Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
2001-06-07
2004-07-27
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C423S437100
Reexamination Certificate
active
06767526
ABSTRACT:
The present invention relates to a process for reducing the harmful emissions of internal combustion engines and significantly limiting their carbonaceous discharges.
It relates more particularly to the conjoint use of at least one oxidation catalyst for the said carbonaceous particles and of nitrogen dioxide for the combustion of the carbonaceous matter issuing from internal combustion-engines.
When fuels are burnt, the carbonaceous products or hydrocarbons form, in their combustion products, carbonaceous particles which are also referred to, in the remainder of the description, by the expression “soot(s)”; which are reputed to be harmful both to the environment and to health. Moreover, these soots deposit on all of the internal walls of the engine and may give rise to malfunctions, especially in turbochargers.
Consequently, the search has long been on for techniques which make it possible to reduce the emission of these carbonaceous particles. This search is, moreover, concommitant with the requirement not to increase the emission of carbon monoxide and of harmful and mutagenic gases such as oxides of nitrogen.
A very great number of solutions have been proposed for reducing these carbonaceous emissions.
Among these solutions, the technique employed most widely consists in adapting in the exhaust circuits a filter capable of halting all or a very great proportion of the carbonaceous particles produced by the combustion of the various fuels. Filters have therefore been produced which, when installed in the exhaust circuits, allow soot emissions to be reduced by at least 85% by mass.
The problem to be solved then moved to the level of these filters. Accummulating progressively in the filters, the soots cause firstly an increase in pressure loss and, secondly, the beginning of an occlusion which leads to a drop in performance of the internal combustion engine.
The research efforts then shifted to the burning of soots collected by these filters.
This so-called burning operation is extremely delicate in terms of its provision and implementation.
It is possible to bring about the combustion of the soots intermittently either by an electrical heater or by a burner or any other technique employing an external energy source.
Another solution consists in drawing the heat required to ignite these soots from the engine itself, so as to heat the soots accummulated in the filter and in so doing to bring about their inflammation (temperature of the order of 500-600° C.).
It has also been proposed to introduce inflammation catalyst precursors into the various fuels so as to lower the inflammation temperature of the soots.
Another solution is aimed at utilizing oxidation catalysts based on noble metals deposited on supports based on alumina or titanium. They make it possible to facilitate, at low temperature, the oxidation of the carbon monoxide and gaseous hydrocarbons emitted by diesel engines. The discharge of carbon monoxide and hydrocarbons unburnt at 300° C. is thereby reduced significantly, to the order of 80 to 90%. However, it is important to note that these catalysts have no oxidizing action whatsoever on the carbonaceous fraction of the soots and, furthermore, have the disadvantage of discharging into the atmosphere not inconsiderable amounts of nitric acid, which give rise to an increase in the acid rains harmful to man and the environment. However, as mentioned before, the search for a solution to the combustion of the soots is concommittant with the requirement not to increase the emission of carbon monoxide nor of gases reputed to be mutagenic and toxic, such as oxides of nitrogen.
One of the most interesting solutions to date consists in adding directly to the fuels an additive deriving from transition metals, alkali metals, alkaline earth metals and/or rare earth metals (EP 0 599 717). The combustion of these soots is significantly improved by this means. However, for the oxidation of these soots to be optimum, it remains necessary for the temperature of the gases to be treated to be at least of the order of 300° C.
Another proposed solution takes advantage of the composition of the exhaust gases emerging from diesel engines. Generally, these gases comprise significant amounts of oxides of nitrogen (NO, NO
2
), oxygen, carbon monoxide, carbon dioxide, water and, if appropriate, sulphur dioxide. Thus the patent EP 341 832 proposes converting this nitrogen monoxide into nitrogen dioxide by catalytic oxidation and using the resultant nitrogen oxide as an oxidizing agent for the particles of carbon accummulated on the filter. However, the temperature conditions within which combustion takes place are narrow. They are limited to a temperature of between 250° C. and 400° C. Finally, it appears that the reaction rate of oxidation of the carbonaceous particles by nitrogen dioxide is reduced not inconsiderably by the sulphur dioxide and when the ratio between the oxides of nitrogen (NO+NO
2
) and the carbon formed by the engine is insufficient.
The object of the present invention is more particularly to provide a novel treatment process which makes it possible specifically to optimize the combustion of carbonaceous particles within a significantly widened temperature range.
More precisely, the present invention provides a process for the combustion treatment of carbonaceous particles collected on a filter situated in an exhaust circuit of an internal combustion engine, characterized in that the combustion of the said particles is brought about by contacting them with a gas mixture comprising at least nitrogen dioxide generated within the exhaust circuit of the said engine, the said particles having been seeded prior to their combustion with at least one catalyst of their oxidation.
Unexpectedly, the inventors found, in fact, that when the oxidation of soots by nitrogen dioxide is conducted in the presence of a soot oxidation catalyst, also denoted hereinbelow by the name “SOC”, It is possible to effect their combustion within a significantly widened temperature range.
The combustion of the particles is advantageously carried out at a temperature lower than that required for the combustion of particles simply seeded with an oxidation catalyst. This temperature of combustion by nitrogen dioxide is also less than that required for the combustion of unseeded particles.
The process claimed also enables the combustion of particles in a wide temperature range, which corresponds to that encountered for diesel engine exhaust gases. Therefore, in contrast to other processes, the process of the invention is effective at a very low exhaust temperature, i.e. at less than 250° C. and, in particular, in the range 200-250° C. However, it remains effective at temperatures exceeding 400° C. as well.
For the purposes of the invention, the term “particles seeded with an oxidation catalyst” is intended to cover carbonaceous particles in and/or on which the soot oxidation catalyst SOC is dispersed in the form of very fine particles. Within the context of the invention, the carbonaceous particles possess the feature of being already combined with the oxidation catalyst when they are placed in the presence of the nitrogen dioxide.
As far as the oxidation catalyst SOC is concerned, it comprises at least one element selected from transition metals, alkali metals and alkaline earth metals, such as manganese, iron, copper, sodium, nickel and scandium, and the rare earth metals. These elements are preferably incorporated in the said catalyst in the form of their oxides. The catalyst can of course comprise a plurality of elements, it being possible for each element to be present, independently of the others, in the form of its corresponding oxide or otherwise.
In accordance with a preferred embodiment of the invention, the oxidation catalyst is a compound containing at least one rare earth.
The expression “rare earth” is intended to denote the elements whose atomic number is between 57 and 71, and also yttrium.
Indeed, the rare earths, and especially the oxides of rare earths, such as, in particular, those
Blanchard Gilbert
Mustel William
Seguelong Thierry
Johnson Edward M.
Rhodia Chimie
Silverman Stanley S.
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