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Gasification Boiler Efficiency

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by jebatty, Nov 11, 2010.

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  1. jebatty

    jebatty Minister of Fire

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    This has been hashed through pretty well, see (among others): Discussion 1, Discussion 2. Also see Energy in Wood.

    Energy in Wood calculates
    My improved data logging is producing repeatable efficiency results in weighed wood burns, based on the Energy in Wood assumptions. That calculation, adjusted for boiler water heated, is coming out at right around 83% net btu's delivered to storage during weighed wood burns. In Horizontal Tank, I calculated that "Net btu’s/lb [delivered to storage] of wood were 4,628," but I did not take into account the fact of about an additional 55 gallons of boiler water being heated. In a burn yesterday, the calculation was 4,680 net btu's/lb delivered to storage and also not taking into account heating of boiler water. These calculations resulted in 77% efficiency, but when 55 additional gallons of boiler water are entered into the calculation, the result is 83% efficiency.

    In making these calculations, I am 1) weighing the wood, including kindling; 2) firing the boiler at a cold start; 3) mixing the 1000 gal storage tank to a uniform temperature before starting the burn; 4) mixing the storage tank to a uniform temperature at the end of the burn; 5) adding 55 gallons of water to account for the boiler; 6) calculating the btu's based on the deltaT in the tank+boiler from beginning to end of burn; and 7) not drawing heat from the system during the burns.

    I believe my calculation remains conservative because it does not account for heat loss from the boiler water and from the tank during the burn period. It also is conservative because my boiler and tank are in the heated space, and during each burn the temperature of the heated space rises by several degrees, as heat from the boiler, flue stack, and tank are released into the heated space.

    The variables which remain assumptions in these calculations are the available energy in the wood of 6,050 btu/lb and flue gases at a constant 400F. My actual flue gas temp varies during the burn, but based on logging stack temp it appears that 400F is a good average.

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  2. Nofossil

    Nofossil Moderator Emeritus

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    Great data!

    This gets at the whole 'Combustion Efficiency' vs. 'Unit Efficiency' vs. 'System Efficiency' discussion.

    Here's my definitions and explanations:

    Combustion Efficiency - how thoroughly does the device burn the fuel? A well operated gasifier can achieve combustion efficiencies very near 100% - that is, virtually all of the combustible material is being fully burned and converted to heat. Almost nothing is lost up the chimney as unburned fuel.

    Unit Efficiency - What percentage of the wood energy is available at the boiler hot water outlet during steady-state burn? Indoor gasifiers run around 90% unit efficiency. About 2% is lost to the surrounding air through the jacket, and about 8% is lost up the chimney, most in the form of energy required to boil the water in the wood and the water that's a byproduct of combustion. Tough to get above 90% unit efficiency without designing a condensing boiler.

    System Efficiency - What percentage of the available wood energy is delivered to the ultimate intended loads (DHW, space heating)? This is a complicated topic. Indoor boilers and storage lose heat to the indoor environment so that it's not really 'lost', but in most cases it didn't go where it was intended - lost from storage, used to heat combustion air, lost from plumbing, less efficient combustion during startup and shutdown, energy lost from hot boiler after fire is out, and so on. People with deliberately underinsulated storage that is intended to heat the surrounding room get a pass on that part of the loss. It's rare in my experience to see residential system efficiencies above 60-70% even with the best gasifiers. By that token, the 77% reported for efficiency in converting wood to hot water in storage is pretty impressive.

    I weigh the wood and deduct the actual water weight to determine how many pounds of dry wood are available. I use 8600 BTU/lb of dry wood weight, which works out to 6880 BTU/lb of wood at 20%.
  3. jebatty

    jebatty Minister of Fire

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    Nofo, review this and see what you think. Is your calculation in effect the same as that in Energy in Wood [6930 btu/lb for 20% MC wood] when it says:
    Energy in Wood goes on to state:
    What do you think?
  4. Nofossil

    Nofossil Moderator Emeritus

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    My calculation is the same as Energy in Wood - I use a very slightly different number to start with, but the theory is the same. There is some variation in wood, by the way. Not much, but some.

    As an engineer and a purist, I think it's appropriate to use the High Heat Value. It's theoretically possible to recover the latent heat of vaporization, and even if it weren't I'd still want to be reminded that it's going up the chimney. IMHO choosing to ignore losses can lead to missed opportunities.

