U.S. patent application number 09/898737 was filed with the patent office on 2003-01-23 for photo-remediation of no2 plume.
This patent application is currently assigned to Siemens Westinghouse Power Corporation. Invention is credited to Rising, Bruce.
Application Number | 20030015413 09/898737 |
Document ID | / |
Family ID | 25409973 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030015413 |
Kind Code |
A1 |
Rising, Bruce |
January 23, 2003 |
PHOTO-REMEDIATION OF NO2 PLUME
Abstract
An photo-remediation method for reducing a visible NO.sub.2
plume is provided. The method uses an illumination source having a
wavelength of at least 350 nm, and preferably between 350-400 nm to
irradiate gases within an exhaust path, and requires no additional
steps or processing requirements for reduction of the visible
plume.
Inventors: |
Rising, Bruce; (Oviedo,
FL) |
Correspondence
Address: |
Siemens Corporation
Intellectual Property Department
186 Wood Avenue South
Iselin
NJ
08830
US
|
Assignee: |
Siemens Westinghouse Power
Corporation
|
Family ID: |
25409973 |
Appl. No.: |
09/898737 |
Filed: |
July 3, 2001 |
Current U.S.
Class: |
204/157.3 ;
422/186 |
Current CPC
Class: |
B01D 53/56 20130101;
B01D 2259/804 20130101 |
Class at
Publication: |
204/157.3 ;
422/186 |
International
Class: |
B01D 053/00; B01J
019/08 |
Claims
In the claims:
1. A method of reducing NO.sub.2 emissions in an exhaust gas
comprising: exposing the exhaust gas to a visibility-affecting
amount of illumination from an illumination source, wherein the
visibility of the exhaust gas is substantially reduced.
2. The method of claim 1, wherein the illumination source is
located within the exhaust path.
3. The method of claim 1, wherein the illumination source is
optically coupled to the exhaust path.
4. The method of claim 1, wherein the wavelength of said
illumination is at least 350 nm.
5. The method of claim 1, wherein the wavelength of said
illumination is between about 350 nm and 400 nm.
6. The method of claim 1 adapted to treat exhaust gasses produced
by a power generation facility.
7. The method of claim 1, wherein the exhaust gas includes an
amount of NO.sub.2 sufficient to visibly absorb light having a
wavelength between about 350 nm and 400 nm, before said exposure
occurs.
8. The method of claim 1, wherein said method reduces an amount of
NO.sub.2 in said exhaust gas to below about 20 parts per
million.
9. The method of claim 1, wherein said method reduces an amount of
NO.sub.2 in said exhaust gas to below about 10 parts per
million.
10. The method of claim 1, wherein said method minimizes the
formation of pollutants.
11. A photo-remediation method using optical sources and coupling
devices as an emission control technology.
12. A photo-remediation system suitable for reducing NO.sub.2
within an exhaust gas, said system comprising: a source of exhaust
gas containing an amount of NO.sub.2 sufficient to act as a visible
colorant within said gas; at least one illumination source in
optical communication with said exhaust gas, said at least one
illumination source adapted to produce light having a wavelength of
at least about 350 nm; whereby NO.sub.2 within said exhaust gas is
decomposed, thereby reducing the visibility of said colorant.
13. The photo-remediation system of claim 12 wherein: said at least
one illumination source produces light having a wavelength between
about 350 nm to 400 nm.
14. The photo-remediation system of claim 13 further including: at
least one optical fiber connection between said at least one
illumination source and said exhaust gas.
15. The photo-remediation system of claim 14 further including: a
cleaning system to maintain said optical communication between said
at least one illumination source and said gas.
16. The photo-remediation system of claim 15 wherein said cleaning
system includes a purged-air system.
17. The photo-remediation system of claim 12 constructed and
arranged to treat exhaust gasses produced by a power generation
facility.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a method for reducing a visible
NO.sub.2 plume in an exhaust stack of a combustion or "gas" turbine
power plant, other power plants or other combustion systems,
including those in stationary or mobile applications.
