Patent Application
20090068601
The present invention relates to warming up large utility-type furnaces, such as are used, for example, to generate steam for major electrical power generating plants, particularly but not limited to coal-fired plants, prior to the start-up of such furnaces to commence their production phase of operation.
Such large furnaces can have the size of a large building, and they often employ dozens of spaced-apart burners to provide the needed heat for generating large amounts of electrical power. These furnaces are fired with all types of fuels, including, but not limited to, oil, gas, coal, bio-mass, etc. The furnaces require an initial heat-up to bring their interior to the required operating temperature at which all burners can be fired fully without causing flameouts, generating large amounts of smoke and other pollutants, potentially damaging portions of the furnace due to excessive heat differentials, and the like.
Operators of such furnaces desire that the warm-up period is as short as possible because during warm-up phase, expensive fuel is consumed without generating any power. In the past, so-called warm-up guns were preferred over smaller pilot burners, especially for coal-fired furnaces, to generate sufficient amounts of heat over a relatively short period of time so that the production phase of the furnace can commence as soon as possible.
While natural gas and oil can be quickly ignited and do not require long warm-up periods, coal-fired furnaces encounter the problem of having to heat the furnace sufficiently so that the large mass of coal consumed by the furnace during its production phase can be ignited and will burn cleanly and completely without emitting excessive pollutants. Conventional burner pilot lights that are also used for the main burner ignition and flame stabilization either have too low a heat output to accomplish the required warm-up over a desired period of time, or require an additional air supply besides the air passing through the main burners, which makes their installation more complicated and expensive.
It is not normally feasible to increase the size of the pilot burners to enhance their heat output because pilots are arranged in the relatively small annular passage between the main burner and the surrounding combustion air duct. This limits the size of pilots, for most industrial installations, to no more than about four to five inches in diameter. With such size limitations, the maximum heat output of pilots not using additional air supply is typically limited to around 3 to 7 million BTUs per hour.
Increasing the fuel flow rate through the pilots beyond that range results first in unstable operation sensitive to the regime of air flow through the main burner. The operation becomes unsatisfactory because the resulting high velocity fuel jets can snuff out the pilot flame.
Further, the high capacity pilots need to be located at the main burner discharge end to generate the flame in the furnace interior and prevent the flame from burning the main burner. At the same time the pilots need to be protected from the main burner flame. In order to meet both these requirements, such installations often require complicated mechanisms to subsequently retract the pilot rearwardly out of the heat and away from fuel particles into the combustion air duct, which are costly, require much maintenance, and are subject to early failures.
Separate warm-up guns not tasked with the main burner ignition were therefore widely employed for warming up furnaces. Although such guns are capable of generating large amounts of heat and, therefore, can significantly shorten the warm-up period for even coal-fired utility-type furnaces, they require their own combustion air supplies as well as relatively complicated installations including their own piping, fans, motors, controls, gun retracting mechanism and the like, all of which make separate warm-up guns expensive to install and maintain.
The present invention improves the manner in which large furnaces, such as are used for commercial power generation, and particularly coal-fired furnaces, are preheated during the initial start-up phase of furnace operation when the interior furnace temperature must be raised sufficiently to commence the production firing of the furnace. This is accomplished in accordance with the present invention by placing a pilot burner (hereafter typically “pilot”) with a much higher heat output in the same limited annular passage between the fuel, e.g. coal supply conduit, and the surrounding combustion air supply duct, where prior art burners have been commonly placed in the past. The pilot of the present invention is operated with combustion air for the main burner and eliminates the need for a separate air supply for the pilot. The pilot flame is ignited and stabilized by injecting a portion of gaseous fuel delivered to the pilot in a spinning pattern that creates intense recirculation and mixing of the discharged pilot fuel with appropriate amounts of air passing through the main burner around the pilot.
Such pilots can provide a heat output in the range between about 4 to 50 million BTU per hour, which is much higher than the heat output that could be achieved with prior art pilots operating without the additional air, and assures a rapid heat-up of the furnace and a relative quick start-up of its production phase. Substantial amounts of fuel otherwise used by the pilot without producing useable steam or electricity are thereby saved.
