

Figure 1: Schematic overview of the magnetic suspension balance setup.

2.3.Particle model

The conversion of the solid is described by a numerical particle model that assumes a

spherical oxygen carrier particle with a uniform porosity, fixed particle diameter and a

uniform pore size 6. The model describes the gas transport inside the particle from the

moment that the particle is exposed to a reactant at certain operating conditions. For the gas

transport, the reaction kinetics and molecular and Knudsen diffusion and external mass

transfer limitations are taken into account. Based on this description, the solid conversion is

simulated as a function of time and these data is compared with TGA results. The equations

applied in the model are listed in Table 1. The kinetics for CuO are same as used by Hamers

et al. 7 (which are based on García-Labiano

1,8) and the kinetics for the NiO/CaAl2O4 are

taken from Medrano et al. 9. The particle properties are based on pycnometer (Quantachrome

Micro-ultrapyc1200) and BET (Thermscientific Surfer) measurements and are listed in Table

2.

Pre

ssu

re v

esse

l

N2

Air

H2

CO

CO2

H2O

Ar

PC

vent

sample

magnet

5

Table 1: The equations of the particle model 6.

Continuity equation

2

21

1 gNtotg g

i i

i

r nr M

t rr

Gas phase components 2

,

2

1 ig g g i

i i

r nr M

t rr

where

1

,

, , ,

1

gN

g k

i i g i tot g eff ik g i tot

k

n j n D nr

Solid phase components

,

1

gNs s s j

i j

i

r Mt

where , , , , ,1 1ox ox s MeO

s MeO s act s Me s Me s act

Me

v M

M

Energy balance g,reactants

2

, , ,21

1N

g g p g s s p s eff i R i

i

T TC C r r H

t r rr

Kinetics

copper: , 0 5exp

10

q

nact tot

i g p g

E pr k C

R T

nickel:

0

,s p s act

i

j

dXr

b M dt

0

2 1

0 03 3

3

1(1 ) (1 )

n

g

s

C

b r CdX

dt r rX X

k D D

0 5exp

10

q

totApE

Rk k

T

0 exp expD

x

ED D k X

RT

Diffusion

- Binary molecular diffusion: Knudsen diffusion:

-

1.75 1 1

, 23 3

0.01013 i k

Bin ik

i k

T M MD

p v v

,

8

3

pore

Kn i

i

d R TD

M

- Maxwell-Stefan diffusion matrix 1

D B

that consists of the elements dik and n gaseous

components

with: 1, , ,

( )

1ni k

ii

kBin in Bin ik Kn ii k

y yB

D D D

and , ,

1 1ik i

Bin ik Bin in

B yD D

Effective diffusivity ,

,

g p

eff ikD D

Table 2: Particle properties for the NiO/CaAl2O4 particle 10

.

Oxygen carrier 17-18.5wt% NiO on CaAl2O4 TGA experiments

Particle diameter, mm 1.7 sieved

Particle porosity, m3

gas/m3

particle 0.55 derived from combination

dry and liquid pycnometer

Average pore size, Å 130 BET porosimetry

6

3. Results and discussion

3.1.Pressure effect on kinetics

Experiments were carried out varying the total pressure (1-20 bar), while the partial pressure

of the reactant was kept constant at 1 bar. In this way, the reactant gas concentration and the

temperature were fixed, so that solely the influence of the total pressure is measured. Redox

cycles have been measured with CuO/Al2O3 and NiO/CaAl2O4 as oxygen carriers at 600 and

800 °C. In case of CO as reactant, a CO2/CO-ratio of 1 and 3 was used at 800 °C and 600 °C

respectively.

The results plotted as solid conversion (defined in equation (1)) as a function of time for the

experiments at constant temperature and different total pressures are shown in Figure 2 and

Figure 3. During the reduction cycles, full solid conversion was not reached. The maximum

solid conversion depends on the reduction temperature and the type of the support material 11,12

. The support material could be present as inert layer in the solid structure, which might

influence the accessibility of the oxygen and thus the degree of reduction. The maximum

degree of reduction depends on the operating temperature. The lowest conversion is reached

for the H2 experiments with CuO at 600 °C. For demonstration of the pressure effect, the

curve has been zoomed in on the first 60 seconds. But after that moment, the particle keeps

on reacting and a conversion of 80% is reached after longer times 7. From the experiments it

can be concluded that the maximum solid conversion does not depend on the operating

pressure, but only on the temperature.

observed mass changesolid conversion

maximum mass change for the assumed active weight content (1)

As can be observed from Figure 2 and Figure 3, where the partial pressure of the reactant was

fixed, but the total pressure was varied, the reaction rate decreases with increasing total

pressure. It has to be noted that in the experiments with a higher total pressure, the reactant

was thus more diluted, because the partial pressure of the reactant was kept constant. The

decreased reactivity with the pressure is observed for all reactants with both oxygen carriers.

