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Technical knowledge
Discussion on automobile exhaust gas treatment system
As a modern means of
transportation, automobiles bring convenience to people's production and
life, and at the same time, their exhaust gas has caused serious
pollution to the atmospheric environment. Therefore, we must strengthen
and raise awareness of environmental protection. According to the
formation mechanism of exhaust pollutants, a method to study the main
harmful components co, ch, nox and so2 in the exhaust gas is proposed.
This paper describes the two most mainstream exhaust gas aftertreatment
systems in the world today, and describes in detail the processing
methods of gasoline and diesel engines.
Three-way catalytic pollution of automobile exhaust purification
Preface
Although the car is the most important means of transportation in the 21st century, it has many drawbacks. Automobile exhaust pollution is environmental pollution caused by exhaust gas emitted by automobiles. It can be said that cars are a mobile source of pollution. In countries around the world, automobile pollution is no longer a new topic. Since the 1940s, photochemical smog incidents have occurred many times in cities such as Los Angeles, the United States, and Tokyo, Japan, where exhaust gas from internal combustion engines discharged from automobile exhaust pipes has caused many casualties and huge economic losses.
In the 21st century, automobile pollution has increasingly become a global problem. With the increasing number of cars and the wider range of use, their negative effects on the world environment are also increasing, especially harming the urban environment, causing respiratory diseases, causing excessively high levels of surface air ozone, and aggravating cities. The heat island effect makes the urban environment worse. According to statistics from relevant experts, by the beginning of the 21st century, automobile exhaust emissions accounted for 30-60% of air pollution. With the increase in motor vehicles, exhaust pollution has become more and more intense, changing from locality to continuous and cumulative, while citizens of various countries have become direct victims of automobile exhaust pollution.
1. The current mainstream exhaust gas aftertreatment system in the world
Due to the different characteristics of the exhaust gas of gasoline engine and diesel engine, the exhaust gas aftertreatment technology of the two is very different.
1.1 Technical route of gasoline engine exhaust gas aftertreatment
Gasoline engine exhaust emission characteristics: conventional pollutants are hc, co, nox, exhaust temperature sometimes exceeds 1000℃, high airspeed (30,000-100,000h-1), high water vapor (about 10%) concentration and extreme conditions of sox It has high activity and durability requirements of 100,000 kilometers, and requires low precious metals.
At present, the technical route of gasoline engine exhaust purification that meets Euro ⅳ and above emission standards is widely adopted in the world as close-coupled catalyst (ccc) + three-way catalyst (twc).
2) Three-way catalyst (twc):
After the close-coupled catalyst (ccc), it is generally installed in the lower part of the car chassis, with low precious metals, high performance and high temperature aging resistance (10-16 million km durability test).
1.2 Technical route of diesel engine exhaust gas aftertreatment
Diesel engine exhaust emission characteristics: emission pollutants are four kinds of co, hc, nox, pm; emission temperature is low (200-450℃), o2 is excessive, high airspeed (30,000-100,000h-1) and sox exists.
Since the content of hc and co is lower than that of gasoline engines, the focus of diesel engine exhaust purification is on the two components of nox and pm, which are in a trade-off relationship. At present, there are several types of diesel engine exhaust purification technology routes that meet the Euro ⅳ emission standards:
1) egr (exhaust gas recirculation system) +doc (diesel catalytic oxidizer)
The function of doc is: oxidation of hc and co; oxidation of soluble organic matter (sof) in PM; partial oxidation of carbon particles (soot) in PM, with a very low ignition temperature. The system uses delayed fuel injection with egr technology to reduce nox.
2) egr+doc+dpf (particle trap)
dpf is generally a wall-flow particle trap. There are sic and cordierite. sic has good thermal shock resistance, high price, and difficult preparation. Cordierite has a lower price and is inferior to sic in thermal shock resistance. dpf can reduce pm by 80%-95%. The purification efficiency of pm is the highest among the four technical routes, but an additional regeneration device is required, otherwise it will block the dpf.
