NASA Technical Reports Server (NTRS) 20080007451: Method for removal of nitrogen oxides from stationary combustion sources

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by  NASA Technical Reports Server (NTRS)

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United States Patent
(12) (io) Patent No.: US 6,676,912 B1
Cooper et al. (45) Date of Patent: Jan. 13,2004
METHOD FOR REMOVAL OF NITROGEN By John M. Kasper, Christian A. Clausen, and C. David
OXIDES FROM STATIONARY COMBUSTION Cooper “Control of Nitrogen Oxide Emissions by Hydrogen
SOURCES Peroxide-Enhanced Gas-Phase Oxidation of Nitric Oxide”
Journal of the Air & Waste Management Association vol. 46,
Inventors: Charles D. Cooper, Maitland, FL (US);
Feb. 1996.
Christian A. Clausen, 111, Chuluota,
FL (US); Michelle M. Collins, Authors: Michelle Collins, David C. Cooper, Christian
Washington, DC (US)
Clausen “a PilotScale Evaluation of A New Technology To
Control Nitrogen Oxide Emissions From Boilers At
Assignee: The United States of America as
Kennedy Space Center” Given at Annual NASA-KSC Part-
represented by the Administrator of
the National Aeronautics & Space ners in Education Conference Oct. 6-8, 1998.
Administration, Washington, DC (US)
By: M. M. Collins, D. D. Cooper, D. A. Clausen, J. D. Dietz,
J. Tenney and D. Bonislawski “A Pilot-Scale Evaluation of
Notice: Subject to any disclaimer, the term of this
a New Technology to Control Nitrogen Oxide (NOx) Emis-
patent is extended or adjusted under 35
U.S.C. 154(b) by 10 days. sions from Stationary Combustion Sources” NASA Techni-
cal Memorandum (Oct. 1998) #207196.
Appl. No.: 09/698,607
By: Michell M. Collins, C. David Cooper, Christian A.
Filed: Oct. 27, 2000 Clausen Loubna M. Tazi, Quang D. Nguyen “New Control
Technology for Nitrogen Oxide Emissions from Stationary
Related U.S. Application Data Combustion Sources” Annual Conference for PNWIS Con-
Provisional application No. 601163,045, filed on Oct. 28, ference (Nov. 1998).
1999.
Int. C1.7 ................................................ CO1B 21/00 By: Michelle M. Collins, C. David Cooper, Christian A.
U.S. C1. ........................ 4231235; 4231393; 4231402 Clausen Loubna A. Tazi, and Quang D. Nguyen, “New
Field of Search ................................. 4231235, 402, Control Technology for Nitrogen Oxide Emissions from
4231393 Stationary Combustion Sources” Paper published at AWMA
Annual Conference (Jun. 1999).
References Cited
By: C. David Cooper “New Control Technology for Nitro-
U.S. PATENT DOCUMENTS gen Oxide Emissions from Stationary Combustion Sources”
3,733,393 A 511973 Couillaud et al. 2nd Annual Partners in Education and Research Conference
3,991,167 A 1111976 Depommier et al. Oct. 5-7, 1999.
4,182,278 A 111980 Coakwell
4,208,386 A 611980 Arand et al. By: Michelle Collins, C. A. Clausen, C. D. Cooper, J. D.
4,213,944 A * 711980 Azuhata et al. ............. 4231235 Dietz J. Tenney and D. Bonislawski “PilotScale Evaluation
4,426,364 A 111984 Cooper et al. of a New Technology to Control NOx Emissions from
4,995,955 A * 211991 Kim et al. ............... 2041157.2
Stationary Combustion Sources” NASA Technical Memo-
5,120,508 A 611992 Jones
randum #208545 (Jul. 1999).
5,151,258 A 911992 Gubanc et al.
5,180,567 A 111993 Yoshimoto et al.
5,230,870 A 711993 Johnson * cited by examiner
5,489,421 A 211996 Velzen et al.
5,520,895 A 511996 Sharma et al.
5,589,147 A 1211996 Farnos et al. Primary ExaminerStanley S. Silverman
5,647,304 A 711997 Nyberg et al.
Assistant Examineradward M. Johnson
5,670,122 A 911997 Zamansky et al.
