Understanding the role of AdBlue® in SCR pollution control systems

AdBlue® is an aqueous solution used in diesel vehicles equipped with Selective Catalytic Reduction (SCR) systems to reduce emissions of pollutant gases, including nitrogen oxides (NOx). The solution is composed of 32.5% high-purity urea (NH₂-CO-NH₂) dissolved in demineralized water, with a simplified chemical formula: CO(NH₂)₂. Its use thus allows compliance with Euro 4, Euro 5 and Euro 6 standards for the reduction of pollutant emissions. AdBlue® triggers a chemical reaction that significantly reduces pollutant emissions from exhaust gases. This chemical reaction occurs when AdBlue® is injected into the exhaust gases, inside the catalyst of a diesel engine. Untreated exhaust gases contain nitrogen oxides (NOX), which are among the main air pollutants.

AdBlue® is specially formulated to limit emissions of this pollutant. AdBlue® therefore contains pure urea. At high temperatures, the injection of urea into the exhaust gases causes the formation of ammonia. This causes a chemical reaction that breaks down all the toxic nitrogen oxides (NOX) into water vapour (H2O) and nitrogen (N) which are harmless to the environment.

Dans le système SCR, l'AdBlue® est injecté dans le flux des gaz d'échappement, où il se décompose thermiquement pour produire de l'ammoniac (NH₃). Cette réaction se déroule à haute température dans les catalyseurs SCR et peut être simplifiée comme suit :

1. Décomposition thermique : CO(NH₂)₂+H₂O→2NH₃+CO₂

2. Réduction des NOx : Les NOx présents dans les gaz d'échappement réagissent ensuite avec le NH₃ dans le catalyseur SCR,donnant de l'azote (N₂) et de l'eau (H₂O) : 4 NO+4NH₃+O₂→4N₂+6H₂O6 NO₂+8NH₃→7N₂+12H₂O

Ces réactions réduisent significativement les émissions de NOx, contribuant à la conformité aux normes Euro VI.

Le système SCR est conçu avec un réservoir d'AdBlue®, une pompe, un injecteur et un système de chauffage.

Voici le parcours :

• Stockage et distribution : L'AdBlue® est stocké dans un réservoir spécifique et acheminé à la pompe.

• Injection : Une pompe dose l'AdBlue® et l’injecte dans le flux d’échappement avant le catalyseur.

• Transformation chimique : L’AdBlue® se décompose dans le gaz d'échappement chaud pour libérer de l'ammoniac, qui entre en réaction avec les NOx dans le catalyseur SCR.

• Retour au réservoir : Les systèmes incluent des éléments chauffants pour maintenir l'AdBlue® à une température optimale, évitant le gel en hiver et la cristallisation à certaines conditions.

Although AdBlue® is an effective solution, certain risks exist:

• Crystallization : AdBlue® can crystallize, especially under low usage or variable temperature conditions, forming deposits that block injectors. When the temperature of these gases is not sufficient to create the reaction or the temperature is below -11°C, it crystallizes.

• Sensor faults : Sensors can become dirty and affect injection accuracy.

• Freezing : At low temperatures, AdBlue® can freeze. Systems are often equipped with heating resistors to alleviate this problem, but this equipment is prone to failure.

Crystallization of AdBlue® occurs when the solution comes into contact with air or undergoes variations
temperature. This phenomenon is due to the formation of urea crystals and ammonia deposits, resulting from the evaporation of the water contained in the AdBlue® solution. These crystals, by accumulating, obstruct the injectors and deteriorate the performance of the SCR system.

Faced with these challenges, Bardahl’s Optiblue additive offers an innovative solution. This additive has been designed to stabilise the AdBlue® solution and prevent crystallisation, even during irregular use or in extreme conditions. Thanks to specific surfactants, Optiblue prevents the accumulation of crystals in injectors and circuits, thus increasing the service life and reliability of the SCR system, while complying with ISO 22241.

AdBlue®, when used optimally, is a valuable solution to reduce NOx emissions. However, crystallization remains a major challenge. With products such as Bardahl’s Optiblue , crystallization risks can be significantly reduced, making SCR systems more robust and durable to meet increasingly stringent emission reduction requirements.