Gasifier Design & Practice

[Peter B.]

Hello:

Newcomer here, with a lot of questions...

I'm interested in the operation and design of downdraft gasifier burners in general, and note that most successful applications these days seem to be limited to furnaces and boilers.

I'm wondering if most/all furnaces and boilers available now use some form of power induced draft - either intake or exhaust?

Do most/all use refractory materials for the outlet from the main firebox to the secondary chamber?

Do most/all exhaust through the 'absolute' bottom of the firebox floor?

I'm also curious about the configuration of the outlets from firebox to secondary chamber... the methods used to introduce secondary air... and the placement of the secondary nozzles.

Also whether firebox outlets (or grates) tend to become clogged with ash.

Also whether the daily maintenance is relatively trouble free... or dammed finicky.

In short, I'm looking for a little more detail than the manufacturers' sites tend to offer. A lot to ask in one post, but what the hey.

I don't have plans of building my own, though I'd love to have the skills, resources and shop to do so.

I don't know an awful lot about this stuff, so I might not have much more to say or offer, but all responses and insights are welcome and appreciated.

Thanks for your time... and for this forum.

Peter B.

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[chrisfallis]

I used to have a 1997 vintage Tarm installed in my basement. I would burn mostly dimensional wood from construction debris and pallet scraps. This was a downdraft machine and the ash (and screws, nails, hinges, bolts, etc) fell down into secondary burn chamber at the bottom of the firebox. There was a separate door down there that I opened every few days. Insert the provided rake and pull the ashes and other debris forward and out into a metal ash-pan.. A two minute job every few days. The induced draft fan created a hot fire that reduced everything woody into very fine ash. This was a super easy boiler to maintain.

 

[Eric Johnson]

You are asking for a lot of information, but since you're from Wisconsin (I'm originally from Coloma) and you have a Round Oak (had one of those, too), I guess I'll give it a shot.

Most gasifiers use forced draft, and most use blowers for positive draft. Some of the Seton-style refractory-heavy units, like Greenwood, use natural draft and some conventional gasifiers, like Wood Gun and BioMax, induce negative draft with fans.

Gasification boilers that have two chambers (Econoburn, EKO, Tarm, BioMax, Wood Gun, etc.) have the nozzles located below the main firebox and above the gasification chamber. The Seton-style gasifiers (I believe) don't really have a secondary burn chamber--they do it all in the same chamber.

Most exhaust through the bottom. The Seton design usually exits out the bottom at the back. The EKO, Tarm, etc., design run the exhaust gasses through vertical firetubes and out the top.

The EKO-style Euro gasifiers have ceramic nozzles through which the wood gas passes and is ignited when superheated air is mixed in. This secondary air is pushed through steel tubes embedded in the refractory mass on either side of the nozzles, which gets it hot. It's actually not "superheated," but that's the term that's often used. It's just very hot air.

I've never had the nozzles become clogged in my EKO. The current generation of EKOs, Econoburns and BioMaxes have turbulators in the firetubes that can be moved up and down with a lever. This keeps them clean and eliminates the need for routine cleaning during the heating season. Other than moving the handle back and forth when you load the stove, the only maintenance required is to empty the ashes out of the secondary burn chamber periodically. In my case, that's a five-minute job about once a week.

They burn clean--no creosote at all and no smoke except during startup. They require very dry wood to work properly. I've been running mine succesfully all winter with no storage, but it's a nice thing to have. Typically, when the temps are in the teens (F), I get 8-10 hour burns, keep the house between 75 and 80 and never have to start a fire from scratch.

Hello:

Newcomer here, with a lot of questions...

I'm interested in the operation agnd design of downdraft gasifier burners in general, and note that most successful applications these days seem to be limited to furnaces and boilers.

I'm wondering if most/all furnaces and boilers available now use some form of power induced draft - either intake or exhaust?

Do most/all use refractory materials for the outlet from the main firebox to the secondary chamber?

Do most/all exhaust through the 'absolute' bottom of the firebox floor?

I'm also curious about the configuration of the outlets from firebox to secondary chamber... the methods used to introduce secondary air... and the placement of the secondary nozzles.

Also whether firebox outlets (or grates) tend to become clogged with ash.

Also whether the daily maintenance is relatively trouble free... or dammed finicky.

In short, I'm looking for a little more detail than the manufacturers' sites tend to offer. A lot to ask in one post, but what the hey.

I don't have plans of building my own, though I'd love to have the skills, resources and shop to do so.

I don't know an awful lot about this stuff, so I might not have much more to say or offer, but all responses and insights are welcome and appreciated.

Thanks for your time... and for this forum.

Peter B.

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[Peter B.]

Chris & Eric:

Thanks to both of you for your replies...

--

Eric:

The additional detail you provided is much appreciated.

I'm originally from the East (D.C.), moved to Wisconsin in the late '60's and have owned my Round Oak for over thirty years. I modified it internally about twenty years ago for (somewhat) improved efficiency, installing a baffle chamber above the main firebox that houses a catalyst. The chamber is fed by a heavy steel duct that is dropped nearly to the firebox floor, so that all exhaust leaving the firebox has to pass near (though not through) the coals. I also added a (primitive) secondary air supply that feeds the main fire when the primaries are closed. Not a true gasifier by any means, but it burns pretty cleanly with a hot bed of coals.

After reading a number of threads here, I've gotten the idea that a clean burning boiler setup is best achieved with (a lot of) buffer storage, burning hot, and making use of the stored water heat.

But I also wonder if there are hot air gasifier furnaces?

One more question: do the natural draft models give more problems - i.e. smoke on reloading, etc.?

Again, thanks.

Peter B.

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[slowzuki]

I would say yes. This type is sensitive to draft and everyones chimney is different producing different draft. Most people don't have a manometer to check the draft. They are easy to make with a ruler and some vinyl clear tubing.

Chris & Eric:

Thanks to both of you for your replies...

One more question: do the natural draft models give more problems - i.e. smoke on reloading, etc.?

Again, thanks.

Peter B.

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[muleman51]

slowzuki- What's your design for a manometer for draft when you are looking at hundredths of an inch. I have made them before for other applications that were mearured by the inch but not that small.

 

[slowzuki]

You're really only shooting for a 1/16 of an inch in the Greenwood models. Clear tubing taped onto a machinists scale with a magnifying glass can get you to a precision of +/- 0.5 of a 1/32 of an inch easily. If you have a good lens and a pair of calipers, 1/64 or better is possible for small values. Once the pressure gets too large the paralax error from the lens position starts to cause error.

For really accurate readings, just lay the tube on a slope and take measurements. Measure the slope on the board and do the trig on the readings to convert to vertical readings. The flatter the slope, the more precise your readings are. A 1/2" rise (measure with your manmeter as a water level if you want really good accuracy!) over a yard will, if you say a 1/4" is about as good as you can measure in the tube on an angle, 0.0035" increments of water column. If you could flatter your tube with something laid on top you might get better than a quarter. Flattening the slope more isn't so hot as the surface tension pulls the water so far up the tube the meniscus is hard to read.