Common Failure Analysis and Prevention Measures of Zirconia Oxygen Analyzer

　　To ensure that the boiler reaches the combustion condition, the power plant introduces the oxygen signal into the combustion automatic control system as a correction signal to control the air volume. This requires that the measurement of the oxygen content in the boiler flue gas must be accurate and reliable. However, the zirconia oxygen analyzer, which works in a high-temperature, severely flue gas-eroded environment for a long time, is prone to failure, so we must strengthen the operation and maintenance of the zirconia oxygen analyzer.

　　The coal consumption of the unit is directly related to the quality of boiler combustion. When the boiler is in a normal combustion state, it has a certain excess air coefficient, and the excess air coefficient is related to the oxygen content in the flue gas. Therefore, the excess air coefficient can be understood by monitoring the oxygen content in the flue gas to judge whether the combustion is in a normal state. Even the oxygen content signal is introduced into the combustion automatic control system as a correction signal to control the air supply to ensure the economic combustion of the boiler. Therefore, measuring the oxygen content of boiler flue gas is of great significance to thermal power plants, which requires that the measurement of oxygen content in boiler flue gas must be accurate and reliable.

　　The basic principle of the zirconia oxygen analyzer is: using zirconia as a solid electrolyte, when the oxygen concentration on both sides of the high-temperature electrolyte is different, a concentration difference battery is formed. The potential generated by the concentration difference battery is related to the oxygen concentration on both sides. If the oxygen concentration on one side is fixed, the oxygen content on the other side can be measured by measuring the output potential. At a high temperature of 600～1200℃, the zirconia material after high-temperature calcination has good conductivity for oxygen ions. When the oxygen concentrations on both sides of the zirconia tube are unequal, oxygen molecules on the side with high concentration combine with two electrons on the zirconia tube surface electrode on that side to form oxygen ions, and then move to the side with low oxygen concentration through the oxygen ion vacancies in the zirconia material lattice. When it reaches the low-concentration side, it releases two electrons on the electrode to form oxygen molecules and release them. As a result, charges accumulate on the electrodes, and a potential is generated between the two electrodes. This potential hinders the further progress of this migration until a dynamic equilibrium state is reached. This forms a concentration difference battery, and the potential generated by it, which is related to the difference in oxygen concentration on both sides, is called the concentration difference potential.

　　On the air-side (reference side) electrode: O2+4e→2O-2

　　On the low-oxygen side (measured side) electrode: 2O-2→O2+4e

　　The potential E generated at both ends of the battery conforms to the Nernst equation:

　　In the formula: n—the number of electrons transported during the reaction, for oxygen n=4;

　　T-thermodynamic temperature (K)

　　F-Faraday constant, F=96500C/mol

　　R-gas constant, R=8.315J/(mol.K)

　　PX-oxygen partial pressure of the gas to be measured

　　PA-oxygen partial pressure of the reference gas

　　In this way, if the zirconia tube is heated to a stable temperature greater than 600℃, and the gas to be measured and the reference gas with the same total pressure flow through both sides of the zirconia, the generated potential has a fixed relationship with the working temperature of the zirconia tube and the oxygen concentration on both sides. If the reference gas concentration is known, the oxygen concentration of the gas to be measured can be calculated according to the oxygen potential on both sides of the zirconia tube and the working temperature of the zirconia tube. When analyzing the oxygen content of the furnace flue gas, air is usually used as the reference gas, that is, the percentage of the reference gas oxygen concentration is 20.8%, then the above formula can be changed to: If the working temperature T is constant, the oxygen concentration difference potential is inversely proportional to the logarithm of the oxygen content in the gas to be measured. In order to correctly measure the oxygen content in the flue gas, the following points must be noted when using the zirconia oxygen analyzer:

　　①To ensure that the output is not affected by the temperature, the zirconia tube should work at a constant temperature or add temperature compensation measures in the instrument circuit.

　　②During use, the pressure of the gas to be measured and the reference gas should be kept equal. Only in this way can the ratio of the oxygen partial pressure in the two gases represent the ratio of the percentage volume content (i.e., oxygen concentration) of the two gases. Because when the pressure is different, even if the oxygen concentration is the same, the oxygen partial pressure is also different.

　　③It is necessary to ensure that both the gas to be measured and the reference gas have a certain flow rate so that they can be continuously updated.

　　3.1 Zirconia probe aging In most cases, when the probe ages, the internal resistance will be greater than 1kΩ, so the degree of probe aging can be judged by measuring the probe internal resistance. Generally, under reasonable installation point selection and moderate harsh flue gas conditions, the probe will not show obvious aging until one year after use. However, if the smoke temperature at the installation point is too high, or the sulfur dioxide content in the flue gas is too large, it will accelerate the aging of the probe and shorten the life of the probe.

