Patent Application
20110252827
The present invention relates to a method of capturing carbon dioxide in a fluid comprising at least one compound more volatile than carbon dioxide CO2, for example methane CH4, oxygen O2, argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2.
The invention can be notably applied to units producing electricity and/or steam from carbon fuels such as coal, hydrocarbons (natural gas, fuel oil, petrochemical residue, etc), household waste, biomass but can also be applied to gases from refineries, chemical plants, steel-making plants or cement works, to the treatment of natural gas as it leaves production wells. It could also be applied to the flue gases from boilers used to heat buildings or even to the exhaust gases from transport vehicles, and more generally to any industrial process that generates CO2-containing flue gases.
Carbon dioxide is a greenhouse gas. For environmental and/or economic reasons, it is becoming increasingly desirable to reduce or even eliminate discharges of CO2 into the atmosphere by capturing it and then, for example, storing it in appropriate geological layers or by realizing it as an asset in its own right.
A certain number of techniques for capturing carbon dioxide, for example methods based on scrubbing the fluids with solutions of compounds that separate the CO2 by chemical reaction, for example scrubbing using MEA, are known. These methods typically have the following disadvantages:
high energy consumption (associated with the regeneration of the compound used to react with the CO2),
degradation of the compound that reacts with the carbon dioxide,
corrosion due to the compound reacting with the carbon dioxide.
In the field of cryo-condensation, that is to say of cooling until solid CO2 appears, mention may be made of document FR-A-2820052 which discloses a method allowing CO2 to be extracted by anti-sublimation, that is to say by solidification from a gas without passing via the liquid state. The cold required is provided by means of fractionated distillation of refrigerating fluids. This method consumes a great deal of energy.
Document FR-A-2894838 discloses the same type of method, with some of the liquid CO2 produced recirculated. The cold may be supplied by vaporizing LNG (liquefied natural gas). This synergy reduces the specific energy consumption of the method, although this remains high despite this, and requires an LNG terminal.
Document U.S. Pat. No. 3,614,872 describes a separation method in which the adiabatic and isentropic expansion of the carbon dioxide yields a refrigerating fluid.
It is one object of the present invention to provide an improved method of capturing carbon dioxide from a fluid containing CO2 and at least one compound more volatile than the latter.
The invention relates first of all to a method for producing chilled water, at least one CO2-lean gas and one or more CO2-rich primary fluids from a process fluid containing CO2 and at least one compound more volatile than CO2 and industrial water, comprising the following steps:
The process fluid generally comes from a boiler or any plant that produces flue gases. These flue gases may have undergone various pre-treatments, notably with a view to removing NOx (oxides of nitrogen), dust, SOx (oxides of sulfur) and/or water.
Prior to separation, the process fluid is either monophasic, in gaseous or liquid form, or polyphasic. What is meant by “gaseous” form is “essentially gaseous” form, that is to say that it may notably contain dust, solid particles such as soot and/or droplets of liquid.
The process fluid contains CO2 that is to be separated from the other constituents of said fluid by cryo-condensation. These other constituents comprise one or more compounds more volatile than carbon dioxide in terms of condensation, for example methane CH4, oxygen O2, argon Ar, nitrogen N2, carbon monoxide CO, helium He and/or hydrogen H2. The process fluids generally comprise predominantly nitrogen or predominantly CO or predominantly hydrogen.
In step a) the process fluid is separated into at least one CO2-lean gas and one or more CO2-rich primary fluids using methods known to those skilled in the art. For example, this may be a separation method using low-temperature solid cryo-condensation of the CO2.
In step b) the CO2-lean gas produced in step a) is brought into direct contact with one or more streams of industrial water. This contact between a gas that is not saturated with water and water causes some of said industrial water to vaporize. The heat of vaporization of this water and any possible heating-up of the CO2-lean gas serves to cool the non-vaporized water.
A direct-contact tower may simply consist of a spray system so as to form water droplets which, under the action of gravity, will drop down against the flow of the CO2-lean gas. In order to improve the performance of the direct-contact tower, use may also be made of gas-liquid contactors of the plates or packings (loose or structured) type.
Depending on circumstances, the method according to the invention may comprise one or more of the following features:
In step a1) the process fluid is first of all cooled without a change in state. This cooling may advantageously take place at least in part by exchange of heat with CO2-rich fluids from the separation process. In addition or as an alternative, it may advantageously take place at least in part by exchange of heat with the CO2-lean gas from the separation process. These cold fluids from the separation process are heated up, while the process fluid is cooled down. This makes it possible to reduce the amount of energy required for the cooling operation.
