METHODS OF CONTROLLING MERCURY EMISSION
TECHNICAL FIELD
[0001] The present disclosure relates generally to methods for controlling
mercury emissions, and more particularly, to methods for controlling mercury reemissions
from wet flue gas desulfurizers.
BACKGROUND
[0002] The Environmental Protection Agency (EPA) recently published the
Mercury and Air Toxics Rule (MATS Rule) that will require all electricity
generating units (EGUs) that burn fossil fuels to reduce mercury emissions levels
within the next three years. Many of these units currently use or will use wet flue
gas desulfurizers (wFGDs) to meet acid gas or SOx emission limits. A wFGD
contacts combustion gas with an aqueous alkaline solution, which solution may be
composed of magnesium compounds, sodium compounds, and slurries of lime or
limestone to capture and neutralize acid gases, such as sulfur dioxide. The aqueous
alkaline solution is commonly referred to as "wFGD liquor" or "scrubber liquor." In
a forced oxidation system, oxygen may be introduced into the wFGD liquor to
oxidize sulfite to sulfate. In many cases, this forms gypsum (calcium sulfate), as the
final byproduct of scrubbing. Other systems may utilize inhibited or natural
oxidation scrubbing which results in sulfite salts or mixed sulfite/sulfate salts as
byproduct.
[0003] Mercury entering EGUs as a contaminant of the fuel is released during
combustion. Combustion gases exiting the boiler may contain mercury in three
forms: particulate, oxidized, and elemental. Particulate mercury can be captured by
particulate control devices such as electrostatic precipitators (ESPs) and fabric filters
(FF). Oxidized mercury is water-soluble and as such wFGDs can absorb the
oxidized mercury from the combustion gas into the liquid phase. Elemental
mercury, which is insoluble in water, is difficult to capture using existing air quality
control devices. Consequently, mechanical methods such as fixed bed catalysts
(e.g., SCRs), and chemical additives (e.g., calcium bromide, hydrogen bromide,
ammonium chloride) have been developed that oxidize elemental mercury in the gas
phase for subsequent capture with a wFGD. The captured mercury leaves the
process via wFGD blow down.
[0004] As oxidized mercury is water soluble, wFGDs are theoretically capable of
capturing nearly 100% of the oxidized mercury in a combustion gas. However, data
collected by the Department of Energy (DOE) as well as numerous laboratory and
commercial studies have shown lower capture efficiencies. The lower efficiencies
are the result of reduction of oxidized mercury to elemental mercury (e.g., Hg2+ to
Hg°) within the wFGD scrubber liquor. For example, one reduction reaction
involves the oxidation of sulfite by ionic mercury in the wFGD to provide sulfate
and elemental mercury. The result is an increase across the wFGD of elemental
mercury content in the scrubbed combustion gas, and thus a decrease in total
mercury capture as measured from fossil fuel to stack. This reduction of oxidized
mercury in the scrubber and subsequent release is known in the industry as mercury
re-emission. The loss in wFGD mercury capture efficiency due to mercury reemission
will prevent some EGUs from meeting the MATS Rule, necessitating
installation of additional capital equipment.
[0005] Mercury re-emission is currently addressed with addition of sulfur-based
additives, both organic and inorganic, or sulfur-based modified inorganics to chelate
ionic mercury in the scrubber liquor, or through addition of absorbents such as
activated carbon. In all these cases, the additive is introduced into the scrubber at an
excess rate that has previously been shown to reduce re-emission. However, in none
of these cases, have there been any development of control methodology that
automatically controls the rate of addition of these re-emission control additives
relative to normal operation. Variation in mercury concentration found in the fossil
fuel, and therefore the combustion gas to be treated, results in periods of over and
underfeeding of additive. Thus, in certain instances the lack of direct control results
in overfeeding of the additives, which in turn leads to increased operating costs and
higher waste generation.
[0006] There is a need in the art for methods of controlling the rate of addition of
mercury re-emission control additives to provide the appropriate amount of additive
to a scrubber without over- or underfeeding of additive.
SUMMARY
[0007] In one aspect, disclosed are methods for controlling mercury re-emission
from a combustion gas. The method may include measuring either ionic mercury
concentration, oxidation-reduction potential (ORP), and/or sulfide ion concentration
within a scrubber liquor of a wet flue gas desulpherizer (wFGD); correlating the
mercury concentration, ORP, and/or sulfide concentration with an amount of
mercury re-emission additive required in the scrubber liquor to reduce and/or
prevent mercury re-emission to a selected level; and adjusting the rate of addition of
mercury re-emission control additive into the scrubber liquor to attain the selected
level of mercury re-emission (e.g., a desired level of mercury re-emission to meet
MATS limits).
[0008] In certain embodiments, ionic mercury concentration is measured,
oxidation-reduction potential is measured, or sulfide ion concentration is measured.
In certain embodiments, a combination of ionic mercury concentration, oxidationreduction
potential, and sulfide ion concentration are measured.
[0009] In certain embodiments, the rate of addition of mercury re-emission
control additive is adjusted to reduce the scrubber liquor ionic mercury
concentration to less than 500 ppt, or less than 200 ppt. In certain embodiments, the
rate of addition of mercury re-emission control additive is adjusted to reduce the
scrubber liquor ionic mercury concentration to less than 200 ppt and the percent
mercury re-emission from the scrubber liquor to 10% or less. In certain
embodiments, the rate of addition of mercury re-emission control additive is
adjusted to reduce the scrubber liquor mercury concentration to less than 100 ppt
and the percent mercury re-emission from the scrubber liquor to 1% or less.
[0010] In certain embodiments, the ORP value is measured, and the rate of
addition of mercury re-emission control additive is adjusted to reduce the scrubber
liquor oxidation-reduction potential by up to 100 mV. In certain embodiments, the
ORP value is measured, and the rate of addition of mercury re-emission control
additive is adjusted to reduce the scrubber liquor oxidation-reduction potential by up
to 300 mV. In certain embodiments, the ORP value is measured, and the rate of
addition of mercury re-emission control additive is adjusted to reduce the scrubber
liquor oxidation-reduction potential by up to 400 mV.
[0011] In certain embodiments, the oxidation-reduction potential of the scrubber
liquor is measured prior to adjusting the rate of addition of mercury re-emission
control additive to provide a baseline ORP, wherein the rate of addition of mercury
re-emission control additive is thereafter increased to reduce the scrubber liquor
oxidation-reduction potential by 50-400 mV. In certain embodiments, the reduction
of scrubber liquor oxidation-reduction potential by 50-400 mV reduces mercury reemission
to 20% mercury re-emission or less, 10% mercury re-emission or less, or
1% mercury re-emission or less.
[0012] In certain embodiments, the rate of addition of mercury re-emission
control additive is adjusted to increase the scrubber liquor sulfide ion concentration
to greater than 20 ppm, greater than 50 ppm, or greater than 70 ppm. In certain
embodiments, the rate of addition of mercury re-emission control additive is
adjusted to increase the scrubber liquor sulfide ion concentration to a range of about
20 ppm to about 100 ppm, or about 30 ppm to about 70 ppm.
[0013] In certain embodiments, the mercury re-emission control additive is a
polydithiocarbamic compound. The mercury re-emission control additive may be an
ethylene dichloride ammonia polymer having a molecular weight of from 500 to
10,000, and containing from 5 to 55 mole % of dithiocarbamate salt groups.
Alternatively, the mercury re-emission control additive may be a composition
comprising a polymer derived from at least two monomers: acrylic-x and an
alkylamine, wherein said acrylic-x has the following formula:
wherein X = OR, OH and salts thereof, or NHR2 and wherein R1 and R2 is H or an
alkyl or aryl group, wherein R is an alkyl or aryl group, wherein the molecular
weight of said polymer is between 500 to 200,000, and wherein said polymer is
modified to contain a functional group capable of scavenging one or more
compositions containing one or more metals. The functional group may be a sulfur
containing functional group.
[0014] In certain embodiments, measurement of mercury concentration,
oxidation-reduction potential, and/or sulfide concentration is automated. In certain
embodiments, adjustment of the rate of addition of mercury re-emission control
additive is automated. In certain embodiments, measurement of mercury
concentration, oxidation-reduction potential, and/or sulfide concentration is
automated, and adjustment of the rate of addition of mercury re-emission control
additive is automated. In certain embodiments, at least one of mercury
concentration, oxidation-reduction potential, and sulfide concentration are
continuously monitored.
[0015] The methods and processes are further described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Figure l a depicts a percent mercury re-emission controlled by measuring
mercury concentration and adjusting addition rate of a mercury re-emission control
additive.
[0017] Figure lb shows the correlation between mercury re-emission (flue gas
mercury emissions) and mercury concentration in the scrubber liquor.
[0018] Figure 2a depicts a percent mercury re-emission controlled by measuring
scrubber liquor oxidation-reduction potential and adjusting addition rate of a
mercury re-emission control additive.
[0019] Figure 2b depicts the relationship between the change in oxidationreduction
potential (i.e., delta scrubber liquor ORP), and resulting mercury reemission.
[0020] Figure 2c depicts data showing a correlation between ORP and percent
mercury re-emission with a correlation coefficient of 0.999.
[0021] Figure 3 depicts electrode responses of oxidation-reduction potential
(ORP) electrode (diamonds) and sulfide ion-selective electrode (ISE, squares) as a
function of increasing wFGD additive in a 0.1 M Na2SO4 electrolyte solution.
