Biomass screening for the production of biofuel

Biomass is cleaned and calibrated with a disc screen, which in a single pass separates the material from contaminants and splits it into fractions of the size you want. The undersize fraction collects fine materials such as soil, sand, and any small plastic fragments; the mid-size fraction represents the ideal size to be used as wood fuel, while the oversize material contains oversized pieces that are sent back to the shredder to be reduced and reprocessed.

These three fractions serve different downstream purposes: for wood fuel (chips, pellets), a uniform, clean particle size means stable combustion, lower ash content and no auger feed blockages in the boiler; for biogas plants, removing soil, stones and plastics before digestion protects the reactor and improves digestate quality. Ecostar disc screens cover 10 to 200 t/h with fractions selectable from 10 to 200 mm, even on wet material: the range includes the Hexact stationary screen and the Hextra and Nhexa mobile screens.

Wet biomass can be screened without stopping the plant thanks to DDS (Dynamic Disc Screening) technology, which handles wet material without clogging: the discs rotate with a motion that reduces clogging and wrapping, while the low pulling force keeps the flow steady.

This is exactly the weak point of traditional screens, where wet material causes frequent downtime and constant cleaning. With DDS the practical result is fewer stops, less cleaning and lower maintenance. Hexact, Hextra and Nhexa handle biomass or green waste even at high moisture content; for the most difficult, long and stringy material, HDDS (Hyper Dynamic Disc Screening) technology is available, with the screening cut adjustable in real time (±30%) by changing the shaft speed.

Wood chips for boilers need a uniform size, with no outsized pieces and no excess fine dust, in line with the ISO 17225 standards, which classify wood chips by particle size and moisture content: uneven sizing is the leading cause of jams in feed systems, especially on small and medium boilers.

The Ecostar disc screen splits biomass into two or three fractions in a single pass: it recovers the calibrated, ready-to-use fraction, isolates fine dust and sends the oversize back to the shredder. Fractions selectable among 10, 20, 30, 50, 80, 100 and 200 mm let calibration match the boiler type.

“The digester is protected from contaminants by screening biomass before digestion, removing inert material, plastics and stones before they enter the reactor: these foreign bodies take up useful digester volume, cause blockages and downtime, and end up in the digestate later spread on fields as fertiliser.

Screening upstream removes these fractions before they reach the digester, resulting in cleaner feedstock, a protected reactor and better digestate. Ecostar screens also handle wet, heterogeneous material, the normal condition for green and organic waste headed to biogas.”

A disc screen uses up to 70% less power than traditional separation systems such as trommels and vibrating screens, because DDS technology works at low pulling force: a single 7.5 kW electric motor per 4 metres of screening surface is enough.

On top of that come the indirect costs: Hardox discs resist wear and cut maintenance and downtime, where trommels and vibrating screens suffer the abrasion and clogging typical of biomass. The range covers different volumes: Hexact and Hextra reach up to 200 t/h, while Nhexa is the compact mobile screen up to 50 t/h, with a 5 metre screening surface and cuts from 15 to 150 mm, suited to small and medium throughput plants or tight spaces.