METHOD AND SYSTEM FOR mMOVAL OF MERCURY FROM A FLUE GAS
5 PRlOMTY STATEMENT UNDER 35 U.S.C. § 119
[0001] The present U.S. Patent Application is a continuation of U.S. Patent Application
Serial No. 141291,707, filed May 30, 2014, now U.S. Patent No. 8,865,099 issued October 21,
2014, in the names of Sterling M. Gray, James B. Jarvis, and Steven W. Kosler, entitled
10 "METHOD AND SYSTEM OF REMOVAL OF MERCURY FROM A FLUE GAS," which
claims priority pursuant to 35 U.S.C. 5 119(e) to U.S. Provisional Application Serial No
611935,884, tiled February 5, 2014, in the names of Sterling M. Gray, James B. Jarvis, and
Steven W. Kosler, entitled "METI-IOD AND SYSTEM OF MERCURY REMOVAL FROM A
FLUE GAS," the disclosure of each of which is hereby incorporated by reference in its entirety.
15
BACKGROUND OF THE INVENTION
[0002] This invention relates to a method for removing mercury from flue gas produced
in a fossil fuel energy conversion plant. Mercury and mercury-containing compounds are
present in varying amounts in fossil fuels. As the fuels are burned, the mercury enters the flue
20 gas stream, and a portion of this mercury can ultimately be emitted from the stack. While the
concentrations of mercury in the flue gas are usually low and of little concern, emitted mercury
ultimately finds its way to surface water, where it is converted to more toxic compounds and can
be concentrated in fish and other species in the food supply. As a result, even low levels of
mercury pose a significant risk to public health, and regulations will increasingly require fossil
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[0003] Various forms of mercury can exist within the flue gas, and the form of the
mercury plays a lcey role in determining how much mercury is emitted. Mercury compounds in
the fuel are converted to vapor-phase, elemental mercury in the boiler. Once the flue gas leaves
the boiler, some of the elemental mercury can be oxidized to a form such as ~ g " , or
alternatively, adsorbed onto fly ash. Many factors are involved, but as a consequence, the flue
gas contains a mixture of varying levels of elemental, oxidized and particulate mercury.
[0004] Some of the mercury in the flue gas can be removed using the pollution control
equipment often found in coal-fired power plants. Particulate mercury would be removed in
equipment that is used to collect the fly ash, and electrostatic precipitators or fabric filters are
examples of equipment that can accomplish this. Similarly, oxidized mercury is very soluble and
is easily removed in equipment that is used to control sulfur dioxide (SO2) emissions. Wet or dry
flue gas desulfurization systems are examples of equipment that can control oxidized mercury
In addition, oxidized mercury is more easily converted to particulate mercury along the flue gas
path.
[0005] While particulate and elemental mercury can be controlled using methods that are
well known in the art, elemental mercury is not as easily controlled. Unlike oxidized mercury,
elemental mercury is not soluble and is thcrefore not captured in the SO2 control step.
Consequently, elemental mercury tends to pass through the emission control equipment and is
emitted through the stack. Thus, a common strategy for controlling mercury emissions is to
oxidize the elemental mercury in the flue gas so that it can then be efficiently removed in
downstream emission control equipment. Indeed, this is the primary purpose of the invention
discussed in this document.
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Attorney Dockct No.: 3387-709USCl
[0006] Halogens play an important role with respect to the form of the mercury in the
flue gas. Halogens, which include the elements chlorine, bromine, iodine and fluorine, occur
naturally in coal. They serve the important function of promoting the oxidation of elemental
mercury along the flue gas path. There are a variety of mechanisms that can accomplish this.
5 For example, some halogen-containing compounds are oxidizing agents that can directly oxidize
I elemental mercury. Alternatively, halogen-containing compounds can work in combination with
other materials to help catalyze mercury oxidation. One example of this is the beneficial effect
of halogens on mercury oxidation within a selective catalytic reduction (SCR) system. Similarly,
halogen compounds can help oxidize and then retain mercury when present as a component of a
10 sorbent material such as activated carbon.
