Identification of the mechanism of coal char particle combustion by porous structure characterization

Abstract

Two limiting models—the shell progressive mechanism and the homogeneous mechanism—can describe combustion of a single coal particle. Some information about the real mechanism can be obtained from investigation of the porous structure development during combustion. Using the principles of gas adsorption and mercury penetration, the porous structure of a partially combusted particle was estimated. Experiments were carried out in an equipment by applying the thermogravimetric method and using a single devolatilized coal particle. The inlet concentration of oxygen was 5 and 15 mol%. The initial temperature of combustion was in a range from 450 to 800 °C. The mechanism of coal char particle combustion depends on the initial temperature and the inlet concentration of oxygen. At low temperature and low inlet concentration of oxygen, the rate of principal chemical reactions is comparable with the rate of diffusional transport of oxygen inside the particle. Combustion is governed by the diffusion mechanism. This is evident from the values of the specific surface area of pores and proportional representation of individual pore types. At higher temperatures and low inlet concentrations of oxygen, combustion proceeds by the shell progressive mechanism. The specific surface area is lower in comparison with the previous case. There is a sharp interface between the particle core and the ash shell. The core exhibits a higher value of specific surface area than in the case of a non-combusted coal char particle. This fact can be explained by the consecutive reaction of carbon dioxide with carbon in the core of the particle. The rate of this reaction is sufficiently high at temperatures above 800 °C.