Abstract: Significance　Stationary pollution sources are significant contributors to CO2 emissions. Accurately assessing CO2 emissions is an important guarantee for achieving the goals of carbon peak and carbon neutrality. Online carbon emission monitoring, featuring high accuracy, good timeliness of emission data, and data traceability, is a crucial method for carbon emission accounting. It has been widely applied in some developed countries. In recent years, China has also carried out pilot monitoring of carbon emissions from stationary sources in the power generation, steel, and cement industries. To quantify the carbon emissions of stationary pollution sources through continuous online monitoring, it is necessary to simultaneously monitor the CO2 concentration and the flow rate of flue gas. The uncertainty in measuring the CO2 concentration and flow rate of flue gas largely depends on the accuracy of measuring equipment and the setting of sampling points. Therefore, aiming at the online monitoring requirements for CO2 concentration and flow rate of flue gas from stationary pollution sources, this paper elaborates on the technical principles, applicable conditions, and domestic and international application status of common methods for monitoring CO2 concentration and measuring the flow rate of flue gas from stationary pollution sources, based on the online monitoring and calculation method for CO2 emissions from stationary pollution sources. Furthermore, based on the progress of the domestic pilot work on carbon emission monitoring of stationary pollution sources, the development trend of China's carbon monitoring system for stationary sources, optical measuring instruments for flue gas CO2 concentration, and flue gas flow meters are prospected. Progress　The direct monitoring method obtains CO2 emissions by online monitoring of flue gas parameters, such as CO2 concentration, flow rate, temperature, humidity, and pressure, at the measuring cross-section inside the flue or chimney. The key lies in the monitoring of the volume concentration of CO2 in the flue gas and the flow rate of flue gas. CO2 concentration and flow rate of flue gas can be continuously measured by CO2 analyzers and flue gas flow meters suitable for the flue gas environment of stationary pollution sources. Currently, the optical methods commonly used for detecting greenhouse gases from stationary pollution sources mainly include non-dispersive infrared spectroscopy (NDIR), Fourier transform infrared spectroscopy (FTIR), and tunable diode laser absorption spectroscopy (TDLAS). The three commonly used optical detection methods are absorption spectroscopy techniques, and the theoretical basis for quantitative analysis of which is the Lambert- Beer law. NDIR technology is currently widely used domestically and internationally for detecting CO2 concentration in flue gas from stationary pollution sources. FTIR technology is currently mainly applied to the high-precision detection of multicomponent flue gas pollutants from stationary pollution sources. TDLAS, with its strong anti-interference and online in-situ detection capabilities, shows promising development prospects in the online monitoring of CO2 concentration from stationary pollution sources. NDIR, FTIR, and TDLAS are all recommended reference methods for online monitoring of CO2 in flue gas from stationary pollution sources according to international standards. Among them, NDIR and FTIR are recommended for extractive measurement, and TDLAS is recommended for in-situ measurement. The above three optical detection technologies generally have a measurement accuracy of less than 2% for CO2 concentration. However, unlike the relatively low uncertainty of CO2 concentration data (within 2%), the typical error for flue gas flow rate is in the range of 10%–20%, and can exceed 50% in extreme cases. According to the measurement principle, the methods for measuring the flow rate of flue gas from stationary pollution sources can be divided into three categories: differential pressure flow meter, ultrasonic flow meter, and thermal flow meter. Pitot tube flow meter (a typical type of differential pressure flow meter) and ultrasonic flow meter are the main methods for measuring flue gas flow rate, and their accuracies can generally reach 5%–10% and 5% respectively. Currently, in power plants in the United States, the proportion of ultrasonic flow meters in use is about 63%, while in power plants in Japan and South Korea, Pitot tube flow meters are mainly used (accounting for 66%). As for the pilot enterprises for carbon emission monitoring in China, NDIR technology is mainly used for CO2 measurement, and Pitot tube flow meters are mainly used for flue gas flow measurement (accounting for 83%), while the proportion of ultrasonic flow meter usage is relatively low (11%). Conclusions and Prospects　Most of China's key stationary pollution sources have installed continuous emission monitoring systems (CEMS), which lays a solid foundation for online monitoring of carbon emissions. On the basis that the current domestic NDIR equipment basically meets the online monitoring accuracy requirements for CO2 concentration in flue gas, independent research and development of instruments such as FTIR and TDLAS can be strengthened to further improve the measurement accuracy of CO2 in flue gas. In addition, since CEMS currently measures flue gas flow mainly based on the Pitot tube principle, which has relatively low accuracy, it is recommended to strengthen the independent research and development of ultrasonic flow meters. This would help to reduce costs while improving measurement stability and accuracy, thereby promoting their wider application in flue gas flow monitoring. Moreover, it is essential to strengthen the construction of the technical system and quality management system for online monitoring of CO2 in flue gas from stationary pollution sources.

Key words: stationary sources, CO2 emissions, on-line monitoring, CO2 concentration, flue gas flow