Patent Publication Number: US-2020300140-A1

Title: SELECTIVE CATALYTIC REDUCTION SYSTEM AND A METHOD FOR NOx REDUCTION

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT Application No. PCT/DK2018/050315, having a filing date of Nov. 29, 2018, which is based on DK Application No. PA 2017 00681, having a filing date of Dec. 1, 2017, the entire contents both of which are hereby incorporated by reference. 
    
    
     FIELD OF TECHNOLOGY 
     The following relates to a selective catalytic reduction system for NOx reduction. The following also relates to a method for NOx reduction. The system and method are configured for selective catalytic reduction of NOx in an exhaust stream either after a four-stroke diesel engine or between the exhaust receiver and the exhaust turbines on a two-stroke diesel engine by using a multifunctioning catalyst with oil as reductant. 
     BACKGROUND 
     Selective catalytic reduction systems are used for converting nitrogen oxides, typically referred to as NOx by means of a catalyst into diatomic nitrogen, N 2  and water, H 2 O. The conventional art teaches that a gaseous reductant may be selected among the following: anhydrous ammonia, aqueous ammonia or urea, and may be added to a stream of flue or exhaust gas and be adsorbed onto a catalyst. 
     Commercial selective catalytic reduction systems are applied in diesel engines, such as those found on ships, diesel locomotives, gas turbines, large utility boilers, industrial boilers, and municipal solid waste boilers and have been shown to effectively reduce NOx. 
     The increasing environmental awareness regarding NOx emissions has introduced higher requirements for NOx reduction equipment. 
     It is known that the International Maritime Organization (IMO) gradually has introduced more restrained NOx emission standards. Likewise, NOx emission tax in countries like Norway and Sweden has raised the demand for NOx reduction equipment. Selective Catalytic Reduction systems using urea as reactant followed by Exhaust Gas Recirculation are commonly used. 
     For the automobile industry, the requirements for the NOx reduction equipment is far more demanding than for the marine industry including NOx reduction at temperatures up 700° C. and here zeolites are more stable than conventional material for catalyst powder typically V 2 O 5 /WO 3 —TiO 2 , therefore the interest in using zeolite-based catalysts. 
     Research has been carried out in order to find a method, in which diesel oil can be used as reductant instead of urea. In 1990, Dr. M. Iwamoto published information about using Cu-ZSM-5 (A Cu exchanged zeolite catalyst) as catalyst and diesel oil as reductant. (Dr. M. Iwamoto (M Iwamoto, Decomposition of NO on copper ion-exchanged zeolite catalysts, Proceedings of meeting on Catalytic technology for removal of nitrogen monoxide, Tokyo, January 1990, p. 17) found that Cu-ZSM-5 (Copper ion-exchanged zeolites) is fairly active in NO x  reduction, if O 2  is present, and hydrocarbons used in the tests were C 2 H 4 , C 3 H 6  and C 3 H. 
     In 1996, S. Matsumoto (S. Matsumoto, Toyota, DeNOx catalyst for automotive lean burn engine; Catalysis Today 29 (1996) 43-45) concluded that, “the durability of Cu-ZSM-5 is insufficient for practical use” and introduced the NOx Storage Reduction catalyst (NSR catalyst) consisting of: “precious metals, alkaline earth metals, alumina and some other metal oxides”. Toyota has continued to develop the NSR system now called DiAir System (K. Yoshida et al., Toyota, Development of NSR and DiAir System to Achieve Clean Emission under Transient Cycle; SAE International 2014-01-2809) with pulsating HC injection during desulfation. However, others have continued developing the ZSM-5 catalyst. 
     It was disclosed (S. A. Yashnik et all, The Cu-ZSM-5 Catalysts Washcoated on Monolith for Diesel Emission Control; Chemistry for Sustainable Development 11 (2003) 309-319), that the Cu-ZSM-5 catalyst has a number of weaknesses, including losses of activity in gas with water and high temperature as well as losses, gradually, of activity with sulphur in the gas. Besides, it might be a problem, that half oxidized carbonous connections stick to the catalyst. 
     Research has found that, adding of Ce to Cu-ZSM-5 will increase the wet gas activity and hereby prevent formation of CuO, which reduces the activity. 
     US20070149385A1 discloses a system for reduction of NOx emission from diesel engines using catalysts with diesel oil as reductant. The system applies a multi-functioning catalyst for cracking of diesel oil, NOx reduction and a catalytic partial oxidation material including a platinum-group metal comprises an element selected from the group consisting of rhodium, platinum, iridium, palladium, osmium, and ruthenium to convert coke deposits to hydrogen and carbon monoxide. One disadvantage of this system is, that it requires use of precious metals which are expensive. 
     Thus, there is a need for an improved selective catalytic reduction system and selective catalytic reduction method for NOx reduction based on application of diesel oil as reductant, wherein a precious metal is not a requirement. 
     EP1111212A2 describes a selective catalytic NOx reduction system for Diesel engine exhaust applying Diesel oil as reductant. The system comprises a diesel oil injector, a selective catalytic reduction reactor and several selective catalytic reduction sections arranged in series. The catalyst sections each contain several catalyst layers. The distance between adjacent sections are, however, very short. Therefore, the system does not provide an optimum solution for NOx reduction. 
     SUMMARY 
     An aspect relates to an improved selective catalytic reduction system and selective catalytic reduction method for NOx reduction based on application of diesel oil as reductant, wherein the system and the method reduces or even eliminates the above-mentioned disadvantages of the conventional art. 
     The selective catalytic reduction system according to embodiments of the invention is a selective catalytic reduction system configured for applying diesel oil as reductant for converting nitrogen oxides (NOx) by means of a catalyst into diatomic nitrogen (N 2 ) and water (H 2 O) in a diesel engine, wherein the selective catalytic reduction system comprises:
         an oil injection system;   a reactor;   a number of selective catalytic reduction catalysts provided in at least a first section comprising a plurality of layers and a second section comprising a plurality of layers, wherein the distance between the sections is larger than the thickness of the layers.       

     Hereby, it is possible to provide an improved selective catalytic reduction system and selective catalytic reduction method for NOx reduction based on application of diesel oil as reductant. 
     Having diesel oil as reactant eliminates the demand for tanks for storing urea on the ship, and eliminates potential complex logistics for supplying urea. The cost for oil injected is around the cost for urea injected. On four-stroke medium speed diesel engines around 75% of the energy content of the injected oil can be recovered in a down-stream installed exhaust gas boiler and used for production of water, electricity or heat, meaning the actual operation costs are down to 2% of the oil used for the diesel engine. 
     By diesel oil is meant a liquid fluid (containing fuel oil) configured to be used in diesel engines. Accordingly, the term diesel oil includes marine diesel oil, also referred to as “distillate marine diesel”, which is a blend of gasoil and heavy fuel oil, with less gasoil than intermediate fuel oil. The term diesel oil includes all fuel oil containing marine fuels. 
     Furthermore, the use of oil as reactant eliminates the risk of blocking the catalysts due to formation of ammonia-bisulphate (NH 4 )HSO 4  or ammonium sulphate (NH 4 ) 2 SO 4 . 
     The reactor of the selective catalytic reduction system according to embodiments of the invention is weighing around 40% less than a conventional urea-SCR reactor with catalysts. Accordingly, embodiments of the invention makes it possible to reduce the reactor weight. 
     The process starts at an exhaust temperature of about 320° C. to about 360° C., such as about 350° C., but can continue operating at an exhaust temperature down to 310° C., because the process causes a temperature increase. 
     The length of the pipe for reactant injection and evaporation can be as short as one fifth of the urea selective catalytic reduction system pipe and can be made in conventional steel. 
     Two-stroke diesel engines, due to the temperature increase in the reactor according to embodiments of the invention require a marginal increase in diesel engine specific fuel consumption. 
     The selective catalytic reduction system according to embodiments of the invention is configured to convert nitrogen oxides (NOx) by means of a catalyst into diatomic nitrogen (N 2 ) and water (H 2 O) in a diesel engine. The chemical process includes three integrated catalysing steps:
         Cracking of the injected oil;
           Heterogeneous NOx conversion and formation of radicals used for a following reaction.   
           Homogeneous NOx conversion downstream of the catalyst.       

