fig1 shows an installation 1 for the combustion of lean fuel gas , using a burner 2 mounted between 4 main parts za , zb , zc , zd that are separated by three partitions 3 , 4 , 5 . the parts represent , respectively , a zone za firebox where combustion takes place , a zone zb containing or in communication with the lean fuel gas , a zone zc containing or in communication with combustion air , a zone zd that is exterior to the installation and accessible to personnel . the installation is , for example , a production facility for superheated steam at a rate of 40 t / hour in which blast furnace gas is to be burnt , at ambient temperature , ( humidity = 2 . 5 % of h 2 o by volume ), low - pressure fed (& lt ; 300 mm ce relative pressure ) and an average composition on dry gas : n2 = 58 %, h 2 = 1 . 7 %, co 2 = 20 . 3 %, co = 20 % ( pci = 660 kcal / m 3 n ). the products of combustion must contain less than 50 ppm of co with less than 1 % of oxygen in these fumes . ignitibility of this gas occurs when there is 35 % to 73 % of gas in the mixture . the burner includes a gas port nose 6 ending in zone a of the chamber . the nose is centered on a central axis x which happens to be , under the circumstances , the main axis of the burner to the extent that the latter has a general circular shape around this axis . the burner also includes means to feed this nose which are capable of ejecting a flux of air and fuel gas in rotation around a central axis centered on the gas port nose . this nose which constitutes the front end of the burner is intended to receive , in front of or on it , to the left of the figure , a flux of fuel gas and combustion air which is put into rotation around the central axis , with means to feed this nose provided for this purpose and which are described subsequently . the burner also comprises a central chamber 7 connected to the nose and upstream of the burner ( relative to the direction of the flux discharge ) and mounted in zone zb between partitions 3 and 4 , with at least one opening 8 ending in this zone zb . in zone zc , there is a back end 7 b of the burner linked to chamber 7 , upstream of the latter , and presenting at least one access for at least one inflow of combustion air of zone zc . the air supply in the preferred example is done entirely by the rear face of the burner for several advantages : guarantee the tightness of the assembly ; facilitate access to the pre - mix air controls ; be able to install the burner in the air chamber ; and be able , depending on the application , to realize separate supplies of different types of combustion air . in zone zd , various pipes discharge which extend between the chamber and the exterior , while crossing the burner and among which pipes one finds , if applicable , a rich gas supply pipe , a flame control pipe , an ignition pipe or other pipes or equipment known to the experts ( not shown ). according to a way of carrying out the invention , the method may include a first stage in which a flux of air intended for combustion is divided into at least one flux of ore - mix air and a flux of complementary air . the complementary air is constituted of at least one central flux of air and / or one flux of peripheral air . in the example shown , one uses both the central and the peripheral flux of air for improved efficiency and versatility in usage and the division is made by different air inlets at the back of the burner or routing of the air in the burner . to this effect , in the example of execution shown , the burner is configured to divide the air coming from the space zc into several flux ( flows ). it comprises a number of intakes or access on its back end : a central access 9 to receive an intake of a flux of central air , a peripheral access 10 to receive an intake of peripheral air , and at least one main access 10 a to receive an intake of pre - mix air . more access points can be added as indicated later on . in a variant execution , this dividing step could be done differently , for example by external pipes outside the burner , and each flux of air could be supplied by independent and external pipes . in a second step of this type of execution , in front of the gas port nose , a pre - mix flux is ejected containing a mixture of pre - mix air and of fuel gas , in rotation around the central axis . the pre - mix flux is nonflammable to the extent that it is mixed at a rate far from the ignitibility ranges , for example above an ignitibility threshold . in effect , in the example described here , one goes from a lean gas rate of 100 % to a rate of 80 - 85 % ( in the gas + air mixture ) whereas the limits of ignitibility are between 30 and 73 % in the mixture . this assumes that the pre - mixture and its rotation are carried out beforehand , as described below . in order to improve combustion and to ensure good flame retention , it is of interest and importance to achieve the most thorough pre - mixture and at the earliest moment possible . for this second step , the burner , in the example described , is configured to achieve the preceding pre - mix inside itself , in this particular case in a so - called pre - mix space 16 of the chamber 7 . it is also configured to place the pre - mix into rotation . this rotation , in the example described , is also preferably achieved in the chamber upstream of the gas port nose . to this effect , the pre - mix air access points 10 a mentioned above lead to the chamber for the same reason as the fuel gas access ports 8 for the purpose of obtaining a pre - mixture using the mixing devices 11 described