At higher pressures, more fluctuations in the experimental results can be seen which is related

to limitations of the experimental set-up. These fluctuations could in principle be decreased

by reducing the total flow rate, but in that case external mass transfer limitations could occur.

Despite these fluctuations, a clear trend can still be discerned from the results at 20 bar.

External mass transfer limitations cannot be the cause for the decrease in the reaction rate,

because the same conversion curves were obtained from experiments with a lower gas flow

rate (320 mLn/min instead of 480 mLn/min). This experiment demonstrates that the reactant

flow rate was sufficiently high to refresh the gas around the sample and to supply a sufficient

amount of reactants for the gas/solid reactions. This has been validated for all the operating

conditions investigated.

7

0 50 100 150 200

0.0

0.2

0.4

0.6

0.8

1.0

a) H2

ptot

=

pH2

=1 bar

T= 800 °C

5bar

10bar

20bar

so

lid c

on

ve

rsio

n (

-)

time (s)

model exp

0 50 100 150 200

0.0

0.2

0.4

0.6

0.8

1.0

b) CO

2bar

5bar

10bar

20bar

so

lid c

on

ve

rsio

n (

-)

time (s)

pCO

=1 bar

T=800 °C

ptot

=model exp

0 50 100 150 200

0.0

0.2

0.4

0.6

0.8

1.0

c) O2

pO2

=1 bar

T= 800 °C

5bar

10bar

20bar

so

lid c

on

ve

rsio

n (

-)

time (s)

ptot

=model exp

Figure 2: The effect of pressure on the redox kinetics of NiO/CaAl2O4 at 800 °C with H2,

CO and air. The markers show the experimental data and the lines the model

predictions.

8

0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

a) H2

pH2

=1 bar

T=600 °C

5bar

10bar

20bar

so

lid c

on

ve

rsio

n (

-)

time (s)

ptot

=model exp

0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

b) CO

2bar

5bar

10bar

20bar

so

lid c

on

ve

rsio

n (

-)

time (s)

pCO

=1 bar

T=800 °C

ptot

=model exp

0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

c) O2

pO2

=1 bar

T=600 °C

5bar

10bar

20bar

so

lid c

on

ve

rsio

n (

-)

time (s)

ptot

=model exp

Figure 3: The effect of pressure on the redox kinetics with CuO/Al2O3 with H2, CO and

air. The markers show the experimental data and the lines the model predictions.

Moreover, experiments have been carried out with different particle sizes and again the same

conversion curves have been obtained. This is illustrated in Figure 4, where the oxidation of

Cu/Al2O3 is displayed at 20 bar (with pO2=4 bar and 1 bar) and the particle size was varied

between 0.15 mm (lines) and 1.1 mm (markers). The same trends can be observed and for

that reason, internal mass transfer limitations can also be ruled out as cause for the observed

decrease in reactivity at elevated pressures.

9

0 50 100 150 200

0.0

0.2

0.4

0.6

0.8

1.0

25% air/N2

so

lid c

on

ve

rsio

n (

-)

time (s)

Air

Figure 4: The particle size effect on the solid reaction rate for the oxidation of Cu/Al2O3

at 800°C and 20 bar. The reactivity of 1.1 mm particles is represented by markers and

the 0.15 mm particles by lines.

Thus, the decrease in reactivity with increasing pressure has to be kinetically controlled. An

expression for the reaction rates including a pressure correction factor has been introduced by

García-Labiano et al. 1 and the kinetic term is demonstrated in equation (2).

0

5

exp

10

nact

q

tot

k Er C

R Tp

(2)

The same method has been followed here. By fitting the experimental data, a number for the

parameter q was determined. The determined numbers for q are displayed in Table 3 together

with the data from literature for comparison. In general, the same trend is observed as by

Garcia-Labiano, but somewhat different values for q have been found. There may be different

reasons for the observed discrepancies; firstly, the experiments reported in literature may

have been carried out with different materials or supports. Furthermore, the quality of the

fitting is different especially at high pressures.

