US3872836A

US3872836A - Coal-fired generator of medium to large capacity

Info

Publication number: US3872836A
Application number: US398518A
Authority: US
Inventors: Walter P Gorzegno; William D Stevens
Original assignee: Foster Wheeler Inc
Current assignee: Foster Wheeler Inc
Priority date: 1973-09-18
Filing date: 1973-09-18
Publication date: 1975-03-25
Anticipated expiration: 1992-03-25
Also published as: IT1033095B; JPS5076401A; CA982893A; ES429881A1; AR203304A1; BR7407773D0; GB1475350A; AU7341474A

Abstract

A supercritical forced-flow once-through vapor generator circuitry in which the furnace enclosure consists essentially of three flow passes in a series-flow relationship. The first pass occupies the enclosure front wall, and the second and third passes occupy the enclosure rear wall and intermediate side walls. The invention is particularly useful for coal-fired generators of medium to large capacity wherein the three passes provide sufficient mass flow rates to cool the enclosure while at the same time employing normal sized tubes.

Description

Cite States atent [191 Gorzegno et a1.

[451 Mar. 25, 1975 COAL-FIRED GENERATOR 0F MEDIUM T0 LARGE CAPACITY [75] Inventors: Walter P. Gorzegno, Florham Park; William D. Stevens, North Caldwell,

both of NJ.

[73] Assignee: Foster Wheeler Corporation,

Livingston, NJ.

[22] Filed: Sept. 18,1973

[21] Appl. No.: 398,518

[51] Int. Cl. F22b 27/04 [58] Field at Search 122/406 S, 406 ST, 448 S, 122/451 S [56] References Cited UNITED STATES PATENTS 3,172,396 3/1965 Kane 122/406 3,324,837 6/1967 Gorzegno et al. 122/406 3,343,523 9/1967 Gorzegno et a1. 122/406 3,548,788 12/1970 Altman et al.. 122/406 3,771,498 11/1973 Gorzegno [22/406 Primary Examiner-Kenneth W. Sprague Attorney, Agent, or Firm-Marvin A. Naigur; John E. Wilson [57] ABSTRACT A supercritical forced-flow once-through vapor generator circuitry in which the furnace enclosure consists essentially of three flow passes in a series-flow relationship. The first pass occupies the enclosure front wall, and the second and third passes occupy the enclosure rear wall and intermediate side walls. The invention is particularly useful for coal-fired generators of medium to large capacity wherein the three passes provide sufficient mass flow rates to cool the enclosure while at the same time employing normal sized tubes.

8 Claims, 9 Drawing Figures Pmmmmasm 3,872,836

sum 1 of PATENIED HARZ 51975 SHEET 2 0F 6 PAIENTED IIIIRZ 5 I975 sum 3 95 INLET ECONOMIZER DIVISION WALLS PATENTED MR 2 5 I975 FROM ECONOMIZER PATENTED 51975 SHEET6UF FURNACE SIDE W 4 I06 I08 ||o FURNACE REAR WALL n6 6 FURNACE 2 "8 SIDE WALLS ,T

we we no ECONOMIZER |NLET IDIVI sum WALLS FIG. 8

BACKGROUND OF THE INVENTION cal forced-flow once-through generator which is particularly suitable for coal firing. A slower heat release is required with the firing of coal than with the firing of either gas or oil; and because of this, the furnace enclosure for a coal-fired generator has to be larger than that required with gas or oil firing. In the above-mentioned patent, a partialdivision wall is employed within the furnace enclosure to absorb a portion of the heat released. This permits a partial reduction in the size of the enclosure. The enclosure is rectangular in shape and is divided into two vertically extending flow passes which'make up the full periphery of the enclosure. One of the flow passes constitutes the enclosure front and rear walls, and the second pass makes up the enclosure side walls.

