Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Utility Patent
1997-04-30
2001-01-02
Dunn, Tom (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
Utility Patent
active
06168770
ABSTRACT:
Most combustion systems incorporating combustion engines emit exhausts containing nitrogen oxides. Emission of nitrogen oxides into the atmosphere is a potential danger to the environment that has many aspects, such as excessive fertilization, acidification, direct poisonous effects and secondary formation of poisonous substances, such as photo-chemical oxidants, for example.
The exhausts from combustion systems mostly contain a certain oxygen surplus originating from the combustion stage and they are oxidants by nature, which makes it difficult to reduce the nitrogen oxides by using ordinary reducing agents. However, one has found that it is quite possible to employ various method of what is known as selective reduction of the nitrogen oxides. As the reducing agent is used ammonia, carbamide or urea or other substances that contain nitrogen, such as amines. To simplify, the method could be said to involve reacting positive valence nitrogen in nitrogen oxides with negative valence nitrogen to form nitrogen of zero valence, i.e. harmless nitrogen gas molecules.
To allow such reactions to be utilized does, however, require either medium high temperatures (200-500° C.) and the effects of a stationary catalyst in honeycomb or particle-bed form, a method known as selective catalytic reduction, SCR, or the effect may be achieved without stationary catalysts but in this case considerably higher temperatures (500-900° C.) are required. By analogy, the latter method usually is referred to as selective non-catalytic reduction, SNCR.
Whichever method is adopted, it is necessary to pay attention to and carefully control the amount of reducing agent that is added in order to avoid excessive effluents of such agents, too, which are not either quite harmless. The reducing agents not spent in the reaction with nitrogen oxides will exit together with the gas stream as so called “slip”. Sometimes, particularly in the case of SNCR, it is disadvantageous to have to work at comparatively high temperatures. Heating of large volumes of gas might be very expensive and entail considerable operation costs.
In accordance with the teaching of the subject invention a combustion exchanger (European Patent 218 590) may be used to reduce the nitrogen oxide contents in a gas flow. This eliminates the problem of slip in reducing agents while at the same time the heating costs are kept low.
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Dunn Tom
Heed Bjorn
Orum & Roth
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