With the trumpet of “winning the blue sky defense war”, the state issued a series of relevant policies in 2018, which put forward higher requirements for atmospheric particulate matter emission. From cement, steel and power industries to all polluting industries, ultra-low emission and ultra-clean emission standards must be strictly implemented. At present, bag dust removal is still the most direct and effective of many dust removal methods. It can meet the strict emission requirements and exert certain functionality at the same time.
1. Development of bag dust removal technology
1.1 Membrane filter material
When the filter cake layer is not formed at the initial stage of the use of conventional filter materials, it mainly depends on the interception effect of the fiber itself. When facing ultra-fine and small particles, the capture ability is poor. Polytetrafluoroethylene microporous membrane is covered on the surface of filter material through adhesive or high-temperature hot pressing technology. Because the particle size of dust particles is much larger than that of micropores covered with membrane, particles are directly intercepted on the surface of microporous membrane. The filtration efficiency is very high and is very suitable for the field of high-precision filtration.
1.2 Design of felt bag mouth
The traditional “rubber strip” bag mouth is thin, and it is very easy to drop the bag after installation and in the long-term injection process. The use of felt bag mouth increases the contact area between the bag mouth and the flower plate of the dust collector by about 2 times, and their high adhesion gives better sealing performance, so as to effectively avoid the rise of dust and the accumulation and hardening at the bottom of the flower plate. The felt bag mouth design can adapt to more complex and changeable filtration conditions. For example, it can withstand higher filtration wind speed, enhance the load-bearing capacity of the filter bag, and is more conducive to ultra-low emission, longer service life and higher value.
1.3 Electrostatic and bag composite dust removal technology
In order to meet the increasingly stringent ultra-low emission standards, the single use of bag dust removal or electrostatic dust removal is facing the problems of increased energy consumption and increased pressure drop. Electrostatic and bag composite dust collector came into being and developed rapidly. The electrostatic and bag composite dust removal technology can better adapt to complex flue gas conditions. Most dust particles can be completely captured after the dual effects of electrostatic attraction and physical filtration, so as to achieve the emission concentration of less than 5 mg / Nm3, reduce the operation resistance and improve the service life of the filter bag.
1.4 Synergistic control of multiple pollutants
The bag filter has changed from the original single dust removal function to the collaborative control of multiple pollutants. For example, combined with the use of some catalysts, it can filter dust and remove harmful substances such as sulfur dioxide, nitrogen oxides and dioxins in flue gas.
2. Failure cause analysis of filter bag
2.1 Blockage failure
When the filter bag is used for filtration, the particulate matter and dust are continuously deposited on the surface of the filter material to form a filter cake layer, which will increase the filtration efficiency, but at the same time, the resistance of the dust removal equipment will be greater and greater. Although when the resistance increases to a certain value, the dust on the surface of the filter bag can be cleaned to a certain extent by means of back blowing and mechanical vibration, when the flue gas humidity is too high or the working temperature is lower than the condensation temperature, the water in the flue gas and dust are mixed and adhered to the filter material surface plate for solidification, which will block the pores between the fibers, increase the filter pressure drop and block the filter bag.
2.2 Damage failure
The filter bag will contact and rub with the metal keel and plate during use, resulting in certain wear over time. If the operation of workers is not standardized during the installation process, resulting in too tight or loose installation of the filter bag, it will aggravate the collision between the filter bag and the bag cage and cause damage. In addition, when the high-pressure pulse air flow is used for back blowing and soot cleaning, the instantaneous air pressure will impact the fiber in the filter material. With the increase of pulse times, the Fiber Cohesion decreases, resulting in stress defects and damage.
2.3 Chemical failure
In the actual process of the filter bag, the filter material is often faced with a variety of complex working conditions such as temperature change, humidity difference, oxidation corrosion, acid-base corrosion and so on, which can easily lead to the change of functional groups and chemical bonds in the fiber, resulting in the failure of the filter bag. For example, polyphenylene sulfide fiber has poor oxidation resistance and is prone to bag breaking caused by oxidation or acid-base corrosion; Polyimide fiber can be applied to high temperature conditions, but it is prone to hydrolysis in case of high humidity and alkaline conditions, which greatly reduces the service life of the filter bag.
Conclusion
To sum up, in the dust removal technology of the cement industry, bag dust removal is still the mainstream. With the development of environmental protection technology and increasingly strict emission requirements, bag dust removal technology has also developed. The commonly used technologies such as the use of membrane filter material, felt bag mouth design, electric and bag composite dust removal technology and collaborative control of various pollutants provide a new scheme for flue gas treatment. The failure caused by dust blockage, friction damage and chemical corrosion is the main reason affecting the service life of the filter bag.