Method and apparatus for preventing erosion in a nozzle tip

Abstract

 An improved nozzle tip (10) for a pulverized coal-fired burner for preventing erosion of the inner surface of the inner shell (12) through which the pulverized coal-air stream passes after discharging from coal delivery pipe (20) before entering the furnace. A portion of the air stream passing through annular duct (26) in the region of the interface between coal delivery pipe (20) and the inner shell (12) is diverted to form a layer of coal-free air along the inner surface of inner shell (12).

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to burner nozzle tips adapted for use on pulverized coal-fired furnaces and more particularly to tilting nozzle tips for burners on pulverized coal-fired furnaces utilizing tangential firing.

[0002] One common method of firing coal in a coal-fired steam generating furnace is known as tangential firing. In this method, pulverized coal is introduced into the furnace in a primary air stream through burners, termed fuel-air emission assemblies, located in the corner windboxes of the furnace. The pulverized coal-air streams discharging from the burners are aimed tangentially to an imaginary circle in the middle of the furnace combustion chamber to create a massive flame therein termed a fireball. Upon leaving the furnace combustion chamber, the combustion products formed in the fireball pass through a convective heat transfer section typically housing a superheater, a reheater, and other heat absorption surfaces to cool the combustion products and generate superheated steam. By changing the vertical position of the fireball formed in the furnace combustion chamber upon convergence of the fuel-air streams emanating from the burners, control of the temperature of the steam leaving the superheater or reheater is achieved. By tilting the burner nozzle tips in unison the fireball can be physically raised or lowered within the furnace combustion chamber to decrease or increase the heat absorption by the furnace waterwalls thereby raising or lowering the temperature of the combustion products leaving the furnace combustion chamber to pass over the superheater and reheater surfaces. As the temperature of the combustion products entering the convective heat transfer section changes, the temperature of the steam generated in the heat absorption surface disposed therein changes proportionally.

[0003] A typical coal-air emission assembly or burner employed heretofore on a tangentially fired furnace comprises a coal delivery pipe, often termed a coal nozzle, through which pulverized coal entrained in a primary air stream is delivered to the furnace, an air conduit surrounding the coal delivery pipe through which additional air is delivered to the furnace, and a nozzle tip pivotally mounted on the coal delivery pipe so as to be tiltable in a vertical plane whereby the pulverized coal-air stream being delivered to the furnace through the coal delivery pipe and the additional air passing through the air conduit can be directly discharged into the furnace combustion chamber as dictated by steam temperature requirements.

[0004] A typical prior art burner nozzle tip is formed of an open-ended inner shell defining a flow passageway through which the pulverized coal-air stream from the coal delivery pipe is delivered into the furnace combustion chamber and an open-ended outer shell spaced from and surrounding the inner shell so as to define an annular duct through which the air leaving the air conduit is directed into the furnace. Additionally, one or more baffles, termed splitter plates, are typically disposed within the inner shell of the nozzle tip and aligned parallel to the longitudinal axis thereof to impart additional directional force to the coal-air stream discharging through the inner shell and to ensure a uniform distribution of the coal-air stream particularly when the nozzle tip is tilted away from the horizontal.

[0005] A problem encountered in using such nozzle tips has been the erosion of the inner wall of the inside shell due to impingement of coal particles entrained in the primary air stream as they pass from the coal delivery pipe through the flow passageway within the inner shell into the furnace combustion chamber. This results in increased wear of the inner surface of the inner shell particularly when the nozzle tips are tilted away from the horizontal. Consequently, the prior art nozzle tips must be replaced more often than preferred and more frequently than would be necessary if the inner surface of the inner shell of the nozzle tip had not been exposed to the erosive effect of the impinging coal particles.

SUMMARY OF THE INVENTION

[0006] The present invention provides an improved nozzle tip which has a significantly longer life in the extremely erosive environment associated with pulverized coal firing.

[0007] In accordance with the present invention, an improved nozzle tip has means for air passage connecting the annular duct between the inner shell and the outer shell and the flow passageway within the inner shell in the region where the inner shell interfaces with the coal delivery pipe.

