| Q1 |
What are advantages and disadvantages of oxygen analyzers? |
| A1 |
The measurement methods of the oxygen analyzers currently available in the market can be classified into the following categories.
Zirconia type measurement system :
Concentration cell system (YOKOGAWA's model name :
ZR22G/ZR22S/ZR402G, ZR202G, ZR202S, OX400)
Limiting Current type
(Yokogawa Electric's model name : OX100, OX102）
Paramagnetic type :
Magnetic flow ratio method (YOKOGAWA's model name : MG8G, MG8E）
Magnetic wind method
Magnetic force method dumbbell type
Electrochemical type :
Galvanic cell type (YOKOGAWA's model name : OX51, OX61）
Polarographic type
Optical type : TDLS System
(YOKOGAWA's model name : TDLS200, TDLS200SJ)
Since each of the measurement methods has its advantages and disadvantages, it is important to select an oxygen analyzer of an appropriate method for your application and usage.
The following describes an overview of each of the measurement methods and their advantages and disadvantages.
(1) Zirconia type measurement system : Concentration cell system
A solid electrolyte like zirconia exhibits conductivity of oxygen ions at high temperature.
As shown in the figure below, when porous platinum electrodes are attached to both sides of the zirconia element to be heated up and gases of different partial oxygen concentrations are brought into contact with the respective surfaces of the zirconia, the device acts as an oxygen concentration cell. This phenomenon causes an electromotive force to be generated between both electrodes according to Nernst's equation.
| Advantages : |
Can be directly installed in a combustion process such as a boiler's flue and requires no sampling system, and response is faster. |
| Disadvantages : |
If the sample gas contains a flammable gas, a measurement error occurs (combustion exhaust gas causes almost no problem because it is completely burned). |
(2) Zirconia type measurement system : Limiting Current type
As shown in the figure below, if the flow of oxygen into the cathode of a zirconia element heated to high temperature is limited, there appears a region where the current becomes constant even when the applied voltage is increased.
This limited current is proportional to the oxygen concentration.
| Advantages : |
•
• |
Capable of measuring trace oxygen concentration.
Calibration is required only on the span side (air).
|
| Disadvantages : |
• |
If the sample gas contains a flammable gas, a measurement error occurs. |
| • |
The presence of dust causes clogging of the gas diffusion holes on the cathode side; a filter must be installed in a preceding stage. |
(3) Magnetic type measurement system : Paramagnetic system
This is one of the methods utilizing the paramagnetic property of oxygen.
When a sample gas contains oxygen, the oxygen is drawn into the magnetic field, thereby decreasing the flow rate of auxiliary gas in stream B. The difference in flow rates of the two streams, A and B, which is caused by the effect of flow restriction in stream B, is proportional to the oxygen concentration of the sample gas.
The flow rates are determined by the thermistors and converted into electrical signals, the difference of which is computed as an oxygen signal.
| Advantages : |
• |
Capable of measuring flammable gas mixtures that cannot be measured by a zirconia oxygen analyzer.
|
| • |
Because there is no sensor in the detecting section in contact with the sample gas, the paramagnetic system can also measure corrosive gases. |
| • |
Among the magnetic types, the paramagnetic system offers a faster response time than other systems. |
| • |
Among the magnetic types, the paramagnetic system is more resistant to vibration or shock than other systems. |
| Disadvantages : |
|
Requires a sampling unit corresponding to the sample gas properties or applications. |
(4) Optical type : Tunable Diode Laser measurement system
Tunable Diode Laser (or TDL) measurements are based on absorption spectroscopy. The TruePeak Analyzer is a TDL system and operates by measuring the amount of laser light that is absorbed (lost) as it travels through the gas being measured. In the simplest form a TDL analyzer consists of a laser that produces infrared light, optical lenses to focus the laser light
through the gas to be measured and then on to a detector, the detector, and electronics that control the laser and translate the detector signal into a signal representing the gas concentration.
Gas molecules absorb light at specific colors, called absorption lines. This absorption follows Beers law.
TDL Analyzers are effectively infrared analyzers which obey the Beer-Lambert Law.
