Patent Application
20080302133
The present invention relates to a method and a system for capturing the carbon dioxide (CO2) contained in flue-gas or in other gaseous effluent coming from industrial installations.
Capturing carbon dioxide and storing it geologically presents an opportunity for reducing the emission of greenhouse-effect gases, in addition to efforts at improving energy efficiency and inciting the use of non-fossil resources.
At present, the cost of capturing CO2 constitutes about three-quarters of the total cost of the system for geologically sequestering CO2, including capturing, transporting, and storing CO2.
Furthermore, energy consumption represents about 50% of the cost of capture.
In particular, amongst the technologies that can be envisaged for capturing CO2, low-temperature or “cryogenic” distillation suffers from the drawback of consuming a large amount of energy in order to achieve low temperatures.
There thus exists a need to rationalize the operation of CO2 capture in order to improve the effectiveness of that operation and reduce its cost.
The term “anti-sublimation” is used below to designate the physical phenomenon of solid condensation whereby a gas changes state and passes directly into the solid phase without liquefaction taking place, i.e. without passing via the liquid state. Anti-sublimation thus constitutes the physical phenomenon that is the inverse of sublimation which designates a body passing directly from the solid state to the gaseous state without passing via the liquid state.
Various methods and systems for capturing CO2 by liquefaction or anti-sublimation have already been proposed.
When carbon dioxide is at a partial pressure of more than 5.18 bar, it is possible to obtain direct liquefaction of CO2 by cooling flue-gas. That type of method nevertheless presents the drawback of requiring effluents to be available under pressure or flue-gas to be compressed.
When CO2 is available at a partial pressure of less than 5.18 bar, as is true of combustion gas, cooling the flue-gas will lead to anti-sublimation of CO2. The solid carbon dioxide can then be handled in solid form, e.g. after being separated in a cyclone, or it can be sublimed prior to being subsequently liquefied downstream, or else it can be melted directly merely by being heated.
Document EP 1 355 716 B1 and patent application WO 2004/080558 disclose a method of extracting CO2 from flue-gas by cooling and solidifying CO2 at atmospheric pressure by extracting heat by means of fractioned distillation.
Nevertheless, in some circumstances, in particular for flue-gas coming from a gas turbine or a gas boiler, the CO2 content of the flue-gas can be quite low, e.g. about 1% to 5%, which implies a starting temperature for anti-sublimation at atmospheric pressure that can be of the order of −110° C. to −120° C.
Furthermore, this CO2 content in flue-gas decreases with increasing target capture ratio.
When forming a solid from a gaseous phase it is also observed that anti-sublimation is delayed at low temperature and at low partial pressure, in particular with CO2. This phenomenon gives rise to anti-sublimation of the compound at a temperature lower than the temperature specified by thermodynamics.
Because of this phenomenon, in order to anti-sublime CO2, it is therefore necessary either to cool the compound below its thermodynamic anti-sublimation temperature, which amounts to having an even lower operating temperature in the heat exchanger, or else to increase the heat exchange surface area, thereby increasing the amount of contact between the compound and cold surfaces. Both of those two conditions increase the cost of the method.
The present invention seeks to remedy the above-mentioned drawbacks and to enable CO2 to be captured in a manner that is effective, but at reduced cost, and with installations that are simplified, and with efficiency that is potentially improved.
According to the invention, these objects are achieved by a method of capturing the carbon dioxide present in flue-gas, the method comprising the following steps:
a) first cooling of the flue-gas in order to eliminate a fraction of the water present therein by condensation;
b) dehydrating the flue-gas in order to eliminate the residual water;
c) second cooling of the flue-gas by heat exchange so as to bring it to a temperature such that carbon dioxide passes directly from the gaseous state to the solid state by anti-sublimation;
d) after removing the flue-gas, heating the solidified carbon dioxide in a closed enclosure up to the triple point where a liquid phase appears; and
e) drawing off or pumping out the liquid and/or gaseous carbon dioxide to a thermally insulated tank;
the method being characterized in that a fraction of the recovered liquid carbon dioxide is recycled, and after expanding to atmospheric pressure, is reinjected during the second cooling step in continuous or intermittent manner in order to be mixed with the flue-gas previously delivered by the dehydration step.
In order to combat the delay in anti-sublimation, it is possible to encourage the kinetics of the anti-sublimation process by injecting CO2 crystals that perform a seeding function for solid formation. Such fine solid particles constitute nucleation centers on which gaseous CO2 solidifies. The reinjection can take place within the heat exchanger, starting from the point where the temperature of the flue-gas is close to the theoretical anti-sublimation temperature.