[0003] 2. Background Information
[0004] Combustion turbines used in power plants comprise a
compressor section, a combustion section, and a turbine section. A
supply of air is compressed in the compressor section and directed
into the combustion section. Fuel enters the combustion section by
means of a nozzle. The compressed air enters the combustion inlet
and is mixed with the fuel, which is then combusted to produce a
high-temperature, high-pressure gas. This gas then exits past the
combustor section via a transition section and is injected into the
turbine section to run the turbine.
[0005] The hot gases produced by the combustion section exit the
power turbine and pass into a duct designed to convey these gases
either to a heat exchanger (where additional energy is extracted)
or directly to the exhaust stack. These exhaust gases will range in
temperature from 600.degree. F. to 1200.degree. F. depending upon
the type of fuel, the load on the engine, and the ambient
temperature.
[0006] As a result of combustion, NO.sub.x (a mixture of NO and
NO.sub.2) is formed. The NO.sub.x produced by the gas turbine, or
any subsequent post-combustion firing, is released into the
atmosphere unless some control mechanism is used. The NO.sub.2
component is a strong colorant in exhaust gases, and absorbs light
in the blue region, including light in the wavelength between about
350-400 nm, resulting in a plume that appears from yellowish to
orange-red.
[0007] The NO.sub.2 plume is evident in many DLN (dry low NO.sub.x)
combustion systems, and determination of the actual source and
mechanism of formation of NO.sub.2 in DLN systems has proven to be
an intractable problem, limiting options for controlling it.
NO.sub.2 may even be produced in conventional combustion turbine
systems and also from some chemical processing facilities, such as
nitric acid production plants. Control of NO.sub.2 is important
because, among other reasons, concentrations of only 20-30 ppm can
produce a very objectionable plume.
[0008] NO.sub.2 plumes may also be present in other combustion
systems different from the gas turbine. These include pulverized
coal (pc) fired boilers (operating according to the Rankine cycle),
diesel engines, and gasoline engines (operating according to the
Otto cycle). In particular, coal/oil energy systems and diesel
cycle energy conversion systems produce significant NO.sub.x levels
and may have a noticeable plume.
[0009] In gas turbine applications, NO.sub.x emissions are most
often reduced using emission control systems which employ chemical
means, such as ammonia injection in an SCR (selective catalytic
reduction) system. Photometric methods have been studied in coal
combustion where NO.sub.x is found in combination with other
pollutants such as sulfur dioxide (SO.sub.2). SO.sub.2 and NO.sub.x
are combustion products related to fuel burning associated with
heat or power production. Photometric remediation methods include
illumination or irradiation of the exhaust gases prior to
subsequent processing steps, but all require additional steps or
processing parameters during the reduction process in order for
NO.sub.2 to be reduced. Many of the photometric methods reviewed
result in formation of undesirable byproducts such as sulfuric acid
mist, ozone, and particulates.
[0010] For example, U.S. Pat. No. 4,995,955 (Kim et al.) discloses
a process for reducing NO.sub.x contamination within an effluent
stream using an ultraviolet light source having a wavelength of
less than 220 nm. This method requires particle filtration or
electrostatic precipitators to remove particles formed from the
chemical processes used to clean the exhaust gases. Using a
wavelength in this region causes the formation of ozone, a
well-known component of urban smog.
[0011] U.S. Pat. No. 3,869,362 (Machi et al.) discloses a process
for reducing NO.sub.x/SO.sub.2 emissions by controlling the ratio
of NO.sub.x to SO.sub.2 before introducing the gas mixture into an
irradiation chamber. This method requires the presence of SO.sub.2
and also additionally requires collecting chambers to cleanse the
exhaust stream of mist and solid particles.
[0012] U.S. Pat. No. 3,984,296 (Richards) discloses a photochemical
process for removing gaseous pollutant compounds (including
NO.sub.x) from a contaminant gas stream, by introducing positive
and/or negative ions into the flue gas prior to irradiation The
ions form complexes with the pollutants which must be removed by
precipitation or other methods.
[0013] U.S. Pat. No. 4,146,450 (Araki et al.) discloses a method
for reducing or removing NO.sub.2 from exhaust gases containing
NO.sub.x using a catalytic reduction method with ammonia which has
been previously excited by ultraviolet radiation.
