Start with no more than a steel drum whose top and bottom has been removed. Place a layer of inch thick branches down as a packed floor for the kiln. Place the kiln end up on to this floor. Air will be able to pass under the edge of the drum through the packed branches at a moderate speed.
Pack the drum with chipped wood or chopped biomass. Do not create tight layering that could cut of air flow entirely. Yet get the packing level up to fifty percent. Once the drum is filled, place a charge of dry wood to act as a starter on top of this load. You may already place six inches of soil around the edge to reduce the center diameter to a quarter of the total. Fire it and let the fire burn most of the wood layer in order to be fully engaged.
At this point smother the top of the fire by throwing six inches of dirt over the center. Or alternately, place a metal lid with a six inch chimney pipe with a damper for fine control.
What we have done here is very familiar to those of us who experienced the methods of the nineteenth century. We have actually banked the fire. The surface cannot flare up losing both heat and fuel and only a limited amount of fuel is burning at any one time and it is mostly in the form of the volatiles in the early going.
Over several hours, the burn front will migrate down to the ground and burn out the floor of this kiln, allowing the edge of the kiln to connect to the earth, and cutting off the air flow eventually. In practice, I consider this to be more of a fail safe to prevent a total burn out of the fuel charge as eventually happens in a banked stove.
I like the dirt layer idea, with or without a metal lid. It acts like a filter for the escaping gases and likely maximizes their combustion. In addition, it will end up been blended with the end product to produce a dry easy to handle mixture if the fire is not quenched with water which is likely necessary.
I suspect that it will simply be better practice to water the fire before it fully engages the floor. Once that point is reached we are very close to running out of volatiles and the charcoal then becomes the primary fuel.
Thus, in lieu of naturally packable materials such as corn stover and bagasse able to produce an earthen kiln, we have a simple metal kiln design that is easily expandable and able to work on the modern farm. There a square set metallic box can be set up in the same manner and material loaded in and packed. This is all rough and ready and certainly will not achieve the optimal thirty percent yield, but it will produce twenty percent or better quite handily.
The key idea is to have the bottom edge set on a layer of branches or any other material able to sustain a fifty percent air flow in through the bottom. The top layer of dirt might be dispensed with if a holding layer does not exist. The Indians had palm fronds to work with. A metal sheet with a chimney closing it all off will do the rest.
My most important point is that this is easy to assemble in some form or the other anywhere and regardless of the local economy. Old rusty galvanized sheet steel is very suitable. You may even get away with using rope on the outside to hold it together. After all the core temperature will be mostly in the core and still be around 300 to 400 degrees. Hot spots on the wall will need an unusual source of air and that really means a fully engaged fire. If that is happening, you have plenty of other problems and it is not working at all.
I really do not know, how much char is to be applied per hectar. But I can tell you how to make char out of your burnable organic waste. The simplest device is a top-lit updraft kiln. It consists of a vertical cylinder, having relatively small holes near its base for primary air. You fill the cylindrical body of the kiln with the material to be charred and then light it from the top. Once the fire gets going, you place a lid on the cylinder. There is a chimney built into the lid. The lid does not sit flush on the kiln, but there is a gap between the lid and the kiln. The draft created by the chimney sucks secondary air into the chimney, where it gets mixed with the pyrolysis gas to burn it. The biomass burns downwards, leaving a layer of charcoal on top. As the primary air comes upwards, it meets the burning front which traverses downwards. The burning biomass utilises all the oxygen in the primary air, so that the air going up through the layer of char has only carbon dioxide, carbon monoxide, nitrogen and the pyrolysis gas left in it. As there is no oxygen left in the updraft air, it cannot burn the char that has formed above the burning biomass.The pyrolysis gas and carbon monoxide burn in the chimney, because of the secondary air that is sucked in through the gap between the chimney and the kiln. You have to find out by trial and error, how long it takes to char the material loaded in the kiln. After that much time is over, you remove the lid, and extinguish the fire by sprinkling water over the burning material. This particular device is portable and manually operated. There are larger charring kilns, based on the oven and retort process. Prof. Yuri Yudkevich, a Russian scientist, has made them for charring useless material generated by the timber industry in Russia. We are already using both types of kilns under field conditions in India for charring agricultural waste as also urban waste. We have a video CD that describes the kilns and you can fabricate them by watching the video CD. I have not used Prof. Antal's kiln and have absolutely no idea how it operates. Our web site www.arti-india. org would show you how to get our CDs by paying us through Pay Pal.