Wood gas fuel is dry hardwood cut into uniform chunks of 3-5 cm, seasoned to 20% moisture or below on a wet basis (25% on a dry basis) and ideally 10-15% wet basis, and carried in a covered bin so rain does not undo the drying. A 4 cm hardwood cube weighs about 45 g, so the 25-40 kg a car uses per 100 km is roughly 556-889 pieces that someone has to cut, dry and handle.
Part of Wood Gas Vehicles
Fuel preparation and carrying covers the size, moisture, species and storage of the wood chunks that feed a vehicle gasifier.
What Fuel The Gasifier Wants
The carrier’s requirements are:
- Size: uniform 3-5 cm chunks. Too small clogs the grate; too large bridges in the hopper.
- Dry: below 20% moisture, because wet wood makes excess tar that clogs everything.
- Hardwood preferred: denser, with more energy per volume. Softwood works but produces more tar.
- Not to be used: treated wood (toxic fumes), green wood, sawdust and fines (they block airflow).
State the moisture basis every time. Wet basis is water ÷ total mass; dry basis is water ÷ dry mass. The conversion is dry = wet ÷ (1 − wet), so 20% wet basis is 25% dry basis, and the 10-15% wet-basis target is 11-17.6% dry basis. The two bases differ by a factor of two in green wood: fresh wood at 50% wet basis is 100% dry basis, the same wood. See Seasoning & Drying. The charcoal-yield article quotes dry basis (Moisture Content), so do not mix its table with these numbers.
What A Piece Weighs, And How Many You Need
Air-dry hardwood has a solid density of about 700 kg/m³ (an assumption; oak is near that).
- A 4 cm cube is 4³ = 64 cm³ = 6.4 × 10⁻⁵ m³.
- Mass: 6.4 × 10⁻⁵ × 700 = 0.045 kg = 45 g.
- For 25 kg per 100 km: 25 ÷ 0.045 = 556 pieces. For 40 kg: 889 pieces.
Preparing that is real work, and it recurs every day. The cost of fuel for a vehicle is the saw and the splitting maul as much as the hopper.
Drying
The carrier’s own timings: air-dry under cover for at least 6 months for hardwood and 3 months for softwood. The corpus seasoning article gives longer, 6 months for softwoods, 12 for hardwoods and 18-24 for oak, and it aims at 20% wet basis, not the carrier’s tighter 10-15% target. Where they disagree, use weighing, not the calendar.
Measure it, because the carrier’s snap-the-piece test is not reliable. Weigh a chunk, dry it in a warm oven or hot place until its weight stops falling, and weigh again. Wet-basis moisture = (wet mass − dry mass) ÷ wet mass. A 100 g chunk that dries to 85 g is (100 − 85) ÷ 100 = 15% wet basis, or 17.6% dry basis.
Carrying
- Bin: a covered bin that sheds rain, ventilated so damp wood does not sweat.
- Quantity: carry the hopper load plus a margin, and plan resupply points rather than carrying everything (Range & Fuel Consumption).
- Volume: 50-80 kg is 0.14-0.32 m³ at 250-350 kg/m³ bulk density (Weight & Space Constraints).
- Away from heat: not against the reactor, and not in the cab (Heat Shielding).
Method
- Season the wood under cover for the stated months, then check by weighing.
- Split and cut to 3-5 cm; discard bark-heavy scraps, fines and dust.
- Store in dry sacks or a covered bin; re-wetting ruins the fuel.
- Load the hopper in dry weather where possible.
Common Mistakes
- Treated or painted wood. It gives off toxic fumes, and its ash is a hazard.
- Judging dryness by looks. Weigh it.
- Rain on the load. The fuel is no longer 20%.
Safety
Refuelling opens the hopper onto a bed of hot gas
Producer gas is about 18-22% carbon monoxide by volume, and the hopper is the highest-concentration point at the vehicle. Refuel outdoors and upwind, with the draw on the unit so air moves down into the hopper, with your face turned away, and never in a garage, shed or enclosed yard. Treated wood gives off toxic fumes. A hot bed can flash at the lid. See Carbon Monoxide Risks.
What’s Next
- Learn next: Operating Procedures — using the fuel day to day
- Combine with: Coppicing & Fuel Management — growing the wood