Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2001-09-14
2004-03-30
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C060S274000, C422S177000, C422S211000, C423S213200
Reexamination Certificate
active
06713030
ABSTRACT:
This invention concerns combatting air pollution from the exhaust gas of a lean burn engine. In particular, it concerns apparatus for, and a method of, reducing the content of nitrogen oxides (NOx) in such gas.
Lean burn engines (which have an air-fuel ratio greater than 14.7, generally in the range 19-50) exhibit higher fuel economy and lower hydrocarbon emissions than do stoichiometrically operated engines and are increasing in number. Emissions from diesel engines are now being regulated by legislation, and whilst it is not too difficult to meet regulations on hydrocarbon or CO emissions, it is difficult to meet regulations on NOx emissions. Since exhaust gas from lean burn engines such as diesel engines is high in oxygen content throughout the engine cycle, it is more difficult to reduce NOx to nitrogen than in the case of stoichiometrically operated engines. The difficulty is compounded by the lower gas temperature. Various approaches are being considered to reduce NOx under the oxidising conditions. One approach is that of selective catalytic reduction (SCR) with hydrocarbon, but a catalyst of sufficient activity and durability to achieve the required conversion has not been found. Another approach is to adsorb the NOx by an adsorbent when the exhaust gas is lean (ie when there is a stoichiometric excess of oxygen) and release and reduce the adsorbed NOx when the exhaust gas is rich, the exhaust gas being made rich periodically. During the lean operation, NO is oxidised to NO
2
which can then react readily with the adsorbent surface to form nitrate. This approach, though, is constrained at low temperature by restricted ability to form NO
2
and by adsorbent regeneration and at high temperature by sulphur poisoning. Most adsorbents operate in a certain temperature window and are deactivated by sulphate formation. The approach of the present invention is that of SCR of NOx by NH
3
. This approach has been applied to static diesel engines using a V
2
O
5
—TiO
2
type catalyst.
The application of NH
3
SCR technology to the control of NOx emission from lean burn vehicles, however, requires a suitable NH
3
supply strategy, especially at low temperature, for various reasons. The engine-out NOx varies with temperature, so the amount of NH
3
supplied must be well controlled as a function of the temperature to maintain the appropriate stoichiometry for the reaction; an insufficient supply of NH
3
results in inadequate NOx reduction, whilst an excess may cause NH
3
to slip past the catalyst Whilst at sufficiently high temperature, the catalyst can selectively oxidise that excess NH
3
to N
2
, at low temperature, the unreacted NH
3
will be emitted as such. Even if the proper stoichiometry of NH
3
is provided, the catalyst may not be sufficiently active at low temperature to react all the NH
3
with the NOx. For example,
FIG. 1
shows the reaction of NH
3
with NOx over a non-metallised zeolite as a function of temperature at a stoichiometry of 1:1 at an inlet concentration of 200 ppm. It can be seen that at temperatures below 300° C. the reduction does not proceed to any significant extent. Furthermore, it has been reported that the presence of excess NH
3
at low temperature could lead to the formation of NH
4
NO
3
and (NH
4
)
2
SO
4
. There is also evidence that the presence of excess gas phase NH
3
can inhibit the NH
3
SCR reaction over some catalysts at low temperature. Urea is usually the preferred form of storing NH
3
on a vehicle. Urea is readily available and is stable in water solution. However, it only hydrolyses readily to NH
3
at temperatures greater than 150° C., and may not be a suitable source of NH
3
at low temperature. Exhaust gas temperatures, though, vary over an engine cycle and for the average light duty diesel car a significant fraction of that cycle is at low temperature. Thus, the control of NOx at low temperature is a problem.
Methods have been suggested to mitigate this problem. For instance, U.S. Pat. No. 5,785,937, JP-A-07136465 and U.S. Pat. No. 4,963,332 all suggest the use of ammonia as a reductant to convert NO
x
to nitrogen over a catalyst EP-A-0773354 also describes the use of ammonia to reduce NO
x
to nitrogen. However, ammonia is synthesis in situ over a three-way catalyst during the rich burning phase of the engine and the supply of ammonia is triggered as a function of the stoichiometry of the fuel in terms of the fuel to air ratio not as a function of temperature.
