Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
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This invention relates to a fluidized bed combustion
system and method and, more particularly, to such a system
and method in which sulfides formed as a result of the
adsorption of the sulfur from the fossil fuel material are
converted to sulfatesO
The use of fluidized beds has long been recognized as
an attractive way of generating heat. In a normal fluidized
bed arrangement, a bed of granular particulate materials is
supported by a non-sifting grid through which an upward flow
of air is passed with a sufficient velocity to separate and
float, or suspend, the particles. The bed is heated with an
ignition device, or the like, to a temperature that will
support combustion of particulate fuel material which is
continuously introduced into the bed. The temperature of
the bed can be controlled by the regulation of air input to
the bed and the quantity of fuel introcluced into the bed.
These type of fluidized beds enjoy several advantages
including excellent heat transfer characteristics, which
result in a reduction of the total steam generator heat
transfer surface requirements, substantially ~miform bed
temperatures, ease of handling the fuel materials, and a
reduction in corrosion and boiler fouling.
Due to the increased emphasis on maintaining stack
emissions within regulated limits, the fluidized bed has
become increasingly important since low quality, high sulfur
coals can be burned directly in a fluidized bed while the
low process temperatures and chemical activity within the
bed can be utilized to limit nitrous oxides and sulfur
dioxides emissions.
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For example, in order to reduce the emissions of sulfur
dioxides generated during the combustion process, an adsorbent,
or acceptor, for the sulfur formed as a result of the combustion
of the particulate fuel material can be added to the fluidized
bed material. This adsorbent can be in several forms, such as
raw limestone, dolomite, or the like, and functions to adsorb,
or capture, the sulfur in the fluidized bed before the latter
is discharged from the stack.
In these type of arrangements, an oxidation atmosphere
is normally maintained through a substantial portion of the
bed and, assuming limestone is used as the acceptor, the sulfur
released during combustion of the coal will react with the lime-
stone to form calcium sulfates which, along with the spent
particulate coal, which is usually in the form of ashes, is
discharged from the bed and is relatively easy to dispose of.
However, as disclosed in applicant's U. S. Patent No. 4,308,810
entitled "Apparatus and Method For Reduction of NOX Emissions
From A Fluid Bed Combustion System Through Staged Combus-tion"
granted January 5, 1982~ it has been discovered that if sub-
stoichiometric (reducing) conditions are maintained at the bed
and oxygen is added to the off-gases from the bed at a location
above the bed, a considerable reduction of nitrous oxides is
achieved. The maintenance of the bed at the sub-stoichiometric
conditions, however, results in the formation of calcium sulfides,
in addition to calcium sulfates, in the bed. This is undesirable
since calcium sulfides decompose in water producing hydrogen
sulfide gas and calcium oxides which, if present in large
guantities in the waste bed materialv can create significant
disposal problems.
SU~MAR~ OF THE INVENTION
Accordingly the present invention seeks to provide a
fluidized bed combustion system and method in which the sulfur
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formed during the combustion of fossil fuel is adsorbed by an
adsorbant, or acceptor, material in the fluidized bed.
The invention in one aspect comprehends a fluidized bed
combustion system comprising a housing with grate means
supported in the housing and adapted to receive a bed of
particulate material at least a portion of which is fossil fuel
and an adsorbent for the sulfur formed as a result of combus-tion
of the fuel. Means are provided for passing air through the
grate means and the particulate material to fluidize the
particulate material and promote the combustion of the fuel.
Means are also provided for withdrawing from the bed the spent
fuel material sulfates and sulfides formed as a result of the
adsorption, and means are provided for exposing the withdrawn
sulfides to oxygen at a temperature sufficient to convert the
sulfides to sulfates.
The invention also pertains to a method of operating a
fluidized bed combustion system compric;ing the steps of forming
a bed of particulate materlal at least a portion of whi.ch is
fossil fuel and an adsorbent for the sulfur formed as a result
of combustion of said fuel, passing air through the particulate
material to fluidize the particulate material and promote the
combustion of the fuel, withdrawing from the bed the spent fuel
material, sulfates and sulfides formed as a result of the ad-
sorption, and exposing the withdrawn sulfides to oxygen at a
temperature sufficient to convert the sulfides to sulfates.
DESCRIPTION OF THE DRAWINGS
The above description, as well as further objects, features,
and advantages, of the present invantion will be more fully
appreciated by reference to the following detailed description of
the presently preferred but nonetheless illustrative embodiment
in accordance with the present invention, when taken in conjunction
with the accompanying drawing which is a vertical sectional view of
a fluidized bed combustion system of the present invention.
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DESCR.IPTION F THE PREFERRED EMBODIMENTS
Referring to the drawing, the reference numeral 10
refers in general to an enclosure forming a major portion of
a fluidized bed combustion system which may be in the form
of a boiler, a steam generator or any similar type device.
The enclosure 10 consists 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 enclosure 10 is not
shown for the convenience of presenta-tion, it being u.nderstood
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
reerence numeral 18, is disposed within the enclosure 10 !
and rests on an air distributor 20 extending horizontally in
the lower portion of the boiler. The bed 18 can conslst of
a mixture o discrete particles of inert material, a fuel
material, such as bituminoùs coal, and an adsorbent for the
sulfur released by the combustion of the fuel material, such
as limestone or dolomite.
