Burning is an oxidation reaction that proceeds at a high speed, which is accompanied by the release of heat in large quantities and, as a rule, by a bright glow, which we call a flame. The process of combustion is studied by physical chemistry, in which it is customary to classify all exothermic processes that have a self-accelerating reaction as combustion. Such self-acceleration can occur due to an increase in temperature (i.e., to have a thermal mechanism) or the accumulation of active particles (to have a diffusion nature).
The combustion reaction has a clear feature - the presence of a high-temperature region (flame), spatially limited, where most of the conversion of the starting substances (fuel) to combustion products occurs . This process is accompanied by the release of a large amount of thermal energy. To start the reaction (the appearance of a flame), it is required to spend a certain amount of energy on ignition, then the process goes on spontaneously. Its speed 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 are 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 state of aggregation of the oxidizing agent and fuel, the combustion process can be attributed to one of three types. The combustion reaction may be:
- homogeneous if the fuel and oxidizing agent (pre-mixed) are in a gaseous state,
- heterogeneous, in which solid or liquid fuel interacts with a gaseous oxidizing agent,
- the combustion reaction of gunpowder and explosives.
Homogeneous combustion is the simplest, has a constant speed, 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. In liquid fuels, the rate of evaporation has a great influence on the rate of combustion, and in solid fuels, the rate of gasification. For example, when coal is burned, the process forms two stages. On the first of them (in the case of relatively slow heating), volatile components of the substance (coal) are released, on the second, the coke residue burns out.
Combustion of gases (for example, ethane combustion) has its own characteristics. In a gaseous medium, flames can spread 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 a finely divided mixture of liquid and solid combustible particles mixed with an oxidizing agent. To ensure sustainable combustion in such cases, a special design of the furnace device is required.
The consequences of the combustion reaction in a gaseous medium are of two types. The first is the turbulization of the gas stream, leading to a sharp increase in the speed of the process. The acoustic disturbances of the flow that arise in this case 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 size of the system and the propagation parameters.
Combustion is used in engineering and industry. The main task in this case is to achieve the maximum completeness of combustion (i.e., optimization of heat release) for a given period. Combustion is used, for example, in mining - methods for developing various minerals are based on the use of a fuel process. But under certain natural and geological conditions, the phenomenon of combustion can become a factor that poses a serious danger. The real danger, for example, is the process of spontaneous combustion of peat, leading to the occurrence of endogenous fires.