How does the combustion reaction proceed?

The combustion is called the oxidation reaction, which proceeds at a high rate, accompanied by the release of heat in large quantities and, as a rule, a bright glow, which we call a flame. The combustion process is studied by physical chemistry, in which all exothermic processes that have a self-accelerating reaction are considered to be combustible. Such self-acceleration can occur due to an increase in temperature (i.e., to have a thermal mechanism) or accumulation of active particles (to have a diffusion nature).

The combustion reaction has a graphic feature - the presence of a high-temperature region (flame), limited spatially, where most of the transformation of the initial substances (fuel) occurs into combustion products. This process is accompanied by the release of a large amount of thermal energy. To start the reaction (the appearance of a flame), it takes some energy to ignite, then the process goes on spontaneously. Its rate depends on the chemical properties of the substances involved in the reaction, as well as on gas-dynamic processes during combustion. The combustion reaction has certain characteristics, the most important of which is the calorific value of the mixture and that temperature (called adiabatic), which theoretically could be achieved with complete combustion without taking into account heat loss.

According to the aggregate state of the oxidant and fuel, the combustion process can be classified into one of three types. The combustion reaction can be:

- homogeneous, if the fuel and oxidizer (premixed) are in a gaseous state,

- heterogeneous, in which a solid or liquid fuel interacts with a gaseous oxidant,

- reaction of burning of gunpowders and explosives.

Homogeneous combustion is the simplest, has a constant rate, depending on the composition and molecular thermal conductivity of the mixture, temperature and pressure.

Heterogeneous combustion is most common both in nature and in artificial conditions. Its speed depends on the specific conditions of the combustion process and on the physical characteristics of the ingredients. At liquid combustibles, the rate of evaporation has a large influence on the combustion rate, while for solid fuels the rate of gasification is significant. For example, when coal burns, the process forms two stages. In the first of these (in the case of comparatively slow heating), the volatile components of the substance (coal) are released, and the coke residue remains burned to the second.

Burning of gases (for example, burning ethane) has its own characteristics. In a gaseous medium, the flame can propagate over a wide distance. It can move through the gas at a subsonic speed, and this property is inherent not only in the gaseous medium, but also in the finely dispersed mixture of liquid and solid combustible particles mixed with the oxidant. To ensure sustainable combustion in such cases, a special design of the furnace device is required.

The consequences of the combustion reaction in the gas environment are of two kinds. The first is the turbulence of the gas stream, which leads to a sharp increase in the speed of the process. The resulting acoustic disturbances of the flow can lead to the next stage - the generation of a shock wave leading to detonation of the mixture. The transition of combustion to the detonation stage depends not only on the intrinsic properties of the gas, but also on the dimensions of the system and the propagation parameters.

Combustion of fuel is used in engineering and industry. The main task at the same time is to achieve the maximum completeness of combustion (i.e., optimization of heat release) for a given interval. Burning is used, for example, in mining - the methods of developing various minerals are based on the use of a fuel process. But in certain natural and geological conditions, the phenomenon of combustion can become a factor that carries a serious danger. A real danger, for example, is the process of spontaneous combustion of peat, leading to the occurrence of endogenous fires.