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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. slowzuki

    slowzuki Feeling the Heat

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    Ok a quick example/problem, skipping the data conversions:
    100 kBTU of dry basis wood is burned at 95% combustion efficiency. 20 kBTU is required to free the water contained due to the latent heat of evaporation and the specific heat of the same water. With a 95% efficient heat transfer to the boiler jacket (assume no heat loss in the boiler), how many kBTU are delivered to the water jacket? What is the total efficiency of the conversion of the wood to usable heat?

    100 kBTU x 0.95 = 95 kBTU
    95 kBTU - 20 kBTU = 75 kBTU
    0.95 x 75 kBTU = 71.25 kBTU Delivered to water jacket

    71.25 / 100 = 71.25% Of the energy in the wood is useable.

    Now as I understand, most of the methods account for the water due to latent heat so you can compare two devices using different fuel.

    Now remember, there is also the water from the combustion to deal with as well. Natural gas condensing boilers condense out the water from combustion to gain most of the few % efficiency they gain. We have a wet fuel to begin with. I've got to read up on my thermo studies again, been awhile. We even had to compensate for if it was 0% to 100% humidity in the combustion air being supplied.

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  2. Nofossil

    Nofossil Moderator Emeritus

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    Moved this to a new thread.

    From the initial post, we have 94 lbs wood at average 30% moisture = 65.8 pounds ‘bone’ dry at 8600 BTU/lb = 565880 potential BTU

    Calculating the energy lost to the water in the wood:

    It takes 1 btu per pound to raise water 1 degree. It takes 970 BTU to boil one pound of water that's at 212 degrees. It takes .5 BTU per pound to raise steam 1 degree.

    We have about 30 pounds of water in our wood. If the wood starts out at 70 degrees and the steam goes up the flue at 600 degrees, it will require the following energy which will be lost to the system:

    30 lbs water, 60 to 212 degrees = 4560 BTU
    30 lbs water to 30 lbs steam = 29,100 BTU
    30 lbs steam 212 to 600 degrees = 5820 BTU

    Total loss = 39,380 BTU at 30% moisture. This represents 7% of the energy available in the wood itself. Thus, I'd expect a 7% loss compared to bone dry wood. Wood at 20% should represent a 4.6% loss.

    The theoretical difference between the energy loss due to water at 30% vs. water at 20% is less than 2.5% assuming the same piece of wood in both cases.

    Not a big number. I should have done the math earlier.

    Of course, if you flip that around, this 7% number could account for a sizable majority of your combustion efficiency losses if you're somewhere near 90%. From a system level, it's not such a big deal. According to these numbers, I need to burn another tenth of a cord if it's 30% instead of 20%.
  3. slowzuki

    slowzuki Feeling the Heat

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    Thats 39,380 BTU of the theoretical max in the log, which is higher than we could hope to extract. Say 50% efficiency getting from log to house, that 39380 becomes a direct 14% difference.
  4. Nofossil

    Nofossil Moderator Emeritus

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    I suspect that the other losses are proportional to the total amount of energy that you get from your combustion. I think that there are three main losses that occur during combustion: Incomplete combustion, flue gas heat, and water related losses. For every 100 potential BTU that you have, you'll lose 7 to water if your wood is at 30%. Another few percent is lost due to incomplete combustion.

    Air has a specific heat of .24BTU/lb. Making a wild assumption that the specific heat of all the non-water flue gas is about that value, we can calculate heat loss up the flue.

    For the original example, 6 pounds of air per pound of wood = 455 pounds of flue gas at 600 degrees = 59,000 BTU of non-water flue gas loss for another 10%. At this point, we're down to around 80% depending on the unburned fuel losses. I actually got 56% measured, so I'm losing energy elsewhere.

