Abstract:
An emissions control system utilizes otherwise wasted heat to efficiently reduce emissions in a main exhaust flow. Heat stored in exhaust from an auxiliary generator (i.e., auxiliary exhaust) may be used to convert urea to ammonia used by a Selective Catalytic Reducer (SCR) system, and/or the auxiliary exhaust may be used to heat the main exhaust flow before entry into an SCR. Additionally, a heat exchanger may be used to transfer heat from a hot clean flow out of the SCR, to the main exhaust flow entering the SCR. Previously, mobile emissions control systems have not used SCR systems to reduce NOx because of the cost and space required for heater fuel. The efficient use of otherwise wasted heat reduces fuel cost and fuel storage requirements, and thereby makes an SCR systems feasible for use in mobile emissions control systems.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to improving the efficiency of emissions control equipment, and more particularly to a use of auxiliary generator exhaust to provide heat required for emissions control processes.  
         [0002]     A variety of activities produce exhaust having harmful levels of emissions (or pollution.) Large stationary emissions sources may have co-located emissions control systems. However, some emissions sources are mobile, and require similarly mobile emissions control systems. An example of a significant mobile emissions source is an ocean going vessel. A single container ship may produce as much emissions as 12,500 automobiles. U.S. patent application Ser. No. 10/835,197 for “Maritime Emissions Control System,” assigned to the assignee of the present invention, describes a mobile emissions control system which may be transported to a ship within a harbor, and which mobile emissions control system captures and processes a main exhaust flow from the ship to reduce emissions. The main exhaust flow may be from the ship&#39;s engine(s), auxiliary generators, or any other source of exhaust from the ship. The emissions control system of the &#39;197 patent includes a shroud which is lowered over the ship&#39;s stack, and a rather long duct for carrying the main exhaust flow from the ship&#39;s stack to emissions processing equipment carried by a barge alongside the ship. As a result of the distance the main exhaust flow must travel before reaching the emissions processing equipment, the temperature of the main exhaust flow is much lower that it&#39;s temperature upon being exhausted from an engine or engines. The &#39;197 applications is herein incorporated by reference.  
         [0003]     The emissions control system processing equipment described in the &#39;197 patent includes a first system for reduction of nitrogen oxides (NOx) as its primary purpose. The first system comprises four stages. The first stage comprises a Pre Conditioning Chamber (PCC) quench vessel. The second stage comprises oxidation column. The third stage comprises reduction column. The fourth stage comprises a caustic (or polishing) column.  
         [0004]     Although a preferred system for reducing NOx emissions is a Selective Catalytic Reducer (SCR) system, the first system does not include an SCR system because using known SCR systems would require the addition of substantial heat. The main exhaust flow would require heating to a high temperature before introduction into the SCR system. Also, ammonia used by SCR systems is preferably generated by heating urea. The cost and space required for an energy source for heating, made such known SCR systems impractical for a mobile emissions control system.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The present invention addresses the above and other needs by providing an emissions control system utilizing otherwise wasted heat to efficiently reduce emissions in a main exhaust flow. Heat stored in exhaust from an auxiliary generator (i.e., auxiliary exhaust) may be used to convert urea solution ammonia used by a Selective Catalytic Reducer (SCR) system, and/or the auxiliary exhaust may be used to heat the main exhaust flow before entry into an SCR. Additionally, a heat exchanger may be used to transfer heat from a hot clean flow out of the SCR, to the main exhaust flow entering the SCR. Previously, mobile emissions control systems have not used SCR systems to reduce NOx because of the cost and space required for heater fuel. The efficient use of otherwise wasted heat reduces fuel cost and fuel storage requirements, and thereby makes an SCR systems feasible for use in mobile emissions control systems.  
         [0006]     In accordance with one aspect of the invention, there is provided an emissions control system including an auxiliary generator and a reaction chamber for converting urea to ammonia. The exhaust from the auxiliary generator is ducted into the reaction chamber to provide heat for conversion of the urea to ammonia. A Selective Catalytic Reducer (SCR) system is used to processes a combination of a main exhaust flow from the stack of a ship and the ammonia. The emissions control system may further include a heat exchanger loop between the output and the input of the SCR for pre-heating the main exhaust flow. The heat exchanger loop preferably uses a liquid for conducting heat between the heat exchangers.  
         [0007]     In accordance with another aspect of the present invention, there is provided a method for reducing the energy required to operate an SCR system. The method includes capturing hot exhaust gases of an auxiliary engine and mixing the hot exhaust gases with an atomized urea solution to convert the urea to ammonia. A main exhaust flow from the stack of a ship is collected for processing by the SCR system. The ammonia is mixed with main exhaust flow, and the resulting mixture is processed in an SCR to reduce NOx emissions. The method may further include capturing heat from the cleaned flow out of the SCR in a heat exchanger hot side, and releasing heat into the mixed flow at a heat exchanger cold side. The use of the hot exhaust gases from the auxiliary engine to convert the urea to ammonia, and the use of the heat exchanger to heat the mixture of ammonia and main exhaust flow, substantially reduces the cost of using the SCR system to reduce NOx emissions. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0008]     The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:  
         [0009]      FIG. 1  is a maritime emissions control systems suitable for application of the efficient emissions control system of present invention.  
