Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A PROCESS AND A PLANT FOR RECYCLING CARBON DIOXIDE
EMISSIONS FROM POWER PLANTS INTO USEFUL CARBONATED
SPECIES
FIELD OF THE INVENTION
The present invention relates generally to processes and apparatuses
used for energy production in fossil-fuel power plants. More particularly, it
concerns a process and a plant for the sequestration of carbon dioxide
emissions emanating from fossil-fuel power plants, and for the production
of useful carbonated species.
BACKGROUND OF THE INVENTION
Fossil-fuel power plants produce the main part of the energy actually
consumed worldwide. Energy is generated from. the combustion of fossil-
fuels such as coal, natural gas and fuel oil. The use of biomass to fuel the
power plant is also within the scope of this invention. Main exhaust gases
formed from such processes may be C02, SO2 and NOx depending on the
nature of the fuel used. Treatment systems are already available for
reducing SO2 and NOx emissions. However to date, CO2 emissions from
fossil-fuel power plants are generally not contained or reduced. These CO2
emissions thus contribute to increase the atmospheric concentration of
CO2, the most important greenhouse gas. It is known that such an increase
in greenhouse gases causes climate changes which could lead to various
environmental problems, such as an increase in violent weather events,
significant temperature warming in specific *areas, changes in the
precipitation pattern trends and a rise of ocean level.
Moreover, in the next century, a significant increase of carbon dioxide
concentrations is expected, unless energy production systems reduce their
emissions into the atmosphere. Carbon sequestration consisting of carbon
capture, separation and storage or reuse represents potent ways to
stabilize and eventually reduce concentration of atmospheric CO2.
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Several technologies, based on carbon sequestration, are being studied by
academic and industrial groups. These are: transformation by algae,
sequestration in oceans, storage in depleted oil and natural gas wells and
dissolution of pressurized CO2 in water tables. CO2 can also be
transformed into more geologically stable forms, such as calcium
carbonate.
Transformation of CO2 with algae involves the use of algal photosynthesis.
The gas emitted by power stations is thus directly introduced in basins
located nearby. The selected algae must therefore support these
environments with harsh conditions. The algae produced could be dried up
and used as fuel to supply the power station. This approach reduces the
required fuel to supply power, but does not eliminate CO2 production
completely.
Sequestration in oceans consists in pumping the carbon dioxide to be
disposed of to depths of 1,000 metres below sea level. The technique is
based on the fact that CO2 possesses a higher density than water. It is
believed that CO2 will sink to the bottom of oceans where lakes of liquid
carbon dioxide will be formed. However, as yet, the environmental impact
of this technology has not been demonstrated (US 6,475,460). Another
way is to bring carbon dioxide and seawater or fresh water into contact to
form carbon dioxide hydrate and sinking it in the seawater, fresh water or
geological formation under conditions for the stability of carbon dioxide
hydrate (CA 2,030,391, patent application US 2003/0055117, patent
application US 2003/0017088; US 6,254,667).
Oil and natural gas wells are capable of supporting enormous pressures
without leakage. They are therefore an ideal location for the storage of
compressed CO2 (patent application CA 2,320,216; US 6,598,398;
US 6,389,814; US 6,170,264). In the petroleum industry, the injection of
CO2 in wells to enhance oil recovery is a widely used technique. However,
this method only constitutes a temporary storage, since in the medium
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term, the displacements of the earth crust are capable of bringing about a
release of C02. Moreover, although there are hundreds of depleted sites
around the world, their total capacity is after all limited, and there is an
obligation to land case the geological formations involved.
The deep water tables are distributed throughout the globe. They generally
include salt water and are separated from the surface water tables which
constitute the drinking water supplies. The water contained in these natural
reservoirs can dissolve the pressurized CO2 and even disperse it in the
geological formations. However, the implementation of this technology
must always imply a strong concern regarding the proximity of the water
tables with the CO2 emission sources.
