1. Characteristics of dust explosion

Dust explosion is a kind of oxidation reaction that the dust particles suspended in the air fully contact with the oxygen in the air, which is completed instantaneously under specific conditions. A lot of heat is released in the reaction, and then high temperature and pressure are produced. Any dust explosion must have three conditions: ignition source; combustible fine dust; dust suspended in the air and reaching the explosion concentration limit range.

(1) Dust explosion is more complex than combustible materials and gases. Combustible dust is suspended in the air to form a dust cloud within the explosion concentration range. Under the action of ignition source, part of the dust in contact with ignition source is ignited first and forms a small fireball. Under the action of the heat released by the combustion of this small fireball, the surrounding dust is heated, the temperature rises, and the phenomenon of ignition and combustion occurs, so that the fireball will expand rapidly and form dust explosion.

The difficulty and intensity of dust explosion are closely related to the physical and chemical properties of dust and the surrounding air conditions. Generally, the greater the combustion heat, the finer the particles and the higher the activity of the dust, the greater the risk of explosion; the greater the risk of explosion of light suspended combustible materials; when the oxygen content in the air is high, the dust is easy to be ignited and the explosion is more intense. Because water has the function of inhibiting explosion, the drier the dust and gas are, the more dangerous the explosion will be.

(2) After the dust explosion, there is often a secondary explosion. This is because there are a lot of dust deposited together in the first explosion, and its concentration exceeds the upper limit value of dust explosion and can not be exploded. However, when the deposited dust is raised again by the shock wave or gas wave formed by the first explosion, it will mix with the air in the air, and the concentration is within the range of dust explosion, then the second explosion may occur immediately. The disaster caused by the second explosion is often much more serious than that caused by the first explosion.

The direct cause of dust explosion in 1963 in a domestic aluminum production plant is the friction between the exhaust fan impeller and the end face of the suction port. The air flow of the shrimp bend and the three-way underpants at the suction port of the fan is not smooth, which is easy to accumulate dust. In particular, it is easier to retain dust during shutdown. Once started, the deposited dust will be lifted up, and soon reach the lower explosion limit, causing dust explosion.

(3) Mechanism of dust explosion: when combustible dust burns in the air, it will release energy, and a lot of gas will be produced in the well. The speed of energy release, that is, the size of combustion speed, is related to the area of powder exposed in the air. Therefore, for the same solid powder, the smaller the particle size is, the larger the specific surface area is, and the faster the combustion diffusion is. If the particle size of this solid is very fine. It can even be suspended. Once ignited by a ignition source, a large amount of energy can be released in a very short time. On the other hand, when the powder is burned, a large amount of gas will be produced, which will make the system form a local high pressure, resulting in explosion and propagation, which is commonly known as dust explosion.

(4) The difference between dust explosion and combustion: the combustion of large solid combustibles is carried out in parallel layers, such as coal combustion. The release of combustion energy is relatively slow. The heat and gas produced can escape quickly. In the case of good ventilation, the stack combustion of combustible dust forms open fire combustion, while in the case of poor ventilation. It can form smoke-free or hidden flame.

There are several stages in the combustion of combustible dust: in the first stage, the surface powder is also heated; in the second stage, the surface layer is gasified, and the volatiles are overflowed; in the third stage, the volatilized and distributed gas phase is burned.

The explosion of ultra-fine powder is also a complex process. Because the scale of dust cloud is generally small, and the flame propagation speed is fast, hundreds of meters per second, ignition occurs in the dust center, and the whole dust cloud can be ignited in less than 0.1s. In this process, if the dust has been burnt out, the highest pressure will be generated; if not, the lower pressure will be generated. Whether combustible particles can burn out depends on particle size and burning depth.

(5) Classification of combustible dust: powder can be divided into two categories according to its flammability: one is combustible and the other is non combustible. The classification methods and standards of combustible powder are different in different countries.

In the United States, combustible powder is classified as class II dangerous goods. Meanwhile, metal powder, carbon dust and grain dust are classified into different groups. The classification method developed in the United States is based on the pressure rise speed of the tested powder in the standard test device, and it is divided into three grades. At present, there is no ready-made standard for classification of combustible dust in China.

2. Influence of dust concentration and particles on Explosion

(1) The upper and lower limits of dust concentration also exist in the explosion of combustible dust. The value is affected by many factors such as ignition energy, oxygen concentration, powder size, powder variety and water content. Using the simplified formula, the explosion limit can be estimated. Generally speaking, the lower limit concentration of dust explosion is 20-60g / m3, and the upper limit is 2-6kg / m3. The upper limit is affected by many factors, and its value is not as easy to determine as the lower limit, and it is usually not easy to reach the upper limit concentration. Therefore, the lower limit value is more important and useful.

In physical sense, the upper and lower limit values of dust concentration reflect the influence of the distance between particles on the flame propagation of particle combustion. If the distance between particles reaches to make the combustion