U.S. patent application number 11/045389 was filed with the patent office on 2005-06-16 for product and process to reduce mercury emission.
Invention is credited to Booth, Charles M., Lanier, William Steven, Lissianski, Vitali V., Maly, Peter M., Seeker, William Randall, Zamansky, Vladimir M..
Application Number | 20050129600 11/045389 |
Document ID | / |
Family ID | 27609154 |
Filed Date | 2005-06-16 |
United States Patent
Application |
20050129600 |
Kind Code |
A1 |
Lanier, William Steven ; et
al. |
June 16, 2005 |
Product and process to reduce mercury emission
Abstract
In a method to decrease emission of mercury, a factor is
selected to control a combustion process to generate a flue gas
comprising fly ash with enhanced unburned carbon; the combustion
process is controlled according to a factor selected from reburning
fuel, flue gas temperature, OFA injection, coal particle size, LNB
flow, LNB design, combustion zone air, stoichiometric ratio of
fuel, fuel/air mixing in a primary combustion zone and fuel/air
mixing in a secondary combustion zone to produce the flue gas
comprising fly ash with enhanced unburned carbon and to vaporize
mercury; and the flue gas is allowed to cool to collect fly ash
with enhanced unburned carbon with absorbed mercury. A system to
decrease emission of mercury; comprises a combustion zone that is
controlled to generate a flue gas comprising fly ash with enhanced
unburned carbon and that produces vaporized mercury; and a post
combustion zone to cool the flue gas to collect fly ash with
enhanced unburned carbon with absorbed mercury.
Inventors: |
Lanier, William Steven;
(Durham, NC) ; Booth, Charles M.; (Raleigh,
NC) ; Lissianski, Vitali V.; (San Joan Capistrano,
CA) ; Zamansky, Vladimir M.; (Oceanside, CA) ;
Maly, Peter M.; (Lake Forest, CA) ; Seeker, William
Randall; (San Clemente, CA) |
Correspondence
Address: |
PHILIP D FREEDMAN PC
P. O. BOX 19076
ALEXANDRIA
VA
22320
US
|
Family ID: |
27609154 |
Appl. No.: |
11/045389 |
Filed: |
January 31, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11045389 |
Jan 31, 2005 |
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10724251 |
Dec 1, 2003 |
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6863005 |
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10724251 |
Dec 1, 2003 |
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10054850 |
Jan 25, 2002 |
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6726888 |
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Current U.S.
Class: |
423/239.1 ;
422/171; 422/173; 423/210 |
Current CPC
Class: |
B01D 53/64 20130101 |
Class at
Publication: |
423/239.1 ;
423/210; 422/171; 422/173 |
International
Class: |
B01D 053/64 |
Claims
1-18. (canceled)
19. A system to decrease emission of mercury; comprising: a
combustion zone that is controlled to generate a flue gas
comprising fly ash with enhanced unburned carbon and that produces
vaporized mercury; and a post combustion zone to cool the flue gas
to collect fly ash with enhanced unburned carbon with absorbed
mercury.
20-25. (canceled)
26. A composition comprising fly ash with absorbed in situ produced
carbon.
27. The composition according to claim 26, comprising fly ash with
at least greater than 10 weight percent absorbed in situ produced
unburned carbon.
28. The composition according to claim 26, comprising fly ash with
12 to 30 weight percent absorbed in situ produced unburned
carbon.
29. The composition according to claim 26, comprising fly ash with
14 to 18 weight percent absorbed in situ produced unburned
carbon.
30. The composition according to claim 26, comprising fly ash with
absorbed unburned carbon formed in situ by controlling a combustion
process according to a factor selected from reburning fuel, OFA
injection, coal particle size, LNB flow, LNB design, stoichiometric
ratio of fuel, fuel/air mixing in a primary combustion zone and
fuel/air mixing in a secondary combustion zone.