[0022] Figure 4 depicts data collected in the field in real-time during a wFGD
additive demonstration to control mercury re-emission across a wFGD scrubber.
The solid line data on the lower portion of the graph represents the feed rate of the
wFGD additive and corresponds to the secondary y-axis. The data points shown in
squares on the top portion of the graph correspond to the primary y-axis and
represent the sulfide concentration in the wFGD scrubber liquor as measured by an
in-line sulfide ISE.
DETAILED DESCRIPTION
[0023] Disclosed herein are methods for controlling mercury emissions from a
scrubber process. More particularly, the inventors have discovered a method of
measuring ionic mercury concentration and/or oxidation-reduction potential (ORP)
and/or sulfide ion concentration within a scrubber liquor of a wet flue gas
desulpherizer (wFGD); correlating the mercury concentration and/or ORP and/or
sulfide ion concentration with an amount of mercury re-emission additive required
to reduce and/or prevent mercury re-emission; and thereafter appropriately adjusting
the rate of addition of mercury re-emission control additive into the scrubber liquor.
The methods disclosed herein provide process efficiency and economic advantages
over conventional methods of reducing mercury re-emission.
[0024] The methods disclosed herein also provide continuous, in-line monitoring
of mercury re-emissions and allow for automatic adjustment of the rate of addition
of mercury re-emission control additive to compensate for changes in fuel
composition and/or scrubber liquor composition. The measurement of ionic mercury
concentration, oxidation-reduction potential, and/or sulfide ion concentration may be
automated, and the resulting measurement(s) used to automatically and in real-time
adjust the rate of addition of mercury re-emission control additive. The methods
disclosed herein therefore reduce the incidence of over- and underfeeding of additive
to the scrubber liquor.
1. Definition of Terms
[0025] Unless otherwise defined, all technical and scientific terms used herein
have the same meaning as commonly understood by one of ordinary skill in the art.
In case of conflict, the present document, including definitions, will control.
Preferred methods and materials are described below, although methods and
materials similar or equivalent to those described herein can be used in practice or
testing of the present invention. All publications, patent applications, patents and
other references mentioned herein are incorporated by reference in their entirety.
The materials, methods, and examples disclosed herein are illustrative only and not
intended to be limiting.
[0026] As used in the specification and the appended claims, the singular forms
"a," "and" and "the" include plural references unless the context clearly dictates
otherwise. The terms "comprise(s)," "include(s)," "having," "has," "can,"
"contain(s)," and variants thereof, as used herein, are intended to be open-ended
transitional phrases, terms, or words that do not preclude the possibility of additional
acts or structures. The present disclosure also contemplates other embodiments
"comprising," "consisting of and "consisting essentially of," the embodiments or
elements presented herein, whether explicitly set forth or not.
[0027] The term "percent mercury re-emission", as used herein, refers to:
%Hg Re - emission o u t l e t ^inlet
T 0 x 100 Equation 1
f
>Inlet Inlet J
where "outlet" refers to EGU stack gas mercury measurement, "inlet" refers to gas
concentrations at the inlet to the wFGD, "0" refers to the concentration of elemental
mercury in the gas, and "T" refers to the total concentration of mercury in the gas.
The "outlet" measurement may refer to mercury gas measurements made at any
location after the gas has exited the wFGD.
[0028] The term "percent mercury oxidation", as used herein, refers to:
%Hg Oxidation Equation 2
where the super- and sub-scripts have the same meaning as defined in Equation 1
above.
[0029] The term "percent mercury capture", as used herein, refers to:
%Hg Capture = x 100 Equation 3
where the super- and sub-scripts have the same meaning as defined in Equation 1
above.
[0030] The term "oxidation-reduction potential", as used herein, refers to the
summation of all the oxidation and reduction potentials in a given solution or
scrubber liquor. As such, the oxidation-reduction potential varies depending on a
liquor composition.
2. Methods of Controlling Mercury Emission
[0031] The methods disclosed relate to controlling the rate of addition of mercury
re-emission control additives to wet flue gas desulpherizers. The rate of addition
may be adjusted based on either ionic mercury concentration in the wFGD scrubber
liquor, changes in the wFGD scrubber liquor oxidative reduction potential, and/or
sulfide ion concentration in the wFGD scrubber liquor.
[0032] In certain embodiments, the ionic mercury concentration in a scrubber
liquor may be measured, and the rate of addition of mercury re-emission control
additive increased to reduce the ionic mercury concentration, thereby reducing the
percent mercury re-emission from the scrubber liquor. In certain embodiments, the
ionic mercury concentration of a scrubber liquor may be measured, and the rate of
addition of mercury re-emission control additive decreased while maintaining an
ionic mercury concentration, thereby maintaining the percent mercury re-emission
from the scrubber liquor without using excess mercury re-emission control additive.
[0033] In certain embodiments, the oxidation-reduction potential of a scrubber
liquor may be measured, and the rate of addition of mercury re-emission control
additive increased to reduce the ORP, thereby reducing the percent mercury reemission
from the scrubber liquor. In certain embodiments, the oxidation-reduction
potential of a scrubber liquor may be measured, and the rate of addition of mercury
re-emission control additive decreased to increase the ORP to an acceptable level to
maintain the percent mercury re-emission from the scrubber liquor without using
excess mercury re-emission control additive.
[0034] In certain embodiments, the sulfide ion concentration in a scrubber liquor
may be measured, and the rate of addition of mercury re-emission control additive
increased to increase the sulfide concentration, thereby reducing the percent mercury
re-emission from the scrubber liquor. In certain embodiments, the sulfide
concentration of a scrubber liquor may be measured, and the rate of addition of
mercury re-emission control additive decreased to decrease the sulfide concentration
while maintaining a percent mercury re-emission from the scrubber liquor without
using excess mercury re-emission control additive.
[0035] In certain embodiments, any combination of ionic mercury concentration,
oxidation-reduction potential, and sulfide ion concentration may be used to monitor
the scrubber liquor and guide adjustment of the addition rate of mercury re-emission
control additive. In one preferred embodiment, all of ionic mercury concentration,
oxidation-reduction potential, and sulfide ion concentration are monitored and used
together to guide adjustment of the rate of addition of mercury re-emission control
additive.
[0036] In certain embodiments, monitoring of the scrubber liquor composition
and subsequent adjustment of the rate of addition of mercury re-emission control
additive may be automated. For example, the ionic mercury concentration, the
oxidation-reduction potential, and/or the sulfide ion concentration may be monitored
by an automated process, and depending on the measured value(s), the rate of
addition of mercury re-emission control additive may be automatically adjusted in
real time to compensate for changes in the fuel, plant load, and/or scrubber liquor
composition, thereby continuously maintaining a desired mercury re-emission level
without over- or under feeding the wFGD with mercury re-emission control
additive.
a. Mercury Concentration
[0037] Mercury concentration in a scrubber liquor can be used to monitor wFGD
operations, and the rate of addition of mercury re-emission control additive can be
adjusted accordingly to adjust mercury capture. In certain embodiments, the rate of
addition of mercury re-emission control additive can be increased to compensate for
higher concentrations of ionic mercury in the scrubber liquor, thereby reducing
mercury re-emission. In certain embodiments, the rate of addition of mercury reemission
control additive can be decreased to compensate for lower concentrations
of ionic mercury in the scrubber liquor, thereby reducing excessive use of reemission
control additive.
[0038] The addition rate of the mercury re-emission additive may be selected
based on the measured concentration of ionic mercury in the scrubber liquor. In
certain embodiments, the percent mercury re-emission from the scrubber liquor can
be reduced by adjusting the rate of addition of mercury re-emission control additive
such that the scrubber liquor ionic mercury concentration is reduced to 1000 parts
per trillion (ppt) or less, 900 ppt or less, 800 ppt or less, 700 ppt or less, 600 ppt or
less, 500 ppt or less, 400 ppt or less, 300 ppt or less, 250 ppt or less, 200 ppt or less,
150 ppt or less, 100 ppt or less, or 50 ppt or less. In certain embodiments, the
percent mercury re-emission from the scrubber liquor can be reduced by adjusting
the rate of addition of mercury re-emission control additive such that the scrubber
liquor ionic mercury concentration ranges from about 0 ppt to about 1000 ppt, from
about 5 ppt to about 900 ppt, from about 10 to about 800 ppt, from about 15 ppt to
about 700 ppt, from about 20 ppt to about 600 ppt, from about 25 ppt to about 500
ppt, from about 30 ppt to about 400 ppt, from about 35 ppt to about 300 ppt, from
about 40 ppt to about 250 ppt, from about 45 ppt to about 200 ppt, or from about 50
ppt to about 150 ppt.
[0039] The percent mercury re-emission may be reduced to 20% or less, 19% or
less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less,
12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less,
5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain
embodiments, at ionic mercury concentration levels less than 200 ppt, mercury reemission
may be reduced to zero or near zero.
