Resilient and sustainable homesteads would be growing a lot of trees and other plants and would thus have both source material and a number of uses for charcoal:
- Charcoal as a soil amendment, to increase the Cation Exchange Capacity
- Charcoal filters for water recycling
- Cooking fuel
Last but not least, any use other than burning sequesters carbon, is carbon-negative, and helps stave off global warming. As F. Gunther describes, making charcoal by the retort method configured to burn the gases prevents the emission of methane, a powerful greenhouse gas, during the process.
Executive summary: It does work, but the results are quite variable in terms of how much of the starting material gets charcoalified and how much is left unconverted. I'm having to use more fuel than I'd like to ensure robustness against these variations. I suspect the main culprit is dampness. So there's a tradeoff between robustness and fuel efficiency. Seems like there's also a tradeoff between clean burning (very hot) and retort life. The bootstrapping or positive feedback effect of burning the pyrolysis gas is dramatic. There can be hardly any embers left between the barrels and yet the thing is roaring like an oil furnace.
There's several tons of moldy flax straw here by the cottage and I kept trying to use as much of that as possible for both the source material inside the retort and the outboard fuel. I think as I dug further into the pile there was more damp stuff. I'm also trying to use up the large quantities of snagwood and lumber scrap that are around campus. It takes a ridiculous amount of time and some energy to cut this stuff up into little pieces for charcoaling. I have one measurement of 0.24 kWh (0.86 MJ) of electric chainsaw energy to cut up 15.4 kg of snagwood into fist-sized pieces. Realistically dry biomass has an energy content of ~18 MJ/kg so about 280 MJ for my wheelbarrow load.
So far my experience is that if the piece of wood is roughly cubical or spherical it can't be any bigger than a tennis ball. Pieces can be long and thin, or thin and flat, but the thin dimension can't be much more than one, one and a half inches, or it won't cook all the way through.
So far my experience is that if the piece of wood is roughly cubical or spherical it can't be any bigger than a tennis ball. Pieces can be long and thin, or thin and flat, but the thin dimension can't be much more than one, one and a half inches, or it won't cook all the way through.
The biomass books warned me about this - drying and comminution are headaches.
Here are my notes on the first series of tests. (See previous post for photos of first test.)
1) 7-Aug-08
Configuration notes
Outer barrel has open top and bottom. Outer barrel has removable lid with 6 inch chimney 2 feet high, chimney has damper and rain cap. Outer barrel is spaced off the ground on 4 thin firebricks, inner barrel directly on ground, lid down. Fiberglas insulation was wrapped on partway through the burn.
Source material (in retort)
flax straw, packed tight by jumping on it.
Fuel material (between inner and outer barrels)
~4 gal lumber scraps (about 1/4 of the way up the side) with flax straw on top.
Burn description
Lit the top, let it get burning and then put the chimney/lid on (didn't fit and sat crooked.) Fiberglas insulation melted and sagged partway off. Hissing and gentle roaring noise. Inner can split up the side. Incomplete conversion.
Discussion/what next
Inner can clearly needs more venting than that provided by the gap between the can & lid.
2) 7-Aug-08
New inner can - drilled fourteen 1/4 inch holes around the rim just below the lid. Outer barrel chimney/lid flared to fit better. No insulation.
Source material
flax straw, packed tight
Fuel material
~5 gal lumber scraps (about 1/3 of the way up the side) with flax straw on top
Burn description
Much smoke when lid was put on. Let it burn down more first? Roaring and pitchy coal-like smoke seems to characterize the pyrolyzing of the flax in the retort. Some ash near the top of the can and some unconverted material near the bottom. 
Discussion/what next
Not as good as #1. Damper straw? Ground keeping bottom of retort cold? Partial insulation on first trial helped? How little starting fuel can you get away with?
3) 8-Aug-08
Propped up inner can on 3 firebricks on edge & put wood underneath. No insulation.
Source material
flax straw, packed tight 5.4 kg
Fuel material
like #2 but with extra wood underneath the inner can.
Burn description
Let burn longer before putting lid on. Left damper open the whole time. Incomplete conversion.
Discussion/what next
Next try insulating outer barrel again, this time with refractory insulation.
4) 11-Aug-08
Insulated barrel with high-temp mineral wool insulation held on by metal stucco lath (I think is what this is) & wire.
Source material
lumber scraps and snagwood, chopsaw cut
Fuel material
wood all around to about the top of the inner can? flax straw kindling
Burn description
Hot - lower half of barrel glowing orange thru gaps in insulation. First one to finish clean without acrid odor. Complete conversion, volume down by about half.
Discussion/what next
That worked great for wood inside & out, what about straw? Concerned about the amount of smoke produced. The straw on the outside can't be packed too tight or air won't draw up through it. Maybe if there was less straw inside the retort it would work better.
