Abstract:
The invention concerns a method involving the intake (through  16 ) of air in an intake of an air separation plant ( 14 ), supplying (though  16 ) at least a fraction of the intake air from a gas turbine compressor section ( 2 ), and extracting from the air separation plant ( 16 ) two gas streams (through  20, 24, 32 , and  36 ) respectively nitrogen-enriched and oxygen-enriched streams; detecting (through  18 ) a substantial decrease in the flow rate of the air fraction from the gas turbine compressor section ( 2 ), then recycling (through  28, 42 ), to the inlet of the air separation plant ( 14 ) at least part of one of the two gas streams, and thus avoiding any major fluctuation of the intake air flow rate of said air separation plant.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method and a plant for feeding an air separation unit by means of a gas turbine. 
   RELATED ART 
   Conventionally, a gas turbine comprises a compressor, a combustion chamber and an expansion turbine coupled to the compressor in order to drive the latter. This combustion chamber receives a combustion gas, and a certain amount of nitrogen, intended to lower the flame temperature in this combustion chamber, which makes it possible to minimize the discharge of nitrogen oxides to the atmosphere. 
   In a known manner, the combustion gas may be obtained by gasification, that is to say by oxidation of carbon products, such as coal or else oil residues. This oxidation is carried out in an independent unit, called a gasifier. 
   Conventionally, it is possible to combine this gas turbine with an air separation unit. The latter, which is usually a cryogenic unit comprising at least one distillation column, enables at least one gas stream mostly consisting of one of the gases of air, especially oxygen or nitrogen, to be supplied from air. 
   Combining this air separation unit with the gas turbine involves making use of at least one of the two aforementioned gas streams. For this purpose, the oxygen and the nitrogen produced in the air separation unit are admitted respectively into the gasifier and the combustion chamber. 
   The aim of the invention is more particularly the combined implementation of a gas turbine and of an air separation unit, in which the inlet air delivered to this separation unit is at least in part supplied by the gas turbine. 
   To this end, the delivery circuit of the compressor of this gas turbine is brought into communication with the inlet of the separation unit, replacing or in addition to an external feed compressor. This arrangement is in particular described in FR-A-2 690 711. 
   However, this known solution has some drawbacks. 
   This is because it has been noticed that feeding the air separation unit by means of the gas turbine is likely to lead to a loss of purity of the gases produced in the separation unit, such as oxygen, nitrogen or argon, or even to an inadvertent shutdown of this separation unit. 
   In the latter case, this unit should then be restarted, which involves a consequent loss of time, together with considerable energy consumption. 
   SUMMARY OF THE INVENTION 
   The invention proposes overcoming these drawbacks. For this purpose, the subject of the invention is a method for feeding an air separation unit by means of a gas turbine, in which inlet air is admitted into an inlet of the said separation unit, at least a portion of the said inlet air is supplied from the said gas turbine, and two gas streams, enriched respectively with nitrogen and with oxygen, are extracted from the separation unit, characterized in that an appreciable decrease is detected in the flow rate of the portion of air coming from the gas turbine, then at least part of at least one of the two gas streams is recycled, towards the inlet of the separation unit. 
   According to other characteristics of the invention:
         at least part of each of the two gas streams is recycled towards the inlet of the separation unit;   the appreciable decrease in the flow rate of the said portion of air coming from the gas turbine is detected by detecting when this flow rate falls below a predetermined value;   the predetermined value corresponds to an instantaneous decrease of at least 5% in the flow rate of the portion of inlet air coming from the gas turbine;   the appreciable decrease in the flow rate of the said portion of air coming from the gas turbine is detected by detecting a shutdown of the gas turbine;   during normal operation of the gas turbine, substantially all the inlet air is supplied from the gas turbine;   after having detected the appreciable decrease in the flow rate of the portion of air coming from the gas turbine, substantially all of the or each extracted gas stream is recycled towards the inlet of the separation unit;   a portion of make-up air is dispatched, the flow rate of which is substantially less than the flow rate of the or each recycled gas stream;   during normal gas turbine operation, only part of the inlet air is supplied from the gas turbine;   after having detected the appreciable decrease in the flow rate of the portion of air coming from the gas turbine, only part of the or each extracted gas stream is recycled towards the inlet of the separation unit;   a gasifier is fed by means of the other, non-recycled, part of the oxygen-rich gas stream which is not recycled;   oxygen make-up is supplied to the gasifier, in addition to the said other part of the oxygen-rich gas stream;   the other, non-recycled, part of the nitrogen-rich gas stream is discharged to the atmosphere.       

