[Weekly Topic] What measures are taken in the powder grinding system during startup and shutdown to prevent explosions? (10 November 26th---December 4th)

During startup and shutdown, flashback can easily occur due to pressure differences; adding a flame arrester will solve this issue.

This post was last edited by zgj2405 on 2011-11-26 at 16:31. During the start-up and shutdown of the coal grinding system, the coal dust concentration changes significantly; there are frequent occasions when the concentration approaches or reaches dangerous levels, increasing the likelihood of an explosion. 1. During startup and shutdown, the outlet temperature of the coal grinder must be strictly controlled to remain within the specified limits ； 2. When shutting down the coal grinding system, all dust accumulated inside the system should be removed to prevent coal dust from accumulating ； 3. During design and installation, avoid the formation of dead corners within the system, which could lead to the accumulation of dust and coal ； 4. Install explosion-proof doors to prevent equipment damage in the event of an explosion.

During the start-up and shutdown of the pulverization system, the coal powder concentration changes significantly, and there are frequent occasions when it approaches or reaches a dangerous level, increasing the likelihood of an explosion. During startup and shutdown, the outlet temperature of the coal grinder must be strictly controlled to remain within the specified limits ； When shutting down the coal grinding system, all the coal dust accumulated inside the system should be removed to prevent its deposition ； During design and installation, avoid the formation of dead corners within the system, which could lead to the accumulation of dust and coal ； Install explosion-proof doors to prevent equipment damage in the event of an explosion.

1. Regularly check for air leakage in the explosion-proof doors of the coal mills and coal feeders; if any leakage is detected, shut down the pulverization system immediately for repair； After the work to address air leakage in the explosion-proof door is completed and the powder production system is started, a thermometer should be used to measure the temperature of various parts of the explosion-proof door, in order to prevent increased air leakage from causing the safety door to activate accidentally. The seal of the explosion-proof door should be checked regularly, and it should be replaced if necessary. 2. Before major and minor repairs of the unit as well as before major and minor repairs of the coal grinding system, it is necessary to properly arrange for the coal hopper to be emptied, and to clean any coal deposits on its inner walls, in order to prevent spontaneous combustion due to such deposits. 3. During major and minor repairs of the boiler, a thorough inspection of the coal grinding system is carried out to eliminate any leaks in the hot air isolation valves, hot air valves, and main air valves, as well as uneven openings in the baffles of the coal powder distributors. The thickness of the coal drop pipes inside the upper part of the separator’s coal outlet pipes is measured, and welding repairs are performed if necessary, in order to prevent raw coal from leaking into the coal outlet pipes due to wear of those pipes. 4. After cleaning the coal on the feeder’s maintenance tray, it is necessary to promptly remove any accumulated coal from inside the mill and the primary air ducts, to prevent explosions when the mill is started up for ventilation. 5. The coal gate valve of the coal feeder should operate smoothly; it must remain fully open during normal operation and fully closed when not in use. In case special circumstances arise that require the use of one pair of coal gates, the valve should be switched once per shift. The coal cut-off signal of the coal feeder is normal, ensuring proper operation; the indicator for adjusting the coal layer thickness in the coal feeder is accurate, and the adjustment functions properly. 6. Before starting the coal grinder and during ventilation, it is necessary to close the exhaust valve. First, open the cold air valve and the hot air valve, then slightly open the main inlet valve to carry out warm-up grinding ； It is strictly prohibited to open the main inlet air valve first and then the hot air valve. When adjusting the primary air volume of the coal grinder, it is necessary to strictly control the \"air-coal\" ratio; the operation must be smooth, with no sudden increases or decreases. 7. When starting the coal feeder, the operator must check from the side of the feeder to prevent any threat to personal safety. 8. When starting the coal grinding system, supply the fire protection steam to the coal feeder 1 minute before starting it; then open the coal gate. Stop supplying the fire protection steam to the coal feeder once the gate oil pump stops operating (the decision to stop this steam supply depends on the specific circumstances. If an abnormal increase in the temperature of the coal feeder’s housing is detected, or if smoke emerges from gaps in the gate when it is opened, continue to supply the fire protection steam until the housing temperature returns to normal or the smoking stops) ； The coal mill fire suppression steam is activated 1 minute before the coal mill starts to be ventilated, and it is turned off 5 minutes after the coal mill has started operating normally. 9. When shutting down the pulverization system normally, after closing the feed conveyor gate, fire suppression steam is introduced to the feed conveyor and the coal mill; after stopping the operation of the coal mill, the fire suppression steam to the feed conveyor is also turned off ； After the coal grinder has been shut down for 5 minutes, the steam used for protection of the coal grinder is stopped, and then the coal grinder and primary air ducts are thoroughly ventilated and cleaned, after which the ventilation process is ceased. 10. When the grinding system is to be shut down normally, the coal shutter of the raw coal hopper should be closed to empty the coal stored in the feeder. The grinder should be ventilated for 5–10 minutes to ensure thorough cleaning, so that any remaining coal dust inside the grinder and in the primary air ducts is removed, thereby preventing spontaneous combustion due to the accumulation of dust in these areas. 11. Before starting up and after shutting down the mill, it is essential to remove the slag accumulated in the mill’s slag discharge tank in a timely manner. During normal operation, the slag from the mill should be discharged twice per shift to prevent spontaneous combustion in the slag chamber.

