Lean NOx Trap

As a lean NO _x trap , in short LNT , nitrogen oxide trap or NO _x storage catalytic converter , short NSK , or NSC is referred to a device, the produced during combustion with oxygen excess oxides of nitrogen (NO _x ) adsorbed . The English part of the name lean (literally “lean”) indicates hyperstoichiometric combustion, i.e. lean combustion with excess oxygen, while trap (literally “capture”) describes the adsorption of nitrogen oxides. In passenger cars, LNT are used for exhaust gas purification in order to remove nitrogen oxides from the exhaust gas, which are undesirable combustion products. Adsorption does not mean conversion of nitrogen oxides, instead they are "stored" in the LNT. Since the “storage capacity” of the LNT is limited, it must be regenerated. To this end, the nitrogen oxides are reduced .

As early as 1996, Toyota offered a vehicle for the Japanese market that was equipped with a NO _x storage catalytic converter. In 2000, the PSA Group introduced the NO _x storage catalytic converter to the European market ; Volkswagen followed suit in the same year and was the first manufacturer to use a NO _x sensor to monitor the conversion of nitrogen oxides.

In 2017, an LNT was used for the majority of diesel vehicles to reduce NO _x emissions.

functionality

Technically, LNT are usually implemented as part of a three-way catalytic converter . In addition to the platinum and rhodium catalyst materials commonly used for oxidation and reduction , corresponding catalysts have an additional layer for storing the nitrogen oxides (the actual LNT). This storage layer consists of heavy ( earth ) alkali metals and lanthanoids (or their basic metal oxides ). For example, potassium oxide or barium oxide are suitable . The nitrogen oxides are stored in the storage layer as nitrates . In order for this to be possible, nitrogen monoxide must first be oxidized to nitrogen dioxide. The partial oxidation of NO to NO ₂ takes place in an upstream oxidation catalytic converter .

When operating with excess oxygen ( ), the LNT is completely filled after approx. 1–2 minutes, so that it must be regenerated for approx. 1–2 seconds when operating with an oxygen deficiency ( ). This intermittent operating mode makes the LNT unsuitable for commercial vehicles. During operation with a lack of oxygen, hydrocarbons and carbon monoxide as well as hydrogen , which are used for reduction, are produced in the exhaust gas . This is an advantage of the LNT, since no additional fuel is required, but the artificial lack of oxygen increases fuel consumption. ${\ displaystyle \ lambda> 1}$ ${\ displaystyle \ lambda <1}$

The storage times are significantly longer in the diesel engine. With a storage capacity of 1 g, a journey of up to 10 minutes can be covered.

In order for the LNT to work optimally, the temperature of the exhaust gas must be around 573–673 K (300–400 ° C): If the temperature is too low, not enough nitrogen dioxide is produced from nitrogen monoxide (only nitrogen dioxide can be sufficiently stored) If the temperature is too high, the conversion rate is reduced, since nitrates are no longer stable at too high temperatures. In the case of the diesel engine, the entire temperature range of the LNT is around 473–773 K (200–500 ° C), and work is being done to lower it.

Chemical equations of an LNT with a storage layer made of barium carbonate

Oxidation of nitric oxide to nitrogen dioxide: ${\ displaystyle 2NO + O_ {2} \ to 2NO_ {2}}$

Adsorption of nitrogen dioxide (formation of nitrates) with the release of carbon dioxide: ${\ displaystyle BaCO_ {3} + 2NO_ {2} + {\ frac {O_ {2}} {2}} \ to Ba (NO_ {3}) _ {2} + CO_ {2}}$

Release of nitrogen monoxide and nitrogen dioxide through the fall of nitrate: ${\ displaystyle Ba (NO_ {3}) _ {2} + CO \ to BaCO_ {3} + 2NO + O_ {2}}$

Reduction of nitrogen dioxide using hydrocarbons, hydrogen and carbon monoxide: ${\ displaystyle HC + 2NO + O_ {2} + {\ frac {H_ {2}} {2}} + CO \ to H_ {2} O + 2CO_ {2} + N_ {2}}$

Source:

Sulphurization

The sulfur burnt from the fuel and the engine oil gradually makes the LNT less effective: Sulfur is not decomposed at low temperatures, such as those that occur in overstoichiometric operation, so that stable barium sulfate is formed. It therefore only makes sense to use fuel with low sulfur content. In the EU, sulfur-free fuel is prescribed and the upper limit is 10 mg / kg. Sulphates have a higher thermal stability than nitrates, which is why a separate sulphate regeneration process is necessary for desulphurisation. To do this, the engine is operated with a rich mixture at a relatively high load, so that hot, low-oxygen exhaust gas is formed. The desulfurization process can then take place at exhaust gas temperatures above about 923-1023 K (650-750 ° C) (according to other sources: 873 K (600 ° C)) and takes about 5 minutes.

