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Steam-assisted pelletization and torrefaction of lignocellulosic biomass Ghiasisis, Bahman


A lignocellulose biomass when roasted in an oxygen free environment at temperatures higher than 250°C loses parts of its mass in volatiles. The solid fraction that is rich in carbon is called torrefied biomass or “biocoal”. Biocoal represents a renewable energy commodity that can substitute for coal. Pelletization improves the handling of a low density torrefied biomass. The common pathway to produce torrefied pellets involves torrefaction of loose biomass prior to pelletisation. It is shown, that producing durable pellets from torrefied biomass is difficult. To produce durable pellets, adding binders or making pellets under excessive temperatures (>140°C) might be possible remedies. Generally adding binders reduces the hydrophobicity of torrefied pellets. The current pellet mills operate at temperatures around or under 100°C. In the present research, several strategies such as modification of biomass feedstock particle size, increasing L/D ratio of the press mill, steam treatment of feedstock biomass, and finally torrefying pellets in the presence of steam or nitrogen are investigated prior to pelletisation. Size reduction of wood chips is done on either a knife mill or hammer mill. Three types of wood pellets are examined: 1-regular commercial pellets from a manufacturer in BC, 2- pellets manufactured using a small scale CPM pellet mill, 3- pellets made in a single pellet press device. The pellets are then torrefied in a large reactor under controlled temperature (200°C-300°C) and oxygen free or deficit environment (N₂ or steam). The quality of torrefied pellets are evaluated in terms of durability, density, hydrophobicity, and grindability. Experiments showed that torrefaction with either steam or N₂ is feasible. The rate of heat transfer increases when steam is the medium in the torrefaction chamber. A severe steam treatment of biomass at 210°C for 5 minutes, produced very dense pellets with a density similar to commercial pellets. The steam treated and torrefied pellets are hydrophobic, preserving their structure and form when immersed in water or exposed to humid air (35°C, 90% RH). The results of this research shows the need for evolution of an integrated system consisting of steam pre-treatment of wood chips, pelletizing, and torrefying pellets with dry steam.

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Attribution-NonCommercial-NoDerivatives 4.0 International