Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-10-27
2004-01-13
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S393000, C423S402000
Reexamination Certificate
active
06676912
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of pollution control, and, more particularly, to removing nitrogen oxides (NO
X
) from effluent gases.
BACKGROUND OF THE INVENTION
Nitrogen oxides (NO
X
) are criteria air pollutants which are emitted in large quantities from high temperature processing sources, such as fossil-fueled power plants, industrial boilers, waste incinerators, and manufacturing plants for the production of nitric acid, fertilizer, explosives, plastics, cement and metal products, for example. Two major constituents of NO
X
are nitric oxide (NO) and nitrogen dioxide (NO
2
), which are considered to be large contributors to smog, acid rain and other deleterious environmental effects when discharged to the atmosphere. The quantity of NO
X
which may be discharged by a source is (or is expected to be) generally limited by governmental regulations. Because of the environmental concerns posed by air pollution, much research time and money has been expended to develop methods for controlling NO
X
emissions.
The reduction of NO
X
emissions from motor vehicle engines has been relatively successful, using catalytic converters. Improvements resulting from further developments appear to have diminishing benefits and incur high installed costs.
Inasmuch as a large portion of flue gas NO
X
is generated at stationary sources, removal efforts in the United States and elsewhere are now being directed to significantly reduce such NO
X
emissions. Current government enforced emission limits, which are often difficult to meet, are expected to become increasingly more stringent.
Stationary fossil-fuel fired power plants comprise a major source of flue gas emissions which contain both sulfur dioxide (SO
2
) and nitrogen oxides (NO
X
). Currently, emissions of SO
2
are much reduced at many coal-fired power plants by wet scrubbing of the flue gases with an alkaline water stream, but removal of NO
X
by scrubbing is largely unsuccessful. Nitric oxide (NO), a primary constituent of NO
X
, has a very low water solubility and is not amenable to water scrubbing. As shown below in a comparison of values of Henry's constant in water at 25° C., nitrogen dioxide NO
2
has a much greater solubility than nitric oxide NO, and the nitrogen acids HNO
2
and HNO
3
are in turn much more soluble than NO
2
.
Values of Henry's Constant H, atmosphere/mol fraction
N
2
86,400
NO
28,700
NO
2
113
N
2
O
4
0.71
HNO
2
0.02
HNO
3
4.8E
−6
SO
2
44
Because scrubbing of NO
X
from fossil-fuel power plant flue gases is largely ineffective, current NO
X
control methods primarily comprise combustion modifications, e.g. burners which are controlled to either limit the quantity of NO which is formed or reduce NO and NO
2
to elemental nitrogen gas N
2
. Typically, such methods reduce NO
X
emissions by only about one-half, generally far less than is required to meet governmental restrictions. Furthermore, the burners are relatively costly.
A variety of post-combustion NO
X
removal methods which have been used or proposed may be classified as Selective Catalytic Reduction (SCR), Selective Non-Catalytic Reduction (SNCR) or Non-Selective Catalytic Reduction (NSCR). The Selective Catalytic Reduction (SCR) method involves the use of a catalyst system which selectively converts NO
X
to elemental nitrogen N
2
, optionally using an added reductant such as ammonia, urea, etc. Examples of SCR methods are described in U.S. Pat. No. 5,520,895 of Sharma et al., U.S. Pat. No. 5,589,147 of Farnos et al. and U.S. Pat. No. 5,180,567 of Yoshimoto et al.
In a related process described in U.S. Pat. No. 5,489,421 of Van Velzen et al., NO in the flue gas is absorbed in a scrubbing liquor containing FeII-EDTA, desorbed and concentrated by vapor stripping, and catalytically converted to hydroxylamine.
