A. Fundamentals

Baling has long been used to densify hays, straws, and other agricultural crops such as cotton to simplify removal from the field and to reduce storage space and transportation costs. Baled straw has a density of 70 to 90 kg/m3 at 10 to 15 wt % moisture content, whereas the bulk density of piled straw is about 5 to 15% of this density range. When straws are compressed to form pellets, briquettes, or cubes in specially designed dies and presses, the density can be increased to 350 to 1200 kg/m3. In contrast, dried wood has a density of 600-700 kg/m3 and a bulk density of about 350 to 450 kg/m3, whereas the bulk densities and densities of wood briquettes are 700 to 800 kg/m3 and up to 1400 kg/m3, respectively.

One of the original uses of biomass pellets in the United States was as fodder. Alfalfa, other grasses, and some straws were pelletized and sold as livestock feed. Biomass densification appears to have the greatest use for up­grading agricultural and forestry residues that might otherwise be lost or that require disposal at additional cost. The potential advantages for energy and feedstock applications of densified waste biomass are evident. High-density, fabricated biomass shapes simplify the logistics of handling and storage, im­prove biomass stability, facilitate the feeding of solid biomass fuels to furnaces and feedstocks to reactors, and offer higher energy density, cleaner burning solid fuels that in some cases can approach the heating value of coals. However, the basic problem often encountered in the use of densified biomass fuels and feedstocks is production cost. Some of the economic factors are discussed in Part D.

The heating value depends on the moisture and ash contents of the densified material and is usually in the range of 15 to 17 MJ/kg. The use of asphaltic binders or pelletizing conditions that result in some carbonization can yield densified products that have higher heating values. Pellets, briquettes, and logs have been manufactured by densification methods from biomass for many years. Prestologs” made from waste wood and sawdust were marketed before 1940 in North America, and the market for pellet fuels made from wood sawdust, shavings, and chips for residential pellet-burning stoves has grown significantly since the 1980s (Pickering, 1995; Folk and Govett, 1992). Numer­ous commercial processes for production of densified fuels in the form of logs, briquettes, and pellets from a wide range of biomass provide domestic fuels for space heating; industry uses the pellets and briquettes as boiler fuels (Edwards, 1991). In Europe, briquettes made from waste biomass are commer­cially available and are used for both residential and industrial applications. In Spain, households consume 80% of the total production of briquettes for use in furnaces, fireplaces, and barbecues. Bakeries use them in furnaces, and small industries such as ceramic plants use them as boiler fuel (Ortiz, Miguez, and Granada, 1996).