Working principle and maintenance of zirconia oxygen analyzer

Structure type and working principle of zirconia oxygen probe

According to the different detection methods, zirconia oxygen probes are divided into two categories: sampling detection oxygen probe and direct-inserting oxygen probe.

1.Sampling detection type oxygen probe

The sampling detection mode is that the measured gas is led into the zirconium oxide detection chamber through a guide pipe, and the zirconium oxide is heated to the working temperature (above 750°C) through a heating element. Zirconia generally adopts tubular shape, and the electrode adopts porous platinum electrode. The invention has the advantages that the invention is not affected by the temperature of the detected gas, and the oxygen content in various temperature gases can be detected by adopting different flow guiding pipes, and the flexibility is applied to many industrial on line detection. The disadvantage is slow reaction time. The structure is complex, which can easily affect the detection accuracy. The sampling tube is easy to be blocked when there are more impurities in the detected gas. The porous platinum electrode is susceptible to the corrosion of sulfur and arsenic in gas and the blockage of fine dust. The heater is usually heated by electric wire, and its life is not long.

When the temperature of the detected gas is low (0-650°C) or the detected gas is clean, the sampling type detection method is suitable, such as the oxygen measurement by a nitrogen making machine and the oxygen measurement in a laboratory, etc.

2.Direct insertion detecting oxygen probe

The direct-inserting type detection is that zirconia is directly inserted into the high-temperature measured gas to directly detect the oxygen content in the gas, the detection method is suitable for the temperature of the detected gas at 700°C~1150°C (the special structure can also be used for the high temperature of 1400°C), the high temperature of the measured gas is utilized to lead the zirconia to reach the working temperature without using a heater (figure 3). The key technology of direct-inserted oxygen probe is high temperature sealing and electrode problem of ceramic material. The structure of two kinds of direct-inserted oxygen probes are listed.

（1）Integral zirconia tube

The form is developed from the form of the zirconium oxide tube used in the sampling detection way, that is, the original zirconium oxide tube is lengthened, so that the zirconium oxide can be directly extended into the high temperature measured gas. This structure does not need to consider the high temperature sealing problem.

（2）Straight-inserting type zirconia oxygen probe

Because of the need to directly insert zirconia into the detection gas, the length of oxygen probe has a higher requirement, its effective length is about 500mm~1000mm, and the special environmental length can reach 1500mm. The testing precision, working stability and service life are all very high requirements, so the direct-inserted oxygen probe is difficult to adopt the whole zirconia tubular structure of the traditional zirconia oxygen probe, and more adopt the structure of the connection between the zirconia and the alumina tube with higher technical requirements. The sealing performance is one of the most important technologies of this zirconia oxygen probe. At present, the most advanced connection way in the world is to weld zirconia and alumina tube permanently together, its sealing performance is excellent, compared with the sampling detection method, the direct-inserting detection has obvious advantages: The zirconium oxide is directly contacted with the gas, and has the advantages of high detection precision, quick reaction speed and small maintenance amount.

Industrial application of oxygen probe

1.Application of Industrial Boiler and Heating Furnace

When the oxygen probe is used, there are two ways of introducing the measured gas, namely direct inserting and sampling detecting. The direct-inserting type response time is short, the heater is not needed, the structure is simple, the size is small and portable, but the temperature of the measured gas is required to be detected simultaneously. Because the temperature of the oxygen probe is controlled by the heater, the measuring precision is high and the operation is reliable, but the response time depends on the flow rate of the gas.

The direct-inserted Oxygen Analyzer has been widely used in the determination of oxygen content in flue gas of boilers and reheating furnaces (Figure 4). The oxygen probe used for this purpose mainly adopts a tubular structure, and the tube can be opened at both ends or at one end, the latter is the most common in the market at present. Porous Pt electrodes are coated on the inner and outer walls of the ZrO2 tube, and the inner and outer electrodes respectively extend to the end of the tube, and Ni Cr wires are connected at the end of the tube as a signal output, thereby controlling the combustion system to realize low oxygen combustion, achieving the purposes of reducing heat loss and saving energy.

