Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MAGNETIC ADSORBENTS, METHODS FOR MANUFACTURING
A MAGNETIC ADSORBENT, AND METHODS OF REMOVAL OF
CONTAMINANTS FROM FLUID STREAMS
BACKGROUND
Amongst the numerous hazardous air pollutants (HAPs) currently regulated by
the EPA,
mercury and mercury-containing compounds have been a source of significant
concern due to
their increasing rate of release and the lack of adequate control
technologies. Although the
resulting quantity in the environment is usually low, it can transfer to
various organisms, and
then magnify up the food chain. For example, the concentration of accumulated
mercury in
some fish can reach levels that are millions of times greater than that in the
water. The
consumption of such fish by humans, and the resulting buildup of mercury in
various tissues
may lead to serious neurological and developmental effects such as losses of
sensory or
cognitive ability, tremors, inability to walk, convulsions, and even death.
Methylmercury, the
most common form of organic mercury, is almost completely incorporated into
the blood
stream, and can be transferred through the placenta and into all of the
tissues of the fetus,
including that of the brain. Because of the health concerns related to eating
mercury
contaminated fish, bans on fishing in certain regions such as in the Great
Lakes have resulted
in considerable losses to the economy.
The EPA has estimated that nearly 87% of the anthropogenic mercury emissions
are from
sources such as waste (as in waste-to-energy facilities) and fossil fuel
combustion (as in coal-
fired power plants). Recognizing this, control technologies have been employed
in an effort
to capture and dispose of the mercury found in combustion exhaust gases.
Currently,
Powdered Activated Carbon (PAC) injection into the flue gas stream is the best
demonstrated
control technology for mercury removal. The demand for Powdered Activated
Carbon for
Mercury capture is expected to grow to approximately 260,000 tons per year
based on
Freedonia estimates when the regulations for mercury removal, based on the
Mercury and Air
Toxics Standards, are implemented in the year 2015. The increased
implementation will
exact a significant economic burden on regulated facilities. Currently,
brominated activated
carbons have been shown to have the highest mercury removal rate per pound of
product.
However, these products have a higher cost margin ¨ and therefore would
increase the
economic impact ¨ and may cause corrosion of plant equipment. Furthermore,
PAC's
generally low mercury adsorption efficiency and lack of adequate regeneration
technologies
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have sparked an interest in modifying the material to either decrease costs or
improve
performance.
Another shortcoming in using PAC injection systems is the accumulation of the
waste PAC
in the fly ash. Fly ash, the fine particulate fraction of the Coal Combustion
Byproducts
(CCBs) (i.e., noncombustible inorganics and uncombusted carbon), is collected
from flue gas
and then commonly sold for beneficial reuse in the production of concrete and
other
materials. Replacing lime, cement, or crushed stone materials that are
typically used in
construction materials with fly ash can conserve energy and resources, while
providing an
alternative to landfill disposal of the waste. However, when typical fly ash
collection devices
are coupled with PAC injection systems, the quality of the collected fly ash
deteriorates
because of the large fraction of carbon in the ash. Such fly ash cannot be
resold for beneficial
reuse and must, instead, be landfilled. Current research geared towards
separation
technologies has yet to find an adequate method to isolate the PAC from the
fly ash.
Therefore, a method that can easily separate PAC from the fly ash is
desirable. Such a
method will (a) maintain the quality of the fly ash for subsequent sale and
reuse, and (b)
permit the reuse of the PAC for additional mercury capture.
U.S. Patent No. 7,879,136 teaches a method to recover PAC from fly ash by
creating a
magnetic activated carbon through a wet precipitation method. This method is
similar to US
Patents 2,479,930, 6,914,034, and 8,097,185B2, which also teach wet methods
using iron
precursors to make a magnetic activated carbon. Others have created magnetic
adsorbents by:
combining the sorbent with a magnetic material using a binder (US Patent No.
7,429,330),
mixing a sorbent with a magnetic material (US Patent No. 4,260,523), or mixing
a magnetic
material with an organic material, followed by activation (US Patents
4,260,523, 4,201,831,
and 7,429,330).
While the methods described by the referenced patents serve well for bench-
scale
applications, they introduce challenges for full-scale production including,
but not limited to,
high energy costs. Therefore, dry production methods are critical in
translating the magnetic
activated carbon technology to full-scale. The use of physical methods to
create a co-mingled
product for mercury capture has been taught by US Patent No. 8,057,576 ("`576
Patent").
