U.S. patent application number 12/366776 was filed with the patent office on 2010-08-12 for combustion systems and processes for burning fossil fuel with reduced emissions.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Peter Martin Maly, William Randall Seeker, Larry William Swanson.
Application Number | 20100203461 12/366776 |
Document ID | / |
Family ID | 42126421 |
Filed Date | 2010-08-12 |
United States Patent
Application |
20100203461 |
Kind Code |
A1 |
Maly; Peter Martin ; et
al. |
August 12, 2010 |
COMBUSTION SYSTEMS AND PROCESSES FOR BURNING FOSSIL FUEL WITH
REDUCED EMISSIONS
Abstract
A combustion system includes a combustion zone comprising a
burner for converting a fuel, under fuel rich conditions, to a flue
gas; an intermediate staged air zone downstream from the combustion
zone for supplying intermediate staged air to the flue gas and
producing fuel lean conditions; a reburn zone downstream from the
intermediate staged air zone for receiving the flue gas; and an
inlet downstream from the combustion zone for supplying a mixture
of air and a reduction reagent to the flue gas, wherein the
reduction reagent is configured to reduce an amount of a pollutant
species in the flue gas.
Inventors: |
Maly; Peter Martin; (Lake
Forest, CA) ; Seeker; William Randall; (San Clemente,
CA) ; Swanson; Larry William; (Laguna Hills,
CA) |
Correspondence
Address: |
CANTOR COLBURN, LLP
20 Church Street, 22nd Floor
Hartford
CT
06103
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
42126421 |
Appl. No.: |
12/366776 |
Filed: |
February 6, 2009 |
Current U.S.
Class: |
431/9 ;
431/115 |
Current CPC
Class: |
F23L 9/02 20130101; F23C
6/045 20130101; F23J 7/00 20130101; F23C 2201/101 20130101 |
Class at
Publication: |
431/9 ;
431/115 |
International
Class: |
F23C 9/00 20060101
F23C009/00 |
Claims
1. A combustion system, comprising: a combustion zone comprising a
burner for converting a fuel, under fuel rich conditions, to a flue
gas; an intermediate staged air zone downstream from the combustion
zone for supplying intermediate staged air to the flue gas and
producing fuel lean conditions; a reburn zone downstream from the
intermediate staged air zone for receiving the flue gas; and an
inlet downstream from the combustion zone for supplying a mixture
of air and a reduction reagent to the flue gas, wherein the
reduction reagent is configured to reduce an amount of pollutant
species in the flue gas.
2. The combustion system of claim 1, wherein the inlet is disposed
in the intermediate staged air zone and configured to supply a
mixture of the intermediate staged air and the reduction reagent
into the flue gas.
3. The combustion system of claim 1, wherein the intermediate
staged air comprises boosted air.
4. The combustion system of claim 1, further comprising an overfire
air inlet configured to supply overfire air to a burnout zone
downstream from the reburn zone and a boosted overfire air inlet
configured to supply boosted overfire air at a pressure higher than
a pressure of the overfire air.
5. The combustion system of claim 4, wherein the overfire air inlet
is configured to supply a mixture of the overfire air and the
reduction reagent into the flue gas.
6. The combustion system of claim 4, wherein the boosted overfire
air inlet is configured to supply a mixture of the boosted overfire
air and the reduction reagent into the flue gas.
7. The combustion system of claim 4, wherein the boosted overfire
air inlet is disposed in the intermediate staged air zone.
8. The combustion system of claim 1, further comprising a boiler
nose downstream from the reburn zone, wherein the boiler nose
comprises a plurality of ports disposed therein configured to
supply a mixture of overfire air and the reduction reagent into the
flue gas.
9. The combustion system of claim 1, further comprising a burner
disposed in the combustion zone, wherein the burner comprises a
first duct configured to channel a fuel flow into the combustion
zone; and a second duct substantially concentrically-aligned with
and extending through the first duct, wherein the second duct is
configured to channel a mixture of air and the reduction reagent
into the combustion zone.
10. The combustion system of claim 9, wherein the air comprises
intermediate staged air.
11. The combustion system of claim 1, wherein the reduction reagent
comprises a sorbent composition, a nitrogen oxides reduction agent,
an oxidizing composition, or a combination comprising at least one
of the foregoing.
12. The combustion system of claim 1 1, wherein the sorbent
composition comprises calcium carbonate, limestone, calcium oxide,
calcium hydroxide, calcium phosphate, cement, cement kiln dust,
lime kiln dust, sugar beet lime, clay, talc, or a combination
comprising at least one of the foregoing.
13. A process for using a combustion system, said process
comprising: supplying a fuel and air under fuel rich conditions to
a combustion zone comprising a burner to form a flue gas; supplying
intermediate staged air to the flue gas through an intermediate
staged air inlet downstream of the combustion zone in an amount
effective to produce fuel lean conditions; channeling the flue gas
to pass to a reburn zone downstream from the intermediate staged
air inlet; and supplying a reduction reagent to the flue gas,
wherein the reduction reagent is configured to reduce an amount of
pollutant species in the flue gas.
14. The process of claim 13, wherein supplying the reduction
reagent further comprises mixing the reduction reagent with the
intermediate staged air.
15. The process of claim 13, wherein supplying intermediate staged
air to the flue gas further comprises supplying intermediate staged
air to the flue gas as boosted air.
16. The process of claim 13, further comprising: supplying fuel to
the reburn zone through a reburn inlet; and supplying overfire air
to a burnout zone downstream from the reburn zone through an
overfire air inlet.
17. The process of claim 16, wherein supplying the reduction
reagent further comprises mixing the reduction reagent with the
overfire air.
18. The process of claim 13, further comprising supplying overfire
air through a port in a boiler nose downstream from the reburn
zone.
19. The process of claim 18, wherein supplying the reduction
reagent further comprises mixing the reduction reagent with the
overfire air from the boiler nose.
