Method and apparatus for burning fuel

Abstract

 Methods and apparatus for the combustion of fuel-air mixtures while inhibiting the formation of nitrogen oxides are provided. The fuel is discharged from one or more nozzles (38) disposed within a housing, air is caused to flow into the housing whereby it mixes with the fuel and the resulting fuel-air mixture is ignited and combusted. The nozzle or nozzles (38) each include one or more ignition orifices (56) for discharging a first portion of fuel in an ignition zone, one or more primary combustion orifices (58) for discharging a second portion of fuel in a primary combustion zone containing excess air and one or more secondary combustion orifices (60) arranged for discharging the remaining portion of fuel within and downstream of the primary combustion zone whereby the fuel is burned in a secondary combustion zone substantially insola- ted from direct contact with incoming air by the primary combustion zone.

Description

[0001] The present invention relates to a method and burner apparatus for combusting fuel-air mixtures, while inhibiting the formation of nitrogen oxides.

[0002] Known methods and burner apparatus are used in a great variety of applications, e.g. heating process streams, generating steam, drying materials, etc. The burning of fuels, however, can result in the formation of nitrogen oxides (NO_x) which when released to the atmosphere constitute pollutants.

[0003] Various methods and burner apparatus for combusting fuel-air mixtures while suppressing the formation of nitrogen oxides have been developed, to meet environmental emission standards imposed by various government authorities. For example, U.S.-A-4,004,875 is directed to a low NO burner wherein the fuel is first burned in a zone in which there is less than a stoichiometric concentration of air, thereby producing a reducing environment that suppresses NO formation with the deficiency in air being made up in a subsequent burning zone.

[0004] Fuel staging has also been used (see for example U.S.-A-4,395,223), in which a portion of the fuel is burned in a first zone with air being supplied at a rate in excess of the stoichiometric rate required with the remaining fuel being burned in a second zone. The presence of excess air in the first zone lowers the temperature of the combustion reaction and suppresses NO formation. The fuel in the second zone reacts with the excess oxygen from the first zone and is diluted with surrounding combustion gases which lowers the combustion reaction temperature and suppresses the formation of NO in the second zone.

[0005] Such known staged combustion methods have required elaborate burner apparatus including a plurality of fuel nozzles and/or complex air or recycle gas distribution systems, making the apparatus expensive to install and operate.

[0006] According to the present invention, there is provided a method of combusting a fuel-air mixture, said method comprising the steps of discharging a first portion of said fuel from each of said one or more nozzles through one or more first orifices therein whereby said fuel mixes with air and provides an ignition zone adjacent said nozzle; discharging a second portion of said fuel from each of said one or more nozzles through one or more second orifices therein whereby said second portion of fuel is distributed in a turbulent pattern which causes said fuel to mix with a rate of air in excess of that required for the stoichiometric burning thereof and to burn in a primary combustion zone; and discharging the remaining portion of said fuel from each of said one or more nozzles through one or more third orifices and said fuel is distributed within and downstream of said primary combustion zone, is mixed with air from said primary combustion zone which is diluted with combustion products and is burned in a secondary combustion zone substantially shielded from direct contact with incoming air by said primary combustion zone.

[0007] Such a method can inhibit the formation of nitrogen oxides in a manner which is simple and inexpensive as compared to prior art methods.

[0008] The invention also relates to apparatus for combusting a fuel-air mixture, said apparatus comprising one or more fuel nozzles disposed within a chamber, air inlets to cause air to flow into the chamber whereby it mixes with the fuel and the resulting fuel-air mixture is ignited and combusted, characterised in that each of said one or more nozzles includes one or more ignition orifices disposed therein positioned to discharge a first portion of said fuel therethrough, whereby said fuel mixes with air and provides an ignition zone adjacent said nozzle; one or more primary combustion orifices positioned to discharge a second portion of said fuel therethrough whereby said fuel is distributed in a turbulent pattern which causes said fuel to mix with a rate of air in excess of that required for the stoichiometric burning thereof and to burn in a primary combustion zone; and one or more secondary combustion orifices disposed in each of said one or more nozzles interiorly of said primary combustion orifices and positioned to discharge the remaining portion of said fuel therethrough, whereby said fuel is distributed within and downstream of said primary combustion zone is mixed with air from said primary combustion zone which is diluted with combustion products and is burned in a secondary combustion zone substantially isolated from direct contact with incoming air by said primary combustion zone.

[0009] In order that the invention may more readily be understood, the following description is given, merely by way of example, reference being made to the accompanying drawings, in which:-

Figure 1 is a side cross-sectional view of one embodiment of apparatus according to the present invention;

Figure 2 is a top plan view of the apparatus of Figure 1;

Figure 3 is an enlarged partly sectional view of a portion of the apparatus of Figure 1 including the fuel discharge nozzle thereof;

Figure 4 is a top plan view of the portion of apparatus of Figure 3; and

Figure 5 is a side cross-sectional view of the burner apparatus of Figure 1 illustrating the operation of the apparatus.