     During the cracking process, the reactant is injected into the exhaust as droplets upstream of the catalyst layer (of the sections) in a manner in which minimum 80% of the droplets have evaporated before reaching the first catalyst layer (of the first section). 
     The selective catalytic reduction system comprises structures configured to evenly disperse the oil into the exhaust. 
     The evaporated oil is cracked on the surface of the catalyst layers. A portion of the evaporated oil may, however, continue as un-cracked evaporated oil until it reaches the next catalyst layer. The temperature increases inside the reactor during the process carried out. Due to the increased temperature, only a minimum of the injected oil is passing through the system. The cracking takes place mainly on the surface of the catalyst layer. The cracking on the first layer is expected to be minimum 40%. Accordingly, it is advantageous to have several layers provided in a distance from each other. 
     The diesel oil contains relative long chained hydrocarbon connections. These hydrocarbon connections are cracked to short-chained hydrocarbon connections, during the catalyst process. These hydrocarbon connections are short enough to enter the catalyst layer of the sections. Accordingly, NO is converted to N 2  and H 2 O, whereas radicals (NH4 + ) reacts with NO 2  that is converted to N 2  and H 2 O. The likely chemical reactions (of the NOx conversion) are shown in the following. 
     The NO oxidation carried out may be expressed as: 
       NO+1/2O 2 ⇒NO 2   (1)
 
       NO+NO 2 +2H + ⇒2NO++H 2 O  (2)
 
     The heterogeneous catalyzing carried out may be expressed as: 
       2C 4 H 8 +10O 2 +2NO + ⇒N 2 +2CO+6CO 2 +8H 2 O  (3)
 
       C 3 H 6 +NO + +2.5O 2 ⇒NH 4   + CO+2CO 2 +H 2 O  (4)
 
     The homogeneous catalyzing carried out may be expressed as: 
       NH 4   + +NO 2 ⇒N 2 +2H 2 O  (5)
 
     In an embodiment of the invention, the selective catalytic reduction system comprises catalyst powder being coated on substrates of honeycomb or corrugated material type. In an embodiment of the invention, the substrate comprises a ceramic material. In an embodiment of the invention, the substrate is zeolite. In embodiments of the invention, the substrate comprises a metal. In embodiments of the invention, the substrate is a metal. In embodiments of the invention, the substrate comprises a ceramic material. In embodiments of the invention, the substrate is a ceramic material. 
     The catalyst powder is typically added to the substrate by using a binder. The catalyst powder may comprise a ZSM-n type with n=5, 11 and others as well as modifications thereof. E.g. the copper (Cu) of the ZSM-n type catalyst may be exchanged with either iron (Fe) or magnesium (Mg). In an embodiment of the invention, the catalyst powder is provided as a coating at least covering the substrate. 
     In embodiments of the invention, the selective catalytic reduction system comprises catalyst powder, where one or more transition metals and one or more stabilizing metals are supported on a zeolite. 
     It may be beneficial that the zeolite is an alumina silicate. 
     The zeolite may include ZSM-5, ZSM-11, ZSM-12, Mordenite or Ferrerite. 
     The transition metal may include one or more of Cu, Fe and Mg. 
     The transition metal may further include one or more of Ce and Zr as stabilizing metals. 
     Cu may be added first and Ce and/or Zr may be added after a calcination or Ce and/or Zr may be added in the same operation. 
     Cu and Ce and/or Zr may be added in the same operation followed by a calcination at a temperature below 550° C. 
     Catalyst powder may be coated on a substrate of a honeycomb shape or corrugated plate shape, by using of a binder. The substrate may be of a metal type or a ceramic type. 
     In an embodiment of the invention, cerium (Ce) or zirconium (Zr), or cerium (Ce) and zirconium (Zr) is used either in one or two sequences optionally with calcination in between the addition in order to provide a more stable (more efficient as function of time) method and reactor used to carry out the method. The binder may comprise up to 50% TiO 2 . The Cu content (in the catalyst slurry) may suitably be about 1 wt % to 4 wt %, or in the range 1.7 wt % to 3.5 wt %. 
     In an embodiment of the invention the Cu content (in the slurry) is in the range 2.8 wt %-3.1 wt %. The thickness of the coating may be in the range 10-200 μm, or 20-150 μm such as 30-100 μm, e.g. approximately 50 μm. 
     It may be an advantage that the Ce is present in the catalyst powder in the range 1-5 wt %, or in the range 2.2 wt % to 3.0 wt %, such as about 2.3 wt %. It may be beneficial that the Zr is present in the catalyst powder in the range 1-3 wt %, or about 1.8 wt %. 
     The amount of carbonized deposits may depend on the sequence of adding supporting metals. Cu may be added first, and Ce added after a calcination. Alternatively, Cu and Ce may be added in the same sequence without calcination in between, reducing the carbonized deposits significantly. Instead of using Cu, it is possible to use iron (Fe) or Mangan (Mg). 
     It may be an advantage that the selective catalytic reduction system comprises a programmable logic controller. 
     In an embodiment of the invention, the amount of injected oil is controlled either by using combined air-oil injection or an “on-off pulse” injection. In case of combined air-oil injection, the air pressure may be adjusted to change the oil amount, as function of the engine load controlled by the programmable logic controller. When the oil is injected in pulses (“on-off”), the amount of injected oil may be controlled by changing the frequency of the pulse. The frequency may be adjusted as function of the engine load controlled by the programmable logic controller. The diesel oil may be injected into a pipe guiding exhaust into the reactor. The pipe may be arranged before the reactor so that the injection takes place outside the reactor. Diesel oil may, however, also be injected into the reactor as long as the exhaust is capable of evaporating the injected diesel oil before the diesel oil enters the layers of the section(s) inside the reactor. 
     It may be an advantage that the oil is injected in droplets either by using combined air-oil injection or a pulse injection or a combination thereof. 
     The programmable logic controller may be configured to control the operation following the diesel engine load and rotational speed. The programmable logic controller may include an integrated safety system enabling the programmable logic controller to generate an alert in case that one or more parameters (e.g. pressure or temperature) no longer are within a predefined range. 
     It may be an advantage that a hardwired safety system is paralleled to the programmable logic controller to ensure the safety in case of programmable logic controller trips. 
     The selective catalytic reduction system comprises an oil injection system. The oil injection system is arranged and configured to inject oil into the reactor, hereby enabling the above-mentioned NO oxidation process to be carried out. The oil may be injected below the first catalytic layer (section), or both below the first catalytic layer (section) and between two catalytic layers (section). The oil injection system may have a suitable number of nozzles with a certain size. 
     It may be an advantage that the selective catalytic reduction system comprises a soot blower arranged to provide air (e.g. with fixed time intervals) under the first catalyst layer in every section in order to keep the catalyst entrance surface clean. 
     The selective catalytic reduction system comprises a reactor in which the above-mentioned NO oxidation process can be carried out. The reactor is configured to house the sections in order to facilitate the NO oxidation process. 
     The selective catalytic reduction system comprises a number of selective catalytic reduction catalysts provided in separated layers in a first section, wherein the selective catalytic reduction system comprises at least one additional section comprising a number of selective catalytic reduction catalysts in separated layers, wherein the at least one additional section is provided in a non-zero distance from the first section. 
     The selective catalytic reduction system may comprise one, two or more sections. 
     In embodiments of the invention, the selective catalytic reduction system comprises three sections. 
     The selective catalytic reduction system may be used in a two-stroke diesel engine or a four-stroke diesel engine. It may be advantageous that the selective catalytic reduction system is configured to be installed and used in a two-stroke diesel engine or a four-stroke diesel engine for marine purposes. 
     It may be an advantage that a non-zero distance is within the range 5-500 mm, or between 10-400 mm, such as 20-250 mm is provided between the layers. Hereby, it is possible to facilitate that the required chemical processes will be carried out. The non-zero distance may further depend on the thickness of the catalyst layer or catalyst layers as well as on the catalyst or catalysts applied. 
     It may be an advantage that the selective catalytic reduction system comprises a number of doors moveably attached to the reactor. 
     Hereby, the sections can be replaced or turned upside down by accessing the sections through the doors. 
     The doors may be either rotatably attached to the reactor, slidably attached or detachably attached to the reactor or a structure attached thereto. 
     In an embodiment of the invention the non-zero distance between the layers is within a range, in which the residence time for moving a gas molecule from one layer to the next layer is between 0.01 seconds to 10.0 seconds, or between 0.1-5.0 seconds, such as 0.2-2.5 seconds. 
     Hereby, it is possibly to gain the positive effect by applying several layers provided in a distance from each other. Experiments have shown that by choosing non-zero distances defined in the above-mentioned manner, provides a very efficient selective catalytic reduction system. 
     It may be advantageous that a plate member is provided between at least some of the adjacent sections. The plate member can improve the distribution of oil, hereby providing a more even oil distribution and thus a more effective NOx reduction process. 
     It may be an advantage that a plate member is provided between the first section and the additional section. 
     It may be an advantage that a plate member is provided between all adjacent sections. 
     Hereby, it is possible to provide an even distribution of the oil. 
     It may be beneficial that the plate member is configured to disperse the oil injected into the interior of the reactor. The plate member may be shaped to guide the evaporated oil radially outwardly to achieve an optimised flow path. 
     It may be beneficial that the selective catalytic reduction catalysts are arranged in layers each having a thickness within the range 5-200 mm, or 10-150 mm, such as 40-75 mm. It may be an advantage that the reactor comprises one or more sections each comprising several catalyst layers. 
     It may be advantageous that the selective catalytic reduction catalysts are selected among the following: Ce/Cu-ZSM-5 or Ce—Zr/Cu-ZSM-5, wherein the selective catalytic reduction catalysts are calcinated respectively after Cu adding and after Ce and Zr adding. In one embodiment, the selective catalytic reduction catalysts are dried before being calcinated. 
     It may be an advantage that the selective catalytic reduction catalysts are selected among the following: Ce/Cu-ZSM-5 or Ce—Zr/Cu-ZSM-5, wherein Cu and Ce and Zr is added at same time and the powder is calcined after the adding of Cu and Ce and Zr. 
     In one embodiment, Cu, Fe or Mg are added and dried and thereafter Ce and Zr is added and the powder is dried and hereafter calcined. 
     For both types, TiO 2  may be added to the binder, binding the catalyst powder to the substrate. The substrate consists of either corrugated steel plates or corrugated ceramic plates with 81-256 Cells Per Square Inch (CPSI). 
     In embodiments of the invention, the selective catalytic reduction system comprises an oxidation catalyst. The oxidation catalyst may be arranged after the selective catalytic reduction system in order to oxidize CO to CO 2  and HC to CO 2  and H 2 O. 
     It may be an advantage that the selective catalytic reduction system comprises a tubular structure extending centrally along the longitudinal axis of the reactor. Hereby, oil can be injected into the central portion of the reactor in order to provide an optimum distribution of the oil. 
     The tubular structure extends centrally along the longitudinal axis of the lower conical portion of the reactor. 
     It may be beneficial that the tubular structure extends through the first section and protrudes therefrom. 
     The tubular structure may be formed as a pipe, e.g. made of metal or a ceramic material. 
     It may be advantageous that the selective catalytic reduction system comprises a diffuser. Hereby, the diffuser can be used to optimize the distribution of evaporated oil entering the reactor or oil being injected into the reactor (wherein the injected oil is evaporated inside the reactor). 
     It may be advantageous that the diffuser is arranged at the inlet portion of the tubular structure. The diffuser may be adapted and arranged to diffuse injected oil towards and onto the first section. 
     It may be an advantage that the selective catalytic reduction system comprises a heat recovering unit. Hereby, it is possible to recover the heat released in the catalysts. The heat can be used for producing steam that might be used for production of drinking water or electricity to be used onboard. 
     During the process, the temperature increases over the reactor. Up to 75% of the energy content of the reactant can be recovered because of the increase in exhaust temperature after the reactor. 
     By way of example, a 2500 kW, four-stroke diesel engine, 720 rpm at 75% load with an exhaust temperature of 340° C. will have around 230 kW-heat extra for steam production, with a better quality (higher degree of superheating). The steam might be used for production of drinking water or electricity. 
     In embodiments of the invention, the system comprises a cooling unit configured to cool down at least a portion of the system. Hereby, it is possible to keep the temperature below a predefined upper temperature limit. 
     The heat content Q oil  in the oil injected can be expressed in a sum of the following components: 
     Heat loss from the reactor to the ambient: Q heat loss    
     Heat for increasing inlet temperature to the reactor: Q inlet temp increase    
     Heat for increasing the temperature over the reactor: Q temp over reactor    
     Heat loss through the oxidation catalyst: Q oxidation    
     Accordingly, this can be expressed as: 
     
