Patent Publication Number: US-2007110642-A1

Title: Exhaust gas purifying apparatus

Description:
This application is a continuation of PCT/JP2004/012747, filed on Sep. 2, 2004. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to an apparatus for reducing and purifying nitrogen oxides (NOx) in an exhaust gas exhausted from diverse kinds of power generators such as, for example, a vehicle engine and so forth, and more particularly to a technology for enhancing efficiency of hydrolysis of a urea aqueous solution used as a reducing agent in the exhaust gas purifying apparatus in order for reducing and purifying NOx component in an exhaust gas exhausted from an engine.  
      2. Description of the Related Art  
      Japanese Unexamined Patent Publication No. 2000-27627 (JP-A-2000-027627) discloses a conventional exhaust gas purifying apparatus for reducing and purifying NOx included in the exhaust gas exhausted from an engine. In this exhaust gas purifying apparatus, a NOx reducing catalytic converter is disposed in an exhaust passage of the engine so as to convert NOx into harmless nitrogen (N 2 ), oxygen (O 2 ), and the like in an oxygen-excessive atmosphere. Moreover, in order to enhance NOx purifying efficiency in the NOx reducing catalytic converter, a configuration for injecting and adding a urea aqueous solution into an exhaust pipe positioned on the exhaust upstream side of the catalytic converter is employed. The urea aqueous solution injected and added into the exhaust pipe is hydrolyzed to ammonia by exhaust heat and water vapor in the exhaust gas and is supplied in a flow of the exhaust gas toward the NOx reducing catalytic converter. Then, in the NOx reducing catalytic converter, the ammonia is used as a reducing agent to reduce and purify the NOx in the exhaust gas.  
      In such an exhaust gas purifying apparatus, a part of the urea aqueous solution injected and added into the exhaust pipe adheres to an inner wall of the exhaust pipe and forms liquid droplets on the surface of the inner wall. Because an outer wall of the exhaust pipe is in contact with the atmospheric air and is at a low temperature as compared with the exhaust gas, the liquid droplets formed on the inner wall by the urea aqueous solution are not sufficiently heated and become difficult to hydrolyze to the ammonia. For this reason, there is a possibility that the ammonia to be supplied to the NOx reducing catalytic converter becomes insufficient so that the NOx purifying efficiency must be reduced. A person skilled in the art might conceive to increase an amount of the urea aqueous solution so as to be additionally injected into the NOx for the purpose of solving such a problem. In this case, however, there might be possibilities that a consumption of the urea aqueous solution increases to reduce cost efficiency and that the urea aqueous solution that has not been hydrolyzed is deposited inside the exhaust pipe or emitted as it is into the atmosphere.  
     SUMMARY OF THE INVENTION  
      Therefore, in view of the above problems or defects which might be encountered by the conventional technology, it is an object of the present invention to provide a novel technology applicable to an exhaust gas purifying apparatus, in which efficiency of hydrolysis where ammonia is produced from a urea aqueous solution is enhanced by suppressing decrease of the temperature of the urea aqueous solution that has formed liquid droplets.  
      To achieve the object, in accordance with the present invention, there is provided an exhaust gas purifying apparatus, which Includes: an exhaust pipe capable of permitting an exhaust gas to flow therein from an upstream side to a downstream side, a nitrogen oxide (NOx) reducing catalytic converter disposed in the exhaust pipe to reduce and purify nitrogen oxides in the exhaust gas; an internal tubular member disposed on an exhaust upstream side of the NOx reducing catalytic converter while leaving a clearance between the internal tubular member and an inner wall of the exhaust pipe and having open opposite ends spaced apart form each other in a direction along a flow of the exhaust gas; a storage tank for storing therein a urea aqueous solution; and an injection nozzle for additively injecting the urea aqueous solution supplied from the storage tank into the exhaust gas in a direction along the flow of the exhaust gas within the internal tubular member.  
      Preferably, a heating device is further provided for heating the urea aqueous solution to be additively injected within the internal tubular member. Here, as a heat source of the heating device, an electric heater, a coolant for cooling an engine, or the exhaust gas per se that has passed through the nitrogen oxide reducing catalytic converter may be used for heating the urea aqueous solution stored in the storage tank or the urea aqueous solution in piping that provides a fluid connection between the storage tank and the injection nozzle. Further, in order to efficiently heat the urea aqueous solution to minimize energy consumption, it is preferable to provide a heat insulator for suppressing radiation of heat from the urea aqueous solution heated by the heating device into the atmospheric air.  
      A further preferably, there may be provided an operating state detecting device configured to detect an operating state of the engine and an addition controlling device configured to control an addition flow rate of the urea aqueous solution to be additively injected from the injection nozzle based on the operating state detected by the operating state detecting device.  
      With the exhaust gas purifying apparatus according to the invention, the internal tubular member is heated from outside and inside by the exhaust gas flowing through the exhaust pipe, and temperature of the internal tubular member rises to substantially the same temperature as that of the exhaust gas in an extremely short time. Therefore, temperature of the urea aqueous solution additively injected into the internal tubular member does not decrease even if the solution adheres to an inner wall of the internal tubular member to form liquid droplets, to thereby enhance efficiency of hydrolysis of the urea aqueous solution. Since the efficiency of the hydrolysis of the urea aqueous solution is enhanced, the ammonia to be supplied to the nitrogen oxide reducing catalytic converter does not become insufficient and enhancement of the purifying efficiency of the nitrogen oxides (NOx) can be achieved.  
