Okay, I am just going to try to address some issues as I read through this list. Sorry for not quoting stuff.
Building stuff round has it's own problems that I won't get into but the biggest problem is just finding suitable round material. Water heater tanks are still a little thinner than I would like. As for scrap metal yards, for some reason our's does not allow anything to be sold out of the yard. I have no idea why but I hope it is not a trend for everyone else. Also, my experience with the water heater tank shows that it is a lot harder to get an air tight seal on a curved surface and I really want this to be an airtight unit.
With the air coming out the bottom I am really just copying the Eko design again. I think for the most part the ash should be able to settle into the ashpan while the rest of the air moves around. It will depend greatly on the velocity of the airflow which I will look into later. I am not against swirling or doing something like the Tarm but I just haven't given it much thought yet.
AS for the previous thread, I was just thinking about a regular wood furnace with a totally cooled and filtered exhaust. I said that I didn't want to get into gasifiers but who am I kidding? I already had plans to build an outdoor kitchen that featured a woodgas cooking range. The problem is that it's already December and I still need a cleaner form of heat in my house. I can't keep our woodstove from dumping a bunch of smoke into the house several times a day. I think we just have one of those bad microclimates around our chimney.
We're on the grid and subject to it's wanes and wants but in reality it's really never off more than a few hours. We're lucky if the woodstove is still hot at 3am.
According to the Paul Anderson and Tom Reed paper on small gasifiers a 1:5 ratio of primary to secondary air is required. The restriction of the fuel in the pyrolisis chamber also has to be factored in. That's easy to say but harder to do. We may be looking at designing some low tech flow meters to place on the air inlets. I have two designs in mind already. One is to make something roughly like one of these http://www.dwyer-inst.com/htdocs/flow/SeriesRMPrice.cfm It's and egg shaped steel ball inside a slighty larger diameter cylinder. When air flows past it the egg rises in proportion to the flow. The simpler and more practical solution is to just use a couple of monometers to measure the static pressure of the airflow. We just need to see the 1:5 ratio so it doesn't need to be any more complicated than that.
It would also be nice to be able to see the flame but that is an optional detail that can be worked out by the builder.
What concerns me about the temperature of the secondary air is not the combustion temperature but the cooling of the surface that it runs through. If I understood how the unit should be lit, I might not be concerned.
Some amount of water is necessary for pyrolisis. Just thought I would throw that in; don't really know much about it.
Maybe I should design a cleanable heat exchanger capable of collecting creosote in a safe place. Self scrubbing? I would really like to know more about scrubbing methods.
I'm thinking downdraft because I want to be able to add wood continuously for as long as I can. I can shut her down and clean the ashes out once a week. Maybe make a batch of charcoal once a week too.
With inlets at center and top and fan failure, disaster is sure to ensue. Or maybe not. I have some numbers to crunch on airflow before I can be sure but maybe the inlet holes are too small to allow enough airflow without the blower's assistance. Just to be safe though, I could probably add some flaps to the primary air inlets so that air can only flow in and not out. This might be enough to starve any potential runaway fire.
The way that I would intend to use this furnace is to place it in an out building and circulate the heated air under the house. I think I have also been designing in my mind a unit that could work inside the house as well. Just like a really efficient wood stove or furnace.
The Eko uses plate steel that is 6mm and 4mm thick or .236" and .157" respectively. I think the most important part about using plate steel is the use of refractory cement, fire brick, and insulation. I think these drawings will need to be changed to reflect the use of refractory and fire brick. I don't think I will worry about insulating the outside.
The Paul and Tom paper refrenced above also states that for each kg of fuel burned, approximately 6 cubic meters of air needs to be delivered. This is equal to 2.2 lbs and 211.9 ft³ or 1 lb to 96.3 ft³. I think we can just say that we need 100 ft³ of air for every pound of wood.
The small Eko can use wood with a 7" diameter and 20" long which is equal to .445 ft³. A good average for split wood around here might be a 5" diameter by 16" piece of wood which would be equal to .182 ft³.
Oak has a density of 45 lbs/ft³ and pine has a density of 25 lbs/ft³.
The Paul and Tom paper was talking about small bits of wood though so I don't know if there will be a direct correlation. Perhaps available surface area will make a difference in the amount of airflow needed.
The Eko claims it can burn sawdust. Any ideas on how this might be done?
Some designs call for having enough air space within the fuel in order to operate properly. Do you think this is necessary in all designs? If you were making charcoal in a 55 gallon drum, couldn't you throw a big ole tree trunk in there and turn it into a big ole hunk of coal?
Sorry for the painfully long post.