5,674,459 A 1011997 Gohara et al. (74) Attorney, Agent, or FirmDandall M. Heald; Gary C.
5,863,413 A 111999 Caren et al. Borda; John G. Mannix
6,039,783 A * 312000 Lueck et al. ................ 4221170
(57) ABSTRACT
OTHER PUBLICATIONS
A method for removing NO, from gas streams emanating
Article by Michelle Collins and Pictures by Quang Nguyen
from stationary combustion sources and manufacturing
“KSC Joins Local University in NOx Study” Environmental
plants utilizes the injection of hydrogen peroxide into the
Newsletter Apr. 1997 (NASA).
gas stream for rapid gas-phase oxidation of NO to NO, and
By Jordan M. Haywood and Dr. C. David Cooper “The
water-soluble nitrogen acids HNO, and HNO,. The nitrogen
Economic Feasibility of Using Hydrogen Peroxide for the
acids may be removed from the oxidized gas stream by wet
Enhanced Oxidation and Removal of Nitrogen Oxides from
scrubbing or by contact with a particulate alkaline material
Coal-Fired Power Plant Flue Gases” Technical Paper ISSN
to form a nitritelnitrate salt.
1047-3289 J. Air & Waste Manage. Assoc. 48:238-246 (vol.
48/Mar. 1998) Journal of the Air & Waste Management
Association). 15 Claims, 5 Drawing Sheets
US. Patent
Jan. 13,2004
Sheet 1 of 5 US 6,676,912 B1
I s
4
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ACID
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REMOVAL
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FLUE GAS
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FIG. I
US. Patent
Jan. 13,2004 Sheet 2 of 5 US 6,676,912 B1
TREATED
20
FLUE G’AS
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ACID
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REM0:VAL
APPARATUS
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REACTOR
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H202
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SOURCE
FLUE
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PRODUCT
FLUE GAS
SOURCE
FIG. 2
US. Patent Jan. 13,2004 Sheet 3 of 5 US 6,676,912 B1
28
32
H202
SOURCE
30
b
I
APPARATUS
FIG- 3
US. Patent
Jan. 13,2004 Sheet 4 of 5 US 6,676,912 B1
GAS/L IQUID
68- S E PARATO R
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4 72
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SCRUBBING
1
-- LIQUOR CONTACT
66 70 74
APPARATUS
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FIG. 4
36 '
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86'
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FIG, 5
56'
US. Patent
Jan. 13,2004 Sheet 5 of 5 US 6,676,912 B1
n
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US 6,676,912 B3
1 2
METHOD FOR REMOVAL OF NITROGEN water scrubbing. As shown below in a comparison of values
OXIDES FROM STATIONARY COMBUSTION of Henry's constant in water at 25" C., nitrogen dioxide NO,
SOURCES has a much greater solubility than nitric oxide NO, and the
nitrogen acids HNO, and HNO, are in turn much more
RELATED APPLICATION 5 soluble than NO,.
This application is based upon prior filed copending Values of Henry's Constant H, atmosphereimol fraction
provisional application Serial No. 601163,045 filed Oct. 28,
1999.
N2 86,400
ORIGIN OF INVENTION NO 28,700
NO2 113
The invention described herein was made in the perfor-
N204 0.71
mance of work under a NASA contract and by an employee HNO, 0.02
of the United States Government and is subject to the HNO, 4.8E-6
so2 44
provisions of Public Law 96-517 (35 U.S.C. 9202) and may
be manufactured and used by or for the Government for
Because scrubbing of NO, from fossil-fuel power plant
governmental purposes without the payment of any royalties
flue gases is largely ineffective, current NO, control meth-
thereon or therefore. In accordance with 35 U.S.C. 9202, the
ods primarily comprise combustion modifications, e.g. burn-
contractor elected not to retain title. ''
ers which are controlled to either limit the quantity of NO
which is formed or reduce NO and NO, to elemental
FIELD OF THE INVENTION
nitrogen gas N,. Typically, such methods reduce NO, emis-
The present invention relates generally to the field of sions by only about one-half, generally far less than is
pollution control, and, more particularly, to removing nitro- 2s required to meet governmental restrictions. Furthermore, the
gen oxides (NO,) from effluent gases. burners are relatively costly.
