US2293737A - Method of and apparatus for burning pulverized fuel - Google Patents

Description

Aug. 25, 1942. R; M. HARDGROVE METHOD OF AND APPARATUS FOR BURNING PULVERIZED FUEL Filed Sept. 8, 1934 INVENTOR I I al v12 M Hardy/ave A R NE Patented Aug. 25, 1942 METHOD OF AND APPARATUS FOR BURNING PULVERIZED FUEL Ralph M. Hardgrove, Westfleld, N. J., assigmor to The Babcock 8r Wilcox Company, Newark, N. L, a corporation of New Jersey Application September 8, 1934, Serial No. 743,276

22 Claims.

This invention pertains to combustion apparatus and more particularly to methods and apparatus for burning pulverized fuel in suspension in furnace chambers.

An object of the invention is to provide an improved method and apparatus for preparing and burning pulverized fuel in suspension, whereby a fuel which is comparatively diflicult to ignite, for example, a fuel low in combustible volatile matter, readily ignites and burns with a steady and stable flame.

The above and other objects are effected by my invention as will be apparent from an examination of the following description taken inconnection with the accompanying drawing in which:

Fig. 1- is a vertical sectional view illustrating my improvements as applied to a boiler furnace;

Fig. 2 is an enlarged horizontal sectional view of the classifying device used in connection with my invention, the section being taken on the line 22 of Fig. 1.

In apparatus and methods for burning pulverized fuel in suspension in furnace chambers heretofore known to me, it is customary to mix air with the fuel in sufficient quantities to effect the transportation of the fuel to a burner, through which it is introduced into the furnace chamber. In direct-firing systems, the transporting air flows through the pulverizing mill wherein the fuel is entrained in the air as the fuel is pulverized and the stream of fuel and air .thus formed is conducted directly to the burner. In indirect-firing systems, where the fuel is stored in bins or bunkers after it is pulverized and from there conducted to the furnace, the transporting air is mixed with the fuel as it comes out of the bin and the mixture conducted to the burner.

After a furnace of the above type is placed in operation, the fuel entering the furnace chamber through the burner is heated to ignition temperature and ignited by the heat from the flame of previously introduced fuel and also by the heat reflected from hot bodies in the furnace, such, for example, as the furnace walls. In a properly operating furnace the fuel is positively ignited as it enters the furnace chamber and-bums with a steady and stable flame. The effectiveness of ignition and degree of stableness of the flame depends upon the rate of flame propagation; and the latter depends upon the temperature of the furnace chamber, the particular characteristics of the fuel burned, and the amount of air present in the entering stream. The flame propagating rate is highest when the furnace is operated at ture, when the fuel burned contains a substantial amount of combustible volatile matter, and when there is a relatively high concentration of fuel in the stream.

Certain types of fuel which are low in combustible volatile matter are difficult to ignite and keep burning when the apparatus and methods -for burning the fuel heretofore known to me are used. I have found that when such a fuel is introduced into a furnace with a minimum of air, or .in other words, if the entering stream is rich in fuel particles, the fuel can be ignited and burned with a steady and stable flame.

An example of a fuellow in combustible volatile matter is pulverized anthracite coal. Heretofore, in pulverized anthracite fuel installations considerable difficulty has been experienced in the ignition of the stream of pulverized anthracite and air, due to the fact that the amount of air normally used to convey the fuel is in excess of that desirable to produce proper flame stability. It is difficult, however, to reduce the supply of air because of its necessity as a vehicle for the movement of the fuel. Furthermore, in the conventional direct-fired installation for pulverized anthracite, air in excess of what would normally be required for transportation of other fuels, has to be used. This latter is because of the fact that the amount of air circulated through a pulverizits transportation to the furnace.

According .to the present invention, I obviate the above difficulties by classifying the mixture of fuel and air coming from the pulverizer or bin into two streams, one stream having a high concentration of fueLi. e., relatively low primary aircoal ratio and the other stream being a relatively leaner mixture, i. e., a relative high primary air-coal ratio; and the two streams are introduced into the furnace through separate burners and in separate paths and, according to a specific aspect of my invention, in parallel paths of different lengths relative to the heating gas outlet. The stream having the greater concentration of fuel ignites and burns with a steady and stable flame, and the lean mixture stream mixes with the rich mixture stream after the high ratings resulting in a high furnace temperalatter is ignited.

In the drawing:

Fig. 1 is a side elevation, partly in section, of a pulverized coal fired steam boiler unit constructed in accordance with the invention; and

Fig. 2 is a horizontal section taken on the line 2--2 of Fig. 1.