20. A process for reducing at least sulfur oxides and/or mercury in
a flue gas of a combustion system, the method comprising: supplying
a fuel and air under fuel rich conditions to a combustion zone
comprising a burner to form a flue gas; supplying intermediate
staged air to the flue gas through an intermediate staged air inlet
downstream of the combustion zone to produce fuel lean conditions;
channeling the flue gas to pass to a reburn zone downstream from
the intermediate staged air inlet; supplying overfire air to a
burnout zone downstream from the reburn zone through an overfire
air inlet; and mixing a sorbent composition with the overfire air
and/or the intermediate staged air before supplying the air to the
flue gas.
Description
BACKGROUND OF THE INVENTION
[0001] This disclosure relates generally to combustion systems for
power plants, and more particularly to combustions systems having
reduced emissions.
[0002] During a typical combustion process within a furnace or
boiler, for example, a flow of combustion gas, or flue gas, is
produced. Known combustion gases contain combustion products
including, but not limited to, carbon, carbon dioxide, carbon
monoxide, water, hydrogen, nitrogen oxides (NOx), sulfur oxides
(SOx), chlorine, and/or mercury generated as a result of combusting
fuels. Various technologies have been applied to combustion systems
to reduce the emissions of pollutant species, however, further
improvements are needed.
[0003] At least some known furnaces use air/fuel staged combustion,
such as a multi-stage combustion, to facilitate reducing the
production of at least some of the combustion products, such as
NOx. For example, a three-stage combustion process includes
combusting fuel and air in a first stage, introducing fuel into the
combustion gases in a second stage, and then introducing air into
the combustion gases in a third stage. In the second stage, fuel is
injected, without combustion air, to form a sub-stoichiometric, or
fuel-rich, zone. During the second stage, at least some of the
fuels combust to produce hydrocarbon fragments that react with NOx
that may have been produced in the first stage. As such, the NOx
may be reduced to atmospheric nitrogen in the second stage. In the
third stage, air is injected to consume the carbon monoxide and
unburnt hydrocarbons exiting the second stage. Although such
air/fuel staging may achieve relatively high NOx reduction,
combustion products such as SOx and mercury continue to exist in
the flue gas.
[0004] One strategy for reducing or eliminating SOx and mercury
emissions in flue gas is to install wet scrubbers, selective
catalytic reduction, or activated carbon systems to capture the
sulfur and mercury before they are emitted into the atmosphere.
These technologies, however, can have their disadvantages. For
example, it can be cost prohibitive to install wet scrubbers to an
existing plant, and the energy required to run the scrubbers can
affect the efficiency and environmental impact of the plant. The
use of activated carbon systems can lead to carbon contamination of
the fly ash collected in exhaust-air treatments, such as the bag
house and electrostatic precipitators.
[0005] An alternative method for the removal of sulfur and mercury
is the application of sulfur sorbing and stabilizing materials to
the fuel itself (e.g., the coal), or injection of the materials
into the combustion process. The sorbent material particles adsorb
sulfur from the coal, or SOx, mercury, and other contaminants from
the flue gas, and the particles are captured in the solids
collection system of the combustion process.
BRIEF DESCRIPTION OF THE INVENTION
[0006] According to one aspect of the invention, a combustion
system includes a combustion zone comprising a burner for
converting a fuel, under fuel rich conditions, to a flue gas; an
intermediate staged air zone downstream from the combustion zone
for supplying intermediate staged air to the flue gas and producing
fuel lean conditions; a reburn zone downstream from the
intermediate staged air zone for receiving the flue gas; and an
inlet downstream from the combustion zone for supplying a mixture
of air and a reduction reagent to the flue gas, wherein the
reduction reagent is configured to reduce an amount of a pollutant
species in the flue gas.
[0007] According to another aspect of the invention, a process for
using a combustion system includes supplying a fuel and air under
fuel rich conditions to a combustion zone comprising a burner to
form a flue gas; supplying intermediate staged air to the flue gas
through an intermediate staged air inlet downstream of the
combustion zone to produce fuel lean conditions; channeling the
flue gas to pass to a reburn zone downstream from the intermediate
staged air inlet; and supplying a reduction reagent to the flue
gas, wherein the reduction reagent is configured to reduce an
amount of a pollutant species in the flue gas.
[0008] According to yet another aspect of the invention, a method
of reducing at least sulfur oxides and/or mercury in a flue gas of
a combustion system includes supplying a fuel and air under fuel
rich conditions to a combustion zone comprising a burner to form a
flue gas; supplying intermediate staged air to the flue gas through
an intermediate staged air inlet downstream of the combustion zone
to produce fuel lean conditions; channeling the flue gas to pass to
a reburn zone downstream from the intermediate staged air inlet;
supplying overfire air to a burnout zone downstream from the reburn
zone through an overfire air inlet; and mixing a sorbent
composition with the overfire air and/or the intermediate staged
air before supplying the air to the flue gas.
[0009] These and other advantages and features will become more
apparent from the following description taken in conjunction with
the drawings.
BRIEF DESCRIPTION OF THE DRAWING
[0010] The subject matter, which is regarded as the invention, is
particularly pointed out and distinctly claimed in the claims at
the conclusion of the specification. The foregoing and other
features, and advantages of the invention are apparent from the
following detailed description taken in conjunction with the
accompanying drawings in which:
[0011] FIG. 1 is a schematic diagram showing a side,
cross-sectional embodiment of a multi-stage reburn combustion
system.
[0012] FIG. 2 is a schematic diagram showing a side,
cross-sectional embodiment of the multi-stage reburn combustion
system of FIG. 1 having a reduction reagent storage and delivery
system.
[0013] FIG. 3 is a schematic view of an exemplary multifunctional
burner that may be used with the multi-stage reburn combustion
system shown in FIG. 1.
[0014] FIG. 4 is a schematic diagram showing a side,
cross-sectional embodiment of the multi-stage reburn combustion
system having hybrid boosted overfire air.
[0015] FIG. 5 is a schematic diagram showing a side,
cross-sectional embodiment of the multi-stage reburn combustion
system having boiler nose overfire air.
[0016] The detailed description explains embodiments of the
invention, together with advantages and features, by way of example
with reference to the drawings.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Disclosed herein are multi-stage reburn combustion systems
utilizing reduction reagents to aid in reducing pollutant emissions
caused by fossil fuel combustion. As will be described in greater
detail below, multi-stage reburn combustion systems apply
intermediate staged air (ISA) between combustion and reburn zones
to help reduce the initial NOx formation entering the reburn zone.
