JP4696039B2 - Urea water injection device for exhaust treatment equipment - Google Patents

Description

  The present invention relates to an engine exhaust treatment device, and more particularly to an exhaust treatment device that can efficiently remove nitrogen oxides in exhaust gas using urea water as a reducing agent.

Conventionally, in a diesel engine, a selective reduction type catalyst having a property of selectively reacting nitrogen oxide (hereinafter referred to as NOx) with a reducing agent even in the presence of oxygen is provided in the middle of an exhaust pipe through which exhaust gas flows. Equip and add the required amount of reducing agent (hydrocarbon, ammonia or its precursor) to the upstream side of this selective catalytic reduction catalyst, and this reducing agent is reduced on the selective catalytic reduction catalyst with NOx in the exhaust gas. In this way, the NOx emission concentration can be reduced. SCR (Selective Catalytic) is a NOx reduction method using this selective reduction catalyst.
Reduction), and urea that uses urea as a reducing agent is particularly called urea SCR. In order to apply this urea SCR to a vehicle, urea water is stored in a tank, and during operation, urea water supplied from this tank is injected into the exhaust passage, and urea is hydrolyzed using exhaust heat, A technique for reducing NOx by ammonia generated thereby is known (Patent Document 1).

Urea injected from the urea injection device generates ammonia (NH ₃ ) from urea by the following chemical reaction.

(NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂
There are several types of reactions in which NOx is reduced by ammonia on a denitration catalyst, but the following reaction has the highest reaction rate at a relatively low temperature.

NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O
Since NOx in the exhaust gas immediately after being discharged from the engine is mostly NO, in order to cause the denitration reaction, it is necessary that the half of the NO to NO _2. For this reason, the denitration reaction is promoted by placing an oxidation catalyst upstream of the urea injection device and making the NOx component NO: NO ₂ = 1: 1 and then performing the denitration reaction. In addition, the oxidation catalyst can also function to raise the exhaust gas temperature by oxidizing hydrocarbons and the like.

  In addition, when the exhaust gas temperature is low, the denitration reaction is difficult to occur, and the denitration performance is reduced. However, when adding urea water, the urea water is vaporized by heating with an electric heater, and the urea is converted into ammonia. There is also known a technique for effectively utilizing a denitration catalyst and enhancing NOx reduction performance by promoting the hydrolysis of NOx (Patent Document 2).

  When injecting urea water into the exhaust gas, if the temperature of the exhaust gas is high, even if the urea water is directly injected, the urea water is vaporized by receiving heat from the exhaust gas, and the urea is contained in the exhaust gas. The NOx can be treated with ammonia generated from urea. However, when the temperature of the exhaust gas is low, it is difficult to completely vaporize the urea water only by the heat of the exhaust gas, and the sprayed urea water spray adheres to the wall surface of the flue and may cause precipitation. Occurs. Moreover, even if it does not adhere to the wall surface, the hydrolysis reaction of urea hardly proceeds, and the hydrolysis reaction of urea occurs on the denitration catalyst. When a denitration catalyst is used in the hydrolysis reaction, NOx reduction reaction with ammonia (= denitration reaction) is not performed, and the denitration reaction is performed on the downstream catalyst where the hydrolysis reaction has occurred. The area of denitration catalyst that can be used is reduced. If the temperature of the chemical reaction is low, the reaction rate is low, and if the exhaust gas temperature is low, the reaction rate of the denitration reaction also decreases, making it difficult to denitrate. It becomes difficult to denitrate. For this reason, when the exhaust gas temperature is low, the denitration performance is greatly reduced. In order to solve this problem, when the temperature of exhaust gas is low, it is effective to hydrolyze urea in advance and inject it into exhaust gas in the form of ammonia gas, and a urea water injection device equipped with an electric heater Is an effective means for improving the NOx removal performance at low exhaust temperatures.

JP 2000-027627 A JP 2005-344597 A

  When vaporizing urea water with an electric heater, in order to consume electric power, it is necessary to increase the amount of electric power generated by the generator attached to the engine as compared with the case where this device is not used. Increasing the amount of power generation requires a larger generator. In addition, since the electric power consumed by the electric heater is likely to be larger than that of other electric components, the supplied current also increases, and the power supply device needs to be compatible with a large current. In order to reduce these problematic factors, it is necessary to reduce the power consumption of the electric heater for vaporizing the urea water.

  On the other hand, since the amount of heat necessary for vaporizing the urea water per unit mass is determined, it is extremely difficult to reduce the power consumption of the electric heater without changing the amount of urea water to be vaporized.

  In addition, when the current supplied to the electric heater is reduced to reduce power consumption without changing the urea water supply amount, the vaporization rate of the urea water is lowered and the temperature of the heat transfer surface in contact with the urea water is lowered. . When urea water is gently heated on the solid surface at about 200 ° C., only moisture is vaporized and solid urea is deposited. If further gentle heating is continued, solid urea is transformed into a stronger solid material such as cyanuric acid, and it becomes more difficult to remove the precipitate from the solid surface. If this deposition occurs on the heat transfer surface of the electric heater, solid matter may be deposited to block the flow path, and the function as the urea water supply device may not be performed.

  In addition, when a droplet is heated by bringing a droplet (a spray is composed of a large number of droplets) into contact with the solid surface, two heat transfers are performed when a liquid film is formed on the solid surface and when it is not formed. There is a form. The former is in the same state as so-called nucleate boiling, and the latter is in the same state as film boiling. In the case of film boiling, when the droplet contacts the heat transfer surface of the solid surface, the vapor generated by vaporization bounces off the droplet, so the droplet does not continue to contact the solid surface, A liquid film is not formed. In this case, since the droplet does not enter a state where it is gently heated on the surface of the solid, urea does not precipitate. However, since film boiling has a characteristic that the heat transfer coefficient is lower than that of nucleate boiling, if a heater is designed to cause film boiling, a problem arises that a larger heat transfer area is required.

