CA1186951A - Venturi burner nozzle for pulverized coal - Google Patents

Abstract

VENTURI BURNER NOZZLE FOR PULVERIZED COAL
Abstract of the Disclosure A new and improved nozzle for pulverized coal comprises a tubular nozzle having an inlet for receiving a primary, flowing stream of coal/air mix-ture and an outlet for discharging the stream into a furnace. A venturi-like, flow constrictor is coaxially disposed in the nozzle adjacent the outlet end with a divergent flow section terminating at a maximum diameter adjacent the outlet end of the nozzle in the furnace. A convergent venturi section is mounted upstream for concentrating the pulverized coal toward the central portion of the flowing stream and a flow spreader is mounted in coaxial alignment in the diver-gent flow section of the venturi and includes a maximum diameter, open end adjacent the outlet end of the nozzle . Wall surf aces of the spreader cooperate with those of the divergent flow section to form an annular pattern of discharge from the outlet end of the nozzle.
A stream of secondary air is introduced to flow in a rapidly swirling annular pattern around the discharge from the nozzle, and by properly matching the secondary air velocity to that of the primary air/coal discharge velocity, the flame is increased in length and an outer, torroidal recirculation zone is formed around the outside of the coal/air discharge pattern. An inner, recirculation zone is created adjacent the large open end of the flow spreader inside the primary air/coal discharge pattern. A plurality of swirl vanes are provided between the outer surface of the spreader and the adjacent facing surface of the diver-gent flow section for imparting a swirling action for stabilizing the annular discharge pattern of the pri-mary coal/air stream from the conduit into the furnace.
This stabilized discharge pattern forms a wall or shield from the secondary air to provide a high tem-perature, reducing stagnant area adjacent the open end of the flow spreader. The inner and outer recir-culation zones draw a portion of the hot combustion products back toward the open end of the flow spreader and volatiles in the coal are driven off rapidly in this high temperature, reducing atmosphere, minimizing the formation of NOX.

Description

BAC~CROIIIID or 1~5 In lo ~LÇA~ b.` ~lvertlon l'he present invention relates to venturi nozzles for pulverized ::oal, and a novel method for introducing pulverized coal to a f urnace in a manner which minimizes the formation of oxides of nitrogen in the burnirlg process.

A wide variety of burner designs hav~ been developed over the years and some of the burners used in f urr~aces I boiler~ and ~he like have beerl especially suited f or burnin~ pulveri zed coal . One of the major problems in burning pulverized coal as well as o~cher fossil fuels is ~he production of oxides of nitrogen in the combustion process. Many attempts at burner desi gn have been di rected toward reducing ~he amount of oxides oE ni~rogen tha~c are Eormed. Such oxides, known as NOX cause air pollution and are generally ohj ect i onabl e ~, . The following U.S. Patent~ are directed towards burners for iEurnaces and the like which employ pulverized coal or other hydrocarbon fossil fuels as a source o:E energy for combustion:
246,321 Litchfield et al 3,007,084 Thomasian et al 251,342~135 Sc~midt 3,150,710 Miller 1,817,911 Andrews et al 3,450,504 Korwin 1,9531090 Vroom 4,019,851 Smith et al 1,993,901 Silley 4,089,628 B}ac:kburn

2,158/521 Nahigyan 4,147,116 ~;raybill 302,325,318 Hendri~ 4,157,,889 130nnel 2,823,628 Poole ek al 41228,747 Smirlock èt al I t is an ob; ect of the present invention to provide a new and improved, venturi nozzle for use in 35 burnin~ pulveriæed coal, hydrocarbons and other fossil fuels.
More particularly, it is an object of the present inven-tion to provide a nozzle of the character described and which pro-duces a reduced or minimal amount of oxides of nitrogen in the combustion process.
The improved venturi nozzle disclosed herein is especially well suited for burning pulverized coal. Applied to a conventional burner for pulverized coal it provides an extra long flame pattern and means for drawing back the flame toward a stagna-tion area of high temperature and reducing atmosphere wherein thevolatiles in the coal are driven off rapidly without any substan-tial formation of oxides of nitrogen. The burning time interval is increased with a resultant lower peak flame temperature. The improved pulverized coal burner is relatively simple and straight forward in construction and operation, and is extremely economical to operate in a variety of different coal burner applications.

