Densification, biomass pelletizing

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... 

The overall energy efficiency of densification systems may be misleading. Only 10 to 25of input energy is lost in making a biomass fuel pellet, even if electrical energy is thermally generated from the same residue when used as feedstock. 

The destructive effects of expanding compressed air and relaxation of elastic deformation can be also reduced if the maximum pressure is held for some time, called dwell time, before it is released. Fig. 8.2 shows, that, without special technical provisions, this is only achieved in ram extruders (Fig. 8.2b, see also Section 8.4.3). In such equipment, a number of briquettes is retained in the long pressing channel and is redensified during each stroke. After the wall friction is overcome and the entire line of briquettes moves forward, the pressing force remains almost constant. A similar, but much smaller effect is obtained in pellet presses (see also Section 8.4.2). Since, as mentioned before, a dwell time and, particularly, the application of several densification cycles also helps to convert temporary elastic deformation into permanent plastic deformation, these techniques are especially suitable for the densification of elastic materials such as, for example, biomass. 

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