Abstract: Nitrogen oxide contained in the flue-gas from a heating unit is removed in a nitrogen oxide removal unit by injection of nitrogen oxide removing agent in an amount determined by the amount of the nitrogen oxide contained in the flue-gas. Such amount of nitrogen oxide contained in the flue-gas is detected, on the one hand, by the estimation obtained through calculation using factors such as rate of supply of fuel to the heating unit and concentration of oxygen in the flue-gas, and on the other hand, by direct measurement, for a fail-safe system.

Abstract: Method and apparatus for control of input to an apparatus for removal of nitrogen oxides from flue-gas by injection of a nitrogen oxide reducing agent according to which the level of nitrogen oxide concentration in the flue-gas can be selectively determined in reference to different factors relating to a combustion process, a first value indicating the amount of nitrogen oxide reducing agent to be injected is calculated on the basis of the determined level of nitrogen oxide concentration, and this first value is modified in accordance with nitrogen oxide removal efficiency actually achieved, and the rate of supply of the nitrogen oxide reducing agent is adjusted in accordance with the modified value obtained, whereby more precise and reliable input control is achieved.