EP0737236A1

EP0737236A1 - Filtration and combustion process for carbon particulate matter from an internal combustion engine

Info

Publication number: EP0737236A1
Application number: EP95905693A
Authority: EP
Inventors: Philippe Barthe; Jacques Lemaire; Denis Petta
Original assignee: Rhone Poulenc Chimie SA
Current assignee: Rhodia Chimie SAS
Priority date: 1993-12-31
Filing date: 1994-12-30
Publication date: 1996-10-16
Also published as: US5813223A; CN1139951A; WO1995018198A1; MX9500019A; JPH09507278A; AU1420595A; CA2180181A1; HU9601778D0; BR9408456A

Abstract

The object of the invention is a filtration and combustion process for carbon particulate matter from an internal combustion engine. The process is characterized by the following steps: a rare earth or rare earth mixture derivative is introduced into the fuel at a concentration of 10 ppm to 500 ppm (by weight), and preferably of 20 ppm to 200 ppm; the soot produced by the internal combustion engine is collected on a filter, the temperature of the gases entering the filter being selected in the range of 100 °C to 350 °C; the soot is allowed to build up until a significant fraction or rate of incoming soot is balanced by the combustion of soot in the soot cake on the filter, and no regeneration is effected as long as the head loss caused by the soot does not exceed a preselected value and is not higher than 400 millibars. This invention is useful for organic synthesis.

Description

A METHOD OF FILTERING AND COMBUSTING CARBONACEOUS MATERIALS FROM A COMBUSTION ENGINE

INTERNAL.

The subject of the present invention is a process for the filtration and combustion of carbonaceous materials from internal combustion engines. The invention relates more particularly to the regulation of the pressure drop caused in the filters by the accumulation of soot in the filters.

During the combustion of fuels in internal combustion engines and in particular when that of diesel in the diesel engine, carbonaceous materials are formed, hereinafter called "soot", which are reputed to be harmful both to the health of higher mammals only for the environment.

This soot is particularly abundant in the case of diesel engines and constitutes a handicap for this type of engine. Most of the solutions envisaged consisting in changing the engine speed or its operating parameters come up against another constraint, that of not increasing the emission of carbon monoxide and / or of gases reputed to be toxic and mutagenic such as oxides of 'nitrogen.

In view of the above, the most effective technique seems to be the adaptation to the exhaust of a filter capable of stopping all or at least most of the soot formed by the combustion of the various fuels.

We have thus succeeded in producing filters, in particular cordiόrite, which make it possible to reduce the emissions of soot by at least 85% by mass.

The problem to be solved resides in the accumulation of this soot in the filters which firstly causes an increase in pressure drop and, secondly, a start of blockage which leads to a loss of efficiency of the combustion engine. internal.

We have tried many times techniques for burning this soot. It has thus been proposed to cause the combustion of this soot intermittently either by electric heating or by heating with a fuel. fossil igniter. We also considered the possibility of drawing the heat necessary for the ignition of this soot in the engine itself by a skilful management of gas flows so as to heat the soot accumulated in the filter and ipso facto to cause their ignition ( temperature of the order of 500-600 ° C).

It has also been proposed to introduce ignition catalyst precursors into the various fuels, so as to lower the temperature of the ignition of the soot. These techniques, in particular that of the combination of increasing the temperature of the soot using an adapted and transient exhaust gas circuit with the addition of oxidation catalyst precursors, made it possible to resolve the least partially the problem.

However, on the one hand the temperature of ignition of the soot remains relatively high (of the order of 500 ° C.) and on the other hand intermittent and brutal combustions are likely to cause significant deterioration of the filter and of its capacities. filtration, either by cracking due to thermal shock, or even by fusion. This alteration of the filters can take the form of a loss of capacity to retain a high percentage of soot, while the initial percentage is sometimes considered insufficient.

This is why one of the aims of the present invention is to provide a process for burning the soot which is as continuous as possible. Another object of the present invention is to provide a method of burning soot which does not result in sudden, violent and sudden ignitions leading to damage to the filter. To achieve such a result, it should be avoided that during the ignition phase, at no point in the filter does the temperature reach 1000 ° C., advantageously 900 ° C., preferably 700 ° C.

It is difficult to measure local temperatures, so it has been established that these latter constraints correspond to maximum gas temperatures, at the outlet of the filter, of at most approximately 600 ° C. ("approximately" means here that zeros , which are here position zeros, are not significant figures), preferably at around 500 ° C.

Another object of the present invention is to provide a method of filtration and combustion of soot, which makes it possible to improve the proportion of soot retained on the filters and then subsequently burned. These aims, and others which will appear subsequently, are achieved by means of a process for treating soot containing one or more rare earths, process in which said soot is brought into contact with a gas containing oxygen at a temperature between 100 ° C and 400 ° C, advantageously between 150 ° C and 350 ° C, preferably between 200 ° C and 300 ° C, the partial oxygen pressure of the oxygen-containing gas being at least equal to 3% atmosphere, or 3.10 ³ Pascals, advantageously 4% atmosphere, or

4, 10 ³ Pascals.

It is even desirable that the partial oxygen pressure be at least equal to 6.10 ³ Pascals, preferably 8 kilo Pascals.

It is also desirable that the contact between the oxygen-containing gas and the soot be maintained for a period of time sufficient to burn at least 90% of the soot.

Indeed, completely unexpectedly, it has been found that rare earths and in particular rare earth oxides, especially those due to cerium catalyze the oxidation of carbonaceous materials at temperatures as low as 100 ° C.

(this value is a rounded value indeed, it could even be noted an oxidation at temperatures of the order of 80 ° C).

This phenomenon only occurs when the oxygen content of the gas is sufficiently high. The threshold from which soot oxidation occurs depends to some extent on the value of the other parameters. It increases when the rare earth content of the soot decreases and / or when the temperature approaches the low values of the ranges mentioned above.

It should be noted that during the previous tests, this regeneration could not be observed because the oxygen content of the exhaust gases is under the usual conditions for testing such a process is chosen so as to maximize the soot and is much lower. to that required for this oxidation which can be described as gentle.

The method according to the invention can be used in many ways to reduce the emissions of solid carbonaceous rejection from all sooty combustions and in particular from internal combustion engines.

We can thus distinguish continuous implementations from those which are intermittent.