   The subject of the invention is also a plant for feeding an air separation unit by means of a gas turbine, comprising a gas turbine having means of supplying compressed air, in particular a compressor, an air separation unit comprising inlet air feed means, these feed means comprising at least first feed means, combined with the supply means of the gas turbine, together with first and second means, outside the said unit, of removing two gas streams respectively enriched with nitrogen and with oxygen, characterized in that it furthermore comprises means of recycling at least one of the two gas streams, capable of bringing at least the first or second removal means into communication with the air feed means of the air separation unit. 
   According to other characteristics of the invention:
         the recycling means are means of recycling each of the two gas streams, capable of bringing the first and second removal means into communication with the air feed means;   the plant also comprises means of detecting an appreciable decrease in the flow rate of air flowing in the first feed means, these detection means being put in connection with the control means, in particular valves, capable of controlling the flow rates of gas flowing in the first and/or second removal means and the recycling means;   the detection means comprise means for measuring the flow rate of air flowing in the first feed means;   the detection means comprise means of detecting a shutdown of the gas turbine;   the recycling means comprise at least one line, which connects the outlet of a compressor of a respective gas stream with the air feed means of the separation unit.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described below, with reference to the appended drawings given solely by way of non-limiting example, in which: 
       FIG. 1  is a schematic view, illustrating a plant according to a first embodiment of the invention, during normal operation of the gas turbine; 
       FIG. 2  is a view similar to  FIG. 1 , illustrating the plant of  FIG. 1 , when the gas turbine is shut down; 
       FIG. 3  is a view similar to  FIG. 1 , illustrating a plant according to a second embodiment of the invention, during normal operation of the gas turbine; and 
       FIG. 4  is a view similar to  FIG. 3 , illustrating the plant of  FIG. 3  when the gas turbine is shut down. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   The plant shown in  FIGS. 1 and 2  comprises a gas turbine, denoted overall by the reference  2 , which conventionally comprises an air compressor  4 , an expansion turbine  6  coupled to the compressor  4 , and a combustion chamber  8 . This gas turbine  2  is also provided with an alternator  10 , driven by a shaft  12  common to the compressor  4  and to the turbine  6 . 
   The plant of  FIG. 1  also comprises an air separation unit, of known type, denoted overall by the reference  14 . The inlet of this separation unit  14  is fed with air by a line  16 , brought into communication with the delivery circuit  5  of the compressor  4 . 
   This line  16  is equipped with a valve  17 , and with a flow rate sensor  18 . The separation unit operates cryogenically and comprises, for this purpose, several distillation columns (not shown). 
   A line  20 , outside the unit  14 , enables a first stream W of waste nitrogen to be extracted. This stream contains at least 90 mol %, preferably at least 95 mol % nitrogen, as well as a few percent oxygen. 
   This line  20  emerges in a compressor  22 , downstream of which extends a line  24 , which is provided with a valve  26  and emerges into the combustion chamber  8 . A line  28 , fitted with a valve  30 , connects the lines  16  and  24 . 
   Outside the unit  14 , a line  32  enables an oxygen-rich gas stream GOX, which contains at least 70 mol %, preferably at least 80 mol %, oxygen to be extracted. This line  32  emerges in a compressor  34 , downstream of which extends a line  36 , fitted with a valve  38 . 
   This line  36  emerges in a gasifier  40 , of conventional type, which is fed by a tank (not shown) containing carbon products such as coal. A line  42 , fitted with a valve  44 , connects the line  16  and the line  36 . 