31. The composition according to claim 26, wherein the absorbed
carbon is enhanced unburned carbon with absorbed mercury
32. A method to decrease emission of mercury, comprising: selecting
a factor to control a combustion process to generate a flue gas
comprising fly ash with enhanced in situ-formed unburned carbon,
wherein the factor is selected from the group consisting of
reburning fuel, OFA injection, coal particle size, LNB flow, LNB
design, stoichiometric ratio of fuel, fuel/air mixing in a primary
combustion zone and fuel/air mixing in a secondary combustion zone;
controlling the combustion process according to a the factor to
produce the flue gas comprising fly ash with enhanced unburned
carbon and to vaporize mercury; and allowing the flue gas to cool
to collect fly ash with enhanced unburned carbon with absorbed
mercury.
33. The method of claim 32, comprising forming the fly ash with
enhanced unburned carbon by fuel staging comprising firing the
furnace in a main burner in the combustion zone in the presence of
unlimited air and injecting additional fuel for reburning in a fuel
rich subsequent combustion zone.
34. The method of claim 32, comprising forming the fly ash with
enhanced unburned carbon by fuel staging comprising: firing the
furnace in a main burner in the combustion zone in the presence of
unlimited air; injecting additional fuel for reburning in a fuel
rich subsequent combustion zone; and subsequently applying overfire
air to burn out remaining combustibles.
35. The method of claim 32, comprising forming the fly ash with
enhanced unburned carbon by fuel staging comprising: firing the
furnace in a main burner in the combustion zone in the presence of
unlimited air; injecting additional fuel for reburning in a fuel
rich subsequent
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a process and system to reduce
emissions of nitrogen oxides and mercury and to reduce the level of
carbon in combustion fly ash. More specifically, the present
invention provides a process and system to increase use of fly ash
and to decrease nitrogen oxides and mercury from flue gases from
combustion systems such as boilers, furnaces and incinerators.
[0002] Production of air pollution by combustion systems is a major
problem of modern industrial society. The pollution can include
particulates such as fine fly ash particles from solid fuel
combustion (for example, pulverized coal firing), and gas-phase
species, such as oxides of sulfur (SO.sub.x, principally SO.sub.2
and SO.sub.3), carbon monoxide, volatile hydrocarbons, nitrogen
oxides (mainly NO and NO.sub.2 collectively referred to "NO.sub.x")
and volatile metals such as mercury (Hg).
[0003] The nitrogen oxides are the subject of growing concern
because of their toxicity and their role as precursors in acid rain
and photochemical smog processes. NO.sub.x is emitted by a variety
of sources, including mobile sources (such as automobiles, trucks
and other mobile systems powered by internal combustion engines),
stationary internal combustion engines and other combustion sources
such as power plant boilers, industrial process furnaces and waste
incinerators. Available NO.sub.x control technologies include
Selective Catalytic Reduction (SCR) and Combustion Modification.
SCR systems can be designed for most boilers and may be the only
approach for high NO.sub.x units such as cyclones. Combustion
Modification achieves deep NO.sub.x control by integrating several
components. Typically, Low NO.sub.x Burn (LNB) is the lowest cost
Combustion Modification technique. It is usually applied as a step
towards low cost deep NO.sub.x control. Other Combustion
Modification techniques include Overfire Air (OFA), Reburning and
Advanced Reburning
[0004] Mercury is identified as a hazardous air pollutant and is
the most toxic volatile metal in the atmosphere. Elemental mercury
vapor can be widely dispersed from emission sources. Other forms of
mercury pollutants include organic and inorganic compounds that
accumulate in plants and animals. Mercury is a constituent part of
coal mineral matter. Its emission from coal-fired power plants is
suspected to be a major source of environmental mercury.
[0005] Thus, there is a need to continue to use low NO.sub.x
technologies but to effectively control mercury emission.
BRIEF DESCRIPTION OF THE INVENTION
[0006] The invention provides an integrated method and system for
reducing NO.sub.x environment emissions and mercury environment
emissions. In the method, a factor is selected to control a
combustion process to generate a flue gas comprising fly ash with
enhanced unburned carbon; the combustion process is controlled
according to a factor selected from reburning fuel, flue gas
temperature, OFA injection, coal particle size, LNB flow, LNB
design, combustion zone air, stoichiometric ratio of fuel, fuel/air
mixing in a primary combustion zone and fuel/air mixing in a
secondary combustion zone to produce the flue gas comprising fly
ash with enhanced unburned carbon and to vaporize mercury; and the
flue gas is allowed to cool to collect fly ash with enhanced
unburned carbon with absorbed mercury.