  On the other hand, in the form in which a liquid film is formed on the solid surface, the heat transfer rate is improved by causing nucleate boiling, but the urea water is likely to be heated gently on the solid surface and may cause precipitation. There is a problem of improving the sex.

  In general, the spray density from the spray device decreases as the distance from the spray point increases. For this reason, in the place near the injection point, the density of spray is high, and the amount of spray that collides with the heat transfer surface may increase. However, when the spray is vaporized by film boiling, the spray rebounds immediately upon hitting the heat transfer surface and repeats contact with the downstream heat transfer surface again, so that the density of the spray reaching the heat transfer surface directly from the injection point And the density of sprays actually contacting the heat transfer surface does not match. For this reason, when the urea water spray is vaporized while causing film boiling on the heat transfer surface, there is a problem that the behavior of the spray becomes complicated and the optimum heating method for the heat transfer surface is unclear.

  In order to solve the above problems, the present invention mainly adopts the following configuration.

  An injection device that injects urea water in a spray form; and an electric heater that has a heat transfer surface that contacts the urea water injected from the injection device and heats and vaporizes the urea water. In the urea water injection device for an exhaust treatment device for injecting an ammonia component generated from urea water, the electric heater has a heat transfer surface in contact with the urea water injected from the injection device, and the electric heater The heat generation amount per unit area of the heat transfer surface is set to be lower in a place far from the injection point of the injection device than in a close place.

  At the same time, the amount of heat generated per unit area is made higher in a place near the injection point of the injection device than in a place far away. In the case of film boiling of the spray on the heat transfer surface, there has been a problem that the optimum heating method of the heat transfer surface becomes unclear because the droplets bounce downstream, but the inventors have made trial and error. Through experiments, we found that the distance from the injection point governs the amount of heat taken from the heat transfer surface per unit area for urea water spray. From this knowledge, we decided to increase the calorific value per unit heat transfer area near the injection point.

  Further, the electric heater uses a heating wire as a heating element, and the heating wire is a continuous heating wire, and the amount of heat generated per unit space at a location far from the injection point is close to the injection point. It is preferable that the heat generation amount per unit space is lower.

  In addition, the electric heater has a structure in which a heating wire is wound around the heat transfer tube, and the heat generation amount per unit space is increased by reducing the pitch of the heating wire in a place near the injection point of the injection device, rather than in a place far away. It is good to have a configuration.

  In addition, when using a heat transfer tube, a coiled object is provided on the inner surface of the heat transfer tube far from the injection point of the injection device, or a spiral groove is formed on the inner surface of the heat transfer tube far from the injection point of the injection device. It is preferable that the urea water spray be easily retained at a location far from the injection point.

  Moreover, when using a heat exchanger tube, it is good to make it the structure which made the internal diameter of the heat exchanger tube smaller than the near place in the place far from the injection point of an injection apparatus.

  In addition, in the case of using a heat transfer tube, the heat transfer tube is a straight tube parallel to the injection axis of the injection device in a place near the injection point of the injection device, and the heat transfer tube is bent in a place far from the injection point of the injection device. It is good to have a configuration.

  Further, in the case where a heat transfer tube is used, a part of the exhaust gas is divided into the heat transfer tube, and a part of the exhaust gas flows into the inlet portion of the heat transfer tube, and urea water spray is supplied from the injection device. It is preferable to have a structure in which the inlet portion of the heat transfer tube is provided close to the injection point of the urea water spray by providing a chamber to be injected, and providing the inlet portion of the heat transfer tube in a recessed portion recessed toward the inside of the chamber. .

  Further, when a heat transfer tube is used and the exhaust gas divided into the heat transfer tube is flowed, the central axis of the portion into which the diverted gas flows is eccentric with respect to the central axis of the heat transfer tube, thereby It is good to make it the structure which produces a swirl in the flow of the shunt gas in a pipe.

  Also, it is preferable that two heating wires are provided, and that each heating wire has a structure in which the heat generation amount per unit volume in each space is equal.

  When the urea water spray is brought into contact with the heat transfer surface and heated / vaporized by the electric heater, the heat generation amount per heat transfer area of the electric heater is made lower at a location far from the injection point than at a nearby location, so that The temperature of the electric heater itself does not rise too much, it becomes possible to reduce the heat loss that inevitably occurs according to the temperature of the electric heater and the outside, and it is consumed to vaporize the same amount of urea water It becomes possible to reduce the electric power of the electric heater.

  At the same time, by reducing heat loss at a location far from the injection point, in the case of the same power consumption as a whole, it is possible to increase power consumption at a location near the injection point. Increasing the heat generation amount per unit area of the heat transfer surface increases the heat transfer surface temperature, and the heat transfer mode between the urea water droplets and the heat transfer surface can be maintained by film boiling. Become. In the case of film boiling, a urea water liquid film is not formed on the heat transfer surface, and urea precipitation can be prevented. As a result, it is possible to avoid a situation in which urea is clogged by the urea water injection device and injection cannot be performed.

  Also, in the place near the injection point, even if the heat generation amount per unit heat transfer area is increased, the amount of heat taken away by the urea water is large, so the temperature of the electric heater does not rise more than necessary, and the increase in heat loss is suppressed. It is done.

