Note: Descriptions are shown in the official language in which they were submitted.
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System for gas processing
Technical Field
The present invention relates to systems for processing gas resulting from
fossil
fuel fired power plants for the generation of electric energy. It relates in
particular
to a system for gas processing to purify such gas in order to facilitate the
transport
and storage of carbon dioxide.
Background Art
In view of reducing the emission of the greenhouse gas carbon dioxide (002)
into
the atmosphere, the flue gases of fossil fuel fired power plants for the
generation
of electrical energy are typically equipped with so-called 002-capture
systems.
002 gases contained in the flue gases is first separated, then compressed,
dried,
and cooled and thus conditioned for permanent storage or a further use such as
enhanced oil recovery. For safe transport, storage or further use, the 002 is
required to have certain qualities. For example, for enhanced oil recovery the
gas
is to have a 002 concentration of at least 95%, a temperature of less than 50
C
and a pressure of 13.8 Mpa. Flue gases from fossil fuel fired power plants
comprise not only 002 but also a number of further contaminants such as water
vapor, oxygen, nitrogen, argon, as well as S03, S02, NO, NO2, which must be
removed in order to fulfill the environmental regulations and requirements for
transport and storage of 002. All of these contaminants and the 002 itself can
appear in various concentrations depending on the type of fossil fuel,
combustion
parameters, and combustor design. The percentage of 002 contained in the flue
gases can range from 4% in the case of combustion of gases for a gas turbine
to
60% -90% in the case of a coal fired boiler with air separation unit providing
additional oxygen to the combustion process. The removal of contaminants from
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flue gases is not limited by technical barriers but rather by the additional
cost and
energy requirements and subsequent reduction in the overall power plant
efficiency.
Minish M. Shah, "Oxyfuel combustion for CO2 capture from pulverized coal
boilers", GHGT-7, Vancouver, 2004, discloses an example of a system for
handling the flue gases resulting from a fossil fuel fired boiler. The system
includes a recycle line for a portion of the flue gas to be returned to the
coal-fired
boiler together with oxygen from an air separation unit. The flue gas is led
through
a filter for removal of ash and dust, such as a fabric filter or electrostatic
precipitator, furthermore through a flue gas desulphurization unit for the
removal
of SOx and finally through a gas processing unit for CO2 purification and
compression. This unit comprises a system for removal of incondensable gases
such as 02, N2, and Ar, a dehydration system for removal of water vapor, and a
series of compression and cooling systems. These include a first low-pressure
compression systems of the non-purified flue gases and a high-pressure
compression system of the purified 002, each with coolers integrated.
For the compression, such systems comprise for example two multistage
centrifugal compressors, a low-pressure compressor and a high-pressure
compressor and apparatuses for dehydration and cryogenic removal of inert
gases arranged between the low- and high-pressure compressors. The multistage
centrifugal compressors have intercoolers following each compressor stage in
order to minimize the power consumption of the compression. The multistage
centrifugal compression typically includes 4-6 compression stages. Because of
the
large number of compressor stages, the low-pressure and high-pressure
compressors are each arranged on independent shafts with a separate driver.
The
heat resulting from the intercoolers is low-level heat of 70-80 C, which is
typically
not recovered but instead dissipated in the cooling water system of the power
plant. The cryogenic system for removal of inert gases generates an inert gas
flow
under pressure, which is typically expanded in a suitable turbine, which in
turn
drives a generator or is arranged to provide a part of the mechanical power
for
driving a compressor.
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Furthermore, Bin Xu, R.A. Stobbs, Vince White, R.A. Wall, "Future CO2 Capture
Technology
for the Canadian Market", Department for Business Enterprises & Regulatory
reform, Report
No. COAL R309, BERR/Pub, URN 07/1251, March 2007, discloses on pages 124-129 a
system for processing the flue gases including dehydration, compression,
cooling, and
cryogenic processing. The compressors used are adiabatic compressors, which
allow an
improvement in terms of power consumption and cooling requirements.
