JP2013019390A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce NOx over a wide temperature range from the low temperature range to the high temperature range of an exhaust gas.SOLUTION: A first selective reduction type catalyst 21 composed of a silver based catalyst is provided in the exhaust pipe 16 of an engine 11 and a second selective reduction type catalyst 22 composed of a copper based catalyst or the like is provided in an exhaust pipe more on an exhaust gas downstream side from the first selective reduction type catalyst. A NOx adsorption catalyst 18 adsorbing the NOx in the exhaust gas in a state of NO or NOis provided in an exhaust pipe more on the exhaust gas upstream side from the first selective reduction type catalyst. A liquid injection nozzle 26 that can inject a hydrocarbon-based liquid 24 toward the NOx adsorption catalyst is provided in the exhaust pipe more on the exhaust gas upstream side from the NOx adsorption catalyst, and the liquid is supplied via a liquid injection amount regulating valve 31 to the liquid injection nozzle by a hydrocarbon-based liquid supply means 27. A controller 38 controls the liquid injection amount regulating valve based on the detection output of a first temperature sensor for detecting the temperature of the exhaust gas related to the NOx adsorption catalyst.

Description

  The present invention relates to an apparatus for purifying exhaust gas by reducing nitrogen oxide (hereinafter referred to as NOx) contained in exhaust gas of a diesel engine.

  Conventionally, as this type of exhaust gas purification device, a hydrocarbon storage material and a NOx reduction catalyst are arranged in series in an exhaust pipe system of a diesel engine, and the hydrocarbon storage material is provided with a plurality of series-arranged refractory ceramic honeycomb carriers. A diesel engine exhaust gas purification device configured by coating with zeolite having different pore sizes is disclosed (for example, see Patent Document 1). In this exhaust gas purifying apparatus, the pore size of the zeolite used for coating each upstream honeycomb carrier is formed larger than the pore size of the zeolite used for coating the adjacent honeycomb carrier on the downstream side. Specifically, the hydrocarbon occlusion material is configured by connecting three cordierite first to third honeycomb carriers in series, and the first honeycomb carrier from the upstream side is a Y-type zeolite (pore diameter of about 10 mm). ), The second honeycomb carrier is coated with mordenite (pore diameter of about 8 mm), and the third honeycomb carrier is coated with ZSM-5 (pore diameter of about 5 mm). As the NOx reduction catalyst, a catalyst such as platinum-alumina or copper-zeolite is used.

  In the diesel engine exhaust gas purification apparatus configured as described above, zeolites having different pore diameters are coated on separate honeycomb carriers, and arranged so that the pore diameter gradually decreases from the upstream side of the exhaust gas flow, and the hydrocarbon storage material Then, HC in the exhaust gas is occluded at a low temperature, retained, and desorbed at a high temperature to react with NOx, whereby both HC and NOx in the exhaust gas can be efficiently reduced. Further, zeolite having a larger pore size is arranged on the upstream side of the exhaust gas flow, and zeolite having a smaller pore size is arranged toward the downstream side to provide a step difference in pore size. Storage and desorption of various HCs up to about 30 can be performed smoothly without causing problems such as clogging.

  In addition, a selective reduction type NOx catalyst that selectively reduces nitrogen oxides in the presence of an ammonia-derived reducing agent is provided in the exhaust passage of the internal combustion engine, storing nitrogen oxides in the exhaust gas in an oxygen-excess atmosphere and having an oxygen concentration of A NOx storage reduction catalyst that releases / reduces the stored nitrogen oxides when reduced is provided in the exhaust passage of the internal combustion engine, the selective reduction NOx catalyst reducing agent supply means supplies the reducing agent to the selective reduction NOx catalyst, and An exhaust purification device for an internal combustion engine is disclosed in which the storage reduction type NOx catalyst reducing agent supply means supplies a reducing agent to the storage reduction type NOx catalyst (see, for example, Patent Document 2). In this exhaust gas purification apparatus for an internal combustion engine, when the amount of reducing agent supplied to the NOx storage reduction catalyst exceeds a threshold value, the NOx storage reducing agent supply means stops the supply of the reducing agent and the selective reduction type The NOx catalyst reducing agent supply means is configured to supply an ammonia-derived reducing agent to the selective reduction type NOx catalyst. Specifically, the NOx storage reduction catalyst is supported on the particulate filter, and the selective reduction NOx catalyst using urea as a reducing agent is disposed downstream thereof. The particulate filter with the NOx storage reduction catalyst uses alumina as a carrier, and an alkali metal such as K, Na, Li or Cs, an alkaline earth such as Ba or Ca, La or Y, etc. on the carrier. And at least one selected from rare earths of the above and a precious metal such as Pt. Examples of the selective reduction type NOx catalyst include a catalyst in which a transition metal such as Cu is ion-exchanged and supported on zeolite as a support, a catalyst in which titania and vanadium are supported, a catalyst in which noble metal is supported on zeolite or alumina as a support, and the like. .

