SEPTEMBER 2012 | VOL. 11 NO. 9 | TAPPI JOURNAL 31 S ootblowers are used to remove fireside deposits from heat transfer surfaces in kraft recovery boilers. As such, they are of vital importance for the thermal perfor- mance and production capacity of the boilers. The ability of a sootblower jet to remove a deposit is directly related to the jet strength (force) exerted on the deposit during blowing. Many studies have been performed over the past two decades to examine jet characteristics and interaction with tubes and deposits; most studies were performed under well controlled laboratory conditions [1-3]. In late 2007, a collaborative project was initiated by Andritz to systematically measure sootblower jet strength in-situ [4]. The study involved designing and constructing two force- measurement systems to use in determining the jet force di- rectly in operating recovery boilers. Prior to this project, no similar study has been reported in the literature, owing un- derstandably to the harsh environment in the boilers. The objectives of this study were to obtain field data on jet strength, compare them with laboratory data, and use them to validate the SJT (Sootblower Jet Turbulence) model that has been developed through the years at the University of Toron- to for predicting the behavior of sootblower jets [5,6]. A total of four field trials were conducted. The first three trials (Trials 1, 2 and 3) were carried out in a recovery boiler at the SCA packaging mill in Obbola, Sweden. The fourth trial (Trial 4) was in a recovery boiler at the Södra Cell mill in Värö, Sweden. The force-measurement probe design, test proce- dures, and results obtained from the first two trials (Part I of the project) have been discussed in detail in a paper published recently in TAPPI Journal by Saviharju et al. [4]. The main conclusions obtained in Part I were that, at a given sootblowing lance pressure, the jet force diminished markedly with an increase in distance between the sootblow- er nozzle and the target. At 1 m from the nozzle, the jet ex- erted only 10% of its maximum possible force on the same target. At a longer distance, the jet struck the target not only with a weaker force, but also for a shorter period of time. These trials also showed that the jet strength fluctuated widely, particularly when the jet was close to the target. The fluctuation was due mainly to the vibration of the target as it was struck by the jet and to a lesser extent, the change in jet strength caused by platen swinging and tremor of the system that held the target. The surrounding flue gas temperature was found to have an insignificant effect on jet strength. This paper concerns Part II of the collaborative project in- volving Trial 3, which used different target sizes and shapes, and Trial 4, which used a different recovery boiler with a dif- ferent force-measurement system design. TRIAL 3: OBBOLA MILL Trial 3 was performed in November 2008 in the same recov- ery boiler as Trials 1 and 2 at the Obbola kraft mill. This is a small recovery boiler, with a distance between the front wall and the nearest sootblower in the superheater region of only 3.5 m. A 4.4 m long force-measurement probe inserted through the front wall of the boiler (Fig. 1) made it possible to deter- Measuremets of sootblower jet strength in kraft recovery boilers – Part II: Results of the third and fourth field trials HONGHI TRAN, AMEYA POPHALI, MARKUS BUSSMANN, AND PASI MIIKKULAINEN RECOVERY BOILER PEER-REVIEWED ABSTRACT: Two additional field trial studies were conducted in different kraft recovery boilers in Sweden using force-measurement probes to determine the force of sootblower jets under various blowing conditions. The results confirm the findings of previous trials that, at a given distance, the force exerted on a target by a fully expanded sootblower jet increases nearly linearly with an increase in lance pressure. At a given lance pressure, the jet force decreases drastically as the distance between the sootblower nozzle and the target increases. At a distance farther than 1 m from the nozzle, the jet retains less than 10% of its original strength. The studies also show clearly that the size and shape of the target have a significant effect on the force exerted on it by the jet. A flat surface receives a greater force from the jet than a target with an inclined surface. Application: Understanding how a sootblower jet behaves and factors that affect the jet strength assist recov- ery boiler operators and engineers in developing effective sootblowing strategies to maximize deposit removal and boiler availability.