Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02793326 2012-10-25
HOT-SIDE METHOD AND SYSTEM
FIELD
The disclosure relates generally to controlling mercury emissions and
particularly
to controlling mercury emissions using halogen-containing additives.
BACKGROUND
In response to the acknowledged threat that mercury poses to human health and
the
environment as a whole, both federal and state/provincial regulation have been
implemented in the United States and Canada to permanently reduce mercury
emissions,
particularly from coal-fired utilities (e.g., power plants), steel mills,
cement kilns, waste
incinerators and boilers, industrial coal-fired boilers, and other coal
combusting facilities.
For example, about 40% of mercury introduced into the environment in the U.S.
comes
from coal-fired power plants. New coal-fired power plants will have to meet
stringent
new source performance standards. In addition, Canada and more than 12 states
have
enacted mercury control rules with targets of typically 90% control of coal-
fired mercury
emissions and other states are considering regulations more stringent than
federal
regulations. Further U.S. measures will likely require control of mercury at
more stringent
rates as part of new multi-pollutant regulations for all coal-fired sources.
The leading technology for mercury control from coal-fired power plants is
activated carbon injection ("ACI"). ACI is the injection of powdered
carbonaceous
sorbents, particularly powdered activated carbon ("PAC"), upstream of either
an
electrostatic precipitator or a fabric filter bag house. Activated or active
carbon is a
porous carbonaceous material having a high adsorptive power.
Activated carbon can be highly effective in capturing oxidized (as opposed to
elemental) mercury. Most enhancements to ACT have used halogens to oxidize gas-
phase
elemental mercury so it can be captured by the carbon surface. ACI technology
has
potential application to the control of mercury emissions on most coal-fired
power plants,
even those plants that may achieve some mercury control through control
devices
designed for other pollutants, such as wet or dry scrubbers for the control
sulfur dioxide.
AC1 is a low capital cost technology. The largest cost element is the cost of
sorbents. However, AC1 has inherent disadvantages that are important to some
users.
First, ACI is normally not effective at plants configured with hot-side
electrostatic
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CA 02793326 2012-10-25
precipitators or higher temperature cold-side electrostatic precipitators,
because the
temperature at which the particulates are collected is higher than the
temperature at which
the carbon adsorbs the mercury. Second, activated carbon is less effective for
plants
firing high- or medium-sulfur coal, plants using selective catalytic reduction
(SCR)
systems to control nitrogen oxide emissions where sulfur dioxide may be
converted to
sulfur trioxide at the catalyst surface and plants using sulfur trioxide flue
gas conditioning
due to the interference of sulfur trioxide with capture of mercury on the
carbon surface.
Another technique to control mercury emissions from coal-fired power plants is
bromine injection with Ad. Such a mercury control system is sold by Alstom
Power Inc.
under the trade names MerCureTM or KNXTM and by Nalco Mobotec Company under
the
trade name MerControl 7895TM. Bromine is believed to oxidize elemental mercury
and
form mercuric bromide. To remove mercury effectively, bromine injection is
done at high
rates, typically above 100 ppmw of the coal. At 100 ppmw without ACI or other
factors
such as high unburned carbon from coal combustion or the presence of a flue
gas
desulfurization system, bromine has been reported as resulting in a change of
mercury
emissions of about 40% lower than the uncontrolled mercury.
Bromine, when added at high concentrations such as 100 ppmw of the coal feed,
is
problematic for at least two reasons. It can form HBr in the flue gas, which
is highly
corrosive to plant components, such as ductwork. ln particular, cold surfaces
in the gas
path, such as air preheater internals, outlet ductwork, scrubber and stack
liners, are very
susceptible to corrosion attack. Also at such high injection rates, a
significant amount of
bromine will be emitted from the stack and into the environment. Bromine is a
precursor
to bromomethane, hydrobromofluorocarbons, chlorobromomethane and methyl
bromide,
which are known ozone depletors in the earth's upper atmosphere.
SUMMARY
These and other needs are addressed by the various aspects, embodiments, and
configurations of the present disclosure. The aspects, embodiments, and
configurations
are directed generally to the conversion of gas-phase mercury to a form that
is more
readily captured.
In one aspect, a method is provided that includes the steps:
(a) generating from a mercury-containing feed material a mercury-
containing
gas stream comprising vapor-phase elemental mercury and a vapor-phase halogen;
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CA 02793326 2012-10-25
(b) contacting at least one of the mercury-containing feed material and the
mercury-containing gas stream with a reactive surface agent upstream of an air
preheater,
reactive surface agent and vapor phase halogen converting elemental mercury
into ionic
mercury and collecting the ionic mercury on the reactive surface agent; and
(c) removing the ionic
mercury active agent and reactive surface agent from
the mercury-containing gas stream.
An advantage of this aspect is temperature stability. Introducing halogens
with the
coal with subsequent carbonaceous reactive surface agent (e.g., activated
carbon) injection
can result in superior temperature stability of the collected particulate
compared to the use
of bromine-treated reactive surface agent, for example. This can particularly
be true when
the activated carbon is brominated with a salt.
In one aspect, a method is provided that includes the steps:
(a) providing
a mercury-containing gas stream comprising vapor-phase
elemental mercury;
(b) contacting the
mercury-containing gas stream with a carbonaceous reactive
surface agent upstream of an air preheater, the reactive surface agent
comprising at least
one of iodine and bromine to collect the ionic mercury on the reactive surface
agent; and
(c) removing the mercury-loaded reactive surface agent from the gas stream.
The combined halogen and reactive surface agent can not only be cost effective
but
also efficacious, at surprisingly low concentrations, in promoting the removal
of both
elemental and speciated mercury from mercury-containing gas streams. Compared
to
bromine and iodine in the absence of a reactive surface agent, the reactive
surface agent
has been found to cost effectively promote the formation of particle-bound
mercury
species at relatively high temperatures.
Very low levels of halogen can enable or facilitate removal of mercury
effectively
in coal-fired systems, if excessively high acid gas species can be controlled.
Mercury will
generally not be removed effectively by carbon sorbents or on fly ash in the
presence of
higher sulfur trioxide and/or nitrogen dioxide concentrations in the mercury-
containing
gas stream. A high concentration of acid gases (which high partial pressure
typically
refers to a trioxide concentration of at least about 5 ppmv in the mercury-
containing gas
stream and even more typically of at least about 10 ppmv and/or a nitrogen
dioxide
concentration of at least about 5 ppmv and even more typically at least about
10 ppmv).
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The higher sulfur trioxide concentration can be due to sulfur levels in the
feed material,
catalytic oxidation of sulfur dioxide to sulfur trioxide across the SCR and/or
where SO3 is
injected to improve performance of the particulate removal device. The
condensation
temperature of sulfur trioxide and/or nitrogen dioxide on a collection or
sorbent surface
can be lower than the condensation temperatures of mercuric iodide and
periodic acid. As
noted, condensed acid can displace sorbed mercury from a carbon sorbent
particle surface.
By forming a mercury-containing particulate that can be collected in an
electrostatic precipitator or baghouse or reacting with collected particulate,
the mercury
can be removed prior to entering the wet scrubber. This can eliminate the
potential for re-
emission of elemental mercury from the scrubber, which is extremely difficult
to control
for variable process conditions. It can also reduce or eliminate mercury from
the scrubber
sludge.
When halogens are introduced with the feed material, the collected mercury
appears to be much more temperature stable than collected mercury caused by
introduction of halogens into the flue gas. Introducing halogens with coal,
for example,
with subsequent ACI injection seems to result in much better temperature
stability of the
mercury associated with the collected particulate than when bromine-treated
activated
carbon is used, particularly when the activated carbon is brominated with a
salt.
Stability of captured mercury in fly ash or other retained particulate solids
is
related to leachability and solubility of the mercury. Mercuric iodide, HgI2,
has a very low
solubility in water, which is significantly different from (less soluble than)
other oxidized
mercury species such as HgC12 and HgBr2. The solubility in water is more than
two
orders of magnitude lower than bromide or chloride species: HgC12 is 73.25
g/1, HgBr2 is
6.18 el, Hg12 is 0.06 g/land He is 5.73 x g/l. The
lower solubility of captured HgI2
will reduce the leachability in fly ash and other solid particulates compared
to other
oxidized mercury species.
These and other advantages will be apparent from the disclosure of the
aspects,
embodiments, and configurations contained herein.
"A" or "an" entity refers to one or more of that entity. As such, the terms
"a" (or
"an"), "one or more" and "at least one" can be used interchangeably herein. It
is also to be
noted that the terms "comprising", "including", and "having" can be used
interchangeably.
"Absorption" is the incorporation of a substance in one state into another of
a
different state (e.g. liquids being absorbed by a solid or gases being
absorbed by a liquid).
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Absorption is a physical or chemical phenomenon or a process in which atoms,
molecules,
or ions enter some bulk phase - gas, liquid or solid material. This is a
different process
from adsorption, since molecules undergoing absorption are taken up by the
volume, not
by the surface (as in the case for adsorption).
"Adsorption" is the adhesion of atoms, ions, biomolecules, or molecules of
gas,
liquid, or dissolved solids to a surface. This process creates a film of the
adsorbate (the
molecules or atoms being accumulated) on the surface of the adsorbent. It
differs from
absorption, in which a fluid permeates or is dissolved by a liquid or solid.
Similar to
surface tension, adsorption is generally a consequence of surface energy. The
exact nature
of the bonding depends on the details of the species involved, but the
adsorption process is
generally classified as physisorption (characteristic of weak van der Waals
forces) or
chemisorption (characteristic of covalent bonding). It may also occur due to
electrostatic
attraction.
"Ash" refers to the residue remaining after complete combustion of the coal
particles. Ash typically includes mineral matter (silica, alumina, iron oxide,
etc.).
"At least one", "one or more", and "and/or" are open-ended expressions that
are
both conjunctive and disjunctive in operation. For example, each of the
expressions "at
least one of A, B and C", "at least one of A, B, or C", "one or more of A, B,
and C", "one
or more of A, B, or C" and "A, B, and/or C" means A alone, B alone, C alone, A
and B
together, A and C together, B and C together, or A, B and C together. When
each one of
A, B, and C in the above expressions refers to an element, such as X, Y, and
Z, or class of
elements, such as Xi-Xo, Yi-Yro, and Z1-Z0, the phrase is intended to refer to
a single
element selected from X, Y, and Z, a combination of elements selected from the
same
class (e.g., X1 and X2) as well as a combination of elements selected from two
or more
classes (e.g., Y1 and Zo).
"Biomass" refers to biological matter from living or recently living
organisms.
Examples of biomass include, without limitation, wood, waste, (hydrogen) gas,
seaweed,
algae, and alcohol fuels. Biomass can be plant matter grown to generate
electricity or
heat. Biomass
also includes, without limitation, plant or animal matter used for
production of fibers or chemicals. Biomass further includes, without
limitation,
biodegradable wastes that can be burnt as fuel but generally excludes organic
materials,
such as fossil fuels, which have been transformed by geologic processes into
substances
such as coal or petroleum. Industrial biomass can be grown from numerous types
of
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plants, including miscanthus, switchgrass, hemp, corn, poplar, willow,
sorghum,
sugarcane, and a variety of tree species, ranging from eucalyptus to oil palm
(or palm oil).
"Carbonaceous" refers to a carbon-containing material, particularly a material
that
is substantially rich in carbon.
"Coal" refers to a combustible material formed from prehistoric plant life.
Coal
includes, without limitation, peat, lignite, sub-bituminous coal, bituminous
coal, steam
coal, anthracite, and graphite. Chemically, coal is a macromolecular network
comprised
of groups of polynuclear aromatic rings, to which are attached subordinate
rings connected
by oxygen, sulfur, and aliphatic bridges.
A "composition" refers to one or more chemical units composed of one or more
atoms, such as a molecule, polyatomic ion, chemical compound, coordination
complex,
coordination compound, and the like. As will be appreciated, a composition can
be held
together by various types of bonds and/or forces, such as covalent bonds,
metallic bonds,
coordination bonds, ionic bonds, hydrogen bonds, electrostatic forces (e.g.,
van der Waal's
forces and London's forces), and the like.
"Halogen" refers to an electronegative element of group VIIA of the periodic
table
(e.g., fluorine, chlorine, bromine, iodine, astatine, listed in order of their
activity with
fluorine being the most active of all chemical elements).
"Halide" refers to a binary compound of the halogens.
"High alkali coals" refer to coals having a total alkali (e.g., calcium)
content of at
least about 20 wt.% (dry basis of the ash), typically expressed as CaO, while
"low alkali
coals" refer to coals having a total alkali content of less than 20 wt.% and
more typically
less than about 15 wt.% alkali (dry basis of the ash), typically expressed as
CaO.
