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Here's our index of machines relevant for preparing gasifier fuels. Please add more that you find. The below is all the options "other" than spending $50,000 on a Laimet spiral screw chipper.
APL / GEK 55gal drum top shear proposal
55gal drum top circle shear chunker
Inside circle shear firewood processors
http://www.youtube.com/watch?v=nZq9OLLA0tI
http://www.sixfold.ee/?go=gallery&g=1
Self-feeding spiral cylinder shear
Wayne Keith's truck axle cylinder shear
http://www.youtube.com/kb8ooe#p/a/u/2/djaSTzDKb3k
Jussi Äystö cylinder shear
http://www.youtube.com/watch?v=bg37Ub9CYfg&feature=related
http://www.youtube.com/user/quafka#p/a/u/1/BjOUk3Hh-1M
Ed Burton's Spiral Shear
USDA Teardrop Chunker
http://gekgasifier.pbworks.com/f/teardropchunker.pdf
Rojek chunker from Czeck Republic
http://www.youtube.com/watch?v=QEzwdKSIyoU&feature=related
Jenson Chopper via Rolf Uhle
Manual wheel guillitine
http://www.fluidynenz.250x.com/_framed/250x/fluidynenz/simplechip.htm
Urban TR10 firewood chunker
http://www.youtube.com/watch?v=XjRqrfS3plc&feature=mfu_in_order&list=UL
Indian gang saw with feeder video
http://www.youtube.com/watch?v=kV6ShmGQ4HM&feature=related
http://www.youtube.com/watch?v=nhcz1fCHeLQ&NR=1
Wood chippers come in two main types: drum chippers and disk chippers. Drum chippers tend to make a long curved slice. Disk chippers make a more square chunk. Disk chippers are highly preferred to drum chippers for gasification purposes. Even with a disk chipper the fuel is not a thick symmetrical chunk as ideal, but it can be made to work. The fuel still needs to be screened/sorted after chipping to remove the fines and too large pieces.
Greg Manning in Canada has built the most impressive DIY wood chipper and sorter i've seen to date. Here's his photoset on the woodgas forum which shows the set up. http://groups.yahoo.com/group/WoodGas/photos/album/560053423/pic/list?mode=tn&order=ordinal&start=1&&count=20&dir=asc
Greg's set up is based on the common low cost chinese disk chipper, and a rotating mesh cylinder to size separate the resulting chips. These chinese disk chippers are available for around $1500-2000 new and are commonly driven off tractor pto's or 20hp and above engines. These are the smallest disk chippers that will produce a reasonable fuel. The common garden scale disk chippers, like the Troybuilt chippers, do not produce a chip of adequate size. It is more of a shredded wood.
the low cost disk chippers are usually called 4" or 5" chippers. here's one example http://www.libertychippers.com/model/liberty-5-pto-chipper.html#top See other examples below.
Wallenstein BX42 Wood Chipper
http://www.woodwardcrossings.com/Pages/Pages_11-04/Farm/chipper_guide/bx42.html
Starchi chippers with embedded screening. Directed at gasifier use
http://www.rsbiomass.com/rsstarchl.html
DR Power:
http://search.drpower.com/search?w=wood+chipper&p=Q
Mack Kissic:
Redneck Axe Wheel Hand Remover
http://www.youtube.com/watch?v=40sCGb678sQ&feature=related
http://www.youtube.com/watch?v=2bVAAx3mMKY&NR=1&feature=fvwp
Cone splitters
http://www.youtube.com/watch?v=zlpcnCsDBeo&feature=related
http://www.youtube.com/watch?v=X3Rypb1dcUE&NR=1
Small disk splitters from Sweedish Volvo Gasifier
http://www.vedbil.se/dagbok/bygge/films/vedtugg.wmv
V-8 Chain saw
http://www.youtube.com/watch?v=EwyMX7PLlcQ&feature=related
References
Analysis of Wood Chipper Chips, Particle Size Distribution, Moisture Content, etc:
http://www.woodenergy.ie/iopen24/pub/4_physical_analysis.pdf