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Semi dry vs. 20% ish

Post in 'The Wood Shed' started by James02, Nov 2, 2011.

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  1. Battenkiller

    Battenkiller Minister of Fire

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    Only the latent heat of evaporation is unrecoverable during the entire process. That is because it can only be regained by condensation of those vapors, which just doesn't (or shouldn't, at any rate) occur inside the flue. As long as the flue walls are above 212ºF, all of that valuable heat is lost forever. Same thing for the water formed by combustion itself (which is a much higher amount), but that heat loss was covered in my calculations by using the low heating value of the wood instead of the high heating value.

    All other energy put into the system will come back during the cooling of the gases as they make their way from the firebox and through the stack. In other words, it doesn't make any difference how high you need to get the gases up to temp or how much energy it takes to do so. As long as the flue gas temps are not excessively high, all of that heat will need to be transferred back into the living space somewhere in order to satisfy the law of conservation of energy. In short, you cannot claim an increase in energy input without accounting for it at the output end. That's why research and test labs use the "stack-loss" method to determine stove efficiency. What goes in must come out, and they count every blessed thing that comes out of that stack.... including the exact amount of flue gases, their rate of removal, and their temperature all along the stack.

    With wood stoves, you have to separate what in going on combustion wise with what is going on heat transfer wise. On several occasions I have posted a simple chart that shows the interrelationship between these two things and their effects on overall efficiency. Without a doubt, there are other heat losses involved in burning wood at higher moisture content. Nowhere did I say that there are not significant heat losses involved, it's just a matter of pinning them all down accurately. It is overall system efficiency that we should be concerned with.

    Wood actually burns slower, cleaner, more predictably, and more efficiently as moisture content rises up to a certain point, but the increased rate of air delivery needed to burn it passes more heat up the flue. Above 20% water by weight (that is 25% MC as read on a resistance meter), overall efficiency begins to drop, even as combustion efficiency continues to increase. So burn wood at or around 20% water content for the very best overall efficiency, but let's not exaggerate the energy losses. They are certainly there, but twice as much heat from wood that is just a few points lower in MC is a total fallacy, and a very disheartening bit of misinformation for a new burner sitting on four cord of wood that measures a few points higher than "ideal".

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  2. Creekyphil

    Creekyphil New Member

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    Its not just as simple as calcuating the heat of vaporization required to move the water up the flue, and then subtracting it from the chemical potential energy of the wood.

    If that ten pound split burns at 80% effieciency at 20% MC, you'll get your 64,000 BTU. with 1920 BTUs lost to vaporizing the water, not net BTUs of 62,080. That I agree with.

    I would guess that the split at 27% MC, however, burns signifgantly less effeciently. The actual numbers are beyond the scope of this discussion, but lets say 60%. You now get 48,000 BTUs from that same split, less the 2880 BTUs lost to heat the water, for a net BTU output of 45,120. Compare that to the output of 62,080 on the first split, and we're talking only 73% of the heat. Thats a big difference for 448 grams of water to make. Multiply by 5 splits per load, and a couple hundred loads per season, and its easy to see why Backwoods Savage's wood piles are something to be envied.

    Stepping back from the numbers, this makes sense when you think of it in terms of new EPA stoves. I can load my stove and get raging secondaries for hours with good wood, and get piles of heat. Load it with the same amount of wet wood, and no light show. Nada. I know theres still the same amount of heat in the wet split, but my stove cannot pull it out effectively. It is the loss of efficiency that causes the difference in the heat I feel, not the pint of water that has to be cooked off.
  3. jimbom

    jimbom Combustion Analyzer

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    Sorry. I understood you were not sure the OP was trolling.

    The Thermodynamic Properties of Steam by Keenan and Keyes thirty sixth printing 1964 lists specific enthalpy of steam at 212 °F , 14.696 psia as 1150.4 BTU/lbm. It seems to me specific enthalpy at 312 °F , 14.696 psia would be about 1250.4 BTU/lbm. Is this what you were thinking also?

