Page 303 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 8 Design of Biomass Gasifiers 279

1200

1000
Bed temperature (°C) 800

600

400

200
0
0.15 0.2 0.25 0.3
Equivalence ratio
FIGURE 8.21 Gasifier temperature in a CFB riser increases with ER.

conversion efficiency of a CFB gasifier for wood dust against the ER. The
efficiency increases with equivalent ratio and then it starts declining. The
optimum value here is 0.26, but it may change depending on many factors.
The bed temperature of a fluidized-bed gasifier increases with the ER
because the higher the amount of air, the greater the extent of the combus-
tion reaction and the higher the amount of heat released (Figure 8.21).
Example 8.1 illustrates the calculation procedure for ER.

Oxygen
Oxygen is used primarily to provide the thermal energy needed for the endo-
thermic gasification reactions. The bulk of this heat is generated through the
following partial and/or complete oxidation reactions of carbon:

C 1 0:5O 2 -CO 2 111 kJ=mol (8.13)
C 1 O 2 -CO 2 2 394 kJ=mol (8.14)
It can be seen that for the oxidation of 1 mol of carbon to CO 2 , the oxy-
gen requirement is (2 3 16)/12 5 2.66 mol, while that for carbon to CO is
(16/12) 5 1.33 mol. Thus, the reaction in Eq. (8.13) is more likely to take
place in oxygen-deficient regions.
Besides supplying the energy for the endothermic gasification reactions,
the gasifier must provide energy to raise the feed and gasification medium
to the reaction temperature, as well as to compensate for the heat lost
to the reactor walls. For a self-sustained gasifier, part of the chemical energy
in the biomass provides the heat required. The total heat necessary comes
from the oxidation reactions. The energy balance of the gasifier is thus the
main consideration in determining the oxygen-to-carbon (O/C) ratio.