Note: Descriptions are shown in the official language in which they were submitted.
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Ba_ grOlllld of the Inventioll
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I`llis inventioll relates to fluidizcd bed type reactors as used for
the combus~ion of pulverized coal in industrial or utility applications, and
more particularly, to a method of feeding particulate material, particularly
particulate fuel or a mixture of particulate fuel and particulate sulfur
absorbent material, to the fluidized bed.
In conventional fluidized bed combustors, particulate fuel, usually
coal, is burned in a bed of fluidized particulate material commonly including
sulfur absorbent such as limestone and inert material such as ash. The bed
is housed in a chamber, the floor of which is formed of a perforated or slotted
plate. A fluidized gas is introduced into the bed upwardly through the floor
of the chamber. The velocity of the fluidizing air is maintained above the
minimum fluidization velocity so that the entire bed of particulate material
is suspended or floated above the floor of the chamber.
In the conventional state-of-the-art method of feeding coal and/or
other particulate material into the bed, the feed material is underfed to the
bed through a standpipe which projects upwardly through the floor of the
chamber into the fluidized bed. The coal merely overflows from the open top
of the standpipe and is mixed into the bed via the turbulence inherently
present in the fluidized bed. ~lowever, bed turbulence cannot be relied upon
to laterally distribute feed material more than a few feet away from the feed
point. Accordingly, it is conventional practice to provide one feed point,
i.e., standpipe, for approximately each ten square feet of bed surface. For a
coal-fired fluidized bed of 500 MW capacity, conceptual design studies have
indicated that approximately 600 feed points may be required to provide an
adequate mixing throughout an entire bed. Clearly, the provision of such a
system is a complex and expensive undertaking, and a feed system not limited
by poor lateral mixing would be desirable.
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Such underbed feed techniques for feeding particulate material such
as coal and sulfur absorbent into the fluidized bed presents some significant
problems. If the coal is not evenly distributed throughout the bed, uneven
combustion rates can result. Such uneven heat distribution can cause materials
failure in the regions of excessive heat liberation and overcooling in regions
of insufficient combustion. Additionally, uneven distribution of sulfur
absorbent can result in significantly reduced overall sulfur retention in the
bed and excessive discharge of sulfur oxides in the atmosphere.
Furthermore, if enough feed points are used to ensure even distribu-
tion of the coal and sulfur absorbent, the bed can become clogged with stand-
pipes. As a result, proper fluidization of the bed is hindered with an
attendant drop in bed turbulence and lateral mixing resulting.
One solution ~o this probelm is to overfeed rather than underfeed
particulate material to the bed. In overfeeding particulate material to a
fluidized bed, the material is literally flung outwardly into the chamber
by one or more feeders located in the wall of the chamber at an elevation
above the top surface of the fluidized bed. The momentum imparted to the
particles by the feeder carries the particles out over the surface of the bed
with the force of gravity causing the particles to drop to the bed surface.
Large particles because of their greater momentum as compared to smaller
particles are carried further into the bed than the smaller particles thus
providing a good lateral distribution of particles across the bed surface.
Overbed feeders are capable of uniformly distributing particulate material
over an area of approximately 100 square feet.
A major problem associated with such an overbed feed system is that
fine particulate material is frequantly entrained in the upwardly flowing
combustion products evolving from the fluidized bed and subsequently carried
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out of the combustion chamber without ever having passed through the bed.
Consequently, combustion efficiency and sulfur removal efficiency decrease as
fine particulates of coal and absorbent are carried away. For effective
sulfur retention, the sulfur dioxides must be formed in the bed as the coal
burns in intimate contact with the particles of sulfur absorbent.
Summary of the Invention
The present invention provides a method of feeding particulate
material to a fluidized bed which achieves even distribution of the particulate
material over the bed with a minimal loss of fines.
A bed of particulate material consisting of crushed coal,and,
preferably, a sulfur absorbent, such as ground limestone or dolomite, is
fluidized in a portion of the air being supplied for combustion of the coal
thereby establishing a fluidized bed. In accordance with the invention, a
turbulent downflow region is established within the fluidized bed near the
periphery thereof. To supply additional crushed coal and/or ground limestone
or dolomite to the fluidized bed, the coarse particulate content thereof is
directed across the surface of the fluidized bed, while the fine particulate
content thereof is directed into the turbulent downflow region established
within the bed.
In one mode of carrying out the method of the present invention,
the fine particulate is separated from the coarse particulate after they have
initially been fed together into the chamber housing the fluidized bed at a
location above the surface of the bed, while in an alternate mode, the fine
and coarse particulates are separated externally to the chamber and independ-
ently fed to the bed.
In the mode involving separation of the fines within the bed chamber,
the mixed fine and coarse portions of the particulate feed material are
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projected across the surface of the bed from a location above the surface
of the bed and adjacent thc periphery of the bed wherein the turbulent downflow
region is established. A curtain of high pressure downwardly directed air
is established near the periphery of the bed such that the incoming particulate
material must traverse the curtain as it is projected across the surface
of the bed. In traversing the air curtain, the fine particulate material is
entrained in the curtain and directed downwardly to the bed surface at the
periphery thereof in the turbulent downflow region. The coarse particulate
material penetrates the air curtain and is distributed across the surface of
the bed.
In the mode involving separation of the fines externally of the
chamber, the coarse and fine particulate feed material are first separated and
then independently fed to the bed. The coarse material is projected across
the surface of the bed from a location above the surface of the bed as in
conventional overbed feeding. The fine material, however, is entrained in a
stream of high pressure air and fed downwardly directly into the turbulent
downflow region along the bed periphery at a location below the surface of
the bed.
