9
$\begingroup$

This page on Adiabatic temperatures helpfully lists the theoretical maximum temperatures that flames will reach under certain conditions for a number of fuels. They assume initial atmospheric conditions of 1 bar and 20 °C.

However, I am remain confused about what would happen if the fuels (and air) were pre-heated to temperatures close to these maximums before they were mixed and ignited.

Would the flame temperature then be higher than these listed maxima?

Would the reaction be slowed down by the higher temperature? (I understand that this might happen for exothermic reaction in equilibrium with an endothermic one, but I don't think that describes burning fuel.)

Is there some other mechanism that prevents the flame temperature from exceeding these minima?

This question was inspired by discussion on a Skeptics.SE question similar to this closed Chemistry.SE question about jet fuel and steel beams. I think it is sufficiently focussed and apolitical to be on-topic here. My background level is an ancient undergraduate science degree; I have forgotten more chemistry than I remember.

Improve this question
$\endgroup$
4
  • $\begingroup$ I'm not really sure the concept of "pre-heating" makes sense... like if I have a block of charcoal and burn it starting at room temperature, then the claim is that it cannot go above some temperature T. But now let's say I were to cut it in half and burn each half in succession. Then technically the second half would've been in a "pre-heated" environment, and hence the max temperature would be... different? Even though it's equivalent to the first situation? Something doesn't make sense here. Either the initial temperature is irrelevant, or (probably) "adiabatic" doesn't give an upper bound. $\endgroup$
    – user541686
    Sep 14, 2017 at 12:10
  • $\begingroup$ @mehrdad Why don't you start a separate thread with your question(s)? $\endgroup$
    – Chet Miller
    Sep 14, 2017 at 12:47
  • $\begingroup$ @ChesterMiller: I'd have to see what the answer to this question is first, it's somewhat of a duplicate... $\endgroup$
    – user541686
    Sep 14, 2017 at 18:56
  • $\begingroup$ I think that ruling out further reaction (for instance water dissociation, that has been mentioned) a simple energy balance tell me that the in-flame temperature will be higher if combustibles are pre-heated. $\endgroup$
    – Alchimista
    Sep 17, 2017 at 15:35

1 Answer 1

Reset to default
4
$\begingroup$

Yes and no: preheating a mix a little bit may increase the final temperature somewhat, but adding even more initial heat has a decreasing effect on the final temperature.

There are two competing factors that limit the final temperature in a closed system:

For example, $\ce{2H2 + O2 -> 2H2O}$ + 286 kJ/mole, but water dissociates more as the temperature rises. At 2,200 °C water is about 3% dissociated. At 3,000 °C it is about 50% split, so half the chemical enegy is unavailable.

BTW, thermodynamically, a heat engine produces power based on the difference in temperature, so preheating a fuel-air mixture actually decreases efficiency, though it might be needed to vaporize the fuel (e.g. in Otto-cycle engine in cold weather).

Improve this answer
$\endgroup$
11
  • $\begingroup$ When you say "final temperature" do you mean "peak temperature"? Because I'd expect the final temperature to be room temperature... $\endgroup$
    – user541686
    Sep 14, 2017 at 18:57
  • $\begingroup$ @Mehrdad: "factors that limit the final temperature in a closed system" implies closed thermally as well as chemically. Of course, the final temperature in an open system decreases to that of the environment... in infinite time;-) $\endgroup$
    – DrMoishe Pippik
    Sep 14, 2017 at 22:22
  • $\begingroup$ Okay, well then I'm completely confused. In a closed system, with a (presumably unlimited) supply of oxygen and fuel, why should there be a maximum flame temperature at all? The only reasons I can see are if other non-thermodynamic effects come into play (e.g. matter turns into plasma, or molecule is split before it can react, etc.). You mention something about reactions being forced backwards but I do not see how heating up an already-burning piece of wood would suddenly turn it back into unburnt wood. I must be misunderstanding a claim becuase my (layman) eyes just can't make sense of this. $\endgroup$
    – user541686
    Sep 14, 2017 at 22:30
  • $\begingroup$ A closed system has limited fuel, oxygen and no thermal flow. If any of those are not true, it is not closed. $\endgroup$
    – DrMoishe Pippik
    Sep 14, 2017 at 22:38
  • $\begingroup$ Oops, yes, you're right. But then wouldn't it matter (...no pun intended) what the distribution and density of the fuel and oxygen are? How can they be completely irrelevant? $\endgroup$
    – user541686
    Sep 14, 2017 at 22:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.