U.S. patent application number 11/055526 was filed with the patent office on 2005-08-04 for method for control of mercury.
Invention is credited to Bailey, Ralph T., Downs, William.
Application Number | 20050169824 11/055526 |
Document ID | / |
Family ID | 23083247 |
Filed Date | 2005-08-04 |
United States Patent
Application |
20050169824 |
Kind Code |
A1 |
Downs, William ; et
al. |
August 4, 2005 |
Method for control of mercury
Abstract
A method and apparatus for reducing mercury in industrial gases
such as the flue gas produced by the combustion of fossil fuels
such as coal adds hydrogen sulfide to the flue gas in or just
before a scrubber of the industrial process which contains the wet
scrubber. The method and apparatus of the present invention is
applicable to installations employing either wet or dry scrubber
flue gas desulfurization systems. The present invention uses kraft
green liquor as a source for hydrogen sulfide and/or the injection
of mineral acids into the green liquor to release vaporous hydrogen
sulfide in order to form mercury sulfide solids.
Inventors: |
Downs, William; (Alliance,
OH) ; Bailey, Ralph T.; (Uniontown, OH) |
Correspondence
Address: |
BWX TECHNOLOGIES, INC.
LAW DEPARTMENT - INTELLECTUAL PROPERTY
91 STIRLING AVENUE
(MAIL STATION BWO11E)
BARBERTON
OH
44203-0271
US
|
Family ID: |
23083247 |
Appl. No.: |
11/055526 |
Filed: |
February 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11055526 |
Feb 10, 2005 |
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09894558 |
Jun 28, 2001 |
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09894558 |
Jun 28, 2001 |
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09282817 |
Mar 31, 1999 |
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6284199 |
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Current U.S.
Class: |
423/210 |
Current CPC
Class: |
F23J 15/003 20130101;
F23J 2219/60 20130101; F23J 2217/102 20130101; F23J 2219/40
20130101; B01D 47/06 20130101; F23J 2215/60 20130101; B01D 53/78
20130101; B01D 53/64 20130101; F23J 15/02 20130101 |
Class at
Publication: |
423/210 |
International
Class: |
B01D 053/64 |
Goverment Interests
[0002] The subject matter of the present invention was developed
under a research contract with the U.S. Department of Energy (DOE),
Contract No. DE-FC22-94PC94251, and under a grant agreement with
the Ohio Coal Development Office (OCDO), Grant Agreement No.
CDO/D-922-13. The governments of the United States and Ohio have
certain rights in the invention.
Claims
We claim:
1. In a method for receiving and scrubbing an industrial flue gas
containing mercury with an aqueous alkali reagent in a scrubber,
the improvement comprising: generating hydrogen sulfide proximate
to the scrubber by providing alkali to a solution comprising sodium
sulfide or potassium sulfide; and supplying the hydrogen sulfide to
the scrubber to remove mercury from the industrial flue gas.
2. The method according to claim 1, further comprising supplying
air to the hydrogen sulfide to produce a mixture of air and
hydrogen sulfide and wherein the step of supplying the hydrogen
sulfide comprises providing the mixture of air and hydrogen sulfide
to the industrial flue gas.
3. The method according to claim 1, further comprising controlling
the supplying the hydrogen sulfide step by adjusting at least one
of the supply of acid and alkali to said solution to alter the pH
of said solution and, correspondingly, a vapor pressure of the
generated hydrogen sulfide.
4. The method according to claim 2, wherein the mixture of air and
hydrogen sulfide is supplied to the industrial flue gas at multiple
locations by way of at least one of a plurality of pipes and a
plurality of air foils, each having apertures therein, for
discharging the mixture of hydrogen sulfide and air into a flue
conveying the industrial flue gas to the scrubber.
5. The method according to claim 1, wherein kraft green liquor is
used to generate the hydrogen sulfide.
6. The method according to claim 5, further comprising controlling
the supplying the hydrogen sulfide step by adjusting the pH of the
kraft green liquor and, correspondingly, a vapor pressure of the
generated hydrogen sulfide.
7. The method according to claim 3, wherein kraft green liquor is
used to generate the hydrogen sulfide.
8. The method according to claim 1, wherein the scrubber is a wet
scrubber.
