Just now, we made two conclusions about fire smoke.

Fire has brought civilization, light and warmth to mankind, but fire has also brought tremendous harm to human life and property.

According to the statistics of the global fire survey conducted by the International Fire Protection Technology Commission, 600 to 7 million fires occur each year , and approximately 65,000 to 75,000 people are killed in the fire.

The statistics of a large number of domestic and foreign fires show that most of the casualties caused by fires are fatal due to smoke. This shows that the hazard of fire smoke is extremely serious and must be controlled.

Understanding the composition and hazards of fire smoke is an important foundation for smoke control design. This article mainly introduces the composition and hazards of fire smoke.

The first section   Composition of fire smoke

First, the generation of fire smoke

Flammable smoke is generated when combustible materials are pyrolyzed or burned.

Fire smoke is a mixture of two parts: the majority of which is the air that is mixed with the combustion products and heated by the flame. This part of the air is relatively free from the chemical reactions that occur during the fire.

The second part is fire pyrolysis and chemical reaction products, which mainly include gases such as carbon monoxide, carbon dioxide, water vapor and methane, as well as solid (smoke) and liquid (such as hydrocarbon) particles. The quality and volume of these components are very small. .

Therefore, the physical properties of fire smoke are basically the same as those of hot air. When assessing the fire risk of a building, the flow of fire smoke can be equivalent to the flow of hot air.

However, the chemical nature of fire smoke is obviously different from that of air, such as its reactivity, combustibility, and toxicity. The particles in fire smoke are very irritating to the eyes and respiratory tract.

The composition of fire smoke is related to the supply of air during a fire. The ratio of combustibles and air is different, and the combustion products produced are also different.

For normal combustion conditions, the air supply is well-guaranteed, the combustion is relatively complete, and the resulting product can no longer be burned. This combustion is called complete combustion, and its combustion products are called complete combustion products.

In the complete combustion state, the combustion products exist mainly in gaseous form and their composition mainly depends on the composition of the combustibles.

For abnormal combustion conditions, there is no good combustion conditions, combustion is not complete, called incomplete combustion, the corresponding combustion products are called incomplete combustion products.

In the incomplete combustion state, the combustion products contain organic compounds such as alcohols and ethers. These combustion products are mostly toxic gases, which cause various degrees of damage to the human respiratory system, circulatory system, and nervous system, affecting people's normal breathing and ability to move.

When a fire occurs in a building, due to limited space, if there is a sprinkler system or other external media involved in extinguishing the fire, most of its combustion is incomplete combustion.

During the fire suppression process, due to different measures and extinguishing agents, different gases will be generated accordingly.

Under normal circumstances, when water is used to save, only a large amount of water vapor is produced, but if some of the combustion materials themselves can react chemically with the fire extinguishing agent, some other harmful substances such as hydrogen sulfide, sulfur dioxide, etc. will be generated. There has been a heavy lesson in the history of personnel poisoning and casualties.

Second, the composition of fire smoke

The composition and properties of fire smoke primarily depend on the chemical composition of the pyrolysis and combustion of the material itself, and secondarily on the combustion conditions.

The so-called burning conditions refer to the heating conditions of the environment, the space-time conditions of the environment, and the oxygen supply conditions.

Because of the complexity of the substances involved in the fire during a fire, especially the environmental conditions of the fire, the composition of the fire smoke is also quite complicated.

On the whole, fire smoke consists of three parts: gas (vapor) generated by pyrolysis and combustion, suspended particles, and remaining air.

(a) Pyrolysis and combustion of the generated gas (gas) body

Most of the flammable substances belong to organic compounds, and their main components are carbon, hydrogen, oxygen, sulfur, phosphorus, nitrogen and other elements.

Under normal temperature conditions, nitrogen does not participate in chemical reactions and precipitates in a free state during the combustion process, and oxygen is consumed as an oxidant during the combustion process.

The elements such as carbon, hydrogen, sulfur, and phosphorus are oxidized to produce the corresponding oxides, ie, carbon dioxide, carbon monoxide, water vapor, sulfur dioxide, and phosphorus pentoxide. In addition, there is a small amount of hydrogen and hydrocarbons produced.

Modern buildings are usually complicated to decorate, and there are more and more indoor products and furniture.

Except some interior furniture and doors and windows made of wood materials, most of the rest of the decoration and furniture use polymer synthetic materials, such as building plastics, polymer coatings, polystyrene foam insulation materials, composite flooring, epoxy resin insulation layer, Chemical fiber, sofas, bedding, etc. The combustion and pyrolysis products of these macromolecular synthetic materials are much more complex than single wooden materials.

(b) Pyrolysis and combustion of suspended particulates

Suspended particles generated by pyrolysis and combustion in fire smoke are called smoke particles.

