Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS AND CONTRIVANCE FOR THE RECYCLING OF WASTE GAS FROM A GAS
TURBINE WITH DOWNSTREAM WASTE HEAT BOILER
[0001] The invention relates to a process for the recycling of waste
gas from a gas tur-
bine with downstream waste heat boiler, the said waste gas being metered to
the supply air
stream of a gas turbine in such a way that the temperature and the composition
of the waste
gas can be controlled and in this way highly concentrated carbon dioxide (002)
is obtained
which can be injected into a storage site so that the carbon dioxide balance
for the entire pro-
cess can be kept low or is negligible. By metered recycling of the waste gas
it is possible to
decrease the temperature in the gas turbine and to considerably increase the
carbon dioxide
content in the waste gas so that, after combustion and heat exchange, gas
scrubbing will be
possible and, on the one hand, the carbon dioxide can be recovered and, on the
other hand,
the content of free oxygen in the waste gas can be decreased. In another
embodiment of the
invention an oxygen-enriched gas is fed together with a fuel gas to a gas
turbine for combus-
tion and then diluted with waste gas so that the temperature can be kept low
despite oxygen
enrichment, and, after combustion and heat exchange, a highly concentrated
carbon dioxide
is obtained.
[0002] Many processes for the generation of energy use the combustion
of combustible
gases in a gas turbine which converts the direct combustion energy to
mechanical energy.
The hot waste gases are then cooled in a heat exchanger, with steam being
generated and
used, in turn, for driving a second turbine which also generates mechanical
energy. The me-
chanical energy can, in turn, be used for various purposes; it is frequently
used for driving
auxiliary units or for generating electric energy. Such processes which are
frequently used in
gas and steam power plants and operate by the principle of combined heat and
power gener-
ation are of high efficiency.
[0003] As a fuel gas for such processes all gases that are suitable for
driving gas tur-
bines can be used, which are ultimately gases which can be fed to the gas
chamber of a tur-
bine and do not produce any corrosive residues or combustion products during
combustion.
These are, for example, natural gas, refinery gases, biogases or synthesis
gases. Refinery
gases are particularly understood to be such gases that form during the
processing of liquid
fossil fuels, such as butane, hydrogenous gases or liquid gas, also referred
to as LPG (Lique-
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fied Petroleum Gas'). If, for example, synthesis gas is used, it can be
produced in any way. A
process for the production of synthesis gas is, for example, the coal
gasification where a fine-
ly ground carbonaceous fuel is gasified with an oxygenous gas in an entrained
flow gasifica-
tion. The synthesis gas thus produced can be used for driving gas turbines by
means of com-
bustion. To ensure the usability of the fuel gas in a gas turbine, gas
scrubbing is normally car-
ried out prior to combustion so that the fuel gas does not produce any
corrosive gases during
combustion and a cost-efficient service life of the gas turbine can be
achieved.
[0004] The temperature in the combustion of fuel gases in gas turbines
is normally up to
2200 C. After combustion the hot waste gas is fed to a waste heat boiler so
that the sensible
heat of the waste gas can be used for the recovery of steam. During combustion
carbon diox-
ide (CO2) and water (H20) form so that - except for these gases - the gas will
only contain ni-
trogen (N2) if the fuel gas is subjected to gas scrubbing prior to combustion.
If pure oxygen is
used for combustion, the waste gas virtually only contains carbon dioxide and
water.
[0005] Carbon dioxide is a greenhouse gas which contributes to global
warming. For this
reason, many countries aim at keeping the amount of carbon dioxide emitted
into the earth's
atmosphere at a low level. It is therefore technically feasible to design
processes in such a
way that they produce less or no carbon dioxide right from the beginning. As
the use of pure
hydrogen as fuel gas is normally not cost- efficient, efforts are made to
provide processes
which render a low or negligible carbon dioxide emission, which is normally
achieved by gas
scrubbing. This process serves to remove the carbon dioxide from the
combustion gases by
absorption of the carbon dioxide with the aid of an absorbing solvent. The
carbon dioxide is
then recovered during the regeneration of the absorbing solvent.
[0006] In order to avoid discharge of the carbon dioxide obtained from
gas scrubbing to
the atmosphere, the carbon dioxide can be compressed and injected into a
storage site. In
this way, this gas is permanently kept from entering the atmosphere. An
example of a pro-
cess for the re-injection of compressed carbon dioxide into a storage site is
given in
EP1 258595A2.
