Auto-ignition Temperature

Detailed Description

The auto-ignition temperature (AIT), also known as the autoignition temperature, self-ignition temperature, or spontaneous ignition temperature, is the lowest temperature at which a substance will spontaneously ignite in normal air without an external source of ignition, such as a flame or spark. At this temperature, the rate of heat generated by the exothermic oxidation reaction exceeds the rate of heat lost to the surroundings, leading to a rapid temperature increase and ignition.

Auto-ignition is distinct from other ignition processes:

Flash point: The lowest temperature at which vapors above a liquid will ignite when exposed to an ignition source.
Fire point: The temperature at which vapors will continue to burn after ignition by an external source.
Auto-ignition: Spontaneous ignition without any external ignition source.

Key concepts related to auto-ignition temperature include:

Thermal runaway: The self-accelerating process where heat generation exceeds heat loss, leading to rapid temperature increase and ignition.
Activation energy: The minimum energy required to initiate the oxidation reaction.
Induction period: The time delay between reaching the auto-ignition temperature and the actual ignition event.
Surface effects: The influence of container materials and surface area on auto-ignition temperature.
Pressure dependence: Auto-ignition temperature generally decreases with increasing pressure.

Importance in Safety Data Sheets

Auto-ignition temperature information in an SDS is important for several reasons:

Process safety: Critical for designing safe processes, especially those involving heating of materials.
Storage requirements: Informs appropriate storage temperature conditions and thermal protection needs.
Hazard assessment: Helps evaluate the risk of spontaneous ignition in various scenarios.
Hot work permits: Essential information for determining safe conditions for hot work near flammable materials.
Equipment design: Guides selection of appropriate equipment and materials for handling and processing.
Emergency response: Provides critical information for firefighting and emergency planning.
Transportation: Informs appropriate transportation conditions and hazard classifications.
Regulatory compliance: Required for various hazard communication and chemical safety regulations.

Measurement Methods

Several standardized methods are used to determine auto-ignition temperature:

Method	Description	Applicable Materials
ASTM E659	Uses a heated flask to determine the lowest temperature at which a substance will spontaneously ignite	Liquids and solids with relatively high vapor pressure
EN 14522	European standard method using a heated vessel	Gases and vapors
IEC 60079-20-1	International standard for determining auto-ignition temperature of gases and vapors	Gases and vapors
DIN 51794	German standard method using a heated vessel	Liquids and gases
ASTM D2155	Uses a heated metal surface to determine ignition temperature	Petroleum products
Godbert-Greenwald Furnace	Uses a heated furnace to determine dust cloud auto-ignition temperature	Combustible dusts
BAM Oven	Used for determining self-ignition temperature of solids in bulk	Solids in bulk form
UN Test N.4	Test for self-heating substances	Solids and pastes

Auto-ignition Temperature Ranges

Auto-ignition temperatures vary widely depending on the chemical structure and physical properties of substances:

Material Category	Typical AIT Range (°C)	Risk Level	Examples
Simple Hydrocarbons (C1-C4)	450-600	Moderate	Methane (537°C), Propane (450°C)
Higher Hydrocarbons (C5+)	200-450	High	n-Hexane (225°C), Gasoline (280-456°C)
Alcohols	300-500	Moderate	Ethanol (363°C), Methanol (464°C)
Ethers	160-280	High	Diethyl ether (160°C), THF (224°C)
Ketones	450-550	Moderate	Acetone (465°C), MEK (516°C)
Vegetable Oils	300-450	Moderate	Linseed oil (343°C), Corn oil (393°C)
Combustible Dusts	200-600	High	Coal dust (500°C), Flour (380°C)
Self-Reactive Substances	<200	High	Some organic peroxides, nitro compounds
Non-Flammable Substances	N/A	Very Low	Water, nitrogen, carbon dioxide

Auto-ignition Temperatures of Common Substances

Substance	Auto-ignition Temperature (°C)	Physical State	Notes
Hydrogen	500	Gas	Wide flammable range (4-75% in air)
Methane	537	Gas	Main component of natural gas
Propane	450	Gas/Liquid	Common LPG component
Butane	405	Gas/Liquid	Common LPG component
Acetylene	305	Gas	Used in welding
Gasoline	280-456	Liquid	Varies by composition
Diesel Fuel	210-315	Liquid	Varies by composition
Kerosene	220	Liquid	Jet fuel, heating oil
Ethanol	363	Liquid	Common alcohol
Methanol	464	Liquid	Industrial alcohol
Acetone	465	Liquid	Common solvent
Diethyl Ether	160	Liquid	Very low auto-ignition temperature
Benzene	498	Liquid	Aromatic hydrocarbon
Toluene	480	Liquid	Common solvent
Paper	218-246	Solid	Varies by type
Wood (pine)	220-260	Solid	Varies by species and moisture content
Coal	400-600	Solid	Varies by type
Carbon Disulfide	90	Liquid	Extremely low auto-ignition temperature

Factors Affecting Auto-ignition Temperature

Chemical Structure

The molecular structure significantly affects auto-ignition temperature:

Chain length: For hydrocarbons, AIT generally decreases with increasing chain length.
Branching: Branched hydrocarbons typically have higher AITs than their straight-chain isomers.
Unsaturation: Unsaturated compounds (alkenes, alkynes) often have lower AITs than corresponding saturated compounds.
Functional groups: The presence of certain functional groups (e.g., -OH, -CHO, -COOH) can significantly affect AIT.
Aromatic rings: Aromatic compounds generally have higher AITs than aliphatic compounds of similar carbon number.

Physical Factors

Several physical factors can influence the measured auto-ignition temperature:

Pressure: AIT generally decreases with increasing pressure.
Oxygen concentration: Higher oxygen concentrations typically lower the AIT.
Container material: Catalytic effects of container materials can lower the measured AIT.
Container size: Larger containers often result in lower measured AITs due to reduced heat loss.
Surface-to-volume ratio: Higher ratios generally result in higher measured AITs due to increased heat loss.
Residence time: Longer residence times in the heated zone can lower the apparent AIT.
Impurities: Certain impurities can catalyze oxidation reactions, lowering the AIT.

Safety Considerations

Understanding auto-ignition temperature is critical for safety for several reasons:

Process temperature limits: Process temperatures should be maintained well below the AIT of any flammable materials present.
Hot surface hazards: Equipment surfaces, pipes, heating elements, and other hot surfaces should be kept below the AIT of any flammable materials in the vicinity.
Electrical equipment: Maximum surface temperature ratings for electrical equipment in hazardous areas are based on the AITs of potentially present flammable substances.
Self-heating materials: Some materials can undergo self-heating at temperatures well below their AIT, eventually reaching the AIT and igniting.
Dust layers: Accumulated dust on hot surfaces can ignite at temperatures lower than the dust cloud AIT.
Confined spaces: Heat accumulation in confined spaces can lead to conditions approaching the AIT.
Safety margins: Safety systems should incorporate appropriate margins below the AIT to account for measurement uncertainties and variations in conditions.

Examples of Auto-ignition Temperature Descriptions in SDSs

"Auto-ignition temperature: 363°C"
"Auto-ignition temperature: 450°C (ASTM E659)"
"Self-ignition temperature: >400°C"
"Auto-ignition temperature: Not applicable for non-flammable substances"
"Auto-ignition temperature: 220-260°C (varies with composition)"
"Auto-ignition temperature: Not determined"
"Auto-ignition temperature: 480°C (gas); 210-215°C (dust layer)"
"Auto-ignition temperature: Decomposes before auto-ignition"

Regulatory Requirements

According to GHS and various regional regulations (EU CLP, US OSHA HazCom, etc.), the auto-ignition temperature should be indicated in Section 9 of the Safety Data Sheet as part of the description of basic physical and chemical properties. This information is considered mandatory for flammable substances, though it may be reported as "not applicable" for non-flammable materials.

For electrical equipment in hazardous areas, temperature classification (T-class) is based on auto-ignition temperatures:

T1: AIT > 450°C
T2: 300°C < AIT ≤ 450°C
T3: 200°C < AIT ≤ 300°C
T4: 135°C < AIT ≤ 200°C
T5: 100°C < AIT ≤ 135°C
T6: 85°C < AIT ≤ 100°C

Best Practices

When reporting auto-ignition temperature in an SDS:

Specify the value in degrees Celsius (°C)
Indicate the test method used (e.g., ASTM E659, EN 14522)
For substances with variable composition, provide an appropriate range
For substances that decompose before auto-ignition, clearly state this behavior
For non-flammable substances, indicate "not applicable" rather than leaving the field blank
For dusts, provide both cloud and layer auto-ignition temperatures when available
For substances where auto-ignition temperature is highly pressure-dependent, consider providing values at different pressures
Ensure consistency between auto-ignition temperature information and flammability classification in the SDS
When the auto-ignition temperature is particularly low (e.g., <200°C), consider highlighting this as a special hazard in Section 2 of the SDS
For substances that can undergo self-heating, provide information on self-heating properties in addition to the auto-ignition temperature