The arrangement of the above-mentioned prior pa tent is satisfactory for very large generators, for instance, a generator of about 1,300 megawatts. With normal sized tubes, for instance, of one and onequarter inches in outside diameter to one and one-half inches, the overall flow area of each pass is still small enough to obtain an adequate mass flow rate in the tubes, despite the very large dimensions of the enclosure required to accommodate the coal heat release.

However, for medium to large sized generators, for instance 600 to 1,000 megawatts, the use of normal sized tubes in a pass consisting of about half the periphery of the furnace enclosure provides a disproportionately large flow area for the total flow in the generator. Under certain load conditions, this could result in insufficient mass flow rate in the enclosure tubes for safe cooling of the tubes. It is possible to employ smaller diameter tubes, for instance, one-inch tubes; but these smaller diameter tubes have the disadvantage that they are readily plugged. In addition, they are also susceptible to magnetite build-up and pressure drop increase requiring more frequent cleaning. Further, it is difficult to manufacture all-welded panels with such small sized tubes.

For purposes of the present application, the mass flow rate is defined aspounds per hour-square foot. For a particular flow in pounds per hour, a large flow'area gives a relatively small mass flow rate, and vice versa. In a generator of the type to which the present invention is directed (wherein the furnace enclosure is rectangular in shape, and is constructed of parallel, vertical tubes in a plurality of adjacent panels with burners in the front and rear walls ofthe enclosurelsafe operation of the generator requires particularly in the burner zone tubes a mass flow rate of at least about 2 X SUMMARY OF THE INVENTION Accordingly, it;is an object of the present invention to overcome the above problems and in particular to provide a circuit geometry for coal-fired generators of medium to large capacity in which the mass flow rates are sufficient to cool the boundary walls of the enclosure, the latter being constructed with normal sized tubes.

Preferably the generator includles an enclosure'division wall which is located in the generator circuitry upstream of the furnace passes. The division wall is sized to absorb a portion of the heat liberated in the furnace and reduces the enthalpy pick-up of the enclosure passes. As a result, the temperature increase in each pass is less than about F, a theoretical maximum which prevents undue thermal stresses in the furnace enclosure, in particular at the welded junction between passes.

The enclosure panels extend the full height of the enclosure. In the upper part of the enclosure, the tubes preferably are larger in diameter; but are on the same tube centers, for reducing the overall pressure drop in the enclosure. The transformation to larger diameter 'occurs at that elevation in the furnace enclosure wherein the heat intensity has been reduced to the extent or degree such that high mass flow rates are no longer required.

In accordance with illustrative embodiments demonstrating features and advantages of the present invention, there is provided a supercritical forced-flow oncethrough vapor generator of the type having a rectangular upright furnace enclosure formed of a plurality of panels of upright tubes welded together lengthwise. In this manner, a gas tight construction is formed of front and rear walls separated by a pair of side walls. A plurality of downcomers serially connect the panels together into first, second and third flow passes. The first pass defines substantially the full extent of the front wall of the enclosure. The second and third passes define substantially the full extent of the side walls and rear wall, respectively, or vice-versa.

BRIEF DESCRIPTION OF THE DRAWINGS The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention, when taken in connection with the accompanying drawings wherein:

FIG. 1 is an elevation section side view of a vapor generator employing a flow circuitry in accordance FIG. 5 is an enlarged perspective, partial view of a furnace enclosure wall illustrating an aspect" of the invention;.

FIG. 5a is an enlarged elevation, partial, section view of an enclosure wall further illustrating an aspect of the present invention;

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, and in particular to FIG. 1 and FIG. 2, the generator in accordance with the present invention is broadly indicated by the number 10. It comprises a'vertically extending rectangular shaped radiant furnace enclosure 12 defined by front and

rear walls

14 and 16, respectively, and

side walls

18 and 20 which extend between the front and rear walls. The front and side walls form continuous panels extending vertically from a bottom hopper 22 to roof 24, the rear wall panels 16 however terminate somewhat short of the roof to define with the roof a gas exit 26 which leads to a vestibule 28 in gas flow communication with a downwardly extending convection zone 30.