    For instance, my brother's wood gasifier is a condensing design with a flue temp of around 120°F (not stovepipe surface - actual flue gas temp). If you use the LHV, his unit efficiency could well be over 100%.

    If we all use HHV and understand that non-condensing designs can't get over about 90% then all is good. I like that a lot better than choosing to ignore a class off energy losses and then claiming some really high efficiency number.

    When I calculate my own system efficiency, I use HHV and I don't take credit for heat that goes into my house from anything other than the baseboards.
  5. tom in maine

    tom in maine Minister of Fire

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    Steady state testing is nice to do and certainly allows us an opportunity to compare boilers to some degree.
    I have been in some boiler rooms that are obviously yielding a lot more than 2% heat loss through the jacket.

    The issue that may be a consideration in a lot of installations is the residual heat that is in the refractory and the water jacket after the house is warm and storage is
    full.
    This is a lot of energy and if the system is not running steady state, it is not necessarily useful heat. It is going up the fuel and through basement walls.

    It is not the end of the world when burning wood, but it is not as efficient and someone is lugging more wood than they might otherwise
    need to.

    Like I said, it is not a big deal, but the reality could be that performance is a lot less than 70% net efficiency.
  6. Chris Hoskin

    Chris Hoskin TarmSalesGuy

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    "For instance, my brother’s wood gasifier is a condensing design with a flue temp of around 120°F...." WHA ?! tell me more!
  7. jebatty

    jebatty Minister of Fire

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    ...and tell me more, tell me more....

    Mu current system deals pretty well with some of these issues. First, my boiler, flue to the 14' ceiling, and storage are in the heated space. Prior posts have shown the very considerable btu's which are shed to the ambient environment during a burn. For example, my burn which ended at 9:00 pm Wed evening, plus loss from the insulated tank, etc., has added heat to the shop and the concrete floor which as of right now, 5:00 am Friday, has resulted in no draw from storage to the radiant floor, and during this time average outside temp was about 40F, with inside temp ranging between 61-68F. That's 44 hours of no direct heat draw. Locating the boiler/tank in a structure which could not use this heat for a useful purpose would represent a very considerable loss in system efficiency.

    So in a sense my system is "steady state" as all available btu's, except loss through insulation and venting up the flue, provide useful heat.

    Second, monitoring the rise in tank temperature at various points allows me to load my burns so that, with just a little care, my boiler never goes into idle mode and yet heat the tank to about 185 top to bottom. Except through error I am not firing the boiler when storage is full.

    I have thought that where it might be useful to do better would be to achieve burns with a more consistent flue temperature. Yet, based on my research a 100F drop in flue temp represents about a 2-3% increase in efficiency. If my burns now range between about 380-420F, there is not much gain in efficiency possible by, saying, having a steady 380F flue temp.

    Based on a comment from Nofo previously, another area of efficiency for me would be lower flow (and fewer watts) on Side A of my plate HX. The 007 is providing far more gpm's than needed. At about 85 watts for the 007, there is room for an efficiency gain. The cost of replacing this circ with a very low wattage circ, which I have not investigated, may not make the change economical, though.
  8. Nofossil

    Nofossil Moderator Emeritus

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    We had a thread on this a few years ago. My family is [del]notorious[/del] famous for wild inventions - rocket propelled radio controlled catapult launched gliders, cam operated trebuchet, spring-powered dragsters (0-15' in .88 seconds). It's no surprise that when my brother's conventional boiler died, he built his own gasifier. Not a job for the faint of heart. It's an open system heating a radiant floor slab. LOTS of flame tube surface area with a water exit temp around 95 °F. It has a second shell-and-tube HX for DHW, and that's encased in an enclosure that provides lots of surface area for heating air that's ducted upstairs. Finally, the cold air inlet comes down the inside of the chimney so that combustion air is preheated using flue gas heat.

    Basic nozzle geometry is based on the EKO 25. There are adjustments for fan speed, primary air, and secondary air. He keeps a meticulous log, including all settings and wood weight for each burn. He operated his old boiler as efficiently as possible - full throttle burns with dry wood. He saw a 40% reduction in wood consumption with the gasifier. He heats a decent sized two story house in Vermont with 3.2 cords a year including DHW.

    There's a gallery of pictures on my web site.

    This MAY be an example of an over-the-top quest for efficiency.
  9. Chris Hoskin

    Chris Hoskin TarmSalesGuy

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    as we say in New Hampshire, that is WICKED cool! Thanks NoFo.
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