Note: Descriptions are shown in the official language in which they were submitted.
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
1
SYSTEMS AND METHODS FOR POST COMBUSTION MERCURY CONTROL
USING SORBENT INJECTION AND WET SCRUBBING
CROSS-REFERENCE TO RELATED APPLICATIONS
(0001] This application claims the benefit of priority
to U.S. Provisional Patent Application No. 62/481,916,
filed April 5, 2017, the entirety of which is
incorporated herein by reference.
TECHNICAL FIELD OF THE INVENTION
[0002] This invention relates, in general, to a system
for cleaning flue gas, and, in particular, to a system
and method for removing mercury with a powdered sorbent
injection prior to or into a wet scrubbing system.
BACKGROUND OF THE INVENTION
[0003] Without limiting the scope of the present
invention, its background will be described in relation
to systems and methods for post combustion mercury
control using sorbent injection and wet scrubbing, as an
example.
[0004] With the introduction of the first national
standards for mercury pollution from power plants in
December of 2011, many facilities will turn to sorbent
injection to meet the EPA Mercury and Air Toxics
Standards (MATS) requirements. Sorbent injection is a
technology that has shown good potential for achieving
mercury removal to the MATS standards.
[0005] While several sorbents are viable for sorbent
injection, activated carbon (AC) has been proven to the
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
2
largest extent. Activated carbon is a high surface area
sorbent typically created from the activation of coal (or
other material high in carbon content) in a controlled
environment to create a porous network. This porous
network and chemical activity of the AC can be
manipulated during activation/manufacturing to create an
AC that will preferentially adsorb certain contaminants
of concern (e.g., mercury from power plant flue gas to
meets MATS standards).
Additionally, post activation
treatment can be performed to enhance the chemical
reactivity of the AC for the target compound(s) of
interest. For sorbent
injection, the AC is ground and
sized to produce powdered activated carbon (PAC), most
typically to 95% passing the 325 mesh for mercury capture
from flue gas.
[0006] Many
efforts have been made to improve PAC
materials to increase the mercury capture potential and
thereby decrease the PAC loading to reduce materials
handling and cost burdens. For
example, U.S. Pat. No.
6,953,494 describes treating a carbonaceous substrate
with an effective amount of a bromine-containing gas;
U.S. Pat. No. 8,551,431 describes a sorbent with halogens
applied with washing; U.S. Pat. No. 8,057,576 describes a
dry admixture of activated carbon and halogen-containing
additive; and U.S. Pat. No. 8,512,655 describes a carbon
promoted by reaction with a halogen or halide and
possibly other components to increase the reactivity of
the sorbent. Other
attempts have been made to improve
the mercury removal from power plant flue gas using
halogen additives to the power plant process itself. For
example, U.S. Pat. No. 8,524,179 describes adding iodine
or bromine to the feed material; and U.S. Pat. No.
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
3
8,679,430 describes injecting a halogen compound into the
combustion chamber and/or exhaust stream.
[0007] All of
these presented disclosures rely on
halogen additives to improve mercury capture. Since
bromine is a strong oxidant, it can also cause oxidation
and corrosion of the duct system and other equipment with
which it comes into contact, causing increased
maintenance and cost. Further,
there are currently no
monitoring requirements for bromine compounds; but if
emitted to the atmosphere, it would be detrimental to the
environment (e.g., ozone depletion in the air and
reaction to form carcinogenic compounds in water).
Therefore, it would be advantageous to use alternative
methods to reduce sorbent injection rates and still
achieve low mercury emissions.
[0008] Other
efforts made to improve PAC performance
has been to target smaller and smaller median (d50)
particle size, thereby increasing the available surface
area. For example, U.S. patent application 2015/0,202,594
describes a PAC with d95 particle size distribution
ranging. from 1 - 28 microns with a d95/d50 ratio of 1.5 -
3; U.S. patent application 2015/0251139 describes a
sorbent with median particle size not greater than 20
microns; and U.S. patent applications 2016/0193587 and
2016/0220945 describe a super fine powdered sorbent with
no more than 10% of the particles having a size greater
than 5 microns.
[0009] Smaller
particle size sorbents have negative
operational effects. For instance, particle sizes less
than 6 microns are difficult to capture with particulate
control devices. Small particles escaping capture can
lead to opacity issues and compliance issues with
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
4
particulate emissions (PM) standards. Furthermore, super
fine sorbents laden with pollutants may be released to
the environment.
[0010] Sorbent
injection, as applied for control of
mercury for MATS compliance, typically involves the
pneumatic conveyance of a powdered sorbent from a storage
silo into the process gas of a power plant's flue duct
downstream of the boiler and upstream of a particulate
control device such as an electrostatic precipitator
(ESP) or fabric filter (FF). Once
introduced to the
process gas, the powdered sorbent disperses and adsorbs
mercury and other unwanted constituents in the flue gas.
The powdered sorbent with adsorbed mercury (and other
constituents) then is captured and removed from the gas
by a particulate control device.
[0011] In coal-
fired power plants, mercury capture
sorbents typically will be co-collected with other
particle matter such as fly ash in an electrostatic
precipitator, fabric filter, an
electrostatic
precipitator in series with a fabric filter, two
electrostatic precipitators in series, two fabric filters
in series, or similar devices. At this typical injection
location (upstream of a particulate collection device),
the sorbents capacity for mercury is limited by the
temperatures naturally present (e.g., greater than 350
F) as the injected sorbents physically and chemically
adsorb mercury through endothermic processes. In such a
configuration, the time between the injection point and
collection point typically is less than three seconds.
