NOx reduction in a combustion product flue gas stream is greatly improved by the efficient mixing of the reducing agent, e.g., NH3 or an NH3 precursor, with the stream in the absence of a catalyst, such as CaO or CaCO3, that promotes undesirable combination of NH3 and O2 to form additional NOx. By employing a high-temperature cyclone separator of the like and by injecting the reducing agent into the stream within such hot cyclone at a location where there is a strong vortex region, extremely efficient mixing of the NOx reducing agent and the combustion product flue gas is promoted after particulates which are harmful to the reaction that promotes NH3 and NOx reduction have been separated from the flue gases. A refractory-lined cyclone separator may be employed, and a plurality of injectors used which discharge liquid or gas into the strong vortex region.
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