Since July 1, China’s national phase VI motor vehicle pollutant emission standard for heavy diesel vehicles has been officially implemented throughout the country. It is prohibited to produce and sell heavy diesel vehicles that do not meet the national VI emission standard, and imported heavy diesel vehicles shall meet the national VI emission standard. It is reported that the national six emission standards have upgraded and improved the automobile exhaust test procedures compared with the national five, and the automobile exhaust pollutant emission limits have been stipulated. The severity has been increased by 40% ~ 50%, and the emission of main pollutants has been nearly doubled. From this point of view, the upgrading of automobile exhaust emission products is an essential work for major automobile enterprises.
1. Composition of automobile exhaust treatment system
Automobile exhaust system mainly includes four parts: exhaust branch pipe, exhaust pipe, muffler and exhaust purification device (three-way Catalysis). Automobile exhaust catalytic purifier is an external purification device, which is mainly used in gasoline engine. It is composed of shell, carrier and catalyst. The performance of the support has a great impact on the activity and service life of the catalyst, so it plays an important role in the purification efficiency of the whole catalytic purifier. The catalyst on the carrier surface oxidizes and reduces the pollutants NOx, HC, CO and other harmful substances and harmful gases in the exhaust gas discharged by the automobile engine, and converts them into harmless CO2, H2O and N2, so as to achieve the role of tail gas purification.
The performance requirements of the carrier include:
1) It has high heat resistance to adapt to the wide exhaust temperature range of automobile engine;
2) It has good mechanical strength and can withstand the thermal impact of high-temperature air flow, as well as the severe impact caused by uneven road surface and cylinder vibration;
3) It has large specific surface area, large specific surface area of carrier and uniform distribution of active components, which is conducive to improving the utilization rate of active components and reducing the cost;
4) It has low heat capacity and high thermal conductivity, which is conducive to shorten the reaction time of the catalyst and reduce the exhaust emission during vehicle cold start;
5) Have appropriate pore structure or porosity;
6) It has certain water absorption and low price.
2. Material type of catalyst support
At present, there are two main types of catalyst supports for automobile exhaust purification at home and abroad: metal and porous ceramics, among which cordierite honeycomb ceramics in porous ceramic support is the most widely used.
2.1 metal carrier:
Advantages: thin wall, large specific surface area, high mechanical strength, low exhaust resistance to automobile, good matching performance between carrier and metal shell, etc. It has small heat capacity, high thermal conductivity and fast corresponding characteristics, which can improve the heating rate of catalyst during cold start. Therefore, it has a good effect on the purification of automobile exhaust during cold start.
Disadvantages: the molding process is complex, the production cost is high (about twice that of ceramic carrier), and the thermal shock resistance is not as good as cordierite honeycomb ceramics. In addition, the surface coating process is not perfect, which limits its development.
2.2 cordierite honeycomb ceramic carrier:
Advantages: honeycomb ceramic products have many parallel flow channels, so the pressure loss of gas flow is small, and the flow distribution in the whole component is good. In addition, honeycomb ceramics can be made of a variety of materials, mainly cordierite, aluminum titanate, silicon carbide, zirconia, silicon nitride, etc. Cordierite (2MgO · 2al2o3 · 5sio2) honeycomb ceramics are widely used because of their excellent thermal shock resistance, good adsorption performance, high fire resistance and appropriate mechanical strength.
Since the extrusion production of honeycomb ceramic carrier by Corning company in 1973, cordierite honeycomb ceramic has become an important part of exhaust purification components such as automobile exhaust catalytic purifier and diesel exhaust particulate trap. Cordierite honeycomb ceramics are generally synthesized from high-quality kaolin, talc and alumina in industry.
Disadvantages: generally, in order to increase the surface area of honeycomb ceramic carrier, its surface will be coated with a uniform high specific surface area alumina coating, and then the precious metal active components will be loaded on the surface. However, the thermal expansion coefficients of cordierite matrix and alumina coating are quite different, which makes the catalyst and coating easy to peel off from the carrier and block the pores, resulting in the failure of the catalyst. Low thermal conductivity also makes the catalyst ignite slowly, and the purification efficiency of automobile exhaust emission during cold start is poor.
2.3 new porous ceramic catalyst support:
① Fiber porous ceramic: refers to a kind of high porosity ceramic material prepared with ceramic fiber as the main raw material.
Advantages: most of the materials have open pore structure, and the porosity can reach more than 90%, while the porosity of general porous ceramic materials is only about 50%. It mainly uses the textile characteristics of fiber for three-dimensional weaving, and the porosity and pore diameter are controllable.
Disadvantages: limited by the length and weaving performance of ceramic fiber, high requirements for three-dimensional weaving technology and expensive product cost limit its development.
② Conductive SiC Porous Ceramic Material: its pores are in a three-dimensional connected network structure, with excellent heat and corrosion resistance, high thermal conductivity, service temperature and good regeneration performance. The thermal shock resistance is stronger than cordierite honeycomb ceramics, which is suitable for applications where the temperature often changes sharply.
The product adopts conductive carrier and uses electric preheating method to raise the temperature of the purifier. The catalyst can reach the ignition temperature of the catalyst in a short time and effectively reduce the emission of harmful substances in the first few minutes of vehicle cold start.