    The specific heat of air is approximately 0.24 BTU/lbm °F . There is about 13 ft ³ of dry air in a lbm at 50 °F. Is this significant with regard to wood stove combustion efficiency?

    I am the position of not being a combustion expert and hope to learn something here.
  4. Battenkiller

    Battenkiller Minister of Fire

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    Your guess would be wrong. And if the real numbers are beyond the scope of this discussion, why are you making up hypothetical ones to try to prove your point?

    Fact:

    In controlled conditions, the measured combustion efficiency of wood is at its highest up around the fiber saturation point of the wood - 28% MC on a meter. Now, that is just under 22% MC wet-basis (% water of total weight). Curiously, that is not quite two points higher than the maximum allowable range for moisture in the EPA test procedure. Why is this fact consistently ignored on this board? It's like all the stove makers are struggling to get stoves that pass the EPA standard with flying colors, but the folks buying and using them are getting different (better) results than the folks designing and testing them. This makes zero sense to me. I say, if you can't get your stove to operate correctly using the same MC that they use to design and test the stove with, you are doing something wrong.

    BTW I personally tested the Backwoods Savage white ash right at the Woodstock plant, just after it had been re-split. It measured 28% MC on a fresh face using my meter. No one was complaining about the lack of heat pouring off that Progress Hybrid using the same wood, and the secondaries? Just spectacular.
  5. Danno77

    Danno77 Minister of Fire

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    The thing that I do not feel has been adressed is that these BTUs referenced are stored energy, and say nothing about the rate at which they are extracted.

    Sure it takes XX BTUs to vaporize the water. Sure a split has YY amount of stored BtUs in it depending on wood fiber amounts. BUT, at what rate are those BtUs released? A very high MC split will not release it's energy at a rate that can boil off that moisture at an acceptable rate. So, it may take 1500BTUs to get rid of that water, but it may take that split 40 minutes to do so.

    Extreme example is that a 70% MC sample of wood has the same BtUs in it as a 10% sample of the same wood. good luck getting that 70% sample to burn. Even after you use your math there is not much difference in NET BTUs (amount needed to get rid of the water subtracted from the wood fiber's stored energy).
  6. jimbom

    jimbom Combustion Analyzer

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    So very true.
    But for a learner burner like me, give me a little hope that if I burn smaller loads in hotter fires, I can get by the first year with not so good wood. Kind of the "Bummer. Your wood is wet. This is what you can do this year while you are gathering future years wood." approach.
  7. Creekyphil

    Creekyphil New Member

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    I said that I don't have efficiency numbers for wet wood, because I'm unaware of any stove being tested with anything other that the EPA test load. If your stove was, I'd love to see the link to the numbers for a load at 20%, 25%, 30% and 40%. Just so we could compare.

    What controlled conditions are we talking about? And even if wood showing 28% on a meter did burn most efficiently in a particular set of conditions (ie your stove), what about wood at higher MC? You think you can burn that efficiently as well, so that the only difference in heat output is the amount of energy required to boil off another pint of water? By that logic, you could put slightly more green oak in your stove (where the additional wood fibers have enough energy to account for the additional water) and I could put slightly less dry oak in, and your house would be as warm as mine.

    Its a nice thought, but I live in the real world.
  8. jimbom

    jimbom Combustion Analyzer

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    Shelton Energy Research. Published in 1983, Solid Fuels Encyclopedia, Jay Shelton, figure 14-2 on page 215.
  9. Thistle

    Thistle Minister of Fire

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    +1 YES. Split it stack it forget about it for 1 to 3 yrs depending on species,wood condition,your location/climate. Its not rocket science.Your nose,eyes & sense of smell/touch can tell you whether its ready or not.
  10. Battenkiller

    Battenkiller Minister of Fire

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    Of course, burn rate is highly important, but you are using hypotheticals to make your point. Fact is that the system can be tweaked to deliver higher burn rates with wetter wood... up to a point. We aren't talking burning an oak log that was just bucked and shoved into the stove, we are talking about 20% water vs. a little more than 20%. Why must people use gross exaggerations to bolster their arguments? My claim is that there is little difference across the range that will actually burn well in the stove.

