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Torrefaction

Torrefaction is a process for the thermal pre-treatment of biomass and involves heating the biomass in a range between 200 - 300°C, in an inert atmosphere. The process occurs in two stages:

  1. Drying at 125°C biomass undergoes a slight breakdown and release of physical water (moisture), with some light volatiles resulting in a weight loss of below 10%.
  2. At temperatures up to 300°C chemical water is removed as a product of by-chemical reactions through thermo-condensation along with carbon dioxide formation. In the range of 180-270°C exothermic reactions occur along with the partial degradation of hemicellulose within the biomass. Subsequently the biomass attains its brown colour whilst releasing carbon dioxide, chemical moisture, some phenols and large amounts of acetic acid.

 A typical biomass torrefaction process has four stages:

  1. Biomass preparation (grinding and drying) 
  2. Reactor (conversion of biomass into torrefied material and combustible gases) with condensation system
  3. Cooling section (cooling of torrefied material)
  4. Combustion section (non-condensable gases are burned with an excess of oxygen in the combustor  and the generated heat is used the torrefaction process)  
Torrefaction

Benefits of Torrefaction of biomass over untreated biomass: 

  • The calorific value (energy per unit of weight) is considerably increased 
  • Torrefied biomass is easy to grind and can easily be compacted into a product with high volumetric energy density (energy per unit of volume) 
  • The torrefied material becomes hydrophobic (i.e. water repellent), meaning it can be stored for a long time without disintegrating 
  • The physical properties of torrefied biomass, such as durability and homogeneity are improved significantly, while the biological activity is strongly reduced. 

This technology produces sustainable alternatives to fossil fuels, thus reducing fossil carbon dioxide emissions. The torrefied biomass can be pressed into pellets to create a fuel with homogenous feeding and conversion properties.

Currently, EBRI is evaluating torrefaction mass and energy balances. This is part of EBRI's Supergen Bioenergy Hub project: Torrefaction integrated assessment.  Data from previous research work will be used to develop full mass-energy balances for incorporation of torrefaction into selected bioenergy systems, including recovery of chemicals as part of an integrated processing scheme.    

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