  In the exhaust gas purification apparatus for an internal combustion engine configured as described above, when NOx in the exhaust gas is absorbed by the NOx storage reduction catalyst and it becomes necessary to supply a reducing agent to the NOx storage reduction catalyst, the NOx storage reduction type The catalyst reducing agent supply means supplies the reducing agent to the exhaust passage upstream of the NOx storage reduction catalyst. The reducing agent supplied to the exhaust passage flows into the NOx storage reduction catalyst together with the exhaust gas flowing from the upstream of the exhaust passage. The NOx storage reduction catalyst uses a reducing agent to reduce and purify harmful gas components in the exhaust gas. Further, when it becomes necessary to supply a reducing agent to the selective reduction type NOx catalyst, the selective reduction type NOx catalyst reducing agent supply means supplies an ammonia-derived compound as a reducing agent into the exhaust gas. The reducing agent supplied to the exhaust passage flows into the selective reduction type NOx catalyst together with the exhaust gas flowing from the upstream of the exhaust passage. The selective reduction type NOx catalyst uses a reducing agent to reduce and purify harmful gas components in the exhaust gas. Which lean NOx catalyst is supplied with the reducing agent to purify NOx is determined by whether or not the amount of reducing agent supplied to the NOx storage reduction catalyst by the storage reduction NOx catalyst reducing agent supply means exceeds a threshold value. Is done. That is, if the amount of the reducing agent supplied to the NOx storage reduction catalyst by the storage reduction NOx catalyst reducing means exceeds the threshold value, the fuel consumption deteriorates, so the reducing agent is supplied to the selective reduction NOx catalyst. When the amount of reducing agent supplied to the NOx storage reduction catalyst by the storage reduction NOx catalyst reducing agent supply means is equal to or less than the threshold value, the required amount of reducing agent is small and does not cause deterioration in fuel consumption. A reducing agent is supplied to the catalyst. By selecting a lean NOx catalyst that supplies a reducing agent under such conditions, the consumption of the reducing agent can be reduced.

JP 2000-192810 A (Claim 1, paragraphs [0008], [0014], [0022]) JP 2002-188429 A (Claim 1, paragraphs [0019] to [0027], [0047], [0061], [0068])

  However, in the diesel engine exhaust gas purification device disclosed in the above-mentioned conventional Patent Document 1, when a platinum-alumina catalyst is used as the NOx reduction catalyst, for example, the activation temperature range is around 200 ° C., and the temperature activation range is narrow. There was a problem. In addition, in the exhaust gas purification apparatus for an internal combustion engine shown in the above-described conventional patent document 2, urea is used as a reducing agent. Therefore, when the exhaust gas purification apparatus is mounted on a vehicle, a urea water tank that stores urea water in the vehicle. There is a problem that the operation of replenishing the urea water tank with the urea water is relatively troublesome.

  A first object of the present invention is to provide an exhaust gas purification device that can reduce NOx over a wide temperature range from a low temperature range to a high temperature range of exhaust gas. A second object of the present invention is to provide an exhaust gas purifying apparatus in which a hydrocarbon-based liquid such as a fuel that is relatively easy to handle can be used as a reducing agent instead of urea water.

  When the exhaust gas component of the engine was analyzed using an analyzer capable of measuring various gas components, it was found that ammonia was contained in the exhaust gas of the engine that passed through the silver catalyst. For this reason, when it was investigated whether the above-mentioned ammonia could be combined with various catalysts to improve NOx reduction performance, etc., when a copper-based catalyst, iron-based catalyst or vanadium-based catalyst was installed on the exhaust gas downstream side of the silver-based catalyst, the copper-based catalyst The present inventors have found that NOx can be reduced by the reaction of ammonia and NOx on an iron-based catalyst or vanadium-based catalyst, and the present invention has been made.

As shown in FIG. 1, the first aspect of the present invention is a first selective reduction catalyst 21 made of a silver catalyst provided in the exhaust pipe 16 of the engine 11, and a downstream side of the exhaust gas from the first selective reduction catalyst 21. A second selective reduction catalyst 22 made of a copper-based catalyst, an iron-based catalyst or a vanadium-based catalyst and an exhaust pipe 16 provided upstream of the first selective reduction-type catalyst 21 in the exhaust gas. NOx adsorption catalyst 18 that adsorbs NOx in the state of NO or NO ₂ , and liquid injection that is provided in the exhaust pipe 16 upstream of the exhaust gas from the NOx adsorption catalyst 18 and can inject the hydrocarbon-based liquid 24 toward the NOx adsorption catalyst 18. A nozzle 26, a hydrocarbon-based liquid supply means 27 that supplies the liquid 24 to the liquid injection nozzle 26 via the liquid injection amount adjustment valve 31, and a first temperature that detects the temperature of exhaust gas related to the NOx adsorption catalyst 18. The degree sensor 41, an exhaust gas purifying apparatus that includes a controller 38 for controlling the liquid injection amount adjusting valve 31 based on the detection output of the first temperature sensor 41.

The second aspect of the present invention is an invention based on the first aspect, and further, as shown in FIG. 2, the exhaust gas upstream side of the first selective reduction catalyst 21 and the exhaust gas downstream side of the NOx adsorption catalyst 18. The exhaust pipe 16 is further provided with a NOx adsorption catalyst-equipped filter 51 that adsorbs NOx in the exhaust gas in the state of NO or NO ₂ and collects particulates in the exhaust gas.