These microparticles generally include free carbon (carbon black particles), condensate droplets of tar-like particles and high-boiling substances, and the like. These solid or liquid particles, suspended in the gas phase, drift with them.

Due to the different nature of the smoke particles, the color of the smoke is different at different stages of fire development.

In the smoldering stage before the fire, due to the decomposition of pyrolysis, the condensed particles of some high-boiling substances are mainly produced, and the color of the smoke is often white or blue-white. In the fire stage, carbon black particles are mainly produced. The color of the smoke is black, forming black smoke.

(c) hot air

In an indoor fire, in the process of vertical rise of fire smoke in the form of a buoyant plume, the surrounding air is continuously sucked to form a plume with a large volume.

The amount of fire smoke generated is mainly determined by the amount of air sucked by the plume of the flue gas, that is, the fire smoke is mainly heated air.

At present, the most commonly used fire smoke generation calculation model is based on the amount of air entrainment, without considering the consumption of combustibles.

Section 2 Harmfulness of Fire Smoke

In a building fire, the fire smoke rises with the hot air flow. When it encounters the ceiling or other obstructions, it begins to decline and gradually fills the entire room. The process of dropping the neutral surface often occurs very quickly.

The hazards caused by fire smoke in buildings are numerous. In summary, there are mainly oxygen deficiency, poisoning, light reduction, dust and high temperature.

The fire smoke generated during the fire will hinder the safe evacuation of the personnel, hinder the firefighters from entering the fire site to search and rescue and extinguish the fire, and will reduce the possibility of the trapped person surviving.

One, lack of oxygen

Oxygen is the key substance for the human body to carry out metabolism, and it is the first need for human life activities.

When the oxygen content in the air is reduced to 15% , people's muscle activity is decreased; when it is reduced to 10 to 14% , people have weakness in their limbs, mental confusion, and unclear direction; when they fall to 6 to 10% , people will Will faint. For people in a burning room, the short-term lethal concentration of oxygen is 6% .

The combustion consumes a large amount of oxygen, so that the oxygen content in the fire smoke is often lower than the physiologically required normal value. Studies have shown that in the fierce development stage of the fire, the concentration of O2 is often only about 3% .

Therefore, in the event of a fire, it is very dangerous for people in the building to escape the fire.

Second, poisoning

There are many types of combustibles in building fires, including various natural materials such as wood materials, paper, wool, and silk, as well as various synthetic materials such as plastics and rubber, and the ever-changing combustion conditions can generate a variety of toxic substances. harmful gas.

If the content of these gases exceeds the minimum concentration allowed by normal physiological conditions, poisoning and even death will occur.

At present, there are dozens of types of toxic gases or toxic gases in known fires, including inorganic toxic and poisonous gases ( CO , CO2 , NOx , HCl , HBr , H2S , NH3 , HCN , P2O5 , HF , SO2, etc. ) And organic toxic and harmful gases (phosgene, aldehyde gases, hydrogen cyanide, etc.).

(a) The effect of carbon monoxide ( CO ) on people

CO is a common product of incomplete combustion in fires and is a toxic gas. It usually occupies a large proportion in the fire and is not easily excluded.

In the fire accidents, the number of people who died from the toxic effects of CO accounted for more than 40 % of the total number of deaths, which was one of the major factors causing deaths in the fire.

The main poisonous effect of CO is that it combines with hemoglobin to produce carboxyhemoglobin, which greatly weakens the binding capacity between hemoglobin and oxygen, and reduces the oxygen content in the blood, resulting in insufficient oxygen supply and hindering the blood from sending oxygen to various parts of the body.

(II) Effect of Hydrogen Cyanide ( HCN ) on Human Body

HCN is a colorless, toxic gas slightly almond odor, toxicity of about 20 times the CO.

Combustion of nitrogen-containing fuels often produces HCN . Such materials include natural materials and synthetic materials such as wool, silk, nylon, polyurethane dimer, and urea resins. In particular, cotton smoldering generates HCN .

HCN is an extremely toxic substance that, although it does not substantially bind to hemoglobin, can inhibit the production of enzymes in the human body and prevent normal cellular metabolism. The HCN concentration and symptoms of poisoning are shown in Table 1.3 .

The interior decoration of modern buildings uses a large number of plastics, and these materials will react and generate a large amount of HCN in the fire. The poisonous effect of this gas on the human body has drawn increasing attention.

By examining the blood components of the victims of fire, it was found that more than 30% of the deaths were due to HCN poisoning.

(III) Influence of other toxic gases on people

Other toxic gases in fire smoke include carbon dioxide ( CO2 ), acrolein ( C3H4O ), and hydrogen chloride ( HCL ).

