Abstrict A power plant utilizing combustion of fuel in a fluidized bed contains
a particulate sulfur-absorbing material. A discharge device for
bed material and ashes includes a crusher which breaks down the
material so that it can be transported away or so that unconsumed
absorbent in the bed material is exposed for reuse in the bed. The
crusher has a stationary grinding portion and a rotating grinding
portion receiving bed material. Scrapers selectively scrape off
layers of material during rotation of the rotating grinding portion
thus distributing the material around the periphery of the grinding
portions.
Claims What is claimed is:
1. In a power plant for the combustion of a fuel in a fluidized
bed including a bed vessel, a bed material within the bed vessel,
supply means for adding sulfur-absorbing material and fuel to the
bed material, means for supplying the bed vessel with a compressed
air for fluidization of the bed material and for combustion of the
fuel supplied to the bed and a discharge means with controllable
discharge capacity for removal of the bed material from the bed
vessel through an outlet means extending from the bed vessel, said
discharge means including:
a housing;
a vertical shaft journalled in said housing;
a driving motor for rotating said shaft;
means for controlling the speed of said driving motor;
a substantially horizontal plate supported by said shaft;
a connecting means in communication with said outlet tube of the
bed vessel for supplying the bed material to be discharged, to a
space above said plate, said connecting means being positioned above
said plate and eccentrically with respect to the axis of rotation
of said shaft;
a plurality of stationary scrapers arranged in said housing above
said plate, said scrapers upon rotation of said plate distributing
material radially over the edge of said plate; and
crushing means located in said housing and including an annular
rotating crushing member supported by said shaft below the peripheral
edge of said plate and a stationary crushing member surrounding
said rotating crushing member and horizontally spaced therefrom,
said crushing means reducing the size of slag lumps in the bed material
which are in excess of the size of a gap formed between said rotating
and stationary crushing members.
2. A power plant according to claim 1 in which said scrapers are
arranged at different heights above said plate.
3. A power plant according to claim 1 in which said scrapers have
different vertical extensions in the radial direction of said plate.
4. A power plant according to claim 2 in which said scrapers have
different vertical extensions in the radial direction of said plate.
5. A power plant according to claim 3 wherein said scrapers trace
arcuate lines over the surface of said plate.
6. In a power plant for the combustion of a fuel in a fluidized
bed including a bed vessel, a bed material within the bed vessel,
supply means for adding sulfur-absorbing material and fuel to the
bed material, means for supplying the bed vessel with a compressed
air for fluidization of the bed material and for combustion of the
fuel supplied to the bed and a discharge means with controllable
discharge capacity for removal of the bed material from the bed
vessel through an outlet means extending from the bed vessel, said
discharge means including:
a housing;
a vertical shaft journalled in said housing;
a driving motor for rotating said shaft;
means for controlling the speed of said driving motor;
a substantially horizontal plate supported by said shaft;
a connecting means in communication with said outlet tube of the
bed vessel for supplying the bed material to be discharged, to a
space above said plate, said connecting means being positioned above
said plate and eccentrically with respect to the axis of rotation
of said shaft;
a plurality of stationary scrapers arranged in said housing above
said plate, said scrapers having different extensions in the radial
direction of said plate, said scrapers upon rotation of said plate
distributing material radially over the edge of said plate; and
crushing means located in said housing and including an annular
rotating crushing member supported by said shaft below the peripheral
edge of said plate and a stationary crushing member surrounding
said rotating crushing member and horizontally spaced therefrom,
said crushing means reducing the size of slag lumps in the bed material
which are in excess of the size of a gap formed between said rotating
and stationary crushing members.
7. A power plant according to claim 6 in which said scrapers trace
arcuate lines over the surface of said plate.
8. A power plant according to claim 6 in which said scrapers trace
arcuate lines over the surface of said plate.
9. A power plant according to claim 6 wherein an outlet of said
crushing means is connected to said bed vessel by a pipe for returning
crushed bed material to said bed vessel.
