Selective catalytic reduction is a flue-gas emission control process that converts nitrogen oxides, typically referred to as NOX, into nitrogen (N2) and water (H2O).
Selective catalytic reduction uses a catalyst to assist in creating the desired reaction between the reducing agent and the nitrogen oxides of the flue gas. The most common reductants are anhydrous ammonia, aqueous ammonia, and urea. These reductants are injected into the flue gas stream in such a way to promote a homogenous mixture before coming in contact with the catalyst. As the flue gas and ammonia mixture passes through the catalyst, the following reactions occur to produce the intended result of N2 and H2O:
Many different variations of catalyst materials and configurations exist that each have their own set of advantages and disadvantages and should be selected based upon the operating conditions and site restrictions.
There are several advantages of selective catalytic reduction systems, including:
On the other side, there are a few disadvantages of SCR systems, such as:
There are some factors that must be considered before installing an SCR system. Firstly, the SCR systems function best at higher temperatures - 400°F and above. Keeping this in mind, the specific catalyst manufacturer will determine the desired range. Secondly, watertube boilers are better suited for SCR systems than firetube boilers. The reason for this is attributed to the flue gas temperature ranges and expected NOX output. However, both boilers can benefit from an SCR system when properly designed.
You should also consider the following factors with regards to reductants: