Fremgangsmåde og indretning til dosering af en reaktant i et strømningsmedium

To achieve high degrees of separation of nitrogen oxides which are contained in the exhaust gas of diesel or lean-burn engines, in accordance with the SCR method and with simultaneous suppression of the leakage of ammonia, a method for metering a reagent (26), for example ammonia, into the flow medium (10) is provided in which a reagent quantity to be metered per unit time into the flow medium (10) is set as a function of the thickness ( delta a, delta b, delta c) of a reaction zone in a catalytically active material (16), especially proceeding from a desired value ( delta b) for the thickness. In this arrangement, the reagent (26), following adsorption on the catalytically active material (16, 16a), is converted together with at least one further reagent of the flow medium (10). In addition, the thickness ( delta a, delta b, delta c) of the reaction zone is defined as the depth in the catalytically active material at which the variation of the concentration of the reagent (26) adsorbed on the catalytically active material (16, 16a) exhibits a local minimum. The invention is in principle suitable for all catalytic conversions of reagents of a liquid or gaseous flow medium (10), of which at least one is present adsorbed on the catalytic converter (16).