9. The method according to claim 1, wherein the scrubber is a dry
scrubber.
10. The method according to claim 1, wherein the step of supplying
the hydrogen sulfide to the industrial flue gas occurs at a rate
sufficient to produce a concentration of hydrogen sulfide in the
industrial flue gas of about 0.05 to 10 ppm.
Description
[0001] This is a divisional application of U.S. Ser. No. 09/894,558
filed Jun. 28, 2001, which is a divisional application of U.S. Ser.
No. 09/282,817 filed Mar. 31, 1999, now U.S. Pat. No. 6,284,199 B1,
and the text of the aforementioned patent applications are hereby
incorporated by reference as though fully set forth herein.
FIELD AND BACKGROUND OF THE INVENTION
[0003] The present invention relates generally to the field of
combustion and flue gas cleanup methods and apparatus and, in
particular, to a new and useful method and apparatus for removing
mercury from the flue gases generated during the combustion of
fossil fuels such as coal, or solid wastes, through the use of
hydrogen sulfide.
[0004] In recent years, the U.S. Department of Energy (DOE) and the
U.S. Environmental Protection Agency (EPA) have supported research
to measure and control the emissions of Hazardous Air Pollutants
(HAPs) from coal-fired utility boilers and waste to energy plants.
The initial results of several research projects showed that the
emissions of heavy metals and volatile organic carbons (VOCs) are
very low, except for mercury (Hg). Unlike most of the other metals,
most of the mercury remains in the vapor phase and does not
condense onto fly ash particles at temperatures typically used in
electrostatic precipitators and fabric filters. Therefore, it
cannot be collected and disposed of along with fly ash like the
other metals. To complicate matters, mercury can exist in its
oxidized (Hg.sup.+2) form, principally as mercuric chloride,
(HgCl.sub.2), or in its elemental (Hg.sup.0) form as vaporous
metallic mercury. The relative amount of each species appears to
depend on several factors such as fuel type, boiler combustion
efficiency, the type of particulate collector installed, and
various other factors.
[0005] The search for industrially acceptable methods for the
capture of mercury from industrial flue gases has included a
significant effort to determine how much mercury can be removed by
existing, conventional air pollution control equipment. One device
used in air pollution control is the wet scrubber, which is
designed for the capture of sulfur oxides and other acid gases.
Tests have been performed on several commercial scale and pilot
scale wet scrubbers. These tests have produced some expected and
some surprising results. It was generally expected that the
oxidized mercury would be easily captured and the elemental mercury
would be difficult to capture. These expectations were based on the
high solubility of mercuric chloride in water and the very low
solubility of elemental mercury in water. This expectation was
generally fulfilled.
[0006] The surprising result concerned elemental mercury. Repeated
tests during which the concentration of elemental mercury in the
flue gas was measured revealed that more elemental mercury was
leaving the wet scrubber than was entering.
[0007] One postulate proposed to explain the cause of the elemental
mercury generation in the wet scrubber is described for example, by
the following general reactions:
M.sub.e.sup.0+Hg.sup.+2.fwdarw.M.sub.e.sup.+2+Hg.sup.0
2M.sub.e.sup.++Hg.sup.+2.fwdarw.2M.sub.e.sup.+2+Hg.sup.0
[0008] M.sub.e is any number of transition metals such as Fe, Mn,
Co, Sn, . . . .
[0009] Transition metal ions are generally present in wet scrubber
slurries as impurities in the industrial applications of concern.
Thus, as the mercuric chloride is absorbed, a portion reacts with
and becomes reduced by trace levels of transition metals and metal
ions and because of its low solubility the elemental mercury is
stripped from the liquid and returned to the flue gas.
[0010] Most of the recent efforts to capture and remove mercury
from the flue gas produced by coal-fired units have concentrated on
gas-phase reactions with introduced reagents such as activated
carbon.
[0011] The subject of mercury emissions by the utility and waste to
energy industries is a new area being investigated by both the DOE
and EPA.
