Low NOx burners

Abstract: A burner (10) for the combustion of a fuel plus air mixture comprises a central nozzle pipe (16) having an inner surface (32) with a portion which diverges outwardly. An axially movable plug (38) is positioned within the nozzle pipe (16) and includes an outer wall (34) with a diverging section extending within the diverging section of the nozzle pipe (16). By axially moving the plug (38), the cross sectional area of the space between the diverging surfaces increases and decreases, respectively decreasing and increasing the velocity of the fuel plus air mixture passing through the nozzle space. This reduces the formation of NO x and the length of the flame produced by the burner (10).

Description: This invention relates to low NO x burners. Low NO x coal-fired burners rely on principles of air staging and/or fuel staging to reduce formation of nitric oxides during combustion. In either case, it becomes necessary to permit a portion of the combustion process to take place in fuel-rich/oxygen-deficient conditions such that reactions can take place to form N₂ rather than NO or NO₂. A good example is the burner disclosed in US patent number US-A-4,836,772 which achieves very low NO x emissions by the use of air staging and fuel staging. Air staging is achieved by a dual air zone burner barrel arrangement which enables regulation of air introduction to the fuel. Consequently, not all the air introduced through the burner is permitted to mix immediately with the fuel, but rather its introduction is controlled to take place gradually. Fuel staging is achieved by the introduction of the fuel in a controlled fuel rich zone, which results in partial combustion and generation of hydrocarbon radicals. These radicals proceed to mix with the products of combustion and reduce NO x formed earlier in the flame. The combined effects are achieved by introducing the fuel jet axially into the combustion chamber, with sufficient momentum to delay the mixing between fuel and air. An undesirable attribute of such a burner/process is the relatively long flame which results. Delayed air/fuel mixing tends to cause flames to become much longer than rapid-mixed high NO x flames. Elongated flames may then impinge on furnace walls leading to slag deposition, corrosion, and higher levels of unburned combustibles (flame chilling). These effects can have significant impacts on the operation, service life, and efficiency of combustion, respectively. Fuel staging is disclosed in US patent number US-A-4,206,712. To reduce flame length in low NO x burners, impellers can be installed at the exit of the coal nozzle. These serve to deflect the fuel jet, reducing axial fuel momentum and reducing flame length. However, NO x increases significantly. Another known burner disclosed in US patent number US-A-4,400,151 separates the fuel jet into several streams which are accelerated and deflected at the nozzle jet. NO x performance is again impaired, like the burner of US-A-4,836,772 which uses an impeller. In addition, the burner in US-A-4,400,151 provides for some fuel jet velocity control with questionable effectiveness. This design suffers from poor mechanical reliability. Tests have shown the burner of US-A-4,836,772 can produce a short flame with very low NO x . However, very high secondary air swirl is required to counteract the fuel jet momentum. The high secondary air swirl requires prohibitively high burner pressure drop. US-A-4,768,948 discloses an annular nozzle burner which produces a compact flame parallel to the burner axis. US-A-4,428,727 discloses a burner for solid fuels having an axially moveable element which can vary the size of an annular outlet gap from the nozzle. An axially adjustable impeller is disclosed in US patent number US-A-3,049,085. US-A-1,779,647 discloses a burner in accordance with the precharacterising portion of claim 1. According to the present invention there is provided a burner for the combustion of a fuel plus air mixture, the burner comprising: a nozzle pipe having an inlet for receiving a fuel plus primary air mixture, an outlet for discharging the fuel plus primary air mixture, and an inner surface which diverges along at least part of the length of said pipe between said inlet and said outlet; a plug extending axially in said nozzle pipe and defining an annular nozzle space in said pipe for the passage of the fuel plus primary air mixture, said plug having an outer surface which diverges along at least part of the length of said plug in said annular nozzle space and opposite the diverging portion of said pipe for diverting the fuel plus primary air mixture outwardly along said nozzle space; and drive means connected between said pipe and said plug for moving said pipe and plug axially with respect to each