Note: Descriptions are shown in the official language in which they were submitted.
CA 02487843 2004-11-16
MERCURY REMOVAL FROM ACTIVATED CARBON AND/OR FLY ASH
FIELD OF THE INVENTION
This invention relates to the treatment of a sorbent, such as activated
carbon, and/or
fly ash to remove mercury that contaminates the sorbent and fly ash as part of
post-
combustion treatments of exhaust gases from a coal-fired power plant.
DESCRIPTION OF THE RELATED ART
In 1990, the United States Environmental Protection Agency ("EPA") put into
place the Clean Air Act Amendments which were designed to reduce the emissions
of
"greenhouse gases". Among the emissions covered are the nitrogen compounds NO
and
NO2, referred to generically as NOx. NOx is generated through the combustion
of coal
and its generation is directly affected by combustion temperature, residency
time, and
available oxygen. Several technologies have been developed to meet the
mandated NOx
reduction limits. One category includes technologies that are employed after
combustion
has taken place. These technologies include selective non-catalytic reduction,
selective
catalytic reduction, and amine enhanced fuel lean gas rebum. These
technologies involve
adding ammonia to the exhaust gases, and a significant amount of the ammonia
finds its
way onto the fly ash, typically by combining with available sulfur and other
compounds
that attach to the ash particles.
Fly ash is a marketable product if it is not contaminated. The ash may be
used, for
example, in concrete products as a replacement for a portion of the cement.
However, fly
ash that has been treated to reduce NOx and which is contaminated either by
unburned
carbon or ammonia compounds is not marketable. Systems have been developed
which
may be used to reduce the amount of ammonia compounds affixed to fly ash. For
example, PCT Inteinational Patent Publication No. WO 01/12268 describes a
process for
the reduction of ammonia residues from the recovered fly ash of a coal fired
power plant.
In addition, the emission of mercury compounds from all sources, including
coal-
fired power plants, has drawn national and international attention due to the
fact that
certain forms of mercury have deleterious effects on wildlife and are toxic to
humans.
Mercury is a naturally occurring element in the environment; however, human
activities
over the centuries have released large quantities of this element from its
sequestered forms
CA 02487843 2004-11-16
2
(mercury-containing ores, soils, rocks, including all forms of coal).
Currently, scientists
believe that most of the mercury entering the environment results from air
emissions.
Large scale releases from certain mining activities (e.g. gold mining), coal
burning,
medical and municipal waste incineration appear to be the largest
anthropogenic sources.
However, natural degassing from the oceans, soils, and rocks is thought to be
the largest
overall source of mercury to the atmosphere.
Mercury emitted from the above sources is transported and transformed by
atmospheric processes that are only partially understood. However, it is known
that when
the oxidized form of mercury (currently believed to constitute a very small
percentage of
all mercury in the atmosphere) deposits to certain aquatic systems, such as
wetlands, salt
marshes, and certain lakes, this form of mercury undergoes chemical
transformation by
certain microbes. These microbes convert the inorganic form to methylmercury,
a very
potent neurotoxin. While this form is typically present in very low
concentrations in the
environment, it can be bioaccumulated via the food chain. The mercury levels
in the top
members of the food chain are often present at concentrations thousands of
even millions
of times greater than what can be found in natural waters. These higher
concentrations are
found in fish or mammals that occupy the top of ecosystem food chains. Persons
who eat
large quantities of these fish are thought to be at risk from developing mild
to severe forms
of mercury poisoning. Women who eat large quantities of inercury-contaminated
fish or
seafood and are pregnant, run the risk of giving birth to a child who may
experience
learning disabilities.
The United States Environmental Protection Agency (EPA) is focusing on
mercury, because mercury has been identified as a toxic of great concern among
all the air
toxics emitted from power plants. To reduce the risk mercury poses to people's
health, the
EPA is announcing that it will regulate emissions of mercury and other air
toxics from
coal- and oil-fired electric utility steam generating units (power plants).
The data indicates
that coal-fired power plants are the largest source of human-caused mercury
emissions in
the United States. It has been reported that the EPA is likely to propose
mercury
regulations by December 15, 2003 and issue final regulations by December 15,
2004.
CA 02487843 2004-11-16
3
Physical forms of mercury in ambient air can be divided into two categories:
vapor
phase, which is dominant in the atmosphere, and particulate phase (associated
with
aerosols), which only comprises a few percent of total airborne mercury
emissions.
Chemical forms determine the transport of mercury between different
environmental
media (air, water and soil). The mercuric compounds can be classified into
elemental and
divalent forms. The elemental form of mercury (Hg ) is the dominant form
(>98%) of
vapor-phase mercury in the atmosphere, and following dissolution in cloud
water or
rainwater, is readily converted to more soluble mercury species. Elemental
mercury
possesses relatively high vapor pressure and low solubility. The former
property leads to
considerable mercury evaporation into the ambient air, while the latter makes
it difficult
for the existing air pollution control devices to remove mercury from the
emission sources.
Divalent mercury forms include inorganic (Hg2+, HgO, HgCl2) and organic
oxidized forms
(CH3Hg, CH3HgCl, CH3HgCH3). Divalent forms possess higher solubility and
readily
combine with a variety of reactants, such as sulfite, chloride and hydroxide
ions, in the
aqueous phase. The boiling points of elemental mercury and some mercury
compounds
are as follows: Hg, 356.58 C; HgC12, 303 C; and HgS, 580 C.
