here's the things i find required to make a gasifier work, in order of importance. there are of course many other factors of relevance, but for getting started, these are the highlights for what you need to get right.
1. use a chunky fuel with low moisture content.
low void space fuel, or a high void space fuel mixed with lots of small and fine pieces, will make even the best reactors fail. moisture too will help you find failure where success should be happening. fuel issues trump all other issues with a fixed bed downdraft type gasifier. use a known good fuel in the beginning.
2. pull it at the right rate.
if you do not have a manometer on your gasifier, and are running an unknown blower, you are shooting at a hummingbird, blindfolded. nothing will work correctly until you are pulling the reactor within reasonable bounds. this seems to be between 1 and 10" of h2o. we find the 4-6" the ideal on ours, but we run rather small nozzles.
3. get the nozzles the correct size.
this is usually smaller than you think. for most of us on these small units they will be between 1/4 and 3/8 for 5-7 nozzles. i suggest using the traditional imbert chart and reducing the nozzle size by 25%. too small is better than too big until you are clear on things. as your fuel gets smaller, you need smaller nozzles. remember the traditional imbert chart assumes nice chunked wood with lots of void space. most of us are trying to run screened wood chips. there is more resistance to blast penetration. you need higher velocity blast, thus smaller nozzles than the chart indicates.
4. properly size the rest of the hearth
make sure you have a properly sized constriction to your flow rates. somewhere between 3 and 4 inches is what most of us start with. again, start too small. under 3 and bridging will be a big problem. over 4 and the needed flow rates start to get quite high for early runs. nozzle height i find not terribly important within the common bounds we work in usually. the book height dimensions are fine. nozzle ring diameter seems more important in terms of bridging. too small a nozzle ring and you will have increased bridging problems. again, the book numbers on nozzle ring dimensions are good.
with these things in order, any gasifier will produce reasonable gas.
with these things out of order, any gasifier will produce a mess.
ps- the imbert sizing chart with all the "book" dimensions is here:
Top 4 ways to make your gasifier work! (see here for cheat sheet)
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