(1) Necessary conditions for combustion
The occurrence and development of material combustion process must meet the following three necessary conditions: combustible, oxidant and temperature (ignition source). Only when these three conditions are met at the same time can combustion occur, and no matter which condition is lacking, combustion cannot occur. However, the above three conditions do not necessarily exist at the same time, and the combustion phenomenon will certainly occur. These three factors also need to interact.
1. Combustible: Any substance that can react with oxygen or other oxidants in the air is called combustible. Combustible substances can be divided into gas combustible substances, liquid combustible substances and solid combustible substances according to their physical states. Combustible substances are mostly compounds containing carbon and hydrogen. Some metals such as magnesium, aluminum and calcium can also burn under certain conditions, and many substances such as hydrazine and ozone can release light and heat through their own explanation at high temperature.
2. Oxidant: A substance that helps and supports the combustion of combustible substances, that is, a substance that can undergo an oxidation reaction with combustible substances is called an oxidant. The oxidant in the combustion process is mainly free oxygen in the air, and other oxidants such as fluorine and chlorine can also be used as oxidants in the combustion reaction.
3. Temperature (ignition source): refers to the energy source of combustion reaction between combustible substances and oxygen or combustion improver. Common is heat energy, as well as other heat energy converted from chemical energy, electric energy and mechanical energy.
4. Chain reaction: There is a chain reaction in flame combustion. When the fuel is heated, it will not only vaporize, but also the molecules of the fuel will decompose to produce free radicals. Free radical is a very active chemical form, which can react with other free radicals and molecules to keep the combustion going. This is a chain reaction of burning.
(2) Sufficient conditions for combustion: (1) a certain combustible concentration; (2) A certain oxygen content; (3) a certain ignition energy; (4) Unrestricted chain reaction. The minimum ignition energy of gasoline is 0.2mJ, ether is 0. 19mJ, and methanol is 0.2 15mJ. For flameless combustion, the first three conditions exist and interact at the same time, and combustion will occur. For flame combustion, in addition to the above three conditions, there are unrestrained free radicals (free radicals) in the combustion process, forming a chain reaction, which is also one of the sufficient conditions for combustion.
Third, the definition and classification of fire
Fire is defined as a disaster caused by uncontrolled burning in time and space.
Fires are divided into four categories: A, B, C and D.
Class A fire refers to solid material fire. Such as wood, cotton, wool, hemp and paper;
Class B fire refers to liquid fire and meltable solid fire. Such as gasoline, kerosene, crude oil, methanol, ethanol, asphalt, paraffin wax on fire;
Class C fire refers to gas fire. Such as fires caused by gas, natural gas, methane, ethane, propane and hydrogen;
Class D fire refers to metal fire such as potassium, sodium, magnesium, titanium, zirconium, lithium, aluminum-magnesium alloy fire, etc.
Four, several commonly used concepts in combustion
1. flashover: liquid (solid) can produce enough flammable vapor, and the combustion phenomenon that can extinguish the flame in case of fire is called flashover.
2. smoldering: The slow burning phenomenon without flame is called smoldering.
3. Deflagration: An explosion propagating at subsonic speed is called deflagration.
4. Spontaneous combustion: Combustible substances spontaneously combust due to heating or self-heating and heat storage without external open flames and other fire sources, which is called spontaneous combustion. That is to say, in the absence of an external ignition source, a substance generates heat due to its internal biological, physical and chemical processes, which raises its temperature and eventually burns itself.
5. Flash point: The lowest temperature at which a liquid (solid) surface can flash under specified test conditions is called flash point. The flash point of isomers in homologues is lower than that of normal isomers; The flash point of homologues increases with the increase of molecular weight and boiling point. The flash point of gasoline, kerosene and other mixed liquids increases with the increase of boiling range; The flash point of mixed liquid formed by low flash point liquid and high flash point liquid is lower than the average value of these two liquids. The flash point of wood is about 260 degrees Celsius.
Significance of flash point: (1) flash point is an important basis for fire hazard classification of production equipment; (2) Flash point is the basis of fire hazard classification of storage warehouses; (3) Flash point is the basis for the classification of Class A, B and C dangerous liquids; (4) According to the classification of Class A, B and C liquids, fire resistance grade, number of layers, floor space, safe evacuation, fire prevention spacing and explosion-proof settings, etc. Detailed description of workshop and warehouse; (5) According to the classification of Class A, Class B and Class C liquids, the layout of liquid storage tanks and storage yards, the fire prevention spacing, the fire prevention spacing of combustible and combustion-supporting gas storage tanks, the fire prevention spacing of liquefied petroleum gas storage tanks, etc.
6. Ignition point: refers to the lowest temperature at which liquid or solid can continuously burn under specified test conditions, which is called ignition point. The ignition point of all liquids is higher than the flash point.
7. Self-ignition point: refers to the lowest temperature at which combustible substances spontaneously ignite, which is the self-ignition point of the substance under specified conditions.
The main ways of combustible spontaneous combustion are: (1) oxidation heating; (2) exothermic decomposition; (3) exothermic polymerization; (4) adsorption and heat release; (5) exothermic fermentation; (6) the active substance meets water; (7) Mixing of combustible and strong oxidant.