[0040] In certain embodiments, an ionic mercury scrubber liquor concentration of
1000 ppt or less may correspond to a mercury re-emission of 20% or less, 19% or
less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less,
12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less,
5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain
embodiments, an ionic mercury scrubber liquor concentration of 900 ppt or less may
correspond to a mercury re-emission of 20% or less, 19% or less, 18% or less, 17%
or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or less, 11% or less,
10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less,
3% or less, 2% or less, 1% or less, or 0%. In certain embodiments, an ionic mercury
scrubber liquor concentration of 800 ppt or less may correspond to a mercury reemission
of 20% or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or
less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8%
or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or
less, or 0%. In certain embodiments, an ionic mercury scrubber liquor concentration
of 700 ppt or less may correspond to a mercury re-emission of 20% or less, 19% or
less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less,
12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less,
5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain
embodiments, an ionic mercury scrubber liquor concentration of 600 ppt or less may
correspond to a mercury re-emission of 20% or less, 19% or less, 18% or less, 17%
or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or less, 11% or less,
10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less,
3% or less, 2% or less, 1% or less, or 0%. In certain embodiments, an ionic mercury
scrubber liquor concentration of 500 ppt or less may correspond to a mercury reemission
of 20% or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or
less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8%
or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or
less, or 0%. In certain embodiments, an ionic mercury scrubber liquor concentration
of 400 ppt or less may correspond to a mercury re-emission of 20% or less, 19% or
less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less,
12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less,
5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain
embodiments, an ionic mercury scrubber liquor concentration of 300 ppt or less may
correspond to a mercury re-emission of 20% or less, 19% or less, 18% or less, 17%
or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or less, 11% or less,
10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less,
3% or less, 2% or less, 1% or less, or 0%. In certain embodiments, an ionic mercury
scrubber liquor concentration of 250 ppt or less may correspond to a mercury reemission
of 20% or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or
less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8%
or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or
less, or 0%. In certain embodiments, an ionic mercury scrubber liquor concentration
of 200 ppt or less may correspond to a mercury re-emission of 20% or less, 19% or
less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less,
12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less,
5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain
embodiments, an ionic mercury scrubber liquor concentration of 150 ppt or less may
correspond to a mercury re-emission of 20% or less, 19% or less, 18% or less, 17%
or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or less, 11% or less,
10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less,
3% or less, 2% or less, 1% or less, or 0%. In certain embodiments, an ionic mercury
scrubber liquor concentration of 100 ppt or less may correspond to a mercury reemission
of 20% or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or
less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8%
or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or
less, or 0%. In certain embodiments, an ionic mercury scrubber liquor concentration
of 50 ppt or less may correspond to a mercury re-emission of 20% or less, 19% or
less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less,
12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less,
5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%.
[0041] In certain embodiments, the addition rate of the mercury re-emission
additive may be adjusted based on targeting a scrubber liquor ionic mercury
concentration of 1000 ppt or less, 900 ppt or less, 800 ppt or less, 700 ppt or less,
600 ppt or less, 500 ppt or less, 400 ppt or less, 300 ppt or less, 250 ppt or less, 200
ppt or less, 150 ppt or less, 100 ppt or less, or 50 ppt or less. The feed of the
scrubber additive may then be controlled to maintain a selected level of mercury
scrubber liquor concentration during process variations, such as load and coal
changes. Hence, active control can be set to follow plant load, changes in flue gas
mercury content due to fuel changes, and scrubber liquor changes.
[0042] In certain embodiments, the rate of addition of mercury re-emission
control additive to a scrubber liquor may provide an additive concentration from
about 0.0001 ppm to about 50,000 ppm, or from about 0.01 ppm to about 5,000
ppm. The rate of addition of mercury re-emission control additive may be adjusted
to target a selected scrubber liquor ionic mercury concentration, and in turn a desired
mercury re-emissions level. Factors that affect addition rate besides concentration
include coal composition which includes but is not limited to mercury and sulfur
concentration of the coal; halogen content of the coal; the type of fuel (e.g.,
anthracite, lignite, bituminous or subbituminous); the megawatt size of the plant
(e.g., 100 to 1000 MW), or capacity of the plant; the presence of other air quality
control devices ahead of the scrubber such as fabric filters or electrostatic
precipitators; the application of other flue gas mercury reduction technologies such
as activated carbon or inorganic sorbents prior to the scrubber; the design type of the
scrubber, (e.g., spray tower, chiota also known as a jet bubbler, or horizontal type);
the scrubber liquor volume; blow down rate (i.e., the rate at which spent liquor is
removed from the scrubber); liquid to gas ratio used in the scrubber; the presences of
trays or liquor dispersion techniques such as trays or baffles; the particle size and
concentration of lime or limestone being added to the scrubber to neutralize acid
gases; the load or demand (i.e., the percent of maximum generating load of the
plant); the quality of the water (e.g., concentration of impurities such as cations and
anions as well as process byproducts when water is reclaimed from cooling tower
blow down); and the relative amount of oxygen introduced into the scrubber slurry
for forced oxidation systems.
b. Oxidation-Reduction Potential
[0043] Oxidation-reduction potential (ORP) of a scrubber liquor can be
monitored as related to mercury capture, and the rate of addition of mercury reemission
control additive can be adjusted accordingly. By reducing the scrubber
liquor ORP through addition of mercury re-emission control additives, mercury reemissions
can be reduced, optionally to zero, and controlled directly based on the
ORP of the treated wFGD liquor. These results are particularly surprising
considering that conventional methods of controlling mercury re-emission rely on
increasing the existing scrubber liquor ORP to more positive or more oxidizing in
order to stabilize ionic mercury species in solution.
[0044] Although not wishing to be bound by theory, it is believed that addition of
mercury re-emission control additives results in the complexation of ionic mercury
in the scrubber liquor, thereby reducing mercury re-emission. In so doing, the
scrubber liquor ORP moves in a more negative direction and becomes more
reducing as ionic mercury is removed from the scrubber liquor via complexation
with control additive. Moving the scrubber liquor ORP toward a more reducing
condition is counter intuitive when faced with stabilizing oxidized mercury in the
liquor.
[0045] The scrubber liquor baseline ORP may vary widely due to variations in
composition. Such variations are influenced by, for example, EGU operations, fuel
composition, boiler additives, supplemental oxidants, and scrubber additives used to
enhance sulfur capture efficiency. Hence, the starting ORP of the scrubber liquor
may vary from, for example, oxidative at +400 mV to slightly oxidative at +90 mV.
Preferably, the mercury re-emission control additives used with the methods of the
invention decrease the liquor ORP, leading to reduction in mercury re-emissions.
[0046] The addition rate of mercury re-emission control additive may be selected
based on the measured concentration ORP of the scrubber liquor or a targeted ORP
value of the scrubber liquor relative to baseline. In certain embodiments, the percent
mercury re-emission from the scrubber liquor can be reduced by adjusting the rate of
addition of mercury re-emission control additive such that the scrubber liquor ORP
is reduced by a value of 500 mV or less, 400 mV or less, 300 mV or less, 250 mV or
less, 200 mV or less, 150 mV or less, 100 mV or less, or 50 mV or less. In certain
embodiments, the percent mercury re-emission from the scrubber liquor can be
reduced by adjusting the rate of addition of mercury re-emission control additive
such that the scrubber liquor ORP reduction ranges from about 50 mV to about 500
mV, about 100 mV to about 300 mV, or about 150 mV to about 250 mV. In certain
embodiments, the value of ORP change may be greater than 50 mV, greater than
100 mV, greater than 150 mV, greater than 200 mV, greater than 250 mV, greater
than 300 mV, greater than 350 mV, greater than 400 mV, greater than 450 mV, or
greater than 500 mV.
[0047] The percent mercury re-emission may be reduced to 20% or less, 19% or
less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less,
12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less,
5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%.
[0048] In certain embodiments, a reduction in ORP of 50 mV or greater may
correspond to a percent mercury re-emission of 20% or less, 19% or less, 18% or
less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or less,
11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less,
4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain embodiments, a
reduction in ORP of 100 mV or greater may correspond to a percent mercury reemission
of 20% or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or
less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8%
or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or
less, or 0%. In certain embodiments, a reduction in ORP of 150 mV or greater may
correspond to a percent mercury re-emission of 20% or less, 19% or less, 18% or
less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or less,
11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less,
4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain embodiments, a
reduction in ORP of 200 mV or greater may correspond to a percent mercury reemission
of 20% or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or
less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8%
or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or
less, or 0%. In certain embodiments, a reduction in ORP of 250 mV or greater may
correspond to a percent mercury re-emission of 20% or less, 19% or less, 18% or
less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or less,
11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less,
4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain embodiments, a
reduction in ORP of 300 mV or greater may correspond to a percent mercury reemission
of 20% or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or
less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8%
or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or
less, or 0%. In certain embodiments, a reduction in ORP of 350 mV or greater may
correspond to a percent mercury re-emission of 20% or less, 19% or less, 18% or
less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or less,
11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less,
4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain embodiments, a
reduction in ORP of 400 mV or greater may correspond to a percent mercury reemission
of 20% or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or
less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8%
or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or
less, or 0%. In certain embodiments, a reduction in ORP of 450 mV or greater may
correspond to a percent mercury re-emission of 20% or less, 19% or less, 18% or
less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or less,
11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less,
4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain embodiments, a
reduction in ORP of 500 mV or greater may correspond to a percent mercury reemission
of 20% or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or
less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8%
or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or
less, or 0%.
[0049] In certain embodiments, the scrubber liquor ORP may be continuously
monitored or monitored at intervals, wherein if the measured ORP increases by 50
mV or more, 100 mV or more, 150 mV or more, 200 mV or more, 250 mV or more,
300 mV or more, 400 mV or more, or 500 mV or more over a desired ORP, the rate
of addition of mercury re-emission control additive may be increased to reduce the
scrubber liquor ORP by 50 mV or more, 100 mV or more, 150 mV or more, 200 mV
or more, 250 mV or more, 300 mV or more, or 400 mV or more. The reduction in
scrubber liquor ORP may result in a percent mercury re-emission of 20% or less,
19% or less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or
less, 12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or
less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%.
