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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. heaterman

    heaterman Minister of Fire

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    Raw Combustion efficiency vs system efficiency

    Combustion efficiency is relatively easy to measure if you have the tool I.E., a combustion analyzer.

    System efficiency, which is what nofossil is trying to measure is a completely different animal and orders of magnitude harder to calculate. There are a host of areas that present parasitic losses on a system, some of which are mentioned in the posts above.

    Trying to calculate system efficiency would involve starting with your combustion numbers and then measuring heat transfer throughout the entire system. This is virtually impossible to do because the whole time you are measuring heat output into your structure in order to compute temp rise, the structure is at the same time losing heat into the great outdoors. So how do you actually know the amount of heat you put into the structure? (This factor alone may be what is skewing your numbers so low.)
    Tough tough tough to do. You're talking lab grade equipment to do the measurements with and a Cray to crunch the numbers.

    One could roughly estimate what's going on if you have a good handle on your flow rate and temp drop from supply to return. This will give you an approximation of the total heat transfered outside the boiler. Then you'd have to factor in the jacket losses of the boiler, the cubic feet and net temp rise of your flue gas going up the stack. Actual btu content of the fuel being burned is obviously a matter of differing opinion...............but you'd have to start there to determine actual input. Without that as your starting number you're just "blowing smoke". (feeble attempt at joke there) AHHHHHHHHHHHHHHHHHHH!!!!! my head is starting to spin just thinking about it.

    I went to a training class on Riello burners last year and while there talked to a couple of their lab guys doing an AFUE rating test on a boiler with their burner installed on it. They spent the better part of 4 hours just dialing in the flow rate to get the 20* drop called for in AFUE testing protocol. AFUE specs call for testing at 140 supply and 120 return. (Not reflective of 99% of the boiler systems actually out there but hey.....it's a government spec so what did you expect.....) At that point you can begin to measure what the actual output of the boiler is. Next they hooked up their gear to precisely measure fuel input (this one was oil) which was measured to the milligram.........you get my drift........ Then there were flue gas measurements which were factored in to the equation. And on and on and on and this is just measuring boiler/burner efficiency. All I can say is in less than lab conditions, I don't know how you would get anything close to valid.

    Gotta say it is fun exercising the ol noggin thinking about it.

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  2. Nofossil

    Nofossil Moderator Emeritus

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    So... is that a good thing or a bad thing?

    I'm starting out trying to look at it as a 'black box' - I know what goes in in terms of fuel, and I can measure to some reasonable degree of accuracy what comes out as usable heat energy.

    I've long since given up trying to analyze every detail, but I'm hoping I can get my hands around any chunks of missing energy that are big enough to be interesting. I'm hoping to learn, and I'm hioping to share whatever I get out of this with the community.
  3. Buck1200

    Buck1200 Member

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    One thing you could do to measure the losses from your piping and boiler would be to calculate the steady state heat loss of your basement and, assuming it's the same temperature as the rest of the house, you'll likely find that the number of btu's heading that way through the distribution system is significantly less than what is actually required to maintain that temperature. The difference is your losses from the boiler jacket, piping, and smoke pipe (as well as light bulbs, transformers, pets, etc).

    BTW- I've been through your site from top to bottom: great job.
  4. barnartist

    barnartist Minister of Fire

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    Nofo,
    Im really glad your working on this. At the same time, you heat more with your tiny stove than me with my monster, and I used freakin 5 cords of wood already, AND I did not start burning until Halloween, AND I have turbulators!!!!!! Id give my left !@%&^ to only burn what you burn.
    Keep working on the "problem" so I can learn from it man. Ypu house must be R-1000.
  5. bbb123

    bbb123 New Member

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    Thats close to the % I got NoFo when I weighed the wood. I came up with around 50% when I did it.
  6. Nofossil

    Nofossil Moderator Emeritus

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    That's encouraging. Now if we all can figure out how to close the gap between combustion efficiency and system efficiency, life will be better.

    Here's the data from last night's burn. First hard data that I have that might start to quantify the difference between dry and really dry wood.

    119.5 lbs wood at average 20% moisture = 95.6 pounds ‘bone’dry at 8600 BTU/lb = 822,160 potential BTU

    Top floor: 182 minutes at 180 BTU/min = 32,760 BTU
    Main floor: 214 minutes at 300 BTU/min = 64,200 BTU
    Bottom floor: 188 minutes at 380 BTU/min = 71,440 BTU

    Hot Tub: 85 minutes at 587 BTU/min = 49,867 BTU
    Hot water: 47134 BTU -> 50979 BTU = 12,507 BTU
    Storage: 81717 BTU -> 300619 BTU = 233,608 BTU

    Total delivered heat energy = 464,382 BTU

    System efficiency works out to 56%. I would have thought that wasn't too good if I hadn't gotten 48% on the previous burn. This works out to about 16% more delivered energy per pound of bone dry wood, and a 32% increase in BTUs per 'as measured' pound.

