Purification of exhaust gases

Details Purification of exhaust gases Purification of exhaust gases

2002-11-27

2004-08-17

Denion, Thomas (Department: 3748)

Power plants

Internal combustion engine with treatment or handling of...

By electrolysis, electrical discharge, electrical field, or...

C060S295000, C060S311000

Reexamination Certificate

active

06775972

ABSTRACT:

This invention concerns the purification of exhaust gases, especially the purification of such gases from diesel and other “lean-burn” engines.
BACKGROUND OF THE INVENTION
Diesel engines are in widespread use in all types of vehicles, stationary power sources and naval and commercial shipping. They are very fuel-efficient, but because of their combustion characteristics generate particulate matter (soot, often called “PM”) on which a variety of organic substances may be absorbed, including unburnt hydrocarbons (HC) and sulphuric acid produced by oxidation of sulphur dioxide derived from sulphur species present in the fuel or in lubricants. Other engines, such as gasoline direct injection (“GDI”), can also produce significant quantities of PM, and we consider that the need for removing such PM will soon be expressed in legislation. Nonetheless, the invention may be applied to combustion processes generally, as well as potentially to chemical process stacks/exhausts, and to combustion engines operating at &lgr;=1 or greater, or lean-burn engines operating at stoichiometric or rich in order to regenerate some exhaust gas aftertreatment device. For simplicity, however, we concentrate on diesel engines hereinafter.
BRIEF SUMMARY OF THE INVENTION
In order to meet various regulations concerning the level of pollutants, it has become commonplace to fit vehicles with an oxidation or three-way catalyst, which only achieves partial removal of PM. The removal of particulates is generally achieved by using some form of filter or trap, which may be cleaned or regenerated intermittently. It has been suggested to include a catalyst in the fuel to the engine, and as well as platinum group metals (“PGMs”), iron, copper or cerium compounds have been suggested. A particulate trap may be catalysed to lower the soot combustion temperature, and some form of external heating, for example electric heating of the trap or of air fed thereto, may be used to initiate soot combustion.
A particularly successful soot trap is marketed by Johnson Matthey PLC as the “CRT” (“Continuously Regenerating Technology”) and is described in U.S. Pat. No. 4,902,487. This system uses a conversion of NO in the exhaust gas to NO
2
, which was discovered to be much more effective at typical low diesel exhaust gas temperatures in the combustion of soot than air or any other exhaust gas component. Thus, NO
2
is typically effective to combust PM at about 250° C., whereas oxygen is effective at about 650° C.
It has been suggested to use a plasma generator for exhaust gas purification (see for example GB 2,274,412 and 2 270 013, UK Atomic Energy Authority). Although it was probably not previously recognised in connection with exhaust gas treatment, such a system produces considerable quantities of NO
2
. Systems such as previously described do not include any filter or trap in combination with a plasma generator, but we believe that this may be a particularly effective system for treating diesel and similar lean-burn exhaust gases.
Accordingly, the present invention provides a system for treating exhaust gases including NO, nitrogen and particulate matter, which system comprising:
(a) a catalyst for generating NO
2
from the NO;
(b) a plasma generator for generating at least one of: (1) NO
2
from the NO or nitrogen or both; and (2) ozone; and
(c) a filter for trapping a desired proportion of the particulate matter,
wherein trapped particulate matter is combusted by reaction with at least one of NO
2
or ozone. In an illustrative embodiment, the exhaust gases are from a diesel engine.
We believe, although we do not wish to be bound by any theory, that in the present invention NO
2
may be generated not only by oxidation of NO in the exhaust gases, but also by oxidation of nitrogen to yield NO, which is itself converted to NO
2
. In the latter case, there is no reliance upon the quantities of NOx leaving the engine. It is also believed that the present invention is especially valuable in that it is not adversely affected by the presence of sulphur in the fuel or in lubricants, which can poison conventional catalysts.
The invention further provides a method of reducing exhaust gas emissions from diesel engines, comprising oxidising NO in the exhaust gas to NO
2
over an oxidation catalyst at oxidation catalyst temperatures within an optimum oxidation catalyst temperature range and using a plasma generator to generate NO
2
from NO or N
2
in the exhaust gas or both at oxidation catalyst temperatures outside of the optimum oxidation catalyst temperature range, trapping particulate matter on a filter and combusting the trapped particulate matter by reaction with NO
2
.
In a further aspect, the invention provides a method of maintaining increased levels of NO
2
in diesel exhaust gas over exhaust gas emitted from the engine independent of exhaust gas temperature, which NO
2
is for combusting particulate matter filtered from the exhaust gas, which method comprising oxidising NO in the exhaust gas to NO
2
at an oxidation catalyst temperature within an optimum oxidation catalyst temperature range and using a plasma generator to generate NO
2
from NO or N2 in the exhaust gas or both at oxidation catalyst temperatures outside of the optimum oxidation catalyst temperature range.
In yet a further aspect, the invention provides a method of increasing levels of NO
2
in an exhaust system to combust particulate matter trapped on a filter in the system, which system includes an oxidation catalyst effective to oxidise NO to NO
2
over an optimum oxidation catalyst temperature. The method according to this aspect comprises using a plasma generator to generate additional NO
2
from one or both of NO and N
2
during engine conditions which are pre-determined to generate increased amounts of particulate matter even when the oxidation catalyst temperature is within the optimum oxidation catalyst temperature range.
The oxidation catalyst can be any catalyst suitable for oxidising NO to NO
2
in the presence of oxygen. Typical catalysts for this reaction include platinum on alumina or silica-alumina. The loading of the platinum can be between 0.5 to 200 g ft
−3
, such as 50 g ft
−3
. A washcoat of the platinum and the alumina support can be coated on a ceramic, e.g. cordierite, substrate of 100-600 cells per square inch (cpsi), typically 400 cpsi. Alternatively a metal substrate can be used, in which case typical cpsi values can be up to 1000. The oxidation catalyst can be positioned to treat all or a portion of the exhaust gases upstream of the filter, or may be fitted downstream of the filter to treat all or a portion of the filtered exhaust gas, with recirculation of the treated gas to the filter. The oxidation catalyst can be positioned upstream or downstream of the plasma generator.
The plasma generator may be any suitable type producing a non-thermal plasma, and may be enhanced by electromagnetic radiation. Suitable plasma generators include high voltage (e.g. 20 kV or more) alternating current, preferably pulsed, generators, suitably using two dielectric plates positioned in the gas flow, and piezoelectric devices such as piezoceramic transformers. It may be positioned to treat all or a portion of the exhaust gases upstream of the filter, or may be fitted downstream of the filter to treat all or a portion of the filtered exhaust gas, with recirculation of plasma-treated gases to the filter. In one embodiment of the present invention, a predetermined proportion of the exhaust gases is treated by the plasma to cause substantially all of the NO present to be converted into NO
2
, and the resulting gases blended with untreated exhaust gases, thus resulting in a desired blend of NO and NO
2
, which according to some studies, may be more effective for the purposes of the present invention than a gas containing substantially only NO
2
in admixture with other exhaust gas components.
The filter used may be a woven or knitted wire filter, a gas-permeable metal or ceramic foamed mass or a wall flow filter of generally known type (ho

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