In the latter case, if the oxidation of carbonaceous materials occurs as soon as all the soot and gas is at the appropriate temperature, it is preferable that the partial pressure of oxygen and temperature be maintained for a sufficiently long period to oxidize all of the soot. produced in order to prevent them from accumulating or being rejected. So on average or statistically or continuously, it is appropriate that, for a defined duration, the sum of the periods of time when the conditions of the present invention are met is sufficient for the soot produced during said defined duration to oxidize in their entirety. From this defined duration depends the constraint imposed on the soot producing and purifying system. In the case where, as in the particle filters, the residence time is large or even infinite with regard to their life expectancy, it is possible to provide said long defined durations. Said defined duration can then reach ten minutes or even 1/2 hour. In the case of internal combustion engines fitted with a filter, a defined long period is paid for by periods during which the pressure drop caused by the filter can be relatively high. However, the accumulation of particles in the filters improves the quality of filtration. So there is a compromise to be found. Thus, it is therefore preferable to choose a defined duration which prevents the carbonaceous particles from causing a pressure drop greater than approximately half an atmosphere. More precisely, it is desirable that 2/5 (or 2 / 5.10 ^ Pascals), advantageously at 1/4 of an atmosphere (ie 1 / 4.10 ⁵ Pascals), preferably 0.2.10 ^ Pascals, and, if one does not wish to improve the filtration characteristics of soot, more preferably 0.15.10 ⁵ Pascals.

According to an implementation of the present invention, it is possible to play on the parameters of the engine and the possible introduction of an oxygen-containing gas so that the constraints of temperature and of oxygen content are fulfilled so that during a so-called "European" cycle (EEC standard), there is no accumulation of soot in the filter.

It is also possible to subject the implementation of the method according to the present invention to a limit value of pressure drop. The time required for gentle oxidation of a given amount of carbonaceous matter obviously decreases with temperature and with the oxygen content. This gives a degree of freedom in the implementations of the present invention.

The present invention makes it possible to envisage the use of systems which are less restrictive than the particle filter. One can in particular envisage systems having the characteristic of cyclones or any system allowing an increase in the duration of the residence time in an environment where the conditions prevail, for the implementation of this catalytic system. Under these conditions, if it is desired to eliminate the soot emitted by the combustion systems, said defined duration is at most equal to the residence time of the particles in the space where the conditions according to the present invention prevail constantly or intermittently.

We can thus envisage several implementations of the present invention. According to one of the embodiments of the present invention, it is possible to use a filter collecting the soot produced by the combustion engine, however, if necessary, air pulses, enriching the exhaust gases with oxygen, allow time in time the partial month regeneration of the filter. This oxygen enrichment of the exhaust gases to reach a value where the method according to the present invention is implemented is not necessary for all operating modes of the engine and can be obtained by any means.

It can in particular be enriched by injecting air into the flow of exhaust gases. This air can advantageously be heated by contacting different heat sources and in particular with hot parts of the engine directly or through a heat exchanger. It is also conceivable to use thermal flywheels which would be heated during the period of non-regeneration of the filter by the exhaust gases and would restore the heat stored in the air in charge of enriching the exhaust gases with oxygen during the periods of use of the method according to the present invention.

In certain cases, the simple mixing of the outside air with the exhaust gases makes it possible to obtain a temperature and an oxygen content sufficient to be placed in the zone where the catalytic oxidation takes place in good conditions.

The conditions prevailing in the turbo of a diesel engine are sufficient to obtain combustion of the carbonaceous particles which, in general, contaminate the turbo of the turbo engines. Indeed, completely unexpectedly, it has been shown that the addition of cerium, or any other rare earth, makes it possible to avoid any carbonaceous deposit in the parts of the turbo, thus facilitating their implementation and their operation. And this, although the temperature conditions prevailing in the turbo are very significantly lower than those which allow the ignition of soot even doped with rare earth.

The preferred rare earths are cerium, neodymium and lanthanum.

They can be used alone or in admixture with potentiating elements including with other rare earths one can refer mutatis mutandis to the part of the description directed more particularly on lanthanum. However, to date, the best results have been obtained using cerium and lanthanum. Alone or in combination with other rare earths.

In their catalytic function, these metals are preferably in the form of their oxide IV in the case of cerium (and for the good rule for praseodymium, but it is not very economical), bivalent oxide in the other cases.

It is possible to use mixtures of rare earths but, in this case, it is preferable that cerium and / or lanthanum are the majority.

It is also possible to use mixtures of rare earths of the preceding type, doped with non-rare earth elements (cf. part on doping lanthanum, which part is transposable for any rare earth or mixture of rare earth).

Rare earths can be introduced into soot by the introduction into the fuel of a derivative such as their salts or their soils.

The introduction of compounds based on rare earth (s) can be carried out in particular by the introduction of compounds based on rare earth (s) in the fuel intended to be introduced into the engine.

Another possibility is to introduce the rare earth (s) in various forms through the air, and in particular air when it is mixed with engine exhaust gases. , when a fraction of the exhaust gases is recycled in the engine. In this case, the rare earth compounds incorporated in the recycled soot are introduced into the engine.

It should however be emphasized that the form in which the rare earth (s) is introduced into the fuel is not without effect on the size of the crystallites and aggregates which are found in the soot, size which plays a role in the catalytic capacity of rare earths present in soot (cf. French patent application N ° 92/14158 filed on 25.11.1992 and entitled "aggregate of ceric oxide crystallites, process for obtaining it and its use to reduce combustion residues ").

Furthermore, the lifespan of cerium salts and soils in organic medium is often short and can be the cause of poor results in the catalytic effect. So when you want to use cerium it is better to use either the soil or the salts which are the subject of the European patent application filed under No. 93 / 304760.7 and published under No. 0575189. The extemporaneous addition to the fuel of the non-stable compounds makes it possible to at least partially overcome the difficulties linked to the non-stability (case of cerium 3 salts not stabilized). Advantageously, the quantity of rare earths introduced into the engine is determined so that the content of rare earth (s) (by mass of metal contained) in the soot reaches a level of between 1000 ppm and 30%, advantageously at least 5000 ppm preferably at least 5% and advantageously at most 25%, preferably at most 10%. On average during a European cycle, a satisfactory value is of the order of 5 to 10%.

It should however be stressed that the form in which the rare earth (s) is introduced into the fuel is not without effect on the size of the crystallites and aggregates which are in the soot, size which plays a role in the catalytic capacity of rare earths present in soot (cf. French patent application N ° 92/14158 filed on 25.11.1992 and entitled "aggregate of ceric oxide crystallites, process for obtaining it and its use to reduce combustion residues "). Contrary to what one could initially believe, this teaching is valid not for the only cerium, but for all the rare earths, alone or in mixture, doped or not, and not as only reserved for the cerium. Thus in the following passage which deals with the aggregates and crystallites of cerium (ceric) oxide which compose them, cerium oxide plays the role of paradigm (in the same way as the verb "to love" is the paradigm of the first French conjugation or that "dominus" is the paradigm of the second Latin declination) of rare earth oxides.