Table 3: Determined values for q (in Eqn. 2) from the experimental data and

comparison with values found in literature.

q CuO/Al2O3 NiO/CaAl2O4

Gaseous

reactant

García-Labiano

et al. 1

This work García-Labiano

et al. 1

This work

H2 0.53 1.0 0.47 0.75

CO 0.83 1.2 0.93 0.85

O2 0.68 1.3 0.46 1.05

10

3.2. Discussion

In the previously described experiments the reactant partial pressure was kept constant, while

the total pressure was varied. The partial pressure was fixed by increasing the dilution at

higher pressures. During both the oxidation and the reduction reactions, the reaction rates

decrease with increasing pressure, which might to a large extent have been caused by the

dilution of the reactant gas.

The following reaction mechanism has been proposed in the literature for redox reactions 13,14

. First, the reactant adsorbs on the oxygen carrier and subsequently, an oxygen atom is

transferred from the adsorbed gas to the oxygen carrier or vice versa 13

. CO2 or H2O is

formed during reduction and this molecule is desorbed from the oxygen carrier. In principle,

not only the reactant could adsorb to the oxygen carrier surface, but also the other gases that

are considered to be inert in the reaction. In case of competitive adsorption, the reactant gas

fraction is a relevant parameter for the kinetics.

Furthermore, the surface is not expected to be flat, also because the metal (for example Ni has

an atomic radius of 125 pm 15

) has a different atomic radius than the oxide (atomic radius of

66 pm 15

). Due to this difference, cavities could be present on the solid surface. The gas

molecule that is present in the cavities could be inert or reactive with the solid. If the space of

a cavity is occupied by an inert gas, it blocks the pathway of the reactive gas. Therefore a

reactive spot on the solid remains unoccupied and this results in a lower reaction rate. These

diffusion limitations are not dependent on the particle size, because the gases are distributed

in the particle by pores that are much bigger, so that the gas close to the solid surface still has

the feed composition.

Other experiments have been carried out to exclude some effects and to prove what could be

the reason for the observed behavior in the experiments. Reductions have been carried out

with varying CO2/CO-ratios at different pressures and no effect on the kinetics was observed.

Therefore it seems that the desorption of gaseous products is not the limiting step.

Furthermore, oxidations have been carried out with air (25% air) that is diluted either by N2

or CO2 (so a 25% air/75%N2 vs. a 25% air/75% CO2 mixture). It is expected that CO2

adsorbs on the solid surface and therefore competitive adsorption with O2 is expected.

However, the experiments showed that the reaction rates did not change significantly, when

air was diluted with CO2. This means that either N2 adsorbs on the surface by physisorption

with the same impact as CO2 or that the adsorption is not a rate limiting step. In the latter

case, the blocking of the reactant gases in the cavities could explain the decrease in reactivity

when the mixture is more diluted.

When the reactant gas fraction is fixed, while the pressure is increased, a small decrease in

reactivity is observed. The temporal evolution of the solid conversion of NiO during

reduction with 20% CO in 20% CO2 and 60% N2 at 1, 5 and 20 bar is illustrated in Figure 5.

The same trend was observed for reductions with H2 and oxidations with air.

11

0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

ptot

=5bar

ptot

=20bar

so

lid c

on

ve

rsio

n (

-)

time (s)

ptot

xCO

=0.2

xCO2

=0.2

T=800°C

ptot

=1bar

Figure 5: The influence of the pressure on the reduction reactivity of NiO/CaAl2O4 with

CO, while the CO-fraction is fixed.

Another point to be taken into account is that, during oxidation, not all the metal is available

at the surface. In fact, the metal does not form a monolayer between the support material and

the pore. According to the Wagner oxidation theory, metal ions and electrons migrate to the

surface of the metal grain, while oxygen ions move to the bulk. This transport is carried out

by vacancies in the metal oxide structure 16

. If the solid structure contains more metal than

oxygen according to the stoichiometry (a so-called oxygen deficient situation), the formation

of oxygen vacancies is possibly the rate limiting step. These vacancies are formed according

to equation (3). In this equation, V0 is an oxygen vacancy, O(s) is an oxygen atom in the solid

matrix and O2(g) is gaseous oxygen molecule.

O 21O s V O g

2 (3)

Equation (3) describes that if an oxygen gas molecule reacts with the metal, two oxygen

vacancies in the solid matrix are replaced by oxygen atoms. If the oxygen pressure is

increased, the equilibrium shifts to the left and the number of vacancies can decrease.