　　3.2 Oxygen fluctuation The oxygen operation curve is a fluctuating line with burrs. Burrs and fluctuations are short-cycle noise and long-cycle noise, respectively, caused by furnace pressure fluctuations and air-coal ratio fluctuations. Therefore, the size of burrs and fluctuations depends on the quality of the furnace, not the probe itself. Normal burrs are about ±0.4%. If the burrs are close to ±1%, it is a small fluctuation, and if it is greater than ±1%, it is a large fluctuation. Probe aging is one of the reasons for the fluctuation.

　　3.3 Abnormal oxygen display There are many reasons for abnormal oxygen display, mainly including aging or damage of zirconia components, heating wire breakage, thermocouple breakage, and the need to replace the corresponding instrument parts or probes; in addition, poor contact or disconnection of connecting wires will also cause abnormal oxygen display, and the wires need to be reconnected or replaced.

　　For example: when the oxygen content of the gas to be measured is not zero, the oxygen content measurement of the oxygen analyzer shows zero.

　　①Temperature fault. Reasons for low temperature:

　　1) Thermocouple damage. It can be judged by measuring the thermocouple resistance.

　　2) Heater damage. Cut off the power supply, measure the resistance of the heater, and measure the insulation resistance between the heater lead and the probe shell should be greater than 2MΩ.

　　3) Poor contact of thermocouple or heater leads. ②Lead wire and converter fault.

　　③Poor contact of electrodes inside the probe.

　　④A large amount of combustible material accumulates inside the probe.

　　3.4 High Oxygen Reading The reasons for a high oxygen reading are numerous, mainly including furnace wall air leakage, installation flange leakage, loose standard gas inlet nut of the probe, improper sealing at the connection between the probe's internal tube component and external tube, or between the internal tube and the zirconia component, and aging of the zirconia. The method of judgment is: if calibration with standard gas is normal, but the oxygen reading is significantly high during operation, it can be judged as air leakage, i.e., furnace wall air leakage, installation flange leakage, loose standard gas inlet nut of the probe, or improper sealing at the connection between the probe's internal tube component and external tube, or between the internal tube and the zirconia component.

　　① Furnace wall air leakage. If the furnace wall near the probe leaks, the negative pressure flue will draw in air. The flue gas around the oxygen analyzer will mix with air, and at this time, the measured oxygen content in the flue gas will include oxygen from the air, so the displayed value will be high. Measures should be taken to seal the furnace wall near the probe.

　　② Installation flange leakage. This can be due to: first, unsealed welding of the flue flange; second, an incomplete sealing gasket between the flue flange and the probe flange; third, loose probe installation bolts. The flue is under slight negative pressure, and in this case, air will leak into the flue from the installation flange. At this time, the measured oxygen content in the flue gas will include oxygen from the air, so the displayed value will be high. Measures should be taken to seal the installation.

　　③ Standard gas inlet nut of the probe is loose. If the standard gas inlet nut of the probe is not tightened, air will leak from the nut into the measuring end of the zirconia measuring element. At this point, air and flue gas mix, increasing the oxygen content in the flue gas, leading to a high oxygen reading. The standard gas inlet nut of the probe should be tightened.

　　④ Improper sealing at the connection between the probe's internal tube component and external tube, or between the internal tube and the zirconia component. This mainly occurs due to damaged sealing gaskets or loose connecting bolts. When the oxygen analyzer is measuring online, the flue is under negative pressure while the outside is under positive pressure. Improper sealing at the connection between the internal and external tubes, or between the internal tube and the zirconia component, will allow oxygen from the air to enter the measuring end of the zirconia measuring element through gaps. At this point, air and flue gas mix, increasing the oxygen content in the flue gas, causing the oxygen analyzer's measured value to read high.

　　3.5 Ash Clogging

　　When the probe is installed at a constricted section where the flue gas velocity is too high, not only is the probe prone to wear, but it is also susceptible to ash clogging. When ash clogging occurs, oxygen readings change very slowly.

　　3.6 Abnormal Temperature. There are many reasons for abnormal temperature, mainly including thermocouple breakage, heating furnace wire breakage, power supply issues, over-temperature protection circuit activation, heating drive signal failure, temperature control circuit fault, etc.

　　4.1 Incorrect Installation Point Selection

　　① Flue gas temperature too high. If a flue gas temperature point of 600-750℃ is chosen, the excessive flue gas temperature will accelerate probe aging.

　　② Selecting inside the furnace or a dead corner. Although the probe's service life may exceed 1 year, the response will be slow, making it unable to guide air adjustment operations.