  A third aspect of the present invention is an invention based on the first or second aspect, and further, as shown in FIG. 1, the first selective reduction catalyst 21 coats the honeycomb carrier with silver zeolite or silver alumina. The second selective reduction catalyst 22 is formed by coating the honeycomb carrier with copper zeolite, iron zeolite or vanadium-based oxide, and the NOx adsorption catalyst 18 is either a rare earth element or a titanium group element on the honeycomb carrier. Alternatively, both oxides are coated.

In the exhaust gas purification apparatus of the first aspect of the present invention, when engine exhaust gas flows into the NOx adsorption catalyst in a low temperature range of the exhaust gas, a part of NOx in the exhaust gas is adsorbed on the NOx adsorption catalyst in the state of NO or NO _2. Is done. Thereby, NOx discharged to the atmosphere is reduced. When the exhaust gas reaches a high temperature, the hydrocarbon liquid injected from the liquid injection nozzle causes a reduction reaction with NO or NO ₂ adsorbed on the NOx adsorption catalyst, thereby detoxifying NOx. Further, when excess hydrocarbon liquid that has passed through the NOx adsorption catalyst out of the hydrocarbon liquid injected from the liquid injection nozzle flows into the silver-based first selective reduction catalyst, ammonia is produced on the first selective reduction catalyst. When the exhaust gas containing ammonia becomes hot due to the heat generated by the oxidation reaction of the hydrocarbon liquid in the NOx adsorption catalyst and flows into the second selective reduction catalyst of copper type, iron type or vanadium type, this second Ammonia and NOx react on the two-selective reduction catalyst to promote NOx reduction reaction and ammonia oxidation reaction. This detoxifies NOx. As a result, NOx in the exhaust gas can be reduced over a wide temperature range from a low temperature range to a high temperature range of the exhaust gas. In addition, when installed in a vehicle, it is necessary to provide a urea water tank for storing urea water as a reducing agent separately from the fuel tank, and the operation of replenishing urea water to the urea water tank is relatively troublesome. Compared to an exhaust purification device, in the present invention, a hydrocarbon-based liquid such as light oil is used as a reducing agent. Therefore, when installed in a vehicle, light oil stored in a fuel tank can be used. There is no need to provide a new tank, and handling of the reducing agent becomes relatively easy.

  In the exhaust gas purification apparatus of the second aspect of the present invention, when the exhaust gas of the engine flows into the NOx adsorption catalyst in the low temperature range of the exhaust gas, a part of the NOx in the exhaust gas is adsorbed by the NOx adsorption catalyst, and the NOx adsorption catalyst is used. When the exhaust gas containing the remaining NOx residue flows into the filter with the NOx adsorption catalyst, a part of the remaining NOx in the exhaust gas is adsorbed by the filter with the NOx adsorption catalyst, and the particulates in the exhaust gas become the NOx adsorption catalyst. It is collected. As a result, in the low temperature region of the exhaust gas, NOx discharged to the atmosphere can be further reduced, and particulates discharged to the atmosphere can be reduced. When the exhaust gas reaches a high temperature and the hydrocarbon liquid injected from the liquid injection nozzle flows into the NOx adsorption catalyst, the hydrocarbon liquid causes a reduction reaction with NOx adsorbed on the NOx adsorption catalyst, thereby detoxifying NOx. When the hydrocarbon liquid that has passed through the NOx adsorption catalyst flows into the filter with the NOx adsorption catalyst, the hydrocarbon liquid undergoes a reduction reaction with the NOx adsorbed on the filter with the NOx adsorption catalyst, thereby detoxifying NOx. At the same time, the particulates in the exhaust gas are collected by the NOx adsorption catalyst. In addition, when excess hydrocarbon liquid that has passed through the NOx adsorption catalyst and the NOx adsorption catalyst-containing filter flows into the silver-based first selective reduction catalyst among the hydrocarbon-based liquids, ammonia is produced on the first selective reduction catalyst. The NOx adsorption catalyst and the NOx adsorption catalyst-attached filter become a high temperature by the heat generated by the oxidation reaction of the hydrocarbon-based liquid, and the exhaust gas containing ammonia is a copper-based, iron-based or vanadium-based second selective reduction catalyst Then, ammonia and NOx react on the second selective catalytic reduction catalyst to promote the NOx reduction reaction and the ammonia oxidation reaction. This detoxifies NOx. As a result, the NOx in the exhaust gas can be more efficiently reduced and the particulates in the exhaust gas can be reduced over a wide temperature range from the low temperature range to the high temperature range of the exhaust gas.

It is a block diagram which shows the exhaust gas purification apparatus of 1st Embodiment of this invention. It is a block diagram which shows the exhaust gas purification apparatus of 2nd Embodiment of this invention. It is a figure which shows the change of NOx reduction rate when attaching the exhaust gas purification apparatus of Example 1, the comparative example 1, and the comparative example 2 to the exhaust pipe of the diesel engine of the same type, respectively, and changing exhaust gas temperature. It is a figure which shows the total NOx reduction rate when attaching the exhaust gas purification apparatus of Example 1, Comparative Example 1, and Comparative Example 3 to the exhaust pipe of the diesel engine of the same type, respectively, and changing exhaust gas temperature.

