Method and installation for liquefying flue gas from combustion installations

Abstract

A method and a plant for producing liquid CO.sub.2 from flue gas as described with reduced energy consumption and a stable behavior.

Claims

1. A method for producing liquid CO.sub.2 product out of combustion flue gas, the method comprising: partially condensing the flue gas in sequence through at least two separation stages to form a two phase mixture of an off gas to be vented and a liquid CO.sub.2 stream; separating the off gas to be vented from the liquid CO.sub.2 stream in at least the last separation stage; wherein the off gas to be vented and the liquid CO.sub.2 stream are expanded and the at least two separation stages are cooled by the expanded off gas to be vented and the expanded liquid CO.sub.2; and wherein the expanded liquid CO.sub.2 is further separated after having passed the last separation stage of the at least two separation stages in an additional separation drum; and wherein the liquid CO.sub.2 moving out of the additional separation drum is split into a part of the liquid CO.sub.2 and a remainder of the liquid CO.sub.2; and wherein gaseous CO.sub.2 moving out of the additional separation drum and the part of the liquid CO.sub.2 moving out of the additional separation drum are expanded to a first pressure level for cooling the last separation stage; and wherein the pressure of the remainder of the liquid CO.sub.2 moving out of the additional separation drum is raised to a second pressure level and the remainder of the liquid CO.sub.2 moving out of the additional separation drum is then expanded for cooling the CO.sub.2 in the at least two separation stages and conveyed to a compression stage of a multistage compressor; and wherein the gaseous CO.sub.2, the part of the liquid CO.sub.2 moving out of the additional separation drum, and the remainder of the liquid CO.sub.2 moving out of the additional separation drum are compressed with the multistage-compressor to produce liquid CO.sub.2.

2. The method according to claim 1, wherein the remainder of the liquid CO.sub.2 moving out of the additional separation drum is expanded to a pressure of approximately 15 bar to 25 bar.

3. The method according to claim 1, wherein the liquid CO.sub.2 from separation stage before the last separation stage is expanded to the pressure of the remainder of the liquid CO.sub.2 moving out of the additional separation drum and the liquid CO.sub.2 from the separation stage before the last separation stage and the remainder of the liquid CO.sub.2 moving out of the additional separation drum are used for cooling purposes in the separation stage before the last separation stage.

4. The method according to claim 3, wherein the liquid CO.sub.2 from the separation stage before the last separation stage and the remainder of the liquid CO.sub.2 moving out of the additional separation drum are fed to a second or third stage of a multistage-compressor.

5. The method according to claim 1, wherein the flue gas is compressed in a first compressor, cooled in a first cooler and/or dried in a drier before entering a first separation stage of the at least two separation stages.

6. The method according to claim 1, wherein offgas from the last separation stage of the at least two separation stages is expanded to approximately 17 bar and resulting in a temperature of approximately −54° C. before entering a heat exchanger of the last separation stage of the at least two separation stages.

7. The method according to claim 1, wherein the offgas is superheated in a superheater after having passed all separation stages of the at least two separation stages and expanded in an expansion machine and subsequently fed again to a heat exchanger of the last separation stage of the at least two separation stages.

8. A plant for producing liquid CO.sub.2 product out of combustion flue gas, comprising: at least two separation stages where the flue gas is partially condensed and divided into off gas to be vented and liquid CO.sub.2 at least in a last separation stage thereof; pressure reducing valves configured to expand the off gas to be vented and the liquid CO.sub.2; and wherein the at least two separation stages are arranged to be passed through in sequence by the flue gas and are cooled by the expanded off gas to be vented and by the expanded liquid CO.sub.2; an additional separation drum configured and arranged to receive the expanded liquid CO.sub.2 from the first expansion device and separate the expanded liquid CO.sub.2 into liquid CO.sub.2 and gaseous CO.sub.2; a splitter wherein the liquid CO.sub.2 moving out of the additional separation drum is split into a part of the liquid CO.sub.2 and a remainder of the liquid CO.sub.2, and wherein the gaseous CO.sub.2 moving out of the additional separation drum and the part of the liquid CO.sub.2 moving out of the additional separation drum are expanded to a first pressure level for cooling the last separation stage; a pump and a pressure reducing valve, where the pressure of the remainder of the liquid CO.sub.2 moving out of the additional separation drum is raised to a second pressure level and expanded for cooling the last separation stage of the at least two separation stages; and a multistage compressor, wherein a stage of the multistage compressor is configured and arranged to compress the gaseous CO.sub.2 moving out of the additional separation drum, the part of the liquid CO.sub.2 moving out of the additional separation drum and the remainder of the liquid CO.sub.2 moving out of the additional separation drum to produce liquid CO.sub.2.