A variety of post-combustion NO, removal methods
BACKGROUND OF THE INVENTION
which have been used or proposed may be classified as
Nitrogen oxides (NO,) are criteria air pollutants which Selective Catalytic Reduction (SCR), Selective Non-
are emitted in large quantities from high temperature pro- 30 Catalytic Reduction (SNCR) or Non-Selective Catalytic
cessing sources, such as fossil-fueled power plants, indus- Reduction (NSCR). The Selective Catalytic Reduction
trial boilers, waste incinerators, and manufacturing plants (SCR) method involves the use of a catalyst system which
for the production of nitric acid, fertilizer, explosives, selectively converts NO, to elemental nitrogen N,, option-
plastics, cement and metal products, for example. Two major 35 ally using an added reductant such as ammonia, urea, etc.
constituents of NO, are nitric oxide (NO) and nitrogen Examples of SCR methods are described in U.S. Pat. No.
dioxide (NO,), which are considered to be large contributors 5,520,895 of Sharma et al., U.S. Pat. No. 5,589,147 of
to smog, acid rain and other deleterious environmental Farnos et al. and U.S. Pat. No. 5,180,567 of Yoshimoto et al.
effects when discharged to the atmosphere. The quantity of In a related process described in U.S. Pat. No. 5,489,421
NO, which may be discharged by a source is (or is expected
40 of Van Velzen et al., NO in the flue gas is absorbed in a
to be) generally limited by governmental regulations.
scrubbing liquor containing FeII-EDTA, desorbed and con-
Because of the environmental concerns posed by air centrated by vapor stripping, and catalytically converted to
pollution, much research time and money has been hydroxylamine.
expended to develop methods for controlling NO, emis-
45 SCR methods are used at only a few major power plant
sions.
installations, because of very high capital costs and substan-
The reduction of NO, emissions from motor vehicle tial operating expenses. The SNCR and NSCR methods have
engines has been relatively successful, using catalytic con- found little practical application because of low conversion
verters. Improvements resulting from further developments efficiencies. It has been proposed in U.S. Pat. No. 5,120,508
appear to have diminishing benefits and incur high installed so of Jones to convert NO to nitrogen dioxide NO, by injecting
costs. a peroxyl initiator and oxygen into a flue gas stream, and
Inasmuch as a large portion of flue gas NO, is generated removing the NO, from the treated flue gas with a particu-
at stationary sources, removal efforts in the United States late sorbent. The initiator is any of a great number of
and elsewhere are now being directed to significantly reduce 5s materials including (a) compounds containing only carbon
such NO, emissions. Current government enforced emis- and hydrogen, (b) compounds containing only carbon,
sion limits, which are often difficult to meet, are expected to hydrogen and oxygen, (c) compounds containing only
become increasingly more stringent. hydrogen and oxygen, and (d) hydrogen H,. Test results
Stationary fossil-fuel fired power plants comprise a major cited in the reference show NO conversions of up to about
source of flue gas emissions which contain both sulfur 6o 83 percent, using propane as the peroxyl initiator. There is
dioxide (SO,) and nitrogen oxides (NO,). Currently, emis- no indication in this reference of required concentrations of
sions of SO, are much reduced at many coal-fired power other initiators, or their effectiveness. The use of hydrocar-
plants by wet scrubbing of the flue gases with an alkaline bon initiators is expensive and consumptive of natural
water stream, but removal of NO, by scrubbing is largely 6s resources.
unsuccessful. Nitric oxide (NO), a primary constituent of There are various references to the use of hydrogen
NO,, has a very low water solubility and is not amenable to peroxide in removing NO, from various source gases. For
US 6,676,912 B3
3 4
example, U.S. Pat. Nos. 4,182,278 of Coakwell and 5,647, Another aspect of the invention relates to an apparatus for
304 of Nyberg et al. describe methods and apparatus for removing NO, from a flue gas stream. The apparatus may
improving gasoline mileage and reducing emissions of an include a reactor for injecting an oxidizing stream of hydro-
automobile engine by injecting water and an oxidant such as ’ gen peroxide (H20,) into the flue gas stream under gas-
hydrogen peroxide into the engine’s combustion chambers. phase reaction conditions at which nitrogen oxides are
Engine exhaust gases are passed through a catalytic burner. oxidized to NO, and at least one of water-soluble nitrogen
Similarly, in U.S. Pat. No. 5,863,413 of Caren et al., oxyacids HNO, and HNO,, and an acid remover down-
hydrogen peroxide is partially dissociated into hydroxyl stream from the reactor for removing the water-soluble
radicals and injected into an automobile engine carburetor, nitrogen oxyacids from the flue gas stream.
an engine exhaust manifold, or into the catalytic burner.