Step a2) consists in solidifying the initially gaseous CO2 by raising the process fluid to a temperature below the triple point for CO2 while the partial pressure of the CO2 in the process fluid is below that of the triple point for CO2. For example, the total pressure of the process fluid is close to atmospheric pressure. This solidification operation is sometimes known as “cryo-condensation” or “anti-sublimation” of the CO2 and, by extension, of the process fluid.
Certain compounds more volatile than CO2 do not solidify and remain in the gaseous state. Together with the non-solidified CO2 these will constitute said CO2-lean gas, that is to say will constitute said gas that comprises less than 50% CO2 by volume and preferably less than 10% CO2 by volume. According to one particular embodiment, said CO2-lean gas contains less than 1% CO2 by volume. According to another particular embodiment, it contains more than 2% thereof. According to another particular embodiment, it contains more than 5% thereof. A solid comprising predominantly CO2, that is to say containing at least 90% by volume if considered in the gaseous state, preferably containing at least 95% by volume, and more preferably still containing at least 99% CO2 by volume, is formed.
This solid may comprise other compounds than CO2. Mention may, for example, be made of other compounds which might also have solidified, or alternatively of bubbles and/or drops of fluid contained within said solid lump. This explains how the solid could potentially consist of not only solid CO2. This “solid” may contain non-solid parts such as fluid inclusions (drops, bubbles, etc).
This solid is then isolated from the compounds that have not solidified after cryo-condensation and recovered. Next, in step a3), it is returned to temperature and pressure conditions such that it changes into a fluid, liquid and/or gaseous, state. At least part of said solid may then liquefy. This then gives rise to one or more CO2-rich primary fluids. These fluids are said to be “primary” to distinguish them from treatment fluids which are said to be “secondary”. What is meant by “CO2-rich” is something “comprising predominantly CO2” within the meaning defined hereinabove.
The inventors have demonstrated that it is particularly advantageous to carry out the first and/or the second cooling of the process fluid using one or more refrigerating cycles each comprising at least one near-isentropic expansion of a gas. These refrigerating cycles consist of several steps which cause a so-called “working” fluid to pass via several physical states characterized by given composition, temperature, pressure, etc conditions. The presence, among the steps of the cycle, of at least one near-isentropic expansion, that is to say of an expansion that causes the entropy of the expanded fluid to increase by less than 25%, preferably less than 15% and more preferably still, less than 10% makes it possible to improve the energy consumption of the separation process.
To provide another part of the cold required to carry out the first and/or second coolings, recourse may be had to one or more cycles comprising an expansion of a fluid that is not a near-isentropic expansion, for example reverse-Rankine cycles. These cycles are said to be reversed because they are used as refrigerating cycles. Their benefit, as a supplement to the refrigerating cycles employing near-isentropic expansion, is that they do not require a large amount of working fluid. By contrast, they are less energy-efficient.
According to one embodiment, some of the near-isentropic expansions of the refrigerating cycle or cycles provide work. They are, for example, carried out by introducing working fluid into a turbine.
The working fluids may be of varying kinds. According to various embodiments, these fluids may comprise nitrogen and/or argon. They may also comprise all or part of the CO2-lean gas obtained or of the process fluid. These fluids may be mixed with other fluids or have undergone intermediate steps of compression, expansion, etc.
When the working fluid of the refrigerating cycle comprises all or part of the process fluid, the near-isentropic expansion or expansions that do not provide external work may give rise to a cooling of the working fluid such that solid CO2 appears. This may constitute all or part of the second cooling of the process fluid. According to one particular embodiment, these near-isentropic expansions are carried out through a Venturi (a throat with Venturi effect).
The abovementioned causing of the fluid to rotate can be obtained by any conventional means, for example by suitably oriented vanes. The increase in speed is achieved through a Venturi effect. The temperature of the working fluid drops. Solid particles of CO2 appear. The fluid has a rotational movement about an axis substantially parallel to the direction of the flow, like a corkscrew. This creates a centrifugal effect allowing these solid particles to be recovered at the periphery of the flow.
According to a preferred embodiment, any work that might be produced by the near-isentropic expansion or expansions serves in part to compress the fluids in other steps of the method.
The invention also relates to the method applied to industrial flue gases with a view to capturing CO2.
According to one particular embodiment, these flue gases come from a plant producing energy (steam, electricity) and may have undergone pretreatments.
