Abstract:
A segmented surface-stabilized gas burner features wide modulation of thermal output simply by the independent control of fuel gas flow to each burner segment. The burner also features a porous fiber burner face, preferably having dual porosities, and a metal liner positioned to provide a compact combustion zone adjacent the burner face. The segmented surface-stabilized burner is ideally suited for use with gas turbines not only because of its compactness and broad thermal modulation but also because only the flow of fuel gas to each burner segment requires control while the relative flow of compressed air into the segments of the burner remains unchanged.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a broadly modulated radiant gas burner that yields minimal emissions of air-pollutants, especially nitrogen oxides (NOx). More particularly, the burner face of this invention is a porous mat of metal and/or ceramic fibers which is divided into segments that can be individually fired. 
     Radiant, surface-combustion gas burners are fed fuel gas admixed with enough air to ensure complete combustion of the fuel gas. Because these burners function without secondary air, their modulation of heat output is limited. Yet, there are important uses of surface-combustion gas burners in tight spaces, such as in the casings of gas turbines, where adding spare burners to increase heat delivery is not a practical solution to broad heating modulation. 
     Assignee&#39;s U.S. Pat. No. 6,199,364 to Kendall et al discloses compact surface-stabilized gas burners that are well suited for use with gas turbines. Surface-stabilized gas burners are therein defined as having burner faces with dual porosities so that surface combustion from the lower porosity areas serves to keep the blue flames from the higher porosity areas attached to the burner face when fired at rates of at least about 500,000 BTU/hr/sf (British Thermal Units per hour per square foot) of burner face. 
     A principal object of this invention is to provide compact surface-stabilized gas burners featuring a broad range of heat delivery. 
     Another important object is to provide such surface-stabilized gas burners with internal walls that divide each burner into two or more segments that can be individually and independently fired to vary the thermal output, while maintaining the adiabatic flame temperature of the fired segments in a range yielding low emissions. 
     Still another object is to provide segmented surface-stabilized gas burners that are simple in construction as well as operation. 
     These and other features and advantages of the invention will be apparent from the description which follows. 
     SUMMARY OF THE INVENTION 
     Basically, the segmented surface-stabilized gas burner of this invention which has a combustion surface formed of metal and/or ceramic fibers may have a unitary body with internal partitions to provide independent burner segments, or it may have two or more burner modules that are compactly fitted together. 
     U.S. Pat. No. 4,543,940 to Krill et al describes a segmented radiant surface burner formed of large cylindrical segments that are bolted together in axial alignment. This arrangement of large burner segments was conceived to fit the peculiar shape of combustion chambers of fire tube boilers. The serial alignment involves sealing between the abutted ends of contiguous burner sections and requires an individual duct to supply fuel gas and air to each burner segment. The complex ducting of fuel gas and air to each burner segment is antithetical to this invention&#39;s objective of burner compactness that is essential to burners used with gas turbines. 
     The combustion surface may be formed of ceramic fibers as taught by U.S. Pat. No. 4,746,287 to Lannutti, of metal fibers as set forth in U.S. Pat. No. 4,597,734 to McCausland, or of mixed metal and ceramic fibers according to U.S. Pat. No. 5,326,631 to Carswell et al. For high surface firing rates, say, at least about 500,000 BTU/hr/sf of burner face, a rigid but porous mat of sintered metal fibers with interspersed bands or areas of perforations is preferred. Such a burner face is shown in FIG. 1 of U.S. Pat. No. 5,439,372 to Duret et al. Still another form of porous metal fiber mat sold by N.V. Acotech S.A. of Zwevegem, Belgium, is a knitted fabric made with a yarn formed of metal fibers. In the rigid porous and perforated burner of Duret et al, radiant surface combustion is interspersed with blue flame combustion from the perforations. Similarly, the yarn of the knitted metal fiber fabric provides radiant surface combustion and the interstices of the knitted fabric naturally provide interspersed spots of increased porosity that yield blue flames. 
     At the aforesaid high surface firing rates, the flames from the areas of increased porosity produce such intense non-surface radiation that the normal surface radiation from the areas of lower porosity disappears. However, the dual porosities make it possible to maintain surface-stabilized combustion, i.e., surface combustion stabilizing blue flames attached to the burner face. Burner faces with dual porosities are referred to as surface-stabilized burners. With such burners, flaming is so compact that visually a zone of strong infrared radiation appears suspended close to the burner face. It is noteworthy that with at least about 40% excess air, surface-stabilized combustion yields combustion products containing low emissions as little as 2 ppm (parts per million) NOx and not more than 10 ppm CO and UHC (unburned hydrocarbons), combined. 
     In as much as the segmented burner of this invention is particularly valuable in uses where the combustion zone is spatially limited, it is seldom a flat burner. Cylindrical burner faces and variations thereof, e.g., tapered or conical, are the usual forms of the segmented burner. 
