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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
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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
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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
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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
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