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Edinburgh Research Explorer
Materials with desirable Thermodynamics Properties for theStorage of Hydrogen Energy using Sponge Metals
Citation for published version:Mignard, D 2011, 'Materials with desirable Thermodynamics Properties for the Storage of Hydrogen Energyusing Sponge Metals' Paper presented at 4th World Hydrogen Technologies Convention, 2011, Glasgow,United Kingdom, 14/09/11 - 16/09/11, .
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Download date: 07. Sep. 2018
Sponge Metals With Desirable Thermodynamic Properties
For The Storage Of Hydrogen Energy
D. Mignard
Institute for Energy Systems The University of Edinburgh
School of Engineering
How do we store hydrogen using sponge metal?
We do not store hydrogen itself, rather we store its energy
in the form of a metal (a porous metal called a ‘sponge’).
During storage:
H2 + Metal Oxide → Metal + H2O
During release:
Metal + H2O → H2 + Metal Oxide
High temperature reaction (for good kinetics), 600-1000oC.
Renewable
power
Water
Heat
Renewable
power
Water
Heat
2 mm
Suitable particles
made by e.g.
mechanical mixing,
precipitation,
sol-gel, etc.
Why using sponge metal?
Up to 3-6 wt.% H2 capacity – compact stationary storage
No need for pressurized containment
Cheap materials, can be non-toxic (e.g. iron)
Reduction step is a well known metallurgic process
(‘Direct Reduced Iron’), and the reverse oxidation is facile.
H2 production from sponge iron was also common practice in
the early chemical industry.
Delivery of pure hydrogen. For example, carbon monoxide
would be screened out during the storage of hydrogen, via
CO + iron oxides ↔ CO2 + iron
Iron sponge factory, near Alibaug, Maharashtra (India) – (C) Lepley, 2007.
How energy efficient?
Fe reduction and
cooling
Fe2O3 warming
Fe2O3
store
Fe store
Reduction
reactor
(with electrical
heating)
H2, 440oC
Fe, T < 702oC
90% H2O,
10% H2,
450oC
950oC
90% H2O,
10% H2,
950oC
(1-x)Fe + xFeO,
950oC
Fe2O3, 450oC
Fe2O3, 25oC
Efficient chemical storage of
wind / solar / etc. energy via hydrogen
With
'cold energy'
recovery
0%
10%
20%
30%
40%
50%
60%
70%
co
mp
resse
d
H2
, 2
00
ba
r
Liq
uid
H2
MT
H
Mg
hyd
rid
e
(slu
rry)
Me
tha
no
l
fro
m C
O2
H2
via
Sp
on
ge
Iro
nCh
em
ica
l E
ne
rgy
Sto
red
, k
Wh
/ k
Wh
e
(Mignard and Pritchard, Int. J. of Hydrogen Energy, 32 (2007), 5039 – 5049)
With suitable heat integration, iron sponge has the best energy efficiency
amongst all processes for compact H2 storage
Why so efficient?
Storing and releasing hydrogen tends to waste energy:
During storage,
H2 + empty carrier → stored H2 + energy (LOST)
During release,
stored H2 + energy (PARASITIC DEMAND)
→ H2 + regenerated carrier
The sponge-iron process works the other way round:
H2 + iron oxides + heat ↔ iron + water,
which enables co-storage and co-generation of H2 + heat.
Current research in the field
Usually, research groups stick to iron, and seek to achieve
- Improved resilience to sintering and attrition
- Maintained activity over 1000’s cycles
- Improved kinetics
- Lower operating temperature.
Typically, a hard support (alumina, silica, etc.) is mixed with
the iron, together with some metallic additives. Fabrication
technique is also important for obtaining stable but active
contact mass.
But what about the process? - Our proposal
H2 storage step requires high conversion per pass!
Low conversion:
High conversion:
A high H2 conversion per pass ≥ should make the
system cheaper, simpler, more efficient and safer.
Sponge reactor Condenser
Compressor
Water
product
H2 feed
H2 recycle
Sponge reactor Condenser
Compressor
Water
product
H2 feed
H2 recycle
Sponge reactor Condenser
‘Acceptable’
H2 loss
Water
product
H2 feed
Sponge reactor Condenser
‘Acceptable’
H2 loss
Water
product
H2 feed
A high conversion per pass is not as critical for the H2 release
step (unreacted steam can be used for: heat recovery; proper
emission control in H2 gas turbine; hybrid fuel cell / turbine
power generation systems, etc.
