Patent Publication Number: US-2023160631-A1

Title: System for generating an inert gas for an aircraft using liquid hydrogen

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of the French patent application No. 2111432 filed on Oct. 27, 2021, the entire disclosures of which are incorporated herein by way of reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for generating an inert gas for use in an aircraft and to a system carried on board an aircraft and configured to implement this method. The invention more specifically relates to a method for generating an inert gas executing fractional distillation of compressed air in the aircraft, in order to be obtain the inert gas, in an aircraft using liquid hydrogen as an energy source, and to a system executing this method and carried on board an aircraft. 
     BACKGROUND OF THE INVENTION 
     Liquid hydrogen (or more accurately liquid dihydrogen of formula H2) is a cryogenic fluid that can be used as an energy source for producing electricity. Thus, for example, it is possible to use a hydrogen fuel cell to power all of the flight control and communications systems of an aircraft, and for the lighting onboard the aircraft and for powering various accessories used in the aircraft. The liquid hydrogen may also act as a power source for the propulsion of an aircraft, by being supplied to a fuel cell or else by direct combustion, which offers the advantage that only water is discharged into the atmosphere. The hydrogen stored in liquid form in a tank of the aircraft needs to be warmed in order to be used in this way. Furthermore, when the liquid hydrogen is used as a fuel, an inert gas is used for removing the gaseous hydrogen present in certain parts of the fuel system, in order to avoid the risks of fire and explosion, particularly during phases of starting or shutting down the aircraft engines. The inert gas is generally nitrogen, obtained by isolating it from the other components of air collected from outside the aircraft. The separation of the nitrogen from the other components of the air is performed using one or more membranes in a system referred to as an “inerting” system. The membrane or membranes used thus make it possible to obtain nitrogen from the air around the aircraft. However, these membranes are susceptible to air contamination. 
     The situation has room for improvement. 
     SUMMARY OF THE INVENTION 
     In the present description, a dihydrogen fluid is referred to as “hydrogen” according to common parlance. 
     It is an object of the present invention to propose an inerting system that does not require membrane separation of the nitrogen and the oxygen while at the same time making it easier to warm the liquid hydrogen stored and used in the aircraft as a source of energy. 
     To this end, what is proposed is a method for generating an inert fluid, the method being performed in an aircraft, the inert fluid being obtained by successive separations of components of a primary fluid initially collected in the form of compressed hot air, the method comprising at least a separation of components by change of phase of one component of the primary fluid, using a heat exchanger configured to execute a cooling of the primary fluid using liquid hydrogen and fed with liquid hydrogen collected from a tank of the aircraft. 
     Advantageously, it is thus possible to execute judicious warming of the liquid hydrogen so that it can be used on board the aircraft, while at the same time using it as a coolant through one or more heat exchangers in order to perform fractional distillation of a compressed air, and ultimately generating inert gas. 
     Advantageously, the inert gas can then be used to remove gaseous hydrogen from certain parts of the aircraft fuel system. 
     The method for generating an inert fluid according to the invention may also comprise the following features, considered alone or in combination:
         The method comprises a first step of separating components of the primary fluid, by cooling the primary fluid, suitable for extracting water from the primary fluid, and a second step of separating components of the primary fluid, by cooling the primary fluid, suitable for extracting carbon dioxide from the primary fluid.   The method comprises a third step of separating components of the primary fluid, by cooling the primary fluid, after the first step and second step and suitable for extracting dioxygen from the primary fluid, in liquid form.   The method comprises a step of liquefying the primary fluid, after the first step, second step and third step of separating components, and followed by a step of pumping the primary fluid into a tank, in the form of liquid nitrogen.   The method further comprises a step of liquefying the primary fluid, after the first step and second step and followed by a step of warming the liquefied primary fluid, suitable for extracting nitrogen in gaseous form from the primary fluid.   The successive steps of separating components of the primary fluid, by cooling the primary fluid, each use a heat exchanger, the heat exchangers used being fed in parallel, each via a motorized valve from a liquid-hydrogen tank of the aircraft.   Successive steps of separating components of the primary fluid, by cooling the primary fluid, each use a heat exchanger, the heat exchangers being arranged in series so that the liquid hydrogen used at the inlet of a heat exchanger for cooling the primary fluid during a component-separating step comes at least in part from an outlet of another heat exchanger used for cooling the primary fluid in another step of separating components of the primary fluid.   The first step of separating components of the primary fluid comprises a cooling of the primary fluid in a heat exchanger using ambient air, the second step of separating components of the primary fluid comprises a cooling of the primary fluid in a heat exchanger by means of the liquid-form dioxygen extracted during the third step, and the third step of extracting dioxygen in liquid form comprises a cooling of the primary fluid using liquid nitrogen taken from the liquid-nitrogen tank.   The method further comprises a step of liquefying the primary fluid, after the first step and second step, wherein the first step and second step of separating components of the primary fluid each comprise a cooling of the primary fluid in a heat exchanger by means of the liquefied primary fluid, and wherein the step of liquefying the primary fluid comprises a cooling of the primary fluid in a heat exchanger by means of the liquid hydrogen.       