BRIEF DESCRIPTION OF THE DRAWINGS
The addition of hydrogen peroxide to scrubbing liquors
FIG. 1 is a flow chart generally illustrating the steps in a
for pollutant removal is shown in U.S. Pat. No. 3,733,393 of
process for removing NO, from a stationary source flue gas
Couillaud et al. and in U.S. Pat. No. 5,151,258 of Gubanc et
is in accordance with the present invention.
al. The Couillaud et al. reference indicates that the incoming
scrubbing liquors contain about 41% H,O, for removal of FIG. 2 is a schematic block diagram of an apparatus for
removing NO, from a stationary source flue gas in accor-
SO,. The effectiveness of N0,removal is not indicated. The
dance with the present invention.
Gubanc et al. reference also indicates that a high concen-
tration (0.5 to 10% or more) of hydrogen peroxide is added FIG. 3 is a schematic view of a gas-phase reactor as
20
to the scrubbing liquor. shown in FIG. 2 for oxidizing flue gas NO, to NO, and
nitrogen oxyacids.
In a similar process described in U.S. Pat. 5,674,459 of
Gohara, et al., flue gases are bubbled through diluted FIG. 4 is a schematic block diagram of an embodiment of
(18-20%) hydrogen peroxide containing recycled sulfuric oxyacid remover step as used in the apparatus of FIG. 2.
acid and nitric acid. Aportion of the liquors is drawn off and 2s FIG. 5 is a schematic view of an alternative oxyacid
treated with limestone to produce gypsum. remover as may be used in the apparatus of FIG. 2.
In U.S. Pat. No. 5,670,122 of Zamansky et al., hydrogen FIG. 6 is a graphical presentation of controlled variables
peroxide or a mixture of hydrogen peroxide and methanol is and results obtained in tests of the invention described in
injected into a flue gas stream. NO is converted to nitrogen 3o Example 2 of the following specification.
dioxide NO, which is subsequently reduced to N, and
DETAILED DESCRIPTION OF THE
removed. PREFERRED EMBODIMENTS
Each of the processes indicated above has severe limita-
The present invention will now be described more fully
tions. Those processes which achieve a relatively high
3s hereinafter with reference to the accompanying drawings, in
removal of NO, have high capital and/or operating costs,
which preferred embodiments of the invention are shown.
making them generally unattractive. Processes with lower
However, this invention may be embodied in many different
total costs do not achieve the desired high removal rates of
forms and should not be construed as limited to the embodi-
NO,. In the United States, the payment of fines for excessive
ments set forth herein. Rather, these embodiments are pro-
NO, emissions is the norm for operating plants in many
40 vided so that this disclosure will be thorough and complete,
industries.
and will fully convey the scope of the invention to those
SUMMARY OF THE INVENTION skilled in the art. Like numbers refer to like elements
In view of the foregoing background, it is therefore an throughout.
object of the invention to provide a method and apparatus for 4s The term “flue gas” as used herein refers to a gaseous
achieving high removals of NO, from flue gases of station- stream from which it is desired to remove nitrogen oxides
ary combustion sources and manufacturing plants at lower (NO,), from a fixed or stationary source, as opposed to
cost. exhaust gas discharged from an internal combustion engine,
This and other objects, features and advantages in accor- for example. Stationary sources of flue gases containing
dance with the present invention are provided by a method 50 NO, include for example, power plants, boilers,
for removing NO, from gas streams emanating from sta- incinerators, and/or manufacturing facilities in which nitric
tionary combustion sources and manufacturing plants acid is generated or utilized.
wherein hydrogen peroxide is injected into the gas stream In the present invention, nitric oxide NO is reacted with
under conditions which will rapidly oxidize NO, species in ss hydrogen peroxide and its radicals to form NO, which
gas-phase reactions. Nitric oxide NO is rapidly oxidized to further reacts to form nitrogen oxyacids. Then, NO, and the
nitrogen dioxide NO,. NO, is further oxidized to nitrous oxyacids, e.g. nitrous acid HNO, and nitric acid HNO,, may
acid HNO, and nitric acid HNO,. These nitrogen oxyacids be readily removed because of their high water solubility or
are much more water-soluble than nitric oxide NO (and even their high reactivity with alkaline compounds to form salts.