The ISA stream is a high-energy stream that mixes the air rapidly
into the flue gas of the combustion system. Other optional air
streams such as overfire air, including boosted overfire air,
hybrid boosted overfire air, and nose overfire air, as well as
burner air injection can also be used to mix air into the flue gas.
These air streams can be used to introduce reduction reagents into
the combustion system in order to further reduce pollutant
emissions in the flue gas beyond the NOx reduction already inherent
to the multi-stage reburn system. Integrating reduction reagent
injection with one or more of the system air streams provides a
means of utilizing the energy of the air stream or streams to
rapidly and thoroughly mix reagents within the flue gas, thereby
providing simultaneous control of NOx and other pollutant
species.
[0018] Examples of pollutant species to be reduced and/or removed
from the flue gas can include, without limitation, carbon, carbon
dioxide, carbon monoxide, hydrogen, NOx, SOx, chlorine, metals such
as mercury, arsenic, and nickel, and other like pollutant species
generated as a result of combusting fuels. As used herein,
reduction reagents are intended to generally include any compound
configured to reduce the amount of pollutant species in a furnace
flue gas. Exemplary reduction reagents can include, without
limitation, sorbent species for reduction of SOx, hydrochloric acid
(HCl), metals, and other like emissions; NOx reduction agents such
as, ammonia, urea, cyanuric acid, and the like; and oxidizing
species such as halogen-based compounds.
[0019] FIG. 1 illustrates an exemplary embodiment of a multi-stage
reburn combustion system 100, which incorporates reduction reagent
injection. In this particular embodiment, the reduction reagent is
injected as a mixture with the ISA into the flue gas. Other
embodiments discussed below illustrate different reduction
reagent-air stream mixtures and injection points. The combustion
system 100 can be used for various applications such as in a
fossil-fuel fired boiler, furnace, engine, incinerator, etc. One
exemplary application of combustion system 100 is as the source of
power generation in a power plant. The flue gas can enter the
system at 110 and travels to the main combustion zone 120. The main
combustion zone 120 is equipped with one or more main burners (not
shown) such as specially designed burners for producing low levels
of NO.sub.X. In one embodiment, the main combustion zone 120
includes two or more burners arranged in two or more rows. Fuel and
primary air are supplied together to the main combustion zone 120
through one or more inlets 128. Secondary air is also generally
supplied to the main combustion zone 120 through inlets 128. The
amounts of fuel and air supplied to the main combustion zone 120
are selected to achieve fuel rich conditions therein. The exact
stoichiometric ratio (SR) in the main combustion zone 120 will vary
depending on the fuel type and furnace design, but will be less
than about 1.0. In one embodiment, the SR in the main combustion
zone 120 is about 0.90 to about 0.95. Examples of suitable fuels
for use in the main combustion zone 120 include, but are not
limited to, fossil fuels, such as lignite coal, bituminous coal,
sub-bituminous coal, anthracite coal, oil, or gas, such as natural
gas or gasified coal, various types of biomass, and combinations
including at least one of the foregoing fuels. Any suitable form of
fuel can be supplied to the main combustion zone 120, including
pulverized coal that is ground using a coal mill. Within the main
combustion zone 120, the fuel undergoes combustion and forms a flue
gas that flows upwardly toward the intermediate staged air zone
122.
[0020] The flue gas produced in the main combustion zone 120 flows
to the ISA zone 122. Air is added to the flue gas in this zone
through one or more intermediate staged air inlets 132. The amount
of ISA supplied to zone 122 is effective to produce fuel lean
conditions, i.e., SR of greater than about 1.0. In one embodiment,
sufficient ISA is supplied to zone 122 to produce an SR of about
1.05 to about 1.10. Flow into the ISA inlet 132 maybe regulated by
an ISA damper 131.
[0021] The reduction reagent can be mixed with the ISA at any point
prior to the air entering the ISA zone 122. Mixing the reduction
reagent with the air at some distance prior to entry into the ISA
zone 122 will provide residence time for the reduction reagent to
substantially disperse throughout the ISA. FIG. 2 illustrates a
reduction reagent system 150, which includes a reduction reagent
storage system 152, such as a hopper, in operative communication
with a reduction reagent metering system 146. The metering system
146 is configured to control the amount of reduction reagent being
fed into an ISA stream 147. For example, the metering system 146
could be a screw feeder configured to feed the reduction reagent
into an inlet in the ISA stream 147. The reduction reagent is
introduced to the ISA stream 147 prior to the ISA inlet 132, and in
this way, the ISA acts as a carrier fluid for the reduction
reagent. As show in FIG. 2, a booster fan 148 can be configured to
increase the pressure of the ISA feedstream 147. The ISA is an
advantageous means for effectively mixing and dispersing the
reduction reagent into the flue gas of the combustion system,
because of the energy with which the ISA stream flows and is
injected into the flue gas.
[0022] After the ISA zone 122, the fuel-lean flue gas then enters
the reburn zone 124 and fuel is added to the flue gas through one
or more reburn inlets 134. The fuel is typically accompanied by
carrier gas. The carrier gas may be carrier air, boosted flue gas
recirculation (FGR), or any other appropriate gas for the specific
fuel and furnace design. The amount of fuel added through the
reburn inlets 134 is effective to produce fuel rich conditions in
the reburn zone 124. The exact SR in the reburn zone 124 of the
combustion system 100 varies depending on the fuel type and
combustion system design but generally ranges from about 0.85 to
about 0.95.
[0023] The flue gas formed in the reburn zone 124 then proceeds
through the combustion system 100 and is subjected to optional
operations and treatments. In one embodiment the flue gas formed in
the reburn zone 124 flows upwardly to the burnout zone 126, which
is downstream from the reburn zone 124. Overfire air (OFA), also
known as separated overfire air (SOFA), is supplied to the burnout
zone 126 through inlet 136. OFA flow through inlet 136 may be
regulated by an OFA damper 135. The OFA restores the system to
overall fuel lean conditions, i.e., SR of greater than about 1.0.