[0007] Those familiar with the art understand that the halogens must be present in a
reactive form to promote mercury oxidation. Reactive forms include the hydrogen halide (e.g.,
hydrobromic acid - HBr), the atomic Sorm of the halogen (e.g., atomic bromine - Br), or the
molecular form of the halogen (e.g., molecular bromine - Brz). Consequently, the prior art
15 focuses on introducing or producing these reactive halogen forms. There are a variety of ways to
accomplish this. For example, halogen-containing compounds can be added to the boiler system
and/or flue gas at a location where they are thcrmolabile (i.e., at a location where high
temperatures cause decomposition to form reactive halogen species). Alternatively, halogencontaining
compounds can decomposed to reactive halogen species at high temperature in
20 equipment external to the flue gas duct and the reactive halogen species can then be added to the
flue gas at any location. Another option is to add reactive halogen species, such as HBr or Br2,
directly added to the flue gas, and yet another option is for various halogen-containing species to
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Attorney Docket No.: 3387-709USC1
bc used in conjunction with a sorbent material such as activated carbon and the combined
material can serve to catalytically oxidize and then adsorb mercury.
[00081 One way to produce reactive halogens is to use a fuel (or fuel blend) with a higher
halogen content. Atthe high temperatures that exist within the boiler itself, the halogens are
5 converted to reactive forms (although the proportions of the various forms depend on which
halogen is being considered). The reactive halogen species leave the boiler and then help to
promote mercury oxidation via the mechanisms discussed above. Extensions of this concept
include the addition of halogen-containing additives to the fuel and the injection of halogencontaining
additives within the furnace. As with the naturally-occurring halogens, the halogens
10 in the additives are decomposed at high temperatures to form the reactive halogen species.
[0009] The prior art contains many examples where the elevated temperature in the boiler
is used to produce reactive halogen species. For example, U.S. Patent No. 7,507,083 B2 issued
to Comrie describes a method in which sorbent compositions containing halogens such as
bromine and iodine are injected onto the fuel or into the combustion chamber where the
15 temperature is higher than about 1,500 OF. Similarly, U.S. Patent No. 6,878,358 issued to
Vosteen, et al. describes a process in which a bromine compound is fed to a multistage furnace
and/or the flue gas in the plant section downstream of the furnace, the temperature during contact
of the bromine con~poundw ith the flue gas being at least 500 OC and preferably at least 800 "C.
Finally, U.S. Patent Application No. 201 1102501 11 A1 tiled by Pollack, et al. describes a method
20 of removing mercury from a flue gas using ~nolecularh alogen or halogen precursors.
[0010] While the described invention is not limited by the zone where the molecular
halogen or halogen precursor is introduced into the exhaust gas stream, the temperature in the
injection zone must be sufficiently high to allow dissociation andlor oxidation of the eleme~ital
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Attorney Docket No.: 3387-709USCl
I halogen from the halogen precursor, meaning that the temperature at the injection zone must be
greater than about 1,000 O F , and in some embodiments, greater than about 1,500 "F
[0011] The above examples use high temperatures along the flue gas path for
decomposition of the halogen salts. It is also possible to use high-temperature systems external
5 to the flue gas path to accomplish the same objective. Here, the reactive halogen species would
be produced in a separate device and then introduced into the fluc gas stream for reaction with
the mercury. Examples of this approach include U.S. Patent Application No. 200710051239 A1
filed by I-Iolmes, et al., which describes a method of producing atomic halogen radicals using a
high-temperaturelhigh-energy chamber for creating dissociated halogen, to be supplied to the gas
10 stream, with or without carbonaceous material.
[0012] Similarly, U.S. Patent Application No. 201010284872 A1 filed by Gale, et al.
describes a two-step process that first produces an acid halide by reaction of a halogen salt with
I
steam at temperatures from about 650 to 1,000 "C (temperatures from 700 to 800 "C being
preferred). This is followed by catalyzed reaction of the acid halide to the molecular halogen,
15 which is then injected into the flue gas stream.
[0013] Many halide salts (CaBrz and HBr being examples) cannot be thermally
decomposed at temperatures below l,OOO°F. That is, these salts are not thermolabile, and this
property explains why they are employed at higher temperatures for the purpose of generating
reactive halogen species. There are, however, halogen-containing compounds that are
20 thermolabile at temperatures below 1,000 OF. These include the ammonia halides (e.g., NHdCI),
the so-called interhalogens, and a variety of organic, halogen-containing species. These
compounds share the common characteristic of decomposing into reactive halogen species as a
result of being thermolabile at the temperature at the injection location.
