FR2797321A1 - LOW-EMISSION NITROGEN OXIDE SMOKE RECIRCULATION BURNERS AND GAS HEATERS COMPRISING SUCH BURNERS - Google Patents

Abstract

The present invention relates to improvements made to industrial burners with liquid or gaseous fuel. The technical sector of the invention is the field of manufacturing industrial burners. According to the invention, a burner (1) with gaseous fuel comprises a central fuel transport conduit (10), a central air transport conduit (2) surrounding said central fuel conduit and a plurality of peripheral fuel transport conduits (13); said peripheral conduits extend at least partly outside the central air transport conduit, the burner further comprises a straight section of flue gas conduit (22) which is arranged in the extension of each nozzle ( 14) equipping the peripheral conduits (13), and the smoke suction conduits (22) are fixed to the burner, outside the air transport conduit (2), by a connection (101) promoting their cooling.

Description

Burners with smoke recirculation and low emission of nitrogen oxides

  and gas heaters comprising such burners The present invention relates to improvements made to industrial burners with liquid or gaseous fuel.

  The technical sector of the invention is the field of manufacturing industrial burners.

  The present invention relates more particularly to improvements in gaseous fuel burners with very low 0o emission of nitrogen oxide comprising primary means for injecting combustible gas disposed in the center of the burner and peripheral secondary injection means, and intended to be mounted either in an orifice provided for this in the wall of an enclosure, such as the combustion chamber or hearth of an oven or a boiler, or in the central part of a gas circulation duct turbine exhaust from an electrical power co-generation facility and

  to heat these exhaust gases.

  To reduce the nitrogen oxide content of the flue gases, it is known to reduce the free oxygen content of the gases in the flame to avoid too strong a combination thereof with the nitrogen in the air, and to limit the maximum flame temperature. To reduce the oxygen content in the flame and to reduce the flame temperature, a solution is to recycle part of the smoke present in the hearth: it is known to carry out this recirculation by means of ejection systems which use the vacuum created by ejecting the fuel. Furthermore, it is known that, in order to limit the maximum flame temperature, combustion can be staggered, that is to say that instead of bringing all of the fuel and of the oxidant into one zone, proceeds in stages: either the injection of the oxidant is staged without modifying the fuel injection, or on the contrary, the fuel injection is staged without staging that of the oxidant; the stepwise injection of oxidizer also causes a drop in the free oxygen content, unlike the stepwise injection of the fuel alone. The present invention relates more particularly to a burner in which the above two methods of recirculating smoke are combined, on the one hand, and of staging the fuel supply,

on the other hand.

  US Patent 5,073,105 describes a burner mounted on a wall of an oven, which comprises a block pierced with openings receiving primary and secondary fuel jets, and delimiting a primary combustion zone; the air enters this zone through a perforated plate and mixes with horizontal and / or radial jets of fuel mixed with the smoke sucked in by the vacuum resulting from these jets through the corresponding openings provided in the block of vertical jets of mixed fuel the smoke sucked through other openings, exit at the downstream end of the block, in another

combustion zone.

  US Pat. No. 5,284,438 describes a burner of similar but more complex structure, in which the burner block delimits a convergent and a diverging part separated by a throttle, and comprises smoke suction ducts by fuel jets which, by their dimensions, their flared shape at the inlet, their inclined position and their outlet located in the divergent part of the block, are supposed to ensure a defined dosage of the fuel-smoke mixture and its injection into a zone of mixing with the air, which is located in the diverging part; this burner comprises a central fuel injection pipe equipped with a nozzle, and a flame stabilizer; this structure causes turbulent and swirling jets (of a combustible mixture - fumes) to form, and causes them to mix with air in the areas of

mixture delimited by the block.

  These burners, the smoke suction channels of which are formed in thick blocks generally made of refractory concrete, are

  fragile, bulky, expensive, and difficult to maintain.

  Document EP 675 321 describes a gaseous fuel burner which comprises several primary fuel injection pipes, arranged in the central part of an air supply channel delimited by the tubular body of the burner; the ends of the canes are folded outwards, arranged under the slots of a flame stabilizer, and the ends of the canes are pierced with holes through which the combustible gas is injected into the hearth through the slots of the stabilizer; the burner further comprises (secondary) canes for injecting gaseous fuel, which are distributed around the canes 10 of primary injection, and which terminate in a single nozzle each opening into a recirculation pipe for the fumes produced in the hearth ; these secondary injection pipes, as well as the associated recirculation conduits, are located inside the channel

  air, to ensure their cooling by the air which circulates there.

