DEVICE DESIGNED TO REDUCE ATMOSPHERIC POLLUTION FROM

EXHAUST GASES ** *.**

The present invention relates to a device for reducing atmospheric pollution from exhaust gases.

In particular, the subject of the present invention is a device designed to reduce atmospheric pollution from exhaust gases produced by internal - combustion engines and from the engine gas itself, which is recirculated at intake, in particular of diesel engines.

It is understood that the invention finds, however, application in the treatment of exhaust gases of different kinds, for example, in the field of industrial, commercial, and domestic heating plants and systems.

It is known that in exhaust gases produced by internal -combustion engines, and in particular diesel engines, pollutant substances, such as carbon particulate, unburnt hydrocarbons, nitrogen oxides, carbon oxides, etc., are present in variable amounts.

There have already been proposed various devices aimed at reducing the content of pollutant agents present in the exhaust gas of internal - combustion engines, suited in particular to reducing

the content of carbon particles (particulate) and of unburnt hydrocarbons .

The document No. EP-A-1123728 describes a device comprising a cylindrical casing having an inlet for the exhaust gases that are to be purified, an outlet for the purified exhaust gases, and a tubular cartridge housed in the casing and set between the inlet and the outlet. The cartridge comprises mineral fibres of catalytic material, the chemical composition of which comprises at least 70wt% of silica (SiO ₂ ) and at least one of the following metals: platinum, rhodium, and palladium.

The cartridge of the device described in EP-A-1123728 is formed starting from a pad including a layer of mineral fibres sandwiched between two gauzes made up of thin steel wire. Said pad is wound in a spiral in order to form an annular cartridge, in which, in a section lying in a plane orthogonal to the longitudinal axis of the cartridge, in a radial direction, there alternate layers of fibres and layers of woven filiform material formed by the thin- wire gauzes.

Experimental tests and analyses have shown that the composition of catalytic material described in the document No. EP-A-1123728 is quite effective for

obtaining the conversion of carbon and unburnt hydrocarbons into carbon dioxide and water. In particular, the composition of catalytic material described in this document presents the advantage of enabling catalysis at relatively low temperatures (in the region of 200 ⁰ C) .

The solution described in the document No. EP-A-1123728, however, albeit having enabled good results to be obtained as regards the percentages of abatement of pollutant substances of exhaust gases, in particular, particulate and unburnt hydrocarbons, does not enable a total abatement of carbon particles to be achieved.

The document No. EP-A-1340890 describes a device for reducing atmospheric pollution from exhaust gases of the type described in the patent No. EP-A-1123728, but in which the layer of catalytic fibres has an undulated arrangement in order to present to the exhaust gas a larger exposed surface in a direction orthogonal to the direction of the flow of gas.

The above solution originated from the realization that the rate of flow of gas through the catalytic material is of determining importance for the process of catalysis that converts the carbon

particles and the unburnt hydrocarbons contained in the gas into carbon dioxide and water.

The undulated form of the layers of catalytic material enables a considerable reduction in the rate of traversal of the flow of gas to be achieved given the same size of the cartridge. This is due to the increase in the active surface of the catalytic material .

Whereas in the solution according to EP-A-1123728 the cartridge had a very large thickness and an active surface equal to the area of the outer cylindrical surface of the cartridge, in the solution according to the patent application No. EP-A-1340890 the thickness of the catalytic material is very reduced, equal only to the thickness of the needled layers, whereas the area of the active surface increases. This arrangement theoretically enables better results to be obtained as compared to the solution proposed by the document No. EP-A-1123728 from the point of view of the efficiency of abatement of the pollutant components contained in the exhaust gases .

However, experimental tests have shown that the layer of filtering material also in the case of the document No. EP-A-1340890 is too large, and the

surface exposed to the gas is too limited, even though the needled or woven material is arranged in an undulated way. In fact, on account of the high rates of flow of the gas, in a short time the gas remains to a large extent trapped in the skein of needled material, which is unable to carry out proper abatement of the carbon particulate and of unburnt hydrocarbons. Indeed, within a relatively short period of time, abatement of the carbon particulate and unburnt hydrocarbons is no longer carried out even in minimal percentages because the filter remains completely choked.

The purpose of the present invention is to provide an improved solution that will enable a further increase in the percentages of abatement of pollutant substances and will bring the content of carbon particulate, hydrocarbons, and carbon monoxide in the purified exhaust gas as close as possible to zero .

According to the present invention, said purpose is achieved by a device having the characteristics forming the subject of the main claim.

The advantages of the present invention will emerge clearly from the ensuing detailed description,

which is provided purely by way of non-limiting example, with reference to the attached plate of drawings, in which:

- Figure 1 is a schematic axial-sectional view of a device according to the present invention;

- Figure 2 is a cross section according to the line II-II of the cartridge of catalytic material used in the device according to the present invention;

- Figure 3 is a detail at an larger scale of the part A enclosed by a circle in Figure 2; and

- Figure 4 is a further enlarged view of a part of Figure 3.

With reference to Figure 1, designated by 10 is a device for reducing the content of pollutants in a flow of exhaust gases. The figure schematically illustrates a device that can be applied to the exhaust system of lorries or buses with diesel engines, but it is understood that the shape and dimensions of the device according to the present invention can be modified to adapt the device to vehicles of other types, such as, for example, motor vehicles, earth-movers, electric-generating sets, steam rollers, fork-lift trucks, or else heating plants and systems of an industrial, commercial, or

domestic use .