The product is an admixture of an adsorptive material and an additive that
either complexes
with the mercury, oxidizes it, or both. The additive is not implanted on the
adsorbent surface.
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This is highlighted by the fact that one embodiment of the patent teaches
injecting the
adsorbent and additive separately into the flue gas. The invention of the '576
Patent does not
include a magnetic additive, and therefore the product cannot be magnetically
recovered from
fly ash.
SUMMARY
By physically implanting magnetic additives on the adsorbent through dry
production
methods a cost-effective manner in which an adsorbent for mercury capture can
be generated
at a large scale.
Disclosed herein are processes to manufacture a magnetic adsorbent, a method
using the
.. magnetic adsorbent for the removal of contaminants from fluid streams, and
the recovery of
the magnetic adsorbent after use.
More specifically, a magnetic adsorbent with sufficient oxidizing power,
affinity, and surface
area for the capture of mercury from the flue gas of coal combustion devices
is provided.
This material may also be applied for the capture of other target contaminants
such as arsenic
and selenium. The magnetic adsorbent can then be recovered from the coal
combustion
flyash, and re-injected into the flue gas for additional mercury capture.
A method of manufacturing the magnetic adsorbent involves combining the
selected
adsorbent with a magnetic additive and in some cases an oxidizing additive.
The precursor
adsorbent may be an activated carbon, reactivated carbon, silica gel, zeolite,
alumina clay, or
other solid material with sufficient surface area for mercury capture. The
magnetic additive is
preferably one of the following: magnetite, hematite, goethite, or maghemite.
The oxidizing
additives may include, but are not limited to halides of alkali metals,
alkaline earth metals,
and ammonium (i.e., NH4Br, KBr, LiBr, NaBr, NaCl, KC1, LiC1, KI, LiI, Nal),
and
semiconductors (TiO2, ZnO, Sn02, V02, and CdS).
A method for the manufacture of a magnetic adsorbent (Magnetic Adsorbent
Creation
Method), its application to remove contaminants from a fluid stream, and its
recovery after
use is provided.
A method to manufacture the magnetic adsorbent, involves combining the
selected adsorbent
with a magnetic additive and in some cases additional additives to improve the
oxidation
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capacity. The additives may be implanted on the adsorbent using a variety of
means,
including but not limited to: mixing, milling, or grinding the adsorbent and
the additives
together until some fraction of the material is physically implanted on the
surface of the
activated carbon.
Furthermore, a method for removing a contaminant or contaminants from a fluid
stream is
provided (Contaminant Removal Method). The method includes contacting the
fluid stream
with the magnetic adsorbent whereby the contaminant is adsorbed on the
magnetic adsorbent,
and then removing the magnetic adsorbent having the contaminant adsorbed
thereon from the
fluid stream.
Also provided is a method to recycle the collected composite back into contact
with the fluid
stream for further contaminant removal (Composite Recycling Method).
In one embodiment, a magnetic adsorbent includes an adsorbent and iron oxide
implanted
onto a surface of the adsorbent, wherein a total surface area of the magnetic
adsorbent is not
substantially less than a total surface area of the adsorbent. Optionally, the
adsorbent is
activated carbon. In one configuration, an additive selected from the group
consisting of a
halogen, a photocatalyst, and a binder is added. In one alternative, the
magnetic adsorbent
does not include secondary deposits. Optionally, a ratio of the weight of the
iron oxide to a
total weight of the magnetic adsorbent is between 1% to 20%. In one
configuration, a ratio of
the weight of the iron oxide to a total weight of the magnetic adsorbent is
between 5% to
.. 15%. Alternatively, a ratio of the weight of the iron oxide to a total
weight of the magnetic
adsorbent is 10%. Optionally, the iron oxide is highly crystalline after
implantation. In one
configuration, a crystalline nature of the iron oxide is maintained after
implantation.
In another embodiment, a magnetic adsorbent consists essentially of an
adsorbent; and an
iron oxide implanted onto the surface of the adsorbent, wherein a surface area
of the magnetic
adsorbent is not substantially less than the surface area of the adsorbent.
In one
configuration the adsorbent is activated carbon.
In one embodiment, a method of making a magnetic adsorbent includes combining
an
adsorbent and a magnetic material using mechanical mixing equipment.
Optionally, the
adsorbent is activated carbon and the magnetic material is iron oxide. In one
alternative, the
mechanical mixing equipment is selected from the group consisting of a ball
mill, a jet mill,
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and a conical mill. In another alternative, the mechanical mixing equipment
encourages
friction and collision between particles. Optionally, the method includes
implanting the
magnetic material on the surface of the adsorbent. In another alternative, the
combining
includes grinding and is performed until the magnetic adsorbent will pass
through a 325-
5 mesh sieve.
