US3788796A

US3788796A - Fuel burner

Info

Publication number: US3788796A
Application number: US00358779A
Authority: US
Inventors: B Krippene; M Peterson; D Marshall
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1973-05-09
Filing date: 1973-05-09
Publication date: 1974-01-29
Anticipated expiration: 1991-01-29
Also published as: JPS58173304A; AU473125B2; DE2421452B2; ZA742939B; IT1036513B; JPS5069633A; DE2421452A1; JPS5236609B2; ES425827A1; CA1004920A; GB1465173A; AU6872374A; AR200443A1; JPS6039923B2

Abstract

An improved pulverized fuel burning method and apparatus having means for enhancing the fuel-air mixture passing therethrough and including three separately controlled passageways delivering the air necessary for combustion of the fuel.

Description

United States Patent 1191 Krippene et al.

[451 Jan. 29, 1974 1 FUEL BURNER [75] Inventors: Brett C. Krippene, Barberton;

MorrisW. Peterson, Medina; David M. Marshall, Akron, all of Ohio [73] Assignee: The Bolcock & Wilcox Company,

New York, NY.

[22] Filed: May 9, 1973 [21] Appl. No.: 358,779

[56] References Cited UNITED STATES PATENTS 2,244,821 6/1941 Bloom 431/181 1 1949 Urquaart 431/188 x 2,690,795 10 1954 Webb 3,672,812 15/1972 Bendixen 431/1 53 x Primary Examiner-Edward G. Favors Attorney, Agent, or Firm.l. Maguire; Robert J.

Edwards [57] ABSTRACT An improved pulverized fuel burning method and apparatus having means for enhancing the fuel-air mixture passing therethrough and including three separately controlled passageways delivering the air necessary for combustion of the fuel.

' 12 Claims, 3 Drawing Figures FUEL BURNER BACKGROUND OF THE INVENTION The present invention relates to fuel burners and more particularly to an improved pulverized fuel burner for reducing the formation of nitric oxides by lowering the combustion zone temperature and providing a reducing atmosphere in the ignition zone.

There is a present day growing concern with the immediate and long term problems created by the rapid increase in air pollution resulting from the rise in the industrial civilization level throughout the world. With this concern comes an acute awareness that immediate steps must be taken to reverse this upward trend in pollution and great efforts are now being made by the public and private economic sectors to develop measures for preventing potentially polluting particles and gases from being discharged into the atmosphere. One such source of atmospheric pollution is the nitrogen oxides (NO present in the stack emission of fossil fuel fired steam generating units. Nitric oxide (NO) is an invisible, relatively harmless gas. However, as it passes through the vapor generator and comes into contact with oxygen, it reacts to form nitrogen dioxide (N or other oxides of nitrogen collectively referred to as nitric oxides. Nitrogen dioxide is a yellow-brown gas which, in sufficient concentrations is toxic to animal and plant life. It is this gas which may create the visible haze at the stack discharge of a vapor generator.

Nitric oxide is formed as a result of the reaction of nitrogen and oxygen and may be thermal nitric oxide and/or fuel-nitric oxide. The former occurs from the re action of the nitrogen and oxygen contained in the air supplied for the combustion of a fossil fuel whereas the latter results from the reaction of the nitrogen contained in the fuel with the oxygen in the combustion air.

The rate at which thermal nitric oxide is formed is dependent upon any or a combination of the following variables; l flame temperature, (2) residence time of the combustion gases in the high temperature zone and (3) excess oxygen supply. The rate of formation of nitric oxide increases as flame temperature increases. However, the reaction takes time and a mixture of nitrogen and oxygen at a given temperature for a very short time may produce less nitric oxide than the same mixture at a lower temperature, but for a longer period of time. In vapor generators of the type hereunder discussion wherein the combustion of fuel and air may generate flame temperatures in the order of 3,700F, the time-temperature relationship governing the reaction is such that at flame temperatures below 2,900F no appreciable nitric oxide (NO) is produced, whereas above 2,900F the rate of reaction increases rapidly.

The rate at which fuel nitric oxide is formed is principally dependent on the oxygen supply in the ignition zone and no appreciable nitric oxide is produced under a reducing atmosphere; that is, a condition where the level of oxygen in the ignition zone is below that required for a complete burning of the fuel.

It is apparent from the foregoing discussion that the formation of thermal nitric oxide can be reduced by reducing flame temperatures in any degree and will be minimized with a flame temperature at or below 2,900F and that the formation of fuel nitric oxide will be inhibited by providing a reducing atmosphere in the ignition zone.