5) 12-Aug-08
Raised outer barrel onto fat bricks instead of thin - more airflow underneath may reduce smoke?
Source material
flax straw, not packed super tight
Fuel material
flax straw, not packed super tight
Burn description
Only converted about halfway.
Discussion/what next
Not having much success cooking flax straw with flax straw. Try using a lot of fuel wood to cook the hell out of the straw.
6) 13-Aug-08
Insulated barrel on fat bricks.
Source material
flax straw, packed tight
Fuel material
snag wood, almost all the way up to the top of the inner can, flax straw kindling
Burn description
Complete conversion, volume down by about half. 4:15 start, cool by 6:25
Discussion/what next
Finally got a complete conversion of the flax straw. Next let's try a wood inside & out burn, see if we can get away with long thin pieces.
7) 13-Aug-08
Configured like 5 & 6.
Source material
snagwood, with some longer pieces. 9.8 kg
Fuel material
lumber scrap & snagwood all the way up, flax straw kindling.
Burn description
Barrel glowed orange again, smoke ceased a few minutes later. Conversion complete. 2.2 kg charcoal (22%)
Things retain their shape but shrink, as they are charcoaled. Here's a branch with pine cones, isn't it pretty? Careful it's extremely fragile.
Discussion/what next
Have successfully converted straw and wood including long/thin and thin/flat pieces, by using a lot of outboard fuel (filling the gap between the barrels almost all the way up.) Let's try making an afterburning chimney to reduce the smoke.
8) 14-Aug-08
New 55-gal afterburning chimney.
Source material
snagwood including one large damp piece
Fuel material
lumber scrap, flax straw kindling.
Burn description
As usual I lit the flax straw kindling and then put the chimney on. Putting the new chimney on immediately accelerated the flax burning. However the fuel wood got a slower start (almost went out) but the thing burned like a rocket once it got going. Lower barrel glowing orange even at top of insulation. Everything but large lump converted. Inside of barrel blistered.
Discussion/what next
Burned cleaner, hotter, and faster with large chimney. It draws so much better I think I can pack the straw quite tight around the outside and still have it burn. Let's try converting straw with straw fuel, using new chimney.
9) 19-Aug-08
Outer can back on thinner fire bricks (3 of them). 55-gal afterburner.
Source material
mostly flax straw, a few branches, reusing unconverted lump. 5.2 kg
Fuel material
mostly flax straw packed tighter. 6.0 kg
Burn description
Lit @ 3:30 and took right off. Almost done by 3:45. Burned quite clean. Near complete conversion. Large lump still not finished. 1.2 kg charcoal (27%)
Discussion/what next
That worked pretty well. Can the amount of outboard fuel be reduced?
10) 20-Aug-08
Configured like 9).
Source material
flax straw, packed tight 5.2 kg
Fuel material
flax straw, less than in 9) 3.0 kg
Burn description
Failure - did not catch, incomplete conversion even after refueling & refiring.
Discussion/what next
That didn't work. Can I get away with refiring it with flax straw, adding more underneath and more to the inner can?
11) 20-Aug-08
Mineral wool insulation retucked.
Source material
char & unconverted from 10), plus new flax straw. 6.4 kg
Fuel material
Flax straw underneath as well as on the sides, tamped tight with pole. 4.4 kg
Burn description
Started okay but still didn't finish clean. May need large chunks of wood in the bottom to help it finish? Some of this straw may be more damp.
Discussion/what next
I think there's a lot of variation in the dampness of the flax straw causing trouble. It may be important to have large chunks of wood in the bottom to help a flax burn finish clean. Let's see if I can finish converting this mass by firing it with a lot of snagwood outside, not adding any more to the inside.
12) 22-Aug-08
Added spark-arresting screen of "expanded metal" I think it's called.
Source material
Do-over, mostly flax straw & char from 11).
Fuel material
snagwood
Burn description
Had to rekindle, after which it burned very hot still didn't convert all the straw. Inner can burned through in one spot. Less than 3.8 kg charcoal.
Discussion/what next
Not sure whether dampness, the hole in the can, or the presence of unremoved char is responsible for the incomplete conversion.
13) 22-Aug-08
Configured like 12).
Source material
Layered top to bottom: fine sawdust 4 gal, straw, pine branches. 5.0 kg
Fuel material
Layered top to bottom: straw, pine branches, lumber scrap. 6.0 kg
Burn description
There was a small detonation early on. Started to catch and then went out. Refired while hot with strips of plywood (about 8 square feet of half-inch altogether). It took off and burned hot. 0.4 kg charcoal
Discussion/what next
Very low yield. There was ash inside the can. Either the sawdust blew out or the holes in the inner can let in too much air. Inner and outer barrels are both perforated and distorted, need replacing. The fast/hot burns with the large chimney are much less smoky but harder on the metal and may be less fuel efficient / harder to get going.
No comments:
Post a Comment