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When SCR or catalyst is used for NOx control,it has to be located in an optimum gas temperature zone in the boiler or HRSG.Typically for most catalysts, 600 to 800 F is a good starting point, though there are catalysts which can operate efficiently around 450-500 F. CO catalysts can operate at higher temperatures;a higher gas temperature reduces the size or amount of catalyst due to the higher efficiency of conversion or activity,however oxidation concerns and catalyst life have to be considered,while low gas temperatures may require a larger size and hence resulting in more cost and higher gas pressure drop across the catalyst. Very low gas temperatures,in the region of 450 F or below, may be a concern when oxides of sulfur are present in the flue gas stream. There is a potential for formation of ammonium sulfate at low gas temperatures(below 450 F typically),when excess ammonia interacts with sulfur trioxide.These particles when deposited on downstream heating surfaces can affect heat transfer process,particularly when extended surfaces are used.
It is important to evaluate the boiler or HRSG performance at various modes of operation when a catalyst is used and ensure that the gas temperatures are within limits set by the catalyst supplier. Charts below give an idea of the boiler or HRSG performance under different loads. Note the variations in gas temperatures at different locations.Figure below show the temperature profiles in a package boiler firing gas at various loads and the bottom figure shows the temperature profiles in a fired HRSG at various steam generation rates. Suitable location for the SCR is obtained by splitting up the HRSG evaporator section or by using a gas bypass system in a package boiler to ensure a reasonable gas temperature at various loads. For the example below,it appears that if a SCR is located between the evaporator and economizer,the gas temperature is reasonable from 100 % to 50 % load,below which gas bypass system must be operated to raise the gas temperature.By splitting up the evaporator of the HRSG below into two portions,a suitable gas temperature range may be arrived at for the SCR.The two evaporators connect with the same drum and have a common downcomer,riser system.

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