With the advent of stricter emission controls, manufacturers of fuel burning equipment have been actively seeking methods of limiting the amount of pollutants which are formed from the combustion of fossil fuel. Heretofore, their efforts have been generally directed at either of the following two methods; one which is commonly referred to as two-stage combustion and calls for initial firing with a deficiency of air and the admission of the remaining air needed for complete combustion at a location remote from the burners, and another which calls for the addition of cooling surface in the combustion zone. While both methods have achieved reductions in nitric oxide formation, they have also resulted in appreciable carryover of unburned combustibles with concomitant loss of operat ing efficiency.

SUMMARY OF THE INVENTION The present invention provides an improved method and apparatus for reducing the formation of nitric oxide while achieving a more complete burning of pulverized fuel than has heretofore been possible.

Accordingly, an improvement is made on pulverized fuel burners of the type disclosed in U. S. Pat. No. 3,049,085 by providing an arrangement whrein at least a part of the fuel burning apparatus is disposed within a windbox to which a portion of the necessary combustion air is supplied and which is formed between the adjacently disposed burner and furnace walls of a vapor generating unit. The burner wall is formed with an access opening for admitting that portion of the fuel burning apparatus which normally resides in the windbox whereas the furnace wall is formed with a burner port which accommodates the combining of fuel and air into a combustible mixture and the ignition thereof. The fuel burning apparatus includes a tubular nozzle which is concentrically disposed about the central axis of the burner and has its outlet end opening adjacent the burner port and its inlet end extending through the burner wall and terminating outside of the windbox. The nozzle defines a central passageway and serves to convey a mixture of pulverized fuel and combustion air for discharge through the burner port into the combustion chamber of the vapor generating unit. A first and second sleeve member are disposed within the windbox to direct combustion air therefrom to the burner port. The first sleeve member has a portion thereof concentrically spaced about the nozzle to form an inner annular passageway therebetween and the second sleeve member has a portion thereof concentrically spaced about the first sleeve member to form an outer annular passageway therebetween. Separate damper or register means are provided for apportioning the flow of windbox air between the inner and outer passageways. A venturi is located in the inlet section of the nozzle and is operatively associated with the conical end portion of an adjustable rod to improve fuel air distribution and provide a fuel discharge pattern which maximizes the effectiveness of the tripartite introduction of combustio'n air to the burner port.

An object of the invention is to provide a pulverized fuel burning apparatus wherein the initial burning of the fuel is conducted under a reducing atmosphere thereby inhibiting the formation of fuel nitric oxide and providing the lower peak flame temperatures required to minimize the formation of thermal nitric oxide.

Another object of the invention is to limit the initial mixing of the fuel and air to cause a recirculating zone which creates a flame stabilizing effect.

A further object of the invention is to admit the remaining air required for complete combustion along a flow pattern which surrounds the fuel-rich mixture and eventually mixes with the fuel for complete combustion.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional elevation view of a vapor generator using fuel burning apparatus embodying the invention.

FIG. 2 is a sectional elevation view of the pulverized fuel burner embodying the invention.

FIG. 3 is a transverse cross-sectional view taken along line 33 of FIG. 2.

DESCRIPTION OF TI-IE PREFERRED EMBODIMENT OF THE INVENTION Referring to FIG. 1 there is shown a vapor generator including water cooled walls 12 which define a furnace chamber or combustion space 14 to which a coal and air mixture is supplied by a pulverized coal burner 16. After combustion has been completed in the furnace chamber 14, the heated gases flow upwardly around-the nose portion 18, over the tubular secondary superheater 20, and thence downwardly through the convection pass 22 containing the tubular primary superheater 24 and the economizer 26. The gases leaving the convection pass 22 flow through tubes of an air heater 28 and are thereafter discharged through a stack 30. It will be understood that the heated gases passing over the superheaters and 24 and the economizer 26 give up heat to the fluid flowing therethrough and that the gases passing through the air heater 28 give up remaining heat to the combustion air flowing over the tubes. A forced draft fan 32 supplies combustion air to the vapor generator and causes it to flow over the air heater tubes and around a plurality of baffles 34 and thence through a duct 36 for apportionment between

branch ducts

38 and 40 respectively.

The air passing through duct 38 is delivered into a windbox 42 and represents a major portion of the air necessary for combustion of the coal being discharged from the nozzle 44 associated with the fuel burner 16. The windbox air is apportioned between an inner annular passageway 95 and an outer annular passageway 96 for discharge through a burner port 50 and into the furnace 14.

