Gas purification techniques

Selective Catalytic reduction (SCR)

The optimum process temperature lies in the area 320 - 500 °C dependent on the catalyst. Mostly used as a catalyst are the oxides of vanadium , tungsten, molybdenum or other metals, with titanium dioxide as carrier material. Optimum mixture for the catalyst bed and a molar proportion NH₃/NO_x lower than 1.1 are the most important process parameters to run the reaction as smoothly as possible and this way avoid undesirable ammonia emission ("NH₃-silt"). The life span of the catalyst is mainly stipulated by erosion caused by the fly ash, deactivation by poisoning (NH₄HSO₄), deposition and clogging of the catalyst. Special attention at start up and shut-down must be given to the installation. It is important that the NH₃-injection is stopped when the temperature decreases below a certain value, to prevent that ammoniumhydrogensulfate itself deposit on the catalyst.

SCR-installation can be set in series directly behind the kettle ("high-dust" - connection ). A second possibility is setting the SCR-unit in series behind substance filters or scrubber (low-dust"-connection ). Reheating from the smoke gases to the response level is necessary.
The advantages of SCR are:

• Higher NO_x-reduction output then SNCR

• Relatively simple installation

The disadvantages are:

• At relatively high SO₃-level must be carried out the process at
  relatively high temperature to avoid condensation

• At "high-dust" connection the fly ashes are full with NH₃.

• Expensive catalyst

• Possibility on constipation and poisoning of the catalyst

• At low-dust"-connection a reheating of the industrial gasses has been
  required

• Possible erosion of the catalyst by the Fly ash

• Higher investment costs then SNCR

Selective catalytic reduction is applied at combustion installations
in following sectors:

• Detritus combustion

• Energy call centers

• Metallurgy

• Glass construction and horticulture

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