    Of course, that wood in my avatar photo was at about 57% MC when I put it in there a few minutes before I opened the door and snapped that shot. Stove was up to about 750ºF, so no denying the heat output. I have a video of three points during that burn cycle that shows a smoke-free stack, even after re-load. For some reason I can't get it up on YouTube (error message). Maybe the world just isn't ready for it yet.

    No, no, no.... it is not an EPA stove (but it is one that uses secondary combustion technology), but most folks think such an incendiary display would not be possible burning green wood in any wood burning appliance. I know how to do it, and that knowledge comes from a bit of understanding coupled with a lot of experience doing it. Tricks of the trade, you might say.
  11. Creekyphil

    Creekyphil New Member

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    I don't have it in front of me. Can you describe the conditions? Can I assume its not an EPA approved stove?
  12. ewdudley

    ewdudley Minister of Fire

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    Well it doesn't have to be as bleak as "10 splits wet equals 5 splits dry", and and it's not so rosey as "all you're losing is a cupla too tree percent".

    FWIW here's my tips for burning not so dry wood, where not so dry is defined as up to about four parts water, ten parts dry matter, i.e., 40% DB, 29% WB:

    -Save your coals. Put a timer on the draft fan or otherwise do whatever you have to to shut the fire down while there is a big layer of coals. Tape off the draft port with aluminum tape if you have to.

    -When restarting, start with a layer of flat splits placed tightly over the layer of reignited coals. Let them bake out and start gasifying, then add more wood sparingly throughout the burn on a pay-as-you-go basis, especially until the water jacket is heated up. If you put too much wood on at once the process collapses. Open primary air up some. The trick of coarse is to prevent a small fire from having to dry out a big pile of wood just to get started.
  13. jimbom

    jimbom Combustion Analyzer

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  14. Battenkiller

    Battenkiller Minister of Fire

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    Yes, it is found there, but I believe it was taken from an EPA study that was originally published in the early 80s. I have the report on my computer but I can't find a specific link to it. Burn rates were also examined. Maximum burn rates are lower with green wood (obviously), so peak output is not achievable at high MC. However, very few folks ever achieve (and rarely need) peak stove output. I live in the real world as well, and my stove operates well below it's peak output at almost all times. Not only am I unwilling to stand there and stick-feed the stove with the air wide open, I don't think the stove would last a single season run at peak output 24/7.
  15. jimbom

    jimbom Combustion Analyzer

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    Sorry that didn't come out correctly. My part starts with "I will type it... and ends with Probably on this site."
  16. Creekyphil

    Creekyphil New Member

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    So this is in an old smoke dragon, like a fisher? And when the wet wood wasn't burning as well, they opened the air way up? Thats supplying much more air than is provided through secondary baffles on todays stoves.

    I get that wet wood contains almost as much potential energy as dry wood. I'm not debating it. But thats only after complete combustion, or in test conditions like a calorimeter, etc. Thats not in a modern stove, where the effeciency is hampered by the additional moisture content.

    I'm not sure the point you're trying to make. Are you saying that the heat output difference in burning wet wood and dry wood in a modern stove is the BTUs required to vaporize the water?
  17. jimbom

    jimbom Combustion Analyzer

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    That is good info. Thanks for sharing.
  18. pen

    pen There are some who call me...mod. Staff Member

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    Well said.

    pen
  19. Battenkiller

    Battenkiller Minister of Fire

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    You are attempting to put implications into my words that I never intended. I never said that the only losses are due to the evaporation of water, that is the claim that others have used to explain the reduced efficiency. I think I did a good job of pointing out the actual major source of heat loss (up the flue), but that doesn't mean you can't get enough heat out of the stove to warm yourself quite adequately, just that by doing so you will have to cut and burn more wood. And I never said "slightly more", but it is certainly not a whole hell of a lot more to produce the same amount of heat.