[0007] Even though such a re-injection of carbon dioxide into a storage
site keeps the
emission of carbon dioxide into the atmosphere low or at a negligible level,
it reduces the
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cost-efficiency of the process. The gas scrubbing for the removal of carbon
dioxide, the com-
pression of the carbon dioxide, possible transport of the compressed carbon
dioxide and the
re-injection into a storage site result in additional costs which have an
impact on the cost-
efficiency of the process. For this reason, efforts are made to keep the costs
for the additional
process steps required for the downstream processing of the carbon dioxide as
low as possi-
ble.
[0008] A starting point for this is to keep the composition of the waste
gas from a gas
turbine such that gas scrubbing requires as little effort as possible. This
means primarily to
keep the carbon dioxide content in the waste gas as high as possible so that
gas scrubbing
has to render only little enrichment. In addition, the oxygen content of the
waste gas to be
treated should be as low as possible because oxygen impairs the operability of
most absorb-
ing solvents. Many absorbing solvents used for the removal of carbon dioxide
by gas scrub-
bing contain amino groups which react with oxygen. For this reason, the
composition of the
waste gas from a gas turbine is important for the cost-efficiency of the
entire process.
[0009] It is therefore of advantage if a process for operating a gas
turbine with down-
stream heat recovery system produces a waste gas which has a high carbon
dioxide content
and a very low oxygen content (02) right from the beginning. In addition, the
content of nitro-
gen as ballast gas should be as low as possible. Other gases should also be
present in minor
quantities only. However, this is normally the case anyway if gas scrubbing is
carried out prior
to combustion and combustion is carried out stoichiometrically.
[0010] It is therefore the objective to provide a process which provides
a highest possible
carbon dioxide content in percent by volume and a lowest possible oxygen
content in percent
by volume. In addition, the process is to allow keeping the nitrogen content
in percent by vol-
ume at a low level.
[0011] The present invention achieves this objective by a process which
exists in two
embodiments which, to a certain extent, represent peripheral areas of a main
process step,
this main process step consisting in metering a part-stream of the cooled
waste gas leaving
the waste heat boiler to the combustion air of the gas turbine after heat
exchange so that a
higher content of carbon dioxide is obtained and, after combustion, heat
exchange for the re-
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covery of thermal energy and gas scrubbing are carried out, carbon dioxide
(CO2) being ob-
tained. This method, to a certain extent, represents a peripheral area, the
other peripheral ar-
ea consisting in avoiding gas scrubbing by using pure oxygen as oxidising
agent in the gas
turbine. As a result, only carbon dioxide and water are produced during
combustion so that
pure carbon dioxide (CO2) is obtained after condensation of the water.
[0012] The waste gas is metered to the combustion air of the gas
turbine in such a way
that as much waste gas as possible is recycled but combustion can nevertheless
be per-
formed easily. The latter is preferably controlled on the basis of measuring
parameters, a
measuring parameter consisting in the measurement of the combustion
temperature in the
gas turbine. Proper handling of this method will yield a waste gas which will
contain only little
oxygen. It is also possible to use an oxygen-enriched gas for the combustion
in the gas tur-
bine and to carry out gas scrubbing after heat recovery. In this case, the
oxygen content in
the waste gas is advantageously maintained at such a level that there is no
notable impair-
ment to gas scrubbing.
is [0013] In so doing, carbon dioxide is preferably obtained in high
concentration. It can be
pure or technically pure but can actually be of any concentration.
[0014] Claim is particularly laid to a process for the metered
recycling of cooled waste
gas from the waste heat boiler of a gas turbine by burning a fuel gas suitable
for combustion
with an oxygenous gas in a gas turbine so that mechanical energy is generated
and the
waste gas evaporates water in a waste heat boiler by indirect heat exchange so
that hot
steam is generated, and which is characterised in that a part-stream of the
cooled waste gas
is metered to the combustion air of the gas turbine after having left the
waste heat boiler, the
said waste gas being fed to the gas turbine for combustion, and another part-
stream of the
cooled waste gas is fed to a gas scrubber for the absorption of acid gases
after having left the
waste heat boiler, with carbon dioxide (CO2) being recovered from the said gas
scrubber.
[0015] Claim is also laid to a process for the metered recycling of
cooled waste gas from
the waste heat boiler of a gas turbine by burning a fuel gas suitable for
combustion with an
oxygenous gas in a gas turbine with an oxygen-enriched gas so that mechanical
energy is
generated and the waste gas evaporates water in a waste heat boiler by
indirect heat ex-
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change so that hot steam is generated, characterised in that a part-stream of
the cooled
waste gas is metered to the combustion air of the gas turbine after having
left the waste heat
boiler, and the other part-stream is cooled so that water condenses and carbon
dioxide (CO2)
is recovered.
5 [0016] Processes for the use of gas turbines including the
recycling of waste gas part-
streams are basically known from EP0453059B1 or JP4116232A. The latter,
however, do not
include the recovery of carbon dioxide and do not meter the recycled waste
gas.
