Hydrogen Flame temperature
Hydrogen Flame temperature
(OP)
Can anyone please tell, why is the adiabatic flame temperature of Hydrogen greater than Methane?
Ideally, I would think that since methane can release a higher BTU/SCF, it would have a higher flame temperature than Hydrogen, but it's not so. I am not able to reason out this fact.
Ideally, I would think that since methane can release a higher BTU/SCF, it would have a higher flame temperature than Hydrogen, but it's not so. I am not able to reason out this fact.





RE: Hydrogen Flame temperature
Here a few plots for an ideal and complete combustion of carbon, sulfur, hydrogen, coal, oil and gas (methane -- CH4) when oxidant is standard air (79% N2 and 21% O2 on volume/mole basis).
I do hope that you will find the above plots useful ...
Thanks,
Gordan Feric, PE
Engineering Software
http://members.aol.com/engware
RE: Hydrogen Flame temperature
An adiabatic flame temperature involves keeping all the developed heat in the products of combustion. Thus, the answer rests on the amount of heat taken up by the moles of gaseous products obtained per mol of fuel.
The stoichiometry of combustion with air tells us that the products are:
for hydrogen 2.88 moles/mol
for methane 10.53 moles/mol
I hope the above answers your query.
RE: Hydrogen Flame temperature
I did not understand the 2.88 moes/mol of H2
and 10.53 moles/mole of CH4(methane) in your reply.
Can you please explain it in a bit of detail?
RE: Hydrogen Flame temperature
When you burn the CH3, a greater total amount of energy may indeed be released in the reaction that that released by burning the H2 (I don't know one way or the other off the top of my head). However, you're talking about the adiabatic flame temperature, and you're not looking solely at the energy liberated. What you're considering when you talk about the adiabatic flame temperature is how that liberated energy is absorbed by the products of combustion and figuring out what the highest flame temperature would be if everything went perfectly during the combustion. So, when you consider that the CH3 will produce a much larger mass in its products of combustion than the H2 will, the CH3 can burn with a lower flame temperature than the H2 will even though the combustion reaction itself may liberate more energy because it has much more "stuff" to heat up.
RE: Hydrogen Flame temperature
Here are the basic H2 and CH4 combustion reactions balanced on mole (volume) basis:
1 H2 + (1/2) O2 + (1/2)*3.76 N2 = 1 H2O + 1.88 N2
Therefore, for 1 mol of H2, one gets 2.88 moles of combustion products.
1 CH4 + 2 O2 + 2*3.76 N2 = 1 CO2 + 2 H20 + 7.52 N2
Therefore, for 1 mol of CH4, one gets 10.52 moles of combustion products.
Note, for standard air the mole composition is as follows:
79% N2 and 21% O2
or for 1 mol of O2, there come 3.76 moles of N2.
Even though this is just basic engineering combustion reaction balancing, it is not quite simple to get it right because of mole/volume and mass basis that get thrown into the subject matter.
I do hope that this helps connect the dots provided by other Eng-Tips.com contributors ...
Thanks,
Gordan Feric, PE
Engineering Software
http://members.aol.com/engware