The recovered fraction of the liquid carbon dioxide that is recycled is thus reinjected during the second cooling step, preferably in the form of fine solid particles, and also preferably into the inside of a heat exchanger.
In a particular implementation, the method further comprises a step of pre-cooling the flue-gas prior to said first cooling step, the pre-cooling step being performed by exchanging heat with at least one of the fluids comprising the liquid water recovered during the first cooling step and the flue-gas dehydration step, and the non-condensable compounds from the flue-gas that are recovered after said second cooling step.
In a particular advantageous implementation, the first cooling of the flue-gas, the dehydration of the flue-gas, and the second cooling of the flue-gas make use of heat exchange with the flue-gas via cooling loops operating by heat exchange with the liquefied natural gas (LNG) that is present in a methane terminal for regassification, and that is used as a cold source.
According to a particular characteristic, the first cooling of the flue-gas and the dehydration of the flue-gas are performed by exchanging heat with the flue-gas via at least one cooling loop making use of glycol-containing water.
According to another particular characteristic, the second cooling of flue-gas makes use of heat exchange with the flue-gas via at least one cooling loop making use of methane or of nitrogen.
Under such circumstances, according to yet another particular characteristic, the second cooling of flue-gas makes use of heat exchange with the flue-gas via at least one additional cooling loop making use of ethylene or ethane.
When LNG is available at a methane terminal for regassifying natural gas, making use of the LNG as a cold source is particularly advantageous since the low temperature of the LNG is thus used advantageously for purposes of industrial and energy optimization, with the flue-gas from which the CO2 is to be extracted then constituting the hot source for intermediate-fluid heat exchangers that enable the LNG that is stored in liquid form at −161° C. and at a pressure of 80 bar to be regassified.
When an industrial installation that gives off CO2, such as a fossil fuel fired power station is located close to a methane terminal in which LNG is regassified, it is therefore entirely appropriate to make use of the LNG as a cold source in the operation of capturing CO2 from the combustion gas or from gaseous effluents, by anti-sublimation at approximately atmospheric pressure, followed by melting at a pressure of a few bar, which can be obtained merely by heating the solid CO2.
The invention is applicable to any flue-gas from power stations and other thermal installations (steel works, cement works, . . . ) that make use of a variety of fossil fuels (natural gas, coal, oil, . . . ) and that contain various concentrations of CO2, even concentrations that are low and less than 1%.
Furthermore, by using various cooling loops involving different fluids and producing staged cooling of the flue-gas, it is possible to reduce very significantly the dimensions of the pipes in the final cryogenic loop that involves very low temperatures (e.g. using nitrogen), even if the distance between the methane terminal and the CO2 capture installation is several hundreds of meters or several kilometers.
The method of the invention may present various other advantageous characteristics depending in different particular implementations:
Flue-gas dehydration includes a step of exchanging heat with the non-condensable compounds of the flue-gas recovered after the second cooling step.
The flue-gas dehydration step is performed discontinuously, alternating between a step of cooling the flue-gas to solidify water on the walls of a heat exchanger, and a step of heating the solidified water in order to enable it to be recovered in liquid form.
The solidified water is heated by exchanging heat with flue-gas, prior to gas being cooled during the step of cooling flue-gas with solidification of water.
The second flue-gas cooling step for giving rise to anti-sublimation of carbon dioxide, and the step of heating the solidified carbon dioxide are performed discontinuously and in alternation.
The step of heating the solidified carbon dioxide up to the triple point where a liquid phase appears can be performed by exchanging heat with the flue-gas prior to cooling it during the second flue-gas cooling step.
The method further includes a step of recovering sulfur oxides contained in the flue-gas by anti-sublimation and by heating up to the triple point where a sulfur oxide liquid phase appears.
