Technical field

This invention in general concerns plants for Selective Catalytic Reduction

(SCR) of nitrogen oxides in the exhaust gases which are produced by combustion, in particular, by the combustion of the air/fuel mixture in an internal combustion engine of a vehicle.

Background of invention

Selective Catalytic Reduction (SCR) is a chemical process for reducing emissions of nitrogen oxides (NO _x ), in which a chemical reagent, typically a reducing agent containing nitrogen, such as ammonia or urea, is added to exhaust gases so that it can react with the nitrogen oxides contained in the exhaust gases, thus giving rise to molecular nitrogen (N ₂ ) and water (H ₂ O).

The chemical reducing agent is added to the exhaust gases in the presence of an appropriate catalyst, which permits reduction of the nitrogen oxides to take place at lower temperatures (in any case above 250°) and with higher reduction performance, in comparison with what would be the case if reduction were to take place spontaneously without catalysis.

On vehicles with an internal combustion engine, the reducing agent is generally supplied in the form of an additive which goes by the name of

AdBlue.

This additive is a stable, colourless water-like liquid, 32.5% of which is urea, the rest being de-ionised water. It exhibits low toxicity, does not entail the risk of chronic diseases, is neither flammable nor explosive, and is therefore easily transportable in a simple tank located on board a vehicle.

The additive is injected into the stream of exhaust gases through a supply line which flows directly into the exhaust pipe, upstream of an SCR catalyst which accelerates reduction of the nitrogen oxides, and which is integrated into the silencer.

The stream of exhaust gases enters circulation at the entrance to the catalyser, thus heating the catalyser rapidly after the engine is started, until the relatively high temperatures (above 250°) which are necessary to trigger the reducing reaction of nitrogen oxides are reached.

When the liquid additive enters the exhaust pipe, the urea contained in the exhaust pipe breaks down into ammonia and carbon dioxide. Ammonia is the reagent which reduces the NO _x giving rise to molecular nitrogen and water.

The supply line of the additive schematically comprises a storage tank, and a pump which entrains the additive contained in the tank, conveying it towards a dosing valve, which injects the additive in dosed quantities into the exhaust pipe.

A filter is also located along the supply line, usually upstream of the pump, which filters the liquid additive.

The filter comprises an external casing provided with an inlet and an outlet for the additive, which casing houses a filter element through which the additive passes, and which traps impurities contained in the additive.

The filtering element is usually a membrane with a pleated surface and a star-like geometry, and is made from a cellulose-based material.

This cellulose-based material tends however to soften and deteriorate rapidly when wetted with water or with aqueous solutions, so that its effectiveness and duration in filtering the additive used in the SCR process are in actual fact very low.

In particular, a membrane exhibiting a star-like geometry and made with a material of this kind tends to deform rapidly, losing the pleating configuration, and assuming an undefined form which strongly limits the available filtering surface.

To overcome this drawback, the cellulose material of the filter membrane is often treated with substances to increase its compatibility with the additive, that is to say, substances which increase its resistance and stability over time when it is used with the particular additive.

Since however the urea and water solution is corrosive, the stability over time of the filter membrane cannot be ensured even if the cellulose-based material is treated with the above-mentioned substances. The aim of this invention is to obviate this drawback, in the ambit of a simple, rational and inexpensive solution.

This aim is achieved by the characteristics of the invention described in the independent claims. The dependent claims delineate preferred and/or particularly advantageous aspects of the invention.

Disclosure of the invention

In particular the invention provides a plant for treating exhaust gases produced by combustion, comprising a conduit for the exhaust gases into which a supply line of an additive opens, which additive reduces the nitrogen oxides contained in the exhaust gases, and at least a filter element, which is made of a polymer material, to filter the additive.

The polymer material is perfectly compatible with the water and urea additive which is normally used in the SCR process, and is not significantly affected by the additive's corrosiveness: this makes the filter element much more resistant and stable compared with the filters at present in use for this type of application.

Preferably, the filter membrane is made from polypropylene, which is one of the available polymers exhibiting the above-mentioned characteristics of compatibility with the additive to the greatest extent.

In a preferred aspect of the invention, the filter element is a depth filter membrane, which in addition to ensuring high filtering efficiency, also exhibits great accumulation capacity, which ensures a longer working life.

Alternatively, the filter element can be a surface filter membrane with a pleated surface, possibly having a star-shaped geometry.

The filter element can be produced from polymer fibres by means of a meltblown process.

Brief description of the drawings

Further characteristics and advantages of the invention will emerge from the following description which is provided as a non-limiting example, with the aid of the appended figures of the drawings, in which: figure 1 is a diagram of a scheme of a plant for treating exhaust gases of a vehicle internal combustion engine, according to the invention. - A -

figure 2 is a section view along a vertical plane of a filter for the additive, which is installed in the plant of figure 1. Description of the preferred embodiment

Figure 1 schematically shows a plant 1 for treating the exhaust gases produced by an internal combustion engine 2, typically but not exclusively a

Diesel engine, which is mounted on board a vehicle, such as a motor car, truck, motor bus or the like.

