Patent Publication Number: US-2007119153-A1

Title: Superheated urea injection for aftertreatment applications

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates in general to internal combustion engine technology and to reducing the level of NO x  emission by selected catalytic reduction (SCR). More specifically, the present invention relates to the use of urea injection upstream from the SCR site, in the vehicle exhaust stream, as part of an urea SCR exhaust aftertreatment system and approach for a diesel engine.  
      Reducing NO x  (NO and NO 2 ) emission in diesel exhaust gas has been a major challenge during the past several years and will continue to be a major focus in the future due to the continuing stringent emission requirements for diesel engines. Engine exhaust NO x  reduction can be achieved in part by more optimal or more complete combustion and in part by exhaust gas aftertreatment. While these two approaches remain compatible, combustion optimization has NO x  reduction limits and alone will likely not be able to meet the EPA proposed levels for the future, such as the U.S. 2007 heavy-duty diesel emission standard. Accordingly, some degree of exhaust gas aftertreatment will be required in order to achieve the requisite reductions.  
      Considering some of the available aftertreatment technologies, urea SCR exhaust aftertreatment may be one of the more cost-effective approaches while still providing effective NO x  reduction. Selective Catalytic Reduction (SCR) technology injects a reduction agent into the exhaust upstream of a catalyst. On the catalyst, the NO x , is reduced to N 2  (nitrogen) and H 2 O (water). One of the first reduction agents used was anhydrous ammonia (NH 3 ).  
      In terms of urea SCR reduction, the basic chemical reactions are well known. When mixing with the exhaust gas beyond a certain temperature, the urea solution atomizes and dissolves as ammonia (NH 3 ) and carbon dioxide (CO 2 ). Then, the NH 3  reacts with NO and NO 2  in the exhaust gas to reduce to N 2  (nitrogen) and H 2 O (water).  
      While atomization of the urea solution is important, the approach described in terms of the prior art requires an exhaust temperature sufficient for the urea solution to atomize and dissolve as ammonia and carbon dioxide. Some time must elapse following injection for this process to occur, and thus the physical site of urea injection must be far enough upstream from the catalyst for the urea solution to be heated and reach the necessary temperature for atomization and for dissolving into ammonia and carbon dioxide. It is also recognized that the finer or small the urea droplets due to atomization, the more effective and efficient the NO x  reduction as a result of this urea SCR technology.  
      In U.S. Patent No. 6,922,987, issued Aug. 2, 2005 to Mital et al., it is stated that NO x  adsorber catalysts have the potential for great NO x  emission reduction (70-90%) and for extending engine life. However, low temperature operation of adsorbers seems to be a problem. The main reason for this problem seems to be the fact that the reductants, especially D2 fuel, starts to boil at around 180 degrees Celsius. At temperatures below this, the injected fuel has a strong tendency to condense and does not participate in the release and reduction step of NO x  adsorbers (catalytic converters). Also, if the droplet size is large, atomization is not good and the fuel does not vaporize easily even at the higher temperature, thereby limiting adsorber performance.  
      Current devices on their own, such as spraying system nozzles and injectors cannot do anything about the condensation at low temperature. Also, the droplet size measurement shows that they have a sauter mean diameter (SMD) in the range of 30-60 μm. If this droplet size can be reduced further, the vaporization will become faster and easier. This will improve the reductant participation in the catalyst reactions and improve the NO x  adsorber capacity and NO x  conversion efficiency.  
      The approach selected to address these concerns, as described in the &#39;987 patent, is to use superheated fuel injection. The system includes a fuel supply upstream of a pressurized fuel injector and a heater for heating the fuel in the pressurized fuel supply. While this approach offers one option for NO x  reduction, it does not provide a urea-based system. This approach also requires a fuel supply, fuel injector, heater, and controller to monitor the temperature. Prior art urea SCR systems, as noted, rely on the exhaust to heat the urea solution to achieve atomization and dissolving of the urea solution into ammonia and carbon dioxide.  
      In order to eliminate any concerns regarding whether the exhaust temperature will be sufficient and the length of time in the exhaust stream adequate to atomize and dissolve the urea solution, the present invention proposes a novel and unobvious approach. According to the present invention, the urea solution is preheated and pressurized to superheated conditions prior to injection into the exhaust, thereby avoiding any dependency or reliance on heat from the exhaust in order to vaporize the urea solution. Another benefit from this approach is that the atomized urea solution is in the form of a very fine mist (steam), providing improved distribution as compared to the results using larger liquid droplets of urea.  
      Yet another benefit from this approach is that the vaporized urea solution, now as a very fine mist or steam, can be injected closer to the catalyst and conceivably at lower temperatures. This also helps to reduce or minimize the package length. Yet another benefit is that the nozzle hole can be larger and it will thereby not be as prone to plugging as when it functions as an essential part of the atomization process.  
     BRIEF SUMMARY OF THE INVENTION  
      An urea SCR system for enhancing internal combustion engine aftertreatment applications according to one embodiment of the present invention comprises a supply of urea solution, a delivery pump for delivering urea solution into an exhaust stream at an injection location, and a heating device positioned between the supply of urea solution and the injection location for preheating the urea solution prior to injection into the exhaust stream.  
      One object of the present invention is to provide an improved urea SCR system.  
      Related objects and advantages of the present invention will be apparent from the following description.  
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       FIG. 1  is a schematic illustration of an urea SCR system according to one embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.  
      Referring to  FIG. 1 , there is illustrated an urea SCR system for enhancing internal combustion engine aftertreatment applications. Urea SCR system  20  includes a supply  21  of aqueous urea, a pressurizing delivery pump  22 , a heating location or station  23 , and a valve-controlled injection nozzle  24 . Also illustrated in  FIG. 1  is an exhaust conduit  25  of a diesel engine (not illustrated) and a selective catalytic reduction (SCR) site  26 . Briefly, the operation or functioning of system  20  begins with an amount of aqueous urea being drawn from supply  21  and delivered downstream to or through heating station  23  by means of delivery pump  22 . This delivery or pumping action increases the pressure of the aqueous urea to approximately 50 psi as it enters the heating station  23 . Heating station  23  defines a flow passage around a glow plug that functions to heat (preheat) the aqueous urea to an elevated temperature between approximately 130° C. and 220° C., preferably 200° C.  
      More specifically, the aqueous urea (i.e., urea solution) needs to be preheated to a temperature above the boiling point of the solution at the pressure that exists in the exhaust system or exhaust stream at the point of injection into the exhaust. While the preferred embodiment selects a glow plug as the heating device, the urea solution can also be heated prior to injection by heat from the exhaust, by the engine coolant, or by other electrical resistance devices.  
      When the heated and pressurized urea solution is injected into the lower pressure of the exhaust stream, the urea solution instantly flashes to steam and the fine mist of urea that is desired is thereby achieved. While the heated urea solution remains in the connecting flow conduit upstream of the injection nozzle  24 , the elevated pressure keeps the urea solution in a liquid state. As noted, the liquid state changes to steam when the urea solution hits the lower exhaust pressure.  
      A urea solution temperature of approximately 200° C. is considered to be a superheated condition. When the superheated urea solution flashes to steam in the exhaust stream, it uniformly mixes with the exhaust gas, and the urea solution dissolves as ammonia (NH 3 ) and carbon dioxide (CO 2 ) according to the following: 
 
