US3110291A

US3110291A - Steam generating unit

Info

Publication number: US3110291A
Application number: US166573A
Authority: US
Inventors: Earle C Miller
Original assignee: Riley Power Inc
Current assignee: Riley Power Inc
Priority date: 1962-01-16
Filing date: 1962-01-16
Publication date: 1963-11-12
Anticipated expiration: 1980-11-12
Also published as: GB975388A

Description

Nov. 12, 1963 E. c. MILLER 3,110,291

STEAM GENERATING UNIT Filed Jan. 16, 1962 2 Sheets-Sheet 1 2p g g a g 78 INVENTOR. z EasrZe C. UZI'ZZe'r H oney Nov. 12, 1963 E. c. MILLER 3,

'STEAM GENERATING UNIT Filed Jan. 16, 1962 2 Sheets-Sheet 2 ,2 h. Il

I I V 4 INVENTOR.

'jarZe C. miller United States Patent 3,110,291 STEAM GENERATING UNIT Earle C. Miller, Worcester, Mass, assiguor to Riley Stoker Corporation, Worcester, Mass, a corporation of Massachusetts Filed Jan. 16, 1962, Ser. No. 166,573 Claims. (Cl. 122-479) This invention relates to a steam generating unit and more particularly to apparatus arranged to produce steam by the burning of a combustible fuel.

In the generation of steam, particularly for the purposes of power generation in a steam turbine, the control of the temperature of the superheated steam is a considerable problem. This comes about because of the need to maintain this temperature as close to a predetermined design temperature for efficient operation without exceeding it and thus encountering the problem of failure of materials. Another problem which is often encountered with large steam generating units is that, during the combustion of the fuel, nitrous oxides are formed which are projected into the air from the stack and are thought to produce a considerable amount of the irritants in the air in certain parts of the world. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the present invention to' provide a steam generating unit having a novel means for maintaining superheat at a constant, pre-determined value.

Another object of this invention is the provision of a steam generating unit having a novel means for reducing the amount of nitrous oxide in the products of combustion.

A further object of the present invention is the provision of a steam generating unit having means for controlling superheat and at the same time regulating the amounts of nitrous oxide in the products of combustion.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings in which:

FIG. 1 is a vertical sectional view of the steam generating unit embodying the principles of the present invention,

FIG. 2 is an enlarged elevational view of a portion of the apparatus taken on the line 11-11 of FIG. 1,

FIG. 3 is a schematic view of the apparatus showing the mode of operation at high load, and

FIG. 4 is a schematic view of the apparatus showing the mode of operation at low load.

Referring first to FIG. 1, wherein are shown the general features of the invention, the steam generating unit, indicated generally by the reference numeral 19, is shown as consisting of a furnace 11 and a boiler 12 suspended from a supporting structure 13. The furnace is of the elongated vertical type and is provided with a front wall 14, a rear wall 15, side walls 16, a roof 17, and a bottom 18 defining a combustion chamber 19. An inwardlydirected abutment 21 extends inwardly from the front wall 14, while a similar abutment 22 extends inwardly from the rear wall 15, the abutments serving to divide the combustion chamber 19 into a lower high-temperature cell 23 and an upper portion 24. Mounted in a downwardly-directed surface of the abutment 21 is a directional-flame burner 25, While a similar burner 26 is mounted on a downwardly-directed surface of the abutment 22. The burners and general furnace arrangement are those shown in the patent of Miller No. 2,947,289.

At the upper part of the furnace, the rear wall 15 is provided with a forwardly-directed nose 27. Between the upper surface of the nose 27 and the roof 17 is defined a horizontal pass 28 leading to a gas off-take opening 29. The off-take opening leads to a back-pass 31 having a vertical dividing wall 32 separating the back pass into a forward portion 33 and a rearward portion 34. The lower end of the back pass 31 is connected through a regenerative air heater 35 to a breaching 36 leading to the stack.

