SO₂ emission measurement with the European standard reference method, EN 14791, and alternative methods–observations from laboratory and field studies

EN 14791 is a European Standard Reference method for the measurement of SO₂ in emissions. This standard is based on a wet-chemical method in which SO₂ present in flue gases is absorbed into an absorption solution containing hydrogen peroxide, and analyzed as sulfates after sampling. This study presents the results obtained when three portable automated measuring systems (P-AMS), based on Fourier-transform infrared (FTIR) spectroscopy, non-dispersive infrared (NDIR) and ultraviolet-fluorescence (UV) techniques, were compared to the Standard Reference Method for SO₂ (EN 14791) in order to verify whether they could be used as alternative methods (AM) to EN 14791. In the case of FTIR, the measurements were performed from hot and wet gas, without any conditioning. UV-fluorescence analyzers were equipped with dilution probes and one NDIR applied a permeation dryer, whereas the other had a chiller. Tests were carried out at concentration ranges from 0 to 200 mg/m³(n) and from 0 to 800 mg/m³(n) for testing of equivalency according to CEN/TS 14793 using a test bench. Equivalency test criteria were met for all tested P-AMS except for NDIR at the lower range. The SO₂ results measured with NDIR and the chiller were lower compared to the set-up with NDIR and permeation. This was most probably due to the chiller causing absorption of SO₂ in the condensate. Tests were also carried out at field conditions, measuring the SO₂ emissions from a boiler combusting mainly bark. The same phenomena were observed in these tests as during the test bench study, i.e. the measurement set-up with NDIR and the chiller gave the lowest results. These data demonstrated that the tested alternative methods (FTIR, UV-fluorescence, and NDIR) could be used instead of the standard reference method EN 14791, thus providing real-time calibration of automated measuring systems. It must however be emphasized that when measuring water-soluble gases, such as SO₂, the choice of suitable conditioning technique is critical in order to minimize losses of the studied component in the condensate. Implications: Portable automated measuring systems (P-AMS) provide real-time information about emissions and their concentrations, thus offering significant advantages compared to wet-chemical methods. This study presents results which can be used as a validation protocol to show that the tested P-AMS techniques (FTIR, NDIR, UV-fluorescence) could be used instead of EN 14791 (CEN 2017a) as alternative methods (AM), when paying attention to the selection of an appropriate conditioning technique.