CO2 sequestration in solid carbonates and/or bicarbonates has already
been reported in Lee et al. (US 6,447,437). However, CO2 chemical
transformation into bicarbonate fertilizer requires methane, hydrogen and
nitrogen. Kato et al. (US 6,270,731) reported CO2 sequestration into
carbon powder. However, methane and hydrogen are required. Shibata et
al. (US 5,455,013) reported CO2 sequestration into CaCO3. However, the
chemical process enables C02 sequestration into CaCO3 only. Other
carbonates cannot be obtained.
Although some solutions have been proposed in the past for reducing C02
emissions in general, few of them have shown to be efficient or
commercially viable for different reasons. Moreover, a very few, if not none,
of the solutions proposed specifically apply to CO2 emissions from fossil-
fuel power plants. Thus, there is still a need for a solution for reducing
those CO2 emissions from fossil-fuel power plants. With the general
concern throughout the world with respect to the urgency of finding a
solution to the problem of emissions of greenhouse gases, this need is
even more obvious.
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SUMMARY OF THE INVENTION
An object of the present invention is to provide a process and a fossil-fuel
power plant that satisfy the above mentioned need.
Accordingly, the present invention provides a process comprising:
a) combustion of a fossil fuel, thereby generating heat and an exhaust
gas containing C02;
b) converting said heat into energy;
the process being characterized in that it comprises the steps of:
c) cooling at least a portion of the exhaust gas to produce a cooled
exhaust gas; and
d) reducing the amount of CO2 contained in the cooled exhaust gas by
biologically transforming said CO2 into carbonated species; thereby
obtaining a low CO2 exhaust gas, wherein step d) comprises:
contacting an aqueous liquid phase with the cooled exhaust gas to
cause at least a portion of the CO2 to dissolve into the aqueous
liquid phase and catalyzing the hydration of at least a portion of the
dissolved CO2 and producing a solution containing hydrogen ions
and carbonate ions, wherein said hydration is catalyzed by an
enzyme comprising carbonic anhydrase or an analogue thereof.
The present invention also provides a power plant for producing energy from
fossil
fuel and recycling carbon dioxide emissions into carbonated species, the plant
comprising:
a combustion unit for burning fossil fuel, thereby producing heat and an
exhaust gas containing C02;
means for converting the heat into energy;
the plant being characterized in that it comprises:
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means for cooling at least a portion of the exhaust gas to produce a cooled
exhaust gas;
means for contacting an aqueous liquid phase with the cooled exhaust gas
to cause at least a portion of the CO2 to dissolve into the aqueous liquid
phase;
biological catalyzing means for biologically transforming at least a portion
of
the dissolved CO2 into hydrogen ions and carbonate ions and producing a
solution containing the same, wherein the hydration is catalyzed by an
enzyme comprising carbonic anhydrase or an analogue thereof; and
precipitation means for precipitating carbonated species from the carbonate
ions.
The invention may comprise a step where the CO2 emissions from the fossil-fuel
power plant are transformed by means of a biological process into different
useful
carbonated species, such as calcium carbonate, a geological natural product.
More particularly, in accordance with the present invention, there is provided
a
process for recycling carbon dioxide emissions from a fossil-fuel power plant
into
useful carbonated species, which process primarily comprises the steps of: a)
combustion of a fossil fuel, thereby generating heat and a hot exhaust gas
containing CO2; and b) converting the heat into energy. The process is
characterized in that it further comprises the steps of: c) cooling the
exhaust gas;
and d) biologically transforming at least a portion of the CO2 contained in
the cooled
exhaust gas into carbonated species, thereby obtaining a low CO2 exhaust gas
and
producing useful carbonated species. The low CO2 exhaust gas obtained in step
d)
can be released in the atmosphere without increasing the problem of greenhouse
effect.
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By biological process, it is meant a process involving the activity of living
organisms.
More particularly, the step d) of biologically transforming the CO2 defined
above preferably comprises the steps of. catalyzing the hydration of at
least a portion of the CO2 contained in the exhaust gas, and producing a
solution containing hydrogen ions and carbonate ions. Then, metal ions are
added to the solution, and the pH is adjusted to precipitate a carbonate of
that metal. These metal ions are preferably selected from the group
consisting of calcium, barium, magnesium and sodium ions. More
preferably, Ca++ is used and the carbonate obtained is CaCO3.
