Monday, July 6, 2009

The English Kiln, Take 1

That's not "English" as in country of origin; we named our kiln after a charcoal-making design that Alex English had experimented with while attending an appropriate technology conference in India some nine years ago (see English Kiln). Unlike our can-can kiln (previous post), this is a "direct" burn design, i.e. the feedstock is partially combusted directly and reduced to char, rather than being cooked "indirectly" from without. The knock on direct burn designs is the smoke they produce--including potent greenhouse gases. Alex added a chimney and afterburner to his, intending to flare-off the smoke, ideally reducing it to mostly heat, water vapor, and CO2.

(CO2?! Isn't that bad? Only if the "C" comes from burning fossil fuel.

The fate of most all living biomass is to wind up as CO2 in the atmosphere; quickly if it is burned, or more slowly as it gets metabolized through the lower reaches of the food chain. Since we're making biochar, we'll be carbon negative--reversing global warming--even if we spill a bit of "C" along the way.)

Our experiences with the can-can kiln told us that the high moisture content of the wood was problematic. We also observed that the board-like form of the Melina scrap often led to broad surface contact between pieces, effectively reducing the useful surface area. In the interests of improving our chances for a decent burn the first time out, we cut the wood into smaller chunks to increase surface area and reduce contact between pieces.

Still concerned about moisture content, we decided to fill the kiln with our little wood chunks and build a small fire underneath to drive off moisture before formally igniting it from the top, as intended. We thought we were pretty clever as we watched a cloud of steam wafting out the top. When the cloud became denser, hotter, and tainted with a brownish color and creosote odor, we started feeling less clever. Wood just doesn't conduct heat very well, and things got a bit too toasty down in the bottom. We had unwittingly initiated pyrolysis in the load.

Of course, pyrolysis is what it's all about; but the design calls for initiating pyrolysis in the top and controlling intake air so the pyrolysis front will move slowly downward through the load. By initiating pyrolysis in the bottom with our external drying fire, we were faced with the prospect of the entire load going off at once. Sort of a green-biofuel version of the "China Syndrome". Not good.

We tried to suffocate the load it by collaring the intake vents, and put on the lid and chimney stack in an attempt to shunt smoke away from the building. But the damned thing had the bit in its mouth and was making a break for it. Dense smoke was pouring out with no sign of abating.

Reluctantly, we decided to abort the mission and overturn the load.


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  2. Look at that smoke!!
    Back to the drawing Bord.
    Use the heat from secondary burning to cook dinner or have a hot shower at the end of the day.

  3. Hmmm... I use a top lit approach as you describe, including with very wet green wood scraps. In my experience green wood will char nicely with this approach, but only provided you light the top surface with plenty of very dry kindling.

    You end up with a sort of double front moving down through the pile - a drying front where the heat from above is drying the fuel below and above that the pyrolysis front where char is being formed.

    In my rudimentary designs we had very little control over the air intakes and ended up with some huge flare flames from the top of the stack, but very very little smoke as all the gases pass through the hottest part of the fire before being burned in the flare.

    I have a page on how I make biochar here.