Dust is the main pollutant from the cement industry. Many links in the cement production process, such as raw material crushing, raw meal grinding, clinker calcination, cement grinding, etc., have different degrees of dust spillover. Among them, the clinker calcination stage has the most serious dust emission degree, accounting for more than 70% of the total dust emission of the cement plant.
A large amount of dust will cause air pollution in the local area and are harmful to human bodies. Therefore, when building a new cement plant, we must design a complete dust control system for each production link and select reasonable dust collection equipment to minimize dust emission in future cement production.
Dangers of Cement Dust
The dust emitted from cement plants has the same chemical composition as cement raw materials and kiln fuels. Take portland cement as an example, its raw materials are mainly limestone and clayey (clay, fluorite, sandstone, etc.), and the main chemical compositions of emitted dust are SiO2, Al2O3, CaO, K2O, Na2O, SO3, etc. The cement dust is harmful to the human body, environment, industrial equipment, and the cement plant itself.
Human Health: Frequent inhalation of cement dust can cause respiratory diseases. Dust deposits in human alveolus may cause silicosis, lung sclerosis, and other diseases, causing irreversible harm to human health.
Industrial Equipment: Dust falls into the running machine will accelerate the wear of machine parts, resulting in shorter service life and running rate of production equipment. It also reduces the accuracy and reliability of equipment, and reduces or even destroys the insulation of electrical equipment.
Cement Production: Though dust is harmful, it is a type of recyclable resource if collected. If cement dust is not recycled, it will increase the consumption of raw materials, fuels, and energy, increase production costs and reduce the economic benefits of the cement plant.
Challenges in Cement Plant Dust Control
Coal Grinding Dust Control
The dust emitted from coal grinding contains a certain amount of coal powders. Coal-bearing dust is flammable and explosive, and piles of coal powder will spontaneously ignite under certain conditions. Therefore, the dust collection equipment of the coal-grinding system must have flame-proof and explosion-proof devices, such as an explosion-proof valve on the dust collector. If the bag filter is used in the coal grinding system, then the filter material of the dust collector must be antistatic material, such as fiber antistatic blended fabric, to prevent explosion accidents caused by static electricity.
Cement Kiln Dust Control
In the dry process of clinker calcination, the gas-solid separation of raw meal is completed in the preheater, and raw meal transfers heat thoroughly with hot gas in the kiln. The exhaust gas discharged from the end head of the kiln has a very complex composition; it contains not only fly ash brought in by raw meal, but also a large amount of flue gas formed by a series of physical and chemical reactions.
The characteristics of dry process rotary kiln dust are high temperature, high dust concentration, fine and sticky, high specific resistance, and containing corrosive flue gas such as acid-base oxides.
Dust Collector Selection
According to the mechanism, the commonly used dust collector in the industry can be divided into mechanical dust collectors, wet dust collectors, fabric dust collectors, electric precipitators, and bag filters. Among them, electrostatic precipitators and pulse jet bag filters are two kinds of high-efficiency dust collection devices that are widely used in the cement industry. Their advantages and disadvantages are compared as follows.
Electrostatic Precipitator
Electrostatic precipitator (ESP) is a kind of dust collector which uses static electricity to separate particles from airflow. ESP is suitable for small particle capture. Its dust removal efficiency is above 99% in industrial applications. It has a large flue gas treatment capacity, high automation degree, and low resistance.
However, due to a large amount of moisture and corrosive gas in the flue gas of cement kiln, the electric resistivity of dust varies greatly, and the high-temperature discharge of the electrode plate exists, which is easy to cause corrosion and condensation of ESP and shorten its service life. Moreover, it is difficult for the ESP to adapt to the changing electric resistivity; in this case, its dust removal efficiency will decline.
Bag Filter
Bag filter is a kind of filtration equipment that separates dust from gas. The most notable advantage of a bag filter is its high dust removal efficiency, which usually reaches 99.9% in industrial applications. After purification by bag filter, the dust emission concentration can reach below 10mg / Nm3.
The disadvantages of a bag filter are that its filter bag needs to be replaced and it needs regular cleaning. The resistance of the bag filter is higher than 1200 PA on average, which makes the daily maintenance cost a little bit expensive.
Conclusion
Overall, the bag filter has a better performance than the electrostatic precipitator in the dust control of the cement kiln process. Due to the characteristics of cement kiln flue gas, the ESP should not only solve the problems of corrosion, condensation, and explosion, which are rather difficult to do, but also have lower stability than bag filters. Though the bag of bag filter needs to be replaced regularly, with the continuous improvement of bag filter technology, the service life of a single filter bag has been increased to more than two years. Moreover, the dust removal efficiency of bag filters is not limited by the electric resistivity of dust.