The device 10 comprises a casing 12 having a central portion 14, for example, of a cylindrical shape, a section 16 for inlet of the gas to be purified, and a section 18 for outlet of the purified gas. The casing 12 preferably comprises two metal sheets 20 with a layer of thermally insulating material 22 set in between.

A catalytic cartridge 24 is housed in the central body 14 of the casing 12. The device 10 is built so that, in operation, the cartridge 24 is traversed by the gas in a radial direction from outside inwards, following a path indicated by the arrows X in Figure 1.

As may be noted more clearly in Figures 3 and 4, the cartridge 24 comprises an outer cylindrical containment wire gauze 26 and an inner cylindrical wire gauze 28, once again for containment, which define the outer boundary and the inner boundary, respectively, of the cartridge 24.

The entire cartridge is contained by the gauzes 26, 28, which form an integral part with opposed end plates 30, 32 (see Figure 1) for hermetic sealing and gripping of the end pieces.

The active material of the catalytic cartridge

24 is .formed by one or more layers of mineral fibres 34 in the form of mattress or fabric, the chemical composition of which is advantageously 99 wt% pure silica fibre (SiO ₂ ) and in any case not less than 80 wt%, together with at least one of the following noble metals: platinum, in an amount ranging between 0.0003 g to 0.5 g per gram of fibres, and iridium, again in an amount ranging between 0.0003 g to 0.05 g per gram of fibres.

The mineral fibre is in the form of a mattress or fabric arranged in a pleated form, i.e., folded so as to form pleats 36, 38, arranged alternately adjacent to the outer surface 26 and the inner surface 28 of the cartridge 24. The pleats 36, 38 of the layer or layers of fibres 34 are connected to one another by rectilinear stretches 40.

The mattress or fabric made of silica-fibre metallic material 34 is coupled to a mesh 42, which has the purpose of strengthening it and supporting it in the pleated condition.

According to the invention, the distance between adjacent undulations 44, the length 1. of the stretches joining the arched stretches 36, 38, and the thickness and the density of each layer of silica-fibre mattress or fabric 34 are elements of

fundamental importance for obtaining the maximum result from the catalytic filter in -so far as, taken together, they determine the active surface exposed to the gas.

In particular, the thickness of the layer or layers of catalytic fibres can range between 0.3 and 1.0 mm, whilst the density of the mattress or fabric can range between 300 g/m ³ to 1000 g/m ³ .

Since the dimensions of the filter (i.e., the distance between the inner containment gauze 28 and the outer containment gauze 26) , as well as the overall width, depend upon the type of vehicle on which the filter is installed, the distance between the undulations and the length of the stretches for joining the undulations must be expressed in a ratio that, in the example illustrated in Figures 3 and 4, is JVd = 9, where 1 is the length of each stretch, and d is the mean distance between two adjacent undulations 44.

Depending upon the different filter dimensions according to the applications, it may be stated that the ratio JVd must be between 5 and 12 to enable, together with the aforesaid values of thickness and density of the layer of material, the filter to work in an optimal way, i.e., to enable a further

improvement in the percentages of abatement of the pollutant substances and to bring the content of carbon particulate in the purified exhaust gas as close as possible to zero.

To the above there is to be added the fact that the flow of exhaust gas acts from the outside towards the inside of the cartridge 24 (see arrows X in Figure 1 ) .

In fact, as already mentioned, the present invention originated from the realization that the rate of flow of gas through the catalytic material is of determining importance for the purposes of the catalytic process that converts the carbon particulate and the unburnt hydrocarbons contained in the gas into carbon dioxide and water.

Experimental tests have shown that, with rates of passage of the gas through the fibres of between 0.2 and 2.5 m/s, and in particular less than 1 m/s, it is possible to obtain almost total abatement of particulate and unburnt hydrocarbons, guaranteeing a long duration. The undulated shape of the layers of catalytic material of the patent No. EP-A-1340890 theoretically enabled a considerable reduction in the rate of traversal of the flow of gas given the same dimensions of the cartridge, thanks to the increase

of the active surface of the catalytic material. But this undulated shape was not sufficient, but rather yielded somewhat poor results in so far as the layer of material had a thickness (from 1 to 20 mm) and density (from 20 to 180 kg/m ³ ) which are too high, and a ratio _l/d not higher than 3, and consequently, on account of the high rates of flow of the gas, the latter remained within a short time to a large extent trapped in the skein of needled material, which was no longer able to carry out abatement of the carbon particles and of the unburnt hydrocarbons.

As compared to the solution described in the document No. EP-A-1340890, the present invention hence envisages a considerable reduction in the thickness and density of the catalytic material and an increase in the active surface, thus increasing considerably the aforesaid ratio 1/d in order to reduce the rate of passage of the gas.

The above arrangement enables unexpected results to be achieved from the point of view of efficiency of abatement of the pollutant components contained in the exhaust gases that can reach up to an almost total abatement of the carbon particles and the unburnt hydrocarbons, guaranteeing a long service life.

It should moreover be considered that, with the sizings of the filter given above, the filter works with a catalytic effect also in regard to levels of NO _x lower than 0.2 g/kWh, and works also with operating temperatures of between 200 ⁰ C and 600 ⁰ C that lie outside of the effect of NO ₂ . Consequently, this filter, unlike current commercially available ones, does not require conversion of NO _x into NO ₂ to guarantee proper operation of the filter itself.