Because of the decrease in oxygen vacancies, the diffusion through the solid matrix decreases

in case the transport via oxygen vacancies is rate determining. In the some cases with oxygen

deficient materials, the dependency of the diffusion flux inside the solid matrix is with pO2-

1/6. In such a situation, a decrease in kinetics can be observed when the pressure is increased,

while the reactant gas fraction is fixed. During reduction the oxygen has to be transported in

the opposite direction.

If the reactant gas fraction and the oxygen vacancies in the oxygen carrier have influence on

the kinetics, the Arrhenius approach might not be the right approach to describe these

gas/solid reactions at different pressures. García-Labiano et al. proposed to include an

additional term in the equation that is dependent on the total pressure with which the

experiments could be described, equation (2). This can be rewritten using Dalton’s law to an

expression that has a dependency on the reactant gas fraction, x, and the partial pressure of

the reactant as reported in equation (4).

12

5 5

1~ ~

10 10

n nn q n n q q n q

tot totq q n n

tot tot

C pr p p x p x p

R Tp p

(4)

During the experiments with a fixed reactant partial pressure, the gas fraction and the total

pressure was varied, which can be fitted by q. The obtained values for q are in general a

factor 0.2-0.4 larger than the reaction order, n. This results in a negative number for n-q with

an order of magnitude of p-0.2

to p-0.4

. It should be noted that the same trend is observed in the

number of vacancies at different pressures. This indicates that the similar trend is observed in

the above mentioned diffusion flux inside a solid matrix. This indicates that the decrease in

reactivity with increasing pressure might be related with the reduced rate in the formation of

oxygen vacancies at higher pressures.

Concluding, the lumped expression given by García-Labiano et al. can capture the effect of

observed phenomena, but a more detailed study preferably with in situ analysis should be

carried out to elucidate the phenomena prevailing at elevated pressures in more detail. The

experimentally determined pressure effect will be used in the next section to investigate its

implications for pressurized packed-bed CLC applications with relatively large particles.

3.3.Pressure effect in large particles for packed bed applications

An increase in pressure results in a decrease in the reaction rates by about a factor 3 at 20 bar

relative to atmospheric pressure. In this section, the effect of reduced reaction rates and the

extent of internal mass transfer limitations for the relatively large particles used in packed-

bed reactors will be studied in more detail.

For packed-bed applications, larger particles have to be used to avoid an excessive pressure

drop over the reactor, which would reduce the overall process efficiency. In case the particle

size is increased, the influence of internal diffusion limitations could increase, which could

result in a decrease of the effective reaction rates. Experiments have been carried out with

different particle sizes to evaluate the impact on the operation of packed bed reactors. The

NiO/CaAl2O4-particles were available with a larger particle size and have been used in this

part of the investigation.

A particle model has been developed to describe the effective reaction rates inside oxygen

carrier particles considering reaction kinetics, molecular diffusion and Knudsen diffusion

through the pores. In the previous section, it has been shown that the redox kinetics have a

negative pressure dependency. The molecular diffusion coefficient is inversely dependent on

the pressure, whereas the Knudsen diffusion coefficient is independent on pressure. The

overall diffusion coefficient is multiplied by the gas density that linearly increases with the

pressure. Hence, no pressure dependency is expected in the molecular diffusion limited

regime and a positive pressure dependency if Knudsen diffusion is the limiting step. At

atmospheric pressure the Knudsen diffusion is by far the most important limitation for

gas/solid reactions with oxygen carriers 6. So, the pressure might reduce the diffusion

limitations in the particles and increase the effective reaction rates.

13

The measured solid conversion as a function of time during the reduction with CO and the

oxidation of a NiO/CaAl2O4 particle with a relatively large particle size of 1.7 mm is

displayed in Figure 6 by the markers. The operating conditions were at 800 °C and the

reactant partial pressure was fixed, while the total pressure was varied. The same experiments

were simulated with the particle model and the model results are shown by the lines in Figure

6. The experiments are described quite reasonably by the model. The effectiveness factor that

is obtained from the model, increases with increasing pressure, meaning that the reaction

becomes more kinetically controlled. This is caused by two reasons. First, the decrease in the

redox kinetics with increasing pressure and second, the increase in the Knudsen diffusion flux

with increasing pressure due to the density increase.

