Auto Ignition Temperature (AIT) is the lowest temperature at which a substance (solid, liquid, or gas) spontaneously ignites in air without an external ignition source, such as a spark or flame. 🔥
Explanation
Think of it as a substance reaching a temperature so hot that it simply catches fire on its own. This is different from the flash point, which is the temperature at which a substance gives off enough vapor to be ignited by an external source (like a match).
The auto-ignition temperature is a critical safety parameter because it represents a hazard that doesn’t require a visible spark or flame. A hot surface, such as an overheated motor, a steam pipe, or even a hot plate, can be enough to ignite flammable materials if the surface temperature reaches the material’s auto-ignition point.
- Every combustible material has a specific AIT.
- Autoignition (also called spontaneous combustion) often occurs due to heat buildup within a material. This heat is usually generated by the oxidation of organic substances.
- At or above this temperature, the heat energy is sufficient to start the oxidation (burning) process without needing an external flame.
- AIT is critical in fire safety, chemical handling, and process safety because it helps determine safe storage, transportation, and operating conditions.
- In some cases, bacteria and decomposition accelerate the process, especially in organic and agricultural products.
- Materials with high vegetable oil content are particularly vulnerable. The slow oxidation of oils generates heat, and when ventilation is poor (e.g., rags in a pile), the heat cannot dissipate, leading to autoignition.
- Factors affecting AIT include:
- Chemical composition (e.g., hydrocarbons vs. metals)
- Pressure (AIT decreases with higher pressure)
- Concentration of oxygen
- Presence of impurities or catalysts
- Surface area and vessel design
- Exposure duration (longer exposure to heat increases ignition risk)
In Summary, auto-ignition temperature is the self-ignition point of a substance. Understanding it helps prevent uncontrolled fires in occupational settings by ensuring materials are stored and handled below their ignition thresholds.
Example
A common example is paper. The auto-ignition temperature of paper is famously cited as 451°F (233°C). If paper is left under a mirror or glass surface where sunlight is concentrated, the constant buildup of heat can eventually raise the temperature of the paper to its ignition point. At that stage, the paper may catch fire on its own without any spark or open flame present.
Another Example
- Gasoline (Petrol): Auto Ignition Temperature ≈ 280–470 °C
- This means if gasoline vapors reach around 280 °C (without any flame or spark present), they can ignite spontaneously.
Another Example
Rags soaked with linseed oil, vegetable oil, or varnishes may ignite at room temperature over time if left in open piles, due to gradual oxidation and heat buildup. Many workshop and industrial fires have started this way.
This is why industrial facilities that handle flammable solvents or combustible dusts have strict controls on potential heat sources to ensure they never approach the auto-ignition temperature of the materials being handled.
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