IE45946B1 - A process for purifying the exhaust gases of diesel-type internal combustion engines - 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. (From CH615979 A5) [FR2373675A1]

Description

The present invention relates to a process for purifying the exhaust gases given off by diesel-type internal combustion engines.

It is known that there are significant differences in the nature and composition of exhaust gases given off by petrol engines and those given off by diesel engines.

In the former case, carbon dioxide and water vapour are found in addition to nitrogen as a result of the· combustion of hydrocarbons. Also found are carbon monoxide ( 4 to 6c/o), nitrogen oxides (from 500 to 2000 ppm), unburned hydrocarbons (500 to 2000 ppm), sulphur oxide derivatives, and lead, the latter emanating from the tetraethyl lead which is added to petrol as an anti-knock agent.

In the latter case, carbon dioxide and water vapour are found in the exhaust gases in addition to nitrogen, also as a result of the combustion of the hydrocarbons.

Also found are a very small proportion of carbon monoxide (500 to 800 ppm,that is to say, about 100 times less than in the first case), nitrogen oxides (from 500 to 2000 ppm), unburned hydrocarbons (from 100 to 500 ppm), oxygen (from 7 to 15 Ψ) owing to the fact that a very large excess of air is injected into the combustion chambers, and, very fine black particles in a proportion of from 40 to 70 mg per litre of fuel injected in to the engine which are responsible for the characteristic colour and the opacity -24894« of the fumes given out by the diesel exhaust.

The exhaust gases of diesel engines have serious harmful effects on the environment owing to the already substantial and rapidly increasing number of vehicles equipped with diesel engines and the frequent use of air compressors or generating sets driven by diesel engines in enclosed premises or underground galleries. Although their relatively low carbon monoxide content and the absence of lead from them makes their toxicity less than that of petrol engine exhaust, their nauseating smell which is caused in particular by aldehydes, the presence of polycyclic aromatic derivatives, some of wnich are suspected to be carcinogenic, and also the presence of carbonaceous particles which rapidly make the atmosphere opaque if the exhaust is produced in a confined space, make methods for purifying these exhaust gases highly desirable.

It has been found that it is possible, under certain conditions, to take advantage of the properties of alumina deposited on a metallic substrate with a large surface area for purifying the exhaust gases from diesel engines by eliminating the major proportion of the unburned hydrocarbons and carbonaceous particles which are the two most harmful constituents.

Thus according to the present invention there is -34 5 9 4 6 provided a process for purifying exhaust gases given off by a diesel type internal combustion engine comprising the steps of: a) depositing alumina trihydrate from a hot solution (as herein defined) of alkaline aluminate on criss-crossed metal wires; b) calcinating the said alumina so as to give it a specific surface area of at least 120 square metres per gram; c) installing the said criss-crossed metal wires covered with alumina in the exhaust circuit of the diesel type internal combustion engine; and d) contacting the exhaust gases with the said criss-crossed wires covered with alumina for a period of at least 0.3 second at a temperature of at least 250°C.

By a hot solution is meant a solution at a temperature within the lb range of from 80°O. to 10U°C., inclusive.

It is known that aqueous solutions of alkaline aluminates can decompose into alumina which precipitates in the form of the trihydrate AIgO^, 3H20, i.e. aluminium hydroxide Q AI (OH) 3 or ΑΙ2(0Η)θ] either spontaneously or by adding seeds which initiate the decomposition reaction. These reactions are used, for example, in the Bayer process for obtaining pure alumina from bauxite intended for the manufacture of aluminium by igneous electrolysis. An example of the reaction is as follows: 2A10zNa + 4H20 + A^Oj ,3H20 + 2NaOH 25 It is also known that certain physical forms of alumina have catalytic and absorbent properties which are 45346 exploited in numerous industrial operations.

More recently, as a result of research into the reduction of the level of pollution hy exhaust gases from thermal engines, it has been found that certain forms of alumina can not only act as catalysts but also adsorb the majority of the lead carried in the form of volatile components in the exhaust gases, the said lead emanating from decomposition in the combustion chambers of the-engine of the tetra-alkyl lead derivatives added to the fuel to improve its octane rating, that is to say its capacity of resisting self-knocking.