[0008] The air passage means divert air that would otherwise pass to the furnace combustion chamber through the annular duct between the inner shell and outer shell to pass along the inner surface of the inside shell thereby providing a layer of coal-free air along the inside wall of the inner shell which reduces erosion of the inner surface of the inner shell. BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a perspective view, partly in section, showing the nozzle tip of the present invention incorporated into a typical fuel-air emission assembly employed on a pulverized-coal furnace utilizing tangential firing;

Figure 2 is a perspective view, partly in section, of the improved nozzle tip of the present invention;

Figure 3 is a cross-sectional plan view, partly in section, of the improved nozzle tip of the present invention;

Figure 4 is a cross-sectional plan view, partly in section, showing an alternate embodiment of the nozzle tip of the present invention; and

Figure 5 is a cross-sectional plan view, partly in section, of the nozzle tip of the present invention showing yet another alternate embodiment.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

[0009] Referring to the drawing, there is depicted therein an improved nozzle tip 10 constructed in accordance with the present invention as best seen in Figure 1. The burner nozzle tip 10 is shown incorporated into a fuel-air emission assembly of the type typically employed on a pulverized coal-fired furnace utilizing tangential firing, although variations of the improved nozzle tip of the present invention may readily be incorporated in other fuel-air configurations without departing from the spirit and scope of the present invention.

[0010] Nozzle tip 10 comprises an open-ended inner shell 12 surrounded by an open-ended outer shell 14 which is spaced away from inner shell 12 by a plurality of ribs 16 disposed therebetween. Ribs 16 are disposed between the inner surface of outer shell 14 and the outer surface of inner shell 12 thereby forming the structural framework of nozzle tip 10. As shown in Figure 2, nozzle tip 10 is recessed into the air delivery conduit 18 of the fuel-air emission assembly and pivotally mounted to the discharge end of coal delivery pipe 20 so as to be tiltable about axis 22 transverse to the longitudinal axis of the coal delivery pipe 20.

[0011] Nozzle tip 10 provides a means for imparting a directional force to the pulverized coal discharging from coal delivery pipe 20 and the air delivered to the furnace through air delivery conduit 18. Inner shell 12 is adapted to fit around the discharge end of coal delivery pipe 20 and define a duct, chamber 24, which serves as a flow passageway through which the pulverized coal-air stream discharging from coal delivery pipe 20 traverses before entering the furnace. An annular duct 26 is defined in the space between inner shell 12 and outer shell 14 which serves as a flow passageway through which the additional air discharging from air delivery conduit 18 must traverse before entering the furnace.

[0012] As mentioned above, nozzle tip 10 is tiltable about an axis transverse to the longitudinal axis to coal delivery pipe 18 in order that the position of the fireball within the combustion chamber of the furnace may be changed to effect steam temperature control. In its normal position, nozzle tip 10 is positioned with its longitudinal axis aligned with the longitudinal axis of coal delivery pipe 18, which is generally horizontally disposed. In order to raise the fireball within the furnace, nozzle tip 10 is rotated about axis 22 so as to tilt upward, thereby causing both the pulverized coal-air stream traversing chamber 24 and the air traversing annular duct 26 to be directed upward. Similarly, if the fireball is to be lowered within the combustion chamber of the furnace, nozzle tip 10 is rotated about axis 22 so as to tilt downward thereby causing both the pulverized coal-air stream traversing chamber 24 and the air traversing annular duct 26 to be directed downward.

