Applied CFD research for gas flow in gas conditioning towers

Niels Finderup Nielsen, Particulate Process & Development, F.L.Smidth Airtech A/S, Denmark

In process plants, for example those operated in cement works, Gas Conditioning Towers (GCTs) are often placed upstream of electrostatic precipitators as an important component of after-treatment. Fig 1(below) shows a GCT used to cool flue gases from temperatures of 300 - 400˜C down to approximately 140˜C, which is a suitable temperature for the electrostatic precipitator. To obtain optimum GCT performance, wet dust build-up on the tower wall must be avoided which can be achieved by introducing gas flow without wall separation. A special swirler unit was recently developed and optimized with the help of a STAR-CD simulation of the swirling type of flow. The swirler, installed just above the diffuser, is a totally new type, which replaces traditional gas distribution screens. Better control enables the gas distribution to achieve enhanced gas conditioning with lower pressure drop.

Model validation
Extensive validation investigations were carried out with various differencing schemes and turbulence models, and the results were also tested for mesh independency. Experimental data included high accuracy Laser Doppler Anemometry (LDA) measurements and full-scale data. The LDA and full-scale measurements showed good agreement with the numerical results (Fig 2). It was concluded that second order upwind difference and the high Reynolds number form of the standard k-epsilon turbulence model give accurate solutions for this type of swirling flow, if the swirl number is less than approximately 0.8.

Improvements achieved
The performance of the swirler was investigated using a standard FLS Airtech GCT design including a low abrupt bend inlet section with guide vanes. A pronounced effect on the velocity profile was found including a central back-flow region in the diffuser section ensuring down-streaming flow at the tower wall with a gas distribution tending towards uniformity further down the tower (Fig 3). If large droplets are in play, the design can be extended by installation of a flow straightener in the outlet of the diffuser. This effectively reduces the swirl component in the tower and prevents wet dust build-up on the tower wall. The key advantages of the new GCT design are: lower pressure drop compared to constructions with gas distribution screens; hot down-streaming of gases at walls; no dust build-up on the tower wall and gas distribution internals; and vibration or rapping are not needed.

Conclusions
The study has demonstrated that a STAR-CD simulation of turbulent swirling gas flow with low swirl numbers is an accurate and effective investigation tool that can partially replace model and full scale testing, resulting in shorter delivery time from problem formulation to result. Prospects are promising as the new GCT design can be applied for new units and optimization of existing installations. From a broader perspective, it is expected that similar swirler designs can achieve optimal operation in many other industrial processes.

Comparison between numerically calculated, model scale, and full-scale measured axial velocities. Thin vertical line indicates zero velocity



Typical FLS Airtech GCT inlet and swirler section and calculated axial velocity distribution in
diffuser and tower. Note that pink to red colors indicate back flow