It has been found, as disclosed in U.S. Patents Nos. 3,227,659, 3,231,520, 3,495,950, and 3,362,783, that the catalytic and adsorbent properties of alumina are particularly effective when deposited on a metallic substrate formed from criss-crossed fine wires such as metal wool or metal shavings, the texture of this criss-cross being such that it only resists the passage of the exhaust gases slightly and that it only reduces the efficiency of the engine to an insignificant degree.

In our Patent Specification No. 2725/76 there is claimed an improvement to the processes for depositing alumina on a metallic substrate for continuously obtaining very adherent deposits having excellent adsorbent properties with simultaneous regeneration of the aluminate liquor. -54 S 3 4 6 However, if attempts are made to use such deposits for purifying the exhaust gases from diesel engines, it is observed that the effectiveness of purification is very slight and, in particular, the disagreeable smell and the opaqueness of the fumes largely remain.

These difficulties are overcome, however, by use of the process of the present invention. Thus it is possible to effectively purify the exhaust gases given off by diesel engines by passing them through criss-crossed fine metal wires covered with alumina, providing that the said, alumina has a specific surface area of at least 120 m /g (measured by the conventional B.E.T. method for adsorbing nitrogen at low temperature in accordance with French standard AFNOR X 11.621), and that it is at a temperature of at least 250°C. Furthermore, it is essential that the period of contact between the said gases and the alumina is at least 0.3 second.

To ensure a minimum temperature of 250°C it is often preferable for the process of the present invention to insulate the exhaust circuit of the diesel engine so that its equilibrium temperature is maintained above this valu e.

If the above conditions are satisfied, it is observed that, on the one hand, the unburned hydrocarbons mixed with the excess air which is still present in the exhaust gases -64584B undergo almost total post-combustion and in all cases at a sufficient rate to eliminate any perceptible smell and that, on the other hand, the carhon particles responsible for the opaqueness of the fumes are retained in a proportion of at least 70% and often as high as 80%.

Quite surprisingly, it seems that this post-combustion takes place in the absence of any metallic catalysing element whereas in the former processes, particularly those forming the subject-matter of French patents Nos. 1,047,802, and 1,400,504 and U.S. patents Nos. 3,231,520 and 3,495,950, it was necessary to incorporate into the alumina, metallic catalysing elements such as copper, vanadium chromium, manganese, platinum or palladium which gradually lost their effectiveness by the so-called poisoning phenomenon which limited the service life in a manner which was economically unacceptable.

The temperature of at least 250 °C required for producing post-combustion of the unburned hydrocarbons is obtained without difficulty as soon as the engine starts up. The temperature of the exhaust gases is normally above 500°C, thus bringing the exhaust chamber to the required temperature. The temperature may even reach about 380°C when the motor operates at full load.

If the ambient conditions are able to cool the purifying device substantially, it is preferable to provide -7thermal Insulation as previously mentioned.

The specific surface area of the alumina, which should he at least 120 mVg» is obtained in known manner by suitably selecting the conditions for drying and calcinating the trihydrate AlgO^, 3HgO which is deposited on the metallic substrate from the alkaline aluminate solution.

A gradual rise in'temperature from ambient temperature to between 530°and 55O°C allows the required specific.surface area to be obtained.

It is also essential for the period of contact between the exhuast gases and the purifying device to be sufficient and, in all cases, at least 0.3 second so that the double process for retaining the carbon particles and for the post-combustion of the unburned hydrocarbons can take place integrally. In practice, this period is obtained by giving the purifying device a volume which is proportional to the volume of exhaust gas given out per second.

The following examples serve to further illustrate the present invention, but should not be contrued as limiting the invention to the specific procedures described therein, EXAMPLE 1 A device for purifying the exhaust gases given off by a 6 cylinder 149 klf (about 202 CV) diesel engine was produced and mounted on a fixed test bed. The fixed test bed of a cylinder 350 mm in diameter and 880 mm long, into which were -843946 introduced 8500 grams of ferritic stainless steel wool containing 17% of chromium composed of . elementary strands of substantially rectangular cross-section (0.1 x 0.4 mm) onto which were deposited 17000 grams of alumina obtained in known manner from an alkaline aluminate solution and having a BET specific surface area of about 140 m per gram, after calcination. The cylinder was closed at both ends by two metal elements expanded to a wide mesh, intended only for immobilising the metal wool, and was connected in the exhaust circuit of the diesel engine.

