80 / 100
compressor which compresses a significantly larger air flow has a
considerably lower maximum speed.
The greater speed of the axial flow compressor AG-MAX1 is
also a benefit for the tail gas expander whose footprint can now
be reduced by up to two sizes. In addition, the increase in speed
now opens an array of design opportunities for arranging the
various pieces of turbomachinery within the train: as all machines
can operate at the same speed, the intermediate gear, the
related CAPEX, frictional losses and also maintenance costs are
all to be omitted. Except for the tail gas expander, which can be
decoupled from the process at times and therefore needs to be
placed at one end of the train, all remaining units can be freely
arranged in the train. Compressors in the middle of the train can
be interchanged as the customer wishes when the steam turbine
is located at the second end of the train. Moreover, the reduced
size of the axial flow compressor and the tail gas expander, as
well as the omission of a gear unit, allow for a significantly
reduced footprint of the modular nitric acid train package
NAMAX.
Using this train design, compressor trains for plant sizes of
400 tpd to over 3000 tpd nitric acid can be implemented. The
first NAMAX train was delivered in April 2019 to thyssenkrupp
Industrial Solutions (tkIS) for a new nitric acid plant in Poland
(Figure 4).
Next step: CO
2
compression
The next aim is to utilise the benefits offered by the new blading
generation not only for air as operating medium, but also for
compressing gases with a lower and higher molecular weight.
A potential field of application results from the demand to
protect the environment from rising levels of CO
2
emissions all
around the world. A promising technology in this regard, CCS has
potential uses in CO
2
intensive economic sectors such as
cement, steel and aluminium production, as well as
petrochemistry and the global expansion of natural gas
extraction. A distinction is made between various methods of
CO
2
separation such as separation after carbon gasification
(pre-combustion/IGCC) or separation following the combustion
process (post combustion). Nevertheless, there is one thing
these methods all have in common: the requirement for CO
2
compressors to transport greenhouse gas from the power plant
to the storage location and to inject the resulting masses of CO
2
into reservoirs.
The resulting flow of CO
2
must be pressurised to over 70 bar
to be able to transport the gas from the separation to the
storage location in an effective manner. At the same time, flow
rates above 100 000 m
3
/hr of CO
2
are occurring in large-scale
power plants. A smart approach to transporting such massive
volumes of CO
2
might be to pre-compress the gas to about
6 bar using an axial compressor and to reduce the flow rate and
subsequently compress it to the final pressure by means of a
radial compressor.
The benefits of using an axial CO
2
compressor are the very
high levels of efficiency, the option to compress large volume
flow in a single compressor, the utilisation of heat resulting from
compression in power plant processes, and the mechanical
reliability of the compressor. The combination of high levels of
efficiency, intermediate coolers and introducing waste heat into
the process reduce the energy consumption of the compression
to a minimum.
When compressing CO
2
instead of air, the different
molecular weight means the pressure is built up in the
compressor in a different manner. This will result in flow
incidences onto the individual blade cascades (stage mismatch),
as the geometrical flow path contraction does not correspond
to the intended compression. Therefore, the main focus of a
new research project at MAN Energy Solutions is to optimise the
flow path contraction for compressing carbon dioxide in an
axial-type compressor. Blade profiles should be designed
essentially the same way but using the modified flow path. The
main challenges are relating to aerodynamics and structural
mechanical properties of the blading.
Furthermore, structural components such as seals are
adapted. In terms of aerodynamical design, fluid mechanics
related designing and validation tools such as streamline
geometry or CFD methods need to be adapted and validated.
In terms of structural mechanical properties, two key aspects
are of importance: both cut of blading (Figure 5) and the
modified aerodynamical damping have an influence on the
resonance behaviour of the blading in the resonance points. This
demands special requirements on the simulation tools of the
fluid-structure interaction, which must also be validated
experimentally.
Compressor design validation is again carried out in a scaled
R&D test rig, which has a flow path contour optimised to match
the molecular weight of CO
2
. During the test, speed and
vane-control performance maps are conducted, which are then
used to derive performance data and define input parameters
for subsequent forced-response analysis. Upon completion of
this development stage, axial compressors with MAX1 blading for
compressing lightweight and heavy gases at pressure ratios up to
25 and flow rates up to 1.5 million m
3
/hr will be available.