       
      
       Q 
       oil 
       =Q 
       heat loss 
       +Q 
       inlet temp increase 
       +Q 
       temp over reactor 
       +Q 
       oxidation  
      
     
     To reduce the ambient Q heat loss  the cross section of the reactor may be circular. 
     Accordingly, in an embodiment of the invention, the reactor comprises a portion that has a circular cross section. It may be an advantage that the reactor comprises a portion that is cylindrical and has a circular cross section. 
     The method of embodiments of the invention is a method for nitrogen oxides (NOx) reduction, the method comprising the step of using a selective catalytic reduction system and applying diesel oil as reductant for converting nitrogen oxides (NOx) by means of a catalyst into diatomic nitrogen (N 2 ) and water (H 2 O) in the exhaust of a diesel engine, wherein the method comprises the step of providing diesel oil in a reactor, wherein the method comprises the step of carrying out a NOx conversion including a NO oxidation followed by a heterogeneous catalyzing process followed by a homogeneous catalyzing process by applying a number of selective catalytic reduction catalysts provided in a first section having a plurality of layers and a second section comprising a plurality of layers, wherein the distance between the sections is larger than the thickness of the layers. 
     Hereby, it is possible to carry out an improved selective catalytic reduction process for NOx reduction based on application of diesel oil as reductant. 
     The selective catalytic reduction process uses diesel oil as reactant. 
     It may be beneficial that the method comprises the step of cracking the injected oil, carrying out a heterogeneous NOx conversion that causes formation of the radical NH4 + , wherein the radical NH4 +  is used for carrying out homogeneous NO x  conversion. 
     It may be an advantage that the method comprises the step of providing the second section at a non-zero distance from the first section, wherein the non-zero distance is selected in such a manner that the radical NH4 +  is allowed to react in the above-mentioned chemical process: NH 4   + +NO 2 ⇒N 2 +2H 2 O, so that the activity is increased. 
     It may be advantageous that the method is applied for selective catalytic reduction of NOx in an exhaust stream either after a four-stroke diesel engine or between the exhaust receiver and the exhaust turbine(s) on a two-stroke diesel engine. 
     The method may be applied for selective catalytic reduction of NOx in diesel engines for marine purposes. 
    
    
     
