Abstract:
To reduce the number of particles in an exhaust gas (A), in particular in the exhaust gas (A) from an internal-combustion engine, sulfur dioxide SO 2  which is contained in the exhaust gas (A) is at least partially catalytically oxidized, in the presence of ammonia NH 3  in a catalytic converter system having a catalytic activity which increases in the direction of flow, to form sulfur trioxide SO 3 . The simultaneous presence of sulfur trioxide SO 3  and ammonia NH 3  in the exhaust-gas stream which is brought about in this way results in an agglomeration of the small and very small particles and reduction in the number thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending International Application PCT/DE98/03152, filed Oct. 28, 1998, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method and a device for reducing the number of particles in an exhaust gas formed during the combustion of a fuel, in particular in the exhaust gas from an internal-combustion engine. 
     The exhaust gas which is formed during the combustion of a fossil, organic fuel generally contains organic particles (soot) and inorganic particles (ash) of different sizes. Soot particles are formed in the event of incomplete combustion or thermal cleavage during the combustion of carbon-containing substances, in particular in the combustion of diesel fuel in badly adjusted diesel engines or as chimney soot at fireplaces. 
     To reduce the levels of nitrogen oxide contained in the exhaust gas from an internal-combustion engine, particularly in the case of internal-combustion engines which are operated with excess air, the selective catalytic reduction (SCR) process has proven particularly useful. In this process, the nitrogen oxides are reacted with ammonia on a catalytic converter to form nitrogen and water. The ammonia which is required for this purpose is preferably transported in liquid form, as an aqueous urea solution, in the vehicle. From this aqueous urea solution, the ammonia is generated by hydrolysis in a hydrolysis catalytic converter in each case in the particular quantities required at that time for conversion of the nitrogen oxides. 
     The aqueous urea solution is mixed with compressed air in a mixing chamber, for example in a manner which is known from published PCT application WO 96/36797. The urea solution is then injected into the exhaust-gas flow via an atomizer nozzle arranged in the exhaust-gas flow. 
     Furthermore, it is known from German published patent application DE 43 10 962 to use an oxidation catalytic converter, in which carbon monoxide and hydrocarbons contained in the exhaust gas are oxidized to form carbon dioxide or carbon dioxide and water, following an SCR catalytic converter, in order to reduce the levels of organic pollutant fractions contained in the exhaust gas. 
     The advantageous effects of the presence of sulfur trioxide SO 3  and ammonia NH 3  through the formation of ammonium bisulfate are known, for example, from the company brochure “Wahlco Dual Flue Gas Conditioning System” produced by Wahlco Environmental Systems, Inc., Santa Ana, Calif., November 1990 edition. In that document, sulfur trioxide SO 3  and ammonia NH 3  are injected into the exhaust gas in order to increase the efficiency of electrostatic soot or ash separators. This so-called “Dual Conditioning” means that an effective level of separation is ensured in the electrostatic separator even under difficult operating conditions, for example if there is a high carbon content in the ash. The properties of sulfur trioxide which are favorable for the separation level of electrostatically operating separators are also known, for example, from U.S. Pat. Nos. 5,024,171 and 5,320,052; in the first of these patents, the injection of ammonia serves primarily to remove excess sulfur trioxide SO 3  which is formed during the sulfur dioxide conditioning. 
     To reduce the levels of particle emissions, it is in principle known to use filters. The particles are trapped in such filters, which are designed as purely mechanical separators or, in large installations, as electrostatic separators. Owing to their limited capacity for trapping particles these filters have to be emptied or regenerated at regular intervals and the solid material which has been separated out has to be disposed of. In the case of soot, this may, for example, be achieved by post-combustion. In addition, filtering processes run the constant risk of becoming blocked. Furthermore, the properties required for a filter in the exhaust-gas flow, namely a high level of separation, a high capacity to trap particles, and a low flow resistance are only technically achievable at the same time with a high level of outlay. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method and device for reducing the number of particles in an exhaust gas formed during the combustion of a fuel, that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, and which is technically not complex and is reliable, i.e. enables the number of particles to be reduced efficiently with little outlay on maintenance. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method for reducing the number of particles in an exhaust gas formed during the combustion of a fuel, comprising the steps of providing an exhaust gas containing sulfur dioxide and particles having a first average particle diameter, admixing ammonia with the exhaust gas, and flowing the mixture of ammonia and exhaust gas through a catalytic converter system having oxidation activity such that the catalytic activity increases, in terms of the oxidation of sulfur dioxide SO 2 , in the direction of flow, whereby sulfur dioxide is at least in part catalytically oxidized to form sulfur trioxide, and ammonia and sulfur trioxide interact on the surface of particles, leading to agglomeration of the particles and affording exhaust gas with diminished content of sulfur oxides and particles diminished in number and having a second average particle diameter greater than said first average particle diameter. 
