The SCR reactions occur at temperatures of nominally 500–8508F (260–4558C) for catalysts of practical use on coal-fired boilers. At temperatures less than 5008F (2608C) the reactivity decreases and there can be reactions between NH3 and SO3. Above 8508F (4558C) the sintering rates of the catalyst increase, and the reactions can begin to be non-selective with NH3 reacting with O2 [33]. Practical limitations to NOx removal include limitations on mixing of NOx and NH3 ahead of the catalyst, emission of unacceptable levels of NH3 slip, loss of catalyst activity over time, and cost and physical constraints on the amount of catalyst that can be used.