The present invention relates to a method for producing at least one argon-enriched fluid and at least one oxygen-enriched fluid from a residual fluid resulting from a method for purifying a residual gas, the residual gas containing carbon dioxide and either argon or oxygen or both. A particular example would be the production of argon from the incondensables of a method for low-temperature separation of a residual gas produced by an installation consuming oxygen, the residual gas being oxy-fuel combustion fumes.
Thermal power stations make it possible to release heat, which can be used to produce steam and mechanical or electrical energy, by combustion of fuels. The combustion fumes release large quantities of CO2 into the atmosphere. In order to resolve this environmental problem, the current solution consists in carrying out the combustion inside the boiler in the presence of a gas which is rich in oxygen and above all depleted of nitrogen. This combustion produces combustion fumes having a high concentration of CO2, which is advantageous because the current technologies for removing CO2 from the combustion fumes make it possible to remove the CO2 more easily from fumes with a high concentration of CO2 than from fumes with a low concentration of CO2. This CO2 then needs to be purified and compressed before being sequestered.
It is an object of the present invention to provide a method for producing argon and oxygen from a residual gas rich in carbon dioxide and also containing argon and/or oxygen, which are incondensables of a unit for purifying fumes with respect to CO2 at low temperature.
One subject of the invention provides a method for producing at least one argon-enriched fluid and at least one oxygen-enriched fluid from a residual fluid resulting from a method for purifying a residual gas containing carbon dioxide and oxygen and/or argon, the residual gas being derived from an installation supplied with oxygen containing argon, which is an oxy-fuel combustion installation, comprising the following steps:
- recovering residual gas, consisting of fumes resulting from the oxy-combustion of a fuel by means of a gas rich in oxygen and in carbon dioxide and containing argon in the combustion chamber of a boiler;
- purifying the residual gas, in particular fumes leaving the boiler, by a purification method in particular at low temperature, so as to produce a fluid enriched with carbon dioxide and a residual fluid depleted of carbon dioxide;
- pretreatment of the residual fluid depleted of carbon dioxide in order to obtain a flow enriched with carbon dioxide and a flow lean in carbon dioxide; and
- cryogenic treatment of the flow lean in carbon dioxide so as to extract therefrom at least one fraction enriched or rich in argon, a fraction enriched or rich in oxygen and at least one fraction depleted of argon and/or oxygen.
According to other optional aspects:
- the cryogenic treatment of the flow lean in carbon dioxide comprises a step of cooling in at least one exchanger, optionally a reboiler, optionally a condenser, optionally an exchanger of the reversible type or of the regenerator type, and a step of distillation in a distillation column;
- the purification method is preferably carried out at low temperature, although other known purification methods may be substituted therefor (for example washing with amines);
- the flow lean in carbon dioxide is substantially free of carbon dioxide, and may contain for example a few ppm of carbon dioxide;
- air is separated in an air separation apparatus, preferably by cryogenic distillation, in order to produce an oxygen-rich flow containing at most 99% oxygen, preferably at most 98% oxygen, or at most 97 mol % of oxygen, and argon, preferably at least 2 mol % of argon, or at least 3 mol % of argon, and the oxygen-rich flow is sent to the installation consuming oxygen, preferably the oxy-fuel combustion;
- the oxygen-enriched fraction is used for the oxy-combustion of the fuel and/or for the pretreatment of the residual gas depleted of carbon dioxide;
- the treatment also makes it possible to recover a fraction enriched or rich in nitrogen;
- one or more fluid(s) which come from a unit for separating gas from air or the unit for separating the gases in air, delivering at least part of the oxygen for the oxy-fuel combustion, are used in the treatment of the flow lean in carbon dioxide;