"High iron coals" refer to coals having a total iron content of at least about
10 wt.%
(dry basis of the ash), typically expressed as Fe2O3, while "low iron coals"
refer to coals
having a total iron content of less than about 10 wt.% (dry basis of the ash),
typically
expressed as Fe2O3. As will be appreciated, iron and sulfur are typically
present in coal in
the form of ferrous or ferric carbonates and/or sulfides, such as iron pyrite.
"High sulfur coals" refer to coals having a total sulfur content of at least
about 1.5
wt.% (dry basis of the coal) while "medium sulfur coals" refer to coals having
between
about 1.5 and 3 wt.% (dry basis of the coal) and "low sulfur coals" refer to
coals typically
having a total sulfur content of less than about 1.5 wt.% (dry basis of the
coal), more
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typically having a total sulfur content of less than about 1.0 wt.%, and even
more typically
having a total sulfur content of less than about 0.8 wt.% of the coal (dry
basis of the coal).
"Ion exchange medium" refers to a medium that is able, under selected
operating
conditions, to exchange ions between two electrolytes or between an
electrolyte solution
and a complex. Examples of ion exchange resins include solid polymeric or
mineralic "ion
exchangers". Other exemplary ion exchangers include ion exchange resins
(functionalized
porous or gel polymers), zeolites, montmorillonite clay, clay, and soil humus.
Ion
exchangers are commonly either cation exchangers that exchange positively
charged ions
(cations) or anion exchangers that exchange negatively charged ions (anions).
There are
also amphoteric exchangers that are able to exchange both cations and anions
simultaneously. Ion exchangers can be unselective or have binding preferences
for certain
ions or classes of ions, depending on their chemical structure. This can be
dependent on
the size of the ions, their charge, or their structure. Typical examples of
ions that can bind
to ion exchangers are: 1-1-h (proton) and OW (hydroxide); single-charged
monoatomic ions
like Na, K+, and Cl; double-charged monoatomic ions like Ca2+ and Mg2+;
polyatomic
inorganic ions like S042- and P043-; organic bases, usually molecules
containing the amino
functional group - NR2H+; organic acids often molecules containing -COO-
(carboxylic
acid) functional groups; and biomolecules that can be ionized: amino acids,
peptides,
proteins, etc.
Mercury Active Agent refers to an additive that oxidizes elemental mercury
and/or
catalyzes the formation of diatomic halogens.
Neutron Activation Analysis ("NAA") refers to a method for determining the
elemental content of samples by irradiating the sample with neutrons, which
create
radioactive forms of the elements in the sample. Quantitative determination is
achieved
by observing the gamma rays emitted from these isotopes.
"Oxidizing agent", "oxidant" or "oxidizer" refers to an element or compound
that
accepts one or more electrons to another species or agent that is oxidized.
In the
oxidizing process the oxidizing agent is reduced and the other species which
accepts the
one or more electrons is oxidized. More specifically, the oxidizer is an
electron acceptor,
or recipient, and the reductant is an electron donor or giver.
"Particulate" refers to fine particles, such as fly ash, unburned carbon, soot
and fine
process solids, typically entrained in a gas stream.
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The phrase "ppmw X" refers to the parts-per-million, based on weight, of X
alone.
It does not include other substances bonded to X.
The phrase "ppmv X" refers to the parts-per-million, based on volume, of X
alone.
It does not include other substances bonded to X.
"Separating" and cognates thereof refer to setting apart, keeping apart,
sorting,
removing from a mixture or combination, or isolating. In the context of gas
mixtures,
separating can be done by many techniques, including electrostatic
precipitators,
baghouses, scrubbers, and heat exchange surfaces.
A "sorbent" is a material that sorbs another substance; that is, the material
has the
capacity or tendency to take it up by sorption.
"Sorb" and cognates thereof mean to take up a liquid or a gas by sorption.
"Sorption" and cognates thereof refer to adsorption and absorption, while
desorption is the reverse of adsorption.
The preceding is a simplified summary of the disclosure to provide an
understanding of some aspects of the disclosure. This summary is neither an
extensive nor
exhaustive overview of the disclosure and its various aspects, embodiments,
and
configurations. It is intended neither to identify key or critical elements of
the disclosure
nor to delineate the scope of the disclosure but to present selected concepts
of the
disclosure in a simplified form as an introduction to the more detailed
description
presented below. As will be appreciated, other aspects, embodiments, and
configurations
of the disclosure are possible utilizing, alone or in combination, one or more
of the
features set forth above or described in detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings are incorporated into and form a part of the
specification to illustrate several examples of the present disclosure. These
drawings,
together with the description, explain the principles of the disclosure. The
drawings
simply illustrate preferred and alternative examples of how the disclosure can
be made and
used and are not to be construed as limiting the disclosure to only the
illustrated and
described examples. Further features and advantages will become apparent from
the
following, more detailed, description of the various aspects, embodiments, and
configurations of the disclosure, as illustrated by the drawings referenced
below.
Fig. 1 is a block diagram according to an embodiment;
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CA 02793326 2012-10-25
Fig. 2 is a block diagram according to an embodiment;
Fig. 3 is a block diagram according to an embodiment;
Fig. 4 is a block diagram according to an embodiment;
Fig. 5 is a block diagram according to an embodiment;
Fig. 6 Is a block diagram according to an embodiment;
Fig. 7 is a plot of total mercury emissions (pg/wscm) (vertical axis) against
time
(horizontal axis);
Fig. 8 is a block diagram according to an embodiment;
Fig. 9 is a block diagram according to an embodiment; and
Fig. 10 is a block diagram according to an embodiment.
DETAILED DESCRIPTION
Mercury Removal by Iodine-Containing Additive
The current disclosure is directed to the use of an iodine-containing
additive,
typically present in relatively low concentrations, to control mercury
emissions from vapor
phase mercury evolving facilities, such as smelters, autoclaves, roasters,
steel foundries,
steel mills, cement kilns, power plants, waste incinerators, boilers, and
other mercury-
contaminated gas stream producing industrial facilities. Although the mercury
is typically
evolved by combustion, it may be evolved by other oxidation and/or reducing
reactions,
such as roasting, autoclaving, and other thermal processes that expose mercury
containing
materials to elevated temperatures.
There are a number of possible mechanisms for mercury capture in the presence
of
iodine.
While not wishing to be bound by any theory, a path for oxidation of mercury
appears to be initiated by one or more reactions of elemental mercury and an
iodine
molecule in the form of 12. The oxidation reactions may be homogeneous,
heterogeneous,
or a combination thereof. For heterogeneous reactions, the reaction or
collection surface
can, for example, be an air preheater surface, duct internal surface, an
electrostatic
precipitator plate, an alkaline spray droplet, dry alkali sorbent particles, a
baghouse filter,
an entrained particle, fly ash, carbon particle, or other available surface.
It is believed that
iodine can oxidize typically at least most, even more typically at least about
75%, and
even more typically at least about 90% of the elemental mercury in the mercury-
containing gas stream.
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CA 02793326 2012-10-25
Under most flue gas conditions, the mercury reaction kinetics for iodine
appear to
be faster at higher temperatures than mercury reaction kinetics for chlorine
or bromine at
the same temperature. With chlorine, almost all the chlorine in the flame is
found as HC1,
with very little Cl. With bromine, there are, at high temperatures,
approximately equal
amounts of HBr on the one hand and Br2 on the other. This is believed to be
why
oxidation of Hg by bromine is more efficient than oxidation by chlorine.
Chemical
modeling of equilibrium iodine speciation in a subbituminous flue gas
indicates that, at
high temperatures, there can be one thousand times less HI than I (in the form
of 12) in the
gas. In many applications, the molecular ratio, in the gas phase of a mercury-
containing
gas stream, of elemental iodine to hydrogen-iodine species (such as HI) is
typically at least
about 10:1, even more typically at least about 25:1, even more typically at
least about
100:1, and even more typically at least about 250:1.
While not wishing to be bound by any theory, the end product of reaction can
be
mercuric iodide (HgI2 or Hg2I2), which has a higher condensation temperature
(and boiling
point) than both mercuric bromide (HgBr2 or Hg2Br2) and mercuric chloride
(HgC12 or
Hg2C12). The condensation temperature (or boiling point) of mercuric iodide
(depending
on the form) is in the range from about 353 to about 357 C compared to about
322 C for
mercuric bromide and about 304 C for mercuric chloride. The condensation
temperature
(or boiling point) for iodine (I2) is about 184 C while that for bromine
(Br2) is about 58
C.
While not wishing to be bound by any theory, another possible reaction path is
that
other mercury compounds are formed by multi-step reactions with iodine as an
intermediate. One possible multi-step reaction is that iodine reacts with
sulfur oxides to
form reduced forms of sulfur, which reduced forms of sulfur then react with
mercury and
form capturable particulate mercury-sulfur compounds.
As will be appreciated, these theories may not prove to be correct. As further
experimental work is performed, the theories may be refined and/or other
theories
developed. Accordingly, these theories are not to be read as limiting the
scope or breadth
of this disclosure.
Fig. 1 depicts a contaminated gas stream treatment process for an industrial
facility
according to an embodiment. Referring to Fig. 1, a mercury-containing feed
material 100
is provided. In one application, the feed material 100 is combustible and can
be any
synthetic or natural, mercury-containing, combustible, and carbon-containing
material,
CA 02793326 2012-10-25
including coal and biomass. The feed material 100 can be a high alkali or high
iron coal.
In other applications, the present disclosure is applicable to noncombustible,
mercury-
containing feed materials, including without limitation metal-containing ores,
concentrates, and tailings.
The feed material 100 can natively include, without limitation, varying levels
of
halogens and mercury. Typically, the feed material 100 includes typically at
least about
0.001 ppmw, even more typically from about 0.003 to about 100 ppmw, and even
more
typically from about 0.003 to about 10 ppmw mercury (both elemental and
speciated)
(measured by neutron activation analysis ("NAA")). Commonly, a combustible
feed
material 100 includes no more than about 5 ppmw iodine, more commonly no more
than
about 4 ppmw iodine, even more commonly no more than about 3 ppmw iodine, even
more commonly no more than about 2 ppmw iodine and even more commonly no more
than about 1 ppmw iodine (measured by neutron activation analysis ("NAA")). A
combustible feed material 100 generally will produce, upon combustion, an
unburned
carbon ("UBC") content of from about 0.1 to about 30% by weight and even more
generally from about 0.5 to about 20% by weight.
The feed material 100 is combusted in thermal unit 104 to produce a mercury-
containing gas stream 108. The thermal unit 104 can be any combusting device,
including, without limitation, a dry or wet bottom furnace (e.g., a blast
furnace, puddling
furnace, reverberatory furnace, Bessemer converter, open hearth furnace, basic
oxygen
furnace, cyclone furnace, stoker boiler, cupola furnace and other types of
furnaces), boiler,
incinerator (e.g., moving grate, fixed grate, rotary-kiln, or fluidized or
fixed bed,
incinerators), calciners including multi-hearth, suspension or fluidized bed
roasters,
intermittent or continuous kiln (e.g., ceramic kiln, intermittent or
continuous wood-drying
kiln, anagama kiln, bottle kiln, rotary kiln, catenary arch kiln, Feller kiln,
noborigama kiln,
or top hat kiln), oven, or other heat generation units and reactors.
The mercury-containing gas stream 108 includes not only elemental and/or
speciated mercury but also a variety of other materials. A common mercury-
containing
gas stream 108 includes at least about 0.001 ppmw, even more commonly at least
about
0.003 ppmw, and even more commonly from about 0.005 to about 0.02 ppmw mercury
(both elemental and speciated). Other materials in the mercury-containing gas
stream 108
can include, without limitation, particulates (such as fly ash), sulfur
oxides, nitrogen
oxides, carbon oxides, unburned carbon, and other types of particulates.
11
The temperature of the mercury-containing gas stream 108 varies depending on
the
type of thermal unit 104 employed. Commonly, the mercury-containing gas stream
temperature is at least about 125 C, even more commonly is at least about 325
C, and even
more commonly ranges from about 325 to about 500 C.
The mercury-containing gas stream 108 is optionally passed through the
preheater
112 to transfer some of the thermal energy of the mercury-containing gas
stream 108 to
air input to the thermal unit 104. The heat transfer produces a common
temperature drop
in the mercury-containing gas stream 108 of from about 50 to about 300 C to
produce a
mercury-containing gas stream 116 temperature commonly ranging from about 100
to
about 400 C.
The mercury-containing gas stream 116 is next subjected to particulate removal
device 120 to remove most of the particulates from the mercury-containing gas
stream and
form a treated gas stream 124. The particulate removal device 120 can be any
suitable
device, including an electrostatic precipitator, particulate filter such as a
baghouse, wet
particulate scrubber, and other types of particulate removal devices.
The treated gas stream 124 is emitted, via gas discharge 128, into the
environment.
