FR2719081A1 - 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

A METHOD OF FILTERING AND COMBUSTING CARBONACEOUS MATERIALS

  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

  accumulation of soot in the filters.

  During the combustion of fuels in internal combustion engines and in particular during that of diesel in the diesel engine, carbonaceous materials are formed, hereinafter called "soot", which are reputed to be harmful both to health

  higher mammals than 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.

  carbon and / or gases known to be toxic and mutagenic, such as nitrogen oxides.

  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 cordierite, which allow

  reduce soot emissions by at least 85% by mass.

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

internal combustion.

  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 either by heating with a fossil fuel 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 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 risk of causing a significant deterioration of the filter and its filtration capacities. , is

  by cracking due to thermal shock, even by fusion.

  This is why one of the aims of the present invention is to provide a method of

  combustion of 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 600 C approximately ("approximately" means here that zeros, which here are zeros of

  position, are not significant figures), preferably at around 500 C.

  These aims, and others which will appear subsequently, are achieved by means of a process for filtering the gases of an internal combustion engine, a process which consists in: introducing into the fuel a derivative of rare earths or of mixture of rare earths at a concentration 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 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 pressure drop caused by the soot does not exceed a value chosen in advance and

not exceeding 400 millibar.

  Surprisingly, it has been observed that there is, in general, no need to cause the regenerations when the above conditions are respected. Allogeneic (or exogenous) regeneration can only be envisaged when it is desired maintain a particularly low pressure drop level. As an allogeneic regeneration, it is therefore advisable to envisage regenerations carried out by a point overheating, and not global over the entire filter; this possibility is only interesting because, according to the invention, a specific overheating does not lead to regeneration

global and complete filter.

  The point overheating can be achieved by any means known to those skilled in the art, such as micro-resistance distributed over the surface of the filter, metallic particles

  heated by eddy current, mini-arc or equivalent.

  The filtration mode according to the invention generally works only during part of the engine speeds, generally diesel, because some of the systems generate gases of temperature of the order of 500 C which ipso facto performs regeneration

  progressive and often complete of the filter.

  It should be noted that the temperature variations, in particular inside the above temperature range of the incoming gases favor the regenerations and make it possible to

  maintain a low value pressure drop.

  The preferred rare earths are cerium, lanthanum and mixtures containing cerium and lanthanum. The most common fuel content of rare earths (metal)

  content 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. One 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, from

  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) and at most 5% on average. 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

  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 number 93 / 304760.7 and published under number 057 5189.

  Indeed, completely surprisingly, it has been found that when additives based on transition metals are used (that is to say metals of which one of the sublayers d is being filled) regenerations at low temperature were either nonexistent or gave rise to inflammations

  resulting in high temperatures which could 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 variations in loss of

much lower charges.

  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 makes it possible to conserve all of its properties in the filtering material 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.

  The following nonlimiting examples illustrate the invention.

Examples:

  A - Description of the experimental conditions

  The engine used is an atmospheric, four-cylinder, indirect injection diesel engine, 1.696 liters of displacement and developing 50 kilowatts at 4400 rpm.

  minute. This engine is sold under the Volkswagen brand.

  The filters used are cordierite filters produced by the Coming 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. The following are measured continuously 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 2,000 rpm, keeping the temperature of the gas entering the filter constant over time. The tests carried out 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 the content of which in fuel oil was 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; element content of the land

  rare (cerium) in 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 n 1: case of the 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 oxide content. of carbon in the gas leaving the filter, as well as the change in the temperature of the gas leaving the filter as a function of time. It is noted that the periods of accumulation 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 of up to 700 ° C. or even 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 250 millibar very quickly during 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 regenerations, which leaves

  assume that the engine will likely be affected by these abrupt variations in counter

downstream 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

  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 n '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 variations in counter

downstream pressure.

  Example no 3: case of a cerium-based additive (compound described in the European patent application filed under the number 93 / 304760.7 and published under the number 057 5189) The results are shown in Figure 3. This figure gives d 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 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 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 conduct will not be affected.

  Example 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 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 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 at the inlet of the filter. 250 C. These graphs show the moderate and permanent behavior of regenerations, which suggests that the engine, the safety

  and the driving pleasure are not altered in any way.

Claims (6)

  1- A method of filtering and burning the soot produced by an internal combustion engine, characterized in that it includes the following provisions: * introducing into the fuel a rare earth derivative or mixture of rare earths at a concentration included 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 or 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 not exceeding 400 millibar.   2- Method according to claim 1, characterized in that the temperature at   which the gases are filtered is between 200 and 350 C.   3- Method according to one of claims 1 and 2, characterized in that the content   in rare earth of said fuel is between 50 and 150 ppm.   4- Method according to one of claims 1 to 3, characterized in that said   rare earth is cerium or a compound of cerium.   - Method according to one of claims 1 to 4, characterized in that one chooses   the surface of the filter so as to maintain the pressure drop in said filter at a   value at most equal to 300 millibar, advantageously 200 millibar.   6- Method according to one of claims 1 to 5, 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 300 millibar, advantageously at millibar.   7- Method according to one of claims 1 to 4, characterized in that one chooses   the content of rare earths in the fuel so as to maintain the pressure drop in said filter at a value at most equal to 300 millibar, advantageously at millibar.