  In addition, the heating wire is a heating element, and in one continuous heating wire, in a place far from the injection point, the heat generation density is lowered as compared with the above, thereby reducing power consumption and urea. In addition to being able to avoid the occurrence of clogging due to water precipitation, it is not necessary to install separate heating wires to make the heat density different, and the number of heating wires used can be reduced. become. When the number of heating wires is reduced, the heater can be easily manufactured and the reliability as a device is improved.

  Moreover, when realizing the above electric heater, the heating wire is wound around the heat transfer tube and the winding pitch of the heating wire is managed so that the heat generation density can be managed. It becomes possible to manufacture a heater whose density varies depending on the location.

  Even when a droplet is film-boiling on the heat transfer surface by providing a coiled object or a spiral groove on the inner surface of the heat transfer tube far from the injection point of the injection device, the coil The spray is trapped at the location of the groove, and the spray does not flow immediately downstream, and the urea water spray stays in contact with the heat transfer surface. Since urea water spray is overwhelmingly easier to transfer heat when it is in contact with the heat transfer surface than when it is floating in the space, heat transfer through the heat transfer tube at a location far from the injection point is improved. When the heat transfer is improved, the thermal resistance from the heating element to the urea water spray decreases, and the temperature difference required to transfer the same amount of heat can be reduced. This prevents the temperature of the heating element from rising too much. When the temperature of the heating element is lowered, heat loss to the outside is reduced, so that the power consumption of the electric heater can be reduced.

  At the same time, the structure that actively retains the urea water spray is not provided at a location near the injection point of the injection device, so that a decrease in heat transfer surface temperature at a location close to the injection point can be prevented, and urea is deposited. Can be avoided.

  Further, by reducing the inner diameter of the heat transfer tube far from the injection point, the amount of spray collision per unit area increases, and heat transfer between the spray and the heat transfer surface is improved. As a result, the temperature of the heating element at a location far from the injection point is lowered, and heat loss to the outside can be reduced. At the same time, by increasing the inner diameter of the heat transfer tube near the injection point, the amount of spray collision per unit area can be reduced, the temperature of the heat transfer surface can be prevented from decreasing, and urea can be prevented from precipitating. .

  Also, in a place near the injection point, the heat transfer tube should be a straight tube parallel to the injection axis, so that the droplet that collided with the inner surface of the heat transfer tube bounces back and increases the distance until it collides with the next heat transfer surface. Thus, the collision amount of the entire spray per unit area can be reduced, the temperature drop near the injection point can be prevented, and urea can be prevented from precipitating. At the same time, by bending the heat transfer tube at a location far from the injection point, the spray sprayed along the injection axis in the center of the heat transfer tube will surely collide with the inner surface of the heat transfer tube. The temperature of the heating element is lowered, and heat loss to the outside can be reduced.

  In addition, by bringing the tip of the heat transfer tube closer to the injection point than the opening into which the diverted gas flows, the heat transfer surface closest to the injection point among the surfaces to which the spray reaches can be heated with a heating wire. This makes it easy to prevent a temperature drop at the heat transfer surface closest to the injection point and to prevent precipitation of urea.

  In addition, the center axis of the location where the diverted gas flows in is decentered with respect to the central axis of the heat transfer tube, and the flow of the diverted gas in the heat transfer tube is swirled, so that the movement of the spray is swirled. The spray is pressed against the inner surface of the heat transfer tube by centrifugal force. Thereby, the heat transfer between the spray and the heat transfer tube is improved, the temperature of the heating element is lowered, the heat loss to the outside is reduced, and the power consumption can be reduced.

  Moreover, even if one heating wire is disconnected by providing two heating wires that change the heat generation density according to the distance from the injection point, even if one heating wire breaks, While preventing the precipitation of urea, it is possible to suppress the power consumption and continue operating the electric heater.

  Exhaust treatment apparatuses according to first to fourth embodiments of the present invention will be described in detail below with reference to FIGS.

  An exhaust treatment apparatus according to a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a diagram showing an overall configuration of an exhaust treatment apparatus according to the first embodiment, in which a part of an exhaust outer wall is cut away in order to make the inlet of a diverted gas passage easier to see. FIG. 2 is a cross-sectional view in the present embodiment, and shows a relationship in a case where two methods are provided, that is, a method in which urea water is directly injected into exhaust gas and a method in which urea water is vaporized by an electric heater. FIG. 3 is a cross-sectional view in the present embodiment, and takes a cross section at a location where the structure of the heat transfer tube can be understood.

In FIG. 1, exhaust gas 201 discharged from a diesel engine is injected with urea water 202 or 203 and passes through a downstream denitration catalyst (not shown), so that nitrogen oxides (NOx) in the exhaust gas. ) Is reduced. In diesel engine exhaust,
Since there is a problem that a large amount of NOx and particulate matter (PM) are contained, both of them may be reduced in the exhaust aftertreatment system. As an example of the aftertreatment system in that case, exhaust gas is purified by providing an oxidation catalyst, a filter, a urea injection device, a denitration catalyst, and a slip ammonia treatment catalyst in this order from the upstream. If there is a filter upstream of the urea injection device, soot in the exhaust gas is removed by the filter, so that it is possible to reduce soot from adhering to the urea injection device. Generally, since passing a filter is at a large pressure loss, in order to reduce this pressure loss, it is better to make the diameter of the exhaust pipe where the filter is installed larger than the normal exhaust pipe portion. This is a case where a urea injection device is installed immediately after the location where the diameter is increased, and a method in which urea water is directly injected into the exhaust and a method in which urea water is vaporized by an electric heater and then injected. In the case of an apparatus having two, the structure of the embodiment of the present invention is effective.