  Next, an embodiment for carrying out the present invention will be described with reference to the drawings.

<First Embodiment>
As shown in FIG. 1, an intake pipe 13 is connected to an intake port of a diesel engine 11 via an intake manifold 12, and an exhaust pipe 16 is connected to an exhaust port via an exhaust manifold 14. The intake pipe 13 is provided with a compressor housing 17a of the turbocharger 17 and an intercooler 18 for cooling the intake air compressed by the turbocharger 17, and the exhaust pipe 16 is provided with a turbine of the turbocharger 17. A housing 17b is provided. Compressor rotor blades (not shown) are rotatably accommodated in the compressor housing 17a, and turbine rotor blades (not shown) are rotatably accommodated in the turbine housing 17b. The compressor rotor blades and the turbine rotor blades are connected by a shaft (not shown), and the compressor rotor blades are rotated via the turbine rotor blades and the shaft by the energy of the exhaust gas discharged from the engine 11, and the compressor rotor blades are rotated. Thus, the intake air in the intake pipe 13 is compressed.

  A first selective reduction catalyst 21 made of a silver catalyst is provided in the middle of the exhaust pipe 16, and a copper catalyst, an iron catalyst, or vanadium is provided in the exhaust pipe 16 on the exhaust gas downstream side of the first selective reduction catalyst 21. A second selective reduction catalyst 22 made of a system catalyst is provided, and a NOx adsorption catalyst 18 is provided in the exhaust pipe 16 on the exhaust gas upstream side of the first selective reduction catalyst 21. The NOx adsorption catalyst 18, the first selective reduction catalyst 21 and the second selective reduction catalyst 22 are accommodated in a case 23 having a larger diameter than the exhaust pipe 16. The first selective reduction catalyst 21 is a monolith catalyst and is configured by coating a cordierite honeycomb carrier with silver zeolite or silver alumina. Specifically, the first selective reduction catalyst 21 made of silver zeolite is configured by coating a honeycomb carrier with a slurry containing zeolite powder obtained by ion exchange of silver. The first selective reduction catalyst 21 made of silver alumina is configured by coating a honeycomb carrier with a slurry containing γ-alumina powder or θ-alumina powder supporting silver.

  The second selective reduction catalyst 22 is a monolith catalyst, and is configured by coating a cordierite honeycomb carrier with copper zeolite, iron zeolite, or vanadium oxide. Specifically, the second selective reduction catalyst 22 made of copper zeolite is configured by coating a honeycomb carrier with a slurry containing zeolite powder obtained by ion exchange of copper. The second selective reduction catalyst 22 made of iron zeolite is configured by coating a honeycomb carrier with a slurry containing zeolite powder obtained by ion exchange of iron. Further, the second selective reduction catalyst 22 made of vanadium oxide contains a slurry containing a vanadium oxide powder alone or a mixed oxide powder containing titanium oxide and tungsten oxide in the vanadium oxide. Each carrier is coated and configured.

NOx trap catalyst 18 is configured to reduction in the presence of adsorbed and the adsorbed NOx (NO or NO ₂₎ hydrocarbons NOx in the exhaust gas in the form of NO or NO _2. The NOx adsorption catalyst 18 is formed by coating a cordierite honeycomb carrier with a NOx adsorbent. The NOx adsorbent is an oxide of one or more rare earth elements selected from the group consisting of La, Ce, Pr, Nd, Sc and Y, or a titanium genus consisting of one or both of Ti and Zr. An oxide of an element or a mixed oxide of the rare earth element and the titanium group element is included.

  On the other hand, a liquid injection nozzle 26 capable of injecting the hydrocarbon-based liquid 24 toward the NOx adsorption catalyst 18 is provided in the exhaust pipe 16 upstream of the NOx adsorption catalyst 18. A hydrocarbon-based liquid supply means 27 is connected to the liquid injection nozzle 26. The hydrocarbon-based liquid supply means 27 includes a liquid supply pipe 28 having one end connected to the liquid injection nozzle 26, a fuel tank 29 connected to the other end of the liquid supply pipe 28 and storing the liquid 24, and a liquid injection nozzle. And a liquid injection amount adjusting valve 31 that adjusts the injection amount of the liquid 24 injected from 26. The hydrocarbon-based liquid 24 is a fuel such as light oil in this embodiment. The liquid injection amount adjustment valve 31 is provided in the liquid supply pipe 28 and adjusts the supply pressure of the liquid 24 to the liquid injection nozzle 26. The liquid injection amount adjustment valve 31 is provided at the base end of the liquid injection nozzle 26. And a nozzle opening / closing valve 33 for opening and closing the valve. Furthermore, a pump 30 capable of supplying the liquid 24 in the fuel tank 29 to the liquid injection nozzle 26 is provided in the liquid supply pipe 28 between the pressure regulating valve 32 and the fuel tank 29.

