| The accumulation of fireside deposits on heat exchange surfaces in a kraft recovery boiler drastically reduces the boiler thermal performance, restricts the flue gas flow, and in severe cases plugs the flue gas passages leading to unscheduled boiler shutdowns for cleaning. High-pressure steam sootblower jets are used to remove these deposits.; Deposits may be brittle or “plastic”, depending on their temperature, and may be removed by sootblowers by two main mechanisms: brittle fracture and debonding. Brittle fracture has been previously studied and applies only to brittle and low-strength deposits. Debonding is likely to be the main mechanism by which strong deposits are removed by breaking the weak bond between the deposit and the tube surface. Deposit removal by a debonding mechanism is the main subject studied in this thesis.; To study the effects of aerodynamic drag and lift forces and flow induced vibrations on deposit debonding, the instantaneous drag and lift forces generated by a supersonic jet impinging on artificial deposits were measured in laboratory experiments. By numerical analysis, the fluctuation components of the drag and lift forces were decoupled from the flow-induced vibrations.; Results showed that the lift force fluctuations and flow-induced vibrations acting on deposits increased and reached maxima at a distance of about 45–50 nozzle diameters downstream. The flow-induced vibrations were found to be much larger in the lateral direction than in the flow direction. These vibrations increase the lift force fluctuations by a maximum factor of 0.872 /β0.5, where β is the damping coefficient of the tube-deposit assembly. This is in good agreement with the results of the experiments, where the fluctuating lift forces were observed to be as effective as the mean drag force in removing artificial deposits.; An analysis of the dimensionless power spectra of lift forces exerted on cylindrical deposits showed a high degree of similarity under a broad range of operating conditions. The information can be used to estimate the power spectra of lift forces exerted on deposits in full-scale boilers. The results would provide a basis for estimating the role of vibration in deposit removal in full-scale boilers. |
