Abstract: A method and apparatus for reducing the quantity of pollutant in waste gases, in particular for reducing the quantity of nitrogen oxide in the flue gas of a coal-fired power plant is provided. The process includes the steps of, introducing the waste gas into a catalyzer, to which a quantity of catalytic reduction agent is supplied, measuring the quantity of pollutant at the outlet of the catalyzer, and setting the quantity of pollutant to a setpoint value smaller than a limit value to be complied with, by varying the quantity of reduction agent supplied, wherein the setting of the quantity of pollutant is dynamically regulated as a function of mean values over time, for example half-hour or quarter-hour mean values, of the measured quantity of pollutant determined regularly beforehand.