Condensation and treatment methods of cement mill dust collection
1 Example of high temperature type mill
The cement mill with a diameter of 4.5m × 12.96m in a factory is a circular flow mill with a cyclone separator, and the end of the mill is dedusted by a flat-bag type dust collector. The mill system is indoors, the air convection between the mill and the outside world is weak, the air leakage of the system is low, and the dust collector has no insulation layer.
During production, it was found that the pressure difference between the inlet and outlet of the bag filter is large; the cement temperature at the outlet of the mill is up to 170°C (the design upper limit is 120°C); . After stopping the grinding, it was found that the grate joints at the grinding outlet were almost all covered with cement; the flat bags were seriously blocked by condensation.
The ventilation resistance of the system is large, which weakens the air cooling effect in the mill. In addition, the temperature of the clinker entering the mill is high. In summer, the high temperature alarm of the hollow shaft of the mill tail is frequent. During the day, the mill can only be started from 0:00 to 11:00, and the rest Time had to stop grinding and cooling due to high temperature. Later, the bag filter was expanded and transformed, the cold air leakage was strictly blocked, the filter bag with water-repellent property was used, and the water spray of the grinding head was stopped before passing the inspection and acceptance.
The results of a thermal test are: the output per hour is 120t/h; the clinker temperature is 110 ℃; the static pressure of the grinding head is -15 ~ 20Pa; Under standard conditions, the outlet air volume is 9882m3/h; the cement temperature at the outlet is 140°C, the exhaust gas temperature at the grinding tail is 127°C, and the air volume is 12480m3/h; the cement product temperature is 116°C; Production has been maintained by frequent filter bag changes.
2 Examples of low temperature type mill
The φ 3m × 11m cement mill in Plant T is also a circular flow mill with a cyclone separator, and an air box type bag filter is used. The clinker is purchased from outside, and the temperature of the clinker into the grinding is similar to the atmospheric temperature in winter, but is slightly higher than 15 ℃ in summer, and the grinding waste gas treatment system has no external insulation layer.
The mill is semi-open, and the indoor and outdoor temperatures are roughly the same. In normal operation, the exhaust gas discharge temperature is as low as 18 ℃. In winter, the dust collector often pastes the bag, and the fan current is reduced from above 75A to below 60A at normal time; the wind speed in the head cover of the belt conveyor at the grinding inlet is reduced from about 0.5m/s to 0.2m/s.
When the mill is running, check the top shell of the bag filter, and it is found that the temperature of the box chamber is equivalent to the atmospheric temperature. Open the upper cover of the dust collector air box, and the four walls of the clean air chamber see water in many places, and the inside of the cover plate is wet; The filter bags are almost filled with wet cement dust; the filter bags are extremely difficult to extract and clean due to the hard cement; after the filter bags are pulled out, it is found that the surface of the entire bag is covered with wet ash, in a slurry or hard state, and the bottom of the bag is mostly Hard shell; the filter bag close to the dust collector shell is more seriously blocked by condensation due to low temperature, especially in the corners.
1 Stop grinding and condensation
For N plants with high-temperature exhaust gas, the moisture in the mill mainly comes from the introduction of materials and the water spray from the grinding head. The average moisture content of slag and gypsum in the mill is 8%, so the amount of water brought into the mill is 1.517t/h, the water spray rate of the grinding head is 1.500t/h, and the total moisture of the two items is 3.017t/h. Due to the high cement temperature (140 ℃), the air entering the grinding and the water taking away from the cement offset each other, so the 3.017t/h water is all put into the waste gas at 92 ℃. Under the standard condition, the outlet air volume of the bag filter is 9882m3/h, and the moisture content of the wet gas is 305g/m3. Looking up the table, the corresponding dew point is 75 ℃. If other conditions remain unchanged, only the change of the water spray into the mill is made, the corresponding dew point is 59 ℃. According to the usual requirements at home and abroad, the design value of the dew point of the bag filter is 60 ~ 70 ℃, both of which generally meet the requirements.
