Talking about the safe production of coal mill system in cement factory

Date: 2019-06-20 Views:

Every year, the coal mill workshop of the factory has accidents such as fire or explosion during use. These accidents not only cause great losses, but are also unacceptable from a safety perspective.

There are several reasons for these accidents: inadequate inert gas protection, incorrect installation of the test equipment or lack of maintenance, design defects, and so on. In the absence of safe operating procedures, many factors combine to cause an accident. It is hoped that through the discussion here, the cement factory can better achieve or maintain safe operation.

1 Concerns in production


1) Control of the pile: The design and management of the pile should be based on the degree of coal and the proportion of volatiles. For example, a very fine and volatile coal pile needs to be compacted and limited in height.

2) Maintain stable operation status through reliable instrument detection and control: the accuracy of the instrument and the perfect function of the device are determined by routine inspection.

3) When the workshop is operated, the oxygen content of the kiln exhaust gas should be lower than 12%, or it can be controlled lower according to the proportion of volatile matter in the coal: firstly, the relationship between the limit value of oxygen content and the volatile matter should be mastered; The air leakage of the inspection control system, in a special occasion similar to the absence of kiln exhaust gas, requires reasonable low moisture to avoid overheating and fire when the temperature of the inlet gas is low.

4) In the case of satisfying the drying, the lowest temperature operation of the coal mill: the inlet temperature of the mill needs to be limited to 200~250 °C, or lower, depending on the proportion of the volatile matter.

5) Bag filter monitors the concentration of O2 and CO: some systems only detect the CO of the dust collector. For safe operation, it is necessary to limit the concentration of O2. The operation of the mill requires the addition of a chain of gas analyzer O2 concentrations.

6) Monitoring the raw coal bunker and pulverized coal bin CO concentration: The carbon monoxide concentration is the early warning signal of the raw coal bunker and the coal bunker, and the mill operation must have a chain of CO concentration.

7) Grounding bar and dust collector filter bag grounding: Design and install the grounding bar in a region where a near point may occur to ensure no ignition due to static electricity.

8) Explosion-proof door: Experts participate in the inspection to ensure that the function and maintenance of the explosion-proof door are maintained at a high standard; the explosion-proof door should have an automatic reset function. The well-designed explosion-proof door can release the impact of the explosion and avoid damage to personnel and equipment; the new factory should avoid the use of rupture disk and non-automatic reset type vent; the application mark indicates the dangerous area.

9) Prepare adequate fire-fighting facilities: If liquid CO2 is available, the liquid CO2 to be filled should be selected as the fire extinguishing agent; if there is no liquid CO2, the bottled CO2 equipped with the weight measuring device can be used. Remote control and automatic control of CO2 fire extinguishing system; the storage capacity of fire extinguishing agent should be sufficient, and the bag dust collector should be equipped with a quick shut-off valve to improve the fire extinguishing efficiency.

10) Practical use of temperature, gas analyzer, O2 and CO content linkage in factory operations.

11) Avoid the deposition of pulverized coal in the equipment to avoid uneven surface of the equipment; the horizontal angle between the chute and the pipeline for conveying pulverized coal is not less than 70 degrees.

2 Use of high-risk fuels in production


Fuels with air dry radicals greater than 36% are high-risk fuels; fuels with more than 20% volatile fuels are high-risk operations. In order to reduce the risk of use, it is necessary to mix high-risk fuel and low-volatile fuel by weight to make the volatile matter of the mixed fuel less than 20%. The use of high-risk fuels requires the following tasks:

1) Professional third party verification of the explosion-proof pressure of the inspection equipment and system.

2) All venting doors need to be self-resetting and need to be led out.

3) Only use a liquid bulk tank to store CO2 fire extinguishing system, and can be controlled remotely and automatically.

4) The mill, dust collector and pulverized coal bin are equipped with raw material powder spraying device.

5) Test the fuel characteristics, including:

1 testing the ignition temperature of the contact between the fuel dust gas and the hot surface, and the test result determines the alarm value of the inlet gas temperature of the mill and sets a chain;

2 The oxygen content in the fuel dust gas is used to set the oxygen content value of the outlet of the dust collector;

3 The maximum explosion pressure Pmax and the pressure rise rate Kst were evaluated by the explosion test to evaluate the explosion-proof design of the equipment and system of the coal mill.

3 Raw coal treatment and material pile control

In order to avoid spontaneous combustion of coal piles, the relationship between deposit period and volatile matter of raw coal stored at 20 °C and above is as follows:

The storage period of coal with volatile matter <15% and anthracite and petroleum coke is 4 months; the storage period of coal with 15% < volatile matter <25% is 2 to 3 months; the storage period of coal with 25% < volatile content < 35% 1 month; the storage period of coal with volatile matter >35% is 1 to 4 weeks; in addition, the spontaneous combustion of coal pile is also related to the particle size distribution of raw coal, moisture, foreign matter composition, compaction degree, mining time, etc., powdered coal High proportion, high water content, containing pyrite (greater than 4%) or high iron and phosphorus content, loose piles, etc. will increase the risk of spontaneous combustion of coal piles.

The height control principle of the raw coal pile in the cement factory is as follows: 15% < volatile matter <25% coal pile height should be controlled at 8~10 m; 25% < volatile matter <35% coal pile height should be controlled at 6~8 m The coal reserve with a volatile content >35% should be controlled below 6 m.

4 fire extinguishing system


The following are the raw material powder or inert gas fire extinguishing commonly used in cement mills in cement plants.