  The pressure regulating valve 32 is a three-way valve having first to third ports 32a to 32c, the first port 32a is connected to the discharge port of the pump 30, the second port 32b is connected to the liquid jet nozzle 26, and the third The port 32 c is connected to the fuel tank 29 via the return pipe 34. When the pressure adjustment valve 32 is turned on, the liquid 24 pumped by the pump 30 flows into the pressure adjustment valve 32 from the first port 32a, and after being adjusted to a predetermined pressure by the pressure adjustment valve 32, from the second port 32b. The liquid jet nozzle 26 is pumped. When the pressure regulating valve 32 is turned off, the liquid 24 pumped by the pump 30 flows from the first port 32a into the pressure regulating valve 32 and then returns from the third port 32c to the fuel tank 29 through the return pipe 34.

  The exhaust pipe 16 upstream of the exhaust gas from the NOx adsorption catalyst 18 has a first temperature sensor that detects the temperature of the exhaust gas related to the NOx adsorption catalyst 18, in this embodiment, the temperature of the exhaust gas immediately before flowing through the NOx adsorption catalyst 18. 41 is provided. The rotation speed of the engine 11 is detected by the rotation sensor 36, and the load of the engine 11 is detected by the load sensor 37. The detection outputs of the first temperature sensor 41, the rotation sensor 36, and the load sensor 37 are connected to the control input of the controller 38, and the control outputs of the controller 38 are connected to the pressure regulating valve 32, the pump 30, and the nozzle opening / closing valve 33, respectively. . The controller 38 is provided with a memory 39. The memory 39 stores the engine speed, the engine load, the pressure of the pressure adjustment valve 32 according to the exhaust gas temperature at the inlet of the NOx adsorption catalyst 18 (immediately before flowing through the NOx adsorption catalyst), the number of times of opening and closing the nozzle on / off valve 33, and the pump 30. The presence / absence of the operation is stored in advance.

  In this embodiment, the first temperature sensor is provided in the exhaust pipe on the exhaust gas upstream side of the NOx adsorption catalyst. However, the case is on the exhaust gas downstream side of the NOx adsorption catalyst and on the exhaust gas upstream side of the second selective reduction catalyst. The first temperature sensor is provided in the exhaust pipe on the exhaust gas upstream side of the NOx adsorption catalyst and the case on the exhaust gas downstream side of the NOx adsorption catalyst and on the exhaust gas upstream side of the second selective reduction catalyst. May be provided. When the first temperature sensor is provided in the case downstream of the exhaust gas from the NOx adsorption catalyst and upstream of the first selective reduction catalyst, the NOx adsorption catalyst outlet (immediately after flowing through the NOx adsorption catalyst) by the first temperature sensor. ) Of the exhaust gas is detected. Further, when the first temperature sensor is provided in the exhaust pipe upstream of the NOx adsorption catalyst and in the case of the exhaust gas downstream of the NOx adsorption catalyst and upstream of the first selective reduction catalyst, that is, NOx adsorption. When the first temperature sensor is provided immediately before and after the catalyst, the exhaust gas temperature at the NOx adsorption catalyst inlet (immediately before flowing through the NOx adsorption catalyst catalyst) and the exhaust gas temperature at the NOx adsorption catalyst outlet (immediately after flowing through the NOx adsorption catalyst catalyst) Therefore, the temperature of the exhaust gas flowing through the NOx adsorption catalyst can be detected by calculating the average value of both temperatures.

The operation of the exhaust gas purification apparatus configured as described above will be described. When the exhaust gas temperature is as low as less than 180 ° C., for example, the controller 38 turns off the pump 30 of the hydrocarbon-based liquid supply means 27 based on the detection outputs of the first temperature sensor 41, the rotation sensor 36 and the load sensor 37. The pressure control valve 32 and the nozzle opening / closing valve 33 are turned off, and the hydrocarbon-based liquid 24 is not injected from the liquid injection nozzle 26. When the exhaust gas at this time the engine 11 flows into the NOx adsorption catalyst 18, a part of NOx in the exhaust gas are adsorbed in the form of gaseous NO or NO ₂ in the NOx adsorption catalyst 18. As a result, exhaust of NOx into the atmosphere in the low temperature range of exhaust gas can be reduced.

When the exhaust gas temperature rises to, for example, 180 ° C. or higher, the controller 38 operates the pump 30 of the hydrocarbon-based liquid supply means 27 based on the detection outputs of the first temperature sensor 41, the rotation sensor 36, and the load sensor 37 to adjust the pressure. By turning on the valve 32 and repeatedly turning on and off the nozzle opening / closing valve 33, the hydrocarbon-based liquid 24 is intermittently injected from the liquid injection nozzle 26. At this time, the injection amount of one injection is set to be relatively small, and the hydrocarbon-based liquid 24 is placed in a state before the excess air ratio λ in the exhaust gas decreases to 1 (for example, about λ = 1.3). Be injected. That is, the hydrocarbon liquid 24 injected from the liquid injection nozzle 26 is gasified to generate hydrocarbon (HC), and the hydrocarbon (HC) reacts with oxygen (O ₂ ) in the exhaust gas to react with carbon dioxide. By generating (CO ₂ ) and water (H ₂ O), the excess air ratio λ in the exhaust gas decreases to, for example, 1.3. Here, it is not necessary to reduce the excess air ratio λ to 1 or less because the adsorption power of NOx (NO or NO ₂ ) to the NOx adsorption catalyst 18 is relatively weak, and the exhaust gas temperature rises and the arrival of hydrocarbons. This is because NOx (NO or NO ₂ ) is easily separated from the NOx adsorption catalyst 18 while being reduced.