10. A power plant according to claim 6 wherein the combustion
in said bed vessel takes place at a pressure considerably exceeding
atmospheric pressure and wherein said crushing means is connected,
on its outlet side, to a pneumatic pressure-reducing transport device,
in which pressure reduction is brought about by passing the material
flow along a plurality of series-connected tube parts with sharp
bends between adjacent tube parts.
11. A power plant according to claim 6 wherein said bed vessel
is enclosed in a pressure vessel with a space between the pressure
vessel and the bed vessel for compressed combustion air, from which
space the bed vessel is supplied with air for fluidization of the
bed and combustion of the fuel, and wherein said crushing means
is connected on its outlet side to a pneumatic, pressure-reducing
transport device, in which pressure reduction is brought about by
passing the material flow along a plurality of series-connected
tube parts with sharp bends between adjacent tube parts.
12. A power plant according to claim 6 wherein the bed vessel
is enclosed within a pressure vessel with a space between the pressure
vessel and the bed vessel for compressed combustion air, from which
the bed vessel is supplied with air for fluidization of the bed
and combustion of the fuel, and wherein said crushing means is connected
on its outlet side to said bed vessel by means of a pneumatic conveying
pipe for returning the crushed bed material to said bed.
Description TECHNICAL FIELD
The invention relates to a power plant, primarily a Pressurized
Fluidized Bed Combustion (PFBC) power plant, involving combustion
of a sulfur-containing fuel in a fluidized bed containing a particulate
sulfur-absorbing material in a bed vessel. A discharge device for
bed material and ashes includes a crusher having a controlled capacity.
The crusher breaks down slag lumps so that the bed material can
be transported away via a pressure reducing pneumatic transport
device, or grinds down bed material so that unconsumed absorbent
in the bed material is exposed and, after having been fed back into
the bed, is utilized.
BACKGROUND ART
Absorbent must be permanently supplied to the bed in dependence
on the sulfur contents in the fuel. Bed material and ashes formed
from the fuel or slag particles which do not accompany the combustion
gases must be continuously or intermittently removed to prevent
the bed level from rising. Fine-grained material, such as finer
fractions of ashes and particles abraded off lumps of bed material,
accompany the combustion gases, and are separated in a cleaning
plant and transported away therefrom. Bed material, ashes and slag
have to be removed by being tapped off from the bed vessel through
a controllable discharge device.
Brannstrom et al U.S. patent application Ser. No. 917649 (filed
Oct. 10th, 1986) describes a power plant with a fluidized bed, in
which the bed level is controlled by grinding down bed material
and directly returning the ground down bed material to the bed.
The grinding takes place to such a small particle size that the
ground bed material leaves the bed vessel together with the combustion
gases, is separated in a cleaning plant and is removed, suitably
through a pneumatic pressure reducing discharge device for example
of the kind described in European Pat. No. 0 108 505 Swedish patent
application No. 8602486-6 describes a PFBC power plant with a discharge
device for bed material with a slag crusher in which slag lumps
above a certain size are crushed in such a dimension that they can
be transported to a collecting container through a pneumatic pressure
reducing transport device.
SUMMARY OF THE INVENTION
According to the invention, the discharge device of the plant includes
a crusher with a vertical shaft located below the bed vessel. This
crusher is supplied with bed material from the bed vessel via a
tube extending from the bed vessel. The crusher comprises a housing
with a ring forming the stationary crushing or grinding portion
of the crusher. A vertical shaft journalled in the housing supports
a conical, circular body, forming the rotating grinding portion
of the crusher, and a substantially horizontal disk. The plate and
the grinding portion may be constructed as one unit and formed as
a truncated cone. The upper surface of the truncated cone forms
the disk. A supply tube for bed material in the upper part of the
housing is connected to the outlet tube and opens out above the
plate, eccentrically in relation to the axis of rotation of the
crusher. In the housing, above the plate supported by the shaft,
there are a number of stationary scrapers which distribute the bed
material coming in through the outlet tube such that bed material
falling down between the grinding portions of the crusher is evenly
distributed along the periphery of the plate. This results in uniform
wear around the stationary grinding portion.
