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Purpose and function of a NOx adsorber
A NOx 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 NOx 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 NOx adsorber was designed to avoid the problems that EGR and SCR experienced as NOx 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 NOx 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.
NOx 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 NOx 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. They were to be marketed as part of the BlueTec program from Audi, Daimler-Chrysler, and Volkswagen.
The NOx adsorber is based on a catalytic converter support that has been coated with a special washcoat containing zeolites.
Alkali/alkaline oxide (carbonate) can also be used as a NOx absorber.
Their effective storage changes with temperature. They start to store NOx between 150 and 200C and at 500C the storage diminishes rapidly.
- "NOX REDUCTION WITH NATURAL GAS FOR LEAN LARGE-BORE ENGINE APPLICATIONS USING LEAN NOX TRAP AFTERTREATMENT" (PDF). Retrieved 2012-02-07.
- "NAIAS Detroit 2007: Volkswagen continues BLUETEC offensive". Archived from the original on 2007-03-20. Retrieved 2007-03-16.