FR2714717A1 - Reduction of carbonaceous particulate emissions in vehicle exhausts - Google Patents

Abstract

Carbonaceous particulates ('soot') containing rare-earths are treated by contact at 200-400 deg.C with an O2-contg. gas whose O2 partial pressure is at least 3% of atmospheric. Also claimed are: (1) Fuel additive comprising a cpd. of a rare-earth of oxidation state greater than three and a cpd. of a transition metal or another rare-earth; and (2) soot made by combustion of fuel containing this additive.

Description

PROCESS FOR OXIDIZING IMBRULES OF MATERIALS
HYDROCARBONES
The present invention relates to a process for treating soot containing one or more rare earths in which said soot is brought into contact with a gas containing oxygen.

 The present invention relates to a process for the oxidation of unburnt hydrocarbon materials.

 It relates more particularly to a process allowing the oxidation of unburned combustion engines and in particular, the soot emitted by diesel engines during their operation.

 During the combustion of fuels and in particular that of diesel in the diesel engine, carbonaceous or hydrocarbon products tend to form soot which is reputed to be harmful both to the environment and to the health of higher mammals.

 For a long time, techniques have been sought which would make it possible to reduce the emission of these carbonaceous particles which will be designated in the following description by the expression soot. This research must take into account the desire not to increase the emission of carbon monoxide and / or gases known to be toxic and mutagenic such as nitrogen oxides.

 Many solutions have been proposed to reduce these carbon emissions. We are moving more and more towards the adaptation to the exhaust of a filter capable of stopping all or at least a large part of the carbonaceous particles induced by the combustion of the various fuels.

 This technique is however greatly limited by the storage capacity of the filter which must be regenerated either by emptying it or by burning the soot present therein. This regeneration operation is extremely costly to plan and to implement. One of the most commonly proposed solutions is the combustion of this soot, combustion which is caused intermittently either by electric heating or by the use of a fossil igniter fuel.

 However, this technique has many drawbacks, not the least of which is the risk of thermal shock leading to a fracture or cracking of the ceramic filter, reducing its rattle of environmental protection to nothing.

 A more satisfactory solution consists in introducing catalysts into the soot which allow frequent self-ignition of the soot collected in the filters. For this, it is necessary that these soots have a sufficiently low auto-ignition temperature to be relatively frequently reached during normal engine operation.

 In recent decades, it has been proposed to use various "rare earths" as the oxidation promoter and some trials have been carried out with cerium-based compounds. During the vast research program which led in particular to the present invention, It has been shown that this solution, consisting in reaching the self-ignition temperature to regenerate the filter, did not make it possible to solve all the problems caused by the presence of soot.

 Despite the progress made in rare earth derivatives, the ignition temperature remained relatively high.

 In addition, this inflammation, although it constitutes progress vis-à-vis other catalysts such as copper, remained relatively brutal.

 This state of affairs implied the need to provide a system making it possible to ensure that the temperature in the filter reaches the low ignition limit value, in order to prevent regeneration from occurring too randomly.

 Despite these devices, the progressive accumulation of soot in the filter led to a pressure drop, a source of both power loss, loss of efficiency, and even greater fuel consumption.

 In addition, diesel engines equipped with a turbo hardly deliver exhaust gas at the right temperature. Which led specialists in the field to consider that none of the techniques envisaged for simple diesel engines could be transposed to turbo-diesel engines. And this, even to the point that no test had been carried out before the present invention.

 Finally, the problems of particles that pass any filter, whatever their finesse, remained whole.

 This is why one of the aims of the present invention is to provide a process which allows the regeneration of filters at a lower temperature.

 Another object of the present invention is to provide a method which makes it possible to reduce the average pressure drop induced by the accumulation of fine particles in the filter.

 Another object of the present invention is to provide a process which allows the elimination of fine particles even when there is a turbo in the diesel engine.

 Another object of the present invention is to provide a process which makes it possible to oxidize the finest particles and reduce the amount of unburnt soot emitted by the system.