The exact SR varies depending on the fuel type and furnace design.
In one embodiment, the SR in the burnout zone 126 is about 1.15 to
about 1.3. The OFA can be added to the burnout zone 126 at a
relatively higher pressure through inlet 136, such as with boosted
overfire air (BOFA). This may be accomplished using one or more
rotating booster fans. The BOFA can be in the form of cool ambient
air, heated air, or both cool ambient air and heated air, with
heated air being preferred. The introduction of the BOFA can
achieve desired levels of air jet penetration and mixing in the
burnout zone 126.
[0024] The flue gas in the burnout zone 126 passes downstream to an
outlet 144, where the flue gas exits the combustion system 100. As
the flue gas passes to outlet 144, the flue gas flows past the tip
of the boiler nose 140 and can flow through one or more heat
exchangers 142 to serve as a heat source.
[0025] The residence time of the substances flowing through various
regions of the combustion system 100 varies depending on fuel and
air flow rates. As used herein, the term "residence time" refers to
the average time the flue gas spends in a defined region of the
furnace. Operation of the exemplary furnace is conducted such that
there is sufficient residence time to enable conversion of the
NO.sub.X to take place, as well as sufficient time for the
reduction reagents to absorb, reduce, or the like the remaining
pollutant species in the flue gas. The exact residence time
required depends on the furnace design, primary fuel type, reburn
fuel type, and/or reduction reagent injection location. In one
embodiment, a residence time of flue gas in a region of the
combustion system 100 between a centerline of the intermediate
staged air inlet 132 and a centerline of the reburn inlet 134 is
about 100 to about 400 milliseconds. In an alternative embodiment,
a residence time of flue gas in a region of the combustion system
100 between the centerline of the reburn inlet 134 and a centerline
of the overfire air inlet 136 is about 300 to about 1000
milliseconds. In general, fuels that devolatilize and mix quickly
require relatively low average residence times. In another
alternative embodiment, a residence time of the flue gas in a
region of the combustion system 100 between the centerline of the
OFA inlet 136 and the tip of the boiler nose 140 is greater than
about 300 milliseconds. In still another alternative embodiment, a
residence time of the flue gas in a region of the combustion system
100 between a centerline of a top burner row and the centerline of
the tip of the boiler nose 140 (i.e., the total residence time of
the combustion system) is greater than about 1,300 milliseconds. As
used herein, the term "centerline" refers to an imaginary line
running through the middle of an object.
[0026] The use of intermediate staged air in the exemplary
combustion system 100 enables the main combustion zone 120 to
operate at fuel rich conditions. This reduces the initial NO.sub.X
flowing into the reburn zone 124 to improve overall NO.sub.X
emissions by, for example, about 10% to about 25%, as compared to
reburn without intermediate staged air. In at least some known
combustion system, both air and fuel staging usually have the
unwanted side effect of increasing the emissions of CO and unburned
carbon in fly ash as measured by loss-on-ignition (LOI). In the
exemplary embodiment, the use of ISA and air-carried reduction
reagent injection provides additional flexibility and control of CO
and LOI, while maintaining low NO.sub.X, SO.sub.X, and metal
levels. The use of ISA combined with BOFA can also help restore the
CO and unburned carbon emissions to more acceptable levels by
improving the penetration of air into, and mixing with, the
combustion gas. This type of integrated technology can reduce
NO.sub.X emissions to less than or equal to about 100
milligram/normal meters cubed (mg/Nm.sup.3) at about 6% O.sub.2
dry, or about 0.163 pound/million Btu (lb/MMBtu), thus meeting the
NO.sub.X emissions requirement of the European Union Large
Combustion Plant Directive (LCPD), Phase 2. The combustion system
100 also can maintain the LOI at a sufficiently low level, such as
levels that permit the fly ash waste to be sold in Europe for
example.
[0027] In one embodiment, the ISA inlet 132 is a burner out of
service (BOOS) through which cooling air is injected. In this way,
an existing furnace may be adapted to incorporate ISA by running
cooling air through the existing top row of burners, making them
the ISA inlets 132. This has a minimal cost impact and avoids
additional wall penetrations in the furnace of the combustion
system 100.
[0028] In another embodiment, the existing burners in the top row
of the main combustion zone 120 are replaced with injectors
specifically designed to inject ISA. In this way the velocity and
mixing of the ISA in the ISA zone 122 may be better configured for
the system, but new furnace wall penetrations are not required.
Alternatively, the existing burners in the top row of the main
combustion zone 120 are blocked off and new injectors specifically
designed to inject ISA and reduction reagent are placed at an
elevation below, equal to, or above the top burner row. This does
require additional wall penetrations for the ISA inlets 132. In
another embodiment, the ISA inlet 132 is above (downstream) of the
upper burner row of the main combustion zone 120. This enables the
use of all of the existing burners in the main combustion zone 120,
but does require additional wall penetrations for the ISA inlets
132.
[0029] In an exemplary embodiment, at least one burner in the
combustion zone 120 is a multi-function burner. Alternatively,
combustion zone 120 can include a row and/or array (not shown) of
multi-function burners. The multi-function burner either burns the
fuel/air mixture or injects air into the combustion zone 120. In
this particular embodiment, the multi-function burner either burns
the fuel/air mixture or injects a mixture of ISA and reduction
reagent into the combustion zone 120 of the combustion system 100.
The multi-function burner can occupy the top row of the combustion
burner, or it can be included anywhere within the combustion zone
120 that enables the system to function as described herein. FIG. 3
is a schematic view of an exemplary multi-function burner 200 that
may be used to combust the fuel/air mixture in the combustion zone
120 or inject the ISA-reduction reagent mixture into the flue gas.
In this embodiment, burner 200 has a substantially cross-sectional
shape that enables burner 200 to function as described herein.
