Products of a combustion process, the exhaust gases from internal combustion engines contain atmospheric pollutants such as nitrogen oxides (NOx):
- nitric oxide NO
- nitrogen dioxide NO2
It is well known that these molecules are toxic, posing serious health problems (e.g. concerning the respiratory system of humans), and that they have harmful effects for the environment (e.g. contributing to the phenomenon of acid rain).
The depollution of combustion engine exhaust gases is possible in particular through Selective Catalytic Reduction (SCR).
It is a question of causing chemical reactions specific to recombining (with nitrogenous compounds) the molecules constituting the undesirable gases that NOx are, to obtain compounds as harmless as nitrogen (N2) and water (H2O) usually contained in (wet) air. 
Urea CO(NH2)2 in aqueous solution is a nitrogenous compound widely used for the Selective Catalytic Reduction (SCR) of nitrogen oxides (NOx) contained in the exhaust gases of industrial engines.
A precious alloy e.g. tungsten W or vanadium V on a support based on titanium oxide TiO2 often serves as a catalyst for chemical reactions which generally occur at temperatures between 250°C and 520°C.
The depollution of exhaust gases from internal combustion engines can then be carried out by means of metal cassettes whose internal, porous parts allow the circulation of their mixture with the nitrogenous compound.
A Selective Catalytic Reduction (SCR) module - requiring a continuous supply of urea, with dosage managed by the means of an automaton and of sensors - can thus complete the noise reduction devices of an exhaust line that reactive (i.e. whose operating principle is based on changes in the geometry of the internal parts with chambers) or dissipative (i.e. whose effectiveness is linked to the presence of porous materials that absorb sound) silencers are.
Although it is generally a very distinct sub-assembly, the Selective Catalytic Reduction (SCR) module can, in somes cases, be combined with a silencer to obtain an assembly offering the features of limitation of emissions of polluting compounds and of noise.
The dimensioning of such components of an industrial combustion engine exhaust line (including for gensets) must be the subject of an overall study, taking into account the combination of :
- in terms of acoustics: the insertion loss of each component (for silencers, depending on their design: at low and/or medium and/or high frequencies; for the SCR Selective Catalytic Reduction device: only at high frequency)
- in terms of aerodynamics: the total pressure loss of each component (it is not uncommon for it to be of the same order of magnitude)
- in terms of general arrangement: the size of each component (it is not uncommon for it to be of the same order of magnitude, and for the part dedicated to the mixture between the nitrogen compound and the exhaust gases - upstream of the reactor – to have to be combined with a primary silencer in view of compactness)
Experience counts to offer in all contexts an optimized solution for high-performance equipment built to last, for the pollution control of industrial combution engines exhaust gases with a Selective Catalytic Reduction (SCR) process of nitrogen oxides (NOx) such as for noise reduction.
 the principle of depollution of combustion engine exhaust gases by means of a Selective Catalytic Reduction (SCR) process is based on the following typical chemical equations :
2 NO + 2 Nacfq + 1/2 O2 → 2 N2 + 3 H2O
NO2 + 2 Nacfq + 1/2 O2 → 3/2 N2 + 3 H2O
NO + NO2 + 2 Nacfq → 2 N2 + 3 H2O