NOx adsorber

A NO_x adsorber or NO_x trap (also called Lean NO_x trap, abbr. LNT) is a device that is used to reduce oxides of nitrogen (NO and NO₂) emissions from a lean burn internal combustion engine.

Purpose and function of a NOx adsorber

A NO_x adsorber is designed to reduce oxides of nitrogen emitted in the exhaust gas of a lean burn internal combustion engine. Lean burn engines, particularly diesels, present a special challenge to emission control system designers because of the relatively high levels of O₂ (atmospheric oxygen) in the exhaust gas stream. The 3-Way catalytic converter technology that has been successfully used on stoichiometric internal combustion engines (typically fueled by petrol but also sometimes fueled by LPG, CNG, or ethanol) since the middle 1980s will not function at O₂ levels in excess of 1.0%, and does not function well at levels above 0.5%. Because of the increasing need to limit NO_x emissions from diesel engines technologies such as exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) have been used, however EGR is limited in its effectiveness and SCR requires a reductant, and if the reductant tank runs dry the SCR system ceases to function.

The NO_x adsorber was designed to avoid the problems that EGR and SCR experienced as NO_x reduction technologies. The theory is that the zeolite will trap the NO and NO₂ molecules—in effect acting as a molecular sponge. Once the trap is full (like a sponge full of water) no more NO_x can be absorbed, and it is passed out of the exhaust system. Various schemes have been designed to "purge" or "regenerate" the adsorber. Injection of diesel fuel (or other reactant) before the adsorber can purge it—the NO₂ in particular is unstable and will join with hydrocarbons to produce H₂O and N₂. Use of hydrogen has also been tried, with the same results, however hydrogen is difficult to store. Some experimental engines have mounted hydrogen reformers for on board hydrogen generation, however fuel reformers are not mature technology.

NO_x adsorbers are experimental technology as of early 2006, and thus extremely expensive. Whether or not this technology will be successfully commercialized is open to question—only time will tell. A NO_x trap is used on the Volkswagen Jetta Clean TDI and the Volkswagen Tiguan concepts. Both are projected to be introduced into the American market by 2008.^[1] They were to be marketed as part of the BlueTec program from Audi, Daimler-Chrysler, and Volkswagen.

Technical details

The NO_x adsorber is based on a catalytic converter support that has been coated with a special washcoat containing zeolites.

Alkali/Alkaline Oxide(Carbonate) can also used as a NOx absorber.

Their effective storage changes with temperature. They start to store NOx between 150-200C and at 500C the storage dimishes rapidly.

References

1. ^ "NAIAS Detroit 2007: Volkswagen continues BLUETEC offensive". Archived from the original on 2007-03-20. http://web.archive.org/web/20070320084304/http://eng.volkswagen-media-services.com/medias_publish/ms/content/en/pressemitteilungen/2007/01/08/naias_detroit_2007.standard.gid-oeffentlichkeit.html. Retrieved 2007-03-16. 

2) "Lean NOx Trap Catalysis for Lean Natural Gas Engine Applications" PDF format. http://inspire.ornl.gov/.../2046106b-fadf-47b4-aff3-332803f46a3b