Abstract:
The air distillation unit ( 2 ) has a low-pressure column ( 11 C) which operates clearly above atmospheric pressure. The waste nitrogen coming from this column serves to purge, under low pressure, the air purification adsorbers ( 10 A,  10 B) in order to desorb them of the contaminants (water and CO 2 ), then is mixed with the impure nitrogen stream sent directly to the nitrogen compressor ( 14 ) which feeds the combustion chamber ( 8 ) of the gas turbine ( 1 ). At the start of regeneration, each adsorber is decompressed down to atmospheric pressure, then brought back up to the low pressure by impure nitrogen.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of feeding, with impure nitrogen, the combustion chamber of a gas turbine combined with an air distillation unit, the air distillation unit being equipped with an apparatus for purifying air of water and of CO 2  by adsorption, this apparatus comprising two adsorbers, each one of which follows an operating cycle which comprises, alternately and cyclically, a co-current adsorption phase at a high pressure of the cycle, and a regeneration phase, the regeneration phase comprising a decompression step (a), a step (c) of counter-current purging by waste gas from the air distillation unit coming from a low-pressure column of this unit, and a step (d) of recompressing the adsorber up to the high pressure of the cycle. 
     The pressures involved here are absolute pressures. 
     BACKGROUND OF THE INVENTION 
     The gas turbines used to drive an alternator for generating electricity are fed by a gas coming from a combustion chamber. This chamber is mainly fed by air compressed by an air compressor, which is, for example, coupled to the turbine, and by a fuel gas. The temperature of the flame, and therefore the generation of NO z , is adjusted by additional injection of nitrogen into the combustion chamber. 
     The fuel gas is often produced by a gasifier fed, on the one hand, by oxygen, and on the other hand, by carbon products, such as coal. An air distillation unit then produces the oxygen consumed by the gasifier and the nitrogen sent to the combustion chamber. 
     The waste gas from the low-pressure column of an air distillation unit, for example, a double air distillation column, is impure nitrogen typically containing 0.5 to 1% oxygen, and may therefore provide nitrogen for the combustion chamber, the maximum permissible oxygen content for this gas typically being 2%. In some plants, where all the products from the air distillation unit are utilized under pressure and where the nitrogen flow rate demanded by the gas turbine is high, it is advantageous to choose a low pressure clearly greater than atmospheric pressure (typically 3 to 4 bar, which corresponds to a medium distillation pressure greater than 1C bar), and to purge the purification adsorbers under this low pressure. This is because this makes it possible to recover the gas having carried out the purging and to mix it with impure nitrogen sent directly from the air distillation unit to the gas turbine, without carrying out expansion or recompression which are costly in terms of power. 
     As is well known, purging the adsorber has the effect of desorbing the contaminants (mainly water and CO 2 ) which have been removed from the air during the previous adsorption phase. 
     However, this option comes up against a problem at the end of the decompression step, the adsorber still contains much oxygen, such that the gas coming from the adsorber at the start of the purging phase has an oxygen content greater than the maximum permissible content for the feeding of the combustion chamber. 
     SUMMARY OF THE INVENTION 
     The aim of the invention is to solve this problem, that is to say to make it possible, simply and economically, to recompress the purging gas for feeding the combustion chamber of the gas turbine. 
     To this end, the subject of the invention is a method of the aforementioned type, characterized in that: 
     the low-pressure column operates at a low pressure which is clearly greater than atmospheric pressure; 
     the waste gas is used almost at the low pressure for the purging step (c); 
     the decompression step (a) is continued down to a minimum pressure of the cycle which is clearly less than the low pressure; 
     between the decompression step (a) and the purging step (c) a partial repressurization step (b) is inserted, in which the adsorber is repressurized by means of waste gas up to almost the purging pressure; and 
     the waste gas coming from the adsorber during the purging step (c) is mixed with the waste gas produced by the air distillation unit, and the mixture is compressed by a nitrogen compressor in order to feed the combustion chamber of the gas turbine. 
     The method according to the invention may comprise one or more of the following characteristics: 
     the minimum pressure of the cycle is almost atmospheric pressure; 
     after the partial repressurization step (b), an additional step (b′) of depleting the adsorber of oxygen is carried out at least once, which step comprises the following substeps: 
     (b′1) additional decompression of the adsorber down to the minimum pressure, and 
     (b′2) additional partial repressurization up to the purging pressure, 
     then the purging step (c) is carried out; 
     the purging step (c) comprises a substep (c2) during which the waste gas is heated followed by a substep (c3) during which the waste gas is not heated; 
     a nitrogen-rich gas, having at least the nitrogen content of the impure nitrogen, not coming directly from the low-pressure column, is used in order to carry out the or each partial repressurization step; 
     the nitrogen-rich gas comes from a nitrogen distribution network; and 
     the nitrogen-rich gas comes from an auxiliary tank fed by part of the gas delivered by the nitrogen compressor. 
