US1762462A - Method of burning powdered fuel - Google Patents

Description

June 10, 1930. L v. ANDREWS 1,762,462

METHOD OF BURNING POWDERED FUEL Filed March 6 1925 (fig. 2

INVENTOR Lllflndrews Y @AO EY; 2

30 by variations in the ash or moisture contents.

-On the other hand, the amount of air which Patented June 10, 1930 UNITED STATESPATENT OFFICE L. ANDREWS, OF. WORCESTER, MASSACHUSETTS, ASSIGNOB TO RILEY STOKER CORPORATION, OF GHUSETTS WORCESTER, MASSACHUSETTS, A CORPORATION OF MASSA- METHOD or BURNING rownnnnn FUEL pA p plication filed March 6, 1925. Serial No. 13,467.

My invention relates to a method of burning powdered fuel and more particularly to a method of burning such fuel safely and. with a flame close to the burner and to the construction ofthe fuel burner and the associated furnace parts through which the pulverized fuel is introduced into the heating zone of the furnace.

In furnaces of this type, a mixture of air and powdered fuel is introduced into thecombustion chamber and there fired. There are two main types of burner employed for this purpose, one in which the fuel and air are completely mixed before introduction into the furnace and the other in which a jet of powdered fuel having lIlSLlfl'lClQIl't air for perfect combustion is introduced into the furnace and there permitted to intermingle with larger quantities of air.

It has been found that the density of the fuel and air mixture plays an important part in determining the rate of flame propagation throughout the dust cloud. As shown by experimentathe greatest rate of flame propagation of powdered coal and air mixtures is obtained when from 80 to 100 cu.

ft. of airis supplied per pound of coal.

This ratio holds constant over a considerable range of coal and is not greatly changed .13 on the chemical analysis of the coal and 1 the amount of impurities present.

The burner system commonly used at present is that in which the full amount of air is mixed with the fuel before it is 1ntroair is sufiicient for complete combustion of the fuel. Hence as above explained, there is toomuchair present in the mixture for rapid propagation of the flame, andthere is atendency for the flame. to be drawn out id ignition may be expected due to therapid flame propagation in the rich mixture, but combustion cannot be completed until all the secondary air has comeinto contactwith all of the fuel. This system results in the formation of a long flame the length of which depends upon the time taken to tho oughly combine the secondary air and the fuel. It, however, is diflicult, in theaveragc furnace construction, to provide sufficient room for the flame, or vice verse, to make the flame sufficiently short so that all of the fuel present in the powdered fuel mixture Will be completely burned before the flame has reached the end of its path of travel.

It is accordingly the main object of my invention to overcome these diiflculties and to provide a method of burning pulverized fuel which maybe used for fuel mixtures correctly proportioned fol-perfect combus tion and which will permit the formation ofa short flame close to the burner or point of introduction "of the fuel mixture into the V the flame from striking back through the burner and yet insures the flame starting closely adjacent to the burner.

With these and otherobjects in view as will be apparent in the following disclosure, my inventionresides in the steps of the method set forth in the specification and covered by the claims appended hereto.

Referringto the drawings, in which like reference numerals lndlcate llke parts:

Fig. l is a sectional and interior view of a.

furnace and fuel burner, partly broken away; 7

Fig. 2 is an enlarged sectional detail showing the operation of the burner;

Fig. 3 is an interior view of the furnace front and Fig. 4 is a section on the line -l4 of Fig. 3, partly broken away.

In accordance with my discovery, I have found that if the'burner is so constructed and the process of combustion is so carried on that a rich mixture of powdered fuel and air is introduced at one point in the burner adjacent to the leaner mixture, the latter will be ignited due to the fact that the flame will be propagated back through the rich mixture to a point adjacent to the burner. Or, if jets or eddies of a slowly moving fuel mixture are formed in the rapidly moving stream of fuel and air, the flame will strike back to a point where the fuel is traveling ahead faster than the rate of flame propagation. This serves to formpilotj. lights, as it were, close to, the burner and intimately in contact with therapidly traveling'lean mixture of fuel and air. r 1

One feasible way of securing this idealresult is to provide a mixture of powdered fuel and air in correct proportions for com-, plete combustion of the fuel and to pass this mixture into the furnace at a rate any suitable source through' a' pipe 10 .of

substantiallyuniform and rectangular cross section. passed first into a small expansion chamber 11 formed in the front wall of the furnace and thence into the combustion chamber proper of the furnace. At the point where the fuel issues from the pipe 10 into the expansionchamber, Iprovide a grid orseries' of baffles 12.. so constructed and positlonedv astoobstruct the flow of the gas and fuel mixture. The expansion chamber flares inwardly as illustrated so as to provide for. the increasing volume'of gas, and theffuel inlet pipe preferably inclines downwardly so as to direct the flame towards the bottom of the furnace.

