Wet electrostatic scrubbers for the abatement of submicronic particulate

Water electrostatic scrubber (WES) represents an alternative technology for the abatement of that submicronic fraction of particulate – belonging to the so-called Greenfield gap – usually hardly captured with other cleaning techniques. The promising potentialities of WES are recognized by the scientific and industrial communities, but the design of this kind of reactor is far from being optimized. This work reports a mathematical model to evaluate the particle removal efficiency in wet electrostatic scrubbers. The model is used to find out optimal working condition of WES units, through the maximization of the particle collection efficiency in function of different process parameters: contact time, specific water consumption, water/gas relative velocity, size and charge of sprayed droplets. The model has been validated by comparison with different experimental data available in literature, both for charged and uncharged scrubbers. Then it is applied to a reference case study to obtain generalizable results. The model shows that the process optimization for micronic and submicronic size particles follows different criteria. For micronic particles, the collection efficiency increases for higher water/gas relative velocity, with a small effect of droplet diameter and a moderate increase with the droplet charge. On the contrary, in the Greenfield gap, the water/gas velocity plays a secondary role in the capture mechanisms, while a substantial increase of collection efficiency by improving the droplet charge level and reducing the droplet size has been observed. With reference to the actual performances of water spraying and charging devices, the model predicts that a collection efficiency as high as 99.5% can be reliably obtained in few seconds with a water consumption of 100 ml/m3 by adopting droplet diameters around 100 μm and charge to mass ratio from 1 to 3 mC/kg, corresponding to droplet charge equal to 10–30% of Rayleigh limit.