     Another subject of the invention is a system for generating an inert fluid, the system being carried on board an aircraft, the generation system comprising a plurality of devices configured each, in succession, to execute a separation of components of a primary fluid initially collected in the form of compressed hot air, the system comprising at least one heat exchanger configured to execute a separation of components, by change of phase of a component of the primary fluid, the heat exchanger executing a cooling of the primary fluid using liquid hydrogen, being supplied with liquid hydrogen collected from a tank of the aircraft. 
     Another subject of the invention is an aircraft comprising a system for generating an inert fluid as hereinabove or comprising an inert-fluid generation system configured to execute an inert-gas generation method as described hereinabove. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The abovementioned features of the invention, together with others, will become more clearly apparent from reading the following description of one exemplary embodiment, the description being given in connection with the attached drawings, among which: 
         FIG.  1   a    is a block diagram illustrating an inert-gas generation system carried on board an aircraft, according to one embodiment; 
         FIG.  1   b    is a diagram illustrating an inert-gas generation method executed in the system depicted in  FIG.  1     a;    
         FIG.  2   a    is a block diagram illustrating a first variant of the inert-gas generation system carried on board an aircraft, depicted in  FIG.  1     a;    
         FIG.  2   b    is a diagram illustrating an inert-gas generation method executed in the system depicted in  FIG.  2     a;    
         FIG.  3   a    is a block diagram illustrating a second variant of the inert-gas generation system carried on board an aircraft, depicted in  FIG.  1     a;    
         FIG.  3   b    is a diagram illustrating an inert-gas generation method executed in the system depicted in  FIG.  3     a;    
         FIG.  4   a    is a block diagram illustrating a third variant of the inert-gas generation system carried on board an aircraft, depicted in  FIG.  1     a;    
         FIG.  4   b    is a diagram illustrating an inert-gas generation method executed in the system depicted in  FIG.  4     a;    
         FIG.  5   a    is a block diagram illustrating a fourth variant of the inert-gas generation system carried on board an aircraft, depicted in  FIG.  1     a;    
         FIG.  5   b    is a diagram illustrating an inert-gas generation method executed in the system depicted in  FIG.  5   a   ; and, 
         FIG.  6    illustrates an aircraft comprising an inert-gas generation system as already illustrated in one of figures  FIG.  1   a   ,  FIG.  2   a   ,  FIG.  3   a   ,  FIG.  4   a   , and  FIG.  5     a.    
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG.  1   a    schematically depicts an inert-gas generation system  10 , referred to as an inerting system, configured to generate nitrogen, which is an inert gas, from compressed air, in an aircraft using liquid hydrogen as a power source and according to one embodiment. The system  10  comprises a liquid-hydrogen tank  1000  to which there is connected a tank-outlet pipeline  1001  intended for distributing liquid hydrogen to various points of the inert-gas generation system  10 . The liquid hydrogen is used as a refrigerant in the system  10  to cool compressed air and execute fractional distillation of this compressed air with a view to obtaining nitrogen. In the present description, the use of the term “downstream” refers to a relative position whereby one element is further away from the source of a fluid than is another element, as opposed to the term “upstream” which refers to a relative position whereby one element is closer to the source of a fluid than is another element. Thus, by way of example, the inlet of a system operating on a fluid constitutes the most “upstream” element of the system and the outlet of this system constitutes an element “downstream” of the inlet. Similarly, a tank of a fluid feeding a system that handles or uses this fluid is situated “upstream” of the elements of this system, with respect to the path that the fluid flows along and the elements through which the fluid passes are situated “downstream” of this tank. 