Burners

32 and 34 are disposed in the front and rear walls 14 and-l6 immediately above the generator enclosure hopper 22. The flow of gases in the enclosure is upwardly through the gas exit 26 into the vestibule 28 and downwardly in the convection zone to the generator outlet 36 and to a conventional air heater 38 for heat exchange between the hot exhaust gases and incoming air for the burners.

The rear wall 16 in the zone of'the gas exit is divided or branched to provide an exit screen 40 at'the gas exit and a second array of screen tubes 42 at the rear end of the vestibule. By bending a selected number of tubes from the rear wall of the enclosure for the screen 42, there is adequate spacing across the

screens

40 and 42 for the flow of hot gases into the vestibule and into the convection zone 30.

The vestibule 28 and convection zone 30 house a primary superheater 44, a finishing superheater 46, a bank of reheater tubes 48 and an economizer 50.

It is a feature of the generator that a membrane-type wall construction, illustrated in detail in FIG. 5, is used especially throughout the walls of the generator. This construction is formed by welding together a plurality of finned tubes 52 along their lengths so that the walls are substantially gas-tight. By virtue of the membranetype wall construction, use of a conventional refractory and casing-type construction, with accompanying cost, is avoided.

. In a preferred embodiment in accordance with the present invention, referring to FIGS. 1, 2 and 4 in particular, the high pressure fluid flow circuitry routing through the furnace enclosure of the generator consists of three series connected pass sections joined through downcomers and headers. The first fluid pass 52 consists of the front wall tubes of the generator connected between a lower inlet header 54, adjacent the front side of hopper 22, and an upper outlet header 56 above the roof '24 in the plane of the front wall. This pass occupies the entire front wall of the enclosure, for essenheader 62 in the plane of the rear wall but also above the roof. As with the front wall, essentially the entire rear wall constitutes the second pass of the furnace en closure. As previously mentioned, some of the tubes of the rear wall are bent rearwardly to embrace the vestibule and these tubes also are part of the second pass terminating in the header 64. The third flow pass 66 constitutes the side walls of the enclosure, and com prises the tubes making up both side walls for essentially the full extents thereof between the front and rear walls. These pass tubes also extend for the full height of the enclosure from lower headers 68 to upper exit headers 70 in the planes of the side walls. In all instances, the headers for a wall are essentially coextensive with the width of the passes.

FIGS. l and 2 illustrate the flow sequence in the passes. The first and second passes are connected together bya downcomer 72 which leads from upper first pass exit header 56 to the lower inlet header 60 of the second pass. The second and third passes are connected by downcomer 74 which leads from the upper

second pass headers

62 and 64 to the lower third pass inlet headers 68. The downcomer is branched so that the flow is equally into the lower side wall headers. This downcomer also receives the flow from both of the

screen tube panels

40 and 42 of the rear wall.

The generator illustrated in FIGS. 1 and 2 also includes a partial division wall 76 which consists of a plurality of L-shaped division wall panels. Each panel comprises a plurality of parallel tubes defining a substantially horizontally extending leg penetrating the rear wall of the furnace enclosure immediately beneath the floor of the vestibule, and a substantially vertically extending leg penetrating the roof of the enclosure, the panels being arranged in spaced-apart vertical planes parallel with the generator side walls and between the front and rear walls. The division wall panels may be six in number occupying a substantial area of the furnace upper area above the plane of the burners. The division wall is connected in the generator circuitry upstream of the aforementioned enclosure passes by an inlet header 78 (shown schematically) receiving flow from the generator economizer 50 and by an outlet header 80 also shown schematically from which the flow is transmitted via downcomer 82 to the first pass inlet header 54.

It should be noted that the exit flow from the furnace side wall outlet headers 70 is into the roof 24, then into the heat recovery area rear wall, and from there into other heat recovery area circuits, primary and finishing superheating s'ectionsin a conventional manner.