[0030] The multi-function burner 200 includes a first duct 206, a
second duct 208, a third duct 210, and a fourth duct 212 that are
each substantially concentrically aligned with a centerline 214 of
the burner 200. More specifically, first duct 206 is the radially
outermost of the ducts 206, 208, 210, and 212 such that a radially
outer surface 216 of first duct 206 defines the outer surface of
burner 200. Furthermore, in the exemplary embodiment, first duct
206 includes a convergent and substantially conical section 218, a
substantially cylindrical section 220, and a divergent and
substantially conical section 222. Second duct 208, in the
exemplary embodiment, is spaced radially inward from first duct 206
such that a first passageway 224 is defined between first and
second ducts 206 and 208. Moreover, second duct 208 includes a
substantially cylindrical section 226 and a divergent and
substantially conical section 228.
[0031] In the exemplary embodiment, third duct 210 is spaced
radially inward from second duct 208 such that a second passageway
230 is defined between second and third ducts 208 and 210.
Furthermore, in the exemplary embodiment, third duct 210 is
substantially cylindrical and includes an annular flame regulation
device 232, such as a flame holder, that creates a recirculation
zone 234. Fourth duct 212, in the exemplary embodiment, defines a
center passageway 236 that has a diameter D1 and that is radially
spaced inward from third duct 210 such that a third passageway 238
is defined between third and fourth ducts 210 and 212. In the
exemplary embodiment, fourth duct 212 is substantially cylindrical
including having conical and/or cylindrical shapes, ducts 206, 208,
210, and 212 may each have any suitable configuration or shape that
enables burner 200 to function as described herein.
[0032] First and second ducts 206 and 208, in the exemplary
embodiment, are each coupled in flow communication with a common
plenum 240, which is coupled in flow communication with air source
32 via main air regulation device 62. Alternatively, first and
second ducts 206 and 208 are each coupled separately in flow
communication independently with air source 32 such that first and
second ducts 206 and 208 do not share a common plenum 240. In the
exemplary embodiment, first and second ducts 206 and 208 are
oriented such that ISA-reduction reagent mixture 30 may be injected
into common plenum 240, through first passageway 224 and/or second
passageway 230, and into primary combustion zone 120 (shown in FIG.
1) and/or ISA zone 122 (shown in FIG. 1). In one embodiment, first
passageway 224 and/or second passageway 230 may induce a swirl flow
pattern (not shown) to ISA-reduction reagent mixture 30 injected
through first passageway 224 and/or second passageway 230.
[0033] Furthermore, third duct 210, in the exemplary embodiment, is
coupled in flow communication with fuel source 14 via fuel flow
regulation device 40. In the exemplary embodiment, third duct 210
is oriented such that fuel 12 may be injected through third
passageway 238 and into primary combustion zone 120, when burner
200 is used to combust fuel 12 and air. Moreover, fourth duct 212,
in the exemplary embodiment, is coupled in flow communication with
air source 32 via air flow regulation device 42 and air velocity
control device 44. In the exemplary embodiment, fourth duct 212 is
oriented such that ISA-reduction reagent mixture 30 may be injected
through center passageway 236 and into intermediate air zone 46 at
a predetermined velocity, when burner 200 is used to inject
ISA-reduction reagent mixture 30.
[0034] During a first operation of multi-function burner 200,
burner 200 is used to burn fuel 12 and air. Control system 60
controls fuel flow regulation device 40 to enable fuel 12 to enter
combustion zone 120 through third passageway 238, controls main air
regulation device 62 to inject ISA-reduction reagent mixture 30
into combustion zone 120 or ISA zone 122 through first passageway
224 and/or second passageway 230, and controls air flow regulation
device 42 to prevent air ISA-reduction reagent mixture 30 from
being injected into combustion zone 120 through center passageway
236.
[0035] During a second operation of multi-function burner 200,
burner 200 is used to inject ISA-reduction reagent mixture 30.
Control system 60 controls fuel flow regulation device 40 to
prevent fuel 12 from entering combustion zone 120 through third
passageway 238, controls main air flow regulation device to inject
air 30 into combustion zone 18 through first passageway 224 and/or
second passageway 230 at first velocity V.sub.1, and controls air
flow regulation device 42 and air velocity control device 44 to
inject ISA-reduction reagent mixture 30 into combustion zone 18
through center passageway 236 at second velocity V.sub.2, which is
higher than velocity V.sub.1. As such, the first portion 202 of
ISA-reduction reagent mixture 30 is injected at velocity V.sub.1
and the second portion 204 of ISA-reduction reagent mixture 30 is
injected at velocity V.sub.2. In another embodiment, ISA-reduction
reagent mixture 30 entering through center passageway 236 does not
experience a velocity change through air velocity control device
44, and ISA-reduction reagent mixture 30 entering combustion zone
18 through center, first, and/or second passageways 236, 224,
and/or 230, respectively, enters from air flow regulation device 42
and main air regulation device 62 at substantially the same
velocity.
[0036] The ISA supplied through the ISA inlet 132 or through the
multi-function burner 200 may be in the form of cool ambient air,
heated air, or both cool ambient air and heated air. In one
embodiment, the ISA is boosted such that the ISA is supplied at a
relatively higher pressure. This may be accomplished using one or
more rotating booster fans. For example, a booster fan 148 is
configured to increase the pressure of the ISA feedstream 147 and
can be disposed upstream of the reduction reagent metering system
152 (as shown in FIG. 1) or it can boost the pressure of the
feedstream after the reduction reagent has been added thereto. The
boosting of the ISA can achieve improved levels of air jet
penetration and mixing in the ISA zone 122, thereby providing
improved dispersion of the reduction reagent into the flue gas.
[0037] Air may be fed to the various stages in the combustion
system 100 from a variety of sources. In one embodiment, a windbox
supplies secondary air to the main combustion zone inlets 128, ISA
to the ISA inlets 132, and/or OFA to the OFA inlets 136 through
ducting 138. In another embodiment, air is delivered to one or more
inlets 128, 132, and 136 through separate ducting (not shown).
Control of the flow to the various inlets may be linked, or may be
independent. The source of the air and the configuration of the
ducting is not critical to the combustion system 100 and may be
tailored to suit the particular furnace design.
