Note: Descriptions are shown in the official language in which they were submitted.
CA 02565750 2006-11-03
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METHOD AND APPARATUS UTILISING HYDROGEN PEROXYDE
TO REDUCE SOX, NOX AND HEAVY METAL EMISSIONS
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to emission control and in particular
to the
control of emissions from combustion sources.
BACKGROUND OF THE INVENTION
Control of emissions from fossil fuel combustions sources addresses a major
environmental problem. The Environmental Protection Agency (EPA) through the
Clean Air
Act regulates the emissions from fossil fuel-fire power plants. Initial
regulations were
focused on oxides-of-nitrogen (NOx) and oxides-of-sulfur (SOx) emissions, but
newer
regulations will include provisions to control heavy metals (Hg, etc.) and
carbon dioxide.
Gas streams from combustion processes are often scrubbed, i.e., contacted with
water or water solutions, to remove many of their contaminants. However, these
scrubbing
processes often produce hazardous waste streams that must be dealt with.
For the reasons stated above, and for other reasons stated below that will
become
apparent to those skilled in the art upon reading and understanding the
present specification,
there is a need in the art for alternative methods and apparatus for treating
combustion gas
streams.
SUMMARY
Methods and apparatus utilizing hydrogen peroxide to reduce SOx, NOx and
mercury or other oxides-of-metal emissions are described herein. Continuous
concentration
of hydrogen peroxide (H202) to levels approaching or exceeding propellant-
grade hydrogen
peroxide facilitates increased system efficiency. By utilizing methods and
apparatus in
accordance with the invention, combustion flue gas streams can be treated for
the removal of
SOX, NOx and heavy metals, while isolating useful by-products streams of
sulfuric acid and
nitric acid as well as solids for the recovery of the heavy metals. The
apparatus is modular
and components can be added or removed depending upon the specific
requirements for a
given removal operation.
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The invention further includes methods and apparatus of varying scope.
BRIEF DESCRIPTION OF TBE DRAWINGS
Figure 1 is a block schematic of an eniission control system in accordance
with an
embodiment of the invention.
Figure 2 is a schematic of a hydrogen peroxide concentration control system
for
use in accordance with one embodiment of the invention.
FIG. 3 is a hydrogen peroxide reaction vessel subsystem for use in accordance
with one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
In the following detailed description of the preferred embodiments, reference
is
made to the accompanying drawings that .form a part hereof, and in which is
shown by way of
illustration specific preferred embodiments in which the inventions may be
practiced. These
embodiments are described in sufficie t detail to enable those skilled in the
art to practice the
invention, and it is to be understood that other embodiments may be utilized
and that logical,
/
mechanical and chemical change,s/may be made without departing from the spirit
and scope
of the present invention. It is noted that the drawings are not to scale
unless a scale is
provided thereon. The following detailed description is, therefore, not to be
taken in a
limiting sense, and the scope of the present invention is defined only by the
appended claims
and equivalents thereof.
Emission control systems in accordance with the invention address
environmental
pollutants SOx, NOx, and heavy metals. Such systems are designed to control
emissions of
these environmental pollutants to the levels established by the EPA. This
emission control
system provides a method based on hydrogen peroxide to reduce the SOx, NOx,
and metal
and metal oxide emissions from combustion sources to acceptable levels as
established by the
Environmental Production Agency. In addition, useful by-product streams of
sulfuric acid,
nitric acid, salts of these acids, and feedstock for oxides-of-metal
production may be isolated.
Figure 1 is a block schematic of an emission control system in accordance with
an
embodiment of the invention. The process starts with a gas stream, such as raw
flue gas 150
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after the particulates have been removed. There are several options for this
design and steps
can be omitted or alternate unit operations may be substituted for the general
processes
depending on the requirements of the installation. These alternate steps are
noted during the
description of the process.
The first step of the process is to use a cooling and wash chamber 102 to cool
and
remove some of the particulates in the entering flue gas 150. Process water
from chamber
103 is provided to chamber 102. Particulates, wash water and heat are sent
from chamber
102 to the sedimentation and cooling Pond 104. Water is then re-circulated
from pond 104
back to water chamber 103.
The washed flue gas is fed from wash chamber 102 to a scrubber tower 105 for
the removal of SOx and/or heavy metals. Scrubber tower 105 uses hydrogen
peroxide from
hydrogen peroxide storage 112 to oxidize sulfurous acid (H2S03) to sulfuric
acid (H2SO4) to
prevent reemission of SO2. Hydrogen peroxide storage 112 preferably provides
aqueous
hydrogen peroxide of approximately 50% to 70% by volume, and more preferably
at
approximately 70% by volume. As the scrubber liquor pH decreases due to the
formation of
sulfuric acid, most of the heavy metal oxides, including Hg, etc., are
dispersed as metal
oxides and/or dissolved hydroxides are converted to sulfates. The remaining un-
dissolved
particulates and insoluble sulfates are removed with centrifuge 106, e.g., a
solid-bowl
centrifuge. Centrifuge 106 continuously removes the solids and circulates the
scrubber liquor
through value 109 back to scrubber tower 105 for continuous scrubbing and
cleaning the flue
gas. When the scrubber liquor (sulfuric acid) reaches the desired
concentration, the cleaned
scrubber liquor is discharged from the centrifuge 106 through valve 110 then
drained to
sulfuric acid storage 111, which can then be utilized for fertilizer or
industrial uses. The
solids from centrifuge 106 are discharged to the recycle solids container 107.
