Abstract:
A porous, low-conductivity material formed of metal or ceramic fibers provides the burner face of a gaseous fuel combustor for gas turbines capable of minimizing nitrogen oxides (NO x ) emissions in the combustion product gases. A preferred burner face, when fired at atmospheric pressure, yields radiant surface combustion with interspersed areas of blue flame combustion. A rigid but porous mat of sintered metal fibers with interspersed bands of perforations is illustrative of a preferred burner face that can be fired at pressures exceeding 3 atmospheres at the rate of at least about 500,000 BTU/her/sf/atm. By controlling the excess air admixed with the fuel in the range of about 40% to 150% to maintain an adiabatic flame temperature in the range of about 2600° F. to 3300° F., the NO x  emissions are suppressed to 5 ppm and even below 2 ppm. At all times, carbon monoxide and unburned hydrocarbons emissions do not exceed 10 ppm, combined.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a burner and process for operating gas turbines with minimal emissions of air pollutants, especially nitrogen oxides (NO x ). More particularly, the burner and process permit operation of gas turbine combustors at high excess air and at elevated pressure. 
     The development of a compact burner that would fit in the castings of gas turbines and yield combustion products with a limited content of atmospheric pollutants [NO x , carbon monoxide (CO) and unburned hydrocarbons (UHC)] has long failed to deliver a commercially acceptable product. In 1981, U.S. Pat. No. 4,280,329 of Rackley et al disclosed a radiant surface burner in the form of a porous ceramic V-shaped element. Theoretically, the proposed burner was attractive but, practically, it had serious deficiencies, such as fragility, high pressure drop therethrough and limited heat flux. No advance in the art of radiant surface combustion for gas turbines has appeared since the Rackley et al proposal. 
     Efforts to minimize atmospheric pollutant emissions from the operation of gas turbines have been directed in different approaches. U.S. Pat. Nos. 4,339,924; 5,309,709 and 5,457,953 are illustrative of proposals involving complicated and costly apparatus. Catalytica Inc. Is promoting a catalytic combustor for gas turbines which reportedly (San Francisco Chronicle, Nov. 21, 1996) is undergoing evaluation. None of the proposals provide simple, compact apparatus and catalysts are expensive and have limited lives. 
     A principal object of this invention is to provide compact burners for gas turbines which feature surface-stabilized combustion conducted at high firing rates with high excess air to yield minimal polluting emissions. 
     Another important object is to provide burners for gas turbines which permit broad adjustment of heat flux. 
     A related object is to provide compact burners with low pressure drop and stable operation over a broad pressure range and excess air variation. 
     Still another object is to provide burners for gas turbines which have simple and durable construction. 
     A further primary object of the invention is to provide a method of operating gas turbines to yield combustion products with a very low content of atmospheric pollutants. 
     These and other features and advantages of the invention will be apparent from the description which follows. 
     SUMMARY OF THE INVENTION 
     Basically, the burner face used in this invention is a porous, low-conductivity material formed of metal or ceramic fibers and suitable for radiant surface combustion of a gaseous fuel-air mixture passed therethrough. A preferred burner face is a porous metal fiber mat which, when fired at atmospheric pressure, yields radiant surface combustion with interspersed portions or areas of increased porosity that provide blue flame combustion. Such a burner face is shown in FIG. 1 of U.S. Pat. No. 5,439,372 to Duret et al who disclose a rigid but porous mat of sintered metal fibers with interspersed bands or areas of perforations. One supplier of a porous metal fiber mat is N. V. Acotech S. A. of Zwevegem, Belgium. As shown by the patentees, bands of perforations are formed in the porous mat to provide blue flame combustion while the adjacent areas of the porous mat provide radiant surface combustion. 
     Another form of porous metal fiber mat sold by Acotech is a knitted fabric made with a yarn formed of metal fibers. While the yarn is porous, the interstices of the knitted fabric naturally provide uniformly interspersed spots of increased porosity. Hence, the knitted metal fiber fabric provides surface radiant combustion commingled with numerous spots of blue flames. 
