Patent Application
20230061105
This invention relates to nozzles for feeding combustion media, for example pulverized coal and air, into a furnace. The invention has particular application in nozzles for feeding flowable combustion media into tangentially fired burners of steam generating boilers. Such flowable combustion media may include, for example, pulverized coal entrained in air, other solid fuels such as biomass or refuse-derived fuels, also entrained in air, gaseous fuels, and also air by itself, for example secondary air directed into a furnace to support combustion.
Many solid fuel and coal-fired power plant boilers are designed for tangential firing, i.e., a configuration in which streams of pulverized coal or other solid fuels, along with air, are directed into a rectangular furnace compartment from columns of nozzles located in such a way as to generate a slowly rotating cyclonic fireball, which produces heat. The heat generated by the combustion of fuel boils water in arrays of water tubes lining the walls of the compartment. Tangential firing is described in a number of patents including U.S. Pat. Nos. 4,252,069, 4,634,054, 5,483,906 and 8,413,595.
The columns of nozzles include nozzle tips which protrude into the furnace, and the nozzle tips are typically pivotable about horizontal axes so that the direction of the air and fuel discharged from the nozzle tips can be adjusted to control the position of the fireball.
These nozzle tips commonly have a double shell configuration, comprising an outer shell, and an inner shell coaxially disposed within the outer shell to provide an annular space between the inner and outer shells. The inner shell is connected to a fuel feeding conduit for feeding pulverized coal or other solid fuel, entrained in air flowing through the inner shell, into the furnace. The annular space between the inner and outer shells is connected to a secondary air conduit for feeding secondary air into the furnace. The secondary air not only serves as supplemental combustion air, but also cools the inner and outer shells. The fuel feeding pipe is typically disposed coaxially within in the secondary air conduit. In furnaces fueled by biomass and other solid fuels such as refuse-derived fuels, the fuel nozzles are similar to these coal nozzles.
A furnace will typically have not only several coal nozzle tips at each corner of the rectangular furnace compartment, but also several air nozzle tips, arranged in a column along with the coal nozzle tips, to introduce additional secondary air into the furnace.
The inner shell of the nozzle tip includes splitter plates which divide the interior of the inner shell into plural flow passages for the flow of coal particles, or other solid fuel particles, along with air. The splitter plates control the direction of the stream of coal and air being discharged through the inner shell of the nozzle tip to ensure a uniform distribution of the stream of coal and air, particularly when the nozzle tip is tilted upward or downward.
These nozzle tips are exposed to high temperatures due both to radiation from the fireball and to hot gases circulating in the furnace. In addition, they are exposed to coal particles, and, especially in the case of high-sulfur coal, they are exposed to ash, composed primarily of iron oxide particles which tend to adhere to parts of the nozzle tips and particularly to the exit edges of the splitter plates. Due to continuous exposure to very high temperatures and to the deposition of coal and ash, metallurgical creep occurs and results in distortion of the splitter plates and other parts of the nozzles. Continued distortion eventually causes metal and weld failures, which affect the aerodynamics at the nozzle outlet, lead to more ash deposition, plugging of the nozzle tip outlet area, and premature structural and configurational failure of the nozzle.
Distortion of the splitter plates has been addressed in the past by welding stiffener bars to the exit edges of the splitter plates. The stiffener bars strengthen the plates, shield them from direct radiation, reduce distortion, and prolong the useful life of a nozzle. However, the mixture of coal and air tends to recirculate into low pressure zones as it passes over the stiffener bars, and, especially in the case where high sulfur coal is being utilized, this recirculation can cause flame attachment, and can also cause ash to adhere to the stiffener bars and adjacent parts of the splitter plates. The adhering ash then becomes a heat sink that causes the associated metal parts to overheat, weaken and become distorted.
The primary object of this invention is to strengthen the furnace ends of nozzle tips, to shield them from hot furnace gases and radiation, and to extend their service life. Briefly, the invention resides in a reinforcing stiffener at the exit edge of a splitter plate in a coal nozzle, shaped so that the aerodynamic flow of air and entrained coal particles past the exit edge of the splitter plates exhibits reduced recirculation, and thereby avoids flame attachment and significant accumulation of coal ash on the exit ends of the splitter plates.
The preferred stiffener is in the form of a smoothly curved cylinder secured to, and extending along, the exit edge of a splitter plate. The cylinder may be solid. Alternatively, it may be hollow, and, if hollow, it can be provided with openings at one or both of its ends to receive air, and obliquely directed exit openings for the flow of air from the interior of the tube to the exterior thereof for cooling and improved high temperature creep resistance.
The aerodynamic feature is of primary advantage when firing high sulfur coals (i.e. coals containing more than 1% sulfur) to prevent or reduce coal ash (iron oxide) adhesion onto exposed splitter plate surfaces. Splitter plates with the cylindrical reinforcement can also be used with lower sulfur coals and other fuels, and in nozzles for introducing other combustion media. The cylindrical reinforcement is applicable not only to nozzles with horizontal splitter plates, but also to nozzles with vertical splitter plates.
