MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

oxygen partial pressure (Suuberg et al., 1988; Essenhigh, 1991) while the desorption step
is independent of oxygen partial pressure (Essenhigh, 1991; Croiset et al., 1996).
Different mechanisms led to different rate expressions. These rate expressions can
be classified into two types: simple Langmuir rate equation and complex Langmuir-
Hinshelwood rate expressions. The simple Langmuir rate equation has the form of
or equivalently,
r in ′ = k1 C
1+ KC
1
r in′
′ = 1 1
+
k1 C k0 The simple Langmuir rate equation requires four rate coefficients (2 pre-exponential
63
(5.49)
(5.50)
factors and 2 activation energies). Notice that the global n-th order rate equation requires
3 rate coefficients (n, A, and E obs). The Langmuir-Hinshelwood rate expressions are more
complex (with more terms in the denominator or numerator) than the simple Langmuir
rate equation. Complex Langmuir-Hinshelwood rate expressions are less desirable in
engineering practice since these rate expressions require more constants, most of which
are difficult to determine theoretically or experimentally. For example, Laurendeau (1978)
proposed a mechanism that utilizes 10 constants (5 pre-exponential factors and 5
activation energies), and Ranish and Walker (1993) proposed a mechanism that utilizes 8
constants (4 pre-exponential factors and 4 activation energies). In the long run, complex
Langmuir-Hinshelwood expression might become a viable option with experimental and
theoretical developments on this reaction, but presently the simple Langmuir rate
equation seems to be a better option due to its simplicity.