     The inert-gas generation system  10  comprises a compressed-air inlet  100  via which compressed air at a pressure greater than or equal to 6 bar is introduced. The compressed air introduced into the inert-gas generation system  10  comes from one or more engines of the aircraft carrying the inert-gas generation system  10 . The compressed-air inlet  100  is connected to an inlet filter  101  configured to hold back impurities in the air taken from the engine or engines and prevent these impurities from being able to enter the inert-gas generation system  10 . The outlet of the inlet filter  101  is connected to a pipeline  102  which passes in succession through equipment for separating components of the fluid passing through it. In the present description, the term “primary fluid” is given to the fluid passing through the pipeline  102  and which is initially compressed air at the inlet  100  of the pipeline  102 , and the term “primary pipeline” is given to the pipeline  102  or the portions of pipeline  102  associated with the various pieces of equipment through which it passes. The primary fluid therefore changes as it gradually passes along the pipeline  102  because components of this fluid are successively extracted therefrom during successive steps of fractional distillation, as the fluid gradually passes through the equipment. The pipeline  102  comprises, in succession, between the inlet filter  101  and an inert-gas outlet  139 , a pipeline portion in a first heat exchanger  110 , a pipeline portion in a first purge device  117 , a pipeline portion in a second heat exchanger  120 , a pipeline portion in a second purge device  127 , a pipeline portion in a third heat exchanger  130  and a pipeline portion in a third purge device  137 . These devices are configured to execute fractional distillation of the compressed air introduced via the inlet  100  of the inert-gas generation system  10 . The pipeline  102  is therefore arranged in such a way that:
         the outlet of the inlet filter  101  is connected to the inlet of the portion of pipeline  102  associated with the heat exchanger  110 ,   the outlet of the portion of pipeline  102  associated with the heat exchanger  110  is connected to the inlet of the purge device  117 , or more specifically to the inlet of the portion of pipeline  102  associated with the purge device  117 ,   the outlet of the portion of pipeline  102  associated with the purge device  117  is connected to the inlet of portion of pipeline  102  associated with the heat exchanger  120 ,   the outlet of the heat exchanger  120  is connected to the inlet of the purge device  127 , or more precisely to the inlet of the portion of pipeline  102  associated with the purge device  127 ,   the outlet of the portion of pipeline  102  associated with the purge device  127  is connected to the inlet of the portion of pipeline  102  associated with the heat exchanger  130 ,   the outlet of the portion of pipeline  102  associated with the heat exchanger  130  is connected to the inlet of the purge device  137 , or more specifically to the inlet of the portion of pipeline  102  associated with the purge device  137 ,   the outlet of the purge device  137  is connected to the outlet  139  of the inert-gas generation system  10 .       

     Each of the heat exchangers  110 ,  120  and  130  comprises a portion of the pipeline  102 , referred to as the primary pipeline, through which the primary fluid passes, and a portion of another pipeline, referred to as the secondary pipeline, used for executing an exchange of heat with the portion of pipeline  102  that passes through it. Thus, the heat exchanger  110  comprises a secondary pipeline, in the form of one or more chambers through which the portion of pipeline  102  it comprises passes through, so as to execute an exchange of heat. The same is true of the heat exchanger  120  which comprises a secondary pipeline, in the form of one or more chambers through which the portion of pipeline  102  that it comprises passes through with a view to executing an exchange of heat. The heat exchanger  130  again likewise comprises a secondary pipeline, in the form of one or more chambers through which the portion of pipeline  102  that it comprises passes through, with a view to executing an exchange of heat. 
     The secondary-pipeline inlet of each of the heat exchangers  110 ,  120  and  130  is connected to the outlet of a motorized valve, and the inlet of each of these motorized valves is connected to the pipeline  1001  used for distributing liquid hydrogen from a liquid-hydrogen tank  1000 . The liquid hydrogen is kept at a storage temperature of around 20K in the liquid-hydrogen storage tank  1000 . Thus, there is a motorized valve  115  between the pipeline  1001  and the secondary-pipeline inlet of the heat exchanger  110 , to control the distribution of liquid hydrogen taken from the tank  1000  to the secondary pipeline of the exchanger  110  and to cool the portion of primary pipeline  102  in the heat exchanger  110 . Similarly, there is a motorized valve  125  between the pipeline  1001  and the secondary-pipeline inlet of the heat exchanger  120 , so as to control the distribution of liquid hydrogen taken from the tank  1000  into the secondary pipeline of the exchanger  120  and to cool the portion of