Note: Descriptions are shown in the official language in which they were submitted.
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FLUIDIZED BED REACTOR AND METHOD OF OPE~ATING SAME
Background of the Invention
This invention relates to a fluidized bed reactor
and a method of operating a fluidized bed reactor, and,
more particularly, to such an apparatus and method in
which heat is generated by the combustion of fuel in a
fluidized bed.
Fluidized bed reactors, combustors, or gasifiers,
are well known. In these arrangements, air is passed
through a bed of particulate materials, including a
fossil fuel such as coal and an adsorbent for the sulfur
generated as a result of combustion of the coal, to
fluidize the bed and to promote the combustion of the
fuel at a relatively low temperature. When the heat
produced by the fluidized bed is utilized to convert
water to steam, such as in a steam generator, the
fluidized bed system offers an attractive combination of
high heat release, high sulfur adsorption, low nitrogen
oxides emissions and fuel flexibility.
The most typical fluidized bed combustion system is
commonly referred to as a bubbling fluidized bed in which
a bed of particulate materials is supported by an air
distribution plate, to which combustion-supporting air is
introduced through a plurality of perforations in the
plate, causing the material to expand and take on a
suspended, or fluidized, state. In the event the reactor
is in the form of a steam generator, the walls of the
reactor are formed by a plurality of heat transfer tubes.
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The heat produced by combustion within the fluidized bed
is transferred to a heat exchange medium, such as water,
circulating through the tubes. The heat transfer tubes
are usually connected to a natural water circulation
circuitry, including a steam drum, for separating water
from the steam thus formed which is routed to a turbine
to generate electricity or to a steam user.
In an effort to extend the improvements in
combustion efficiency, pollutant emissions control, and
operatin turndown afforded by the bubbling bed, a
fluidized bed reactor has been developed utilizing the
fast fluidized bed process. According to this process,
fluidized bed densities between 5 and 20~ volume of
solids are attained which is well below the 30~ volume of
solids typical of the bubbling fluidized bed. The
formation of the low density fast fluidized bed is due to
its small particle size and to a high solids throughput,
which requires high solids recycle. The velocity range
of a fast fluidized bed is between the solids terminal,
or free fall, velocity and a velocity which is a function
of the throughput, beyond which the bed would be
converted into a pneumatic transport line. For each
solids circulation rate of flow there is a maximum
velocity, beyond which said conversion of the fluidized
bed to pneumatic transport occurs.
The high solids circulation required by the fast
fluidized bed makes it insensitive to fuel heat release
patterns, thus minimizing the variation of the
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temperature within the combustor or gasifier, and
therefore decreasing the nitrogen oxides formation.
Also, the high solids loading improves the efficiency of
the mechanical device used to separate the gas from the
solids for solids recycle. The resulting increase in
sulfur adsorbent and fuel residence times reduces the
adsorbent and fuel consumption. Furthermore, the fast
fluidized bed inherently has more turndown than the
bubbling fluidized bed.
However the fast fluidized bed process is not
without problems. For example, the particulate fuel and
adsorbent material used in a fast fluidized bed process
must be relatively fine therefore requiring further
crushing and drying of the particulate material, which is
expensive. Also, the bed height required for adequate
adsorption of the sulfur will be greater than that in a
conventional bubbling fluidized bed system, which further
adds to the capital expense and operating costs.
Summary of the Invention
It is therefore an object of the present invention
to provide a fluidized bed reactor and method of
operating a fluidized bed reactor in which a wide range
of fuel and adsorption particle size can be utilized.
It is a still further object of the present
invention to provide a reactor and method of the above
type in which adequate adsorption is achieved with a
reduced bed height.
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It is a still further object of the present
invention to provide a reactor and method of the above
type in which a gas column is formed in the fluidized bed
boiler which is saturated with particulate material.
It is a still further object of the present
invention to provide a reactor and method of the above
type in which the particulate material in the gas column
is collected and essentially the same amount recirculated
to the fluidized bed to maintain the saturated gas
column.