US 6,301,927 discloses a method of separating CO2 from a feed gas by means of
autorefrigeration, where the feed gas is first compressed and expanded in a
turbine. The
CO2 contained in the feed gas is then liquified and separated from its gaseous
components
in a vapor-liquid-separator.
US 4,977,745 discloses a method for recovering low purity CO2 from flue gas
including
compressing flue gas and directing it through a water wash and a dryer and
finally to a CO2
separation unit.
US 7,416,716 discloses a method and apparatus for purifying carbon dioxide, in
particular for
the removal of SO2 and NOx from CO2 flue gas resulting from a coal fired
combustion
process. For this, the flue gas or raw CO2 gas is compressed to an elevated
pressure by
means of a compression train with intercoolers for the cooling of the
compressed gas, where
some of the compression is performed adiabatically. The compressed gas
containing water
vapor, 02, S0x, and NOx is then led into a gas/liquid contact device for
washing the
gaseous CO2 with water for the removal of SOx and NOx.
Summary of Invention
According to an aspect of the present invention, there is provided a system
for processing
flue gases from a fossil fuel fired power plant for the generation of
electrical energy
comprising: an adiabatic compressor for a first low-pressure compression of
the flue gas; a
second low-pressure compression system having one or more stages and one or
more
coolers; and a high-pressure compression system having several stages and one
or more
coolers, where both the second low-pressure compression system and the high-
pressure
compression system are combined in one single machine, arranged on one common
shaft,
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and driven by one common driver, wherein the adiabatic compressor is
configured for a
discharge pressure of the flue gases of a pressure in the range from 5 bar abs
to 9 bar abs.
Some embodiments may provide a fossil fuel fired power plant for the
generation of the
electrical energy with an improved flue gas processing system for the
processing of the flue
gases resulting from the combustion of the fossil fuel for the power plant.
According to another aspect, a fossil fuel fired power plant comprises a post-
combustion flue
gas processing system, where the system comprises
- a first low-pressure flue gas compressor, where the first low-pressure flue
gas compressor
is an adiabatic, axial compressor without intercooling,
- one or more heat exchangers arranged downstream from the first low-pressure
flue gas
compressor and configured and arranged for the transfer of heat from the
compressed flue
gas to the power plant or a system connected with the power plant,
- a second low-pressure flue gas compressor arranged downstream of the one or
more heat
exchangers and having one or more stages and one or more coolers,
- a unit for cryogenic purification of the flue gases by removal of inert
gases from the flue gas
arranged downstream of the second low-pressure flue gas compressor, and
- a high-pressure CO2 compressor system arranged downstream of the unit for
cryogenic
purification and configured and arranged for the compression of a CO2 flow
resulting from
the unit for cryogenic purification, the high-pressure CO2 compressor system
having several
stages and one or more coolers,
- where both the second low-pressure flue gas compressor and the high-pressure
CO2
compression system are combined in one single machine and are arranged on one
common
shaft that is driven by one common driver.
The power plant with the post-combustion flue gas processing system according
to some
embodiments allows, due to the integration of an adiabatic compressor, a
reduction of the
total power consumption necessary for the flue gas compression. Furthermore,
the
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adiabatic compressor without intercoolers allows a recovery of the heat from
the flue gas
and its use in the power plant or in a system connected with the power plant
such as an
industrial consumer or other consumer requiring heat. Thereby, required heat,
for
example for feedwater preheating, that would otherwise be extracted from the
power
5 plant can now be drawn from the compressed flue gases. The system
according to
some embodiments therefore facilitates an improvement in the overall
efficiency of the
power plant thus integrated with the flue gas processing system, however
without an
increase in number of compressor machines.
Additionally, a flue gas processing system according to some embodiments
allows a
reduction in the initial investment cost for the system. The system comprises
a total of
only two compression machines with two drivers and two shafts, i.e. the
adiabatic, flue
gas compressor on one hand and the combination of second low-pressure flue gas
compressor with high-pressure CO2 multi-stage compressor, on the other hand.