Therefore, the adsorption of mercury is limited by
diffusion and reaction kinetics possible in this short
time. Alternatively, if a fabric filter is used as the
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
particulate control device, longer residence times can be
realized. This
technique is not preferred due to the
high cost to install and. operate fabric filters as the
primary particulate control device.
[0012] A drawback
to co-collection of sorbents with
fly ash has arisen in some scenarios when fly ash is sold
as a commodity product. Comingling the sorbent and fly
ash makes the mixture of a quality no longer acceptable
to sell. To
alleviate this issue, two particulate
control devices may be employed in series with the second
being a fabric filter and sorbent injection for mercury
control between the two. This
technique segregates the
sorbent from fly ash collection and allows for longer
contact times for the sorbent to collect mercury. While
effective, the capital expenditure, additional
operational costs, and pressure drop of the additional
fabric filter unit are exorbitant and increase the cost
of control. Similarly,
sorbent might be injected into
the later sections of an electrostatic precipitator so as
to try to segregate fly ash material and sorbent. This
method, however, even further limits residence time for
the carbon to remove mercury, as compared to traditional
injection upstream of the electrostatic precipitator, so
often would not improve mercury removal or injection
rates necessary to substantially reduce mercury
emissions.
[0013] After
exiting the particulate control device,
the process gas continues through flue gas ducts with
decreased levels of mercury and other constituents. At
this point, it is either emitted out of the stack or
perhaps passes through a wet flue gas desulfurization
(WFGD) unit when installed. Wet flue gas desulfurization
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
6
units are currently installed on over 50% of the MW
capacity in the United States to reduce sulfur dioxide
(SO2) emissions. While intended for SO2 capture, mercury
also can be captured in the wet flue gas desulfurization
units. A high percentage of mercury in the flue gas will
partition to a wet flue gas desulfurization liquid when
it is found in the oxidized form, but the elemental
mercury will pass through without capture. Once oxidized
mercury is captured in the liquid, however, it can be
reduced by chemical reactions to elemental mercury and
leave the stack, referred to as "mercury re-emission."
Sorbent introduced in the wet flue gas desulfurization
liquid could sequester mercury species already present in
the liquid stream and minimize re-emission of mercury
from wet flue gas desulfurization units.
[0014] The above-described injection locations in
coal-fired power applications can have some
disadvantages. First, as the powdered sorbent mixes with
the fly ash, it changes the properties of the mixture
that can affect the salability of this byproduct. For
example, fly ash often is sold for use as a cement
additive. During
concrete production, an air-entrained
admixture (AEA) is also added to develop strength
properties. When
powdered sorbents are mixed with fly
ash, especially PAC, they can adsorb the AEA, diminishing
its effectiveness and requiring more AEA to be added.
Increases in AEA add to cost and thereby may prohibit the
sale of fly ash for a cement additive. For
facilities
that sell fly ash, a solution other than a typical PAC
Injection must be applied to preserve these byproduct
sales.
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
7
0 0 15 Second, for
most facilities, sorbent injection
is a retrofit technology applied to the existing
infrastructure. Injection locations have to be installed
within existing duct networks that may have poor mixing
or residence time necessary for high mercury removal.
[0016] Sorbent
injection is a proven effective way to
remove mercury; however, for some applications, the
amount of PAC required can be very high and, therefore,
costly (e.g., because of the high temperatures, short
residence times, and numerous other complicating
factors).
[0017]
Additionally, typical powdered sorbents where
50% of their distribution (d50) having a particle size of
less than 15 micrometers being applied to a WFGD system
creates several issues. First, such
particle sizes
create dusting and opacity issues past the WFGD mist
eliminators due to wetting time constraints. Second, the
necessary wetting time of small particles is long. If
not properly wetted and mixed, these particles will float
at the top of the absorber vessel and be carried out the
stack with the flue gas. Third, such
particles sizes
cause plugging of vacuum filter cloths that increases the
labor and operating expenditures of the application and
causes un-scheduled operational interruptions. Finally,
such particle sizes causes plugging of emissions
monitoring equipment (sorbent traps, CEMS, etc.) because
small particles make it past the mist eliminators and
collect in the sample collection probes of the emissions
monitoring equipment. This can
lead to false readings
and malfunctions of the instruments.
SUMMARY OF THE INVENTION
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
8
0 0 1 8 The present
invention disclosed herein is
directed to systems and methods for post combustion
mercury control using sorbent injection and wet
scrubbing.
[0019] In one
embodiment, the present invention is
directed to a sorbent composition for removing mercury
from flue gas, including a powdered sorbent having a
fifty percent distribution particle size of from about
20 micrometers to about 75 micrometers. In one
aspect,
the powdered sorbent may have a fifty percent
distribution particle size of from about 25 micrometers
to about 75 micrometers. In another aspect, the powdered
sorbent may have a fifty percent distribution particle
size of from about 30 micrometers to about 75
micrometers.
[0020] In another
aspect, the powdered sorbent may be
powdered activated carbon. In yet
another aspect, the
powdered activated carbon may improve mercury removal
without halogens. In still
yet another aspect, the
powdered sorbents may reduce mercury concentrations in
the air phase. And in
another aspect, the powdered
sorbents may reduce mercury concentrations in the liquid
phase.
