Towards a TDLAS based optical gas standard for the absolute HCl measurements in ﬂue gases from combustion process
Creators: name: Qu, Zhechao
name: Werhahn, Olav
name: Ebert, Volker
Publisher: name: Physikalisch-Technische Bundesanstalt (PTB)
Year: 2019
Subjects: Gas metrology
TDLAS
thermal boundary layer
Hydrogen chloride (HCl)

description: European Directives are coming into force setting increasingly stringent Emission Limit Values (ELVs) for regulating emissions from e.g. combustion processes. Optical methods offer promising tools to investigate the dynamic release of molecular species from combustion environments. One of the most widely used optical technique to meet the high sensitivity, online and in situ requirements is laser-based absorption spectroscopy, in particular tunable diode laser absorption spectroscopy (TDLAS), which is frequently applied in combustion research and industry for the determination of species concentrations [1, 2]. Direct tunable diode laser absorption spectroscopy (dTDLAS) is a variant of TDLAS which combines this spectroscopic technique with a special, first principles approach to yield traceable absolute gas species concentrations. Therefore, dTDLAS can be implemented as an optical gas standard (OGS), which comes with no need for an initial or a cyclic calibration with reference gases [3, 4, 5]. Here, we present our new dTDLAS based HCl spectrometer developed within the IMPRESS2 project [6], specially designed and optimized to measure traceable HCl concentration in different matrices (high temperature, high water vapor and CO2 contents) motivated by large-scale power stations or biomass burning domestic boilers. This system is designed to serve as an OGS and thus as a traceable transfer standard to directly quantify HCl emissions, or calibrate HCl sensors or dynamically generated gas standards in the field. It employs a 3.6 µm infrared interband cascade laser (ICL) and is targeting the HCl transition lines in the fundamental band to achieve 1 ppm sensitivity already at 1 m absorption path and meet the compatibility goal set by lowered HCl ELVs in the European legislation such as the Industrial Emissions Directive (IED). Laser spectrometers for combustion applications are frequently implanted as sampling-free cross-stack open-path sensors to avoid systematic errors caused by sampling artefacts. This benefit comes at the price of a new problem, caused by spatial heterogeneities in the cross-stack configuration. In particular gas temperature variation along the (entire!) line-of-sight of the spectrometer need to be carefully considered [7]. In addition to a previous case study for CO2-detection, we have extended our work to CO-TDLAS and are working on the application of our principles to the HCl spectrometer. Key points of our research are a better understanding of the spatial temperature variations and their implications for the absolute accuracy of cross-stack dTDLAS spectrometers. We also discuss the thermal boundary layer effects on HCl concentration measurement and provide a method for quantifying the effects. This work was supported by IMPRESS2 within EMPIR. The EMPIR initiative is co-funded by the European Union’s Horizon 2020 research and innovation programme and the EMPIR Participating States.