other to change the cross-sectional area of the nozzle space at the diverging portions of said pipe and plug so that the fuel plus primary air mixture moves at a different velocity near the diverging portions of the pipe (16) and plug; characterised in that the inner surface of the pipe and the outer surface of the plug include respective cylindrical portions downstream of the diverging portions thereof, and in that the burner includes secondary air means extending around said pipe for supplying secondary air in an annular steam around the fuel plus primary air mixture discharged from said nozzle outlet, and means for supplying ducted air or recirculated gas from the outer surface of the plug at the diverging portion thereof into the annular nozzle space. Embodiments of the invention provide a burner which can simultaneously achieve low NO x emissions with a relatively short flame. A new and useful burner for the combustion of coal, oil or gas is therefore provided. Am embodiment of the burner generally resembles the burner disclosed in US-A-4,836,772 with an axial coal nozzle and dual air zones surrounding the nozzle. However, the coal nozzle is altered to accommodate a hollow plug. A pipe extends from the burner elbow through the nozzle mixing device, which uses a conical diffuser. The coal/primary air (PA) mixture is dispersed by the conical diffuser into a pattern more fuel rich near the walls of the nozzle and fuel lean toward the centre as in US patent number US-A-4,380,202. The nozzle then expands to about twice the flow area compared to the inlet. As the nozzle expands, the central pipe is expanded to occupy an area roughly equivalent to the inlet area of the nozzle. Therefore the fuel/PA mixture travelling along the outside of the hollow plug is at about the same velocity as at the entrance of the nozzle. The centre pipe with hollow plug can be moved fore/aft relative to the end of the burner nozzle and thereby change the fuel/PA exit velocity from the nozzle. Embodiments of the invention provide a burner for the combustion of coal, oil or gas, which comprises an axially moveable plug having a divergent cross section which is positioned within a nozzle pipe for carrying fuel, for example pulverised coal, the pipe also having a divergent cross section. Burners embodying the invention are simple in design, rugged in construction and economical to manufacture. A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing which is a schematic sectional view of a burner embodying the invention. The drawing shows a burner, generally designated 10, which is particularly designed for burning a pulverised coal plus primary air mixture supplied at an elbow member 12 to a nozzle inlet 14. The nozzle inlet supplies the coal/primary air mixture to the inlet of a central nozzle pipe 16 which extends across a secondary air windbox 18 defined between a water wall 20, which acts as a boundary for a combustion chamber 22, and an outer burner wall 24 which has an access opening that is closed by a flange 17 of the nozzle pipe 16. Water tubes 26 from the water wall 20 are bent to form a conical burner port 30 having a diverging wall extending into the combustion chamber 22. A conical diffuser 28 is positioned in the central nozzle pipe 16 for dispersing the coal/primary air mixture into a pattern which is more fuel rich near an inner surface or wall 32 of the nozzle pipe 16, and more fuel lean towards an outer wall 34 of a hollow plug 38 positioned in the central nozzle pipe 16. Although the plug 38 is shown cross hatched, it is in fact hollow and contains various structures including, for example, conduits for ignition means and for oil atomisers, shown only as an atomiser outlet 40 for discharging an atomised oil plus medium mixture 42 into the combustion chamber 22. The atomising medium may be steam or air for example. Drive means, shown schematically at 44, are connected to the plug 38 for moving the plug axially in the fore and aft direction as shown by the double arrow. This causes the outwardly diverging walls of the outer plug surface 34 to move closer to, or further away from, the outwardly diverging walls of the inner nozzle pipe surface 32, to change the velocity of coal/primary air exiting through an annular outlet nozzle 46, defined between the central pipe and the plug, into the combustion chamber 22 in the direction of the arrows 36. Secondary air flows from the windbox 18 in the direction of the arrows 48 into an annular secondary air passage 50 defined between an outer surface of the nozzle pipe 16 and an inner surface of a burner barrel 52. The annular inlet into the secondary air passage 50 can be opened or closed by axially moving a slide damper 54 which is slidably mounted on the outer surface of the pipe 16. The secondary air