In one embodiment, a method of treating an effluent stream includes treating
the effluent
stream by injecting magnetic adsorbent particles and using magnetic field to
recover the
magnetic adsorbent particles. Optionally, the magnetic adsorbent particles are
re-injected
into the effluent stream with additional magnetic adsorbent particles after
recovery.
Optionally, the magnetic adsorbent particles remove mercury from the effluent
stream.
In another embodiment, high quality fly ash is recovered from a stream treated
by a magnetic
adsorbent. The method includes capturing the magnetic adsorbent from fly ash
in the effluent
stream using a magnetic field to generate two products: (1) high quality fly
ash, and (2)
recovered magnetic adsorbent.
In one embodiment, a system for removing mercury from an effluent system
includes an
activated carbon injection system, injecting an activated carbon product into
an effluent. The
system further includes a first electrostatic precipitator positioned after
the activated carbon
injection system, receiving the effluent. Optionally, the first electrostatic
precipitator is
positioned immediately following the activated carbon injection system,
without any
intervening treatments. In one alternative, the activated carbon product is
magnetic. In
another alternative, the activated carbon product includes a photocatalyst.
Optionally, the
first electrostatic precipitator activates the photocatalyst. Alternatively, a
second electrostatic
precipitator immediately precedes the activated carbon injection system.
Optionally, the
activated carbon product has iron oxide implanted on the surface and has a
surface area that is
not substantially less than the surface area without the iron oxide.
In another embodiment, an adsorbent includes an activated carbon portion and a
magnetic
portion joined with the activated carbon portion, wherein magnetic activity of
the magnetic
portion is not shielded by the activated carbon portion. Optionally, the
magnetic portion is
implanted on the surface of the activated carbon portion. Alternatively, a
total surface area of
the activated carbon portion without the magnetic portion is substantially at
least the same as
a total surface area of the adsorbent.
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In one aspect, the present invention provides a magnetic adsorbent for
removing mercury from
a contaminated flue gas stream comprising: an adsorbent; an iron oxide and an
additive
implanted onto a surface of the adsorbent, wherein the additive is a halogen
or halide, the
halogen or halide enhancing oxidation of the mercury in the stream, wherein at
least the iron
oxide is implanted on the adsorbent by surface deposition; wherein the
magnetic adsorbent is
dry and during use is placed into contact with the stream by pneumatic
injection, wherein the
implanted iron oxide has a crystalline structure identical to the crystalline
structure of iron oxide
in maghemite, hematite, or magnetite.
In another aspect, the present invention provides a magnetic adsorbent for
removing mercury
from a contaminated flue gas stream, consisting essentially of: an adsorbent;
an iron oxide and
an additive implanted onto a surface of the adsorbent, wherein the additive is
a halogen or
halide, the halogen or halide enhancing oxidation of the mercury in the
stream; wherein at least
the iron oxide is implanted on the adsorbent by surface deposition; wherein
the magnetic
adsorbent is dry and during use is placed into contact with the stream,
wherein the implanted
iron oxide has a crystalline structure identical to the crystalline structure
of iron oxide in
maghemite, hematite, or magnetite.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows an SEM image and EDS map of the titania and iron signals of the
activated
carbon adsorbent, demonstrating the absence of additives on the adsorbent
surface;
.. Figure 2 shows an SEM image and an EDS map of the iron signal of a magnetic
adsorbent
prepared by milling an activated carbon material with magnetite at a loading
of 10% by weight,
demonstrating the distribution of magnetite throughout the sample, on its
surface;
Figure 3 represents a spot EDS analysis on a particle isolated in Figure 2,
demonstrating the
clear presence of iron on the adsorbent;
Date Recue/Date Received 2020-10-09
81784444
6a
Figure 4 represents an SEM image and an EDS map of the iron and titania signal
of a
magnetic adsorbent prepared through ball-milling an activated carbon adsorbent
with
TiO2 and magnetite at 1% and 10% loading by weight, respectively. The image
demonstrates
the wide distribution of magnetite, as well as the presence of titania, on the
adsorbent surface;
Figure 5 represents a spot EDS analysis on a particle isolated in Figure 4,
demonstrating the
clear presence of iron on the adsorbent;
Figure 6 represents a spot EDS analysis on a particle isolated in Figure 4,
demonstrating the
clear presence of titania on the adsorbent;
Figure 7 represents an SEM image and an EDS map of the iron signal of a
magnetic adsorbent
prepared with magnetite, at a loading of 10% by weight, via mechanofusion. The
figure
demonstrates the distribution of magnetite throughout the sample, on its
surface;
Figure 8 represents a schematic of the bench scale apparatus used to collect
the fixed bed data
presented herein;
Figure 9 represents the mercury removal curve during a fixed bed evaluation
with various
magnetic adsorbent materials, demonstrating the benefit of additives for
mercury removal;
Figure 10 represents the mercury removal curve during actual flue gas
conditions with the
base activated carbon and the produced magnetic adsorbent, demonstrating the
benefit for
mercury removal;
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Fig. 11a and lib show embodiments of an activate carbon injection system up
stream of an
electrostatic precipitator (ESP);
Fig. 12 shows Mercury removal curve for activated carbon injection in a 5 MW
slip stream
flue gas for a PAC readily available for commercial purchase in the industry
(Industry
Carbon), a MPAC coated with 10% Fe304 (MPAC), with 10% Fe304 and 1% TiO2 (MPAC-
TiO2), and another with 10% Fe304 and 2.5% NaBr (MPAC-Na-Br) by weight.
