Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1999-07-02
2001-03-06
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S239100, C423S392000, C423S393000, C423S394200
Reexamination Certificate
active
06197268
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-containing pollutants from gaseous industrial plant effluents. Specifically, the invention relates to the reduction or elimination of nitrogen oxides and ammonia, from industrial waste gas streams.
BACKGROUND OF THE INVENTION
Recent federal and local environmental laws require very significant reduction of discharge of harmful gaseous impurities 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 impurities discharged 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
impurities with ammonia or an ammonia precursor, such as urea, in the absence of a catalytic substance, 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. Pat. Nos. 5,233,934 and 5,453,258, the texts of which are incorporated herein by reference. Unfortunately, SNCR process require very high temperatures, for instance temperatures in the range of about 800 to 120° C., and even at these temperatures 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 significantly lower temperatures than SNCR processes, 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. Pat. Nos. 4,119,703, 4,975,256, 5,482,692, 5, 589,147, 5,612,010 and 5,743,929, the disclosures of which are incorporated herein by reference. 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.
Because of stringent environmental regulations, efforts are continuously made to improve NO
x
removal processes to minimize or eliminate emission of NO
x
and ammonia 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 gas streams by converting the nitrogen oxides to nitrogen by reduction with nitrogen-containing reducing agents, and residual nitrogen oxides and unreacted reducing agent are removed from the gas stream by oxidizing these components to nitric acid with ozone.
According to a broad embodiment, the invention comprises a process for removing at least one nitrogen oxide from a gas stream comprising the steps:
(a) contacting the gas stream with a nitrogen-based reducing agent at a temperature in the range of about 150 to about 1300° C., thereby converting part of the at least one nitrogen oxide to nitrogen and producing a waste gas containing residual nitrogen oxide;
(b) contacting the waste gas with ozone, thereby converting at least part of the residual nitrogen oxide to nitric acid, nitric acid precursors or mixtures thereof; and
(c) contacting the nitric acid, nitric acid precursors or mixtures thereof with an aqueous medium having a pH greater than 7, thereby converting the nitric acid, nitric acid precursors or mixtures thereof to nitrate salt and scrubbing the nitrate salt from the waste gas.
In a preferred embodiment, the nitrogen-based reducing agent is ammonia, urea, melamine, cyanuric acid or mixtures thereof. In a more preferred embodiment, the nitrogen-based reducing agent is ammonia.
In another preferred embodiment, the waste gas contains unreacted nitrogen-based reducing agent and step (c) comprises contacting the waste gas with ozone and the aqueous medium at a temperature in the range of about 25° to about 125° C. in the presence of a catalyst which promotes the oxidation of the nitrogen-based reducing agent, thereby converting at least part of the unreacted nitrogen-based reducing agent to nitrate salt. Step (c) of the above process is preferably carried out at a temperature in the range of about 50 to about 100° C., and is more preferably carried out at a temperature in the range of about 75 to about 95° C.
In a preferred embodiment, the catalyst which promotes the oxidation of the nitrogen-based reducing agent comprises tungsten, palladium, platinum, iridium, rhodium or mixtures thereof.
In a more preferred embodiment, the pH of the aqueous medium with which the waste gas is contacted in step (c) is at least about 9.
In a preferred embodiment, the invention comprises a process comprising the steps:
(a) contacting a gas stream containing nitrogen oxides with ammonia at a temperature in the range of about 150 to about 1300° C., thereby converting part of the nitrogen oxides to nitrogen and producing a waste gas containing residual nitrogen oxides and ammonia;
(b) contacting the waste gas with ozone, thereby converting at least part of the residual nitrogen oxides to nitric acid, nitric acid precursors or mixtures thereof; and
(c) contacting the waste gas with ozone at a temperature in the range of about 25 to about 125° C. in the presence of an aqueous medium having a pH of at least about 9 and a catalyst selected from the group consisting of tungsten, palladium, platinum, iridium, rhodium and mixtures thereof, thereby converting at least part of the ammonia and at least part of the nitric acid, nitric acid precursors or mixtures thereof to nitrate salt and scrubbing the nitrate salt from the waste gas.
In another aspect of any of the above embodiments, steps (b) and (c) are carried out simultaneously, for example in the same vessel.
Step (c) of the above preferred embodiment is preferably carried out at a temperature in the range of about 50 to about 100° C., and is more preferably carried out at a temperature in the range of about 75 to about 95° C.
In one variation of the invention, step (a) is carried out at a temperature in the range of about 600 to about 1300° C. in the absence of a catalyst which promotes the reduction of nitrogen oxides to nitrogen. In another variation, step (a) is carried out at a temperature in the range of about 150 to about 700° C. in the presence of a catalyst which promotes the reduction of nitrogen oxides to nitrogen.
In any of the embodiments of the invention, the aqueous medium with which the waste gas is contacted in step (c) preferably contains ammonium hydroxide, alkali metal hydroxides, alkaline earth metal oxides or mixtures thereof. In more preferred embodiments, the aqueous medium contains sodium hydroxide.
REFERENCES:
patent: 4119703 (1978-10-01), Nishida et al.
patent: 4351811 (1982-09-01), Matsuda
patent:
Anderson Mark H.
Ferrell Robert James
Hwang Shuen-Cheng
Saxena Neeraj
Griffin Steven P.
Medina Maribel
Neida Philip H. Von
Pace Salvatore P.
The BOC Group Inc.
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