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The hydration is catalyzed by a biocatalyst capable of catalyzing the
hydration of dissolved CO2 into hydrogen ions and bicarbonate ions. More
preferably, the biocatalyst is selected from the group consisting of enzyme,
cellular organelle, mammal cells and vegetal cells. Most preferably, the
5 biocatalyst is the enzyme carbonic anhydrase or an analogue thereof.
CO2 transformation takes place inside a bioreactor and is performed by a
biocatalyst which accelerates the transformation of CO2 into bicarbonate in
an aqueous environment. The bicarbonate can then be precipitated into a
stable solid product.
This invention thus proposes the integration of a CO2 transformation
process into a fossil-fuel power plant in order to produce bicarbonate
species which are useful by-products, and thereby reducing at the same
time the CO2 emissions. This CO2 transformation process is based on a
biological reactor which enables CO2 transformation into bicarbonate in an
aqueous environment. The CO2 is then precipitated into a stable solid
product, safe for the environment. As can be appreciated, in the present
invention, only water, a biocatalyst and a cation source are required for
carbon dioxide sequestration.
In accordance with a preferred aspect of the invention, step d) of
biologically transforming the CO2 comprises the step of: feeding liquid H2O
and at least a portion of the exhaust gas, preferably all, into a bioreactor
containing therein a reaction chamber filled with the biocatalyst. The
biocatalyst is optionally immobilized on solid supports packing the
bioreactor or in suspension in a liquid phase. In that latter case, the
biocatalyst may be either free in the aqueous liquid phase, immobilized on
solid supports or entrapped inside a solid matrix.
The present invention also provides a power plant for producing energy
from fossil fuel, and recycling carbon dioxide emissions into carbonated
species. The plant comprises a combustion unit for burning fossil fuel,
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thereby producing heat and an exhaust gas containing C02; and
conventional means for converting the heat into energy. The plant is
characterized in that it further comprises: means for cooling the exhaust
gas; and biological means for biologically transforming at least a portion of
the CO2 from the cooled exhaust gas into hydrogen ions and carbonate
ions, and means for precipitating carbonated species.
The biological means preferably comprises a bioreactor including a
reaction chamber filled with a biocatalyst capable of catalyzing the
hydration of dissolved CO2 into hydrogen ions and bicarbonate ions. The
reaction chamber preferably comprises:
- a liquid inlet for receiving an aqueous liquid;
- a gas inlet for receiving the cooled exhaust gas to be
treated;
- a gas outlet for releasing a low CO2 gas; and
a liquid outlet for releasing a solution containing carbonate
ions.
Also preferably, the precipitation means comprises a precipitation vessel,
wherein the bicarbonate ions can react with metal ions and precipitate a
carbonate of that metal. -
GENERAL DESCRIPTION OF THE INVENTION
CO2 production in a fossil fuel power plant
CO2 is produced during combustion of fossil fuels such as coal, natural gas
or fuel oil (Equation 1). For the purpose of the present invention, fossil-
fuel
power plant is also directed to power plants using biomass as the fuel. In
the case of a coal power plant, the heat released during this combustion is
used to heat water and produce steam which then passes through steam
turbines coupled to electric alternators leading to electricity generation. In
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the case of a natural gas power plant, the fuel is burned directly in gas
turbines coupled to electric alternators.
CxHy + (x+y/4) 02 x C02+y/2 H2O Equation 1
Other gases may also be produced by combustion, namely SO2 and NO5 ,
given the original sulphur and nitrogen content of the used fuel. These
other gases are encountered mainly in coal power plants.
Flue gas exhausting from combustion chambers and containing CO2 is
discharged directly to the atmosphere. In the context of this invention, CO2
emissions are treated and reduced by a biological process.
