Last update of this page : 30/06/2004

Reduction techniques and costs

Nitrogen oxides (NOx)

Any combustion in air (boilers, car engines, cigarettes etc.) causes nitrogen oxide emissions. These NOx emissions are the result of reactions between the nitrogen (from the air or the fuel) and the oxygen in the air.

The two main sources of NOx emissions are the internal combustion engine and stationary combustion plants.

If catalytic converters are fitted to vehicles, they limit NOx emissions. As for combustion plants, two types of technique exist: primary techniques (acting on combustion) and secondary techniques (acting on the waste gases).

Primary techniques

These limit emissions by acting on combustion. For instance, there are low-NOx burners, recycling of combustion gases, gradual injection of the fuel and/or the combustion air. The effectiveness of these techniques varies between 20 and 60% (if several primary techniques are combined).

Secondary techniques

These yield far higher reduction rates. Only the two processes which have really made an impact on the market are indicated here.

Selective non catalytic reduction (SNCR)
NOx are reduced by the action of urea or ammonia, injected into the furnace without a catalyser. In this case, the reaction occurs at high temperature (850-1050°C).
The efficiency is between 50 and 70% with an NH3/NO ratio of 2.5.

Selective catalytic reduction (SCR)
This works on the combustion gases by the reducing action of ammonia at 280-400°C in the presence of a catalyser.
This type of process can reduce the NOx content of the fumes by up to 90% with an NH3/NO ratio of 0.7-1.