It is a still further object of the present
invention to provide a reactor and method of the above
type in which the volume of solids contained in the
boiler furnace is relatively high, compared to the
bubbling fluidized bed.
It is a still further object of the present
invention to provide a reactor and a method of the above
type in which the temperature of the fluidized bed is
varied by varying the amount of air introduced into the
bed.
It is a still further object of the present
invention to provide a reactor and method of the above
type in which cooling surfaces are provided in contact
with the bed and the gas column.
It is a still further object of the present
invention to provide a reactor and method of the above
type which incorporates operating principles and
advantages of both the bubbling fluidized bed and the
fast fluidized bed.
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Toward the fulfillment of these and other objects,
the method of the present invention features the forming
of a gas column above a fluidized bed which contains a
mixture of air, the gaseous products of combustion from a
fluidized bed, and particulate material, a portion of
which is coarse enough to continuously stay in the bed
while the rest ls fine enough to be entrained by the air
and the gaseous products of combustion. The gas column
is saturated with particulate material and the
particulate material is separated from the mixture and
injected back into the bed to maintain the saturation.
Any remaining portion of the separated particulate
material is passed to external equipment.
Brief Description of the Drawings
The above brief description, as well as further objects,
features and advantages of the method of the present
invention will be more fully appreciated by reference to
the following detailed description of presently preferred but
nonetheless illustrative embodiments in accordance with
the present invention when taken in conjunction with the
accompanying drawing in which:
Fig. 1 is a schematic view depicting an atmospheric
fluidized bed reactor of the present invention forming a
part of a natural circulation steam generator;
Fig. 2 is a graph depicting an example of the
relationship between the fluidizing air velocity and the
solids entrainment; and
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Fig. 3 is a graph depicting an example of the
relationship between load, percentage of air, bed
temperature and furnace exit temperature.
Description of the Preferred Embodiment
The present invention will be described in
connection with a fluidized bed boiler forming a portion
of natural water circulation steam generator, shown in
general by the reference numeral 10 in Fig. 1 of the
drawings.
The steam generator 10 includes a steam drum 12
which receives water from a feed pipe 14 and which
discharg~s the steam generated via a plurality of steam
pipes 16.
A fluidized bed boiler 18 is disposed adjacent the
steam drum 12, and includes a front wall 20A, a spaced,
parallel rear wall 20B, and two spaced sidewalls, one of
which is shown by the reference numeral 22, which extend
perpendicular to the front and rear walls to form a
substantially rectangular furnace 24.
The walls 20A, 20B and 22 of the boiler 18 are
formed by a plurality of vertically-disposed tubes
interconnected by vertically-disposed elongated bars, or
fins, to form a contiguous, air-tight structure. Since
this type of structure is conventional, it is not shown
in the drawings nor will it be described ln any further
detail. The ends of each of the tubes of the walls 20A,
20B, and 22 are connected to horizontally-disposed lower
and upper headers 26 and 28 for reasons that will be
explained later.
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An opening 30 is formed in the upper portion of the
rear wall 20B by bending back some of the tubes (not
shown) forming the latter wall to communicate the upper
portion of the furnace 24 with a separating section 32
disposed adjacent the boiler 18. A cyclone separator 33
forms the lower portion of the separating section 32 and
includes a coaxially disposed tubular portion 34 which,
together with the walls of the separator, form an annular
flow path for the gases entering the separator from the
boiler 18. The latter gases swirl around in the annular
chamber to separate the entrained solids therefrom by
centrifugal forces, before the gases pass to the upper
portion of the separating section. The separated solids
fall into a lower hopper portion of the separator 33 and
are passed to a diverter device, or valve 35, which has a
recycle conduit 36 and an extraction conduit 37 extending
therefrom. The valve 35 operates in a conventional
manner to vary the proportional flow of solids between
the conduits 36 and 37. The conduit 36 extends through
the rear wall 20B and into the boiler 18 and the conduit
37 is adapted for connection to external e~uipment (not
shown)
A heat recovery enclosure 38 is formed adjacent the
separating section 32 and has an opening 39 formed in an
upper wall portion which receives the clear gases from
the separating section. A pair of superheaters 40A and
40B are disposed in the heat recovery enclosure 38 in the
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path of the gases, and each superheater consists of a
plurality of tubes connected in a flow circuitry for
passing steam through the tubes in a conventional manner
to remove heat from the gases.