In the case of coal power plants, flue gas has first to be cooled in order to
have a temperature that does not lead to the ,denaturizing (free and/or
immobilized) of the biocatalyst. Gas cooling is obtained with any type of
heat exchanging device, and the recovered energy is preferably used to
increase the process efficiency. The heat could, for example, be used to
pre-heat the air required for combustion, or to supply energy for additional
turbines. The gas is then preferably treated to remove excess ash, SO2
and NOx, in order that the gas be of optimum quality for the biological
process. Ash can be removed using units such as electrostatic
precipitators and fabric filters. SO2 can be removed using scrubber units
and NOx using burners or catalytic systems leading to the conversion of
NOx to N2 and H2O. These units, which are used to remove ash, SO2 or
NOx, are already known in prior art and do not need further description.
CO2 transformation in a biological process
,Gas phase containing CO2 with appropriate level of ash, SO2, NOx and at
appropriate temperature and pressure, is then fed to the biological process,
enabling CO2 transformation into bicarbonate and hydrogen ions, and then
to useful carbonated species. This biological process is preferably
performed in a biological reactor where CO2 transformation takes place.
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This transformation is catalyzed by a biocatalyst accelerating CO2
transformation. The biocatalyst is a biological entity which can transform a
substrate in one or more products. The biocatalyst is preferably an
enzyme, a cellular organelle (mitochondrion, membrane, etc.), and animal,
vegetal or human cells. More preferably, the biocatalyst is the enzyme
carbonic anhydrase but may be any biological catalyst enabling CO2
transformation. CO2 transformation reaction is the following:
C02 +H20 HCO3+H+ Equation 2
This reaction is natural. It is at the basis of CO2 transportation and removal
phenomenon in the human body and in most living beings.
The biological catalyst may be free or immobilized inside the biological
reactor. An example of a bioreactor which could be used for biological
transformation of CO2 is described in "Process and Apparatus for the
Treatment of Carbon Dioxide with Carbonic Anhydrase" (Blais et al.)(CA,
2,291,785; W098/55210). In this process, carbonic anhydrase is
immobilized onto solid supports. Solid supports can be made of various
organic and inorganic material and have shapes proper to packed
columns. The gas phase containing CO2 enters at the bottom of the
packed column and the liquid phase enters at the top of the column. Both
phases flow counter currently and close contact of liquid and gas phases is
promoted by a solid support having immobilized enzymes on its surface.
Gaseous CO2 is then transferred to the liquid phase where it dissolves and
then is transformed according to Equation 2. The liquid flows in and out of
the column and is treated for precipitating the bicarbonate ions produced
by the bioreaction.
Another biological reactor with free and/or immobilized enzymes for CO2
transformation into bicarbonate is the following.
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The bioreactor consists of a chamber containing biocatalyst particles. The
gas to be treated enters at the bottom of the chamber. A diffusion system is
responsible for the uniform distribution of the gas phase at the bottom of
the chamber and is designed for minimum bubble diameter. These
conditions are required to optimize gas-liquid contact. An agitation device
(magnetic or mechanical) can also be used to assure uniform biocatalyst
distribution. Liquid phase enables gas dissolution and thus the biocatalytic
reaction. In this process, the biocatalyst (preferably carbonic anhydrase,
but may be any biological catalyst) is free in the liquid phase and/or
immobilized onto a solid support and/or entrapped inside a solid matrix.
These particles are moving freely in the liquid and are prevented from
exiting the chamber by a membrane or filter. The liquid flows in and out of
the chamber and is treated for precipitation of the bicarbonate ions
produced by the bioreaction.
As mentioned, bicarbonate ions produced in these two types of bioreactors
are preferably precipitated and finally sequestrated. This precipitation is
obtained by combining bicarbonate ions to cations. Cations used are
preferably calcium, barium, magnesium, sodium or any cation that could
lead to the formation of carbonate or bicarbonate salts. As shown in Figure
2, a potential source of cations is the reagent solution coming out of the
SO2 treatment unit. Bicarbonate ions can also be used directly in other
chemical or biological processes.