Installation of an oxygen sensor

Reasonable installation is the key to ensure the reliable operation of oxygen sensor. Many use problems are caused by improper installation of oxygen sensor.

1.sampling measurement point

The determination of the measurement point is the primary work, which should follow the following principles:

（1）The selected measurement point requires that the detected gas in the furnace can be correctly reflected, so as to ensure the authenticity of the output signal of the oxygen sensor and avoid the dead angle of the return air as much as possible;

（2）The measurement point can not be too close to the combustion point or nozzle, and the gas in these parts is in the intense reaction, which will cause the oxygen sensor detection value to fluctuate and distort sharply; Do not be too close to air-generating equipment such as fans, so as not to damage the sensor by the vibration of the motor;

（3）The invention can avoid being placed in the position of possible collision so as to avoid damaging the probe by collision and ensure the safety of the sensor

2.Installation and connection mode of oxygen sensor

（1）The installation of the oxygen probe can adopt the horizontal or vertical mode, wherein the vertical installation is ideal. No matter what way is adopted, the direction of the guide plate of the probe sampling tube should be as far as possible in the direction of the measured airflow, and in the initial installation, the basic direction is determined by knowing the process. After the system is powered on to heat the probe, the direction of the sampling tube is rotated, and the fluctuation of the output oxygen potential is observed by the digital multimeter to finally determine the better guiding direction.

（2）The joint used for the installation of the oxygen sensor is a special flange joint, and the asbestos pad is arranged to be pressed to ensure the sealing, otherwise, because the common furnace is negative pressure, the leakage at the flange joint can affect the measurement accuracy or cause signal fluctuation.　

（3）The signal lead-out line of the oxygen sensor is preferably a shield line to eliminate interference. The best way is to use two two-core cables, one two-core shielded cable to output the oxygen potential, and one two-core KVV control cable to control the probe to heat the connection end. If the field condition does not exist, a 4-core KVV cable can be used directly to connect the probe oxygen potential signal and the heating end.

（4）The standard gas port of the oxygen probe is closed normally, and only used when the gas is calibrated; The blowing air port is connected with an air pump or a compressed air pipeline, the air inlet of the blowing air port is generally controlled by a valve such as an electromagnetic valve, the valve is opened once in a certain period, the gas is introduced into a gas blowing sampling tube, the valve is closed when the probe is normally detected, and no other gas can enter the sampling tube. The compressed air scanning probe of the manufacturer must ensure that the compressed air does not contain water, that is, the compressed air adopted must be separated from water.

Use and maintenance of oxygen sensors

1.connection heating control

The sampling detection type oxygen probe can work normally only after the oxygen sensor is connected with the heating control, and the output in the cold state is a random signal, which does not represent any meaning, after the oxygen sensor is connected with the heating control, the normal gas detection can be started at room temperature. The general probe zero-setting is at room temperature, after heating the probe, through the measurement of air, the digital multimeter is used to measure the output millivolt value of the probe at this time, this value is the value of the zero position deviation of the probe, the zero position deviation needs to be added in the display instrument to correct the oxygen concentration displayed by the instrument.

2.Considerations when installing or replacing an oxygen sensor

When installing or replacing an oxygen sensor, the oxygen concentration display value of an oxygen analyzer shall be corrected. Without this work, the oxygen concentration detected by the oxygen analyzer may deviate from the actual concentration after the replacement of the new sensor, thus affecting the measurement.

3.The correction principle and method of oxygen concentration.

The output of the oxygen sensor is the concentration of the measured gas and the standard air difference potential value, we call the oxygen potential, this potential value at the zero point (that is, air measurement), the different probe initial output potential deviation, and the output potential through the model conversion of the output oxygen concentration may be error, so in the oxygen analyzer to calibrate the probe signal is very necessary work, otherwise, the oxygen concentration and the actual measured gas concentration will exist a large deviation, can not meet the needs of the production in the field, even misleading control the influence of the production.

