Towards a dTDLAS-Based Spectrometer for Absolute HCl Measurements in Combustion Flue Gases and a Better Evaluation of Thermal Boundary Layer Effects

Abstract

This work presents a mid-IR direct tunable diode laser absorption spectroscopy (dTDLAS)-based HCl spectrometer, which is specially designed and optimized to measure HCl concentration in combustion exhaust gas matrices (i.e. elevated gas temperatures, high water vapour and CO2 contents). The work is motivated by (legal) requirements for monitoring combustion emissions from large-scale power stations or biomass burning domestic boilers. In our novel dTDLAS HCl spectrometer we use a low power room temperature mid-IR ICL diode laser to access the HCl P5 line at 2775.76 cm−1 in the 1-0 vibrational band which was especially selected for gas matrixes with high CO2 and H2O admixtures. With this set-up we demonstrate at 77 cm path length, total pressure from 320 hPa to 954 hPa, room temperature and at 110 s of signal averaging an optimal precision of 0.17 µmol/mol (ppm). Gas monitoring in combustion applications and elevated gas temperatures are prone to systematic errors caused by spectroscopic falsifications in colder gas boundary layers (BL) unavoidable in the high temperature gas ducts. These BL lead, e.g. to temperature, matrix composition or target gas concentration gradients near walls, which also influence the spectroscopic raw signal via their temperature and collision partner dependence. Depending on the chosen spectral line these can cause significant systematic deviations in in-situ, line-of-sight (LOS) laser spectrometers. For an improved understanding of the quantitative effects of thermal BL on our LOS HCl dTDLAS spectrometer and better representing real BL, we expanded our spectroscopic BL simulation model to allow for continuous (linear) instead of the previous stepwise changes. From the new simulation results, we deduce systematic relative deviations in the extracted HCl-concentration to be up to 10% depending on the magnitude of the BL changes and the choice of the “representative” single temperature measurement. With this simulation model, a user can now derive the system’s systematic deviation based on assumptions on the present temperature gradients. The model also helps the user to choose the gas temperature measurement location in his process, in order to minimize the spatial heterogeneity effects.

Measurements of N2, CO2, Ar, O2 and Air Pressure Broadening Coefficients of the HCl P(5) Line in the 1–0 Band Using an Interband Cascade Laser

“…The HCl gas molecule is not only reactive but also very sticky, similar as NH 3 . As we have shown in our previous study [20], the system equilibrium condition can only be reached after passivation. Actually, during this passivation process, HCl molecules stick to the inner surface of the system, while those molecules came into the gas flow again during the evacuation process.…”

“…Comparing to other foreign broadening measurements, here the HCl-N 2 broadening coefficient was interfered with the self-broadening contribution in Equation (4) (first term) which was subtracted due to the relative high concentration of HCl. Using the self-broadening coefficient of 0.223 cm −1 /atm from HITRAN [20]. The respective Lorentzian widths show very good linearity with pressures with an intercept of (2.73 ± 3.99) × 10 −4 cm −1 .…”

“…The experimental setup of the spectrometer for the determination of the HCl pressure broadening coefficients is shown in Figure 1. A similar setup was used in our previous thermal boundary layer effects study [20]. A 3.6 µm interband cascade laser (ICL) (Nanoplus GmbH) is used to scan across the P5 HCl absorption line in the HCl 1-0 fundamental band.…”

“…Recently, advances in solid-state laser technologies have provided alternative Mid-IR light sources and enabled access to the strongest HCl transitions in the fundamental band [17], which are about 50 times stronger than at the 1.7 µm HCl first overtone band which we have previously studied [18,19]. As such, we were motivated to develop an optimized Mid-IR absorption diagnostic for HCl [20]. This study focuses on the HCl P(5) transition line in the 1-0 band, which is optimal for HCl measurements in hot flue gases, as it is spectrally well isolated from water and CO 2 .…”

Ultra-Highly Sensitive Hydrogen Chloride Detection Based on Quartz-Enhanced Photothermal Spectroscopy

“…Laser absorption spectroscopy (LAS) with advantages of rapid response, as well as non-invasive, highly sensitive, and selective detection is widely applied for HCl sensing. Tunable diode laser absorption spectroscopy (TDLAS) has been adopted for measuring HCl and a precision of 2 ppm at 1 s averaging time has been obtained [8]. Generally, a multi-pass gas cell has been employed in TDLAS to improve the detection performance; however, this has resulted in the TDLAS sensor being bulky and costly.…”