     The burner segments which fit together may be designed to deliver equal quantities of heat, but it is usually advantageous to have segments of unequal heat delivery capacities. For example, a two-segment burner, can have one segment with 60% and the other segment with 40% of the total heat delivery capacity of the burner. Such unequal segments permit greater heat delivery modulation than if the burner had two equal segments. The same is true of three-segment burners. Three segments of 55%, 35% and 10% of heat delivery capacity permit greater modulation of heat delivery than is possible with three segments of equal heat delivery capacity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To facilitate further description and understanding of the invention, reference will be made to the accompanying drawings of which: 
     FIG. 1 is a schematic representation of a simple two-segment cylindrical burner shown in axial section; 
     FIG. 2 is a similar representation of a three-segment cylindrical burner shown in axial section; 
     FIG. 3 is a left end view of the burner of FIG. 2; 
     FIG. 4 is a left end view of the burner of FIG. 1 modified to provide three burner segments; 
     FIG. 5 schematically represents a hemispherical burner having two burner segments; 
     FIG. 6 is a schematic axial section of a three-segment conical burner adapted for use with a gas turbine; and 
     FIG. 7 shows an alternate form of an element of the burner of FIG.  6 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 schematically depicts a two-segment cylindrical burner  10  having a porous fiber combustion surface  11  which is divided into two separate burning segments by a funnel-like baffle  12 . Tube  13  connected to frusto-conical portion  14  of funnel  12  is fitted co-axially in cylinder  15  to create core plenum  16  and annular plenum  17 . Core plenum  16  expands beyond tapered baffle  14  into plenum  18  which supplies fuel gas and air to segment A of combustion surface  11 . Segment A of surface  11  is the portion to the right of the line where baffle  14  meets an inner support screen (not shown) of fiber surface  11 . Porous fiber combustion surface  11  surrounding annular plenum  17  is segment B contiguous to segment A. 
     It is obvious that fuel gas and air can be supplied to tube  13  for surface combustion on only segment A of porous fiber layer  11 . For increased thermal output, fuel gas and air can be introduced via cylinder  15  to annular plenum  17  for combustion on segment B of fiber layer  11 . Of course, the reverse order of firing can be carried by feeding fuel gas and air to plenum  17  and feeding fuel gas and air to core plenum  16  when increased heat output is desired. 
     The simplicity and compactness of burner  10  of FIG. 1 demonstrates that it can be made with a unitary cylindrical body having a hemispherical closed end and a funnel-like baffle inserted through the opposite open end of the cylindrical body. In fact, that is the construction that has been described in relation to FIG.  1 . However, if each of lines  13 ,  14  in FIG. 1, which form funnel  12 , are considered as two contiguous metal sheets and segments A, B of fiber layer  11  are not united at circumferential line S, burner  10  becomes one having two telescoped burner modules. The module with plenum  16 ,  18  has its tube  13  inserted into a central, similar tube of annular plenum  17 . The insertion is made from the right end of cylinder  15  that supports segment B of porous fiber layer  11 . When tapered wall  14  of plenum  18  is brought into contact with similar tapered wall of annular plenum  17 , the insertion is completed and segment A of combustion surface  11  meets segment B to function essentially as if surface  11  had been pressure or vacuum molded or knitted as a continuous porous fiber layer  11  spanning both plenums  17 ,  18 . 
     FIG. 2 shows an axial section of cylindrical burner  20  that is sealed by metal disk  21  at its right end and open at its opposite end. 
     FIG. 3 is a left end view of burner  20  revealing three radial baffles  22 ,  23 ,  24  which form three plenums  25 ,  26 ,  27  in burner  20 . Plenums  25 ,  26 ,  27  feed three equal segments of porous fiber combustion surface  28  on cylinder  29 . However, it is usually preferable to make the angles between baffles  22 ,  23 ,  24  unequal so that the areas of the three segments of combustion surface  28  are also unequal. Moreover, baffles need not be radial. For example, two baffles at right angles to each other within cylinder  29  can provide three plenums of unequal size. A single baffle that is not a diametrical divider will form two plenums of unequal size in burner  20  with porous fiber layer  28  divided into two segments of unequal areas. 
     FIG. 4, like FIG. 3, is an open end view of a cylindrical burner  30  that, like burner  10  of FIG. 1, has a funnel-like plenum surrounded by an annular plenum. Burner  30  differs from burner  10  in that the annular plenum is divided into two unequal parts by baffles  31 ,  32  extending from tube  33  outwardly to the cylindrical screen (not shown) that supports porous fiber layer  34 . Thus, baffles  31 ,  32  have converted the two-segment burner  10  of FIG. 1 into three-segment burner  30 . 