To control mercury emissions in the mercury-containing gas stream 108, an
iodine-
containing additive 132 is employed. The iodine in the additive 132 can be in
the form of
a solid, liquid, vapor, or a combination thereof. The iodine containing
additive can
comprise a vapor phase iodine, wherein at least 75% of the iodine-containing
additive in
the gas stream is the vapor phase iodine, at least 85% of the iodine-
containing additive in
the gas stream is the vapor phase iodine, and at least 95% of the iodine-
containing additive
in the gas stream is the vapor phase iodine. It can be in the form of
elemental iodine (12),
a halide (e.g., binary halides, oxo halides, hydroxo halides, and other
complex halides), an
inter-halogen cation or anion, iodic acid, periodic acid, periodates, a
homoatomic
polyanion, and mixtures thereof. In one formulation, the iodine in the
additive 132 is
composed primarily of an alkali or alkaline earth metal iodide. In one
formulation, the
iodine-containing additive 132 is substantially free of other halogens and
even more
typically contains no more than about 25%, even more typically no more than
about 10%,
and even more typically no more than about 5% of the halogens as halogen(s)
other than
iodine. In one formulation, the iodine-containing additive 132 contains at
least about 100
ppmw, more commonly at least about 1,000 ppmw, and even more commonly at least
about 1 wt.% iodine. In one formulation, the iodine-containing additive
contains no more
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than about 40 wt.% fixed or total carbon, more commonly no more than about 25
wt.%
fixed or total carbon, even more commonly no more than about 15 wt.% fixed or
total
carbon, and even more commonly no more than about 5 wt.% fixed or total
carbon. In one
formulation, the iodine-containing additive 132 is a high (native) iodine
coal. In one
formulation, the iodine-containing additive 132 is an iodine-containing waste
or byproduct
material, such as a medical waste. In one formulation, the iodine-containing
additive 132
comprises iodine attached to a solid support, such as by absorption,
adsorption, ion
exchange, formation of a chemical composition, precipitation, physical
entrapment, or
other attachment mechanism. The solid support can be inorganic or organic.
Examples
include ion exchange resins (functionalized porous or gel polymers), soil
humus, a porous
carbonaceous material, metal oxides (e.g., alumina, silica, silica-alumina,
gamma-alumina,
activated alumina, acidified alumina, and titania), metal oxides containing
labile metal
anions (such as aluminum oxychloride), non-oxide refractories (e.g., titanium
nitride,
silicon nitride, and silicon carbide), diatomaceous earth, mullite, porous
polymeric
materials, crystalline aluminosilicates such as zeolites (synthetic or
naturally occurring),
amorphous silica-alumina, minerals and clays (e.g., bentonite, smectite,
kaolin, dolomite,
montmorillinite, and their derivatives), porous ceramics metal silicate
materials and
minerals (e.g., one of the phosphate and oxide classes), ferric salts, and
fibrous materials
(including synthetic (for example, without limitation, polyolefins,
polyesters, polyamides,
polyacrylates, and combinations thereof) and natural (such as, without
limitation, plant-
based fibers, animal-based fibers, inorganic-based fibers, cellulosic, cotton,
paper, glass
and combinations thereof). Commonly, the halogen-containing additive 232
contains no
more than about 10 wt.% iodine, more commonly no more than about 5 wt.%
iodine, even
more commonly no more than about 1 wt.% iodine, even more commonly no more
than
about 0.5 wt.% iodine, and even more commonly no more than about 0.1 wt.%
iodine.
The iodine-containing additive 132 can be contacted with the mercury-
containing
gas stream at one or more contact points 136, 140, and 148 (where point 136
can be remote
from the location of the thermal unit, including applying the additive to the
feed at places
such as a mine or in transit to the thermal unit location). At point 136, the
iodine-
containing additive 132 is added directly to the feed material 100 upstream of
the thermal
unit 104. At points 140 and 148, the iodine-containing additive 132 is
introduced into the
mercury-containing gas stream 108 or 116, such as by injection as a liquid,
vapor, or solid
powder. As can be seen from Fig. 1, the additive introduction can be done
upstream or
13
CA 2793326 2019-09-18
downstream of the (optional) air preheater 112. The iodine-containing additive
can be
dissolved in a liquid, commonly aqueous, in the form of a vapor, in the form
of an aerosol,
or in the form of a solid or supported on a solid. In one formulation, the
iodine-containing
additive 132 is introduced as a liquid droplet or aerosol downstream of the
thermal unit
104. In this formulation, the iodine is dissolved in a solvent that
evaporates, leaving behind
solid or liquid particles of the iodine-containing additive 132.
Surprisingly, the iodine-containing additive 132 can allow mercury capture
without
a carbon sorbent, native unburned carbon, or ash being present. In contrast to
bromine,
mercury removal by iodine does not primarily depend on co-injection of
activated carbon
sorbents for vapor-phase mercury capture. In one process configuration, the
mercury-
containing gas stream upstream of the particulate removal device is
substantially free of
activated carbon. The iodine-containing additive 132 can effectively enable or
facilitate
removal of at least about 40%, even more commonly at least about 75%, and even
more
commonly at least about 90% of the elemental and speciated mercury in the
mercury-
containing gas stream when the feed material 100, upon combustion, produces a
UBC of
no more than about 30% and even more commonly of no more than about 5%. When a
higher UBC level is produced, the iodine-containing additive 132 can remove at
least about
40%, even more commonly at least about 75%, and even more commonly at least
about
90% of the elemental and speciated mercury in the mercury-containing gas
stream that is
not natively removed by the unburned carbon particles.
In one plant configuration, sufficient iodine-containing additive 132 is added
to
produce a gas-phase iodine concentration commonly of about 8 ppmw basis of the
flue gas
or less, even more commonly of about 5 ppmw basis or less, even more commonly
of
about 3.5 ppmw basis or less, even more commonly of about 1.5 ppmw or less,
and even
more commonly of about 0.4 ppmw or less of the mercury-containing gas stream.
Stated
another way, the iodine concentration relative to the weight of mercury-
containing,
combustible (e.g., coal) feed (as fed) (whether by direct application to the
combustible
feed and/or injection into the mercury-containing (e.g., flue) gas) commonly
is about 40
ppmw or less, more commonly about 35 ppmw or less, even more commonly about 30
ppmw or less, even more commonly is about 15 ppmw or less, even more commonly
no
more than about lOppmw, even more commonly no more than about 6 ppmw, even
more
commonly about 4 ppmw or less, and even more commonly no more than about 3
ppmw.
Stated another way, the molar ratio, in the mercury-containing (e.g., flue)
gas, of gas-phase
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diatomic iodine to total gas-phase mercury (both speciated and elemental) is
commonly
no more than about 1,200, and even more commonly no more than about 600, even
more
commonly no more than about 250, even more commonly no more than about 150,
and
even more commonly no more than about 80. By way of illustration, an effective
concentration of gas-phase iodine at the air preheater outlet or particulate
removal device
inlet ranges from about 0.1 to about 10 ppmw, even more commonly from about
0.15 to
about 5 ppmw, even more commonly from about 0.20 to about 2 ppmw, and even
more
commonly from about 0.25 to about 1.50 ppmw of the mercury-containing gas
stream.
Commonly, the mercury-containing gas stream includes no more than about 1.0,
even more commonly no more than about 0.5 and even more commonly no more than
about 0.1 ppmw total bromine. The feed material generally includes no more
than about
ppmw and even more commonly no more than about 5 ppmw natively occurring
bromine.
The mercury-containing (e.g., flue) gas temperature for elemental mercury
capture
promoted by iodine commonly ranges from about 150 to about 600 C and even more
commonly from about 180 to about 450 C. The residence time upstream of
particulate
(e.g., fly ash) removal device 120 is commonly about 8 seconds, and even more
commonly
at least about 4 seconds, and even more commonly at least about 2 seconds.
Generally, sufficient iodine-containing additive 132 is added to produce a gas-
phase iodine concentration commonly of about 3.5 ppmw or less, even more
commonly of
about 2 ppmw or less, even more commonly of about 1.5 ppmw or less, and even
more
commonly of about 0.4 ppmw or less. Stated another way, the molar ratio, in
the mercury-
containing (e.g., flue) gas, of gas-phase iodine to total gas-phase mercury
(both speciated
and elemental) is commonly no more than about 1,000, even more commonly no
more
than about 600, even more commonly no more than about 500, even more commonly
no
more than about 250, even more commonly no more than about 150, and even more
commonly no more than about 80.
The above concentration ranges and conditions can, in appropriate
applications,
apply to bromine as a mercury removal additive.
Mercury Removal by Halogen-Containing Additive in Presence of Selective
Catalytic Reduction
In another plant configuration shown in Fig. 2, the halogen concentration
needed
to effect mercury removal is further reduced by coupling halogen with a
selective catalytic
CA 2793326 2019-09-18
reduction ("SCR'') zone prior to particulate removal. As will be appreciated,
SCR converts
nitrogen oxides, or NOx, with the aid of a catalyst, into diatomic nitrogen
(N2) and water.
A gaseous reductant, typically anhydrous ammonia, aqueous ammonia, or urea
(but other
gas-phase reductants may be employed), can be injected into a stream of flue
or exhaust
gas or other type of gas stream or absorbed onto a catalyst followed by off
gassing of the
ammonia into the gas stream. Suitable catalysts include, without limitation,
ceramic
materials used as a carrier, such as titanium oxide, and active catalytic
components, such
as oxides of base metals (such as vanadium (V205), wolfram (W03), titanium
oxide (TiO2)
and tungstate (e.g., W042- ), zeolites, and various precious metals. Other
catalysts,
however, may be used.
The SCR catalyst surface, depending on the design, catalyst and layering, is
active
for reactions other than the primary nitrogen oxide reduction. There are
competing
reactions occurring for available sites to reduce NOx, oxidize SO2 to SO3 and
to promote
the reaction of mercury with various species to result in an increased
fraction of oxidized
mercury species. The SCR ammonia rate is co-variable with load and temperature
and
affects the balance between these competing reactions.
The presence of ultra trace vapor iodine and/or bromine species at the SCR
catalyst
surface can be surprisingly effective for mercury control. While not wishing
to be bound
by any theory, the amount of halogen (e.g., iodine and/or bromine) required to
result in the
formation of an oxidized form of mercury is lower when an SCR is in use. The
surface of
the SCR catalyst is believed to promote the formation of diatomic elemental
halogens
and/or mercury oxidation.
To capture the oxidized mercury in the particulate collection device, vapor
SO3 in
the flue gas should be managed to limit the concentration after the air
preheater. This can
be accomplished by selection of lower-sulfur and higher alkaline coal,
selection of lower
activity SCR catalyst and control of SCR reagent injection rate and
temperature. These
interactive parameters must be managed to achieve the desired deN0x rate.
In one configuration, combustion of high alkali coals and low-sulfur coals is
preferred to further inhibit formation of vapor SO3 species. The SCR catalyst
is typically
selected to yield a specified SO2 to S03 conversion rate for a design coal and
operating
condition. Catalyst activity gradually degrades over a period of years and
must be
replaced. A number of catalytic layers installed at intervals, with different
oxidation rates,
ages and catalyst activity are typically present for coal-fired plant SCRs.
Effective SO2 to
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S03 oxidation rate across the aggregate of catalyst surfaces in an SCR should
be preferably
lower than about 2.0% and more preferably lower than about 1.5% and even more
preferably lower than about 1.2% in order to limit vapor SO3 formation in the
SCR.
Vapor SO3 can also be controlled after formation in the SCR by means of in-
duct
injection of sorbents or by absorption in a dry scrubber. Vapor SO3 can also
be condensed
on intermediate surfaces prior to the particulate control device, in
particular the air
preheater. Lower process temperatures will reduce SCR oxidation rate and
increase SO3
dropout in the air preheater. Mercury oxidation in the SCR benefits when SO3
is reduced
as a competing reaction, even at lower temperatures.
Ammonia reacts within the SCR to reduce nitrogen oxides. Excess ammonia
lowers the ability of the SCR to react with or catalyze reaction (oxidation)
of mercury. In
one configuration, concentration(s) of ammonia and precursors thereof are
maintained at
a level just sufficient for deN0x. That is, the concentration of ammonia and
precursors
thereof is preferably no more than about 125%, more preferably no more than
about 120%,
more preferably no more than about 115%, and even more preferably no more than
about
110% of the stoichiometric amount required to perform deN0x. As a result, the
amount
of ammonia slip will be reduced relative to conventional SCR-based systems.
SCR
reagent rate and/or ammonia and precursor addition is controlled to yield an
average flue
gas ammonia slip immediately downstream of the SCR of preferably less than
about 5
ppmv and more preferably less than about 3 ppmv as ammonia.
To realize the full benefits of mercury oxidation by low concentration halogen
addition with SCR and further achieve substantial removal of mercury in the
particulate
control device, the vapor SO3 in the flue gas at the particulate control
device inlet is
commonly limited to less than about 7.5 ppmv, more commonly to less than about
5 ppmv
and more preferably to less than 2 ppmv at all process conditions by the
methods described
above, singly or in combination.