[0007] In an embodiment, the method decreases emissions of nitrogen
oxide and mercury while decreasing carbon in fly ash. The method
comprises selecting a combination of factors from the group
consisting of fuel type, fuel/air staging and a combustion
condition to control a combustion process to generate a flue gas
comprising fly ash with enhanced unburned carbon; controlling the
combustion process according to the factors to produce the flue gas
comprising fly ash with enhanced unburned carbon, NO.sub.x and
vaporized mercury; removing NO.sub.x from the flue gas; allowing
the flue gas to cool to a lower temperature to collect fly ash with
absorbed mercury; passing the fly ash with absorbed mercury through
an ash burnout unit to remove carbon from the fly ash and to
produce a mercury-containing exhaust gas; and passing the
mercury-containing exhaust gas through a collection unit to capture
the mercury.
[0008] Additionally, the invention relates to a system to decrease
emission of mercury; comprising a combustion zone that is
controlled to generate a flue gas comprising fly ash with enhanced
unburned carbon and that produces vaporized mercury; and a post
combustion zone to cool the flue gas to collect fly ash with
enhanced unburned carbon with absorbed mercury.
[0009] In another embodiment, the invention is a system to decrease
emissions of nitrogen oxide and mercury while decreasing carbon in
fly ash, comprising a combustion zone that is controlled by fuel
type, fuel/air staging or a combustion condition to generate a flue
gas comprising fly ash with enhanced unburned carbon and that
produces vaporized mercury; a post combustion zone to cool the flue
gas to collect fly ash with enhanced unburned carbon with absorbed
mercury; an ash treatment unit that removes carbon from the fly ash
and produces a mercury-containing exhaust gas; and a collection
unit that captures the mercury.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic representation of a coal-fired
combustion device adapted for a method of the invention;
[0011] FIG. 2 is a graph showing effects of in-situ formed carbon
in ash on NO.sub.x and Hg removal; and
[0012] FIGS. 3 and 4 show modeling prediction comparisons.
DETAILED DESCRIPTION OF THE INVENTION
[0013] Commercially available NO.sub.x control technologies for
stationary combustion sources are known to increase carbon content
in fly ash (carbon in ash can be referred to as Loss on Ignition
(LOI)). This is because NO.sub.x control principles in Combustion
Modification are based on fuel and/or air staging. Staging
combustion configurations that require both fuel-rich and fuel-lean
zones to control NO.sub.x emissions do not provide sufficient upper
furnace residence time for complete carbon burnout. The increase in
ash carbon content decreases combustion efficiency. Carbon content
increase can make the ash unsuitable for use in cement. As a
result, the ash must be discarded to landfill at an additional
cost.
[0014] The invention represents an improvement over prior
techniques in that NO.sub.x and mercury are effectively and
efficiently reduced. In an embodiment, this is accomplished without
creating a waste stream of ash. In an embodiment, the invention
surprisingly achieves improvement by synergistically combining the
effects of NO.sub.x reduction in fuel-rich zones, NO.sub.x
reduction on the surface of an enhanced active carbon in fly ash,
mercury absorption on carbon in ash and by the utilization of ash
burnout and mercury recovery systems. The invention allows for the
formation of enhanced active fly ash under controlled conditions of
coal reburning or other fuel or air staging low NO.sub.x
technologies. The invention is particularly applicable to
stationary combustion systems.
[0015] These and other features will become apparent from the
drawings and following detailed discussion, which by way of example
without limitation describe preferred embodiments of the
invention.
[0016] FIG. 1 shows system 10 of the invention. As shown in FIG. 1,
the system 10 comprises a coal-fired boiler 12. The boiler 12
includes a combustion zone 14 and post combustion zone 16, which
includes convective pass 18. System 10 further includes particulate
control device (PCD) 20, ash burnout unit 22 and mercury collection
unit 24 comprising a bed of activated carbon or other reagent. Most
of the coal is burned in a primary combustion zone 26 of the boiler
12. The remaining coal is injected downstream to provide a
fuel-rich reburning zone 28. Overfire air is injected into a
burnout zone 30 to complete combustion.
