The present invention relates to a method for dosing an urea-based reducing agent (e.g. a solution of urea in water) into a gas exhaust stream generated from a combustion engine and addressed to an aftertreatment system (e.g SCR or SCRT device). The method according to the invention allows to improve the mixing of the reducing agent into gas exhaust stream and consequently to improve the catalyst efficiency of the aftertreatment device.
As it is know, a problem in the field of internal combustion engines, in particular with diesel engines, whether turbocharged or not, is the formation of nitrogen oxides during combustion. The nitrogen oxides are discharged with the engine exhaust gases and represent one of the main pollutants. In order to reduce the emissions of nitrogen oxides approximately up to 90%, selective catalyst reduction (SCR) devices have been developed. Depending on the particulate emission limits, these systems can be equipped with a particulate trap (SCRT System).
The functioning of SCR and SCRT device is based on the reaction, promoted by an appropriate catalytic unit, between the nitrogen oxides in the exhaust gases and ammonia specifically introduced as reducing agent. The ammonia is usually introduced in the form of a preferably liquid reagent able to release ammonia, under suitable temperature conditions or by the action of specific catalysts. The preferred source is usually urea in an aqueous solution, for example between 10 and 60% in weight, from which the ammonia is obtained by hydrolysis.
The urea is generally nebulized in a dosing module which is located upstream the SCR-SCRT system.
In
(NH2)2CO[aqueous]→(NH2)2CO[solid]+6.9H2O[gas]
After the evaporation of water the urea decomposition starts according to reactions:
(NH2)2CO[solid]→NH3[gas]+HNCO[gas]
HNCO[gas]+H2O→NH3[gas]+CO2+[gas]
The injection methods proposed by the solutions in
In the solution shown in
Also it has to be noted that decomposition of urea-water solution, may cause formation of other products in particular isocyanic acid. This is an highly reactive compound that tends to form liquid deposits, such as liquid films, or solid deposits on the various parts of the exhaust system (e.g. pipes, deflectors, SCR-SCRT System). This is caused by contacting the reacting agent solution with cold surfaces such as for example the walls of the dosing module housing or those of the gas exhaust pipe.
The arrangements proposed in
Therefore it is the main object of the present invention to provide a method for dosing a reducing agent into a gas exhaust stream generated from a combustion engine and addressed to a aftertreatment system which allows to overcome the above mentioned problems/drawbacks.
Within this aim, a first object of the present invention is to provide a method for dosing a reducing agent into an exhaust gas stream which allows a complete urea decomposition and an uniform mixing of ammonia with the exhaust gas.
Another object of the present invention is to provide a method for dosing a reducing agent into an exhaust gas stream which avoids the interaction downstream the injection position between the reducing agent and cold walls of the exhaust gas system (e.g. dosing module walls, exhaust gas walls, aftertreatment system walls). Not the last object of the present invention is to provide a method which is highly reliable and relatively easy to perform at competitive costs.
These and further objects are achieved by a method as described in the attached claims which form an integral part of the present description. In particular according to the method of the invention, said exhaust gas is conveyed into an dosing module housing which develops along a longitudinal axis. In particular the exhaust gas is conveyed by generating an annular inlet jet inclined with respect to the axis of the dosing housing. Moreover according to the method of the invention, the urea-based reducing agent is dosed by generating an urea-based reducing agent spray which is preferably coaxial to said axis of the housing.
The invention will become fully clear from the following detailed description, given by way of a mere illustrative and non limiting example, to be read with reference to the attached drawing Figures, wherein:
The present invention relates to a method for dosing an urea-based reducing agent into an exhaust gas stream generated from a combustion engine, for example a diesel engine. For the purposes of the present invention, by the expression “urea based reducing agent” it is meant an urea-based solution, e.g. water-urea solution, able to develop ammonia when injected in an exhaust gas current addressed to a after-treatment device such as, for example, an SCR or SCRT device.
The method according to the invention comprises a first step of providing a dosing module 20 having a dosing housing which develops along an axis X (also indicated with longitudinal axis X). In this regard,
According to the method of the invention, the exhaust gas stream coming from the engine is conveyed into said dosing housing 20 by generating an annular inlet jet (indicated with AJ) inclined with respect to the axis X. In other words, the exhaust gas is annularly introduced into the dosing module housing according to a inlet direction Y inclined to said axis X. In this way, the inlet exhaust gas stream presents a radial component perpendicular to the longitudinal axis X and an axial component which is parallel to the longitudinal axis itself.
The method according to the invention also provides for dosing said urea-based reducing agent by generating a urea-based reducing agent spray (indicated with UWS) which is internal to the dosing housing 20 and preferably coaxial to the axis X of the housing itself. More in detail, the reducing agent is sprayed, by means of spraying means, so that the opening cone of the spray UWS is incident to the inlet direction Y of the annular inlet jet AJ of exhaust gas. As shown in
With reference again to
As shown in
It has been shown that the present invention achieves the aim and the objects as set forth above. More in detail, it has been shown that the method for dosing an urea-based reducing agent allows a complete decomposition and an uniform mixing of ammonia with the exhaust gas. Moreover, the method also avoids the formation of liquid deposit on the internal surface of the dosing housing and of the gas exhaust pipe.
Many changes, modifications, variations and other uses and applications of the subject invention will become apparent to those skilled in the art after considering the specification and the accompanying drawings which disclose preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by this invention.
Further implementation details will not be described, as the man skilled in the art is able to carry out the invention starting from the teaching of the above description.
Number | Date | Country | Kind |
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09179455.2 | Dec 2009 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP10/69723 | 12/15/2010 | WO | 00 | 6/1/2012 |