[0038] Overfire air is a well-known technology that is used to
reduce NOx emissions in utility and industrial furnaces. The OFA
system of the combustion system 100 diverts secondary combustion
air from a burner windbox ducting 138 to the OFA injectors at the
inlets 136. The OFA supply pressure in the burner windbox ducting,
determines the maximum dynamic pressure that will be available at
the OFA injector outlet. Sufficient OFA dynamic pressure ensures
effective penetration and mixing of overfire air with combustion
flue gas. In some cases, the available dynamic pressure to the OFA
injector is not high enough to achieve the required penetration and
mixing of the air and combustion gas. If this happens, boosting the
OFA can assist in reducing NOx emissions.
[0039] The multi-stage reburn combustion system can further include
an optional hybrid boosted overfire air system to supplement the
OFA. FIG. 4 illustrates an exemplary embodiment of a combustion
system 300 comprising hybrid boosted overfire air (HBOFA). In this
embodiment, the HBOFA is configured to deliver the reduction
reagent to the combustion flue gas in the system 300. The
combustion system 300 functions in the same manner as the
combustion system 100 of FIG. 1, with the only difference being
that hybrid boosted overfire air is used to combine two discrete
air supply systems, boosted air and secondary combustion air, to
achieve effective penetration and mixing of overfire air and
reduction reagent with combustion flue gas in the burnout zone 324.
In another embodiment, the HBOFA can also supply air to the
multifunction burner of FIG. 3 when the burner is in the desired
operational mode. A portion of the OFA is delivered to the OFA
injectors 336 as either "cold" or "hot" high-pressure air from
booster fans (not shown). The remaining OFA is delivered to the OFA
injectors 336 from the existing "hot" secondary combustion air
(HOFA) system (e.g., burner windbox ducting 338). This approach is
a low-cost alternative to a traditional stand-alone boosted
overfire air system. A feature of HBOFA is that both the boosted
high-pressure air (BOFA) and low-pressure secondary combustion air,
such as OFA, achieve air jet penetration and mixing in an overfire
air system.
[0040] The BOFA can also be injected into the ISA zone 322 through
ISA inlets 332. As such, the BOFA and ISA can be mixed in the ISA
zone to further improve NOx reduction and help lower CO emissions.
As mentioned, in this embodiment the HBOFA is used as a carrier
medium for injection of the reduction reagent into the ISA zone 322
and/or the burnout zone 324 of the combustion system 300. Again,
the penetration and mixing achieved by the HBOFA streams into the
flue gas makes the HBOFA system an excellent carrier fluid for the
reduction reagent into the combustion system 300. The reduction
reagent, therefore, can be introduced into the OFA, the BOFA, the
ISA (as discussed above), or some combination of all three. Because
the HBOFA can be supplied at a higher than normal boost pressure,
it provides a desired level of penetration into, and mixing of the
reduction reagents with the boiler gases. Similar to ISA injection
of the reduction reagents, the desired reducing compounds can be
mixed with the HBOFA at any point prior to the OFA or ISA
injectors. The greater the mixing distance before the injectors,
the greater the time for the reduction reagent to substantially
disperse throughout the HBOFA. For example, the reduction reagent
system 150 of FIG. 2, can also be in operative communication with
the HBOFA streams, wherein the system controls the amount of
reduction reagent being fed into the HBOFA.
[0041] To reiterate, for several reasons HBOFA can be a useful
addition to a multi-stage reburn combustion system having reduction
reagent injection. For example, cold ambient or hot preheated
overfire air can be supplied at a higher than normal boost
pressure, to induce the high temperature, low pressure air, and
provide a desired level of penetration into and mixing of the air
and reduction reagent with the boiler gases. Moreover, boosting a
portion of the OFA lends to smaller fans (for OFA and/or BOFA) with
a reduced weight, reduced power requirements, and lower capital
cost.
[0042] In another exemplary embodiment, the upper furnace arch,
i.e., the boiler nose, is employed as a plenum from which overfire
air is injected into the combustion gases. This can be in addition
to or instead of the BOFA and HBOFA systems described above.
Moreover, if necessary the nose overfire air itself can be either
BOFA or HBOFA. With this configuration, the overfire air need
penetrate only a short distance into the flue gas to provide
optimum mixing performance without the need for higher pressure
boost air fans or higher pressure overfire air. As such, the nose
overfire air (NOFA) provides another means with which to inject the
reduction reagent into the combustion system so that the reagent is
able to penetrate and thoroughly mix with the flue gas for optimum
effect in reducing pollutants in the gas. Particularly, the boiler
nose itself may serve as a plenum in which an overfire
air-reduction reagent mixture is received, preferably through
openings in one or both of the side walls for flow through ports in
the boiler nose and consequent injection into the combustion gases.
The overfire air-reduction reagent mixture can be supplied to ducts
extending from one or both of the side walls of the furnace into
the boiler nose. A plurality of port ducts communicate between the
laterally extending duct(s) in the boiler nose and ports formed
along the one or more inclined surfaces of the boiler nose for
injection into the combustion gases. That is, the boiler nose is
generally comprised of a vertically upwardly inclined lower surface
directed toward the restriction in the flue gas passage formed by
the nose and the opposite boiler wall and an upper inclined surface
directed away from the restriction in the flue gas passage. The
overfire air-reduction reagent mix injection ports may be provided
in the lower or upper or both inclined surfaces of the boiler nose.
In a further embodiment, the overfire air-reduction reagent mixture
may be supplied to the boiler nose in a pair of discrete ducts
respectively extending into the boiler nose from opposite side
walls of the furnace.
[0043] Referring now to FIG. 5, there is illustrated a multi-stage
reburn combustion system generally designated 400, which is similar
in construction to the combustion system 100 of FIG. 1 with the
exception of the nose overfire air injection as set forth below.
Thus, the combustion system 400 includes a combustion zone 420, a
ISA zone 422, a reburn zone 424, and a burnout zone 426. Fuel and
primary and/or secondary air are supplied through the inlets 428.
The flue gas from the burnout zone 426 passes downstream to an
outlet 444 where the flue gas exits the combustion system 400. As
the flue gas passes to outlet 444, it flows past the tip of the
boiler nose 440.
