The differences in elemental composition between raw and
torrefied biomass under the studied conditions (residence time,
temperature) are shown in Fig. 4 known as the Van Krevelen
diagram.
These differences are explained by the O/C and H/C ratios each
of the studied samples. It has been postulated that the decrease in
the O/C ratio improves the torrefied biomass properties compared
to raw biomass. The increase of the reaction temperature and
residence time of torrefaction cause shifting the torrefied biomass
away from raw biomass toward coal (comparable to lignite) due to
intensified the carbon content and calorific value.
The heating value is an important property based on the
chemical composition of fuel and a very important factor for the
design and the control of heat and power plants applied biomass.
Torrefaction process can remarkably improve the heating value of
biomass. As can be seen in Table 1, the HHV of torrefied samples
increases with temperature and residence time comparing to raw
biomass, e.g. for 290 C and 1.5 h the HHV rises c.a. 54% with respect
to the raw sample. The increase of the higher heating value of the
torrefied biomass compared to raw biomass defines the energy
density ratio. The energy density is in the range of 1.18 ÷ 1.53.
According to the weight loss and heating values of torrefied
biomass, the energy yield of torrefaction products can be calculated
by the equation (2):