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Longevity solution for coke oven door lining with refractory castables

Coke oven production furnace doors are opened frequently, the surface temperature is high, the temperature of coal loading and coke discharge changes greatly, the operating environment is poor, and the service life is short. The search for a new furnace door lining material with high strength, light weight, good thermal insulation and excellent thermal stability is a common concern among coking workers. The door lining structures of traditional domestic 4.3m coke ovens and 6m coke ovens are basically built with small refractory bricks such as cordierite bricks and clay bricks. Some coking plants have also tried refractory castables as furnace door lining materials, but Its service life is not long. In recent years, some ultra-large-volume coke oven door linings have adopted castable prefabricated block structures. However, prefabricated blocks produced with domestically produced castables cannot meet the design indicators of imported advanced foreign technologies. Especially in terms of service life, it is not even as good as the performance of used modules that have been removed from abroad.

Damage mechanism and performance requirements of coke oven door lining

The factors causing coke oven door damage are summarized as follows:

1) Chemical attack.

The products of the coking process are complex, mainly including H2, CH4, C2H6, C2H4, C3H8, CO, CO2, N2, benzene, toluene, xylene and some other compounds. The furnace door lining bricks have always been chemically attacked by these products. These active gaseous and liquid pyrolysis products first diffuse into the interior of the brick through the apparent pores of the brick, and eventually chemically react with the substances in the brick and physically diffuse to destroy the crystal lattice structure of the brick and reduce the comprehensive physical properties of the brick.

2) Thermal damage.

In each cycle of coking, the furnace door lining brick is subjected to three thermal shocks, namely 1000°C to room temperature, room temperature to 700°C, and 700°C to room temperature. This leads to cracks on the surface of the lining bricks and concentration of thermal stress at the corners, often causing damage to the brick corners.

3) Mechanical damage caused by carbon deposit cleaning.

During the use of the coke oven, coking products such as tar adhere firmly to the surface of the lining bricks. As the use cycle prolongs, the carbon deposit layer gradually thickens, causing the furnace door to not close tightly, resulting in air leakage and fire. Methods such as mechanical eradication and high-pressure water flushing are commonly used to clean up carbon deposits. The resulting mechanical force accelerates the damage of the furnace door lining bricks.

In order for the furnace door lining bricks to have a long service life, the lining bricks must have the following basic properties: 1) have good thermal shock stability and withstand the thermal shock during the coke oven production process; 2) have high strength ;3) It has extremely low thermal conductivity coefficient and density (can reduce energy consumption, save costs, and improve the working environment);. 4) It has high resistance to chemical erosion; 5) It has the ability to resist carbonization (removal of carbon is labor-intensive and causes environmental pollution).

Formula selection and performance testing

According to the working environment and damage mechanism of the coke oven door, the following three options are generally considered when studying the lining materials of the coke oven door:

1) Option 1, choose clay-fused quartz material;

2) Option 2, choose lightweight insulation materials;

3) Option 3, choose cordierite material.

Each of these three options has its advantages and disadvantages, but overall, option 1 is more ideal. Because thermal insulation materials are used in Option 2, the thermal insulation materials have the characteristics of low thermal conductivity, small thermal expansion coefficient and elastic modulus, and good thermal shock resistance. However, there are defects and disadvantages such as insufficient strength, large apparent porosity, coke and some chemical products of the coking process, which are more likely to penetrate into the interior of the lining bricks and corrode the lining bricks. Option 3 uses cordierite material. The cordierite lining brick has high strength, low thermal conductivity and good thermal shock stability. Its average thermal expansion coefficient is about 2×10-6/℃ (25~1000℃) , which can meet the requirements for furnace door lining bricks, but it is very expensive, has high masonry requirements, and is difficult to replace after partial damage.