later on . however , in a variant , the pre - mixing could be done beforehand outside the burner , for example , in an enclosure provided for this purpose in which a rate above the ignitibility rate is maintained . for the gas concerned of the example , the mixing occurs at a rate between 5 and 20 % above the ignitibility threshold with an insufficient air percentage ( proportions going between 78 and 95 % gas in the mixture ), for reasons of safety and efficiency , one prefers to adopt a rate that is 10 to 20 % below the total air to be supplied . as a variant , one could , for certain applications , implement the method by obtaining a pre - mixture with an insufficient rate of fuel gas in the same proportions between 5 and 20 % or with different proportions for particular applications of biogas or voc ( volatile organic compounds ) burns . in a third step of execution , the complementary flux is ejected a the center of the pre - mix flux by way of the flux of central complementary air and / or around the pre - mix flux by way of the flux of peripheral complementary air , in a manner so as it reaches the threshold of ignitibility at the gas port nose . in the example described , ejection of the complementary flux occurs simultaneously at the center and in the periphery in order to achieve a better final mix . for this purpose , the burner is configured so as to deliver the pre - mixture flux in the form of ring 12 located between a central pipe 13 and the periphery 14 of the front end of the chamber . according to a mode of execution of the method , the pre - mixture flux is obtained by incorporating pre - mixture gas in fuel gas . in effect the lean , and generally residual , combustibles are distributed under very low pressure and in consideration of the large volumes involved , it is important to facilitate the flow of these gases by the effects of mechanical drives . for this application , the burner includes the incorporating devices 11 mentioned earlier which inject air into the fuel gas . incorporation is done directly in an enclosure of the chamber having a pre - mixture space 16 ( fig2 ) located between a central pipe 13 and an internal partition 31 of the chamber . according to a mode of execution , incorporation is achieved by injection of pre - mix air at an entry point of the fuel gas into the burner so as to : drive the fuel gas into a pre - mix space 16 , obtain the pre - mixture by turbulence resulting from the injection , and direct the pre - mixture towards the gas port nose while initiating a rotation around the central axis . for this purpose , the burner includes injection devices including nozzles 17 or calibrated directional high - output orifices located in the incorporation devices 11 that are profiled and directed towards the pre - mixture space 16 at the ports 8 . the gas located near and around the ports 8 is driven by the partial vacuum generated by the air jets at the exits of the nozzles which are directed by the orientation of the jets and [ the gas ] is mixed by the turbulence created by the jets . a rotational movement of the mixture is also initiated in the pre - mixing space by the orientation of the air jets . the incorporating devices may also preferably comprise second means of pre - mix air injection . these means of injection are placed so as to obtain an incorporation of air parallel to the central axis and by directing the pre - mix flux towards the gas port nose . these means of injection have preferably a progressive state condition depending on the level of power used . these second means of injection may be formed , as in the example described , by tubes 21 around orifices 22 in the partition 23 of the back end of the burner ( fig3 , 10 , 11 ). these tubes are preferably of different lengths and are five in number in the example . they extend to the interior of the pre - mixing space from air intakes or orifices 22 located on the partition 23 or back face of the burner . the orifices 22 are preferably capped by flaps ( not shown ) that can be operated by scaled springs or electric controls . the flaps may be located on the orifices with or without tubes . the tubes allow , on the one hand , to avoid the respective flows upsetting each other and , on the other hand , to supply air at different points with a guarantee of its distribution . the orifices have a determined size so as to avoid finding themselves too massively inside the limits of ignitibility and that there may locally be conditions that are favorable to a combustion that would deteriorate the burner . alternatively , one could obtain the incorporation of gas in the air , for example by interchanging the different inflows and regulating the respective outflows . this variant may be considered in particular for heating large volumes of air ( drying applications ) or for burning vocs . in this case , the pre - mixture gas would replace the pre - mixture air and the flows , the pre - mixture could be unchanged and the central and peripheral flows could involve for instance fuel gas instead of combustion air . according to one way of carrying out the invention , the flux of central complementary air is ejected in rotation in front of the gas port nose and in divergent flow to penetrate the pre - mix flux and the flux of peripheral complementary air is ejected in a convergent flow and in strong spiral rotation . to this effect the burner is configured in the example with a cone - shaped deflector 18 at the exit of the central pipe 13 and blades 19 in the pipe which put the central flux of air into rotation . other equivalent means may also be suitable , as for example calibrated directional orifices or oriented ports in a separating partition . preferably the central air is divergent with an angle at the top of 60 to 180 ° or of 30 to 90 ° in relation to the axis of the burner . this ejection produced in this way allows achieving good penetration of the air in the pre - mixture so as to best complete the rate of missing air . the flux of central air of the example has previously penetrated the intake 9 in the internal conduit of pipe 13 , in the ring space around the central pole 51 . if applicable , this central air can have another function that is explained later on , which is to feed at its ejection base a rich gas which would be distributed in ring shape around the central air , during its use , in particular during startups or shortages of lean gas . as to the peripheral complementary air , the burner is configured with injection nozzles 20 a , 20 b located on a ring 14 on the front end or face of chamber 26 a . the nozzles are oriented both tangentially to a circle centered on the central axis and oriented towards the front . spiral rotation is obtained by this dual slant of the nozzles . the peripheral air wraps around the flux of lean gas and enhances its rotation . it is distributed at high speed and optimizes the mixing . nozzles 20 a , 20 b are fed by the space of peripheral pre - ejection 30 located in a double partition of the chamber at the front of the chamber which is itself fed by the intake devices 10 a that have been provided in the vicinity of the back end 26 b of the chamber . the sub - components of the burner are now described below , with reference to the corresponding figures , namely , the chamber , the central tube and the central pole . in reference to fig6 to 9 , the chamber 7 has a general rotational shape and consists of : a dual peripheral wall formed by an external wall 24 and an internal one 25 , a front end or face 26 a , formed by a ring 14 including the means for peripheral injection 20 a , 20 b ; a back end or face 7 b including different intakes or feeds 10 , 10 a at least for a flux of peripheral air and of pre - mixture , a space 27 for air circulation divided by the double wall allowing to let the two ends 26 a and 26 b be in communication with each other , intake ports 8 on the double peripheral wall , these ports being intended to interface each other between an internal space 16 in the so - called pre - mix space chamber and the outside , conduits , in the form of hollow girders 11 , located in the thickness of the double wall , these conduits extending between the ports 8 , between a receiving space for air flux 10 a or intake located at the back end 7 b and a pre - ejection space 30 of the peripheral air located at the front end , and pre - mix air injection nozzles 17 located under the conduits , these nozzles being configured so as to perform said first permanent injection of pre - mix air , these conduits and the nozzles being part of the means of incorporation mentioned previously . the nozzles are in fact exit perforations made in the ring one of the functions of which is to close off the front end of the double wall of the chamber . the other back end of the double wall is closed off by a wall 23 b . these perforations communicate with the pre - ejection space 30 of the double wall and lead to the outside through an internal wall of the chamber . the nozzles are arranged on the ring , being offset relative to the radial axis r of the chamber and slanted towards the front relative to a plane perpendicular to the chamber . the nozzles are offset and slanted in different ways according to an alternation . the angles proposed are specific to this power of the burner , but would be inevitably modified for another size burner . these angles have been determined so that the jets of consecutive orifices do not interfere with each other and do not collide with the end of tube 13 nor impede the flow of fluids coming out of the gas ring contained between 13 and 56 , nor the divergent complementary central air . this divergent cone must practically “ mesh ” with the convergent complementary peripheral jet with the most acute angle ( here 15 °). the angle of the next orifice is more open in order to continue further along in the rotation the work of the preceding orifice . a first series of nozzles ( 20 a ) maybe slanted from 5 ° to 45 ° to the front , ( 15 ° preferred in the example of execution ) and from 30 to 65 ° relative to the radial axis ( r ), ( 44 ° preferred in the example ) and a second series of nozzles ( 20 b ) slanted from 25 to 65 ° to the front ( 45 ° preferred in the example ), and from 30 to 70 ° relative to the radial axis ( 53 ° preferred in the example ). the chamber may also include orifices 55 arranged on the internal wall 25 at the height of the pre - ejection chamber 30 . these orifices permit feeding the blade device 37 from the chamber 30 in order to improve the air / lean gas mixture between the blades . in reference to fig1 and 11 , a central tube 13 , meant to be installed centered on the central axis , is dimensioned to extend longitudinally between the two ends of the chamber and to put in communication with each other . an outside surface 35 for the purpose of delimiting the pre - mix space with the inside wall 25 or the inside face 31 of the double wall of the chamber , and an inside surface 52 ; fasteners at the chamber and reception devices of an air or gas chamber located at the end of the tube ; and a first set of blades 37 , located at the