[0021] In another
embodiment, the present invention is
directed to a method of cleaning flue gas, the method
including removing particulates from flue gas using a
particulate removal system; and injecting a powdered
sorbent into the flue gas, wherein the powdered sorbent
has a fifty percent distribution particle size of from
about 20 micrometers to about 75 micrometers. In one
aspect, the method may also include removing the powdered
sorbent from dewatered slurry in a flue gas
CA 03059203 2019-13-04
WO 2018/186909
PCT/US2017/059606
9
desulfurization system using a hydrocyclone in
communication with the flue gas desulfurization system.
In another embodiment, the hyrocyclone is followed by a.
vacuum filter. In another
aspect, the powdered sorbent
may be powdered activated carbon. In still yet another
aspect, the powdered activated carbon may improve mercury
removal without halogens.
[0022] In still
yet another embodiment, the present
invention is directed to a method of cleaning flue gas,
the method including removing particulates from flue gas
using a particulate removal system; and injecting a
powdered sorbent into the WFGD liquor, wherein the
powdered sorbent has a fifty percent distribution
particle size of from about 20 micrometers to about 75
micrometers. In one aspect, the method may also include
removing the powdered sorbent from dewatered slurry in a
flue gas desulfurization system using a hydrocyclone in
communication with the flue gas desulfurization system.
In another embodiment, the hydrocycione is followed by a
vacuum filter. In another
aspect, the powdered sorbent
may be powdered activated carbon. In still yet another
aspect, the powdered activated carbon may improve mercury
removal without halogens.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] For a more complete understanding of the
features and advantages of the present invention,
reference is now made to the detailed description of the
invention along with the accompanying figures in which
corresponding numerals in the different figures refer to
corresponding parts and in which:
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
0 0 2 4 Figure 1 is an illustration of a post
combustion mercury control using sorbent (in many cases,
activated carbon injection (ACI) system) and wet
scrubbing according to an embodiment;
[0025] Figure 2 is a chart showing improved mercury
capture when using an Improved Sorbent injection System
according to an embodiment;
[0026] Figure 3 is a diagram of the improved results
of mercury removal after injection of the activated
carbon according to an embodiment; and
[0027] Figure 4 is a flowchart of a process for
controlling mercury from process gas according to an
embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0028] While the making and using of various
embodiments of the present invention are discussed in
detail below, it should be appreciated that the present
invention provides many applicable inventive concepts,
which can be embodied in a wide variety of specific
contexts. The specific embodiments discussed herein are
merely illustrative of specific ways to make and use the
invention, and do not limit the scope of the present
Invention.
[0029] Described herein are embodiments for post
combustion mercury control using sorbent (in many cases,
activated carbon injection (ACI) system) and wet
scrubbing (hereinafter "Improved Sorbent Injection
System") and methods of using it and making it. In some
embodiments, the Improved Sorbent Injection System
Includes injecting the sorbent at an improved point in
the post-combustion cleaning system of a coal-fired power
CA 03059203 2019-13-04
WO 2018/186909
PCT/US2017/059606
11
plant (or, in alternatives, other types of power plants
and exhaust systems). In some embodiments, the Improved
Sorbent Injection System includes injecting the sorbent
at a point in the system where it later can be filtered
out without affecting other cleaning processes. In many
embodiments, the sorbent injected is activated carbon;
however, in alternatives, other sorbents may be used.
When the term "sorbent" is used herein, in many
embodiments this may be activated carbon, although other
sorbents may be used.
[0030] The Improved Sorbent injection System
additionally includes the revelation that the available
electrostatic precipitators may be on the hot side of an
air-heater, which is a more challenging environment for
sorbents to remove mercury because of the elevated
temperatures and short residence times. Therefore,
the
Improved Sorbent Injection System includes the use of
alternative injection strategies with longer residence
times, better mixing, and lower temperatures that are
more advantageous.
[0031] For
facilities burning bituminous coal with
substantial levels of sulfur, sulfur trioxide (SO3) will
be generated and be present in the flue gas stream. SO3
also can be found in substantial quantities when power
plants inject it to condition fly ash aiding in its
removal. In
implementing embodiments of the Improved
Sorbent Injection System, it has been noted that PAC and
most sorbents traditionally lose their capacity for
mercury removal with increasing concentrations of S03. In
Implementing embodiments of the Improved Sorbent
Injection System, it has been investigated and determined
that SO3 concentration will be highest right after the
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
12
boiler and will decrease through the duct system as it
sorbs and reacts with fly ash.
Additionally, once the
temperature cools sufficiently, it will condense to
sulfuric acid mist, which does not adversely affect PAC.
In implementing embodiments of the Improved Sorbent
Injection System, it has been discovered that with
typical PAC injection locations before the electrostatic
precipitator/fabric filter, SO3 concentrations are close
to the maximum and will cause the largest detrimental
effect on mercury removal. Previous mitigation methods
are to add a dry sorbent to reduce SO3 concentrations to
improve PAC performance. However, this adds more capital
and operating costs. Therefore,
embodiments of the
Improved Sorbent Injection System have been designed to
circumvent adverse impacts of S03.
