There exists a minimum temperature for fire to survive and ignite, that is if the heat drops below this amount the flames will cease to exist. Without reaching a certain degree of warmth no sparks or embers can be ignited.
Nevertheless, the temperature is not set in stone and differs depending on the fuel or design. As temperatures rise, chemical reactions speed up, meaning that to keep a fire going, the heat must be powerful enough for rapidly occurring chemical processes which will then create even more warmth.
Red flames may appear weaker, but the heat they generate can still range from 525°C up to 1000°C (1832°F). The paler the color of a flame, the lower its temperature; however, when it’s closer to orange in hue, then you know that your fire is burning at near 1,000°C.
If you observe an orange flame, then its temperature may vary from 1100°C to 1200°C. White flames have a higher range of 1300°C to 1500°C (2732°F), with the brighter the white representing a hotter temperature.
If you observe a blue flame, or flames with a blue base, the temperature is likely to climb to an astonishing 2500°C – 3000°C (5432°F)!
Cool Flame Fires
Cool flames can burn at temperatures of 400°C (752°F) or lower. The chemical reactions behind these remarkable phenomena differ from traditional fires and they rarely make an appearance in the real-world since they have a tendency to either quench out or heat up until it eventually converts into regular fire. These fires would be hard to notice with the naked eye as they as practically invisible in sunlight.
In the 1810s, Sir Humphry Davy inadvertently uncovered cool flames by inserting a hot platinum wire into an amalgamation of air and diethyl ether vapor.
When the experiment on slow combustion of ether is done without light, a phosphorescent light appears above the wire that becomes particularly visible once it ceases to be ignited.
Upon realizing that these peculiar flames would not burn his skin or light matches, he did discovered they could transform into ordinary ones.
While traditional flames tend to break down molecules into small pieces that combine with oxygen, creating carbon dioxide (i.e., burning), a cool flame’s particles remain relatively large and easily recombine amongst themselves.
Consequently, less heat and energy is emitted; a self-sustaining oscillatory combustion process that can persist for prolonged periods of time results. While igniting a cool flame will generally cause the temperature to rise by several tens of degrees Celsius, the temperatures associated with regular flames are typically around 1100° Celsius (2012° Fahrenheit).
By burning hydrocarbons and alcohols, you can achieve this unique “Cool Flame” effect. The intensity of the cool flames will differ depending on the oxygen content in your environment. Chances are that most people have encountered this phenomenon while experiencing engine knocking due to incomplete combustion of gas engines.
Under typical conditions, a flame front advances through the combustion chamber from the spark plug in an organized fashion and compresses the fuel/air mixture before it. Nevertheless, due to the pressure and temperature increase that goes hand in hand with this process, a cool flame may form within the last unburned fuel-air mixture (known as end gasses) which can lead to autoignition of these same gases.
The coolest or lowest temperature that a flame is capable of emitting has been measured between 200 and 300°C, with n-butyl acetate as the benchmark at a “cold” 225°C (437°F).
It’s important to consider that cool flames can be difficult to detect at lower temperatures due to their lack of heat and light, which are both generated from the combustion process.
NASA’s exploration into the realm of low temperature combustion involving nano droplets of heptane has been a fascinating scientific journey, and further investigation continues to be done in this area.