1、 Conditions for explosion

　　1. Temperature of coal powder

　　Different types of explosives have their own ignition temperatures, but finer coal powder, due to its larger specific surface area, has an explosion temperature much lower than its ignition point.

　　2. Volatile matter of coal powder

　　The combustible gas produced by coal powder heating burns before coke. Under the same conditions, the higher the volatile fraction, the more intense the explosion of coal powder.

　　3. Concentration of coal powder

　　Only when the concentration of coal powder in the air reaches a certain level, will an explosion occur. The lower limit concentration of coal powder air mixture explosion is 20~30g/m3, and the upper limit is 2000~7000g/m3. If the concentration is outside the two limits, there is no risk of explosion.

　　4. Particle size of coal powder

　　The finer the coal powder, the more likely it is to explode.

　　5. Ash and moisture content of coal powder

　　The higher the ash content in coal powder, the weaker its explosiveness; The larger the water content, the less likely it is to explode.

　　6. Fire source

　　① Pulverized coal accumulates and spontaneously ignites ② sparks are entrained in the system ③ Pulverized coal particles in the system collide with the pipe wall and box, generating electrostatic sparks.

　　2、 Explosion proof measures

　　1. Explosion proof design of process system ③ ⑤⑥

　　① Process pipeline

　　Avoid laying pipelines horizontally. The inclination angle of the pipeline is generally at least 70 ° for upward pipelines and at least 45 ° for downward pipelines (static coal powder accumulation angle of 25 ° to 30 °). The interior of the pipeline should be smooth. The gate valve on the intake duct should be in pneumatic form.

　　② Bag dust collector temperature

　　The coal mill bag dust collector has strict requirements for the inlet air temperature, which not only prevents coal powder explosion caused by overheating, but also prevents condensation caused by low temperature. The suitable operating temperature range is 55-75 ℃. When the inlet temperature is too high, the outlet temperature of the mill should be appropriately reduced. When the inlet temperature is too low, the outlet temperature of the mill should be appropriately increased.

　　③ Process layout

　　If possible, preheater exhaust gas should be used as the hot air source. The oxygen content in the exhaust gas of the preheater is very low, usually 8% to 10%. In this inert atmosphere, coal powder will not burn.

　　If the coal powder preparation system is a dual fan system, when the main fan is stopped and the dust removal fan is running, the thermal engine can directly enter the coal mill dust collector through a short circuit in the connecting pipeline, causing the temperature of the dust collector to quickly rise above the warning temperature. To avoid hot air short circuiting into the dust collector, the main fan is interlocked with the dust removal fan to prevent the dust removal fan from operating independently.

　　④ Reasonable design of air volume for bag dust collector

　　Before selecting a coal mill bag dust collector, strict calculation of the processing air volume should be carried out, fully considering the possible increase in air volume caused by factors such as production increase, so that the dust collector air volume can fully meet the system's needs, and avoid the equipment resistance increasing due to exceeding the processing capacity, insufficient dust cleaning capacity, and dust accumulation.

　　2. Explosion proof design of dust collector

　　① Adopting anti-static filter bags

　　Using anti-static filter bags, their friction potential is lower than that of general filter materials, which is conducive to preventing static electricity accumulation from causing spark discharge and igniting coal powder.

　　Anti static filter media can be divided into three types according to weaving methods: conductive yarn woven into base fabric filter media, carbon fiber blended filter media, and stainless steel fiber blended filter media.

　　In addition, according to the post-processing method, it can be divided into: ordinary type, oil and waterproof type, and ordinary laminated type. Due to the relatively low operating rate of the coal mill system, it often starts and stops, especially when the moisture content of the raw coal is high, which can easily cause smoke condensation. Therefore, it is recommended to use oil and water resistant or film covered anti-static filter materials.

　　② Body design

　　Install a dust blocking plate at the location where dust is prone to accumulate inside the dust collector to eliminate all dust accumulation surfaces and prevent spontaneous combustion of coal powder.

　　The slip angle of the ash hopper is greater than 70 °. In order to prevent the accumulation of ash due to the small angle (valley angle) between the walls of the two ash hoppers, slip plates are added to the adjacent side plates to increase the valley angle and eliminate the deposition of coal powder.

　　Attention should also be paid to ensuring dust cleaning in the design of filter bag length, bag spacing, and cleaning system, in order to avoid blind reduction of equipment size that may cause over density between bags or excessively long filter bags, resulting in dust accumulation on the surface of the filter bags or even between bags.

　　If negative pressure operation is used, the overall airtightness of the dust collector should be ensured.

　　③ Explosion proof valve

　　Explosion proof valves are mainly divided into three types: adjustable explosion-proof doors with locks, heavy hammer explosion-proof doors, and explosion-proof diaphragms. An adjustable explosion-proof door with a lock adjusts the release pressure of the door by adjusting the lock. The heavy hammer explosion-proof door can be reused by adjusting the position of the heavy hammer to set different release pressures. After the explosion-proof door is opened to release pressure, it can be reused. Explosion proof diaphragms correspond to different release pressures based on their materials, sizes, and processing methods. After bursting and releasing the pressure, they need to be installed separately. The bag dust collector is equipped with explosion-proof doors based on its explosion-proof ventilation area and venting pressure. In cases where the explosion-proof requirements are equivalent, all three types of explosion-proof doors can meet the explosion-proof requirements.

　　④ Automatic fire extinguishing device

　　The coal mill bag dust collector should be equipped with corresponding automatic fire extinguishing devices. In the event of an explosion, CO2 or nitrogen extinguishing gases can be sprayed into the bag chamber, ash hopper, and other parts of the dust collector to avoid greater losses.

　　⑤ Temperature measuring components

　　Temperature measuring components are installed on the ash hopper and inlet duct of the coal mill bag dust collector, which can transmit temperature signals from the inlet and ash hopper to the electrical control cabinet or central control room. And upper and lower limit alarms should be set, and connected to the automatic fire extinguishing system. The central control room should be able to automatically and continuously record temperature and pressure curves of various parts, in order to analyze and summarize production process parameters for the purpose of identifying the cause of accidents after they occur.

　　⑥ Ash hopper heating and vibration device

　　Facilities such as ash hopper electric heaters, vibration motors, and level gauges can be selected and configured according to climate and other conditions. In the cold northern regions, an electric ash hopper heater is necessary to prevent coal powder from condensing and sticking on the inner wall of the shell when the temperature of the ash hopper is too low; In the south, there is no need to configure it.

　　If the ash hopper unloading device is equipped with a double-layer pneumatic ash unloading valve, there is no need to configure a vibration motor; But if a feeder is selected, a vibration motor should be installed to avoid ash from the ash hopper wall canopy.

　　In addition, when choosing a material feeding and ash conveying device, attention should be paid to avoiding sparks caused by material friction during transportation. It is recommended to use explosion-proof material feeders or flap valves.

　　⑦ Shell grounding

　　The shell of the coal mill bag dust collector needs to be grounded to eliminate static sparks.

　　3、 Operation and maintenance

　　1. Before stopping the machine, perform * * cleaning operation to remove dust on the surface of the filter bag, and stop the machine only after the ash hopper is completely unloaded.

　　2. Reasonable maintenance procedures and records should be established to ensure the stable and reliable operation of the explosion-proof valve and feeding device of the dust collector. Once a malfunction is found in the feeder, it should be repaired immediately to avoid dust accumulation in the ash hopper.