Many existing air pollution control technologies and several innovative
methods
have been evaluated for the control of vapor-phase mercury emissions from
combustion
processes. Sodium sulfide (Na2S) has been used for vapor-phase mercury control
in
municipal solid waste combustors in Canada, Sweden, Germany and British
Columbia.
Sodium sulfide injection is usually combined with dry sorbent injection and
fabric filters
for acid gas and particulate matter control. It has been reported that
mercuric sulfide
(HgS) is generated as a fine particulate in the process, which may prove
difficult to capture
in less efficient electrostatic precipitators. Other potential problems for
this process
include corrosion, hydrogen sulfide formation and chemical storage and
handling. These
problems, compounded by the lack of test data on full-scale coal-fired power
plants, cloud
the utility of sodium sulfide injection for the control of mercury emissions.
(See Sengupta,
"Environmental Separation of Heavy Metals: Engineered Processes", CRC Press,
2001.)
Wet scrubbers have been routinely used to remove hydrochloric acid and sulfur
dioxide from the flue gases of industrial factories, coal-fired power plants
and municipal
CA 02487843 2004-11-16
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waste combustors. Considerable interest in the use of wet scrubbers systems to
simultaneously remove sulfur dioxide and mercury has recently been expressed.
The
removal of vapor-phase mercury in the wet scrubber system would also occur by
absorption in the scrubbing slurry, whereby the mechanism of mercury removal
depends
on the solubility of mercury in the scrubbing slurry, contact time and
solution chemistry.
Elemental mercury is essentially insoluble in the wet scrubbing slurry, while
some of the
oxidized species, such as mercuric chloride, are highly soluble. Therefore,
oxidized
mercury can be easily absorbed with sufficient gas-liquid contact, while the
removal of
elemental mercury would remain limited. Chang and Owens reported that the
treatment of
a coal-fired power plant flue gas using only a wet scrubber allowed 70-75% of
elemental
mercury to be discharged into the atmosphere (see Chang and Owens, EPRI J.,
16, 2, 183-
189, 1994), while other studies reported 30-70% removal of elemental mercury
by wet
scrubbers. (see, Sengupta, above).
Presently, the most effective and widely used technology for capturing mercury
from flue gas emissions is to inject activated carbon into the gas stream.
Injected activated
carbon binds the vapor-phase mercury through physical adsorption and
chemisorption and
is collected in downstream particulate collection devices, such as fabric
filters (baghouses)
or electrostatic precipitators. Results from several tests indicated that
effectiveness of
activated carbon injection in removing mercury vapor depends on the type and
composition of burned materials, flue gas composition and temperature, mercury
speciation, activated carbon properties and injection rate and operating
conditions.
Because activated carbon can be collected effectively in the existing
particulate control
devices, direct activated carbon injection has several potential advantages
over wet
scrubbing processes.
One method of doing this is to inject powdered activated carbon into the
exhaust
gas upstream of a primary particulate collector (e.g., an electrostatic
precipitator or
baghouse). However, when using this method, the carbon/mercury mixture is
collected
along with fly ash. The collected fly ash has a higher carbon content (from
the activated
carbon) and has increased mercury levels due to the mercury adsorbed on the
activated
carbon. As a result, the collected fly ash becomes unusable in concrete
without
CA 02487843 2004-11-16
beneficiation to remove the additional carbon content. Thus, the value of the
resulting fly
ash declines because of more limited uses and the need for expensive
beneficiation
techniques such as froth flotation, electrostatic separation, or reburning the
fly ash.
Because the primary use of fly ash includes cementitious material for concrete
and
5 concrete products, feed stock for Portland cement manufacture, liquid waste
stabilization,
and lightweight aggregate production, it is essential to maintain the high
quality of fly ash
for use in concrete. Also, mercury adsorbed by the activated carbon may
increase the
potential for release of mercury into the environment during reuse or
landfilling of the fly
ash.
Another method for capturing and removing mercury from exhaust gases involves
injecting powdered activated carbon into the exhaust gas downstream of the
primary
particulate collector and ahead of a secondary particulate collector (e.g., an
electrostatic
precipitator or baghouse). The resulting carbon/mercury mixture is then
collected in the
secondary particulate collector for disposal. Therefore, the quality of fly
ash collected in
the primary particulate collector is warranted for reuse in concrete. However,
the issue
still remains as to what to do with the carbon/mercury mixture collected in
the secondary
particulate collector. As stated above, the mercury adsorbed by the activated
carbon may
increase the potential for release of mercury into the environment during
landfilling or
other disposal of the mixture. Furthermore, the expense associated with
current activated
carbon injection technology can be quite high due to the disposal costs
associated with
mercury contaminated carbon.
Therefore, there is a need for an improved method and apparatus that can
remove
adsorbed mercury from a sorbent, such as activated carbon, that is collected
separately or
collected with fly ash in an exhaust gas treatment process for a coal-fired
power plant.