     FIG. 5 is a diametrical sectional view of hemispherical burner  40  that has a pan plenum  41  with inlet opening  42 . A hemispherical screen which supports a porous layer  43  of metal and/or ceramic fibers is attached to pan  41 . Funnel-like baffle  44  with its tube  45  extending through pan  41  divides combustion surface  43  into two segments, A, B that can be fired separately or together. Fuel gas and air supplied to tube  45  will yield radiant surface combustion on segment A of porous fiber layer  43 . When increased heating is desired, fuel gas and air introduced through inlet  42  to pan  41  will combust on segment B of porous fiber layer  43 . Of course, combustion can be carried out with only segment B of burner  40 . When greater heating is desired, fuel gas and air can be fed to tube  45  for combustion on segment A of porous fiber layer  43 . 
     FIG. 6 demonstrates a three-segment burner  50  of the invention adapted for use with a gas turbine. FIG. 6 is presented as an improved (provides greater thermal modulation) burner for replacement of burner  62  in FIG. 6 of assignee&#39;s U.S. Pat. No. 6,199,364. Whereas prior burner  62  has a single plenum  63 , new burner  50  has three plenums,  51 ,  52 ,  53  which supply fuel gas and air to three segments A, B, C of porous combustion surface  54 . Tubular baffle  55  separates plenum  51  from plenum  52  which is separated from plenum  53  by tubular baffle  56 . Burner  50  of this invention, like burner  62  of assignee&#39;s patent, is surrounded by metal liner  57  that has multiple louvers  58 . Liner  57  spaced from combustion surface  54  serves to confine the combustion zone. 
     Housing  59  is a metal cylinder attached to the casing of a gas turbine (not shown). Three-segment burner  50  is attached to housing cap  63  by spacer bolts (not shown). Inasmuch as prior burner  62  was made with a dual porosity burner face  64 , the new three-segment burner  50  can also have burner face  54  with dual porosity. The tapered cylindrical support of burner face  54  has an impervious cylindrical extension  54 A welded to a circular opening in metal disk  60 . Similarly, baffle  56  is welded to an opening in disk  61  and baffle  55  is connected to an opening in disk  62 . Spacer bolts (not shown) hold disks  60 ,  61 ,  62  in the desired spaced arrangement and spacer bolts between disk  62  and housing cap  63  support the entire assembly of disks  60 ,  61 ,  62  which are components of burner  50 . Cylindrical band  65  is welded to disk  60  and is dimensioned for a slip-fit with collar  64  of liner  57 . Thus, when cap  63  is lifted away from housing  59 , all of burner  50  is withdrawn from housing  59 . 
     Plenums  51 ,  52 ,  53  are each supplied with fuel gas by valved tubes  66 ,  67 ,  68 , respectively. Pipe  69  feeds tubes  66 ,  67 ,  68  which are connected to ring manifolds  70 ,  71 ,  72 , respectively, each manifold having multiple holes positioned to inject fuel gas above disks  62 ,  61 ,  60 , respectively. Compressed air from the compressor section of a gas turbine (not shown) flows into and fills housing  59  which is part of the casing of the turbine. Compressed air in housing  59  flows over disks  60 ,  61 ,  62  and into plenums  53 ,  52 ,  51 , respectively. Compressed air discharges from plenums  51 ,  52 ,  53  through segments A, B, C, respectively, of porous fiber burner face  54  into combustion zone  75 . Compressed air also passes through the multiple louvers  58  of liner  57  into combustion zone  75 . By opening the valve of tube  68 , fuel gas is injected upward as multiple jets from holes in ring manifold  72  into the compressed air flowing over disk  60  and the resulting gas-air mixture flows into plenum  53  from which it exits through segment C of porous burner face  54  and, upon ignition, undergoes radiant surface combustion. Any known igniter  76  positioned below disk  60  near segment C will ignite the gas-air mixture exiting segment C of porous burner face  54 . 
     When greater thermal delivery is required, fuel gas may similarly be fed through valved tube  67  to ring manifold  71 , and injected by manifold  71  as multiple jets into compressed air flowing between disks  61 ,  62 . Thence, the mixture flows through plenum  52  and segment B of burner face  54  to produce more surface-stabilized combustion. For maximum heating, fuel gas is admitted through valved tube  66  to manifold  70  from which it escapes as multiple jets into compressed air passing between disks  62  and housing cap  63 . The gas-air mixture fills plenum  51  and combusts upon exiting segment A of porous burner face  54 . The products of combustion from segments A, B, C mix with compressed air entering combustion zone  75  through louvers  58  of liner  57 . The total hot gases flow from combustion zone  75  through curved duct  77  (partially shown) which channels the hot gases to the turbine (not shown) as the driving force thereof. 