[0010] The burner apparatus 10 is shown in Figures 1 and 2 connected in an opening 14 provided in a wall 12 of a furnace chamber, and is designed for use in applications where gaseous fuels such as hydrocarbon gases are combusted.

[0011] The apparatus 10 includes a housing formed by an external cylindrical housing member 16 attached over the opening 14 by a plurality of bolt members 18, and a heat resistant refractory material member 20 mounted in an opening formed in an insulating layer of refractory material 22 lining the interior of the wall 12. The member 20 can be attached to the wall 12 and/or refractory material 22 of the furnace chamber as illustrated or it can be attached to the cylindrical housing member 16 in any convenient manner.

[0012] The housing member 16 functions as an air register, and for this purpose, includes a plurality of circumferentially spaced air inlet openings 26. A wall 24 closes the end of the housing member 16 and rotatably positioned over the housing member is a cylindrical damper 28 having air openings (not shown) complementary to the air openings 26 in the housing member 16. The damper 28 can be rotated by a handle 30 between a position in which the openings 26 are closed by solid portions of the damper 28 and a position in which the damper penings 28 register with openings 26 to provide full air flow as shown in Figure 1.

[0013] Coaxially disposed within the housing member 16, is a guide tube, the outer end of which is rigidly attached, e.g. by welding in an opening in the wall 24, and the inner end of which has a shielding cone 34 attached thereto. A fuel supply conduit 36 extends through the guide tube 32 and has a fuel discharge nozzle 38 connected at the inner end thereof. The exterior end of the conduit 36 is threaded for connection to a source of fuel and the conduit 36 is sealingly attached to a plate 39 which is in turn removably connected by means of bolt members 40 to the wall 24.

[0014] A pilot 42 is connected to a supply conduit 44 which in turn extends through an opening in the wall 24 and has a removable closure member 46 connected thereto. The outer end of the supply conduit 44 is connected to a pilot fuel-air mixer 48 which is adapted for connection to a source of pilot fuel.

[0015] Figures 3 and 4 show that the shielding cone 34 is dish-shaped and includes a plurality of openings 50 formed therein for allowing the passage of a limited amount of air therethrough. The shielding cone 34 functions to create a protected area adjacent the nozzle 38 when incoming air is flowing in the direction indicated by the arrow 52 of Figure 3. As will be understood, the creation of a protected area adjacent the nozzle 38 can be brought about by various types and shapes of apparatus other than the shielding cone 34.

[0016] The nozzle 38 extends through a central opening in the shielding cone 34 and includes a hemispherical end wall 54 which includes a first set of one or more orifices 56. When more than one orifice 56 are utilized, they preferably are all the same size and are equally spaced around the nozzle 38 in a plane preferably perpendicular to the axis of the nozzle 38, i.e. the angle designated by the letter "c" on Figure 3 is preferably 90°. The axis of the nozzle 38 is parallel to the axis of the housing member 16 whereby the axes of the orifices 56 lie in a plane substantially perpendicular to the direction of air flow through the housing member 16: The first set of orifices 56 discharge a first portion of the fuel supplied to the nozzle 38 which mixes with a portion of the incoming air and provides an ignition zone adjacent the nozzle 38.

[0017] A second set of one or more orifices 58 is disposed in the wall portion 54 of the nozzle 38. When more than one orifice 58 are utilized, they preferably are all of the same size and are equally spaced around the wall 54 interiorly of and above the ignition orifices 56. The axes of the orifices 58 are also preferably inclined in the direction of flow of air at the same angle "b" (Figure 3) as each other which is preferably in the range 15° to 70° therewith. The second set of orifices 58 discharge a second portion of the fuel supplied to the nozzle 38 which is distributed in a turbulent outwardly flaring pattern.

[0018] A third set of one or more orifices 60 is disposed in the wall portion 54 of the nozzle 38 interiorly of and above the primary combustion orifices 58. Again, when more than one orifice 60 are utilized, they are preferably all of the same size and are spaced on a circular pattern in the nozzle 38. The axes of the orifices 60 can be parallel to the axis of the nozzle 38 and to the direction 52 of air flow, or, as shown in Figure 3, the axes of the orifices 60 can be inclined at an angle "a" in the range of 1° to 30° therewith. It is to be noted that angle "a" can be about equal to or less than the angle "b", but should not be greater than the angle "b".

[0019] The orifices 60 discharge the remaining portion of fuel supplied to the nozzle 38, whereby substantially all of such remaining portion of fuel is burned in a secondary combustion zone within and downstream of the primary combustion zone created by the discharge of the second portion of fuel from the orifices 58.

