US1951862A - Apparatus for burning pulverized fuel - Google Patents

Description

March 20, 1934. o. CRAIG 1,951,862

' APPARATUS FOR BURNING PULVERIZED FUEL Filed July 13, 1931 9 23 2a a 6 A9 4 J4 Z2 52 5/ ff 0,, Qawsozv CRAIG 7' Wxi ATTO R N EY Patented Mar; 20, 1934 1,951,862 mm'ros FOR ammo roLvEmzEn Ollison Graig, Worcester, Stoker Corporation,

Mass, assignor to Riley Worcester, Mass, a corporation of Massachusetts Application July 13, 1931, Serial No. 550,494 1 Claims. (01; 110-104) This invention relates to" the combustion of pulverized fuel, and more particularly to appa-' ratus for pulverizing coal or a similar fuel and deliveringit directly to a furnace for combustion E therein.

Pulverized'fuel is ordinarily delivered to a furnace burner by means of a current of primary air which travels at a constant velocity, and any additional. or so-called secondary air necessary 10 to complete the combustion of the fuel is supplied at the furnace, either through the burner or through ports in the furnace walls.- If the demand for heat decreases, it is customary to reduce the quantity of fuel and thequantity of secondary air. If the demand becomes still less, the secondary air may be entirely cut off, and the primary air alone depended upon to support combustion, the rate of fuel supply being reduced to the amount which can be burned by means of the primary air. It will' thus be seen that the amount of the primary air flow determines the minimum rate of heat release in the furnace. The primary air usually represents a substantial proportion, usually at least 15 or 20 per .cent,

'25 of the to al air supplied at the maximum rate of combustion. Hence the range of ratings over which such a furnace can be operated is seriously limited, and it is seldom practical to obtain a ratio of over 5 to 1 between the maximum and minimum combustion rates, although in many installations a very much higher ratio is desired. During periods of low demand in such cases, itis customary to operate the furnace intermittently, which has many practical disadvantages. Such operation not only requires constant supervision on the part of the operators, but it also causes expansion difliculties in the furnace structure and associated parts due to the fluctuating temperatures. Although it has been proposed heretofore to educe the quantity of primary air as the demand or heat decreases, this decreases the velocity of the fuel and air mixture issuing from the burner nbzzle and results in additional difficulties. It is found that if this velocity is made too low, the flame will strike back into the burner nozzle and cause destruction of the burner from overheating. The rate of flame propagation varies with the percentages of volatile matter and ash in the-fuel and with the relative proportions of fuel and air, but in the case of ordinary bituminous coal when burned in pulverizedform it is seldom practical to operate with a primary air velocity at the burner male of less than 2000 feet-per minute. This therefore determines the rate of air flow and prevents further reduction in the combustion rate.

vA still further difliculty is encountered in buming pulverized fuel at low combustion rates. The fuel as it enters the furnace must immediately ignite, or the flame will become extinguished. This ignition is ordinarily obtained largely by means of heat radiated to the incoming fuel from the adjacent walls of the furnace. However, at low combustion rates the furnace temperature decreases, and the heat radiated to the fuel decreases in a much greater ratio, thereby rendering it extremely diflicultto operate under these conditions.

It is accordingly the main object of the invention to provide apparatus for burning pulverized fuel which will make possible operation over a wide range, and particularly to enable the rate of heat release to be reduced to a very low amount when the demand makes it desirable.

It is a further object of the invention to provide apulverizing apparatus connected to supply pulverized fuel directly to a furnace for com- 'bustion, and so arranged that when the demand for heat is low it will supply a finer and hence more easily ignited fuel supply.

It is a further object of the inventionto provide a simple, convenient, and practical construction whereby the rate of flow of the primary air'which transports the pulverized-fuel from a pulverizer to a furnace burner may be controlled as desired and whereby in particular it may be reduced in amount when the demand for heat decreases.

It is a further object of the invention to provide apparatus whereby the primary air; flowing to a furnacemaybe reduced inamount, when the demand for heat is low, without decreasing the velocityof discharge into the furnace to a value below the rate of flame propagation.