     The great range of thermal modulation made possible by the invention is best appreciated if the area of combustion surface  54  of segmented burner  50  and the area of combustion surface  64  of prior burner  62  (U.S. Pat. No. 6,199,364) are made equal. Burner  62  can be thermally modulated over a range that is characteristic for the selected type of combustion surface. If the same type of combustion surface is used on segmented burner  50 , then all three segments A, B, C can be individually and independently modulated to the same extent as combustion surface  64  of prior burner  62 . But segmented burner  50  can have any one or two of segments A, B, C turned off by closing valved tubes  66 ,  67 ,  68 , respectively, to achieve a great turn-down of heat output to a small fraction of the lowest turn-down possible with prior burner  62 . 
     A two-segment burner that still permits substantially broader thermal modulation than prior burner  62  can be visualized by eliminating either tubular baffle  55  along with disk  62 , ring manifold  70  and valved tube  66 , or tubular baffle  56  along with disk  61 , manifold  71  and valved tube  67 . Segmented burner  50  is shown in FIG. 6 in a preferred cone-like shape, i.e., a conical form with a convex end in lieu of a pointed apex. This term, cone-like shape, as herein used, shall also include truncated conical forms. Of course, other forms of segmented burners, such as those shown in FIGS. 1,  2 ,  4 ,  5  may be adapted for use with gas turbines. 
     The unique feature of segmented burners of this invention for gas turbines is that compressed air from the compressor of a gas turbine flows into and around the segmented burner continuously whether one or all the segments are being fed fuel gas. The percentage of compressed air going into each segment and around the burner being fixed by the dimensions given the various parts of the burner. For example, if the space between disks  61 ,  62  is reduced, less compressed air will flow into plenum  52 . In short, while a burner is in operation, at any rate of compressed air flow, the flow of compressed air into any plenum cannot be varied. Only the flow of fuel gas can be varied to each plenum. 
     While burner  50  is shown in FIG. 6 with a louvered liner  57 , an alternate liner is known as a backside-cooled liner (ASME Paper 99—GT-239). FIG. 7 is a schematic representation of backside-cooled liner  57 A as a substitute for louvered liner  57  of FIG.  6 . FIG. 7 shows only the right profile of liner  57 A inasmuch as the left profile is only a mirror image of FIG.  7 . Liner  57 A is without louvers or other openings except for a few louvers  58 A in the end portion of liner  57 A which is connected to curved duct  77 . A cylindrical metal shell  57 B, called convector in the ASME Paper, surrounds liner  57 A and is spaced therefrom to provide a narrow annular gap. Convector  57 B extends over substantially the full length of liner  57 A and is connected and sealed to liner  57 A at  57 C where liner  57 A meets curved duct  77 . 
     Thus, compressed air flowing between housing  59  and convector  57 B will, besides entering the spaces between disks  60 ,  61 ,  62  and housing cap  63 , flow through the gap between convector  57 B and liner  57 A exiting through a few rows of openings or louvers  58 A in the portion of liner  57 A adjacent to curved duct  77 . Accordingly, any liner that serves to confine the combustion zone close to the burner surface and to moderate the combustion temperature can be used with the segmented burner. 
     Moreover, each burner need not have an individual liner. U.S. Pat. No. 6,199,364 shows a circular array of five burners in FIG. 3 which have a pair of metal liners that confine the combustion of all five burners in an annular zone. Such a collective liner may be used for several burners of this invention. Inasmuch as the collective liner is in two concentric parts, it is possible to cool each part with compressed air in a different way. For example, the inner liner may be louvered and the outer liner may be backside-cooled, or vice versa. 
     As known, the metal screen which supports the porous fiber layer of surface combustion burners usually has a perforated back-up plate that helps to ensure uniform flow of the fuel gas-air mixture though all of the porous fiber burner face. In a unitary (not modular) segmented burner of this invention, each internal baffle can be held in place by welding to a back-up plate. In the absence of a back-up plate, a baffle can be welded to the screen that supports the porous fiber layer. 
     While natural gas is a fuel commonly used with gas turbines, the burner of this invention may be fired with higher hydrocarbons, such as propane. Liquid fuels, such as alcohols and gasoline, may be used with the burner of the invention, if the liquid fuel is completely vaporized before it passes through the porous burner face. The term, gaseous fuel, has been used to include fuels that are normally gases as well as those that are liquid but completely vaporized prior to passage through the burner face. Another feature of the invention is that the burner is effective even with low BTU gases, such as landfill gas that often is only about 40% methane. 
     The term, excess air, has been used herein in its conventional way to mean the amount of air that is in excess of the stoichiometric requirement of the fuel with which it is mixed. 
     Those skilled in the art will visualize variations and modifications of the invention in light of the foregoing teachings without departing from the spirit or scope of the invention. For example, circular manifold  70  in FIG. 6 can be eliminated if valved fuel tube  66  is extended so that it discharges through a mixing nozzle into the opening where baffle  55  is joined to disk  62 . Accordingly, only such limitations should be imposed on the invention as are set forth in the appended claims.