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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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    EDIT: I mis-clicked and left out part of the burn data. Corrected efficiency is 56%. I changed numbers to match corrected data. Balance of post stands, but improved efficiency from dry wood in later posts is blown out of the water.

    Pretty sobering data. I ran a detailed analysis of the live burn that I did two nights ago. Worked out to 48% efficiency. Either I'm missing something big, or I have a lot of room for improvement. Here's how it came out:

    94 lbs wood at average 30% moisture = 65.8 pounds 'bone' dry at 8600 BTU/lb = 565880 potential BTU

    Top floor: 94 minutes at 180 BTU/min = 11,700 BTU EDIT: 17010
    Main floor: 93 minutes at 300 BTU/min = 15,900 BTU EDIT: 27750
    Bottom floor: 71 minutes at 380 BTU/min = 7410 BTU EDIT: 26980

    Hot Tub: 36 minutes at 586 BTU/min = 21,120 BTU
    Hot water: 47134 BTU -> 50979 BTU = 3845 BTU
    Storage: 81717 BTU -> 301817 BTU = 220,100 BTU

    Total delivered heat energy = 316,805 BTU

    Storage BTU is calculated as usable BTU, assuming that 120 degrees is the lowest usable temperature. In the storage tank, the top and bottom temperature probe covers the top and bottom quarter of the tank, while the middle covers the middle half.

    The hot water tank is 40 gallons.

    Hot tub heat load is based on the previously measured ability to raise the temperature of the 550 gallon hot tub by 8 degrees per hour.

    I've clearly lost some efficiency in that the EKO, refractory, steel, and water, is at 140 degrees when the circ shuts down. That heat energy is lost to these calculations, although some percentage of it ends up heating the basement as opposed to going up the stack. This was also a really short burn, with the house needing very little heat and the storage and hot water tanks not depleted at all.

    Am I missing something, or am I really only getting 56%?

    My wood consumption has been right exactly on track from previous seasons - a bit under two cords from October 15th until now.

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  2. wdc1160

    wdc1160 New Member

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    I'll call you crazy if you use the number 8600
    I never use the number 8600 usable btu's-- because the water gets boiled off and that isn't ineffcient on your part -- is it?
    Also on the best day your wood wound't have 8600 with 0 moisture. I think the most I have ever seen manufactuerer use is 7K.




    @ 20% moisture your looking at 5800 btu
    @ 60% moisture 4000 btu

    but,


    Another way to look at it



    Heating Value per Air-Dried Cord in BTUs
    White Oak 30,600,000 Poplar 17,260,000
    White Elm 24,500,000 White Pine 17,100,000
    Tamarack 24,000,000 Basswood 17,000,000
    White Birch 23,400,000 White Cedar 16,300,00
    Black Ash 22,600,000 White Spruce 16,200,000
    Maple 19,300,000 Balsam Fir 15,500,000


    I don't have a source for this info.....

    but my estimate earlier was gross btu's in the 480k range, not 560...
  3. SciGuy

    SciGuy New Member

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    NOFOSSIL,

    As the latent heat of vapoization is 970BTUs per lb of water, the 28.2 lb of water your unit evapoated required an additional 27,354 BTUs to do that task.
    Hmmmmm that bumps you up to 52.5% energy recovered.

    Hugh
  4. wdc1160

    wdc1160 New Member

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    I am also curious how you came to the btu/min absorbtion rates you calculated. I know your probably being cautious/conservative in your calcs, but

    I don't think the absorbtion in your house is going to be a very agressive version of "what could be"

    Recalc with 10-8 btu per min per foot -- Unless your telling me upstairs in your house you only have 18ft of baseboard....
  5. Nofossil

    Nofossil Moderator Emeritus

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    I used 8600 BTU/lb for wood at 0% moisture. I think that number is good. I had 94 pounds of wood at about 30% moisture. If the right number for potential BTUs is not somewhere around 566,000 BTU, then I'm wrong. I started trying to convert it to the equivalent weight if it had been 20% moisture and then using the 6880 number, but I skipped the intermediate step. I think I had about 66 pounds of actual wood in my load.

    I figured that I'm losing a few percent from vaporizing the extra water, but it doesn't come close to explaining the numbers.