9. The plant according to claim 8, wherein each separation stage of the at least two separation stages comprises a heat exchanger and a separation drum for separating liquid CO.sub.2 from gaseous CO.sub.2.

10. The plant according to claim 8, further comprising at least one expansion machine and/or at least one offgas superheater.

11. The plant according to claim 8, further comprising a first compressor, a first cooler, and a drier.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Referring to the drawings, wherein like items are numbered alike in the various Figures:

(2) FIG. 1 depicts an installation for CO.sub.2 liquefaction out of flue gases according to the prior art and

(3) FIG. 2 depicts an embodiment of the installation for CO.sub.2 liquefaction according to the invention. In FIG. 2 identical components are designated with identical reference numerals. The statements concerning FIG. 1 correspondingly apply.

DETAILED DESCRIPTION

(4) Referring to FIG. 2, treatment of the flue gas stream 3 in the first compressor 1, the first cooler 5, the drier 7, the first heat exchanger 11 and the first separation drum 13 exactly takes place as described by means of FIG. 1. As well, the gaseous phase 3.2 is extracted at the head of the first separation drum 13, as explained in FIG. 1, transported through the second heat exchanger 17 and then conveyed to the second separation drum 19. The two phases (liquid and gaseous) of the partial stream 3.2 are divided in the second separation drum 19 into the offgas stream 23 and liquid CO.sub.2. At the bottom of the second separation drum 19 this partial stream is extracted and has the reference numeral 3.3 such as in FIG. 1.

(5) As already explained by means of FIG. 1, the partial stream 3.3 is expanded to a pressure of 15,5 bar in a third pressure reducing valve 15.3, thereby cooling down to −54° C. The partial stream 3.3 streams through the second heat exchanger 17, thereby taking up heat from the partial stream 3.2 of the flue gas and enters with a temperature of approximately −47° C. (cf. flag no. 8) and is conveyed into a third separation drum 33.

(6) There the partially liquid and partially gaseous CO.sub.2 has a pressure of approximately 16,5 bar and a temperature of −47° C. (cf. flag no. 9).

(7) At the head of the third separation drum 33 the gaseous phase is extracted and expanded in a fourth pressure reducing valve 15.4. The gaseous partial stream being extracted at the head of the third separation drum 33 is designated with reference numeral 3.4 in FIG. 2. At the foot of the third separation drum 33 a smaller liquid partial stream 3.5 is extracted and expanded in a fifth pressure reducing valve 15.5. Subsequently the partial streams 3.4 and 3.5 are brought together again. Then they have a pressure of approximately 5 to 10 bar and a temperature of −54° C. (cf. flag no. 7d).

(8) The liquid CO.sub.2 present in the third separation drum 33 is brought to an increased pressure level of approx. 20 bar to 23 bar in a sixth partial stream 3.6 by a second CO.sub.2 pump 35 (cf. flag no. 7e)

(9) In a sixth pressure reducing valve 15.6 the CO.sub.2 which has been liquid so far is expanded to a pressure of approximately 20 bar, with a temperature of −45° C. With this partially liquid, partially gaseous CO.sub.2 the flue gas stream 3.2 in the second heat exchanger 17 is cooled. As the entrance temperature of the partial stream 3.6 is higher than the entrance temperatures of the offgas 23 as well as the partial stream 3.3, the partial stream 3.2 first is cooled with the partial stream 3.6. Thus it is possible to take up heat from the partial stream 3.2 even with this higher temperature of −47° C. In FIG. 2 as well this fact can be graphically clearly seen.

