Surfactant adsorptivity of solid products from pulverized-coal combustion under controlled conditions

Most kinetic studies of pulverized coal char combustion have focused on the measurement or prediction of rates of solid-phase mass loss, which determine the ultimate degree of conversion of the solid fuel for a given residence time and gas environment. The strongest motivation for improving the degree of conversion in practical coal combustion systems, however, is not related to the mass of residual carbon, but to its detrimental behavior in concrete mixtures. When fly ash is added to air-entrained concrete as a partial replacement for Portland cement, the specialty surfactants, or air entraining admixtures, can be rendered ineffective by adsorption onto the surfaces of the porous residual carbon particles. This paper presents the first set of measurements of the surfactant adsorptivity of carbon-containing solid products generated during pulverized coal combustion under controlled laboratory conditions. The specific adsorptivity of the carbon is found to vary dramatically over the course of conversion, exhibiting a sharp maximum in the early stages of char combustion. The main features of the data can be explained by considering the concurrent evolution of surface area, pore structure, and elemental composition, especially oxygen content. It is also shown that surfactant adsorptivity can be greatly altered by post-combustion modification of the carbon surfaces in the form of air oxidation at temperatures above about 300°C (reduced activity) or heat treatment at 900°C in argon (increased activity). The combined results are used to describe a mechanism for the competitive adsorption process in concrete mixtures and to discuss implications for combustion systems.