Chipping is the most common method of comminuting biomass in preparation for combustion or other form of energy conversion. The two predominant chipping types are disc chippers and drum chippers (Fig. 6.2).

The working principle of the disc chipper is that 2-4 bevelled knives are fixed radially in a fast rotating disc. The knives, which can be adjusted for desired chip size (measured in the fibre direction) cut the biomass perpendicular or slightly offset to the feeding direction, and run up against an anvil to ensure the material is severed. A fan blade mounted on the rear of the disc creates a pneumatic force that blows the chips out of the spout and into a container or onto the ground. In larger chippers (>40 cm intake), the disc can have a diameter of over 120 cm and weigh more than 1,000 kg. Because the disc always cuts at a constant angle to the material, the disc chipper can produce very uniform chips. Disc chippers produce more ‘stickers’ than others (long slivers which cause stoppages in conveyor systems) as these are pulled into a parallel orientation to the knives. Various solutions have been found to reduce that problem which is more pronounced in small material (small trees, tops and branches).

The drum chipper consists of a number of knives mounted along the longitudinal axis of a steel cylinder, with a smaller diameter than the disc chipper. It is therefore more compact and can be built into smaller spaces (e. g., on chipper trucks). By nature of its design, the knives on a drum chipper cut into the material at different angles, depending on the size of the log or branches. This produces slightly more heterogeneous chips. The drum chipper can generally be built for larger diameter logs, or larger bunches of smaller material, as the disc chipper intake has to be limited to less than the radius of the disc. Provided enough power can be delivered to the drum, it is possible to build drum chippers with larger intake capacities. The length of the knives reduces the negative consequences of hitting dirt or a stone as this would represent a smaller proportion of the knife than the same damage on a shorter disc knife. The knives on both disc and drum chippers have to be maintained (sharpened or reversed) at least once a day, and even more frequently when working with material that has been contaminated with soil, sand or stones, or cutting carbonized bark.

Auger or conical screw chippers are robust and produce homogenous chips of good quality. However, they require much higher power drivers, due to the high forces required in severing the material that is fed in the same direction as the axis of rotation. Chip size is adjusted by exchanging the screw for one with a different pitch, while the whole screw needs to be sharpened in place, or exchanged for a newly sharpened one. An advantage of screw chippers is that large material (chunks — up to 150 mm) can be made for e. g., thermal-gasification. Irrespective of the working principle, chippers are deployed in many sizes and configurations. Some are built onto terrain going base machines, others on trucks or trailers for mobility, while others are located centrally at terminals or conversion plants.