Densification, relaxation, and combustion characteristics of densified Biomass fuels: a comparative study with wood combustion.

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University of Dar es Salaam
Densification of loose biomass waste such as agricultural, forest and industrial residues to form Densified Biomass Fuels (DBF) is an attractive option in areas where these resources remain unutilized. The potential of DBF to substitute fuelwood or coal in industrial combustion systems depends on how closely its handling and combustion behaviour match the existing facilities. In this work, experiments were conducted to study densification, relaxation, and combustion characteristics of rice straw and sawdust DBF. Experiments on wood combustion were also conducted for comparison to DBF. Densification tests were performed in uniaxial closed-die mode. Die diameters of 2.5 and 3.8 cm were used with die pressure up to 134 MPA. Results showed that an exponential relationship in the form: P=Ae(Bp), seems to best represent the uniaxial densification process for the material tested. Where: P is the die pressure, and p is the density of DBF. The values for coefficients `A' and `B' depend on die size, material type, and storage relative humidity. The relaxation behavior of DBF was studied in low, medium, and high, relative humidity conditions. DBF made of rice straw showed 52-68% increase in length in the pressure range tested, which is 6-18% higher than for sawdust. DBF from the larger diameter die showed 3-9% higher expansion compared to the smaller die. DBF showed higher percentage increase in length when stored in high humidity conditions. The change of moisture content of DBF during storage delays the stabilization period and increases their expansion considerably. Combustion tests were carried out in an isothermal thermogravimetric analyser. Results demonstrate that, during pyrolysis the cylindrical particles of both DBF and wood loose mass in a thin reaction zone. DBF were found to expand during combustion by up to 50% of its original volume. The anisotropic properties in wood have an influence on the direction of volatile gases coming out of the particle during pyrolysis. Model predictions show that drying and pyrolysis overlaps. Model predictions on the advancement of pyrolysis zone in radial and axial directions, and burning rate of the particle are in good agreement with experiment results.
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Biomass energy, Comparison, Fuelwood, Tanzania
Rajabu, H. M. (2001). Densification, relaxation, and combustion characteristics of densified Biomass fuels: a comparative study with wood combustion. Doctoral dissertation, University of Dar es Salaam. Available at (