SUMMARY OF THE INVENTION
The foregoing needs are met by a method according to the invention for
reducing
the amount of mercury affixed to a sorbent. The method includes the steps of
providing an
amount of sorbent wherein at least a portion of the amount of sorbent has
particulates
having mercury compounds affixed to the particulates; and exposing the amount
of sorbent
to heated flowing air until mercury compounds are liberated from at least some
of the
CA 02487843 2004-11-16
6
particulates. Preferably, the arnount of sorbent is maintained in the heated
flowing air until
the sorbent reaches a temperature of at least 700 F (372 C). When the sorbent
is activated
carbon, it is preferred that the amount of sorbent is maintained in the heated
flowing air
until the activated carbon reaches a temperature in the range of 700 F (372 C)
to 1000 F
(538 C).
In another aspect, the invention provides a method for reducing the amount of
mercury in an amount of particulate matter including fly ash and activated
carbon. The
method involves providing an amount of particulate matter including fly ash
and activated
carbon wherein at least a portion of the fly ash or activated carbon has
adsorbed mercury
compounds; and exposing the amount of particulate matter to heated flowing air
until
mercury compounds are liberated from at least some of the particulate matter.
Preferably,
the particulate matter is exposed to heated flowing air until the particulate
matter reaches a
temperature of at least 700 F (372 C), and when the sorbent is activated
carbon, the
particulate matter is exposed to heated flowing air until the particulate
matter reaches a
temperature in the range of 700 F (372 C) to 1000 F (538 C).
It is therefore an advantage of the invention to provide a method for reducing
the
amount of mercury affixed to a sorbent, such as activated carbon.
It is another advantage of the invention to provide a method for reducing the
amount of mercury affixed to particulate matter including fly ash and
activated carbon.
It is yet another advantage of the invention to provide a method for reducing
the
amount of mercury affixed to a sorbent such that the sorbent can be reused in
a mercury
reduction process.
These and other features, aspects, and advantages of the present invention
will
become better understood upon consideration of the following detailed
description,
drawings, and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a diagrammatic illustration of an exhaust gas treatment apparatus
for a
coal-fired power plant.
CA 02487843 2004-11-16
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Figure 2 is a diagrammatic illustration of another exhaust gas treatment
apparatus
for a coal-fired power plant.
Figure 3 is a diagrammatic illustration of an embodiment of an apparatus for
removal of mercury from a sorbent such as activated carbon.
Figure 4 is a graph showing the influence of temperature on mercury removal.
Figure 5 is a graph showing the effect of retention time on mercury removal
efficiency.
Like reference numerals will be used to refer to like or similar parts from
Figure to
Figure in the following description of the drawings.
DETAILED DESCRIPTION OF THE INVENTION
This invention relates to a method for reducing the amount of mercury affixed
to a
sorbent. The method includes the steps of providing an amount of sorbent
wherein at least
a portion of the amount of sorbent has particulates having mercury compounds
affixed to
the particulates; and exposing the amount of sorbent to heated flowing air
until mercury
compounds are liberated from at least some of the particulates. Preferably,
the amount of
sorbent is maintained in the heated flowing air until the sorbent reaches a
temperature of at
least 700 F (372 C). When the sorbent is activated carbon, it is preferred
that the amount
of sorbent is maintained in the heated flowing air until the activated carbon
reaches a
temperature in the range of 700 F (372 C) to 1000 F (538 C). After mercury
compounds
are liberated from at least some of the particulates, the mercury-depleted
sorbent may be
reused in a mercury reduction process.
The method according to the invention may be a continuous process in which an
in-process temperature of the sorbent is measured when the sorbent is exposed
to the
heated flowing air. The sorbent being exposed to the heated flowing air is
removed from
the flowing air when the measured in-process temperature reaches at least 700
F (372 C).
Thereafter, a second amount of sorbent comprising particulates having mercury
affixed to
the particulates is exposed to the heated flowing air until the sorbent
reaches a temperature
of at least 700 F (372 C).
CA 02487843 2004-11-16
8
The sorbent may be exposed to the heated flowing air by providing a metal
media
having openings; passing heated flowing air through the openings, and
depositing the
sorbent on the metal media. In one form, the openings are 10 microns or less.
In another
form, the flowing air is passed through the openings at greater than 0 to
about 10 cubic feet
(0.28 cubic meters) per minute. Preferably, the sorbent is preheated to a
temperature of at
least 300 F (148 C) before exposing the amount of sorbent to the flowing air.
Referring to Figure 1, there is shown a diagrammatic illustration of an
example
exhaust gas treatment system for a coal-fired power plant. Components of the
exhaust gas
treatment system which are present in a typical system but not necessary for
an
understanding of the present invention have been omitted from the
illustration. A
combustion chamber 100 is connected to a flue 105 for directing exhaust gases
away from
the combustion chamber 100. A sorbent storage and injection unit 115 is
connected to the
flue 105 via a conduit 110 and can be any means for storing and injecting a
sorbent, such
as activated carbon, to be used in capturing and removing mercury from the
exhaust gases.
The flue 105 is also connected to a primary particulate collector 120 for
collecting
particulate matter from the exhaust gas in the flue 105. The particulate
matter includes,
among other things, fly ash (which may be contaminated with ammonia from an
NOx
reduction process) and sorbent, which typically includes adsorbed mercury. The
particulate collector 120 can be an electrostatic precipitator or baghouse or
any other
means for collecting particulate matter in the exhaust gases in the flue 105.