H2 storageSponge reactor
(reduction of oxide)Condenser
‘Acceptable’
H2 loss
Water
product
H2 feed
Heat (from renewable power)
H2 storageSponge reactor
(reduction of oxide)Condenser
‘Acceptable’
H2 loss
Water
product
H2 feed
Heat (from renewable power)
H2 releasesteam feed
Sponge reactor
(oxidation of metal)
Power
generation
Heat
H2 releasesteam feed
Sponge reactor
(oxidation of metal)
Power
generation
Heat
0
1
2
3
4
5
0 20 40 60 80 100
% H2 at equilibrium
[ R
ec
yc
le f
low
ra
te
/ f
ee
d f
low
ra
te]
rati
o
200
400
600
800
1000
1200
1400
0 20 40 60 80 100
% H2 at equilibrium
T (
oC
)
FeFeO
Fe3O4
Can we operate at lower temperature?
T ≤ 600 oC good for costs (and durability, as we found in the lab!) However, lowering T also lowers the conversion per pass during H2 storage. From that point of view, iron is not the best metal.
TARGETS: Reduction stage (hydrogen storage), must be endothermic with H2 equil. concentration ≤ 5 % at T = 600 oC Oxidation stage (hydrogen release), must be exothermic with H2 equil. concentration ≥ 50 % at T = 300 oC No single, common enough element can do this. We need an alloy of at least two metals M and M’, such that for the reaction MxM’y + 2H2O → MxM’yO2 + 2H2
the targets set above can be met.
A bit of thermodynamics...
The previous targets are equivalent to setting for the reaction MxM’y + O2 → MxM’yO2
the following requirements: DSɵ
r ≤ - 236 J/mol (standard entropy of reaction) DHɵ
r, 298K ≤ - 572 kJ/mol (standard enthalpy of reaction) Further approximations: DHɵ
r, 298K ~ DHf,ɵ298K (MxM’yO2),
the enthalpy of formation of the oxide. DSɵ
r = -205 + Sɵ(MxM’yO2) - Sɵ(MxM’y)
Hence the target properties of the alloy and the mixed oxide: Sɵ(MxM’yO2) - Sɵ(MxM’y) ≤ - 31 J/mol DHf,ɵ
298K (MxM’yO2) ≤ - 572 kJ/mol (The first condition seems tough, and may need to be relaxed somewhat) Let’s take y = 0 and check a few elements – we visualize this on a Ellingham diagram (next slide)
No single element is suitable... Tentatively, mixtures of Iron, Tin, Tungsten, Molybdenum, Nickel could be considered – beware of melting points!
-700
-600
-500
-400
-300
-200
-100
0 150 300 450 600 750
-DG
f(M
xO2),
kJ/
mo
lT (oC) 4Cu+O2->2Cu2O
4/3Bi+O2->Bi4/3O2
2Pb+O2->2PbO
2Ni+O2->2NiO
2/3Mo+O2->Mo2/3O2
TARGET
Sn+O2->SnO2
2/3W+O2->W2/3O2
3/2Fe+O2->Fe3/2O2
2Zn+O2->2ZnO= Melting points
Can we engineer the right alloy/mixed oxide?
A wealth of literature for the prediction of enthalpies of formation and entropies. Some are accurate within a few %, e.g. Schwitzgebel, Lowell and Parsons, 1971 (J. Chem. Eng. Data, 16(4), 418-423) predict DHf,ɵ
298K of binary oxides from a rather empirical model, but within a few % of exp. values. Holland, 1989 (American Mineralogist, 74, 5-13) predicts Sɵ from molar volumes and coordination within a few %. A universal but less accurate (up to ~ 10% error) method for Sɵ was demonstrated by Jenkins and Glasser, 2003 (Inorg. Chem., 42, 8702-8708) based on no more than the density.
Preliminary results - Experimental Validation
0
20
40
60
80
100
0 2 4 6 8
% h
yd
rog
en
un
rea
cte
d
number of reduction/oxidation cycles
Observed hydrogen equilibrium concentration after cycling at 600oC
NiFe2O4 / NiFe2
CoFe2O4 /CoFe2
Fe3O4 / Fe
Challenges ahead - stability of the mixed oxide and phase segregation of the metal - emissions? - thermal management for maximum efficiency (heat recovery + insulation)
Thank you for your attention