CA2272204A1 - Ozonizer and gas sensor - Google Patents

Description

This invention relates to improvements in the production of ozone and to an ozonizer for said production.
Especially the invention relates to an improved sensor for ozone or other gases. The invention also relates to a method and apparatus to optimize energy efficiency of an ozonizer. Ozonizers including the features of the invention may, for example, be used for extending food shelf life in warehouses or for odour control.
The mechanism, theory, and electrical behaviour of ozonizers have been known for some time. The most efficient method of generating ozone is by using silent discharge in air or pure oxygen. Basically, the feed gas is moved between two electrodes which have high voltage (over 1000 V) across them. In order to prevent an arc across the electrodes, at least one dielectric material such as thin glass is placed between them. A silent discharge is formed to the "streamer discharge" stage, where free electrons are formed and them accelerated to a high enough energy to dissociate OZ into atoms. The O atoms may then react further with 02 to form ozone (03) . Some of the resulting 03 can be destroyed by free electrons, O atoms, or by products such as nitric oxide (NO) if an air feed gas is used. For sufficiently long residence times, a steady state mixing ratio of ozone is produced. If pure Oz is used as a feeding gas, it has been possible to reach an ozone mixing ratio of 5 - 7%. If dry air is used, 1% ozone mixing ratios have been possible.

- 2 -Energy efficiency depends on voltage, type of feed gas (oxygen or air), feed gas flow rate, pressure, and geometry of the electrodes and the gap containing the feed gas.
For any ozonizer geometry and feed gas flow rate, the ozone output can be optimized by controlling the peak voltage across the electrodes. If the voltage can be controlled, the other parameters in ozonizer design become less important. The gap between the electrodes must still be narrow enough to form the silent discharge of the given pressure of feed gas, and the volume of feed gas between the electrodes must be large enough to attain the necessary residence time. The dielectric material should be smooth and stay clean to avoid ozone destruction. A cool temperature helps to prevent unnecessary ozone destruction.
Ozonizer efficiency studies have been carried out on a device that used an automobile electronic ignition system to generate the high voltages. These devices have been designed for delivering a strong spark across a spark plug and have not been optimized for producing ozone. The simple technique tested was taken directly from an automotive ignition system. The ozonizer was intended for use aboard an aircraft where 28VDC is available. For commercial use, this system would require a large power supply. As with most automotive devices, the system is rugged but certainly not efficient.

- 3 -Other commercial ozonizers still use an ignition coil transformer from an automobile to generate the required high voltage, but use it as a pulse transformer. In these cases, a DC voltage is used (typically 160 VDC) and a transistor or a SCR is used to switch a fast 160 VDC pulse to the coil.
In this case, the power supply is very simple.
The simplest high voltage power source for the ozonizer is using a simple high voltage power transformer. These are commercially available up to 6000 VAC. Because this voltage is too low for an ozonizer with cylindrical geometry, flat plate ozonizers must be used. The amount of ozone production is limited because the line frequency is only 50 or 60 Hz, whereas 400 Hz would generate much more ozone.
In order to maximize the energy efficiency of the ozonizer, it would be advantageous to control the feed gas flow rate to be high enough to prevent a steady state to be achieved. Under these conditions, the production of ozone is dependent mostly on the peak voltage produced across the electrodes.
The present inventor has addressed this problem.
Solid state sensors for measuring ozone, carbon monoxide, NOx and other gases are know and are for example, manufactured by Capteur Sensors and Analysers Ltd. The Capteur LGL 52 ozone sensor is, for example, a very small solid state sensor that consists of a heated amorphous

- 4 -semiconductor that changes its electrical resistance whenever ozone is adsorbed onto its surface. In a typical application, the sensor is heated to 500°C and the sample air is blown across the sensor by using a fan. The response of the sensor is that the sensor resistance changes proportionally to the square root of the ozone concentration.
When using the measurement techniques recommended by Capteur, a warm up time of one or two days is required, and the zero signal may drift by several tens of ppbV during a day. It would be advantageous to provide a sensor in which the zero is stabilized so that the measurements can be at ambient concentrations as low as 5 ppbV (parts per billion by volume).
The present inventor has addressed this problem.
According to the invention there is provided a method of generating ozone comprising applying a high voltage AC
waveform between electrodes across a stream of gas including oxygen to produce a silent discharge between the electrodes, extending the time of discharge by applying said high voltage through an energy storage device so that the high voltage alternating current waveform maintaining an output voltage for ozone production for preferably 25% of cycle time, sensing the proportion of ozone generated in the stream of gas and controlling the flow rate of said stream in response to said proportion to provide a gas residence