When the remainder of the hydrocarbons flows into the NOx adsorption catalyst 18, NOx adsorbed to the NOx adsorption catalyst 18 in the state of gaseous NO or NO ₂ reacts with hydrocarbon (HC) to produce carbon dioxide (CO ₂ ). , Water (H ₂ O) and nitrogen (N ₂ ) are produced. That is, hydrocarbons cause a reduction reaction with NOx adsorbed on the NOx adsorption catalyst 18 to detoxify NOx.

  On the other hand, when excess hydrocarbon flows into the silver-based first selective reduction catalyst 21 together with the exhaust gas, ammonia is generated on the catalyst 21. This is because the HC component (alkane) constituting the hydrocarbon-based liquid 24 such as light oil is adsorbed on the first selective reduction catalyst 21, and NOx and oxygen react on the first selective reduction catalyst 21 to generate isocyanate. A reaction intermediate such as a seed (N = C = O, C = N) is formed, and it is presumed that ammonia was generated by adding water to the reaction intermediate.

When the exhaust gas containing ammonia produced by the first selective reduction catalyst 21 flows into the second selective reduction catalyst 22 of copper, iron or vanadium, as shown in the following formulas (1) to (3): Furthermore, NOx (NO and NO ₂ ) in the exhaust gas reacts with ammonia on the second selective reduction catalyst 22 and is reduced to N ₂ , thereby detoxifying NOx.

NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (1)
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (2)
6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O (3)
Here, since the temperature of the exhaust gas flowing into the second selective reduction catalyst 22 is high due to the heat generated by the oxidation reaction of hydrocarbons in the NOx adsorption catalyst 18, the exhaust gas in the second selective reduction catalyst 22 is in the exhaust gas. The NOx reduction reaction proceeds promptly. As a result, the NOx reduction efficiency in the exhaust gas in the high temperature range of the exhaust gas can be improved. Therefore, NOx can be efficiently reduced over a wide temperature range from the low temperature range to the high temperature range of the exhaust gas.

<Second Embodiment>
FIG. 2 shows a second embodiment of the present invention. 2, the same reference numerals as those in FIG. 1 denote the same components. In this embodiment, the NOx adsorption catalyst-equipped filter 51 is provided in the exhaust pipe 16 upstream of the first selective reduction catalyst 21 and downstream of the NOx adsorption catalyst 18. The NOx adsorption catalyst-equipped filter 51 is configured to adsorb NOx in the exhaust gas in the state of NO or NO ₂ , reduce the adsorbed NOx in the presence of hydrocarbons, and collect particulates in the exhaust gas. Is done. The NOx adsorption catalyst-equipped filter 51 is formed by coating a cordierite honeycomb carrier with a NOx adsorbent. The honeycomb carrier has a polygonal cross section partitioned by porous partition walls, and adjacent inlet and outlet portions of a large number of through holes formed in parallel to each other by these partition walls are alternately sealed by a sealing member. Configured by stopping. When the engine exhaust gas introduced from the inlet portion of the honeycomb carrier passes through the porous partition walls, the particulates contained in the exhaust gas are collected and discharged from the outlet portion. In addition, the NOx adsorbent is an oxide of one or more rare earth elements selected from the group consisting of La, Ce, Pr, Nd, Sc and Y, or titanium consisting of one or both of Ti and Zr. A metal element oxide or a mixed oxide of the rare earth element and titanium element. Further, the filter 51 with NOx adsorption catalyst is housed in a case 52 having a diameter larger than that of the exhaust pipe 16 together with the NOx adsorption catalyst 18, the first selective reduction catalyst 21 and the second selective reduction catalyst 22. The configuration other than the above is the same as that of the first embodiment.

The operation of the exhaust gas purification apparatus configured as described above will be described. When the exhaust gas temperature is as low as less than 180 ° C., for example, the controller 38 turns off the pump 30 of the hydrocarbon-based liquid supply means 27 based on the detection outputs of the first temperature sensor 41, the rotation sensor 36 and the load sensor 37. The pressure control valve 32 and the nozzle opening / closing valve 33 are turned off, and the hydrocarbon-based liquid 24 is not injected from the liquid injection nozzle 26. At this time, when the exhaust gas of the engine 11 flows into the NOx adsorption catalyst 18, a part of the NOx in the exhaust gas is adsorbed to the NOx adsorption catalyst 18 in the state of gaseous NO or NO ₂ and passes through the NOx adsorption catalyst 18. of the exhaust gas containing the remainder flows into the NOx trap catalyst with filter 51, with a portion of the NOx in the exhaust gas balance is adsorbed in the form of gaseous NO or NO ₂ in the NOx trap catalyst with filter 51, the exhaust gas Particulates therein are collected by the NOx adsorption catalyst-equipped filter 51. As a result, in the low temperature range of the exhaust gas, NOx emission into the atmosphere can be reduced as compared with the exhaust gas purification apparatus of the first embodiment, and particulate emission into the atmosphere can be reduced.