The function of the division wall panels in the circuitry is to provide substantial enthalpy pick-up upstream of the furnace enclosure passes to limit the enthalpy pick-up in the three furnace enclosure passes. The division wall is sized so that the temperature increases in each of the furnace enclosure passes is within the l25F theoretical maximum limit. In other words, the passes making up the furnace enclosure are in sideby-side relationship. As the fluid temperature increases from pass to pass in the furnace periphery, a temperature differential exists between the passes. One pass panel is welded directly to another, and the higher the temperature differential between the passes, the higher the-thermal stresses'in the panel joints. With advanced welding techniques, a maximum design temperature differential is about F in keeping with good design practice.

The temperature differentials which exist are most severe during start-up conditions at about 25% load. This is illustrated in FIG. 6. In the economizer there is an enthalpy pick-up of approximately 50 BTU/lb. The division wall provides a further enthalpy increase from about 450 BTU/lb. to about 620 BTU/lb. This increases the temperature of the fluid from about 450F at the inlet to about 590F at the outlet. This shifts the enthalpy pick-up of the three furnace enclosure passes to the more horizontal portion of the temperature enthalpy diagram so that the temperature increase in the first pass is from about 590F to about 700F, that in the second pass being from about 700F to 740F, and that in the third pass being from about 740F to 790F. In this way, the thermal stresses within the furnace enclosure are within design limits. The pass two and three panels are welded together, and the temperature difference in these tubes obviously will be below the 125F limit. The pass one and three panels also are welded together. The temperature difference in the upper zone of these passes is much less than 125F. In the lower zone, the temperature difference is about 125F which is still within design limits.

It is possible to employ the buffer circuit arrangement of FIG. 3 in the corners of the furnace enclosure to reduce stresses. Details of this circuit are disclosed in U.S. Pat. No. 3,344,777, assigned to assignee of the present application. Specifically, referring to FIG. 3, the corners of the furnace enclosure are made up of tubes of a circuit or pass 84 which is separate from and interposed between the adjacent circuits of

passes

86 and 88. Flow into the corner circuit is accomplished by providing an inlet header 90 substantially co-extensive with the corner circuit, and transmitting a portion of the flow from each the adjacent passes into the header so that the corner circuit is essentially at a temperature intermediate the temperature of the two adjacent passes. In a preferred embodiment, the buffer circuit corner circuits each comprises an expanse of about 1 /2 feet of the side walls and about five tubes of the front and rear walls.

It is desirable to maintain high mass flow rates in the passes to render the passes relatively insensitive to a flow imbalance, so that should a small flow imbalance occur in one of the passes, the tubes of the passes still will be adequately cooled by the high mass flow rates. At the same time, it is desirable to reduce the pressure drop which increases with increased mass flow rates, to a minimum level. Both criteria are accomplished by dividing the furnace into a high heat release zone A which extends from near the bottom of the furnace to an elevation above the burners, and a lower heat release zone B which constitutes the remainder of the furnace above the zone A to the roof. In accordance with the present invention, the tube diameters are increased, as shown in FIG. 5a, in going from the high heat release zone A to the lower heat release zone B. In the latter zone, high mass flow rates are less critical.

In a particular, embodiment in accordance with the invention, the front and rear walls of the first and second passes have I 4 inch OD tubes in the burner zone, changing at an elevation of abouttwo-thirds of the height of the furnace to l /2 inch OD tubes. The. side walls or third pass have 1 inch OD tubes changing at about the same elevation to l /2 inch OD tubes, the center-tocenter distances in all the passes remaining constant and being about 1 inches. The tube diameters of the buffer circuit panels may also be increased in zone B, for instance, from 1 inches to l /2 inches.

The dimensions of the furnace enclosure have an affect on mass flow rates, and in this respect, the width of the furnace may be about twice the depth. In actual practice, the width may vary from about 1.7 to about 2.5 times the depth and still achieve required mass flow rates with normal sized tubes.

EXAMPLE I.