In summary, CO2 is to be transformed, for example into calcium carbonate,
in the biological process, according to the following reactions:
C02 dissolved + H2O H+ + HCO3
HCO3 = H+ + 0032"
0032 + Ca2+ CaCO3
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The coupling of the biological process for CO2 removal and transformation
to a fossil-fuel power plant leads to a reduction of CO2 emissions into the
atmosphere and an increase energy efficiency of the plant. Furthermore,
the required cooling of the flue gas enabling proper operation of the
5 bioreactor is coupled with energy recovery systems that produce additional
power output for the power plant.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a flow sheet of a preferred embodiment of the process
according to the invention, in the context of power plant processes.
10 Figure 2 is a flow sheet of a further preferred embodiment of the process
according to the invention, in the context of power plant processes.
MORE DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Figure 1 shows a flow sheet where a biological process is integrated to
energy generation processes.
In this diagram, the nature of the fossil fuel (10), either coal (12) or
natural
gas (14), used to power the plant leads to two different branches.
In the case of coal (12), the fuel is burned in a combustion chamber (16);
the heat (17) is ,used to produce steam from water in a heat recovery
steam generator system (18). The steam propels turbines and alternators
(20) producing electric power. The flue gas (22) exiting the combustion
chamber (16) is treated to remove ash, NOx and/or SO2 (23). In the current
configuration of power plants, the gas is finally exhausted by a stack (24).
In the context of this invention, the gas (26) is not exhausted at this stage,
but rather sent to additional heat exchangers and energy recovery systems
(28) to cool it down to an adequate temperature for the biological process.
Energy is produced by this step. The cooled gas (30) is then treated in a
gas treatment unit (32) to remove additional contaminants that may be
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harmful to the biological process, and finally, CO2 is removed by the
bioreactor (34) and the low CO2 gas (36) is blown to the atmosphere.
In the case of natural gas (14), the fuel (14) is burned directly in the
turbine
(38), and the intermediary step of steam production is not present in the
main power production stage, although it may be used in subsequent heat
recovery stages. The rest of the process is analog to that of the left branch
(coal).
Figure 2 is a flow sheet schematically showing the integration of the
biological process (32) to a SO2 treatment unit (40).
This diagram shows the cross-linking that may be performed between the.
biological process, which produces carbonate and/or bicarbonate ions (33),
and the SO2 treatment unit (40) present in the current power plant process.
To remove SO2 from the flue gas (42), a reagent solution (44) is required.
An analog solution is also required for the biological process (32). This
solution (44), readily available from either sub-processes, may be used in
closed loop for both processes.
Experimental results
The feasibility of treating flue gas from power plant by a biological process
has been demonstrated. The lab scale biological process enabled CO2
absorption and its transformation into CaCO3 The biological process was
performed with a 3 operation units each comprising a 3L-bioreactor
containing 2 L of packing material with immobilized carbonic anhydrase for
CO2 absorption. The units also included two ion exchange columns
required for recovering and concentrating the bicarbonate ions and a
precipitation and sedimentation vessel for formation and recovery of solid
CaCO3. The bioreactor used was similar to the one described in "Process
and Apparatus for the Treatment of Carbon Dioxide with Carbonic
Anhydrase" (Blais et al.)(CA 2,291,785; W098/55210), and was operated
at room temperature and atmospheric pressure. Gas phases with CO2
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concentrations ranging from 0.5 to 12% were efficiently treated with the
bioreactor. CO2 removal rate ranged from 1,47 x 10-4 to 4,5 x 10"3 mol
CO2/min. Bicarbonate ions produced were recovered and concentrated in
ion exchange columns. The removal of ions enabled the recycling of the
CO2 absorbent used in the bioreactor. A carbonate/bicarbonate rich
solution was obtained following regeneration of ion exchangers. A calcium
source, CaCl2 was added to the bicarbonate/carbonate rich solution,
conducting to the formation of precipitated calcium carbonate. A carbon
mass balance indicated that carbon dioxide removed from the gas was
recovered as precipitated CaCO3.
These results indicate that the biological process can be used to manage
CO2 emissions from power plants. Moreover, valuable products such as
CaCO3 are produced.
Although the present invention has been explained hereinabove by way of
preferred embodiments thereof, it should be understood that the invention
is not limited to these precise embodiments and that various changes and
modifications may be effected therein without departing from the scope or
spirit of the invention.
