Thermophysical Properties of Ash Deposit on Boiler Heat Exchange Surfaces

A brief overview on boiler tubes ash deposit characterization and thermophysical properties of deposits are given. The structure of an ash deposit on the heat exchange surface of a boiler is usually non-uniform. The thickness of the deposit increases with time and during this process chemical reactions and sintering of ash particles may lead to structural changes. Therefore the thermal conductivity of the ash deposit has to be considered an effective quantity that adjusts energy transfer through the material to a given heat conduction model (for example, planar or cylindrical wall). In this way, an ash deposit may be characterized as a heat-conducting body and compared with other materials. The radiation flux originating from the ash particle layer and deposit layer is spectral in the region of short wavelengths and the emissivity of the ash particulate layer is lower than the emissivity of a smooth one; in contrast, the selectivity of the slag deposit is negligible. This caused by the relatively good transparency of the layer of fine ash particles, the interaction between particles, and the reflection of radiation into the space.