     Still another form of porous burner face suitable for this invention is the perforated, ceramic fiber plate disclosed in U.S. Pat. No. 5,595,816 to Carswell having small perforations effective for radiant surface combustion, which is simply modified to have interspersed areas with larger perforations for blue flame combustion. 
     Another version of a perforated, ceramic or metal fiber plate adapted for this invention is one having uniform perforations that produce blue flame combustion, but such a plate is combined with an upstream configuration that limits flow to selected portions of the plate such that those portions operate with surface combustion in or near a radiant mode. One embodiment of this approach could simply involve another perforated plate, slightly spaced from the upstream side of the main plate. The perforations of the back-up plate are of a size and distribution that some of its perforations are aligned with perforations of the main plate so that the latter perforations support blue flame combustion. The unperforated portions of the back-up plate that are aligned with perforations of the main plate impede the flow of the fuel-air mixture to these perforations so that they yield surface combustion. The back-up plate need not be a low-conductivity plate like the main plate that is the burner face. In this case, the back-up plate obviously serves to diminish the flow of the fuel-air mixture through selected areas of the perforated, ceramic or metal fiber plate. 
     A perforated back-up plate may also be used with the various other forms of burner face previously described; usually the back-up plate helps to ensure uniform flow of the fuel-air mixture toward all of the burner face. With the knitted fabric formed of a metal fiber yarn, the back-up plate provides support for the fabric as well as uniform flow thereto. Hence, a perforated back-up plate can have a different function depending on the burner face with which it is combined. Inasmuch as the burner face will in most cases be cylindrical, as hereinafter described, the back-up plate that may also be cylindrical will hereafter be called perforated shell. 
     The complete burner of the invention has a porous fiber burner face attached across a plenum with an inlet for the injection of a gaseous fuel-air mixture, a perforated shell within the plenum behind the burner face, and a metal liner positioned to provide a compact combustion zone adjacent to the burner face. Such a burner has been successfully operated at high firing rates or high heat-flux and with high excess air to produce combustion gases containing not more than 5 ppm NO x  and not more than 10 ppm CO and UHC, combined. Through the control of excess air, the burner is capable of delivering combustion gases containing not more than 2 ppm NO x  and not more than 10 ppm CO and UHC, combined. All ppm (parts per million) values of NO x , CO and UHC mentioned in the specification and claims are values corrected to 15% O 2 , the gas turbine standard. 
     At the high surface firing rates required for burners that can be fitted in the casings of gas turbines, say at least about 500,000 BTU/hr/sf (British Thermal Units per hour per square foot) of burner face, 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. For brevity, burners having faces with dual porosities will be referred to as surface-stabilized burners. 
     Visually, flaming is so compact that a zone of strong infrared radiation seems suspended close to the burner face. The compactness of flaming is aided by the metal liner that confines combustion adjacent the burner face. Even though this surface-stabilized combustion is conducted with about 40% to 150% excess air depending on inlet temperature, the combustion products may contain as little as 2 ppm NO x  and not more than 10 ppm CO and UHC, combined. 
     The aforesaid firing rate of at least about 500,000 BTU/hr/sf of burner face is for combustion at atmospheric pressure. Inasmuch as gas turbines operate at elevated pressures, the base firing rate must be multiplied by the pressure, expressed in atmospheres. For example, at an absolute pressure of 150 pounds per square inch or 10 atmospheres, the nominal minimum firing rate becomes 5,000,000 BTU/hr/sf. It is entirely unexpected and truly remarkable that stable operation of the surface-stabilized burner at high pressure permits a firing rate or heat flux as high as 15,000,000 BTU/hr/sf. This heat flux is calculated to be at least ten times that of the porous ceramic fiber burner of the aforesaid Rackley et al patent; moreover, the ceramic fiber coating of the burner would disintegrate at high pressure and high gas flow operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To facilitate the description and understanding of the invention, reference will be made to the accompanying drawings of which: 