[0032] In embodiments of the Improved Sorbent
Injection System, alternative injection strategies are
utilized. A standard
power plant setup typically
includes a boiler, followed by an air heater, and
followed by a particulate control device (electrostatic
precipitator or fabric filter) that exits in an exhaust
stack. As air
pollution regulations have become more
stringent, additional pollution control devices have been
added to the standard power plant configuration.
Therefore, selective catalytic reduction (SCR) units
could be added between the boiler and the air heater for
controlling nitrogen oxides (NOxs). For SO2 control, flue
gas desulfurization units (FGD) could be installed
between the electrostatic precipitator and exhaust stack.
[0033] Embodiments of improved Sorbent Injection
System, provide that PAC will no longer accumulate with
the fly ash, since the overwhelming majority of fly ash
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
13
will occur in the traditional particulate capture
equipment (i.e., electrostatic precipitator, fabric
filter). Therefore,
this fly ash byproduct can be used
and sold for various purposes, such as for use in.
concrete. Since the injection point typically is further
downstream, effluent will be cooler. The longer
residence time and cooler temperature will lead to
improved removal of mercury. After the
electrostatic
precipitator or other particulate control device, gases
that might compete with the activity of the PAC in the
removal of the mercury will be lessened.
Furthermore,
the re-emission of mercury likely is reduced, since more
of the mercury will be captured in the PAC and is not
available for the reaction in the slurry. Since the
mercury will not be as available in the slurry, when the
slurry is dewatered, the residual mercury and other
reaction byproducts in the dewatered slurry will be
lessened. By removing
the PAC, the wet flue gas
desulfurization solids byproduct integrity can be
maintained for reuse, recycling, or disposal.
[0034] Embodiments
of the Improved Sorbent Injection
System were not known or expected, since the wet flue gas
desulfurization system is used for control of SO2 gases;
and using it for particulate removal of powdered sorbents
is an unexpected application. The wet
flue gas
desulfurization unit is quite suited for the removal of
powders, even though this is not a typical application.
Mercury removal will occur in the gas phase, and then be
retained during contact in the wet flue gas
desulfurization unit. Those in
the art focus on
capturing mercury from the liquid phase of a wet flue gas
desulfurization unit. In
contrast, the position of the
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
14
injection of powdered sorbent provides gas phase capture
of mercury in parallel with liquid phase mercury capture.
SO3 will be lower downstream of the particulate control
devices, thereby reducing the exposure of the sorbent to
this detrimental acidic compound and thereby eliminate
the need to apply dry sorbent injection to eliminate SO3
before it comes into contact with the sorbent. Also,
since the temperature of the flue gas will be cooler at
the point of injection, the activity of SO3 is reduced.
Also for wet flue gas desulfurization units, the powdered
sorbent materials contribute to the reduction of other
unwanted reactions and constituents in the discharged
liquid (such as heavy metals and nutrients) after contact
with the slurry. In this way, there is the advantage of
serving as two treatment processes (one for mercury
removal and the other for wastewater treatment)
encompassed by one material and system.
[0035] In one
embodiment, specifically engineered PACs
for mercury removal are applied with sorbent injection
for mercury removal from coal-fired power plant flue gas.
In concert with the engineered PACs, complimentary
improvements to the overall system are provided.
[0036]
Furthermore, if PAC is utilized as the sorbent,
it can be engineered also to improve wet flue gas
desulfurization slurry chemistry and improve the quality
of the discharged wastewater. In fact, some systems may
teach that merely the injection of PAC prior to the flue
gas desulfurization is sub-optimal and call for the
injection of additional materials and other treatments.
However, by the proper positioning of the injection site
of the PAC, at proper temperatures and after the removal
CA 03059203 2019-13-04
WO 2018/186909
PCT/US2017/059606
of much particulate, with the proper PAC selection an
advantageous system is achieved.
[0037] Referring
initially to Figure 1, an embodiment
of an Improved Sorbent injection System ("system") is
schematically illustrated and generally designated 100.
System 100 may be a coal-fired electric power generation
plant, in one embodiment. System 100
may include a
boiler 102, such as for a coal-fired power plant.
Although the example described herein applies to coal-
fired power plants, the process gas or flue gas to be
treated may originate from many industrial facilities
such as a power plant, cement plant, waste incinerator,
or other facilities that will occur to one skilled in the
.rt.
[0038] Such gas
streams contain many contaminants
and/or pollutants, such as mercury, that are desirable to
control and/or decrease in concentration for protection
of health and the environment. Nevertheless, system 100
is being described for removing, controlling, and/or
reducing pollutants, such as mercury, from a coal-fired
power plant gas stream using one or more of activated
carbon injection devices/units and additive injection
devices/units as discussed herein. Boiler 102 may be a
coal-fired boiler that burns or combusts coal to heat
water into superheated steam for driving steam turbines
that produce electricity. These types
of power plants
are common throughout the U.S. and elsewhere. Boiler 102
may further include an economizer 104, in one embodiment.
Economizer 104 may be used to recover heat produced from
boiler 102.
[0039] The flue
gas or process gas 106 exiting boiler
102 and/or economizer 104 may then be flowed,
CA 03059203 2019-13-04
WO 2018/186909
PCT/US2017/059606
16
transported, ducted, piped, etc. via one or more process
lines 108 to a selective catalytic reduction unit 110 for
the removal of nitrogen containing compounds, in one
embodiment. Typically,
selective catalytic reduction
unit 110 may convert NO. compounds to diatomic nitrogen
(N2) and water (H20) using a catalyst and a gaseous
reductant, such as an ammonia containing compound.
