NOx adsorber
Encyclopedia
A NOx adsorber or NOx trap (also called Lean NOx trap, abbr. LNT) is a device that is used to reduce oxides of nitrogen (NO and NO2) emissions from a lean burn
Lean burn
Lean burn refers to the use of lean mixtures in an internal combustion engine. The air-fuel ratios can be as high as 65:1, so the mixture has considerably less fuel in comparison to the stoichiometric combustion ratio ....

 internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

.

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
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

. Lean burn engines, particularly diesel
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

s, present a special challenge to emission control system designers because of the relatively high levels of O2 (atmospheric oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

) in the exhaust gas stream. The 3-Way catalytic converter
Catalytic converter
A catalytic converter is a device used to convert toxic exhaust emissions from an internal combustion engine into non-toxic substances. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion are converted to less toxic substances by dint...

 technology that has been successfully used on stoichiometric internal combustion engines (typically fueled by petrol but also sometimes fueled by LPG, CNG
Compressed natural gas
Compressed natural gas is a fossil fuel substitute for gasoline , diesel, or propane/LPG. Although its combustion does produce greenhouse gases, it is a more environmentally clean alternative to those fuels, and it is much safer than other fuels in the event of a spill...

, or ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

) 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
Exhaust gas recirculation
In internal combustion engines, exhaust gas recirculation is a nitrogen oxide emissions reduction technique used in petrol/gasoline and diesel engines. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. In a gasoline engine, this inert exhaust...

 (EGR) and selective catalytic reduction
Selective catalytic reduction
Selective catalytic reduction is a means of converting nitrogen oxides, also referred to as with the aid of a catalyst into diatomic nitrogen , , and water, . A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and is absorbed...

 (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
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...

 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 hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

s to produce H2O
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and N2
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

. Use of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 has also been tried, with the same results, however hydrogen is difficult to store. Some experimental engines have mounted hydrogen reformers
Steam reforming
Fossil fuel reforming is a method of producing hydrogen or other useful products from fossil fuels such as natural gas. This is achieved in a processing device called a reformer which reacts steam at high temperature with the fossil fuel. The steam methane reformer is widely used in industry to...

 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
Volkswagen Jetta
Although the Golf had reached considerable success, in the North American markets, Volkswagen observed that the hatchback body style lacked some of the appeal to those who preferred the traditional three-box configuration...

 Clean TDI and the Volkswagen Tiguan
Volkswagen Tiguan
Volkswagen Tiguan is a compact crossover vehicle initially built by the German automaker Auto 5000, but now built by Volkswagen, based on the PQ35 platform of the Volkswagen Golf. Production started in the winter of 2007 in Wolfsburg, Germany, and started selling shortly after...

 concepts. Both are projected to be introduced into the American market by 2008. They were to be marketed as part of the BlueTec
BlueTec
BlueTec is Daimler AG's marketing name for engines equipped with advanced NOx reducing technology for vehicle emissions control in diesel-powered vehicles...

program from Audi, Daimler-Chrysler, and Volkswagen.

Technical details

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 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.
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