Note: Descriptions are shown in the official language in which they were submitted.
CA 02846324 2014-03-14
REDUCING ENVIRONMENTAL POLLUTION AND FOULING WHEN BURNING
COAL
CROSS REFERENCE TO EARLIER APPLICATIONS
[00011 This application
claims the benefit of U.S. provisional application
serial number 61/788,442 filed March 15, 2013, the entire disclosure of which
is
incorporated by reference.
INTRODUCTION
[00021 The invention
provides compositions and methods for reducing the
levels of mercury, nitrogen oxides, and/or sulfur oxides emitted into the
atmosphere
upon burning of mercury-containing fuels such as coal. In particular, the
invention
provides for addition of various halogen and other sorbent compositions into
the coal
burning system during combustion. Use of the sorbents reduces emission of
pollutants
and prevents fouling in the furnace.
[00031 Significant coal
resources exist around the world capable of meeting
large portions of the world's energy needs into the next two centuries. High
sulfur coal is
plentiful, but requires remediation steps to prevent excess sulfur trom. being
released into
the atmosphere upon combustion. In the United States, low sulfur coal exists
in the form
of low BTU value coal in the Powder R.iver basin of Wyoming and Montana, in
lignite
deposits in the North Central region of North and South Dakota, and in lignite
deposits in
Tex.as. But even when coals contain low sulfur, they still contain non-
negligible levels
of elemental and oxidized mercury and/or other heavy metals.
[00041 Unfortunately, mercury is at least partially volatilized upon
combustion of coal. As a result, the mercury tends not to stay with the ash,
but rather
becomes a component of the flue gases. If remediation is not undertaken, the
mercury
tends to escape from the coal burning facility into the surrounding
atmosphere. Some
mercury today is captured by utilities, for example in wet scrubber and SCR
control
systems. However, most mercury is not captured and is therefore released
through the
exhaust stack.
CA 02846324 2014-03-14
[00051
Mercury emissions into the atmosphere in the United States are
approximately 50 tons per year. A significant fraction of the release comes
from
emissions from coal burning facilities such as electric utilities. Mercury is
a known
environmental hazard and leads to health problems for both humans and non-
human
animal species. To safeguard the health of the public and to protect the
environment, the
utility industry is continuing to develop, test, and implement systems to
reduce the level
of mercury emissions from its plants. In combustion of carbonaceous materials,
it is
desirable to have a process wherein mercury and other undesirable compounds
are
captured and retained after the combustion phase so that they are not released
into the
atmosphere.
[0006] Other
pollutants such as nitrogen oxides (N0x) and sulfur oxides
(S(3x) are released upon combustion of coal. These contribute to environmental
problems such as smog and acid rain. The industry is actively pursuing methods
of
reducing these pollutants as well.
[00071
Certain coal facility operators qualify for tax credits under 45 of the
IRS code for their efforts in reducing emissions of these pollutants. Under
certain
circumstances, these operators have observed an undesirable fouling in their
furnaces.
For example, when low sulfur sub-bituminous coals are burned as fuel, deposits
tend to
form on boiler tube surfaces, which leads to lower heat exchange efficiency
and
increased costs of operation
[0008]
Accordingly, operators need sorbent compositions and methods of use
that will enable them to achieve desirable reduction of atmospheric pollutants
without
compromising on heating efficiency.
SUMMARY
[0009]
Sorbents for use especially- with sub-bituminous and lignite coals are
provided that contain low levels of alkali. The hi.gh levels of alkali in
sorbents of the
prior art can contribute to fouling of a furnace in which they are being
burned. By
reducing the alkalinity of the sorbents, the operator can tninimize the sodium
and
potassium available in the gas phase for unwanted reactions that lead to
formation of
deposits on boiler surfaces and elsewhere.
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[00101 The
sorbents can be used to prepare a refined coal that can be burned
to reduce emissions of one or more of mercury, nitrogen (as N0x), and sulfur
(as S0x).
A method of inaking the refined coai involves combining a sub-bituminous coal
(or a
lignite coal) a.nd sorbent components. in one embodiment, the sorbents are
0.001- 1 wt %
of a liquid sorbent, and 0.1 to 10% by weight of a powder sorbent, wherein the
percentages are by weight based on the total weight of the refined coal. The
liquid
sorbent contains a bromine compound and the powder sorbent contains calcium,
silica,
alumina, and further comprises less than I ,'D Na200 by weight and less than
I% K20 by
weight, and wherein the powder sorbent further comprises less than 0.1% by
weight
chlorine.
[0011] In an
aspect, the powder sorbent contains cement kiln dust (CKD),
which is helpful in reducing emissions of NO-x, with the improvement that,
when the
CKD is high in alkali, some of the CKD is substituted by lower alkali
materials to reach
a specification of less than 2% or less than 1% total alkali. In various
embodiments, the
'15 powder
sorbent also meets a low chlorine specification to reduce fouling in sun-
bituminous and lignite coals.
DESCRIPTION
[0012} ln
various embodiments, the invention provides compositions and
methods for reducing emissions of mercury, nitrogen oxides (N0x), and sulfur
oxides
(S0x) that arise from the combustion of mercury-containing fuels such as coal.
A
commercially valuable enabodiment is use of the i.nvention to reduce nitrogen,
sulfur
and/or mercury emissions from coal burning facilities to protect the
environment and
comply with government regulations and treaty obligations. Improvements to
powder
sorbents provide superior performance by reducing fouling in coal-burning
furnaces,
while removal of environmental pollutants such as NOx and SOx are not
deleteriously
affected.
[00131 in
various embodiments, the methods prevent release of mercury into
the atmosphere from point sources, such as coal-burning utilities by capturing
the
mercury in the ash, while at the same time minimizing furnace fouling that
could
decrease the efficiency of the furnace.. Further, the methods prevent release
of mercury
and other heavy metals into the environment by leaching from solid wastes such
as coal
ash produced by burning the mercury containing coal. In both these ways,
mercury is
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CA 02846324 2014-03-14
kept out of bodies of water. Thus, prevention or reduction of mercury
emissions from
such facilities as coal-burning utilities leads to a variety of environmental
benefits,
including less air pollution, less water polluti.on, and less hazardous waste
production,
with less resulting ground contamination. For convenience but without
innitation,
advantageous features of the invention are illustrated as preventing air,
water, and
ground pollution by mercury or other heavy metals.
[00141 In one embodiment, a method of burning coal in a furnace to
reduce
emissions of NO and at least one of SOx and mercury is provided. The method
involves
burning a refined coal in the furnace. The refined coal in turn is an
admixture of sub-
bituminous coal, a bromine compound and a powder sorbent. The powder sorbent
contains calcium, silica, alumina, and is further characterized by a low
alkali value of
less than I ./0 by weight Na20 and less than 1 4 by weight 1c00, based on the
weight of
the powder sorbent, and preferably also by a low chlorine value of less than
0.5%, less
than 0.3%, or less than 0.1%. Jn various embodiments, treat levels of the
bromine
compound are 0.001 to 1.0% by weight, while typical treat levels of the powder
sorbent
are 0.1 to 10% by weight percentages are based on the weight of the coal.. In
various
embodiments, the powder sorbent contains CKD or a mixture of CKD with other
low
alkali powder described herein. The powder sorbent can also contain
aluminosilicate clay
such a kaolin or metakaolin.
f00151 In another embodiment, there is provided a method of generating
energy through combustion of a mercury containing sub-bituminous coal in the
furnace
of the coal burning facility. The method involves first applying a first
sorbent
composition onto coal and delivering the coal with the applied first sorbent
into the
furnace. .At the same time, a second sorbent is added into the furnace as the
coal with the
applied first sorbent :is being delivered. The coal is then combusted in the
presence of the
first and second sorbents in the furnace to produce heat energy and ash. The
first sorbent
contains a bromine compound and the second has a composition of greater than
40% by
weight CaO, greater than 10% by weight Si02. 2 to 10% .AI203, t to 5% Fe203, 1
to 5%
MgO, less than 1% by weight Na20, and less than 1% Ka-)Ø In embodiments, the
second
sorbent has less than 0.5% chlorine.
[OM] In another embodiment, a method of making a refined coal is
.= provided. The refined coal contains sub-bituminous coal and added
sorbent components.
The method involves add mixing coal, a liquid sorbent (for example 0.001 to 1%
by
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CA 02846324 2014-03-14
weight, based on the coal) and a powder sorbent (for example 0.1 to 10 .10 by
weight
based on the coal), wherein the liquid sorbent comprises a bromine compound
and
powder sorbent comprises Lnieater than 40% CaO, greater than 10% 5102, 2 to
10%
A1703, 1 to 5% Fe203. 1 to 5% Mg0, less than I% Na20, and less than 1% KO. In
various embodiments, the powder sorbent is further characterized as having
less than
0.5% chlorine or less than 0.1% chlorine.
[0017] Advantageously, use of the noted powder sorbents,
especially the
compositions that are low in total alkali, low in chlorine, or both. has been
found to be
effective at reducing fouling in furnaces that are burning sub-bituminous
coals or lignite
coal, such as those of the Powder River Basin (PRB). By using the modified
sorbents
described herein, furnace operators can comply with environmental regulations
and
qualify for certain tax benefits under the United States IRS code, and at the
same time
avoid undesirable fouling of the furnace and associated facilities.
[00181 Further examples of each of thc limitations of the
embodiments are
given in the description that follows. It is to be understood that the various
components
and the method steps described herein can be mixed and matched to provide
other
embodiments not literally recited or exemplified. 'Examples are also given,
enabling the
person of skill in the art to carry out the invention and to achieve the noted
enviromnental and operational benefits.
[0019] Various sorbent components are used in combination to treat coal
ahead of combustion and/or to be added into the flame or downstream of the
frame,
preferably at minimum temperatures to assure complete formation of the
refractory
structures that result in various advantages of the methods. When the
components are
added to coal before combustion, the product is a refined coal, the use of
which lowers
environmental pofiution and may qualify the utility for certain tax benefits
in the United
States.
[0020] The sorbent components include calcium, alumina, silica,
and
halogen. To reduce fouling when burning sub-bituminous or lignite coals such
as those
of the Powder River Basin, it has been found to be advantageous to keep 1<i20
of the
sorbent to a maximum of 1% and to keep Na20 of the sorbent to a maximum. of
1%,
wherein percentages are by weight of the powder sorbent containing calcium,
alumina,
silica, and other components. In embodiments, Na20 and K20 are each less than
0.5% or
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CA 02846324 2014-03-14
are each less than 0_1%. In addition, in various embodiments, it has also been
found
advantageous to provide the powder sorbent with low chlorine, e.g. <0.5%, <
0.3%, or
<0.1%.
[0021]
Calcium is provided by adding to the powder sorbent a compound or
composition that has a non-negligible amount of calcium. For example, many
alkaline
powders contain 20% or more calcium, based. on CaO. Examples are ]imestone,
lime,
calcium oxide, calcium hydroxide (slaked lime), portland cement an.d other
manufactured products or by-products of industrial processes, and calcium-
containing
aluminosilicate minerals. Silica and alumina content is based on SiO2 and
A1,03
equivalents, even though it is appreciated that silica and alumina are often
present in a
more complex chemical or molecular form.