    I do have exactly 18 feet of baseboard in my top floor. 30 on the main floor and 38 in the basement. I assumed 600 BTU/ft/hr for baseboards.

    This is the first time I've weighed the wood. What I'm burning this year is no worse than what I've burned in previous seasons. I did a back-of-the-envelope calculation based on degree days, cords of wood, and historical oil consumption and came up with a delivered efficiency of around 65% or 18,000,000 BTU/cord in previous seasons. This year is exactly on track, so this efficiency calculation really surprised me.
  6. wdc1160

    wdc1160 New Member

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    Sorry, I didn't realize. I thought your upstairs was bigger



    According to the cords of woods I listed up above your aproaching 80% of many of the woods listed per cord. Are those numbers too pessimistic in their calcs??

    In order to calculate getting 65% efficiency from a cord @ 18M BTU. You would have to assume 27.7M BTU per Cord?

    What I have
    is 56% effcient. from your 94 lbs of wood. I figured 5100 btu with your wood types.

    Where do you think your losing it??? or are we figuring usage incorrectly?
  7. webbie

    webbie Seasoned Moderator Staff Member

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    I think you have to adjust for the moisture before you even start figuring on the burn......at least to be fair to the wood!

    20% moisture is considered to have 7000 BTU per pound
    So, if the scale remains the same, 30% moisture would have about 6200 BTU per pound input.

    Additionally, since most units are not built to handle high moisture wood, the burn could be worse than just that. I found that with Tarm boilers, a moisture content over 25% lowered the firebox temps so much that the whole thing was tough to operate. In other words, then the right calcs are done, 48% might seem GOOD.

    Your moisture adjustment seems to consider that the moisture burns off "for free".

    Check the work.
  8. wdc1160

    wdc1160 New Member

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    To be clear. Craig Nofo -- You didn't burn white oak -- If your going to calc 6200 btu per pound we have a very large dispersion in what we think would btu per pound should be.
  9. Buck1200

    Buck1200 Member

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    New guy here... Lurker... love this type of stuff. I also wonder about the measurement of heat delivery. Would you be better off monitoring flow rate and delta T straight from the boiler, rather than attempting to calculate delivery at 5 or 6 downstream destinations? I realize that requires yet another analog in, but it might relieve you from what I see to be quite a few sources of measurement inaccuracy.
  10. webbie

    webbie Seasoned Moderator Staff Member

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    Explain.

    From what I read, air seasoned wood has a normal BTU input of 6200-7000 BTU per pound after adjustment for moisture. It is also claimed that the higher BTU woods (if any) are resinous (softwoods...that is why folks like softwood pellets).

    So if 6200-7000 at 20-24% moisture, I would assume the lower end at 30%.

    In the case of these fuels, I think it is best to always be very conservative in terms of numbers.
  11. Nofossil

    Nofossil Moderator Emeritus

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    Works out perfectly. I calculated that 94 pounds of wood at 30% moisture would yield 566,000 BTU. That works out to 6021 BTU/lb, so my 48% efficiency figure stands. I use really dry wood (<20%) to start the fire, and burn the greener wood once it's going.

    Gasification is sustained and vigorous throughout, with secondary combustion chamber temps running 900 - 1200 outside the labyrinth. No creosote, no visible smoke, and no odor at all.

    I'm quite sure I'd do a bit better with drier wood, but it's not damp enough to interfere with gasification as far as I can see.

    My flue temps are around 500 degrees - much higher than I expected. Have to get moving on those turbulators.

    I'm curious about the real world difference between combustion efficiency and system efficiency. Could I be running 80% combustion efficiency and losing a bunch to heating of the boiler itself, piping losses, and so on? Anyone else have any data on system level efficiency?

    I'd love to add a flowmeter, but I don't have one at the moment. Donations cheerfully accepted ;-)
  12. webbie

    webbie Seasoned Moderator Staff Member

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    It's great to fiddle with this data, because it shows the folly (which I am constantly trying to point out) of people talking about their Pellet Stoves being over 80% efficient their wood stoves being the same. There is a vast different between combustion, steady-state, AFUE and then the REAL AFUE (such as Energuide from Canada)....and, then, the actual delivered efficiency to a single point (indoors of the home).....