Under the condition of exhaust gas discharge temperature of 92 ℃, the reasons for the dust collector to cause condensation and paste bags are:
At the beginning of the grinding stop, the inside of the mill is still hot, and the residual water evaporates continuously, but the exhaust fan has been stopped, and the air volume is extremely low, so it is easy to condense. After the shutdown, the whole system is cooling down, and the bag filter is located at the rear end, and its heat capacity is not large. The air box type bag filter used in many factories is relatively large in size and has a large cooling area. The original temperature is the lowest and the cooling is fast, and it is easy to reach the dew point. In addition, the filter bag has a full filtration “intercept” effect on the tiny water droplets (water mist) in the airflow, so that the water can be concentrated here; If the advance amount is insufficient and the exhaust fan stops too early, the water in the mill is more than when the mill is stopped normally; or the water spray is poorly atomized and the water evaporation is delayed, which will aggravate the condensation. Especially in winter, the cooling is fast and the drop is large, and water is precipitated below the dew point. Assuming the dew point is 80 ℃, the saturated moisture content is 716g/m3 dry gas under standard conditions, and it is 9.8g/m3 dry gas when it drops to 10 ℃, and the water precipitation is about 706g/m3 dry gas, and when it drops to 60 ℃, it is 9.8g/m3 dry gas. The saturated moisture content is 196g/m3 dry gas, and it is 35.1g/m3 dry gas when it is lowered to 30 ℃, that is, the more cooling, the more thorough the water evolution. Therefore, dew condensation in winter is particularly sensitive for baghouses without external insulation. If the grinding is stopped for a long time, there is a process in which the dew point is separated from the dew point during the heating period after the grinding.
The effect of stopping grinding and cooling on condensation can be roughly estimated by the total amount of water entering the mill, ventilation volume, and temperature, but it is difficult to be detailed and accurate, because it is difficult to determine the amount of water that is lagging behind in evaporation, and it can only be obtained from experience.
To sum up, the negative impact of grinding stoppage also exists in the low-temperature exhaust gas type.
2 The amount of ventilation in the mill
The ventilation rate in the mill is a very important operating parameter in the grinding process. A mill has its optimal range of ventilation volume in the mill under certain conditions, and the value of the optimal air volume range is quite different. Hodebank multinational company believes that the wind speed of the ring mill in the clearance area of the mill should be 1.0 ~ 2.0m/s; the air volume of the mill with a larger diameter can be lower, such as 0.45m3/kg cement for a φ 4.8m mill, The φ 3.2m mill is 0.9m3/kg cement. Lude recommends 0.3m3/kg cement for multi-silo mill. It has also been suggested that 0.40-1.20m3/kg of cement should be used to enhance ventilation. N factory calibration data, the wind speed in the mill is 0.3hn/s, which is equivalent to 0.10m3/kg cement. The air volume is obviously low, and as a result, the dew point increases significantly under the condition of certain water entering the mill. According to the above data conditions, if the water entering the mill remains unchanged and the air volume is increased to 4 times the test value, the dew point will decrease from 75 ℃ to 45 ℃. Calculated under standard atmospheric pressure conditions, the former has 232g/m3 moist air more than the latter, while the saturated moisture content at a dew point of 45 °C is only 76g/m3 moist air.
Therefore, an effective way to solve condensation is to increase the ventilation volume in the mill. Huaxin Cement Plant φ 2.7m × 4m secondary cement mill, after water was sprayed in the mill, it was found that a small amount of water vapor condensed in the air duct and bag filter to make the cement into coarse particles, and the ventilation rate was increased from the original 2000 ~ 2500m3/h to 4000m3/h, the phenomenon of coarse grains disappears.
The ventilation volume in the mill is maintained by the exhaust fan. Since there are a series of air leakage points on the way from the mill to the front of the exhaust fan, each air leakage point is slightly loosened and plugged, and the ventilation volume in the mill is greatly affected.
3 temperature changes
In recent years, there have been more and more grinding stations in China. The temperature of the clinker is almost the same as the atmospheric temperature. The temperature of the exhaust gas is low, and it is particularly prone to condensation, which shows that it is very sensitive to changes in the total water content of the mill.
Regarding the problem of the dew point value of the bag filter, it is usually set at 60 to 70 ℃ at home and abroad. Practice in grinding stations has proven difficult to achieve such high temperatures. Because the temperature of the exhaust gas from the mill is only about 70 °C, the temperature of the bag filter is already below 60 °C after passing through the pipeline and adding cold air. In addition, the previous discussion on the condensation of high-temperature exhaust gas has also fully exposed its limitations and has little practical significance. Therefore, it is necessary to re-explore the control indicators and design data.
1 Stop grinding and condensation
For N plants with high-temperature exhaust gas, the moisture in the mill mainly comes from the introduction of materials and the water spray from the grinding head. The average moisture content of slag and gypsum in the mill is 8%, so the amount of water brought into the mill is 1.517t/h, the water spray rate of the grinding head is 1.500t/h, and the total moisture of the two items is 3.017t/h. Due to the high cement temperature (140 ℃), the air entering the grinding and the water taking away from the cement offset each other, so the 3.017t/h water is all put into the waste gas at 92 ℃. Under the standard condition, the outlet air volume of the bag filter is 9882m3/h, and the moisture content of the wet gas is 305g/m3. Looking up the table, the corresponding dew point is 75 ℃. If other conditions remain unchanged, only the change of the water spray into the mill is made, the corresponding dew point is 59 ℃. According to the usual requirements at home and abroad, the design value of the dew point of the bag filter is 60 ~ 70 ℃, both of which generally meet the requirements.