The primary particulate collector 120 is connected via conduit 155 to a
mercury
removal apparatus 9, which will be described in detail below. The particulate
matter
collected in the primary particulate collector 120 is transferred to the
mercury removal
apparatus 9 via the conduit 155. Alternatively, the particulate matter
collected in the
primary particulate collector 120 can be transferred to the mercury removal
apparatus 9 by
alternative means such as loading equipment. The mercury removal apparatus 9
removes
the mercury from the particulate matter collected in the primary particulate
collector 120
as described below. The primary particulate collector 120 is also connected to
a second
flue 125 for directing the exhaust gases, after the removal of particulate
matter, from the
primary particulate collector 120. The second flue 125 is connected to a
secondary
CA 02487843 2004-11-16
9
particulate collector 140 for collecting further particulate matter from the
exhaust gases
from the second flue 125. The secondary particulate collector 140 can be an
electrostatic
precipitator or baghouse or any other means for collecting particulate matter.
The
secondary particulate collector 140 is also connected to a third flue 145 for
directing the
exhaust gases, after the removal of further particulate matter, from the
secondary
particulate collector 140 into the atmosphere. Alternatively, the third flue
145 could also
be removed and the exhaust gases directed to the atmosphere directly from the
secondary
particulate collector 140.
Referring now to Figure 2, there is shown a diagrammatic illustration of
another
example exhaust gas treatment system for a coal-fired power plant. Components
of the
exhaust gas treatment system which are present in a typical system but not
necessary for an
understanding of the present invention have been omitted from the
illustration. Like
reference numerals will be used to refer to like or similar components in
Figure 1 and
Figure 2. A combustion chamber 100 is connected to a flue 105 for directing
exhaust
gases away from the combustion chamber 100. The flue 105 is connected to a
primary
particulate collector 120 for collecting particulate matter from the exhaust
gas in the flue
105. The particulate matter includes, among other things, fly ash (which may
be
contaminated with ammonia from an NOx reduction process). The particulate
collector
120 can be an electrostatic precipitator or baghouse or any other means for
collecting
particulate matter in the exhaust gases in the flue 105. The primary
particulate collector
120 is connected to a particulate storage unit 160 via conduit 155 so that
particulate matter
can be stored after collection in the primary particulate collector 120.
Alternatively, the
particulate matter collected in the primary particulate collector 120 can be
transferred to
the particulate storage unit 160 by alternative means such as loading
equipment.
The primary particulate collector 120 is also connected to a second flue 125
for
directing the exhaust gases, after the removal of particulate matter, from the
primary
particulate collector 120. A sorbent storage and injection unit 135 is
connected to the
second flue 125 via a conduit 130 and can be any means for storing and
injecting a
sorbent, such as activated carbon, to be used in capturing and removing
mercury from the
exhaust gases in the second flue 125. The second flue 125 is also connected to
a
CA 02487843 2004-11-16
secondary particulate collector 140 for collecting further particulate matter
from the
exhaust gases from the second flue 125. The particulate matter collected in
the secondary
particulate collector 140 includes, among other things, sorbent, which
typically includes
adsorbed mercury. However, depending on the level of mercury in the exhaust
gases, a
5 portion of the sorbent collected may not include any adsorbed mercury. The
secondary
particulate collector 140 can be an electrostatic precipitator or baghouse or
any other
means for collecting particulate matter.
The secondary particulate collector 140 is connected via conduit 150 to a
mercury
removal apparatus 9 as described below,. The particulate matter collected in
the secondary
10 particulate collector 140 is transferred to the mercury removal apparatus 9
via the conduit
150. Alternatively, the particulate matter collected in the secondary
particulate collector
140 can be transferred to the mercury removal apparatus 9 by alternative means
such as
loading equipment. The mercury removal apparatus 9 removes the mercury from
the
particulate matter collected in the secondary particulate collector 140. The
secondary
particulate collector 140 is also connected to a third flue 145 for directing
the exhaust
gases, after the removal of particulate matter, from the secondary particulate
collector 140
into the atmosphere. Alternatively, the third flue 145 could also be removed
and the
exhaust gases directed to the atmosphere directly from the secondary
particulate collector
140.
Referring to Figure 3, there is shown in more detail the mercury removal
apparatus,
indicated generally at 9, for removal of mercury from (i) the particulate
matter collected in
the primary particulate collector 120 in Figure 1(which includes fly ash and
sorbent
having adsorbed mercury, and/or (ii) the particulate matter collected in the
secondary
particulate collector 140 in Figure 2 (which includes sorbent having adsorbed
mercury).
The mercury removal apparatus 9 includes a storage unit 10 that holds (i) the
particulate
matter collected in the primary particulate collector 120 in Figure 1 and
delivered to the
storage unit 10 via conduits 155 and 177, and/or (ii) the particulate matter
collected in the
secondary particulate collector 140 in Figure 2 and delivered to the storage
unit 10 via
conduits 150 and 177. Thus, when describing the particulate matter treated in
the mercury
removal apparatus 9, it should be understood that the particulate matter can
be a mixture of
CA 02487843 2004-11-16
11
one or more of any of the following: fly ash, fly ash contaminated with
ammonia, fly ash
having adsorbed mercury, sorbent, sorbent having adsorbed mercury, and other
particulate
matter.