0 50 100 150 200

0.0

0.2

0.4

0.6

0.8

1.0

a)

2bar

5bar

10bar

20bar

so

lid c

on

ve

rsio

n (

-)

time (s)

pCO

=1 bar

T=800°C

ptot

=model exp

0 50 100 150 200

0.0

0.2

0.4

0.6

0.8

1.0

5bar

10bar

20bar

so

lid c

on

ve

rsio

n (

-)

time (s)

pO2

= 1bar

T=800°C

b)

ptot

=

model exp

Figure 6: Effective reaction rates of NiO/CaAl2O4 for reductions with CO (a) and

oxidations (b) at 800 °C varying the operating pressure and constant reactant partial

pressure (particle size=1.7mm).

The good agreement between the model and the experiments with 1.7 mm particles indicates

that the model can be used for the prediction of the behavior of particles in packed bed

reactors. For the chemical-looping combustion process in packed bed reactors, an operating

pressure of about 20 bar is expected to be optimal 5. As an example the modelled solid

conversion as a function of time is displayed in Figure 7 for a reduction cycle with 50% CO

at 800 °C and 20 bar for different particle diameters. It is shown that the particle size does not

14

have a large effect on the effective reaction rates when smaller than about 5 mm, because the

extent of internal diffusion limitations decreases with increasing pressure. Hence, the fact that

larger particles have to be used to reduce the pressure drop in a packed-bed reactor does not

have a negative overall impact on the process performance.

0 50 100 150 200

0.0

0.2

0.4

0.6

0.8

1.0

ptot

=20bar, T=800°C

xCO

=0.5

so

lid c

on

ve

rsio

n (

-)

time (s)

0.2mm

1.7mm

5mm

10mm

dpart

=

Figure 7: The simulated influence of the particle size on the effective reaction rates for a

reduction of a NiO/CaAl2O4 particle with a 50% CO/CO2 mixture at 800°C and 20 bar.

4. Conclusions

The effect of the pressure on the reaction rates of CuO/Al2O3 and NiO/CaAl2O4 particles as

oxygen carriers for chemical-looping combustion has been investigated using a pressurized

magnetic suspension balance and the experimental results have been compared with a

numerical particle model. From the experimental results it was concluded that the pressure

decreases the solid reaction rate and it is proven that this decrease is kinetically controlled

and can be well described in case a pressure factor with a negative exponent is included in the

kinetic description. A possible explanation for the observed pressure effect is the competition

between gaseous species for adsorption on the oxygen carrier and the number of oxygen

vacancies that decreases with the pressure. Furthermore, it was shown that the solid

conversion was independent on the total pressure and thus only related to the temperature.

For packed bed applications, increasing the pressure has also a positive effect on the overall

reaction rates, because the internal diffusion limitations (controlled by Knudsen diffusion

flux) decrease with increasing pressure. For that reason, still reasonably high reaction rates

can be obtained when using relatively large oxygen carrier particles.

Acknowledgements

The research originating these results has been supported by the CATO-2 program under the

project number WP1.3F2.

15

References

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Nomenclature

b gas solid reactant stoichiometric factor, mol solid/mol gas

[B] inversed diffusion matrix

C concentration, mol/m3

Cp heat capacity, J/kg/K

Di,k diffusivity, m2/s

16

D0 pre-exponential factor for diffusion term, m2/s

[D] diffusion matrix

EA activation energy, J/mol

ED activation energy for the diffusive component, J/mol

k0 pre-exponential factor, mol1-n

s-1

for CuO and mol1-n

m3n-3

s-1

for NiO

kx conversion dependent factor in reaction rate, -

j diffusive mass flux, kg/m2/s

M molar mass, kg/mol

N number of components

n reaction order related to gas phase, -

ni gas flux of component i, kg/m2/s

p pressure, Pa

r particle radius, m

T temperature, K

t time, s

R gas constant, J/mol/K

X solid conversion, -

x gas fraction, -

Greek symbols

ΔHR reaction enthalpy, J/mol

ε porosity, m3/m

3

λ heat dispersion, W/m/K

ν reaction stoichiometric factor (negative for reactants), -

νi diffusion volume of component i, m3/mol

νs stoichiometric factor for solids, mol MeO/mol Me

ρ density, kg/m3

τ tortuosity, -

ω mass fraction, kg/kg

Subscripts

act active

Bin binary

eff effective

g gas

i gas component number

j solid component number

k gas component number

Kn Knudsen

MeO metal oxide

Me metal

mol molecular

n number of gas components

p particle

s solid

tot total

Superscripts

ox in oxidized state