primary pipeline  102 , in the heat exchanger  120 . Similarly again, there is a motorized valve  135  between the pipeline  1001  and the secondary-pipeline inlet of the heat exchanger  130 , so as to control the distribution of liquid hydrogen taken from the tank  1000  into the secondary pipeline of the exchanger  130  and cool the portion of primary pipeline  102 , in the heat exchanger  130 . Each outlet of a portion of the primary pipeline  102  associated with a heat exchanger from among the heat exchangers  110 ,  120  and  130  is equipped with a device for measuring the temperature of the primary fluid at the outlet of the exchanger, so as to measure the temperature of the primary fluid at the outlet of the portion of primary pipeline  102  associated with that exchanger and so as to control the motorized valve situated at the inlet of the secondary pipeline of that same exchanger in order to achieve feedback control of the cooling executed in the heat exchanger using the liquid hydrogen stored in the liquid-hydrogen tank  1000 . Thus, the outlet of the primary pipe  102  associated with the heat exchanger  110  comprises a device  116  for measuring the temperature of the primary fluid in the pipeline  102 , the outlet of the primary pipe  102  associated with the heat exchanger  120  comprises a device  126  for measuring the temperature of the primary fluid in the pipeline  102 , and the outlet of the primary pipe  102  associated with the heat exchanger  130  comprises a device  136  for measuring the temperature of the primary fluid in the pipeline  102 . The opening of the motorized valve  115  is therefore controlled as a function of the primary-fluid temperature measured by the temperature-measurement device  116 . The same is true of the openings of the motorized valves  125  and  135  which are respectively controlled as a function of the temperatures measured by the temperature-measuring devices  126  and  136 . According to one embodiment, a control unit (not depicted in  FIG.  1   a   ) controlling the motorized valves controls all of the motorized valves. In a variant, each of the motorized valves has its own control unit controlling its opening. What is meant here by the opening of a valve is a degree of opening, so that control of the opening of a valve implies operations of opening or of closing the valve using the motor coupled to a shut-off element that shuts off the pipeline internal to the valve (a gate, for example). Each of the heat exchangers  110 ,  120  and  130  comprises a secondary-pipeline outlet via which the liquid hydrogen passes as it leaves the secondary chamber or chambers of the heat exchanger. Thus, the heat exchanger  110  comprises a secondary-pipeline outlet  111 , the heat exchanger  120  comprises a secondary-pipeline outlet  121  and the heat exchanger  130  comprises a secondary-pipeline outlet  131 . According to one embodiment, the secondary-pipeline outlets  111 ,  121  and  131  of the exchangers are connected to hydrogen-using installations in the aircraft, such as, by way of example, one or more fuel cells. According to one embodiment, the control unit controlling the motorized valve  115  is configured to obtain a temperature lower than 430K at the measurement point of the primary-fluid temperature-measurement device  116 , so that the water contained in the primary fluid can be separated from the primary fluid using the purge device  117 , which comprises a purge outlet  118 ; the control unit controlling the motorized valve  125  is configured to obtain a temperature of between 195K and 220K at the measurement point of the primary-fluid temperature-measurement device  126 , so that the carbon dioxide (CO2) contained in the primary fluid can be separated from the primary fluid using the purge device  127 , which comprises a purge outlet  128 , and the control unit controlling the motorized valve  135  is configured to obtain a temperature of between 77K and 96K at the measurement point of the primary-fluid temperature-measurement device  136  so that oxygen (or more accurately dioxygen) in liquid form (LO2) contained in the primary fluid can be separated from the primary fluid using the purge device  137 , which comprises a purge outlet  138 . According to this embodiment, nitrogen is then available at the outlet  139  of the inert-gas generation system. Advantageously, all or some of this nitrogen is transferred into the available space of one or more hydrogen systems, so as to fill this or these spaces with inert gas and consequently reduce the flammability of the contents of this or these systems. According to this embodiment, three successive cooling operations executed by the heat exchangers  110 ,  120  and  130  therefore make it possible to obtain nitrogen at the outlet  139  of the inert-gas generation system. Advantageously and judiciously, the use of the liquid hydrogen for generating inert gas in the inert-gas generation system  10  makes it possible both to meet a need to warm the liquid hydrogen so that it can be used on board the aircraft, for example in a fuel cell, and the need to cool the compressed hot air in order to execute fractional distillation yielding an inert gas such as nitrogen. 
       FIG.  1   b    is a diagram illustrating a method for generating nitrogen in the inert-gas generation system  10 . 
