How to achieve ultra-low emissions in the cement industry by bag dust collector

At present, the main equipment for dust control in the cement industry are: bag filter, electrostatic precipitator and electric bag composite dust collector, as well as emerging wet electrostatic precipitators.

(1) Bag filter

It is a dust removal device that relies on filter material as a filter material to achieve "gas-solid" separation. Its working mechanism is trapped by the screening, inertia, adhesion, diffusion and static electricity generated when the dust-containing gas passes through the filter cloth.

(2) Electrostatic precipitator

The strong electric field generated by the high voltage causes the dust-containing gas to be locally ionized, that is, the corona discharge is generated. When the dust-containing gas passes through the electric field, the negative ions collide with the dust particles and adhere thereto, thereby realizing the dust charge and under the action of the electric field force. , the dust particles in the gas are separated.

(3) Electric bag dust collector

The electric bag composite dust collector is a collection of an electric precipitator and a bag type dust collector. The electric field in the front is charged and collected by the electric field. The dust that is difficult to collect from the remaining charge enters the filter bag area uniformly with the flue gas, and is filtered through the filter bag. Flue gas purification.

(4) Wet electrostatic precipitator

The principle of dust collection is the same as that of the dry type electrostatic precipitator, which relies on high-voltage corona discharge to charge the dust, and the charged dust reaches the dust collecting plate/tube under the action of the electric field force. The dust collector of the wet electrostatic precipitator is different from the dry electrostatic precipitator in that it uses a corrosion-resistant conductive material as a dust collecting pole or a non-metallic material which is formed by a water-jet or overflow water to form a conductive water film and is not electrically conductive. Therefore, the collection efficiency is higher than that of the dry electrostatic precipitator.

From a technical point of view, except for the electrostatic precipitator, it is difficult to achieve the ultra-clean emission target of 10mg/Nm3, and the other three types of precipitators are likely to achieve the goal of ultra-clean emission. However, from the perspective of equipment maintenance and stability, the bag filter is the most feasible solution. Therefore, this paper focuses on some key technologies for bag filter to achieve ultra-net emissions of 10mg/Nm3.

1. Thoughts on ultra-clean discharge of bag filter

To develop a bag filter with ultra-clean emission function, we must first discuss the problem of ultra-clean emission filter. The goal of whether the filter itself can achieve ultra-clean emissions is the most critical factor in whether the precipitator can achieve ultra-clean emissions. Of course, the dust removal mechanism of the bag filter not only depends entirely on the filtration performance of the filter bag. After the filter bag captures most of the dust, a part of the dust, that is, the dust layer, is retained inside the filter material and the surface layer. It also captures dust through the screening action of the dust layer, which improves the filtration efficiency and is also an important idea for the ultra-clean discharge of the bag filter. The thicker the dust layer, the lower the dust emission concentration. Therefore, how to intelligently clean the dust and scientifically preserve the thickness of the dust layer on the surface of the filter bag is a key factor in achieving ultra-clean emissions. In addition, the uniform airflow in the precipitator also has a certain influence on the filtration performance. If the airflow distribution is unreasonable, the local airflow velocity is too fast, and the dust that escapes in the area increases, thereby affecting the discharge concentration of the dust collector. In addition to the above-mentioned key factors, the prevention of accidental discharge caused by broken filter bags, the realization of accidental discharge warning and self-treatment technology of bag-type dust collectors are also key technologies for achieving ultra-clean emissions.

2. Key factors for ultra-clean emissions of bag filters

2.1 Ultra-clean discharge filter bag

One of the keys to the development and application of bag filters is the development of filter media. As the "core" of the bag filter, the filter material has the performance and quality, which is directly related to the dust concentration of the bag filter. The performance and quality of the filter bag are related to the structure of the filter material, fiber shape, processing and post-treatment.

In terms of structure, film filter media is generally used as the main measure to reduce emission concentration. The membrane filter material is a filter material which is coated with a polytetrafluoroethylene (PTFE) film on the surface of the filter material. The film can be efficiently filtered and has good filtration performance. In addition to the membrane filter material, the multi-gradient filter bag has also become a new technology filter material for effective dust control. The gradient filter generally consists of four layers, the first gradient consists of fine fibers, and the pores have good filtration performance. Most of the dust is trapped in the first gradient, and it is difficult to stay inside the filter material, thereby ensuring smooth airflow inside the filter material, avoiding the problem of increasing running resistance with the use of the filter material, and ensuring dust removal efficiency. .

From the shape of the filter material, the fine fiber and the shaped fiber have a higher specific surface area than the ordinary fiber under the same gram weight, and can obtain better filtration efficiency. These two fibers are commonly used in cement companies that require very low emission concentrations. In addition, according to the data, the recently developed sea-island fiber has higher specific surface area and better filtration performance, so the island fiber can also be an alternative to ultra-clean emission filter.

2.2 Intelligent cleaning system

The cleaning control methods commonly used at home and abroad have timing and fixed resistance cleaning. Timing cleaning refers to controlling the cleaning mechanism operation according to a preset cleaning cycle and pulse interval. If the time setting is unreasonable, the initial dust layer of the filter material cannot be formed, and the dust discharge concentration is increased. Fixed resistance cleaning is to control the cleaning according to the actual resistance of the dust collector. Usually, the upper and lower limits of the resistance are set. When the resistance is greater than the upper limit, the cleaning is started. The cleaning system starts to cycle repeatedly according to the built-in program, and stops cleaning until the resistance is lower than the lower limit. The two modes cannot adjust the cleaning sequence.

The intelligent cleaning system links the cleaning process to the resistance of the various chambers of the precipitator. According to the pressure difference of each bag chamber, it is sprayed from high to low in order to avoid unnecessary cleaning action, protect the dust layer on the surface of the filter bag, and reduce the escape of ultra-fine dust. Therefore, the use of intelligent cleaning can become the preferred cleaning system for ultra-clean emission precipitators.

2.3 Airflow uniform system

The uniformity of the airflow between each bag and each bag in the dust collector has a certain influence on the filtration performance. At present, CFD software is often used on the market to calculate the internal airflow distribution of the simulated dust collector. By continuously simulating and adjusting the height of the baghouse partition, the angle of the airflow uniform, the size and other parameters, the dusty airflow is evenly distributed and transported to each filter unit of the dust collector, so that the filter load of each filter unit filter bag is consistent. It is ensured that the flow deviation of each filter unit is controlled within 10%, so that the airflow of the entire precipitator is stably distributed, thereby reducing the dust emission concentration of the precipitator due to excessive local airflow. Therefore, perfecting the airflow uniform distribution system of the dust collector is the basic requirement for the ultra-clean emission of the dust collector.

2.4 Accident emission warning and self-processing technology

In the cement enterprise, the damage of the filter bag occurs, which not only causes the accident discharge to affect the environment, but also causes the problem of large-volume filling bags, resulting in no small economic loss. In response to this problem, the Cement Research and Design Institute has found through a large number of tests that it has a characteristic peak concentration after the broken filter bag is cleaned, and quickly finds the bag number and specific location of the filter bag breakage. After the position of the broken bag is accurately positioned, the control system automatically releases the emergency plug, and blocks the filter bag mouth to avoid the escape of the dust after the filter bag is damaged, so as to prevent the use of the surrounding filter bag due to the damage of the partial filter bag.

Under increasingly stringent emission standards, the cement industry bag filter should start with the above four key technologies to achieve ultra-clean emission targets of 10mg/Nm3 and below.