The specific correction is usually carried out by standard gas, the method is that the standard gas confirmed by the metering is introduced into the probe through the standard gas port, the output oxygen potential and the instrument display oxygen concentration at this time are measured, the instrument display oxygen concentration should be the same as the standard gas concentration, if there is any deviation, the instrument linear parameter is corrected, the standard metering requires at least three different concentration standard gas calibration systems, so that the system linearity is corrected repeatedly through three times of calibration, and the normal operation of the system is ensured.

4.Effect of dust accumulation on oxygen sensor and purging and cleaning method

Because the oxygen sensor is a long-term online detection and measurement device, the dust produced by the boiler and other equipment (especially coal combustion furnace or powder burning furnace, etc.) will block the gas-guiding sampling pipeline, resulting in the numerical distortion of the measurement, even unable to measure, at this time, it is necessary to regularly scan the dust in the sampling pipe, the length and length of the scan time to determine the ash deposition degree, this kind of sweeping method requires the oxygen analyzer has the corresponding function or the matching oxygen sensor maintenance device, if not these devices can only install manual valves to control the compressed air or air pump to regularly enter the blowing port to dust the probe, but at this time must pay attention to the following situations:

（1）Because the oxygen potential of the oxygen sensor will decrease in the process of purging, the lowest possible to be reduced to 1, 2 mV, at this time the detected oxygen potential does not represent the atmosphere in the furnace, this point must be noted.

（2）The flow rate of sweeping air should ensure that the soot can be removed, the output value of oxygen potential of oxygen sensor can be noticed in the course of sweeping. If the value of oxygen potential has not been decreased, it shows that the flow rate of air is too small, the dust has not been cleaned up, and the sweeping pipeline should be adjusted or checked.

（3）The channel of the blowing opening is directly communicated with the furnace, and after the blowing is finished, the valve should be closed, the blowing hole should be blocked, and the negative pressure air in the furnace should be prevented from entering, which will affect the detection of oxygen sensor

The oxygen sensor should be considered as a separate detection component when analyzing the quality of the sensor. When detecting the oxygen potential of the oxygen sensor, all wires connected with the oxygen sensor should be disconnected, and the oxygen potential is directly detected at the output end of the oxygen sensor by a digital meter with high internal resistance. By detecting the oxygen potential, the values are compared with those of normal use.

Actual operation

Since 2003, the cracking furnace of our plant has adopted ZGP2+ZDT high-temperature direct-inserted zirconia analyzer, which is mainly used for the measurement of oxygen content in flue gas, participated in the combustion control of the cracking furnace, and the operation is stable and reliable. In May 2005, it was found that the deviation of the indicated value is relatively large, all are positive deviation, it was determined that the gas path is leaking, the zero-point gas is communicated on the standard gas path, and the leakage of the rotor flowmeter was found. Due to the negative pressure in the measurement system, the outside air enters, because the volume fraction of oxygen in the air is high, the measured value is high, and the processing is normal. Several points for attention in practice are summarized:

⑴Zirconium tube is required to work normally at 750°C, so the instrument should keep constant temperature   

⑵No leakage of gas pipelines

⑶Keep the injector pressure stable at 0.15MPa 

⑷ The ejector gas source should be closed when the standard gas is checked and open during the measurement.

⑸The existence of H2, CO, CH4 and other combustible gases in the measurement gas will make the measurement result lower.

Conclusion

Zirconia oxygen measuring instrument has the advantages of simple structure, short response time, wide measurement range, high use temperature, reliable operation, convenient installation, small maintenance, etc. 

Therefore, it has been widely used in metallurgy, chemical industry, electric power, ceramics, automobile, environmental protection and other industrial departments.