Some practical problems are associated with the use of biomass material (sawdust, wood chips or agricultural residues) as fuel. Those problems are mainly related to the high bulk volume, which results in high transportation costs and requires large storage capacities, and to the high moisture content which can result in biological degradation as well as in freezing and blocking the in-plant transportation systems. In addition, variations in moisture content make difficult an optimal plant operation and process control. All those problems may be overcome by densification, which consists in compressing the material to give it more uniform properties.
The main advantages of densified fuels, compared to non-densified ones are the following:
- An increased bulk density (from 80-150 kg/m3 for straw or 200 kg/m3 for sawdust to 600-700 kg/m3 after densification), resulting in lower transportation costs, reduced storage volume and easier handling.
- A lower moisture content (humidity <10%), favouring a long conservation and minor losses of product during the storage period.
- An increased energy density and more homogeneous composition, resulting in better combustion control possibilities and thereby higher energy efficiency during combustion.
The major disadvantage is the relatively high-energy cost for the pelleting process, increasing the price of the end product.
Densified products can be found as briquettes or as pellets. The heating value, moisture content and chemical characteristics are about the same for both but the density and strength are somewhat higher for pellets. The major difference is the size (generally Ø 6 to 12 mm, with a length 4 to 5 times the Ø for pellets), making them easy to use in fully automatic operation, from household appliances to large-scale combined heat and power (CHP) plants.
Comparison between briquettes and pellets
|Raw Material||Dry and grinded wood or agricultural residues||Dry and grinded wood or agricultural residues. Raw material can be more coarse than for pelleting, due to the larger dimensions of final product|
|Shape||Cylindrical (generally Ø 6 to 12 mm, with a length 4 to 5 times the Ø).||Cylindrical (generally Ø 80 to 90 mm) or parallelepiped (150*70*60 mm)|
|Struture||Stable, hard, without dust||Relatively friable, fragile|
|Bulk Density||Min. 650 kg/m3||600 to 700 kg/m3|
|Transport||Bulk, bags, big bags||Unit, palet|
|Handling||Manual or automatic use||Manual use|
|Pellet characteristics for domestic use|
|Heating value||> 4,7 kWh (>17 MJ/kg)|
|Moisture content||Max. 10%|
|Ash content||Max. 0.5%|
|Dimension||Diameter: 6 mm; Length: 25 mm|