Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND DEVICE FOR TREATING A CARBON DIOXIDE-CONTAINING GAS
STREAM
Summary of the Invention
The invention relates to a method for treating a carbon dioxide-containing gas
stream (raw gas stream), in particular a carbon dioxide-containing gas from a
large-scale
furnace plant. The precompressed raw gas stream is partially liquefied in a
cryogenic
carbon dioxide purification stage, and the liquid is separated off from which
a gas stream
having an elevated carbon dioxide content (carbon dioxide gas stream) is
obtained by
reevaporation. In addition, a gas stream having a reduced carbon dioxide
content (vent
gas stream) is obtained from the non-liquefied raw gas. This vent gas stream
is
expanded in at least one expansion turbine and the refrigeration generated in
this
process is recovered for cooling the raw gas stream. The carbon dioxide gas
stream is
compressed to a final pressure and fed to further utilization and/or storage.
The
invention also relates to an apparatus for carrying out the above-described
method.
Carbon dioxide-containing gas streams are produced in all large-scale furnace
plants which are operated with fossil fuels such as coal, oil or natural gas.
These
include, in particular, power plants, but also industrial furnaces, steam
kettles and similar
large-scale thermal plants for power and/or heat generation. In addition,
carbon dioxide-
containing gas streams are also formed in process plants of the chemical or
petrochemical industry, such as cracking furnaces of olefin plants or steam
reformers of
synthesis gas plants. Owing to the harmful climatic effect of carbon dioxide
gas,
solutions are being sought in order to decrease the emissions of carbon
dioxide-
containing exhaust gases into the atmosphere.
Very recently, novel power plant concepts are proposed in which the fossil
fuel,
e.g. coal, is burnt with an oxygen-rich combustion gas, in particular with
technically pure
oxygen, or with oxygen-enriched air (oxygen combustion gas method). The oxygen
fraction of this combustion gas is, e.g., 95 to 99.9% by volume. The resultant
exhaust
gas, which is also termed flue gas, contains principally carbon dioxide (C02)
at a fraction
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of approximately 70 to 85% by volume. The purpose of these novel concepts is
to inject
the carbon dioxide formed in the combustion of the fossil fuels and present in
concentrated form in the flue gas into suitable repositories, in particular
into certain rock
strata or salt water-bearing strata, and thereby to limit the emission of
carbon dioxide
into the atmosphere. The harmful climatic effect of greenhouse gases such as
carbon
dioxide is thereby reduced. Such power plants are termed in the specialist
field "oxyfuel"
power plants.
In the concepts known to date, a dedusting, denitrification and
desuiphurization
of the flue gas proceed in sequential steps. Subsequently to this flue gas
purification, the
carbon dioxide-rich exhaust gas thus treated is compressed and fed to a carbon
dioxide
purification stage. There, typically, a gas substream having a reduced carbon
dioxide
content and another gas substream having an elevated carbon dioxide content
are
generated by way of a cryogenic separation method. The gas substream having an
elevated carbon dioxide content is the desired carbon dioxide product stream,
which is
produced having a carbon dioxide content of, e.g., greater than 95% by volume
and is
provided for further use, in particular for transport to repositories. The gas
substream
having a reduced carbon dioxide content is produced as a subsidiary stream
(what is
termed vent gas) at 15 to 30 bar, preferably 18 to 25 bar, and contains
predominantly
the components not intended for the injection, in particular inert gases such
as nitrogen
(N2) and argon (Ar) and oxygen (02). However, in this gas substream, fractions
of
carbon dioxide are also still present at a concentration of approximately 25
to 35% by
volume. This vent gas is currently blown off into the atmosphere.
Usually, the raw gas stream is precompressed to a desired pressure in upstream
plant parts and dried, e.g., in adsorber stations. This means that the vent
gas also is first
still present in the compressed state. At present this pressure level is
reduced by
expansion valves.
In EP 1952874 Al and EP 1953486 Al (Air Products) it has already been
proposed, after warming the vent gas and further heating by means of waste
heat from
the compression, to carry out a turbine expansion of the vent gas stream.
However,
utilization of the energy liberated in the turbine expansion, in particular of
the
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refrigeration capacity produced in the expansion process, is not envisaged in
this case.