[0013] When an air stream with particles contained therein turns to change direction, the heavier entrained particles tend to pass to the outside of the turn whereas the lighter particles are less resistant to a change in direction and tend to flow with the air stream in making the change in direction. When nozzle tip 10 is tilted away from the horizontal, the coal-air stream exiting coal delivery pipe 20 and passing through chamber 24 is deflected upward or downward by nozzle tip 10. Two problems arise as a result of the deflection of the pulverized coal-air stream. Firstly, the pulverized coal to some extent stratifies as the pulverized coal-air stream is deflected. Secondly, the pulverized coal impinging on the inner surface of inner shell 12 causes erosion of the inner surface. To eliminate stratification, at least one, and usually two or three baffles, termed splitter plates, are positioned within inner shell 12 so as to divide chamber 24 into a plurality of flow passages. Each splitter plate 30 deflects a portion of the pulverized coal-air stream passing through chamber 24. Although there remains some stratification of coal within each flow passage, over all of the flow passages the distribution of pulverized coal is maintained.

[0014] Since some coal particles impinge on splitter plates 30 that would otherwise impinge on the surface of inner shell 12 when the pulverized coal-air stream passing through chamber 24 is deflected, splitter plates 30 reduce but do not eliminate the erosion of the inner surface of inner shell 12. The erosion of the inner surface of inner shell 12 is particularly acute at the top when nozzle tip 10 is tilted downward causing the pulverized coal-air stream traversing chamber 24 to be directed downward. Similarly, the erosion of the inner surface of inner shell 12 is particularly acute at the bottom when nozzle tip 10 is tilted upward causing the pulverized coal-air stream traversing chamber 24 to be directed upward.

[0015] The side walls of inner shell 12 also erode due to impingement of coal particles. Although erosion of the side walls is less acute, there erosion is enhanced when the horizontal cross-section of chamber 24 is trapazoidal with the pulverized coal-air stream passing from the longer parallel line segment of the trapazoidal cross-section across and substantially perpendicular to the parallel line segments, and exiting the trapazoidal cross-section at the shorter parallel line segment. The trapazoidal cross-section further aggrevates coal particles impinging on the inner surface of inner shell 12. When the inner shell 12 becomes too worn, the furnace must be taken off line in order to replace or repair the nozzle tips.

[0016] In accordance with the present invention, an improved nozzle tip 10 is provided having means for passing air from annular duct 26 to inside inner shell 12 to provide a coal-free layer of air along the inner surface of inner shell 12. The air passage means are located just upstream of where coal delivery pipe 20 introduces the pulverized coal-air stream into inner shell 12. As shown in Figure 1, the air passage means in a preferred embodiment comprise a plurality of openings through inner shell 12 angled in the direction of flow through both annular duct 26 and chamber 24. Air passage means 32 as shown in Figure 1 may comprise but are not limited to holes as other air passage means such as slots may be used.

[0017] In a preferred embodiment of the invention, the air passage means 32 located on a surface of inner shell 12 that supports a splitter plate are located adjacent the mounting of splitter plates 30 on the inside surface of inner shell 12. This enables the air passing through such strategically located air passage means 32 to prevent erosion at the side edges of splitter plates 30.

[0018] When the pressure in chamber 24 is greater than or equal to the pressure in annular duct 26, it is necessary to use a deflector to enhance air flow through air passage means 32. An alternate embodiment of the invention employing a deflector 34 is shown in Figure 4. The deflector may be mounted on inner shell 12 or be, as shown, the displaced portion of inner shell 12 that creates air passage means 32.

[0019] An additional alternate embodiment employing a deflector mounted on outer shell 14 is shown in Fi^pure 5.

[0020] Accordingly, the present invention provides an improved nozzle tip 10 which possesses a longer useful lifetime than prior art nozzle tips in the extremely erosive environment associated with pulverized coal firing. The improvement is characterized by air passage means 32 which provide a layer of coal-free air along the inner surface of inner shell 12 to prevent the stream of pulverized coal-air passing through chamber 24 from contacting and eroding the inner surface of inner shell 12.

[0021] While the preferred embodiment of the present invention has been illustrated and described as incorporated into a fuel-air emission assembly of the type typically employed on a tangentially-fired furnace, it is to be understood that the invention should not be limited thereto. The nozzle tip of the present invention could be readily modified by those skilled in the art to be applied within the spirit and scope of the present invention to any number of burner configurations wherein pulverized coal and other abrasive solids are combusted.