     FIG. 1 is a schematic representation of one embodiment of the gas burners of the invention in an annular arrangement positioned between a typical air compressor and gas turbine; 
     FIG.  2  and FIG. 3 are sectional views of different arrays of burners around the shaft connecting the compressor and the turbine; 
     FIG.  4  and FIG. 5 are longitudinal sectional diagrams of different embodiments of the burner of the invention; 
     FIG. 6 differs from FIG. 1 in showing the burner in a outside the casing of the gas turbine; 
     FIG. 7 like FIG. 5 shows still another embodiment of the burner of the invention; and 
     FIGS.  8 , 9 , 10  and  11  illustrate four different embodiments of the burner face used pursuant to the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 schematically depicts a gas turbine  10  with the discharge portion of air compressor  11 , combustion section  12 , and the inlet portion of turbine  13 . Compressor  11  and turbine  13  share a common axle  15 . Burners  16  having a face  18  with dual porosities are disposed in combustion section  12  annularly around shaft  15 . Two burners  16  are shown in FIG. 1 but, depending on the size of gas turbine  10 , usually six to twelve burners  16  will be uniformly spaced from one another in combustion section  12  around shaft  15 . Each burner  16  is cylindrical and has outer metal liner  17  spaced from burner face  18 . 
     Part of the compressed air leaving compressor  11  enters cylindrical neck  19  of each burner  16  and the remainder flows exteriorly of liners  17 . Each burner  16  is supplied gaseous fuel by tube  20  extending through the casing of gas turbine  10 . Tube  20  discharges between two spaced blocks  21  (or through multiple radial holes in a single block  21 )in neck  19 , causing the gaseous fuel to flow radically in all directions into the compressed air flowing through neck  19 . The resulting admixture of fuel and air fills burner plenum  22 . Thence, the fuel-air mixture passes through perforated shell  23  spaced from dual porosity burner face  18 . Shell  23  helps in providing uniform flow through all of burner face  18 . Upon ignition, the mixture exiting burner face  18  burns in the form of a compact zone of combustion that visually seems flameless over the regions of low porosity and has a stable flame pattern over the regions of high porosity (hereinbefore called surface-stabilized combustion). Essential to combustion pursuant to this invention is feeding a fuel-air mixture with 40% to 150% excess air at a firing rate of at least 500,000 BTU/hr/sf/atm. 
     Some of the compressed air from compressor  11  flows through combustion section  12  in the space between and around the several cylindrical metal liners  17  which have multiple openings for the passage of air therethrough. Thus, the compressed air not used for combustion serves to cool metal liners  17  and to cool the products of combustion prior to entry into turbine section  13 . Liners  17  extend to the entrance of turbine section  13  and deliver a still hot pressurized gas mixture to turbine  13  to drive its rotor and produce power. The expanded gas mixture leaving engine  13  may discharge to a waste heat recovery system (not shown). The closed end of burners  16  are shown in FIG. 1 with burner face  18  and perforated shell  23 . Optionally, the end may be sealed with a solid plate but, of course, the burner will then have less combustion capacity. 
     FIG. 2 is a simplified view of five burners  16 , taken parallel to their closed ends, uniformly spaced around shaft  15  within combustion zone  12  of gas turbine  10 . The five burners  16  include individual metal liners  17 . 
     FIG. 3 is identical to FIG. 2 except that individual liners  17  have been replaced by a pair of metal liners  17 A and  17 B that confine the combustion of all five burners  16  in an annular zone. Compressed air to cool liners  17 A and  17 B and to enter the annular combustion zone through openings in liners  17 A, 17 B flows along the length of the outer surface of liner  17 A and along the length of the inner surface of liner  17 B. 