The main reasons for choosing option 1 are:

1)The main materials in Option 1 are mainly clay. Clay materials are low-priced and high-strength. At the same time, they can be manufactured into molded modules or integrally cast. The production process is simple and it is more convenient to use in practice;

2)Although clay materials have shortcomings such as large thermal expansion coefficient and elastic modulus, poor thermal shock resistance, large thermal conductivity, and poor thermal insulation performance, by introducing fused quartz with extremely low thermal expansion coefficient into clay materials, the material can be utilized The dual effects of reducing the overall thermal expansion coefficient and toughening micro-cracks can significantly improve the thermal shock resistance of clay linings; since the volume density and thermal conductivity of fused quartz are lower than clay, introducing fused quartz into traditional clay bricks can Reduce the quality and thermal conductivity of the lining bricks to meet the performance of the coke oven door.

3)Introducing a second phase with a low thermal expansion coefficient into the refractory material, taking advantage of the inconsistent thermal expansion coefficients between the particles and the matrix phase in the refractory material, can cause micro-cracks or micro-pores in the product, which can significantly improve the thermal shock resistance of the refractory material. The thermal expansion coefficient of fused quartz is about 2×10-6/℃, which is much smaller than the thermal expansion coefficient of mullite and cristobalite of clay clinker. If you choose to introduce granular fused quartz into the clay castable to replace the clay particles of the same particle size, it will ensure that the overall particle size distribution of the castable remains unchanged. In order to reduce the impurity content in the matrix and try to suppress the transformation of fused quartz to cristobalite, pure calcium aluminate cement was chosen as the binding agent instead of alumina cement in the experiment. The system was positioned as a low-cement castable system. By changing the fused quartz content, the Comparative analysis of relevant performance indicators, the chemical composition of the main raw materials used in the test is shown in Table 1, and the test plan for the addition of fused quartz is shown in Table 2. The relevant test results are as follows.

Table 1 Chemical composition of raw materials

raw material Al₂O₃ SiO₂ CaO Fe₂O₃ K₂O Na₂O other
clay 46.29 49.58 1.32 0.06 0.08 2.67
Pure calcium aluminate cement 69.36 29.63 17.64

Table 2 Castable formula

Recipe number B1 B2 B3 B4 B5 B6
Jiao gem 0~3 mm 30 28 22 16 8 0
Fused quartz 0~3 mm 0 8 14 20 28 36

1) Line change rate. The influence of the addition amount of fused quartz on the linear change rate of the castables is shown in Figure 1. The castables are in an expansion state after being fired at 800°C, and are in a shrinking state after being fired at 1100°C, and the change rates are not large.

Figure 1 Effect of the addition amount of fused quartz on the change rate of the pouring material line

2) Strength. The effect of the added amount of fused quartz on the strength of the castable is shown in Figure 2. The flexural strength gradually decreases as the added amount of fused quartz increases; while the compressive strength generally shows a downward trend.

Figure 2 Effect of fused quartz addition amount on castable strength

3) Bulk density and apparent porosity. The bulk density and apparent porosity of the castable gradually decreased with the increase in the amount of fused quartz added. The results are shown in Figure 3.

Figure 3 Effect of the addition amount of fused silica on the volume density and apparent porosity of castables

4) Thermal shock resistance. As the amount of fused quartz added increases, the flexural strength retention rate of the castable after five water-cooling thermal shocks gradually increases. The results are shown in Figure 4.

Figure 4 Effect of the addition amount of fused silica on the flexural strength retention rate of the specimen after thermal shock

In order to further confirm the various properties between clay-fused silica castables and ordinary clay castables, a comparative experiment was conducted based on the proportions in Table 3. Comparative test results show that the online changes, compressive strength, and flexural strength of clay-fused quartz castables are similar to those of clay castables, but their thermal shock resistance is significantly better than ordinary clay castables. The measured values of comparative performance are shown in Table 4.

