In order to ensure a better combustion condition of the boiler, it requires that the oxygen content measurement of the boiler flue gas must be accurate and reliable. However, the zirconia oxygen analyzer is prone to failure in the harsh environment of high temperature and severe flue gas erosion, so we should strengthen the operation and maintenance of the zirconia oxygen analyzer.
1 Significance of oxygen measurement
The coal consumption of the unit is directly related to the quality of boiler combustion. The boiler has a certain excess air coefficient when it is in a good combustion state, and the excess air coefficient has a certain relationship with 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 good 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, it is very important to measure the oxygen content of boiler flue gas in thermal power plants, which requires that the oxygen content measurement of boiler flue gas must be accurate and reliable.
2 Working principle of zirconia oxygen analyzer
The basic principle of zirconia oxygen analyzer is: zirconia is used as the solid electrolyte. When the oxygen concentration on both sides of the electrolyte is different under high temperature, a concentration cell is formed. The potential generated by the concentration cell 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. Zirconia materials calcined at high temperature have good conductivity to oxygen ions at 600~1200 ° C. When the oxygen concentration on both sides of the zirconia tube is different, the oxygen molecules on the side with higher concentration will be bonded on the surface electrode of the zirconia tube on that side
Combine two electrons to form oxygen ions, and then swim to the side with low oxygen concentration through the oxygen ion holes in the zirconia material lattice. When reaching the low concentration side, release two electrons on the electrode at this side to form oxygen molecules, so as to cause charge accumulation on the electrode, and generate potential between the two electrodes. This potential prevents the further progress of this migration until the dynamic equilibrium state is reached, which forms a concentration cell, The potential generated by it related to the oxygen concentration difference on both sides is called concentration difference potential.
3 Cause analysis, judgment and elimination of common faults of zirconia oxygen analyzer
3.1 Zirconia probe aging
When most probes are aged, the internal resistance will be greater than 1k Ω, so the aging degree of probes can be judged by measuring the internal resistance of probes. In general, the probe will not age significantly until it has been used for one year when the installation point is reasonably selected and the flue gas is moderately harsh. However, if the installation lighting temperature 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 beat
The oxygen operation curve is a undulating line with burrs. Burr and fluctuation are short cycle noise and long cycle noise respectively, which are caused by furnace pressure fluctuation and air coal ratio fluctuation. Therefore, the size of burr and fluctuation depends on the quality of the furnace and is not caused by the probe itself. The normal burr is about ± 0.4%. If the burr is close to ± 1%, it is a small jump, and if it is greater than ± 1%, it is a big jump. The aging of the probe is one of the reasons for the jump.
3.3 Abnormal oxygen display
There are many reasons for abnormal oxygen display, mainly including aging or damage of zirconia elements, broken heating furnace wires and thermocouples, and replacement of corresponding instrument components or probes; In addition, poor contact or disconnection of the connecting wire will also cause abnormal oxygen display, and the wire needs to be reconnected or replaced.
For example, when the oxygen content of the measured gas is not zero, the oxygen measurement of the oxygen analyzer will display zero.
① Temperature fault. Reasons for low temperature:
1) The thermocouple is damaged. It can be judged by measuring thermocouple resistance.
2) The heater is damaged. Cut off the power supply, measure the resistance of the heater, and measure
The insulation resistance between the lead of the calorimeter and the probe shell shall be greater than 2M Ω.
3) The thermocouple or heater lead is in poor contact.
② Lead wire and converter failure.
③ The electrode in the probe has poor contact.
④ A large amount of combustibles are accumulated in the probe.
3.4 High oxygen display
There are many reasons for high oxygen content display, mainly including furnace wall air leakage, installation flange leakage, untightened probe standard gas inlet nut, loose connection between probe internal tube assembly and external tube, and between internal tube and zirconia element, zirconia aging, etc. The judgment method is: when the calibration with standard gas is normal, but the oxygen content is obviously high during operation, it can be judged as air leakage, that is, the furnace wall leaks, the installation flange leaks, the probe’s standard gas inlet nut is not tightened, or the connection between the probe’s internal tube assembly and the external tube, and the connection between the internal tube and the zirconia element is not tight.
① The furnace wall leaks. If the furnace wall near the probe leaks, the flue with negative pressure will suck in air, and the flue gas around the oxygen analyzer is mixed with air. At this time, the measured oxygen content in the flue gas contains oxygen in the air, so the displayed value will be high. Measures shall be taken to seal the furnace wall near the probe.
② The installation flange leaks. First, the welding of flue flange is not sealed, second, the sealing gasket between flue flange and probe flange is not complete, and third, the probe mounting bolt is not tightened. There is a slight negative pressure in the flue. In this case, the air will leak into the flue from the mounting flange. At this time, the measured oxygen content in the flue gas includes the oxygen in the air, so the displayed value will be high. Measures shall be taken to seal the installation.
③ The standard gas inlet nut of the probe is not tightened. If the standard gas inlet nut of the probe is not tightened, the air will leak into the measuring end of the zirconia measuring element from the nut. At this time, the air and flue gas will mix, and the oxygen content in the flue gas will increase, making the oxygen content display higher. The standard gas inlet nut of the probe should be tightened.
④ The connection between the internal tube assembly and the external tube of the probe and the connection between the internal tube and the zirconia element are not tight, mainly due to the damage of the sealing gasket or the untightening of the connecting bolt. When the oxygen content analyzer is measuring online, the flue is in a negative pressure state and the outside is in a positive pressure state. If the connection between the inner and outer pipes or the connection between the inner pipe and the zirconia element is not tight, the oxygen in the air will enter the measuring end of the zirconia measuring element through the gap. At this time, the air and flue gas will mix, and the oxygen content in the flue gas will increase, making the measured value of the oxygen content analyzer display higher.
4 Faults caused by misoperation and on-site treatment
① The smoke temperature is too high. If the smoke temperature point of 600~750 ° C is selected, the aging of the probe will be accelerated due to the high smoke temperature.
② The inner side of the furnace and the dead angle are selected. Although the service life of the probe is more than 1 year, the response is slow and the air regulation operation cannot be guided.
③ The eddy current is selected, and the oxygen content fluctuates greatly.
④ At the flue necking, the wind speed is high, which is easy to cause probe dust blockage and large scouring.
⑤ It is selected at the place where the negative pressure of the flue is greater than 1000Pa, which is easy to cause gas leakage and high oxygen content.
⑥ The oxygen content under working conditions is unrepresentative, and the oxygen content of normal flue gas is far from that of the probe installation.
5 Preventive measures
① Ensure that the smoke temperature is between 400~500 ° C, the negative pressure of the flue is less than 1000Pa, the measuring point shall be completely burned, and the smoke flow shall be stable without severe vibration and knock sources.
② Install the controller of zirconia oxygen analyzer in a place with good environment.
③ In order to prevent flue gas convection and condensate from entering the zirconium head, the probe should be slightly tilted downward.
④ The measuring point shall be located away from the manhole door and other positions, and shall be located at the sealed and air tight part of the boiler.
Post time: 09-12-22