In DE 102009039898 Al (Linde), for improving the energy efficiency, it is
proposed that the vent gas stream is expanded in at least one expansion
turbine and
both the resultant kinetic energy and also the refrigeration generated are
utilized for
energy recovery. For utilizing the kinetic energy, the expansion turbine can
be coupled to
a compressor (booster) which compresses the raw gas stream and/or the carbon
dioxide
gas stream. For utilizing the refrigeration generated in the expansion, the at
least
partially expanded vent gas stream can be brought into heat exchange with
process
streams that are to be cooled, e.g. the raw gas stream and/or the carbon
dioxide gas
stream.
The carbon dioxide gas stream is usually compressed by way of a final
compressor to the required final pressure of above 80 bar (preferably 120 to
150 bar) for
transport and subsequent sequestration.
Alternatively, for the carbon dioxide gas stream compression, liquefaction of
the
separated-off carbon dioxide-rich gas with subsequent pressure elevation by
way of
pumps is also possible. Here, however, the use of refrigerant is necessary.
When
external refrigeration from a refrigeration plant is used, a liquid carbon
dioxide pure
product is already present after the cryogenic separation, which liquid carbon
dioxide
pure product can be brought to the necessary final pressure by way of a pump.
However, the use of a refrigeration plant (external refrigeration) increases
the necessary
energy consumption.
An object of the present invention is to design a method of the type mentioned
at
the outset and also an apparatus for carrying out the method in such a manner
that the
energy efficiency can be further improved.
Upon further study of the specification and appended claims, other objects and
advantages of the invention will become apparent.
These objects are achieved in terms of the method in that, from some of the
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liquid that is separated off in the cryogenic carbon dioxide purification
stage, a liquid
stream having an elevated carbon dioxide content (carbon dioxide liquid
stream) is
obtained which is fed in a liquid phase to further utilization and/or storage.
Using the invention, an energy-sparing operation of the carbon dioxide
purification stage is made possible without using external refrigeration from
an external
refrigeration plant. The refrigeration necessary for cooling and partial
condensation of
the raw gas stream can be provided via heat exchange with the evaporating
liquid
forming the carbon dioxide gas stream and also by heat exchange with the vent
gas
stream that is cooled by expansion in the expansion turbine. By branching off
a carbon
dioxide liquid stream, the final compression of the carbon dioxide gas stream
can be
relieved, whereby, in total, an improvement of the energy efficiency is
achieved.
Furthermore, in this manner an additional liquid carbon dioxide product can be
provided
without further energy consumption.
It has proved in this case that operation of the carbon dioxide purification
stage
without an external refrigeration plant is expedient when the carbon dioxide
liquid stream
amounts to 5 to 25%, preferably 10 to 15%, of the total liquid that is
separated off in the
cryogenic carbon dioxide purification stage.
According to a particularly advantageous embodiment of the invention, the
carbon dioxide liquid stream is fed to the carbon dioxide gas stream after
compression
thereof to the final pressure. In this case the carbon dioxide liquid stream
is expediently
compressed to the final pressure by way of a liquid pump before it is fed to
the carbon
dioxide gas stream.
Another variant of the invention provides that the carbon dioxide liquid
stream is
temporarily stored in a liquid gas tank for further use, in particular in the
food industry.
Customers' wants can thereby be met by an additional liquid carbon dioxide
product
without additional energy expenditure and without the use of an external
refrigeration
plant.
A further possibility of use is in the use of the carbon dioxide liquid
stream, after
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evaporation, as transport medium for the pneumatic transport (i.e.,
entrainment of solid
particles, such as coal dust, by a gas stream) or as a lock gas of feedstocks,
in particular
coal dust, in large-scale furnace plants. For example, during preparation of
combustible
material (coal/lignite) for coal fired power plants, contact with oxygen has
to be
5 minimized. Carbon dioxide is possible gas to use for displacing air, thereby
acting as a
lock gas.
For utilization of the kinetic energy, the expansion turbine can also be
coupled to
at least one compressor (booster), in such a manner that the expansion turbine
compresses the raw gas stream and/or the carbon dioxide product stream during
the at
least partial expansion of the vent gas stream. For utilization of the
refrigeration
generated in the expansion, the at least partially expanded vent gas stream is
preferably
brought into heat exchange with process streams that are to be cooled, e.g.
the raw gas
stream and/or the carbon dioxide product stream. By expansion of the vent gas,
process-internal refrigeration output can be provided and external
refrigeration can
thereby be spared.
