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Inefficiency of burning wet wood

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by M1sterM, Aug 29, 2008.

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  1. M1sterM

    M1sterM Member

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    I have a couple questions, and before I Google it, I thought I'd ask all you experts. A recurring theme on many of these threads is the issue of wood moisture content. 1) What are the percentages measuring, is it moisture content by weight or by volume?

    2) We know how many BTU's there are per pound of wood, and even per different species of wood. However, if the average moisture content of the sticks are averaging, say, 25%, how many BTU's are you truly getting out of the wood, vs. transporting the moisture up the chimney? Obviously, you need to know the answer to #1 before you do the calculation. If someone answer's #1 for me, I can do some example calculations for #2, but anyone else, feel free.

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  2. Adios Pantalones

    Adios Pantalones Minister of Fire

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    1)It's by weight. We typically use weight of water/total weight of wood + moisture.

    2) you first need to multiply the mass of water in the wood x rise in temp to ignition x heat capacity of water (1 cal/g C).
    Then add the product of the # moles water x molar enthalpy of vaporization.
    Then add any additional heating of the gas (vapor heat capacity x g x temp change).
    Convert to "per pound" and subtract from BTU/pound of wood (which is normally tabulated at something like 20% moisture- so only do the heat calc for the water amount OVER 20% mc in the wood).
  3. Adios Pantalones

    Adios Pantalones Minister of Fire

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    By the way- this does NOT account for inefficiency of burning either! If you get a smokey fire with wet wood- which ya do- then that means not all of the energy is being released (see wood gas thread) as particles are unburnt. You would also need to know the inefficiency of the process to do this calculation properly- it's not that all of the heat is being produced and some is wasted on boiling water- some heat is left on that table in the form of smoke!
  4. BJ64

    BJ64 Minister of Fire

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    Wow! Just think how smart A.P. would have been if they had not kicked him out of school for drawing cartoons!
  5. ScottF

    ScottF New Member

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    Yes that was a very educated answer. Im guessing AP is a chemist or an engineer of some sort by profession?
  6. Jags

    Jags Moderate Moderator Staff Member

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    Naaa- he just stayed in a Holiday Inn Express last night. :)
  7. JPapiPE

    JPapiPE New Member

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    Nah, he just got his paint brush stuck to the pallet and decided to become a rocket scientist
  8. Apprentice_GM

    Apprentice_GM Member

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    Elsewhere AP has posted he is an analytical chemist. By his first post in this thread you'd think he's still trying to prove it!! grin LOL
  9. RedRanger

    RedRanger New Member

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    There is NO efficiency in burning non-seasoned wood with a moisture content more than 25%--PERIOD!!


    Specially with a newer epa certified stove or insert. just equals= smoke and more smoke and very little heat.
  10. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Say what, eh?
  11. cmonSTART

    cmonSTART Minister of Fire

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    moisture content GREATER than????
  12. myzamboni

    myzamboni Minister of Fire

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    Remind me never to get into a war of wits with a Potter :cheese:
  13. Doing The Dixie Eyed Hustle

    Doing The Dixie Eyed Hustle Minister of Fire

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    You are a smart man.
  14. RedRanger

    RedRanger New Member

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    Alright , then I mean greater than 25%. :red:
  15. ROYJ24

    ROYJ24 Member

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    Can u say that in laymans term? Do I divide by 6 carry the 4, HUH? :roll:
  16. Cluttermagnet

    Cluttermagnet Minister of Fire

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    Moles degrees celcius per kilocalorie squared times the velocity of light times the gravity gradient g divided by the levity factor k...

    Gee, I didn't know they had a mole problem in that area.
    I'd hire an exterminator pronto.
  17. fossil

    fossil Accidental Moderator Staff Member

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    A pound of water requires 1 BTU to raise its temperature 1 degree F...until it gets to its boiling (or saturation) point, which depends on the pressure it's under. Since we don't run our woodstoves pressurized, we're normally talking in the expected range of atmospheric pressures spanning our elevations...not a great deal of difference so far as water's concerned. Let's just say we're at sea level and normal atmospheric pressure, then the saturation (boiling) temperature is 212 F. We've expended 1 BTU per degree of temperature change to get the pound of water in our wood up to 212 F. Now, in order to turn that pound of water into steam at 212 F, we have to pump in the latent heat of vaporization, which is roughly 1000 BTU's, if I recall correctly, something in that magnitude, anyway. So, we've burned a bunch of wood to heat a bunch of water, then burned a bunch more wood to turn the hot water into steam, and now all that steam is going to go up our stovepipes, carrying all our heat with it, just itchin' to lose its latent heat and condense back into a liquid, depositing all sorts of nasty things with it (what we call creosote). Water...can't live with it, can't live without it. Water in beer = good. Water in firewood = bad. Rick
  18. BJ64

    BJ64 Minister of Fire

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    I KNOW where you learned all that from. Most of that came straight out of a Navy boiler operating book!
  19. Doing The Dixie Eyed Hustle

    Doing The Dixie Eyed Hustle Minister of Fire

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    This is gonna be good.

    :lol:
  20. fossil

    fossil Accidental Moderator Staff Member

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    Oh no, I've been outed! :ahhh: 30+ years in uniform, propulsion engineer all the way, and 3 degrees in Mechanical Engineering, and that's all I can remember anymore. And that gave me a headache. %-P Rick
  21. fossil

    fossil Accidental Moderator Staff Member

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    And I've named my ship's wheel Eileen. :lol: Rick
  22. Doing The Dixie Eyed Hustle

    Doing The Dixie Eyed Hustle Minister of Fire

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    Aww, how sweet :red:

    Wait, I feel hands upon my throat :snake:

    :lol:

    I got a load of firewood last year in February, the wood dude was away on vacation. His assistant pulled from the front of the pile, making deliveries to customers with freshly split wood, instead of the seasoned stuff out on the lower back 40. It was a PITA to get dry. John made it right when he got back by bringing 1/2 a load of very seasoned... atleast I was able to keep cranking.

    The 1/2 cord that's left of it is standing at the ready, all nice & dried, ready to do it's service, in the line of duty this winter :coolsmile:

    I heart my firewood guy like whoa [​IMG]
  23. fossil

    fossil Accidental Moderator Staff Member

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    Don't worry, I won't steer you wrong. %-P Rick
  24. Doing The Dixie Eyed Hustle

    Doing The Dixie Eyed Hustle Minister of Fire

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    Yeah, but what about Elsie?

    [​IMG]

    "And all I need is a tall ship, and a star to steer her by." John Masefield
  25. begreen

    begreen Mooderator Staff Member

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    She's a cow, you need a steer.
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