   A line  46 , which extends downstream of the gasifier  32 , conveys the fuel gas arising from the aforementioned oxidation of the carbon products. This line  46 , which is equipped with a valve  48 , is brought into communication with the combustion chamber  8  of the gas turbine. 
   Moreover, the sensor  18  is put in connection with the valves  26 ,  30 ,  38  and  44 , by the respective control lines, shown in dot-dash lines, which are allocated references  26 ′,  30 ′,  38 ′ and  44 ′. 
   The operation of the aforementioned plant, during normal operation of the gas turbine  2 , will be described below, with reference to FIG.  1 . 
   The air separation unit  14  receives compressed air from the compressor  4  and conventionally produces two gas streams, respectively enriched with nitrogen and oxygen, which are conveyed by the line  20  and the line  32 . 
   The oxygen-rich gas stream is admitted into the gasifier  40 , which moreover receives the carbon products such as coal. The oxidation carried out in this gasifier  40  leads to the production of fuel gas, delivered by the line  46 , which feeds the combustion chamber  8  of the gas turbine. The latter also receives, via the line  24 , the nitrogen-enriched gas stream W, together with compressed air from the compressor  4 , via the line  5 . 
   The gases arising from the corresponding combustion, mixed with the waste nitrogen, are sent to the inlet of the expansion turbine  6 , where they are expanded while driving the latter. This also enables the compressor  4  and the alternator  10 , which for example feeds an electrical distribution network (not shown), to be driven via the shaft  12 . 
   It should be noted that, during this normal operation of the turbine  2 , the valves  26  and  38  are open, while the valves  30  and  44  are closed. As such, the line  16  is not fed either by the line  28  or by the line  42 , which are thus shown in dotted lines. 
   When the gas turbine  2  detects an incident, especially due to a sudden variation in one of its parameters, this gas turbine shuts down, or undergoes an appreciable malfunction. As such, the flow rate of compressed air flowing in the line  16  is subjected to an appreciable decrease. 
   When this decrease in the flow rate is greater than a predetermined value, which corresponds for example to an instantaneous drop of at least 5%, the sensor  18  detects this drop in flow rate. It then sends signals to the valves  26 ,  30 ,  38  and  44 , via the control lines  26 ′,  30 ′,  38 ′ and  44 ′. 
   The changeover of these four valves may also be initiated by means of a sensor (not shown) indicating the shutdown of the turbine. 
   This then causes valves  30  and  44 , initially closed, to open and the valves  26  and  38 , initially open, to close. In this way, the oxygen-enriched stream no longer feeds the gasifier via the line  36 , while the nitrogen-enriched stream no longer feeds the combustion chamber  8 , via the line  24 . 
   Instead, these two gas streams are recycled towards the inlet of the air separation unit  14 , via the line  28  and the line  42 . 
   The nitrogen-enriched stream, which may be filled with impurities, is advantageously recycled upstream of a conventional purification device. This recycled stream may also undergo prior cooling, before being admitted into the separation unit  14 . 
   On the other hand, the oxygen-enriched stream may be delivered to the inlet of this unit  14  without being subjected to purification or to cooling. 
   It should be noted that the mixture of these two streams, enriched respectively with nitrogen and with oxygen and, admitted to the inlet of the unit  14 , has a composition close to that of air. 
   Also, assuming the gas turbine would still be operating, although detecting an incident, its complete shutdown is carried out. 
   In  FIG. 2 , the line  24 , the lines  36  and  46  and the gas turbine  2  are shown in dotted lines. On the other hand, the line  28  and the line  42  are shown in solid lines. 
   Given that as soon as the separation unit  14  is no longer sufficiently fed by the compressor  4 , the two gas streams are recycled, via the line  28  and the line  42 , towards the inlet of this unit  14 , the latter does not undergo a sudden variation in its inlet flow rate. The latter may thus be kept constant, or be gradually decreased, by reducing the load of this separation unit  14 . 