- 5 -time between said electrodes less than that required to establish a steady state proportion of ozone in the gas.
The gas may be pure oxygen or air or other oxygen containing gas.
The energy storage device, which may be an inductor, causes a modification of a originally simple waveform, for example a square wave, to produce a more complex waveform having multiple peaks and shoulders per cycle due to the spread in discharge.
The frequency of the waveform may be adjustable and may range from 200 to 400 Hz. Conveniently the peak voltage at the output of a transformer may be around 14000 V (ptp).
An energy storage device may be inserted in the circuit to extend the time of discharge of the plasma to generate more ozone and less waste heat.
The sensing step may be carried out using a commercially available Capteur sensor at a constant temperature controlled by circuitry, for example, recommended by Capteur. To stabilize the zero setting of the sensor, AC voltage may be supplied. The AC voltage may be obtained from a DC source, for example, a solid state switch. Thus, instead of measuring the resistance only in one direction, the exciting current for an ohmmeter is passed first in one direction and then another.

- 6 -The circuit works because the semiconductor sensor is never biased by the exciting current because the exciting current always reverses to correct any effect. In the case where the current is allowed to flow only in one direction, the resistance of the sensor if corrupted by the exciting current. Capteur recommends that the duty cycle be held very low or that extremely small currents be used. In either case, the sensor is nevertheless corrupted by the current.
The sensor may thus be modified for greater accuracy.
The response of the sensor may be fed to the computer chip switching regulator and utilized directly to control the flow rate of the gas stream between the electrodes.
The modification of the sensor to stabilize its setting is itself new and is not confined to use with ozonizers.
For example, drifting problems occur with carbon monoxide detectors which may be modified on the same principles.
Supply of AC current may enable the settings to be stabilized for greater accuracy.
Thus the invention also includes a gas sensor comprising a DC power source, variable resistor means responsive to changes in concentration of said gas to change its resistance and switch means to convert direct current to alternating current, the switch means being connected between said DC power source and said variable resistor means. Indicia means may be provided to indicate when the gas concentration reaches a prechosen level, for example, an unacceptable level of carbon monoxide.
An embodiment of the invention will now be described with reference to the drawings in which:
Figure 1 is a sketch of an ozonizer according to the invention;
Figure 2 is a block diagram of an ozonizer according to the invention in cooperation with a sensor according to the invention;
Figures 3A and 3B are more detailed block diagrams of the sensor of Figure 2;
Figure 4 is a schematic plan of the electronics of Figure 3;
Figure 5 is a schematic plan showing control of the gas flow rate through the ozonizer;
Figure 6 is a schematic of the ozonizer high voltage driver;
Figure 7 is a graph showing one suitable waveform between the ozonizer electrodes; and Figure 8 shows how the waveform is modified and its effect on ozone production and waste heat.
Figure 1 shows one example of an ozonizer 10 which may be utilized. The ozonizer 10 comprises a pair of coaxial tubes 12 of borosilicate glass (Pyrex) sealed to each other at each end. Electrodes 14 are coated on the outside of the outer tube and the inside of the inner tube. The electrodes may be formed of a conductive nickel paint. A gas inlet 16 _ g _ to the space between the tubes is provided at one end and a gas outlet 18 at the other end.
Figure 2 shows a block diagram of both a sensor system and ozonizer control according to the invention.
Figure 3A shows how the sensor resistance is monitored in block form. Instead of measuring the resistance only in one direction, the exciting current for the ohmmeter (always required) is passed first in one direction and then another.
The direction is set by the forward and reverse signals to the CMOS switch to pines 11 and 10 respectively. An even simpler block diagram is shown in Figure 3B. In Figure 3A, the actual duty cycle for the excitation is 50%. In Figure 3B, the duty cycle is 100% since current always flows one direction or the other.
The actual circuit, together with the measurement and control current to the heater is shown in Figure 4. The exciting current is generated by U2, a current source, which has been set to deliver about 1.0 V across the sensor when it is subjected to approximately 200 ppbV of ozone. The voltmeter is the PIC 14000 micro computer. Pin 1 monitors the sensor voltage. The forward and reverse signals are also generated by the PIC in its computer program. The duty cycle for measurement is approximately 10%, only. The resistance of the heater is measured periodically and the program controls the duty cycle of the heater power to ensure that the temperature remains at 500°C. A display and a control for the program is present, as well as IZC
communication to the PIC in the ozonizer.
Figure 6 shows the high voltage driver circuit for the ozonizer tube. It is based on using a pulse transformer consisting of an automotive ignition coil. To a first approximation (neglecting inductor L10), the design is typical for a capacitative discharge ignition system.
Briefly, a capacitor (C10) is charges with a DC voltage, in this case 320 VDC by turning in the IGBT transistor Q20.
The Q10 is allowed to "fire", quickly dumping all of the energy of C10 into the primary circuit of the ignition coil.
The energy must go somewhere, and in the case of an ignition system, a very powerful spark is generated across the gap of a spark plug connected to the secondary of the ignition coil.
If now the sparkplug is replaced by an ozonizer electrode, the energy of the capacitor C10 is dumped onto the ozonizer. A large electric field forms between the electrodes of the ozonizer, and electrons are ripped from molecules of the gas between the electrodes, and the silent discharge is formed. Unfortunately, the plasma formed is very conductive, and the energy is expended before significant ozone can be formed, and the main product is waste heat.
An energy storage device may be inserted in the primary circuit of the ignition coil in order to extend the time of the discharge in the plasma in order to generate more ozone and less waste heat. The device is L10, an inductor. Now, when the capacitor C10 is discharged, the inductor must be charged with current. The formation of a high enough electric field in the ozonizer is delayed by this process.
However when the plasma forms, the energy is not lost, but flows at a steady rate into the plasma, greatly extending the time of ozone formation. Tests demonstrate that a factor of two or more ozone is produced under these circumstances.
The voltage waveform (see Figure 7), as measured at the plus terminal of transformer T10, shows considerable structure during a complete cycle. For the purposes of showing the effectiveness of adding L10 to the circuit, a current waveform would more clearly show the energy flowing into the ozone producing plasma because inductors store energy in the form of current. In fact, because inductors store energy in this way, the voltage waveform is severely modified by the action of the inductor. For example, if the flow of current in the inductor were starting to decrease because the ozone forming plasma was collapsing, the voltage at the + terminal of T10 would rise to whatever level was necessary to reform the plasma. The inductor must release its stored energy and it will do so by the laws of physics (Faraday's Law). Conversely at the start of a pulse, the current is zero and the inductor has no energy stored. The large voltage spike expected from dumping energy from the capacitor C10 is slowed by~the buildup of current in the inductor.
The net effect of the inductor's presence on ozone formation can be shown schematically on the voltage waveform (see Figure 8). If the inductor is not present, the waveform becomes a narrow square pulse with an amplitude of 320V. The inductor causes the pulse to be lengthened in time but reduced in pulse height. Ozone is produced for the entire width of the indicator modified pulse, but only for the first part of the square pulse. The rest of the energy is used to create waste heat.
The circuit is efficient in a second way. Because the LOAD transistor IGBT Q20 also turns on to load C10 as fast as Q10 discharges C10, (the FIRE sequence) a second pulse is delivered to the ozonizer in the opposite polarity. Ozone is made in this half cycle as well. In fact because the ozonizer is driven in an alternating current fashion, any impurities that would have formed on the walls by electrolysis are removed. The ozonizer remains clean.
Because modern switching power supplies can handle both 120 VAC and 240 VAC, the above circuit was designed to be compatible with these. In the case of the 240 VAC, the line voltage is rectified directly and the "neutral" leg forms the low voltage (LOW-IN) at 0 VDC. In the case of 120 VAC, the voltage is doubled by also rectifying the low-going part of the input wave, and adding the two voltages by using capacitors. In this case, the LOW-IN is actually minus 160 VDC. The HIGH-IN voltage is plus 160V.
In order to control both voltages by using PIC computer (5 V TTL logic) an optical isolator is used. The circuit is save and is compatible with GFI circuit breakers.
The power supplied to the ozonizer tube may be useful for controlling the stability of the ozonizer. A novel way to get this signal is to use the current flowing into the "grounded" electrode of the ozonizer (see Figure 5). The amount of ozone produced is proportional to the current that flows into the plasma. Therefore, measuring the current is all that needs to be done. Because the ozonizer is driven by an AC waveform, the current returning to the ground electrode is also AC. The power can be derived by rectifying this signal. As shown in Figure 5, the rectifying diode is D30, and the current is determined by the voltage drop across R30. The PIC pin 1 is used to measure the voltage. The PIC generates, in turn the duty cycles for the LOAD and the FIRE sequences in a fuzzy logic control loop.
A front panel LED to indicate ozonizer production is trivial A LED (D31) is used on the opposite phase of D30.
No current limiting resistor is necessary.
The ozonizer is controlled between the sensor and the ozonizer by computer software.

Claims