HEAT TRANSFER THROUGH COAL ASH DEPOSITS

D. W ANDERSON, Purdue University

Abstract

Experiments have been performed in a facility designed for steady state heat transfer through coal ash deposits obtained from utility boilers. A one-dimensional heat flux through the ash deposits was achieved by subjecting one surface to intense irradiation from SiC electrodes and the other surface to allow temperature heat sink. Temperature profiles and "effective" thermal conductivities (k(,e)), which account for both conduction and radiation heat transfer, were measured over a wide range of temperatures. Heat fluxes were measured calorimetrically. The test facility, experimental methods, and experimental results are discussed. Flyash, crushed slag and fouling deposits, and solid fouling deposits were studied, with hot surface temperatures ranging from 425 to 1420(DEGREES)C and surface irradiation from 23 to 500 kW m('-2). Thermal conductivities were low, typically less than 0.5 W m('-1)K('-1) at temperatures below the sintering and fusion regimes. Particle sintering and melting had considerable effect on temperature profiles and thermal conductivities. Samples which underwent partical melting experienced irreversible changes, with k(,e) increasing as much as tenfold. The coupled problems of coal ash deposition onto relatively cool boiler wall steam tubes, the resulting deposit layer growth, and heat transfer through the deposit were modeled numerically. Temperature dependence of slag viscosity, thermal conductivity, density, and ash deposition flux were accounted for. Viscosities were calculated from the correlation of Capps (1977), and results from the experimental program were used to model k(,e)(T) for Southern Indiana coal slag. A simplified ash deposition model was developed for the case of thermophoretically-controlled deposition. Results of a parametric study, in which the effects of ash deposit properties and operating conditions were examined, are presented in dimensionless form. Ash deposition flux, chemical composition, thermal conductivity, surface emissivity, and flame temperature were found to have the greatest effect on heat transfer rates. Variations in ash deposit density, critical viscosity, and tube wall temperature were of secondary importance. Correlations of numerical results for steady state heat flux as a function of the ash silica ratio, and for steady state ash deposit thickness as a function of silica ratio and thermal conductivity, are presented.

Degree

Ph.D.

Subject Area

Mechanical engineering

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