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[0014] The most obvious means for introducing reactive halogen species is to add them
directly to the flue gas. As an example, U.S. Patent 8580214 132, filed by Moore, et al. discusses
introducing a hydrogen halide selected l'rom HBr and HI. One drawback of such technologies is
that the reactive halogen species can be highly toxic, corrosive and difficult to handle.
5 [0015] Some halogen-containing materials can be used at temperatures below which they
are thermolabile when combined with a sorbent material such as activated carbon. The use of
brominated activated carbon is well documented. Here, sorbent materials can be impregnated
with brominated compounds that might not otherwise be reactive at the injection location. The
brominated sorbent matcrial serves to catalytically oxidize the mercury. Then, the sorbent can
10 retain the mercury, where it can be removed along with the sorbent in downstream particulate
removal equipment.
[OOlG] Brominated sorbents can be produced by combining the bromine-containing
compounds and the sorbent before, during or after injection into the flue gas path.
Commercially-available activated carbons are brominated prior to injection. However, this is not
15 always the case. For example, U.S. Patent Application US 201210308454 Al, filed by Heuter, et
al. discusses a method whereby bromine-containing compounds and carbon-containing
adsorbents (activated carbon or activated coke) are added to the flue gas as a mixture, or
upstream relative to the flue gas flow, are brought into contact with carbon-containing adsorbents
introduced in the form of a cloud of flue gas dust into the flue gas stream.
20 [0017] As discussed herein, there are a variety of ways to introduce reactive halogen
species into a flue gas stream. Once introduced, they promote mercury oxidation through
various means, resulting in a form of mercury that can be removed from the flue gas stream
using equipment designed for the removal of other pollutants. Unfortunately, many of the
7
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Attorney Docket No 3387-709USCl
techniques of the prior art have undesirable consequences on the operation of the equipment or
the equipment itself. For example, the addition of halogens to the fuel or within the boiler can
result in corrosion of metal surfaces within the boiler. Similarly, other compounds (such as
ammonia) may be produced, causing equipment fouling or contributing to additional emissions
5 from the facility. Therefore, there is a need for a method that employs reasonably non-toxic
I
I additives, injected downstream of the boiler itself, which does not require the use of
supplemental adsorbents.
SUMMARY OF THE INVENTION
10
I
[0018] Processes and methods are provided for decreasing emissions of mercury upon
1 combustion of fossil fuels such as coal. In some embodiments, a halide salt, such as sodium
bromide, is injected into the flue gas path between the economizer and the stack where the
temperature is typically below around 500 "C. Because many of the halide salts are non-
15 thermolabile at temperatures below around 500 "C, they do not produce reactive halogen species
and are therefore not effective in oxidizing elemental mercury. However, some flue gas streams
contain certain flue gas constituents, such as sulfur trioxide or sulfuric acid, which react with
halide salts to form reactive halogen species. These species can then react with elemental
mercury to produce an oxidized form of mercury by various means. This oxidized mercury can
20 then be captured using downstream flue gas cleaning devices such as the particulate control
device or SO* scrubber.
100191 The foregoing has outlined rather broadly certain aspects ofthe present invention
in order that the detailed description of the invention that follows may better be understood.
Additional features and advantages of the invention will be described hereinafter which forin the
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Attorney Docket No.: 3187-709USC1
subject of the claims of the invention. It should be appreciated by those skilled in the art that the
conception and specific embodiment disclosed may be readily utilized as a basis for modifying
or designing other structures or processes for carrying out the same purposes of the present
invention. It should also be realized by those skilled in the art that such equivalent constructions
do not depart from the spirit and scope of the invention as set forth in the appended claims
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] For a more complete understanding of the present invention and the advantages
thereof, reference is now made to the following description taken in conjunction with the
accompanying drawings in which like reference numerals indicate like features and wherein:
FIG. 1 depicts the layout of a typical coal fired power plant; and
FIG. 2 shows the locations of injection of halide salts of certain emhodiments of the
present invention
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention is directed to improved methods and systems for, among
other things, removing elemental mercury froin a flue gas. The configuration and use of the
presently preferred embodiments are discussed in detail below. It should be appreciated,
however, that the present invention provides many applicable inventive concepts that can be
embodied in a wide variety of contexts other than the removal of mercury from a flue gas.
