To date, humanity uses a wide variety of combustible substances. There are already quite a lot of species and they all have some of their own, unique characteristics. What are these substances? This is the raw material that can continue to burn after the source of ignition has been removed.
Gases and liquids
To date, there are several groups of combustible substances.
You can start the review with gases - the GG group. To this category belong those substances that can mix with air, thus forming an explosive or flammable medium, at a temperature not exceeding 50 ° C. In this group of gases, certain individual volatile compounds can be attributed. It can be ammonia, acetylene, butadiene, hydrogen, isobutane and some others. Separately, it is worth saying that this also includes vapors that are released during the evaporation of flammable liquids (LVH), which represent the following category.
The group of flammable liquids includes those liquid combustible substances that will continue to burn after removal of the ignition source, and their flash point does not exceed the threshold of 61 degrees Celsius for a closed crucible. If this vessel is open, the threshold will increase to 66 degrees. Such liquid substances include acetone, benzene, hexane, heptane, isopentane, styrene, acetic acid and many others.
Flammable liquids and dust
It would seem that a flammable liquid and a combustible are one and the same thing, but in practice this turned out to be different. They are divided into two different categories. Even though the parameters of their ignition coincide and some liquids belong to both this and this group, there is a main difference. Oil-based substances are also referred to as GH. This, for example, can be castor or transformer.
Further, it is worth mentioning such a combustible substance as dust. GP is a solid substance that is currently in a finely divided state. Once in the air, such dust can form an explosive structure with it. If such particles settle on walls, ceilings and other surfaces, they can cause a fire.
GP classes
Separately, it is worth noting that there are classes of combustible substances and materials. For example, dust is divided into three categories depending on the degree of fire hazard and explosion hazard.
- The first class is the most dangerous aerosols, in which the lower concentration limit of explosiveness (ignition) (LEL) up to 15 g / m 3 . These include sulfur, mill, hard rubber or peat dust.
- The second class includes those particles for which the LEL limit is in the range from 15 to 65 g / m 3 . They are considered more explosive.
- The third category is the most fire hazardous. This is a group of liquid aerogels in which the LEL is more than 65 g / m 3 and the autoignition temperature is up to 250 degrees Celsius. Tobacco or elevator dust, for example, has such properties.
General characteristics
What combustibles are they and why? There are several specific characteristics, possessing which, liquid, dust, gases and other substances can be classified as combustible.
For example, a flash point is a value that characterizes the lower temperature limit, upon reaching which the liquid will form flammable vapors. However, it should be noted here that the presence of a fire source near such a vapor-air mixture will cause only its combustion, without a stable effect of burning the liquid itself.
If we previously spoke about the lower concentration limit, then there is also the upper one. NKPV or VKPV - these, respectively, are the values, upon reaching which, ignition or explosion of liquid, dust, gases, etc. can occur. All types of combustible substances have these limits. However, it is important to note here that if the concentration is lower or, conversely, above the specified limits, then nothing will happen even if there is an open flame source in the immediate vicinity of the substance.
Solid raw materials
It is worth mentioning that solid combustible substances behave somewhat differently than dust, liquid or gas. When heated to a certain temperature, this group of raw materials behaves individually, and this depends on its characteristics and structure. For example, if you take sulfur or rubber, then when they are heated, they first melt, and then evaporate.
If you take, for example, wood, coal or paper and some other substances, then they begin to decompose upon heating, leaving behind gaseous and solid residues.
Another very important point: the composition of flammable substances and their chemical formula strongly affects the direct combustion process itself. There are several stages into which this phenomenon is divided. Simple substances, such as anthracite, coke or soot, for example, heat up and smolder without any sparks, since their chemical composition is pure carbon.
Complex combustion products include, for example, wood, rubber or plastic. This is due to the fact that their chemical composition is quite complex, and therefore there are two stages of their combustion. The first stage is the decomposition process, which is not accompanied by the usual release of light and heat, but the second stage is already considered to be burning, and at this time heat and light begin to be released.
Other substances and characteristics
Naturally, solids also have a flash point, but for obvious reasons, it is much higher than that of liquid or gaseous substances. Flash point ranges from 50 to 580 degrees Celsius. It is worth mentioning separately that such a common combustible material as wood has a threshold from 270 to 300 ° C, depending on the species of the tree itself.
The highest rate of combustion among solids has gunpowder and explosives. This is due to the fact that both of these substances have a sufficiently large amount of oxygen, which is enough for their complete combustion. In addition, they may well burn under water, underground, as well as in a completely sealed environment.
Wood
It is worth a little more to say about this combustible solid material, since today it is one of the most common. The reason for this is that it is one of the most affordable. It is worth mentioning that in fact wood is a substance with a cellular structure. All cells are filled with air. The degree of porosity of any rock exceeds 50% and increases, which indicates a not too high concentration of solid in relation to air. It is because of this that it lends itself well enough.
If we conclude, then we can say that there are a large number of various combustible substances in the world that cannot be dispensed with in everyday life, but at the same time it is necessary to be extremely careful in their operation, using only for their intended purpose.