When the exhaust gas temperature rises to, for example, 180 ° C. or higher, the controller 38 operates the pump 30 of the hydrocarbon-based liquid supply means 27 based on the detection outputs of the first temperature sensor 41, the rotation sensor 36, and the load sensor 37 to adjust the pressure. By turning on the valve 32 and repeatedly turning on and off the nozzle opening / closing valve 33, the hydrocarbon-based liquid 24 is intermittently injected from the liquid injection nozzle 26. At this time, the injection amount of one injection is set to be relatively small, and the hydrocarbon-based liquid 24 is placed in a state before the excess air ratio λ in the exhaust gas decreases to 1 (for example, about λ = 1.3). Be injected. That is, the hydrocarbon liquid 24 injected from the liquid injection nozzle 26 is gasified to generate hydrocarbon (HC), and the hydrocarbon (HC) reacts with oxygen (O ₂ ) in the exhaust gas to react with carbon dioxide. By generating (CO ₂ ) and water (H ₂ O), the excess air ratio λ in the exhaust gas decreases to, for example, 1.3. Here, it is not necessary to reduce the excess air ratio λ to 1 or less because the adsorption power of NOx (NO or NO ₂ ) to the NOx adsorption catalyst 18 and the NOx adsorption catalyst-equipped filter 51 is relatively weak, and the exhaust gas temperature This is because as NOx (NO or NO ₂ ) is reduced, the NOx adsorbing catalyst 18 and the NOx adsorbing catalyst-equipped filter 51 are easily separated while rising and the arrival of hydrocarbons.

When the remaining hydrocarbon flows into the NOx adsorption catalyst 18, NO or NO ₂ adsorbed on the NOx adsorption catalyst 18 reacts with hydrocarbon (HC) to produce carbon dioxide (CO ₂ ) and water (H ₂ O). And nitrogen (N ₂ ) is produced. That is, hydrocarbons cause a reduction reaction with NOx adsorbed on the NOx adsorption catalyst 18 to detoxify NOx.

When hydrocarbons that have passed through the NOx adsorption catalyst 18 flow into the NOx adsorption catalyst-equipped filter 51, NOx adsorbed in the NOx or NO ₂ state on the NOx adsorption catalyst-equipped filter 51 reacts with hydrocarbons (HC) to produce CO2. Carbon (CO ₂ ), water (H ₂ O) and nitrogen (N ₂ ) are produced. That is, hydrocarbons cause a reduction reaction with NOx adsorbed by the NOx adsorption catalyst-equipped filter 51, thereby detoxifying NOx. Particulates in the exhaust gas are collected by the filter 51 with NOx adsorption catalyst.

On the other hand, when excess hydrocarbon flows into the silver-based first selective reduction catalyst 21 together with the exhaust gas, ammonia is generated on the catalyst 21 as in the first embodiment. When the exhaust gas containing ammonia generated by the first selective reduction catalyst 21 flows into the copper-based, iron-based or vanadium-based second selective reduction catalyst 22, the second selective reduction is performed as in the first embodiment. NOx (NO and NO ₂ ) in the exhaust gas reacts with ammonia on the type catalyst 22 and is reduced to nitrogen (N ₂ ), thereby detoxifying NOx. Here, the temperature of the exhaust gas flowing into the second selective reduction catalyst 22 is high because of the heat generated by the oxidation reaction of hydrocarbons in the NOx adsorption catalyst 18 and the NOx adsorption catalyst-equipped filter 51. The reduction reaction of NOx in the exhaust gas in the reduction catalyst 22 proceeds promptly. As a result, the NOx reduction efficiency in the exhaust gas in the high temperature range of the exhaust gas can be improved. Therefore, NOx in the exhaust gas can be reduced more efficiently than in the first embodiment over a wide temperature range from a low temperature range to a high temperature range of the exhaust gas, and particulates in the exhaust gas can be reduced.

  In the first and second embodiments, the exhaust gas purifying apparatus of the present invention is applied to a diesel engine. However, the exhaust gas purifying apparatus of the present invention may be applied to a gasoline engine. In the first and second embodiments, the exhaust gas purification apparatus of the present invention is applied to a turbocharged diesel engine. However, the exhaust gas purification apparatus of the present invention is applied to a naturally aspirated diesel engine or a naturally aspirated gasoline. It may be applied to the engine.

  Next, examples of the present invention will be described in detail together with comparative examples.

<Example 1>
As shown in FIG. 1, a NOx adsorption catalyst 18, a first selective reduction catalyst 21, and a second selective reduction catalyst 22 are sequentially disposed in an exhaust pipe 16 of a turbocharged diesel engine 11 having a displacement of 8000 cc from the exhaust gas upstream side. Was provided. A liquid injection nozzle 26 for injecting the hydrocarbon-based liquid 24 is provided in the exhaust pipe 16 upstream of the exhaust gas from the NOx adsorption catalyst 18. The NOx adsorption catalyst 18 was a catalyst prepared by coating a honeycomb carrier with a slurry containing CeO ₂ as a rare earth oxide. The first selective reduction catalyst 21 was a silver catalyst prepared by coating a honeycomb carrier with a slurry containing zeolite powder obtained by ion exchange of silver. The second selective reduction catalyst 22 was a copper catalyst prepared by coating a honeycomb carrier with a slurry containing zeolite powder obtained by ion exchange of copper. This exhaust gas purification apparatus was designated as Example 1.