Other specifics and advantages will become apparent from reading the following description given with reference to the figures in which:
The plant illustrated in
The fluid 24 consisting of flue gases is compressed in a compressor 101, notably to compensate for the pressure losses in the various pieces of equipment in the plant. Let us note that this compression may also be combined with the compression known as the draft compression of the boiler that produces the flue gases. It may also be carried out between other steps of the method, or downstream of the CO2 separation method;
The compressed fluid 30 is injected into a filter 103 to eliminate particles down to a level of concentration of below 1 mg/m3, preferably of below 100 μg/m3.
Certain embodiments of the invention will be described in greater detail hereinafter. The dust-free fluid 32 is cooled to a temperature close to 0° C., generally of between 0° C. and 10° C., so as to condense the water vapor it contains. This cooling is carried out in a tower 105, with water injected at two levels, the cold water 36 and water 34 at a temperature close to the wet-bulb temperature of the ambient air. It is also possible to conceive of indirect contact. The tower 105 may or may not have packings.
The fluid 38 is sent to a unit that eliminates residual water vapor 107, for example using one and/or another of the following methods:
The dried fluid 40 is then introduced into the exchanger 109 where the fluid is cooled down to a temperature close to, but in all events higher than, the temperature at which CO2 solidifies. This temperature can be determined by a person skilled in the art aware of the pressure and composition of the process fluid 40. This temperature is situated at around about −100° C. if the CO2 content of the process fluid is of the order of 15% by volume and for a pressure close to atmospheric pressure.
The fluid 42 which has undergone a first cooling 109 is then introduced into a vessel 111 where it continues to be cooled down to the temperature that provides the desired level of CO2 capture. Cryo-condensation of at least part of the CO2 contained in the fluid 42 occurs producing, on the one hand, a CO2-lean gas 44 and, on the other hand, a solid 62 comprising predominantly CO2. The gas 44 leaves the vessel 111 at a temperature of the order of −120° C. This temperature is chosen as a function of the target level of CO2 capture. At this temperature, the CO2 content of the gas 44 is of the order of 1.5% by volume, namely a capture level of 90% starting out from a process fluid containing 15% CO2. There are various technologies that can be used for this vessel 111:
The fluid 46 is then heated up in the exchanger 109.
The invention in terms of step b) thereof will now be described in greater detail: on leaving the exchanger 109, the fluid 48 can also be used notably to regenerate the unit used for eliminating residual vapor 107 and/or for producing cold water 36a by evaporation in a direct-contact tower 115 into which a dry fluid 50 is introduced which then becomes saturated with water, vaporizing some of it. The cold water could then potentially undergo additional cooling in a refrigerator unit 119.
The solid 62 comprising predominantly CO2 is transferred to a bath 121 of liquid CO2.
This bath 121 needs to be heated in order to remain liquid, to compensate for the addition of cold from the solid 62 (latent heat of fusion and sensible heat). This can be done in various ways:
Liquid 64 comprising predominantly CO2 is tapped from the bath 121. This liquid is split into three streams. In the example, the first is obtained by an expansion 65 to 5.5 bar absolute producing a diphasic, gas-liquid, fluid 66. The second, 68, is obtained by compression 67, for example to 10 bar. The third, 70, is compressed for example to 55 bar. The 5.5 bar level provides cold at a temperature close to the triple point temperature for CO2. The 10 bar level allows the transfer of the latent heat of vaporization of the fluid 68 at around −40° C. Finally, at 55 bar, the fluid 70 does not vaporize during the exchange 109. There is efficient use to be made of the cold energy contained in the fluid 64 during the exchange 109 while at the same time limiting the amount of energy required to produce a purified and compressed stream 5 of CO2.
Part of the cold required for the first cooling 109 and for the second cooling 111 is provided by a refrigerating cycle 200 employing a working fluid 51 which is argon. It comprises, in succession: a compression 129, possibly two compressions 56 and 57, a cooling by indirect exchange 109, a near-isentropic expansion 131 which gives rise to cooling, a heating-up in the vessel 111, and a heating-up 109. During the cooling 109, part of the working fluid is tapped off then undergoes near-isentropic expansion 130, followed by indirect exchange 109 and finally compression 128 before reaching the compression stage 129. The near-isentropic expansions 130 and 131 supply work part of which can be used for the compressions 56 and 57.
This cycle 200 produces cold at between about −100 and −120° C. for the cryo-condensation 111 and between about 5° C. and −100° C. in order to offset the deficit of cold during the exchange 109.
Another part of the cold needed for the first cooling 109 is provided by an additional refrigerating cycle 181, 183, for example of the reverse Rankine type.
