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.
A computer model, RAFT (Reactor Aerosol Formation and Transport), has been developed to predict the size distribution and composition of the particles (aerosols) formed from condensation of the fission product and control rod material vapors released in LWR accidents. The underlying theory of RAFT considers the processes of homogeneous and heterogeneous nucleation, aerosol agglomeration, and aerosol and vapor deposition, in conjunction with the equilibrium chemistry of the Cs-I-Te-0-H-Ag-In-Cd-inert gas system. Calculations using RAFT show that under most accident conditions, the particle size spectrum is determined primarily by the competition between the homogeneous and heterogeneous nucleation mechanisms, rather than the agglomeration mechanism, and that direct vapor deposition on structural surfaces is an important mechanism for the scavenging of fission product vapors.
Mechanistic models have been developed for particle and vapor deposition on the blades of coal-fired gas turbines. The particle deposition models include the simultaneous contribution of Brownian and turbulent diffusion, thermophoresis, eddy impaction, and inertial impingement. The diffusive mechanisms have been validated against experimental data for low-speed cascade flow and particle-laden flow through pipes. The inertial deposition treatment is shown to collapse to the well-known expression for particle capture in a flow turning around a bend. A method is presented for calculating Na2SO4 and K2SO4 vapor deposition on cooled blades. Scaling laws are formulated for estimating the contribution of boundary layer homogeneous and heterogeneous nucleation mechanisms for highly cooled turbine blades.
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