H2Tech - Q2 2022 - 18

SPECIAL FOCUS HYDROGEN INFRASTRUCTURE DEVELOPMENT
Many viable thermodynamic path options,
including refrigeration and liquid
pumping, near-isothermal, and highpressure
ratio compression, exist to move
this compression process from its start to
its endpoint. Put differently, the available
options are to compress the CO2
and reand
main
in the gas state on the right side of
the vapor dome, refrigerate the CO2
pump it in the liquid state on the left side
of the dome or utilize some combination
of these methods.
CO2 sequestration and storage. For
most power plant carbon capture and
sequestration applications, the following
new compression duties are required:
1. Pipeline header injection and
recompression transport
2. Injection into geological storage
reservoirs for sequestration
3. Separation processes
(membrane, thermal or chemical)
4. Power plant cycle compression.
According to industry convention,
Cooler
Compressor
Compressor
CO2 as a supercritical (dense phase) fluid
is well above
above 2,100 psi should be transported in
pipelines. At 2,100 psi, CO2
Centrifugal compressor
and pump combinations
provide the most
promising options for
the efficient transport of
large volumes of CO2
.
its critical point and will be supercritical at
almost all ambient temperatures. Fluids in
a dense phase share some physical properties
of liquids, such as a very low compressibility;
they also share some physical properties
of gases and will expand in space to
fill voids. The advantage of transporting
CO2
at supercritical pressures, therefore,
is that its density does not change much
with pressure: from a thermodynamic
perspective it is essentially pumped rather
than compressed. This significantly reduces
the power demand for the pumping
stations along a CO2
pipeline.
However, there are two disadvantages:
Cooler
Electric
motor
Cooler
Compressor
CO2 out
2,100 psia
Compressor
CO2 in
50 psia
FIG. 3. Integrally geared compressor with
interstage cooling.
the added injection compression ratio required
at the pipeline header station and
the significantly higher material costs
when building a pipeline designed for a
maximum allowable operating pressure
above 2,100 psi. Since the CO2
available
from separation is at low, near-atmospheric
pressures (< 100 psi), the pipeline header
station must always use a compressor;
for a 2,100-psi CO2
pipeline, a high-pressure
ratio header compressor with many
intercooled stages will be needed to handle
the significant volume reduction. In
such a case, however, beyond the header
station the gas is simply pumped.
Fortunately, transport at 2,100 psi is
depends on the
must be transported
pipeline is
not required for all applications: the actual
transport pressure of CO2
separation process outlet starting pressure,
the distance the CO2
and the geological sequestration injection
pressure (which is often well below 2,100
psi). If a lower-pressure CO2
utilized, conventional compressors are preferred
for the header station and for recompression
along the line. The transport pressure
is selectable depending on the carbon
sequestration application; it is not always
advantageous to go with supercritical CO2
Purely from a compression stage thermodynamic
perspective, CO2
.
is a heavy
gas but relatively easy to compress. That
ease notwithstanding, CO2
presents several
technical
challenges that must be
addressed to make its compression or
pumping process efficient and reliable.
These include:
* Most equations of state for CO2
are still inaccurate at high pressures
and temperatures.
* CO2 is a heavy gas, resulting in
amplified rotor dynamic and
impeller-dynamic forces.
* CO2 has a strong thermodynamic
path dependence and multi-phase
behavior.
* CO2 forms carbonic acid in
the presence of water, which
then drives corrosion.
* CO2 is soluble in elastomeric
materials, which can lead to rapid
decompression failures.
* When rapidly expanded, CO2
quickly forms liquids and dry
ice, which can be a problem
at the shaft seals.
* CO2 has a low sonic speed, which
results in higher shock losses and
a reduced operating range.
* CO2 selectively leeches certain
elements from common metals
and has a very low viscosity at
high pressures.
All of these represent manageable, if complex,
engineering and design challenges.
Compression and pumping options.
CO2
has a high pressure ratio per comFIG.
4. An 8-impeller-stage, barrel-type,
centrifugal compressor with multiple nozzles
for intercooling or side streams.
18 Q2 2022 | H2-Tech.com
FIG. 5. A multi-stage, horizontally split
compressor with nozzles for two intercoolers
driven by an electric motor through a gearbox.
pressor impeller stage. Because of this,
it also has a significant specific volume
decrease with pressure along with a very
high heat of compression. This means
that CO2
heats up when compressed and
requires stage intercooling to maintain
Gear
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