The boiler 12 is provided with a steam-and-water drum 37 containing the usual steam purifying equipment and connected by downcomers 38 to

headers

39 and 41 underlying the bottom 18 of the furnace. Water-wall tubes extend upwardly from the header 39 and along the surface of the back wall 15 along the surface of the abutment 22 and the nose 27 to terminate at the steam-and-water drum for the release of steam thereto. In a similar manner, water-wall tubes extend upwardly from the header 41 along the front wall 14, overlying the abutment 21 and also providing protection for the roof 17. The downcomers 38 are also connected to headers 42 underlying the side wall 16 and connected to water-wall tubes which extend over the surface of the side walls and eventually terminate in a steam-and-water drum 37. An economizer 43 extends entirely across the back pass 31 lying in both the bottom parts of the forward portion 33 and the rearward portion 34. Steam tubes 44 leave the upper part of the steam-and-Water drum 37 and some of them support the dividing wall 32, while others line the back wall of the rearward portion 34 of the back pass. In either case, they are connected to the lower end of a low temperature convection superheater 45 which lies in the rearward pass 34.

The upper part of this superheater is connected by a pipe 46 to the lower end of a radiant intermediate temperature superheater 47 which is mounted adjacent the front wall 14 of the furnace between the abutment 21 and the roof 17. The upper end of this last-named superheater is connected by a pipe 48 to the forward part of a high-temperature superheater 49. The superheater 49 is in the form of pendant platens which terminate in a short convection section lying in the horizontal pass 28 and which is ultimately connected to a superheated steam header 51.

A low-temperature convection reheater 52 lies in the forward portion 33 of the back pass and is connected at its upper end to a high-temperature convection reheater 53 which lies in the horizontal pass 28 adjacent the gas offtake opening 29. This high-temperature reheater discharges into a reheated steam header 54.

A forced draft fan 55 provides air to the air heater 35 which, in turn, discharges preheated air through a venturi duct 56, which duct is provided with a damper 57 capable of dividing the air between a plenum chamber 58 for supplying air to the burners 2S and 26 and an overfire air duct 59. A duct 61 leads from the plenum chamber 58 to the burners 26 arranged on the undersurface of the abutment 22 and this duct is provided with a damper 62.

. In a similar manner, a duct 63 leads from the plenum chamber 58 to the burners 25 on the undersurface of the abutments 21, this duct being provided with a damper 64.

The damper 64 is connected by a linkage to a hydraulic 1 linear actuator 65 which is connected by

lines

66 and 67 to the output of a main control 68. The input of the main control 63 is connected by a line 69 to a temperature measuring device 71 located in the reheated steam header 54. A similar temperature measuring device 72 is located in the superheated steam header 51 and is connected by a line 73 to the input side of the main control 68. The damper 62 in the duct 61 is also connected through a linkage to a hydraulic linear actuator 74 which is con trolled by the main control 68 through

lines

75 and 76. The overfire air duct 59 is connected by a first branch duct 77 to an overfire air opening 7 8 located in the front wall 14-.of the furnace just above the abutment 21. The duct Patented Nov. 12, 1963.

59 is also connected through a branch duct 75 to a similar overfire air opening 81 located on the rear wall 15 just above the abutment 22. In the duct 77 is located a damper 82 which is connected through a linkage to a hydraulic actuator 83 which is controlled by the main control 63 through the medium of lines 84 and 35. In a similar manner, the duct 79 is provided with a damper 86 whose setting is controlled by a hydraulic linear actuator 37 which is connected to the main control 63 by lines 88 and 89.

At the bottom of the rearward portion 34 of the backpass 31 is located a set of dampers 91 connected by linkages to a hydraulic linear actuator 92 which is connected for control to the main control '68 by means of lines 93 and 94. At the bottom of the forward portion 33 of the backpass are located dampers 95 which are connected by linkages to a hydraulic linear actuator 96 which is connected to the main control by means of lines 97 and 98. The damper 57, which controls the division of the incoming air between the plenum chamber 58 and the over fire air duct 59, is connected -by linkages to a hydraulic linear actuator 99 which is connected by lines 101 and 162 to the output of the main control 63. Pressure plugs 103 and 104 are mounted in the venturi duct 56 and are connected by lines 1695 and 196 to the input side of the niain control as, the location in this manner giving a pressure differential between the two lines 165 and 166 indicative of the rate of flow of air into the furnace and, therefore, of the load.