DETAILED DESCRIPTION OF THE DRAWINGS
In one embodiment of a Magnetic Adsorbent Creation Method, magnetic adsorbent
composites are prepared, whereby a magnetic material is physically implanted
onto the
exposed surface of an adsorbent. The implantation may be achieved by
simultaneously
combining the adsorbent and iron oxide together and using mechanical mixing
equipment
such as a ball mill, jet mill, conical mill, etc. This mixing environment
encourages friction
and collision between the particles to promote implantation. Forces for
implantation may
include Van der Waals Forces, capillary forces, electrical forces and
electrostatic coulomb
forces. These forces may be promoted during the mixing process.
Surface implantation is an important feature of the magnetic adsorbent
created, in contrast to
some prior art adsorbents where the magnetic material is implanted within the
adsorbent;
implantation on the surface does not shield or block the magnetic forces from
acting on the
magnetic material. This feature provides for the recapture and recycling of
magnetic
adsorbent since magnetic forces may be applied to recapture it after
treatment. This greatly
improves the cost effectiveness of the methods and materials described herein.
In various
places herein the implantation of magnetic materials is discussed. Significant
variation of the
amount and type of magnetic material implanted is contemplated and may be
related to the
implantation techniques used and described herein.
The adsorbent material for the creation of a magnetic adsorbent will have an
appreciable
surface area and developed porosity. It can be: activated carbon, reactivated
carbon, zeolite,
alumina clays, silica gels, etc. For many applications the adsorbent is
activated carbon. The
term "activated carbon" as used herein is meant to reference powdered or
granular carbon
used for purification by adsorption. in many configurations the activated
carbon used has a
surface area between 200 and 1,000012/g, more preferably between 300 and
700m2/g, and
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most preferably between 400 and 600m2/g. In some alternatives, powder
activated carbon
(PAC) is used. For this application, the term "Powdered Activated Carbon"
refers to an
activated carbon, 90% of which passes through a 352-mesh sieve (45 m) (i.e.,
at least 90%
passes through a 352 mesh). Also, the following abbreviations may be used
herein: Activated
Carbon: AC; Powdered Activated Carbon: PAC; and Magnetic Powdered Activated
Carbon:
MPAC.
The magnetic material may be at least one of the following: magnetite (Fe304),
maghemite
(7-Fe203), hematite (a-Fe201) and goethite (Fe0(OH)); and in many embodiments
magnetite.
The amount of magnetic material in the composite is preferably between at
least 1% and less
than 20% by weight based on the total weight of the final composite; more
preferably
between 5% and 15% by weight based on the total weight of the final composite;
most
preferably 5% by weight based on the total weight of the final composite.
Figs. 1-3 show the results of a composite made as described above. Fig. 1
shows an SEM
image and EDS map of the titania and iron signals of the activated carbon
adsorbent at 500
times magnification, demonstrating the absence of additives on the adsorbent
surface
according to one embodiment. In Fig. 1 an SEM image and an EDS (Energy-
dispersive X-
ray spectroscopy) map show the iron signals appearing on the activated carbon.