BRIEF SUMMARY OF THE INVENTION
The invention provides a burner nozzle for pulverized coal and other fuels comprising: tubular nozzle means having an inlet for receiving a primary flowing stream of coal/air mixture and an outlet end for discharging said stream into the combustion zone of a furnace for burning; annular, venturi-like flow con-strictor means in said nozzle means coaxially disposed adjacent said outlet end, said flow constrictor means having a divergent flow section with a maximum diameter outlet adjacent said outlet end of said nozz]e means and a convergent flow section upstream thereof for more evenly distributing said pulverized coal in the central portion of said stream; flow spreader means mounted in coaxial alignment in said divergent flow section for adjustable axial movement in sald nozzle means, said spreader means having a maximum diameter, open end adjacent said ou-tlet and a wall surface cooperating with wall surfaces of said divergent flow section to form a diverging annular-shaped flow passage; and swirl vane means positioned in said passage between said spreader means and surfaces of said divergent flow section for imparting a swirling action to stabilize the combustion of said coal/air stream, said stabilized discharge flow providing a wall for shielding a high tempera-ture stagnation area formed adjacent said open end of said flow spreader means for drawing at leas-t some combustion products back toward said spreader means whereby volatiles in said coal are driven off in a reducing atmosphere thereby reducing the formation of oxides of nitrogen by said burner.
From another aspect, the invention provides a method of burning pulveri.zed coal and other fuels comprising the steps of:
accelerating a flowing stream of coal/air mixture in a convergent venturi sec-tion to distribute the coal particles toward a central portion of the stream; partially decelerating said stream while forming a shallow, annular, conically shaped flow pattern around a hollow spreader cone; directing said annular flow pattern to swirl around the axis of said cone and form a stable, annularly-shaped, flow pattern discharging into a combustion zone; burning said discharging coal with an elongated flame pattern extending from said stabilized conical flow pattern; forming a shielded stagnation area of high temperature, reducing atmosphere adjacent said cone for driving off the volatiles in said coal early in the combustion process; and burning off said volatiles in a continuing : . . . ~ , .
.~

burning process minimizing the formation of oxides of nitroyen.
A flow of secondary air is preferably introduced to form an annular, outer, swirling air pattern surrounding the primary coal/air stream dis-~4 charged from the outlet end of the conduit~. Theflow velocity of the primary and secondary streams are regulated to form a torroidal recirculation zone outwardly surrounding the coal/air stream.

For a better understanding of the present invention, reference should be taken in con~unction with the drawings, in which:
FIG. 1 is a cross-sectional view of a new 10 and improved burner nozzle for pulverized coal con-structed in accordance with the features pre~ent invention;
~ I G . 2 i s a cr os s - sec t i onal vi ew o~ a bur ne r assembly employing the burner noæzle of FIG. l; and FIG~ 3 is a transverse, cross-sectional view taken substantially along lines 3-3 o FIG. 1.
DESCRIPTION OF A PREFERED EMBODIMENT
Referring now more particularly to the drawi ngs; therei n i5 illustrated a new and improved venturi no2zle ~or burning pulverized coal and other fossil fuels generally referred to in FIG. 1 by the reference nwneral 24. A burner assembly 10 adapted to employ the burner nozzle 24 is illustrated ~n FI~
2. The burner nozzle 24 incïudes a primary, hollow, l~ubular, discharge c:onduit or nozzle 12~ preferably formed of steel with a circular, transverse cross--section and mounted to extend into the center of a circular opening 14a fors~ed in the wall 14 of a f ur nac e .
The venturi nozzle discharges pulveriæed coal and primary air into the frustro-conical burner throat 23 and swirling secondary air is in~roduced into the khroat in the anr~ular space surrounding the venturi nozzle 12 along flow lines "B". The swirling action of the secondary air is imparted by a plurality of swi rl vanes 2 9 whi ch are mounted on rotatable support axles 31 " extending between the front and rear annular plates 27, 33 of the secondary air re~ister which surrounds the burner assembly 10, and 5 supplies air indica~ced by arrows C between plates 27, 33 .
~ he vanes 29 are collectively controlled to pivot in unison and for this purpose, a vane ring control assembly 37 is provided adjacent the outer 10 surf ace of the outer regis~er plate 33 . A chain and sprocket drive system 39 driven and controlled by a shaf t 41 and a handwheel 43 positioned outside of the burner front 41 is provided for selectively adjusting the angl e of the vanes 29 .
The burner ~rsnt 47 is forme~ with a central opening A7a in order to accommodate a primary coal/air supply ~onduit 51 which supplies a flow of pulverized coal and prim~ry air to the burner noæzle 12. A~
viewed in FIGo 2, a left-hand (inner) end portion of ~he supply condui~ 51 also provides support for the burner nozzle 12 which is mounted for telescopic longi~
tudinal sllding movement thereon. Control of the relative longitudinal pOSitiOIl of the noæzle on the supply conduit is attained through two control rod~
53 mova~le in the direc~ions indica~ed ~y arrows A cylindrical burner barrel 69 is mounted in coaxial alignment with the primary supply conduit 51 to extend between the secondary air register plate 33 and the burner front ~7.
The incoming flow o the primaxy coal~air mixture from ~he supply piEe 75 is directed into th~
burne~ nozzle head 80. The plurality of adjustable vanes 79 in the burner nozzle head are used to uni-formly distribut~ the coal/air mixture in the coal nozzle head 80.