100221 In
various embodiments, it is advantageous for the powder sorbent to
contain an effective amount of cement kiln dust (CKD), which is believed to
contribute
to the reduction of NOx from the coal-burning facility. Some CKD has a
relatively high
chlorine content, even as high as 10%. If CK.D is used; depending on the
source oI CKD
and its natural content of alkali and chlorine, the resulting powder could
wind up being
too high in alkali and/or chlorine for best results when burning sub-
bituminous or lignite
coals. if so, it is advantageous to blend off some of the CKD with other
materials lower
ìn .sodium and potassium, preferably to achieve, a specification of <1% Na20
and <1%
K20; or even <0.5% Na,0 and <0.5% K20, as well as a low chlorine spec as
noted.
Such low alkali materials include uind outs (cement kiln clinker that may or
may not
meet cement product specification and is subsequently ground for blending with
CKD);
kiln feed (the feed stream going into the cement kiln, including all the
components for
manufacturing cement, e.g. Ca, Mg, Si, Al, Fe, and so on); transition cement
(cement
product. in silo that is emptied to make room for a specific new cement
product;
weathered clinker (clinker that has been impounded on site, recovered and
ground before
adding to the CKD); impound CKD (CKD from on-site impound or waste storage);
and
limestone. To the extent that any of these materials represent waste products
that would
otherwise go to waste or have to be lan.dfilled, additional environmental
benefits are
achieved by their use in the sorbents described herein.
[0023] In various embodiments, together, the components
= reduce emissions of mercury, nitrogen oxides, and sulfur oxides;
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* reduce emissions of elemental and oxidized mercury;
= increase the efficiency of the coal burning process through de-
slagging of boiler tubes;
* prevent the fouling of the furnace by unwanted deposits;
* increase the level of Hg, A.s, Ph, and/or CI in the coal ash.;
= decrease the levels of leachable heavy metals- (such as Hg) in the
ash, preferably to levels below the detectable limits; and
O make a highly cementitious ash product.
[00241 As
used herein, all percentages are on a weight basis, unless indicated
as otherwise. It should be noted that the chemical compositions of various
materials
described herein are expressed in terms of simple oxides calculated from
elemental
analysis, typically determined by x-ray fluorescence techniques. While the
various
simple oxides may be, and often are, present in Mare complex compounds in -the
material, the oxide analysis is a useful method for expressing the
concentration of
compounds of interest in the respective compositions.
[0025] In a
typical coal burning facility, coal arrives in railcars. If sorbents
have already been applied, it is a refined coal. It is a raw coal if sorbents
have not yet
been applied. In a typical ill-ustrative embodiment, the coal is delivered
onto a receiving
belt, which leads the coal into a pug mill. After the pug mill, the coal is
discharged to a
feed belt and deposited in a coal storage area. Under the coal storage area
there is
typically a grate and bin area; from there a belt transports the coal to an
open stockpile
area, sometirnes called a bunker. Stoker furnaces can be fed with coal from
the bunker
or from a crusher. For furnaces burning pulverized coal, the coal is delivered
by belt or
other means to milling equipment such as a crusher and ultimately to a
pulverizer. In a
storage system, coal is pulverized and conveyed by air or gas to a collector,
from which
the pulverized coal is transferred to a storage bin, from which the coal is
fed to the
furnace as needed. In a direct fired system, coal is pulverized and
transported directly to
the furnace. In a semi-direct system, the coal goes from the pulverizer to a
cyclone
collector. The coat is fed directly from the cyclone to the furnace.
[00261 During
operation coal is fed into the furnace and burned in the
presence of oxygen. For high btu fuels, typical flame temperatures in the
combustion
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chamber are on the order of 2700 F (about 1480 C) to about 3000 F (about 1640
C) or
even higher, such as 3300 F (about 1815 C) to 3600 F (about 1982 C).
[00271 .A
refined coal is produced by adding sorbents to coal before
combustion. The sorbents can be added by the coal producer and shipped to the
furnace
operator, or the refined coal can be produced in a separate facility near or
on the property
of the operator. In the case of refined coal, a coal containing all the
sorbent components
is fed to the furnace for combustion.
[00281
various other embodiments, sorbent compositions according to the
invention are added to the raw coal or into various parts of the furnace
during
-10
combustion. In non-limiting fashion, sorbents are added to the coal, in the
pug mill, on
the receiving belt or feed belt, in the coal storage area, in the collector,
in the storage bin,
in the cyclone collector, in the pulverizer before or during pulverization,
and/or while
being transported from the pulverizer to the furnace for combustion.
Conveniently, in
various embodiments the sorbents are added to the coal during processes that
mix the
'15 coal such
as the in the pug mill or in the pulverizer. In a preferred embodiment, the
sorbents are added onto the coal in the pu]verizers.
[00291
Alternatively or in addition, sorbent components are added into the
coal burning system by injecting them into the furnace during combustion of
the fuel. In
a preferred embodiment, they are injected into the fireball or close to the
fireball, for
20 example
where the temperature is above 2000 F, above 2300 F, or above 2700 F.
According to the design of the burners and the operating parameters of the
furnace,
effective sorbent addition takes place along with the fuel, with the !aril-
nary combustion
air, above the flame, with or above the overfire air, and so on. Also
depending on the
furnace design and operation, sorbents are injected from one or more faces of
the furnace
25 and/or
from one or more corners of the furnace. Addition of sorbent compositions and
sorbent components tends to be most effective when the temperature at
injection is
sufficiently high and/or the aerodynamics of the burners and furnace set up
lead to
adequate mixing of the powder sorbents with the fuel arid/or combustion
products.
Alternatively or in addition, sorbent addition is made to the convective
pathway
30
downstream of the flame and furnace. In various embodiments, optimum injection
or
application points for sorbents are found by modeling the furnace and choosing
parameters (rate of injection, place of injection, distance above the flame,
distance from
8
CA 02846324 2014-03-14
the wall, mode of powder spraying, and the like) that give the best mixing of
sorbent,
coal, and combustion products for the desired results.
[0030] in
coal burning systems, hot combustion gases and air move by
convection away from the flame through the convective pathway in a downstream
direction (i.e., downstrearn in relation to the fireball). The convective
pathway of the
facility contains a number of zones characterized by the temperature of the
gases and
combustion products in each zone. Generally, the temperature of the combustion
gas
falls as it moves in a direction downstream froni the fireball. From the
furnace, where
the coal in one example is burning at a temperature of approximately 2'700 F ¨
3600 F
(about 1480 C ¨ 1650 C), the fly ash and combustion gases move downstream in
the
convective pathway to zones of ever decreasing temperature. To illustrate,
downstream
of the -fireball is a zone with temperature less that 2700 F. Further
downstream, a point
is reached where the temperature has cooled to about 1500 F. Between the two
points is
a zone having a temperature -from about 1500 F to about 2700 F. Further
down.stream, a
zone of less than 1500 F is reached, and so on. Further along in the
convective pathway,
the gases and fly ash pass through lower temperature zones until the baghouse
or
electrostatic precipitator is reached, which typically has a temperature of
about 300 F
before the gases are emitted up the stack.
[0031] The
combustion gases contain carbon dioxide as well as various
undesirable gases containing sulfur, nitrogen, and mercury. The convective
pathways
are also filled with a variety of ash which is swept along with the high
temperature gases.
To remove the ash before emission into the atmosphere, particulate removal
systems are
used. A variety of such removal systems, such as electrostatic precipitators
and a bag
house, are generally disposed in the convective pathway. in addition, chemical
scrubbers
can be positioned in the convective pathway. Additionally, there may be
provided
various instruments to -monitor components of the gas such as sulfur (as S0x),
nitrogen
(as N0x), and mercury.
[0032] Thus,
in various embodiments, the process of th.e present invention
calls for the application. of sorbents
directly into the furnace during combustion (addition "co-
combustion")
9
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directly to a fuel such as coal before combustion (addition "pre-
combustion" to make a refined coal);
directly into .the gaseous stream after combustion preferably in a
temperature zone of greater than 500 C and preferably greater than 800 C
(addition
"post-combustion); or
in a combination of pre-combustion, co-combustion, and post-
combustion additions.
{0033]
Application of the sorbents is 'made "into the coal burning system" in
any of pre-combustion, co-combustion., or post-combustion modes, or in any
combination. When the sorbents are added into the coal burning system, the
coal or
other Wel is said to be combusted "in the presence" the various sorbents,
sorbent
compositions, or sorbent components
[00341 In a
preferred embodiment downstream addition is carried out where
the temperature is about I500 F (815.5 C) to about 2700 F (1482,2 C). In some
aspects, and depending upon the specifics of furnace design and the layout of
the
convective pathways, the cutoff point or distinction between "into the
furnace", "into the
fireball", and "into the convective pathways" can be rather arbitrary. At some
point, the
combustion gases leave what is clearly a burning chamber or furnace and enter
a separate
structure that is clearly a flue or convective pathway for gases downstream of
the
furnace. However, many furnaces are quite large and so permit addition of
sorbents
"into the furnace" at a considerable distance from where the fuel and air are
being fed to
form the fireball. For example, some furnaces have overtire air injection
ports and the
like specifically designed to provide additional oxygen at a location above
the fireball to
achieve more complete combustion and/or control of emissions such as nitrogen
oxides.
The overtire air ports ean be 20 feet or higher above the fuel injection. In
various
embodiments, sorbent components or compositions are injected directly into the
fireball
alone with coal being fed, at a location above the coal feed, above or below
the overfire
air ports, or at a higher location within the burning chamber, such as at or
just under the
nose of the tbrnace. Each of these locations is characterized by a temperature
and by
conditions of turbulent flow that contribute to mixing of the sorbents with
the fuel and/or
the combustion products (such as the fiy ash). In embodiments involving
applying
sorbent compositions into the furnace or downstream of the furnace,
application is
preferably made where the temperature is above I 500 F, preferably above 2000
F, more
CA 02846324 2014-03-14
preferably where the temperature is above 2300cF, and most preferably where
the
temperature is above 2700F.
[00351 In
various embodiments described herein, sorbent compositions that
tend to reduce or remediate the release of mercury, nitrogen, and/or sulfur
from coal
burning utilities also have the beneficial effect of rendering the ash
produced by
combustion of the fuel highly cementitious. As a result, the ash is usable in
commerce as
a partial or complete replacement for portland cement in various cement and
concrete
produ.cts.
1 0 [0036) Burning
the coal with the sorbent compositions described herein
results in an ash that has, in various embodiments, increased levels of the
heavy metals
compared to coal burned without the sorbent, but which nevertheless contains
lower
levels of leachable heavy metals than the ash produced without the sorbents.
As a result,
the ash is safe to handle and to sell into commerce, for example as a
cementitious
material.
