A commonly used method to reduce the emissions of NOx in CFBC boilers is to add NH3 (SNCR) in the cyclone inlet. However, experience shows that the conversion of NOx is sometimes ineffective, In this work, it was found that under conditions in which combustion occurs in the cyclone, the reduction of NOx could be improved by at least 30% by shifting the position of ammonia addition to the inner vortex of the cyclone. Under conditions in which no combustion occurs in the cyclone, the CO emission was found to be decreased by 60% by taking the same measures, without any significant change in NOx reduction and NH3 slip. The decrease in CO is explained by competition for OH radicals between NH3 decomposition and CO oxidation. The position for ammonia addition in the cyclone was found from laboratory experiments of the mixing characteristics of such cyclones and modelling of the SNCR reaction under well-stirred conditions.