- the flow lean in carbon dioxide is cooled upstream of the cryogenic treatment, the flow lean in carbon dioxide being substantially free of carbon dioxide;
- the flow lean in carbon dioxide is cooled upstream of the cryogenic treatment, and at the same time it is purified with respect to carbon dioxide, the flow lean in carbon dioxide containing carbon dioxide;
- the flow lean in carbon dioxide is cooled in at least one reversible exchanger or in an exchanger of the regenerator type, and the flow produced is sent to a column of the cryogenic treatment unit;
- one of these fluids is a nitrogen-rich liquid which at least partially keeps cold the cryogenic treatment of the flow lean in carbon dioxide;
- no fluid intended for or coming from a column of the treatment unit is expanded in a turbine;
- the flow lean in carbon dioxide is sent to a first column, optionally having a bottom reboiler, and separated to form an oxygen-enriched fluid and a nitrogen-enriched fluid, and an intermediate flow is drawn off from the first column and sent to the bottom of a second column where it is enriched with argon to form a/the argon-enriched fraction;
- an argon-enriched fraction is drawn off from the second column to be sent to a denitrogenation column in order to form an argon-rich fraction;
- one or more fluid(s) which come from a unit for separating gas from air or the unit for separating the gases in air, delivering at least part of the oxygen for the installation supplied with oxygen, for example the oxy-fuel combustion, are used in the treatment of the flow lean in carbon dioxide;
- at least one column of the unit for separating gas from air and at least one column of the treatment unit are contained in a single coldbox;
- one of these fluids is a nitrogen-rich gas which will be used as a cycle gas for at least one reboiler and/or at least one condenser of the cryogenic treatment;
- the pretreatment removes at least 50%, or even substantially 100%, of the carbon dioxide in the residual gas before the cryogenic treatment;
- the pretreatment is carried out at least partially by antisublimation/sublimation of the carbon dioxide in a plurality of exchangers in parallel;
- the sublimation of the carbon dioxide is carried out in the presence of the oxygen-enriched fraction so as to constitute a carbon dioxide/oxygen mixture used for the oxy-combustion of the fuel;
- the pretreatment is carried out at least partly by a process of the TSA, PSA or VPSA type so as to produce a fraction enriched with carbon dioxide and a fraction depleted of carbon dioxide but enriched with argon;
- the pretreatment is carried out at least partly by an absorption process;
- the absorption process uses an aqueous solution of basic pH;
- the basic pH is obtained by injecting NaOH and/or Na2CO3 and/or NH3;
- the pretreatment is carried out at least partly by an adsorption process;
- the pretreatment is carried out at least in part by permeation;
- the flow enriched with carbon dioxide, produced by the pretreatment, is recycled into the boiler, preferably to the combustion chamber.
Another subject of the invention provides an installation for producing at least one argon-enriched fluid and at least one oxygen-enriched fluid from a residual fluid resulting from a method for purifying a residual gas, the residual gas containing carbon dioxide and argon and/or oxygen, the residual gas being derived from an installation supplied with oxygen containing argon, which is an oxy-fuel combustion installation, comprising:
- a unit for purifying the residual gas, consisting of fumes leaving a boiler for oxy-combustion of a fuel by means of a gas rich in oxygen and carbon dioxide, in which case the purification unit may be a low-temperature purification unit so as to produce a fluid enriched with carbon dioxide and a residual fluid depleted of carbon dioxide;
- a unit for pretreatment of the residual fluid in order to obtain a flow enriched with carbon dioxide and a flow lean in carbon dioxide; and
- a unit for cryogenic treatment of the flow lean in carbon dioxide so as to extract therefrom a fraction enriched with argon, a fraction enriched with oxygen and a fraction depleted of argon and/or oxygen.
