The effects of atmosphere and additives on coal slag viscosity

With the advent of advanced coal-fired power systems operating at higher working fluid temperatures, slag corrosion, erosion, and fouling of heat exchanger surfaces will become even more of a problem than in today`s systems. Laboratory experiments have shown excessive corrosion of candidate alloy and ceramic heat exchanger materials by both calcium-rich subbituminous and iron-rich bituminous coal slags. The viscosity of the slag greatly affects the corrosion rate since it determines the rate of transfer of corrosive species to the materials and corrosion product away from the materials. Slag viscosity is controlled by the composition of the slag and surrounding atmosphere as well as its temperature. In this paper we report the results of investigations of the viscosities and critical temperatures of three coal slags in three atmospheres: air, air plus water vapor, and reducing gas. In addition, the effects of additions of alumina, magnesia, and copper oxide on viscosity, crystallization, and critical temperature of the slags are reported. Conclusions are drawn about appropriate test conditions for determining slag corrosion rates and about ways of modifying slag viscosity to reduce corrosion rates.