The post-combustion systems used at power generating plants and factories are somewhat different.

These systems remove nitrogen oxides from the waste gases (flue gases) processes using classified as selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR).

Oxides of nitrogen are also removed by some systems used to extract sulfur dioxide from flue gases.

Selective Catalytic Reduction (SCR)

In this process, the nitrogen oxides in the flue gases are reduced to nitrogen

During this process, only the NO_x species are reduced

NH₃ is used as a reducing gas

The catalyst is a combination of titanium and vanadium oxides. The reactions are given below :

Selective catalytic reduction catalyst is best at around 300 too 400 oC.

Typical efficiencies are around 80 %

Selective Non-catalytic Reduction (SNR)

At higher temperatures (900-1000^oC), NH₃ will reduce NO_x to nitrogen without a catalyst.
At NH₃: NO_x molar ratios 1:1 to 2:1, about 40-60%reduction is obtained.
SNR is cheaper than SCR in terms of operation cost and capital cost.
Tight temperature controls are needed. At lower temperatures, un-reacted ammonia is emitted. At higher temperatures ammonia is oxidized to NO.

Selective noncatalytic reduction systems make use of the tendency of certain compounds, ammonia (NH₃) and urea (H₂NCONH₂) in particular, to react with and reduce oxides of nitrogen:

When either ammonia or urea is injected into flue gases at temperatures of about 870°C–1,100°C for ammonia and 900°C–1,150°C for urea, NO_x removal can reach as high as 50 percent.

Selective catalytic reduction operates on the same principle as SNCR, using ammonia or urea as the working gas along with a catalyst.

A number of catalysts have been used, the most effective of which appear to be oxides of molybdenum, titanium, tungsten, and vanadium as well as zeolites, which are naturally occurring alumina silicates.

Electron Beam Radiation

This treatment process is under development, and is not widely used. Work is underway to determine the feasibility of electron beam radiation for neutralizing hazardous wastes and air toxics.
- Irradiation of flue gases containing NO_x or SO_x produce nitrate and sulfate ions.
- The addition of water and ammonia produces NH₄ NO₃, and (NH₄)₂SO₄
- The solids are removed from the gas, and are sold as fertilizers.