AdBlue is the name used internationally for an additive developed for the chemical and motoring sectors.
It's an extremely high purity, 32.5% aqueous solution of urea (minimum 31.8% - maximum 33.3%) that transforms nitrogen oxides into nitrogen (N2) and water vapour (H2O) by means of chemical reduction.
Content of various metals must not exceed 0.2 mg/kg for each of them in order not to contaminate the catalytic converter. This means AdBlue cannot be substituted by urea used in agriculture. The composition and quality standards are regulated by DIN 70070.
AdBlue is an odourless, colourless synthetic product: it is not considered a dangerous substance inasmuch as it's neither flammable nor toxic.
Principle underlying catalytic reduction of nitrogen oxides.
SCR technology has been in use since the early '80s in thermal and fossil fuel power stations, on gas turbines, locomotive diesel engines and large marine power units. In all these applications combustion is optimised with the dual aim of achieving improved performance and direct reduction of particulate emissions. Post-treatment is based on a simple principle: the chemical reaction of ammonia (NH3) with the nitrogen oxides NO and NO2 to produce two harmless substances – water vapour (H2O) and nitrogen (N2).
These are the relevant formulas:
NO + NO2 + 2 NH3 2 N2 + 3 H2O
And for any residual oxygen present in the exhaust gases
4 NO + O2 + 4 NH3 4 N2 + 6 H2O
6 NO2 + 8 NH3 7 N2 + 12 H2O
In very large systems ammonia is drawn directly from pressurised tanks. With regards to road vehicles, the use of pure ammonia has been studied but in the end was abandoned because of problems of storing it on board trucks and in refilling stations.
The normalised urea technique in the form of a solution – AdBlue – was preferable for two reasons: this product is not categorised as a dangerous substance, there is no danger in the event of spills and it's easy to store both on board vehicles and at transporter premises, despite the limitation that it crystallises at temperatures below -11°C.