In one configuration, the halogen/mercury mass ratio for iodine as the primary
halogen additive is commonly no more than about 200, more commonly no more
than
about 100, more commonly no more than about 75, more commonly no more than
about
50, and more commonly no more than about 40 and commonly at least about 5,
more
commonly at least about 10, more commonly at least about 15, and even more
commonly
at least about 20.
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CA 2793326 2019-09-18
In one configuration, the halogen/mercury mass ratio for brome as the primary
halogen additive is commonly no more than about 400, more commonly no more
than
about 300, more commonly no more than about 275, more commonly no more than
about
250, and more commonly no more than about 240 and commonly at least about 10,
more
commonly at least about 20, more commonly at least about 30, and even more
commonly
at least about 40.
In one configuration, the maximum amount of halogen added to the feed material
100 is commonly no more than about 40 ppmw, more commonly no more than about
25
ppmw, more commonly no more than about 20 ppmw, more commonly no more than
about 15 ppmw, and even more commonly no more than about 10 ppmw, and the
minimum
amount of halogen added to the feed material 100 is commonly at least about
0.5 ppmw,
more commonly at least about 0.75 ppmw, more commonly at least about 1 ppmw,
and
even more commonly at least about 1.5 ppmw.
The embodiment is directed particularly to the capture of mercury from systems
with conventional SCR firing low sulfur and halogen-deficient coals (e.g.,
coals having no
more than about 6 ppmw bromine and/or iodine). The embodiment can enhance
mercury
capture on the particulate control device and not on any wet flue gas
desulfurization (FGD)
scrubber. Low concentration iodine, in particular, surprisingly achieves
mercury
oxidation with SCR, and the oxidized mercury can be captured in the
particulate-phase on
the fly ash surface, particularly when the sulfur trioxide concentration in
the flue gas at the
particulate control device is managed to an amount of less than about 2 ppmv.
This
embodiment can be used with currently installed SCR catalysts for low sulfur,
low halogen
fueled units. In addition, though for bituminous coals there is ample native
chlorine and
bromine to oxidize mercury to as high as 100% across the SCR, the SCR is
generally
unable to realize any mercury capture until the FGD scrubber (due to the vapor
SO3). Iodine-oxidized mercury species are typically not effectively captured
in an FGD
scrubber because iodine-Hg species are generally not soluble. Accordingly, the
present
embodiment captures mercury (for lower SO3 concentrations) at the ESP or
baghouse.
Referring to Fig. 2, the waste stream 108 optionally flows through an
economizer
200, which transfers some of the heat of the combustion stream 108 to water
for recycle
to other operations. The halogen-containing additive 232 is contacted with the
feed
material 100 upstream of the thermal unit 104 and/or with the mercury-
containing gas
stream 108 inside or downstream of the thermal unit 104.
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The mercury-containing gas stream 108 proceeds to SCR unit 204, where nitrogen
oxides are converted into molecular nitrogen and water.
The mercury-containing gas stream 108 proceeds to the optional air preheater
112
and then is subjected to particulate removal by particulate removal device 120
to form a
treated gas stream 124 that is substantially free of particulates and mercury.
As will be
appreciated, an economizer uses waste heat by transferring heat from flue
gases to warm
incoming feedwater (e.g., which is subsequently converted into steam for power
generation) while a preheater is a heat exchanger that transfers thermal
energy from flue
gas to combustion air before input of the combustion air to the furnace.
The treated gas stream 124 is then emitted from the gas discharge 128.
Although the SCR catalyst is commonly located between the economizer and air
preheater outlet, it may be located at other locations in the mercury-
containing gas stream.
The SCR catalyst is commonly located prior to (or upstream of) the particulate
removal
device (e.g. baghouse and electrostatic precipitator). Commonly, SCR catalysis
is
performed at a temperature ranging from about 250 to about 500 C, more
commonly at a
temperature ranging from about 300 to about 450 C, and even more commonly at a
temperature ranging from about 325 to about 400 C. The degree of SO2 to SO3
oxidation
by the SCR varies within this temperature range depending on plant load,
catalyst
characteristics and other process conditions. At lower temperatures, the SCR
contribution
to total vapor SO3 can be negligible.
Removal of Mercury by Halogen-Containing Additive and Mercury Active Agent
In another configuration, a combination of iodine or bromine on the one hand
with
another mercury active agent on the other is utilized to effect mercury
oxidation and
removal, particularly at higher temperatures than is possible with bromine or
chlorine or
hydrogen chloride alone. As used herein, a "mercury active agent" refers to an
additive
that oxidizes elemental mercury and/or catalyzes the formation of diatomic
halogens.
Conversion of elemental mercury to oxidized species is typically accomplished
by the
combined halogens that are either present in or added to the combustible fuel.
In one plant design, additional halogens, preferably in the form of diatomic
elemental halogens, are, in addition to those in the combustible fuel,
injected post-
combustion.
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In another plant design, oxidants, such as other halogens or non-halogen
mercury
active agents, are added to the combustible fuel in addition to or in lieu of
downstream
flue gas introduction.
Examples of suitable mercury active agents that are oxidants, which are
combined
with iodine or bromine prior to fuel pre-combustion include bromide salts
including
sodium, calcium, magnesium, zinc or potassium bromide, calcium, sodium or
potassium
bromate, iodide salts including sodium, calcium, magnesium, zinc or potassium
iodide,
calcium, sodium or potassium iodate, chloride salts including sodium, calcium,
magnesium, zinc or potassium chloride, calcium, sodium or potassium chlorate,
sodium,
iron oxides sufficient to enrich the fly ash, and diatomic bromine or chlorine
sorbed onto
a suitable sorbent.
Examples of additional mercury active agents that are oxidants added to the
feed
material pre- or to the waste gas post-combustion in combination with iodine
or bromine
added to the feed material include bromine, iodine, or chlorine gas
(preferably as diatomic
elemental halogens), hydrogen bromide, hydrogen chloride, chlorite, chlorate,
perchlorate,
hypochlorite, and other bromine, chlorine and fluorine-containing compounds,
finely
divided iron oxides, copper oxides and other base metal oxides.
As will be appreciated, other mercury active agents can be employed. For
example,
different mercury active agents, that perform differing of the functions of
oxidizing
elemental mercury and/or catalyzing the formation of diatomic halogens, can be
blended,
mixed or otherwise combined or co-injected. For example, mercury oxidants can
be added
upstream, in, or downstream of the SCR zone. In one formulation, a first
mercury active
agent, such as an SCR catalyst, can catalyze the formation of diatomic
halogens, a second
mercury active agent, such as a halogen-containing compound or metal oxide,
can oxidize
elemental mercury and a reactive surface agent, such as circulating fluidized
bed ash,
powdered zeolites, fluidized catalytic cracker (FCC) fines, fumed silicates,
metal oxide
particles or powders, such as iron oxide, re-milled or fine fraction fly ash,
fluidized bed
combustor ash and combinations thereof, provides surface area for removal of
mercury
compounds.
While not wishing to be bound by any theory, it is believed that the halogen
in the
form of diatomic halogen gas is both an efficient mercury oxidizer and is
available for
reaction with oxidized mercury by direct halogenation to form, for example,
HgI2 or
I-IgBr2 In this configuration, the iodine (or iodine and bromine or bromine)
concentration
CA 2793326 2019-09-18
needed to effect mercury oxidation and mercury halogenation is reduced by
addition of an
oxidant to the flue or waste gas.
The mercury active agent and halogen are believed to act synergistically to
effect
mercury oxidation for subsequent removal by fly ash, unburned carbon, or
another suitable
additive. The mercury active agent can be supported or unsupported, with
preferred
carriers being a porous carbonaceous substrate (such as fly or bottom ash from
coal
combustion, carbon black, activated carbon, coke, char, charcoal, and the
like), activated
alumina, ceramic, clay, silica, silica-alumina, silicates, zeolites, fine
fraction fly ash,
bottom ash, FCC fines, fluidized bed combustor ash, and the like. The mercury
active
agent can be introduced either as a liquid, such as a slurry in a vaporizable
carrier liquid
or dissolved in a solvent, as particles or powders, as a gas, or as a
combination thereof.
In either of the above plant configurations, the mercury oxidation, whether by
unburned carbon or mercury active agent addition, is performed preferably
between the
economizer and air preheater outlet or at a preferred temperature of from
about 250 to
about 500 C, a more preferred temperature of from about 300 to about 450 C,
and an even
more preferred temperature of from about 325 to about 400 C.
In one application, iodine and/or bromine is added to the combustible fuel or
otherwise introduced to the furnace or boiler, such as in levels set forth
above, while a
diatomic elemental halogen (such as diatomic elemental iodine, bromine, and/or
chlorine)
is added to the flue gas downstream from the furnace or boiler. In this
configuration, the
flue gas concentration of the injected or otherwise introduced diatomic
elemental halogen
preferably ranges from about 0.1 to about 8 ppmw of the flue gas, even more
preferably
from about 0.25 to about 5 ppmw, and even more preferably from about 0.5 to
about 2
PPm,
In one application, iodine and/or bromine are added to the combustible fuel or
otherwise introduced to the furnace or boiler while a non-halogen oxidant,
such as those
set forth above, is added to the flue gas downstream from the furnace or
boiler. In this
configuration, the flue gas concentration of the injected or otherwise
introduced oxidant
preferably ranges from about 0.1 to about 8 ppmw, even more preferably from
about 0.25
to about 5 ppmw, and even more preferably from about 0.5 to about 2 ppmw.
In either application, the halogen or non-halogen oxidant or mixture thereof
is
typically introduced either as a gas or a liquid droplet or aerosol, with the
oxidant being
dissolved in a vaporizable solvent.
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In one application, halide or interhalogen compound-containing additive 132 is
added to the feed material 100 or otherwise introduced to the thermal unit 104
while
diatomic elemental iodine (12) or bromine (Br2) is added to the flue gas
downstream from
the thermal unit 104. In this configuration, the flue gas concentration of the
injected or
otherwise introduced diatomic iodine commonly ranges from about 0.1 to about 8
ppmw,
even more commonly from about 0.25 to about 5 ppmw, and even more commonly
from
about 0.5 to about 2 ppmw of the mercury-containing gas stream.
Figs. 8-9 provide an example of a plant configuration according to an
embodiment.
The mercury active agent 900 is introduced to the feed material 100, in the
thermal
unit 104, at a point 140 between the thermal unit 104 and optional preheater
112 and/or at
a point 148 between the optional preheater 112 and a particulate removal
device 120 or
between the optional preheater 112 (Fig. 8) and a scrubber 400 (Fig. 9). When
the mercury
active agent 900 is introduced upstream of the preheater 112, the mercury
active agent 900
is typically a non-carbonaceous agent due to the high mercury-containing gas
stream
temperature.
The mercury-containing gas stream 116 is thereafter treated by the particulate
removal device 120 (Fig. 8) and/or by the dry scrubber 400 and particulate
removal device
120 (Fig. 9) to form a treated gas stream. The dry scrubber 400 injects a dry
reagent or
slurry into the mercury-containing gas stream 116 to ''wash out" acid gases
(such as SO2
and HC1). A dry or semi-dry scrubbing system, unlike a wet scrubber, does not
saturate
the flue gas stream that is being treated with moisture. In some cases, no
moisture is added.
In other cases, only the amount of moisture that can be evaporated in the flue
gas without
condensing is added.
Although the scrubber 400 is shown after the preheater 112, it is to be
understood
that the scrubber 400 may be located at several different locations, including
without
limitation in the thermal unit 104 or in the gas stream duct (at a point
upstream of the
particulate control device 120 such as at points 140 and/or 148) (as shown in
Fig. 9).
The particulate control device 120 removes substantially all and typically at
least
about 90% of the particles entrained in the mercury-containing gas stream 116.
As a result,
at least most of the iodine and mercury in the mercury-containing gas stream
116 is
removed by the particle removal device 120.
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Removal of Mercury by Halogen-Containing Additive and Reactive Surface Agent
Addition
Although additional reactive surface particles are normally not required for
iodine
to form a mercury-containing particulate, in other embodiments addition of
carbon- and
non-carbon-containing solid and/or aerosol particles, referred to as "reactive
surface
agents", in favorable regions of the flue gas stream can enhance mercury
removal by the
iodine-containing additive 132, particularly when the feed material 100
produces, upon
combustion, a low UBC level or the mercury-containing gas stream 108 has low
levels of
natively occurring particulates, such as ash, unburned carbon, soot, and other
types of
particulates. Low UBC levels generally comprise no more than about 30, even
more
generally no more than about 5, and even more generally no more than about
0.5% UBC
in the post-combustion particulate. In one configuration, the surface active
agent acts as a
support for iodine or bromine, as discussed above.
