Combatting air pollution

Details Combatting air pollution Combatting air pollution

2000-10-02

2003-04-15

Denion, Thomas (Department: 3748)

Power plants

Internal combustion engine with treatment or handling of...

Methods

C060S289000, C060S301000, C060S300000, C423S213200, C423S239100, C423S244090, C423S244010

Reexamination Certificate

active

06546717

ABSTRACT:

This invention relates to a method of combatting air pollution from the exhaust gas of an internal combustion engine, and to an internal combustion engine whose exhaust apparatus contains means therefor.
One of the pollutants in the exhaust gas of an internal combustion engine is NOx (oxides of nitrogen). NOx is produced by the thermal fixation of nitrogen in the combustion air, leading to thermal NOx, or by the conversion of chemically bound nitrogen in the fuel, leading to fuel NOx. The most abundant nitrogen species in an engine exhaust stream is NO, with low amounts of NO
2
and N
2
O. For example, the nitrogen oxide species in the exhaust stream of a typical diesel passenger car are about 95% NO, 4% NO
2
and a little N
2
O.
Although nitrogen monoxide (NO) is the most abundant nitrogen species in an engine exhaust stream, various processes for the treatment of the exhaust gas proceed more rapidly when the NOx species is NO
2
rather than NO. For example, European patent specification 341832A discloses that in the regeneration of a diesel particulate trap at low temperature, NO
2
is a more effective oxidant than NO or O
2
. Again, to combat NOx emissions from lean-burn engines, it has been proposed to adsorb NOx by an adsorbent when the exhaust gas is lean (i.e. when there is a stoichiometric excess of oxygen) and release the adsorbed NO
2
when the exhaust gas is rich, the exhaust gas being periodically made rich to release the adsorbed NOx; during the lean operation, NO is oxidised to NO
2
which can then readily react with adsorbent surface to form nitrate—see for instance European patent specification 560991A. Yet again, in other strategies to combat NOx emissions from lean-burn engines, such as selective catalytic reduction (SCR), there are indications that the presence of NO
2
may be beneficial, see for instance Applied Catalysis B: Environmental, 2 (1993) 81-100, Elsevier Science Publishers BV and Journal of Catalysis 171, 27-44 (1997), Academic Press.
It would be desirable to be able to oxidise NO to NO
2
in engine exhaust gas at a lower temperature, for instance because the engine is operating at a lower temperature or while the exhaust apparatus is warming up after the engine has been started at ambient temperature.
In addition, it would be desirable to oxidise NO to NO
2
in engine exhaust gas while not oxidising SO
2
to SO
3
. Fuels such as petrol (gasoline) or diesel contain small quantities of organo-sulphur compounds, e.g. thiophene. In the engine operation, these compounds are oxidised to SO
2
. If the SO
2
is oxidised to SO
3
, the SO
3
can readily react to form sulphuric acid or sulphate, which is undesirable. For instance, in diesel engines a small fraction of the SO
2
, typically about 2%, is further oxidised to SO
3
, which condenses with water in the exhaust gas as sulphuric acid, which is then adsorbed on carbonaceous particulates, hence increasing the mass of particulates. Increasing the amount of SO
3
is thus undesirable. Again, in the technique of NOx adsorption discussed above, SO
3
readily adsorbs on the NOx adsorbent to form base metal sulphate, which, since metal sulphates are generally much more stable than nitrates, deactivates the adsorbent and requires high temperature regeneration of it. Yet again, in the SCR technique discussed above, SO
3
can react with reductant ammonia to form ammonium sulphate, which can poison the catalyst.
Catalysts which are active in the oxidation of NO to NO
2
are also very active in the oxidation of SO
2
to SO
3
. The most effective, and most commonly used, catalyst for oxidising NO to NO
2
is based on Pt, see for instance European specifications 560991A and 341832A cited above. Pt, however, is very active in the oxidation of SO
2
to SO
3
, as shown in the accompanying
FIG. 1
, which is a graph showing % conversion of NO or SO
2
