Biomass fuels are a potential source of renewable energy. One of the major barriers to their widespread use is that biomass has a lower energy content than traditional fossils fuels, which means that more fuel is required to get the same amount of energy. When combined – low energy content with low density – the volume of biomass handled increases enormously. Compaction or densification is one way to increase the energy density and overcome handling difficulties.
Bulk density is defined as the weight per unit volume of a material, expressed in kilograms per cubic metre (kg/m3). Most agricultural residues have low bulk densities, as shown in Figure 1.
The low density of biomass materials poses a challenge for the handling, transportation, storage and combustion processes. These problems may be addressed through densification, a process that produces a solid fuel with denser and more uniform properties than the raw biomass.
The main advantages of biomass densification for combustion are:
* uniform combustion in cookstove
* easily measured out and loaded into combustion device
* simplified storage, packing and handling infrastructure
* reduced cost of distribution due to increased energy density
The major disadvantage to biomass densification technologies is the high cost associated with the densification processes. This however is mitigated by using low value biomass waste, not suitable for use as a loose material.
Low moisture results in improved density and durability of the fuel. For most biomass densification processes, the optimum moisture content is in the range of 8%–20% (wet basis). Above the optimum moisture level, the strength and durability of the densified biomass are decreased.
Pellets are very high in density and are formed by an extrusion process, using roller and a die, where finely ground biomass material is forced through round dies and cut to a desired length. The standard shape of a biomass pellet is a cylinder, having a length smaller than 40 mm and a diameter around 6 – 8 mm.