(336d) Cyclones with and without an Eccentrically Positioned Vortex Finder

With fluidized bed, circulating fluidized bed (CFB) and even pneumatic conveying operations, reverse flow cyclones are typically the critical component in a successful process. Although the designing principle for cyclones is straightforward, the hydrodynamics in these units are not. This poor understanding of the hydrodynamics has limited cyclone capabilities which why cyclone designs has not changed significantly in 150 years.

This is especially true with the symmetrical design of the cyclone. With the exception of the inlet, everything else on a cyclone is symmetric. For example, the gas outlet tube immersed into the cyclone, also called the vortex finder, is located concentrically, i.e. its axis coincides with the axis of the cyclone. However, it has been shown in numerous recirculating cyclones of CFB power plants that eccentric positioning of the vortex finder axis improves the cyclone efficiency without affecting the pressure drop [1,2]. By applying this measure the circulating material became finer and its quantity increased resulting in a considerable improvement of the boiler performance: The heat transfer in the combustion chamber and, if installed in the fluid bed cooler, improved the optimum combustion temperature to ~900°C and reduced the amount of unburnt filter ash. In addition, the optimization led to cost savings of operational costs.

Trefz [3] gave a qualitative explanation of the positive effect of eccentrically shifting the vortex finder axis on the cyclone efficiency based on flow measurements in cyclones. At the top of the cyclone barrel region, there is a secondary flow through the boundary layer of about 10% of the total flow carrying particles towards the center. The vortex finder forces this secondary flow around the tube down to the opening allowing to remove particles from that flow by centrifugal force. In cyclones with a tangential inlet and a concentric vortex finder, however, this separation is not optima. The axis of the vortex flow is pushed away from the geometrical center of the cyclone by the incoming gas flow and as a consequence the tangential velocity of the flow around the vortex finder is not uniform. In the section of the vortex finder surface which is closer to the cyclone axis the tangential velocity stagnates and the secondary flow streams vertically downwards with practically no circumferential velocity. In this section, the particle separation is poor. By shifting the vortex finder axis into the “eye” of the vortex a uniform flow around the vortex finder is achieved and its separation efficiency improves considerably.

Recently, numerical granular-fluid simulations of high loaded cyclones with a concentric and with an eccentric vortex finder have been performed by applying the CFD code Barracuda Virtual Reactor^® of CPFD, LLC. The CFD results confirm that eccentrically shifting the vortex finder results in a better collection efficiency without compromising the pressure drop. The calculations are compared with measurement data of commercial high loaded cyclones in CFB boilers and the origin of the improvement due to the eccentric shifting the vortex finder is studied.

References

[1] Muschelknautz U, Muschelknautz E (1999). Separation efficiency of recirculating cyclones in CFB combustions. VGB PowerTech 4/1999, 48-53.

[2] Ipsen C, Roschek D, Muschelknautz U (2014) Optimierung eines Zyklonabscheiders einer zirkulierenden Wirbelschichtfeuerung. VGB PowerTech 4/2014:75-79

[3] Trefz M (1992). Die verschiedenen Abscheidevorgänge im höher und hoch beladenen Gaszyklon unter besonderer Berücksichtigung der Sekundärströmung. VDI-Fortschr.-Ber., VDI-Verlag, Düsseldorf, Reihe 3, No. 295.