         [0010]      FIG. 2  depicts a prior art Selective Catalytic Reducer (SCR) system.  
         [0011]      FIG. 3  shows an SCR system utilizing wasted heat according to the present invention.  
         [0012]      FIG. 4  is a method for processing an exhaust flow according to the present invention. 
     
    
       [0013]     Corresponding reference characters indicate corresponding components throughout the several views of the drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing one or more preferred embodiments of the invention. The scope of the invention should be determined with reference to the claims.  
         [0015]     The present invention provides a system and method for improving emissions control for engine exhaust, chemical process plants, or other pollution sources. The emissions control system and method according to the present invention provides more efficient operation of an emissions control system. The present invention is particularly useful where energy sources used to generate heat or power required for operation of the emission control system are not available or are inadequate, or where providing such sources of energy is not cost effective.  
         [0016]     A maritime emissions control system for reducing emissions from a ship  10  is shown in  FIG. 1 . A shroud  14  is positioned over the ship stack  12  to collect a main exhaust flow. The main exhaust flow may be from the ship&#39;s engine(s), auxiliary generators, or any other source of exhaust from the ship. The main exhaust flow passes through a duct  16  to an emissions processing equipment  17  residing on a barge or smaller ship  18 . Such maritime emissions control system is described in U.S. patent application Ser. No. 10/835,197 for “Maritime Emissions Control System,” incorporated by reference above. Because of the long path from the sources, for example a ship engines or power generating equipment, the main exhaust flow is substantially cooled by the time the main exhaust flow reaches the emissions processing equipment  17 .  
         [0017]     A preferred method of reducing the emissions in exhaust includes a Selective Catalytic Reduction (SCR) system. SCR systems generally use ammonia derived from urea to supplement the catalytic reaction—in essence, giving nitrogen oxides the “extra” molecules needed to convert to harmless nitrogen and water. Such systems have proven very useful and effective.  
         [0018]     A prior art Selective Catalytic Reducer (SCR) system  19  for reducing NOx emissions is shown in  FIG. 2 . The SCR system  19  comprises a urea source  20  for providing a flow of a urea solution  22  atomized and pumped by a compressor  34  into a reaction chamber  28 . A heat source  24  generates a heat flow  30  using energy  26 , which heat flow  30  is also provided to the reaction chamber  28  to heat the flow of urea solution  22  to generate ammonia. A resulting gaseous ammonia flow  32  passes into a mixing chamber  38  to mix with a main exhaust flow  36 . A mixed flow  40  is urged forward by a fan  42  to create an urged flow  44  to a heater  46 . The heater  46  heats the urged flow  44  to create a heated flow  48  into an SCR  50 . The heater  46  is preferably a duct burner, and preferably uses the same fuel as a generator within the SCR system  19 , and more preferably uses the same diesel fuel as a diesel generator within the SCR system  19 . The heater  46  also received the energy  26 . A chemical reaction in the SCR  50  between the ammonia and the NOx converts the NOx to nitrogen gas and water in a clean flow  52 .  
         [0019]     The prior art SCR system  19  thus requires the following energy inputs in order to perform its function: energy  26  to produce the heat flow  30  to convert urea to ammonia, power to atomize and inject the urea solution into the reaction chamber  28 , power for the fan  42  to urge the mixed flow  40  through heater  46  and through the SCR  50 , energy  26  for the heater  46  to raise the temperature of the ammonia and main exhaust flow  36  mixture to the operating temperature of the SCR  50 .  
         [0020]     In summary, the prior art SCR system  19  requires energy to generate heat at different steps of the process and the prior art SCR system  19  needs electrical energy for pumps and the like. This is very typical of many industrial and power generation processes to which the present invention applies. Because of these energy requirements, the prior art SCR system  19  shown in  FIG. 2  requires too much energy to be practical with a mobile emissions control system such as the maritime emissions control system of  FIG. 1 . In general, ammonia is derived from urea because urea is a much safer chemical than aqueous or anhydrous ammonia, and urea is easier to handle than ammonia.  