(NH 3 ) 2 +CO+H 2 O→CO S +2NH 3 . 
 
 Then, NH 3  reacts with NO and NO 2  in the exhaust gas according to the following: 
 
4NH 3 +4NO+O 2 →4N 2 +6H 2 O. 
 
8NH 3 +6NO 2 →7N 2 +12H 2 O. 
 
 These end products or by-products of the reaction are acceptable in terms of emissions, and thus one challenge is to try and estimate, as precisely as possible, the amounts of NO and NO 2  in the exhaust gas. 
 
      The disclosed system  20  provides various benefits to those designing diesel engine systems in order to address emissions reduction. First, the availability and cost of urea, as compared to more expensive alternatives, provides a cost benefit to the customer. Secondly, by not requiring the exhaust stream to provide the heating of the urea after injection, the time in the exhaust stream prior to the SCR site can be shortened. This shortening translates into a shorter distance between the injection location and the SCR site and a reduced package length. Thirdly, since system  20  does not rely on heat from the exhaust stream to vaporize the urea solution, the urea solution could conceivably be injected at a comparatively lower temperature. A related benefit of creating the vaporized urea solution by superheating is that the delivery nozzle opening can be comparatively larger since the nozzle does not need to participate in the atomization or vaporizing process. By making the injection nozzle hole larger, it will not be as prone to clogging or plugging.  
      The component parts and system subassemblies include the supply  21  of aqueous urea, delivery pump  22 , heating station  23 , and valve-controlled injection nozzle  24 . The supply  21  includes a storage tank  21   a  that is flow coupled to the delivery pump  22  by conduit  30 .  
      The proportions or strength of the urea-water solution are not considered to be critical, but the preferred composition is approximately 32.5% urea and 67.5% water. The volume of tank  21   a  varies depending on the application and will typically range from 10 liters to approximately 100 liters or perhaps a little higher. The tank  21   a  includes a heating element (not illustrated) to ensure that the urea solution stays in a liquid state when injection is required. A filter (not illustrated) is included as part of the construction of tank  21   a  to ensure that any contaminants entering tank  21   a  are not allowed to enter the downstream remainder of system  20 .  
      The lines carrying the urea solution to the delivery pump  22  are constructed out of a material that is compatible with the urea solution such as rubber, Teflon®, and stainless steel. It is recommended that these conduits or lines include a method to warm them when the ambient temperature falls below the freezing point for the urea-water solution.  
      The delivery pump  22  draws aqueous urea out of storage tank  21   a  and pushes it forward with an elevated pressure of approximately 50 psi. The pump needs to increase the pressure to a level sufficient to maintain the urea-water solution in a liquid state when the temperature is raised above the boiling point at normal atmospheric pressure.  
      Conduit  31  connects the delivery pump outlet  32  with control valve  33  prior to injection nozzle  24 . The heating station includes a glow plug  34  that is positioned in a section of conduit  31  in order to heat the flow of aqueous urea. The glow plug  34  elevates the aqueous urea temperature to approximately 200° C. and this elevated temperature, combined with the elevated pressure, creates a very fine mist (steam) as the urea solution is injected by nozzle  24  into the exhaust stream.  
      The reference herein to the SCR site describes the location of the catalyst which is in the exhaust stream, downstream from where the reduction agent (urea solution steam) is injected. Contact with the catalyst by the reduction agent and the exhaust, i.e., “on the catalyst”, causes the NO x  to be reduced to N 2  (nitrogen) and H 2 O (water). In terms of available reducing agents, urea is readily available and it is a non-hazardous product. While an aqueous solution of urea has previously been used as a reducing agent, the technique of preheating or superheating the solution is believed to be novel and this approach, as presented by the present invention, very clearly provides benefits to the customer, as described herein.  
      While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.