     FIG. 4 shows a modified form of burner  16 . The closed end E is sealed by an impervious disk protected by insulation (not shown). Short neck  19  is attached to a circular plate  25  having central tapered hole  26 . Metal liner  17  is also attached to plate  25 . Spaced from plate  25  is another circular plate  27  with central hole  28  in which tapered plug  29  is movable to adjust the gap between the tapers of hole  26  and plug  29 . Gaseous fuel supply tube  20  passes through the shell of gas turbine  10  and is connected to an annular bore  30  in plate  27 . Bore  30  has several (only two shown) right-angle openings  31  which discharge the gaseous fuel against plate  25 . Compressed air flowing through the gap between plates  25 , 27  mixes with the gaseous fuel exiting openings  31  and fills plenum  22 . Thence, the mixture passes uniformly through all of cylindrical, perforated shell  23  and burner face  18  to undergo surface-stabilized combustion in the compact zone between face  18  and metal liner  17 . Compressed air that does not flow through the gap between plates  25 , 27  flows along the exterior surface of liner  17  to effect cooling thereof while some of the air passes through multiple openings in liner  17  to mix with the combustion product gases and thereby moderate the temperature thereof. 
     FIG. 4 serves to illustrate one way of ensuring thorough mixing of gaseous fuel and compressed air and one way of controlling the amount of compressed air flowing into plenum  22 . By mechanical or pneumatic or electrical linkage (not shown) that extends from tapered plug  29  to the exterior of the shell of gas turbine  10 , plug  29  can be moved to restrict or widen the gap between the tapers of plug  28  and hole  26 , thereby controlling the amount of air admixed with the fuel. The means for moving plug  29  is not part of this invention and is within the purview of skilled mechanical workers. 
     FIG. 5 shows a burner that differs from that of FIG. 4 in four principal aspects: compressed air flows to the burner countercurrent to the flow of combustion gases; the cylindrical burner fires inwardly instead of outwardly; the metal liner is within the burner instead of around it; the proportion of air from the compressor flowing into the plenum of the burner is indirectly controlled by varying the proportion allowed to bypass the burner, i.e., not enter the plenum of the burner. Burner  35  is within a metal casing  36  which serves to channel compressed air toward the feed end of burner  35  having an annular plenum  37  formed between cylindrical metal wall  38  and cylindrical burner face  39 . The feed end of plenum  37  has wall  38  and burner face  39  connected to an annular disk  40  that has multiple openings  41  circularly spaced from one another to act as inlets to plenum  37 . The opposite end of cylindrical plenum  37  is closed by annular plate A connected to wall  38  and burner face  39 . Perforated shell  42  within plenum  37  surrounds and is spaced from porous burner face  39  to promote uniform flow of fuel-air mixture toward all of burner face  39 . 
     At the entry end of burner  35 , circular block  43  is connected to annular disk  40  and has a central, tapered hole  44  that coincides with the opening of disk  40 . Attached to disk  40  at its central opening is internal cylindrical metal liner  45 . Compressed air flowing toward the entry to burner  35  can enter plenum  37  by flowing through the gap between disk  40  and recessed side  46  of block  43 . Compressed air can simultaneously flow through the gap between tapered hole  44  and tapered plug  47 . As discussed relative to the burner of FIG. 4, plug  47  can be moved to restrict or increase the flow of compressed air into cylindrical liner  45 . In contrast to FIG. 4, the amount of air flowing into plenum  37  of burner  35  is indirectly controlled by allowing a variable proportion of all the air from the compressor to flow into liner  45  simply by moving tapered plug  46  toward or away from tapered hole  44 . 
     Gaseous or vaporized fuel is supplied by tube  48  which passes through the shell of the gas turbine (not shown) in which metal casing  36  is installed. Tube  48  also passes through casing  36  and is connected to an annular bore  49  in circular block  43 . Several uniformly spaced holes  50  from the recessed side  46  of block  43  to bore  49  serve for the injection of fuel into the gap between disk  40  and recessed side  46  of block  43 . Compressed air flowing through that gap mixes thoroughly with the gaseous fuel injected by spaced holes  50  and the mixture flows into burner plenum  37 . The mixture exiting porous burner face  39  undergoes surface-stabilized combustion in the confined annular space between burner face  39  and perforated liner  45 . Compressed air flowing through liner  45  cools both liner  45  and the combustion product gasses by mixing therewith. 
     Gas turbine  55  of FIG. 6 has casing  56  that encloses air compressor  57 , turbine  58  and shaft  59  connecting  57 , 58 . Between compressor  57  and turbine  58  is a channeled section  60  which directs the flow of air from compressor  57  into outer housing  61  attached to casing  56 . Cylindrical burner  62  is suspended in housing  61 . 
