THERMOCHEMICAL CONVERSION: (CO)GASIFICATION AND HYDROTHERMAL GASIFICATION - Biomass as a Sustainable Energy Source for the Future - page 307

treated in Chapter 9), and char gasification. This last step is treated in this chapter in
some depth. Biomass devolatilization leaves a highly porous char with a substantial
specific surface area. The development of the porosity and surface area of a biomass
particle depend on the heating rate. A high heating rate causes volatile biomass
fragments to quickly and easily escape from the remaining porous solid organic
structure. In this case, these mostly organic compounds do not have sufficient resi-
dence time within the charring structure to recondense (forming a secondary char).
In contrast, a slow heating rate for devolatilization results in relatively high amounts
of residual char.
Heterogeneous reactions in coal/biomass gasification include the reaction of char,
the solid residue of pyrolysis, with gas-phase species (mainly O 2 ,H 2 O, CO 2 , and H 2 ).
In this part, the main reactions are presented, and kinetic modeling approaches are
described.
The main heterogeneous reactions playing a role in gasification can be repre-
sented as

Cs ðÞ +1 = ð Þ O 2 g ðÞ ⇄ ð Þ = α 2 α − 2 CO g ðÞ +2 −ð Þ = α CO 2 ð RX : 10 : 1a Þ

Cs ðÞ +H 2 Og ðÞ ⇄ CO g ðÞ +H 2 ð RX : 10 : 3a Þ

Cs ðÞ +CO 2 g ðÞ ⇄ 2CO g ðÞ ð RX : 10 : 4a Þ

3

CO 2

148

0

161

146

(a)

-3

165

(b)

120

-6

(c)

(d)

-9

129

-12

112

116

-15

162

-1 0.78

0.88 0.98 1.08 1.18 1.28

1000/T (K -1 )

FIGURE 10.4 Arrhenius plot for CO 2 -biochar reaction. (a) Cotton wood, (b) Douglas fir, (c)
straw, and (d) spruce. (Source: Reproduced with permission from Di Blasi (2009). © Elsevier;
numbers correspond to references in that publication.)