  FIG. 2 is a cross-sectional view in the direction indicated by the arrows AA in FIG. 1, and illustrates a method of injecting urea water directly into the exhaust gas by including a cross section of the urea water injection device 5. The urea water spray 206 is generated by injecting the urea water 202 from the urea water injection device 5 attached to the exhaust pipe 1, and this spray has the swirl blade 11 together with the exhaust gas, or the downstream exhaust pipe 3. By passing through this section, the exhaust gas 205 is sufficiently mixed with urea and is sent to the denitration catalyst. Further, the component 12 is for attaching the injection device 5 to the exhaust pipe 1. However, since the exhaust pipe 1 may become extremely high in some cases, a cooling medium such as a cooling urea solution is used as the attachment component 12. When the cooling is performed by circulating the air, it is possible to protect the injection device 5 that is an electrical component.

  Next, an injection device of the type in which urea water is vaporized by an electric heater and then injected will be described. This type of injection device includes an electric heater, an injection device 6 and a hydrolysis catalyst 10. A heat transfer tube 8 and a heating wire 9 in FIG. 1 constitute an electric heater in the present embodiment, and vaporize urea water of the urea water 203 injected from the injection device 6. For this purpose, a heat transfer surface in contact with the urea water is formed on the inner peripheral surface of the heat transfer tube 8. The heat transfer tube is often supplied with a urea water spray and a gas transporting it, and a diverted gas obtained by diverting a part of the exhaust gas is taken in. In order to show the intake port 103 for the diverted gas, a part of the exhaust pipe 1 is notched in FIG. 1 but is not notched in an actual product. A diverted exhaust introduction part 7 is connected to the inlet 103 of the diverted exhaust. At the same time, the urea water injection device 6 is attached to this part via an attachment part 13. The attachment component 13 can protect the injection device 6 by circulating the cooling medium in the same manner as the component 12.

  FIG. 3 is a cross-sectional view in the direction indicated by the arrow BB in FIG. 2, and shows a portion where the urea water is vaporized to cause a hydrolysis reaction. A part of the exhaust gas is introduced by the component 7 from the diversion exhaust inlet 103 (not shown because it is in front of the cross section, but the location is indicated by a phantom line), and at the same time, urea water injected from the injection device 6 The spray 207 is sent to the heat transfer tube 8. The component 7 forms a chamber into which the urea water spray 207 is injected from the injection device 6 and a part of the exhaust gas is introduced from the diverted exhaust inlet 103. The urea in the vaporized state here passes through the hydrolysis catalyst 10, whereby the hydrolysis reaction proceeds and ammonia gas is generated. The shunt gas 204 mixed with the ammonia gas fills the space between the outer wall 2 and the inner wall 3 and is discharged from the opening 104 provided near the blades of the fixed swirl vane 11 and mixed with the mainstream exhaust gas. And sent to the denitration catalyst.

  The heat transfer tube 8 is connected to the component 7 via the flange 14. The flange 14 is formed with a recess that is recessed from the connection side of the heat transfer tube 8 toward the injection device 6, and the heat transfer tube 8 is connected to the bottom of the recess. As a result, the connecting portion (inlet of the heat transfer tube 8) between the flange 14 and the heat transfer tube 8 is brought close to the injection point, so that the spray surely enters the heat transfer tube 8. The droplets constituting the urea water spray 207 spread radially due to the inertia at the time of injection. However, when the flange 14 has a flat plate shape, the inlet of the heat transfer tube 8 becomes far from the injection point which is the apex of the spray 207. The spray that protrudes from the heat transfer tube 8 and scatters easily occurs. On the other hand, if the attachment part of the flange 14 and the component 7 is brought close to the injection point, it interferes with the inlet 103 of the diverted gas. For this reason, only the inlet of the heat transfer tube 8 is brought close to the injection point, thereby preventing the spray from being scattered and increasing the vaporization efficiency of the spray. At the same time, when the inlet of the heat transfer tube 8 approaches the injection point, the heating wire 9 can also approach the injection point, and the amount of heat generated near the injection point can be increased. Further, by forming the flange 14 as a thin plate, the flange 14 is elastically deformed in response to thermal expansion generated when the heat transfer tube 8 is heated and the temperature rises, and has a function of alleviating the generation of thermal stress.

  The heat transfer tube 8 starts with a straight tube that is coaxial with the spray axis of the spray 207, but the diverted gas flowing into the heat transfer tube 8 is guided perpendicularly to the paper surface through the inlet 103, but the center of the inlet 103 is the center of the heat transfer tube 8. It is not orthogonal to the central axis. In such a relationship, the flow passing through the inlet 103 has an angular momentum when flowing through the heat transfer tube 8, and the gas flow in the heat transfer tube 8 can be swirled. When the gas flow is swirled, the spray affected by the flow is swirled, and the spray is pressed against the inner surface of the heat transfer tube 8 by the centrifugal force. When the spray is pressed toward the heat transfer surface and comes into contact, the heat is easily transferred and the vaporization performance is improved.

  A single sheath-type heating wire 9 is wound around the heat transfer tube 8, and the cross-sectional structure of the heating wire 9 is such that an appropriate insulating material is used after filling the inside of the surface sheath with an electrical insulating material. When a resistance conductive wire exists and current flows through the conductive wire, heat is generated in proportion to the product of the two currents and the electrical resistance. Further, since the electric wire for supplying current to the heating wire and the terminal portion 101 for connecting the heating wire are provided on only one side, it is possible to provide two conductive wires for generating heat in the cross section of the sheath. . In addition, one conductive wire in the sheath is used, and one end of the conductive wire is electrically short-circuited by coming into contact with the sheath. 101 can be only on one side.