     During a step S 0 , compressed air, taken from one or more engines of the aircraft carrying the inert-gas generation system  10  and at a pressure in excess of 6 bar, is received at the inlet  100  of the system  10 . The compressed air is then filtered by the filter  101 . A first cooling of this air, also referred to here as primary fluid, is executed during a step S 1  in the heat exchanger  110  of which the inlet pipeline, a portion of the primary pipeline  102 , is connected to the outlet of the filter  101 . This cooling of the primary fluid is executed using liquid hydrogen which passes through the secondary pipeline of the heat exchanger  110 . The flow rate of liquid hydrogen in this secondary pipeline is controlled by the motorized valve  115  so that, still during step S 1 , an extraction of water can be performed by the purge device  117 . The water is condensed by keeping the temperature at the outlet of the primary pipeline  102  associated with the heat exchanger  110  at a temperature below 430K. The condensed water is extracted from the primary pipeline  102  via the outlet  118  of the purge device  117 , after which the primary fluid, rid of water, is conveyed into the portion of primary pipeline  102  associated with the heat exchanger  120 . A second cooling of the primary fluid is then executed during a step S 2  using, once again, liquid hydrogen as coolant. In this heat exchanger, the primary fluid, which is to say, the compressed and filtered air introduced into the system  10 , now rid of water, is cooled so that its temperature at the outlet of the portion of primary pipeline  102  associated with the heat exchanger  120  is comprised between 195K and 220K, making it possible to obtain the carbon dioxide contained in the primary fluid in liquid form. This carbon dioxide in liquid form is then extracted from the primary fluid via the outlet  128  of the purge device  127 , still during step S 2 . The primary fluid at this stage is the rest of the compressed air introduced at the inlet of the system, following the extraction of the water and the carbon dioxide. It is conveyed in this form to the heat exchanger  130  where it is subjected, during a step S 3 , to a third cooling using liquid hydrogen, so that its temperature at the outlet of the portion of primary pipe  102  associated with the heat exchanger  130  is comprised between 77K and 96K. At this temperature, the oxygen (or more accurately the dioxygen) is present in the primary fluid in liquid form and is extracted, still during step S 3 , via the outlet  138  of the purge device  137 . The primary fluid available in the primary pipeline  102  at the outlet  139  of the purge device  137  is then supplied, during a step S 4 , to the outlet of the inert-gas generation system  10 , in the form of nitrogen in the gaseous state. Advantageously this can be used for its non-flammability properties in one or more hydrogen systems of an aircraft carrying the inert-gas generation system  10  or else can be put to use somewhere else in the aircraft where its inert-gas properties can be put to beneficial use. 
       FIG.  2   a    schematically depicts a first variant of the inert-gas generation system  10  as previously described in relation to  FIG.  1   a   . This first variant of the system  10  comprises numerous elements in common with the inert-gas generation system described in  FIG.  1   a   . Thus, the heat exchangers  110 ,  120  and  130 , and the motorized valves  115 ,  125  and  135  and the temperature-control devices  116 ,  126  and  136  are arranged in the same way as in the system as described in connection with  FIG.  1   a   . The same is true of the inlet  100 , the inlet filter  101 , the pipeline  102  and the liquid-hydrogen tank  1000  and the pipeline  1001  for distributing liquid hydrogen to the heat exchangers  110 ,  120  and  130  via the motorized valves  115 ,  125  and  135  respectively. However, in this variant, the part of the system downstream of the heat exchanger  130  is arranged differently so that the purge device  137  is replaced by a device  137 ′ for heating the primary fluid which comprises two outlets  139  and  139 ′ and so that the control of the motorized valve  135 , which valve is configured for regulating the temperature of the primary fluid at the outlet of the portion of primary pipeline  102  associated with the heat exchanger  130 , is adapted so that the primary fluid has a temperature of between 63K and 77K at this point, which is to say, so that it is in the form of liquid air. The inlet of the heating device  137 ′ is connected to the portion of primary pipeline  102  leaving the heat exchanger  130 . A warming of the liquid air thus obtained, within the heating device  137 ′, allows the nitrogen to be extracted in gaseous form at the outlet  139 ′ of the heating device  137 ′ so that nitrogen-impoverished air can be supplied at the outlet  139  of the inert-gas generation system  10 . 
       FIG.  2   b    is a diagram illustrating a method for generating nitrogen, in gaseous form, in the inert-gas generation system  10  arranged according to the first variant embodiment described hereinabove. Steps S 0 , S 1  and S 2  remain unchanged in comparison with the method described with reference to  FIG.  1   b   , which is to say, the two first steps of separating components of the primary fluid, which are respectively aimed at extracting the water and the carbon dioxide (also referred to as CO2 gas) from the primary fluid, are performed in the same way as in the method described in connection with  FIG.  1   b   . However, in this variant and after the step of extracting the carbon dioxide, the cooling of the primary fluid that is executed in the heat exchanger  130  is designed so that the primary fluid has a temperature of between 63K and 77K so that, during a step S 3   b , air in liquid form is obtained and so that, during a step S 4   b , nitrogen in gaseous form is extracted from the primary fluid to be made available at the outlet  139 ′ of the system  10 , and nitrogen-impoverished air can be made available at the outlet  139  of the system  10  in a step S 5   b.    