A boiler bank in the heat recovery enclosure 38 is
formed by a series of parallel tubes 42 connecting the
steam drum 12 to a water drum 44 for transferring water
to the latter drum under conditions that will be
described later. A gas flow passage is provided adjacent
the tubes 42 and an outlet 45 is provided for the gas.
The walls forming the upper portions of the
separating section 32 and the heat recovery enclosure are
formed by a plurality of vertically disposed tubes
interconnected by vertically disposed elongated bars, or
fins to form a contiguous, wall-like structure identical
to the walls forming the boiler 18. The upper ends of
these walls are connected to a plurality of horizontally-
extending upper headers 46, and the lower ends of the
walls are connected to a plural:ity of horizontally
extending lower headers, one of which is shown by the
reference numeral 48.
Although not shown in the drawing, it is understood
that water flow circuitry, including downcomers and the
like, are provided to connect the steam drum 12 and/or
the water drum 44 to the headers 26, 28, 46, and 48 to
form a flow circuit for the water and steam through the
steam drum 12, the water drum 44 and the walls forming
the boiler 18, the separating section 32 and the heat
recovery enclosure 38. Since this is a conventional
3i~0~c technique it will not be described any further.
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A plenum chamber 50 is disposed at the lower portion
of the boiler 18 into which pressurized air from a
suitable source (not shown) is introduced by conventional
means, such as a forced-draft blower, or the like.
A perforated air distribution plate 52 is suitably
supported at the lower portion of the combustion chamber
of the boiler 18, and above the plenum chamber 50. The
air introduced through the plenum chamber 50 passes in an
upwardly direction through the air distribution plate 52
and may be preheated by air preheaters (not shown) and
appropriately regulated by air control dampers as needed.
The air distribution plate 52 is adapted to support a bed
54 of a particulate material consisting, in general, of
crushed coal and limestone, or dolomite, for adsorbing
the sulfur formed during the combustion of the coal.
The inner surfaces of the lower portion of the walls
20A, 20B, and 22 of the boiler 18, are lined with a
refractory 56, or other suitable insulating material,
which extends a predetermined distance above the air
distribution plate 52.
A fuel distributor 58 extends through the front wall
20A for introducing particulate fuel onto the upper
surface of the bed 54, it being understood that other
distributors can be associated with the walls 20A, 20B
and 22 for distributing particulate sorbent mate~ial
and/or additional particulate fuel material onto the bed
54, as needq~.
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A drain pipe 60 registers with an opening in the air
distribution plate 52 and extends through the plenum 50
for discharging spent fuel and sorbent material from the
bed 54 to external equipment.
A multiplicity of air ports 62 are provided through
the sidewall 22 at a predetermined elevation from the bed
54 to introduce secondary air into the boiler for reasons
to be described. It is understood that additional air
ports at one or more elevations can be provided through
the walls 20A, 20B, and the other sidewall as needed.
In the operation of the steam generator 10, a
quantity of fuel and sorbent particles, such as coal and
limestone, is introduced through the distributor 58 (and
other distributors as needed) and builds up on the upper
surface of the plate 52. The particles are ignited by
burners (not shown) positioned within the bed and, as the
combustion of the coal progresses, additional air is
introduced into the plenum chamber 50 at a relatively
high pressure and velocity. Alternatively, the particles
can be warmed up by a burner located in the plenum 50.
