Unlike disk and drum chippers that slice and chunk wood into smaller particle sizes through cutting knives that slice fiber, grinders separate wood through a mashing and tearing or fibers. Thus, grinding may be more effective at lower moisture contents. Horizontal grinders such as that shown in Figure 14.8 have a rectangular open top for loading, with a conveyor and feed roller infeed that forces residues against the grinder, and then ejects hog fuel along an in-line conveyor outfeed. Vertical grinders, more commonly called “tub” grinders, have a large, cylindrical open top in which residues are loaded, and rely on gravity to feed the grinder.

Grinders, both of the tub and horizontal varieties, have an important place in the cur­rent infrastructure for woody biomass processing. The quality of the product resulting from grinders is generally of lower quality than a chipped product. Grinders tend to be more forgiving of soil and other contaminants, with the result that a higher proportion of these undesirable materials typically find their way into the product. Material processed in grinders is most often suitable for boiler fuel, in part because large biomass boiler systems tend to be less sensitive to ash content. Grinders are better adapted to locations where cut-to-length logging is common. In these operations, logging slash tends to be dispersed throughout the logging site. Logging residues are generally forwarded to the roadside or other locations where they can be accessed by the grinding equipment. Grinders are paired with a knuckle-boom loader and the outfeed discharges into a chip van of some sort. The ground product tends to be inconsistent in size and shape, and thus is not a preferred fuel or feedstock.

The choice of tub versus horizontal grinder is largely dependent on the type of material being processed. Tub grinders are better adapted to odd-shaped pieces, such as stumps, short bole sections, and the like. Horizontal grinders are more efficient at processing material with a more linear configuration, such as tree-length material or long tops and limbs. Horizontal grinders are capable of very high throughputs, making them efficient options where the product is acceptable.