US2807223A - Method of and apparatus for burning fuel - Google Patents

Description

p 24, 1957 J. M. MILLER ETAL ma'm'on OF AND APPARATUS FOR BURNING FUEL Filed Feb. 25. 1953 2 Sheets-Sheet 1 VII IIIIIIIA ll Ill IN VEN TORS la/w M. M/um W/u MM M. ape/mp1- ypo/V440 M. MCZfl/PEA/ Ai /DRIVE) l 4, 1957 J. M. MILLER ETAL 2,807,223

METHOD OF AND APPARATUS FOR BURNING FUEL INVENTORS" (Jo/0v M- M/AAEA MammMl/RQH DoumaMA/clnfi'av ATTORNEY v Patented Sept. 24, 1957 1 METHOD F AND APPARATUS FOR BURNENG FUEL John Matthewson Miller, Brookfield, and William Mac- Donald Urquhart and Donald M. McLaren, Glasgow, Scotland, assignors to The Bahcock & Wilcox Company, New York, N. Y., a corporation of New Jersey Application February 25, 1953, Serial No. 338,312

Claims priority, application Great Britain February 27, 1952 4 Claims. (Cl. 110-4055) The present invention relates to methods of and apparatus for the combustion of solid fuels, and more particularly to the construction and operation of spreader stokers wherein fuel is projected into the furnace by a jet or jets of elastic fluid.

In accordance with the invention granular solid fuel the initial acceleration of the fuel occurring in a generally horizontal direction followed by an upward deflection to obtain a desirable distribution of the fuel transversely of the furnace. Advantageously, the elastic fluid may consist of air at relatively low pressure and volume, with the air used for mixing and accelerated combustion of the fuel within the furnace. The fines in granular solid fuel burn in suspension, while the coarser particles are, in general, uniformly distributed over the transverse area of the furnace. A moving grate is positioned in the lower portion of the furnace to support the ash and coarse particles of fuel projected into the furnace by the spreader stoker. The fuel and ash supporting surface of the moving grate moves from the front wall of the furnace, beneath the position of the spreader stoker, toward the opposite wall.

The various features of novelty which characterize our invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which we have illustrated and described preferred embodiments of our invention.

Of the drawings:

Fig. 1 is an elevation, in longitudinal section, of the lower portion of a spreader stoker fired furnace constructed and arranged in accordance with the invention;

Fig. 2 is an enlarged view of a portion of the apparatus shown in Fig. 1;

, Fig. 3 is a plan view taken on line 33 of Fig. 1;

Fig. 4 is an enlarged view of another portion of the apparatus shown in Fig. 1; and

Figs. 5 and 6 illustrate modified constructions of the spreader stoker shown in Figs. 1 to 3, inclusive.

Referring to Figs. 1 to 4 of the drawings, the furnace has a front wall 1 and a rear wall 2 at the ends of a combustion space 3 above a travelling grate 4 of the chain grate type arranged to that its upper run moves rearwardly.

In front of and extending laterally of the front wall 1 is arranged a hopper 5, into which coal is led from an overhead bunker 6 through a traversing chute 7. At the bottom of the hopper and arranged side by side in a row extending laterally of the furnace are a number of feeders 8 operating on the endless conveying surface principle. The feeders are arranged during operation to remove coal rearwardly from the hopper. Suitably all the feeders are driven'at the same speed by common driving means. The amounts of'coal fed by the feeders can be adjusted individually by raising or lowering guillois projected into a furnace by a jet of elastic fluid, with tine doors 9 positioned above the rear ends of the feeders and slidable within the rear wall 10, which is a double wall of the hopper.

Coal is prevented from falling in front of the feeders by plates 11 and from falling between the feeders by ridged structures 12.

The coal from the rear ends of the feeders is arranged to fall onto a common chute 13 sloping downwardly and rearwardly. At the bottom of the chute behind each feeder is a plate 14 of a respective spreader unit 15, which is arranged to receive the coal falling from the rear end of the feeder. The chute is provided with structures 16 of refractory between adjacent spreader units, which are formed with sloping surfaces 17 normal to the main surface of the chute and adapted to direct coal falling thereon towards the nearmost spreader unit. The side surfaces 18 of the structures 16 act to confine the falling coal to paths leading them to the spreader units.