Accordingly, the specific emhodiments discussed are merely illustrative of specific ways to
make and use the invention, and do not limit the scope of the invention. In addition, the
following terms shall have the associated meaning when used herein:
9
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[0022j "emission control device" means any device for the removal of emissions from a
flue gas stream, including an electrostatic precipitator, a fabric filter, and a wet scrubber;
100231 "flue gas" means an exhaust gas that is produced from an industrial process and
includes both gas that will be used in connection with the process from which it is produced or
5 even another related process (e.g., to produce heat), which will exit into the atmosphere via a
stack for conveying waste exhaust gases from an industrial process. The flue gas can be
produced from any industrial process such as a power generating process, metal smelting process
and the lilte, wherein any form of mercury is present in the flue gas;
[0024] "fixed structure" means a non-moving, solid object containing chemical agents
10 that is placed into the flue gas stream;
I: 100251 "halide salt" means any salt of a halide including, without limitation, salts that
contain a halogen anion, such as chloride, bromide, fluoride or iodide, and a non-hydrogen cation
such as sodium, magnesium, calciuln or potassium, including by way of example sodium
bromide, sodium chloride, sodium fluoride, sodium iodide, calcium bromide, calcium chloride,
15 calcium fluoride, calcium iodide, magnesium bromide, magnesium chloride, magnesium
fluoride, magnesium iodide, potassium bromide, potassium chloride, potassium fluoride and
potassium iodide;
100261 "injecting" means the introduction of a material into a flue gas from a point
external to the duct work containing the flue gas, and includes the introduction of a liquid phase
20 solution or a powder into the flue gas, and the placement of a solid in the flue gas stream;
100271 "mercury" means any form of mercury, including without limitation, all oxidized
forms of Hg, elemental Hg and particulate-bound mercury;
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[0028] "molecular halogen" means any halogen in molecular form, such as Brz, Clz, and
F2, and products resulting from the disassociation of the molecular halogen, such as the
dissociation of Brz to form a Br radical, Br anion, Br cation, or a combination thereof;
[0029] "reactive halogen precursors" means halogen-containing chemicals that are not
5 reactive halogen species, but that can be decomposed or converted into reactive halogen species
in some manner (such as thermal decomposition);
[0030] reactive halogen species" means halogen-containing species that can cause the
oxidation of mercury by one means or another. Examples of reactive halogen specics include the
atomic form of the halogen (Cl, Br, I or F), the molecular form of the halogen (Br2, Clz, i2 and
10 F2), and the hydrogen halides (HCI, HBr, HI and HF); and
[0031] "sulfuric acid" means sulfuric acid, present in either the vapor phase or
condensed as a liquid, and sulfur trioxide, which is the anhydrous form of vapor-phase sulfuric
acid.
100321 Referring now to FIG. 1 which depicts a typical plant configured to burn fossil
15 fuels to produce energy. For example, for a coal-fired boiler, coal is conveyed 14 from an
external location (a coal pile or barge, etc.) and ground to a very fine powder by large metal
spheres in the pulverized fuel mill 16. The pulverized coal is mixed with preheated air 24 driven
by the forced draft fan 20.
100331 The hot air-fuel mixture is forced at high pressure into the boiler where it rapidly
20 ignites. Water of a high purity flows vertically up the tube-lined walls of the boiler, where it
turns into steam to begin the process of extracting the heat energy from the flue gas. The steam
produced in the boiler is used to produce electrical energy using a system of turbines and
ancillary equipment, and condensate produced from the steam is recycled to the boiler beginning
11
at the economizcr 23. The energy extracted into the boiler water causes the temperature of the
flue gas to decrease, and at the point where the flue gas leaves the economizer, the temperature is
typically within the range of 600 to 800 OF
100341 The temperature of the flue gas at the economizer outlet (600 to 800 OF) is
5 significant. At this temperature, many halide salts are non-thermolabile and are not considered
reactive halogen precursors (which explains why these salts are commonly applied to the fuel or
injected into the boiler, where the temperature exceeds at least 1,000 OF).
