Downdraft gasifiers are notoriously prone to bridging and plugging. The problem gets worse the smaller one makes a gasifier, and the less chunky the fuel is. Wood chips and shavings are particularly poor in their flow characteristics.
Enough GEK owners were having problems with this that starting with v3.0 we began including a motorized stirrer standard with every kit. It isn't terribly fancy or a high dollar right angle drive gearhead motor, but it gets the job done for close enough to free that we can add it to the list of standard includeds.
The first gen stirrer was a rod with ellipse paddles attach at various levels- 3 or 4 of them. While this helped the situation much, it can still bind on the most difficult fuels, as the paddles are moving perpendicular through the fuel and can dig in while pushing the pieces. The paddles can also hollow out a tube down the center of the reactor when the fuel is fingery and wettish (like fresh stringy wood chips), leaving fuel stacked against the wall, and the middle empty from bottom to top.
The new solution we are trying (and liking) is a spiral rod auger oriented vertically down the bed, and driven slowly upward against the fuel. This can't get enough traction to really lift the fuel, but it is enough to "fluff" it. The fuel lifts slightly then tumbles back into and around the auger. This method also results in the stirrer "meeting" the fuel as a long inclined plane, not as a perpendicular "plow". This seems to allow one to get the fuel agitation radius much further off center with less energy needed than the previous paddle methods. I like it. I've never seen this method for stirring and it seems very useful. Maybe we should patent it. Nah . . . We're going to start shipping it with kits as of September, 09.
Both of these methods use our highly simplified stirrer drive mount. We originally did this drive and motor mount with complicated wobble joints and flanges to keep binding loads off the motor. It was a giant amount of work for very little benefit. We finally came up with a very simple method that works somewhat like a drill motor and your hand. The motor now mounts directly on the end of the stirrer shaft, the shaft passes through a dimensioned bushing welded into a pipe plug, and a rod extends upward to provide the "hand" to stop the motor from free rotation.
This whole assembly screws into the plumbing bung in the center of all gek reactor lids. Thus it is backwards compatable to v0.7. (Ignore the vice grips in the right test vesion. It should/will have a mount like you see in the left straight rod version. This is all I had handy to take the picture.) Some high temp grease keeps the tar binding at bay, as well as prevents air leaks, at the through hole into the reactor
This stirrer method is also used on the BEK (Biochar Experimenters Kit) to fluff the biomass/char during pyrolysis to increase gas circulation and heat transfer.
Spiral Rod Stirrer
1 reply to this topic
Posted 03 September 2009 - 05:08 AM
oh, and the motor is a little 2rpm motor and linear gearhead. this is much more powerful than the bbq rotisserie motors of previous testing. the v3.0-3.1 went out with a 4rpm version of this. we just got some more in that are a 2rpm gearhead, with the same motor, so now more torque.
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