The present invention provides both a method and an apparatus for preheating furnaces, particularly large utility-type furnaces that have many burners which often operate with difficult-to-ignite coal during the warm-up phase of furnace operation. Generally speaking, this involves a main production burner that includes a first conduit for directing a fuel, for example coal, into an interior of a furnace. An air duct surrounds the coal conduit to define an annular combustion air passage into the furnace where the coal and combustion air are mixed and ignited during the production phase of furnace operation.
A pilot nozzle is positioned in the air passage of the main burner so that a downstream end of the nozzle is proximate the downstream end of the burner. The nozzle is surrounded by a tubular hood which has an open downstream end proximate the downstream end of the nozzle and an upstream end. Air flowing through the main burner passage for air is prevented from directly entering the hood by placing a flow inhibitor, such as a plate, over the upstream end of the hood while permitting air to enter the hood via a gap formed between the hood and the plate.
A relatively lesser portion of a pressurized fluid fuel, e.g. a gas, is flowed through igniter orifices in the nozzle located inside the tubular hood at a rate commensurate with the amount that can be burned within the limited space inside the tubular hood. The fuel gas jets inside the hood are directed at angles that facilitate entrainment of air through the hood. A major portion of the pilot gas is discharged from main pilot orifices—a plurality of spaced-apart pilot orifices in a downstream end portion of the nozzle immediately adjacent to the hood. The fuel jets from the main pilot orifices are oriented so that the emitted fuel jets angularly diverge in the downstream direction and have a tangential flow component relative to the longitudinal axis of the pilot nozzle. As fuel jets discharging from the main pilot orifices pass in the vicinity of the hood downstream end, they also facilitate the flow of air through the hood.
Fuel emitted by the igniter orifices inside the tubular hood is mixed with air passing through the hood and is ignited to generate an igniter flame that propagates past the downstream end of the hood. The igniter flame in turn ignites the mixture of fuel from the main pilot orifices and air passing through the main burner to generate a pilot flame that extends into and heats the furnace interior. Portions of the pilot flame and its constituent gases recirculate from the furnace interior rearwardly towards the nozzle while the flame as a whole spins relative to the nozzle axis to maintain a stable pilot flame.
One reason why placing high heat output pilot burners inside the combustion air duct of the main burner has heretofore been unsuccessful was that the volume of air flowing through the duct may vary substantially so that the pilot fuel flowing with a fixed rate often fails to ignite, or to maintain the flame, due to unfavorable fuel-to-air ratios, unless the pilot has its own air supply and controls. This is overcome by the present invention because the amount of air entering the hood is substantially proportional to the fuel delivered inside the hood through the igniter orifices and only to a small degree affected by the amount of flow through the duct as its upstream flow inhibitor effectively shields the pilot fuel from the effect of the high velocity air flowing through the duct. The hood forms a small combustion chamber where a relatively minor amount of the pilot fuel is initially ignited to form the igniter flame which propagates in a downstream direction past the downstream end of the hood, where the major portion of the pilot fuel is discharged via the appropriately positioned and oriented pilot orifices.
The hood, including the earlier mentioned flow diverter, also effectively shields sensitive components like the spark electrode inside the hood from the heat of the main pilot flame and the furnace, which allows operating the burner without having to retract the pilot into the burner.
In addition, to maintain a pilot flame, it must be stable and remain anchored to the nozzle. High heat output pilots require high fuel velocities through the burner orifices of as much as 500-1500 ft./sec. Such high fuel jet velocities lead to undesirable flame instabilities which are significantly reduced or entirely eliminated in accordance with the present invention by imparting a spin to the pilot flame downstream of the nozzle that facilitates establishing a recirculating flow downstream of the nozzle. To attain such a spin, the axes of the pilot orifices are tangentially offset relative to the pilot axis as described in more detail below. The tangential flow component of the jets provides the spinning results obtained with common prior art burners by placing relatively large spinners around the nozzles that cannot be applied here due to the earlier mentioned space limitations.
Another important advantage of the present invention is that the amount of air entering the interior of the hood automatically adjusts itself to the amount of fuel emitted by the igniter orifices inside the hood because as the volume of emitted fuel varies, its speed varies correspondingly, which in turn lowers or raises the fuel pressure inside the hood inversely to the velocity of the fuel emitted from the igniter orifices. With the lowered pressure, more air from the air duct is aspirated into the hood interior so that an approximate stoichiometric balance between the fuel and the air in the hood is maintained. This assures an uninterrupted igniter flame to maintain the main pilot flame even in the event of a temporary flameout. The amount of air drawn into the hood is correspondingly lowered as less fuel is emitted from the igniter orifices of the nozzle and the pressure inside the hood rises correspondingly.
