Ken Carloni and Darren McAvoy of Utah State are both working on Big Box biochar kiln designs. I am focussing more on the smaller hand-fed kilns - Oregon Kiln and my new Ring of Fire kiln. All of these kilns are Flame Cap Kilns and they work essentially as continuously fed updraft gasifiers (no bottom air).
All three of us have converged on some basic design principles that include:
1. The right shapes to minimize surface to volume ratio so you don't lose heat out the sides. You don't count the base that is insulated by the ground, or the top where the flame cap is. Cones and troughs are not the best for this. Cylinders and rectangles are better. Sloping sides are handy for stacking kilns (like Oregon kiln) but not needed for operation.
2. Not too tall so you have the option of hand loading. Machines can actually be slow and inefficient and may be best for delivering loads of feedstock to the hand crew.
3. A heat shield makes a big difference. Holds in more heat for faster processing. It also helps a lot to protect workers from radiant heat that causes fatigue and dehydration. And it provides pre-heated air through the gap that contributes to the counterflow fluid dynamics that keep smoke in the hot zone longer for cleaner combustion.
4.Quenching can be with water or with a snuffer lid. If using a snuffer lid, bigger kilns will take more than a day to cool down, so your kiln will be out of action for that period. If using water, you can either flood and stir, or dump, spread and spray. Unburned brands should be removed and set aside for the next burn, or left on the ground for the microbes to eat.
5. Feedstock considerations #1: Size. Most important is uniform size. Start with small brush and let it burn till it chars and falls to the bottom in a layer of glowing coals. Then start adding bigger material if you have it. The flame front will move up and the small brush char you made will be protected from air so it does not burn up. Bigger logs (4 to 6 inches diameter) evolve more gas and heat and can char all at the same time in a stack of uniform material. If you mix smaller stuff it will just burn up to ash before big stuff is charred.This is why the Air Burner demos have shown poor efficiency. An Air Burner is just a flame cap kiln with an active counterflow provided by a blower. It is meant for incineration and people are used to throwing giant logs, stumps and everything in there to burn to ash for waste disposal. This will not make char. The Air Burner needs to be operated like a flame cap kiln. You don't even really need the blower if you do it right. And don't try to char big stumps and giant logs. It won't work and you will get mostly ash.
6. Feedstock considerations #2: Moisture. Dry is best, but green is good. Ken and I are starting to char green brush. Green brush actually has more fuel value than dry brush because it still has volatiles. It also has water, so that takes more heat to evaporate, so it's a trade-off. If the green brush is small enough and you have dry material to get started, you can add green once you have a bed of hot coals and it chars just fine. This is important for labor saving so you don't have to do more than one entry to the site. You can cut, drag and char all on the same day. It is probably also making a somewhat lower temperature char, but there is no data yet.
7. It would be really great to get some funding to test emissions, char conversion efficiencies, char characteristics, and field operations and logistics of different designs. I am sure we can make more improvements if we have a little support beyond our own pocket books.
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