Thus it is preferable to make use of directly or indirectly aggregates of ceric oxide crystallites, aggregates whose largest dimension is between 20 Å (2 nanometers) and 10,000 Å (1,000 nanometers) preferably between 50 Â (5 nanometers) and 5,000 Â (500 nanometers) and where the size of the crystallites measured for the plane (1, 1, 1) by the technique of Debye and Scherrer is between 20 and 250 Â (2 to 25 nanometers) preferably 100 to 200 nanometers. It should be emphasized that these measurements are virtual measurements and that it would probably be more correct to refer to the width of the x-ray peak.

This is the reason why the technique for measuring the crystallite size according to the Scherrer technique will be indicated below. It should also be noted that position zeros are not, except where expressly indicated, significant figures. It is preferable that these aggregates are topologically as close as possible to the soot, this is why it is desirable to introduce into the combustion chamber or to manufacture in situ said aggregates so that these aggregates can form simultaneously serve as germs for soot. To be effective, it is advisable to add, or to form "in situ", cerium oxide in the form of aggregates specified above at a content of at least 10 ppm relative to the carbonaceous fuel, preferably 20 ppm more advantageously 50 ppm.

It is preferable that the cerium oxide thus formed has a particle size such that the dβo (diameter of the mesh allowing 80% by mass of the product to pass) is at most equal to 10,000 Å (1,000 nanometers), preferably 5,000 Å ( 500 nanometers). It is also preferable that the ∑ 0 is greater than 200 Å (20 nanometers) and preferably 500 Å (50 nanometers).

The characteristics of soot containing the above aggregates may play a role in the present invention. It is therefore preferable that the particle size of the soot is such that the grain has a do equal to at least 100 Å and / or a dβo at most equal to 1000 Å and which contain at least 0.01%, advantageously at least 0.1 %, preferably at least 0.5% aggregate according to the present invention.

Advantageously on average the content of rare earth (s) (by mass of metal contained) in the soot reaches a level of between 1000 ppm and 30% advantageously at least 5000 ppm preferably at least 5% and advantageously at most 25 %, preferably at most 10%. In general, the soot grains form clusters with a dso of between 2000 and 5000 Å. According to an implementation of the present invention, the soot thus formed has a total cerium content of between 1 and 5% by weight, preferably from 1.5 to 2.5%.

As has been mentioned above, advantageously the aggregate of crystallites is formed during the combustion of the fuel, or of the fuel, the latter being added with at least one compound of cerium, preferably tetravalent in the form of solution or of soil.

It is also preferable that the d2o be greater than 200 Å (20 nanometers) and preferably 500 Å (50 nanometers). The elements promoting this low temperature combustion (or regeneration) are the oxygen content and the content of hydrocarbon compounds. With regard to the oxygen content (which has already been treated above), it is preferable that the oxygen content is at least equal to 3%, preferably approximately 5%.

As regards the presence of hydrocarbon compounds in the exhaust gases, it is preferable that the volatility of the hydrocarbon compounds, their content in the gases and their temperature are such that, at the temperature where the soot is liable to be subjected upon combustion (for example accumulate in the particulate filter), the content (mass ratio) of volatile hydrocarbon compounds in the soot is at least equal to one tenth, preferably a quarter, more advantageously a half of the mass over dry. By volatile is meant all the hydrocarbon compounds, in particular those which exist in the exhaust gases, which are in the form of gas at 600 ° C., advantageously at 400 ° C., preferably at 350 ° C.

It is desirable that these hydrocarbon compounds have a boiling point of between about 100 and 400 ° C.

This regeneration generally occurs when the content of hydrocarbon compounds in the gases is greater than or equal to 10 ppm, preferably greater than 20 ppm.

The best results are obtained with gas oils of which 95% by mass of the constituents distill under atmospheric pressure at a temperature at least equal to 160 ° C. advantageously at 180 ° C. and of which 95% by mass of the constituents are volatile under atmospheric pressure at 400 ° C preferably at 360 ° C.

The process gives good results with gas oils with a high aromatic content, than with gas oils with a high aliphatic content, provided that the distillation constraints set out above are observed. Most often the compositions are compounds of rare earth (s) liquid under the conditions of use (in particular ambient temperature with the engine), in the form of sol (s), or dissolved, in hydrocarbon diluents, in particular the fuels, including diesel. Thus, the present invention is particularly advantageous for two kinds of fuel, those whose aromatic content is very high [content of aromatic derivative (s) is at least equal to 1/5 advantageously to a third], because it allows to use these fuels which without this invention would lead to excessively disturbing deposits. Furthermore, it is on fuels (that is to say said mixtures) called paraffinic, the paraffin content of which is at least equal to 30% that the effects are most marked. These fuels are being studied to meet new, more stringent standards. It is desirable that for this type of composition, the aromatic content (by mass) is at most equal to 1/5, advantageously 1/10, preferably 1/20.

The invention is particularly well suited to particles emitted by fast diesel engines (as opposed to slow engines). These engines are mainly used in land transport, such as heavy goods vehicles (trucks, coaches, etc.), and light vehicles. The term “fast diesel engine” means engines whose maximum power is reached at rotational speeds at least equal to 1500 revolutions / minute; advantageously at least equal to 1800 revolutions / minute.

A particularly advantageous implementation of the present invention consists in a process for filtering the gases of an internal combustion engine. This process which consists in: introducing into the combustion chamber at least one rare earth derivative or mixture of rare earths at a concentration of between 10 ppm and 500 ppm (by mass), preferably between 20 ppm and 200 ppm;

collecting the soot produced by the internal combustion engine on a filter, the temperature of the gases entering the filter being chosen in the range 100 ° C-350 ° C (position zeros are not significant figures);

and

allow the soot to accumulate until reaching a regime where a significant fraction of the incoming soot is compensated by the combustion of soot in the soot cake on the filter and do not provide for any regeneration as long as the pressure drop caused by the soot n '' does not exceed a value chosen in advance and does not exceed 400 millibar.

The above pressure drop does not incorporate the pressure drop caused by the filter not loaded with soot, pressure drop which is generally less than 100 millibar, and very often less than 50 millibar. For reasons that are not fully understood, metal filters give particularly good results, or more precisely, particularly frequent regenerations.

