Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 108

Within each temperature range, the rate and degree of torrefaction change
from low to high.
Severe torrefaction results in the greatest mass and energy loss, but it gives
highest energy density in the torrefied biomass. Light torrefaction on the other
hand retains maximum amount of mass and energy of the dry biomass but
attains the lowest energy density. The choice of torrefaction regime would
depend on the specific technocommercial need of the torrefaction plant.
The above definition of the degree of torrefaction does not take into
account the residence time of torrefaction, which also affects torrefaction
( Section 4.3.3 ). For example, a biomass torrefied for 1 min at a given tem-
perature will have a substantially lower degree of torrefaction than if it had
been torrefied for 100 min at that temperature. So, the degree of torrefaction
should be a combined product of temperature, residence time, and other
influencing factor like oxygen concentration.
As the quality of roasted coffee is identified by its color and or taste, the
quality of torrefied biomass for energetic use could also be expressed in
terms of the following three attributes:
1. Mass yield
2. Energy density
3. Energy yield.
These are described in some detail in the following sections.
4.5.1 Mass Yield
Mass yield gives a measure of the solid yield of the torrefaction process.
It defines what fraction of the original mass of biomass would remain in the
torrefied product. Torrefaction concerns the change in the hydrocarbon con-
tent of the biomass. So, a simple definition based on a ratio of product mass
and the original (feed) mass may not give a true picture of the process.
Drying is a physical change while torrefaction primarily concerns changes
in the organic component of the biomass. A typical biomass contains physically
bound water, inorganic materials (ash) 1 , and organic substances. As ash and
water do not carry any part of the chemical energy of the biomass their removal
is of little consequence as far as energy content of the product is concerned. The
drying of biomass increases its energy density but that does not bring about any
chemical change in the biomass. Torrefaction on the other hand takes the pro-
cess to a further height where chemical changes make the biomass both physi-
cally and chemically more attractive. Thus, a definition of mass yield on dry
1 Strictly speaking the inorganic materials in biomass are not necessarily equal to its ash content.
For example, Na and K in ash exist as inorganic form as a salt in biomass. On the contrary, P
and S in ash originate from DNA and proteins, respectively.