In a particular example, for an 835 MW coal-fired generator, the furnace enclosure has a width of about 82 feet (the dimension of the front and rear walls) and a depth of about 40 feet (the dimension of the side walls). The total flow at full load is 5,280,000 Ibs./hr. The first and second passes each are made up of 455 tubes, and the third pass is made up of 555 tubes. The tubes in the lower furnace are .l /2 inches OD on 2-inch centers, 0.250 inch wall thickness. Those in the upper furnace are l inches OD on 2-inch centers, 0.280 inch wall thickness. This gives a mass flow rate in lbs./hr.-sq. ft. as follows:

, Table A Panel Mass Flow Passes l and 2 Lower Furnace 2.35 X. 10

Up er Furnace 1.65 X l0 Pass Lower Furnace 1.94 X l0 Upper Furnace 1.36 X 10 The enthalpy pick-up in the three passes increased from 725 BTU/lb. to about 1,135 BTU/1b., broken down as follows:

Table B Panel Enthalpy Pick-up Pass l j I28 BTU/lb. Pass 2 I28 BTU/lb. Pass 3 I54 BTU/lb.

tails of the hopper for such a construction are shown in prior U.S. Pat. No. 3,498,270, also assigned to the assignee of the present application; and it is an aspect of the present invention that the all-welded hopper construction may be employed in the generator of FIG. 1.

An embodiment of the invention is illustrated in FIG. 7 and FIG. 8. This embodiment differs from that of FIGS. 1, 2 and 4 in that the second pass of the furnace enclosure is comprised of the side walls of the enclosure, the rear wall making up the third pass of the enclosure. Referring to FIG. 8 in particular, the flow from the economizer 92 is into the division wall panels 94,

similar to the embodiment of FIGS. 1 and 2, and from there into the furnace front wall 96 via downcomer 98 and inlet header 100. From the outlet header 102 of the furnace front wall, the flow is via downcomer 104 to inlet header 106 for the enclosure side walls 108. Finally from the outlet header 110 of the side walls, the flow is collected in downcomer lll2-and transmitted to inlet header 114 of the furnace rear wall 116, the flow from the rear wall exiting in outlet headers 118.

The embodiment of FIGS. 7 and 8 offers the same advantages as the embodiment of FIGS. 1, 2 and 4. In particular, it permits the use of a properly sized enclosure (to accommodate the coal heat release) having the desired mass flow rates without the need to resort to small sized tubes. This embodiment has the additional advantage in that relatively less stress or distortion would be experienced in the furnace enclosure with heating of the enclosure. In this respect, the coolest pass is the first pass; and the hottest pass is the third pass. The latter will tend to expand more than the first pass, with the second pass having an intermediate expansion. Such differential expansions tend to slightly distort the furnace enclosure. In the present embodiment wherein the intermediate temperature pass is disposed between the low and high temperature passes, there is less tendency for furnace enclosure distortion than perhaps would otherwise be the case.

A further advantage of the present invention, either the embodiment of FIGS. 1, 2 and 4 or the embodiment of FIG. 7 and FIG. 8, which will be apparent to those skilled in the art, is that dividing the front and rear walls of the enclosure into different passes in series obviates the need for balancing the firing'rates of the burners in the front and rear walls heretofore required in certain types of generators wherein the front and rear walls were in the same flow pass. In fact, by the present invention, extreme differences in firing rates between the front and rear wall burners are possible.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.

We claim:

1. In a supercritical forced-flow once-through vapor generator of medium to large capacity for coal firing and of the type having a rectangular upright furnace enclosure formed of a plurality of panels of upright tubes welded together lengthwise to form a gas tight construction, including front, a pair of side and rear walls, the front and rear walls being separated by the pair of side walls, a plurality of burners in the front and rear walls, the improvement comprising a plurality of downcomers serially connecting said panels into first, second and third flow passes, said flow passes extending uninterrupted substantially the full height of the furnace enclosure, the first pass defining substantially the full extent of the front wall of the enclosure, the second and third passes defining substantially the full extents of the rear wall of the enclosure and of the side walls of the enclosure, respectively, or vice versa.