[0050] In certain embodiments, the scrubber liquor ORP may be continuously
monitored or monitored at intervals, wherein if the measured ORP decreases by 50
mV or more, 100 mV or more, 150 mV or more, 200 mV or more, 250 mV or more,
300 mV or more, 400 mV or more, or 500 mV or more below the desired ORP, the
rate of addition of mercury re-emission control additive may be decreased to
increase scrubber liquor ORP by 50 mV or more, 100 mV or more, 150 mV or more,
200 mV or more, 250 mV or more, 300 mV or more, 400 mV or more, or 500 mV or
more. Preferably, when the scrubber liquor ORP is increased by reducing the rate of
addition of mercury re-emission control additive, the percent mercury re-emission is
maintained at or near a level reached before increasing the scrubber liquor ORP.
[0051] In certain embodiments, the rate of addition of mercury re-emission
control additive to a scrubber liquor may provide an additive concentration from
about 0.0001 ppm to about 50,000 ppm, or from about 0.01 ppm to about 5,000
ppm. The rate of addition of mercury re-emission control additive may be adjusted
to target a selected change in oxidation-reduction potential, and in turn a desired
mercury re-emissions level. Factors that affect addition rate besides concentration
include coal composition which includes but is not limited to mercury and sulfur
concentration of the coal; halogen content of the coal; the type of fuel (e.g.,
anthracite, lignite, bituminous or subbituminous); the megawatt size of the plant
(e.g., 100 to 1000 MW), or capacity of the plant; the presence of other air quality
control devices ahead of the scrubber such as fabric filters or electrostatic
precipitators; the application of other flue gas mercury reduction technologies such
as activated carbon or inorganic sorbents prior to the scrubber; the design type of the
scrubber, (e.g., spray tower, chiota also known as a jet bubbler, or horizontal type);
the scrubber liquor volume; blow down rate (i.e., the rate at which spent liquor is
removed from the scrubber); liquid to gas ratio used in the scrubber; the presences of
trays or liquor dispersion techniques such as trays or baffles; the particle size and
concentration of lime or limestone being added to the scrubber to neutralize acid
gases; the load or demand (i.e., the percent of maximum generating load of the
plant); the quality of the water (e.g., concentration of impurities such as cations and
anions as well as process byproducts when water is reclaimed from cooling tower
blow down); and the relative amount of oxygen introduced into the scrubber slurry
for forced oxidation systems.
c. Sulfide Concentration
[0052] Sulfide concentration in a scrubber liquor can be used to monitor wFGD
operations, and the rate of addition of mercury re-emission control additive can be
adjusted accordingly to adjust mercury capture. Using sulfide concentration to
monitor wFGD operations provides an in-line, continuous, and direct means of
measuring the re-emission control additive used to control mercury re-emission
across the wFGD scrubber. In certain embodiments, the rate of addition of mercury
re-emission control additive can be increased to compensate for lower
concentrations of sulfide in the scrubber liquor, thereby reducing mercury reemission.
In certain embodiments, the rate of addition of mercury re-emission
control additive can be decreased to compensate for higher concentrations of sulfide
in the scrubber liquor, thereby reducing excessive use of re-emission control
additive.
[0053] A sulfide ion- selective electrode (ISE) can be used to monitor sulfide
concentration. The sulfide ion-selective electrode may be designed for the
detection of sulfide ions (S ) in aqueous solutions, and may be suitable for use in
both field and laboratory applications. The sulfide ion-selective electrode may have
a solid-state crystal membrane. Where the mercury re-emission control additive
contains sulfide ions, the sulfide ion-selective electrode can be used to monitor and
control the additive feed rate and dosage to a wFGD.
[0054] The addition rate of the mercury re-emission control additive may be
selected based on the measured concentration of sulfide in the scrubber liquor. In
certain embodiments, the percent mercury re-emission from the scrubber liquor can
be reduced by adjusting the rate of addition of mercury re-emission control additive
such that the scrubber liquor sulfide concentration is increased to 20 ppm or greater,
30 ppm or greater, 40 ppm or greater, 50 ppm or greater, 60 ppm or greater, 70 ppm
or greater, 80 ppm or greater, 90 ppm or greater, or 100 ppm or greater. In certain
embodiments, the percent mercury re-emission from the scrubber liquor can be
reduced by adjusting the rate of addition of mercury re-emission control additive
such that the scrubber liquor sulfide concentration ranges from about 100 ppm or
less, 90 ppm or less, 80 ppm or less, 70 ppm or less, 60 ppm or less, 50 ppm or less,
40 ppm or less, 30 ppm or less, or 20 ppm or less. The sulfide concentration may be
determined using a sulfide ion- selective electrode and correlating the measured
electrode response to sulfide concentration.
[0055] The percent mercury re-emission may be reduced to 20% or less, 19% or
less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less,
12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less,
5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain
embodiments, at sulfide levels of 20 ppm or greater, 30 ppm or greater, 40 ppm or
greater, 50 ppm or greater, 60 ppm or greater, 70 ppm or greater, 80 ppm or greater,
90 ppm or greater, or 100 ppm or greater, mercury re-emission may be reduced to
zero or near zero.
[0056] In certain embodiments, a sulfide scrubber liquor concentration of 20 ppm
or greater may correspond to a mercury re-emission of 20% or less, 19% or less,
18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or
less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or
less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain embodiments,
a sulfide scrubber liquor concentration of 30 ppm or greater may correspond to a
mercury re-emission of 20% or less, 19% or less, 18% or less, 17% or less, 16% or
less, 15% or less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less,
9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2%
or less, 1% or less, or 0%. In certain embodiments, a sulfide scrubber liquor
concentration of 40 ppm or greater may correspond to a mercury re-emission of 20%
or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less,
13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or
less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%. In
certain embodiments, a sulfide scrubber liquor concentration of 50 ppm or greater
may correspond to a mercury re-emission of 20% or less, 19% or less, 18% or less,
17% or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or less, 11% or
less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or
less, 3% or less, 2% or less, 1% or less, or 0%. In certain embodiments, a sulfide
scrubber liquor concentration of 60 ppm or greater may correspond to a mercury reemission
of 20% or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or
less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8%
or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or
less, or 0%. In certain embodiments, a sulfide scrubber liquor concentration of 70
ppm or greater may correspond to a mercury re-emission of 20% or less, 19% or
less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less,
12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less,
5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%. In certain
embodiments, a sulfide scrubber liquor concentration of 80 ppm or greater may
correspond to a mercury re-emission of 20% or less, 19% or less, 18% or less, 17%
or less, 16% or less, 15% or less, 14% or less, 13% or less, 12% or less, 11% or less,
10% or less, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less,
3% or less, 2% or less, 1% or less, or 0%. In certain embodiments, a sulfide
scrubber liquor concentration of 90 ppm or greater may correspond to a mercury reemission
of 20% or less, 19% or less, 18% or less, 17% or less, 16% or less, 15% or
less, 14% or less, 13% or less, 12% or less, 11% or less, 10% or less, 9% or less, 8%
or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, 1% or
less, or 0%. In certain embodiments, a sulfide scrubber liquor concentration of 100
ppm or greater may correspond to a mercury re-emission of 20% or less, 19% or
less, 18% or less, 17% or less, 16% or less, 15% or less, 14% or less, 13% or less,
12% or less, 11% or less, 10% or less, 9% or less, 8% or less, 7% or less, 6% or less,
5% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%.
[0057] In certain embodiments, the addition rate of the mercury re-emission
additive may be adjusted based on targeting a scrubber liquor sulfide concentration
of about 20 ppm to about 100 ppm, or about 30 ppm to about 70 ppm. The feed of
the scrubber additive may then be controlled to maintain the selected level of sulfide
concentration during process variations, such as load and coal changes. Hence,
active control can be set to follow plant load, changes in flue gas mercury content
due to fuel changes, and scrubber liquor changes.
[0058] In certain embodiments, the rate of addition of mercury re-emission
control additive to a scrubber liquor may provide an additive concentration from
about 0.0001 ppm to about 50,000 ppm, or from about 0.01 ppm to about 5,000
ppm. The rate of addition of mercury re-emission control additive may be adjusted
to target a selected scrubber liquor sulfide ion concentration, and in turn a desired
mercury re-emissions level. Factors that affect addition rate besides concentration
include coal composition which includes but is not limited to mercury and sulfur
concentration of the coal; halogen content of the coal; the type of fuel (e.g.,
anthracite, lignite, bituminous or subbituminous); the megawatt size of the plant
(e.g., 100 to 1000 MW), or capacity of the plant; the presence of other air quality
control devices ahead of the scrubber such as fabric filters or electrostatic
precipitators; the application of other flue gas mercury reduction technologies such
as activated carbon or inorganic sorbents prior to the scrubber; the design type of the
scrubber, (e.g., spray tower, chiota also known as a jet bubbler, or horizontal type);
the scrubber liquor volume; blow down rate (i.e., the rate at which spent liquor is
removed from the scrubber); liquid to gas ratio used in the scrubber; the presences of
trays or liquor dispersion techniques such as trays or baffles; the particle size and
concentration of lime or limestone being added to the scrubber to neutralize acid
gases; the load or demand (i.e., the percent of maximum generating load of the
plant); the quality of the water (e.g., concentration of impurities such as cations and
anions as well as process byproducts when water is reclaimed from cooling tower
blow down); and the relative amount of oxygen introduced into the scrubber slurry
for forced oxidation systems.
