Characterization of HCl Primary Reference Gas Standards for Emission Monitoring

Hydrogen chloride (HCl) is a key acidic pollutant emitted by waste-to-energy plants, as well as in industrial processes such as cement manufacturing and fossil fuel combustion. Due to its harmful impact on human health and the environment, strict emission limits are enforced under regulations like the European Industrial Emissions Directive (2010/75/EU), revised in 2024, which require regular monitoring of HCl. In Europe, HCl emissions are typically monitored using wet-chemical methods in accordance with EN 1911. These methods exhibit high uncertainty at concentrations below 10 mg m-³, the EU’s daily average emission limit, prompting the exploration of more accurate alternatives. Optical gas analysers, already approved by the U.S. Environmental Protection Agency, offer a promising alternative with lower measurement uncertainty. These instruments require calibration with reference gas standards of established metrological traceability. To meet this requirement, VSL employed an optical gas analyser to investigate key factors influencing the development of primary reference materials (PRMs) with HCl. The study focused on stability, adsorption behaviour, and pressure dependence of HCl PRMs. As a result, VSL successfully produced PRMs for HCl in high-pressure cylinders, achieving amount fractions as low as 5 µmol mol-1 in nitrogen (approximately 10 mg m-³) with an expanded relative uncertainty of 3% (k = 2). These PRMs are essential for ensuring accurate and reliable calibration of optical gas analysers used in emission monitoring applications.