Lowering of the nitrogen oxide levels in exhaust gases by controlled addition of NH.sub.3

摘要

A process for selective catalytic reduction of nitrogen oxides from exhaust gases, preferably those from diesel engines in vehicles, using pulsed superstoichiometric addition of NH.sub.3 or NH.sub.3 - releasing substances, comprises controlling the pulsed superstoichiometric addition of NH.sub.3 in such a way that, after it has started, the addition is interrupted again only when the amount of NH.sub. 3 stored in the catalyst has reached a specific upper threshold value which is predetermined in accordance with the catalyst properties and the catalyst volume, the amount of NH.sub.3 stored being calculated from the difference between the metered amount of NH.sub.3 and the amount of NO. sub.x separated off, which is determined from the NO.sub.x concentration in the exhaust gas and the average degree of separation, and the addition of NH.sub.3 is resumed only when the amount of NH.sub.3 stored in the catalyst, which is determined in the same way, has reached a predetermined lower threshold value, this pulsed addition of NH.sub.3 being interrupted after a predetermined number of cycles until the amount of NH.sub.3 stored in the catalyst, which is determined in the manner described, has completely reacted, this completing one entire cycle of the pulsed addition of NH. sub.3.