The coal mill grinding system is an important part of the dry process cement manufacturing. In cement plants, we usually adopt the air swept coal mill system or vertical mill system as the pulverized coal preparation system, which is arranged at the cement kiln head or kiln tail to provide fuel for clinker calcination. However, due to the flammable and explosive characteristics of pulverized coal, in the process of its preparation, storage, transportation, and combustion, if not handled properly, spontaneous combustion or explosion may occur at any time and cause heavy losses to enterprises. In order to reduce and prevent the occurrence of accidents and ensure the safe production of cement plants, we should fully consider the unstable factors existing in the design of the coal mill system and take a series of measures to eliminate the potential safety hazards.
Properties of Pulverized coal
Pulverized coal is a tiny particle of coal. Its surface area is much larger than that of coal with the same quality. Therefore, after contact with air, it oxidizes and spontaneously ignites more easily. When the pulverized coal suspended in the air reaches a certain limit, it will form an explosive mixture, which will explode and burn in case of an open fire. Once the pulverized coal in the closed system is burned, the pressure will increase rapidly, and the flame will spray out, causing serious personnel burns. The explosion lower limit density of coal powder is 45g/m3 and the upper limit can reach 2000g/m3.
| Data | Bituminous Coal Powder | Anthracite Powder |
| Explosion lower limit density(g/m3) | 110～335 | 45～55 |
| Explosion upper limit density(g/m3) | 1500 | 1500～2000 |
| Explosion upper limit of pulverized coal particle size(mm) | 0.5～0.8 | |
| Ignition point(℃) | 500～530 | |
What Safety Measures Should We Take?
Set explosion-proof valves and explosion-proof doors
- Set explosion-proof valves in the pipeline of the coal mill system so that a large amount of gas accumulated in the pipeline during the explosion can be discharged into the atmosphere in time to avoid more serious explosion accidents.
- Set explosion-proof doors respectively on the pulverized coal silo, cement separator, and dust collector. The area of the explosion-proof door should be calculated according to the volume ratio of the equipment, generally 0.04m2 /m3, but not less than 90cm2. Besides, the door frame should have a certain strength and firm sealing. And its performance must meet the design requirements.
- When designing the explosion-proof valve and explosion-proof door, the passage should be avoided. If necessary, a steel plate enclosure can be set on three sides to avoid the damage caused by pressure leakage during the explosion.
Improve the design of pulverized coal silo and pipeline
The density of pulverized coal flue gas in silos and pipelines is high. All we need to do is make sure that they are always in a flow dynamic state to avoid spontaneous combustion and explosion caused by blockage and long term accumulation.
- Pipelines in the coal mill system should not have a horizontal section except the section leading to the burner.
- Strictly control the reserve volume and storage time of pulverized coal in the silo. Over storage is prohibited, and the storage period is generally within 4 ~ 5h.
- For improving the design of pulverized coal silo, we can fill CO2, N2, and other inert gases, add explosion-proof doors, install automatic monitoring and alarm device that can continuously measure temperature and CO content, and set storage capacity indicator and automatic feeding device.
Select the hot air source and control the temperature
There are two hot air sources for pulverized coal drying, namely kiln tail flue gas and kiln head waste gas. When we take kiln tail flue gas as a drying medium, the oxygen content in flue gas is low, which has the effect of inhibiting pulverized coal combustion. So if the process layout allows, the coal mill should be set near the kiln tail as far as possible.
We also need to strictly control the temperature of the gas-powder mixture at the coal mill exit as reducing the temperature not only ensures the safe and stable operation of the grinding system but also reduces the coking of the burner. Generally, the gas temperature at coal mill inlet is not higher than 400℃, the outlet gas temperature is lower than 90℃.
Install CO2 fire extinguishing system
In the case of gas combustion and explosion occur in equipment and silo, the fire extinguishing system can spray CO2 timely to stop the combustion. This system has an automatic type and manual type. The former is connected with a monitoring system. When abnormal occurs, it can eliminate hidden troubles in a timely and effective manner. The CO2 fire extinguishing system of cement plants can cover coal mills, cement separators, hot air pipelines, coal mill dust collectors, and coal silo dust collectors.
Optimize the process design
- Set up protection and isolation zones separately in the inflammable and explosive zone.
- The maintenance passageway should be spacious and easy to pass, which is convenient to escape in emergency.
- After the coal mill stops running, we must run the other equipment continuously until the pulverized coal in the equipment is consumed.