Adjacent each spreader unit plate 14 the lowermost part of the chute is formed by a metal block 19. The surface of the chute above these blocks, between the refractory structures 16 and above the level thereof, is formed by the surfaces of refractory bricks 20.

A fixed scraper plate 21 is provided adjacent the rear of each feeder to insure the falling from the feeder conveying surface of coal that might otherwise adhere thereto. A screen 22 projecting downwardly and forwardly from the furnace front wall 1 near the bottom thereof is provided for intercepting radiation from the furnace that might damage mechanical parts of the feeders.

In the illustrated embodiment of the invention, the center lines of adjacent spreader units are 1' 9 /2" apart. Each spreader unit plate 14 is bolted to the block 19 and has a shelf 23 extending horizontally and rearwardly from the bottom of the chute a distance of for example 5". Rearwardly of the shelf 23 is a deflector plate 24 providing a guide surface which curves upwardly, the curvature being cylindrical around a laterally extending axis distant 6" from the upper surface of the plate. The plate 24 terminates where the slope thereof has an angle of 33 or thereabouts to the horizontal. The side edges 25 of the plate diverge rearwardly, being 10% apart at the front edge of the shelf and 16" apart at the rear edge of the deflector plate 24. The side edges of the plate are furnished with upstanding guide walls 26 having their upper edges horizontal and at a height of 2 /2" above the level of the shelf 23 and provided with forward and upward extensions 27 defining the path of the coal falling over the part of the chute provided by the block '19. The spreader unit is located so that the shelf 23 is 12 /8" above the grate.

The spreader stoker unit also comprises a horizontally directed elongated orifice 30 formed in the block 19. The orifice is 10" wide and deep with its lower edge /2" above the plane of the shelf 23. The orifice registers with a slot 31 in a duct 32 extending adjacent the lower ends of the chute transversely of the furnace and arranged to supply air to all the spreader units.

The duct 32 is utilized to support the spreader units; to this end the blocks 19 are secured to the duct 32 by screws. The chute 13 is located with respect to the duct 32 by brackets 33 bolted to the plates 34 supporting the refractory blocks 20 and having curved parts conforming to the shape of the duct 32.

The space below much of the upper run of the grate is divided into compartments 35-40 to which undergrate air is supplied under the control of respective dampers 35a-40a. The compartments extend as zones transversely of the grate and control of the respective quantities of air thereto permits the rates of combustion attained at different zones to be varied to suit the character of the coal to be burned.

At the rear end of the furnace the contents of the grate are led beneath an arch 45 at the bottom of the furnace rear wall 2, before being removed from the grate by the usual ash bars 46 at the rear end of the grate. A vertical refractory faced shield 47 is provided in front of the arch 45, which shield has its front surface in the same plane as that of the rear Wall 2 above the arch and acts as a downward extension thereof, and at the lower end of the shield is a forwardly and upwardly inclined ledge 55. The ledge is welded to a pipe 54 whose center line is 1' 1 /2 above the level of the main surface of the grate, and which is pierced just above the level of the upper surface of the ledge to form a numberof orifices 56, each deep, side by side across the width of the furnace adapted for the blowing from the ledge, forwardly into the furnace, of coal falling onto the ledge. The pipe 54 is supplied with air suitably from the same source and under the same pressure as that for the duct 32.

The shield 47 is secured directly to, and the pipe 54 is supported through supporting bars 57 from, supporting blocks 52 clamped at the rear arch 45 between adjacent watertubes 53 of the rear wall 2. The shield 47, the ledge 55, and the pipe 54 are suitably sectionali'zed, the sections being arranged closely side by side across the Width of the furnace.