[0017] The oxygen-enriched gas is preferably taken from an air
separation unit. Howev-
er, the said gas can also be provided by a pressure swing adsorption unit. The
oxygen-
enriched gas can actually be produced in any way desired. The use of an oxygen-
enriched
gas as oxidising agent in the gas turbine causes an increase of the carbon
dioxide content af-
ter combustion and a decrease of the nitrogen content in the waste gas. Gas
scrubbing is
thus simplified as the gas ballast of the nitrogen during gas scrubbing is
low. It will neverthe-
less be required if the nitrogen content in the carbon dioxide of the waste
gas is technically
existent. In one embodiment of the invention where oxygen-enriched combustion
air is used,
a part-stream of the cooled waste gas is fed to a gas scrubber for the
absorption of acid gas-
es after having left the waste heat boiler, with carbon dioxide (CO2) being
recovered from the
said gas scrubber. When using an oxygen-enriched gas as oxidising agent
combustion must
be done properly by metering cooled waste gas in order to keep the residual
oxygen content
in the combustion at a low level.
[0018] In one embodiment of the invention the oxygen-enriched gas is
pure oxygen, the
other part-stream obtained being cooled so that water condenses and carbon
dioxide (CO2) is
recovered. As in the case of the other embodiments the carbon dioxide may then
be com-
pressed and injected into the storage site. If pure oxygen is used, there is
no nitrogen content
in the waste gas. In this case there is no need for gas scrubbing.
[0019] The fuel gas for the gas turbine can be of any type as long as
it is suitable for
combustion in a gas turbine. In this context, it is particularly important
that during combustion
the fuel gas does not deliver any corrosive constituents which may affect the
turbine. In one
embodiment of the invention the fuel gas is synthesis gas.
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[0020] In another advantageous embodiment the synthesis gas is a
synthesis gas which
originates from a coal gasification reaction in which a finely ground
carbonaceous fuel gas is
gasified with an oxygenous gas in an entrained flow reaction. Coal
gasification reactions for
the production of synthesis gas are well known from the state of the art; an
exemplary em-
bodiment of a coal gasification reaction for the recovery of synthesis gas is
given in
EP061602261.
[0021] However, the fuel gas can also be natural gas. Prior to
combustion in a gas tur-
bine, it can be treated so that corrosive constituents and particularly
sulphur compounds are
removed. An example of natural gas treatment is given in EP920901B1. The
treated natural
gas is then used for firing the gas turbine.
[0022] In another embodiment of the invention the fuel gas is a
refinery gas. The treat-
ment of liquid fossil fuels frequently yields gases which can be used for the
heating of gas
turbines. Examples are LPG ("Liquefied Petroleum Gas"), propanes and butanes
and hydro-
gen. In an exemplary embodiment the latter can be added to the combustion gas
of a gas
turbine if it is intended to use the process embodying the invention.
[0023] In another embodiment of the invention the fuel gas is biogas.
This is a fuel gas
produced from biological raw materials such as wood, manure, straw or grasses.
These may,
for example, be obtained by fermentation but also, for example, by
gasification.
[0024] The carbon dioxide obtained can then be compressed and injected
into a carbon
dioxide storage site. Although this is the preferred embodiment within the
framework of the
invention, it is just as well conceivable to use the carbon dioxide for other
purposes or to use
a part-stream for re-injection into a storage site.
[0025] Metering of the cooled and recycled waste gas from a gas turbine
with waste heat
boiler is preferably done on the basis of measured values. This is typically
the temperature of
the waste gas from the gas turbine directly downstream of the gas turbine and
prior to enter-
ing the waste heat boiler. In one embodiment of the invention the portion of
the recycled gas
stream from the waste heat boiler and the amount of the part-stream metered
into the gas
turbine are hence controlled by the values measured for the temperature of the
waste gas
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from the gas turbine. This is a preferred embodiment but it is also feasible,
for example, to
measure the gas constituents in the waste gas and to meter the cooled and
recycled waste
gas on the basis of these measured values. Gas constituents suitable for
measurement are,
for example, carbon dioxide (CO2) or oxygen (02). Controlling is carried out
either manually or
by computer. Claim is also laid to a contrivance for running the process
embodying the inven-
tion, provided there is a corresponding interconnection of plant sections.
[0026] With the aid of the gas turbine, mechanical energy is generated
which can be
used for any purpose. It can, for example, be used for the generation of
electric power. The
thermal energy from the waste heat boiler can also be used for any purpose.
The latter can
preferably be used for the generation of steam and, via a turbine, for the
generation of electric
power. In the process embodying the invention actually as many turbines as
desired can be
used.