[0037] To
make the ash products, a carbonaceous fuel is burned to produce
heat energy from combustion of the carbonaceous material. Unburned material
and
particulate combustion products form ash, some of which collects at the
'bottom of the
furnace, but the majority of which is collected as fly ash from the flue by
precipitators or
fi]ters, for example a bag house on a coal hurnin2 facility. The content of
the bottom ash
and the fly ash depends on the chemical composition of the coai and on the
amount and
composition of sorbent components added into the coal burning facility during
combustion.
[0038] In
various embodiments, mercury emissions from the coal burning
facility are monitored. Emissions are monitored as elemental mercury, oxidized
mercury, or both. Elemental mercury means mercury in the ground or zero
oxidation
state, while oxidized mercury means mercury in the +1 or +2 oxidation state.
Depending
on the level of mercury in the flue gas prior to emission from the plant, the
amount of
sorbent composition added pre-, co-, and/or post-combustion is raised,
lowered, OT is
maintained unchanged. In general, it is desirable to remove as high a level of
mercury as
is practical. In embodiments, mercury removal of at least 40% up to 90% and
greater
can be achieved, based on the total amount of mercury in the coal. This number
refers to
11
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the mercury removed from the flue gases so that mercury is not released
through the
stack into the atmosphere. In this aspect, the num.bers correspond to the
percent
reductions in emissions of mercury from the facility, compared to burning the
coal
without sorbent. Normally, removal of mercury from the flue gases leads to
increased
levels of mercury in the ash. To minimize the amount of sorbent added into the
coal
burning process so as to reduce the overall amount of ash produced in the
furnace, it is
desirable in many embodiments to use the measurements of mercury emissions to
adjust
the sorbent composition rate of addition to one which will achieve the desired
mercury
reduction without adding excess material into the system.
[0039] In various
embodiments of burning coal Or other fuels with the added
sorbent components, mercury and other heavy metals in the coal such as
arsenic,
antimony, lead, and others report to the bag house or electrostatic
precipitator and
become part of the overall ash content of the coal burning plant;
alternatively or M
addition, the mercury and heavy metals are found in the bottom ash. As such,
emissions
of mercury and other heavy metals from the facility are reduced.
[0040] In
general, mercury and other heavy metals in the ash are resistant to
leaching under acidic conditions, even though they tend to be present in the
ash at
elevated levels relative to ash produced by burning coal without the sorbent
components
described herein. Advantageously, heavy metals in the ash do not leach beyond
regulatory levels; in fact, a decreased level of leachable heavy metal is
observed in the
ash on a ppm basis, even though the ash normally contains a higher absolute
level of
heavy metals by virtue of being produced by burning with the sorbents. Because
in
addition the cementitious nature of the ash is enhanced, the ash from the
combustion
(coai ash) is valuable for sale in commerce and use, for example, as a
cementitious
material to make portland cements as well as concrete products and ready
mixes.
[0041] In
preferred embodirrients, leaching of heavy metals is monitored or
analyzed periodically or continuously during combustion. The TCLP procedure of
the
United States Environmental. Protection A.gency is a commonly used method. The
amount of sorbent, particularly of sorbent components with Si (SiCh or
equivalents)
and/or Al (A103 or equivalents), is adjusted based on the analytical result to
maintain
the leaching in a desired range.
12
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[00421 in
one embodiment, a method is provided for burning coal to reduce
the amount of mercury released in-to the atmosphere. The method involves
applying a
sorbent composition comprising a halogen compound into the system in which the
coal is
being combusted. The halogen compound is preferably a bromine compound; in a
preferred embodiment., the sorbent is free of alkali metal compounds so as to
avoid
corrosion on boiler tubes or other furnace components. The coal is combusted
in the
furnace to produce ash and combustion gases. The combustion gases contain
mercury,
sulfur and other components. To accomplish a desired reduction of mercury in
the
combustion gases in order to limit release into the atmosphere, the mercury
level in the
combustion gases is preferably monitored, for example by measuring the level
analytically. In preferred embodiments, the amount of the sorbent composition
applied
is adjusted (i.e., by increasing it, decreasing it, or in some cases deciding
to leave it
unchanged) depending on the value of the mercury level measured in the
combustion
gases. in a preferred embodiment, the sorbent is added into the system by
applying it to
the coal pre-combustion, then delivering the coal containing the sorbent into
the furnace
for combustion.
[0043] In
another embodiment, sorbent components comprising a halogen
(preferably bromine or iodine, and most preferably bromine) compound and at
least one
aluminosilicate material are applied into the coal burning system. The
components are
added separately or as a single sorbent composition, and are optionally added
onto the
coal pre-combustion, into the furnace during combustion, or into the flue
gases
downstream of the furnace at suitable temperatures. In a preferred embodiment,
the
components are added to the coal pre-combustion., and the coal containing the
sorbent is
then delivered into the furnace for combustion. As before, preferably mercury
is
monitored in the flue gases and the sorbent application rate is adjusted
depending on the
value of the measured mercury level. The halogen contributes to lowering the
level of
mercury emissions, while the aluminosilicate contributes to making 'mercury-
captured in
the ash non-leaching.
[00441 In a
related embodiment, a method for reducing leaching of mercury
and/or of other heavy metals from ash produced from the combustion of coal or
other
.fuel in a coal burning system or in an incinerator involves introd.ucing
sorbents
containing silica and alumina i.nto the incinerator or coal burning system
during
combustion, measuring leaching of mercury and/or other heavy metals from the
resulting
13
CA 02846324 2014-03-14
ash, and adjusting the level of silica and alumina added according to the
measured
leaching of heavy metals. If leaching is higher than desired, the rate of
application of the
sorbent can be increased to bring the leaching back down into the desired
range. In a
preferred embodiment, the sorbent further contains a halogen (e.g. bromine)
compound
to enhance capture of mercury in the ash. Advantageously, the sorbent
containing silica
and alumina is added in a powder composition that contains <1% Na20 and <I%
K20, to
reduce or eliminate fouling.
[0045] In
one embodiment, the invention provides a method for reducing the
amount of oxidized mercury in flue gases that are generated .by combustion of
mercury-
containing carbonaceous :Fuel such as coai while at the same time producing a
cementitious ash product. The method comprises burning the fuel in the
presence of an
alkaline powder sorbent wherein the powder sorbent comprises calcium, silica,
and
alumina. The alkaline powder is added to the coal pre-combustion, injected
into the
furnace during combustion., applied into the flue gases downstream of the
furnace
(preferably where the temperature is 1500 F or greater), or in any
combination. The
powders are alkaline, characterized by a pH above 7 when combined with water,
preferably above 8 and preferably above 9. Advantageously, the sorbent
contains less
that l% each, less than 0.5% each, or less than 0.1% each by weight of alkalis
such as
Nal() and K20. In various embodiments, the sorbent further contains iron and
magnesium. In various embodiments, the aluminum content of the sorbent is
higher than
the alumina content of portland cement, preferably above about 5% or above
about 7%
alum in a.
[0046] To
monitor emissions while the fuel is burning, a level of mercury
(oxidized, elemental, or both) is measured in the flue gases downstream from
the
furnace. The measured mercury level is compared to a target level and, if the
measured
level is above the targeted level, the amount of powder sorbent added relative
to the
amount of fuel being burned is increased. Alternatively, if the measured level
is at or
below the target level, the rate of sorbent addition can be decreased or
maintained
unchanged.
[0047] In another
embodiment, the powder composition is an alkaline sorbent
composition that contains an alkaline calcium component as well as significant
levels of
silica and alumina. In a non-limitin.g embodiment, the powder composition
comprises 2
to 50% of an aluminosilicate material and 50 to 98% by weight of an alkaline
powder
1 4
CA 02846324 2014-03-14
comprising calcium. In a preferred embodiment, the alkaline powder comprises
one or
more of lime, calcium oxide, portland cement, cement kiln. dust, lime kiln
dust, and sugar
beet lime, whil.e the aluminosilicate material contains one or more selected
from the
group consisting of calcium montmorillonite, sodium montmorilIonite, and
kaolin. In a
particular embodiment, the powder sorbent comprises CKD and other material to
meet a
low alkali specification and/or a low chorine specification.
{-0-0481 The powder composition is added to the coal at a rate of about 0.1 to
ithout 10% by weight, based on the amount of coal being treated with the
sorbents for a
batch process, or on the rate of coal being consumed by combustion for a
continuous
process. In embodiments, the rate is 0.1-.5%, 0.1 ¨ 2%, 0.1-1.5%, O.1 -1.%,
from 1 to 8%
by weight, 2 to 8% by weight, 4 to 8% by weight, 4 to 6% by weight, or about
6% by
weight. In certain embodiments, the powder composition. is injected to the
fireball or
furnace during combustion and/or is applied to the coal under ambient
conditions, prior
to its combustion. The temperature at the injection -point is preferably at
least about
1000 F or higher. For some low value fuels, this corresponds to injection into
or close to
the fireball.
[00491 In a
further embodiment, a method for reducing mercury and/or sulfur
emitted into the environment during combustion of coal in a coal burning
system
comprises adding sorbent components comprising bromine, cal.cium, silica, and
alumina
into the coal burning system and combusting the coal in the presence of the
sorbent
components to produce combustion gases and fly ash. The amount of mercury in
the
combustion gases is measured and level of com.ponents containing bromine added
into
the system is adjusted depending on the measured value of mercury in the
combustion
gases.
[0050] in various
embodiments, the four components (calcium., silica,
alumina, and bromine) are added together or separately to the coal pre-
combustion, to the
furnace, and/or to the flue gases at suitable temperature as described herein.
Sorbents
containing the components preferably contain a maximum of 1% by weight Na20
and a
maximum of 1% by weight .K20. Preferably, bromine is present at a level
effective to a
capture, in the ash, at least 20%, at least 40%, at least 80% or at least 90%
of the mercury
in the coal, and silica and alumina are present at levels effective to produce
fly ash with a
leaching value of less than 0.2 ppm (200 ppb) with respect to mercury,
preferably less
than 100 ppb Hg, less than 50 ppb, and most preferably less than 2 ppb with
respect to
CA 02846324 2014-03-14
mercury. A level of 2 ppb represents the current lower detectable limit of the
TCLP test
for mercury leaching.
[00511 In
certain. embodiments, the methods provide coal ash and/or fly ash
containing mercury at a level corresponding to capture in the ash of at least
40% or of at
least 900/o of the mercury originally in the coal before combustion. 1.n some
embodiments, the mercury level is higher than in known fly ashes due to
capture of
mercury in the ash rather than release of mercury into the atmosphere. Fly ash
produced
by the process contains up to 200 ppm mercury or higher; in some embodiments
the
mercury content of the fly ash is above 250 ppm. Since the volume of ash is
normally
increased by use of the sorbents (in typical embodiments, the volume of ash
about
doubles), the increased measured levels of mercury represent significant
capture in the
ash of mercury that, without the sorbents, would have been released into the
environment. The content in the fly ash of mercury and other heavy metals such
as lead,
chromium, arsenic, and cadmium is generally higher than in fly ash produced
from
'15 burning coal without the added sorbents or sorbent components.
