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Homework Help: Methane-air combustion

  1. Jun 4, 2009 #1
    1. The problem statement, all variables and given/known data
    I want to figure out what pressure would need to be applied onto a 5% methane 95% air mixture to heat it up to the autoignition temp of 853 K.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 4, 2009 #2
    Are you familiar with LEL (Lower Explosive Limit)?
  4. Jun 4, 2009 #3
    The LEL is the leanest mixture in which it will still be flammable, which I believe for methane air the explosive limts are 5-15% so 5% would be the LEL?
  5. Jun 4, 2009 #4
    Most authorities give LEL as slightly above 5%, but even if it were 5% you'd have a very hard time exploding it. Further, Burgess and Hertzberg think LEL rises with pressure. Here is a link to some of their work, albeit with spark ignition. Note they use LFL - almost the same but more specific to mine safety.
    http://www.cdc.gov/niosh/mining/pubs/pdfs/fompa.pdf [Broken]
    Last edited by a moderator: May 4, 2017
  6. Jun 4, 2009 #5
    Thank you very much for the link. Looks like I will find some useful information in this source!
  7. Jun 5, 2009 #6
    Oops! A thousand pardons. I forgot to compare the lowering of LEL due to elevated temperature against the raising of LEL due to elevated pressure. Both are empirical formulas and I don't know their interaction (if any). Somewhere within the combustion engineering literature may be a graph showing both, but I believe it most likely your mixture WILL be explosive at that temperature.
  8. Jun 5, 2009 #7
    This paper is helpful but I am trying to figure out how much pressure needs to be applied to autoignite, not how much pressure is released when it is ignited by a spark. That seems like the concept of that paper.

    The problem is I am going to have to pipes coming into a T. Compressed in one and methane in the other. My intention is to only have one of them on at a time, but I wanted to make sure in case a valve failed etc, that if the two flows met at pressure, they wouldn't autoignite.
  9. Jun 5, 2009 #8


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    Staff: Mentor

    I don't get you.

    At first I thought you asked about adiabatic compression where temperature rises till it is high enough for autoignition (853 K) - something like is happening in Diesel engine.

    But now you state you will be mixing compressed gases. You can compress them, cool them, mix them. No heating, no autoignition.

    Or am I missing something?
  10. Jun 5, 2009 #9
    That is true. I just needed to numerically prove what temperature pressure relations auto ignition could happen at even though our temps would be no where close. they wanted to see it on paper, but I found what i needed in a combustion physics book but thank you so much for the input!
  11. Jun 5, 2009 #10
    Here is more or less what I was looking for(attachment):

    Attached Files:

  12. Jun 18, 2009 #11
    Hi there..I'm newbie here.I'm doing my MSc of Laminar Combustion of methane/air mixtures. I got some problem so hope u guys can help me out.

    1. If given initial condition of before ignition, with P=100kPa and [tex]\phi[/tex]= 1.2, HOW do I determine the partial pressure of methane needed?

    Ptotal = PCH4 + Pdry air
    100kPa = PCH4 + Pdry air rite??

    correct me if I'm wrong..
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