Another part of the cold needed for the second cooling 111 is provided by an additional refrigerating cycle 191, 193, for example of the reverse Rankine type.
Following the indirect exchange 109, the CO2-rich primary fluids 66, 68, 70 are compressed in stages 141, 142, 143. For example, the first stages compress gaseous streams. If need be, the compressed CO2 75 is cooled by an indirect-contact exchanger to convert it to liquid form. It is then mixed with the stream 73. This liquid mixture is pumped to the transport pressure (fluid 5). As the transport pressure is generally supercritical, the supercritical fluids will, by extension, be considered to be liquid at a temperature below that of the critical point for CO2.
The CO2-lean gas 44 is compressed, for example by a multi-stage compressor 315. On leaving, the fluid 303 is cooled if necessary to the inlet temperature for the exchanger 109 by the exchanger 316. This may be a direct-contact or an indirect-contact exchanger.
The compressed CO2-lean gas 304 is cooled in the exchanger 109 so that it can be expanded in the turbine 312 (near-isentropic expansion) so as to provide some of the cold needed for the exchange 111. The fluid 307 leaving the exchanger 111 is once again expanded (near-isentropic expansion) to provide work and cold for the exchanger 111 via the fluid 308. This loop in which the CO2-lean gas is expanded can be repeated as many times as necessary.
After the exchanger 111, the CO2-lean gas 46 is heated up in the exchanger 109. The outgoing fluid 48 is processed like the fluid 48 in
Some of the cold needed for the exchanger 111 may be supplied by a refrigerating cycle 191, 193 of the Rankine type.
The CO2-lean gas 44 gives up cold energy in the exchangers 111 and 109. It is then compressed by the multi-stage compressor 415. Next, it is cooled if necessary to the inlet temperature of the exchanger 109 in the exchanger 416. This may be a direct-contact or an indirect-contact exchanger.
The CO2-lean gas 404 is once again cooled in the exchanger 109 before it is being expanded by the turbine 412. This near-isentropic turbine produces the cold required to compensate for part of the deficit of cold energy in the exchanger 111.
Next, the fluid 407 is expanded again by the near-isentropic turbine 414. The fluid 408 gives up its cold energy to compensate for part of the deficit of cold energy in the exchanger 111. This loop in which the CO2-lean gas is expanded can be repeated as many times as necessary.
Following the exchanger 111, the CO2-lean gas 46 is heated up in the exchanger 109. Finally, the outgoing fluid 48 is processed as the fluid 48 in
The process fluid 40 is compressed by the compressor 512 which may be a multi-stage compressor. The CO2-lean gas is expanded in a near-isentropic turbine 514. The temperature of the fluid 503 must remain above the cryo-condensation temperature for CO2.
Part of the CO2 contained in the fluid 503 then condenses in the vessel 111. The solid CO2 62 is tipped into the liquid bath 121 and the next steps are the same as those described in
A near-isentropic expansion with production of work is carried out on the fluid 42 in the turbine 612 so as to cool the fluid to a temperature below the cryo-condensation temperature for CO2 and thus produce solid CO2 in the form of carbon dioxide snow together with a CO2-lean gas 602.
This expansion turbine 612 needs to be designed with a great deal of care. It has to be suited to the high flow rates such as those of the flue gases 40 of an industrial plant, have very good isentropic efficiency, and be resistant to potential additional erosion due to the presence of solid CO2. To achieve this, carbon dioxide snow is allowed to be present in the rotor part of the turbine (the region contained between the leading edge 951 and the trailing edge 954 in
The turbine may be a radial turbine (centripetal or centrifugal). It may be a supersonic shockwave turbine. It may be axial.
The latter technology is the best suited to high flow rates, but does require numerous successive stator and rotor stages. To avoid erosion, it will be preferable for the carbon dioxide snow to be separated out downstream of each rotor stage before the fluid enters the next stator stage. The first two technologies have the advantage of remaining effective for high expansion ratios (in excess of 10) thus making it possible to avoid having to perform numerous separation operations.
Moreover, other precautions have preferably to be taken in order to create such a turbine:
This carbon dioxide snow is then separated from the CO2-lean gas in a separator 612 to obtain a solid comprising predominantly CO2 62 and a CO2-lean gas 44.