It is a further object of the invention to provide a pulverizer connected t dischaIge pulverized fuel at will to either a large main burner or a small low capacity burner, and to provide control means whereby the quantity of primary air may be conveniently reduced when the small burner alone is in operation. r

with these and other objects in view, as will be apparent to those "skilled in the art, my invention resides in the combination of parts described 105 in the specification and covere by the claims appended hereto.

Referring to the drawing illustrating one embodiment of the invention, and in which like reference nmnerals indicate like parts:

Fig. 1 is a view of an apparatus arranged to deliver pulverized fuel to a furnace for combustion, certain parts being shown in section on the line 11 of Fig. 3;

5 Fig. 2 is a view of a portion of the apparatus shown in Fig. 1, with certain parts adjusted to different positions; and

Fig. 3 is an elevation of the pulverizing apparatus in a direction looking away from the furnace, the upper part of the casing being broken away for clearness of illustration.

In the drawing I have shown a pulverizing apparatus 10 of the general type disclosed in the I patent to Andrews No. 1,763,496, which is particularly suitable for. the pulverization of coal and similar fuels. Reference may be had to said patent for a detailed description of the pulverizer. This apparatus comprises a casing 11 shaped to provide a pulverizing chamber 12 and a fan chamber 14 which communicate through an axial passage 15 forming a central outlet from the pulverizing chamber. An inlet chute 16 leads into the pulverizing chamber on the side opposite to the outlet 15, and provides a means for introducing the coarse fuel to be pulverized. The upper wall of the chute 16 has an opening 18 therein for the admission of air, and an adjustable damper 19 is provided to control the rate of flow. The peripheral wall of the fan chamber 14 is shaped as a scroll or volute leading to a discharge opening 20.

A horizontal rotatable shaft 22 extends centrally through the casing 11 and supports a fan 23 in the fan chamber 14 and a vertical rotor disk 24 in the pulverizing chamber 12. Agitator pegs 26 are mounted on the inlet side of the disk 24, and pulverizing pegs 27 are mounted on the outlet side of the disk. These pegs 27-interfit with stationary pegs 28 mounted on the casing wall. A series of preliminary breaker hammers 30 are arranged to revolve adjacent to the'inlet 16, these hammers being carried by a small disk 31 mounted on the shaft 22. Radial rejector arms 32 are mounted on the shaft 22 and arranged to revolve adjacent to the outlet 15.

In order to control the rate at which the coarse fuel is introduced into the pulverizing chamber 12, I provide a suitable feed regulating device. In the illustrated embodiment, this device comprises a rotatable star wheel 33 of well-known construction mounted within the upper portion of chute 16. The-feeding rate may be varied by adjusting the position of a plate 34 pivotally mounted on the chute adjacent to the wheel 33.

In operation, the shaft 22 is rotated at a high speed by a suitable source of power, such as an electric motor or steam turbine, and the fan 23 producesa powerful current of air through the pulverizing chamber 12 and outlet. opening 15. Coarse fuel is delivered to the inlet chute 16 at a desired rate by means of the star wheel 33, and is broken up preliminarily by the hammers 30. The air current then carries the fuel around'the periphery of the rotor disk and into contact with the interfitting pegs 2'7 and 28,'which reduce it to a finely pulverized condition. The fine fuel passes with the air current through the outlet 15 into the fan chamber 14, and is delivered to the discharge opening 20. Coarse particles in the stream are caught by the rejector arms 32 and hurled back into the pulverizing chamber.

I have shown the pulverizing apparatus 10 arranged adjacent to a furnace having a refractory wall 35. A large main burner 36 is mounted upon the wall 35 and arranged to discharge fuel and air into the furnace through a circular flaring opening 37 in the wall. The burner 36 comprises a central discharge nozzle 39 for the fuel and primary air, this nozzle being connected to the discharge 20 of the pulverizing apparatus by means of a pipe 40. Around the nozzle 39, a series of guide vanes 41 are mounted to distribute and whirl the secondary air, which enters the furnace around the outside of the fuel stream. The secondary air is supplied to the burner 36 from an air duct 43, the rate of flow being regulatable by means of dampers 44. It will be understood that the duct 43 is supplied with air by a suitable fan (not shown). A suitable shut off valve45 of the well known quarter turn plug type may be provided in the pipe 40.

