i’m not even sure i should be posting this but i had saved a link to this website years ago, and find that it still is active, although i haven’t gotten a reply from the site owner although i sent an email a few months ago.
i wanted to share it because i think it may offer a basis to think in another direction on a couple of issues:
type of fuel (wet wood chips) and category of burn design (“single chamber stratified batch combustion method”)
now the site is about what the author did in 2006/2007, and the research was intended to develop a simple wood chip furnace, but the method involved the gasification of wood chips and apparently was quite successful until they introduced woodchips that were not just wet with moisture content but also environmentally web (rain and snow on the surfaces). it seems to me that there are several possible ways to gasify wood, and if this is yet another way, then perhaps there are possible pros and cons and methods of controlling it as well.
anyways, just throwing this out there and am interested if this has any possible application for what everyone is interested in here: producing wood gas for internal combustion engines.
(i am including this section from the second page to whet any appetites that might be out there:)
"If we were to build an ideal wood fire, as opposed to a natural one, we would have the embers on top. The radiant embers would gasify the wood underneath. Cool air and fuel gas would mix well in some porous insulating medium at this lower level and move upwards until they reached the embers where ignition would take place. If we could just push air through the substance of wood from underneath we’d have nearly complete combustion, a hot transparent flame, and little or no smoke.
Well, wood isn’t a porous spongy insulator. But a bed of wood chips in a container is a close approximation. We can filter air up through an insulating pile of chips. If we can move embers to the top, the fuel gases should burn, and burn cleanly. This is the method used in the VTHR furnace.
While it is a simple concept, the main design challenges in developing the burner were maintaining stratified temperatures for clean even combustion, fuel to air ratio, thermal stresses of the furnace wall, heat exchanger design, fuel loading method, prevention of gas leakage, and compact footprint and form factor.
Why green wood?
Water in fuel can either stop or slow combustion, depending on the amount, the temperature, and the location. Slowing combustion isn’t always a bad thing, however. In fact we slow combustion on purpose in high compression auto and aircraft engines.
High octane fuel is actually fuel that burns slower than low octane fuel. Some racing engines have used water injection to mimic high octane fuel and slow combustion. In an engine this results in a longer period of steady expansion in a cylinder, rather than a sudden damaging explosion of fuel. We call explosion in an engine “ping” or detonation and it is detrimental and power robbing.
In the experimental furnace I’m studying, combustion is aided by being slowed down. Damp fuel means that the air supply can be filtered through the chips to adequately supply the needs of the slow burning upper hot layer. Faster combustion would yield insufficient air supply for the upper layer and a fuel-rich condition.
What this means is, in the VTHR furnace, if we use dry fuel, we get smoke. If we use green wood it burns cleanly. This is just one of the many non-intuitive aspects of this design.
Another thing that happens with moisture in the experimental furnace is that we get a phenomenon called the “water gas reaction”. This basically causes the incandescent charcoal in the hot layer to combine with superheated steam from below to produce two good fuel gases, carbon monoxide and hydrogen, which recombine cleanly with oxygen only above the charcoal where the temperature is a little cooler.
Another reaction which results from the movement upwards of fuel gases from cool to superheated zones is that tars and creosote are broken down by the intense heat before combustion to simpler fuel gases in a process called “cracking”. Like water gas, these cracked fuel gases, well mixed with the right amount of oxygen, burn cleanly only as they move past the dissociating incandescent charcoal layer to a slightly cooler area…"