CN108729997B

CN108729997B - Reactant exhaust mixing device of engine exhaust SCR system

Info

Publication number: CN108729997B
Application number: CN201710243008.4A
Authority: CN
Inventors: 杨延相; 张平; 郑远师; 李伟; 谈秉乾
Original assignee: Wuxi Henghe Environmental Protection Technology Co Ltd; ZHEJIANG FUAI ELECTRONICS Co Ltd
Current assignee: Wuxi Henghe Environmental Protection Technology Co Ltd; ZHEJIANG FUAI ELECTRONICS Co Ltd
Priority date: 2017-04-14
Filing date: 2017-04-14
Publication date: 2020-01-31
Anticipated expiration: 2037-04-14
Also published as: CN108729997A

Abstract

A reactant exhaust mixing device of an SCR system comprises reactant nozzles, exhaust mixing pipes, reflecting plates and mixers, and is characterized in that the exhaust mixing pipes are straight pipes, the mixers and the reflecting plates are positioned in the exhaust mixing pipes, the mixers and the exhaust pipes are coaxially installed and fixed, the reflecting plates are positioned at the upstream of the mixers and are approximately parallel to the axis of the exhaust pipes, nozzle installation surfaces are arranged on the exhaust mixing pipes and at the upstream of the mixers, oblique angles are formed between the nozzle installation surfaces and the axis of the exhaust mixing pipes, the reactant nozzles are installed on the nozzle installation surfaces, the jet direction of ejected reactants forms an oblique angle with the flow direction of exhaust in the exhaust mixing pipes, and conical divergent jets are sprayed to the mixers and the reflecting plates.

Description

Reactant exhaust mixing device of engine exhaust SCR system

Technical Field

The invention belongs to the technical field of engine exhaust aftertreatment, and particularly relates to a reducing agent and engine exhaust mixing device for an engine exhaust selective reduction (SCR) technology.

Background

With the increasing prominence of environmental issues, energy conservation and emission reduction have become the ever-increasing requirements for vehicles and engines, and for this reason, vehicles with internal combustion engine as the power source have come to the series vehicle emission standards and are becoming more and more stringent.

The SCR technology requires that a reductant be quantitatively injected into the exhaust gas of a diesel engine, mixed with the exhaust gas, and then introduced into an SCR catalytic converter. The reducing agent may be a 32.5 wt% urea aqueous solution (also called Diesel exhaust fluid DEF or additive blue), or ammonia gas. The ammonia gas generated by high-temperature decomposition of DEF in exhaust gas or the gaseous ammonia gas directly injected enters the SCR catalyst after being mixed with the engine exhaust gas, and the ammonia gas and NOx in the engine exhaust gas are subjected to catalytic reduction reaction under the action of the catalyst, so that the NOx is decomposed into harmless N2 and H2O. If the reducing reagent is not uniformly mixed with the exhaust gas, the ratio of ammonia to NOx (ammonia-nitrogen ratio) in the SCR catalyst is not uniform, so that a part of the catalyst unit may have excessive ammonia leaking into the exhaust gas, and a part of the catalyst unit may have difficulty in effectively degrading NOx due to lack of ammonia.

In another aspect, if the DEF is not rapidly pyrolyzed into ammonia and mixed with the exhaust gas uniformly after being injected into the engine exhaust pipe, it may become solid crystals remaining in the exhaust pipe due to other physicochemical changes, and accumulate over a long period of time to block the engine exhaust pipe, thereby seriously deteriorating the engine performance.

The crystallization of DEF in the exhaust pipe is affected by many factors, the most significant being temperature and two-phase flow velocity. The temperature determines the chemical reaction product, and the flow rate determines whether solid crystals in the product can be accumulated in the pipeline.

The existing SCR technology is to try to mix the sprayed reactant with the engine exhaust gas uniformly, so that a gas-assisted SCR system for spraying DEF by using compressed air in an atomizing mode, a non-gas-assisted SCR system for spraying DEF by means of pressure, and a solid ammonia SCR system for directly spraying ammonia gas exist.

Disclosure of Invention

The present invention addresses the above-mentioned problems, and aims to provide SCR exhaust mixing devices that can reduce the risk of crystallization in the exhaust pipe of an engine, improve the uniformity of mixing of a reactant and exhaust, and reduce the cost of an SCR system.