The primary air introduced through the plenum chamber 50
comprises a fraction of the total air required for
complete combustion of the coal so that the combustion in
the lower section of the furnace 24 is incomplete. The
latter section thus operates under reducing conditions
and the remaining air required for complete combustion of
the coal is supplied by the air ports 62. When operating
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at maximum capacity, the range of air supplied through
the plenum 50 can be from 40% to 90% of that required for
complete combustion with this amount varying according to
the desired bed temperature, while the remaining air (60%
to 10%) is supplied through the ports 62 to complete the
combustion.
The high-pressure, high velocity, combustion-
supporting air introduced through the air distribution
plate 52 from the plenum chamber 50 causes the
relatively-fine particles of coal and limestone,
including the fine particles of coal ash and spent
limestone, to become entrained within and to thus be
pneumatically transported by the combustion gases. This
mixture of entrained particles and gas rises upwardly
within the furnace 24 to form a gas column containing the
entrained solids and passes from the boiler 18 through
the opening 30 and into the separating section 32.
According to a feature of the present invention,
the amount of relatively fine and course coal and
limestone particles introduced to the bed 54 by the
distributor 58 is such that the gas column formed in the
furnace 24 above the bed 54 is saturated with the solid
particles, i.e. maximum entrainment of the solid
particles by the gas is attained. The maximum particle
entrainment as a function of fluidizing velocity as
determined experimentally, is shown in Figure 2. In
applying Figure 2, the fraction of bed material of size
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that can be transported by the gases has to be taken into
consideration, as well as the partial segregation of
coarser material. As shown in Figure 2, the particle
entrainment at a 12 feet per second fluidizing velocity
is about 28 pounds per pound of gas, but it becomes about
10 pounds once said considerations are made. As a result
of the saturation, a portion of the fine particles are
not entrained by the gas and together with coarse
particles form a discrete bed 54, which exhibits a relatively high
percentage volume of particles, such as 20% to 30~ of the
total volume, when operating at maximum capacity.
The coarse particles are accumulated in the lower
portion of the bed along with a portion of the fine
particles, while the remaining portion of the fine
particles passes upwardly through the gas column. The
entrained relatively fine particles pass upwardly through
the length of the gas column and exit from the boiler 18
through the opening 30. The fine particles are separated
from the combustion gases within the separator 33; and
are recycled back to the fluidi~ed bed through the valve
35 and conduit 36. The position of valve 35 is adjusted
to vary the relative proporti.on of solids entering the
conduits 36 and 37 and therefore the relative amount of
solids entering the boiler 18 and discharging to external
equipment, respectively. In this manner, the volume of
relatively fine particles recycled back to the boiler 18
via conduit 36, and therefore the ratio of fine particles
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to coarse particles being circulated, can be precisely
regulated. The introduction of additional particulate
fuel material, both fine and coarse, through the
distributor 58 is maintained at proper levels to insure
that the gas column above the bed 54 is saturated
notwithstanding the discharge of spent materials from
drain 60 and the discharge of a portion of the fine
particles from conduit 37.
Water is introduced into the steam drum 12 through
the water feed pipe 14 where it mixes with water in the
drum 12. Water from the drum 12 is conducted downwardly
through the tubes 42 into the water drum 44 and, through
downcomers or the like, into the lower headers 26 and the
tubes forming the boiler walls 20A, 20B and 22, as
described above. Heat from the fluidized bed, the gas
column, and the transported solids converts a portion of
the water into steam, and the mixture of water and steam
rises in the tubes, collects in the upper headers 46, and
is transferred to the steam drum 12. The steam and water
are separated within the steam drum 12 in a conventional
manner, and the separated steam is conducted from the
steam drum by the steam pipes 16 to the superheaters 40A
and 40B and thereafter to a steam turbine, or the like.
The separated water is mixed with the fresh supply of
water from the feed pipe 14, and is recirculated through
the flow circuitry in the manner just described. Other
cooling surfaces, preferably in the form of partition
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walls with esssentially vertical tubes, can be utilized
in the furnace 24.
