OUR SAWMILL RUNS ON WOOD!
(Page 2 of 4)
November/December 1982
By the Mother Earth News editors
[2] In order to eliminate the need to purchase some of the 90° pipe elbows and short nipples required, we made an inexpensive tool (see the sidebar for details) to actually bend one end of each of the 3/8" iron air inlet tubes into a J-shape. These pipes were then inserted into the gasification chamber from the bottom (rather than the sides), and right-angle street ells—with 1/8" air holes drilled into their upper surfaces—were threaded to the exposed ends of tubes. As a result, the number of both horizontal and vertical jets has been doubled, thus reducing the potential for solid-fuel "bridging" above the hearth zone. (This last modification has shown us that even the ballpark calculations mentioned in our wood-gas update—see MOTHER NO. 71, page 164—aren't all that critical, and that there's considerable leeway in sizing the nozzle-to-hearth ratio.)
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A "COOLER" DESIGN
Through experience, we've discovered that the effectiveness of the cooler/filter system is probably the single most important factor in any successful wood-gas generator, since the dual-purpose unit must strip soot and tar from the raw gas and cool the smoke to below the dew point of water to remove excess moisture from the fuel.
And although our "old" scrubber was fully capable of performing both of those tasks, its capacity was limited by its dry medium. Furthermore, the new design is less expensive to make and easier to service.
Our latest unit forces the smoke to pass through two chambers, each filled with a mixture of water and a nonfoaming wetting agent (such as automatic dishwasher detergent) that interacts with the ash and tar in the smoke, causing those particles to sink. These filtering chambers are separated by a compartment through which cooling fluid is continuously circulated.
By referring to the illustration, you can see how the apparatus works: Hot gaseous fuel is drawn into the preliminary bath near the base of the scrubber, and bubbles up against the intermediate (cooling) chamber, splashing water as it does so. The smoke is then pulled back through the fluid and up along the outside wall of the cooler (Tank 2). As the gases approach the top of the unit, they're drawn back down through the space between Tanks 2 and 3, and into the fluid reservoir in the secondary filter chamber. Another direction change then takes the smoke back up along the concave bottom of the inner vessel (Tank 3), where it's forced to bubble through a field of 1/4" holes before being released from the unit. (Any excess liquid that accumulates in the center tank-as a result of positive pressure from the inlet ports below-is allowed to restabilize by returning to the secondary filter chamber through four downcomer tubes ... and the sight glasses in the filter chambers allow the water level to be monitored easily.)
Besides simply cleaning the gases, of course, the two filter chambers cool it, since the liquid mediums within them are in contact with the low-temperature intermediate chamber at all times. This central cooling cell is formed by welding an extra bottom onto the base of Tank 2, then directing the coolant inlet fitting at an angle tangential to the chamber's periphery. A coupler, located at the center of the well, serves as an outlet.