     Plenum  63  of burner  62  has dual porosity burner face  64  connected to burner neck  65  that is attached to tapered hole  66  in plate  67 . Perforated shell  68  within plenum  63  is spaced from burner face  64  and promotes uniform flow of the fuel-air mixture toward all of face  64 . Disk  69  with protective insulation (not shown) seals the end of plenum  63  opposite neck or inlet end  65 . Metal liner  70  is spaced from and surrounds burner face  64 , forming therebetween a confined combustion zone. 
     Spaced above plate  67  is block  71  with hole  72  centered over hole  66  in plate  67 . Tapered plug  73  can slide up and down in hole  72  to vary the gap between the tapers of hole  66  and plug  73  and thus vary the quantity of compressed air flowing from housing  61  and between plate  67  and block  71  into plenum  63 . Gaseous or vaporized fuel is supplied to burner  62  by several tubes  74  that pass through housing  61  and connect with nozzles  75  in block  71  which direct the fuel against plate  67  to effect good mixing with compressed air flowing along plate  67  and into plenum  63 . 
     Surface-stabilized combustion takes place in the confined annular space between burner face  64  and liner  70 . Air from compressor  57  filling housing  61  that does not flow into plenum  63  as an admixture with fuel injected through nozzles  75  flows through openings in liner  70  and blends with the combustion product gases. The blended gases are directed by channeled section  60  into turbine  58 . 
     The burner of FIG. 7 like that of FIG. 5 is within a metal casing  80  but air from the compressor enters radially through lateral duct  81  instead of longitudinally as indicated in FIG.  5 . Burner  82 , in contrast to previously described burners, has a flat burner face  83  extending across a pan-like plenum  84  containing perforated shell  85 . This form of burner is well suited for the use of a knitted metal fiber fabric as burner face  83  with perforated shell  85  acting both as support for the fabric and as aid for uniform gas flow over all of face  83 . 
     Lateral wall  86  of plenum  84  connects burner face  83  to plate  87  that has central tapered hole  88  serving as inlet to plenum  84 . Spaced from plate  87  is block  89  with central hole  90 . Tapered plug  91  in hole  90  can be moved toward or away from hole  88  in plate  87  to vary the flow of compressed air into plenum  84 . Several tubes  92  pass through casing  80  and are connected to nozzles  93  in block  89 . Gaseous fuel supplied by tubes  92  impinges on plate  87  and mixes with compressed air flowing from casing  80  into the space between plate  87  and block  89 . The resulting mixture enters plenum  84  and exits through dually porous burner face  83  to undergo surface-stabilized combustion. 
     Attached to lateral wall  86  of pan-like plenum  84  is metal liner  94  with multiple openings which confines combustion in a tubular zone adjacent burner face  83 . Compressed air in casing  80  which does not flow into plenum  84  to support combustion flows around liner  94  to cool it and to pass through the openings in liner  91  to cool the combustion gases by mixing therewith. 
     FIG. 8 is an enlarged illustration of a porous mat  100  of sintered metal fibers which has been perforated along spaced bands  101  as taught in the previously cited patent to Duret et al. This preferred form of burner face is generally used with a metal or ceramic plate  102  spaced from the upstream side of burner face  100 . Perforated shell is the term previously adopted for plate  102  because it is frequently curved, e.g., cylindrical as shown in FIGS. 1 and 2. Perforated shell  102  with comparatively large perforations is disposed in the plenum of the burner to help achieve uniform flow toward all of burner face  100 . 
     FIG. 9 similarly illustrates burner face  103  in the form of a knitted fabric made with a metal fiber yarn. In this case, perforated shell  102  serves to support face  103  as well as promote uniform gas flow thereto. 
     FIG. 10 shows a uniformly perforated burner face  104  and perforated shell  105  with perforations arranged in spaced bands  106 . Face  104  made of sintered metal fibers may have porosity that is too low for providing radiant surface combustion. The perforations in face  104  are chosen to provide blue flame combustion. Perforated shell  105  is designed to reduce gas flow to some of the perforations in face  104 . Specifically, the unperforated areas between perforated bands  106  of shell  105  diminish gas flow to perforations in face  104  Which are aligned with the unperforated areas. Such perforations receiving diminished flow will support surface combustion while other perforations of face  104  in line with perforated bands  106  will yield blue flame combustion. In lieu of the sintered metal fiber face  104 , a uniformly perforated ceramic fiber face may be used to yield surface combustion with spaced bands of blue flame combustion. 
