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Wood Moisture Content - Impact on BTU's

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Joful, Jan 21, 2013.

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  1. Joful

    Joful Minister of Fire

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    So, people here are always talking about dry wood, and how you're not getting your full potential from wood that's less than optimally seasoned. It's been stated more than once here that, "this is because you're spending a lot of your BTU's on boiling water," but that math does not support that statement. Instead, I suspect the majority of the impact has to do with your secondary burn system not firing off (whether it be catalytic or non-catalytic), due to the low temperature steam coming off of the too-wet wood rapidly drying in your stove.

    First, consider a 2 cu.ft. load of oak at 20% MC, with density of roughly 3350 lb./cord, or 26 lb./cu.ft. as typically stacked cordwood. So, there's a roughly 52 lb. load of wood in your stove at 20% moisture content, meaning 43 lb. of wood plus 9 lb. of water. It takes roughly 1000 BTU/lb. of water to get from room temperature to steam, so you're spending roughly 9000 BTU's on converting water in the wood to steam.

    Take the same load green, at 35% moisture content, and now you have 58 lb. for the same 2 cu.ft. load. You're now spending 15,000 BTU boiling off water.

    At 26,000,000 BTU/cord, one oft-quoted value for oak, you have roughly 203,000 BTU/cu.ft. So, for 20% MC, you're spending 2% of your available BTU's on boiling water. For 35% MC, you're still spending less than 4% of your available BTU's on boiling water. Not much difference.

    Sitting here tonight, struggling with a less than optimally seasoned load myself, I think the primary troubles with wet wood are the difficulty in lighting, and the trouble it causes for our secondary burn systems, by lowering firebox temperatures so much during the first half hour or more of the burn cycle. After all, I suspect almost all of the 9000 or 15,000 BTU's quoted above are taken in the first 30 minutes of the burn cycle.
    BrianK likes this.

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  2. Huntindog1

    Huntindog1 Minister of Fire

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    I always found that leaving a row open , in my stove load east west, its a no-cat also, that using row which is up front in front of the door which is also in front of my doghouse air front center just under the front door, that using this row for some small split kindling on hot coals made from some good dry white oak, this is better than loading that last row with big splits. As the small stuff made from good dry high btu wood will raise the temps up in the stove so much quicker and get the secondaries going quicker and not burn up all the wood in the stove trying to get temps up. As its the temps you build in the stove that makes the stove work like it should. Its the quicker way of build temps in the stove and over coming the less than ideal wood your using. If your wood is less than ideal let the stove top get a little hotter than normal before you start shutting the input air back down in increments.
  3. oldspark

    oldspark Guest

    Wet wood does not burn well in a stove without a secondary burn system either, you have to get rid of the water first so it lowers the btus. The best way to burn wet wood is a hot fire and then I guess it is not all that bad.
  4. mepellet

    mepellet Minister of Fire

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    Not true. One Btu is the amount of energy required to raise one pound of water one degree f. Therefore it depends on what you are calling room temp but even if you say it is 60 deg f, it only takes 152 btu/pound of water to convert it to stream.

    I think your number of 1000BTUs is based on a gallon of water which is about 8.3 pounds if I remember correctly.
  5. Joful

    Joful Minister of Fire

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    I think you're ignoring the energy required to convert from water at 212F to steam. This is truly not my area of expertise, but this number came from my father in law, an engineer who spent his career designing and managing large steam turbines and boilers for power generation. Quickly Googling "btu to convert water to steam" turns up countless references that agree with his quoted number. Here's one such, from the Boiler Operator's Handbook By Kenneth E. Heselton: "It takes 970 Btu to convert one pound of water at 212F to steam at the same temperature..."

    So, one pound of water at room temperature would require 142 BTU to get from 70 to 212F, and then an additional 970 BTU to get from water to steam (phase change), or roughly 1112 BTU per pound. Given that we don't typically weigh the wood we're using, or measure it's temperature prior to loading, I figured 1000 was close enough.
    Adios Pantalones likes this.
  6. Huntindog1

    Huntindog1 Minister of Fire

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    I agree that the biggest issue is keeping the stove from getting into the secondary burn mode.

