Selection and determination of filter material for boiler flue gas purification

                                                                              "——Part One of the Special Introduction Series on Berg Filter Materials"

Application field

Bag filters for flue gas purification of heating and power station boilers;

Bag filter for flue gas purification of coal-fired and gas-fired boilers;

Bag filter for flue gas purification of straw direct combustion power generation boiler;

Bag filter for flue gas purification of fuel mixed combustion boiler;

Bag-type dust collector for flue gas purification of garbage incineration and mixed combustion boilers with coal addition;

Bag-type dust collector for flue gas purification of domestic and medical waste incineration boilers.

Berg Recommendation

PPS material filter material;

Composite filter material made of two or three types of fibers: PPS, P84, and PTFE;

PTFE and glass fiber composite filter material.

Analysis of operating conditions and selection and determination of filter materials

The bag filters used for boiler flue gas purification are generally required to be synchronized with the boiler equipment overhaul period, and a service life of 30,000 hours for filter materials (filter bags) has now become the basic requirement for thermal power plants. Except for special requirements such as heating boilers and straw power generation boilers, the general requirements for the service life of filter materials are 16,000 hours for straw power generation boilers and 3 heating periods for heating boilers. However, due to differences in application conditions between domestic and foreign coal-fired power station boiler flue gas dust removal, it is important to analyze the flue gas performance characteristics when selecting filter materials, and the following points should be noted:

(1) Coal type: Foreign boilers usually burn clean coal, while in China, raw coal is predominantly used, resulting in differences in calorific value, ash content, sulfur content, and other aspects;

(2) Equipment technology: The automation level of boilers and other related equipment used in developed countries is relatively high, while domestic equipment generally has a combination of manual and automatic operations, which has a significant impact on the stability of various flue gas characteristics;

(3) Boiler equipment: Boiler equipment in developed countries abroad is relatively advanced, while there are still a large number of small and medium-sized equipment (especially early boilers) in China, and there are also differences in the age of the boilers;

(4) Comprehensive energy utilization: The combustion of blast furnace gas leads to significant fluctuations in flue gas volume and dust concentration, changes in gas composition, and an increase in flue gas temperature.

(5) There are also certain differences in combustion methods, smoke exhaust methods, and other aspects.

Therefore, the application environment of filter materials is characterized by four highs, one large, and one low (high temperature of discharged flue gas, high content of SO2 and NOX in flue gas; high oxygen content in flue gas; high dust concentration; low resistance required for filter bags). Due to the relatively harsh conditions of flue gas emitted from coal-fired boilers, it will affect the service life of filter materials (filter bags). Therefore, when using filter materials in bag filters, we should learn from but not blindly copy foreign peer technology and experience, and consider the actual situation of domestic boilers to select filter materials in a targeted manner.

(I) General operating conditions

Currently, the filter bag material for bag filters in power plant boiler units mostly chooses PPS needle felt filter material. There are also options for PPS+P84 composite fiber needle felt filter material, with a P84 fiber content of around 20%. It is mainly targeted at boilers that burn low-sulfur coal and have boiler flue gas emission temperatures or dust collector inlet temperatures below 170℃.

PPS, whose scientific name is polyphenylene sulfide and English name is Poly Phenylene Sulfide (abbreviated as PPS), is produced globally by only a few large chemical companies, with Japanese companies dominating the market. The registered trademarks of products from Japan's TOYOBO are PROCON/Pukang, those from Japan's TORAY are TORCON/Telitong, and those from the American company PHILIP are RYTON/Layton.

P84, scientifically known as polyimide, exhibits a multi-leaf-like cross-section compared to the rod-like cross-section of PPS fibers. Its irregular fiber cross-section enlarges the surface area of the fibers, enhancing the breathability of needle felt and achieving the effect of reducing resistance and increasing efficiency. P84 fibers are exclusively produced by the Austrian company LENZING.

