Siemens developed the first ozone generator, which was based on corona discharges in 1957. Today ozone is produced by several different methods both commercially and in the laboratory.
The generation of ozone involves the intermediate formation of atomic oxygen radicals which can react with molecular oxygen. All processes that can dissociate molecular oxygen into oxygen radicals have the potential for ozone generation. Energy sources that make this action possible are electrons or photon energy. Electrons can be used from high-voltage sources in the corona discharge, from nuclear sources, and from electrolytic processes. Suitable photon quantum energy includes UV light of wavelengths lower than 200 nm and γ-rays.
In nature, ozone generation occurs when oxygen molecules reacts in the presence of electrical discharges, e.g., lightning, and by action of high energy electromagnetic radiation. Some electrical equipment inadvertently generates levels of ozone that can be easily smelled; this is especially true if there is a spark or a very high voltage.
Ozone Generation by Corona Discharge
Corona discharge in a dry process gas containing oxygen is presently the most widely used method of ozone generation for water treatment. The corona or plasma is created in an ozone generator by applying a high voltage between two electrodes. Ozone is formed by the following reactions:
A 1/2 O2 = O Heat of Reaction A= +59.1 Kcal
B O + O2 = O3 Heat of Reaction B = -24.6 Kcal
AB 3/2 O2 = O3 Heat of Reaction AB= +34.5 Kcal
The overall reaction (AB) that produces ozone requires energy and is an endothermic reaction that obtains energy from the electric discharge. A basic ozone generation system is composed of the following: gas source (compressed air or oxygen), gas dryers, and ozone generators.
It is of utmost importance that a dry process gas is applied to the corona discharge. Limiting nitric acid formation is also important in order to protect the generators and to increase the efficiency of the generation process. In normal operation of properly designed systems, a maximum of 3 to 5 g nitric acid is obtained per kilogram ozone produced with air. If increased amounts of water vapor are present, larger quantities of nitrogen oxides are formed when spark discharges occur. Also, hydroxyl radicals are formed that combine with oxygen radicals and also ozone. Both reactions reduce the ozone generation efficiency. Consequently, the dryness of the process gas is important in order to obtain a good yield of ozone. Moreover, with air, nitrogen oxides can form nitric acid, which can cause corrosion.
The formation of ozone through electrical discharge in a process gas is based on the corona discharge in air or oxygen. In an ozone generator here are numerous distributed micro electrical discharges (arc or plasma) by which the ozone is effectively generated. It appears that each individual micro discharge lasts only several nanoseconds. The current density ranges between 100 and 1000 Amps/cm2. By using oxygen or enriching the process air in oxygen, the generating capacity of a given ozone generator can be increased by a factor ranging from 1.7 to 2.5 versus air alone. Whether using air or oxygen feed energy is lost in the form of heat, cooling of the process gas is very important. In smaller systems this is often down by using ambient air to cool one or both of the electrodes. In larger systems the cooling is typically done with water usually on the ground electrode.
Other methods of ozone generation include:
Photochemical Ozone Generation
The formation of ozone from oxygen exposed to UV light at 140-190 nm was first reported by Lenard in 1900 and fully assessed by Goldstein in 1903. It was soon recognized that the active wavelengths for technical generation are below 200 nm. In view of present technologies with mercury-based UV-emission lamps, the 254-nm wavelength is transmitted along with the 185-nm wavelength, so destruction of ozone occurs simultaneous with its generation. Moreover, the relative emission intensity is 5 to 10 times higher at 254 nm compared to the 185-nm wavelength. Thus only small amount of ozone can be produced.
Attempts to reach a suitable photo stationary state of ozone formation with mercury lamps have failed. The main reason for this failure is that thermal decomposition is concomitant with ozone formation. Except for small-scale uses or synergistic effects, the UV-photochemical generation of ozone has not found widespread use.
Electrolytic Ozone Generation
Electrolytic generation of ozone has historical importance because synthetic ozone was first discovered by Schönbein in 1840 by the electrolysis of sulfuric acid. The simplicity of the equipment can make this process attractive for small-scale users or users in remote areas.
Many potential advantages are associated with electrolytic generation, including the use of low-voltage DC current, no feed gas preparation, reduced equipment size, possible generation of ozone at high concentrations, and generation in the water, eliminating the ozone-to-water contacting processes. Problems and drawbacks of the method include: corrosion and erosion of the electrodes, thermal overloading due to anodic over-voltage and high current densities, need for special electrolytes or water with low conductivity, and with the in-site generation process, incrustations and deposits are formed on the electrodes, and production of free chlorine is inherent to the process when chloride ions are present in the water or the electrolyte used.
Radiochemical Ozone Generation
High-energy irradiation of oxygen by radioactive rays can promote the formation of ozone. Even with the favorable thermodynamic yield of the process and the interesting use of waste fission isotopes, the cheminuclear ozone generation process has not yet become a significant application in water or waste water treatment due to its complicated process requirements.