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How Hot is a Bunsen Burner Flame?
 

According to the North American Combustion Handbook, Vol 1, p. 12

v Adiabatic flame temperature of propane/air is 3572 °F / 1967 °C

 

 

But that doesn't seem right, perhaps the answer to this very simple question is a bit more complicated.  So let’s take a look at some general facts and variables that determine how hot a propane fueled, Bunsen burner flame is.


 

Let's begin with the combustion reaction itself.


 

C3H8 +  5O2  ⇒ 3CO2(g) + 4H2O(g)   -2020 kJ/mol


 

Theoretical combustion is the ideal combustion process during which a fuel is burned completely. To achieve the maximum heat of combustion, for every 1 mole of propane (C3H8)  that enters the reaction 5 moles of oxygen (5O2) must be available.  A complete combustion occurs when all the carbon (C) has formed (CO2) molecules and all the hydrogen (H) has formed (H2O). 


 

1) Fuel rich mixtures, excess propane or insufficient quantities of oxygen will result in a diminished temperature.  If there are unburned molecules in the exhaust gas such as C, Hand CO the combustion process is incomplete.  Un-reacted propane and a yellow flame will result.


 

2) Fuel lean mixtures, insufficient quantities of propane or excess oxygen will also result in a diminished flame temperature.  A short, sharp inner cone, deeper purple, hissing, popping unstable flame will result.


 

3) As the quantity of propane and oxygen reacting increases so does the amount of heat.  However, a larger flame does not burn at a higher temperature but it does provide more heat given the same fuel-air mixture.


 

When consulting textbooks for the temperature of a propane flame one will discover that there is a rather wide range of reported temperatures.

This variance can be due to a number of factors.


 

1)  Instrumentation Error:  Measuring the temperature can effect the flame temperature and not all thermocouples are as accurate one would like.


 

2)  Where in the flame was the temperature measured?


 

3) Fuel flow is it laminar (smooth) or turbulent? (bunsen burners use a laminar flow)


 

4)  Is the fuel premixed?  The propane and oxygen molecules must collide if they are to react.  (fuel is premixed in the bunsen burner tube)

The Flame


 

v Adiabatic Temperature


 

Flame temperatures are reported as adiabatic meaning that there is no loss of heat energy to the environment or to external objects. This is a purely fictional scenario but convenient when computing enthalpies of reactions.  How much of the heat energy of the flame is absorbed by the burner head or radiated out to the environment?


 

v Heat Loss


 

a)  Minerals and metals act as a heat sink.  When placed in a flame they reduce the temperature of the flame.  The amount of temperature change (ΔT) is a function of the quantity of heat transferred (q), the specific heat of the sample and it’s mass.   (ΔT) = (specific heat)(mass) / q


 

b)  Flame temperature can also be affected by the volume of the sample. As the volume of the sample increases, its surface area increases which promotes the radiation of energy to the environment.


 

So perhaps the question of how hot the bunsen burner flame is, really isn’t clear cut.  However you would be on safe ground to assume that if the melting point of the sample metal is at or near 1500 °C that an acorn size sample of the metal will not melt under a Bunsen burner flame.

 

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