Under the condition of exhaust gas discharge temperature of 92 ℃, the reasons for the dust collector to cause condensation and paste bags are:
At the beginning of the grinding stop, the inside of the mill is still hot, and the residual water evaporates continuously, but the exhaust fan has been stopped, and the air volume is extremely low, so it is easy to condense. After the shutdown, the whole system is cooling down, and the bag filter is located at the rear end, and its heat capacity is not large. The air box type bag filter used in many factories is relatively large in size and has a large cooling area. The original temperature is the lowest and the cooling is fast, and it is easy to reach the dew point. In addition, the filter bag has a full filtration “intercept” effect on the tiny water droplets (water mist) in the airflow, so that the water can be concentrated here; If the advance amount is insufficient and the exhaust fan stops too early, the water in the mill is more than when the mill is stopped normally; or the water spray is poorly atomized and the water evaporation is delayed, which will aggravate the condensation. Especially in winter, the cooling is fast and the drop is large, and water is precipitated below the dew point. Assuming the dew point is 80 ℃, the saturated moisture content is 716g/m3 dry gas under standard conditions, and it is 9.8g/m3 dry gas when it drops to 10 ℃, and the water precipitation is about 706g/m3 dry gas, and when it drops to 60 ℃, it is 9.8g/m3 dry gas. The saturated moisture content is 196g/m3 dry gas, and it is 35.1g/m3 dry gas when it is lowered to 30 ℃, that is, the more cooling, the more thorough the water evolution. Therefore, dew condensation in winter is particularly sensitive for baghouses without external insulation. If the grinding is stopped for a long time, there is a process in which the dew point is separated from the dew point during the heating period after the grinding.
The effect of stopping grinding and cooling on condensation can be roughly estimated by the total amount of water entering the mill, ventilation volume, and temperature, but it is difficult to be detailed and accurate, because it is difficult to determine the amount of water that is lagging behind in evaporation, and it can only be obtained from experience.
To sum up, the negative impact of grinding stoppage also exists in the low-temperature exhaust gas type.
2 The amount of ventilation in the mill
The ventilation rate in the mill is a very important operating parameter in the grinding process. A mill has its optimal range of ventilation volume in the mill under certain conditions, and the value of the optimal air volume range is quite different. Hodebank multinational company believes that the wind speed of the ring mill in the clearance area of the mill should be 1.0 ~ 2.0m/s; the air volume of the mill with a larger diameter can be lower, such as 0.45m3/kg cement for a φ 4.8m mill, The φ 3.2m mill is 0.9m3/kg cement. Lude recommends 0.3m3/kg cement for multi-silo mill. It has also been suggested that 0.40-1.20m3/kg of cement should be used to enhance ventilation. N factory calibration data, the wind speed in the mill is 0.3hn/s, which is equivalent to 0.10m3/kg cement. The air volume is obviously low, and as a result, the dew point increases significantly under the condition of certain water entering the mill. According to the above data conditions, if the water entering the mill remains unchanged and the air volume is increased to 4 times the test value, the dew point will decrease from 75 ℃ to 45 ℃. Calculated under standard atmospheric pressure conditions, the former has 232g/m3 moist air more than the latter, while the saturated moisture content at a dew point of 45 °C is only 76g/m3 moist air.
Therefore, an effective way to solve condensation is to increase the ventilation volume in the mill. Huaxin Cement Plant φ 2.7m × 4m secondary cement mill, after water was sprayed in the mill, it was found that a small amount of water vapor condensed in the air duct and bag filter to make the cement into coarse particles, and the ventilation rate was increased from the original 2000 ~ 2500m3/h to 4000m3/h, the phenomenon of coarse grains disappears.
The ventilation volume in the mill is maintained by the exhaust fan. Since there are a series of air leakage points on the way from the mill to the front of the exhaust fan, each air leakage point is slightly loosened and plugged, and the ventilation volume in the mill is greatly affected.
3 temperature changes
In recent years, there have been more and more grinding stations in China. The temperature of the clinker is almost the same as the atmospheric temperature. The temperature of the exhaust gas is low, and it is particularly prone to condensation, which shows that it is very sensitive to changes in the total water content of the mill.
Regarding the problem of the dew point value of the bag filter, it is usually set at 60 to 70 ℃ at home and abroad. Practice in grinding stations has proven difficult to achieve such high temperatures. Because the temperature of the exhaust gas from the mill is only about 70 °C, the temperature of the bag filter is already below 60 °C after passing through the pipeline and adding cold air. In addition, the previous discussion on the condensation of high-temperature exhaust gas has also fully exposed its limitations and has little practical significance. Therefore, it is necessary to re-explore the control indicators and design data.