Fig. 2 shows
a sample created according to the milling with activated carbon described
above. Fig. 2
shows an SEM image and an EDS map of the iron signal of a magnetic adsorbent
at 500
times magnification prepared by milling an activated carbon material with
magnetite at a
loading of 10% by weight, demonstrating the distribution of magnetite
throughout the
sample, on its surface. In Fig. 3 a graph showing the occurrence of primarily
Fe 330 on the
activated carbon is shown as compared to other contaminates. Fig. 3 represents
a spot EDS
analysis on a particle isolated on Fe-particle in ball milled sample (FeOx),
demonstrating the
clear presence of iron on the adsorbent. The graph shows the counts 310 vs.
energy (keV)
320. Fig. 1 shows an SEM image and EDS map of the titania and iron signals of
the
activated carbon adsorbent at 500 times magnification, demonstrating the
absence of
additives on the adsorbent surface according to one embodiment;
Additionally, other additives, such as oxidizers, photocatalysts, and binders,
may be applied.
Oxidizing additives may be selected from halides of alkali metals, alkaline
earth metals, and
ammonium (i.e., NH4Br, KBr, LiBr, NaBr, NaCl, KC1, LiC1, KI, LiI, NaI) and
photocatalysts
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(i.e., TiO2, ZnO, V02, Sn02, and CdS). Some oxidizing additives and
photocatalyts may also
act as a binder, encouraging the magnetic additives to adhere to the adsorbent
surface. Other
separate binders may also be applied (i.e., Binders). In many embodiments
described herein,
the adsorbent is activated carbon, the magnetic material is magnetite, and the
oxidizing
and/or binding additives are NaBr and/or TiO2. The amount of additional
additive material in
the composite is preferably between at least 0.1% and less than 10% by weight
based on the
total weight of the final composite; more preferably between 0.5% and 5% by
weight based
on the total weight of the final composite.
Additional features of embodiments of magnetic adsorbent created include
unique iron oxide
concentration, the crystalline nature of the iron oxide included, the absence
of secondary
deposits or byproducts on the surface, the impact on the physical
characteristics of the
magnetic adsorbent, and the additives that may be added. In some embodiments,
the iron
oxide concentration of the magnetic adsorbent produced is between 1% and 20%
by weight,
more preferably between 5% and 15% by weight, most preferably 10% by weight.
By using
magnetic additives such as (maghemite (7-Fe203), hematite (a-Fe2O3) and
magnetite (Fe304))
that are already crystalline in nature, the Magnetic Adsorbent Creation Method
produces an
adsorbent that maintains the crystalline structure of the magnetic material.
This crystallinity
is likely greater than that of materials produced via wet chemistry methods.
Further, since
heat treatments are not necessary in the Magnetic Adsorbent Creation Method,
the crystalline
nature is not degraded.
The occurrence of secondary deposits is also reduced Or eliminated by the
Magnetic
Adsorbent Creation Method. In contrast, wet chemistry methodologies may
include reactants
that leave byproducts and interact with the adsorbent or iron oxide. The
Magnetic Adsorbent
Creation Method further does not erode the pore volume or pore size of the
magnetic
adsorbent and may result in a slight measurable increase in total surface area
caused by
interstitial spaces created by the adhered particles on the surface of the
activated carbon
adding to the available surface area. In many wet chemistry methodologies the
deposition of
iron oxides may degrade the surface area, pore size, and pore volume. The
magnetic
adsorbent can be treated with a halogen, a photocatalyst, or a binder to
further enhance the
mercury oxidation and therefore adsorption and removal from the contaminated
stream.
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By adding a known magnetic species, the magnetic strength is controlled and
deposited on
the surface of the adsorbent. Surface-deposition of the magnetic material
allows magnetic
forces for recovery to be maximized. Further, the speciation and crystallinity
of the magnetic
material is not altered by production, thereby protecting its magnetic
properties. This is in
5 contrast to those methods that deposit the magnetic material within the
sorbent, where the
sorbent material itself can mask the magnetic forces and hinder recovery.
Additionally, those
methods that teach magnetic doping of a sorbent precursor followed by
activation will likely
face difficulties controlling the speciation and crystallinity, and therefore
the magnetic
properties, of the magnetic compounds.
10 Once production is complete, the material can be applied for contaminant
removal in a fluid
stream. While the said material has the potential to be effective for various
contaminants in a
myriad of fluid streams, it is known to be effective for the contaminant
mercury and the fluid
stream of flue gas. In this representation, the material is removed from the
flue gas by typical
particle collection devices in operation, such as electrostatic precipitators,
fabric filters,
cyclones, and even scrubbers. It will be appreciated by those skilled in the
art that although
embodiments are described in connection with the removal of mercury from flue
gas,
embodiments not limited to the removal of mercury from flue gas and may be
used to remove
other heavy metals including, but not limited to, arsenic, selenium, and
boron.
