Strength Development in Fouling Deposits

When coal is combusted in a boiler, the fly ash that is produced flows with the hot gases through the radiant and convective zones before it is removed. About 10% of the ash collects on the walls and pendant tubes. If this ash is not removed it forms a thick layer that limits the heat transfer that can be achieved to the steam cycle. If the buildup becomes excessive the gas flows can be significantly affected, further impacting the boiler performance. Regular cleaning is required to ensure optimal boiler efficiency and ease of ash removal. The longer an ash deposit sits on a convective tube, the stronger it becomes and the harder it is to remove as the deposited ash sinters together. Timeframes required for cleaning depend on the time it takes for strength to develop in the ash deposits due to this sintering process. At present, to assess a coal ash's expected behaviour on the boiler walls, a boiler operator will compare the ash fusion temperature (AFT) to those of coals with known performance in their boiler. But AFT has no correlation with strength development at sintering temperatures but instead is related to the surface tension of liquid phases present. This project considered the sintering phenomenon of fly ash in terms of mechanical strength and elemental composition to better understand deposition behaviour and provide a predictive tool to estimate the optimum time for soot blowing for deposit removal.

Fourteen fly ash samples were collected and analysed for chemistry and particle size. The sintering initiation temperature of each fly ash and selected blends was measured using pressure drop across a heated packed bed or using a thermomechanical analyser. A model of the sintering initiation temperature based on the ash chemistry was developed using a neural network.

To measure sintering strength, sintered compacts were produced from the fly ash samples by heating above their measured sintering initiation temperature. After cooling, sintered compacts of fly ash were then tested for mechanical shear strength. The closed porosity of each compact was determined before the shear strength was measured. When the closed porosity ratio attains 0.4, the ratio of the shear strength of the sintered compact compared to strength of the unsintered ash increases significantly. Frenkel's equation of sintering due to viscous flow allows the time required to reach the closed porosity ratio of 0.4 to be predicted for a given temperature experienced by the ash. Some ashes never reach the prescibed strength, predominantly due to the ash particle size, with more strength developed in finer particles. The higher the exposure temperature of the ash above the sintering initiation temperature, the faster the sintering will occur and strength will develop, and the shorter the optimum time for soot blowing.