SCR methods are used at only a few major power plant installations, because of very high capital costs and substantial operating expenses. The SNCR and NSCR methods have found little practical application because of low conversion efficiencies. It has been proposed in U.S. Pat. No. 5,120,508 of Jones to convert NO to nitrogen dioxide NO
2
by injecting a peroxyl initiator and oxygen into a flue gas stream, and removing the NO
2
from the treated flue gas with a particulate sorbent. The initiator is any of a great number of materials including (a) compounds containing only carbon and hydrogen, (b) compounds containing only carbon, hydrogen and oxygen, (c) compounds containing only hydrogen and oxygen, and (d) hydrogen H
2
. Test results cited in the reference show NO conversions of up to about 83 percent, using propane as the peroxyl initiator. There is no indication in this reference of required concentrations of other initiators, or their effectiveness. The use of hydrocarbon initiators is expensive and consumptive of natural resources.
There are various references to the use of hydrogen peroxide in removing NO
X
from various source gases. For example, U.S. Pat. Nos. 4,182,278 of Coakwell and 5,647,304 of Nyberg et al. describe methods and apparatus for improving gasoline mileage and reducing emissions of an automobile engine by injecting water and an oxidant such as hydrogen peroxide into the engine's combustion chambers. Engine exhaust gases are passed through a catalytic burner. Similarly, in U.S. Pat. No. 5,863,413 of Caren et al., hydrogen peroxide is partially dissociated into hydroxyl radicals and injected into an automobile engine carburetor, an engine exhaust manifold, or into the catalytic burner.
The addition of hydrogen peroxide to scrubbing liquors for pollutant removal is shown in U.S. Pat. No. 3,733,393 of Couillaud et al. and in U.S. Pat. No. 5,151,258 of Gubanc et al. The Couillaud et al. reference indicates that the incoming scrubbing liquors contain about 41% H
2
O
2
for removal of SO
2
. The effectiveness of NO
X
removal is not indicated. The Gubanc et al. reference also indicates that a high concentration (0.5 to 10% or more) of hydrogen peroxide is added to the scrubbing liquor.
In a similar process described in U.S. Pat. 5,674,459 of Gohara, et al., flue gases are bubbled through diluted (18-20%) hydrogen peroxide containing recycled sulfuric acid and nitric acid. A portion of the liquors is drawn off and treated with limestone to produce gypsum.
In U.S. Pat. No. 5,670,122 of Zamansky et al., hydrogen peroxide or a mixture of hydrogen peroxide and methanol is injected into a flue gas stream. NO is converted to nitrogen dioxide NO
2
which is subsequently reduced to N
2
and removed.
Each of the processes indicated above has severe limitations. Those processes which achieve a relatively high removal of NO
X
have high capital and/or operating costs, making them generally unattractive. Processes with lower total costs do not achieve the desired high removal rates of NO
X
. In the United States, the payment of fines for excessive NO
X
emissions is the norm for operating plants in many industries.
SUMMARY OF THE INVENTION
In view of the foregoing background, it is therefore an object of the invention to provide a method and apparatus for achieving high removals of NO
X
from flue gases of stationary combustion sources and manufacturing plants at lower cost.
This and other objects, features and advantages in accordance with the present invention are provided by a method for removing NO
X
from gas streams emanating from stationary combustion sources and manufacturing plants wherein hydrogen peroxide is injected into the gas stream under conditions which will rapidly oxidize NO
X
species in gas-phase reactions. Nitric oxide NO is rapidly oxidized to nitrogen dioxide NO
2
. NO
2
is further oxidized to nitrous acid HNO
2
and nitric acid HNO
3
. These nitrogen oxyacids are much more water-soluble than nitric oxide NO (and even NO
2
), and may be removed by wet scrubbing of the oxidized gas stream, or by passing the oxidized gas stream through a particulate alkaline material to form a nitrite
itrate salt. For example, electric power plants burning fossil fuels
Clausen, III Christian A.
Collins Michelle M.
Cooper Charles D.
Borda Gary C.
Heald Randall M.
Johnson Edward M.
Mannix John G.
Silverman Stanley S.
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