Here’s a story from some fellows in Amphoe Prao trying to do something about smoke from agricultural field waste and hoping to get carbon for soil improvement  rather than ash :

This shows a film of a makeshift biochar kiln in action:

What is happening? Well, I saw a farmer burning a fallow field that he had strimmed, and racked into piles. Dylan was the first to see the fires, and we then went and took a look, and started talking to the farmer. It was obvious to me that each pile needed more air. This was near Warm Heart and I suggested to the farmer that i would like to see if I could make something to burn the biomass without as much smoke. He said fine, help yourself.

I put mind into gear, and went shopping for bits in Phrao. I found some 5ft. long roofing sheets, several 6m. lengths of angle iron and some self-tapping screws, and made a five ft. cube, open at the bottom  and with a large hole in the top with an even larger chimney, suspended six inches above the hole in the top. Dylan was convinced it needed much  more air, so I put a 6″ gap in the middle of each of the four sides. I borrowed one of Warm Heart’s trucks and collected some biomass from the farmer’s field, but there wasn’t much left, but I thought enough. It was a bad thought. This prototype was called the Star Ship by the kids.

Dylan and I tried a burn, but it was not a real success. Small amount of smoke, but lots of black ash sent up. It caused anger among their neighbours.

We all had a post-mortem, and decided to close the gaps in the side and fill it with something similar to rice straw, namely lemon grass leaves. I collected three truck loads, putting more on each trip. I can now hand you over to Michael Shafer of Warm Heart, and his email about the test which I couldn’t attend.

Gordon Hirst and Michael said:-  That’s what my kid’s call Stuart’s shiny contraption in the middle of the football field.

One and all have been highly skeptical about the UFO’s potential. Today’s tests put all the skeptics to shame. The UFO performed wonderfully. Details below.

Before starting, I made a number of minor adjustments to the original design.

  • I closed all of the side vents, leaving only an opening around the bottom for air inflow.
  • I reduced the gap at the bottom to a single thin cinder block from the original two.

Gordon and I tried to create a test situation close to the anticipated field use. We stuffed the UFO full of dried lemon grass tops. It took 8-9 large bags of fuel, approximately two pickup loads to fill it completely. (Rough estimate of volume: 6 cu m) One of the kids jumped on top of the pile to compress it.

We top lit the pile.

The fire took immediately and burned very hot. For the first ten minutes we had a lot of flame and considerable smoke. During this time, we had flashes of burning gas bursting through the cracks between sheets of roofing metal. Toward the end of the 10 minutes,the fire had reached the bottom of the pile and began to creep out on the ground.

At this point – approximately the same time as with our regular TLUDs – the smoke stopped and the flames around the sides disappeared. Looking in the cracks around the top we could see a fairly chaotic pattern of orange flame floating above the pile. The chimney sucked the flames up. (Never saw the classic purple flame at all. So much space between the ultimate top of the pile and the chimney perhaps the orange was the inevitable consequence of cooling).

At one point we began to get a lot of smoke. When I investigated, I found that part of the pile had collapsed against the side of the UFO, cutting off the upward flow of air from the ground level “vent.” When I adjusted the pile and pushed stuff around some to recreate a flow, the smoke stopped.

We decided to quench the pile after an hour to see what was happening.

We poured a huge amount of water on the pile and even so it reignited an hour later.

When poked open, the pile revealed that pyrolysis had gone in about 6-9 inches on average. (Much more in some areas. Interior airflow was very chaotic and the burn varied all over the place). Within the pile, the lemon grass was entirely untouched.

The outer layer was GREAT biochar. Even wet, it “clinked” when crushed in your hand. Perfect carbonization of the stalks preserving every detail, brittle, essentially no ash on the surface of the pile.

Lessons learned:

  • KISS: keep it simple, stupid.
  • The simple sheet steel was entirely unaffected by the process. No burn holes, little discolorization, no warping, sagging or whatever.
  • The frame was totally unaffected.
  • The “box” kiln (with plenty of leaks, drafts, etc.) produced a very chaotic burn, but pyrolysis preceded without trouble.
  • Filling the container to the max made a huge difference. The mini-piles used previously simply allowed way too much air to give a good test of the kiln.
  • Quenching is as big a problem as we have always thought it was.
  • Burn time is as big a problem as we have always thought it was.

What next:
Given these lessons, Gordon and I have decided to make some big changes in our plans.

  • We have decided to abandon the pyramid model as unnecessarily difficult to make and maintain, and wasteful of materials (all those wasted little triangles). We plan to try with a modified box design that is perfectly cubical to minimize material wastage.
  • We have decided to keep the interior chimney/outside airway concept from the Vietnamese design but to modify it.
  • On the one hand, we believe that the ability to add oxygen to the interior of the pile is critically important to speed the burn process and
  • On the other, we believe that the ability to inject water into the interior of the pile is critically important to speed the quenching process. (I am really hung up about the quenching issue. We poured water onto this pile to very little effect. The problem is that even pyrolyzed, the straw in arrayed in such as way as to sluice the water off, thus keeping it from penetrating. Also, the heat instantly turns the water into steams which explodes outward, thus not touching the burning areas below. I really think that the way to go is going to be to force water up into the inside of the pile. Here it will cool, etc., but more important, when it flashes to steam, it will escape outward through the still pyrolyzing straw, cooling and killing the oxygen. In fields where there is no easy access to water, this is the only way I think that we will be able to quench without busting our springs toting water by truck!)

Tomorrow, Wed., I will test various thicknesses of a “cement board” to see how it stands up to heat. Because it comes in 4×8 sheets, it lends itself to the extremely easy construction of modular wall and top pieces for a cubical kiln in which the board can be easily and quickly replaced if broken or burnt through.

If with testing it turns out that cost/burnout rate make the cement board solution impractical, a second option is to use the corrugated roofing steel Stuart used. Because it comes in much smaller and less convenient sizes, however, it will require much more work in the construction of the frames, and from a practical point of view seems to be a second choice at this point. That said, tomorrow I will test the sheet steel and cement board at the same time to see if I can establish use-life comparisons between them.


I then proposed building the same basic box at the School of Renewable Energy to its Director Dr. Natthawud Dussacee. He gave me the go ahead. It’s built and is ready for a trial early this coming week. I’ll keep you in the loop.