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The ash deposition is one of the major concerns when evaluating the performance and reliability of coal-fired steam generators. In order to take into account the insulation effect of ash deposits, up to now, several models have been reported, but none has taken into account the porous structure of a typical ash deposit under the boiler operational conditions. This work proposes a correlation for the effective thermal conductivity based on a porous structure that represents the growing behavior of a typical ash deposit. The correlation is developed based on a finite volume analysis, which takes into account the rarefaction effects due to the close relation between the geometric scale and the mean free path of the molecules. This effect is considered through the temperature jump, which modifies the boundary condition under which the Fourier equation can still be applied. The found results from the proposed correlation are in good agreement with the measured effective thermal conductivity and gives a good estimative of the energy loss due ash deposition on superheaters of coal-fired steam generators.

An ash deposit growing model based on surface growing theory is developed in order to represent the coal fired ash deposits. The predicted characteristics are compared to physical and chemical measurements of a typical ash deposit collected from an existing coal fired power plant located in the south of Brazil. Several structures were generated and compared to scanning electron images of ash deposits collected from the steam superheater section. The physical and chemical characteristics of the ash deposits, such as porosity, pore size distribution and fractal dimension of a typical ash deposit were measured using SEM image. All the images were analyzed using Imago software. Also, the existing ceramic phases were quantified using EDX detection and X-ray diffraction. The proposed random trajectory particle deposition model led to good agreement when compared to the macro quantities such as porosity and fractal dimension. It is expected that the modeled porous structure can be used for further heat transfer analysis and evaluation of coal-fired steam boiler designs.