The furnace is adapted for the production of hot gases for steam generation. Appropriately the gases are led across heat absorbing surfaces (not shown) comprising steam generating tubes and steam superheating tubes and subsequently through an economizer. Before the gases reach the economizer they may be passed through a classifier (not shown) for the removal of some of the grits in the gases, and these grits, comprising those having a substantial proportion of their weight in the form of combustible material, are added at 60 to the coal entering the traversing chute 7.

In the embodiment of the invention illustrated, the furnace is approximately 14' x 7 6" in horizontal section, and three spreader stoker units are installed in the front wall of the furnace. When burning 3500 to 4000 pounds per hour of coal having 10,000 to 12,000 B. t. u. per pound less than percent of the theoretical combustion air is introduced through the orifices 30, with the remaining combustion air passed upwardly through the grate 4.

In operation, coal prepared for spreader stoker firing, e. g. crushed and capable of passing through a /2" ring, is fed from the bunker 6 through the traversing chute 7 to the hopper 5, and the feeders 8 are operated to discharge coal continuously from the hopper to the spreader units 15. Air is supplied to the duct 32 at a pressure of 12" to 18" water gauge, and after issuing from the orifices 30 continuously projects coal rearwardly from the spreader units. The coal particles or lumps so supplied to the furnace burn partly in suspension in the combustion space above the fuel bed and partly in the fuel bed on the grate 4, which is supplied with undergrate air from the compartments 35-40. The ash arising from the combustion of the coal on the grate is discharged at the rear of the grate. Air is also supplied to the pipe 54 and reprojects forwardly into the furnace coal particles or lumps which find their way to the ledge 55.

The feeders 8 supply coal continuously to the spreader units 15. In falling downwardly over the chute in contact with or near the refractory bricks 20, the coal particles or lumps are subjected to drying effects by the radiation they receive from the furnace combustion space 3 and the heat they receive by contact or radiation from the radi-antly heated refractory bricks 20.

The spreader unit plates 14 at the bottom of the ramp 13 arrest the downward progress of the coal. When the coal particles or lumps are acted upon by the currents of air from the orifices 30, they undergo rearward accelerations. During such accelerations the shelf 23 prevents the coal as a whole from falling. The shelf surfaces are generally horizontal, and the acceleration of a particle or lump while it is on a shelf is larger than if the shelf were directed rearwardly and upwardly. The deflector 24 at the end of the shelf 23 deflects the relatively fast moving lump or particle so that it leaves the spreader unit in a path having an upward inclination to the horizontal, the angle of which Will generally be relatively large. By reason of the large angles to the horizontal and the speed at which the coal lumps or particles leave the spreader units, the coal particles or lumps destined to burn out on the grate tend to have trajectories which carry them large distances rearwardly before they reach the fuel bed on the grate. It is foundthat in this way coal lumps or particles can be distributed over an area which is relatively great from front to rear, so that the furnace can have a deep combustion space. The currents of air from the orifices 30 fan out to a certain extent laterally, while moreover adjacent coal particles or lumps on each spreader unit plate 14 tend to be blown apart. Coal lumps and particles are, therefore, distributed also to each side of each spreader unit at the rear thereof, so that a satisfactory fuel bed is formed on the grate which extends across the whole width thereof.

The air from the orifices 30 and those coal particles or lumps designed to fall on the fuel bed on the grate travel at least initially in the same general direction with the air sweeping past such lumps or particles. The lumps or particles are therefore scrubbed by the air so that ample oxygen for rapid ignition and combustion of such particles or lumps is available.

The larger lumps of coal, inasmuch as they have a smaller ratio of surface area to weight, tend to be less greatly accelerated by the air from the orifices 30 and less sustained by the mass of gases rising generally upwardly from the fuel bed on the grate, and consequently they tend to fall in greater proportion towards the front end of the fuel bed on the grate. Larger lumps require a longer time for their combustion than smaller lumps or particles, and this longer time they receive because the upper run of the grate moves rearwardly. The lumps or particles falling near the rear of the fuel bed on the grate tend to be such as are more greatly accelerated by the air currents from the orifices and more greatly sustained by the gas mass rising generally upwardly from the fuel bed, i. e. they are generally the smaller lumps or particles; consequently although a less time is available to them for combustion in the fuel bed before they are discharged from the grate, they will be generally burnt out on the grate before they reach the ash bars 46, since they require only a shorter time to burn.

