Modeling fragmentation by percolation in combustion of carbons

Abstract

A 3-D model has been developed to investigate the process of combined combustion and percolative fragmentation of carbons in the intermediate regime between intraparticle diffusion and chemical kinetics control. The model is based on the discretization of the field into a cubic structure. Vacancies can be originated in the structure to build a lattice having a given original voidage and void size distribution. A Monte Carlo simulation is used to remove cells from such a lattice by combustion and percolative fragmentation. The fraction of cells removed by percolation with respect to the total number of cells removed by combustion and percolation and the distribution of volumes of percolated fragments have been calculated as functions of model variables. These include: the lattice original voidage and void size distribution, the depth of penetration of combustion and the size of the unit cell of the lattice. The simulation successfully describes carbon combustion efficiency and size distribution of carbon fines as functions of porosity and pore size distribution under conditions for both peripheral and uniform percolation, provided that an appropriate size of the unit cell is used. The choice of this variable is a difficult point in the simulation. The size of the unit cell, in fact, cannot be predicted on a general basis due to its dependence on the physical and chemical properties of the specific carbon being considered.