[0020] Referring now to Figure 5 in operation, fuel under a pressure generally in the range of from about 0.2 to about 2 bar gauge is supplied to the conduit 36. Pilot fuel at a pressure in the range of from about 0.2 to about 1 bar gauge is supplied to the air mixer 48, where it is mixed with air and the resulting fuel-air mixture is discharged from the pilot 42, ignited and burned. The flame from the functions to ignite the fuel discharged from the nozzle 38. However, it is to be noted that other ignition means can be utilized and the use of a pilot burner is optional.

[0021] The pressurized fuel supplied to the conduit 36 flows to the nozzle 38 and is discharged into the furnace chamber through the orifices 56, 58 and 60 therein. The ignition orifices 56, are of a size and/or number whereby the first portion of fuel discharged therethrough is about 1% to about 25% of the total fuel discharged from the nozzle 38. Such portion of the fuel mixes with air in the protected ignition area 62 shielded by cone 34 adjacent the nozzle 38, is ignited by the flame from the pilot 42 or other means and burns.

[0022] The second set of orifices, i.e. the primary combustion orifices 58, are of a size and/or number such that a second portion of fuel is discharged therethrough, is about 1% to about 60% of the total rate of fuel discharged from the nozzle 38. The second portion of fuel is distributed in an outwardly flaring pattern from the nozzle 38 in a turbulent manner which causes the fuel to mix with air flowing into the housing of the burner 10 by way of the openings 26 in the housing member 16. By adjusting the position of the damper 28 on the housing member 16, the total rate of air is adjusted to be substantially equal to or greater than that required for the stoichiometric burning of the total rate of fuel discharged from the nozzle 38. The second portion of fuel and air mixture produced is combusted in a primary combustion zone 64 which flares outwardly from the nozzle 38. Because the second portion of fuel is mixed with air in excess of that required for the stoichiometric burning of the fuel, the temperature in the primary combustion zone 64 is lowered and the formation of NO in the primary combustion zone is inhibited.

[0023] The remaining portion of the fuel supplied to the nozzle 38 is discharged therefrom by way of the third set of orifices therein, i.e. the secondary combustion orifices 60. The portion of the fuel from the orifices 60 is distributed within and downstream of the primary combustion zone 64 into a secondary combustion zone 66 which is substantially shielded from direct contact with incoming air by the primary combustion zone 64. The fuel in the secondary combustion zone is mixed with air from the primary combustion zone which is diluted with combustion products from the primary combustion zone.

[0024] Thus, because the remaining portion of fuel discharged through the secondary combustion orifices 60 is burned in a secondary combustion zone 66 within and downstream of the primary combustion zone 64, and because the air mixed with such remaining portion of fuel is diluted with combustion products, the combustion takes place at a relatively low temperature whereby the formation of NO is inhibited.

[0025] Using the method of the present invention, because the combustion in the primary combustion zone takes place in excess air, the flame temperature in such zone is lowered whereby the formation of NO is inhibited. Combustion in the secondary combustion zone is delayed because the secondary combustion zone is shielded by the primary zone from direct contact with incoming air. This delay in the mixing of the fuel and air allows for dilution of the air with combustion products from the primary combustion zone and from within the combustion chamber, resulting in a lower combustion temperature which inhibits the formation of NO in the secondary combustion zone.

[0026] While the present invention has been described as it relates to a natural draft burner apparatus, it is equally applicable to a wide variety of burner designs, including those utilizing forced draft. In addition, more than one fuel discharge nozzle of the present invention can be utilized in a single burner apparatus, for example, the burner apparatus disclosed in U.S.-A-3,033,273. Further, the fuel discharge nozzle and shielding cone can both take various other forms and shapes so long as the functional limitations described above are met thereby.

[0027] In order to facilitate a clear understanding of the method and apparatus of the present invention, the following example is given.

EXAMPLE

[0028] A burner apparatus 10 designed for a heat release of 1756.8 Kw by burning natural gas having a caloric value of 9.615 Kw.hr/m3 is fired into a furnace chamber. The nozzle 38 includes a first set of 6 orifices 56 of 1.59 mm diameter, a second set of 4 orifices 58 of 3.57 mm diameter and a third set of 4 orifices 60 of 4.76 mm diameter. The axes of the orifices 56 are at an angle of 90° with the axis of the nozzle 38, the axes of the orifices 58 are at an angle of 40° with the axes of the nozzle 38 and the axes of the orifices 60 are at an angle of 10° therewith.

[0029] The fuel is supplied to the nozzle 38 at a pressure of about 15 psig and at a rate of about 185 m3/hr. The first portion of fuel discharged through the ignition nozzles 56 is at a rate of about 16.9 m3/hr, the second portion of fuel discharged through the primary combustion orifices 58 is at a rate of about 56.2 m3/hr, and the remaining portion of fuel discharged through the secondary combustion orifices 60 is at a rate of about 109.6 m3/hr.

