Coal feed system for a fluidized bed combustor

A coal feed system for use in a fluidized bed combustor in which a grate is supported in a housing and is adapted to support a bed of particulate material including a combustible fuel. Air is passed through the grate and into the bed to fluidize the particulate material. The particulate fuel material is separated externally of the bed into a relatively coarse material and a relatively fine material. The fine material is agglomerated and it, along with the coarse material, is passed to a distributor which distributes the material onto selected areas extending across the upper surface of the bed.

BACKGROUND OF THE INVENTION 
This invention relates to a fluidized bed combustor and, more particularly, 
to an improved particulate coal feed system for such a combustor. 
The use of fluidized beds has long been recognized as an attractive way of 
generating heat. In a normal fluidized bed arrangment, air is passed 
through a bed of particulate material, which usually includes a mixture of 
inert material, and a fuel material such as high sulfur bituminous coal. 
As a result of the air passing through the bed, the bed behaves like a 
boiling liquid which promotes the combustion of the fuel. The basic 
advantages of such an arrangement include a relatively high heat transfer 
rate, substantially uniform bed temperature, combustion at relatively low 
temperatures, a reduction in corrosion and boiler fouling and a reduction 
in boiler size. 
In these arrangements, the particulate fuel material must be continuously, 
or at least periodically, distributed into the bed to replenish the spent 
material expended in the combustion process. Many conventional systems 
utilize a feeder, or feeders, for distributing the particulate fuel 
material from a position above the upper surface of the bed where it falls 
by gravity onto the latter surface. However, a great majority of the 
commercial coal that is available is of a relatively large particle size 
range, which may extend from very small particles to particles that will 
pass through a two-inch screen. Therefore, the distribution of the 
particulate fuel material onto the upper surface of the bed presents 
problems since, if the particulate fuel material is relatively small, the 
combination of the upwardly rising combustion gases and air passing 
through the bed will blow the material out of the top of the combustor 
before it is burned. 
It also has been suggested to provide in-bed feeding systems in which the 
particulate fuel material is introduced directly into the bed from a point 
below the upper surface of the bed. Although these systems avoid the above 
problems, they present additional problems since the lateral transfer or 
dispersion of the materials through the bed is relatively poor, requiring 
a multiplicity of feed points to prevent hot spots or over-cool spots. 
This, of course, increases the cost of the installation. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a coal feed 
system for a fluidized bed combustor in which an optimum distribution of a 
particulate fuel material of a relatively large particle size range is 
achieved. 
It is a further object of the present invention to provide a system of the 
above type in which the particulate fuel material is separated into a 
relatively course material and a relatively fine material which is 
agglomerated before being distributed to the bed. 
It is a still further object of the present invention to provide a system 
of the above type in which the coarse particulate material and the 
agglomerated material are both distributed from a single distributor to 
selected areas across the upper surface of the bed. 
Toward the fulfillment of these and other objects, the system of the 
present invention comprises a fluidized bed combustor including grate 
means supported in a housing and adapted to receive on its upper surface a 
bed of particulate material at least a portion of which is combustible. 
The particulate fuel material is separated externally of the bed, with the 
relatively fine particulate fuel material from the separator being 
agglomerated and passed, along with the relatively coarse material, to a 
single distributor located above the upper surface of the bed. The 
distributor is adapted to distribute the coarse material and the 
agglomerated fine material to selected areas extending across the upper 
surface of the bed.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing, the reference numeral 10 refers in general to a 
fluidized bed combuster of the present invention consisting of a front 
wall 12, a rear wall 14, and two sidewalls, one of which is shown by the 
reference numeral 16. The upper portion of the combustor 10 is not shown 
for the convenience of presentation, it being understood that it consists 
of a convection section, a roof and an outlet for allowing the combustion 
gases to discharge, in a conventional manner. 
A bed of particulate material, shown in general by the reference numeral 
18, is disposed within the combustor 10 and rests on a perforated grate 20 
extending horizontally in the lower portion of the boiler. The bed 18 can 
consist of a mixture of discrete particles of inert material and fuel 
material such as bituminous coal. 
An air plenum chamber 22 is provided immediately below the grate 20 and an 
air inlet 24 is provided through the rear wall 14 in communication with 
the chamber 22 for distributing air from an external source (not shown) to 
the chamber. A pair of air dampers 26 are provided in the inlet 24 for 
controlling the flow of air into the chamber 22. The dampers 26 are 
suitably mounted in the inlet 24 for pivotal movement about their centers 
in response to actuation of external controls (not shown) to vary the 
effective openings in the inlet and thus control the flow of air through 
the inlet and into the chamber 22. Since the dampers 26 are of a 
conventional design they will not be described in any further detail. 
A bed light-off burner 28 is mounted through the front wall 12 immediately 
above the grate 20 for initially lighting off the bed 18 during startup 
and a bed tap, or drain pipe 29 extends from a corresponding opening 
formed in the grate 20 to a position below the chamber 22 for discharging 
the spent materials from the bed 18. 