In the usual manner, a desuperheater 1425 is connected in the pipe 46 between the low temperature superheater 45 and the radiant intermediate temperature superheater 47. The amount of water injected into the steam is controlled from the main control 68 by a line 1636 extending between the main control and the desuperheater. It will be understood that the main control '68 is of the usual type which receives signals from various sources such as devices measuring load, such as the plugs 103 and 194, and devices for measuring steam temperatures, such as devices 7 1 and 72, and converting them into signals which pass through the various lines to actuators which operate to correct tendencies for change in the temperatures of superheat and reheat. The nature of this control is not part of the present invention and its design and construction is well known in the art.

Referring to FIG. 2, it can be seen that the opening 78 through the front wall 14 of the furnace is formed by bendin alternate water-wall tubes rearwardly of adjacent water-wall tubes to provide inter-tube openings or slots through the wall. The opening 81 on the rear wall 15 is similarly constructed.

l The operation of the apparatus will now be readily understood in view of the above description and particularly in view of FIG. 3 and FIG. 4. The

burners

25 and 26 operate in the usual manner to produce a mixture of fuel and air which burns to form a mass of products of, combustion 1G7. It will be understood that most of the visible flame portion of this gas mass will terminate slightlyabove the

abutments

21 and 22 because of the nature of the particular furnace involved and the presence of" the high-temperature cell 23 in which a considerable part of the combustion can be completed. The products of combustion flow upwardly through the combustion chamber -19.around the nose 27, through the horizontal pass 28, and through the gas iofiitake 29 into the backpass .3 1. It is divided between the forward portion 33 and the rearward portion 34, depending on the settings of the

dampers

91 and 55. Under certain conditions, some of the gas may be directed through a by-pass section (not shown) of the back passin which there are no superheater or, reheater elements. The gas leaves the backpass, flows through the air heater 35 where it serves to heat the incoming air, and then passes into the breaching 36. Flowing in this manner, water is heated and steam is generated, superheated, and reheatedin the usual manner by the gas mass transmitting heat by radiation to the water- Walls and to the radiant superheater 47, transmitting heat to the superheater 49 by a combination of radiation and convection, and transmitting heat mainly by convection to the

reheater sections

52 and 53 as well as the superheater i S. Heat is recovered in the economizer 43 as well as in the air heater 35 to give the unit optimum effi ciency. If the steam generating unit were operated only in this way at various loads, the superheat passing into the superheated steam header 51 on its way to the turbine wouldrdr-oop, i.e., the curve of superheat versus load wouldbe inclined with lower superheat temperatures at lower loads. The amount of air passing into the

burners

25 and 26 is controlled by the dampers 64 and 62, re-

spect-ively, so as to cause the gas mass 107 to move up the back wall 15 of the furnace or up the front wall 14. This is done through the medium of the actuators 65 and 74 under the regulation of the main control 68 through the medium of lines '66, 67, 75 and 76. This type of flame placement is shown and described in the aforementioned patent of Miller No. 2,947,289. Generally speakiug, this effect is produced by causing a greater flow;

'trol 68 by means of the pressure plugs 103 and 10 4 and their lines Hi5 and 1%, the dampers are set to cause a considerably greater flow of air from the duct 77 through the opening 78 in the front wall than flows through the duct 79 and the opening 81 in the rear wall. The effect of this is to force the gas mass 107 toward the rear wall.

of the furnace. At high load, the total amount of air required by all of the

burners

25 and 26 is quite high, and this is regulated either by the dampers 62 and 64 operating together, or by control of the motor associated with the forced draft fan 55 going at a faster rate. At the same time, the total amount of overfire air passing through the openings 78 and 81 is also higher at high load. However, -the division will be such that the overfire air coming from the front wall through the opening 7 B will domi-' nate the overfire air coming through the opening 81 in the rear wall and the gas mass will be pushed toward the rear wall. This overfire air, of course, has the elfect of protecting the front and rear walls and of producing a complete burning of nitrous oxide in a manner which will be described below. i 7