[0030] The discharged fuel is combined with air in the burner apparatus 10 and burned whereby a heat release in the furnace chamber of about 1756.8 kw is realized. The stack emissions from the furnace chamber contain a NO concentration of less than about 30 ppm. A conventional burner including a conventional nozzle fired in the furnace chamber in the same manner and under the same conditions creats stack emissions containing a NO_x concentration of more than about 70 ppm.

Claims

1. A method of combusting a fuel-air mixture wherein fuel is discharged from one or more nozzles disposed within a burner housing, air introduced into said housing is mixed with the fuel and the resulting fuel-air mixture is ignited and combusted, characterised in that it comprises the steps of discharging a first portion of said fuel from each of said one or more nozzles (38) through one or more first orifices (56) therein$O'whereby said fuel mixes with air and provides an ignition zone (62) adjacent said nozzle; discharging a second portion of said fuel from each of said one or more nozzles (38) through one or more second orifices (58) therein, whereby said second portion of fuel is distributed in a turbulent pattern which causes said fuel to mix with a rate of air in excess of that required for the stoichiometric burning thereof and to burn in a primary combustion zone (68); and discharging the remaining portion of said fuel from each of said one or more nozzles (38) through one or more third orifices (60) is mixed with air from said primary combustion zone (64) which is diluted with combustion products and is burned in a secondary combustion zone (66) substantially shielded from direct contact with incoming air by said primary combustion zone (64).

2. A method according to claim 1, characterised in that said first portion of fuel is discharged at a rate in the range of from 1% to 25% of the total rate of fuel discharged from each of said one or more nozzles.

3. A method according to claim 1 or 2, characterised in that said second portion of fuel is discharged at a rate in the range of from 1% to 60% of the total rate of fuel discharged from each of said one or more nozzles.

4. A method according to any preceding claim, characterised in that the total rate of air introduced into said housing is substantially equal to or greater than the rate required for the stoichiometric burning of the total rate of fuel discharged from said one or more nozzles.

5. A method according to any preceding claim, characterised in that said second portion of fuel is distributed by said second orifices (58) in an outwardly flaring pattern whereby said primary combustion zone is of an outwardly flaring shape.

6. A burner apparatus for combusting a fuel-air mixture, said apparatus comprising one or more fuel nozzles (38) disposed within a chamber, air inlets (26) to cause air to flow into the chamber whereby it mixes with the fuel and the resulting fuel-air mixture is ignited and combusted, characterised in that each of said one or more nozzles (38) includes one or more ignition orifices (56) disposed therein positioned to discharge a first portion of said fuel therethrough, whereby said fuel mixes with air and provides an ignition zone (62) adjacent said nozzle; one or more primary combustion orifices (58) positioned to discharge a second portion of said fuel therethrough whereby said fuel is distributed in a turbulent pattern which causes said fuel to mix with a rate of air in excess of that required for the stoichiometric burning thereof and to burn in a primary combustion zone (64); and one or more secondary combustion orifices (60) disposed in each of said one or more nozzles (38) interiorly of said primary combustion orifices (58) and positioned to discharge the remaining portion of said fuel therethrough whereby said fuel is distributed within and downstream of said primary combustion zone (64), is mixed with air from said primary combustion zone (64) which is diluted with combustion products and is burned in a secondary combustion zone (66) substantially isolated from direct contact with incoming air by said primary cmbustionzone (64).

7. Apparatus according to claim 6, characterised in that it includes means (34) attached thereto for creating a protected area adjacent each of said one or more nozzles (38) and said one or more ignition orifices (56) therein.

8. Apparatus according to claim 6 or 7, characterised in that one or more ignition orifices (56) are of a size whereby the flow of fuel therethrough to produce said first portion constitutes from 1% to 25% of the total flow of fuel discharged from each of said one or more nozzles.

9. Apparatus according to claim 6, 7 or 8, characterised in that said one or more primary combustion orifices (58) are of a size whereby the flow of fuel therethrough to produce said second portion of fuel constitutes 1% to 60% of the total flow of fuel discharged from each of said one or more nozzles.

10. Apparatus according to any one of claims 6 to 9, characterised in that the axes of said one or more ignition orifices (56) are substantially perpendicular to the axis of the associated nozzle.

11. Apparatus according to any one of claims 6 to 10, characterised in that the axes of said one or more primary combustion orifices (58) are inclined to the axis of the associated nozzle at an angle in the range of from 15° to 70° and the axes of said one or more secondary combustion orifices (60) are parallel to or inclined in the axis of the associated nozzle at an angle in the range of from 1° to 30°.

12. Apparatus according to any one of claims 6 to 11, characterised in that the portion of said one or more nozzles containing said orifices is hemispherical in shape.

Drawing