A separator, shown in general by the reference numeral 30, is located 
externally of the boiler 10 and is adapted to receive particulate fuel 
material, such as coal, of a relative large particle size range from an 
external source (not shown), such as coal crusher, via a duct 32. The 
separator 30 adapted to separate the fuel material in a conventional 
manner, such as by the use of a screen or screens, into relatively coarse 
and relatively fine particles. The relatively coarse particles are passed 
from the separator 30 via a duct 34 and the relatively fine particles are 
passed from the separator via a duct 36. As an example, the separator 30 
can be adapted to separate particles greater than 1/16 of an inch in 
diameter from those less than 1/16 of an inch and pass the former to the 
duct 34 and the latter to the duct 36. 
An agglomerator, shown in general by the reference numeral 38, receives the 
fine particles from the duct 36 and is designed to agglomerate the 
particles into coarser particles by any conventional technique. For 
example, if the particles contain any moisture, they can be agglomerated 
by pelletizing in a disc pelletizer, such as the Series 7000 pellet mill 
manufactured by the California Pellet Mill Co. Alternatively, if the fine 
particles are relatively dry they can be agglomerated on a roll briquetter 
or an extruder of a conventional type. 
A duct 39 connects the output of the agglomerator 38 to a distributor, 
shown in general by the reference numeral 40, which also receives the 
coarse particles from the duct 34. The distributor 40 is mounted relative 
to the upper portion of the front wall 12 and operates to distribute the 
mixture of course particles from the duct 34 and the agglomerated 
particles from the duct 39 onto selected areas across the upper surface of 
the bed 18. 
The distributor 40 includes an inlet pipe 42 for receiving the coarse coal 
particles from the duct 34 and the agglomerated material from the duct 39 
where they are mixed and fed, by gravity, onto a distributor tray 44 which 
extends immediately below the outlet end of the pipe 42 and into the 
interior of the combustor 10. The tray 44 is pivotally mounted relative to 
an actuating lever 46 for controlling the angular position of the tray 
relative to the upper surface of the bed as shown, for example, by the one 
position represented by the solid lines and the two positions represented 
by the dashed lines. A control unit for the lever 46 is shown in general 
by the reference numeral 48 and operates in a conventional manner to 
control the pivotal movement of the tray 44. 
The distributor 40 also includes an air distributor unit, shown in general 
by the reference numeral 50, for distributing pressurized air at a 
selected rate through a plurality of vanes, one of which is shown by the 
reference numeral 52, located immediately above the tray 44, to inject the 
air across the coal particles on the tray. As a result, the coal particles 
are distributed to selected areas extending across the upper surface of 
the bed 18 which are determined by the position of the tray 44 under the 
control of the lever 46 and the unit 48. For example, in the uppermost 
position of the free end of the tray 44 as viewed in the drawing, the 
particles falling onto the tray from the pipe 42 would be propelled by the 
air from the unit 50 towards the rear wall 14, and would fall onto the 
rear portion (i.e., the right hand portion as viewed in the drawings) of 
the upper surface of the bed 18. Similarly, in the lowermost position of 
the tray 44 as shown in the drawings, the particles would be distributed 
onto the front portion (i.e., the left hand portion as viewed in the 
drawing) of the upper surface of the bed 18. Since the tray 44 can be 
pivoted to an infinite number of angular positions relative to the upper 
surface of the bed 18 under control of the lever 46 and the control unit 
48, it can be appreciated that an accurate control of the precise location 
of the particulate feed across the upper surface of the bed can be 
achieved. 
In operation, the bed 18 is started up by opening the dampers 26 associated 
with the air inlet 24 to distribute air upwardly through the compartment 
22, through the perforations in the grate 20 and into the bed 18. This 
loosens the particulate material in the bed 18 and reduces material 
packing and bridging. 
The separator 30 and the agglomerator 38 are activated, and operate as 
discussed above, to feed coarse and agglomerated particulate fuel 
material, via the ducts 34 and 39, respectively, to the inlet pipe 42. 
The light-off burner 28 is then fired to heat the material in the bed 18 
until the temperature of the material reaches a predetermined level, at 
which time the distributor 40 is activated to distribute the mixture of 
coarse and agglomerated particulate fuel from the inlet pipe 42 onto 
selected areas extending across the upper surface of the bed 18 as 
determined by the position of the tray 44, to insure a uniform 
distribution across the upper surface. 
After the bed 18 has been fluidized and has reached a predetermined 
elevated temperature, the light-off burner 28 is turned off while the 
distributor 40 continues to distribute the particulate fuel across the 
upper surface of the bed 18 in accordance with predetermined feed rates. 
It is understood that if the combustor is used for the purpose of steam 
generation, a pluraity of heat exchange tubes carrying the fluid to be 
heated, such as water, may be routed through the interior of the combustor 
in a conventional manner, with these tubes being omitted in the drawing 
for the convenience of presentation. In the event that the combustor is 
used for other purposes, such as gasification, or the like, the water 
walls and tubes may be omitted and conventional refractory construction 
used to contain the fluid bed can be added. 
A latitude of modification, change and substitution is intended in the 
foregoing disclosure and in some instances some features of the invention 
will be employed without a corresponding use of other features. 
Accordingly, it is appropriate that the appended claims be construed 
broadly and in a manner consistent with the spirit and scope of the 
invention herein.