| Because of its relatively short half-life, ozone is always generated on-site by an ozone generator. The two main principles of ozone generation are UV-light and corona-discharge. Ozone generation by corona-discharge is most common nowadays and has most advantages. Advantages of the corona-discharge method are greater sustainability of the unit, higher ozone production and higher cost affectivity. |
UV-light can be feasible where production of small amounts of ozone is desired (e.g. laboratories) [3,67]. This chapter will concentrate only on the first mentioned principle.
An ozone production unit with corona-discharge consists of the following parts: oxygen source, dust filters, gas dryers, ozone generators, contacting units and torch destruction .
In the ozone generator, the corona-discharge element is present, which provides a capacitive load. In here ozone is produced from oxygen as a direct result of electrical discharge. This corona-discharge ruptures the stable oxygen molecule and forms two oxygen radicals. These radicals can combine with oxygen molecules to form ozone. To control and maintain the electrical discharge, a di-electric is present, carried out in ceramic or glass. The excessive heat of the electrodes is often cooled by cooling water, or by air (figure 1) .
Figure 1: outline corona-discharge generator
For the production of ozone, ambient air can be used (supplied by a compressor) or pure oxygen (supplied by an oxygen generator, or sometimes by oxygen bottles). To condition this air, air dryers and dust filters are used.
To break down the remaining ozone after use, ozone destructors are applied. The mechanism of an ozone destructor can be based on different principles. Usually a catalyst is applied, which accelerates the decomposition of ozone into oxygen (e.g. magnesium oxide).
The generation of ozone is very energy-intensive, with some 90 % of the power supplied to the generator being utilized to produce light, sound and primary heat [1,3]. Important factors that influence ozone generation are: oxygen concentration inlet gas, humidity and purity of inlet gas, cooling water temperature and electrical parameters. To minimize the energy that is used at a high ozone yield, it is important that these factors are optimal.
Cooling water temperature
The generation of ozone is accompanied by heat formation. This makes it important to cool the generator. An ozone reaction is reversible and this increases when temperatures rise. As a result, more oxygen molecules are formed:
3O2 ⇌ 2O3
Figure 2 illustrates the relation between cooling water temperature and the yield of ozone generation. This figure shows that an increasing cooling water temperature results in a decreasing ozone production [1,5]. To limit the decomposition of ozone, the temperature in the discharge gap should not be higher than 25 °C. The general advise is that cooling water may increase 5 °C to 20 °C maximally. It is important that the temperature of the inlet air is not too high.
Figure 2: influence of water cooling on ozone generation efficiency
Humidity inlet air
Before the feed gas enters the ozone generator, air dryers should dry the air. Ambient air contains moisture, which reacts with ozone. This leads to a reduction of the ozone yield per kWh. An additional problem of high humidity is that undesired reactions occur in the corona unit. When increased amounts of water vapor are present, larger quantities of nitrogen oxides are formed when sparks discharge occurs. Nitrogen oxide can form nitric acid, which can cause corrosion.
Furthermore, hydroxy-radicals are formed that combine with oxygen radicals and with ozone. Al these reactions reduce the capacity of the ozone generator [3,5].
Figure 3 shows the influence of the humidity on the capacity of an ozone generator. The two descending lines illustrate the capacity of the generator: 'oxygen' for an oxygen-fed generator and 'air' for an air-fed generator. At a dew point of -10 °C, the capacity of the air-fed generator is only 60% of the total achievable capacity. For ozone generators that are oxygen-fed, this capacity is higher; about 85% .
Figure 3: influence humidity inlet air on efficiency of ozone production
To prevent these side-reactions, inlet air first passes a drying chamber before ozone is generated. For drying, an aluminum compound can be used, comparable with silica gel. In an ozone generator two or more drying chambers are used alternately. When a drying chamber is used for a certain period of time, humid air is led to the other drying chamber, while the first is regenerated.
Purity of gas (inlet)
The presence of organic impurities in gas feed must be avoided, including impurities arising from engine exhausts, leakages in cooling groups, or leakages in electrode cooling systems. The gas supply of the generator must be very clean. An example is given in figure 4, where the concentration of hydrocarbons is related to the ozone yield. This figure shows that at a hydrocarbon concentration of about 1%, the ozone generation nearly approaches zero .
Figure 4: influence of hydrocarbons on the generation yield of ozone
Produced amount of ozone versus oxygen concentration of inlet air
Ozone is produced from oxygen, so it can be produced from ambient air (21 % oxygen) or nearly pure oxygen (e.g. 95 %). Pure oxygen can be generated from ambient air by an oxygen generator. The ozone concentration an ozone generator delivers is dependant on the oxygen concentration (among other things). This is clarified by figure 7, where the oxygen concentration is outlines against the ozone concentration. The diverse lines demonstrate the ozone generators with different energy use. Summarising, one can claim that the ozone production increases by a factor 1,7 to 2,5 when pure oxygen is used, at constant electrical power [3,5].
Figure 7: influence of oxygen concentration on ozone production at different electrical current