[0044] In an exemplary embodiment, the boiler nose 440 is used as a
plenum for receiving overfire air mixed with the reduction reagent
and injecting the mixture directly into the flue gases passing
through the flue gas passage restriction 443. For example, the
overfire air-reduction reagent mixture may be supplied directly
into the cavity or plenum 441 within the boiler nose 440 for flow
through injection ports 445 directly into the flue gas passage. The
ports 445 can be arrayed in the inclined wall portion of the boiler
nose 440. While the injection ports 445 are illustrated in the
lower wall surface of the boiler nose inclined upwardly toward the
restriction in the passage, it will be appreciated that the
injection ports 445 may be disposed in the upper inclined surface
of the boiler nose extending in a direction away from the
restricted passage 443. The location of the NOFA provides another
exemplary point of injection for the reduction reagent. The flue
gas passage restriction 443 permits the reduction reagent to
substantially penetrate the flue gas in this location and improve
the mixture of the reagent with the flue gas.
[0045] A control system may control reduction reagent application
to any of the system air streams described above, i.e., the ISA,
OFA, BOFA, HBOFA, NOFA, or any combination of the foregoing air
streams. The control system may also manage application of the
air-reagent mixture by the injectors into the combustion system.
The control system may be configured to independently control each
of the injectors. The control system may further be configured to
control reduction reagent application based on at least one input
parameter.
[0046] The control system can be in operative communication, for
example, with the main feed line for the reduction reagent into an
air stream. As mentioned previously, the main feed line is also in
operative communication with a storage system, such as a silo or
hopper (as shown in FIG. 2).
[0047] The control system may automatically control quantity and
frequency of reduction reagent injection as a function of
combustion system operating parameters. For example, reduction
reagent injection may be adjusted by increasing or decreasing the
rate of feed from a blower in communication with the reduction
reagent supply and/or the rotational speed of a star feeder. Input
parameters to control system may include sulfur content in the flue
gas, mercury content in the flue gas, the air flow rate to
combustion system, and the like. Reduction reagent injectors may be
operated independently of one another or as a group to each or all
of the air streams to the combustion system.
[0048] In one embodiment, the measured sulfur content of the flue
gases can be compared to a target sulfur content that is desired to
be achieved for environmental, regulatory, or other reasons. If the
measured sulfur content in the flue gases is above the target, the
rate of addition of the reduction reagent into the combustion
system via one of the available air streams is adjusted
accordingly. If the measured sulfur content is at or below target,
the method includes the step of leaving the addition rate of the
reduction reagent into the system unchanged or reducing it.
[0049] The combustion gases contain carbon dioxide, various
undesirable gases containing sulfur, and mercury species. The
convective pathways of the combustion system are also filled with a
variety of ash which is swept along with the high temperature flue
gases. To remove the ash before emission into the atmosphere,
particulate removal systems are used. A variety of such removal
systems, such as electrostatic precipitators and a bag house, can
be disposed in the convective pathway. In addition, chemical
scrubbers can be positioned in the convective pathway.
Additionally, there may be provided various instruments to monitor
components of the gas, such as sulfur oxides, metals, and the like.
The reduction reagents can be effective in absorbing some of the
components, such that the reduction agents can be removed from the
gas by the particulate removal systems, thereby removing the
pollutant from the flue gas stream.
[0050] In each of the multi-stage reburn combustion system
embodiments described above any suitable reduction reagent can be
injected into the combustion gases by utilizing one or more of the
system air streams. The reduction reagents described herein will
reduce pollutant emissions in the multi-stage reburn combustion
system, such as, without limitation, carbon, carbon dioxide, carbon
monoxide, hydrogen, NOx, SOx, chlorine, metals such as mercury,
arsenic, and nickel, and other like pollutant species generated as
a result of combusting fuels. For example, mercury is at least
partially volatilized upon combustion of coal. When present during
coal combustion, the mercury tends not to stay with the ash, but
rather becomes a component of the flue gases. If remediation is not
undertaken, the mercury tends to escape from the coal-burning
facility into the surrounding atmosphere.
[0051] Exemplary reduction reagents are able to be injected in
locations within the combustion system that may experience
temperatures greater than or equal to 2000.degree. F. during
operation of the furnace. Further, the location may be at a
temperature greater than or equal to 2300.degree. F. Exemplary
reduction reagents can include, without limitation, sorbent species
for reduction of SOx, hydrochloric acid (HCl), metals, and other
like emissions; NOx reduction agents such as, ammonia, urea,
cyanuric acid, and the like; and oxidizing species such as
halogen-based compounds. The reduction reagent composition may be
in the form of a powder, fluid, or any other like form suitable for
being mixed in an air stream as a carrier fluid.
[0052] In one embodiment, the reduction reagent comprises a powder
sorbent composition. The components of the sorbent composition may
be provided as alkaline powders. Without being limited by theory,
it is believed that the alkaline nature of the sorbent components
leads at least in part to the desirable properties that aid in
reducing pollutants in the flue gas as described above. Sources of
calcium for the sorbent compositions can include calcium powders
such as calcium carbonate, limestone, calcium oxide, calcium
hydroxide, calcium phosphate, and other calcium salts. An alkaline
powder sorbent composition may contain one or more
calcium-containing powder such as cement (e.g., Portland cement),
cement kiln dust, lime kiln dust, various slags, sugar beet lime,
and the like, along with an aluminosilicate clay such as, without
limitation, montmorillonite, kaolin, and the like. The sorbent
composition may contain sufficient SiO.sub.2 and Al.sub.2O.sub.3 to
form a refractory-like mixture with calcium sulfate produced by
combustion and with mercury and other heavy metals such that the
calcium sulfate is handled by the particulate removal system of the
combustion system, and mercury and heavy metals are not leached
from the fly ash under acidic conditions. The calcium containing
powder sorbent composition may contain by weight a minimum of 2%
silica and 2% alumina, more specifically a minimum of 5% silica and
5% alumina. Thus, the sorbent compositions may include from about 2
to 50%, more specifically 2 to 20%, and more specifically yet about
2 to 10% by weight aluminosilicate material such as the exemplary
clays.