Various tests were performed corresponding to the different charges of the engine under respective powers of 42, 79 and 149 kW. The consumption of the engine was of the order of 217 grams of fuel per kW per hour. At the maximum rate of 149 kW, the flow-rate of the exhaust gases •T was about 530 Nnr/hour and the temperature of the exhaust chamber was about 380°C.

The gases were passed into the device for about 0.46 second, calculated as a function of their flow-rate and of the volume of the said device.

The exhaust gases leaving the purifying device, irrespective of the charge of the engine, no longer had a smell or appreciable opaqueness. Analysis showed that the unburned hydrocarbon content had dropped from 200 ppm upstream of the filtering device to less than 20 ppm downstream and that 77% of the carbon particles had been -945946 retained in the alumina.

The purifying device maintained its efficiency for the order of several thousand hours. At the end of the period, the loss of charge in the exhaust circuit, which is of the order of 100 mm of water for the case in the example, did not increase substantially.

EXAMPLE 2 A device for purifying the exhaust gases given off by a 150 kW diesel engine was produced and mounted on a power trolley designed for circulating in underground galleries. It was formed by connecting in series two cylindeis of 350 mm diameter and 920 mm total length in which were introduced 8900 grams of ferritic stainless steel wool containing 17?£ of chromium composed of elementary strands of substantially rectangular cross-section (0.1 x 0.4 mm) upon which had been deposited 17000 grams of alumina obtained in known manner from an alkaline aluminate solution and having after calcination a BET specific surface area of about 140m2/gram.

This cylinder was closed at both ends by two metal elements expanded to large mesh, intended only for immobilising the metal wool, and was connected in the exhaust, circuit of the diesel engine with a diffuser which opened out at the inlet, intended for limiting the losses of charge.

The exhaust gases were passed into this purifying device for an average of 0.6 second. -1045926 Under normal operating conditions of the power trolley, the exhaust gases were analysed at the inlet and at the outlet of the purifying device. The results were as follows: (they are given in parts per million).

Inlet Outlet carhon monoxide 185 150 nitrogen oxides 400 270 aldehydes 20 3 to 4 opacity index measured by BOSCH apparatus 3 to 4 <0.5 A slight reduction in the CO content is observed owing to partial oxidation of CO to C02 caused by the nitrogen oxides present, and a reduction of about 85% in the opacity measured with a BOSCH apparatus, corresponding to a reduction in the content of opacifying carbon particles in an identical portion.

The content of aldehydes, which are the main cause of the nauseous smell of the fumes, was reduced by a proportion of between 80 and 85%.

Claims (3)

1. A process for purifying the exhaust gases given off by a diesel type internal combustion engine comprising the steps of; 5 a) depositing alumina trihydrate from a hot solution (as herein defined) of alkaline aluminate on criss-crossed metal wires, b) calcinating the said alumina so as to give it a specific surface area of at least 120 square metres per 10 gram, e) installing the said criss-crossed metal wires covered with alumina in the exhaust circuit of the said diesel type internal combustion engine, and d) contacting the exhaust gases with the said 15 criss-crossed metal wires covered with alumina for a period of at least 0.3 second at a temperature of at least 250°C.

2. A process for purifying the exhaust gases given off by a diesel type internal combustion engine comprising the steps of; a ) depositing alumina trihydrate from a “hot solution (as herein defined) of alkaline aluminate on criss-crossed metal wires, b) calcinating the said alumina so as to give it a specific surface area of at least 120 square metres per gram, c) installing the said criss-crossed metal wires 25 covered with alumina in the exhaust circuit of the said diesel engine, -12d) insulating the exhaust circuit of the said engine so that its equilibrium temperature in operation is always at least 250°C, and e) contacting the exhaust gases with the said criss5 crossed metal wires covered with alumina for a period of at least 0.3 second at a temperature of at least 250°C,

3. A process for purifying the exhaust gases given off by a diesel type internal combustion engine substantially as herein described with particular reference to the 10 Examples.