LNT: SCR’S LITTLE BROTHER
Total explains how the Lean NOx Trap (LNT) works, the alternative to the selective catalytic reduction (SCR) system.
Perhaps you never have heard of an NOx Trap or an LNT. Although the name may sound a bit exotic, in reality this is simply another means used by car manufacturers to capture nitrogen oxide emitted by diesel engines. The reduction of these emissions is a very important objective both for brands and for governments, as they have an impact on the public’s health.
LNT is an alternative to the SCR system, the catalytic converter that uses AdBlue® to eliminate nitrogen oxide, which we have spoken about before. In the case of LNT, it works by absorbing the NOx that is formed in lean mixture, i.e. due to an excess of oxygen.
In concrete terms, nitrogen oxide is trapped on a substrate, which is generally formed of barium nitrate. When this substrate becomes saturated, periodic regeneration takes place during the enrichment of the mixture, so as to eliminate the nitrogen oxide through a chemical reaction known as reduction.
The process used by LNT consists in oxidizing the nitrogen oxide trapped on the substrate in the form of solid nitrate, which involves using a chemical oxidation catalytic converter. As we mentioned, if the substrate is saturated due to high concentration, it can be regenerated in milliseconds thanks to a rich mixture phase – i.e., with a low concentration of oxygen – or stoichiometric phase. This provides the necessary conditions for a three-way catalytic converter, allowing it to continuously capture NOx and transform it into harmless nitrogen.
This process involves different elements such as platinum salts for the initial oxidation, as well as barium oxide and other substrate metals that react with the nitrogen oxide resulting in its absorption, in addition to rhodium salts, which are responsible for transforming NOx to nitrogen.
Differences with the SCR system
At this point, you may be wondering what advantages or disadvantages LNT offers compared to the SCR system. What is certain is that the NOx Trap process is less efficient and more expensive because it uses scarce metals such as platinum and rhodium. That is why the trend suggests that the SCR system will remain the most popular for getting rid of nitrogen oxide.
However, the LNT system is more profitable for small engines than the SCR. That is why manufacturers such as Volvo, Jeep, Renault and Hyundai have all designed diesel vehicles with an LNT system. In addition, cars with NOx Traps do not require a separate tank to be filled with AdBlue® or a similar product, as for SCR vehicles, making them more practical for the driver.
Sulfur oxide: LNT’s kryptonite
Perhaps the greatest weakness of the LNT system is sulfur oxide, which is produced when fuel is burned. It has the same effect on LNT as kryptonite has on Superman! In practical terms, if the NOx Trap absorbs sulfur oxide, it can cause the device to malfunction. Sulfur oxide forms a compound on the substrate that is difficult to remove, which causes the system to work under more severe conditions, making it less efficient.
To avoid these problems, vehicles equipped with LNT must use diesel fuel with a sulfur level of less than 10 ppm. It is also recommended to use quality lubricants tailored to the vehicle’s engine. This helps ensure that the various systems of the car will work better for longer.