Automatic Monitoring System for Boiler Nitrogen Oxide Pollution SourcesUsing liquid ammonia method to prepare denitrification reducing agent, selective catalytic reduction (SCR) method is used as the denitrification device and supporting system transformation. Reduce the NOx concentration from 500 mg/Nm3 to 75mg/Nm3 (with a design SCR efficiency of 85%),
The performance of the denitrification device is mainly as follows:
The NOx removal rate of the denitrification device during the performance assessment test (with the additional layer catalyst not in operation) shall not be less than 85%, ensuring that the outlet is less than 75 mg/Nm3, the ammonia escape rate is less than 2.5 ppm, and the SO2/SO3 conversion rate is less than 1%;
a) Boiler 50% THA~100% BMCR load;
b) The NOX content at the flue gas inlet shall not exceed (500) mg/Nm3;
c) The dust content of the flue gas at the inlet of the denitrification device is less than (42) g/Nm3;
d) The NOX content at the flue gas outlet is lower than (75) mg/Nm3;
e) When the NH3/NOx molar ratio does not exceed the guaranteed value (0.86).
Definition of denitrification efficiency:
Denitrification=C1-C2 × 100%
C1
In the formula: C1- NOX content in the flue gas at the inlet of the denitrification system during operation (mg/Nm3).
C2- NOX content (mg/Nm3) in the flue gas at the outlet of the denitrification system during operation.
The escape rate of ammonia refers to the concentration of ammonia at the outlet of the denitrification device.

2. Composition of analyzer (NH3/NOx/O2) system

2.1 Automatic Monitoring System for Boiler Nitrogen Oxide Pollution Sourcesanalysis
The front-end monitoring probe of the entire monitoring system is installed at the pollution source monitoring point. The monitoring signal is converted and processed by the transmitter into a digital signal, which is transmitted to the local monitoring computer through a standard RS485 serial interface. The local monitoring computer and analysis system cabinet are placed in a dedicated monitoring room. On the monitoring computer, the environmental parameters such as nitrogen oxides (NOX), NH3, temperature, oxygen content, and pressure of the pollution source are collected and processed through an online environmental monitoring network system to achieve automated data reporting and statistical work of environmental parameters. The monitoring data can be transmitted to the environmental monitoring center station or other relevant departments through telephone network or Internet network. Analog ports or dry contacts can also be used for parameter transmission or device control.
The system adopts a complete extraction method to collect sample gas, filters it, and transmits it through a heat tracing pipeline. The sample gas is processed before the analyzer to become a dry gas to be tested and enters the analysis instrument for detection. Gas analysis uses alternating injection method and non dispersive infrared principle to detect the sample gas. The measurement results are input into the data acquisition device through a digital port. Data management software processes raw data, generates various forms of reports, and can be remotely transmitted.
In addition, to ensure the normal operation of the system, various diagnostic and alarm functions have been designed. It can output alarm signals, mark data, or issue control signals, such as stopping sampling, starting blowback, etc. The system has blowback and calibration functions, which can be programmed automatically or manually implemented at any time. Calibration can be performed using standard steel cylinder gas, which can be directly used to calibrate the analytical part or through a probe for overall calibration.
The series adopts an innovative three-stage dehydration system. The system includes a moisture separator and two electronic coolers. The design of the dehydration system ensures that the loss of NOx and other pollutants carried away by condensed water is minimized, thereby ensuring the accuracy of monitoring data.

2.2 Measurement and analysis of nitrogen oxides (NOX)
Monitoring NOx before and after the denitrification system can help us understand the efficiency of the denitrification system. There are generally three principles for measuring nitrogen oxides (NOX): chemiluminescence (CLD), non dispersive infrared absorption (NDIR), and ultraviolet absorption (UV). This system adopts a unique alternating flow modulation chemiluminescence (CLD) method, which theoretically eliminates zero drift. In addition, the sample gas and zero gas alternate into the same pool, further reducing errors caused by different instruments themselves. The NOX monitoring unit adopts a low-temperature NOX converter, which converts NO2 into NO. under the action of a special carbon based catalyst. The working temperature of this converter is about 190 ℃. While ensuring the complete conversion of NO2 to NO, its durability and lifespan are greatly improved. Using semiconductor sensors, it can measure components with a small content of 0-10ppm, which has a longer service life than traditional sensors and further improves sensitivity and reliability.
Under the precise control of the solenoid valve, the sample gas and reference gas (gases with zero or a known concentration of the analyte) are alternately injected into the detection cell at a constant flow rate. The infrared emitted by the infrared light source is detected by the detector after passing through the detection pool. When the sample gas and reference gas are sequentially introduced into the detection cell, the absorption of infrared energy will change, causing displacement of the thin film in the detector. The displacement is converted into an electrical signal, and finally the concentration of the analyte in the sample gas is calculated.