The storage unit 10 may be a standard storage silo, preferably configured with
sufficient height to allow for gravity feed. A maintenance gate 11 closes the
bottom of the
storage unit 10 to halt the flow of the particulate matter for maintenance of
the equipment
downstream. The storage unit 10 may be replaced by a feed directly from the
primary
particulate collector 120 and/or secondary particulate collector 140. The
storage unit 10
may have insulated walls or may include aeration stones 13 which receive air
from an air
supply unit 49 via conduit 63 (having pressure gauge 75 and inline valve 64)
for aeration
of the particulate matter with heated air.
The particulate matter is fed from the storage unit 10 through a rotary air
lock in
the form of a high temperature rotary feeder 12. The rotary feeder 12 receives
air from the
air supply unit 49 via conduit 65 with an inline valve 66 that assists in
maintaining a steady
feed and depth of the particulate matter to a preheating section which
includes a preheater
15. A suitable rotary feeder is available from Delta/Ducon Conveying
Technology, Inc.,
Malvem, Pennsylvania, USA. The particulate matter flows through conduit 16
wherein
preheater 15 preheats the particulate matter using a bulk flow heat exchanger
in the form
of a series of vertical plates. The plates of the preheater 15 receive heat
from a bulk flow
heat exchanger 32 via conduit 59 and from a heat recovery unit 35 via conduit
57 as will
be described further below. The preheater 15 serves to preheat the particulate
matter to a
temperature of at least 300 F (148 C).
The preheated particulate matter is fed into an insulated heating chamber 17
either
as a batch or continuous process. The insulated heating chamber 17 includes a
treatment
bed with a downwardly sloping floor 20 formed of a porous metal material,
preferably an
alloy sold under the designation "Inconel 600". Either a vibratory feeder or a
rotary vane
feeder may be used to deposit the preheated carbon/mercury mixture onto the
floor 20. A
vibratory feeder is preferred due to its economy of operation and high heat
applicability.
Hot air is passed through the porous metal material of the floor 20 to provide
both heat and
fluidization to move the particulate matter deposited on the floor 20. The
result is a
CA 02487843 2007-12-20
12
fluidized bed conveyor 21. A suitable fluidized bed conveyor is available as
an air slide
from Delta/Ducon Conveying Technology, Inc., Malvern, Pennsylvania, USA. The
porous metal media that supports the carbon/mercury mixture bed may be 0.062
inches
(1.57 millimeters) thick and preferably has openings of 10 microns or less. A
suitable
porous metal media is available from Mott Metallurgical Corporation of
Farmington,
Connecticut, USA.
The heated air is passed through the porous metal media of the floor 20 to
provide
for uniform aeration and heating of the particulate matter bed. The hot
aeration air is
provided such that a minimum of particles are carried out of the treatment
area. This is
accomplished through the design of the porous media sizing, the aeration air
pressure and
the air temperature. The heated air is passed through the particulate matter
bed at a
specific flow rate which is designed to maximize the heat uptake by the
particulate matter
and provide for the removal of mercury compounds from the particulate matter.
The same
aeration air provides the fluidization to move the particulate matter through
the heating
chamber in a continuous process or to move the particulate matter out of the
chamber in a
batch system.
The hot air passed through the floor 20 may come from a direct fired natural
gas burner,
oil fired burner, electrical heat source, or waste heat source, such as the
waste heat of a
combustion turbine. In the apparatus of Figure 3, the hot air passed through
the floor 20
comes via a conduit 55 from a gas furnace 45 that receives preheated air from
an air
preheater 47 via a conduit 71. The air preheater 47 receives air from the air
supply unit 49
(such as a compressor) via a conduit 69 with an inline valve 70 that controls
the flow of air
into the air preheater 47. A programmable logic controller 29 receives signals
from a
pressure gauge 73 in the conduit 55 and provides control signals to inline
valve 70 in order
to control air flow into the air preheater 47 and the gas furnace 45.
Preferably, the hot air
is supplied to the floor 20 from the gas furnace 45 at a pressure from greater
than 0 to
about 3 psi (0.0207 MPa). At these pressures, the metal media will pass heated
air into the
particulate matter on the fluidized bed conveyor 21 at approximately 0 to
about 10 cubic
feet (0.28 cubic meters) per minute. One advantage to using flowing air is
that more rapid
heating of the particulate matter and a resulting faster release of mercury
occurs. Mercury
CA 02487843 2004-11-16
13
is liberated from the particulate matter by heats of at least 700 F (372 C),
and when the
sorbent is activated carbon, it is preferred that the mercury be liberated
from the particulate
matter by heats in the range of 700 F (372 C) to 1000 F (538 C). The mercury
is carried
away from the particulate matter by the air. The liberating air also provides
the motive
force or fluidization that allows the particulate matter to move within the
treatment bed.