[0040] Process gas 106 may then be flowed,
transported, ducted, piped, etc. to a heat exchanger,
pre-heater, and/or air heater 112 where heat is
transferred from process gas 106 to a feed of air to be
fed back into boiler 102. Process gas
106 may then be
transferred via process line 108 to an electrostatic
precipitator 114 for removal of particulates contained in
process gas 106, in one example.
[0041] System 100 may also include an additive
injection device/unit 116 for injecting one or more
compounds, chemicals, etc., such as organosuifides,
inorganic sulfides, acids, bases, metal oxides, oxides,
metals, photocatalysts, and/or minerals to aid with
sorbent performance. Preferably, additive injection unit
116 is located downstream of electrostatic precipitator
114 for injecting these compounds and/or chemicals prior
to injection of activated carbon products as discussed
herein. System 100
may further include one or more
activated carbon injection ("ACI") devices, units,
systems, etc. (ACI unit 118). ACI unit 118 may include
an activated storage vessel, such as a powdered activated
carbon (PAC) storage vessel. Such vessels may be silos,
and the like where activated carbon, such as PAC, may be
stored for use in system 100. Activated carbon silo (not
shown) may be any type of storage vessel such that it is
CA 03059203 2019-13-04
WO 2018/186909
PCT/US2017/059606
17
capable of containing a supply and/or feedstock of
activated carbon, such as PAC, for supplying the
activated carbon to process gas 106 of system 100. Some
additional exemplary activated carbon silos may include
supersacs, silos, storage vessels, and the like.
[0042] Activated
carbon may be injected anywhere along
process line 108 downstream of additive injection unit
116, preferably. In one
embodiment, system 100 may
include one or more fluidizing nozzles 120 that may
assist in providing activated carbon in a fluidized
form, such that it may be transported in a substantially
fluid form downstream in system 100.
Additionally,
system 100 may include one or more control valves (not
shown) that may be disposed and/or located substantially
proximal to the exit or outlet of activated carbon and/or
fluidizing nozzles 120 for controlling the flow of
activated carbon from ACI unit 118 to system 100. The
feed of activated carbon can also be controlled by a
series of additional control valves, movable barriers,
etc. (not shown). To assist
the process of fluidizing
activated carbon for exiting ACI unit 118, fluidization
assistance may be applied in the form of physical
agitation or the use of fluidizing nozzles. In addition,
system 100 may include other types of control valves, such
as manual valves (not shown), and the like as would be
known to those skilled in the art.
[0043] The treated
process gas 106 may then be sent to
a flue gas desulfurization unit 122 via process line 108
for removal of sulfur compounds, in one embodiment.
After being treated in flue gas desulfurization unit 122,
treated process gas 106 may then be sent to a stack 124
for emission into the environment. As is known to those
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
18
skilled in the art, flue gas desulfurization unit. 122 may
have an gas/air phase and a liquid/water phase; system
100 described herein reduces mercury concentrations in.
the air phase and liquid phase of flue gas
desulfurization unit 122, such that the discharge water
of flue gas desulfurization unit 122 has a lower
concentration of mercury in the process or flue gas than
prior to upstream of ACI unit 118.
[0044] Additionally, activated carbon is used to
target reduced concentrations of nitrates/nitrites and
heavy metals, such as mercury, arsenic, lead, and
selenium in the liquid or wet phase of flue gas
desuifurization unit 122 such that the discharge water of
flue gas desulfurization unit 122 has lower
concentrations of these contaminants in the process or
flue gas than prior to upstream of ACI unit 113.
[0045] In one
embodiment, activated carbon of system
100 is used to target reduced concentrations of mercury
in the gas/air phase and reduced concentrations of
nitrates/nitrites and heavy metals such as mercury,
arsenic, lead, and selenium in the wet phase of flue gas
desulfurization unit 122. System 100 may also include a
hydrocyclone 126 for further removal of particulates in
the wet flue gas desulfurization unit liquor prior to
discharges.
Hydrocyclone 126 may be used to remove
activated carbon, powdered sorbent, powdered activated
carbon, and the like from the wastewater of the wet flue
gas desulfurization unit 122. Hydrocyclone 126 may also
be followed by a vacuum filter that will further remove
particulates/solids prior to liquor discharge.
[0046] Example 1 - Preparation of PAC
CA 03059203 2019-13-04
WO 2018/186909
PCT/US2017/059606
19
0 0 4 7 A magnetic
activated carbon sample with 6% by
weight of magnetite (Fe300 was prepared with PAC treated
with a wet method to precipitate ferric chloride and
ferrous sulfate in 200 lb. batches followed by dewatering
and drying at 200 C. The dried product was sieved and
resulted in about 95% of the final product passing
through a 325-mesh sieve.
[0048] Mercury Removal
[0049] The product
was tested at the Mercury Research
Center (MRC). The MRC
removes a constant flow of
approximately 20,500 actual cubic feet per minute (acfm)
of flue gas (representative of a 5 mega watt [MW] boiler)
from the Southern Company Plant Christ Boiler (78 MW).