The filters give better results after 3, preferably 5 regeneration cycles.

Surprisingly, it has been observed that there is, in general, no need to cause the regenerations when the above conditions are respected. Allogenic (or exogenous) regeneration is generally not useful and should only be considered when it is desired to maintain a particularly low pressure drop level (generally less than 200, even 150 millibar).

As an allogeneic regeneration, it is therefore advisable to envisage regenerations carried out by overheating (to bring the point to a temperature at month equal to 500 ° C. advantageously at 600 ° C.) non-autogenic punctual, and not global over the entire filter; this possibility is only advantageous because, according to the invention, a point overheating does not lead to an overall and complete regeneration of the filter.

The point overheating can be carried out by any means known to those skilled in the art, such as micro-resistance (s) distributed over the surface of the filter, metal particles heated by eddy current, mini-arc or the like.

The filtration mode according to the invention generally works only during part of the engine speed, since some of the modes generate gases with a temperature of at least 500 ° C (two significant figures) which ipso facto performs a gradual and often complete regeneration of the filter. It should be noted that the temperature of the gases entering the filter can vary widely within the 100-400 ° C temperature range. These variations, which can be deliberately caused, promote regeneration and maintain a low value pressure drop. It is advantageously possible to choose the position of the filter so that the temperature of the filter is for most of the time possible at a temperature included in the above ranges.

As indicated above, the preferred rare earths are cerium, lanthanum and mixtures containing cerium and lanthanum. The most common content of the rare earth (metal) fuel contained is between 50 and 150 ppm.

According to the present invention, the rare earth content of the fuel can be chosen so as to adjust the pressure drop to a value chosen in advance. We can also play on the filtering surface, and while remaining in the range specified above the temperature.

This value chosen in advance is preferably between 100 and 400 millibar, preferably between 150 and 300 millibar.

To obtain such results, it is preferable that the rare earths present in the soot have a concentration between 500 ppm and 10%, preferably between at least 1000 ppm (mass of metal contained relative to the total mass of soot, including including the compounds they have adsorbed) and at most 5% on average.

In most of the engines marketed to date, it is advantageous to introduce the rare earth, or the mixture of rare earths, with its procession of impurity (s) and adjuvant (s), with a content of between 10 and 1000 ppm in the fuel. These values are expressed in contained metal. Advantageously, contents of 20 to 200 ppm are used, preferably from 50 to 150 ppm. Some internal combustion engines, such as gasoline engines and some diesel engines under test and development, produce or should produce less soot. Insofar as only the regeneration effect of the filter is sought (and not the general improvement of combustion), this makes it possible to reduce the quantities of rare earth, or mixture of rare earths to be introduced; in fuel, when this mode of introduction has been favored; and more generally in the combustion chamber. This reduction is made in proportion to the production of soot to maintain the rare earth content, or the mixture of rare earths in said soot.

It is advantageous that the form in which the rare earth, or the mixture of rare earths is introduced induces the formation of aggregates of crystallites of rare earth oxide, or mixture of rare earths, where the largest dimension of said aggregate is between 20 angstroms and 10,000 angstroms, preferably between 100 and 5,000 angstroms, for which the size of the crystallites is between 20 and 250 angstroms, preferably between 50 and 200 angstroms. The introduction of compounds based on rare earths, alone or in mixture, can be carried out in particular by the introduction of compounds based on rare earths, alone or in mixture, in the fuel intended to be introduced into the engine.

Another possibility is to introduce the rare earths, alone or as a mixture, in various forms by means of air, and in particular air when it is mixed with engine exhaust gases, when a gas fraction exhaust is recycled to the engine. In this case, the rare earth compounds, alone or as a mixture, incorporated into the recycled soot are introduced into the engine.

One of the most practical means of introducing rare earths, alone or as a mixture, into the engine circuit consists in introducing it into the fuel either in the form of salt or in the form of soil. These compounds, salts or soils, advantageously contain products which are not harmful to combustion or to the environment. Thus, it is preferable that the salts or the soils are prepared from hydrocarbon compounds such as the carbon acid salts, whether they are of the carboxylic type or of the type with mobile hydrogen compounds such as, for example, acetylacetonates.

It is also possible to envisage compounds of the sulfur-based acid type such as sulfuric acids (alkyl or aryl acid sulfates) or else acids of the sulfonic type. However, the latter acids have the drawback of increasing the sulfur content of the fuel.

Generally for the rare earths having as valence III the highest, the carboxylic acid salts from C2 to C20. preferably from C4 to C15, are among the best suited for this use. For cerium, derivatives of cerium IV are preferred because of their stability and their ability to make few particles. It is preferable that the rare earth oxides or mixtures of rare earth oxides are stable in the fuel. Cerium is the preferred rare earth, alone; or in combination. In fuels, cerium can be introduced either in the form of soils or in the form of various salts provided that the latter are sufficiently stable in the medium. Mention may in particular be made of the salts which are the subject of the European patent application filed under the number 93 / 304760.7 and published under the number 0575189.

In fact, completely surprisingly, it has been found that when additives based on transition metals (that is to say metals with one of the sub-layers d is being filled) ) regeneration at low temperature was either non-existent or gave rise to sudden inflammations leading to high temperatures liable to damage the filters. On the contrary, in the case of elements supplemented with derivatives based on rare earths, one reaches rather quickly, after an accumulation phase in the filter, a state in which the quantity of soot arriving on the filter is compensated by numerous combustions random, but not brutal, having taken place in the mass of soot accumulated on the filter.

This results in slightly marked thermal effects as well as much lower pressure drop variations.

Thus, it has been shown that the use of additives based on rare earth elements leads to a double advantage:

- firstly, the limitation of thermal excursions (due to the regeneration of the filter) makes it possible to preserve the filtering material all of its properties and in particular its filtration efficiency; the system has a more sustainable and better efficiency;

- secondly, the behavior in the presence of additives based on rare earth elements allows more flexible management of the pressure drop phenomena; in fact, in the case of additives based on elements of the transition metals (in particular copper and iron), the regenerations are random, violent and brutal, the pressure drops change very suddenly. This causes variations in engine power and affects driving safety and comfort as well as the proper functioning of the engine; on the other hand, in the case of additives based on rare earth elements, the regenerations are of low amplitude which reduces the effects on the pressure drop and the thermal effects. The pressure drop due to the filter stabilizes and can be managed without penalizing safety and driving pleasure.

According to one of the preferred modes of the present invention, the lanthanum derivatives can be used to practice the present invention. This led to the development of lanthanum derivatives as well as other doped rare earths.

