What is the impact of the torrefaction temperature on the properties of the product biomass?

Torrefaction is a thermochemical process that occurs at temperatures between 200°C and 300°C in an inert atmosphere, typically under low oxygen conditions. During this process, biomass (such as wood, agricultural residues, and other organic materials) is heated, releasing volatile compounds, leaving behind a solid product with improved fuel properties.

The properties of torrefied biomass are influenced by various factors such as the type of biomass, heating rate, reaction time, and, most importantly, the torrefaction temperature.

As the torrefaction temperature increases, several changes occur in the properties of the product biomass: a. The mass yield decreases: With increasing temperature, more volatile compounds are released, leading to a reduction in the mass yield of the torrefied biomass. b. The energy yield decreases: Although the energy content per unit mass increases with increasing temperature, the overall energy yield (product of mass yield and energy content) decreases due to the decrease in mass yield. c. The energy densification increases: Higher temperatures result in a more carbon-dense final product, leading to increased energy densification (energy per unit volume). d. Hydrophobicity increases: The torrefied biomass becomes more hydrophobic (water-resistant) with the increase in temperature, which improves its storage and handling properties. e. Grindability improves: With the increase in temperature, the biomass undergoes structural changes that make it more friable, improving its grindability.

There is no single optimal torrefaction temperature for all applications, as the choice depends on the specific requirements and constraints of the end use. For example, a higher torrefaction temperature provides better grindability and hydrophobic properties but at the cost of reduced energy yield. Therefore, it is essential to consider the desired properties of the final product, as well as the economic and environmental implications, to determine the most suitable torrefaction temperature for a particular application.