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Methane production

  1. Jan 23, 2016 #1
    Dear PF Forum,
    I'm interested in composting.
    I want to understand the benefit of composting.
    Besides
    • Reducing organic waste
    • Producing fertilizer
    This composting process also produce energy (methane) if properly managed.
    I wonder how, without oxygen addition/introduction, in a sealed composter bin, can methane be synthesized?
    Wiki (https://en.wikipedia.org/wiki/Fermentation) says that
    CH3COO− + H+ → CH4 + CO2
    Okay...
    So perhaps without introducing energy from outside of the system. Methane can be synthesized.
    But now I'm wondering about a new thing.

    https://en.wikipedia.org/wiki/Heat_of_combustion
    It is said here that
    CH4 + 2O2 -> CO2 + 2H2O + energy.

    What I can surmise from that is:
    A: "Creating CO2" release energy, is this right?
    B: "Creating H2O" release energy, is this right?
    C: "Creating CH4" needs energy, is this right?

    And then, there is
    https://en.wikipedia.org/wiki/Standard_enthalpy_of_formation
    CO2 -> -393
    H2O -> -242
    D: Why these numbers are minus? Is it because
    "Creating CO2 (or H2O) DOES NOT need energy, but releasing instead"
    Okay, if this is so, than I can (at least) take it

    E: So, if producing CO2 releases energy, then PRODUCING CH4 should consume energy, is this right?
    But also from the same link above: https://en.wikipedia.org/wiki/Standard_enthalpy_of_formation
    F: Methane -> -74.9. What does this mean?
    Does this mean: Creating methane also RELEASES energy?

    So the statements E, F and [A+B+C] seem contradictory to me.
    Can anyone help?
    I'd like to understand this first, before I read fermentation.
    So that I can understand the benefit of composting.

    Thank you very much.
     
  2. jcsd
  3. Jan 23, 2016 #2

    Bystander

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    The "zeroes" are the elements in their standard states.
     
  4. Jan 23, 2016 #3

    Ygggdrasil

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    Whether a chemical reaction requires or releases energy depends on the exact reaction involved. There are some reactions that produce methane that require energy and there are some reactions that produce methane that release energy.

    The standard enthalpy of formation is defined as the energy required (or released) when synthesizing the compound from its constituent elements in their standard state. For methane, this means synthesizing methane from graphite and hydrogen gas:

    C(s) + 2H2 (g) --> CH4 (g) ΔH = –74.9 kJ/mol
    (in this case, producing methane releases energy)

    To determine the amount of heat generated or consumed in any other type of reaction, you would look up the standard enthalpies of formation for all reactants and products, then apply Hess's[/PLAIN] [Broken] law.
     
    Last edited by a moderator: May 7, 2017
  5. Jan 23, 2016 #4

    Borek

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    Staff: Mentor

    Also, what can be confusing here is the fact that the convention is such that if the reaction is exothermic (produces energy) enthalpy change is listed as negative, while enthalpy change for an endothermic reaction (one that consumes energy) is positive.

    Think as if you were looking from the point of view of reactants - when the reaction is exothermic they are releasing energy so their energy change is negative, when the reaction is endothermic they consume the energy, so their energy change is positive.
     
  6. Jan 23, 2016 #5
    Thanks Bystander,
    Thanks Ygggdrasil (shouldn't it be 2 gs? I read yggdrasil in Storm comic),
    Thanks Borek.
    I think I understand what you gentlemen (I assume) tell me.
    So for one last question perhaps.
    Is it possible to produce methane inside a closed composter bin with typical organic kitchen waste only with an exhaust hole, without adding/introducing oxygen and sunlight from outside?
    I just want to know what is and how much is the beneift of garbage processing.

    Thanks again.
     
  7. Jan 23, 2016 #6

    Borek

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    Staff: Mentor

    Definitely.

    Actually methane production in landfills can be a serious problem - https://en.wikipedia.org/wiki/Landfill_gas
     
  8. Jan 23, 2016 #7
    Thanks Borek for the answers.
    Now, this is my ultimate question actually.
    After I understand that producing methane is possible in a closed system without introducing energy from the outside.
    Given that carbon is roughly 10% in plant tissue.
    Is it possible to know that organic material produces methane (if fermented) about 5% of its weight?

    Thank you
     
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