    As there are countless people on here that are new to these type stoves that get on this board and say they cant get any heat out of their stove as they never get the stove into the secondary burn mode.

    I think its because these stove are severely limited on the amount of air you can get into the stove. As these stoves dont need alot of air to burn in the secondary burn mode as when you raise the fire box temps to the level for secondary light off those higher temps let the stove burn with very little air flow. Thus this also allows the stove to radiate alot fo heat out into the room as the air flow is so slow less heat escapes up the flue.

    Here is some EPA info on the subject:

    http://www.epa.gov/burnwise/workshop2011/WoodCombustion-Curkeet.pdf
  7. PassionForFire&Water

    PassionForFire&Water Minister of Fire

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    I think it's plain simple....you can not burn water!
    You need to evaporate the water to steam vapor, this happens at 212F, and this takes energy/heat, what comes out of your firebox.

    30% MC leaves only 70% of biomass to burn
    20% MC leaves 80% biomass to burn
    10% MC well ... you can do the math.

    That's why it's important to buy well seasoned cordwood.
    While wood dries, it shrinks, so you get more wood in the same cord of 8x4x4
  8. Jon1270

    Jon1270 Minister of Fire

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    I think you're right that the loss of secondary combustion is a major reason for loss of efficiency, but I don't think it's as complete an explanation of the problem as you're suggesting. In my recent experience, it seems that the moisture also inhibits *primary* combustion, i.e. there's more smoke and very little flame right near the fuel, let alone up around the secondary tubes. Some of the oak I got in a recent delivery was up around 45% MC (green can be much wetter still) and did little more than smoulder. If it hadn't been mixed with drier fuel it probably wouldn't have burned at all. Some of that oak sat there smoldering and cooling the stove for hours; it certainly didn't shed all its moisture in the first half hour. And during that extended time while it was cooling the stove, it wasn't just inhibiting its own primary and secondary combustion; the lower temps were also inhibiting the secondary combustion of the smoke from the drier wood that was in the stove with it.
  9. Jon1270

    Jon1270 Minister of Fire

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    Reading the EPA document that Huntingdog linked to, I see that I may be misusing the terms 'primary,' and 'secondary.' By their definitions, all flame is secondary combustion regardless of where the flames are or where they're getting oxygen from.
  10. Joful

    Joful Minister of Fire

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    :rolleyes: You clearly work in Marketing, not Engineering.
  11. Joful

    Joful Minister of Fire

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    Yep... my assumption was that things are not so bad you cannot get primary combustion happening. Good point.
  12. oldspark

    oldspark Guest

    Its this simple, water dont burn!
  13. PassionForFire&Water

    PassionForFire&Water Minister of Fire

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    Happy to see that I'm not the only one that believes this;)
    Backwoods Savage likes this.
  14. BrowningBAR

    BrowningBAR Minister of Fire

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    A better title is "Wood Moisture Content - Impact on my sanity"

    Burning wet wood is possible, but, dear god, it sucks.
    milleo, PapaDave, Trilifter7 and 2 others like this.
  15. Slow1

    Slow1 Minister of Fire

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    Consider also that the volume of steam to water is about 1600:1 - so if you are boiling off water you are expanding that out to a lot of steam which is displacing quite a bit of volume that would otherwise be either combustible gasses or air carrying O2. There are so many factors to consider here but in the end the simple "water doesn't burn" does hold true (even if you did fly in a KC-135).
    Backwoods Savage likes this.
  16. jharkin

    jharkin Minister of Fire

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    Another engineer here... and I have been guilty of arguing with math in the past (dont bring up the fans vs. efficiency threads again please dear god)

    But a few years experience has shown me that for all the theory its quite simple in practice:
    >30% - So painful its not even worth trying
    25-30% - Will work if you have no alternative
    20-25% - Not too bad, just keep an eye on the flu
    <20% - Ahhhhhhhhhhhhh heat :)


    So the simple answer for me is.... drumroll...


