Talking about dust explosion

A dust explosion is an explosion that occurs when flammable solid particles suspended in the air come into contact with an ignition source such as an open flame. Once a dust explosion occurs, it often causes serious casualties and causes huge economic losses to the enterprise, which seriously threatens the safety of industrial production. Metal dust, coal powder, plastic dust, organic dust, fiber dust and agricultural and sideline products such as grain flour may cause dust explosion accidents. According to statistics, since the 20th century, there has been at least one dust explosion accident every day in the world. About 75% of such accidents are caused by explosions. The damage caused by fires accounts for 20%, and the remaining losses are caused by toxic gases. Caused by a leak. The polishing shop is a high-risk area for aluminum dust explosions.

1. Conditions for dust explosion

    There are five essential factors for dust explosion: combustible dust, oxidant, ignition source, effective mixing ratio of dust and air, and relatively closed space. According to analysis, 70% of the dust produced by industrial production is flammable. There are also many combustible dusts that are likely to cause dust explosions, mainly magnesium-aluminum metal dust, flour and feed dust, grain dust stored in warehouses, wood chips, coal dust, plastic dust, chemicals, starch, sugar dust, tobacco, etc. The oxidant is a combustion-supporting substance that is supplied to the redox reaction during dust combustion. The most common oxidant in the dust explosion is oxygen in the air. In a few cases, carbon dioxide and nitrogen can also be used as oxidants. For example, carbon dioxide in the air in aluminum dust will also participate in the reaction. Nitrogen in the magnesium dust will also participate in the reaction, which makes carbon dioxide and nitrogen not be used as inhibitors of aluminum-magnesium dust. It is generally believed that Mars with thermal energy higher than 10mJ or overheated surface with temperature greater than 400°C can be the ignition source of dust explosion. Common dust explosion ignition sources have external open flame and external heat source, dust accumulation, spontaneous heat accumulation, high temperature equipment, electricity. Sparks, frictional sparks and heat, static electricity, sparks from collisions, etc.

    The explosion often throws a large amount of deposited dust into the air, so a small-scale dust explosion often triggers a chain reaction, causing a more serious secondary explosion. Equipment and places that are prone to dust and explosions include wood and metal shredders, dust collectors, blowers, electrostatic precipitators, filters, grinders, dryers, mixers, ventilation ducts, storage areas, and the like.

    2. The process of dust explosion

    The suspended dust particles are quickly oxidized by the oxidant under the action of the ignition energy of the ignition source, and a redox reaction occurs, and more heat is released during the reaction to ignite the surrounding combustible dust. The dust deposited on the surface of the object will also be promoted by the strong air flow. This chain reaction is very fast, and a large amount of heat energy is generated instantaneously and the space pressure rises. When the pressure exceeds the pressure value of the closed space shell, it will explosion occurs. It is worth noting that most serious dust explosions are caused by secondary explosions. Dust explosions often occur in a small area in space or equipment. This type of explosion is called an explosion. Its power is not great, but it often raises dust deposited nearby, and the central area will explode in a short time after the explosion. Forming a negative pressure, the surrounding fresh air is filled in from the outside to the inside, mixed with the raised dust to form a dangerous dust cloud. A dust deposit of 1 mm/m2 and a total mass of 500 g can form a 5 m high, 100 m3 combustible dust cloud. If these dust clouds are ignited by the first explosion, they will produce a series of more powerful explosions, namely a second explosion.

    3. Factors affecting the nature of dust explosion

Fine particles generated by mechanical processes (crushing, sieving, transportation, etc.) can be dispersed (suspended) in a gas for a certain period of time, called dust. Dust has a wide range of particle sizes ranging from fine to 1/10 μm to hundreds of microns. In common industrial production, dust generally comes from the crushing action of the crushing machine. At this time, the dust may be the main product of the action, or it may be the secondary product. Among them, the energy consumed by the work done by the machine is partially converted into “surface Gibbs free energy” and stored in a single dust particle. At the same time, the finer the dust particle, the larger the relative surface area and the larger the surface energy, ie the unit volume. The total surface energy stored in the same quality of dust is greater. Taking coal dust as an example, a 1 g heavy coal has a surface area of ​​6 cm 2 and 1 g of coal powder dust (10 μm in diameter), and its surface area can reach 2 m 2 . Therefore, several factors affecting dust explosions can be summarized as follows:

    (1) Particle size: This is an important factor affecting the reaction rate. The smaller the particles, the easier it is to burn and the stronger the explosion. When the particle size is less than 200 μm and the dispersion is large, it is easy to float in the air, and the heat absorption is fast, and it is easy to catch fire. Particles larger than 500 μm and containing a certain amount of large particles are not easy to detonate.

    (2) Chemical composition: If the organic dust contains COOH, OH, NH 2 , NO, C=N, C=N and N=N, the risk of explosion is high; halogen and potassium, sodium The dust, the explosion trend is weakened.

    (3) Explosive concentration - The minimum weight of suspended dust capable of propagating a flame in a given volume is called the explosive concentration. Usually, dust that reaches the dust explosion concentration will explode.

    (4) Air humidity - When the air humidity is high, the hydrophilic dust will absorb the water, so that the dust is difficult to disperse and catch fire, and the speed of spreading the flame will also decrease. Even if the dust with high humidity is on fire, its heat is first consumed by the water in the evaporated dust before it is used in the combustion process. Dust humidity of more than 30% is not easy to detonate.

    (5) There are enough ignition temperatures - most of the dust explosions originate from external open flames, such as mechanical impact, electric welding and cutting, static sparks or electric sparks, friction sparks, matches and heat transfer in high temperature bodies. The minimum ignition temperature for this type of ignition source is 300 to 500 °C.

    4. Prevention of dust explosion

    (1) Reduce dust exposure. Dust is on fire and explosion requires oxidant. The mixture of atmosphere and flammable dust increases the risk of dust explosion. Reducing the large amount of dust exposure is the primary measure to reduce dust explosion accidents. Select quality parts and mature processes and facilities, and strengthen regular maintenance of equipment to reduce the run, run, drip and leakage of pipes and lines, and prevent the leakage of dust materials. Before the hot work, especially in the hot work of confined spaces, the sanitary cleaning of the construction space is extremely important.

    (2) Strict control of all possible ignition sources is the most important means to avoid dust and fire. For example, the work area should avoid or strictly control open flames, such as smoking, electric welding, gas cutting, gas welding, etc. The open flame heating equipment should be far away from the material area, no open flame lighting, the heating equipment and lighting equipment should be restricted as much as possible in the workshop, and the motor vehicle should be equipped with a fireproof cover when entering the factory.

    (3) Improper use and control of materials lead to accidents. The design of the production process should be designed to minimize or eliminate the risk of the material; the refining process of the particles should be carried out as wet or in an inert environment as possible; hazardous materials should be processed and transported in a closed space as much as possible to isolate Air, water vapor; the pulverized material must undergo strict screening, stone removal and iron absorption to avoid sparks; when the equipment is to be emptied, independent dust removal and purification equipment should be used to prevent centralized treatment by pipeline connection.

    (4) Take structural explosion-proof measures. Factory buildings that process flammable dust are as open as possible or outdoor. The selection principle of the building location is: when exploding, do not endanger other working sites and buildings even when the explosion flame is released from the light roof or wall. In addition to this, it is necessary to set up necessary structural explosion-proof facilities such as anti-explosion, venting, explosion suppression and explosion-proof.

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