This separation may be performed downstream of the rotor part by causing the fluid in the rotor part to rotate and by using the centrifugal effect to separate a CO2-rich fraction at the periphery from a CO2-lean fraction at the center. It may also be advantageous to increase the speed and therefore achieve an additional expansion of the fluid in a convergent nozzle 956 (a turbine known as a Laval turbine). By reducing the pressure before decelerating the gas the amount of solidified CO2 can be increased. Most of the CO2-lean gas is recovered at the center of the flow 959 and most of the solid CO2 is recovered at the periphery 958, mixed in with a fraction of the gas.
The benefit of a turbine for performing solid cryo-condensation is that a great deal of solid CO2 can be generated in a very small volume as compared with indirect-exchange systems.
If necessary, an additional refrigerating cycle 191, 193 of the Rankine type or which includes a near-isentropic expansion of a working fluid with or without the production of work provides the separator 612 with cold energy. The solid 62 comprising predominantly CO2 is tipped into the liquid bath 121 and the next steps are the same as those depicted in
The CO2-lean gas 44 is heated up by exchange of heat with the process fluid in the exchanger 109. The fluid 605 is then compressed to a pressure higher than or equal to atmospheric pressure. Finally, the outgoing fluid 48 is processed as in
The process fluid 42 is still cooled to below the cryo-condensation temperature for CO2 in the vessel 111 to produce a cooled CO2-lean gas 701. It is also possible for this vessel to be situated after the “expansion/Venturi” part 702 of the method, and will now be described.
Some of the CO2 to be captured solidifies in the form of a solid containing predominantly CO2 62 and is extracted from the vessel 111. To improve CO2 capture, the fluid 701 is made to rotate about an axis that is substantially parallel to the direction in which it flows using a system of fixed vanes 717.
The fluid 703 is expanded as it leaves the vanes and cools, to below the cryo-condensation temperature for CO2, without producing work. The expansion may take place through the Venturi effect by passing the fluid through a restriction 718. Solid particles comprising predominantly CO2 form and are recovered at the periphery of the flow thanks to the centrifugal effect caused by the rotation of the fluid.
A mixture 705 of solid comprising predominantly CO2 and gas is recovered. The outgoing non-condensables 44, 46 give up their cold energy in the exchangers 111 and 109.
The stream 705 is made up predominantly of solid, although it may be necessary to separate the residual gas from the solid in a separator 731. The non-condensable part then gives up its cold energy in the exchangers 111 and 109.
The solid comprising predominantly CO2 62 is tipped into the liquid bath 121 and undergoes the same steps as those described in
The streams 48 are used to cool the water, in the same way as the stream 50 in
The process fluid 40 is under pressure, for example as much as 60 bar (compression performed by the compressor 101 or by an additional compressor). It may potentially be more concentrated in CO2 than in the other examples, typically containing between 50 and 90% by volume.
The exchange 809 comprises the same features as the exchange 109 in
A CO2-rich liquid 816 is extracted by the separator 812. The residual fluid 802 is made to rotate about an axis substantially parallel to the direction in which it flows by a system of fixed vanes 817. It is expanded as it leaves 803 the vanes having been rotated and cooled to below the cryo-condensation temperature for CO2 without producing work. The expansion may take place through a Venturi effect by passing the fluid through a restriction 818.
Solid particles comprising predominantly CO2 form and are recovered at the periphery of the flow thanks to the centrifugal effect caused by the rotating of the fluid. The stream 805 is made up predominantly of solid, although it may be necessary to separate the residual gas from the solid in a separator 841. The non-condensables 44 give up their cold energy in the exchangers 811 and 809.
In order to improve the level of CO2 capture, a second (or even a third or more) step in which the fluid 806 undergoes a near-isentropic expansion with Venturi effect may be added. This step is identical to the previous one:
The solid 62 comprising predominantly CO2 recovered at the outlet from the separators 841 and possibly 851 is tipped into the liquid bath 121 and processed as in
A primary airflow 15 passes through the unit 3 in which the coal 15 is pulverized and carried along toward the burners of the boiler 1. A secondary airflow 16 is applied directly to the burners in order to provide additional oxygen needed for near-complete combustion of the coal. Feed water 17 is sent to the boiler 1 to produce steam 18 which is expanded in a turbine 8.
