Structure and surface area of pulverized coal during combustion

To investigate the relationship between structure and surface area during combustion both single particles and suspensions of pulverized bituminous coal were heated at rates comparable to those obtaining in large boilers (maximum approximately 10 4 °C sec −1 ). The single-particle behavior was monitored by high-speed cine photography, and samples of chars were examined microscopically and their surface areas measured by xenon adsorption. It was found that during combustion the great majority of particles formed hollow spheres which burned both externally and internally at approximately constant diameter until they fragmented. At the same time their specific surface area progressively decreased in value. Both structural form and surface area were rank and lithotype dependent. Computations were also performed in which the “true” area of reacting surface of the char residue was made a major variable in a mathematical model of single-particle behavior, and the values for the ratio of “true” to external area were manipulated to achieve agreement with the results of combustion trials conducted in a small tube furnace. The best agreement between measured and computed results was obtained when the ratio, while remaining fractional, was allowed to reach a maximum at a time corresponding to fragmentation. These findings are explicable in terms of preferential oxidation of parts of the external and internal surfaces, resulting in an area for reaction smaller than the calculated external surface of the particle. The progressive destruction of the micropores during burnout leads to a decrease in the measured surface but an increase in the surface area available for attack. The latter may be 3 or 4 orders of magnitude less than the former.