[00521
Preferably, the mercury in th.e coal ash is non-leaching in that it
exhibits a concentration of mercury in the extract of less than 0.2 ppm when
tested using
the Toxicity Characteristic Leaching Procedure (TCLP), test. Method 1311 in
"Test
Methods for Evaluating Solid Waste, Physical/Chemical Methods," EPA
Publication SW
846 ¨ Third Edition, as incorporated by reference in 40 CFR 260.11. It is
normally
observed that fly ash from burning coal with the sorbents described herein has
less
leachable mercury than ash produced from burning coal without the sorbent,
even though
the total mercury content in ash produced from the sorbent treated coal is
higher by as
much as a factor of 2 or more over the level in ash produced by burning
without the
sorbents. To illustrate, typical ash from burning of .PRB coal contains about
100-125
ppm mercury; in various embodiments, ash produced by burning PRB coal with
about
6% by weight of the sorbents described herein has about 200-250 ppm mercury or
more.
[00531 In
another embodiment, the invention provides a hydraulic cement
product containing portland cement and from OA% to about 99% by weight, based
on the
total weight of the cement product, of a coal ash or fly ash described above.
16
CA 02846324 2014-03-14
[0054] In a
further embodiment, the invention provides a pozzolanic product
comprising a pozzolan and from 0.01% to about 99% by weight, based on the
total
weight of the pozzolanic product of the ash described above.
[0055] The
invention also provides a cementitious mixture containing the
hydraulic cement product.
100563 The
invention further provides a concrete ready mix product
containing aggregate and the hydraulic cement product.
[0057] In
another embodiment, a cementitious mixture contains coal ash
described herein as the sole cementitious component; in these embodiments, the
ash is a
total replacement for conventional cements such as portland cement. The
cementitious
mixtures contain cement and optionally aggregate, fillers, and/or other
admixtures. The
cementitious mixtures are normally combined with water and used as concrete,
mortars,
grout, flowable fill, stabilized base, and other applications.
[0058] The
methods thus encompass burning coal with the added sorbents to
produce coal ash and energy for heat or electricity- generation. The ash is
then recovered
and used to formulate cementitious mixtures including cements, mortars, and
grouts,
[0059] In a
preferred embodiment, powder sorbent compositions described
herein contain one or more alkaline powders containing calcium, along with
lesser levels
of one or more aluminosilicate materials. The halogen component, if desired,
is added as
a further component of the alkaline powder or is added separately as part of a
liquid or
powder composition. Advantageously, use of the sorbents leads to a reduction
in
emissions or releases of sulfur, nitrogen, mercury, other heavy metals such as
lead and
arsenic, and/or chlorine from the coal burning system.
Sorbent compositions used in various embodiments of the invention
described above and herein contain components that contribute calcium, silica,
and/or
alumina. preferably in the form of alkaline powders. In various embodiments,
the
compositions also contain iron oxide, In a non-limiting example, the powder
sorbent
contains about 2-10% by- weight A1203, greater than 40%, for example about 40-
70%
CaO, >30% Si07, about 1-5% Fe201, and <2% of total alkalis such as sodium
oxide and
potassium oxide, preferably less than 1%. The components comprising calcium,
silica,
and alumina - and other elements if present - are combined together in a
single
composition or are added separately or in any combination as components to the
fuel
CA 02846324 2014-03-14
burning system. In preferred embodiments, use of the sorbents leads to
reductions in the
amount of NOx, S0x, and/or mercury released into the atmosphere
[0061]
Advantageously, the sorbent compositions contain suitable high levels
of alumina and silica. it is believed that the presence of alumina and/or
silica leads to
several advantages seen from use of the sorbent. To illustrate, it is believed
that the
presence of alumina and/or silica and/or the balance of the silica/alumina
with calcium.,
iron, and other ingredients contributes to the low acid leaching of mercur'
andlor other
heavy metals that is observed in ash produced by combustion of coal or other
fuels
containing mercury in the presence of the sorbents.
'I 0 [00621 As noted,
the components that contribute calcium. silica, and/or
alumina are preferably provided as alkaline powders. Without being limited by
theory, it
is believed that the alkaline nature of the sorbent components leads at least
in part to the
desirable properties described above. For example, it is believed the alkaline
nature of
the powders leads to a reduction in sulfur pitting. After neutralization, it
is believed a
geopolymeric ash is formed in the presence of the sorbents, coupling with
silica and
alumina present in the sorbent to form a ceramic like matrix that reports as a
stabilized
ash. The stabilized ash is characterized by very lowing leaching of mercury
and other
heavy metals. In some embodiments, the leaching of mercury is below detectable
limits.
However, for some coals, it is also observed that high alkali in the sorbent
components
tends to contribute to undesirable fouling. Accordingly, the present teachings
describe
how to overcome that disadvantage by using sorbents of lower alkalinity (as
measured by
content of .Na20 and K20) and/or lower chlorine, especially for use with sub-
bituminous
and lignite coal.s.
[00631
Sources of calcium for the sorbent compositions of the invention
include, without limitation, calcium powders such as calcium carbonate,
limestone,
dolomite, calcium oxide, calcium hydroxide, calcium phosphate, and other
calcium salts.
Industrial products such as limestone, lime, slaked lime, and the like
contribute major
proportions of such calcium salts. As such, they are suitable components for
the sorbent
compositions of the invention.
[00641 Other sources
of calcium include various manufactured products.
Such products are com.mercially available, and some are sold as waste products
or by-
products of other industrial processes. In preferred embodiments, the products
further
18
CA 02846324 2014-03-14
contribute either silica, alumina, or both to the compositions of the
invention. Non.-
limiting examples of .industrial products that contain silica and/or alumina
in addition to
calcium include portland cement, cement kiln dust, lime kiln dust, sugar beet
lime, slags
(such as steel slag, stainless steel slag, and blast furnace slag), paper de-
inking sludge
ash, cupola arrester .filter cake; and cupola furnace dust.
[0065] These
and optionally other materials are conibined to provide alkaline
powders or mixtures of alkaline powders that contain calcium, and preferably
also
contain silica and alumina. Other alkaline powders containing calcium, si]ica,
and
alurnina include pozzolanic materials, wood ash, rice hull ash, class C fly
ash, and class
F fly ash. In various embodiments, these and similar materials are suitable
components
of sorbent compositions, especially if the resulting composition containing
them as
components falls within the preferred range of 2 tol 0% by weight A1203,
greater than
40% by weight Ca(), greater than JO% by weight SiO2, about 1 to 5% Fe103, and
less
than 2% by weight total alkali.. Mixtures of materials are also used.. Non-
limiting
examples include mixtures of portland cement and lime, and mixtures containing
cement
kiln dust, such as cement kiln dust and lime kiln dust.
[0066] Sugar
beet lime is a solid waste m.aterial resulting from the
manufacture of sugar from sugar beets. It is high in calcium content, and also
contains
various impurities that precipitate in the liming procedure carried out CM
sugar beets.. It
is an itein of commerce, and is normally sold to landscapers, fanners, and the
like as a
soil amendment.
[0067]
Cement kiln dust (CKD) generally refers to a byproduct generated
within a cement kiln or related processing equipment during portland cement
manufacturing.
[0068] Generally, CKD
comprises a combination of different particles
generated in different areas of .the kiln, pre-treatment equipment, and/or
material
handling systems, including for example, clinker dust, partially to fully
calcined material
dust, and raw material (hydrated and dehydrated) dust. The composition of the
CKD
varies based upon the raw materials and fuels used, the manufacturing and
processing
conditions, and the location of collection points for CKD within the cement
manufacturing process. CKD can include dust or particulate matter collected
from kiln
effluent (i.e., exhaust) streams, clinker cooler effluent, pre-calciner
effluent, air pollution
19
CA 02846324 2014-03-14
control devices, and the like. Commercial CKD has a range of alkalinity,
depending on
its source. In some embodiments, it is possible to meet the low alkali spec of
the powder
sorbents described herein by using the low alkali CKD. If only high alkali CKD
is
available, it may be necessary to blend off or substitute part of the high
alkali CKD
product with the lower alkali material described above.
[00691
Whii.e CKD compositions will vary for different kilns, CKD usually
has at least some cementitious and/or pozzo]anie properties, due to the
presence of the
dust of clinker and calcined materials. Typical CKD compositions comprise
silicon-
containing compounds, such as silicates includlog tricalcium silicate,
dicaicium silicate;
aluminum-containing compounds, such as aluminates including triealcium
aluminate;
and. iron-containing compounds, such as ferrites including tetracalcium
alurninoferrite.
CKD generally comprises calcium oxide (CaO). Exemplary CKD compositions
comprise about 10 to about 60% calcium oxide, optionally about 25 to about
50%, and
optionally about 30 to about 45% by weight. In some embodiments, CKD comprises
a
concentration of free lime (available for a hydration reaction with water) of
about I to
about 10 %, optionally of about 1 to about 5%, and in some embodiments about 3
to
about 5%. Further, in certain embodiments, CKD comprises compounds containing
alkali metals, alkaline earth metals, and sulfur, inter alio.
[0070} Other
exemplary sources for the alkaline powders comprising
calciuna, and preferably further comprising silica and alumina, include
various cement-
related byproducts (in addition to portland cement and CKD described above).
Blended-
cement products are one suitable example of such a source. These blended
cement
products typically contain mixes of portland cement and/or its clinker
combined with
slag(s) and/or pozzolan(s) (e.g., fly ash, silica fume, burned shale).
Pozzolans are
usually silicaceous materials that are not in themselves cementitious, but
which develop
hydraulic cement properties when reacted with free bine (free Ca0) and water.
Other
sources are masonry cement and/or hydraulic lime, which include mixtures of
portland
cement and/or its clinker with lime or limestone. Other suitable sources are
aluminous
cements, which are hydraulic cements manufactured by burning a mix of
limestone and
bauxite (a naturally occurring, heterogeneous material comprising one or more
aluminum
hydroxide minerals, plus various mixtures of silica, iron oxide, titania,
aluminum
silicates, and other impurities in minor or trace amounts). Yet another
example is a
pozzolan cement, which is a blend.ed cement containing a substantial
concentration of
CA 02846324 2014-03-14
pozzolans. Usually the pozzolan cement comprises calcium oxide, but is
substantially
free of portland cement. Common examples of widely-employed pozzolans include
natural pozzolans (such as certain volcanic ashes or tuffs, certain
diatomaceous earth,
burned clays and shales) and synthetic pozzolans (such as silica fume and fly
ash).
[00711 Lime kiln dust
(LKD) is a byproduct from the manufacturing of lime.
LKD is dust or .particulate matter collected from a lime kiln or associated
processing
equipment. Manufactured lime can be categorized as high-calcium lime or
dolomitic
lime, and LKD varies based upon the processes that form it. Lime is often
produced by a
calcination reaction conducted by heating calcitic raw material, such as
calcium
carbonate (CaCO3), to form free lime CaO and carbon dioxide (CO2). High-
calcium
lime has a high concentration of calcium oxide and typically some impurities,
including
aluminum-containing and iron-containing compounds. High-calcium lime is
typically
formed from high purity calcium carbonate (about 95% purity or greater).
