Introduction to filter materials for pulverized coal injection and coal mill purification in blast furnaces

Selection and determination of filter materials for dust source control in blast furnace pulverized coal injection, coal milling, and other systems

                                                                              "——Special Introduction Series of Berg Filter Materials Part III"

Application field

Bag filter for dust source control in pulverized coal injection system of blast furnace in ironmaking process of steel plant;

Bag filter for dust source control in cement plant coal mill (also serving as drying mill) system;

Bag filters are used for dust source control in other coal grinding systems.

Berg Recommendation

Stäf series anti-static filter needle felt;

Metas series anti-static filter needle felt.

Product performance analysis and comparison

(I) Introduction to pulverized coal injection system of blast furnace

Pulverized coal injection into blast furnaces is an ironmaking process that was widely applied in steel production during the 1960s and 1970s, marking a significant technological revolution in blast furnace ironmaking. Since pulverized coal can replace a portion of coke, achieving coal substitution for coke, it can significantly reduce coke consumption, thereby lowering production costs. Additionally, pulverized coal injection into blast furnaces is a central aspect of optimizing the structure of the ironmaking system and a means of adjusting furnace conditions to ensure smooth, stable, and high-yield production. The filter material of the pulverized coal collector plays a crucial role in the system.

The filter material currently used in coal injection systems typically employs polyester anti-static needle felt. In practical applications, there are instances of unstable performance, primarily manifested as powder leakage and dust emission from the collector. After communicating with relevant enterprises, BURKERT has developed an ultra-fine fiber surface layer anti-static needle felt - Stäubli anti-static filter material.

(II) Factors affecting the filtration efficiency of filter materials

1. The influence of fiber diameter on filtration efficiency

It is generally believed that there is a direct relationship between the fiber diameter of filter materials and filtration efficiency. For example, 2.5 denier polyester fibers (with a diameter of 14.1µm) typically exhibit high filtration efficiency for dust particles with diameters above 7µm. During the filtration process, dust tends to enter the filter material initially. Once a dust cake forms on the surface, it blocks smaller dust particles from entering the filter material. BURKERT has adopted finer fibers to manufacture ultra-fine surface layer needle-punched filter materials and made improvements in the post-processing technology, achieving excellent results in practical use. For instance, our developed PET ultra-fine surface layer needle-punched filter material has been produced on a large scale.

2. The influence of fiber specific surface area on filtration efficiency

In practical testing, we found that microfiber exhibits superior filtration efficiency, and the specific surface area is not necessarily a decisive factor in practical use. For instance, some fibers have irregular cross-sections, resulting in a significantly larger specific surface area compared to other fibers of the same denier. When comparing fibers with irregular cross-sections of 1.7 denier to 1 denier PET fibers, we found that the specific surface area of the irregular fibers is 13.1% larger than that of the 1 denier PET fibers. However, the filtration efficiency of the 1 denier PET fiber needle-punched filter felt for 0.7µm dust is significantly higher than that of the 1.7 denier irregular fiber needle-punched filter felt. Based on this understanding, we have developed various high-efficiency filtration needle-punched felts made of microfiber to enhance the filtration efficiency and overall performance of the product. Their specific surface areas are all around 300㎡/kg, which is much smaller than that of multi-leaf cross-section fiber materials with the same linear density.

Due to the high filtration resistance of needle-punched filter felt made entirely of microfiber, which can increase system resistance, we adopted a processing method with a gradient density variation. The filtration surface is made of microfiber, which is conducive to improving filtration efficiency, while the reverse side uses coarser fibers to reduce pressure drop. By comparing various technical parameters, we can see that the main physical indicators of the microfiber surface layer filter material are almost indistinguishable from those of ordinary needle-punched filter felt.

(III) Application comparison between ultra-fine surface needle felt and ordinary needle felt

We conducted tests on the practical application of ultra-fine surface needle felt. By comparing it with ordinary needle felt, we can more intuitively observe the characteristics of ultra-fine surface needle felt. See Table 1 and Figure 1 for details.

Table 1 Comparison of two different filter materials before and after use (laboratory simulation, 5000 hours, average values)

Figure 1 Comparison of performance between Stafford and ordinary filter materials

We can observe that compared to ordinary needle felt, ultra-fine surface needle felt exhibits excellent dust removal performance. In practical use, the system resistance increases much slower with ultra-fine needle felt than with ordinary needle felt. This is attributed to the larger specific surface area and smaller fiber gaps of ultra-fine fibers, which achieve better filtration performance and efficiency than ordinary filter materials. It is more suitable for the powder collection requirements of pulverized coal injection systems.

(IV) Characteristics of ultra-fine surface needle felt

Higher strength: High-strength low-elongation industrial yarn is used as the base fabric, with high needle punching density and tightly packed fibers, significantly enhancing the strength.

Smaller pore size: Generally, products use 2.5D fibers, while the new filter material uses 0.8~1.5D or finer fibers. The fiber diameter is reduced several times, resulting in a smaller pore size, higher porosity, and improved filtration accuracy.

Lower elongation: The fibers of the filter felt are tightly intertwined, resulting in high density and significantly reduced elongation. Using high-strength, low-elongation fibers as the base fabric further minimizes elongation.

High acid and alkali resistance: Due to the use of high molecular weight polyester superior fibers with direct spinning technology, the acid and alkali resistance, as well as hydrolysis resistance, have been improved.

More wear-resistant and fold-resistant: Due to the high strength of the filter felt, the large number of fibers per unit volume, high density, and robustness, it is wear-resistant and has a longer service life.

High uniformity: Multiple opening and mixing processes, coupled with fine carding, enhance the uniformity of the fiber web, resulting in an overall uniform filter felt with a high level of surface flatness.

Smooth surface: Due to its uniformity, fine and dense fibers, the surface can be calendered to achieve a smooth mirror finish.

High filtration accuracy: Due to the fine micropores of the filter felt, particles with diameters smaller than the pore size are blocked on the surface of the filter material, making it difficult for them to enter the interior of the filter material. The smaller the pore size, the higher the accuracy.

Low resistance: The layered composite structure with a gradient of the new filter material is more conducive to resolving the contradiction between filtration accuracy, resistance, and energy consumption. It results in minimal resistance loss during operation and saves energy.

Excellent surface filtration performance: The smooth surface of the filter material, coupled with finer "S"-shaped micropores, prevents finer particles from penetrating, leaking, or clogging the filter felt. This allows these particles to accumulate on the surface of the filter felt, facilitating easy dust removal through shaking, thus achieving a new filtration cycle.

In summary, the comprehensive performance of ultra-fine surface needle-punched filter material is closer to that of surface filter material.

• 技术参数

application performance

A certain steel company currently has two ball mills, one with a nominal output of 16 t/h and the other with 10 t/h, in two separate series. In each series, a disc feeder is located at the bottom of the raw coal bunker to feed coal into the ball mill. Hot flue gas generated by a flue gas generator is drawn into the ball mill to dry and grind the raw coal inside. After being separated by a coarse powder separator, the unqualified coal powder is separated out and sent back to the ball mill for further grinding; the qualified coal powder is sent to a cyclone dust collector and a bag filter for collection. The filtered clean gas is discharged into the atmosphere through an induced draft fan. The collected qualified coal powder is sent to the coal powder bunker via a buried scraper conveyor. The specific process flow is shown in Figure 2.