1 Prevent coal accumulation at the coal mill inlet. Coal accumulation at the coal mill inlet is mainly caused by wet coal sticking there due to the impact of air currents. The accumulated coal can catch fire whether the grinding system is in operation or stopped. By installing a partition above the coal mill inlet (under the coal feed pipe), the coal, powder, and air can be mixed properly, which not only prevents coal accumulation at the upper part but also reduces coal buildup on the slope of the lower hopper. 2 Improvement of the fine powder separator: The powder accumulation at the tangential entry point of the fine powder separator can be addressed by installing guide vanes within the air duct, thereby increasing local turbulence, raising the flow velocity in that area, and enhancing the airflow’s ability to scour away the powder accumulated below. At the same time, the installation of guide vanes results in a more even distribution of the air and powder flow, further improving the separation efficiency. 3 Eliminate hot air leakage: By moving the cold air damper from in front of the hot air damper to behind it, placing it in a negative pressure area, this not only solves the problem of increased temperature at the coal grinder inlet caused by hot air leakage, but also addresses the issue of thermal pollution resulting from cold air leaking outside during operation. In addition, equipment maintenance must be strengthened; when it is found that the hot air valve does not close properly or cannot be closed at all, maintenance personnel should be contacted promptly to address the issue and restore normal operation. 4 Improve the sealing and insulation of the powder silo: (1) Seal the junction between the powder silo and the factory building, and conduct regular inspections; any air leaks detected should be repaired promptly to prevent air leakage from the silo ； (2) Install heating pipes outside the powder bin to enhance its insulation performance. Practice has proven that this method is highly effective in removing dust accumulation on the inner wall of steel plate-type powder silos ； (3) During operation, maintain the powder level in the powder bin within the range of 1.5–4.0 meters, and strictly implement the powder reduction system ； (4) Install negative pressure measurement points in the powder bin; during the operation of the powder production system, the opening degree of the vapor absorption valve in the powder bin should be adjusted promptly to maintain a negative pressure of 20–30 Pa in the bin. 5 Enhance the adjustment of coal powder fineness: While ensuring an economically optimal fineness for the coal powder, and taking into account the actual characteristics of the coal being used, adjust the baffles of the coarse powder separator so that the R90 value of the coal powder remains between 24% and 28%, with it operating at the upper end of this range. 6　Strengthen coal quality analysis. Coal quality reports related to the coal used for combustion should be delivered to the operators in a timely manner, so that the outlet temperature of the coal grinder can be adjusted according to the properties of the coal, the temperature of the air-fuel mixture can be controlled, and an appropriate fineness of the coal powder can be achieved. 7 Improvement of explosion-proof doors: The opening direction of explosion-proof doors should avoid surrounding cables, important equipment, and areas that may pose a risk to human safety; otherwise, necessary protective measures must be taken, such as installing shielding panels and using new types of explosion-proof doors. During simultaneous operation, inspection efforts should be intensified to ensure the tightness of the explosion-proof doors. 8 Strengthen operational management and follow procedures strictly. During the operation of the pulverization system, adjust the outlet temperature of the coal grinder in a timely manner; address any interruptions in coal supply promptly. Use the moisture absorption pipes correctly, and regularly identify and eliminate areas within the system where powder accumulates or leaks. It is essential to remove all remaining powder from the system when shutting it down.