front of the tube , said blades 37 extending into the pre - mix flow space between the outside wall 35 of the tube and the inside wall of vessel 25 or inside face 31 . the blades are profiled so as to create a rotation of the pre - mix flux during its flow towards the exit of the vessel . a space between the vessel and the tube forms a ring - shaped conduit 38 ( fig1 ) intended to convey the flux of pre - mix air . a wall 23 forms a radial collar of the central tube , said wall separating the pre - mix space with the back end of the central tube which is itself in communication with the air chamber . in reference to fig1 , the central pole 51 is intended to be located in the central tube 13 and centered on the central axis . the burner also includes a second set of blades 19 located inside and in proximity to the front end of the central tube . in the example , the blades are attached to the central pole 51 which crosses the central tube . they are meant to extend from the surface of the pole 50 to the inside wall 52 of the central tube . the burner may also include a “ burn cone ” 18 as a deflector located downstream of the central tube and spaced from it so as to provide a divergent outflow of the central flux of air . in the example shown , the burn cone is placed at the front end of the axial pole 51 . the gas is ejected at the end of the pole , at a divergent angle that is defined by a series of calibrated orifices 54 placed in a ring form around the cone shaped deflector 18 which allows to eject this gas over a maximum circumference so that any rich gas jets that may be present and originate as close as possible to the central combustion air and have maximum momentum when colliding with the flow of lean gas . preferably , for best results , the cone shaped deflector maybe a deflector 18 b with a peripheral serration 52 and have central orifices 53 leading to the inside of the conduit of the central pole . in general , the burner is designed to receive , under normal operating conditions , an ejection of complementary flux at a very high speed above 100 m / second whereas the pre - mix flux is ejected at a speed between 40 and 80 m / sec . if applicable in a variant of execution , the burner may include a rich gas supply . in the example , the rich gas is brought in under pressure to the periphery of the central tube directly to the pre - mix space . preferably , for very high capacity burners ( over 20 megawatt ), the rich gas is distributed around the central tube so as to mix thoroughly with the pre - mixture . a ring - shaped vessel 36 for receiving and distributing gas around several orifices crossing the rear wall 23 in the shape of a radial collar of the central tube ; a portion of tube 56 arranged in double wall around the central tube so as to convey the flux entering the pre - mix chamber to essentially half way into the chamber ; and a connection cone 57 from the double wall to the vessel through the intermediary of the collar so as to collect the rich gas ; as an accessory , a ring - shaped deflector 58 placed at a distance from the end of the double wall so as to diverge the rich gas and promote a good stirring with the air ; and alternatively or as a complement to the deflector above , a series of orifices 59 that are calibrated and placed across the central tube is arranged in form of a ring just upstream of the deflector 58 so as to let the complementary central air be ejected into the flux of rich gas and to contribute in this way to make it diverge . according to a variant of execution , the vessel 36 maybe connected to a rich gas supply tube ( the orifices 10 a 2 being blanked off ) or another vessel 36 b ( not shown ) wrapped around vessel 36 and connected to the supply tube . calibrated orifices arranged with a divergent angle may be made in a ring connecting the two tubes 13 and 56 b at the front end . possibly , the double wall 56 tube portion may extend to the end of the central tube 13 , forming a central double wall 56 b so as to eject the rich gas directly to the gas port nose around the central air . on the other hand , for burners of lower capacity ( for instance less than 20 mw ), the rich gas is , still under pressure , brought into the central pole . it is ejected at a defined divergent angle by a series of calibrated orifices 53 arranged in a ring around a particular device ( deflector with peripheral serration 52 ) which allows to eject this gas on a maximum circumference so that the rich gas jets originate as close as possible to the combustion air and have maximum momentum when colliding with the flow of lean gas . the above configurations make it possible to obtain a consistent flame of a continuous structure and maximum surface ( optimization of thermal transfer in the burn chamber ). the rich gas is thus supplied with combustion air at its base , whatever the composition / proportion of the fuels : single and pure gas or gas in mixtures . the burner is designed in mechanic / welded modules which allow for a maximum of flexibility and ease of design , adaptation , construction , installation and maintenance , in the knowledge that : the combustion air may be more or less hot , for installations with multiple burners mounted next to each other , the directions of rotation of the fluids must be coordinated so they won &# 39 ; t upset the combustion and the flows in the burn chamber , it allows to easily replace existing burners , and the rich fuels may be of different qualities . the possible flows of the different flux are described in accordance with one operating mode of the