     FIG. 11 presents burner face  107  with alternating bands  108  of small perforations and bands  109  of larger perforations. The perforations of bands  108  are dimensioned to yield radiant surface combustion when fired at atmospheric pressure while the larger perforations of bands  109  give blue flame combustion. As a rough guide, the open area of each larger perforation is usually about 20 times that of each small perforation. Burner face  107  is made of a low thermal conductivity material formed of metal or ceramic fibers. A preferred embodiment of burner face  107  is the ceramic fiber product of previously cited patent to Carswell provided with perforations of two sizes adapted to give the desired two types of combustion. As indicated in FIG. 11, burner face  107  may frequently be used without a perforated shell. 
     A burner face of the type illustrated in FIG. 8 is preferred in achieving combustion that yields product gases containing as little as 2 ppm NO x  or less and yet no more than 10 ppm CO and UHC, combined. All of the burner faces that have been described, when fired at a pressure of at least 3 atmospheres and at a rate of at least about 500,000 BTU/hr/sf/atm, while controlling excess air in the fuel-air mixture fed to the burner face, are capable of delivering combustion product gases containing not more than 5 ppm NO x  and not more than 10 ppm CO and UHC, combined. Depending on the temperature of the compressed air that is admixed with the gaseous fuel, excess air is varied between about 40% and 150%; the percentage of excess air is increased relative to higher temperatures of the compressed air to maintain an adiabatic flame temperature in the range of 2600° F. to 3300° F. Preferably, excess air is controlled to keep the adiabatic flame temperature in the range of 2750° F. to 2900° F. to drop the content of air pollutants in the combustion gases down to 2 ppm NO x  or lower with not more than 10 ppm CO and UHC, combined. 
     Tests conducted with a burner like that of FIG. 4 with a face as shown in FIG.  8  and fired at 10 atmospheres with natural gas at the rate of 10,000,000 BTU/hr/sf kept the content of NO x  in the combustion product gases below 2 ppm even though the temperature of the fuel-air mixture was increased as long as excess air was also increased. Specifically, the following tests produced less than 2 ppm NO x . 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Fuel-Air Temperature °F. 
                 Excess Air Range 
               
               
                   
                   
               
             
             
               
                   
                   400 
                  55 to 67% 
               
               
                   
                   600 
                  66 to 81% 
               
               
                   
                   800 
                  81 to 98% 
               
               
                   
                 1,000 
                 98 to 118% 
               
               
                   
                   
               
             
          
         
       
     
     The adiabatic flame temperatures of all the tests were maintained in the range of 2750° F. to 2900° F. by controlling excess air in the ranges given above. It is believed that such a high firing rate and the suppression of NO x  to less than 2 ppm has never been even closely approached. Similar outstanding results are attainable when reducing the firing rate to 5,000,000 BTU/hr/sf or increasing that rate to 15,000,000 BTU/hr/sf; that means the operator has the freedom to vary the firing rate to a maximum at least three times the minimum at any given pressure. This operating flexibility is itself noteworthy. 
     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 readily 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, besides the flat and cylindrical forms of burner faces shown in the drawings, conical and domed shapes may be used. Many patents directed to means for controlling the flow of compressed air into the burners of gas turbines are certainly suggestive of substitutes for the movable plug schematically shown in the drawings to control the compressed air entering the burner. Accordingly, only such limitations should be imposed on the invention as are set forth in the appended claims.