Typical
calcium oxide content in an LKD product derived from high-calcium lime
processing is
greater than or equai to about 75% by weight, optionally greater than or equal
to about
85% by weight, and in some cases gyeater than or equal to about 90% by weight.
In
sorne lime manufacturing, dolomite (CaCO3-MgCO3) is decomposed by heating to
primarily generate calcium oxide (CaO) and magnesium oxide (MgO), thus forming
what is known as dolomitic lime. In LKD generated by dolomitic lime
processing,
calcium oxide can be present at greater than or equal to about 45% by weight,
optionally
g-reater than about 50% by weight, and in certain embodiments, &eater than
about 55%
by weight. While LKD varies based upon the type of lime processing employed,
it
generally has a relatively high concentration of free lime. Typical amounts of
free lirne
in LKD are about 10 to about 50%, optionally about 20 to about 40%, depending
upon
the relative concentration of calcium oxide present in the lime product
generated.
[0072) Slags
are generally byproduct compounds generated by metal
manufacturing and processing. The term "slag" encompasses a wide variety of
byproduct compounds, typically comprising a large portion of the non-metallic
byproducts of ferrous metal and/or steel manufacturing and processing.
Generally, slags
are considered to be a mixture of various metal oxides, however they often
contain metal
sulfides and metal atoms in an elemental form.
[00731
Various examples of slag byproducts useful for certain embodiments
of the invention include ferrous slags, such as those generated in blast
furnaces (also
CA 02846324 2014-03-14
kriown as cupola furnaces), including, by way of example, air-cooled blast
furnace slag
(ACBFS), expanded or foamed blast furnace slag, pelletized blast furnace slag,
granulated blast furnace slag (GBFS), and the like. Steel slags can be
produced from
basic oxygen steelmaking furnaces (BOS/B0F) or electric arc furnaces (AF).
Many
slags are recognized for having cementitious and/or pozzolanie properties,
however the
extent to which stags have these properties depends upon their respective
composition
and the process from which they are derived, as recognized by the skilled
artisan.
Exemplary slags comprise calcium-containing compounds, silicon-containing
compounds, aluminum-containing compounds, magnesium-containing compounds, iron-
containing compounds, manganese-containing compounds and/or sulfur-containing
compounds. In certain embodiments, the slag comprises calcium oxide at about
25 to
about 60%, optionally about 30 to about 50%, and optionally about 30 to about
45% by
weight. One example of a suitable slag generally having cementitious
properties is
grounci granulated blast furnace slag (GGBFS).
[00741 As described
above, other suitable examples include blast (cupola)
funace dust collected from air pollution control devices attached to blast
furnaces, such
as cupola arrester filter cake. Another suitable industrial byproduct source
is paper de-
inking sludge ash. As recognized by those of skill in the art, there are many
different
manufactured/industrial process byproducts that are feasible as a source of
calcium for
the alkaline powders that form the sorbent compositions of the invention. Many
of these
well known byproducts comprise alumina and/or silica, as well. Some, such as
lime kiln
dust, contain major amounts of CaO and relatively small amounts of silica and
alumina,
Combinations of any of the exemplary manufactured products and/or industrial
byproducts are also contemplated for use as the alkaline powders of certain
embodiments
of the invention.
[0075] III
various embodiments, desired treat levels of silica and/or alumina
are above those provided by adding materials such as portland cement, cement
kiln dust,
lime kiln dust, and/or sugar beet lime. Accordingly, it is possible to
supplement such
materials with aluminosilicate materials, such as without limitation clays
(e.g.
montmorillonite, kaolins, and the like) where needed to provide preferred
silica and
alumina levels. in various embodiments, supplemental aluminosilicate materials
.inake
up at least about 2%, and preferably at least about 5% by weight of the
various sorbent
components added into the coal burning system. In general, there is no upper
limit from
22
CA 02846324 2014-03-14
a technical point of view as long as adequate levels of calcium are
maintained. However,
from a cost standpoint, it is normally desirable to limit .the proportion of
more expensive
aluminosilicate materials. Thus, the sorbent components preferably comprise
from about
2 to 50%, preferably 2 to 20%, and rnore preferably, about 2 to 10% by weight
aluminosilicate material such as the exemplary clays. A non-limiting example
of a
sorbent is about 93% by weight of a blend of CKD and LKD (for example, a 50:50
blend
or mixture) and about 7% by weight of an. aluminosilicate clay.
[00761 in
various embodiments, an alkaline powder sorbent composition
contains one or more calcium-containing powders such as portland cement,
cement kiln
dust, lime kiln dust, various slags, and sugar beet lime, along with an
aluminosilicate
clay such as, without limitation, montmorillonite or kaolin. The sorbent
composition
preferably contains sufficient Si02 and A1203 to form a refractory-like
mixture with
calcium sulfate produced by combustion of the sulfur-containing coal in the
presence of
the CaO sorbent component such that the calcium sulfate is handled by the
particle
control system; and to form a refractory mixture with mercury and other heavy
metals so
that. the mercury and other heavy metals are not leached from the ash under
acidic
conditions. In preferred embodiments, the calcium containing powder sorbent
contains
by weight a minimum of 10% silica and 2-10% alumina. Preferably, the alumina
level is
higher than that found in portland cement, that is to say higher than about 5%
by weight,
preferably higher than about 6% by weight, based on A1203-
[0077} hi
various embodiments, the sorbent components of the alkaline
powder sorbent composition work together .with optional added halogen (such as
bromine) compound or compounds to capture chloride as well as mercury, lead,
arsenic,
and other heavy metals in the ash, render the heavy metals non-leaching under
acidic
conditions, and improve the cementitious nature of the ash produced. As a
result,
emissions of harmful elements are mitigated, reduced, or eliminated, and a
valuable
cementitious material is produced as a by-product of coal burning.
[00781
Suitable aluminosilicate materials include a wide variety of inorganic
minerals and materials. For example, a number of minerals, natural materials,
and
synthetic materials contain silicon and aluminum associated with an oxy
environment
along with optional other cations such as, without limitation, Na, K, Be, Mg,
Ca, Zr, V,
Zn, Fe, Mn, and/or other anions, such as hydroxide, sulfate, chloride,
carbonate, along
with optional waters of hydration_ Such natural and synthetic materials are
referred to
23
CA 02846324 2014-03-14
herein as aluminosilicate materials and are exemplified in a non-limiting way
by the
clays noted above.
[0079] In
aluminosilicate materials, the silicon tends to be present as
tetrahedra, while the aluminum is present as tetrahedra, octahedra, or a
combination of
both. Chains or networks of aluminosilicate are built up in such materials by
the sharing
of 1, 2, or 3 oxygen atoms between silicon and aluminum tetrahedra or
octahedra. Such
minerals go by a variety of names, such as silica, alumina, aluminosilicates,
geopolymer,
silicates, and aluminates. However presented, compounds containing aluminum
and/or
silicon tend to produce silica and alumina upon exposure to high temperatures
of
combustion in the presence of oxygen
[0080] In
one embodiment, aluminosilicate materials include polymorphs of
Si02-.A1703. For example, silliminate contains silica octahedra and alumina
evenly
divided between tetrahedra and octahedra. .Kyanite is based on silica
tetrahedra and
alumina octahedra. Andalusite is another polymorph of Si 02' A1203.
[0081] In other
embodiments, chain silicates contribute silicon (as silica)
and/or aluminum (as alumina.) to the compositions of the invention. Chain
silicates
include without Ihnitation pyroxene and pyroxcnoid silicates made of infinite
chains of
SiO4 tetrahedra lin.ked by sharing oxygen atoms.
[0082] Other
suitable aluminosilicate materials include sheet materials such
as, without limitation, micas, clays, chrysotiles (such as asbestos), talc,
soapstone,
pyrophillite, and kaolinite. Such materials are characterized by having layer
structures
wherein silica and alumina octahedra and tetrahedra share two oxygen atoms.
Layered
aluminosilicates include clays such as chlorites, glauconite, illite,
polygorski.te,
pyrophillite, sauconite, vermiculite, kaolinite, calcium montmorillonite,
sodium
montmorillonite, and bentonite. Other examples include micas and talc.
[0083]
Suitable aluminosilicate materials also include synthetic and natural
zeolites, such as without limitation the analcime, sodalite, chabazite,
natrolite, phillipsite,
and mordenite groups. Other zeolite minerals include heulandite, brewsterite,
epistilbite,
yagawaralite, laumontite, ferrierite, paulingite, and cli.noptilolite. The
zeoli.tes
are minerals or synthetic materials characterized by an aluminosilicate
tetrahedral.
.fram.ework, ion exchangeable "large cations" (such as Na, K, Ca, Ba, and Sr)
and loosely
held water molecules.
24
CA 02846324 2014-03-14
[00841 ln
other embodiments, framework or 3 silicates, aluminates, and
aluminosilicates are used. Framework aluminosilicates are characterized by a
structure
where SiO4 tetrahedra, A104 tetrahedra, and/or A.106 octahedra are linked in
three
dimensions. Non-limiting examples of framework silicates containing both
silica and
alurnina include feldspars such as albite, anorthite, andesine, bytownite,
labradorite,
microcline, sanidine, and orthoclase.
[00851 in
one aspect, the sorbent powder compositions are characterized in
that they contain a major amount of calcium, preferably greater than 20% or
greater than
40% by weight based on calcium oxide, and that furthermore they contain,
levels of
silica, a.ndfor alumina higher than that found in commercial products such as
portland
cement. In preferred embodiments, the sorbent COTIlpositions comprise greater
than 5%
by weight alumina, preferably greater than 6% by weight alumina, preferably
greater
than 7% by weight alumina, and preferably greater than about 8% by weight
alumina.
[00861 Coal
or other fuel is treated with sorhent components at rates effective
to control the amount of nitrogen, sulfur and/or mercury released into the
atmosphere
upon combustion. In various embodiments, total treatment levels of the sorbent
components ranges from about 0.1% to about 20% by weight, based on the weight
of the
coal being treated or on the rate of the coal being, consumed by combustion,
when the
sorbent is' powder sorbent containing calcium, silica, and alumina. When the
sorbent
components are combined into a single compositic.m, the component treat levels
correspond to sorbent treat levels. In this way a single sorbent composition
can be
provided and metered or otherwise measured for addition into the coal burning
system.
In general., it is desirable to use a minimum amount of sorbent so as not to
overload the
systern with excess ash., while still providing enough to have a desired
effect on sulfur
and/CM' Mercury emissions. Accordingly, in various embodiments, the treatment
level of
sorbent ranges from about 0.1% to about 10% by weight, in some embodiments
from.
about 1 or 2% by weight to about 10% by weight. For many coats, an addition
rate of
6% by weight of powder sorbent has been found to be acceptable.