by oxygen on a 2% by weight platinum on alumina catalyst plotted against temperature. The graph shows also that the conversion of NO to NO
2
at low temperature (e.g. below 250° C.) is very low.
Furthermore, the oxidation of NO to NO
2
must be practicable in the environment of an engine exhaust gas. Although many catalysts, e.g. Cu, Co or Mn, oxidise NO to NO
2
, many are not sufficiently active in an engine exhaust gas or are readily deactivated in this environment. Even Pt catalyst is inhibited by hydrocarbon and SO
2
in this reaction.
The present invention provides a method of combatting air pollution from an internal combustion engine whose exhaust gas contains NO, which method comprises contacting the exhaust gas containing NO with ozone to react the NO with the ozone to produce NO
2
and reducing the NO
2
to nitrogen.
The invention provides also an internal combustion engine whose exhaust apparatus contains means to contact exhaust gas containing NO with ozone to react the NO with the ozone to produce NO
2
and means to reduce the NO
2
to nitrogen.
International patent application publication WO 97/22794 refers to an apparatus for reducing pollutants from the combustion of a fuel, the apparatus comprising:
a combustion chamber having a pre-combustion gas stream to the combustion chamber and a post-combustion gas stream of exhaust from the combustion chamber;
a catalytic converter for treating the exhaust to reduce the amount of at least one pollutant from at least one of incomplete combustion of the fuel and oxides of nitrogen; and
a device for adding ozone to at least one of the pre-combustion gas stream and the post-combustion gas stream to thereby further reduce the amount of at least one pollutant in the exhaust treated by the catalytic converter. There is no mention of reacting ozone with NO in the exhaust gas to produce NO
2
. Indeed, while one can see that enhancing the oxidation of hydrocarbon or carbon monoxide in the exhaust gas might be beneficial, it is not apparent why one would wish to choose conditions so that NO in the exhaust gas is oxidised to NO
2
for treatment with a catalytic converter.
It can be seen that the present invention does not rely on the use of a catalyst to oxidise the NO. Instead, the oxidation is achieved by use of a particular oxidant, ozone. We have demonstrated that ozone reacts with the NO to produce NO
2
even at low temperature. In addition, we have shown that at low temperature ozone is an excellent selective oxidant, i.e. it converts all or much NO to NO
2
while converting no or little SO
2
to SO
3
.
The internal combustion engine is usually in a vehicle. The engine is especially a lean-burn engine. The engine can be a diesel or petrol engine.
In the present invention, the ozone is usually injected into the exhaust gas containing NO. Accordingly, the engine usually includes injection means to inject the ozone into the exhaust gas containing NO.
The engine usually includes an ozone generator to generate the ozone. The ozone can be generated in ways known in themselves. For instance, the ozone generator can be a corona discharge tube through which passes air between two electrodes which are kept at a large potential difference. Alternatively, the ozone generator can be a high energy lamp to ionise oxygen in air to ozone.
Generally, at least 80% and preferably 100%, of the NO in the exhaust gas contacting the ozone is reacted with the ozone to produce NO
2
. Sufficient ozone must be employed to carry out the required degree of conversion of the NO. The conversion is NO+O
3
→NO
2
+O
2
. Generally, the exhaust gas containing NO is contacted with at least 80%, preferably at least 100%, of the amount of ozone required to react stoichiometrically with the NO to produce NO
2
. The amount of ozone is usually 0.04-0.16% by volume of the exhaust gas with which it is contacted. The ozone generator is chosen accordingly. As an example, it has been found that for an exhaust gas flow rate of 7,500 liters per hour, containing 400 ppm NO, complete conversion to NO
2
can be achieved if 1 liter of air per minute is fed to an ozone generator (Thermo Electron) operating at 60% capacity; if the operation is reduced to belo

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