I = Io ·e-E ·G ·L
where I is the radiation intensity after absorption,
I0 is the initial radiation intensity,
E is the extinction coefficient,
G is the gas concentration,
and L is the path length of the measurement area.
| Advantages : |
• |
Capable of measuring a number of near infrared absorbing gases in difficult process applications. |
| • |
Capability of measuring at very high temperature, high pressures and under difficult conditions (corrosive, aggressive, high particulate service). |
| • |
Most applications are measured in-situ, reducing installation and maintenance costs. |
| Disadvantages : |
|
The installation of the flange is necessary for both sides of the process. |
(5) Electrochemical type : Galvanic cell type
If oxygen is dissolved via the diaphragm in an electrolytic solution in which an anode (base metal) and cathode (noble metal) are adjacent to each other, a current proportional to the quantity of dissolved oxygen is generated. The amount of oxygen passing through the diaphragm is proportional to the partial oxygen pressure of the sample gas, therefore, the oxygen concentration can be determined by measuring the current.
| Advantages : |
• |
The detecting system can be made compact; this measurement system is available in portable or transportable form. |
| • |
Relatively inexpensive in comparison with oxygen analyzers of other measurement systems. |
| Disadvantages : |
|
The cell life is limited. As it is a kind of oxygen cell, the galvanic cell deteriorates even if not used.
In general, it should be replaced approximately every year. |
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| Q2 |
What are application examples for which the oxygen analyzer is used? |
| A2 |
Oxygen concentrations are measured for a variety of purposes, such as energy conservation, air pollution prevention, safety management, and quality control.
The following lists major application examples by measurement method.
| Concentration cell system Zirconia Oxygen Analyzer |
| |
Package boiler combustion control, gas fired
Combustion control of power generation boilers, gas fired
Combustion control of pulverized coal boilers
Combustion control of hot stoves for steelmaking
Heating and combustion exhaust gas control of coke ovens for steelmaking
Low-oxygen concentration control of reheating and soaking furnaces for steelmaking
Air leakage detection of sintering furnaces for steelmaking
Lime kiln combustion control
Cement kiln combustion control
Combustion control of heating furnaces for oil refinery & petrochemical industry
Naphtha cracking furnaces
Incinerator combustion control
Oxygen concentration measurement in oxygen enrichment facilities
Oxygen concentration measurement of exhaust gas from activated sludge process equipment |
| Limiting Current type Zirconia Oxygen Analyzer |
| |
Oxygen concentration control of N2 reflow furnaces
Atmospheric control of semiconductor manufacturing equipment
N2 and air purity control for air separators
Oxygen deficiency prevention
Oxygen concentration control of glove boxes for research and development and parts machining
Oxygen concentration control of experimental clean rooms for environment, fermentation, biochemistry, etc.
Continuous measurement of flow gases during food packaging |
| Paramagnetic system Oxygen Analyzer |
| |
Low-oxygen concentration control of CDQ plants for steelmaking
Oxygen concentration control of gas containing a flammable gas
Safety control (explosion prevention) at various plants
Measurement of trace oxygen concentration in various manufacturing processes
City gas quality control |
| Galvanic cell system Oxygen Analyzer |
| |
Oxygen deficiency prevention
Package boiler combustion control, gas fired
|
| Tunable Diode Laser Analyzer(TDLS) |
| |
Combustion control of heating furnaces
Incinerator combustion control
Process control
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| Q3 |
What are the differences between the dry gas base and wet gas base? |
| A3 |
Exhaust gas emitted by combustion of fuel contains water vapor generated by the burning of hydrogen in the fuel.
When directly measuring gases in a flue with a zirconia oxygen analyzer, exhaust gas containing moisture is measured. Values measured under these conditions are "values based on a wet gas."
In contrast, for oxygen analyzers that sample and measure exhaust gases, the sample gas temperature is lowered to normal temperature in the process of being introduced into the analyzer and moisture that would condense into water is also removed from the sample gas.
Values measured after removing the moisture in this way are called "values based on a dry gas."