 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 an oxygen-containing gas at a temperature between 200 C and 400 "C, advantageously between 200 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% of atmosphere, or 3.103 Pascals, advantageously at 4% of atmosphere, or 4.103 Pascals.

 It is even desirable that the partial oxygen pressure be at least equal to 6.103 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 of cerium catalyze the oxidation of carbonaceous materials at temperatures as low as 200 C (this value is a rounded value indeed, it has even been noted oxidation at temperatures of the order of 1800 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 have been observed because the oxygen content of the exhaust gases is under the usual conditions of operation and engine test much lower than that required for this oxidation than the 'we can call it sweet.

 The method according to the invention can be used in many ways to reduce the solid rejection emissions of all combustions and in particular of 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 the soot and gas assembly is at the appropriate temperature, it is preferable that the partial oxygen pressure 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. Thus, 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 duration is paid for by periods during which the pressure drop caused by the filter is very high, U is therefore preferable to choose a defined duration which prevents carbonaceous particles n 'cause a pressure drop greater than 114 atmospheres (i.e. 1 / 4.105 Pascals), advantageously 0.2.105 Pascals, preferably Q, 15.105 Pascals.

 According to an implementation of the present invention, it is possible to play on the parameters of the engine and the 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 pressure drop limit value.

 The time required for gentle oxidation of a given amount of carbonaceous matter obviously decreases in temperature and in 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 atmosphere 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, while pulses of air, enriching the exhaust gases with oxygen, allow regeneration from time to time. of the filter.

 This oxygen enrichment of the exhaust gases to reach a value where the method according to the present invention is implemented, can be obtained by any means.

 It can in particular be enriched by injecting air into the exhaust gas flow. 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 turbos.

 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.

 However, to date, the best results have been obtained using cerium.

 In their catalytic function, these metals are preferably in the form of their cerium oxide IV as part of the cerium, trivalent oxide in the other cases.

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

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

 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 N0 92114158 filed on 25.11.1992 and entitled "aggregate of ceric oxide crystallites, process for obtaining it and its use for reducing 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. Also when it is desired to use cerium it is preferable to use either the soils or the salts which are the subject of the European patent application filed under N "93 / 304760.7 and published under N" 0575189.

 Advantageously, the rare earths are present in the soot at a level of between 500 PPM and 10% preferably at least 1000 ppm (mass of metal contained) and at most 5% on average during a European cycle.

 To be effective, cerium or a rare earth must be added, so as to obtain a rare earth content (expressed as contained metal) of at least 10 ppm relative to the carbonaceous fuel, preferably 20 ppm more advantageously 50 ppm.

 It is preferable not to exceed 500 ppm for cost reasons.

 It is preferable that the cerium oxide thus formed has a particle size such that the d80 is at most equal to 10,000 A (1,000 nanometers), preferably 5,000 A (500 nanometers).

 It is also preferable that the d20 be greater than 200 A (20 nanometers) and preferably 500 A (50 nanometers).

Claims (9)

 or 4.103 Pascalss.  of atmosphere, i.e. 3.1 03 Pascalss advantageously at 4% of atmosphere,  oxygen of said oxygen-containing gas is at least 3%  between 200 and 400 "C, and by the fact that the partial pressure in  oxygen, characterized in that said gas has a temperature  in which said soot is brought into contact with a gas containing  CLAIMS 1- Method for treating soot containing one or more rare earths 2- A method according to claim 1 characterized in that the contact between  the oxygen-containing gas and soot is maintained for a period of  enough time to burn at least 90% of the soot. 3- Method according to one of claims 1 and 2 characterized in that at  at least part of said oxygen-containing gas comes from air heated to  contact with 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 gases  engine exhaust. 5- Method according to one of claims 1 to 5 characterized in that said  engine is an internal combustion engine, preferably an engine  diesel. 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 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 fitted with a particulate filter. 9- Method according to one of claims 1 to 8 characterized in that the  particles in the filter are subjected intermittently  under the conditions of this process.