passage 50, near the combustion chamber 22, is divided into an outer annular passage 56 containing one or more swirling vanes 57, and an inner annular passage 58 containing one or more swirling vanes 59. Secondary air is thus discharged in an annular pattern around the exiting coal/primary air mixture through the burner port 30 into the combustion chamber 22. With the plug positioned as shown, the fuel/air mixture leaves the nozzle at 36 with a velocity similar to that in US patent number US-A-4,836,772 and may pass through a flame stabilising ring 60 to stabilise and accelerate combustion. However, as the fuel/PA leaves the nozzle, the bluff body effect of the hollow plug 30 makes the adjacent flow streams pull in/recirculate to occupy this zone. This acts effectively to reduce the axial momentum of the fuel/PA jet. This zone remains fuel rich to achieve low NO x emissions. The reduced fuel jet momentum tends to reduce flame length for two reasons. Firstly, the coal particles have more time to burn out per unit distance from the burner. Secondly, the reduced fuel jet momentum enables the surrounding swirling secondary air (with combustion by-products) to penetrate more readily and complete mixing with the fuel jet at a moderate distance from the burner. The geometry of this arrangement enables variation of the burner nozzle exit velocity by simple repositioning the hollow plug 38 fore/aft relative to the end of the nozzle, consequently affecting NO x formation and flame length. Lower exit velocities can be achieved by partially retracting the hollow plug 38, shortening the flame. This solves the problem of reducing flame length in a low NO x burner. An alternative sometimes used to reduce flame length is to install an impeller at the exit of the burner nozzle. This causes the coal/PA to be deflected at an angle off the burner axis, thereby reducing axial momentum. Flame length is shortened in proportion to the flare angle of the impeller. The disadvantage of the impeller is that the fuel is unavoidably deflected into the secondary air streams surrounding the fuel jet. This diminishes the fuel rich zone during coal devolatilisation and causes NO x to increase significantly relative to the same burner without an impeller. The embodiment described above reduces fuel jet momentum as the flame develops by collapsing the fuel jet, keeping it fuel rich. Consequently NO x is kept low while the flame is shortened. The principle advantage of the embodiment described above is low NO x with reduced flame length. However, several other advantages are also achieved. One concerns the use of the burner with difficult-to-burn coals. It has been demonstrated that low burner nozzle velocities facilitate combustion of "difficult" pulverised fuels, such as low volatile coal, high moisture lignite, and petroleum coke. The burner can be made to behave like a so-called enhanced ignition dual register burner by retracting the hollow plug somewhat. This results in much lower fuel/PA velocities leaving the nozzle and increases residence time of the fuel on the ignition zone immediately downstream of the nozzle. However, the majority of coals burned in the US and many other countries are readily burned without resorting to very low nozzle velocities. In fact, operation with very low nozzle velocities can result in flame flash-back into the nozzle, damaging the burner and potentially producing a hazardous condition. So another advantage is the ability to change nozzle velocity easily to accommodate changes in coal quality. Therefore this same burner could readily fire a difficult-to-burn coal or easily burned coals by adjusting nozzle velocity. Another advantage of the burner concerns the use of the "pipe and hollow plug" axially positioned in the coal nozzle. This device can serve as the housing for the burner igniter and/or an auxiliary fuel element such as a main oil atomiser 40 or a main gas element. The pipe an plug serve as a convenient location for such equipment and facilitate the use of fuel staging principles for firing natural gas or fuel oil by the axial location. The device shown in the figure has cylindrical walls 64, 65 in the pipe, and 62, 63 on the hollow plug. The walls 62, 64 can be tapered instead to provide more adjustment to nozzle exit velocity. The device as shown has a "hollow plug" the size of the burner nozzle inlet, with the burner nozzle exit being twice the area of the inlet. Other ratios of nozzle and plug areas may prove more efficient in some circumstances, eg a "hollow plug" twice the area of the nozzle inlet and a nozzle exit three times the area of the inlet. The pipe and plug in this embodiment are ducted at 66 and 67 to supply small quantities of air or recirculated flue gas to further reduce NO x or control flame shape.