  Each recirculation duct opens into the hearth parallel to the air channel, and communicates at its other end with the hearth by an orifice drilled in the wall of the air channel, at the outer periphery of the burner; these orifices make it possible, by suction and entrainment effect, to recirculate the fumes which mix with the combustible gas and reduce the formation of nitrogen oxides; to complete this reduction by increasing the staging of the combustion, the burner comprises fuel injection rods (tertiary) passing through the wall of the enclosure of the hearth; each rod is associated with an air intake channel surrounding

the walking stick.

  Patent EP 675 321 also describes a burner devoid of tertiary fuel injection means, the performance of which

are reduced.

  The burner described in this document has certain drawbacks: the gas streams delivered by the canes and the air channels associated with these tertiary injection means, are capable of forming a curtain surrounding the secondary means of injecting combustible gas, this which is likely to disturb and limit the efficiency of the smoke recirculation system by the secondary means of injecting gaseous fuel; another drawback results from the difficulty of regulating and controlling the effect produced by the air jets surrounding the gas jets of the tertiary injection means, and in particular controlling these coaxial jets and their mixture necessary for good combustion with the other jets caused by this burner, which is equipped with three separate systems for injecting gaseous fuel and two separate systems for transporting and injecting air into the burner (and the hearth); in addition, a drawback linked to the rods extending through the walls delimiting the enclosure of the hearth, resides in the large size of this system and in the difficulty in repairing, maintaining and / or modifying such a system partially embedded in the

perimeter wall of the foyer.

  An objective of the present invention is to provide an improved burner and to at least partially remedy the drawbacks of known burners while improving their performance in the

  reduction in the formation of nitrogen oxides.

  An objective of the present invention consists in particular in

  improve the self-recirculation of smoke.

  An objective of the present invention is also to propose a compact, simple, inexpensive burner capable of being used in

  a duct for transporting exhaust gas from turbine (s).

  According to a first aspect, the invention consists in proposing a gaseous fuel burner which comprises a central fuel transport duct, a central air transport duct surrounding said central fuel duct, several peripheral gaseous fuel transport ducts. 'extending partly at least outside the central air transport duct, preferably parallel thereto, and each terminated by a fuel ejection nozzle; the burner further comprises a straight section of cylindrical flue gas recirculation pipe (or tube), which is arranged in the extension of each nozzle and at a distance (in front) from it; the air transport duct and the smoke suction ducts are metallic, substantially parallel to one another, and preferably open substantially in the same plane perpendicular to the longitudinal axis of the burner; the smoke extraction ducts are fixed to the metal body of the burner, preferably on the air transport duct, by a metallic connection which contributes to ensuring, by conduction, the cooling of the suction ducts by the body of burner (in particular through the air duct)

  which is itself cooled by the air it carries.

  Because the smoke circulation ducts are not, unlike the burner described in patent EP 675 321, protruding inside the air duct, they do not disturb the operation of the burner, especially when the latter - equipped for this purpose with a central cane for injecting liquid fuel - is supplied with

liquid fuel only.

  Tests have shown that such a burner causes low

emission of nitrogen oxides.

  The position of the smoke circulation ducts arranged outside the air duct results in a separation of the jets of the fuel / smoke mixture with respect to the air stream transported by the central (main) duct; the mixing of these jets with the flame resulting from the combustion of the fuel delivered by the central duct occurs gradually and further (by comparison with the structure described in patent EP 675 321) downstream of the burner nose,

  which reinforces the staging effect of the fuel injection.

  The effect of suction and entrainment of the fumes by the jets of combustible gas delivered by the peripheral nozzles is improved by the fact that the entry of the fumes at the upstream end of the recirculation conduits takes place over the entire periphery of these; a greater quantity of smoke is thus sucked in and mixed with each jet of

combustible.

  In addition, the jet of fuel / smoke mixture leaving each peripheral recirculation duct causes, due to its

  speed, additional smoke training.