The flue gases 19 resulting from the combustion, comprising nitrogen, CO2, water vapor and other impurities, undergo various treatments to remove some of said impurities. The unit 4 removes the NOx for example by catalysis in the presence of ammonia. The unit 5 removes dust, for example using an electrostatic filter, and the unit 6 is a desulfurization system for removing the SO2 and/or SO3. The units 4 and 6 may be superfluous depending on the composition of the product required. The purified flow 24 from the unit 6 (or 5 if 6 is not present) is then sent to a low-temperature cryo-condensation purification unit 7 to produce a relatively pure flow 26 of CO2 and a nitrogen-enriched residual flow 25. This unit 7 is also known as a CO2 capture unit and implements the method that forms the subject of the invention, as illustrated, for example in
A primary airflow 15 passes through the unit 3 where the coal 15 is pulverized and carried along toward the burners of the boiler 1. A secondary flow of oxidant 16 is supplied directly to the burners in order to provide the additional oxygen needed for near-complete combustion of the coal. This secondary oxidant is the result of the mixing of flue gases 94 recirculated using a blower 91 with oxygen 90 produced by a unit 10 for separating air gases. Feed water 17 is sent to the boiler 1 to produce steam 18 which is expanded in a turbine 8.
The flue gases 19 from the combustion of the coal, comprising nitrogen, CO2, water vapor and other impurities, undergo various treatments to remove some of said impurities. The unit 5 (ESP) removes the dust, for example using an electrostatic filter. The dust-free flow 24 from the unit 5 is sent to a low-temperature cryo-condensation purification unit 7 to produce a relatively pure flow 26 of CO2 and a nitrogen-enriched residual flow 25. This unit 7 is also known as a CO2 capture unit and implements the method that forms the subject of the invention, as illustrated, for example, in
In this case, the presence of a unit for separating the air gases is used to provide cold at low level for the solid cryo-condensation of CO2 in the unit 7 and to carry out cryo-condensation, preferably by direct exchange with the process gas. The fluid 93 may be in liquid, gaseous or diphasic form and consists of a mixture of cooled air gases. For example, this may be cold gaseous nitrogen or air (at between −56° C. and −196° C.), or alternatively liquid nitrogen or air. It is intended to be introduced into the vessel referenced 111 in
The unit 7 may also produce a fluid 92 which will be used in the unit for separating air gases. This may, for example, be a fraction of the lean gas leaving the vessel 111 in
This scheme may prove well suited to instances where existing plants are being modified, where replacing the primary air sent to the coal pulverizers with a mixture of recirculated flue gases plus oxygen could prove complicated, partly because of the increase in water content, the flue gases containing far more water than damp air, and partly for safety reasons, although that should not be overestimated.
Moreover, it may prove advantageous to combine the units 7 and 10 into a single unit, notably by carrying out one (or more) exchange(s) of heat between fluids of the 2 units.
The blast furnace gases 903 made up for example of 47% CO, 36% CO2, 8% N2 and 9% other compounds such as H2 and H2O can be split into two. Most 905 goes to the CO2 capture unit with another proportion 904 used to reduce the nitrogen concentration in the loop. The fluid 905 is cooled beforehand in a direct-contact exchanger 906, has its dust removed in the filter 103, and is then compressed by a compressor 901, is cooled in an exchanger 105 and dried in a drier 107 before entering the low-temperature exchanger 109 where it will be cooled and then partially liquefied to a temperature close to the triple point for CO2 without the formation of solid. The diphasic gas-liquid fluid 912 obtained is separated into a gaseous fraction 502 and a liquid fraction 920 in the separator 928. The gaseous fraction 502 is then cooled by near-isentropic expansion, for example in a turbine 514, so as to obtain a diphasic gas-solid fluid 503. This is separated in the vessel 111 into a gaseous fraction 44 and a CO2-rich solid fraction 62. The solid fraction 62 is compressed, for example by an endless screw and mixed with the liquid 920 in the bath 121, which is heated by gas 72 produced by vaporizing liquid 74 in the exchanger 109. The liquid CO2 64 is compressed by a pump 69 to obtain a pressurized liquid 70 and is heated up in the exchanger 109 without undergoing vaporization or pseudo-vaporization if the pressure is above the supercritical pressure. The lean gas is successively heated up by a compressor 315 and by the exchanger 109.
The invention may also be adapted to types of blast furnace operating on enriched air, for example by adding a CO/N2 separation using cryogenic distillation, cooling the gas 44 to the required temperature.