Soluble metals
and metal oxides present in the cleaned scrubber liquor may further be removed
as insoluble
solids upon neutralization of the sulfuric acid as may be performed, for
example, during the
production of fertilizer.
The concentration of hydrogen peroxide in the first scrubbing mixture is
maintained at a predetermined level, e.g., 0.1 to 5 percent by volume.
Concentration of the
first scrubbing mixture may be maintained using a hydrogen peroxide controller
of the type
described below. Additional detail of such a controller may be found in U. S.
Patent No.
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6,039,783 issued March 21, 2000 to Lueck et al. and U.S. Patent No. 6,641,638
issued
November 4, 2003 to Lueck et al.
When the flue gas exits the scrubber 105 it contacts the demister 108, where
the
mist that contains sulfuric acid coalesces. The coalesced mist is returned to
the scrubber 105
and the desulfurized flue gas flows to the neutralizer 118. Residual acid
gases are neutralized
with a base in neutralizer 118, and then the cleaned and neutralized flue gas
exits through the
exhaust stack. Alternately, an exhaust fan could be used in place at the
outlet of the
neutralizer 118, if desired, to boost the clean and neutralized flue gas out
of the exhaust stack
as processed flue gas 152.
When or if removal of NOx emissions is also desired, the neutralized flue gas
from neutralizer 118 flows to NO oxidation tower 113, where nitric oxide (NO)
is oxidized to
nitrogen dioxide (NO2). Alternatively, the flue gas may be passed directly
from the demister
108 to the NO oxidation tower 113 for later neutralization, eliminating
neutralizer 118.
Hydrogen peroxide (1I202) from the peroxide storage 112 may be concentrated in
hydrogen
peroxide concentrator 114 or pumped directly into NO oxidation tower 113. The
concentrator 114 is used to increase the concentration of H202 from 70 percent
to as high as
85 percent or more as required by the process. While a variety of methods are
known for the
concentration of hydrogen peroxide, examples particularly suited for use with
embodiments
of the invention may be found in U.S. Patent Application Serial No.
10/845,607, entitled
"CONCENTRATION OF HYDROGEN PERO)(IDE" and filed May 11, 2004.
Oxidation tower 113 decomposes hydrogen peroxide into oxidative species that
convert NO into NO2. One example of this process, the catalytic decomposition
of hydrogen
peroxide, is described in U. S. Patent No. 6,793,903. The oxidized NO in the
flue gas flows
from the oxidation tower 113 to the NOx scrubber tower 115 where it is
captured in an acidic
hydrogen peroxide scrubber liquor. The concentration of hydrogen peroxide in
the second
scrubbing mixture ranges from 0.1 to 5 percent by volume and is controlled by
a second
hydrogen peroxide controller. The cleaned flue gas that exits from scrubber
tower 115 passes
into neutralizer 119, where base is added to neutralize any residual acid
gases. Once
neutralized, the cleaned flue gas exits through the exhaust stack as processed
flue gas 152.
Alternately, an exhaust fan could be used in place at the outlet of the
Neutralizer 119, if
desired, to boost the clean and neutralized flue gas out of the exhaust stack
as processed flue
gas 152.
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ASTM methods D-1608 and D-1609 have been used to measure the concentration
of NOx in the gas streams in early field tests of the emission control system.
These methods
were later modified to measure the nitrite and nitrate ions directly by ion
chromatography.
However, these methods required off-line analysis that was not practical to
use in a
5 continuous process. One optical method (Fourier transform infrared, FTIR)
was used to
measure the hydrogen peroxide concentration directly. The FTIR analytical
method,
developed to analyze scrubber liquor samples, measured the hydrogen peroxide
concentrations continuously with an attenuated total-reflectance (ATR) cell. A
zinc sulfide
ATR crystal was used for the alkaline solutions and an AMTIR ATR crystal was
used for the
acid solutions. Although this analytical method was useful for laboratory
samples, it was not
selected for the hydrogen peroxide controller.
The analytical system used for the control system, as initially described in
US
Patent No. 6,039,783, contains a unique process that controls the
concentration of hydrogen
peroxide. The process is controlled by a programmable logic controller (PLC)
designed to
measure the concentration of hydrogen peroxide and to add additional hydrogen
peroxide as
required to maintain the preset concentration. In addition to the hydrogen
peroxide
controller, the control system contains two commercial controllers, one for pH
and the other
to measure the concentration of sulfuric acid, nitric acid, or salts of these
acids. These
commercial controllers are used to maintain a target pH or concentration and
to add reagents
as required. Details of the design and operation of the control system are
given in the
following section.
