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

Description:
BACKGROUND 
     The present disclosure relates to a method and a device for the liquefaction of the CO 2  contained in the flue gases. 
     Most cryogenic methods for the production of CO 2  out of combustion flue gases use conventional separation schemes having two or more separation stages. In  FIG. 1  such an installation is shown as block diagram. 
     In the  FIGS. 1 and 2  the temperature and the pressure at various points of the flue gas stream as well as of the CO 2  are indicated by so-called flags. The temperatures and the pressures belonging to each flag are compiled in a chart in the following. It is obvious for a man skilled in the art that these temperatures and pressures are meant as an example. They can vary depending on the composition of the flue gas, the ambient temperature and the requested purity of the liquid CO 2 . 
     In a first compressor  1  the flue gas is compressed. This compression can be a multi-stage compression process with coolers and water separators between each compression stage (not shown) separating most of the water vapour resp. water from the flue gas. 
     In  FIG. 1  the flue gas stream is designated with reference numeral  3 . When being emitted by the first compressor  1  the flue gas has a temperature significantly higher than the ambient temperature and then is cooled to approximately 13° C. by a first cooler  5 . The pressure is approximately 35.7 bar. 
     The moisture still contained in the flue gas stream  3  is freed from water by a suitable drying process e.g. adsorption dried in a drier  7  and subsequently conveyed to a first separation stage  9 . This first separation stage  9  comprises a first heat exchanger  11  and a first separation drum  13 . The first heat exchanger  11  serves for cooling the flue gas stream  3 . As a result of this cooling a partial condensation of the CO 2  contained in the flue gas stream  3  takes place. Consequently, the flue gas stream  3  enters the first separation drum  13  as a two-phase mixture. There the liquid phase and the gaseous phase of the flue gas stream are separated by means of gravitation. In the first separation drum the pressure is approximately 34,7 bar and the temperature is −19° C. (cf. flag no. 5). 
     At the bottom of the first separation drum  13  liquid CO 2  is extracted and via a first pressure reducing valve  15 . 1  expanded to a pressure of approximately 18.4 bar (cf. ref. No. 3.1). This results in a temperature of the CO 2  between −22° C. and −29° C. (cf. flag no. 10). The partial CO 2  stream  3 . 1  of the flue gases is heated and evaporated in the first heat exchanger  11  by the flue gas stream  3 . At the exit of the first heat exchanger  11  the partial stream  3 . 1  has a temperature of approximately 25° C. and a pressure of approximately 18 bar (cf. flag no. 11). 
     When the second partial stream  3 . 2  being extracted at the head of the first separation drum  13  is followed it becomes clear that this partial stream  3 . 2  being extracted from the first separation drum  13  in a gaseous state is cooled in a second heat exchanger  17  and partially condensed. Afterwards this partial stream  3 . 2  being also present as two-phase mixture is conveyed to a second separation drum  19 . The second heat exchanger  17  and the second separation drum  19  are the main components of the second separation stage  21 . 
     In the second separation drum  19  again a gravity-supported separation between the liquid phase and the gaseous phase of the partial stream  3 . 2  takes place. In the second separation drum  19  there is a pressure of approximately 34,3 bar and a temperature of approximately 
     −50° C. (cf. Flag no. 11). 
     The gaseous phase in the second separation drum  19 , the so-called offgas  23 , is extracted at the head of the second separation drum  19 , expanded to approximately 17 bar in a second pressure reducing valve  15 . 2 , so that it cools down to approximately −54° C. 
     In the figures the offgas is designated with reference numeral  23 . The offgas  23  streams through the second heat exchanger  17  thereby cooling the flue gas  3 . 2  in the counter stream. 
     At the bottom of the second separation drum  19  liquid CO 2  is extracted and expanded to approximately 17 bar in a third pressure reducing valve  15 . 3 , so that it reaches a temperature of −54° C. as well (cf. flag no. 7a). This partial stream  3 . 3  as well is conveyed to the second heat exchanger  17 . Wherein a part of the liquid CO 2  evaporates and a partial stream  3 . 3 . 1  is extracted from the second heat exchanger  17 , expanded to approximately 5 to 10 bar in a fourth pressure reducing valve  15 . 4 , so that here as well a temperature of −54° C. is reached (cf. flag no. 7b), and again conveyed to the second heat exchanger  17 . 
     After the partial stream  3 . 3 . 1  streamed through the second heat exchanger  17 , it again is brought together with the partial stream  3 . 3  and conveyed to the first heat exchanger  11 . At the entrance of the first heat exchanger  11  this partial stream has a pressure of approximately 5 to 10 bar with a temperature of −22 to −29° C. (cf. flag no. 14). 
     This partial stream  3 . 3  takes up heat in the first heat exchanger  11 , so that at the exit of same it has a temperature of approximately −7° C. with a pressure of approximately 5 to 10 bar. The third partial stream  3 . 3  is conveyed to a second compressor  25  at the first compressor stage, whereas the partial stream  3 . 1  having a pressure of approximately 18 bar is conveyed to the second compressor stage at the three-stage compressor  25  shown in  FIG. 1 . 
