Production and processing sites are often accompanied by dust explosions such as aluminum powder, zinc powder, aluminum grinding powder, various plastic powders, organic synthetic drug intermediates, wheat flour, sugar, sawdust, dyes, bakelite ash, milk powder, tea powder, tobacco powder, coal powder and plant fiber dust.
Dust explosion generally has four situations:
(1) Dust itself is flammable or explosive;
(2) Dust must be suspended in the air and mixed with air or oxygen to reach the explosion limit;
(3) There is enough heat energy to cause dust explosion, which is the ignition source;
(4) Dust has certain diffusibility;
(5) Dust will explode in a closed space, such as ethanol and water dust explosion in granulating furnace and boiling dryer.
Explosion principle
Table 1-4 Classification and grouping of explosive dust
Generally, the dusts prone to explosion accidents include aluminum powder, zinc powder, ferrosilicon powder, magnesium powder, iron powder, aluminum processing grinding powder, various plastic powders, organic synthetic drug intermediates, wheat flour, sugar, sawdust, dyes, bakelite ash, milk powder, tea powder, tobacco powder, coal powder, plant fiber dust and so on. The dust of these materials is prone to explosive combustion because of the existence of strong reducing agents such as H, C, N and S. When they exist with peroxide and explosive dust, they will decompose through oxidation reaction to produce a large amount of gas, or the amount of gas is small, but a large amount of combustion heat is released. For example, aluminum powder is in danger of explosion as long as it is in carbon dioxide atmosphere. ? [2]?
The difficulty of dust explosion is related to the physical and chemical properties of dust and environmental conditions. It is generally believed that substances with high combustion heat are more likely to explode, such as coal dust, carbon and sulfur. Substances with fast oxidation rate are easy to explode, such as magnesium powder, aluminum powder, ferrous oxide, dyes, etc. Dust that is easy to be charged is also easy to cause explosion, such as synthetic resin powder, fiber dust and starch. These substances with poor conductivity accumulate due to static electricity generated by friction with machines or air, and when they reach a certain amount, they will discharge to generate electric sparks, which will constitute the fire source of explosion.
Generally, dust that is not easy to cause explosion includes soil, sand, iron oxide, abrasive materials, cement, timely dust and similar dust after combustion. The dust chemical properties of this kind of substance are relatively stable and not easy to burn. However, if this kind of dust is produced in oil mist and combustible gases such as CO, CH4 and coal gas, it is also prone to explosion.
The explosion of dust can be thought to be formed by the following three steps: the first step is the rapid dry distillation or gasification of suspended dust under the action of heat source to produce combustible gas; Step two, combustible gas and air are mixed for combustion; In the third step, the heat released by dust combustion is transferred to nearby suspended or blown dust by means of heat conduction and flame radiation, and these dust are heated and vaporized, so that the combustion cycle can continue. With the advance of each cycle, the reaction speed is gradually accelerated, and finally an explosion is formed through intense combustion. This kind of explosion reaction, as well as the explosion flame velocity, explosion wave velocity and explosion pressure, will be accelerated and increased continuously and develop by leaps and bounds.
Typical danger:
influencing factor
physicochemical properties
Influencing factors of dust explosion
The greater the combustion heat of a substance, the greater the explosion risk of its dust, such as coal, carbon and sulfur dust; The more easily oxidized substances, the more easily their dust explodes, such as magnesium, ferrous oxide, dyes, etc. The more charged the dust, the more likely it is to cause an explosion. In the production process, due to the impact and friction of dust, the generated static electricity is not easy to dissipate, resulting in static electricity accumulation. When it reaches a certain value, electrostatic discharge will occur. Electrostatic discharge sparks can lead to fire and explosion accidents. Dust explosion is also related to the volatiles it contains. For example, if the volatile matter in pulverized coal is lower than 10%, it will not explode, so there is no explosion danger of coke powder.
Limit value of dust explosion concentration-Table 1
grain size
The surface of dust absorbs oxygen in the air. The finer the particle, the more oxygen it absorbs, so it is easier to explode. Moreover, the lower the ignition point, the lower the explosion limit. With the decrease of dust particle diameter, not only the chemical activity increases, but also it is easy to carry static electricity.
dust loading
Similar to combustible gas, dust explosion also has a certain concentration range and upper and lower limits. But most of the general data only list the lower limit of dust explosion, because the upper limit of dust explosion is high. Table 1 (right) lists some characteristics of dust explosion.
Explosion characteristics
(1) Multiple explosions are the biggest characteristics of dust explosion.
When air billow exploded for the first time, it would blow up dust deposited on equipment or the ground. In a short time after the explosion, negative pressure will be formed in the explosion center, and the surrounding fresh air will be poured in from the outside and mixed with the raised dust, thus triggering a secondary explosion. In the second explosion, the dust concentration will be higher.
(2) The minimum ignition energy required for dust explosion is relatively high, generally above tens of millijoules.
(3) Compared with combustible gas explosion, the pressure of dust explosion rises slowly, the duration of higher pressure is long, the released energy is large, and the destructive power is strong.