       BRIEF DESCRIPTION 
       Some of the embodiments will be described in detail, with references to the following Figures, wherein like designations denote like members, wherein: 
         FIG. 1  shows a schematic view of a selective catalytic reduction system according to embodiments of the invention; 
         FIG. 2A  shows a schematic view of a selective catalytic reduction system according to embodiments of the invention integrated in a two-stroke diesel engine; 
         FIG. 2B  shows a schematic view of a selective catalytic reduction system according to embodiments of the invention integrated in a four-stroke diesel engine; 
         FIG. 3A  shows a schematic view of a reactor of a selective catalytic reduction system according to embodiments of the invention; 
         FIG. 3B  shows a schematic, cross-sectional view of a reactor of a selective catalytic reduction system according to embodiments of the invention, wherein the reactor comprises a first section and an additional section provided at a non-zero distance from the first section; 
         FIG. 4A  shows a schematic, cross-sectional view of a reactor of a selective catalytic reduction system according to embodiments of the invention, wherein the reactor comprises a first section, a second section provided in a non-zero distance from the first section and a third section provided at a non-zero distance from the second section; 
         FIG. 4B  shows a schematic, cross-sectional view of a portion of a first section and a second section provided at a non-zero distance from the first section of a reactor of a selective catalytic reduction system according to embodiments of the invention; 
         FIG. 5A  shows a top view of a manifold of a selective catalytic reduction system according to embodiments of the invention; 
         FIG. 5B  shows a cross-sectional side view of the manifold shown in  FIG. 5A ; 
         FIG. 5C  shows a side view of the manifold shown in  FIG. 5A ; 
         FIG. 5D  shows a perspective view of the manifold shown in  FIG. 5A ; 
         FIG. 6A  shows a graph showing the relative activity (two sections relative to one section) as function of the residence time between the sections; 
         FIG. 6B  shows a schematic, cross-sectional view of a reactor of a selective catalytic reduction system according to embodiments of the invention, wherein the reactor comprises a first section and an additional section provided at a non-zero distance from the first section; 
         FIG. 6C  shows a perspective, top view of a layer of a section of a reactor according to embodiments of the invention; Table 1 shows the Cu percentage in slurry, the slurry loading measured in g/L of a various layer configurations; 
         FIG. 7A  shows the catalyst activity as function of the Cu content in the layers described in Table 1; 
         FIG. 8  shows 3 somewhat different embodiments in schematic, cross-sectional view of a reactor of a catalytic reduction system according to embodiments of the invention; 
         FIG. 9  is an enlarged view of a part of the reactor shown in  FIG. 8 ; 
         FIG. 10A  shows a 3D rendering of an embodiment of a cassette with catalysts for insertion in a reactor, 
         FIG. 10B  is a cross sectional view of the cassette seen in  FIG. 10A  and 
         FIG. 10C  is a side view of the reactor in  FIG. 10A . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now in detail to the drawings for the purpose of illustrating embodiments of the present invention, a selective catalytic reduction system  2  of embodiments of the present invention is illustrated in  FIG. 1 . 
       FIG. 1  is a schematic side view of a selective catalytic reduction system  2  according to embodiments of the invention. The selective catalytic reduction system  2  comprises a reactor  10  provided with doors  18 ,  18 ′,  18 ′″ each giving access to a portion of the interior part of the reactor  10 . Hereby, it is possible to replace or turn around the sections inside the reactor  10 . 
     The reactor  10  comprises a cylindrical central portion sandwiched between a lower conical portion  20  and an upper conical portion  20 ′. An end pipe  22  provided with a flange  58  in its distal end is provided in the distal end of the lower conical portion  20 . Likewise, an end pipe  22 ′ provided with a flange  58 ′ in its distal end is provided in the distal end of the upper conical portion  20 ′. The doors  18 ,  18 ′,  18 ″ are moveably attached (e.g. rotatably attached or attachably attached) to the central portion of the reactor  10 . 
     It is important to underline that the reactor  10  can be oriented both horizontally and vertically. It may also be possible to arrange the reactor  10  in an inclined orientation so that the reactor is angled relative to both horizontal and vertical. 
     A pipe  26 ′ provided with a flange  58  in its proximal end is attached to the end pipe  22 ′, by fixing the flanges  58 ,  58 ′ to each other. Similarly, a bent pipe  26  provided with a flange  58  in its proximal end is attached to the end pipe  22 , by fixing the flanges  58 ,  58 ′ to each other. It is important to underline that the pipe configuration can be different. In one embodiment according to embodiments of the invention, the pipe  26  may be straight. 
     An injection unit  24  extends through the wall of the pipe  26  and is configured to inject oil  40  into the pipe  26 . The injection unit  24  is configured to inject oil  40  towards the end pipe  22  of the reactor  10 . Accordingly, the injected oil  40  will enter the reactor  10 , in which the diesel oil is applied as reductant in the selective catalytic reduction system  2 . 
     When oil (in liquid form) is injected into the exhaust in the pipe  26 , the diesel oil droplets will evaporate. Accordingly, in an embodiment according to embodiments of the invention, the system does not require a separate oil evaporation unit. 
     The selective catalytic reduction system  2  comprises a differential pressure sensor  32  connected to a first portion of the reactor  10  by means of a first conduit  34  and to a second portion of the reactor  10  by means of a second conduit  34 ′. Hereby, the differential pressure sensor  32  is capable of measuring the differential pressure across that part of the reactor  10  that extends between the first portion and the second portion. For practical reasons it may be an advantage that the differential pressure sensor  32  is connected to a lower portion of the reactor  10  by means of the first conduit  34  and to an upper portion of the reactor  10  by means of a second conduit  34 ′. 
     Accordingly, the differential pressure sensor  32  can measure the differential pressure across the central portion of the reactor  10 , in which the catalysts are arranged. Therefore, the differential pressure sensor  32  is configured to detect when the differential pressure exceeds a predefined pressure level (e.g. 1-1000 mbar, such as 4-500 mbar, or 8-100 mbar, e.g. 10-20 mbar). The selective catalytic reduction system  2  comprises an alert unit configured to generate an alert when the differential pressure exceeds the predefined pressure level. 
     In embodiments of the invention, two separated pressure sensors are applied to measure the pressure at two different positions of the reactor  10 . By comparing the two detected pressures, it is possible to calculate the pressure difference between the two measurement points. Accordingly, the differential pressure sensor  32  may be replaced with two pressure sensors. 
     The selective catalytic reduction system  2  comprises a first temperature sensor  28  arranged and configured to detect the temperature in the lower (inlet) portion of the reactor  10 . The selective catalytic reduction system  2  comprises a second temperature sensor  30  arranged and configured to detect the temperature in the upper (outlet) portion of the reactor  10 . By comparing the temperatures detected by means of the first temperature sensor  28  and the second temperature sensor  30 , it is possible to measure the temperature increase across the reactor  10 . 
     The selective catalytic reduction system  2  comprises a pump  48  connected to a diesel tank  50  via a pipe  54 ″. The pump  48  is a pump configured to generate a sufficiently high pressure. The pump  48  is in fluid communication with the injection unit  24 . The pump  48  is connected to a flow sensor  46  through a pipe  54 ′. A control valve  44  is connected to the flow sensor  46  via a pipe  54  and the control valve  44  is connected to the injection unit  24  via a pipe  36 . A pipe  55  extends between the pipe  54 ′ and the tank diesel tank  50 . Accordingly, diesel oil can be returned from the pipe  54 ′ to the tank diesel tank  50 . In one embodiment according to embodiments of the invention, the selective catalytic reduction system  2  comprises two pumps  48  (a first pump and a second pump) connected in parallel. Hereby, it is possible to apply the second pump in case that the first pump is malfunctioning, needs to be serviced or be replaced, or vice versa. 
     A tray (for collecting oil) may be arranged under the injection unit  24 . Likewise, a tray may be arranged under the pump for collection of leaking oil. The system  2  may comprise a sensor arranged and configured to generate an alert in case of a sufficiently large leakage. 
     The selective catalytic reduction system  2  comprises a programmable logic controller  60 . The differential pressure sensor  32 , the temperature sensors  28 ,  30 , pump  48 , the flow sensor  46  and the control valve  44  are connected to the programmable logic controller  60  by means of cables  52 ,  52 ′,  52 ″. However, it is possible to replace this wired connection with a wireless connection (by applying corresponding transmitters and receivers). The programmable logic controller  60  receives the measurements (sensor inputs) detected by the differential pressure sensor  32 , the temperature sensors  28 ,  30 , the pump  48 , the flow sensor  46  and the control valve  44 . The programmable logic controller  60  is configured to control the pump  48  and the control valve  44  on the basis of the sensor inputs of the differential pressure sensor  32 , the temperature sensors  28 ,  30  and the flow sensor  46 . The programmable logic controller  60  may be configured to generate an alert when the difference between the detected temperatures exceeds a predefined temperature level or when the difference between the detected temperatures is lower than a predefined temperature level. 
