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Energy plays a vital role in economic and social development, but environmental pollution is an incidental problem that has to be faced. Since the reform and opening up, our economy has achieved a long time of rapid growth, but the economic growth and the stable supply of energy is as inseparable as meat. The amount of fly ash produced by thermal coal consumption in power generation in China is very large every year, and many industrial production processes are also confronted with dust problems. This kind of solid waste composed of countless tiny spheres contains a variety of harmful components, high storage cost and serious dust pollution, so it is very important to effectively recover and recycle in a reasonable way. High temperature flue gas purification is an essential process in fly ash recycling process, which directly relates to the recovery rate of effective components in fly ash and the degree of environmental pollution caused by this process. The high temperature ceramic filtration technology is a relatively advanced high temperature dusty gas purification technology in the world in recent years.
1 Main categories of dust removal technologies on the market and their advantages and disadvantages
At present, the main dust removal technologies at home and abroad include cyclone dust removal, cloth bag dust removal, electric dust removal, electric bag dust removal, particulate bed filtration, porous inorganic membrane filtration and so on. The deficiencies of these dust removal methods are as follows:
Cyclone dust removal efficiency is poor, can not meet the requirements of environmental emissions, generally only used for pre-dust removal.Cloth bag type dust removal is limited by the temperature resistance, corrosion resistance and other properties of filter media, the use of temperature can not be too high, generally used under 250℃, obviously can not meet the needs of high temperature flue gas purification.Electric dust removal has some problems, such as unstable corona discharge, short electrode life, sensitivity to flue gas composition, high temperature insulation, etc., and it is not suitable to be used at high temperature for a long time (generally lower than 380℃).Granular layer filtration dust removal resistance to high temperature, corrosion resistance, but its filtration efficiency is not ideal, dust emission concentration is high. Metal film and metal woven filter media are expensive and limited in application due to corrosion resistance and wear.Among other filtration methods, the dust removal technologies that have been studied or are being studied include ceramic textured filter and ceramic fiber filter, etc. The filtration efficiency of these dust removers can reach more than 99%, but there are many problems such as low strength, easy plugging and failure.The ceramic membrane and support body contain a large number of micro-pore structure, giving it a large specific surface area. The surface of these pores is a good carrier of catalyst. After the catalyst is loaded on the surface, desulfurization and decontamination can be realized at the same time of dust removal. The ceramic membrane filter has the advantages of high temperature resistance, corrosion resistance, high mechanical strength, stable structure and no deformation, and can realize functional regeneration through multiple "cleaning", long service life, so it is considered to be one of the best choices of high temperature dust gas dust removal.
2 High temperature dust removal ceramic material
High temperature dust contains a lot of oxidizing and reducing components, low melting point metal compounds, and so on. This kind of medium has a certain corrosive effect on high temperature filter materials. Therefore, good high temperature resistance and chemical temperature are the basic conditions of high temperature ceramic filter elements. At present, the developed high temperature ceramic filtration materials are mainly divided into two kinds: oxide ceramic materials, such as alumina, spinel, cordierite, mullite, etc.; Non-oxide ceramic materials, such as silicon carbide, silicon nitride, etc. Among them, silicon carbide material has good thermal conductivity, high strength, small coefficient of thermal expansion, good thermal impact resistance and other advantages, is the preferred high temperature ceramic material.
3. Filtration mechanism of ceramic membrane filter
For the filtration and separation of gas-solid system, the dust removal mechanism of ceramic membrane filter is mainly inertial collision, diffusion and interception. Generally, dust with larger particle size is trapped because the particle size is larger than the membrane aperture. When passing through the membrane micropore, the dust of medium size is trapped by inertial collision and contact with the micropore wall. Smaller particles are trapped by Brownian motion in contact with the pore wall.
The dust removal process is mainly divided into three stages:
In the first stage, the dust-containing gas enters the ceramic membrane tube, and the dust particles are blocked by the membrane layer. At this time, the membrane layer on the surface of the filter tube plays a major role.
In the second stage, with the process of filtration, the dust on the surface of the film layer increases and the filter cake is formed on the surface of the film. In this process, the filter cake plays a major role in filtering the dust-containing gas, making the collection efficiency significantly improved, which is also the main stage of filtration.
In the third stage, after a certain period of operation, due to the continuous thickening of filter cake layer, the filtration resistance increases, the filtration speed decreases, and the pressure drop increases. At this time, the dust attached to the surface of the filter tube must be removed in time, and the ceramic membrane filter can be regenerated by backblowing gas, so as to restore its filtration capacity.It should be noted that the cleaning process of ceramic membrane filtration mainly depends on the cake layer formed on the filter tube. During the cleaning and regeneration process, excessive cleaning will reduce the filtration efficiency.
Ceramic membrane used in the field of high temperature gas dust removal can not only protect the environment, achieve clean production, but also has a wide range of economic benefits. Faced with the harsh environment of high temperature gas dust removal, the application of traditional porous ceramic materials is greatly restricted. Therefore, the development of high temperature ceramic membrane with good resistance to high temperature and thermal shock, as well as the research on filtration and reverse blowing of aircraft will be the focus of researchers in the future. At the same time, it is necessary to further optimize the ceramic membrane manufacturing process and reduce the cost, so as to realize the industrialization application of ceramic membrane in the field of high temperature dust removal as soon as possible.
