Corona Discharge Technique
Corona Discharge Technique
A corona discharge is a process by which a current flows from an electrode with a high potential into a neutral fluid, usually air, by ionizing that fluid so as to create a region of plasma around the electrode. The ions generated eventually pass the charge to nearby areas of lower potential, or recombine to form neutral gas molecules.
When the potential gradient (electric field) is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive. If a charged object has a sharp point, the electric field strength around that point will be much higher than elsewhere. Air near the electrode can become ionized (partially conductive), while regions more distant do not. When the air near the point becomes conductive, it has the effect of increasing the apparent size of the conductor. Since the new conductive region is less sharp, the ionization may not extend past this local region. Outside this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized.
Corona discharge only forms when the electric field (potential gradient) at the surface of the conductor exceeds a critical value, the dielectric strength or disruptive potential gradient of the fluid. In air at atmospheric pressure, it is roughly 30 kilovolts per centimeter[1] but decreases with pressure, so Corona is more of a problem at high altitudes.[4]
Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small diameter wires. The high curvature causes a high potential gradient at these locations so that the air breaks down and forms plasma there first. On sharp points in air corona can start at potentials of 2 – 6 kV.[2] In order to suppress corona formation, terminals on high voltage equipment are frequently designed with smooth large diameter rounded shapes like balls or toruses, and corona rings are often added to insulators of high voltage transmission lines.
Mechanism
Corona discharge results when the electric field is strong enough to create a chain reaction: electrons in the air collide with atoms hard enough to ionize them, creating more free electrons which ionize more atoms. The diagrams below illustrate at a microscopic scale the process which creates a corona in the air next to a pointed electrode carrying a high negative voltage with respect to ground. The process is:
- A neutral atom or molecule, in a region of the strong electric field (such as the high potential gradient near the curved electrode), is ionized by a natural environmental event (for example, being struck by an ultraviolet photon or cosmic ray particle), to create a positive ion and a free electron.
The electric field accelerates these oppositely charged particles in opposite directions, separating them, preventing their recombination, and imparting kinetic energy to each of them. The electron has a much higher charge/mass ratio and so is accelerated to a higher velocity than the positive ion. It gains enough energy from the field that when it strikes another atom it ionizes it, knocking out another electron, and creating another positive ion. These electrons are accelerated and collide with other atoms, creating further electron/positive-ion pairs, and these electrons collide with more atoms, in a chain reaction process called an electron avalanche. Both positive and negative coronas rely on electron avalanches. In a positive corona, all the electrons are attracted inward toward the nearby positive electrode and the ions are repelled outwards. In a negative corona, the ions are attracted inward and the electrons are repelled outwards.
The glow of the corona is caused by electrons recombining with positive ions to form neutral atoms. When the electron falls back to its original energy level, it releases a photon of light. The photons serve to ionize other atoms, maintaining the creation of electron avalanches.
- At a certain distance from the electrode, the electric field becomes low enough that it no longer imparts enough energy to the electrons to ionize atoms when they collide. This is the outer edge of the corona. Outside this, the ions move through the air without creating new ions. The outward moving ions are attracted to the opposite electrode and eventually reach it and combine with electrons from the electrode to become neutral atoms again, completing the circuit.
Thermodynamically, a corona is a very nonequilibrium process, creating a non-thermal plasma. The avalanche mechanism does not release enough energy to heat the gas in the corona region generally and ionize it, as occurs in an electric arc or spark. Only a small number of gas molecules take part in the electron avalanches and are ionized, having energies close to the ionization energy of 1–3 ev, the rest of the surrounding gas is close to ambient temperature.
Ozone production from Corona Discharge
How does a Corona Discharge Ozone Generator work?
The heart of every ozone system is the ozone generator. Ozone (O3) is created from Oxygen (O2) in nature and in ozone generators for commercial or industrial applications, however Ozone (O3) quickly reverts back to molecular Oxygen (O2). Ozone cannot be stored due to a short half-life and must be produced on-site and on-demand. Therefore, the ozone generator is the most important component of any successful ozone system.
Industrial and commercial ozone applications use Corona Discharge ozone generators almost exclusively. There is an almost infinite number of variations to the fundamental corona discharge principle, and we will layout many of those variations in this article. However, the fundamentals of a diffused electrical discharge through a dielectric material to create a corona discharge to generate ozone will all be shared.
Fundamentals of Corona Discharge:
Ozone is produced from electrical discharge, commonly referred to as a spark. Great deals of ozone are produced from lightening during a thunderstorm. This is one of the reasons you smell the “fresh” smell after a thunderstorm.
Any electrical discharge, or spark will create ozone. The spark will split the oxygen molecule (O2) found in ambient air into elemental oxygen (O). These Oxygen atoms will quickly bind to another oxygen molecule (O2) to form ozone (O3).
The electrical energy used in ozone generation splits the oxygen molecule. The theoretical energy required to split the oxygen molecule is described below:
- 0.82 kWh of electrical power for every 1 kg of ozone generated
- 0.372 kWh of electrical power for every 1 lb of ozone generated
In actual ozone production, the energy required to produce ozone will be 10 – 20 times the mathematical figures shown due to the ozone generation inefficiencies.
In a corona discharge ozone generator, the electrical discharge will take place in an air gap within the corona cell designed specifically to split the oxygen molecule and produce ozone. In this air gap a dielectric is used to distribute the electron flow evenly across this gap to spread the electron flow to as great a volume of oxygen as possible.
Dielectric used to create Corona:
A single spark from an anode to cathode will find a few oxygen molecules in-between and will produce some ozone. However, if this spark is spread out over a greater area, more oxygen molecules will be contacted. This is the reason for the dielectric barrier used in an ozone generator to create a corona. Using a dielectric the spark is spread over a greater area and creates a true corona.
High Voltage Transformer:
To push the electrical discharge through the dielectric material a higher voltage is required. Therefore, an ozone generator will implement some type of transformer to increase the voltage from line voltage up to 600 – 20,000 volts depending upon the dielectric, and air gap between the anode and cathode.
Using this high voltage corona discharge as oxygen molecules are passed through the gap between the dielectric and the anode or cathode ozone will be produced. These are the fundamentals of corona discharge. Below we will examine the common components used in these ozone generators and the benefits of each.
COMMERCIAL OZONE PRODUCTION FROM ELECTRICAL DISCHARGE
The most common method of producing ozone commercially and industrially is electrical discharge, or corona discharge. A corona discharge is simply a diffused spark through a dielectric to spread out that electrical discharge to a large area for maximum efficiency.
There are many types and styles of corona discharge ozone generators. These go by many names, but are fundamentally the same, using these components:
Corona cell using a dielectric Dielectric material may be glass, ceramic, or quartz
Dielectric may be conical, or flat plate
High voltage transformer to increase voltage of the electrical discharge
Power supply to regulate power to transformer 60Hz machines will only regulate the voltage to the transformer
High frequency machines (greater than 60 Hz) will regulate frequency and/or voltage to transformer
ADVANTAGES OF CORONA DISCHARGE OZONE GENERATOR
Scalable and can create very large amounts of ozone
Creates ozone at medium to high ozone concentrations (up to 30% by weight)
Cost effective for long term operation
Low maintenance
DISADVANTAGES OF CORONA DISCHARGE OZONE GENERATOR
High cost for initial capital investment
Creates excess heat that must be removed for efficient operation
Requires very clean, dry air/oxygen feed-gas for reliable operation
OZONE GENERATION IN NATURE
