Torrefaction process definitions

In biomass energy applications, torrefaction aims for the production of a fuel having improved properties compared to the original biomass feed. However, this should be achieved without losing too much chemical energy to volatile products during the process. Hence, the mass yield and energy yield are considered to be crucial parameters. In the open literature much focus is spent on the mass and energy yield of the torrefaction as a function of residence time and torrefaction temperature (see e.g. Pach et al., 2002). It is often unclear which definitions are used to calculate both yields and how the torrefaction temperature and time are defined. In this work these definitions are carefully formulated.

Mass and energy yield

In considering the macro composition of biomass, it can be expressed as physically bound water, organics and ash. The organic part of the biomass contains all the (reactive) chemical energy and during torrefaction part of this energy is removed in the form of reaction products. Therefore, the definition of the mass and energy yield are expressed on the basis on the organic part of the biomass, thus on dry and ash-free basis (daf):

Use is made of the lower heating value (LHV), although the higher heating value can be used as well. However, the LHV value of biomass represents the amount of usable energy to be utilised effectively from the biomass and is generally used in the evaluation of many applications.