    In theory, if a wood stove is making one room 78 degrees, that could be a big loss if the room does not need to be that hot...if you only wanted it to 70. The extra 8 degrees becomes a waste.

    As an example of this, look at the canadian Energuide ratings of freestanding gas stoves. There we are dealing with a very stable flame and heat exchange system. Some unit which are rated at over 80% efficiency by their makers, come in at 60% in Energuide.....that is a 25% difference on the way down, or a 33% difference on the way up - substantial.

    I get the feeling that folks think I am being pessimistic when I always figure LOW for the claimed efficiencies of much of this stuff. Yes, the turbulators will help and I think a couple runs with 20-22% wood might make a big difference also....but if your existing numbers are accurate, it might be hard to get to over 60% system efficiency.

    I think the boiler itself may exceed 80% in steady state when it is running. After that, everything goes downhill.
  13. wdc1160

    wdc1160 New Member

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    According to the oregon dept of ag.-- department of measurments standards division
    dry hickory has 6.4k btu @ 20 percent
    dry black locust "I don't know what locust was used" @ 20 percent = 6.8Kbtu <- didn't realize this was so high

    @ 30 percent you could do 6k. thats believable.
  14. Nofossil

    Nofossil Moderator Emeritus

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    I teach my clients that they should look at problems as 'gold', because the only way to get better is by finding problems and solving them. I'm looking at this as a problem, and trying to convince myself that it's a good thing in the spirit of practicing what I preach. So here goes:

    If my 48% number is correct or even close, that's a good thing because there's a lot of room for improvement. I'm currently burning about 4.5 cords/year to completely replace my oil from mid-October to mid-April for heat, hot water, and hot tub.

    If that's at a system efficiency of 48%, then it might be reasonable to think that if I can get to 65% or so, I could get by on 3.5 cords per year. That would be amazing.

    So.... where am I losing 50% of my potential BTUs?

    - 15% heat up the flue and incomplete combustion based on EKO claim of 85%
    - 5% additional loss due to latent heat of vaporization for additional water

    Where's the other 30%? That's about 180,000 BTU during the course of this fire - not peanuts.
  15. leaddog

    leaddog Minister of Fire

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    maybe I missed it but I think there is a very sizable loss of heat from the boiler and piping. I know that if you stand very close to the door or put your finger on the edge you will notice. They are insulated but there is parts that aren't and they radiate alot of heat. I have mine is an uninsulated shed and there is a noticeable difference inside and outside.
    leaddog
  16. cbrodsky

    cbrodsky Member

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    Interesting thread - I haven't been following all the details, but I'm curious to understand more on how the storage value is measured since it seems to dominate the production side of the calculation.

    As for delivery loss, assuming you have a standard baseboard system that is turning on/off regularly, you might be able to estimate some value for how much piping you have going to the zones through unconditioned space, then assume it goes to ambient after each heating cycle, and see how many BTUs are being lost to that water/copper cooling down on each cycle. I've never calculated it - but it's probably simpler than calculating forced air delivery losses.

    -Colin
  17. kuribo

    kuribo Feeling the Heat

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    [/quote]


    "My flue temps are around 500 degrees - much higher than I expected. Have to get moving on those turbulators.

    I'm curious about the real world difference between combustion efficiency and system efficiency. Could I be running 80% combustion efficiency and losing a bunch to heating of the boiler itself, piping losses, and so on? Anyone else have any data on system level efficiency?"

    [/quote]


    I would think that it would be easy to have 80%+ combustion efficiency and only half of that on a system basis in light of pumping losses. frictional losses, less than perfect heat exchange, heat losses through piping, storage tank, etc., AND 500F flue temps. A lot of heat is being lost up your flue; if your temps outside the combustion chamber are 1200F, you are losing maybe 20-30% up the flue... . The 90%+ efficiency gas condensing boilers have extremely low flue exhaust temps....
  18. leaddog

    leaddog Minister of Fire

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    If you take your ir thermometer and take some readings all over your boiler you will get some very sobering readings. Be sure and check the bottem legs and top surface. I put some foil/bubble/foil on mine and dropped the readings down but I haven't followed up on it more because of lack of time. maybe I should. There is alot of surface area there. You have it inside so it isn't "heat loss" but I think you will see that it is something that you need to figure into your figures.
    leaddog
  19. Buck1200

