How do I use a blast furnace - part 2

Blast furnace's air supply system

The blast quantity, pressure, temperature, humidity, oxygen enrichment rate, wind speed at the tuyere (or blast kinetic energy), flame temperature in front of the tuyere and other parameters shall be determined according to the furnace capacity, equipment conditions, raw materials, fuel conditions, tuyere injection conditions and smelting iron.

Air volume

Volume of air entering blast furnace in standard state per unit time (m3 / min or m3 / h). Under the same conditions, the greater the air volume, the higher the output. Blast furnace air volume first depends on the blast furnace volume, which is generally 2.0 ~ 2.2m/min per cubic meter. Because the measurement of air volume is often inaccurate due to air leakage and instrument error, and the air volume is directly proportional to the consumption of coke and injection fuel, blast furnace operators are used to estimating the air volume based on smelting intensity. Under the same conditions, the number of charging batches of blast furnace is directly proportional to the air volume, so the blast furnace operator actually controls the air volume according to the number of charging batches. The smelting intensity depends on the quality of raw materials, fuel and the type of iron to be smelted, which is generally between 0.9 ~ 1.2t / (m • d). Take the upper limit when the quality of raw materials and fuels is good; Otherwise, take the lower limit. The smelting intensity of smelting cast pig iron should be lower than that of smelting steel-making pig iron, and that of SMELTING FERROMANGANESE should be lower than that of smelting cast pig iron. The higher the volume of gas passing through the furnace column, the more difficult it is to pass through the actual furnace temperature. When the production of blast furnace needs to be limited, the smelting intensity and air volume shall be determined according to the rated pig iron output. The air volume blown into the blast furnace and the number of charging batches per hour (charging speed in the furnace) shall be stable. The fluctuation of air volume will affect the fluctuation of material speed and furnace temperature, and further cause the fluctuation of air pressure and instability of furnace condition. Therefore, the blast furnace air volume should not be increased or decreased at will after being selected at an appropriate level. Only when the furnace is cool, the blanking is not smooth, or the equipment fault requires air reduction, can the air be reduced. Once the conditions permit to restore the air volume after air reduction, it shall be restored step by step in time.

Wind speed

The speed of blast at the outlet of tuyere, usually in M / s. The wind speed has an important influence on the gas flow distribution in the lower part of the blast furnace. High wind speed and strong penetration are conducive to extending the tuyere raceway, increasing the central gas flow and increasing the central temperature. However, the higher the wind speed, the better. It has an appropriate range according to different conditions. If the wind speed is too small, it is easy to cause accumulation in the center of the hearth; If the wind speed is too high, it is easy to form edge accumulation. The wind speed can be divided into standard wind speed and actual wind speed: the former is calculated according to the air volume under the standard state, and the latter is calculated according to the air volume under the actual blast furnace air temperature and air pressure. The standard wind speed of blast furnace is mostly between 80 ~ 200m / s. The larger the blast furnace, the more or shorter the tuyere, the smaller the height diameter ratio of the blast furnace, the lower the smelting intensity, the higher the oxygen enrichment rate, the less fuel injected, and the closer the wind speed is to the upper limit; Otherwise, it is close to the lower limit. The wind speed is properly selected, the hearth is active, and the furnace condition is stable and smooth. Therefore, after determining the air volume, the blast furnace operator should carefully select the wind speed according to the specific conditions, and determine the tuyere diameter accordingly. Some blast furnace workers use blast kinetic energy to measure the penetration ability of blast in front of tuyere. The influence of blast kinetic energy on the raceway is more practical than the wind speed, but the calculation is more complex. Both are common.

Wind temperature

Blast temperature of blast furnace. The higher the air temperature, the more heat brought into the furnace by the blast, and the lower the fuel ratio of the blast furnace. Therefore, the air temperature is usually used to the highest level that the blast furnace may accept. The degree of blast furnace receiving air temperature mainly depends on smelting conditions. The better the quality of raw materials and fuels, the more fuel injected, the higher the blowing humidity, the more stable and smooth the furnace condition, and the higher the acceptable air temperature of the blast furnace. The blast temperature of blast furnace in China is mostly between 900 ~ 1250 ℃; Blast furnace air temperature in industrial developed countries is mostly between 1150 ~ 1350 ℃. Increasing or decreasing the air temperature is an important means to adjust the furnace condition. Increasing the air temperature can increase the furnace temperature, and reducing the air temperature can reduce the furnace temperature. However, most advanced blast furnaces stabilize the air temperature at the highest level, and adjust the furnace condition by adjusting the fuel injection amount or blast humidity. Reduce the air temperature only when it is very necessary. In this way, a lower fuel ratio can be obtained.

Blowing humidity

Water vapor content in blast. Mostly in g / m3. The humidity of natural blast varies with the atmospheric humidity, and the fluctuation of blast humidity has an impact on the material speed and furnace temperature, so the natural fluctuation of blast humidity cannot be allowed. Two methods are usually adopted: dehumidification blast is used to control the blast humidity at the lowest level; Or control the blowing humidity at an appropriate level through humidifying blast. Dehumidification blast should be used when injecting fuel; Humidification blast should be used when fuel is not injected or the injection volume is small. When humidifying blast is adopted, changing the blast humidity can be used as a means to adjust the furnace condition.

Theoretical combustion temperature

The maximum flame temperature of coke and injected fuel burning in front of the tuyere. This temperature is difficult to measure directly and is mostly calculated theoretically, so it is called theoretical combustion temperature. The theoretical combustion temperature has a suitable range, about 2000 ~ 2350 ℃. Too low and low hearth temperature will easily lead to abnormal operation of hearth; If it is too high, more SiO will be generated, which is easy to cause unstable furnace condition. The quality of raw materials and fuels is good, the amount of slag is small, the coke ratio is low, and the upper limit can be controlled when the furnace condition is stable; On the contrary, the lower limit of deviation is controlled. Modern blast furnaces mostly adopt comprehensive blast, and the factors affecting the theoretical combustion temperature increase. The theoretical combustion temperature has a positive relationship with air temperature and oxygen enrichment rate, and an inverse relationship with the amount of injected fuel and blast humidity. Several factors should be reasonably coordinated to keep the theoretical combustion temperature within a reasonable range. In order to reduce the fuel ratio of blast furnace, the air temperature is usually maintained at the highest level, the humidity is controlled at the lowest level, and the fuel injection amount and oxygen enrichment rate are taken as adjustment factors.

Wind pressure

The pressure before the blast enters the blast furnace. The air pressure is related to the furnace top pressure, furnace capacity and material column permeability. The top pressure of modern blast furnace with furnace capacity of 1000 ~ 5000m3 is mostly between 0.1 ~ 0.25MPa, and the air pressure is mostly between 0.2 ~ 0.45MPa. The wind pressure is equal to the sum of the resistance of the material column in the furnace and the pressure on the furnace top. Blast furnace operators first determine the furnace top pressure according to the equipment conditions. The higher the furnace top pressure, the more conducive to blast furnace production. The furnace top pressure is automatically adjusted and its value is relatively stable. Therefore, the level and change of air pressure can reflect the level and change of material column resistance in the furnace. It is an important index for blast furnace operators to judge the furnace condition. The more stable the wind pressure is, the more smooth the furnace condition is.