SUMMARY OF THE INVENTION
[0012] The present invention provides a means in the wet scrubber
to rapidly precipitate the oxidized mercury at the gas/liquid
interface in the wet scrubber before it can be reduced by the
transition metals. One of the most insoluble forms of mercury is
mercuric sulfide, which in mineral form is cinnabar. One means for
supplying a source of sulfide for the oxidized mercury to react
with is hydrogen sulfide. Thus, at the gas/liquid interface in the
scrubber, the following reaction is proposed for the absorption and
precipitation of ionized (oxidized) mercury:
H.sub.2S(g)+HgCl.sub.2(g).fwdarw.HgS(s)+2HCl(aq)
[0013] HgS has a solubility product of 3.times.10.sup.-52 and
therefore precipitates essentially completely.
[0014] There is good reason to expect that the precipitation
reaction proceeds faster than the reduction reactions.
Specifically, in the case of the precipitation reaction, both
reactants are well mixed in the gas phase. Thus, as they diffuse
from the gas to the gas/liquid interface both reactants can react
instantly at that interface. By contrast, the reduction reactions
require that the reactants, i.e., the Hg.sup.+2 and the transition
metal ion, diffuse in the liquid phase to a reaction plane in the
liquid. Liquid phase diffusion is orders of magnitude slower than
gas phase diffusion. Therefore, the oxidized mercury will rapidly
precipitate as cinnabar in the scrubber and thereby prevent the
reduction of that mercury back to vaporous elemental mercury. The
precipitation of mercury as cinnabar has a distinct advantage over
other mercury sequestering methods in that it converts mercury to a
very insoluble form. In this way, the mercury should be inert and
effectively removed from the food chain.
[0015] Accordingly, one aspect of the present invention is drawn to
an improvement in a method using a wet scrubber for receiving and
scrubbing an industrial gas containing mercury with a wet scrubber
slurry, the improvement comprising: adding hydrogen sulfide to the
industrial gas; and scrubbing the industrial gas in the wet
scrubber. The method according to the present invention is
particularly suited to the task of reducing mercury emissions in an
industrial process which burns coal in a furnace to produce an
exhaust flue gas, including conveying the exhaust flue gas through
a dust collector and adding hydrogen sulfide to the flue gas before
it enters the wet scrubber, or within the wet scrubber.
[0016] Another aspect of the present invention is drawn to an
apparatus using a wet scrubber for receiving and scrubbing an
industrial gas containing mercury with a wet scrubber slurry, and
particularly the improvement comprising: means for generating
hydrogen sulfide; and means for supplying the hydrogen sulfide to
the industrial gas upstream of the wet scrubber. The present
invention is again particularly suited to utility installations
which burn fossil fuels such as coal, or solid wastes, and which
use, in addition to the wet scrubber, an electrostatic precipitator
or a fabric filter and other conventional components for reducing
emissions to the atmosphere.
[0017] Particularly, the present invention contemplates provision
of a hydrogen sulfide generating system which produces the hydrogen
sulfide from a reaction of adding an acid to a solution of aqueous
sodium and/or potassium sulfide to generate the hydrogen sulfide.
Advantageously, the hydrogen sulfide generating system may use
equipment and methods wherein the acid is added to green liquor
from the Kraft pulping process to generate the hydrogen
sulfide.
[0018] The system has an inherent safety advantage in that no gas
phase H.sub.2S is accumulated or stored. All H.sub.2S that is
generated is immediately injected.
[0019] The various features of novelty which characterize the
invention are pointed out with particularity in the claims annexed
to and forming a part of this disclosure. For a better
understanding of the invention, its operating advantages and
specific benefits attained by its uses, reference is made to the
accompanying drawings and descriptive matter in which a preferred
embodiment of the invention is illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] In the drawings:
[0021] FIG. 1 is an illustration of the present invention as
applied to a coal-fired utility boiler installation of the type
used by utilities in the generation of electric power;
[0022] FIG. 2 is an illustration of a hydrogen sulfide H.sub.2S
generation system according to the present invention as
particularly applied to a wet scrubber of FIG. 1;
[0023] FIG. 3 is an enlarged partial view of the circled area in
FIG. 2, illustrating one embodiment of a system for injecting
H.sub.2S into flue gases for mercury control according to the
present invention;
[0024] FIG. 4 is an enlarged partial view of the circled area in
FIG. 2, illustrating another embodiment of a system for injecting
H.sub.2S into flue gases for mercury control according to the
present invention;
[0025] FIG. 5 is a chart illustrating the results of tests
comparing performance without H.sub.2S injection to performance
with H.sub.2S injection; and
[0026] FIG. 6 is an illustration of the present invention as
applied to a coal-fired utility boiler installation of the type
used by utilities in the generation of electric power which employs
a dry scrubber flue gas desulfurization system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] Referring to the drawings generally, wherein like reference
numerals designate the same or functionally similar elements
throughout the several drawings, and to FIG. 1 in particular, FIG.
