Note: Descriptions are shown in the official language in which they were submitted.
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SYSTEM AND METHOD OF DECREASING NOX
EMISSIONS FROM A FLUIDIZED BED REACTOR
8ACKGROUND OF THE lN V ~:N'l'lON
This invention relates to a system and method of
decreasing nitrogen oxides (''NOx'') emissions from a
fluidized bed reactor. More particularly, this invention
relates to the selective injection of a reactant into the
reactor for reducing NOx levels in gaseous products of
combustion in the reactor.
Fluidized bed combustion systems are well known and
include a furnace section in which an oxygen-containing
gas such as air is passed through a bed of particulate
materials, including nitrogen-cont~;ning, carbonaceous
fuel material, such as coal. Sorbent particles, such as
limestone, lime, or dolomite may be added for the capture
of oxides of sulfur generated during combustion. The
oxygen-containing gas fluidizes the particulate materials
in the furnace section and promotes the combustion of the
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particulate fuel material at a relatively low
temperature. These types of combustion systems are often
used in steam generators in which a cooling fluid, such as
water, is passed through a fluid flow circuit in a heat
e~h~nge relationship to the fluidized bed reactor to
generate steam and to permit high combustion efficiency,
fuel flexibility, high sulfur adsorbtion, and relatively
low NOx emissions.
A typical fluidized bed reactor utilized in the
generation of steam is commonly referred to as a
"bubbling" fluidized bed in which the fluidized
particulate materials form a bed having a relatively high
density and a well-defined or discrete upper surface. A
more commonly used fluidized bed reactor is referred to as
a "circulating" fluidized bed in which the fluidized
particulate materials form a lower dense bed having a
density below that of a typical bubbling fluidized bed and
in which the primary gas has a fluidizing velocity which
is equal to or greater than that of a bubbling bed. The
primary gas passing through the lower dense bed entrains a
substantial amount of fine particulate materials to form
an upper dispersed bed of particulate materials, often to
the extent that the primary gas is substantially saturated
with the particulate materials in the dispersed bed.
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It is generally considered desirable to operate these
circulating fluidized beds using relatively high internal
and external solids recycling so that they are insensitive
to fuel heat release patterns, thus minimizing temperature
variations and stabilizing the sulfur emissions at a low
level. The high external solids recycling is achieved by
disposing a separator, such as a cyclone separator, at the
furnace section outlet to receive the flue gases, and the
particulate materials entrained thereby, from the
dispersed bed of the furnace section. The entrained
particulate materials are separated from the flue gases in
the separator, and the cleaned flue gases are passed to a
heat recovery section while the separated particulate
materials are recycled bac~ to the furnace section. This
recycling improves the efficiency of the separator, and
the increased residence times of the fuel and sorbent
particles result in more efficient use of the fuel and
sorbent particles and, therefore, reduced consumption of
the same.
Bubbling and circulating fluidized bed reactors also
offer advantages in pollution control. For example, the
emissions of NOx from fluidized bed reactors are
relatively low compared to emissions from other
conventional systems such as gas-fired systems and
coal-fired power plants. To obtain even lower NOx
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emission levels, selective non-catalytic reduction
( "SNCR" ) methods and selective catalytic reduction methods
("SCR") are employed. In SNCR methods, a reactant such as
urea or ammonia, is injected into the reactor to react
with the NOx, forming N2 and H20. The reactant is
typically injected through numerous ports at various
locations across the reactor including the furnace
section, the separator, and the duct connecting the
furnace section and separator. SNCR methods thereby allow
even lower NOx emission levels to be obt~; ne~ .
However, SNCR methods are not without problems. For
example, inefficient utilization of the added reactant
often prevents the SNCR methods from obtaining the desired
degree of decrease in NOx levels. For more efficient
usage of the reactant, it is desirable to have a high
residence time of the reactant in the system, a high
degree of mixing of the reactant with the NOx-cont~ining
flue gases, and a low degree of mixing of the reactant
with the particulate materials circulating in the system.
Present systems often suffer from inefficient use of the
reactant. For example, systems which inject the reactant
into the furnace section and systems which inject the
reactant into various locations across the duct may suffer
from too much mixing of the reactant with the particulate
materials and insufficient mixing of the reactant with the
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NOx-containing flue gases. Similarly, systems which
inject the reactant into the separator may suffer from
insufficient residence time and from insufficient m; Yi ng
of the reactant with the NOx-containing flue gases.