     The exhaust gas formed during the combustion of a fuel is in particular the exhaust gas from an internal-combustion engine. 
     The simultaneous presence of sulfur trioxide SO 3  and ammonia NH 3  in the exhaust-gas flow which is brought about in this way results in agglomeration of the small and very small particles. This is because acidic sulfur trioxide SO 3  and basic ammonia NH 3  become attached to the surface of the particles. The associated electrostatic or chemical interaction of the adsorbates formed during the attachment or adsorption leads to increased agglomeration. The formation of ammonium bisulfate NH 4 HSO 4  and/or ammonium sulfate (NH 4 ) 2 SO 4  which also takes place on the surface of the particles contributes to additional bonding of the particles. 
     It has been found that the toxicity of the particles contained in the exhaust gas depends, inter alia, on the size of the particles. The size of the particles has a substantial influence on the function of the lungs and the toxic effects associated with intake in the lungs. In other words, the toxic effect of the particles decreases as the size of the particles increases. 
     Moreover, the invention makes use of the fact that both ammonia NH 3  and sulfur dioxide SO 2  are contained in sufficient quantities in the exhaust gas from an internal-combustion engine which is run in particular on diesel fuel and downstream of which there is a catalytic converter operated with ammonia NH 3  as the reducing agent for the selective reduction of nitrogen oxide NO x . 
     In an advantageous configuration of the method, the catalytic converter system therefore comprises an SCR catalytic converter in which the exhaust gas is subjected to an SCR process. As a result, the presence of ammonia NH 3  in the exhaust gas is ensured from the outset. 
     In a further advantageous configuration, the catalytic converter system comprises an oxidation catalyst which is separate from, in particular connected downstream of, the SCR catalytic converter and in which the oxidation of the sulfur dioxide SO 2  takes place. As a result of this procedure, it is possible for existing exhaust-gas cleaning installations which are equipped with a controlled diesel catalytic converter to be retrofitted easily and at low cost. 
     To reduce the number of catalytic converters which are required in the catalytic converter system, in an alternative configuration the oxidation of the sulfur dioxide SO 2  takes place in a partial area, which is situated in particular on the outlet side in the SCR catalytic converter, of the SCR catalytic converter. 
     The catalytic converter system preferably contains, as the catalytically active component used for the oxidation of the sulfur dioxide SO 2 , at least one element selected from the group consisting of Pt, Pd, Rh, Ir, Au, Ag, Ru. 
     In an alternative configuration, the catalytic converter system contains, as the catalytically active component used for the oxidation of the sulfur dioxide, at least one oxide or mixed oxide of an element selected from the group consisting of the transition metals or the lanthanides or the actinides. 
     In a particularly preferred configuration of the method according to the invention, the catalytically active component used for the oxidation of the sulfur dioxide is vanadium V or a compound which contains vanadium V. 
     In particular, the catalytic converter system contains, as a promoter used for the oxidation of the sulfur dioxide SO 2 , at least one compound containing an alkali metal or alkaline-earth metal. 
     With the above object in view, there is also provided in accordance with the invention a method for reducing the number of particles in an exhaust gas formed during the combustion of a fuel, in particular in the exhaust gas from an internal-combustion engine, wherein the gas contacts a device for admixing ammonia to the exhaust gas and a device having a catalytic converter system disposed in an exhaust pipe, for at least partial catalytic oxidation of sulfur dioxide SO 2  to form sulfur trioxide SO 3 , with a catalytic activity which increases in the direction of flow. 