NO,), and may be removed by wet scrubbing of the oxidized 6o Depending on the specific conditions, H,O, may decom-
gas stream, or by passing the oxidized gas stream through a pose to (a) OH and OH, (b) HO, +H, (c) H,O+O,, or (d)
particulate alkaline material to form a nitritehitrate salt. For H,O+HO,. In the present invention, it is beneficial to
example, electric power plants burning fossil fuels and using provide conditions such that H,O, largely decomposes to
wet scrubbing to remove SO, from the flue gas may be 6s hydroxyl radical OH.
retrofitted so that enhanced simultaneous removal of NO, In NO, treatment, a large number of chemical reactions
and SO, is achieved. may be hypothesized. In the present invention, the general
US 6,676,912 B3
5 6
reactions leading to NO, oxidation are believed to include H,O, from a H,O, source 32 is illustratively injected into
the following: the flue gas 22 i.e. into the reactor 28, in a manner which
results in rapid effective gas-phase mixing and reaction as
- will be appreciated by those skilled in the art. The hydrogen
- peroxide from the source 32 is typically a water solution,
H,O, 20-H (Reaction 1)
H,O, + OH- HO, + H,O (Reaction 2) and may be injected into the reactor 28 as a liquid stream 30
NO + HO, - NO, + OH (Reaction 3) which rapidly evaporates upon injection. For example, the
NO, + OH HNO, (Reaction 4)
NO + OH HNO, (Reaction 5) H,O, concentration may be less than about 70% by weight,
and equal to or greater than about 35% as is a typical
commercially available concentration, depending on local
At temperatures above about 400" C., reaction kinetics
economic and safety considerations.
and equilibrium favors the production of NO, and HNO,. At
lower temperatures, the conversion of NO is somewhat The concentration of the H,O, in the stream 30 may be
lower. adjusted to achieve the desired H,O,/NO, ratio at a volu-
15
Referring now to the flowchart of FIG. 1, the steps of the metric injection rate which produces rapid distribution and
process for treating flue gas are now described. From the contact of the H,O, stream with the flue gas 22.
start (Block 6) the stationary source is operated, generating Alternatively, the hydrogen peroxide stream 30 may be
the flue gas at Block 8. General sequential treatment steps injected as an atomized stream or be first pre-vaporized in
exemplary of a method of the present invention include: flue 20 pre-vaporization apparatus such as is known in the art and
gas introduction (Block 10) comprising passing a flue gas described in greater detail below in an alternate embodi-
stream from the stationary source to a hydrogen peroxide ment.
injection step at Block 12 wherein a stream of H,O, is
The molar ratio of H,O, to NO, required in the oxidation
injected into the flue gas. At Block 14 a reaction is per-
25 reaction may vary from about 0.5 to about 3.0 or more, for
formed in which the stream of H,O, is intimately mixed
reaction temperatures of about 40Ck650" C. At lower reac-
with the flue gas and reacted therewith. Accordingly, a major
tion temperatures, the H,O,/NO, ratio may be somewhat
portion of the NO is oxidized to NO, and the nitrogen acids.
higher, with a maximum value of about 8.0.
The method further illustratively includes acid removal
(Block 16) wherein NO, and nitrogen acids are separated The minimum reaction time in the reactor 28 to attain
and removed from the flue gas. Thereafter the flue gas is 30 adequate completion of the reaction is relatively short,
discharged having a low NO, content, at Block 18, before typically in a range from about 0.1 seconds to about 3
stopping at Block 19. seconds at 400 to 650" C., and from about 0.1 seconds to
Combustion of fossil fuels, e.g. coal or oil may lead to somewhat longer at lower reaction temperatures, of up to
large quantities of sulfur dioxide SO, in the flue gases. The 35 about 5 seconds.
