High Temperature Solid Cycles Research Topics at IC:

Reactions of Sulphur during chemical looping

combustion using Iron

Zili Zhanga, Nick Florinb, Paul S. Fennella

p.fennell@imperial.ac.uk

a Dept. of Chemical Engineering and Chemical Technology

Imperial College London

Cambridge, September 2013

Determine the effect and fate of sulphur in chemical looping

combustion system

When sulphur (in the form of H2S) introduced to the system:

-Sulphur products distribution in air and fuel reactor?

-Effect of sulphur on kinetics and long term reactivity of Iron oxide?

H2S

3𝐹𝑒2𝑂3 + 𝐶𝑂 ↔ 2𝐹𝑒2𝑂3 + 𝐶𝑂2 ∆𝐻°1023𝐾 = −44.3𝑘𝐽/𝑚𝑜𝑙

2𝐹𝑒3𝑂4 +1

2𝑂2 ↔ 3𝐹𝑒2𝑂3 ∆𝐻

°1023𝐾 = −238.4𝑘𝐽/𝑚𝑜𝑙

Detail of closed Fluidised Bed Reactor

Steam

Inlet tubes

Quartz Tube

Supporting Ring

Quartz

Lining Tube

Central Bed Thermal Couple

48.3mm od x 7.13mm wt

Incolloy Tube

504 mm Heating section

304mm

Copper Electrode

Copper Electrode

Half Moon Positioning

Ring

Half Moon Positioning Ring

Flat copper o-ring

Flat copper o-ring

Investigation of kinetics of Iron oxide reduction with CO in

the presence of H2S

Type of

Experiments

Fe2O3selection

Acid treated

Sand

T P F Reduction

gas

Oxidation

gas

Fe2O3 to

Fe3O4kinetics with

CO0.5g(300-

425µm)

40g(500-

710µm)

723-973K

1bar 2.5 ×Umf

N2+CO+CO2(80-85%, 1-

5%, 15%

N2+Air(82

%, 18%)

Fate and

effect of H2S

addition

773-923K

N2+CO+CO2+H2S(80-

85%, 1-5%,

15%,300-

450ppm)

N2+Air(82

%, 18%)

Typical cycles profile of Iron oxide reduction with CO in the

presence of H2S at 823 K, 1 barCalibration

&Ambient InputInert Input

Inject Fe2O3

10 redox cycles

*CO,CO2 on the left axes (vol%), all sulphur compounds on the right axes(ppm).

Detailed look at cycle 4 of Iron oxide reduction with

CO in the presence of H2S at 823 K, 1 bar

Procedure:

Reduction Cycle:

N2 purge 120s

N2+CO2 120s

N2+CO2+CO+H2S 180s

Oxidation cycle:

N2 purge 120s N2+Air 240s

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6

rate

(1

0-6

mo

l/(g

*s)

)

Inlet mole fraction CO (vol. %)

maximum rate method(used in this work)

extrapolation method

literature results (Bohnet al., 2010)

Determine Rates of the reduction of Fe2O3 to Fe3O4

Left: a typical rate versus conversion graph for the reduction of Fe2O3 to Fe3O4 with

3 vol% CO 15 vol%CO2 at 823K

Right: Comparison of the maximum rate method used in this work with extrapolation

method and literature reference for reduction of Fe2O3 to Fe3O4 at 823KRef: BOHN, C. D., CLEETON, J. P., MÜLLER, C. R., DAVIDSON, J. F., HAYHURST, A. N., SCOTT, S. A. & DENNIS, J. S. 2010. The kinetics of the

reduction of iron oxide by carbon monoxide mixed with carbon dioxide. Aiche Journal, 56, 1016-1029.

0 0.2 0.4 0.6 0.8 10

20

40

60

80

Conversion(X)

r’(m

ol/

(s*g)

Rate,CO2

Rate,CO

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6

rate

(1

0-6

mo

l/(g

*s)

)

Inlet mole fraction CO (vol. %)

cycle3

cycle4

cycle5

Intrinsic rate constant Ki =565 (s-1), Effectiveness Factor=0.71 (Koverall was derived

using cycle 3-5).

Dependence of overall rate of Fe2O3 reduction on CO vol% at

823K

Arrhenius plot showing the activation energy based on

overall rate constant k for the reduction of Fe2O3 to Fe3O4

4.00

5.00

6.00

7.00

8.00

0.12 0.14 0.16 0.18

lnk

(s-1

)

103/RT (mol/J)

cycle2, E=50.1 kJ/mol

cycle3, E=64.4 kJ/mol

cycle4, E=64.0 kJ/mol

cycle5, E=60.2 kJ/mol

0 200 400 6000

3

6

9

12

15

Time (s)

Con

centr

ation (

%)

0

500

1000

1500

2000

2500

3000

Con

cen

tra

tion

(p

pm

)

H2S

SO2

COS

CO2

CO

O2

Reduction Oxidation

0 200 400 6000

3

6

9

12

15

Time (s)

Con

centr

ation (

%)

0

500

1000

1500

2000

2500

3000

Con

cen

tra

tion

(p

pm

)

H2S

SO2

COS

CO2

CO

O2

Reduction Oxidation

The different fates of sulphur before and after completion of

reduction of Fe2O3 to Fe3O4 at 823K, with 3vol%CO,300ppm H2S

*CO,CO2 on the left axes (vol%), all sulphur compounds on the right axes(ppm).

