we've been debating for some time how small we can go in gas flow rates before we can't reasonably keep up the temps needed for good tar conversion. clearly smaller is more difficult in woodgas, as the smaller restrictions needed for good tar cracking start to prevent reasonable fuel flow.
if you have perfect not sticky chunk fuel, you can go down to about 2.5" restriction. the wwII imbert chart goes down to 2.3", suggesting that was the end they saw as tolerable. many of us have personally found that 3" is about the min limit. thus this is what we deliver the gek with, as most are trying to run smallish engines.
but how low can a 3" restriction go. well, we're closing in on a real answer with bear's summer and fall of tests. bear has now completed three fully instrumented parallel tests of the gek running on walnut shells, softwood chips and pellets, all using the standard 3" gek inverted V bell. each datalogged run is a few hour endeavor over the full range of pull rates, with temps, pressures, gas flow rates, and tar production logged in parallel. then a week of fussing with the resulting raw data to make it visual.
we know that 2m3/hr gas is needed for each 1hp/hr. so reading through the tests we should be able to see where the floor falls out from underneath us and gas flow is no longer high enough to create the needed internal conditions.
what is the tentative answer?
well, the raw data is now up for all three runs now, but the tar vials are still in process for the last two tests as of friday, sept 25, 09. the tar/soot soluble separation takes a few days, as does the pictures and mapping the hue density. thus only the walnut shell run has the run conditions mapped to tar produciton. the woodchip and pellet run pix will be coming out over the next few days. you can watch the progress at the links below
even without all the tar data here, there are some very interesting things to see happening between the different fuels. there will likely be some answers here regarding long-standing debates. watch here as more particulars emerge . . .
walnut shell run:
wood chip run:
may the satisfactions of knowledge soon bring peace to the fervent speculations of the gasifying mind.
thank you bear.
Smallest engine for gasifier? Here's where one answer will be
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