# TDLAS Monitoring of Carbon Dioxide with Temperature Compensation in Power Plant Exhausts
## 1 Introduction  
Carbon dioxide (CO\({}_{2}\)) is a major component of greenhouse gases. Measurements of CO\({}_{2}\) in large emission sources such as coal-fired power plants provide an effective support of carbon emission statistics. Outlet ducts in power plants provide a challenging detection environment for gas absorption detection due to the instability and poor timeliness. Compared with the current spectroscopic techniques such as Fourier transform infrared (FTIR) [1] and non-dispersive infrared (NDIR) spectroscopy [2], tunable diode laser absorption spectroscopy (TDLAS) with high sensitivity and short response time [3] offers significant advantages for in situ CO\({}_{2}\) measurements in the hot, humid, and dusty environments of power plants. With the rapid advancements in semiconductor laser industry, TDLAS has been widely applied in different fields such as combustion diagnostics [4; 5; 6; 7; 8], isotopic analysis [9; 10; 11; 12; 13], and atmospheric gas detection [14; 15; 16; 17; 18]. Because of their exquisite constituents and compact packaging, as well as low cost, tunable semiconductor diode lasers are nearly the ideal source for highly-sensitive and highly-spectral-resolved measurements in industrial fields [19]. However, Doppler-broadening at high temperatures alters a larger portion of the integrated absorbancefor a particular transition to the far-wing region. For in-situ CO\({}_{2}\) measurements in outlet ducts, the temperature-varied absorption line intensity leads to measurement error in both direct absorption (DA) and wavelength modulation spectroscopy (WMS) signals. Numerous studies have been performed to investigate the influence of temperature variation on TDLAS measurement. Many of them used spectroscopic parameters such as line intensity [20] from databases like HITRAN (High Resolution Transmission) or HITEMP (High Temperature Molecular Spectroscopic). Nevertheless, these databases, which mainly derive from theoretical calculation, may have some uncertainty while applied in high-temperature measurement. Several of previous studies [21; 22; 23; 24; 25] measured spectral parameters such as line intensity, line position, and broadening coefficients in a harsh experimental condition. Under a certain range of pressure and temperature, they employed these spectroscopic parameters for temperature compensation. Empirical corrections to the temperature dependence of the absorption for CO\({}_{2}\)-CO\({}_{2}\), CO\({}_{2}\)-N\({}_{2}\), and CO\({}_{2}\)-O\({}_{2}\) at 193-300 K and the wavelength near 4.3 \(\upmu\)m are discussed [26; 27]. These corrections are applied via the corrective shape \(\chi\)-function to the line shape function of individual absorption features. [PERSON] et al. [28] continued the studies at 4.3 \(\upmu\)m to develop a temperature dependent \(\chi\)-function for corrections of CO\({}_{2}\)-CO\({}_{2}\) and CO\({}_{2}\)-N\({}_{2}\) collisions at temperatures up to 800 K. Additionally, [PERSON] et al. [29] proposed a 1f ratio method to correct the harmonic signals affected by temperature fluctuations. They employed a DFB (Distributed Feedback) laser at 760.77 nm to monitor 21% oxygen in the range of 300-900 K and the results showed that the proposed method was effective for minimizing the influence of temperature changes. [PERSON] et al. [30] introduced a temperature correction coefficient for HCl concentration using TDLAS at 1.7 \(\upmu\)m. [PERSON] et al. [31] studied the temperature influence on the detection of ammonia slip based on TDLAS. The empirical formula for temperature compensation was proposed and the error was reduced to about 5.1%. [PERSON] et al. [32] simulated the process of direct TDLAS measurement of H\({}_{2}\)O based on the HITRAN spectra database and gave the temperature correction curve in atmosphere detection. A temperature correction function was optimized based on open-path TDLAS for determining ammonia emission rules in soil environments [33]. [PERSON] et al.