An air plenum chamber 22 is provided immediately below
the distributor 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 suitably mounted in
the inlet 24 for pivotal movement about their centers in
response to actuation of external cont.rols (not shown) to
vary the effective opening in the inlet and thus control the
flow of air into the chamber 22. Slnce 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 rear wall 14 immediately above the distributor 20
for initially lishting off the bed 18 during startup.
A plurality of overbed feeders 30 are provided which
extend through the side wall 16. The feeders 30 receive
crushed coal from inlet ducts or the like, and are adapted
to feed the crushed coal particles onto the upper surface of
the bed 18 in a conventlonal manner. It is understood that,
alternatively, feeders similar to 30 could be located below
the upper surface of the bed to feed relatively fine coal
and that feedars identical to the feeders 30 could also be
provided through one or more of the front walls 12, the rear
walls 14 and the other side wall.
An insulated discharge-conversion duct 34 is pro-
vided adjacent the enclosure 10 and has an angularly-
extending upper portion 34 and a vertically extending lower
portion 34b. The upper duct portion 34a is connected to the
front wall 12 in any known manner and registers with the
lower portion of the fluidized bed 18.
A sparger ring 36 is disposed on the free end of the
lower duc~ portion 34b in communi~ation with the interior of
the duct 34 to introduce pressurized air from an external
source (not shown) into the duct whereby it flows in a
counterflow relationship to the materials discharging
through the duct as shown by the arrows, for reasons that
will be explained in detail later.
A metering device 38 is supported by the lower duct
portion 34b for metering -the discharge of the bed material
passing through the duct 34 to external apparatus for dis-
posal. Since the metering device 38 is o~ a conventional
design it will not be described in any further detail.
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In operation, a bed of particulate material is formed
on the distributor 20 and can consist of a mixture of crushed
fossil fuel, an inert material and an adsorbent ~or the
sulfur produced as a result of the combustion of the fossil
fuel. Although any known type of adsorbent can be used,
limestone will be referred to hereinafter for the purposes
of example. The bed 18 is initially fluidized by opening
the dampers 26 associated with the air inlet 24 to distri-
bute air up through the compartment 22, through the distri~
butor 20 and into the bed 18~
The burner 28 is fired to heat the material in the bed
until the temperature of the material reaches a predetermined
value at which time additional crushed fossil fuel is dis-
charged from the feeders 30 onto the upper surface of the
bed 18 as needed to insure a continuous replanishing of the
fuel as it burns off.
After the bed 18 has been fluidized and has reached a
predetermined elevated temperature, the burner 28 is turned
o while the feeders 30 continue to distribute particulate
fuel to the upper surface of the bed 18 in accordance with
predetermined feed rates~
The temperature of the bed 18 is controlled by
regulating the antount of air passing to the bed 18 through
the dampers 26 and the amount of particulate fuel material
introduced to the bed from the feeders 32. The bed tempera-
ture is maintained in t~e range of 1500F-1600F which
insures a capture of a great majority of the sulfur in the
form of sulur dioxide (SO2) formed during the combustion-
The bed 1~ is maintained at sub-stoichiometric (reduc;ng)
conditions for the reasons indicated above by control of the
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air and/or fuel input to the bed, and as a result, the
reactlon of the fossil fuel with the limestone will form
calcium sulfide in addition to calcium sulfate throughout the bed.
The mixture of spent fuel material, primarily in the
form of ashes, and the calcium sulfides will discharge
through the discharge-conversion duct 34 and to the metering
device 38 for disposal.
Air is passed from an external source through the
sparger ring 36 and upwardly through tl~e duct 34 in a
counterflow relation to the materials flowing downwardly
through the duct. Since the particulate material is at a
temperature of 1500F-1600F, this counterflow of the air
and particulate material results in a cooling of the latter
and a heating of the air to near-bed temperatures. As the
air temperature exceeds apprcximately 1200F/ conversion of
the calcium sulfide to calcium sulfate begins and continues
as the sulfides pass through the duct 34. As a resultr
a great majority of the sulfides are converted to sulfates
- before reaching the metering device 38 and are thus dis-
charged from the system in the form of sulfates, which
eliminates the disposal problems associated with the sulfides
as discussed above.
In addition, a minimal formation of nitrogen oxides is
inherent in the above process due to the relatively low bed
temperature of 1500-16Q0F, and due to the maintenance of
sub-stoichiometric conditions in -the bed. As a result, a
stack emissions are reduced to minimal amounts, notwith-
standing the fact that a high sulfur fossil fuel is combusted
in the bed.
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Although not shown in the drawings, it is understood
that heat exchange tubes can be provided in the system of the
present invention either separately or as an integral part
of the walls 12, 14 and 16 to provide for the passage of
water in a heat exchange relationship to the bed for the
purpose of converting the water to steam, or the like.
It is also understood that several variations can b~
made in the foregoing without departing from the`scope of
the invention. For example, the adsorbent material is not
limited to the use of limestone but can be any other
material that results in the formation of the sulfates and
sulfides discussed above.
Other modifications, changes and substitutions are
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.