Referring now in detail to the construction illustrated, the reference numeral l indicates a pulverizing mill, to which thefuel to be pulverized is supplied by a suitable hopper communicating at its lower end with a feeder l2 connected to the inlet of the mill. Fuel may be supplied to the hopper from any suitable source (not shown). I have shown the mill as being driven by anelectric motor l3 operatively connected to the mill by a flexible drive I4.

A stream of air under pressure is supplied to the mill from a power-driven fan IS, the discharge of which is connected to a conduit l6 which in turn is connected at itslower end to the interior of the mill. If the fuel to be pulverized is wet, the inlet of the fan l will be connected by a conduit H to a source of hot air, for example, a conventional air preheater II, as indicated in Fig. 1. The fuel-laden air flows out of the mill through an outlet disposed at the upper end thereof and connected to the lower end of a conduit [8.

In operation, the fuel in the hopper is fed into the mill by the feeder l2 and is pulverized. The stream of air under pressure flowing through the mill from the fan l5 entrains therein the particles of pulverized fuel and carries such particles out of the mill through an outlet into the conduit l8. The apparatus thus far described and the operation thereof, is well known to those skilled in the art, and further showing or description, therefore is deemed unnecessary.

The upper end of the conduit I8 is connected to a centrifugal classifying device which, according to the present invention, may be of any suitable design. I have shown the device 20 as comprising a casing 2| having a tangentiallyarranged inlet 22, to which the upper end of the conduit I8 is connected (see Fig. 2). Suspended within the casing from the top thereof and arranged coaxially with respect thereto, is a cylindrical member 23 which is open at its lower end and communicates at its upper end with one ranged to direct a stream of fuel-and air vertically-downward in the furnace chamber and in a path approximately parallel to the path of the stream introduced through the burner 25. The side-by-side arrangement of the burners 25 and at one side of but unequally spaced from the. heating gas outlet of the furnace provides initially parallel U-shaped flow paths for the two fuel-air streams of different lengths of travel in the furnace to the furnace gas outlet. Specifically, the rich fuel-air stream will have a shorter path of travel in the furnace than the lean fuel-air stream.

From the above description of the classifying devicev 20 it will be apparent that the mixture of fuel and air entering the casing 2| at anappreciable rate through the tangentially-arranged inlet opening 22 will result in the mixture of fuel and air moving within the casing in a circular direction and about the vertical axis thereof. By virtue of the centrifugal force, incident to the movement of the mixture in a circular direction in the chamber, the heavier constituents of the stream, which will be the major portion of the fuel mixed with a smallamount of air, will concentrate at the outside of the casing and flow downward along the walls of the casing and through the pipe 29 to the burner 30. A relatively small amount of the fuel and the major portion of the air will collect at the center of the centrifuge and pass upwardly out of the casing 2| through the open lower end of the cylindrical member 23 and through the pipe 24 to the burner 25.

It is to be observed, that the air entering the mill from the fan l5 and serving as a vehicle for the pulverized particles of fuel, is what is' usually referred to as primary air and the amount of primary air used is a fraction of the total air required for the completion of combustion. I additional or secondary air necessary for the completion of combustion may be introduced into the furnace in any suitable manner. I have shown the secondary air as being introduced into the furnace from the hollow interior of the front wall 35 of the furnace 21 through openings 28 provided in the front wall. The secondary air is supplied to the hollow interior of the front wall from the hollow side walls 31 (only one appearing in the drawing). A fan 38 supplies air under sired type of pulverizing apparatus can be used for reducing the fuel to proper size for burning in suspension, and air in any amount required for transportation or drying of the fuel can be used without interfering with the ignition of the fuel and the maintenance of a stable flame in the furnace chamber. As stated, the mixture of fuel and air is classified by the centrifugal device 20, to provide a stream relatively rich in fuel particles, and a relatively lean stream. As the two streams are introduced into the furnace in approximately parallel paths, the rich stream easily ignites due to its good flame-propagating characteristic and burns with asteady and stable flame and without harmful interference from the adjacent lean mixture stream. The lean mixture stream merges with the rich mixture stream after the latter has been ignited and is burning, and the mixing aids in the combustion of the fuel in. both streams.

While I have shown my invention in only one form, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various changes and modifications without departing from the spirit thereof, and I desire, therefore, that only such limitions shall be placed thereupon as are imposed by the prior art The forth in the appended separate streams, one consisting of a rich fuelair mixture and the other containing a relatively lean fuel-air mixture, and said conduit means delivering the stream containing the relatively lean fuel-air mixture to the fuel burner having the greater distance from its discharge end to said outlet.