    Buck1200 Member

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    Since nofossil's boiler is within the conditioned space, these losses add to the heating of his house, and so should not necessarily be considered 'wasted'. Counting them up and controlling them (this heat energy) is another matter, however. This is where knowing the mass flow rate and composition of your flue gas, as well as accurate temperature, of course, over the entire burn cycle from dead cold to blazing hot to dead cold, and subtracting that from the theoretical energy capacity of the wood being burned would be the only way to accurately determine the absolute efficiency of the boiler, at least as far as liberating heat energy from the wood is concerned. Again, you might not know where it's all going, but if it's not going up the stack then at least it's heating the house.
  20. Eric Johnson

    Eric Johnson Mod Emeritus

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    That's an excellent point, leaddog. My little boiler room probably stays around 120 from boiler/chimney connector radiant, and the vermiculite-filled cinderblock wall is noticeably warm to the touch. The wood stacked against it is extra dry.

    Did you factor the heat produced by your boiler/stovepipe/chimney into your calculation, nofossil? You're recovering a lot of radiant heat that I'm not.

    By the way, I agree that a lower-than-expected number is nothing more than an opportunity for improvement. Sounds almost Buddhist. That's my old song about the U.S. energy situation: We waste so much energy that just tightening up our wasteful habits would generate a huge drop in demand, with no perceptible effect on our (relatively lavish) lifestyles.
  21. Nofossil

    Nofossil Moderator Emeritus

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    I'll take a crack at it, but the heating zones didn't cycle much during the burn.


    I don't seem to see so much heat from the boiler and stovepipe. My boiler room is in the basement, and it's pretty much the same temp as the rest of the basement.



    I don't know what flue temp EKO used when calculating their 85% or 87% efficiency. I'm sure that you would need really low flue temps to get above 90%, but I don't think the ratio of combustion and flue temps tells the whole story - a lot of heat comes from the primary chamber as well. If I had a way to estimate the flue gas flow rate, I could calculate the losses there.

    Maybe the 180,000 missing BTUs are scattered around, a little here and a little there...



    I'm doing another burn tonight. As it happens, it's MUCH drier wood - about 20% average this time. Also smaller pieces.
  22. Bob Rohr

    Bob Rohr Minister of Fire

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    A btu meter on the output of your tank to the distribution system would be a great place to measure DELIVERED energy. I think Caleffi has one about to be introduced. It's designed for solar, not sure the capacity.

    BTU metering is very common in Europe, both for district heat systems and apartments.

    Most are located in a cabinet in the hallways. They measure cold water, heat, and sometimes DHW.

    In some rental properties you swipe a credit card to prepay an amount of water. Even the drinking fountains in Germany, when you can find one, have water meters.

    You think oil is expensive, just wait. Water will soon be the most precious resource.

    hr

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  23. kuribo

    kuribo Feeling the Heat

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    "I don’t know what flue temp EKO used when calculating their 85% or 87% efficiency. I’m sure that you would need really low flue temps to get above 90%, but I don’t think the ratio of combustion and flue temps tells the whole story - a lot of heat comes from the primary chamber as well. If I had a way to estimate the flue gas flow rate, I could calculate the losses there."


    There are several different ways used to calculate "efficiency". You might talk to your manufacturer to find out which method they are using...steady state efficiency. cycle efficiency, or combustion efficiency....

    By the way according one text, using wood with a high moisture content can cut the efficiency figures in half...
  24. Nofossil

    Nofossil Moderator Emeritus

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    New fire burning now with dry wood - check it out using 'voyeur' link in my signature. Secondary combustion and flue temps are both higher. Way too soon to calculate efficiency, but it will be interesting.

    One thing that's easy to see in the graph is the seven minute match-to-gasification time. When I close the lower door and turn on the fan, secondary combustion jumps to 900 right away, and is over 1200 within 15 minutes.
  25. brad068

    brad068 Feeling the Heat

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    You know NoFo, your graphing reminds me of my job. I use a SCADA system to monitor DO, temps, RAS rates, cycles, etc. If only I could employ this system to my boiler ops.
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