Ethanol, commonly known as ethyl alcohol, is a clear, flammable liquid that is a significant fuel source and solvent used globally. Its burning properties are important for applications ranging from portable camping stoves and laboratory spirit lamps to its use as a gasoline additive in fuel mixtures like E85. The combustion of ethanol releases a substantial amount of thermal energy, which is why it is used as a fuel, but this process also presents distinct hazards related to the flame’s visibility.
The Chemistry Behind Ethanol Combustion
The heat and light produced by an ethanol flame are the result of a rapid oxidation process, known as combustion. This chemical reaction combines ethanol fuel with oxygen from the air, releasing energy in the form of heat and light. Ethanol’s chemical formula is C2H5OH, and when it undergoes complete combustion, the reaction consumes three molecules of oxygen (O2) for every molecule of ethanol.
The balanced chemical equation for this reaction is C2H5OH + 3O2 \(\rightarrow\) 2CO2 + 3H2O. The products are carbon dioxide (CO2) and water (H2O), which are both gases at the flame’s high temperature. This process is highly exothermic, meaning it releases a large amount of energy.
The energy released is quantified by the heat of combustion, which for ethanol is approximately 1367 kilojoules per mole (kJ/mol) under standard conditions. This energy release provides the foundation for the flame’s high maximum theoretical temperature. The efficiency of this chemical mechanism, which produces minimal byproducts other than carbon dioxide and water vapor, contributes to ethanol’s reputation as a relatively clean-burning fuel.
Measured Flame Temperatures and Variables
The question of how hot ethanol burns has two different answers: the theoretical maximum and the measured working temperature. The adiabatic flame temperature represents the theoretical maximum temperature the flame could reach if combustion were perfectly complete and no heat was lost to the surroundings. For ethanol burning in air, this calculated value is approximately 2082°C (3779°F).
In real-world applications, the flame temperature is consistently lower than the theoretical maximum because of heat loss and incomplete combustion. The measured working temperature of a typical ethanol flame, such as in a spirit lamp or small burner, usually falls within the range of 1200°C to 1500°C (2200°F to 2700°F). This range is often cited for the hottest part of the flame where the reaction is most efficient.
Several factors cause this temperature variation in practice, most importantly the air-fuel mixture, also called stoichiometry. Achieving the maximum temperature requires an optimal ratio of oxygen to fuel, and any deviation, such as a fuel-rich or fuel-lean mixture, will lower the temperature. Fuel purity is another factor, as denatured ethanol contains additives that can slightly alter the combustion process and temperature. Furthermore, the ambient temperature and humidity of the surrounding air influence the flame temperature by absorbing some of the heat produced during the reaction.
Unique Physical Properties of Ethanol Flames
One of the most distinguishing and hazardous characteristics of an ethanol flame is its near-invisibility, especially in bright ambient light. Unlike flames from fuels such as wood or candle wax, which have a bright yellow or orange color, pure ethanol flames are typically a pale blue. The yellow color in most flames comes from the incandescence of hot soot particles, which are solid bits of unburned carbon.
Ethanol’s molecular structure is simpler than long-chain hydrocarbons, allowing it to combust very cleanly and produce very little soot. Because there are few glowing carbon particles, the flame lacks the bright, easily visible yellow light. The faint blue color that is present comes from the specific light emitted by excited molecules and radicals, like C2 and CH, that form temporarily during the combustion process.
This low soot production results in low emissivity, meaning the flame does not radiate heat as effectively as a sooty flame. A person can be relatively close to an ethanol fire without feeling the intense radiant heat that a yellow flame produces, making it difficult to detect its presence. This combination of low visibility and low radiant heat makes an ethanol flame a significant safety risk, as a fire can be actively burning without being immediately noticeable.
Essential Handling and Fire Safety Protocols
Given the stealth nature of the invisible flame, handling ethanol requires strict adherence to safety protocols. Proper storage is necessary, which means keeping the fuel in sealed, approved containers away from any heat source or direct sunlight. Because ethanol vapor is denser than air and can travel along the floor to an ignition source, any area where ethanol is used must have adequate ventilation to prevent vapor buildup.
If an ethanol fire occurs, it is imperative to use the correct extinguishing agent. Ethanol is miscible with water, meaning adding water can actually spread the burning liquid, making the fire larger. Extinguishing an ethanol fire should be done by smothering it, such as by using a lid or a snuffer to cut off the oxygen supply.
Extinguishing and Post-Fire Safety
For larger spills or fires, a Class B fire extinguisher, such as a CO2 or dry chemical type, is appropriate, or an alcohol-resistant foam (ARF) may be used. After extinguishing, it is important to visually confirm that the flame is completely out by carefully passing a non-flammable object, like a metal rod, over the surface to ensure no heat is present. Never attempt to refuel a burner until it has completely cooled down, as pouring fuel onto a hot surface can instantly ignite the vapors.