MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

= q obs
q max
The factor can be used as a criterion of whether combustion occurs in Zone III.
12
(2.15)
Combustion is considered occurring in Zone III when the factor is close to unity. For
example, Mitchell et al. (1992) set this threshold at 0.9. Of course, the factor is
affected by the stoichiometric coefficient of oxygen ( o), which lies between 0.5 and 1 and
is sometimes difficult to determine accurately.
The Global n-th Order Rate Equation
An empirical n-th order rate equation is often used to describe kinetics of char
oxidation at typical industrial boiler temperatures (Smith, 1982; Hurt and Mitchell, 1992;
Monson et al., 1995). The most common form of the n-th order rate equation
is
n −Eobs qrxn = k sPos = Aexp(
RT p
n
)Pos , (2.16)
where q rxn is the global char oxidation rate in gram C per unit time per unit external surface
area, n is the apparent reaction order, Pos is the oxygen partial pressure at the external
surface, A is the pre-exponential factor, E obs is the observed activation energy, T p is the
particle temperature in K. This simple equation provides a basis for estimating char
oxidation rates, and has often been adequate for practical use at atmospheric pressure over
small temperature ranges (Smoot and Smith, 1985). Due to its simplicity, this equation is
often used in comprehensive computer models. However, the global n-th order rate
equation does not explicitly account for pore diffusion effects on kinetics. Pore diffusion
effects are implicitly included in the observed activation energy and the pre-exponential
factor. One weakness of this equation is that it cannot be extrapolated between Zone I
and Zone II. In addition, the reaction order is often observed to vary as experimental