3. Mercury Re-emisson Control Additives
[0059] Mercury re-emission control additives that can be used with the methods
of the invention include any additive suitable to reduce and/or prevent mercury reemission
from combustion processes, and in particular, scrubber liquors.
[0060] In certain embodiments, the mercury re-emission control additive may be
a poly-dithiocarbamic compound (e.g., MerControl 8034, also referred to herein as
"poly-DTC"), or another sulfur-containing additive such as sodium sulfide, sodium
hydrosulfide, sodium bisulfide, or a poly-sulfide.
[0061] In certain embodiments, the mercury re-emission control additive may be
diethyldithiocarbamate or a sodium salt thereof. In certain embodiments, the
mercury re-emission control additive may be dimethyldithiocarbamate or a sodium
salt thereof.
[0062] In certain embodiments, the mercury re-emission control additive may be
an inorganic poly-sulfide or blend, such as PRAVO, a product from Vosteen.
[0063] In certain embodiments, the mercury re-emission additive may be a
sodium or calcium salt of l,3,5-triazine-2,4,6(lH,3H,5H)-trithione (also referred to
as trimercapto-S-triazine), such as TMT-15, a product from Degussa.
[0064] In certain embodiments, the mercury re-emission control additive may be
an activated carbon, such as disclosed in US Patent 7,727,307 B2.
[0065] In certain embodiments, the mercury re-emission control additive may be
a dithiol, a dithiolane, or a thiol having a single thiol group and either an oxygen or a
hydroxyl group. Suitable dithiols include, but are not limited to, 2,3-
dimercaptopropanol, dimercaptosuccinic acid, and 1,8-octanedithiol. Suitable
dithiolanes include, but are not limited to, l,2-dithiolane-3-valeric acid and 2-methyl
1,3-dithiolane. Suitable thiols include, but are not limited to, mercaptoacetic acid
and sodium salts thereof.
[0066] In certain embodiments, a combination of mercury re-emission controlled
additives may be used. In one preferred embodiment, the mercury re-emission
control additive comprises a poly-dithiocarbamic compound.
a. Ethylene Dichloride Ammonia Polymer Containing
Dithiocarbamate Groups
[0067] The mercury re-emission control additive may be a water-soluble
ethylene dichloride ammonia polymer having a molecular weight of from 500 to
10,000, and containing from 5 to 55 mole % of dithiocarbamate salt groups to
prevent re-emission of mercury across a wFGD.
[0068] The polymer may be prepared by the reaction of ethylene dichloride and
ammonia to provide a polyamine or polyimine. The polyamine or polyimine may
have a molecular weight range of 500-100,000. In a preferred embodiment, the
molecular weight may be 1,500 to 10,000, with the most preferred molecular weight
range being 1,500 to 5,000.
[0069] The dithiocarbamate groups of the polymers may be introduced by the
reaction of the polyamines or polyimines with carbon disulfide to produce
polydithiocarbamic acid or their salts. Such reaction is preferably carried out in a
solvent such as water or alcohol at a temperature of from 30 °C and 100 °C for
periods of time ranging between 1 and 10 hours. Good conversion may be obtained
at temperatures between 40 0 and 70 °C for 2 to 5 hours.
[0070] The mole % of dithiocarbamate salt groups in the finished polymer may
be within the range of 5 to 55%, 20 to 40 mole %, or 25 to 30 mole . The salts
include, but are not limited to, alkaline and alkali earth such as sodium, lithium,
potassium or calcium.
[0071] The finished polymer may be applied to a combustion process at a ratio of
1:1 to 2000: 1 weight copolymer to weight of mercury being captured. One preferred
ratio may be from 5:1 to 1000:1 more preferably from 5:1 to 500:1.
b. Acrylic-x and Alkylamine Polymer
[0072] The mercury re-emission control additive may be a composition
comprising a polymer derived from at least two monomers: acrylic-x and an
alkylamine, wherein said acrylic-x has the following formula:
wherein X = OR, OH and salts thereof, or NHR2 and wherein R1 and R2 is H or an
alkyl or aryl group, wherein R is an alkyl or aryl group, wherein the molecular
weight of said polymer is between 500 to 200,000, and wherein said polymer is
modified to contain a functional group capable of scavenging one or more
compositions containing one or more metals.
[0073] The metals can include zero valent, monovalent, and multivalent metals.
The metals may or may not be ligated by organic or inorganic compounds. Also, the
metals can be radioactive and nonradioactive. Examples include, but are not limited
to, transition metals and heavy metals. Specific metals can include, but are not
limited to: copper, nickel, zinc, lead, mercury, cadmium, silver, iron, manganese,
palladium, platinum, strontium, selenium, arsenic, cobalt and gold.
[0074] The molecular weight of the polymers can vary. For example, the target
species/application for the polymers can be one consideration. Another factor can
be monomer selection. Molecular weight can be calculated by various means
known to those of ordinary skill in the art. For example, size exclusion
chromatography, as discussed in the examples below can be utilized. When
molecular weight is mentioned, it is referring to the molecular weight for the
unmodified polymer, otherwise referred to as the polymer backbone. The functional
groups that are added to the backbone are not part of the calculation. Thus the
molecular weight of the polymer with the functional groups can far exceed the
molecular weight range. In one embodiment, the molecular weight of the polymer is
from 1,000 to 16,000. In another embodiment, the molecular weight of said
polymer is from 1,500 to 8,000.
[0075] Various functional groups can be utilized for metal scavenging. The
following phraseology would be well understood by one of ordinary skill in the art:
wherein said polymer is modified to contain a functional group capable of
scavenging one or more compositions containing one or more metals. More
specifically, the polymer is modified to contain a functional group that can bind
metals. In one embodiment, the functional group contains a sulfide containing
chemistry. In another embodiment, the functional group is a dithiocarbamate salt
group. In another embodiment, the functional groups are at least one of the
following: alkylene phosphate groups, alkylene carboxylic acids and salts thereof,
oxime groups, amidooxime groups, dithiocarbamic acids and salts thereof,
hydroxamic acids, and nitrogen oxides.
[0076] The molar amounts of the functional group relative to the total amines
contained in the unmodified polymer can vary as well. For example, the reaction of
3.0 molar equivalents of carbon disulfide to a 1.0: 1.0 mole ratio acrylic acid / TEPA
copolymer, which contains 4 molar equivalents of amines per repeat unit after
polymerization, will result in a polymer that is modified to contain 75 mole %
dithiocarbamate salt group. In other words, 75 % of the total amines in the
unmodified polymer have been converted to dithiocarbamate salt groups.
[0077] In one embodiment, the polymer may have between 5 to 100 mole % of
the dithiocarbamate salt group. In a further embodiment, the polymer has from 25 to
90 mole % of the dithiocarbamate salt group. In yet a further embodiment, the
polymer has from 55 to 80 mole % of the dithiocarbamate salt group.
[0078] Monomer selection will depend on the desired polymer. In one
embodiment, the alkylamine is at least one of the following: an ethyleneamine, a
polyethylenepolyamine, ethylenediamine (EDA), diethylenetriamine (DETA),
triethylenetetraamine (TETA) and tetraethylenepetamine (TEPA) and
pentaethylenehexamine (PEHA). In another embodiment, the acrylic-x is at least
one of the following: methyl acrylate, methyl methacrylate, ethyl acrylate, and ethyl
methacrylate, propyl acrylate, and propyl methacrylate. In another embodiment, the
acrylic-x is at least one of the following: acrylic acid and salts thereof, methacrylic
acid and salts thereof, acrylamide, and methacrylamide.
[0079] The molar ratio between monomers that make up the polymer, especially
acrylic-x and alkylamine can vary and depend upon the resultant polymer product
that is desired. The molar ratio used is defined as the moles of acrylic-x divided by
the moles of alkylamine. In one embodiment, the molar ratio between acrylic-x and
alkylamine is from 0.85 to 1.5. In another embodiment, the molar ratio between
acrylic-x and alkylamine is from 1.0 to 1.2. Various combinations of acrylic-x and
alkylamines are encompassed by this invention as well as associated molecular
weight of the polymers.
[0080] In one embodiment, the acrylic-x is an acrylic ester and the alkylamine is
PEHA or TEPA or DETA or TETA or EDA. In a further embodiment, the molar
ratio between acrylic-x and alkylamine is from 0.85 to 1.5. In yet a further
embodiment, the molecular weight can encompass ranges: 500 to 200,000, 1,000 to
16,000, or 1,500 to 8,000. In yet a further embodiment, the acrylic ester can be at
least one of the following: methyl acrylate, methyl methacrylate, ethyl acrylate, and
ethyl methacrylate, propyl acrylate, and propyl methacrylate, which is combined
with at least one of the alklyamines, which includes PEHA or TEPA or DETA or
TETA or EDA. In yet a further embodiment, the resulting polymer is modified to
contain the following ranges of dithiocarbamate salt groups: 5 to 100 mole , 25 to
90 mole , or 55 to 80 mole %.
[0081] In another embodiment, the acrylic-x is an acrylic amide and the
alkylamine is TEPA or DETA or TETA or EDA. In a further embodiment, the
molar ratio between acrylic-x and alkylamine is from 0.85 to 1.5. In yet a further
embodiment, the molecular weight can encompass ranges: 500 to 200,000, 1,000 to
16,000, or 1,500 to 8,000. In yet a further embodiment, the acrylic amide can be at
least one or a combination of acrylamide and methacrylamide, which is combined
with at least one of the alklyamines, which includes PEHA or TEPA or DETA or
TETA or EDA. In yet a further embodiment, the resulting polymer is modified to
contain the following ranges of dithiocarbamate salt groups: 5 to 100 mole , 25 to
90 mole , or 55 to 80 mole %.