         [0021]     An improved SCR system  52  according to the present invention is shown in  FIG. 3 . An auxiliary engine  54  provides auxiliary exhaust  56  as a heat source for the reaction chamber  28 . The auxiliary engine  54  is preferably a diesel generator, a gas turbine generator, or a gasoline engine driven generator, and is more preferably a diesel generator. The auxiliary exhaust  56  both provides heat to convert the urea to ammonia, and remains mixed with the ammonia flow  32  to add heat to the mixed flow  40 . The urea is preferable in solution, and is more preferably an approximately 35 percent to approximately 40 percent urea aqueous solution. The urea flow  22  is pumped into the reaction chamber  28  by the compressor  23 , and preferably, the urea solution is atomized by compressed air from the air compressor  34  and sprayed into a flow of the hot auxiliary exhaust  56 . The auxiliary exhaust  56  is typically at a temperature of approximately 1000 degrees Fahrenheit and a temperature of approximately 650 degrees Fahrenheit is required to convert urea to ammonia. The ammonia mixed with the auxiliary exhaust  56  passes into mixing chamber  38 , where the ammonia mixes with the main exhaust flow  36 .  
         [0022]     The mixed flow  40   b  is urged forward by the fan  42  and through the heater  46  into the SCR  50 . The heater  46  is preferably a duct burner, and preferably uses the same fuel as a generator within the SCR system  52 , and more preferably uses the same diesel fuel as a diesel generator within the SCR system  52 . The heater  46  may alternatively receive power from a generator within the SCR system  52 . A chemical reaction in the SCR  50  between the ammonia and the NOx converts the NOx to nitrogen gas and water in the clean flow  52 . The auxiliary exhaust  56  may further be used to preheat the SCR  50  and the heat exchangers  58 ,  60  before the main exhaust gas  36  is introduced into the improved SCR system  53 .  
         [0023]     Continuing with  FIG. 3 , the improved SCR system  52  may further include a heat exchanger for transferring heat normally exhausted from the SCR  50 , and using that heat to pre heat the mixed flow into the SCR  50 , thereby reducing the heating (and therefor energy) required by the heater  46 . A first mixed flow  40   a  passes from the mixing chamber  38  to a heat exchanger cold side  58 , and a second mixed flow  40   b  continues from the heat exchanger cold side  58  to the fan  42 . The cleaned flow  52  enters a heat exchanger hot side  60  where a fluid is heated, and the cleaned flow  52  exits the heat exchanger hot side  60  as a second cleaned flow  62 . The fluid flows through a rearward heat transfer tube  64  to the heat exchanger cold side  58 , where heat is transferred from the fluid to the mixed flow  40   a . The fluid then flows through a forward heat transfer tube  66  back to the heat exchanger hot side  60 . A second pump  34   b  may reside in either the rearward heat transfer tube  64  or the forward heat transfer tube  66  to pump the fluid. The hot clean flow  52  is thus used to reduce the energy required by the heater  46  to heat the heated flow  48  to approximately 600 degrees Fahrenheit at entry to the SCR  50 . Rather than use the typical industrial process gas-to-gas heat exchanger, the present invention preferably uses a gas-to-liquid heat exchanger, thereby increasing the heat transfer efficiency. Preferably, a heat transfer oil or fluid is used.  
         [0024]     Selective Catalytic Reducer systems are well know and available from: Argillon LLC, Alpharetta, Ga., Babcock Power Environmental, Worchester, Mass., CRI, Inc., Houston, Tex., Englehard Corp, Iselin, N.J., Haldor-Topsoe, Houston, Tex., Mitsubishi Power Systems, Newport Beach, Calif., and Johnson Matthey, San Diego, Calif.  
         [0025]     A method for processing an exhaust flow according to the present invention is described in  FIG. 4 . The method includes the steps of capturing hot exhaust gases of an auxiliary engine at step  80 , mixing the hot exhaust gases with urea at step  82 , converting the urea to ammonia at step  84 , Collecting a main exhaust flow from the stack of a ship at step  86 , Mixing the ammonia with main exhaust flow at step  88 , and Processing the mixture in a Selective Catalytic Reducer (SCR) system at step  90 . The method may further include capturing heat from the cleaned flow  52  out of the SCR  50  in a heat exchanger hot side, and releasing heat into a mixed flow  40   a  at a heat exchanger cold side  58 .  
         [0026]     The new and unique arrangements and processes of the present invention result in a significant reduction in the amount of energy that must be supplied for emission control system operation, and are capable of reducing the diesel fuel usage by a factor of two or three, depending on the temperature rise required for the incoming exhaust gas stream. In the particular example of  FIG. 3 , the incoming gas temperature may be as low as 300 degrees Fahrenheit, while the SCR requires a minimum of 600 degrees Fahrenheit for operation. With an exhaust gas flow rate of 10,000 standard cubic feet per minute (SCFM), this invention reduces the energy required for heating the exhaust and the urea by more than a factor of three.  
         [0027]     While a system including an SCR  50  was described above, the present invention is suitable for use with any mobile system having an auxiliary engine, and requiring heating of any flow within the system, or of an element of the system. The present invention is particularly suitable to any mobile emissions control system having an auxiliary engine and requiring a heating capability, and more particularly useful for any maritime emissions control system which must be mobile and self contained.  
         [0028]     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.