With diesel engines, such temperatures can only be reached with strong heating measures. Usually the periodically necessary desulfurization is combined with the regeneration of the particle filter , for which the exhaust system must also be heated. If the vehicle is parked in such a phase, the engine compartment must be cooled with the vehicle fan after it has been switched off.

Individual evidence

↑ Günter P. Merker (Ed.), Rüdiger Teichmann (Ed.): Fundamentals of Combustion Engines Function and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th edition, Springer, Wiesbaden 2019, ISBN 978-3-658-23556-7 . S. XVI
↑ Günter P. Merker (Ed.), Rüdiger Teichmann (Ed.): Fundamentals of Combustion Engines Function and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th edition, Springer, Wiesbaden 2019, ISBN 978-3-658-23556-7 . P. 440
↑ Günter P. Merker (Ed.), Rüdiger Teichmann (Ed.): Fundamentals of Combustion Engines Function and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th edition, Springer, Wiesbaden 2019, ISBN 978-3-658-23556-7 . P. 55
↑ ^a ^b ^c ^d Günter P. Merker (Hrsg.), Rüdiger Teichmann (Hrsg.): Fundamentals of Combustion Engines Functionality and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th Edition, Springer, Wiesbaden 2019, ISBN 978-3-658 -23556-7 . P. 990
^ NOx Storage Catalyst (NSC). Accessed April 22, 2019 .
↑ ^a ^b ^c Günter P. Merker (Ed.), Rüdiger Teichmann (Ed.): Fundamentals of Combustion Engines Functionality and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th Edition, Springer, Wiesbaden 2019, ISBN 978-3-658- 23556-7 . P. 991
↑ Richard van Basshuysen (Ed.): Otto engine with direct injection and direct injection: Otto fuels · Natural gas · Methane · Hydrogen . 4th edition, Springer, Wiesbaden 2017, ISBN 978-3-658-12215-7 . P. 418f.
↑ Richard van Basshuysen (Ed.): Otto engine with direct injection and direct injection: Otto fuels · Natural gas · Methane · Hydrogen . 4th edition, Springer, Wiesbaden 2017, ISBN 978-3-658-12215-7 . P. 419f.
↑ Reinhard Ratzberger, Eberhard Schutting, Helmut Eichlseder, Hadl, Martin Wieser, Horst Mitterecker: Combination of LNT and Passive SDPF - A System Assessment for Small Passenger Car Diesel Engines under RDE Conditions . In: 26th Aachen Colloquium Automobile and Engine Technology . 2017, p. 1038 (English).
↑ Landsberg, Zink, Müller-Stach, Albarracin-Caballero, Wittka, Fiebig, Wilkes, Robb, Schönen: Investigations on Exhaust Aftertreatment Systems for Euro 6d with Specific Focus on RDE Driving Scenarios . In: 27th Aachen Colloquium Automobile and Engine Technology . 2018, p. 246 .
↑ Wulf Hauptmann, Thomas Utschig, Benjamin Barth, Christian Tomanik, Ulrich Göbel, Ina Grisstede, Wilfried Müller, Johannes Hipp, Christian Beidl: Challenge RDE: Sustainable Diesel Exhaust Systems for Euro 6d . In: 26th Aachen Colloquium Automobile and Engine Technology . 2017, p. 1068 .
↑ Andreas Hertzberg: Operating strategies for a gasoline engine with direct injection and NOx storage catalytic converter , dissertation. University of Karlsruhe (2001). P. 16
↑ ^a ^b ^c Günter P. Merker (Ed.), Rüdiger Teichmann (Ed.): Fundamentals of Combustion Engines Functionality and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th Edition, Springer, Wiesbaden 2019, ISBN 978-3-658- 23556-7 . P. 992
↑ ^a ^b Horst Bauer: Exhaust technology for gasoline engines . Ed .: Robert Bosch GmbH. 6th edition. Stuttgart 2002, ISBN 3-7782-2020-9 , pp. 59 .
↑ Richard van Basshuysen (Ed.): Otto engine with direct injection and direct injection: Otto fuels · Natural gas · Methane · Hydrogen . 4th edition, Springer, Wiesbaden 2017, ISBN 978-3-658-12215-7 . P. 479:
"There are still higher requirements for the fuel, which has to be sulfur-free for the NO _x reduction catalysts"
↑ Standard DIN EN 228 Fuels for motor vehicles - Unleaded petrol - Requirements and test methods ( online ).
↑ Richard van Basshuysen (Ed.): Otto engine with direct injection and direct injection: Otto fuels · Natural gas · Methane · Hydrogen . 4th edition, Springer, Wiesbaden 2017, ISBN 978-3-658-12215-7 . P. 270:
"The problem with sulfur poisoning is the fact that the sulphate has a higher thermal stability than the nitrate and therefore a separate sulphate regeneration is required at a higher catalyst temperature."