While not wishing to be bound by any theory, it is believed that reactive
surface
agents provide surface area that iodine, mercury, and/or mercuric iodide or
bromide can
chemically react with (e.g., provides surface area for heterogeneous mercury
reactions)
and/or otherwise attach to. The reactive surface agent can be any carbon- or
non-carbon-
containing particle that provides a nucleation or reaction site for iodine,
bromide, mercury,
mercuric iodide, and/or mercuric bromide. Suitable solid or liquid reactive
surface agents
300 include, without limitation, zeolites, silica, silica alumina, alumina,
gamma-alumina,
activated alumina, acidified alumina, amorphous or crystalline
aluminosilicates,
amorphous silica alumina, ion exchange resins, clays (such as bentonite), a
transition metal
sulfate, porous ceramics, porous carbonaceous materials, such as coal ash
(e.g., fly or
bottom ash), unburned carbon, charcoal, char, coke, carbon black, activated
carbon, other
hydrocarbon and coal derivatives, and other forms of carbon, trona, alkali
metal
bicarbonates, alkali metal bisulfates, alkali metal bisulfites, alkali metal
sulfides, elemental
sulfur, limestone, hydrated or slaked lime, circulating fluidized bed ash,
fluidized catalytic
cracker (FCC) fines, fumed silicates, metal oxide particles or powders, such
as iron oxide
and those comprising labile anions, re-milled or fine fraction fly ash, bottom
ash, fluidized
bed combustor ash, and mixtures thereof. The reactive surface agent 300 may be
introduced as a solid particle (powder) and/or as a dissolved or slurried
liquid composition
comprising a vaporizable liquid carrier.
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The mean, median, and 1390 sizes of the particles are typically no more than
about
100 microns, even more typically no more than about 50 microns, even more
typically no
more than about 25 microns, even more typically no more than about 10 microns,
and even
more typically no more than about 5 microns. Unlike iodine additives, micron-
sized non-
carbon particles have not been consistently effective with bromine or chlorine-
based coal
additives.
In some configurations, the reactive surface agent is a porous carbonaceous or
non-
carbonaceous material, such as coke, fly ash, bottom ash, pet coke, carbon
black, activated
carbon, charcoal, char, beneficiated unburned carbon derived from fly ash, and
mixtures
thereof. In some applications, the porous carbonaceous or non-carbonaceous
material is
powdered and typically has a P85 size of no more than about 1 mm in size and
more
typically of no more than about 0.75 mm in size and an average diameter
typically between
about 0.10 to about 0.75 and more typically between about 0.15 to about 0,25
mm. In
some applications, the porous carbonaceous or non-carbonaceous material is
granular and
typically has a P85 size of more than about 1 mm in size and more typically of
in the range
of from about 1 mm to about 2.5 mm in size and an average diameter typically
between
about 0.75 to about 1.25 mm.
The porous carbonaceous or non-carbonaceous material may be impregnated with
a chemical agent, such as a mercury active agent. Porous carbonaceous or non-
carbonaceous materials can contain a variety of inorganic impregnants, such as
ionic,
elemental, or compounded halogens (e.g., iodine, iodide, bromine, bromide,
chlorine,
chloride, iodine-containing salts, bromine-containing salts, chlorine-
containing salts, and
mixtures thereof) silver, and cations such as alkali earth metals, alkaline
earth metals, and
transition metals. In one formulation, the porous carbonaceous or non-
carbonaceous
material is impregnated with a mercury active agent or SCR catalytic material.
The amount of chemical agent in the porous carbonaceous or non-carbonaceous
material can vary widely. Commonly, the impregnated porous carbonaceous or non-
carbonaceous material comprises at least about 0.1 wt.%, more commonly at
least about
0.5 wt.%, and even more commonly at least about 1 wt.% chemical agent and no
more
than about 5 wt.%, more commonly no more than about 4 wt.%, and even more
commonly
no more than about 2 wt.% chemical agent.
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CA 2793326 2019-09-18
The porous carbonaceous material or non-carbonaceous can have a high surface
area. Typically, the porous carbonaceous or non-carbonaceous material has a
surface area
of at least about 500 m2/g, more typically of at least about 750 m2/g, and
even more
typically of at least about 1,000 m2/g and no more than about 2,500 m2/g, more
typically
no more than about 2,000 m2/g, and even more typically no more than about
1,500 m2/g.
The ash content of the porous carbonaceous or non-carbonaceous material can
determine the efficiency of reactivation. The porous carbonaceous or non-
carbonaceous
material typically has an ash content in the range of from about 10% to about
95% and
even more typically in the range of from about 20% to about 70%.
Commonly, the reactive surface agent is introduced downstream of the iodine-
containing additive 132, more commonly downstream of the economizer 200, and
more
commonly downstream of the air preheater 112 and upstream of particulate
removal device
120 and/or scrubber 400.
In other embodiments, the additive 132 is combined with other pollution
control
technologies that provide suspended solid and/or aerosol particles or other
reaction
surfaces at favorable location and temperature. Exemplary embodiments include,
without
limitation:
1. Spraying slurried solids or solutions of dissolved solids at a point
upstream
to allow sufficient evaporation. In a utility boiler, this region would
normally be prior to,
or upstream of, any air preheater 112 to allow sufficient residence time.
2. Providing a downstream slurry spray such as by conventional flue gas
desulfurization ("FGD") spray dryer absorber ("SDA"). The slurry spray would
normally
downstream of any air preheater 112.
3. Providing alkaline liquid spray, such as wet FGD, to capture residual
mercury past the ESP rather than allowing re-emission of mercury as elemental
mercury -
as can happen with bromine or chlorine.
4. Providing intimate particulate contact for the iodine-mercury or bromine-
mercury compounds, such as filtering the flue gas through a fabric filter.
5. Providing additional submicron aerosol at the inlet to an air preheater
112
to take advantage of the temperature differential across the air preheater to
boost surface
reaction.
Examples of these alternatives will be discussed with reference to Figs. 3-6
and 8.
CA 2793326 2019-09-18
Referring to the embodiments of Figs. 3 and 4, the reactive surface agent 300
is
introduced at a point 140 between the thermal unit 104 and optional preheater
112 and/or
at a point 148 between the optional preheater 112 and a particulate removal
device 120
(Fig. 3) or between the optional preheater 112 and a scrubber 400 (Fig. 4).
When the
reactive surface agent 300 is introduced upstream of the preheater 112 and
vapor phase
halogens are present in the gas stream, the reactive surface agent 300 is
believed to increase
the maximum temperature where mercury removal begins and increase the overall
mercury removal effectiveness.
The mercury-containing gas stream 116 is thereafter treated by the particulate
removal device 120 (Fig. 3) and/or by the dry scrubber 400 and particulate
removal device
120 (Fig. 4) to form a treated gas stream.
Although the scrubber 400 is shown after the preheater 112, it is to be
understood
that the scrubber 400 may be located at several different locations, including
without
limitation in the thermal unit 104 or in the gas stream duct (at a point
upstream of the
particulate control device 120 such as at points 140 and/or 148) (as shown in
Fig. 4).
The particulate control device 120 removes substantially all and typically at
least
about 90% of the particles entrained in the mercury-containing gas stream 116.
As a result,
at least most of the iodine and mercury in the mercury-containing gas stream
116 is
removed by the particle removal device 120.
In another embodiment shown in Fig. 6, the reactive surface agent 300 is
introduced at one or more points 140, 148, and/or 600 to the mercury-
containing gas
stream. The mercury-containing gas stream treatment process includes first and
second
particulate removal devices 120A and B positioned on either side or on a
common side
(e.g., downstream) of the preheater 112. Due to the higher
reaction/condensation
temperature of iodine compared to bromine, the halogen-containing additive 232
can be
introduced to the feed material 100, in the thermal unit 104, between the
thermal unit 104
and first particulate removal device 120A and/or between the first and second
particulate
removal devices 120A and B to enable or facilitate removal of a first portion
of the evolved
elemental and speciated mercury in the mercury-containing gas stream 108. The
reactive
surface agent 300 may optionally be introduced between the first and second
particulate
removal devices 120A and B to enable or facilitate removal of additional
elemental and
speciated mercury in the second particulate removal device 120B. The first
portion
represents typically at least most of the mercury in the mercury-containing
gas stream 108
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upstream of the first particulate removal device 120. In one configuration,
the reactive
surface agent 300 is typically a non-carbon agent due to the high mercury-
containing gas
stream temperature upstream of the preheater 112.
Fig. 5 shows a mercury-containing gas stream treatment system according to
another embodiment.
The treated gas stream 504 is further treated by a scrubber 500 prior to
discharge
by gas discharge 126 to remove speciated mercury compounds, not removed by the
particulate removal device 120, and sulfur oxides. The scrubber 500 is
typically a wet
scrubber or flue gas desulfurization scrubber. Wet scrubbing works via the
contact of
target compounds or particulate matter with the scrubbing solution. The
scrubbing solution
comprises reagents that specifically target certain compounds, such as acid
gases. A
typical scrubbing solution is an alkaline slurry of limestone or slaked lime
as sorbents.
Sulfur oxides react with the sorbent commonly to form calcium sulfite and
calcium sulfate.
The scrubber 500 has a lower dissolved mercury and/or halogen concentration
than
conventional treatment systems, leading to less corrosion and water quality
issues.
Although mercury vapor in its elemental form, Hg , is substantially insoluble
in the
scrubber, many forms of speciated mercury and halogens are soluble in the
scrubber.
Diatomic iodine, however, has a very low solubility in water (0.006g/100 ml),
which is
significantly different from (less soluble than) C12 and Br2.
Because mercuric iodide is significantly less soluble than mercuric chloride
or
bromide and because a greater fraction of mercury is removed by particulate
removal
devices (e.g. baghouse and electrostatic precipitator) prior to the wet
scrubber, soluble
mercury present in the scrubber slurry will be reduced. As will be
appreciated, mercuric
chloride and bromide and diatomic chlorine and chloride, due to their high
solubilities,
will typically build up in the scrubber sludge to high levels, thereby
requiring the scrubber
liquid to be periodically treated. In addition, mercury contamination of by-
product FGD
gypsum board is a problem that this disclosure also addresses by reducing
mercury present
in scrubber slurry.
In some applications, the total dissolved mercury concentration in the
scrubber is
relatively low, thereby simplifying treatment of the scrubber solution and
reducing
mercury contamination of by-product materials. Typically, no more than about
20%, even
more typically no more than about 10%, and even more typically no more than
about 5%
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CA 2793326 2019-09-18
of the total mercury in the mercury-containing gas stream is dissolved in the
scrubber
solution.
As set forth below, test data show that the iodine is surprisingly and
unexpectedly
effective compared to what was previously thought achievable from injection of
halogens
including, bromine or chlorine. Whereas other halogens, such as bromine,
generally
require additive rates between 30 and 100ppmw of feed material 100, iodine
appears to be
at least 10 times more effective. Applicant has measured 70 to 90% mercury
capture with
just 3 ppmw iodine in the feed material.
A further plant configuration is shown in Fig. 10.
A combustible feed material 100 and halogen-containing additive 232 (which may
be formed by the process above) are combusted in the thermal unit 104 to
produce the
mercury-containing gas stream 108. The mercury-containing gas stream 108 is
treated by
optional (hot-side) particulate removal device 120 to remove at least most of
any
particulate material in the mercury-containing gas stream 108 and produce a
(treated)
mercury-containing gas stream 116. The mercury-containing gas stream 116 is
passed
through optional preheater 112 to produce a cooled gas stream 804. The cooled
gas stream
804 is subjected to optional (cold-side) particulate removal device 120 to
remove at least
most of any particulates in the cooled gas stream 804 and form the treated gas
stream 124.
Porous carbonaceous or non-carbonaceous material 800 is introduced, typically
by
injection, into the mercury-containing gas stream 108 at one or more contact
points 140,
148, and 600. The porous carbonaceous or non-carbonaceous material 800
collects
gaseous contaminants, including oxidized mercury, speciated mercury, acid
gases, and
halogens and halides, prior to be removed by the optional particulate removal
device 120
and/or 120.
The porous carbonaceous or non-carbonaceous material 800 can be entrained in a
carrier gas or in the form of a slurry when introduced, under pressure, into
the gas stream.
The rate of addition of the porous carbonaceous or non-carbonaceous material
800 to the
gas stream typically is in the range of from about 6 to about 0.1, more
typically in the range
of from about 4 to about 0.25, and even more typically in the range of from
about 2 to
about 0.5 lb material/MMacf gas.
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EXPERIMENTAL
The following examples are provided to illustrate certain embodiments of the
invention and are not to be construed as limitations on the invention, as set
forth in the
appended claims. All parts and percentages are by weight unless otherwise
specified.
Experiment 1
A trial of mercury control by addition of coal additives was completed on a
cyclone-fired boiler rated at 280 MW gross electricity production, but capable
of short-
term peak production of 300 MW. The boiler configuration was six cyclone
combustors
arranged three over three on the front wall. Each cyclone burns approximately
54,000 lb/h
of Powder River Basin (PRB) coal at full load. The typical coal sulfur content
is 0.3%
(dry basis) and the coal ash calcium expressed as CaO (dry basis) averages
20%.