By a method for reducing the emission of soot from an internal combustion engine in which exhaust gases are passed through a particle filter and into which an additive containing lanthanum is introduced into the combustion chambers.

According to one of the embodiments of the present invention, the lanthanum is added to the fuel in the form of a salt or a stable soil.

The use of rare earths as combustion aids has been described for a long time in the state of the prior art, but lanthanum has never been cited except incidentally, or rather accidentally, as a member of this family. More recently, in an exhaustive study carried out at the French Petroleum Institute on different rare earths and on their ability to catalyze the oxidation of soot, M. DESOETE showed that lanthanum had no catalysis property on carbonaceous particles .

This article published in English and entitled "Catalysis of soot combustion by metals oxides" dated February 1988 and published under the reference 35991 (available on public request) clearly shows in its figure 10 the absence of effect of the oxide of lanthanum.

Completely surprisingly, the soot formed by the introduction of lanthanum-based compounds into the circuits of internal combustion engines, and in particular diesel engines, have the property of being significantly more flammable, that is to say with a lower ignition temperature than soot prepared without additives.

The introduction of lanthanum-based compounds can be carried out in particular by the introduction of lanthanum-based compounds into the fuel intended to be introduced into the engine.

Another possibility is to introduce lanthanum in various forms through the air, and in particular air when it is mixed with engine exhaust gases, when a fraction of the exhaust gases is recycled in the engine. In this case, the lanthanum compounds incorporated in the recycled soot are introduced into the engine.

One of the most practical ways to introduce lanthanum into the engine circuit is to introduce it into the fuel either in the form of salt or soil form. These compounds, salt or be, advantageously contain products which are not harmful to combustion or to the environment.

Thus, it is preferable that the salts or the soils are prepared from hydrocarbon compounds such as the carbon acid salts, whether they are of the carboxylic type or of the type with mobile hydrogen compounds such as, for example, acetylacetonates.

It is also possible to envisage compounds of the sulfur-based acid type such as suic acids (alkyl or ary acid sulfates) or else acids of the sulfonic type. However, the latter acids have the drawback of increasing the sulfur content of the fuel.

In general, the carboxylic acid salts from C2 to C20. preferably from C4 to C-j 5, are among the best suited for this use.

To obtain a good catalysis of the combustion of soot, it is advisable to ensure in the soot a satisfactory concentration of lanthanum and of its possible potentiating additives; this concentration is advantageously between 500 ppm and 10%, preferably between at least 1000 ppm (mass of metal contained relative to the total mass of soot, including the compounds which they have adsorbed) and at most 5% by average.

In most engines marketed to date, it is advantageous to introduce lanthanum, with its procession of impurity (s) and additive (s), at a content of between 10 and 1000 ppm in the fuel. These values are expressed in contained metal. Advantageously, contents of 20 to 200 ppm are used, preferably from 50 to 150 ppm.

Some internal combustion engines, such as gasoline engines and some diesel engines under test and development, produce or should produce less soot. Insofar as only the regeneration effect of the filter is sought (and not the general improvement of combustion), this makes it possible to reduce the quantities of lanthanum to be introduced; in fuel, when this mode of introduction has been favored; and more generally in the combustion chamber. This reduction is made in proportion to the production of soot to maintain the lanthanum content in said soot.

It is advantageous that the form in which the lanthanum is introduced induces the formation of aggregates of lanthanum oxide crystallites where the largest dimension of the aggregate is between 20 angstroms and 10,000 angstroms (2 and 1000 nm), preferably between 100 and 5000 angstroms (10 and 500 nm), for which the size of the crystallites is between 20 and 250 angstroms (2 and 25 nm), preferably between 50 and 200 angstroms (5 and 20 nm). According to the present invention, it has also been shown that lanthanum is an element capable of potentiating, or of being potentiated by, other elements in particular capable of catalyzing the oxidation of carbonaceous products and those inducing defects in the crystal lattice of lanthanum oxide. It has thus been shown that the transition elements, that is to say metals of which one of the sublayers d is being filled, gave marked synergistic effects with lanthanum. This synergistic effect is also demonstrated with the other elements of the layers f being filled, and in particular with the other rare earths including yttrium. The most marked results are those due to lanthanum associated with manganese, copper, cobalt and / or iron. Also of particular interest are other rare earths, including yttrium, alone or in admixture.

The lanthanum content relative to the sum of the metallic elements contained in the adjuvant is generally between 5% and 95%. Advantageously, it is at least equal to 50%, preferably to 80%. The elements potentiating, or potentiated by, lanthanum are introduced as this element can be.

As mentioned above, another object of the present invention is to provide a process which allows regeneration which can be described as low temperature of the particulate filters.

This object is achieved by means of a process using the lanthanum-based compounds mentioned above.

This process consists of:

- Introducing into the fuel a lanthanum derivative as specified above at a concentration between 10 ppm and 500 ppm (by mass), preferably between 20 ppm and 200 ppm (in metal content);

- collect the soot produced by the internal combustion engine on a filter, the temperature of the gases entering the filter being chosen in the range 100 ° -400 ° C (in this description the position zeros are not significant figures , except where specified), and allow the soot to accumulate until reaching a regime where a fraction significant of the incoming soot is compensated by the combustion of the soot in the soot cake on the filter and does not provide for any forced regeneration as long as the pressure drop caused by the soot does not exceed a value chosen in advance and advantageously does not not exceeding 500 millibars.

Indeed, according to the present invention, it has been shown that there could be regenerations at temperatures as low as about 100 ° C. During a cycle of a European engine, there can be sudden variations in temperature in the exhaust gases which can, without reaching the maximum value above (i.e. the minimum temperature fatal to the filter (s)), allow partial or complete regeneration.

The following nonlimiting examples illustrate the invention.

EXAMPLES:

A - Description of the experimental conditions

The engine used is an atmospheric diesel engine, with four cylinders, with indirect injection, of 1.696 liters of displacement and developing 50 kilowatts at 4400 rpm. This engine is sold under the Volkswagen brand.

The filters used are cordierite filters produced by the Corning Company, of the EX 4-7 type (5.66 inches in diameter, 6 inches in length, with a cell density of 100 cpi / 17 mil). Each additive has been tested on a new filter. We continuously measure during the tests:

- the pressure drop linked to the filter (pressure drop between inlet and outlet of the filter);

- the temperature of the gases entering the filter;

- the temperature of the gases leaving the filter;

- carbon monoxide emissions.

The tests are carried out at 2000 rpm, keeping the temperature of the gas entering the filter constant over time. The trials conducted at a temperature of 250 ° C are reported below but similar results have been obtained at other temperatures.