[Merker_XVI-1] Günter P. Merker (Ed.), Rüdiger Teichmann (Ed.): Fundamentals of Combustion Engines Function and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th edition, Springer, Wiesbaden 2019, ISBN 978-3-658-23556-7 . S. XVI

[Merker_440-2] Günter P. Merker (Ed.), Rüdiger Teichmann (Ed.): Fundamentals of Combustion Engines Function and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th edition, Springer, Wiesbaden 2019, ISBN 978-3-658-23556-7 . P. 440

[Merker_55-3] Günter P. Merker (Ed.), Rüdiger Teichmann (Ed.): Fundamentals of Combustion Engines Function and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th edition, Springer, Wiesbaden 2019, ISBN 978-3-658-23556-7 . P. 55

[Merker_990-4] Günter P. Merker (Hrsg.), Rüdiger Teichmann (Hrsg.): Fundamentals of Combustion Engines Functionality and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th Edition, Springer, Wiesbaden 2019, ISBN 978-3-658 -23556-7 . P. 990

[5] NOx Storage Catalyst (NSC). Accessed April 22, 2019 .

[Merker_991-6] Günter P. Merker (Ed.), Rüdiger Teichmann (Ed.): Fundamentals of Combustion Engines Functionality and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th Edition, Springer, Wiesbaden 2019, ISBN 978-3-658- 23556-7 . P. 991

[7] Richard van Basshuysen (Ed.): Otto engine with direct injection and direct injection: Otto fuels · Natural gas · Methane · Hydrogen . 4th edition, Springer, Wiesbaden 2017, ISBN 978-3-658-12215-7 . P. 418f.

[8] Richard van Basshuysen (Ed.): Otto engine with direct injection and direct injection: Otto fuels · Natural gas · Methane · Hydrogen . 4th edition, Springer, Wiesbaden 2017, ISBN 978-3-658-12215-7 . P. 419f.

[9] Reinhard Ratzberger, Eberhard Schutting, Helmut Eichlseder, Hadl, Martin Wieser, Horst Mitterecker: Combination of LNT and Passive SDPF - A System Assessment for Small Passenger Car Diesel Engines under RDE Conditions . In: 26th Aachen Colloquium Automobile and Engine Technology . 2017, p. 1038 (English).

[10] Landsberg, Zink, Müller-Stach, Albarracin-Caballero, Wittka, Fiebig, Wilkes, Robb, Schönen: Investigations on Exhaust Aftertreatment Systems for Euro 6d with Specific Focus on RDE Driving Scenarios . In: 27th Aachen Colloquium Automobile and Engine Technology . 2018, p. 246 .

[11] Wulf Hauptmann, Thomas Utschig, Benjamin Barth, Christian Tomanik, Ulrich Göbel, Ina Grisstede, Wilfried Müller, Johannes Hipp, Christian Beidl: Challenge RDE: Sustainable Diesel Exhaust Systems for Euro 6d . In: 26th Aachen Colloquium Automobile and Engine Technology . 2017, p. 1068 .

[Hertzberg-12] Andreas Hertzberg: Operating strategies for a gasoline engine with direct injection and NOx storage catalytic converter , dissertation. University of Karlsruhe (2001). P. 16

[Merker_992-13] Günter P. Merker (Ed.), Rüdiger Teichmann (Ed.): Fundamentals of Combustion Engines Functionality and Alternative Drive Systems Combustion, Measurement Technology and Simulation, 9th Edition, Springer, Wiesbaden 2019, ISBN 978-3-658- 23556-7 . P. 992

[Bosch_Otto-14] Horst Bauer: Exhaust technology for gasoline engines . Ed .: Robert Bosch GmbH. 6th edition. Stuttgart 2002, ISBN 3-7782-2020-9 , pp. 59 .

[15] Richard van Basshuysen (Ed.): Otto engine with direct injection and direct injection: Otto fuels · Natural gas · Methane · Hydrogen . 4th edition, Springer, Wiesbaden 2017, ISBN 978-3-658-12215-7 . P. 479:
"There are still higher requirements for the fuel, which has to be sulfur-free for the NO _x reduction catalysts"

[EN228-16] Standard DIN EN 228 Fuels for motor vehicles - Unleaded petrol - Requirements and test methods ( online ).

[17] Richard van Basshuysen (Ed.): Otto engine with direct injection and direct injection: Otto fuels · Natural gas · Methane · Hydrogen . 4th edition, Springer, Wiesbaden 2017, ISBN 978-3-658-12215-7 . P. 270:
"The problem with sulfur poisoning is the fact that the sulphate has a higher thermal stability than the nitrate and therefore a separate sulphate regeneration is required at a higher catalyst temperature."