Percolation model for simulation of coal combustion process

A pulverized coal combustion simulation model applying a percolation theory based on a Monte Carlo method has been developed to predict swelling and fragmentation behaviors during a coal combustion process. The shape of pulverized coal particles before reaction was assumed as a three-dimensional cube arranged in large number of small lattices. These lattices were classified into char, volatile, ash, or macropore, depending on the coal's industrial analysis value, and they were arranged randomly in the cubic. The coal combustion processes were classified as devolatilization and char combustion, and they were assumed to occur simultaneously. In the devolatilization process, the dual-competing reaction model determined the devolatilization time of a volatile lattice. With coal devolatilization, the coal particles swelled due to increase in internal pressure. With char combustion, O₂ lattices were arranged around a coal particle and the random walk model was applied to represent the O₂ diffusion behavior. Furthermore, the char reaction model was applied to determine the char reaction time. Thus, a char lattice was lost when its total reaction time was longer than the O₂ diffusion time and the reaction time. With char combustion, ash agglomeration occurred. Char combustion finished when all of char lattices were burned out or the char lattices were completely surrounded by ash lattices, preventing oxygen lattices from coming into contact with the char lattices. It was shown that this model well represents the difficulty of char burnout caused by increase in diffusion resistance in the latter period of the reaction. A particle temperature profile was determined by a model calculation in an environment in which atmospheric temperature was 1500 K. Using only the industrial coal properties, this model predicts detailed variations of reaction rate, porosity, and maximum relative particle diameter with particle conversion in the pulverized coal combustion process.