       FIG. 2A  illustrates a schematic view of a selective catalytic reduction system  2  according to embodiments of the invention integrated in a two-stroke diesel engine  4 . The two-stroke diesel engine  4  comprises a cylinder  66  with a piston  70 . A compressor  80  is arranged to deliver compressed air to a scavenge air receiver  76  that is in fluid communication with the cylinder  66 . The exhaust gas leaves the cylinder  66  via a pipe that is connected to an exhaust gas receiver  78 . The exhaust gas receiver  78  is connected to an exhaust turbine (expander)  82 . The exhaust gas receiver  78  is connected to both the inlet and the outlet of the exhaust turbine  82 . A control valve  68 ′″ is arranged to control the flow through the pipe connecting the gas receiver  78  to the inlet of the exhaust turbine  82 . A control valve  68  is arranged to regulate the flow from a pipe leaving the gas receiver  78 . 
     The selective catalytic reduction system  2  comprises an oil pump  48 , an oil injector  6  and a reactor  10 . The pump  48  is arranged to deliver pressurised oil to the oil injector  6 . The oil injector  6  is arranged and configured to inject pressurised oil into the reactor  10 . A blower  72  is arranged to blow (compressed) air into the reactor  10 . In an embodiment according to embodiments of the invention, the blower  72  is configured to blow air under the first catalyst layer in every section of the catalyst in a predefined manner, with fixed time intervals to keep the catalyst entrance surface clean. 
     The reactor  10  is arranged between the outlet of the exhaust gas receiver  78  and the exhaust turbine  82 . An oil injection system  6  is arranged between the exhaust gas receiver  78  and the reactor  10 . 
     The selective catalytic reduction system  2  comprises a control valve  68 ′ arranged between the exhaust gas receiver  78  and the oil injection system  6 . 
     In embodiments of the invention, the blower  72  may be an integrated part of the selective catalytic reduction system  2 . 
     In embodiments of the invention, the blower  72  may be a separate unit not being an integrated part of the selective catalytic reduction system  2 . 
     A control valve  68 ″ is arranged after the outlet of the reactor  10 . The control valve  68 ′ is configured to regulate the flow from the reactor  10 . 
     The scavenge air receiver  76  is connected to the exhaust turbine  82 . A control valve  74  is provided between the scavenge air receiver  76  and the exhaust turbine  82 . The control valve  74  may be of any suitable type and size. 
     The selective catalytic reduction system  2  comprises a control unit  62  configured to control a number of units of the selective catalytic reduction system  2 . In one embodiment according to embodiments of the invention, the selective catalytic reduction system  2  comprises a control unit  62  configured to control the pump  48 , and at least a selection of the control valves  68 ,  68 ′,  68 ″,  68 ′″ and the brake resistor  74 . In an embodiment according to embodiments of the invention, the selective catalytic reduction system  2  comprises a control unit  62  configured to control the pump  48 , the control valves  68 ,  68 ′,  68 ″,  68 ′″ and the brake resistor  74  and the oil injection system  6 . 
     The selective catalytic reduction system  2  may comprise a number of sensors (e.g. as shown in  FIG. 1 ) and an alert unit configured to generate an alert when one or more parameters detected by one or more of the sensors exceed a predefined level or is smaller than a predefined level. 
     As shown in  FIG. 2A , the selective catalytic reduction system  2  is configured for selective catalytic reduction of NOx between the exhaust receiver  78  and the exhaust turbine  82  on a two-stroke diesel engine  2  (e.g. for marine purpose). 
       FIG. 2B  illustrates a schematic view of a selective catalytic reduction system  2  according to embodiments of the invention integrated in a four-stroke diesel engine  4 . The four-stroke diesel engine  4  is symbolised by a cylinder  66  with a piston  70 . A compressor  80  is arranged to deliver compressed air to an air cooler  30  being in fluid communication with the cylinder  66 . The exhaust gas leaves the cylinder  66  via a pipe that is connected to an exhaust gas receiver  78 . The exhaust gas receiver  78  is connected to an exhaust turbine  82 . 
     The reactor  10  is arranged after the exhaust turbine  82 . The selective catalytic reduction system  2  comprises an oil injector  6  and a reactor  10 . A diesel tank  50  is in fluid communication with the oil injector  6 . Accordingly, the tank  50  is configured to deliver oil to the oil injector  6 . The exhaust turbine  82  is connected to the oil injector  6 , which is arranged and configured to inject pressurised oil into the reactor  10 . A blower  72  is arranged to blow (compressed) air into the reactor  10 . In an embodiment of the invention, the blower  72  is configured to blow air under the first catalyst layer in every section of the catalyst in a predefined manner, with fixed time intervals to keep the catalyst entrance surface clean. 
     In embodiments of the invention, the blower  72  may be an integrated part of the selective catalytic reduction system  2 . 
     In embodiments of the invention, the blower  72  may be a separate unit not being an integrated part of the selective catalytic reduction system  2 . 
     The selective catalytic reduction system  2  comprises a control unit  62  configured to control a number of units of the selective catalytic reduction system  2 . In embodiments of the invention, the selective catalytic reduction system  2  comprises a control unit  62  configured to control the oil injection system  6  as well as one or more structures of the selective catalytic reduction system  2 . 
     The selective catalytic reduction system  2  may comprise one or more control valves (not shown) and the control unit  62  may be configured to control one or more of these control valves. In embodiments of the invention, the control unit  62  is configured to receive one or more signals from one or more sensors and to regulate one or more devices on the basis of the received signal(s). The control unit  62  may be configured to control one or more valves and/or the oil injection system  6  on the basis of temperature and/or differential pressure detections. 
     The selective catalytic reduction system  2  may comprise a number of sensors (e.g. as shown in  FIG. 1 ) and an alert unit configured to generate an alert when one or more parameters detected by one or more of the sensors exceed a predefined level or is smaller than a predefined level. 
       FIG. 3A  illustrates a schematic view of a reactor  10  of a selective catalytic reduction system according to embodiments of the invention. The reactor  10  comprises a cylindrical central portion provided with a first door  18  arranged in the lower end (with respect to the flow direction indicated by an arrow) of the central portion and a second door  18  arranged in the upper end (with respect to the flow direction indicated by an arrow) of the central portion. The doors  18 ,  18 ′ are configured to be opened or removed in order to access the interior of the reactor  10 . This enables an easy replacement of the layers (see  FIG. 3B ) arranged inside the reactor  10 . 
     The central portion is sandwiched between a lower conical portion  20  and an upper conical portion  20 ′. An end pipe  22  provided with a flange  58 ′ in its distal end is provided in the distal end of the lower conical portion  20 . Likewise, an end pipe  22 ′ provided with a flange  58 ′ in its distal end is provided in the distal end of the upper conical portion  20 ′. 
     A bent pipe  26  provided with a flange  58  in its proximal end is attached to the end pipe  22 , by attachment of the adjacent flanges  58 ,  58 ′ to each other. The pipe  26  may have another configuration. The pipe  26  may by way of example be straight. 
     A pressure tank  31  configured to deliver pressurised air is connected to the reactor  10  by means of a first conduit and a second conduit. A valve  33  is arranged in the first conduit between the pressure tank  31  and the reactor  10 , whereas another valve  33  is arranged in the second conduit between the pressure tank  31  and the reactor  10 . 
     The reactor  10  is provided with a first support leg  90 , a second support leg  90 ′ and a third support leg (not shown). The reactor  10  may be applied in a selective catalytic reduction system that comprises a control unit configured to control the one or more oil injectors and/or one or more control valves (not shown). The control unit may be configured to receive information from one or more sensors including the differential pressure sensor  32 . 
       FIG. 3B  illustrates a schematic, cross-sectional view of a reactor  10  of a selective catalytic reduction system according to embodiments of the invention. The reactor  10  basically corresponds to the one shown in  FIG. 3A . The reactor  10  comprises a cylindrical central portion that is sandwiched between a lower conical portion  20  and an upper conical portion  20 ′. An end pipe  22  provided with a flange  58 ′ in its distal end is provided in the distal end of the lower conical portion  20 . Likewise, an end pipe  22 ′ provided with a flange  58 ′ in its distal end is provided in the distal end of the upper conical portion  20 ′. 
     Inside the interior of the cylindrical central portion a first section  14  and an additional section  16  are provided in a non-zero distance D 3  from the first section  14 . A plate member is provided between the first section  14  and the additional section  16 . The plate member  84  is configured to disperse the oil injected into the interior of the reactor  10 . 
     Each section  14 ,  16  comprises several layers S 1 , S 2 , S 3  and S 1 ′, S 2 ′, S 3 ′, respectively. The distance between adjacent layers S 1 , S 2 , S 3 , S 1 ′, S 2 ′, S 3 ′ and the distance D 3  between the first section  14  and the additional section  16  provides time for the generated free radical NH 4   +  to have an impact through its reaction during the previously mentioned homogeneous catalyzing step: 
       NH 4   + +NO 2 ⇒N 2 +2H 2 O  (5)
 