2. Pulverized fuel burning apparatus comprising a fuel pulverizer of the air-swept type, a furnace having a heating gas outlet, a plurality of separate pulverized fuel burners arranged to discharge fuel into said furnace in separate paths having different lengths of travel in said furnace relative to said outlet, conduit means for delivering pulverized fuel-laden air from said pulverizer simultaneously to said fuel burners including a classifier for separating the fuel-laden air into two separate streams, one consisting of a rich mixture of the heavier fuel particles and the other consisting of a lean mixture of relatively lighter fuel particles and air, said conduit means delivering the rich mixture stream to the fuel burner having the shorter distance from its discharge and to said outlet.

3. Pulverized fuel burning apparatus comprising a fuel pulverizer of the air-swept type, a furnace having a heating gas outlet, a plurality of separate pulverized fuel burners arranged to discharge fuel into said furnace in separate paths having different lengths of travel in said furnace relative to said outlet, conduit means for delivering pulverized fuel-laden primary air from said pulverizer simultaneously to said fuel burners including a centrifugal classifier for separating the fuel-laden primary air into two separate streams, one consisting of a rich mixture of the heavier fuel particles and primary and the the other consisting of a lean mixture of relativeley lighter fuel particles and primary air, said conduit means'delivering .the rich mixture stream to the fuel burner having the shorter distance from its discharge end to said outlet, andmeans for supplying secondary air to said furnace along the path of fuel travel.

4. Pulverized fuel burning apparatus comprising a fuel pulverizer of the air-swept type, a

furnace having a plurality of separate pulverlean mixture of relatively lighter fuel particles l air-fuel proportioning in said streams.

5. The process of pulverizing and direct firing anthracite coal having a low volatile content which comprises pulverizing the coal, supplying sufiicient primary air during pulverization to carry the pulverized coal in suspension upwardly out of the pulverizing zone and form a primary air-coal stream having a relatively high primary air-coal ratio and a relatively ,low velocity of flame propagation, classifying the primary aircoal'stream to substantially change its primary air-coal ratio and divide the stream into two separate streams, one having a relatively low primary air coal ratio and the other a relatively high primary air-coal ratio, then separately delivering the two primary air-coal streams to a combustion zone, discharging the low primary air-coal ratio stream into the combustion zone through a U-shaped path of travel therein, separately introducing the high primary air-coal ratio stream in a direction to mix with the low primary air-coal ratio stream after the latter has been ignited, and supplying secondary air to the combustion zone at points spaced along the path of travel of the low primary air-coal ratio stream therein.

6. The process of pulverizing and burning anthracite coal having a low volatile content which comprises pulverizing the coal, supplying sufficient primary air during pulverization to carry the pulverized coal in suspension out of the pulverizing zone. and form a primary air-coal stream having a relatively high primary air-coal ratio and a relatively low velocity of flame propagation, classifying the primary air-coal stream to substantially change its primary air-coal ratio and divide the stream into two separate streams, one having a relatively low primary air-coal ratio and the other a relatively high primary air-coal ratio, then separately delivering the two primary air-coal streams to a combustion zone, discharging the low primary air-coal ratio stream in a vertically downward direction into the combustion zone through a U-shaped path of travel therein, separately introducing the high primary air-coal ratio stream in a direction to mix with the low primary air-coal ratio stream after the latter has been ignited, and supplying secondary air to the combustion zone at spaced points along the downward path of travel of the low primary aircoal ratio stream therein.

7. The process of pulverizing and direct firing anthracite coal having a relatively low volatile content which comprises pulverizing the coal,

supplying suflicient primary air during pulveriza-.

ity of flame propagation, centrifugally classify ing the primary air-coal stream to substantially change its primary air-coal ratio and dividing the stream into two separate streams, one having a relatively low primary air-coal ratio and the other a relatively high prima y air-coal ratio, then separately introducing the two primary air-coal streams into a combustion zone and introducing the high primary air-coal ratio stream in a direction to mix with the low primary air-coal ratio stream after the latter has been ignited, and supplying secondary air to the combustion zone at points spaced along the path of travel of the low primary air-coal ratio stream therein.

