Fireproof and explosive-proof measures for coal grinding mill

One: The combustion characteristics of pulverized coal

1. Pulverized coal is more easily combustible than coal chunk. Pulverized coal is made up by coal particles, which enjoy way much large surface area than that of same amount of coal chunks. It could be easily resulted in spontaneous combustion after its contact with the air.

2. Pulverized coal would be transformed into explosive compound while hovering in the air for long. The explosion limit varies from coal by coal. The lowest limit is 45g / m3, and the highest limit is 2000g / m3, which is quite large. The explosion is strongest when the intensity is at 300 400s / m3. May it be in closed space, such as coal grinding system or open space, such as a boiler room, a coal powder will cause explosion or combustion when reaching the explosion limit?

3. In the isolated coal grinding system, when the pulverized coal is burning, the pressure will increase rapidly which will cause the damage on the whole system, and result in the flame spraying outside, burning the personnel and destroy other equipment.

Two: Fireproof requirements for coal grinding system

1. Installing explosive-proof valves and explosion-proof doors

1) Explosive-proof valve should be installed on the pipeline of coal grinding system, so that when the explosion occurs, the gas pressure in the pipeline would be ventilated to avoid the possibility of more serious explosion accidents.

2) Explosion-proof doors should be installed on silos, classifier and cyclone equipment respectively. The area of the explosion-proof door should be set based on the ratio of the volume of the equipment, normally at 0.04m2 / m3, but it should not be less than 90cm2. Thin iron sheet, no more than 0.5mm, should be attached on the explosive-proof sheet. A cross - shaped mark should be inscribed on the outside-facing surface of the explosive-proof piece. The frame of the explosion-proof door should be tough enough, firm and sealed to meet the design requirements.

3) When explosive-proof valve and explosive-proof door are blown up, the operation of the coal grinding mill must be stopped immediately, the fire put out and find out the cause of the fire. Only if the valve (door) is repaired, the operation can be restored.

4) Temperature monitor should be installed on the key parts like coal bunker, classifier, cyclone and other important parts of coal grinding system to detect the abnormal temperature of each part at any time, so as to prevent the spontaneous combustion and explosion caused by high temperature.

2. Pipes and equipment

1) In order to prevent and reduce the accumulation of pulverized coal in the pipe, pipe lying should by no means be horizontal, except the section leading to burner;

2) At the end of the branch pipe of the coal grinding system, only one pulverized coal collecting pipes are allowed.  

3) A well-functioning chain reaction device should be installed in the grinding system for the all devices could be stopped simultaneously in the event of accident.

4) Only electrical equipment meeting the explosive-proof requirements can be in place by the grinding area.

5) Nitrogen or carbon dioxide fire extinguishers should always be in place by the coal grinding mill and bunker.

6) The stock of pulverized coal in the coal bunker should be strictly regulated to avoid excessive storage of pulverized coal while storage of the pulverized in the bunker for longtime is not recommended.

3. Inspection, cleaning and fire prohibition

1) Coal grinding mill should be checked shift by shift so as to find out and eliminate the potential risk of air leakage and coal leakage in time.

2) The surface of the equipment and pipes of the coal grinding system should be cleaned on regular basis to prevent dust accumulation.

3) The steam fire extinguishing device should be checked on regular basis to make sure it’s functional when needed.

Three: Remedial action for the explosion at coal grinding site.

1. Shut the hot air door of the coal mill entrance immediately, and stop the operation of the grinding mill when necessary.

2. Put out the fire with nitrogen or carbon dioxide fire extinguishing agent.

3. If there were sparks at the exit of the mill, we can put it out by increasing the coal feeding, shutting the hot air door, then stop the coal grinding mill, and carefully clean up the coal.

4. If the coal bunker were on fire, coal feeding should be stopped immediately while putting out the fire with nitrogen or carbon dioxide extinguishing agent. When necessary, water can be used to cool down the shell of the fire equipment, but putting out the fire directly by fire is strictly forbidden.

Hot work activities are strictly prohibited around the grinding area. "hot work permit" should be granted before any hot work. Effective preventive measures should be taken before any hot work, and special staff should be in place to monitor the operation.