     The latter device comprises a catalytic converter system within an exhaust pipe, for at least partial oxidation of sulfur dioxide SO 2  to form sulfur trioxide SO 3 , with a catalytic activity for the oxidation of the sulfur dioxide SO 2  which increases in the direction of flow, and a device for admixing ammonia to the exhaust gas. 
     In particular, the catalytic converter system contains an SCR catalytic converter which, at least in a partial area, is fitted as an oxidation catalytic converter for oxidation of sulfur dioxide SO 2 . 
     In particular, the partial area is located on the outlet side in the SCR catalytic converter, since the oxidation catalytic converter used for the catalytic oxidation of the sulfur dioxide SO 2  would also promote the catalytic oxidation of ammonia NH 3 , so that the latter would no longer be available for the SCR process. 
     In an alternative embodiment, the catalytic converter system comprises an oxidation catalytic converter which is separate from the SCR catalytic converter. This allows simple retrofitting of exhaust-gas cleaning installations which already comprise an SCR catalytic converter. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a method and device for reducing the number of particles in an exhaust gas, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  each show a diagrammatic, outline illustration of an exemplary embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with  FIG. 1 , exhaust gas A which is formed during the combustion of a fuel which contains sulfur S is guided inside an exhaust pipe  2 . The sulfur dioxide SO 2  which is formed in the exhaust gas A as a result of the combustion of the sulfur flows through a catalytic converter system  4  which comprises an SCR catalytic converter  6   a.  The SCR catalytic converter  6   a  is used both for oxidation of the sulfur dioxide SO 2  to form sulfur trioxide SO 3  and for the selective catalytic reduction of nitrogen oxide NO x  contained in the exhaust gas A. The reducing agent RM, which contains ammonia NH 3  required for the reduction of the nitrogen oxides NO x , which in the exemplary embodiment is aqueous urea solution, is introduced into the exhaust pipe  2  upstream of the SCR catalytic converter  6   a,  as seen in the direction of flow of the exhaust gas A, and is mixed with the exhaust gas A. In the exemplary embodiment, an atomizer nozzle  8  is provided for this purpose, by means of which the aqueous urea solution is injected into the exhaust gas flow as a fine mist. The urea is hydrolyzed in a hydrolysis catalytic converter  10  connected upstream of the SCR catalytic converter, so that gaseous ammonia NH 3  is formed, which serves as a reducing agent for the SCR-process to be carried out in the SCR catalytic converter  6   a.    
     The SCR catalytic converter  6   a  comprises two partial areas  12 ,  14 , the chemical compositions of which differ in terms of the catalytically active substances. In an inlet-side or front partial area  12 , the catalytic converter contains, as the catalytically active components, 70-95% by weight titanium dioxide TiO 2 , 2-10% by weight tungsten oxide WO 3  or molybdenum oxide MoO 3 , 0-2% vanadium pentoxide V 2 O 5 , 0.1-6% by weight aluminum oxide Al 2 O 3  and 0.1-6% by weight silicon dioxide SiO 2 . In the outlet-side or rear partial area  14 , the vanadium pentoxide concentration is higher, being approximately 4% by weight in the exemplary embodiment. The vanadium pentoxide has a significant influence on the catalytic activity of the SCR catalytic converter  6   a  with regard to the conversion of sulfur dioxide SO 2  to sulfur trioxide SO 3 . The BET surface areas of the SCR catalytic converter  6   a  are between 30 and 150 m 2 /g. Its pore volume, measured using the mercury penetration method, is between 100 and 1000 ml/g, with a monomodal or polymodal pore radius distribution. 
     It is essential that the vanadium pentoxide concentration in the inlet-side partial-area  12  be lower than in the outlet-side partial area  14 , so that the catalytic activity in terms of the oxidation of the sulfur dioxide SO 2  increases in the direction of flow. Moreover, an alkali metal or an alkaline-earth metal or a compound containing an alkali metal or an alkaline-earth metal, in the exemplary embodiment 5% by weight of potassium sulfate K 2 SO 4 , is added as a promoter to the partial area  14  in order to increase this catalytic activity. 
