1

THE PRODUCTION OF HYDROGEN AND THE CAPTURE OF CARBON DIOXIDE USING CHEMICAL LOOPING

Jason Cleeton1, Chris Bohn2, Christoph Müller1,2, Stuart Scott1, John Dennis2

1Department of Engineering University of Cambridge

Trumpington StreetCambridge CB2 1PZ

2Department of Chemical Engineering University of Cambridge

Pembroke StreetCambridge CB2 3RA

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OVERVIEW

H2 PRODUCTION AND CO2 CAPTURE VIA REDUCTION AND OXIDATION OF IRON OXIDES

BACKGROUND: • Phase equilibria and reaction chemistry• Thermodynamics and estimation of exergetic efficiency• Trade-off between heat output and H2 output

EXPERIMENTAL:• CO2 capture

• H2 production• Effect of temperature• Effect of transition• Carbon contamination

3

PHASE EQUILIBRIA

0.01

100

1000000

700 800 900 1000 1100 1200 1300 1400

0.01

100

1000000

Temperature (K)

Kp =

pC

O2 /p

CO =

pH

2O /p

H2·K

W

Kp =

pH

2O /p

H2Fe3O4

Fe2O3

FeO

Fe

Triple Point, 848 K

4

REACTION CHEMISTRY

• Introduction to the reduction and oxidation reactions• Overall, heat is generated

Reduction OxidationFe2O3

Fe3O4

FeO

H2O

CO CO2

CO CO2

Fe

FeO

Fe3O4

air N2

Thermallyneutral

Exothermic

H2H2

ΔH < 0

5

Coal

RED1 RED2

OX21193.15K

GASIFIER1173.15K

HX

Fe2O

3

Fe3O

4

Fe3O

4

FeO

Water Sat. steam

H2/H

2O to

cooling/ condensing

Syngas

Air

N2 to cooling

CO2/H

2O to

cooling/ condensing

Turb 1

Turb 2

Comp 1

Pump 1

Fuel stream

H2/H

2O stream

OC stream

Air stream

(3)

FLOW SHEET

OX1

Biomass

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DEGREES OF FREEDOM

• Each reactor is modelled using Gibbs free energy minimisation.

• Oxidation with air releases most of the heat. This reactor must be hotter than the gasifier, which fixes its temperature.

• Other reactors aim to run adiabatically (simplify heat integration).

• Supply a stoichiometric amount of air to complete the re-oxidation to Fe2O3.

• Control variables are the recycle rate of iron around the loop and the amount of steam added to the reactor producing the H2. A basis of 1kg/s of coal is assumed.

• Aiming for a system which

• Does not require heating utility

• Produces pure H2 (avoids carbon deposition during reduction)

• Maximises the production of H2 or exergetic efficiency

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Coal

RED1 RED2

OX21193.15K

GASIFIER1173.15K

HX

Fe2O

3

Fe3O

4

Fe3O

4

FeO

Water Sat. steam

H2/H

2O to

cooling/ condensing

Syngas

Air

N2 to cooling

CO2/H

2O to

cooling/ condensing

Turb 1

Turb 2

Comp 1

Pump 1

Fuel stream

H2/H

2O stream

OC stream

Air stream

(3)

FLOW SHEET

OX1

Biomass

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HEAT INTEGRATION

RED = HOT COMPOSITE

BLUE = COLD COMPOSITE

GasifierSteam supply

Oxidation reactorProduct coolingCooling +

condensation

Heat Released (MW)

Tem

pera

ture

of

Hea

t Cur

ve (

K)

ΔT

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OPERATING REGIME

Molar Flowrate of Steam into OX1 (mol/s)

Oxy

gen

Car

rier

Rec

ycle

Rat

e (m

ol F

e/s)

+ HeatHeat Integrated

Incomplete Syngas Conversion

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EXERGY

Molar Flowrate of Steam into OX1 (mol/s)

Oxy

gen

Car

rier

Rec

ycle

Rat

e (m

ol F

e/s)

utilfuelgfuel

netHex EE

WE

,,

2

Definition of Exergy:

Condition : Efficiency1 atm: 48%1 atm + steam cycle: 54%10 atm + steam cycle: 58%

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THERMODYNAMIC SUMMARY

• The reaction equilibria of iron allow the production of H2 via a cyclic process.

• Theoretical exergetic efficiencies are competitive with steam reforming.

• There is a trade off between producing heat and hydrogen, which can be controlled by how much of the re-oxidation is done by air vs. steam.

