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Trying to Understand Modulation

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Eric Johnson, Feb 1, 2008.

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  1. Gooserider

    Gooserider Mod Emeritus

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    Maybe a dumb question - but has anyone experimented with using an automotive O2 sensor in a gasifier stack to see what the combustion exhaust looks like? Would this work as a way to function as a control variable that would account for the varying amounts of combustibles in the wood - say by modulating the fans and / or possibly a stack damper to maintain an "optimum" level of O2 in the exhaust stream?

    Another possible place where it might be possible to modulate, though it would probably be difficult to find an appropriate material and control system, would be the size of the gas orifice between the wood chamber and the 2ndary chamber - If it were feasible to adjust that on the fly - possibly by sliding some sort of plate into the opening - then reducing the volume of gas going into the secondary chamber should reduce the heat output, which should eventually reduce the heat back to the wood chamber and slow down the gas output, nice negative feedback loop? Might want to have some sort of interlock on the door to the wood chamber though, as it wouldn't be good to open the door on a chamber full of backed up wood gas - a potentially explosive combo...

    Gooserider

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  2. Nofossil

    Nofossil Moderator Emeritus

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    Automotive O2 sensor is on my list of projects. I don't think it will last long, though, and I think that there are some issues with the sensor electronics that may make it difficult to get much data from it. I'm hoping to at least get some comparative data as I play with the adjustments.

    The EKO has adjustments that restrict the amount of air going to the secondary combustion nozzle(s). I'm particularly interested in the following:

    1) As you open the nozzles, at what point do you start seeing excess O2? At ideal settings, the flue gas should be about 8% oxygen.

    2) As you slow down the fan (or restrict the fan inlet), do you need to change the setting for the secondary inlets?

    3) Do the secondary inlets need different settings based on the surface area of the fuel? (small sticks/splits vs large rounds)

    If there's a body of hard data out there on these questions, I haven't found it yet. I do know that optimum settings for the EKO looks like about 1.6 x ideal (stoichiometric) oxygen. What does that mean in real life? No idea yet....
  3. sled_mack

    sled_mack New Member

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    In it's simplest form, we are all modulating to some extent - none of us are running the fans wide open. At least I don't think so?

    I think if you could set min/max limits on the fan speed, you could do something simple with modulation. It would take some experimentation to find out what is the min speed that you can operate and not lose gassification. The max could be set to limit the stack temp when the water is at max temp. It would work, I think, and provide more heat when the water is cold and less heat when the water is hot. Is it enough to make a difference? I have no idea.

    The next level of sophistication would be to monitor water and stack temp. If the stack gets too hot, cut the fan speed to prevent too much loss up the stack.

    But I think that is only part of it. I think it needs to include independent primary/secondary air control. And, I think it needs to analyze the gasses to know how to adjust the ratio. It may also be helpful to know the secondary chamber temp, to ensure that it is kept hot enough to keep secondary burn going. All of this leading to a true "set it and forget it" system.

    As HR stated, this could be a lot of work for very little gain. If I had the instrumentation available, I'd be tempted to play with it. But I'm not likely to make the investment, when an equal investment in insulation would get me solid results.
  4. Eric Johnson

    Eric Johnson Mod Emeritus

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    I think you're describing something in a whole different category than a $6,000 or $7,000 boiler, especially considering the vagaries in the fuel quality.

    I don't think you'd need much fan pressure to keep gasification going--once you get it going good. I can shut off my blower and load the stove, then close the bypass damper and get flames coming out of the bottom of the nozzles from the draft alone.
  5. Gooserider

    Gooserider Mod Emeritus

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    Another thought that occurs to me along the same lines is that does the monitoring miss something if it looks ONLY at flue temps, without looking at the exhaust VOLUME as well? I'm not sure how to phrase it from a math standpoint, but if you have an exhaust stream at temperature X, the energy contained in that exhaust is a function of both the temperature and the volume - if you doubled the volume of gas, at the same temp, you've doubled the energy, right?

    So if you turn up the fans, thus increasing the volume of air going through the boiler, and get no change, or only a slight decrease in exhaust temps, doesn't that mean you are increasing the total amount of energy going up the stack?

    I'm just wondering about this because I see a lot of threads where there is a great deal of importance being placed on stack temps, and a presumption being made that lower stack temps = greater efficiency / lower heat loss up the stack, even if the temp is lowered by increasing the fan speeds... I'm wondering if this is accurate?