According to other optional aspects:
- the unit for cryogenic treatment of the flow lean in carbon dioxide comprises at least one exchanger and at least one distillation column;
- at least one exchanger is a reboiler;
- at least one exchanger is a condenser;
- the treatment unit makes it possible to recover a fraction enriched or rich in argon and a fraction enriched or rich in oxygen;
- the installation comprises means for sending the oxygen-enriched fraction to the boiler and/or to the pretreatment unit;
- the treatment also makes it possible to recover a nitrogen-enriched fraction;
- the installation comprises means for sending one or more fluid(s), which come from the unit for separating the gases in air, delivering at least part of the oxygen for the installation supplied with oxygen, for example the oxy-fuel combustion, to the unit for cryogenic treatment of the flow lean in carbon dioxide;
- one of these fluids is a nitrogen-rich liquid for keeping the treatment cold;
- one of these fluids is a nitrogen-rich gas which will be used as a cycle gas for at least one reboiler and/or at least one condenser of the unit for cryogenic treatment of the flow lean in carbon dioxide;
- the pretreatment unit is/comprises a carbon dioxide antisublimation/sublimation unit comprising a plurality of exchangers in parallel;
- the sublimation unit is connected to a conduit for conveying the oxygen-enriched fraction so as to form a mixture of carbon dioxide and oxygen, and optionally means for sending the mixture to the fuel oxy-combustion unit;
- the pretreatment unit is/comprises an installation of the TSA, PSA or VPSA type which produces a fraction enriched with carbon dioxide and a fraction depleted of carbon dioxide but enriched with argon;
- the pretreatment unit is/comprises an absorption installation;
- the absorption process uses an aqueous solution of basic pH;
- the basic pH is obtained by injecting NaOH, Na2CO3, NH3;
- the absorption process is a process of washing with methanol;
- the pretreatment unit is/comprises a permeation unit;
- the installation comprises means for recycling the flow enriched with carbon dioxide from the pretreatment unit into the boiler.
The oxygen sent by the air separation apparatus to the oxy-fuel combustion comprises at most 98 mol % of oxygen, preferably at most 97 mol % of oxygen, or at most 96 mol % of oxygen.
The oxygen sent by the air separation apparatus to the installation, for example the oxy-fuel combustion, comprises at least 1 mol % of argon, preferably at least 2 mol % of argon, or at least 3 mol % of argon.
The argon-enriched gas produced by the apparatus comprises at least 50 mol % of argon, preferably at least 70 mol % of argon, or at least 90 mol % of argon.
The invention will be described in more detail with reference to the figures. FIG. 1 shows an oxy-fuel combustion installation comprising units for purifying the fumes, FIG. 2 shows the units for purifying the fumes in more detail, FIG. 3 shows a unit for purifying the fumes with respect to CO2 at low temperature, FIG. 4 shows an apparatus for recovering nitrogen and/or oxygen and/or argon from a residual gas of the unit of FIG. 4, and FIG. 5 shows a variant of FIG. 4.
FIG. 1 is a schematic view of an oxy-fuel combustion installation. An air separation apparatus 2 produces an oxygen flow 10 with a typical purity of 95 mol % so as to maximize its argon content, and a residual nitrogen flow 13. The apparatus also produces gaseous nitrogen 13, and liquid nitrogen 159 which is intended for treatment of the incondensables. The oxygen flow 10 is divided into two factions 11 and 12. The main fume recycle flow 15 passes through the units 3 in which the coal 14 is converted into powder. The fraction 11 is mixed with the recycle flow downstream of the unit 3, and the mixture is sent to the combustion chamber of the boiler 1. The fraction 12 is mixed with a secondary fume recycle flow 16, which provides the burners with ballast in order to maintain the temperatures at acceptable levels. Water 17 is sent to the boiler 1 in order to produce steam 18, which is expanded in a turbine 8. Fumes 19 rich in CO2, typically containing more than 70 mol % (not counting the steam) undergo several treatments in order to remove impurities. The unit 4 removes the NOx, for example by catalysis. The unit 5 subsequently removes the dust, and after this the unit 6 is a desulfurization system for removing the SO2 and/or the SO3. The units 4 and 6 may be superfluous, depending on the composition of the product required. The purified flow 24 coming from the unit 6 (or 5 if there is no 6) is sent to a compression and purification unit 7 in order to produce a relatively pure flow of CO2 25 and a residual flow 26.
FIG. 2 is a schematic view of the compression and purification unit 7 of FIG. 1. A flow 110 (corresponding to the flow 24 of FIG. 1) enters a unit 101 in which it is prepared upstream of the compression in the unit 102. In the unit 101, the flow 110 may be purified with respect to dust, SO2 and/or SO3, and/or cooled.
The residual flow 111 produced by the unit 101 may be condensed water, dust or H2SO4, HNO3, Na2SO4, CaSO4, Na2CO3, CaCO3, etc.