  The distance separating each peripheral fuel nozzle from the inlet of the associated recirculation tube is preferably of the same order of magnitude as the radius or the diameter of the recirculation tube section, for example of the order of 2 at 20 centimeters; thus, the free space between the nozzle and the inlet (the upstream end) of the recirculation duct facilitates the suction of the fumes and the mixing of the combustible gas with the fumes present in the hearth, by delimiting a passage section for fumes of dimensions considerably larger than that delimited by the peripheral opening of the recirculation tubes described in document EP 675 321; in addition, this arrangement allows and / or facilitates the adjustment, during assembly or installation of the burner, of this distance, which makes it possible to optimize the suction, mixing and entrainment of the fumes by the jets of combustible gas. It has been found that, surprisingly, the cooling of the smoke recirculation conduits by conduction with the central air transport conduit is sufficient to avoid deterioration of these conduits although these smoke conduits are arranged in the hearth ( and / or are subjected to intense heating by the direct radiation of the flame); preferably, for this purpose, the metal connection serving as a heat sink extends over a large surface, in particular substantially over the entire length of the

  recirculation; this connection is preferably obtained by welding.

  In fact, in the burner according to the invention, the cooling of the smoke circulation tubes cannot be obtained by convection with an air flow (relatively cold) as is the case in patent EP 675 321 and in patent application FR 98/10350 filed on August 7, 1998 by the Applicant; in this document, in fact, peripheral air injection nozzles are arranged nested with the nozzles

  fuel injection devices.

  According to other preferred characteristics, which contribute to improving the compactness, simplicity and efficiency of the burner - the ratio of the length of the recirculation conduits associated with the gas ejection nozzles, to their diameter, is greater than 1 and less than or equal to 10, in particular close to 2 to 4; - The peripheral gas ejection nozzles are arranged substantially flush or slightly protruding on the internal face of the walls of the enclosure of the hearth; the outlet orifice of the central air duct protrudes inside the enclosure of the hearth, and the front ends of the recirculation ducts located in the extension of the fuel gas ejection nozzles are arranged substantially in the same plane as said outlet port of the central air duct; - the recirculation tubes associated with the fuel gas ejection nozzles are welded to the shell of the central air duct, on the

external surface thereof.

  Said peripheral nozzles and the associated suction ducts are preferably identical, arranged at equal distance from the longitudinal axis of the burner (that is to say on a circle) and according to a symmetry

  of revolution (that is to say angularly equidistant).

  Preferably, the peripheral nozzles for injecting combustible gas, as well as the associated smoke tubes, are three to twelve in number, in particular six to eight, arranged in a crown, regularly (angularly) distributed around the axis. longitudinal (from

general symmetry) of the burner.

  Although the longitudinal axes of the peripheral nozzles and the associated smoke ducts can extend in a diverging conical manner (at most 5 degrees), it is preferable that these axes be

  substantially parallel to the longitudinal axis of the burner.

  According to another aspect, the invention consists of a turbine gas heater, which comprises a burner according to the invention which is mounted in the central part of a turbine gas transport duct which

  is fitted with partial closing flaps.

  In this particular use of the burner according to the invention, the external position of the smoke circulation tubes avoids disturbing the

  combustion of the fuel delivered in the central part.

  The advantages of the invention will be better understood from

  through the following description which refers to the accompanying drawings, which

  illustrate without any limiting character preferential modes of

realization of the invention.

  In the drawings, identical or similar elements bear,

  unless otherwise indicated, the same references from one figure to another.

  FIG. 1 illustrates in longitudinal section view a burner with

  liquid and gaseous fuel according to the invention.

  Figure 2 is a front view of the front end of the burner

  Figure 1 and is a view along II of Figure 1.

  Figure 3 illustrates in longitudinal section view a heater

  turbine outlet gas according to the invention.

  The burner 1 (FIGS. 1 and 2) comprises a first cylindrical shell 2 with a longitudinal axis 3 horizontal, and a second cylindrical shell 4 coaxial with the first, which delimits a duct 5; the ferrule 2 delimits a central air passage duct 31, which extends the

air inlet duct 5.

  The burner extends through a cylindrical orifice 6 provided in

  the wall 7 separating the hearth 8 from the outside 9 of the enclosure.

  The burner comprises in the central part a central rod 10 for transporting liquid fuel (in particular such as heavy fuel oil, gas oil, petrol, oils, waste liquids, etc.) up to the burner head equipped with a central nozzle 11 and a flame stabilizer 12, which are arranged at the front end of the duct 31 and of the shell 2, and are projecting inside the hearth 8; the burner further includes a

  cane 100 gas fuel injection plant.

  The burner comprises a bundle of six peripheral tubes 13 for transporting combustible gas, which are each fitted at their front end with a nozzle 14 arranged projecting (for example protruding from 1

  centimeter) on the internal face 15 of the wall 7.