  The pitch for winding the heating wire 9 around the heat transfer tube 8 is most honey at the upstream side of the heat transfer tube 8, and when the heat generation amount per unit length of the heating wire is constant, the pitch is wound as honey. However, the calorific value per unit surface area of the heat transfer tube increases. In the case of the present embodiment, the broken line representing the spray 207 in FIG. 3 is the inner surface of the heat transfer tube 8 as the location where the spray injected from one injection point directly reaches and the spray collides with the heat transfer tube. It is useful for preventing precipitation of urea by having the highest collision density at the intersection and making the winding pitch of the heating wire at this place the most nectar. When vaporizing a liquid consisting of a single component such as water, vaporization occurs depending on the applied heat, so it is desirable to use a uniform heat flux because the entire heat transfer surface can be used effectively. The urea water has a special problem that precipitation occurs if the vaporization process is poor. For this reason, the amount of heat generated per unit area of the heat transfer surface, which is the inner surface of the heat transfer tube 8, is made non-uniform so that a portion with a high heat flux of heat flowing in the thickness direction of the heat transfer tube is formed, and the temperature necessary for heat conduction Even if the difference is increased, it is effective to give priority to the prevention of precipitation. When the collision density of the spray that collides with the heat transfer surface is high, the temperature tends to decrease because the vaporization heat is taken away from the heat transfer surface with the collision of the spray. Increasing the heat generation density of the heating wire in such a place helps increase the temperature of the heat transfer surface, and if the temperature can be maintained above a certain level, the form of heat transfer between the droplet and the solid surface will cause film boiling. It is possible to prevent the urea water from becoming a liquid film and adhering to the heat transfer surface, thereby producing an effect of preventing the precipitation. Further, since there is a harmful effect on increasing the heat generation density, in addition to the heat generation density, in order to reduce the collision amount per unit area of the spray that collides with the heat transfer tube near the injection point, the inner diameter of the heat transfer tube 8 is set downstream. It is thicker than the side. Changing the inner diameter of the heat transfer tube and increasing the diameter near the injection point is useful for maintaining the heat transfer surface temperature above the film boiling temperature, and has the effect of preventing urea precipitation.

  The heat transfer tube 8 is tapered after the straight tube portion to reduce the diameter, so that it becomes possible to increase again the collision density where the spray directly colliding from the injection point decreases away from the injection point. . This is because the inner surface of the taper faces the injection point, and the heat transfer performance is improved because the spray easily hits the heat transfer surface.

  The heat transfer tube 8 is tapered so that the inner diameter becomes smaller after that. However, when spraying is caused by film boiling on the heat transfer surface, the droplets repelled from the heat transfer surface face each other. Colliding with the heat transfer surface, the small tube diameter means that the heat transfer surfaces facing each other are close, and the droplets tend to collide with the heat transfer surface frequently, improving heat transfer performance. To do.

  Furthermore, by providing the heat transfer tube 8 with a bent portion, the liquid droplets that have passed through the center of the straight tube portion also collide with the inner wall surface at the bent portion, and may contact the heat transfer surface once. It can help to improve the vaporization rate of the spray. Further, by providing a bend away from the injection point, it compensates for a decrease in spray collision, and lowers the heat transfer resistance for transferring heat from the heating wire 9 to the spray, thereby suppressing the temperature rise of the heating wire 9. Can be connected to reduce heat loss. Moreover, the temperature reduction of the heating wire 9 is achieved by setting the winding pitch of the heating wire 9 to a larger pitch than near the injection point. At the same time, the bend is included in the heat transfer tube, thereby reducing the thermal stress and increasing the durability of the heat transfer tube.

  The heat transfer tube 8 is connected to the flue member 2 through the outlet flange 15 and promotes the hydrolysis reaction of urea by passing a gas through the hydrolysis catalyst 10 disposed there. If the chemical reaction in which urea is decomposed is described in more detail, it can be divided into the following two-stage reactions.

(NH ₂ ) ₂ CO → HNCO + NH ₃
HNCO + H ₂ O → NH ₃ + CO ₂
That is, urea is once decomposed into isocyanic acid (HNCO) and ammonia (NH ₃ ), and further, isocyanic acid (HNCO) reacts with water (H ₂ O) to form the remaining ammonia and carbon dioxide. It is. Among these reactions, the first half is a thermal decomposition reaction, and the second half is a hydrolysis reaction. The pyrolysis reaction is an endothermic reaction and can occur by heating urea. On the other hand, the hydrolysis reaction is an exothermic reaction, and it is difficult for the reaction to proceed simply by raising the temperature, and the presence of the catalyst greatly improves the reaction rate. When performing the thermal decomposition reaction and the hydrolysis reaction together on the hydrolysis catalyst, the heat generated by the exothermic reaction can be passed to the endothermic reaction, which is efficient. I need. In the form of FIG. 3, the hydrolysis catalyst 10 is not heated. However, if the thermal decomposition reaction of urea is caused in the heat transfer tube 8, only an exothermic reaction is caused on the catalyst. The reaction is accelerated even if not performed.

  The hydrolysis catalyst 10 includes a method in which a catalyst is applied to a ceramic honeycomb and a method in which a catalyst is applied to a metal sheet that has been finely bent to increase the surface area. Ceramics have the advantage that a structure with an increased surface area is easy to make, and metal catalysts have the advantage that they are not easily broken even when subjected to external forces such as vibration. In any structure, since it is necessary to increase the surface area, a pressure loss occurs when the shunt gas flows. When this pressure loss is large, the flow rate of the diverted gas decreases. If the volume of the diverted gas flow path is determined, if the flow rate of the gas flowing through it decreases, it will take a long time to enter and exit, and until the urea injected there comes out as ammonia It will take time at the same time. If this time delay is large, controllability of ammonia is controlled when the amount of ammonia added is controlled according to changes in exhaust gas. For this reason, it is effective to reduce the pressure loss by increasing the cross-sectional area of the hydrolysis catalyst. For this reason, although the internal diameter of the heat exchanger tube 8 in front is small, the diameter of the hydrolysis catalyst 10 is larger than that.