The synthesis gases 903 chiefly comprise the compounds CO, CO2, H2 and H2O. The CO can be converted (in a so-called shift reaction) into CO2 and H2 in the presence of water vapor: CO+H2O<−>CO2+H2 in the unit 907. This unit may also include one or more exchangers for cooling the gas prior to compression. The fluid 905 may possibly have its dust removed in a filter 103, then be compressed by a compressor 101, cooled in an exchanger 105 and dried in a dryer 107 before entering the low-temperature exchanger 109 where it may be partially liquefied at a temperature close to that of the triple point for CO2. This diphasic gas-liquid fluid 912 is separated into a gaseous fraction 502 and a liquid fraction 920 in the separator 928. The gaseous fraction 502 is then cooled by near-isentropic expansion, for example in a turbine 514, to obtain a diphasic gas-solid stream 503. This is separated into a gaseous fraction 44 and a CO2-rich solid fraction 62 in the vessel 111. The solid fraction 62 is mixed with the liquid 920 in the bath 121, which is heated with gas 74 produced by the vaporizing of the liquid 72 in the exchanger 109. The liquid CO2 64 is compressed by a pump and heated up in the exchanger 109 without vaporizing, or pseudo-vaporizing if the pressure is above the supercritical pressure. The lean gas 44 is successively heated up via a compressor 924 and the exchanger 109. This lean gas essentially consisting of hydrogen may be sent to a gas turbine to be combusted without the emission of CO2. The unit 10 may supply hot nitrogen 90a which is introduced downstream of the dryers 910, and/or cold nitrogen 90b, introduced directly into the vessel 111 to increase the amount of CO2 captured. In the first instance, the expansion in the turbine 514 of the hot nitrogen present in the stream 502 provides additional cold energy for solid cryo-condensation of CO2 in the turbine 514; in the second instance, the cold nitrogen 90b, by heating up upon contact with the fluid 503, leads to solid cryo-condensation of the CO2. The other benefit of hot nitrogen 90a is that it increases the molecular weight of the gas 502, something that may prove advantageous in reducing the cost of the expansion 514 and/or of the compression 924. What actually happens is that when these gases are very rich in hydrogen, it is not easy for these gases to be compressed/expanded using the technologies best suited to high flow rates, namely technologies of the axial, radial or supersonic shockwave type. It then becomes necessary to use technologies of the positive-displacement type, for example using pistons or screws, which are very expensive to implement.
The synthesis gases 903 produced in the reactor 900 chiefly comprise the compounds CO, CO2, H2 and H2O. The fluid 905 may potentially be compressed by a compressor 101, cooled in an exchanger 105 and dried in a dryer 107 before entering a low-temperature exchanger 109 where it may be partially liquefied at a temperature close to that of the triple point for CO2. The diphasic gas-liquid fluid 912 obtained is separated into a gaseous fraction 502 and a liquid fraction 920 in the separator 928. The gaseous fraction 502 is then cooled by a near-isentropic expansion, for example in a turbine 514, so as to obtain a gas-solid diphasic mixture 503. This is separated into a gaseous fraction 44 and a CO2-rich solid fraction 62 in the vessel 111. The solid fraction 62 is mixed with the liquid 920 in the bath 121, which is heated with gas 74 produced by the vaporizing of the liquid 72 in the exchanger 109. The liquid CO2 64 is compressed by a pump and heated up in the exchanger 109 without vaporizing or pseudo-vaporizing if the pressure is above the supercritical pressure. The lean gas 44 can then be purified in terms of CO2 at a low temperature, for example by adsorption using a molecular sieve 13X before being introduced into a cryogenic unit 924 for the production of CO. This unit operates, for example, by methane scrubbing or partial condensation of the CO. This unit 924 produces a hydrogen-enriched gas 929 and a CO-enriched gas 925. One or more fluids of this unit may be compressed at low temperature, then reintroduced into the heat exchanger 926.
In this case, solid cryo-condensation replaces elimination of CO2 by absorption with amines (MDEA or MEA). If there is a desire to produce pure hydrogen, then it is possible to add an H2 PSA into this scheme either upstream of this solid cryo-condensation purification, that is to say on the outlet side of the reformer 900 after the cooling of the synthesis gas, or on the H2-rich gas 929.
It might be supposed that these solid cryo-condensation methods are deficient in cold. In actual fact, this is not the case at all. On the contrary, these solid cryo-condensation methods with near-isentropic expansion of the process gas produce excessive amounts of cold, especially if the method also provides external work. The problem is then that the CO2-rich fluids and the CO2-lean gas exit at low temperature, which represents an appreciable energy loss. In order to minimize the energy consumption of this method, one or more of the following operations may be carried out:
The fluid 24 consists of flue gases and may be at a temperature of the order of 150° C. and is injected into a filter 103 to remove the particles down to a concentration level of below 1 mg/m3, preferably below 100 μg/m3.