Thus, the pilot burner of the present invention is relatively inexpensive because it has no moving parts and needs no internal or external controls.
A further advantage attained with the present invention is that the pilot burner is shielded from the high temperature and abrasive/corrosive/contaminating influences of the gases, dust and particles on the furnace interior because the pilot is located inside the air duct, which reduces maintenance costs and prolongs the life of the burner. Still further, since the pilot burner of the present invention requires no external controls, separate air supply lines and the like, it can be made relatively larger in the limited space available in the air ducts of industrial burners. This in turn makes it possible to increase the heat output of the burner and thereby shorten the warm-up period for the furnace, all of which reduces operating costs for the furnace warm-up and pilot burner maintenance.
The construction and operation of such main burners is well known to those of ordinary skill in the art and, therefore, is not further described herein.
Burner installation 4 includes a pilot burner 36 constructed in accordance with the present invention to initiate combustion in the furnace interior and, during a start-up phase of operation of the furnace, to warm up the furnace interior until main burners 16 can be fired after the furnace interior has reached the required temperature for maintaining a stable and complete combustion of the coal (or other fuel). The pilot has a feed tube 38 through which a fluid fuel, such as natural gas for example, is supplied from an appropriate source (not shown) to a pilot nozzle 40. The nozzle is surrounded by a tubular shield or hood 42, the ends of which are open, and an igniter, e.g. an electrical spark igniter 44, is provided for igniting the fuel, as is further described below.
Tubular hood 42 has open upstream and downstream ends 66, 68, respectively. A flow straightener and conditioner 70 (shown also in
Pilot 36 is further fitted with a damper plate 84 (also shown in
The axial position of damper plate 84 relative to the upstream end of the hood can be adjusted by moving the plate along fuel supply tube 38 of the pilot burner to vary the width of a gap 90 between the upstream end of the hood and the damper plate to accommodate specific characteristics of the fuel and provide a range of air flows through the burner 32.
The downstream end of igniter support pipe 76 ends at a bluff body 92 (
In a presently preferred embodiment of the invention, pilot nozzle 40 is configured as a cap attached to the downstream end of fuel feed tube 38 and has a multiplicity of fuel discharge orifices 52 arranged in a plurality of, e.g. two, rows 52A, 52B that are spaced apart in the axial direction of the nozzle, as illustrated in
In addition, each orifice 52 is arranged so that its center line 98 is offset relative to a radius line 100 with its origin at the center 96 of the nozzle so that each orifice is also tangential relative to this center, as is illustrated in
In a presently preferred embodiment, the pilot nozzle 40 additionally includes relatively small-diameter center holes 102. In use, gas flows through the center holes which cools the nozzle center.
Referring to
Turning to the operation of pilot 36 for starting up a cool furnace, combustion air flows through annular passage 34 of burner 32 in a downstream direction past tubular hood 42 and then into the furnace interior 8. The gas for the pilot is flowed through feed tube 38 to orifices 46 and pilot nozzle 40. Sizing of the orifices 46 is such that a relatively minor portion of the fuel exits through igniter orifices 46 in the feed tube 38 which are oriented to direct resulting fuel jets into the enlarged space 104 inside the hood and in the vicinity of igniter 44. At the same time, air from annular passage 34 of the main burner enters the interior of hood 42 via gap 90 between the upstream end of the hood and damper plate 84. Flow straightener 70 straightens out the incoming air so that it flows generally in the direction of the pilot axis and becomes mixed with fuel from igniter orifices 46. The resulting mixture is ignited by spark igniter 94 to form an igniter flame 47 in the enlarged space 104 which propagates in a downstream direction past downstream end 68 of the hood, as is illustrated in
The bulk of the fuel for preheating the furnace is ejected through orifices 52 in nozzle 40 as gas jets 53 which diverge outwardly in the downstream direction so that the ejected fuel becomes mixed with combustion air that flows through the annular passage 34 of the main burner. This mixture is ignited by the igniter flame 47 exiting from the downstream end of the hood which maintains the main pilot flame 54.