Inefficient utilization of the reactant results in
excessive use of the reactant which adds to the cost of
the SNCR method. Additionally, adding excessive amounts
of reactant can generate new pollution problems.
SUMMARY OF THE lNV~N'l'lON
It is therefore an object of the present invention to
provide a system and method of operating a fluidized bed
reactor in which NOx emission levels are lowered.
It is a further object of the present invention to
provide a system and method of operating a fluidized bed
reactor in which NOx emission levels are lowered using a
selective non-catalytic reduction method.
It is a still further object of the present invention
to provide such a system and method in which a reactant is
efficiently used to decrease NOx emission levels in
gaseous products of combustion.
It is a still further object of the present invention
to provide a system and method of the above type which
permits increased residence time of the reactant,
increased mixing of the reactant with gaseous products of
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combustion, and decreased mixing of the reactant with
particulate materials to provide for highly efficient use
of the reactant.
It is a still further object of the present invention
to provide a system and method of the above type in which
a reactant is selecti~ely injected into the system at a
particular location for efficiently decreasing NOx
emission levels in gaseous products of combustion.
Toward the fulfillment of these and other objectives,
the system and method of the present invention permits the
lowering of NOx levels in flue gases from a fluidized
bed reactor through selective non-catalytic reduction. A
reactor is connected to a separator ~y a duct, and a
reactant is introduced into the duct for decreasing NOx
levels in the flue gases passing from the reactor, through
the duct, and into the separator. The reactant, such as
ammonia or urea, is injected into a gaseous-rich region of
the duct, near an upper, inner portion of the duct, so
that a high degree of mixing of the reactant with flue
gases is achieved while maintaining a low degree of m; ~i ~q
of the reactant with the particulate materials. The
reactant is also injected into the duct at a point nearer
to the reactor than to the separator to provide for
increased residence time. In this manner, the reactant is
used efficiently while obtaining the desired lowering of
NOx levels in the flue gases.
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BRIEF DESCRIPTION OF THE DRAWINGS
The above brief description, as well as further
objects, features and advantages of the present invention
will be more fully appreciated by reference to the
following detailed description of the presently preferred
but nonetheless illustrative embodiments in accordance
with the present invention when taken in conjunction with
the accompanying drawings wherein:
FIG. 1 is a schematic, elevational view of a portion
of a fluidized bed combustion system for practicing the
present invention.
FIG. 2 is a schematic, side elevational view of a
fluidized bed combustion system for practicing the present
invention.
FIG. 3 is an enlarged, schematic, plan view taken
along the line 3-3 of FIG. 2.
FIG. 4 is an elevational view of the system of FIGS.
2 and 3, taken along the line 4-4 of FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIGS. 1-3, the reference numeral 10
refers in general to a fluidized bed reactor used for the
generation of steam. The reactor 10 includes an enclosure
12 having a front wall 14, a spaced, parallel rear wall
16, two spaced side walls 18 and 20 (FIG. 3) which extend
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perpendicular to the front and rear walls, a roof 22, and
a floor 24, which together form a substantially
rectangular enclosure.
A lower portion of the enclosure 12 is divided by a
perforated distribution plate 26 into a furnace section 28
and a plenum chamber 30. The distribution plate 26 is
suitably supported at the lower portion of the enclosure
12 and supports a bed of part-culate materials which may
include nitrogen-containing carbonaceous fuel particles,
such as coal, for combustion; sorbent particles, typically
a calcium-cont~ining sulfur acceptor such as limestone,
lime, or dolomite, for the capture of Sx released
during combustion of the fuel particles; and solid
products of combustion.
A conduit 31 supplies the plenum chamber 30 with a
fluidizing, oxygen-containing gas such as air from a
conventional, suitable source (not shown), such as a
forced-draft blower or the like. The fluidizing gas
introduced into the plenum chamber 30 passes in an upward
direction through the distribution plate 26 to support
combustion and to fluidize the particulate materials in
the furnace section 28.
A conduit 32 supplies the furnace section 28 with
particulate materials which may include
nitrogen-containing particulate fuel material, such as
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coal, and sorbent particles. It is understood that more
than one conduit 32 may be used and any number of
arrangements for providing fuel and sorbent particles to
the furnace section 28 of the enclosure 12 may be used.
Examples of a few arrangements that may be used are
disclosed in U.S. Patent No. 4,936,770, assigned to the
assignee of the present invention, the disclosure of which
is hereby incorporated by reference.
