MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

While much research has been conducted on char oxidation at atmospheric
pressure, relatively few char oxidation experiments have been conducted at elevated
pressures. Among these experiments, drop tube experiments and TGA (or fixed-bed)
experiments have provided the most useful kinetic data for char oxidation at elevated
pressures. Other high pressure experiments, such as pressurized, fluidized bed
combustion, are less helpful in providing kinetic data due to the complex flow patterns
involved.
A global n-th order rate equation is often used to model char oxidation rates at
atmospheric pressure. The global n-th order approach, however, was recently shown to
be inadequate in modeling char oxidation rates when total pressure is varied (Monson,
1992; Monson et al., 1995). It is generally thought that in order to model the effects of
total pressure over a broad temperature range, an intrinsic modeling approach (i.e., pore
diffusion effects are accounted for explicitly) is required, and a Langmuir-Hinshelwood
type expression is needed (Essenhigh, 1988; Essenhigh, 1991; Banin et al., 1997; Du et
al., 1991; Croiset et al., 1996). A Langmuir-Hinshelwood type expression can reflect the
physical processes involved in the carbon-oxygen reaction and thus hold more potential in
modeling char oxidation rates at elevated pressures. A significant challenge of using a
Langmuir-Hinshelwood type expression lies in accounting for the pore diffusion effects in
a numerically economic manner. Methods of accounting for pore diffusion effects on
such rate expressions will be explored in this project.
Organization of this Dissertation
First, literature pertinent to high pressure char oxidation is presented in Chapter 2.
The objective and approach used in this study are explained in Chapter 3. Analytical
expressions are developed to accurately predict the effectiveness factors for m-th order
rate equations and the Langmuir rate equation in Chapter 4. Some theoretical
developments related to high pressure char oxidation are presented in Chapter 5. Next,
2