At low load,the dampers 62 and 64 will be set to force the gas mass up the front wall of the furnace, for instance, by causing the air flowing through the burner 25 to be much greater than that flowing from the burner 26 so that the flame is thrown toward the front wall. It will be understood that this same type of flame placement can also be accomplished by causing the vanes in the inter-tube burners to occupy suitable positionsrelative to one an-.

other. At the same time, the dampers 86 and 82 are set to cause the air proceeding fromthe opening 81 in the) rear wall to dominate the situation, thus forcing the gas flow also toward the front wall. This movement of the flame from the front to the rear of the furnace as selected by the main control 63 has the effect of maintaining the superheat in the superheated. steam header 51 at a con- This is beoauseathigh load,

27; the eifect is a longer residence time in the furnace and cooler gases passing over the convection surfacesof the boiler. At the same time, the flame is furthest away from the radiant superheater 47 and heat transfer to the radiant superheater is at its lowest value. On the other hand, at low load, as is evident, in FIG. 4, the flame passes up the front wall 14 where it transmits the greatest amount of heat by radiation to the superheater 47 and takes the shortest path around the nose 2.7; that is to say, has the shortest residence time in the main combustion chamber 19, thus producing highest temperature of gases passing into the convection passes. it will be understood that at various loads between high load and low load the flame may occupy intermediate positions between the front and rear walls of the furnace. This method of controlling the superheat might, in a practical boiler, be combined with other methods. For instance, if, at any time, at higher loads the temperature of superheat exceeds the predetermined value despite the best efforts by the flame placement technique, the temperature may be reduced by a signal passing through the line 166 to the desuperheater 3195. Additionally, the relative temperatures of the reheated steam in the reheated steam header 54 and the superheated steam in the superheated steam header 51 may be adjusted by regulating the proportions of gas which pass through the forward portion 33 of the back pass over the convection reheater 52 and the rearward portion 34 of the back pass over the convection superheater 45. Additional lowering of the superheat may be accomplished in a well-known manner by shunting some of the gases through the dead by-pass section of the back pass so that they pass over neither the reheater 52 or the superheater 45.

Now, the damper 57 is regulated by the actuator 99 operating from the main control 68 through the lines 1G1 and 102. This damper divides the incoming air between -the

burners

25 and 26 on the one hand and the overfire air openings 78 and 81 on the other hand. At high load, although considerable air is needed for combustion through the burners, the damper is set to permit the burners to have a lower proportion of the air. On the other hand, at low load, where air is necessary in the burners to maintain ignition, the greater proportion of air goes to the burners. At the same time, it is not necessary to provide as great amount of overflre air through the

openings

78 and 31.

In general, then, the present invention involves dividing the combustion air between the overflre air openings which are above the

abutments

21 and 22 and the directional-flame burners mounted on the undersurfaces of the abutments. With ordinary operation of the furnace, the percentage of nitrous oxide in the flue gases increases with load in a more or less straight line relationship. It has been discovered that the slope of this curve (or the absolute value of the nitrous oxide) can he reduced by lowering the velocity of the fuel and air to the burner. At high load, where the velocity of gases through the burner would normally be quite high, some of the combustion air is introduced independently into the furnace as overfire air, this point of introduction being quite separated from the portion in which air is introduced which combines immediately with the fuel from the main fuel gun. At low load, where it is necessary to mix the air immediately with the fuel to maintain ignition, a considerable portion of the air would be introduced through the main burners in the usual way rather than as overfire air in a separate stream. At low load, the unit will be operating on the lower part of the nitrous oxide curve where the formation of nitrous oxide is quite low. In the preferred embodiment, at low load the greater amount of the air will go through the burners and the flame placement will be such that the flame will go up the front wall to provide longer residence time and be closer to the radiant superheater. by the greater proportion-of the overflre air being introduced to the back wall to assist in pressing the flame forwardly. At high load, however, a greater amount of air is introduced as overfire air and the burners are set to produce a flame which would be positioned toward the rear wall producing longer residence time and less radiant heat transmission to the radiant superheater. The

This will be assisted overfire air is biased so that the front wall would receive the greatest stream of overfire air to assist in positioning the flame to the rear of the furnace.