4) scr (selective catalytic reduction)
The system achieves the purpose of purifying PM inside the machine in advance through fuel injection, and then uses urea solution to catalytically purify nox outside the machine. The reaction temperature is above 200°C, and the ignition temperature of the composite oxide low-temperature scr catalyst can reach below 150°C. The system uses urea mixed with water to form a 32.5% solution. The recognized industrial trade name for this solution is adblue. Its composition is specified in din standard no.70070. Urea (adblue) in solid or aqueous solution is classified as non-hazardous. adblue is a transparent liquid with a slight smell of ammonia. If it splashes, the water evaporates and crystals form.
The system structure diagram is as follows, adblue is stored in a urea tank mounted on the chassis. The urea tank supplies solution to a dosing unit (du) mounted on the chassis. du is controlled by the engine control module (ecm). du uses compressed air from the vehicle system to produce adblue spray, which is delivered to the nozzle in the engine exhaust system through a very precise metering and pumping system. The amount of adblue injected into the exhaust is controlled by ecm, which can match the engine's nox output at any speed and load. When it comes into contact with high-temperature exhaust gas, water quickly evaporates and urea becomes ammonia. Ammonia reacts with nox in the catalytic converter, and the result of this process is to emit harmless n2 and h2o from the exhaust pipe.
This system can most effectively reduce the content of automobile exhaust and reduce the pollution caused by nox. However, the scr system has many shortcomings, resulting in its low penetration rate.
(1) The scr system is complicated to install, and the equipment cost is high, and it must be installed before the car leaves the factory;
(2) The scr system needs to consume the adblue solution made of urea, which greatly increases the cost of car use, and the production and consumption of urea will also cause secondary pollution;
(3) The scr system has no effect on co, ch, particles and other pollutants in automobile exhaust, and other purification equipment must be installed;
(4) The scr system has high requirements for automobile intelligence, which can easily cause the engine to fail to ignite.
2. Influencing factors of conversion efficiency of three-way catalytic converter
2.1 The influence of temperature
Each catalyst has its upper and lower limits of activity temperature. Too high a temperature can speed up the growth of the catalyst surface, reduce the surface area, and reduce its activity. In severe cases, it will cause fission and make the catalyst lose its activity; if the temperature is too low, the catalyst Activity cannot be exerted. The exhaust temperature of the engine directly affects the reaction temperature in the catalytic converter.
2.2 The effect of airspeed
Space velocity (ie, space velocity, sv, space velocity) is the ratio of the volumetric flow rate of exhaust gas flowing through the catalyst per hour (converted to a standard state) to the volume of the catalyst, and its unit is h-1. The size of the space velocity actually represents the residence time t of the reaction gas in the catalyst (generally, the unit is s). For the same catalyst, under certain conditions, the higher the engine speed, the higher the airspeed, the shorter the residence time, and the lower conversion rate; on the contrary, the lower the engine speed, the lower the airspeed, the longer the residence time, and the higher the conversion rate.
2.3 Influence of emission concentration
Emission concentration refers to the concentration of pollutants at the entrance of the catalytic converter. For gasoline engines, the emission concentration of co and hc is high at low speed and low load, and lower at high speed and high load. Under a certain working condition, the conversion efficiency is relatively low when the emission concentration at the inlet of the catalytic converter is high, and the conversion efficiency is relatively high when the emission concentration at the inlet is low.
2.4 The influence of air-fuel ratio
When twc is working at the air-fuel ratio near the theoretical air-fuel ratio, the conversion efficiency of co and hc oxidation and nox reduction can be both high at the same time. In order to ensure the precise control of the air-fuel ratio, the engine generally adopts an air-fuel ratio feedback (endless loop) electronic control system with an oxygen sensor as the feedback component.
2.5 Quality of fuel and lubricating oil
The lead contained in gasoline and the sulfur, zinc, phosphorus and other elements contained in the lubricating oil and its additives easily react with the catalyst active material to cause phase change or cover the active surface of the catalyst to cause the catalytic converter to become poisoned and invalid. Experiments have shown that only one box of leaded gasoline will completely invalidate a new twc. Therefore, unleaded gasoline or lubricating oil with low sulfur, zinc, and phosphorus should be selected in use.
