Page 307 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 8 Design of Biomass Gasifiers 283

heat supplied to the gasifier depends on the heat requirement of the endother-
mic reactions as well as on the gasification temperature. The latter is a
design choice, and it is discussed next.

8.6.3.1 Gasification Temperature
The choice of gasification temperature is an important process choice.
Because lignin, a refractory component of biomass, does not gasify well at
lower temperatures, thermal gasification of lignocellulosic biomass prefers a

minimum gasification temperature in the range 800 900 C. For biomass, an
entrained-flow gasifier typically maintains a peak temperature well exceed-

ing 900 C. For coal, the minimum is 900 C for most gasifier types (Higman
and van der Burgt, 2008, p. 163).
A higher peak gasification temperature is chosen for an entrained-flow
gasifier. A higher ash-melting temperature requires a higher choice of the
gasifier temperature. This temperature is raised through the gasifier’s exo-
thermic oxidation reactions, so a high reaction temperature also means a
high oxygen demand.
In entrained-flow gasifiers, the peak gasification temperature is typically

in the range of 1400 1700 C, as it is necessary to melt the ash; however,
the gas exit temperature is much lower. The peak temperature of a fluidized-
bed gasifier is in the range of 700 900 C to avoid softening of bed materi-

als. It is about the same as the gas exit temperature in a fluidized-bed gas-
ifier. In a crossdraft gasifier, the mean gasification temperature is about
1250 C, whereas the peak temperature is about 1500 C. The gas exit temper-

ature of a downdraft gasifier is about 700 C, but its peak gasifier tempera-

ture at the throat is 1000 C. The updraft gasifier has the lowest gas exit tem-

perature (200 400 C), while its gasification temperature may be up to

900 C (Knoef, 2005). Once the gasification temperature is known, the
designer can turn to the heat balance on this basis.
8.6.3.2 Heat of Reaction
Heat of reaction is the heat gained or lost in a chemical reaction. To calcu-
late it for gasification, we consider an overall gasification reaction where
1 mol of biomass (C a H b O c ) is gasified in α moles of steam and β moles of
oxygen. The overall equation is:
0 0 0 0
C a H b O c 1 αH 2 O 1 βO 2 5 A UC 1 B UCO 2 1 C UCO 1 D UCH 4
(8.17)
0
0
1 E UH 2 O 1 F UH 2 1 Q
The equilibrium analysis of Section 7.5.2 gives the mole fraction A ,B ,
0
0
0
C , D , E , and F in the flue gas for given values of α and β. The chosen S/
0
0
0
B ratio defines α while the ER defines β. The heat of reaction, Q, for the