In order to achieve the purpose, the invention adopts the following technical scheme:

According to the above technical solution, although the reactant jet can only be obliquely sprayed to the mixer, and because the reactant jet may have a large diffusion cone angle, is just that the upper half of the reactant can directly reach the mixer, but the reactant in the lower half of the reactant can collide with the reflection plate first and then rebound after reflection to reach the mixer, thereby preventing the reactant in the lower half of the reactant from being excessively concentrated on the lower wall surface of the mixing tube.

When the DEF is heated by the engine exhaust gas or the exhaust pipe wall surface, water is lost first to analyze urea crystal particles, but if the temperature is sufficiently high, that is, higher than the vaporization temperature of urea, the urea crystal particles do not appear and become gaseous urea directly. The high temperature urea reacts with the water vapor and is pyrolyzed into ammonia and carbon dioxide. Meanwhile, urea also undergoes other chemical reactions to generate crystals such as biuret, cyanuric acid, melamine and the like, but the reaction speed is slower than that of urea pyrolysis into ammonia gas, and the crystals cannot be accumulated when DEF does not contact with the solid wall surface at a lower temperature. When the flow velocity of the two-phase flow is large enough, even if formed, the crystal can flow away with the exhaust gas and can not accumulate.

Therefore, according to the technical scheme of the invention, the reflecting plate with higher temperature in the middle of exhaust prevents the reactant from being directly sprayed onto the pipe wall of the mixing pipe, and meanwhile, the reactant which reaches the mixer after being reflected is relatively uniformly distributed on the mixer, and then is mixed by the mixer, so that the exhaust reactant in the exhaust mixing pipe at the downstream of the mixer can be uniformly mixed.

The following technical solution further defines or optimizes the present invention.

The central line of the jet flow of the nozzle is intersected with the central line of the exhaust mixing pipe, the reflecting plate and the mixer are divided into two parts which are bilaterally symmetrical by a formed plane, the intersection point of the central point of the nozzle orifice and the virtual extension line of the rear edge connecting line of the reflecting plate and the mixer is positioned at the lowest part of the mixer, so that the reactant sprayed by the nozzle cannot be directly sprayed to the wall surface at the bottom of the mixing pipe, all the reactant is divided into two parts, is mostly directly sprayed to the mixer, and is sprayed to the reflecting plate and then reaches the mixer after being reflected.

, the nozzle is installed near the intersection of the installation surface and the exhaust mixing pipe, and the spray cone formed by the nozzle spray is completely free and not blocked by any solid wall surface before reaching the mixer or the reflecting plate, which is to prevent the nozzle spray from spraying to other wall surfaces, especially the wall surfaces with lower temperature, such as the surrounding wall surface of the pit where the installation surface of the nozzle is located, so as to avoid unnecessary concentrated distribution or crystallization of the reactant.

In order to prevent stagnant areas from occurring on the back of the baffle plate, which can lead to crystal accumulation, it is preferable to bend at an angle of not more than 15 ° to the lower side (in the direction of the mixing tube wall on the side opposite of the nozzle) of the baffle plate or the second half of the baffle plate that effectively receives the reactant spray (the exhaust gas flow outlet end).

In order to reduce the phenomenon that the jet flow on the reflecting plate is reflected to the side mixing pipe and then is gathered to the included angle of the wall surface with lower temperature, the reflecting section (the rear half section) of the reflecting plate is designed into a suspended tongue shape which is not contacted with any solid wall surface, so that the heat dissipation of the reflecting plate to the wall surface of the mixing pipe is reduced, the temperature of the reflecting plate is improved, the front half section of the reflecting plate is fixed with the exhaust mixing pipe by adopting a welding method and the like, the direction of the reflecting plate and the axis of the exhaust mixing pipe can form an angle within 5 degrees, the front end is higher and lower, the front end is closer to the side of the nozzle , namely the inlet end of the exhaust flow is higher, and the outlet end is lower.

The thickness of the reflector is not more than 3mm, and the rear end, or the rear end and the front end, is chamfered from the upper side, i.e., the side of the nozzle , to form a knife edge, which also facilitates the DEF liquid sprayed onto the reflector to completely separate from the reflector from the knife edge without spreading to the back side of the reflector due to capillary phenomenon to form crystals.