The very fine particles tend not to fall onto the fuel bed on the grate, but to be carried upwardly in the space above the fuel bed. Such particles tend to follow in a particularly high degree the air from the orifices 30 as it enters the mass of gases rising upwardly from the fuel bed. Much of the air penetrates horizontally or nearly hori- Zontally into the said upwardly rising gas mass, while other parts of the air eddy above the spreader units, as shown at 70, being drawn again towards the original direction of motion from the orifices 30. The exit of the very fine particles from the furnace is thus delayed so that their burning out in suspension before they leave the combustion space is promoted.

Shield 47 presents to the combustion space a surface from which coal rebounds onto the grate in a forward direction, or which leads to the ledge 55 many coal lumps or particles falling from the rear wall 2 and from the shield itself. From the ledge 55 air discharging forwardly through the orifices 56 projects the coal lumps or particles forwardly into the furnace. The coal lumps or particles which impinge onto the shield at the end of their trajectories from the spreader units might not, if the shield were absent and they fell on the grate, have time to burn out before being carried by the grate to the ash bars 46, but by virtue of their re-delivery in a forward direction so that they fall onto the fuel bed on the grate in front of the plane of the rear Wall 2 sufiicient time is given to such lumps or particles to make possible the completion of their combustion in the furnace. Moreover, the shield 47 reduces the possibility of particles or lumps in the rain of coal projected rea'rwardly from the spreader units pursuing trajectories which would carry them onto or beyond the ash bars 46; at the same time the shield 47 constrains air flowing upwardly through the grate below the rear arch 45 to flow in a forward direction above the grate and through the relatively narrow opening between the pipe 54 and the fuel bed on the grate, and this movement of air and gases is adapted to shorten the trajectories of lumps or particles which might otherwise fall too near the end of the grate.

We have found that with the apparatus described most grades of coal adapted for spreader stoker firing may be satisfactorily spread onto grates for carrying fuel beds having lengths of from 14 to 20 feet.

The amount of air delivered to the furnace through the orifices 56 is only a small fraction of the total air supplied. The amount of air supplied by the orifices 30 is greater, but to provide it requires a minimum of auxiliary power, for although the air is admitted through the orifices 30 with a velocity suflicient to achieve a satisfactory distribution of fuel by the spreader units 15 over the grate, the aggregate area of the orifices 30 is limited. It is found that the air quantity is substantially less than the normal requirements for overfire air, so that no inefficiency arises by the presence of an excessive proportion of excess air in the gases leaving the furnace. Overfire air that may be required in addition to that from the orifices 30 is provided through suitable secondary air nozzles. The high velocity of the air from the orifices 30 is a factor causing an advantageous degree of turbulence in the gas mass rising from the fuel bed, and thus promoting complete combustion. The pressure of air required in the duct 32 is not however greater than that readily obtainable from a single stage fan.

The turbulence above the fuel bed and the capability of admitting more air to some zonw of the grate than to others are features rendering the furnace particularly adapted for efiiciently burning high ash coals.

When the fresh coal supplied is wet, the addition thereto at 60 of the grits collected from the gases by the classifier above referred to, and .to be refired, increases the ease with which the coal is distributed over the grate, no doubt by reducing, since the grits are very dry, the tendency of wet fine coal particles to adhere to one another.

For the rest, the advantages of e. g. unrestricted ignition of the coal, flexibility in following load fluctuations, little tendency to form large clinkers, facility of burning highly swelling coals, and other advantages associated with spreader stoker firing are achieved in the furnace de scribed.

It is found that, as compared with spreader stoker firing by mechanical coal sprinkler means, fewer grits are carried over from the furnace, while the proportion of combustible material in the grits carried over is smaller.

The means at the lower end of the furnace rear wall 2 arranged for re-directing coal particles forwardly within the furnace are disclosed and claimed in a copending application Serial No. 338,797, filed February 25, 1953, in the name of William M. Urquhart.

