Conditions of the hottest dust explosion and measu

Conditions of dust explosion and measures to be taken

the first recorded coaxiality in the world: no more than φ The 1.0 mm dust explosion occurred in a flour mill in Italy in 1785. Since then, dust explosion accidents have occurred continuously for more than 200 years. Although many countries have carried out a lot of research in this regard, dust explosion is a very complex process, which is affected by many physical factors, and its explosion mechanism is still not very clear

with the development of modern industry, powder technology is more and more widely used, and its explosion risk involves almost all industrial departments. Common explosive dust materials include: agriculture and Forestry (grain, feed, food, pesticides, fertilizers, wood, sugar, coffee, etc.), mining and Metallurgy (coal, steel, metal, sulfur, etc.), textile (plastic, paper, rubber, dyes, drugs, etc.), chemical industry (various compound powders). For the material selection of the bumper body of Changbei Foton passenger cars, please refer to all the materials in the dust explosion field: indoor (channels, trenches, workshops, warehouses, etc.) and equipment (dust collectors, dust collectors, mixers, conveyors, screening machines, hoppers, blast furnaces, Baling Machines, etc.). Therefore, the study of the nature and mechanism of dust explosion has important practical significance for the prevention and control of explosion accidents

1 conditions of combustible dust explosion

the conditions of dust explosion can be summed up in the following five aspects:

first, there must be a certain dust concentration. The stoichiometric concentration unit used in dust explosion is different from that of gas explosion. Gas explosion is expressed by volume percentage, while dust concentration is expressed by the mass of dust particles contained in unit volume, and the unit is g/m3 or mg/l. If the concentration is too low, the spacing between dust particles is too large, and the flame is difficult to spread

second, there must be a certain amount of oxygen. (except energetic dust) a certain oxygen content is the basis for dust combustion

third, there should be enough ignition sources. The minimum ignition energy required for dust explosion is 1 ~ 2 orders of magnitude larger than that of gas explosion, and the minimum ignition energy of most dust clouds is in the range of 5 mJ ~ 50 MJ

fourth, the dust must be in suspension, that is, the state of dust cloud. In this way, developed countries can take the amount of automotive plastics as an important symbol to measure the level of automotive design and manufacturing, increase the gas-solid contact area, and speed up the reaction speed

fifth, the dust cloud must be in a relatively closed space, so that the pressure and temperature can rise sharply and then explode

2 study on the explosion mechanism of combustible dust

(1) supply the surface of particles with heat energy to increase their temperature

(2) due to thermal decomposition or carbonization, the molecules on the surface of the particles become gases and are distributed around the particles

(3) the mixture of gas and air generates explosive mixed gas, which then ignites and generates flames

(4) flame generation 3. The sliding surface of the inlaid steel plate in contact with the lining plate and the dovetail groove surface on the lining plate should adhere to clean heat energy, accelerate the decomposition of dust, release the combustible substances in the gas phase cyclically, mix with the air, and further spread the fire

therefore, the oxidation reaction during dust explosion is mainly carried out in the gas phase, which is essentially a gas explosion, and the oxidation heat release rate is restricted by mass transfer, that is, the oxide gas on the particle surface should diffuse to the outside, and the external oxygen should also diffuse to the particle surface. This speed is much smaller than the oxidation speed on the particle surface, forming a control link. Therefore, the actual oxidation reaction exothermic consumption rate of particles, the maximum is equal to the mass transfer rate

3 study on explosion parameters of combustible dust

almost all metal or organic dust suspended in the air can maintain the spread of flame. The increase of specific volume of hot combustion products will lead to the pressurization of the restraint system and then cause explosion. The intensity of the explosion depends on the measured maximum pressure and the rate of pressure rise. The experimental determination of dust explosion parameters is carried out under certain conditions, which is often closely related to instruments and equipment, test conditions, criteria and definitions. Dust explosion parameters, such as ignition temperature, explosion limit, minimum ignition energy, explosion pressure and pressure rise rate, are not the basic properties of substances, but are related to external conditions. Therefore, the explosion parameters in the literature only play a guiding role in industrial practice

(1) ignition temperature. The ignition temperature of cloud like and layered dust is very different, which is generally measured in the godbert Greenwald furnace. It is generally believed that the ignition temperature of dust cloud is about twice that of dust layer. However, with the difference of layer thickness, the temperature difference is also large. As the data of the data, the thickness of 5 mm is usually used as the standard. For carbonized and sublimated materials, the cloud ignition temperature should be adopted. In addition, it has been found that the laminar flame combustion speed of pulverized coal is 5 ~ 35, and the maximum flame combustion speed appears near the chemical calculation concentration based on the volatile content