At present, the widely used gas oxygen content analyzers are mainly divided into two categories: one is made by electrochemical method, such as zirconia analyzer, etc.; the other is made by physical method, such as electromagnetic type, Magnetic mechanical, etc. This article mainly introduces the principle and application of zirconia analyzer.
1 Fuji Zirconia oxygen analyzer
1.1 Principle
1.2 Because zirconia itself is a solid electrolyte, it has the characteristics of conducting oxygen ions at high temperature. Porous platinum electrodes are coated on both sides of it. When the concentrations of the gases on both sides are different, the following reactions occur:
High side of oxygen concentration: O2+4e→2O2-
Low side of oxygen concentration: 2O2--4e→O2
In this way, a concentration battery with a zirconia tube as the electrolyte is formed, and the potential between the electrodes obeys the Nernst equation:
E=(RT/Nf) Inco/Px (1)
When the total gas pressure on both sides is the same, the above formula can be written as:
E=(RT/nF) Inco/cx (2)
In the formula: R is the gas constant; T is the absolute temperature, K; n is the electronic parameter (4) participating in the reaction; F is the Faraday constant; Po, co are the partial pressure and concentration of oxygen on the participating side, respectively; To measure side oxygen partial pressure and concentration.
When the oxygen concentration co of the participating side is a known constant, the potential E and cx have a single-valued function relationship. Therefore, the principle of the zirconia analyzer is to use zirconia as a solid electrolyte to form a concentration difference battery. The generated concentration difference potential is related to the oxygen concentration on both sides. When the oxygen concentration on one side is fixed, by measuring the output potential E, the other can be measured. oxygen concentration on one side. It can be seen from the formula (2) that the output potential is also proportional to the absolute temperature T, so the temperature must be constant during the measurement process, or temperature compensation measures should be taken.
The sensor of the flow-type zirconia analyzer is generally made into a tubular structure. It is directly inserted into the high temperature gas to be measured, and air is used as a reference gas to pass into the outside of the zirconia tube, and the gas on the north side enters the tube after removing mechanical impurities through a filter. The measured gas sample and air can be pumped to make their flow rate constant and the total pressure of the two-phase flow to be basically the same. A thermocouple for measuring the working temperature of the zirconia tube is installed outside the tube, and the output thermoelectric potential signal is sent to the thermostat to realize constant temperature control. The thermoelectric potential of the measured working temperature can also be sent to the trigger circuit without the need for a constant temperature heating furnace to perform temperature compensation for the output concentration difference potential. Therefore, the in-line zirconia analyzer is characterized by rapid response, and the response time after adding a filter is only about 3s.
1.2 Application
The measurement range of zirconia oxygen analyzer is generally 0~10% O2 or ppm level, and the error is about ±5. It is used for flue gas analysis in the combustion process and gas oxygen analysis in high temperature furnaces, and is suitable for monitoring and control in the combustion process; especially for the determination of oxygen content in boiler flue gas.
Taking Fuji zirconia analyzer as an example, it mainly consists of two parts: detector (sensor) and converter (transmitter). Secondly, there is a calibration box used with these two parts, which is to complete the calibration of the zero point and span of the analyzer.
In the actual application selection process, pay attention to:
(1) If there is flammable gas (CO, H2, etc.) in the measurement gas, the measurement may be inaccurate due to the possibility of combustion at the sensor.
(2) If there is corrosive gas (such as alkali metal, silicon vapor, Pb, etc.) in the measurement, it may corrode the pick and shorten the service life of the sensor.
(3) When the gas temperature is high (at 300°C), the detector flange should be separated from the furnace wall to ensure that the surface temperature of the detector flange is lower than the specified value of 125°C. The flow of the gas being measured into the detector is reduced. When the measured gas temperature is higher than 600°C, a heat-dissipating guide tube is required.
(4) When there is a lot of dust in the gas to be tested, the installation of the guide pipe should have an inclination angle, so that the flow direction of the flue gas is from bottom to top; reducer flow. To sum up, when choosing a zirconia analyzer, two points should be paid attention to, one is the composition of the sample gas; the other is the temperature of the sample gas.