Carbon dioxide is the most abundant gas produced during a fire. The increase in its content directly leads to a decrease in the oxygen content, which in turn increases the respiratory rate of humans and increases the intake of other toxic and harmful gases.

Acrolein is a toxic substance produced during the smoldering process of fibrous substances. It has a strong stimulatory effect on the senses and the lungs of humans. Exposure to it for a long time will cause severe pulmonary dysfunction.

For example, when 5.5 ppm of acrolein is contained in the flue gas , it causes irritation to the upper respiratory tract; if the concentration is greater than 10 ppm , it can cause changes in the lungs and death can occur within minutes.

The content of acrolein in the wood-burning flue gas is as high as about 50 ppm , and the presence of formaldehyde, acetaldehyde, hydroxide, and hydrogen cyanide in the flue gas is extremely harmful to the human body. In the fire of PVC and other substances, gases such as hydrogen chloride are quite common. This kind of material can affect people's feelings and lungs.

With the wide application of polymer synthetic materials in construction, decoration and furniture manufacturing, the components of toxic and harmful fumes produced in fires are more complex and more toxic, which requires people's attention.

Third, reduce light

Wavelength of visible light is 0.4 ~ 0.7 μ m, typically in fire smoke tobacco particle diameter of several μ m to several tens μ m, i.e. the particle size is larger than the wavelength of visible smoke particles, which smoke particles are opaque to visible light It has a complete shielding effect on visible light.

When the smoke is diffused, the visible light is shielded by the smoke particles and the visibility is greatly reduced.

At the same time, some gases in the flue gas are extremely irritating to the naked eye, such as HCl , NH3 , HF , SO2 , Cl2, etc., so that people's speed in the evacuation process is greatly reduced, which is the light reduction of smoke Sex.

It not only hinders safe and rapid evacuation activities, but also hinders firefighters from carrying out normal fire fighting activities.

IV. Dust

In fire smoke is harmful aerosols, are most harmful airborne dust particle diameter of less than 10 μ m, which are invisible, long-term float in the atmosphere, then at least several hours in length.

In particular fly ash particles is smaller than 5 μ m, since the diffusion of the gas, the body can enter the lungs, adhesion and aggregation in the alveolar walls, causing respiratory diseases, increased heart disease mortality, cause direct harm to the human body.

When firefighters extinguish fire, personal protection must be worn.

Fifth, high temperature

In a fire room, the fire smoke has a high temperature, sometimes as high as several hundred degrees. In an underground building, the temperature of the fire smoke may even be as high as 1000 °C or more. Such a high temperature is for people, objects, or the environment. , will have serious adverse effects.

The effects of high-temperature smoke on people can be divided into direct-contact effects and thermal radiation effects.

The human body's tolerance to high-temperature smoke is limited. At 65 °C , it can endure for a short time. At 120 °C , it will produce irreparable damage within 15 minutes . At 140 °C , the time for damage to the human body is about 5 minutes. At 170 °C , it can only tolerate about 1min ; and in a few hundred degrees of high-temperature smoke, the human body can't stand for one minute.

Although the air permeability and thermal insulation of clothing can limit the temperature effect, most people cannot breathe in air at temperatures higher than 65 °C .

When the human body inhales high-temperature toxic fumes, it will severely burn the respiratory tract, and “heavily” the respiratory system. The light stimulates the respiratory mucosa and causes chronic bronchitis. Even if the person is rescued from the fire, it is difficult to escape from the danger of life.

If the smoke layer is above the person's head height, the person is mainly exposed to the thermal radiation effects of high-temperature smoke. At this time, the harm caused by high-temperature flue gas is lower than the harm caused by direct human exposure to high-temperature flue gas.

The effect of thermal radiation intensity is attenuated with increasing distance. It is generally believed that in ordinary buildings with a floor height of no more than 5m , the smoke layer will pose a threat to people when the temperature of the smoke layer reaches 180 °C or more.

In addition to the threat to the human body, the high temperature of the flue gas can seriously affect the properties of the material. For example, the mechanical properties of reinforced concrete materials will be severely reduced with increasing temperature. For buildings using reinforced concrete materials, attention should be paid to high temperature smoke. The impact of gas, and take appropriate protective measures.

Large-span steel roofs are often used in large-space buildings, and the mechanical properties of steels are also significantly reduced with increasing temperatures. Collapses beyond a certain limit have occurred repeatedly in building fires. .

Therefore, minimizing the effect of high-temperature smoke in fires is an important aspect of reducing fire losses.

In addition, in the event of a fire, especially when a flashover occurs, flames and smoke rush out of the windows and doors, and the heavy smoke will also cause serious terror, often causing chaos to the evacuation process. Some people even Lost reason and panic.

Therefore, the harm caused by fire smoke is very large.

This article is reproduced from WeChat public number: Orange Rescue.