     Accordingly, by providing a non-zero distance between adjacent layers S 1 , S 2 , S 3 , S 1 ′, S 2 ′, S 3 ′ and between the first section  14  and the additional section  16 , it is possible to increase the effect and efficiently of the reactor  10 . The distance between adjacent layers S 1 , S 2 , S 3 , S 1 ′, S 2 ′, S 3 ′ and the distance D 3  between the first section  14  and the additional section  16  is selected in such a manner that maximum effect of the free radical NH 4   +  can be achieved. The distance D 3  is between 5-1000 mm, or in the range 50-500 mm, such as 100-400 mm. 
     A diffuser  88  is provided in the lower conical portion  20 . The diffuser is configured to mix and diffuse the injected oil  40  towards and onto the first layer S 1  of the first section  14 . 
     The exhaust to be treated enters the reactor  10  through the inlet provided in the first end  92 , passes the first section  14 , the second section  16  and leaves the reactor  10  through the outlet provided in the second end  94 . The exhaust carries the injected oil  40  in gaseous form. The gaseous oil  40  is initially guided outwardly by means of the diffuser  88 . A closing structure  134 ,  134 ′,  134 ″ extends radially in extension of each layer S 1 , S 2 , S 3 . Accordingly, the closing structures  134 ,  134 ′,  134 ″ force the evaporated diesel oil  40  to pass (axially) through the passages provided in the layer S 1 , S 2 , S 3 . 
     The first section  14  comprises three layers S 1 , S 2 , S 3  arranged above each other separated by a gap. Likewise, the second section  16  comprises three layers S 1 ′, S 2 ′, S 3 ′ arranged above each other, wherein a distance is provided between adjacent layers S 1 ′, S 2 ′, S 3 ′. 
     The layers S 1 , S 2 , S 3  of the first section  14  and the layers S 1 ′, S 2 ′, S 3 ′ of the second section  16  may form a Ce/Cu-ZSM-5 type catalyst or a Ce—Zr/Cu-ZSM-5 type catalyst. 
     When a Ce/Cu-ZSM-5 type catalyst is used, the catalyst may be calcinated respectively after Cu adding and after Ce and Zr adding. 
     When a Ce—Zr/Cu-ZSM-5 type catalyst is used, Cu and Ce and Zr may be added at same time and powder may be calcined after the adding of Cu and Ce and Zr. 
     For both types, TiO 2  may be added to the binder, binding the catalyst powder to the substrate. The substrate may consist of either corrugated steel plates or corrugated ceramic plates with a CPSI (Cells Per Square Inch) in the range 81-256. 
     The layers S 1 , S 2 , S 3  of the first section  14  and the layers S 1 ′, S 2 ′, S 3 ′ of the second section  16  may have essentially the same thickness. The thickness of each layer may be 5-500 mm, or 10-250 mm such as 40-150 mm, e.g. 75 mm. 
     In an embodiment of the invention, the layer S 1  has a thickness of 5-500 mm, or 10-250 mm such as 40-150 mm, e.g. 75 mm. In an embodiment of the invention, the layer S 2  has a thickness of 5-500 mm, preferably 10-250 mm such as 40-150 mm, e.g. 75 mm. In an embodiment of the invention, the layer S 3  has a thickness of 5-500 mm, or 10-250 mm such as 40-150 mm, e.g. 75 mm. 
     In an embodiment of the invention, the layers S 1 ′, S 2 ′ and S 3 ′ have a thickness of 5-500 mm, or 10-250 mm such as 40-150 mm, e.g. 75 mm. 
     In an embodiment of the invention, the layers S 1 , S 2 , S 3  are of equal type and dimension. 
     In embodiments of the invention, the layers S 1 , S 2 , S 3  are of different equal type and/or thickness. 
     A plate member  84  is arranged between the first section  14  and the second section  16 . A tubular structure  86  formed as a centrally arranged tube extends along the longitudinal axis of the lower conical portion  20  of the reactor  10 . Evaporated oil  40 ′ is introduced into the space between the two sections  14 ,  16 . The distal end of the tubular structure  86  is provided in a distance h to the plate member  84 . The plate member  84  is configured and arranged to guide the evaporated diesel oil  40 ′ radially towards the periphery of the reactor  10 , from which the evaporated diesel oil  40 ′ is guided towards the first layer S 1 ′ of the second section  16 . 
     The selective catalytic reduction system according to embodiments of the invention may comprise an additional oxidation catalyst reactor (not shown) configured to oxidize CO to CO 2  and HC to CO 2  and H 2 O. The additional oxidation catalyst reactor may be arranged after the reactor  10 . 
     Under the first catalyst layer S 1  (arranged closest to the inlet  92  of the reactor  10 ) in the first section  14  and the first catalyst layer S 1  in the second section  16 , air may be blown with fixed time intervals to keep the catalyst entrance surface clean. This may be done by means of a blower (as illustrated in  FIG. 2A  and  FIG. 2B ). 
     The catalysts of the sections  14 ,  16  ensure three integrated catalyzing processes including:
         Cracking of oil;   Heterogeneous NOx conversion and formation of radicals used for a following reaction;   Homogeneous NOx conversion downstream the catalyst.       

