Densification involves compacting loose density biomass in a die into a solid compact such as a briquette or pellet. Biomass arrives at the plant in chops or bales. The bales are cut into short pieces using a hydraulic piston pressing the hay against a grid of knives. The bales can also be shredded using a roller and knife arrangement. If the moisture is more than 15%, the chopped biomass is dried in a drum dryer.

Figure 7.5 shows the relationship between particle size and bulk density of biomass for an industrial grinder. The spread in data can be attributed possibly to variations in actual particle size distribution in various size groups. Note that in this particular example size groups vary from 1 to 3 mm. The bulk density for 2.5 mm is slightly more than 100 kg m-3. Thebulkdensity increases to more than 160 kg m-3 as particle size decreases to less than 1 mm. Table 7.3 indi­cates that bulk density can be increased by almost 25% by tapping (vibrating) the container.

In preparation for pelleting, the dried chops are ground in a hammer mill. For cubing, the chops are not ground. For pelleting, the ground biomass is mixed with saturated steam—in a paddle mixer located on top of the mill. Steam heats and moisturizes grind biomass. For cubing, small quantities of water are added to biomass. The steam or water acts as a lubricant to enhance binding. The moisture content of mash before pelleting is usually in the 10% range and that of chops before cubing is 12%.

Pellet mills are equipped with a large diameter short screw, a die ring, and from one to three press rolls. The feed screw pushes the biomass uniformly toward the openings in the die ring. Press wheel forces the feed through the die openings in the ring. The pressures in the mill range from 24 to 34MPa (Tabil et al. 1997). Pellets and cubes exit the mill warm and moist. They are cooled and dried to a moisture content of roughly 10% for cubes and 8% for pellets. The cooled pellets and cubes are stored under roof in a flat storage or in a hopper bottom silo. Pellets and cubes are loaded into rail cars or trucks using a front-end loader or from self-unloading overhead bins.

In some cases, the preprocessing of biomass may consist only of grinding (Mani et al.

2006) . The grind will have a bulk density of 180 kg/m3 in the truck box. This density is suit­able for short hauls. For longer hauls and long term storage, it is preferred to densify biomass to pellets or cubes.

Densified biomass requires less area and volume for storage and transport than loose biomass. In addition to savings in transportation and storage, densified biomass lends itself to easy and cost — effective handling. Dense cubes and pellets have the flowability charac­teristics similar to those of cereal grains. Bulk handling equipment for granular material is well developed and available commercially (Fasina and Sokhansanj 1996).