I have to say, I come from the agroecology and gardening end of the permaculture community, so when it comes to engineering topics, or basically has anything to do with generating power or heat, I'm much less technically inclined. But I am trying to break through these walls a bit and understand them! Still, I'm confused about a number of things, even having poked around these and other forums quite a bit. I have a lot of questions, and they sort of jump around but please bare with me!
My thinking was that a simple project to introduce me to understanding this world would be to build a simple charcoal maker/kiln/retort (first question: is there any difference between these three things, or are they basically synonyms? I've read conflicting things on different pages!).
In a resource-constrained, too-frequently polluted world, before settling on a design, I wanted to understand what retort/kiln design would be most efficient (both in terms of minimal fuel input and maximum charcoal out) and generate the least harmful emissions (e.g. particulate matter or other nasty gasses). (Second question: What IS the most efficient design? From what I've read both TLUD and Rocket Stove designs can work well, but what are their pluses and minuses? Are there others? After reading about several different types of designs, I'm still unclear, probably because I don't understand the basic concepts well enough)
In trying to read about charcoal making, I ended up frequently on pages that discuss wood gasification. I did not understand before today that in charcoal production, the "smoke" exiting the chimney is actually wood gas (at least that's what I've read and heard in articles and videos trying to figure this out!). But it was my impression that pyrolysis (which is what we're looking to encourage to make coal) occurs at lower temperatures and lower oxygen levels than gasification (which is what you want to be at to produce usable wood gas). And my understanding was that in gasification, your goal is to burn the fuel so hot that you're NOT left with charcoal (or at least as much of it), and that it essentially all gasifies. So I guess third question: Could one design a charcoal maker/kiln/whatever from which one also harvested the wood gas exiting the system? Or are these two mutually incompatible processes, where efficiency in one (e.g. good at producing coal) would be bad at producing the other (e.g. low quality or low amounts of gas)?.
If they're incompatible, from what I've read, being a pragmatic hobbyist on a suburban-size lot, there doesn't seem to be much reason for me to develop a wood gasification system. In my short time exploring this topic, I've learned I have NO interest in storing wood gas (it seems like a good way to accidentally make a bomb, if you're as non-tech savvy as I am...), and so wood gasification is only really valuable if you can use it right away, say hooked up to a gas-powered generator. And those wood gas cars are cool, but I feel like my bike is infinitely more efficient, and I at least know how that works :p So at best, I'd really only be putting all of this work into developing something that could power a back up generator? Would wood gas make more sense to use as a cooking fuel since it burns cleaner, or can I just use my charcoal grill and feel pretty good about that? I guess I'm just trying to understand: in what ways would/could wood gas fit in more urban/suburban permaculture homestead settings, if at all?
For that matter, from my limited understanding, the power density (as described by Vaclav Smil) seems bad for wood gas and even worse for coal gasification. From a global perspective, does gasification really make sense as a form of sustainable power compared to solar, wind and hydro? It seems like the land footprint that would be required to gather sufficient amounts of wood would eventually lead to deforestation, as it did in the 19th and early 20th centuries? Is the idea that it makes more sense in areas with comparatively little solar exposure?
Finally, getting back to the charcoal making, putting on my permaculture hat of "designing for multiple yields," it seems that the yields of a charcoal maker are the charcoal, potentially the wood gas (see questions above), and heat, and a large number of the designs and videos I see people routinely not taking advantage of the latter (burning in the middle of summer, outside, and no mass around the kiln). Are there any charcoal retort/kiln designs that have thermal mass around them to catch and slowly release that heat, a la the rocket mass heater? Is this another case where doing one thing well (capturing the heat) would make another function work less well (e.g. producing less charcoal, or a less efficieny burn?). At the very least, perhaps it makes sense to just save your charcoal making until winter, so that you're at least getting a use out of the heat you're generating?