The invention also provides a system for capturing carbon dioxide present in flue-gas, the system comprising:
a) first cooler means for cooling flue-gas and comprising at least one heat exchanger for eliminating a fraction of the water present in the flue-gas by condensation;
b) flue-gas dehydration means;
c) second cooler means for cooling flue-gas and comprising at least one heat exchanger for bringing the flue-gas to a temperature that causes anti-sublimation of the carbon dioxide present in the flue-gas;
d) heater means for heating the solidified carbon dioxide in a closed enclosure in order to cause it to melt; and
e) means for drawing off or pumping the liquid and/or gaseous carbon dioxide to a thermally insulated tank;
the system being characterized in that it further comprises expander means for expanding a fraction of the recovered liquid carbon dioxide to atmospheric pressure and for reinjecting said fraction of the carbon dioxide into the flue-gas at said second flue-gas cooler means. The system preferably further includes means for reinjecting said recovered fraction of the liquid carbon dioxide in the form of fine solid particles into the flue-gas.
In a particular embodiment, the first cooler means comprise a heat exchanger between the flue-gas and at least one of the fluids comprising the liquid water recovered in the first cooler means or in the flue-gas dehydration means, and the non-condensable compounds of the flue-gas recovered at the inlet to the second cooler means.
In an advantageous application, the system of the invention includes cooling loops using heat-transferring fluids flowing firstly in heat exchangers present in a methane terminal for exchanging heat with the liquefied natural gas subjected to a regassification process, and secondly in heat exchangers placed in at least one of the first cooler means, the dehydration means, and the second cooler means in order to exchange heat with the flue-gas, giving rise to capture of carbon dioxide.
According to a particular characteristic, the system includes at least one cooling loop using glycol-containing water as its heat-transferring fluid and including at least one heat exchanger disposed in the first cooler means to the dehydration means.
According to another particular characteristic, the system includes at least one cooling loop using methane or nitrogen as its heat-transferring fluid and including at least one heat exchanger disposed in the second cooler means.
Under such circumstances, in a particular embodiment, the system may further comprise at least one cooling loop using ethylene or ethane as its heat-transferring fluid and including at least one heat exchanger disposed in the second cooler means.
The system may comprise means for recovering non-condensable compounds from the flue-gas at the outlet from the second cooler means, and means for exchanging heat with at least one of the flue-gas dehydration means.
In a particular embodiment, the flue-gas dehydration means comprise at least first and second enclosures provided with heat exchangers and capable of receiving flue-gas discontinuously so that each of them can act in turn to cool flue-gas and solidify the water contained therein on the walls of the corresponding enclosure, and to heat the solidified water in order to enable it to be recovered in liquid form.
According to another particular aspect of the invention, the second flue-gas cooler means and said heater means comprise at least first and second enclosures provided with heat exchangers and capable of receiving flue-gas discontinuously in such a manner that each of them in turn cools flue-gas with anti-sublimation of the carbon dioxide that is deposited on the walls of the corresponding enclosure, and heats the solidified carbon dioxide in order to cause it to melt.
The system may also include means for recovering sulfur oxides in the heater means in a closed enclosure.
Other characteristics and advantages of the invention from the following description of particular embodiments, given with reference to the accompanying drawings, in which:
An embodiment of the present invention is described with reference to
Nevertheless, reference is made initially to
As mentioned above, this phenomenon is applicable to the flue-gases from fuel-burning installations and power stations that make use of a variety of fossil fuels (natural gas, coal, oil, . . . ), the flue-gases containing varying concentrations of CO2 that may lie in the range less than 1% to concentrations of several tens of percent.
Table 1 gives examples of typical compositions for gas turbine flue-gases.
As can be seen, after water has been eliminated, the flue-gas contains about 4% CO2 (at a partial pressure of 0.04 bar).
The flue-gas can then be cooled in a heat exchanger until CO2 condenses as from −110° C. (see horizontal dashed line in
The solid CO2 trapped in the heat exchanger can then be heated and taken to the conditions of the CO2 triple point where liquid CO2 can be eliminated so as to move the thermal equilibrium in favor of producing liquid CO2 (see top dashed-line curve in
The temperature at which the anti-sublimation process begins depends on the CO2 content of the flue-gas. Thus, it varies over the range −78.5° C. for pure CO2 at atmospheric pressure, to −121.9° C. for effluent containing CO2 at a partial pressure of 0.01 bar (see Table 2).
In accordance with the invention, a fraction of the liquid CO2 obtained at the outlet from the method is recycled by being expanded until fine solid particles are formed in the last heat exchanger so as to create nucleation centers.
This recycling enables the anti-sublimation heat exchanger to be optimized in terms of exchange area and final operating temperature.
In the example of
The invention applies to capturing CO2 from the combustion gas, however it can also be applied to capturing CO2 from other gaseous effluents, for example synthesis gas obtained in a hydrogen-production context.