Three-way catalytic pollution of automobile exhaust purification
Preface
Although the car is the most important means of transportation in the 21st century, it has many drawbacks. Automobile exhaust pollution is environmental pollution caused by exhaust gas emitted by automobiles. It can be said that cars are a mobile source of pollution. In countries around the world, automobile pollution is no longer a new topic. Since the 1940s, photochemical smog incidents have occurred many times in cities such as Los Angeles, the United States, and Tokyo, Japan, where exhaust gas from internal combustion engines discharged from automobile exhaust pipes has caused many casualties and huge economic losses.
In the 21st century, automobile pollution has increasingly become a global problem. With the increasing number of cars and the wider range of use, their negative effects on the world environment are also increasing, especially harming the urban environment, causing respiratory diseases, causing excessively high levels of surface air ozone, and aggravating cities. The heat island effect makes the urban environment worse. According to statistics from relevant experts, by the beginning of the 21st century, automobile exhaust emissions accounted for 30-60% of air pollution. With the increase in motor vehicles, exhaust pollution has become more and more intense, changing from locality to continuous and cumulative, while citizens of various countries have become direct victims of automobile exhaust pollution.
1. The current mainstream exhaust gas aftertreatment system in the world
Due to the different characteristics of the exhaust gas of gasoline engine and diesel engine, the exhaust gas aftertreatment technology of the two is very different.
1.1 Technical route of gasoline engine exhaust gas aftertreatment
Gasoline engine exhaust emission characteristics: conventional pollutants are hc, co, nox, exhaust temperature sometimes exceeds 1000℃, high airspeed (30,000-100,000h-1), high water vapor (about 10%) concentration and extreme conditions of sox It has high activity and durability requirements of 100,000 kilometers, and requires low precious metals.
At present, the technical route of gasoline engine exhaust purification that meets Euro ⅳ and above emission standards is widely adopted in the world as close-coupled catalyst (ccc) + three-way catalyst (twc).
1) Close-coupled catalyst (ccc):
Close to the engine to solve the exhaust emission when the engine is cold started. The main function is to reduce the emission of hc during cold start. Most of the hc is discharged during cold start. At this time, the catalyst cannot react without reaching the light-off temperature and the engine is in rich oil condition when starting, and the oxidation process is not due to oxygen depletion. complete. The key is the low-temperature activity, high-temperature stability of the catalyst, the inhibition of the conversion of co and the high conversion rate of hc.2) Three-way catalyst (twc):
After the close-coupled catalyst (ccc), it is generally installed in the lower part of the car chassis, with low precious metals, high performance and high temperature aging resistance (10-16 million km durability test).
1.2 Technical route of diesel engine exhaust gas aftertreatment
Diesel engine exhaust emission characteristics: emission pollutants are four kinds of co, hc, nox, pm; emission temperature is low (200-450℃), o2 is excessive, high airspeed (30,000-100,000h-1) and sox exists.
Since the content of hc and co is lower than that of gasoline engines, the focus of diesel engine exhaust purification is on the two components of nox and pm, which are in a trade-off relationship. At present, there are several types of diesel engine exhaust purification technology routes that meet the Euro ⅳ emission standards:
1) egr (exhaust gas recirculation system) +doc (diesel catalytic oxidizer)
The function of doc is: oxidation of hc and co; oxidation of soluble organic matter (sof) in PM; partial oxidation of carbon particles (soot) in PM, with a very low ignition temperature. The system uses delayed fuel injection with egr technology to reduce nox.
2) egr+doc+dpf (particle trap)
dpf is generally a wall-flow particle trap. There are sic and cordierite. sic has good thermal shock resistance, high price, and difficult preparation. Cordierite has a lower price and is inferior to sic in thermal shock resistance. dpf can reduce pm by 80%-95%. The purification efficiency of pm is the highest among the four technical routes, but an additional regeneration device is required, otherwise it will block the dpf.