　　③ Selecting a swirling flow area, oxygen readings fluctuate greatly.

　　④ Selecting a constricted section of the flue. High wind velocity here can easily cause probe clogging and significant erosion.

　　⑤ Selecting a location where flue negative pressure is greater than 1000Pa. This easily leads to air leakage faults and high oxygen readings.

　　⑥ Unrepresentative operating oxygen content. The normal flue gas oxygen content and the oxygen content at the probe installation point differ significantly.

　　4.2 Wiring Errors

　　① Connecting the positive and negative poles of the thermocouple incorrectly, resulting in a negative signal being fed to the temperature control system, which will burn out the probe.

　　② Connecting the positive and negative poles of the oxygen potential signal incorrectly, leading to abnormal measurement.

　　③ Broken connection in the middle. Putting it into operation without checking will result in failure to start and inability to run.

　　④ Worn signal cable leading to a short circuit with the furnace body, resulting in abnormal measurement.

　　4.3 Improper Installation Causing Air Leakage

　　① Standard gas inlet nut of the probe is loose, causing high oxygen readings.

　　② Probe installation is not sealed, or the furnace body flange welding is not sealed.

　　③ There are leakage points near the installation point, such as an unsealed soot blower hole upstream of the installation point, or significant air leakage from the furnace body, which will cause high oxygen readings.

　　④ During component replacement or maintenance, maintenance personnel may assemble incorrectly or leave probe installation bolts loose, causing air leakage.

　　4.4 Examples of Handling Special On-site Situations.

　　If the flue gas erosion at the installation point is severe, the outer tube is prone to wear through. An anti-wear cover can be welded to the outer tube; if the erosion is very

　　severe, leading to the probe's outer tube wearing through in less than a year, an installation point with less erosion must be found.

　　5.1 Rational Selection of Installation Location

　　① The installation point should ensure that the measured flue gas temperature is between 400-500℃, the flue negative pressure is less than 1000Pa, combustion at the measurement point should be complete, and flue gas flow should be stable without severe vibration or knocking sources.

　　② Install the zirconia oxygen analyzer controller in an area with good environmental conditions.

　　③ To prevent flue gas convection and condensation water from entering the zirconia head, the probe should be tilted slightly downwards.

　　④ The measurement point should avoid locations like manhole doors and should be in a well-sealed, air-tight part of the boiler.

　　5.2 Issues to Note During Probe Installation

　　① Zirconia probes are typically flange-mounted. An asbestos sealing gasket must be used between the flue flange and the probe flange, and the probe installation bolts must be tightened.

　　② Avoid knocking or colliding with the probe head to prevent damage to ceramic components.

　　③ When inserting the probe into a hot flue, do not do it too quickly; generally, inserting it at 10cm per minute is advisable.

　　④ The standard gas inlet nut of the probe must be tightened.

　　5.3 Maintain constant zirconia sensor temperature. Only when the temperature is constant will the output potential be related solely to the oxygen partial pressure of the gas being measured. Too low or too high a temperature will adversely affect the final result, so maintaining a constant zirconia sensor temperature is very important.

　　5.4 Operators should adjust air volume and fuel quantity in a timely manner

　　Sometimes, a significant deviation in the oxygen indication of the zirconia oxygen analyzer does not necessarily mean the analyzer is faulty; it is often due to operators failing to make timely adjustments after changes in combustion conditions. In such cases, simply adjusting the air volume and fuel quantity promptly can restore normal operation.

　　5.5 Strengthen daily maintenance and management

　　Newly commissioned zirconia oxygen analyzers should be calibrated at least once every two weeks and cleaned regularly. The standard gas tube should be clear, and the probe should be calibrated online quarterly. Any issues found should be dealt with promptly. If the boiler is out of service for an extended period, the heating furnace power should be cut off to prolong its lifespan. 1 to 2 days before ignition, connect the power first to avoid sudden cooling or heating that could cause the zirconia tube to fracture. To prevent probe damage, the probe should be removed from the flue, cleaned of ash, and properly stored during boiler maintenance. Daily technical management should be strengthened, and detailed records should be kept regarding the replacement of zirconia oxygen analyzers, causes of faults, and handling results.

　　and so on should be meticulously recorded. By strengthening the fault analysis, summary, and maintenance of zirconia oxygen analyzers, the accuracy and reliability of boiler flue gas oxygen content measurement signals have been significantly improved, thereby greatly enhancing the safety and economic efficiency of boiler combustion. This not only saves fuel costs but also reduces environmental pollution, extends boiler life, and provides strong assurance for the safe and economic operation of large-capacity, high-parameter thermal power generating units.