Method and apparatus for metering a reagent into a flowing medium

An apparatus for metering an ammonia reagent into a flowing medium and for reacting the ammonia reagent with an agent contained in the medium to produce a catalytic conversion of said agent, comprising:

• a) a catalytic converter through which a medium is passed, said catalytic converter including a first catalytically active material in which an agent is reacted with the ammonia reagent to produce a catalytic conversion of said agent;
• b) a metering device for metering the ammonia reagent into the flowing medium upstream of said catalytic converter;
• c) a control unit comprising:
• i) a sensor means for detecting the electrical conductivity within said first catalytically active material having the ammonia adsorbed therein;
• ii) a means for calculating the thickness of the reaction zone from the electrical conductivity measured by said sensor means, the thickness of the reaction zone being defined as a depth at which the ammonia is adsorbed in said catalytically active material;
• and
• iii) a means for controlling said metering device to adjust the quantity of ammonia to be metered into the flowing medium per unit time in response to said calculated thickness of the reaction zone.