In spite
of the addition of an adiabatic compressor, the system's total number of
machines is still
the same. Finally, the combination of the second low-pressure flue gas
compressor and
high-pressure CO2 compressor into one machine results not only in a reduction
in
investment cost but also allows space efficiency in the power plant
construction.
In a particular embodiment of the invention, the second low-pressure flue gas
compression system and the high-pressure CO2 compression system combined into
one
machine arranged on one shaft comprises two low-pressure compressor stages and
four
to six high-pressure compressor stages.
In a further particular embodiment of the invention, the flue gas processing
system
comprises a dehydration unit arranged downstream of the second low-pressure
flue gas
compressor. This allows greater possibilities in the handling and use of the
resulting
002.
In a further particular embodiment of the invention, the flue gas processing
system
comprises one or more heat exchangers for cooling of the flue gas downstream
from the
adiabatic compressor, where the heat exchanger(s) is/are configured for heat
exchange
with a water flow that can be part of the water/steam cycle of a power plant
or any other
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water flow system for heat recovery within the power plant or in a system
connected with
the power plant. For this embodiment, the adiabatic flue gas compressor is
configured
for a discharge pressure of the flue gases of a selected pressure range. This
pressure
range is selected for example in consideration of an optimal heat recovery in
connection
with the water/steam cycle of the power plant, an optimally minimized power
consumption of the adiabatic compressor, and the integration of the low- and
high-
pressure compression stages downstream from the adiabatic flue gas compressor.
In an embodiment, the adiabatic flue gas compressor discharge pressure can be
set to 7
to 9 bar abs. Above this pressure range the adiabatic compression would
require more
power consumption than the compression in an intercooled centrifugal
compressor. With
this discharge pressure the temperature at the discharge of the adiabatic
compressor is
in the range from 170 to 280 C. This allows an efficient heat recovery for
instance by
heating condensates from the power plant steam/water cycle through the use of
a
dedicated heat exchanger. After the heat recovery, the flue gas is at a
temperature of
about 50 C. It is then further cooled in a second exchanger, where heat is
dissipated. It
is then compressed to 30 to 40 bar abs by two stages of the second low-
pressure flue
gas compressor, a centrifugal compressor with intercoolers. These two stages
can be
easily combined with the high-pressure CO2 compressor having 4 to 6 stages,
for
instance by the use of one integral gear compressor with 6 to 8 stages. The
adiabatic
compressor facilitates an improved recovery of the heat resulting from the
cooling of the
compressed flue gas. This can further improve the overall efficiency of a
power plant
integrated with this type of flue gas processing system. A further advantage
of the power
plant according to some embodiments is in that the number of flue gas
compressors,
these being adiabatic and centrifugal, remains constant compared to power
plants of the
prior art having only centrifugal compressors.
In a further particular embodiment of the invention, the first, low-pressure
flue gas
compressor and second low-pressure flue gas compressors are configured such
that the
ratio of the discharge pressure of the adiabatic compressor to the discharge
pressure of
the first stage of the low-pressure flue gas compressor is in the range from
1.5 to 2.5.
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The power plant can be any kind of fossil fuel fired power plant, including a
steam turbine
power plant with a coal-fired boiler, where this boiler can be operated with
or without
additional oxygen provided by an air separation unit. The fossil fuel fired
power plants
can also include gas turbine or combined cycle power plants.
In a further embodiment, the system according to the invention further
comprises a
system for the removal or reduction of the SOx and NOx. Such system can be
arranged
either in the low-pressure flue gas treatment system, that is upstream of the
flue gas
compression or downstream from the adiabatic compressor. If the SOx and NOx
removal system is arranged downstream from the adiabatic flue gas compressor,
the
proposed embodiment can still be realized by combining the remaining
centrifugal stages
required for flue gas compression with the stages required for CO2 compression
in one
machine driven by one driver. The SOx and NOx removal reaction kinetics as
well as
reactor sizing will affect the choice of the adiabatic compressor discharge
pressure. For
instance, the discharge pressure can then be raised to around 15 bar abs, thus
leaving
one stage of flue gas compression to be combined with the CO2 compression in
one
multistage centrifugal compressor.
