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Selective catalytic reduction (SCR) for boilers

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What is selective catalytic reduction?

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).

How does selective catalytic reduction work?

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:

Selective catalytic reduction for boilers formula

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.

What are the advantages of selective catalytic reduction systems?

There are several advantages of selective catalytic reduction systems, including:

  1. SCR is one of the most effective tools available for lowering NOX emissions. Often by 90% or more!
  2. Compared to replacing the old burner completely, SCR offers a relatively simple installation for retrofit applications.
  3. SCR provides high turndown capabilities that typically exceed that of an ultra Low NOX burner.
  4. Custom duct orientations can be modeled to ensure proper installation and performance at any facility.

What are the disadvantages of selective catalytic reduction systems?

On the other side, there are a few disadvantages of SCR systems, such as:

  1. Additional components and systems will need to be controlled as well as properly maintained and tuned.
  2. Expensive catalysts will need to be maintained and replaced for optimal performance.
  3. Additional site regulations required for ammonia handling, storage, and utilization.

Selective catalytic reduction considerations

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:

Anhydrous Ammonia

Anhydrous ammonia is the easiest and most cost-effective reductant to utilize but requires extreme caution in handling and normally requires a permit.

Aqueous Ammonia

Aqueous ammonia is an ammonia-water solution (19 - 29%). It's safer to handle than the anhydrous form but requires a vaporizer in order to properly inject into the flue gas stream. As a result, aqueous ammonia typically has the highest operational cost when factoring in transportation.


Urea is the safest option but tends to have the highest capital costs and consume the most energy due to the complex processing equipment required to extract the ammonia.

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