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Fact or Fiction

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by jebatty, Jul 26, 2013.

  1. jebatty

    jebatty Minister of Fire

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    One prominent OWB of advertised gasification design has been a frequent target of negative criticism, maybe deserved, but at the same time other mfrs, whether OWB, gasification, or other, make ambiguous and unsupported claims about efficiency, output, rate of burning wood, etc. which go unchallenged. This is common hype, "buyer beware," not unique to the wood boiler industry. It seems to me that negative criticism directed at one mfr needs to be read with the proverbial "grain of salt," unless the criticism is based on objective fact, measured performance, fully disclosed and with a method repeatable by others. Peer review possibility would be appropriate. Also very helpful would be the background test, method and data information used by the mfr to support the advertising claims. This is woefully lacking, even from the mfrs with excellent reputations.

    I can't claim to be fully objective in everything I say or do, but when my comments have been directed at performance, efficiency, btu output, and the like, I have based those comments on identified sources and personally measured performance and data logging, such as pounds of wood burned, delta-T and flow rates both calculated and based on differential pressure, digital DS18B20 temperature measurements, etc.

    Perhaps rather than basing comments on advertising, it would be better to base them on actual operation and performance of the boiler which is the object of the comments in a real world environment, not the laboratory. That would be helpful information for all of us. I would enjoy access to any high profile wood boiler in a real world install and be given the opportunity to run it through the paces and report, much as I already have done with the Tarm Solo 40, Wood Gun E500 and Garn WHS 3200. Separating fact from hype and fiction is a worthy goal.
    hobbyheater likes this.

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  2. ewdudley

    ewdudley Minister of Fire

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    I think a good starting point would be to publish a straightforward description of the Balenthin Batch-Mode Constant-Volume Calorimetry Method that most any (batch-mode boiler) manufacturer could follow and report their results. It seems to me that non-batch-mode wood boilers are non-starters, and therefore the difficulty of measuring flow rates and delta-T are therefore non-issues.

    So all we need to know is weight of wood, MC of wood, volume of tank, starting mixed temperature, and ending mixed temperature, and a common formula for comparing the results.
  3. jebatty

    jebatty Minister of Fire

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    Realistically I think we also need to know or agree on btu's available from the wood. As has been reported previously, the available energy in seasoned (20% moisture content) wood used in an actual usage environment (400°F flue gases) is about 6050 Btu/pound.
  4. hobbyheater

    hobbyheater Minister of Fire

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    What would be the BTU value per pound at 7% moisture content ?
  5. jebatty

    jebatty Minister of Fire

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    Wood Heat 1 LB is pretty good at explaining the heat value of wood and the down-rating due to moisture content, and you can estimate the heat value at 7% MC from the chart at the end of the document. While the down-rating in the document considers a stack temperature sufficient to prevent condensation (300F), it does not take into account a further down-rating based on stack temperature above about 300F. So the chart shows 6480 btu/lb with stack temperature of 300F.

    Most of the gasification boilers I have worked with have a variable stack temp, from high burn into a range around 500F, mid burn around 400F, and then lower as the fire/coals die down further. If 400F is used as an average stack temp, then Energy in Wood shows that 6050 btu/lb is the available heat in one pound of wood at 20% MC.
    hobbyheater likes this.
  6. heaterman

    heaterman Minister of Fire

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    One thing + an observation.

    If you go to the DOE/EIA.gov website and look up their fuel cost calculator they list cordwood at 20,000,000 btu/cord. They also offer a disclaimer stating that figure is a rough approximation due to density and MC. If one uses the 20MM number that comes out to 10,000 btu per pound. Where's the discrepancy?

    RE: flue gas temp. I noted with great wonder the correlation between flue gas temp and moisture content while observing the Windhager pellet boiler I ran in my house last winter.
    The readout on the boiler display showed flue temps as low as the 175-180* range depending on firing rate and water temp. I thought the display read out was whacky until I double checked it with the Testo. Turns out it was spot on but instead of seeing condensation and creosote dripping from the boiler and flue there was just a trace of dry ash in there.
    So I'm thinking how can that be? The answer is moisture content of the fuel. When you deal with a product that is at or below 10% MC flue gas dew points drop to around 110*-120* range. When I adjusted the analyzer to consider 10%MC it would routinely read dew point (condensation point) below 120. The MC of the fuel (along with a couple other factors) determines dew point of the flue gas.
    I had never really seen that illustrated so graphically and found it interesting.
  7. jebatty

    jebatty Minister of Fire

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    Could the DOE/EIA.gov website use the 20MM number based on average wood mix? From other sources I see white oak at 24.2MM and white pine at 13.2MM, red maple and paper birch both at 20MM, for example.

    I never have looked into MC data for wood less than 20% MC, so I would say that all of my info is for cord wood assumed 20% MC. Intuitively I can see why things change as MC decreases.
  8. heaterman

    heaterman Minister of Fire

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    I'm guessing that is the case Jim. Using cords rather than weight introduces a lot of variables.

    20%MC is about as low as anyone will see cord wood on a practical basis. Real world for most is more like 25% unless you get a full two years of seasoning. I just brought up the dew point/flue gas temp correlation as an item of interest.
  9. jebatty

    jebatty Minister of Fire

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    I agree on the 20% as a practical matter. This site shows for Mpls-St. Paul that wood exposed to outdoor atmosphere reaches equilibrium MC between about 12-15%. I assume that is covered and not exposed to the elements. Also, cord wood air dried can take a long time to reach equilibrium, and the drying process is not uniform. I have had covered red oak sizzle at 2+ years of air drying, and aspen and some pine appear to be at or about 20% with one good summer of air drying.

    http://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf
  10. DaveBP

    DaveBP Minister of Fire

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    Oh Good! I was beginning to think there weren't enough variables in these discussions.

    More camouflage for whatever ( if any ) truth is behind some of the advertising claims.
  11. heaterman

    heaterman Minister of Fire

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    ????????? Camoflage?
    Color me dumb but I don't understand your statement. ;?;hm
    It is Monday morning though and it's possible all my gray matter is not fired up yet.

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