The tests were carried out in particular with:

- an iron-based additive with a fuel oil content of 20 ppm (mass);

- a copper-based additive; the copper content in the fuel oil is 20 ppm;

- two additives based on cerium and rare earths; the content of rare earth elements (cerium) in the fuel oil is 50 ppm.

Taking into account the respective molar masses of the elements, the molar contents, or more exactly atomic contents, are substantially of the same order for all the additives, namely:

iron: 0.36 moles / 1000 kg of fuel oil; copper: 0.32 moles / 1000 kg of fuel oil; cerium: 0.36 moles / 1000 kg of fuel oil.

B - Results

Example ^p # 1; iron-based additive

The results are shown in FIG. 1. This figure gives on the one hand the evolution of the pressure drop as a function of time as well as the carbon monoxide content in the gas leaving the filter, as well as the evolution of the temperature of the gas leaving the filter as a function of time. It is noted that the accumulation periods can reach a duration of 35,000 seconds. The pressure drop easily reaches 300 millibar. During the regenerations which correspond to the sudden decrease in the back pressure, there is a sharp increase in the carbon monoxide content for the gases leaving the filter. Simultaneously with these variations in carbon monoxide content, there is a very sudden increase in the temperature of the gases leaving the filter, with temperatures up to 700 ° C., even 800 ° C., while the temperature of the gases at filter inlet is only 250 ° C. The variation in back pressure is very abrupt. You can lose 250 millibar very quickly when of these regeneration phases, and the duration of the accumulation zones as well as the amplitude of the variations in back pressure seems to vary randomly. These graphs highlight the random and brutal behavior of the regenerations, which suggests that the engine will undoubtedly be affected by these sudden variations in back pressure.

In this example, the iron-based additive is ferrocene.

Example 2: case of the copper-based additive

The copper used is a cupric carboxylate. The results are shown in FIG. 2. This figure gives on the one hand the evolution of the back pressure as a function of time as well as the carbon monoxide content in the gas leaving the filter, and the evolution of the temperature of the gas leaving the filter as a function of time.

We note that the accumulation periods can reach 54,000 seconds, or almost 20 hours. There is a pressure drop, or back pressure, of up to 350 millibar. During the regenerations which correspond to the sudden decrease in the pressure drop, there is a sharp increase in the carbon monoxide content in the gases leaving the filter. In parallel with these variations in the carbon monoxide content, there is a very sudden increase in the temperature of the gases leaving the filter with temperatures which can reach more than 800 ° C., while the temperature of the gases entering the filter is only 250 ° C. The variation in back pressure is very abrupt. You can lose 300 millibar very quickly during these regeneration phases. The duration of the accumulation zones as well as the amplitude of the variations in back pressure seem to vary randomly. These graphs highlight the random and brutal behavior of these regenerations, which suggests that the engine will be affected by these sudden downstream variations in back pressure.

Example No. 3: Case of a cerium-based additive (compound described in the European patent application filed under number 93 / 304760.7 and published under number 0575189)

The results are shown in FIG. 3. This figure gives on the one hand the evolution of the back pressure as a function of time as well as the CO content in the gas leaving the filter, and the evolution of the temperature of the gas leaving the filter as a function of time. There is a filter loading period of 25,000 seconds. Beyond this, it can be seen that the back pressure is practically stable. This back pressure stabilizes around 200 millibar. During the regenerations which correspond to very small decreases in the back pressure (less than 50 millibar), variations in the carbon monoxide content in the gases leaving the filter are observed. These variations testify constant regeneration activity over time. In parallel with these variations in the carbon monoxide content, there is a moderate increase in the temperature of the gases leaving the filter with temperatures which can reach at most 400 ° C., and this for a temperature of the gases entering the filter. 250 ° C. These graphs show the moderate and permanent behavior of the regenerations, which suggests that the engine and therefore the violent driving will not be affected.

Example n ° 4: example of an additive based on cerium sol.

These results are shown in FIG. 4. This figure gives on the one hand the evolution of the back pressure as a function of time as well as the carbon monoxide content in the gas leaving the filter, and the evolution of the temperature of the gas leaving the filter as a function of time. There is a filter loading period of 45,000 seconds. Beyond this, it can be seen that the back pressure is practically stable. This back pressure stabilizes around 200 millibar. During the regenerations which correspond to very small reductions in the back pressure (less than 50 millibar), variations in the carbon monoxide content in the gases leaving the filter are observed. These variations bear witness to a constant regeneration activity over time. In parallel with these variations in the carbon monoxide content, there is a moderate increase in the temperature of the gases leaving the filter with temperatures which can reach at most 350 ° C., and this for a temperature of the gases entering the filter. 250 ° C. These graphs show the moderate and permanent behavior of the regenerations, which suggests that the engine, safety and driveability are not affected in any way. The number of fine particles formed is significantly lower than that of the preceding examples.

Example 5: tests on engine bench in isoregime

Tests have been carried out on a new F8Q 706 type engine of 1870 cm ³ , and the particle filter used is an EBERSPACHER particle filter number 2626000415. This filter is equipped with two thermocouples, one upstream, the other in downstream. The tests were carried out on particulate filters which have undergone several loads and several regenerations. Indeed, the first effects are relatively erratic.

- For each additive: a new particle filter (F.A.P.) is used.

- The particulate filter must be initially stabilized, that is to say it must undergo a minimum of 3 cycles: fouling and regeneration.

• fouling on point 1500 rpm 3/4 load, loading of particulate filter with 70 g of soot;

• regeneration on point 4000 rpm, exhaust temperature: 600 ° C. - On each particle filter, during the stabilization phase during fouling and on the fouling point, the law is noted: mass of soot deposited: f (ΔP FAP) (the relationship is constructed by removal and successive weighing of the filter element.

The self-regeneration test is carried out as follows: - Initial conditions:

• the particle filter is dirty with 70 g of soot,

• before the test, the particle filter is cold (room temperature), the tests below are tests on stabilized particle filters (FAP). The tests are carried out in isoregime, the speed being expressed in number of revolutions per minute.

The results obtained either without additive, or with an additive based on various rare earth salts introduced into the fuel at the rate, unless otherwise indicated, of 100 ppm of metal contained, are collated in the following tables: the filters are previously charged with particles under conditions where they accumulate even in the presence of rare earths, these conditions are: 1500 revolutions / minute; 3/4 of the full load; richness of the mixture between 0.8 and 0.9; • oxygen content of gases: between 2 and 4%; the particulate filters are loaded with 70 g of soot per filter.

