Off Grid Power in Liberia 

Appropriate Fuels


For more detailed information about feedstocks, download the Full Feedstock Section of the Technical Handbook.

For your gasifier to operate properly, you must use the correct biomass fuels which have been properly prepared and sifted.
Feedstock fuel must be dry, of the correct shape and size to flow through the reactor, and free of dust, sand and contaminants:

  • Particle size: 1 cm – 4 cm (0.5 in. – 1.5 in.)
  • Moisture content (% by dry weight): 5% – 30%
  • Ash content <5%

The following Table of Feedstocks shows the most common ones that have been tested and are known to work, which ones are known to be unusable, and which ones need more testing. For more information on fuel preparation, contact APL For more information on fuel preparation, contact APL.

Download printable version of this chart

GreenKnown to work with standard operations and maintenance effort
YellowKnown to work with increased operations and maintenance effort
GrayNot enough testing to approve at this time. Use voids warranty
RedKnown to not work. Use voids warranty
Wood Chips
e.g: Oak, Rubber, Pine
Use only chips; chunks or long shards can bind auger or bridge in the reactor
Nut Shells
e.g: Coconut, Walnut, Hazelnut
Not all shells will work, please contact us to discuss your particular feedstock
Corn CobsMust be broken to size and must not include husks. Increased chance of slagging
Palm Kernel ShellsRisk of high temperatures. May need to be blended or other steps taken to lower temperatures
Macadamia Nut ShellsExcellent shape, not enough testing
Cashew Nut ShellsKnown toxicity, not enough testing
Wood PelletsMay work depending on size & makeup, pellets prone to decompose
Coffee GroundsToo fine, not physically compatible, pelletization may allow use
Saw DustToo fine, not physically compatible, pelletization may allow use
Corn stoverHigh ash content; silica content leads to slag
Rice HuskHigh silica content leads to slagging
BambooDifficult to prepare to correct size and shape
Grasses: Switchgrass, Miscanthus, etc.High silica and low bulk density.
Paper, Sugarcane Bagasse, Coconut HuskShapes not physically compatible
Municipal Solid Waste / TrashSlag risk; heavy metals; plastic content not suitable
CoalBurns too hot, releases sulfur and heavy metals
PlasticsMelts and fouls auger/reactor
Manure - Cow, Pig, Chicken, etcHigh slag, low energy density
TiresNot chemically compatible

Feedstock Processing and Handling

The PP30 requires its pieces of feedstock be larger than ½ inch (1cm), but smaller than 1 ½ inches (4cm), with a moisture content between 10% and 30%. Feedstock that is too big can jam the internal feed system. A max of 10% of the feedstock may be smaller than ½ inch because these small pieces can clog the reactor and prevent the syngas from flowing, as well as causing bridging jams in the hopper.

The ideal feedstock has smooth-surfaced chips, such as found on many nut shells, which make the most ideal feedstock. However, most Power Pallet operators use wood chips, even though chips that are very rough can cause fuel jams when the pieces lock together to form bridges rather than flowing.

Side by side comparioson of Bad: Shredded Wood and Good: Wood Chips


If you intend to use wood chips with the PP30, you will probably want to chip your own with a wood chipper. The two main options are auger chippers and disc chippers. Auger chippers (such as the Laimet brand from Finland) chip to a very regular size depending on the size of the screw auger installed.

Disc vs. auger chipper blades
Disc vs. Screw Auger Chipper Blades

While these make very consistent chips, they are much more expensive than the more common disc chippers. While less expensive to purchase, chips from disc chippers need to be sifted to select the right-sized chips, which results in the loss of the feedstock that is either too big or too small. For efficient sifting of large amounts of feedstock, we recommend using a mechanized sifter.

Drying Feedstock

The 5%-30% moisture is measured on a dry-weight basis. Wetter fuel works best at high power levels, because there is more waste heat to dry the feedstock. Moisture levels under 10% can cause too-high temperatures in the reactor, which can fuse the ash and cause clinkers that can clog the reactor.

Large Clinker  of Fused Ash
Large Clinker (Fused Ash)

During start up it’s best to use dryer feedstock (<15% moisture) in the bottom of the hopper and top of the reactor.

The best drying method is to chip green wood and then dry the chips by spreading them out on a tarp or screen and leaving them exposed to the sun and wind. This is because dried wood tends to splinter and form rough pieces which tends to cause bridging and jams. We can supply simple plans to build drying racks that use the radiator exhaust to help speed drying.