In order to degrade organic pollutants in aqueous solutions a number of advanced oxidation methods such as direct Fenton's reactions, electrochemical methods, DC corona discharge, pulse corona discharge, photocatalysis, and UV photolysis have been applied. Gliding arc or glidarc technology has been demonstrated to be effective at removing organic compounds from aqueous solutions and gases. A gliding arc is an electrical discharge formed between two or more thin “knife-edge” divergent electrodes with a high velocity (for example, >1 m/s) gas flowing between the electrodes to prevent sparking. The electrical discharge is formed in the gas phase between two or three divergent electrodes at the smallest gap between the electrodes and the discharge spreads as it glides along diverging electrode edges with an increasingly larger gap until it dissipates as it clears the electrodes. The gas flow maintains a near non-thermal characteristic of the plasma. This discharge leads to the formation of positive ions, negative ions, electrons and other chemically active species. Usually one or two high voltage AC transformers energize the gliding arc reactors (ACG). Although, nearly all previous work with the gliding arc discharge has used AC power, in early studies DC voltage was examined using a high voltage, 3000V, obtained by rectifying a secondary voltage of a transformer at 50 Hz. The usual AC power supply uses the alternating voltage of the high voltage transformer's secondary, which makes it more reliable and robust then the DC power supply that uses high voltage diodes to rectify the voltage. Both configurations have significant energy losses by thermal effect.
Gliding arc discharges have been investigated as a potential technology for gas phase pollution treatment and for liquid phase pollution treatment. While fundamental studies of gas phase gliding arc discharges have been conducted, a detailed understanding of how to apply gliding arc technology for water treatment is still evolving. One mode of operation is to apply the gliding arc above a liquid solution, generally water, whereby the high-velocity gas and some regions of the plasma impinge upon the liquid surface causing reactive species formed in the gas phase to transfer into the liquid and to possibly form reactive species in the liquid or liquid-gas interface. Measurements of OH radicals and NO formed in humid air gliding discharges and the analysis of the pH changes induced in the liquid phase below the discharge from nitrates formed in humid air plasma have been conducted.
An alternative electrode configuration has been examined where the liquid is sprayed through the plasma zone. Since the efficiency of aqueous solution treatment by gliding arcs depends in part on the gas-liquid interfacial contact area between the solution treated and the plasma zone, spraying the solution via a special two-way nozzle directly into the plasma is an effective method to enhance liquid phase treatment using a gliding arc. This alternate configuration has been shown to enhance dye decolorization more effectively than using a reactor configuration with the discharge over a planar water surface. Also in contrast to the discharge above water, when water is sprayed through the discharge with oxygen as the carrier gas, significant amounts of hydrogen peroxide have been formed.
Many other technologies exist for the production of hydrogen peroxide. For example, hydrogen peroxide is made industrially in very large-scale chemical processes that require large quantities of chemical feedstocks. However, there are many applications where small-scale systems and where generation using only readily available materials (water, oxygen, electricity) are of interest. Other competing technologies for the small-scale generation include electrochemical processes. Those electrochemical processes require more complicated membrane and electrode systems than does gliding arc technology. The gliding arc reactor can produce the hydrogen peroxide directly in the spray and it uses only water, oxygen, and electricity. In fact, the use of oxygen is optional, as hydrogen peroxide can be formed with a noble gas, water and electricity in such discharges, although greater amounts can be formed in the case with oxygen as a carrier gas. Gliding arc technology permits the use of small reactors and power supplies that are portable and easy to construct.
Ultimately, the practical use of gliding arc technology to promote chemical transformations, such as the removal of organic pollutants in water or the generation of hydrogen peroxide, other reactive oxygen species, or reactive nitrogen species for treatment of potentially contaminated foods, depends on the efficiency that can be achieved. The efficiency can be measured as the specific energy yield of the chemical conversion of interest.