CH615979A5 - Process for purifying exhaust gases emitted by internal combustion engines of the diesel type - Google Patents

Abstract

By passing through an absorbing mass, consisting of alumina with a high specific surface area deposited on metal wool, and maintained at at least 250 DEG C, an almost complete post-combustion of the unburnt hydrocarbons is achieved, as well as retention of the carbon-containing particles which are responsible for the opacity of the exhaust smoke. Application to all types of vehicles, to worksite compressors and diesel engine electricity generating units, with a view to reducing the problems due to the exhaust gases.

Description

The present invention relates to a process for purifying the exhaust gases emitted by an internal combustion engine of the Diesel type, in which these gases pass through a device constituted by a tangle of metal wires, coated with alumina deposited at from an alkaline aluminate solution then calcined, and introduced into a metal enclosure placed in the engine exhaust circuit.

We know that there are significant differences in nature and composition between the exhaust gases emitted by petrol and diesel engines.

In the first case, in addition to nitrogen, there is carbon dioxide and water vapor from the combustion of hydrocarbons, carbon monoxide (4 to 6%), nitrogen oxides ( from 500 to 2000 ppm), unburnt hydrocarbons (500 to 2000 ppm), oxidized sulphide derivatives, and lead, the latter coming from tetraethyl lead added as an antiknock.

In the second case, in the exhaust gases, in addition to nitrogen, carbon dioxide and water vapor also coming from the combustion of hydrocarbons, a very small proportion of carbon monoxide (500 to 800 ppm, almost a hundred times less than in the first case), nitrogen oxides (from 500 to 2000 ppm), unburnt hydrocarbons (from 100 to 500 ppm), oxygen (from 7 to 15% ) due to the fact that a very large excess of air and very fine black particles are always injected into the combustion chambers, at a rate of approximately 40 to 70 mg per liter of fuel injected into the engine which are responsible for the characteristic color and opacity of the fumes emitted by the exhaust.

Diesel engine exhaust is a serious nuisance to the environment due to the large and rapidly increasing number of vehicles fitted with diesel engines (transporting goods and passengers) and the frequent use of air compressors. air or generator sets powered by Diesel engines, used in closed rooms or underground galleries.

And, although, due to their low carbon monoxide content and the absence of lead, their toxicity is relatively low, their foul odor due, in particular, to aldehydes, the presence of polycyclic aromatic derivatives, some of which are suspected of being carcinogenic, and the presence of carbonaceous particles which quickly cloud the atmosphere when the exhaust is in a confined environment, make it essential to clean up these exhaust gases.

It has been discovered that it is possible, under certain conditions, to use the properties of alumina deposited on a metal substrate having a large surface area, to purify the exhaust gases of diesel engines, by eliminating most of the unburnt hydrocarbons and carbonaceous particles, which are the two most annoying constituents.

It is known that aqueous solutions of alkaline aluminate can, under certain conditions, decompose into alumina, which precipitates in the form of trihydrate AI2O3,3H2O, and into alkaline hydroxide, either spontaneously or by addition of germs which serve as primer for the decomposition reaction. These reactions are implemented, for example, in the Bayer process which makes it possible to obtain, from bauxite, pure alumina intended for the manufacture of aluminum by igneous electrolysis.

It is also known that certain physical forms of alumina have catalytic and adsorbent properties which are used in numerous industrial operations.

More recently, as a result of work aimed at reducing the level of pollution by exhaust gases from heat engines, it has been found that certain forms of alumina could not only act as a catalyst, but also adsorb most of the lead entrained in the form of volatile compounds in the exhaust gases, said lead coming from the decomposition, in the engine combustion chambers, of tetraalkyl lead derivatives added to the fuel to improve its octane number, it that is, its ability to resist self-detonation.

It has been found, and this is the subject of US Pat. Nos. 3227659, 3231520,3495950, 3362783 in the name of Texaco Inc., that the catalytic and adsorbent properties of alumina were particularly effective when it was deposited on a metallic substrate formed by an entanglement of fine threads, such as metallic wool or metallic straw, the texture of this entanglement being such that it offers only a low resistance to the passage of exhaust gases, and that it only insignificantly reduces the efficiency of the engine.

In French patent application No. 75.39408, an improvement to the processes for depositing alumina on a metal substrate was claimed, making it possible to obtain continuously, with simultaneous regeneration of the aluminate liquor, very adherent deposits and having excellent adsorbent properties.

However, if one attempts to use such deposits to purify the exhaust gases of diesel engines, it is found that the efficiency of the purification is very low and, in particular, that the unpleasant odor and the opacity of the smoke remains largely.

The invention aims to remedy the aforementioned drawbacks. To this end, the method according to the invention is characterized in that said alumina has a specific surface at least equal to 120 m2 / g, that the temperature of the purification device is maintained at least equal to 250 ° C and that the duration of contact between the exhaust gases and the alumina is at least equal to 0.3 s.

Thus, the invention stems from the observation that it was possible to effectively purify the exhaust gases emitted by diesel engines, by passing through a tangle of fine metal wires covered with alumina, under the conditions that said alumina has a specific surface at least equal to the value indicated above (measured by the conventional BET method of nitrogen absorption at low temperature according to French standard Afnor X 11.621), than its temperature, and that the duration of contact between said gases and alumina are at least equal to the limit values specified above.

When these conditions are fulfilled, it can be seen that, on the one hand, the unburnt hydrocarbons which are mixed with the excess of air always present in the exhaust gases, undergo an almost total afterburning and, in all case, of a rate sufficient to remove any perceptible odor and that, on the other hand, the carbonaceous particles, responsible for the opacity of the fumes, are retained in a proportion at least equal to 70% and up to 80%.