       FIG.  3   a    schematically depicts a second variant embodiment of the inert-gas generation system  10  as already described in connection with  FIG.  1   a   . In this variant, the various heat exchangers used are not connected to the liquid-hydrogen tank  1000  in parallel via the pipeline  1001 , as in the embodiment variants described in connection with  FIG.  1   a    and  FIG.  2   a   . In this second variant, the secondary pipelines of the heat exchangers  110 ,  120  and  130 , through which pipelines the liquid hydrogen used for cooling the primary fluid circulates, are arranged in series. Furthermore, a fourth heat exchanger  140  is used downstream of the heat exchanger  130  on the primary pipeline  102  to liquefy the nitrogen obtained at the outlet of the portion of primary pipeline  102  associated with the heat exchanger  130 . At this point, a temperature-control device  146 , configured to operate a motorized valve  145 , allows regulation of the temperature of the primary fluid. According to this embodiment variant, the outlet of the portion of primary pipeline  102  associated with the heat exchanger  140  is connected to a pump  137 ″ the outlet of which is connected to a liquid-nitrogen storage tank  150  via a non-return valve  151  positioned at the outlet  139  of the inert-gas generation system  10 . 
     In this variant, the liquid hydrogen stored in the liquid-hydrogen tank  1000  passes through a secondary distribution pipeline  1002 . The secondary hydrogen-distribution pipeline  1002  passes in succession through the motorized valve  145  (used for regulating the temperature of the primary fluid at the outlet of the primary pipeline  102  of the heat exchanger  140 ) and the portion of secondary pipeline  1002  associated with the heat exchanger  140 , and then the motorized valve  135  (for regulating the temperature of the primary fluid at the outlet of the primary pipeline  102  associated with the heat exchanger  130 ), the portion of secondary pipeline  1002  associated with the heat exchanger  130 , and then the motorized valve  125  (used for regulating the temperature of the primary fluid at the outlet of the primary pipeline  102  associated with the heat exchanger  120 ) and the portion of secondary pipeline  102  associated with the heat exchanger  120 , and then finally the motorized valve  115  (for regulating the temperature of the primary fluid at the outlet of the primary pipeline  102  associated with the heat exchanger  110 ) and the portion of secondary pipeline  1002  associated with the heat exchanger  110 . Each of these motorized valves  135 ,  125  and  115  is associated with a so-called “bypass” pipeline allowing the portion or portions of secondary pipeline  1002  downstream (and therefore the valve or valves arranged downstream on the secondary pipeline  1002 ) to be fed with fluid when the valve that it bypasses is configured to allow hydrogen to pass only at a limited flow rate with a view to regulating the temperature at the outlet of the primary pipeline of an exchanger. Thus, a “bypass” portion  1002   a  of the secondary pipeline is designed to bypass the heat exchanger  140  when the motorized valve  145  is configured to limit the flow rate of hydrogen distributed in the secondary pipeline  1002 , a “bypass” portion  1002   b  of secondary pipeline is designed to bypass the heat exchanger  130  when the motorized valve  135  is configured to limit the flow rate of hydrogen distributed in the secondary pipeline  1002 , a “bypass” portion  1002   c  of secondary pipeline is designed to bypass the heat exchanger  120  when the motorized valve  125  is configured to limit the flow rate of hydrogen distributed to the secondary pipeline  1002 , and a “bypass” portion  1002   d  of secondary pipeline is designed to bypass the heat exchanger  110  when the motorized valve  115  is configured to limit the flow rate of hydrogen distributed in the secondary pipeline  1002 . 
     Such an arrangement of the exchangers  140 ,  130 ,  120  and  110 , arranged in series on the hydrogen distribution pipeline  1002 , so as to sequentially execute successive cooling operations in these exchangers, advantageously allows the hydrogen initially stored at a temperature of 20K to be warmed up progressively as it successively passes through the various portions of the secondary pipeline  1002  so that it can be used later on board the aircraft, for example in a fuel cell. Advantageously, the exchanger positioned furthest upstream on the distribution pipeline  1002  is the one at which the lowest temperature is required to execute cooling of the primary fluid, and so on. 
       FIG.  3   b    is a diagram illustrating a method for generating nitrogen, in aqueous form, in the inert-gas generation system  10  arranged according to the second variant embodiment described above. The steps S 0 , S 1 , S 2  and S 3  are similar to those described previously according to the embodiment or variant described in relation to  FIG.  1   a    and  FIG.  1   b   . That means to say that the operations of filtering, of separating components of the primary fluid with a view to extracting the water from the primary fluid, then the carbon dioxide, then the oxygen in liquid form, are similar to what was described earlier. Thus, the temperatures of portions of primary pipeline  102  at the measurement point at which the temperature measurement devices  116 ,  126  and  136  are located are identical to those described hereinabove in the method in relation to  FIG.  1   a   . However, the temperature of the primary fluid is regulated at the outlet of the portion of primary pipeline  102  associated with the heat exchanger  140  in order there to obtain nitrogen in liquid form in a step S 4   c , which nitrogen is then pumped by the pump  137 ″ before being stored in the nitrogen tank  150  in a step S 5   c.    