[0035] To improve thermal efficiency, the flue gas from the economizer is further cooled
by the incoming combustion air in the air preheater 24, where the flue gas temperature is
10 typically reduced to within the range of 220 to 400 "F
100361 The flue gas path between the economizer and the stack 28 typically contains
emission control equipment to remove various flue gas contaminants. Equipment typically
found upstream of the air preheater can include a selective catalytic reduction (SCR) system to
reduce NO, emissions. Equipment typically found downstream of the air preheater can include a
15 dry or wet electrostatic precipitator (ESP or WESP) for removal of particulate, a fabric filter (bag
house) and a wet or dry flue gas desulfurization (FGD) system. Other common emission control
systems include equipment for rcmoval of sulfuric acid and equipment for injecting activated
carbon. All of these emission control systems play a role in removing mercury from the flue gas
and are affected by the presence of reactive halogen species.
20 [0037] The composition of the flue gas leaving the boiler depends on what is being
burned, but it will usually consist of mostly nitrogen (typically more than two-thirds) derived
from the combustion air, carbon dioxide (C02), and water vapor as well as excess oxygen (also
derived from the combustion air). The flue gas also typically contains a small percentage of a
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Attorney Docket No.: 3387-709USC1
number of pollutants, such as particulate matter, carbon monoxide, nitrogen oxides, and sulfur
oxides and mercury
100381 Sulfuric acid is a pollutant that can be found at widely varying levels in the flue
gas from coal-fired boilers and other combustion sources and processes. For coal-fired boilers,
:i
5 most of the sulfur in the fuel is converted to sulfur dioxide. However, a small fraction of the fuel
sulfur is further oxidized to sulfur trioxide (the anhydrous form of sulfuric acid). The amount of
sulfur trioxide (SO?) that is present in the flue gas is a function of many variables, including the
fuel sulfur content, the design of the boiler, the excess oxygen concentration ancl the chemical
composition of the fly ash. Further, SO3 can be produced within the SCR reactor at levels that
10 depend on the catalyst type, the temperature and the operating conditions for the system. As a
result of these factors, the SO3 concentration can vary widely.
100391 In some cases, it is desirable to remove the sulfuric acid from the flue gas. This is
typically accomplished through a reaction between the So3 (or the vapor-phase sulfuric acid) and
an alkaline solid material such as hydrated lime or sodium carbonate. In the case of sodium
15 carbonate, a substitution reaction causes the absorption of sulfuric acid with the liberation of
coz.
100401 Many of the halide salts are inert at the temperatures that exist downstream of the
economizer (typically below 800 OF). This explains why these salts are typically added to the
fuel or to the boiler (where temperatures exceed at least 1,000 OF) or are used in combination
20 with an absorbent such as activated carbon. In the presence of certain flue gas species, an
example being sulfuric acid, substitution reactions and/or redox reactions, depending on which
halide salt is present, can cause the otherwise inert halide salts to liberate reactive halogen
species. This effect has been noted in the laboratory, and it occurs at temperatures characteristic
13
1920181 1
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of that downstream of the boiler economizer and also at temperatures characteristic of that
downstream of the air preheater.
[0041] Referring now to FIG. 2, which shows points of injection of a halide salt, such as
sodium bromide, into the flue gas path at any point between the economizer, where the
5 temperature is typically below around 800 O F , and the final emission control equipment prior to
the stack. The halide salt may be injected or placed, for the purpose of example, between the
economizer 23 and the SCR 29, between the SCR 29 and the air preheater 24, between the air
preheater 24 and the particulate removal 25, or between the particulate removal 25 the flue gas
desulfurization system 27. In each case, the halide salt would be non-thermolabile at the
I 10 temperature of the injection location. In those cases where the composition of the flue gas
I I stream included constituents that are reactive with the halide salts, such as sulfuric acid, reactive
I halogen species will be formed, which may affect the form of the mercury, or may affect the
operation of downstream emission control equipment, causing a net reduction in mercury
emissions from the stack.
15 100421 An effective pathway for production of reactive halogen species is to react the
solid form of the halide salt with a vapor-phase flue gas constituent. Thus, halide salts can be
injected in the form of a solid (e.g., a powder), or placed into the flue gas stream as part of a
fixed structure containing the halide salt with or without other materials, or injected into the duct
as a solution of the halide salt, whereby the water portion of the solution is evaporated to leave
20 the halide salt in the solid form.
