Torrefaction of biomass Part V

Torrefaction of biomass

Torrefaction is a thermochemical treatment method in the first place earmarked by an operating temperature within the range of 200 to 300 ?°C. It is carried out under atmospheric conditions and in the absence of oxygen. The name torrefaction is adopted from the roasting of coffee beans, which is, however, done at lower temperature and does allow the presence of oxygen. Nevertheless, the mechanical effects of torrefaction on biomass are supposed to be similar to its effect on coffee beans, which is their brittle structure afterwards. Torrefaction has many synonyms. Some examples are roasting, slow and mild pyrolysis, wood cooking and high-T drying. Especially the link with pyro lysis is easy to make as torrefaction covers part of the initial decomposition reactions of pyrolysis.

The main torrefaction product is the solid phase, which is - similar to pyrolysis - referred to as the charred residue (or char). In the field of torrefaction the solid product is also frequently called torrefied wood or torrefied biomass. Similar to pyrolysis, during torrefaction the chemical structure of biomass is altered. This leads to the formation of a variety of volatile (decomposition) products of which some are liquids at room temperature (condensables). By mass, important reaction products other than char are carbon dioxide, carbon monoxide, water, acetic acid and methanol (Bourgeois and Guyonnet, 1988). After condensation, liquid products manifest themselves as a yellowish liquid. All these non-solid reaction products contain relatively more oxygen compared to the untreated biomass. Hence the O/C ratio of torrefied biomass is lower than untreated biomass, resulting in an increase of the calorific value of the solid product. Torrefaction of wood has attracted more interest recently. Pentananunt et al (1990) carried out torrefaction experiments at the Asian Institute of Technology in Bangkok. They compared combustion characteristics of torrefied wood produced after 2-3 hours of torrefaction and concluded that torrefied wood produces less smoke compared with untreated wood. In Brazil, a bench unit was used to determine the effect of raw material, temperature, and residence time on the properties of torrefied wood at the University of Campinas (Felfli et al., 1998). Torrefaction research is currently carried out at the National Renewable Energy Centre (NREL) in Golden, Colorado (Nimlos et al, 2003) and the Royal Institute of Technology (KTH) in Stockholm, Sweden (Pach et al, 2002).