Comparison of direct absorption and wavelength modulation methods for online measurement of CO₂ by TDLAS

According to the statistics, 39.65 billion tons of coal has been consumed up from China in 2015. Obviously, CO2 emission from power generation and thermal supply industry segment contributes a large part to global warming. Therefore, it is of great importance for online monitoring of CO2 emissions from coal-fired power plants. The CO2 concentration was measured by tunable semiconductor laser absorption spectrum(TDLAS), which had the benefits of high accuracy, rapid response, non-contact, etc. In order to prevent interference from absorption of other gases, a distributed feedback laser with a center wavelength of 1 580 nm was chosen as the light source. Simulated flue gas was made referring to the concentration of CO2 in the flue gas of the power plant. The direct absorption(DA) and wavelength modulation(WM) methods were used to measure the CO2 concentration. The results show that the relative standard deviation(RSD) for DA and WM are 0.94% and 0.22%, while the maximum relative errors and detection limits for DA and WM are 2.64% and 1.65%, 0.013 6% and 0.001 4%. It is found that WM is better than DA in measurement qualities. But the WM method is influenced by the calibration method in field application and the line width is interfered by the fluctuations of the parameters such as pressure and temperature, resulting in reduced long-term stability. In the case of the DA method, the concentration can be calculated directly by measuring the absorbance, line strength and pressure without calibration. It has a high signal-to-noise ratio for the measurement of gas with high concentration. Meanwhile, the measurement accuracy is high enough to meet the requirement for CO2 online measurement in power plants. Therefore, the DA method is a better choice for the CO2 measurement in the flue gas of power plant boiler.