The scrapers can be designed and arranged in different ways. They
are suitably located at different levels above the plate and have
an orientation which deviates from the radial direction, suitably
arcuate or helical as the blades of the impeller of a centrifugal
pump. Desirably the gap between the plate and each scraper is decreased
in the direction of rotation, each one of the scrapers thus removing
a material layer when the disk is rotating. This results in a substantially
even distribution of material along the periphery and in uniform
wear in the crusher. It would also be possible to use scrapers at
the same level but having different lengths in the radial direction.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in greater detail, by way of
example, with reference to the accompanying drawings, wherein:
FIG. 1 schematically shows a PFBC power plant incorporating the
invention,
FIG. 2 shows, in sectional elevation, the crusher included in the
plant of FIG. 1 and
FIG. 3 shows a cross sectional view taken along line A--A in FIG.
2 indicating the disposition of the scrapers in the crusher.
DESCRIPTION OF PREFERRED EMBODIMENT
In FIG. 1 10 designates a pressure vessel. In it are arranged
a bed vessel 12 and a gas cleaning plant symbolized by a cyclone
14. In reality the cleaning plant may consist of parallel-connected
groups of series-connected cyclones. The lower part of the bed vessel
12 includes an air distributor 16 which divides the bed vessel
12 into an upper combustion chamber 18 and a lower ash chamber 20.
The air distributor 16 consists of a number of elongated air distribution
chambers 22 with air nozzles 24. The chambers 22 communicate with
a space 25 between the pressure vessel 10 and the bed vessel 12.
This space 25 contains combustion air under pressure. Through the
nozzles 24 the combustion chamber 18 is supplied with air for fluidizing
a bed 26 of particulate material containing a sulfur absorbent and
for combustion of a fuel which is supplied to the bed 26 from a
fuel storage (not shown) through a fuel pipe 28. Fresh bed material
is supplied to the bed 26 from a reservoir of bed material (not
shown) via a pipe 30. Above the upper surface 32 of the bed there
is a freeboard 34 for collecting the combustion gases. From the
freeboard 34 the combustion gases are passed, via a pipe 36 to
the cleaning plant 14 and the cleaned gases are led, via a pipe
38 to a turbine 40. The turbine 40 drives a generator 41 and a
compressor 42. Combustion air compressed in the compressor 42 is
supplied to the space 25 through a pipe 44. Dust separated in the
cleaning plant 14 is transported away to a collecting container
(not shown) through a pipe 37. The bed 26 contains tubes 46 for
generating steam for driving a steam turbine (not shown) and for
cooling the bed 26.
Between the elongated air distribution chambers 22 there are openings
48 through which bed material from the bed 26 may pass down to
the ash chamber 20. The material 50 in the ash chamber 20 consists
of ashes, slag and consumed and unconsumed sulfur absorbent. The
ash chamber 20 includes tubes 52 for cooling the material 50. The
coolant in these tubes 52 may, for example, consist of combustion
air, steam or water. The material 50 is removed from the ash chamber
20 through an outlet tube 54 and a crusher 56. The tube 54 may include
a valve 55. The crusher 56 has an outlet 58.
In one embodiment of the present invention, the crusher 56 may
be constructed in such a way that it only crushes slag lumps to
a grain size of about 7-10 mm, whereas all other material having
a grain size of about 3-6 mm may pass freely without being reduced
in size. In this case slag is broken down so that the material fed
out through the crusher 56 is given an appropriate maximum grain
size for example that which is required for pneumatic transportation.
