Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1999-08-10
2001-05-15
Dunn, Tom (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S239100, C423S390100, C423S393000, C423S394000
Reexamination Certificate
active
06231824
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the removal of pollutants from gas streams, and more particularly to the reduction or elimination of nitrogen oxides from gaseous industrial plant effluents. Specifically, the invention relates to the reduction or elimination of nitrogen oxides from waste gas streams by oxidizing the nitrogen oxides to nitric acid or precursors thereof.
BACKGROUND OF THE INVENTION
Recent federal and local environmental laws require very significant reduction of discharge of harmful gaseous substances into the atmosphere. Chief among such harmful air pollutants are nitrogen oxides (NO
x
). In response to strict enforcement efforts of these laws industrial air polluters have made considerable efforts to reduce the amount of these harmful substances into the air in gaseous effluents from industrial or municipal sources. Successful efforts to reduce the concentration of NO
x
in gaseous effluents often involve reacting the NO
x
in waste gases with nitrogen-based reducing agents. One commercially used method of reducing NO
x
from gas streams involves contacting the NO
x
with ammonia or an ammonia precursor, such as urea, in the absence of a catalyst, a technique known as selective non-catalytic reduction (SNCR). The ammonia reduces the NO
x
to nitrogen while itself being oxidized to nitrogen and water. Typical SNCR-based processes are disclosed in U.S. Pats. Nos. 5,233,934 and 5,453,258. SNCR processes require very high temperatures, for instance temperatures in the range of about 800 to 1200° C., and even at these temperatures only low conversions of NO
x
are achieved. For example, it is not uncommon to attain NO
x
reductions in the range of 40 to 50% by SNCR-based processes.
Another technique for removing NO
x
from waste gas streams involves contacting the waste gas with ammonia or an ammonia precursor in the presence of a substance which catalyzes the reduction of NO
x
to nitrogen, as in SNCR processes. These catalytic reduction processes are referred to as selective catalytic reduction (SCR). SCR processes have a few advantages over SNCR processes. They can be carried out at temperatures significantly lower than the temperatures at which SNCR processes are carried out. For example, they are quite effective at temperatures in the range of about 250 to 600° C. Typical SCR processes are described in detail in U.S. Pats. Nos. 4,119,703, 4,975,256, 5,482,692, 5,589,147, 5,612,010 and 5,743,929. Although SCR processes are much more efficient than SNCR processes in the reduction of NO
x
to nitrogen, they have the disadvantages of being more costly than SNCR processes, the catalyst can be poisoned or deactivated and often they do not remove all of the NO
x
from the gas stream being treated.
Another disadvantage of both SCR and SNCR processes is that ammonia, which itself is regarded as an environmentally unacceptable pollutant, is often released into the atmosphere in the gaseous effluent from the reactor because the reactions are often conducted in the presence of excess ammonia and/or because of sudden changes in the process that produces the NO
x
. Ammonia may also be released because of depletion or masking of the catalyst by contamination over time.
Another known method of removing NO
x
from gas streams involves contacting the NO
x
with ozone, thereby oxidizing them to higher nitrogen oxides, such as N
2
O
5
and removing the higher oxides from the gas stream by means of aqueous scrubbers.
Specific details of ozone-based NO
x
oxidation processes are disclosed in U.S. Pat. Nos. 5,206,002 and 5,316,737, the disclosures of which are incorporated herein by reference. Ozone-based NO
x
oxidation processes are quite expensive because of the high cost of producing ozone.
It is also known to oxidize NO to NO
2
and remove the NO
2
from gas streams by contacting a gas stream containing one or both of NO and NO
2
with oxygen, particularly in the presence of an NO
2
-selective adsorbent which contains metal cations. The oxygen oxidizes the NO to NO
2
, and the NO
2
is adsorbed from the gas stream by the adsorbent. Typical NO
2
adsorption processes are disclosed in U.S. Pat. Nos. 3,674,429, 4,153,429, 4,160,013, 5,417,950, 5,514,204, 5,670,125 and 5,670,127, the disclosures of which are incorporated herein by reference.
Because of stringent environmental regulations, efforts are continuously made to improve NO
x
removal processes to minimize or eliminate emission of NO
x
into the atmosphere. This invention provides a process which accomplishes this objective.
SUMMARY OF THE INVENTION
The invention provides a process for removing nitrogen oxides from a gas stream by a multi-step process comprising a first step comprising converting NO in the gas stream to NO
2
by contact with oxygen; a second step comprising oxidizing NO
2
and any NO remaining in the gas stream to nitric acid precursors and/or nitric acid with ozone; and a third step comprising scrubbing the nitric acid precursors and/or nitric acid from the gas stream with an aqueous liquid.
According to a broad embodiment, the invention comprises a process for removing nitric oxide from a gas stream comprising the steps:
(a) contacting the gas stream with oxygen at a temperature in the range of about 150 to about 1000° C., thereby oxidizing at least part of the nitric oxide in the gas stream to nitrogen dioxide;
(b) contacting the gas stream with ozone, thereby converting at least part of the nitrogen dioxide to nitric acid, nitric acid precursors or mixtures thereof; and
(c) contacting the nitric acid, nitric acid precursors or mixtures thereof with an aqueous liquid, thereby scrubbing at least part of the nitric acid, nitric acid precursors or mixtures thereof from the gas stream.
In a preferred embodiment of the invention, step (a), above, is carried out in the presence of a catalyst which promotes the oxidation of nitric oxide to nitrogen dioxide. In a more preferred embodiment, the catalyst is a metal cation-exchanged zeolite. In another more preferred embodiment, the zeolite has as exchangeable cations sodium ions, calcium ions or mixtures thereof. In another more preferred embodiment, the zeolite is type A zeolite, type X zeolite, type Y zeolite or mixtures thereof. In another more preferred embodiment the zeolite is 4A zeolite, 5A zeolite, 13X zeolite, 10X zeolite, calcium-exchanged type Y zeolite, sodium-exchanged type Y zeolite or mixtures thereof.
In another preferred embodiment the aqueous liquid used in step (c) has a pH greater than 7 and step (c) comprises converting the nitric acid, nitric acid precursors or mixtures thereof to nitrate salt. In a more preferred embodiment, the pH of the aqueous liquid is at least about 9.
In another preferred embodiment, step (a) of the process is carried out at a temperature in the range of about 200 to about 600° C.
In another preferred embodiment, the aqueous liquid contains ammonium hydroxide, alkali metal hydroxides, alkaline earth metal oxides or mixtures thereof. In a more preferred embodiment, the aqueous liquid contains sodium hydroxide.
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Anderson Mark H.
Hwang Shuen-Cheng
Tseng James K.
Workosky Robert Francis
Dunn Tom
Medina Maribel
Neida Philip H. Von
Pace Salvatore P.
The BOC Group Inc.
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