The invention further relates to an apparatus for treating a carbon dioxide-
containing gas stream (raw gas stream), in particular from a large-scale
furnace plant,
having a carbon dioxide purification appliance that is charged with the
precompressed
raw gas stream. The carbon dioxide purification appliance comprises an outlet
line for a
gas stream having an elevated carbon dioxide content (carbon dioxide gas
stream) and
an outlet line for a gas stream having a reduced carbon dioxide content (vent
gas
stream). The outlet line for the carbon dioxide gas stream is connected via a
final
compressor to a utilization appliance and/or repository, whereas the outlet
line for the
vent gas stream is connected to at least one expansion turbine which comprises
an
outlet line for the at least partially expanded vent gas stream. The outlet
line for the at
least partially expanded vent gas stream is connected to a heat-exchange
appliance
which is chargeable with the precompressed raw gas stream, the carbon dioxide
gas
stream and the vent gas stream.
The objects are achieved in terms of the apparatus in that the carbon dioxide
purification appliance additionally comprises an outlet line for a liquid
stream having an
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elevated carbon dioxide content (carbon dioxide liquid stream). This outlet
line
bypasses the heat-exchange appliance and the final compressor and is connected
directly to a utilization appliance and/or storage appliance for liquid having
an elevated
carbon dioxide content.
Preferably, the outlet line for the carbon dioxide liquid stream comprises a
liquid
pump and, downstream of the final compressor, the carbon dioxide liquid stream
is
introduced into the outlet line for the carbon dioxide gas stream.
The invention is suitable for all conceivable large-scale furnace plants in
which
carbon dioxide-containing gas streams are produced. These include, e.g.,
fossil-fuel-
fired power plants, industrial furnaces, steam kettles and similar large-scale
thermal
plants for power and/or heat generation. The invention can be used
particularly
advantageously in large-scale furnace plants which are supplied with
technically pure
oxygen or oxygen-enriched air as combustion gas and in which, accordingly,
exhaust
gas streams having high carbon dioxide concentrations are produced. In
particular, the
invention is suitable for what are termed low-CO2 coal power plants which are
operated
with oxygen as combustion gas ("oxyfuel" power plants) and in which the carbon
dioxide
that is present in the exhaust gas in high concentration is separated off and
injected
below ground ("CO2 capture technology").
The invention is associated with a large number of advantages.
The final compressor for the carbon dioxide gas stream is relieved, which
leads
to energy savings and also to a reduction of capital costs. The demands on the
operation of the final compressor are reduced so, for example, a smaller
compressor
can be used thereby reducing capital costs. In addition, the energy balance at
the heat-
exchange appliance is optimally utilized. In some circumstances, the intake
temperature
at the final compressor can be adjusted in such a manner that simpler
materials (no
high-alloy steels) can be used. Furthermore, a prepurified liquid carbon
dioxide product
can be provided from the system which can be utilized, for example, for
treatment to give
a food-specific carbon dioxide product (external utilization) or else also as
liquid store in
a tank system.
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The carbon dioxide liquid stream can also be used for other applications (e.g.
treatment to give purified seal gas for the oxyfuel process, use as transport
medium for
the pneumatic transport of coal dust in the oxyfuel process, storage of liquid
carbon
dioxide for use as start-up gas or charge gas after evaporation).
When multistage compression is used to bring the pressure of the carbon
dioxide
gas stream to the required final pressure, the carbon dioxide liquid stream,
after a
supercritical compression of the carbon dioxide gas stream (> 72 bar), can be
fed (in the
supercritical state) upstream of the suction side of the next-following
compressor stage
(or pump). The temperature falls and the density increases thereby. Owing to
the higher
density, the energy requirement of the subsequent compressor stages/pumps for
achieving the required final pressure falls.
Brief Description of the Drawings
The invention and further embodiments of the invention are described in more
detail hereinafter with reference to working examples shown schematically in
the figures,
in comparison with the previous prior art. Various other features and
attendant
advantages of the present invention will be more fully appreciated as the same
becomes
better understood when considered in conjunction with the accompanying
drawings, in
which like reference characters designate the same or similar parts throughout
the
several views, and wherein:
Figure 1 shows a block diagram of a carbon dioxide treatment plant with
expansion of the vent gas via an expansion turbine as per the prior art
according to EP 1952874 Al;
Figure 2 shows a block diagram of a carbon dioxide treatment plant with
expansion of the vent gas via expansion turbines with energy recovery as
per the prior art according to DE 102009039898 Al;
Figure 3 shows a block diagram of a carbon dioxide treatment plant with
removal of a separate carbon dioxide liquid stream with subsequent
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compression by way of a liquid pump to the required final pressure for
pipeline transport; and
Figure 4 shows a block diagram of a carbon dioxide treatment plant with
removal of a separate carbon dioxide liquid stream with subsequent
temporary storage for external use.