In addition to the main burner 36, I preferably provide a small low capacity burner 4'7, which is mounted upon the wall 35 and arranged to discharge fuel and air into the furnace through a circular flaring opening 48 in the 1 wall. This burner 47, as illustrated, is similar to the large burner, and comprises a central discharge nozzle 49 for the fuel and primary air, which is connected to the discharge 20 of the pulverizing apparatus by means of a pipe 51. Around the nozzle 49, a series of guide vanes 52 are mounted to distribute and whirl the secondary air, which enters the furnace around the outside of the fuel stream. The secondary air is supplied to the burner 47 fromthe air duct 43, the rate of flow being regulatable by means of dampers 53. A shut off valve 54 is provided in the pipe 51.

It will now be seen that with the apparatus as so far described, the entire fuel and air stream discharged by the fan 23 may be directed to either of the burners as desired. If burner 36 is to be operated, valve 45 will'be opened and valve 54 closed. If burner 4'7 is to be operated, valve 54 will be opened and valve 45 closed. A supplementary switching valve 56 may be provided to prevent the pulverized fuel from piling up in a solid mass against either of the plug valves which may be closed. This supplementary valve 56 is shown as a fiat plate pivotally mounted at one edge about an axis located at the intersection of the adjacent edges of pipes 40 and 51. With burner 36 in operation, the valve 56 will be in the position illustrated in Fig. 1, while if burner 47 is to be operated, the valve will be raised to the 125. position shown in Fig. 2. It will be understood that an external handle (not shown) or similar device is provided whereby valve 56 may be adjusted and held in a desired position.

If now the entire fuel and air stream'passing through the discharge opening 20 is directed to the low'capacity burner 47, certain serious difiiculties will arise. The burner nozzle 49 is much smaller than the nozzle 39, so that theresistance to the flow will be greatly increased and the pressure atthe fan outlet 20 will likewise increase to a very high value. This increased pressure will not only cause fine coal to be blown out from the pulverizer through clearance spaces and openings into the surrounding atmosphere, but it will also greatly increase the velocity of flow through the nozzle 49. This discharge velocity will greatly exceed the rate of flame propagation and will result in blowing out the flame. At low rates of heat release, the furnace temperature is low, and it is necessary, if ignition is to be maintained, that the velocity of fuel discharge shall only slightly exceed the rate of flame propagation. This enables the fuel to ignite close to the nozzle 49 and within the refractory 150 lined opening 48, so that the flame will maintain the surrounding refractories at a high temperature and the refractories in turn will. radiate heat to the entering fuel stream.

- If an attempt is made to reduce the velocity in burner nozzle 49 by partially closing valve 54, the undesirable back pressure will be still further increased. It has furthermore been found impractical to reduce the primary air flow by means of the air inlet damper 19, since there is a great deal of air leakage into the pulveriz'er past the coal feeding device. Moreover any reduction in the primary air flow obtained by throttling either the inlet or outlet of the pulverizer will reduce the air velocity through the pulverizing chamber and cause fuel to pile up in the bottom of the pulverizing chamber and drag against the rotor. Operation of the machine is thus rendered unsatisfactory.

