Densified Biomass as a Fuel Source: A New Pellet For New Possibilities
Duncan, William Andrew
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Current biomass pellets are susceptible to fines production during handling and transportation, moisture absorption, biological degradation, low specific energy content, off-gassing and poor flow and handling characteristics. The goal of this thesis was to create a novel pellet that addressed these issues. A new mould and die assembly was created to form spherical pellets using hybrid poplar that was torrefied in-situ. The new pellets were subjected to crushing, abrasion and tests of their hydrophilic performance. Additionally, ultimate and proximate analysis was performed. A “batch” torrefier was constructed to investigate the properties of torrefied poplar in order to compare the results to in-situ torrefied pellets. Mechanical and chemical tests which examined failure strength in compression, abrasion resistance, hydrophobic nature, gross calorific value, density, and size were used to determine where the new pellets ranked in comparison to standard cylindrical pellets. A set of designed experiments which used the aforementioned responses was created to formulate linear models which could predict the same responses based upon three two-level “factor” inputs: biomass particle size, moisture content of biomass, and temperature of torrefaction. The new spherical pellets withstood compressive forces of up to 17.06 MPa, did not abrade, absorbed up to 29.4% moisture yet returned to an “air-dry” moisture content of <5% on a mass basis within 24 hours and obtained specific energies up to 22.09 MJ/kg. In addition, spherical pellets did not disintegrate when immersed, but instead remained intact and possessed much of their original physical characteristics once dried. The designed experiments showed that torrefaction temperature was the most important factor which affected test responses.