The particulate matter should be retained on the fluidized bed conveyor 21
until it
reaches a minimum temperature of 700 F (372 C). A blocking means such as a
weir or a
dam 25 is used to retain the particulate matter in the insulated heating
chamber 17. As the
particulate matter is fed into the beginning of the fluidized bed conveyor 21,
the fluidized
particulate matter level rises. As the particulate matter level rises, the
mercury-depleted
particulate matter at the exit area spills over the weir or dam 25 and drops
through conduit
31 into a heat recovery zone 27. A series of thermocouples 28 in electric
communication
with the programmable logic controller 29 may be used to control the inline
valve 66 that
controls air input to the rotary feeder 12 to control particulate matter input
to the insulated
heating chamber 17 in response to the measured particulate matter exit
temperature. As
the target temperature is reached by the particulate matter, more particulate
matter is fed
into the fluidized bed conveyor 21 via the rotary feeder 12. The programmable
logic
controller 29 uses data from the thermocouples 28 and a level probe 30 to
monitor and
control heat exchange rates, to control particulate matter feed rates from the
rotary feeder
12, to control treatment bed air flow from the gas furnace 45 (by control of
inline valve 70)
and to monitor particulate matter temperatures within the system.
An alternative to using a weir or dam 25 as the particulate matter outflow
blocking
means is the use of a gate to retain particulate matter in the insulated
heating chamber 17.
As the particulate matter in the insulated heating chamber 17 reaches the
designated
temperature as determined by the thermocouples 28 in electrical communication
with the
programmable logic controller 29, the programmable logic controller 29 causes
the gate to
raise allowing a portion of mercury-depleted particulate matter to exit the
fluidized bed
conveyor 21. The programmable logic controller 29 also causes particulate
matter to be
fed into the fluidized bed conveyor 21 via rotary feeder 12. The use of a gate
mechanism
aids in preventing incoming particulate matter from short circuiting the
fluidized bed
CA 02487843 2007-12-20
14
conveyor 21, allowing only heated particulate matter from the bottom of the
particulate
matter bed and in close contact with the incoming hot air, to exit the
insulated heating
chamber 17.
The heat recovery zone 27 is equipped with a bulk flow heat exchanger 32 which
transfers heat from the mercury-depleted particulate matter to the plates of
the preheater
via conduit 59 as described above. The mercury-depleted particulate matter
leaving the
system is cooled and may possibly be reused in the mercury removal process.
The upper
portion of the insulated heating chamber 17 is domed to provide a static
pressure area.
The gases from the insulated heating chamber 17 pass through conduit 37 into a
heat
10 recovery unit 35 in the form of a heat exchanger which transfers heat from
the gases to the
plates of the preheater 15 via conduit 57 as described above. The heat
recovery unit 35
cools the gases and provides heat to the particulate matter in the
pretreatment area via
preheater 15. After passing through the heat recovery unit 35 the gas and
entrained
mercury-depleted particulate matter then passes to a high temperature baghouse
36, which
15 receives heat from the gas furnace 45 via conduit 61, operated at a
temperature to ensure
that the mercury remains in a gaseous state thereby inhibiting reformation and
deposition
of the mercury on the particulate matter and equipment surfaces. The high
temperature
baghouse 36 receives air from the air supply unit 49 via conduit 67 and
captures fugitive
particles, and the mercury-depleted particulate matter collected by the high
temperature
baghouse 36 is transferred via conduit 33 to the heat recovery zone 27 where
the bulk flow
heat exchanger 32 transfers heat from the mercury-depleted particulate matter
to the plates
of the preheater 15 via conduit 59 as described above. After passing through
the bulk flow
heat exchanger 32, the mercury-depleted particulate matter is transferred to a
storage unit
42. The mercury-depleted particulate matter leaving the system is cooled and
can, under
certain circumstances, be reused in the mercury removal process. The remaining
gaseous
mercury also exits the high temperature baghouse 36 and may be transferred to
a mercury
condenser 38.
EXAMPLE
The following Example has been presented in order to further illustrate the
invention and is not intended to limit the invention in any way.
CA 02487843 2004-11-16
(A) OVERVIEW
The experiments described in this example focus on the liberation of mercury
from
a blend of activated carbon and fly ash. In the laboratory, the samples were
heated up
under the various temperatures from 260 C to 649 C (from 500 F to 1200 F) and
the
5 general desorption trend curves showed that more mercury can be removed with
higher
temperature and longer treatment of time. In the laboratory conditions, 80% of
mercury
can be liberated when the heating time is set to 5 minutes. A pilot study on
several
different samples was carried out afterwards. The mercury liberation
efficiency and mass
balance were analyzed in detail. The liberation efficiency for these samples
collected
10 under the air slide ranged from 74.44% to 85.57% separately.
(B) BACKGROUND
Several factors may influence the adsorption and desorption of mercury by
activated carbon. The adsorption ability of untreated activated carbon can be
affected by
temperature as demonstrated by the work of other researchers.. Lowering the
flue gas
15 temperature from 345 F to 250 F with direct injection of virgin activated
carbon was
shown to improve mercury removal efficiency from 0% to 37%. Further tests
showed that
virgin activated carbon injection at 200 F resulted in greater than 90%
mercury removal
(See, Sengupta, above.) Results from several tests indicated that
effectiveness of activated
carbon injection in removing mercury vapor also depends on the type and
composition of
burned materials, flue gas composition and temperature, mercury speciation,
activated
carbon properties and injection rate and operating conditions (See, Menon,
"Adsorption of
Mercury Vapor by Granular Activated Carbon", Master thesis, Utah State
University,
1991).