SO, may be simultaneously removed as H,SO, together Prior to injection of the hydrogen peroxide stream 30, the
with the nitrogen acids in the acid removal step (Block 16). flue gas temperature may be adjusted to a desired reaction
Optionally, the flue gas may be first subjected to a flue gas temperature ranging from ambient, i.e. about 20" C., to
desulfurization (FGD) processing at Block 9 prior to the flue about 650" C., depending upon the NO, concentration and
gas introduction (Block lo), whereby a large portion of the 40 concentration of other constituents, such as SO,, in the flue
sulfur dioxide SO, is removed before treatment to remove gas stream. For most power plant applications, the flue gases
the NO,. Various FGD systems are well known in the art and are hot, and a reaction temperature of about 300" C. to about
typically include scrubbing of the flue gas with an alkaline 600" C. is preferred. A more preferred reaction temperature
liquor. 45 may typically be from about 450" C. to about 550" C. For
An exemplary embodiment of the apparatus 20in accor- flue gases from sources where the flue gas temperature is
dance with the present invention is further explained with lower, the steady-state NO conversion may be lower.
reference to FIG. 2. A flue gas 22 containing NO, is As depicted in FIG. 3, the reactor 28 may be a separate
generated by a stationary source 21, such as a fossil-fuel item of equipment specially constructed, or it may simply
so
fired power plant or boiler. The flue gas 22 from stationary comprise a portion 60 of an existing exhaust gas flue 52,
source 21 may optionally be first passed through flue gas modified by installation of H,O, injection apparatus 42. The
desulfurization (FGD) stage or apparatus 24 to convert SO, injection apparatus 42 may comprise one or more nozzles 44
to an FGD product 25, e.g. sulfuric acid H,SO, or gypsum spaced within a cross-section of the reactor 28 to uniformly
CaSO,, which is separated from the flue gas. 5s disperse the H,O, stream 30 from an H,O, source 32 into
As indicated above, in an alternative arrangement, sulfur the incoming flue gas 22. The velocity of the H,O, stream
dioxide (SO,) in the flue gas 22 may be converted to sulfuric 30 at the nozzles 44 should be sufficient to achieve rapid
acid simultaneously with NO, oxidation in a reaction step intimate mixing in the reactor 28. The injection nozzles must
and reaction products of both sulfur and nitrogen removed serve at least three functions. First, the liquid hydrogen
together from the oxidized flue gas stream 56 in a subse- 6o peroxide must be kept cool enough while inside the nozzle
quent acid removal apparatus 36. In any case, the flue gas 22, that it does not decompose before being sprayed into the flue
with or without SO,, is passed to reactor 28 for NO, gas stream 22 within the reactor 28. Therefore the nozzle
conversion to nitrogen acids before ultimately being exterior must be insulated from the hot flue gas. The nozzle
released as the treated flue gas 40. 65 may also be cooled, for example, to about 100" C. Second,
To achieve the desired conversion of NO to NO, and the nozzles must be placed in the proper location within the
nitrogen acids, a stream 30 including hydrogen peroxide flue gas so that the temperature is right for the desired
US 6,676,912 B3
7 8
reactions to occur. The optimum temperature for the desired The scrubbing liquor 70 may be simply water 72 that may
reactions is about 930" F., and temperature can vary with or may not be recycled, or may include an alkaline material
position in the flue gas stream (both radially and 74 to control the liquor at a particular pH value, for example
longitudinally). The nozzles must be placed so that the spray at a value within a range of pH 4 to pH 10. The pH may be
of peroxide droplets does not impinge on the walls, and is 5 adjusted with any suitable alkaline material 74, e.g. sodium
not in a zone of gases that is too hot nor too cold. Third, the hydroxide, calcium hydroxide, or potassium hydroxide,
nozzles must eject the hydrogen peroxide so that it forms depending upon the intended disposition of the final product
tiny droplets that can evaporate rapidly when exposed to the as will be readily appreciated by those skilled in the art.
hot flue gas stream, and allow the evaporated vapors to mix
10 Asecond embodiment of an acid removal apparatus 36' is
thoroughly with the flue gas. This atomization may be
illustrated in FIG. 5, in which the oxidized flue gases 56' are
caused by an ultrasonic means (as was used in Example 2,
contacted with particulate alkaline materials 84' which
described several pages following) or by a more traditional
absorb the nitrogen acids. For example, the alkaline mate-
means as explained in the next paragraph.
