Page 247 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 7 Gasification Theory 223

The water gas reaction, R2, is dominant in a steam gasifier. In the
absence of steam, when air or oxygen is the gasifying medium, the
Boudouard reaction, R1, is dominant. However, the steam gasification reac-
tion rate is higher than the Boudouard reaction rate. Another important gasi-
fication reaction is the shift reaction, R9 (CO 1 H 2 O2CO 2 1 H 2 ), which
takes place in the gas phase. It is discussed in the next section.
A popular form of the gas solid char reaction, r, is the nth-order
expression:

1 dX 2 E
n 21
r 5 m 5 A 0 e RT P s (7.42)
ð12XÞ dt i
where X is the fractional carbon conversion, A 0 is the apparent preexponen-
21
tial constant (s ), E is the activation energy (kJ/mol), m is the reaction order
with respect to the carbon conversion, T is the temperature (K), and n is the
reaction order with respect to the gas partial pressure, P i . The universal gas
constant, R, is 0.008314 kJ/mol K.

7.4.1.4 Boudouard Reaction
Referring to the Boudouard reaction (R1) in Eq. (7.6), we can use the
Langmuir Hinshelwood rate, which takes into account CO inhibition (Cetin
et al., 2005) to express the apparent gasification reaction rate, r b :

r b 5 k b 1 P CO 2 s 21 (7.43)
1 1 ðk b 2 =k b 3 ÞP CO 1 ðk b 1 =k b 3 ÞP CO 2
are the partial pressure of CO and CO 2 , respectively, on
where P CO and P CO 2
the char surface (bar). The rate constants, k i , are given in the form, A exp
(2E/RT), where A is the preexponential factor (bar 2n 2n
s ). Barrio and
Hustad (2001) gave some values of the preexponential factor and the
activation energy for Birch wood (Table 7.6).
When the concentration of CO is relatively small, and when its inhibiting
effect is not to be taken into account, the kinetic rate of gasification by the
Boudouard reaction may be expressed by a simpler nth-order equation as:
E
r b 5 A b e 2 RT P n s 21 (7.44)
CO 2
For the Boudouard reaction, the values of the activation energy, E, for
biomass char are typically in the range of 200 250 kJ/mol, and those of the
exponent, n, are in the range of 0.4 0.6 (Blasi, 2009). Typical values of A,
E, and n for char from birch, poplar, cotton, wheat straw, and spruce are
given in Table 7.7.
The reverse of the Boudouard reaction has a major implication, especially
in catalytic reactions, as it deposits carbon on its catalyst surfaces, thus deac-
tivating the catalyst.