Certain features of the invention will now be described in greater detail: the dust-free fluid 30 is cooled to a temperature of close to 0° C., generally of between 0° C. and 10° C., so as to condense the water vapor it contains. This cooling is carried out in a tower 105b, with water injected at two levels, cold water 36b and water 34b at a temperature close to the wet bulb temperature of the ambient air. It is also possible to conceive of indirect contact. The tower 105 may or may not have packings. This tower may also serve as a scrubbing tower for the SO2.
On leaving this first tower, the fluid that may have been desaturated, is compressed to a pressure of between 5 and 50 bar abs in the compressor 101. The fluid 32 is cooled to a temperature close to 0° C. and generally of between 0° C. and 10° C. so as to condense the water vapor it contains. This cooling is carried out in a tower 105 with water injected at two levels, cold water 36 and water 34 at a temperature close to the wet bulb temperature of the ambient air. It is also possible to conceive of indirect contact. The tower 105 may or may not have packings.
The fluid 38 is sent to a unit 107 that eliminates the residual water vapor, for example using one and/or another of the following methods:
Adsorption on fixed beds, fluidized beds and/or rotary dryer, it being possible for the adsorbent to be activated alumina, silica gel or a molecular sieve (3A, 4A, 5A, 13X, etc.);
Condensation in a direct-contact or indirect-contact exchanger.
The process fluid 40 is cooled then brought into contact in a distillation column 79 with pure CO2, so as to recover the compounds less volatile than CO2 in the form of a liquid containing CO2 and, for example, NO2 (or its dimer N2O4). This liquid can be pumped and vaporized in the unit 78, then sent either to a combustion chamber to reduce the NO2 or to the unit for purifying the stream 30 by low-pressure scrubbing of the SO2, where it acts as a reagent, either directly in the form of NO2 or in the form of nitric acid having been reacted with water.
The process fluid 74a is then cooled and partially condensed into liquid form and sent to the separator 76. The liquid fraction 76a is sent to the bath 121. The gaseous fraction 76b is sent to an expansion turbine so as there to produce a gas-solid diphasic stream 42 which is then sent to the vessel 111 where it is separated into a CO2-lean gas 44 and solid CO2 62. An auxiliary fluid 93, for example from an air gas separation unit, may potentially supply additional cold for solid cryo-condensation. When it does, it may be advantageous to tap from the CO2-lean gas 44 a fluid 92 which returns to the unit that supplied the fluid 93. The solid 62 is compressed for example by an endless screw and injected into the bath 121 of liquid CO2, from which a liquid 64 is tapped. This liquid may potentially be pumped and introduced into a distillation column 75 where its compounds more volatile than CO2 are eliminated. The pure liquid 68 is heated up without vaporizing or pseudo-vaporizing if it is supercritical. It may once again be pumped to obtain the fluid 5 ready for transport. A part of the fluid 5 may be tapped off to be vaporized or pseudo-vaporized in a unit 72. This unit 72 is, for example, any arbitrary source of heat of the plant that produces the process fluid. This part of the fluid 80 is then expanded in a turbine 73 used to produce electricity or mechanical power and is then cooled in the exchanger 109 and condensed by direct exchange in the bath 121, at the same time melting the solid CO2.
The invention, in respect of step b) thereof, will now be described in greater detail: on leaving the exchanger 109, the fluid 48 can still notably be used to regenerate the unit that eliminates residual vapor 107 and/or for producing cold water 36a by evaporation in a direct-contact tower 115 into which a dry fluid 50 is introduced and becomes saturated with water, vaporizing part of it. Potentially, the cold water may undergo additional cooling in a refrigerating unit 119. Thereafter, this cold water can be used in one and/or other of the towers 105 and 105b to cool the process gas before and/or after compression.
Downstream of the rotor part, if centrifugal force is not to be used on the solid parts, the rotor part may consist of a simple deceleration cone.
If the downstream stator part 961 is to be used to achieve a first separation, then the fact that the fluid has been made to rotate in the rotor part and the centrifugal effect can be used to separate a CO2-rich fraction at the periphery from a CO2-lean fraction at the center. It may also be advantageous to increase the speed and therefore perform an additional expansion of the fluid in a convergent nozzle 956 (a so-called “Laval” turbine). By reducing the pressure before decelerating the gas, the amount of solidified CO2 can be increased. Most of the CO2-lean gas is recovered at the center of the flow 959 and most of the solid CO2 is recovered at the periphery 958, mixed in with a fraction of gas.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0858866 | Dec 2008 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2009/052509 | 12/14/2009 | WO | 00 | 7/7/2011 |
