Flour will explode, so will flour explode? What are the requirements?

I remember that the previous chemistry textbook said that powder would explode in a closed space.

Random talk on dust explosion

Shi Jianye (Mannheim, Germany)

Flour will explode? This is an incredible thing for most people. Believe it or not, most powdery substances in our life are explosive to some extent.

I remember watching "100,000 Why" when I was a child. There was a short article about how flour exploded: put some flour in a jar, shake it a few times, and then light it with fire. Bang, the pressure from burning flour will lift the lid off. Many years later, dust explosion-proof became the theme of my works, from China to German.

1785 12 14, a flour explosion occurred in a flour mill in Turin, Italy. This is the first recorded dust explosion in the world. In the future, gunpowder explosion accidents were frequently reported in newspapers. According to statistics, at present, there is an explosion accident of grain powder in the world about one day, and it is quite serious about ten times a year. 1February 4, 979, a flour explosion occurred in the Roland M L Party in Bremen, Germany. 14 people died, 17 people were injured, and property loss 1 100 million marks (the author visited the factory 1993, watched the film shot after the explosion, and had a discussion with the employer of the factory). It can be seen that the accident, especially the death of someone, hit him hard. Although the insurance company compensated all the losses of the factory, he even wanted to give up running the factory. 1987 March 15, an explosion accident of flax dust occurred in Chinese yam textile factory in Harbin, killing 58 people and injuring 177 people. 13,000 square meters factory was destroyed, and the direct loss exceeded10 million RMB. From 1983 to 1992, there are 15 grain dust explosion accidents in the United States every year, with an average annual loss of nearly12 million US dollars.

In order to avoid the disaster caused by gunpowder explosion, western countries have begun to study the phenomenon of gunpowder explosion in recent decades. Because gunpowder explosion involves many disciplines such as chemical reaction kinetics, gas-solid two-phase fluid mechanics, explosion mechanics, thermodynamics, static electricity, machinery and even biology, its mechanism is very complicated. Although some scientists have put forward some dust ignition and propagation models from different disciplines, their application scope is very small or just a set of qualitative mathematical equations. There is no universal meaning, and there is no universally accepted model. In fact, the dust as the research object itself is very complicated-it is a general term for all powdery substances. Can be divided into organic dust (such as grain and food, cosmetics, medicines, textile fibers, coal and tobacco, etc. ) and inorganic dust (such as aluminum powder, magnesium powder and sulfur, etc.). ). According to its shape, it can be divided into sheet, spherical and fibrous. Different kinds and shapes of dust have different explosion behaviors. It is generally believed that dust explosion has three necessary conditions:

1. An ignition source with sufficient energy;

2. There is a relatively closed oxygen-containing space;

3. The dust in this space reaches a low explosion concentration.

When these three elements are available at the same time, the outbreak will be inevitable. For grain dust, when the concentration is about 40g/m3 (equivalent to reaching five fingers), if someone lights a cigarette or carries out welding and cutting operations, the consequences will be unimaginable! I advise you to leave this trouble spot as soon as possible. By the way, the domestic environmental hygiene standards stipulate that the dust concentration in the workplace shall not exceed 5mg/m3. This concentration is quite low. When you dance unrestrained disco in a disco, you will vent your anger to your heart's content. Suddenly, colorful smoke will come out from somewhere, which will aggravate the drama atmosphere. I didn't know that this kind of smoke would do potential harm to your health, especially to your lungs.

The burning of corn starch observed under high-speed camera is an interesting process: slow and lazy. It's a bit like the sugar cooked in the pot when cooking shredded sweet potatoes in the kitchen. With the increase of furnace temperature, sugar is melting, color is deepening, and countless bubbles, large and small, are tumbling. Only when corn starch is heated, the surface of "boiling" bubbles will have flame. However, the real powder explosion is a violent process. It is an instantaneous chain reaction, which has gone through the process of gestation, occurrence, development and final disaster. Its propagation is similar to dominoes: under the action of ignition source, one or several burning dust particles transfer energy to their adjacent unburned particles in three-dimensional space, forming a small fireball. When the burning energy of this small fireball can raise the surface temperature of the fireball to the natural point of the surrounding dust, the adjacent dust will continuously produce a chain reaction, which will make the fireball expand rapidly and form an explosion. If the burning temperature of the small fireball can't make the temperature of the surrounding dust reach above the natural point, the fireball will extinguish itself and there will be no explosion danger.

The severity of dust explosion is determined by the distribution and concentration of dust in the space and the disturbance degree of airflow to this space. The more uniform the dust distribution, the closer to the optimal explosion concentration, the higher the airflow disturbance, and the more violent the explosion. In industrial production, it is not the primary explosion of dust that causes the greatest harm, but its secondary explosion. The scale of the initial explosion may be small, but the resulting shock wave, vibration or air churning aggravate the disturbance of airflow, which makes the dust deposited on the solid surface fly away and form a new dust cloud. The newly formed dust cloud is ignited by the existing explosion flame, which will lead to a disastrous secondary explosion, that is, a secondary explosion. As long as the conditions for the second explosion can be provided in front of the explosion flame, the explosion will spread and blow up the good factory beyond recognition. Fire, toxic gas, house collapse and out-of-control of water and electricity caused by secondary explosion will cause secondary injuries to people.

In order to prevent gunpowder explosion and reduce its harm, scientists and engineers have done a lot of research, and various technologies have emerged because of transportation. There are two main ideas: one is to avoid explosion. Such as eliminating static electricity, heat generated by friction of mechanical equipment, metal impact sparks, and hot surfaces generated by welding and cutting; Adding inert gas to reduce oxygen concentration; Set up ventilation and dust removal system to reduce the concentration of air dust. The second is to limit the development and spread of gunpowder explosion and reduce its harm. For example, equipment with high explosion risk is relatively isolated; Install explosion-proof doors, windows and conduits on equipment and building structures to quickly release explosion pressure; Set up mechanical and chemical explosion-proof devices to isolate the explosion in a certain area; Set chemical explosion suppression device to put out the initial explosion and so on. It should be pointed out that there are still many shortcomings in industrial application of these technologies, and this problem has not been completely solved.

Powdered substances exist widely as intermediates, final products or by-products of most products. Where there is dust, there is a potential danger of dust explosion. Although powder explosion is very dangerous and causes great disaster, there is no need to talk about "powder" discoloration. Compared with gas and liquid explosion, its explosion conditions are relatively harsh. Powdered substances are widely used in production and life because of their strong plasticity and convenient transportation, and become an indispensable part of human beings. When people take advantage of its advantages, they often ignore or don't know its potentially dangerous side. With the adoption of new machines and the improvement of production capacity, the explosion risk in places that were not dangerous or not dangerous suddenly increased. For example, after the textile industry used combers and other equipment in the 1960 s, an explosion accident occurred. Once the hazards are recognized, people will take preventive measures. Generally speaking, the benefits of using powdery substances far outweigh the losses caused by powder explosion. This knife cuts bread and fingers. There is almost no technology and product in modern society that does not have the nature of a double-edged sword. The problem is that we can use its advantages to avoid its shortcomings, foster strengths and avoid weaknesses, and make it benefit human society well.