PPS fibers are widely used in various working conditions due to their excellent acid and alkali corrosion resistance. Needle-punched filter felts made from PPS fibers play a significant role in areas such as flue gas purification in power plant boilers. Due to their complete strength retention and inherent chemical resistance, PPS fibers can maintain good filtration performance in certain environments and achieve an ideal service life. They are widely used in pulse bag filters in areas such as coal-fired boilers, waste incinerators, and dust collection and treatment of fly ash in power plants. PPS filter felts exhibit excellent filtration performance under general working conditions, and their application conditions mainly depend on the properties of PPS fibers. The suitable conditions for coal-fired flue gas dust removal are as follows:

Operating temperature. Long-term operating flue gas temperature: 120-160℃;

Oxygen content. It is applicable to situations where the oxygen content in flue gas is 8% or below;

Sulfur and nitrogen oxide content. The content of SO2 in flue gas is less than 800mg/m3; the content of NOx is less than 700mg/m3, and the content of NO2 is less than 15mg/m3;

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

Flue gas humidity. The moisture content in the flue gas is less than 12%;

Before starting the boiler operation, pre-coating with ash is required;

When there are issues such as oil spraying points, boiler tube bursting, or flue gas overtemperature, it is not advisable to directly enter the bag filter. A bypass flue should be established;

The bag filter is designed with a filtration wind speed of approximately 1.0 m/min and a design resistance of 1500 Pa.

The filter bag of a normally operating bag filter has a guaranteed service life of 30,000 hours. To reduce the resistance of the filter material and enhance its temperature resistance, PPS+P84 composite fiber needle felt is used.

(II) Special operating conditions

In the traditional field of high-temperature dust collection, PPS fibers mainly appear in the form of needle-punched PPS fiber felts with a single specification. The filter bags made from them exhibit the good properties of PPS, mainly including resistance to acid and alkali corrosion and good hydrolysis resistance. However, due to the inherent characteristics of China's coal-fired boilers, such as high sulfur content, high dust content, high oxygen content, and high moisture content, conventional PPS needle-punched felts do not perform well in industrial and mining applications such as power plants in China. To address these issues, filter material manufacturers around the world have successively developed various improved PPS needle-punched felt products, such as PPS coated filter materials and the HBT/PPS gradient special filter materials from BURKERT. These products are mainly targeted at the characteristics of the inlet flue gas of dust collectors, which are high temperature, high humidity, high sulfur content, or the simultaneous operation of desulfurization devices and dust collectors, with flue gas exhaust temperatures above 170°C.

In order to reduce the resistance of filter material, the needle-punched felt made of PPS fibers of a single fineness (referred to as conventional filter material) has been further improved to a needle-punched felt with a fiber gradient structure made of PPS fibers of different finenesses (referred to as gradient filter material), as shown in Figure 1. It is named HBT/PPS needle-punched felt filter material, abbreviated as HBT by Huabote.

The main technical features are as follows:

(1) The cross-sectional structure of the needle-punched felt adopts a gradient layered structure with increasing fiber fineness from layer to layer. It consists of four layers: an ultra-fine fiber layer on the surface, a fine fiber layer, a base cloth, and a coarse fiber layer. The finer the fibers, the denser the ventilation channels. Characteristics of gradient filter material: The fibers in the filter material gradually increase in fineness from the dust-facing side to the clean air side, forming fluid channels that are dense at the front and sparse at the back. This changes the traditional needle-punched felt's uniform-width bypass channels to trapezoidal bypass channels that are narrower at the front and wider at the back. This makes the filtration pore size in the filter material resemble a curved trumpet shape, with a very small pore size on the dust-facing side, similar to that of membrane filter materials, achieving high filtration accuracy. The closer to the clean air side, the larger the pore size. This makes it less likely for dust to clog the tiny pores of the filter material during filtration, resulting in good ventilation, thereby reducing the pressure drop across the filter material, decreasing the frequency of dust cleaning, saving energy, and extending the service life of the filter material.

Figure 1 Schematic diagram of gradient filter material structure

(2) Drawing inspiration from the surface filtration technology of membrane-covered filter materials, we have transformed traditional needle-punched felt made of single fineness fibers or mixed fibers into gradient fiber needle-punched felt. The filtration method has shifted from "surface + deep layer" to "surface filtration", and the ability of the filter material surface layer formed by ultra-fine fibers to block fine dust particles has been enhanced.

The trapezoidal channel, narrow at the front and wide at the back, makes it difficult for dust to accumulate in the inner layer of the filter material, ensuring long-term operation of the filter material while slowing down the rate of change in its inherent resistance.

The surface layer is made of microfiber, which makes the needle felt surface denser, improving its ability to block fine dust particles, and the surface is easy to clean. The schematic diagram of different filtration methods is shown in Figure 2.

