The zirconia analyzer is suitable for Oxygen measurements from ppm up to % levels in a flue gas or other inert gas (free from combustibles).

The zirconia cell is an electrochemical galvanic cell employing a high temperature ceramic sensor containing stabilized zirconium oxide.

General Description:

The zirconia cell is a high temperature ceramic sensor. It is an electrochemical galvanic cell comprising of two electrically conducting, chemically inert. A zirconia cell, is a kind of solid electrolyte, it is coated on the outside (measuring electrode) and inside (reference electrode) with a thin porous layer of metal, generally platinum. At temperatures above 600°C, zirconium oxide becomes an Oxygen ion conductor, which generates an EMF between the platinum electrodes. This EMF depends on the difference between the partial pressure of oxygen in the sample gas and the Oxygen in the reference gas (using ambient air with 20.94% Oxygen).

ADVANTAGES OF THE ZIRCONIUM OXIDE PROBE

Risultato immagini per icona vantaggi
Zirconium oxygen cells have numerous advantages over other existing methods of measuring oxygen concentration:
  •  Resistant to corrosion by acid & aggressive substances, to abrasion and to the alternation of oxidant and reducing conditions.
  • The cell can be placed directly in the exhaust gas flow, which increases the sensitivity of the gas analyzer and reduces response times.
  • An oxygen cell can be used in the analysis of wet gases, since it operates at a high temperature that exceeds the dew point temperature of the combustion gas.
  • The preliminary preparation of a gas sample for analysis (cleaning, cooling and drying) is not necessary, thus reducing the acquisition and maintenance costs of the equipment.
  • The zirconia cell is not affected by vibrations.
  • The cell output signal increases with the decrease of the oxygen concentration in the analyzed gas.
  • -The shelf life of a backup cell is practically unlimited.

We explain the physical background, operating principle and construction of non-consumptive zirconium dioxide oxygen sensors

Nernst voltage:
Risultato immagini per nernst
Two different ion concentrations on either side of an electrolyte generate an electrical potential known as the Nernst voltage. The larger the difference in the ion concentration ratio, the greater the voltage. Zirconium dioxide At high temperatures >600°C zirconium dioxide (ZrO2) exhibits two mechanisms:
  • ZrO2 partly dissociates to produce oxygen ions which can be transported through the material when a voltage is applied.
  • ZrO2 behaves like a solid electrolyte for oxygen. If two different oxygen pressures exist on either side of an ZrO2 element a voltage (Nernst voltage) can be measured across that element.
Where: E is the potential difference (volts) R is the gas constant (8.314 J mol-1 K-1) T is the absolute temperature (K) F is the Faraday constant (96484 coulomb mol-1) P1 & P2 are the partial pressures of the oxygen on either side of the zirconia tube The Nernst equation can therefore be reduced to:
The sensor output voltage in mV is amplified and processed by the analyzer electronics.

If the cell communicates with the surrounding air on both sides, the sensor output signal is 0 mV.

With an increase in the concentration of combustible components in the analyzed gases, the oxygen concentration decreases and the output signal of the zirconium cell increases sharply. The output signal of the zirconium cell changes when oxygen is absent in the combustion gas. Indeed, the cell becomes a sensor of purely combustible components. This property of the zirconium cell is extremely useful in controlling combustion processes, since it allows you to measure excess air or excess fuel.
The image shows the trend of the signal produced by the sensor with changing Oxygen concentrations.
As a rule, in the combustion gases of ovens and boilers, due to incomplete combustion of fuel and air, oxygen and residual combustible components are simultaneously present. In this case, due to the high temperature, the residual combustible components are oxidized by the oxygen contained in the combustion gases on the surface of the zirconium cell. Zirconia-based analyzers measure net oxygen, i.e. the concentration of oxygen remaining after the combustion of the fuel in the oven and the oxidation of the combustible components on the surface of the heated cell. Other analyzers measure gross oxygen, only the concentration of oxygen remaining after burning in the oven. Typically, the difference between the net and gross values ​​is small, the concentration of combustible components is in the ppm range and oxygen is in the percentage range. However, situations are sometimes possible in which this difference becomes significant. The differences may also arise from the fact that the zirconium oxide cell measures oxygen using a wet base, when the analyzed waste gas contains water vapor. All other measurement methods require cooling and drying of the sample and therefore operate on a dry basis. The difference between dry and wet measurements can be up to 0.5% by volume. O2, since the concentration of water vapor in the combustion gas can be high. Both measurement methods (wet or dry) are standard.

Principle of Operation

The figure below shows a schematic of the zirconia cell produced by Adev srl and also used in OxyPink.
Molecular oxygen is ionized at the porous platinum electrodes.

PtO → Pt + ½ O2

½ O2 + 2 e- → O2

The platinum electrodes on each side of the cell provide a catalytic surface for the change in oxygen molecules, O2, to oxygen ions, and oxygen ions to oxygen molecules. Oxygen molecules on the high concentration reference gas side of the cell gain electrons to become ions which enter the electrolyte. Simultaneously, at the other electrode, oxygen ions lose electrons and are released from the surface of the electrode as oxygen molecules. The oxygen content of these gases, and therefore the oxygen partial pressures, is different. Therefore, the rate at which oxygen ions are produced and enter the zirconium oxide electrolyte at each electrode differs. As the zirconium oxide permits mobility of oxygen ions, the number of ions moving in each direction across the electrolyte will depend on the rate at which oxygen is ionised and enters the electrolyte at each electrode. The mechanism of this ion transfer is complex, but it is known to involve vacancies in the zirconia oxide lattice by doping with yttrium oxide. The result of migration of oxygen ions across the electrolyte is a net flow of ions in one direction depending upon the partial pressures of oxygen at the two electrodes. For example in the Nernst equation:
  • If P1>P2   ion flow will be from P1 to P2   i.e. a positive E.M.F.
  • If P1<P2   ion flow will be from P2 to P1   i.e. a negative E.M.F.
  • If P1=P2   there will be no net ion flow     i.e. a zero E.M.F.