<Comparative Example 1>
The configuration was the same as Example 1 except that the NOx adsorption catalyst was not used. This exhaust gas purification apparatus was designated as Comparative Example 1.

<Comparative example 2>
The configuration was the same as in Example 1 except that the NOx adsorption catalyst and the second selective reduction catalyst were not used. This exhaust gas purification apparatus was designated as Comparative Example 2.

<Comparative test 1 and evaluation>
The NOx reduction rate by the exhaust gas purification apparatuses of Example 1, Comparative Example 1, and Comparative Example 2 was measured when the exhaust gas temperature was gradually increased from room temperature to 600 ° C. by changing the engine speed and load. The result is shown in FIG.

  As is clear from FIG. 3, in the exhaust gas purification apparatuses of Comparative Examples 1 and 2, the NOx reduction rate gradually increases from a relatively high temperature of about 200 ° C., whereas in the exhaust gas purification apparatus of Example 1, It was found that the NOx reduction rate gradually increased from a relatively low temperature of about 150 ° C. The exhaust gas purification apparatus of Comparative Example 1 has a maximum NOx reduction rate of about 40%, and the exhaust gas purification apparatus of Comparative Example 2 has a maximum NOx reduction rate of about 30%, whereas Example 1 In the exhaust gas purification apparatus, the NOx reduction rate was found to be as high as about 50% at the maximum.

<Comparative Example 3>
Other than the NOx adsorption catalyst and the second selective reduction catalyst, and the platinum selective catalyst prepared by coating the honeycomb carrier with a slurry containing zeolite powder ion-exchanged platinum as the first selective reduction catalyst. The same configuration as in Example 1 was used. This exhaust gas purification apparatus was designated as Comparative Example 3.

<Comparative test 2 and evaluation>
Measures the total NOx reduction rate by the exhaust gas purification apparatuses of Example 1, Comparative Example 1 and Comparative Example 3 when the exhaust gas temperature is gradually increased from room temperature to 600 ° C. by changing the engine speed and load. did. The result is shown in FIG.

  As apparent from FIG. 4, the exhaust gas purification device of Comparative Example 1 had a total NOx reduction rate of about 40%, and the exhaust gas purification device of Comparative Example 3 had a total NOx reduction rate of about 30%. In contrast, the exhaust gas purification apparatus of Example 1 has a higher total NOx reduction rate of about 50%, and the exhaust gas purification apparatus of Example 1 has a higher total NOx reduction ratio than the exhaust gas purification apparatuses of Comparative Examples 1 and 3. I found out.

11 Diesel engine (engine)
16 Exhaust pipe 18 NOx adsorption catalyst 21 First selective reduction catalyst 22 Second selective reduction catalyst 24 Hydrocarbon liquid 26 Liquid injection nozzle 27 Hydrocarbon liquid supply means 31 Liquid injection amount adjustment valve 38 Controller 41 First temperature sensor 51 Filter with NOx adsorption catalyst

Claims (3)

A first selective reduction catalyst (21) made of a silver catalyst provided in an exhaust pipe (16) of the engine (11);
A second selective reduction catalyst (22) comprising a copper-based catalyst, an iron-based catalyst or a vanadium-based catalyst provided in the exhaust pipe (16) on the exhaust gas downstream side of the first selective reduction-type catalyst (21);
A NOx adsorption catalyst (18) provided in the exhaust pipe (16) upstream of the exhaust gas from the first selective reduction catalyst (21) and adsorbing NOx in the exhaust gas in a state of NO or NO ₂ ;
A liquid injection nozzle (26) provided in the exhaust pipe (16) upstream of the exhaust gas from the NOx adsorption catalyst (18) and capable of injecting a hydrocarbon-based liquid (24) toward the NOx adsorption catalyst (18);
Hydrocarbon liquid supply means (27) for supplying the liquid (24) to the liquid injection nozzle (26) via a liquid injection amount adjustment valve (31);
A first temperature sensor (41) for detecting the temperature of exhaust gas related to the NOx adsorption catalyst (18);
An exhaust gas purifying apparatus comprising: a controller (38) for controlling the liquid injection amount adjusting valve (31) based on a detection output of the first temperature sensor (41). The NOx in the exhaust gas is in the state of NO or NO ₂ provided in the exhaust pipe (16) upstream of the first selective reduction catalyst (21) and downstream of the NOx adsorption catalyst (18). The exhaust gas purifying apparatus according to claim 1, further comprising a filter (51) with a NOx adsorption catalyst that adsorbs at the exhaust gas and collects particulates in the exhaust gas.   The first selective reduction catalyst (21) is configured by coating a honeycomb carrier with silver zeolite or silver alumina, and the second selective reduction catalyst (22) is formed on the honeycomb carrier with copper zeolite, iron zeolite or vanadium oxide. The exhaust gas purification apparatus according to claim 1 or 2, wherein the NOx adsorption catalyst (18) is formed by coating a honeycomb carrier with an oxide of either a rare earth element or a titanium group element or both. .

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