⑶ The needle felt fibers are treated with PTFE (polytetrafluoroethylene) impregnation, resulting in a PTFE film coating on the fiber surface, as shown in Figure 4. This treatment enhances the corrosion resistance, oxidation resistance, and temperature tolerance of the fibers, and improves the water repellency of the filter material.

（4）

The needle felt fibers are treated with silicone impregnation to enhance their wear resistance, hydrophobic properties of the filter material, and temperature resistance.

After various technical improvements, the performance of the filter material has been significantly enhanced, allowing for an extended service life. The main technical indicators for the improved filter material are as follows: flue gas temperature ranging from 120 to 180℃, SO2 content in the flue gas less than 2000mg/m3, NOx content less than 1200mg/m3, oxygen content less than 10%, and dust collector resistance at 1200Pa.

（三）

Complex working conditions

To further expand the application scope of filter materials for boiler flue gas purification in coal-fired power plants and other applications: boiler combustion involves significant variations in coal quality, such as high sulfur coal; there are substantial fluctuations in temperature, humidity, dust content, sulfur content, etc.; the boiler experiences frequent start-ups and shutdowns (resulting in low temperatures ranging from 60-120℃, acid condensation corrosion); coal and gas are co-fired (with exhaust gas temperatures reaching 180-220℃); excessive air blasting in the boiler, air leakage in the air preheater, leading to increased oxygen content, and other issues. If PPS is used exclusively, it will inevitably affect the service life. Therefore, it is necessary to select HBT/PTFE composite needle felt filter materials, replacing PPS and P84 fibers with PTFE fibers, to meet the application requirements of filter materials under the aforementioned complex working conditions.

PTFE, also known as "plastic king", is one of the best corrosion-resistant materials in the world today. Its scientific name is polytetrafluoroethylene. It exhibits high chemical stability and excellent resistance to chemical corrosion, such as strong acids, alkalis, and oxidants. It also boasts outstanding heat resistance, cold resistance, and wear resistance, and possesses characteristics such as non-stickiness, non-absorption of water, and non-flammability.

Given the relatively high price of PTFE fiber, it is generally used as the main component, with an appropriate amount of other materials such as glass fiber mixed in. The price of this filter material is influenced by factors such as the amount of PTFE mixed, the other fibers used, and their mixing proportions.

Glass fiber exhibits excellent properties such as high temperature resistance, corrosion resistance, dimensional stability, minimal elongation and shrinkage, high strength, and chemical stability, with a normal operating temperature above 260°C. The filter material made from it possesses a three-dimensional microporous structure, high porosity, and low resistance to gas filtration, making it an efficient high-temperature filter material. Compared to other high-temperature resistant chemical fiber felts, it has the special advantages of low price and higher temperature resistance.

The HBT/PTFE composite needle felt filter material typically adopts the following structure:

Front layer: PTFE100 microfiber

Base layer: (PTFE/GLASS) fine fibers

Base cloth: (PTFE/GLASS) base cloth

Back layer: (PTFE/GLASS) coarse fiber

among which

(1) Adopt a filter material structure primarily composed of PTFE fibers, with a grammage of not less than 750g; the ratio of PTFE fibers to high-strength glass fibers is determined according to the actual requirements of the user. Do not use PPS fibers to accommodate frequent use at high temperatures around 220℃.

(2) The moderate use of high-strength glass fiber is entirely aimed at ensuring that the performance of the filter material is not compromised under working conditions, while also meeting the user's demand for filter material price.

(3) Compared with PTFE membrane-coated fiberglass filter material, the structure replaces the PTFE membrane with a PTFE microfiber filtration layer on the dust-facing side, enhancing the reliability of PTFE usage. In terms of filtration mechanism, it maintains surface filtration to ensure the stability and long-term durability of the filter material's filtration performance.

Additionally, the experience of using coated filter media in a certain power plant indicates that the actual operating resistance reaches above 1600-2000Pa, making it difficult to meet the low resistance requirements under the gas-to-cloth ratio filtration velocity specified by the customer's technical specifications. The coating on the coated filter media is damaged under complex operating conditions such as dusty airflow scouring in dust collectors (see Figure 5).

• 部分过滤材料技术参数

1、PPS PTFE覆膜滤料与梯度特种滤料参数对比

2、华博特（HBT）产品性能参数

• 应用业绩