U. S. standards for biomass pellet fuels have been developed and recommended by the Pellet Fuels Institute in the United States; they are shown in Table 6.5. The older standards included recommendations for moisture content and heating value, but these do not. Instead, it is recommended that the heating value be certified by the pellet manufacturer, so whatever the pellet material

and its moisture content, the consumer should be able to estimate the energy cost. The national standards in Table 6.5 make it possible for the manufacturers of pellet stoves, most newer versions of which auger-feed the pellets from the top, to produce units designed to accept the standardized pellets.

B. Economic Factors

The wholesale cost in the United States of wood waste pellets is in the range of $85 to $140/t (mid-1997). This cost range effectively precludes their use as feedstocks for most conversion processes, and it limits residential fuel applications. The production cost exclusive of biomass cost is estimated to be about 30 to 60% of the wholesale cost and depends on production rates and the amount of processing needed. For example, in Spain, the increase in electric energy consumption required to mill wood wastes to 5- to 8-mm sizes is almost totally compensated for by the decrease in electric energy consumption during densification (Ortiz and Gonzalez, 1993). Exclusive of wood cost, the cost of manufacturing densified wood residues in small units operated by one person is about $22/t at a production rate of 1250 t/year. Smaller particles in the 2-mm size range can increase production rates by 50% or more, but the energy cost is excessive. Industrial manufacturing costs in Spain of densified wood wastes exclusive of wood cost are about $32 to $48/t at production rates of 1.0 t/h (Ortiz, Miguez, and Granada, 1996). In Finland, the cost of producing straw fuel pellets on farms in small, portable pelletizers is estimated to be about $54 to $84/t (Wilen et ah, 1987). Note that the hardware cost can be a major factor in the cost of producing densified biomass. Biotruck 2000, described earlier, for producing pellets or briquettes from agricultural wastes in Europe has a production rate of about 8 t/h in the field and costs about $400,000 (700,000 DM) (Sutor, 1995).