After the composite is separated from the fluid stream and collected, it can
be recovered and
reused. The recovery utilizes the magnetic properties of the material. Using
the above
scenario as an example, the magnetic material is collected in an electrostatic
precipitator with
other flue gas particles (fly ash). A magnetic recovery system is applied
after the electrostatic
precipitator collection to separate the magnetic material from the fly ash.
The magnetic
material is then stored for reuse. Additionally, before reuse, the material
may be regenerated
using chemical or thermal techniques. The material may then be reapplied for
further
contaminant removal from the fluid stream. Utilizing this technique results in
significant cost
savings for the user and reduces the quantity of waste materials.
In one embodiment the composite is treated with a halogen known for oxidizing
Hg. In this
regard, the halogenated composite may be formed by (i) mechanically mixing a
halogen
compound, a magnetic material and adsorbent; (ii) exposing the composite of
adsorbent and
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magnetic material to a halogen gas; or (iii) reacting the magnetic material
and a halogen, then
co-milling the resultant with adsorbent.
In another embodiment a photocatalyst, for example, titanium dioxide (TiO2),
is included in
the magnetic adsorbent. Hydroxyl radicals can be generated on the surface of
TiO2 in an
excited state; these powerful oxidants enhance mercury capture by oxidizing
elemental Hg to
form, for example, Hg0. The oxidized mercury (e.g., Hg0) can then serve as
additional
sorption sites for elemental Hg, increasing mercury capture as a whole.
Furthermore, as the
adsorbent is re-injected for mercury capture, the gradual buildup of Hg0 on
the sorbent may
improve mercury uptake over the injection cycles. In those scenarios where
electrostatic
precipitators (ESP) are used for particulate capture, the energy required to
excite TiO2's
electrons to generate hydroxyl radical formation is provided by the ESP
itself. For bag house
installations, UV lamps generating wavelengths less than about 365 nm would be
required to
provide the required energy for TiO2 excitation. As would be recognized by one
skilled in
the art, UV radiation includes invisible radiation wavelengths from about 4
nanometers, on
the border of the x-ray region, to about 380 nanometers, just beyond the
violet in the visible
spectrum.
Figs. 4-6 show the results from preparing a ball milled sample as described
above with
Ti02/Fe01. Fig. 4 represents an SEM (Scanning Electron Microscope) image and
an EDS
map of the iron and titania signal of a magnetic adsorbent (at 500 times
magnification)
prepared through ball-milling an activated carbon adsorbent with TiO2 and
magnetite at 1%
and 10% loading by weight, respectively, demonstrating the wide distribution
of magnetite,
as well as the presence of titana, on the adsorbent surface. Fig. 5 represents
a spot EDS
analysis on a particle isolated on Fe-particle in ball milled sample
(TiO2/FeO),
demonstrating the clear presence of iron on the adsorbent. Fig. 5 demonstrates
the existence
of iron with peak 530. The graph shows the counts 510 vs. energy (keV) 520.
Fig. 6
represents a spot EDS analysis on a particle isolated on Ti-particle in ball
milled sample
(Ti02/Fe01), demonstrating the clear presence of titania on the adsorbent.
Fig. 6
demonstrates the existence of Ti with peak 630. The graph shows the counts 610
vs. energy
(keV) 620. Fig. 7 represents an SEM image and an EDS map of the iron signal of
a magnetic
adsorbent (at 500 times magnification) prepared with magnetite, at a loading
of 10% by
weight, via mechanofusion; demonstrating the distribution of magnetite
throughout the
sample, on its surface.
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The magnetic adsorbent will have a specific fraction of magnetized particles
depending on
the manufacturing technique. In some embodiments, this fraction is recoverable
from fly ash
or other non-magnetic particles from fluid streams (such as air and water).
The magnetic
recovery is achieved by passing a mixed particle stream through a magnetic
recovery device.
