[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 substoichiometric,
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.
[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.
[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:
FIG. 1 is a schematic diagram showing a side, cross-sectional embodiment of a multi-stage
reburn combustion system.
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.
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.
FIG. 4 is a schematic diagram showing a side, cross-sectional embodiment of the multi-stage
reburn combustion system having hybrid boosted overfire air.
FIG. 5 is a schematic diagram showing a side, cross-sectional embodiment of the multi-stage
reburn combustion system having boiler nose overfire air.
[0011] The following description explains embodiments of the invention, together with advantages
and features, by way of example with reference to the drawings.
[0012] 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.
[0013] 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.
[0014] 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
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.
[0015] 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 may be regulated by an ISA damper 131.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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
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.
[0021] 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
X flowing into the reburn zone 124 to improve overall NO
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
X, SO
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
X emissions to less than or equal to about 100 milligram/normal meters cubed (mg/Nm
3) at about 6% O
2 dry, or about 0.163 pound/million Btu (lb/MMBtu), thus meeting the NO
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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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
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
2, which is higher than velocity V
1. As such, the first portion 202 of ISA-reduction reagent mixture 30 is injected at
velocity V
1 and the second portion 204 of ISA-reduction reagent mixture 30 is injected at velocity
V
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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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).
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] Exemplary reduction reagents are able to be injected in locations within the combustion
system that may experience temperatures greater than or equal to 2000° F. during operation
of the furnace. Further, the location may be at a temperature greater than or equal
to 2300° 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.
[0048] 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
2 and Al
2O
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.
[0049] 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.
[0050] 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.
[0051] Aluminosilicate materials may include polymorphs of SiO
2.Al
2O
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
2.Al
2O
3.
[0052] 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
4 tetrahedra linked by sharing oxygen atoms.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] Various aspects and embodiments of the present invention are defined by the following
numbered clauses:
- 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 clause 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 any preceding clause, wherein the intermediate staged
air comprises boosted air.
- 4. The combustion system of any preceding clause, 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 any preceding clause, 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 any preceding clause, 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 any preceding clause, wherein the boosted overfire air
inlet is disposed in the intermediate staged air zone.
- 8. The combustion system of any preceding clause, 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 any preceding clause, 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 any preceding clause, wherein the air comprises intermediate
staged air.
- 11. The combustion system of any preceding clause, 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 any preceding clause, 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 clause 13, wherein supplying the reduction reagent further comprises
mixing the reduction reagent with the intermediate staged air.
- 15. The process of clause 13 or 14, 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 any of clauses 13 to 15, 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 any of clauses 13 to 16, wherein supplying the reduction reagent
further comprises mixing the reduction reagent with the overfire air.
- 18. The process of any of clauses 13 to 17, further comprising supplying overfire
air through a port in a boiler nose downstream from the reburn zone.
- 19. The process of any of clauses 13 to 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.
1. A combustion system (100, 300, 400), comprising:
a combustion zone (120, 420) comprising a burner for converting a fuel, under fuel
rich conditions, to a flue gas;
an intermediate staged air zone (122, 322, 422) downstream from the combustion zone
for supplying intermediate staged air to the flue gas and producing fuel lean conditions;
a reburn zone (124, 424) downstream from the intermediate staged air zone for receiving
the flue gas; and
an inlet (128, 132, 134, 136, 200, 332, 336, 338, 428, 445) 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 (100, 300) of claim 1, wherein the inlet (132, 332) is disposed
in the intermediate staged air zone (122, 322) and configured to supply a mixture
of the intermediate staged air and the reduction reagent into the flue gas.
3. The combustion system (100, 300) of any preceding claim, further comprising an overfire
air inlet (136, 336) configured to supply overfire air to a burnout zone (126, 324)
downstream from the reburn zone (124) and a boosted overfire air inlet (132, 332)
configured to supply boosted overfire air at a pressure higher than a pressure of
the overfire air.
4. The combustion system (100, 300) of claim 3, wherein the overfire air inlet (136,
336) is configured to supply a mixture of the overfire air and the reduction reagent
into the flue gas.
5. The combustion system of claim 3 or claim 4, wherein the boosted overfire air inlet
(132, 332) is configured to supply a mixture of the boosted overfire air and the reduction
reagent into the flue gas.
6. The combustion system (400) of any preceding claim, further comprising a boiler nose
(440) downstream from the reburn zone (424), wherein the boiler nose comprises a plurality
of ports (445) disposed therein configured to supply a mixture of overfire air and
the reduction reagent into the flue gas.
7. The combustion system (100, 300, 400) of any preceding claim, further comprising a
burner (200) disposed in the combustion zone (120, 420), wherein the burner comprises
a first duct (206) configured to channel a fuel flow into the combustion zone; and
a second duct (208) 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.
8. A process for using a combustion system (100, 300, 400), said process comprising:
supplying a fuel and air under fuel rich conditions to a combustion zone (120, 420)comprising
a burner (200) to form a flue gas;
supplying intermediate staged air to the flue gas through an intermediate staged air
inlet (132, 332) downstream of the combustion zone in an amount effective to produce
fuel lean conditions;
channeling the flue gas to pass to a reburn zone (124, 424) 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.
9. The process of claim 8, wherein supplying the reduction reagent further comprises
mixing the reduction reagent with the intermediate staged air.
10. The process of claim 8 or claim 9, further comprising:
supplying fuel to the reburn zone (124, 424) through a reburn inlet (134); and
supplying overfire air to a burnout zone (126, 324, 426) downstream from the reburn
zone (124, 424) through an overfire air inlet (136, 336).