Conditions of Combustion
The system of combustion conditions is widely known as the so-called „combustion triangle”. The term includes that three factors must be available simultaneously in space and time to occurring combustion (Figure 1).
Combustible material can be a gas (e.g. methane), vapour (e.g. ethanol), or powder (e.g. flour). The oxidizing medium is most often provided by the oxygen content of the air, although other oxidizing media are also effective in aiding inflammation (e.g., nitric oxide or chlorine).
Among all of the ignition sources, the Fire Protection Technical Directive summarized by the National Directorate General for Disaster Management of the Ministry of the Interior defines a total of 13 types, from which the most common are: hot surfaces (e.g furnace surfaces, friction of bearings, falling screws), open flame or spark (e.g. smoking), electric sparks (e.g. electronic devices), electrostatic charge (e.g. synthetic fibre clothing), chemical reactions and self-ignition (e.g. presence of solvents or chemically unstable substances), etc. To summarize, almost anything that can provide enough energy to cause inflammation can function as a source of ignition. The amount of ignition energy required for ignition varies from powder to powder.
Figure 1.: Conditions of combustion
In case of fire, flammable material and the oxidant must be at the appropriate concentration (i.e. within ignition limits) and the ignition source must have sufficient energy to ignite the fuel-oxidant mixture. In open-air, the duration of fire is usually determined by the required time to consume fuel (i.e. to fully oxidize). In the combustion of solids, such as coal or wood, the rate of combustion is determined by diffusion processes that limit the rate of access of the oxidizing medium to the remaining unburned fuel. Depending on the size of the fuel particle, the combustion process can take hours.
Properties of static dust layer have a special role in the planning of combustion safety measures.
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