In order to overcome these difficulties I provide means whereby enough air is admitted to the pulverizing. chamber not only to insure efficient circulation .of the fuel but also to maintain the fuel in suspension where it maybe struck by the impact beaters or revoluble pegs of the pulverizer and so be satisfactorily pulverized. I furthermore make possible satisfactory operation of both the furnace and thepul verizer at low rates of heat release; For this 3 purpose I provide a by-pass conduit or pipe 58 through which a portion of the fuel and air discharged by the fan may be recirculated, preferablyby returning it to the pulverizing chamber. This pipe 58-, as illustrated, extends from the fan 3 discharge opening 20 and isconnected "to the pulverizing chamber 12 on the inlet side of the disk .24 and near the shaft 22. A shut off valve 59, similar -to valve 45, may be provided in the pipe 58. For the purpose of dividing the fuel stream in desired proportions between pipes 58' and 51, I preferably provide a valve 60, which is shown as a fiat plate pivotally mounted at one edge about an axis located at the intersection 0fthe=adjacent edges of the pipes. This valve may be adjusted manually by an external handle (not shown). When it is desired to-operate the furnace at high rates of heat release, valve will be opened, valves 54 and 59 closed; and valves 56 and will be adjusted to the positions shown in Fig. 1. All the air and pulverized fuel discharged by the fan 23 will therefore be delivered to'the nozzle 39 of burner 36. Dampers 53 will be closed, anddampers 44 opened to sup- 55 ply the secondary'air needed to burn the fuel at the.desired rate. As the demand for heat decreases, the fuel will be supplied to the chute 16' at a decreased rate and dampers 44 will be partially closed to decrease the secondary air 60. supply. The pointwill eventually be reached at which the primary air supplied with the fuel through pipe 40 issufiicient to burn the fuel, and dampers 44 will then be completely closed. If the demand decreases still further, the fuel supply will be decreased accordingly, valve 59 will be opened, and valves 56 and 60 adjusted slightly upwardly to permit a portion of the fuel and airdischarged by the fan to flow through the by-pass conduit 58 and thus back to the pul verizing chamber. This will decrease the quantity of primary air delivered to the burner nozzle 39 without increasing the back pressure at the fan discharge 20 or varying the air flow through the pulverizing chamber 12, and thus the rate of heat release may be reduced.

'hurled outwardly in the fan chamber under the The velocity of discharge at the nozzle 39 cannot be reduced below the rate of flame propagation, however, without causing the flame to strike back into the nozzle and thus overheat the burner. Hence, if the demand decreases still further, I open valve 54, close valve 45, and adjust valves 56 and 60 to the 'positions shown in Fig. 2, thus placingthe low capacity burner 47 into operation. Valve 60 will be so positioned as to supply primary air to the nozzle 49 at a velocity slightly exceeding the rate of flame propagation, and dampers 53 will be adjusted to supply any secondary air necessary to burn the fuel at the desiredrate. It 'will be apparent that by dischargingthe fuel into the furnace through the small nozzle 49 at low ratings instead of through the large nozzle 39 I have reduced the cross-sectional area of the jet of fuel and primary air, thereby permitting a considerable decrease in the quantity of primary air without reducing thedischarge velocity below the rate of flame propagation. At the very lowest combustion rate, the dampers 53 will be ,entirely closed, and only the.primary air from nozzle 49 will be depended upon to burn the fu el. n will thus be seen that with a sufliciently small burner nozzle 49, any desired minimum combustion rate may be maintained continuously, with the limitation that suflicient heat must be released to make up for radiation losses through the furnace walls and to maintain the refractories surrounding the opening 48 at a sufliciently high temperature to ignite the incoming fuel. The pipe 58 isshown of substantially the same size as pipe 40, so that it will be capableof handling most of'the air discharged by the fan when these low ratings are obtained.

his a fact that the finer the pulverized fuel is ground, the easier it is to ignite in a furnace. Since at low ratings I i-e-circulate a large part of the fuel through the pulverizing chamber and subject it for a second time to the action of the pulverizing elements, I am able to deliver a much finer product to the burner and can-maintain ignition at much lower ratings. In the embodiment illustrated I am able to deliver an even finer product for combustion by segregating the coarser particles of fuel and returning'them for further pulverization. Since these coarser particles are influence o'f centrifugal force, ,I so arrange the by-pass conduit that it will directly receive these coarse particles as they move along the peripheral wall 65 of the fan chamber. in approaching the discharge opening 20. The construction is shown particularly in Fig. 2 in whichthe lower wall of the by-pass conduit 58 forms a continuation of the fan chamber peripheral wall 65. With this arrangement, the-coarse particles will enter the by-pass conduit, while the fine particles will be'135 carried above the valve 60 by the air current andthus enter the pipe 51.