    I'm copying this to my new efficiency thread - sorry for the double posting.
  5. kuribo

    kuribo Feeling the Heat

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    at standard pressure and temp, a cubic foot of air weighs about 0.0807 lb/cf. If your blower is providing 100 cfm, that is 484.2 lbs of air per hour the blower is running. How many cfm and for how long does the blower run???
  6. Nofossil

    Nofossil Moderator Emeritus

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    Got no data. It's shut down to a smallish opening (a pair id pie-shaped wedges about 1.25" wide at the ends). I don't have a good way to measure flow, and I haven't been able to find specs on the blower.
  7. kuribo

    kuribo Feeling the Heat

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    100 cfm is pretty small, I think a hair dryer probably outputs more, so I would guess your blower is putting out at least 150-200 cfm....How long does the blower run during a burn?
  8. Nofossil

    Nofossil Moderator Emeritus

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    It runs pretty continuously, but at varying speeds. I also have no idea what the effective pressure drop is. Again, best estimate that I can justify is that EKO might have set it up to be somewhere near the 1.6 x stoichiometric that's supposed to be optimum for gasifiers. Of course, the setting would have to assume something about the surface area of wood that's pyrolizing.
  9. Eric Johnson

    Eric Johnson Mod Emeritus

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    Here's the label on one of my blowers (mine has two of these). I have no idea what these numbers mean, but I bet you guys do. I was pleased to note that they're made in Germany, so they should be relatively easy to replace if it ever came to that.

    Attached Files:

  10. kuribo

    kuribo Feeling the Heat

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    The fellow at New Horizons tells me that he calculated an 85% efficiency at 300F flue gas temp and a 45% efficiency at a 650F flue gas temp when testing the EKO (EKO 25 I think he said...)

    That doesn't answer the question about efficiency and moisture content, but it does show how high flue temps can drastically reduce efficiency....
  11. kuribo

    kuribo Feeling the Heat

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    I have here a factory test report on the EKO 80. It seems it used about 140cfm during their test cycle.....

    Another thing that needs to be figured in is the water in the combustion air....This is also heated to steam and goes out the flue....
  12. Nofossil

    Nofossil Moderator Emeritus

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    That's curious. Given that it's the same boiler in both tests. I wonder what you can do to it that would raise the flue temp and affect efficiency that drastically. I calculate less than 20% total heat loss at 600 degrees flue temp, even assuming that the fan flow rate is twice stoichiometric. Worse still, a too-high fan rate would lower the flue temp, not raise it. Poor combustion would lower it as well. The only thing I can think of that would raise the flue temp and lower efficiency is a thick ash buildup on the HX surfaces.
  13. kuribo

    kuribo Feeling the Heat

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    He said he varied the airflows and "played" with it.....Maybe give him a call and ask him....Zenon at New Horizons....
  14. sled_mack

    sled_mack New Member

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    Make sure you're all talking about the same efficiency. By removing the turbulators and increasing the fan openings, you could get a much higher stack temp - even with the same COMBUSTION efficiency. But, efficiency measured from raw wood to storage tank could be much different. Not knowing what Zenon was referring to, means you have no idea how to compare it to your discussion.
  15. Eric Johnson

    Eric Johnson Mod Emeritus

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    I don't know how inclined you are to hustle for a buck, nofossil, but if I were you, I'd consider putting that thermocouple temp monitoring kit into a marketable package. It's something I'd like to have, but I lack the knowledge and motivation to do it myself. I bet others are in the same boat.

    That's the kind of thing dealers should offer as an option with a new boiler, along with clear instructions on how to dial these things in. I think the more you understand about how the boiler works from the get-go, the happier you're going to be.
  16. wdc1160

    wdc1160 New Member

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    Eric, I am convinced people nearly kill themselves before they figure out they need to make an adjustment or a tweak. Especially if the tweak may be more technical than they are conforatable with. I think that its realistic to believe you could cut your learning curve in 1 year proportional to what it would take someone tweaking 3 years to find. And, even then your more likely to find things the tweaker won't.
    There are 100 sayings like: If you can't measure it you can't control it, if you can't quantify it you really don't know much about it, and you can't measure improvement if you don't measure.
    Flashbacks from every math class I have ever taken.
    I think if you could quantify a boiler system easily its probably worth doing.

