Controlling the injection of ammonia in a dry type exhaust gas

Abstract

In a first processor unit the relationship between the amount of nitrogen oxides in a combustion exhaust gas and a numerical value representing an amount of combustion, e.g. - fuel flow rate or the like, is stored. In a second processor unit the ratio of ammonia to nitrogen oxides as a function of the combustion exhaust gas temperature is stored. A numerical value corresponding to the actual measured amount of combustion is inputted to the first processor unit to derive the amount of nitrogen oxides in the combustion exhaust gas. The actual measured combustion exhaust gas temperature is inputted to the second processor unit to derive the ratio of ammonia to nitrogen oxides. The optimum amount of ammonia to be injected is derived by multiplying the amount of nitrogen oxides, derived by means of the first processor unit, by the ratio of ammonia to nitrogen oxides derived by means of the second processor unit. In a preferable embodiment of the present invention, the ratio of ammonia to nitrogen oxides is corrected before the multiplication by means of a correction factor which is stored in a third processor unit and which corresponds to a time variation rate of the combustion exhaust gas temperature.