The compression unit 102 compresses the flow 112 coming from the unit 101, from a pressure close to atmospheric pressure to a high pressure of between 15 and 60 bar abs, preferably around 30 bar abs. This compression may be carried out in a plurality of steps with intermediate cooling. In this case, condensates 113 may be produced. The heat of compression may be recovered in order to preheat the water 17. A hot flow 114 leaves the compression unit 102 and enters the unit 103. This unit cools the flow 114, dries it and optionally purifies it with respect to mercury, producing residuals 115, 116 and 117.
The unit 104 is a low-temperature purification unit. In this case, “low-temperature” means a minimum temperature in the cycle of the purification process below 0° C. and preferably below −20° C., or even as close as possible to the triple point of pure CO2 at −56.6° C. In this unit, the flow 118 is cooled and partially condensed in one or more steps. One or more flows enriched with CO2 are expanded and vaporized in order to obtain a product enriched with CO2 119. A high-pressure flow of incondensables 120 is recovered from the unit 104 and sent to a pretreatment unit 122. The pretreated flow 123 is sent to a treatment unit 124 in which one or more fluids are produced, which may be liquid and/or gaseous nitrogen 125 and/or liquid and/or gaseous oxygen 126 and/or gaseous and/or liquid argon 127.
The product rich in CO2 119 is compressed in a compression unit 105. In the unit 105, the compressed flow 121 is condensed and may be pumped.
FIG. 3 shows a low-temperature purification apparatus which corresponds to the unit 104 of FIG. 2. The flow 118 comprising fumes at about 30 bar and at a temperature of between 15° C. and 43° C. is filtered at 3 in order to form the flow 5. The flow 118 comprises above all carbon dioxide as well as NO2, oxygen, argon and nitrogen. It may be produced directly at high pressure by the unit 103, or it may be compressed by a compressor (in dashes) 2. The flow 5 is cooled in an exchange line 9 and is partially condensed. A part 7 of the flow 5 is not cooled in the exchange line 9, but instead mixed with the rest of the flow 5 downstream of the exchange line in order to vary the temperature of the mixture. The partially condensed flow is sent to a first phase separator 11 and separated into a gas phase 13 and a liquid phase 17. The gas phase 13 is divided into two in order to form a flow 15 and a flow 21. The flow 21 is used for reboiling the column 43 in the exchanger 25, then is sent to a second phase separator 22. The flow 15 short-circuits the reboilers in order to regulate the reboiling.
The liquid 17 from the first phase separator 11 is expanded in a valve 19 and the liquid flow 29 of the second phase separator 22 is expanded in a valve 31, and the two expanded flows are then sent to the head of the column 43. The column 43 is used principally to remove the incondensable components (oxygen, nitrogen and argon) from the feed flow 118.
A flow depleted of carbon dioxide 33 is drawn off from the head of the column 43 and sent to the compressor 35. The compressed flow 37 produced in this way is recycled to the flow 5.
A flow enriched or rich in carbon dioxide 67 is drawn off from the bottom of the column 43 and divided into two. One part 69 is pumped by the pump 71 in order to form a flow 85, subsequently pumped in the pump 87 and then removed from the system. The flow 85 corresponds to the flow 25 of FIG. 1. The remainder 73 of the flow 67 is used to keep the apparatus cold.
It is recommendable to purify the flow 118 with respect to NO2.
The incondensables may be separated before or after the NO2 separation.
In FIG. 3, after the split from the flow 69, the remainder 73 of the flow enriched with carbon dioxide is vaporized in the exchange line 9 and sent to a column for purification with respect to NO2 105.
This column may have a head condenser and a bottom reboiler, the flow 73 being delivered to an intermediate point. Otherwise, if there is no bottom reboiler, the flow is delivered to the bottom.
A flow lean in NO2 79 is drawn off from the column 105 and returned to the exchange line 9. This flow 79 is heated, compressed in the compressors 75, 77, sent to the exchanger 65, drawn off as the flow 78, cooled in the exchangers 81, 83 and mixed with the flow 69 in order to form the flow 85. The exchanger 81 may be used to heat the water intended for a boiler. The exchanger 83 is cooled by a flow of refrigerant 185, which may be R134a, ammonia, water, etc., the heated refrigerant being denoted as 187. A flow enriched with NO2 84 is drawn off from the bottom of the column 105. This flow 84 is recycled to a point upstream of the filters 3.