8. The process of pulverizing and direct firing wet anthracite coal having a relatively low volatile content which comprises pulverizing the coal, supplying sufiicient preheated primary air to dry the coal during pulverization and to carry the pulverized coal in suspension upwardly out of the pulverizing zone and form a primary air-coal stream having a relatively high primary air-coal ratio and a relatively low velocity of flame propagation, centrifugally classifying the primary aircoal stream to substantially change its primary air-coal ratio and dividing the stream into two separate streams, one having a relatively low pri- ;mary air-coal ratio and the other a relatively high primary air-coal ratio, then separately introducing the two primary air-coal streams into a combustion zone and introducing the high primary air-coal ratio stream in a direction to mix with the low primary air-coal ratio stream after the latter has been ignited, and supplying preheated secondary air to the combustion zone at points spaced along the path of travel of the low primary air-coal ratio stream therein.

9. Pulverized fuel burning apparatus comprising a fuel pulverizer of the air-swept type, walls forming a pulverized fuel burning furnace including a front wall having a series of vertically spaced secondary air ports opening-to the furnace, a pulverized fuel burner arranged to discharge downwardly into said furnace along said front wall, a centrifugal classifier, a conduit delivering a stream ofpulverized fuel laden primary air having a relatively high air-fuel ratio from said pulverizer to said classifier, said classifier being arranged to deliver a stream of fuel and primary air having a relatively low air-fuel ratio to said fuel burner, and a separate conduit arranged to convey a stream of fuel and primary air having a relatively high air-fuel ratio from said classifier and discharge the same into said furnace at a point spaced from said fuel burner.

10. Pulverized fuel burning apparatus comprising a fuel pulverizer of the air-swept type, walls forming a pulverized fuel burning furnace including a front wall having a series of vertically spaced secondary air ports opening to the furnace, a pulverized fuel burner arranged to discharge in a vertically downward direction into said furnace along said front wall and a second pulverized fuel burner arranged to discharge into said furnace in a direction to mix with the discharge from said first burner at a point spaced from said burner, a centrifugal classifier, a single conduit delivering pulverized fuel laden primary air having a relatively high air-fuel ratio from said pulverizer to said classifier, a conduit arranged to deliver a stream of fuel and primary air having a relatively low air-fuel ratio from i said classifier to said first burner, and a separate conduit arranged to deliver a stream of fuel and primary air having a relatively high air-fuel ratio from said classifier to said second burner.

11. In a direct fired pulverized fuel burning furnace installation, the process of pulverizing and burning anthracite coal having a low volatile content which comprises pulverizing the coal, supplying sufficient primary air during pulverization to carry the pulverized coal in suspension upwardly out of the pulverizing zone in a primary air-coal streamlhaving a primary air-coal ratio higher than the desired ratio for producing rapid ignition and maintaining stable flame conditions in the combustion zone, centrifugally classifying the primary air-coal stream to facilitate the separation and removal of a portion of the primary air therefrom and correspondingly reduce the primary air-coal ratio to the desired value, then discharging the reduced primary aircoal ratio stream into the combustion zone, and separately introducing the separated primary air into the combustion zone in a direction to mix with the reduced primary air-coal ratio stream after the latter has been ignited.

12. In a direct fired pulverized fuel burning furnace installation, the process of pulverizing and burning anthracite coal having a low volatile content which comprises pulverizing the coal, supplying sufficient primary air during pulverization to carry the pulverized coal in suspension out of the pulverizing zone in a primary air-coal stream having a primary air-coal ratio higher than the desired ratio for producing rapid ignition and maintaining stable flame conditions in the combustion zone, centrifugally classifying the primary air-coal stream to facilitate the separation and removal of a portion of the primary air therefrom and correspondingly reduce the primary air-coal ratio to the desired value, then discharging the reduced primary air-coal ratio stream in a vertically downward direction into the combustion zone, separately introducing the separated primary air into the combustion zone in a direction to mix with the reduced primary air-coal ratio stream after the latter has been ignited, and supplying secondary air to the combustion zone at spaced points along the downward path of travel therein of the reduced primary air-coal ratio stream.

13. The method of pulverizing and direct firing low volatile fuels which comprises entraining the fuel in suspension in a stream of air, separating part of the air from said stream thereby enriching the remaining fuel and air mixture, delivering the enriched mixture to a furnace for combustion in an ignition zone and delivering the separated air to the furnace outside said ignition zone to support combustion of the ignited enriched mixture.

14. The method of supplying pulverized low volatile fuel to a furnace for combustion which comprises entraining the fuel in suspension in a stream of air, delivering the fuel and air mixture to a separating zone, regulably separating part of the air from said stream thereby variably enriching the remaining mixture, delivering the enriched mixture to the furnace for combustion in an ignition zone and delivering the separated air to the furnace outside said ignition zone to support combustion of the ignited enriched mixture.

