Page 246 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

222 Biomass Gasification, Pyrolysis and Torrefaction

Here, ΔG is the standard Gibbs function of reaction or free energy change
for the reaction, R is the universal gas constant, and T is the gas temperature.

Example 7.2
Find the equilibrium constant at 2000 K for the reaction
CO 2 -CO 1 1=2O 2
Solution
Enthalpy change is written by taking the values for it from the NIST-JANAF ther-
mochemical tables (Chase, 1998) for 2000 K:
o o o
ΔH 5 ðh 1ΔhÞ 1 ðh 1ΔhÞ 2 ðh 1ΔhÞ
f CO f O 2 f CO 2
5 1 molð2110; 527 1 56; 744Þ J=mol 1 1=2 mol ð0 1 59; 175Þ J=mol
2 1 molð2393; 522 1 91; 439Þ J=mol 5 277; 887 J
The change in entropy, ΔS, is written in the same way as for taking the values
of entropy change from the NIST-JANAF tables.
ΔS 5 1 3 S CO 1 1=2 3 S O 2 21 3 S CO 2
5 ð1 mol 3 258:71 J=mol KÞ 1 ð1=2 mol 3 268:74 J=mol KÞ
2 ð1 mol 3 309:29 J=mol KÞ
5 83:79 J=K
From Eq. (7.34), the change in the Gibbs free energy can be written as:
ΔG 5 ΔH 2 TΔS
5 277:887 kJ 2 ð2000 K 3 83:79 J=KÞ 5 110:307 kJ
The equilibrium constant is calculated using Eq. (7.37):
ΔG 110:307
K 2000 K 5 e 2 RT 5 e 2 0:008314 3 2000 Þ 5 0:001315 (7.38)
ð

7.4.1.3 Kinetics of Gas Solid Reactions
The rate of gasification of char is much slower than the rate of pyrolysis of
the biomass that produces the char. Thus, the volume of a gasifier is more
dependent on the rate of char gasification than on the rate of pyrolysis. The
char gasification reaction therefore plays a major role in the design and per-
formance of a gasifier.
Typical temperatures of the gasification zone in downdraft and fluidized-
bed reactors are in the range of 700 900 C. The three most common

gas solid reactions that occur in the char gasification zone are as follows:
Boudouard reaction: ðR1:C1CO 2 -2COÞ (7.39)
Water gas reaction: ðR2:C 1 H 2 O2CO 1 H 2 Þ (7.40)

Methanation reaction: ðR3:C 1 2H 2 2CH 4 Þ (7.41)