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Show INTRODUCTION Among the issues that detennine the design and operation of coal utilization equipment, ash deposition on heat transfer surfaces plays a significant, in many cases dominant, role. A brief survey of the development of coal conversion technologies identifies the historical role of ash deposition as a motivation for major new technologies and concludes that "it is ... inevitable that there will be always some problems with ash, whatever the system of coal combustion" W. Nevertheless, the fate of inorganic material associated with coal remains less well understood than the behavior of the organic material during coal combustion. While there are several indices of ash behavior, there is no complete model that describes ash deposition in a comprehensive way. This paper describes progress toward development and validation of such a model. Significant experimental and theoretical work has been directed at developing a better understanding of ash deposition and the resulting deposit properties. Quantitative data addressing deposit properties as a function of coal properties, location within an experimental facility, and operating conditions have been published by several investigators <2=8). However, there are fewer published ash deposition data from facilities larger than pilot scale. Fly ash formation models are in various states of development by several researchers. Investigators at the MIT Energy Laboratory (2) describe a model for the generation of fly ash capable of predicting the size and composition distributions of fly ash from detailed descriptions of coal mineral matter. Similar models are under development by other investigators (2,lQ) building on published fundamental results (i, 11, 12.). The output of these models is a description of the size and elemental composition distributions of the entrained particulate phase resulting from the combustion of pulverized coal. A great deal of infonnation is available on rates and mechanisms of ash deposition. In this paper, we consider four major mechanisms of deposition, or mass transport to a surface: (1) inertial transport including impaction and sticking, (2) thennophoresis, (3) condensation, and (4) chemical reaction. In general, the rates of inertial impaction on cylinders in cross flow are well established. Rates on walls with parallel flows are less well established. The capture efficiency a measure of the propensity of material to stick to a surface after impaction, is far less well established. The rates of thennophoretic deposition on heat transfer surfaces are reasonably well established when local temperature gradients and the functional fonn of the thennophoretic force on the particle (or the thermophoretic velocity) are known. Condensation rates can be predicted reasonably well given accurate vapor pressure and concentration data. The accuracy to which rates of chemical reaction are known is often inadequate: especially those involving sulfation and alkali adsorption in silicates. The results discussed in this paper rely in part on an engineering model that predicts relevant aspects of ash deposition in pulverized coal boilers UJ). This model is called ADL VIC (Ash Deposit Local Viscosity, Index of refraction, and Composition). This model is based on both rust-principle derivations and a series of experimental results, supplementary to those indicated above, that allow specification of critical parameters in the mechanism of ash deposition. Predictions from this model have been compared with experimental result from combustion systems of several different sizes and coals of many different ranks (U-H). This paper presents the results of a joint project between Central Illinois Public Service Co. (CIPS) and Sandia to anticipate the deposition-related consequence of switching from midwestern/eastern coals to a western coal in a 600 MW e utility boiler. The ere ult , based on a three-week test bum, represent the frrst application of ADLVIC to a utility boiler. 2 |