Oxygen measurement principle and application of th

2022-09-30
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The oxygen measurement principle and application of zirconia analyzer

1 preface

a comprehensive analysis is made on the problems of 1 unit's air supply unit consumption increasing and high oxygen load not going up. Through the inspection, it is found that the air supply fan is fully open, the oxygen content is only about 4.0%, at the same time, the unit consumption of the air supply fan increases by 0.45kwh/tq, and the unit consumption of the induced draft fan is also 0 With the increase of 2 kwh/tq and the decrease of exhaust gas temperature in the flue on the side a, the exhaust gas temperature on the side a is about 21 ℃ lower than that on the side B. It is judged that there is air leakage on the side a of the preheater. Thermal test is arranged for air leakage test, and it is found that the air leakage rate of the preheater on the side a is as high as 18%

The air leakage at the first side of the preheater of boiler 1 is serious, which seriously affects the unit consumption of air supply, and the high load oxygen cannot go up. Take the opportunity of boiler shutdown to check and eliminate the internal air leakage point of the preheater in the opinions of Shanghai Municipality on the energy efficiency design of affordable housing buildings (hereinafter referred to as the "opinions"). During the shutdown and standby of #1 boiler this time, the air leakage pipe of preheater was blocked. After starting up on August 24, 2004, it was observed that under the same load and the same oxygen content, the power consumption rate of air supply decreased by 0.029 percentage points, the power consumption rate of induced draft decreased by 0.009 percentage points, and the power consumption rate of air supply and induced draft decreased by 0.038 percentage points in total

the 125MW Unit of Heze Power Plant uses oxygen signal to correct the air volume command, as shown in Figure 1 below

the air volume command is sent to the air supply control system to control the opening of the scoop tube of the coupling of the air supply fan a and B, so as to control the air supply volume; The deviation between the air volume command and the actual air volume is output through the PI operation of the rafc block and introduced into the induced draft control system as a feedforward signal to control the opening of the scoop tube of the coupling of induced draft fan a and B, so as to control the negative pressure of the furnace

2 oxygen measurement principle

as shown in Figure 2, there are two platinum electrodes on the inner and outer surfaces of the bottom of the zirconia tube, namely the reference electrode and the measuring electrode, with two platinum leads respectively, forming a zirconia oxygen measurement battery, namely the oxygen concentration battery

this type of probe is mainly composed of the following components: zirconia element is the key component of the probe, and it is the main body to form an oxygen measuring

battery, including zirconia tube and molybdenum electrode and electrode lead coated on the bottom of the tube. The electrode lead can lead out the signal; The heating

furnace is used to heat the zirconia tube to keep it constant at the set temperature (780 ± 10 ℃); The standard gas pipe is used to connect the standard gas and calibrate the

probe; The thermocouple is used to measure the temperature in the oxygen battery and is connected to the transmitter temperature control system; The terminal block is equipped with three pairs of terminals for signals, thermocouples

and heating furnaces, as well as filters, mounting flanges and probe housings

4 oxygen transmitter

the schematic diagram of the original zo-12 (q) oxygen transmitter is shown in Figure 4, which consists of two parts

4.2 temperature control part

the function of the temperature control part is to set the temperature of the oxygen measuring battery in the probe within the range of (780 ± 10 ℃), and the thermocouple installed in the probe transmits the pool temperature signal et to the temperature control circuit. After cold end compensation and decoupling protection, it is amplified by 100 times and added to the proportional integral circuit, and compared with the pool temperature setting potential of 3.24v. The comparison result is sent to the phase-shift trigger circuit to generate variable cycle pulses to trigger the solid-state relay, Due to different pulse periods, the conduction angle of the thyristor in the solid-state relay can be controlled, so as to change the heating power of the probe heating furnace and achieve the purpose of constant temperature

4.3 front panel function keys

the front panel function keys of the transmitter are shown in Figure 5. It is equipped with seven function keys and a background adjustment, including: four parameter display keys: oxygen content - firm and indomitable enterprising will - display flue gas oxygen content, signal - display probe signal, background - display background potential, pool temperature - display probe zirconia battery temperature; The three keys in the upper row are independent operation keys, which do not affect each other, including the digital display switch. However, since there are about 10PF capacitance range conversion (10% and 20%) between the traditional 74ls193 pin and the PCB board, the standard signal is set in the self-test instead of the probe signal to check whether the oxygen conversion part of the transmitter is normal; When the background potential is used for calibration of standard gas, adjust the potentiometer to calibrate the influence of the background potential

the transmitter is equipped with two protection functions: the disconnection protection is used to input a pseudo thermocouple signal of about 34mv into the line part after the thermocouple is disconnected, and automatically disconnect the power supply of the heating furnace to protect the probe from burning out; Overtemperature protection is used when the temperature control system fails and loses control, and the pool temperature rises to 820 ℃. The overtemperature protection system will keep the control power supply at 820 ℃ to avoid burnout of the probe

5 calibration:

zirconia battery aging, ash deposition, SO2 and SO3 corrosion of the battery and many other interference factors. During the operation process, the instrument parameters will gradually change, which will bring errors to the measurement. Battery aging is manifested in two main parameters: the increase of internal resistance and the increase of background potential. When the internal resistance is greater than 800 Ω or the background potential increases to (― 25 ~ ― 30) MV, the oxygen display jumps and the response is slow. In order to make the measurement accurate, we also need to know that it must be calibrated regularly with standard gas

first adjust the standard gas flow to 500ml/min, then remove the standard gas inlet plug on the probe, connect the standard gas with a conduit to the standard gas inlet, and seal it

when the standard gas is introduced for about (3 ~ 5) minutes, the display is stable. At this time, adjust the background adjustment potentiometer on the front panel of the transmitter until the oxygen display is equal to the standard gas value

remove the standard gas connecting pipe, plug the standard gas inlet again, after the calibration of the instrument is completed, press the background key to read and record the value

[References]

[1] Guan Yihe, Ruan Yong, Zhou Jianli JJG, zirconia oxygen analyzer [S]

[2] Ding Weidong Zo series

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