Head gas 32 from the second phase separator 22 is cooled in the exchanger 55 and sent to the third phase separator 133. A part of the liquid from the third phase separator 133 is sent to the column 43 and the remainder, as a flow with intermediate purity 45, is divided into two flows 47, 141. The flow 47 is vaporized in the exchanger 55 and sent to the head of the column 43 or mixed with the flow 33.
The flow 141 is expanded in a valve, heated in the exchangers 55, 9, compressed in the compressor 59, cooled as a flow 91 in the exchanger 60 and mixed with the compressed flow 5. The valve which is used to expand the flow 141 may be replaced by a liquid turbine.
The head gas from the third phase separator 133 is cooled in a heat exchanger 55, optionally after compression in a compressor 134, and sent to a fourth phase separator 143. The head gas, lean in carbon dioxide 157, from the fourth phase separator 143 is heated in a heat exchanger 55, then in the exchanger 9, heated in the exchanger 65 and expanded as a flow 23 in the exchanger 63 coupled to the compressor 35. The gas lean in carbon dioxide 157 comprises between 30 and 45% of carbon dioxide and between 30 and 45% of nitrogen. It also comprises substantial quantities of oxygen and argon. The bottom liquid 51 from the phase separator 143 is sent to the column 43 with the flow 47.
The flow expanded in the turbine 63 is mixed with the flow 115 which does not pass through the turbine, and subsequently reheated at 89. A part 97 of the heated flow is expanded in the turbine 61 and sent to the atmosphere as a flow 99.
A flow 120 rich in incondensables (oxygen and/or argon and/or nitrogen) and containing CO2 is recovered in the unit 104 in order to recover at least one of its components as a product. This flow 120 may be a part of the flow 101 coming from the turbine 61 and/or a part of the head gas 157 from the fourth phase separator 143 upstream of the exchanger 55 and/or a part of the flow expanded in the turbine 63 and/or a part of the flow 157 downstream of the exchanger 9.
TABLE 1
|
|
Molar fractions in percentages (example)
|
for O2, N2, Ar, CO2.
|
FLUIDS/
|
Components
118
33
67
84
157
141
78
|
|
O2
2.5
4.8
0
0
13.3
2.3
0
|
N2
7.8
11
0
0
43.8
0.1
0
|
Ar
1.9
4.9
0
0
9.5
2.6
0
|
CO2
87.8
79.3
99.95
99
33.4
95
100
|
NOx
250
50 ppm
500
1
5 ppm
500 ppm
0
|
ppm
ppm
|
|
FIG. 4 shows an apparatus for pretreatment and an apparatus for separation by cryogenic distillation of the flow 120. This flow 120 is first pretreated in the pretreatment unit 122. This pretreatment unit removes at least 50 mol % of the carbon dioxide in the residual gas 120, before the cryogenic treatment producing a flow 169 enriched with CO2 which can be recycled to the unit 104 with the flow 118.
The pretreatment may be carried out by antisublimation/sublimation of the carbon dioxide in a plurality of exchangers in parallel. As an alternative, the pretreatment may be carried out by absorption (for example washing with methanol), adsorption, permeation or several of these techniques.
The sublimation of the carbon dioxide is carried out, for example, in the presence of an oxygen-enriched fraction so as to form a mixture of carbon dioxide and oxygen used for the oxy-combustion of the fuel. By virtue of the anti-sublimation, the temperature of the treated gas falls from −56.6° C. (triple point of CO2) to −170° C/−175° C., a temperature at which cryogenic distillation of the gases in air can be carried out.
Otherwise, the pretreatment may be carried out by a process of the TSA, PSA or VPSA type so as to produce a fraction enriched with carbon dioxide and a fraction depleted of carbon dioxide but enriched with argon.
The pretreatment may be carried out by an absorption process, using for example an aqueous solution of basic pH. The basic pH is optionally obtained by injecting NaOH and/or Na2CO3 and/or NH3. The absorption process may also use a non-aqueous fluid such as methanol. In this case, the absorption will be carried out at low temperature and preferably under pressure.
As an alternative, the pretreatment is carried out by permeation or by a combination of the various processes mentioned.