NOx emissions are controlled on this Unit by Overfire Air (OFA) ports located
on
the rear wall, and by a Selective Catalytic Reduction (SCR) system located
upstream of
the air preheater. There are no mercury controls on this boiler, but a portion
of the mercury
released during combustion is retained by unburned carbon particles captured
in the
electrostatic precipitator.
A liquid-phase iodine-containing additive, that was substantially free of
bromine
and chlorine, and a solid-phase iron-containing additive were added to the
furnace. While
not wishing to be bound by any theory, the iodine-containing additive is
believed to control
Hg emissions by enhancing the amount of particle-bound mercury captured. The
iron-
containing additive is believed to thicken the molten slag layer contained in
the cyclone so
that more combustion occurred in the fuel-rich region. Increasing the fuel-
rich combustion
leads to lower NO emissions in the flue gas leaving the boiler. The iodine-
containing
additive contained from about 40 to about 50 wt.% iodine. The iron-containing
additive
contained from about 60 to about 70 wt.% total iron, of which from about 30 to
about 70
wt.% was ferrous oxide (FeO), and the remaining portion was substantially all
either
ferrous ferric oxide (magnetite, Fe304), ferric oxide (Fe2O3), or a mixture
thereof.
Enrichment of the fly ash with reactive iron may function as a catalyst for
heterogeneous
mercury oxidation.
Depending on access and/or coal yard operational procedures, the additives
were
applied to the coal either upstream or downstream of the crusher house. The
solid-phase
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CA 2793326 2019-09-18
iron-containing additive was provided in granular form that was stored in a
bulk storage
pile located in close proximity to the iron-containing additive conveying
equipment. The
iron-containing additive was transferred from the storage pile to a feed
hopper via front-
end loader and added to the coal belt via a series of screw feeders and bucket
elevators.
The liquid iodine-containing additive was delivered in Intermediate Bulk
Container (IBC) totes. The liquid material was metered by a chemical pump to a
dispensing nozzle at the top of the bucket elevator where it was combined with
the iron-
containing additive prior to being dropped onto the coal supply belt. The feed
rate of both
the solid iron-containing additive and the liquid iodine-containing additive
was controlled
to an adjustable set-point based on the weight of coal being fed on the coal
belt. The
hopper of the conveyor was filled several times a day during normal
operations.
This goal of this trial was to demonstrate 20 percent NO reduction and 40
percent
mercury reduction over a three-hour period at full load. The test period
included several
days of operation with and without additive coal treatment. The initial test
period was
deemed the "Baseline Tests" conducted to quantify the native or untreated Hg
emissions
in the stack and the baseline NOx emissions. Then, additive treatment using
both additives
began, and combustion improvements were confirmed by measuring higher cyclone
temperatures with an infrared pyrometer. After a few days of operation with
both
additives, the expected NO reduction was recorded during a one-day combustion
tuning
test designed to demonstrate that the iron-containing additive would allow
more aggressive
cyclone operation than was previously possible. Boiler performance was
monitored
carefully during the emissions test to assure that the emission reductions did
not cause
other problems. Hg reduction was demonstrated using data from a Thermo Fisher
Mercury
CEM on the stack (downstream from the ESP) and further validated using a
modified EPA
Method 30-B, "Determination of Mercury from Coal-Fired Combustion Sources
Using
Carbon Sorbent Traps", the Sorbent Trap Method (STM). Finally, the unit was
operated
for several days in load dispatch mode to demonstrate the long term
operability of the
treated fuel.
Based on historical coal analyses, the uncontrolled Hg emissions without the
iodine-containing additive were expected to vary between 5 and 10 ug/wscm
(0.004 to
0.008 ppmw total Hg in flue gas). Uncontrolled emissions calculated from
average coal
mercury analysis were 6 lig/wscm (0.005 ppmw) at the air preheater outlet.
However,
due to the high amount of unburned carbon in the fly ash (10-20%) and low flue
gas
CA 2793326 2019-09-18
temperatures (<300 F), there was significant native mercury removal without
the iodine-
containing additive. During the test period, baseline Hg concentrations as
measured at the
outlet continuous emission monitor ("CEM") ranged from 1.0 to 1.5 g/wscm
(0.0008 to
0.0013 ppmw).
Prior to iodine-containing additive addition, the total Hg emission averaged
about
1.1 g/wscm (0.0009 ppmw). After this baseline period, both the iron- and
iodine-
containing additives were added to the coal at various concentrations. The
iron-containing
additive was added at between about 0.3% and 0.45% by weight of the coal feed.
The
iodine-containing additive was added at a rate ranging from about 2 to 7 ppmw
of the
operative chemical to the mass feed rate of the coal. Hg emissions measured at
the stack
dropped to the range of 0.1 to 0.4 g/wscm (0.0001 to 0.0003 ppmw). Therefore,
Hg
reduction ranged from about 60 to 90 percent additional removal compared to
the baseline
removal with just the high-UBC fly ash, with an average of 73 percent
additional reduction
when the additive rate was optimized. Overall mercury removal based on the
uncontrolled
mercury concentration from coal mercury was more than 95%. Table 1 summarizes
the
results achieved at each iodine treatment rate.
The STM results confirmed the Hg-CEM results. Three pairs of baseline mercury
("Hg") samples were obtained. The Hg concentrations ranged from about 1.1 to
1.6
g/wsem (0.0009 to 0.0013 ppmw), with an average of 1.36 g/wscm (0.0011 ppmw).
Three pairs of sorbent traps were also pulled during iodine-containing
additive use. These
Hg values ranged from about 0.3 to 0.4 g/wscm (0.0002 to 0.0003 ppmw), with
an
average of 0.36 g/wscm (0.00026 ppmw). The average Hg reduction, compared to
baseline mercury removal, as determined by the STM Method, was 73 percent,
exactly the
same as the additional Hg reduction determined by the Hg-CEM.
Even though the electrostatic precipitator was already removing about 71
percent
of the Hg without iodine addition, treatment with the iodine-containing
additive caused
removal of an additional 73 percent of the Hg. With iodine addition, the total
Hg removal
based on the Hg content of the coal was 96 percent with a treatment rate of 7
ppmw iodine
to the feed coal. Surprisingly, with a treatment of just 2 ppmw iodine and
added
iodine/mercury molar ratio of only 30, the total mercury removal was 90%.
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Table 1: Experiment 1, Results with SCR1'2
Iodine Mercury Removal
Added Uncontrolled Controlled Total Mercury
Addition to above Baseline
Coal 0
Iodine/Fig Mercury Mercury (13/) Removal
(PPIlaw)
Molar Ratio ( g/wscm)I ( g/wscm) (%)
0 0 4.0 1.1 0% 71%
7 106 4.0 0.15 86% 96%
75 4.0 0.2 82% 95%
3 45 4.0 0.3 73% 93%
2 30 4.0 0.4 64% 90%
I Average uncontrolled mercury concentration based on average coal analysis of
72 ng/g at full load coal rate and APH outlet gas flow.
2 Unit load was 280 MW or more for all of the tests with gas temperature at
the
APH outlet ranging from about 285 to 300 F.
Experiment 2
Further mercury control testing on the cyclone boiler described above was
completed during summer while the SCR unit was out of service and the flue gas
redirected
around the SCR unit such that the flue gas was not exposed to the SCR
catalytic surface.
During the tests described, only the iodine-containing additive was applied
and the iron-
containing additive feed system was entirely shut down. Mercury stack
emissions were
monitored by the unit mercury CEM as previously discussed.
Testing was performed over a period of two months at several different
concentrations of iodine-containing additive and with a bromine-containing
salt added
onto the coal belt. A reference condition with no coal additives applied was
also tested.
Test coal during the entire period was the same as for previous testing, an
8,800 BTU PRB
coal. Flue gas temperatures measured at the air preheater outlet varied from
320 to 350 F,
significantly higher than during the previous tests described in Experiment 1.
For this
coal, a number of coal mercury analyses averaged 71.95 ng/g total mercury
content. This
average coal value was used as the basis for mercury removal percentages at
all conditions
over the entire unit from boiler to stack. Note that there may have been some
variation in
coal mercury by coal shipment even though the same mine supply was maintained
throughout the tests.
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Each test condition was monitored for a period of days to a full week to
ensure that
the coal supply to each of the cyclones was 100% treated and mercury emissions
were
stabilized. Table 2 summarizes the data obtained with the unit at full load
conditions. The
iodine-containing additive was applied at the listed concentrations. The
bromine-
containing additive was applied at two concentrations.
Table 2: Experiment 2, Results with SCR Bypassedu
Iodine or Added Mercury Removal Total
Uncontrolled Controlled
Bromine Iodine or above Baseline Mercury
Mercury Mercury
Addition to Bromine:Hg (%) Removal
1
Coal (ppmw) Molar Ratio 0,1g/wscm) (1.tg/wscm) (%)
0 0 6.0 2.9 0% 51%
20 302 6.0 0.5 83% 92%
12 181 6.0 0.9 69% 85%
8 121 6.0 1.1 62% 82%
6 91 6.0 0.9 69% 85%
15 (Br) 359 (Br) 6.0 1.0 66% 83%
6 (Br) 144 (Br) 6.0 1.4 52% 77%
1.Average uncontrolled mercury concentration based on average coal analysis of
72 ng/g at full load coal rate and APH outlet gas flow.
2. Unit load was 280 MW or more for all of the tests with gas temperature at
the
APH outlet ranging from 320 to 350 F.
During the tests, the unit fly ash UBC percentage varied from 6% to 25% as
measured post-test by fly ash taken from the electrostatic precipitator
hoppers. Exact UBC
during each test could not be determined based on hopper UBC content post-
test, since
hopper ash may not be entirely evacuated until days after it is removed from
the ESP
collection plates. Flue gas temperature at the inlet to the particulate
control (ESP) varied
from about 320 to 350 F. This was higher than the previous tests with the SCR
in service,
primarily due to summer vs. winter ambient conditions and the need to maintain
peak load
for extended periods.
Mercury removal, as calculated by the total from coal analysis to measured
outlet
mercury CEM, varied from 85 to 92%. With no treatment, mercury removal was
approximately 51%.
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CA 2793326 2019-09-18
This result shows that treatment by the iodine-containing additive is
effective at
higher process temperatures (e.g., from about 320 to 350 F at the ESP inlet)
and without
the benefit of an SCR catalyst.
Higher UBC is known to assist with native mercury capture by physisorption of
oxidized mercury onto UBC carbon. However, at greater than 320 F, the
physisorption of
vapor mercury declines significantly. Thus, the addition of the iodine-
containing additive,
by itself, with no SCR catalysis effect was shown to improve higher
temperature mercury
removal to 90% or higher, but the form of mercury removed (particle-bound or
vapor
species) was not determined.
The bromine-containing additive treatment also increased mercury removal from
77 to 83% compared to 51% with no treatment. This result was unexpected on the
basis
of previous experience and industry understanding from other test sites. The
expectation
was that a significantly higher level of bromine addition would be required to
realize a
high rate of mercury removal. Higher UBC carbon in the cyclone boiler ash may
be
responsible for the excellent bromine performance with no SCR, but data on
real-time in-
situ UBC was not available to confirm this hypothesis.
Since mercury emission was measured at the stack, the speciation and form of
mercury upstream was not explicitly measured, so the differences in mercury
speciation
as a result of iodine and bromine treatment were not evaluated by these tests.
Experiment 3
A series of tests were performed at Site A, a 360 MW coal-fired power plant
firing
Powder River Basin ("PRB") coal. The tests compared mercury removal when
iodine was
added to the coal at two concentrations (Experiment 3) and when a bromide
additive was
applied to the PRB coal (Experiment 4). The plant was firing 100% PRB coal
before the
tests began. The plant was equipped with a lime spray dryer ("SDA") followed
by a fabric
filter ("FF") baghouse (collectively "SDA/FF") for control of SO2 and
particulates. During
the trial, semi-continuous mercury analyzers were located at the outlet of the
air preheater
upstream of the SDA and FF baghouse at the stack outlet.
The iodine content of the coal feed was provided by coal blending. Two blend
ratios of PRB Black Thunder coal ("Black Thunder" or "BT") and higher iodine
coal
("Coal B") were tested to evaluate the influence of the bituminous coal on
mercury removal
by native fly ash. The first blend ratio was nominally 92.7% Black Thunder and
the
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CA 2793326 2019-09-18
balance was Coal B. The second blend ratio consisted of 85.6% Black Thunder
and the
balance Coal B. Coal sulfur content for both blends was 0.4% dry basis. Coal
ash calcium
(CaO, dry basis) was 17.2% for the first and 18.4% for the second blend. The
unit operated
normally during the week except that one of the five coal mills, Mill C, was
out of service.
Vapor-phase mercury concentrations were monitored at the outlet of the air
preheater on the A-side of the unit and at the stack. A summary of the tests,
including the
blending ratios and the average mercury concentrations, is presented in Table
3 and Fig.