[0014] It is desirable, therefore, to provide a method of exhaust
gas NO.sub.2 decomposition without the need for additional
processing requirements such as additional chemicals, temperature
control, particle collection of precipitates, or other processing
steps.
SUMMARY OF THE INVENTION
[0015] Accordingly, the present invention provides a method of
reducing a visible NO.sub.2 plume in an exhaust stack by
irradiating the exhaust gases within the duct or stack with at
least one illumination source. The visible plume is substantially
reduced while minimizing the formation of additional pollutants.
This reduction may result in NO.sub.2 concentrations of below 20-30
ppm, and may represent an NO.sub.2 reduction of 90% or more in some
cases. The reduction process is effective irrespective of the
source of the NO.sub.2 (including the combustion of fuel gas, oil,
or solid fuel). The degree of reduction will be a function of,
among other things, the initial NO.sub.2 concentration, lamp
intensity, and path length.
[0016] The illumination source can be placed directly within the
exhaust duct/stack or optically coupled to these, and has a
wavelength of light of at least 350 nm and preferably between 350
and 400 nm. The method does not require the presence of other
chemicals or additional processing steps to achieve its desired
reduction.
[0017] It is an object of the invention therefore to provide a
method of reducing a visible NO.sub.2 plume within the exhaust.
[0018] It is a further object of the invention to provide a method
of reducing a visible NO.sub.2 plume in exhaust gasses by
irradiating the exhaust stack gasses with a source of illumination
within the stack or optically coupled thereto.
[0019] It is an additional object of the invention to provide such
a method of reducing a visible NO.sub.2 plume without requiring the
presence of other chemicals or other processing steps to achieve
the desired reduction.
[0020] It is a further object of the invention to reduce a visible
NO.sub.2 plume in exhaust without producing undesirable
by-products.
[0021] These and other objects of the invention will be apparent
from the following description and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The invention is further illustrated by the following
non-limiting drawing, in which:
[0023] FIG. 1 is a schematic representation of an exhaust path in a
turbine engine having at least one illumination source to reduce
the visible plume.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0024] The present invention uses photo-remediation methods for
reduction or elimination of a visible NO.sub.2 plume in exhaust
gases emitted from turbine engines, power plants, or any chemical
process which releases NO.sub.x into the environment. More
specifically (and referring now to FIG. 1), visible NO.sub.2
emissions in an exhaust duct 1 of a combustion turbine engine,
having a compressor section 2, a combustion section 3 and a turbine
section 4. The turbine engine can optionally be equipped with a
duct burner or auxiliary burner 6 which is another source of
NO.sub.x. The NO.sub.2 is substantially reduced by
photo-irradiating the exhaust gases 5 with at least one
illumination source 7 located within the exhaust duct and/or stack
or optically coupled thereto. The formation of secondary pollutants
or byproducts (including sulfates and nitrates) is minimized, and
additional processing steps are not required for removal.
[0025] The method takes advantage of low bonding energy that exists
between the NO molecule and the additional oxygen (O) atom. This
bonding energy is low in comparison to other species present in the
exhaust (CO.sub.2, N.sub.2 etc.). A moderately short wavelength
light source is sufficient to decompose the NO.sub.2 compound and
reduce the plume's visual appearance.
[0026] The source of illumination 7 is one or more lamps which can
be located within the exhaust duct and/or stack 1 or optically
coupled to either of these. Mercury vapor lamps are one class of
source that produces the necessary wavelength of light, although
other irradiation sources such as lasers or other high-energy
sources may fulfill this requirement. Cooling and maintenance of
the lamps located within the stack or duct may be required
depending upon the exhaust gas conditions and the location of the
illumination source. Optical coupling of the illumination source to
the stack is an approach that allows placement of the illumination
source in a location outside the duct where it can be easily
maintained, cleaned and replaced as necessary. An optical fiber
network can be used to connect the illumination source with the
duct, with one end of the fiber lead placed at the illumination
source and the other end of the fiber lead placed in the duct or
stack. Preferably, the optical fiber network is fabricated from a
material (such as silica-based fibers) exhibiting minimal losses in
the UV region (about 300-400 nm), and with a high thermal
stability. Optical coupling may also be accomplished through use of
a focusing lens, reflective materials, or similar techniques known
to produce, transmit, and direct visible light. Any method of
optical coupling known to those skilled in the art may be used to
link the illumination source to the duct or stack.