A duct 34 is connected to the rear wall 16 of the
enclosure 12 near the roof 22 and side wall 18. As best
shown in FIGS. 2 and 3, the duct has a roof or top wall
36, a floor or bottom wall 38, an outer wall 40 and an
inner wall 42. The duct 34 is disposed 50 that the outer
wall 40 is aligned with and falls in the same vertical
plane as the side wall 18 of the enclosure 12, and so that
the top wall 36 is aligned ~7ith and falls in the same
horizontal plane as the roof 22 of the enclosure 12. An
opening 44 in the rear wall 16 of the enclosure 12 places
the duct 34 in gas flow communication with the furnace
section 28 of the enclosure 12. For reasons to the be
described, a port 46 is provided for injecting a reactant
into an upper portion of the duct 34 through the top wall
36 of the duct. The port 46 is located near the opening
44 in the rear wall 16 of the enclosure 12 and is also
located closer to the inner wall 42 of the duct 34 than to
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-- 10 --
the outer wall 40 thereof. Although the duct 34 depicted
and described is subtantially rectangular, the duct 34 may
have any number of shapes, including but not limited to a
cylindrical configuration.
A cyclone separator 48 extends adjacent the enclosure
12 and is connected thereto by the duct 34 which extends
to an upper portion of the separator 48. An opening 49 in
an outer wall of the separator 48 places the duct 34 in
gas flow communication with the separator 48 so that flue
gases and particulate materials may pass from the
enclosure 12, through the duct 34, and into the separator
48. The lower portion of the separator 48 includes a
conically shaped hopper section 50 which is connected at
its lower end to a conduit 52 which has a branch conduit
52a extending back to the enclosure 12 and a branch
conduit 52b extending externally from the separator.
The separator 48 receives flue gase~ and entrained
particulate materials from the furnace section 28 and
operates in a conventional manner to disengage the
entrained particulate materials from the flue gases. The
separated particulate materials fall to the hopper section
50 of the separator 48 and pass to the conduit 52 for
recycle to the furnace section 28, via the branch conduit
52a, or for disposal via the branch conduit 52b. Although
reference is made to one separator 48, it is understood
that one or more additional separators (not shown~ may be
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used with the reactor 10. The number and size of
separators 48 used is determined by the capacity of the
steam generator and economic considerations.
The separated flue gases, which are substantially
free of particulate materials, pass via z duct 54, located
immediately above the separator 48, into a heat recovery
section shown in general by the reference numeral S6. A
plurality of heat exchange surfaces 58A, 58B, 58C are
disposed in the heat recovery section 56, all of which are
formed by a plurality of heat exchange tubes which extend
in the path of the separated flue gases as the separated
flue gases pass through the heat recovery section 56. The
heat exchange surfaces 58A, 588, 58C may serve as
reheaters, superheaters, economizers, or the like, as
desired. After passing across the heat exchange surfaces
58A, 58B, 58C, the separated flue gases exit the heat
reco~ery section 56 through outlet 60.
The walls of the enclosure 12, the duct 34, the
separator 48, and the heat recovery section 56 are
preferably formed by a plurality of spaced, parallel tubes
interconnected by fins to form contiguous airtight
structures. Since this type of structure is conventional,
it will not be shown or described in further detail. The
ends of each of these finned tubes are connected to a
plurality of horizontally disposed upper and lower headers
(not shown), respectively.
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.
- 12 -
- A steam drum (not shown) is located above the
enclosure 12, the duct 34, the separator 48, and the heat
recovery section 56. The steam drum receives a cooling
fluid such as water from a feed pipe, and a plurality of
downcomers and pipes extend from the steam drum and are
utilized, along with connecting feeders, risers, headers,
etc., to establish a fluid flow circuit which includes the
finned tubes forming the aforementioned walls and the heat
exchange surfaces 58A, 58B, 58C in the heat recovery
section 56. Water may be passed in a predetermined
se~uence through this fluid flow circuitry to convert the
water to steam and to heat the steam with the heat
generated by the combustion of the fuel particles.
In operation, particulate materials, including
nitrogen-containing carbonaceous fuel particles, such as
coal, and sorbent particles, typically a
calcium-containing sulfur acceptor such as limestone,
lime, or dolomite, are introduced into the furnace section
28 via the conduit 32 (FIG. 2). An oxygen-containing gas,
such as air, from an external source is introduced at a
relatively high pressure via the conduit 31 into the
plenum chamber 30 and is passed upwardly through the
distribution plate 26 at a relatively high fluidizing
velocity to fluidize the particulate materials in the
furnace section 28. A light-off burner (not shown) or the
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~ike ignites the fuel particles, and thereafter the fuel
particles are self-combusted by the heat in the furnace
section 28, thereby generating gaseous and solid products
of combustion.
