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AUS 32 (aqueous urea solution) is an aqueous solution of urea composed of 32.5% urea and 67.5% demineralised water. It is used in the selective catalytic reduction (SCR) process. It is a diesel exhaust fluid (DEF), also called AdBlue (registered trademark). The AUS 32 is standardized according to ISO 22241. Heavy vehicles built from October 2006 are equipped with a system that uses the AUS 32.

This colorless solution is biodegradable, non-toxic, non-explosive, non-flammable, and poses no major danger to the environment or to health. It is unstable and corrosive to certain metals and requires specific storage and transport conditions1, which do not tolerate extreme temperatures well. It is not subject to the regulation on the transport of dangerous goods. Its odor is weak (generally considered non-existent) in the stable state, although an intense smell of ammonia can be noticed in case of failure of the SCR system.

AdBlue is a registered trademark of the German Association of the Automotive Industry (VDA) which maintains the DIN 70070 quality standards, supplemented since by the ISO 222415 standard.


Schematic diagram of RCS technology

Schematic diagram of the operating mode of a pollution control technology:

  • diesel motor ;
  • oxidation of exhaust gases;
  • DPF delete that retains almost all the particles;
  • injection of the urea solution by the dosing unit into the exhaust stream and hydrolysis under the effect of heat;
  • selective reduction of NO and NO2 oxide and nitrogen dioxide; oxidation of ammonia NH3.

During the combustion process in the diesel engine, more air is admitted than diesel in the combustion chamber, in order to avoid the release of unburned fuel. Nitrogen oxides are formed as a greenhouse gas and are dangerous for health.

The first element constituting the SCR process consists of oxidation of the exhaust gases by oxygen injection. After the particulate filter (DPF), the urea solution, contained in a dedicated tank8 of vehicles equipped with RCS technology, is injected.


Tank and AdBlue unit of a truck.

The low dosage (32.5% urea solution) allows for safe transport, storage and use (apart from the risk of contact or inhalation allergy9). It is a stable compound in time, easily handled, unlike pure ammonia, which is a dangerous component for people, materials, animals and the environment. It is therefore a technically very advantageous solution.

The low consumption required for the reaction allows infrequent refilling and minimizes the impact of this reservoir in terms of space on the chassis (consumption compared to diesel of 3 to 8% of urea solution10,4). These systems are currently in use in Europe as well as in Japan and Singapore. Almost all European truck manufacturers (trucks, buses, dumpsters, etc.) offer vehicles equipped with the SCR system to meet the Euro 5 standard that this system achieves. However, these systems are sensitive to impurities in the urea solution, which requires maintaining a high quality in the preparation of the solution, quality defined in the ISO 22241 standard which has four parts (quality requirements, test methods; , transport and storage, filling interface).

The use of AUS 32 has been developed with the implementation of Euro 4, Euro 5 and Euro 6 emission standards designed to reduce emissions of nitrogen oxides (NOx) from heavy goods vehicles.

Public works machinery, agricultural machinery and commercial and private vehicles will also be affected in the coming years.

The increasing use of SCR technology in Europe has forced the creation of efficient distribution networks. AdBlue can be purchased both as a tanker from manufacturers and as a service station pump. The main European producers are Yara1, Borealis (former GPN of the Total group), BASF, Greenchem9,1, Kruse, Dehon SA, Dureal and AMI.

Private vehicles are now subject to EU regulations to limit nitrogen dioxide emissions. Each vehicle has its own specificities for filling (eg tank size and location, alert, etc.)

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