It is possible to remove all the carbon dioxide in the pretreatment unit in order then to deliver a flow containing a few ppm of carbon dioxide. This makes it possible to use a plate exchanger and a finned exchanger as exchangers.
On the other hand, if the flow 123 still contains carbon dioxide, it is necessary to continue the pretreatment by using reversible exchangers or exchangers of the regenerator type, as described on page 475 of “Tieftemperaturtechnik” [Low-temperature technology], sections 9.4.2.3 and 9.4.2.4, pub. Springer Verlag. Thus, the remaining carbon dioxide may be removed by passing through an exchanger 130 of one of these two types. Clearly, the feed flow 123 should no longer contain more than a few ppm of carbon dioxide at the entry of the columns.
After the pretreatment, the flow depleted of carbon dioxide 123 is sent to a cryogenic distillation unit 124 as illustrated in FIG. 4. The flow 123 is cooled to a cryogenic temperature in an exchanger 130 and sent to the middle of a column 131 having a bottom reboiler 133. As an alternative, the flow 123 could be cooled by expansion in a turbine with production of work (isentropic expansion). Gaseous oxygen GOX is drawn off from above the bottom of the column 131, heated in the exchanger 130 and used as a product 126 and/or recycled to the pretreatment 122 and/or to the boiler 1. Liquid oxygen 136 may be drawn off from the bottom of the column 131, for example as a product. An argon-enriched flow 141 is sent from the column 131 to the column 137, and a flow of impure argon 145 is drawn off from below the condenser 155 of this column 137. A flow of bottom liquid 143 is returned to the column 131. The impure argon 145 is purified in a denitrogenation column 139 comprising a head condenser 153 and a bottom reboiler 151. Liquid argon 127 is produced at the bottom of the denitrogenation column 139. The apparatus is kept cold at least partially by injecting liquid nitrogen 159 coming from the air separation apparatus 2 supplying the oxy-fuel combustion. The liquid nitrogen 159 is sent to the head of the column 131. This air separation apparatus 2 also delivers gaseous nitrogen 13, which is cooled in the exchanger 130 and heats the bottom reboiler 133 of the column 131 in order to form a condensed flow. The condensed flow is sent in part 147 after expansion to the head condenser 153 of the denitrogenation column 139, in part 165 to the head condenser 155 of the column 137 and in part 157 after expansion to the head of the column 131. The nitrogen 163 vaporized in the condenser 153 is mixed with the head gas 135 of the column 131, heated in the subcooler 160 and the exchanger 130, and forms the gaseous nitrogen 165. The nitrogen 161 vaporized in the condenser 155 forms the nitrogen flow 161.
At least one column of the apparatus 124 may optionally be contained in the same coldbox as at least one column of the apparatus 2. The transfers of nitrogen 13 and/or 159 can thus take place without having to reheat and cool the nitrogen. For the case of FIG. 4, the columns 137, 139 may be omitted if it is not necessary to recover argon.
FIG. 5 shows a variant of the cold part of FIG. 4, in which the cold mixture 123 coming from the exchanger 130 is sent to an intermediate level of a column 163 without a reboiler or head condenser. The head gas 171 from the column 163 constitutes the gaseous nitrogen, and the bottom liquid 173 is sent to a column 165 at an intermediate position. Gas 175 is returned from the intermediate position of the column 165 to the bottom of the column 163. The column 165 has a bottom reboiler 175 and a head condenser 177. Gaseous oxygen 126 and/or liquid oxygen 136 is recovered at the bottom of the column 165, and the head liquid 145 is sent to a denitrogenation column 167, the liquid argon being formed 127 in the bottom of the latter. The denitrogenation column has a bottom reboiler 151 and a head condenser 153.
Liquid nitrogen 159 coming from the air separation apparatus 2 is sent to the head of the column 163. The column 163 has a head condenser which, like all the reboilers and condensers of FIG. 5, operates by a cycle of gaseous nitrogen coming from the air separation apparatus 2, which cycle is not illustrated but is similar to that of FIG. 4.
Optionally, the delivery of liquid nitrogen 159 may constitute the only source of cooling for the process.
Ways of separating the flow 123 by cryogenic distillation other than those illustrated in FIGS. 4 and 5 may of course be envisaged.
Patent Application
20120067082