One example is using a design similar to an electrostatic precipitator (ESP)
with
electromagnets to collect the magnetic adsorbent while allowing the other
particles to pass
through the collection device. The recovered magnetic adsorbent can then be
regenerated or
reused, depending on the application. In flue gas treatment for mercury,
magnetic adsorbent
is separated from the other particles (fly ash) in the flue gas airflow. The
recovered, used
magnetic adsorbent would be mingled with fresh magnetic adsorbent, and then
injected again
for in-flight mercury capture. This has the added benefit of improving the
quality of the
flyash for potential salability. Fig. 1 1 a and lib show two embodiments of an
activated
carbon injection system positioned before and between an ESP. Boiler 1110
feeds to a
selective catalytic reduction system 1115. Then the effluent flows to an air
heater 1120. At
this point, activated carbon is injected from ACT 1125. The activated carbon
then passes
through the ESP 1130 which produces an electrostatic discharge, which, in some
embodiments, can excite the properties of the activated carbon to result in
enhanced mercury
removal. Finally, the flue gas passes through the flue gas desulfurization
1135 and out of
exhaust stack 1140. In Fig. 1 lb a first ESP 1130 precedes the injection of
activated carbon
and a second ESP 1132 follows. The injection may also occur before both ESPs.
Other
instances would exclude the use of a selective catalytic reduction system 1115
or a flue gas
desulfurization 1135. The activated carbon in many of these cases, as
described above,
includes a photocatalyst. The advantage of this system is that similar results
may be achieved
as compared to an activated carbon system with a fabric filter, without the
same pressure drop
as would be experienced with a fabric filter. In some cases an ACT system and
an added ESP
system may be used to retrofit existing plants. As mentioned above, this
system may provide
synergy with the magnetic adsorbent for mercury removal but be less costly
than other well
established retrofits known by those skilled in the art, such as a fabric
filter installation for
AO, and may be easily integrated into existing systems. In some
configurations, the
positioning of the ESP in a typical system may enhance the activity of the
activated carbon as
compared to systems injecting activated carbon at an earlier point in the
system. Typically,
ESP systems are located near the end of an effluent cleaning system as shown
in Figs. 1 I a
and 1 lb. Therefore, activated carbon may be injected immediately before the
ESP and may
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have higher effectiveness since the effluent will have cooled significantly by
that point in the system.
Also, fewer other constituents may exist in the effluent immediately before
the ESP, therefore allowing
the activated carbon to work primarily to remove Mercury. The specific
configuration of the system will
determine the exact operating parameters and removal capabilities. In some
embodiments halogens may
be substituted for the photocatalyst.
Example 1
Preparation of Activated Carbon/Iron Composite
A magnetic activated carbon sample with a 10% by weight concentration of
magnetite (Fe304) was
prepared by simultaneously grinding 9g of activated carbon with lg of
magnetite in a ball mill. Grinding
continued until 90% of the final product would pass through a 325-mesh sieve.
A virgin product was
also prepared using the same activated carbon, but with no additive, milled to
the same specification.
Hg Removal
Figure 8 presents the bench-scale test stand that was used to quantify the
adsorption capacity of the
inventive MPAC.
Air 815 and High-grade nitrogen gas 861 were passed through mass flow
controllers 820, to control the
flow of air representing effluent into the system. The nitrogen gas 861 from
reservoir was passed
through an elemental mercury permeation tube 825 to create a mercury vapor
laden air with 10 ug/m3 of
Hg. The mercury vapor was then transported through a heating tube 830 to the
fixed-bed reaction
column. The temperature of the Hg gas was monitored and maintained at 150 C
upstream of the MPAC
835. The sorbent was evenly dispersed within a matrix of silica sand, and
supported on a quartz frit. The
temperature of the sorbent bed was monitored and maintained at 110 C using
heating tape. Effluent gas
from the sorbent bed was cooled using a series of impingers 840, 845 in a
water bath prior to monitoring
elemental Hg by an inline RA-915 Zeeman Mercury Spectrometer (Ohio Lumex) 855.
Effluent
concentrations of mercury from the stand were recorded for comparison of
composite PAC samples. A
carbon trap and exhaust system 860 collected remaining waste from the system.
The Hg adsorption capacities of the composite and the virgin counterpart were
quantified using the test
stand shown in Figure 8. Table 1 summarizes the test results. As shown, the
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addition of iron oxide produced a sorbent with greater Hg removal capacity.
Table 2 shows
the characteristics of the virgin AC and the composite product.
