Furnaces – Including fluid fuel burner – Powdered solid fuel
Reexamination Certificate
1999-09-13
2001-07-17
Ferensic, Denise L. (Department: 3749)
Furnaces
Including fluid fuel burner
Powdered solid fuel
C110S265000, C239S418000, C239S423000, C239S424000
Reexamination Certificate
active
06260491
ABSTRACT:
The present invention relates to nozzles feeding combustion providing medium into furnaces. The present invention thereby typically, but not exclusively, relates to pulverized coal feeding nozzles and secondary air nozzles in tangentially fired burners in steam generation boilers. Tangential firing is described in U.S. Pat. No. 4,252,069, U.S. Pat. No. 4,634,054 and U.S. Pat. No. 5,483,906.
Pulverized coal feeding burners typically have pivotably arranged coal nozzle tips protruding into the furnace. The coal nozzle tips have a double shell configuration, comprising an outer shell and an inner shall. The inner shell is 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 or pipe, for feeding pulverized coal entrained in an air flow through the inner shell into the furnace. The annular space is connected to a secondary air conduit for feeding secondary air through said channel into the furnace. The secondary air is meant to provide combustion air and cool the outer shell. The fuel feeding pipe is typically disposed axially in the secondary air conduit.
The nozzle tip is located in an opening in a nozzle supporting wall, typically in the outlet of the secondary air box. The external cross section of the nozzle tip is typically rectangular and mainly corresponds to the internal cross section of the outlet end of the air conduit. Narrow gaps typically remain between the peripheral walls of the nozzle tip and the walls of the air conduit. Secondary air is allowed to leak through the narrow gaps. The air typically flows horizontally into the furnace. When the nozzle tips are arranged to discharge fuel and air horizontally into the furnace, the air leaking through the gaps will flow mainly in the direction of the external walls of the nozzle tips and thus protects its wall plates from furnace radiation heat.
The coal nozzle tip is typically pivotably connected to the fuel feeding pipe, in order to be able to control the level of the fire ball in tangential firing. Thus, when the nozzle tip is tilted to provide an upward or downward flow of fuel and air into the furnace, one of its walls will be bent away from the air flow leaking out and thus be more or less unprotected.
Fuel, as well as secondary air nozzle tips of tangential fired boiler units are exposed to severe furnace conditions that can lead to thermal distortion and/or high temperature oxidation. This problem requires operators to annually replace many of their coal and air nozzle tips at a fairly high cost. Especially on tangentially fired boiler units, the conditions of the nozzle tips play a key role in sustaining long term optimal combustion performance.
It has been noticed that the cooling air flow flowing within the nozzle tip of fuel or air feeding nozzles cannot at certain high temperature conditions provide a sufficient cooling of the external walls of the nozzle tips. Thus, the external wall plates may be heavily damaged, leading to the above mentioned problems.
Exposure to direct radiation, particularly when nozzle tips are up- or downward tilted induces thermal gradients through thick stainless steel plates, ¼ to ¾ inch thick. The thermal gradient causes distortion and eventually closure of the passages in the nozzles, leading to performance degradation. Exposure to high radiation also results in operating temperatures exceeding material limits and eventual oxidation and thinning effect of the plate resulting in “burnback” and eventual performance degradation.
It is an object of the present invention to provide an improved nozzle with which the above problems may be avoided or at least minimized.
It is particularly an object of the present invention to provide a nozzle the external walls of which are well protected from heat radiation.
The objects of the present invention are achieved by nozzles comprising the characterizing features mentioned by the appended claims.
The present invention provides a nozzle, for feeding combustion maintaining medium into a furnace at high temperature conditions. A nozzle according to the present invention includes, according to a preferred embodiment, a nozzle tip and fuel and/or air feeding means.
The nozzle tips may be pivotably mounted, e.g., to fuel feeding pipes, air feeding boxes, such as windboxes, furnace wall constructions or any other suitable conveniently located constructions. The nozzle tips are disposed so as to protrude at least partly into the furnace. Typically, several nozzles may be disposed one on top of the other and be connected to a vertical box mounted to the furnace wall, preferably in a corner area thereof.
A combustion maintaining medium, such as pulverized coal and air, may be fed through the feeding means and the nozzle tips into the furnace. Typically, pulverized coal is fed as a mixture with transport air. Secondary air may be fed separately from the coal. The nozzles may be used to feed other suitable fuels and gases, as well.
The nozzle tip according to a preferred embodiment of the present invention typically includes a mainly open-ended outer shell and a shroud means covering a portion of the outer shell. At the first end of the outer shell the passage inside the outer shell is in flow connection with the air feeding means. The other end of the outer shell typically protrudes into the furnace. The outer shell typically is of a square or rectangular cross section, having rounded corners.
The shroud means is typically made of a shroud plate which is disposed to cover a portion of the first end of the outer shell. A gas space is formed between the shroud plate and the covered portion of the outer shell. Shroud air, such as secondary air, is led through the gas space and discharged along the uncovered surface of the outer shell, thus providing protection against radiation heat to the outer shell. The shroud, i.e., the plate work thereof, may be recessed, to form a bulbous shape and, therefore, be self protected from much of the radiation. Some leak air will also flow rather close to the first end of the shroud even if the nozzle tip is tilted. The leak air only later deviates from the nozzle tip and thus, the leak air also provides some protection close to the windbox.
Shroud plates are typically mounted to cover a portion of the upper and bottom sides of the outer shell. The shroud plate may be formed to guide the shroud air in a desired direction and to provide the desired form of shroud air flow. The shroud channels or directs cooling air along the outer shell, outer plate work, of the coal of air nozzle tips, thereby providing additional cooling to those sections more exposed to radiation.
The nozzle tips further include an air cooling zone formed peripherally on the interior side of at least a portion of the outer shell. An air flow is maintained along the interior side of the outer shell in the air cooling zone.
The nozzles according to the present invention are especially suitable for feeding fuel and air into tangentially fired furnaces, as the nozzles tips may be pivotably mounted, so as to allow the direction of the flow from the nozzles to be changed. The flow may be directed upward or downward in order to control the combustion process in the furnace. Nozzle tips may be tilted either up or down typically ±30°. The present invention maintains an air shroud and cooling along the outer shell surface even in extreme tilted positions.
The shroud means suggested by the present invention may be used to protect air nozzles from radiation in furnaces, as well. Then, the air flowing through the nozzle provides the interior cooling of the outer shell and an additional air flow guided by the shroud means provides the outer protection of the nozzle tip.
The present invention provides effective radiation heat protection. High velocity jets, 85 ft/sec to 250 ft/sec, of air are strategically directed from specifically designed channels and blanket the nozzle tip with cooling air. The air shroud provides added cooling of the no
Ferensic Denise L.
Fitzpatrick ,Cella, Harper & Scinto
Foster Wheeler Corporation
Rinehart K. B.
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