The plant 1 is designed to subject the exhaust gases to a Selective Catalytic

Reduction (SRC) process, during which the nitrogen oxides (NO _x ) contained in the exhaust gases undergo a chemical reaction of reduction, giving rise to molecular nitrogen and water.

The plant 1 comprises a conduit 3 for the exhaust gases, in this case an exhaust pipe which connects the exhaust manifold of the engine 2 with the silencer, and a supply line 4 for an additive which operates the reduction of the nitrogen oxides contained in the exhaust gases.

The additive is a reducing agent, generally in the form of a nitrogenous liquid, approximately 32.5% of which is urea, the rest being de-ionised water, and the additive goes by the name of AdBlue.

The supply line 4 comprises a tank 40, and a pump 41 which sucks up additive contained in the tank 40 and supplies it to a dosing valve 42, which injects dosed quantities of the additive directly into the conduit 3 of the exhaust gases.

In particular, the dosing valve 42 injects the additive into the stream of exhaust gases upstream of a catalyser 30, which is incorporated into the silencer, where appropriate catalysing substances accelerate the reduction of the nitrogen oxides.

After the engine 2 has been started, the stream of exhaust gases rapidly heats up the catalyser 30, until the relatively high temperatures (above

250 ⁰ C) which are necessary to trigger the reaction of reduction of the nitrogen oxides are reached. In these conditions, when the additive enters the exhaust pipe 3, the urea dissolved in the additive breaks down into ammonia and carbon dioxide. Thanks the high temperatures and the presence of the catalyser 30, the ammonia reduces the NO _x , giving rise to molecular nitrogen and water, which are then discharged into the atmosphere through the silencer.

For the reduction reaction to take place correctly, the additive must be injected in calibrated quantities which are based on the temperature of the exhaust gases, and on the percentage of NO _x contained in the exhaust gases. This calibration is effected automatically by an electronic control unit 9, which controls the operation of the pump 41 and of the dosing valve 42, in response to signals coming from a temperature sensor 90 and from an exhaust gas sensor 91 , which are mounted in the exhaust pipe 3 preferably downstream of the catalyser 30.

The supply line 4 further comprises a filter 5 which filters the additive, and which is preferably positioned between the tank 40 and the pump 41. As shown in figure 2, the filter 5 comprises an external casing 50, which is preferably made from polymer material, the internal volume of which is divided by a cartridge 6 into two distinct chambers, of which a first chamber 7 communicates with an input pipe 51 for the additive to be filtered, and a second chamber 8 communicates with an output pipe 52 for the filtered additive.

More in particular, the external casing 50 comprises a cylindrical beaker- shaped lower container 53, the mouth of which is hermetically closed by an openable upper cover 54, to which both the input conduit 51 and the output conduit 52, arranged respectively in a central and a peripheral position, are constrained.

The cartridge 6 comprises a central cylindrical cage 60 positioned between two coaxial support plates, of which a lower plate 61 is completely closed, and an upper plate 62 is provided with a central hollow bushing 63, which is removably inserted into an internal portion of the input pipe 51 , by interposing a ring seal 64.

The central cage 60 and the support plates 61 , 62 are all made of polymer material, preferably of the same type which is used for the external casing 50. A substantially cylindrical filter element 65 is coaxially inserted into the central cage 60 and is constrained between the support plates 61 and 62 of the cartridge 6. The stream of additive flows from the first chamber 7 to the second chamber 8 such as to trap any impurities contained in the additive. The filter element 65 is made from a polymer material, preferably polypropylene, which is optimally compatible with the additive to be filtered, that is to say, it does not suffer corrosion, damage or become excessively soft, when the additive flows through it, wetting it.

In this way, the filter element 65 has a much longer working life than the filter elements of the prior art which are used for this kind of application, in comparison with which it is also more efficient and stable over time. In a preferred aspect of the invention, the filter element 65 is made of the same polymer material of which the external casing 50 and possibly the central cage 60 and the support plates 61 and 62 are made. In this way, although the filter element 65 and of the other components of the casing 50 and of the cartridge 6 require different manufacturing processes, it is advantageously possible to reduce both the supply costs for the raw materials, and the costs of eliminating the filter 5, since the filter 5 can be entirely disposed of by simply following the procedures which are prescribed for polymer materials, and requiring neither dismantling nor waste separation. In particular, the filter element 65 can be made from a polymer fibre non- woven fabric, obtained for example using a meltblown procedure. In the illustrated example, the filter element 65 is a depth filter membrane which, besides ensuring superior filtering efficiency, allows a substantial accumulation of impurities before it is saturated, thus significantly prolonging the working life of the filter 5.

However the invention does not exclude the possibility that the filter element 65 may have the form of a thinner, pleated surface membrane, possibly wound around a central axis, thus acquiring a star-like geometry. Obviously, a person skilled in the sector might introduce numerous modifications of a technical and applicational nature to the plant 1 described above, without thereby forsaking the ambit of the invention as claimed below.