The pressure in the crusher 56 should, in a PFBC power plant, be
approximately equal to the pressure in the bed vessel 12. The outlet
58 of the crusher 56 is connected, either directly or through an
ejector 60 to a pressure-reducing discharge device 64 having a
plurality of series-connected tube parts with sharp bends between
adjacent tube parts, such as for example, a device described in
EP No. 0108505. This ejector 60 may be fed with propellant gas directly
from the space 25 or through a booster compressor, which on its
suction side is connected to the space 25 in the pressure vessel
10. In another embodiment of the invention, the crusher 56 may be
designed in such a way that it crushes both slag lumps and other
fed-out bed material. Unconsumed sulfur absorbent in the interior
of bed material particles is then exposed, and the crushed material
is returned to the bed 26. The outlet 58 of the crusher 56 is then
connected to the combustion chamber 18 through the ejector 60 and
the dash-lined return pipe 62.
For measuring the depth of the bed 26 differential pressure sensors
75 77 are provided, which are connected on one side through pipes
74 76 to the bed vessel 12 at different heights at points 70 72
and which are connected on their other side, through a common pipe
78 to the bed vessel 12 at a point 80 approximately on a level
with the air distributor 16. In reality more than two differential
pressure sensor would normally be provided. These pressure sensors
70 72 are connected to a signal processing unit 82 through pipes
84 86. A temperature sensor 88 in the bed 26 is also connected,
through a line 90 to the signal processing unit 82. A desired value,
determined by the power level, is supplied to the signal processing
unit 82 from a power control equipment (not shown) through a line
92. A control unit 96 for a supply device (not shown) for fresh
bed material is connected to the signal processing unit 82 by a
line 94 and to the supply device by a line 98. A control unit 100
is connected to the signal processing unit 82 by a line 102 and
to a drive motor 106 of the crusher 56 by a line 104. At constant
power output, the feeding out of material 50 from the ash chamber
20 is controlled by controlling the speed of the motor 106 and hence
of the crusher 56 so that the upper surface 32 of the bed is maintained
at a constant level. The amount of removed bed material is equal
to the amount of supplied bed material and fuel residue, such as
coal ashes formed which does not accompany the combustion gases
out from the freeboard 34. When the power is reduced, the level
of the upper surface 32 is lowered by increasing the speed of the
crusher 56 and when the power is increased, the level of the surface
32 is raised by stopping the crusher of reducing its speed and increasing
the rate of supply of fresh bed material.
FIG. 2 shows, on an enlarged scale, a section of the vertical crusher
56 included in the plant of FIG. 1. The crusher 56 comprises a housing
110 with a stationary grinding portion 112 and a rotor 114 with
a rotating grinding portion 116. The housing 110 consists of a lower
part 110a and an upper part 110b, which parts are formed with flanges
110c and 110d, respectively, and are joined together by means of
a bolted joint 118. Despite cooling in the ash chamber 20 the material
50 has a relatively high temperature, up to 400.degree. C., when
leaving the ash chamber 20. The parts of the crusher 56 are thus
subjected to a high temperature, which must be taken into account
during the design work. The stationary grinding portion 112 is constructed
as a replaceable ring resting on a flange 120 in the lower part
110a of the housing 110 in such a way that it may expand radially.
The stationary grinding portion 112 is retained by a resilient centering
ring 122 with a flange 122a which is clamped between the flanges
110c and 110d. The space between the stationary grinding portion
112 and the housing 110 is filled with heat insulating material
124.
The rotor 114 of the crusher 56 is journalled in an upper bearing
126 in a sleeve 128 which slides into the lower part 110a of the
crusher housing 110 and in a lower bearing in the drive motor 106.
The upper bearing 126 is suitably positioned on a level with the
lower edge of the stationary grinding portion 112. The rotor 114
comprises a shaft 130 which is connected at its bottom end to the
output shaft of the motor 106. At its upper end the shaft 130 supports
a suitably annular rotating grinding portion 116 and a suitably
plane plate 132 connected to the grinding portion 116. The grinding
portion 116 the shaft 130 and the plate 132 jointly define a volume
which is filled with a heat-insulating material 134. The shaft 130
and the grinding portion 116 are joined together by a bolted joint
133. The shaft 130 is internally water-cooled, cooling water being
introduced and discharged through a rotating connection 136. The
shaft 130 is provided with a bore 138. The cooling water is introduced
into a central tube 140 in the bore 138 so as to obtain satisfactory
cooling of the upper part of the shaft 130 and of the bearing 126.