[0053] Suitable aluminosilicate materials include a wide variety of
inorganic minerals and materials. For example, a number of
minerals, natural materials, and synthetic materials contain
silicon and aluminum associated with an oxy environment along with
optional other cations such as, without limitation, Na, K, Be, Mg,
Ca, Zr, V, Zn, Fe, Mn, and/or other anions, such as hydroxide,
sulfate, chloride, carbonate, along with optional waters of
hydration. Such natural and synthetic materials are referred to
herein as aluminosilicate materials and are exemplified in a
non-limiting way by the clays noted above.
[0054] In aluminosilicate materials, the silicon tends to be
present as tetrahedra, while the aluminum is present as tetrahedra,
octahedra, or a combination of both. Chains or networks of
aluminosilicate are built up in such materials by the sharing of 1,
2, or 3 oxygen atoms between silicon and aluminum tetrahedra or
octahedra. Such minerals go by a variety of names, such as silica,
alumina, aluminosilicates, geopolymer, silicates, and aluminates.
However presented, compounds containing aluminum and/or silicon
tend to produce silica and alumina upon exposure to high
temperatures of combustion in the presence of oxygen.
[0055] Aluminosilicate materials may include polymorphs of
SiO.sub.2.Al.sub.2O.sub.3. For example, silliminate contains silica
octahedra and alumina evenly divided between tetrahedra and
octahedra. Kyanite is based on silica tetrahedra and alumina
octahedra. Andalusite is another polymorph of
SiO.sub.2.Al.sub.2O.sub.3.
[0056] Chain silicates may contribute silicon (as silica) and/or
aluminum (as alumina) to the sorbent compositions. Chain silicates
can include, without limitation, pyroxene and pyroxenoid silicates
made of infinite chains of SiO.sub.4 tetrahedra linked by sharing
oxygen atoms.
[0057] Other suitable aluminosilicate materials include sheet
materials such as, without limitation, micas, clays, chrysotiles
(such as asbestos), talc, soapstone, pyrophillite, and kaolinite.
Such materials are characterized by having layer structures wherein
silica and alumina octahedra and tetrahedra share two oxygen atoms.
Layered aluminosilicates include clays such as chlorites,
glauconite, illite, polygorskite, pyrophillite, sauconite,
vermiculite, kaolinite, calcium montmorillonite, sodium
montmorillonite, and bentonite. Other examples include micas and
talc.
[0058] Suitable aluminosilicate materials also include synthetic
and natural zeolites, such as without limitation the analcime,
sodalite, chabazite, natrolite, phillipsite, and mordenite groups.
Other zeolite minerals include heulandite, brewsterite,
epistilbite, stilbite, yagawaralite, laumontite, ferrierite,
paulingite, and clinoptilolite. The zeolites are minerals or
synthetic materials characterized by an aluminosilicate tetrahedral
framework, ion exchangeable "large cations" (such as Na, K, Ca, Ba,
and Sr) and loosely held water molecules.
[0059] In various embodiments, the alkaline powder sorbent
compositions that form the reduction reagent can further comprise
an optional halogen (such as bromine) compound or compounds to
capture chloride as well as mercury, lead, arsenic, and other heavy
metals in the ash, thereby rendering the heavy metals non-leaching
under acidic conditions, and improving the cementitious nature of
the ash produced. As a result, emissions of pollutants are
mitigated, reduced, or eliminated, and a valuable cementitious
material is produced as a by-product of the fuel combustion.
[0060] Sorbent compositions comprising a halogen compound contain
one or more organic or inorganic compounds that contain a halogen.
Halogens include chlorine, bromine, and iodine. The halogen
compounds are sources of the halogens, especially of bromine and
iodine. For bromine, sources of the halogen include various
inorganic salts of bromine including bromides, bromates, and
hypobromites. In various embodiments, organic bromine compounds are
less preferred because of their cost or availability. However,
organic sources of bromine containing a suitably high level of
bromine are considered within the scope of the invention.
Non-limiting examples of organic bromine compounds include
methylene bromide, ethyl bromide, bromoform, and carbon
tetrabromide. Non-limiting inorganic sources of iodine include
hypoiodites, iodates, and iodides, with iodides being preferred.
Organic iodine compounds can also be used.
[0061] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. Ranges disclosed herein are inclusive and combinable
(e.g., ranges of "up to about 25 wt %, or, more specifically, about
5 wt % to about 20 wt %", is inclusive of the endpoints and all
intermediate values of the ranges of "about 5 wt % to about 25 wt
%," etc.). "Combination" is inclusive of blends, mixtures, alloys,
reaction products, and the like. Furthermore, the terms "first,"
"second," and the like, herein do not denote any order, quantity,
or importance, but rather are used to distinguish one element from
another, and the terms "a" and "an" herein do not denote a
limitation of quantity, but rather denote the presence of at least
one of the referenced item. The modifier "about" used in connection
with a quantity is inclusive of the stated value and has the
meaning dictated by context, (e.g., includes the degree of error
associated with measurement of the particular quantity). The suffix
"(s)" as used herein is intended to include both the singular and
the plural of the term that it modifies, thereby including one or
more of that term (e.g., the colorant(s) includes one or more
colorants). Reference throughout the specification to "one
embodiment", "another embodiment", "an embodiment", and so forth,
means that a particular element (e.g., feature, structure, and/or
characteristic) described in connection with the embodiment is
included in at least one embodiment described herein, and may or
may not be present in other embodiments. In addition, it is to be
understood that the described elements may be combined in any
suitable manner in the various embodiments.
[0062] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which the
embodiments of the invention belong. It will be further understood
that terms, such as those defined in commonly used dictionaries,
should be interpreted as having a meaning that is consistent with
their meaning in the context of the relevant art and the present
disclosure, and will not be interpreted in an idealized or overly
formal sense unless expressly so defined herein.
[0063] While the invention has been described in detail in
connection with only a limited number of embodiments, it should be
readily understood that the invention is not limited to such
disclosed embodiments. Rather, the invention can be modified to
incorporate any number of variations, alterations, substitutions or
equivalent arrangements not heretofore described, but which are
commensurate with the spirit and scope of the invention.
Additionally, while various embodiments of the invention have been
described, it is to be understood that aspects of the invention may
include only some of the described embodiments. Accordingly, the
invention is not to be seen as limited by the foregoing
description, but is only limited by the scope of the appended
claims.
* * * * *