[0027] The invention has the advantage to provide treated carbon dioxide
(CO2) from a
gas turbine for compression and re-injection into a storage site, the cost
efficiency of the pro-
cess being improved by recycling a part-stream of the waste gas from the gas
turbine in gas
flow direction downstream of the waste heat boiler to the gas turbine and
metering it to the
combustion air so that the carbon dioxide content in the waste gas is
increased in such a way
that either gas scrubbing for the removal of carbon dioxide from the waste gas
can be carried
out in a cost efficient way or, in an ideal configuration, completely omitted
by using an oxy-
gen-enriched oxidising agent.
[0028] The invention is explained by means of two drawings showing
exemplary embod-
iments only and not being restricted to the latter.
[0029] FIG. 1 shows the process embodying the invention in which a first
part-stream of
the waste gas is recycled downstream of the waste heat boiler and metered to
the gas tur-
bine, and the second part-stream of the waste gas is fed downstream of the
waste heat boiler
to a gas scrubber for carbon dioxide. FIG. 2 shows the process embodying the
invention in
which a first part-stream of the waste gas is recycled downstream of the waste
heat boiler
and metered to the gas turbine which is heated with pure oxygen as oxidising
agent, and the
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second part-stream of the waste gas condenses downstream of the waste heat
boiler and is
used as pure carbon dioxide stream.
[0030] FIG.1 shows a gas turbine (1) heated with a carbonaceous fuel
gas (2) and com-
bustion air (3), the combustion air (3) being added via a mixing valve (4) and
mechanical en-
ergy being generated by the combustion in the gas turbine (1). The waste gas
(5) from the
gas turbine (1) is fed to a waste heat boiler (6) where the waste gas (5)
dissipates its sensible
heat via indirect heat exchange to water (6a) supplied and as a result steam
(6b) is generat-
ed. A part-stream of the waste gas (5a) is recycled and added to the
combustion air (3) via
the mixing valve (4). As a result, the carbon dioxide content in the waste gas
(5) increases. In
addition, the temperature of the combustion gas and the waste gas (5) is
lowered, this being
of non-impairing effect on the gas turbine (1). The other part-stream of the
waste gas (5b) is
fed to a gas scrubber (7) containing an absorbing solvent used to remove the
carbon dioxide
(CO2, 8) by scrubbing, a tail gas free of carbon dioxide (7a) being obtained.
This is recovered
during the regeneration (9) of the solvent and can be compressed and injected
into a storage
site. The recycled amount (5a) is metered by controlling the mixing valve (4)
on the basis of
the measurement of the temperature of the waste gas (5) with the aid of a
sensor (10) and is
controlled by a computer (10a).
[0031] FIG.2 also shows a gas turbine (1) heated with a carbonaceous
fuel gas (2) and
pure oxygen (11) from an air separation unit (11a) which separates the exhaust
air (3a) into
oxygen (11) and the residual air constituents (11b), with pure oxygen (11)
being added as ox-
idising agent via a mixing valve (4) and mechanical energy being generated by
the combus-
tion in the gas turbine (1). The waste gas (5) from the gas turbine (1) is fed
to a waste heat
boiler where the waste gas (5) dissipates its sensible heat via indirect heat
exchange to water
(6a) supplied and as a result steam (6b) is generated. A part-stream of the
waste gas (5a) is
recycled and added to the oxygen (11) via the mixing valve (4). As pure oxygen
(11) is used
as oxidising agent, the waste gas (5) contains only water (H20) and carbon
dioxide (CO2).
The second part-stream (5b) of the cooled waste gas is further cooled for
condensation (5c)
so that virtually pure carbon dioxide (8) is obtained after separation of the
condensed water
(5d). In addition, the temperature of the combustion gas and the waste gas (5)
is lowered by
the recycled amount, this being of non-impairing effect on the gas turbine
(1). The carbon di-
oxide (8) can be compressed and injected into a storage site. The recycled
amount is me-
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tered by controlling the mixing valve (4) on the basis of the measurement of
the temperature
of the waste gas (5) with the aid of a sensor (10) and is controlled by a
computer (10a).
[0032] List of reference numbers and designations
1 Gas turbine
2 Carbonaceous fuel gas
3 Combustion air
3a Air for the air separation unit
4 Mixing valve
Waste gas
5a First part-stream of the waste gas
5b Second part-stream of the waste gas
5c Cooler or condenser
5d Condensed water
6 Waste heat boiler or heat exchanger
6a Water
6b Steam
7 Gas scrubber
8 Carbon dioxide (CO2)
9 Regeneration unit
Temperature sensor
10a Computer
11 Oxygenous gaseous oxidising agent
lla Air separation unit
llb Residual air constituents