1 illustrates a coal-fired utility boiler installation of the type
used by utilities in the generation of electric power, generally
designated 10, and which represents one type of industrial process
to which the present invention is applicable. In its broadest form,
the present invention comprises a method for removing mercury from
the flue gas generated during the combustion of fossil fuels or
solid wastes through the use of trace amounts of hydrogen sulfide.
Of course, while the aforementioned coal-fired utility boiler
installations are but one example, and the method of the present
invention will likely first find commercial application to the
removal of mercury from the flue gases produced by such utility
boiler installations which combust such fossil fuels, any
industrial process using a wet scrubber type of absorber module to
purify such flue gases may benefit. Such processes could include
incineration plants, waste to energy plants, or other industrial
processes which generate gaseous products containing mercury. Thus
for the sake of convenience, the terms industrial gas, flue gas, or
simply gas will be used in the following discussion to refer to any
gas from an industrial process and from which an objectionable
component, such as mercury, is to be removed.
[0028] As will be described infra, an alternate embodiment of the
present invention involves methods and apparatus for the addition
of trace amounts of hydrogen sulfide to industrial gases which are
treated by dry scrubber flue gas desulfurization systems. Thus,
while the majority of the following description is presented in the
context of the present invention as being applied to wet scrubber
systems, it will be appreciated that the present invention is not
limited thereto. Further, since both wet and dry scrubbers remove
sulfur species from the flue gas by introduction of an alkali
sorbent, some common terminology may be used as appropriate for the
sake of convenience. In the case of wet scrubbers, the alkali
sorbent can be provided as an aqueous alkali solution or slurry; in
dry scrubbers, the alkali sorbent is usually provided as an aqueous
alkali slurry. Thus, for the sake of convenience in the following
description, the term aqueous alkali reagent will be used to
encompass both aqueous alkali solutions and/or aqueous alkali
slurries as appropriate to the type of scrubber means being
used.
[0029] As illustrated in FIG. 1, and proceeding in the direction of
flue gas flow generated during the combustion process, the boiler
installation 10 includes a furnace 12 having a gas outlet 14 which
conveys flue gases, generally designated 16, to an air heater 18
used to preheat incoming air 20 for combustion. Pulverizers 22
grind a fossil fuel 24 (e.g., coal) to a desired fineness and the
pulverized coal 24 is conveyed via burners 25 into the furnace 12
where it is burned to release heat used to generate steam for use
by a steam turbine-electric generator (not shown). Flue gas 16
produced by the combustion process are conveyed through the gas
outlet 14 to the air heater 18 and thence to various types of
downstream flue gas cleanup equipment. The flue gas cleanup
equipment may comprise a fabric filter or, as shown, an
electrostatic precipitator (ESP) 26 which removes particulates from
the flue gas 16. H.sub.2S generation system 50 is optimally
provided along flue 28. Generation system 50 permits the injection
of H.sub.2S into flue gas 16 via injection apparatus 76. Both
generation system 50 and injection apparatus 76 are described in
greater detail infra. A flue 28 downstream of the ESP 26 conveys
the flue gas 16 to a wet scrubber absorber module 30 which is used
to remove sulfur dioxide and other contaminants from the flue gas
16. Flue gas 16 exiting from the wet scrubber absorber module 30
or, simply, the wet scrubber 30, is conveyed to a stack 32 and
exhausted to atmosphere. Forced draft fans 34 and induced draft
fans 36 are used to propel the air 20, fuel 24, and flue gases 16
through the installation 10. For further details of various aspects
of such installations 10, the reader is referred to STEAM its
generation and use, 40th Ed., Stultz and Kitto, Eds., Copyright
.COPYRGT. 1992 The Babcock & Wilcox Company, particularly to
Chapter 35--Sulfur Dioxide Control, the text of which is hereby
incorporated by reference as though fully set forth herein. While
the aforementioned STEAM reference contains a description of one
form of wet scrubber 30 produced by The Babcock & Wilcox
Company (B&W) and to which the present invention is applicable,
the present invention is not limited to such B&W wet scrubber
designs. Persons skilled in the art will appreciate that the
principles of the present invention apply equally well to other
types of wet scrubber designs, available from other
manufacturers.