The invention has the beneficial effects that: when the straight cylinder type exhaust mixing pipe with simple structure and convenient arrangement is adopted, the crystallization risk in the exhaust pipe of the engine is reduced, the mixing uniformity of the reactant and the exhaust is improved, and the cost of the SCR system can be reduced.

Drawings

FIG. 1 is a two-dimensional schematic representation of an embodiment of the invention.

FIG. 2 is a two-dimensional schematic diagram of a second embodiment of the present invention.

Fig. 3 is a three-dimensional cross-sectional view of a third embodiment of the present invention.

Fig. 4 is a schematic view of embodiments of the reflective sheet of the present invention.

Detailed Description

The invention is further illustrated in the following description with reference to the figures and examples.

Referring to fig. 1, there are shown embodiments of the invention illustrating a segment mixing tube in series with the engine exhaust pipe in which the NOx reducing reagent (e.g., DEF) of the SCR system is mixed with the engine exhaust, including an exhaust mixing tube front segment 1, reagent nozzles 2, baffles 8, mixers 6, and a mixing tube rear segment 9.

The exhaust mixing pipe front section 1 and the rear section 9 are straight pipes arranged coaxially, the reflecting plate 8 is located near the pipe axis in the exhaust mixing pipe front section 1 and is fixed approximately parallel to the pipe axis, the mixer 6 is located at the junction of the mixing pipe front section 1 and the rear section 9 and is fixed in a coaxial installation mode with the mixing pipe, and therefore the reflecting plate 8 is located at the upstream of the mixer 6. The mixer 6 can make local exhaust flow direction generate great difference change, so that it can promote the exhaust gas downstream to make quick strong mixing, at the same time the smooth surface of mixer 6, higher temperature and higher surface flow rate are favorable for evaporation flow of liquid reactant, and its anti-crystallization capacity is strong. On exhaust mixing pipe anterior segment 1, be equipped with nozzle installation face 5, nozzle installation face 5 becomes the oblique angle with exhaust mixing pipe anterior segment 1 axis, reactant nozzle 2 installs on nozzle installation face 5, the reactant efflux direction that erupts becomes the oblique angle with the exhaust flow direction in exhaust mixing pipe anterior segment 1, reactant fluidic border is 11, spout to blender 6 and reflecting plate 8, the efflux that spouts to reflecting plate 8 forms reflection flow 12 after its reflection, also reach the middle part region of blender 6 at last, and can not gather in the bottom wall of exhaust mixing pipe anterior segment 1 then reentrant blender edge. The mixer therefore strongly promotes the mixing of almost all of the reactants with the exhaust gas, and the uniform distribution of the reactant exhaust gas inside the exhaust-gas mixing pipe rear section 9 is achieved more quickly.

The central line 4 of the jet flow of the nozzle is intersected with the central line of the front section 1 of the exhaust mixing pipe, and the reflecting plate 8 and the mixer 6 are divided into two parts which are symmetrical left and right by the formed plane. In the plane of symmetry of reflecting plate 8 and blender 6, because the nozzle is close to the handing-over position of nozzle installation face 5 and exhaust mixing pipe anterior segment 1, nozzle efflux front end space is wide, and installation angle guarantees that the reactant efflux can not touch the boundary of nozzle installation department pit 10, and the nozzle efflux is not blockked by any solid wall before reaching blender 6 or reflecting plate 8, just can not produce the crystallization deposit at the solid wall yet.

The virtual extension line 7 of the connecting line of the nozzle central point 3 and the back edge of the reflecting plate 8 and the mixer 6 are positioned at the lowest part of the mixer 6, so that the nozzle jet flow positioned above the extension line 7 is directly sprayed to the mixer 6, and the rest part of the nozzle jet flow is sprayed to the reflecting plate 8 and reaches the mixer 6 after being reflected. This almost completely avoids the nozzle reactant being directly sprayed onto the wall of the mixing tube where the temperature is relatively low and the exhaust gas flow rate is also low, while at the same time enabling the reactant to be distributed over as large an area as possible on the mixer. The crystallization risk of the liquid reactant on the wall surface of the exhaust pipe is favorably reduced.