While a value based on a dry gas regards the total gas composition available after moisture removal as 100%, a value based on a wet gas assumes that the gas composition including water vapor is 100%. Therefore, the results differ between both measurement types even when the oxygen concentration of the same exhaust gas is measured (see the bellow Figure).
This difference between the measurement types is largest when gas containing much hydrogen is burned (since much water vapor is generated). Special care is required when the output (based on a wet gas) of the zirconia oxygen analyzer is calculated in combination with other analytical values measured based on a dry gas (for example, CO or NOx concentration measured using an infrared gas analyzer).
Note:
YOKOGAWA model ZR402G Zirconia Oxygen Analyzer incorporates a "dry gas-based" calculation function; it can output dry gas-based values.
 Values based on a wet gas or dry gas → Return to questions
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| Q4 |
The relationship between the air ratio and heat efficiency |
| A4 |
"Air ratio" means the ratio of the amount of air theoretically required to completely burn all the fuel to the amount of air actually supplied for combustion. The air ratio, m, is calculated by:
The air ratio can thus be obtained by measuring the oxygen concentration in exhaust gas.
Attached Figure 1 Relationship between Air Ratio and Thermal Efficiency
Attached Figure 1 shows the relationship between the air ratio and thermal efficiency.
In combustion control, if there is insufficient air for combustion (region where the air ratio is small), the fuel is burned incompletely, giving off black smoke. This causes an energy loss and pollution. On the other hand, if excess air for combustion is supplied (region where the air ratio is large), air not used for combustion is overheated and emitted from the stack, causing a heat loss. This also increases the emissions of NOx and SO2, which cause air pollution and global warming.
Thus, there is a narrow range of air ratio for optimum combustion. The ideal air ratio differs with fuel type, but is typically 1.02 to 1.10. In general, however, fuel is burned slightly on the excess side to prevent black smoke emissions.
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| Q5 |
Explosion protection (flame-proof) |
| A5 |
The following explosionproof oxygen analyzers are available.
ZR22S/ZR202S Explosionproof Direct In-situ Zirconia Oxygen Analyzers
Both the ZR22S separate type probe and the ZR202S integrated type Zirconia oxygen analyzers do not need a sampling device, and allow direct installation of the probe in the wall of a flue or furnace to measure the concentration of oxygen in the stack gas.
| Explosionproof approval : |
ATEX, FM, CSA, IECEx |
| Sample Gas Temperature : |
0 to 700℃ |
| Sample Gas Pressure : |
-5 to +5 kPa |
TDLS200 Tunable Diode Laser Analyzer
With the capability of measuring at very high temperature, high pressures and under difficult conditions (corrosive, aggressive, high particulate service), the TDLS200 is one of the most robust process analyzers available. Most applications are measured in-situ, reducing installation and maintenance costs.
| Explosionproof approval : |
ATEX Group II for zone 1 (Cat 2G) or 2 (Cat 3G) with purge systems |
| Sample Gas Temperature : |
up to 1500℃ |
| Sample Gas Pressure : |
less than 1MPa |
MG8E Paramagnetic Oxygen Analyzer
| Flameproof standard : |
TIIS ExdⅡBT4 |
| Measurement Gas : |
Class A and B hazardous gases or vapor
Gas or vapor with ignition temperature of 135℃ or greater
Hydrogen concentration must be below 4%.
Not applicable for gases containing acetylene, carbon disulfide and ethyl nitrate. |
| Used condition of Flameproof : |
| |
(a) |
Before opening the cover, remove power and make sure of non-hazardous atmospheres. |
| (b) |
When the ambient temperature of the converter exceeds 50℃ , use wire resistant to 70℃ or greater for external wiring. |
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| Q6 |
Which oxygen analyzers bear the CE mark? |
| A6 |
The following oxygen analyzers comply with the CE mark.
| • ZR402G |
Separate Type Zirconia Oxygen Analyzer |
| • ZR202G/ZR202S |
Integrated Type Zirconia Oxygen Analyzer |
| • AV550G |
Zirconia Oxygen Averaging Converter |
| • OX400 |
Low Concentration Zirconia Oxygen Analyzer |
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