Process for removing nitrogen oxides from exhaust gas streams

A process for removing nitrogen oxides from a combustion gas stream which process comprises: (a) heating an aqueous solution of urea, or a mixture of urea, containing biuret or ammonium carbamate on site, in a hydrolysis reactor such that a gaseous ammonia-containing product is obtained which is essentially free of urea, biuret, or ammonium carbamate, the temperature and pressure being maintained by the input of heat to the reactor; (b) separating the gaseous ammonia-containing product from the liquid phase aqueous reaction media at the operating pressure; (c) retaining the liquid phase reaction medium in the reactor for further conversion to gaseous ammonia and carbon dioxide, and/or recycling at least a portion of the said reaction medium back into the reactor, a urea dissolver, or the feed solution to the reactor for further conversion; (d) withdrawing the gaseous ammonia and carbon dioxide-containing product separated in step (b) at a controlled rate; and (e) contacting the gaseous ammonia-containing product with the combustion gas stream at a rate which essentially matches the demand requirement of nitrogen oxides removal in said combustion gas stream; characterised in that the temperature in the hydrolysis reactor is not controlled but is allowed to vary to match the demand requirement for ammonia and in which the pressure is varied as function of the demand requirement for ammonia or reactor operating temperature.