Selective non-catalytic reduction

From Wikipedia, the free encyclopedia

Selective Non Catalytic Reduction (SNCR) is a method for reducing nitrogen oxide emissions in conventional power plants that burn biomass, waste and coal. The process involves injecting either ammonia or urea into the firebox of the boiler at a location where the flue gas is between 1600 °F and 2100 °F (870 °C and 1150 °C) to react with the nitrogen oxides formed in the combustion process. The resulting product of the chemical reaction is elemental nitrogen (N₂), carbon dioxide (CO₂), and water (H₂O).

Urea (NH₂CONH₂) is easier to handle and store than the more dangerous ammonia (NH₃). In the process it reacts like ammonia:

NH₂CONH₂ + H₂O -> NH₃ + CO₂

The reduction happens according to (simplified)^[1]

4 NO + 4 NH₃ + O₂ -> 4 N₂ + 6 H₂O

The reaction mechanism itself involves NH₂ radicals that attach to NO and then decompose.

The reaction needs a certain minimum temperature to happen, otherwise the NO and the ammonia don't react. Ammonia that hasn't reacted is called ammonia slip and is undesirable.

At too high temperatures ammonia decomposes:

4 NH₃ + 5 O₂ -> 4 NO + 6 H₂O

In that case NO is formed instead of reduced. Thus the reaction needs a specific temperature window to be efficient. The reaction also needs sufficient reaction time in that temperature window.

A further complication is mixing. Generally more NO will form in the center and less near the walls, as the walls are cooler than the center. Thus optimally more ammonia must find its way to the center and less near the walls, otherwise NO in the center meets insufficient ammonia for reduction and excess ammonia near the walls slips through.

Though in theory selective non-catalytic reduction can achieve the same efficiency of about 90 % as selective catalytic reduction, these practical constraints of temperature, time, and mixing often lead to worse results in practice. However, selective non-catalytic reduction has an economical advantage over selective catalytic reduction, as the cost of the catalyst isn't there.

[edit] References

This category is for articles related to NOx (oxides of nitrogen) control techniques and systems.