    BTW there were hundreds of wood burning appliance tests of all sorts being done for decades before the EPA even came into existence. Shelton and others were the pioneers whose work the EPA used as the starting point to figure how to reduce emissions. But let's be clear, the EPA was never concerned with greater overall efficiency, only cleaner air. That these two things are highly correlated is a no brainer, but no one (at least not me) is advocating using green wood rather than properly dried wood. In fact, I'll put my artificially-dried wood up against just about any outdoor-dried wood out there, no matter how long it sat in a stack.
  20. Creekyphil

    Creekyphil New Member

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    You never said that the only losses are due to the evaporation of water? This post, and the math contained within it, seem to do exactly that. Apologies if I'm misunderstanding, but the way it is written, I think everyone else reading this probably is too.
  21. Battenkiller

    Battenkiller Minister of Fire

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    No, I am not saying that, nor have I ever said that. Why do you keep hammering away at that silly point?

    What I am saying is that wood that measures 28% MC on a modern moisture meter will burn fine in a modern EPA stove because it is real close to the MC that the stoves are designed to run on. The Progress Hybrid that we put Dennis' stuff into uses hybrid technology, with a big secondary air manifold producing awesome secondaries with wood that measured 28% on the meter (and later on, measured in an oven-dry test). Is the PH a super stove that can handle this 9 year-old "wet" wood? I think not. Everything right up to enzymatic changes in the wood over time has since been hypothesized to explain a phenomenon that really needs no explanation. The simple truth is that the wood was still well within the limits of moisture that the stove was designed to run on, so it burned perfectly fine.

    Anyway, I've got to go pack my shop up now. Should take me about a week, so I'm done here, maybe for good (that ought to thrill some folks). You can go ahead and put more meanings into my words than I agree with myself. That's the beauty of the Internet... no one is accountable for their foolishness... and why I don't take any of these comments too seriously. I'll be staying warm with some nice black birch that has been sitting outside for only a year now. Hope it keeps me warm enough, it's only down in the mid-20s on my meter.
  22. Battenkiller

    Battenkiller Minister of Fire

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    Between the two MCs I used as an example there, there will be very overall little heat loss due to other factors. Combustion efficiency will actually be greater, but heat loss up the stack will be greater as well, so it's basically a wash (give or take a few efficiency points). All that changes dramatically as MC increases to higher levels. Read the rest of my posts regarding that matter. Now, over and out, sir.
  23. Creekyphil

    Creekyphil New Member

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    I did not mean to offend you, I thought it was just a friendly discussion between two science minded people. And regarding accountability for what is posted, I'd happily send my name, address and phone number via pm, if you would like to discuss standards of internet conduct further.
  24. Battenkiller

    Battenkiller Minister of Fire

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    I'm not offended at all, and I didn't mean to offend you, either. I sincerely don't have time for this right now. I've just been wasting time here trying to avoid the very depressing task of disassembling my workshop of 21 years. Time is up now, got to move on.

    I'm selling my stove, and I really don't know if I'll even want to burn anything but pellets in the new place. I'll be 60 in March. Every hour spent handling firewood is one less hour I get to play with the grandkids. They will be grown before I know it, and won't really want to hang with Papa at that point. No matter what I do, I certainly won't be going back to trying to heat my home 100% with wood now that I'm moving away from electric baseboards (Yay!).

    FWIW most all of what I post here is either to point out paradoxical information in the pursuit of intellectual curiosity, or simply to help new burners cope better and not give up. If I have failed in any way by giving this information, or in the manner in which I've presented it, then I can say I've failed in my objective.

    I am not a wood combustion authority, nor have I ever knowingly claimed I was. I am pretty good at understanding what the real authorities are saying, although I'm never sure if my explanations are clear or not. I do my best. That's another beautiful thing about the Internet. Nobody gets to fire you for screwing up.
  25. Backwoods Savage

    Backwoods Savage Minister of Fire

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    KISS! I have to agree. Too much analyzing can be harmful. Keep it simple. Don't try to burn wood that you cut this year. If one decides to burn wood then one must abide by Mother Nature's rules. Get the wood ahead of time, git it split and get it stacked so it has time to dry. No more analyzing needs to be done to that formula.

    For those who have not tried, if you ever experience the difference between good dry wood and marginal wood, you will be simply amazed and will never go back.
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