The amount of combustion air typically flowing through the annular passage 34 depends on the operational needs of the regime and is substantially independent of the pilot burner operation. The rate at which fuel is needed for the pilot also may be changed for operational reasons. To maintain the igniter flame 47, the amount of air fed to the burner must reflect the amount of fuel ejected by the igniter orifices to maintain an overall flammable mixture inside the hood 42 on the downstream part of bluff body 92.
To properly control the flow of air into hood 42, damper plate 84 blocks combustion air flowing through annular passage 34 directly into the hood. Instead, combustion air must first flow from the annular passage in a radial direction (relative to hood 42) through gap 90 and is then redirected past flow straightener 70 into the interior of the hood, thus minimizing the effects of air flow velocity through the passage 34 onto the amount of air flow entering the hood 42. The axial position of damper plate 84 relative to the upstream hood end can be adjusted by moving the plate, including its flange 86, along feed tube 38 to set the proper width for gap 90 to permit a sufficient air flow into the hood while preventing variations in the combustion air flow in the annular passage from materially affecting the air flow rate through the hood.
In use, the position of the damper plate is not normally changed. The air intake via gap 90 into the hood is nevertheless automatically varied as a function of the gas flow rate through igniter orifices 46 because as the gas velocity through the igniter orifices increases or decreases, the pressure inside the hood changes inversely to the pressure changes. An increase in the gas velocity through the igniter orifices lowers the pressure in the hood, which causes an increase in the air flow rate through gap 90 into the hood and vice versa. This air flow variation occurs automatically and requires no controls of any type.
Accordingly, the pilot burner of the present invention is self-regulating and maintains the igniter and pilot flames 47, 54 regardless of changes in the combustion air flow rate while stabilizing the pilot flame 54 and anchoring it to the end of the pilot burner. This assures a continuing, uninterrupted, self-regulating operation of the pilot burner to fully heat up the furnace as quickly as possible.
It is typically preferred to maintain the igniter flame 47 inside hood 42 for the duration of the pilot burner operation so that in the event the main flame generated by the pilot becomes extinguished, it is immediately reignited by the pilot flame.
In order to assure reliable flame propagation from the flame 47 through the high velocity fuel jets 53, flammable mixtures in substantial parts of the flow immediately adjacent to the nozzle 40 have to be achieved and maintained over the duration necessary to ignite the fuel. This is accomplished by placing orifices 52 about the circumference of the nozzle 40 in two or more staggered rows axially spaced from each other and by the tangential positioning of the orifices spinning off fuel emitted from pilot orifices 52. In each row, the orifices are typically spaced by about one to three times the diameter of the orifices. In a presently preferred embodiment, the spacing between the orifices is approximately twice the nozzle diameter.
Propagation of the flame through gas jets 53 is not sufficient for the flame 54 stabilization. Flow recirculation 58 enhanced by the spinning of fuel emitted from pilot orifices 52 caused by the tangential positioning of the orifices makes the pilot operation efficient and reliable.
As is well known to those skilled in the art, a tangential component imparted to fuel jets to form a forward-directed spiral motion facilitates the formation of gaseous recirculation patterns. The greater the spiral effect, the better the recirculation. The recirculation component of the gas is a function of the so-called “swirl number” S according to the following formula:
For certain fuels, such as oil, for example, pilot nozzle 40 can extend past the downstream end of main burner 4 into burner throat 26. However, for coal-fired burners, the pilot is recessed into the annular space 34 between coal supply conduit 20 and combustion air conduit 32 to keep the pilot away from the heat, smoke, dust, particulates and the like that are typically present on the interior of coal-fired furnaces, but which are kept out of annular passage 34 and therefore also away from the pilot nozzle by the flow of combustion air.
The combustion of fuel from pilot 36 is continued until the furnace interior has reached the desired temperature, at which time the production fuel, e.g. coal, can be ignited and stably combusted without generating large amounts of pollutants as would occur if combustion were commenced before the required furnace temperature has been reached.
This application claims the priority of Provisional Patent Application No. 60/967,915 filed on Sep. 6, 2007, the disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60967915 | Sep 2007 | US |
