US2840053A - Combustion zone control - Google Patents

Description

June 24, 1958 v. z. CARACRISTI COMBUSTION ZONE CONTROL Fiied Jan! 16, 1953 2 Sheets-Sheet 1 Steam to Turbine R o T N E V m r IH. 2H 4 9 a F w 2 u l m m m m 2 mmmm mwwm p O U L Virginius Z. Coracristi WM ATTORNEIY 2 Sheets-Sheet 2 Filed Jan. 16, 1953 Upper Firing Olrcle Intermediate Firing Circle Lower Firing Circle Fig.

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coMnUsTIoN zoNE CONTROL Virginius Z. Caracristi, Bronxville, N. Y., assignor to Combustion Engineering, Inc., New York, N. Y., a corporation of Delaware Application January 16, 1953, Serial No. 331,605

' 2 Claims. Cl. 122478) This invention relates to an improvement in the regulation of the temperature of the steam leaving the superheater of a steam generator. It is particularly applicable. to a superheater so located in a boiler that substantially all the heating effect is by convection from the products of combustion leaving the furnace whose walls are lined with water coo-led surfaces and in which turbulent fuel firing is employed.

It is well known that in the operation of such a steam generating unit the temperature of the superheated steam varies with fluctuations in steam output; a reduction in superheated steam temperature occurring with a reduction in rating. Because it is undesirable to furnish steam of different temperature to the turbine various means have been devised to correct for steam temperature fiuctuations.

One method of regulating the temperature of the superheated steam is to cause variation in heat absorption within the furnace from the fuel burned therein so that the gases passing over the superheated surface are at a temperature capable of producing the desired degree of superheat. A method of this nature is disclosed in the patent to Kreisinger et al., No. 2,363,875 of November 28, 1944.

This method provides for directing streams of fuel and air tangentially towards the periphery of an imaginary cylinder generally co-axial with the furnace chamber, and for tilting the streams of fuel and air upwardly towards the furnace chamber outlet or downwardly away from the furnace chamber outlet. A variation in heat absorption by the furnace heating surface is thereby obtained. 4 This results in a higher or lower temperature of the gases leaving the furnace and consequently a higher or lower superheat depending on whether the burners are tilted upwardly or downwardly. At any given tilting position the streams of fuel and air issuing from the burner D nozzles are tangentially directed towards an imaginary horizontal firing circle the diameter of which is held constant.

In theroperation of steam generators of this type it has been found that with burners as presently designed, a tilt downwardly from the horizontal will not cause as much variation in gas temperature as a tilt of equal angularity when directed upwardly from the horizontal. Consequently the controlled superheated steam temperature is eifected considerably more by tilting the burners upwardly than when tilting the burners downwardly an equal angular degree.

It is an important object of this invention to overcome the above disadvantage and to make the tilting of tangentially fired burners downwardly equally as effective as the tilting of these burners upwardly.

Furthermore, I have found that the effect of burner tilt upon the temperature of the gases and consequently upon the temperature of the superheated steam can be increased by a reduction in firing circle diameter when States Patent 2 tilting upwardly, and by an increase in firing circle diameter when tilting downwardly.

Accordingly another important object of my invention is to increase the steam temperature control range considerably by an appropriate and gradual change in the diameter of the firing circle from a relatively small diameter at the maximum upward tilt position to a relatively large diameter circle at the maximum downward tilt position of the burners.

Additional objects and advantages will become apparent from the following description of an illustrative embodiment of the invention when read in conjunction with the accompanying drawings wherein:

Figure l is a schematic side view of a steam generator illustrating a preferred application of the invention.

Figure 2 is a horizontal schematic plan view taken on line 22 of Fig. 1.

Figure 3 is a simplified elevational view of a burner unit taken on line 3-3 of Fig. 2 showing one application of my invention to a set of burners tilted with respect to the vertical axis of the furnace chamber.

Figure 4 is a set of curves which illustrate graphically how the application of my invention extends the steam temperature control range.