burner . an ignition flame is brought to the nose of the burner through the intermediary of a guide tube 60 ( fig1 ). the permanent air system is then activated by a pump ( not shown ) which blows combustion air into the back end of the burner by putting the air supply vessel zc under pressure . a fraction of the combustion air penetrates into the double wall 27 of the vessel ( fig6 ) across the inlet orifices 10 that are for example rectangular and made in the ring - shaped wall 23 b closing the double wall at the rear . whereas another fraction penetrates directly into the double vessel towards the nozzles 20 a , 20 b . a portion of this fraction penetrates into the girders 11 ( fig8 ), whereas the other portion feeds directly into a pre - ejection chamber of peripheral air 30 by way of a partial , so - called deflection double wall of the vessel ( fig7 ) which features no ports and which extends at an angle of approximately 90 ° between the radial walls 32 and 33 . the girders are put under pressure and combustion air escapes from the nozzles in a tangential direction ( fig7 ) to a circle centered on the central axis and towards the blades of the first rotation device . the combustion gas which may be under light pressure ( generally less than 200 ce ) enters crosswise into the vessel under an effect of entrainment of the air jets at the level of the ports 8 between the girders 11 . the turbulence results in a pre - mixing or stirring inside the pre - mix space 16 of the vessel at the entry of the blade device ( fig7 ), especially by deflection against the deflection wall 31 . since the girders also open into the pre - ejection chamber 30 of peripheral air , they contribute to the air supply there in addition to the air conveyed by the inside of the deflection or guide double wall 24 , 25 . combustion air also penetrates by entry 9 of the central tube 13 ( fig1 , 11 ) and opens directly at the level of the nose 6 , after having entered the space between the blades of the second blade device 19 ( fig2 ) where it assumes a rotational movement . this air re - exits in front of the nose in a deflecting manner by way of the cone shaped deflector 18 placed in front of it . during this time the peripheral air is ejected from the chamber 30 ( fig9 , 14 - 16 ) in the form of two swirls by way off the peripheral nozzles 20 a , 20 b in front of the gas port nose . when the pre - mixture arrives at the exit in front of the burner where it is ejected in a ring shaped swirl , it is squeezed and stirred between the central and peripheral flux of air which penetrate it thoroughly . the rotational direction of the different flux of air may be opposite that of the pre - mix flux , but preferably they should be in the same direction . if applicable , supplemental air may enter the pre - mixture chamber by tubes 21 or flaps ( fig1 , 11 ) arranged on a ring - shaped wall 23 coming from the collar of the central tube and helping to enrich the air mixture . air may also come from the back end of the vessel 36 through orifices 10 a 2 and enrich the pre - mixture . if applicable ( fig6 ), air may escape from the vessel beginning from the pre - ejection chamber 30 through orifices 55 made in the inside wall of the vessel and it penetrates radially in the blade device 37 between the blades . this helps to improve the stirring of the gas mixture with air . if several burners are used which are arranged near each other in a combustion chamber of an installation , care must be taken to ensure that the different swirls mesh . to this effect , the orientation of different nozzles and blades must be adapted . for example , the peripheral swirls should be working in opposite between two burners . the flame is stable and has caught on well , and one eliminates all the vibrations caused by unstable combustion ; no adjustment is required ; the combustion air can be divided into more than two , even more than three fractions ; there is no need for a support fuel to compensate for irregularities in the mixture or in the leanness of the fuel gas , nor for devices or associated equipment which allows you to save rich gas if there is a shortage of lean gas ; possibility to function normally with pure rich gas and rich gas only ; elimination of the need for heating gas or combustion air , as a result of the burner &# 39 ; s capability to properly burn gases with very low ncv (& lt ; 750 kcal / m 3 ) in cold gas and cold air ; generally heating of combustion air of a 20 mw occurred at 200 ° c . [ 392 ° f . ]; reduction in the oxygen content of fumes because of very good combustion due to an optimized air - fuel mixture . the oxygen content in fumes has been reduced to 0 . 6 - 1 % instead of 2 %; there is a rise in the flame temperature from 60 to 80 ° c . [ 140 to 176 ° f .] bringing about a significant increase of thermal transfers in the combustion chambers (+ 15 %), productivity of the boiler being thereby improved , if the re - superheating can follow ; there is a significant reduction of losses to fumes ( at constant temperature ) because the volume of fumes drops in the same proportion as the air factor , by 10 to 15 %, thereby the boiler output being improved by at least 1 point , for a 100 mw boiler , representing more than 10 gwh / year of fuel ; and reduction of electric power consumption of ventilators for air and fumes to be stirred to the extent that the volumes of air and fumes to be stirred are smaller , this also resulting in smaller size ( by 15 to 20 %) blower and draught fans and a reduction in their power consumption of more than 10 %.