Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Patent
1991-10-07
1993-12-21
Heller, Gregory A.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
423239, C01B 2100, B01J 800
Patent
active
052719154
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The present invention relates generally to a method and an apparatus for processing nitrogen oxide gas (NO.sub.x gas) More specifically, the present invention relates to a method of reducing NO.sub.x gas contained in exhaust gases discharged from, such as, a diesel engine, a gasoline engine, a gas-turbine engine and other burning fuel systems, such as, a thermoelectric generator, and to an apparatus to be used for performing such a method.
2. Background Art
A NO.sub.x gas reduction processing is known, for example, as a flue gas denitration technique which has been brought into practice. The flue gas denitration technique is roughly classified into a dry process and a wet process In general, the most advanced technique is a selective contact reduction method of the dry process which has the following advantages:
(1) A high denitration rate can be attained.
(2) The NO.sub.x gas is reduced and decomposed into nitrogen (N.sub.2) and water (H.sub.2 O) which are both harmless to human beings. Accordingly, no further processing unit is necessary for further processing the decomposed components before exhausting processed flue gas.
The selective contact reduction method typically utilizes ammonia gas (NH.sub.3), hydrocarbon, or carbon monoxide (CO) as a reducing agent. Since ammonia selectively reacts with NO.sub.x even in the presence of oxygen (O.sub.2) while the other two reducing agents react solely with oxygen, the ammonia gas is generally utilized particularly in case of reducing the NO.sub.x gas contained in the exhaust gas discharged from the diesel, gasoline or gas-turbine engine or the like.
In the above-noted selective contact reduction method, noble metals, such as, platinum or various metallic oxides carried by, such as, aluminum oxide (Al.sub.2 O.sub.3) and titanium dioxide TiO.sub.2, are respectively used as a catalyst in the reaction between the NO.sub.x gas and the ammonia gas. Since most of the NO.sub.x gas contained in the exhaust gas is nitrogen monoxide (NO) gas with only about 5% of NO.sub.2 gas, the main reaction generated in the selective contact reduction method is represented by the following formula, wherein the NO gas is reacted with the ammonia gas on the catalyst so that the NO gas is reduced and decomposed to N.sub.2 and H.sub.2 O:
However, the above-noted selective contact reduction method has the following problems
(1) Ammonia gas which is used for decomposing NO.sub.x is, as is well known, highly toxic and corrosive, and very dangerous to handle.
(2) Capability of the catalyst tends to be degraded with use due to contact with applied ammonia gas as well as with various components contained in the exhaust gas Accordingly, it is required to change the catalyst relatively in a short term, which is troublesome.
(3) The range of working temperature is limited. Specifically, at a high temperature of about 450.degree. C., sintering of the catalyst which follows phase transition in the catalyst is advanced to an undesired degree so that the catalyst is degraded. On the other hand, at a temperature less than about 320.degree. C., ammonia gas and water content react with sulfur oxide (SO.sub.x) contained in the exhaust gas to generate some compounds, such as, ammonium sulfate ((NH.sub.4).sub.2 SO.sub.4) so as to lower the denitration efficiency. Accordingly, in the above-noted selective contact reduction method, the range of working temperature is generally limited to that of 320.degree. to 450.degree. C. In practice, denitration below 320.degree. C. is commonly required, in particular for a co-generation system.
(4) The entire system inevitably becomes large as the catalyst requires a large surface to contact reactants. A ample space is required to handle large volumes of dangerous ammonia gas to match NO.sub.x volume.
DISCLOSURE OF INVENTION
Therefore, it is an object of the present invention to provide a method of processing nitrogen oxide gas that can eliminate the above-noted defects inherent in the background art.
It is another object of the present
REFERENCES:
patent: 5020457 (1991-06-01), Mathur et al.
patent: 5192516 (1993-03-01), Correa
Greenwood N. N. & Earnshaw A.-Chemie der Elemente (Translation of Chemistry of the Elements, Pergamon 1984)-De, Weinheim, VCH Verlagsgesellschaft, 1988-1707 blz.: DE, pp. 553-554.
Asano Yoshihiko
Haba Masanori
Ishizaka Yuji
Syougi Yoshito
Syouzi Fukuzi
Heller Gregory A.
Kabushiki Kaisha Meidensha
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