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Torrefaction of Biomass Best Before or After Densification?

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An article in an upcoming issue of Applied Energy looks at torrefaction treatment of biomass in terms of its effectiveness before and after densification. Torrefaction, as the article explains, is a thermochemical treatment of biomass at 200°C to 320°C. It is carried out under normal pressure in the absence of oxygen. 
 
The article notes that during the torrefaction process, water contained in the biomass as well as superfluous volatiles are released, and the biopolymers (cellulose, hemicellulose, and lignin) partly decompose. The final product is the remaining solid, dry, blackened material referred to as "torrefied biomass" or "bio-coal." Torrefied biomass represents a high quality renewable energy commodity that can be used to substitute for fossil fuels such as coal. However, densification processes such as pelletization is necessary to improve the tradability of "low-dense" torrefied biomass.
 
In a recent experiment, the article continues, two process pathways were assessed for energy and mass balance in making torrefied pellets from softwood chips. Qualities of the resulting torrefied pellets were to be compared. Pathway I involved drying the wood chips, torrefaction, and grinding, followed by densification. In pathway II, wood chips were dried, ground, densified, and went through torrefaction in the final stage. 
 
Results demonstrated that it was difficult to bind the torrefied biomass particles and a binding agent was necessary to enable their effective pelletization with reasonable energy consumption. In contrast, pelletization of raw materials was possible without using binding agents when "raw wood pellets" were sent through torrefaction. These pellets surprisingly stayed intact and had several promising properties such as higher energy/carbon value, reduced moisture content, and higher stability in water. In addition, pathway II was more efficient in terms of overall energy and material balance, the Applied Energy article concluded. 

 

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