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3132 Wilford Drive, Toledo, Ohio 43617
Phone: 419.841.9984    Fax: 419.841.9535

A Quick Guide to Selective Catalytic Reduction Systems

Nitrogen Oxides abated with Catalysts and Ammonia.


One of the means to abating nitrogen oxides (NOx) from flue gas as a result of steam generation in commercial boilers includes selective catalytic reduction (SCR). Simply, SCR is a mix of an ammonia source – typically anhydrous ammonia or aqueous ammonia – with a flue gas as it enters an SCR chamber before descending over a catalyst bed where the nitrogen oxides react with the ammonia to form nitrogen and water before exiting the chamber harmless. In this fashion, many energy producers from traditional steam generators to bio-mass energy stations are made compliant with pollution standards by use of this precision pollution control equipment.

But how is it done in detail? For those interested in the chemistry of the reaction, the stoichiometry appears as follows for use of ammonia in the reaction:

4NO + 4NH3 + O2 –> 4N2 + 6H20
2NOs + 4NH3 + O2 –> 3N2 + 6H20
NO + NO2 + 2NH3 –> 2N2 + 3H20

As the equations demonstrate, the catalytic reduction of the nitrogen oxides results in harmless exhaust. The destruction rate of the nitrogen oxides can reach up to 95 percent if the reactions are tuned correctly the outflow of NOx in the manufacturing process. Also important is the standard operating temperatures of the reaction, which should stay within the rough range of 675-840 degrees Fahrenheit.

Another factor in the efficiency of pollution control equipment such as the SCR is the quality and type of catalyst components in the catalyst bed. These catalysts vary from ceramic vessels used as a carrier for metals such as titanium oxide, or active catalysts such as oxides of base metals like vanadium and tungsten. Other catalysts include zeolites and varying precious metals. In industrial settings, such as steam generation running at the prescribed temperatures listed above for the ideal reaction, the less expensive base metal oxides will work fine, despite their lower resistance to heat.

But in addition to the standard uses of SCR to combat NOx emissions in the industrial environment, pollution controls for smaller power generation such as diesel motors are also seeing SCR development. Currently, some diesel engines on large container ships are exploring SCR as pollution control equipment, albeit with modifications. In these smaller devices, the operating temperatures are much higher and the outflow of NOx less predictable. This requires a buffer to the SCR chamber to control the flow of NOx, as well as the more expensive zeolite catalyst to function at the higher temperatures.

The small problems not withstanding, there could be a time in the near future where SCR pollution control equipment becomes the standard for diesel engines large and small, effectively eliminating one of the largest sources of smog-causing NOx pollution in the world.

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