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.
Highlights
European Directives are coming into force setting increasingly stringent emission limit values (ELVs) for regulating emissions from, e.g. combustion processes
In order to address these issues and to derive good estimates for possible systematic deviations caused by these problems as described above, we propose the use of approximate, assumed boundary layers (BL) properties in realistic application scenarios in combination with a direct tunable diode laser absorption spectroscopy (dTDLAS)-instrument with a full physical model and complete set of spectral parameters for the target absorption line
In the paper we present a novel first-principles Hydrogen chloride (HCl) spectrometer especially for applications in CO2-rich exhaust gases, e.g. from combustion or even oxyfuel combustion processes
Summary
European Directives are coming into force setting increasingly stringent emission limit values (ELVs) for regulating emissions from, e.g. combustion processes. For spatially heterogeneous distributions of the target species (in an otherwise physically homogenous gas sample) LOS methods have the advantage that they essentially measure the “spatially averaged” concentration along the path. The temperature dependence of the transition line) by the manufacturer or end-user in order to quantify the effects of spatial heterogeneities of the target species and the physico-chemical boundary conditions. Further the user would need the exact instrument wavelength, the (often manufacturer proprietary) spectral parametrization and information on the signal evaluation process used This severe lack of information makes it impossible to quantify or at least estimate the systematic deviations (and its variability) of the open-path LOS TDLAS instrument. Note that as the principle of the simulation method has already been validated before (Qu et al 2018, 2020), here we use it as a toolbox to quantify the BL effects
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