[0082] In another embodiment, the acrylic-x is an acrylic acid and salts thereof
and the alkylamine is PEHA or TEPA or DETA or TETA or EDA. In a further
embodiment, the molar ratio between acrylic-x and alkylamine is from 0.85 to 1.5.
In yet a further embodiment, the molecular weight can encompass ranges: 500 to
200,000, 1,000 to 16,000, or 1,500 to 8,000. In yet a further embodiment, the
acrylic acid can be at least one or a combination of acrylic acid or salts thereof and
methacrylic acid or salts thereof, which is combined with at least one of the
alklyamines, which includes TEPA or DETA or TETA or EDA. In yet a further
embodiment, the resulting polymer is modified to contain the following ranges of
dithiocarbamate salt groups: 5 to 100 mole , 25 to 90 mole , or 55 to 80 mole .
[0083] Additional monomers can be integrated into the polymer backbone made
up of constituent monomers acrylic-x and alkylamine. A condensation polymer
reaction scheme can be utilized to make the basic polymer backbone chain. Various
other synthesis methods can be utilized to functionalize the polymer with, for
example, dithiocarbamate and/or other non-metal scavenging functional groups.
One of ordinary skill in the art can functionalize the polymer without undue
experimentation .
[0084] In certain embodiments, the composition can be formulated with other
polymers such as those disclosed in U.S. Patent No. 5,164,095, herein incorporated
by reference, specifically, a water soluble ethylene dichloride ammonia polymer
having a molecular weight of from 500 to 100,000 which contains from 5 to 55 mole
% of dithiocarbamate salt groups. In one embodiment, the molecular weight of the
polymer is from 1,500 to 10,000 and contains 15 to 50 mole % of dithiocarbamate
salt groups. In a preferred embodiment, the molecular weight of the polymer is from
1,500 to 5,000 and contains 30 to 55 mole % of dithiocarbamate salt groups.
[0085] In certain embodiments, the composition can be formulated with other
small molecule sulfide precipitants such as sodium sulfide, sodium hydrosulfide,
TMT-15® (sodium or calcium salts of trimercapto-S-triazine),
dimethyldithiocarbamate, and/or diethyldithiocarbamate.
c. Dosage
[0086] The dosage of the disclosed mercury re-emission control additives may
vary as necessitated to reduce or prevent mercury re-emission. The dosage amounts
can be selected based on a desired ionic mercury concentration, change in ORP,
and/or sulfide concentration in the scrubber liquor, which correspond to a percent
mercury re-emission.
[0087] Process medium quality and extent of process medium treatment are a
couple of factors that can be considered by one of ordinary skill in the art in
selecting dosage amount. Ajar test analysis is a typical example of what is utilized
as a basis for determining the amount of dosage required to achieve effective metals
removal in the context of a process water medium, e.g. wastewater.
[0088] In one embodiment, the amount of mercury re-emission control additive
for effectively removing metals from contaminated waters may be within the range
of 0.2 to 2 moles of dithiocarbamate per mole of metal, or 1 to 2 moles of
dithiocarbamate per mole of metal contained in the water. According to one
embodiment, the dosage of metal removal polymer required to chelate and
precipitate 100 ml of 18 ppm soluble copper to about 1ppm or less was 0.011 gm
(11.0 mg) of polymer. The metal polymer complexes formed are self-flocculating
and quickly settle. These flocculants are easily separated from the treated water.
[0089] In the context of applying the polymer to a gas system, such as a flue gas,
the polymer can be dosed incrementally and capture rates for a particular metal, e.g.
such as mercury, can be calculated by known techniques in the art. In certain
embodiments, a mercury re-emission control additive, such as a water-soluble
ethylene dichloride ammonia polymer with dithiocarbamate salt groups, may be
applied to a scrubber liquor at a ratio of 1:1 to 2000:1 weight of polymer to weight
of mercury being captured. One preferred dosage ratio is from 5:1 to 1000:1, more
preferably from 5:1 to 500:1.
4. Applications
[0090] Methods of the present invention can be used in any process in which it is
desirable to remove mercury from a flue gas. For example, the methods of the
present invention can be used in waste incineration plants (e.g., domestic waste,
hazardous waste, or sewage sludge incineration plants), power stations (e.g.,
bituminous coal-fired, or lignite-fired power stations), other plants for hightemperature
processes (e.g., cement burning), and high-temperature plants co-fired
with waste or combined (multistage) high-temperature plants (e.g., power stations or
cement rotary kilns having an upstream waste pyrrolysis or waste gasification). In
certain embodiments, the sulfide ion-selective electrode may be used in wastewater
treatment for dosage control of metals scavenging polymers.
[0091] Methods of the present invention can be used in processes of any
dimension. The methods can be used in processes having a flue gas volumetric flow
rate of 15x103 m3 S.T.P. db/h, for example for sewage sludge incineration, or of
50x103 m3 S.T.P. db/h, for example in hazardous waste incineration plants, or of
150x103 m3 S.T.P. db/h, for example in domestic waste incineration, and also in
large power stations having, for example, 2-3x106 m S.T.P. db/h.
[0092] The mercury re-emission control additives may be added to scrubbers
currently used in the industry, including spray towers, jet bubblers, and co-current
packed towers. These types of particulate control devices are provided as examples
and are not meant to represent or suggest any limitation. In general the mercury reemission
control additives may be introduced into a scrubber and thereby into the
scrubber liquor via several routes. For example, a mercury re-emission control
additive may be added to a virgin limestone or lime slurry prior to addition to a
scrubber, to the recirculation loop of a scrubber liquor, or to a "low solids" return to
a scrubber from the scrubber purge stream. The addition of a mercury re-emission
control additive, such as a polydithiocarbamic acid compound, can be made in any
suitable location in a scrubber process, wholly or fractionally (i.e. a single feed point
or multiple feed points), including but not limited to the make-up water for the lime
or limestone slurry or the scrubber liquor.
[0093] In certain embodiments, the mercury re-emission control additive may be
added to a wet scrubber via a "low solids" liquor return. A portion of the liquor is
usually continuously removed from the scrubber for the purpose of separating
reaction byproducts from unused lime or limestone. One means of separation that is
currently used is centrifugation. In this process the scrubber liquor is separated into
a "high solids" and "low solids" stream. The high solids stream is diverted to
wastewater processing. The low solids fraction returns to the wet scrubber and can
be considered "reclaimed" dilute liquor. The mercury re-emission control additives,
such as polydithiocarbamic acid compounds, can conveniently be added to the
reclaimed low solids stream prior to returning to the scrubber.
[0094] In certain embodiments, the mercury re-emission control additive may be
added to the wet scrubber via a "virgin liquor." Virgin liquor is the water-based
dispersion of either lime or limestone prior to exposure to flue gas and is used to add
fresh lime or limestone while maintaining the scrubber liquor level and efficiency of
the wet FGD. This is prepared by dispersing the lime or limestone in water. Here
the mercury re-emission control additive , such as a polydithiocarbamic acid
compound, can be added either to the dispersion water or the virgin liquor directly.
[0095] In certain embodiments, the mercury re-emission control additive, such as
a polydithiocarbamic compound, may be added to scrubber liquor injected directly
into the flue gas prior to the scrubber for the purpose of controlling relative humidity
of the flue gas or its temperature.
[0096] The scrubber liquors referred to herein may be water-based dispersions of
calcium carbonate (limestone) or calcium oxide (lime) used in a wet Flue Gas
Scrubber to capture SOx emissions. The liquor may also contain other additives
such as magnesium and low-molecular weight organic acids, which function to
improve the sulfur capture. One example of such an additive is a mixture of lowmolecular
weight organic acids known as dibasic acid (DBA). DBA consists of a
blend of adipic, succinic, and glutaric acids. Each of these organic acids can also be
used individually. In addition, another low-molecular weight organic acid that can
be used to improve sulfur capture in a wet scrubber is formic acid. Finally, the
scrubber liquor may also contain byproducts of the interaction between the lime or
limestone and sulfur species, which leads to the presence of various amounts of
calcium sulfite or calcium sulfate. The scrubber liquor may include the make-up
liquor, return liquor, the reclaimed liquor, virgin liquor, and/or liquor injected
directly into flue gasses.
5. Examples
[0097] The foregoing may be better understood by reference to the following
examples, which are presented for purposes of illustration and are not intended to
limit the scope of the invention.
Example 1
Correlation between scrubber mercury levels and re-emission and additive
[0098] Mercury determination is based on reducing all aqueous ionic mercury to
elemental mercury followed by its quantitative release into air being swept through
the sample. The resulting gas-phase elemental mercury is quantified indirectly by
atomic absorption spectroscopy. Finally, the gas-phase elemental mercury is
captured on an activated carbon packed filter.