CO and H2S input for 50s (approaching

completion of transition from Fe2O3 to

Fe3O4)

CO and H2S input for 180s (120s after

complete completion of transition from

Fe2O3 to Fe3O4)

No SO2 release SO2 Release

0

0.2

0.4

0.6

0.8

1

Ine

rt2

cycl

e1

cycl

e2

cycl

e3

cycl

e5

cycl

e8

cycl

e1

0

Inert Reaction cycle

mo

l/m

ol H

2S

inp

ut

Elemental sulphur SO2 release in oxidation

Conversion to COS Conversion to SO2

H2S unreacted

0

0.2

0.4

0.6

0.8

1

Ine

rt2

cycl

e1

cycl

e2

cycl

e3

cycl

e5

cycl

e8

cycl

e1

0

Inert Reaction cycle

mo

l/m

ol H

2S

inp

ut

SO2 release in oxidation Conversion to COS

Conversion to SO2 H2S unreacted

Sulphur product for the reduction of of Fe2O3 to Fe3O4 at 823K

with 3vol%CO, 300ppm H2S for 60s (left) and 180s (right)

Most of H2S was converted into SO2 in the fuel reactor in the 60s case (left),

while in the 180s case (right) most of the H2S was converted into FeS which

was released as SO2 later in air reactor.Elemental sulphur was detected in the downstream trap of the reactor using quantitative XRF analysis after being

dissolved in Toluene solvent.

Sulphur reaction mechanism with iron

12Fe2O3+CO+H2S8Fe3O4+CO2+SO2+H2O 2Fe2O3+CO+H2S Fe3O4+CO2+FeS+H2O

0

20

40

60

80

100

0 3 6 9 12

rate

(1

0-6

mo

l/(g

*s))

Cycles

Without H2S

50s, 300ppm H2S

60s, 300ppm H2S

180s, 300ppm H2S

60s, 450ppm H2S

Effect of H2S addition on the overall rate of Fe2O3 reduction at

823 K, 3% CO

• Increasing the residence time of H2S only has significant effect on the

reduction rate if the increase is after complete reduction

• Increasing H2S concentration has an adverse effect on the rate of

reduction.

y = 0.9188x - 8.8762R² = 0.9994

y = 1.2985x - 8.976R² = 0.952

-12.0

-11.0

-10.0

-9.0

-8.0

-1.50 -1.00 -0.50 0.00

ln r

ate

(1

0-6

mo

l/(g

*s))

ln [CO] (mol m-3)

cycle4

cycle5

cycle8

cycle10

without H2S cycle3

without H2S cycle4

without H2S cycle5

Dependence of overall rate of Fe2O3 reduction on CO vol% at

823 K in the presence of 300 ppm H2S

The linear dependence of rate of reduction on [CO] is still satisfactory. The

rates of reaction in the presence of H2S are in general lower than that without

H2S presence

Arrhenius plot based on overall rate constant k for the

reduction of Fe2O3 to Fe3O4 in the presence of H2S

A similar activation energy was obtained compared to the case without H2S

presence. The pre-exponential factor, however, was lower when H2S was

present.

5.00

5.50

6.00

6.50

7.00

0.12 0.13 0.14 0.15 0.16 0.17

lnk

(s-1

)

103/RT (mol/J)

cycle4, E=55.5 kJ/mol

cycle5, E=48.8 kJ/mol

cycle8, E=52.7 kJ/mol

cycle10, E=57.0 kJ/mol

without H2S cycle4

without H2S cycle3

without H2S cycle5

Key Summary and Implications

• A closed-system fluidised-bed reactor has been designed and

constructed for the study of the effect and fate of sulphur in the CLC

cycle

• Experiments concerning Iron oxide reduction agree with the previous

work in Cambridge.

• The study of iron oxide reduction in the presence of H2S showed that

sulphation is nearly reversible.

• Good process control could avoid the production of SO2 in the air

reactor

Summary and Implications

• The rate of iron oxide reduction started to be adversely

affected by H2S addition when reduction of Fe2O3 to Fe3O4was nearly completed (FeS formation became dominant).

• The linear dependency of rate of reduction of Fe2O3 to Fe3O4for CO was still satisfactory in the presence of H2S.

• Similar activation energies were obtained compared to the

case without H2S presence. The pre-exponential factor,

however, was lower when H2S was present.

Acknowledgment

This work was financially supported by :

• The EPSRC under the “Joint UK – China Hydrogen Production

Network”. Project reference: EP/G06265X/1

Thank you!

Q&A

Sulphur product for the reduction of of Fe2O3 to Fe3O4 at 823K

with 3 vol%CO and 300ppm H2S for 60s (left) and 180s (right)

Most of H2S was converted into SO2 in the fuel reactor in the 60s case (left),

while in the 180s case (right) most of the H2S was converted into FeS which

was released as SO2 later in air reactor.Elemental sulphur was detected in the downstream trap of the reactor using quantitative XRF analysis after being

dissolved in Toluene solvent.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Inert1

Inert2

cycle1

cycle2

cycle3

cycle4

cycle5

cycle6

cycle7

cycle8

cycle9

cycle10

Inertcycle

Reac oncycle

mol/molH

2Sinput

H2Sunreacted ConversiontoSO2

ConversiontoCOS SO2releaseinoxida on

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Inert1

Inert2

cycle1

cycle2

cycle3

cycle4

cycle5

cycle6

cycle7

cycle8

cycle9

cycle10

Inertcycle

Reac oncycle

mol/molH

2Sinput

H2Sunreacted ConversiontoSO2

ConversiontoCOS SO2releaseinoxida on

Elementalsulphur