Brief Description of the Drawings
Figure 1 shows a diagram of an embodiment of a flue gas processing system
according
to an embodiment of the invention that may be integrated in a power plant for
the
generation of electricity.
Best Modes for Carrying out the Invention
Figure 1 shows a flue gas processing system 1 for the processing of flue gases
resulting
from a fossil fuel fired power plant. The power plant itself is not shown save
for a line 2
directing the flue gas resulting from the combustion of fossil fuels for the
generation of a
working medium to drive a turbine. The processing system 1 comprises
essentially a flue
gas line 2, directing flue gases to a first compressor system Cl, heat
recovery system
HR, a second compressor system C2, all
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arranged in series in the sequence mentioned, and a 002-line 3 for directing
the
separated 002 to a facility for further use. The flue gas line 2 leads from a
power
plant to the first compressor system Cl, which comprises an adiabatic flue gas
compressor 5. The heat recovery system HR comprises heat exchangers for the
cooling of the compressed flue gases released by the compressor Cl and
transfer
of heat from the flue gases to the power plant. The second compressor system
02
comprises a combined multi-stage and intercooled compressor system for the low-
pressure compression of flue gases and the high-pressure compression of
purified
002. Finally, the line 3 leads purified and compressed 002 away from the
system
1 to a further system 4 for transport, storage or further use of the 002 such
as
enhanced oil recovery.
Flue gases are led to system 1 as shown via the line 2, where the flue gases
can
result for example from a coal-fired boiler, from a gas combustion chamber, or
oxyfired coal-fired boiler. As such, they can contain 002 gas of various
concentrations, such as 4% or more in the case of a gas turbine power plant
with
or without flue gas recirculation, or up to 60-90% in the case of oxyfired
coal
burning boilers for steam turbine power plant. Following the boiler or
combustion
chamber, the flue gases may have been pre-treated in a filter such as an
electrostatic precipitator or a fabric filter or any other process unit for
the removal
of sulphur. Furthermore, the flue gases may have been treated in an apparatus
for
the removal of NOx or mercury.
The flue gas line 2 carries the 002-containing flue gas to the low-pressure,
adiabatic flue gas compressor 5 driven by a driver 6 and configured to
compress
the flue gas to a discharge pressure of 5 to 20 bar abs. A minimized power
consumption for the compression can be reached with a configuration for a
discharge pressure of 5 to 8 bar abs, for example 7 bar abs. The adiabatic
compressor 5 is configured for a compression to a discharge pressure of no
more
than 20 bar. Compression to a discharge pressure higher than this limit would
increase the power consumption such that there would no longer be any benefits
from the use of an adiabatic compressor. This is due to the fact that after a
pressure of around 8 bar abs, the adiabatic (axial) power consumption becomes
higher than that of an intercooled centrifugal compressor. After this pressure
the
benefit of having more efficient wheels in the axial machine is more than
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compensated by the increase of power consumption due to the gas temperature
increase in the absence of intercooling. At the compressor discharge the
compressed flue gas may have a temperature of ca. 20000-28000.
The optimum discharge pressure of the adiabatic compressor will be set by the
minimization of power consumption, but also by additional parameters such as
water/steam cycle integration, intermediate removal of SOx and NOx if any, as
well as machine selection.