       FIG.  4   a    schematically depicts a third variant embodiment of the inert-gas generation system already described in connection with  FIG.  1   a   . According to this embodiment variant, the portions of secondary pipeline associated with the various heat exchangers  110 ,  120 ,  130  and  140  successively used to participate in fractional distillation of the compressed air, which is to say, the portions of pipeline through each of which a fluid that is to cool, by exchange of heat, the primary fluid present in the primary pipeline  102  passes, are connected to different sources of refrigerant. According to this variant embodiment, the inlet  115   b  of the motorized valve  115  is connected to an ambient-air inlet (identified as “AIR” in  FIG.  4   a   ), the inlet of the motorized valve  125  is connected to the outlet  138  of the purge device  137 , configured to extract oxygen in the liquid state (identified as “LO2” in  FIG.  4   a   ) from the primary fluid downstream of the heat exchanger  130 , and the inlet of a motorized valve  165  configured to regulate the temperature of the primary fluid obtained from the outlet  139  of the system  10 , in collaboration with a heat exchanger  160 , is connected to the hydrogen tank  1000  (identified as “LH2” in  FIG.  4   a   ) by a distribution pipeline  1003 . An outlet  121  of the secondary pipeline associated with the heat exchanger  120  allows the liquid oxygen used as a coolant in the heat exchanger  120  to be directed onwards, with a view to potential subsequent use, for example in a fuel cell carried onboard the aircraft. The temperature at the outlet of the portion of primary pipeline  102  associated with the heat exchanger  160  is controlled via the motorized valve  165  which is controlled on the basis of a temperature-measurement device  166 . 
     Furthermore, the portion of secondary pipeline associated with the heat exchanger  130  is connected to the outlet  168  of a pump  167  configured to pump the primary fluid in the form of liquid nitrogen (identified as “LN2” in  FIG.  4   a   ) available at the outlet of the heat exchanger  160  along the primary pipeline  102 , downstream of the heat exchanger  160  designed to control the temperature of the primary fluid taken from the outlet  139 . The pump  167  thus allows nitrogen in the liquid state to be pumped in the portion of secondary pipeline associated with the heat exchanger  130  so as to cool the primary fluid, and then allows this liquid nitrogen to be stored in a tank  169 , by way of a pipeline  163  comprising a nonreturn valve. 
     A motorized and controlled two-outlet valve  162  allows the nitrogen still in the gaseous state before being completely cooled to be directed from an outlet  131  of the portion of secondary pipeline associated with the heat exchanger  130  towards an outlet  164  when the system is started up, and then allows nitrogen in the liquid state to be directed towards the liquid-nitrogen tank  169  when the system is operating under nominal conditions. The directional valve  162  is controlled by a controller which has not been depicted in the figure, which is suitable for determining the state of the primary fluid at various points of the system  10 , by temperatures taken by temperature-measuring devices. Thus, the successive operations of separating components of the primary fluid can be carried out by obtaining suitable temperatures in the various portions of the primary pipeline  102  and fluids, the proximity of which could be detrimental to safety if an element of the system should leak or break, are advantageously kept away from one another. Advantageously, the liquid oxygen extracted at the outlet  138  of the purge device  137  could be used to operate one or more fuel cells used in the aircraft. 
       FIG.  4   b    is a diagram illustrating a method for generating nitrogen, in liquid form, in the inert-gas generation system  10  arranged according to the third variant embodiment described hereinabove. In the inert-gas generation system  10  arranged according to this third variant, the step S 0  of filtering the compressed air conveyed to the inlet of the system  10  remains unchanged in comparison with the variants described previously. Then, in a way similar to that executed in the method described in connection with  FIG.  1   b   , a cooling of the primary fluid is executed during a step S 10  in order to extract the water from the primary fluid, downstream of the heat exchanger  110 . In this third variant of the method, this first cooling of the primary fluid is carried out using ambient air taken from outside the aircraft carrying the inert-gas generation system  10 . The ambient air is introduced for this purpose into the portion of secondary pipeline associated with the heat exchanger  110  after its flow rate through this pipeline has been regulated by the motorized valve  115 . The ambient air is then recovered via an outlet  111  of the secondary pipeline associated with the heat exchanger  110  so as possibly to be used somewhere else in the aircraft. A second cooling of the primary fluid is then executed during a step S 20 , using in the heat exchanger  120  liquid oxygen collected from the outlet  138  of the purge device  137 . Thus, judiciously, a fluid extracted from the primary fluid during one step of fractional distillation for generating an inert gas is used as a refrigerant in another step of the fractional distillation. A third cooling of the primary fluid is then executed in a step S 30 , in the heat exchanger  130 , using liquid nitrogen obtained from a fourth cooling of the primary fluid, which cooling is executed during a step S 40  aimed at liquefying the nitrogen available in the gaseous state in the outlet pipeline  139 . This fourth cooling of the primary fluid is executed using liquid hydrogen from the hydrogen tank  1000 , used as coolant in the heat exchanger  160 . The liquid nitrogen thus obtained is then pumped into and stored in the liquid-nitrogen tank  169 , during a step S 50 . 
