H2Tech - Q4 2022 - 24

SPECIAL FOCUS FUTURE OF HYDROGEN ENERGY
~40%/1,000 hr to ~0.4%/1,000 hr for
steam electrolysis.3
Conversely, alkaline
electrolyzer degradation rates are at 1% per
10,000 hr-to reach this level many more
improvements are required for SOECs.
Stack level improvement. SOEC stack
performance is determined by the performance
of cells and other stack components.
The properties of each component
change during long-term operation under
the influence of high temperature. FIG. 8
shows improvement in stack performance
from 2009-2019. For steam electrolysis,
the stack lifetime tested was limited to
< 4 mos in 2009, whereas stack lifetimes of
nearly 2.5 yr were experimentally demonstrated
in 2019. This is mainly due to an
increase in stack durability-SOEC stacks
are now less prone to sudden performance
failure and degrade less rapidly.
Integrating the SOEC and the heat recovery
unit remains a challenge. In addition,
the efficiency and cost of the overall
plant must be considered for design optimization.
In the next section, potential
integration schemes are discussed.
SOEC HEAT INTEGRATION
WITH WASTE HEAT SOURCES
SOECs operate with high efficiency,
especially if fed with high-temperature
waste heat. The electrochemical conversion
of water permits the storage of both
heat and electricity in the produced H2
form. Green H2
produced by SOECs
can be further processed into synthetic
natural gas, methanol, green ammonia,
etc., and thermally integrated with a wide
range of exothermic chemical syntheses,
resulting in further efficiency improvements.
Heat integration is also possible
with energy sources like nuclear reactors,
coal-fired power plants, biomass, domestic
waste incinerators, etc. Some of them
are discussed here.
SOEC heat integration with diesel engines.
Exhaust gas is a high-grade waste
heat with temperatures that can exceed
500°C for diesel engines. Therefore, if a
diesel engine is integrated with the SOEC
system as a heat recovery steam generator
(HRSG), it will significantly reduce the
power consumption of the SOEC.
FIG. 7. Cell-level improvements; currentvoltage
curves for cells fabricated in 2006
and 2020 at 750°C, measured in H2
O/H2
Coupling SOEC and ammonia (NH3
production plants. NH3
= 1.
)
synthesis is
an exothermic chemical reaction at relatively
high temperatures and pressure
(FIG. 9), so a large amount of heat is available
for recovery in the plant's energy balance.
In addition, the produced H2
used as feed for NH3
can be
production if mixed
with a proper amount of nitrogen (Eq. 3).
1/2N2
+ 3/2H2
} NH3
∆H298K = -45.7 kJ/ mol
(at standard condition)
(3)
System integration with an SOE and
an NH3 plant by the Haber-Bosch process
operated at high pressure (150 bar-200
bar) and temperature (300°C-500°C) is
possible. The ammonia reaction can supply
a significant amount of heat energy
required by the electrolysis reaction, considering
that for each ammonia synthesis,
1.5 mols of H2
are needed.
Heat integration of high-temperature
electrolysis and methanation. The heat
of the exothermic methanation reaction
can be used entirely for the evaporation of
the process water for electrolysis.
The methanation reaction catalyzes
H2 with CO or CO2 into methane [synthetic
natural gas (SNG)] and water
(Eqs. 4 and 5).
CO + 3H2
} CH4
+ H2
∆H298K = -206 kJ/ mol
CO2 + 4H2 } CH4
∆H298K = -165 kJ/ mol
O
+ 2 H2
O
(4)
(5)
The exothermic reaction heat of
methanation, the cooling of the product
stream after methanation and the heat
quantity generated by overvoltage (exothermic
operation) are greater than the
required heat quantity for vaporization
and preheating reactant water. Therefore,
high efficiencies are achieved by coupling
high-temperature electrolysis and methanation
(FIG. 10).
Versatile application of SOECs. SOECs
can operate reversibly, enabling efficient
cyclic conversion between electrical and
chemical energy and providing long-term
and high-capacity energy storage. In fuel
cell mode operation, electricity is generated
by oxidizing fuels. In SOEC mode,
electricity generates H2
, syngas, etc.
Development is ongoing to direct.
For exO
and O2
ly produce speciality and commodity
chemicals other than green H2
FIG. 8. Stack development over time: (A) stack test duration since 2009, and
(B) corresponding degradation rates.
24 Q4 2022 | H2-Tech.com
ample, syngas can be produced from
co-electrolysis of H2
high-temperature SOECs. Syngas
using
can
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