The air passing through duct 40 is the remaining portion of air necessary for combustion and is delivered into a primary air fan 52 wherein it is further pressurized and thereafter conveyed through a duct 54 into an air-swept type pulverizing apparatus 56.

The coal to be burned in the vapor generator 10 is delivered in raw form via pipe 58 from the raw coal storage bunker 60 to a feeder 62 in response to the load demand on the vapor generator 10 in a manner well known in the art. The pulverizer 56 grinds the raw coal to the desired particle size. The pressurized air from primary air fan 52 sweeps through the pulverizer 56 carrying therewith the ground coal particles for flow through a pipe 64 and thence to the burner nozzle 44 for discharge through the port 50 into furnace 14.

A damper 66 is associated with the forced draft fan 32 to regulate the total quantity of air being admitted to the vapor generating unit 10 in response to the load demand. A damper 68 is associated with the primary air fan 52 to regulate the quantity of air being introduced through the burner nozzle 44.

It will be appreciated that for the sake of clarity the drawings depict one coal burner associated with one pulverizer wherein in actual practice there may be more than one burner associated with a pulverizer and there may be more than one pulverizer associated with the vapor generating unit.

Referring to FIG. 2 there is shown the pulverized coal burner 16 arranged to fire through the burner port 50, the latter being lined by refractory and formed as a frusto-conical throat diverging toward the furnace side of the wall 12 and being fluid cooled by the tubes 70. An outer burner wall 72 having an access opening 74 is spaced from the furnace wall 12. The space between the burner and furnace walls forms the windbox 42.

The pulverized coal burner 16 includes the cylindrical nozzle 44 having an inlet and outlet portion 44A and 448 respectively. The nozzle 44 defines a central passageway 45 and extends through the access opening cover plate 76, across the windbox 42 to a point adjacent the burner opening 50. An open-ended elbow member 78 is flow connected at one end to the nozzle inlet portion 44A and at the other end to the coal burner pipe 64..

In accordance with the invention there is shown a venturi section 80. disposed within the central passageway 45 and having an inlet and

outlet portion

80A and 80B respectively. The trailing edge of the inlet portion 80A lies in the same transverse plane as the trailing edge of the nozzle inlet portion 44A. The outlet portion 808 of venturi 80 has its leading edge terminating at a point intermediate of the ends of nozzle 44, preferably within the inlet half of the central passageway 45.

A guide tube 82 extends through an end plate 84 of elbow 78 along a longitudinal axis co-axial with that of the venturi section 80. The guide tube is supportedly fixed to the plate 84 and in turn provides support for rod member 86 which extends therethrough in slidable co-axial relationship therewith. The rod member 86 has both ends thereof protruding past the ends of the guide tube 82, one end being located outside of the burner nozzle 45 and including a stop 88 and the other end projecting into the inlet portion 80A of venturi section 80, the latter end being formed with a conical end piece 90 whose side surface is spaced from and parallel to the inner peripheral surface of the venturi inlet portion 80A. The rod member 86 is sized so that its conical end piece 90 may be positioned at any desired location along the longitudinal axis of the venturi inlet portion 80A. The rod 86 is held fixed in the selected position by a lock-bolt 92 which extends through the wall of guide tube 82.

A first and

second sleeve member

94 and 96, respectively, are disposed within the windbox 42 to direct combustion air to the throat section formed within burner port 50. The first sleeve member 94 has a portion 94A concentrically spaced about the outlet portion 44B of nozzle 44 to form an inner annular passageway 95 therebetween. The remaining portion of sleeve 94 is in the form of a flange plate 948 extending laterally outward from the inlet end of portion 94A. An annular wall plate 98 encircles the nozzle portion 448 and is connected thereto. The plates 94B and 98 are spaced from one another to form the inlet 95A to passageway 95 which extends normal thereto. The inner periphery of annular plate 98 is also connected to a sleeve-like section 100 extending along a segment of the outlet portion 44B in contiguous surrounding relationship thereto. The second sleeve member 96 has a portion 96A concentrically spaced about the outlet end of sleeve portion 94A to form an outer annular passageway 97 therebetween. The remaining portion of sleeve 96 is in the form of a flange plate 96B extending laterally outward from the inlet end of portion 96A. An annular wall plate 102 encircles the sleeve portion 94A and is connected thereto. The plates 96B and 102 are spaced from one another to form the inlet 97A to passageway 97 which extends normal thereto.