    Dry wood was not the only difference. For this burn, the house, outside temp, hat water tank, and storage tank were all cooler. The burn was longer as well. with about 50% more actual fuel.

    This was the driest wood I've burned this year. It created another problem: The delta T through the boiler was so high that when the controller tried to keep the inlet above 140, it caused the outlet to exceed 180, which shuts down the EKO fan.

    I think this means that I need to either install my three speed Grundfos which at high speed pumps more than my Taco 007, or I need to throttle back the EKO when burning wood that's this dry. Am I on the right track here?

    Attached Files:

  7. atlarge54

    atlarge54 New Member

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    I've wondered about nofossil's home insulation, windows etc. All the glass people I talk to say don't buy the argon filled windows, they claim seals will go bad. Are those flat roofs? I can't recall, wasn't living space over 3000 sq ft.?
  8. bbb123

    bbb123 New Member

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    Nofo I was lookin at your graphs your starting your fire when tank is around 120 and it starts idleing a couple hours after that. I start mine at around the same temp and mine will not idle until the second load of wood and tank gets up around 170 top 160 bottom (4-6 hours). I believe I'm running the basic taco cirulator I'm thinkin you HX in your tank isn't workin fast enough. Or maybe your circulator isn't running 100% it would def. be easier to change that out. What's the gpm rating on these circulators?
  9. slowzuki

    slowzuki Feeling the Heat

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    The 85% would be a steady state efficiency of the boiler, a short burn cycle would be considerably less. This efficiency would not be seen without turbulators, 500F flue gas is tossing lots of btu's up the chimney.
  10. Eric Johnson

    Eric Johnson Mod Emeritus

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    Would it be possible to attain and sustain a steady state of operation with one of these boilers, or does the need to refuel periodically practically blow that possibility out of the water?

    And presumably, your efficiency will vary during the course of a burn cycle. I'm guessing it's a lot lower at the start than in the middle or the end, just because of the moisture content of the wood.
  11. Donl

    Donl Feeling the Heat

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    I may have this all wrong, but the way I'm seeing it is that yes the potential for 95.6 pounds of bone dry is 822,160 BTU. However, I don't see where there is a loss included to account for the 23.9 ponds of water that needs to be taken care of.
  12. Nofossil

    Nofossil Moderator Emeritus

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    I'm using a Taco 007. These circs are very sensitive to pressure drop. I'm planning on replacing it with a Grundfos 3 speed, which is less sensitive and should pump a bit more at its highest speed. I'd agree that my in-tank HX could be bigger. I'm thinking of a retrofit where I form a bunch of copper fins that clip over the HX tubing.

    There's been some discussion of HX losses. In a system like this, those aren't really losses - they're just failure to move energy from one place to another. The energy in this case doesn't exit the system except to the extent that a higher boiler temp results in higher flue temps.
  13. Nofossil

    Nofossil Moderator Emeritus

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    Another data point - last night's fire (1/14/08).

    Ran a much shorter fire last night - 70 lbs of wood at 20% moisture. Bottom line - 56% again. I did make a small software change to try and reduce the intermittent idling so it's not a perfect comparison, but it looks like moisture makes more of a difference than short vs. longer burns.

    Got some cold weather coming, so maybe I can get some data on a longer burn. Might have to give back the bathroom scale, though.

    As of last night, I've burned exactly 2 cords since October 15th.
  14. barnartist

    barnartist Minister of Fire

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    im going to weigh my wood I load in today and try to borrow your numbers. If your 56%, I must be 10%. I dont have a meter though.
    Your right some cold on the way. I wonder what effect insulating the bottom of the boiler would have on efficiancy?
  15. Nofossil

    Nofossil Moderator Emeritus

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    My bad - I messed up on the initial calculation and left out some data. Actual efficiency was 56%, not 48%.

    Still lots of room for improvement, but it raises an interesting question. I thought that the dry wood was giving me better numbers, but apparently not.

    Wood at 30% moisture - 56.0%
    Wood at 20% moisture - 56.5% burn 1, and 56.6% burn 2.

    Nothing if not consistent......

    Hard to get excited over 1/2%.

    I'm doing a longer burn tonight, and I closed the fan inlet shutter from 1.35" to 1.25" hoping to reduce overtemp idling.