[0030] The wet scrubber 30 contains, in a lower portion thereof, an
inventory of wet scrubber slurry 38. During operation of the wet
scrubber 30, recirculation pumps 40 pump and recirculate the wet
scrubber slurry 38 up through pipes 42 and into absorber spray
headers 44 located in an upper portion of the wet scrubber 30. The
wet scrubber slurry 38 is sprayed counter currently into the flue
gas 16 where it absorbs SO.sub.2. The wet scrubber slurry 38 falls
down through various devices and drains back into the lower portion
of the wet scrubber 30. The scrubbed flue gas 16 then exits from a
wet scrubber outlet 46 and is eventually conveyed to the stack
32.
[0031] Referring now to FIG. 2 in particular, there is shown an
embodiment of a system for accomplishing the method of injecting
small amounts of H.sub.2S into flue gas for mercury removal
according to the present invention. An H.sub.2S generation system,
generally referred to as 50, is provided and includes a
well-stirred tank containing a liquid section 54 comprising sodium
and/or potassium sulfide and a gas section 52 where air and
H.sub.2S are mixed and the mixture 74 of air and H.sub.2S is
transferred to an injection apparatus 76, described infra. The
H.sub.2S vapor pressure in the tank 51 is controlled by pH. The pH
in the tank 51 liquid solution 54 is controlled by the addition of
a strong mineral acid 56, such as hydrochloric or sulfuric acid
(HCl or H.sub.2SO.sub.4) from a tank or container 58, or by the
addition of an alkali solution 57 such as sodium carbonate or
sodium hydroxide (NaOH or Na.sub.2CO.sub.3) from a tank or
container 85. The acid is added to lower the pH and increase the
H.sub.2S vapor pressure in tank 51. The alkali is added to raise
the pH and lower the H.sub.2S vapor pressure in tank 51. The
H.sub.2S produced is immediately transported to the injection
system or injection apparatus 76. This is an inherent safety
feature since no gaseous H.sub.2S is allowed to accumulate.
Stirring or mixing means, advantageously comprising a motor 64 and
driven stirring shaft with two paddles 66, keep the tank sections
52 and 54 well stirred. As a result, the constituents in the liquid
zone 54 are well mixed to yield the H.sub.2S at the desired vapor
pressure and the air 68 and generated H.sub.2S are well mixed in
the gas section 52. Pumping means 60 conveys the mineral acid 56 to
the tank 51 via line 62; pumping means 61 conveys the alkali
solution 57 to the tank 51 via line 63. Suitable control valves in
lines 62 and 63 would be used as needed to control the flow of acid
56 and alkali 57.
[0032] Air 68 is provided by fan (blower or compressor) means 70
into the upper section 52 of tank 51 where it mixes with the
H.sub.2S. Line 72 from the upper section of the tank 51 conveys the
mixture 74 of air and H.sub.2S to an injection system 76 in flue 28
for injecting the H.sub.2S-air mixture 74 into the flue gas 16.
[0033] The rate of hydrogen sulfide generation is controlled by the
rate of acid addition. The rate of air 68 provided into the tank 52
is controlled by the fan means 70 that supplies the air 68 in the
quantity and at the pressure necessary for rapid mixing of the
H.sub.2S-air mixture 74 with the flue gas 16 at an inlet 78 to the
wet scrubber 30.
[0034] The H.sub.2S-air injection system 76 can comprise one or
more pipes 78 arranged in a simple grid, each of the pipes 78 being
provided with a plurality of apertures 80, the pipes 78 arranged
across a width W and height H of flue 28, as shown in FIG. 3.
