[PDF] A Laboratory Study Of Deposit Removal By Debonding And Its Application To Fireside Deposits In Kraft Recovery Boilers eBook

Author : Seyed Abdolreza Ebrahimi-Sabet
Publisher :
Page : pages
File Size : 27,53 MB
Release : 2001
Category :
ISBN :

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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 '/'ß0.5', where 'ß ' 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.

Author : Liming Zhao
Publisher :
Page : 154 pages
File Size : 39,28 MB
Release : 2011
Category :
ISBN : 9780494766156

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Magnesium oxide-based additives have been used in kraft recovery boilers to minimize deposit accumulation on superheater and generating bank tubes. Due to inconclusive results observed in boilers and claims made for additive effectiveness backed by little scientific evidence, only a few mills still use the additives for fouling control. A systematic study was performed to examine the effect of MgO-based additives on deposit accumulation and removal under controlled laboratory conditions, and to evaluate the additive effect in a kraft recovery boiler. The results show that while the additive coating has no effect on deposition, it could ease deposit removal if a coating greater than 5 mum can be applied on the tube surface effectively. Separate injection of additive and black liquor had insignificant effect on deposition and removal. Results of the field study show that the amount of additive collected on the tube is too small to have an impact.