     The temperature increases across the first section  14 . This temperature increase ΔT 1  is indicated in  FIG. 3B . Likewise, the temperature increases across the second section  16 . This temperature increase ΔT 2  is also indicated in  FIG. 3B . 
     In an embodiment of the invention, the selective catalytic reduction system comprises a waste heat recovering system arranged after the reactor  10 . Hereby, the waste heat recovering system can recover the heat released in the catalysts of the reactor  10 . 
     The recovered heat may be used for producing steam for production of drinking water or electricity (that may be used onboard if the selective catalytic reduction system is applied for selective catalytic reduction of NOx in an exhaust stream in a marine diesel engine). 
       FIG. 4A  illustrates a schematic, cross-sectional view of a reactor  10  of a selective catalytic reduction system according to embodiments of the invention. The selective catalytic reduction system comprises a reactor  10  provided with a first section  14 , a second section  16  provided in a non-zero distance D 1  from the first section  14  and a third section  16 ′ provided in a non-zero distance D 2  from the second section  16 . The reactor  10  basically corresponds to the one shown in  FIG. 3A  and  FIG. 3B . The reactor  10  comprises a cylindrical central portion that is sandwiched between a lower conical portion  20  and an upper conical portion  20 ′. An end pipe  22  provided with a flange  58 ′ in its distal end, is provided in the distal end of the lower conical portion  20 . Likewise, an end pipe  22 ′ provided with a flange  58 ′ in its distal end is provided in the distal end of the upper conical portion  20 ′. 
     Inside the interior of the cylindrical central portion, however, a first section  14  and an additional section  16  are provided in a non-zero distance D 1  from the first section  14 . A second plate member  84 ′ is provided in a non-zero distance D 2  between the additional (second) section  16  and the third section  16 ′. The plate members  84 ,  84 ′ are configured to disperse the oil  40 ′ injected into the interior of the reactor  10 . 
     The distance D 1  between the first section  14  and the additional section  16  as well as the distance between adjacent layers S 1 , S 2 , S 3 , S 1 ′, S 2 ′, S 3 ′ provides time for the generated free radical NH 4   +  to have an impact through its reaction with nitrogen dioxide (NO 2 ) forming diatomic nitrogen gas (N 2 ) and water (H 2 O). Likewise, the distance D 2  between the additional section  16  and the third section  16 ′ as well as the distance between adjacent layers S 1 ″, S 2 ″, S 3 ″ provides time for the generated free radical NH 4   +  to have an impact through the above-mentioned reaction with nitrogen dioxide (NO 2 ). 
     By providing a non-zero distance D 1 , D 2  between the adjacent sections  14 ,  16 ,  16 ′ and between adjacent layers S 1 , S 2 , S 3 , S 1 ′, S 2 ′, S 3 ′, S 1 ″, S 2 ″, S 3 ″, it is possible to improve the effect and efficiently of the reactor  10 . The distances may be selected in such a manner that maximum effect of the free radical NH 4   +  can be achieved. The distances D 1 , D 2  may be between 5-1000 mm, or in the range 50-500 mm, such as 100-400 mm. In an embodiment of the invention, the distances D 1 , D 2  are equal. The distance between adjacent layers S 1 , S 2 , S 3 , S 1 ′, S 2 ′, S 3 ′, S 1 ″, S 2 ″, S 3 ″ may be between 5-1000 mm, or in the range 25-500 mm, such as 50-400 mm. 
     A tubular structure  86  extends centrally along the longitudinal axis of the lower conical portion  20  of the reactor  10 . The tubular structure  86  extends through the first section  14  and protrudes therefrom. A diffuser  88  is arranged at the inlet portion of the tubular structure  86 . The diffuser  88  is adapted and arranged to mix and diffuse injected oil towards and onto the first layer C 1  of the first section  14 . 
     Exhaust from the engine in which the selective catalytic reduction system is installed, enters the reactor  10  through the inlet provided in the first end  92 , passes the first section  14 , the second section  16 , the third section  16 ′ and leaves the reactor  10  through the outlet provided in the second end  94 . 
     The first section  14  comprises three layers S 1 , S 2 , S 3  arranged above each other, wherein adjacent layers are axially spaced from each other. The second section  16  comprises several layers S 1 ′, S 2 ′, S 3 ′ arranged above each other and are spaced from each other. The third section  16 ′ comprises several layers S 1 ″, S 2 ″, S 3 ″ spaced from each other. 
     The sections  14 ,  16 ,  16 ′ may be of the same type as the sections explained with reference to  FIG. 3B . The sections  14 ,  16 ,  16 ′ may have the same geometry (including thickness) as the sections explained with reference to  FIG. 3B . 
     The temperature increases across all sections  14 ,  16 ,  16 ′. The temperature increase ΔT 3 , ΔT 4 , ΔT 5  across each of the segments  14 ,  16 ,  16 ′, is indicated in  FIG. 4A . 
     In an embodiment of the invention, the selective catalytic reduction system comprises a waste heat recovering system (not shown) arranged after the reactor  10 . Hereby, the waste heat recovering system can recover the heat released in the catalysts of the reactor  10 . Accordingly, the recovered heat may be used for producing steam for production of drinking water or electricity (that may be used onboard if the selective catalytic reduction system is applied for selective catalytic reduction of NOx in an exhaust stream in a marine diesel engine). 
       FIG. 4B  illustrates a schematic, close-up view of a portion of a first layer S 1  and a second layer S 2  provided in a non-zero distance from the first layer S 1 . The adjacent layers S 1 , S 2  may be layers of the same section of adjacent sections of a reactor of a selective catalytic reduction system according to embodiments of the invention. Each layer S 1 , S 2  comprises a structure provided with a plurality of passages  112 ,  112 ′ extending through the layer S 1 , S 2 . Each layer S 1 , S 2  comprises a substrate  108 ,  108 ′ at least partly, or completely, covered by a catalyst powder  109 ,  109 ′ attached to the substrate as a coating. 
     The cracking process represents the first process step inside the reactor. The diesel oil used as reactant is injected into the exhaust as small droplets upstream (in the direction indicated by the arrow) the catalyst layer of the first layer S 1 . The injection of oil is carried out in such a manner that minimum 80% of the oil droplets are evaporated before reaching the first catalyst layer S 1 . The selective catalytic reduction system is configured to evenly disperse the oil into the exhaust. 
     The evaporated oil is cracked on the surface of the catalyst layer S 1 . Accordingly, cracked oil  110  will be present on the distal portion of the substrate  108 ,  108 ′. A portion of the oil may flow un-cracked towards the passages  112 ,  112 ′ of the first catalyst layer S 1  and the second catalyst layer S 2 . Due to the increased temperature (as explained with reference to  FIG. 4A ), only a minimum of the injected oil is passing through the system. 
     When the selective catalytic reduction system is used in marine engines, the diesel oil contains relative long-chained hydrocarbon (HC) connections. These connections are cracked to short-chained HC connections during the catalyst process. Accordingly, the short-chained HC connections will be short enough to enter the catalyst layer and convert NO to N 2  and H 2 O and further produce radicals (NH4+). The chemical reactions (of the NOx conversion) are show in the following. 
     The NO oxidation carried out may be expressed as: 
       NO+1/2O 2 ⇒NO 2   (1)
 