The velocity of the fluidizing gas is then controlled
to maintain a dense bed of particulate materials in a
lower portion of the furnace section 28 and to pass or
entrain an amount of the particulate materials upwardly
from the dense bed to form a dispersed bed above the dense
bed.
The fluidizing gas mixes with the gaseous products of
com~ustion to form flue gases which pass upwardly through
the upper region of the furnace section 28 with the
entrained particulate material. The flue gases and at
least a portion of the entrained particulate materials
pass from the furnace section 28, through the duct 34, and
to the separator 48. In the separator 48, the particulate
materials are separated from the flue gases and fall to
the hopper section 50 of the separator 48 before passing
to the conduit 52 for recycle to the furnace section 28,
via branch conduit 52a, or for disposal via the branch
conduit 52b.
The separated flue gases exit the separator 48 via
the duct 54 and pass to a heat recovery section 56. In
the heat recovery section 56, the separated flue gases
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- 14 -
pass through the heat exchange surfaces 58A, 58B, 58C
before exiting via outlet 60.
Water is passed through the feed pipe to the steam
drum and is then passed through the fluid flow circuit so
that the heat generated by com~ustion is used to convert
the water to steam and to superheat the steam.
As the flue gases, with the entrained particulate
materials, pass from the furnace section 28, through the
opening 44, and into the duct 34, the particulate
materials tend to move toward an upper, outer portion of
the duct 34, whereas the flue gases, including undesired
N0x, tend to be concentrated more toward an upper, inner
portion of the duct. Due to this action, a gaseous-rich
region is formed in the upper, inner portion of the duct.
A reactant for lowering NOX levels, such as ammonia
or urea, is selectively injected into the gaseous-rich
region of the duct. The reactant is typically chosen for
its ability to provide an NH2 radical which, through a
series of complex reactions, reacts with the NOX to
yield N2 and H20. The reactant is injected into the
gaseous-rich region of the duct 34, in an upper portion of
the duct nearer to the inner wall 42 of the duct 34 than
to the outer wall 40 of the duct 34, to provide a high
degree of mixing of the reactant with the flue gases,
including NOX, while avoiding a high degree of mixing of
the reactant with the particulate materials in the upper,
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outer portion of the duct 34. The point of injection of
the reactant into the duct 34 is also at a location near
the opening 44 to provide for increased residence time of
the reactant. The high degree of mixing of the reactant
with the flue gases, the low degree of mixing of the
reactant with the particulate materials, and the high
residence time of the reactant in the system allow for
efficient use of the reactant while obtaining a large
decrease in NOx levels in the flue gases.
Several advantages result from the foregoing system
and method. For example, emissions of NOx are lowered
while making efficient use of P~pP~ive reactants. Also,
problems associated with excessive reactant use are also
avoided. The selective injection of the reactant is also
advantageous from the standpoint of ease and cost of
fabrication and operation.
It is understood that variations may be made in the
system and method of the present invention without
departing from the scope of the present invention. For
example, the injection point of the reactant may be in any
number of locations along the duct 34, as long as the
reactant is injected into a gaseous-rich region of the
duct 34 near an upper, inner portion of the duct 34. In
that regard, the injection port 46 may pass through the
upper 36, lower 38, outer 40, or inner 42, walls of the
duct 34 and may extend into the duct 34 or terminate in a
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- 16 -
duct wall. Although it is preferred that the duct 34 be
formed by finned, cooling tubes, the duct 34 may be of any
conventional construction. Additionally, the separator 48
may, but need not, be a cyclone separator, and one or more
separators may be associated with the furnace section.
Although only a single source of fluidizing gas is
discussed in the detailed description, it is understood
that the method and system of the present invention may be
used in connection with multi-staged combustion in which
fluidizing and combustive gases may be introduced into the
furnace section at various locations and at various levels.
Other modifications, changes, and substitutions are
intended in the foregoing disclosure and, in some
instances, some features of the invention can be employed
without a corresponding use of other features. Various
modifications to the disclosed embodiment as well as
alternative applications of the invention will be
suggested to persons skilled in the art by the foregoing
specification and drawing. Accordingly, it is appropriate
that the appended claims be construed broadly and in a
manner consistent with the scope of the invention therein.
