# TDLAS Monitoring of Carbon Dioxide with Temperature Compensation in Power Plant Exhausts
## 3 Experiment Details  
The exhaust of a power plant has a complicated composition, possibly including H\({}_{2}\)O, SO\({}_{2}\), N\({}_{2}\), O\({}_{2}\), CO, and CO\({}_{2}\). According to the HITRAN2012 database [38], O\({}_{2}\) and SO\({}_{2}\) have no transitions near 1579 nm, and the strongest transition of H\({}_{2}\)O is up to \(10^{-26}\) in magnitude. Figure 1 shows the simulated absorbance of H\({}_{2}\)O (0.8%), CO (0.001%), and CO\({}_{2}\) (10%) in the near-infrared at a typical condition of the power plant exhaust from 1578 to 1580 nm (6330-6335.5 cm\({}^{-1}\)). The CO\({}_{2}\) absorbance near 1579.12 nm (6332.7 cm\({}^{-1}\)) selected in this study is approximately 1000 times stronger than the transitions of other gases and isolated from CO interference transitions, while it is still accessible by distributed feedback diode lasers. Thus, this transition is sufficiently strong and ideal for in situ CO\({}_{2}\) measurement in the outlet ducts. Figure 1: Simulated absorbance of CO\({}_{2}\) (10%), CO (0.001%), and H\({}_{2}\)O (0.8%) from 6330–6335.5 cm\({}^{-1}\) based on the HITRAN (High Resolution Transmission) database (298 K, 1 atm). The experimental setup for CO\({}_{2}\) concentration measurement is schematically shown in Figure 2. The setup consists of two main parts: gas preparation and laser detection. For the gas preparation, the CO\({}_{2}\) mixtures were generated by mixing the pure N\({}_{2}\) and CO\({}_{2}\) (0%-20%, interval of 2%) continuous gas flows controlled by two mass flow controllers (Seven Stars, Beijing, China). A gas mixer was used to obtain the uniform gas mixture. For the laser detection, the 1.58 \(\upmu\)m DFB laser with \(\sim\)5 mW output power was used as the laser source to exploit the CO\({}_{2}\) absorption line. A photodetector (SM05 PD5A InGaAs Detector, THORLABS) was used to detect the transmitted laser beam through the test gas. Laser intensity and wavelength were varied by a combination of temperature and injection current using a commercial controller (PCI-1 DA) with the real time spectrum display and capture features. The electrical signals received by the controller were processed into real-time spectral images. The wavelength was scanned (laser temperature at 35 \({}^{\circ}\)C, injection current scan between 30 and 120 mA) with a linear ramp of current from a function generator. The temperature and current tuning coefficients were measured to be 0.413 cm\({}^{-1}\)/K and 0.049 cm\({}^{-1}\)/mA, respectively. For the second harmonic signals, the laser controller contains lock-in amplification and demodulator for 2f signal extraction. The laser beam was directed into a Herriot cell (29.5 cm long, 571 mL volume, effective optical path length of 20 m) by a collimator and detected by the InGaAs detector. The 20-meter optical path allowed the system to have sufficient detection sensitivity. The cell was mounted with two wedged (1.5\({}^{\circ}\)) sapphire windows to avoid unwanted interference fringes. Three type-K thermocouples were equally spaced along the center section of the Herriot cell to determine the temperature of CO\({}_{2}\); the maximum temperature difference observed among these thermocouples was \(<\)0.1 K from 298 K to 338 K. The gas pressure was continuously measured by a pressure gauge (YN60, full range 0.6 MPa) with an accuracy grade of \(\pm\)0.015 MPa. Before each experiment, the Herriot cell was purged with pure N\({}_{2}\) for 5 minutes to remove the residual gas. The cell was operated at 298-338 K (interval of 10 K) at an operational pressure of 1 atm. When a stabilized temperature was obtained, the test gases at six different CO\({}_{2}\) concentrations (10%-20%, interval of 2%) were measured with a single sweep of the laser. The continuous tuning range (at constant temperature) was within 5 cm\({}^{-1}\). The diode laser was scanned at a 5 Hz repetition  
Figure 2: Schematic diagram of the experimental setup. rate continuously, and the data were sampled at a frequency of 24 kHz to provide 4800 data points/sweep.