Figure 1 shows a conventional treatment of a carbon dioxide-containing raw gas
stream from a coal power plant as per the prior art according to EP 1952874 Al
(Air
Products). The raw gas stream, after compression in the raw gas compressor 1,
is fed
via a heat-exchange unit 2 to a primary separator 3 for separating off carbon
dioxide.
The vent gas from the primary separator 3 is introduced into the heat-exchange
unit 2
and then fed to a secondary separator 4. The carbon dioxide product stream is
withdrawn, respectively, from the bottoms of primary carbon dioxide separator
3 and
secondary separator 4, introduced into the central heat-exchange unit 2 (and
in the case
of the vent gas from the primary separator 3 additionally via a CO2 product
compressor
7), and then subjected to a final compression 8 in order finally to be fed via
a pipeline
(carbon dioxide pipeline) 9, e.g. to an injection below ground. The vent gas
of the
secondary separator 4 is withdrawn from the top of the secondary separator 4,
likewise
introduced into the central heat-exchange unit 2 and finally, downstream of a
further
warming in the heat exchanger 5, expanded via a turbine 6 in order to be
delivered to
the atmosphere.
In contrast to the method shown in Figure 1 for carbon dioxide treatment, the
method according to DE 102009039898 Al (Linde), shown in Figure 2, offers the
advantage of energy recovery in the expansion of the vent gas. In this method,
as in the
method shown in Figure 1, two carbon dioxide separators 3 and 4 and also a
central
heat-exchange unit 2 are provided. In contrast to the method as per Figure 1,
however,
simple expansion of the vent gas via a single turbine does not occur, but
instead a
stepwise expansion via two expansion turbines 5 and 6 is performed. By way of
the
stepwise expansion of the vent gas stream, the formation of solid carbon
dioxide in the
vent gas can be prevented. Downstream of the expansion in the first expansion
turbine
5, the vent gas stream is warmed in the central heat-exchange unit 2 and then
expanded
further to close to atmospheric pressure in the second expansion turbine 6 and
again
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warmed in the central heat-exchange unit 2. The available pressure level of
the vent gas
can thereby be completely exploited. The vent gas that is cold after the
expansion is
warmed in the central heat-exchange unit 2 against the process streams that
are to be
cooled. The vent gas thereby provides some of the refrigeration capacity
necessary in
the process.
Figure 3 shows a carbon dioxide treatment according to the invention. The
process procedure differs from that shown in Figure 2 in that some of the
liquid carbon
dioxide separated off in the primary separator 3 is branched off and fed via a
carbon
dioxide liquid pump 9 downstream of the final compressor 8 to the CO2 pipeline
10. In
this procedure the carbon dioxide liquid stream is brought to the required
final pressure
for the pipeline transport by way of the carbon dioxide liquid pump 9. Since
this carbon
dioxide liquid stream by passes the final compressor 8, the operational
demands on the
final compression 8 are reduced thereby increasing energy efficiency and
reducing
capital costs. The energy balance at the heat-exchange unit 12 can be used
optimally.
The intake temperature at the final compressor 8 can be adjusted in such a
manner that
simpler materials (e.g. no high-alloy steels) can be used.
In the variant of the invention shown in Figure 4, the liquid carbon dioxide
that is
separated off from the primary separator 3 is first temporarily stored in a
liquid carbon
dioxide tank 11. The liquid carbon dioxide can be fed, as required, from tank
11 via the
liquid pump 9 to the CO2 pipeline 10 and/or loaded into a transport vehicle 12
for further
external utilization (e.g. in the food industry) and/or evaporated in a CO2
evaporator 13
and provided for internal utilization (e.g. as start-up or feed gas).
The entire disclosure[s] of all applications, patents and publications, cited
herein
and of corresponding German Application No. 10 2011 014 678.4, filed March 22,
2011,
are incorporated by reference herein.
The preceding examples can be repeated with similar success by substituting
the
generically or specifically described reactants and/or operating conditions of
this
invention for those used in the preceding examples.
