Numerous devices and methods of fabricating solid fuel pellets and briquettes from a variety of biomass, especially RDF, wood, and wood and agricultural residues, have been developed and patented. The pellets and briquettes are manufactured by extrusion and other techniques. A binding agent such as a thermoplastic resin may be incorporated during fabrication. A ring-die extru­sion or die and roller mill is the most widely used machine type in wood pelleting, although punch and die technology has been developed (Folk and Govett, 1992). Other types of pelleting machines include disk pelletizers, drum and rotary cylinder pelletizers, tablet presses, compacting and briquetting rolls, piston-type briquetters, cubers, and screw extruders.

An exemplary method for production of pellets was developed in 1977 (Gunnerman, 1977). A raw material of random particle size such as sawdust or other wood residue, from which rocks, tramp metal, and other foreign materials are removed, is conveyed to a hammermill where particle size is adjusted to a uniform maximum dimension that is about 85% or less of the minimum thickness of the pellets desired. The milled product is then dried in a rotary drum dryer to a moisture content of about 14 to 22 wt % and fed through a ring-shaped die capable of generating pressures between 55 and 275 MPa to afford the desired shape and diameter. The pellet mill die and roller assembly must be capable of producing sufficient compression within the die to raise the temperature of the material to about 160-177°C. The products from the mill have a low, uniform moisture content, a maximum cross-sectional dimension of 13 mm, a density of 400 kg/m3, and a heating value of 19.8 to 20.9 MJ/kg. It is not necessary to add a binder to the particles, providing the pressure during pelleting produces the necessary temperature increase. During extrusion, the lignins in the biomass migrate to the pellet surface and form a skin on cooling that protects the pellet from shattering and from any rapid change in moisture content before use. This same basic procedure has been used over the years in several different hardware designs.

Briquettes are formed by similar procedures except the products are usually larger in diameter and length than pellets. Briquetting is described to consist of subjecting wood residues containing 8 to 15 wt % moisture at a maximum particle size of 0.5 to 1.0 cm to a pressure of about 200 MPa, which increases the temperature about 100-150°C (Ortiz, Migues, and Granada, 1996). The major machine types used to manufacture briquettes are impact, extrusion, hydraulic, pneumatic, and double-roll presses, and die presses that can also be used for pellet production. Briquette production rates are 200 to 1500 kg/ h for impact presses, but some models can produce 2000 to 6000 kg/h; 500 to 2500 kg/h for extrusion presses; and up to 5000 kg/h for hydraulic and pneumatic presses. The pellet machines suitable for pellet or briquette pro­duction contain annular or flat dies. The production rates are as high as 25,000 kg/h.

A few examples of typical biomass densifiers, feedstocks, and densified products are shown in Table 6.4. The first six examples in this table are commercial or commercially available systems, the last of which, Biotruck 2000, is unique (Sutor, 1995). It is a moving vehicle of special design that

continuously performs all of the operations in the field from harvesting agricul­tural virgin biomass to pellet production. The operating sequence consists of the integration into one machine of continuous crop harvesting, size reduction to about 0.6-mm pieces, heating the pieces to temperatures between 80 and 120°C using the waste heat of the engine, and compressing the heated pieces in a toothed-wheel pelleting press. No binder is used. The production rate of pelletized cereal crops is about 8000 kg/h and the bulk density is 500 to 700 kg/m3. In addition to cereal crops, the agricultural biomass suitable for harvesting and conversion to pellets by this system include grasses such as Chinese silvergrass, switchgrass, and hays and straws. Pellets for both feed and fuel applications are produced with Biotruck 2000.

Another unique example of densification listed in Table 6.4 is the production of high-density, moisture-resistant briquettes from wet wood residues without predrying or the use of binders (Edwards, 1991). The briquettes do not disinte­grate when wet and retain a maximum of about 40 wt % moisture after immersion in water. They are made from wood and bark alone or from mixtures in a pilot extruder at operating ram pressures typically ranging from 30 to 50 MPa at a maximum surface temperature of about 210°C. Moisture-resistant briquettes were made in tests from Western hemlock sawdust, a 50:50 mixture of Western hemlock and red cedar sawdusts, and Western hemlock bark hog fuel. The feed contains up to about 65 wt % moisture and must be sized so that the maximum size is less than 80% of the barrel diameter. The key to using wet biomass appears to be the simultaneous removal of excess moisture in the initial portion of the extruder while the feedstock is heated under pressure as it moves through the barrel, and the reduction of the temperature to less than 100°C before the briquettes leave the barrel to avoid the risk of explosive flash evaporation. Briquettes made in this manner contained about 35 wt % moisture. Over a 24-h period of immersion in water, they exhibited 0% swelling and only small changes in density and moisture content. The upper limit of the moisture content after immersion was consistently near 40 wt %.

Although not listed in Table 6.4, the combustible fraction of municipal solid waste, RDF, is commercially available as pellets that are similar to those produced from agricultural and woody residues (cf. Davis and Koep, 1990). The pellets have heating values of about 16.3 to 18.6 MJ/kg, moisture contents of 8 to 10 wt %, less than 10 wt % ash, and densities of 600 to 700 kg/m3.

Development of other densification methods for certain agricultural residues is expected to lead to improvements in soil growth characteristics as well as advanced residue recovery systems for energy applications. For example, cotton is a major crop in the state of Arizona. State law requires that cotton plant residue must be buried to prevent it from serving as an overwintering site for insect pests such as the pink bollworm. Research is underway to develop two systems for collecting and densifying this residue to facilitate removal from the field (Coates, 1995). The stalks are first pulled with an implement developed for the purpose. They are then baled using equipment that produces large round bales, or chopped with a forage harvester and converted into modules. The bales are either 1.2 m in diameter x 1.2 m long, or 1.8 m in diameter x 1.5 m long, depending on the baler used. The modules measure 2.1 m x 2.2 m in cross section and are up to 9.6 m long. The densities of the round bales are 93 to 168 kg/m3. The modules have densities of 168 to 252 kg/m3. The energy required to harvest and densify the residues is 9.2 kWh/t for the bales, and 8.6 kWh/t for the modules, and the heating values of the densified residues are about the same as those of wood. The module system produced a denser package than the baling system, and also made loading easier using truck-mounted module movers.