    Gooserider
  6. Nofossil

    Nofossil Moderator Emeritus

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    You're absolutely right. Blowing a bunch of extra air through lowers both stack temp AND efficiency. That's why I went through the work of measuring the actual heat output from the boiler for a given weight of wood consumed.
  7. TCaldwell

    TCaldwell Minister of Fire

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    My interpretation of stack temps is that it can be a factor for determining efficiency , however it is very specific to each boiler, due to many factors , stack piping, internal boiler heat exchange design and potential, both fixed and make up the "dna" for each boiler installation, then you have the variables, wood types, return water temps and associated flow rates, internal or external storage, and so on that in any day are constantly changing. the complexity is represented by the threshold of the engineering dept or deep pockets thereof. I read that herlt has a controller that is adjusted at installation with a thermocouple at the top of the fluestack set just above condensing temp, modulating the induced draft , they state , that this is not necessarily the optimum effiency for the boiler, but due to on site differences in install , they must at least meet this , claiming only costs a few percentage points in effiency, the herlt is a large downdraft gassifier. With the garn , draft induced, and no way to manipulate the air at all, and you happen to put too much dry wood in the chamber,and its starving for air they tell you to partially block, disrupt the air flow in front of the primary air inlet with a brick, sometimes works. Each manufacturer seems to have their own 90x to a moving target, and it seems most of us want that last 10%. I am not sure what this will actually do , but i am going to modulate the garn airflow with a 2 hp 3 ph motor controlled by a variable speed drive, from 1 rpm to 4600 rpm, it currently runs a 3450 rpm.On the low side for a batch burner i can reduce rpm at the end of the burn not shed heat up the stack, on the high side i am curious if it will help with the puffing by supplying more combustion air. this is a start, i am afraid it wont end untill the the vsd is controlled by a fixed mount oxygen trim kit!
  8. Gooserider

    Gooserider Mod Emeritus

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    Sounds reasonable, so the next question would be how does one simplify that into something potentially useful for modulating? I would think that getting a handle on the total number of BTU's getting pushed around would help, I see the following as major categories....
    1. Heat loss through boiler exterior - presumably this could be figured off an experimentally derived lookup table, or possibly calculated based on a known thermal transmission property of the boiler insulation and the difference between the firebox / water jacket temp and the exterior temps.

    2. Heat being sent out the plumbing - should be a very straightforward number - temperature difference times the volume of the water getting moved - presumably you could get this by measurment in some way, or looking at the pressure and calculating against the pumps flow/head output curve.

    3. Heat going out the stack - The challenge would be to figure the volume - stack temps are easy to measure, but volume would be much more of a challenge - something on the order of making sure all air enters through the fans, then trying to figure something on the fan's output curve per RPM, along with the static head that the fan is pushing against to get a volume of air going in, which would need to be corrected for the expansion due to heating, to give a volume going out... Seems like a very ugly equation...

    Gooserider
  9. sled_mack

    sled_mack New Member

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    There are flow meters available that work for air. The trick would be in the placement. Either make some ducting like a cold air intake for the fans and put the flow meter there, or mount the fans away from the boiler and push through the ducting and the flow meter.

    I know that only gets you air in, not the volume of air out. But are you concerned with volume of mass of air exiting? I'm just thinking a small flowmeter on the inlet measuring cold air will be far less expensive than one on the stack that has to stand up to the heat and other goodies in the exhaust.
  10. Nofossil

    Nofossil Moderator Emeritus

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    A few of the pros here have meters that measure CO, O2, and other flue gas components. I think you'd come close to ideal if you gradually crank up your secondary air until the CO stops dropping (at something pretty close to zero). We know for the EKO 80 at least that an oxygen concentration of 8% is pretty close to ideal, and that's probably true for gasifiers in general. Any more than that means you're blowing extra air through that's carrying away heat and not contributing to improved combustion.
  11. Gooserider

    Gooserider Mod Emeritus

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    Most of the airflow meters have a certain level of head loss themselves, so measuring with them will change the flow characteristics of what you are trying to measure... IOW, you'd have to turn up the fans and burn more energy to make up for the flow meter, though it might not be a significant difference...

    As far as air-in vs. air-out, I agree it would be far easier to measure the air-in, just because of the issues you mention on dealing with the exhast. However the key numbers for looking at heat up the stack ARE temp and air-out... For that the air-in is only useful to the extent it allows you to calculate, as opposed to measuring, the air-out. If we just had the air blowing through a heat source, say a water-air heat exchanger, then it would be a very straightforward calculation. But we don't, the air is going through a fire, where there are a bunch of chemical reactions occurring and we are adding in all the gasses from the wood breakdown, so I'm not sure how, or if it would be, possible to calculate air-out from simply air-in, especially if the amount of wood gasses aren't readily knowable - weighing the wood is a PITA at best, and would only give you info about the entire burn cycle, I don't see an easy way to figure instantaneous wood gas production...