Alternatively, the H.sub.2S-air injection system 76 could utilize a
more sophisticated structure incorporating air foil mixing
technology such as that illustrated in FIG. 4. In the system of
FIG. 4, one or more air foils 82 would be provided, each having a
plurality of slots or apertures 84 for introducing the H.sub.2S-air
mixture 74 into the flue gas 16 as the flue gas 16 flows across the
air foils 82. In either case, the flue gas 16 flowing past the
pipes 78 or air foils 82 picks up the H.sub.2S-air mixture 74 and
conveys it to the wet scrubber 30 to increase the H.sub.2S content
in the flue gas 16 to a desired level, preferably between about
0.05 and 10 ppm, or more preferably 2 ppm or below.
[0035] FIG. 5 depicts test results obtained when a pilot scale wet
scrubber was used to capture mercury. In FIG. 5, the legends "WS
Inlet" and "WS Outlet" refer to conditions at the wet scrubber 30
inlet and outlet, respectively. The left two bars represent
baseline conditions of the elemental mercury content of the flue
gas 16 at the wet scrubber inlet and outlet, without H.sub.2S
addition. The large increase in elemental mercury at the wet
scrubber outlet is due to the chemical reduction of oxidized
mercury within the wet scrubber 30 according to the reactions
previously noted. The right two bars represent the improved
performance achieved when H.sub.2S was injected at a concentration
of about two parts per million (2 ppm). As shown, the chemical
reduction of oxidized mercury to elemental mercury was completely
prevented.
[0036] As described earlier and as illustrated in FIG. 6, the
present invention is also applicable to combustion systems
employing dry scrubbers for flue gas desulfurization. Again, like
reference numerals designate the same or functionally similar
parts, including installation 10, forced draft fans 34 and induced
draft fans 36. Flue gas 16 produced by the combustion process are
conveyed through the gas outlet 14 to the air heater 18 and thence
to various types of downstream flue gas cleanup equipment. A flue
28 conveys the flue gas 16 to a dry scrubber absorber module or dry
scrubber 150 which is used to remove sulfur dioxide and other
contaminants from the flue gas 16. Flue gas 16 exiting from the dry
scrubber 150 is conveyed to a fabric filter or, as shown, an
electrostatic precipitator (ESP) 26 which removes particulates from
the flue gas 16 and then the flue gas 16 is conveyed to a stack 32
and exhausted to atmosphere. A waste disposal system 160 can also
be included to handle unwanted materials from the ESP 26 and/or the
dry scrubber 150. Forced draft fans 34 and induced draft fans 36
are used to propel the air 20, fuel 24, and flue gases 16 through
the installation 10 as before.
[0037] Advantages of the present invention include the fact that
the cost of control of mercury emissions according to the present
invention is relatively insignificant compared to the costs for
control of other hazardous air pollutants. Further, the amount of
H.sub.2S required should be below the threshold odor level. The
cost and operating expenses of a system as depicted in FIG. 2
should be substantially less compared to any other system proposed
to date. According to a preferred embodiment of the present
invention, one source of the sodium sulfide used to generate the
H.sub.2S can comprise green liquor, an intermediate chemical used
in the Kraft pulp and paper industry and which is readily available
from pulp and paper companies. As is known to those skilled in the
art, green liquor consists of an aqueous mixture of sodium sulfide
and sodium carbonate. Green liquor is relatively easy and safe to
handle and should be widely available.
[0038] According to the present invention, the mercury in the flue
gas 16 ends up as mercuric sulfide (also known as cinnabar). This
is the chemical form that mercury is most often found in nature and
is probably the most desirable chemical form to sequester mercury.
Much of the mercury in this form is present as a fine particulate
in the scrubber slurry and for that reason, it is possible to
separate much of the mercury from the gypsum crystals.
[0039] While a specific embodiment of the invention has been shown
and described in detail to illustrate the application of the
principles of the invention, it will be understood that the
invention may be embodied otherwise without departing from such
principles. By way of example and not limitation, while the
principles of the present invention were described as being
particularly applicable to fossil-fired boiler installations, it
will be appreciated by those skilled in the art that the present
invention can be used to remove mercury from industrial gases
produced by refuse incinerators, refuse boilers, hazardous waste
incinerators, or ore roasters.
* * * * *