(10) The partial stream 3.2 leaves the second heat exchanger 17 with a temperature of approximately −22° C. to −29° C. and is brought together with the partial stream 3.1 extracted before from the first separation drum 13. As there is a pressure of approximately 34.5 bar in the first separation drum 13, the liquid partial stream 3.1 from the first separation drum 13 is expanded to approximately 20 bar in a seventh pressure reducing valve 15.7. These two partial streams 3.1 and 3.6 brought together enter the first heat exchanger 11 with a temperature of approximately −22° C. to −29° C. (cf. flag no. 10), thereby taking up heat from the flue gas stream 3. They leave the first heat exchanger (cf. flag no. 11) with a temperature of approximately 25° C. and a pressure of approximately 18 bar and can thus be conveyed to the second compression stage of the second compressor 25.

(11) As the partial streams 3.1 and 3.6 can be conveyed to the second compression stage of the second compressor 25, the partial stream 3.3, which has to be conveyed to the first compression stage of the second compressor 25, is correspondingly reduced. Consequently the power required by the second compressor 25 is smaller. This has positive effects on the energy demand of the installation according to the invention.

(12) A second possibility of reducing the energy demand of the CO.sub.2 liquefaction plant can be seen in not only overheating the offgas 23 in the offgas superheater 19 after the exit from the first heat exchanger 11, but also re-conveying it to the second heat exchanger 17 after the expansion in the expansion turbine 31. After the overheating the offgas has a temperature of approximately 80° C. to approximately 100° C. with a pressure of approximately 26 bar (cf. flag no. 17). By the expansion in the expansion machine 31 the pressure drops to 2.3 bar and the offgas reaches a temperature of −54° C. Thus the offgas can once more contribute to the cooling of the flue gas stream 3 resp. the partial stream 3.2. Afterwards the offgas can be emitted to the surroundings with a low pressure and approximately surrounding temperature. It is also possible to carry out a multi-stage expansion and overheating of the offgas 23 (not shown in FIG. 2).

(13) This as well results in a considerable reduction of the energy demand of the installation according to the invention, as on the one hand the offgas 23 contributes to a greater amount to the cooling of the flue gas stream 3 resp. the partial stream 3.2 and the expansion machine 31 generates mechanical work, which e.g. can be used for driving the first compressor 1 or the second compressor 25. All in all it can be stated that the method according to the invention and the installation for CO.sub.2 liquefaction required for carrying out the method according to the invention are still relatively simple in their design in spite of the considerable advantages.

(14) A further advantage is that the partial stream 3.6 is expanded to a pressure with which it is possible to bring it together with the partial stream 3.1 being extracted as liquid phase from the first separation drum 13. So that these two partial streams can be brought to common pressure and temperature level and conveyed to the second compression stage of the second compressor.

(15) Furthermore, this setup clearly improves the control over the flue gas condensation. With adjustment of the flow rate over the CO.sub.2 pump 35 the driving force for heat transfer, the Logarithmic Mean Temperature Difference (LMTD), is varied. In this way the performance of the second separation stage 21 can be adjusted. This is especially important, when operating at condensation temperatures near the sublimation and freezing point of CO.sub.2.

(16) In order to maximize the described effect, the heat recovery out of the offgas from separation can be increased by having the vent gas recirculated to the cold box, after expansion, at least once before releasing it to the atmosphere.

(17) TABLE-US-00001 Table of flags, pressures and temperatures. Temperature, approx. Pressure, approx. Flag no. [° C.] [bar]  1   13 35.7  2   13 35  3 — —  4 — —  5 −19 34.7  6 −50 34.3  7 −53° C. 5 to 10  7a −54 27  7b −54 5 to 10  7c −54 15.5  7d −54 5 to 10  7e −45 ≈20 to 23   7f −45 20  8 −47 16.5  9 −47 16.5 10 −22 to −29 18.4 11   25 18 12  −7 5-10 13 −22 to −29 20 14 −22 to −29 5-10 15 — 16   26 to 30 26 17   80 to 100 25.8 18 −54 2.3 19   80 to 130 60 to 110 The tolerances for The tolerances for the temperatures are the pressures are ±5 ±5° C. bar

(18) While the invention has been described with reference to a number of preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.