The cooling water is returned through a gap 142 between the wall
of the bore 138 and the tube 140. Between the shaft 130 and the
sleeve 128 there is formed an annular gap 144. At the lower part
of the sleeve 128 there is a sealing device 147 between the sleeve
128 and the shaft 130. Through the connection 146 the gap 144 is
supplied with air or other gas, partly for cooling the sleeve 128
and partly for cleaning the space 148 between the rotating grinding
portion 116 and a disk 152 which is joined to the sleeve 128 by
a bolted joint 150. As shown by the arrows the gas flows up through
the gap 144 through openings 154 an annular gap 156 a space 148
and an annular gap 158 to a collecting space 160 for the crushed
material. The sleeve 128 is externally provided with heat insulation
162. The lower part 110a of the housing 110 is internally provided
with a layer of abrasion-resistant, heat insulating material 164.
The upper part 110b of the crusher housing 110 is internally provided
with a disk 168 which is joined by means of a bolted joint 170 to
an annular flange 172. Thermal insulating material 174 is provided
in the space above the disk 168. Through the upper part 110a of
the crusher housing 110 there passes an inlet tube 176. The inlet
tube 176 is joined to a flange on the outlet tube 54 by means of
a flange 178 and a bolted joint. The inlet tube 176 is located eccentrically
in relation to the vertical axis C--C of the crusher 56. The inlet
tube 176 opens out in an orifice 180 at such a height above the
plate 132 of the rotor 114 that the largest slag lumps, 40-60 mm,
that may pass the gap 48 or rectangular openings between the chambers
22 of the air distributors, are able to pass between the orifice
180 and the plate 132. The distance from the edge 182 of the plate
132 is so large that bed material falling down through the tube
176 does not of its own run over the edge 182 of the plate 132.
The disk 168 is provided on its lower side with a number of scrapers
184a-184g which have an orientation deviating from the radial. The
scrapers 184a-184g are suitably shaped to trace arcuate lines over
the surface of the plate 132 as shown in FIG. 3 and have different
vertical extensions so that between them and the plate 132 there
are formed gaps 186a-186g with a successively diminishing size.
When the rotor 114 rotates, as indicated by the arrow 188 in FIG.
3 material on the plate 132 will be scraped off by an appropriate
one of the scrapers 184a-184g. The first scraper 184a scrapes off
a first material layer, the second scraper 184b scrapes off a second
material layer, and so on, because of the fact that the gaps 186a-g
between the scrapers 184a-184g successively decrease in size. In
this way the material is distributed relatively evenly along the
periphery 182 of the plate 132 to the conical gap 190 between the
two grinding portions 112 and 116 of the crusher 56. In the conical
gap 190 the slag lumps are crushed. In those cases where the crusher
56 is employed only to ensure that larger pieces of material are
crushed to such a size, suitably 7-10 mm, that they may pass freely
through a pressure-reducing pneumatic discharge device, the gap
192 between the grinding portions 112 and 116 has such a size that
the main part of the bed material having a size of about 3-6 mm
is able to fall freely through the gap 192 into the space 160 without
reduction in size and thus without giving rise to wear of the grinding
portions. The outlet 194 is connected to the outlet pipe 58. In
those cases where the crusher 56 is also used for crushing bed material
particles so that absorbent which has not been utilized in inner
parts of the particles is exposed, the gap 192 has such a size as
to obtain a fine-grained material with a large effective absorption
surface of the yet-to-be-utilized absorbent.
It will be appreciated that the form of crusher described with
reference to FIGS. 2 and 3 is open to wide variations and all such
variations with the scope of the following claims are intended to
be within the scope of the invention.
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