The significance of NH3 monitoring and analysis of SCR ammonia escape measurement
Due to the need to inject NH3 during the denitrification process, it is necessary to monitor the residual NH3 after the denitrification process to ensure that the final emission concentration is within the emission standards. The data from the online monitoring system can not only be reported to relevant departments, but also directly used as process control parameters in the denitrification process, preventing excessive NH3 from reacting with SO3 to form NH4HSO3, and reducing denitrification operating costs by effectively utilizing NH3.
Due to the high solubility of NH3 in water, the measurement is inaccurate. The main solution is to use a probe reduction reaction method to measure NH3. The temperature at the probe is relatively high, which can prevent NH3 loss. As the probe penetrates deep into the flue, it is easy to maintain the required temperature for the reaction. The online analysis of nitrogen oxide monitoring at the inlet and outlet of flue gas denitrification in this project adopts the direct extraction method. The difficulty lies in the high temperature, high dust, high humidity, and high corrosion of the measured flue gas, which makes the sampling probe easy to block and the system easy to corrode. Therefore, multi-stage filtration and dust removal, two-stage dehumidification, and aerosol filtration and droplet removal measures are adopted for the sampling and sample gas processing system to improve the system's dust removal and dehumidification capabilities and ensure reliable operation.

3. Daily maintenance and inspection items
In order to ensure the normal operation of the system, regular inspections and maintenance must be carried out

4. Corresponding common faults
Due to the harsh working environment of the analysis system, some faults may occur. Timely and rapid elimination of faults can not only ensure the safe operation of the main system, but also extend the operating life of the analyzer.
4.1 Low Traffic - Traffic Alarm
Phenomenon: The concentration of the sample gas or standard gas cannot reach the normal flow rate.
corresponding:
① Adjust the needle valves (NV-1, NV-2);
② Confirm the operation status of the sampling pump (P-1), replace the pump membrane or pump;
③ Check if the secondary filter is clogged (F-1/F-2) and replace the filter paper;
④ Check the operation of P-2 and replace the pump membrane;
⑤ Confirm if the air filter (FA-1) is clogged and replace the air filter;
⑥ Check the set pressure and operating condition of the pressure regulator (R-1)
Set pressure: -0.01MPa; Reset the pressure or replace the pressure regulator;
⑦ Check if there are any blockages or leaks in other related components of the pneumatic process.

4.2 Abnormal Sampling Temperature
Phenomenon: 'SAMPLING temperature abnormality' on the operation panel turns red
corresponding:
① Check if the electronic coolers (C-1, C-2) are functioning properly, and replace them if there are any abnormalities;
② Confirm if the ozone decomposer heater (DO-1) is running, and replace it if it is abnormal.

4.3 Abnormal NH3 measurement data
Phenomenon: Abnormal fluctuations in NH3 measurement values or abnormal test values;
corresponding:
① Adjust the coefficients of the NOx and NOx NH3 gas pipelines to ensure that the test value * is obtained when measuring the same gas in both pipelines;
② Calibrate the analyzer;
③ Replace the probe NH3 conversion catalyst;
④ Replace the NOx gas path and NOx NH3 gas path conversion catalyst tubes (COM-1, COM-2).

4.4 Unable to calibrate normally
Phenomenon: The calibration coefficient of zero gas or range gas exceeds the set range, and the operation panel 'cannot calibrate' turns red
corresponding:
① Confirm if the standard gas flow rate is normal. If the flow rate is too low, troubleshoot as described above;
Confirm the cylinder pressure. If the cylinder pressure is too low or there is no pressure, please replace the cylinder
② Check whether the set concentration value of the calibration gas matches the concentration value of the gas cylinder *;
③ Confirm the operation of the solenoid valve (SV-1,2,3,6): If the solenoid valve stops running, the 'solenoid valve stop' on the operation panel will turn red, replace the solenoid valve.

5 Conclusion
This system has been running reliably for one year, and through precise monitoring of flue gas components (NH3/NOx/O2), it ensures the qualified emission of nitrogen oxides (NOX) from boilers, improves the local atmospheric environment, and its environmental and social benefits will be significant in the long run.