How to choose the filter media for boiler flue gas purification

Application areas:

Heat supply, power station boiler flue gas purification bag filter.

Coal-fired, gas-fired boiler flue gas purification bag filter.

Straw direct-fired power boiler flue gas cleaning bag filter.

Flue gas cleaning baghouse for fuel-burning boilers.

Waste incineration and waste and coal mixed boiler flue gas cleaning baghouse.

Flue gas cleaning baghouse for domestic and medical waste incineration boilers.

SINO recommends:

Filter materials made of PPS.

Filter materials made of a combination of two or three fibres, PPS, P84 and PTFE.

PTFE, glass fibre composite filter material.

Analysis of the working conditions and the selection and determination of the filter material

Boiler flue gas purification using bag filter generally requires synchronisation with the boiler equipment overhaul period, 30,000 hours of filter media (filter bag) life has now become the basic use period required by thermal power plants. In addition to special requirements such as heating boilers, straw power boilers, etc., the service life of the filter media is generally required for straw power boilers for 16,000 hours and for heating boilers for 3 heating periods. However, the same coal-fired power station boiler flue gas dedusting, the domestic application conditions and foreign differences, so in the selection of filter media to focus on the analysis of flue gas performance characteristics, and should pay attention to the following points.

(1) The type of coal used: foreign boilers usually burn fine coal, while China is mainly burning raw coal, there are differences in calorific value, ash, S content, etc..

(2) Equipment technology: boilers and other related equipment used in developed countries abroad have a higher degree of automation, while domestic equipment generally has a combination of manual and automatic conditions, which has a great impact on the stability of various characteristics of the flue gas.

(3) Boiler equipment: boiler equipment in developed countries abroad are relatively advanced, while there are still a large number of small and medium-sized equipment in China (especially early boilers), in addition to the difference between old and new boilers.

(4) Comprehensive use of energy: the combustion of blast furnace gas, resulting in larger fluctuations in flue gas volume and dust concentration, changes in gas composition, higher flue gas temperature, etc.

(5) There are also certain differences in combustion methods, smoke exhaust methods, etc.

Therefore, resulting in the application of filter media environment for the four high a low (high flue gas emission temperature, flue gas in SO2, NOX content is high; high oxygen content in the flue gas; dust concentration is large; requires low filter bag resistance). Due to the relatively harsh flue gas conditions of coal-fired boiler emissions, the service life of the filter media (filter bags) will be affected. Therefore, when using filter media in the bag filter, learn from but not blindly copy the technology and experience of foreign counterparts, the actual situation of domestic boilers should be taken into account and the filter media should be selected in a targeted manner.

(a) General working conditions

Most of the current power plant unit boiler bag filter material selection PPS needle felt filter media. There is also the choice of PPS + P84 composite fibre needle felt filter media, P84 fibre content in about 20%. Mainly for boiler flue gas emission temperature or dust collector inlet temperature below 170°C, boilers burning low sulphur coal.

PPS scientific name polyphenylene sulfide, English name Poly Phenylene Snlfide (PPS for short), PPS fiber worldwide only a few large chemical companies to produce, the Japanese company's products occupy the dominant market. Japan Toyobo (TOYOBO) company's registered trademark for PROCON / Pu resistance, Japan Toray (TORAY) company products registered trademark for TORCON / Triton, the United States Philips (PHILIP) company's registered trademark for RYTON / Leighton.

P84 is the scientific name of polyimide, P84 fiber cross-section and PPS fiber rod compared to the multi-leaf shape, its irregular fiber cross-section to increase the fiber surface area, increase the needle felt permeability, with the effect of reducing resistance to increase efficiency. P84 fiber is the exclusive production of Austrian LENZING company.

PPS fibres are used in many applications due to their good resistance to acid and alkali corrosion, and needled felts made from them play an important role in the purification of power plant boiler flue gases. In coal-fired boilers, waste incinerators, power plant fly ash dust collection and treatment in the field of pulse bag filter is widely used. PPS filter felt in general working conditions show excellent filtration performance, the application conditions mainly depend on the PPS fibre performance, coal-fired flue gas dust removal is more suitable for the use of the following conditions.

Operating temperature. Long-time working flue gas temperature of 120-160°C.