[0017] Combustion in the primary zone 26 generates NO.sub.x. Most
mercury content of the coal is transferred to gas phase during
combustion. In reburning zone 28, NO.sub.x from primary combustion
zone 26 is reduced to N.sub.2. During the reburning process, carbon
in the reburning coal does not burn out as completely as in a
boiler environment that has excess air. Therefore, coal reburning
increases the level of unburned carbon in the flue gas. By
selecting coal type and specific conditions for injection of fuel
and air, the combustion process can be controlled to produce a flue
gas with increased carbon-containing fly ash. The flue gas is
cooled at convective pass 18 where mercury is absorbed by the fly
ash carbon. The fly ash with mercury is then collected in the PCD
20. Fly ash collected in the PCD 20 is treated in an ash treatment
unit 22. Ash treatment unit can be an electrostatic separator, a
burnout unit or the like. If a burnout unit is used, then excess
heat can be can be partially recovered, for example by the plant by
preheating water used for boiler heat exchange. Mercury released
from the fly ash carbon is absorbed by activated carbon as the ash
burnout products pass through mercury collection unit 24.
[0018] In the FIG. 1 embodiment, concentrations of nitrogen oxides,
mercury, and carbon in ash are reduced by a three-step process. In
the first step, the concentration of NO.sub.x is decreased in the
fuel-rich zone of coal reburning (in other embodiments this step
can be accomplished by LNB or by another fuel/air staging low
NO.sub.x Combustion Modification technology). The combustion zone
of the particular technology is controlled to form enhanced carbon
in fly ash. The enhanced carbon in fly ash can be formed by
optimizing the fuel staging and air staging conditions and
combustion conditions, for example, by changing the amount of the
reburning fuel, temperature of flue gas at the location of
reburning fuel and/or OFA injection. Also, more active carbon in
fly ash can be formed by selecting a coal type or particle size.
Also, enhanced carbon can be controlled by adjusting LNB flow, by
selecting a specific LNB design, by regulating excess air in the
main combustion zone, adjusting the stoichiometric ratio of fuel
and adjusting fuel/air mixing in primary and secondary combustion
zones. Other approaches to form and enhance the formation of active
carbon in fly ash can be used. The enhanced carbon in fly ash is
formed "in-situ," i.e. in the burner, in the main combustion zone
or in the reburning zone. The fly ash can have a concentration of
carbon of about 1 to about 30 weight percent, desirably 3 to 20
weight percent and preferably 5 to about 15 weight percent.
[0019] In the second step, the carbon-containing fly ash is cooled
to below 450.degree. F., desirably below 400.degree. F. and
preferably below 350.degree. F. At these levels, NO.sub.x is
further reduced in a reaction with carbon, and mercury is absorbed
by the enhanced carbon in the fly ash. A PCD can collect the ash
with carbon and absorbed mercury.
[0020] In the third step, the carbon is burned out from the fly
ash. At the same time, mercury is desorbed from fly ash and
collected in an activated carbon bed or a bed of other reagents,
for example, gold or other metals that form amalgams. Currently,
carbon burnout reactors are designed for effective removal of
carbon. In the invention, the burnout reactor can be used in
combination with a mercury capture reactor.
[0021] It is beneficial to use in-situ formed carbon in fly ash for
mercury removal instead of activated carbon injection for a number
of reasons. Activated carbon is produced by pyrolysis of coal, wood
and other materials at relatively low temperatures and in a time
consuming process that can take from many hours to several days. In
the invention, enhanced carbon in fly ash can be produced in a
matter of seconds at combustion temperatures. Since the stream of
gas through the carbon burnout reactor is much smaller than the
stream of flue gas, the amount of activated carbon needed to
collect mercury can be about two orders of magnitude lower than the
amount of injected activated carbon to accomplish the same result.