A plurality of dampers or registers 104 are located within the inlet 95A to passageway 95 and are circumferentially and equidistantly spaced and pivotally connected between and adjacent the outer periphery of the plates 94B and 98. The dampers 104 are adapted to pivot between open, closed and intermediate positions and are preferably interconnected through a linkage train 105 so as to be collectively and simultaneously adjustable through a shaft member 106 operatively connected thereto and terminating outside of the windbox 42 and connected to a manually operated handle 108.

A plurality of dampers or registers 110 are located within the inlet 97A to passageway 97 and are circumferentialiy and equidistantly spaced and pivotally connected between and adjacent the outer periphery of the plates 96B and 102. The dampers 110 are adapted to pivot between open, closed and intermediate positions and are preferably interconnected through a linkage train 107 so as to be collectively and simultaneously adjustable through a shaft member 112 operatively connected thereto and terminating outside of the windbox 42 and connected to a manually operated handle 115.

A plurality of vanes 114 are arranged in surrounding relationship to the sleeve-like section 100 and are located within the inner annular passageway 95, the vanes 114 are equidistantly spaced and preferably linked to one another so as to be collectively and simultaneously adjustable through a shaft member 1 l6 operatively connected thereto and terminating outside of the windbox 42 and connected to a manually operated handle 118'.

If desired, the

shaft members

106, 112 and 116 may be suitably geared or otherwise connected to an operating means (not shown) which would be responsive to an automatic control.

An ignitor assembly 120 of known type extends through cover plate 76 and through the back plate 98 and terminates at the discharge end of annular space 95. An observation tube 122 extends through the cover plate 76 and through the back plate 98 and terminates adjacent to the inside of back plate 98.

FIG. 3 shows a fragmented portion of the windbox side of cover plate 76 and includes the flange plate 968 with the pivots 110A of the dampers 110 extending therethrough. The

sleeve portions

96A and 94A cooperate with one another to form the outer annular passageway 97 therebetween and the nozzle portion 448 and sleeve portion 94A'cooperate to form the inner annular passageway 95 therebetween. The passageway 95 houses the vanes 114 and the discharge end of ignitor 120. The nozzle portion 44B defines the outlet portion of central passageway 45.

In the operation of the preferred embodiment, the coal to be burned in the furnace 14 is delivered in raw form via pipe 58 from the raw coal storage bunker 60 to the pulverizer feeder 62, which regulates the quantity of coal supplied to the pulverizer 56 in response to the load demand on the vapor generator in a manner well known in the art. The pulverizer 56, being of the air-swept type, is supplied with pressurized combustion air from a primary air fan 52, the quantity of air supplied being regulated by a damper device 68 to provide sufficient air to initiate ignition at the burner discharge and provide adequate flow velocity to insure a thorough sweeping of the pulverizer 56, coal burner pipe 64 and nozzle 16. The conical end-shaped rod member 86 is axially adjustable to vary the net' effective area across a section of the venturi 80, thus varying the velocity of the coal-air mixture passing therethrough and providing the means for enhancing fuel-air distribution and the pattern of fuel discharge from nozzle 44.

The total air required for combustion is delivered to the vapor generator by a forced draft fan 32 including a damper device 66 which regulates the quantity of air in response to the load demand on the vapor generator 10 in a manner well known in the art. The combustion air is heated as it comes into indirect contact with the flue gases flowing through the tubes of an air heater 28 and is thereafter conveyed through a duct 36 to be apportioned between

branch ducts

40 and 38, the former leads to the pulverizer 56 as afore-described and the latter leads to the windbox whence the air is apportioned between the inner and outer passageways and 96 respectively.

From the foregoing, it will be noted that three separate flow paths are provided for admitting combustion air to the burner port 50; i.e., the central flow path through the central passageway 45 of nozzle 44 including the venturi section 80, the inner annular flow path from windbox 42 through the inner annular passageway 95 and the outer annular flow path from windbox 42 through the outer annular passageway 97. The regulation of the proportional amounts of air passing through these flow paths coupled with the enhancement of fuelair distribution and the shaping of the fuel discharge pattern constitute major features of the present inven tion. i

Under actual operation, it has been found that main-' taining the combustion air which flows through the central passageway 45 within a range of 15 to 30 percent of stoichiometric air and that which flows through the inner annular passageway 95 within a range of 35 to 45 percent of stoichiometric air creates a stable ignition zone under a reducing atmosphere and provides lower peak flame temperatures. The combustion air which flows through the outer annular passageway 97 is maintained within a range of 55 and 65 percent of stoichiometric air and represents the air needed to complete the combustion of the fuel.