[0099] Mercury quantification may be accomplished using a RA-915+ Mercury
Analyzer, manufactured by Ohio Lumex. The RA-915+ is a portable atomic
absorption spectrometer (AAS) with a 10-meter (m) multipath optical cell and
Zeeman background correction. The operation of the RA-915+ is based on the
principle of differential Zeeman AAS combined with high-frequency modulation of
polarized light. This combination eliminates interference and provides the highest
sensitivity. The combination of RA-915+ features leads to the direct detection of
mercury without preliminary accumulation on a gold trap. The RA-915+ includes a
built in test cell for field performance verification. For the purposes of site
measurement RA-915+ was used with the optional RP-91C. The RP-91C consists
of a gas-phase pump, flow meter and gas/liquid impingers. AAS uses the absorption
of light to measure the concentration of gas-phase mercury. The mercury absorbs at
254 nm and makes a transition to higher electron energy state. The gas-phase
mercury concentration is determined from the amount of absorption. Concentration
measurements are determined from a working curve after calibrating the instrument
with standards of known mercury concentration. Site requirements include: a
temperature range of 5 to 40 °C, relative humidity of up to 98%, and atmospheric
pressure of 84 to 106.7 kilopascals. Sensitivity of the instrument is reportedly not
affected by up to 95% background absorption caused by interfering components
(e.g., dust, moisture, organic and inorganic gases).
[00100] The laboratory unit is setup sequentially similar to the impingers used in
conditionally flue gas samples for continuous mercury monitors. Two impingers are
assembled in series. The second impinger is dry and used as a liquid catch prior to
the detector. The first impinger of the Ohio Lumex is filled with about 10 mL of
deionized water and 2 mL of 2% stannous chloride in 10% hydrochloric acid
solution. Stannous chloride, di-hydrate is used to prepare the solution. Analytical
grade hydrochloric acid is used to prepare the solutions to insure non-detectable
mercury content. The stannous chloride, di-hydrate and concentrated hydrochloric
acid are obtained from VWR. Air is pulled through the two impingers to the
detector and through the activated carbon filter at a constant rate of 2 mL per hour.
To the first impinger containing the reducing agent, an appropriate sample volume is
quickly added. The sample's absorption is then converted to mercury concentration
using a constructed mercury standard curve.
[00101] A standard calibration curve is constructed by use of secondary standards.
The normal dynamic analytical range is from 1 - 100 mg/kg by direct determination
without dilution. The standard solutions are prepared in 2% hydrochloric acid
solution from Mercury Stock Solution, 1.00 mg/mL mercury in 2% hydrochloric
acid, Teledyne Leeman Labs. A standard curve is constructed each day. The
secondary standard solution contains 218 ppt mercury as determined by Naperville
Analytical laboratory. The overall average slope during the demonstration is 4.74 X
10-5 with R2-values typically around 0.998. The percent relative standard
deviation, RSD, of the slopes is 3%.
[00102] Samples were prepared by filtering scrubber samples through a 0.45m
filter. The filtrate was diluted 1 g to 50 g with deionized water and then digesting
with 1mL of BrCl reagent (available from Leeman) at ambient temperature for at
least 18 hours. Excess BrCl was quenched with 0.075 mL of 12% hydroxylamine
hydrochloride before analysis.
[00103] The information of Example 1 was collected at a commercial energy
generation unit (EGU) consisting of two boilers burning high chlorine bituminous
coal and equipped with SCR (selective catalytic reduction) catalysts. The
combustion gases are combined into a cold-side ESP (electrostatic precipitator) and
then through a LSFO wFGD (limestone forced oxidation wet flue gas desulfurizer)
before being released to the atmosphere. The chlorine content in the coal is 1200
ppm on an as received basis. The units were monitored using continuous mercury
monitors at the inlet of the wFGD and at the stack. Method 30B carbon traps were
used to verify the results. The findings are provided in Table 1 below.
Table 1
[00104] In this example, poly-dithiocarbamate (poly-DTC) was added to the
wFGD basin to effect reduction of the mercury levels from about 10,000 ppt to less
than 100 ppt. It is clear from this example that as the concentration in the basin is
reduced, mercury re-emissions is reduced. Subsequently, the addition of poly-DTC
was terminated. As can be seen, the mercury levels in the basin returned to near
baseline, i.e. 17,000 ppt and re-emissions increased to 100%. This demonstrates a
correlation between wFGD liquor concentration and mercury re-emission.
Example 2
Correlation between scrubber mercury levels and re-emission and additive
[00105] This information was collected on a commercial EGU generating
513MWg. The boiler was burning bituminous coal with 400 ppm chloride content.
The air quality control devices or AQCDs consisted of a cold-side ESP and LSFO
wFGD. Mercury CMMs were used to measure mercury at the inlet of the wFGD
and at the stack. A poly-DTC was fed to the wFGD to affect mercury re-emission
control. The results are provided in Table 2 below.
Table 2
[00106] In this example, the EGU had only 38% mercury capture due to reemissions
of nearly 56%. The baseline wFGD basin liquor concentration was
51,000 ppt. Upon addition of poly-DTC to control mercury re-emissions, the wFGD
mercury levels decreased to less than 200 ppt. When this occurred the mercury reemissions
decreased to zero and the total capture of mercury increased to 70%.
When addition of poly-DTC was terminated, the wFGD mercury levels increased to
28,000 ppt with a similar increase in mercury re-emission to 63%. This example
shows the link between wFGD mercury concentration and re-emission which serves
as the control strategy.
Example 3
Correlation between scrubber mercury levels and re-emission and additive
[00107] This example was conducted at an EGU consisting of a 550 MWg boiler
firing sub-bituminous coal. The unit air quality control devices (AQCDs) consisted
of an electrostatic precipitator for particulate removal and three wet Flue Gas
Desulfurization (wFGD) scrubbers for SOx removal. The wFGD scrubbers use
sodium-based solutions to capture SOx. In order to increase the flue gas oxidized
mercury concentration, a halogen-based boiler additive such as hypochlorite was
added. The results are provided in Table 3 below.
Table 3
[00108] Baseline, i.e., normal operation without wFGD additives, conditions are
seen above. The mercury concentration in the wFGD liquor was nearly 15,000 ppt
with a measured mercury re-emission of 23%. Upon addition of poly-DTC to
control mercury re-emissions, the wFGD mercury levels decreased to less than 200
ppt with the measured mercury re-emission decreasing to zero percent. This shows
that the mercury level in the wFGD can be used to control the addition of poly-DTC,
scrubber additive.
Example 4
Laboratory results demonstrating correlation between lower ORP due to additive
and re-emissions control
[00109] A laboratory experiment was carried out in a laboratory wFGD simulator.
The conditions of the test were: pH = 4.5, [S0 3
2 ] = 0.2 mM, [CI ] = 300 mM. A
poly-DTC scrubber additive was introduced to control mercury re-emissions from
the laboratory jet bubbler unit at 100 ppm. The results are provided in Table 4
below.
Table 4
[00110] As can be seen Table 4, the addition of poly-DTC to complex mercury
eliminates mercury re-emission while the ORP of the solution becomes more
negative or changes in the direction of a reducing potential.
Example 5
ORP and Re-emissions and Control
[00111] The information was collected at a commercial energy generation unit
(EGU) consisting of two boilers burning high chlorine bituminous coal and
equipped with SCR (selective catalytic reduction) catalysts. The combustion gases
are combined into a cold-side ESP (electrostatic precipitator) and then through a
LSFO wFGD (limestone forced oxidation wet flue gas desulfurizer) before being
released to the atmosphere. The chlorine content in the coal is 1200 ppm on an as
received basis. The units were monitored using continuous mercury monitors at the
inlet of the wFGD and at the stack. Method 30B carbon traps were used to verify
the results. The results are provided in Table 5 below.
Table 5
[00112] As can be seen Table 5, at baseline, i.e. before addition of a wFGD
additive, mercury re-emission is high at 71% and the ORP is +540 mV. Counter to
conventional approaches, the addition of a poly-DTC to control the re-emission
results in a decrease in ORP. Indeed, as shown in the above table, there is a
correlation between the observed decrease in ORP and mercury re-emission that
allows the control of poly-DTC addition.
Example 6
ORP and Re-emissions and Control
[00113] This information was collected on a commercial EGU generating
513MWg. The boiler was burning bituminous coal with 400 ppm chloride content.
The air quality control devices or AQCDs consisted of a cold-side ESP and LSFO
wFGD. Mercury CMMs were used to measure mercury at the inlet of the WFGD
and at the stack. A poly-DTC was fed to the wFGD to affect mercury re-emission
control. The results provided in Table 6 below.
Table 6
[00114] As is seen Table 6, prior to addition the EGU was only capturing 38% of
the mercury in the combustion gas due to in part 57% mercury re-emission across
the wFGD. The ORP at this condition is about +473 mV. The addition of the
wFGD additive to control mercury re-emission resulted in a decrease, i.e. a more
reducing potential, in ORP (i.e. +16 mV) and reduction of mercury re-emission to
near zero percent. The overall capture increased to 70%, which matched the total
oxidized mercury in the combustion gas at the inlet to the wFGD. As expected,
upon cessation of addition of the poly-DTC, the ORP increased (i.e. more oxidative
potential), mercury re-emission increased to 64%, and the mercury capture
decreased to 23%. This shows the use of ORP to control the addition of poly-DTC.
Example 7
ORP and Re-emissions and Control
[00115] This example was conducted at an EGU consisting of a 550 MWg boiler
firing sub-bituminous coal. The unit air quality control devices (AQCDs) consist of
an electrostatic precipitator for particulate removal and three wet Flue Gas
Desulfurization (wFGD) scrubbers for SOx removal. The wFGD scrubbers use
sodium-based solutions to capture SOx. In order to increase the flue gas oxidized
mercury concentration, a halogen-based boiler additive such as hypochlorite was
introduced. The results provided in Table 7 below.