Table 3 Castable ratios in comparative experiments

Item Linear change rate (1250℃)/% Flexural strength/MPa Compressive strength/MPa Linear change rate after burning (1250℃)/%  Burned bulk density (g.cm-3) Apparent porosity/% Thermal conductivity ( 800℃)/(W.m-1.K-1)  Thermal shock resistance/time (1100℃ water cooling)
index ≥0.4 ≥5.7 ≥55 ≥0.4 ~2.15 ≤0.78 ≥30
110℃ 110℃
≥2.7 ≥20
1200℃ 1200℃
Recipe A After baking at 110℃×24h -0.06 7.61  61.3 2.26 15.8 0.637 33
After burning at 800℃×3h -0.22   5.31   50.5 2.20 20.8
After burning at 1100℃×3h -0.16   4.21   33.4 2.18 21.5
After burning at 1200℃×3 h -0.14   5.05   36.3 2.13 24.1
Recipe B After baking at 110℃×24h -0.03    6.97     56.0 2.18 14.1 0.625 >50
After burning at 800℃×3h +0.11   4.72    48.7 2.13 17.6
After burning at 1100℃×3h +0.38   5.73   64.5 2.13 17.6
After burning at 1200℃×3 h +0.16   5.90   60.5 2.15 16.2

Table 4 Comparison of test block performance in the laboratory stage and actual measured values

Item Linear change rate (1250℃)/% Flexural strength/MPa Compressive strength/MPa Linear change rate after burning (1250℃)/%  Burned bulk density (g.cm-3) Apparent porosity/% Thermal conductivity ( 800℃)/(W.m-1.K-1)  Thermal shock resistance/time (1100℃ water cooling)
index ≥0.4 ≥5.7 ≥55 ≥0.4 ~2.15 ≤0.78 ≥30
110℃ 110℃
≥2.7 ≥20
1200℃ 1200℃
Recipe A After baking at 110℃×24h -0.06 7.61  61.3 2.26 15.8 0.637 33
After burning at 800℃×3h -0.22   5.31   50.5 2.20 20.8
After burning at 1100℃×3h -0.16   4.21   33.4 2.18 21.5
After burning at 1200℃×3 h -0.14   5.05   36.3 2.13 24.1
Recipe B After baking at 110℃×24h -0.03    6.97     56.0 2.18 14.1 0.625 >50
After burning at 800℃×3h +0.11   4.72    48.7 2.13 17.6
After burning at 1100℃×3h +0.38   5.73   64.5 2.13 17.6
After burning at 1200℃×3 h +0.16   5.90   60.5 2.15 16.2

Application and effect

According to the experimental data, a castable prefabricated block with 4 furnace doors was produced according to Formula B after optimization and was used in the No. 2 7.63m coke oven of Wuhan Iron and Steel Co., Ltd. Compared with the castable prefabricated blocks produced by other domestic manufacturers, the prefabricated blocks produced according to formula B are well formed and there is no corner drop phenomenon. The appearance quality after the oven is also very good and there are no fine cracks (see Figure 6). , has good thermal insulation performance (as shown in Table 5) and can meet the requirements of coke oven production. This product was promoted in two 7.63m coke ovens in Shagang. It has been used for 4 years and the furnace door lining is currently in good condition.

Figure 6 Pictures of the molding module before and after the oven

Table 5 Comparison of thermal insulation performance with similar products from other domestic manufacturers °C

 

Furnace conditions

 

Machine side coke oven door (fused quartz-clay)
Ordinary furnace door No. 52 outer surface temperature Test furnace door (No. 70) outer surface temperature Ordinary furnace door No. 68 outer surface temperature
Furnace temperature 200℃ 64 54 64
Furnace temperature 500 ℃ 110 78 107
Furnace temperature 700℃ 115 95 110
Furnace temperature 800 ℃ 120 100 116
Coke oven normal production status 125 105 126

Conclusion

1) Introduce fused quartz with extremely low thermal expansion coefficient into ordinary clay castables, and use the dual effects of reducing the overall thermal expansion coefficient of the material and toughening micro-cracks to significantly improve the thermal shock resistance of the clay-based lining and solve the problem of strength There is a conflict between high strength and thermal shock resistance and thermal insulation performance. Practice has proved that the strength, thermal shock resistance and thermal insulation performance of clay-fused quartz castables can meet production requirements.

2) The castables can be made into large modules. The installation process is simple, the installation process is highly mechanized, and the labor intensity is low. The damaged furnace door lining can be partially replaced in time. At the same time, it has thermal insulation performance, strength, and resistance to rapid cooling and heating. It has relatively good properties, does not require high-temperature roasting, is energy-saving and environmentally friendly, and has a long service life.

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