Intro
Environmental protection plays an important role in the modern world. Cement production is inevitably associated with formation of large amounts of dust, which is harmful to the health of personnel, residents of nearby settlements, flora and fauna, and environment.
Dedusting of waste gas and aspiration air in cement industry.
Waste gas and aspiration air dedusting is one of key tasks of every enterprise.
Final dust content must ensure environmental safety and compliance with sanitary standards.
Features of waste gas dedusting in cement production.
A rotary kiln in cement industry is one of the main sources of dust. The kiln is operated at high temperatures, and a significant amount of dust particles is released during the clinker burning process. These particles may contain inorganic compounds such as silicon oxide, aluminum, calcium, and other components of the raw mix.
The main method of cleaning a rotary kiln waste gases in cement production is the use of electrical filters of various types. They are capable of cleaning gas to acceptable parameters before releasing it into the atmosphere.
To create safe and comfortable operating conditions, an air dedusting system is provided, which uses bag filters as cleaning devices.
M HEAVY TECHNOLOGY is capable to design and implement all methods of waste gas and aspiration air dedusting, considering the best possible solutions for the enterprise.
Environmental Emissions Limit for cement kilns in the European Union.
1 BAT – AEL – EU BREF (2009) – Best Available Technologies – Total Emissions Based on Daily Averages.
2 IFC EHS Guidelines for cement and lime production. Specified level of emissions based on daily averages.
To ensure the required degree of waste gas and aspiration air dedusting one-, two- or three-stage dust cleaning systems are used.
To achieve maximum dust catching efficiency, two- or three-stage configurations are used, which combine various cleaning devices. For example, the first stage is a cyclone, the second stage is an electrostatic precipitator or a bag filter. In a three-stage system, a cyclone, an electrostatic precipitator and a bag filter for final fine cleaning are used in sequence.
The choice of a specific dust cleaning system design depends on the production specifics, the type of pollutants, the requirements for the degree of purification, as well as on economic factors.
M HEAVY TECHNOLOGY solutions for cement industry.
Electrostatic precipitators effectively separate dust that is generated during various technological processes.
We have already previously touched upon the use of electrostatic precipitators and bag filters in various industries.
In this article we will mainly review purification of cement kiln off-gases in electrostatic precipitators.
Key indicators for using electrostatic precipitators to clean kiln off-gases from dust.
- Low pressure drops
- Low sensitivity to overtemperature
- Low sensitivity to high gas humidity
- Easy maintenance
Electric filters are also divided into several types, depending on the amount of gas being purified, its properties and dust composition. The main difference between the types of electric filters is in the length and profile of the precipitation electrodes, electrode spacing and calculated vacuum of the housing.
Electrostatic precipitators can also have different design of distribution grids, precipitation electrodes and corona electrodes.
The dust that is generated in the rotary kiln during the dry-type cement production process does not have completely required electrical properties and therefore, an air conditioning tower (evaporative cooler) is installed before the electrostatic precipitator, where dust cooling and humidification are performed simultaneously.
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Description of the operating principle of electrostatic precipitators.
Combustion products are sucked off from the dust source by means of an exhaust fan and then pass through an electrostatic precipitator. Contaminated gases enter the filter housing through an input element with dividing walls and pass through a system, formed by precipitation electrodes, between which corona electrodes are suspended. The corona electrodes are connected to a source of ultra-high voltage. Under the influence of a strong electric field gas ionization and charging of dust particles with negative ions occur. As a result of the electric field influence charged dust particles are attracted to the precipitation electrodes, on which they deposit.
Drw. Operating principle of electrostatic precipitators
For cleaning precipitation and corona electrodes rapping equipment is used. The dust falls into the discharge funnel of the electrostatic precipitator, where from it is transported for further processing by means of extracting equipment.
Cleaning of cement gas.
Let us see, how cement gas dedusting in an electric filter is performed, at the example of one of the projects, which was implemented by M HEAVY TECHNOLOGY.
Gas cleaning plant for rotary kiln and raw material mill.
M HEAVY TECHNOLOGY experts took part in designing and commissioning of a “dry”-type gas cleaning system for a rotary cement kiln and a raw material mill at Verkhnebakansk cement plant. For cleaning gases being discharged into the atmosphere from the rotary kiln and raw material mill the project provided for installation of an electrostatic precipitator, designed to clean 674940 m3/h of gases with to = 150÷121oС and an initial dust content of 47÷37 g/m3.
The residual dustiness of the air released into the atmosphere does not exceed 30 mg/m3. After the cyclone heat exchanger hot gases with a temperature of to = 300oС are sent to the gas conditioning tower or to the raw mill using a behind-the-furnace smoke exhauster with a drive power of 3040 kW. The gases from the raw mill pass the conditioning tower without cooling. If the raw material mill stops, all waste gases from the cyclone heat exchanger are sent to the conditioning tower, where they undergo evaporative cooling. Gas conditioning in the tower takes place by spraying water from a special pumping unit, equipped with 30 kW pumps and screw-type compressors with a drive power of 160 kW each.
The dust, deposited in the tower, is discharged into the dust transport system from the electrostatic precipitator by a special reversible screw conveyor Ø500 mm, with a drive power of 5.5 kW. In cases where the dust is wet because of unevaporated water, it is unloaded by the same conveyor into tipper trucks for disposal. The movement of gases in the electrostatic precipitator takes place due to the vacuum, created by the end smoke exhauster DHRV 71N-2500/K, with a drive power of 1250 kW.
According to M HEAVY TECHNOLOGY design solutions the collected dust is unloaded from the electrostatic precipitator by drag chain conveyers and reloaded to a 39.5 m long screw conveyor Ø500 mm with a conveying capacity of up to 48 t/h, and a drive power of 7.5 kW. Then the dust is transferred to Ø630 mm and Ø800 mm screw conveyors, with a drive power of 18.5 kW each. The process flow diagram also provides for the possibility of accumulating dust in a 400 m3 bin for its subsequent uniform return to a continuous homogenization silo or a cyclone heat exchanger. The bin is filled with dust by a belt bucket elevator with a capacity of up to 50 t/h and a drive power of 30 kW.
For dedusting of dust junction units from the electrostatic precipitator and loading the storage bin efficient bag filters are installed with a filter surface area of 81 m2, capacity of 8750 m3/h, fans Q = 11400 m3/h and a drive power of 18.5 kW. The residual dust content of the air, discharged into the atmosphere after the bag filters, does not exceed 15 mg/m3.
After the plant start-up all equipment, including dust cleaning system, was brought to design parameters. The extensive experience and knowledge of M HEAVY TECHNOLOGY allowed us to successfully complete and implement this project at a high level.