Dry shredders are commercially used for reducing the size of biomass. The two most common types of machines are vertical and horizontal shaft hammermills. Metal hammers on rotating shafts or drums reduce particle size by impacting the feed material until the particles are small enough to drop through grate openings. Hammermills are commonly used in MSW-processing systems to reduce the size of the components before separation of RDF (i. e., refuse — derived fuel or the combustible fraction of MSW), and other materials. Ham­mermills are also used as agricultural choppers and tree chippers. Rotating cutters equipped with knife blades that reduce particle size by a cutting or shearing action are used for the same applications, although they usually have smaller capacities than hammermills.

Early biomass grinders were used to produce wood pulps from roundwood for the manufacture of paper (Riegel, 1933). Logs are positioned with the sides against a rotating grindstone so that damage to the fibers is minimized. Water is passed over the stone to wash the wood meal into a storage tank. In the original designs, the logs were held against the grindstone by springs. Later versions were hydraulic magazine grinders that automatically replaced the ground logs with fresh logs. In the 1950s, attrition mills were introduced to produce mechanical pulps from wood chips. Rotating, opposing discs, either one stationary or both moving in opposite directions, are used. The chips are fed to the mill near the central, rotating shaft and move outward to the periphery of the discs through a series of successively smaller channels that progressively reduce the feed to pulp-size particles.

Hydropulpers are wet shredders in which a high-speed cutting blade pulver­izes a water suspension of the feed over a perforated plate. The pulped material passes through the plate and the nonpulping materials are ejected. The action is similar to that of a kitchen waste disposal unit. Hydropulpers can also be used for the simultaneous size reduction and separation of the combustible fraction of MSW from the inorganic materials. But since the product is a water slurry of small particles, which can have the consistency of a heavy cream, the hydropulper is quite suitable for the preparation of RDF for microbial conversion after passage through a liquid cyclone to remove gritty, mostly inert material. Fiber recovery operations where long fibers are removed for resale can be performed before microbial processing. Experimental studies have shown that hydropulpers can also supply good feedstocks for microbial processing from other biomass. Maintenance costs for wet shredders are lower than those for dry shredders.

Agricultural choppers that are operated as stationary cutters and as moving choppers in the field separately from the harvesters or that are part of forage

harvesters that chop the crop during the harvesting process are commonly employed for preparing hay and other forage crops for ensiling. When a chopper is used in the field, more stems pass through the chopper lengthwise and a shorter average cut is obtained than when the same setting is used on a stationary chopper. Forage chopping in the field usually supplies material 25 mm or less in length. Silage systems offer several advantages for the size reduction of herbaceous biomass energy crops. The major disadvantage for high-yielding biomass species is the inability of the forage chopper to effectively harvest severely lodged (fallen) crops or plants (Coble and Egg, 1989). Sugar­cane harvesters, which are designed for harvesting high-yield, thick-stemmed, lodged biomass, have been used to harvest such crops. A separate size-reduction step is needed prior to storage or conversion.

In contrast to mechanical pulps, chemical wood pulps are often made from bark-free wood chips. One form of chipper has four knives fastened at 90° from each other on a rotating disc in such a way that only the edge of the blade projects beyond the disc. Each knife cuts a thin strip about 10 mm thick from the logs, which are fed along their long axis to the chipper at about a 38° angle to the disc. The mechanical action is similar to that of a sausage slicer. The denser hardwoods produce thinner chips than softwoods, but both types of chips are suitable feedstocks for most fixed — and fluid-bed gasifiers and most other thermal conversion processes.

Chipping has been the traditional mechanical method of size reduction to prepare wood fuels for direct combustion. It is an energy-intensive operation, but it does improve bulk density, handling, and transportation costs. Disc chipping and hogging are two preferred means of preparing wood fuels (Suadi — cani and Heding, 1992). Hammer hogs with free-swing hammers break the feed into small pieces, whereas knife hogs cut the feed with blades. The least desirable option seems to be chipping in the field at the time of harvest, which requires that a power chipper accompany the harvester through the field. Whole-tree chips are also reported to lose approximately 10% of their oven dry weight after storage for 6 months (с/. Curtin and Barnett, 1986). A variety of machines are available for producing wood chips in the field. One of the notable developments in North America is a swath harvester called “Jaws” that produces fuel chips while clearing 2.4-m wide paths through young, crowded stands of pine (Ranney et al., 1985). Another is the large mobile wood chipper, “Chiparvestor,” that can handle trees up to 0.76 m in diameter, and the medium — size unit that chips trees 0.56 to 0.69 m in diameter (Biomass Energy Research Association, 1990). The latter unit can produce 544 t of chips in 8 h, while another model for small-diameter trees can produce 91 t of chips in 8 h. Commercial wood chippers, both mobile and stationary, and chip harvesting methods are far advanced in Finland, where the usage of wood chip fuels has increased greatly (Seppanen, 1988).

Among the other options that can be considered for producing wood are chunking, billeting, and crushing. For smaller trees, chunking and billeting are similar processes that cut stems and limbs into 6- to 20-cm long pieces. Machines have been designed that chop small-diameter stems into smaller chunks or slightly longer billets. Chunkwood is approximately fist-sized. The production of chunkwood requires less energy than chip production, but it is not certain that the cost is competitive (Suadicani and Heding, 1992). Crushing is carried out by passing the stems between two or more metal rolls of varying size, rotational speed, and surface. Tests have shown that crushing rates of approximately 15 linear m/min can be achieved on stems up to 21 cm in diameter using only 11.2 kW (15 HP) of power (с/. Ranney et ai, 1987). The crushing and bunching of wood may offer significant advantages over chipping. This technique is flexible and is able to process large stems and stem lengths to yield bolts of crushed wood that exhibit relatively rapid drying. For reactor feeding purposes, however, further size reduction would be necessary. The feedstock characteristics required for the combustion or conversion process used determine which of these methods of size reduction may be applicable.