      Moreover, if the additive flow rate of the urea aqueous solution to be additively injected from the injection nozzle is controlled based on the operating state of the engine, a just enough required amount of the urea aqueous solution according to the operating state of the engine may be additively injected. Therefore, it is possible to minimize a consumption of the urea aqueous solution while maintaining a necessary level of purifying efficiency of the nitrogen oxides. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a first embodiment of an exhaust gas purifying apparatus according to the present invention.  
       FIG. 2  is a detail view of a vicinity of an injection nozzle.  
       FIG. 3  is a block diagram of a second embodiment of the exhaust gas purifying apparatus according to the invention.  
       FIG. 4  is a block diagram of a third embodiment of the exhaust gas purifying apparatus according to the invention.  
       FIG. 5  is a block diagram of a fourth embodiment of the exhaust gas purifying apparatus according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention will be described hereinbelow in more detail with reference to the accompanying drawings.  
       FIG. 1  shows a first embodiment of an exhaust gas purifying apparatus according to the invention.  
      In an exhaust pipe  2  forming an exhaust passage of an engine  1 , a NOx reducing catalytic converter  3  for reducing and purifying NOx is disposed. The NOx reducing catalytic converter  3  is formed by supporting a zeolite active component, for example, on a monolith-type catalyst carrier made of ceramic cordierite or Fe—Cr—Al heat resisting steel and having a honeycomb-shaped cross section. The active component supported on the catalyst carrier is activated when ammonia as a reducing agent is supplied, and effectively converts NOx to harmless substances.  
      The exhaust gas purifying apparatus is provided with a storage tank  4  for storing a urea aqueous solution. The storage tank  4  is connected to a urea aqueous solution adding device  6  through piping  5 . On the other hand, the urea aqueous solution adding device  6  is connected to a pump  8  through piping  7 . The pump  8  pressurizes air and supplies the pressurized air to the urea aqueous solution adding device  6 . The urea aqueous solution adding device  6  is connected to an injection nozzle  10  provided in the exhaust pipe  2  positioned on an exhaust upstream side of the NOx reducing catalytic converter  3  through piping  9 , and mixes the urea aqueous solution into the pressurized air from the pump  8  to supply the mixture to the injection nozzle  10 .  
      Around the piping  9  connecting the urea aqueous solution adding device  6  and the injection nozzle  10 , an electric heater  11  as a heating device is provided and a heat insulator  12  is provided around the electric heater  11 .  
      Furthermore, a urea aqueous solution addition control unit  13  including a computer and functioning as an addition controlling device is provided. The urea aqueous solution addition control unit  13  can receive a signal of an engine operating state outputted from an engine control unit  14  for controlling the engine  1  through a controller area network (CAN), and controls actuation of the urea aqueous solution adding device  6 , the pump  8 , and the electric heater  11 , respectively. It is also possible to obtain the engine operating state from various sensors for detecting a rotating speed, an intake air flow rate, a load, and the like of the engine. Here, the engine control unit  14  or the various sensors correspond(s) to an operating state detecting device.  
      Here, with reference to  FIG. 2 , a specific configuration near the injection nozzle  10  will be described.  
      In the exhaust pipe  2  on the exhaust upstream side of the NOx reducing catalytic converter  3 , a cylindrical internal tubular member  20  with open opposite ends is provided along the flow of the exhaust gas. The internal tubular member  20  is supported substantially concentrically with the exhaust pipe  2  by two annular-plate-shaped support plates  21  provided in the exhaust pipe  2  while leaving a clearance between the internal tubular member  20  and an inner wall of the exhaust pipe  2 . Each of the support plates  21  has a large number of holes. Therefore, the exhaust gas flowing through the exhaust pipe  2  is able to flow on inner and outer sides of the internal tubular member  20 . The injection nozzle  10  connected to an extreme end of the piping  9  can inject and add the urea aqueous solution along the flow of the exhaust gas in the internal tubular member  20 .  
      Next, operation of the exhaust gas purifying apparatus will be described.  
      As the engine  1  is brought into operation, the exhaust gas of the engine  1  is exhausted into the exhaust pipe  2 . At this time, the urea aqueous solution addition control unit  13  receives the operating states such as the rotating speed, the intake air flow rate, the load, and the like of the engine  1  from the engine control unit  14 . Then, the urea aqueous solution addition control unit  13  calculates an addition flow rate of the urea aqueous solution required to reduce and purify the NOx in the exhaust gas based on the received operating states, and controls actuation of the urea aqueous solution adding device  6 , the pump  8 , and the electric heater  11 , respectively. As a result, the urea aqueous solution is mixed with air and passes through the piping  9  to thereby be heated by the electric heater  11  and injected and added from the injection nozzle  10  into the internal tubular member  20 .  