The boiler runs on a low-sulfur bituminous coal blend
from varying sources. The typical
SO3 concentration of
the fuel blends resulted in about 2 parts per million
(ppm) of S03. Figure 2
shows improved mercury capture
when using an embodiment of an Improved Sorbent Injection
System. The product
was pneumatically injected at
increasing injection rates upstream of the electrostatic
precipitator (ACT 1 in Figure 2) and downstream of the
electrostatic precipitator (ACT 2 in Figure 2).
Particulate removal was achieved with the electrostatic
precipitator for ACI 1. Particulates remained uncaptured
for ACT 2, and returned to the Christ process train.
Mercury concentrations were monitored at the MRC inlet
and the MRC outlet, and the observed concentrations were
converted to pounds per trillion British thermal units
(1b/Tbtu) using the standard EPA Method 19. Mercury
removal by the AC was calculated as the inlet mercury
concentration minus the outlet mercury concentration and
is illustrated in Figure 2. At typical injection rates
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
and above, less AC is necessary to remove the same amount
of AC which would result in significant cost savings for
the utility.
[0050] Example 2 - Preparation of PAC
[0051] A coal-
fired power plant with a 540 MW unit was
conducting a trial to inject a powdered sorbent into the
wet scrubber sump which was subsequently pumped into the
absorber vessel. The powdered sorbent met the typical 95%
passing the 325 mesh with a d50 particle size of 15
microns. Albeit vapor phase mercury emissions went down
based on continuous emissions monitoring equipment, the
unit began experiencing elevated levels of mercury in
their sorbent traps. This was because Hg bound to the
fine PAC particles was escaping past the mist eliminators
and being captured in the sorbent trap. Furthermore, the
fine PAC particles began clogging the rotary vacuum
filters, causing the system to shut down.
[0052] In a second
trial of the Improved Sorbent
Injection System, a powdered sorbent with 50% passing the
325 mesh and a d50 particle size of 45 microns was added
to the sump and subsequently injected into the absorber.
[0053] Turning now to Figure 3, data shows the
improved mercury capture when using an embodiment of an
Improved Sorbent Injection System. The larger
particle
size sorbent decreased dusting during handling and
injection of the sorbent into the sump. Furthermore, the
rotary vacuum filters remained operational without any
issues. In
addition, sorbent trap data matched the
continuous emissions monitor data to indicate that no
opacity issues with sorbent past the mist eliminators was
observed.
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
21
(0054] Turning now to Figure 4, a method for
controlling mercury removal in flue gas or process gas is
schematically illustrated and generally designated 400.
In step 402, process or flue gas may be transferred to a.
pre-heater for heat transfer to an air source to be fed
back into a particular unit, such as boiler 102. This
step may also include transferring the process or flue
gas to an economizer prior to transferring it to a SCR,
such as selective catalytic reduction unit 110.
[0055] In step
404, the process or flue gas may be
transferred to a particulate collection device/unit, such
as electrostatic precipitator 114. This step may include
removing particulates from the process or flue gas. In
step 406, a chemical and/or compound may be injected into
process or flue gas downstream of the particulate
collection device/unit, such as
electrostatic
precipitator 114. This step
may include contacting the
process and flue gas with one or more of organosuifides,
inorganic sulfides, acids, bases, metal oxides, oxides,
metals, photocatalyst and/or minerals to aid with
activated carbon/sorbent performance.
[0056] In step
408, the process or flue gas may be
contacted with activated carbon, such as from Ad I unit
118. In this
step, activated carbon may be PAC.
Preferably, such contact occurs downstream of the
Injection described in step 406. Such contacting of the
process or flue gas with activated carbon after the
Injection described in step 406 reduces the mercury
concentration of the process or flue gas. In step
410,
the process or flue gas is transferred to a wet flue gas
desulfurization unit, such as flue gas desulfurization
unit 122, where the powdered sorbent material contributes
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
22
to the reduction of other unwanted reactions and
constituents in the discharged liquid (such as heavy
metals and nutrients) after contact with the slurry in.
flue gas desulfurization unit 122. In this way, there is
the advantage of serving as two treatment processes (one
for mercury removal and the other for wastewater
treatment) encompassed by one material and system.
[0057]
Additionally in this step, activated carbon is
used to target reduced concentrations of
nitrates/nitrites and heavy metals, such as mercury,
arsenic, lead, and selenium in the liquid or wet phase of
flue gas desulfurization unit 122 such that the discharge
water of flue gas desuifurization unit 122 has lower
concentrations of these contaminants in the process or
flue gas than prior to upstream of ACI unit 118. In step
412, the process or flue gas may be transferred to a
stack for emitting to the environment.
[0058] In another
embodiment of the present invention,
the particle size of the sorbent may be increased to
reduce or eliminate the issues of increased dusting and
opacity issues, long wetting times, plugging of vacuum
filters, and the dike. In one aspect, the particle size
for environment 50% distribution (d50) of the sorbent
particles may be from about 20 micrometers to about 75
micrometers. This means
that approximately 50% of the
sorbent particles have a particle size of less than this
range and 50% of the sorbent particles have a particle
size of more than this range. Additionally, the systems
and sorbents described herein may decrease the
distribution and/or amount of sorbent having particle
sizes of less than 20 micrometers, less than 15
CA 03059203 2019-13-04
WO 2018/186909
PCT/US2017/059606
23
micrometers, less than 10 micrometers, and less than 5
micrometers.
[0059] In one
embodiment, such sorbents having a d50
of from about 20 micrometers to about 75 micrometers may
be injected in the flue gas just upstream of flue gas
desulfurization unit 122. In another
embodiment, such
sorbents may be injected into the absorber vessel of flue
gas desulfurization unit 122.