rials 84' may be supported on fabric in a conventional
15
The nozzles 44 may be configured to atomize the stream
"baghouse" 86' and react with the nitrogen acids to form
30 comprising a solution of H,O, in water, using
nitrate and nitrite salts.
mechanical, pressure, steam-assisted or air-assisted atomi-
zation. Alternatively, pre-vaporized H,O, generated by the It should be noted that where the incoming flue gas
pre-vaporization apparatus 58 may be injected. The dimen- 20 contains SO,, the resulting oxidized liquors 48 of FIG. 4 will
sions of the reactor 28 should provide the desired residence also contain sulfuric acid and/or its reaction products.
time to complete the nitrogen oxidation and transport the Likewise, the salts of both sulfur and nitrogen will form in
oxidized flue gas 56 to the acid removal apparatus 36. a baghouse operation.
In one embodiment, the reactor 28 may be configured to
expose the gaseous reaction mixture 62 to ultraviolet (UV) 2s EXAMPLE 1
radiation. Agroup of UV lamps 46 may be spaced across the
Asmall scale test apparatus included a narrow bore quartz
reactor cross-section to provide UV radiation to the reaction
tube reactor enclosed in an electrically heated oven. The
mixture 62 within the reactor 28. Use of broad-spectrum UV
reactor was approximately 2.5 meters long and had an inside
radiation has been found to stimulate the NO oxidation 30
diameter of about 6 mm. A stream of air augmented with
reactions and may be particularly useful where the degree of
nitric oxide (NO) was passed through the tube reactor. The
oxidation is otherwise less than desired, e.g where the
system was designed so that H,O,, water, or H,O, and water
temperature of the flue gas 22 is at a lower value, e.g. from
could be injected into the heated air stream to test the effects
ambient to about 400" C. When the reactor 28 is operating
3s on conversion of NO to NO, and/or HNO, and/or HNO,.
at elevated temperatures, the UV lamps 46 may be enclosed
Gases exiting from the reactor were diluted with room
within cooling passages and air cooled by a stream 64 of air
temperature air to quench the reactions, and then were
blown over the lamps.
analyzed for NO, NO,, HNO,, and HNO,.
As shown in FIG. 2, the nitrogen acids and a portion of the
residual NO, are removed from the oxidized flue gas 56 by 40 In a first series of tests, the effect of reaction temperature
an acid removal apparatus 36. Various acid removal pro- upon overall NO conversion was evaluated. H,O, was
injected through a hypodermic needle into the incoming gas
cesses are known, but two particular examples will be
containing 500 ppm NO,, achieving an H,O, concentration
discussed herein.
of 1200 ppm. The results were as follows:
In a first acid removal embodiment, depicted in FIG. 4,
4s
the stream of oxidized flue gases 56 is passed to a contact or
scrubber apparatus 66 where the oxidized flue gases are
intimately contacted with an aqueous scrubbing liquor 70 Temp. ' C. % Oxidation of NO
into which the NO, and nitrogen acids become dissolved. so 300 24
The scrubbed flue gases and scrubbing liquor 80 containing 350 29
400 39
nitrogen acids are then separated in a gasiliquid separator 68. 450 80
The contact apparatus 66 and separator 68 of FIG. 4 together 500 98
550 98
comprise the acid removal apparatus 36 shown in FIG. 2, 600 93
and may be a conventional wet scrubber apparatus as known 55 650 89
700 79
in the art. The wet scrubber may comprise a plurality of
stages, with countercurrent operation.
In a second series of tests, sampling indicated that the
The separator 68 may also include apparatus for removing
any remaining liquid droplets from the scrubbed oxidized 60 maximum conversion of NO under the particular reactor
conditions was complete in about 0.3 seconds.
separated flue gases before they are discharged as cleaned
flue gas 40 to the atmosphere or directed to a further In a third series of tests, the effect of hydrogen peroxide
treatment step. As shown in FIG. 4, the separated liquors 82 concentration upon NO conversion was evaluated, all tests
may be totally discharged as stream 48, or a portion 76 may conducted at 500" C. (932" F.) and a reactor residence time
65
be recycled to the contact apparatus 66 to become part of the of 0.7 seconds. The operating conditions and species con-
scrubbing liquor 70. centrations in parts per million (ppm) were as follows: