Why are Methanol Flames Less Visible than other Flames?

Does methanol fire really exist or is it just a myth? This article explores this much-requested topic and gives our deep insights.

Introduction:

Methanol CH₃OH is a very common alcohol and is also known to be the simplest alcohol in its class of hydrocarbons. It has a methyl group CH₃ connected with a hydroxy group OH in its molecular structure. It has a strong smell and appears as a colorless liquid.

Methanol has an extremely low weight as compared to other alcohols which makes its vapors travel a considerable distance to an ignition source before flashing back. Any buildup of vapors in enclosed areas, like sewers or buildings, may result in a methanol fire if it was ignited.

This alcohol also finds many applications in industry where it is used as a solvent for paints and plastics, to produce chemicals, to extract water from gasoline for cars and airplanes, and as a component in many other industrially important items.

Methanol is also used as fuel in racing cars due to its high-octane rating and the feature to produce less pollution. It has been seen that such vehicles if involved in collision produce “invisible fires” that are hard to spot, even during the day and are also hard to extinguish by fire crew. This type of fire is known to severely burn crew members and drivers of racing cars where cars accident occurred on a racing circuit but what makes it invisible?  

How is Methanol fire Differnet?

To explain how it is different, it is important to first understand why different colors of flame occur during the burning of such chemicals. Hot soot, an aerosol made of carbon monoxide, elemental carbon, and radicals, is what gives these flames distinct red and yellow hues. Moreover, the combustion of impure hydrocarbon chemicals produces hot soot.

In essence, the elemental makeup of the substance being burned and the temperature at which it is heated directly affects the flame's color and also its visibility. This is because heat will cause the element’s electrons to get excited and upon returning to the ground state, visible light is released. The color of this light depends on the energy and number of electrons going back to the original shells.

Now coming back to Methanol, burning it produces a transparent faint blue or completely colorless flame, this flame is usually not easily visible to the human eye even in the dark. This is because of the fact that this alcohol can undergo complete combustion due to its simple structure and produces carbon dioxide (CO₂) and water (H₂O). Comparing its combustion to ethanol, it takes twice as much air to burn ethanol as compared to methanol, which means higher percentage of unburned ethanol is there in its flame, resulting in more color in its flame. Moreover, this alcohol burns at a fairly lower temperature when compared to other fuels which also results in a faint flame.

2CH₃OH + 3O₂  → 2CO₂ + 4H₂O₂

C₂H₅OH + 3O₂  → 2CO₂ + 3H₂O

Another reason of faint flames for methanol fire is that it has lower carbon atom density in its molecular structure which is also related to flame color. When methanol and ethanol's carbon contents are examined, a fair difference can be seen in their heats of combustion and carbon mass (methanol: 38% carbon mass and -726 kJ/mol, ethanol 52% carbon mass, and -1368 kJ/mol). The flame of ethanol is much more visible than of methanol flame. This is explained by the increased carbon mass in ethanol and the higher heat of combustion.

Finally, another reason why these flames are not visible is due to the scientific observation which finds that flames of other alcohols and hydrocarbons like ethanol have more carbon supply in their flame due to their molecular structure having more carbons in them. Methanol being the simplest hydrocarbons in all of them has less supply of carbon in its flame which doesn’t allow it to produce that yellow flame that other hydrocarbons are capable of producing.

Wrapping Up:

This article explored the color of methanol fire and the reason why methanol fire invisible flames exist as compared to other fuels that have significantly visible flames. The presence of less carbon density in its molecular structure results in its different flame color.