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48% gasifier efficiency - Ooops, 56%

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Nofossil, Jan 13, 2008.

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  1. Nofossil

    Nofossil Moderator Emeritus

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    I'm getting turbulators made, and trying to get a quote from Zenon as well.

    This design is great - I considered inserts that would have a similar effect, and be thermally coupled to the tubing walls. I think cleaning the fly ash would be a challenge.

    Here's a photo - the page seems to change and didn't have the article when I looked at it.
    [​IMG]

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  2. barnartist

    barnartist Minister of Fire

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    That looks great! but maybe a few less fins, and a spiral motion, but yeah, to clean the things. Man, that is VERY interesting.
  3. TCaldwell

    TCaldwell Minister of Fire

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    barnartist, I know a boiler maker in ny. that has experimented with 6'' dia auger bits slid into horizontal heat exchanger flue tube, claims it reduces the need for about 3 times its length of conventional heat exchange pipe. to clean, just pull out of the pipe!
  4. Eric Johnson

    Eric Johnson Mod Emeritus

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    A couple more points on turbulators:

    Joe Brown made the point awhile back that turbulators should be made of some kind of tempered (or maybe it was annealed) steel so that they don't warp in the tubes and get jammed up so that you can't slide them out.

    Slowzuki's JetStream gasifier has little stainless steel turbulators that, as I recall, are about four inches long and slide into the ends of the tubes. Ken said that they're not hard to make and that he has the pattern (or design). I'm not sure if they could be adapted to the EKO, however, since I suspect they go into the bottoms of the tubes, not the tops, which are easily accessible on the EKO. But I'm sure you could modify the basic design to work.

    The turbulators-as-hx-cleaners that come on the current EKO models is such a cool design that I'm surprised that it's not an option on other high-end gasifiers like the Tarm. As far as I know, only the EKO, Econoburn and BioMax do it that way. All I can say is that it sure beats cleaning them by hand.
  5. slowzuki

    slowzuki Feeling the Heat

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    Although we only pulled them part way out, they are actually about 2 ft long. They are just twisted strips of sheet metal with the right clearance to pull out of the boiler tubes. They do go in from the top and have a little hook to keep them from dropping too far.

  6. Nofossil

    Nofossil Moderator Emeritus

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    I got my turbulators. Haven't had a chance to take pictures or install them yet - probably Friday. That will give me a chance to do a comparison run lloking at flue temps. Detailed efficiency will have to wait a bit longer.
  7. jebatty

    jebatty Minister of Fire

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    Reviewed the thread and a few comments and considerations.

    Wood moisture content as related to efficiency
    There is a factor which I did not see discussed which is the extra energy required to reduce wood moisture from around 25-35% to a lower level. Moisture in wood is not the same as a wet sponge. Water in green wood exists between the cells and within the cells. The water between the cells is pretty much free water and is lost relatively quickly, resulting in an initial moisture content of around 25-35% depending on species. This is similar to a wet sponge. To reduce water content further, the water bound up within the cellular structure needs to be released. This takes extra energy. It is somewhat similar to the extra energy needed or released in a phase change. There are resources on kiln drying of woods that explain this in detail and likely provide energy calculations. My understanding is that this extra energy is material and if not considered will result in an otherwise unexplained error factor.

    Stack temp vs efficiency – heat loss up the chimney
    This source cites a 1% efficiency gain for each 40 degree reduction in stack temp, as might be achieved through use of tubrulators.
    http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Turbulators.asp
    Another source, which now I cannot find, claimed each 100 degree drop in stack temp related to a 5% efficiency gain. I assume there is a "sweet spot" in these calculations and the number changes above or below a specified stack temp.

    Efficiency related to water temp
    This source states that system efficiency rises as water temp falls, and to achieve maximum system efficiency to achieve the purpose of the boiler: For hot water boilers used in a space heating application, reduce water temperature to the lowest temperature that will meet the demand
    http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Efficiency_Tips.asp

    Interesting sources on boiler efficiency and turbulator science
    http://mistug.tubitak.gov.tr/bdyim/abs.php?dergi=muh&rak=97077
    http://www.wargaboiler.com/services.html
    http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=5782217

    Maybe some hyperbole or an agenda here, but some interesting points on efficiency in general
    http://www.boilerspec.com/EmmisEffic/boiler_efficiency_facts.pdf
  8. Eric Johnson

    Eric Johnson Mod Emeritus

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    Great information and observations, Jim. Gracias!

    I could be wrong, but while your observations about bound vs. free water in wood is correct when it comes to air- or kiln-drying wood, I'm not sure that the same logic applies to wood being burned. It seems to me (just thinking about it), that once you toss that chunk into the firebox, water is water and wood is wood......
  9. jebatty

    jebatty Minister of Fire

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    Eric, what you say seems to make sense on a first look. But the cell wall needs to breakdown to release all of the water, and that takes energy.

    Wood moisture content as related to efficiency
    I think I was in error though in drawing the distinction as water between the cells and water in the cells. The proper distinction is water in the cells (free water) and water in the cell walls (bound water). The bound water is chemically attached to the cell wall. It is the release of the bound water which requires extra energy. Drying Lumber Hardwood, USDA, General Technical Report FPL-GTR-118, pg 2.

    Because MC of wood cells varies throughout the log, on average approximately 1,100 BTU/lb (2.6MJ/kg) of water are required to evaporate water from green wood. Same source, pg. 2.

    Using this number and first calculating the MC in the wood being burned, and converting that to lbs, one can calculate the approximate BTU's needed to exhaust all water from the wood during the burn process.

    Relative humidity as related to efficiency
    Another factor which I did not see discussed is the effect of relative humidity on efficiency. The higher the relative humidity, the more energy will be required in the burn process. This is one more variable, or error factor, to be considered in determining various levels of efficiency.
  10. slowzuki

    slowzuki Feeling the Heat

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    Just found Dr. Hill's tables of energy content in wood, I'll type out a few:

    Available Heat Per Cord in Millions of BTU

    Ash, Green-16.5, Air dry-20.0
    Aspen/Poplar - Green-10.3, Air Dry-12.5
    Red oak - Green 17.9, Air dry - 21.3
    White Pine - Green 13.1, Air dry - 13.3
    Douglas Fir - Green 13.0, Air Dry 18.0

    There is no desription of the calc to arrive at these values though.
  11. wdc1160

    wdc1160 New Member

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    slow where were you 3 1/2 weeks ago. Is that how you got the name slowzuki?
  12. slowzuki

    slowzuki Feeling the Heat

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    3 1/2 weeks ago I was working 80 hours a week at work and plowing 10-15 hours a week at home and driving 10 hours a week to get to work!
  13. FishHawk 210CC

    FishHawk 210CC New Member

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    I'm new to this site and have been reading up on the discussion of overall efficiency of the boiler. Another way to look at this is based strictly on flue gas temps and how it relates to the process of heat extraction.

    You begin with wood and air at 70 deg F, utilize the energy of the wood (minus the heat for the mositure, wood quality, etc) and heat this mass up to a final temperature of 1800 deg F and then begin the process of recovering the heat for use in the multiple uses around the house. Your exit temp is 600 deg F

    1800 - 600 is a delta T of 1200 deg of temperature removed. This would yield be a maximum efficiency of 66% without taking into account boiler heat losses, moisture in the wood, combustion efficiency, etc. If you are getting 56% out a possible 66% you are doing well @ 85% capture of heat not going up the chimney.

    If you decrease your flue temp to 200 deg F your potential efficiency would go up to 89% from 66%.
  14. buickpwr

    buickpwr New Member

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    Wow what a read, some guys here have it figured.