Table 1: Hg Removal Results for the Virgin AC and MPAC Products
Sorbent Loading
(mg Hg/g sorbent)
t = 30 s t = 1 min t = 5 min
Virgin AC 1.7 3.4 11.2
MPAC, 5% Fe304 7.8 16.5 39.2
Table 2: Characteristics of Virgin AC and MPAC Products
Sample BET Pore size Pore Vol. BJH P. Vol.
m2/g A cc/g cc/g
Base AC 382 10.6 0.20 0.05
MPAC, 5% Fe304 370 30.6 0.28 0.13
It is clear from the data in Table 1 that the iron oxide coating improved the
ability of the
sorbent to trap Hg from the air stream. This is likely attributable to the
iron oxidizing the
elemental Hg to Hg(II), which is more amenable for adsorption by activated
carbon.
Example 2
Preparation of Activated Carbon/Iron Composite
A magnetic activated carbon sample with a 10% by weight concentration of
magnetite
(Fe304) was prepared by simultaneously milling 18 lbs. of activated carbon
with 2 lbs. of
magnetite in a ball mill. Grinding continued until 95% of the final product
would pass
through a 325-mesh sieve. Two additional sorbents were made by adding
additional
oxidants. The first was prepared by simultaneously milling 18 lbs. of
activated carbon with
2 lbs. of magnetite and 0.2 lb. of TiO2 in a ball mill to the same size
specification as the first.
The second was prepared by simultaneously milling 18 lbs, of activated carbon
with 2 lbs. of
magnetite and 0.5 lb. of NaBr in a ball mill to the same size specification as
the first. A
fourth carbon was procured from a commercial activated carbon supplier
designed for the
mercury removal from flue gas application.
Following from the above examples, Fig. 9 represents the mercury removal curve
during a
fixed bed evaluation with various magnetic adsorbent materials, demonstrating
the benefit of
additives for mercury removal. The y-axis 910 shows the normalized mercury
concentration
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in C/Co. The x-axis 915 represents time in minutes. The results for various
additives are shown
including PAC 920, MPAC 930, PAC-Br 935, MPAC-Br 940, and MPAC-TiO2 945 in
accordance with
the various embodiments described herein.
Fig. 10 represents the mercury removal curve during actual flue gas conditions
with the base activated
5 carbon (injection in a 5 MW slip stream flue gas for a Base AC (PAC) 1030
and a MPAC 1040 coated
with 5% Fe304 by weight) and the produced magnetic adsorbent, demonstrating
the benefit for mercury
removal. The y-axis 1010 shows the total percentage of mercury removal and the
x-axis 915 shows the
injection rate in lb/MMacf.
Mercury Removal
10 The four products were tested at the Mercury Research Center (MRC). The
MRC removes a constant
flow of approximately 20,500 acfm of flue gas (representative of a 5 MW
boiler) from the Southern
Company Plant Christ Boiler (78 MW). The boiler runs on low-sulfur bituminous
coal blend from
varying sources. While typical SO3 concentrations of previous fuel blends
resulted in less than 1 ppm of
SO3, the current coal blend lead to SO3 concentrations between 2 - 3 ppm
downstream of the air heater
15 (AH). The products were pneumatically injected at 3, 5, and 7 lb/MMacf
injection rates upstream of the
electrostatic precipitator (ESP). Particulate removal was achieved with the
ESP. Mercury concentrations
were monitored at the MRC inlet and just downstream of the ESP and the
observed concentrations were
adjusted to 3% oxygen concentration for the purpose of standardization for
comparison. Total mercury
removal was calculated as the inlet mercury concentration (in ug/m3 at STP and
3% 02) minus the outlet
mercury concentration (in ug/m3 at STP and 3% 02) divided by the inlet and is
illustrated in Figure 12.
Compared to the commercially available activated carbon, the MPAC carbons
1241, 1242, 1251, 1252,
1253, 1261, 1262, 1263, show significant advantage in higher mercury removal
percentages. In the
figure showing the activated carbon injection in a 5 MW slip stream flue gas
the bars shown are as
follows: a PAC 1231, 1232, 1233, readily available for commercial purchase in
the industry (Industry
Carbon), a MPAC coated with 10% Fe304 (MPAC) 1241, 1242 with 10% Fe304 and 1%
TiO2 (MPAC-
TiO2) 1251, 1252, 1253, and another with 10% Fe304 and 2.5% NaBr (MPAC-Na-Br)
1261, 1262, 1263
by weight.
The previous detailed description is of a small number of embodiments for
implementing the
compounds and methods related to magnetic adsorbents, such as magnetic
activated carbon,
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and are not intended to be limiting in scope. The following claims set forth a
number of the
embodiments of the compounds and methods related to magnetic adsorbents, such
as
magnetic activated carbon, disclosed with greater particularity.
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