Pollution: cleaner coronas
May 20, 1998
Corona discharge filters are widely used to remove various sulphur and nitrogen-based pollutants from discharges at gas and coal power stations. Most of these filters rely on "negative-charge" coronas to oxidise the impurities. Now Paul Feldman and Krishnaswamy Kumar of Environmental Elements Corporation in the US have designed a new type of electrode that could improve the efficiency of such filters.
Coronas form when a stream of gas passes between two electrodes maintained at different voltages. The potential difference causes energetic electrons to collide with the gas, producing a glowing plasma that consists of negative ions and radicals. This is a negative discharge corona.
However, if dust builds up on the equipment, a positive discharge corona is produced. Dust is a poor conductor and causes an intense electric field to build up on the electrode. This field is proportional to the resistivity of the dust. As the intensity of the field approaches the breakdown strength of the gas, it generates a plasma containing positive ions and radicals. However, these particles do not oxidise the impurities in the gas.
Patent 5733360 describes a new electrode design that optimises all parts of a corona discharge reactor to achieve maximum efficiency. One electrode consists of an array of wires embedded in a single metal sheet. The second electrode is a porous metal plate covered by a woven fibreglass cloth. Rapid voltage pulses are applied to the first electrode as the gas containing the pollutants is pumped through the pores in the second electrode towards the first. The pores are positioned almost exactly opposite the wires in the first electrode, which maximises the number of electrons hitting the covered plate to generate a positive corona. As the positive corona is generated by the electrodes, an intense negative corona current is created in the porous material. This increases the contact between the gas containing the impurities and the ions and radicals in the plasma, and therefore increases the efficiency of the system.