    Nofo so what kind of monitoring plans you selling?
  17. Nofossil

    Nofossil Moderator Emeritus

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    Got no plans to sell, but I'll give away my design. I have a thermocouple signal conditioner that I found - the Analog Devices AD595 - that makes it really simple and cheap to get a signal from a thermocouple that reads degrees on a standard voltmeter. Chip is $12. I'll post suppliers and part numbers as soon as I get a few minutes. If there's enough interest, I'll see if I can draft a college student to wire up a few and figure out a package price. Data logging is a little more complicated, but there's a pretty reasonable device called the LabJack that would do the job, I think.
  18. wdc1160

    wdc1160 New Member

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    What will your business slogan be...

    Here at Nofo's Controller's Limited-- if we can't measure it it don't exist.
  19. barnartist

    barnartist Minister of Fire

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    I second what Eric said. I'll buy one right now.
  20. barnartist

    barnartist Minister of Fire

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    Also once again, can a higher flow rate lower the stack temp. Nofossil, im not sure how a higher fan rate lowers stack, do you mis type that? I think Zenons right on this one. Sled_Mack has great results in keeping his stack a hair above the 200 mark. I cant seem to run below 300 and get enough heat.

    Are those for sale yet Fossil?
  21. Nofossil

    Nofossil Moderator Emeritus

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    I KNEW this thread would get some discussion going!

    If the amount of material that's burning is constant, the more air you push through, the lower the stack temp. I think this could happen two ways. First, any excess air added to the secondary combustion can't increase burning, so all it can do is dilute the hot combustion gases with cooler air. Second, if you have a bunch of large chunks of wood in your primary chamber, there's not much surface area for gas generation to happen. I can imagine in that case you could be blowing a bunch of extra air though that has nothing to react with and just passes through the nozzle without adding to gas generation. That would also have the effect of lowering the flue temperature. That happens as the fire dies down - fan running on high, not enough fuel surface area to react with it. Slowing the fan down would increase the flue temperature in that case for sure.

    When you've got a good bed of coals and plenty of fuel, adding more primary air increases gas generation and thereby generates higher output, presumably raising flue temperature, all other things being equal.

    Heat loss up the chimney is a big percentage of total loss for these units. I'm still wrapping my head around the factors that affect flue temp.

    An observation: The HX pipes on the EKO 40, 60, and 80 are much longer than mine. I wonder if that makes them inherently more efficient. The test results that I've seen are for the 80.

    I'm having a hard time imagining what I can do besides turbulators to drop my stack temp without throttling the boiler way back.
  22. kuribo

    kuribo Feeling the Heat

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    One sould also be able to lower the flue temps by removing more heat from the combustion gases via the water jacket....
  23. wdc1160

    wdc1160 New Member

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    I thought from my watching the thread, you guys could go over some of the assumptions you have made. Some of the ones that seem obvious may not be.


    These are three that I thought could use more thought
    One example would be the outlet temp being lower = more efficient / lower stack temp.
    An influx of air can be to much of a good thing if it pushes uncombusted gases out the stack.
    This may also be said about the surface area of the fuel.
  24. Nofossil

    Nofossil Moderator Emeritus

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    For those inclined to roll their own, here's a picture of my two-channel thermocouple signal conditioner. Next to it is an Omega SMCJ-K which does the same thing, one channel only, for $100. I use it as a reference for calibrating mine, but it's a simple solution.

    List of parts required: Analog Devices AD-595, 6.8K resistor, prototyping board, type K thermocouple, DC power source. A 9V transistor battery would work, but you'd want something a bit higher for high temps. The output is 10mv/ degree C, so 9 volts would limit you to 900 degrees C, or about 1600 F. The chip will tolerate up to 30V supply.

    Digi-key provides data sheets, which are a bit thick for non-electrical-engineer types. I managed to slog through. The resistor is to scale the output to fit the range of my controller's inputs.

    Attached Files:

  25. TCaldwell

    TCaldwell Minister of Fire

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    nofo, maybee something like this would be a little more potent than the turbulators in the eko, www.hpac.com. compact efficient heat transfer.
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