7. There were some operational problems associated with the inlet mercury
analyzer
immediately prior to beginning the first coal blending test that may have
compromised the
inlet concentrations measured. Therefore, a triplicate set of EPA Draft M324
(sorbent
trap) samples were collected at the preheater outlet location for secondary
mercury
measurement. During the second test, simultaneous M324 samples were collected
at the
air pre-heater and stack.
Table 3. Vapor-Phase Mercury during Coal Blending Tests at Site A
Test Coal Inlet Hg Inlet He Outlet Hg Outlet Iodine Total Hg
0.1g/Nne) (p.g/Nne (p.g/Nm3) He enrichment Iodine Removal
( g/Nm3) (ppmw of (ppmw of ({Y0)
coal feed) coal feed)
100% JR PRB 9.8 8.1 10.4 9.6 0.0 0.4 -6b
7.3 4 Coal B NA 7.7 3.6 3.3 0.4 0.8 NA'
92.7% BT (7.24) M324 (50) M324
14.4% Coal B 5.8 5.4 1.4 1.4 0.7 1.1 76
85.6% BT (5.28) "324 (0.97) "324 (81) M324
All concentrations shown corrected to 3% molecular oxygen.
a Analyzer operational problems ¨ data suspect
b Analyzer calibration drift, 0% Hg removal.
M324
Mercury concentration measured with EPA Draft M324
There was no measurable vapor-phase mercury removal measured while firing
100% Jacobs Ranch coal. At the first blend ratio, the mercury removal across
the SDA-
FF increased to 50%. The mercury removal during the second blend test
increased to 76%
(81% based upon M324 sorbent trap samples).
CA 2793326 2019-09-18
Coal B samples were tested for mineral and halogen constituents after the
trial.
Coal B samples were tested at 4.9 ppmw iodine in the coal by neutron
activation analysis
(NAA). The baseline PRB samples typically average 0.4 ppmw iodine. The
resulting
enrichment of iodine is shown in Table 2 above.
Experiment 4
One additional test at Site A was to add sodium bromide (NaBr) to the coal to
increase the bromine concentration in the flue gas in an attempt to enhance
mercury
capture. No activated carbon was injected during this test.
NaBr was applied to the coal at the crusher house prior to entering the
transfer
house and coal bunkers. At this chemical injection location, it was estimated
that it would
take 4-5 hours before the "treated" coal would be fired in the boiler. The
chemical additive
was applied to the coal continuously for a period of 48 hours prior to
injecting activated
carbon to ensure that the entire system was "conditioned" with the additive.
During testing with NaBr injection, the unit was burning coal from the Jacobs
Ranch mine. At normal operating conditions, the coal yielded a total vapor-
phase mercury
concentration of about 18 to about 22 jig/Nm3 at the outlet of the air
preheater with 70-
90% in elemental form. During the chemical additive tests, the fraction of
elemental
mercury at the air preheater outlet decreased to about 20-30%.
Although the fraction of oxidized mercury at the inlet of the SDA increased
substantially, no increase in mercury removal across the system was noted. The
fraction
of oxidized mercury at the outlet of the fabric filter was also lower
(nominally 80%
elemental mercury compared to typically >90% elemental mercury when NaBr was
not
present with the coal).
Experiments 3 and 4 illustrate the difference between the two halogen
additives.
In the case of iodine added by means of the blend coal, the mercury was being
removed
across the SDA-FF at up to 76% of total mercury, even though there was less
than 1%
UBC content in the ash/spray dryer solids. In the case of the bromine
additive, there was
increased vapor oxidized mercury at the SDA inlet but mainly elemental vapor
mercury
measured at the outlet with no increased mercury capture. In combination with
iodine
treatment on the coal, the SDA-FF provides fine spray solids and full mixing
in a
temperature range where heterogeneous reaction can occur.
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Experiment 5
Coal blending tests were completed at other PRB coal-fired power plants, using
various western bituminous coals in blend ratio to PRB of up to 20%. The
results are
shown in Table 4 below. None of the western bituminous blend coals in these
trials that
exhibited any significant mercury removal except the Coal B that is described
in
Experiments 3 and 4 above.
Table 4: Results of Western Bituminous Blend Tests For Mercury Control
Test/Unit Blend Coal in PRB APC UBC Carbon Blended Coal Mercury
Equipment (/0 of ash) Iodine ppm Removal
(%)
Site B ColoWyo, 20% SDA/ESP <1.0 <0.5 (I) 0
Site B TwentyMile, 16% SDA/ESP 0.6 <0.5 (I) 0
I Native iodine in western bituminous coals typically is less than 0.5 ppmw.
SDA - Spray Dryer Absorber, SO2 Control
ESP - Electrostatic Precipitator
Experiment 6
Another test site for coal blending, Site D, fires subbituminous PRB coal and
is
configured with low-NOx burners and selective catalytic reduction ("SCR") unit
for NOx
control, a spray dryer absorber ("SDA") for SO2 control, and a fabric filter
("FF") for
particulate control. The test matrix included evaluating each coal at 7% and
14% higher
iodine coal (Coal B) mixed with a balance of PRB. Each blend test was
scheduled for
nominally 16 hours with eight hours of system recovery time between tests.
Coal A had a
native iodine content of less than about 0.5 ppmw while coal B had a native
iodine content
of about 4.9 ppmw. Coal A sulfur content was 0.6 to 0.8%, dry basis. The blend
coals
averaged 0.65% sulfur. The coal ash calcium content, based on typical analysis
of Coals
A and B, was between 17 and 20% (CaO, dry basis).
For the first blend test (Coal B at 7.2%), there was a significant decrease in
both
the SDA inlet and stack mercury concentrations at the beginning of the test.
However,
there was no increase in oxidized mercury (Hg+2), which would suggest that, if
this
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=
decrease were due solely to the coal blend, mercury removal occurred in the
particulate
phase before reaching the SDA inlet sampling location. Based on this
assumption, the
mercury removal for the first test was about 50%, calculated using the mercury
concentration at the beginning of the test and at its lowest point during the
test. If removal
is calculated strictly based on SDA inlet and outlet mercury concentrations,
then removal
increased from 10% to 27% due to coal blending.
During the second test (Coal B at 13.2%), the stack mercury levels gradually
decreased, but the inlet did not. Based on the SDA inlet and stack
concentrations, the
mercury removal for the second test increased from about 15% to 51%. The
iodine content
of the coals was not analyzed at the time of testing, but the iodine content
of Coal B has
since been analyzed. Iodine enrichment compared to the baseline PRB coal was
approximately 0.7 ppmw at the 14% blend ratio, based on typical iodine
analysis for Coal
B. The iodine/mercury molar ratio was approximately 30. Surprisingly, mercury
removal
was more than 50% even at this low additive rate.
Experiment 7
A trial of mercury control when firing an iodine treated coal was completed on
a
70 MW, wall-fired unit firing a Powder River Basin coal. The purpose of this
test was to
compare the mercury removal of the treated coal product on mercury emissions
compared
to the identical coal at the same process conditions without treatment. The
coal was treated
remotely by application of an aqueous iodine-containing solution by spray
contact with
the coal. A unit train was loaded with about half untreated and half treated
coal. The level
of treatment based on coal weight and chemical applied was 7.6 ppmw of iodine
in the as-
loaded coal. The concentrated chemical spray was applied to substantially all
of the coal
and was well-distributed.
At the power plant, the untreated coal from this unit train was fired for six
days and
then the first treated coal was introduced. Treated coal was then burned
exclusively in this
unit for another seven days. Coal sulfur content averaged 0.35%, dry basis.
Coal ash
calcium content was 20.9%, dry basis, CaO.
Coal samples taken at the plant from the coal feed to the boiler were analyzed
for
halogen content by neutron activation analysis (NAA). Samples during the
baseline period
averaged 26.014g chlorine as-received, 1.2 g/g bromine and 0.4 g/g iodine.
Samples
taken while firing treated coal averaged 18.9 g/g chlorine as-received, 1.1
g/g bromine
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and 3.0 ug/g iodine. The results for iodine indicated loss during transit and
handling (7.6
ug/g as loaded and 3.0 as-received). However, the coal sampling and analytical
frequency
was lower than necessary to conclusively determine this.
The plant pollution control equipment consisted of a cold-side electrostatic
precipitator operating at an inlet flue gas temperature of 360 F to 400 F. The
level of
unburned carbon (loss-on-ignition) was 0.7% or essentially none in the PRB fly
ash. In
addition, the mercury speciation as measured by the outlet mercury monitor was
initially
almost all elemental mercury. These conditions were expected to be extremely
problematic for conventional mercury control such as activated carbon
injection (ACI) or
bromine treatment of coal. For AC!, the temperature was too high for
substantial elemental
mercury sorption except at higher injection rates with halogenated activated
carbon. This
would be expensive and would add carbon detrimentally into the fly ash.
Bromine
treatment of coal would be expected to increase the oxidation of mercury when
applied as
typically practiced at 30 to 100 ppm on the coal, but the lack of unburned
carbon in the fly
ash would limit capture of the oxidized mercury species. It would not be
unexpected to
see no mercury capture for this condition with bromine added to the coal.
A modular rack Thermo Fisher mercury continuous emission monitor (Hg-CEM)
was installed at the ESP outlet (ID fan inlet) to measure the total and
elemental mercury
in the flue gas. The monitor directly read mercury concentration in the flue
gas on one-
minute average intervals in units of micrograms mercury per standard cubic
meter of flue
gas, wet basis (ug/wscm).
The treated coal first reached the boiler from only one of 3 bunkers and the
mercury
concentration at full load rapidly decreased from 5 to 2.6 g/wscm (0.0041 to
0.0021
ppmw in the flue gas) or about 50% reduction. After all the coal feed switched
to treated,
the mercury decreased slightly more and remained lower. Overall, the average
baseline
mercury concentration measured at the stack outlet when initially burning the
coal with no
iodine treatment was about 5.5 g/wscm (0.0045 ppmw) at high load above 70 MW
and
1.7 g/wscm (0.0014 ppmw) at low load of about 45 MW. When firing treated
coal, the
high load Hg concentration averaged about 2.6 g/wscm (0.0021 ppmw) and the
low load
about 0.8 ug/wscm (0.0006 ppmw). The use of treated coal reduced mercury
emission by
about 53%. In addition, episodes of extreme mercury spikes during high
temperature
excursions related to soot blowing were substantially eliminated. After the
unit came back
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from an outage, the regular coal feed (untreated) was resumed and the mercury
emissions
returned to baseline of about 5.5 g/wscm (0.0045 ppmw) at full load.
In addition to reducing the total mercury by converting to a particulate form,
the
additive also appears to have converted the majority of the remaining vapor
phase mercury
to an oxidized form. This creates an opportunity to obtain additional mercury
capture with
the injection of a low-cost untreated sorbent. If the mercury were not
converted to an
oxidized form, additional trimming of the mercury emissions would require a
more
expensive brominated sorbent.
In order to further validate the mercury measurements, a set of independent
emissions tests were completed using a sorbent trap method (EPA Method 30B).
The
sorbent trap emissions agreed well with the Hg-CEM throughout the trial.
Total mercury removal in this trial was more than 50% for a difficult process
condition (PRB coal, gas temperature 350 to 400 F, no UBC and undersized
electrostatic
precipitator) for which zero or minimal removal would be expected by either
injection of
activated carbon or bromine treatment of feed coal.
Experiment 8
A trial of mercury control by addition of coal additives was completed on a
cyclone-fired boiler rated at 600 MW gross electricity production, but capable
of short-
term peak production of 620 MW. The boiler configuration was 14 cyclone
combustors
arranged three over four on the front and rear walls. Each cyclone burns
approximately
50,000 lb/h of Powder River Basin (PRB) coal at full load. The coal sulfur
content was
0.3% (dry basis) and the coal ash calcium content (CaO) averaged 22% (dry
basis).
NO. emissions are controlled on this unit by Overfire Air (OFA) ports located
on
the front and rear walls, and by a Selective Catalytic Reduction (SCR) system
located
upstream of the air preheater. There are no Hg controls on this boiler, but a
portion of the
mercury released during combustion is retained by unburned carbon particles
captured in
the electrostatic precipitator.
A liquid-phase iodine-containing additive, that was substantially free of
bromine
and chlorine, and a solid-phase iron-containing additive were added to the
furnace. The
additives were applied to the coal upstream of the crusher house. The solid-
phase iron-
containing additive was provided in granular form that was stored in a bulk
storage pile
located in close proximity to the iron-containing additive conveying
equipment. The
CA 2793326 2019-09-18
liquid iodine-containing additive was delivered in Intermediate Bulk Container
(IBC)
totes. The liquid material was metered by a chemical pump to a dispensing
nozzle at the
top of the bucket elevator where it was combined with the iron-containing
additive prior
to being dropped onto the coal supply belt. The feed rate of both the solid
iron-containing
additive and the liquid iodine-containing additive was controlled to an
adjustable set-point
based on the weight of coal being fed on the coal belt.