Additionally, the cost of controlling conditions to optimize
production of enhanced carbon in fly ash from a coal-fired boiler
typically, on a mass basis, is much less than the cost of injected
activated carbon. Further in the invention, since the carbon is
produced "in situ," no extra costs are incurred in respect of
handling of the activated carbon and delivering it to the boiler.
Thus mercury control in accordance with the invention, represents
only a small incremental cost above and beyond the cost of NO.sub.x
control.
[0022] The following EXAMPLES are illustrative and should not be
construed as limitations on the scope of the claims unless a
limitation is specifically recited.
EXAMPLE 1
[0023] Tests were performed in a Boiler Simulator Furnace (BSF).
The BSF was a down-fired combustion research facility that had a
nominal firing capacity of 1.times.10.sup.6 Btu/hr. The BSF was
designed to simulate chemical and thermal characteristics of a
utility boiler. The BSF was equipped to fire natural gas, oil or
coal. The BSF had two main sections: a vertical down-fired radiant
furnace and a horizontal convective pass. The furnace was
constructed of eight modular refractory lined sections with access
ports. It was cylindrical in shape and had an inside diameter of 22
in. The convective pass contained air-cooled tube bundles to
simulate boiler heat transfer banks. The BSF was equipped with both
a baghouse and an electrostatic precipitator for particulate
control at the end of the convective pass.
[0024] The BSF is well-suited to process development studies
leading to utility boiler applications because it accurately
simulates boiler thermal environments. Flame characteristics,
gas-phase sampling, gas temperature, continuous monitoring of
combustion products and pollutants, particulate mass loading,
particle size and resistivity and particle deposition rates onto
heat transfer surfaces are typical types of studies that can be
made in the BSF.
[0025] A continuous emissions monitoring system (CEMS) was used for
on-line flue gas analysis. The CEMS components included a
water-cooled sample probe, sample conditioning system (to remove
water and particulate) and gas analyzers. The CEMS was capable of
determining O.sub.2: paramagnetism to 0.1% precision, NO.sub.x:
chemiluminescence to 1 ppm precision, CO: nondispersive infrared
spectroscopy to 1 ppm precision, CO.sub.2: nondispersive infrared
spectroscopy to 0.1% precision, SO.sub.2: nondispersive ultraviolet
spectroscopy to 1 ppm precision and Total Hydrocarbons (THC): Flame
ionization detection to 1 ppm precision.
[0026] High purity dry nitrogen was used to zero each analyzer
before and after each test. EPA protocol span gases were used to
calibrate and check linearity of the analyzers. Test data was
recorded on both a chart recorder and a personal computer based
data acquisition system employing Labview.RTM. software. A suction
pyrometer was used to measure furnace gas temperatures. A high
volume filter was used to obtain ash samples. The samples were sent
to a contract laboratory for residual carbon analysis.
[0027] Carbon in fly ash was formed using two approaches: by
limiting the amount of air in the combustion zone and by fuel
staging (reburning). An on-line mercury analyzer from PS Analytical
was used in these tests to monitor mercury emissions. The analyzer
measured both elemental (Hg) and oxidized (Hg.sup.+2) mercury in
flue gas. In the reburning tests, coal was fired through a main
burner under normal excess air conditions. A second coal stream
(reburning fuel--see FIG. 1) was injected into the furnace to
produce a fuel rich-zone in which NO.sub.x emissions were reduced
to N.sub.2. Overfire air was then added to burn out any remaining
combustibles. Fly ash generated by this process contained an
increased amount of carbon that effectively captured mercury
emissions.
[0028] Mercury measurements were conducted in a slip-stream using a
fabric filter to collect fly ash. This set up was used to simulate
a baghouse. Temperature of flue gas at the location where
slip-stream was separated from the main stream was about
500.degree. F. The fabric filter surface area was 0.56 ft.sup.2.
Flue gas flow passing through the fabric filter varied between 1.9
scfm and 2.3 scfm. Temperature of the filter varied from300.degree.