More particularly, the invention is a nozzle for feeding a flowable combustion medium i.e., a combustible medium such as particles of coal or biomass, entrained in air (usually referred to as “primary air”), or air alone (usually referred to as “secondary air,” into a furnace.
The nozzle comprises a nozzle tip for directing flow of the combustion medium into the combustion chamber of a furnace. The nozzle tip includes a shell having an inlet for receiving the combustion medium, an outlet for directing the combustion medium into the combustion chamber, and an interior space located between the inlet and the outlet.
The nozzle further comprises a splitter located within the shell. The splitter comprises one or more splitter plates, each extending in a forward direction, i.e., away from the inlet and toward said outlet. The splitter plates divide the interior space of the shell into two or more channels, each allowing for the flow of part of the combustion medium from the inlet to the outlet.
Each splitter plate has a planar first surface, an opposite planar second surface, and a downstream edge extending across the shell adjacent the outlet of the shell.
The nozzle further comprises a stiffener extending along the downstream edge of each splitter plate. The external cross-section of each stiffener, transverse to the direction in which the stiffener extends along the downstream edge of its splitter plate, is in the form of a continuous curve proceeding outward and forward from the first surface to a first location, inward from the first location to a second location beyond the plane of the second surface of the splitter plate, and inward and rearward from the second location to the second surface of the splitter plate. The shape of the stiffener exerts an aerodynamic effect which results in a reduction of recirculation of the combustion medium flowing past the stiffener, reduces deposition of ash. The aerodynamic effect significantly extends the useful life of the nozzle.
The external cross-section of the stiffener has a cylindrical shape, i.e., the cross sections throughout most of its length are uniform. The part of the external cross-section of the stiffener that proceeds from the first location to the second location should be convex and preferably, the cross-sectional shape of the stiffener is circular or elliptical, except at the locations of welds that secure the stiffener to the exit edge of the splitter plate.
The stiffener can have a hollow interior, openings at one of both of its ends for receiving secondary air from one or more channels in the nozzle tip, and plural openings distributed along the length of the stiffener for releasing the secondary air from the interior of the stiffener for the purpose of cooling. These plural openings preferably extend from the hollow interior to the exterior of stiffener in two groups. A first group is positioned to direct air from the interior of the stiffener to the exterior thereof in a direction forward and outward from the first surface of the splitter plate, and a second group of is positioned to direct air from the interior of the stiffener to the exterior thereof in direction forward and outward from the second surface.
The aerodynamic effect of the improved stiffener in accordance with the invention, along with its reinforcement of the splitter plates, results in a significant reduction in thermal distortion and warping, and extend the useful life of the nozzle. When the stiffener is hollow, and provided with openings for the flow of cooling air outward from its interior into the path of flow of the combustion medium, still further improvements in the useful life of the nozzle can be achieved.
The nozzle tip 10 in
The outer shell is typically, but not necessarily, tapered, and is composed of two vertical side walls 22 and 24, and upper and lower walls 26 and 28, respectively.
An array 30 of holes is provided in the upper wall 26 of the nozzle tip, and a similar array 32 of holes is provided in the lower wall 28. The arrays are located adjacent the front opening of the outer shell and extend rearward to an intermediate location between the front and rear openings of the outer shell. The holes in these arrays allow flow of secondary air from the space between the inner and outer shells, through the outer shell, to the outer surface of the outer shell. Air passes through the holes from the interior of the nozzle tip to the exterior, reducing the temperature difference between the inner and outer surfaces, thereby reducing thermal distortion and resulting damage. When the nozzle is tilted, the flow of air through the holes in the wall facing the flame increases so that a greater cooling effect is achieved at the parts of the nozzle tip having the greater exposure to radiant heat. The flow of air through the arrays of holes washes the exposed outer surface of the nozzle tip with cool air in a film or boundary layer. The air flow also reduces direct contact between the flame and the nozzle tip. Details of the arrays of holes and their function are explained in U.S. Pat. No. 8,413,595, granted on Apr. 9, 2013. The disclosure of U.S. Pat. No. 8,413,595 is here incorporated by reference.
The nozzle tip includes an outer shroud 34 forming channels 36, bounded by the outer shroud, the upper wall 26 of the nozzle tip, and shroud-supporting partitions 38. A similar shroud structure is provided on the bottom side of the nozzle tip. The channels 36 direct secondary air along the outer surface of the upper wall 26 of the nozzle tip, and similar channels (not shown) direct air along the outer surface of the lower wall 28. Cooling is achieved by flow of air though the arrays of holes and by the flow of secondary air flow through the shrouds.
The upper and lower shrouds are convex so that the gap between the nozzle tip and the nozzle (not shown) in which it fits remains substantially the same regardless of the angle of tilt.
As shown in
As will be apparent from
Arrows 60 in
As shown in
In summary, the continuous curvature of the exteriors of the stiffeners in cross-section allows the stiffeners to strengthen the exits end of the splitter plates without creating conditions that promote ash adhesion, and the flow of air from the interior of the stiffeners through their openings promotes cooling and reduces high temperature creep.
