The invention relates to a method for substantially decreasing nitrogen oxides (NO.sub.x) in waste furnace gases during the combustion of fossil fuels.
To decrease the emission of nitrogen oxides (NO.sub.x) in the waste gases from power plant and industrial furnaces, furnace or primary measures or waste gas purification or secondary measures can be taken, such as are evident, for example, from "Technische Mitteilungen" (Technical Communications), volume 80, No. 9, November/December 1987, pages 566 to 572.
The furnace or primary measures for decreasing nitrogen oxide emission, however, at present have limits, where the formation of other undesirable polluting gas components, especially carbon monoxide and hydrocarbons, occurs to an increased extent. For this reason, the emission of nitrogen oxides from power plant and industrial furnaces is generally decreased by NO.sub.x waste gas purification, that is, by secondary measures, as can be understood for example, from the German Offenlegungsschriften 3,335,917, 3,406,657 and 3,431,961.
In nitrogen oxide emission reduction by NO.sub.x waste gas purification, the power plant and industrial furnaces are operated so that only the least possible amounts of polluting gas components, particularly carbon monoxide and hydrocarbons, are discharged from them together with the waste gas. This is accomplished, moreover, independently of the proportions of nitrogen oxides, which are formed in the furnaces and which are composed of combustion nitrogen oxides (thermal NO.sub.x) and fuel nitrogen oxides (fuel NO.sub.x).
To reduce the emission of the relatively high proportion of nitrogen oxides (NO.sub.x) in the waste gases leaving power plant and industrial furnaces, these gases are then passed for selective catalytic reduction (SCR method) through or over catalysts. With the help of these catalysts and the addition of ammonia, the nitrogen oxides are converted to nitrogen and water and removed or eliminated with the waste gases. Depending on the design of the catalysts, NO.sub.x conversions of 70 to 90% can be attained. In any case, however, the catalysts require an operating temperature of 300.degree. C. and work best only at waste gas temperatures above 350.degree. C.