Wood processing machinery for biomass: four machines and what they decide

A biomass fuel line has four main machines. Get one wrong and the cost per ton changes for the whole line. Here is what each does and what the decision rests on.

The four machines

Chipper. Turns logs and offcuts into chips at a set size. The first point that determines whether the output is consistent. Blunt knives produce more oversized, long slivers, take about 30% longer to cut and burn about 39% more fuel. The share of fines does not rise.

Hammer mill. Used when making pellets. Grinds chips fine enough to press. Screen size sets the fineness.

Dryer. Brings moisture from roughly 40–50% down to about 10% before pressing. The heaviest energy consumer on the line, at roughly 70% of its energy, and the main reason pellets cost more than chips.

Pellet mill. Presses wood powder through a die under high pressure. Friction heat softens the lignin in the wood, which binds the pellet without any added glue.

Why chips cost so much less than pellets

Count the stages.

Chips: screen → chip → ship.

Pellets: screen → chip → mill → dry → press → cool → grade → ship.

Drying alone consumes enormous energy, because tens of percent of water has to be evaporated. In one set of published figures, drying took about 350 kWh per ton against about 49 kWh for pelletising.

The feedstock itself also costs more per ton of pellets. One ton of pellets at about 9% moisture holds roughly 910 kg of dry wood, which takes about 1.6 tons of wood at around 43% moisture. Raw material alone is roughly 1.6 times the chip figure per ton, before any drying.

Chips against pellets

	Wood chips	Rubberwood pellets
Calorific value	~2,200 Kcal/Kg	~4,000 Kcal/Kg
Moisture	~43%	~9%
Density	Low, needs yard space	High, easy to store and ship
Production stages	3	8
Suits	Boilers and power plants with yard space	Tight space, automatic feeding

For the decision criteria in detail see wood chips versus pellets.

What pellet-line investors most often get wrong

Underestimating feedstock moisture. ROI modelled on 30% wood when 45% is what actually arrives. The dryer works harder than designed and output never reaches target.

Forgetting the cost of screening. If the feedstock carries soil, knives and other parts that touch the material wear faster. Allow extra for maintenance.

Comparing machine price instead of cost per ton. A cheap machine that draws heavily and stops for repairs costs more across its life than an expensive one that runs.

Ask before investing

We run this line ourselves in Trang every day and are not an equipment dealer, so we can advise from feedstock screening through to machine specification. Call +66 92 392 2292.

Read on: why deciding too fast costs more

Sources

• FAO (Krajnc), Wood Fuels Handbook (2015)
• Tumuluru, Idaho National Laboratory, INL/JOU-15-34655
• Nati & Spinelli, FORMEC 2010 proceedings paper (2010)
• Nati, Eliasson & Spinelli, Croatian Journal of Forest Engineering paper
• Roy et al., Oak Ridge National Laboratory, Oak Ridge National Laboratory report (2020)
• Naimi et al., Biomass Conversion and Biorefinery paper