[0060] Embodiments
of this disclosure may be further
illustrated by the following Items:
[0061] Item 1. A system
for cleaning flue gas, the
system comprising: a particulate removal system; an
additive injector positioned downstream of the
particulate removal system, for injecting an additive
into the flue gas; a powdered sorbent injector positioned
downstream of the additive injector, for injecting
powdered sorbents, wherein no powdered sorbent injectors
are positioned upstream of the particulate removal
system; and
a flue gas desulfurization system positioned downstream
from the powdered sorbent injector, wherein no other
processing apparatus is located between the powdered
sorbent injector and the flue gas desulfurization system.
[0062] Item 2. The system
of Item 1, wherein the
particulate removal system is a fabric filter.
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
24
[0063] Item 3. The system
of any of the preceding
items, wherein the particulate removal system is an
electrostatic precipitator.
[0064] Item 4. The system
of any of the preceding
items, wherein no other substance is injected between the
powdered activated carbon injector and the flue gas
desulfurization system.
[0065] 5. The system
of any of the preceding items,
wherein the flue gas desulfurization system is a wet flue
gas desulfurization system.
[0066] 6. The system
of any of the preceding items,
further comprising: an air heater located upstream from
the particulate removal system.
[0067] 7. The system
of any of the preceding items,
further comprising: a selective catalytic reduction
system located upstream of the air heater.
[0068] 8. The system
of any of the preceding items,
further comprising: a hydrocyclone in communication with
the flue gas desulfurization system, the hydrocycione
being used for removing the activated carbon from
dewatered slurry resulting from the flue gas
desulfurization system.
CA 03059203 2019-13-04
WO 2018/186909
PCT/US2017/059606
(0069] 9. The system
of any of the preceding items,
wherein the powdered sorbent is powdered activated
carbon.
[0070] 10. The system of any of the preceding items,
wherein the powdered activated carbon improves mercury
removal without halogens.
[0071] 11. The system of any of the preceding items,
wherein the additive injector injects one or more of the
group consisting of organosulfides, inorganic sulfides,
acids, bases, metal oxides, oxides, metals,
photocatalysts, and minerals into the flue gas.
[0072] 12. The system of any of the preceding items,
wherein the powdered sorbents reduce mercury
concentrations in the air phase.
[0073] 13. The system of any of the preceding items,
wherein the powdered sorbents reduce mercury
concentrations in the air phase and water phase of the
wet flue gas desulfurization system such that the
discharge water of the wet flue gas desulfurization
system has a lower mercury concentration than prior to
the injection of the powdered sorbents into the flue gas
upstream of the wet flue gas desulfurization system.
[0074] 14. The system of any of the preceding items,
wherein the powdered sorbents reduce concentrations of
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
26
one or more of the group consisting of nitrates and
nitrites, heavy metals, mercury, arsenic, lead, and
selenium in the wet flue gas desulfurization system such
that the discharge water of the wet flue gas
desulfurization has lower concentrations than prior to
the injection of the powdered sorbents into the flue gas
upstream of the wet flue gas desuifurization system.
[0075] 15. The system of any of the preceding items,
wherein the powdered sorbents reduce mercury
concentration in the air phase and reduce concentrations
of one or more of group consisting of nitrates, nitrites,
heavy metals, mercury, arsenic, lead and selenium in the
wet flue gas desuifurization system.
[0076] 16. The system of any of the preceding items,
wherein the powdered sorbents are without halogens and
impregnated or admixed with one or more of the group
consisting of organosuifides, inorganic sulfides, acids,
bases, metal oxides, oxides, metals, photocatalysts, and
minerals.
[0077] 11. The system of any of the preceding items,
wherein the powdered sorbent has a fifty percent
distribution particle size of from about 20 micrometers
to about 75 micrometers.
CA 03059203 2019-13-04
WO 2018/186909
PCT/US2017/059606
27
[0078] 18. A method of cleaning flue gas, the method
comprising: removing particulates from flue gas using a
particulate removal system, injecting an additive into
the flue gas downstream of the particulate removal
system, injecting powdered sorbent into the flue gas
downstream of said injection of the additive, wherein no
powdered sorbent is injected upstream of the particulate
removal system, and treating the flue gas in a flue gas
desulfurization system positioned downstream from a point
where the powdered sorbent is injected, wherein no other
processing is done between the powdered sorbent injector
and the flue gas desulfurization system_
[0079] 19. The method of Item 18, wherein the
particulate removal system includes an electrostatic
precipitator.
[0080] 20. The method of Items 18-19, wherein no
other substance is injected between the point where the
powdered sorbent is injected and the flue gas
desulfurization system.
[0081] 21. The method of Items 18-20, wherein the
flue gas desulfurization system is a wet flue gas
desulfurization system.
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
28
(0082] 22. The method of Items 18-21, wherein an air
heater is located upstream from the particulate removal
system.
[0083] 23. The method of Items 18-22, wherein a
selective catalytic reduction system is located upstream
of the air heater.
[0084] 24. The method of Items 18-23, further
comprising: removing the powdered sorbent from dewatered
slurry in the flue gas desulfurization system using a
hydrocyclone in communication with the flue gas
desulfurization system.
[0085] 25. The method of Items 18-24, wherein the
powdered sorbent is powdered activated carbon.