Table 7
[00116] This example demonstrates that an "absolute" value of ORP of greater
than +200 mV and up to +2000 mV is not applicable here. At baseline, the solution
ORP is -65 mV with mercury re-emission at 23% and total capture at only 52%.
The addition of a poly-DTC to reduce the ORP to around -212 mV results in near
zero mercury re-emission and mercury capture of 90%. The ORP became more
negative or more reducing compared to baseline.
[00117] While not wanting to be limited by theory, the above observation is
consistent with the removal of ionic mercury from wFGD liquor. The removal of
the ionic mercury reduces potentially oxidizing species from the solution being
monitored. In theory, this should result in a decrease in the solution ORP or a more
negative number. This is indeed observed in the above examples and is unexpected.
The correlation between feed of poly-DTC and ORP or mercury concentration in the
wFGD demonstrates the link that is unexpectedly found in this disclosure.
Example 8
Use of soluble mercury concentration in the scrubber liquor
[00118] Figures l a and lb demonstrate that controlling mercury concentration in a
scrubber liquor is an effective method of controlling mercury re-emission. The
lower the mercury concentration in the scrubber, the lower the mercury re-emission
and hence the lower the unit mercury emissions.
[00119] One method of controlling mercury re-emission includes adding sufficient
mercury re-emission control additive to reduce the soluble mercury level in a basin
liquor to or below 500 ppt, preferably below 200 ppt. An upper limit may be set to
1000 ppt as a starting point. Once the level gets to below this limit, a maintenance
dose may be included that is based on the incoming total mercury in the flue gas or
coal source. The maintenance dose may vary depending on the scrubber retention
time or the negative of the blow down rate. The control ratio may be between 2000
to 1, 1500 to 1, 900 to 1, 500 to 1, 100 to 1, or 50 to 1 weight to weight ratio of
active to mercury total at the inlet.
Example 9
Use of Oxidation-reduction Potential or ORP
[00120] Figures 2a and 2b demonstrate that ORP is an effective, indirect, measure
of mercury re-emission in a wet forced oxidation scrubber. Dose of polydithiocarbamic
compound (e.g., MerControl 8034) can be directly tied to changes in
the ORP. Once the ORP is less than 100 mV, the maintenance feed rate can be
started based on the incoming mercury concentration found in the fuel (e.g., coal).
The ORP value may be different for each scrubber type, amount of forced oxidation,
scrubber efficiency and so on. The higher the delta value (i.e., change in ORP), the
more reducing the liquor may become.
[00121] Figure 2c shows data collected from a commercial 500+ MW coal-fired
electricity generating unit (EGU). The boiler was fired with eastern bituminous
coal. The EGU was equipped with a selective catalytic reduction device (SCR) and
a wet flue gas desulfurizer (wFGD) employing limestone in a forced oxidation
operation. Flue gas mercury concentration and speciation were determined using
continuous mercury monitors (CMMs) located at the inlet and exit of the wFGD.
All measurements were made at full load of the unit and confirmed periodically with
Method 30B carbon traps. The ORP (oxidation-reduction potential) was measured
continuously using commercially available instrumentation and reported relative to a
silver/silver chloride reference electrode at wFGD operating temperature. Percent
mercury re-emission was calculated using the following equation:
[00122] The results show a correlation between ORP and percent mercury reemission
with a correlation coefficient of 0.999. This is a good correlation using
plant data showing the predictive nature of ORP relative to mercury re-emission.
Example 10
Use of Sulfide Concentration
[00123] Data from the laboratory and the field demonstrate the usefulness of a
sulfide ion-selective electrode (ISE) for wFGD additive dosage control for
preventing mercury re-emission and thus lowering overall mercury emissions at
coal-fired power plants. Figure 3 shows data collected in the laboratory in which the
mercury re-emission control additive was added in aliquots to an electrolyte solution
(0.1 M sodium sulfate, Na2S0 4, in deionized water). The responses of an oxidationreduction
potential (ORP) electrode and a sulfide selective ion- selective electrode
(ISE) were compared. It can be seen that both react to the addition of the mercury
re-emission control additive in a similar manner.
[00124] Figure 4 shows data collected in the field in real-time during a wFGD
additive demonstration to control mercury re-emission across the wFGD scrubber.
The solid line data on the lower portion of the graph represents the feed rate of the
wFGD additive and corresponds to the secondary y-axis. The data points shown in
squares on the top portion of the graph correspond to the primary y-axis and
represent the sulfide concentration in the wFGD scrubber liquor as measured by the
in-line sulfide ion-selective electrode. It can be seen that increases of the mercury
re-emission control additive feed lead to increases in sulfide concentration.
[00125] The foregoing examples demonstrate that scrubber liquor ionic mercury
concentration, oxidation-reduction potential, and sulfide ion concentration may be
used individually or collectively to monitor mercury re-emission and adjust the rate
of addition of mercury re-emission control additive to reduce mercury re-emission
or maintain a selected level of mercury re-emission.
[00126] Any ranges given either in absolute terms or in approximate terms are
intended to encompass both, and any definitions used herein are intended to be
clarifying and not limiting. Notwithstanding that the numerical ranges and
parameters setting forth the broad scope of the invention are approximations, the
numerical values set forth in the specific examples are reported as precisely as
possible. Any numerical value, however, inherently contains certain errors
necessarily resulting from the standard deviation found in their respective testing
measurements. Moreover, all ranges disclosed herein are to be understood to
encompass any and all subranges (including all fractional and whole values)
subsumed therein.
[00127] Furthermore, the invention encompasses any and all possible
combinations of some or all of the various embodiments described herein. Any and
all patents, patent applications, scientific papers, and other references cited in this
application, as well as any references cited therein, are hereby incorporated by
reference in their entirety.

We claim:
1. A method for preventing the re-emission of mercury across a flue gas
desulfurization
process comprising:
(a) a polydithiocarbamic compound of one or more polydithiocarbamic
material,
(b) a scrubber liquor,
(c) a scrubber.
2. The method of claim 1 wherein the polydithiocarbamic compound is water
soluble.
3. The method of claim 1 wherein the polydithiocarbamic compound contains 5
to 50 mole
percent of dithiocarbamate salt groups.
4. The method of claim 1 wherein the polydithiocarbamic compound contains
15 to 50 mole percent of dithiocarbamate salt groups.
5. The method of claim 1 wherein the polydithiocarbamic compound contains
25 to 40 mole percent of dithiocarbamate salt groups.
6. The method of claim 1 wherein the polydithiocarbamic compound has a
molecular weight of 500 to 100,000.
7. The method of claim 1 wherein the polydithiocarbamic compound has a
molecular weight of 1,500 to 10,000.
8. The method of claim 1 wherein the polydithiocarbamic compound has a
molecular weight of 1,500 to 5,000.
9. The method of claim 2 wherein the slurry is water based.
10. The method of claim 1 wherein the scrubber is a spray tower system.
11. The method of claim 1 wherein the scrubber is a jet bubblers system.
12. The method of claim 1 wherein the scrubber is a co-current packed tower
system.
13. The method of claim 1 wherein the scrubber is a dry scrubber.
14. The method of claim 1 wherein the polydithiocarbamic compound is added
to the system in a weight ratio of 1:1 to 2000:1 in relation to the mercury
content.
15. The method of claim 1 wherein the polydithiocarbamic compound is added
to the system in a weight ratio of 5:1 to 500:1 in relation to the mercury content.
16. The method of claim 1 wherein the polydithiocarbamic compound is added
to the slurry and then added to the scrubber.
17. The method of claim 1 wherein the polydithiocarbamic compound is added
to the scrubber containing the slurry.
18. The method of claim 1 wherein the polydithiocarbamic compound is added
to a virgin liquor then added to the scrubber.
19. The method of claim 1 wherein the polydithiocarbamic compound is added
to a make-up liquor then added to the scrubber.
20. The method of claim 1 wherein the polydithiocarbamic compound is added
to a return liquor then added to the scrubber.
21. The method of claim 1 wherein the polydithiocarbamic compound is added
to a reclaimed liquor then added to the scrubber.
22. The method of claim 1 wherein the polydithiocarbamic compound is added
to a liquor injected directly into flue gasses then added to the scrubber.
23. The method of claim 1 wherein the polydithiocarbamic compound is added
to a lime slurry then added to the scrubber.
24. The method of claim 1 wherein the polydithiocarbamic compound with
dithiocarbamate salt groups is added to a recirculation loop of the scrubber
liquor.
25. The method of claim 1 wherein the polydithiocarbamic compound with
dithiocarbamate salt groups is added to a low solids return to the scrubber from
the scrubber purge stream.
26. A method for preventing the re-emission of mercury, produced during the
burning
of coal, across a flue gas desulfurization process comprising:
(a) a polydithiocarbamic compound with dithiocarbamate salt groups,
(b) a water based scrubber liquor ,
(c) a scrubber.
27. The method of claim 26 wherein the polydithiocarbamic compound is added
to a virgin liquor then added to the scrubber.
28. The method of claim 26 wherein the polydithiocarbamic compound is added
to a lime slurry then added to the scrubber.
29. The method of claim 26 wherein the polydithiocarbamic compound with
dithiocarbamate salt groups is added to a recirculation loop of the scrubber
liquor.
30. The method of claim 26 wherein the polydithiocarbamic compound with
dithiocarbamate salt groups is added to a low solids return to the scrubber from
the scrubber purge stream.