2. The generator of claim I wherein the second pass defines substantially the full extent of the furnace enclosure rear wall and the third pass defines substantially the full extents of the furnace enclosure side walls.

3. The generator of claim 1 wherein the second pass defines substantially the full extents of the furnace enclosure side walls and the third pass defines substantially the full extent of the furnace enclosure rear wall.

4. The generator of claim 1 further including means defining a division wall, and downcomer means connecting the division wall upstream ofthe furnace enclosure first pass.

5. The generator of claim 1 including buffer circuit means at the corners of the furnace enclosure comprised of panels parallel and between the panels of the enclosure passes, and means for transmitting to said buffer circuit panels portions of the flow to the adjacent pass panels so that the buffer circuit panels are at a temperature intermediate the temperatures of the adjacent pass panels.

6. The generator of claim I wherein the furnace enclosure comprises a high heat release zone in the area of the burners and a lower heat release zone above the h gh heat release zone, the tube diameters being greater in the low heat release zone.

7. The generator of claim 6 wherein the furnace enclosure front and rear walls define the generator first and second passes respectively, the side walls defining the generator third pass, the high heat releasezone extending to an elevation of about two-thirds the height of the generator.

8. The generator of claim 7 wherein the width of the furnace enclosure between the side walls equals about 1.7 to about 2.5 times the enclosure depth, the side wall tubes in the high heat release zone being larger in diameter than the front and rear wall tubes in said zone.

i l =l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,872,836 March 25, 1975 Patent No. Dated Walter P. Gorze no et a1. Inventor(s) g Page 1 Of 2 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Abstract, line 6, after "side walls" change the period to a comma and insert thereafter respectively, or vice-verse Column 2, line 35, after "flow passes" insert which extend uniterrupted substantially the full height of the furnace enclosure Column 2, line 38, "extent of the side walls and rear wall" to extent of the rear wall and intermediate side walls Column 2, line 62, "5a" to 5A Column 3,

line 20, the line should read 24. The rear wall Column 3, line 22, after "26" insert shown best perhaps in Fig. l, Column 3, line 57, after "52" insert (Fig. 4)

Column 6, line 65, "header" to headers In the drawings,

Figure 2, should appear as shown on the attached sheet.

Signed and Sealed this twenty-fourth D ay Of February 1 9 76 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner uj'Parents and Trademarks Page 2 of 2 Patent No. 3,872,836

UNITED STATES PATENT AND TRADEMARK OFFICE Certificate Patent No. 3,872,836 Patented March 25, 1975 Walter P. Gorzegno and William D. Stevens Application having been made by Walter P. Gorzegno and William D. Stevens, the inventors named in the patent above identified, and Foster Wheeler Energy Corporation, a corporation of Delaware, the assignee, for the issuance of a certificate under the provisions of Title 35, Section 256, of the United States Code, adding the name of Jan L. Friedrich as a joint inventor, and a showing and proof of facts satisfying the requirements of the said section having been submitted, it is this 5th day of August 1980, certified that the name of the said Jan L. Friedrich is hereby added to the said patent as a joint inventor With the said Walter P. Gorzegno and William D. Stevens.

FRED W. SHERLING,

Associate Solicitor.

Claims

2. The generator of claim 1 wherein the second pass defines substantially the full extent of the furnace enclosure rear wall and the third pass defines substantially the full extents of the furnace enclosure side walls.

4. The generator of claim 1 further including means defining a division wall, and downcomer means connecting the division wall upstream of the furnace enclosure first pass.

6. The generator of claim 1 wherein the furnace enclosure comprises a high heat release zone in the area of the burners and a lower heat release zone above the high heat release zone, the tube diameters being greater in the low heat release zone.

7. The generator of claim 6 wherein the furnace enclosure front and rear walls define the generator first and second passes respectively, the side walls defining the generator third pass, the high heat release zone extending to an elevation of about two-thirds the height of the generator.