15. Apparatus for producing combustion of low volatile fuel in a furnace comprising a burner, means for supplying fuel suspended in a stream of air to the burner, regulatable means for regulably withdrawing part of the air from said stream to thereby variably enrich the remaining fuel and air mixture, means for delivering the enriched mixture to the furnace for combustion in an ignition section thereof, and means for delivering the air withdrawn from the stream to the furnace outside said ignition section to support combustion of the enriched mixture.

16. The method of producing combustion of low volatile fuels which comprises entraining the fuel in a stream of air, causing said fuel and air stream to flow into a separating zone, separating part of the air from said stream thereby enriching the remaining fuel and air, mixture to an extent that ignition may be easily produced and maintained, delivering the enriched mixture in a vertically downward direction to the furnace for combustion in an ignition zone, and delivering the separated air to the furnace outside said ignition zone to support'combustion of the enriched mixture.

17. The methodof producing combustion of low volatile fuels which comprises entraining the fuel in suspension in a stream of air, causing said fuel and air stream to flow in a curved path, withdrawing part. of the air from the fuel and air stream as it flows through the curved path to enrich the remaining mixture to an extent that it may be readily ignited, delivering the enriched mixture to a furnace for combustion in an ignition zone, regulating the degree to which said remaining mixture is enriched, and delivering the separated air to the furnaceoutside said ignition zone to support combustion of the enriched mixture.

18. The method of producing combustion of low volatile fuels which comprises entraining the fuel in a stream of air, causing said fuel and air stream to flow in a curved path; withdrawing part of the air from the fuel and air stream as it flows through the curved path to enrich the remaining mixture to an extent that it may be readily ignited, delivering in a vertically downward direction the enriched mixture to a furnace for combustion in an ignition zone, controlling the degree to which said remaining mixture is enriched, and delivering the separated air to the furnace outside said ignition zone to support combustion of the enriched mixture.

19. Apparatus for producing combustion of low volatile fuel in a furnace comprising means for supplying the fuel entrained in suspension in a stream of air to a separator, means for withpart of the air from said mixture to drawing thereby enrich the remaining mixture to an extent that it can be readily ignited, a separate burner nozzle through which the enriched mixture is delivered to the furnace for combustion in an ignition section thereof, and means for delivering the air withdrawn from said mixture to the furnace outside said ignition section to support combustion of said enriched mixture.

20. Apparatus for producing combustion of low volatile fuel in a furnace comprising means for supplying the fuel entrained in a stream of air to a separator, means for withdrawing part gravity, and means for delivering the air withdrawn from said mixture to the furnace outside said ignition section to support combustion of said enriched mixture.

21. Apparatus for producing combustion of, low volatile fuel in a furnace comprising means for supplying the fuel entrained in a stream-of air to a separator, means for withdrawing part of the air from said mixture to thereby enrich the remaining mixture to an extent that it can be readily ignited, a burner nozzle through which the enriched mixture is delivered to the furnace for combustion in an ignition section thereof, a valve for controlling the nozzle to thereby control the degree to which the mixture is enriched, and means for delivering the air withdrawn from said mixture to the furnace outside said ignition section to support combustion of said enriched mixture.

22. Apparatus for producing combustion of low volatile fuel in a furnace comprising means for supplying the fuel entrained in a stream of air to a separator, means for withdrawing part of the air from said mixture to thereby enrich the remaining mixture to an extent that it can be readily ignited, a burner nozzle through which the enriched mixture is delivered to the furnace for combustion in an ignition section thereof, said nozzle being disposed so that the enriched mixinire flows therethrough by the action of gravity, a valve for controlling the nozzle to thereby control the degree to which the mixture is enriched, and means for delivering the air withdrawn from said mixture to the furnace outside said ignition section to support combustion of said enriched mixture.

RALPH M. HARDGROVE.

Patent No. 2,295,757.

cERT'IFIcATE 0F CORRECTION. 4

August 5 9 2- RALPH m. EARDGROVE.

1 It is hereby certified that error apjpears in the printed specification of the above numbered patent requiring correctionas follows Page 5', first column, line 52, claim 2, after "particlefl'lnsert --and sir--; line 511. before "saicl' insert "and"; line 57, ior"'a nd" read --end-; line 50,

claim 3, after 'primsry ineert --e.ir--- and that the said Letters Patent should be readwith this: cbrreotion therein that the same may conform to the reeord of the case in the Patent oijfice. 4

Signed. and. sealed this 50th day of may, A. 1:. 19%.

Leslie Frazer (Seal) I 4 Acting Commissioner of Patents. '7

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