     The subject of the invention is also an electricity generation plant adapted to implement such a method. 
     This plant, of the type comprising a gas turbine, an air distillation unit producing impure nitrogen, the air distillation unit being equipped with an apparatus for purifying air of water and of CO 2  by adsorption, this apparatus comprising at least two, preferably only two, adsorbers, each one of which follows an operating cycle which comprises, alternatively and cyclically, a co-current adsorption phase at a high pressure of the cycle, and a regeneration phase, the regeneration phase comprising a decompression step (a), a step (c)of counter-current purging by waste gas from the air distillation unit coming from a low-pressure column of this unit, and a step (d) of recompressing the adsorber up to the high pressure of the cycle, and a nitrogen compressor, the intake of which is connected to the air distillation unit by means of a feed pipe and the delivery side of which is connected to the combustion chamber of the gas turbine, in which: 
     the air distillation unit produces impure nitrogen under a low pressure which is clearly greater than atmospheric pressure; 
     a first impure nitrogen pipe fitted with a valve connects the air distillation unit to the purified air outlet of each adsorber; and 
     a second impure nitrogen pipe fitted with a valve connects the air inlet of each adsorber to the feed pipe. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary implementations of the invention will now be described with regard to the appended drawings in which: 
     FIG. 1 shows very schematically an electricity generation plant according to the invention; 
     FIG. 2 shows in more detail part of this plant; 
     FIG. 3 is a diagram which illustrates the operating cycle of an adsorber; 
     FIG. 4 is a view, similar to FIG. 1, of a variant; and 
     FIG. 5 is a view, similar to FIG. 3, corresponding to the plant of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows very schematically an electricity generation plant which comprises a gas turbine  1  combined with an air distillation unit  2 , and a gasifier  3 . 
     The gas turbine mainly comprises a turbine  4  whose shaft  5  is coupled to an alternator  6  and to an air compressor  7 , and a combustion chamber  8 . 
     The air distillation unit  2  mainly comprises an air compressor  9 , an apparatus  10  for purifying air of water and of CO 2  by adsorption, this apparatus comprising two adsorbers  10 A and  10 B in parallel, and a cold box  11  which mainly comprises a heat exchange line  11 A and a double air distillation column consisting of a medium-pressure column  11 B and of a low-pressure column  11 C. 
     The word “compressor” may mean a single compressor or several compressors in parallel. Similarly, the word “adsorber” may refer to a single adsorption bottle or to several bottles in parallel, and there may be several air distillation units  2  in parallel, if the demand for oxygen by the gasifier  3  is very high, which is generally the case. 
     In operation, the atmospheric air intended to be distilled and compressed at  9  has a medium distillation pressure greater than 10 bar, typically 13 bar. The compressed air is purified by one of the two adsorbers, the adsorber  10 B in FIG. 1, then is cooled in the heat exchange line  11 A with counter-current of the distillation products. In the double column, the low-pressure column  11 B of which operates at 3 to 4 bar, the air is separated into pressurized oxygen which is sent in gas form, via a pipe  12 , to the gasifier  3  and into impure nitrogen WN 2 , which exits the cold box via a pipe  13 . This pipe is connected to the intake of a nitrogen compressor  14 , the delivery side of which is connected to the combustion chamber  8 . 
     Furthermore, the gasifier is fed with carbon products via a pipe  15  and produces a pressurized fuel gas, which is sent via a pipe  16  to the combustion chamber  8 . The latter is further fed with compressed air by the compressor  7  of the gas turbine. 
     A portion of the impure nitrogen is taken off in the pipe  13  via a tap-off  17  in order to regenerate the adsorber which is not in the adsorption phase, that is the adsorber  10 A in FIG.  1 . As described below in more detail, the impure nitrogen having passed through this adsorber is recovered via a pipe  18  and reinjected into the pipe  13 , the mixture thus arriving at the compressor  14 . 
     FIG. 2 shows the purification apparatus  10  in more detail. 