  A part of the tubes 13 extends parallel to the axis 3 inside the air transport duct 5; the tubes 13 are supplied by a common manifold 16, and pass through an annular flange 17 connecting the ferrules 2 and 4; as illustrated in FIG. 2, the six identical nozzles 14 for ejecting combustible gas are spaced apart

angularly 60 degrees.

  The longitudinal axes 20 of the nozzles 14 which are parallel to the axis

  3, are located on identical or neighboring spokes.

  Six straight sections 22 of circular section tubes, cut at right angles at their two ends 23, 24, are arranged in the extension of the gas ejection nozzles 14, and are welded along one of their generatrices to a bar solid metal 101, associated with each of the conduits, which extends parallel to the conduit 22 and which is itself welded (over its entire length) to the external face of the part

  front end of the shell 2, according to generators thereof.

  The axis of the tubes 22 coincides with the axis of the nozzles 14 and

I TO \

  the rear end 24 of identical sections 22 is disposed at a distance 25 from the corresponding nozzle 14, which distance is close

  50 millimeters, and corresponding to the diameter of the tubes 22.

  The front end 23 of the tubes 22 extends in the plane 112

  containing the outlet of the central duct 31.

  With reference to FIG. 3, the burner 1 is mounted in a duct, of axis coincident with the axis 3 of the burner, delimited by walls 201 (in the case where several burners are mounted in the duct 200, their

  axes 3 are parallel to that of the conduit).

  The conduit 200 transports (according to the arrows 103) gas leaving a turbine, the temperature of which is of the order of 500 C or else, when the turbine is stopped, air; in the latter case, flaps 104, which are pivotally mounted in the conduit 200 along axes 105, partially close - in the position shown in solid lines - the passage section of the conduit 200 leaving the passage free - according to the arrows 102 - through the burner pipe 2; the position (open) of the flaps illustrated in broken lines corresponds to the case where the gases of

  turbine are present. The burner is fixed in the duct 200 by supports not shown.

  In both operating cases, an outside air supply (corresponding to 5% of the total air) is introduced through a duct 106 to cool the central part of the burner where the stabilizer 105 for the central flames (main ) combustion of the gas injected through the central conduits 100 which pass through a rose window

  o10 deflector 104; the conduits 100 are supplied by a collector 107.

  Furthermore, the peripheral conduits 13 are supplied with combustible gas by a second manifold 16; the flow of gaseous fuel injected by the peripheral nozzles generally represents around 90 to 95% of the total fuel flow. An advantage of the burner is that the mixing jets leaving the tubes 22 help to lengthen the flame, which improves the efficiency of the gas heater.

  and the heat recovery installation, located downstream of the burner.

II

Claims (9)

  1. A gaseous fuel burner (1) which comprises a central conduit (10) for transporting fuel, a central conduit (2) for air transport surrounding said central conduit for fuel and several peripheral conduits (13) for transporting fuel gaseous, characterized in that said peripheral conduits extend at least partly outside the central air transport conduit, in that the burner further comprises a straight section of conduit (22) for the circulation of smoke which is arranged in the extension of l0 each nozzle (14) equipping the peripheral conduits (13), and in that the smoke suction conduits (22) are fixed to the burner, outside the conduit (2) of air transport, via a link (101) favoring their cooling.   2. Burner according to claim 1, in which the conduits (22) for circulating smoke and the conduit (2) for air transport are metallic. 3. Burner according to claim 2, in which the conduits (22) are welded by one of their generators to a metal connecting rod (101) which is welded to a generator of the conduit (2), parallel to this one.   4. Burner according to claim 3, in which the upstream orifice (24) of each duct (22) is disposed at a distance (25) from the nozzle   (14) corresponding, which is of the order of the diameter of the conduit (22).   5. Burner according to any one of claims 3 or 4,   in which the outlet (23) of the conduits (22) extends substantially   in the transverse plane (112) of outlet from the duct (2).   6. Burner according to any one of claims 3 to 5,   in which the nozzles (14) and the conduits (2, 22) protrude inside the hearth (8), and in which the conduits (22) are not air flow contact.   7. Burner according to any one of claims 3 to 6, which comprises a flame stabilizer (105) and a deflecting rosette   (106) arranged in the air duct (2, 31) in the vicinity of its plane end (112).   8. Turbine gas heater transported by a conduit (200) equipped with shutters (104) shutter, characterized in that   comprises a burner according to any one of claims 1 to 7.   9. The heater according to claim 8, which comprises a   air supply duct (106) for cooling the burner.