      The urea aqueous solution injected and added from the injection nozzle  10  is hydrolyzed to ammonia by the exhaust heat and the water vapor in the exhaust gas, and is supplied to the NOx reducing catalytic converter  3  together with the exhaust gas. Then, in the NOx reducing catalytic converter  3 , the NOx in the exhaust gas are reduced and purified by using the ammonia as the reducing agent.  
      At this time, because the urea aqueous solution is heated by the electric heater  11  before it is injected and added, the solution is injected and added into the exhaust pipe  2  after temperature thereof has been increased. Therefore, the decrease in temperature of the exhaust gas due to injection and addition of a low-temperature urea aqueous solution is suppressed. As a result, catalytic reaction in the NOx reducing catalytic converter  3  is maintained active, and the NOx are reduced and purified efficiently. Moreover, because only the additively injected urea aqueous solution, i.e., a minimum necessary amount of the urea aqueous solution is heated, an amount of energy necessary for heating the urea aqueous solution can be suppressed.  
      Further, because the exhaust gas flows also on the outer side of the internal tubular member  20 , temperature of the internal tubular member  20  rises to substantially the same temperature as that of the exhaust gas in an extremely appreciably short time. Accordingly, the urea aqueous solution additively injected from the injection nozzle  10  receives heat from the internal tubular member  20  and is heated even if it adheres to the inner wall of the internal tubular member  20 . Therefore, the temperature of the urea aqueous solution does not decrease even if the solution adheres to the inner wall of the internal tubular member  20  and forms liquid droplets. Thus, the hydrolysis is carried out efficiently, and consequently, the ammonia to be supplied to the NOx reducing catalytic converter  3  does not become insufficient and the NOx in the exhaust gas are reduced and purified efficiently.  
       FIG. 3  shows a second embodiment of the exhaust gas purifying apparatus according to the invention. Because a basic configuration of the exhaust gas purifying apparatus of the present embodiment is the same as that in the previous first embodiment, only differences will be described (hereafter the same shall apply).  
      In the present embodiment, in order to heat the urea aqueous solution stored in the storage tank  4 , a configuration is employed in which the electric heater  11  and the heat insulator  12  for heating and keeping the heat in the urea aqueous solution are provided around the storage tank  4 . In this way, in addition to the operation and effects of the first embodiment, efficiency of the hydrolysis of the urea aqueous solution can be stabilized because the urea aqueous solution at a substantially constant temperature is added into the exhaust gas even if the additive flow rate of the solution per se varies.  
       FIG. 4  shows a third embodiment of the exhaust gas purifying apparatus according to the invention.  
      In the embodiment, in order to heat the urea aqueous solution stored in the storage tank  4  by a coolant of the engine  1 , a configuration in which piping  22  for the coolant is made to pass through the storage tank  4  is employed. In this way, without using the electric heater, the urea aqueous solution stored in the storage tank  4  is heated and the temperature of the solution is increased by utilizing waste heat. Accordingly, it is possible to suppress an energy consumption required to increase the temperature of the urea aqueous solution. In the embodiment, the piping  22  corresponds to the heating device.  
       FIG. 5  shows a fourth embodiment of the exhaust gas purifying apparatus according to the invention.  
      In the embodiment, in order to heat the urea aqueous solution to be supplied to the urea aqueous solution adding device  6 , a configuration is employed in which an intermediate portion of the piping  5  connecting the storage tank  4  and the urea aqueous solution adding device  6  is wound around the exhaust pipe  2  positioned on the exhaust downstream side of the NOx reducing catalytic converter  3 . In this way, without using the electric heater, the urea aqueous solution flowing through the piping  5  can receive heat from the exhaust gas flowing through the exhaust pipe  2  and the temperature of the solution is increased by utilizing wasted heat. Therefore, it is able to suppress the energy consumption required to increase the temperature of the urea aqueous solution. At this time, because the exhaust heat on the exhaust downstream side of the NOx reducing catalytic converter  3  is utilized, the temperature of the exhaust gas supplied to the NOx reducing catalytic converter  3  is not reduced and the NOx purifying efficiency is not reduced.  
      As the exhaust gas purifying apparatus, the first through fourth embodiments may be used not only separately but also in proper combination. In each embodiment, it is also possible to use a pump for pressurizing the urea aqueous solution instead of the urea aqueous solution adding device  6 , to directly inject and add the pressurized urea aqueous solution from the injection nozzle  10 . In this way, the pump for pressurizing and supplying the air becomes unnecessary. In this case, actuation of the pump for pressurizing the urea aqueous solution may be controlled by the urea aqueous solution addition control unit  13  to control the addition flow rate of the urea aqueous solution.  
     INDUSTRIAL APPLICABILITY  
      As described above, the exhaust gas purifying apparatus according to the invention is extremely useful because the decrease of temperature of the urea aqueous solution injected and added into the exhaust gas is suppressed and the efficiency of the hydrolysis where the ammonia is produced is enhanced to thereby prevent a shortage of the ammonia to be supplied to the nitrogen oxide reducing catalytic converter to facilitate reduction and purifying of the nitrogen oxides.