Recovery Boiler Fireside Deposit Removal by Debonding

A. Ebrahimi-Sabet, Ph.D.

Fireside deposits on heat transfer surfaces in kraft recovery boilers are removed by sootblowers through two main mechanisms: brittle fracture and debonding. Brittle fracture applies only to brittle, low-strength deposits in the region downstream of the generating bank. In the superheater region, deposits are usually dense and hard, and may have a liquid surface due to the high flue gas temperature. These hard and/or plastic deposits may still be removed by a sootblower if there is a weak bond at the deposit/tube interface. This deposit removal mechanism is called debonding.

The present work examines the effects of principal aerodynamic parameters of a sootblower jet on deposit removal by debonding. Blow-off experiments were carried out on model deposits of different sizes, shapes and adhesion strengths. The results suggest that the jet peak impact pressure (PIP), the deposit adhesion strength, and the tube-deposit contact area are the principal parameters affecting deposit debonding. The jet angle of attack and the deposit shape have an insignificant effect on debonding.

Experiments were also carried out to measure the lift and drag acting on model deposits (Figure 1). Deposits were connected to a load cell and were placed in front of a 4:1 scaled-down fully expanded laboratory nozzle. Lift and drag forces were measured at different distances from the nozzle by the load cell and used to calculate the maximum stress generated at the deposit/tube interface. The effect of stress generated by the lift force fluctuations at the deposit/tube interface was examined as a function of the damping coefficient b of the tube/deposit assembly in a tube bundle. The results suggest that depending on the damping coefficient b, vibrations caused by lift fluctuations may be the dominant forces in debonding deposit.

Figure 1: The experimental setup and the deposit-nozzle orientation during lift force measurements

Figure 2: Comparison of the mean drag force decay and the estimated effect of maximum vibration amplification of the stress generated by the lift force in recovery boilers for different damping coefficients

Abdolreza Ebrahimi-Sabet (Reza Sabet) graduated from the University of Tehran in 1989 with a B.A.Sc. and from the University of New Brunswick in 1995 with a M.Sc.E. both in mechanical engineering. Reza has more than four years experience as a mechanical and process consultant engineer in petroleum engineering, marine engineering, plant design, and feasibility studies. He joined the Pulp & Paper Centre in May 1996. Reza is currently working as a Ph.D. candidate under the supervision of Professors D.E. Cormack and H.N. Tran on "Recovery Boiler Fireside Deposit Removal by Debonding". He joined Rowan Williams Davies & Irwin Inc. as a Senior Technical Coordinator in July 2000 and has finished his studies in Fall 2000.