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ABSTRACT
The pore structure in char particles changes greatly during combustion, which significantly affects the char combustion characteristics. This article describes a model for the evolution of the char pore structure. A fractal pore model is used to generate the porous char particles to simulate the char combustion for various porosities, , specific surface areas,
, fractal dimensions,
, and particle diameters,
. Then, expressions are given to associate these pore structure parameters with the char conversion,
, and the operating conditions, using a modulus,
, indicating the pore diffusion resistance. As
increases,
increases,
and
decrease, and
increases to a maximum first and then decreases. In some cases,
only decreases with increasing
when the reaction rates are too high or the conditions are such that the O2 cannot diffuse into the particle. The results compare well with the pore structure parameters of three Chinese chars combusted in a drop tube furnace at different conversions measured by a mercury porosimeter.
Nomenclature
| = | constants in Eq. (17) | |
| = | char particle ash content | |
| = | initial char particle ash content | |
| = | constants in Eq. (20) | |
| = | constants in Eq. (21) | |
| = | char pore fractal dimension | |
| = | initial char pore fractal dimension | |
| = | constants in Eq. (18) | |
| = | molecular diameter (m) | |
| = | char particle diameter (m) | |
| = | initial char particle diameter (m) | |
| = | largest distance between the solid elements in three independent directions (m) | |
| = | molecular translational energy (J/mol) | |
| = | translational energy distribution function for gas molecules | |
| = | reaction heat (kJ/mol) | |
| = | Boltzmann constant | |
| = | side length of the elements in the fractal pore model (m) | |
| = | length of the overlapped pore system (m/m3) | |
| = | initial length of the overlapped pore system (m/m3) | |
| = | scaling factor | |
| = | constants in Eq. (40) | |
| = | molecular mass (kg) | |
| = | total number of molecules without intermolecular collisions | |
| = | initial total number of molecules in one gas element | |
| = | number of gas elements in the fractal pore model | |
| = | number of solid element faces in contact with gas elements in the fractal pore model | |
| = | number of solid elements in the fractal pore model | |
| = | total number of elements in the fractal pore model | |
| = | molecular number density (m–3) | |
| = | number of molecules | |
| = | O2 partial pressure at the particle external surface (atm) | |
| = | initial O2 partial pressure at the particle external surface (atm) | |
| = | universal gas constant | |
| = | char particle pore radius (m) | |
| = | accumulated surface area in mercury intrusion measurements (m2) | |
| = | specific surface area of the char particle (m2/kg) | |
| = | initial specific surface area of the char particle (m2/kg) | |
| = | specific surface area corresponding to the 50- | |
| = | maximum value of | |
| = | temperature (K) | |
| = | initial temperature (K) | |
| = | time (s) | |
| = | volume (m3) | |
| = | char conversion | |
| = | char conversion when combustion stops | |
| = | critical char conversion corresponding to the maximum | |
| = | a short distance (m) | |
| = | constant in Eq. (13) | |
| = | constant in Eq. (14) | |
| = | modulus defined in Eq. (22) | |
| = | initial modulus | |
| = | gas molecule mean free path (m) | |
| = | circular constant | |
| = | char particle porosity | |
| = | initial char particle porosity | |
| = | apparent char particle density (kg/m3) | |
| = | initial apparent char particle density (kg/m3) | |
| = | true char particle density (kg/m3) | |
| = | dimensionless pore structure parameter defined in the RPM |