Energy & Fuels 1995,9, 277-283
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Viscosity of Ash Particles in Combustion Systems for Prediction of Particle Sticking C. L. Senior" and S. Srinivasachar? PSI Technologies, 20 New England Business Center, Andover, Massachusetts 01810 Received June 20, 1994. Revised Manuscript Received November 23, 1994@
A new model has been developed for predicting the viscosity of individual coal ash particles in the range of 104-108 Pa-s. Previous experimental work has demonstrated that there is a critical viscosity for adhesion of ash particles. At temperatures and velocities typically found in coalfired utility boilers, this critical viscosity has been shown to lie between lo5 and lo7Paas. Previous models of coal slag viscosity were derived from data at much lower viscosities (less than lo2 P e s ) and, while such models accurately predict viscosity of coal slag at high temperatures, they do not perform well in predicting the viscosity associated with particle adhesion to heat-transfer surfaces in utility boilers. Using viscosity data taken from the literature, a new model has been formulated. The data sets are described in detail as well as the coefficients needed to calculate viscosity. Finally, the results of the model are compared to results from previous coal slag viscosity models. The new model is shown to produce a better fit between calculated and measured values of viscosity in the range of 104-108 Paas. Furthermore, the new model does a better job of predicting viscosity for certain compositions which are commonly found in ash particles.
Introduction During the combustion of pulverized coal, ash particles formed from inorganic species transformations may deposit on heat-transfer surfaces, causing costly decreases in heat-transfer rates and system efficiencies. Traditionally power generating facilities have attempted to predict the severity of coal ash deposition by calculating empirical indices based upon the overall ash composition of the coa1.l While suitable for narrow ranges of coal-boiler combinations, these indices are less accurate predictors when extrapolated t o other coals or operating conditions.2 This can be attributed to their use of overall ash composition, when in fact deposition is initiated and propagated by individual ash particles with varying compositions and sizes. It has been shown that, during deposit initiation, sticking efficiencies of particles to heat-transfer surfaces are significantly less than unity at temperatures and velocities associated with utility boiler^.^,^ Research has produced convincing evidence that at short residence
* Author to whom correspondence should be addressed.
' Present address:
ABB-Combustion Engineering, P.O. Box 500, 1000 Prospect Hill Road, Last Windsor, CT 06095-0500. Abstract uublished in Advance ACS Abstracts. Januarv 15. 1995. (1)Winegakner, E. C. ASME Research Committee on Co&os