[00871
Sorbent compositions comprising a halogen compound contain one or
more organic or inorganic compounds that contain a halogen. Halogens include
chlorine, bromine, and iodine. Preferred halogens are bromine and iodine. The
halogen
compounds are sources of the halogens, especially of bromine and iodine. For
bromine,
CA 02846324 2014-03-14
=
sources of the halogen include various inorganic salts of bromine including
bromides,
bromates, and bypobromites. In various embodiments, organic bromine compounds
are
less preferred because of their cost or availability. However, organic sources
of bromine
containin.g a suitably high level of bromine are considered within the scope
of the
invention. Non-limiting examples of organic bromine compounds include
methylene
bromide, ethyl bromide, bromofoure and carbon tetrabromide. Non-limiting
inorganic
sources of iodine include hypoiodites, iodates, and iodides, with iodides
being preferred.
Organic iodine compounds can also be used.
[0088} When
the halogen compound is an inorganic substituent, it is
preferably a bromine or iodine containing salt of an alkaline earth element.
Exemplary
alkaline earth elements include beryllium, magnesium, and calcium. Of halogen
compounds, particularly preferred are broinides and iodides of alkaline earth
metals such
as calcium. Alkali metal bromine and iodine compounds such as bromides and
iodides
are effective in reducing mercury emissions. But in some embodiments, they are
less
preferred as they tend to cause corrosion on the boiler tubes and other steel
surfaces
and/or contribute to tube degradation and/or firebrick degradation. in
various
embodiments, it has been found desirable to avoid potassium salts of the
halogens, in
order to avoid problems in the furnace.
[00891 in
various embodiments, it has been found that the use of alkaline
earth salts such as calcium tends to avoid such problems with sodium and/or
potassium.
Thus i.n various embodiments, the sorbents added into the coal burning system
contain
essentially no alkali metal-containing bromine or iodine compounds, or more
specifically
essentially no sodium-containing or potassium-containing bromine or iodine
compounds.
[0090] .in
various embodiments, sorbent compositions containing halogen
are provided in the form of a liquid or of a solid composition. In various
embodiments,
the halogen-containing composition is applied to the coal before combustion,
is added to
the furnace during combustion, and/or is applied into flue gases downstream of
the
furnace. When the halogen composition is a solid, it can further contain the
calcium.
silica, and alumina components described herein as the powder sorbent.
Alternatively, a
solid halogen composition is applied onto the coal and/or elsewhere into the
combustion
system separately from the sorbent components comprising calcium, silica, and
alumina.
When it is a liquid composition it is generally applied separately.
26
CA 02846324 2014-03-14
[0091] In
various embodiments, liquid mercury sorbent comprises a
solution containing 5 to 60% by weight of a soluble bromine or iodine
containing salt.
Non-limiting examples of preferred bromine and iodine salts include calcium
bromide
and calcium iodide,. In various embodiments, liquid sorbents contain 5-60% by
weight
of calcium bromide andlor calcium iodide. For efficiency of addition to the
coal prior to
combustion, in various embodiments it is preferred to add mercury sorbents
having as
high level of bromine or iodine compound as is feasible. In a non-limiting
embodiment,
the liquid sorbent contains 50% or more by weight of the halogen compound,
such as
calcium bromide or calcium iodide.
[00921 In various
embodiments, the sorbent compositions containing a
halogen compound further contain a nitrate compound, a nitrite compound, or a
combination of nitrate and nitrite compounds. Preferred nitrate and nitrite
compounds
include those of magnesium and calcium, preferably calcium.
[00931 To
further illustrate, one embodiment of the present invention
involves the addition of liquid mercury sorbent directly to raw or crushed
coal prior to
combustion. For example, mercury sorbent is added to the coal in the coal
feeders.
Addition of liquid mercury sorbent ranges from 0.01 to 5%. In various
embodiments,
treatment is at less than 5%, less than 4%, less than 3%, or less than 2%,
less than 1%,
less than 0.5%, and less than 0.2% where all percentages are based on the
amount of coal
being treated or on the rate of coal consumption by combustion. Higher
treatment levels
are possible, but tend to waste material, as no further benefit is achieved.
Preferred
treatment levels are from 0.025 to 2.5% by weight On a wet basis. The amount
of sol.id
bromide or iodide salt added by way of the liquid sorbent is of course reduced
by its
weight fraction in the sorbent. In an illustrative embodiment, addition of
bromide or
iodide compound is at a low level such as from 0.01 % to 1% by weight based on
the
solid. When a 50% by weight solution is used, the sorbent is then added at a
rate of
0.02% to 2% to achieve the low levels of addition. For example, in a preferred
embodiment, the coal is treated by a liquid sorbent at a rate of 0.02 to 1%,
preferably
0.02 to 0.5 % calculated assuming the calcium bromide is about 50% by weight
of the
sorbent. In a typical embodiment, approximately I%õ 0.5%, or 0.25% of liquid
sorbent
containing 50% calcium bromide is added onto the coal prior to combustion, the
percentage being based on the weight of the coal.. in a preferred embodiment,
initial
treatment is started at low leveis (such as 0.01% to 0.1%) and is
incrementally increased
27
CA 02846324 2014-03-14
until a desired (low) level of mercury emissions is achieved, based on
monitoring of
emissions. Similar treatment levels of halogen are used when the halogen is
added as a
solid or in multi-component compositions with other components such as
calcium, silica,
alumina, iron oxide, and so on.
[0094} When used,
liquid sorbent is sprayed, dripped, or otherwise.
delivered onto the coal or elsewhere into the coal burning system. In various
embodiments, addition is made to the coal or other fuel at ambient conditions
prior to
entry of the Iliellsorbent composition into the furnace. For example, sorbent
is added
onto powdered coal prior to its injection into the furnace. .Alternatively or
in addition,
liquid sorbent is added into the furnace during combustion and/or into the
flue gases
downstream of the furnace. Addition of the halogen containing mercury sorbent
composition is often accompanied by a drop in the mercury levels measured in
the flue
gases within a minute or a few minutes; in various embodiments, the reduction
of
mercury is in addition to a reduction achieved by use of an alkaline powder
sorbent
based on calcium, silica, and alumina..
[0095} in
another embodiment, the invention involves the addition of a
halogen component (illustratively a calcium bromide sol.ution) directly to the
-furnace
during combustion. In another embodiment, the invention provides for an
addition of a
calcium bromide solution such as discussed above, into the gaseous stream
downstream
of the furnace in a zone characterized by a temperature in the range of 2700 F
to 1500 F,
preferably 2200 F to 1500 F. In various embodimentsõ treat levels of bromine
compounds, such as calcium bromide are divided between co-, pre- and post-
combustion
addition in any proportion.
[0096] in
one embodiment, various sorbent components are added onto
coal prior to its combustion to make a so-called refined coal. The coal onto
which the
sorbents are applied is preferably particulate coal, and is optionally
pulverized or
powdered according to conventional procedures. In a non-limiting example, the
coal is
pulverized so that 75% by weight of the particles passes through a 200 mesh
screen (a
200 mesh screen has hole diameters of 75 inn). In various embodiments, the
sorbent
components are added onto the coal as a solid or as a combination of a liquid
and a solid.
Generally, solid sorbent compositions are in the form of a powder. If a
sorbent is added
as a liquid (illustratively as a solution of one or more bromine or iodine
salts in water), in
one embodiment the coal remains wet when fed into the burner. In
various
2.8
CA 02846324 2014-03-14
embodiments, a sorbent composition is added onto the coal continuously at the
coal
burning facility by spraying or mixing onto the coal while it is on a
conveyor, screw
extruder, or other feeding apparatus. In addition or alternatively, a sorbent
composition
is separately mixed with the coal at the coal burning facility or at the coal
producer. In a
preferred erribodiment, the sorbent composition is added as a liquid or a
powder to the
coal as it is being fed into the burner. For example, in a preferred
commercial
embodiment, the sorbent is applied into th.e pulverizers that pulverize the
coal prior to
injection. if desired, the rate of addition of the sorbent composition is
varied to achieve a
desired level of mercury emissions. in one embodiment, the level of mercury in
the flue
gases is monitored and the level of sorbent addition adjusted up or down as
required to
maintain the desired mercury level.
[0097] In
preferred embodiments, nitrogen, mercury, and sulfur are
monitored using industry standard methods such as those published by the
American
Society for Testing and Materials (ASTM) or international standards published
by the
International Standards Organization (ISO). An apparatus comprising an
analyticai.
instrument is preferably disposed in the convective pathway downstream of the
addition
points of the mercury and sulfur sorbents. In a preferred embodiment, a
inercury
monitor is disposed on the clean side of the particulate control system
Alternatively or
in a.ddition, the flue gases are sampled at appropriate locations in the
convective pathway
without the need to install an instrument or monitoring, device.. In various
embodiments,
a measured level of mercury or sulfur is usecl to provide feedback signals to
pumps,
solenoids, sprayers, and other devices th.at are actuated or controlled to
adjust the rate of
addition of a sorbent composition into the coal burning system. Alternatively
or in
addition, the rate of sorbent addition Can be adjusted by a human operator
based on the
observed levels of mercury and/or sulfur.
[OO98 Iu
various embodiments, the ash produced by burning coal in the
presence of the sorbents described herein is cementitions in that it sets and
develops
strength when combined with water. The ash tends to be self-setting due its
relatively
high level of calcium. The ash serves alone or in combination with portland
cement as a
hydraulic cement suitable for formulation into a variety of cementitious
mixtures such as
mortars, concretes, and grouts.
[0099] The
ceinentitious nature of ash produced as described herein is
demonstrated for example by consideration of the strength activity index of
the ash, or
29
CA 02846324 2014-03-14
more exactly, of a cementitious mixture containing the ash. As described in
ASTM
C311-05, measurement of the strength activity index is made by comparing the
cure
behavior and property development of a 100% portland cement concrete and a
test
concrete wherein 20% of the portland cement is replaced with an equal weight
of a test
cement. ln the standard test, strength is compared at 7 days and at 28 days. A
"pass" is
considered to be when the strength of the test concrete is 75% of the strength
of the
pordand cement concrete or greater. In various embodiments, ashes of the
invention
exhibit of strength activity of 100% to 150% in the AST3V1 test, indicating a
strong
--pass". Similar high values are observed when tests are run on test mixtures
with other
than an 80:20 blend of portland cement to ash. In various embodiments, a
strength
activity index of 100% to 150% is achieved with blends of 85:15 to 50:50,
where the first
number of the ratio is portland cement and the second number of the ratio is
ash prepared
according to the invention. In particular embodiments, the strength
development of an
all-ash test cementitious mixture (i.e., one where ash represents 100% of the
cement in
the test mixture) is greater than 50% that of the all-portland cement control,
and is
preferably greater than 75%, and more preferably 1.00% or more, for example
100 --
150%. Such results demonstrate the highly cementitious nature cif ash produced
by
burning coal or other fuel in the presence of the sorbent components described
herein.
