Research On Fixed Source Carbon Dioxide Emission Monitoring System Based On Infrared Absorption Spectroscopy

With the progress of industrial civilization,global warming has become the most concerning ecological issue for all countries,and energy conservation and emission reduction have become one of the trends in technological development today.Among them,carbon dioxide（CO₂）as a key factor of the greenhouse effect has received much attention,and monitoring of CO₂ emissions from fixed sources has become a research hotspot.Compared with gas sensing technologies such as electrochemistry,surface acoustic waves,and optics,non-dispersive infrared（NDIR）detection technology analyzes gas by measuring the energy absorbed by the gas in the infrared spectrum range,and has the advantages of good selectivity,short response time,high integration,high accuracy,and non-destructive testing,making it easy to deploy and monitor on-site,and has become the most direct and promising gas detection technology among all optical gas sensing technologies.This paper combines NDIR and infrared differential detection technology to develop a highly accurate,miniaturized,and low-power gas sensor.An upper computer monitoring platform is designed,and a fixed-source CO₂ emission monitoring system is achieved.The main work of this paper is as follows:Firstly,the differential detection model is introduced into the hardware system of the NDIR gas sensor,and the optical gas chamber of the sensor is fabricated and simulated.Electrical modules such as the light source modulation drive circuit,detector signal acquisition circuit,main control circuit,power supply circuit,and temperature and humidity sensor circuit are designed.Under the control of the light source modulation drive circuit,the light source emits infrared light into the gas chamber.The infrared light is attenuated by gas absorption in the chamber and captured by the infrared detector after filtering.The captured signal is converted into an analog voltage signal,which is processed by the amplification bias circuit and differential circuit.The processed signal is collected by the ADC and transmitted to the main control for lock-in amplification processing.The characteristic parameters of CO₂ concentration,namely the first harmonic amplitude ratio,are obtained by comparing the differential signal and reference signal results.Finally,the obtained characteristic parameters are uploaded to the upper computer for further processing.Then,based on the hardware design,software programs were developed to implement reasonable scheduling of various modules through the main program.Specific design work includes the development of an infrared light source square wave driving program,an analog-to-digital（AD）acquisition program,an orthogonal lock-in amplifier program based on a differential detection model,communication programs for the main control and upper computer,and temperature and humidity sensor modules.A CO2 emission monitoring platform based on Lab VIEW was designed,which can calculate,display,and store data transmitted by sensors.The software program can convert electrical signals into concentration values and visualize the data.Subsequently,considering the working temperature range permitted by the pyroelectric detector,a temperature control system for the detector was developed under low-temperature conditions.In terms of hardware,the temperature signal acquisition circuit and heating film driving circuit were mainly targeted,and a simplified inductance-resistance（LR）model based on the traditional inductance-resistance-capacitance（LRC）model of a buck converter was established to determine the electrical parameters of the driving circuit.In terms of software,temperature data programs and proportional-integral-derivative（PID）programs were mainly written.The temperature control system provides assurance for the normal operation of the sensor under low-temperature conditions.Finally,the system integration and application were carried out,and the sensors were integrated into the monitoring system for experimental testing and application.Firstly,a low-concentration gas experiment was conducted by continuously monitoring the 1100 ppm CO₂ standard gas for 24 min.The fluctuation range was between 1091.1ppm and 1108.8 ppm.With an integration time of 0.25 s,the system detection limit was0.15 ppm,and the response time for concentration changes in the range of 1000 ppm was only 2 s.For the high-concentration gas experiment,the system continuously monitored the 12%CO₂ standard gas for 26 min,with a fluctuation range between 11.82%and 12.16%.With an integration time of 0.25 s,the system detection limit was 0.0027%,and the response time for concentration changes in the range of 4%was 3.5 s for rising and 2.25 s for falling.The performance of the temperature control system was also tested.When the ambient temperature was-20°C,the response time for a temperature change of 1°C was 13 s.The target temperature was set to 21°C,and the temperature fluctuation range was 20.95-21.05°C,with a stability of 0.0161°C（1σ）.Finally,the monitoring system was used for mobile gas monitoring application experiments,which confirmed that the designed CO₂ monitoring system could accurately and quickly detect CO₂ emissions and had good detection performance.The innovation of this paper can be summarized as follows:1.Combining differential detection and orthogonal lock-in amplification techniques,a nonlinear calibration curve function was obtained based on the detection concentration range and system output constraints,achieving wide-range gas detection.2.To address the issue of abnormal operation of pyroelectric detectors at low temperatures,a temperature control system was designed,and a heating film BUCK drive circuit model was established.The traditional LRC model was simplified into an LR model,and this model was used to investigate the relationship between pulse width modulation frequency,inductance resistance parameters,and the quality of output voltage.