I am far less comfortable using various oven designs and pressure chamber converters to achieve largely the same end with a marginally better yield, yet with an order of magnitude jump in handling costs. My best design concept of the two lung incinerator, while maximizing yield will also demand to be fed year round in order to be possibly economic. And that also applies to pyrolyzers and the like. This means that a minimal 1000 ton per day operation will require at least a 1000 square miles of supply area and all the trucking that goes with that. Tom Miles is certainly not wrong on this.
My single farm modified container will only operate for around a month during the appropriate season and very little in between. It must be cheap and I do not know if that will actually be achievable. The second lung and its controls could turn out to be commercially crippling, principally because an expensive high grade fire brick must be used.
I keep coming back to the simplicity of carefully field stacking corn stover to produce the biochar. We know that this will yield a mix of char and soil representing a twenty percent yield with only a small increase in handling effort. With equipment we can actually build windrows, even driving on top of them to compact the stack properly before covering with dirt and igniting.
The only drawback, which seems to make some folks hysterical is that we lose the volatiles into the atmosphere. Most of this is CO2, while the rest is in the form of a wide range of organic molecules, similar to that produced from a forest fire or slash and burn agriculture. The heavy end falls back onto the soil, while the lights are typically degraded sooner or later in the upper atmosphere. Methane and probably ethane even end up in the troposphere above our atmospheric circulation system.
Unlike forest fires and their like, this process sequesters a great deal of carbon. Which returns us to the whole point of the exercise. Adding charcoal to the soil appears to vastly improve and stabilize the majority of soils. Right now we do not know were it does not work.
This is because charcoal is a strong acid, yet is insoluble. That allows it to grab nutrients year after year and recycle them back to the plants. A minimum amount of maintenance ensures maximal fertility anywhere once the initial effort is made to create the soils.
I suspect that, while terra preta soil manufacturing was the dominant culture in the Amazon, that there is no reason for it to be a continuously applied system in most soils. After all we know that a season's corn production will generate around a ton of charcoal per acre which is actually a lot already. Fifty tons per acre is likely the maximum that you would ever want in the soil.
Thus doing corn with terra preta in normal field rotation is very plausible everywhere. Europe and North America are the most glaring examples that I am familiar with, and I am very sure that this will be another green revolution in both India and China. Fifty years of effort and all crop lands will be well on the way to be terra preta soils and their permanent fertility will be secure. I can tell you that from a farmers perspective, that this is almost too good to be true. Fertility has been foremost on their thoughts forever.
Even more exciting, this looks like a method to restore fertility in despoiled lands were past practice has destroyed fertility and with it the soil's water holding ability. Mesopotamia particularly leaps to mind. Why should the Garden of Eden be covered with blowing salt ladened dust and treeless hillsides.
I am hopeful that the simple restoration of irrigation, can allow a corn crop to be nursed into full growth. Remember that the root practically lies on the surface, so working the top three inches of soil with biochar should quickly restore these soils. The important question is whether the charcoal will progressively sequester the salts and as a result to gently sweeten the soils. If it does not, there are still practical options because of the soil improvement brought on. They will simply take longer to have effect.