In my last post on biochar we discussed the best way to accumulate a sufficient inventory of corn stover. It must be kept dry in a large bulk storage shed and allowed to continue air drying to produce a uniform and predictable feedstock. The next challenge is to produce the biochar itself.
My suggestion is to build a modified incineration device that is rigged for top down charging unto a well raised grill. The maximum temperature will be 600 degrees. The idea is to allow the heat to break down the organics allowing the produced carbon to crumble and fall through the grill. The operating temperature brings this about. Some ignition may occur but it is minimized by a lack of oxygen.
As the charge roasts off, it dramatically compresses on a one for ten ratio and is easily shaken through the grill allowing recharging through the top loading device. One can envisage a ten ton charge, been recharged five tons at a time as the produced carbon passes the grill. Thus once established, the process becomes fairly continuous although there will be variation in the produced gases. We are avoiding significant ignition within the chamber as much as possible and using heat to reduce the plant waste. This will be a slow process taking some time and may be as much as half a day, though The nature of the plant waste suggests that it may be much quicker.
The produced carbon can be built up in the chamber underneath the grill and should be designed to store a fair amount. The lower portion of the carbon will cool down, perhaps with an assist from a little water or steam. However the lower layers of carbon powder should naturally cool to allow an auger to remove the lowest layers. The main danger is that if the temperature is too hot, it is capable of spontaneously igniting. Thus removal must be monitored and preferably kept in an airless environment during the process.
The heat is produced by the process gas itself which contains all the produced volatiles and is passed into a second chamber for combustion at high temperature at around 2000 degrees. This is high enough to reduce all the components safely and produce a hot flow of CO2 loaded air that is partially fed back into the main chamber under the grill. This delivers high quality process heat into the charge from the bottom up. Once established, I see no reason to introduce oxygen at this stage. Even it the charge climbs well past the 600 degree mark, the lack of oxygen will keep it under control.
I suspect that operating experience will suggest lower temperatures closer to 400 degrees as sufficient and even preferable in terms of product quality. This design allows such experimentation.
Also such a system might be operated automatically after final charging and allowed to cool down over night in order discharge the carbon in the morning. After all it will simply run out of fuel in the form of process gas.
This becomes a simple system. The first chamber handles the bulk and operates at a fairly low temperature range so it can be constructed with low cost fire bricks and ordinary sheet steel. It is really a large wood stove. The main thing is to keep it fairly air tight. This prevents any of the produced carbon from burning unnecessarily.
The second chamber receives production gases at a temperature of 600 degrees. It is blended with air and burned immediately bringing the temperature to 2000 degrees. Some of this well oxygenated output gas to fed back into the first chamber to deliver heat. The remainder is sent through a boiler to strip the excess heat out of the gas before it is vented. That heat may be used then to produce power or operate a greenhouse.
In fact, the needs of green house operations suggest that this can be best integrated with biochar production. Green houses need hot water during the fall, winter and early spring. The corn stover is delivered during September and October, and its consumption is easily fitted into the winter schedule of the greenhouse. The system can then be left idle during the growing season when plant waste is not available, temperatures are high and no biochar is needed for planting.
Greenhouses are always looking for energy sources that are outside the hydrocarbon regime, and most never quite solve the problem. I think that this is a solution in corn growing country.
I have not discussed the possibility of developing a cash economy around this whole process. I first had to make it internally profitable for each participant. For the farmer, he trucks his ten tons of chopped and gathered corn stover to the biochar facility in exchange for at least a ton of biochar in the winter in preparation for spring sowing. He solves a disposal problem and way more importantly, he receives a powerful soil amendment in sufficient quantity to do some good and encourage repetition.
The operator has the capital cost of the plant and storage shed. He does not have the cost of building up inventory. He gains a revenue stream from power production and that should be significant and also attract financing support. If the heat is additionally fed into a greenhouse operation, it is reasonable that the whole process will turn out to be profitable. Again, it is all working regardless of the biochar market itself.
The operator will also produce a surplus of biochar depending on his terms of trade. It could be as much as is handed back to the farmer. There is presently no market, but one should evolve rather quickly as farmers see the value of blending it with fertilizer. Farmers not producing corn will quickly begin buying up the surplus for their fields. Thus we develop a biochar market.