       FIG.  5   a    schematically depicts a fourth variant embodiment of the inert-gas generation system  10  already depicted in  FIG.  1   a   . In this fourth variant, the first cooling of the primary fluid introduced into the inlet  100  of the system  10  and then filtered in the filter  101  as it enters the primary pipeline  102  is carried out in the heat exchanger  110  using nitrogen in the gaseous state. The nitrogen in the gaseous state that is used for this first cooling is obtained by extracting liquid oxygen from the primary fluid in a purge device  157 . The primary fluid rid of liquid oxygen is conveyed towards the portion of secondary pipeline associated with the exchanger  110  via a pipeline  159 . The liquid oxygen is extracted via a purge outlet  158  of the purge device  157  and the residual gaseous nitrogen is conveyed via a pipeline  159  from the purge device  157  to the portion of secondary pipeline associated with the exchanger  110 . The gaseous nitrogen used as a coolant in the heat exchanger  110  is then available at the outlet  111  of the heat exchanger  110  for use as an inert gas. Once again, the liquid oxygen can be used to operate a fuel cell carried onboard the aircraft. 
     According to this variant embodiment, the inlet of the portion of secondary pipeline associated with the heat exchanger  120 , used for cooling the primary fluid with a view to extracting the carbon dioxide, is connected to the outlet of the primary pipeline associated with the heat exchanger  130 , which is used here for obtaining liquid air by cooling the primary fluid, after the successive extraction of the water and of the carbon dioxide, which extraction operations are executed respectively in the purge devices  117  and  127 . The cooling executed in the exchanger  130  is executed using liquid hydrogen, taken from the liquid-hydrogen tank  1000  via a pipeline  1004  and then via the motorized valve  135  which passes into the portion of secondary pipeline associated with the heat exchanger  130 . The motorized valve  135  that regulates the flow rate of the liquid hydrogen in the portion of secondary pipe associated with the heat exchanger  130  is controlled on the basis of a temperature measured by the primary-fluid temperature-measuring device  136  arranged at the outlet of the portion of primary pipeline  102  associated with the heat exchanger  130 . The flow of liquid air is transmitted between the outlet of the primary pipeline associated with the heat exchanger  130  and the inlet of the secondary pipeline associated with the heat exchanger  120  through a pipeline  139 ″. 
       FIG.  5   b    is a diagram illustrating a method for generating nitrogen, in gaseous form, in the inert-gas generation system  10  arranged according to the fourth variant embodiment described above. 
     The step S 0  of receiving compressed air in the system  10  and of filtering this air is still identical to the step S 0  executed in the other variants. Then, in this variant, the first cooling of the primary fluid is executed in a step S 100  using gaseous nitrogen obtained by the extraction of liquid oxygen during a step S 400  from liquid air used as a coolant in the second cooling of the primary fluid, which is executed in a step S 200 . This liquid air is itself obtained after a third cooling of the primary fluid, in the heat exchanger  130  and during a step S 300 , using liquid nitrogen. After the water and the carbon dioxide have been extracted, liquid air has been obtained and liquid oxygen has been extracted from the primary fluid, the nitrogen in the gaseous state is then available at the outlet  111  of the heat exchanger  110 . 
       FIG.  6    depicts an aircraft  1  comprising the inert-gas generation system  10  according to one of the embodiment variants described hereinabove. The use of such an inert-gas generating system is particularly advantageous on board an aircraft in as much as the hydrogen stored at a temperature of 20K in one or more tanks needs to be warmed in order to be able to be used in a fuel cell, and its initial temperature allows it to successively cool compressed hot air so that fractional distillation operations can be performed and an inert gas such as nitrogen generated in order to reduce the flammability of the contents of certain parts of the hydrogen-distribution system. 
     While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.