It appears that, quite surprisingly, this afterburning takes place in the absence of any metal catalyst element5

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lique, while in the previous processes, in particular those which are the subject of patents FR No. s 1047802, 1400504, US 3231520, 3495950, it was necessary to introduce, into the alumina, metallic catalyst elements, such as copper, vanadium, chromium, manganese, platinum or palladium, which gradually lost their effectiveness by the phenomenon known as poisoning, which limited their life in an economically unacceptable manner.

The temperature at least equal to 250 ° C., which is necessary to produce the afterburning of unburnt hydrocarbons, is obtained without difficulty, as soon as the engine is started, the temperature of the exhaust gases normally being greater than 500 ° C. which brings the exhaust pipe to a temperature of up to around 380 ° C when the engine is operating at full load.

In the event that the ambient conditions could lead to significant cooling of the purification device, it is possible to provide for thermal insulation by all known means.

The specific surface of the alumina which must be at least equal to 120 m2 / g is obtained in a known manner by a suitable choice of the drying and calcination conditions of the trihydrate AI2O3, 3H2O which is deposited on a metallic substrate from the alkaline aluminate solution.

A gradual rise in temperature, from ambient to 530-550 ° C, makes it possible to obtain such a specific surface.

It is also essential that the contact time between the exhaust gases and the purification device is sufficient and, in any case, at least equal to 0.3 s so that the double process of retention of carbonaceous particles and afterburning of unburnt hydrocarbons can take place entirely. In practice, this duration is obtained by giving the purification device a volume in relation to the volume of exhaust gas emitted per second.

Example 1:

A device for purifying the exhaust gases emitted by a 6-cylinder diesel engine, 149 kW (approximately 202 CV) mounted on a fixed test bench, consisting of a cylinder 350 mm in diameter and 880 mm long, into which 8,500 g of ferritic stainless steel wool containing 17% chromium was introduced, composed of elementary strands with a substantially rectangular section of 0.1 x 0.4 mm, on which we deposited from an alkaline aluminate solution, in known manner, 17000 g of alumina having, after calcination, a BET specific surface about 140 m2 / g. This cylinder was closed at both ends by two elements of expanded metal with large mesh, the sole purpose of which is to immobilize the metallic wool, and was connected to the exhaust circuit of the diesel engine.

Different tests have been carried out, corresponding to different engine loads, at respective powers of 42, 79 and 149 kW. The consumption of the engine was about 217 g of fuel per kilowatt per hour. At the maximum speed of 149 kW, the exhaust gas flow was 530Nm3 / h, and the temperature of the exhaust was approximately 380 ° C.

615,979

The duration of passage of the gases in the purification device, calculated as a function of their flow rate and of the volume of said device, was approximately 0.46 s.

The exhaust gases leaving the purification device,

whatever the engine load, no longer had any appreciable odor or opacity. Analysis showed that the content of unburnt hydrocarbons had dropped from 200 ppm upstream of the filtration device to less than 20 ppm downstream and that 77% of the carbonaceous particles had been retained.

The period during which the purification device retains its effectiveness is of the order of several thousand hours. At the end of this time, the pressure drop in the exhaust circuit, which is of the order of 100 mm of water, for the example taken, has not increased appreciably.

Example 2:

We made a device for purifying the exhaust gases emitted by a 150 kW diesel engine, mounted on a loco-tractor intended to circulate in underground galleries, constituted by the series connection of two cylinders of 350 mm in diameter and 920 mm in total length, into which 8900 g of ferritic stainless steel wool containing 17% chromium was introduced, composed of elementary strands of substantially rectangular section of 0.1 x 0.4 mm, on which deposited, from an alkaline aluminate solution, in a known manner, 17,000 g of alumina having, after calcination, a BET specific surface about 140 m2 / g.

This cylinder was closed at both ends by two elements of expanded metal with large meshes, having the sole purpose of immobilizing the metallic wool, and was plugged into the exhaust system of the Diesel engine, with a flared diffuser at the inlet, intended to limit pressure losses.

The duration of passage of the exhaust gases in this purification device was, on average, 0.6 s.

Under normal operating conditions of the locomotive, the exhaust gases at the inlet and outlet of the purification device were analyzed. The results were as follows:

(they are given in parts per million)

Entrance

Exit

Carbon monoxide

185

150

Nitrogen oxides

400

270

Aldehydes

20

3 to 4

Opacity index measured at the Bosch device

3 to 4

<0.5

There is a slight decrease in the CO content, due to a partial oxidation of CO2, in the nitrogen oxide content, and a reduction of approximately 85% in the opacity, measured with the Bosch device, which corresponds a reduction, in an identical proportion, of the content of opacifying carbonaceous particles.

The content of aldehydes, mainly responsible for the foul odor of smoke, has been reduced by 80 to 85%.

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Claims (2)

615,979 2 CLAIMS 1. Process for purifying the exhaust gases emitted by an internal combustion engine of the Diesel type, in which these gases pass through a device constituted by a tangle of metal wires, coated with alumina deposited from a solution of alkaline aluminate then calcined, and introduced into a metal enclosure placed in the exhaust circuit of the engine, characterized in that said alumina has a specific surface at least equal to 120 m2 / g, that the temperature of the device d purification is maintained at least equal to 250 ° C. and that the contact time between the exhaust gases and the alumina is at least equal to 0.3 s. 2. Method according to claim 1, characterized in that the metal enclosure is insulated, so as to maintain the temperature of the purification device, in operation, at least equal to 250 ° C.