The present invention provides an improved apparatus and method for reducing the content of NOx.
Accordingly, the invention provides an apparatus for reducing the content of nitrogen oxides (NO
x
) in the exhaust gas of a lean burn engine, which apparatus comprises:
(a) an exhaust capable of allowing exhaust gases to flow therethrough;
(b) a selective catalytic reduction catalyst located in the flow-path of the exhaust gas and being capable of (i) catalysing the reduction of the NO
x
by ammonia to nitrogen and (ii) adsorbing and desorbing ammonia during the engine cycle;
(c) means for supplying ammonia from an ammonia source to the catalyst; and
(d) switching means for intermittently supplying ammonia during an engine cycle thereby enabling (i) the catalyst to adsorb ammonia when ammonia supply is switched on and (ii) the adsorbed ammonia to react with NO
x
when ammonia supply is switched off,
characterised in that the catalyst comprises a zeolite and the switching means is triggered on and off at pre-set temperature levels of the catalyst.
The invention provides also a method of reducing the content of nitrogen oxides (NOx) in the exhaust gas of a lean burn engine, which method comprises passing the exhaust gas over a selective catalytic reduction catalyst which catalyses the reduction of the NOx by ammonia to nitrogen and which adsorbs and desorbs ammonia during the engine cycle, ammonia being supplied intermittently to the catalyst during the engine cycle, the catalyst adsorbing ammonia during its supply and the ammonia which has been adsorbed reacting with the NOx when the ammonia is not supplied.
We have discovered that ammonia can be absorbed on a SCR catalyst and thereafter used in the NOx reduction when ammonia is not being supplied. It is an advantage to be able to achieve the NOx reduction while supplying the ammonia intermittently. In particular, the ammonia supply can be halted and yet NOx reduction occur when the temperature of the catalyst is low and supply would have the problems referred to above. The stored ammonia can be used as reductant for NOx over the same catalyst without the presence of gas phase NH
3
.
The ammonia can be supplied without the exhaust gas so that the catalyst adsorbs the ammonia and then the exhaust gas passed over the catalyst for the NOx reduction to occur Preferably, however, the exhaust gas is passed continuously over the catalyst.
The invention uses adsorption and desorption characteristics of the required catalyst. A higher amount of NH
3
will be adsorbed, and hence be available for subsequent reaction, if adsorption is at a lower temperature at which the catalyst adsorbs less NH
3
. Preferably NH
3
is adsorbed at a temperature at which a large amount is adsorbed; the temperature is preferably below that of maximum desorption. The temperature, however, is preferably above that at which any significant formation of ammonium salts occurs.
FIG. 2
shows the desorption profile from zeolite ZSM5 (non-metallised) of NH
3
which had been pre-adsorbed at 100° C. It can be seen that at say 300° C. more NH
3
is retained, adsorbed, than at say 400° C., and that the temperature of maximum desorption is about 370° C. Bearing in mind that the desorption of NH
3
is endothermic, it can also be seen that if NH
3
were adsorbed at say 300° C. and then heated, NH
3
would be desorbed in accordance with the graph so that less would be available for subsequent reaction, while if NH
3
were adsorbed at the same temperature, 300° C., and cooled, NH
3
would not be desorbed so the adsorbed NH
3
would be available for subsequent reaction
Chandler Guy Richard
Goersmann Claus Friedrich
Rajaram Raj Rao
Self Valerie Anne
Johnson Matthey Public Limited Company
Medina Maribel
RatnerPrestia
Silverman Stanley S.
LandOfFree
Process and apparatus for reducing the nitrogen oxide... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process and apparatus for reducing the nitrogen oxide..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process and apparatus for reducing the nitrogen oxide... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3249165