Fig. 5 is a sectional view similar to Fig. 2 but illustrating a modification of the spreader unit in which the deflector 24 instead of being curved and integral with the shelf 23 comprises a flat plate welded to the rear end of the shelf 23 and extending upwardly and rearwardly at an angle of 33 or thereabouts.

Fig. 6 is another sectional view similar to Fig. 2, illustrating a modification in which the lower lip of the orifice 30 is in the same plane as the upper surface of the shelf 23. It is found appropriate to use this construction should 6 the coal have too great a tendency, by reason of undue wetness, to adhere to the shelf 23.

While in accordance with the provisions of the statutes we have illustrated and described herein the best form of the invention now known to us, those skilled in the art will understand that changes maybe made in the form of the apparatus disclosed without departing from the spirit of the invention covered by our claims, and that certain features of our invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. The method of operating a spreader stoker fired furnace which comprises continuously gravitating crushed solid fuel downwardly adjacent an inner wall face of said furnace to a position intermediate the furnace height, initially accelerating said solid fuel in a substantially horizontal direction by a jet of air and thereafter deflecting said fuel and jet of air in an upwardly inclined direction across said furnace, collecting coarse fuel and ash in the lower portion of said furnace, and continuously moving said coarse fuel and ash in a generally horizontal direction across said furnace from a position adjacent the fuel inlet, and in a direction generally along the axis of the jet, whereby the length of the path of travel of fuel on the fuel bed through the furnace is in a substantially direct proportion to the particle size.

2. The method of operating a spreader stoker fired furnace which comprises continuously gravitating crushed solid fuel downwardly adjacent an inner wall face of said furnace to a position intermediate the furnace height, initially accelerating said solid fuel in a substantially horizontal direction by a fluid jet and thereafter deflecting said fuel and fluid jet in an upwardly inclined direction across said furnace, collecting coarse fuel and ash in the lower portion of said furnace, and continuously moving said coarse fuel and ash across said furnace and in a direction generally parallel to the axis of the jet, whereby the length of the path of travel of fuel on the fuel bed through the furnace is in a substantially direct proportion to the particle size.

3. Combustion apparatus comprising walls defining a furnace, a horizontally disposed shelf positioned along the wall of said furnace intermediate the height thereof, an upwardly inclined deflector merging with said shelf at a position spaced from said wall inwardly of said furnace, means for continuously delivering solid fuel at a selected rate to said shelf, air jet means for projecting said fuel across said shelf and deflector into said furnace, and a moving grate positioned in the lower portion of said furnace to collect ash and solid material thereon, said grate adapted to move in a horizontal direction generally along the axis of the jet whereby the length of the path of travel of fuel through said furnace on said grate is in a substantially direct proportion to the particle size.

4. Combustion apparatus comprising walls defining a furnace, a horizontally disposed shelf positioned along one wall of said furnace intermediate the height thereof, an upwardly inclined deflector merging with said shelf at a position spaced from said wall inwardly of said furnace, means for continuously delivering solid fuel at a selected rate to said shelf, means for discharging an elastic fluid jet horizontally across the upper surface of said shelf, and a moving grate positioned in the lower portion of said furnace to collect ash and solid material thereon, said grate adapted to move in a horizontal direction generally along the axis of the jet whereby the length of the path of travel of fuel through said furnace on said grate is in a substantially direct proportion to the particle size.

References Cited in the file of this patent UNITED STATES PATENTS 803,805 Brewster Nov. 7, 1905 979,850 Hanna Dec. 27, 1910 (Other references on following page) 7 Shults Sept. 1, 1914 Jackson et a1. Apr. 15, 1919 Kraemer Mar. 31, 1925 Kohout Jan. 8, 1929 Crow et a1 June 18, 1929 Kohout Oct. 31, 1939 Brown Jan. 23, 1940 & Clausen et a1 Feb. 19, 1946 Glaeser Oct. 4, 1949 FOREIGN PATENTS Great Britain July 1, 1935 France Sept. 10, 1934 Grmany July 3, 1937

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