The test period included several days of operation with and without additive
coal
treatment. The initial test period was deemed the "Baseline Tests" conducted
to quantify
the native or untreated Hg emissions in the stack and the baseline NO
emissions. Then,
additive treatment using both additives began.
Mercury reduction was demonstrated using data from a Thermo Fisher Mercury
CEM on the stack (downstream from the ESP). Based on historical coal analyses,
the
uncontrolled Hg emissions were expected to vary between 5 and 10 g/wscm. Coal
mercury content was analyzed during the trial and averaged 68.7 ng/g. Based on
this and
the flue gas flow rate, the expected mercury concentration in the flue gas at
the air
preheater outlet was 5.8 !..tg/wscm (0.0005 ppmw).
Due to the high amount of unburned carbon in the fly ash (10-20%) and low flue
gas temperatures (< 300 F), there was significant native mercury removal
without the
iodine additive. During the baseline period, vapor-phase Hg concentrations as
measured
by the stack outlet Hg-CEM ranged from 0.2 to 1.1 1.tg/wscm (0.0002 to 0.0009
ppmw)
with an average of about 0.6 ilg/wscm. Iodine was then added to the coal feed
at various
concentrations and mercury emissions dropped to the range of 0.03 to 0.13
Kg/wscm
(0.00002 to 0.0001 ppmw). Overall mercury removal, coal pile to stack, at this
condition
was > 98%. Additional mercury reduction from the baseline condition ranged
from 78 to
95 percent, with an average of 78 percent reduction at a feed rate equivalent
to 3 ppm by
weight of iodine on the coal.
Sorbent Trap method (STMs) using a modified EPA Method 30-B were conducted
during baseline tests to substantiate the Hg-CEM measurements. The STMs all
agreed
with the Hg-CEM agreed within specified limits (%Relative Accuracy < 20%).
During
additive injection, STMs were not conducted at the extremely low mercury
conditions, due
to the prohibitively long STM sample times in order to collect enough mercury
to be above
the detection limit of the analysis.
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This experiment demonstrates the ability to economically achieve a critical
90%
mercury removal with only 3 ppmw iodine in combination with iron additive
added to the
coal feed, without the need for expensive additional mercury control
equipment.
Table 5: Experiment 8 Results
Iodine Mercury Total
Added Uncontrolled Controlled
Addition to Removal above Mercury
Iodine/Hg Mercury Mercury
Coal Baseline (%) Removal
Molar Ratio ( g/wscm)1 ( g/wscm)
(ppmw) (%)
0 0 5.8 0.6 0% 90%
3 47 5.8 0.13 78% 98%
I Average uncontrolled mercury concentration based on average coal analysis of
69 ng/g at full load coal rate and APH outlet gas flow.
Experiment 9
The objective of three tests, described below, was to assess the stability of
elemental iodine adsorbed on a porous carbonaceous material (i.e., activated
carbon) by
exposing the loaded carbon to different temperatures for extended periods of
time.
In the first test, elemental iodine was formed by oxidizing 1000 ppm iodide
solution by acidifying it, and adding a small amount of hydrogen peroxide. The
iodine
solution was then passed through a bed of activated carbon in a filter funnel.
The carbon
is considered to be loaded when the yellow color of iodine in water breaks
through the
carbon bed. Four hundred twenty five milligrams of iodine per gram of carbon
was
adsorbed on SabreTM powdered activated carbon ("PAC") with this method.
The loaded carbon was dried, and a weighed amount placed in an Erlenmeyer
flask
with the neck plugged with a filter paper saturated with soluble starch
solution. The
temperature of the flask was raised to ¨50 C and held for one hour. Only a
faint purple
color developed on the starch paper indicating that only a trace of iodine was
driven off
the carbon at this temperature
In a second test, SabreTM PAC was loaded with iodine using the iodine number
procedure. This test indicated that 1,079 mg of iodine was sorbed per gram of
carbon.
Note that in this procedure the carbon filter cake is not washed; thus some
the iodine can
be held interstitially and is not strictly sorbed to the carbon.
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A slight purple color formed on the starch paper at a temperature of 30 C and
significant color and small iodine crystals developed on the paper when the
temperature
reached 50 C. The temperature was raised to about 80 C, and dense iodine
vapors formed
above the carbon.
Once cooled, a 0.25 gram portion was reacted with 50 ml of an alkaline
hydrogen
peroxide solution for 15 minutes to remove the iodine from the carbon by
reducing it to
iodide. The percentage of iodine retained on the carbon after heating to ¨80
C was
calculated to be about 200 mg/g of carbon.
In a third test, an iodine solution was made so that up to 500 mg/g was
available
when reacted with two grams of SabreTM PAC. After washing, filtering, and
drying, a
portion was analyzed to determine the amount of iodine adsorbed. About 360 mg
I2/g
carbon was adsorbed.
The method of stripping the iodine from activated carbon by reducing it to
iodide
with hydrogen peroxide appears to be quantitative as tests performed on the
stripped
carbon with isopropyl alcohol, which has previously been shown to strip iodine
from
activated carbon, were negative.
Another portion of the loaded carbon was heated to 50 C and held at that
temperature for 1.5 hours with only a small amount of iodine liberated as
indicated by the
starch paper. The carbon was cooled and stripped of iodine. The iodide
concentration of
the strip solution was measured. The amount of iodine retained by the carbon
after heating
to 50 C was found to be 280 mg 12/g SabreTM PAC.
Another portion of iodine loaded carbon was heated to more than 80 C to drive
off more iodine, and the carbon analyzed as in the above paragraph. The amount
of iodine
retained after this extreme heating was 320 mg 12/g SabreTm PAC.
These tests indicate that SabreTM PAC holds between 200 and 300 mg of iodine
per gram of carbon with sufficient tenacity to be stable up to about 80 C and
greater
amounts are fairly stable at 25 C.
Significantly, loadings of up to 360 mg/g were achieved. In later tests with
some
high microporosity carbons, loadings of 50 to 100% were obtained. While the
iodine
becomes less energetically adsorbed and may evolve some vapor off the carbon
at very
high loadings, these tests demonstrate that a high quality carbon can be used
since the
quantities required are no more than about 3 lbs. per lb. of iodine. Also, in
some
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configurations, the iodine could be desorbed off the carbon and into the flue
gas by heating,
for example, and the carbon re-used.
Experiment 10
A trial was conducted to evaluate mercury removal at a 450 MW coal-fired
cyclone
boiler during treatment of coal with an iodine-containing additive. The plant
was
configured with an SCR, rotary air preheater (APH) and a cold-side ESP. The
plant was
burning a mixture of 85% PRB and 15% low-sulfur Eastern bituminous coal. Coal
sulfur
content was 0.4%, dry basis and coal ash calcium content was about 17% as CaO,
dry
basis.
A liquid-phase iodine-containing additive, substantially free of bromine and
chlorine, was added to the coal at 6 to 13 ppm by weight of halogen to the
coal feed for a
period of three days. Total and speciated mercury (elemental and oxidized) was
measured
continuously using a Thermo Fisher Mercury CEM on the stack (downstream from
the
ESP). Prior to halogen addition, the total mercury was 2 ug/wscm and the
oxidized
mercury fraction was approximately 50%. Iodine was added to the coal at a
first rate of 6
ppmw and then at a second rate as high as 13 ppmw. Injection of halogen
immediately
increased the Hg oxidation at the stack from 50% to 90%. However, total
mercury
emissions as measured at the stack were not substantially affected even after
3 days of
injection. Table 6 summarizes the trial results.
Table 6: Experiment 10 Results
Iodine Addition to Coal Total Mercury Oxidized Mercury Removal
(ppmw) (pg/wscm)' Mercury (%) above Baseline (%)
0 (Baseline) 2.0 30 - 60 0%
6 1.5 ¨ 2.0 70 - 90 < 25%
13 1.5 ¨ 2.0 90 - 100 <25%
The S02 emissions during the testing were monitored by the stack continuous
emission monitors. The SO2 emissions during the trial were 0.75 to 0.8
lb/MMBtu. The
air preheater exit temperature fluctuated between about 260 F at 350 MW and
320 F at
450 MW. The oxidation rate of SO2 across the SCR was estimated to be 1.9% at
full load
and 0.3% at low load, based on prior measurements of S03 made at the SCR inlet
and
outlet. The formation and deposition of S03 through the SCR to the stack was
modeled to
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estimate S03 concentrations after the SCR at low load and full load. At full
load, the SO
at the APH outlet was estimated to be about 7 ppmvd, but at low load (350 MW),
less than
1 ppmvd. Table 7 summarizes the model results.
Table 7: SO3 Model Prediction for 85% PRB blend
Load, MW 450 350
ESP Inlet Temperature 320 260
S03, ppmvd at 3% 02
SCR in let 2.9 2.9
SCR outlet 11.3 4.2
ESP inlet 6.9 0.4
Stack 4.4 0.3
The S03 at the outlet of the APH (ESP inlet) is lower at low load for two
reasons
(1) lower temperatures in the SCR mean less SO2 will be oxidized and (2) lower
temperatures at the APH outlet mean more removal of S03 across the APH. At low
load
with iodine treatment of the coal, reduced SO2 to SO3 oxidation plus reduced
ammonia for
deN0x, the mercury was 100% oxidized. At full load, however, there was an
excess of
SO3 of as much as 7 ppmv that blocks mercury reaction on ESP fly ash surface
sites.
Doubling the rate of halogen coal additive in this case increased the mercury
oxidized fraction, but was not effective in increasing mercury removal (due to
the excess
SO3). This trial illustrates again that very low levels of halogen (6 ppmw of
coal feed) are
required with SCR to oxidize the majority of mercury, but it also demonstrates
the
sensitivity of mercury capture to SO3 levels at the ESP and, in this case, the
undesirable
SO2 to SO3 oxidation occurring in the SCR. Even though a low-sulfur coal is
being fired,
the overall emissions of SO2 is low and the mercury is present almost entirely
in an
oxidized form when a halogen coal additive is present, the catalytic formation
of SO3
across the SCR and its deposition on mercury capture surfaces, especially in
the ESP, are
still very detrimental to mercury retention in the fly ash.
A number of variations and modifications of the disclosure can be used. It
would
be possible to provide for some features of the disclosure without providing
others.
For example in one alternative embodiment, coal containing naturally high
concentrations of iodine (e.g., greater than about 2 ppmw, even more typically
greater than
about 3 ppmw, and even more typically greater than about 4 ppmw) is blended
with the
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feedstock coal having no or low concentrations of iodine (e.g., no more than
about 2 ppmw
and even more commonly no more than about 1 ppm by weight) to increase mercury
removal. The coal, when fired, can have high or low UBC content without
adversely
impacting mercury removal.
The present disclosure, in various aspects, embodiments, and configurations,
includes components, methods, processes, systems and/or apparatus
substantially as
depicted and described herein, including various aspects, embodiments,
configurations,
subcombinations, and subsets thereof Those of skill in the art will understand
how to
make and use the various aspects, aspects, embodiments, and configurations,
after
understanding the present disclosure. The present disclosure, in various
aspects,
embodiments, and configurations, includes providing devices and processes in
the absence
of items not depicted and/or described herein or in various aspects,
embodiments, and
configurations hereof, including in the absence of such items as may have been
used in
previous devices or processes, e.g., for improving performance, achieving ease
and \ or
reducing cost of implementation.
The foregoing discussion of the disclosure has been presented for purposes of
illustration and description. The foregoing is not intended to limit the
disclosure to the
form or forms disclosed herein. In the foregoing Detailed Description for
example, various
features of the disclosure are grouped together in one or more, aspects,
embodiments, and
configurations for the purpose of streamlining the disclosure. The features of
the aspects,
embodiments, and configurations of the disclosure may be combined in alternate
aspects,
embodiments, and configurations other than those discussed above. This method
of
disclosure is not to be interpreted as reflecting an intention that the
claimed disclosure
requires more features than are expressly recited in each claim. Rather, as
the following
claims reflect, inventive aspects lie in less than all features of a single
foregoing disclosed
aspects, embodiments, and configurations. Thus, the following claims are
hereby
incorporated into this Detailed Description, with each claim standing on its
own as a
separate preferred embodiment of the disclosure.
Moreover, though the description of the disclosure has included description of
one
or more aspects, embodiments, or configurations and certain variations and
modifications,
other variations, combinations, and modifications are within the scope of the
disclosure,
e.g., as may be within the skill and knowledge of those in the art, after
understanding the
present disclosure. It is intended to obtain rights which include alternative
aspects,
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embodiments, and configurations to the extent permitted, including alternate,
interchangeable and/or equivalent structures, functions, ranges or steps to
those claimed,
whether or not such alternate, interchangeable and/or equivalent structures,
functions,
ranges or steps are disclosed herein, and without intending to publicly
dedicate any
patentable subject matter.
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