Brief Description of the Drawing
Figure 1 is a vertical section of a fluidized bed reactor e~ploying
one mode of carrying out the present invention, and
Figure 2 is a vertical section of a fluidized bed reaction employing
an alternate mode of carrying out the present invention.
Description of the Preferred Embodiment
Referring to the drawing, there is shown a chamber 10 having a bed
of discrete material 12 that is supported upon a horizontally disposed screen
or perforated plate 14 to provide a lower plenum chamber 16. The chamber 16
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has an inlet 18 for the supply of air for combustion from a suitable outside
source for passage upward through the interstic0s of the plate 14 and the
free area between the discrete fuel particles supported thereon.
The upward flow of air and the downward movement of particulate
matter produces a circulation within the bed that thoroughly mixes all
particles with the incoming fluidizing air. According to the present invention,
downflow of particles within the bed is designed to be maximum near the
perimeter of the housing so there will be a natural circulation of particles
thereby establishing a turbulent downflow region 28 along the periphery of
the fluidizing bed 12. To attain this pronounced downflow at predetermined
areas of the bed, a portion of the perimeter of the distribution plate 14 is
formed with fewer or smaller openings therethrough that tend to decrease the
effect of upward moving air.
Above the surface of the bed 12 in a wall of the chamber 10, adjacent
to the periphery of the bed 12 wherein the turbulent downflow region 28 has
been established, an inlet 20 is provided through which the particulate material
being fed to the bed can be projected across the surface of the bed. The coal
and limestone are supplied from a source 22 to a rotary feeder 24 that supplies
the fuel to a distributor 26 that, in turn, flings the granular material out-
ward across the housing 10 to fall upon the top of the bed 12, the rotary
feeder 24 being operated at a rate that is determined as necessary to carry
the load of the boiler.
The particulate material falls in a continous stream into the path
of the revolving distributor wheel 26 that flings the particles outwardly over
the bed. These blades are mounted in approximately four rows which are spaced
apart, parallel to the axis of the distributor rotor. One set of rows preferably
has the blades of the distributor set at an angle that throws the fuel to the
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right, while the next pair is set so the fuel will be thrown to the left. This
criss-crossing of particulate fuel and limestone results in consistently uniform
distribution of such granular material to the bed. The lateral distribution
may be varied by adjusting the speed of rotation or the angularity with which
the blades of the distributor are set. The distributor speed and angular
setting of the blades are adjusted by means considered conventional in the art.
One of the characteristics of a rotary feeder/distributor of the type
defined is the inherent classification of particles as they are flung out across
the housing. Large particles of coal and limestone would be flung furtner away
from the feeder/distributor because of their greater momentum as compared to
smaller particles. However, if particles are too small, they will remain
airborne due to the velocity of the combustion product gases rising through
the bed and escaping thru the freeboard area above the bed, and subsequently
to the atmosphere.
To ensure capture of the fine particulate material in the bed 12,
the coarse particulate and fine particulate must be separated and the fines
directed into the bed to be drawn into the turbulent downflow region 28
thereof before the fines become entrained in the gases evolving from the bed.
The method of the present invention contemplates separation of the fines from
the coarse particulate either internally of the chamber lO after the homo-
geneous mixture of fine and coarse particulate is fed into the chamber, or
externally of the chamber lO before the particulate material is fed to the
bed 12.
To accomplish internal separation, the present invention contemplates
establishing an air curtain which the incoming particulate feed must traverse
as it is projected into the chamber. As shown in Figure 1, there is provided,
a header 32 with downward directed nozzles 34 over the inlet 20. The header
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is supplied with pressurized air whereby the air stream exhausting from
nozzles 34 forms a curtain that impinges upon the airborne particulate matter
being thrown by distributor 26. By virtue of their lower momentum, the smaller
particles (fines) will deviate from their original trajectory and be carried
down to the bed before major devolatilization and/or entrainment in the product
gas has occurred. The high pressure air curtain is directed downwardly so as
to intersect the surface of the bed 12 at the periphery thereof wherein the
turbulent downflow region 28 has been established. The fines carried to the
bed in the air stream are drawn into the downflow and carried down into the
bed for combustion therein. The high pressure air curtain also serves to
enhance the formation of the downflow region 28 at the periphery of the bed.
Alternately, the fine particulate material can be separated from the
coarse particulate material externally to the chamber 10 with the coarse
material and the fine material then being independently fed to the bed.
One arrangement for separating the particulate coal and limestone
involves integrating an arcuate classifying screen 36 and a rotary feeder in
the manner shown by Figure 2. The screen may be vibrated continuously by a
conventional apparatus whereby the fines will drop through the screen 36 to
the throat of a venturi 38 where they will be picked up by a stream of
transport air supplied through duct 42 directly to the bed. To assure maximum
reaction time for the fines, they are delivered to the fluidized bed 12 at a
location below the surface into the downflow region 28. The end of duct 42
supplying the particles to the bed is bent down to insure a natural downward
movement of the particles above the restricted portion of the distributor
plate and enhance the establishment of the downflow region 28 within the bed 12.
To more thoroughly burn any combustible gases that have been generated
-- by the devolatilization of coal in the freeboard area, and to burn unburned
gases given off from coal in the bed itself, a source of tertiary combustion
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air may be introduced tl~rough tangential inlets 44 at spaced sides of the
combustion chamber, above tl)e incoming coal and limestone.
~ ecause particulate coal is supplied to the upper surface of the
bed where the oxygen content is significantly lower, the potential for NO
is greatly reduced. Moreover, the arrangement defined permits feeding large
coal sizes that result in obvious economies of operation. Furthermore, the
feeder may be made to respond quickly to a change in conditions as called for
by any condition responsive device.