It can be seen that at high load, when there is a much greater mass of gases which will be more diflicult to position, a greater amount of ovenflre air is used, thus assisting in pushing the flame to the rear of the furnace. At low load, where the mass of gases is smaller and it is easier to position the flame, the overfire air is smaller in amount, also, and is not needed in the flame-positioning to as great an extent. At the same time, at high load where nitrous oxide is a problem, the air velocity through the burner is reduced, thus reducing the nitrous oxide formation without loss of completeness of combustion because of the overfire air being provided at that load.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent, is:

l. A steam generating unit, comprising (a) front, rear, and side walls defining a verticallyelongated furnace,

(b) a pair of abutments on opposed walls defining a high-temperature cell at the lower end of the furnace,

(c) a nose on one of the opposed walls defining a gas off-take at the upper end of the furnace,

(d) a fuel-burning apparatus mounted in the high-temperature cell adapted to form an upwardly-directed flame,

(e) overfire air openings located on both opposed walls above the abutments,

(f) a source of air including proportioning dampers connected to the apparatus and to the openings,

(g) -a radiant superheater associated with the other of the opposed walls,

(It) and a control means responsive to an increase in the temperature of superheated steam to decrease the proportion of air flowing to the openings on the said one of the opposed walls relative to that flowing to the openings on the said other of the opposed walls to cause the said flame to change to a position between the said opposed walls such that the temperature of superheated steam will be maintained at a predetermined value.

2. A steam generating unit as recited in claim 1 wherein the fuel-burning apparatus consists of vane-type directional-flame burners mounted on the undersurfaces of the abutments.

3. A steam generating unit as recited in claim 1 wherein a back pass containing a convection superheater is connected to the said gas off-take.

4. A steam generating unit as recited in claim 1 wherein means is provided to regulate the division of the air bet-ween the overfire air openings and the fuel-burning apparatus.

5. A steam generating unit, comprising (a) front, rear and side walls defining a vertically-elongated furnace,

(b) a convection pass connected to the upper end of the furnace,

(c) a radiant superheater associated with one of two opposed walls,

(a') a fuel-burning apparatus mounted in the lower end of the furnace and adapted to form an upwardlydirected flame,

(e) overfire air openings located in the said opposed walls above the fuel-burning apparatus,

(g) a first means for regulating the apparatus so that the flame is directed along the other of the opposed walls at high load and along the said one of the 0pposed walls at low load,

(h) and a second damper means dividing the total flow of air to the openings so that a higher proportion flows to the openings on the said one of the 5 opposed walls at high load to assist the first means in directing the flame along the other of the said opposed walls and so that a, higher proportion flows to the openings ont the othertof the opposed Walls at low load to assist the first means in directing the flame along the said one of the opposed walls.

References Cited in the file of this patent UNITED STATES PATENTS 2,869,519 Schroedter et al J an. 20, 1959 2,947,289 Miller Aug. 2, 1960 FOREIGN PATENTS 724,934 Great Britain Feb. 23, 1955

Claims

1. A STEAM GENERATING UNIT, COMPRISING (A) FRONT, REAR, AND SIDE WALLS DEFINING A VERTICALLYELONGATED FURNACE, (B) A PAIR OF ABUTMENTS ON OPPOSED WALLS DEFINING A HIGH-TEMPERATURE CELL AT THE LOWER END OF THE FURNACE, (C) A NOSE ON ONE OF THE OPPOSED WALLS DEFINING A GAS OFF-TAKE AT THE UPPER END OF THE FURNACE, (D) A FUEL-BURNING APPARATUS MOUNTED IN THE HIGH-TEMPERATURE CELL ADAPTED TO FORM AN UPWARDLY-DIRECTED FLAME, (E) OVERFIRE AIR OPENINGS LOCATED ON BOTH OPPOSED WALLS ABOVE THE ABUTMENTS, (F) A SOURCE OF AIR INCLUDING PROPORTIONING DAMPERS CONNECTED TO THE APPARATUS AND TO THE OPENINGS, (G) A RADIANT SUPERHEATER ASSOCIATED WITH THE OTHER OF THE OPPOSED WALLS,