Selective Catalytic Reduction System (SCR)

What is SCR (Selective Catalytic Reduction)?

The selective catalytic reduction (SCR) removes nitrogen oxides (NOx) from flue gas emitted by power plant boilers and other combustion sources.

The location of the system within the flue gas cleaning process depends on the type of fuel involved.

In thermal power plant SCR is located between economizer and air-preheater.

Basic Working Principle of Selective Catalytic Reduction

WHAT IS SCR (selective catalytic reduction)

The flue gas entering the reactor passes through the ammonia injection grid where ammonia is injected into the flue gas. The mixture passes through the catalyst layer where conversion of the NOx into N2 and H2O take place.

Chemical Reactions

  • 4NO + 4NH3 + O2 → 4N2 + 6H2O
  • 6NO2 + 8NH3 → 7N2 + 12H2O

Application of SCR

  • Power plants
  • Energy from waste plants
  • Industrial Plants (Steel industry, Oil Industry, Pulp and paper industry)


Flue-gas desulfurization (FGD)