[0027] It may be desirable to use a purged-air system to maintain
the lamps or the optical fiber interface, either on a continuous or
periodic basis.
[0028] The bond dissociation energy of the NO--O bond is 305
KJ/kg-mole. Using the conversion E=1.2.times.E.sup.-4
kJ/mole/lambda, where lambda is in meters, 305 kJ/Mole corresponds
to 393 nm radiation. Thus, a UV source with strong emission between
350 and 400 nm would be an appropriate method of irradiation. Use
of wavelengths between 350-400 nm also decreases undesirable
secondary reactions such as the formation of ozone.
[0029] The method of the present invention can be accomplished at
any temperature, from ambient temperature up to temperatures of
about 1,500.degree. F. As the temperature increases, the wavelength
of the light necessary to initiate the reaction to disassociate
NO.sub.2 into NO and O may be increased (the energy requirement of
the light source is decreased).
[0030] In one embodiment, at least one illumination source 7 is
installed in the exhaust path 1 prior to the gases exiting to the
atmosphere 8. Alternatively, the illumination source 7 may be
located outside the exhaust path and optically coupled to locations
in the exhaust path. A fiber optic cable, not shown, may be used to
provide such an optical coupling. In either arrangement, the
exhaust gases should be in optical communication with the source of
illumination 7.
[0031] Due to the prevention of undesirable secondary reactions and
particulate matter, the need for other processing steps is
advantageously eliminated. Physical methods such as scrubbers,
temperature control, electrostatic precipitators and the like are
unnecessary. It is also unnecessary to add other chemicals which
facilitate precipitation or reduction of the pollutants by other
mechanisms. While there can be some formation of ozone, ozone is
unstable at the temperatures contemplated and is expected to break
down; thus production of ozone is not expected to be a significant
problem. Similarly, recombination of NO and O to form NO.sub.2 is
not expected to be a problem because the concentrations of NO and O
will be very low; thus the likelihood of recombination is also very
low.
[0032] Test results have shown that there is a strong relationship
between the intensity of the illumination source (as measured in
watts) and the decomposition rate of NO.sub.2. Higher intensities
revealed more rapid decomposition of the NO.sub.2: when plotted,
the results show a log-linear relationship between NO.sub.2
concentration and time. Quantum efficiency (the number of photons
required per molecule of NO2 dissociated) peaks at 390 nm;
wavelengths much longer than this will not have sufficient energy
to cause dissociation, while shorter wavelengths will not be as
efficient in causing the dissociation of NO and O. The following
example provides an estimate of lamp size requirements for a common
exhaust, say from a combustion turbine power plant. Using a gas
flow of 25 kg/hour of NO.sub.2 and the above bond dissociation
energy of 305 kJ/kg-mole, 1 25 kg hour .times. kg - mole 46 kg
.times. 305 , 000 Joules kg - mole .times. hr 3 , 600 sec = 46
watts
[0033] A light source having 46 watts of radiative power near 393
nm is required for illumination of an exhaust stack having a flow
rate of 25 kg/hour. Thus, it can be seen that the power required to
accomplish the desired objectives is not significant.
[0034] Additionally, test results show that the rate of
photo-dissociation of NO.sub.2 to NO is temperature sensitive. At
150.degree. C., the rate of dissociation using photometric
techniques is approximately an order of magnitude greater that at
25.degree. C.
[0035] Temperature may also play a role in the suitable wavelength.
For example, the light source appropriate for gases having
temperatures over 25.degree. C. may have a wavelength longer than
400 nm.
[0036] This method is effective to reduce the presence of NO.sub.2
by about 50%-90%, depending upon operational considerations. As a
result, the concentration of NO.sub.2 in exhaust gases may reduced
to below 20 ppm and below 10 ppm, or even less.
[0037] While particular embodiments of this invention have been
described above for purposes of illustration, it will be evident to
those skilled in the art that numerous variations of the details of
the present invention may be made without departing from the
invention as defined in the appended claims.
* * * * *