It will now be clear to those skilled in the art that I- have provided a very simple apparatus whereby pulverized fuel may be burned in a furnace-over an extremely wide range\ of ratings. By means of my invention, it is possible to operate a furnace fired by pulverized fuel at a very low rating continuously, and ready for operation at a high combustion rate upon a moments notice. When the demand for heat is low, there is no necessity for operating the burners intermittently, as has been the practice heretofore, and constant supervision is not required.

Having thus described my invention, what I 150 claim as new and desire to secure by Letters Patent is:

1. A pulverized fuel burning system comprising walls forming a combustion chamber, walls forming a fan chamber, a rotatable fan within the fan chamber, means to deliver pulverized fuel and air to said fan chamber, a large furnace burner and a small furnace burner associated with the combustion chamber, means to conduct air and pulverized fuel from the fan chamber to said burners, means including a by-pass conduit to divert a portion of the air and fuel discharged by the fan and cause it to recirculate through the fan chamber, and valve means to control the relative velocities of fiow to each of said burners and in said by-pass conduit.

2. A pulverized fuel burning system comprising walls forming a combustion chamber, walls forming a pulverizing chamber and a fan chamber which are connected by a passage forming an outlet from the pulverizing chamber, means in the pulverizing chamber to pulverize material, a rotatable fan mounted in the fan chamber, a large furnace burner and a small furnace burner associated with the combustion chamber, means to conduct air and pulverized fuel from the fan chamber to said burners, means including a bypass conduit to divert a portion of the air and fuel discharged by the fan and cause it to re- .circulate through the fan chamber, and valve means to control the relative velocities of flow to each' of: said burners and in said by-pass conduit.

3. A pulverized fuel burning'system comprising a casing shaped to provide a pulverizing chamber and a fan chamber which are connected by a passage forming a central inlet to the fan chamber, a rotatable fan mounted in the fan chamber, means in the pulverizing chamber to pulverize material, the fan'chamber having a discharge opening, a large furnace burner, a small furnace burner, pipes connecting the burners with the fan chamberdischarge, a by-pass conduit connecta casing shaped to provide a pulverizing chamber and a fan chamber which are connected by a passage forming a central inlet to the fan cham-- peripheral wall of the fan chamber, and valve means to control the relative velocities of flow in said pipes and conduit.

5. A pulverized fuel burning system comprising walls forming a pulverizing chamber and a fan chamber which are connected by a passage forming an outlet from the pulverizing chamber, means in the pulverizing chamber to pulverize material, a rotatable fan mounted in the fan chamber, walls forming a furnace chamber, means to conduct air and pulverized fuel from the fan chamber to the furnace chamber, means to vary the cross-sectional area available for the flow of the pulverized fuel and air into the furnace, means including a by-pass conduit to divert a portion of the fuel and air discharged by thefan and cause it to recirculate through the pulverizing chamber and the fan chamber, and valve means to control the relative quantities of air and fuel flowing to the furnace and through the bypass conduit.

6. A pulverized fuel burning-system comprising walls forming a combustion chamber, walls forming a fan chamber, a rotatable fan within the fan chamber, means to deliver pulverized fuel and airto said fan chamber, burner means arranged to project pulverized fuel and air into the combustion chamber, means to conduct air and pulverized fuel from the fan chamber to the burner means, means to vary the cross-sectional area available for the flow of the fuel and primary air into the combustion chamber, means including a by-pass conduit to divert a. portion of the air and fuel discharged by the fan and cause it to recirculate through the fan chamber, and valve means to control the relative velocities of flow to duce air and coarse fuel into the pulverizing,

chamber, a rotor mounted within the pulverizing chamber and provided with impact members arranged to pulverize the ,fuel'as it is carried in suspension in air flowing through the pulverizing chamber, a fan chamber communicating with theoutlet of the pulverizing chamber, a rotatable fan within the fan chamber arranged to withdraw fine fuel and air from the pulverizing chamber, a furnace burner, apipe connecting the fan chamber with the burner, a by-pass conduit connecting the fan chamber with the pulverizing chamber, and valve means to control the relative velocities of flow in said pipe and by-pass conduit, whereby the air 1 elocity in the pipe may be varied as desired while still maintaining a substantially constant air velocity through the pulverizing chamber to maintain the fuel in suspension therein.

OLLISON CRAIG.

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