" :The baffles: illustrated comprise strips of The fuel mixture .is preferably.

metal having plane rectangular faces of considerable width. Two of these baffles have beenused in one installation'in a horizontal position, as illustrated in Figs. 1 and 2, where the inlet pipe is inclined downwardly. I prefer to mount them: as: shown in'Fig. i so that they be separately adjusted to different angles by means of the damper handles 1 f fastened thereto and projecting through a slot 15 suitably formed in the furnace wall. This permits one to spread-the flame and give it a desired length of--travel, as well as to fillthe furnace with the flame, if desired, and to control the condition of the slag formed by'burning the fuel. The arrangement of baffles illustrated in Fig. 4;

permits-one to spread the flame. The ad justability of the baffles particularly desirable if the furnace construction and the nature of the coal are such as to be conducive to the formation of slag. For example, if the burner pipe is directed downwardly and the flame would ordinarily strilrethe floor and so melt the light ash and forma slag, the bafliesmaybe adjusted to position the flame higher in the furnace and preventslagging the ash, or the shape of the flame may be changed to that found most effective.

' While I do not care to limitthiscase to any particular theory of operation, and it of course is impossible to observewhat is happening within the furnace, it is my belief that these baffles cause fuel particlesto become segregated from the uniform mix ture and to spill over the edges of the baffles in thin sheets, as light snow blows from a roof. It maybe, however, that the frictional retardation of the baffle on the mixture slows down the rate of travel of these thin fuel streams. However, these streams of concen-1 trated fuel or slowly moving bodies tend to ignite easily and more rapidly than the leaner mixture of the surrounding cloud of air and fuel,- and insure a high temperature or a pilot flame nearer the inlet pipe than is obtainable vif the baffles are not present.

Consequently the flame strikes back into this territoryadjacent the fuel burner and. in theexpansion chamber. The heat of these pilot flames causes the gas immediately around'them to expand and to ignite in this chamber andcons'equently by the time the fuel has reached the combustion chamber of: the furnace it is burning fully.

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I moreover preferably soform this grid or baffle construction that it will be selfcleaning and capable of'being protected from the intense heatof the furnace. This is ordinarily insured by making the upper surfaces thereof smooth. The baffles are spaced apart sufrlciently so as not to ob struct seriouslythe introduction of fuel into the furnace. While it is feasible to use water-cooled baffles, 1 preferably so locate the baffle or grid construction that the incoming mixture of gas and fuel will keep the grid cool and prevent its destruction.

It will be readily understood from the above disclosure that various other arrangements may be utilized within the scope of the claims appended hereto. It will also a be understood that this baflie construction is not intended to be used with an imperfect mixture of fuel and air, in order to give a more uniform 'mixture, but it is employed with a fuel and air body which has the desired uniformity of composition, and is intended to destroy locally that uniformity and to segregate fuel particles from the incoming mass. The fuel may be mixed pre viously in a construction of the type shown in the reissued patent to Blyth, No. 16,229 of Dec. 15, 1925.,

Having thus described my invention, what I desire to secure by Letters Patent and claim as new, is

1. The method of burning pulverized fuel comprising the steps of feeding through a burner to a combustion chamber a supply of pulverized fuel and air which are intimately mixed and proportioned for complete combustion of the fuel, while maintaining a rate a of fuel movement through the burner sufficiently greater than the rate of flame propagation to prevent the flame from striking back into the burner and such as would normally cause the flame to burn only at a distance from the burner, causing the fuel miX- ture to issue rapidly into and expand in a largecombustion space, and maintaining a localized fuel zone in the fuel stream in said combustion space but adjacent to the burner which has a more rapid rate of flame propagation than the major portion of the fuel stream and sufficient to cause said major portion to start burning close to the burner.

2.' The method of burning pulverized fuel comprising the steps of feeding to a furnace burner a supply of pulverized fuel and air which are intimately mixed and proportioned for complete combustion of the fuel,

-while maintaining a rate of fuel movement through the burner suficiently greater than the rate of flame propagation to prevent the flame from striking back into the burner and such as would cause the flame to burn only at a distance from the burner, and causing a small portion only of the fuel mixture,

as it enters the combustion space, to contact with an extensive surface arranged at an angle to and in the path of the stream to provide a restricted zone of rapid flame propagation near said surface which burns close to the burner and ignites the main stream of fuel and air and causes it to burn close to the burner.

2nd day of March, 1925.

Signed at Worcester, Massachusetts, this L. v. Annnnws.

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