Characterization of the self-regeneration power of soot with a hearing oil on f8q a 7Q6 engine The test lasts approximately 130 minutes (8000 seconds). It was found that when there had been regeneration the total pressure was maintained at a value less than about 200 millibar during the rest of the test. The pressure drop linked to the single filter is of the order of 50 to 100 millibar.

tsoréαimβ

The self-regeneration tests are carried out on three isoregimes in the order: 2500 -

1500 and 4000 rpm. - The engine is started and adjusted to the operating point (example:

2500 rpm PME (Average Effective Pressure: 0 bar), without waiting for the engine to warm up, after 10 minutes, the acquisition of the first point begins.

The test is carried out by increasing the load every ten minutes according to predefined stages (PME: 0; 1; 2; 3; 4; 4,5; 5; 5,5; 6,5; 7; 7,5 bar ). Average effective pressure (PME) is given from the couple by the PME relation

= (40 x π x torque Vcylindrée.

Once the particulate filter is loaded, the engine is stalled on a certain speed and as specified above, the load is gradually increased in steps of 10 minutes (increment of the order of 9 Nm per step) until reaching the value of around 100 Nm once the regeneration has been obtained, the values of the various variables are determined just before the trigger.

Yide test

The engine is run empty at the desired speed.

TABLE 1 Tests without additives

REGIME T. ech. T. PME dp O2 HC CO With upstream FAP or

FAP without additive rpm ° C ° C bar mbar% ppm g / h ppm g / h

1500 503 4.15 769. . . without

2500 432 382 4.15 853 9.2 without

4000 451 419 1, 55 1154 11.2 without TABLE 2

VACUUM TEST with 150 ppm (metal content) of cerium salt (cerous octoate)

with

REGIME T. ech. T. PME dp O2 HC CO or upstream FAP without

FAP additive rpm ° C ° C bar mbar% ppm g / h ppm g / h

800 98 79 0 240 17.50 138 2.8 300 12.3 with

1000 104 87 * 0 258 17.45 62 1.5 230 11.5 with

1500 126 114 * 0 360 17.70 38 1, 60 250 21.70 with

1500 125 119 * 0 426 17.55 27 1.1 240 1.2 with

2500 239 227 * 0 1156 15.55 58 3.8 320 42.6 with

4000 362 336 * 0 1133 13.45 46 4.6 220 44.7 with

Lanthanum results

The results obtained with an additive based on lanthanum octoate introduced into the fuel at the rate of 100 ppm of lanthanum are collated in the following tables:

TABLE 3 TESTS IN ISOREGIME WITH LANTHANE SALT

T. With

REGIME T. ech. upstream PME dp O2 HC CO or

FAP FAP without additive rpm ° C ° C bar mbar% ppm g / h ppm g / h

1500 460 388 6.4 705 5 10 0.4 150 12 with

There is no re-generation

1500 d 295 241 * 2.5 663 13.5 46 1.9 160 13.4 with a 2500 371 331 * 3.5 821 10.8 29 1.9 9 18 18.3 with b 2500 285 251 * 2.2 943 13.8 32 2.2 170 23.1 with

4000 431 383 * 1, 6 1060 10.9 8 0.8 220 46.8 with TABLE 4 VACUUM TESTS WITH LANTHANE SALT

T. ech. T. PME dp 02 HC CO With

FAP or FAP upstream REGIME without additive rpm ° C ° C bar mbar% ppm g / h ppm g / h

1500 498 124 0 705 17.5 46 2.00 265 23.00 with

1500 189 161 0 892 16.8 410 16.8 660 54.8 with

A O

No regeneration

1500 170 145 * 0 1020 17 82 3.5 240 20.5 with

spontaneous regeneration

ARTIFICIAL SOOT TESTS

Soot production method

The soot is obtained by pyrolysis of fuel oil containing the additive. A tube called a "reactor tube" is traversed by a gas flow consisting of a mixture of nitrogen and oxygen 98/2 (by volume). The reactor tube is heated by an oven and the gas flow is thus brought to 1200 ° C. Upstream of the oven, the fuel oil, with or without additive, is sprayed very finely. The oil droplets are transported by the gas flow and are also brought to 1220 ° C.

Under these conditions the fuel burns partially and forms soot. The soot is then collected downstream from the pyrolysis oven by filtration of the gas flow. If the fuel oil contains a metallic additive, the metal is found in the form of an oxide intimately mixed with the soot.

The fuel injection rate upstream of the pyrolysis oven is adjusted to 10 ml / h. Under these conditions, the average diameter of the elementary particles of the soot produced by pyrolysis is identical to that of the soot produced by a diesel engine.

According to the studies carried out by the applicant, the metal additives are used at a concentration in the diesel fuel such that the metal concentration in the diesel is between 0 and 200 ppm. Under these conditions, the metal concentrations in the soot produced by an engine supplied with an additive diesel fuel are between 0 and 4%. as the final catalyst concentration in the soot is one of the important parameters which condition the ignition temperature, it was necessary to find out what metal concentration in the fuel oil it was necessary to have in order to obtain the same metal concentration in the soot obtained by pyrolysis. It has been shown that it is necessary for the metal concentration in the fuel oil to be multiplied by 20 with respect to the additive concentration in the diesel fuel. The interval is thus of the order of 0 to 4000 ppm by mass. In conclusion, for the soot obtained by pyrolysis to be representative of the motor soot, the concentration of additive in the fuel oil must be multiplied by 20.

The study of soot by Thermoqravimetric analysis or ATG

The soot produced by the pyrolysis oven can be recovered in ATG.

The conditions are as follows:

Sétaram material: model TG92

Gas flow 2.3 l / h

Air gas Temperature increasing from 20 to 900 ° C due to

10 ° C / min

Mass of soot analyzed 20 mg

ATG records the remaining mass of soot as a function of time. By convention we define the ignition temperature as the abscissa of the point defined as the intersection of the baseline at the origin with the tangent to the curve where the combustion speed is maximum (i.e. at inflection point of the so-called "S" curve).

Study of the reactivity of soot obtained by pvrolvse of αazoles containing additives composed of lanthanum, copper and cobalt

Copper and cobalt are two metals which are known for their ability to lower the ignition temperature, are known to be toxic and cause dangerous thermal shock to the filter. One of the objects of the present study was to show the synergy that exists between a metal like copper or cobalt with respect to lanthanum.