    Just stack your wood for two years and forget about it :ZZZ
    PapaDave likes this.
  17. semipro

    semipro Minister of Fire

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    Its seems more complicated than "water don't burn"
    I was thinking the same as Slow1 as I read through this thread...the steam is displacing other gases and preventing the additional combustion that would otherwise occur in the stove and flue.
  18. Lumber-Jack

    Lumber-Jack Minister of Fire

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    And yet, even after two (or 3) years you will still have moisture in that wood, so therefore, you will in effect, still be trying to burn water.
    Joful likes this.
  19. Slow1

    Slow1 Minister of Fire

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    Ok.. so here I go playing with the numbers even more... If we go with the OP's original estimates of 9lbs of water that is just over a gallon. So with the 3600:1 ration that is about 481cuft of steam - now in my stove that would completely fill the stove more than 160 times, but then again I don't know what the flow rate of my flue is... perhaps I should look that up somewhere and see how much time it takes to evacuate all that steam, but clearly that has to make a difference somewhere in the burning equation.
  20. mellow

    mellow Resident Stove Connoisseur

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    Here is some simple math:

    Cat Stove + Wet Wood = Combustor Failure
    Adios Pantalones likes this.
  21. Adios Pantalones

    Adios Pantalones Minister of Fire

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    A couple of things- those that said it takes more energy to boil water than just the energy to get it to 212F are correct. There's the "specific heat"- which is the energy to heat it up, and there is the heat of vaporization (I won't get more technical and confuse things).

    Further, I see these calculations of energy lost in boiling water here all the time. They ignore a big factor- which is inefficient burning. That is energy lost beyond the extra energy of boiling water. Smoke particles that are not fully combusted are basically unburnt fuel lost up the stack. If you have a chimbly fire- all that creosote burning was energy that should have been produced in the stove, and that doesn't count the uncondensed smoke. These calculations are a start, but are inadequate estimates, IMO.
    Joful and Jon1270 like this.
  22. Adios Pantalones

    Adios Pantalones Minister of Fire

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    Another point- below a certain moisture level the efficiency of the burn is LESS than that for an "optimum moisture" wood. The wood will gasify faster than it can efficiently burn, and energy will be lost as smoke up the chimney. I occasionally do this on purpose in the wood kiln so that the smoke, in essence, steals oxygen from stuff in the kiln; simple redox chemistry in a primitive solid fuel burning appliance :)
    Joful likes this.
  23. oldspark

    oldspark Guest

    I think it is as simple as water dont burn.
    "Today's EPA approved woodstoves provide pre-heated oxygen to the secondary burn chamber directly, through a separate intake controlled by the chimney updraft, enabling you to turn your draft control all the way down to control the primary fire without extinguishing the secondary burn. This technology results in fewer particulate emissions, longer burn times and cleaner chimneys, but it is important to note that steam is still not combustible: even these state-of-the-art stoves won't burn the gases in the secondary burn chamber if the fuelwood is wet. "
    Backwoods Savage likes this.
  24. TradEddie

    TradEddie Minister of Fire

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    Good explanation, but even your 1000BTU may still be underestimating. I can't seem to find numbers but I suspect it will take far more BTUs to heat that 212F steam to secondary combustion temperature, than to heat air to the same temperature, although I think the effect of that is what you're suggesting causes wet wood to burn badly.

    Another contributing factor may be that steam is ruining your draft, or at least releasing gas to satisfy that negative pressure, instead of drawing air from outside.

    TE
    Adios Pantalones likes this.
  25. Adios Pantalones

    Adios Pantalones Minister of Fire

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    It's more than that. Slower starts to get the secondaries hot enough, less primary flame early on, etc- too much water in the wood contributes to inefficiency in several ways.
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