A line 7 leads from the discharge of the low-pressure flue gas compressor 5 to
a
first heat exchanger 8, through which the compressed and hot flue gases flow
in
counterf low to a flow of water or another cooling medium. The cooling medium
is
led from the heat exchanger 8 via line 9 to a system for heat recovery in a
system
within the power plant or in a system connected with the power plant. The
adiabatic/axial flue gas compressor 5 allows the recovery of heat from the
flue
gases at a higher temperature (170-24000) compared to the case if a
centrifugal
compressor were used instead in this position. This heat can be effectively
used in
the power plant. For example, in the embodiment shown, the heat recovery
system is the water/steam cycle 9 of a steam turbine system. In a particular
example, this water flow is connected to a feedwater preheater or to the
condensate extraction pump. A part of the condensates can be heated directly
by
the flue gas, thus by-passing the low-pressure heaters. The steam consumption
of
the low-pressure heaters is reduced and, as a consequence, more steam is
expanded in the cycle steam turbine and the plant can produce more electrical
power. Due to the use of the adiabatic /axial flue gas compressor a gain of
the net
power output of the power plant of 0.5% to 1% can be achieved over the net
output of a power plant having only centrifugal flue gas compressors. The
power
plant according to the invention achieves a greater output although having the
same number of compressor machines as a power plant with only centrifugal
compressors.
After having passed through the heat exchanger 8, the flue gases have a
temperature of for example 50 C. On the flue gas side, the heat exchanger 8 is
connected via a line 10 to a further heat exchanger or cooler 11, where the
flue
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gases are further cooled to a temperature of for example 30 C. The heat
resulting
from this cooling is of low-grade and can be dissipated.
A line 13 leads from the cooler 11 to the combined compression system 02
driven
by driver 17 and comprising a low-pressure flue gas compressor 14, a high-
5 pressure 002 compressor 15 arranged on shaft 16 and driven by driver 17.
The
low-pressure flue gas compressor can have for example two stages of a
centrifugal compressor with intercooler, whereas the high-pressure 002
compressor can have for example four to six stages with intercoolers. If the
discharge pressure of the adiabatic compressor is lower, that is within the
10 discharge pressure range given between 5 to 20 bar abs, the centrifugal
low-
pressure flue gas compressor can also have three instead of two stages. The
flue
gases, compressed to a pressure of for example 30 bars abs by the low-pressure
compressor 14, are led via line 18 to a dehydration unit 19 and thereafter to
a
cryogenic unit 20. In the cryogenic unit, the flue gas is separated resulting
in a
purified 002 gas flow and a vent gas containing inert gases like nitrogen,
oxygen
and argon. The vent gas is sent via line 21 to an expander 22, which can be
mounted on the same shaft 16 or mounted on an independent shaft. In the flue
gas processing system according to the invention, the low-pressure flue gas
compression system 14 and high-pressure 002 compression system 15 are
arranged on the same shaft, whereas the low-pressure flue gas compression
system is arranged up-stream of the cryogenic purification system and the high-
pressure 002 compression system is arranged down-stream from the purification
system.
The cryogenically purified flue gas, now containing mainly 002 of a
concentration
sufficient for transport and storage, is led from the unit 20 to the high-
pressure
compressor system 15 for further compression to a pressure of 110 bar abs,
from
where it is finally led via line 3 to a system 4 for further use of the 002.
The
cryogenic process can be optimized in that the purified 002-gas is fed in two
separate flows to the compressor system 15 at two different pressures
respectively, by which the compressor power consumption is minimized. One
first
low-pressure line feeds the purified 002 gas to the front inlet of the
compressor
system 15 and a second medium pressure line feeds the purified 002 gas to an
intermediate stage of the compressor system 15.
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Terms used in Figures
1 system for processing flue gases
2 flue gas line from power plant
3 line for purified CO2 gas
4 system for transport, storage or further use of purified CO2
5 adiabatic compressor
6 driver
7 flue gas line
8 heat exchanger
9 system for cooling medium
10 flue gas line
11 heat exchanger
12 system for cooling medium
13 flue gas line
14 low-pressure compressor for flue gas
15 high-pressure compressor for CO2 gas
16 shaft
17 driver for combined low- and high-pressure compressor
18 flue gas line
19 dehydration unit
20 cryogenic unit
21 line for inert gases
22 expander for vented inert gases
Cl adiabatic compressor
02 combined compressor machine
HR heat recovery system