Fig. 2a and 2b show a second embodiment of the present invention, in which the front half 13 of the baffle plate 8 is fixed to the exhaust mixing pipe 1, the direction of installation of the baffle plate 8 is at a small angle β within 5 ° with respect to the axis of the exhaust mixing pipe 1, the front half being higher and lower, the structure being such that the gas flow on the back of the baffle plate will not separate and create stagnant areas, thereby preventing the liquid DEF in the exhaust gas from crystallizing and growing and accumulating in these stagnant areas, the baffle plate 8 is fixed by its front half 13 to the wall of the exhaust mixing pipe 1, for example by welding, and its effective reflecting surface is its rear half 14, and in order to reduce the heat dissipation of the baffle plate 8 to the wall of the mixing pipe to increase the temperature of the effective part of the baffle plate 8, the sides of the rear half 14 are narrowed, and the rear half 14 of the baffle plate is in the shape of a suspended tongue not in contact with any solid wall, which forms a large gap 15 with the inner wall of the exhaust mixing pipe 1, which at the same time reduces the flow rate of the reactant reflected on the baffle plate to the rear wall of the mixing pipe, and thus preventing the reactant from collecting at a lower temperature.

Fig. 3 shows a third embodiment of the present invention, in which the reflective plate 8 is designed as a bending plate with small angle bends at the front and back, and the rear half 14 is bent at downward with an angle α of no more than 15 °.

Fig. 4 shows another kinds of structures of the reflecting plate, the thickness of the reflecting plate 8 is not more than 3mm, and in order to reduce the exhaust gas flow resistance and prevent the liquid on the reflecting plate 8 from flowing backward to the back of the reflecting plate 8 due to capillary action, both the front inlet 16 and the rear outlet 17 are chamfered from above into a knife edge.

The above examples are only for illustrating the essence of the present invention, but not for limiting the present invention. Any modifications, simplifications, or other alternatives made without departing from the principles of the invention are intended to be included within the scope of the invention.

The present invention is not concerned with parts which are the same as or can be implemented using prior art techniques.

Claims

A reactant exhaust mixing device of SCR system is composed of reactant nozzles, exhaust mixing tubes, reflecting plates and mixers, and is characterized in that the exhaust mixing tubes are straight tubes, the mixers and the reflecting plates are arranged in the exhaust mixing tubes, the mixers and the exhaust mixing tubes are coaxially and fixedly arranged, the reflecting plates are arranged at the upstream of the mixers and are approximately parallel to the axis of the exhaust mixing tubes, nozzle mounting surfaces are arranged on the exhaust mixing tubes and at the upstream of the mixers, the nozzle mounting surfaces form oblique angles with the axis of the exhaust mixing tubes, the reactant nozzles are mounted on the nozzle mounting surfaces, the direction of the reactant jet is oblique angles with the exhaust flowing direction in the exhaust mixing tubes, the divergent jet is sprayed to the conical mixers and the reflecting plates, the nozzle jet center line is intersected with the exhaust mixing tube center line, the formed plane divides the reflecting plates and the mixers into two parts which are symmetrical left and right, the nozzle jet center points and the virtual line of the reflecting plates are connected with the rear edge of the reflecting plates, the intersection point of the mixer is located at the lowest point of the mixer, so that the reactant jet completely sprays to the bottom of the mixer, and the two parts of the mixer directly spray nozzle, and the two parts of the mixer, wherein the two parts of the mixer are divided into two parts 46 parts.
2. The exhaust mixing device for SCR system of claim 1, wherein said nozzle is installed near the end of the intersection line between the installation surface and said exhaust mixing pipe, and the spray cone formed by the nozzle spray is completely free and not blocked by any solid wall surface before reaching the mixer or the reflector.
3. The exhaust mixing device for reactants of an SCR system of claim 2, wherein the rear half of said deflector plate effective to receive the reactant spray is angled downwardly at an angle of no more than 15 °.
4. The SCR system reactant exhaust mixing device of claim 3, wherein: the reflecting section of the reflecting plate is in a shape of a suspended tongue which is not contacted with any solid wall surface, so that the heat dissipation of the reflecting plate to the wall surface of the mixing pipe is reduced, the temperature of the reflecting plate is improved, and the front half section of the reflecting plate is fixed with the exhaust mixing pipe.
5. The exhaust mixing device of SCR system as recited in claim 4, wherein the direction of the first half of the reflector plate may be within 5 ° of the axis of the exhaust mixing pipe, and the front end is closer to the nozzle .
6. The exhaust mixing device for SCR system according to claim 4, wherein the reflector has a thickness of not more than 3mm and at least a rear end chamfered from the nozzle side to form a blade.