  Ammonia is produced from urea by passing through the hydrolysis catalyst, and the gas 204 in a state where the ammonia and the split exhaust gas are mixed exits from the opening 104 near the fixed swirl vane 11, and enters the inner exhaust passage 4. Mix with the mainstream exhaust gas flowing through. The fixed swirl 11 itself is stationary, but when gas passes through it, swirl occurs in the gas flow. In the present embodiment, the swirl vane 11 is composed of eight blades. When the gas flows around the blades, the flow is bent depending on the angle of the blades, and the collected flow becomes a swirl flow. However, a separation vortex is generated around each blade because the flow cannot be bent. . The opening 104 which is the outlet of the diverted gas is preferably provided at the place where the separation vortex is generated, and by the effect of the separation vortex, the mainstream exhaust gas and the divided gas containing ammonia can be mixed well in a short section. For this reason, it is preferable to provide the opening part 104 for every blade | wing of the turning blade 11, and if it is an 8-blade, it is preferable to provide 8 places.

  In addition, if the concept of the present invention is applied, it is possible to inject urea water into a heat transfer tube having the same shape without introducing a diverted gas, to vaporize the urea water, and to mix it with exhaust gas. .

  Also, as a means of changing the amount of heat generated per unit area of the heat transfer surface, the electric resistance per unit length of the conductive wire that generates heat is made different depending on the location, instead of changing the winding pitch of the heating wire. But it is feasible. In this case, since the flowing current is the same between the continuous heating wires, heat generation in proportion to the electric resistance can be obtained. That is, the heating wire near the injection point has a smaller electrical resistance per unit length by reducing the diameter of the conductive wire inside the heating wire, and the amount of heat generated per unit length increases. When the heating wire configured as described above is wound around the heat transfer tube at an equal pitch, the heat generation amount per unit heat transfer surface can be increased near the injection point.

  In addition, it is possible to reduce heat loss by installing separate heaters at locations near and far from the injection point so that the heat generation per heat transfer area of the heater far from the injection point is reduced. Is possible.

  An exhaust treatment apparatus according to a second embodiment of the present invention will be described with reference to FIG. FIG. 4 shows only the portion related to the electric heater in the urea water injector, and is a diagram at the same cross-sectional position as FIG. Also, parts having the same function as in the first embodiment are distinguished from the first embodiment by adding the same number to the sign, followed by the letter b, but the description is omitted because the function is the same. .

  In the present embodiment, two heating wires 16 and 17 are used, and the pitch of each heating wire is niche near the injection point and the winding pitch is increased at a location far from the injection point. With this configuration, the heating wires 16 and 17 usually share and consume half of the required power, generate the necessary heat, and if one of the heating wires breaks, The heating wire consumes all the necessary power and generates the necessary heat. With such a configuration, even when a trouble that one heating wire is disconnected occurs, it can serve as an electric heater without impairing the ability to prevent deposition and reduce power consumption.

  Furthermore, in the present embodiment, a spiral coil 18 is inserted inside the heat transfer tube 19. The coil 18 has the same effect as providing a projection on the inner surface of the heat transfer tube 19, and the droplet that collides with the coil 18 has the effect of jumping downstream and making it difficult to stay in place. If the droplet stays in place, even if it is film boiling, the number of times of contact with the heat transfer surface increases, so that heat is transmitted well and the evaporation rate is improved. At the same time, the temperature of the heating wires 16 and 17 is also lowered, so that heat loss can be reduced. For this reason, in order to effectively utilize the effect of the coil 18, a linear portion is provided after the tapered portion where the diameter of the heat transfer tube 19 is reduced, and the coil 18 is disposed on the inner surface from the linear portion. If this coil is provided near the injection point, urea may be deposited due to excessive spray retention, so it is preferable that the coil be installed from a location away from the injection point. Further, the diverted gas flowing in the heat transfer tube 19 flows in from a position eccentric to the left side from the central axis, and therefore, when flowing through the vertical portion of the heat transfer tube 19, it is swung in the direction of the right screw. For this reason, it is preferable that the coil 18 is also twisted in the right-handed direction, and the flow along the coil 18 enhances the swirl, thereby increasing the centrifugal force applied to the spray and increasing the contact between the droplet and the heat transfer surface. Leads to. The swirling flow generated in the diverted gas is weakened by passing through the bent portion. However, since the coil 18 is still present, the function of retaining the spray is generated, so the coil 18 extends from the straight portion of the heat transfer tube 19 to the bent portion. It is preferable to install.

  The diverted gas and spray after passing through the heat transfer tube 19 are sent to the hydrolysis catalyst 10b. However, since the hydrolysis catalyst preferably has a larger diameter to reduce pressure loss, in this embodiment, the heat transfer tube Finally, a tube expansion part is provided. Moreover, in order to vaporize the spray which could not be vaporized by the time it reaches the hydrolysis catalyst 10b, the heating wires 16 and 17 are wound to the outside of the pipe containing the hydrolysis catalyst so that all the sprays can be vaporized. I have to. When heating is also performed from the outside of the hydrolysis catalyst, it is preferable to configure a catalyst using a metal as a carrier in order to improve heat transfer characteristics.