In Figure 1 the boiler illustrated includes a steam and water drum 10. This drum supplies water through the downcomers 12 to the lower water wall headers 14, 16 and 18 to which are connected the lower ends of water cooled tubes 20 forming the walls and roof of the furnace chamber 22. The upper ends of some of the aforesaid water cooled furnace tubes 20 are connecteddirectly to steam and water drum 10. Others terminate in upper water wall header 24 which in turncommum'c'ates with drum 10 through connecting tubes 26. In the lower portion of the furnace chamber tangentially firing burners 28 are arranged in each corner such as shown in Fig. 2. In the illustrative embodiment shown each of these burners is equipped with three fuel and air ports 28a, 28b, and 28c. Fuel and air is discharged through each of these ports into the furnace in a direction along a line which is tangential to the periphery of an imaginary firing circle. The combustion gases follow a spiral flow path upwardly towards the furnace outlet 29, while giving off a substantial portion of the heat contained in the gases to the water wall tubes 20 for steam generation. In leaving the furnace the gases enter a generally horizontal gas pass 30 in which are suspended a high temperature superheater section 31 and a low temperature superheater section 32. A vertical gas pass 34 extends downwardly from the rear portion of said horizontal passage 30. Additional heat absorbing surfaces such as economizer 36 or air heater surface (not shown) may be arranged in gas pass 34 through which the gases flow on their way to a stack (not shown).

The steam generated in water wall tubes 20 enters drum 10 together with a certain amount of recirculated water. The steam is separated from the water by suitable apparatus located within the steam and water drum 10 and the separated dry saturated steam enters superheater inlet header 38 via superheater connecting tubes 40. In passing through low temperature section 32 and high temperature section 31 heat is applied to the saturated steam and its temperature raised to a desire-d degree of superheat. The superheated steam thereupon leaves high temperature section 31 through outlet header 42 and is conveyed to a steam turbine (not shown) by steam pipe 44.

In operation it frequently becomes necessary to change the steam capacity of the unit. The superheated steam temperature generally varies with the amount of steam generated by the unit. Other operating conditions such as slagging-over of furnace heating surface may also affeet the superheated steam temperature. .In orderto operate the turbine at maximum efficiency it is most desirable to supply the turbine with superheated steam of a constant temperature regardless of-steam load' or other varying operating conditions. :As' pointed out earlier herein one method of controlling and maintaining a constant superheated steam temperature regardless of variations in steam generator capacity, providesfor changing the heat absorbed by the-water walls by altering the location of the combustion zone within the furnace. The temperature of the gases leaving the furnace is thereby controlled and accordingly the degree ofsupcrheat.

. Although this method when applied to a modern steam generator generally solves the problem caused by variations in steam temperature with change in steam load it has been found that the effect of temperature change when tilting the burners upwardly a given angular degree far exceeds that when tilting the burners downwardly an equal angular degree when firing tangentially towards a firing circle of relatively small diameter. In other words, under. these conditions a downward tilt is considerably less effective in changing the superheated steam temperature than an upward tilt of equal angularity. l

In attempting to overcome the above difficulties it wasfound that a substantial equality in effectiveness of temperature change may be obtained by increasing 'the diameter of the firing circle. However, a portion of the steam temperature control range must thereby be sacri ficed. A still further increase in firing circle diameter revealed that a downward tilt under these conditions may become more effective in changing the steam temperaturethgin an upward tilt of equal angularity.

To overcome the above difficulty I have made use of the fact that the effectiveness of burner tilting upon the steam temperature can be increased both by a reduction in firing circle diameter when tilting upwardly, and by an increase in firing circle diameter when tilting downwardly. Accordingly, my method and apparatus as disclosed herein for firing fuel tangentially, provides for a gradual decreasegof the firing circlewhen tilting burners upwardly and a gradual increase of the firing circle when tilting the burners downwardly.