     The intake of each adsorber is connected to four pipes each fitted with a valve: one pipe  19 A or  19 B for venting to atmosphere, connected to a vent pipe  20 ; one partial repressurization pipe  21 A or  21 B, connected to the pipe  18 ; one pipe  22 A or  22 B for removing purging gas, also connected to the pipe  18 ; and one air inlet pipe  23 A or  23 B, connected to the delivery side of the compressor  9 . 
     The outlet of each adsorber is connected to three pipes each fitted with a valve: one impure nitrogen feed pipe  24 A or  24 B, connected to the pipe  17 ; one purified air outlet pipe  25 A or  25 B, connected to a pipe  26  which leads to the cold box  11 , and one first repressurization pipe  27 A or  27 B, connected to the outlet of the other adsorber. 
     As indicated in FIG. 2, the pipe  17  is equipped with a heating device  28 . 
     The operating cycle of each adsorber, offset by a half period with respect to that of the other adsorber, is shown in FIG.  3 . In this diagram, where the time t is plotted on the x-axis and the absolute pressures P on the y-axis, the lines indicated by arrows show the direction of flow in the adsorber: when an arrow is in the direction of increasing pressure (upwards in the diagram), the current is called “with a co-current”, in the adsorber. If the arrow directed upwards is located below the line showing the pressure in the adsorber, the current enters the adsorber by the inlet end of the adsorber; if the arrow, directed upwards, is located above the line showing the pressure, the current leaves the adsorber by the outlet end of the adsorber, the inlet and outlet ends being respectively those of the air to be treated and of the purified air withdrawn in the adsorption phase; when an arrow is in the direction of decreasing pressure (downwards in the diagram), the current is called “with a counter-current”, in the adsorber. If the arrow directed downwards is located below the line showing the pressure of the adsorber, the current leaves the adsorber by the inlet end of the adsorber; if the arrow directed downward is located above the line showing the pressure, the current enters the adsorber by the outlet end of the adsorber, the inlet and outlet ends being still those of the gas to be treated and of the gas withdrawn in the production phase. 
     The cycle of FIG. 3 comprises the following successive steps. 
     At the end of the adsorption phase at the high pressure of the P M  cycle, which is almost the medium distillation pressure, the adsorber undergoes the following steps: 
     (a) depressurization with a counter-current down to atmospheric pressure P A  by venting to atmosphere via the pipe  19  and the vent  20 ; 
     (b) partial repressurization by unheated impure nitrogen (device  28  inactive), with a co-current, via the pipes  18  and  21 , up to the pressure P W  of this gas. This step decreases the oxygen content in the adsorber; (b′) depressurization with a counter-current down to atmospheric pressure P A  by venting to atmosphere via the pipes  19  and  20  (b′1), then another partial repressurization by unheated impure nitrogen, with a co-current, via the pipes  18  and  21 , up to the pressure P W  (b′2). Thus, the oxygen content of the adsorber is again reduced, and goes below the maximum permissible value in the combustion chamber  8  of the gas turbine; 
     (c) purging with a counter-current, via the pipes  17  and  24  for the entry,  22  and  18  for the withdrawal. This purging is carried out first of all with unheated gas (c1), then with gas heated at  28  (c2), then again with unheated gas (c3). Throughout this step, impure nitrogen coming from the adsorber is recovered via the pipes  22  and  18  and reinjected into the pipe  13 , where it is mixed with the impure nitrogen carried by this pipe; 
     (d) repressurization with a counter-current by purified air coming from the other adsorber, via the pipe  27 , up to the high pressure P M  of the cycle. 
     It will be noted that this cycle involves consumption of impure nitrogen for the partial repressurizations (b) and (b′2), this impure nitrogen no longer being available to feed the nitrogen compressor  14 . The corresponding flow rate of impure nitrogen is about 5% of the air flow rate, which reduces the flow rate available for the nitrogen compressor by the same amount. 
     In order to reduce this drawback, it is possible to use, in order to carry out the partial repressurizations (b) and (b′2), pressurized nitrogen not coming directly from the cold box  11 . This nitrogen may be supplied by a nitrogen supply network, or else by an auxiliary tank  29  of impure nitrogen, shown in FIGS. 4 and 5. This tank is fed by a tap-off  30  provided at the delivery side of the compressor  14  and equipped with an expansion valve  31  driven by a flow rate controller  32  (FIG.  5 ). As shown in FIG. 5, the pipes  21  are no longer connected to the pipe  18 , but they are connected to the tank  29  via a pipe  33 . 
     By virtue of the presence of the tank  29 , the flow rate of impure nitrogen needed for steps (b) and (b′2) is distributed throughout the duration of the cycle, which makes it possible to increase the flow rate available for the compressor  14 .