HP March 2022 eBook—Energy Transition - 97

Special Focus
Sustainability
J. B. HANSEN and M. S. SKJØTH-RASMUSSEN,
Haldor Topsoe, Copenhagen, Denmark
Can ammonia be a future energy storage solution?
Ammonia is well known as a fertilizer. However, it is also a
potential carbon-free fuel and an excellent solution for storage
of renewable electricity, especially if the syngas needed for producing
the ammonia is made by electrolysis.
The basic idea is that low-value electricity-produced by
wind turbines or solar panels during surplus periods-is stored
efficiently as a high-value chemical (i.e., ammonia). In turn, that
chemical may be used as fuel or converted back to electricity
when demand for power is high. This is a highly efficient step to
possibly expand power production from wind and solar, as well
as to secure the best possible storage of energy.
The authors' company is leading a development project
funded by the Danish Energy Agency EUDP in cooperation
with Aarhus University, The Technical University of Denmark,
Energinet.DK, Vestas Wind Systems, Ørsted and Equinor.
The energy storage challenge. In a fossil-free future, energy
scenarios with large proportions of electricity production from
intermittent renewable sources like wind and solar energy storage
and sector coupling are seen as crucial enabling technologies.
However, sufficient amounts of energy are provided to our
globe via electricity production; the big challenges lie in how it
is stored and distributed.
Energy storage technologies analyses have shown that chemical
storage is by far the best option for large scales.1
in a Dutch study of power to ammonia2
Moreover,
, a comparison was made
to other chemical storage options. The analysis showed that:
* Liquid hydrogen is much more expensive than
ammonia and has a lower energy density.
* Compressed hydrogen lacks the desired scale
for seasonal storage.
* Methane, methanol and ammonia showed similar
capital expenditures (CAPEX), while the first two
rely on expensive carbon dioxide (CO2
Ammonia only needs air, water and power.
) sources.
In addition, the logistics of handling ammonia are well known
and much simpler than those of the other carbon-free energy
vector, hydrogen.
The electrified ammonia plant. Most of the world's ammonia
production is still based on fossil fuels and production based
on the Haber Bosch Synthesis. The front end of a conventional
ammonia plant involves carbon monoxide (CO) and CO2
-containing
gases, but the synthesis section itself is free of these. During
the last 50 yr, the authors' company has worked to increase
the performance of ammonia catalyst and come up with smarter
FIG. 1. The integration of the electrified ammonia production
with the air separating SOEC technology eliminates the need
for an air separation unit.
Hydrocarbon Processing | JANUARY 2020 | HydrocarbonProcessing.com
ammonia reactor and synthesis loop layouts to reduce energy
consumption. It is now close to the thermodynamic limit.
Therefore, there seems to be very little room for improvement
in the refined conventional fossil-based ammonia process,
and when looking into the future, the demands for renewable
solutions will increase. However, while the front-end part of the
conventional ammonia plant inevitably releases CO2
, which is
expensive to handle sustainably by sequestration, an optimized
and CO2
-free ammonia synthesis can have a revival. Previously,
electrolysis of water by alkaline electrolysis coupled with an air
separation plant was actually used at large scale. Interest in this
technology has reemerged as concerns about CO2
emissions increase,
and is further boosted by the drastic price drop of electricity
from intermittent, renewable sources.
The energy intensity and operating expenses (OPEX) using
classical, alkaline electrolysis will, however, be higher compared
to the conventional fossil-based ammonia processes. CAPEX
will also be high due to the air separation unit and state-of-theart
electrolyzers.
It is often stated that ammonia production is an energyintensive
process. This is true; however, the fact is that more
than 94% of energy is used for production of the synthesis gas,
whereas the ammonia loop consumes only a minor fraction to
compress and recirculate the synthesis gas and provide refrigeration
to condense out the product.
As an alternative, the electrolysis may be done in solid oxide
electrolysis cells (SOEC), offering much higher efficiency-
more than 70% from DC electricity input to ammonia output
on a lower heating value basis. However, the unique abilities of
the SOEC to separate oxygen from the fuel side come into play,
as shown in FIG. 1.
The fine details. The project's purpose is to demonstrate the
principles of this new process for ammonia synthesis gas generation.
In more detail, the new process utilizes the unique capabilities
Oxygen
Air
H2O
EI
SOEC
H2+N2
Haber-Bosch
synthesis
Steam
Ammonia
H2O
http://www.Energinet.DK http://www.HydrocarbonProcessing.com
HP March 2022 eBook—Energy Transition

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