p~

The venturi nozzle 24 in FIGo 1 provides a shallow slope~ venturi ~tructure having a generally frustro-conically shaped, divergent~ nozzle outlet - section 28 secured at its minimum diameter (inner) end to a cylindrical, intermediate, throat section 30~ The inlet of the venturi nozzle i5 a frusto-conically-shapedg inlet or convergent, nozzle section 32 having a minimum diameter (inn~r) end joined to the upstream end of the intermediate throat section 30. ~he maximum diameter, upstream end of the conver-gent nozzle section 3~ is mounted within the inside wall surface of the conduit 51 and i5 secured to a .
cylindrical shell 36.
The 1ame pattern issuing from the burner 15 10 is indic:ated in an animated f ashion in the drawings and is referred ~o generally by the reference numeral 34 in FIG. 2. The flame pattern is considerably ~2 to 3. 5 ~imes ) lsnger than th~ flame pattern ormed by typical p~ior art burners. Tbe venturi nozzle 24, 20 mounted in a position adjacent the the outlet end of the conduits 51, is believed to cause a doubling or tripling of ~he average flame length over that normally attalned for a giverl 10w and coal/air discharge velocity. The long flame pattern provides 25 16:)nger residence time for the coal in the coal/air mixture in the furnace to burn so that the coal burns at an lower peak temperature which reduces 2~0X forma-tion .
As the coal/ai r mixture f lows through ~he 30 venturi nozzle the coal particles in the stream are concentrated toward the central portion of the flowing stream and are more uniformly distributed in the primary fuel/air mixture. The venturi nozzle provides an inner shell (arrows E) of coal and air formed aroun 35 the outside of a central, or inner recirculation zone 5~

F. This recirculation zone ls formed at the end of the conic~l coal spreader 40~ The resulting discharge pattern is shown by the divergent arrows E (FIG. 2) which graphically illustrate a generally shallow, frustro annular discharge pattern of the fuel/air s~ream as it en~er~ the c~nbustion zon within ~he f urnace .
In accordance with the present invention, the novel venturi no~zle 24 includes a frustro-conically shaped, hollow, divergent flow spreader 4û, shown in FIGSo 1 and 3, mounted in coaxial alignment within the divergent section 28 s~enturi nozzle. The slopes of the venturi nozzle divergence section 28 and coal spreader 40 define an annular, generally frustr~conically shaped flow passage 38 for directing the discharge of the coal/air stream outwardly into the combustion zone in a shall.ow, f rustro-conical shaped discharge pattern as indicated by the arrows E
in FIG. 2.
In a prototype embodliment of the venturi nozzle 24, the outer dia~eter o~ an outer shell 36 of the f low constrictoi: 24 was constructed to be approxi-mately 7-l/2 i nches and the overall length of the venturi 1102Zle was 17-1/2 inches. The inside diameter of the ~hroat section 30 of the vellturi nozæle was 5-