[00100]
Because the ash resulting from combustion of coal according to
the invention contains mercury in a non-leaching form, it is available to be
sold into
commerce. Non-linaiting uses of spent or waste fly ash or bottom ash include
as a
component in a cement product such as portland cement. In various embodiments,
cement products contain from about 0.1% up to about 99% by weight of the coal
ash
produced by 'burning compositions according to the invention. In one aspect,
the non-
leaching property of the -mercury and other heavy metals in the coal ash makes
it suitable
for all known industrial uses of coal ash.
[01011 Coal
ash according to the invention, especially the fly ash
collected by the particle control systems (bag house, electrostatic
precipitators, etc.) is
used in portland cement concrete (PCC) as a partial or complete replacement
for portland
cement. In various embodiments, the ash is used as a -mineral admixture or as
a
component of blended cement. As an admixture, the ash can be total or partial
replacement for portland cement and can be added directly into ready mix
concrete at the
CA 02846324 2014-03-14
batch plant. Alternatively, or in addition, the ash is inter-ground with
cement clinker or
blended with portland cement to produce blended cements.
[01021 Class
F and Class C fly ashes are defined for example in U.S.
Standard ASTM C 618. The ASTM Standard serves as a specification for fly ash
when
it is used in partial substitution for portland cement. It is to be noted that
coal ash
produced by the methods described herein tends to be higher in calcium and
lower in
silica a.nd alumina than called for in the specifications for Class F and
Cla.ss C fly ash in
ASTM C 618. Typical values for the fly ash of the invention is >50% by weight
CaO,
and <25% Si02/A1103/Fe203. In various embodiments, the ash is from 51 to 80 %
by
weight CaO and from about 2 to about 25% of total silica, alumina, and iron
oxide. It is
observed that fly ash according to the invention is highly cementitious,
allowing for
substitutions or cutting of the portland cement used in such cementitious
materials and
cementitious materials by 50% or more. In various applications, the coal ash
resulting
from burning coal with sorbents described herein is sufficiently eementitious
to be a
complete (100%) replacement for portland cement in such compositions.
[0103] To
further illustrate, the American Concrete institute (AC1)
recommends that Class F fly ash replace from 15 to 25% of portland cement and
Class C
fly ash replace from 20 to 35%. It has been found that coal ash produced
according to
the methods described herein is sufficiently cementitious to replace up to 50%
of the
portland cement, while maintaining 28 day strength development equivalent to
that
developed in a product using 100% portland cement. That is, although iiì
various
embodiments the ash does not qualify by chemical composition as Class C or
Class F ash.
according to ASTM C 618, it nevertheless is useful for formulating high
strength
concrete products.
[0104] Coal ash made
according to the invention can also be used as a
component in the production of flowa.ble fill, which is also called controlled
low strength
material or CLSM. CLSM is used as a self leveling, self compacting back fill
material in
place of compacted earth or other fill. The ash described herein is used in
various
embodiments as a 100% replacement for portland cement in such CLSM materials.
Such
compositions are formulated with water, cement, and aggregate to provide a
desired
fiowability and development of ultimate strength. For example, the ultimate
strength of
towable fill should not exceed 1035 kPa (150 pounds per square inch) if
removability of
the set material is required. If formulated to achieve higher ultimate
strength, jack
31
CA 02846324 2014-03-14
hammers may be required for removal. However, when it is desired to formulate
flowable fill mixes to be used in higher icia.d bearing applications, mixtures
containing a
greater range of compressive strength upon cure can be designed.
[0105] Coal
ash produced according to the methods described herein is
also usable as a component of stabilized base and sub base mixtures. Since the
1950's
111.13I1CTOUS variations of the basic lime/fly ash/aggregate formulations have
been used as
stabilized base mixtures. An example of the use of stabilized base is used as
a stabi.l.ized
road base. To illustrate, gravel roads can be recycled in place of using ash
according to
the composition. An existing road surface is pulverized and re-deposited in
its original
location. Ash such as produced by the methods described herein is spread over
the
pulverized road material and mixed in. Following compaction, a seal coat
surface is
placed on the roadway. A.sh according to the invention is useful in such
applications
because it contains no heavy inetals that leach above regulatory requirements.
Rather,
the ash produced by methods of the invention contains less leachable mercury
and less
leachable other heavy metals (such as arsenic and lead) than does coal ash
produced by
burning coal without the sorbents described herein.
[0106] Thus,
the invention provides various methods of eliminating the
need to landfill coal a.sh or fly ash resulting from combustion of coal that
contains high
levels of mercury. Instead of a costly disposal, the material earl be sold or
otherwise
used as a raw material.
[0107] hi a
preferred embodiment, use of the sorbents results in a.
cementitious ash that can replace portland cement in whole or in part in a -
variety of
applications. Because of the reuse of the cementitious product, at least some
portland
cement manufacture is avoided, saving the energy req-uired to make the cement,
and
avoiding the release of significant amounts of carbon dioxide which would have
arisen
from the cement manufacture. Other savings in carbon dioxide emissions result
from the
reduced need for lime or calcium carbonate in desultbrization scrubbers. The
invention
thus provides, in various embodiments, methods for saving energy and reducing
green
house emissions such as carbon dioxide. Further detail of various embodiments
of this
aspect of the invention are given below.
EXAMPLES
Example 1
32
CA 02846324 2014-03-14
[OM] The following are the required specifications for powder
sorbent
and halide sorbent for use in production of Refined Coal using sub-bituminous
coal.
Powder Sorbent:
Content
Content
Constituent Constituent
(Mass %) Mass
(%)
Calcium Oxide (CaO) >40% Potassium Oxide (KO) <1%
Silicon Oxide (Si0) >1(% Sodium Oxide (Na20) ¨ <1%
Aluminum Oxide (A.I203) 2 - 10% Sulfur Oxide (S03) <7%
Iron Oxide (Fe2Q,) T- 5% Chloride (CI)
Magnesium Oxide (MgO' 1 - 5% Mercury (H.g) <0,1
ug/g
Size >80% passing 200 Size distribution shall be determined by
fine -wire
Distribution mesh screen analysis
In addition to the hard limit of 7% 803, the ratio between CaO and S03 should
not
fall below 6:1 and preferably should remain greater than SI. This ensures
sufficient
CaO to absorb the added. sulfur.
2.
.As practical matter, the lig content of the powder sorbent should be
maintained less
than or equal to that of the coal being treated.
Acceptable test methods for Oxide Analysis are:
ASTM D3682 Standard Test Methods for Major and Minor Elements in
Combustion Residues from Coal Utilization Processess
ASTM C114 Standard Test Methods for Chemical Analysis of Hydraulic
Cement
Acceptable test methods for Mercury content:
ASTM D6414 Standard Test Method For Total Mercury in Coal and Coal
Combustion Residues by Acid Extraction or Wet OxidatiorilCold
Vapor Atomic Absorption
ASTM D6722 Standard Test Method for Total Mercury In Coal and Coal
Combustion Residues by Direct Combustion ..Analysis
CA 02846324 2014-03-14
EPA 7473 Mercury in Solid or Semisolid Waste (Manual Cold-Vapor
Technque)
Halide Sorbent
Constituent Content J Determination Method
(Mass /0)
Calcium Brornide (CaBe>) 52 - 54 Process blending control
and specific gravity
measurement
Water (H20) 46 -48 Process Blending control
and specific gravity
measurement
Example 2
[01091 A series of tests were performed in the Energy &
Environmental
Research Center's (EERC's) combustion test facility (CTF) to determine the
effect of
sorbents on the emissions of NO, and Hg during combustion of a Powder River
Basin
(PRB) :sub-bituminous coal. Testing was conducted to support efforts to
confirm that the
process employed produces "refined coal," as is defined in Section 45 of the
Internal
Revenue Code. Section 45(c)(7)(A) defines refined coal to include a fuel which
1) is a
solid fuel produced from coal, 2) is sold by the taxpayer with the reasonable
expectation
that it will be used for the purposes of producing steam, and 3) is certified
by the
taxpayer as resulting (when used in the production of steam) in a "qualified
emission
reduction."
Section 45(c)(7)(B) defines the term "qualified emission reduction" to mean a
reduction of at least 20% of the emissions of NO, a.nci at least 40% of the
emissions of
either SO, or Hg released when the refilled coal is burned as compared to the
emissions
released when the feedstock coal is burned.
Description of Facilities and Procedures
34
CA 02846324 2014-03-14
[01101 The
CIF is useci extensively to investigate SO,. and NO
emissions and the transformation of toxic trace metals (Hg, As, and Pb) during
the
combustion of coal and other fuels or waste materials. The CTF is capable of
producing
gas and particulate samples that are representative of those produced in
industrial-and
full-scale pulverized coal (pc)-fired boilers. The test facility has several
pollution control
devices that may be used to reduce emissions, including an electrostatic
precipitator
(ESP) or fabric-filter baghouse for particulate control, a selective catalytic
reduction
(SCR) column for NO, control, and a wet scrubber for control of sulfur
emissions. The
CTF was designed to replicate almost all types and configurations of full-
scale pc-fired
boilers used by U.S.-based utilities to generate electricity from steam. For
example, the
CTF can fire pc at a rate between 550,000 and 750,000 Btu/hr, depending upon
desired
operating conditions. Although the CTF is adjusted to simulate conditions of
full-scale
pc-tired boilers, because of tbe numerous variables that can impact combustion
effects in
a commercial boiler, it is not possible to exactly replicate what will be
observed in
regular commercial operations. The tiring rate is typically a function of the
coal rank,
with low-rank coals fired at the low end of the range and higher-rank coals
fired at the
mid- to upper level of the indicated range. Because the CTF furnace is
refractory-lined,
the tiring rate is set based. on the furnace exit gas temperature (FEGT)
desired to simulate
a specific boiler that would be used at a coal-fired power plant. For sub-
bituminous
coals, a firing rate between 550,000 and 600,000 BTU/hr will typically produce
a FECiT
between 21000 and 22000 F, which is typical of many coal-fired power plants
that burn
sub-bituminous coals such as the coal tested here.
[01111
Combustion air in the CTF is provided by a forced-draft fan in this
balanced-draft system. The induced-draft fan at the back of the system is used
to
maintain a slight vacuum in the combustion zone and exhaust the combustion
flue gases
to a stack. Combustion air is typically preheated using an electric air heater
and is split
between primary, secondary, ancl overfire air (OFA.).
[0112]
Combustion gas analysis is provided by continuous emission
monitors (CE,Ms) at two locations: the furnace exit, which is used to monitor
and
maintain a specified excess air level for all test periods, and the outlet of
the particulate
control device, which is used to access any air inleakage that may have
occurred so that
emissions of interest sampled at the back end of the system can be corrected
for the
dilution caused by the inleakage. For this test series, flue gas analyses were
obtained
CA 02846324 2014-03-14
from the duct at the outlet of the ESP. Each CM rack contains five modules for
determination of 07, C.02, CO, S02, and NO,. With the exception of SO,, each
of the
modules was manufactured by Arnetek. Each of the analyzers uses a flue gas
conditioner
to remove moisture from the gas stream prior to analysis. All data reported
here are on a
dry gas basis. All gas analyses are continuously monitored and recorded by the
CTE's
data a.cquisition system. National Instruments provided both the hardware and
software
(LabView) used to collect all data presented here.