    I need the magical btu/lb number and finally found it, here goes;

    - 8660 btu/lb may be possible in a laboratory. problem is the number is often used in factoring outputs and
    efficiencies of appliances/boiler

    This "high heat value" is only obtained in perfectly dry wood 0% moisture and only in an environment of pure oxygen.
    so our practical world 8660 is completely unrealistic.

    After being cut split and stacked to sit for a year or two you reach about or around 20%.
    - 1.25 lbs of well seasoned wood(20%) is made up of;
    1 lb of fiber and 0.25 lb of water resulting in 6928 btu/lb (8660 x 0.80) of actual total weight.

    problem of 0.25 lb of water in the piece?
    - evaporate the water and raise it to the flue gas temp.

    - additional water in the air used to combust our piece.
    - additional water/moisture produced as a by-product of the combustion cycle
    (hydrogen atoms from the wood combine with the oxygen atoms in the induced air to form water vapour).

    By using the High Heat Value (8660 btu/lb or 6928 btu/lb 20%) by way of thought that all latent heat can be recovered to produce more
    useable output... so in reality to use those figures you would have to have flue gas temp exhaust at the temp of the initial incoming
    induced air, which this would be near 0 degrees F.
    The latent heat put into the 3 water vapours will be ignored due to not practical in recovery and removed from calculations.

    So account for the Latent heat effect. because its much closer to the actual conditions of actual use that we go thru daily.

    1050 btu to boil/evaporate 1 lb of water
    1 btu additional is needed to raise the pound's temp 1 degree

    Total weight of moisture given off by fire:
    .25 lb of moisture in piece
    add about .54 lb of water vapour as product of combustion
    @ 60 degrees have 0.79 lbs of water vapour
    Heat to flue temp of say 400 degree
    0.79 x [1050 btu x (400 - 60)] = 1098 btu per 1.25 pound piece; and 879 btu/lb (1098 x 0.80)

    6050 btu/lb (6930 - 879) is show to be realistic and that is what the average Joe should use if he want to weight his wood and toss
    it in the firebox, seeking some boiler efficiency numbers that are accurate and real.

    Be careful of charts described as output per cord or pound of wood. There lies a assumption that is based upon some efficiency of
    the boiler used, when assuming and stack temp changes etc then the chart figures become inaccurate. or your boiler has to match test unit
    in-order to use chart.

    Comparison using High heat value or Low heat value; LHV gives 8% higher than the same results using HHV
    example 80% boiler efficiency (LHV) using 6050 btu/lb is 74% efficient using (HHV) 8660 btu/lb

    Lastly
    Green piece say 50% that weights 2 lbs
    2 lb piece = 1 lb wood fiber = 8660 btu
    1.54 lb of water to vapourize and heat ( taking away 2200 btu)
    2 lb piece has a net available energy of 6460 btu or 3230 btu/lb

    reference: Technical Series; The amount of energy in wood

    thx
    just some thoughts on everyone using the same factor would be nice
  15. Nofossil

    Nofossil Moderator Emeritus

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    Nice summary.

    I include the latent heat of vaporization since it's possible to reduce the amount of water in the wood, and it's possible to build condensing boilers.

    I take the actual weight of the wood, subtract the weight of the water, and multiply the result by 8660 to give me the theoretical heat value. To actually attain that, you'd have to have 100% perfect combustion and the flue temperature at ambient - obviously not possible, but a worthy goal nonetheless.

    By this criteria, the EKO 80 can achieve 90% efficiency under ideal conditions.
  16. jebatty

    jebatty Minister of Fire

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    nofo -- when you have an extra minute or so, look at this site on boiler efficiency. It deals directly with coal, but you might find it useful anyway.
    http://www.firecad.net/boilerblog/
  17. Nofossil

    Nofossil Moderator Emeritus

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    Thanks - nice link. Tmonter sent me a spreadsheet that he uses to do the same calcs for other solid fuel boilers.

    Eric was threatening to create a sticky thread with links to other high-value threads. Might be nice to collect efficiency related resources like the one you referenced.
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