3/8 inches and the ~xial length of this throat was 3 inches. The axial length of the divergen~ nozzle section 28 was constructed to be B-1/2 inches and the convergerlt nozzle section 32 was 6 inches in length.
3û Accordingly, the angle of convergent slope in section 32 was scmewhat greater than ~he angle of diver~ence in the ~ection 28. The flow spreader 40 was constructed with a maximum diameter at a~ outer open end 40A of about 2-3/4 inches a~d the minimum diameter at the inner end of the spreader was about 1 inch. The spreader had a length in an axial direction of ahout 4 inches. ~es~s wi~h a prototype of the size noted provided excellent results in terms of reduced NOX formation, and acceptable CO emisslon.
The small diameter end of the conical flow spreader 40 .is supported and secured at tbe outer end of thP central support tube 42 mounted in coaxial alignment on the center axis A-A in the burner nozzle 12. The support tube is moveable longitudinally in axial sliding movement in either direction as indicated by the arrows "G" (FIG. 1) by precise positioning of the outer end in the packing gland 770 When the spreader cone 40 is moved inwardly (toward the right as shown in FIG. 2) the annular flow area 38 and the flow cross~section of the diver-gent discharge stream of coal/air mixture may be reduced slightly as the spreader cone is moved closer and cl oser to the ~chroat section 30 of the venturi like, flow constrictor 24. Conversely~ when the support tube 42 is moved in an opposite direction (~o the left ), the flow area i~ increased . The velocity of the stream discharged f rom the out].et end 28a of the divergent flow section 28 may be readily controlled by movement of the spreader cone relative to the flow constrictor 24~
In order to stabilize combustiQn,venturi nozzle sec~ion 28 is provided with a plurality of swirl vanes 52 mounted on the outer sur~ace of tbe spreader cone 40. These vanes impart a swirling action (arrows K~ FIG~ o the primary coal~air stream in the passage 38 between the spreader CORe and the in~ide surf ace of the divergent noz~le section 28 adjacent ~he outlet end 28a, The swirling action of the discharging coal/air stre~ imparted by the swirl vanes 52 increases the stability of the flane 6~

pattern 34 in the combustion 20ne and in the area immediately adjacent the outlet end 12b of the nozæle 12.
The swirling primary coal/air strPam forms a wall surrounding a stagnant area (labeled F in FI~. 2), immediately adjacent the hollow outer end of the cone 40. The stagnant area F has a relatively low pressure and provides a reducing atmosphere Gf high temperature resulting in the volatiles in the pulverized coal beinq driven off and burned with minimal formation of oxides of nitrogen or NOx. This is accomplished because of the reducing atmosphere, and the high tem-peratures in this area.
The proper matching of velocities between swirling secondary air (arrows B) and the swirling primary coal/air stream E discharged from the outlet end of the burner nozzle 12 is believed to provide a second or outer recirculation zone H of torroidal sonfiguration outside and around the stagnant area Fo The entry of secondary or outsi.de air into the primary coal/a.ir mixture is minimi2ed so that a reducing atmo-sphere of high temperature is maintained. The concen-tric inner and outer recirculat:ion zones cause a por-tion of the combustion products to be drawn back towards the burner nozzle outlet 12b as indicated by - the inner and outlet flame path arrows. A rapid devolitilization and combustion of ~he coal is thus accomplished wi~hout forming excessive quantitles of oxides of nitro~en (NOX) which are polluting to the atmosphere.
The venturi no~zle 24 applied to a conven-tional swirl stabilized burner provides stratified combustion because of ~he novel geome~ry of he burner venturi nozzle as described above, and ln a manner resulting in a reduced production of nitrogen oxide emissions. The venturi nozzle 24 is positioned adja-cent the outlet end of the nozzle 12 and this forms a long flame pattern to provide a lower peak flame tem-perature~ The average time interval for burning of the coal is substantially increased thereby to maintain efficient combustion~
The convergent or entry section 32 of the venturi nozzle 24 tends to concentrate the coal par-ticles toward the central portion of the accelerating coal/air stream and more evenly distributes the coal in the primary flow. This stream passes into a condi-tion of low pressure and high velocity in the throat.
section 30 and subsequently, the coal/air stream is decelerated whi.le forming an annularly shaped, swirling flow pattern around the hollow spreader cone 40. The annular stream is caused to swirl by the swirl vanes ~2 in ~he outlet passage 38 between the confining annular surfaces of the spreader cone 40 and the inner surface of the divergent nozzle section 28. The swirling action tends to stabilize combustion. The swirling action also helps to establish the stagnation area F early in the combustion process at the open end of the spreader cone 40~ In thi~ area volatiles in the coal are evolved and burned in a high tempera-ture, reducing atmosphere without significant forma-tion of oxides of nitrogen. The coal burning process in the flame pattern 34 takes place over a relatively long time period and the peak flame during the combus-tion process i~ considerably lower tha~ in prior art burners, These factors are also believed to contribute to the reduced amount of oxides of nitrogen that are formed.
Although the present invention has been described ~ith reference to a single illustrated embodi-ment thereof r it should be understood that numerous other modifications and embodimersts can be made by~hose skilled in ~he art that wiLl fall within the spirit and scope of ~he principles of this invelltion.
What is claimed as new and is desired to be 5 secur d by Letters Patent i5:

Claims (12)

1. A burner nozzle for pulverized coal and other fuels comprising:
tubular nozzle means having an inlet for receiving a primary flowing stream of coal/air mixture and an outlet end for discharging said stream into a the combustion zone of a furnace for burning;
annular, venturi-like flow constrictor means in said nozzle means coaxially disposed adjacent said outlet end, said flow constrictor means having a diver-gent flow section with a maximum diameter outlet adjacent said outlet end of said nozzle means and a convergent flow section upstream thereof for more evenly distributing said pulverized coal in the central portion of said stream;
flow spreader means mounted in coaxial alignment in said divergent flow section for adjust-able axial movement in said nozzle means, said spreader means having a maximum diameter, open end adjacent said outlet and a wall surface cooperating with wall surfaces of said divergent flow section to form a diverging annular-shaped flow passage; and swirl vane means positioned in said passage between said spreader means and surfaces of said divergent flow section for imparting a swirling action to stabilize the combustion of said coal/air stream, said stabilized discharge flow providing a wall for shielding a high temperature stagnation area formed adjacent said open end of said flow spreader means for drawing at least some combustion products back toward said spreader means whereby volatiles in said coal are driven off in a reducing atmosphere thereby reducing the formation of oxides of nitrogen by said burner.

2. The burner nozzle of claim 1 wherein said flow constrictor means includes a generally cylin-drical, intermediate throat section of minimum diameter joined at opposite ends to minimum diameter ends of said divergent flow section and said convergent flow section, respectively.

3. The burner nozzle of claim 1 wherein said convergent and divergent flow sections of said flow constrictor means are substantially frustro-conical in shape.

4. The burner nozzle of claim 1 wherein said convergent flow section is shorter in axial length than said divergent flow section and is adapted for distributing the pulverized coal in said stream in a central portion thereof, said divergent flow section substantially maintaining said distribution as said stream diverges in said divergent section.

5. The burner nozzle of claim 1 wherein said flow constrictor means includes a cylindrical outer shell mounted for axial displacement in coaxial alignment in said tubular nozzle means.

6. The burner nozzle of claim 1 wherein said spreader means is generally frustro-conical in shape with a minimum diameter end upstream of said open end.

7. The burner nozzle of claim 6 including support means secured to said upstream end of said spreader means and mounted for axial displacement in coaxial alignment along a central axis of said tubular nozzle means.

8. A method of burning pulverized coal and other fuels comprising the steps of:
accelerating a flowing stream of coal/air mixture in a convergent venturi section to distribute the coal particles toward a central portion of the stream;
partially decelerating said stream while forming a shallow, annular, conically shaped flow pattern around a hollow spreader cone;
directing said annular flow pattern to swirl around the axis of said cone and form a stabile, annularly-shaped, flow pattern discharging into a combustion zone;
burning said discharging coal with an elongated flame pattern extending from said stabilized conical flow pattern;
forming a shielded stagnation area of high temperature, reducing atmosphere adjacent said cone for driving off the volatiles in said coal early in the combustion process; and burning off said volatiles in a continuing burning process minimizing the formation of oxides of nitrogen.

9. The method of claim 3 including the step of forming a zone of low pressure adjacent the center axis of said discharging flow pattern for drawing back a portion of the combustion products toward said cone to establish a recirculation area of high temperature to rapidly volatilize and burn off said volatiles.

10. The method of claim 9 including the step of forming a wall of swirling coal and air around said recirculation area to provide a reducing atmo-sphere at high temperature.

11. The method of claim 11 including the step of creating a torroidal recirculation zone around said stabilized annular flow pattern by introducing a swirling flow of secondary air.

12. The method of claim 11 including the step of adjusting the velocity of said secondary air flow to establish said outer torroidal recirculation zone around said stabilized, annular flow pattern.