The hot clean gases from the separating section 32
pass over the superheaters 40A and 40B and the tubes 42
to remove additional heat from the gases and add heat to
the water flowing through the latter tubes, before the
gases exit from the steam generator, via the outlet 45.
If the air which is introduced into the plenum 50 is at a
relatively high pressure on the order of 10 atmospheres,
the gases from the outlet 39 may be directed to a gas
turbine, or the like (not shown).
In response to changes in load of the steam turbine,
the temperature of the bed 54 is maintained at a preset
acceptable value by changing the amount of air supplied
to the boiler via the air plenum 50, with the remaining
air necessary to complete combustion being supplied
through the air ports 62. This is shown in the graph of
Fig. 3, which depicts variations, as a function of load,
of the temperatures and of the percentage of theoretical
air for combustion supplied to the air plenum 50. The
curve referred to by the reference letter A shows the
variations in the air added to the lower portion of the
bed via plenum 50 as a percentage of the theoretical air
for combustion, with changes in load. From this it can
be appreciated that, at constant load, variations in the
air added to the bed 54 will vary the temperature of the
bed.
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Curve B in Fig. 3 is a plot of the -temperature of
the mixture of gases and entrained solid particles as the
exit from the gas column in the boiler 18 through the
opening 30, while curve C depic~s the substantially
direct proportional relationship between bed temperatures
and load.
It is understood that the weight ratio of fine
particles in the bed to the coarse solid particles in the
bed 54 can vary from approximately 55-45 to 93-7, with
this weight ratio being substantially the same in all the
fluidized bed locations. Also, the bed volume occupied
by the solids (which is approximately 20-30~ of the total
volume) is substantially the same in all fluidized bed
locations and does not vary with height.
It is thus seen that the method of the present
invention incorporates operating principles of both the
bubbling fluidized bed system and the fast fluidized bed
system and therefore results in several advantages. For
example, the relatively high amount of lateral mixing of
the particles within the fluidized bed is similar to the
mixing attained by the bubbling fluidized bed. In
addition, the fine particles are retained in the reacting
zone, as in the case of a fast fluidized bed, and fuel
and adsorbent having a wider range of particle size can
be utilized. Also, a smaller static bed height and much
smaller expanded bed height than those of the fast
fluidized bed are possible. This, in conjunction with
the air discharging from the air ports 62 above the
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fluidized bed results in a smaller power requirement for
the air fans and less important mechanical forces due to
bed pressure variations. Further, the temperature of the
bed can be varied by varying the amount of air supplied
to the bed, and the majority of the reactions between
solid and gas, including the combustion in particular,
occur only below the overfire air ports 62 therefore
minimizing carbon monoxide and hydrocarbon emissions.
Also in conjunction with the preceding advantage, staging
of the air with an important overfire aix fraction
reduces the nitrogen oxides emissions. In addition, the
use of refractory material, preferably of the high
conductivity type, is possible below the overfire air
ports 62, where the surfaces face reducing gases, and in
erosion-prone locations elsewhere. Further, no active
control of the solids circulation rate of flow by the
solids recycle system is necessary, because the
continuous maintanance of the saturated gas column limits
the solids circulation. Also, by providing for
extraction via the conduit 37 of relatively small amounts
of the particles from the recycle system, as well as the
drain pipe 60, the residence time of the coarse and fine
partiCu~ate solids in the system can be adjusted to suit
their reacting characteristics.
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Although not specifically illustrated in the
drawings, it is understood that other additional and
necessary equipment and structural components will be
provided, and that these and all of the components
described above are arranged and supported in an
appropriate fashion to form a complete and operative
system.
It is also understood that variations may be made in
the method of the present invention without departing
from the scope of the invention. For example, the fuel
supplied to the boiler can be in liquid or gaseous form
rather than in the particulate solid form as described.
Of course, other variations can be made by those skilled
in the art without departing from the invention as
defined in the appended c'aims.
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