[0086] 26. The method of Items 18-25, wherein the
powdered activated carbon improves mercury removal
without halogens.
[0087] 27. The method of Items 18-26, wherein
injecting an additive comprises: injecting into the flue
gas one or more of the group consisting of
organosulfddes, inorganic sulfides, acids, bases, metal
oxides, oxides, metals, photocatalysts, and minerals.
[0088] 28. The method of Items 18-27, further
comprising: reducing mercury concentration in the air
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
29
phase of the wet flue gas desulfurization system with the
powdered sorbent.
[0089] 29. The method of Items 18-28, further
comprising: reducing mercury concentrations in the air
phase and water phase of the wet flue gas desulfurization
system such that the discharge water of the wet flue gas
desulfurization system has a lower mercury concentration
than prior to the injection of the powdered sorbents into
the flue gas upstream of the wet flue gas
desulfurization system with the powdered sorbent.
[0090] 30. The method of Items 18-29, further
comprising: reducing concentrations of one or more of the
group consisting of nitrates and nitrites, heavy metals,
mercury, arsenic, lead, and selenium in the wet flue gas
desulfurization system such that the discharge water of
the wet flue gas desulfurization system has lower
concentrations than prior to the injection of the
powdered sorbents into the flue gas upstream of the wet
flue gas desulfurization system with the powdered
sorbent.
[0091] 31. The method of Items 18-30, further
comprising: reducing mercury concentration in the air
phase and reduce concentrations of one or more of group
consisting of nitrates, nitrites, heavy metals, mercury,
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
arsenic, lead, and selenium in the wet flue gas
desulfurization system with the powdered sorbent.
[0092] 32. The
method of Items 18-31, wherein the
powdered sorbent is without halogens and impregnated or
admixed with one or more of the group consisting of
organosulfides, inorganic sulfides, acids, bases, metal
oxides, oxides, metals, photocatalysts, and minerals.
[0093] 33. The
method of Items 18-32, wherein the
powdered sorbent has a fifty percent distribution
particle size of from about 20 micrometers to about 75
micrometers.
[0094] 34. A
sorbent composition for removing mercury
from flue gas, comprising: a powdered sorbent having a
fifty percent distribution particle size of from. about
20 micrometers to about 75 micrometers.
[0095] 35. The sorbent composition of Item 34,
wherein the powdered sorbent has a fifty percent
distribution particle size of from about 25 micrometers
to about 75 micrometers.
[0096] 36. The
sorbent composition of Items 34-35,
wherein the powdered sorbent has a fifty percent
distribution particle size of from about 30 micrometers
to about 75 micrometers.
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
31
(0097] 37. The sorbent composition of items 34-36,
wherein the powdered sorbent is powdered activated
carbon.
[0098] 38. The sorbent composition of items 34-37,
wherein the powdered activated carbon improves mercury
removal without halogens.
[0099] 39. The sorbent composition of items 34-38,
wherein the powdered sorbents reduce mercury
concentrations an the air phase.
[00100] 40. The sorbent composition of items 34-39,
wherein the powdered sorbents reduce mercury
concentrations an the water phase.
[00101] 41. A method of cleaning flue gas, the method
comprising: injecting a powdered sorbent into the flue
gas, wherein the powdered sorbent has a fifty percent
distribution particle size of from about 20 micrometers
to about 75 micrometers; and collecting the powdered
sorbent in a flue gas desulfurization system.
[00102] 42. The method of Item 41, further comprising:
removing the powdered sorbent from a dewatered slurry in
a flue gas desulfurization system using a hydrocyclone in
communication with the flue gas desulfurization system.
CA 03059203 2019-13-04
WO 2018/186909
PCT/US2017/059606
32
0 0 1 03] 43. The method of Items 41-42, further
comprising: removing particulates/solids with a vacuum
filter after the hydrocyclone prior to liquor discharge.
[00104] 44. The method of Items 41-43, wherein the
powdered sorbent is powdered activated carbon.
[00105] 45. The method of Items 41-44, wherein the
powdered activated carbon improves mercury removal
without halogens.
[00106] 46. A. method of cleaning flue gas, the method
comprising: injecting a powdered sorbent into the
liquor/slurry of a flue gas desuifurization system,
wherein the powdered sorbent has a fifty percent
distribution particle size of from about 20 micrometers
to about 75 micrometers.
[00107] 47. The method of Item 46, further comprising:
removing the powdered sorbent from dewatered slurry in a
flue gas desulfurization system using a hydrocyclone in.
communication with the flue gas desulfurization system.
[00108] 48. The method of Items 46-47, further
comprising: removing particulates/solids with a vacuum
filter after the hydrocyclone prior to liquor discharge.
[00109] 49. The method of Items 46-48, wherein the
powdered sorbent is powdered activated carbon.
CA 03059203 2019-10-04
WO 2018/186909
PCT/US2017/059606
33
0 0 1 1 0 50. The method of Items 46-49, wherein the
powdered activated carbon improves mercury removal
without halogens.
[00111] While this invention has been described with
reference to illustrative embodiments, this description
is not intended to be construed in a limiting sense.
Various modifications and combinations of the
illustrative embodiments as well as other embodiments of
the invention will be apparent to persons skilled in the
art upon reference to the description. It is, therefore,
intended that the appended claims encompass any such
modifications or embodiments.