    Turbulators next!
  16. barnartist

    barnartist Minister of Fire

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    what did you notice Nofossil as far as ash and such with the 30% wood? Much difference compared to 20%?
  17. slowzuki

    slowzuki Feeling the Heat

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    I think you should find you're heat output is much lower on wetter wood but the calc, once accounting for the water, should show about similar efficiency. With your boiler idling all the time the dry wood may even penalize you a bit more.
    Ken
  18. Nofossil

    Nofossil Moderator Emeritus

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    You're exactly right. I'm deducting the weight of the water before calculating the fuel value.

    I've been preaching the gospel of really dry wood, but based on this one data point, perhaps there's very little actual advantage to having wood that's dryer than 30%.

    I ran a longer burn last night, still with dry wood. I closed down the fan shutter a bit (from 1.35" to 1.25") and made a software tweak to try and reduce idling. Still got idling, but system efficiency was 58.5%. Flue temps were down slightly.

    I'm having a hard time resisting the temptation to change several things at once, so this isn't as scientific a series of experiments as it might be.
  19. slowzuki

    slowzuki Feeling the Heat

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    I don't think you're reading into it right, the efficiency is based on the max theroetical energy you can get out of the wood and what you actually got out. So wet wood the max you can get out of a pound of it is lower. So burning wet wood wastes wood, even if your boiler can burn it efficiently!

    Trying to think of how to state this right, there is probably a wetness of wood say 150% where all the energy in wood fiber would be used to evaporate the water in the wood, so there is 0 net energy available in the wood. If you could theoreticly get this wood to burn, and get no heat off it, you would have a 100% efficient boiler with no heat output, you follow?
  20. Eric Johnson

    Eric Johnson Mod Emeritus

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    You would have 100 percent combustion efficiency and 0 percent heat transfer efficiency, right?

    What would the "overall efficiency" of the boiler be in that instance? One way to look at it would be 50%, but we know that can't be. I'd have to say zero overall.

    I agree that more moisture = less efficiency. I don't see any way around it.
  21. Nofossil

    Nofossil Moderator Emeritus

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    You're right about getting less out of a pound of wood. However, my question is how much benefit there is in getting an individual piece of wood dry before burning it. Of course, it weighs less when dried. Do I get mor eusable heat out of it?

    I assumed that a log burned wet would generate less usable heat than the same log burned after drying. Note that the dry weight of the log is the same either way, and the dry weight is what I use to calculate potential heat energy.

    It's not a huge difference, by the way, but I expected noticeably better efficiency with dry wood. I've also assumed that the excess moisture would dampen the secondary combustion - it certainly makes it hard to get secondary combustion started. As you point out, there has to be a moisture level high enough to cause problems. All I can say is that 30% doesn't seem to hurt.

    However, the little data that I have suggests essentially no difference in usable heat from the same log at 20% or 30%. I really need to run another few burns with 30% wood. I just don't have any right now.

    I've actually put a lot of time and effort into planning to cut and store wood covered two years in advance. Maybe there's no reason to do so.
  22. wdc1160

    wdc1160 New Member

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    You may be onto something big here nofo

    I have read a great number of threads by people who swear their 2 year old wood is the efficiency king. Maybe not in your boiler -- maybe its an EKO thing?
  23. Nofossil

    Nofossil Moderator Emeritus

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    I'd be cautious about overstating conclusions at this point - we have ONE data point for wetter wood, and that's 30% - hardly dripping. Actually, most of the wood was 32-34%, but I had some 20% mixed in. It's seasoned for at least a year. In the case of the locust, it's seasoned 8 years (uncovered) before being split, and half a year (covered) afterwards. Don't want to get carried away and assume that we can burn it the same day we cut it.

    I guess it's fair to say that I don't have all the answers yet.
  24. wdc1160

    wdc1160 New Member

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    Yah, I hope no one threw away their old wood or even traded it in for newer wood.
  25. slowzuki

    slowzuki Feeling the Heat

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    On a lb basis completely dry, you get more energy out of dry wood. Evaporating water uses a lot of your energy.

    Eric, combustion efficiency is basically how much fuel gets xfered completely to H20 and CO2 and how much fuel is lost out the stack as CO and soot and unburned tars and HC's.

    Heat transfer efficiency is how much of the energy inside the boiler combustion area makes it out into the water (outlet of the boiler I think is fairly std to account for heat loss of the boiler).

    So to keep with the same example, my extra wet wood that only had enough energy to evaporate the water in it, we can't evaluate the xfer efficiency because we had no spare heat to xfer. But I said all the wood was converted to heat so we had 100% combustion efficiency.

    I know this is a made up situation but it is easier to understand that way. I'll try to come up with a good example here.
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