Claims: A burner for the combustion of a fuel plus air mixture, the burner comprising: a nozzle pipe (16) having an inlet (14) for receiving a fuel plus primary air mixture, an outlet (36) for discharging the fuel plus primary air mixture, and an inner surface (32) which diverges along at least part of the length of said pipe (16) between said inlet (14) and said outlet (36); a plug (38) extending axially in said nozzle pipe (16) and defining an annular nozzle space in said pipe (16) for the passage of the fuel plus primary air mixture, said plug (38) having an outer surface (34) which diverges along at least part of the length of said plug (16) in said annular nozzle space and opposite the diverging portion of said pipe (16) for diverting the fuel plus primary air mixture outwardly along said nozzle space; and drive means (44) connected between said pipe (16) and said plug (38) for moving said pipe (16) and plug (38) axially with respect to each other to change the cross-sectional area of the nozzle space at the diverging portions of said pipe (16) and plug (38) so that the fuel plus primary air mixture moves at a different velocity near the diverging portions of the pipe (16) and plug (38); characterised in that the inner surface (32) of the pipe (16) and the outer surface (34) of the plug (38) include respective cylindrical portions downstream of the diverging portions thereof, and in that the burner includes secondary air means (50) extending around said pipe (16) for supplying secondary air in an annular steam around the fuel plus primary air mixture discharged from said nozzle outlet (36), and means (67) for supplying ducted air or recirculated gas from the outer surface of the plug (38) at the diverging portion thereof into the annular nozzle space. A burner as claimed in claim 1, including means (66) for supplying ducted air or recirculated gas from the inner surface (32) of the pipe (16) at the diverging portion thereof into the annular nozzle space. A burner according to claim 1 or claim 2, wherein the inner surface (32) of said nozzle pipe (16) includes a cylindrical portion upstream of the diverging portion of said pipe (16). A burner according to claim 3, wherein the outer surface (34) of the plug (38) includes a cylindrical portion upstream of the diverging portion of the plug (38). A burner according to any preceding claim, including a conical diffuser (28) positioned within said nozzle space downstream of the nozzle inlet (14) and upstream of the portions of the plug and pipe surfaces which diverge. A burner according to any preceding claim, wherein said secondary air means (50) comprises a barrel (52) positioned around said nozzle pipe (16) and defining an annular secondary air chamber around said nozzle pipe (16), and wherein the burner includes means in said secondary air chamber for dividing said chamber into an inner annular chamber (58) and an outer annular chamber (56), and at least one swirling vane (59) in at least one of said inner and outer chambers. A burner according to any preceding claim, wherein the inner surface (32) of said nozzle pipe (16) diverges to expand the nozzle pipe outlet (36) to about twice the flow area of the nozzle pipe inlet (14). A burner according to any preceding claim, wherein the outer surface (34) of said plug (38) diverges to expand the plug to occupy an area roughly equivalent to the area of the nozzle pipe inlet (14). A burner according to any one of claims 1 to 6, wherein the inner surface (32) of said nozzle pipe (16) diverges to expand the nozzle pipe outlet (36) to about three times the flow area of the nozzle pipe inlet (14). A burner according to claim 9, wherein the outer surface (34) of said plug (38) diverges to expand the plug to occupy an area about twice the area of the nozzle pipe inlet (14).

Inventors: LARUE ALBERT DANIEL, LARUE, ALBERT DANIEL
Applicants: BABCOCK & WILCOX CO, THE BABCOCK & WILCOX COMPANY
Application year: 1992
Main classification: F23D1
IPC codes: F23D1, F23C99