F. to 370.degree. F.
[0029] Mercury concentration was measured behind fabric filter to
avoid interference of fly ash with the mercury analyzer. Mercury
measurements were done first for baseline coal firing (SR=1.16),
which resulted in a carbon in ash content of less than 2%. BSF
conditions were then changed to form high carbon fly ash. In the
reburning tests, carbon in ash content varied from 8% to 14% by
changing heat input of the reburning fuel and temperature of the
reburning fuel injection between 2000.degree. F. and 2500.degree.
F. It is believed that carbon in ash increased with decrease in
injection temperature because of lower residence time. The lower
residence time results in incomplete combustion of the reburning
coal. In tests where high carbon fly ash was formed by reducing
excess air, stoichiometric ratio (SR) in the combustion zone varied
from 1.03 to 1.16 resulting in carbon in ash content between 1% and
7%.
[0030] A series of tests was conducted to demonstrate the invention
under a variety of process conditions. Kittanning coal from
Pennsylvania was used as the main fuel in all tests. Three coals
were included in the tests--Kittanning, a North Antelope coal from
the Powder River Basin and a Ukrainian coal. These coals were
selected to provide a range of analytical properties, including
fixed carbon, volatile matter and ash characteristics. The tests
were performed to generate various levels of NO.sub.x control and
fly ash carbon amount and activity. In the tests, the following
parameters were varied: (1) coal type: Kittanning, North Antelope,
and Ukrainian; (2) reburn coal grind: 55-90% passing through a U.S.
size 200 mesh sieve; (3) reburn heat input: between 20% and 30% of
the total; and (4) residence time in the reburning zone: between
0.75 and 1.0 s.
[0031] An analysis was performed for each coal, including ash and
mercury contents. Fly ash samples were collected from fabric filter
and analyzed. Mercury was analyzed by double gold amalgamation/cold
vapor atomic absorption. Carbon in fly ash was also analyzed. This
allowed mercury capture to be defined as a function of fly ash
carbon.
[0032] FIG. 2 presents reburning and straight firing data obtained
with Kittanning coal. FIG. 2 shows good agreement between two
approaches (on-line mercury emissions measurements and measurements
of mercury captured by fly ash) to determine mercury removal. FIG.
2 also demonstrates that carbon in ash content is one of the
important parameters that affect activity of fly ash. Mercury
removal increases almost linearly with carbon in ash increase for
LOI less than 7% and then levels off. The data demonstrate that in
situ formed carbon in ash in contents of 5 to 12 weight percent can
effectively reduce mercury emission.
[0033] TABLE 1 shows the effect of carbon in ash content on
NO.sub.x reduction and mercury capture by fly ash formed in coal
reburning obtained with Kittanning and Ukrainian coal. Although the
amounts of carbon in fly ash are close for these coals, Kittanning
coal produces much more active carbon in fly ash providing more
efficient mercury control and NO.sub.x control.
1TABLE 1 Reburn Fuel Ash LOI Inlet Hg Hg in Fly Ash Hg Capture by
Fly Ash NO.sub.x Reduction Type % dry mg/hr mg/hr % of inlet % None
1.66 2.53 0.085 3.4 Ukrainian 2.38 3.07 0.314 10.2 50.8 Kittanning
3.15 3.16 1.38 43.7 55.3
EXAMPLE 2
[0034] A process model was developed and used to predict NO.sub.x
and mercury control in coal reburning. The process model included a
detailed kinetic mechanism of coal reburning combined with gas
dynamic parameters characterizing mixing of reagents and global
reactions of carbon burnout and mercury absorption. In the
modeling, a set of homogeneous and heterogeneous reactions
representing the interaction of reactive species was assembled.
Each reaction was assigned a certain rate constant and heat release
or heat loss parameter. Numerical solution of differential
equations for time-dependent concentrations of the reagents
permitted prediction of concentration-time curves for all reacting
species under selected process conditions. Modeling revealed the
process conditions required for improvements in NO.sub.x and
mercury removal.