By varying the firing circle diameter inthis manner in a steam generator equipped with my inventive improvement it is therefore possibleto obtain a' substantially equal increaseand decrease in, steam temperature with tilting angles of substantially equal degree upwardly or downwardly from the horizontal. Furthermore, and this is even more important, the control of the superheated steam temperature is extended over a wider load range of the steam generator.

This is graphically illustrated in Fig. 4. There steam temperature variations are plotted against various angles of vertical burner tilt over a tilting range extending from minus degrees (downwardly) to plus 30 degrees (upwardly) from the horizontal. This is indicated by curves X, Y and Z. Curve X represents temperature variations plotted for various angles 'of burner tilt-upwardly and downwardly from the horizontal-when firing tangentially towards a fixed and relatively small firing circle such as of 2 feet diameter. Curve Y represents temperature variations plotted for various angles of burner tilt upwardly and downwardly from the horizontalwhen firing tangentially towardsia fixed and relatively large firing circle such as of 6 feet diameter. a

It will be noted thatcurve X (small diameter firing circle) drops from the horizontal-burner position e to the lowermost 30 degree position a and rises from the horizontal position e to the uppermost 30 degree position b. In a similar manner 'cur ve Y (large diameter firing circle) drops from a horizontal position g to a lowermost position 0 and rises from the horizontal position g to the uppermost position d. i a i I It will further be noted that the temperature variation 4 (e-b) obtained by an upward tilt when usinga small firing circle as indicated by curve X substantially exceeds that (e-a) obtained by a downward tilt with the same small firing circle. Conversely the temperature variation (g-a') obtained by an upward tilt when using a large firing circle as indicated by curve Y is considerably less in extent than that (gc) obtained by a downward tilt when directed tangentially at the same large firing circle.

My inventive improvement in burner tilting as herein disclosed takes advantage of the inequalities in temperature variation so clearly shown by curves X and Y. This is accomplished by varying the diameter of the firing circle from a relatively small diameter such as two feet at the maximum upward tilt to a relatively large diameter such as 6 feet at the lowermost downward tilt position.

Thus, in a steam generator equipped with my invention, the temperature variations when plotted against the angle of burner tilt will follow a line represented by curve Z. The uppermost point I) of curve Z is common with curve X indicating that in the uppermost tilting position (+30) :1 small firing circle diameter is utilized. The lowest point 0 of curve Z is common with curveY indicating that in the lowermost tilting position (-30) a large firing circle is utilized.

It will also be noted that when gradually increasing the firing circle diameter (from 2 feet to 6 feet for instance) as represented by curve Z the temperature variation (hb) between the horizontal position 12 and the uppermost position b approximately equals the temperature variation h-cbetween position I1 and lowermost tilting position 0. 7

Furthermore, it will be noted by studying Fig. 4 that when employing my improved burner tilting method and apparatus in the water cooled furnace of a steam generator having superheater heating surface absorbing heat primarily by convection, the burner tilting effect upon the steam temperature is substantially increased over that obtainable when using either a small or large but fixed firing circle diameter. This is clearly demonstrated by comparing the overall temperature variation (0 to b) of curve Z (using variable firing circle) with the temperature variations (a to b) of curve X or (c to d) of curve Y, both based on a fixed firing circle diameter.

Thus my invention provides that the set of burners and air nozzles 28 in the furnace chamber be mounted so that tilting of each discharge port 28a, 28b, and 28c occurs individually about respective axes Ta, Tb, and Tc each of which forms an angle A with the horizontal H, as schematically illustrated in Figure 3. When discharging fuel and air through these ports in a horizontal direction they are set so that these streams of fuel and air are directed tangentially at the periphery of intermediate firing circle I. The angle A between the tilting axes Ta, Tb and Tc of the burners and of the horizontal H is such that when the'fuel and air streams are directed upwardly they will follow a line tangent to the periphery of an upper firing circle U which is smaller than the aforementioned intermediate firing circle I. In discharging fuel and air streams through these burners downwardly said streams will flow in a direction tangential to the periphery of a lower firing circle L which will be correspondingly larger than the intermediate firing circle I. Angle A is preferably of a magnitude which results in an upper firing circle diameter of about two feet and a lower firing circle diameter of about six feet, when the burners are tilted upwardlyor downwardly approximately 30 degrees. Obviously, angle A may vary with the geometrical shape of the furnace chamber, with the arrangement of water wall heating surface, or may be affected by other factors such as gas new set of operating conditions that may be encountered such as change in fuel and change in slagging conditions of the furnace walls.