The invention also makes it possible, by using the same anti-sublimation method, to capture sulfur oxides (SOx) that might be present in the flue-gas together with the CO2.
In the
The flue-gas delivered via the line 101 is cooled in several stages.
In a cooling device 120 comprising a heat exchanger 122, with a cooling loop 210, e.g. glycol-containing water, the combustion gas is cooled from 40° C. to 1° C. so as to cause a fraction 123 of the water present in the gas to condense as a liquid (liquefaction) in the enclosure 121.
The condensed water 123 is removed via a pipe 124 and may be sent to a heat exchanger 111, for example, in order to perform pre-cooling of the flue-gas prior to its entry into the cooling device 120. The water heated in the heat exchanger 111 serves to bring the flue-gas temperature to 30° C., for example, and it is then removed via a line 113 at a temperature that is close to ambient (30° C.).
The flue-gas coming from the cooling device 120 is introduced via a line 125 into a gas dehydration device 130.
It is necessary to dehydrate the gas in order to eliminate residual water (about 0.6% water in the gas, assuming that the vapor pressure of water at 1° C. is 6.6 millibar (mbar)).
The residual water can solidify and might therefore block the installation downstream, and might then be found in the captured CO2.
The dehydration operation can also be performed by using the LNG from the methane terminal 200 as a cold source, e.g. in a heat exchanger 133 possibly suitable for being inserted in the same cooling loop 210 (e.g. using glycol-containing water) as the heat exchanger 122.
The residual water can thus be solidified, e.g. at −30° C., on the walls of at least one (131) of the two enclosures 131, 132 that operate discontinuously in alternation (i.e. in batch mode).
When the water has solidified on the walls of one of the enclosures 131, the gas is switched to the inlet of the other enclosure or of one of the other enclosures 131 so as to cause residual water to solidify in the same manner. At the same time, the water that has solidified on the walls of the first enclosure 131 is heated, e.g. making use of the heat in the gas by causing it to pass through the first enclosure 131 prior to penetrating into the second enclosure 132 where water capture is to take place.
This discontinuous operation in alternation of the enclosures 131 and 132 is shown in greater detail in
In
In the enclosures 131, 132, it is possible to use a heat exchanger 134 as described above, forming part of a heat exchange cooling loop, e.g. using the LNG of a methane terminal as the cold source. It is also possible to make use of a heat exchanger 134 having non-condensable gas (nitrogen, oxygen, . . . ) flowing therethrough and recovered from the gas at the outlet 145 of the CO2 capture process.
The non-condensable gas can likewise subsequently be transferred to a heat exchanger 112 acting, like the heat exchanger 111, to pre-cool and eliminate condensed water in a stage 110 situated upstream from the cooler device 120 and the dehydration device 130. The residual non-condensable gas (O2, N2, . . . ) can then be rejected into the atmosphere via a line 114 at a temperature of about 30° C. (
The liquid water removed by the tubes 136 can be used, like the water recovered by the pipe 124, for pre-cooling in the heat exchanger 111.
The gas present in the pipe 135 at the outlet from the dehydration stage 130 can present a temperature of about −30° C. and it penetrates into another cooler device 140 that may comprise one or more heat exchangers 143 forming part of cooling loops such as the loop 220 using the LNG present in the methane terminal 200 as a cold source.
The cooler device 140 comprises at least two enclosures 141, 142, each having a heat exchanger 143 forming part of the cooling loop 220, together with means 144, 155 for drawing off or pumping liquid and/or gaseous CO2, and possibly also sulfur dioxide.
The enclosures 141, 142 operate in discontinuous manner in alternation in turns (i.e. they operate in batch mode) for capturing CO2 (and possibly SO2) by anti-sublimation and then for causing it to melt. Operation is similar to that described above with reference to
Thus, after CO2 (or SO2) has been deposited on the walls of the enclosure 141 in which the heat exchanger 143 is active, the gas is switched to the enclosure 142. Within the enclosure 141, the heat exchanger 143 is deactivated and energy from the gas can be used to cause the temperature of the solid CO2 to rise, e.g. from a temperature of −130° C. to a temperature of about −56.6° C., with the CO2 having a partial pressure of 5.18 bar, which corresponds to the triple point where the liquid and gaseous phases appear and coexist simultaneously (see
As mentioned above, an important characteristic of the invention lies in the fact that a fraction of the liquid CO2 obtained on the line 144 at the outlet from the cooler device 140 and recovered in the tank 150 is recycled by a pipe 152 provided with a valve 153 either (dashed line) to the pipe 135 feeding dehydrated flue-gas to the inlet of the cooler device 140, or preferably (continuous lines) directly to the enclosures 141 and 142 in order to create nucleation centers for the anti-sublimation of CO2.
