Removing nitrogen oxides during a lean-burn engine cold start
View Patent ↗An apparatus and a method for removing NOx from an exhaust stream of a lean-burn engine during an engine cold start are provided. The apparatus includes a close-coupled LNT coupled to a main LNT by a hollow conduit. NOx from the cold exhaust is initially stored on the close-coupled LNT. As the engine exhaust warms up, the close-coupled LNT reaches its operating temperature and reduces the stored NOx to N 2 . When the temperature of the close-coupled LNT reaches approximately 350° C., NOx is released from the close-coupled LNT, is absorbed on the main LNT, and is converted to N 2 during fuel-rich conditions, The close-coupled LNT can be regenerated after the engine is turned off by providing heat. The released NOx during the regeneration of the closed-coupled LNT is absorbed by the main LNT. The stored NOx on the main LNT can be reduced to N 2 during the next vehicle operation.
1. A method for the selective removal of NOx from a gaseous stream using lean NOx traps containing a catalyst that catalyzes the reduction of NOx to N 2 and a compound that stores NOx as nitrates, comprising:
heating a first lean trap to a temperature at which stored nitrates decompose and are released;
cooling the first lean NOx trap in an environment that does not contain NOx;
passing the gaseous stream through the first lean NOx trap; and
passing the gaseous stream through a second lean NOx trap, the second lean NOx trap storing NOx released from the first lean NOx trap.
2. The method of claim 1 , comprising passing the gaseous stream through the first lean NOx trap at a temperature below 250° C.
3. A method according to claim 1 , comprising passing the gaseous stream through the first lean NOx trap at a temperature below 140° C.
4. The method of claim 1 , comprising passing the gaseous stream through the first lean NOx trap at a temperature below 200° C.
5. The method of claim 4 , wherein the temperature in the heating step is approximately 600° C.
6. A method for reducing the total NOx released into the environment during operation from cold start to shut down of a vehicle containing an internal combustion engine comprising:
operating the engine to produce an exhaust stream comprising NOx;
passing the exhaust stream comprising NOx from the engine sequentially through a first LNT and a second LNT, wherein the LNTs comprise a catalyst that can cayalyze the reduction of NOx to N 2 and compounds that can store NOx as nitrates;
shutting off the engine; and
after shutting off the engine, regenerating the first LNT by heating it to a temperature at which it releases stored NOx.
7. A method according to claim 6 , comprising operating the internal combustion engine with alternating cycles of rich and lean fuel.
8. A method according to claim 6 , further comprising cooling the LNT in an environment that does not contain NOx.
9. A method according to claim 6 , wherein the engine is a diesel engine.
10. A method according to claim 6 , wherein the engine is a gasoline engine.
11. A method according to claim 6 , wherein regenerating the first LNT comprises heating it to 600° C.
12. A method according to claim 11 wherein the heating step is carried out for about 30 seconds.