Claims

1. Apparatus for preventing erosion in a nozzle tip of the type having an open-ended inner shell for delivering a mixture of fuel and air to a furnace, an open-ended outer shell spaced from and surrounding the inner shell so as to define an annular flow passageway therebetween through which additional air is directed into the furnace, apparatus for preventing erosion of the inner surface of the inner shell comprising means for passing air from the annular flow passageway through the inner shell to within the inner shell, whereby erosion of the inner surface of the inner shell is prevented by providing a fuel-free layer of air along the inner surface of the inner shell.

2. Apparatus for preventing erosion in a nozzle tip as described in Claim 1 wherein the means for passing air from the annular flow passageway through the inner shell to within the inner shell is upstream of where the mixture of fuel and air is introduced into the inner shell.

3. Apparatus for preventing erosion in a nozzle tip as described in Claim 1 wherein the means for passing air from the annular flow passageway through the inner shell to within the inner shell comprises a plurality of holes spaced around the perimeter of the inner shell.

4. Apparatus for preventing erosion in a nozzle tip as described in Claim 1 wherein the means for passing air from the annular flow passageway through the inner shell to within the inner shell comprises holes passing through the inner shell angled in the direction of flow from the outer surface of the inner shell to the inner surface of the inner shell, adapted to pass air along the inside surface of the inner shell substantially in the direction of flow.

5. Apparatus for preventing erosion in a nozzle tip as described in Claim 1 further comprising means for deflecting a portion of the air flow passing through the annular passageway into the air passing means.

6. Apparatus for preventing erosion in a nozzle tip as described in Claim 2 further comprising means for deflecting a portion of the air flow passing through the annular passageway into the air passing means.

7. Apparatus for preventing erosion in a nozzle tip of the type having an open-ended inner shell for delivering a mixture of fuel and air to a furnace, an open-ended outer shell spaced from and surrounding the inner shell so as to define an annular flow passageway therebetween through which additional air is directed into the furnace, and a splitter plate positioned within the inner shell along a line parallel to the longitudinal axis thereof so as to divide the inner shell into a plurality of flow passages, apparatus for preventing erosion of the inner surface of the inner shell and the side edge of the splitter plate comprising means in the region where the splitter plate is mounted on the inside of the inner shell for passing air from the annular flow passageway through the inner shell to within the inner shell, whereby erosion of the inner surface of the inner shell and erosion of the splitter plate where mounted on the inner surface of the inner shell is prevented by providing a fuel-free layer of air along the inner surface of the inner shell.

8. Apparatus for preventing erosion in a nozzle tip as described in Claim 7 wherein the means for passing air from the annular flow passageway through the inner shell to within the inner shell is upstream of where the mixture of fuel and air is introduced into the inner shell.

9. Apparatus for preventing erosion in a nozzle tip as described in Claim 7 further comprising means for deflecting a portion of the air flow passing through the annular passageway into the air passing means.

10. Apparatus for preventing erosion in a nozzle tip as described in Claim 8 further comprising means for deflecting a portion of the air flow passing through the annular passageway into the air passing means.

11. A method of preventing erosion in nozzle tips of the type having an open-ended inner shell for delivering a mixture of fuel and air to a furnace, an open-ended outer shell spaced from and surrounding the inner shell so as to define an annular flow passageway therebetween through which additional air is directed into the furnace, and a splitter plate positioned within said inner shell along a line parallel to the longitudinal axis thereof so as to divide the inner shell into a plurality of flow passages, the method of preventing erosion of the inner surface of the inner shell comprising:

(a) passing an air-fuel mixture through the inner shell;

(b) dividing the air-fuel mixture passing through the inner shell into a plurality of flow streams;

(c) passing additional air into a furnace through the annular flow passageway; and

(d) diverting a portion of the air passing through the annular flow passageway through the inner shell, whereby erosion of the inner surface of the inner shell is prevented by providing a fuel-free layer of air along the inner surface of the inner shell.

Drawing