[01131 The
CEM analyzers are individually calibrated prior to every test
conducted on the CIT. Nitrogen is used as the zero gas, while several span
gases are
used to calibrate each instrument over the range used during testing.
Typically, 02 is
measured over a 0% to 10% range, CO, is measured over a 0% to 20% range, CO is
measured over a 0 to 500 ppm range, and NO, is measure over a 0 to 1000 ppm
range.
SOs measurements are made over various ranges, depending upon the sulfur
content of
the coal being tested. During this test series, the SO, measurement instrument
was
calibrated over a range from 0 to 1000 ppm, which is appropriate for the
sulfur content of
the subbituininous PRB coal tested here.
[0114] Flue
gas mercury (Hg) measurements were obtained separately by
a continuous Hg monitor (CMM) manufactured by Tekra.n Instrum.ents
Corporation.
The system draws a gas sample from the flue gas ducting at the exit of the
particulate
control device. Moisture is removed from gas stream prior to analysis. The
flue gas
conditioning system uses a 10% NaOH solution to remove CO2 and SO, to prevent
interference with the ability of the analyzer to accurately measure the flue
gas Hg
concentration. Since a11 Hg analyzers can only measure elemental mercury, Hg ,
the total
mercury, Hg(-rì, concentration is obtained by reducing the oxidized mercury,
portion with a 10% NaOH solution containing stannous chloride. The Tekran
instrument
traps Hg from the conditioned sample onto a cartridge containing an ultrapure
gold
adsorbent. The amalgamated Hg is then thermally desorbed and detected using
cold-
vapor atomic fluorescence spectrometry. A dual-cartridge design enables
alternate
sampling and desportion, resulting in continuous mea.surement of the sample
stream.
Similar to the CEIVI calibration described above, the CMM is also zeroed and
spanned
prior to testing and checked at the completion of testing. No drift was noted
during the
tests conducted and reported here.
36
CA 02846324 2014-03-14
The CTI: configuration utilized during these tests included only an ESP for
particulate
control, with both Hg and NO, measurements obtained from the duct at the
outlet of the
ESP. At the completion of the feedstock and refined coal test periods, fuel
and fly ash
samples were collected and submitted for analysis. The samples collected
during testin9,
are described within the f011owing discussion.
Fuel Preparation and Analysis
{0115] The
sub-bituminous coal tested was a sample obtained from a coal
pile. The coal is a PRB sub-bituminous coal with a gross calorific value
approximately- in
the range from 8500 to 10,000 Btu/lb, depending on moisture content, which is
sourced
from several mines located in Wyoming.
[0116] The
as-received coal was inspected for surface moisture upon
receipt and floor-dried as necessary. The air-dried sample was crushed to 1/4-
inch top
size and fed to a hammer mill pulverizer, creating a size distribution of
approximately
70% passing 200 mesh for use during testing, typical of the coal processing
achieved at
most coal-fired power plants. This size distribution is typical of that
achieved by the
pulverizers at most full-scale utility boilers. The refined coal sample used
during this test
series was produced by the EERC and is considered comparable to the refined
coal
produced at the Section 45 fa.ciii.ties. The sorbents used to prepare the
refined coal were
applied to the pulverized fuel as described below. This differs from full-
scale
application., where the sorbents are applied to the coal as it is reclaimed
from stockpiles
in the coal yard, mixed, crushed, and then sent to the pulverizers. The
primary reason for
application of the sorbents to the pc for utilization in the pilot-scale tests
is the potential
loss of material to the dust collection system utilized during pulverization
of fuel
samples. All pilot-scale fuel samples are remotely crushed and pulverized
prior to
utilization. By preparing the refined fuel front the pc sample, the potential
loss of the
sorbents to the dust collection system is avoided and results in the best
simulation of
what occurs at full scale.
[01171 The
pulverized fuel was split into two parts: a feedstock sample
and a second coal sample that is processed into refined coal. The refined coal
was
prepared by laying out a weighed quantity (about 500 lb.) on the floor of the
coal
preparation facility. Weighed quantities of halide sorbent and powder sorbent
were
carefully applied to the coal, which was 'periodically mixed while the
sorbents were
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CA 02846324 2014-03-14
applied. The powder solvent was distributed by hand, making several passes
over the
extent of the coal pile, with mixing of the fuel after each pass. The halide
sorbent was
placed in a small pressurized metal spray canister such that the spray
canister muzzle
produced a mist that was applied to the exposed surface of the pile. Treatment
required
several passes to completely distribute the sorbent. After each pass, a rake
was used to
turn the pile over, exposing new surface for the next treatment pass. in each
case, several
small portions of the sorbents were distributed over the coal pile, followed
by mixing
until the spec,ified treatment rate was achieved 0.008 wt% halide sorbent and
0.25 wt%
powder sotbent.
[01181 Each of the
samples (feedstock coal and refined coal) was
transferred to storage hoppers for use in the pilot-scale testing described
below. These
storage hoppers sit directly above the coal feed hopper during testing. A.
rotary valve is
used to transfer the coal and refined coal samples, respectively, from the
storage hoppers
to the feed hopper. The storage hoppers and feed 'hopper are cleaned with a
dilute acid
'15 solution after each test to remove any trace of the treated fuel.
Fuel analysis
[0119]
During each test period, a coal sample is conveyed from the
storage hopper through a small tube that penetrates the sidewall of the feeder
at a 70%
angle, with the open end situated immediately below the rotary valve between
the
storage hopper and the feed hopper. This rube intercepts a small portion of
the fuel each
time the feed hopper fills. J.n this manner, a true as-fired sample of the
fuel is obtained.
The coal sample fans by gavity into a sample bag attached to the end of the
sample tube.
A new bag is attached to the sample tube prior to each new test period,
separating the
fuel samples representing the feedstock and refined coal test periods.
[0120] The
as-fired coal is continuously sampled to determine the
emission baseline from combusting the feedstock. coal and the emissions from
similarly'
combusting the relined coal. The feedstock coal and refined coal were
submitted
separately for determination of proximate and ultimate analyses, heating -
value, inorganic
elementai oxide analysis (by x-ray fluorescence), and chlorine and mercury
contents.
Results from those analyses are provided in Table i . Fuel samples undergo
several
handling steps that tend to allow evaporation of some portion of the as-
received moisture
content. The greatest reduction occurs during pulverization of the fuowl. The
hammer
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CA 02846324 2014-03-14
min pulverizer creates an induced draft th.at tends to dry the newly exposed
surfaces of
the fine coal particle resulting from the pulverization. The extent of the
drying that
occurs is -primarily a ftinction of ambient atmospheric conditions
(temperature and
relative humidity) at the time of fuel preparation. As a result, the
composition of the as-
fired analyses so that comparisons between the feedstock coal and the refined
coal can be
readily made.
[0121j The
feedstock coal (Test AF-CTS-1461) was determined to have
an as-fired heating value of 9621 Btu/lb at a moisture content of 20.03 wt%.
Moisture-
free heating value and ash content were determined to be 12,031 Btuilh and
4.91 wt%,
respectively. The feedstock coal sulfur content was determined to be 0.37 wt%
on a
moisture-free basis (0.624 lb S02/MM.Btu). The powder sorbent and liquid
halide
sorbent have no heating value, and the liquid halide sorbent introduces
additional
moisture into the refilled coal because of the water content of the liquid, so
a reduction in
the heating value (Btu/Ib) of the refined coal is generally expected in
comparison to the
Btu/lb of the feedstock coal. Ash anaiysis of the inorganics contained in each
fuel
indicate that the refined coal is enriched in CaO and SO3, while depleted in
Si02, A1203,
and Fe,03 relative to the feedstock coal. Mercury content was determined to be
0.0570
nig (5.924 ikb/TBtu, dry basis) and 0.0556 pg/g (5.908 lb/TBM, dry basis) in
the
feedstock and refined coal samples, respectively. The chlorine content of the
feedstock
and refined coal samples was determined to be 19.4 and 30.0 ug/g,
respectively.
[0122] Dry
sieve analyses completed on feedstock and refined coal
samples collected during each test are presented in Example 2 Table. Results
for the
feedback coal indicate 84.3 wt% passes 200 mesh and 69.2 wt% passes 325 mesh,
while
87.1 wt% passes 200 mesh and 73.1 wt% passes 325 mesh for the refined coal
sample.
Example 2
[0123] The
feedstock coal is PRB coal. The refined coal is feedstock PRB
coal plus 0.008% by weight halide sorbent and 0.25% powder sorbent. The
powd.er
sorhent is 15% CKD and 85% grindouts. The halide sorbent is from Example 1.
NOx
and Hg emissions were measured for the feedstock and refined coals.
Example '2a
NO, Results
39
CA 02846324 2014-03-14
. .
02, NO, NOx, ppm NO, NO, Reduction
% ppm Corrected to 2.5% 02 113/MIABt
Feedstock Coal 2.71 151 152 0.197
NA*
Refined Coal 2.75 116 117 0.154
21.83
lig Results
0-); c07, 1-1gm, ps/d.Nm' Hor
Hs Reduction
0.
yo corrected to 2.5% 02 lb/TBtu
Feedstock Coal 2.71 1 s .97 2.052 1.392 NA
efi n ed Coal 2.75 16.08 0.826 0.576
58.62
* not applicable
[0124] Mercury concentration was 0.558 uglg in the ash of
the feedstock
coal and 0.833 ugig in the ash of the refined coal
Example 3 --- mid-American sub-bitumin.ous coal
[0125] The feedstock coal is PRB coal. The refined coal is
feedstock PRB
coal plus 0.005% by weight halide sorbent and 0.25% powder sorbent. The powder
sorbent is 15% CKD and 85% grindouts. The halide sorbent is from Example 1.
NOx
and Hg emissions were measured for the feedstock and refined coals.
[0126] The morning hours were used to establish baseline
emissions from
combustion of feedstock coal fired at an average rate of 60.281b/hr to achieve
a FEGT of
approximately 2139 F. Excess oxygen was controlled to 3.05% ( about 16.98%
excess
air) at the furnace exit, with OFA utilized at 15.13%simulating the NOx
controls used at
the generating station.
[01271 The refined coal was fired at a rate of 58.91
lb/hr, achieving a
FEGT of 2139 Fat 3.1% excess ox.ygen(about 17.33% excess air) at the furnace
exit,
with OFA maintained at 15.18%. Resultant emission reduction are given in the
following
table.
40
CA 02846324 2014-03-14
. .
NO, Results
02, N0x7 NOx, ppm NO,
NO, Reduction
% ppm Corrected to 2.5% 01-7 1b/MMBt
Feedstock Coal 3.86 243 256 0.327 NA*
Refined Coal 3.84 191 200 0.252 22.94
Hg Results
02, CO,, ug/dNin3 Fitt,
lig Reduction
% corrected to 2.5% 07, 11111-Btu
Feedstock Coal 3.86 17.07 2.877 2.018 NA
Refined Coal 3.84 18.22 1.618 1.119 44.55
* not applicable
41