The soot was all produced by the method described above and its ability to burn was measured by ATG. the metal concentrations in the fuel oil are certainly very high, but they give indications on an implementation on an engine test bench with concentrations in the diesel oil used as fuel corresponding to approximately i / 20 ^θmθ of the concentrations in the fuel oil.

Additives used

The names of the additives used for this study, and their characteristics are grouped in Table 1: TABLE 5 Characteristics of the additives used

Name of additive Concentration of metal in additive in%

Copper cekanoate 9.2

Cobalt octoate 10

Lanthanum octoate 10.23

The tables below give the ignition temperatures of the soot as a function of the metal concentration in the fuel oil.

TABLE 6 Ignition temperature as a function of the concentration of metal acting alone in the fuel oil (comparative examples)

TABLE 7

Ignition temperature as a function of the lanthanum concentration acting alone in the fuel oil

TABLE 8

Ignition temperature depending on the composition of the additives in the fuel oil

Claims

1) Method for treating soot containing one or more rare earths in which said soot is brought into contact with a gas containing oxygen, characterized in that said gas has a temperature of between 200 and 400 ° C., and by the fact that the partial oxygen pressure of said gas containing oxygen is at least equal to 3% of atmosphere, or 3.10 ³ Pascals advantageously to 4% of atmosphere, or 4, 10 ³ Pascals.

2) Method according to claim 1 characterized in that the contact between the oxygen-containing gas and the soot is maintained for a period of time sufficient to burn at least 90% of the soot.

3) Method according to one of claims 1 and 2 characterized in that at least a portion of said oxygen-containing gas comes from heated air in contact with a part of the engine or its annexes.

4) Method according to one of claims 1 to 3 characterized in that said oxygen-containing gas at least partially contains the exhaust gases of an engine.

5) Method according to one of claims 1 to 5 characterized in that said engine is an internal combustion engine, preferably a diesel engine.

6) Method according to one of claims 1 to 5 characterized in that the soot is that emitted by an internal combustion engine, preferably diesel, equipped with a "turbo".

7) Method according to one of claims 1 to 6 characterized in that the rare earths are present in the soot at a level between 500 PPM and 5%.

8) Method according to one of claims 1 to 7 characterized in that said engine is equipped with a particle filter.

9) Method according to one of claims 1 to 8 characterized in that the particles contained in the filter are subjected intermittently to the conditions of the present process.

10) Method according to claim 1, for treating soot produced by an internal combustion engine, characterized in that it comprises the following provisions:

introducing into the combustion chamber at least one rare earth derivative or mixture of rare earths at a concentration of between 10 ppm and 500 ppm (by mass), preferably between 20 ppm and 200 ppm;

collecting the soot produced by the internal combustion engine on a filter, the temperature of the gases entering the filter being chosen in the range 100 ° C-350 ° C;

and

• allow the soot to accumulate until reaching a speed where a significant fraction of the incoming soot is compensated by the combustion of soot in the soot cake on the filter and do not provide for any regeneration as long as the loss of charge caused by the soot does not exceed a value chosen in advance and does not exceed 0.5 bar (5.10 Pa).

11) Method according to claims 3 to 8, and 10, characterized in that said rare earth is introduced into the motor through the air.

12) Method according to claims 3 to 8, and 10, characterized in that said rare earth is introduced into the engine through the fuel.

13) Method according to claim 12, characterized in that the rare earth content of said fuel is between 50 and 150 ppm.

14) Method according to claim 10 to 13, characterized in that the temperature at which the gases are filtered is between 200 and 350 ° C.

15) Method according to one of claims 10 to 15, characterized in that said rare earth is cerium or a compound of cerium.

16) Method according to one of claims 10 to 16, characterized in that the filter surface is chosen so as to maintain the pressure drop in said filter at a value at most equal to 0.3 bar, advantageously at 0.2 bar.

17) Method according to one of claims 10 to 17, characterized in that one chooses the temperature of the gases in contact with the filter so as to maintain the pressure drop in said filter at a value at most equal to 0 , 3 bar, advantageously at 0.2 bar.

18) Method according to one of claims 10 to 18, characterized in that the rare earth content in the fuel is chosen so as to maintain the pressure drop in said filter at a value at most equal to 0, 3 bar, preferably 0.2 bar.

19) A method according to claim 1 for reducing the emission of soot from an internal combustion engine in which the exhaust gases are passed through a particle filter, characterized in that a additive containing lanthanum.

20) A treatment method according to claim 19, characterized in that the lanthanum is added to the fuel in the form of a salt or a stable sol in said fuel.

21) Method according to claim 21, characterized in that the lanthanum-containing compound is directly introduced into the engine.

22) Method according to one of claims 19 and 22, characterized in that the lanthanum content of the fuel is between 10 and 1000 ppm, advantageously between 20 and 200 ppm, preferably between 50 and 150 ppm.

23) Method according to one of claims 1, 2 and 4, characterized in that the soil and / or salt are introduced so as to form aggregates of crystallites whose largest dimension is between 20 and 10,000 angstroms (2 and 1000 nm) and the size of the crystallites of which is between 20 and 250 angstroms (2 and 25 nm).

24) Method according to claim, characterized in that the lanthanum is introduced into the engine through the air.

25) Adjuvant for internal combustion engine fuel, characterized in that it comprises:

a) a rare earth compound, the highest oxidation state of which is the trivalent state,

b) a compound comprising at least one transition element or another rare earth.

26) Adjuvant according to claim 25, characterized in that the content of said rare earth whose highest oxidation state is the trivalent state, relative to the sum of the metallic elements contained in the adjuvant is at least equal to 50%, preferably 80%.

27) Soots based on trivalent rare earth, characterized in that they can be obtained by combustion in an internal combustion engine of a fuel containing an adjuvant based on trivalent rare earths (that is to say of which the highest oxidation state is the trivalent state)

28) Soot according to claim 27 characterized in that said adjuvant is an adjuvant according to claims 25 and 26.

29) Soot according to claim 27 and 28 characterized in that it has a particle size such that the do is at least equal to 200 A and that the dβo is at most equal to 1000 A, characterized in that they contain at least 0.01%, advantageously at least 0.1%, preferably at least

0.5% aggregate of Pascals crystallites. 23

30. Soot according to claim 28, characterized by the fact that said ErrorI Undefined Bookmark, has a total content of rare earth (s) of between 1000 ppm and 30%, preferably from 5% to 10% ( en masse).

31. Soot according to claim 28 and 29, characterized in that the grains of said soot form clusters whose dso is between 2000 and 5000 Å.

32. Soot according to claim 28 to 30, characterized in that said trivalent rare earth is lanthanum.