MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

coal-general kinetic rate constants instead of the coal-specific and condition-specific
constants used in the global models.
Intrinsic models vary in levels of sophistication and can be classified into two
subcategories: macroscopic and microscopic (Laurendeau, 1978; Reade, 1996).
Macroscopic models use average properties of the particle to estimate the effective
diffusivity in the porous structures in the char particle, and usually do not model the
evolution of pore structure with burnout. In other words, macroscopic models assume
spatially uniform properties (porosity, pore size and surface reactivity) and temporally
invariant pore structures. Spatially uniform properties are required to solve for the
analytical solutions of the effectiveness factor, which saves substantial computational
time over the numerical solutions. Microscopic models involve the development of a
reaction model for a single pore and then the prediction of the overall particle reactivity
by an appropriate statistical description of the pore size distribution (Laurendeau, 1978).
Microscopic models use a pore structure model to approximate the spatial and temporal
variations in local diffusivity, pore structure, and surface area. If the pore structure is not
allowed to change with conversion, and the properties of particle are assumed to be
uniform, then the microscopic approach becomes equivalent to the macroscopic
approach. Microscopic models have the potential to capture the effects of burnout on
reactivity. However, these models are numerically combersome and generally less
desirable as submodels in comprehensive combustion codes (Cope, 1995).
Microscopic models can be further classified into discrete and continuum models,
depending on whether the pore space and solid are treated as discrete phases or as
continuum phases (Sahu et al., 1989; Sahimi, 1990). Generally, the discrete models are
8