Visitors to UK filling station forecourts will likely have seen AdBlue® dispensers and containers on sale. Developed as part of the push to lower vehicle pollutant emissions, these products – formally known as nitrogen oxide (NOx) reduction agents – are more commonly known as AdBlue® and diesel exhaust fluid (DEF). When used in diesel vehicles designed for them, such NOx reduction agents enable significant reductions in a vehicle’s NOx emissions. 

Whilst drivers should always consult their vehicle handbook for specific information related to their make and model, the following information is nonetheless intended to help consumers and forecourt operators understand what a NOx reduction agent like AdBlue® is, how it works and other important advice such as housekeeping guidance for this pollutant reducing technology.

What are NOx reduction agents?

NOx reduction agents are a high purity aqueous solution of urea used by many modern diesel vehicles – Heavy Goods Vehicles (HGVs), light commercial vehicles and passenger vehicles – which feature a Selective Catalytic Reduction (SCR) system.

Via the SCR, these significantly reduce the level of NOx emissions from a vehicle’s exhaust. Widely mistaken to be a fuel additive, NOreduction agents never come into contact with fuel, having a separate (usually blue) filter cap located either next to your fuel filler, in the boot or under the bonnet of your vehicle.

NOx reduction agents are available at filling stations, garages, motor retailers and supermarkets in different quantities – from dedicated dispensers to up to 10 litre containers.

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How do NOx reduction agents work?

The most common and widely available NOx reduction agent is known as AdBlue®, a registered trademark of VDA, the German Association of the Automotive Industry. They are a water/urea-based fluid and are injected into the exhaust gases of a vehicle through the SCR system.

The  urea in the NOx reduction agent vaporises and decomposes to form ammonia and carbon dioxide (via isocyanic acid), with the ammonia then reducing the harmful NOx emissions into nitrogen and water.

4NO + 4NH3 + O 4N2 + 6H2O

2NO2 + 4NH3 + O2 → 3N2 + 6H2O

NO + NO2 + 2NH→ 2N2 + 3H2O

Figure 1: possible ammonia-to-NOx chemical reactions following the introduction of NOx reduction agents like AdBlue®

Why are they used?

SCR technology and NOx reduction agents enable makers of new diesel vehicles to meet stricter NOx emissions regulations. NOx reduction agents like AdBlue® are generally considered to reduce NOx emissions by over 90%.

Handling guidance and housekeeping

Correct and safe housekeeping of NOx reduction agents is very important and when being stored, handled and used, it should be done according to the manufacturer’s instructions and regulator’s guidance.

Incorrect handling can lead to risks, including:

  • Corrosion of metalwork in tanks and pipework
  • Impurities entering the vehicle’s SCR system and causing damage
  • Damage to fuel tanks, pumps and injection systems if accidently put into diesel fuel tanks
  • Pollution risks from accidental releases (particularly to groundwater and watercourses)

Customers at filling stations should also be aware of different flow rates for HGV dispensers and passenger vehicles. Passenger car customers must not use HGV refilling equipment, as it is unsafe to do so.

If in doubt, vehicle owners should always consult their vehicle handbook.

Further information

For more information on AdBlue® and other NOx reduction agents, visit the AA or RAC websites.


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