A burner designed according to my invention and operating as hereinafter set forth will therefore in the preferred embodiment illustrated discharge fuel and air tangentially towards the periphery of a frustum of a circular cone having a lower base formed by lower firing circle L and an upper base formed by the upper firing circle U. The plane within which the tilting of the burner nozzles occurs would be the tangent plane which touches the conical surface of the imaginary frustum. This is schematically illustrated in Fig. 1.

Although I have described herein a preferred embodiment of my invention wherein the burners are tilted to follow the outline of an imaginary conical surface, my invention is also applicable to the tilting of burners following an irregular or curved outline such as the periphcry of any solid of rotation located within the furnace chamber.

Furthermore, although in my preferred embodiment as herein described the imaginary frustum of a cone is coaxial with the furnace chamber, my invention is not restricted thereto but could also be practiced in an organiza-' tion wherein the axis of the imaginary solid of rotation (such as the frustum of a cone) is neither coinciding with nor parallel to the longitudinal axis of the furnace. The shape of the furnace chamber, the arrangement of the water wall heating surface or other heating surfaces located therein may be such as to dictate the form and axial location of the imaginary solid of rotation by which the tilting of the burner nozzles is guided for the control of the temperature of superheated steam.

Furthermore my improved tilting burner superheat control means can be employed in combination with, or can be supplemented by other superheat control means such as bypass damper control, desuperheating control or gas recirculation control.

In Fig. 3 of the illustrative embodiment of my invention three burner nozzles such as 28a, 28b and 280 are shown to constitute'one set of burner nozzles in each corner. To accomplish the object of my invention the axis SS of the entire set of burner nozzles is tilted from the vertical V. Other means of tilting could be employed to achieve the object of my invention such as tilting each individual burner nozzle independently of all the others and following its own imaginary outline located within the furnace such as the conical surface of a frustum or the surface of any other solid of rotation.

.Such a furnace may be placed in any position, upright or horizontal and the above described imaginary solid of rotation may have the shape of an hourglass or may be composed of two frustums joined end to end with the smaller base in common.

I claim:

1. In a steam generator having an elongated furnace chamber equipped with water cooled walls and an outlet for the furnace gases at one end thereof; a superheater for superheating steam located adjacent said furnace outlet in position to absorb heat from said gases; at least two burner nozzles oppositely mounted in said furnace walls each firing a fluid fuel in directions tangential to an imag inary firing circle located within said furnace chamber; means for mounting said burner nozzles for tilting in an are extending from an uppermost burner tilt position to a lowermost burner tilt position for increasing or decreasing the temperature of said superheated steam when tilt-.

ing said burner nozzles upwardly or downwardly respectively; said means including a pivotally supported axis for tilting each nozzle in an arcfalling in a plane located at one side of the longitudinal axis of said furnace and inclined in a direction toward said furnace axis at the furnace outlet side of said burner nozzles, whereby said firing circle decreases gradually in diameter as the burner nozzle is tilted toward the furnace outlet end from the lowermost position to the uppermost position thereof.

2. An organization as defined in claim 1 wherein said firing circle decreases gradually to a diameter of approximately one half, when tilting the burner nozzles 30 degrees upwardly from the horizontal, as compared to the firing circle that prevails when tilting the burner nozzles downwardly 30 degrees from the horizontal.

References Cited in the file of this patent UNITED STATES PATENTS 2,243,909 Kruger June 3, 1941 2,363,875 Kreisinger et a1 Nov. 28, 1944 2,575,885 Mittendorf Nov. 20, 1951 2,590,712 Lacerenza Mar. 25, 1952

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