     The BET surface areas of the SCR catalytic converter  6   a  are between 30 and 150 m 2 /g. Its pore volume, measured using the mercury penetration method, is between 100 and 1000 ml/g, with a monomodal or polymodal pore radius distribution. 
     The base substance for the SCR catalytic converter  6   a  is produced by mixing, milling and kneading of the oxides or their precursor compounds, if appropriate with the addition of customary ceramic auxiliaries and fillers. This base substance is either processed into solid extrudates or is used to coat ceramic or metallic supports in honeycomb or plate form. The catalytic converter  6   a  is then present in the form of an intermediate. The partial area  14  which is provided for sulfur dioxide oxidation is then produced by partially impregnating the intermediate in a solution containing the component which has a catalytic activity for the sulfur dioxide oxidation, in the exemplary embodiment vanadium pentoxide V 2 O 5 . After drying at 20-100° C., calcining then takes place in the temperature range between 200 and 800° C. 
     In a further embodiment, the partial area  14  of the SCR catalytic converter contains at least one of the transition metals platinum Pt, palladium Pd, rhodium Rh, iridium Ir, gold Au, silver Ag or ruthenium Ru preferably 2 g of platinum Pt per liter of catalytic converter volume. Moreover, 0-100% by weight of an alkali metal or alkaline-earth metal or of an alkali metal compound or alkaline-earth metal compound or of a mixture thereof, for example 5% by weight of potassium sulfate K 2 SO 4 , is added in the partial area  14  as a promoter. 
     In the exemplary embodiment, the catalytic activity of the sulfur dioxide SO 2  for the oxidation increases suddenly at the boundary between the two partial areas  12  and  14 . In an alternative embodiment, instead of a sudden change in the vanadium pentoxide content, this content is increased continuously over the entire length of the SCR catalytic converter, so that the catalytic activity with regard to the sulfur dioxide oxidation increases continuously in the direction of flow of the exhaust gas A. In an alternative embodiment of this nature, the SCR catalytic converter therefore does not comprise adjoining partial areas  12  and  14 , at the boundary between which the composition changes suddenly. 
     As a result of the oxidation of the sulfur dioxide SO 2  to form sulfur trioxide SO 3  which takes place in the outlet-side partial area  14 , the small and very small particles agglomerate to form larger pieces. This is caused by an attachment of acidic sulfur trioxide SO 3  and basic ammonia NH 3  to the surface of the particles, which, owing to the electrostatic or chemical interaction of the adsorbates and due to the formation of ammonium bisulfate NH 4 HSO 4  or ammonium sulfate (NH 4 ) 2 SO 4 , become joined together. In other words, the exhaust gas A, upstream of the catalytic converter arrangement  4  as seen in the direction of flow, contains particles R 1  which on average have a smaller diameter than the particles R 2  contained in the exhaust gas A downstream of the catalytic converter arrangement  4  as seen in the direction of flow. This increase in the mean size of the particles consequently also leads to a reduction in the number of particles. 
     Moreover, the presence of the partial area  14  which has a higher catalytic activity with regard to the oxidation action reduces the levels of hydrocarbons which remain behind in the exhaust gas A in the incomplete combustion process, even if they are situated on the particles R 1 . Since ammonia NH 3  becomes attached to the particles R 1 , moreover, the amount of excess ammonia NH 3  in the exhaust gas, i.e. the ammonia slip, is reduced. 
     In accordance with  FIG. 2 , in an alternative configuration of the device, a catalytic converter system  4   b  is provided, in which, in addition to a pure SCR catalytic converter  6   b,  which is provided only for the catalytic reduction of nitrogen oxides NO x , there is also an oxidation catalytic converter  16 , which is formed spatially and structurally separate from the SCR catalytic converter, i.e. as a structurally separate unit, and can have the same composition as the partial area  14  in the exemplary embodiment shown in FIG.  1 . This makes it easy to retrofit exhaust-gas cleaning installations in which a conventional SCR catalytic converter is already fitted.