While in accordance with provisions of the statutes there is illustrated and described herein a specific embodiment of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

The embodiments of the invention inwhich an exclusive property or privelege is claimed are defined as follows:

1. In combination with a boundary wall of a furnace, at least one burner port formed in the boundary wall, a burner wall spaced from the boundary wall to form a windbox therebetween to which a portion of the necessary combustion air is supplied, an improved pulverized fuel burner comprising a tubular nozzle having an inlet and an outlet end, said nozzle having at least a portion thereof disposed within the windbox and having the outlet end opening adjacent said port, means for supplying a mixture of pulverized fuel and the remaining portion of the necessary combustion air to the nozzle for discharge into said port, a first and second sleeve member disposed within the windbox to direct the air therefrom to said port, the first sleeve member having a portion thereof concentrically spaced about the nozzle to form an inner annular passageway therebetween, the second sleeve member having a portion thereof concentrically spaced about the first sleeve member to form an outer annular passageway therebetween, and separate damper means associated with each of said passageways for apportioning the flow of windbox air therebetween.

2. The combination according to claim 1 including damper means for regulating the quantity of air flowing through said nozzle.

3. The combination according to claim 1 including separate means disposed outside of the windbox for positioning each of said separate damper means.

4. The combination according to claim 1 including an inlet communicating with said inner annular passageway and housing the damper means associated therewith.

5. The combination according to claim 1 including an inlet communicating with said outer annular passageway and housing the damper means associated there- I with.

6. The combination according to claim 1 including a venturi section disposed within said nozzle.

7. The combination according to claim 6 including an adjustable rod means extending through at least a portion of said venturi section and cooperating therewith to enhance the fuel-air mixture exiting from said nozzle.

8. A method of inhibiting the formation of nitric oxide from the combustion of fuel including at least one burner port, means for supplying pulverized fuel and combustion air to said port, a pulverized fuel burner having a nozzle defining a central passageway, means forming an inner annular passageway about said nozzle, means forming an outer annular passageway about said inner passageway and means apportioning the flow of combustion air between said passageways and comprising the steps of:

conveying a portion of the necessary combustion air through the inner and outer annular passageways for discharge into said burner port,

conveying a mixture of pulverized fuel and the remaining portion of the necessary combustion air through the central passageway for discharge into said burner port, regulating the quantity of combustion air passing through the central passageway'to provide a fuel ignition zone within a reducing atmosphere,

regulating the quantity of air passing through the inner annular passageway to stabilize the ignition zone, and

regulating the quantity of combustion air passing through the outer annular passageway to achieve complete combustion of the fuel.

9. The method according to claim 8 wherein the quantity of combustion air passing through said central passageway is between 15 and 30 percent of stoichiometric air.

10. The method according to claim 8 wherein the quantity of combustion air passing through said inner annular passageway is between 35 and 45 percent of stoichiometric air.

11. The method according to claim 8 wherein the quantity of combustion air passing through said outer annular passageway is between 55 and 65 percent of stoichiometric air.

12. The method according to claim 8 including the step of varying the fuel-air mixture velocity through V UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,188,796 H Dated January 29, 9F

Brett C. Krippene, Morris W. Peterson, and Inv fls) David M. Marshall It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the Title page, the named Assignee should appear as follows:

'The Bahcock 8c Wilcox Company Signed and sealed this 9th day of July 1974.

(SEAL) Attest:

MCCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer v Commissioner of Patents FORM PO-1 0 (1 p p USCOMM-DC 60376-P69.

' a U.5. GDVERNMENT PRINTING OFFICE: IQ, 0-35-3!

Claims

5. The combination according to claim 1 including an inlet communicating with said outer annular passageway and housing the damper means associated therewith.

8. A method of inhibiting the formation of nitric oxide from the combustion of fuel including at least one burner port, means for supplying pulverized fuel and combustion air to said port, a pulverized fuel burner having a nozzle defining a central passageway, means forming an inner annular passageway about said nozzle, means forming an outer annular passageway about said inner passageway and means apportioning the flow of combustion air between said passageways and comprising the steps of: conveying a portion of the necessary combustion air through the inner and outer annular passageways for discharge into said burner port, conveyinG a mixture of pulverized fuel and the remaining portion of the necessary combustion air through the central passageway for discharge into said burner port, regulating the quantity of combustion air passing through the central passageway to provide a fuel ignition zone within a reducing atmosphere, regulating the quantity of air passing through the inner annular passageway to stabilize the ignition zone, and regulating the quantity of combustion air passing through the outer annular passageway to achieve complete combustion of the fuel.

12. The method according to claim 8 including the step of varying the fuel-air mixture velocity through said nozzle.