[0043] As might be expected, the formation of reactive halogen species, as evidenced by
changes in the concentration and form of flue gas mercury, depends on various factors including
the concentration of the halide salt and the concentrations of flue gas constituents that cause the
14
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Attorney Docket No : 3387-709USC1
formation of reactive halogen species. It is noteworthy, however, that reactive halogen species
can be formed even commensurate with the process of sulfuric acid removal. In one test, halide
salts were injected into the duct of a coal-fired plant as part of a solution containing variable
levels of sodium carbonate, and including no sodium carbonate. The injection of sodium
5 carbonate is known to cause the removal of sulfuric acid from the flue gas. A net reduction in
mercury emissions was noted both with and without the presence of the sodium carbonate in
solution.
[0044] While the present device has been disclosed according to the preferred
embodiment of the invention, those of ordinary skill in the art will understand that other
10 embodiments have also been enabled. Even though the foregoing discussion has focused on
particular embodiments, it is understood that other configurations are contemplated. In
particular, even though the expressions "in one embodiment" or "in another embodiment" are
used herein, these phrases are meant to generally reference embodiment possibilities and are not
intended to limit the invention to those particular embodiment configurations. These terms may
15 reference the same or different embodiments, and unless indicated otherwise, are combinable
into aggregate embodiments. The terms "a", "an" and "the" mean "one or more" unless
expressly specified otherwise. The term "connected" means "communicatively connected"
unless otherwise defined
[0045] When a single embodiment is described herein, it will be readily apparent that
20 more than one embodiment may be used in place of a single embodiment. Similarly, where more
than one embodiment is described herein, it will be readily apparent that a single embodiment
may be substituted for that one device
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[0046] In light of the wide variety of possible mercury removal methods and systems
available, the detailed embodiments are intended to be illustrative only and should not be taken
as limiting the scope of the invention. Rather, what is claimed as the invention is all such
modifications as may come within the spirit and scope of the following claims and equivalents
5 thereto
[0047] None of the description in this specification should be read as implying that any
particular element, step or function is an essential element which must be included in the claim
scope. The scope of the patented subject matter is defined only by the allowed claims and their
equivalents. Unless explicitly recited, other aspects of the present invention as described in this
10 specification do not limit the scope of the claims.
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Attorney Docket No.: 3387-70YUSCl
WHAT IS CLAIMED IS:
1. A method for treating a flue gas stream, comprising:
injecting a halide salt into a flue gas stream, wherein the halide salt is not thermolabile at
the temperature ofthe flue gas stream at thc point of injection.
2. A method for treating a flue gas stream, comprising:
injecting a halide salt solution into a flue gas stream in combination with other salts,
wherein the halide salt is not thermolabile at the temperature of the flue gas stream at the point of
hjection.
3. A method for treating a flue gas stream, comprising:
1
I injecting a halide salt into a flue gas stream, wherein the halide salt is not thermolabile at
the temperature of the flue gas stream at the point of injection; and
chemically reacting the halide salt with constituents in the flue gas stream to form
reactive halogen species.
4. A method for treating a flue gas stream, comprising:
injecting a halide salt into a flue gas stream at a location between an economizer and a
scrubber.
5. The method of Claim 1, wherein the halide salt is sodium bromide.
6. The method of Claim 1, wherein the halide salt is sodium chloride.
7. The method of Claim 1, wherein the halide salt is dissolved into a liquid-phase solution
before injection into the flue gas stream.
8. The method of Claim 1, wherein the halide salt is injected into the flue gas stream as a
powder.
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9. The method of Claim 1, wherein the halide salt is injected into the flue gas stream by
placing a solid structure containing the halide salt in the flue gas stream.
10. The method of Claim 1, wherein the temperature of injection is less than about 1,000 F.
11. The method of Claim 1, wherein the temperature of injection is less than about 1,000 F
and greater than about 200 OF.
12. The method of Claim 1; wherein a constituent in the flue gas stream is sulfur trioxide or
sulfuric acid.
13, The method of Claim 1, wherein the halide salt reacts with sulfur trioxide or sulfuric acid
in the flue gas stream to produce the reactive halogen species.