As a result, it is possible to optimize the dimensioning of the cooling loop 220 operating with nitrogen or methane, the dimensioning of an optional additional loop using ethylene or ethane, and the dimensioning of the anti-sublimation heat exchangers 142 and 141 in terms of heat exchange area or CO2 capture rate.
By using different cooling loops 210, 220 that implement different heat-transferring fluids and that lead to cooling in stages, it is possible to reduce the size of the cryogenic loop that uses nitrogen.
Thus,
On the assumption that the energy needed for capturing 0.600 kilowatt hours per kilogram (kWh/kg) of CO2 (which corresponds to exhaust gas from a gas turbine with recovery of energy, water, and non-condensables), the cooling power needed for capturing 320 (metric) tonnes (t) of CO2 per hour is 192 megawatts (MW). On the basis of a nitrogen loop at 25 bar (−155° C. on the liquid side, and 30° C. on the gas side), the flow rate of nitrogen to be conveyed is more than one million normal cubic meters per hour (Nm3/h), giving a pipe diameter of 0.40 meters (m) for the pipe 31 on the liquid side assuming the speed of the liquid is 10 meters per second (m/s), (or of 0.70 m if the speed of the liquid is 3 m/s, for example), and a pipe diameter of 1.60 m for the pipe 32 on the gas side (10 m/s). A nitrogen loop of such a size can give rise to problems in operation (cryogenic fluid to be kept cold) and in terms of investment, in particular over distances of several kilometers.
In contrast, when use is made firstly of a cooling loop, e.g. with glycol-containing water 210 having go-and-return pipes 41 and 42, and also a nitrogen cooling loop 220 comprising a pipe 43 on the liquid side and a pipe 44 on the gas side, the dimensions of the various pipes, including the pipe 44 can be reduced.
As mentioned with reference to
Table 3 gives the water contents of the flue-gas and of the captured CO2 as a function of the cooling temperature.
With a dew point at −30° C., the water content of the flue-gas is thus 490 parts per million (ppm), i.e. about 5 grams (g) of water per kilogram (kg) of captured CO2 (1.3%).
In order to avoid problems of corrosion and of hydrate formation during transport and injection into storage, it is preferable to dehydrate the flue-gas to a greater extent in order to obtain a water content in the CO2 that is as low as 50 ppm.
For this purpose, the flue-gas can be cooled to about −75° C. by using an additional cooling loop in the gas dehydration portion.
This additional cooling loop may be a loop using LNG as its cold source and a heat-transferring fluid such as methane or ethylene.
Nevertheless, and as shown in
Table 4 gives numerical values for an example of a flue-gas dehydration installation in a cycle that combines 800 MW of natural gas, with a cooling power of 164 MW for performing dehydration, shared between the heat exchanger 122 (99 MW) and the heat exchanger 133 (65 MW). On the basis of a glycol-containing water loop at 1 bar (−40° C. cold side and 30° C. hot side), the flow rate of water that needs to be transported is about 2500 m3/h, giving a diameter of about 0.30 m for the pipes 41 and 42 in
Table 4 gives the temperature, the pressure, and the flow rate for nitrogen, oxygen, argon, CO2, and water at various points in the installation of
The cooling power needed for cooling the flue-gas from −75° C. to −90° C. and for CO2 anti-sublimation (going from the vapor state at atmospheric pressure and −75° C., to the liquid state at the triple point) is 50 MW (comprising 21 MW for cooling the flue-gas from −75° C. to −90° C., and 29 MW for CO2 anti-sublimation). On the basis of a nitrogen loop at 25 bar (−155° C. on the liquid side and 30° C. on the gas side), the pipe diameter will be 0.20 m for the pipe 43 situated on the liquid side if the flow speed of the liquid is 10 m/s (or 0.30 m if the flow speed of the liquid is 3 m/s), and the diameter will be 0.80 m for the pipe 44 on the gas side (10 m/s) (
The dimensions of such a nitrogen cooling loop are thus entirely acceptable for easy practical implementation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0513078 | Dec 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2006/051400 | 12/20/2006 | WO | 00 | 6/20/2008 |