       NO+NO 2 +2H + ⇒2NO++H 2 O  (2)
 
     The heterogeneous catalyzing carried out may be expressed as: 
       2C 4 H 8 +10O 2 +2NO + ⇒N 2 +2CO+6CO 2 +8H 2 O  (3)
 
       C 3 H 6 +NO + +2.5O 2 ⇒NH 4   + +CO+2CO 2 +H 2 O  (4)
 
     The homogeneous catalyzing carried out may be expressed as: 
       NH 4   + +NO 2 ⇒N 2 +2H 2 O  (5)
 
       FIG. 5A  illustrates a top view of a manifold  100  of a selective catalytic reduction system according to embodiments of the invention.  FIG. 5B  illustrates a cross-sectional side view of the manifold  100  shown in  FIG. 5A .  FIG. 5C  illustrates a side view of the manifold  100  shown in  FIG. 5A  and  FIG. 5D  a perspective view of the manifold  100  shown in  FIG. 5A . 
     The manifold  100  comprises a cylindrical tubular body portion that is provided with a first flange  102  in the inlet end of the manifold  100  and a second flange  104  in the opposite outlet end of the manifold  100 . The flanges  102 ,  104  are configured to be attached to an adjacent pipe (not shown) provided with a matching flange. 
     The manifold  100  comprises four connection pipes  96 ,  96 ′,  96 ″,  96 ′″ evenly distributed along the circumference of the manifold  100 . The connection pipes  96 ,  96 ′,  96 ″,  96 ′″ are angled relative to the longitudinal axis of the cylindrical tubular body portion. 
     Each connection pipes  96 ,  96 ′,  96 ″,  96 ′″ is provided with a flange  98  in its distal end. A nozzle  106  extends through the interior of each connection pipes  96 ,  96 ′,  96 ″,  96 ′″. The nozzles  106  are arranged and configured to inject oil into the upper, central portion of the manifold  100 . 
       FIG. 6A  illustrates two graphs each showing the relative activity  118  (two sections relative to one section) as function of the residence time  116  (measured in seconds) between the sections for the homogeneous NOx reduction process as indicated in the above-mentioned equation (5). 
       FIG. 6A  illustrates two examples of how the NOx reduction activity  118  varies as function of distance between two layers (shown as the time  116  for a gas particle to move from one section to the other section). When one layer is placed in a very small distance from the adjacent layer, the increase in activity  118  is marginal, whereas when the distance is increased, the activity is increased. The activity  118 , however, depends on the catalyst specification (the layers making up the section). 
     The uppermost graph comprises points  122  representing the activity when two layers of different types are applied. The activity is measured relative to a reference activity in a reactor comprising only a single layer. 
     The lowermost graph comprises points  120  representing the activity when the catalyst layer thickness is low. The activity is measured relative to the activity in a reactor comprising only a single section. 
     It can be seen that the relative activity  118  is very low compared to the situation wherein the catalyst layer thickness is larger. Both graphs increase as function of residence time  116 . Accordingly, it is possible to achieve an increased efficiency of the selective catalytic reduction system according to embodiments of the invention by introducing two sections being provided in a non-zero distance from each other. 
     The lowermost graph, however, illustrates that the effect of several layers is minimal when the catalyst layer is thin. Accordingly, the catalyst layer exceeds a predefined minimum layer thickness. The predefined minimum catalyst layer thickness may depend on the relative content of Cu, Fe or Mg. 
       FIG. 6B  illustrates a schematic, cross-sectional view of a reactor  10  of a selective catalytic reduction system according to embodiments of the invention, wherein the reactor  10  comprises a first section  14  and an additional section  16  provided at a non-zero distance D 3  from the first section  14 . The reactor  10  basically corresponds to the one shown in  FIG. 3B . The reactor  10 , however, does not comprise a tubular structure. 
       FIG. 6C  illustrates a perspective, top view of a layer S 1  of a section of a reactor according to embodiments of the invention. The layer S 1  is provided with a plurality of longitudinal extending passages  112 . The passages  112  have rectangular, square, cross sections, or of equal size. Each passage  112  is, at least partly, covered with a catalyst coating. In an embodiment (not show), corrugated plates are applied. In this embodiment, the passages are not rectangular. 
     The height H of the layer S 1  may be in the range 20-300, such as 40-150 mm, e.g. 60-120 mm. 
     Table 1 illustrates the Cu percentage in slurry, the slurry loading measured in g/L of a first layer configuration called 1A-1B, a second layer configuration called 1Bb and a third layer configuration called 1B-1C. In the first layer configuration, 1A-1B, the layers are arranged on the top of each other in the two sections. In the second layer configuration, 1Bb, there is no space between the layers in two sections, whereas the third layer configuration, 1B-1C, a space between adjacent layers. 
       FIG. 7A  illustrates the catalyst activity  126  (NOx reduction measured in ppm) as function of the Cu content in the layers described in Table 1.  FIG. 7A  depicts the NOx reduction as function of the number of layers  124 . A layer, 1B, arranged under predefined reference conditions is indicated with open squares. The first layer configuration, 1A-1B is indicated with solid circles, the second layer configuration 1Bb is indicated with open circles, whereas the third layer configuration, 1B-1C is indicated with open triangles. 
       FIG. 7A  illustrates that 1, the layers in the first layer configurations 1A-2B having a 1.7% Cu content, can be placed on top of each other. This, however, provides a relative low activity  126 . Increased Cu loading increases the activity  16 , however, also the production of radical&#39;s, explained with reference to the previously mentioned equation (4) and (5), which has to enter a homogeneous reduction process after the catalyst not inside the catalyst. 
     In  FIG. 7A , the layers in the second layer configuration, 1Bb are placed on top of each other in two adjacent sections.  FIG. 7A  shows, that with the relative high Cu content (2.9%) there is no outcome of adding an extra layer on top of the first layer in each section. 
     Type 1B and 1C are placed in one section with space in between. An increase in activity due to the homogeneous catalysing process after the catalyst can be observed. 
       FIG. 8  discloses  3  different reactors  10 , where the reactor far left comprises five sections  14 ,  16 ,  16 ′,  16 ″ and  16 ′″ separated by distances D 1 , D 2 , D 4  and D 4 . The reactor depicted centrally in  FIG. 8  has 4 sections, where the reactor at the far-right side of the figure has 3 sections. In each section in the reactors shown in  FIG. 8  there are 5 spaced apart layers S 1 , S 2 , S 3 , S 4 , S 5 . (See  FIG. 9 ). In other respects, the reactors shown in  FIG. 8  are similar to the reactors depicted in  FIGS. 3B and 4A . 
     In  FIG. 9  an enlarged view of a section  16  with accompanying distance D 1  is provided. Layers S 1 -S 5  are disclosed and the distances d 1 -d 4  between the layers are also indicated. As seen the distance D 1  is significantly larger than each one of the thicknesses L 1 -L 5  of the layers S 1 -S 5 . This corresponds to the distance relations previously mentioned. The distances d 1 -d 4  indicated between the layers, may be in the order of the thickness L 1 -L 5  of the individual layers. 
     In the embodiment shown in  FIG. 9  an arrow w indicates a movement which the diffuser  88  may perform in order to get closer to or further away from the first section  14  in a reactor  10 . By adjusting this distance correctly, the time allowed between injection of oil  40  and the contact between oil and the first layer of the first section may be either reduced or diminished. 
       FIGS. 10A, 10B  an  10 C shows an embodiment wherein the layers E 1  and E 2  are shaped differently than in the previously mentioned embodiments. Each of the layers E 1  and E 2  are basically made from the same material as previously mentioned layers S 1 -S 5 ; S 1 ′-S 5 ′; S 1 ″-S 5 ″ and are also structured with through going canals running straight through the thickness from one side to an opposed side. However, as seen in  FIG. 10A , each of the layers E 1 , E 2  are composed of square blocks, which are assembled in a 2×2 flat upper layer E 1 , and a 2×2 lower layer E 2 . Upper layer E 1  and lower layer E 2  are assembled in a nearly cubic shaped cassette  140 .  FIG. 10B  shows a sectional view of a cubic shaped cassette along line A-A in  FIG. 10C . 
     An example of measures for a cassette adapted for a specific catalytic reduction system is provided in  FIG. 10C  and  FIG. 10B . As can be seed in  FIG. 10B  a distance of 150 is donated between the layers E 1  and E 2 , whereas each of the layers has a thickness of 75. The nearly cubic cassette has a height of  302  and a measure  307  along each side. The provided measures are given in mm; but could well be given in inches or scaled with another scaling size according to the use of the cubic cassette. The cassette  140  is particularly well suited for systems where regular exchange of the cassettes are foreseen, and where shipment of cassettes from stock to place of use will frequently be undertaken. 
     Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
     For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. The mention of a “unit” or a “module” does not preclude the use of more than one unit or module. 
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               2  Selective catalytic reduction system 
               4  Diesel engine 
               6  Oil injection system 
               10  Reactor 
               11 ,  11 ′,  11 ″ Selective catalytic reduction catalyst 
               12 ,  12 ′,  12 ″ Selective catalytic reduction catalyst 
               14  First section 
               16 ,  16 ′ Additional section 
               18 ,  18 ′,  18 ″ Door 
               20 ,  20 ′ Conical portion 
               22 ,  22 ′ End pipe 
               24  Injection unit 
               26 ,  26 ′ Pipe 
               28  Temperature sensor 
               30  Temperature sensor 
               31  Pressure tank 
               32  Differential pressure sensor 
               33 ,  33 ′ Valve 
               34 ,  34 ′ Conduit 
               36  Pipe 
               38  Cable 
               40  Oil 
               42  Cable 
               44  Control valve 
               46  Flow sensor 
               48  Pump 
               50  Diesel tank 
               52 ,  52 ′,  52 ″ Cable 
               54 ,  54 ′,  54 ″ Pipe 
               55  Pipe 
               56  Cable 
               58 ,  58 ′ Flange 
               60  Programmable Logic Controller (PLC) 
               62  Control unit 
               64  Air cooler 
               66  Cylinder 
               68 ,  68 ′,  68 ″,  68 ′″ Valve 
               70  Piston 
               72  Blower 
               74  Control valve 
               76  Scavenge air receiver 
               78  Exhaust gas receiver 
               80  Compressor 
               82  Exhaust turbine (expander) 
               84 ,  84 ′ Plate member 
               86  Tubular structure 
               88  Diffuser 
               90 ,  90 ′ Support leg 
               92  First end 
               94  Second end 
               96 ,  96 ′,  96 ″,  96 ′″ Connecting pipe 
               98  Flange 
               100  Manifold 
               102  Flange 
               104  Flange 
               106  Nozzle 
               108 ,  108  Substrate 
               109 ,  109 ′ Catalyst 
               110  Cracked diesel oil 
               112 ,  112 ′ Passage 
               114  Graph 
               116  Residence time between the catalyst layers 
               118  Relative activity (two layers relative to one layer) 
               120  Point 
               122  Point 
               124  Number of layers 
               126  Activity (NOx reduction [ppm]) 
               134 ,  134 ′,  134 ″ Closing structure 
               140  Cubic cassette 
             D 1 , D 2 , D 3  Distance 
             d 1 , d 2 , d 3 , d 4  Distance 
             ΔT 1 , ΔT 2 , ΔT 3  Temperature increase 
             ΔT 4 , ΔT 5  Temperature increase 
             S 1 , S 2 , S 3 , S 4 , S 5  Layer of selective catalytic reduction catalysts 
             S 1 ′, S 2 ′, S 3 ′, S 4 ′, S 5 ′ Layer of selective catalytic reduction catalysts 
             S 1 ″, S 2 ″, S 3 ″ Layer of selective catalytic reduction catalysts 
             S 4 ″, S 5 ″, E 1 , E 2  Layer of selective catalytic reduction catalysts 
             L 1 , L 2 , L 3 , L 4 , L 5  Thickness 
             h Distance 
             H Height 
             W arrow