  In addition, by providing a spiral groove on the inner surface of the heat transfer tube 19 instead of providing the coil 18, it is possible to retain the spray and improve the heat transfer performance.

  An exhaust treatment apparatus according to a third embodiment of the present invention will be described with reference to FIGS. FIG. 5 is a front view of the structure of the exhaust treatment apparatus according to the third embodiment, in which only the portion that vaporizes urea water is taken up. FIG. 6 is a cross-sectional view taken along the line AA in FIG. 5, and FIG. 7 is an external view showing only the components around the heating wire. In addition, as in the past, parts having the same function are identified by the same reference numeral, followed by c.

  In FIG. 5, the split exhaust gas flows in from the inlet 103 c provided in the part 7 c, receives urea water injection and vaporization, passes through the hydrolysis catalyst 10 c, and then mixes with mainstream exhaust gas (not shown). Perform denitration treatment. In the present embodiment, the heating wire 9c is arranged inside the outer tube 23 that forms the branch passage. The inlet side of the outer tube 23 is joined to the flange 14c, and this flange 14c is connected to the component 7c. Further, the outlet side of the outer tube 23 is joined to a flange 21, and this flange 21 is connected to the flange 22 of the tube 24 holding the hydrolysis catalyst. Further, the heating wire 9c is sandwiched between the flanges 21 and 22 so as to be connected from the outside to the inside of the pipe line while being sealed, and is connected to the electric wire for supplying power at the terminal portion 101c.

  The cross-sectional view of FIG. 6 shows the inside of the outer tube 23, and the outer tube 23 is provided with a heating wire 9 c, an inner cylinder 19, and a terminal plate 20 of the inner cylinder. The heating wire 9c has a structure wound around the inner cylinder 19, and is wound around the pitch near the injection point and with a large pitch near the injection point. Further, the inlet side of the outer tube 23 extends ahead of the flange 14c and serves as a guide vane that imparts a swirl to the flow of the diverted gas at the front end portion 105. There is a urea water injection device (not shown) at the upper part of the component 7 c, from which urea water is injected to form a spray 207.

The inner cylinder 19 and the inner cylinder end plate 20 are constituted by a perforated plate, and a part of the spray colliding with the inner cylinder 19 can pass through. In this way, in addition to being able to use the inner surface of the inner cylinder 19 as a heat transfer surface, the surface of the heating wire 9c, the inner surface of the outer tube 23, and the outer surface of the inner tube 19 are also used as heat transfer surfaces. It will be usable. For this reason, a large heat transfer area can be obtained in a short flow path section, and the heat transfer performance can be dramatically improved. At the same time, the heating wire is adjusted by adjusting the pitch of 9c according to the spray collision density, so that the heating wire itself has a constant calorific value per unit length, while preventing precipitation of urea and preventing overheating of the heating wire. Is planned. In addition, since the heating wire 9c is on the inner side of the heat transfer surface of the outer tube 23, it is not necessary to heat the outer surface of the outer tube 23, the temperature of the outer surface can be lowered, and heat loss can be reduced. Figured. Moreover, increasing the heat transfer area by the inner cylinder 19 formed of a perforated plate also leads to a reduction in the outer area where heat loss occurs. Furthermore, compared with the case where the spray 207 directly collides with the heating wire 9c, the heat load applied to the heating wire 9c is reduced when the inner cylinder 19 is present. That is, at the local surface of the heating wire 9c, there is a time when the droplet collides and the other heat transfer surface is dry. At the moment when the droplet collides, the surface temperature of the heating wire 9c is instantaneously lowered due to the heat being taken away, and when it passes, the temperature rises again. For this reason, a temperature change is large, and repeated thermal stress is generated. Although this becomes a factor which reduces the durability of the heating wire 9c, the durability of the heating wire 9c is improved by reducing the spray in which the inner cylinder 19 directly collides with the heating wire 9c. Further, the inner cylinder end plate 20 is a perforated plate, so that the shunt gas can pass therethrough, and at the same time, the spray at the center of the inner cylinder 19 can collide to improve the vaporization rate.

  The pipe 24 holding the hydrolysis catalyst 10c has the same outer diameter as that of the outer pipe 23 so as to share parts, but the cross-sectional area through which the fluid can pass is wider than the inner side of the inner cylinder 19. This is because the thickness of the inner cylinder 19 and the thickness of the heating wire 9c can also be used for gas circulation, thereby reducing the pressure loss in the hydrolysis catalyst and increasing the surface area of the catalyst. Therefore, sufficient ammonia is produced.

  FIG. 7 is an external view in which only parts around the heating wire are extracted, but the shape of the distal end portion 105 of the outer tube 23 is shown particularly clearly. The distal end portion 105 is provided with four notches in the outer tube 23, and these notches are eccentric with respect to the central axis of the tube. For this reason, the diverted gas flowing in through the notch flows eccentrically with respect to the center, and this creates a swirling flow of gas. Further, the turning direction can be synergistically caused by matching with the turning caused by the eccentricity of the inlet portion 103c shown in FIG.

An exhaust treatment apparatus according to a fourth embodiment of the present invention will be described with reference to FIGS. FIG. 8 is a front view of the structure of the exhaust treatment apparatus according to the fourth embodiment, in which only the portion that vaporizes urea water is taken up. 9 represents a cross-sectional view taken along the line B-B in FIG. 8, and FIG. 10 is an external view showing only parts around the heating wire. In addition, as in the past, parts having the same function are identified by the same reference numeral, followed by d.

  In FIG. 8, the diverted exhaust gas flows from the inlet 103 d provided in the component 7 d, is subjected to urea water injection / vaporization, passes through a hydrolysis catalyst (not shown) after the pipe 27, and then illustrated. Denitration treatment is performed by mixing with the mainstream exhaust gas.