4.1 Raw carpet


Raw mats are mainly used in factories where there is a common practice when planning to stop production or unplanned shutdowns (such as emergency repairs in the firing system) but not emptying the pulverized coal bin. The raw meal layer (raw material blanket) after 25~50 cm is formed on the upper surface of the pulverized coal in the pulverized coal bin to keep the pulverized coal in the warehouse safely stored for 48 hours. There are two ways to form a raw carpet in a pulverized coal bin:

1) Adding to the wind path of the coal grinding system through the pneumatic conveying system, the raw meal powder is collected by the dust collector and enters the coal powder bin.

2) Feed the raw meal powder into the mill through the grinding belt conveyor, and then enter the coal dust collector through the wind system of the mill system to enter the coal powder bin. Regardless of the type of addition, the wind system of the mill needs to be operated. The addition of raw meal requires proper consideration of the balance.

4.2 Inert gas


The injection of CO2 gas can greatly reduce the oxygen content in the protection zone. The selection principles for CO2 fire suppression systems are as follows:

1) The mill, the classifier, the cyclone, the dust collector, the wind path, and the coal powder bin are in accordance with 2 kg × 2 / m ⊃ 3; determine the storage capacity of CO2.

2) The available gas reserves should be considered in accordance with the volume of 4 kg CO2 per cubic meter of protected area.

3) The injection time is about 30 minutes. Avoid pulverized coal suspension caused by rapid injection.

4) The liquid CO2 storage pressure is 18 bar, and the liquid CO2 in the storage tank should be maintained at -40 °C through the refrigeration unit. It is vaporized by an evaporator during use.

5) The injection pressure of CO2 is kept below 4 bar.

6) The CO2 storage tank needs to be installed on the weighing scale to measure the weight, and 2 tCO2 is the lower limit of the reserve.

7) The CO2 system needs to be able to implement remote drive and automatic drive.

5 temperature control


In order to avoid the risk of explosion or fire of the coal mill system, the temperature of each position needs to be strictly controlled according to the design requirements: normal operating temperature, high alarm temperature, limit temperature, etc.

6 CO and O2 control


1) Set position of the CO gas analyzer probe (original coal bunker top, mill inlet, dust collector outlet, pulverized coal bunker top, preheater outlet) and set high alarm value and limit value.

2) O2 gas analyzer probe setting position (preheater outlet, mill inlet, dust collector outlet) and set high alarm value and limit value.

7 Explosion-proof pressure of venting doors and equipment


The coal mill system needs to set the venting port at the appropriate position, including: the original coal bunker top (the closed coal bunker), the pipe mill to the classifier pipe, the mill, the fan, the screw conveyor, the unloader, the mill To the dust collector pipe, dust collector gas chamber, pulverized coal warehouse top. The vents need to be connected to the outside through pipes.

8 System analysis and treatment methods that may cause fire in coal mill system


1) Operations that may cause the system to catch fire

1 feed moisture is too large to increase the mill inlet temperature.

2 The mill outlet temperature is too high, causing the coal to be over-baked.

3 The system has too much air leakage or the preheater requires too much hot air to dry the outlet of the dust collector. The O2 content is high. At this time, the system should be minimized, and the amount of hot air in the preheater should be increased by using the water spray inside the mill.

4 The system air volume is too low during operation.

5 When the mill is restarted, the startup interval is too short.

6 The pulverized coal deposited in the mill causes the dust collector to explode at startup.

7 The system is shut down but the door or vent door is opened without cooling.

8 The high volatile content of coal has a high fineness.

2) Dust collector operation


When the CO concentration is too high or the temperature difference between the inlet and outlet of the dust collector is greater than 10 °C, it can be judged that the inside of the dust collector has ignited. The emergency operation at this time should include:

1 system automatic emergency stop, the dust collector inlet and outlet rapid shut-off valve automatically closes.

2 According to 2 kg/m⊃3; spray CO2 into the dust collector.

3 Monitor the changes in the concentration of O2 and CO. If there is no decrease, continue to inject CO2.

4 workshop water fire system to prepare for follow-up ambulance.

3) Operation of fire in pulverized coal bin


The potential pulverized coal fire can be detected by the temperature in the warehouse and the change in CO concentration. If the fire occurs in the warehouse, the system operation steps are as follows:

1 Stop feeding pulverized coal into the bin; stop the work of the top dust collector, and switch the compressed air system from compressed air to CO2.

2 Maintain the work of the warehouse discharge system and feed the hot coal powder into the firing system.

3CO2 system starts, no material space is 2 kg/m⊃3; injection; injection time should be kept above 15~30 min to avoid airflow fluctuation and pipeline freezing.

4 If the temperature in the chamber still rises, you need to increase the injection volume of CO2 to reduce the time interval.

4) The shutdown of the firing system is the operation of the fire in the pulverized coal bin

If the firing system is shut down and the inert gas suppression effect is not significant enough, the following operations are required:

1Inject the raw meal into the pulverized coal bin to form a raw carpet of at least 25 cm thick in the upper layer of the pulverized coal.

2 The top dust collector is stopped.

3 If the pulverized coal temperature in the warehouse reaches the limit temperature of 200 °C, the outer side of the silo wall should be sprayed with water to lower the temperature (the insulation layer needs to be removed).

4 Contact the fire department.

5) Grinding operation inside the mill

If the temperature of the integrated mill inlet and outlet is too high and the CO concentration of the dust collector can judge the fire in the mill, the following operations should be performed:

1 system emergency stop.

2 Inject CO2 into the mill.

3 Continuously monitor the condition inside the mill. If there is no improvement, increase the CO2 injection amount and inject the raw meal into the mill.

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