Behavior of Coal Ash and Alkali Matter in a Coal-Fired Gas Turbine System

Abstract

The behavior of coal ash and corrosive alkali species in a gas turbine fueled by an ultra-clean coal water mixture (UCCWM) is investigated. A thermochemical equilibrium analysis is first conducted to study the effect of coal cleaning on the extent of vaporization of ash constituents. It is found that for a selected bituminous coal, cleaned up to 10% of the initial ash content, the amount of Ca, Mg, Si and Al vaporized is independent of coal cleaning and that of Na, K and Fe directly proportional to the degree of coal cleaning. In order to study the fate of the vaporized constituents, a gas-to-particle conversion (condensation) model is formulated. The aerosol processes of homogeneous nucleation, heterogeneous nucleation, particle agglomeration, particle deposition, as well as direct vapor deposition on boundary surfaces are included in the formulation. The aerosol formation calculations are driven by the gas phase equilibrium chemistry of twelve elements and twenty seven gaseous species. The model is partially validated by comparison against available laboratory data on ash nucleation in a laminar flow furnace. Application of the model to UCCWM gas turbine system indicates that the characteristic condensation time of sodium and potassium sulfates is much smaller than the residence time in gas turbine, that in spite of the presence of numerous nucleated ash particles the alkali sulfates undergo self-nucleation, and that thermophoresis and Brownian motion are expected to be the primary mechanisms of sulfate deposition on turbine blades.