• Two well-mixed reduction reactors were modelled to enable to complete oxidation of CO to CO2. If this is to be achieved in a single reactor, spatial gradients in concentration are needed.

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PACKED BED: IDEALIZED CASE

• Separation of oxides due to concentration gradient

Gas

ifie

r

Fe2O3

(CO2, Steam, Air)

Solid fuel (e.g. wood)

(CO, H2, N2, tars)

Fe3O4

Distance along reactor

FeO CO2 for capture

Moving front

Sche

mat

ic,

Con

cent

rati

on

Fe3O4 + CO ↔ 3 FeO + CO2

CO

CO2

Kp1Kp2

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EXPERIMENTAL SET-UP

12.7 O.D.10.2 I.D.

6.4

O.D

.4

30

20

0

302

0

6.4 O.D.

1.6

O.D

.Packed bed of iron oxide particles

Sand plug

Gas outlet tocondenser

Gas inlet: (CO+CO2+N2), N2 or air

H2O(l) inlet

Thermocoupletype K

Chamber heatedby tape

Perforated plate

Wire mesh

GASES:•N2

•CO2

•10 vol. % CO + N2

•Air

FLOW:• 1 – 2 L/min

MEASUREMENT:•H2 (0-30 vol. %)

•CO2, CO (0-20 vol. %)•CO (0-2000 ppm vol.)NDIR Analysers

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EXPERIMENTAL: LONG BED

• Complete conversion of entering CO to CO2

• Very high purity H2 at outlet after sufficiently long purge

purgeCO, CO2

Mol

e fr

acti

on [

%]

Time [s]

Con

cent

rati

on [

ppm

]

H2

CO2

CO

CO2

ProductionH2

Production

steam purgepurge

N2

Purge

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EXPERIMENTAL: SHORT BED

0

2

4

6

8

10

12

14

16

18

20

0 200 400 600 800

Slower kinetics for transition from Fe3O4 – FeO

Time [s]

Mol

e fr

acti

on in

eff

luen

t [M

ole

%]

CO2

COH2

Kp = pCO2 /pCO = 1.87

60 75

Total conversion of CO

Drop to KP for Fe3O4 – FeO transition

• Kink in effluent gas curve at Kp

• Reduced rate for Fe3O4 → FeO compared to Fe2O3 → Fe3O4

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MOVING FRONTS

• SHARPNESS OF FRONTS:A. Time Constant Fe3O4 to FeO

A. Time Constant Fe2O3 to Fe3O4

B. Revised Schematic

s 2.0v

L

s 125kinetic

L, length of bed; v, interstitial velocity at given conditions

Fe2O3

Fe3O4FeO

Gradual Transition Sharp Front

kinetic

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Fe2O3-Fe

• High initial production of H2, exponential decrease• Decrease over entire temperature range

Cycle Number

H2 [

µm

ol]

900°C

600°C

18

Fe2O3-FeO

• Lower initial production of H2, but marginal decreaseH

2 [µ

mol

]

Cycle Number

900°C750°C600°C

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EFFECT OF OXIDATION IN AIR

• Reoxidation of Fe3O4 to Fe2O3 produces 50°C temperature rise

• No thermal sintering or adverse effect of oxidation

Cycle Number

H2 [

µm

ol]

Fe2O3 – FeOFe2O3 - Fe

Fe3O4 – FeOFe3O4 - Fe

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CONTAMINATION

• 600°C, possibility of Boudouard reaction:2CO ↔ C(s) + CO2

• Deposited C reoxidised in steam, air • Little deposition if FeO lowest oxide

Time [s]

H2

[CO] ≈ 4600 ppm vol.[CO2] ≈ 1500 ppm vol.

Mol

e fr

acti

on in

eff

luen

t gas

, dr

y ba

sis

[Mol

e %

]

Fe - Fe2O3

FeO - Fe2O3

Cycle 2

Cycle 2

Time [s]

CO

2 mol

e fr

acti

on [

Mol

e %

]

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CONCLUSION

PROCESS CHARACTERISTICS• Exergetically competitive.• Easy trade-off between heat and H2.• Full heat integration possible; no external heating utility

requirements.EXPERIMENTAL• CO2 suitable for carbon capture

• High purity H2

• Several cycles possible with FeO• Additional oxidation with air aides process by generating

heat• Carbon contamination not a problem for FeO

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ACKNOWLEDGEMENTS

• EPSRC

• Gates Cambridge Trust