Because oxygen is readily chemisorbed by activated carbon to form carbon-
oxygen
complexes that are important in determining surface reactions and adsorptive
behavior, it
is necessary to study the possibility of carbon-oxygen complex formation
during the tests
conducted in the presence of oxygen and their impact on mercury removal. In a
set of
experiments, adsorption capacity in a variety of oxygen concentrations was
tested and the
test results are listed as Table 1 below. It shows that the oxygen can enhance
the
adsorption of mercury into the activated carbon.
CA 02487843 2004-11-16
16
Table 1- Effect of oxygen on the adsorption capacity of activated carbon
Concentrations (02) 0% to 3% 6% and 9%
Adsorption Capacity Unchanged 16 and 32%
In another series of experiments done upon SIAC (sulfur-impregnated activated
carbon), the effect of CO2 on the adsorption capacity was examined. When the
concentration of CO2 was increased from 5% to 15%, identical breakthrough
curves were
obtained, which indicated that COZ behaves like an inert gas and does not
affect the
performance of SIAC (see, Sengupta, above).
Sinha and Walker reported that sulfur-impregnated carbon exhibits faster
initial
breakthrough at room temperature than the virgin activated carbon due to the
reduction in
surface area induced by the impregnation process (see, Sinha and Walker,
Carbon 10:754-
756, 1972). However, at higher temperatures (302 F), the adsorptive capacity
of sulfur-
impregnated carbon greatly surpassed the capacity of virgin activated carbon
due to
chemisorptions of mercury and formation of mercuric sulfide (see, Sengupta,
above.)
Furthermore, they reported that water vapor reduces adsorption of mercury for
sulfur-
impregnated carbon.
Matsumura used steam-activated carbon surface on the removal efficiency for
mercury vapor (see, Matsumura, "Adsorption of mercury vapor on the surface of
activated
carbons modified by oxidation or iodization", Atmospheric Environment, 8:1321-
1327,
1974.) He concluded that oxidized or iodized activated carbon adsorbed mercury
vapor
20-60 times more than untreated activated carbon when exposed to mercury vapor
in
concentrations of up to 40 mg/m3 in a nitrogen stream at 86 F. Oxidized
carbons were
successfully regenerated with hydrochloric acid. Iodized activated carbons
were shown to
be suitable adsorbents for mercury vapor though adsorbed mercury was not
proportional to
the amount of iodine adsorbed on the carbon.
Teller and Quimby evaluated the performance of activated carbon impregnated
with copper chloride or sulfur for the removal of mercury under the conditions
CA 02487843 2004-11-16
17
representative of solid waste incinerators (see, Teller and Quimby, "Mercury
Removal
from Incineration Flue Gas", Somerville, NJ: Air and Water Technologies, Co,
1991.)
They concluded that moisture content of the carrier gas and temperatures
tested in their
study had no effect on the copper chloride-impregnated carbon's capacity for
mercury
removal. They also concluded that as the impregnate concentration increases
(for copper
chloride), mercury removal increases, but they were not able to correlate
these two
parameters. They observed that copper chloride-impregnated carbon exhibits as
much as
300 times higher capacity for mercury removal as compared to untreated
activated carbon.
Sulfur-impregnated carbon exhibited only a 60% improvement in the breakthrough
time.
While all these factors can influence the absorption of mercury by activated
carbon,
the effects of temperature and content of 02 were tested in the laboratory
tests and pilot
studies.
(C) EXPERIMENTS
Bench scale experiments were conducted to evaluate the mercury recovery rate
from activated carbon under various operating conditions. Since adsorption is
an
exothermal process, it is expected the adsorptive capacity of activated carbon
will decrease
significantly as temperature increases, which means higher mercury recovery
rate. In the
meantime, carbon loss is likely to increase considerably as temperature goes
up. Thus, it is
essential to select an optimum combination of temperature and detention time,
that takes
into consideration both the mercury recovery rate and carbon regeneration
rate.
Samples containing both activated carbon sorbent and fly ash were taken from
the
Presque Isle Power Plant (PIPP) of Wisconsin Electric Power Company,
Wisconsin, USA
and used in the bench scale studies. The mercury concentration in the sample
before the
thermal treatment was 0.512 ppm. Samples were treated under different
temperatures
ranging from 500 F to 1200 F (from 260 C to 649 C) in an oven for 5 to 6
seconds. The
tests were done in a pure nitrogen environment. Figure 4 shows the efficiency
of mercury
desorption from the activated carbon and fly ash mixture as the temperature
increases.
Mercury started to be liberated from the samples at around 700 F and the
mercury
removal efficiency stayed around 30% from 900 F to 1200 F. The retention
time of 5 to
6 seconds was chosen to preserve as much carbon as possible. An 80% removal
efficiency
CA 02487843 2004-11-16
18
was obtained at 1100 F after the sample retention time was lengthened to 5
minutes as
shown at point C in Figure 4. However, it was suspected that most of the
carbon was lost
during the elongated heating process. Since the retention time plays an
important role on
the treatment process, another set of bench scale experiments were carried out
to test the
effect of retention time on mercury removal efficiency. In these experiments,
samples
from PIPP, with an original mercury concentration of 0.42 ppm, were heated in
the pure
nitrogen gas filled oven for retention times ranging from one to five minutes.
The
operating temperature of the oven varied from 700 F to 1000 F. As shown in
Figure 5,
more mercury can be removed with higher temperature and longer treatment.