The block flow diagram for a hydrogen peroxide controller suited for use with
embodiments of the invention is shown in Figure 2. The hydrogen peroxide PLC
that
controls the operations starts the first sequence by pulling a sample into the
system from
sample point 231 with pump 232 and pumping it through valve 233. The pH probe
247 and
conductivity probe 248 are exposed to the sample before passing the sample
into metallic
filter 234 where a portion of the sample passes through the filter 234 and the
remainder
bypasses the filter 234 and washes the residues back to sample return 245. The
filtrate that
passed through filter 234 continues through valve 235, multiport valve 236,
and sample loop
237. From sample loop 237 the sample returns to multiport valve 236, and then
exits and
returns back to sample return 245. The sequence of events described above
serves to collect
a sample from the sample point 231, filter the sample, fill the sample loop
that has a known
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volume, and returns the unused sample to sample return 245. While the sample
loop 237 is
being filled, the metering pump 238 is pulls sodium hypochlorite from
container 239 and
injects a known volume into reaction cel1240 through multiport valve 236. The
second
sequence is triggered by the PLC, which sends a signal to rotate multiport
valve 236 and
pump a second quantity of sodium hypochlorite, but this time it is used to
displace the sample
from the sample loop 237, which is pumped into the reaction cell 240. The
reaction of
sodium hypochlorite with hydrogen peroxide produces water, sodium chloride,
and oxygen,
which causes an increase in pressure in the reaction cell that is sensed by
the pressure
transducer 241. Calibration data programmed into the PLC for pressure change
as a function
of hydrogen peroxide concentration is used to control the concentration of
hydrogen peroxide
in the system. If the measured concentration is below the set concentration, a
pump is
activated to transfer hydrogen peroxide from storage. The hydrogen peroxide
pump stops,
when the measured concentration is greater than the set concentration. This
cycle is repeated
continuously to maintain the set concentration of hydrogen peroxide. While the
concentration is being measured, the filter 234 is back-flushed with water 243
through valve
244 to remove particulates captured by the filter 234. When the back-flush
starts, valve 233
closes and pump 232 stops and the water 243 pushes through the filter 234 and
back to the
sample return 245. In addition, the contents of the reaction cell may be
expelled through
valve 242 to sample waste 246 at this time. Once the back-flush is complete,
the PLC returns
the process to the first sequence and sampling process start over again.
Figure 3 shows the details of the reaction ce11240. The reaction cell body 351
has
an internal chamber 352 that is attached to a gauge guard 353 that protects
the pressure
transducer 241. Sodium hypochlorite and the sample are pulled through sodium
hypochlorite
inlet 355 by metering pump 238. Once the reaction in the pressure cell is
complete, it is
discharged through valve 242 to sample waste 246. The reaction cell is
fabricated preferably
from a machinable corrosion-resistant polymer. While the foregoing brief
description of the
control of hydrogen peroxide concentration is included to aid the reader, a
more detailed
description is provided in U.S. Patent No. 6,039,783 and U.S. Patent No.
6,641,638 as noted
previously.
The pH may be controlled with commercially-recognized devices, such as
Rosemont's model 0054pH/ORP-08 pH controller and a model 306P-02010055 pH
probe.
The pH probe is item 247 in Figure 2 and it is used to continuously measure
the pH. The pH
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controller system has proportional algorithms that adjust the pump feed rate
as the pH set-
point is approached. For one embodiment, the pH is controlled to a level of
between 7.0 and
<0.1 by adding a base, e.g., potassium hydroxide. The concentration of
sulfuric acid, nitric
acid, or salts of these acids, is controlled with the Rosemont model 1054B%1-
99 controller.
The conductivity probe model 228-02-21-54-61 is item 248 in Figure 2. Once
calibrated for
the specific ion used in the system, the proportional control algorithms
adjust pumping rate
for the base used to form the salts.
CONCLUSION
Methods and apparatus for controlling emissions have been described. The
methods utilize hydrogen peroxide to reduce SOx and mercury (or other oxides-
of-metal)
emissions. Continuous concentration of hydrogen peroxide to levels approaching
or
exceeding propellant-grade hydrogen peroxide facilitates increased system
efficiency. By
utilizing methods and apparatus in accordance with the invention, combustion
flue gas
streams can be treated for the removal of SOx and oxides-of-metal, while
isolating useful by-
products streams of sulfuric acid and as well as solids for the recovery of
the heavy metals.
The methods and apparatus may also be extended to reduce NOx emissions. One of
the
significant advantages of the present invention is the fact that the process
can be run
continuously, with measuring and adjustments made in real time while the
process is being
performed. Computer monitoring can initiate flow changes of reagents in
response to
automatic measurements to maintain desired process conditions.
Although specific embodiments have been illustrated and described herein, it
will
be appreciated by those of ordinary skill in the art that any arrangement that
is calculated to
achieve the same purpose may be substituted for the specific embodiment shown.
This
application is intended to cover any adaptations or variations of the present
invention.
Therefore, it is manifestly intended that this invention be limited only by
the claims and the
equivalents thereof.