    Gooserider
  12. TCaldwell

    TCaldwell Minister of Fire

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    I have a anemometer, and know the cfm in and out of the garn at the static 3450 rpm, along with the 2 inches of w/c with the 3/4 hp motor, I have been told that it will take 2 hp to modulate the motor to 4600 rpm, power required to move the added volume of air at that rpm, physically i should be able to move more air, combustion wise i will be anxious to see what happens, with respect to combustion analysis as nofossil suggests, hopefully then it might need some sort of turbulator for longer heat transfer. I have 3 thermocouples to monitor secondary burn, at the back of the appliance ,and at the top of the fluestack. first i want to know whats going on in the combustion chamber, then figure what parameters to mod by, temp, pressure, time, 02 ect. and the associated controller. when installed i will post results, and welcome all opinions
  13. brad068

    brad068 Feeling the Heat

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    Hey Tom,

    How did the leaf blower work? I'm curious to see how you setup works too. The idea of installing turbulators might offset the more cfms that you are trying to get. Did you receive you parts yet?
  14. TCaldwell

    TCaldwell Minister of Fire

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    garnification, the leafblower setup was not one of the more meaningful experiments, over pressureized the burn chamber, backed up at the draft inducer motor, pretty good flame out the door! all stuff is in, will install friday , i will keep you posted
  15. TCaldwell

    TCaldwell Minister of Fire

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    just hooked up the new motor and variable speed drive to the garn, some first observations, experimenting and using the potentometer or hertz control dial, to increase or decrease fan rpm. keep in mind the stock baseline of the original setup was 60 hz representing 3450 rpm from beginning to end of burn, the conversion is 1 hz=57.5rpm of fan speed. as i increased the hz also did the thermocouple reading in the secondary burn chamber, even more so the thermocouple at the rear of the appliance, stack temp increased alot. with the hz at 75 =4312rpm the burn chamber temp dropped off and the stack temp rose, basically i was blowing alot of heat up the stack. when decreased hz to 55 the burn chamber temp climbed to max temp and the stack temp dropped to about 335 , as the burn cycle came to a close and the burn chamber temps dropped , i lowered hz to 45 and the burn chamber tenp would rise and the stack temp would stay at 300. thus by decreasing the hz as the burn was ending ,i was extracting more heat , than if i kept the hz at 60 from start to finish, i honestly thought i would get more performance at the high end , so far more at the low end. i wonder if following a target stack temp over 80 percent of the burn might be more efficient than a shorter higher burn temp coupled with a higher stack temp at the first half of the burn followed by lower burn temp coupled with a lower stack temp caused by too much air flow. bring it on
  16. Jimxt88

    Jimxt88 New Member

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    ABG, this reminds me of something I read yesterday in a book called "The Rational Construction of Frunaces" by W.E. Groume-Grimailo, Petrograd, Russia, 1911. In which he describes the naturally occuring stratification of wood combustion gases. He states that in a chamber, carbon dioxide which is heavier than air will stay low and be pushed lower by Hydrogen which is lighter than air and floats. He describes a furnace design in which carbon dioxide is displaced by Hydrogen and forced out a low exaughst opening while the upper chamber stays hot and filled with hydrogen. In the quote above you describe slowing the gasses (by use of baffles, I assume) and SS pipes that allow the gas to "settle". It reminds me of the double bell masonry heater design which exploits this characteristic of gases of different densities to stratify. I am wondering if it is possible to store the hydrogen gasses from a low temperature burn in the boiler and reintroduce them to the gassifying chamber when the charge has been reoxiginated and temperatures in the gassifier restored.

    Can you say more about what you have learned regarding this modulating technology?
  17. wdc1160

    wdc1160 New Member

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    As I learn more about the strategies used to modulate wood combustion I have found that the most efficient method of storing the gases is leaving them in the wood. Consequently a great deal of thought in the design of the vito is put into making sure that no excess wood is cooked.
    So although stratification, separation through baffle, and storage are accomplished its not the ideal measure. (the reintroduction of the gases at a more convenient time when demand and conditions are optimum.)

    Some of the best examples of “high tech” wood burning are from WWII. They had some pretty clever folks using wood gas.

    However, I have stopped doing work on modulation. I have found that if I take the same amount of extra money and invest that in storage. I could also accomplish modulation only through the buffering of heat in a tank. Additionally, I could accomplish a very efficient burn.

    It seems that the natural way that wood likes to be burnt is like a forest fire. We can use math and science to stop this natural process, but it takes great expense and complication. Although, it isn’t technically beyond our capabilities, It currently doesn’t make fiscal sense to work against natures wish to burn fuel in one uninterrupted duration.

    I had made this determination when I decided that solar during the winter could also be helpful-- Again nature not wanting to fit output to demand.

    I have redoubled my efforts into storage e.g. charging, discharging, insulating, burying, constructing.
  18. Jimxt88

    Jimxt88 New Member

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    ABG, Do you have pictures of your energy system? I am interested in anything that is working for you. In particular, I would like to see examples of water storage systems. I also want to recognize TCaldwell's innovation on his GARN described above.
  19. dfergx

    dfergx New Member

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    From reading through this thread I think it seams important to size the gasifier properly. to big and it idles wasting fuel. is this correct?
  20. Nofossil

    Nofossil Moderator Emeritus

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    That's always been my hunch, and there are lots of reasonable theories that would support that conclusion. However, I'll have to admit that I have no hard data to prove it. One thing I've learned through all of this is that when you get around to taking real data, you will be surprised by something.
  21. wdc1160

    wdc1160 New Member

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    I can't see any reason why oversizing couldn't be considered "wasteful"

    In the sense that it loses alot of the effciencies you buy a gasifier for.
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