Based upon the test results obtained from the bench scale experiments, a test
program was designed to generate experimental data from a pilot scale
apparatus according
to the invention. Four batches of samples from three different power plants
were tested.
These samples were obtained from Presque Isle Power Plant (PIPP) of Wisconsin
Electric
Power Company, Wisconsin, USA, the Valley Power Plant (VAPP) of Wisconsin
Electric
Power Company, Wisconsin, USA, and the Pleasant Prairie Power Plant (PPPP) of
Wisconsin Electric Power Company, Wisconsin, USA. Table 2 lists the analysis
results
for these samples:
Table 2
Experiment Sequence 15` 2nd 3rd 4th
Sample Description PIPP PPPP VAPP PIPP(l) PIPP(2)
Samples collected Hg Content (ppm) 0.18 0.97 0.20 0.17 0.15
before Experiment Loss on ignition (%) 26.7 3.2 33.5 25.0 21.7
Hg content (ppm) 0.046 0.14 0.031 0.18 0.031
Samples collected
under the air slide Hg Removed (%) 74.44 85.57 84.50 -5.88 79.33
Loss on Ignition (%) 38.1 9.8 36.9 33.5 26.1 =
Hg content (ppm) 0.38 1 0.38 0.32 0.32
Samples collected
under the Bag house Hg Increased (%) 111.11 3.09 90.00 88.24 113.33
Loss on Ignition (%) 22.6 10.5 26.9 27.4 22.0
Before the test, the initial temperature for the inlet of the air slide is
1000 F. Loss
on Ignition is used to measure the carbon content in the samples, where we
assume that all
the loss of the mass is due to carbon combustion.
CA 02487843 2004-11-16
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For the first 3 runs of the experiment, since the mercury concentration for
the
samples collected under the slide decreased from the initial concentration, we
can assume
that mercury has been separated from the fly ash. However, we noticed that the
mercury
content for those samples collected under the bag house become even higher.
This might
be due to the lower temperature of the bag house, which is cooled down by the
cooling
water that flows through.
Under this consideration, in the last experiment, the cooling water was cut
off to
increase the temperature of the bag house. We had expected lower
concentrations of
mercury in both collectors under the air slide and the bag house. However, in
the 4th run
of the test, we got the abnormal results as shown in the Table 3. One possible
explanation
is that the sample collected under the air slide was contaminated by the
samples from
VAPP, which was tested before the 4th run.
For the first 3 runs of the tests, cooling water was used to cool down the
baghouse.
According to recorded data, the highest temperature for the baghouse of among
these tests
was around 300 F. The baghouse temperature was increased to 600 F in the 4th
comparison test, assuming more mercury could be liberated from the sample at a
higher
temperature. We noticed that for sample PIPP, the mercury removal efficiency
for the
sample below the air slide increased from 74.44% to 79.33, while the mercury
content for
the sample collected under the baghouse jumped from 111.11% to 113.33%. This
is
inconsistent with the original assumption that the higher temperature in the
baghouse will
liberate more mercury. On the other hand, it further proves the explanation
that some part
of mercury desorbed under the airside was reabsorbed onto the sample under the
baghouse.
For the last two experiments, the weights of the samples were recorded to
calculate
the mass balance of mercury.
Table 3 - Recorded weights of all samples
Sample's Weight (lb) VAPP (3T run) PIPP (4t run)
Before the test (Total) 16.5 18.9
After the test Under the slide 14.5 9.9
CA 02487843 2004-11-16
Under the bag house 4.3 1.64
Table 4 shows the mass balance analysis on mercury and carbon for the VAPP
sample:
Table 4
Sample weight (lb) Hg content (ppm) Total Hg (10' lb)
Before the test 16.5 0.20 3.3
Under the air slide 14.5 0.031 0.4495
Under the bag house 4.3 0.38 1.634
5
Mercury desorbed = (Total Mercury before the test)
- (Total mercury under the air slide + Total mercury under the bag
house)
= (3.3-(0.4495+1.634)) x 10"61b
10 =1.2165 x 10-61b
Percentage of mercury removed = Mercury desorbed (lb) x 100%
Total Mercury content before the test (lb)
=1.2165 x 10'Zb x 100
3.3 x 10-b lb
=36.86%
Table 5
Sample's weight Loss on Ignition (%) Total Carbon
(lb) (lb)
Before the test 16.5 33.5 5.5275
Under the air slide 14.5 36.9 5.3505
Under the bag house 4.3 26.9 1.1567
The temperature change for the system was recorded by a data logger and a
laptop.
Five thermal couples were installed in the system. They are connected to the
burner; bag
house; and inlet, midway and outlet of the air slide. The temperature changes
for the test
CA 02487843 2007-12-20
21
on the PIPP sample showed a temperature drop that was observed at the air
slide as sample
was fed into the system.
Thus, there has been provided a method and apparatus for removing adsorbed
mercury from a sorbent, such as activated carbon, collected separately or
collected with fly
ash in the exhaust gas treatment process in a coal-fired power plant.
Although the present invention has been described in considerable detail with
reference to certain embodiments, one skilled in the art will appreciate that
the present
invention can be practiced by other than the described embodiments, which have
been
presented for purposes of illustration and not of limitation. Therefore, the
scope of the
appended claims should not be limited to the description of the embodiments
contained
herein.