     Intercooler between the various stages of the second compressor  25  and an aftercooler for the compressed CO 2  are not shown in  FIG. 1 . 
     At the exit of the second compressor  25  the compressed CO 2  has a pressure of between 60 bar and 110 bar with temperatures of 80° C. to 130° C. In the aftercooler, which is not shown, the CO 2  is cooled down to ambient temperature. 
     If necessary the CO 2  can be either fed directly into the pipeline or liquefied and conveyed from a first CO 2  pump  27  e.g. into a pipeline (not shown). The first CO 2  pump  27  raises the pressure of the liquid CO 2  to the pressure given in the pipeline. 
     Going back to the offgas  23  it can be seen that the offgas streams through the second heat exchanger  17  and the first heat exchanger  11 , thereby taking up heat from the flue gas stream  3 . At the exit of the first heat exchanger  11  the offgas has a temperature of approximately 26° C. to 30° C. with a pressure of approximately 26 bar (cf. flag no. 16). 
     For maximising the energy recovery it is known to overheat the offgas  23  with an offgas superheater  29  and then convey it to a expansion turbine  31  or any other expansion machine. Wherein mechanical energy is recycled and afterwards the offgas is emitted into the surroundings with a low pressure approximately corresponding to the surrounding pressure. 
     This installation described by means of  FIG. 1  for liquefying CO 2  is relatively simple and works without problems. The disadvantage of this method and this installation for the production of liquid CO 2  out of flue gas of power plants e.g. fuelled with fossils is its high energy demand having negative effects on the net efficiency degree of the power plant. 
     SUMMARY 
     The present invention provides a method and an installation for liquefying the CO 2  contained in the flue gas operating with a reduced energy demand and thus increasing the net efficiency degree of the power plant. 
     At the same time the method is simple and the operation technique favourably controllable in order to guarantee a robust and trouble-free operation. 
     According to an embodiment of the present invention, these advantages are accomplished by conveying the partial stream  3 . 2  of the liquid CO 2  after the exit out of the second heat exchanger  17  to a third separation drum having a pressure of approximately 16,5 bar with a temperature of −47° C. Here again a separation of the liquid and the gaseous phase takes place and a considerable part of the liquid phase is increased in pressure by a second CO 2  pump (cf. flag no. 7e), afterwards expanded and can thus be used for cooling in the second heat exchanger. However, this partial stream must be expanded to only 20 bar, so that it can be conveyed together with the liquid phase from the first separation drum to the first heat exchanger and afterwards conveyed to the second compressor stage of the second compressor. 
     One advantage of this method is that only a smaller part of the liquid CO 2  of the liquid CO 2  present at the last separation stage has to be expanded to a pressure of 5 to 10 bar. It is rather possible to expand a considerably bigger part of the liquid CO 2  to a pressure of approximately 18 bar so that this increased part can be injected in the second compression stage of the second compressor. This results in a considerable reduction of the required power for the second compressor  25  having the direct effect of an improved net efficiency degree of the upstream power plant. The same applies to the method claims  8  to  10 . The advantages of the subclaims are explained in connection with  FIG. 2  in the following. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings, wherein like items are numbered alike in the various Figures: 
         FIG. 1  depicts an installation for CO 2  liquefaction out of flue gases according to the prior art and 
         FIG. 2  depicts an embodiment of the installation for CO 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 
     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 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 . 
     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 . 
     There the partially liquid and partially gaseous CO 2  has a pressure of approximately 16,5 bar and a temperature of −47° C. (cf. flag no. 9). 
     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). 
     The liquid CO 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 2  pump  35  (cf. flag no. 7e) 
     In a sixth pressure reducing valve  15 . 6  the CO 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 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. 
     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 . 
     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. 
     A second possibility of reducing the energy demand of the CO 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 ). 
     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 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. 
     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. 
     Furthermore, this setup clearly improves the control over the flue gas condensation. With adjustment of the flow rate over the CO 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 2 . 
     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. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 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 
               
               
                   
                   
               
             
          
         
       
     
     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.