Oxygen content. Flue gas oxygen content of 8% or less can be applied.

Sulphur and nitrogen oxide content. SO2 content in flue gas <800mg/m3; NOx content <700mg/m3 and NO2 content <15mg/m3.

Ash content. Ash content of coal combustion <30%.

Flue gas humidity. Water content in flue gas <12%.

Pre-dusting required before boiler operation start-up.

Oil injection and ignition of the furnace, boiler burst pipe, flue gas over temperature should not be directly into the bag filter, need to set up a bypass flue.

Baghouse dust collector design filtration air speed around 1.0m/min, dust collector design resistance 1500Pa.

In order to reduce the resistance of the filter material and increase the temperature resistance, PPS+P84 composite fibre needled felt is used.

(ii) Special working conditions

PPS fibres are traditionally found in the field of high temperature dust collection, mainly in the form of single gauge PPS fibres needled into felts. The filter bag made of it shows the good characteristics of PPS, mainly acid and alkali corrosion resistance, good resistance to hydrolysis. However, due to the special inherent characteristics of China's coal-fired boilers, such as high sulphur content, dust, high oxygen content, moisture content, etc., conventional PPS needle felt in China's power plants and other industrial mines when used in the results are not very good. In order to solve these problems, the world's filter media manufacturers have developed a variety of improved PPS needle felt products, such as PPS coated filter media and Borg's HBT/PPS gradient special filter media. The main focus is on the high temperature, high humidity and high sulphur characteristics of the flue gas inlet to the dust collector, or the simultaneous operation of the desulphurisation unit and the dust collector, with an exhaust temperature of 170°C or more.

In order to reduce the resistance of the filter media, the filter media needle felt made of a single fineness of PPS fibres (referred to as conventional filter media) is further improved by using different finenesses of PPS fibres made of filter media needle felt with a gradient structure of fibres (referred to as gradient filter media).

The main technical characteristics are

(1) Needle felt cross-sectional structure using a gradient hierarchy of increasing fibre fineness layer by layer, it consists of a superfine fibre layer on the surface, a fine fibre layer, a base cloth and a coarse fibre layer in 4 layers. The finer the fibres, the more dense the air passages. The gradient filter media features: the fibres in the filter media take a gradual increase in fineness from the dust surface to the net air surface, forming a dense fluid channel before and after the sparse, from the traditional needle felt equal width around the flow channel changed to the narrow trapezoidal flow channel before and after the wide, making the filter media in the filter aperture like a curved trumpet, the dust surface aperture is very small, close to the laminated filter media, high filtration accuracy, the closer the net air surface, the larger the aperture, so that in The closer to the net air surface, the larger the pore size, so that the dust in the filter is not easy to block the tiny pores of the filter media, good permeability, thus reducing the pressure drop of the filter media, reducing the number of times to clean the ash, saving energy, but also to extend the service life of the filter media.

(2) Drawing on the surface filtration technology of the laminated filter media. The traditional single fineness fibre or mixed fibre needle felts are transformed into gradient fibre needle felts, the filtration method is changed from "surface + deep" to "surface filtration", the ultra-fine fibres form the surface layer of the filter media to improve the ability to block microscopic dust particles.

The narrow front and wide back trapezoidal channel makes the inner layer of the filter media less prone to dust collection and keeps the filter media running for a long time while the change in the inherent resistance of the filter media is slowed down.

The surface layer is made of ultra-fine fibres, the ultra-fine surface layer makes the needle felt surface more dense and increases the ability to block fine dust particles, and the surface is easy to clean. The different filtration methods are illustrated in Figure 2.

(3) The needle felt fibres are treated with PTFE (polytetrafluoroethylene) impregnation, so that the surface of the fibres is coated with a PTFE film, see Fig. 4.

(4) Needle felt fibres are treated with silicone impregnation to enhance the wear resistance, water repellency and temperature resistance of the fibres.

With the use of various technical improvements, the performance of the filter media has been significantly improved and the service life of the filter media can be extended. The main technical indicators of the improved filter media are: flue gas temperature 120-180°C, SO2 content <2000mg/m3, NOx content <1200mg/m3, oxygen content <10%, dust collector resistance 1200Pa.