  The cross-sectional view of FIG. 9 shows the internal structure, in which there is a tapered inner cylinder 25 inside the outer tube 23d, and the heating wire 9d is wound around it. Between the heating wire 9d and the outer tube 23d, there are four plates 29 for fixing the pitch of the heating wires, every 90 degrees. An end plate 26 is provided at the end of the inner cylinder 25.

  10, the relationship between the heating wire 9d, the tapered inner cylinder 25, and the pitch fixing plate 29 is shown in an external view.

In the present embodiment, since the inner cylinder 25 has a tapered shape whose diameter decreases as it goes first, spraying is easier to hit even on a surface far from the injection point. The inner cylinder 25 and the end plate 26 are composed of perforated plates, and the surface of the heating wire 9d and the inner surface of the outer tube 23d can be used as a heat transfer surface. Further, since the inner cylinder 25 is tapered and the outer tube 23d is a straight tube, a gap is generated between the heating wire 9d and the outer tube 23d, but the pitch fixing plate 29 is used to heat the heating wire 9d. Since it also serves to transmit to the inner surface of the outer tube 23d, the outer tube 23d can sufficiently serve as a heat transfer surface, and at the same time, precipitation of urea can be avoided.

  Further, by using the gasket 30 for the flanges 21d and 28 for drawing the heating wire 9d to the outside, the sealing performance is improved and the shunt gas does not leak outside.

  The gas that has passed through the heater section passes through the bent pipe 27 connected to the flange 28 and is sent to a hydrolysis catalyst (not shown). However, by utilizing the fact that the inner cylinder 25 is tapered, The bent tube 27 has a smaller diameter than the tube 23d. A bent tube with a small diameter is easier to reduce the bending radius, which helps to reduce the size of the equipment. For this reason, if the tube diameter in the bending passage is reduced, the size can be reduced, and if the passage diameter is increased thereafter, the pressure loss does not need to increase. Further, since the pipe line 27 is narrowed, it is difficult for the shunt gas to pass through the inner cylinder 25 and then flow into the pipe line 27. Thereby, most of the diverted gas flows inside the inner cylinder 25. In this case, the flow velocity of the diverted gas increases as the diameter of the inner cylinder 25 becomes tapered, and the swirling speed increases if swirling occurs at the inlet. This also increases the centrifugal force against the spray, so the force that presses the spray against the heat transfer surface is increased, and it is possible to have high heat transfer performance while adopting the heat transfer form of film boiling. It is possible to improve performance and reduce power consumption.

1 is a diagram illustrating an overall configuration of an exhaust treatment apparatus according to a first embodiment of the present invention. It is AA arrow sectional drawing of the exhaust-gas treatment apparatus which concerns on the 1st Embodiment of this invention. FIG. 3 is a cross-sectional view of the exhaust treatment device according to the first embodiment of the present invention as viewed in the direction of arrows BB in FIG. 2. It is sectional drawing of the location similar to FIG. 3 of the urea water vaporization part which concerns on the 2nd Embodiment of this invention. It is a front view of the urea water vaporizer which concerns on the 3rd Embodiment of this invention. It is AA arrow sectional drawing in FIG. 5 of the urea water vaporizer which concerns on the 3rd Embodiment of this invention. It is a bird's-eye view of only the components around a heating wire among urea water vaporizers concerning a 3rd embodiment of the present invention. It is a front view of the urea water vaporizer which concerns on the 4th Embodiment of this invention. It is BB arrow sectional drawing in FIG. 8 of the urea water vaporizer which concerns on the 4th Embodiment of this invention. It is a bird's-eye view of only the components around a heating wire among urea water vaporizers concerning a 4th embodiment of the present invention.

Explanation of symbols

1, 2, 3 Exhaust pipe 4 Exhaust inner flow path 5, 6 Urea water injector 7 Divided exhaust introduction part 8, 19 Heat transfer pipe 9, 16, 17 Heating wire 10 Hydrolysis catalyst 11 Swirling blades 12, 13 Injector attachment part 14 Heat transfer tube inlet flanges 15 and 21 Heat transfer tube outlet flange 18 Coil 25 Inner tube 20 and 26 Inner tube end plate 22 Hydrolysis catalyst holding portion flange 23 Split flow passage outer tube 24 Hydrolysis catalyst hold portion 27 Split gas bending passage 28 Split flow Gas bending passage flange 29 Heating wire pitch fixing plate 30 Gasket 101 Heating wire terminal connection portion 103 Split exhaust inlet 104 Split exhaust outlet 105 Split flow swirl generator 201, 205 Exhaust gas 202, 203 Urea water 204 Split gas 206 containing ammonia 207 Urea water spray

Claims (1)

An injection device that injects urea water in a spray form; and an electric heater that has a heat transfer surface that contacts the urea water injected from the injection device and heats and vaporizes the urea water. In the urea water injection device for the exhaust treatment device that injects the ammonia component generated from the urea water,
The amount of heat generated per unit area of the heat transfer surface of the electric heater is lower at a place far from the injection point of the injection device than at a close place ,
The electric heater is configured to heat urea water spray through a heat transfer tube in which the heat transfer surface is formed, and to distribute a part of the exhaust gas to the heat transfer tube,
A portion of the exhaust gas flows into the inlet of the heat transfer tube and a chamber in which urea water spray is injected from the injection device;
The urea water for an exhaust treatment apparatus , wherein the inlet portion of the heat transfer tube is provided in a recess recessed toward the interior of the chamber, thereby bringing the inlet portion of the heat transfer tube closer to the injection point of the urea water spray. Injection device.

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