Fire

What is a fire

The visible external side effect of a combustion is called fire. Depending on the aggregate state of the fire material, flames and/or embers can form and heat and light are released. Substances such as gases and substances that become liquid (e.g. wax or fat) only produce flames, while other substances (e.g. metals) only produce embers. Solid materials (such as wood and paper) burn with both flames and embers. The resulting combustion gases (the smoke) are hot and therefore lighter than the surrounding air. As a result, they rise, which leads to a negative pressure in the flame. To compensate for the negative pressure, the fresh air required for combustion is sucked in (chimney effect). In extreme cases, the resulting draft can reach hurricane force and a so-called firestorm develops. In contrast to a fire, a fire is a harmful fire that spreads uncontrollably.

How is fire created?

For a fire to occur, combustible material (fire load) and an oxidizing agent (usually oxygen) must be present. A fire only starts when the ignition temperature of the respective flammable material is reached. The so-called combustion triangle (see figure) summarizes these prerequisites for the development of a fire. The ignition temperatures of some fire loads are summarized below. The exemplary list illustrates well that the ignition temperatures vary greatly. The ignition temperature can be reached, for example, as a result of a lightning strike, a spark or electricity.

• Wood: 280°C to 340°C
• Straw: 250°C to 300°C
• Newsprint: 175°C
• Charcoal: 300°C
• Phosphorus: 34°C

When does a fire go out?

The combustion triangle is also instructive when it comes to determining when a fire will go out. In addition to being extinguished by human hands, a fire goes out as soon as there is a lack of fuel, heat or oxygen. This can be used to fight fires.

What are the chemical and physical mechanisms involved in a fire?

Combustion theory

Combustion theory deals with the chemical and physical processes that take place in a fire. This scientific subject is important, among other things, for the optimal design of combustion engines, jet engines and gas and coal-fired power plants. Because a very complex interplay of chemistry, fluid mechanics and thermodynamics takes place in a fire, there is still a need for research here, even 1.5 million years after fire was tamed by humans. This is also due to the fact that e.g. fuels such as petrol and diesel consist of many chemicals that react differently when burned.

Chemical mechanisms

From a chemical point of view, fire is an oxidation reaction with the appearance of a flame. The reaction of two substances with each other releases energy that can be perceived as light and heat. Fire is an exothermic reaction that releases more energy than is needed to ignite (and thus start the chemical chain reaction).

The most common oxidizing agent is oxygen from the air. However, a fire can also occur when two solid substances react with each other. The best-known example is certainly lighting a match, where a little phosphorus reacts with a salt on the match head. The resulting combustion releases energy that ignites a sulphurous substance on the match head.s

When organic materials, i.e. chemical compounds based on carbon, are complete combusted, they are converted with the oxidizing agent oxygen to form carbon dioxide and water. If the combustion is incomplete, other, only partially oxidized substances and non-oxidized substances (e.g. soot) are produced in addition to carbon dioxide.

Combustion can only be complete if there is a certain minimum quantity ratio of oxygen and fuel (the so-called stoichiometric ratio). This means that all oxidizable components of the fuel have bound the maximum amount of oxygen (or another oxidizing agent). In addition to the correct proportions, fuel and oxidant must also be completely mixed so that each reaction pair can be found. For this, solid fuels would have to be as comminuted as possible and liquid fuels would have to be as atomized as possible. However, this state is not achieved in practice.

Physical mechanisms

In addition to the chemical mechanisms, many physical mechanisms are at work in a fire. These include the mass transfer in the flame (e.g. the above-mentioned mixing of fuel and oxidant), the heat transfer within the fire and to the environment as well as flow-mechanical and dynamic effects, such as the turbulence that occurs as a result of a fire.

How hot is a fire?

Within the flame of a fire there are different hot temperature areas. This can best be studied using the example of a candle flame. Blue and dark areas can be seen near the wick, showing temperatures of around 600°C. The yellow and orange glowing regions in the outer area of the flame, on the other hand, have very high temperatures of up to 1500°C. The inner areas are cooler than the outer areas due to a lack of oxygen (see combustion triangle).

Man and the fire

It is unclear when humans started using fire. Some researchers suspect that humans have already used wild fire (e.g. from lightning) more than two million years ago. At that time, fire could be used sensibly in many ways: Meat could be preserved longer and made more edible and provided more energy than plant food. In addition, the fire warmed and dangerous wild animals could be kept at a distance.

It is also unclear when fire could be used as planned. The oldest lighter found so far is around 790,000 years old. In any case, the use of fire increased as people became more sedentary. Here, for example, it was used in agriculture for slash and burn and new professions emerged that used fire, such as bakers, cooks, blacksmiths and also potters.

With the increasingly dense settlement, people also recognized the ambivalence of fire. In addition to its useful side, it also harbors the danger of destruction, which manifested itself, for example, in the burning down of entire settlements. In this respect, it was obvious to control the fire by using stoves. At the same time, fire protection regulations were issued in Europe from the 15th century and fire brigades were established.