[0035] The chemical kinetic code ODF, for "One Dimensional Flame"
was employed to model BSF experimental data. ODF is designed to
march through a series of well-stirred or plug flow reactors,
solving a detailed chemical mechanism. The kinetic mechanism
consisted of over 500 reactions, including both gas-phase and
heterogeneous reactions. The gas phase reactions described chemical
behavior of 94 C--H--O--N species. The heterogeneous reactions
included devolatilization of the coal, soot and char; char
oxidation by O.sub.2; soot oxidation by O.sub.2 and
oxygen-containing radicals; reduction of NO on the char and soot
surfaces; and radical recombination on the char and soot surfaces.
The mechanism was supplemented with reactions describing
interactions of gas-phase mercury-containing species with other
gas-phase components and char.
[0036] The reaction between mercury and char was introduced using
an effective reaction describing mercury absorption and desorption
on the carbon surface:
Hg(g)+C(s)Hg--C(s) (1)
[0037] where Hg--C(s) indicates carbon with bound mercury on its
surface. Rate coefficient of this reaction was calibrated against
pilot-scale data (Brown, T. D., Smith, D. N., Hargis, R. A., Jr.,
and O'Dowd, W. J. "Mercury Measurement and Its Control: What We
Know, Have Learned, and Need to Further Investigate," J. Air &
Waste Manage. Assoc., 1999, pp. 1-97) on mercury removal by fly ash
collected in a particulate control device and re-injected into flue
gas. The activation energy of this reaction was adjusted to
describe the temperature dependence of the mercury
absorption/desorption rate, while the pre-exponential factor was
fitted to describe the absolute values of the absorption/desorption
rates on fly ash with different carbon content. The model was used
to describe experimental data on NO.sub.x reduction and mercury
removal in Kittanning coal reburning obtained in the BSF. The
reburning fuel was injected into flue gas at 2500.degree. F. The
initial amount of NO.sub.x was 600 ppm. The temperature of flue gas
decreased in the model at a linear rate of -550.degree. F./s,
approximately as in the experiments.
[0038] A comparison of BSF experimental data on Kittanning coal and
modeling predictions on NO.sub.x reduction and the amount of carbon
in ash is presented in FIG. 3. Modeling predicts that an increase
in the amount of the reburning fuel from 20% to 25-30% can result
in additional .about.10% NO.sub.x reduction. An increase in the
amount of the reburning fuel also results in a carbon in ash
increase, which can be used to reduce mercury emissions.
[0039] FIG. 4 shows a comparison of modeling predictions with BSF
experimental data on mercury removal by carbon in ash of Kittanning
coal. Vertical lines in FIG. 4 represent uncertainty of mercury
concentration measurements in experiments, which was estimated to
be .+-.15%. The space between the two curves in FIG. 4 represents
modeling results obtained with two expressions of the rate
coefficient for reaction (1) that fit to higher and lower
efficiency of mercury absorption by fly ash. FIG. 4 demonstrates
that modeling predictions agree with experimental data within
uncertainty of experimental data. Modeling predicts that the
efficiency of mercury removal increases as the amount of carbon in
ash increases. Modeling predicts that about 90% mercury reduction
can be achieved at approximately 10 to 15 weight percent carbon in
ash under optimum conditions.
[0040] Thus modeling predicts that an increase in the amount of the
reburning fuel will improve the efficiency of NO.sub.x reduction
and will result in significant mercury removal. Modeling indicates
that 90% mercury removal and 10% increase in NO.sub.x reduction can
be achieved.
[0041] In known processes, fly ash collected in a PCD (ESP or
baghouse) and subsequently re-injected shows little or no affinity
for mercury absorption. It may be that the active carbon absorption
sites of this fly ash are occupied by competing species such as
SO.sub.2, HCl, and even H.sub.2O, thus reducing the available
surface for mercury capture. Fly ash, once collected in a PCD, is
likely to be "deactivated" for subsequent mercury absorption.
[0042] On the other hand in accordance with the invention, freshly
formed or "in-situ" carbon in fly ash is quite active toward
mercury absorption. Fly ash with a carbon content of 5%-15% has
comparable mercury capture efficiency to injected activated
carbon.
[0043] While preferred embodiments of the invention have been
described, the present invention is capable of variation and
modification and therefore should not be limited to the precise
details of the EXAMPLES. The invention includes changes and
alterations that fall within the purview of the following
claims.
* * * * *