3) egr+doc+poc (particle catalytic oxidizer)
Poc can capture and oxidize part of the particulate matter. Its structure is to allow exhaust gas to pass through a wrinkled channel without clogging. The exhaust resistance is small, and the volatile organic components in the particulate matter can be reduced. The conversion efficiency of the particulate matter can reach more than 60%. Use poc and doc together, doc provides a higher regeneration temperature (250°c-500°c) for poc, which can realize continuous passive regeneration4) scr (selective catalytic reduction)
The system achieves the purpose of purifying PM inside the machine in advance through fuel injection, and then uses urea solution to catalytically purify nox outside the machine. The reaction temperature is above 200°C, and the ignition temperature of the composite oxide low-temperature scr catalyst can reach below 150°C. The system uses urea mixed with water to form a 32.5% solution. The recognized industrial trade name for this solution is adblue. Its composition is specified in din standard no.70070. Urea (adblue) in solid or aqueous solution is classified as non-hazardous. adblue is a transparent liquid with a slight smell of ammonia. If it splashes, the water evaporates and crystals form.
The system structure diagram is as follows, adblue is stored in a urea tank mounted on the chassis. The urea tank supplies solution to a dosing unit (du) mounted on the chassis. du is controlled by the engine control module (ecm). du uses compressed air from the vehicle system to produce adblue spray, which is delivered to the nozzle in the engine exhaust system through a very precise metering and pumping system. The amount of adblue injected into the exhaust is controlled by ecm, which can match the engine's nox output at any speed and load. When it comes into contact with high-temperature exhaust gas, water quickly evaporates and urea becomes ammonia. Ammonia reacts with nox in the catalytic converter, and the result of this process is to emit harmless n2 and h2o from the exhaust pipe.
This system can most effectively reduce the content of automobile exhaust and reduce the pollution caused by nox. However, the scr system has many shortcomings, resulting in its low penetration rate.
(1) The scr system is complicated to install, and the equipment cost is high, and it must be installed before the car leaves the factory;
(2) The scr system needs to consume the adblue solution made of urea, which greatly increases the cost of car use, and the production and consumption of urea will also cause secondary pollution;
(3) The scr system has no effect on co, ch, particles and other pollutants in automobile exhaust, and other purification equipment must be installed;
(4) The scr system has high requirements for automobile intelligence, which can easily cause the engine to fail to ignite.
2. Influencing factors of conversion efficiency of three-way catalytic converter
2.1 The influence of temperature
Each catalyst has its upper and lower limits of activity temperature. Too high a temperature can speed up the growth of the catalyst surface, reduce the surface area, and reduce its activity. In severe cases, it will cause fission and make the catalyst lose its activity; if the temperature is too low, the catalyst Activity cannot be exerted. The exhaust temperature of the engine directly affects the reaction temperature in the catalytic converter.
2.2 The effect of airspeed
Space velocity (ie, space velocity, sv, space velocity) is the ratio of the volumetric flow rate of exhaust gas flowing through the catalyst per hour (converted to a standard state) to the volume of the catalyst, and its unit is h-1. The size of the space velocity actually represents the residence time t of the reaction gas in the catalyst (generally, the unit is s). For the same catalyst, under certain conditions, the higher the engine speed, the higher the airspeed, the shorter the residence time, and the lower conversion rate; on the contrary, the lower the engine speed, the lower the airspeed, the longer the residence time, and the higher the conversion rate.
2.3 Influence of emission concentration
Emission concentration refers to the concentration of pollutants at the entrance of the catalytic converter. For gasoline engines, the emission concentration of co and hc is high at low speed and low load, and lower at high speed and high load. Under a certain working condition, the conversion efficiency is relatively low when the emission concentration at the inlet of the catalytic converter is high, and the conversion efficiency is relatively high when the emission concentration at the inlet is low.
2.4 The influence of air-fuel ratio
When twc is working at the air-fuel ratio near the theoretical air-fuel ratio, the conversion efficiency of co and hc oxidation and nox reduction can be both high at the same time. In order to ensure the precise control of the air-fuel ratio, the engine generally adopts an air-fuel ratio feedback (endless loop) electronic control system with an oxygen sensor as the feedback component.
2.5 Quality of fuel and lubricating oil
The lead contained in gasoline and the sulfur, zinc, phosphorus and other elements contained in the lubricating oil and its additives easily react with the catalyst active material to cause phase change or cover the active surface of the catalyst to cause the catalytic converter to become poisoned and invalid. Experiments have shown that only one box of leaded gasoline will completely invalidate a new twc. Therefore, unleaded gasoline or lubricating oil with low sulfur, zinc, and phosphorus should be selected in use.