   It should be noted that, during the phase of recycling the two aforementioned gas streams towards the inlet of the separation unit  14 , it is possible to resort to a make-up compressor  50 , which can be seen in FIG.  2 . The latter thus makes it possible to compensate for gas losses, associated with recycling of this sort. 
   This compressor  50  may also be used to start up the separation unit  14 , without resorting to the gas turbine  2 , which enables this turbine and this separation unit to be started up in parallel, as required. This make-up compressor  50  is capable of having a very small size, such that it is of low cost and does not involve much energy expenditure. 
   When the gas turbine is again able to operate normally, the various valves  26 ,  30 ,  38  and  44  are placed in their initial configuration. This enables the plant to be set up again in its arrangement of FIG.  1 . 
     FIGS. 3 and 4  show a second embodiment of the plant according to the invention. 
   This variation differs from the plant shown in  FIG. 1 , in that it is provided with a compressor  52 , allowing the separation unit  14  to be fed with air via a line  54 . 
   During normal operation of the gas turbine  2 , operation of the plant is identical to that described with reference to  FIG. 1 , given that the compressor  52 , in combination with the compressor  4  of the gas turbine  2 , enables the separation unit  14  to be fed with air. 
   On occurrence of an incident at this gas turbine  2 , the reduction in air flow rate in the line  16  is detected, in a similar manner to that described above. The valves  26  and  38  are then closed, and the valves  30  and  44  are opened so as to recycle the gas streams conveyed by the line  28  and the line  42  to the inlet of the separation unit  14 . 
   It should be noted that these gas streams are recycled only in part, so as to compensate only for the lack of feed by the compressor  4  of the turbine  2 , given that the external compressor  52  continues to direct the air towards the separation unit  14 . 
   Also, it should be noted that the mixture of the air coming from the compressor  52  and the two gas streams respectively enriched with nitrogen and with oxygen, has a composition close to that of air. 
   The other portion of the oxygen-rich gas stream, which is not recycled, is sent to the gasifier  40 , in a manner similar to the arrangement of FIG.  3 . Also, a unit  56  is provided, enabling oxygen make-up to be supplied, such that the flow rate of oxygen admitted at the inlet of the gasifier is not subjected to a sudden decrease. This makes it possible not to shut down this gasifier, which is advantageous in terms of savings in time and in energy consumption. 
   The portion of nitrogen which is not recycled, flowing via the line  24 , is discharged to the atmosphere. The valve  26  is closed, while the gas turbine is shut down. 
   Given that part of the nitrogen- and oxygen-enriched gas streams are recycled via the line  28  and the line  42 , this makes it possible to prevent any sudden decrease in the flow rate of air admitted to the inlet of the separation unit  14 . 
   The respective loads of the separation unit  14  and of the gasifier  40  can be progressively decreased, once these recycling operations are implemented. In this way, it is possible to reduce the flow rate of the gas streams recycled via the line  28  and the line  42  progressively, and also the flow rate of oxygen provided by the make-up unit  56 . Once this recycling is stopped, the gasifier can again be fed just by the oxygen flowing in the line  36 . 
   When the gas turbine is able to operate normally again, the various valves are placed in their initial configurations, such that the plant returns to its arrangement of FIG.  3 . 
   The invention enables the aforementioned objectives to be achieved. 
   This is because the applicant has noticed that the loss in purity in the products extracted from the separation unit, and the inopportune shutdowns of the latter, are mainly due to the sudden decreases in the flow rate of air admitted to the inlet of this separation unit. Now, such sudden decreases are connected to the malfunctions, or even to the shutdown of the gas turbine, the compressor thereof then no longer feeding the separation unit. 
   Recycling at least part of each of the gas streams which are extracted therefrom towards the inlet of the separation unit enables any appreciable fluctuation of this inlet air flow rate to be prevented. Thus, it is possible to keep the latter constant, or to decrease it progressively, such that satisfactory operation of this separation unit is permanently guaranteed. 
   It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.